1,4-Digermacyclopenta-1,3-diene: Continuous Cyclic σ*-π Delocalization Enables Aromatic Stabilization in a...

Aromaticity in heavy-element systems can arise from unconventional modes of cyclic electron delocalization that are not accessible in lighter-element frameworks. Herein, we report 1,4-digermacyclopenta-1,3-diene as an unusual aromatic system in which a formally 4π electron framework is stabilized through cooperative σ*-π interactions. X-ray crystallography reveals a nearly planar [Ge₂C₃] ring, while magnetic and electronic analyses (NICS, GIMIC, ACID, and EDDB) consistently indicate significant aromatic stabilization. In particular, GIMIC calculations show a pronounced diatropic ring current with an average strength of +8.38 nA T^-1. Natural bond orbital analysis reveals that this stabilization originates from σ*-π delocalization involving the Ge-Ge bond and adjacent π orbitals. Despite a low-lying open-shell contribution (y ≈ 0.11), the system remains predominantly closed-shell. Notably, this system represents a rare case in which σ*-π interactions form a continuous cyclic delocalization pathway, resulting in a distinct form of aromatic stabilization.