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 productio n 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 al kyl/hydride exchange with the second equivalent of HB(C6F5)(2). The product of HB(C6F5)(2) addition to the methylene ligand, methyl hydride complex Cp 2Ta(CH2B(C6F5)(2))(mu -H)(CH3), 2, can be intercepted by carrying out the r eaction in hexane at low temperature, a medium in which it is nearly totall y insoluble. This complex eliminates methane at higher temperatures in a fi rst-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 bor ataalkene complex formulated as Cp2Ta[CH2B(C6F5)(2)], 3, is unstable and ca nnot be isolated. DFT calculations support its formulation and show that it is present as a singlet/triplet mixture, accounting for the observed param agnetism of solutions containing 3. While 3 cannot be isolated or spectrosc opically probed, it can be trapped if 2 is allowed to decompose in the pres ence 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 structur ally characterized, and the structural and spectroscopic data for these com pounds support an eta (2) bonding description for the borataalkene ligand w hich is reminiscent of the commonly held Dewar-Chatt-Duncanson model for al kene bonding to transition metals. DFT calculations on 5 and the model comp lex Cp2Ta[eta (2)-CH2BH2](CO), 6, provide further support for this descript ion. The facile conversion of this ligand from an eta (2) to an eta (1) bon ding mode is proposed to account for some H/D exchange processes observed i n both dihydride I and methyl hydride complex 2. Plausible mechanisms for b oth of these processes are proposed.