Hydrido{(acylphosphine)(diphenylphosphinous acid)}rhodium(III) Complexes. Catalysts for the Homogeneous Hydrolysis of Ammonia- or Amine-Boranes under Air

The reaction of [RhCl(COD)](2) (COD = 1,5-cyclooctadiene) with PPh2(o-C6H4CHO) (Rh/P = 1:1) in the presence of 4-methylpyridine (4-pic) or isoquinoline (iquin) leads to hydridoacyl complexes [RhHCl(PPh2(o-C6H4CO))L-2] (L = 4-pic, 1; iquin, 2). Their reaction with diphenylphosphine oxide affords neutral complexes [RhHCl{(PPh2(o-C6H4CO))(PPh2O)H}L] (L = pyridine (py), 4; 4-pic, 5; iquin, 6) with trans P-ligands belonging to a formally PCP pincer ligand due to the formation of a O---H---O hydrogen bond between the oxygen atom of the diphenylphosphinous acid and the oxygen atom of the coordinated acyl. The reaction of 4 with chelating N-donors (LL) leads to displacement of pyridine and chloride to afford cationic [RhH{(PPh2(o-C6H4CO))(PPh2O)H}(LL)]BPh4 (LL = 2,2'-bipyridine, 7; biacetyldihydrazone, 8; biacetyloximehydrazone, 9; 2-aminomethylpyridine, 10), which undergo cleavage of the O---H---O hydrogen bond in solution. Complex 9, containing a nonsymmetric ligand, exists in solution as an equimolar mixture of two isomers in equilibrium leading to fluxional behavior for which ?G = 52 +/- 1 kJ mol1 has been determined using the Eyring equation. The complexes were fully characterized spectroscopically. Single crystal X-ray diffraction analysis was performed on 8. Complexes 1-10 have been tested as homogeneous catalysts for the hydrolysis of amine- or ammonia-borane (AB) to generate up to 3 equiv of hydrogen in the presence of air. When using 0.5 mol % loading of 5, at 40 degrees C, complete hydrogen release from 0.46 M solutions of AB, TBAB (tert-butylamine-borane), and DMAB (dimethylamine-borane) requires 12.5, 9, or 15 min, respectively. The catalyst maintains its activity in consecutive runs. Kinetic studies indicate a first-order behavior in substrate in every case. The catalytic hydrolysis of AB shows first-order dependence on the catalyst concentration.