Hydrogenation mechanisms in (boratacycle)tantalum analogues of dimethylzirconocene

The hydrogenation of Cp*[C4H4B-N(i-Pr)(2)]TaMe2 (1) (Cp* = C5Me5) in the pr esence of PMe3 affords Cp*[C4H4B-N(i-Pr)(2)]Ta(H)(2)(PMe3) (2) in essential ly quantitative yield. Similarly, the hydrogenation of Cp*[C4H4B-Me]TaMe2 ( 3) in the presence of PMe3 affords Cp*[C4H4B-Me]Ta(H)(2)(PMe3) (4). Hydroge nation of 1 and 3 is accompanied by the reversible: formation of side produ cts. The most important of these complexes, Cp*[C4H4B-N(i-Pr)(2)]Ta(PMe3)(2 ) (5) and Cp*[C4H4B-Me]Ta(PMe3)(2) (6), react slowly with dihydrogen formin g 2 and 4, respectively. In the early stages of the hydrogenation of 1, the C-H activation product Cp*[C4H4B-N(i-Pr)(2)]Ta(H)(CH2PMe2) (7) is also pre sent. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hy drogenation of [C5H5B-Ph][C4H4B-N(i-Pr)(2)]TaMe2 (8) in the presence of PMe 3 affords [C5H5B-Ph][C4H4B-N(i-Pr)(2)]Ta(PMe3)(2) (9) as the exclusive prod uct. The use of a bulkier phosphine, P(i-Pr)(3), gives [C5H5B-Ph][C4H4B-N(i -Pr)(2)]Ta(H)(2)[P(i-Pr)(3)] (10). Changing the phosphine to one of interme diate bulk, PEt3, leads to the formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr)( 2)]Ta(H)(2)(PEt3) (11t). The cis isomer (11c) is observable during early re action times. 11c is a classical dihydride, perturbed by an unsymmetric thr ee-center/two-electron interaction with the boron of the boratabenzene liga nd. Isomerization of 11c to 11t proceeds via phosphine loss followed by kin etically detectable rearrangement of the unsaturated intermediate prior to phosphine recoordination. Treatment of 11c with excess PMe3 results in the formation of 9 via a mixed-phosphine intermediate, [C5H5B-Ph][C4H4B-N(i-Pr) (2)]Ta(PEt3)(PMe3) (12). The addition of [H(OEt2)(2)][B(C6H3(CF3)(2)] to 11 c results in the protonation of the nitrogen atom of the borollide ligand ( H-11c(+)). H-11c(+) is stable at room temperature for over a week. Treatmen t of 10 with excess PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)(2)]Ta(H)(2)(PMe3) (13). Upon thermolysis in the presence of a large excess of PMe3, 13 is co nverted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.