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 sta ble but undergoes clean photochemical insertion with alkynes to give v inyltantalum species, in contrast to the thermal reactivity of isoelec tronic group 4 metallocenes which give methylidene intermediates. Cert ain 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 a pparent alkyne deinsertion and ejection of the tantalum fragment to gi ve R2Et2C4B4H4 (R = Me, Et, Ph) carborane clusters. The analogous diph enyl complex (Et2C2B4H4)CpTaPh2 (8) is thermally reactive, eliminating benzene and undergoing trapping reactions of the derived benzyne inte rmediate with alkynes. The structures of the resulting metallaindene c omplexes are supported by X-ray crystallography, protonolysis, and spe ctroscopy. Insertions occur with good regioselectivity, controlled by steric and stereoelectronic factors that differ somewhat from those ob served for zirconocene and titanocene analogues. Reaction of complex 8 with excess styrene results in a novel triple-insertion process in wh ich 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 a ctivation 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 i ntramolecular 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 polymerizat ion and related processes.