Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)₃[PMo₁₂O₄₀] was reacted with <i>n</i> equiv of lithium metal (<i>n</i> = 1-24) to generate <b>PMo</b>_<b>12</b><b>/<i>n</i></b> products which were shown to be mixtures of electron-rich <b>PMo</b>_<b>12</b><b>Li</b>_<i>x</i> 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 <i>n</i> ∼ 8 and a significant structural change at <i>n</i> > 12. Successive Mo^VI reductions were monitored by XANES and XPS, and at <i>n</i> = 24, results were consistent with the formation of at least one Mo^IV-Mo^IV bonded {Mo^IV₃} triad together with Mo^V. Upon dissolution, the <b>PMo</b>_<b>12</b><b>Li</b>_{<b><i>x</i></b>} species present in the solid <b>PMo</b>_<b>12</b><b>/<i>n</i></b> products undergo electron exchange and single-peak ³¹P NMR spectra were observed for <i>n</i> = 1-12. For <i>n</i> ≥ 16, changes in solid state and solution ³¹P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with Mo^V-Mo^V and {Mo^IV₃} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}⁺ and 2-electron-reduced <b>PMo</b>_<b>12</b> in (TBA)₄[PMo₁₂O₄₀{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 Mo₂C at <i>n</i> ≥ 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.