pH-Guided Design of Cu(II) Phosphonopropionate Coordination Polymers: Synthesis, Structures, Proton Conductivity, and Catalytic Properties

Abstract

A series of new Cu(II) phosphonopropionate coordination polymers (CPs) was obtained from reaction of CuCl2 and H3PPA (3-phosphonopropionic acid) in water under reflux conditions at different pH values (2, 3, 4, and 6). The resulting compounds, formulated as [Cu(μ4-HPPA)(μ-H2O)]n (Cu@pH2), [Cu(μ4-HPPA)(H2O)]n (Cu@pH3), [Cu(μ5-HPPA)]n (Cu@pH4), and [Cu2(PPA)(OH)]n·3nH2O (Cu@pH6), were isolated as stable microcrystalline solids and fully characterized. The crystal structures of the first three compounds were solved from laboratory powder X-ray diffraction data. Their pH-guided structural differences are related to coordination modes of the bridging or terminal water ligand (or its absence), as well as to bridging modes of μ4-HPPA2–/μ5-HPPA2– blocks, resulting in 2D layers with 4,6L26, sql, or 5,5L4 topologies. Proton transport properties of the obtained compounds were investigated, revealing the highest proton conductivity for Cu@pH6 (1.2 × 10–3 S·cm–1 at 90 °C and 95% relative humidity), with proton transport occurring predominantly through a water-mediated Grotthuss-type mechanism. In addition, Cu(II) phosphonopropionates were screened as efficient catalysts in the mild oxidation of cycloalkanes and propane. The effects of different reaction parameters, substrate scope and selectivity features were investigated. The present study expands the family of Cu(II) phosphonate CPs and highlights the prospective use of these compounds in proton conduction and catalysis.