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

A series of Cu4X4(PPh2py)2 compounds (X = Cl (1), Br (2), I (3), 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 2·2CH2Cl2, 2·2CHCl3, and 3·0.5CH2Cl2 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.