Reactions of Bis(pentafluorophenyl)borane with Cp₂Ta(CH₂)CH₃:  Generation and Trapping of Tantalocene Borataalkene Complexes

Reaction of Cp 2 Ta( CH 2 ) 3 with 2 equiv of HB(C 6 F 5 ) 2 results in production of the dihydride Cp 2 Ta(CH 2 B(C 6 F 5 ) 2 )(μ-H)(H), 1, plus 1 equiv of H 3 CB(C 6 F 5 ) 2 . The pathway to 1 involves stepwise attack of borane first at the methylene group, followed by attack at the methyl group, which undergoes alkyl/hydride exchange with the second equivalent of HB(C 6 F 5 ) 2 . The product of HB(C 6 F 5 ) 2 addition to the methylene ligand, methyl hydride complex Cp 2 Ta(CH 2 B(C 6 F 5 ) 2 )(μ-H)(CH 3 ), 2, can be intercepted by carrying out the reaction in hexane at low temperature, a medium in which it is nearly totally insoluble. This complex eliminates methane at higher temperatures in a first-order decomposition process (Δ H ⧧ = 20.4(5) kcal mol - 1 and Δ S ⧧ = −2.0(2) cal mol - 1 K - 1 ). The product, a borataalkene complex formulated as Cp 2 Ta[CH 2 B(C 6 F 5 ) 2 ], 3, is unstable and cannot be isolated. DFT calculations support its formulation and show that it is present as a singlet/triplet mixture, accounting for the observed paramagnetism of solutions containing 3 . While 3 cannot be isolated or spectroscopically probed, it can be trapped if 2 is allowed to decompose in the presence of t BuNC or CO, giving the products Cp 2 Ta[η 2 -CH 2 B(C 6 F 5 ) 2 ](L) (L = t BuNC, 4; CO, 5 ). Both of these compounds have been structurally characterized, and the structural and spectroscopic data for these compounds support an η 2 bonding description for the borataalkene ligand which is reminiscent of the commonly held Dewar−Chatt−Duncanson model for alkene bonding to transition metals. DFT calculations on 5 and the model complex Cp 2 Ta[η 2 -CH 2 BH 2 ](CO), 6, provide further support for this description. The facile conversion of this ligand from an η 2 to an η 1 bonding mode is proposed to account for some H/D exchange processes observed in both dihydride 1 and methyl hydride complex 2 . Plausible mechanisms for both of these processes are proposed.

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