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

Three monomeric polyoxometalates [M(C10H8N2)3][α-PMoMoO40Zn2(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-PMoMoO43] (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 MoV 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 MoV centers into the PMo12 units at high temperature, allowing the rationalization of the experimental observations.