The secondary coordination sphere of metalloenzymes is implicated in controlling nuclearity, enhancing substrate selectivity, and stabilizing reactive intermediates, and is thus essential in promoting the reactivity of the metal center in the active site. Inspired by the small molecule activation performed by biological systems and the work of others in the area of bioinorganic chemistry creating biomimetic systems, our group designed a tripodal ligand platform featuring a secondary coordination sphere. The ligand is tautomerizable, which allows for versatility in both the primary and secondary coordination spheres. Anionic coordination positions a hydrogen-bond acceptor in the secondary sphere, whereas dative coordination positions a hydrogen-bond donor in the secondary sphere. Previous reports detailed the metallation routes to access both tautomeric forms of the ligand in iron(II) complexes. Moreover, the ability of each arm of the ligand to tautomerize independently was observed in an iron(II)-hydroxo complex that included both tautomeric forms. The secondary coordination sphere also displayed a propensity to engage in hydrogen-bonding interactions with ancillary ligands and allowed for the isolation of a terminal iron(III)-oxo complex. Our lab has previously demonstrated the accessibility of the iron(III)-oxo species from the reduction of nitrite. Herein, the reduction of other oxyanions, the halogen and selenium oxyanions and chromate, to furnish the iron(III)-oxo complex was explored. In Chapter 2, the reduction of the halogen oxyanions to form the iron(III)-oxo complex and an iron(II)-halide complex is detailed. The series of iron(II)-halide complexes was independently synthesized to confirm the formulation of the oxyanion reduction products. The reduction of perchlorate was made catalytic, albeit with low turnover number. The synthesis of a zinc-perchlorate complex gave insight into the binding of the oxyanions to a metal center and the role of the secondary coordination sphere.In the reduction of the selenium oxyanions in Chapter 3, elemental selenium was formed along with the iron(III)-oxo complexes. The identification of the elemental selenium was confirmed through the formation of triphenylphosphine selenide. 31P NMR spectroscopy also enabled the quantification of the elemental selenium upon formation of the phosphine selenide.The reduction of the iron(III)-oxo complex by 1,2-diphenylhydrazine, a two proton/two electron donor, was investigated in Chapter 4 and led to the isolation of a new iron(II)-hydroxo complex with three hydrogen-bond donors in the secondary coordination sphere. The reactivity of this complex was briefly explored, as was an alternative route to access the iron(III)-oxo complex. Addition of acid and water to an octahedral iron(III) species cleanly furnished the iron(III)-oxo complex. A new tripodal ligand platform, which featured only two arms capable of hydrogen-bonding interactions, was synthesized. Its metallation to furnish iron(II) and cobalt(II) complexes was investigated in Chapter 5. The impact of reducing the number of hydrogen-bonding interactions in the secondary coordination sphere was studied. The structures of the metal(II) complexes were as expected, with anionic or dative coordination of the ligand accessible and hydrogen-bonding interactions to an axial ligand present. However, upon oxidation of the iron(II) complexes, formation of a μ-oxo iron(III) dimer was observed, illustrating a limitation of this system. Overall, this work demonstrated the importance of the secondary coordination sphere in achieving small molecule activation in a biomimetic system. The ability of our tripodal ligand system to support the terminal iron(III)-oxo complex allowed for the deoxygenation of many oxyanion substrates, most notably perchlorate. Reducing the number of hydrogen-bonding interactions in the secondary coordination sphere resulted in the formation of a dimeric complex upon oxidation.
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