Synthesis, Structure, and Magnetic Properties of a Mn21 Single-Molecule Magnet

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Abstract
The reaction of [Mn3O(O2CMe)6(py)3](ClO4) (1; 3MnIII) with [Mn10O4(OH)2(O2CMe)8(hmp)8](ClO4)4 (2; 10MnIII) in MeCN affords the new mixed-valent complex [Mn21O14(OH)2(O2CMe)16(hmp)8(pic)2(py)(H2O)](ClO4)4 (3; 3MnII−18MnIII; hmp- is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3·7MeCN crystallizes in the triclinic space group P1̄. The structure consists of a low symmetry [Mn21(μ4-O)4(μ3-O)12(μ-O)16] core, with peripheral ligation provided by 16 MeCO2-, 8 hmp-, and 2 pic- groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1−0.8 T and 1.8−4.0 K ranges gave S = 17/2, D ≈ −0.086 cm-1, and g ≈ 1.8, where S is the molecular spin of the Mn21 complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10−997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (χM‘ ‘) signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (Ueff) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.

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