Rare-Earth Metal(II) Aryloxides: Structure, Synthesis, and EPR Spectroscopy of [K(2.2.2-cryptand)][Sc(OC₆H₂tBu₂-2,6-Me-4)₃]

The suitability of aryloxide ligands for stabilizing +2 oxidation states of Sc and Y has been examined and EPR evidence indicating the first O-donor complexes of Sc^II and Y^II has been obtained, as well as an X-ray crystal structure of a Sc^II aryloxide complex. The trivalent rare-earth metal aryloxide precursors, Ln(OAr′)₃, 1-Ln (Ln=Sc, Y, Gd, Dy, Ho, Er; OAr′=OC₆H₂tBu₂-2,6-Me-4), were synthesized from the corresponding rare-earth metal trichlorides and LiOAr′⋅OEt₂. Reduction of THF solutions of 1-Ln with potassium graphite in the presence of 2.2.2-cryptand (crypt) yielded dark-colored solutions, 2-Ln, whose EPR spectra at 77 K are characteristic of the Ln^II ions: a two-line spectrum (g_∥=1.99, g_□=1.97, A_ave=154 G) for 2-Y and an eight-line spectrum (g_ave=2.01 and A_ave=291 G) for 2-Sc. Solutions of 2-Y decompose within one minute at room temperature, wheras 2-Sc persists up to 40 min at room temperature. 2-Sc was identified by X-ray crystallography as [K(crypt)][Sc(OAr′)₃], which has a trigonal-planar arrangement of oxygen-donor atoms around Sc^II. Analogous reductions of 1-Ln for Ln=Gd, Dy, Ho, and Er also gave dark solutions of limited stability. Theoretical analysis using time-dependent density functional theory (TD-DFT) along with complete active space self-consistent field (CASSCF) methods, and structural analysis with the Guzei ligand solid angle G-parameter method are presented.