Observation of new intermediates in the reaction of dihydrogen with iridium, rhodium, and mixed metal A-frame complexes with parahydrogen-induced polarization

The reaction of hydrogen with iridium and rhodium mu-S A-frame complexes ha s been investigated using parahydrogen-induced polarization (PHIP) NMR spec troscopy. The reaction of Ir-2(mu-S)(CO)(2)(dppm)(2) where dppm = bis(diphe nylphosphino)methane with hydrogen proceeds by two addition pathways corres ponding to oxidative addition inside (endo) and outside (exo) the pocket of the A-frame complex. The exo addition pathway yields a kinetically signifi cant 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(mu-S)(CO)(2)(dppm)(2), formed as a contam inant from rhodium present in the iridium trichloride starting material, To confirm this observation, IrRh(mu-S)(CO)(2)(dppm)(2) was prepared independ ently and its reaction with parahydrogen was investigated, leading to resul ts supporting endo addition Hz as the major pathway and exo addition as a m inor reaction channel. The complex Rh-2(mu-S)(CO)(2)(dppm)(2), which had no t previously been found to react with dihydrogen, generates polarized hydri de signals under para-enriched hydrogen, showing that an equilibrium is est ablished between the complex and its Ha 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.