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.