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) — together 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, whereas ICR-27 is a nonporous layered structure. In the structure of ICR-25 there are numerous acidic –OH groups, whereas ICR-26 is characteristic with the presence of uncoordinated N heteroatoms. Proton conductivities of ICR-25 and ICR-26 were measured at the relative humidity of 75% and 92%, yielding maximal values of 4.6 × 10–6 and 2.0 × 10–4 S cm⁻¹, respectively. Activation energies indicate vehicle mechanism of proton transport in ICR-25 and proton-hopping (Grotthuss) mechanism in the case of ICR-26. We propose that using N-heteroaromatic phosphonate ligands is an efficient way for the construction of proton-conducting coordination polymers.