Alkaline-Earth-Metal-Induced Liberation of Rare Allotropes of Elemental Silicon and Germanium from N-Heterocyclic Metallylenes.

Abstract

The synthesis and striking reactivity of the unprecedented N-heterocyclic silylene and germylene ("metallylene") alkaline-earth metal (Ae) complexes of the type [(η(5)-C5Me5)2Ae←:E(N(t)BuCH)2] (3, 4, and 7-9; Ae = Ca, E = Ge 3; Ae = Sr, E = Ge 4; Ae = Sr, E = Si 7; Ae = Ba, E = Si 8; Ae = Ba, E = Ge 9) are reported. All complexes have been characterized by spectroscopic means, and their bonding situations investigated by density functional theory (DFT) methods. Single-crystal X-ray diffraction analyses of examples revealed relatively long Si-Ae and Ge-Ae distances, respectively, indicative of weak E:→Ae (E = Si, Ge) dative bonds, further supported by the calculated Wiberg bond indices , which are rather low in all cases (∼0.5). Unexpectedly, the complexes undergo facile transformation to 1,4-diazabuta-1,3-diene Ae metal complexes of the type [(η(5)-C5Me5)2Ae(κ(2)-{N(t)Bu═CHCH═N(t)Bu})] (Ae = Sr 10, Ae = Ba 11) or in the case of calcium to the dinuclear complex [(η(5)-C5Me5)2Ca←:N((t)Bu)═CHCH═((t)Bu)N:→Ca(η(5)-C5Me5)2] (12) under concomitant liberation of elemental silicon and germanium. The formation of elemental silicon and germanium is proven by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray spectroscopy. Notably, the decomposition of the Si(II)→Ba complex 8 produces allo-silicon, a rare allotropic form of elemental silicon. Similarly, the analogous Ge(II)→Ba complex 9, upon decomposition, forms tetragonal germanium, a dense and rare allotrope of elemental germanium. The energetics of this unprecedented alkaline-earth-metal-induced liberation of elemental silicon and germanium was additionally studied by DFT methods, revealing that the transformations are pronouncedly exergonic and considerably larger for the N-heterocyclic germylene complexes than those of the corresponding silicon analogues.