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

One-electron reduction of the complex [(TPyA)₂Fe^II₂(^NPhL^2–)]²⁺ (TPyA = tris­(2-pyridylmethyl)­amine, ^NPhLH₂ = azophenine = N,N′,N″,N‴-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)₂Fe^II₂(^NPhL^3–•)]⁺. X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on ^NPhL^2– 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 Fe^II centers and ^NPhL^3–• 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 U_eff = 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.