Fluorescent metal-organic polymers of zinc and cadmium from hydrothermal in situ acylation reaction

A series of metal-organic complexes based on d¹⁰ metals and the ligand H₄bbh (H₄bbh = benzene-1, 2, 4, 5-biformhydrazide), formed through hydrothermal in situ acylate reaction of H₄bta (H₄bta = benzene-1, 2, 4, 5-tetracarboxylic acid) with hydrazine hydrate (N₂H₄·H₂O), have been prepared and structurally characterized by single-crystal X-ray diffraction. Compounds [Zn(μ₂-H₂bbh)(phen)(H₂O)]₂ (1) (phen = 1, 10-phenanthroline) and [Zn(μ₂-H₂bbh)(2, 2′-bpy)]₂ (2) (2, 2′-bpy = 2, 2′-bipyridine) are both dinuclear complexes in which bridging ligands H₂bbh^2- display different μ₂- coordination modes. [Zn(μ₂- H₂bbh)_1/2(μ₂-H₂bbh) _1/2(H₂O)]_n (3) exhibits a two-dimensional (2-D) layer structure containing simultaneously two kinds of different coordination modes of H₂bbh^2-: μ₂-bidentate and μ₄-tetradentate. [Cd(μ₃-H₂bbh)(phen)] _n (4) consists of one-dimensional (1-D) double-metal chains. The crystal structures of these compounds are stabilized by hydrogen bonds and π⋯π interactions, forming three-dimensional supramolecular networks. All of the compounds were characterized by IR, UV-vis spectra and elemental analysis and they show good fluorescence properties in the solid state at room temperature. In order to understand the emission mechanism, we carried out TDDFT calculations on the excited electronic states of compound 2.