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, γHS1 = 93%) with a small admixture of low-spin (LS) state (S = 1/2, γLS1 = 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 (γHS2 = 76%, γLS2 = 24%), but also by the appearance of AFM interactions (θII = −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 (γHS ~ 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.