The First 1,3,4-Oxadiazole Based Dinuclear Iron(II) Complexes Showing Spin Crossover Behavior with Hysteresis

Three new dinuclear complexes [FeII2(μ‐L)2]X4 (L is the bis‐tridentate ligand 2,5‐bis{[(2‐pyridylmethyl)amino]methyl}‐1,3,4‐oxadiazole and X = ClO4–, BF4– and CF3SO3–) have been synthesized and fully characterized by single‐crystal X‐ray diffraction, Mössbauer spectroscopy and magnetic susceptibility measurements. Upon cooling, a trapped [high‐spin–low‐spin] state of the iron(II) centres is detected. Depending on the counterion, a pronounced thermal hysteresis is found. In one case, it was possible to observe a space group change that accompanies the spin transition. This is the first system showing spin crossover based on an oxadiazole ligand. Herein we present a new 1,3,4‐oxadiazole bridging ligand and three spin‐crossover active dinuclear iron(II) compounds. For two of the complexes a trapped high‐spin/low‐spin state at temperatures below 170 K was proven. Moreover, the spin state switching could be monitored crystallographically for one complex. The other compound shows a 26 K wide hysteresis in the region of 150–175 K.