Single-Molecule Magnets: Preparation and Properties of Low Symmetry [Mn4O3(O2CPh-R)4(dbm)3] Complexes with S = 9/2

The preparation and properties of [Mn(4)O(3)(O(2)CPh-R)(4)(dbm)(3)] (R = H, p-Me, p-OMe, and o-Cl; dbm(-) is the anion of dibenzoylmethane) single-molecule magnets (SMMs) with virtual C(S) symmetry are reported. They were prepared by controlled potential electrolysis in 26-80% yields. The structures comprise a distorted-cubane core of virtual C(S) symmetry, in contrast to the other, more common complexes of this type with virtual C(3)(V) symmetry. Solid-state magnetic susceptibility data establish the complexes have S = 9/2 ground-state spins, and ac susceptibility studies indicate they are single-molecule magnets (SMMs). Magnetization vs dc field sweeps below 1.00 K reveal hysteresis loops confirming a SMM, with a very large step at zero applied field diagnostic of fast quantum tunneling of magnetization (QTM) through the anisotropy barrier. The fast QTM rate suggested a significant rhombic ZFS parameter E, as expected from the low (virtual C(S)) symmetry. This was confirmed by high-frequency electron paramagnetic resonance spectroscopy on polycrystalline and single-crystal studies. The results confirm the importance of symmetry on the QTM rates.