N-Heterocyclic Carbene Stabilized Bisacylgermylenes.

In this contribution, a metal-free synthetic approach toward isolable bisacylgermylenes, a novel class of germylenes, is described. Starting from tetra(2,4,6-trimethylbenzoyl)germane 1 and bromo-tris(2,4,6-trimethylbenzoyl)germane 3, we demonstrate that simple treatment with NHCs leads to two distinct types of reactivity: acyl abstraction and germylene stabilization. Reaction of 1 with NHCs produces imidazolium-substituted germenolates 2a,b via a hydrogen atom transfer (HAT) mechanism. In contrast, compound 3 undergoes stepwise substitution to afford isolable NHC-stabilized bisacylgermylenes 4a,b, which were structurally characterized by NMR and single-crystal X-ray diffraction. The UV/Vis spectra of 4a,b show two absorption bands in the visible region of the light, which do not overlap and have different characters. Reactivity studies revealed its nucleophilic character and resulted in the formation of several germylene/iron and germylene/boron complexes. The removal of the NHC at room temperature with triphenylborane leads to a degradation, mainly based on the instability of the unstabilized bisacylgermylene. To determine its reactivity, an alternative synthetic strategy was developed based on the reaction of the geminal bisgermenolates 7 with the bischloro-bis(2,4,6-trimethylbenzoyl)-germane 9. Trapping reactions with NHCs results in the formation of 4a,b in near quantitative yields. This work expands the synthetic scope of main group low-valent chemistry and introduces germylenes with tunable reactivity.