Bis(pyrazolato) Bridged Diiron Complexes: Ferromagnetic Coupling in a Mixed‐Valent HS‐Fe II /LS‐Fe III Dinuclear Complex

Using a new bis(tridentate) compartmental pyrazolate‐centered ligand HL, the bis(pyrazolato)‐bridged diiron complex [L2FeII2][OTf]2 (1) and its singly oxidized mixed‐valent congener [L2FeIIFeIII][OTf]3 (2) have been synthesized and structurally characterized. While 1 features two HS‐FeII ions coordinated to two cis‐axial pyridine moieties in a highly distorted octahedral environment, the metal ions in 2 are coordinated by the ligand strand in a trans‐axial configuration. Very different Fe–N bond lengths and distinctly different coordination polyhedra are associated with pronounced valence localization in the case of 2. The electronic structures and magnetic properties of 1 and 2 have been further investigated by Mössbauer spectroscopy and variable temperature magnetic susceptibility measurements. In the case of 1, weak antiferromagnetic coupling is observed between the two HS‐FeII ions (J = –0.6 cm–1), while the HS‐FeII and LS‐FeIII ions in 2 are ferromagnetically coupled (J = +5.2 cm–1) to give an ST = 5/2 ground state with significant zero‐field splitting (DFe(II) = 2.3 cm–1). The findings are rationalized with the help of DFT computations.

Dinuclear iron complexes of a new pyrazolate‐based ligand with flexible side arms were synthesized in both the FeII2 and mixed‐valent FeIIFeIII forms. Upon oxidation, large structural changes were observed, associated with a high‐spin to low‐spin transition and charge localization. The different magnetic signatures of the two compounds were evidenced by a suite of experimental methods and investigated by DFT calculations. image John Wiley \u0026 Sons, Ltd.