Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal.

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)[PMoO] was reacted with equiv of lithium metal ( = 1-24) to generate products which were shown to be mixtures of electron-rich species. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at ∼ 8 and a significant structural change at > 12. Successive Mo reductions were monitored by XANES and XPS, and at = 24, results were consistent with the formation of at least one Mo-Mo bonded {Mo} triad together with Mo. Upon dissolution, the species present in the solid products undergo electron exchange and single-peak P NMR spectra were observed for = 1-12. For ≥ 16, changes in solid state and solution P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with Mo-Mo and {Mo} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)} and 2-electron-reduced in (TBA)[PMoO{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for MoC at ≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.