Synthesis, structural and spectroscopic characterization of mono- and binuclear copper(I) complexes with substituted diimine and phosphine ligands

The reaction of [Cu(CH3CN)4]BF4, 6-(4-methoxyl)phenyl-2,2′-bipyridine (designated as MeO-CNN), and/or tricyclohexylphosphine (PCy3) and diimine ligands derived from 4,4′-bipyridine gave four mono- and binuclear copper(I) complexes, [Cu(MeO-CNN)2]BF4 (1), [Cu2(MeO-CNN)2(PCy3)2(4,4′-bipy)](BF4)2 · 1.5CH2Cl2 (2) (bipy = bipyridine), [Cu2(MeO-CNN)2(PCy3)2(bpete)](BF4)2 · 4CH2Cl2 (3) (bpete = trans-1,2-bis(4-pyridyl)ethene) and [Cu2(MeO-CNN)2(PCy3)2(4,4′-azpy)] (BF4)2 · 1.5CH2Cl2 (4) (azpy = azobispyridine). Crystallographic studies of complexes 1–4 show that each copper(I) center adopts a pseudo-tetrahedral coordination geometry. Complexes 2–4 consists of –Cu(MeO-CNN)(PCy3) units which are linked through 4,4′-bipy, bpete and 4,4′-azpy, respectively. The UV–Vis spectra of these four complexes all exhibit intense high-energy absorptions at λmax < 340 nm and broad visible bands in a range of 430–550 nm, ascribed to intraligand (IL π → π∗) transitions and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The density functional theory calculation was used to interpret the absorption spectrum of 1, which further supports the assignment of MLCT character. The binuclear complexes 2 and 3 both display red solid-state emissions centred at 620 and 660 nm from metal-to-ligand charge-transfer excited state, respectively. Interestingly, the electron paramagnetic resonance (EPR) spectral measurements confirm copper(I) complexes oxidized to corresponding copper(II)–halide product upon excitation at 355 nm in dichloromethane solution.