P6Mo18O73 heteropolyanion and its four-copper complex: theoretical and experimental investigation

Abstract

The non-classical KP₆Mo₁₈O₇₃ heteropolyanion has been studied by the density functional theory (DFT) method, and the calculated geometry compares well with the experimental one. In fully oxidized [KP₆Mo₁₈O₇₃]⁷⁻ state, the d_xy-orbitals centered at eight “belt” Mo sites in the lower part of the “basket” are the major contributors to the LUMO and LUMO+1, while the LUMO+2 orbital is mostly focused on the two polar parts. In contrast, the HOMOs indicates that the coordination of the KP₆Mo₁₈ heteropolyanion to metal ions favorably occurs at the oxygen atoms from four external phosphates and two molybdates of the handle of the “basket”. Compared with Wells–Dawson [P₂Mo₁₈O₆₂]⁶⁻, the HOMO–LUMO gap in fully oxidized [KP₆Mo₁₈O₇₃]⁷⁻ is much smaller, indicating much easier reduction that is consistent with the cyclic voltammogram. Both frontier orbitals and Mulliken analysis indicate that two of three blue electrons in [KP₆Mo₁₈O₇₃]¹⁰⁻ (KP₆Mo₁₈-3e) have spin alpha while third blue electron has spin beta, in agreement with magnetic data. The four-copper complex of the non-classical KP₆Mo₁₈-3e heteropolyanion has been synthesized and structurally characterized; its structure supports the theoretical results such as reactivity and basicity of external oxygen sites.