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-pi interactions involving N-9-methyl-adenine, N-6-methyl-adenine, N-9-methyl-hypoxanthine, a dimer of N-9-methyl-adenine, and N-9,N-9'-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. N-9-Methyl-adenine, N-6-methyl-adenine, and the dimer of M-methyl-adenine interact with the anion via the six-membered ring more strongly than adenine due to cooperativity effects between the noncovalent pi-pi and anion-pi interactions. This pattern, i.e., coexistence of pi-pi and anion-pi bonding, is observed experimentally in the solid state. Finally, we report the solid-state characterization of two new compounds N-6,N-6-dimethylene-bisadenine hydrochloride and an outer-sphere complex of protonated N-9,N-9-trimethylene-bishypoxanthine with zinc tetrachloride anion, that exhibit interesting anion-pi interactions. They are in strong agreement with high-level theoretical calculations.