Room-Temperature Stable Ln(II) Complexes Supported by 2,6-Diadamantyl Aryloxide Ligands

The sterically bulky aryloxide ligand OAr* (OAr* = ^–OC₆H₂-Ad₂-2,6^tBu-4; Ad = 1-adamantyl) has been used to generate Ln(II) complexes across the lanthanide series that are more thermally stable than complexes of any other ligand system reported to date for 4fⁿd¹ Ln(II) ions. The Ln(III) precursors Ln(OAr*)₃ (1-Ln) were synthesized by reacting 1.2 equiv of Ln(NR₂)₃ (R = SiMe₃) with 3 equiv of HOAr* for Ln = La, Ce, Nd, Gd, Dy, Yb, and Lu. 1-Ce, 1-Nd, 1-Gd, 1-Dy, and 1-Lu were identified by single-crystal X-ray diffraction studies. Reductions of 1-Ln with potassium graphite (KC₈) in tetrahydrofuran in the presence of 2.2.2-cryptand (crypt) yielded the Ln(II) complexes [K(crypt)][Ln(OAr*)₃] (2-Ln). The 2-Ln complexes for Ln = Nd, Gd, Dy, and Lu were characterized by X-ray crystallography and found to have Ln–O bond distances 0.038–0.087 Å longer than those of their 1-Ln analogues; this is consistent with 4fⁿ5d¹ electron configurations. The structure of 2-Yb has Yb–O distances 0.167 Å longer than those predicted for 1-Yb, which is consistent with a 4f¹⁴ electron configuration. Although 2-La and 2-Ce proved to be challenging to isolate, with 18-crown-6 (18-c-6) as the potassium chelator, La(II) and Ce(II) complexes with OAr* could be isolated and crystallographically characterized: [K(18-c-6)][Ln(OAr*)₃] (3-Ln). The Ln(II) complexes decompose at room temperature more slowly than other previously reported 4fⁿ5d¹ Ln(II) complexes. For example, only 30% decomposition of 2-Dy was observed after 30 h at room temperature compared to complete decomposition of [Dy(OAr′)₃]⁻ and [DyCp′₃]⁻ under similar conditions (OAr′ = OC₆H₂-2,6-^tBu₂-4-Me; Cp′ = C₅H₄SiMe₃).