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

The cationic Au^III pincer complex [(^<i>t</i>BuPCP)Au^III-OH]OTf (<b>AuOH</b>, ^<i>t</i>BuPCP = 2,6-(CH₂P^<i>t</i>Bu₂)₂C₆H₃) was prepared and characterized. In contrast to hydrogenolysis reactions of other late metal hydroxides, the hydrogenolysis of <b>AuOH</b> to form the corresponding Au^III-H and water is shown to require an acid catalyst. The kinetic data, collected in acetone-<i>d</i>₆, are consistent with a reaction mechanism involving initial protonation to form the Au^III aquo complex, [(^<i>t</i>BuPCP)Au^III-OH₂](OTf)₂ (<b>AuOH</b>_<b>2</b>), which was isolated and characterized. Following substitution of the water by a coordinating solvent, the dicationic Au^III center undergoes electrophilic activation of H₂ to generate the [(^<i>t</i>BuPCP)Au^III-H]OTf product (<b>AuH</b>). The mechanism of hydrogenolysis is supported by experimental and computational (density functional theory) results. A Au^III-dihydrogen complex (<b>Au(H</b>_<b>2</b><b>)</b>) was located along the potential energy surface for the reaction, and computations indicate that deprotonation of <b>Au(H</b>_<b>2</b><b>)</b> by acetone solvent is barrierless. The bonding and properties of the proposed Au^III-H₂ complex are consistent with predictions made for high-valent, electrophilic metal-dihydrogen complexes.