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Synthesis, Magnetic Properties, and STM Spectroscopy of Cobalt(II) Cubanes [Co II 4 (Cl) 4 (HL) 4 ]
Author(s) -
Scheurer Andreas,
Ako Ayuk M.,
Saalfrank Rolf W.,
Heinemann Frank W.,
Hampel Frank,
Petukhov Konstantin,
Gieb Klaus,
Stocker Michael,
Müller Paul
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902578
Subject(s) - cubane , chemistry , cobalt , magnetization , crystallography , tetragonal crystal system , magnetic anisotropy , ferromagnetism , magnetic susceptibility , crystal structure , condensed matter physics , inorganic chemistry , magnetic field , physics , quantum mechanics
Abstract Reaction of cobalt(II) chloride hexahydrate with N‐substituted diethanolamines H 2 L 2–4 ( 3 ) in the presence of LiH in anhydrous THF leads under anaerobic conditions to the formation of three isostructural tetranuclear cobalt(II) complexes [Co II 4 (Cl) 4 (HL 2–4 ) 4 ] ( 4 ) with a [Co 4 (μ 3 ‐O) 4 ] 4+ cubane core. According to X‐ray structural analyses, the complexes 4 a , c crystallize in the tetragonal space group I 4 1 / a , whereas for complex 4 b the tetragonal space group P $\bar 4$ was found. In the solid state the orientation of the cubane cores and the formation of a 3D framework were controlled by the ligand substituents of the cobalt(II) cubanes 4 . This also allowed detailed magnetic investigations on single crystals. The analysis of the SQUID magnetic susceptibility data for 4 a gave intramolecular ferromagnetic couplings of the cobalt(II) ions ( J 1 ≈20.4 K, J 2 ≈7.6 K), resulting in an S =6 ground‐state multiplet. The anisotropy was found to be of the easy‐axis type ( D =−1.55 K) with a resulting anisotropy barrier of Δ ≈55.8 K. Two‐dimensional electron‐gas (2DEG) Hall magnetization measurements revealed that complex 4 a is a single‐molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep‐rate dependencies below a blocking temperature of about 4.4 K. The hysteresis loops collapse at zero field owing to fast quantum tunneling of the magnetization (QTM). The structural and electronic properties of cobalt(II) cubane 4 a , deposited on a highly oriented pyrolytic graphite (HOPG) surface, were investigated by means of STM and current imaging tunneling spectroscopy (CITS) at RT and standard atmospheric pressure. In CITS measurements the rather large contrast found at the expected locations of the metal centers of the molecules indicated the presence of a strongly localized LUMO.