Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged Fe Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine.

A series of Fe(L) compounds were structurally and physically characterized (L = 2,6-bis(2-methylthiazol-4-yl)pyridine). A crystal structure phase transformation from dihydrate compound 1 to anhydrous compound 3 through partially hydrated compounds 2 and 2' upon dehydration was found. Compounds 1 and 3 exhibited a gradual spin crossover (SCO) conversion, whereas compounds 2 and 2' demonstrated two-step and one-step abrupt SCO transitions, respectively. An X-ray single-crystal structural analysis revealed that one-dimensional and two-dimensional Fe cation networks linked by π stacking and sulfur-sulfur interactions were formed in 1 and 3, respectively. A thermodynamic analysis of the magnetic susceptibility for 1, 2', and 3 suggests that the enthalpy differences may govern SCO transition behaviors in the polymorphic compounds 2' and 3. A structural comparison between 1 and 3 indicates that the SCO behavior variations and crystal structure transformation in the present Fe(L) compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF anions as in typical ionic crystals.