Hydrogenation Mechanisms in (Boratacycle)tantalum Analogues of Dimethylzirconocene

Abstract

The hydrogenation of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1) (Cp* = C5Me5) in the presence of PMe3 affords Cp*[C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (2) in essentially 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). Hydrogenation of 1 and 3 is accompanied by the reversible formation of side products. 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 forming 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 present. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (8) in the presence of PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PMe3)2 (9) as the exclusive product. 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 intermediate 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 reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine loss followed by kinetically 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 11c 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. Treatment 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 converted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.