Gold(III) Semiquinone Complexes: Synthesis, Structure, and Application in Photocatalysis.

Recently, hemilabile ligands have been found to enable two-electron Au(I)/Au(III) redox cycling. Yet, single-electron processes and the use of redox-active ligands in gold chemistry remain largely underexplored. Here, we report the synthesis, isolation, full characterization and first photocatalytic application of Au(III) semiquinone complexes. The latter are easily obtained via one-electron oxidation of Au(III) catecholate precursors. Experimental and DFT studies establish that oxidation occurs at the O^O ligand while the gold center retains its +3 oxidation state. Owing to their square-planar geometry and electron-rich character, Au(III) catecholate complexes readily engage into electron donor-acceptor (EDA) interactions with organic substrates. Upon visible-light irradiation, these EDA adducts undergo photoinduced single-electron transfer, enabling the generation of aryl radicals from aryldiazonium salts. This reactivity translates into efficient photocatalytic C-H arylation of heterocycles under mild conditions, with broad substrate scope and high functional group tolerance. Variation of the redox-active and ancillary ligands provides a simple means to tune the system, enabling catalytic optimization as substantiated with challenging electron-rich aryldiazonium partners. Mechanistically distinct from established gold photoredox systems, this work delineates a new strategy for standalone Au(III)-mediated photoredox catalysis and highlights the broader potential of redox-active ligands in expanding the reactivity landscape of gold complexes.