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Sloan ME, Clark TJ, Manners I: Homogeneous Catalytic Dehydrogenation/Dehydrocoupling of Amine-Borane Adducts by the Rh (I) Wilkinson's Complex Analogue RhCl (PHCy (2))(3) (Cy = cyclohexyl). Inorg Chem. 2009 Feb 17.
The Rh (I) complex RhCl (PHCy (2))(3) (1) (Cy = cyclohexyl, C (6) H (11)) has been investigated as a catalyst for the dehydrogenation/dehydrocoupling of dimethylamine-borane adduct Me (2) NH.BH (3) (3) at 20 degrees C to afford the cyclic dimer [Me (2) N-BH (2)](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, C (8) H (12), OTf = OSO (2) CF (3)) with weakly electron-donating ligands at the metal center, which are reduced to catalytically active Rh (0) species, catalytic dehydrogenation of 3 using 1 was found to occur in a homogeneous manner according to nanofiltration experiments, Hg (0) poisoning and kinetic studies. Moreover, the presence of the sterically bulky ligand PHCy (2) in complex 1 has been found to significantly increase the rate of reaction previously reported for Wilkinson's catalyst RhCl (PPh (3))(3). The catalytic activity of 1 toward a range of other amine-borane adducts RR'NH.BH (3) (e.g., RR' = (i) Pr (2), MeBz, MeH) at 20 degrees C was also investigated. The third row transition metal analogue of 1, the Ir (I) complex IrCl (PHCy (2))(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 (C (6) F (5))(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 Ph (2) PH.BH (3) (14) at 60-90 degrees C to afford linear dimer Ph (2) PH-BH (2)-PPh (2)-BH (3) (15). |
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