Polynuclear transition metal complexes and single-molecule magnets

The synthesis and physical characterization of various polynuclear vanadium and manganese complexes are reported. A new family of single-molecule magnets, (V{dollar}sb4{dollar}O{dollar}sb2{dollar}(O{dollar}sb2{dollar}CEt){dollar}sb7{dollar}(L){dollar}sb2{dollar}) {dollar}sp{lcub}+/-{rcub}{dollar} (L = bpy, cation, L = picolinate, anion) with a butterfly core, was established with DC and AC magnetization studies. A S{dollar}sb{lcub}rm T{rcub}{dollar} = 3 spin ground state was determined with zero-field splitting parameter D = {dollar}-{dollar}1.5 cm{dollar}sp{lcub}-1{rcub}.{dollar} The results from magnetization hysteresis, DC and AC magnetic susceptibility studies indicate that there are two forms of Mn{dollar}sb{lcub}12{rcub}{dollar} molecules in the solid state. One has a S{dollar}sb{lcub}rm T{rcub}{dollar} = 10 ground state with negative zero-field splitting parameter D = {dollar}-{dollar}0.5 cm{dollar}sp{lcub}-1{rcub}{dollar} and the other has either a S = 8 or 9 ground state with a negative zero-field splitting parameter D {dollar}approx -{dollar}0.3 cm{dollar}sp{lcub}-1{rcub}.{dollar} The origin of two different spin ground states is due to the different magnetic exchange interaction between the metal ions reflected by orientation of anisotropic axis for single Mn{dollar}sp{lcub}rm III{rcub}{dollar} ion in the molecules. The Mn{dollar}sp{lcub}12{rcub}{dollar} molecules with a S{dollar}sb{lcub}rm T{rcub}{dollar} = 10 are those where all of the Jahn-Teller distortion axes of the Mn{dollar}sp{lcub}rm III{rcub}{dollar} ions are essentially parallel to each other in the same molecule. The Mn{dollar}sb{lcub}12{rcub}{dollar} molecules with S{dollar}sb{lcub}rm T{rcub}{dollar} = 9 or 8 ground states where the Jahn-Teller axes of one or two Mn{dollar}sp{lcub}rm III{rcub}{dollar} ions are close to be perpendicular to those of the others. Various experiments have shown that the intermolecular exchange interactions are negligible between the Mn{dollar}sb{lcub}12{rcub}{dollar} molecules. Clear indications of magnetization quantum tunneling have also been found for all Mn{dollar}sb{lcub}12{rcub}{dollar} single-molecule magnets based on the presence of steps in hysteresis loops, dips in field dependent relaxation rate curves, as well as zero-field cooled and field-cooled magnetization experiments. Results from detailed studies on half-integer Mn{dollar}sb{lcub}12{rcub}{dollar} anions concluded that these molecules function as single-molecule magnets. Steps are also seen on the hysteresis loops for these S = 19/2 molecules due to magnetization tunneling.