Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.

The use of high magnetic field and high frequency in an unconventional spectrometer has provided very informative EPR spectra of a manganese(III) octahedral complex for the first time. The parameters of the spin Hamiltonian operator are in fair agreement with those calculated with ligand‐field theory. High‐frequency EPR is thus a powerful tool for the structural investigation of complexes that contain metal ions with integer spins.

Rastertunnelmikroskopie wurde zur Detektion von individuellen Einzelmolekülmagneten auf Goldsubstraten eingesetzt (siehe schematische Formel). Das Adsorbat, [Mn12O12(L)16(H2O)4], wurde durch Abscheidung eines abgeschirmten Dodecamangan(III,IV)‐Clusters aus verdünnten Lösungen in THF/H2O/NH4OH erhalten. Das System kann als erste Stufe hin zu Informationsspeichern mit ultrahoher Speicherdichte auf der Grundlage von Einzelmolekülmagneten angesehen werden.

The reaction of ferric chloride and /?-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(1n) clusters [MFe,-(OCH,),,(L),]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(n1) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe,(OCH,),,] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by "Na and 'Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange inter-

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Two novel polynuclear manganese(II,III) complexes have been synthesized by exploiting controlled methanolysis. A one-pot reaction of MnCl(2), NaOMe, dibenzoylmethane (Hdbm), and O(2) in anhydrous methanol, followed by recrystallization from MeOH/CHCl(3) mixtures, afforded the alkoxomanganese complexes [Mn(7)(OMe)(12)(dbm)(6)].CHCl(3).14MeOH (2) and [Mn(2)(OMe)(2)(dbm)(4)] (3). Complex 2 crystallizes in trigonal space group R&thremacr; with a = 14.439(2) Å, alpha = 86.34(1) degrees, and Z = 1. Complex 3 crystallizes in triclinic space group P&onemacr; with a = 9.612(1) Å, b = 10.740(1) Å, c = 13.168(1) Å, alpha = 80.39(1) degrees, beta = 87.66(1) degrees, gamma = 83.57(1) degrees, and Z = 1. The solid-state structure of 2 comprises a [Mn(6)(OMe)(12)(dbm)(6)] "crown" with crystallographically imposed 6-fold symmetry plus a central manganese ion. The layered Mn/O core mimics a fragment of the manganese oxide mineral lithiophorite. Conductivity measurements confirmed the nonionic character of 2 and suggested a mixed-valence Mn(II)(3)Mn(III)(4) formulation. The metrical parameters of the core were analyzed with the aid of bond-valence sum calculations. The central ion is essentially a valence-trapped Mn(II) ion, whereas the average Mn-O distances for the manganese ions of the "crown" are consistent with the presence of two Mn(II) and four Mn(III) ions. However, (1)H NMR spectra in solution strongly support valence localization and suggest that the observed solid-state structure may be a result of static disorder effects. Magnetic susceptibility vs T and magnetization vs field data at low temperature are consistent with an S = (17)/(2) ground state. Complex 3 is a symmetric alkoxo-bridged dimer. The two high-spin Mn(III) ions are antiferromagnetically coupled with J = 0.28(4) cm(-)(1), g = 1.983(2), and D = -2.5(4) cm(-)(1).

The structure of hexadeca-mu-acetato-tetraaquadodeca-mu(3)-oxo-dodecamanganese bis(acetic acid) tetrahydrate, [Mn(12)O(12)(CH(3)COO)(16)(H(2)O)(4)] x 2CH(3)COOH x 4H(2)O, known as Mn(12)-acetate, has been determined at 83 (2) K by X-ray diffraction methods. The fourfold (S(4)) molecular symmetry is disrupted by a strong hydrogen-bonding interaction with the disordered acetic acid molecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn(12) isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen-bonded acetic acid molecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B36, 2042-2046].

The tailoring of the structure and properties of singlemolecule magnets (SMMs) is a very active branch of modern coordination chemistry. Investigations on classes of structurally related SMMs, such as those of the Mn 12 family, have helped unravel the mechanisms underlying slow magnetic relaxation in high-spin molecules, a key step in both fundamental and application-oriented research.[1] At the same time, the search for new SMMs with higher blocking temperatures has fueled synthetic efforts aimed, on one side, at increasing structural control on cluster architectures [2] and, on the other side, at developing the so-called serendipitousassembly approach.[3] An elegant strategy for structural design is based on site-specific modification of preformed clusters.[4] Carboxylate abstraction from Mn 12 clusters, for instance, has been carried out site specifically using a variety of incoming ligands, such as nitrates, [4a] phosphanates, [4b] phosphates, [4c] or different carboxylates.[4d] However, these substitutions are accompanied by only small perturbation of the magnetic properties.The SMM behavior is associated with the magnetic anisotropy of clusters, which in turn depends on local anisotropies and on the way they vectorially add to give a resulting total anisotropy.[5a] Herein we show that site-specific ligand replacement provides a means to raise the symmetry of Fe 4 clusters from C 2 to D 3 , which results in a dramatic increase of magnetic anisotropy and energy barrier. Fe 4 clusters are among the simplest inorganic systems showing SMM behavior. [5,6] The archetypal member of this class is the tetrairon(iii) compound [Fe 4 (OMe) 6 (dpm) 6 ] (1) (Hdpm = dipivaloylmethane). The six m-methoxide ligands bridge a central iron(iii) ion to three peripheral iron centers arranged at the vertices of an isosceles triangle [5a] with a crystallographic C 2 symmetry, and some disorder which yields three different isomers in the lattice. At low temperature, the cluster has a high-spin state (S = 5) and an easy-axis magnetic anisotropy, two requisites for the observation of slow magnetic relaxation. For the major component, the second-order zero-field splitting (ZFS) parameters are D = À0.206(1) cm À1 and E = 0.010(3) cm À1 . In addition, the presence of sizeable fourth-order contributions has been demonstrated.[5b]