Luminescent tetranuclear Ag(I)/Cu(I) cubane clusters: Investigating argentophilicity, cuprophilicity, and mixed argento-cuprophilicity

In this article, we have investigated the luminescence properties and metallophilic interactions in the heterobimetallic Ag(I)-Cu(I) acetylide cubane cluster, [Ag₂Cu₂(CCPh)₄(PPh₃)₄]. Single-crystal X-ray diffraction analysis reveals unusually short Ag∙∙∙Ag, Cu∙∙∙Cu, and Ag∙∙∙Cu separations, ranging from 2.5763(9) to 2.9546(8) Å, indicative of strong d¹⁰⋯d¹⁰ argentophilic, cuprophilic, and mixed argento-cuprophilic interactions. Density Functional Theory (DFT) calculations, combined with Electron Localization Function (ELF) analysis, show significant electron localization along these metal–metal contacts, along with electron density delocalization around the silver and copper centers. Quantum Theory of Atoms in Molecules (QTAIM) analysis provides further insight into the electron density at bond critical points (BCP), quantifying the strength of the metallophilic interactions. Additionally, Natural Bond Orbital (NBO) analysis identifies donor–acceptor orbital interactions involved in the Ag∙∙∙Ag, Cu∙∙∙Cu, and Ag∙∙∙Cu contacts, further highlighting the nature of these closed-shell interactions. These combined analyses offer a detailed understanding of the structural, electronic, and photophysical properties of the complex, shedding light on the role of metallophilic interactions in its luminescent behavior.