Ring-Opening Polymeric Single-Crystal-to-Single-Crystal Transformation in a Co(II) Compound

The reaction of the flexible ligand 2,5-bis((pyridin-2-ylmethyl)thio)-1,3,4-thiadiazole (bptz) with cobalt(II) chloride afforded a mononuclear cyclic compound [CoCl₂(bptz)]·1/2MeCN (1), which in the solid state quickly transformed through an irreversible ring-opening polymeric process to a one-dimensional compound [CoCl₂(bptz)] (2), triggered by the loss of lattice solvent molecules at room temperature. The transformation involved the reconstruction of coordination bonds and drastic changes in the ligand conformation, which thus resulted in vigorous lattice variations. Self-consistent-charge density-functional tight-binding (SCC-DFTB) calculations confirmed that this transformation was spontaneous. This result not only means the realization of solid-state single-crystal-to-single-crystal transformation, but also gives a deep insight into the mechanism of bond reconstruction.