Secondary Coordination Sphere Effects on the Properties of High Valent Iron and Manganese Complexes with Oxido and Hydroxido Ligands
In nature, metalloproteins have evolved a precise control over the primary and secondary coordination sphere of their active sites to perform chemical transformations with high selectivity and efficiency. The approach developed in the Borovik lab to emulate these properties has been to synthesize molecular constructs that are capable of stabilizing reactive species through control of the primary coordination sphere of a metal ion with a rigid ligand platform and the secondary coordination sphere through hydrogen-bonding (H-bonding) interactions. This thesis describes how modifications of the secondary coordination sphere by incorporating both H-bond acceptors and donors affects the chemistry of metal complexes. Chapter 2 describes the protonation and oxidation of the previously reported [FeIVH3buea(O)]– complex. Addition of either a proton or oxidant to the FeIV-oxido complex resulted in the formation of a new FeIV species, characterized as the protonated congener of [FeIVH3buea(O)]–. Density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) provided insight into the structural properties of the complex and suggested that the [H3buea]3– was the site of protonation, resulting in a tautomerization with the additional proton being H-bonded to the oxido ligand. These results demonstrated the difficulty in preparing high valent FeIV-hydroxido species. Chapter 3 explores the proton-coupled electron transfer (PCET) process for the oxidation of an FeIII-hydroxido complex to generate the corresponding FeIV-oxido complex. It was shown that the choice of solvent greatly affected the yield of the FeIV-oxido complex and that the decreased yield could be attributed to loss of the starting FeIII-hydroxido complex by acting as a base in the reaction. Chapter 4 explores how modifications of the secondary coordination sphere of a metal complex via replacement of one H-bond donor with an acceptor affected the stability of high valent Fe complexes. The [H2bupa]3– ligand incorporates an H-bond accepting group, an anionic amidate, within the secondary coordination sphere, which produced a unique “hybrid” FeIII-oxido/hydroxido complex with a proton likely positioned between the amidate nitrogen and oxido ligand. When oxidized to the FeIV oxidation state, the complex is best characterized as an FeIV-oxido complex with the proton positioned on the ligand.
The last two chapters examine the effect of group 2 and 3 metal ions on high valent Mn-oxido complexes. Chapter 5 describes the alteration of the redox properties of a series of Mn-oxido complexes within the [H3buea]3– ligand by the addition of CaII, SrII, or ScIII ions. Addition of these Lewis acidic metal ions to a MnV-oxido complex resulted in spontaneous electron transfer to [FeCp2] to generate the analogous MnIV-oxido Lewis acid adduct. The MnIV adduct could be independently generated by the addition of Lewis acids to the MnIV-oxido complex. Of the metal ions examined, addition of ScIII ions showed the greatest attenuation of the redox potential of the Mn-oxido complexes. Chapter 6 describes efforts to identify a putative MnIII-peroxido species observed in parallel-mode electron paramagnetic resonance (EPR) spectroscopy that was generated by the addition of CaII ions and base to a MnIV-oxido complex within the [H2bupa]3– ligand.