An Azophenine Radical-Bridged Fe₂ Single-Molecule Magnet with Record Magnetic Exchange Coupling

One-electron reduction of the complex [(TPyA)2FeII2(NPhL2–)]2+ (TPyA = tris­(2-pyridylmethyl)­amine, NPhLH2 = azophenine = N,N′,N″,N‴-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)2FeII2(NPhL3–•)]+. X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on NPhL2– to give an S = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between S = 2 FeII centers and NPhL3–• in the reduced complex, giving an S = 7/2 ground state with an estimated coupling constant magnitude of |J| ≥ 900 cm–1. Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of Ueff = 50(1) cm–1. To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet.