Rhodium Phosphines Partnered with the Carborane Monoanions [CB11H6Y6]- (Y = H, Br). Synthesis and Evaluation as Alkene Hydrogenation Catalysts

Addition of Ag[ closo -CB 11 H 12 ] to [(PPh 3 ) 2 RhCl] 2 affords the new exo polyhedrally coordinated complex [(PPh 3 ) 2 Rh( closo -CB 11 H 12 )] ( 1 ), which has been characterized by multinuclear NMR spectroscopy and X-ray crystallography. Using the less nucleophilic [ closo -CB 11 H 6 Br 6 ] - anion afforded the arene-bridged dimer [(PPh 3 )(PPh 2 -η 6 -C 6 H 5 )Rh] 2 [ closo -CB 11 H 6 Br 6 ] 2 ( 2 ) with poor compositional purity. However, with the new precursor complexes [(PPh 3 ) 2 Rh(nbd)][Y] (Y = closo -CB 11 H 12 ( 3 ), closo -CB 11 H 6 Br 6 ( 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 [(PPh 3 ) 2 Rh(nbd)][BF 4 ] and Crabtree's catalyst, [(py)(PCy 3 )Ir(cod)][PF 6 ] (cod = 1,5-cyclooctadiene). A clear counterion effect is observed. For the hydrogenation of cyclohexene the [BF 4 ] - and [ closo -CB 11 H 12 ] - salts are broadly similar, but the [ closo -CB 11 H 6 Br 6 ] - 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, [(PPh 3 ) 2 HRh(μ-Cl) 2 (μ-H)RhH(PPh 3 ) 2 ][CB 11 H 12 ] ( 5 ), has been characterized by multinuclear NMR spectroscopy and X-ray crystallography.