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

Three monomeric polyoxometalates [M(CHN)][α-PMoMoOZn(CHN)]·2HO (M-PMoZn, M = Fe, Co, Ru) with {Zn(bpy)} units capped on reduced α-Keggin polyanions and [M(bpy)] counter-ions were synthesized under hydrothermal conditions. The 1D polymer [N(CH)][Ru(CHN)][α-PMoMoO] (Ru-PMo) 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)] 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 P NMR spectroscopy. P diffusion ordered NMR spectroscopy (DOSY) indicates that the diffusion coefficient of the dissolved species of Ru-PMo corresponds to a dimeric structure. Magnetic susceptibility measurements performed on Ru-PMo show the existence of antiferromagnetic interactions between the d electrons of the six Mo 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 centers into the PMo units at high temperature, allowing the rationalization of the experimental observations.