Alkene and alkyne insertion reactions with tantalum metallacarborane complexes:: The Et2C2B4H42- borane ligand as a spectator and participant

The tantalum carborane complex (Et2C2B4H4)CpTaMe2 (1) is thermally stable but undergoes clean photochemical insertion with alkynes to give vinyltantalum species, in contrast to the thermal reactivity of isoelectronic group 4 metallocenes which give methylidene intermediates. Certain tantalum vinyltitanium products display NMR resonances indicative of gamma-agostic Ta-H3C interactions sufficiently strong to stabilize two different regioisomers. Decomposition of these species occurs by apparent alkyne deinsertion and ejection of the tantalum fragment to give R2Et2C4B4H4 (R = Me, Et, Ph) carborane clusters. The analogous diphenyl complex (Et2C2B4H4)CpTaPh2 (8) is thermally reactive, eliminating benzene and undergoing trapping reactions of the derived benzyne intermediate with alkynes. The structures of the resulting metallaindene complexes are supported by X-ray crystallography, protonolysis, and spectroscopy. Insertions occur with good regioselectivity, controlled by steric and stereoelectronic factors that differ somewhat from those observed for zirconocene and titanocene analogues. Reaction of complex 8 with excess styrene results in a novel triple-insertion process in which styrene units are added to both ortho positions of an aryl ligand and to the central boron atom of the C2B3 ring. The proposed mechanism (supported by the use of styrene-d(8) and alkylated metallacarborane starting materials) features two benzyne intermediates, derived from activation of both ortho-CH bonds, and one insertion into a putative Ta-B bond. An important hypothesis is that a Ta-C fragment can undergo intramolecular insertion into a carborane B-H bond, a step unknown for cyclopentadienyl C-H bonds and one that is potentially relevant to the use of metallacarborane complexes as catalysts for olefin polymerization and related processes.