Vanadium imido complexes

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. 

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

Isocyanate formation through multiple bond metathesis of C02 with carbodiimide has been also demonstrated [112]. This transformation can be promoted by titanium isopropoxide, at 383 K, in THF as solvent. It is worth noting that the reverse process, which opens an entry into carbodiimide synthesis, is a well-known process that is catalyzed by several other systems, including trialkylphosphine oxides [113] or vanadium-oxo or -imido complexes [114]. 

Heating of N-isopropyl-N phenylcarbodiimide at atmospheric pressure affords a mixture of N,N -diisopropyl-, N,N diphenyl- and the starting carbodiimide. " Apparently, a thermal metathesis reaction occurs. Vanadium 0x0 [V(0)(0-i-Pr)3] andimido vanadium complexes [V(NPhMe)Cl3] are found to be catalysts for this metathesis reaction. Likewise, mixtures of aromatic and aliphatic carbodiimides undergo the carbodiimide metathesis reaction. Vanadium 0x0 complexes V(0)(0-i-Pr)3 are precatalysts which are converted into the reactive imido complexes V(NR)(0-i-Pr)3 during the reaction. 

The lack of stability of terminal oxo compounds of the heavier Group 5 metals led us naturally to the door of the isoelectronic imido (NR) ligand, where the availability of a substituent attached to the multiply-bonded group would allow both steric and electronic modulation of the products stability and reactivity. There had been a handful of half-sandwich imido complexes of the Group 5 metals synthesised by other workers, especially for vanadium and tantalum, but at that time none were known for niobium. A half-sandwich imido compound of niobium we considered, therefore, a prime target. 

F. Montilla, A. Pastor, A. Galindo, Guanylation of aromatic amines catalyzed by vanadium imido complexes. J. Organomet. Chem. 689, 2004 993. 

It was reported that one of the trimethylphosphine ligands in the vanadium(III) imido complex [CpV(NAr)(PMe3)2] (Ar = 2,6-C6H3Pr 2) could be substituted with ethene or diphenylethyne to afford complexes of the type [CpV(NAr)(PMe3)(T 2-L)]. Spectroscopic data were consistent with considerable back-donation from the vanadium(m) centres to the alkene and alkyne ligands. Photolysis of a mixture of tri-fcrt-butylcyclopropenium tetrafluoroborate and sodium hexacarbonylvanadate afforded the structurally characterised complex [(C3But3)V(CO)4], which represented the first example of partial insertion of an early transition metal atom into a cyclopropenyl ring. 

Figure 63. Examples of vanadium(V) tris(dithiocarbamate) imido complexes.

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). ... 

Imido-thiolato-complexes of vanadium of the type [V(Bu N)(SR)3] have been made by the reaction of [V(Bu N)Cl3] with Li[SR] (R = Bu , SiPhj) 18). Comproportionation reactions of the trithiolate with the trihalide precursor were then used to derive the related complexes... 

A vanadium(II) tmeda complex with 2,6-dichlorothiophenolate has been reported with both S- and Cl-donor atoms (217).874 The thiolate donor atoms are trans to each other while the Cl atoms are trans to the N atoms in the tmeda ligand.874 The complex will react with azobenzene to give the phenylimido complex where one of the chloro-donor ligands is now trans to the imido nitrogen (218) 874 this complex is structurally similar to other imidovanadium(IV) complexes.643-646 A V11 cysteine methyl ester complex has been shown to reduce water to H2 but the mechanism of this process is not known.875... 

Recently, efforts have been made to expand the number of homogeneous vanadium catalysts using ancillary, nonreactive ligands. These catalysts may be in the -e5, +4 or q-3 oxidation states. The dianionic imido ligand is isolobal with cyclopentadienide and has been used to prepare diamagnetic, formally d° metal compounds like (10) (Figure 13) which, when activated by MAO, display modest activity for ethylene polymerization. VCI3 complexes of Q -diimines... 

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