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.

Surface-supported arrays of Fe(4)-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe(4)(L)(2)(dpm)(6)] (1) on Au, an isomorphous compound [Fe(3)Cr(L)(2)(dpm)(6)] was synthesized and structurally and magnetically characterized (H(3)L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr(3+) ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L(2,3) edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L(2,3) spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe(3+) ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.

Iron(III)-based single-molecule magnets have been covalently grafted on Au(111) in one step using 1,2-dithiolan-3-yl side groups. Reaction with the substrate quantitatively affords a monolayer of electronically intact clusters doubly linked to the surface via Au-S bonds, as demonstrated by a combination of STM, XAS/XMCD and XPS studies.