Abstract
Complexes featuring lanthanide silicon bonds represent a research area still in its infancy. Herein, we report a series of Cp-free lanthanide (+II) complexes bearing σ-bonded silyl ligands. By reactions of LnI2 (Ln = Yb, Eu, Sm) either with a 1,4-oligosilanyl dianion [K-Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2-K)] (1) or with 2 (Me3Si)3SiK (3) the corresponding neutral metallacyclopentasilanes ({Me2Si(Me3Si)2Si}2)Ln·(THF)4 (Ln = Yb (2a), Eu (2b), Sm (2c)), or the disilylated complexes ({Me3Si}3Si)2Ln·(THF)3 (Ln = Yb (4a), Eu (4b), Sm (4c)), were selectively obtained. Complexes 2b, 2c, 4b, and 4c represent the first examples of structurally characterized Cp-free Eu and Sm complexes with silyl ligands. In both series, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding lanthanide metals. Density functional theory calculations were also carried out for complexes 2a–c and 4a–c to elucidate the bonding situation between the Ln(+II) centers and Si.
Highlights
Complexes with the metal in the oxidation state 3+ bearing Cp or substituted Cp ligands are dominating the organometallic chemistry of the rare-earth elements
In the meantime the number of elements has increased substantially, and quite recently Evans et al were successful in completing the series of crystalline examples of divalent molecular complexes of all lanthanides.[3−6] This enabled for the first time a comparison of all lanthanides in a single uniform coordination environment and postulation of their electronic ground states.[6]
The few reported samarium silyl compounds, which have been characterized by X-ray diffraction analysis, feature Sm with Cp* ligands almost exclusively in oxidation state 3+7−9 with the exception of some divalent silylene complexes reported by Evans and co-workers[10] and our group.[11]
Summary
Complexes with the metal in the oxidation state 3+ bearing Cp or substituted Cp ligands are dominating the organometallic chemistry of the rare-earth elements. Reaction of YbI2·(THF)[2] with 1 showed formation of 2a, but again the generation of several side products was observed These results are similar to what was previously described for titanium[37] and yttrium[38] complexes bearing tris(trimethylsilyl)silyl groups. Another aspect that makes a comparison with oligosilanyl zinc compounds tempting is the observed upfield shift for a distorted geometry at Zn from δ = −123.9 ppm for the linear Si−Zn−Si arrangement in [(Me3Si)3Si]2Zn45 to δ = −150.8 ppm for the bent Si−Zn−Si arrangement in [(Me3Si)3Si]2Zn·(bipy).[45] For the Si−Yb−Si complexes 4a and 2a a similar behavior can be observed, where the shift of the undistorted complex 4a of δ = −144.8 ppm is raised to δ = −154.0, −158.5 ppm for the constrained geometry of complexes 2a and 2a·DME with much smaller Si−Yb−Si angles than found for 4a From these NMR data it seems likely that the polarization of the Si−Yb bond should be somewhat more pronounced than that of a Si−Zn bond but less than a Si−Mg bond. HOMO orbitals of 2a−c and 4a−c support our analysis (Figure 3 and Supporting Information Figures S26 and S27) as they resemble lone pairs situated on silicon centers with no extension toward the lanthanide centers
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