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Mono- and binuclear complexes of group 5 metals with diimine ligands: synthesis, reactivity and prospects for application

September 2020
Russian Chemical Reviews 89(9)

September 2020
89(9)

DOI:10.1070/RCR4949

Authors:

Ya. S. Fomenko

Nikolaev Institute of Inorganic Chemistry

Artem Leonidovich Gushchin

Nikolaev Institute of Inorganic Chemistry

Published data on the coordination compounds of group 5 metals with diimine type ligands are surveyed. Methods of synthesis, structural types, reactivity and properties of these compounds are summarized and systematized. Particular attention is paid to the redox, magnetic and catalytic properties and to biological activities of these complexes, which is important for understanding the areas of their potential application. The bibliography includes 177 references.

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Russian Chemical Reviews

Mono- and binuclear complexes of group 5 metals with diimine ligands:

synthesis, reactivity and prospects for application

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1. Introduction

The coordination chemistry of group 5 metals Ð vanadium,

niobium and tantalum Ð is an actively developing area of

modern chemistry and is closely connected with other areas

such as materials science, catalysis, biology and medi-

cine.

1±4

Coordination compounds of vanadium attract

considerable attention, which is due, first of all, to their

catalytic properties in diverse organic reactions

5±14

and

biological activity (insulin-enhancing effect, antidiabetic

and antitumour activities).

15 ± 19

These aspects of vanadium

chemistry are addressed in quite a few reviews and book

chapters that appeared in the last 20 years. One of the latest

reviews published by American scientists

surveys the

papers of the period from 2008 to 2018 devoted to homoge-

neous and heterogeneous catalysts based on vanadium

compounds. On the other hand, paramagnetic vanadium(

III

and

) complexes are used as building blocks for the design

of bi- and polynuclear structures exhibiting ferro- and

antiferromagnetic exchange interactions, which is promis-

ing for the development of new magnetic materials.

21, 22

Niobium and tantalum complexes attract much less

attention than vanadium complexes, although an obvious

interest in the coordination chemistry of these metals has

appeared in recent years.

23 ± 25

This is indicated by recent

reviews concerning applications of these compounds in

catalysis

26, 27

and for activation of small molecules, includ-

ing dinitrogen,

white phosphorus

and carbon dioxide.

https://doi.org/10.1070/RCR4949

Mono- and binuclear complexes of group 5 metals with diimine

ligands: synthesis, reactivity and prospects for application

Iakov S. Fomenko, Artem L. Gushchin*

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,

prosp. Akad. Lavrentieva, 630090 Novosibirsk, Russian Federation

Contents

1. Introduction 966

2. Metal complexes with 1,4-diazabuta-1,3-dienes and acenaphthene-1,2-diimines 968

2.1. Vanadium 968

2.2. Niobium and tantalum 970

3. Mixed-ligand metal complexes containing the {(ZZ

-C

)M(DAD)} moiety (M =Nb, Ta) 976

3.1. Niobium 976

3.2. Tantalum 976

4. Complexes with 2,2-bipyridine, 1,10-phenanthroline and their derivatives 979

4.1. Vanadium 979

4.1.1. Complexes without an oxovanadium group 979

4.1.2. Oxovanadium complexes 982

4.1.3. Oxovanadium complexes VOL

982

4.1.4. Oxovanadium complexes VOL 985

4.1.5. Binuclear complexes 988

4.2. Niobium and tantalum 993

4.3. Oxoniobium complexes 995

5. Conclusion 995

Published data on the coordination compounds of group 5 metals with diimine type ligands are surveyed. Methods of

synthesis, structural types, reactivity and properties of these compounds are summarized and systematized. Particular

attention is paid to the redox, magnetic and catalytic properties and to biological activities of these complexes, which is

important for understanding the areas of their potential application.

The bibliography includes 177 references.

#2020 Uspekhi Khimii, ZIOC RAS, Russian Academy of Sciences and IOP Publishing Limited

Received 13 February 2020

I.S.Fomenko. PhD student, Junior Researcher at the Laboratory of the

Chemistry of Complex Compounds, NIIC SB RAS.

Telephone: +7(383)316 ±5845, e-mail: fomenko@niic.nsc.ru

Current research interests: chemistry of vanadium coordination and

cluster compounds

A.L.Gushchin. Doctor of Sciences, Chief Researcher, Head of the

Laboratory of the Chemistry of Complex Compounds, NIIC SB RAS.

Telephone: +7(383)316 ±5845, e-mail: gushchin@niic.nsc.ru

Current research interests: chemistry of coordination and cluster com-

pounds of transition metals, chalcogenide clusters of early transition

metals, metal complexes with redox active ligands.

Translation: Z.P.Svitanko

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

Being class A elements (Pearson's hard Lewis acids),

group 5 metals form stable coordination compounds,

mainly with O- and N-donor ligands. In this respect, note-

worthy is the pronounced proneness of vanadium to form

oxo derivatives. More than 5000 structures containing a

O group are available in the Cambridge Crystallo-

graphic Data Centre alone. Vanadium is highly prone to

form complexes with O,N-donor chelating Schiff bases.

These complexes mimic the active sites of vanadium-

dependent haloperoxidases and nitrogenases and catalyze

reactions accompanied by oxygen transfer and C7H acti-

vation.

31 ± 36

Niobium and tantalum complexes containing

N-donor ligands (amides, imides, etc.) attract attention as

precursors of NbN and TaN thin films, which are used in

various types of detectors.

25, 37 ± 43

Vanadium, niobium and tantalum complexes with

N-donor diimine ligands are also widely used. A distinctive

feature of these ligands is their redox-active nature, i.e.,they

can occur in different oxidation states within a complex.

Therefore, they are often used to stimulate the redox

reactivity of metal complexes.

44 ± 56

Group 5 metals can

also exist in different oxidation states: for vanadium,

oxidation states from +3 to +5 are rather common, but

lower oxidation states down to 71 are also possible;

niobium and tantalum usually exist in the high oxidation

state (+5), but can also exist in +3 and +4 oxidation

states. Therefore, combinations of group 5 metal ions and

redox-active diimine ligands are highly demanded for cata-

lytic processes involving electron transfer.

57 ± 72

Currently,

there are no integrating publications devoted to diimine

complexes of group 5 metals; only niobium and tantalum

complexes with 1,4-diazabuta-1,3-dienes are mentioned in a

review

of 2001, dealing with half-sandwich complexes of

early transition metals. The present review summarizes all

currently known data on the coordination compounds of

vanadium, niobium and tantalum with diimine type redox-

active ligands, particularly, 1,4-diazabuta-1,3-dienes (DAD)

and acenaphthenequinone-1,2-diimines (acenaphthene-1,2-

diimines, bian), and with heterocyclic compounds Ð

2,2

-bipyridine (bpy), 1,10-phenanthroline (phen) and their

derivatives. The information is arranged according to the

class of ligands (a-diimines and heterocyclic compounds),

the type of metal, nuclearity (mono- and binuclear com-

plexes) and to the presence of the M

O oxo group.

Polynuclear compounds are beyond the scope of this review.

The emphasis is placed on the properties (redox, magnetic,

catalytic) and biological activities of these complexes.

The following designations and abbreviations are used:

Ad Ð 1-adamantyl,

AIBN Ð azobis(isobutyronitrile),

Ar-bian Ð bis(aryl)acenaphthenequinone-1,2-diimine,

bian Ð acenaphthenequinone-1,2-diimine,

bpyÐ2,2

-bipyridine,

Cat Ð catalyst,

C.N. Ð coordination number,

Cp Ð cyclopentadienyl,

Cp* Ð pentamethylcyclopentadienyl,

cur Ð curcumin,

Cy Ð cyclohexyl,

DAD Ð 1,4-diazabuta-1,3-diene,

dbbpy Ð 4,4

-di-tert-butyl-2,2

-bipyridine,

dcbpy Ð 2,2

-bipyridine-3,4-dicarboxylic acid,

DFT Ð density functional theory,

dmbpy Ð 4,4

-dimethyl 2,2

-bipyridine,

DME Ð 1,2-dimethoxyethane,

dmphen Ð 4,7-dimethyl-1,10-phenanthroline,

dnbpy Ð 4,4

-di(non-5-yl)-2,2

-bipyridine,

dpp Ð 2,6-diisopropylphenyl,

DPPH Ð 1,1-diphenyl-2-picrylhydrazyl,

dppz Ð dipyrido[3,2-a:2

-c]phenazine,

dpq Ð dipyrido[3,2-d:2

-f]quinoxaline,

EAS Ð electronic absorption spectrum,

EG Ð ethylene glycol,

esv Ð (2R,4R,9R,11R)-3,3,10,10-tetra-

methyl-1,2,3,4,6,7,9,10,11,12-decahydro-2,4,9,11-dimetha-

nodibenzo [b,j][1,10]phenanthroline,

Fc Ð ferrocene,

GSSG Ð glutathione,

Hacac Ð acetylacetone,

Hbmp Ð bis(4-methoxyphenyl)phosphinic acid,

bta Ð benzene-1,2,4,5-tetracarboxylic acid,

ca Ð chloranilic acid,

cht Ð cis,cis-cyclohexane-1,3,5-triol,

Hdppa Ð diphenylphosphinic acid,

dtbc ± 3,5-di-tert-butylcatechol,

HER Ð hydrogen evolution reaction,

HFS Ð hyperfine structure,

fum Ð fumaric acid,

Hglyc Ð glycolic acid,

HlactÐlacticacid,

HMDSO Ð hexamethyldisiloxane,

HOMO Ð highest occupied molecular orbital,

Hpbz Ð 2-(2-pyridyl)benzimidazole,

peol Ð 2,2-bis(hydroxymethyl)propane-1,3-diol

(pentaerythritol),

Hpiv Ð pivalic acid,

HTcac Ð trichloroacetic acid,

HTCSQ Ð tetrachlorosemiquinone,

HTfac Ð trifluoroacetic acid,

thme Ð 1,1,1-tris(hydroxymethyl)ethane,

tmp Ð 1,1,1-tris(hydroxymethyl)propane,

IDA Ð iminodiacetate,

MAO Ð methylaluminoxane,

MMA Ð methyl methacrylate,

PCA Ð pyrazine-2-carboxylic acid,

PDT Ð photodynamic therapy,

phen Ð 1,10-phenanthroline,

phen Ð 4,7-diphenyl-1,10-phenanthroline,

Si-Me-CHD Ð 1-methyl-3,6-bis(trimethylsilyl)cyclo-

hexa-1,4-diene,

TBHP Ð tert-butyl hydroperoxide,

TEMPO Ð (2,2,6,6-tetramethylpiperidin-1-yl)oxyl,

TfO Ð trifluoromethanesulfonate (triflate),

UE Ð unpaired electron,

VHPO Ð vanadium-dependent haloperoxidase,

iso

Ð isotropic hyperfine coupling constant,

Ð hyperfine splitting constants,

BÐ magnetic field induction,

EÐ electrode potential,

Red

1=2

Ð half-wave reduction potential,

1=2

Ð half-wave oxidation potential,

eÐ molar absorption coefficient,

iso

Ð isotropic g-factor,

Ð g-factors along the x,y,zaxes, respectively,

I Ð nuclear spin of the metal,

eff

Ð effective magnetic moment,

Ð Bohr magneton,

Ð molar magnetic susceptibility,

yÐ Weiss constant.

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 967

2. Metal complexes with 1,4-diazabuta-1,3-dienes

and acenaphthene-1,2-diimines

Unique redox properties of diimine ligands are key param-

eters determining their high utility in coordination chem-

istry.

74 ± 85

When incorporated in metal complexes, they can

occur in three main redox states: neutral, radical anion and

dianion states.

52, 86, 87

Furthermore, some metal complexes

tend to undergo reversible intramolecular electron transfer

(redox isomerism),

77, 82, 84, 85, 88, 89

which opens up prospects

for the design of new magnetic materials based on these

complexes.

2.1. Vanadium

Vanadium complexes with ligands of this type are not

abundant. The two known vanadium(

III

) complexes,

[V(Me-Ph-DAD)(THF)Cl

](1) and [V(H-dpp-DAD)Cl

]

(2)[Me-Ph-DAD

{

is 2,3-dimethyl-1,4-diphenyl-1,4-diaza-

buta-1,3-diene, dpp is 2,6-diisopropylphenyl, H-dpp-DAD

is 1,4-bis(2,6-diisopropylphenyl)-1,4-diazabuta-1,3-diene]

were prepared from [V(THF)

]in68%and59%yields,

respectively.

In complex 2, the H-dpp-DAD ligand with

bulky substituents at the nitrogen atoms stabilizes the

coordination number (C.N.) equal to five for the metal. In

this case, the vanadium atom has a trigonal-bipyramidal

environment, instead of the octahedral one present in

complex 1. In both complexes, the DAD ligands occur in

the neutral form.

Complexes 1and 2exhibit catalytic properties towards

ethylene polymerization after methylaluminoxane (MAO)

or AlEt

Cl activation. They show relatively high activity

when used in combination with AlEt

Cl in toluene at

740 8C. The catalyst efficiency increases with temperature

rise and with increasing concentration of the aluminium co-

catalyst.

Danish researchers

reported the synthesis of octahe-

dral tris-chelates with the acenaphthene-1,2-diimine deriva-

tive Ar-bian (Ar is 3,5-dimethylphenyl): [V(Ar-bian)

](3)

and [V(Ar-bian)

](PF

)(4). Complex 3was synthesized by

the reaction of [V(THF)

] with Li(Ar-bian) taken in a

3 equiv. amount, while compound 4was formed upon the

one-electron oxidation of complex 3with FcPF

(Fc

ferrocenium) (Scheme 1).

Figure 1 shows the cyclic voltammograms of solutions

of complexes 3and 4in THF.

The complexes demonstrate

similar redox behaviours, including four quasi-reversible

one-electron processes. Nevertheless, two additional quasi-

reversible processes were detected for the latter complex in a

wider potential range, at 73.62, 72.65, 71.97, 71.07,

70.68 and +0.46 V (versus an Fc

/Fc electrode). The

observed pattern of redox transitions for compounds 3and

4is very similar to that for the analogous titanium complex

[Ti(Ar-bian)

] (Ar is 3,5-dimethylphenyl).

This means that

these redox processes are mainly ligand-centred. However,

the moderate positive shift of potentials observed for

vanadium complexes implies that the energy of the highest

occupied molecular orbital (HOMO) is lower for complexes

3and 4than for the analogous titanium complex. The

processes at 70.68 and +0.46 V refer to the [VL

]

[VL

]

and [VL

]

/[VL

]

pairs, respectively, whereas

the transitions at 71.07 71.97, 72.65 and 73.62 V

correspond to the [VL

]

/[VL

]

,[VL

]

/[VL

]

,[VL

]

[VL

]

and [VL

]

/[VL

]

pairs, respectively (L means a

ligand).

The above complexes differ in the magnetic behaviours.

Compound 3shows temperature-independent paramagnet-

ism (see curve 1in Fig. 1), while compound 4demonstrated

a wide spin transition at a temperature of *150 K. The

effective magnetic moment (m

eff

)at5Kis1.33m

is the

Bohr magneton), indicating the presence of antiferromag-

netic exchange. Upon spin transition, the m

eff

value

increases to approximately 3.41 m

. This is due to the fact

that complex 4exists as two valence tautomers depending

on temperature: at <150 K, complex 4can be described by

the formula [V

(Ar-bian

)(Ar-bian.

)

]

, while at

>150 K, it is described as [V

(Ar-bian.

)

]

. This phe-

nomenon is called valence tautomerism (or redox isomer-

ism) and has been considered in detail by Bendix and

Clark.

A recently reported series of isostructural mononuclear

vanadium(

III

) complexes 5±10 has the general formula

[V(Ar-bian)(THF)Cl

], where Ar = 2,6-Me

(5),

NCl

dpp

Structures 1, 2

[V(THF)

]

Li(Ar-bian)

(3 equiv.)

73 LiCl

FcPF

7Fc

PFÿ

3(59%)

Ar Ar

Ar =

PFÿ

4(76%)

Scheme 1

{In the acronyms of symmetrical diazabutadienes, the first substituent is

attached to carbon atoms and the second one is attached to nitrogen

atoms.

I.S.Fomenko, A.L.Gushchin

968 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

2,6-Et

(6), 2,6-Pr

(7), 3,5-(F

(8),

4-MeOC

(9), 2,6-(Ph

CH)

-4-MeOC

(10). All com-

plexes were synthesized by a procedure that included the

reaction of [V(THF)

] with the appropriate diimine. In

these compounds, the vanadium atom has a distorted

octahedral geometry formed by two diimine nitrogen

atoms, the oxygen atom of the solvent molecule and three

chlorine atoms in the mer-orientation.

Complexes 5±10

are structurally similar to the compound mer-

[V(L

)(MeCN)Cl

], where L

is 2,2

-bipyridine or 1,10-

phenanthroline (see Section 4.1.1).

These vanadium(

III

) complexes catalyze the polymer-

ization of ethylene in the presence of AlEt

Cl to give linear

polymers with high molecular mass. Complex 7with 1,2-

bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian)

proved to be a more active catalyst than other com-

pounds.

There are four oxovanadium complexes with bian type

ligands. Three similar complexes, [VO(Ar-bian)(acac)]Cl

[Ar = Ph (11), 4-MeC

(12)and4-ClC

(13); Hacac is

acetylacetone], were not structurally characterized

(Scheme 2). They catalyze the oxidation of alkenes such as

cyclohexene, cis-cyclooctene, styrene, (S)-(7)- and (R)-(+)-

pinenes and (S)-(7)- and (R)-(+)-limonenes with tert-butyl

hydroperoxide (TBHP) or H

. In the case of enantiopure

alkenes, the reaction proceeds selectively to give the corre-

sponding epoxides. Complex 13 with Ar = 4-ClC

proved to be most active in the styrene epoxidation on

treatment with H

, while its analogue 12 with

Ar = 4-MeC

showed the highest activity in the epox-

idation of cis-cyclooctene with TBHP. Kinetic data indicate

that the first step (reaction of the catalyst with the oxidant)

followsanassociativepathway,andinthecase

of complex 11, the product of this reaction

[VO(Ph-bian)(acac)(TBHP)]

was isolated. Acording to

density functional theory (DFT) calculations, the

vanadium(

) complex [VO(Ph-bian)(MeOO)]

is the most

likely active species of this process. Two competing path-

ways are possible, one involving the reaction of the alkene

with the coordinated peroxide and another one involving

the reaction of the peroxide with the coordinated alkene.

One more example of oxovanadium diimine complexes

is [VO(dpp-bian)Cl

](14), prepared recently by the reaction

of VCl

with dpp-bian in acetonitrile in air. Unlike com-

plexes 11 ±13, this compound was studied by X-ray diffrac-

tion. The coordination environment of vanadium is a

distorted square pyramid, in which two N atoms and two

Cl atoms are located in the equatorial plane, while the

terminal oxygen atom occupies the axial position.

Complex 14 is paramagnetic and has a typical EPR

spectrum consisting of eight lines, which is usual for

vanadium(

) complexes. At 300 K, the efficient magnetic

moment is 1.67 m

; this value does not change as the

temperature decreases down to 2 K, which attests to the

absence of considerable exchange interactions and to sepa-

ration of paramagnetic centres.

0.25

0.50

0.75

1.00

1.30

1.50

T/cm

Kmol

50 100 150 200 250 T/K

20 mA

747372710E/V

1+ 2+

Figure 1. Temperature dependences of w

T(w

is the molar

magnetic susceptibility) (a) and cyclic voltammograms of THF

solutions versus Fc

/Fc (b) for complexes 3(1)and4(2).

The concentration of (Bun

4N)PF

is 0.1 mol L

, the sweep rate is

200 mV s

.Eis the electrode potential.

Ar = 2,6-Me

(5, 82%),

2,6-Et

(6, 81%),

2,6-Pri

(7, 87%),

3,5-(F

(8, 76%),

4-MeOC

(9, 78%),

2,6-(Ph

CH)

-4-MeOC

(10, 82%)

Structures 5 ± 10

+HCl, MeOH

7Hacac

7 7

11 ±13

Ar = Ph (11, 88%), 4-MeC

(12, 86%), 4-ClC

(13, 90%)

Scheme 2

dpp

Structure 14

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 969

The cyclic voltammogram of complex 14 in CH

exhibits two quasi-reversible reduction steps at 70.32 and

71.05 V (versus Ag/AgCl); presumably, these reactions are

ligand-centred and can be assigned to the bian/bian.

and

bian.

/bian

pairs in view of the ability of the acenaph-

thene diimine moiety to successively accept two electrons to

give the acenaphthylene diamide dianion.

Complex 14, when used in combination with pyrazine-2-

carboxylic acid (PCA) as a co-catalyst, shows a high

catalytic activity towards the oxidation of cyclohexane

with hydrogen peroxide at moderately elevated temperature

(40±50 8C) (Scheme 3). The yields of the resulting cyclo-

hexanol and cyclohexanone are 28.2% and 8.7%, respec-

tively;

this is comparable with the catalytic activity of the

complexes [VO(L

](L

= bpy, phen; X = Cl, Br)

discussed in Section 4.1.4.

2.2. Niobium and tantalum

Niobium and tantalum are more prone to form complexes

with 1,4-diazabuta-1,3-diene ligands than vanadium. As

noted above, these ligands are redox active and can occur

in coordination compounds in neutral, radical anion and

dianion states. Each state is characterized by particular

bond lengths of the diimine moiety (Scheme 4; the bond

lengths are given in nm). For this reason, metal complexes

with DAD ligands are actively used as catalysts in various

redox reactions.

Apart from the unique redox properties, the DAD

ligands can be coordinated to the metal in a great variety

of modes, as shown by structures A±F(Fig. 2). The neutral

diimine form can be coordinated in the Z

(N,N) (C)and

(C,N) (E) modes. Structure Cexists in the complexes

[V(Me-Ph-DAD)(THF)Cl

] and [V(H-dpp-DAD)Cl

which have been considered above.

The DAD dianion

tendstooccurintheZ

(A)andZ

(s,p)(B) coordination

modes,andinrarecases,them-Z

bridging coordination

is possible (F). In the case of the radical anion form,

structure Dwith Z

(N,N) coordination mode is formed.

For early transition metal complexes, structure B, char-

acterized by additional p-interaction between the metal and

the C

C bond, is formed most frequently. This interaction

makes the DAD ligand non-planar (the C, N and metal

atoms do not lie in one plane). This gives rise to different

possible orientations for the non-planar DAD ligand, supine

(facing upwards) or prone (facing downwards). The supine

and prone orientations are mainly typical of half-sandwich

cyclopentadienyl complexes (see Section 2.3). In the case of

supine conformation, the substituents R at the nitrogen

atoms point upwards, towards the cyclopentadienyl (Cp)

ligand. Conversely, the prone conformation implies that the

groups R point away from this ligand (Fig. 3).

The niobium complex [Nb(DAD)X

](n=3, 4) is

prepared from niobium(

) halides NbX

(X = F, Cl, Br)

by ligand exchange reactions. Thr complexes

[Nb(Z

-supine-R-dpp-DAD)Cl

][R=Me(15), H (16)]

were obtained by reactions of NbCl

with substituted

DAD in 1 : 1 ratio in the presence of 1-methyl-3,6-bis(tri-

methylsilyl)cyclohexa-1,4-diene (Si-Me-CHD) in toluene

(Scheme 5). The diimine ligand is coordinated to niobium

in the dianion form to form structure B.

Complex 15 catalyzes the radical addition of CCl

various alkenes in toluene at 100 8C (Scheme 6). The

product yield and the degree of substrate conversion con-

siderably depend on the nature of substituent. In the case of

R=n-C

, these values approach 100%. High activity

(the yield and conversion of >90%) is also observed for

R=4-MeC

,4-ClC

and 3-MeC

; in other cases,

these characteristics are <85%. The reaction involving

cyclopentene has a high stereoselectivity giving only trans-

1-chloro-2-(trichloromethyl)cyclopentane in 62% yield.

(a)H

, Cat, MeCN, 40 8C, 6 h; Cat is the catalyst

OOH O

Scheme 3

*1.5

*1.3

71e

*1.4

*1.35

71e

*1.35

*1.4

Scheme 4

NRR N

NRR

ABC

NRR

MN R

DEF

Figure 2. Coordination modes of the DAD ligand to the metal (M).

Figure 3. Supine (a)andprone orientations (b) of the DAD ligand.

R = Me (15, 65%), H (16, 91%) 15,16

dpp

NbCl

dpp

(Si-Me-CHD)

SiMe

Scheme 5

I.S.Fomenko, A.L.Gushchin

970 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

Therefore, the authors suggest that the reaction occurs

within the niobium coordination sphere. For comparison,

when [Nb(DME)Cl

] (DME is 1,2-dimethoxyethane) is used

as the catalyst, this reaction affords a mixture of cis-and

trans-isomers in 1 : 5 ratio.

A mechanism has been proposed for this reaction; this

mechanism, typical of niobium complexes with DAD

ligands, is considered in relation to the reaction of styrene

with CCl

(Scheme 7). It is noteworthy that styrene can

react with initial complex 15 to give p-complex I.The

coordination of styrene stimulates the electron transfer

from the DAD

ligandtothemetal,whichchangesthe

coordination of the DAD ligand from mode Bto mode D

(see Fig. 2). Nevertheless, complex Iis not involved in the

catalytic cycle; upon the addition of CCl

, the initial

complex is regenerated. First, CCl

reacts with

[Nb(Me-dpp-DAD)Cl

] to give niobium(

) complex II

containing the DAD ligand in the radical anion form,

which is followed by reductive cleavage of the C7Cl bond

to give niobium(

) derivative III and the .CCl

radical. The

latter reacts with styrene within the coordination sphere of

complex III to form structure IV and the .CHPhCH

CCl

radical. Then the organic radical abstracts a chloride ion

from the metal centre of complex IV,whichgivesthe

reaction product and regenerates the initial complex. The

key role in the generation of organic radicals is played by

the redox-active DAD ligand, which facilitates the electron

transfer from DAD to the substrate via the metal centre.

The formation of the intermediate complexes

[Nb(Z

-R-dpp-DAD)Cl

][R=Me(17), H (18)], which

correspond to intermediates III and IV in Scheme 7, was

confirmed experimentally. Moreover, these complexes were

isolated and structurally characterized in the reactions of

NbCl

with R-dpp-DAD (R = Me, H) in the presence of

Si-Me-CHD (0.5 equiv.).

Despite the similar composition and structure, these

complexes have different EPR spectra (Fig. 4). The spec-

trum of compound 17 exhibits a signal consisting of 10 lines

as a result of hyperfine coupling with the

Nb nucleus

(nuclear spin I = 9/2). The isotropic hyperfine coupling

constant (A

iso

) equal to 105 G indicates that the unpaired

electron (UE) is not completely localized on the metal, but

has a contribution of the ligand. However, the EPR spec-

trum of complex 18 consists of 9 lines, and lower A

iso

value

(6.56 G) attests to complete localization of the unpaired

(a) CCl

, Cat, 100 8C; R = 4-MeC

, 4-ClC

, 4-F

3-MeC

2-MeC

, n-C

, Cy, Bn; Cy is cyclohexyl

CCl

Scheme 6

Me Cl

dpp

ClCCl

7ClCCl

CCl

dpp

CCl

dpp

.CCl

III

Scheme 7

R N

dpp

R = Me (17, 59%), H (18, 65%)

17,18

Structures 17, 18

3440 3460 3480 3500 3520 3540 B/G

2800 3000 3200 3400 3600 3800 4000 B/G

Figure 4. EPR spectra of complexes 17 (1)and18 (2)inthe

2800 ± 4200 and 3000 ± 4000 G ranges, respectively ( a), and of

complex 18 in the 3440 ± 3560 G range (b).

Bis the magnetic field induction.

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 971

electron on the ligand.

A similar difference in the UE

localization is also observed for tantalum complexes dis-

cussed below.

Complexes of the [Nb(R

-R

-DAD)Cl

] type also acti-

vate the C7OandC7Cl bonds. In particular, they catalyze

the chlorination of benzyl ethers with silicon tetrachloride

to give the corresponding benzyl chloride (Scheme 8). In

these reactions, SiCl

acts simultaneously as an oxygen

scavenger and the source of chloride ions. The reaction

mechanism includes two successive etherification reactions,

the first step giving one benzyl chloride molecule and silyl

ether and the second step giving the other benzyl chloride

molecule and silicon dioxide, which precipitates. Symmet-

rical and unsymmetrical substrates with a benzyl group

were tested; in the latter case, the yields of products sharply

decreased.

Analogous tantalum complexes also catalyze

this reaction, but they are less efficient.

The authors note that benzyl groups in the ether

molecule play an important role in stabilization of the

radicals that are formed in the catalytic reaction. The

reaction does not occur for substrates without these groups.

The above-mentioned complex [Nb(Z

-supine-Me-dpp-

DAD)Cl

](15) was most active in the chlorination of the

substrates shown in Fig. 5.

This complex catalyzes hydrodehalogenation of cyclic

and acyclic halohydrocarbons in the presence of PhSiH

(Scheme 9; the values in parentheses are product yields).

The putative mechanism of hydrodehalogenation is

depicted in Scheme 10. The key role in the formation of

hydrocarbon radicals is played by the redox-active DAD

ligand. The first step is coordination of halohydrocarbon to

the metal centre to give Nb

complex Vupon the transfer of

one electron from the DAD ligand to the metal. The ligand

is thus converted to the radical anion with a planar type of

coordination (mode D,seeFig.2).Thisisfollowedby

electron transfer from Nb

to halohydrocarbon to give

complex VI, cleavage of the C7X bond and formation

of the hydrocarbon radical. In the next step, the hydro-

carbon radical reacts with PhSiH

to give the product RH

and the PhSiH

.radical. In the last step, intermediate VII

disproportionates to give PhSiH

and complex 15.

As noted above, the complex [Nb(Z

-Me-dpp-DAD)Cl

]

(17), corresponding to intermediate VII, was obtained by

the reaction of NbCl

with Me-dpp-DAD in the presence of

Si-Me-CHD (Scheme 11).

The reaction of compound 17

with PhSiH

affords [Nb(Z

-supine-Me-dpp-DAD)Cl

](15)

aCl

(a) SiCl

, Cat, CDCl

= H, 4-Me, 2,4,6-Me

, 4-OMe, 2-OMe, 4-Ph,

4-Cl, 4-F, etc.; R

= H, Me, Pr

, Ph; R

= Et, All, n-C

, Ph, CPh

Scheme 8

MeMe

Me Me

(95%) (96%)

OSiMe

F F

(90%) (93%)

Figure 5. Benzyl type substrates used in the chlorination reactions

catalyzed by complex 15 (the values in parentheses are the yields of

the chlorination product).

Examples of substrates

(72%) (82%)

R = Me (87%),

(86%)

X = Cl (62%),

Br (99%)

R X aR H

(a) Cat, PhSiH

, 120 8C

Scheme 9

Ar = dpp, X = Hal

R H

SiPhH

XNb

.SiPhH

SiPhH

VII

R X

Scheme 10

(a) PhSiH

(7PhSiH

Cl); Ar = dpp

Me Me

ArAr

NbCl

Si-Me-CHD

PhMe

17 (59%)

15 (80%)

Scheme 11

I.S.Fomenko, A.L.Gushchin

972 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

in high yield (80%). Thus, PhSiH

serves as not only the

source of an H atom, but also as the reducing agent needed

for reduction of intermediate VII to complex 15.

The hydrodehalogenation reactions catalyzed by com-

plex 15 can be used to convert toxic organic compounds,

which have low reactivity by themselves, to less toxic

products under mild conditions.

Professor Scholz's research team published a series of

studies on niobium and tantalum complexes with

DAD ligands. The reaction of NbCl

with

(H-Bu

-DAD)(THF)

gave complexes with different

compositions and structures depending on the reactant

ratio (Scheme 12). When the ratio was 1 : 3 and 1 : 2.5,

binuclear complexes 19 and 20 were isolated. Compound

19 is a heterometallic complex where the Nb and Li atoms

are connected by two bridging DAD ligands. They are

coordinated to niobium in the Z

(C,N) mode (see struc-

ture E in Fig. 2) and to lithium in the Z

(N,N) mode

(structure A). The third DAD ligand completes the octahe-

dral environment of niobium, being coordinated in the

(s,p)mode(B).

Three different coordination modes of

the H-Bu

-DAD ligand are present simultaneously in binu-

clear complex 20, in particular, Z

(s,p)(B), Z

(C,N) (E)

and m-Z

(F).

The mononuclear complexes mer-[Nb(H-Bu

DAD)(THF)Cl

](21) and [Nb(H-Bu

-DAD)

Cl] (22)are

formed if the reactants are taken in 1 : 1 and 1 : 2 ratios,

respectively. In addition, the complex (Bu

)[Nb(H-Bu

DAD)Cl

](23) was isolated as a by-product in the synthesis

of compound 21. In these complexes, the ligand is coordi-

nated in a mode B(see Scheme 12).

Later, a similar procedure was applied to prepare com-

plexes fac-[Nb(H-R-DAD)(THF)Cl

][R=Pr

(24), Cy (25)]

(Scheme 12) with strong C7H X hydrogen bonds

(*2.7



A distance), retained in solution, between the chlor-

ide ligand and hydrogen at the a-carbon atom of the Pr

Cy substituent.

This gives rise to facial isomers instead of

mer-forms, which are produced for compound 21.One

more distinction is that no additional p-interaction between

the metal and the ligand is present in complexes 24 and 25,

and the Nb, C and N atoms are located nearly in the same

plane (coordination mode A) (see Scheme 12).

A series of binuclear niobium and tantalum complexes

(RNC)

(RNC

CNR)][M=Nb,R=Bu

(26), Cy

(27); M = Ta, R = Pr

(28)] were obtained in 60% or higher

yields by reactions of [M

(SMe

)

]withRNCin1:10

ratio in toluene.

65, 99

This is a rare example of mixed-valence

binuclear complexes containing M

III

and M

centres.

For [Nb

(Bu

NC)

(Bu

CNBu

)] (26), the crys-

tal structure was determined. The RNC

CNR ligand

formed upon isocyanide dimerization is coordinated to the

{NbCl

(RCN)

} moiety via two central carbon atoms and

to the {NbCl

}moietyvia nitrogen atoms. In the {NbCl

(RCN)

} moiety, niobium has a distorted pentagonal-bipyr-

amidal environment, whereas the niobium environment in

{NbCl

} is a distorted octahedron.

65, 99

The chemistry of tantalum complexes with 1,4-diaza-

1,3-butadiene ligands is similar to that of the niobium

complexes. Tantalum also tends to form compounds of

type [Ta(Z

-R

-DAD)Cl

], in which DAD ligands are

bound to the metal in amode B(see Fig. 2). They are

prepared by reactions of TaCl

with R

-R

-DAD in the

presence of excess reducing agent. If the reducing agent is

taken in deficiency, the reaction may give the complexes

[Ta(Z

-R

-DAD)Cl

] with the DAD ligand in the radical

THF Li

(Cy-DAD)

(Pr

-DAD)

THF

(THF)

NbCl

(Bu

)

72 LiCl

75 LiCl

21 (75%)

20 (43%)

24 (58%)

(n=1) +

25 (55%)

(n=1)

THF

Me Me

(n=3)

23 (5%)

Li N

19 (48%)

(n=2)

74 LiCl

(n= 2.5)

75 LiCl

N N

22 (81%)

Scheme 12

Nb Nb

NCl

CNR

RNC

RNC Cl

RNC

R=Bu

(26), Cy (27)

26,27

Structures 26, 27

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 973

anion form (mode D). In addition, half-sandwich complexes

[(Z

-C

)Ta(Z

-R

-DAD)Cl

], which are prepared

most often by displacement of chloride ligands from [(Z

)TaCl

], are often encountered in the chemistry of

tantalum (Scheme 13). For niobium, such complexes are

less frequent and will be considered in more detail in

Section 3.2.

Systematic research in this field is carried out by

Professor Mashima's group in Japan.

The authors pre-

pared a wide range of high-valent tantalum complexes

[Ta(L

)Cl

][L

= R-dpp-DAD (R = Me, H) and

dpp-bian] (Scheme 14). The reactions of Me-dpp-DAD

and H-dpp-DAD diimines with TaCl

in toluene in the

presence of 1-methyl-3,6-bis(trimethylsilyl)cyclohexa-1,4-

diene as a reducing agent give the complexes [Ta(Z

supine-R-dpp-DAD)Cl

][R=Me(29), H (30)] in high

yields. During the reaction, diimine is reduced and coordi-

nated to the metal in the ene-diamide dianion form (see

Scheme 14).

Complexes 29 and 30 activate the C7Cl bond in

reactions with alkyl halides (CHCl

, CCl

) giving com-

pounds 31 (for R = Me) or 32 and 33 (for R = H). The

formation of complex 31 is accompanied by insertion of

methyl halide into the C7N bond followed by elimination

and formation of the vinylidene group. In the case of

complexes 32 and 33, the insertion into the C7Cbondcan

also take place, resulting in the formation of amide imine

complexes. The authors suggest that the reaction starts with

the electron transfer from the ene-diamide dianion ligand to

CYCl

(Y = H, Cl), resulting in generation of {TaCl

}

complexes with the DAD ligand in the radical anion form

and also .CYCl

radicals followed by their into the C7C

bond.

In addition, [Ta(Z

-supine-R-dpp-DAD)Cl

]

[R = Me (29), H (30)] catalyze the radical polymerization

of styrene in the presence of chloroalkanes to give poly-

styrene in a yield of 26% and a polydispersity [the ratio of

N N

Cl Cl

-R

-DAD

TaCl

=H)

-R

-DAD,

Si-Me-CHD

[(Z

-C

5)TaCl

-R

-DAD

=H)

Scheme 13

7NaCl

CYCl

(Y = H, Cl)

N N

HCYCl

Cl Cl

dppdpp

N N

Me CH

dpp

ClCl Cl

R = Me, H

R = Me (29, 92%),

H(30, 86%)

31 (89%)

Y = Cl (32, 83%),

H(33, 62%)

(R = H)

{TaCl

}

R = Me (35, 35%),

H(36, 77%)

38 (45%)

NaBPh

CHCl

dpp

(R = H)

CYCl

(Y = H, Cl)

(R = Me)

7CYHCl

Ph,

CCl

N N

Cl Cl

dppdpp

CCl

AIBN, PhMe, 60 8C

,7HCMe

TaCl

, Si-Me-CHD

PhMe, 12 h NN

dpp

N N dpp

dpp

ClCl Cl

WCl

,778 8C

Co, 778 8C

(R = H) WCl

N N dpp

dpp

ClCl Cl

dpp

6

[BPh

R = Me, H

AIBN is azobis

(

isobut

ronitrile

)

N N

Cl Cl

dppdpp

CCl

34 (54%)

37 (92%)

N N dpp

dpp

ClCl Cl

(R = H)

7BPh

7PhPh

Scheme 14

I.S.Fomenko, A.L.Gushchin

974 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

weight-average to number-average molecular masses (M

)] of 1.3 in the case of compound 29. When homologue

30 was used in a similar reaction, only complex 34 was

isolated (see Scheme 14).

The reaction of {TaCl

}

(obtained by the reaction of

TaCl

with Si-Me-CHD in 2 : 1 ratio) with Me-dpp-DAD

and H-dpp-DAD affords the complexes

[Ta(Z

-R-dpp-DAD)Cl

][R=Me(35), H (36)], in which

DAD occurs in the radical anion form (mode Din Fig. 2)

(see Scheme 14). The cyclic voltammograms of these com-

plexes exhibit two reversible one-electron redox processes:

the reduction of [Ta(DAD)Cl

]/[Ta(DAD)Cl

]

[half-wave

reduction potentials E

Red

1=2

=70.47 (35), 70.39 V (36)] and

the oxidation of [Ta(DAD)Cl

]/[Ta(DAD)Cl

]

[half-wave

oxidation potentials E

1=2

=70.01 (35), 0.11 V (36)]. There-

fore, the complexes can be reduced with Cp

Co; for exam-

ple, complex 37 was prepared and then converted back to

compound 36 via the reaction with WCl

. Further oxidation

of 36 with WCl

gives complex 38, which can be reduced

back to compound 36 on treatment with Cp

Co.

Com-

plexes 35 and 36 react with NaBPh

to be converted to

compounds 29 and 30, respectively (see Scheme 14). During

this reaction, the DAD ligand accepts an electron from the

BPh

anion, generating the ene-diamide dianion form of the

ligand (coordination mode Bin Fig. 2) with the supine

orientation. In addition, complex 35 reacts with AIBN to

afford compound 31.

It is noteworthy that the use of acenaphthene-1,2-

diimine (dpp-bian) instead of DAD does not give

[Ta(dpp-bian)Cl

] under similar conditions, but

affords instead the binuclear complex

[{Ta(dpp-bian)Cl}

(m-Cl)

][TaCl

](39) (Scheme 15). This

compound contains the [{Ta(dpp-bian)Cl}

(m-Cl)

]

cation

in which each tantalum atom has a distorted octahedral

geometry, with the ligand being in the dianion form. No

formation of binuclear complexes was detected in the case

of DAD ligands. Similarly to the synthesis of [Ta(Z

-R-dpp-

DAD)Cl

][R=Me(35), H (36)], the reaction of {TaCl

}

with dpp-bian resulted in the isolation of [Ta(dpp-bian)Cl

]

(40).

The diimine complex [Ta(Ar

NC(R)

C(R)NAr

)(OAr)

]

(Ar

= 2,6-dimethylphenyl, R = Bn) (41) is obtained by

reactions of [Ta(OAr)

] with aryl isocyanide 2,6-

NC (Scheme 16).

Thesamecomplexisalso

formed upon thermolysis of the bis(iminoacyl) derivative

[Ta(OAr)

(Ar

NCR)

] in toluene.

100

The coordination environment of tantalum in complex

41 can be described as a trigonal bipyramid. The nitrogen

atoms of the ene-diamide ligand occupy one axial and one

equatorial positions. The TaN

metallacycle has a nearly

planar orientation (mode Ain Fig. 2). Niobium complexes

of this type are rather prone to nonplanar ligand coordina-

tion due to additional p-interaction between the metal and

the C

C bond.

100

The reactions of the binuclear complex [Ta

(SMe

)

]

with the isocyanides RNC (R = Bu

,Pr

and Cy), like

these reactions of niobium, are accompanied by dimeriza-

tion of isocyanide to give binuclear the complexes

[Ta

(RNC)

(m-RNC

CNR)] (42 ±44) (Scheme 17).

The mechanism of formation of the bridging pseudo-

diimine ligand in these compounds is not entirely clear. The

authors suggest that the reaction starts with the binding of

two isonitrile molecules to give a linear heterocumulene

molecule (Scheme 18).

In this molecule, the p-orbital of the C

N bond should

be perpendicular to the plane of the drawing, while the

p-orbital of the C

C bond should lie in this plane. Upon

interaction with a Lewis acid (in this case, the metal centre),

it is coordinated to the C

C bond. As this takes place, the

ligand conformation changes to enable interaction with one

more metal ion via nitrogen.

[TaCl

]

N N

ClCl Cl

dppdpp

dpp

N N dpp

dpp

39 (84%) 40 (89%)

a b

(a) TaCl

, Si-Me-CHD, PhMe, 12 h; (b) {TaCl

}

, PhMe, 15 h

Scheme 15

Ar0= 2,6-Me

,R=Bn

Ta(OAr)

Ar0

ArO

+2Ar

0NC

OAr

ArO

NAr0

OAr

Scheme 16

RNC Ta

CNR

RNC

RNC CNR

ClCl

TaTa

ClCl

S SMe

MeMe

42 ±44

R=Bu

(42), Pr

(43), Cy (44)

Scheme 17

C C NN

R R

2 RNC

Scheme 18

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 975

The formally zerovalent 17-electron complex

[Ta(H-Pr

-DAD)

](45) is formed in the reaction of TaCl

with 5 equiv. of sodium naphthalenide and 3.5 equiv. of

H-Pr

-DAD in dimethoxyethane. (In the case of niobium,

the reaction proceeds in a similar way.) Tantalum complex

45 was structurally characterized. All DAD ligands incor-

porated in the complex have a planar conformation and are

coordinated to the metal in the Z

(N,N) mode (see structure

Cin Fig. 2). This compound is paramagnetic (m

eff

=1.66 m

at 11 K) and has one unpaired electron (d

electron config-

uration).

101

The oxidation of complex 45 on treatment with AgBPh

gives rise to the diamagnetic 16-electron complex

[M(H-Pr

-DAD)

][BPh

](46)(d

electron configuration).

101

3. Mixed-ligand metal complexes containing the

{(ZZ

-C

)M(DAD)} moiety (M = Nb, Ta)

Cyclopentadienyl complexes of transition metals are exten-

sively studied in relation to their high catalytic activity, in

particular in polymerization reactions. The presence of 1,4-

diazabuta-1,3-diene as an additional ligand in these com-

plexes improves their catalytic characteristics because of

redox-active nature of the DAD ligand and the possibility

of varying the nature of substituents at nitrogen and carbon

atoms. Niobium and tantalum, unlike vanadium, tend to

form mixed-ligand half-sandwich high-valent complexes.

Usually, the structures of these complexes have a `piano

stool' geometry, which is typical of half-sandwich cyclo-

pentadienyl complexes. Furthermore, as noted above, DAD

ligands can occur in different conformations, supine and

prone (see Fig. 3).

3.1. Niobium

The complex [(Cp)Nb

(H-Bu

-DAD)Cl

](47)wasprepared

in 45% yield by the reaction of [(Cp)Nb

III

(PMe

)

]with

H-Bu

-DAD in 1 :1.5 ratio in toluene under light irradiation

with a power of 125 W (Scheme 19). The DAD moiety is

(s,p)-coordinated to niobium (see structure Bin Fig. 2)

in the supine orientation relative to the Cp ligand. The back

p-donation from the ligand to the metal is necessary to

counterbalance the electron density deficit on pentavalent

niobium.

102

One more method for the preparation of half-sandwich

niobium(

) complexes is based on reactions of an appro-

priate compound of pentavalent metal with the DAD ligand

reduced to the dianion taken as the lithium salt. For

example, the complexes [(Z

-C

)Nb(Z

,p)-supine-H-

Ar-DAD)Cl

] [Ar = 4-MeOC

;R=Me(48), H (49)]

are formed in reactions of [(Z

-C

)Nb

] with 1 equiv.

of Li

[H-(4-MeOC

)-DAD] in 32% and 18% yields,

respectively (see Scheme 19).

103

Dialkylation of compound

48 with MgBn

affords [Cp*Nb(Z

-prone-H-Ar-

DAD)(CH

Ph)

](Cp*=Z

-C

Ar = 4-MeOC

)

(50), while the orientation of the DAD ligand changes

from supine to prone. This complex is unstable in solution

and decomposes to the benzylidene complex

[Cp*Nb(

CHPh)(Z

-prone-H-Ar-DAD)] (Ar = 4-

MeOC

)(51).

103

Reactions of this type are also typical

of tantalum compounds (see Section 3.2.).

The reactions of complexes 48 and 49 with a minor

excess of magnesium buta-1,3-dienyl [Mg(C

)] afford

[(Z

-C

)Nb(Z

-H-Ar-DAD)(Z

-supine-buta-1,3-diene)]

[Ar = 4-MeOC

;R=Me(52), H (53)] (see Scheme 19).

In these compounds, coordination mode Ais imple-

mented.

103

The complex [CpNb(H-Pr

-DAD)

](54) was prepared in

63% yield by the reaction of CpNbCl

,H-Pr

-DAD ligand

and sodium amalgam in THF at 740 8C. Both DAD

ligands occur as dianions, but they are coordinated to

niobium in different modes. One ligand is coordinated in a

mode Bin the supine orientation, while the other is virtually

planar and is coordinated in a mode A.

104

3.2. Tantalum

The reaction of [(Z

-C

)Ta

](R=H,Me)with

1equiv. of Li

(H-R

-DAD)inTHFgivesthecomplexes

[(Z

-C

)Ta

-supine-H-R

-DAD)Cl

][R

=H,

=4-MeOC

(55); R

=Me, R

=4-MeOC

(56),

4-MeC

(57), 2-MeC

(58), Cy (59), Pr

(60)] and

Structure 45

R = Me (52, 76%), H (53, 48%)

52,53

Ar = 4-MeOC

=Bu

=H(47); R

= Ar:

=Me (48), H (49)

[(C

5)Nb

]

[CpNb

III

(PMe

)

]

MgBn

Ar Ph

Mg(Z

-C

)

Nb Cl

= Ar)

47 ±49

= Ar,

= Me)

50 (96%)

PhMe, 35 8C

Ar Ph

H-Bu

-DAD

Li Li

Ar Ar

Scheme 19

N N

Structure 54

I.S.Fomenko, A.L.Gushchin

976 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

[Cp*Ta

-prone-H-Bu

-DAD)Cl

](61). The complex

[Cp*Ta

III

-H-dpp-DAD)Cl

](62) results from the oxida-

tion of the binuclear tantalum(

III

) complex [Cp*TaCl

]

the presence of dpp-DAD (Scheme 20).

103, 105

In complexes 55 ±61, DAD ligands exist in the ene-

diamide dianion form and are Z

-coordinated to the metal

centre by two nitrogen atoms and two carbon atoms via

additional p-interaction (mode Bin Fig. 2), but have differ-

ent conformations. In compounds 55 ±60,theR-DAD

ligands adopt a supine conformation relative to the cyclo-

pentadienyl ligand. Conversely, the prone conformation is

observed in [Cp*Ta(H-Bu

-DAD)Cl

](61)becauseofsteric

crowding (the Bu

groups point away from Cp*). In the

complex [Cp*Ta(H-dpp-DAD)Cl

](62), rare Z

(C,N) coor-

dination mode is implemented (see structure Ein Fig. 2);

the same coordination mode was found in

[Nb

(H-Bu

-DAD)

](20).

96, 103, 105

The halogen atoms in [(Z

-C

)Ta

(H-R

-DAD)Cl

]

can be replaced by other ligands via reactions depicted in

Scheme 20. Dialkylation reactions give coordination

compounds [Cp*TaMe

-prone-H-R-DAD)] [R =

4-MeOC

(63), 2-MeC

(64), Pr

(65), Bu

(66)] and

[Cp*Ta(CH

Ph)

-prone-H-R-DAD)] [R = 4-MeOC

(67), Cy (68)], while the orientation of DAD ligands changes

from supine to prone. The replacement of one chlorine

atom by the benzyl group gives the derivative

[Cp*Ta(CH

Ph)(Z

-supine-H-(2-MeC

)-DAD)Cl] (69),

in which the supine orientation is retained. The complexes

[Cp*Ta(CH

Ph)

-prone-H-R-DAD)] decompose in a

toluene solution to give the benzylidene complexes

[Cp*Ta(

CHPh)(R-DAD), which are considered below

in more detail.

103

The butadiene complexes [(Z

-C

)Ta(Z

-H-R

DAD)(Z

-buta-1,3-diene)] [R

=H, R

= 4-MeOC

(70); R

=Me, R

= 4-MeOC

(71), 4-MeC

(72), Cy

(73)] were obtained by reactions of [(Z

-C

)Ta

(H-R

DAD)Cl

](55 ±57,59)withMg(C

)

.Inthiscase,the

coordination of the DAD ligand changes from Z

-supine to

the virtually planar Z

-(N,N) (mode A), while the butadiene

ligand is coordinated in the Z

-mode with supine orientation

(similarly to mode Bin the case of DAD ligands) (see

Scheme 20).

103, 106

The reactions between [Cp*Ta(H-Bu

-DAD)Cl

](61)

and LiC

CBu

generate the acetylide complexes

[(Z

-C

)Ta(Z

-H-R

-DAD)(C

CBu

)

][R

=Me,

=Bu

(74); R

=H, R

=Pr

(75)]. In both complexes,

the DAD ligand exists in the ene-diamide form, follows

coordination mode B, and has the prone-orientation in

complex 74 and supine orientation in complex 75 (see

Scheme 20).

105

Compound 61 reacts with LiSBu

to give the complex

[Cp*Ta(H-Bu

-DAD)(SBu

)

](76), in which DAD is

(C,N)-coordinated (mode Ein Fig. 2). Heating of deriv-

ative 76 in toluene induces C7S bond cleavage to afford

[Cp*TaS(H-Bu

-DAD)] (77), in which the DAD ligand has

,p) conformation (mode B)withtheprone orienta-

tion, like in the initial complex 61 (see Scheme 20).

105

Complexes 71 and 73 catalyze polymerization of methyl

methacrylate (MMA) in the presence of aluminium-based

co-catalyst VIII (Scheme 21). The 73 ±VIII ± MMA system

(1 : 10 : 100) showed the best results in this reaction, provid-

ing a quantitative product yield and low polydispersity

=1.4).

107

A similar polymerization reaction is also catalyzed by

the complexes [Cp*Ta(Z

-H-R-DAD)(Z

-supine-MMA)]

CBu

NTa

Cp*

CBu

NTa

Cp*

Cl Ph

Cp*

dpp

= Me)

LiSBu

, THF

Cp*

SBu

NTa

Cp*

NTa

Mg(Z

-C

)MgBn

MeMgI or MgMe

SiMe

TaCl

[(Z

-C

5)Ta(H-R

-DAD)Cl

]

[(Z

-C

5)TaCl

]

(H-R

-DAD)

[Cp*TaCl

]

H-dpp-DAD

76 (58%)

74 (66%) 75 (45%)

77 (84%)

55 ±60

NTa

Cp*

61 (54%)

70 ±73

NTa

Cp*

67,68 69

= Me)

=H,R

= 4-MeOC

(55;70, 56%); R

= Me: R

= 4-MeOC

(56;63, 81%; 67, 91%; 71, 52%), 4-MeC

(57;72, 60%),

2-MeC

(58;64, 76%; 69, 43%), Cy (59;68, 83%; 73, 59%), Pr

(60;65, 40%), Bu

(61;66, 28%)

NTa

Cp*

63 ±66

LiC CBu

Scheme 20

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 977

[R = 4-MeOC

(78), Cy (79)], which are formed upon the

reaction of [Cp*Ta(Z

-supine-MMA)Cl

]with

(R-DAD)(THF)

. The complex [Cp*Ta(Z

-supine-

MMA)Cl

] can be prepared in two different ways, as

shown in Scheme 22. Compounds AlR

(R = Me, Et) or

AlEt

Cl were used as co-catalysts. The authors note that

without a co-catalyst, the complexes do not exhibit catalytic

activity. The best results were shown for the

78 ±AlMe

± MMA system (1 : 1 : 100); the quantitative

product yield was attained in 10 min and the polydispersity

index was relatively low (M

=1.1).

108

As noted above, the bis-benzyl complexes

[Cp*Ta(CH

Ph)

-prone-DAD)] are converted to the ben-

zylidene complexes [Cp*Ta(

CHPh)(Z

-DAD)].

103

Subse-

quently, the same group of authors extended the range of

benzylidene complexes of tantalum and obtained

products [Cp*Ta(

CHPh)(Z

-prone-H-R-DAD)] (80a ±d)

(Scheme 23).

109

These complexes proved to be fairly reactive. Their

characteristic reactions are summarized in Scheme 24 in

relation to [Cp*Ta(

CHPh)(Z

-prone-H-(4-MeOC

DAD)] (80a).

109

The reaction of compound 80a with neo-

pentyl alcohol in toluene at 778 8C includes protonation

of the benzylidene group, resulting in the

formation of [Cp*Ta(OCH

)(CH

Ph)(Z

-N,N

-prone-

H-(4-MeOC

)-DAD)] (81). In complex 81,theDAD

ligand is coordinated in the Z

-N,N

mode with a partial

contribution of the Z

-prone coordination.

Similarly, compound 80 reacts with pyrrole in toluene to

give the complex [Cp*Ta(Z

-NC

)(CH

Ph)(Z

-supine-H-

(4-MeOC

)-DAD)] (82) in a moderate yield. In this case,

the DAD ligand orientation changes from prone to supine.

The reaction with CO affords [Cp*Ta(Z

-OC

CHPh)(Z

supine-H-(4-MeOC

)-DAD)] (83) in a quantitative yield,

while the reactions with Bu

CN and carbodiimides

(Pr

NPr

and CyN

NCy) give the imide

complex {Cp*Ta[

NC(Bu

)

CHPh](Z

-prone-H-

(4-MeOC

)-DAD)} (84) and the azametallacyclobutane

complexes {Cp*Ta[NR

)CHPh](Z

-supine-H-(4-

MeOC

)-DAD)} [R = Pr

(85), Cy (86)]. In addition,

compound 80 reacts with acetophenone and methyl ben-

zoate. In the former case, the binuclear complex

[Cp*Ta(m-O)(Z

-prone-H-(4-MeOC

)-DAD)]

(87)is

formed, while in the latter case, the complex

[Cp*Ta(OMe)(OC(Ph)

CHPh)(Z

-prone-H-(4-MeOC

)-DAD)]

(88)isproducedasaresultofC7O bond cleavage in the

ether (see Scheme 24).

MeO O

Cat, VIII

CMe

MeO O

VIII =

Scheme 21

OMe

[Cp*TaCl

][{Cp*TaCl

}

]

Na/Hg (2 equiv.)

72NaCl

OMe

Cp*

Ta O

MeO

Cp*

OMe

(H-R-DAD)(THF)

72 LiCl

78,79

R = 4-MeOC

(78), Cy (79)

Na/Hg (2 equiv.),

72NaCl

Scheme 22

NTa

RCp*

MgBn

THF

NTa

RPh

Cp*

NTa

Cp*

80a ±d

PhMe

R = 4-MeOC

(a, 83%), 4-MeC

(b, 75%), 4-FC

(c, 80%),

4-ClC

(d, 81%)

Scheme 23

(a)Bu

OH, PhMe, 778 8C; (b) pyrrole, PhMe, 80 8C; (c)CO

(1 atm), PhMe; (d)Bu

CN, PhMe, 778 8C; (e)R

;

(f) PhC(O)Me, PhMe, 778 8C; (g) PhCO

Me, hexane, 50 8C;

= 4-MeOC

NTa

Cp*

87 (82%)

NTa

Cp*

83 (96%)

Cp*

82 (74%)

NTa

Cp*

81 (91%)

NTa

Cp*

80a

NTa

Cp*

OMe

OPh

88 (87%)

NTa

Cp*

85 (R

=Pr

, 66%),

86 (R

= Cy, 88%)

NTa

Cp*

NTa

Cp*

84 (91%)

Scheme 24

I.S.Fomenko, A.L.Gushchin

978 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

One more half-sandwich tantalum complex [Cp*Ta(Z

H-(4-MeOC

)-DAD)(CH

CMe

)] (89) was obtained

by the reaction of [Cp*Ta(CH

CMe

Me)Cl

]with

1equiv.ofLi

[H-(4-MeOC

)-DAD] in THF at 778 8C

(Scheme 25).

110

This reaction starts with the transformation of the

homoenolate moiety into the lactone one via C7O bond

cleavage of the ether group, which is followed by chloro-

form elimination to give complex 89. This product is a rare

example of metallalactone complexes of early transition

metals. The crystal structure of [Cp*Ta(Z

-supine-H-(4-

MeOC

)-DAD)(CH

CMe

Al(C

)

)] (90), resulting

from the reaction of compound 89 with Al(C

)

,was

determined by X-ray diffraction (Scheme 26).

110

4. Complexes with 2,2-bipyridine,

1,10-phenanthroline and their derivatives

4.1. Vanadium

4.1.1. Complexes without an oxovanadium group

Octahedral vanadium tris-chelates [V(L

)

]

are 2,2

bipyridine and 1,10-phenanthroline derivatives) have been

studied more extensively than their a-diimine or acenaph-

thene-1,2-diimine analogues. The complexes [V(bpy)

]

(n=73, 71, 0, +1, +2) synthesized under supervision

of Professor Herzog in Germany have been known since the

1950s. Much later, the vanadium(

)complex

{[V(phen)

](fum)}(OH)

10 H

O(91)(H

fum is fumaric

acid) was synthesized in 23% yield by the reaction of

sodium orthovanadate, malic acid and 1,10-phenanthroline

in the presence of hydrazine under hydrothermal condi-

tions.

111

In air, this compound rapidly loses water molecules

of solvation. Under the reaction conditions, the malate

anion is dehydrated to fumarate.

In addition, several vanadium complexes with

4,4

-di-tert-butyl-2,2

-bipyridine (dbbpy) with the composi-

tion [V(dbbpy)

]

, where n=0(92), 71(93), + 2 (94), + 3

(95), have been prepared.

112

None of these compounds was

structurally characterized. The neutral complex [V(dbbpy)

]

(92) is formed upon the reaction of vanadium(

III

) chloride

with 3 equiv. of the ligand and sodium amalgam and THF

(Scheme 27).

Complex 92 is unstable and is oxidized and hydrolyzed

in air. Charged complexes were prepared from the neutral

compound. The reduction of [V(dbbpy)

] with sodium

naphthalenide (NaNaph) in THF furnishes the highly

unstable anionic complex Na[V(dbbpy)

](93). Complexes

with a charge of +2 (94)and+3(95) were prepared by the

oxidation of the neutral complex with tropylium

tetrafluoroborate and FcBF

, respectively (see Scheme 27).

Whereas [V(dbbpy)

]

is sensitive to oxygen,

[V(dbbpy)

]

proves to be stable in air. The complex

[V(dbbpy)

]

is not produced under these conditions, but

disproportionates to [V(dbbpy)

] and [V(dbbpy)

]

(Ref. 112).

According to spectral data, vanadium in [V(dbbpy)

]

(95) is in the trivalent state, i.e., its electron configuration is

. The formation of the doubly charged complex

(dbbpy)

]

can be represented as a result of metal-

centred one-electron reduction of [V

III

(dbbpy)

]

.The

next one-electron reduction is ligand-centred and yields the

compound [V

(dbbpy.

)(dbbpy)

]

with the radical anion

form of one of the three diimine ligands. Finally, the neutral

complex [V

(dbbpy.

)

(dbbpy)] contains two ligands in the

radical anion form. In the negatively charged complex

(dbbpy.

)

]

, all three ligands are radical anions,

while vanadium is still in +2 oxidation state.

112

The cyclic voltammogram of [V(dbbpy)

] in THF exhib-

its three reversible one-electron oxidation waves at 71.78,

71.66 and 70.09 V (versus the Fc

/Fc pair), which

correspond to the +1/0, +2/+1 and +3/+2 redox pairs

of vanadium (Fig. 6). In addition, a reversible one-electron

(a)Li

(H-(4-MeOC

)-DAD), THF, 778 8C;

[Ta] = Cp*Ta(H-(4-MeOC

)-DAD), Ar = 4-MeOC

[Ta]

OMe

[Ta]

7MeCl

Cp*

89 (83%)

Cp* Scheme 25

(a) Al(C

)

; Ar = 4-MeOC

Cp*

90 (66%)

aTa

Cp*

)

Scheme 26

COÿ

ÿO2C

2HO

Structure 91

VCl

+ 3 dbbpy + NaHg

[V(dbbpy)

]

92 (73%)

THF

Na[V(dbbpy)

]

[V(dbbpy)

](BF

)

[V(dbbpy)

](BF

)

FcBF

95 (89%)

NaNaph

93 (85%)

]

BFÿ

94 (91%)

Scheme 27

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 979

reduction takes place at 72.38 V (corresponds to the 0/71

pair) and is followed by irreversible reduction at 72.79 V.

This reduction is apparently associated with the formation

of the [V(dbbpy)

]

anion, unstable under these condi-

tions,

112

which is also confirmed by the absence of data on

dianionic and trianionic complexes, despite the reported

isolation of Na

[V(bpy)

]

7THF.

113

For all paramagnetic compounds Ð [V(dbbpy)

](92),

[V(dbbpy)

]

(94) and [V(dbbpy)

]

(95) Ð the temper-

ature dependence of the magnetic susceptibility was studied

in the 4 ± 290 K range. The authors detected that

[V(dbbpy)

](93) is diamagnetic. For neutral complex 92,

the magnetic moment is 1.80 m

and does not change in the

range of 50 ± 290 K. This is in line with the ground state

with the spin S = 1/2, as was shown previously for

[V(bpy)

]. The considerable decrease in the magnetic

moment at T< 50 K is caused by the weak intermolecular

antiferromagnetic interaction. The effective magnetic

moment of compound 94 amounts to 3.84 m

, which corre-

sponds to the ground state with S = 3/2. In the case of

complex 95, the effective magnetic moment is 2.80 m

in the

range from 50 to 290 K, while at 50 K, the magnetic

moment markedly decreases due to large zero-field splitting

(Fig. 7).

112

Complexes 96 and 97 described as [V(L

)

]with

4,4

-dimethyl-2,2

-bipyridine (dmbpy) and 5,6-dimethylphe-

nanthroline (Me

phen) ligands, respectively, were prepared

in a similar way. Compound 96 was studied by X-ray

diffraction. The magnetic moments of these complexes do

not change considerably with temperature and are 1.7 (96)

and 1.75 m

(97), which is indicative of the ground state

with S = 1/ 2.

The EPR spectrum of a frozen solution of compound 96

in THF at 30 K (Fig. 8) is typical of [V(L

)

](L

= bpy,

dbbpy, Me

phen). The isotropic g-factor (g

iso

)is1.984,

while |A

iso

|=78610

. The axial splitting is sub-

stantially anisotropic (hyperfine splitting constants

|*|A

|>|A

|), which is characteristic of systems in

which the unpaired electron occupies a metal-centred orbi-

tal (vanadium d

orbital). In view of the above, the

structure of [V(Me

bpy)

]

(96) is best described as

(Me

bpy.

)

(Me

bpy

)]

, where the ground state with

S = 1/2 appears due to the antiferromagnetic interaction of

three unpaired electrons of V

with two p-radical anions

(Me

bpy.

A similar situation is observed for phenan-

throline derivative 97.

The synthesis of the mononuclear vanadium(

III

)com-

plex cis-[V

III

(bpy)Cl

(acac)] (98) was reported; the complex

was prepared in 60% yield by the reduction of

(acac)

]

0.25 CH

in the presence of bpy in ace-

tonitrile. The diffusion of diethyl ether vapour into an

acetonitrile solution containing 98, together with 3,5-di-

tert-butylcatechol (H

dtbc) and Et

N, induces the oxidation

of vanadium to give [Et

NH]

(m-dtbc)]

114

Two complexes mer-[V

III

)(MeCN)Cl

][L

= bpy

(99), phen (100)] were prepared by the reaction of stoichio-

metric amounts of [V(THF)

]withbpyandphenligands

in acetonitrile under solvothermal conditions in an argon

atmosphere (Scheme 28). In the solid state, the complexes

were stable in air for a month.

115

The presence of labile chloride and nitrile ligands makes

these compounds highly attractive for the preparation of

polynuclear structures with desired bridging ligands.

2mA

0.0 70.5 71.0 71.5 72.0 E/V

Figure 6. Cyclic voltammogram of [V(dbbpy)

](92)inTHF

(0.1 M Bun

4NPF

) for sweep rates of 100 (1), 200 (2)and

400 mV s

(3).

112

050 100 150 200 250 T/K

eff

[V(dbbpy)

]

[V(dbbpy)

]

[V(dbbpy)

]

Figure 7. Temperature dependences of the effective magnetic

moment for powdered samples of complexes 92 (1), 94 (2)and

95 (3).

112

300 350 B/mT

2.4 2.2 2.0 1.8 g-Scale

Figure 8. Experimental (1) and simulated (2) EPR spectra of

[V(dmbpy)

](96) in THF at 30 K.

98 (60%)

Structure 98

I.S.Fomenko, A.L.Gushchin

980 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

Although complexes 99 and 100 are poorly soluble in most

organic solvents, this problem was solved by conducting the

reactions under solvothermal conditions.

115, 116

Polyalco-

hols and carboxylates were chosen as bridging ligands; the

reactions resulted in the synthesis of tri-, tetra- and octanu-

clear coordination compounds.

The reaction of 3 equiv. of mer-[V(bpy)(MeCN)Cl

](99)

with 2 equiv. of Na(O

CPh) in EtOH at 150 8C furnishes

product [V

(O)Cl

CPh)

(bpy)

(OEt)

](101)in36%

yield (see Scheme 28). This product consists of three V

III

atoms coordinated by bridging benzoate and ethoxide ions

and chloride ligands. The vanadium atoms are linked

through the m

-O atom lying in the {V

} plane, resulting in

the formation of a pseudo-T-shaped structure.

115

The reaction of 1 equiv. of compound 99 with 2 equiv.

of 1,1,1-tris(hydroxymethyl)ethane in MeCN at 150 8C

gives [V

(thme)

(bpy)

]

2.5 MeCN (102).

115

The double

complex salt [V(bpy)Cl

][V(bpy)

](103)isformedasa

by-product.

117

Complex 102 consists of four V

III

atoms,

which form a star-shaped {V

} plane. The central vanadium

atom is surrounded by six alkoxide groups of two thme

anions, one anion being located above the other below the

} plane; they bind the central vanadium atom with

peripheral atoms. The octahedral coordination of the

peripheral vanadium atoms is completed by the coordina-

tion of one bpy molecule and two chloride ligands located in

the cis-positions.

115

Other tetranuclear complexes Ð

(tmp)

(bpy)

](104), [V

(Hpeol)

(bpy)

](105)and

(thme)

(dbbpy)

](106) Ð with identical structure

were prepared in a similar way (see Scheme 28).

115

When two types of bridging ligands are used in the

reaction, more complex structures are formed. For example,

the reaction of 4 equiv. of mer-[V(bpy)(MeCN)Cl

](99),

2equiv. of cis,cis-cyclohexane-1,3,5-triol and 3 equiv. of

Na(O

CPh) in MeCN at 150 8C gave the compound

(Hcht)

(bpy)

]Cl

2MeCN (107) (see Scheme 28).

The structure of this compound resembles that of

(thme)

(bpy)

](102) from which one peripheral

{V(bpy)Cl

} group was removed.

115

At a different reactant

ratio [99 :H

cht : Na(O

CPh) = 1 : 2 : 2], the octanuclear

complex [V

(OH)

(cht)

CPh)

(bpy)

](108) is formed

(see Scheme 28).

115

The structure of complex 108 is a

combination of two tetrahedral moieties linked by bridging

hydroxyl and benzoate groups.

115

For [V

(O)Cl

CPh)

(bpy)

(OEt)

](101), the molar

magnetic susceptibility sharply decreases with decreasing

temperature (Fig. 9). This attests to strong antiferromag-

netic interaction between the V

III

ions. At *70 K, the plot

has a plateau at w

T=1.08 cm

Kmol

, which is indica-

tive of the ground state (S = 1) with a strong antiferromag-

netic exchange. In the case of [V

(thme)

(bpy)

](102),

the situation is markedly different. At room temperature,

T=3.38 cm

Kmol

, which corresponds to four V

III

ions with a g-factor of 1.9. It slowly decreases as the

Cl V

VOV

Cl Cl

[V(THF)

]

NCl

107 (32%)

Na(O

CPh) (2 equiv.) (L

= bpy)

101 (36%)

102,104 ±106

99,100 (99%)

(a)H

thme (2 equiv.), H

tmp (2 equiv.) or H

peol; (b)H

cht (2 equiv.),

Na(O

CPh) (3 equiv.); (c)H

cht (2 equiv.), Na(O

CPh) (2 equiv.); R

=H:

=Me (102, 56%), Et (104, 57%), CH

OH (105, 18%); L

=N N=

bpy (99), phen (100); R

=Bu

=Me (106, 11%); H

thme is 1,1,1-tris-

(hydroxymethyl)ethane, H

tmp is 1,1,1-tris(hydroxymethyl)propane,

peol is 2,2-bis(hydroxymethyl)propane-1,3-diol (pentaerythritol),

cht is cis,cis-cyclohexane-1,3,5-triol

MeCN

Cl Cl

O O

VOH

O O

108 (42%)

Scheme 28

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 981

temperature decreases to 100 K. This attests to a weak

antiferromagnetic exchange interaction. Further lowering

of the temperature induces a more pronounced decrease in

Tdown to 0.78 cm

Kmol

at 2 K (see Fig 9).

115

4.1.2. Oxovanadium complexes

Currently ever increasing attention is being paid to oxova-

nadium complexes containing the highly stable V

group. This is due, first of all, to the fact that they possess

important catalytic properties and biological activities.

In nature, vanadium-dependent haloperoxidases

(VHPO)

118 ± 120

catalyze the oxidation of halides with

hydrogen peroxide, while vanadium-dependent nitroge-

nases

121, 122

are involved in nitrogen fixation. Furthermore,

oxovanadium complexes are very important for homo- and

heterogeneous catalysis. They behave as oxidants in various

transformations: epoxidation of alkenes, aromatization of

a,b-unsaturated cyclohexanone derivatives, oxidation of

alcohols, C7C bond cleavage in glycols giving the corre-

sponding ketones, oxidation of alkanes, hydroxy esters and

amines and so on (Scheme 29).

5±12

4.1.3. Oxovanadium complexes VOL

Oxovanadium complexes described as VOL

represent a

rather abundant class of compounds with octahedral struc-

tures. In these complexes, two diimine ligands occupy four

coordination sites, the oxo ligand occupies the fifth site,

while the sixth one is occupied most often by a solvent

molecule, halide ion, etc. The ligands may be either identi-

cal, giving rise to the most frequently encountered homo-

leptic complexes VOL

, or different, giving rise to

heteroleptic complexes VOL

The following compounds of this type have been

prepared: cis-[V

O(L

)

(OH)]Y [Y = BF

= bpy

(109), dbbpy (110), dmbpy (111), phen (112); Y = ClO

;

=bpy (113); Y = SbF

= dbbpy (114)],

cis-[V

O(L

)

Cl]Cl [L

= bpy (115), dbbpy (116)],

cis-[V

O(L

)

(BF

)](BF

)[L

= dmbpy (117),

dbbpy (118)], cis-[V

O(bpy)

(Tcac)](OTf)

(119), cis-[V

O(bpy)

(SO

)] (120)and

cis-[V

O(dbbpy)

(SbF

)](SbF

)(121).

Complexes 109 ±114 were synthesized by reactions of

[VO(THF)

] with 2 equiv. of AgBF

or AgSbF

and

2 equiv. of the ligand in water. cis-[V

O(bpy)

(OH)](ClO

)

(113) was also formed upon the reaction of VOSO

with bpy

in the presence of Ba(ClO

)

. All compounds contain a

cis-{V

O(OH)} moiety. In non-aqueous media,

[VO(THF)

]wasconvertedtocis-[V

O(L

)

Cl]Cl

= bpy (115), dbbpy (116)] and cis-[V

O(L

)

(Y)]Y

[Y = BF

= dmbpy (117), dbbpy (118); Y = SbF

= dbbpy (121)].

123

A complex of a similar composi-

tion, [VO(bpy)

(Tcac)](OTf) (119), was synthesized by the

reaction of [VO(H

](OTf)

and bpy with LiTcac in

1:1:2 ratio.

124, 125

Complex 120 was formed in 56% yield

upon the reaction of VOSO

with bpy in 1 : 2 ratio.

Due to the strong trans-effect of the oxo ligand, the

diimine ligand is unsymmetrically coordinated to vanadium

in all of the complexes. For example, in complex 120,the

length of the V7Nbondinthetrans-position relative to

is 2.247



A, while the V7Nbondinthetrans-position

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

T/cm

K mol

0 50 100 150 200 250 T/K

0.5

1.0

1.5

2.0

2.5

3.0

3.5

T/cm

K mol

0 50 100 150 T/K

Figure 9. Temperature dependences of w

Tfor

(O)Cl

CPh)

(bpy)

(OEt)

](101)(a)and

(thme)

(bpy)

](102)(b).

115

RSR

O O

RSR

, ROH

RSR

ORO

OPh

R = Alk

HO O

Scheme 29

Y=BF

: N N = bpy (109, 86%), dbbpy (110, 85% ), dmbpy (111,

76% ), phen (112); Y = ClO

, N N = bpy (113, 76%); Y = SbF

N N = dbbpy (114, 78%); N N = bpy (115, 80%), dbbpy (116,

97%); X = Y = BF

: N N = dmbpy (117, 95%), dbbpy (118, 97%);

X = Tcac, Y = OTf, N N = bpy (119, 20%); X = SO

Y is absent, N N = bpy (120, 56%); X = Y = SbF

N N = dbbpy (121, 96%); HTcac is trichloroacetic acid,

TfO is trifluoromethanesulfonate (triflate)

109 ±114

115,116

117 ±121

Structures 109 ± 121

I.S.Fomenko, A.L.Gushchin

982 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

relative to HO

is 2.175



A long. A similar situation is

observed for other complexes of this type.

123 ± 125

All complexes are paramagnetic: the magnetic moments

vary in the range of 1.66 ± 1.75 m

, which is close to the spin-

only magnetic moment for one unpaired electron (1.73 m

The EPR spectra of the complexes in acetonitrile show a

signal composed of eight lines, which is typical of vanadiu-

m(IV) complexes with the d

configuration and the nuclear

spin I(

V)=7/2.

123

According to another study,

126

(dbbpy)

]BF

(122) participates in the H.transfer from organic substrates

HX with activated O7HandC7H bonds, being converted

to [V

O(dbbpy)

(OH)]BF

(110) (Scheme 30).

For instance, 2,6-Bu

-4-MeO-C

OH (ArOH) and

2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPO-H) are

converted to the corresponding ArO.and (2,2,6,6- tetrame-

thylpiperidin-1-yl)oxyl (TEMPO) radicals, hydroquinone is

converted to benzoquinone, and dihydroanthracene is

transformed into anthracene (Scheme 31). All these reac-

tions are slow and usually proceed for several hours at

ambient temperature. It was shown by DFT calculations

that the hydrogen atom transfer entails substantial struc-

tural changes, which affect the V7OandV7OH bond

lengths and the d orbital occupancy, thus giving rise to high

activation barriers of such reactions.

126

The vanadium complexes cis-[VO(L

)

Cl]Cl

= phen (123), dipyrido[3,2-d :2

-f ]quinoxaline

(dpq, 124a), dipyrido[3,2-a:2

-c]phenazine (dppz, 125)}

have been reported.

127

These compounds resemble the

complexes described above in the structure and composi-

tion, but are formed in the reaction of vanadium trichloride

with L

in methanol. During the reactions, vanadium(

III

)

is oxidized by air oxygen to give the VO

group. Accord-

ing to X-ray diffraction data, the chloride ligand in

[VO(dpq)

Cl](PF

)(124b)iscis-oriented with respect to

the oxovanadium oxygen atom.

These complexes are of interest for biomedicine. They

show activity in DNA cleavage upon irradiation in the near-

IR range (>750 nm) via the formation of hydroxyl radicals

and can bind to DNA. The compound [VO(dppz)

Cl]Cl

(125) has a pronounced photocytotoxicity in HeLa cells

under visible and ultraviolet light, but is not cytotoxic in the

dark.

127

Later, the same group of authors published a

study

128

describing, apart the above mentioned com-

pounds, a complex of the same composition with the

phenanthroline ligand. It was shown that all these com-

plexes can bind to DNA molecules.

The reactions of VOSO

with phen or dmphen in 1 : 2

ratio in water yield compounds [VO(L

)

(SO

)]

BFÿ

4+HX

122

BFÿ

110

N N = dbbpy

Scheme 30

R =H: X = Cl (124a), PF

(124b); R ±R is benzo (125 )

124a,b,125

Structures 124, 125

OMe

O O

BFÿ

HO OH

(0.5 equiv.)

NMe

(0.5 equiv.)

122

Me Me

Scheme 31

R=H(126), Me (127)

126,127

OSO

Structures 126, 127

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 983

= phen (126), 4,7-dimethyl-1,10-phenanthroline

(dmphen, 127)], which are structurally and compositionally

similar to complex 120. It was shown that these compounds

have high cytotoxic activities against human acute lympho-

blastic leukemia cells. The complex [VO(dmphen)

(SO

)]

(127) has a higher cytotoxicity than complex 126. Presum-

ably, the former can be promising for the therapy of acute

lymphoblastic leukemias.

129

All of the above-described complexes are vanadium(

)

compounds. On the other hand, the complexes

O(L

)

(Y)]

areknowntobeslowlyoxidizedbyair

oxygen to give oxo peroxo complexes with the V

O(O

)

moiety. For example, compounds [V

O(O

)(L

)

](BF

)

= bpy (128), dmbpy (129), dbbpy (130)] were prepared

by the oxidation of [V

O(L

)

(OH)](BF

)intetra-

hydrofuran (Scheme 32). The cis-dioxo compound

(O)

)

]BF

is an intermediate of this reaction. The

reaction is reversible: in the presence of hydrogen donors

(hydroquinone, TEMPO-H or triphenylphosphine), the

complexes [V

O(O

)(L

)

](BF

) are slowly reduced to

O(L

)

(OH)](BF

130

The self-oxidation of a mixture of bipyridine and oxo-

vanadium triflate V

O(OTf)

in acetonitrile in air affords

cis-[V

(bpy)

](OTf) (131). The structure of this product

comprises two asymmetric units, each consisting of a pair of

D-andL-enantiomers related by an inversion centre. The

complex has an octahedral structure and consists of the cis-



moiety with an average V7O bond length of 1.622



and two cis-coordinated bipyridine ligands. The V7N bond

lengths, which are trans to each other, are markedly shorter

(2.108



A) than those in the trans-positiontoV

(2.292



A) because of the stronger trans-effect of the oxo

group mentioned above.

131

The synthesis of analogous compounds,

(phen)

]

O(132)and

(bpy)

](H

)

O(133), with cis-orientation of

the oxo ligands has been reported.

132

The reaction of VCl

with bipyridine in water under

inert conditions gives the binuclear complex

[{V

III

(bpy)

Cl}

O]Cl

(134), in which two {V

III

(bpy)

Cl}

moieties are linked by the bridging O

ligand.

133

The

magnetic moment of this complex is 3.176 m

(at 300 K)

per vanadium centre, and it does not change with lowering

the temperature. This value is close to the spin-only mag-

netic moment (2.83 m

)forthed

configuration. The com-

plex is oxidized in air to afford [V

(bpy)

]Cl (135).

The isostructural series cis-[V

O(L

)

also

includes more rare complexes with X =F:

134

cis-[VO(L

)

F]A [A = BF

= bpy (136), dmbpy

(137), dbbpy (138), phen (139); A = ClO

= bpy

(140), dbbpy (141); A = SbF

= bpy (142)]. These

compounds are formed in reactions of VOSO

with the

corresponding ligands in aqueous solutions of HBF

or HF

and/or KF.

135

In relation to cis-[V

O(dbbpy)

F]ClO

(141), it was shown that the V7F bond is mainly ionic.

The reaction of this complex with Me

SiCl giving Me

SiF

leads to 100% conversion of the substrate within several

minutes. The second product of the reaction is the chloride

complex cis-[V

O(dbbpy)

Cl]. The reactivity of complex

141 towards fluorination reactions of various substrates was

BFÿ

128 ±130

BFÿ

(a)O

, THF, MeCN; R = H (128, 40%), Me (129, 60%),

(130, 98%)

Scheme 32

TfO

131

Structure 131

134 (80%)

2Cl

Structure 134

141 =[V

O(dppby)

ClOÿ

Br OH

Br OBu

141 Bu

141

Scheme 33

I.S.Fomenko, A.L.Gushchin

984 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

studied (Scheme 33); these reactions gave products in quan-

titative yields (based on vanadium).

135

4.1.4. Oxovanadium complexes VOL

Oxovanadium complexes containing one diimine ligand are

more abundant than VOL

. These compounds are described

by the general formula [VO(L

)L] (L

= bpy, phen and

their derivatives; L are carboxylates, triazoles, hydroxide,

alkoxides, halides, Schiff bases, etc.) and have an octahedral

structure.

A large series of mixed-ligand complexes [VO(phen)X]

containing various Schiff bases, along with bpy or phen

derivatives, have been obtained. Complexes 143 ±146 show

moderate antitumour activity against human lung cancer

and hepatoma cells.

136

Similar complexes 147 and 148

containing 4,7-diphenyl-1,10-phenanthroline (Ph

phen) as

one of the ligands also exhibit antitumour activity. These

complexes are considered as potential components for the

development of new anticancer agents.

137

Oxovanadium complexes with Schiff bases based

on vitamin B

Ð compounds 149 ±152 Ð have been

reported.

138

These complexes can be used as photoactivated

anticancer agents. Their photoirradiation generates singlet

oxygen, which induces programmed cell death (apoptosis),

while in the dark the complexes are inactive. Complexes

149 ±152 selectively target cancer cells, while their uptake

by normal cells is low. In addition, these complexes are

located in the endoplasmic reticulum, whereas other por-

phyrin-based agents for photodynamic therapy are mainly

accumulated in the cancer cell mitochondria. Thus, this

study opens up new opportunities for the photochemother-

apy targeting the endoplasmic reticulum of cancer cells

instead of the nuclear or mitochondrial DNA of a normal

cell.

The iminodiacetate (IDA)- and bipyridine-containing

complex [VO(bpy)(IDA)]

O(153) exhibits antioxidant

activity against superoxide radicals (O

.) and stable

organic radicals Ð 1,1-diphenyl-2-picrylhydrazyl (DPPH)

and 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid).

139

Complex 154, containing a vanadium-coordinated imine

ligand based on 2-hydroxy-1-naphthaldehyde apart from

phenanthroline, can form intercalates with DNA molecules

of cancer cells.

140

Similar complexes have been obtained with other

ligands, e.g., tryptamine and Schiff bases derived from

2-hydroxy-1-naphthaldehyde (compound 155 as a MeOH

solvate) and ortho-vanillin (compound 156 as a MeOH and

O solvate). Both complexes have high affinity for the

tumour DNA and also promote DNA cleavage. They could

find application as anticancer agents able to bind to the

DNA of cancer cells.

141

In addition, several complexes [VO(L

)(L)]

= bpy, phen) have been synthesized with Schiff bases

obtained by condensation of amino acids (such as glycine,

alanine, valine, leucine, isoleucine, methionine, phenylala-

N O

144

N O

MeS

148

N O

MeS

143

N O

MeS

145 O

N O

146,147

R=H(146), Ph (147)

Structures 143 ± 148

149,151 H

150

H152

NN=

(151,152)

(149,150),

Structures 149 ± 152

153

154

Structures 153, 154

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 985

nine, threonine, aspartic acid and histidine) with salicylal-

dehyde or its analogues.

68, 142

For example, in compound

157, oxovanadium is coordinated to the adduct

of 3-hydroxy-5-hydroxymethyl-2-methylpyridine-4-carbal-

dehyde and isoleucine.

In the complex [VO(bpy)(sal-

-Ser)]

O(158)

(sal-

-Ser stands for the Schiff base derived from salicylal-

dehyde and

-serine), weak ferromagnetic ordering takes

place at 12 K with the Weiss constant (y)of0.3K.The

effective magnetic moment equal to 1.67 m

virtually does

not change as temperature decreases.

142

A similar complex

[VO(phen)(sal-

-val)] (159) mimics the action of the VHPO

enzyme by efficiently catalyzing the in vitro bromination of

alkene alcohols, for example 3-butenyl alcohol, in the

presence of KBr and H

to give the corresponding

bromine-containing products at room temperature

(Scheme 34).

All complexes described above contain ONO-donor

Schiff bases; complexes with SNO-donor ligands are less

abundant. For example, mixed-ligand complexes

O(L

)L]havebeenprepared,whereH

L is S-benzyl-

b-N-(2-hydroxyphenylethylidene)dithiocarbazate (the

adduct of 2-hydroxyacetophenone and dithiocarbazate)

and L

=bpy (160) or phen (161). For compounds 160

and 161, quasi-reversible one-electron oxidations were

detected at 0.421 and 0.418 V (against a calomel electrode),

respectively.

143

The curcumin (cur) dye and its vanadium complexes

[VO(L

)(cur)Cl] [L

= phen (162), dppz (163)] show

photocytotoxic behaviour; their cytotoxicity sharply

decreases in the dark. The photocytotoxicity of the dppz

complex is comparable with that for Photofrin used for the

photodynamic therapy (PDT). These complexes are also

active in DNA cleavage in the near-IR range (*785 nm)

and are stable over a wide range of redox potentials.

Curcumin binding to the VO

group increases its hydro-

lytic stability and enhances the biological activity.

144

Complexes 164 and 165 contain a salicylaldehyde adduct

with 2-aminomethylbenzimidazole or with its 1-(anthracen-

9-ylmethyl) derivative as the NO-ligand and phenanthroline

or its analogue annulated to naphthalene substituted by the

Gly-Gly-OMe dipeptide as the NN-ligand. These complexes

N O

155

N O

MeO

156

157

Structures 155 ± 157

Cat

Scheme 34

160,161

N N = bpy (160, 73%),

phen (161, 70%)

Structures 160, 161

VO Cl

MeO OMe

HO OH

O O

162,163

N N = phen (162, 87%), dppz (163, 74%)

Structures 162, 163

MeO

NNH

164

165

Structures 164, 165

I.S.Fomenko, A.L.Gushchin

986 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

were prepared in *70% yield by the reaction of VOSO

and

BaCl

with the ligands in methanol.

145

Complex 165 was found to bind to DNA and to show

photocytotoxic activity in the near-IR range, specifically

targeting the cancer cell mitochondria. The uptake of the

complex by mitochondria induces apoptosis, which is

related to the formation of HO.radicals on exposure to

light. The activity of the complex is comparable with the

activity of Photofrin, a clinically approved PDT agent. It is

expected that these results would open up new prospects for

the development of mitochondria-targeting photochemo-

therapeutic agents based on metal complexes with potential

in vivo applications.

The possibility of binding to DNA was also studied for

[VO(phen)(Naph-

-Ser)]

1.5 MeOH (166) (Naph-

-Ser

stands for the Schiff base derived from

-serine and

2-hydroxy-1-naphthaldehyde). The results demonstrated

that this complex can bind to tumour DNA via intercalation

and weak electrostatic interactions.

146

The complexes [VO(L

)

(SO

)] [L

=phen (126),

dmphen (127)] are described in the previous Section.

129

the same study, monosubstituted analogues of these com-

plexes, that is, [VO(L

)(H

(SO

)] [L

= phen (167),

dmphen (168)], were prepared by a similar procedure. Both

the former and the latter complexes exhibited cytotoxic

activities against human acute lymphoblastic leukemia cells.

The reaction of VCl

with dbbpy in acetonitrile in air

gives [VO(dbbpy)(H

O)Cl

](169) in 95% yield. This com-

plex is paramagnetic and shows a characteristic EPR signal,

in a frozen dichloromethane solution, consisting of eight

lines with S = 1/2, g-factors along the axes

= 1.978 and g

= 1.945, and hyperfine splitting

(HFS) constants A

=6.5 mT and A

17.86 mT.

147

Complex 169 exhibits a moderate activity as a catalyst

for the oxidation of cyclooctene with tert-butyl hydroper-

oxide under mild conditions in chloroform. The major

reaction product is cyclooctene epoxide (IX), while cis-

cyclooctane-1,2-diol (X) is formed in trace amounts. In

addition, considerable amounts of trans-1,2-dichlorocy-

clooctane (XI) are formed (Scheme 35). Presumably, the

source of chlorine is chloroform, which reacts with hydro-

peroxide to give the Cl

C.radical.

The synthesis of complexes with a chiral

dihydrophenanthroline derivative, (2R,4R,9R,11R)-

3,3,10,10-tetramethyl-1,2,3,4,6,7,9,10,11,12-decahydro-2,4,-

9,11-dimethanodibenzo[b,j][1,10]phenanthroline (esv) Ð

O(esv)(H

O)Cl

](170)and[V

O(esv)(OMe)Cl

](171)

Ð has been reported. The latter complex is formed upon

oxidation of the former in methanol. Compounds 170 and

171 are rare examples of chiral oxovanadium complexes;

they can be considered as potential catalysts for oxidation

reactions in the enantioselective organic synthesis.

148

The reactions of VX

and V(THF)

(X = Cl, Br) with

bpy or phen in 1 : 1 ratio in acetonitrile under solvothermal

conditions afford the polymeric complexes [VO(L

]

=bpy;X=Cl(172), Br (173); L

= phen; X = Cl

(174), Br (175)]. Their polymeric structure is formed via

V=O_V=O contacts. They are insoluble in weakly coor-

dinating solvents. Meanwhile, in strongly coordinating

solvents, such as dimethylformamide, the polymeric struc-

ture {V=O_V=O} is destroyed to give mononuclear com-

plexes [VO(L

)(DMF)X

Complexes 172 ±175 have high catalytic activities

towards the oxidation of alkanes with hydrogen peroxide

to the corresponding alcohols and ketones under mild

conditions (Scheme 36). Complex 172 is the most active in

this series, providing the 34.7% and 4.3% yields of the

alcohol and the ketone, respectively, upon oxidation of

cyclohexane. On the basis of experimental and theoretical

data, it was shown that the reaction follows the radical

Fenton mechanism (see Scheme 36), involving the decom-

position of hydrogen peroxide to give HO.species (rate-

limiting step), which then induce the homolytic cleavage of

the R7H bond of the alkane producing H.and R.radicals.

The latter react with molecular oxygen to afford alkyl

peroxide ROO.radicals, which react with H.to be con-

166

Structures 166

N N

ClCl

169

Structure 169

TBHP,

Cat (2%)

55 8C

IX XIX

Scheme 35

X=H

O(170), MeO (171, 10%)170,171

N N

Me Cl

Structures 170, 171

N N

172 ±175

N N =bpy: X = Cl (172, 90%),

Br (173, 65%);

N N = phen: X =Cl (174, 66%),

Br (175, 43%);

Structures 172 ± 175

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 987

verted to alkyl hydroperoxide ROOH, as has been detected

experimentally.

It is assumed that under the reaction conditions, com-

plex 172 is converted to molecular complexes 172a and 172b

containing coordinated acetonitrile and water molecules,

respectively (Scheme 37). In the presence of hydrogen per-

oxide, these molecules can be replaced by H

to give

[VO(bpy)(H

)Cl

](172c). The H

coordination to V

activates the molecule and facilitates the homolysis of the

O7O bond to give intermediate 172d and HO..The

activation barrier for the generation of HO.from com-

pound 172c via the transition state TS1 is 11.8 kcal mol

The overall activation barrier for the formation of the HO.

radical with respect to the most stable complex 172a is

21.5 kcal mol

in terms of the Gibbs energy (DG

6

)and

19.6 kcal mol

in terms of the enthalpy (DH

6

). The latter

value is in good agreement with the experimentally obtained

activation energy for the oxidation of cyclohexane

with hydrogen peroxide catalyzed by complex

172 (20 2kcalmol

Fluoride derivatives are also known among the com-

plexes with the general formula VOL. The reaction of VOF

with phen in 30% HF resulted in the synthesis of

O(phen)(H

O)F

](176) in 86% yield.

149

This complex

and its bipyridine analogue [V

O(bpy)(H

O)F

](177)can

be obtained from VF

under hydrothermal conditions.

150

the same study, the complex [VO(H

]

O was fully

characterized for the first time; this is a convenient starting

compound for the synthesis of other oxofluoride com-

plexes.

150

The isostructural complexes fac-[V

O(L

]

= bpy (178), phen (179)] were prepared by the reac-

tion of VOF

with L

in dry acetonitrile in *80% yield.

The crystal structure was determined only for the complex

with phenanthroline. An attempted crystallization of

[VO(bpy)F

] was accompanied by hydrolysis giving [VO

(b-

py)F] (180).

151

Direct synthesis of [VO

)F]

=(py)

(181), phen (182)] from VOF

has been

reported.

152

4.1.5. Binuclear complexes

Along with mononuclear complexes, binuclear oxovana-

dium complexes are also abundant in vanadium chemistry.

They can be conceived as two oxovanadium groups linked

by bridging ligands. In these compounds, magnetic

exchange interactions often take place, which is of interest

to researchers working in the field of molecular magnetism.

Binuclear compounds can be obtained from mononuclear

precursors by ligand exchange reactions in solution under

ambient or solvothermal conditions. In addition, binuclear

complexes are formed upon self-assembly reactions of

simpler vanadium compounds (chlorides, oxides) in the

presence of diimine and bridging ligands.

The reaction of [VO(H

](OTf)

with bpy and lithium

trimethylacetate [Li(Piv), Hpiv is pivalic acid] in 1 : 1 : 2

ratio in acetonitrile affords the binuclear complex

[(VO)

(m-OH)(m-Piv)

(bpy)

](OTf) (183). A change in the

reactant ratio to 1 : 1 : 3 induces the formation of the

bimetallic compound [Li

(VO)

(m-Piv)

(bpy)

](184a),

which is isolated as two solvates with either MeCN or

. The replacement of Li(Piv) by lithium trifluoro-

HO.

Cat

OOH

OO.

.Scheme 36

7OOH.

TS1 (21.5)

172b (1.61)

MeCN

NCMe

172a (0.0)

Cl O

172c (9.7)

HO.

Cl OH

172d (0.0)

Cl OH

TSI =

1.705



6

Scheme 37

N N

176,177

N N = phen (176,179),

bpy (177,178)

N N

178,179

Structures 176 ± 179

183

TfO

R=Bu

(a), CF

(b)

VOLi

Li OV

184a,b

Structures 183, 184

I.S.Fomenko, A.L.Gushchin

988 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

acetate Li(Tfac) (HTfac is trifluoroacetic acid) results in the

formation of [Li

(VO)

(m-Tfac)

(bpy)

](184b), whereas a

similar reaction with lithium trichloroacetate Li(Tcac)

in ethanol produces the mononuclear complex

[VO(bpy)

(m-Tcac)](OTf) (184c).

124

The structure of the complex cation

[(VO)

(OH)(Piv)

(bpy)

]

in compound 183 is composed

of two crystallographically nonequivalent vanadium(

)

atoms coordinated by two 2,2

-bipyridine molecules and

linked to one another by two bridging carboxylates and one

hydroxyl group. The complexes [Li

(VO)

(m-Piv)

(bpy)

]

(184a)and[Li

(VO)

(m-Tfac)

(bpy)

](184b) are structurally

similar, they contain two crystallographically equivalent

vanadium(

) atoms, coordinated by two bpy molecules,

and two equivalent lithium atoms. The vanadium and

lithium atoms are connected by six bridging carboxylate

anions. The complex [Li

(VO)

(m-Piv)

(bpy)

](184a)was

found to have additional interactions between the V and Li

atoms via the oxygen atoms of the oxo ligand to form the

Li7O

V moieties, resulting in some elongation of the

O bond.

124

The molar magnetic susceptibility of

[(VO)

(m-OH)(m-Piv)

(bpy)

](OTf) (183) (Fig. 10) at room

temperature is 0.47 cm

Kmol

, which is markedly lower

than the value (0.75 cm

Kmol

) expected for two iso-

lated V

ions with S = 1/2. This value decreases with

decreasing temperature to reach 0.04 cm

Kmol

at2K,

which is attributable to intramolecular antiferromagnetic

interactions. The magnetic behaviours of

[Li

(VO)

(m-Piv)

(bpy)

](184a)and

[Li

(VO)

(m-Tfac)

(bpy)

](184b) are similar (see Fig. 10),

but they markedly differ from that of complex 183.Thew

value in the temperature range of 8 ± 300 K, equal to

0.73 ± 0.75 cm

Kmol

for [Li

(VO)

(m-Piv)

(bpy)

], and

in the temperature range of 18 ± 300 K, equal to

0.70 ± 0.74 cm

Kmol

for [Li

(VO)

(m-Tfac)

(bpy)

], vir-

tually does not depend on the temperature and is close to

the spin-only value of 0.75 cm

Kmol

for two isolated

ions. On further cooling, w

Tdecreases to 0.61 and

0.28 cm

Kmol

for compounds 184a,b, respectively, as a

result of antiferromagnetic exchange.

124

One more heterometallic binuclear complex

[Ru

(NO)(NO

)

(OAc)(O)V

O(dbbpy)] (185) was prepared

by the reaction of [VO(dbbpy)(H

O)Cl

](169)with

[Ru(NO)(NO

)

(OH)]

O in acetonitrile

(Scheme 38). The bridging acetate ion is presumably formed

upon the hydrolysis of acetonitrile. In this reaction, vana-

dium(

) is oxidized to vanadium(

When crystals of this complex are irradiated with light in

the 365 ± 405 nm range at 10 K, the ground state Ru7NO

isomerizes into the metastable state Ru7ON (stable up to

140 K); this is accompanied by the formation of free NO,

which does not take place for the initial ruthenium nitroso

complex. Laser irradiation of complex 185 in acetonitrile at

445 nm induces release of NO and formation of the para-

magnetic Ru

III

complex with g

= 2.627, g

=2.20.

Binuclear vanadium complexes with oxidized N-(2-sul-

fanylpropionyl)glycine (H

mpgSS, XII) containing the S7S

bond, structurally resembling the oxidized form of gluta-

thione (GSSG, XIII), have been synthesized.

The successive addition of bpy or phen, H

mpgSS and

NaOAc to a solution of VOSO

in methanol in the presence

of oxygen results in the formation of [(V

(m-OH)(m-

OAc)(m-H

mpgSS)

)

]

xMeOH

O[L

= bpy

(186), phen (187)] containing the oxidized dimer

mpgSS

]. When glutathione is used instead

of H

mpgSS, the complex [(V

(m-OH)(m-

OAc)(OAc)

(bpy)

](188) is obtained.

0.2

0.4

0.6

T/cm

K mol

0 50 100 150 200 250 T/K

Figure 10. Temperature dependences of w

Tfor compounds 183

(1), 184a (2)and184b (3).

124

169

MeCN, 60 8C

NRu

185 (99%)

Scheme 38

NCO

XIII (GSSG)

NCO

XII (H

mpgSS)

Structures XII, XIII

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 989

Lactic acid (Hlact) in the S- and/or R-form can also

serve as a bridging ligand capable of linking two oxovana-

dium moieties. A series of chiral and achiral complexes

(bpy)

(S-lact)

](189), [V

(phen)

(S-lact)

](190),

(bpy)

(R-lact)(S-lact)] (191)and[V

(phen)

(R-

lact)(S-lact)] (192) were prepared by the reaction of V

bpy or phen and excess lactic acid under solvothermal

conditions. Complexes are structurally similar and contain

a bidentate lactate anion, which is coordinated to vanadium

via alkoxide and carboxylate groups; the other coordination

sites of vanadium are occupied by diimine nitrogen and a

terminal oxygen atom. Compounds 189 ±192 are paramag-

netic, which is consistent with the presence of two V

centres with d

configuration, but the hyperfine coupling is

not observed in their EPR spectra because of pronounced

spin ± spin coupling between the unpaired electrons within

the molecule.

153

The binuclear complex [(VO)

-O)

-SeO

)(phen)

]

(193) with a bridging selenite ligand was prepared from

,SeO

,Et

NOH and phen under hydrothermal con-

ditions. When bpy was taken instead of phen, the polymeric

chain complex [VO(bpy)(m-SeO

)]

(194) was obtained.

The reaction of VOSO

with dbbpy in methanol yields

the binuclear complex [(VO)

-SO

)

(dbbpy)

(CH

OH)

]

4MeOH (195) (Scheme 39). It is a centrosymmetric dimer

consisting of two {VO(dbbpy)(MeOH)} moieties linked by

two bridging sulfate anions. The subsequent reaction of this

complex with sodium pivalate [Na(Piv)] results in the

vanadium oxidation giving the complex

[(VO)

-O)

-SO

)(dbbpy)

]

2MeCN (196), which

does not contain pivalate ligands, but has one bridging

sulfate anion and two bridging O

ligands. The structure

of compound 196 is similar to the structure of complex 193

described above.

Complex 195 dissociates in ethanol giving the mono-

nuclear form [VO(m

-SO

)(dbbpy)(MeOH)]. Meanwhile, in

dichloromethane, the binuclear structure is retained, and

the EPR spectrum exhibits an HFS consisting of 15 lines

due to spin exchange interactions between the two para-

magnetic moieties [I(

V) = 7/2]. The effective magnetic

moment decreases from 2.62 to 0.57 m

with decreasing

temperature. This behaviour is characteristic of binuclear

compounds of this type and is related to antiferromagnetic

exchange interaction within the molecule.

154

Organic molecules with several donor heteroatoms can

also serve as bridging ligands for the formation of binuclear

structures. For example, the binuclear complex with a

histidine bridge, [V

(

-His)

(bpy)

]

[(EG)

(bpy)] (197)

(His is histidine, EG is ethylene glycol) was obtained in

65% yield under hydrothermal conditions. In this com-

pound, two VO

moieties are linked via the pyrazole

nitrogen atom on one side of the histidine molecule and

via the amino group nitrogen and carboxyl oxygen on the

other side of the His molecule.

155

The temperature dependence of w

T(Fig. 11) for com-

plex 197 has a minimum at *180 K; further lowering of the

temperature induces a sharp increase in w

T. The depend-

ence of (w

)

on temperature in the range of 4 ± 180 K

N N = bpy (186, 71%),

phen (187, 66%)

188 (40%)

Me Me

NH HN

186,187

Structures 186 ± 188

N N = bpy (189, 28%), phen (190, 59%)

189,190

VOV

193

Structures 189, 190, 193

VOSO

, MeOH

HMe

195 (74%)

196 (42%)

OOO

(a) Na(Piv) (4 equiv.), EtOH, MeCN

Scheme 39

197

Structure 197

I.S.Fomenko, A.L.Gushchin

990 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

obeys the Curie ± Weiss law with the Curie constant of

0.257 cm

Kmol

and the Weiss constant of 6.8 K.

Thus, ferromagnetic exchange interaction takes place

between the V

ions. At 300 K, the calculated effective

magnetic moment is 1.67 m

for each vanadium atom.

155

The bridging ligands can be represented by molybdate

and tungstate ions. For example, the salt

[VO(dbbpy)

](SO

) reacts with Na

or Na

MoO

water to give the binuclear complexes [V

(m-MeO)

(m-

)

(dbbpy)

](198)or[V

(m-MeO)

(m-

MoO

)

(dbbpy)

](199). The complexes are structurally

similar, each containing two {VO(dbbpy)}

moieties

bridged by m-MO

2ÿ

anions (M = W, Mo) and methoxy

groups. Notably, the complexes exhibit catalytic activities

towards the oxidation of 1-phenylethanol with hydrogen

peroxide in water (Scheme 40), and the use of safe and

cheap H

and H

O reagents complies with green chem-

istry principles. With the use of W catalyst 198, the yield of

the ketone is 63%, while in the case of Mo analogue 199,the

yield is 28%.

156

The reaction of V

(SO

)

, benzene-1,2,4,5-tetracarbox-

ylic acid (H

bta) and bpy or phen in methanol

under solvothermal conditions was used to obtain

the complexes [(VO)

(bpy)

(bta)(H

](200)and

[(VO)

(phen)

(bta)(H

](201) with the bridging bta

anion.

157

These complexes can be considered as model compounds

for studying the mechanism of action of vanadium-depend-

ent haloperoxidases that catalyze the oxidative bromination

of organic substrates. Compounds 200 and 201 catalyze the

oxidative bromination of phenol red (XIV) with hydrogen

peroxide in the presence of KBr and phosphate buffered

saline to give bromophenol blue (XV) (Scheme 41).

157

A redox-active ligand, which promotes magnetic

exchange interactions due to the ability to accept or release

electrons, can act as a bridging ligand in oxovanadium

complexes. An example of such ligands is the chloranilic

acid anion ca

ca is chloranilic acid), which can be

converted to the semiquinone form. The reaction of

O(bpy)(TCSQ)Cl] (HTCSQ is tetrachlorosemiquinone)

with Bu

NOH gives the binuclear complex

[Cl(bpy)V

O(ca)OV

(bpy)Cl] (202)in*40% yield. In

this reaction, the semiquinone radical anion (TCSQ.

)ina

basic medium (Bu

NOH) undergoes nucleophilic substitu-

tion of HO

by Cl

to give the ca

anion, which links two

oxovanadium moieties into a binuclear structure.

158

A binuclear complex of a similar structure

[VO(dbbpy)Cl(ca)Cl(dbbpy)VO] (203) was synthesized by

the direct method based on the reaction of

[VO(dbbpy)(H

O)Cl

](169) with chloranilic acid in aceto-

nitrile in the presence of Et

N (79% yield). This method

doubles the product yield. In both complexes, strong anti-

ferromagnetic exchange exists between the two V

para-

magnetic centres. The molar magnetic susceptibility

monotonically decreases with decreasing temperature. At

300 K, the w

Tvalues are 0.71 and 0.72 cm

Kmol

for

complexes 202 and 203, while at temperatures below 5 K,

0 50 100 150 200 250 T/K

200

400

600

800

1000

0.26

0.28

0.30

0.32

0.34

0.36

0.38

T/cm

K mol

wÿ1

m/cm

mol

Figure 11. Temperature dependences of (w

)

(1)andw

T(2)for

complex 197.

155

M=W(198, 40%), Mo (199, 37%)

O O

Structures 198, 199

Ph Me

OH H

,198 or 199,H

90 8C, 17 h Ph Me

Scheme 40

200,201

N N = bpy (200, 53%),

phen (201, 63%)

Structures 200, 201

O O

BrBr

O O

OH a

XIV XV

(a)H

, KBr, 200 or 201

Scheme 41

202

Structure 202

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 991

the w

Tvalues are 0.01 and 0.02 cm

Kmol

, respectively

(Fig. 12).

158

The cyclic voltammogram of a solid sample of

[VO(dbbpy)Cl(ca)Cl(dbbpy)VO] (203) measured using a

paste electrode shows a quasi-reversible reduction process

with the half-wave potential E

1/2

=70.83 V (versus

Ag/AgCl), which presumably corresponds to the V

III

pair.

There are reported vanadyl complexes with bridging

phosphinate ligands with the general formula

[(VO)

)

(X)

](NO

)[L

=bpy; X=dppa (204),

bmp (205); L

= phen; X = dppa (206), bmp (207)]

[Hdppa is diphenylphosphinic acid, Hbmp is bis(4-meth-

oxyphenyl)phosphinic acid]. The EPR spectra of these

compounds show a signal consisting of 15 lines, which is

typical of binuclear V

complexes. Only weak exchange

interactions were detected. Complexes 204 ±207 catalyze the

oxidation of cinnamic alcohol with molecular oxygen

(Scheme 42). The highest activity was found for

[(VO)

(bpy)

(bmp)

](NO

)(205); the yield of the resulting

aldehydes was 61% within 7 h.

159

The {O

V7O7V

O} unit is a fairly widespread

structural motif of binuclear oxovanadium complexes. A

series of mixed-valence binuclear vanadium(

)com-

plexes with glycolate and lactate anions (Hglyc is glycolic

acid) have been reported.

160

The complexes [V

(phe-

Y]Cl [Y = glyc (208), S-lact (209)], (NH

)

(b-

py)

]Cl [Y = glyc (210), R,S-lact (211)] and

(NH

)

(bpy)

(S-lact)

](212) contain the mixed-

valence moiety {O

7O7V

O}. In the complexes

(phen)

(glyc)]Cl (208)and[V

(phen)

(S-lact)]Cl

(209), the {VO(phen)

}moietyislinkedtothe

{VO(phen)(glyc)} and {VO(phen)(S-lact)} moieties, respec-

tively, via the m

-O

bridge. In complexes 210 ±212,

the{VO(bpy)(glyc)} and {VO(bpy)(S-lact)} moieties are

linked to the symmetrical binuclear complex also by the

-O

bridge.

These complexes show similar redox behaviours. Both

oxidation (in the 0.4 ± 0.5 V range corresponding to the

transition) and reduction (at potentials

below 70.2 V corresponding to the V

tran-

sition) waves were detected in the cyclic voltammograms.

These compounds catalyze the oxidation of thioanisole to

afford methyl phenyl sulfoxide and methyl phenyl sulfone

(Scheme 43); with moderate amounts of the catalyst and

relatively short reaction time, 98% yield of methyl phenyl

sulfoxide and *50% conversion of the substrate can be

attained.

160

In the heterometallic compound

[Cu

(bpy)

)]

[(VO)

O(bpy)

)

]

10 H

O(213),

the anionic part is a mixed-valence binuclear vanadium

complex structurally similar to the above-described

(bpy)

(L)

]

anions (L = glyc, lact) (210 ±212).

161

100 K, the w

Tvalue for complex 213 is 1.53 cm

Kmol

This value is consistent with the presence of one vanadiu-

) centre and two isolated copper(

) centres. The w

value increases with decreasing temperature, which is due to

ferromagnetic exchange between Cu

ions.

161

An attempted crystallization of the complex

O(bpy)Cl

](214) obtained by the reaction of VOCl

with bpy in 1 : 1 ratio in dichloromethane affords crystals

of the binuclear complex [(VO)

(m-O)(bpy)

](215). This

complex consists of two V

O groups bridged by an oxygen

atom.

162

0 50 100 150 200 250 T,K

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.002

0.004

0.006

0.008

0.010

0.012

T/cm

K mol

/cm

mol

Figure 12. Temperature dependences of w

T(1)andw

(2)for

[Cl(bpy)V

O(ca)OV

(bpy)Cl] (202).

158

Ph OH +O

Cat (5 mol.%), MeCN, 7 h Ph H

Scheme 42

208

210

Structures 208, 210

Me +

(a)H

, Cat, H

O, EtOH

Scheme 43

215

Structure 215

I.S.Fomenko, A.L.Gushchin

992 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

Binuclear vanadium(

III

) complexes are also known. The

symmetrical complexes [V

III

(m-O)(bpy)

]Cl

(216)

133

and [V

III

(m-O)(phen)

]Cl

(217) have been reported.

The bpy-containing complex is formed by the reaction of

VCl

with bpy in 1 : 3 ratio in water, while the phen complex

is formed in ethanol. In both compounds, weak antiferro-

magnetic interactions take place at room temperature with

eff

=3.24 and 3.18 m

, respectively. Complex 216 is oxi-

dized by air oxygen to give [V

O(bpy)

Cl](ClO

)(218)and

(bpy)

]Cl (219).

Complex 217 is able to bind to DNA molecules and

initiate their cleavage.

Hydrolysis of (Et

N)[V

III

(dmbpy)(SPh)

] furnishes the

binuclear complex [V

O(dmbpy)

(SPh)

](220), in which

two vanadium atoms are bridged by the m-O

and

m-SPh

groups. The V7Vdistanceis2.579(3)



A, which is

consistent with the V

III

single bond length. In this

complex, strong antiferromagnetic interactions take place.

The effective magnetic moment is 1.35 m

(for two V

III

centres) at 300 K and gradually decreases to 0.095 m

5 K, which is much lower than expected for two d

ions (for

one d

ion, m

eff

=2.83 m

163

Fluorine-containing binuclear vanadium complexes are

very rare. For example, the compounds

[(V

(m-O)

(bpy)

](221)and[(V

(m-O)

(phen)

]

(222) have been reported.

59, 152

4.2. Niobium and tantalum

Niobium and tantalum complexes with bpy and phen type

ligands are much less typical than vanadium complexes.

Professor Christe's research team

obtained the

unique niobium and tantalum azide complexes

[M(L

)

][M(N

)

][M=Nb;L

= bpy (223), phen

(224); M = Ta; L

= bpy (225), phen (226)], which were

formed from pentafluorides (MF

)via the exchange reac-

tion with Me

SiN

in the presence of the appropriate ligand

in acetonitrile (Scheme 44). The yields of tantalum com-

plexes 225 and 226 were quantitative (99%).

In the [M(L

)

]

cation, the metal atom has a

distorted square-antiprism geometry, that in the [M(N

)

]

anion has a distorted octahedral environment.

The formation of these complexes is stepwise and, in the

case of bpy, it was possible to isolate the structurally similar

intermediate compounds [M(bpy)

][M(N

)

][M=Nb

(227), Ta (228)] (Scheme 45).

The square-antiprismatic complexes [Nb

)

]

=bpy,X=NCSe(229); L

=dmbpy; X=NCS

(230), NCSe (231)] are formed upon the reactions of

[NbX

] with 2 equiv. of an NN ligand in acetonitrile

(Scheme 46) or upon the reduction of K[NbX

]inthe

presence of bpy.

164, 165

Also, the structurally similar com-

plex [Nb(bpy)

]Br (232) was obtained by the reaction of

NbBr

with bpy at 0 8C. It is of interest that an analogous

reaction of NbCl

with bpy gives the complex

[Nb(bpy)Cl

]

MeCN (233)withC.N.7.

166

The reactions of K[TaX

] with 1 equiv. of bpy produce

the complexes [Ta

(bpy)X

][X=NCS(234), NCSe (235)],

which are structurally similar to complex 233

(Scheme 47).

164

In the presence of bipyridine, TaCl

reacts with O-meth-

ylhydroxylamine or bis(trimethylsilyl)-O-methylhydroxyl-

amine to give mer-[(Ta

NOMe)(bpy)Cl

](236)in70%

yield.

167

2Cl

216,217

N N = bpy (216),

phen (217, 80%)

Structures 216, 217

VOV

220

221,222

N N = bpy (221),

phen (222)

Structures 220 ± 222

bpy (2 equiv.)

phen (2 equiv.)

223,225

M = Nb (223,224), Ta (225,226)

MeCN

2MF

+10Me

SiN

710 Me

SiF

[M(phen)

)

][M(N

)

]

224,226

Scheme 44

2MF

+6Me

SiN

76Me

SiF

bpy (2 equiv.)

M = Nb (227), Ta (228, 99%)

[M(bpy)

][M(N

)

]

227,228

Scheme 45

[NbX

]

(2 equiv.), MeCN Nb

229 ±231

N N = bpy (229), dmbpy (230,231); X = NCSe (229,231),

NCS (230)

Scheme 46

X = NCS (234), NCSe (235)

K[TaX

]bpy, MeCN [Ta

(bpy)X

]

234,235

Scheme 47

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 993

The reaction of [Ta

(OC(Ad)C

-3,5)

](Adis

1-adamantyl) with excess pyridine in the presence of H

leads unexpectedly to dimerization of two pyridine rings in

the bpy ligand, giving rise to [TaMe(OCH(Ad)C

3,5)

(py)(bpy)] (237)in45%yield.

168

The niobium(

) complexes [Nb(L

)(Cp)

](PF

)

= bpy (238), phen (239)] were synthesized in *50%

yields by the reactions of [Nb(Cp)

], the appropriate

ligand and TlPF

in 1 : 1 : 3 ratio in dibromoethane. The

crystal structure of compound 238 was determined; the

coordination environment of niobium is pseudo-tetrahedral

and includes two Z

-Cp ligands coordinated in an angular

fashion and one bpy molecule.

169

The cyclic voltammogram of complex 238 in acetonitrile

exhibits two reversible redox peaks at E

1/2

=70.305 and

71.350 V (against a calomel electrode). The first peak

refers to the one-electron reduction of Nb

to Nb

III

, while

the second one is associated with the bpy reduction. In the

case of compound 239, a reversible process attributable to

the reduction of Nb

takes place at 70.310 V, whereas the

reduction of phen at 71.340 V is fully irreversible.

169

Niobium and tantalum tris-chelates [M(L

)

]

(M = Nb, Ta; L

= bpy, dbbpy; n=0, 71) have been

reported. The complex [Nb(bpy)

](240) was prepared by

the reaction of NbCl

with bpy in 1 : 3 ratio in cold THF in

the presence of Li as a reducing agent. The anionic complex

Li[Nb(bpy)

](241) was obtained in 54% yield by the

reaction of NbCl

withbpyin1:3ratioinTHF,which

also proceeds in the presence of Li.

170

The neutral complex

[Nb(dbbpy)

](242) is formed when NbCl

is allowed to

react with dbbpy in 1 : 3 ratio in THF with sodium amalgam

as a reducing agent (65% yield). The complex

Na[Ta(dbbpy)

](243)isobtainedinasimilarway.The

addition of an equimolar amount of tropylium tetrafluor-

oborate in THF to compound 243 results in the formation

of the neutral complex [Ta(dbbpy)

](244) in 92% yield.

171

For tantalum, the complexes [Ta(bpy)

](245)and

Na[Ta(bpy)

](246) are also known.

172

Anionic complexes 241,243 and 246 are diamagnetic

(S = 0), but they have different electronic structures.

According to DFT calculations and X-ray diffraction data,

compound 241 is best described by the formula

[Nb

(bpy.)(bpy

)

]

, whereas in the case of complex

246, all bpy ligands are equivalent and occur in the reduced

dianion form: [Ta

(bpy

)

]

. Like in the case of vana-

dium, neutral complexes 242 and 244 are paramagnetic

(S = 1/2). The EPR spectrum of niobium complex 242

tends to show the HFS, namely, a signal consisting of 10

lines with the parameters g

iso

=1.985 and A

iso

=88.3 G,

which implies strong interaction of the electron with the

Nb nucleus (I = 9/2). The structure of 242 is best

described as [Nb

(bpy

)

(bpy

)]

. Conversely, the EPR

spectrum of a similar tantalum complex 244 exhibits no

HFS; instead, a broad single peak is present. Presumably, in

compound 244, tantalum is in the pentavalent state, while

one bipyridine ligand is in the radical anion state, which is

described as [Ta

(dbbpy.)(dbbpy

)

]

171

Binuclear niobium and tantalum complexes are scarcely

known. Only one publication

173

considers binuclear sulfide

clusters {Nb

} with one or two coordinated diimine

molecules. These are mixed-ligand niobium(

) complexes:

(Et

[Nb

)(NCS)

][L

=bpy(247), dcbpy (248),

dnbpy (249), phen (250)], (Bu

[Nb

(Hpbz)(NCS)

]

(251)and[Nb

)

(NCS)

][L

= phen (252), Hpbz

(253)]. They are formed in the reactions of

[Nb

(NCS)

](R=Et,Bu

) with appropriate

ligands in acetonitrile. Even in the presence of excess ligand,

the isothiocyanate is not completely displaced; only one or

two diimine molecules can be introduced into the com-

plexes.

All of these complexes are intensely coloured [the molar

absorption coefficient (e)&10

Lmol

]. Their elec-

tronic absorption spectra (EAS) show broad bands in the

visible range, which apparently refer to the charge transfer

from the {Nb

} cluster core to the diimine ligand. It was

shown that compounds 247 ±249 are capable of reversible

absorption saturation and show nonlinear light transmis-

sion, i.e., they possess optical limiting properties and can be

considered as potential broad-band optical limiters. In

addition, P25 type TiO

nanoparticles modified by

(Et

[Nb

(NCS)

(bpy)] (247) demonstrate a moderate

photocatalytic activity towards the hydrogen evolution

reaction (HER) on irradiation at >410 nm. The photo-

catalyst activity is 12 mol of H

per gram of the catalyst per

hour (or *15 H

molecules per formula unit per hour). The

results are modest in comparison with efficient systems

based on molybdenum disulfide, but demonstrate the con-

ceptual applicability of niobium sulfides as the catalysts for

hydrogen evolution.

173

N N

TaCl Cl

Cl N

OMe

236

237

N N

TaO O

NMe

Structures 236, 237

238,239

N N = bpy (238), phen (239)

Structures 238, 239

N N = bpy (247, 99%), dcbpy (248, 80%), dnbpy (249, 55%),

phen (250, 30%), hpbz (251, 30%); R = Et (247 ±250), Bu

(251);

N N = phen (252, 40%), Hpbz (253, 24%);

dcbpy is 2,20-bipyridine-3,4-dicarboxylic acid,

dnbpy is 4,40-di(non-5-yl)-2,20-bipyridine,

Hbpz is 2-(2-pyridyl)benzimidazole

NCS

SCN

NbNb

SCN

N2R

247 ±251

252,253

NCS

SCN

NbNb

Structures 247 ± 253

I.S.Fomenko, A.L.Gushchin

994 Russ. Chem. Rev., 2020, 89 (9) 966 ± 998

4.3. Oxoniobium complexes

Generally, niobium is less prone to form oxo derivatives

than vanadium. Professor Christe's research team

174

pre-

pared the complex [NbO(bpy)(N

)

](254) in 94% yield by

the reaction of [NbO(N

)

] with bpy in acetonitrile.

A series of oxoniobium(

) complexes [NbO(L

]

[X = Cl; L

= bpy (255), phen (256); X = Br; L

= bpy

(257), phen (258)] have been synthesized in high yields by

the reaction of NbX

, hexamethyldisiloxane (HMDSO) and

the appropriate ligand in 1 : 1: 1 ratio. These complexes are

structurally similar to [NbO(bpy)(N

)

174

If no HMDSO is

added, the reaction gives the complexes

[Nb(L

)

][NbF

][L

= bpy (259), phen (260)], which

presumably have the same structure as azide complexes

223 ±226. Analogous tantalum complexes

[Ta(L

)

][TaF

][L

= bpy (261), phen (262)] have

also been obtained (Scheme 48), but oxo derivatives

[TaO(L

] are not formed under these conditions.

175

The oxoniobium(

) sulfide complexes

[Et

N][NbO(S

)

(bpy)] (263), [Et

N][NbO(CS

)(S

)(bpy)]

(264)and[Et

N][NbO(S

(CO

Me)

)(S

)(bpy)] (265)have

been prepared. Complex 263 is formed upon the reaction of

NbO(SPh)

with elemental sulfur in the presence of bpy in

DMF. Complexes 264 and 265 are formed upon the reaction

of compound 263 with carbon disulfide and dimethyl

acetylenedicarboxylate, respectively. In all of these com-

pounds, the niobium atom has a distorted pentagonal-

bipyramidal geometry.

176

The crystal structures of the peroxoniobium complex

K[Nb(O

)

(phen)]

O(266) and its hydrogen peroxide

adduct K[Nb(O

)

(phen)]

(267) have been

reported.

177

The niobium atom has a dodecahedral ligand

environment.

5. Conclusion

The present review demonstrates the great diversity of

group 5 metal complexes with diimine ligands of various

nature, which possess practically valuable properties,

including redox ability, magnetic behaviour and catalytic

and biological activities. Vanadium tends to form com-

plexes with 2,2

-bipyridine and 1,10-phenanthroline ligands

and their derivatives. The known vanadium compounds

include both mononuclear complexes with and without

oxovanadium group and binuclear structures with different

bridging groups. Most often, vanadium is in the tetra- or

pentavalent state and, more rarely, in the trivalent state.

Oxovanadium derivatives are most abundant, because of

high oxophilicity of vanadium. The coordination environ-

ment of vanadium in these compounds is, most often, an

octahedron and, more rarely, a square pyramid. In binu-

clear vanadium(

III

) complexes, ferro- and antiferromag-

netic exchange interactions are often present, which attracts

the attention of researchers working in the field of molec-

ular magnetism. On the other hand, high-valent vanadium

diimine complexes are of interest as catalysts for various

oxidation reactions of organic substrates (alkenes, alkanes,

organic sulfides, alcohols, etc.). They are considered as

model compounds for processes catalyzed by vanadium-

containing enzymes. In addition, oxovanadium complexes

containing diimine and Schiff base ligands can form inter-

calates with DNA molecules. In some cases, these com-

plexes selectively bind to the tumour DNA. This fact, in

combination with their phototoxicity, makes these com-

pounds promising for the development of anticancer drugs

capable of competing with clinically approved drugs.

Niobium and tantalum, unlike vanadium, tend to form

mononuclear complexes with redox-active a-diimines, in

particular, 1,4-diazabuta-1,3-dienes, in which the DAD

ligands occur, most often, in the reduced form. The ability

of the DAD ligand to exist in different oxidation states and

the ease of its modification account for extensive applica-

tion of metal complexes based on these ligands as catalysts

in various redox transformations, including polymerization

of alkenes, hydrodehalogenation of halohydrocarbons,

chlorination of benzyl ethers, radical addition of CCl

alkenes and so on.

Thus, the chemistry of vanadium, niobium and tantalum

coordination compounds with diimine ligands is a rapidly

developing area both for fundamental research and for

solving applied problems, including the development of

new materials, the search for new biologically active com-

pounds and catalysts.

N N

254

Structure 254

X = Cl: L

= bpy (255, 52%), phen (256, 61%); X = Br: L

= bpy

(257), phen (256,*50%); M = Nb: N N = bpy (259, 86%),

phen (260, 89%); X =Ta: N N = bpy (261, 86%), phen (262,83%)

[NbO(MeCN)X

][NbO(L

]

HMDSO

MeCN

(M = Nb) 255 ±258

MeCN M

259 ±262

Scheme 48

N N

263 (51%)

[Et

N N

[Et

264 (63%)

[Et

O O

N N

266

N N

MeO

265 (65%)

Structures 263 ± 266

I.S.Fomenko, A.L.Gushchin

Russ. Chem. Rev., 2020, 89 (9) 966 ± 998 995

The review was written with the financial support of the

Russian Foundation for Basic Research (Project No. 18-03-

00155).

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Iridium Complexes with BIAN-Type Ligands: Synthesis, Structure and Redox Chemistry

Article

Full-text available

Jun 2023
INT J MOL SCI

A series of iridium complexes with bis(diisopropylphenyl)iminoacenaphtene (dpp-bian) ligands, [Ir(cod)(dpp-bian)Cl] (1), [Ir(cod)(NO)(dpp-bian)](BF4)2 (2) and [Ir(cod)(dpp-bian)](BF4) (3), were prepared and characterized by spectroscopic techniques, elemental analysis, X-ray diffraction analysis and cyclic voltammetry (CV). The structures of 1–3 feature a square planar backbone consisting of two C = C π-bonds of 1,5-cyclooctadiene (cod) and two nitrogen atoms of dpp-bian supplemented with a chloride ion (for 1) or a NO group (for 2) to complete a square-pyramidal geometry. In the nitrosyl complex 2, the Ir-N-O group has a bent geometry (the angle is 125°). The CV data for 1 and 3 show two reversible waves between 0 and -1.6 V (vs. Ag/AgCl). Reversible oxidation was also found at E1/2 = 0.60 V for 1. Magnetochemical measurements for 2 in a range from 1.77 to 300 K revealed an increase in the magnetic moment with increasing temperature up to 1.2 μB (at 300 K). Nitrosyl complex 2 is unstable in solution and loses its NO group to yield [Ir(cod)(dpp-bian)](BF4) (3). A paramagnetic complex, [Ir(cod)(dpp-bian)](BF4)2 (4), was also detected in the solution of 2 as a result of its decomposition. The EPR spectrum of 4 in CH2Cl2 is described by the spin Hamiltonian Ĥ = gβHŜ with S = 1/2 and gxx = gyy = 2.393 and gzz = 1.88, which are characteristic of the low-spin 5d7-Ir(II) state. DFT calculations were carried out in order to rationalize the experimental results.

Mononuclear Oxidovanadium(IV) Complexes with BIAN Ligands: Synthesis and Catalytic Activity in the Oxidation of Hydrocarbons and Alcohols with Peroxides

Article

Full-text available

Oct 2022

Reactions of VCl3 with 1,2-Bis[(4-methylphenyl)imino]acenaphthene (4-Me-C6H4-bian) or 1,2-Bis[(2-methylphenyl)imino]acenaphthene (2-Me-C6H4-bian) in air lead to the formation of [VOCl2(R-bian)(H2O)] (R = 4-Me-C6H4 (1), 2-Me-C6H4 (2)). Thes complexes were characterized by IR and EPR spectroscopy as well as elemental analysis. Complexes 1 and 2 have high catalytic activity in the oxidation of hydrocarbons with hydrogen peroxide and alcohols with tert-butyl hydroperoxide in acetonitrile at 50 °С. The product yields are up to 40% for cyclohexane. Of particular importance is the addition of 2-pyrazinecarboxylic acid (PCA) as a co-catalyst. Oxidation proceeds mainly with the participation of free hydroxyl radicals, as evidenced by taking into account the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, as well as the dependence of the reaction rate on the initial concentration of cyclohexane.

Copper(II) complexes with BIAN-type ligands: Synthesis and catalytic activity in oxidation of hydrocarbons and alcohols

Article

Feb 2024
INORG CHIM ACTA

A New Cobalt(II)-Lithium(I) Carboxylate Complex with N,O-Donor Mono-iminoacenaphthenone Ligand: Synthesis, Structure and Magnetic Behavior

Article

Oct 2023

A new carboxylate complex [Co2Li2(Piv)6(L)2]·CH3CN (1) (Piv is the anion of pivalic acid) have been synthesized and structurally characterized. According to ac magnetic measurements, compound 1 exhibits a field induced slow magnetic relaxation which is approximated by a combination of the Raman and direct mechanisms. These data are confirmed by ab initio calculations indicating easy-plane anisotropy of Co(II) ions in the complex.

Novel Copper(II) Complexes with BIAN Ligands: Synthesis, Structure and Catalytic Properties of the Oxidation of Isopropylbenzene

Article

Full-text available

May 2023

Two new isomeric complexes [CuBr2(R-bian)] (R = 4-Me-Ph (1), 2-Me-Ph (2)) were obtained by reacting copper(II) bromide with 1,2-bis[(2-methylphenyl)imino]acenaphthene ligands and characterized. The crystal structure of 2 was determined by X-ray diffraction analysis. The copper atom has a distorted square-planar environment; the ω angle between the CuN2 and CuBr2 planes is 37.004°. The calculated ω parameters for optimized structures 1 and 2 were 76.002° and 43.949°, indicating significant deviations from the ideal tetrahedral and square-plane geometries, respectively. Molecules 2 form dimers due to non-covalent Cu···Br contacts, which were analyzed by DFT calculations. The complexes were also characterized by cyclic voltammetry and UV-Vis spectroscopy. A quasi-reversible Cu(II)/Cu(I) redox event with E1/2 potentials of 0.81 and 0.66 V (vs. SHE) was found for 1 and 2, respectively. The electronic absorption spectra showed the presence of Cu(I) species as a result of the partial reduction of the complexes in the acetonitrile solution. Both complexes were tested as homogenous catalysts for the oxidation of isopropylbenzene (IPB) in acetonitrile at low temperatures. Differences in the mechanism of the catalytic reaction and the composition of the reaction products depending on the oxidizing ability of the catalyst were revealed.

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

Article

Full-text available

Dec 2022

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L 1 ) with the general formula [NbL 1 (N t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H 3 L 1 ][Cl] and subsequent reaction with [Nb(N t Bu)Py 2 Cl 3 ]. The pyridine ligand in 1-Py can be removed by the addition of B(C6F5)3 as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL 1 (N t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC8, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including 1H-, 13C{1H}-, and 1H-15N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Trapping of Ag+ into a Perfect Six-Coordinated Environment: Structural Analysis, Quantum Chemical Calculations and Electrochemistry

Article

Full-text available

Oct 2022
MOLECULES

Self-assembly of (Bu4N)4[β-Mo8O26], AgNO3, and 2-bis[(2,6-diisopropylphenyl)-imino]acenaphthene (dpp-bian) in DMF solution resulted in the (Bu4N)2[β-{Ag(dpp-bian)}2Mo8O26] (1) complex. The complex was characterized by single crystal X-ray diffraction (SCXRD), X-ray powder diffraction (XRPD), diffuse reflectance (DR), infrared spectroscopy (IR), and elemental analysis. Comprehensive SCXRD studies of the crystal structure show the presence of Ag+ in an uncommon coordination environment without a clear preference for Ag-N over Ag-O bonding. Quantum chemical calculations were performed to qualify the nature of the Ag-N/Ag-O interactions and to assign the electronic transitions observed in the UV–Vis absorption spectra. The electrochemical behavior of the complex combines POM and redox ligand signatures. Complex 1 demonstrates catalytic activity in the electrochemical reduction of CO2.

Redox-Active Tin(IV) Complexes Based on Sterically Hindered Catecholate Ligands

Article

Jan 2024
RUSS J COORD CHEM+

Gold( i ) complexes with redox active BIAN and MIAN ligands: synthesis, structure and electrochemistry

Article

Jan 2023

This article reports on the synthesis and structure of novel cationic gold( i ) complexes with BIAN/MIAN ligands, displaying redox activity and the antichelate effect.

Mono(arylhydrazino)acenaphthenones as a platform for the design of NIR chromophores based on Pd(II)-BIAN complexes

Article

Jan 2023

A new synthetic procedure for mono(arylhydrazino)acenaphthenones Ph-mhan (1) and 2-tol-mhan (2), based on the reaction of acenaphthenquinone with an arylhydrazonium salt, has been developed. These compounds were used further to...

Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

Article

Full-text available

Dec 2019

Ligands are innocent when they allow oxidation states of the central atoms to be defined. A noninnocent (or redox) ligand is a ligand in a metal complex where the oxidation state is not clear. Dioxygen can be a noninnocent species, since it exists in two oxidation states, i.e., superoxide (O2−) and peroxide (O22−). This review is devoted to oxidations of C–H compounds (saturated and aromatic hydrocarbons) and alcohols with peroxides (hydrogen peroxide, tert-butyl hydroperoxide) catalyzed by complexes of transition and nontransition metals containing innocent and noninnocent ligands. In many cases, the oxidation is induced by hydroxyl radicals. The mechanisms of the formation of hydroxyl radicals from H2O2 under the action of transition (iron, copper, vanadium, rhenium, etc.) and nontransition (aluminum, gallium, bismuth, etc.) metal ions are discussed. It has been demonstrated that the participation of the second hydrogen peroxide molecule leads to the rapture of O–O bond, and, as a result, to the facilitation of hydroxyl radical generation. The oxidation of alkanes induced by hydroxyl radicals leads to the formation of relatively unstable alkyl hydroperoxides. The data on regioselectivity in alkane oxidation allowed us to identify an oxidizing species generated in the decomposition of hydrogen peroxide: (hydroxyl radical or another species). The values of the ratio-of-rate constants of the interaction between an oxidizing species and solvent acetonitrile or alkane gives either the kinetic support for the nature of the oxidizing species or establishes the mechanism of the induction of oxidation catalyzed by a concrete compound. In the case of a bulky catalyst molecule, the ratio of hydroxyl radical attack rates upon the acetonitrile molecule and alkane becomes higher. This can be expanded if we assume that the reactions of hydroxyl radicals occur in a cavity inside a voluminous catalyst molecule, where the ratio of the local concentrations of acetonitrile and alkane is higher than in the whole reaction volume. The works of the authors of this review in this field are described in more detail herein.

New Oxidovanadium(IV) Complexes with 2,2′-bipyridine and 1,10-phenathroline Ligands: Synthesis, Structure and High Catalytic Activity in Oxidations of Alkanes and Alcohols with Peroxides

Article

Full-text available

Feb 2019

Reactions of [VCl3(thf)3] or VBr3 with 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio in air under solventothermal conditions has afforded polymeric oxidovanadium(IV) four complexes 1–4 of a general formula [VO(L)X2]n (L = bpy, phen and X = Cl, Br). Monomeric complex [VO(DMF)(phen)Br2] (4a) has been obtained by the treatment of compound 4 with DMF. The complexes were characterized by IR spectroscopy and elemental analysis. The crystal structures of 3 and 4a were determined by an X-ray diffraction (XRD) analysis. The {VOBr2(bpy)} fragments in 3 form infinite chains due to the V = O…V interactions. The vanadium atom has a distorted octahedral coordination environment. Complexes 1–4 have been tested as catalysts in the homogeneous oxidation of alkanes (to produce corresponding alkyl hydroperoxides which can be easily reduced to alcohols by PPh3) and alcohols (to corresponding ketones) with H2O2 or tert-butyl hydroperoxide in MeCN. Compound 1 exhibited the highest activity. The mechanism of alkane oxidation was established using experimental selectivity and kinetic data and theoretical DFT calculations. The mechanism is of the Fenton type involving the generation of HO• radicals.

An oxidovanadium(IV) complex with 4,4′-di-tert-butyl-2,2′-bipyridine ligand: Synthesis, structure and catalyzed cyclooctene epoxidation

Article

Dec 2019
POLYHEDRON

The interaction of vanadium(III) chloride (VCl3) with 4,4′-di-tert-butyl-2,2′-bipyridine (dbbpy) in air resulted in the monomeric oxidovanadium(IV) complex [VOCl2(dbbpy)(H2O)] (1) in high yield. The complex was characterized by IR and EPR spectroscopies, by elemental analysis, and by single crystal X-ray diffraction analysis. The vanadium atom has a distorted octahedral coordination environment. The EPR spectrum of 1 in CH2Cl2 demonstrates an eight-line signal typical of vanadium(IV) with a d¹ electronic configuration. Complex 1 exhibits catalytic activity in the cyclooctene oxidation with tert-butyl hydroperoxide (TBHP) in CHCl3. Detailed EPR, NMR and GC–MS studies of the reaction revealed a few mechanistic details and the nature of by-products that are generated by involvement of the chloroform solvent.

Synthesis and ε-Caprolactone Polymerization Activity of Electron-Deficient Gallium and Aluminum Species Containing a Charged Redox-Active dpp-Bian Ligand

Article

Nov 2019

The synthesis of electron-deficient gallium- and aluminum-centered species containing a redox-active dpp-Bian ligand (dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) is described. The reaction of digallane [(dpp-Bian)Ga-Ga(dpp-Bian)] with [Ph3C][PF6] or AgPF6 resulted in polyoxidized species [(dpp-Bian)GaF2]2 (1), [(dpp-Bian)H2][PF6] (2), and [(dpp-Bian)GaF(O2PF2)]2 (3). The reaction of digallane with B(C6F5)3 led to electron-deficient gallylene [(dpp-Bian)GaB(C6F5)3] 4 of a dpp-Bian radical anion. The soft oxidation of digallane with tosyl cyanide gave the trinuclear cationic species [(dpp-Bian)Ga(Tos)3Ga(Tos)3Ga(dpp-Bian)][Ga(CN)4] (5) containing dpp-Bian radical anions. The reaction of [(dpp-Bian)AlEt2] with 1 equiv of [Ph3C][B(C6F5)4] resulted in the cationic complex [(dpp-Bian)AlEt2][B(C6F5)4] (6) of neutral dpp-Bian, while the treatment of [(dpp-Bian)AlEt(Et2O)] with 1 equiv of [Ph3C][B(C6F5)4] resulted in the compound [(dpp-Bian)AlEt(Et2O)][B(C6F5)4] (7) of a dpp-Bian radical anion. The reaction of diethylaluminum derivative [(dpp-Bian)AlEt2] with 1 equiv of B(C6F5)3 gave the cationic complex [{(dpp-Bian)AlEt}2F][EtB(C6F5)3] (8) containing radical-anion dpp-Bian ligands. The paramagnetic compounds 1, 2, 4, 5, 7, and 8 were characterized by electron paramagnetic resonance spectroscopy, and the diamagnetic complex 6 was characterized by NMR spectroscopy. The molecular structures of 1-6 and 8 were established by single-crystal X-ray diffraction analysis. Compounds 4 and 6-8 were found to be active initiators for immortal ring-opening polymerization of ε-caprolactone.

α-Diimine-Niobium Complex-Catalyzed Deoxychlorination of Benzyl Ethers with Silicon Tetrachloride

Article

Sep 2019

α-Diimine niobium complexes serve as catalysts for deoxygenation of benzyl ethers by silicon tetrachloride (SiCl4) to cleanly give two equivalents of the corresponding benzyl chlorides, where SiCl4 has the dual function of oxygen scavenger and chloride source with the formation of a silyl ether or silica as the only byproduct. The reaction mechanism has two successive trans-etherification steps that are mediated by the niobium catalyst, first forming one equivalent of benzyl chloride along with the corresponding silyl ether intermediate that undergoes the same reaction pathway to give the second equivalent of benzyl chloride and silyl ether.

Hydrodehalogenation of Alkyl Halides Catalyzed by a Trichloroniobium Complex with a Redox Active α-Diimine Ligand

Article

May 2019

A high-valent d ⁰ niobium(V) complex, (α-diimine)NbCl 3 ( 1 ), bearing a dianionic redox-active α-diimine ligand served as a catalyst for a hydrodehalogenation reaction of alkyl halides in the presence of PhSiH 3 . During...

Chelate rings of different size with non-innocent ligands

Article

May 2019

Wolfgang Kaim

Redox-active unsaturated chelate ligands can be realised with different ring sizes of the resulting metallacycles. An overview is presented, starting from an exposition of non-innocent behaviour and chelate effects. A systematic approach is used to describe the most familiar situation, the metal complexes of 1,4-hetero-1,3-dienes in established forms (e.g. o-quinone, α-dithiolene, α-diimine ligands) and with less common combinations of O, S, and N heteroatoms. The different steric and electronic conditions in six-membered chelate ring systems derived from the β-diketonate structure will be discussed by example of substituted and π extended ligands, including 9-oxidophenalenyl, formazanate, and anions derived from indigo or 9,10-anthraquinone. Four-membered chelate rings existing in at least two ligand-based oxidation states are available through steric and electronic stabilisation in amidinate or triazenide complexes. Three-membered and seven-membered chelate ring situations are being discussed briefly as further alternatives.

Recent developments in penta-, hexa- and heptadentate Schiff base ligands and their metal complexes

Article

Jun 2019
COORDIN CHEM REV

Novel ligand platforms that promote reactivity are of long standing and continued interest in coordination chemistry, with Schiff base ligands and their metal complexes representing one of the most versatile and long standing topics of interest. The synthesis and structure of polydentate Schiff bases and their metal complexes is fascinating, because it reveals a great richness of structural, physico-chemical and catalytic properties. Given the simplicity and ease of access to multidentate Schiff bases and their metal complexes, investigation of such compounds is essential to precise and understand structure–property relationships in order to optimize and improve their use in a wide range of fields, including catalysis, supramolecular chemistry, magnetism, electrochemistry, nanoscience, energy materials, and biological applications. This review highlights the recent developments of pentadentate, hexadentate, heptadentate and macrocyclic Schiff base ligands containing various donor sets made of different combinations of N, O, S or P donor atoms and their metal complexes (essentially mononuclear), as well as presenting synthetic methods and interesting structures of complexes formed by first-to-third row transition metals (from group 4–12), main group elements, lanthanides and actinides. This review is divided into three main sections, each of them corresponding to one type of denticity of the Schiff base under consideration. Each category is described with representative examples according to a periodic order, and emphasis is given to the coordination aspects. Their catalytic, magnetic and biological properties are also outlined. This review that contains 359 references should act as a source of information to researchers interested to work in this domain and stimulate further investigation in this fascinating area of Schiff base coordination compounds.

Critical assessment of the electrocatalytic activity of vanadium and niobium nitrides for the reduction of dinitrogen to ammonia

Article

Feb 2019

Previous theoretical work has predicted vanadium and niobium nitrides to be catalytically active towards the electrochemical reduction of dinitrogen to ammonia and inactive for the hydrogen evolution reaction. The present experimental study investigates the electrocatalytic activity of vanadium(III) nitride, niobium(III) nitride and Nb4N5 for the nitrogen reduction reaction in aqueous electrolyte solutions of different pH under ambient conditions using a robust testing protocol and thoroughly controlled experimental conditions to exclude any contamination with adventitious sources of ammonia and nitrogen oxides. VN and Nb4N5 (supported on carbon cloth) were synthesised by annealing of hydrothermally produced hydroxide precursors in NH3 atmosphere at 600-1100 °C; NbN was obtained by solid state reac-tion between niobium(V) chloride and urea at 1000 °C. Comprehensive testing of the materials under a wide range of conditions unam-biguously demonstrates their inability to catalyse the electrosynthesis of ammonia from dinitrogen, as well as the propensity of VN (synthesised at 600 °C) and Nb4N5 to release lattice nitride in a non-catalytic process, which leads to the formation of ammonia under reductive conditions. Thus, polycrystalline nitrides of vanadium and niobium are concluded to be catalytically inactive towards the ammonia electrosynthesis from N2 dissolved in water. The present work additionally emphasises the compulsory requirement for the implementation of reliable testing and analysis procedures for the assessment of the catalytic properties of materials for the nitrogen reduction reaction.

Vanadium Chlorides Supported by BIAN (BIAN = bis(arylimo)-acenaphthene) Ligands: Synthesis, Characterization, and Catalysis on Ethylene Polymerization

Article

Feb 2019
POLYHEDRON

Several vanadium chlorides bearing bis(arylimino)-acenaphthene (BIAN) ligands, (2,6-Me2C6H3-BIAN)V(THF)Cl3 (1), (2,6-Et2C6H3-BIAN)V(THF)Cl3 (2), (2,6-ⁱPr2C6H3-BIAN)V(THF)Cl3 (3), [3,5-(CF3)2C6H3-BIAN]V(THF)Cl3 (4), [4-OMe-C6H4-BIAN]V(THF)Cl3 (5), and [2,6-(Ph2CH)2-4-OMeC6H2-BIAN]V. (THF)Cl3 (6) were synthesized by direct reaction of VCl3(THF)3 with corresponding BIAN ligands. All these complexes were characterized by elemental analyses, and FT-IR spectroscopy. The molecular structures of 1, 2, and 4 were identified by X-ray crystallography, in which the six-coordinated vanadium metal centers were in distorted octahedral geometry with the oxygen atom of the coordinated THF, two nitrogen atoms of the diimine ligand and one chlorine atom in the same plane. When activated with AlEt2Cl these vanadium complexes showed high catalytic activities for ethylene polymerization affording linear polyethylene with high molecular weight. However, when MAO was used as co-catalyst, ultra high molecular-weight polymers were obtained albeit with decreased activity.

Mono- and binuclear complexes of group 5 metals with diimine ligands: synthesis, reactivity and prospects for application

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