Titanocene borane σ-complexes

Article abstract of DOI:10.1021/ja9900757

The chemistry of titanocene bisborane complexes Cp₂Ti(HBcat')₂ (1a-g) (HBcat' = catecholborane or a substitued catecholborane) and monoborane complexes Cp₂Ti(HBcat')(L) (2-4) (L = PMe₃, PhSiH₃, or PhCCPh) is reported. These complexes are unusual σ-complexes. The B-H bond in the catecholborane of 1 acts as a two-electron-donor ligand. The 4-tert-butyl version 1a was studied in depth and underwent ligand substitution reactions with PMe₃, CO, PhSiH₃, and PhCCPh. The products of the reaction of 1a with PMe₃ and PhSiH₃ are the novel monoborane σ-complexes Cp₂Ti(HBcat')(PMe₃) (2a; HBcat' = HBO₂C₆H₃-4-t-Bu) and Cp₂Ti(HBcat')(PhSiH₃) (3; HBcat' = HBO₂C₆H₃-4-t-Bu), in which the catecholborane remains a two-electron- donating ligand. Reaction with CO formed Cp₂Ti(CO)₂. Reaction with PhCCPh formed Cp₂Ti(HBcat')-(PhCCPh) (4; HBcat' = HBO₂C₆H₃-4-t-Bu), which was observed in solution and reductively eliminated the vinyl boronate ester (Ph)(Bcat')C=C(Ph)(H). The rates for the reactions of 1a with these substrates showed a first-order dependence on the concentration of 1a and a zero-order dependence on the concentrations of both the departing HBcat' and the incoming ligand. The substitution reaction proceeded at the same rate ((3.8 ± 0.3) x 10^-4) regardless of the identity of the incoming ligand. The entropy of activation was +30 ± 5 eu. These data are consistent with a dissociative substitution mechanism for the reaction of 1a with these substrates. The ΔH(+) value of 25 ± 3 kcal mol^-1 for these reactions provides an upper limit for the strength of the borane-metal interaction. Electronic effects on the reaction rate support a bonding model involving back-donation from titanium to the borane, and the unusual steric effects allow a proposal for the geometric changes that occur upon formation of the transition state.