Benzene triimide (BTI, or mellitic triimide) is a C 3-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.

A triangular prism cage 2 with benzene triimide (BTI) as the bases, 2,7-naphthalene dimethyl as the supporting pillars was designed and synthesized efficiently. The two parallel BTI planes constitute a cavity of 7.58 Å which allows inclusion of anions of various sizes. Cage 2 showed so far the strongest binding affinity to a series of polyhedral (tetrahedral and octahedral) anions including PF 6 -(24651 M -1 ), ClO 4 -(28234 M -1 ), CH 3 SO 3 -(30571 M -1 ), BF 4 -(31613 M -1 ) and HSO 4 -(84623 M -1 ). The obtained eleven crystal structures of 2⸧Xcomplexes with different anions demonstrated that multiple and cooperative anion- interactions contribute synergistically to the strong complexation. The series of complex structures systematically suggested that cage 2 is a good, general tool for probing anion- binding motifs that have only been predicted in theoretical calculations. For anions of triangular (NO 3 -) and polyhedral (BF 4 -, ClO 4 -, PF 6 -) shapes, the as-predicted most stable anion- binding motifs with charge-neutral  systems were experimentally observed for the first time.