Slow Magnetic Relaxation from Hard-axis Metal Ions in Tetranuclear Single-Molecule Magnets

We report the synthesis of the novel heterometallic complex [Fe3Cr(L)2(dpm)6]⋅Et2O (Fe3CrPh) (Hdpm=dipivaloylmethane, H3L=2-hydroxymethyl-2-phenylpropane-1,3-diol), obtained by replacing the central iron(III) atom by a chromium(III) ion in an Fe4 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 Fe3Cr (S=6) and Fe4 (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 (Ueff/kB=7.0 vs. 15.6 K). The origin of magnetic anisotropy in Fe3CrPh has been fully elucidated by preparing its Cr- and Fe-doped Ga4 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 Dc=0.470(5) cm−1, Ec=0.029(1) cm−1, Dp1=0.710(5) cm−1, Ep1=0.077(3) cm−1, Dp2=0.602(5) cm−1, and Ep2=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 Fe3CrPh 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.