Vanadium complexes acetonitrile

A vanadium complex can be precipitated from a concentrated acetonitrile solution of (HPS)VO(OEt)(EtOH) by the addition of a small amount of water. This complex has an elemental analysis consistent with the formulation [(HPS)V0]20 MeCN. Its 51V NMR spectrum shows only the three upheld resonances. Addition of ethanol results again in complete conversion to the —530 ppm form (HPS)VO(OEt) (35). 

Anhydrous vanadium trichloride or tribromide reacts with acetonitrile to yield green or red-brown solutions. Complexes [VX3(MeCN)3] (X = Cl or Br) crystallize with varying amounts of acetonitrile in the lattice,212 where the IR spectra show the free (2253.5 cm-1) and coordinated (2292 cm-1) acetonitrile. 

Several solvated vanadium(III) halides have been described in the previous sections and those with water, alcohol, acetonitrile or THF have been useful starting materials for the synthesis of vanadium(III) complexes. 

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. 

Figure 3.166 shows the reactivity and potential of these early transition metal pincer carbene complexes taking vanadium as an example. The broad range of compounds becomes possible owing to the rich redox chemistry of vanadium that makes different oxidation states easily accessible. Here, examples for vanadium(ll), vanadium(III) and vanadium(IV) are shown. Abstraction of halogen is facile making the introduction of different TT-donor ligands possible, even weak ones like acetonitrile. 

Acetonitrile is a convenient solvent in which to study, by pulse radiolysis, the one-electron reduction of transition metal complexes that are not stable in water or hy-droxylic media. For example, the tantalum compound [Ta2Cl6(4-methylpyridine)4] has been shown to be reduced by CH3CN with A = 1.2 x 10 dm mol s [24], The absorption spectrum of the product shows the characteristic features of d-d transitions and it was suggested that the added electron is delocalized over the double-bonded Ta=Ta moiety. Another example is the radiolytic reduction of the vanadium(III) complex [VCl3(y-pic)3], where y-pic is 4-methylpyridine, to the derivative via the V complex [25]. It was shown by pulse radiolysis that the electron adduct of the complex decayed in a first-order process with k= 1.3 x 10 s which is thought to involve loss of Cl". The intermediate V complex had a... 

Vanadium(V) mono-, di-, tri-, and tetra(phosphoraniminato) complexes (83)-(86) have been prepared and an acetonitrile solvate of [V(NPPh3)4]Cl structurally characterized.443 A tridentate pyrazolylcyclohexanol complex of vanadium(V) has also been synthesized.444 A derivatized pyridine ligand is chlorinated by VOCl3 in the presence of t-BuOOH to yield the vanadium(V)... 

The ability of phosphines to act as both a metal oxidation states and as a 7r acceptor to relieve electron density from lower metal oxidation states explains, in part, the observation that diphosphines stabilize the +3, +1, and 0 oxidation states at the expense of the +2 oxidation state.870 Vanadium(II) diphosphine halides, [V(dmpe)2X2] (dmpe = dimethyl-dimethylphosphinoethane, X = Br or I), were prepared as side products in the synthesis of [XV(CO)2(dmpe)2].885 The structurally characterized diphosphino V11 tetrahydroborate complex, tra .s-[V(BH4)2(dmpe)2], (223) was synthesized via the reduction of similar vanadium(III) precursors.832 The reaction of a vanadium(II) chloride salt with dmpe afforded the tra s-[VCl2(dmpe)2] complex which can be alkylated with MeLi or MgMe2 to afford the organometallic trans-[V(Me)2(dm pcVJ compound, which in turn reacts with thiocyanate to form tnms -[V(NCS)2(dmpe)2].8 87 Acetonitrile or propionitrile readily displace the chlorides from trans- WCl2(dmpe)2] to form // tf/ ,v-[V( NCR)2(dmpc)2][BI>h4]2 (R = Me or Et) the acetonitrile derivative reacts with HCPh(S02CF3)2 in acetonitrile solvent to yield the hexaacetonitrile species [V(NCMe)6]2+.887... 

Aromatic hydrocarbons can be oxidized to the corresponding phenols by transition metal peroxo complexes and, in particular, vanadium(V) peroxo complexes, which act either as electrophilic oxygen transfer reagents or as radical oxidants -, depending on the nature of the ligands coordinated to the metal and on the experimental conditions. Vanadium picolinato peroxo complex (V0(02)PIC(H20)2) (39) (PIC = picoUnic acid anion) has been reported to be particularly effective in the hydroxylation of benzene and substituted benzenes (equation 50) . Accordingly, 39 smoothly oxidizes substituted benzenes 38 to the corresponding monophenols 40 in acetonitrile at room temperature. 

Vanadium-based complexes and salts have been used for oxidation either with oxygen in the presence of coreductants [7i], with peroxyacetic acid in acetonitrile [7j], or with HP/O2 combined oxidants [7k-mj. 

The use of a heterogeneous system under nitrogen consisting of a stirred suspension of silica gel with adsorbed ferric-catechol complex in benzene treated with 35% hydrogen peroxide has been reported to result after 2.5 hours in a 60% yield of phenol (ref. 14). The formation of phenol in 56% yield resulted from a mixture of benzene and vanadium(V) catalyst in acetonitrile under nitrogen when reacted for 2 hours at ambient temperature (ref. 15). More recent studies have involved the conversion of benzene in trifluoromethanesulphonic acid to phenol by the electroreduction of dioxygen (ref. 16) and from generation... 

Trace levels of vanadium(V) were extracted from water and coal fly ash samples and analyzed as the 2-(8-quinolylazo)-5-(dimethylamino)phenoI complex [922]. A Cjg column (2 = 550nm) and a 50/50 acetonitrile/water (lOmmol/kg tetrabutyl-ammonium [TEA] bromide) mobile phase generated the separation. A number of potentially co-eluting metal ions were studied as well Ni, Co, and Fe. Detection limits of 3 pg were reported. Vanadium(IV) and (V), Pb(II), Cu(II)and Cr(VII) were resolved on a Cg colunrn (2 = 245nm) with a 12/78 acetonitrile/water (50 mM TEA with 2mM EDTA to pH 6) mobile phase [923]. Elution was complete in <20 min and peak shapes were good. Detection limits of 1 ng and linear concentration ranges of 1-30 pg/mL were reported. 

In acetonitrile vanadium(III) chloride gives VCI3 (AN)3, titanium(III) chloride o gives TiCl3(AN)3 and chromium(III) chloride CrCl3(AN)3. These complexes in acetonitrile solution were all found to act as chloride ion donors in the presence of strong chloride ion acceptors i ... 

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