Trigonal Planar Heteroleptic Lanthanide(III) Bis(silyl)amide Complexes Containing Aminoxyl Radicals and Anions

Modulation of the crystal field (CF) in lanthanide (Ln) complexes can enhance optical and magnetic properties, and large CF splitting can be achieved with low coordination numbers in specific geometries. We previously reported that the homoleptic near-linear Sm²⁺ complex [Sm^II{N(SiⁱPr₃)₂}₂] (1-Sm) is oxidized by the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO^•) radical to give the heteroleptic, approximately trigonal planar Sm³⁺ complex, [Sm^III{N(SiⁱPr₃)₂}₂(TEMPO^-)] (2-Sm). Here, we report the synthesis of homologous [Ln^III{N(SiⁱPr₃)₂}₂(TEMPO^-)] (2-Ln; Ln = Tm, Yb) complexes by the oxidation of the parent [Ln{N(SiⁱPr₃)₂}₂] (1-Ln; Ln = Tm, Yb) with TEMPO^•; complexes 2-Ln all contain TEMPO^- anions. The homoleptic bent Ln³⁺ complexes [Ln^III{N(SiⁱPr₃)₂}₂][B(C₆F₅)₄] (3-Ln; Ln = Sm, Tm, Yb) were also treated with TEMPO^• to yield the heteroleptic, approximately trigonal planar Ln³⁺ complexes [Ln^III{N(SiⁱPr₃)₂}₂(TEMPO^•)][B(C₆F₅)₄] (4-Ln; Ln = Sm, Tm, Yb); the cations of 4-Ln all contain TEMPO^• radicals. We have compared the electronic structures of the two geometrically similar families of Ln³⁺ complexes with the TEMPO^- anion (2-Ln) or TEMPO^• radical (4-Ln) using a combination of UV-vis-NIR and EPR spectroscopy, magnetic measurements, and ab initio calculations. These studies revealed no single-molecule magnet behavior for 2-Yb despite evidence for sizable CF splitting and a high degree of purity of the ground stabilized m_J = |±7/2⟩ state, while the radical TEMPO^• in 4-Yb did not significantly improve performance.