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.
- 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
Available at: http://works.bepress.com/fernande-grandjean/208/