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.