Copper(I)-based metal-metal-to-ligand charge transfer excited state with halogen-atom transfer photo-reactivity and photocatalysis

Metal-metal-bonded excited states of Cu(I) complexes have rarely been studied, although such excited states of d10 noble metal complexes have been well documented to cleave C–H and C–X bonds. We describe here a panel of air-stable two-coordinate binuclear Cu2(I,I) N-heterocyclic carbene complexes with short intramolecular Cu–Cu (2.75–2.88 Å) and Cu–arene (2.61–2.65 Å) distances. The triplet metal-metal-to-ligand charge transfer excited states of these Cu2(I,I) complexes are highly emissive and long-lived (Φem up to 0.67, τ 2.9–36.1 μs in solution) and can cleave strong R–X (X = Br or Cl) bonds to give mixed-valence [X–Cu1.5Cu1.5–Y]+/2+ (Y = X or solvent) species and carbon-centered radicals via an excited-state halogen-atom transfer mechanism. The spin-delocalized [X–Cu1.5Cu1.5–X]+ species (X = Br or Cl) have been characterized by single-crystal XRD, EPR spectroscopy, and density functional theory (DFT) calculations. Cu3 is an efficient photocatalyst for C–C coupling reactions with aryl/alkyl halides under 390 nm LED irradiation.