In this manuscript we report a high-level ab initio study of anion-pi 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 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 N6,N6'-dimethylene-bisadenine hydrochloride and an outer-sphere complex of protonated N9,N9'-trimethylene-bishypoxanthine with zinc tetrachloride anion, that exhibit interesting anion-pi interactions. They are in strong agreement with high-level theoretical calculations.
The synthesis of a metalled double-helix containing exclusively silver-mediated C*-C* base pairs is reported herein (C*=N hexylcytosine). Remarkably, it is the first crystal structure containing infinite and consecutive C*-Ag -C* base pairs that form a double helix. The Ag ion occupies the center between two C* residues with N(3)-Ag bond lengths of 2.1 Å and short Ag -Ag distances (3.1 Å) suggesting an interesting argentophilic attraction as a stabilization source of the helical disposition. The solid-state structure is further stabilized by metal-mediated base-pairs, hydrogen bonding and π-stacking interactions. Moreover, the angle N(3)-Ag-N(3) is almost linear in the [Ag(N hexylcytosine) ] motif and the bases are not coplanar, thus generating a double-strand helical aggregate in the solid state. The noncovalent and argentophilic interactions have been rationalized based on DFT calculations.
We report the first gold(iii) complex with a cytosine derivative since 35 years. In the crystal structure, the complex stacks by reciprocal regium-bonding interactions. These interactions appear to be common in the Cambridge structure database.
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