Coordination networks of Ag(i) and N,N′- bis(3-pyridinecarboxamide)-1,6-hexane: structures and anion exchange

Abstract

A bidentate ligand N,N′- bis(3-pyridinecarboxamide)-1,6-hexane (L) and its silver complexes have been synthesized and characterized by single crystal X-ray diffraction. Reaction of AgClO₄ and L in H₂O/EtOH gives rise to a coordination polymer [Ag₂L₃OH][ClO₄]·2.5H₂O. The X-ray crystal structure of the compound shows honeycomb-like networks in which four-coordinated Ag ions are linked to three-coordinated Ag ions via three ligands L. The coordination of the long ligands L to the Ag ions creates tube-like structures and the tubes of adjacent honeycomb layers are interlocked, leading to an interpenetrating network. The compound [AgL][ClO₄], synthesized from CH₃OH, is composed of twisted zigzag coordination polymers in which ligands L are linked by Ag ions. Ligand L displays two different conformations A and B within a single strain of polymer. The two conformers differ in the orientation of the two pyridyl-groups which are arranged periodically in the polymer in the sequence ABBABBA. The polymer chains assemble into 2-D undulating sheets via amide hydrogen bonds. Reaction between AgNO₃/AgCF₃SO₃ and L leads to polymeric compounds [AgL][NO₃] and [AgL][CF₃SO₃]. The compounds are composed of coordination polymers in zigzag conformation and the polymer chains assemble into undulating sheets via inter-chain hydrogen bonds. The inter-sheet Ag–Ag distances of the compounds are in the order [AgL][CF₃SO₃] > [AgL][ClO₄] > [AgL][NO₃]. The anion exchange properties of the compounds are monitored by using X-ray powder diffraction, infrared spectroscopy and elemental analysis. Our results show that the anions in [AgL][NO₃] and [AgL][CF₃SO₃] can be totally replaced with ClO₄⁻. However, the exchange is not reversible. In additional, inter-conversion between [AgL][NO₃] and [AgL][CF₃SO₃] by anion exchange is shown to be unfeasible. Anion selectivity could be due to the different hydration energy of the anions and the structural reorganization involved in the conversion.