A hydridotantalum(V)-carborane analogue of Schwartz's reagent: synthesis and reactivity

The dichlorotantalum metallacarborane CpCl2Ta(Et2C2B4H4) (1) on treatment with LiAlH4 in THF gave a hydridotantalum dimer [CpTa(H)(Et2C2B4H4)](2)(mu-Cl)(2) (2), which was found to undergo alkyne insertion with p-tolyl acetylene to generate exclusively trans-CpCl(p-MeC6H4-CH=CH)Ta(Et2C2B4H4) (3); the reaction of the latter complex with anhydrous HCl afforded p-tolylstyrene and 1. In contrast, reaction of 2 with diphenyl acetylene or methyl phenyl acetylene gave hydridotantalum-alkyne complexes, which are apparently the first examples of alkyne pi-coordination to a formal d(0) metal. Treatment of 2 with styrene produced the alkyl-tantalum species CpTaCl(CH2CH2Ph)(Et2C2B4H4) (5), but no reactivity was observed between 2 and the sterically hindered olefins cis-stilbene, trans-stilbene, and cyclohexene. Addition of anhydrous HCl to 5 generates 1 and styrene via an apparent beta-elimination mechanism. (C) 1998 Elsevier Science S.A.