Homogeneous Catalytic Dehydrogenation/Dehydrocoupling of Amine-Borane Adducts by the Rh(I) Wilkinson's Complex Analogue RhCl(PHCy2)3 (Cy = cyclohexyl)

The Rh(I) complex RhCl(PHCy2)(3) (1) (Cy = cyclohexyl, C6H11) has been investigated as a catalyst for the dehydrogenation/dehydrocoupling of dimethylamine-borane adduct Me2NH center dot BH3 (3) at 20 degrees C to afford the cyclic dimer [Me2N-BH2](2) (4). Unlike previously studied neutral and cationic Rh(I) precatalysts such as [{Rh(mu-Cl)(1,5-cod)}(2)] and [Rh(1,5-cod)(2)]OTf (1,5-cod = 1,5-cyclooctadiene, C8H12, OTf = OSO2CF3) with weakly electron-donating ligands at the metal center, which are reduced to catalytically active Rh(O) species, catalytic dehydrogenation of 3 using 1 was found to occur in a homogeneous manner according to nanofiltration experiments, Hg(O) poisoning and kinetic studies. Moreover, the presence of the sterically bulky ligand PHCy2 in complex 1 has been found to significantly increase the rate of reaction previously reported for Wilkinson's catalyst RhCl(PPh3)(3). The catalytic activity of 1 toward a range of other amine-borane adducts RR'NH center dot BH3 (e.g., RR' = Pr-i(2), MeBz, MeH) at 20 degrees C was also investigated. The third row transition metal analogue of 1, the Ir(I) complex IrCl(PHCy2)(3) (2), was also explored as a catalyst for the dehydrocoupling of 3 and was found to exhibit much reduced catalytic activity compared to 1 but proved significantly more active for sterically encumbered substrates. Addition of the strong Lewis acid B(C6F5)(3) as a co-catalyst to both 1 and 2 has been found to significantly increase the rate of the dehydrocoupling reactions in all cases. The Rh(I) complex 1 (but not the Ir(I) analogue 2) was also found to be active for the catalytic dehydrocoupling of the phosphine-borane adduct Ph2PH center dot BH3 (14) at 60-90 degrees C to afford linear dimer Ph2PH-BH2-PPh2-BH3 (15).