Titanocene Borane σ-Complexes

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')CC(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) × 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.