Date of Award

12-16-2013

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Daniel L Reger

Abstract

The semi-rigid ligands, p-[CH(pz)2]2C6H4, Lp, m-[CH(pz)2]2C6H2, Lm, and m-[CH(3,5-Me2pz)2]2C6H4, Lm* (pz = pyrazolyl ring), link two bis(pyrazolyl)methane units into a single molecule by a rigid phenylene spacer and are used to study the structural variations and magnetic interactions in self-assembled first row, divalent transition metal complexes in polymeric, dinuclear and dinuclear metallacyclic settings. The formation of these architectures highly depend on the geometry of these ligands: anti, the two -CH(pz)2 units are on the opposite side of the plane defined by the phenylene spacer, or syn, both -CH(pz)2 units are on the same side of the phenylene spacer.

The first chapter focuses on structural modifications in dinuclear or polymeric copper(II) complexes of Lp, in anti conformation, induced by slight changes in the solvent of crystallization, and the effect of these changes on the supramolecular organization and weak hydrogen bonding patterns in these complexes.

Opposite to the structural diversity generated with Lp in anti conformation, Lm and Lm* prefer to self-assemble into dinuclear metallacycles, by adopting syn conformation. In these units the divalent metal centers are also linked by a small anionic bridge, X = F-, Cl-, Br-, OH-, CN-, N3-, to generate [M2(μ-X)(μ-L)2]3+, where L = Lm or Lm*.


The tendency of these ligands to form a single structural type prompted us to tackle a fundamental problem in magnetism: carefully designed systems that control the geometry around the metal centers are lacking, therefore magneto-structural correlations are based on compounds where several structural parameters vary simultaneously. As demonstrated in chapters II-V, this [M2(μ-X)(μ-L)2]3+ metallacyclic system, for the first time, uniquely allows the overall structure to be maintained constant while a single structural feature, directly affecting the antiferromagnetic superexchange interactions, is selectively altered.

The copper(II) series, [Cu2(μ-X)(μ-Lm*)2](ClO4)3, represents the first series of dinuclear complexes with a strict linear Cu-X-Cu bridging arrangement, enabling extremely strong antiferromagnetic superexchange pathways. The magnetic susceptibilities of the copper(II) complexes are close to 0, even at room temperature, allowing the study of these compounds in solution by different NMR techniques. In chapter VI, I show that the structure, geometry and magnetic interactions in solution and solid state are similar.

Chapter VII is centered around the dynamic behavior of [Zn2(μ-OH)(μ-Lm)2](ClO4)3 in solution. The VT-NMR and spin saturation transfer experiments reveal an unprecedented example of concerted Berry pseudorotation at two metal sites accompanied by the simultaneous 180° ring flip of the phenylene spacer - termed the "Carolina Twist and Flip" mechanism. This process is hindered by the methyl groups on the pyrazolyl rings for [M2(μ-OH)(μ-Lm*)2](ClO4)3, M = Zn(II), Cd(II). Chapter VIII discusses the synthesis and characterization of an unusual cubane type structure, [M4(μ-OH)4(μ-Lm)2(Solvent)4](ClO4)4 where M = Ni(II) or Cd(II). The nickel(II) centers are ferromagnetically coupled and the cadmium(II) compound undergoes a similar rearrangement to [Zn2(μ-OH)(μ-Lm)2](ClO4)3.

Rights

© 2013, Andrea Eva Pascui

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