Titanocene borane sigma-complexes

The chemistry of titanocene bisborane complexes Cp2Ti(HBcat')(2) (1a-g) (HB cat' = catecholborane or a substitued catecholborane) and monoborane comple xes Cp2Ti(HBcat')(L) (2-4) (L = PMe3, PhSiH3, or PhCCPh) is reported. These complexes are unusual sigma-complexes. The B-H bond in the catecholborane of 1 acts as a two-electron-donor ligand. The 4-tert-butyl version la was s tudied 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 sigma-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. Reactio n with CO formed Cp2Ti(CO)(2). Reaction with PhCCPh formed Cp2Ti(HBcat')(Ph CCPh) (4; HBcat' = HBO2C6H3-4-t-Bu), which was observed in solution and red uctively eliminated the vinyl boronate ester (Ph)(Bcat')C=C(Ph)(H). The rat es for the reactions of 1a with these substrates showed a first-order depen dence on the concentration of 1a and a zero-order dependence on the concent rations of both the departing HBcat' and the incoming ligand. The substitut ion 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 mechan ism for the reaction of la with these substrates. The Delta H double dagger 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 th e reaction rate support a bonding model involving back-donation from titani um to the borane, and the unusual steric effects allow a proposal for the g eometric changes that occur upon formation of the transition state.