Modification of the Spin Transition Properties in Hetero-Anionic Iron(II)-Triazole Complexes

The phenomenon which is called spin crossover is known to occur in some coordination compounds with an octahedral ligand field and electron configurations from 3d4 to 3d7. Thereby, a reversible transition between spin states (high spin and low spin state) is possible, through several external stimuli. Iron(II) triazole complexes exhibit this phenomenon at a wide range of temperatures depending on the ligands and anions used. For this reason, they are often considered for several possible practical applications. It is also possible to combine ligands or anions to modify the transition temperature. The latter of which was rarely discussed in the past. In this study we synthesized a series of iron(II)-4-Aminotriazole complexes, with different ratios of chloride- and tetrafluoroborate-anions, of the formula [Fe(Atrz)3]Cl2-X(BF4)X. We show that the combination of these anions leads to transition temperatures between those of their corresponding pure anion complexes. We furthermore present that a simple modification of the synthesis leads to a possible easy way of fine-tuning transitions temperatures.