Mechanistic Insights into Molecular Oxygen Reactivity with Late Transition Metal–Hydride Bonds

A gold complex, [(^tBuPCP)Au–H]⁺ (^tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene), has been recently reported to insert O₂ into the Au(III)–H bond, leading to a stable Au(III)–OOH complex with an observed kinetic behavior sharing similarities with those of previously reported Pd(II)–H (nonradical) and Pt(IV)–H (autoaccelerated radical chain) reactions with O₂. In this work, we computationally investigate, by inclusion of spin–orbit coupling (SOC) effects, along the adiabatic PES, this elusive reaction mechanism in connection with the Au(III)–H bond nature and in comparison with recent case studies involving isostructural Pd(II)–H ([(^tBuPCP)Pd–H]) or different ligand supported Au(III)–H ([(CNC)Au–H], CNC = 2,6-bis(alkylimidazol-2-ylidene)-pyridine) bonds. The M–H (M = Au, Pd) bonds in these complexes are shown to be mainly of electron-sharing nature, featuring, however, a decreasing degree of M(δ+)–H(δ−) polarization in the order [(^tBuPCP)Pd–H] > [(^tBuPCP)Au–H]⁺ > [(CNC)Au–H], which we propose to be related to their reactivity with dioxygen, with the M–H bond displaying no reactivity ([(CNC)Au–H]), a radical chain ([(^tBuPCP)Au–H]+), and a nonradical ([(^tBuPCP)Pd–H]) reactivity. The decisive factors in dictating the M–H bond polarity and, consequently, the preferred pathway lie in the nature of both the ligand and the metal, demonstrating how the fine-tuning of the electronic structure of these complexes causes mechanistic pathways to diverge.