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

Abstract

Treatment of [(PPh₃)₂Ir(COD)][<i>closo</i>-CB₁₁H₆Br₆] with H₂ in CH₂Cl₂ solution affords crystallographically characterized (PPh₃)₂Ir(H)₂(<i>closo</i>-CB₁₁H₆Br₆), 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\nlower temperatures (−80 °C) a solvent-stabilized complex [(PPh₃)₂Ir(H)₂(CH₂Cl₂)][<i>closo</i>-CB₁₁H₆Br₆] is suggested to also be present. (PPh₃)₂Ir(H)₂(<i>closo</i>-CB₁₁H₆Br₆) reacts with ethene\nto give the tris-ethene complex [(PPh₃)₂Ir(η²-C₂H₄)₃][<i>closo</i>-CB₁₁H₆Br₆]. Subsequent addition\nof hydrogen returns (PPh₃)₂Ir(H)₂(<i>closo</i>-CB₁₁H₆Br₆). This cycle can be repeated a number of\ntimes without apparent decomposition, with the anion acting in a “catch and release” manner,\nstabilizing the metal center when needed. This stabilization is also apparent for the hydrogenation of cyclohexene with (PPh₃)₂Ir(H)₂(<i>closo</i>-CB₁₁H₆Br₆) as a catalyst. The complex\nmay be reused up to five times, without decomposition to di- and trimeric iridium hydride\nspecies. This is in contrast to other reported iridium hydrogenation systems with other weakly\ncoordinating anions that, on consumption of olefin, decompose to inactive complexes. The\nnew complexes reported here represent intermediates in the catalytic cycle of olefin\nhydrogenation by cationic group 9 catalysts.