Dimeric vanadium complex

These dimeric complexes involve, in their neutral state, two metal atoms in the (III) oxidation state. In the vanadium complexes such as [CpV(bdt)]2 and [CpV(tft)]2, the V—V bond length, 2.54 A in [CpV(bdt)]2, are shorter than observed in model complexes with a single V—V bond, indicating a partial double-bond character, also confirmed by a measured magnetic moment of 0.6 fiB in [CpV(tfd)]2, lower than expected if the two remaining unpaired electrons contribute to the magnetic susceptibility [20, 49]. This class of complexes most probably deserves deeper attention in order to understand their exact electronic structure. 

Figure 10.4 Structure of the self-dimerized V complex on Si02 modeled by DFT. (a) Top view (b) side view. Red vanadium green nitrogen blue oxygen gray Ph ring yellow isobutyl group derived from L-leucine.

Amino acids (L) have also been shown to form a dimeric-type complex of (V0(00)2L)2(00)L coordination, for which it has been proposed that one peroxo group and the carboxylate group of one amino acid form a bridge between the two vanadium nuclei, after the fashion displayed in Scheme 6.6 [24], The complex was found not to retain its integrity when dissolved in aqueous solution. 

Tab. 38. Enantioselective dimerization of 2-naphthol (and 2-naphthylamine) derivatives promoted by vanadium complexes 219 and 220.

Arylimido)vanadium(V) complexes have been prepared from the reaction of VOCl3 with various /wa-substituted aryl isocyanates. The reactivity and structure of some of these have been investigated (78).431 The reactivities of a series of vanadium(V) chalcogens have been reported. The structure of [(Me3Si)2N]2V(Se)[SeSi(SiMe3)3] 432 (79) shows that the V=Se double bond contracts by 8% compared to the single bond in a related dimeric vanadium(IV) structure (vide infra). The... 

A cyanohydridoborato vanadium(II) complex, tra s-[V(NCBH3)2(thf)4], has been synthesized and structurally characterized (212).858 Additional oligomeric complexes have been prepared complexes that have been characterized include trinuclear vanadium complexes,859-862 dinuclear V-Fe complexes,863 and the first tetranuclear V11 species [V4(/x3-Cl)2(M-Cl)2(M-CF3C02)2(thf)6].864 The addition of tris(pyrazolyl)borate ligand to the tetranuclear species results in the dissociation of the complex to form a neutral dimeric species [V(tpb)-(/x-Cl)(/x-CF3CO2)2V(th03] (213) in which one of the vanadium centers is facially coordinated by tpb.864... 

In 2006, Khan reported the dimeric vanadium(v)-salen complex S,S,S,S)-5 (Figure 9.2), which could catalyse the asymmetric addition of a cyanide ion to aldehydes using potassium cyanide with acetic anhydride. The reaction of aromatic aldehydes as substrates with 1-5 mol% of complex 5 afforded adducts 4 in excellent yields (90-99%) and high enantioselectivities (88-95%). In addition, the catalyst was able to be precipitated and recycled by adding -hexane to the postcatalytic reaction mixture. In this way, catalyst 5 was readily recovered and reused four times. 

Bu Bu (ssss)-5 Figure 9.2 Chiral dimeric vanadium(v) salen complex 5, reported by Khan. ... 

In 2011, Khan also reported the novel, recyclable, dimeric vanadium(v)-salen complex [R,R,R,R)-6. Dinuclear chiral complex 6 had a flexible polyether linkage and was efficient for the stereoselective O-acetylcyanation of a variety of aliphatic and aromatic aldehydes with potassium cyanide as a cyanide source, providing the chiral O-acetyl cyanohydrins 4 in excellent yields (93%-quant.) with high enantioselectivities (72-98%). As shown in Scheme 9.2, these results were achieved using only a 1 mol% catalyst loading. In addition, Khan investigated the use of ethyl cyanoformate for... 

In 2010, Kahn found that dimeric vanadium(v) salen complex [S,S,S,S)-5, which had previously been reported as a catalyst for the asymmetric synthesis of 0-acetyl cyanohydrins 4, was an effective catalyst for the asymmetric addition of cyanide to N-benzyl-protected aldimines to give (J )-amino nitriles 16 in 65-92% yields with up to 94% enantiomeric excess. Addition of a small amount of water and low temperature were essential to achieve both high conversion and high asymmetric induction for asymmetric catalysis. In this system, catalyst 5 could be reused approximately four times while maintaining its high activity. 

SCHEME 6 Gong s catalytic, asymmetric oxidative dimerization of naphthols using vanadium complex 35. Reboot of viriditoxin/pigmentosin A S5mthetic plan. 

Stereoselective auto-oxidations have also been described in the reactions of dimeric vanadium(iv)-tartrate complexes. The rate of uptake of Oa in the presence of these complexes is dependent on the isomeric form (d, l, or meso) of the ligand, the influence of pH being observed to be important. The overall stoicheiometry conforms to the equation... 

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

The role of steric influences on the formation of various vanadium amidinate complexes in the oxidation states +2 and +3 has been studied in detail. The reaction of VCl2(TMEDA)2 and of VCl3(THF)3 with 2 equivalents of formamidinate salts afforded dimeric V2[HC(NCy)2l4 (cf. Section IV.E) with a very short V-V multiple bond and [ [HC(NCy)2 V(/i-Cl)l2 which is also dimeric (Scheme 107). The formation of V2[HC(NCy)2l4 was shown to proceed through the intermediate monomeric [HC(NCy)2l2V(TMEDA), which was isolated and fully characterized. The dinuclear structure was reversibly cleaved by treatment with pyridine forming the monomeric [HC(NCy)2l2V(py)2. ... 

Scheme 10.2 Chiral self-dimerization of V-Schiff-base monomer complexes on Si02. Two vanadium monomers are dimerized through two sets of hydrogen bonds between Ph-OH and COO.

Mono- and diphenylphosphine also give the corresponding dimeric complexes with vanadium hexacarbonyl. 

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