Homologues of N-heterocyclic carbenes: Detection and electronic structure of N-bridgehead pyrido[a]-anellated 1,3,2-diazagermol-2-ylidenes

Novel N-bridgehead pyrido[a]-anellated 1,3,2-diazagermol-2-ylidenes 1a,b were obtained from GeCl2 · dioxane and dilithium reagents formed from N-tert-butyl pyridine-2-aldimines and excess lithium in THF whereas attempts to generate the analogous silylene by reduction of the dichloro-pyrido[a]-1,3,2-diazasilole 4a, synthesized from SiCl4 and the new dilithium reagent, failed. Characteristic chemical shifts of the pyrido 1H and 13C nuclei between those of pyridine compounds and the not fully cyclodelocalized electron-rich 4a with dihydropyridine substructure hint to a cyclodelocalized 10π electron system in 1a,b. Quantum chemical investigations of a series of pyrido[a]- and benzo-anellated imidazol-2-ylidenes and their silylene and germylene homologues show for all compounds cyclodelocalized 10π-systems but for pyrido[a]-anellation an increase of the energy of the π-MO’s relative to those of element(II) lone electron pairs which leads to destabilization compared to the benzo-anellated isomers.

Graphical abstractCyclodelocalized N-bridgehead pyrido[a]-anellated N-heterocyclic germylenes 1a,b and the electron-rich, partly delocalized dichloro-pyrido[a]-1,3,2-diazasilole 4a were prepared from GeCl2 · dioxane or SiCl4, respectively, and dilithium reagents, formed from N-tert-butyl pyridine-2-aldimines and excess lithium in THF. Quantum chemical investigations of pyrido[a]- and benzo-anellated model germylene and homologous silylenes and carbenes illuminate the relative stabilities and electronic properties.Figure optionsDownload full-size imageDownload as PowerPoint slide