Synthesis and structural characterisation of bulky heptaaromatic (hetero)aryl o-substituted s-aryltetrazines

An expedient two-steps synthesis produces in good yield polyaromatic heptacyclic (hetero)arylated o-substituted s-aryltetrazines (s-Tz) directly from diphenyl s-tetrazine. This methodology overcomes the steric limitations of classical Pinner-like syntheses encountered for o-functionalized s-Tz. A single step palladium-catalyzed N-directed C–H bond tetrahalogenation is followed by a Pd-catalyzed Suzuki (hetero)arylation that is achieved simultaneously on four sites. The single crystal X-ray diffraction structure of the resulting typical polyaromatic heptacyclic aromatic compound 3,6-bis(2,6-diphenyl)-1,2,4,5-tetrazine (3) is analyzed, together with R-functionalized peripheral phenyl derivatives [R = p-t-Bu (4), and m-OMe (10)]. Generally, stacking interactions between aromatic rings can be considerably stronger if electron-depleted rings are combined with electron-rich ones. Thus, electron-poor heteroaromatic aryltetrazines are expected to interact with electron-rich phenyl aromatic rings from reduced π…π repulsion, rendering the formation of stacking networks more favorable. Herein, despite the presence of strongly electron-deficient heteroaromatic tetrazine cores, the disruption of planarity between the various aromatic rings involved precludes the expected stacking arrangement. Thus, packing organization is driven by weak hydrogen bonding with C–H…N short contacts (or C–H…O when possible). These new heptaaromatics, which incorporate for the first time an electron-attracting nitrogen-rich core are easily modifiable from cross-coupling reactions, and constitute a relevant structural and electronical alternative to the well-known and high value class of hexaphenylbenzenes.