http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Investigation of Molecular Magnetic Compounds Incorporating 4d and 5d Transition Metal Cyanometallates
The field of molecular magnetism has expanded rapidly since the discovery of single molecule magnets (SMMs) in the 1990's and has witnessed extraordinary advances in the last several decades. One of the current trends in molecular magnetic research is to incorporate metal ions that have pronounced single-ion anisotropy in an effort to improve magnetic exchange interactions. The 4d and 5d transition metal ions have large spin-orbit coupling parameters which contribute to the orbital angular momentum effects that lead to anisotropic behavior. The work herein describes efforts to synthesize and characterize new cyanide-bridged molecular materials incorporating 4d and 5d transition metal ions, specifically the [Os(CN)_(6)]^(3-), [Mo(CN)_(6)]^(3-) and [W(CN)_(8)]^(3-) ions. The 5d hexacyanometallate [Os(CN)_(6)]^(3-) was incorporated into a trinuclear cyanide bridged molecule and the [Fe(CN)_(6)]^(3-) analog was prepared as a reference compound for assessing the effect of the 5d versus 3d metal ion on the magnetic properties. Both molecules exhibit SMM bistability with a pronounced increase (~90 %) in the blocking temperature (TB) of the OsIII analogue. In addition to typical SMM behavior, both compounds exhibit exchange-biased SMM behavior, a shift in the quantum tunneling of the magnetization (QTM) from zero field. This exchange-bias can be turned "on" or "off" depending on the presence of interstitial methanol molecules. New trigonal bipyramidal (TBP) molecules incorporating the rarely studies hexacyanomolybdate(III) ion are presented in chapter III of this dissertation. The molecules of general formula [M(tmphen)_(2)]_(3)[Mo(CN)_(6)]_(2) (M = V^(II), Mn^(II) and Fe^(II); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline), represent additions to a large homologous family of TBP molecules reported by the Dunbar group over the years. The [Mo(CN)_(6)]^(3-) ion was prepared in situ by loss of one cyanide ligand from [Mo(CN)_(7)]^(4-). Of particular interest among the compounds reported is the V_(3)Mo_(2) analog which exhibits extraordinarily strong antiferromagnetic coupling (estimated J = -134 cm^(-1). The observed exchange coupling parameter is more than twice the current record for the antiferromagnetic coupling parameter for a cyanide-bridged magnetic molecule. Another set of results were obtained using the octacyanometallate anion [WV(CN)_(8)]^(3-) as a building block for the synthesis and magnetic studies of a family of new cyanide-bridged magnetic materials. The compounds exhibit several different structural motifs including three 0-D molecular compounds (two pentanuclear molecules and a linear trinuclear molecule) and a 1-D chain, findings that illustrate the structural versatility of the octacyanotungstate(V) ions. The TBP molecule, [Mn(tmphen)_(2)]_(3)[W(CN)_(8)]_(2), exhibits evidence for an out-of-phase signal when subjected to ac measurements in zero applied field. The 1-D chain also reveals evidence for the beginning of an out-of-phase signal under zero applied field which hints at single chain magnet behavior. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151035
Doctor of Philosophy Texas A & M University 2013 http://hdl.handle.net/1969.1/151035