Anion−π Interactions in Bisadenine Derivatives: A Combined Crystallographic and Theoretical Study

In this manuscript we report a high-level ab initio study of anion−π interactions involving N9-methyl-adenine, N6-methyl-adenine, N9-methyl-hypoxanthine, a dimer of N9-methyl-adenine, and N9,N9‘-trimethylene-bisadenine. DNA
bases like adenine are electron-deficient arenes that are well suited for interacting favorably with anions. We
demonstrate that these compounds are able to interact favorably with anions. N9-Methyl-adenine, N6-methyl-adenine,
and the dimer of N9-methyl-adenine interact with the anion via the six-membered ring more strongly than adenine
due to cooperativity effects between the noncovalent π−π and anion−π interactions. This pattern, i.e., coexistence
of π−π and anion−π bonding, is observed experimentally in the solid state. Finally, we report the solid-state
characterization of two new compounds N6,N6‘-dimethylene-bisadenine hydrochloride and an outer-sphere complex
of protonated N9,N9‘-trimethylene-bishypoxanthine with zinc tetrachloride anion, that exhibit interesting anion−π
interactions. They are in strong agreement with high-level theoretical calculations.