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

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