Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.

The preparation, characterization, and X-ray structure are reported for the single-molecule magnet (PPh4)[Mn12O12(O2CPh)16(H2O)4]·8(CH2Cl2) (2). Complex 2 crystallizes in the triclinic space group P1̄, which at 213 K has a = 17.2329(2), b = 17.8347(2), c = 26.8052(2) Å, α = 90.515(2), β = 94.242(2), γ = 101.437(2)°, and Z = 2. The salt consists of PPh4 + cations and [Mn12O12(O2CPh)16(H2O)4]- anions. The (Mn12O12)15+ core of the anion is formed by an external ring of eight Mn atoms bridged by μ3−O2- ions to an internal tetrahedron of four Mn atoms. Because of disorder in both phenyl rings and solvate molecules, it was difficult to use bond valence sum values to determine definitively the oxidation state of each Mn atom. There is a Mn4O4 cubane unit in the internal part of the molecule and these Mn atoms are all MnIV ions. For the eight “external” Mn atoms the bond valence sum values did not define well their oxidation states. For these eight Mn atoms, it was not possible to determine whether a trapped-valence MnIIMnIII 7 or an electronically delocalized description is appropriate. High-frequency EPR (HFEPR) data were collected for the previously structurally characterized MnIV 4MnIII 7MnII valence-trapped salt (PPh4)[Mn12O12(O2CEt)16(H2O)4] (1) at 328.2 and 437.69 GHz. In the high magnetic field the crystallites orient and the HFEPR spectra are pseudo−single-crystal like, not powder patterns. The spectral features are attributed to the fine structure expected for a S = 19/2 complex experiencing axial zero-field splitting D Ŝ z 2, where D = −0.62 cm-1. The sign of D was definitively determined by the temperature dependence of the spectrum. Complex 2 exhibits one out-of-phase ac magnetic susceptibility (χ‘ ‘M) signal in the 3−6 K range. The temperature of the χ‘ ‘M peak is frequency dependent, as expected for a single-molecule magnet. The rate at which the direction of magnetization reverses from “up” to “down” was evaluated from χ‘ ‘M data collected at various frequencies (1−1512 Hz) of oscillation of the ac magnetic field. This gives magnetization relaxation rates in the 2.86−4.51 K range for complex 2 and in the 3.2−7.2 K range for complex 1. Rates were also determined in the 1.80−2.50 K range for complex 1 via magnetization decay experiments. In this latter case, the polycrystalline sample is magnetically saturated in a large dc field. After the magnetic field is rapidly decreased to zero, the decay of the magnetization to zero is monitored. The rates evaluated by both the frequency dependence of the out-of-phase ac signal and dc relaxation decay experiments for complex 1 fit on an Arrhenius plot to give an activation energy of U eff = 57 K and a preexponential rate of 1/τ0 = 7.2 × 107 s-1. From the HFEPR data, complex 1 has a S = 19/2 ground state with D = −0.62 cm-1. This gives a potential-energy barrier of U = 79 K for the double-well potential-energy diagram. The value of U eff is less than the barrier height U, because when individual [Mn12 -] anions convert from spin “up” to spin “down”, they can not only...

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The magnetic properties of j[(tacn)6Fe8(M3-0)2(M2-0H)12]Br7(H20)}Br'H20, Fe8, tacn = 1,4,7-triazacyclononane, a molecule comprising eight iron(III) ions bridged by oxo and hydroxo groups, are reported. The magnetic susceptibility, both dc and ac, and the magnetization indicate that at low-temperature spin levels with 8 < S < 10 are populated. EPR confirms that levels of high spin multiplicity are populated at 4.2 K. For the first time an attempt is made to calculate the energy of the spin levels of such a large cluster within a spin Hamiltonian formalism. The 1 679 616 states originating from the coupling of eight S = s/2 spins are classified by using total spin and point group symmetry, and the Hamiltonian matrix is calculated using an irreducible tensor operator approach. In this way the susceptibility can be calculated, and satisfactory fits of the experimental magnetic susceptibility are achieved. The values of the required parameters compare well with those previously reported in analogous iron complexes.