Acid Catalyst Is Required for Hydrogenolysis of a Late Metal Hydroxo Complex

The cationic AuIII pincer complex [(tBuPCP)­AuIII–OH]­OTf (AuOH, tBuPCP = 2,6-(CH2PtBu2)2C6H3) was prepared and characterized. In contrast to hydrogenolysis reactions of other late metal hydroxides, the hydrogenolysis of AuOH to form the corresponding AuIII–H and water is shown to require an acid catalyst. The kinetic data, collected in acetone-d6, are consistent with a reaction mechanism involving initial protonation to form the AuIII aquo complex, [(tBuPCP)­AuIII–OH2]­(OTf)2 (AuOH2), which was isolated and characterized. Following substitution of the water by a coordinating solvent, the dicationic AuIII center undergoes electrophilic activation of H2 to generate the [(tBuPCP)­AuIII–H]­OTf product (AuH). The mechanism of hydrogenolysis is supported by experimental and computational (density functional theory) results. A AuIII–dihydrogen complex (Au­(H2)) was located along the potential energy surface for the reaction, and computations indicate that deprotonation of Au­(H2) by acetone solvent is barrierless. The bonding and properties of the proposed AuIII–H2 complex are consistent with predictions made for high-valent, electrophilic metal–dihydrogen complexes.