Light-Driven Proton-Coupled Two-Electron Ligand Reduction Causes the Rearrangement of the Coordination Sphere in Cu(I) 4 H -Imidazolato Complexes

The direct conversion of solar light into chemical energy, inspired by natural photosynthesis, presents a promising strategy for energy storage and chemical transformation. We investigate this approach using photoactive Cu(I) complexes with the functional 4H-imidazolato ligand for storage of multiple photoredox equivalents. This study demonstrates that these complexes undergo light-driven reduction under a variety of reaction conditions involving different irradiation wavelengths, solvents, and electron/proton donors. We used single-crystal X-ray analysis together with NMR spectroscopy to characterize the isolated photoreduction product. This analysis supports a proton-coupled two-electron-transfer mechanism and reveals a significant structural transformation of the complex. By employing UV–vis spectroscopy alongside DFT calculations, we elucidated the mechanisms underlying the photoreduction process, including protonation of both exocyclic nitrogen atoms at the chelating binding site as a consequence of the two-electron reduction, resulting in the migration of the Cu(I) bisphosphine fragment from the chelating binding site of the imidazolato ligand to form a trigonal-planar Cu(I) 1H-imidazolato complex. This rearrangement drastically changes the shape of the molecule, exposing an additional binding site on the Cu(I) center and making the N,N-binding site available for subsequent reactions.