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 π‐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 H2‐TAP. In addition, the electrophilic C=N moieties in TAP led to the formation of a dense π‐stacked structure. These results highlight the effect of partial replacement of C=C bonds with C=N bonds in antiaromatic π‐electron systems. Best of both worlds: The thiophene‐fused 1,4‐diazapentalene with two C=N bonds (TAP) was synthesized as a stable compound. The fused thiophene rings restored the antiaromatic character of the 8π‐electron system diminished by C=N doping while imparting thermal stability. TAP readily underwent hydrogenation owing to the pronounced aromaticity of the hydrogenated product. Moreover, the high electrophilicity of C=N bonds of TAP led to dense π‐stacking in the solid state.