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 (Φ_em 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–Cu^1.5Cu^1.5–Y]^+/2+ (Y = X or solvent) species and carbon-centered radicals via an excited-state halogen-atom transfer mechanism. The spin-delocalized [X–Cu^1.5Cu^1.5–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.