Investigation into the Effects of a Trigonal-Planar Ligand Field on the Electronic Properties of Lanthanide(II) Tris(silylamide) Complexes (Ln = Sm, Eu, Tm, Yb)

In recent work we have reported the synthesis and physical properties of near-linear Ln(II) (Ln = lanthanide) complexes utilizing the bulky bis(silylamide) {N(Si i Pr 3 ) 2 }. Herein, we synthesize trigonal-planar Ln(II) complexes by employing a smaller bis(silylamide), {N(Si BuMe 2 ) 2 } (N**), to study the effects of this relatively rare Ln geometry/oxidation state combination on the magnetic and optical properties of complexes. We show that the charge-separated trigonal-planar Ln(II) complexes [K(2.2.2-cryptand)][Ln(N**) 3 ] (Ln = Sm (1), Eu (2), Tm (3), Yb (4)) can be prepared by the reaction of 1.5 equiv of [{K(N**)} 2 ] with LnI 2 THF 2 (Ln = Sm, Yb) or LnI 2 (Ln = Eu, Tm) and 1 equiv of 2.2.2-cryptand in E 2 O. Complex 3 is the first structurally characterized trigonal-planar Tm(II) complex. In the absence of 2.2.2-cryptand, [K(DME) 3 ][Sm(N**) 3 ] (5) and [Ln(N**) 2 (μ-N**)K(toluene) 2 ] (Ln = Sm (6), Eu (7)) were isolated in the presence of DME (dimethoxyethane) or toluene, respectively. The 1:1 reaction of [{K(N**)} 2 ] with LnI 2 THF 2 (Ln = Sm, Yb) in THF gave the four-coordinate pseudo-tetrahedral Lewis base adducts [Ln(N**) 2 (THF) 2 ] (Ln = Sm (8), Yb (9)) and the cyclometalated complex [Yb(N**){N(Si BuMe 2 )(Si BuMeCH 2 )}(THF)] (10). Complexes 1-10 have been characterized as appropriate by single-crystal XRD, magnetic measurements, multinuclear NMR, EPR, and electronic spectroscopy, together with CASSCF-SO and DFT calculations. The physical properties of 1-4 are compared and contrasted with those of closely related near-linear Ln(II) bis(silylamide) complexes.