title: Electron Hopping through Double-Exchange Coupling in a Mixed-Valence Diiminobenzoquinone-Bridged Fe2 Complex.
abstract: 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(II)2(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(II)2 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.
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
authors.forename: Alexandra I
authors.initials: AI
authors.lastname: Gaudette
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
authors.forename: Ie-Rang
authors.initials: IeR
authors.lastname: Jeon
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
authors.forename: John S
authors.initials: JS
authors.lastname: Anderson
authors.affiliation: Department of Chemistry, Missouri University of Science and Technology, University of Missouri , Rolla, Missouri 65409-0010, United States.
authors.forename: Fernande
authors.initials: F
authors.lastname: Grandjean
authors.affiliation: Department of Chemistry, Missouri University of Science and Technology, University of Missouri , Rolla, Missouri 65409-0010, United States.
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Long
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
authors.forename: T David
authors.initials: TD
authors.lastname: Harris