No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Synthesis and Characterization of Pyridine Amide Cation Radical Complexes of Iron: Stabilization Due to Coordination with Low-Spin Iron(III) Center †
Abstract
We reported the synthesis and characterization of peptide complexes of low-spin iron(III) [Fe(bpb)(py)2][ClO4] (1) and Na[Fe(bpb)(CN)2] (2) [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene; py = pyridine], where iron is coordinated to four nitrogen donors in the equatorial plane with two amide nitrogen anions and two pyridine nitrogen donors (Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J. Chem. Soc., Dalton Trans. 1993, 2451). Chemical oxidation of 2 and a new low-spin iron(III) complex Na[Fe(Me6bpb)(CN)2].H2O (4) [synthesized from a new iron(III) complex [Fe(Me6bpb)(py)2][ClO4] (3) (S = 1/2)] [H2Me6bpb = 1,2-bis(3,5-dimethylpyridine2-carboxamido)-4,5-dimethylbenzene) by (NH4)2Ce(NO3)6 afforded isolation of two novel complexes [Fe(bpb)-(CN)2] (5) and [Fe(Me6bpb)(CN)2].H2O (6). All the complexes have been characterized by physicochemical techniques. While 1-4 are brown/green, 5 and 6 are violet/bluish violet. The collective evidence from infrared, electronic, Mössbauer, and 1H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that 5 and 6 are low-spin iron(III) ligand cation radical complexes rather than iron(IV) complexes. Cyclic voltammetric studies on isolated oxidized complexes 5 and 6 display identical behavior (a metal-centered reduction and a ligand-centered oxidation) to that observed for complexes 2 and 4, respectively. The Mössbauer data for 6 are almost identical with those of the parent compound 4, providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (-2J > or = 450 cm(-1)) of the S = 1/2 iron atom with the S = 1/2 ligand pi-cation radical leads to an effectively S = 0 ground state of 5 and 6. The oxidized complexes display 1H NMR spectra (in CDCl3 solution), characteristic of diamagnetic species.
... The values of the bond confirm the oxidation state of the metal centre distances Fe À O of 1.956 (3) ± 0.001 Å and Fe À N of 1.891 (4) ± 0. 005 Å (the X-ray diffraction experiment was performed at T = 100 K). Mössbauer spectra of 119 recorded at T = 80, 200, and 298 K have the form of symmetric quadrupole doublets with the close parameters (Table S9) and correspond to the low-spin state of the Fe III ion (d 5 , S Fe = 1/2) [168,169]. Considering the valent and spin states of the metal centre and the singlet ground state of the compound (S = 0), which demonstrates a well-resolved 1 H NMR spectrum in CDCl 3 , the authors have interpreted complex 119 as containing ligands with spins S = 0 and 1/2. However, the metric parameters of the ligands are not unambiguous (MOS = -1.23/-1. ...
The present review covers the coordination compounds of the ‘‘late” transition metals of the 3d row (Mn,
Fe, Co, Ni, Cu) based on sterically hindered functionalized o-iminobenzoquinone type ligands featured by
the presence of various functional groups. These functions cause the changes of the molecular and electronic structures of metal complexes and the degree of steric shielding of the metal centre. The latter
parameter affects the mutual arrangement of interacting magnetic orbitals and introduces specific
changes to the channels of magnetic exchange interactions in the coordination compound. The following
aspects are discussed in detail in the present review: the relationship between the structural modifications and the magnetic behaviour of o-iminobenzoquinonato complexes of 3d row elements; the effect
of the molecular structure transformations both on the oxidation state and the spin state multiplicity
of the metal centre, as well as on the oxidation state of the redox-active organic ligand. The systematization of the literature published mainly from 2007 to 2020 has been carried out. The bibliography contains 173 references.
... Used solvents were dried and puried as reported in literature. [38][39][40] The ligand N 0 -[(E)-phenyl(pyridin-2-yl) methylidene]furan-2-carbohydrazide was synthesized as before 24 and N 0 -[(E)-phenyl(pyridin-2-yl)methylidene]acetohydrazide synthesized using similar method. ...
The two alkoxo bridged complexes [Cu2(L¹)2(HL²)2(H2O)](NO3)2·2H2O 1 and [Cu2(L¹)2(HL²)2](NO3)2·H2O 2 have been synthesized by metal assisted hydrolysis of N′-[(E)-phenyl(pyridin-2-yl)methylidene]furan-2-carbohydrazide and N′-[(E)-phenyl(pyridin-2-yl)methylidene]acetohydrazide and characterized by various physicochemical techniques, where L¹ is 2-benzoylpyridine and HL² is phenyl(pyridin-2-yl)methanediol. The molecular structures of the complexes have been determined by single crystal X-ray diffraction analyses. The distances between the two metal centers, viz., Cu(1)...Cu(2) are 3.027 Å for 1 and 3.023 Å for 2. The molecular structures of both complexes consist of gem-diols. Low temperature magnetic susceptibility measurements reveal antiferromagnetic interactions with J values of -12.90 cm⁻¹ for 1 and -12.97 cm⁻¹ for 2. X-band ESR spectra showed typical S = 1 signals for both complexes. The zero-field splitting parameter (D) values estimated from the spectra of complexes 1 and 2 are 0.0030 and 0.011 cm⁻¹, respectively in the polycrystalline state. Electrochemical studies of binuclear complexes evidence two irreversible one electron transfer reduction waves (Epc1 = 0.142 to -0.126, Epc2 = -0.480 to -0.188 V). The electronic spectra of the complexes have been explained by TD-DFT calculations. Both complexes 1 and 2 were tested as catalysts for the oxidation of the model substrate 3,5-di-tert-butylcatechol (3,5-dtbc) to 3,5-di-tert-butylquinone (3,5-dtbq) and can be considered as functional models for catechol oxidase. These complexes also catalyze the dismutation of superoxide at biological pH; complex 2 is more SOD active than 1.
A structural basis for Cu( ii ) metallocycle formation is presented. Design requirements for the NNN bis-aryl pincer ligand have been delineated, paving the way for development of a novel class of metal–organic frameworks.
A novel SOD‐like macrocycle “H2DPD” and its trivalent chromium, iron and divalent manganese complexes have been isolated and characterized using the conventional tools. The macrocycle was prepared by 2:2 condensation of P‐phenylenediamine with 5,5‐dimethyl1,3‐cyclohexanedione. IR and electronic spectral data suggested that H2DPD coordinates to the metal ion as N4 tetradentate donor with two Cl− occupying the remaining two sites of the distorted octahedron. XRD spectrum of Cr3+ complex indicated that the complex crystallizes in a face‐centered monoclinic structure with lattice parameters: a = 10.9380 Å; b = 12.4870 Å; c = 12.4600 Å, α = γ = 90° and β = 111.430 with space group P 1 21/c 1 (14). The energy gap (EHOMO‐ELUMO), molecular electrostatic potential map (EPM) of title compounds, bond length, bond angle, as well as global and local reactivity were estimated using DFT method. The Eg values obtained from electronic spectra of Cr3+, Mn2+ and Fe3+ complexes were found to be 1.284, 1.220 and 1.138 eV, respectively which are in accordance with those evaluated by DFT revealing semiconductor nature. Also, the thermal degradation of all title compounds was carried out and the kinetic parameters were evaluated using Coats‐Redfern and Horowitz‐Metzger equations. Moreover, the compounds have screened for antibacterial as well as superoxide mimic activities. Cr3+ complex exhibited the most significant potent activity against all bacterial strains. With respect to SOD‐like activity, the macrocycle showed the most remarkable SOD‐like activity comparable to the standard drug, ascorbic acid. H2DPD and its Cr3+, Mn2+ and Fe3+complexes were synthesized and characterized. The DFT calculations, antibacterial as well as SOD‐activities were evaluated for the isolated compounds SOD activity.
The selective installation of phosphinoyl and carbamoyl moieties on the pyridine scaffold is an important transformation in synthetic and medicinal chemistry. By employing quinolinone as an efficient organic photocatalyst, we developed a catalytic system driven by visible light that forms phosphinoyl and carbamoyl radicals, which react with various heteroarenium derivatives under mild, transition-metal-free conditions. This straightforward and environmentally friendly synthetic method represents a new approach to site-divergent pyridine functionalization that offers considerable advantages in both simplicity and efficiency. Ambient temperature is sufficient for the formation of the reactive radicals, and the site-selectivity can be switched from C2 to C4 by changing the radical coupling sources. Under standard reaction conditions, phosphinoyl radicals give access to C4 products, while carbamoyl radicals selectively give C2 products. We found that the carbamoyl radical overcomes the intrinsic preference for forming the para-product by allowing the oxo functionality of the carbamoyl radical to electrostatically engage the nitrogen of the pyridinium substrate, which preferentially gives the ortho-product. The phosphinoyl radical cannot engage in the same interaction, because the phosphorus is too large. This novel synthetic route tolerates a broad range of substrates and provides a convenient and powerful synthetic tool for accessing the core structures of numerous privileged scaffolds.
N,N',N'',N''',N'''',N'''''‐(triphenylene‐2,3,6,7,10,11‐hexayl)hexapicolinamide (H6hptp), a triangular ligand containing three non‐innocent coordination sites, and its trinuclear iron(III) complex (PPh₄)₃[Fe₃(hptp)₆(CN)₆] were prepared. In this complex, three low‐spin Fe3+ ions are coordinated by the triangular hptp6‐ bridging ligand at its apices. Cyclic voltammetry of (PPh₄)₃[Fe₃(hptp)₆(CN)₆] in PhCN solution showed one reductive wave for 3Fe2+/3Fe3+ and three oxidative waves for the hptp6‐ ligand. The controlled‐potential electronic spectra of the solution presented broad absorption bands in the near‐IR (NIR) region for oxidations because a π‐radical is generated on the hptp6‐ ligand. The electron paramagnetic resonance spectra of frozen solutions also showed large oxidization‐state dependent differences caused by the strong d‐π interactions between the electrons in the d orbitals of the Fe3+ ions and those of the π‐radicals in the hptp6‐ ligand. The electronic and magnetic states of the oxidized species were investigated using density functional theory.
In the presence of FeCl3 and Et3N, ligand H4Ldtda(AP) underwent S-S bond cleavage and generated pincer non-innocent H3LONS ligand, which formed homoleptic, six-coordinate, low-spin Fe(III) complex (1). The complex was...
Using deprotonated forms of tetradentate bis(phenol) amine ligands 2–(((3,5–di–tert–butyl–2–hydroxybenzyl)(2–hydroxyethyl)amino)methyl)–4,6–di–tert–butylphenol (2LH2) and newly synthesized Methyl–2–(bis(3,5–di–tert–butyl–2–hydroxybenzyl)amino) propanoate (1LH2), dinuclear copper(II) complexes were synthesized. These ligands yielded two binuclear complexes of composition [(nL)2CuII2][{n = 1, (1) and n = 2, (2)] which have been characterized by X–ray crystallography, UV–Vis and magnetic susceptibility measurements. The phenolate moieties of the copper complexes were electrochemically oxidized to phenoxyl radicals. The antiferromagnetic exchange coupling of two copper centers of these complexes has been investigated by magnetic susceptibility (2–300 K) measurements. The weak exchange coupling constants (J) of these complexes, when compared to the literature values, is most likely attributed to the smaller Cu…Cu distance and Cu–O(Ph)–Cu angle. [1LCuII(py)] (4) and [2LCuII(py)] (5) show rhombic spectra typical of d9 configuration. The dimer complex can be converted into the corresponding monomeric Cu(II) complex, [1L2CuII2(X)] (X = py), by adding an exogenous ligand such as pyridine (py) into a CH2Cl2 solution of the dimer. All EPR spectra have been simulated and analyzed. The cyclic voltammograms observed with 1L2Cu2 and 2L2Cu2 revealed two oxidations with a similar small difference in their redox potentials. Since a copper(II)–oxidation is not feasible at such low potentials, these redox processes were assigned to ligand–centered oxidation yielding phenoxyl radical in the complex. Density Functional Theory (DFT) at the B3LYP level and Time-Dependent (TD)-DFT calculations rationalize the electronic structure of the complexes and throw light on the origin of observed electronic transitions.
Three new copper (II) and nickel (II) complexes viz. [Cu(L)2](1a), [Cu(L)2](1b) and [Ni(L)2].DMF(2), where HL = 2-((E)-(2, 4-dibromophenylimino) methyl)-4-bromophenol, have been synthesized and characterized by using various physico-chemical and spectroscopic techniques. The crystal structures of Schiff base (HL) and their metal complexes (1a), (1b) and (2) were determined by single crystal X-ray diffraction. IR and UV–Vis spectra and magnetic susceptibility measurements agree with the observed crystal structures. The crystallographic and spectroscopic studies confirmed four coordinate environments around the metal (II) ions. The synthesized Schiff base ligand (HL) crystallizes in the orthorhombic system of the space group Pbca. Complex (1a) of HL was crystallized in the monoclinic system of the space group P21/c, a = 10.1712(9) Å, b = 10.9299(10) Å,c = 12.7684(11) Å,α = 90̊,β = 104.649(2)̊, γ = 90̊ and Z = 2 whereas complex (1b) and (2) crystallized in the triclinic system of the space group P-1, a = 11.499(5)Å, b = 11.598(5)Å, c = 12.211(5)Å, α = 98.860(5), β = 115.653(5),γ = 100.906(5) and Z = 2 for (1b), a = 9.080(6) Å, b = 9.545(8)Å, c = 9.545(8)Å, α = 101.43(4)º,β = 99.63(3)̊, γ = 117.71(2)º and Z = 1 for (2). The synthesized ligand (HL) was behaved as monobasic bidentate Schiff base ligand having N and O donor sites. The electron paramagnetic resonance spectra indicate a dx²–y²ground state (g|| > g⊥> 2.0023) for (1a) and (1b). Copper (II) complexes display X-band EPR spectra in 100% DMSO and 77 K, giving indicating dx²–y²ground state. Superoxide dismutase-like activities of HL and its complexes were investigated by nitrobluetetrazolium chloride-DMSO assay and IC50 values were evaluated. These complexes were also tested for their in vitro antimicrobial activities against two bacteria (E. coli and Salmonella typhi) and two fungi (Pencillium, Aspergillus sp.) comparing with the Schiff base. The antimicrobial results showed that the complexes were more biologically active compounds to the Schiff base (HL).
Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamides ligands 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of FeII/CoII and FeIII/CoIII have been synthesized. Molecular structures of six of them reveal that these six-coordinate complexes utilize all the available donor sites of the ligands and assume MII/IIIN2(pyridine/pyrazine)N'2(amide)- N''2(azo) coordination. Complexes of FeII and CoIII are diamagnetic and FeIII and CoII are paramagnetic (S= 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CH2Cl2 reveal facile metal-centred FeIII/FeII and CoIII/CoII redox responses and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E1/2 values for MIII/MII redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with the values of the corresponding E1/2 values for six-coordinate MIII(bpy)3]³⁺/[MII(bpy)3]²⁺ or [MIII(trpy)2]³⁺/[MII(trpy)2]²⁺ or [MIII(L)]⁺/[MII(L)]⁰ or [MIII(L')2]⁺/[MII(L')2]⁰ (bpy = 2,2'-bipyridine, trpy = 2,2':6',2''-terpyridine and L(2–) = 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane) and tridentate L'(-) = {2-[2-(arylimino)phenylazo]-pyridine}) couples. Density Functional Theory (DFT) at the B3LYP level and Time-Dependent (TD)-DFT calculations rationalize the electronic structure of the present complexes and throw light on the origin of observed electronic transitions.
Transition metal complexes with so-called non-innocent ligands, whose specific feature is the impossibility of exact a priory determination of the metal oxidation state and the electronic structure of the ligands, are considered. The results of experimental studies (spectroscopic, electrochemical, diffraction) of the complexes with different types of non-innocent ligands are analysed. The attention is focused on their structure and redox properties.
(1,3) Bis (2-hydroxybenzamido)propane (H2L) forms mononuclear iron(III) complex, Fe(HL)(OH2)22+. The kinetics and mechanism of its formation has been investigated at 25.0 ≤ t/°C ≤ 40.0 (I = 0.3 mol dm-3, 10% (v/v) MeOH + H 2O). For the reaction, Fe(OH2)5(OH) 2+ + H2L⇌ Fe(HL)(OH2)22+, k2 (25°C) = (2.14 ± 0.15)×10 3 dm3 mol-1 s-1, ΔH ‡ = 39.7 ± 1.3 kJ mol-1, ΔS ‡ =-48 ± 4 J K-1 mol-1; k -2 (25°C) = 0.168 ± 0.0014 s-1, ΔH ‡ = 76.0 ± 1.5 kJ mol-1, ΔS ‡ =-4 ± 5 J K-1 mol-1 where k2 and k -2 denote the forward and reverse rate constants respectively. Fe(HL) (OH2)22+ is a moderate acid (pK = 2.25 ± 0.05, 25.0°C, I = 0.5 mol dm-3, 20% (v/v) MeOH + H2O). ΔpK (HL--Fe(HL)2+) = 8 is in conformity with the fact that both the phenolic groups are coordinated to iron(III). The metal ion promoted ionisation of the amide function also points to the N-coordination of this tetradentate ligand in Fe(HL) 2+/FeL+. The high value of the stability constant of FeL+ (ca log Kstab ≈ 18), which is further enhanced by the amide deprotonation, is favourable for this ligand to be an efficient iron(III) sequestering agent. Fe(L) (OH2)2+ undergoes facile aqua ligand substitution forming Fe(L)(X)(OH2) (n-1)- (Xn- = N3-, NCS-, imidazole, SO32-); with ascorbate anion (HAsc -) replacement of both H2O occurs leading to Fe(L)(HAsc)/Fe(L)(Asc)- in which HAsc-/Asc2- is chelated (ΔpK(HAsc--Fe(L)(HAsc)) = 6). No significant labilising action of the coordinated amide (even after N-H deprotonation) and phenoxide/phenol functions is evident. Fe(L)(SO3)-, Fe(L)(HAsc)/Fe(L)(Asc)- undergo facile internal redox despite the stabilising action of the multidentate ligand.
The potentially bidentate hard and soft acid containing (Te,N) compounds, 2-(2-aryltelluroethyl)3-methyl pyridines (L) [C14H15NTe] and tridentate(N,Te,N) 2-[2-(3-methyl pyridoethyltelluro)ethyl]-3methyl pyridine (L) [C18H20N2 Te] have been synthesized in good yield (> 75%) and characterized by physical, analytical and spectroscopic [1H, 13C(1H) and 125Te(1H) NMR and IR ] methods.The reaction of methyl iodide with L [C14H15NTe] results in the formation of the salt of the type ArTe (CH2)2 -2(C5H4N) +. CH3l with affecting the tellurium moiety.
The present work discusses copper(II) complexes of 13-membered macrocyclic ligands carrying electronic substituents on the ligand periphery. The complexes have been thoroughly characterized including crystallographic investigations of two representative compounds. The crystal structures confirm that the metal center is coordinated by two deprotonated Namide and two neutral Namine atoms in an N4 basal plane within the macrocyclic cavity. The electrochemical studies suggest that the different electronic substituents present on the ligand periphery (-H, -Cl, -CH3, and -OCH 3) significantly alter the CuIII-CuII potential values. Based on the electrochemical findings, the transient Cu(III) species generated coulometrically are characterized by their absorption spectra.
Four water soluble copper(II) complexes, [Cu(HL)2(H2O)2]Cl2 (1), [Cu(HL)2(ClO4)2] (2), [Cu(HL)2(SCN)2] (3) and [CuL2]8H2O (4), where HL is pyridine 2-carboxamide, have been synthesized and characterized by various spectroscopic techniques. Structures have been determined by single crystal X-ray crystallography. The pH induced inter-conversion of Cu(HL)2(H2O)2]Cl2 (1) and [CuL2]8H2O (4) through co-ordination mode switching was investigated thoroughly with the help of absorption spectroscopy. Complexes 1-3 were found to be active catalysts for the oxidation of toluene, ethyl benzene and cyclohexane in the presence of hydrogen peroxide as the oxidant under mild conditions. Toluene was oxidized to benzyl alcohol and benzaldehyde, ethyl benzene was oxidized to 1-phenylethanol and acetophenone and cyclohexane was oxidized to yield cyclohexanol and cyclohexanone Antimicrobial activities have been investigated with these copper(II) complexes against gram + ve bacteria, gram -ve bacterial and fungal species. [Figure not available: see fulltext.]
The asymmetric redox-active ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) is prepared and metalated to afford the hexanuclear complex [Mn6((N,O)L)6](6-). Structural analysis and magnetic measurements reveal this complex to feature Mn(II) ions bridged by (N,O)L(3-)˙ radicals, which are antiferromagnetically coupled to give an S = 12 ground state.
In order to gain insight into the coordination site and oxidative activity of the CuM site of hydroxylases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM), we have synthesized, characterized and studied the oxidation chemistry of copper complexes chelated by tridentate N2Sthioether, N2Osulfoxide or N2Osulfone donor sets. The ligands are those of N-2-methylthiophenyl-2'-pyridinecarboxamide (), and the oxidized variants, N-2-methylsulfenatophenyl-2'-pyridinecarboxamide (), and N-2-methylsulfinatophenyl-2'-pyridinecarboxamide (). Our studies afforded the complexes [(L1)Cu(II)(H2O)](ClO4)·H2O (·H2O), {[(L1(SO))Cu(II)(CH3CN)](ClO4)}n (), [(L1)Cu(II)(ONO)] (), [(L1(SO))Cu(II)(ONO)]n (), [(L1)Cu(II)(NO3)]n (), [(L1(SO))Cu(II)(NO3)]n () and [(L1(SO2))Cu(II)(NO3)] (). Complexes and were described in a previous publication (Inorg. Chem., 2013, 52, 11084). The X-ray crystal structures revealed either distorted octahedral (in , ) or square-pyramidal (in , ) coordination geometry around Cu(II) ions of the complexes. In the presence of H2O2, conversion of → , → and → occurs quantitatively via oxidation of thioether-S and/or Cu(ii) coordinated NO2(-) ions. Thioether-S oxidation of L1 also occurs when [L1](-) is reacted with [Cu(I)(CH3CN)4](ClO4) in DMF under O2, albeit low in yield (20%). Oxidations of thioether-S and NO2(-) were monitored by UV-Vis spectroscopy. Recovery of the sulfur oxidized ligands from their metal complexes allowed for their characterization by elemental analysis, (1)H NMR, FTIR and mass spectrometry.
A new potentially tetradentate redox-active o-aminophenol-based ligand, H2L = 2-(2-ethylthio)pyridine-anilino-4,6-di-tert-butylphenol, reacts with Pd(II)(O2CCH3)2 in CH3OH in the presence of air and Et3N affording isolation of a green crystalline solid of composition [Pd(L)] 1. When examined by cyclic voltammetry (CV), 1 exhibits two quasireversible oxidative responses at E1/2 = 0.16 (peak-to-peak separation, ΔEp = 100 mV) and 0.89 V (ΔEp = 90 mV) vs SCE (saturated calomel electrode). Chemical oxidation of 1 by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of two crystalline solids, red [Pd(L)][PF6]·CH2Cl2 2 and dark green [Pd(L)][BF4]2·2CH2Cl2 3, respectively. Single-crystal X-ray crystallography at 100(2) K unambiguously established that the O,N,S,N-coordinated ligand is present in the square-planar complexes [Pd(II){(L(AP))(2-)}] 1, [Pd(II){(L(ISQ))(•-)}][PF6]·CH2Cl2 2, and [Pd(II){(L(IBQ))(0)}][BF4]2·2CH2Cl2 3, as dianionic (L(AP))(2-), monoanionic o-iminobenzosemiquinonate(1-) π-radical (Srad = (1)/2) (L(ISQ))(•-), and neutral o-iminobenzoquinone (L(IBQ))(0) redox level. Reaction of 1 and 2 with PPh3 afforded isolation of two crystalline complexes: dark green [Pd(II)(L)(PPh3)] 4 and red [Pd(II){(L(ISQ))(•-)}(PPh3)][PF6]·CH2Cl2 5. X-ray structure determination of 5 at 100(2) K revealed Pd(II)ON2P coordination environment. The square-planar complexes 1-5 possess an S = 0, (1)/2, 0, 0, and (1)/2 ground-state, respectively, as was established by (1)H NMR and EPR spectroscopy, and room-temperature magnetic moment data. All redox processes are thus shown to be ligand-based. Absorption spectral measurements were done for all complexes. DFT calculations at B3LYP-level of theory adequately describe the electronic structures of 1-3, and 5, containing a spin-paired d(8) Pd(II) ion. Time-dependent-DFT calculations on 1-3 and 5 shed light on the origin of UV-vis-NIR spectral absorptions.
The crystal structure of the title compound, [Et3NH][(Me2bpb)FeCl2] or (C6H16N)[FeCl2(C20H16N4O2)], has been determined. Four N atoms of the Me2bpb [1,2-bis(2-pyridinecarboxamido)-4,5-dimethylbenzene] ligand and two chloro ligands are coordinated to the iron(III) ion. The geometry of the complex anion is distorted octahedral, with a Cl—Fe—Cl angle of 156.59 (3)°. The 4,5-dimethylbenzene ring of the Me2bpb ligand is tilted from the N4 plane by 9.5 (1)°.
In the molecule of the title complex, [Ni(C21H17N3O2S2)], the Ni atom is coordinated by three N atoms and one S atom of the pyridine-2,6-dicarboxamide ligand. The geometry around the NiII atom is approximately square planar. The crystal structure is stabilized by weak – and C—H…O intermolecular interactions.
Reactions of anhydrous CoX2 (X = Br−, SCN−) and Ni(ClO4)2 with N,N,N′,N′-tetraisobutylpyridine-2,6-dithiocarboxamides (S-dbpt), N,N,N′,N′-tetraisopropyl pyridine-2,6-dithiocarboxamides (S-dppt), and N,N,N′,N′-tetraethylpyridine-2,6-dithiocarboxamides (S-dept) lead to the formation of [Co(S-dbpt)Br2] (1), [Co(S-dppt)(SCN)2] (2), and [Ni(S-dept)2]·(ClO4)2·H2O (3), respectively. The X-ray crystal structures of the three S-dapt ligands and three complexes along with spectroscopic analyzes are presented. The molecular structure investigations of the S-dapt ligands show that the thiamide planes are twisted with respect to the pyridine ring, which is more in the case of phenyl groups. The structures of the Co(II) complexes reveal that an increase in steric crowding on the amide side arms of the ligands has no substantial effect on the geometry adopted by the corresponding complexes. The Co(II) gives only 1 : 1 five-coordinate, ion-paired complexes with a distorted square pyramidal geometry. Ni(II), on the other hand, prefers an octahedral geometry with 1 : 2 metal–ligand ratio. The coordination behavior of S-dapt has been compared to the analogous oxo(O-daap) ligands. Lesser propensity of S atom to get involved in H-bonding interactions ensures an S-N-S type of tridentate coordination by S-dapt.
A brownish-black complex [Fe(III)(L)2] (1) (S = 0), supported by two tridentate redox-active azo-appended o-amidophenolates [H2L = 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol], has been synthesized and structurally characterized. In CH2Cl2 1 displays two oxidative and two reductive 1e(-) redox processes at E1/2 values of 0.48 and 1.06 V and -0.42 and -1.48 V vs SCE, respectively. The one-electron oxidized form [1](+) isolated as a green solid [Fe(III)(L)2][BF4] (2) (S = 1/2) has been structurally characterized. Isolation of a dark ink-blue one-electron reduced form [1](-) has also been achieved [Co(III)(η(5)-C10H15)2][Fe(III)(L)2] (3) (S = 1/2). Mössbauer spectral parameters unequivocally establish that 1 is a low-spin (LS) Fe(III) complex. Careful analysis of Mössbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major LS Fe(III) and a minor LS Fe(II) component (redox isomers): [Fe(III){(L(ISQ))(-•)}2](+) and [Fe(II){(L(IBQ))(0)}{(L(ISQ))(-•)}](+) (2) and [Fe(III){(L(AP))(2-)}2](-) and [Fe(II){(L(ISQ))(-•)}{(L(AP))(2-)}](-) (3). Notably, for both at 8 K mainly the major component exists. Broken-Symmetry (BS) Density Functional Theory (DFT) calculations at the B3LYP level reveals that in 1 the unpaired electron of LS Fe(III) is strongly antiferromagnetically coupled with a π-radical of o-iminobenzosemiquinonate(1-) (L(ISQ))(-•) form of the ligand, delocalized over two ligands providing 3- charge (X-ray structure). DFT calculations reveal that the unpaired electron in 2 is due to (L(ISQ))(-•) [LS Fe(III) (SFe = 1/2) is strongly antiferromagnetically coupled to one of the (L(ISQ))(-•) radicals (Srad = 1/2)] and 3 is primarily a LS Fe(III) complex, supported by two o-amidophenolate(2-) ligands. Time-Dependent-DFT calculations shed light on the origin of UV-vis-NIR spectral absorptions for 1-3. The collective consideration of Mössbauer, variable-temperature (77-298 K) electron paramagnetic resonance (EPR), and absorption spectral behavior at 298 K, and DFT results reveals that in 2 and 3 the valence-tautomerism is operative in the temperature range 80-300 K.
The use of accurate model Hamiltonians is crucial to extract information on the magnetic interactions from the raw data obtained from magnetic susceptibility measurements, EPR, or NMR spectroscopy. Combining information from different experimental techniques can be used to propose topologies and magnitudes of the magnetic couplings in the system. It provides a rigorous and rational way to study the magnetic interactions in molecular complexes and extended systems without the need of fitting a set of parameters of an a priori defined model Hamiltonian. Theoretical treatments, able to discriminate between different physical effects, permit one to understand the various mechanisms involved in the magnetic couplings and to establish magneto-structural correlations. It has enabled theoreticians to discriminate between through space and through ligand interactions and to establish the balance between direct exchange, kinetic exchange, and spin polarization contributions to the magnetic coupling.
The synthesis of anionic amido/carbocyclic carbene ligated Pd (II) complex has been reported. The palladium complex is an active catalyst for the cross-coupling reaction between aryl bromides and phenylboronic acids under phosphine-free conditions at room temperature.
Pyridine-2-carboxamide and pyridine-2,6-dicarboxamide-based chelating ligands form a variety of coordination complexes with a number of metal ions, providing varying coordination geometry and nuclearity. Recent years have seen considerable interest in the designing of this class of ligands and to study their structural properties to serve specific stereochemical requirement of a particular metal-binding site. Notably, this class of ligands has been extensively utilized by Mascharak and co-workers to provide low-molecular-weight representations of metallo-proteins/enzymes such as bleomycins, nitrile hydratase. Moreover, the transition metal complexes of this class of ligands are being used as various exogenous nitric oxide (NO) donors. Using over 60 this class of chelating carboxamide ligands, the stereochemical properties of over 150 discrete coordination complexes, studied by single-crystal X-ray crystallography have been analyzed. Various bonding modes for a given chelating ligand are involved, and are reviewed with reference to ligand structure and the resulting coordination complexes. It is shown that the complexes synthesized have served to address notable issues such as effect of ligand structure/donor atom on metal-centered redox potentials, change of spin-state of iron(III), ligand-radical coordinated metal-complexes, interesting chemical reactivity studies, and catalytic potential. The ligands are introduced systematically as a function of their denticity, making easy access to information on specific type of ligands and coordination complexes thereof. X-ray crystallographically determined bond lengths of various donor atoms/groups are collected in a table, thus providing an accessible source for reference purposes. Source material for the review amounts to about 90 references.
An improved method for the synthesis of carboxamide ligands containing thioether donor sites is described. This replaces the pyridine as the reaction medium used in the classical method by tetrabutylammonium bromide. The desired products, 1,4-bis[o-(R-2-carboxamidophenyl)]-1,4-dithiobutane, R = quinoline (1), furan (2), thiophene (3), pyridine (4), and pyrazine (5), were obtained in good yields and shorter reaction times.
The present work shows three new amide-based ligands H2L1, H2L2 and H2L3 and their nickel and copper complexes. The X-ray structural analysis substantiate that the ligands constitute a square-based basal plane around the metal center. The crystal structures also show interesting solid state packing due to hydrogen-bonding and various weak C⋯H interactions. The solution-based spectral studies support the solid-state geometry observed for these complexes. The electrochemical results show that the Ni3+/2+ and Cu3+/2+ redox couple primarily depends on the N4 donors composed of Namide and Namine atoms. It was observed that the ligands H2L1 and H2L2 are better suited to stabilize the Cu(III) species whereas ligand H2L3 is ideal for the stabilization of Ni(III) species. On the basis of electrochemical findings, transient Ni3+ species were generated and characterized by the absorption spectroscopy.
Reaction of N-(phenyl)-2-pyridinecarboxamide (HL1) and N-(p-tolyl)-2-pyridinecarboxamide (HL2) ligands with MnCl2·4H2O affords complexes [(HL1)2MnCl2] 1 and [(HL2)2MnCl2] 2. The structures of 1 and 2 were determined by three-dimensional X-ray crystallography revealing that the MnII ions assume distorted octahedral geometry with coordination by two HL1/HL2 ligands providing two pyridine N and two amide O and two chloride ions. Notably, secondary interactions [C-H···Cl (pyridine
3-H hydrogen) and N-H···Cl (amide NH hydrogen)] triggered by MnII-coordinated chloride ions acting as hydrogen bonding acceptors generate self-complementary dimeric tectons, which lead to
2D supramolecular architectures.
Two new cyano complexes, K[Fe(bipy)(CN)4]·H2O (1) and (μ-bipym)[Mn(H2O)3{Fe(bipy)(CN)4}]2[Fe(bipy)(CN)4]2·12H2O (2), have been synthesised and their structures determined by single-crystal X-ray diffraction. Complex 1 is made up of mononuclear [Fe(bipy)(CN)4]− anions, potassium cations and water molecules of crystallization. The iron(III) is six-coordinated, being surrounded by two nitrogen atoms of a chelating bipy and four carbon atoms of four cyanide groups [Fe–N and Fe–C 1.991(3)–1.990(3) and 1.958(5)–1.914(5) Å, respectively]. Complex 2 consists of centrosymmetric tetranuclear (μ-bipym)[Fe(H2O)3{Fe(bipy)(CN)4}]22+
cations, [Fe(bipy)(CN)4]− anions and water molecules of crystallization. The cyano-containing iron(III) complex of 1 is present in 2 but in the latter it acts not only as a counterion but also as a monodentate ligand towards the manganese atom through one of its four cyanide groups. Bond lengths and angles around the iron atoms in 2 are practically identical to those observed in 1. The manganese atom in 2 is six-coordinated with two bipym- and one cyanide-nitrogen atoms and three mer-water molecules comprising a distorted octahedral environment [Mn–N(bipym) 2.35(1) and 2.29(1) Å, Mn–N(cyanide) 2.19(2) and Mn–O 2.21(1)–2.12(2) Å]. The manganese–manganese and manganese–iron separations across
the bis-chelating bipym and single-cyano bridge are 6.131(6) and 5.092(4) Å, respectively. Studies of the magnetic behaviour of 1 and 2 in the temperature range 1.9–300 K reveal that compound 1 is a magnetically isolated low-spin iron(III) complex with an important orbital contribution whereas significant antiferromagnetic interactions occur in 2 between the manganese(II) ions across bis-chelating bipym (J
=
−1.2 cm−1) and between the manganese(II) and iron(III) ions through the single-cyano bridge (j
=
−3.0 cm−1). The use of [FeIII(AA)(CN)4]− (AA = bidentate ligand) as a ligand towards metal ions appears very promising in designing new cyano-bridged
polynuclear compounds.
Using a pyridine amide ligand 4-methyl-2-{N-(2-pyridyl)carbamoyl}pyridine (HL), in its deprotonated form, binuclear mixed-valence homonuclear bimetallic complexes [CoIII,II2(L)3(X)]X·S [X = Cl (1); X = Br, S = CH3OH (2)] and heteronuclear bimetallic complex [CoIIIZnII(L)3(Cl)]Cl·CH3OH·5H2O (3) have been synthesized. Structural analysis revealed that trivalent cobalt is in distorted octahedral and bivalent cobalt/zinc is in distorted tetrahedral environment. Three L(−) ligands provide six-coordination by utilizing three pyridine amide units (pyridineN and amideN donor set) in a facial mode, which in turn places three 4-methylpyridine nitrogens to coordinate to another metal center, which completes four-coordination by a chloride/bromide ion. Temperature-dependent magnetic susceptibility measurements in the solid state of a representative complex 1 revealed the spin-states as CoIII (S = 0) and CoII (S = 3/2), with a zero-field splitting parameter for CoII (g = 2.20 and D = 3.9 cm−1; 2D is the energy gap between the two Kramers doublets Ms = |±3/2〉 and |±1/2〉). To the best of our knowledge, this is for the first time that a pyridine amide ligand has been utilized to accommodate two cobalt ions or a cobalt and a zinc ion in different geometry, oxidation state, and spin-state (class I mixed-valence dimer). The crystal packing diagram reveals interesting non-covalent interactions involving C–HO/Cl/Br, leading to the generation of 1D, 2D and 3D supramolecular architectures. Absorption spectral and redox properties of the complexes have also been investigated.
The new cyano complexes of formulas PPh4[FeIII(bipy)(CN)4]·H2O (1), [{FeIII(bipy)(CN)4}2MII(H2O)4]·4H2O with M = Mn (2) and Zn (3), and [{FeIII(bipy)(CN)4}2ZnII]·2H2O (4) [bipy = 2,2‘-bipyridine and PPh4 = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)4]- anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [{FeIII(bipy)(CN)4}2MII(H2O)4] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)4]- entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H2O)4 units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)4]- units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)4]- complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe−M distances are 5.126(1) and 5.018(1) Å in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)4]- units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1−3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron−zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) Å at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) Å at Fe(2). The magnetic properties of 1−3 have been investigated in the temperature range 2.0−300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the −CN−Zn−NC− bridging skeleton (iron−iron separation larger than 10 Å) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.
The mononuclear PPh4[Fe(phen)(CN)(4)]. 2H(2)O (1) complex and the cyanide-bridged bimetallic [{Fe(phen)(CN)(4)}(2)M-(H2O)(2)]. 4H(2)O compounds [M = Mn(II) (2) and Zn(II) (3); phen = 1,10-phenanthroline; PPh4 = tetraphenylphosphonium cation] have been synthesized and structurally and magnetically characterized. Complex 1 crystallizes in the monoclinic system, space group P2(1)/c, with a = 9.364(4) Angstrom, b 27.472(5) Angstrom, c = 14.301(3) Angstrom, beta = 97.68(2)degrees, and Z = 4. Complexes 2 and 3 are isostructural and they crystallize in the monoclinic system, space group P2(1)/n, with a = 7.5292(4) Angstrom, b = 15.6000(10) Angstrom, c 15.4081(9) Angstrom, beta = 93.552(2)degrees, and Z = 2 for 2 and a = 7.440(1) Angstrom, b = 15.569(3) Angstrom, c = 15.344(6) Angstrom, beta = 93.63(2)degrees, and Z = 2 for 3. The structure of complex 1 is made up of mononuclear [Fe(phen)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization, The iron(III) is hexacoordinate with two nitrogen atoms of a chelating phen (2.018(6) and 2.021(6) Angstrom for Fe-N) and four carbon atoms of four terminal cyanide groups (Fe-C bond lengths varying in the range 1.906(8)-1.95(1) Angstrom) building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral double zigzag chains of formula [{Fe(phen)(CN)(4)}(2)M(H2O)(2)] and crystallization water molecules. The [Fe(phen)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a bridging ligand toward M(H2O)(2) units [M = Mn(II) (2) and Zn(II) (3)] through two cyanide groups in cis positions, the other two cyanide remaining terminal. Two water molecules in trans positions and four cyanide-nitrogen atoms from four [Fe(phen)(CN)(4)](-) units build a distorted octahedral surrounding Mn(II) (2) and Zn(II) (3). The M-O bond lengths are 2.185(3) (2) and 2.105(3) Angstrom (3), whereas the M-N bond distances vary in the ranges 2.210(3)-2.258(3) Angstrom (2) and 2.112(3)-2.186(3) Angstrom (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is as in 1. The shorter intrachain Fe-M distances through bridging cyano are 5.245(5) and 5.208(5) Angstrom in 2 and 5.187(1) and 5.132(1) Angstrom in 3. The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. Complex 1 is a low-spin iron(III) complex with an appreciable orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the magnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10.2 Angstrom) being negligible. More interestingly, 2 exhibits one-dimensional ferrimagnetic behavior due to the noncompensation of the local interacting spins (S-Mn = 5/2 and S-Fe = 1/2) which interact antiferromagnetically through bridging cyano groups. A comparison between the magnetic properties of the isostructural compounds 2 and 3 allow us to check the antiferromagnetic coupling in 2.
The preparation, crystal structures, EHT band calculation and optical properties of two new charge transfer salts, namely (DIET)2[Fe(bpca)(CN)3] (1) and (DIEDO)2[Fe(bpca)(CN)3] (2), where bpca=bis(2-pyridylcarbonyl)amide anion, DIET=diiodoethylenedithotetrathiavalene and DIEDO=diiodoethylenedioxotetrathiavalene are reported. The magnetic properties of 2 and those of the low-spin iron(III) precursor of formula (PPh4)[Fe(bpca)(CN)3]·H2O (3) were also investigated in the temperature range 1.9–205 K. Crystal data; (1): monoclinic P21, a=8.8238(2)Å, b=13.2891(3) Å, c=18.5042(5) Å, β = 91.115(1)°, Z=2, R=0.0710 for 7021 independent reflections with I>2 σ(I) and (2): Monoclinic P21/c, a=8.6870(1) Å, b=12.6122(2) Å, c=36.0277(11) Å, β=90.380(5)°, Z=4, R=0.0602 for 4633 independent reflections with I>2σ(I). The crystal structures for both compounds consist of alternating organic and inorganic layers. Compounds 1 and 2 exhibit semiconductive behavior. Simple tight-binding band calculations indicate quasi one- (1) and two-dimensional (2) electronic band structures. The magnetic properties of 2 compared to that of the low-spin iron(III) precursor 3 (which was used as a blank) reveal the occurrence of a relative large antiferromagnetic interaction between the DIEDO radical units, however, below 30 K, the magnetic behavior of 2 is indistinguishable from 3. To cite this article: L Ouahab et al., C. R. Chimie 8 (2005).
A new potentially tridentate ligand HL11 consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L11(−) affords a bis-complex [NiII(L11)2] (1). In 1, the two L11(−) ligands bind to the NiII center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The NiIIN2(pyridyl)N′2(amide)N″2(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni–Nam (am=amide) bond lengths are the shortest and the Ni–Nazo bond lengths are the longest. Complex 1 exhibits a quasireversible NiIII/NiII redox process. Moreover, the complex displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results.
A heterogeneous catalyst prepared by chemically binding [1,2-bis(salicylidene amino)-phenylene] zirconium complex to modified carbamated silica gel catalyst has been synthesized. This binding of the complex with silica gel has been demonstrated by carrying out the same reaction with a small molecular homologue compound t-butanol, modified similarly. The oxidation of cyclohexane using molecular oxygen was studied and the product distribution was found to be entirely different from that reported in the literature. The product distribution consisted of cyclohexanol (major product) together with small amounts of cyclohexene in the ratio 6.6:1. At 200°C, the conversion was 21% and cyclohexanol was formed with a selectivity of 73%. The thermogravimetric analysis shows that the catalyst is unstable only beyond 383°C.
Four low-spin dicyanodicarboxamidocobalt(III) complexes have been prepared from N,N′-bis(8-quinolyl)malonamide derivatives, the malonyl fragment of which was either unsubstituted (Et4N)[Co(BQM)(CN)2] (3a), monosubstituted by a benzyl group (Et4N)[Co(mono-BenzBQM)(CN)2] (3b), or disubstituted by two benzyl or two phenylethyl groups, (Et4N)[Co(di-BenzBQM)(CN)2] (3c) and (Et4N)[Co(di-PhEtBQM)(CN)2] (3d), respectively. The X-ray crystal structures of 3a and 3c reveal that the tetradentate ligand adopts a helical or a planar configuration around the cobalt centre of 3a and 3c, respectively. Complexes 3a−3d were characterised in solution by 1H NMR spectroscopy by 1D and 2D COSY H-H techniques. The solid-state structures of 3a and 3c are retained in solution, and complexes 3b and 3d adopt structures similar to 3a and 3c, respectively, showing that the level of the malonic substitution determines the configuration of the tetradentate ligand around the cobalt centre and forces the cyanide ligands to adopt either cis or trans orientations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
Synthesis and crystal structure of two coordination polymers of composition [MnII(H2bpbn)1.5][ClO4]2·2MeOH·2H2O (1) and [CoII(H2bpbn)(H2O)2]Cl2·H2O (2) [H2bpbn=N,N′-bis(2-pyridinecarboxamido)-1,4-butane], formed from the reaction between [Mn(H2O)6][ClO4]2/CoCl2·4H2O with H2bpbn in MeCN, are described. In 1 each MnII ion is surrounded by three pyridine amide units, providing three pyridine nitrogen and three amide oxygen donors. Each MnII center in 1 has distorted MnN3O3 coordination. In 2 each CoII ion is coordinated by two pyridine amide moieties in the equatorial plane and two water molecules provide coordination in the axial positions. Thus, the metal center in 2 has trans-octahedral geometry. In both 1 and 2, the existence of 1D zigzag network structure has been revealed. Owing to π–π stacking of pyridine rings from adjacent layers 1 forms 2D network; 2 forms 2D and 3D network assemblies via N–H⋯Cl and O–H⋯Cl secondary interactions. Both the metal centers are high-spin.
Two low-spin Fe(III) dicyano-dicarboxamido complexes have been prepared from N,N′-bis(8-quinolyl)malonamide derivatives. Crystal structures show that the four nitrogen donors available to complex the metal are arranged in the equatorial plane with the two cyanides trans to each other in the axial positions when the malonyl moiety is disubstituted. In contrast, the unsubstituted malonyl results in only three nitrogens in the equatorial plane with the fourth in an apical position and the two cyanides occupying cis sites, one equatorial and the other axial. NMR analyses show that the solid state structure of both complexes is retained in solution. Both types of configurational complexes catalyze cyclic olefin oxidations with H2O2 but only the cis-dicyano complex catalyzes stilbene oxidation with formation of epoxides, diols and benzaldehyde.
Two series of complexes of the types trans-[CoIII(Mebpb)(amine)2]ClO4 {Mebpb2−=N,N-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine=pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[CoIII(cbpb)(amine)2]X {cbpb2−=N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine=pyrrolidine (prldn), X=PF6 (1b), piperidine (pprdn), X=PF6 (2b), morpholine (mrpln), X=ClO4 (3b), benzylamine (bzlan), X=PF6 (4b)} have been synthesized and characterized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy. The crystal structure of 1a has been determined by X-ray diffraction. The electrochemical behavior of these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on the redox properties, is also reported. The reduction potential of CoIII, ranging from −0.53V for (1a) to −0.31V for (3a) and from −0.48V for (1b) to −0.22V for (3b) show a relatively good correlation with the σ-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox processes.
Four half-sandwich ruthenium(II) complexes [(η6-C6H6)Ru(L1-O)][PF6] (1), [(η6-C6H6)Ru(L2-O)][PF6] (2), [(η6-C6H6)Ru(L3-O)][PF6] (3), [(η6-C6H6)Ru(L4-O)][PF6] (4a), and [(η6-C6H6)Ru(L4-O)][BPh4] (4b) [L1-OH, 4-nitro-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L2-OH, 2,4-di-tert-butyl-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L3-OH, 2,4-di-tert-butyl-6-{[2′-((pyridin-2-yl)benzylamino)methyl}-phenol; L4-OH, 2,4-di-tert-butyl-6-{[(2′-imethylaminoethyl)methylamino]methyl}-phenol (L4-OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported. The molecular structures of 1–3, 4a, and 4b have been elucidated in solution using 1H NMR spectroscopy and of 1, 3, and 4b in the solid state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central amine nitrogen also becomes chiral. The 1H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely diastereoselective [1: SRu,SN/RRu,RN; 2: RRu,RN/SRu,SN; 3: SRu,RN/RRu,SN; 4b: SRu,RN/RRu,SN]. The crystal packing in 1 and 3 is stabilized by C–H…O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a one-electron oxidized phenoxyl radical coordinated species [2]2+, [3]2+, and [4a]2+, respectively. The radical species are characterized by CV, UV–Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant (UV–Vis spectroscopy).
The tetradentate dianionic ligand 1,2-bis(pyridine-2-carboxamido)benzenate(2−) (bpb)2-, is noninnocent in the sense that it can be coordinated as a dianion, (bpb)2-, a monoanionic radical, (bppox1)-, and a neutral species, (bpbox2)0. Photolysis at 20 °C of high-spin [Fe(bpb)(N3)2]- yields the dinuclear iron(IV) species [FeIV2(μ-N)(bpb)2(N3)2]- and N2. The same reaction in frozen acetonitrile solution produces the mononuclear low-spin ferric species [FeIII(bpbox2)(N)(N3)]-, which was characterized by EPR and Mössbauer spectroscopy. The putative intermediate [FeV(bpb)(N)(N3)]- was not detected.
A heterogeneous catalyst consisting of [1,2-bis(salicylidene amino)-phenylene] zirconium complex chemically bonded to carbamate-modified silica gel catalyst has been synthesized and the oxidation of n-heptane using molecular oxygen was studied in a batch reactor system. The study was conducted in the temperature range of 160–200°C. The product distribution was found to be entirely different from that reported in literature. Cyclohexanone was the only oxygenated product formed, the rest being isomerized products of hexane and heptane (2-methylpentane, methylcyclopentane, 4-methyl, 1-pentene and toluene). Experiments suggest that in the oxidation of heptane carried out in this work, reforming reaction occurred first and cyclohexane formed undergoes oxidation to give cyclohexanone. The thermogravimetric analysis (TGA) shows that the catalyst starts breaking only beyond 383°C.
A Mn(iii) complex [Mn(pmpa)2](PF6) (pmpaH = N-(2-picolyl)picolinamide) has been synthesized and structurally characterized by X-ray crystallography. The complex undergoes one electron oxidation and reduction at 1.05 V and -0.20 V (versus Ag/AgCl electrode) respectively. DFT calculations show that both redox processes have significant ligand character. DFT calculations also show that there is strong electronic coupling between the central metal ion and the amide ligands, which leads to higher ligand-to-metal charge transfer and thus higher metal-ligand covalency with increasing oxidation state on the central metal ion. This implies that amide ligands are redox non-innocent.
New bis (pyridylurea) ligand, H2L, was synthesized by the reaction of ethylpyridine-2-carbamate (EPC) and p-phenylenediamine. The ligand was characterized by elemental analysis, IR, (1)H NMR, electronic and mass spectra. Reaction of the prepared ligand with Co(2+), Ni(2+), Cu(2+), Fe(3+), VO(2+) and UO2(2+) ions afforded mono, bi- and trinuclear metal complexes. Also, new mixed ligand complexes of the ligand H2L and 8-hydroxyquinoline (8-HQ) with Co(2+), Ni(2+), Cu(2+) and Fe(3+) ions were synthesized. The ligand behaves as bi- and tetradentate toward the transition metal ions, coordination via the pyridine N, the carbonyl O and/or the amidic N atoms in a non, mono- and bis-deprotonated form. The complexes were characterized by elemental and thermal analyses, IR, electronic and mass spectra as well as conductance and magnetic susceptibility measurements. The results showed that the metal complexes exhibited different geometrical arrangements such as square planar, tetrahedral, octahedral and square pyramidal arrangements. The Coats-Redfern equation was used to calculate the kinetic and thermodynamic parameters for the different thermal decomposition steps of some complexes. 3D molecular modeling of the ligand, H2L and a representative complex were studied.
The crystal structure of a novel dimeric zinc(II) complex, [ZnL(H2O)]2(ClO4)2·4H2O (L = N-(bis(2-pyridyl)methyl)-2-pyridinecarboxamide), has been determined by X-ray diffraction. In this complex each planar Npy–Namido–Npy moiety of the ligand coordinates to one zinc ion and the pendant pyridine of one [ZnL] unit completes the coordination sphere of a [ZnL] neighbor. Units of the complex are connected in a two-dimensional network by intermolecular hydrogen bonds. The thermodynamic properties of the ligand with bivalent metal ions Co(II), Ni(II), Cu(II) and Zn(II) were studied by potentiometric titration and the order of the stability constants is in agreement with the Irving–Williams series. The dimeric complex is stabilized through ligand sharing, as confirmed by the crystal structure and thermodynamic properties.
Two heterobimetallic 2-dimensional layer complexes, {[Fe(bpy)(CN)(4)](2)M(4,4'-bipyridine)}·4H(2)O [bpy = 2,2'-bipyridine, M = Mn (), Cu ()] have been prepared by diffusion and their structures determined by single crystal X-ray diffraction. and are isomorphous, made of neutral bimetallic [Fe(bpy)(CN)(4)](2)M(4,4'-bipyridine)] layers and uncoordinated water molecules located between the layers. Interestingly, complex shows the compression of the Jahn-Teller distortion around the copper(ii) ion. Magnetic investigation shows antiferromagnetic coupling between the manganese(ii) and iron(iii) ions mediated by bridging CN(-) in , while ferromagnetic coupling between the copper(ii) and iron(iii) ions is seen in . Both complexes and reveal a metamagnetic-like behaviour with different critical fields (H(c), at 1.8 K): 2.0 kOe () and 3.2 kOe (). The weak interchain antiferromagnetic interaction can be illustrated by the spin-polarization mechanism.
Three cobalt(III) complexes of the type trans-[CoIII(Mebpb)(amine)2]X {Mebpb2−=N,N′-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine=N-methylimidazole (N-MeIm) (1), 3-methylpyridine (3-MePy) (2), 3-acetylpyridine (3-AcPy) (3), X=BPh4- (1), ClO4- (2, 3)} were synthesized and characterized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy. The structure of 1·CH3OH was determined by X-ray crystallography and was found to have a distorted octahedral geometry around Co. The electrochemical behavior of these complexes with the goal of evaluating the effect of axial ligation on the redox properties is also reported. The reduction potential of Co(III), ranging from −0.63V for (1) to −0.20V for (3) shows a relatively good correlation with the σ-donor ability of the axial ligands.
Two new pentacoordinate ligands containing two pyridine 2-carboxamide units connected by -(CH2)(n)-N(Me)-(CH2)(n)- links have been synthesized. The ligand with n = 3 affords a monomeric Cu(II) complex, while the one with n = 2 gives rise to a ligand-shared dimer.
The homonuclear macrocyclic complex involving zirconium on both sites is prepared starting from 2,6-diformyl-4-methyl-phenol and 1,2-phenylenediamine, and is chemically bound to carbamate-modified alumina. The UV-Vis spectrum shows the presence of the metal on the catalyst surface. The TGA of the catalyst suggests that the catalyst is stable up to 250 °C. Reformation of n-hexane has been carried out in its presence under nitrogen atmosphere at considerably low temperature and pressure (423 K, 27 atm). We obtained a high conversion of 72% and the GC-MS of the product showed the forming of 2-methylpentane (2MP), 3-methylpentane (3MP), methylcyclopentane (MCP) and cyclohexane (CH). Experimental results reveal that there are no dehydrogenation (forming alkenes), dehydrocyclization (forming benzene or toluene), and cracking (forming C1–C5 compounds) reactions and a mechanism forming the products has been suggested.
Complexes of general formula M(bpbH2)Cl2· nH2O, where bpbH2 = N,N′-bis(2′-pyridinecarboxamide)-1,2-benzene and M = Mn (n = 0), Co (n = 1), Co and Ni (n = 2) and Cu (n = 1.5) have been prepared. As well, the complexes Ag(bpbH2)(NO3) and Co(bpbH2)(NCS)2·2.5H2O were isolated. The copper and nickel complexes exhibit clear evidence of neutral amide-N coordination. Several of the metal ions used promote amide deprotonation on coordination, and a series of complexes of formula M(bpb)·H2O (M = Co, Ni, Cu, Zn and Pd) are reported. A planar MIIN4 system is proposed for these complexes. Stereochemistries for the complexes have been proposed utilising microanalyses, magnetic susceptibilities, thermogravimetric analyses and infrared and visible spectral data.
A series of low-spin iron(III) complexes [Fe(bpc)(L)2]+ (H2bpc=4,5-dichloro-1,2-bis(2-pyridinecarboxamido)- benzene; LBu3P, Im, 1-MeIm, tBupy) have been synthesized. These iron complexes display reversible one-electron oxidation and reduction couples. The stable one-electron-oxidized species [Fe(bpc)(Bu3P)2]2+ has been generated electrochemically. The complex trans-[Fe(bpc)(1-MeIm)2](ClO4) has been characterized by X-ray crystallography: space group P; a=8.425(1), b=9.512(1), c=18.413(19) Å; α=96.91(4), β=95.83(4), γ=90.44(1)°; V=1457(2) Å3; Z=2.
The shifts of the proton resonance lines of inert reference molecules in solution caused by paramagnetic substances are accurately given by the theoretical espression ΔH/H = (2π/3)ΔK where ΔK is the change in volume susceptibility. For aqueous solutions of paramagnetic substances, about 2% of t-butyl alcohol is incorporated, and an aqueous solution of t-butyl alcohol of the same concentration is used as an external reference. Values obtained for the susceptibility of a variety of paramagnetic molecules are in satisfactory agreement with those in the literature. Less than 0.03 ml. of a dilute solution can be studied.
Using a bidentate ligand N-(2-chloro-6-methylphenyl)pyridine-2-carboxamide (HL5), in its deprotonated form, two new five-co-ordinate complexes of composition [M(L5)2(H2O)]·H2O (M = CoII 1 or CuII 2) have been prepared and characterized including X-ray crystallography. The co-ordination geometry at CoII and CuII is approximately trigonal bipyramidal (two deprotonated amide nitrogens and a water molecule in the equatorial plane and two pyridines in the axial positions), being more distorted in the case of CuII. The observed distortion is caused by (i) a small bite angle of the chelating ligand and (ii) the presence of two ortho substituents, a chloro and a methyl group, on the phenyl ring (steric effect). To the best of our knowledge, 1 represents the first structurally characterized mononuclear high-spin cobalt(II) complex with a pyridine amide ligand. The magnetic moments of 1 and 2 at 300 K reveal that the compounds are paramagnetic (1 has S = 3/2 and 2 has S = 1/2), both as solids and in dmf solution. Temperature dependent magnetic susceptibility measurements confirmed their spin state. The stereochemistry of the cobalt(II) centre in 1 does not change to any measureable extent on dissolution in dmf (cf. solid and solution state absorption spectra). The geometry of the copper(II) centre in 2 observed in the solid state is not retained in dmf solution (absorption spectra), changing to a tetragonal stereochemistry. Cyclic voltammetric measurements (dmf solution; glassy carbon electrode) on 1 reveal an oxidative response at 0.48 V vs. saturated calomel electrode (SCE) and a reductive response at –1.66 V corresponding to CoIII–CoII and CoII–CoI redox couples, respectively. For 2 the CuII–CuI process was observed at –0.53 V vs. SCE.
Mononuclear iron(III) complexes trans-[Fe(bpb)X2]+/–[H2bpb = 1,2-bis(pyridine-2-carboxamido)-benzene; X = pyridine (py)1, N3–2, MeCO2–3 or CN–4] and trans-[Fe(bpc)X2]+/–[H2bpc = 4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene; X = py 5, Cl–6 or MeCO2–7] have been synthesised and characterized. Complexes 1 and 4 are low-spin (µeff= 2.50 and 2.10 at 298 K, solution phase) whereas 2, 3, 6 and 7(µeff= 5.86–5.98 at 298 K, solution phase) are high spin. Solid-state magnetic susceptibility (40–300 K) and Mössbauer spectral measurements at 300 and 77 K of 1 and 7 confirmed the spin state. The complex [NEt4][Fe(bpc)(MeCO2)2]·CHCl37 has been characterized by X-ray crystallography: space group P21/c, a= 12.283(3), b= 18.819(3), c= 16.437(3)Å, β= 101.02(2)°, Z= 4, R=R' = 0.041 for 5069 observed reflections. The solid-state EPR spectra (300 and 77 K) of 1, 4 and 5 are typical of low-spin iron(III) and have been analysed to determine the ligand-field parameters. All the complexes display ligand-to-metal charge-transfer transitions in the visible region (550–800 nm). Cyclic voltammetric measurements of 1 in pyridine and 4 in dimethylformamide revealed a quasi-reversible FeIII–FeII reduction [E½=–0.06 V, ΔEp= 150 mV for 1 and E½=–0.83 V, ΔEp= 100 mV for 4vs. saturated calomel electrode (SCE)]. The high-spin complexes exhibit an electrochemically irreversible but chemically reversible FeIII–FeII reduction (Epc=–0.44 to –0.83 V) in acetonitrile solution. Complexes 2, 4 and 6 show a reversible one-electron oxidation (E½=+0.66 to +1.05 V vs. SCE).
Reactions of anhydrous CrCl3 with H2bpb and H2bpc [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene; H2bpc = 4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene] in dimethylformamide and manganese(III) acetate with H2bpc in methanol, yielded the respective [CrIII(bpb)Cl]·xH2O, [CrIII(bpc)Cl]·H2O and [MnIII(bpc)(O2CMe)] complexes. The electrochemistry of the chromium(III) amide complexes has been studied by cyclic voltammetry. They exhibit reversible oxidation couples with E° values ranging from 0.28 to 0.80 V vs. ferrocenium–ferrocene. Except for [CrIII(bpb)(CN)2]– which has a quasireversible reduction couple at –1.97 V, the electrochemical reductions are irreversible. The complexes [CrIII(bpb)(H2O)2]ClO4, [CrIII(bpb)Cl(MeOH)] and [MnIII(bpc)(O2CMe)] have been shown to catalyse olefin epoxidation by iodosobenzene. With [Mn(bpc)(O2CMe)], catalytic oxidation of alkanes by PhIO has also been observed. An oxygen-rebound mechanism involving a MnvO or a CrvO intermediate is proposed for the PhIO oxidation reactions.
A series of organo and non-organo cobalt complexes of bpb and bpc ligands [H2bpb = 1,2-bis(2-pyridinecarboxamido)benzene, H2bpc = 4,5-dichloro-1,2-bis(2-pyridinecarboxamido)benzene] have been synthesised. Complexes prepared include Na[CoL(CN)2], Na[CoL(N3)2], [CoL(py)2]ClO4 and [CoL(R)(H2O)](L = bpb or bpc; py = pyridine; R = Me, Et, Pri or CH2CH2CMeCH2). The complex [CoL(CH2CH2CMeCH2)(H2O)] was formed as a rearrangement product from the reaction of 3,3-dimethylallyl bromide and [CoL]·H2O in the presence of NaBH4 and NaOH in methanol. The complexes [Co(bpc)(CH2CH2CMeCH2)(H2O)]1 and [Co(bpb)Et(H2O)]2 have been characterized by X-ray crystallography: 1, space group Pnma, a= 13.934(2), b= 12.204(2), c= 13.173(2)Å, Z= 4, and R= 0.046 for 1129 observed reflections; 2, space group Pnma, a= 15.072(1), b= 12.119(2), c= 9.884(3)Å, Z= 4, and R= 0.034 for 1387 observed reflections. Electrochemical studies on the one-electron oxidation of these complexes suggest the involvement of the equatorial ligand in these processes.
Die Darstellung der Titelkomplexe erfolgt nicht über den Diiminliganden direkt, sondern über spezifische Oxidation entsprechender o-Phenylendiamin-Komplexe.
The controlled nucleophilic halide displacement reaction of [NEt4][Fe(bpc)Cl2] [H2bpc=4,5-dichloro-1,2-bis(pyridine-2-carboxamido) benzene] with AgClO4 in MeCN afforded a crystalline iron(III) complex Fe(bpc)Cl·H2O 1. The mixed chloro-dimethylformamide (DMF) axially ligated complex [Fe(bpc)Cl(DMF)] (obtained during recrystallization of 1 from DMF; however, it loses DMF quite readily to revert back to 1) has been structurally characterized. It belongs to only a handful of mononuclear high-spin iron(III) complexes having deprotonated picolinamide ligand. The iron(III) centre is co-ordinated in the equatorial plane by two pyridine nitrogens and two deprotonated amide nitrogens of the ligand, and two axial sites are co-ordinated by a chloride ion and a DMF molecule. The metal atom has a distorted octahedral geometry. Reaction of 1 with [nBu4N][OH] in MeOH afforded a μ-oxo-bridged diiron(III) complex, [Fe(bpc)]2O·DMF·2H2O, 2. The spin state and the co-ordination environment of the iron(III) centres in 1 and 2 have been determined by temperature-dependent (25–300 K) magnetic susceptibility measurements in the solid state (Faraday method) and Mössbauer spectral studies at 300 K. Complex 1 behaves as a perfect S=5/2 system, in the solid-state as well as in DMF solution. The two iron(III) centres in 2 are antiferromagnetically coupled (J=−117.8 cm−1) and the bridged dimeric structure is retained in DMF solution. Bridge-cleavage reactions of 2 have been demonstrated by its ready reaction with mineral acids such as HCl and MeCO2H to generate authentic S=5/2 complexes, [Fe(bpc)Cl2]− and [Fe(bpc)(O2CMe)2]−, respectively.
A series of non-electrolytic low-spin ruthenium(III) complexes [μeff (in dichloromethane solution): 1.79–1.93 μB at 298 K] have been synthesized using closely-related encapsulating Schiff-base ligands having an aliphatic nitrogen and a phenolate oxygen as donor sites. The structural behaviour of the complexes in CDCl3 solution has been examined by 1H NMR spectroscopy utilizing paramagnetically-shifted ligand proton resonances. Cyclic voltammetric studies in dichloromethane solution at a platinum working electrode reveal the presence of a reversible-to-quasi-reversible RuIII-RII couple (Ef = −0.62 to −0.74 V) and an RuIV-RuIII couple (Ef = 0.53–0.70 V) vs S.C.E. (saturated calomel electrode). Additionally, an RuV-RuIV couple (Ef/Epa = 1.02–1.18 V) is also observed.
For corrigendum see DOI:10.1002/anie.199516491
A series of organo and non-organo rhodium and iridium complexes of bpb and bpc ligands [H2bpb = 1,2-bis(2-pyridinecarboxamido)benzene, H2bpc = 4,5-dichloro-1,2-bis(2-pyridinecarboxamido)benzene] have been synthesized. These complexes display reversible one-electron oxidation couples. Stable one-electron-oxidized species have been generated both chemically and electrochemically. The oxidation potentials are affected dominantly by the charge effect but are relatively independent of the nature of the central metal ions and axial ligands. On the contrary, [Rh(bpe)R]·H2O complexes [R = Me or Et; H2bpe = 1,2-bis(2-pyridinecarboxamido)ethane] can only be oxidized irreversibly at a potential of about 0.3 V more anodic than that of the corresponding bpb complexes. The involvement of the equatorial ligand in the oxidation of the bpb and bpc complexes has been suggested. The complex [Rh(bpb)(py)2]ClO4 has been characterized by X-ray crystallography: space group P, a= 8.316(1), b= 9.620(2), c= 17.405(6)Å, α= 98.87(2), β= 99.57(2), γ= 90.55(2)°Z= 2, and R= 0.039 for 4 788 observed reflections.
The synthesis, via a triphenyl phosphite intermediate, of a series of bis(amides) of pyridine-2-carboxylic acid and various diamines is reported. Products isolated are of the form (C5H5NCO)2R where R = (I) -NH(CH2)2NH-, (II) -NH(CH2)3NH-, (III) -NHCH(CH2)4CHNH-, (IV) -NH(O-C6H4)NH-, and (V) H2CN(CH2)2NCH2. The compounds were characterized by microanalysis, melting point, and NMR, IR, and mass spectral data.
The collective evidence from electronic, infrared, Mössbauer, and 1H NMR spectroscopies, from X-ray crystal structure determinations, from electrochemical studies, and from magnetic susceptibility data provides unambiguous evidence that [FeCl(TPP·)]+ and Fe(OClO3)2(TPP·) (TPP = meso-tetraphenylporphinate) are iron(III) porphyrin π-cation radical species rather than iron(IV) complexes. The distinction is real, not semantic, and greatly affects the physical and chemical properties of high-valent iron porphyrins. Strong antiferromagnetic coupling (|-2J| ≳ 500 cm-1) of the S = 5/2 iron atom with the S = 1/2 porphyrin radical leads to an overall S = 2 state for [FeCl(TPP·)][SbCl6] (1) (μeff300K = 4.8 μB). By contrast, ferromagnetic coupling (2J ≃ +80 cm-1) in Fe(OClO3)2(TPP·) (2) leads to an S = 3 ground state (μeff300K = 6.5 μB). X-ray crystal structure determinations of the p-tolyl analogue of 1, [FeCl(TpTP·)][SbCl6]·2C2H2Cl 4 (3), and of 2 provide an orbital symmetry rationale for this sharply contrasting magnetic behavior and give new insight into the magnetic coupling of a metal to a porphyrin cation radical. An explanation is offered for the insignificant coupling (J ∼ -2 cm-1) in compound I of horseradish peroxidase. Crystal data for 3: monoclinic, space group P21/c, Z = 4, a = 10.98 (1) Å, b = 22.57 (1) Å, c = 23.65 (1) Å, β = 97.73 (5)° at -140°C, R = 0.088, Rw = 0.097, Fe-Nav = 2.07 (1) Å, Fe-Cl = 2.168 (5) Å. Crystal data for 2: monoclinic, space group P21/c, Z = 2, a = 12.132 (1) Å, b = 14.622 (2) Å, c = 13.153 (1) Å, β = 127.84 (1)° at 25°C, R = 0.116, Rw = 0.130, Fe-Nav = 2.045 (10) Å, Fe-O = 2.13 (1) Å.
The first crystallographically characterized neutral square-planar complex of cobalt in an oxidation state higher than 2+, Co(eta-4-1), is reported. Structural data for this new class of compounds indicate that the macrocycle in Co(eta-4-1) is [GRAPHICS] noninnocent; however, EPR data in toluene at 5.9 K (S = 1/2; g1 = 2.558, g2 = 2.170, g3 = 2.017; A2 almost-equal-to 15 G) show that the metal center is the primary residence site of the unpaired electron. Co(eta-4-1) is a stable, yet potent, oxidant which is soluble in benzene and slightly soluble in pentane. The Co(eta-4-1)/[Co(III)(eta-4-1)]- couple is reversible and found at 0.550 V vs Fc+/Fc in CH2Cl2 (ca. 1.26 V vs NHE). Co(eta-4-1) slowly oxidized water, yielding H[Co(III)(eta-4-1)], which may also be prepared by the reaction of [Co(III)(eta-4-1)]- with HBF4. Both the redox and the acid/base chemistries of [Co(III)(eta-4-1)]- are reversible. Electrochemical and EPR data are also presented for Co(eta-4-2) and Co(eta-4-3).
The reaction of Cu, Cu(BF4)2, PPh3, and 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in the appropriate amounts gave the stable radical complex [(Ph3P)2Cu(η4,μ-bptz)Cu(PPh3)2](BF4), whose crystal structure demonstrates π(Ph)/π(bptz•-)/π(Ph) interactions and the localization of the unpaired electron in the central tetrazine ring.
Two nonmacrocyclic ligands with pyridine and carboxamide nitrogens as donors readily form his complexes of the type [(FeL2)-L-III](+) in which the Fe(III) centers exhibit preference for coordination to carboxamido nitrogens over pyridine nitrogens. These Fe(III) complexes with deprotonated carboxamido nitrogens are very stable in water.
Using a dipeptide ligand 2,6-bis(N-phenylcarbamoyl)pyridine, in its dianionic form L(2−), [Et4N]2[NiIIL2]·H2O (1), [Et4N][NiIIIL2]·H2O (2), and [NiIVL2]·0.75H2O (3) have been synthesized. X-ray structures of 1 and 3 reveal tetragonally compressed octahedral geometry around the metal center. Magnetic studies (63−300 K) reveal the spin states of 1 and 2 as S = 1 and S = 1/2, respectively. EPR (77 K) spectrum reveals a dz2 ground state for 2. Complex 3 is diamagentic. These complexes give rise to a clean chemically reversible three-member electron-transfer series (cyclic voltammetry).
Reaction of N-(2-(4-imidazolyl)ethyl)pyrimidine-4-carboxamide (PrpepH, 4), a ligand that resembles a portion of the metal-chelating locus of the antitumor drug bleomycin (BLM), with FeCl 2·2H 2O and 1,8-bis(dimethylamino)naphthalene in methanol affords the "bis" complex [Fe(Prpep) 2]·2CH 3OH (5) in good yield. The Fe(III) complex [Fe(Prpep) 2]ClO 4·2CH 3OH·CH 3CN (6) is obtained from a reaction mixture containing [Fe(DMSO) 6](ClO 4) 3·DMSO, PrpepH, and 1,8-bis(dimethylamino)naphthalene in acetonitrile/methanol. The Fe(II) complex 5 crystallizes in the triclinic space group P1 with a = 8.939 (2) Å, b = 9.777 (2) Å, c = 14.953 (3) Å, α = 79.29 (2)°, β = 72.72 (2)°, γ = 85.30 (2)°, V = 1225.6 (5) Å 3, and Z = 2 while 6 crystallizes in the monoclinic space group P2 1/c with a = 11.499 (2) Å, b = 16.544 (4) Å, c = 15.816 (3) Å, β = 100.06 (2)°, V = 2963 (1) Å 3, and Z = 4. In both complexes, the ligand is anionic and tridentate. The most noteworthy observation in these two complexes is the coordination of the deprotonated amido nitrogen to the iron center in two oxidation states. Along with [Fe(Pypep) 2]Cl·2H 2O (3), a complex reported previously from this laboratory, 5 and 6 constitute the first examples of complexes containing Fe(II)/ Fe(III)-N pep (pep = deprotonated amido nitrogen) bonds. Successful isolation of these complexes demonstrates the possibility that the β-hydroxyhistidine moiety of BLM remains coordinated to iron in Fe-BLM during the redox cycles. The role of the pyrimidine ring of BLM in the stabilization of the iron center in Fe(II)-BLM is also discussed.
The temperature dependence of the magnetic susceptibility (4.2-300 K) is reported for a series of crystalline salts of first-row transition-metal ions encapsulated by hexaamine ligands of the sar type, namely, [M(sar)]n+, where sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane, or [M((X,Y)sar)]n+, where X = Y= 1,8-NH2 1,8-NH3+ or X = 1-CH3, Y = 8-H. The cage complexes of FeIII, CoIII, and NiIII all exhibit low-spin ground states [of 2T2g and 1A1g (Oh) and 2A1g (D4h) origin] whereas that of MnIII is high-spin [5A1g or 5B1g (D4h)]. The complexes of MnII, CoII, and NiII are high-spin [of 6A1g, 4T1g, and 3A2g (Oh) origin], but surprisingly, those of FeII exhibit either a low-spin (1A1g) or a high-spin (5T2g) ground state depending on the nature of the apical substituent and the lattice. Clearly, the magnitude of the ligand field parameters (10Dq and B)† generated by these saturated macrobicycles for the 3d6 FeII is that required for the high-spin/low-spin crossover for six saturated amine ligands. The ground states for VIV (2T2g), VIII (3T1g), CrIII (4A2g), CuII (2B1g in D4h), and ZnII (1A1g) appear to be unambiguous. The magnitude of the zero-field-splitting parameter has been estimated from the low-temperature χ(T) data for the CrIII and high-spin MnIII, MnII, FeII, CoII, and NiII cage complexes. The low-spin (2Eg origin) state for CoII is stabilized by the (aza)capten ligand because of the larger nephelauxetic effect of sulfur combined with appreciable Jahn-Teller splitting of the 2Eg ground state. The magnetic properties in general reflect the reduction in symmetry observed crystallographically for the individual metal ions.
Reaction of the peptide ligand PypepH (1), which resembles part of the metal-chelating section of bleomycins (BLM), with (Et4N)[FeCl4] in ethanol affords the iron(III) complex [Fe(Pypep)2]Cl·2H2O (2). The structure of this synthetic analogue of Fe(III)-BLM is reported. The complex crystallizes in the triclinic space group P1 with a = 11.080 (5) Å, b = 9.319 (4) Å, c = 13.665 (5) Å, α = 112.75 (3)°, β = 103.92 (3)°, γ = 73.53 (3)°, and Z = 2. The structure was refined to R = 4.46% by using 1938 unique data (Fo2 > 3σ(Fo2)). The coordination geometry around iron(III) is octahedral with average Fe-N(imidazole) = 1.952 (4) Å and Fe-N(pyridine) = 1.982 (4) Å, respectively. The Fe(III)-N(peptide) bond is 1.957 (4) Å long. The complex is isolated as the mer isomer. Variable-temperature Mössbauer spectra and magnetic susceptibility measurement at ambient temperature reveal that the iron is in the +3 oxidation state with a low-spin electronic configuration. The dark red ferric complex (2) can be electrochemically and chemically reduced to purple ferrous species. The electronic absorption spectrum of the reduced species is reported.
(Phenylazo)benzaldoxime, PhN=NC(Ph)=NOH (Hpbo), reacts with K2PtCl4 in alkaline media affording trans-Pt(pbo)(2). One-electron-one-proton electroprotic transformation converts it to a cis complex isolated as the dimer [cis-Pt(pbo)(Hpbo)](2). Both complexes are diamagnetic in the solid state. In trans-Pt(pbo)(2), the PtN4 coordination sphere is planar, the metal atom being a center of inversion. In the cis dimer one monomer is stacked on the other such that the Pt...Pt (3.235(1) Angstrom) midpoint is a center of inversion. Each metal atom is displaced by 0.088 Angstrom from its N-4 plane toward its neighbor. Unsymmetrical and nearly linear O...H-O bridging is present within each monomer. In dichloromethane solution, the equilibrium dimer (s = 0) reversible arrow 2 monomer (s = 1/2) exists. The paramagnetic monomer is EPR-active (g = 1.987). The intensity of the EPR-signal as well as the paramagnetic moment progressively diminishes with decreasing temperature due to the shift of the equilibrium to the left. The Delta H and Delta S values associated with dissociation are 8.3(+/- 2) kcal mol(-1) and 23.5(5) cal mol(-1) K-1, respectively. EHMO studies have revealed that the unpaired electron in the cis monomer (idealized point group C-s) resides in an azoimine orbital (3A'') with a large azo-pi(*) contribution. The EPR spectrum is consistent with this. The cis monomer is essentially a stable free radical. In dimer (C-i) formation 3A''-3A'' and d(z)2-d(z)2 interactions contribute significantly to binding. The doubly occupied 6A(g) HOMO of the dimer corresponds to the bonding 3A''-3A'' combination. Accordingly the average N=N length in the cis dimer (1.31 Angstrom) is longer than that in the trans complex (1.28 Angstrom) in which the azo-pi(*) orbitals are empty and constitute the LUMO. The cis monomer displays quasireversible one-electron oxidation (E(1/2), 0.52 V vs SCE) and in presence of NEt(3) the oxidized complex is deprotonated affording the sterically favorable trans-Pt(pbo)(2). The latter is reconverted to the O...H-O stabilized cis complex via one-electron reduction (E(1/2), -0.28 V) and proton addition. In this remarkable family, metal geometry is controlled by electroprotic transfer at ligand sites: azo-pi(*) (electron transfer) and oximato oxygen (proton transfer). Crystal data for the complexes are as follows. trans-Pt(pbo)(2): crystal system monoclinic; space group P2(1)/n; a = 5.646(4) Angstrom, b = 10.784(7) Angstrom, c = 18.367(14) Angstrom; beta =,98.11(5)0; V = 1107(1) Angstrom(3); Z = 2; R = 2.74%; R(W) = 3.20%. [cis-Pt(pbo)(Hpbo)](2): crystal system monoclinic; space group C2/c; a = 25.183(8) Angstrom, b = 8.849(4) Angstrom, c = 20.839(8) Angstrom; beta = 90.09(3)degrees; V = 4644(3) Angstrom(3); Z = 4; R = 2.42%; R(W) = 2.90%.
The reaction rates of the conversion of various substituted picolinic and quinaldinic acids to the corresponding trichloromethylpyridines and quinolines by treatment with phosphorus pentachloride in refluxing thionyl chloride have been measured, and the apparent second-order kinetic constants determined. The logarithmic rate constants (log k) were found to be linear with the basicities (pKa) of the unsaturated ring nitrogens in the substrates. The reaction constant, p, of the conversion was estimated.
A ferric complex, (Et3HN)Fe(III)(bpb)Cl2, has been synthesized, and its structure has been determined by X-ray crystallography. This complex and its triflate derivative, (Et3HN)Fe(III)(bpb)(OTf)2, are found to catalyze the epoxidation of a variety of olefins by iodosylbenzene, OIPh. These reactions give little allylic oxidation of cyclohexene and stereochemical retention with cis-stilbene. Al(OTf)3, a nonredox metal salt, has also been found to catalyze the epoxidation of cyclohexene by iodosylbenzene, and the reactivity is quite similar to that of Fe(OTf)3, which we studied previously. In addition to epoxides, other products were observed. For the reactions containing Fe(OTf)3, Al(OTf)3, or (Et3HN)Fe(III)(bpb)(OTf)2, cis-1,2-cyclohexanediol ditriflate and 3-acetamidocyclohexene were found. The amide oxygen in 3-acetamidocyclohexene was derived from iodosylbenzene as verified by isotopic labeling using (OIPh)-O-18. For the reactions containing (Et3HN)Fe(III)(bpb)Cl2, FeCl3, or AlCl3, trans-1,2-dichlorocyclohexane and 3-chlorocyclohexene were observed. 1,4-Diiodobenzene was found in all of the reactions. The presence of these products suggests strongly that the mechanisms of these reactions are related to those occurring between soluble iodine(III)-containing compounds and olefins in the absence of any metal catalysts. A new mechanism that accounts for all of the products is proposed which involves electrophilic attack on the olefin by the iodine(III) center in a metal-iodosylbenzene complex. The reactions of PhI(OAc)2 with norbornenecarboxylic acid or norbornene in different solvents were also investigated. The products isolated were shown to be 5-(acetyloxy)-3,3a,4,5,6,6a-hexahydro[3-beta,3a-alpha,5-alpha,6-beta,6a-alpha]-3,6-methano-2H-cyclopenta[b]furan-2-one (1), 5-acetamido-3,3a,4,5,6,6a-hexahydro[3-beta,3a-alpha,5-alpha,6-beta,6a-alpha]-3,6-methano-2H-cyclopenta[b]furan-2-one (2), and exo-2-acetoxy-syn-7-acetamidonorbornane (3). The structures of 1 and 3 were determined by X-ray crystallography. The formation of these products provides additional evidence for the electrophilic character of iodine(III) compounds.
One iron(II) complex of N-2-pyridinylcarbonyl-2-pyridinecarboximidate monoanion (bpca), [Fe(bpca)2] . H2O (1), and three iron(III) complexes, [Fe(bpca)Cl2(H2O)] . (CH3)2CO (2), [Fe(bpca)2](NO3) . 1.67H2O (3), [Fe(bpca)2](ClO4) (4), have been prepared and characterized by means of IR, UV-Vis and EPR spectroscopy and magnetic susceptibility measurements, where appropriate. The crystal structures of 1-3 have been determined from single crystal X-ray data, showing that the metallic ions are surrounded in all cases by distorted octahedral coordination spheres. From magnetic measurements it is evident that compounds 1 and 3 are practically in the low-spin state at room temperature, compound 2 is in the high-spin state in all the range of measured temperatures and compound 4 shows spin-crossover behaviour. Crystal data for 1: space group P2(1)/c, a = 8.827(1), b = 8.941(2), c = 28.957(4) angstrom, beta = 95.90(1)-degrees, Z = 4; for 2: space group P2(1)/c, a = 8.293(2), b = 13.779(2), c = 15.884(4) angstrom, beta = 98.22(1)-degrees, Z = 4; for 3: space group P1BAR, a = 8.840(1), b = 10.713(1), c = 14.989(1) angstrom, alpha = 71.16(1), beta = 73.43(1), gamma = 82.67(1)-degrees, Z = 2.
Synthetic iron(IV) complexes are attracting interest both as models for putative intermediates of biochemical reactions as well as for proposed catalytic entities. We have previously reported the X-ray structure of [Et4N][Fe(IV)Cl(eta4-MAC)] (H-4[MAC] = 1,4,8,11-tetraaza-13,13-diethyl-2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocy-clotetradecane), where iron is coordinated to a plane of four amide nitrogen anions of a macrocyclic ligand and to one axial chloride (Collins, T. J.; Kostka, K. L.; Munck, E.; Uffelman, E. J. Am. Chem. Soc. 1990,112, 5637-5639). In zero magnetic field, the 4.2 K Mossbauer spectrum of [Et4N] [FeIVCl(eta4-MAC)] was a single quadrupole doublet with DELTAE(Q) = 0.89 mm/s and delta = -0.02 mm/s, consistent with an iron(IV) assignment. Here we present full synthetic and characterization results together with detailed Mossbauer and integer spin EPR studies of [FeIVCl(eta4-MAC)]- prepared with [Ph4P]+, [Et4N]+, [n-Bu4N]+, and [PPN]+ counterions. In strong applied fields, the Mossbauer spectra exhibit magnetic hyperfine patterns typical of complexes with integer electronic spin. The zero-field splitting parameters (D = -2.6 cm-1 and E/D = 0. 1 3) are such that the two lowest spin levels of the ground multiplet are nearly degenerate (DELTA = 0.16 cm-1). Correspondingly, [FeIVCl(eta4-MAC)]- exhibits an integer spin EPR resonance at X-band with g(eff) = 8.0, indicative of a high-spin (S = 2) ground configuration. Quantitative analysis of the integer spin EPR spectra observed from both frozen CH3CN solution and from polycrystalline samples shows that the principal g-values are less than 2, as expected for high-spin iron(IV), and that the spin concentration of the S = 2 species agrees within 12% with the concentration determined by optical spectroscopy. We also report synthetic details and the X-ray structure of [Et4N]2[FeIIICl(eta4-MAC)].CH2Cl2.H2O. Crystals of [Et4N]2[FeIIICl(eta4-MAC)].CH2Cl2.H2O at 293 K are monoclinic, space group P2(1)/n, with a = 11.797(9) angstrom, b = 18.662(6) angstrom, c = 21.984(8) angstrom, beta = 102.75(6)degrees, V = 4708.6 angstrom3, Z = 4 (d(calcd) = 1.423 g cm-3), mu(alpha)(Mo Kalpha) = 3.92 cm-1, and R1 (unweighted, based on F) = 0.086 for 3298 observed reflections [I > 3sigma(I)]. Mossbauer and EPR studies show that this complex has an intermediate-spin (S = 3/2) ground state with hyperfine parameters similar to those reported for the porphyrin complex [FeIII(TPP)(FSbF5)] (Gupta, G. P.; Lang, G.; Reed, C. A.; Shelly, K.; Scheidt, W. R. J. Chem. Phys. 1987,86,5288-5293). However, the zero-field splitting tensor of [Et4N]2[FeIIICl(eta4-MAC)] (D = -3.7 cm-1, E/D = 0.05) is rotated by 90-degrees relative to the magnetic hyperfine and electric field gradient tensors. Analysis of the high-field Mossbauer spectra for both [FeIIICl(eta4-MAC)]2- and [FeIVCl(eta4-MAC)]- reveals magnetic hyperfine interactions substantially smaller than expected for an ionic complex, suggesting substantial covalency for both redox states.
The redox parameters of [Fe(PyPepS)(2)](-), the first low-spin Fe(III) complex with both carboxamido N and thiolato S donors in the first coordination sphere, indicate significant stabilization of the +3 oxidation state of iron. Such stabilization could account for the reactivity of the iron center in nitrile hydratase.
Two novel low-spin complexes, [Et4N][FeL2.15H(2)O (1) and [Et4N[CoL2].2H2O (2) (L is a deprotonated bis-amide ligand), have been synthesized and characterized. Four relatively longer equatorial M-N-amide bonds and two significantly shorter axial M-N-py bonds are the noteworthy features of their X-ray structures. EPR and (34-300 K) magnetic studies of 1 confirm its low-spin character. Cyclic voltammetric studies reveal a highly stabilized M(III) state. For I a linear correlation between the Fe(III)-Fe(II) reduction potentials and the reciprocal of solvent dielectric constants is obtained.
- Ray M.