Titanocene Borane σ-Complexes

The chemistry of titanocene bisborane complexes Cp 2 Ti(HBcat‘) 2 ( 1a − g ) (HBcat‘ = catecholborane or a substitued catecholborane) and monoborane complexes Cp 2 Ti(HBcat‘)(L) ( 2 − 4) (L = PMe 3, PhSiH 3, 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 PMe 3, CO, PhSiH 3, and PhCCPh. The products of the reaction of 1a with PMe 3 and PhSiH 3 are the novel monoborane σ-complexes Cp 2 Ti(HBcat‘)(PMe 3 ) ( 2a; HBcat‘ = HBO 2 C 6 H 3 -4- t -Bu) and Cp 2 Ti(HBcat‘)(PhSiH 3 ) ( 3; HBcat‘ = HBO 2 C 6 H 3 -4- t -Bu), in which the catecholborane remains a two-electron-donating ligand. Reaction with CO formed Cp 2 Ti(CO) 2 . Reaction with PhCCPh formed Cp 2 Ti(HBcat‘)(PhCCPh) ( 4; HBcat‘ = HBO 2 C 6 H 3 -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) × 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 Δ H ⧧ 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.