Reversible Colossal Barocaloric Effect of a New Fe II Molecular Complex with Low Hysteretic Spin Crossover Behavior

Barocaloric cooling, that is, lowering the temperature of a material under pressure action, is an attractive solid‐state effect that can potentially compete with volatile gas‐based cooling. To observe the barocaloric effect (BCE), it is necessary for materials to have high‐entropy, low‐hysteretic phase transitions with a large volume change between phases. Here details on a new Fe II complex [Fe(L)(NCS) 2 ], L = N 1 , N 3 ‐bis((1‐propyl‐1 H ‐1,2,3‐triazol‐4‐yl)methylene)‐2,2‐dimethylpropane‐1,3‐diamine) possessing spin crossover (SCO) behavior near room temperature with large entropy and volume change are reported, which provides high sensitivity to external pressure. The observed BCE effect, characterized using variable pressure calorimetry, powder X‐ray diffraction, UV–vis, IR, and Raman spectroscopy, shows a colossal isothermal entropy change of >100 J kg −1 K −1 and a reversible adiabatic temperature change of ≈16 K at a pressure of 1 kbar, demonstrating a high refrigerant efficiency compared to other solid‐state materials. These results stimulate further investigations of SCO materials as barocaloric refrigerants, which depend on the proper design of their constituent organic ligands.