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.