Thiophene-Fused 1,4-Diazapentalene: A Stable C=N-Containing π-Conjugated System with Restored Antiaromaticity

A thiophene-fused 1,4-diazapentalene (TAP) was rationally designed and synthesized as a C=N-containing 4n pi-electron system that exhibits restored antiaromaticity impaired by the doping with C=N bonds. X-ray crystallographic analysis and quantum chemical calculations revealed that the annulation of thiophene rings with the 1,4-diazapentalene moiety resulted in a much higher antiaromaticity than the pristine 1,4-diazapentalene. These effects can be ascribed to the reduced bond alternation of the eight-membered-ring periphery caused by stabilization of the less-stable bond-shifted resonance structure upon increasing the degree of substitution of imine moieties. Consequently, TAP underwent facile hydrogenation even under mild conditions because of its pronounced antiaromaticity and the high aromaticity of the corresponding hydrogenated product H-2-TAP. In addition, the electrophilic C=N moieties in TAP led to the formation of a dense pi-stacked structure. These results highlight the effect of partial replacement of C=C bonds with C=N bonds in antiaromatic pi-electron systems.

Automated summary

The study focused on the rational design and synthesis of a thiophene-fused 1,4-diazapentalene (TAP) with enhanced antiaromaticity. By reducing bond alternation at the eight-membered-ring periphery and stabilizing bond-shifted resonance structures, TAP displayed increased hydrogenation reactivity due to its pronounced antiaromaticity and the high aromaticity of the hydrogenated product H-2-TAP. Additionally, the electrophilic C=N moieties in TAP promoted the formation of a dense pi-stacked structure.