Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal
Magda Pascual‐Borràs, Elisabetta Arca, Hirofumi Yoshikawa, Thomas J. Penfold, Paul G. Waddell, R. John Errington
Abstract
High Resolution Image Download MS PowerPoint Slide 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.