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Slow Magnetic Relaxation from Hard‐Axis Metal Ions in Tetranuclear Single‐Molecule Magnets
Author(s) -
Tancini Erik,
RodriguezDouton Maria Jesus,
Sorace Lorenzo,
Barra AnneLaure,
Sessoli Roberta,
Cornia Andrea
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.201001040
Subject(s) - chromium , electron paramagnetic resonance , crystallography , single molecule magnet , magnetization , magnetic anisotropy , magnet , chemistry , metal , ion , molecule , manganese , materials science , nuclear magnetic resonance , magnetic field , physics , organic chemistry , quantum mechanics
We report the synthesis of the novel heterometallic complex [Fe 3 Cr(L) 2 (dpm) 6 ]⋅Et 2 O ( Fe 3 CrPh ) (Hdpm=dipivaloylmethane, H 3 L=2‐hydroxymethyl‐2‐phenylpropane‐1,3‐diol), obtained by replacing the central iron(III) atom by a chromium(III) ion in an Fe 4 propeller‐like single‐molecule magnet (SMM). Structural and analytical data, high‐frequency EPR (HF‐EPR) and magnetic studies indicate that the compound is a solid solution of chromium‐centred Fe 3 Cr ( S =6) and Fe 4 ( S =5) species in an 84:16 ratio. Although SMM behaviour is retained, the | D| parameter is considerably reduced as compared with the corresponding tetra‐iron(III) propeller ( D =−0.179 vs. −0.418 cm −1 ), and results in a lower energy barrier for magnetisation reversal ( U eff / k B =7.0 vs. 15.6 K). The origin of magnetic anisotropy in Fe 3 CrPh has been fully elucidated by preparing its Cr‐ and Fe‐doped Ga 4 analogues, which contain chromium(III) in the central position (c) and iron(III) in two magnetically distinct peripheral sites (p1 and p2). According to HF‐EPR spectra, the Cr and Fe dopants have hard‐axis anisotropies with D c =0.470(5) cm −1 , E c =0.029(1) cm −1 , D p1 =0.710(5) cm −1 , E p1 =0.077(3) cm −1 , D p2 =0.602(5) cm −1 , and E p2 =0.101(3) cm −1 . Inspection of projection coefficients shows that contributions from dipolar interactions and from the central chromium(III) ion cancel out almost exactly. As a consequence, the easy‐axis anisotropy of Fe 3 CrPh is entirely due to the peripheral, hard‐axis‐type iron(III) ions, the anisotropy tensors of which are necessarily orthogonal to the threefold molecular axis. A similar contribution from peripheral ions is expected to rule the magnetic anisotropy in the tetra‐iron(III) complexes currently under investigation in the field of molecular spintronics.