This contribution describes the synthesis, structural systematics, absolute configurations, and structural interconversions of a series of C1-symmetric lanthanide chloro, hydrocarbyl, and amide complexes/precatalysts based on chiral chelating Me2Si(ηs-Me4Ci)(η5-C5H3R*)2-ligands [Me2SiCpʺ(R*Cp)p]2-, where R* = (+)-neomenthyl, (-)- menthyl, and (-)-phenylmenthyl. The ligands are prepared in three steps from known chiral cyclopentadienes. Metalation of the chiral dienes followed by condensation with Me4C5Si(CH3)2Cl and in situ lithiation provides the dianions in nearly quantitative yield. Transmetalation of the lithiated ligands with anhydrous lanthanide trichlorides followed by ambient temperature ether workup provides Me2SiCpʺ(R*Cp)LnCl2Li(OEt2)2 complexes in high yield. For (R)-Me2- SiCpʺ[(+)-neomenthylCp]Lu(µ-Cl)2Li(OEt2)2: space group P2X2X2X\a = 12.240(2), b = 12.876(2), and c = 24.387(5) Å (24 °C); Z = 4; R(F) = 0.0509. As established by NMR and circular dichroism, the diastereomerically pure chloro complexes can be epimerized in appropriate donor solvents to afford mixtures of (R)- and (S)-configurational isomers with the isomer ratio dependent on solvent, R*, and lanthanide ion. Selective epimerization allows enrichment in either antipode with diastereomerically pure complexes obtained in a single recrystallization. Li+sequestering crown ethers inhibit epimerization. The temperature dependence of the (R) ⇌ (S) equilibrium constant in THF yields ΔH = 1.7 ± 0.3 kcal/mol and ΔS = 3.6 ± 0.8 eu for Me2SiCpʺ[(+)-neomenthylCp)Lu(µ-Cl)2Li(OEt2)2 and ΔH = 4.8 ± 0.5 kcal/mol and ΔS = 13.4 ± 0.5 eu for Me2SiCʺ[(-)-menthylCp]Sm(µ-Cl)2Li(OEt2)2. The mechanism is proposed to involve reversible ring detachment to an intermediate LiCpR* complex. Alkylation or amidation with ME(SiMe3)2(M = Li or K, E = CH; M = Na or K, E = N) yields the corresponding chiral hydrocarbyls and amides in high yield. For (/f/S)-Me2SiCʺ[(+)-neomenthylCp]YCH(SiMe3)2: space group P21; a = 19.178(4), b = 8.736(1), and c = 21.391(5) Å β = 97.62(2); Z = 4; R(F) = 0.071. For (S)-Me2SiCʺ[(-)-menthylCp]SmCH(SiMe3)2: space group PI; a = 8.993(3), b = 12.738(2), and c = 16.549(4) Å α = 86.04(2)° β = 82.81(2)°, γ = 72.91(2)°; Z = 2; R(F) = 0.026. For (R)-Me2SiCʺ[(-)-menthylCp]YCH(SiMe3)2; space group P21; a = 12.319(3), b = 15.707(4), and c = 18.693(5) Å 0 = 91.59(2)°; Z = 4; R(F) = 0.054. For (S>Me2SiCʺ[(+)-neomenthylCp]SmN(SiMe3)2: space group P21; a = 9.122(2), b = 10.112(3), and c = 18.478(3) Å β = 90.58(2)° Z = 2; R(F) = 0.029. For (S)-Me2- SiCʺ[(-)-menthylCp]SmN(SiMe3)2: space group P212121; a = 10.217(3), b = 19.103(6), and c = 19.456(7) Å Z = 4; R(F) = 0.044. For (R)-Me2SiCʺ[(-)-menthylCp]YN(SiMe3)2: space group P1; a = 8.937(3), b = 12.397(6), and c = 16.673(7) Å α = 85.53(2)°, β = 82.17(2)°, γ = 74.78(2)°; Z = 2; R(F) = 0.065. The preferred planar chiral configurations of these complexes can be largely understood on the basis of significant, crystallographically identifiable, nonbonded interactions between R* and the remainder of the molecule. The hydrocarbyl and amide complexes are configurationally stable in toluene at 60 °C for many hours but undergo facile epimerization in the presence of primary alkyl amines, presumably via reversible Cp protonation/detachment. The hydrocarbyl complexes undergo rapid hydrogenolysis at ambient temperature, with retention of configuration, to yield the corresponding hydrides.
ASJC Scopus subject areas
- Colloid and Surface Chemistry