Mixed-Valent Octanuclear Iron Oxide Complexes and a Fresh Look at the “Verwey Transition”

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Venue:OE 134

Dr. Raphael Raptis

Department of Chemistry and biochemistry Florida International University

Refreshments will be served at 1:15 PM

Abstract:

We will present structural, electrochemical and spectroscopic studies of a family of octanuclear complexes, [Fe8(µ4-O)4(µ-4-R-pz)12X4]—R = H, Cl, Br, Me, Et, Ph; X = Cl, Br, NCS, N3, OAr—containing a Fe4O4-cubane core, which can be reversibly reduced in four consecutive steps from an all-ferric, [Fe8], to four mixed-valence ferric/ferrous states, [Fe8]–2–3–/4– . The iron-oxide, Fe8(µ4-O)4-core of these complexes is a structural model of the repeat unit of the minerals magnetite, maghemite and ferrihydrite.

The [Fe8] species are antiferromagnetically coupled with a diamagnetic ground state. Spectroscopic (electronic, vibrational, Mössbauer, XPS) analysis of mixed-valent [Fe8]– species indicates partial charge delocalization over the four Fe4O4-cubane Fe- sites. The magnetic susceptibility data of [Fe8]– complexes have been modeled with the help of magnetization measurements at pulsed magnetic fields of 65 T and 100 T, allowing an estimation of their double-exchange parameters.

The twice reduced [Fe8]2– species with X = Cl, containing an iron-oxo core isoelectronic to magnetite (Fe3O4, an inverse spinel) shows upon cooling a transition from a delocalized to localized valence, over a temperature range overlapping with that of the “Verwey transition” of magnetite. In contrast the isoelectronic X = NCS species is valence-trapped with an electronic structure corresponding to a normal spinel. The spectroscopic and magnetic analysis of this species places the 100-year old issue of the Verwey transition of bulk magnetite into a new, molecular level perspective.

Biography:

Dr. Raphael Raptis received his B.S. in Chemistry from Aristotle Univ. of Thessaloniki, Greece and Ph.D. in Inorganic Chemistry from Texas A&M Univ., College Station. Dr. Raptis was a faculty at the Chem. Dept. of Univ. of Crete, Greece from 1993-1997, a visiting professor at the Chem. Dept. of Univ. of Texas at El Paso, TX from 1997-1998 and an Assistant, Associate and Full Professor at Chem. Dept. of Univ. of Puerto Rico, San Juan from 1998-2013. He joined the Department of Chemistry and Biochemistry at Florida International University in 2013. Dr. Raptis has authored over 100 peer-reviewed scientific publications and review articles and holds 3 patents. His research interests include synthetic and structural Inorganic Chemistry, polynuclear transition metal complexes, relationships between structure and redox properties, MRI contrast agents, X-ray crystallography.

The event is free and open to the public.