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(SiiPr3)2}. Herein, we synthesize trigonal planar Ln(II) complexes by employing a smaller bis(silylamide), {N(SitBuMe2)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 LnI2THF2 (Ln = Sm, Yb) or LnI2 (Ln = Eu, Tm) and one equiv. of 2.2.2-cryptand in Et2O.
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 LnI2THF2 (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 cyclometallated complex [Yb(N**){N(SitBuMe2)(SitBuMeCH2)}(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 closely related near-linear Ln(II) bis(silylamide) complexes.