Heteroleptic β-Diketonate Fe3+ Complex: Spin-Crossover and Optical Characteristics

The heteroleptic halogen-substituted Fe³⁺ complex of formula [FeL₂Bipy]Cl, where L is 1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)-propane-1,3-dione and Bipy is 2,2′-bipyridine, was synthesized, and its optic and magnetic properties were studied. Magnetic measurements showed that the complex at high temperatures (T > 75 K) is predominantly in the high-spin (HS) state of Fe³⁺ ions (S = 5/2, γ_HS¹ = 93%) with a small admixture of low-spin (LS) state (S = 1/2, γ_LS¹ = 7%). At T* ≈ 46 K, a partial spin-crossover transition (SCO, 5/2↔1/2) occurs. This process is accompanied not only by a change in the magnetic state (γ_HS² = 76%, γ_LS² = 24%), but also by the appearance of AFM interactions (θ_II = −2.3 K) between neighboring Fe³⁺ ions. A theoretical model was proposed to describe magnetic susceptibility, χ(T). As a result of the analysis of the ground spin state M(H) at 2.0 K, it was established that the majority of the LS states, as well as part of the HS states of Fe³⁺ ions (γ_HS ~ 53%), do not participate in exchange interactions. EPR studies confirmed the presence of HS and LS Fe³⁺ centers and made it possible to isolate I-type and II-type HS centers, corresponding to strong low-symmetry and weak distorted octahedral fields. SCO was also detected and the temperature dependences of the EPR intensities, I(T), of the HS and LS centers were analyzed.