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 FeII Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine

摘要

A series of [Fe~(II)( L )_(2)](BF_(4))_(2) 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~(II)( L )_(2)](BF_(4))_(2) compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF_(4) anions as in typical ionic crystals.