Abstract
In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)(3)[PMo(12)O(40)] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo(12)/n products which were shown to be mixtures of electron-rich PMo(12)Li(x) 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 n ∼ 8 and a significant structural change at n > 12. Successive Mo(VI) reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one Mo(IV)-Mo(IV) bonded {Mo(IV)(3)} triad together with Mo(V). Upon dissolution, the PMo(12)Li(x) species present in the solid PMo(12)/n products undergo electron exchange and single-peak (31)P NMR spectra were observed for n = 1-12. For n ≥ 16, changes in solid state and solution (31)P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with Mo(V)-Mo(V) and {Mo(IV)(3)} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}(+) and 2-electron-reduced PMo(12) in (TBA)(4)[PMo(12)O(40){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 Mo(2)C at n ≥ 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.