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

Addition of high pressures of H2 to five-coordinate [(tBu)4(POCOP)Ir(CO)(H)]OTf [(tBu)4(POCOP) = κ3-C6H3-2,6-(OP(tBu)2)2] 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 (tBu)4(POCOP)Ir(CO) species show an Ir–H 1H 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 H2 and CD3OD. The existence of the dihydrogen complexes, as well as isotope exchange reactions, provide evidence for proposed ionic hydrogenation intermediates for glycerol deoxygenation.