title: Double exchange in a mixed-valent octanuclear iron cluster, [Fe8(μ4-O)4(μ-4-Cl-pz)12Cl4](-).
abstract: A combination of SQUID and pulsed high-field magnetometry is used to probe the nature of mixed valency in an Fe(II)Fe7(III) cluster. DFT-computed spin Hamiltonian parameters suggest that antiferromagnetic coupling dominates, and that electron transfer both between the four irons of the cubane core (t1) and between a cubane and three neighboring irons (t2) is significant. Simulations using the computed parameters are able to reproduce the key features of the measured effective magnetic moment, μeff(T), over the 2 < T < 300 K temperature range. In contrast, the field dependence of the molar magnetization, Mmol, measured at 0.4 K is inconsistent with substantial electron transfer: only values of t2∼ 0 place the separation between ground and first excited states in the region indicated by experiment. The apparent quenching of the cubane-outer electron transfer at very low temperatures indicates that vibronic coupling generates one or more shallow minima on the adiabatic potential energy surfaces that serve to trap the itinerant electron in the cubane core.
authors.affiliation: Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK. john.mcgrady@chem.ox.ac.uk.
authors.forename: Ekaterina M
authors.initials: EM
authors.lastname: Zueva
authors.forename: Radovan
authors.initials: R
authors.lastname: Herchel
authors.forename: Serguei A
authors.initials: SA
authors.lastname: Borshch
authors.forename: Evgen V
authors.initials: EV
authors.lastname: Govor
authors.forename: W M C
authors.initials: WM
authors.lastname: Sameera
authors.forename: Ross
authors.initials: R
authors.lastname: McDonald
authors.forename: John
authors.initials: J
authors.lastname: Singleton
authors.forename: Jurek
authors.initials: J
authors.lastname: Krzystek
authors.forename: Zdeněk
authors.initials: Z
authors.lastname: Trávníček
authors.forename: Yiannis
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authors.lastname: Sanakis
authors.forename: John E
authors.initials: JE
authors.lastname: McGrady
authors.forename: Raphael G
authors.initials: RG
authors.lastname: Raptis