Complexes vanadium

Inorganic complexes of vanadium are normally found in the following forms  

The role played by the electron-donating or electron-withdrawing effects of the various ligands in rendering the oxidation process easier for [VO(salen)] compared to [VO(saloph)] or [VO(acen)] is evident. The influence of the solvent on the redox potentials is also clear. 

It should be emphasized that [VO(salen)] in its oxidized form [VO(salen)]+ ([C104]- counteranion) maintains the original square-pyramidal geometry, but, in accord with the electrochemical quasireversibility of the couple Viv/Vv, there are some variations in bond 

In agreement with the oxidation state V(IV)-d, it has a magnetic moment /ieff = 1.75 /iB- The V-S bond lengths are substantially identical, but the bonding angles are grouped as four smaller and two larger. 

From a speculative viewpoint, the quasireversibility of the electrochemical reduction Viv/Vm could account for such structural rearrangements. 

Fujita et al. used a catalytic amount of a binuclear titanium(IV) complex in an attempt to find an efficient system to oxidize sulfides with high enantioselectivity [102]. Prior to this study, they investigated other systems with several transition metals. A similar asymmetric sulfoxidation was discovered [105] using a catalytic amount of nonracemic Schiff base oxovanadium complex (Table 1.4) under atmospheric conditions at room temperature in dichloromethane. With 0.1 mol% of catalyst and cumene hydroperoxide as oxidant, oxidation produces sulfoxides in excellent yields. However, the reaction is limited to alkyl aryl sulfide substrates, and the best enantioselectivity obtained was 40% ee, for (S)-methyl p-methoxy phenyl sulfoxide. 

Molecular Metal-Metal Bonds Compounds, Synthesis, Properties, First Edition. 

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Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

A chiral vanadium complex, bis(3-(heptafluorobutyryl)camphorato)oxovana-dium(IV), can catalyze the cycloaddition reaction of, mainly, benzaldehyde with dienes of the Danishefsky type with moderate to good enantioselectivity [21]. A thorough investigation was performed with benzaldehyde and different activated dienes, and reactions involving double stereo differentiation using a chiral aldehyde. 

Figure 6.5 Proposed structure for the vanadium complex prior to the oxygen transfer from peroxide to the allylic olefin.

Table 6.4 Comparison of the performances showed by the vanadium complexes with ligands 4—7 in the AE reaction generating the epoxides A and B, respectively.

Vanadium complexes, 3,453-569 acetylacetone exchange reactions, 2,380 alkyl peroxides... 

C° total concentration of pyrosulfate and sulfate anions Cv concentration of biatomic vanadium complexes C3l concentration of S03 in the melt phase Cp specific heat (kcal/kg °C)... 

For the two metallocene systems, V(Cp)2 (d3), and Ni(Cp)2 (ds), the only acceptable assignments locate the 4> level between the two II levels in each case, and thus, using the data of Prins and Van Voorst (4 7), yield the parameters Ds = 3543 cm"1, Dt = 2074 cm"1 for the vanadium complex, and Ds = 3257 cm 1,Dt = 1806 cm-1 for the nickel derivative. Moreover, these authors, treating the available data for Fe(Cp)2, derived parameters corresponding to Ds = 5100 cm-1 and Dt = 2740 cm-1, from which it may be noted that, although the individual values of Ds and Dt vary appreciably from one complex to another, the ratio Dt/Ds remains approximately constant and shows the values 0.585,0.554,and 0.537 for the V, Ni, and Fe compounds respectively. More recently Sohn, Hendrickson, and Gray (48, 49) have successfully analysed the d-d spectra of several d6 systems in terms of parameters which lead to the ratios 0.525, 0.540, and 0.585 for Dt/Ds in the complexes Fe(Cp)2, Co(Cp)2+, and Ru(Cp)2 respectively, and it therefore seems reasonable to adopt an average value of 0.55 for this ratio for all metallocene systems. 

The chromium, tungsten, and vanadium complexes can be prepared by an entirely analogous procedure. 

The isoelectronic relationship has been applied to the vanadium complex with hydrotris(pyrazolyl)borato ligand, which possesses the same electronic features as the Cp ligand thus the combination of complex VCl2(NAr)Tp (158) (Ar = 2,6-diisopropylphenyl, Tp = hydrotris(3,5-dimethylpyrazolyl) borate) with MAO was reported to catalyze the polymerization of ethylene (at atmospheric pressure) and propylene (at 7 bar), giving polyethylene (14kg/mol-h, Mw = 47000, Mw/Mn = 3.0) and polypropylene (1 kg/mol h, Mw = 3800, Mw/Mn = 2.0), respectively [256]. 

The vanadium complex 159, bearing oligometallasilsesquioxanes, is an interesting model complex for surface species on silica [257]. Polymerization of ethylene with 159/MAO affords polyethylene with an Mn value of 2100 and an Mw value of 47900. An aluminum-vanadium adduct 160 polymerized ethylene without any cocatalysts, affording similar catalytic activity with the 159/MAO system [258]. 

The eight-coordinate vanadium complex V(S2CMe)4 contains both dodecahedral and square prismatic eight-coordinate molecules in the same crystal.322 Of particular interest is the chain-like, mixed valence platinum complex [Pt2(S2CMe)4( i-I)]A, which displays unusual electrical properties metallic conduction between 300 and 340 K and semiconducting properties below 300 K,323 whereas the analogous nickel complex, [Ni2(S2CMe)4(p-I)]x is a semiconductor.324... 

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. 

Vanadium complexed with tetraphenyl-arsonium chloride and tetramethylene dithio-carbamate, extracted with chloroform... 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

Several anionic metal carbonyl hydrides stoichiometrically convert acyl chlorides to aldehydes. The anionic vanadium complex [Cp(CO)3VH] reacts quickly with acyl chlorides, converting them to aldehydes [44]. Although no further reduction of the aldehyde to alcohol was observed, the aldehydes reacted further under the reaction conditions in some cases, so a general procedure for isolation of the aldehydes was not developed. 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

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