Reduction of Rare-Earth Metal Complexes Induced by γ Irradiation

The utility of γ irradiation for generating unstable, low oxidation state molecular species containing rare-earth metal ions in frozen solution has been examined. The method was evaluated by irradiating Ln­(III) precursors (Ln = Sc, Y, and La) in a solid matrix of 2-methyltetrahydrofuran at 77 K with a 700 keV 137Cs source to generate free electrons capable of reducing the Ln­(III) species. These experiments yielded EPR and UV–visible spectroscopic data that matched those of the known Ln­(II) species [(C5H4SiMe3)3YII]1–, [(C5H4SiMe3)3LaII]1–, and {ScII[N­(SiMe3)2]3}1–. Irradiation of the La­(III) complex LaIII[N­(SiMe3)2]3 by this method gave EPR and UV–visible absorption spectra consistent with {LaII[N­(SiMe3)2]3}1–, a species that had previously eluded preparation by chemical reduction. Specifically, the irradiation product exhibited an axial EPR spectrum split into eight lines by the I = 7/2 139La nucleus (g ⊥ = 1.98, g || = 2.06, A ave = 519.1 G). The UV–visible absorption spectrum contains broad bands centered at 390 and 670 nm that are consistent with a La­(II) ion in a trigonal ligand environment based on time-dependent density functional theory which qualitatively reproduces the observed spectrum. Additionally, the rate of formation of the [(C5H4SiMe3)3YII]1– species during the irradiation of (C5H4SiMe3)3YIII was monitored by measuring the concentration via UV–visible spectroscopy over time to provide data on the rate at which a molecular species is reduced in a glass via γ irradiation.