Complexes of vanadium

Two separate publications (125, 126) described the synthesis of a number of carbonyl complexes of vanadium. The mononuclear species V(CO) , n = 1-6, have all been identified by using CO matrix-dilution experiments and mixed CO- CO isotope experiments while main-... 

Recently, several papers have appeared that report the isolation of stable complexes of vanadium(IV) with dithiocarboxylato ligands (77, 80). Dithiocarboxylato salts (PhCS, P-CH3C6H4CS2,... 

Macrocyclic ligands 1,4,7-triazacyclonona-iV-acetate 221 and iV-(2-hydroxybenzyl)-l,4,7-cyclononane 225 have been prepared from l,4,7-triazacyclio[5.2.1.04 10]decane 40 and were subsequently used for the synthesis of a series of mono- and dinuclear complexes of vanadium(rv) and (v) (Scheme 35) <1995ICA(240)217>. 

Inorganic complexes of vanadium are normally found in the following forms ... 

Dobson, J.C. Taube, H. Coordination Chemistry and Redox Prop ies of Poly-pyridyl Complexes of Vanadium(II). Inorg. Chem. 1989,25, 1310-1315. 

Fewer electrochemical studies have been performed on low-oxidation-state complexes of vanadium, compared to higher oxidation states. Complexes in oxidation states lower than V(I) are limited to those which contain ligands such as strong 7T acids (CO and RNC), arenes, or tropy-lium. For example, the [Cp2Co][V(CO)6] salt contains V(-I) which undergoes a reversible oxidation at —0.54 V in CH2CI2 to form the 17-electron V(0) species [3]. 

Congeneric Nb species are known [4]. A (trimpsi)V(NO)(CO)2 complex (trimpsi = BuSi(CH2PMe2)3) has been reported [5] which undergoes a reversible oxidation to the cationic V(0) complex at —0.74 V versus Cp2Fe/CH2Cl2 and a second, irreversible oxidation at 0.61 V. More highly reduced complexes of vanadium have been prepared, but their electrochemistry has evidently not been explored. The electrochemical synthesis of CpV(CO)4 from a vanadium anode, cyclopentadiene, and 115 bar of CO at 100 °C in DMF has been reported [6]. 

Table 4 Complexes of Vanadium(II) with Pyridine and Picolines...

Table 5 Data on the Complexation of Vanadium(II) with Thiocyanate in Water at 25 °C...

A similar preparative procedure was used to isolate tetrahedral complexes of bidentate donors (L = bipy or phen) [VL2](SCN)3. However, complexes of vanadium(lll) chloride or bromide with these same bidentate donors have stoichiometries depending on conditions.199 With excess of bipy or phen, complexes [VX JX were obtained. In EtOH or in MeCN two series of complexes were isolated. VC13LL (L = bipy or phen L = EtOH or MeCN) and [VC12L2]+ + [VCUL]-. In the reaction of vanadium(III) with phenanthroline202 203 or bipyridine204 there is evidence for extensive hydrolysis. Spectrophotometry was used to estimate the equilibrium constant (Kcq = 0.43) for the reaction in equation (7). 03... 

Most complexes of vanadium(IV) with group IV ligands are normally organometallic compounds, which have been reviewed.9... 

More recently p-tolyl isocyanate has been used to prepare a simple imido complex of vanadium(V)... 

The vanadium(V) oxidation of the sulfide PhCH=CHSPh has been studied in aqueous acetic acid containing perchloric acid. The reaction is first order in vanadiiun(V) and fractional order in sulfide. An intermediate complex of vanadium and the sulfide forms and its decomposition is the slow step of the reaction.181 Two Indian groups have reported on the use of ruthenium(VI) and ruthenium(III).182 183 The kinetics and mechanism of the oxidation of diethylene glycol by aqueous alkaline potassium bromate in the presence of Ru(VI)182 and the Ru(III)-catalysed oxidation of aliphatic alcohols by trichloroisocyanuric acid183 have been examined. 

Rehder, D. (1977) Vanadium-51 chemical shift and vanadium-51-phosphorus-31 coupling in carbonyl complexes of vanadium./. Mag. Res., 25, 177-181. 

Complexes of vanadium(III) typically have octahedral coordination, though other coordinations are certainly not unusual, particularly with bulky ligands where trigonal bipyramidal coordination is adopted. Nitrogen- and oxygen-containing mul-... 

Schwendt, P., P. Svancarek, I. Smatanova, and J. Marek. 2000. Stereospecific formation of a-hydroxycarboxylato oxo peroxo complexes of vanadium(V). Crystal structure of (NBu jfVjOjtPjXL-lactXj FIjO and (NBu lA C fC lD-LactXL-lact)] H20. J. Inorg. Biochem. 80 59-64. 

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

Treatment with either vanadium salts or organic complexes of vanadium have decreased plasma insulin levels and improved insulin sensitivity in animal models of both insulin resistance and type 2 diabetes. This work has recently been reviewed [13]. The Zucker Diabetic Fatty (ZDF) rat develops overt hyperglycemia in the presence of hyperinsulinemia followed by [3-cell depletion. This is a type 2 diabetic rat model developed from the Zucker Fatty (fa/fa) rat. In these animals, chronic treatment with vanadium reduced the elevated plasma glucose levels [152,153], The effect in the type 2 models of diabetes can take weeks to develop, whereas the effect in the type 1 models of diabetes are seen within 3 to 4 days. 

Nxumalo, F., N.R. Glover, and A.S. Tracey. 1998. Kinetics and molecular modelhng studies of the inhibition of protein tyrosine phosphatases by N,N-dimethylhydroxy-lamine complexes of vanadium(V). Journal of Biological Inorg. Chem. 3 534—542. 

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