Novel highly electron-deficient quinoxaline-annulated 1,3,2-diazagermol- and diazastannol-2-ylidenes, stabilized as LiCl adducts

Reaction of 2,3-bis(neopentylamino)quinoxaline (1) with nBuLi and GeCl4 or GeCl2·(dioxane) (molar ratio 1:2:1) in THF furnished highly moisture-sensitive Ge(IV) and Ge(II) heterocycles 2a and 3a, respectively. The quinoxaline-annulated N-heterocyclic germylene (quinNHGe) 3a is stable only in the presence of Li(THF)x and exhibits electrophilic properties associated with the strongly electron-withdrawing annulation. Coordination of chloride at Ge(II) and of Li+ at nitrogen is assumed, as found in crystals of a bis(quinoxaline)-annulated eight-membered NHGe LiCl adduct. Addition of dineopentyl-benzimidazol-2-ylidene (bnNHC) provides a labile bnNHC-quinNHGe adduct 4 as indicated by strong downfield coordination shift of the NMR signal for the carbene donor atom. Attempts to grow single crystals led to decomposition and protonation of the carbene forming the bis(benzimidazolium) salt 5 with Li2(THF)2Cl42- anion. Introduction of 2-methoxyethyl or 2-dimethylaminoethyl side arms as chelating functional groups into the diaminoquinoxalines 6 and 7 and subsequent reaction with 2 nBuLi/GeCl2·(dioxane) did not markedly stabilize the resulting donor-substituted quinNHGe 8 and 9. Related silicon (2b) and tin heterocycles (3c) were synthesised for comparison. The quinoxaline-annulated N-heterocyclic stannylene 3c exhibits an extreme 119Sn upfield shift compared to other N-heterocyclic stannylenes, suggesting higher coordination at tin.

Graphical abstract2,3-Bis(neopentylamino)quinoxaline reacts with nBuLi (2 equiv.) and GeCl2·(dioxane) or SnCl2 to the corresponding strongly electron-withdrawing annulated N-heterocyclic germylene and stannylene which display dominant electrophilic character and need LiCl(THF)x or interaction with a strong Lewis base for stabilization. Related heterocycles with fourvalent germanium and silicon were prepared for comparison of NMR data and properties.Figure optionsDownload full-size imageDownload as PowerPoint slide