Electron Hopping through Double-Exchange Coupling in a Mixed-Valence Diiminobenzoquinone-Bridged Fe₂ Complex

The ability of a benzoquinonoid bridging ligand to mediate double-exchange coupling in a mixed-valence Fe₂ complex is demonstrated. Metalation of the bridging ligand 2,5-di(2,6-dimethylanilino)-3,6-dibromo-1,4-benzoquinone (LH₂) with Fe^II in the presence of the capping ligand tris((6-methyl-2-pyridyl)methyl)amine (Me₃TPyA) affords the dinuclear complex [(Me₃TPyA)₂Fe^II₂(L)]²⁺. The dc magnetic measurements, in conjunction with X-ray diffraction and Mössbauer spectroscopy, reveal the presence of weak ferromagnetic superexchange coupling between Fe^II centers through the diamagnetic bridging ligand to give an S = 4 ground state. The ac magnetic susceptibility measurements, collected in a small dc field, show this complex to behave as a single-molecule magnet with a relaxation barrier of U_eff = 14(1) cm^-1. The slow magnetic relaxation in the Fe^II₂ complex can be switched off through one-electron oxidation to the mixed-valence congener [(Me₃TPyA)₂Fe₂(L)]³⁺, where X-ray diffraction and Mössbauer spectroscopy indicate a metal-centered oxidation. The dc magnetic measurements show an S = ⁹/₂ ground state for the mixed-valence complex, stemming from strong ferromagnetic exchange coupling that is best described considering electron hopping through a double-exchange coupling mechanism, with a double-exchange parameter of B = 69(4) cm^-1. In accordance with double-exchange, an intense feature is observed in the near-infrared region and is assigned as an intervalence charge-transfer band. The rate of intervalence electron hopping is comparable to that of the Mössbauer time scale, such that variable-temperature Mössbauer spectra reveal a thermally activated transition from a valence-trapped to detrapped state and provide an activation energy for electron hopping of 63(8) cm^-1. These results demonstrate the ability of quinonoid ligands to mediate electron hopping between high-spin metal centers, by providing the first example of an Fe complex that exhibits double-exchange through an organic bridging ligand and the largest metal-metal separation yet observed in any metal complex with double-exchange coupling.