Supramolecular Tuning of Spin Crossover Properties in Isostructural Cocrystal Solvates

In order to improve strategies for the design of spin-crossover materials, it is necessary to develop and understand structure–property relationships, and this is best achieved by obtaining isostructural materials. In this work we synthesized a series of four isostructural cocrystal solvates using the Fe(3-bpp)2A2 complex and the 2,2-dipyridyl disulfide coformer in methanol and ethanol (A = BF4– or PF6–). The spin-crossover properties of the materials were determined by variable temperature single-crystal X-ray diffraction. They show the same SCO behavior but with shifted spin-crossover temperatures, which has been related to the hydrogen bond basicity, pKBHX of the anions, and solvents present. This relationship between hydrogen bond basicity and the spin-crossover transition temperature offers a design strategy for the supramolecular tuning of spin-crossover properties in specific isostructural materials.