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)₃Y^II]^1-, [(C₅H₄SiMe₃)₃La^II]^1-, and {Sc^II[N(SiMe₃)₂]₃}^1-. Irradiation of the La(III) complex La^III[N(SiMe₃)₂]₃ by this method gave EPR and UV-visible absorption spectra consistent with {La^II[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)₃Y^II]^1- species during the irradiation of (C₅H₄SiMe₃)₃Y^III 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.