Influence of the Halogen Anion Combination on the Spin Crossover Properties of Iron(II) Triazole Complexes

In this paper we study a series of solid iron(II)-triazole spin crossover complexes of the formula [Fe(NH₂trz)₃]Br_2(x−1)Cl_2x, where the counterions bromide and chloride are present in the same structure in different ratios. Thus, the spin transition temperature also changes in the temperature range between the pure complexes [Fe(NH₂trz)₃]Br₂ and [Fe(NH₂trz)₃]Cl₂. Since these switching temperatures of the pure complexes are rather far apart, this allows us to examine how the SCO behavior of iron(II) triazole complexes with mixed counterions changes with different ratios. To achieve this, the complexes were synthesized with both counterions simultaneously using the salts FeBr₂ and FeCl₂ ⋅ 4 H₂O. The complexes were analyzed by IR and XRD measurements to confirm a successful synthesis. Energy dispersive X-ray spectroscopy (EDX) was further used to validate the results and to verify a homogeneous distribution of the mixed anions in all samples. This study reveals a linear switching behavior of the complexes depending on the substitution ratio with a temperature range between the homo ionic samples.