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 (Ka = 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 (Ka < 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.