Small molecule activation and reduction using non-heme iron complexes
In Nature, metalloenzymes are responsible for carrying out a vast array of different reactions. The secondary coordination sphere of these play an intricate role in controlling nuclearity, enhancing substrate selectivity, stabilizing reactive intermediates, and controlling the redox potential of the active site metal. Many of these secondary sphere interactions are hydrogen bonding networks formed by strategically located amino acid residues. Inspired by these intricate networks, our group designed a tripodal, tetradentate ligand featuring a secondary coordination sphere through a tautomerizable framework. Each ligand arm incorporates a pyrrole-imine, which upon anionic coordination to a metal center places a hydrogen-bond acceptor in the secondary sphere. Tautomerization to the azafulvene-amine enables dative coordination and places a hydrogen-bond donor in the secondary coordination sphere. Previous work details metalation strategies allowing for both tautomeric forms of the ligand and demonstrating the independent tautomerization of each arm to form mixed tautomer species. Further work investigated the role of the secondary coordination sphere in stabilizing reactive species, such as a terminal iron(III)-oxo species. Using this ligand framework, our lab previously reported the syntheses of a series of metal complexes featuring pseudohalide ligands. In each of these complexes, hydrogen bonding interactions between the bound substrate and the ligand arms stabilized each species. In Chapter 2, the role of these interactions and the ability of the secondary coordination sphere protons to transfer to a substrate is explored through the aspatial reduction of an iron-azide complex. Reduction of the azide substrate results in fragmentation to N2 and NH3. The protons necessary to afford ammonia derive directly from the secondary coordination sphere. Isotopic labeling studies confirm the source of ammonia as the bound nitrogen atom of the azide substrate. Additionally, density functional theory (DFT) calculations were carried out in order to elucidate information about the mechanism of this reduction. These calculations were consistent with our observation of an intermediate octahedral iron(III) complex, arising from loss of NH3 from the iron center. Previous work in the group had shown the ability of an iron(II) bis(triflate) complex to catalytically reduce nitrite and nitrate. In Chapter 3, the intermediate of these reductions, an iron-nitrosyl complex, is further studied through reductive means in an effort to mimic the reactivity of the enzyme cytochrome c nitrite reductase (ccNiR). Similar to the reactivity observed in the reduction of azide, the addition of exogenous acid and strong reductants results in the release of NH3 and the formation of an iron(II)-hydroxo complex. Mechanistic studies were conducted showing the plausibility of a hydroxylamine intermediate similar to that of the enzymatic process. In Chapter 4, selenium oxyanion reduction by an iron(II) bis(triflate) complex was revisited. Previous work had shown that this iron system was capable of carrying out the stoichiometric reductions of selenite and selenite, resulting in an iron(III)-oxo species and elemental selenium. In this work, stoichiometric selenate reduction run at higher temperatures resulted in an increased yield of elemental selenium compared to that of the previous work. Methods to reduce selenate and selenite catalytically were also developed, taking advantage of biphasic solutions to avoid the deactivation of the iron catalyst. While neither oxyanion was able to be reduced catalytically with high turnover numbers or rates, this is the first homogeneous synthetic system to catalyze these reductions. Additionally, the selenium(0) produced from these reductions was further characterized as amorphous red elemental selenium through TEM imaging, UV-visible spectroscopy, and PXRD studies. The synthesis and characterization of early transition metal (M = Ti, V, Cr, Mo) complexes was explored in Chapter 5. Metal complexes structurally analogous to a previously reported octahedral iron complex bearing our tripodal ligand were successfully synthesized. While only two complexes were structurally characterized by single-crystal X-ray diffraction, the structures of the remaining two complexes were proposed based off similar spectroscopic data and solubilities. Additionally, a Cr(II) bis(triflate complex analogous to the previously reported iron complex was synthesized. Overall, this work demonstrated the role secondary coordination sphere interactions, primarily hydrogen bonding, can play on reductive processes. The built-in proton donors of the secondary coordination sphere helped promote reductions of NO and N3- to afford NH3. The proximity of the proton donors to bound substrates stabilized the formation of an iron(III)-oxo species, allowing for successful stoichiometric and catalytic reductions of selenium oxyanions. Furthermore, the versatile binding pocket of the ligand allowed for metalation of the ligand using early transition metals to afford octahedral metal complexes.
chemistry, inorganic chemistry, bioinorganic chemistry, oxyanions, small molecule activation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign