Inverting the Electronic Structure of Diylidylgermylenes by Backbone Modification

Owing to the strong electron-donating ability of ylide substituents, diylidyltetrylenes are usually highly nucleophilic species with strong donor capacities. Here, we demonstrate that their electronic properties are in fact highly flexible and can be effectively tuned through variation of the substituent in the ylide backbone. Initial density functional theory studies showed that cyano groups are particularly capable in lowering the LUMO energy of diylidyl germylenes thus turning these usually highly nucleophilic species into electrophilic compounds. This was confirmed by experimental studies. Attempts to synthesize the germylene (Y_CN )₂ Ge [with Y_CN =Ph₃ P-(C)-CN] from the corresponding metalated ylide Y_CN K selectively led to germanide [(Y_CN )₃ Ge)K]₂ thus reflecting the electrophilic nature of the intermediate formed germylene. XRD analysis of single crystals of (Y_CN )₂ Ge - serendipitously obtained through protonative cleavage of one ylide from the germanide - revealed a monomeric structure with rather long Ge-ylide linkages, which corroborates well with a pure single bond and no stabilization of the empty p_π orbital at germanium through π bonding. The germanide exhibits methanide-like reactivity towards chalcogens but a likewise weak Ge-C bond as demonstrated by the insertion of carbon dioxide.