A Ferric Semiquinoid Single-Chain Magnet via Thermally-Switchable Metal–Ligand Electron Transfer

We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally induced metal–ligand electron transfer. Reaction of FeCl3 with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) in the presence of (NMe4)Cl gave the compound (NMe4)2[LFeCl2]. Together, variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L2–)­FeII units to one with (L3–•)­FeIII upon decreasing temperature, with a transition temperature of T1/2 = 213 K. The dc magnetic susceptibility measurements show strong metal–radical coupling within the chain, with a coupling constant of J = −81 cm–1, and ac susceptibility data reveal slow magnetic relaxation, with a relaxation barrier of Δτ = 55(1) cm–1. To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet.