Investigating the potential of flexible and pre-organized tetraamide ligands to encapsulate anions in one-dimensional coordination polymers: Synthesis, spectroscopic studies and crystal structures

Abstract

The synthesis of amide-based molecules, possessing pre-organized structures, has re-ceived significant attention due to their potential applications as molecular receptors and as compo-nents of nanomaterials. In this study, four extended tetraamide ligands incorporating ethylene and propylene spacers, namely 1,2-bis[N,N′-6-(3-pyridylmethylamido)pyridyl-2-carboxyamido]ethane (L1), 1,2-bis[N,N′-6-(4-pyridylmethylamido)pyridyl-2-carboxyamido]ethane (L2), 1,2-bis[N,N′-6-(3-pyridylmethylamido)pyridyl-2-carboxyamido]propane (L3) and 1,2-bis[N,N′-6-(4-pyridylmethylam-ido)pyridyl-2-carboxyamido]propane (L4), were successfully synthesized. Further, reaction of L2 and L4, incorporating pendant 4-pyridyl donors as the metal coordinating sites, with cadmium salts, pro-duced two close-packed one-dimensional coordination polymers, {[Cd3 (L2)4 (H2O)10 ](NO3)6·12H2O-·CH3OH}n and {[Cd(SO4)(L4)(H2O)2 ]·4H2O·CH3OH}n. X-ray crystallography reveals that the flexible tetraamide ligands fold upon themselves in the coordination polymer structure. As a consequence, the anion pocket in {[Cd(SO4)(L4)(H2O)2 ]·4H2O·CH3OH}n incorporating the ligand with the propylene spacer was blocked from encapsulating charge-balancing anions, which were involved in bridging the di-cadmium units. Interestingly, a strong interaction between the 2,6-pyridine dicar-boxamide moiety with the nitrate anions was found in {[Cd3 (L2)4 (H2O)10 ](NO3)6·12H2O·CH3OH}n, showing potential for materials made from these ligands to serve as anion receptors.