Facile synthesis of alkynyl- and vinylidene-niobocene complexes. Unexpected eta(1)-vinylidene-eta(2)-alkyne isomerization

Treatment of Nb(eta 5-C5H4SiMe3)(2)(Cl)(L) (1) with Mg(C=CR)(2) in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(e ta(5)-C5H4SiMe3)(2)(C=CR)(L) (2: L = CO, R = Ph (2a); L = CO, R = SiMe3 (2b ); L = CO, R = Bu-t (2c); L = PMe2Ph, R = Ph (2d); L = P(OEt)(3), R = Ph (2 e)). The alkynyl-containing niobocene species 2 can be chemically or electr ochemically oxidized to give the corresponding cation-radical alkynyl compl exes [Nb(eta(5)-C5H4SiMe3)(2)(C=CR)(L)](.+) [BPh4](-) (3: L = CO, R = Ph (3 a); L = CO, R = Bu-t (3c); L = PMe2Ph, R = Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d(2) niobocene species [Nb(eta(5)-C5H4SiMe3)(2)(=C=CHR)(L)][BPh4] (4: L = CO, R = Ph (4a); L = CO, R = Bu-t (4c); L = PMe2Ph, R = Ph (4d)) with a hyd rogen atom by abstraction from the solvent or, for 3a, the binuclear diviny lidene d(2) niobocene complex [(eta(5)-C5H4SiMe3)(2)(Co)Nb=C=C(Ph)(Ph)C=C=N b(CO)(eta(5)-C5H4SiMe3)(2)][BPh4](2) (4a') from a competitive ligand-ligand coupling process. Complexes 4 were also prepared by an alternative procedu re in which the corresponding complexes 2 were reacted with HBF4. Finally, in solution the GO-containing vinylidene mononuclear complexes 4a and 4c un dergo an unexpected isomerization process to give the eta(2)-alkyne derivat ives [Nb(eta(5)-C5H4SiMe3)(2)(eta(2)(C,C)-HC=CR)](+) (5: R = Ph (5a); R = B u-t (5c)). The structure of 5a was determined by single-crystal diffractome try. DFT calculations were carried out on [NbCp2(=C=CHCH3)(L)](+)/[NbCp2(HC =CCH3)(L)](+) (Cp = eta(5)-C5H5; L = CO, PH3; exo, endo) model systems in o rder to explain the eta(1)-vinylidene-eta(2)-alkyne rearrangement observed. Calculations have shown that in both carbonyl-niobocene and phosphine-niob ocene systems the eta(1)-vinylidene and the eta(2)-alkyne complexes are iso energetic, in marked contrast with the systems previously considered in the oretical studies. The reaction takes place through an intraligand 1,2-hydro gen shift mechanism where eta(2)(C,H)-alkyne species are involved. The ener gy barrier for the isomerization process in the phosphine-containing nioboc ene systems is almost 10 kcal mol(-1) higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation b arrier indicates that the different experimental behavior between 4a, 4c, a nd 4d has a kinetic rather than a thermodynamic origin. Finally, the interc onversion between exo and endo isomers has been studied.