Anion-pi and pi-pi cooperative interactions regulating the self-assembly of nitrate-triazine-triazine complexes

We theoretically investigate the cooperative enhancement of the interactions between anions and electron-deficient aromatics by pi-pi stacking, focusing on the recent crystallographic observation of anion-pi-pi interactions in a synthesized coordination compound based on 1,3,5-triazine moieties. Using a combination of state-of-the-art dispersion-corrected density functional and quantum Monte Carlo calculations, we rationalize the unusual structural features observed in this nitrate-triazine-triazine complex. We show that the triazine rings are staggered and bent and slip with respect to each other with the nitrate bound off-center in a T-like configuration. Our results indicate that this pi-pi stacking is not simply enforced by the coordination of the triazines within the particular crystal structure but is regulated by cooperative anion-pi and pi-pi interactions. In the nitrate-triazine-triazine complex, this cooperative effect amounts to 6% of the total binding energy. Ways to further increase this energetic enhancement in the design of anion-host architectures are discussed.