NMR Studies of the Reaction Path of the o-H-2/p-H-2 Spin Conversion Catalyzed by Vaska's Complex in the Solid State

We have studied the o/p spin conversion of dihydrogen in contact with frozen solutions of Vaska's complex Ir(CO)Cl(PPh3)(2) (1) in C6D6 and with polycrystalline 1 at 77 K. The main purpose of this study was to elucidate the mechanism of this type of reactions found accidentally previously (Eisenschmid et al JACS 109:8089-8091, 1987 and Eisenberg ACS 24:110-116, 1991). The formation of p-H-2 was followed after thawing of the samples by H-1 nuclear magnetic resonance (NMR) spectroscopy at 298 K, where the oxidative addition of dihydrogen to 1 occurs leading to Vaska's dihydride Ir(CO)ClH2(PPh3)(2) (2) which is known to exhibit para-hydrogen-induced polarization (PHIP). The PHIP signal was shown to be proportional to the concentration of p-H-2 as elucidated from the decrease of the signal of dissolved o-H-2. The reaction was found to be faster for the frozen solution as compared to the polycrystalline powder. Optical microscopy showed that small particles of 1 are separated from the solution during the freezing process, exhibiting a larger surface area as compared to the polycrystalline powder. When a mixture of H-2 and D-2 was exposed to the frozen solutions or to the polycrystalline powder, the formation of HD was observed by H-1 NMR. This finding indicates the presence of a chemical spin conversion involving two dihydrogen molecules. Additional H-1 NMR experiments of dihydrogen in frozen C6D6 at 110 K indicated the formation of larger pores containing gaseous H-2 as well as dihydrogen sites in interstitial sites between benzene molecules. Moreover, in the presence of 1, a signal at -4.5 ppm was observed which was attributed to a dihydrogen in close contact with Ir