Zinc(ii) and cadmium(ii) carboxyphosphonates with a 3D pillared-layered structure: synthesis, crystal structures, high thermal stabilities and luminescent properties

Abstract

Two novel divalent metal carboxyphosphonates with a 3D pillared-layered structure, [Zn(HL)] (1) and [CdCl(H₂L)] (2) (H₃L = 4-HOOCC₆H₄CH₂NHCH₂PO₃H₂), have been synthesized under hydrothermal conditions. Compounds 1 and 2 both feature three-dimensional (3D) framework structures with two-dimensional (2D) inorganic layers pillared by H₃L. In compound 1, each {ZnO₄} tetrahedron is linked via corner-sharing by three {CPO₃} tetrahedra to form a two-dimensional (2D) inorganic layer in the ac-plane. The adjacent layers are further pillared by the organic backbone {–C₆H₄CH₂NHCH₂–} of the carboxyphosphonate ligand, generating a 3D pillared-layered structure. For compound 2, each Cd(II) cation is six-coordinated, and the interconnection of two {CdCl₂O₄} octahedra forms a dimer via edge-sharing. The dimers are interconnected by {CPO₃} tetrahedra via corner-sharing to form a 2D inorganic layer in the ab-plane. Such neighboring 2D inorganic layers are further cross-linked through the organic pillars of the carboxyphosphonate ligand, thus resulting in a 3D pillared-layered structure. An interesting feature of compounds 1 and 2 is the presence of alternate left- and right-handed helical chains in their structures. It is of note that two title compounds are both stable up to high temperature. The luminescent properties of compounds 1 and 2 have also been studied.