Addition of Ag ] to [(PPh 3 ) 2 RhCl] 2 affords the new exopolyhedrally 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.

Treatment of [(PPh 3 ) 2 Ir(COD)][closo-CB 11 H 6 Br 6 ] with H 2 in CH 2 Cl 2 solution affords crystallographically characterized (PPh 3 ) 2 Ir(H) 2 (closo-CB 11 H 6 Br 6 ), in which the weakly coordinating carborane anion is bound to the metal center. In solution the anion rapidly dissociates/recombines with the metal center, and this process can be frozen out at -50°C. At lower temperatures (-80°C) a solvent-stabilized complex [(PPh 3 ) 2 Ir(H) 2 (CH 2 Cl 2 )][closo-CB 11 H 6 Br 6 ] is suggested to also be present. (PPh 3 ) 2 Ir(H) 2 (closo-CB 11 H 6 Br 6 ) reacts with ethene to give the tris-ethene complex [(PPh 3 ) 2 Ir(η 2 -C 2 H 4 ) 3 ][closo-CB 11 H 6 Br 6 ]. Subsequent addition of hydrogen returns (PPh 3 ) 2 Ir(H) 2 (closo-CB 11 H 6 Br 6 ). This cycle can be repeated a number of times without apparent decomposition, with the anion acting in a "catch and release" manner, stabilizing the metal center when needed. This stabilization is also apparent for the hydrogenation of cyclohexene with (PPh 3 ) 2 Ir(H) 2 (closo-CB 11 H 6 Br 6 ) as a catalyst. The complex may be reused up to five times, without decomposition to di-and trimeric iridium hydride species. This is in contrast to other reported iridium hydrogenation systems with other weakly coordinating anions that, on consumption of olefin, decompose to inactive complexes. The new complexes reported here represent intermediates in the catalytic cycle of olefin hydrogenation by cationic group 9 catalysts.

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