Physicochemical Properties of Near-Linear Lanthanide(II) Bis(silylamide) Complexes (Ln = Sm, Eu, Tm, Yb)

Following our report of the first near-linear lanthanide (Ln) complex, [Sm(N^††)₂] (1), herein we present the synthesis of [Ln(N^††)₂] [N^†† = {N(SiⁱPr₃)₂}; Ln = Eu (2), Tm (3), Yb (4)], thus achieving approximate uniaxial geometries for a series of "traditional" Ln^II ions. Experimental evidence, together with calculations performed on a model of 4, indicates that dispersion forces are important for stabilization of the near-linear geometries of 1-4. The isolation of 3 under a dinitrogen atmosphere is noteworthy, given that "[Tm(N″)(μ-N″)]₂" (N″ = {N(SiMe₃)₂}) has not previously been structurally authenticated and reacts rapidly with N₂(g) to give [{Tm(N″)₂}₂(μ-η²:η²-N₂)]. Complexes 1-4 have been characterized as appropriate by single-crystal X-ray diffraction, magnetic measurements, electrochemistry, multinuclear NMR, electron paramagnetic resonance (EPR), and electronic spectroscopy, along with computational methods for 3 and 4. The remarkable geometries of monomeric 1-4 lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln(N″)(μ-N″)]₂ complexes. EPR spectroscopy of 3 shows that the near-linear geometry stabilizes m_J states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on 1-4 did not yield Ln^II reduction potentials, so a reactivity study of 1 was performed with selected substrates in order to benchmark the Sm^III → Sm^II couple. The separate reactions of 1 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azobenzene, and benzophenone gave crystals of [Sm(N^††)₂(TEMPO)] (5), [Sm(N^††)₂(N₂Ph₂)] (6), and [Sm(N^††){μ-OPhC(C₆H₅)CPh₂O-κO,O'}]₂ (7), respectively. The isolation of 5-7 shows that the Sm^II center in 1 is still accessible despite having two bulky N^†† moieties and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the Sm^III coordination spheres of the products.