Bicapped Keggin polyoxomolybdates: discrete species and experimental and theoretical investigations on the electronic delocalization in a chain compound

Abstract

Three monomeric polyoxometalates [M(C₁₀H₈N₂)₃][α-PMo^VI₉Mo^V₃O₄₀Zn₂(C₁₀H₈N₂)₂]·2H₂O (M-PMo₁₂Zn₂, M = Fe, Co, Ru) with {Zn(bpy)₂}²⁺ units capped on reduced α-Keggin polyanions and [M(bpy)₃]²⁺ counter-ions were synthesized under hydrothermal conditions. The 1D polymer [N(C₄H₉)₄][Ru(C₁₀H₈N₂)₃][α-PMo^VI₈Mo^V₆O₄₃] (Ru-PMo₁₄) was prepared by a similar strategy, in the absence of 2,2′-bpy ligands. In this chain capped reduced Keggin anions are linked via Mo–O–Mo bridges and are surrounded by both tetrabutylammonium cations and [Ru(bpy)₃]²⁺ counter-ions. The compounds were characterized in the solid state by single crystal and powder X-ray diffraction and IR spectroscopy and in solution by ³¹P NMR spectroscopy. ³¹P diffusion ordered NMR spectroscopy (DOSY) indicates that the diffusion coefficient of the dissolved species of Ru-PMo₁₄ corresponds to a dimeric structure. Magnetic susceptibility measurements performed on Ru-PMo₁₄ show the existence of antiferromagnetic interactions between the d¹ electrons of the six Mo^V centers, with a singlet spin ground state. However, attempts to fit the data in the 2–300 K temperature range with Heisenberg Hamiltonians adapted for 0 or 1D systems suggest that these electrons are delocalized. Density Functional Theory (DFT) and Wave Function Theory (WFT) calculations indicate a migration of the electrons of the capping Mo^V centers into the PMo₁₂ units at high temperature, allowing the rationalization of the experimental observations.