Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Fiedler, Adam T.

Second Advisor

Gardinier, James R.

Third Advisor

Kincaid, James R.

Abstract

Redox-active p-(hydro)quinones work in concert with transition-metal centers to facilitate electron transfers in numerous biological contexts. While p-(hydro)quinones are known to interact with both heme and nonheme iron cofactors, the nonheme systems are particularly relevant to photosynthetic and bioremediation processes. In photosynthesis, two p-quinones facilitate an electron transfer away from the photoexcited P680 cofactor via a non-heme Fe(II) center. Based on EPR results, this interaction results in formation of transient Fe(II)/p-semiquinone (pSQ) species. In addition, a superoxo-iron(II)-pSQ species has been proposed as an intermediate of the oxidative cleavage mechanism of hydroquinone dioxygenases (HQDOs), which play a central role in the catabolism of aromatic pollutants. Despite the prevalence of iron-(hydro)quinone interactions observed in nature, there is a dearth of reported synthetic analogs. We therefore aimed to synthesize five-coordinate monoiron(II) complexes featuring a variety of substituted p-quinone ligand or p-hydroquinone ligands. These complexes contain a tris(3,5-diphenylpyrazolyl)borate (Ph2Tp) or tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP) supporting ligand to mimic the different types of facial triads found in nonheme iron dioxygenases. The corresponding Fe(II)-pSQ intermediates were generated via two methods: (i) chemical reduction of the mononuclear Fe(II)-p-quinone complexes, and (ii) proton-coupled electron transfer from an iron-hydroquinonate precursor. The presence of a pSQ radical coupled to a high-spin Fe(II) center was confirmed by spectroscopic (UV-vis, EPR, resonance Raman) and computational (DFT) methods. Recent O2 reactivity studies have examined the ability of these complexes to serve as functional HQDO models. Additionally, we synthesized a diiron(II) species that, upon treatment with a chemical oxidant, yields a stable complex in which two Fe(II) centers are bridged by a p-semiquinone radical. The unique S=7/2 electronic structure of this complex was studies extensively by spectroscopic and computational methods and represents the first complex to feature Fe(II) centers bound to a semiquinonate radical. Further studies were focused on the development of additional dimetal(II) species bridged by varying hydroquinonate ligands to explore their potential of generating novel species that display a high degree of electronic coupling upon one-electron oxidation.

Available for download on Thursday, April 26, 2018

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