Benzene Triimide Cage as a Selective Container of Azide

Benzene triimide (BTI, or mellitic triimide) is a C₃-symmetric backbone with a highly electron-deficient, extended π surface and three easy functionalization sites. Here, we report the first BTI-based cage composed of two face-to-face BTIs pillared by three m-xylylene spacers and efficient and selective binding of azide through cooperative anion-π interactions. The cage was easily synthesized in two steps from benzene triimide. Crystal structures showed that the two BTI planes can be separated at about 5-6 Å and form a well-defined electron-deficient cavity. Among a series of anions tested, the cage was found able to bind N₃^-, SCN^-, and I^-. In particular, the binding toward N₃^- is very strong (K_a = 11098 ± 46 M^-1) and highly selective, over 150 and 250 times higher than SCN^- and I^-, respectively. The control single BTI, however, showed only very weak binding (K_a < 5 M^-1). The crystal structure showed that N₃^- is tightly trapped within the cavity through multiple, very short anion-π interactions. The slow enter-release of N₃^- from the cavity was observed in the NMR. The charge-transfer and electron-transfer character of the interactions was also discussed.