Bipyridine-triggered modulation of structure and properties of zinc-diphosphonates: coordination role vs. template rule

Abstract

Two zinc-diphosphonates driven by 4,4′-bipyridine (4,4′-bipy), [Zn₄(HEDP)₂(4,4′-bipy)₂]·4.5(H₂O) (1) and (H₂-4,4′-bipy)_0.5·[Zn₃(HEDP)(H₂EDP)(H₂O)]·1.5(H₂O) (2), were solvothermally prepared (HEDP = CH₃C(OH)(PO₃)₂, 1-hydroxyethylidenediphosphonate). Compound 1 features a layered structure with the Zn-HEDP chains as supramolecular units and 4,4′-bipy as interchain linkers. Compound 2 possesses a Zn-HEDP layer with protonated 4,4′-bipy as template. The structure differences between the two compounds are mainly ascribed to the distinct role of 4,4′-bipy in the process of in situ assembly. In compound 1, the 4,4′-bipy moiety acts as coordinative unit to participate in the formation of the resulting architecture. However, in compound 2, the protonated 4,4′-bipy serves as template to direct the formation of the final structure. The luminescence and proton conduction of the two compounds were investigated. Compounds 1 and 2 exhibit intense blue fluorescence; compound 2 has a proton conductivity of 2.61 × 10⁻⁴ S cm⁻¹ at 100% relative humidity and 70 °C.