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

The sterically bulky aryloxide ligand OAr* (OAr* = - OC 6 H 2 -Ad 2 -2,6 t Bu-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 4f n d 1 Ln(II) ions. The Ln(III) precursors Ln(OAr*) 3 ( 1-Ln ) were synthesized by reacting 1.2 equiv of Ln(NR 2 ) 3 (R = SiMe 3 ) 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 (KC 8 ) 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 4f n 5d 1 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 4f 14 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 4f n 5d 1 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' = OC 6 H 2 -2,6- t Bu 2 -4-Me; Cp' = C 5 H 4 SiMe 3 ).