O₂ activation has been observed for homogeneous gold-based materials and the reaction mechanisms have been evaluated, including O₂ binding and reduction processes. Gold(I)–porphyrin complexes exhibit halide-dissociation-induced O₂binding without the distinct formation of conventional gold–hydride species. Acid-promoted O₂ reduction reactions have been observed in gold nanoclusters, which are systematically analyzed based on kinetics, to reveal the controlling factors.

Gold (Au) exhibits distinct reactivities with O₂ from the nanometer to atomic scales, whereas bulk Au is chemically inert. Since the discovery of catalytic reactivities in Au-based materials, including nanoparticles, molecular complexes, and nanoclusters (AuNCs), mechanistic insights into the interaction and reductive activation of O₂ by Au have been pursued by researchers. However, atomic-level understanding of the reaction mechanism remains elusive compared with that for other widely explored transition metal–O₂ systems. This article briefly discusses recent progress in clarifying the reaction mechanisms of Au–O₂ chemistry for homogeneous molecular Au complexes and atomically precise AuNCs. Research focuses on (1) O₂ activation by Au complexes without the distinct formation of conventional metal-hydride species, and (2) the application of kinetic analyses to the reaction of thiolate-protected AuNCs with O₂ to quantify parameters such as O₂-binding constants on the Au surface. These findings help elucidate both the Au–O₂ interaction and reductive activation of O₂, which are essential in catalytic systems using O₂ as a terminal oxidant that can contribute to the developments of ecofriendly oxidation Au-based catalysts with high performance.