Chiral and achiral imidazole-linked tetrahedral zinc phosphonate frameworks with photoluminescent properties

Presented here are three imidazole-linked tetrahedral zinc phosphonate frameworks based on a special ligand 2-(1-imidazole)-1-hydroxyl-1,1'-ethylidenediphosphonic acid (= ImhedpH4) with both an organic imidazole linker and inorganic phosphonate groups. Three new compounds, namely, [Zn2(ImhedpH2)2(ox)](H2ppz) (1), Zn3(ImhedpH)2(H2O)4 (2) and [Zn3(Imhedp)2](H2ppz) (3) (ox = oxalate and ppz = piperazine), were obtained by hydrothermal synthesis and structurally characterized by single-crystal X-ray diffraction. Compound 1 shows a chain structure with the ox ligands linking the [Zn2(ImhedpH2)2] units, and the resulting chains are further connected to form a 3D structure through the strong N-H···O and O-H···O hydrogen bonds between the neighboring chains. Compound 2 displays an inorganic layer structure with dangling imidazole units, where these protonated imidazoles also act as pillars between adjacent inorganic layers. Compound 3 features an unusual chiral three-dimensional (3D) Zn-Imhedp framework with guest H2ppz(2+) cations. It can be topologically represented as a 4-connected qtz network. Furthermore, the luminescent properties of the three compounds were investigated in the solid state.