Coordination chemistry of flexible benzene-1,3,5-tricarboxamide derived carboxylates; notable structural resilience and vaguely familiar packing motifs

Abstract

Flexible benzene-1,3,5-tricarboxamides (BTAs), organic species well-known for their tendencies to form functional soft-materials by virtue of their complementary hydrogen bonding, are explored as structurally reinforcing supramolecular building blocks in porous coordination polymers. We report the synthesis and characterisation of two related, carboxylate-terminated BTA derivatives, and the structure and functionality of their polymeric Cd(II) complexes. The polycarboxylate ligand benzene-1,3,5-tricarboxamide tris(phenylacetic acid) H₃L1 was prepared, and the analogous trimethyl benzene-1,3,5-tricarboxamide tris acetate Me₃L2 was prepared and its single crystal structure elucidated. On reaction with cadmium nitrate in a DMF/H₂O mixture, each BTA compound yielded coordination polymer species with columnar packing motifs comparable to the familiar BTA triple helix seen in purely organic systems. In the case of Me₃L2, this transformation was achieved through a convenient in situ ester hydrolysis. Complex 1 is a 2-dimensional layered material containing tubular intralayer pores, in which amide–amide hydrogen bonding is a notable structural feature. In contrast, the structure of 2 contains no amide–amide hydrogen bonding, and instead a columnar arrangement of ligand species is linked by trinuclear Cd(II) cluster nodes into a densely packed three-dimensional framework. The crystal structures revealed both materials exhibited significant solvent-accessible volume, and this was probed with thermal analysis and CO₂ and N₂ adsorption experiments; complex 2 showed negligible gas uptake, while compound 1 possesses an unusually high CO₂ capacity for a two-dimensional material with intralayer porosity and surprising structural resilience to guest exchange, evacuation and exposure to air.