Rhodium phosphines partnered with the carborane monoanions [CB11H6Y6](-) (Y = H, Br). Synthesis and evaluation as alkene hydrogenation catalysts

Addition of Ag[closo-CB11H12] to [(PPh3)(2)RhCl](2) affords the new exopolyhedrally coordinated complex [(PPh3)(2)Rh(closo-CB11H12)] (1), which has been characterized by multinuclear NMR spectroscopy and X-ray crystallography. Using the less nucleophilic [closo-CB11H6Br6](-) anion afforded the arene-bridged dimer [(PPh3)(PPh2-eta(6)-C6H5)Rh](2)[closo-CB11H6Br6](2) (2) with poor compositional purity. However, with the new precursor complexes [(PPh3)(2)Rh(nbd)] [Y] (Y = closo-CB11H12 (3), closo-CB11H6Br6 (4); nbd = norbornadiene) as starting materials, treatment with H-2 affords 1 and 2 in good yield and compositional purity. Complex 2 has been characterized by multinuclear NMR spectroscopy and X-ray diffraction. The new complexes 3 and 4 have been evaluated as internal alkene hydrogenation catalysts using the substrates cyclohexene, 1-methylcyclohexene, and 2,3-dimethylbut-2-ene under the attractive conditions of room temperature and pressure. These new catalysts have also been compared with [(PPh3)(2)Rh(nbd)] [BF4] and Crabtree's catalyst, [(py)(PCy3)Ir(cod)] [PF6] (cod = 1,5-cyclooctadiene), A clear counterion effect is observed. For the hydrogenation of cyclohexene the [BF4](-) and [closo-CB11H12](-) salts are broadly similar, but the [closo-CB11H6Br6](-) salt is significantly better, matching Crabtree's catalyst in hydrogenation efficiency. This pattern is mirrored in the hydrogenation of 1-methylcyclohexene and the sterically hindered 2,3-dimethylbut-2-ene, although with the latter substrate Crabtree's catalyst does outperform 4. Nevertheless, these results are excellent for a rhodium complex, which have traditionally been considered as ineffectual catalysts for the hydrogenation of internal alkenes at room temperature and pressure. The deactivation product in the catalytic cycle, [(PPh3)(2)HRh(mu-Cl)(2)(mu-H)RhH(PPh3)(2)] [CB11H12] (5), has been characterized by multinuclear NMR spectroscopy and X-ray crystallography.