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[Bis(tetrahydrofuran–O)–bis(1,3–dialkyl–2–diphenylphosphanyl–1,3–diazaallyl)calcium] – Synthesis and Crystal Structures of Calcium Bis[phospha(III)guanidinates] and Investigations of Catalytic Activity
Abstract
The reaction of [(thf)4Ca(PPh2)2] (1) with diisopropyl– and dicyclohexylcarbodiimides yields the phospha(III)guanidinates [(thf)2Ca{RNC(PPh2)NR}2] with R = isopropyl (2) and cyclohexyl (3). The metathesis reaction of K{RNC(PPh2)NR} with anhydrous CaI2 also allows the synthesis of these phospha(III)guanidinate complexes 2 and 3. For 2 a cis arrangement is observed whereas 3 crystallizes as trans isomer. The phospha(III)guanidinates act as bidentate chelate bases with an average Ca–N distance of 242.5 pm. The C–P bond length between the PPh2 fragment and the 1,3–diazaallyl unit is with values above 190 pm very large. The complexes 2 and 3 show a moderate catalytic activity in hydrophosphanylation reactions of dialkylcarbodiimides with diphenylphosphane.
... The C-P bond length between the PPh 2 fragment and the 1,3iazaallyl unit is with values above 1.90 Å very large. [47] t of C(N i Pr) 2 B(C 6 F 5 ) 2 with CO 2 or excess carbodiimide gave HC(N i Pr) 2 (CO 2 )B(C 6 F 5 ) 2 or HC(N i Pr) 2 C(N i Pr) 2 B(C 6 F 5 ) 2 , respectively (Scheme 31). ...
... [43] The calcium-bis(phosphaguanidinates) shown in Scheme 30 were found to exhibit a moderate catalytic activity in hydrophosphination reactions of dialkylcarbodiimides with diphenylphosphane (Scheme 381). [47] Scheme 381 Ca-phosphaguanidinate-catalyzed hydrophosphination of dialkylcarbo- ...
This review provides a comprehensive overview of the most recent progress in chemistry and applications of metal complexes containing heteroallylic ligands such as amidinates and guanidinates. Clearly, the coordination chemistry of amidinates and guanidinates has reached a state of maturity and continues to be a highly popular area of research. These heteroallylic ligand systems allow a wealth of variations and modifications, making a larger ligand library available than in cyclopentadienyl chemistry. Exciting results have been obtained in recent years for almost any metallic elements in the Periodic Table. Truly remarkable developments include, for example, the chemistry of cyclic amidinate-based silylenes and the stabilization of metal–metal quadruply and quintuple bonds by amidinate and guanidinate ligands. The range of applications for metal amidinates and guanidinates in homogeneous catalysis has considerably broadened in recent years. In materials science, alkyl-substituted metal amidinates and guanidinates are now well established as volatile precursors for a variety of ALD and MOCVD processes. Without doubt the chemistry of metal amidinates and guanidinates and related complexes will continue to produce exciting results and applications in the years to come.
Organophosphorus Chemistry provides a comprehensive and critical review of the recent literature. Coverage includes phosphines and their chalcogenides, phosphonium salts, low coordination number phosphorus compounds, penta- and hexa- coordinated compounds, quiquevalent phosphorus acids, nucleotides and nucleic aicds, ylides and related compounds, phosphazenes and the application of physical methods in the study of organophosphorus compounds.
This is the 40th in a series of volumes which first appeared in 1970 under the editorship of Stuart Trippett and which covered the literature of organophosphorus chemistry published in the period from January 1968 to June 1969, citing some 1370 publications. The present volume covers the literature from January 2009 to January 2010, citing more than 2200 publications, continuing our efforts to provide an up to date survey of progress in an area of chemistry that has expanded significantly over the past 40 years.
The terminal antimony phosphanides Sb(NONDipp)(PR2) (NONDipp=[O(SiMe2NDipp)2]²⁻; Dipp=2,6‐iPr2C6H3; R=Cy, Ph) were assessed for catalytic hydrophosphination reactivity. Stoichiometric reactions showed that phenylisocyanate inserts into the Sb−P bond to afford the phosphanylcarboxamidate complex, and subsequent reaction with Cy2P−H confirmed liberation of Cy2PC(O)NHPh. Catalytic hydrophosphination of PhN=C=O by Ph2P−H was demonstrated using 1 mol % catalyst at room temperature. A mixture of mono‐ and di‐insertion products were produced under these conditions.
In the space of 20 years, molecular complexes of the large alkaline earths calcium, strontium and barium have gone from being mere oddities to some of the most exciting catalysts in the landscape of homogenous metal-mediated catalysis. This article reviews the main achievements in the area of organic transformations catalyzed by alkaline-earth complexes. It encompasses a broad range of hydrofunctionalization reactions (e.g., hydroamination, hydrophosphination and hydrogenation) and dehydrocouplings, as well as a range of other reactions that do not fit into any specific category. A survey of the sophisticated processes catalyzed by alkaline-earth Lewis acids is included.
A catalyst-free, low-solvent method for the hydrophosphinylation of isocyanates and isothiocyanates is reported. A range of phosphorus nucleophiles including secondary phosphine oxides HP(O)R2 (R = Ph, ⁱPr), phosphites HP(O)(OR)2 (R = Me, Et), and methyl phenylphosphinate were tested. The procedure tolerated isocyanates and isothiocyanates featuring a wide range of substituents and, with use of 4 equiv of 2-methyltetrahydrofuran (2-MeTHF), solid substrates can be utilized. Twenty-five compounds were prepared with improved functional group tolerance compared to previous methods allowing access to new compounds (16 are novel). Facile scale up and simple reaction conditions make this a straightforward and practical methodology for obtaining phosphorus analogues of ureas and thioureas, which are challenging to synthesize by other methods.
Hydrofunctionalization (hydroelementation) of alkynes and heterocumulenes such as iso(thio)cyanates and carbodiimides requires a catalyst. Complexes of the essential, nontoxic, and globally abundant s‐block metals potassium and calcium represent ideal catalyst candidates with a sufficiently high reactivity. In this overview, catalytic addition of amines (hydroamination), phosphanes (hydrophosphanylation and hydrophosphination), and phosphane oxides (hydrophosphorylation) across alkynes and heterocumulenes mediated by s‐block metal species is evaluated. Generally, anti‐Markovnikov products are obtained, which undergo subsequent transformations depending on the reaction conditions. Certain requirements with respect to electronic and steric properties of the substrates and catalysts as well as solvent prerequisites and reaction conditions are addressed. Mechanisms of hydrofunctionalization reactions are discussed.
Metalation of the formamidine Dipp−N=C(H)−N(H)−C2H4−Py (1a) and benzamidine Dipp−N=C(Ph)−N(H)−C2H4−Py (1b) with [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) yields the corresponding homoleptic complexes [M{Dipp−N=C(R)−N−C2H4−Py}2] (M/R = Ca/H (2a), Ca/Ph (2b), and Sr/Ph (3b)) regardless of the applied stoichiometry. Only during calciation of Dipp−N=C(H)−N(H)−C2H4−Py (1a), the heteroleptic intermediate [{(Me3Si)2N}Ca{Dipp−N=C(R)−N−C2H4−Py}]2 (2a’) has been observed. The formamidinate complex of strontium crystallizes as a tmeda adduct of the type [(tmeda)Sr{Dipp−N=C(H)−N−C2H4−Py}2] (3a). Metalation of the pivalamidine Dipp−N=C(tBu)−N(H)−C2H4−Py (1c) leads to the formation of heteroleptic mononuclear [{(Me3Si)2N}M{Dipp−N=C(tBu)−N(H)−C2H4−Py}] (M = Ca (2c) and Sr (3c)) with a side-on bonding of the Dipp group to the alkaline earth metals. Calciation of chiral Dipp−N=C(tBu)−N(H)−CH2CH(Me)−Py (R)-1d and (S)-1d with [(thf)2Ca{N(SiMe3)2}2] yields the homoleptic complexes [Ca{Dipp−N=C(tBu)−N−CH2CH(Me)−Py}2] with the enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. The complexes 2c and 2d catalyze the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-methyl-4,4-diphenylpyrrolidine but the stereochemistry cannot be influenced by the chiral compounds (R,R)-2d and (S,S)-2d.
Phenyl isocyanate inserts into the Bi-P bond of the terminal phosphanide Bi(NON(Ar))(PCy2) (NON(Ar) = [O(SiMe2NAr)2](2-) Ar = 2,6-iPr2C6H3) to afford the κ(2)N,O-phosphanylcarboxamidate complex. Liberation of the hydrophosphination product from the metal is achieved by reaction with HPPh2. The diphenylphosphanide product, Bi(NON(Ar))(PPh2), is however unstable and decomposes to generate reduced species, preventing catalytic turnover.
The preparation of the hydrophosphorylation catalysts succeeds via the metalation of dimesitylphosphane oxide and diphenylphosphane sulfide with potassium hydride in ethereal solvents such as tetrahydropyran (THP) and tetrahydrofuran (THF) yielding the tetramers [(thp)K(OPMes2)]4 (1a) and [(thf)3{K(OPMes2)}4] (1b) as well as [(thp)KSPPh2]∞ (2) with a strand-like structure in the crystalline state. In ethereal solution these complexes very slowly degrade into KPAr2 and KE2PAr2 (E = O, S). The catalytic conversion of iPr-N═C═E' (E' = O, S) and of R-N═C═N-R (R = iPr, cHex) to the addition products Ar2P(E)-C(=E')-NHR (Ar = Ph, Mes; E = O, S; E' = O, S, NR) was studied in the presence of catalytic amounts of Ar2PEK (Ar = Ph, Mes; E = O, S). Steric hindrance prevents the addition of dimesitylphosphane oxide to N,N'-diisopropylcarbodiimide, whereas diphenylphosphane oxide and sulfide smoothly add to iPr-N═C═N-iPr yielding Ph2P(E)-C(═N-iPr)-NHiPr (E = O, S).
Despite their highly electropositive character and almost universally divalent metal centers, the coordination and organometallic compounds of the alkaline-earth (group 2) elements (beryllium to barium) display remarkable diversity. A variety of synthetic methods can be used to produce them, from direct oxidation of the metals to ligand-exchange reactions. The change in radius from Be2+ to Ba2+ is nearly sixfold, and there is a correspondingly large range of coordination numbers and structural types known - from monomers to three-dimensional coordination polymers. Widely used sterically bulky ligands have greatly improved the solubility and modified the reactivity of alkaline-earth compounds, and computations increasingly have been used to describe their metal-ligand interactions, including the involvement of metal d orbitals in bonding.
The synthesis of the guanidine MesN{C(NCy2)}N(H)Mes (LH; Mes = 2,4,6-Me3C6H2, Cy = cyclohexyl), and its use as a proligand for the synthesis of alkaline earth metal complexes are reported. Described herein are (i) an unusual Hauser base cubane, (ii) a homoleptic and a base-stabilized magnesium complex featuring the same guanidinate ligands, and (iii) the comparison of a series of alkaline earth (Mg, Ca, Sr, Ba) bis(guanidinate) complexes, which allows the opportunity to compare the changing trends in bonding as the Group is descended. The reaction between LH and MeMgI(OEt2)2 yields the Hauser base as a mixture of the tetramer [Mg4L4(μ3-I)4] (1a) and dimer [Mg2L2(μ-I)2(OEt2)2] (1b), and the reaction with two equivalents of MgnBu2 leads to the formation of four-coordinate [MgL2] (2), which features a square-planar geometry for the magnesium cation, or five-coordinate [MgL2(THF)] (3), depending on the solvent used. 1a is the first crystallographically-characterized cubane structure to consist of four LAeX (L = ligand, X = halide) units. The complexes [AeL2(THF)2] (Ae = Ca, 4; Ae = Sr, 5) and [BaL2] (6) were synthesized via redox transmetallation/ligand exchange reactions. Complex 6 is the first example of a homoleptic, monomeric barium complex of the NCN ligand family, with the structure stabilized by a number of barium-arene interactions in the solid state.
A catalytic heavier alkaline earth chemistry somewhat reminiscent of d0 transition metal catalysis has emerged in the first decade of the twenty-first century. As complexes of these elements are effectively redox inactive, catalytic reactivity is dependent on the viability of sequential σ-bond metathesis and/or polarized insertion steps. This chapter reviews this reactivity with an emphasis upon the types of processes which have been reported as well as the fundamental insights into alkaline earth reactivity that have been deduced from catalytic and stoichiometric mechanistic studies.
The hydroamination of diphenylbutadiyne with primary
arylamines requires a reactive catalyst. In the presence of heterobimetallic
K2[Ca{N(H)Dipp}4] (Dipp = 2,6-diisopropylphenyl) the performance of this
reaction in THF yields 2-tert-butyl-6,7,10,11-tetraphenyl-9H-cyclohepta[c]-
quinoline (1a) and 2-fluoro-6,7,10,11-tetraphenyl-9H-cyclohepta[c]quinoline
(1b) within 3 days at room temperature when 4-tert-butyl- and 4-
fluoroaniline, respectively, have been used. During this catalysis o-CH
activation occurs and quinoline derivatives are formed. Blocking the o-CH
positions by methyl groups and use of 2,4,6-trimethylaniline under similar
reaction conditions leads to the formation of N-mesityl-7-(E)-((mesitylimino)(phenyl)methyl)-2,3,6-triphenylcyclohepta-1,3,6-
trienylamine (2) containing a β-diketimine unit with a N−H···N hydrogen bridge. NMR experiments with labeled 4-tertbutylaniline
verify the transfer of N-bound hydrogen atoms to the newly formed cycloheptatriene ring. If the s-block-metalmediated
hydroamination of diphenylbutadiyne is performed in refluxing THF for 6 days, N-aryl-2,5-diphenylpyrroles 3a−d (3a,
R = tBu, R′ = H; 3b, R = F, R′ = H; 3c, R = R′ = Me; 3d, R = R′ = H) are obtained regardless of the substitution pattern of the
arylamines.
Reaction of the phospha(III)guanidine, Ph2PC{NCy}{NHCy}, with tert-butyl hydroperoxide resulted in isolation of [HC(NHCy)(2)][O2PPh2] via cleavage of the P-C-amidine bond. The crystal structure shows chains of ion-pairs linked by NH center dot center dot center dot O hydrogen-bonding in the solid-state. Protonation of the starting material using anhydrous HCl afforded the phospha(III)guanidinium chloride, [Ph2PC(NHCy)(2)][Cl] in which the integrity of the P-C-amidine bond was maintained. The crystal structure reveals a single hydrogen-bonding interaction between NH and the chloride ion.
Metalation of N,N′-bis(2,6-diisopropylphenyl)benzamidine (1a) and -pivalamidine (1b) in tetrahydrofuran (THF) with n-butyllithium and potassium bis(trimethylsilyl)amide gives the corresponding N,N′-bis(2,6-diisopropylphenyl)benzamidinates and -pivalamidinates of lithium, [(thf)2Li{(Dipp-N)2C-Ph}] (2a) and [(thf)Li{(Dipp-N)2C-tBu}] (2b), as well as of potassium, [(thf)3K{(Dipp-N)2C-Ph}] (3a) and [(thf)3K{(Dipp-N)2C-tBu}] (3b) (Dipp = 2,6-diisopropylphenyl). Because metalation of these amidines is not possible with [(thf)2Ca{N(SiMe3)2}2], a metathetical approach has been chosen. Hence, the reaction of 3a with calcium iodide in THF yields (tetrahydrofuran)calcium bis[N,N′-bis(2,6-diisopropylphenyl)benzamidinate] (4a). For the bulkier N,N′-bis(2,6-diisopropylphenyl)pivalamidinate, heteroleptic [(thf)Ca{(Dipp-N)2C-tBu}{N(SiMe3)2}] (4b) forms. Depending on the softness and charge-to-radius ratio, syn configuration of the amidinate ligands is observed for 2a, 4a, and 4b, whereas the anti configuration is realized in complexes 2b, 3a, and 3b the latter being stabilized by intramolecular metal–π interactions.
An attempt to prepare Tmp-C(NDipp)2Li by treatment of lithium tetramethylpiperidide (Li-Tmp) with N,N′-bis(2,6-diisopropylphenyl)carbodiimide (Dipp-N=C=N-Dipp) failed, and ether degradation occurred instead, allowing the isolation of a few crystals of O=CH–CH=C(NH-Dipp)2 (1) after hydrolytic work-up. We then investigated calcium and strontium compounds. Metalation of PrisoH [Priso = iPr2N–C(N-Dipp)2] with [(thf)2Ca{N(SiMe3)2}2] in refluxing hexane for 18 h yielded [{(Me3Si)2N}(thf)Ca(Priso)] (2). At room temperature, very few crystals of [Ca(Priso)2]·hexane (3) precipitated after several months from a saturated hexane solution of 2. KN(SiMe3)2 was added to a THF solution of [(thf)4CaI2] and PrisoH and heated to 60 °C for several hours yielding [(thf)Ca(Priso)(μ-I)]2 (4). Metalation of PrisoH with [(thf)2Sr{N(SiMe3)2}2] led to formation of [Sr(Priso)2] (5) regardless of the applied stoichiometry. The reaction of N,N′-bis(2,6-diisopropylphenyl)benzamidine with KN(SiMe3)2 and [(thf)4CaI2] in THF followed by recrystallization from 1,2-dimethoxyethane (dme) solution yielded [(dme)Ca{(Dipp-N)2C–Ph}2] (6). The molecular structures of these compounds are discussed and compared with those of other encumbered complexes of divalent metal ions.
The synthesis, characterization, and decomposition pathway of the 18-crown-6 adduct of bis(allyl)calcium [Ca(η1-C3H5)(η3-C3H5)(18-crown-6)] (2) are reported. The solid-state structure of adduct 2 features one σ- and one π-bonded allyl ligand at the metal center. 2 is the first structurally characterized example of a mononuclear calcium complex bearing a purely σ-bound allyl ligand. DFT calculations indicate that [Ca(η1-C3H5)(η3-C3H5)(18-crown-6)] and [Ca(η1-C3H5)2(18-crown-6)] conformations are close in energy. In THF solution, η3-allyl ligands are observed exclusively at both ambient temperature and −95 °C. 2 undergoes rapid cleavage of the crown ether to give vinyl-terminated alcoholates of different chain length with a half-life of t1/2 = 2.5 h.
Metalation of 1-(2-furanylmethyl)- (1a) and 1-(2-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaprop-2-ene (1b) with n-butyllithium and potassium bis(trimethylsilyl)amide in tetrahydrofuran (THF) yields the corresponding lithium (2a,b) and potassium derivatives (3a,b), with the alkali metals binding to the furanylmethylamido and pyridylmethylamido pockets. The calcium derivatives are accessible via a metathetical approach of the potassium complexes with calcium iodide in THF. Whereas bis[1-(2-furanylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaallyl]calcium (4a) precipitates as a thf adduct, bis[1-(2-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaallyl]calcium (4b) crystallized without solvent coligands. Instead of coordinated solvent molecules, strong calcium−π interactions to an aryl group saturate the coordination sphere. Bidentate 1,2-dimethoxyethane (DME), however, is able to replace this side-on bound aryl group, leading to the dme adduct of bis[1-(2-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diaallyl]calcium (4c). In all of these s-block metal complexes, strong agostic interactions between the cations and the tert-butyl groups stabilize these complexes.
Two coordination complexes, [Mg(HPDC)2(H2O)2] (1) and [Dy(PDC)2(H2O)2]·5H2O·NH4 (2), were synthesized under hydrothermal conditions. The structures and fluorescent properties of the complexes were studied by elemental analysis, IR, thermal analysis, powder X-ray diffraction, and X-ray crystallography. In 1, Mg(II) coordinated with six oxygens from H2PDC and water. For 2, there were NH4+ ions in the holes of the metal quarternary rings to balance electronic charges. Compared to 1, 2 constructed from Dy(III) had larger void space and potential applications on gas adsorption. The two complexes show similar fluorescence in the visible region.
The synthesis and catalytic properties of a series of magnesium compounds consisting of monoanionic, N,N'-chelating ligands (N∩N = amidinates, guanidinates, phosphaguanidinates) is reported. The compounds were synthesized by (i) insertion of a carbodiimide into an existing Mg-C or Mg-N bond, or (ii) protonolysis of an organomagnesium compound by a neutral pre-ligand. Structural analyses of mono- or bis-(chelate) compounds with general formula Mg(N∩N)X(L)n and Mg(N∩N)2(L)n (X = halide, aryloxide, amide; L = Et2O, THF; n = 0, 1 or 2) have been performed and the influence that the ligand substituent patterns have on the solid-state structures has been probed. Selected examples of the compounds were tested as (pre)catalysts for the polymerization of lactide, the dimerization of aldehydes and the hydroacetylenation of carbodiimides.
The synthesis and characterization of magnesium and calcium complexes of sterically demanding aminopyridinato ligands is reported. The reaction of the 2-Me3SiNH-6-MeC5H3N (L(1)H), 2-MePh2SiNH-6-MeC5H3N (L(2)H), and 2-Me3SiNH-6-PhC5H3N (L(3)H) with KH in tetrahydrofuran (THF) yielded potassium salts L(1)K(thf)0.5 (1), L(2)K (2), and L(3)K(thf)0.5 (3), which, through subsequent reaction with MgI2 and CaI2, afforded the homoleptic complexes (L)2Ae(thf)n [L = L(1), Ae = Mg, n = 1 (4); L = L(2), Ae = Mg, n = 0 (5); L = L(3), Ae = Mg, n = 0 (6); L = L(2), Ae = Ca, n = 2 (7)] and heterobimetallic calciates {[(L)3Ca]K}∞ [L = L(1) (8); L = L(2) (9)]. The solid state structure of 8 reveals a polymeric arrangement in which the calciate units are interlocked by bridging potassium ions. Metalation reactions between L(1)H or L(2)H and ((n)Bu)2Mg lead to the solvent-free compounds (L)2Mg [L = L(1) (10); L = L(2) (5)]. The bridged butyl mixed-metal complex [(L(1))Li(μ2-(n)Bu)Mg(L(1))]∞ (11) was also obtained via a cocomplexation reaction with (n)BuLi and ((n)Bu)2Mg. 11, which adopts a monodimensional polymeric array in the solid state, is a rare example of an alkyl-bridged Li/Mg complex and the first complex to feature an unsupported bridging butyl interaction between two metals. Changing the cocomplexation reaction conditions, the order of reagents added to the reactions mixture, and with the use of a coordinating solvent (tetrahydrofuran) formed the magnesiate complex (L(1))3MgLi(thf) (12).
This chapter provides a non-critical overview of research advances involving the elements of Groups 1 and 2 during the calendar year 2009. As was the case in previous years, coverage will concentrate upon topics centred around the synthesis, structures and applications of coordination compounds of these elements. This self-imposed restriction will necessarily result in the omission of numerous advances in other branches of chemistry and, for this, the author wishes to apologise in advance.
Redoxtransmetallation ligand exchange reactions involving a heavy alkaline earth metal, bis(pentafluorophenyl)mercury and 2,6-di-iso-propylphenol (HOdipp) in dme (1,2-dimethoxyethane) afforded alkaline earth aryloxo complexes which were structurally characterised. All complexes obtained were mononuclear, but the ionic radius of the alkaline earth metal was found to influence the number, and binding mode, of ligated dme molecules. Using Ca metal in the reaction afforded [Ca(Odipp)2(dme)2]1 containing a six coordinate Ca metal atom and the phenolate ligands oriented in a cisoidal arrangement. Using Sr metal in the reaction yielded {3[Sr(Odipp)2(dme)2]·[Sr(Odipp)2(dme)3]}2 containing both six and seven coordinate Sr metal atoms. The six coordinate species [Sr(Odipp)2(dme)2] contains phenolate ligands positioned in a cisoidal geometry. The seven coordinate species [Sr(Odipp)2(dme)3] contains a unidentate dme ligand, a rare binding mode for dme in alkaline earth chemistry. Using Ba metal in the reaction afforded [Ba(Odipp)2(dme)3]3, which contains an eight coordinate Ba metal atom.
Bicycle race: Structurally complex bis(imidazolidine-2,4-dione) molecules may be synthesized with complete atom-efficiency from simple building blocks by a kinetically controlled magnesium-mediated cascade of intermolecular isocyanate insertion and intramolecular alkyne hydroamination reaction steps.
In this review phosphorus-containing anions and molecules acting as ligands at calcium cations are discussed. Limitation to calcium derivatives is justified on the basis of its advantageous properties, its electropositive character ensuring high reactivity, and particularly the tremendous recent interest in calcium-based organometallics. Detailed discussions focus on substance classes such as phosphanides, phosphane oxides, phosphinites, phosphinates, and phosphonates. Isoelectronic relationships based on Grimm's hydride displacement law (such as O/NH/CH2 and also O/N−/CH−/C2−) allow categorization of calcium derivatives with phosphorus-containing anionic and neutral ligands. Applications of these compounds in stoichiometric and catalytic reactions are presented including very recent developments in calcium-mediated intermolecular hydrophosphanylation of alkynes.
Two procedures for the synthesis of group 4 phosphaguanidine compounds M(R2PC{NR′}2)(NR″2)3 (M=Ti, Zr; R=Ph, Cy; R′=iPr, Cy; R″=Me, Et) are described. Spectroscopic characterization indicated symmetrical bonding of the phosphaguanidinate ligand in solution for the P-diphenyl derivatives whereas the P-dicyclohexyl analogs adopt a more rigid geometry with inequivalent Namidine substituents within the phosphaguanidinate ligand. X-ray diffraction studies show exclusively monomeric tbp metal centers for a series of derivatives, with a chelating phosphaguanidinate ligand that spans an axial and an equatorial position. Two different conformers have been identified in the solid-state that differ in the relative orientation of the phosphorus R2P–C substituents with respect to the equatorial plane of the tbp metal. The synthetic protocol was extended to the bimetallic complex, [PhP(C{NiPr}2Ti{NMe2}3)CH2–]2, which was characterized by crystallography as the meso-form.
The tetrahydrofuran adducts [(thf)(4)M(PPh(2))(2)] (M = Ca, Sr) are air sensitive and can easily be oxidized by chalcogens. Metalation of diphenylphosphane oxide, diphenylphosphinic acid, and diphenyldithiophosphinic acid as well as salt metathetical approaches of the potassium salts with MI(2) allow the synthesis of [(thf)(4)Ca(OPPh(2))(2)] (1), [(dmso)(2)Ca(O(2)PPh(2))(2)] (2), [(thf)(3)Ca(O(2)PPh(2))I](2) (3), [(thf)(3)Ca(S(2)PPh(2))(2)] (4), [(thf)(2)Ca(Se(2)PPh(2))(2)] (5), [(thf)(3)Sr(S(2)PPh(2))(2)] (6), [(thf)(3)Sr(Se(2)PPh(2))(2)] (7), and [(thf)(2)Ca(O(2)PPh(2))(S(2)PPh(2))](2) (8), respectively. The diphenylphosphinite anion in 1 contains a phosphorus atom in a trigonal pyramidal environment and binds terminally via the oxygen atom to calcium. The diphenylphosphinate anions act as bridging ligands leading to polymeric structures of calcium bis(diphenylphosphinates). Therefore strong Lewis bases such as dimethylsulfoxide (dmso) are required to recrystallize this complex yielding chain-like 2. The chain structure can also be cut into smaller units by ligands which avoid bridging positions such as iodide and diphenyldithiophosphinate (3 and 8, respectively). In general, diphenyldithio- and -diselenophosphinate anions act as terminal ligands and allow the isolation of mononuclear complexes 4 to 7. In these molecules the alkaline earth metals show coordination numbers of six (5) and seven (4, 6, and 7).
AbstractThis chapter describes recent advances in metal-catalyzed C–P bond formation, which may be classified into two types of reactions.
In hydrophosphination and related processes, P–H groups add across unsaturated C–X (X = C, N, O) bonds. Phosphination of electrophiles
typically results in substitution at sp2 or sp3 carbon; the P–H group is removed, often by a base. The scope of both nucleophilic and electrophilic partners in these processes
is surveyed, and the proposed mechanisms and intermediates in the metal-catalyzed reactions are described.
Graphical Abstract
KeywordsCross-coupling-Hydrophosphination-Mechanism-Palladium-Phosphination-Phosphine
A series of homoleptic guanidinate-type complexes of the heavier alkaline earth (Ae) metals calcium and strontium have been prepared. The six-coordinate compounds [Ae{(i)PrNC(NPh(2))CN(i)Pr}(2)(THF)(2)] (Ae = Ca and Sr) were synthesised through reactions of the appropriate THF-solvated hexamethyldisilazide [Ae{N(SiMe(3))(2)}(2)(THF)(2)] with two molar equivalents of diphenylamine and 1,3-di-iso-propylcarbodiimide. Both compounds were shown to crystallise with a cisoid arrangement of the two THF molecules at the metal centres. In contrast, the (Me(3)Si)(2)N-substituted calcium guanidinate, [Ca{CyNC{N(SiMe(3))(2)}CNCy}(2)(THF)(2)], contains two coordinated THF molecules with a trans disposition. Further reactions of the free amidine [{(2-FC(6)H(4))N}C(NH(i)Pr)(2)] with either [Ca{N(SiMe(3))(2)}(2)(THF)(2)] or its dimeric unsolvated analogue provided monomeric or dimeric derivatives respectively in which the ligands had tautomerised to an anisobidentate form. A further reaction of the phosphaguanidine [CyN=C(PPh(2))N(H)Cy] with [Sr{N(SiMe(3))(2)}(2)(THF)(2)] provided the first example of a phosphaguanidinate complex of this heavier alkaline earth metal. This compound has also been characterised in the solid state and shown to exist with a transoid configuration of the coordinated THF molecules.
An extensive review covering all aspects of Calcium-mediated reactions, which include application of calcium compounds as homogeneous catalysts, is presented. Considerable contributions is made to an understanding of Schlenk equilibria in organocalcium chemistry and it is found that the stability of heteroleptic calcium complexes depend on temperature, concentration, presence of polar (co)solvents, amd ligand bulk. Studies also found that the homoleptic complex of calcium is fast in ε-CL polymerization and slow in the polymerization of L-lactide. Investigation of calcium catalysts based on Schiff base ligands shows that polymerizations are initiated either by a salen-Ca complex or by well-defined single-site catalysts. Successful use of organocalcium species as initiators for syndioselective and living styrene polymerization can be attributed to the right balance of Lewis-basic and Lewis acidic properties.
The chiral β-diketimine ligand [(A)-Ph(Me)CH-N=C(Me)]CH2was prepared by condensation of acetylacetone with the commercially available chiral building block (S)-Ph(Me)CH-NH2. Reaction of bis(o-Me 2N-α-Me3Si-benzyl)calcium with this β-diketimine led to double deprotonation. Reaction of bis(o-Me 2N-α-Me3 Si-benzyl)calcium with the commercially available chiral bis-oxazoline (S)-Ph-BOX gave diastereopure [(S)-Ph-BOX](o-Me2N-α-Me3Si-benzyl)calcium which in solution slowly decomposed with formation of o-Me2N-α-Me 3Si-toluene. The corresponding amide complex [(S)-Ph-BOX] CaN(SiMe3)2(THF)2 is stable and the crystal structure has been determined. In solution, this heteroleptic amide is in Schlenk equilibrium with the homoleptic species [(S)-Ph-BOX]2Ca and Ca[N(SiMe3)2]2-(THF)2. This Schlenk equilibrium can be steered to the heteroleptic side. Use of the enantiopure calcium amide catalyst for the hydrosilylation of styrene with PhSiH3 or in the intramolecular hydroamination of aminoalkenes gave good product yields, but only small ee-values were observed (5 -10 %). From stoichiometric reactions of the catalyst with the substrates it is concluded that the "true" catalytically active species is mainly present as a homoleptic calcium complex, which explains the poor enantioselectivities.
The reaction of calcium-bis[bis(trimethylsilyl)amide]-THF (1/2) with benzonitrile in THF nearly quantitatively yields the title compound calcium-bis[N,N'-bis(trimethylsilyl)- benzamidinate]-THF (1/2). Under similar conditions pivalonitrile substitutes an ether ligand of calcium-bis[bis(trimethylsilyl)amide]-DME (1/2) without the formation of the corresponding amidinate derivative. The calcium-di(benzamidinate) [H 5 C 6 -C(NSiMe 3 ) 2 ] 2 Ca · 2THF crystallizes in the space group Pbcn with {a = 188.8(6); b = 1286.0(3); c = 1802.5(5) pm; Z = 4}. The calcium atom is hexa-coordinate with a distorted octahedral trans-configuration and with Ca - O and Ca - N bond distances of 238 and 243 pm, respectively. The bond lengths within the NCN moiety with values of 132 pm are characteristic of a diazaallylic system. The short N - Si bond lengths of about 170.5 pm as well as the high field shift of the ²⁹ Si{ ¹ H} NMR signal are evidence for an effective backdonation of the anionic charge from the nitrogen to the silicon atoms.
The transmetalation of bis[bis(trimethylsilyl)amino]stannylene in tetrahydrofuran with the alkaline earth metals yields the bis(tetrahydrofuran) adducts of calcium (1a), strontium (1b) and barium bis[bis(trimethylsilyl)amide] (1c). The metalation of 6-methyl-6-phenylfulvene with 1a, 1b and 1c gives 1,1′-bis(1-phenyl-ethen-1-yl)calcocene 2a, -strontocene 2b and -barocene 2c, respectively. The reaction of 1b with equimolar amounts of 6-methyl-6-phenylfulvene and acetophenone leads to the formation of bis[μ-[η 5-1-(3-hydroxy-1-methyl-1,3-diphenyl-2-propen-1-yl)-2,4-cyclo -pentadien-1-yl](2-)-C,O:O]tetrakis(tetrahydrofuran-O)distro ntium 3. The metathesis reaction of 2a with YCl3 and SnCl2 yields the corresponding yttrocenechloride 4 and stannocene 5. Crystallographic data of 1a: monoclinic, P21/c, a = 841.6(4), b = 1936.9(6), c = 1938.0(10)pm, β = 100.84(4)°, Z = 4, wR2 = 0,1199); 1b: orthorhombic, Pbca, o = 1924.8(4), b = 1747.8(3), c = 1930.1(4) pin, Z = 8, wR2 = 0.1248; 3: monoclinic, P21/c, a = 1230.42(11), b = 1343.08(11), c = 1597.34(14) pm, β = 104.610(7)°, Z = 4, wR2, = 0.0980.
The transmetalation of bis[bis(trimethyIsilyl)amino]stannylene in tetrahydrofuran with the alkaline earth metals yields the bis(tetrahydrofuran) adducts of calcium (1a), strontium (1b) and barium bis[bis(trimethylsilyl)amide] (1c). The metalation of 6-methyl-6-phenylfulvene with 1a, 1b and 1c gives 1,1′-bis(l-phenyl-ethen-l-yl)calcocene 2a, -strontocene 2b and -barocene 2c, respectively. The reaction of 1b with equimolar amounts of 6-methyl-6-phenylfulvene and acetophenone leads to the formation of bis[μ-[η
Due to their reactivity, the alkaline earth metal bis[bis(trimethylsilyl)amides] are valuable synthons to incorporate the metal atoms in numerous molecules. The easy access of these compounds by metathesis as well as transmetalation reactions led to a vast development of the molecular alkaline earth metal chemistry in the last decade. Due to a pKa value of 25.8 for hexamethyldisilazane, metalation of more acidic molecules is a useful reaction to metallocenes, acetylides, phosphanides, arsanides, alcoholates, and others. Special interest is given to the dimeric alkaline earth metal bis[bis(trialkylsilyl)phosphanides] due to their unexpected structures in the solid state and in solution. These molecules crystallize with a central trigonal M2P3 bipyramide in the case of M=Ca, Sr and Ba. These are the minimum structures according to ab initio calculations for calcium and strontium for gaseous [M(PH2)2]2, whereas for barium the structure of the type M(μ-PH2)4Ba has to be considered. Addition reactions of the homoleptic alkaline earth metal bis(amides) and bis(phosphanides) to benzonitrile yield the corresponding benzamidinates and 1-aza-3-phosphapropenides, respectively. Both ligands bind as bidentate ligands to the metal center. The nitrogen atoms show a trigonal planar surrounding in contrast to the phosphorus atoms. Reaction of the alkaline earth metal bis(amides) with 6,6-dialkylfulvenes gives the corresponding 1,1′-bis(alkenyl)alkaline earth metallocenes. Heterobimetallic molecules crystallize from the metalation of phosphanes with a mixture of alkaline earth metal and tin(II) bis(bistrimethylsilyl)amides. The wide preparative use of these compounds less than 10 years after the first synthesis of a bis(trimethylsilyl)amide of a heavier alkaline earth metal demonstrates the still growing importance of this compound class.
Bis-silylated phosphines react with carbodiimides giving phosphaguanidines through insertion into the PSi-bond. Depending on the substituents R′ and R″ the N,P-bis-silylated derivatives (I) or the N′,N′-bis-silylated isomers (II) are formed, the latter results from P,N-silyl migration and formation of a PC double bond. (R = Me, Cy, t-Bu, Ph, p-Me2NC4H4;R′ = Alkyl, R″ = Aryl e.g. Cy, Ph, pClC6H4The structures of compounds I and II are discussed on the basis of their NMR spectra and the reaction with methanol leads to PH-functional derivatives . Treatment of RP(SiMe3)2 with PhNCO is also discussed.ZusammenfassungBis-silylated phosphines react with carbodiimides giving phosphaguanidines through insertion into the PSi-bond. Depending on the substituents R′ and R″ the N,P-bis-silylated derivatives (I) or the N′,N′-bis-silylated isomers (II) are formed, the latter results from P,N-silyl migration and formation of a PC double bond. (R = Me, Cy, t-Bu, Ph, p-Me2NC4H4;R′ = Alkyl, R″ = Aryl e.g. Cy, Ph, pClC6H4The structures of compounds I and II are discussed on the basis of their NMR spectra and the reaction with methanol leads to PH-functional derivatives . Treatment of RP(SiMe3)2 with PhNCO is also discussed.
Diphenyltrimethylsilylphosphine was found to be an effective catalyst for cyclization of phenyl isocyanate, affording cyclic dimer and trimer at moderate temperature. An abnormal decarboxylation took place to give diphenylcarbodi-imide and its cyclic trimer above 50°. Evidence for the equilibrium reaction, 3 diphenylcarbodi-imide cyclic trimer, was obtained. Phenylbis(trimethylsilyl)phosphine forms a stable 1 : 1 adduct with phenyl isocyanate; its structure is discussed.
Hydrophosphination reactions of phenyl substituted alkynes are catalyzed effectively by [(thf)4Ca(PPh2)2] (1). The reactions of diphenylethyne and diphenylbutadiyne with diphenylphosphane in THF in the presence of catalytic amounts of 1 (approx. 5mol%) yield quantitatively the cis-addition products trans-1,2-diphenyl-1-diphenylphosphanylethene (2) and 1,4-diphenyl-1,4-bis(diphenylphosphanyl)buta-1,3-diene (3), respectively. The phenyl groups in 3 are oriented nearly perpendicular to the butadiene backbone and therefore, the π-systems of the phenyl groups and the butadiene unit show no interaction with each other.
The neutral phospha(III)guanidine compounds, Ph2PC{NR}{NHR}
(1a, R = Cy; 1b, R =
iPr), are reported. NMR spectroscopic data indicated the presence of the Esyn isomer in solution and structural studies confirmed this in the solid-state. Reaction of 1a with M(CO)4(pip)2
[M = Mo, W; pip = piperidine] proceeded via displacement of the piperidine ligands to generate the complexes M[Ph2PC{NCy}{NHCy}](CO)4
(2a, M = Mo; 3a, M = W). The X-ray structure of 2a and 3a were solved, showing the first structurally characterised examples of 1-aza-3-phospha-4-metallacyclobut-1-ene rings.
From an historical perspective, much of the chemistry associated with the s-block elements has centred on the synthesis and structural characterization of an eclectic selection of compounds, with less effort directed towards developing their chemistry with regards to synthetically useful applications. Whilst obvious exceptions to this general statement exist, and the fundamental academic studies are invaluable in their own right, it is only in recent years that a renewed interest in the application of metal-ligand compounds of these elements has been evident. This short review is focussed on progress made in the molecular1 chemistry of the s-block metals, as presented in the literature in the period 2006-2007. Each section is divided into two parts: (i) advances in the synthesis of new complexes and ligand design, leading to a better understanding of the chemistry of these elements is initially presented; (ii) the latter part of each section concentrates on the application of these compounds in a number of synthetically important areas. Clearly with such a broad topic as this, not all of the literature can be covered, and it is important to state that this review does not include the vast literature concerning the organic transformation chemistry of organolithium and related reagents. Where possible, however, the author has selected examples that he feels will be of interest to the readers of Current Organic Chemistry.
Reaction of SrI2, EuI2(THF)2, and YbI2(THF)2 with KN(SiMe3)2 and [{(iPr)2ATI}K] ((iPr)2ATI = N-isopropyl-2-(isopropylamino)troponiminate) led to monoaminotroponiminate complexes of the heavier alkaline earth elements and the related divalent lanthanides of composition [{(iPr)2ATI}M{N(SiMe3)2}(THF)2)] (M = Sr, Eu, Yb). Diaminotroponiminate complexes of composition [{(iPr)2ATI}2M(THF)2)] (M = Sr, Ba) were obtained by the reaction of SrI2 and BaI2 with 2 equiv of [{(iPr)2ATI}K]. All new compounds were characterized by single-crystal X-ray diffraction. Independent of the ionic radius of the center metal all monoaminotroponiminate complexes [{(iPr)2ATI}M{N(SiMe3)2}(THF)2] including the previously reported Ca analogue are isostructural. The same phenomenon is observed for [{(iPr)2ATI}2M(THF)2)]. The heterolepetic compounds [{(iPr)2ATI}M{N(SiMe3)2}(THF)2)] were used as catalysts for the intramolecular hydroamination/cyclization reaction of nonactivated aminoalkenes. A decrease in the rate with increasing ion radius of the center metal is observed for the alkaline earth elements. The ytterbium complex, which is oxidized during the catalytic conversion, is also an efficient precatalyst for the hydroamination/cyclization reaction.
Amides of the heavier group 2 elements Ca, Sr, and Ba are effective precatalysts for the atom-efficient addition of phosphine P−H bonds to carbodiimides. A number of intermediates within the catalytic cycle have been identified by in situ NMR methods and by stoichiometric synthesis.
The β-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}(THF)] effects intermolecular hydrophosphination of a range of alkenes and alkynes. In behavior reminiscent of lanthanocene(III) catalysis, a more electrophilic alkene is polymerized to phosphine-terminated macromolecules.
The effectiveness of calcium complexes for ring opening polymerization of L-lactide or D-lactide was examined and melt polymerizations were performed at a monomer catalyst ratio of 350:1 for 15 min under an argon atmosphere. The molecular weights of poly( L-lactide) measured by gel permeation chromatography (GPC) are found to increase with increasing M/I ratios maintaining narrow polydispersity (PDI) index. The rate of polymerization of coordinated solvent tetrahydrofuran is found to be faster in chlorinated solvent as the complexation of calcium ion by coordinating solvents enhances the nucleophilicity of the initiator. The results also show that the rate of enrichment of carbonate monomer is slower than the lactide during random copolymerization reactions. The calcium complexes containing bulky borate ligands and phenolate initiators polymerize lactide with a high degree of hetroactivity in THF.
Schiff base derivatives of the biometals (Zn, Mg, Ca) in the presence of anion initiators have been shown to be very effective catalysts for the ring-opening polymerization of trimethylene carbonate (TMC or 1,3-dioxan-2-one) to poly(TMC) devoid of oxetane linkages. The order of catalytic activity as a function of metal was found to be Ca(II) Mg(II) > Zn(II). Optimization of the calcium system was achieved utilizing a salen ligand with tert-butyl substituents in the 3,5-positions of the phenolate rings and an ethylene backbone for the diimine along with an azide ion initiator. These conditions led to a TOF of 1286 h-1 for a melt polymerization carried out at 86 °C. Solution studies in tetrachloroethane demonstrated the polymerization reaction to proceed via a mechanism first order in [monomer], [(salen)Ca], and [anion initiator] and to involve TMC ring-opening by way of acyl−oxygen bond cleavage. The activation parameters were determined to be ΔH = 20.1 kJ/mol and ΔS = −128 J/(mol K).
A novel and efficient calcium alkoxide initiating system, generated in situ from bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] and an alcohol, for the ring-opening polymerization of cyclic esters has been developed. The solution polymerization in THF using mild conditions is living, yielding polyesters of controlled molecular weight and tailored macromolecular architecture. The polymerizations initiated with the 2-propanol−Ca[N(SiMe3)2]2(THF)2 system are first-order in monomer with no induction period. At high 2-propanol/Ca[N(SiMe3)2]2(THF)2 ratios, complete conversion of 2-propanol occurs due to fast and reversible transfer between dormant and active species.
A number of extensions to the multisolution approach to the crystallographic phase problem are discussed in which the negative quartet relations play an important role. A phase annealing method, related to the simulated annealing approach in other optimization problems, is proposed and it is shown that it can result in an improvement of up to an order of magnitude in the chances of solving large structures at atomic resolution. The ideas presented here are incorporated in the program system SHELX-90; the philosophical and mathematical background to the direct-methods part (SHELXS) of this system is described.
Die Organocalciumchemie steht noch immer am Anfang ihrer Entwicklung. Die Direktsynthese aus Iodarenen und aktiviertem Calciumpulver eröffnet einen einfachen Zugang zu Arylcalciumiodiden in guten Ausbeuten. Der Einfluss des Substitutionsmusters der Phenylgruppe, des Halogens und des Lösungsmittels wird diskutiert. Die Arylcalciumhalogenide zeigen ein Schlenk-Gleichgewicht, das die Isolierung von Diarylcalciumderivaten ermöglicht. Wegen der hohen Reaktivität der Arylcalciumhalogenide müssen während der Herstellung und Handhabung tiefe Temperaturen aufrechterhalten werden. Eine geringere Reaktivität und höhere Beständigkeit können durch sterisches Abschirmen des Calciumatoms erreicht werden, z. B. durch Erhöhung der Koordinationszahl des Metallzentrums oder Substitution des Iodidions durch sperrige Gruppen.
The equimolar reaction of calcium bis[bis(trimethylsilyl)amide] with 2,2,6,6-tetramethylheptane-3,5-dione (Htmhd) in THF yields liquid mononuclear [Ca(tmhd){N(SiMe3)2}(THF)3] (1). A similar reaction in toluene with a stoichiometric ratio of 2:3 gives the dinuclear complex [{(THF)Ca}2(tmhd)2(µ-tmhd){µ-N(SiMe3)2}] (2). The calcium atoms of these complexes are in a distorted octahedral environment. In 2 the complex consists of two octahedra connected by a common face of one nitrogen base and two oxygen atoms; the bridging Ca−N bond lengths are extremely large. The metalation of the tetradentate Jacobsen reagent with calcium bis[bis(trimethylsilyl)amide] in 1,2-dimethoxyethane (DME) gives the corresponding calcium complex 3, nearly quantitatively, as its DME adduct. The calcium atom is in an unusual trigonal prismatic coordination sphere. The metathesis reaction of 3 with tin(II) chloride yields the corresponding yellow tin(II) complex with the metal atom in a distorted square-pyramidal environment. Complexes 2 and 3 show catalytic reactivity in the ring-opening polymerization of cyclic esters such as lactones and lactides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
The heteroleptic calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] (Ar = 2,6-diisopropylphenyl) and the homoleptic heavier alkaline earth amides, [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr and Ba) are reported as competent pre-catalysts for the hydroamination of 1,3-carbodiimides. Whilst the reaction scope is currently limited to reactions of aromatic amines with 1,3-dialkylcarbodiimides, in most cases preparations in hydrocarbon solvents proceed rapidly at room temperature with catalyst loadings as low as 0.2 mol-% and the guanidine reaction products crystallize directly from the reaction mixture. Initial studies are consistent with the intermediacy of heavier group-2 guanidinate complexes.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
The metathesis reaction of CaI2 with KPPh2 in THF yields (thf)4Ca(PPh2)2 (1). A metallation of HPPh2 with alkaline earth metals succeeds with strontium and barium and gives (thf)4Sr(PPh2)2 (2) and (thf)5Ba(PPh2)2 (3). From crystal structure determinations of 1 and 2 Ca-P and Sr-P bond lengths of 298.65(6) and 314.29(9) pm, respectively, were obtained. Extremely large Ba-P distances of 332.8(2) and 334.5(2) pm were found for 3. In order to investigate the influence of the bulkiness of the phosphanides on the molecular structure, (thf)4Ba[P(Mes)2]2 (4) and (thf)2Ba[P(Et)Ph]2 (5) were prepared in a similar manner. Derivative 4 is monomeric with Ba-P distances of 318.72(9) pm, whereas 5 crystallizes as a one-dimensional polymer with bridging ethyl(phenyl)phosphanide groups with Ba-P bond lengths between 324.8(1) and 336.0(1) pm.
The quasiliving characteristics of the ring-opening polymerization of ϵ-caprolactone (CL) catalyzed by an organic amino calcium were demonstrated. Taking advantage of this feature, we synthesized a series of poly(ϵ-caprolactone) (PCL)–poly(L-lactide) (PLA) diblock copolymers with the sequential addition of the monomers CL and L-lactide. The block structure was confirmed by 1H-NMR, 13C-NMR, and gel permeation chromatography analysis. The crystalline structure of the copolymers was investigated by differential scanning calorimetry and wide-angle X-ray diffraction analysis. When the molecular weight of the PLA block was high enough, phase separation took place in the block copolymer to form PCL and PLA domains, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2654–2660, 2006
The organocalcium chemistry developed vastly during the last decade. The preparation of the organocalcium compounds via direct synthesis (insertion of Ca into a C-X bond of phenyl halides, Grignard reaction) affords skilful procedures due to the inertia of the calcium metal and the extreme reactivity of the organocalcium derivatives. Further suitable preparative methods include metathesis reactions of CaX2 with KR or LiR, metallation reactions of H-acidic substrates, metal-halogen exchange reactions, and transmetallation of heavy main group atoms in their compounds with calcium metal. Possibilities to stabilize organocalcium compounds include steric shielding by bulky ligands at the periphery and electronic reduction of the nucleophilicity of the calcium-bound carbanions. Selected applications in catalysis such as hydrophosphination are also mentioned. Very recent developments and challenges in the preparation of alkaline earth metal(I) compounds are presented as well. Concepts to overcome the rather large atomization energies of the metals are discussed.
Since their initial development in the early 1900’s, Grignard reagents have proven to be immensely useful and are among the most common organometallic reagents. The nature of the reagents in solution is complex and depends on substituents, solvent, concentration and temperature. Despite continuing questions about their solid state and solution composition and conformation, organomagnesium reagents find new applications continually. In contrast, little information about the heavier alkaline earth organometallic compounds RMX and R2M (R = alkyl, aryl; M = Ca, Sr, Ba, X = halide) exists. High reactivity due to the predominantly ionic character of the metal-ligand bond and increased lability complicates synthetic access. Recent interest in the organometallic chemistry of the alkaline earth metals calcium, strontium and barium has been sparked by the realization that heavy alkaline earth metal organometallics play unique roles in synthetic applications, such as polymerization initiators and as reagents to modify polymers. As a result, the organometallic chemistry of calcium, strontium and barium has received significant attention over the last few years. This article summarizes recent results in this emerging area of chemistry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)
The ring-opening polymerization of l-lactide with calcium alkoxides generated in-situ from bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] and 2-propanol are presented. The polymerization in THF at room temperature proceeds rapidly and in a living manner, giving poly(l-lactide)s of controlled molecular weight, low polydispersity, and tailored end-functionalities. Kinetic studies show the absence of an induction period and a pseudo-first order rate constant of 6.41 L mol–1 min–1, which is significantly higher than for related Y5(-O)(O
i
Pr)13– or aluminum alkoxide-initiated polymerizations. The initiation involves a two-step process: (1) alcoholysis of bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] to give the corresponding calcium alkoxide and (2) ring-opening of l-lactide via acyl-oxygen cleavage and insertion into the calcium-alkoxide bond. In the presence of excess alcohol, fast and reversible exchange between free alcohol molecules and coordinated alkoxide ligands takes place. This allows tuning of the poly(l-lactide) molecular weight over a wide range.
Single-site calcium initiators containing chelating tmhd
(H-tmhd = 2,2,6,6-tetramethylheptane-33-dione) ligands
[(THF)Ca(tmhd)]2[-N(SiMe3)2](-tmhd)
(2) and
[(THF)Ca(tmhd)]2[-OCH(Me)Ph](-tmhd)
(3) have been synthesized and
applied for the ring-opening polymerization of L-lactide and
-caprolactone. Both 2 and
3 were highly reactive and
promoted a fast polymerization of L-lactide and -caprolactone
to high monomer conversions under mild conditions (THF as a
solvent, room temperature). More importantly, results showed
that the ring-opening polymerizations of lactides and lactones
initiated by either 3 or
2 in the presence of equivalent
2-propanol are living, to provide polymers and block copolymers
of controlled molecular weights and tailored end-groups. The
polymerizations were first-order in monomer up to high
conversions, in which the in
situ initiating system 2/2-propanol revealed no induction period
and much faster polymerization kinetics as compared to
3.
This review describes recent development of ring-opening polymerization of lactides and related cyclic esters using main group metal complexes as catalysts/initiators. The complexes described here are classified according to metal groups. Most attention is devoted to the well-defined metal complexes.
The use of sterically demanding ligands has allowed the organometallic compounds of the heavy alkaline-earth metals calcium, strontium and barium to emerge from the shadow cast by the far better studied organomagnesium Grignard reagents. Metallocenes and other cyclopentadienyl-based complexes have been the most intensively investigated, but in the past decade a wealth of new non-cyclopentadienyl compounds have been characterized. A broad range of structure types are known, encompassing σ- and π-bound anionic ligands, and Lewis base adducts with neutral donors. In this review, crystallographically characterized complexes are discussed, and current interpretations of the bonding in heavier Group 2 element compounds are examined. Recent applications of non-cyclopentadienyl compounds in organic synthesis are surveyed.
A new dibenzylcalcium compound with chiral anionic centers has been prepared and was structurally characterized. The compound, which shows slow inversion of the chiral carbanions, is an active initiator for the living polymerization of styrene.
Alkali and alkaline-earth metal complexes with sterically demanding hypersilyl-substituted cyclopentadienyl or fluorenyl ligands [hSi-Cp or hSi-Flu; hSi = (Me3Si)3Si] were prepared. The crystal structures of [Li(hSi-Flu)(THF)2], [K(hSi-Flu)(THF)], [Ca(hSi-Flu)2(THF)2], [Sr(hSi-Flu)2(THF)2] and [Ca(hSi-Cp)2(THF)] were determined. All structures show the enormous steric bulk of the hypersilyl substituent, which either results in a reduction in the number of solvent molecules in the coordination sphere of the metal or in a distortion of the ligand-metal interaction. Heteroleptic calcium and strontium complexes with a benzyl ligand (2-Me2N-α-Me3Si-benzyl) and a sterically demanding hSi-Flu ligand were prepared and used as initiators in the polymerization of styrene. The polymers produced with the heteroleptic benzylcalcium initiator are enriched in syndiotactic sequences, although they show the same tacticities as polymers obtained with benzylcalcium initiators containing the less bulky Me3Si-Flu ligand. Introduction of the bulky hypersilyl group in heteroleptic benzylstrontium initiators resulted in a slight increase of the syndiotacticity of the polymers.
The guanidinate complex [Ca{(Cy)N–C{N(SiMe3)2}–N(Cy)}2] can be prepared in good yields and in crystalline purity by two different routes: 1) addition of [Ca{N(SiMe3)2}2] to (Cy)N=C=N(Cy) or 2) addition of KN(SiMe3)2 to (Cy)N=C=N(Cy) followed by a metathesis reaction of the obtained potassium guanidinate complex with CaI2. Crystallization from Et2O yielded [Ca{(Cy)N–C[N(SiMe3)2]–N(Cy)}2·(Et2O)] (1), which shows a C2-symmetric structure in the crystal. A Sr analogue could be prepared likewise and also crystallizes as the mono-etherate [Sr{(Cy)N–C{N(SiMe3)2}–N(Cy)}2·(Et2O)] with a very similar crystal structure. The Schlenk equilibrium between (1) and [Ca{α-(Me3Si)-o-(Me2N)-benzyl}2·(THF)2] (3) in benzene is not completely on the heteroleptic side; however, an excess of (1) yields predominantly the heteroleptic benzylcalcium complex [Ca{(Cy)N–C{N(SiMe3)2}–N(Cy)}{α-(Me3Si)-o-(Me2N)-benzyl}]. Styrene polymerization with this mixture results largely in atactic polystyrene of which the molecular weight distribution shows a tailing in the lower range. This could be due to decomposition of the guanidinate ligand under the given polymerization conditions. It is shown that heating a mixture of 1 and 3 gives the crystalline decomposition product [Ca{(Cy)N–C{N(SiMe3)2}–N(Cy)}{(Cy)(TMS)N}], which crystallizes as a dimer with bridging amide ligands.
New heteroleptic benzylcalcium complexes with modified fluorenyl ligands were prepared which include a complex with a chelating dimethylamino substituent, [9-(2-Me2N-ethyl)fluorenyl][2-Me2N-α-Me3Si-benzyl]Ca (4), and a series of complexes with bulky substituents like tBu-, Ph(Me)2C-, Me(Ph)2C- and Me(4-tBuC6H4)2C- in the 2- and 7-positions of the fluorenyl ligand (complexes 5 through 9, respectively). Crystal structures of 4, [(2,7-tBu-9-Me3Si-fluorenyl)(2-Me2N-benzyl)Ca]2 (5) and [2,7-Ph(Me)2C-9-Me3Si-fluorenyl][2-Me2N-α-Me3Si-benzyl]Ca·THF (6) show that modification of the fluorenyl ligand hardly affects the coordination mode of the benzylic ligand. Also, in solution all compounds are of heteroleptic nature and it was shown that fluorenyl modifications hardly affect the barriers for inversion at the chiral benzylic carbon atom [18.7(2)–19.2(2) kcal/mol]. Complex 4 does not initiate styrene polymerization, which underscores the importance of a free coordination site at calcium. Complexes 5–9 initiate styrene polymerization, and the syndiotacticity of the obtained polymers increases with the bulkiness of the substituents on the fluorenyl ligands. Syndiotacticities up to r = 95% (rr = 90%) were obtained.
A series of dinucleating bis(β-diketiminate) ligands with rigid bridges has been prepared. In all cases the β-diketiminate unit is 2,6-iPr2C6H3NC(Me)C(H)C(Me)N-(bridge), and the bridges are either para-phenylene, meta-phenylene or 2,6-pyridylene (the dinucleating ligands are abbreviated as PARA-H2, META-H2 and PYR-H2, respectively). These ligands have been converted to heteroleptic bimetallic calcium and zinc complexes. For calcium, only the PARA-phenylene bridged ligand led to a heteroleptic bimetallic calcium amide complex PARA-[CaN(SiMe3)2 · THF]2. For the other ligands, homoleptic complexes have been isolated: META-Ca and PYR-Ca. For zinc, the whole range of heteroleptic amides could be isolated: PARA-[ZnN(SiMe3)2]2, META-[ZnN(SiMe3)2]2, and PYR-[ZnN(SiMe3)2]2. Analogue ethylzinc complexes have been prepared in quantitative yields: PARA-(ZnEt)2, META-(ZnEt)2, and PYR-(ZnEt)2. Reactions of the ethylzinc complexes with SO2 gave access to the ethylsulfinate complexes META-(ZnO2SEt)2 and PYR-(ZnO2SEt)2; for the para-phenylene bridged ligand no products could be isolated. Crystal structures of the following complexes are presented: (META-Ca)2, PARA-[CaN(SiMe3)2 · THF]2, PARA-[ZnN(SiMe3)2]2, META-[ZnN(SiMe3)2]2, PYR-(ZnEt)2, and PYR-(ZnO2SEt)2. All heteroleptic complexes have been tested for activity in the copolymerization of cyclohexene oxide (CHO) and CO2. The bimetallic complex PARA-[CaN(SiMe3)2 · THF]2 is not active. For zinc, the PARA and META complexes were found to be active under highly concentrated conditions (Zn/CHO ratio of 1/1000; no solvent), but no significant polymer yields could be achieved with PYR complexes. This is either due to conformational changes of the complex or to coordination of the pyridylene N atom to the catalytic centers. The order of activity found in the bimetallic zinc complexes is META > PARA. This is likely due to a more advantageous Zn · · · Zn distance in the meta-phenylene bridged bimetallic catalysts. There are several indications for bimetallic action. First of all, the ethylzinc systems PARA-(ZnEt)2 and META-(ZnEt)2 initiate CHO/CO2 copolymerization, whereas monometallic ethylzinc catalysts are not reactive. Second, the bimetallic zinc catalysts are also highly active under diluted conditions: a low metal/CHO ratio of 1/3000 gave high-MW polymers (Mn > 100.000, PDI = 1.33) with essentially only carbonate linkages. In contrast, monometallic systems show a drastic loss of activity upon dilution. Finally, the bimetallic catalyst META-[ZnN(SiMe3)2]2 shows a significantly higher activity than a comparable monometallic model system.
Reactions of the dimeric calcium hydride complex [(DIPP-nacnac)CaH·thf]2 {1; DIPP-nacnac = CH[(CMe)(2,6-iPr2C6H3N)]2} with the α-hydrogen containing ketones acetophenone, acetone, dibenzylketone and 2-adamantone are smooth. In most cases not only addition but also substantial enolization is observed as a side reaction and in some cases also aldol condensation was found. Despite this unselectivity, the addition products could be isolated crystalline pure. Crystal structures of [(DIPP-nacnac)CaOCH(Me)Ph]2, [(DIPP-nacnac)CaOCH(CH2Ph)2]2 and [(DIPP-nacnac)-Ca(2-adamantoxide)]2 have been determined. The calcium hydride complex 1 is an effective catalyst in the hydrosilylation of ketones. Independent from the silane/ketone ratio, a strong preference for formation of bis-alkoxy silanes [PhSiH(OR)2] is observed. In most cases no enoxy groups have been found in the product. This indicates that the mechanism does not involve addition of the calcium hydride to the ketone functionality. A concerted addition of silane to ketone through a six-coordinate hypervalent silicon intermediate is proposed.
Interest in the utility of polylactide as a commodity polymer has increased significantly in recent years due to numerous environmental advantages over conventional petrochemically derived plastics. As such, the development of novel catalyst systems for the ring opening polymerization of lactide has seen tremendous progress in the past decade. In particular, divalent metals (i.e. Mg, Ca and Zn) supported by monoanionic ancillary scaffolds are appealing because of their low toxicity and cost. A much less common approach involves the use of neutral ligands in combination with the aforementioned divalent metal centres. The additional valence thus renders it possible, upon reaction with traditional Lewis or Brønsted acid activators, to generate sterically and electronically unsaturated species, akin to the most widely employed olefin polymerization catalysts. This Perspective is not intended as a comprehensive review, but rather a systematic highlight of key contributions, which have served to extend the forefront of this exciting field.
(Chemical Equation Presented) Transition-metal-free hydrogenation of alkenes can be carried out with simple organocalcium catalysts (20 bar H 2, 20°C). Both steps in the proposed catalytic cycle, hydride addition to the double bond and σ-bond metathesis with H2, have been confirmed. Alkenes sensitive to polymerization are also hydrogenated in good yields.
The heteroleptic calcium amides [{ArNC(Me)CHC(Me)NAr}Ca(NR(2))(THF)] (Ar=2,6-di-iso-propylphenyl, R=SiMe(3), Ph) and the homoleptic heavier alkaline earth amides, [M{N(SiMe(3))(2)}(2)] (M=Ca, Sr and Ba) are reported as pre-catalysts for the hydroamination of isocyanates.
Structural investigation of Li-complexes of the phospha(III)guanidinate anion [Ph2PC[NiPr]2]- revealed variable coordination to lithium; synthesis of the dimethyl aluminium compound, (Ph2PC[NiPr]2)AlMe2, which behaves as a metal-functionalised phosphine ligand towards platinum, is reported.
Amide and alkoxide coordination complexes of calcium supported by beta-diiminato and bulky trispyrazolylborate complexes are reported together with their activity in lactide ring-opening polymerization; some are amongst the most active systems discovered to date.
A series of monomeric amide or aryloxide complexes of the form LCaX, where L = CH[CMeNC(6)H(3)-2,6-](2), CH[CMeNC(6)H(4)-2-OMe](2), a bulky tris-pyrazolyl borate, Tp(iPr) or Tp(tBu) or 9-BBN-pz(2) and X = N(SiMe(3))(2) or OC(6)H(3)-2,6-, has been prepared and characterized and investigated in the ring-opening polymerizations of lactide. The compounds (Tp(tBu))CaX in THF are shown to be highly active and stereoselective. The propylene oxide complex (Tp(tBu))Ca(OC(6)H(3)-2,6-).(PO) has been isolated and structurally characterized (single-crystal X-ray) and shown to be inert to the polymerization of PO.
Different isomeric forms of the amidine unit have been identified in Ph2P(E)C[NR'][NHR'] (E = S, Se; R' = iPr, Cy), using both solid- and solution-state techniques.
To date, enormous progress has been made in the ring-opening polymerization (ROP) of lactide and glycolide. This review focuses on the different approaches reported in the literature for the controlled ring-opening polymerization of lactide and glycolide until 2003. The catalytic systems for coordination polymerization are presented first, following the nature of their ancillary ligands. Following this, the anionic, nucleophilic and cationic strategies are discussed successively. Focus is on the mechanistic and stereochemical aspects of the various processes, and whenever possible comparisons between the different systems are attempted.
The calcium-catalyzed intramolecular hydroamination of alkenes and alkynes is reported. The beta-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}-Ca{N(SiMe3)2}(THF)] affects catalytic cyclization of a range of aminoalkenes and aminoalkynes with activities that are broadly commensurate to those of established rare earth catalysts.
Diisopropylcarbodiimide, (i)PrN[double bond, length as m-dash]C[double bond, length as m-dash]N(i)Pr, inserts into the lithium-phosphorus bond of in situ prepared "Ph(2)PLi(THF)(n)" to afford the lithium salt, [Li(Ph(2)PC{N(i)Pr}(2))(THF)(n)](x)(2a); alternatively, this compound can be made by deprotonation of the neutral phosphaguanidine, Ph(2)PC{N(i)Pr}{NH(i)Pr}(1a) with (n)BuLi. Displacement of the THF solvate in 2a is readily achieved with TMEDA to afford Li(Ph(2)PC{N(i)Pr}(2))(TMEDA)(3a). X-Ray crystallographic analyses show that 2a exists as a dimer in the solid state with a folded ladder structure and an N,N' chelating phosphaguanidinate, while 3a is monomeric with N,P-coordination of the ligand to lithium. Compound 2a reacts via a transmetallation pathway with AlMe(2)Cl to afford the dimethylaluminium complex, Al(Ph(2)PC{N(i)Pr}(2))Me(2)(4a), which can also be prepared by protonation of a methyl group of AlMe(3) using 1a. The formation of a series of dialkylaluminium compounds has been investigated employing this latter pathway using both 1a and the N,N'-dicyclohexyl analogue, Ph(2)PC{NCy}{NHCy}(1b), affording Al(Ph(2)PC{NR}(2))Et(2)(5a,b), Al(Ph(2)PC{NR}(2))(i)Bu(2)(6a,b) and the diphenylaluminium compound Al(Ph(2)PC{N(i)Pr}(2))Ph(2)(7a). The oily nature of most of the dialkyl compounds and high sensitivity to oxygen and moisture lead to difficulty in manipulation and characterization; however, NMR spectroscopy indicated highly pure products (>95%) upon removal of the solvent. The molecular structures of the crystalline examples 4a and 7a are reported, showing monomeric aluminium species with symmetrically chelating phosphaguanidinate ligands. The series of aluminium compounds AlLCl(2){L=[EC{NiPr}(2)](-): A, E=Me; B, E=Me(2)N; C, E=(Me(3)Si)(2)N and D, E=Ph(2)P} were investigated using density functional theory. In the more simple cases A and B, the delocalized electron density of the metallacycle was represented by a combination of the HOMO and an orbital of lower energy (A, HOMO-5; B, HOMO-6). The HOMO-1 in B was pi-bonded across the Me(2)N-C bond suggesting delocalization of electron density into the metallacycle. In the more complex systems C and D, delocalization within the metallacycle was less extensive due to the (Me(3)Si)(2)N- and Ph(2)P-moieties. A number of occupied orbitals in D, however, display phosphorus 'lone-pair' characteristics, indicating that these species have the potential to behave as Lewis bases in the formation of poly(metallic) systems.
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Organo alkali metal compounds such as (n)BuLi and (Me3Si)2NK act as excellent catalyst precursors for the addition of phosphine P-H bonds to carbodiimides, offering a general and atom-economical route to substituted phosphaguanidines, with excellent tolerability to aromatic C-Br and C-Cl bonds.
Heterobimetallic compounds of the alkaline-earth metals show a wide structural variety with strongly differing reactivity patterns. The combination of magnesium and alkali metal amides yields cyclic molecules with an extreme high reactivity which often are considered as "inverse crowns" with the metal atoms as coordination sites for Lewis bases. In other metallates of the alkaline-earth metals an activation of alkyl groups succeeds. In alkaline-earth metal zincates an inverse coordination of the type M(2)[(mu-R)(2)ZnR](2) is observed and the alkyl groups are in bridging positions between zinc and the s-block metals thus forming a very reactive M-C-Zn three-center-two-electron bond. Furthermore, the metals of the carbon group form alkaline-earth metal-silicon, -germanium and -tin bonds or, in the presence of very strong Lewis bases, even solvent-separated ion pairs. For electronegative substituents at tin an inverse coordination mode such as M[(mu-R)(2)SnR](2) is observed.
Spectroscopic, crystallographic, and computational studies of the substituent distribution about the "NCN" unit in a series of phospha(III)- and phospha(V)-guanidines, R(2)PC{NR'}{NHR'} and R(2)P(E)C{NR'}{NHR'} (R = Ph, Cy; R' = (i)Pr, Cy; E = S, Se), are reported. In the phosphorus(III) systems, the P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be the E(syn)-(alpha) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E(syn)-(alpha) and Z(anti) in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the alpha- or beta-conformer relative to rotation about the P-C(amidine)() bond; however, DFT calculations indicate a low-energy structure for the N,N'-dimethyl model complex in the beta-conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the E(syn)-(beta) and Z(syn)-(beta) isomers in solution ( approximately 3:2 ratio), whereas for the P-dicyclohexyl analogues, the latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement about the imine double bond) is the dominant form. Intramolecular E...HN (E = S, Se) interactions are observed in solution for the Z(syn)-(beta) configuration of both P-substituted species, characterized by J(SeH) coupling in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the infrared spectrum. An X-ray crystallographic analysis of representative Ph(2)P(E)- and Cy(2)P(E)-substituted species shows exclusively the E(syn)-(beta) configuration for the P-diphenyl substituted compounds and the Z(syn)-(beta) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents. Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the observed preferences in substituent orientation, suggesting that steric factors play an important role in determining the subtle energetic differences at work in these systems.
Organocalcium chemistry is still in its infancy. The direct synthesis of activated calcium and (substituted) iodobenzenes allows for the large-scale and high-yield synthesis of aryl calcium iodides. The influence of the substitution patterns of the phenyl group, halogen atom, and solvent is discussed. Aryl calcium iodides show a Schlenk equilibrium that enables the isolation of diaryl calcium derivatives. Owing to the high reactivity of aryl calcium halides, low temperatures have to be maintained throughout the preparative procedures in order to avoid side reactions. A decrease of reactivity and, hence, an enhanced stability at higher temperatures can be achieved by shielding of the calcium atom by increasing the coordination number of the metal center or by substitution of the iodide anion by bulky groups.