Proton-conducting coordination polymers formed from bipyridine phosphonate ligands

Abstract

Phosphonate-based linkers remain underexplored for constructing proton-conductive coordination polymers. We synthesized three new coordination polymers, ICR-25, ICR-26, and ICR-27, employing two new phosphonate ligands derived from 4,4′-bipyridine: 4,4′-bipyridine-2,2′,6,6′-tetraphosphonic acid (H₈BipyTP) and 4,4′-bipyridine-2,2′-diphosphonic acid (H₄BipyDP), with Ca²⁺ (ICR-25), Al³⁺ (ICR-26), and Mg²⁺ (ICR-27) ions. These materials represent new members of the limited family of N-heteroaromatic phosphonate frameworks. ICR-25 and ICR-26 contain hydrophilic voids with bound water molecules, while ICR-27 is a nonporous layered structure. In the structure of ICR-25, there are numerous acidic –OH groups, while ICR-26 is characterized by the presence of uncoordinated N heteroatoms. The proton conductivities of ICR-25 and ICR-26 are measured at relative humidity values of 75% and 92%, yielding the maximum value of 2.0 × 10⁻⁴ S cm⁻¹, which is achieved by ICR-26 at 92% humidity and a temperature of 306 K. Activation energies indicate the vehicle mechanism of proton transport in ICR-25 and the proton-hopping (Grotthuss) mechanism in the case of ICR-26. We thereby propose that using the N-heteroaromatic phosphonate ligands is an efficient method for the construction of proton-conducting coordination polymers.