Reactions of bis(pentafluorophenyl)borane with Cp2Ta(=CH2)CH3:: Generation and trapping of tantalocene borataalkene complexes

Reaction of Cp2Ta(=CH2)(3) with 2 equiv of HB(C6F5)(2) results in production of the dihydride Cp2Ta(CH2B(C6F5)(2))(mu -H)(H), 1, plus 1 equiv of H3CB(C6F5)(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(C6F5)(2). The product of HB(C6F5)(2) addition to the methylene ligand, methyl hydride complex Cp2Ta(CH2B(C6F5)(2))(mu -H)(CH3), 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 (DeltaH(double dagger) = 20.4(5) kcal mol(-1) and DeltaS(double dagger) = -2.0(2) cal mol(-1) K-1). The product, a borataalkene complex formulated as Cp2Ta[CH2B(C6F5)(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 (BuNC)-Bu-t or CO, giving the products Cp2Ta[eta (2)-CH2B(C6F5)(2)](L) (L = (BuNC)-Bu-t, 4; CO, 5). Both of these compounds have been structurally characterized, and the structural and spectroscopic data for these compounds support an eta (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 Cp2Ta[eta (2)-CH2BH2](CO), 6, provide further support for this description. The facile conversion of this ligand from an eta (2) to an eta (1) bonding mode is proposed to account for some H/D exchange processes observed in both dihydride I and methyl hydride complex 2. Plausible mechanisms for both of these processes are proposed.