Intrinsic and thermodynamic influences on hydrogen atom transfer reactions involving transition metal complexes

Hydrogen atom (H·) transfer occurs in a vast number of chemical processes, from stoichiometric organic synthesis to enzyme catalysis. Yet, a fundamental understanding of such reactivity is lacking. This dissertation explores mechanisms of net H· transfer reactions and probes factors that influence reaction rates.; More than 50 years ago, Bell, Evans, and Polanyi proposed an empirical were linearly related to the strengths of bonds broken and formed. The simple reactions of main group radicals. However, it is unable to account for the reactivity patterns of structurally different species; for example, the observation orders of magnitude faster than from C–H bonds at the same driving force.; The focus of the dissertation is an iron(III) coordination complex in which one bi-imidazoline ligand is deprotonated at a terminal amine position. Despite the three bond separation of proton and electron accepting sites, this complex acts as a H· acceptor and donor. Redox reactions involving hydrocarbons, hydroxylic acids, and oxyl radicals with this and other metal di-imine complexes are described here. Kinetic and thermodynamic studies show H· transfer reactivity that is influenced by both thermodynamic and intrinsic factors.; The systems outlined here permit a test of the applicability of Marcus Theory to hydrogen atom transfer reactions. Originally devised to explain outer-sphere electron transfer, Marcus Theory has been increasingly applied to reactions characterized by stronger electronic interactions between reactant species. Self-exchange reactions of iron bi-imidazoline complexes were investigated and the barrier to H· transfer in the absence of driving-force elucidated. Using this information, the Marcus Cross Relation was used to predict rate constants for net H· transfer. For reactions of the various transition metal complexes studied and hydroxylic acids or oxyl radicals, calculated and experimentally determined rate constants were found to agree over twelve orders of magnitude. These results strongly suggest that H· transfer can be accurately modeled by classical Marcus Theory.