Light-Driven Proton-Coupled Two-Electron Ligand Reduction Causes the Rearrangement of the Coordination Sphere in...

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.