Synthesis, structures, luminescent and molecular recognition properties of three new alkaline earth metal carboxyphosphonates with a 3D supramolecular structure

Abstract

Three new alkaline earth metal carboxyphosphonates with a 3D supramolecular structure, namely, Ca[(H₃L)(H₂O)] (1), Sr[(H₃L)(H₂O)₂] (2) and Ba[(H₃L)(H₂O)] (3) (H₅L = 4-{[bis(phosphonomethyl)amino]methyl}benzoic acid), have been synthesized under hydrothermal conditions. For compound 1, {CaO₆} and {CPO₃} polyhedra are interconnected into a 1D double-chain via corner-sharing. Adjacent chains are further cross-linked via the organic backbone {–C₆H₄CH₂N(CH₂)₂–} of the carboxyphosphonate ligands, generating a 2D layer. These neighboring 2D metal phosphonate layers are connected through hydrogen bonding interactions to form a 3D supramolecular structure. In compound 2, two edge-sharing {SrO₇} polyhedra form centrosymmetric {Sr₂O₁₂} units. Then, these {Sr₂O₁₂} units are connected by phosphonate groups to form a 1D chain, which is further cross-linked by N atoms from carboxyphosphonate ligands to form a 2D inorganic layer. These 2D inorganic layers interdigitate through hydrogen bonding interactions to generate a 3D supramolecular structure. In compound 3, the connections of {BaO₁₀} and {CPO₃} polyhedra form a 2D layer via corner-sharing and edge-sharing. Then adjacent layers are further assembled into a 3D supramolecular structure through hydrogen bonding interactions. The thermal stabilities and luminescence properties of compounds 1–3 were investigated. Interestingly, compounds 1–3 are selective and reversible for sensing of 1-butanol, 1-propanol and ethanol, respectively.