Unusual structural effects of intermolecular π-bonding in the tetracyanopyrazine (ion-radical) dimer

The facile reduction of tetracyanopyrazine (TCP) by tetrakis(dimethylamino)ethylene (TDAE) leads to black crystals of the donor/acceptor salt: TDAE2+·(TCP)22−·2CH3CN in which the vertical anionic TCP stacks consisting of distinct π-(TCP)22− units are surrounded by TDAE dications. The monomers within these supramolecular π-(TCP)22− complexes are arranged co-facially at interplanar separation of ∼3.17 Å characteristic for ion-radical π-dimers and show unusual ∼30° rotation relative to each other. The structural scrutiny of the π-dimers together with the X-ray crystallographic analysis of the neutral tetracyanopyrazine acceptor, its charge-transfer complexes, and the monomeric anion-radical TCP−˙ reveals the unique “quasi-quinonoidal” distortion of the π-bonded tetracyanopyrazine moieties. In contrast to the molecular bending and intermolecular charge-transfer that are commonly observed in various ion-radical π-dimers and in conventional donor/acceptor complexes, the specific electron density redistribution and bond length alternation in π-bonded (planar) tetracyanopyrazine moieties (relative to the neutral molecule and isolated anion-radical) can only be accommodated by the bonding molecular orbital (HOMO) of the (TCP)22− dimer that is notably different from the corresponding SOMO of the monomeric TCP−˙.