Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal.

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.