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)2RhCl]2 affords the new exopolyhedrally coordinated complex [(PPh3)2Rh(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-η6-C6H5)Rh]2[closo-CB11H6Br6]2 (2) with poor compositional purity. However, with the new precursor complexes [(PPh3)2Rh(nbd)][Y] (Y = closo-CB11H12 (3), closo-CB11H6Br6 (4); nbd = norbornadiene) as starting materials, treatment with H2 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)2Rh(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)2HRh(μ-Cl)2(μ-H)RhH(PPh3)2][CB11H12] (5), has been characterized by multinuclear NMR spectroscopy and X-ray crystallography.