Proton conductivity studies on two non-porous coordination complexes with different proton densities

Abstract

Two new complexes based on the 2,6-dicarboxy-4-hydroxypyridine ligand, formulated as [NaCu₂(CAM)(HCAM)(H₂O)]·H₂O (1) and [Mn(HCAM)(bim)] (2) (H₃CAM = chelidamic acid or 2,6-dicarboxy-4-hydroxypyridine, bim = 2,2′-biimidazole), were synthesized. The two compounds were characterized via their IR spectra, thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. Compound 1 features a 1-D zigzag chain constructed by μ₂-κO1:κO2:κO3:κN1 HCAM²⁻ and μ₂-κO6:κO8:κO10:κN2 CAM³⁻ ligands and Cu(II) ions. Complex 2 also shows a 1-D zigzag chain connected by μ₂-κO6:κO7:κO8:κN2 HCAM²⁻ ligands, Mn(II) ions and terminal H₂bim ligands. Both complexes are further linked via H-bonding interactions to form 3-D supramolecular networks. Complexes 1 and 2 display a relatively high proton conductivity (σ) values of 6.72 × 10⁻⁵ and 1.70 × 10⁻⁴ S cm⁻¹, respectively, at 85 °C under 100% relative humidity (RH). Notably, although complexes 1 and 2 possess similar structures with 1-D chains, the proton conductivity of 2 is higher than that of 1 because complex 2 has a higher proton density and richer hydrogen bonds stemming from two undeprotonated imidazole rings of the H₂bim molecules. This work revealed that increasing the proton density of a material by incorporating proton-rich-carriers as auxiliary ligands is an effective strategy to achieve high-performance non-porous coordination polymer-based proton conductors.