Date of Award

Spring 2016

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Fiedler, Adam T.

Second Advisor

Gardinier, James R.

Third Advisor

Kincaid, James R.


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.

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