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 ScII and YII has been obtained, as well as an X‐ray crystal structure of a ScII 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 LnII ions: a two‐line spectrum (g∥=1.99, g□=1.97, Aave=154 G) for 2‐Y and an eight‐line spectrum (gave=2.01 and Aave=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 ScII. 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. Definitive EPR: A long‐known ligand broadens the scope of LnII chemistry! A bulky aryloxide ligand supports the second example of a crystallographically‐characterized ScII complex. TD‐DFT calculations, EPR, and UV/Visible spectroscopy are consistent with the existence of other LnII aryloxides whose steric undersaturation may contribute to their thermal instability.