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

We report the synthesis of the novel heterometallic complex [Fe(3)Cr(L)(2)(dpm)(6)]center dot 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 vertical bar D vertical bar parameter is considerably reduced as compared with the corresponding tetra-iron(III) propeller (D = -0.1.79 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.