A study of magnetostructural parameters related to spin crossover and single molecule magnetism
Date
2013
Authors
Fiedler, Stephanie R., author
Shores, Matthew P., advisor
Kennan, Alan J., committee member
Anderson, Oren P., committee member
Crans, Debbie C., committee member
Patton, Carl E., committee member
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Abstract
Herein are described several methods to probe transition metal complexes that were designed by systematic structural modifications to allow for comparison of the resultant magnetic properties. In Chapter 1, a brief introduction is presented to introduce the broader goal of our research: controlling spin on the synthetic level. The introduction provides background regarding spin crossover and single molecule magnetism as well as some previous research to put our projects in context relative to endeavors by other researchers. In Chapter 2, heteroleptic complexes of the form [Fe(H2bip)2(pizR)]Br2 and [Fe(H2bip)2(pizR)](BPh4)2 are described, which have the opportunity to chelate an anion via hydrogen bonding to the H2bip ligand. The third ligand, pizR, is varied between two ligands that we predict will have similar ligand field strengths: pizH and pizMe. Because pizH has an additional hydrogen-bonding site, while pizMe does not, we selected these ligands in order to understand the effect of hydrogen bonding on the anion-binding/spin-state switching event independent from ligand field strength. From these studies, the pizH anion hydrogen bond is observed in crystallographic studies, but does not affect the anion-binding or spin-state switching properties in solution. In Chapter 3, we further investigate the geometry of the pizR ligand in Fe(II) complexes. What began as attempts to study hydrogen bonding in solution revealed unexpected structural distortions of the ligand that are correlated to the spin state of the complexes. The R-substituted nitrogen atom on the imidazoline moiety of the pizR ligand switches between a planar geometry, which is observed for high-spin species, and a pyramidalized geometry, which is observed for low-spin species. We reason that this occurs as a result of the weak-field, non-pizR ligands that influence the ligand field in the high-spin species. Chapters 4 and 5 delve deeper into understanding the relationship between structural parameters and magnetic properties in complexes with non-covalent interactions. In Chapter 4, a series of complexes with metallophilic Pt-Pt interactions show antiferromagnetic magnetic coupling of non-bonded transition metals through a Pt-Pt bond. By comparing complexes with Pt-Pt interactions to those without Pt-Pt interactions, we are able to determine that the Pt-Pt bond is a unique superexchange pathway for the transition metal coupling. Off-set complexes, exhibiting two Pt S interactions instead of one Pt-Pt interaction, do not show evidence of magnetic coupling between transition metals. Furthermore, by comparing magnetic properties of complexes where the apical ligand varies, we determine that the presence or absence of intermolecular interactions is largely independent from the strength of coupling through the Pt-Pt bond. In Chapter 5, an asymmetric trinuclear manganese complex with unique magnetic exchange properties and two high-spin square planar complexes of iron and cobalt, are investigated. The trinuclear manganese complex consists of a central octahedral Mn(II) ion that is coupled antiferromagnetically to another octahedral Mn(II) ion and ferromagnetically to a terminal tetrahedral Mn(II) ion. The different coupling is rationalized as a result of the change in geometry, which affects the orbital overlap that is predicted for each pair of ions. The high-spin square-planar Fe(II) and Co(II) complexes illustrate an unusual pairing of spin-state with square-planar geometry. Moreover, the Fe(II) complex exhibits signs of easy-axis molecular anisotropy and slow-relaxation of magnetization, albeit in the presence of a magnetic field. Lastly, in Chapter 6, we investigate a trinuclear Fe(III) complex bridged by a triethynylmesitylene ligand. The magnetic properties of the complex are compared to a previous Fe(III) complex bridged by a triethynylbenzene ligand. Steric interactions between the aromatic core of the ethynylmesitylene ligand and the auxiliary dimethylphosphinoethane ligands on Fe(III) are predicted to engender a ligand conformation to promote strong orbital overlap. Magnetic susceptibility data for the two complexes both exhibit ferromagnetic coupling between metal centers as expected. Further studies are necessary to confirm the observed behavior, but the new triethynylmesitylene complex appears to have slightly stronger coupling than the previous triethynylbenzene complex.
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Subject
metallophilic interactions
structural correlations
anion-binding
iron
magnetic properties
spin-state switching