Slow Phase Transition-Induced Scan Rate Dependence of Spin Crossover in a Two-Dimensional Supramolecular Fe(III) Complex

Spin crossover (SCO) is commonly accompanied by a synchronous phase transition. A few phase transitioncoupled SCO compounds have been reported, yet the synergy between SCO and phase transition on different time scales has not been explored. Herein, we report an [Fe(H-5-Cl-thsa-Et)(5-Cl-thsa-Et)]center dot H2O (1 center dot H2O; H-2-5-Cl-thsa-Et = 5-chloro-salicylaldehyde ethylthiosemicarbazone) Fe(III) complex that displays a two-dimensional supramolecular structure and SCO behavior above room temperature. Its dehydrated form1 exhibits a two-step spin transition with aplateau in the temperature-dependent magnetization (M-T) curve at room temperature and a 51 K thermal hysteresis loop (T-c up arrow down arrow = 299/248 K) at a rate of 5 K/min. The improved SCOperformance in 1 could be attributed to the stronger intralayer but weaker interlayer interactions, which is supported by single-crystal structural analysis and density functional theory calculations. Remarkably, complex 1 displays an unusual scan rate-dependent SCO behavior at rates of 0.5-30 K/ min, in whichM-T curveplateaus appear at lower scan rates (<10 K/min) but vanish at faster scan rates (>= 10 K/min). Scan rate-dependent differential scanning calorimetry, powder X-ray diffractometry, time-dependent magnetic moment decays, and infrared spectroscopy consistently reveal that the slow structural relaxation is coupled with a slowcrystallographic phase transition, which is the mechanism for the unusual scan rate-dependent SCO. [GRAPHICS] .