Vanadium complexes carbonates

A particularly striking structural feature of the vanadium complex is the exceptional long distance (R) of 171 pm between the ring carbons closest to the metal compared to that in the Pt and Pd complexes (158 pm)315. This suggests that the vanadium complex approaches a metallacyclobutadiene structure, found in the analogous trigonal-bipyrami-dal rhenium complexes315 (Section V. A.2). 

Vanadyl salen complexes epoxidized cyclohexene presumably through intermediate hydroperoxides formed by radical chain autoxidation <1997105927, 1998TL5923>. Reactions using vanadium complexes have been carried out in liquid carbon dioxide <19990M4916>. 

Vanadium. A residual oil was desulphurized (673 K, 115 atm) with a non-stoicheiometric vanadium sulphide (S/V, 0.8-1.8) formed in situ from VS4 Vanadium sulphide catalysts have been prepared by in situ sulphiding of vanadium complexes, e.g., bis(acetylacetonato)oxovanadium(IV), dissolved in crude petroleum.Vanadium compounds occurring in heavy oils have been activated as desulphurization and demetallization catalysts by treatment with triethylaluminium. Catalysts consisting of vanadium promoted by nickel can be prepared in situ by deposition of the metals from heavy crude oils. Ni-V hds and hdm catalysts on silica or carbon have been claimed. 

The complexes derived from simple monodentate carboxylates are not stable, and only one mononuclear complex with a sterically hindered carboxylate has been successfully isolated, [VCKCbC Bujs] (13).103 Attempts to prepare carboxylates in organic solvents often led to the formation of oligomeric cluster compounds with a variety of interesting structural types.103 In aqueous solutions, these vanadium complexes appear to contain six-coordinate vanadium.60,104 Even carbonate has been found to form a weak vanadiumtV) complex in solution.105... 

The low-valency cyclopentadienyl vanadium complexes are usually stabilized by carbon monoxide, and VCp(CO)4 is the most useful precursor to various low-valency organovanadium complexes. There are a number of synthetic routes to VCp(CO)4. One convenient method is to carry out the reaction between NaCp and VCl3 in THF in situ and then to carbonylate under 60 atm CO pressure at 120 °C [19]. Reduction of preformed vanadocene by potassium, and subsequent carbonylation also gives rise to VCp(CO)4 [20]. These methods, however, cannot be applied to alkyl-substituted cyclopentadienyl derivatives. It is necessary to treat alkyl-substituted cyclopentadiene... 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

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. 

A variety of such ternary catalytic systems has been developed for diastereoselective carbon-carbon bond formations (Table). A Cp-substituted vanadium catalyst is superior to the unsubstituted one,3 whereas a reduced species generated from VOCl3 and a co-reductant is an excellent catalyst for the reductive coupling of aromatic aldehydes.4 A trinuclear complex derived from Cp2TiCl2 and MgBr2 is similarly effective for /-selective pinacol coupling.5 The observed /-selectivity may be explained by minimization of steric effects through anti-orientation of the bulky substituents in the intermediate. 

Insertions of isocyanide into niobium-carbon bonds follow a path similar to that with vanadium, resulting in the formation of the 7]2-iminoacyl complexes, which can then be involved in further chemistry.175 176 The reaction of acetone with cyclopentadienyl complex 110 under a carbon monoxide atmosphere gives the if -acetone compound 111. Complex 111 subsequently undergoes either stepwise insertion of two isocyanides via 112 or double insertion of the isocyanide to give complex 113 (Scheme 48).177... 

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