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)

Abstract

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(SiPr)}. Herein, we synthesize trigonal-planar Ln(II) complexes by employing a smaller bis(silylamide), {N(SiBuMe)} (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**)] (Ln = Sm (1), Eu (2), Tm (3), Yb (4)) can be prepared by the reaction of 1.5 equiv of [{K(N**)}] with LnITHF (Ln = Sm, Yb) or LnI (Ln = Eu, Tm) and 1 equiv of 2.2.2-cryptand in EtO. Complex 3 is the first structurally characterized trigonal-planar Tm(II) complex. In the absence of 2.2.2-cryptand, [K(DME)][Sm(N**)] (5) and [Ln(N**)(μ-N**)K(toluene)] (Ln = Sm (6), Eu (7)) were isolated in the presence of DME (dimethoxyethane) or toluene, respectively. The 1:1 reaction of [{K(N**)}] with LnITHF (Ln = Sm, Yb) in THF gave the four-coordinate pseudo-tetrahedral Lewis base adducts [Ln(N**)(THF)] (Ln = Sm (8), Yb (9)) and the cyclometalated complex [Yb(N**){N(SiBuMe)(SiBuMeCH)}(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.

Structural Characterization of Lithium and Sodium Bulky Bis(silyl)amide Complexes

“…Low carbon values were consistently obtained in microanalysis experiments of 4 and 7 . As <5% protic impurities were seen in the 1 H-NMR spectra of these samples, we attribute this observation to the formation of silicon carbides; we have commented previously that this can lead to incomplete combustion of complexes of these bis(silyl)amides [17,18,19,20,21,22]. …”

“…The ability of N′′ to bridge metal centres is well-documented [4,5,6]; we have previously only seen this mode for the bulkier {N(Si t BuMe 2 ) 2 } ligand on a limited number of occasions to date [14,19]. Structurally characterized complexes containing unsupported bridging n -butyl anions are rare [28,29,30] and to the best of our knowledge this is the first example where two lithium amides are bridged by n -butyl anions.…”

“…The synthesis of lithium salts is facile, with n BuLi effecting deprotonation at room temperature; sodium salt preparation is less straightforward as either refluxing conditions or the use of a non-standard reagent such as n BuNa is required. We envisage that these lighter congeners will find synthetic utility as alternative ligand transfer agents to the potassium bis(silyl)amides that we have used extensively in the preparation of f-block complexes [14,15,16,17,18,19,20,21]. Whilst the potassium salts have proved effective in salt metathesis reactions with metal iodide precursors due to facile potassium iodide elimination, we envisage that the lithium and sodium reagents herein could give improved yields when metal chlorides are used as starting materials.…”

“…We have utilized the potassium salts of these ligands to synthesize a catalogue of low-coordinate s- and f-block complexes by salt metathesis methodologies [15,16,17,18,19,20,21,22]. In these previous studies, we found that the potassium bis(silyl)amides generally react with metal iodide precursors to give target complexes in moderate crystalline yields but for metal chloride precursors poor yields have resulted.…”

Reversible temperature-induced polymorphic phase transitions of [Y(OAr)3] and [Ce(OAr)3] (Ar = 2,6-tBu2-4-MeC6H2): interconversions between pyramidal and planar geometries

“…1). [18][19][20][21][22][23][24] These explanations are also likely to underpin the non-linearity of several two-coordinate group 2 compounds as well. 19,[25][26][27][28][29] Despite many years of disagreement, it has more recently been proposed that the polarisable-ion model (Fig.…”