Synthesis and Characterization of Biomimetic Co and Fe Complexes with Trispyrazolylborate Ligands
Date of Award
Doctor of Philosophy (PhD)
Gardinier, James R.
Numerous reactions of biological and environmental significance are catalyzed by dioxygenases that incorporate both atoms of O2 into the substrate. These enzymes require a transition-metal cofactor for activity, typically a mononuclear nonheme iron center, although dioxygenases with first-row transition metal ions (Mn, Co, Ni) have been discovered. This dissertation focuses on synthetic studies based on mononuclear dioxygenases found in bacterial pathways for the breakdown and assimilation of inert organic compounds, including human-generated pollutants. Such enzymes are essential for bioremediation technologies used to restore contaminated soils and groundwaters. While crystallographic studies have revealed the active-site structures of various types of dioxygenases, the associated catalytic mechanisms are not fully understood, largely due to difficulties in isolating and characterizing transient intermediates. In particular, there is little clarity regarding the structural and electronic factors that govern the reactivity of iron-superoxo and -alkylperoxo intermediates, which play pivotal roles in the putative mechanisms. Here, these challenges are addressed by generating a series of synthetic cobalt and iron containing complexes that replicate key structural, electronic, and chemical features of environmentally important dioxygenases. This “bio-inspired” approach combines coordination chemistry, reaction kinetics, spectroscopic techniques, and computational methods. As the properties of synthetic models can be modified in a straightforward and systematic manner, through their design and synthesis it is possible to illuminate relationships that exist between geometric/electronic structures, spectroscopic parameters, and O2 reactivity. Moreover, since the biomimetic approach permits a great deal of control over ligand properties and reaction conditions, it is possible to trap and spectroscopically characterize catalytic intermediates that have been postulated, but never observed, in studies of the metalloenzymes
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