Facile Synthesis of Alkynyl− and Vinylidene−Niobocene Complexes. Unexpected η1-Vinylidene−η2-Alkyne Isomerization

Abstract

Treatment of Nb(η5-C5H4SiMe3)2(Cl)(L) (1) with Mg(C⋮CR)2 in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η5-C5H4SiMe3)2(C⋮CR)(L) (2: L = CO, R = Ph (2a); L = CO, R = SiMe3 (2b); L = CO, R = tBu (2c); L = PMe2Ph, R = Ph (2d); L = P(OEt)3, R = Ph (2e)). The alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give the corresponding cation-radical alkynyl complexes [Nb(η5-C5H4SiMe3)2(C⋮CR)(L)]•+[BPh4]- (3: L = CO, R = Ph (3a); L = CO, R = tBu (3c); L = PMe2Ph, R = Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d2 niobocene species [Nb(η5-C5H4SiMe3)2(CCHR)(L)][BPh4] (4: L = CO, R = Ph (4a); L = CO, R = tBu (4c); L = PMe2Ph, R = Ph (4d)) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d2 niobocene complex [(η5-C5H4SiMe3)2(CO)NbCC(Ph)(Ph)CCNb(CO)(η5-C5H4SiMe3)2][BPh4]2 (4a‘) from a competitive ligand−ligand coupling process. Complex...