NMR Studies of the Reaction Path of the o-H2/p-H2 Spin Conversion Catalyzed by Vaska’s Complex in the Solid State

Abstract

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-H2 was followed after thawing of the samples by 1H 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-H2 as elucidated from the decrease of the signal of dissolved o-H2. 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 H2 and D2 was exposed to the frozen solutions or to the polycrystalline powder, the formation of HD was observed by 1H NMR. This finding indicates the presence of a chemical spin conversion involving two dihydrogen molecules. Additional 1H NMR experiments of dihydrogen in frozen C6D6 at 110 K indicated the formation of larger pores containing gaseous H2 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.