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. Complexes 4 were also prepared by an alternative procedure in which the corresponding complexes 2 were reacted with HBF4. Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η2-alkyne derivatives [Nb(η5-C5H4SiMe3)2(η2(C,C)-HC⋮CR)(CO)]+ (5: R = Ph (5a); R = tBu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp2(CCHCH3)(L)]+/[NbCp2(HC⋮CCH3)(L)]+ (Cp = η5-C5H5; L = CO, PH3; exo, endo) model systems in order to explain the η1-vinylidene−η2-alkyne rearrangement observed. Calculations have shown that in both carbonyl−niobocene and phosphine−niobocene systems the η1-vinylidene and the η2-alkyne complexes are isoenergetic, in marked contrast with the systems previously considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift mechanism where η2(C,H)-alkyne species are involved. The energy barrier for the isomerization process in the phosphine-containing niobocene systems is almost 10 kcal mol-1 higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation barrier indicates that the different experimental behavior between 4a, 4c, and 4d has a kinetic rather than a thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.