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München, Germany

Eichhofer A.,Karlsruhe Institute of Technology | Buth G.,Karlsruhe Institute of Technology | Dolci F.,European Commission | Fink K.,Karlsruhe Institute of Technology | And 2 more authors.
Dalton Transactions | Year: 2011

The first examples of polymeric homoleptic iron chalcogenolato complexes 1∞[Fe(SePh)2] and 1∞[Fe(SeMes)2] (Ph = phenyl = C6H5, Mes = mesityl = C6H2-2,4,6- (CH3)3) have been both prepared by reaction of [Fe(N(SiMe3)2)2] with two equivalents of HSeR (R = Ph, Mes) while 1∞[Fe(SePh)2] was found to be also easily accessible through reactions of either FeCl2, Fe(OOCCH 3)2 or FeCl3 with PhSeSiMe3 in THF. In the crystal, the two compounds form one-dimensional chains with bridging selenolate ligands comprising distinctly different Fe-Se-Fe bridging angles, namely 71.15-72.57° in 1∞[Fe(SePh)2] and 91.80° in 1∞[Fe(SeMes)2]. Magnetic measurements supported by DFT calculations reveal that this geometrical change has a pronounced influence on the antiferromagnetic exchange interactions of the unpaired electrons along the chains in the two different compounds with a calculated magnetic exchange coupling constant of J = -137 cm-1 in 1∞[Fe(SePh)2] and J = -20 cm-1 in 1∞[Fe(SeMes)2]. In addition we were able to show that the ring molecule [Fe(SePh)2]12 which is a structural isomer of 1∞[Fe(SePh)2] behaves magnetically similar to the latter one. Investigations by powder XRD reveal that the ring molecule is only a metastable intermediate which converts in THF completely to form 1∞[Fe(SePh)2]. Thermal gravimetric analysis of 1∞[Fe(SePh)2] under vacuum conditions shows that the compound is thermally labile and already starts to decompose above 30 °C in a two step process under cleavage of SePh2 to finally form at 250 °C tetragonal PbO-type FeSe. The reaction of 1∞[Fe(SePh)2] with the Lewis base 1,10-phenanthroline yielded, depending on the conditions, the octahedral monomeric complexes [Fe(SePh)2(1,10-phen)2] and [Fe(1,10-phen)3][Fe(SePh)4]. © 2011 The Royal Society of Chemistry.

Boeer A.B.,University of Manchester | Barra A.-L.,CNRS French National High Magnetic Field Laboratory | Chibotaru L.F.,Catholic University of Leuven | Collison D.,University of Manchester | And 7 more authors.
Angewandte Chemie - International Edition | Year: 2011

Structurally similar, magnetically diverse: The exchange and Zeeman parameters of two dimetallic, six-coordinate cobalt(II) compounds-i.e., with orbitally degenerate metal ions-have been determined by a combination of high-resolution EPR and INS spectroscopies, together with ab intio calculations. It is suggested that the correlation between the local crystal field about the CoII centers and the superexchange pathway controls the magnetic interaction. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mayer J.,ZWE FRM II | Mayer J.,TU Munich | Hugenschmidt C.,ZWE FRM II | Hugenschmidt C.,TU Munich | And 2 more authors.
Physical Review Letters | Year: 2010

Density functional theory calculations predict the surface segregation of Cu in the second atomic layer of Pd which has not been unambiguously confirmed by experiment so far. We report measurements on Pd surfaces covered with three and six monolayers of Cu using element selective positron-annihilation-induced Auger electron spectroscopy (PAES) which is sensitive to the topmost atomic layer. Moreover, time-resolved PAES, which was applied for the first time, enables the investigation of the dynamics of surface atoms and hence the observation of the segregation process. The time constant for segregation was experimentally determined to τ=1.38(0.21)h, and the final segregated configuration was found to be consistent with calculations. Time-dependent PAES is demonstrated to be a novel element selective technique applicable for the investigation of, e.g., heterogeneous catalysis, corrosion, or surface alloying. © 2010 The American Physical Society.

Mayer J.,ZWE FRM II | Mayer J.,TU Munich | Hugenschmidt C.,ZWE FRM II | Hugenschmidt C.,TU Munich | And 2 more authors.
Surface Science | Year: 2010

We present a high resolution positron annihilation induced Auger Electron Spectroscopy (PAES) of the CuM2,3VV-transition with the unprecedented energy resolution of δE/E<1%. This energy resolution and the highly intense positron source NEPOMUC enabled us to resolve the double peak structure with PAES for the first time within a measurement time of only 5.5 h. In addition, sub-monolayers of Cu were deposited on Feand Pd-samples in order to investigate the surface selectivity of PAES in comparison with EAES. The extremely high surface selectivity of PAES due to the different positron affinity of Cu and Fe lead to the result that with only 0.96 monolayer of Cu on Fe more than 55% of the emitted Auger electrons stem from Cu, whereas with EAES the Cu Auger fraction amounted to less than 6%. © 2010 Elsevier B.V. All rights reserved.

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