The {FeIII[FeIII(L1)2]3} star-type single-molecule magnet

Rolf W. Saalfrank; Andreas Scheurer; Ingo Bernt; Frank W. Heinemann; Andrei V. Postnikov; Volker Schünemann; Alfred X. Trautwein; Mohammad S. Alam; Holger Rupp; Paul Müller

doi:10.1039/B515980F

The {Fe^III[Fe^III(L¹)₂]₃} star-type single-molecule magnet†‡§

Rolf W. Saalfrank,*^a Andreas Scheurer,^a Ingo Bernt,^a Frank W. Heinemann,^b Andrei V. Postnikov,^cg Volker Schünemann,^d Alfred X. Trautwein,^e Mohammad S. Alam,^f Holger Rupp^f and Paul Müller*^f

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* Corresponding authors

^a Institut für Organische Chemie, Universität Erlangen-Nürnberg, Henkestr. 42, 91054, Erlangen, Germany
E-mail: saalfrank@chemie.uni-erlangen.de
Fax: +49-9131-852-1165
Tel: +49-9131-852-2554

^b Institut für Anorganische Chemie der Universität Erlangen-Nürnberg, 91058 Erlangen, Germany

^c Institut für Festkörperforschung, Forschungszentrum Jülich, 52452 Jülich, Germany

^d Fachbereich Physik, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany

^e Institut für Physik der Medizinischen Universität Lübeck, 23538 Lübeck, Germany

^f Physikalisches Institut III, Universität Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
E-mail: phm@physik.uni-erlangen.de
Fax: +49-9131-15249
Tel: +49-9131-852-7272

^g Institute of Metal Physics, S. Kowalewskoi 18, 620219 Yekaterinburg, Russia

Abstract

Star-shaped complex {Fe^III[Fe^III(L¹)₂]₃} (3) was synthesized starting from N-methyldiethanolamine H₂L¹ (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mössbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3·4CHCl₃ contains the enantiomers (Δ)-[(S,S)(R,R)(R,R)] and (Λ)-[(R,R)(S,S)(S,S)], whereas in case of 3·3CH₂Cl₂ four independent molecules, forming pairs of the enantiomers [Λ-(R,R)(R,R)(R,R)]-3 and [Δ-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

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The {Fe^III[Fe^III(L¹)₂]₃} star-type single-molecule magnet†‡§

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