Triple Pancake Bonding in Neutral π-Conjugated Dimers: a Computational Study

High-order pancake bonding arising from the overlap of multiple π-type orbitals in π-conjugated molecules is exceedingly rare. Recently reported cofacially stacked hexaazatrinaphthylene trianions ([HAN]^3-) stabilized by tetravalent actinides exhibit six-electron triple pancake bonds, but the presence of counterions hides the intrinsic nature of the bonding. Here, we designed a neutral HAN derivative via hydrogen coordination, 1,5,9-trihydro-1,4,5,8,9,12-hexaazatriphenylene (HATH₃), which features a quartet ground state with three π-type singly occupied molecular orbitals. The HATH₃ monomer dimerizes both in trans- and cis-cofacial arrangements, with ultrashort intermolecular separation of 2.968 and 2.971 Å with substantial interaction energy of -158.6 and -135.1 kJ/mol, respectively. The stability of these dimers occurs primarily through orbital interactions, three electron-sharing π-orbitals between two HATH₃ fragments. Electrostatic interactions and dispersion make smaller but significant bonding contributions to the overall stability. These neutral dimers exhibit a genuine triple pancake bond, providing new insight into the nature of high-order π-stacking interactions. These strong intermolecular interactions can be important in aggregate formation and crystal formation.