Synthesis, crystal structure and water-induced reversible crystal-to-amorphous transformation property of [Co2(2,4-pydc)2(bpa)(H2O)6](H2O)2

The dinuclear CoII coordination compound with empirical formula [Co2(2,4-pydc)2(bpa)(H2O)6](H2O)2 (1); bpa = 1,2-bis(2,4-pyridyl)ethane, 2,4-pydc = 2,4-pyridinedicarboxylate anion, has been synthesized and õcharacterized by elemental analysis, single crystal X-ray diffraction, IR and UV–vis spectra, TGA and XRPD. Compound 1 is a symmetry-related dinuclear compound consisting of two six coordinated cobalt atoms, one bpa ligand, two 2,4-pydc ligands and two lattice water molecules. Each CoII ion is coordinated by one oxygen atom and one nitrogen atom from 2,4-pydc bidentate chelate ligand, one nitrogen atom of bpa bidentate bridging ligand and three water molecules, giving a distorted octahedral geometry with CoN2O4 chromophore. The crystal packing of this compound reveals a novel 3D supramolecular network, formed by hydrogen-bonding and C–H⋯π interactions. Investigations of the dynamic structural transformation behavior demonstrate that the title compound exhibits a solvent-induced reversible crystal-to-amorphous transformation with chromotropism when exposed to water vapor. This indicates that the dehydrated amorphous form, Co2(2,4-pydc)2(bpa)(H2O)2 (1A), may be utilized as an indicator for humidity.