Detection of an Iridium-Dihydrogen Complex: A Proposed Intermediate in Ionic Hydrogenation.

Addition of high pressures of H to five-coordinate [(POCOP)Ir(CO)(H)]OTf [(POCOP) = κ-CH-2,6-(OP(Bu))] complexes results in observation of two new iridium-dihydrogen complexes. If the aryl moiety of the POCOP ligand is substituted with an electron withdrawing protonated dimethylamino group at the para position, hydrogen coordination is enhanced. Five-coordinate Ir-H complexes generated by addition of triflic acid to (POCOP)Ir(CO) species show an Ir-H H NMR chemical shift dependence on the number of equivalents of acid present. It is proposed that excess triflic acid in solution facilitates triflate dissociation from iridium, resulting in unsaturated five-coordinate Ir-H complexes. The five-coordinate iridium-hydride complexes were found to catalyze H/D exchange between H and CDOD. The existence of the dihydrogen complexes, as well as isotope exchange reactions, provide evidence for proposed ionic hydrogenation intermediates for glycerol deoxygenation.