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” LnII 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 mJ states with oblate spheroid electron density distributions, validating
our previous suggestions. Cyclic voltammetry experiments carried out
on 1–4 did not yield LnII reduction potentials, so a reactivity study of 1 was
performed with selected substrates in order to benchmark the SmIII → SmII 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 SmII 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 SmIII coordination
spheres of the products.