Subtle Modulation of Cu₄X₄L₂ Phosphine Cluster Cores Leads to Changes in Luminescence

A series of Cu₄X₄(PPh₂py)₂ compounds (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>), PPh₂py = 2-(diphenylphosphino)­pyridine) were prepared and characterized using X-ray crystallography, NMR, UV–vis, and luminescence spectroscopy. The copper chloride and bromide clusters have Cu₄X₄ octahedral cores while the copper iodide clusters contain an unprecedented butterfly shaped core. Crystallization of the copper bromide and iodide clusters from the appropriate solvent produced the solvates <b>2</b>·2CH₂Cl₂, <b>2</b>·2CHCl₃, and <b>3</b>·0.5CH₂Cl₂ where the presence of the lattice solvate influences the overall structural properties. Using TD-DFT calculations, the emission was assigned to a mixed metal- and halide-to-ligand charge transfer, (M + X)­LCT. Subtle differences in the copper core geometry and μ-halide bonding perturb the emissions of these copper­(I) halide clusters