Cationic Iridium Phosphines Partnered with [<i>closo</i>-CB₁₁H₆Br₆]^-:  (PPh₃)₂Ir(H)₂(<i>c</i><i>loso</i>-CB₁₁H₆Br₆) and [(PPh₃)₂Ir(η²-C₂H₄)₃][<i>c</i><i>loso</i>-CB₁₁H₆Br₆]. Relevance to Counterion Effects in Olefin Hydrogenation

Abstract

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.