Effects of mono- or di-fluoro-substitution on spin crossover behavior in a pair of Schiff base-like FeII-coordination polymers

Abstract

Spin crossover (SCO) has long been a hot topic in the field of molecular magnetism owing to its unique bistability character. Rational control of thermal hysteresis and transition temperature (T_1/2) is crucial for their practical applications, which rely on precise manipulation of the substituents of SCO coordinating ligands and molecular packing interactions. In this study, we designed two different bridging ligands (2-FDPB: 4,4′-(2-fluoro-1,4-phenylene)dipyridine; 2,3-FDPB: 4,4′-(2,3-difluoro-1,4-phenylene)dipyridine) featuring one and two fluoro substitution on the central benzene ring and applied a Schiff base-like equatorial tetradentate ligand {diethyl(E,E)-2,2′-[4,5-difluoro-1,2-phenyl-bis(iminomethylidyne)]bis(3-oxobutanoate)-(2-)-N,N′,O3,O3′} (H₂L) to coordinate with the Fe^II ion. Two Fe^II-coordination chain polymers [Fe^II(L)(2,3-FDPB)]·0.25CH₂Cl₂ (1) and [Fe^II(L)(2-FDPB)]·0.5CH₃OH (2) were obtained. 1 crystallizes in the monoclinic P2₁/n space group with only one Fe^II center, while 2 crystallizes in the triclinic P space group with two independent Fe^II centers. Unlike the identical 2D layer stacking in 1, 2 exhibited alternating stacking of the extending 2D layers and meshed chains. Magnetic measurements revealed the typical thermally induced spin crossover behavior (SCO): 1 exhibited a 41 K-wide thermal hysteresis with transition temperatures of T_1/2↑ = 245 K and T_1/2↓ = 204 K, while 2 showed a higher transition temperature (T_1/2 = 330 K) with no thermal hysteresis. Magneto-structural correlation studies suggest that the electron-withdrawing effect present in the fluoro substituents does not have a significant impact on the SCO behaviors. Despite the fluoro substituents having a similar atomic radius of hydrogen atoms, variations in the number of these substituents can alter the crystallization behavior of these complexes, which in turn affects the solvents, molecular stacking patterns, and intermolecular interactions, ultimately influencing the SCO behaviors.