| Abstract
| - The homoleptic ytterbium(II) tetraalkylaluminate complexes {Yb[AlR4]2}n have beenobtained according to a silylamide elimination reaction from Yb[N(SiMe3)2]2(THF)2 and excessAlR3 (R = Me, Et, iBu). While the tetramethylaluminate derivative is a pyrophoric powderwhich is insoluble in aliphatic and aromatic hydrocarbons, the ethyl and isobutyl congenersare readily soluble in n-hexane. Perethylated polymeric {Yb[AlEt4]2}n is constituted formallyof the two molecular fragments [Yb(AlEt4)]+ and [Yb(AlEt4)3]-, forming an intricate three-dimensional network in the solid state. Both fragments are linked by bridging α-carbonatoms and secondary Yb···H−C agostic interactions combining μ,η1, μ,η2, and μ,η3 coordinationmodes which result in remarkably short Yb···Al (2.809(2) Å) and a large range of Yb···C(2.649(5)−3.364(6) Å) distances. DFT calculations on the molecular fragments [Yb(AlEt4)3]-and [Yb(AlEt4)]+ reproduced the X-ray geometry remarkably well. Moreover, the theoreticalinvestigations on model systems for the aluminate coordination support the highly fluxionalnature of the aluminate coordination (ΔE(η2→η3) = −8 kcal/mol), which is also indicated bysolution NMR spectroscopy. A topological analysis of the total electron density of the μ,η2-bonded aluminate ligand in the benchmark systems Y(AlR4)3 (R = Me, Et) revealed thepresence of two bond critical points between the Y−Cb and Cb−Al bonds (Cb = bridgingcarbon atom) and thus suggests a hypervalent character of the bridging carbon atom.
- Mild alkylation of Yb[N(SiMe3)2]2(THF)2 with organoaluminum compounds AlR3 (R = Me, Et, iBu) has been achieved via Yb → Al silylamide transfer to afford homoleptic {Yb[AlR4]2}n. In the solid state, {Yb[AlEt4]2}n forms an intricate three-dimensional network revealing μ,η1-, μ,η2-, and μ,η3-aluminate coordination modes and Yb···(C−H) α- and β-polyagostic interactions. DFT calculations at the BPW91/I level of theory and a topological analysis of the total electron density were applied to further investigate the aluminate bonding.
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