Bowles Alex W J, Liu Yu, Stevens Matthew P, Vitorica-Yrezabal Iñigo J, McMullin Claire L, Ortu Fabrizio
School of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK.
Department of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
Chemistry. 2023 Sep 26;29(54):e202301850. doi: 10.1002/chem.202301850. Epub 2023 Aug 14.
The study of sub-valent Group 2 chemistry is a relatively new research field, being established in 2007 with the report of the first Mg(I) dimers. These species are stabilized by the formation of a Mg-Mg covalent bond; however, the extension of this chemistry to heavier alkaline earth (AE) metals has been frustrated by significant synthetic challenges, primarily associated with the instability of heavy AE-AE interactions. Here we present a new blueprint for the stabilization of heavy AE(I) complexes, based upon the reduction of AE(II) precursors with planar coordination geometries. We report the synthesis and structural characterisation of homoleptic trigonal planar AE(II) complexes of the monodentate amides {N(SiMe ) } and {N(Mes)(SiMe )} . DFT calculations showed that the LUMOs of these complexes all show some d-character for AE = Ca-Ba. DFT analysis of the square planar Sr(II) complex [Sr{N(SiMe ) }(dioxane) ] revealed analogous frontier orbital d-character. AE(I) complexes that could be accessed by reduction of these AE(II) precursors were modelled computationally, revealing exergonic formation in all cases. Crucially, NBO calculations show that some d-character is preserved in the SOMO of theoretical AE(I) products upon reduction, showing that d-orbitals could play a crucial role in achieving stable heavy AE(I) complexes.
亚价态第2族元素化学的研究是一个相对较新的研究领域，它始于2007年首个Mg(I)二聚体的报道。这些物种通过形成Mg-Mg共价键而得以稳定；然而，将这种化学扩展到更重的碱土（AE）金属却因重大的合成挑战而受阻，这些挑战主要与重AE-AE相互作用的不稳定性有关。在此，我们基于具有平面配位几何结构的AE(II)前体的还原，提出了一种稳定重AE(I)配合物的新方案。我们报道了单齿酰胺{N(SiMe ) }和{N(Mes)(SiMe )}的同配体三角平面AE(II)配合物的合成及结构表征。密度泛函理论（DFT）计算表明，对于AE = Ca-Ba，这些配合物的最低未占分子轨道（LUMO）都显示出一些d特征。对平面正方形Sr(II)配合物[Sr{N(SiMe ) }(二氧六环) ]的DFT分析揭示了类似的前沿轨道d特征。通过还原这些AE(II)前体可得到的AE(I)配合物进行了计算建模，结果表明在所有情况下都能以放能方式形成。至关重要的是，自然键轨道（NBO）计算表明，理论上的AE(I)产物在还原后，其单占据分子轨道（SOMO）中仍保留一些d特征，这表明d轨道在实现稳定的重AE(I)配合物方面可能起着关键作用。
