Cyclohexane-Based Phosphinite Iridium Pincer Complexes: Synthesis, Characterization, Carbene Formation, and Catalytic Activity in Dehydrogenation Reactions

Metalation of two cyclohexane-based phosphinite pincer ligands, <i>cis</i>-POCyPO (<b>4</b>) and <i>trans</i>-POCyPO (<b>5</b>) (POCyPO = {1,3-bis-[(di-<i>tert</i>-butylphos­phinito]­cyclohexane}⁻), is reported. In line with previously published results (Dalton Trans. 2009, 8626, DOI: 10.1039/B910798C), ligand <b>4</b> undergoes aromatization to give benzene-based complex (POCOP)­IrHCl (<b>3</b>) at high temperatures in the presence of [Ir­(COD)­Cl]₂. However, here we present the isolation of carbene complex (POCyOP)IrCl (<b>6</b>) which is an intermediate in the aromatization process; upon reaction with H₂, <b>6</b> can be readily transformed to the corresponding hydrido-chloride <b>8</b>. Metalation of <i>trans</i>-POCyOP ligand <b>5</b> gives hydrido-chloride <b>13</b> which only upon further heating can be converted to the corresponding carbene <b>14</b>. A mechanistic study of hydrogenation of carbene <b>6</b> is reported, as well as interesting ambient temperature CO-induced C–H activation in β-position of <b>6</b>, a process that under other circumstances takes place around 200 °C. The cis complex (POCyOP)­IrHCl (<b>8</b>), upon activation with base, revealed moderate activity in transfer dehydrogenation of cyclooctane (144 turnover numbers (TON)), while the performance of trans analog <b>13</b> was much better (up to 1684 TON). Carbene complex <b>6</b> and <i>in situ</i> generated <b>14</b> demonstrated promising activity in acceptorless dehydrogenation of alcohols, presumably operating via a novel metal–ligand cooperation type mechanism. Some of the alcohol dehydrogenations generated large amounts of polystyrene