Secondary Coordination Sphere Effects on Properties and Reactivities of Metal complexes

In nature, metalloproteins can perform difficult transformations with high selectivity and efficiency through precise control of the primary and secondary coordination sphere. Synthetic chemists have developed biomimetic ligand scaffolds to better understand the coordination environments in the active sites of metalloproteins. However, besides the ligands covalently bound to the metal center, secondary coordination sphere that is comprised of non-covalent interactions also has significant influence in complex properties. This dissertation describes how modulation of the secondary coordination sphere by incorporating different H-bond donors or acceptors affects the chemistry of metal complexes. Chapter 2 describes a series of MnIII-oxido complexes within the hybrid tripodal framework, [H3bpuea-R]3‒, which one of the ligand arms contains a para-substituted phenyl ring. Varying the R-groups on the phenyl ring allowed modulation on the strength of one single H-bond donor in the secondary coordination sphere without perturbing the primary coordination sphere. This modulation showed influences on basicity and reactivity of the MnIII-oxido complexes. Chapter 3 describes newly synthesized FeII/III-NH3 and FeIII-amido complexes in C3-symmetric tripodal phosphinic amido ligand framework, [poat]3–. Comparing the FeII/III-NH3 complexes in the [poat]3– ligand framework and analogous complexes in [MST]3– ligand framework showed significant differences in their structural and redox properties, suggesting [poat]3– can serve as a stronger donor ligand and provide stronger H-bonding interactions. Chapter 4 describes the reactivity of a high spin FeIV-oxido complex, [FeIVPOP(O)]–, in C3-symmetric tripodal phosphoryl amido ligand framework. Taking inspiration from TauD having substrate anchored in close proximity of the metal center, benzyl alcohol was used as a substrate that can participate in possible H-bond interaction with the P=O in phosphoryl amido arms of the complex. Reactivity of [FeIVPOP(O)]– with benzyl alcohol was observed. Hammett analysis and determination of kinetic isotope effect (KIE) for the reactions were performed. The KIE values suggest the cleavage of the C–H bond is involved in the rate determining step whereas the cleavage of the O–H bond is not. Furthermore, the reaction rate of the [FeIVPOP(O)]– complex with benzyl alcohol was ~100 times larger than that of the [FeIVpoat(O)]– complex. This result showed two complexes in similar primary coordination sphere can have significantly different reactivities by modification of the secondary coordination sphere around the FeIV-oxido moiety. In chapter 5, extending the study of the C3-symmetric tripodal phosphoryl amido ligand framework, [POP]3–, binding of α-keto acids such as sodium phenylpyruvate (NaPhP) and phenylgloxylic acid (PGA) were studied as exogenous ligand for iron complex. The Na(NMe4)[FeIIHPOP(PhP)] complex was isolated and characterized. Intramolecular tautomerization was observed in the FeII-PhP complex with the [PhP]2– ligand bound as enolate form and one of the phosphoryl amide arms protonated. Preliminary UV-vis and EPR results for reactivities of the α-keto acids bound iron complexes with O2 showed possible oxidation of the iron cente