pH changes induce an axial ligand effect on nonheme iron(IV)-oxo complexes with an appended aminopropyl functionality

Nonheme iron enzymes often utilize a high-valent iron(IV)-oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV)-oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity towards olefin epoxidation upon changing the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen atom transfer reaction with substrates. In particular, these changes have occurred due to protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that the replacement of the pendant amine with a neutral or anionic ligand lowers the rate-determining barriers of cyclooctene epoxidation dramatically. The computational work further establishes that the change in oxygen atom transfer is due electrostatic interactions of the pendant amine cation that favorably affects barrier heights.