Polynuclear manganese single-molecule magnets

The chemistry and physical properties of single-molecule magnets are explored. Three molecules having the chemical composition: [Mn 4(pdMH)6(O2CCH3)2](ClO 4)2 (1), [Mn4(hmp) 6(H2O)2Br2]Br2 ( 2), and [Mn18O14(O2CCH 3)18(hep)4(hepH)2(H2O) 2](ClO4)2 (3) are shown to be new examples of single-molecule magnets. Detailed magnetic characterizations employing AC and DC magnetic susceptibility and HFEPR measurements are shown. These complexes have large spin ground states in addition to a large Ising type magnetic anisotropy. This results in a barrier to magnetization reversal leading to slow magnetization relaxation. Frequency dependent out-of-phase AC magnetic susceptibility signals are observed for all three complexes. Magnetization measurements show that the relaxation process occurs via two mechanisms: thermal activation and quantum mechanical tunneling. At high temperatures, magnetization relaxation occurs via a thermally activated process with temperature dependent relaxation rates that follow the Arrhenius law. However, at lower temperatures the relaxation rate shows deviation from Arrhenius behavior. Temperature independent relaxation rates are observed suggesting a quantum mechanical tunneling in the magnetization relaxation. Temperature independent relaxation rates on the order of 10-4, 10-3, and 10 -8 s-1 were calculated for complexes 1, 2, and 3 respectively. Magnetization vs. field measurements are presented for complex 1. Hysteresis in the magnetization is observed below 1 K with steps at increments of magnetic field suggesting resonant magnetization tunneling.; The coordination chemistry of Mn complexes is explored using a series of structurally related ligands. Results are presented detailing how chemical changes affect the magnetic properties of single-molecule magnets. The results show that the magnetic properties are influenced by the level of hydration of the molecule in addition to intermolecular magnetic exchange interactions.