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

Treatment of Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (Cl)(L) ( 1 ) with Mg(C⋮CR) 2 in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (C⋮CR)(L) ( 2: L = CO, R = Ph ( 2a ); L = CO, R = SiMe 3 ( 2b ); L = CO, R = t Bu ( 2c ); L = PMe 2 Ph, 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(η 5 -C 5 H 4 SiMe 3 ) 2 (C⋮CR)(L)] • + [BPh 4 ] - ( 3: L = CO, R = Ph ( 3a ); L = CO, R = t Bu ( 3c ); L = PMe 2 Ph, R = Ph ( 3d )). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d 2 niobocene species [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 ( C CHR)(L)][BPh 4 ] ( 4: L = CO, R = Ph ( 4a ); L = CO, R = t Bu ( 4c ); L = PMe 2 Ph, R = Ph ( 4d )) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d 2 niobocene complex [(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)Nb C C(Ph)(Ph)C C Nb(CO)(η 5 -C 5 H 4 SiMe 3 ) 2 ][BPh 4 ] 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 HBF 4 . Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η 2 -alkyne derivatives [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 ( C, C )-HC⋮CR)(CO)] + ( 5: R = Ph ( 5a ); R = t Bu ( 5c )). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp 2 ( C CHCH 3 )(L)] + /[NbCp 2 (HC⋮CCH 3 )(L)] + (Cp = η 5 -C 5 H 5; L = CO, PH 3; exo, endo ) model systems in order to explain the η 1 -vinylidene−η 2 -alkyne rearrangement observed. Calculations have shown that in both carbonyl−niobocene and phosphine−niobocene systems the η 1 -vinylidene and the η 2 -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 η 2 ( 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.