Observation of New Intermediates in the Reaction of Dihydrogen with Iridium, Rhodium, and Mixed Metal A-Frame Complexes with Parahydrogen-Induced Polarization

Abstract

The reaction of hydrogen with iridium and rhodium μ-S A-frame complexes has been investigated using parahydrogen-induced polarization (PHIP) NMR spectroscopy. The reaction of Ir2(μ-S)(CO)2(dppm)2 where dppm = bis(diphenylphosphino)methane with hydrogen proceeds by two addition pathways corresponding to oxidative addition inside (endo) and outside (exo) the pocket of the A-frame complex. The exo addition pathway yields a kinetically significant dihydride that has not been previously reported. This species is a key intermediate in the formation of the thermodynamic product of the reaction system. Also present are polarized resonances for the addition of hydrogen to the mixed metal A-frame, IrRh(μ-S)(CO)2(dppm)2, formed as a contaminant from rhodium present in the iridium trichloride starting material. To confirm this observation, IrRh(μ-S)(CO)2(dppm)2 was prepared independently and its reaction with parahydrogen was investigated, leading to results supporting endo addition H2 as the major pathway and exo addition as a minor reaction channel. The complex Rh2(μ-S)(CO)2(dppm)2, which had not previously been found to react with dihydrogen, generates polarized hydride signals under para-enriched hydrogen, showing that an equilibrium is established between the complex and its H2 adduct. In all of these cases, PHIP is used to detect species not readily seen by normal NMR spectroscopy and to investigate the mechanism of these reactions.