Oxalate-extended Cd2+–acylhydrazidate coordination polymers: synthesis, structure and fluorescence property

Abstract

Two new oxalate-propagated Cd²⁺–acylhydrazidate coordination polymers [Cd₂(ox)_0.5(Hpth)(pth)(bpy)₂] (ox = oxalate, pth = phthalhydrazidate, bpy = 2,2′-bypyridine) 1 and [Cd₂(ox)(pdh)₂]·H₂O (pdh = pyridine-2,3-dicarboxylhydrazidate) 2 were obtained by simple hydrothermal self-assemblies of Cd(CH₃COO)₂, aromatic dicarboxylic acids, N₂H₄, and oxalic acid with or without bpy. The acylhydrazidate molecules (pth, pdh) originated from the acylation of N₂H₄ with aromatic dicarboxylic acids. X-ray analysis revealed that in both compounds, ox acts as the second linker, extending the metal–acylhydrazidate oligomers (tetranuclear for 1; dinuclear for 2) into the high-dimensional coordination polymers. In compound 1, the monoacylhydrazidate molecule with a −2 oxidation state was observed for the first time. In the solid state, only compound 2 emits light, while in an aqueous solution, both compounds emit light. The density functional theory (DFT) calculations indicate that whether in the solid state or in an aqueous solution, the emissions for compound 2 are both assigned to the charge transfer within the pdh molecule, corresponding to the charge transfer from the π* orbitals of the pyridine ring moiety to the π orbitals of the acylhydrazidate ring moiety. The solvent effect of water causes the blue shift from 525 nm to 405 nm. The blue-light emission (465 nm) for compound 1 in an aqueous solution is assigned to the charge transfer between the pth molecules, corresponding to the charge transfer from the π* orbitals of the benzene ring moiety of one pth molecule to the π orbitals of the acylhydrazidate ring moiety of another pth molecule. The Cd²⁺ center provides the path for charge transfer.