Phosphonocarboxylates in Mixed-Ligated Approach with Diverse Structural Features (0D to 3D), and Studies of Non-Covalent Interactions for Revealed Proton Conduction.

Coordination compounds are an essential group of functional materials with different dimensionalities (0D, 1D, 2D, and 3D) depending on the metal atoms or organic linkers/ligands used. In this study, two different phosphonocarboxylates were combined with 4,4'-bipyridine (bpy) to create mixed-ligated coordination frameworks, resulting in three distinct coordination compounds with different dimensionalities: [Co(H₂O)₄(4,4'-bpy)₂][H₃L1]₂∙10H₂O (1) with 0D, {[Zn(H₂L1)(4,4'-bpy)]·H₂O}_n (2) with 2D, and [Co(H₂O)Co₂(H₂L2)₂(4,4'-bpy)₃]_n (3) with 3D structure. Here, H₄L1 = 5-Phosphonoisophthalic acid, and H₅L2 = 3,5-Diphosphonobenzoic acid. Compound 1 had the most guest water molecules connected to the protonated phosphonate moieties and coordinated water molecules, creating extended hydrogen-bonding interactions among these three structures. Thereby, compound 1 was subjected to proton conductivity (PC) measurement, and revealed a PC value of 0.84 × 10^-4 S cm^-1 at 50 °C and 95% relative humidity (RH). A structural correlation was conducted with the previously reported similar guest-framework system, and a single point energy (with counterpoise = 2, basis set superposition error <0.003%) was calculated for the supramolecular architecture formed by the guest organic moiety and lattice water. Furthermore, non-covalent interactions were checked and compared.