Magnetism, Mössbauer Spectroscopy, and Proton Conductivity of Coordination Polymers Based on Phosphonate and Phosphinate Linkers

Coordination polymers (CPs) are versatile materials formed by metal ions and organic ligands, offering a broad range of structural and functional possibilities. Phosphonates and phosphinates are particularly attractive ligands for CPs due to their multiple binding sites, varied coordination geometries, and ability to form robust network structures. Phosphonates, considered harder ligands, form strong bonds with hard metals such as Fe³⁺, while phosphinates offer additional versatility due to the varied pendant groups on phosphorus. This study presents a series of six new coordination polymers, ICR-20 and ICR-21, incorporating Fe²⁺, Co²⁺, and Ni²⁺ metal centers, using phosphinate (H₂PBP-(Me)) or phosphinate-phosphonate (H₃PPP-(Me)) ligands in combination with 4,4'-bipyridine. The materials are isoreticular despite the incorporation of different functional groups, demonstrating the interchangeability of the phosphinate and phosphonate groups in their design. These polymers were characterized structurally and investigated for their magnetic properties. The combination of local insights from Mössbauer spectroscopy and bulk magnetic data provides complex information on crystal field parameters and magnetic interactions in Fe-based polymers. Additionally, their proton conductivity was evaluated, showing promising results.