Abstract
The synthesis of amide-based molecules, possessing pre-organized structures, has received significant attention due to their potential applications as molecular receptors and as components 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-pyridylmethylamido)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, produced 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 dicarboxamide 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.
Highlights
Studies concerning the synthesis and applications of neutral organic molecules equipped with hydrogen-bond donors for binding or separating anions have become a topical area in supramolecular chemistry [1,2,3,4,5,6]
We examine the conformational flexibility of the new tetraamides and utilize compounds L2 and L4 for the synthesis of coordination polymers
The potential for formation of a larger anion pocket created by two 2,6-pyridine dicarboxamide units was proposed to be a source of interesting anion-encapsulation behavior
Summary
Studies concerning the synthesis and applications of neutral organic molecules equipped with hydrogen-bond donors for binding or separating anions have become a topical area in supramolecular chemistry [1,2,3,4,5,6]. Incorporation of ligands possessing hydrogen-bond-donor moieties has been a strategy of first choice to generate coordination frameworks capable of interactions with anions [12,13,14]. Custelcean and co-workers have successfully synthesized coordination polymers from ligand-possessing urea moieties [18]. These compounds form weak hydrogen-bonding interactions with the anions, through the urea moieties, allowing Custelcean et al to examine selective anion separations via crystallization [19]. The roles of non-covalent interactions in coordination compounds have been reported in many other relevant works [20,21,22,23,24,25,26]
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