Polarization Change via Spin Crossover and Valence Tautomerism Behavior in [FeGa] Complexes

Abstract

Macroscopic polarization switching achieved through directional electron transfer or spin crossover (SCO) has attracted considerable attention because of its potential applications in data storage devices, sensors, and energy converters. Here, by substituting the central metal and anion of a reported compound, [Fe(RR-cth)Co(SS-cth)(μ-dhbq)](AsF6)3, we obtained two dinuclear compounds that crystallize in the polar space group, P21, [Fe(RR-cth)Ga(SS-cth)(μ-dhbq)] (X)3 (X = AsF6 and ClO4). Among them, complex 1(AsF6)3 exhibited a two-step magnetic transition, including SCO behavior near 50 K and a gradual valence tautomerism (VT) behavior that occurred over a wide temperature range up to at least 250 K. The SCO behavior of complex 1(AsF6)3 can be triggered by changes in temperature or the application of magnetic fields, leading to a polarization change of approximately 0.5 μC cm−2. The transition entropy of 55.9 J K−1 mol−1 during the SCO process was obtained using the Clausius–Clapeyron equation. Contrarily, complex 1(ClO4)3 only exhibited gradual VT behavior from a low temperature to room temperature. Single-crystal X-ray diffraction, variable-temperature infrared spectroscopy, and pyroelectric measurements confirmed the gradual VT process that occurs over a wide temperature range in both complexes. This study broadens the range of polarization switching molecular systems by replacing the central metal and anion, thereby facilitating the development of multifunctional molecular materials. (Figure Presented)