MECHANISTIC STUDIES OF THE REDUCTIVE COUPLING OF CARBON-MONOXIDE IN 7-COORDINATE NIOBIUM(I) AND TANTALUM(I) DICARBONYL COMPLEXES

The mechanism by which the two carbonyl ligands in the seven-coordinate complexes [M(CO)2(dmpe)2Cl] [M = Ta, Nb; dmpe = 1,2-bis(dimethylphosphino)ethane] are reductively coupled has been elucidated. The first step involves two-electron reduction to afford the [M(CO)2(dmpe)2]- anion, the tantalum analogue of which was structurally characterized as the tetra-hexylammonium salt. The geometry of this anion is distorted octahedral with cis-carbonyl ligands. Reaction of the anion with 1 equiv of trialkylsilyl chloride yields the siloxycarbyne species [M(= COSiR3)(CO)(dmpe)2] (M = Ta or Nb), formed by electrophilic attack of the silyl halide on the carbonyl oxygen atom. The tantalum triisopropylsiloxycarbyne complex, the X-ray structural analysis of which is described, reacts with 1 equiv of trimethylsilyl chloride to afford the asymmetric acetylene complex, [Ta{Me3SiOC = COSi(iota-Pr)3}(dmpe)2Cl], which was also structurally characterized. The transition state between the carbyne and coupled products may be an eta2-ketenyl of the type [Ta{eta2-(O)CCOSiR3}(dmpe)2X]- (X = halide). Such an assignment is supported by a series of experiments involving addition of different silyl halide reagents (Me3SiX; X = Cl, I, OTf) to (n-C6H13)4N[Ta(CO)2(dmpe)2]. The generality of the reaction mechanism is discussed.