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Elucidating the Electronic Structure of Transition Metal Complexes Featuring Redox Active Ligands

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Thesis type
(Thesis) Ph.D.
Date created
In this thesis a number of projects involving the design and characterization of complexes bearing redox active ligands are described. Focusing on the phenolate containing ligands, the properties and electronic structure of their corresponding metal complexes were studied by a series of experimental (i.e. electrochemistry, UV-Vis-NIR, EPR, rR etc.) and theoretical (DFT) methods. Specifically, the redox processes of these metal complexes were tuned by varying the para-ring substituents. In one study, nickel-salen (salen is a common abbreviation for N2O2 bis-Schiff-base bis-phenolate ligands) complexes were investigated, where the oxidation potentials of the ligand were predictably decreased as the electron donating ability of the para-ring substituents was increased (NMe2 > OMe > tBu > CF3). Interestingly, the oxidation of these geometrically-symmetric complexes afforded an asymmetric electronic structure in a number of cases, in which the ligand radical was localized on one phenolate rather than delocalized across the ligand framework. This difference in electronic structure was found to be dependent on the electron donating ability of the substituents; a delocalized ligand radical was observed for electron-withdrawing substituents and a localized ligand radical for strongly donating substituents. These studies highlight that para-ring substituents can be used to tune the electronic structure (metal vs. ligand based, localized vs. delocalized radical character) of metallosalen complexes. To evaluate if this electronic tuning can be applied to the metal center, a series of cobalt complexes of these salen ligands were prepared. Indeed, the electronic properties of the metal center were also significantly affected by para-ring substitution. These cobalt-salen complexes were tested as catalysts in organometallic radical-mediated polymerizations, where the most electron rich complexes displayed the best conversion rates. With a firm understanding of the role that the para-ring substituent can play in influencing the electronic structure and reactivity of metallosalen complexes in catalysis, two novel iron complexes, which contain a number of redox active phenolate fragments, were prepared. In addition, these iron-complexes feature a chiral bipyrrolidine backbone. Ligands with this chiral diamine backbone bind metals ions diastereoselectively owing to its increased rigidity, which is critical to stereoselectivity in catalysis. A symmetric (with two phenolates) ligand was prepared by reported methods, and a novel route to synthesize an asymmetric ligand (one phenolate and one pyridine) from symmetric starting materials was established. The neutral iron-complexes were found to be high spin (S = 5/2), and can undergo ligand based oxidation to form an antiferromagnetically-coupled (Stotal = 2) species. The results presented will serve as the basis for catalyst development using complexes of similar ligands.
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Thesis advisor: Storr, Tim
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