Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

Homoleptic tris­(alkyl) rare earth
complexes Ln­{C­(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high
yield from LnI3THFn and 3 equiv
of KC­(SiHMe2)3. X-ray diffraction studies reveal 1a–d are isostructural, pseudo-C3-symmetric molecules that contain two secondary
Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic
assignments are supported by comparison with Ln­{C­(SiDMe2)3}3 and DFT calculations. The Ln↼HSi
and terminal SiH exchange rapidly on the NMR time scale at room temperature,
but the two motifs are resolved at low temperature. Variable-temperature
NMR studies provide activation parameters for the exchange process
in 1a (ΔH⧧ =
8.2(4) kcal·mol–1; ΔS⧧ = −1(2) cal·mol–1K–1) and 1a-d9 (ΔH⧧ = 7.7(3) kcal·mol–1; ΔS⧧ = −4(2) cal·mol–1K–1). Comparisons of lineshapes, rate constants
(kH/kD), and
slopes of ln­(k/T) vs 1/T plots for 1a and 1a-d9 reveal that an inverse isotope
effect dominates at low temperature. DFT calculations identify four
low-energy intermediates containing five β-Si–H⇀Ln
and one γ-C–H⇀Ln. The calculations also suggest
the pathway for Ln↼HSi/SiH exchange involves rotation of a
single C­(SiHMe2)3 ligand that is coordinated
to the Ln center through the Ln–C bond and one secondary interaction.
These robust organometallic compounds persist in solution and in the
solid state up to 80 °C, providing potential for their use in
a range of synthetic applications. For example, reactions of Ln­{C­(SiHMe2)3}3 and ancillary proligands, such
as bis-1,1-(4,4-dimethyl-2-oxazolinyl)­ethane (HMeC­(OxMe2)2) give {MeC­(OxMe2)2}­Ln­{C­(SiHMe2)3}2, and reactions with disilazanes provide solvent-free
lanthanoid tris­(disilazides).