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 FeII in the presence of the capping ligand tris((6-methyl-2-pyridyl)methyl)amine (Me3TPyA) affords the dinuclear complex [(Me3TPyA)2FeII2(L)]2+. The dc magnetic measurements, in conjunction with X-ray diffraction and Mössbauer spectroscopy, reveal the presence of weak ferromagnetic superexchange coupling between FeII 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 Ueff = 14(1) cm-1. The slow magnetic relaxation in the FeII2 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.



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Center for High Performance Computing Research

Keywords and Phrases

Activation energy; Charge transfer; Electrons; Exchange coupling; Ferromagnetic materials; Ferromagnetism; Ground state; Infrared devices; Lanthanum compounds; Ligands; Magnetic susceptibility; Magnetic variables measurement; Magnetism; Metal complexes; Metals; Nematic liquid crystals; X ray diffraction; AC Magnetic susceptibility; Dc magnetic measurements; Double exchange coupling; Ferromagnetic exchange couplings; Intervalence charge transfer; Mixed valence complexes; Slow magnetic relaxations; Super-exchange coupling; Iron compounds; 2,5 di(2,6 dimethylanilino) 3,6 dibromo 1,4 benzoquinone; amine; benzoquinone derivative; diiminobenzoquinone; iron complex; ligand; lignd; quinone derivative; tris[(6methyl 2 pyridyl)methyl)amine; unclassified drug; electron; energy; magnetism; measurement; oxidation; spectroscopy; temperature; X ray diffraction

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