Tetranuclear Au2Cu2 Clusters with Butterfly- and Planar-Shaped Metal Cores: Strong Rigidochromism Induced by Jahn–Teller Distortion in Two-Coordinated Gold(I) Centers

Two tetranuclear Au2Cu2 cluster complexes [Au2Cu2(μ-(PPh2)2py)2(μ-OH)]­(PF6)3, 2, and [Au2Cu2Cl2(μ-(PPh2)2 py)2]­(OTf)2, 4, have been prepared by the reactions of precursor complexes [Au2(μ-(PPh2)2py)2]­(OTf)2, 1, and [Cu2(μ-(PPh2)2py)2(μ-SMe2)­(OTf)2], 3, with [Cu­(NCCH3)4]­PF6 and AuCl­(SMe2), respectively. The crystal structures of complexes 2 and 4 were determined by X-ray crystallography, indicating a butterfly-shaped Au2Cu2 metal core for 2 and a planar-shaped Au2Cu2 metal core for 4. In complex 2, the Cu atoms occupy the edge-sharing bond, while in complex 4, alternating Au and Cu atoms occupy the tetragon vertices. The optical properties of the complexes were investigated by experimental and computational methods. Although complex 2 displayed a luminescence vapochromic behavior in the presence several volatile organic compounds, complex 4 indicated only an distinguishable change in its emission color when it was exposed to vapor of hydrogen-bond donor solvents. The calculations showed that 2 undergoes an unsymmetrical distortion in its two-coordinated gold­(I) centers upon excitation to the first triplet excited state. This distortion induces a large Stokes shift and a strong rigidochromism behavior that is unprecedented for two-coordinated gold­(I) complexes.