Spin Crossover in Dimethyl Sulfoxide Solutions of an Iron(II) Perrhenate Complex with 2,6-Bis(Benzimidazol-2-yl)Pyridine

Abstract—Dimethyl sulfoxide (DMSO) solutions of [FeL₂](ReO₄)₂⋅1.5H₂O (1), the iron(II) perrhenate complex with 2,6-bis(benzimidazol-2-yl)pyridine (L), are characterized by physicochemical methods, namely, by static magnetic susceptibility measurements, conductometry, and UV–Vis spectrophotometry. As found previously, complex 1 exhibits a sharp high-temperature spin crossover (SCO): ¹А₁ ↔ ⁵Т₂. The temperature-dependent μ_eff(Т) study of complex 1 in DMSO showed that the SCO also manifests itself in solution. The 298 K electrical conductivity of DMSO solutions implies that complex 1 is almost completely dissociated in the range of concentrations studied from 3.6 × 10^–6 to 9.12 × 10^–4 mol/L. An absorption peak is found in the UV region of the spectrum, which is almost independent of temperature. In the visible region, two combined absorption peaks are observed at 520–560 nm, responsible for FeL²⁺ and \({\text{FeL}}_{{\text{2}}}^{{2 + }}\) complex formation and varying with temperature and L concentration.