Browsing by Subject "Single Molecule Magnets"
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Item Enhancing Magnetic Properties of Molecular Magnetic Materials: The Role of Single-Ion Anisotropy(2013-07-09) Saber, Mohamed Rashad MohamedConsiderable efforts are being devoted to designing enhanced molecular magnetic materials, in particular single molecule magnets (SMMs) that can meet the requirements for future technologies such as quantum computing and spintronics. A current trend in the field is enhancing the global anisotropy in metal complexes using single-ion anisotropy. The work in this dissertation is devoted to the synthesis and characterization of new building blocks of the highly anisotropic early transition metal ion V(III) with the aim of incorporating them into heterometallic molecular materials. The results underscore the importance of tuning the local coordination environments of metal ions in order to ensure enhanced single ion anisotropy. A family of mononuclear axially distorted vanadium (III) compounds, A[L_(3)VX_(3)] (3-9) (X = F, Cl or Br, A^(+) = Et_(4)N^(+), nBu_(4)N^(+) or PPN^(+) , L_(3) = Tp or Tp* (Tp = tris(-1-pyrazolyl)borohydride), Tp* = tris(3,5-dimethyl-1-pyrazolyl)borohydride)), and [Tp*V(DMF)_(3)](PF_(6))_(2) were studied. Replacement of the Tp ligand in 3 with the stronger ?-donor Tp* results in a near doubling of the magnitude of the axial zero-field splitting parameter D_(z) (D_(z) = -16.0 cm^(-1) in 3, and -30.0 cm^(-1) in 4) as determined by magnetic measurements. Such findings support the idea that controlling the axial crystal field distortion is an excellent way to enhance single-ion anisotropy. High Field-High Frequency EPR measurements on 4 revealed an even higher D value, -40.0 cm^(-1). Interestingly, compound 4 exhibits evidence for an out-of-phase ac signal under dc field. In another effort, a new series of vanadium cyanide building blocks, PPN[V(acac)_(2)(CN)_(2)]?PPNCl (13) (acac = acetylacetonate), A[V(L)(CN)_(2)] (A^(+) = Et_(4)N^(+), L = N,N'-Ethylenebis(salicylimine) (14), A = PPN^(+), L = N,N'-Ethylenebis(salicylimine) (15), L = N,N'-Phenylenebis(salicylimine) (16), and L = N,N'-Ethylenebis(2-methoxysalicylimine) (17)) were synthesized. Magnetic studies revealed moderate Dz values (-10.0, 5.89, 3.7, 4.05 and 4.36 cm^(-1) for 13-17 respectively). The first family of cyanide-bridged lanthanide containing molecules with a trigonal bipyramidal (TBP) geometry, (Et_(4)N)_(2)[(Re(triphos)(CN)_(3))_(2)(Ln(NO_(3))_(3))_(3)]-?4CH_(3)CN (19-27 with Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy and Ho) were prepared using the [(triphos)Re(CN)_(3)]^(-) building block, results that add valuable information to our database of compounds with a TBP geometry. Magnetic studies revealed diverse magnetic responses including slow relaxation of the magnetization at zero field for 25 and 26 , an indication of SMM behavior.Item Investigation of Molecular Magnetic Compounds Incorporating 4d and 5d Transition Metal Cyanometallates(2013-05-31) Southerland, Heather IreneThe 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.