Quantum Tunneling of Magnetization in a New [Mn18]2+ Single-Molecule Magnet with S = 13

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Abstract
The reaction between 2-(hydroxyethyl)pyridine (hepH) and a 2:1 molar mixture of [Mn3O(O2CMe)6(py)3](ClO4) and [Mn3O(O2CMe)6(py)3](py) in MeCN leads to isolation of [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) in 10% yield. The complex is 2MnII,16MnIII and consists of a Mn4O6 central unit to either side of which is attached a Mn7O9 unit. Magnetization data collected in the 2.0−4.0 K and 20−50 kG ranges were fit to yield S = 13, g = 1.86, and D = −0.13 cm-1 = −0.19 K, where D is the axial zero-field splitting parameter. AC susceptibility studies in the 0.04−4.0 K range at frequencies up to 996 Hz display out-of-phase (χM‘ ‘) signals, indicative of a single-molecule magnet (SMM). Magnetization vs applied DC field scans exhibit hysteresis at Ueff) of 14.8 cm-1 = 21.3 K. Below ca. 0.25 K, the relaxation rate is temperature-independent at 1.3 × 10-8 s-1, indicative of quantum tunneling of magnetization (QTM) between the lowest energy Ms = ±13 levels of the S = 13 state. Complex 1 is both the largest spin and highest nuclearity SMM to exhibit QTM.