Vanadium carbene complexes

The bis(neopentyl)vanadium(III) complex V(CH2CMe3)2(Cp)(PMc3) catalyzes the ROMP of norbomene in benzene at 20°C. The initiating species is presumably a vanadium carbene complex formed by the decomposition of the catalyst. The vanadium(III) carbene complex V(=CHCMe3)(Cp)(dmpe), where dmpe = l,2-bis(dimethylphosphino)ethane, is a poor initiator (Hessen 1993). 

Reaction of the diene complexes Cp2M(butadiene) (M = Zr, Hf) with CpV(CO)4 leads to the vanadium carbene complex 23. In the case of M = Hf, complex 23 is in equililnium widi the carbene 24. These complexes react with various ketones to form nine-membeied mctallaoxy vanadium carbenes.2W... 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

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. 

The acylvanadium complex (78) is converted to the Fischer type (see Fischer-type Carbene Complexes) vanadium complex (79) on treatment with Me3+Bp4 (Scheme 42). The N-substituted carbene analog is formed from Na2[VCp(CO)3] and iminium chloride (80) as shown in... 

Scheme 43. The Fischer-type see Fischer-type Carbene Complexes) vanadium complexes are prepared by treatment of (butadiene)hafhocene (81) or (butadiene)zirconocene with CpV(CO)4 (Scheme 44). The equilibrium mixture reacts with cyclopentanone to give the adduct complex (82). - ...

Thermolysis of ()) -cyclopentadienyl)bis(neopentyl)(tri-methylphosphine)vanadium(I) (82) in the presence of 1,2-bis(dimethylphosphuio)ethane leads to the formation of the Schrock-type see Schrock-type Carbene Complexes) alkylidene vanadium complex (83), which is supported by X-ray crystallographic analysis (Scheme 45). " The imido vanadium complex (84) is converted to the corresponding alkylidene complex (86) on treatment with ben-zylidene(triphenyl)phosphorane (85) via substitution of the phosphine ligand (Scheme 46). ... 

Downing, S.P., Guadano, S.C., Pugh, D. et al. (2007) Indenyl- and fluorenyl-functionahzed N-heterocyclic carbene complexes of titanium, zirconium, vanadium, chromium, and yttrium. Organometallics, 26, 3762. 

Then the hydrogen transfer from methyl group occurs to produce ethane and a carbene complex of vanadium. 

Simple addition of a NHC to yield an early transition metal carbene complex in a high oxidation state was applied to synthesize [(IMes)VCl3(0)] 15 (Figure 6.2). The NHC ligand conferred a high stability to the complex, and while other tri-chloro-oxo-vanadium(V) adducts e.g. [VCl3(0)(MeCl ]) were easily hydrolyzed in air, compound 15 was air-stable over months even in a dichloromethane solution. A major contribution to this stability may arise from the highly unusual Cl-Ccarbene interactions determined from the crystal structure (vide infra). 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Bis(guanidinato)bis(benzyl) complexes, with Zr(IV), 4, 776 Bis(heteroarene) vanadium complexes, preparation, 5, 48 Bis(heteroatom) polysilanes, synthesis, 3, 584 Bis(iV-heterocyclic carbene) ligands, in silver(I) complexes,... 

Interest has again continued in studies of the generation and reactivity of phosphinidene species (RP ), phosphorus analogues of carbenes, and this area has been reviewed. A review has also appeared of the use of terminal phosphinidene complexes in the formation of phosphorus-element bonds. Further studies of phosphanylidene-a -phosphoranes, RP = PR3, (regarded as phosphine-complexed phosphinidenes, i.e., ArP -PRs), have shown that the species Me3P=PAr (Ar = Mes or 2,6-Mes2C6H3) are good vehicles for the delivery of the terminal phosphinidene moiety ArP to zirconium and vanadium sites. Terminal phosphinidene complexes of cobalt, ruthenium, rhodium and osmium, and iridium, have also been... 

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