To gain insight into the ligand effects of heme and non-heme transition metal active sites, a range of biomimetic model complexes have been developed and designed. For several non-heme iron oxidants, the iron(IV)-oxo species was spectroscopically characterized and further studied with a variety of representative substrates which gave reactivity patterns, rate constants and product distributions of oxygen atom transfer processes. Over the years, effects of the axial and equatorial ligand on the structure, spectroscopic properties and reactivity patterns of a range of nonheme iron(IV)-oxo species have been investigated, although questions remain regarding the exact nature of the effect of all ligands.<br/>In this work a combined computational and experimental study is presented on a nonheme iron(IV)-oxo complex and its reactivity patterns upon changing its axial ligand coordination. Thus, in heme enzymes the axial ligand has a major contribution to reactivity patterns due to the associated push-effect. In nonheme iron chemistry the effects of axial ligands are lesser pronounced; however, here we show that displacement of an axial group and replacement with a solvent molecule or anionic ligand ca have a dramatic effect on reactivity patterns. In particular, a pendant amide group affects axial ligand binding and reactivity patterns. Thus, cyclooctene epoxidation rates are seen to be enhanced in the presence of this pendant aminde group. To understand the reactivity patterns, a density functional theory study into the electronic properties and reactivity of aminopyridine ligated macrocycles was performed. In particular, we focus on the effect of the axial ligand and replace the pendant amino group with alternative groups. We show here that there is a considerable axial ligand effect that affects the rate constants of substrate (cyclooctene) epoxidation dramatically. Thus, replacing the pendant amino-arm by a neutral or anionic axial ligand lowers the rate determining barriers of cyclooctene epoxidation by values of more than 10 kcal mol<sup>–1</sup>. We have analyzed the observations and show that the change in axial ligand affects the electron affinity of the oxidant as well as its acidity.

Axial ligand effect on nonheme iron(IV)-oxo reactivity through the addition of an appended aminopropyl functionality to the ligand structure

ACS Spring Meeting 2021

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