Incorporation of Cation Affects the Redox Reactivity of Fe–NNN Complexes on C–H Oxidation

Abstract

Cations such as Lewis acids have been shown to enhance the catalytic activity of high-valent Fe−oxygen intermediates. Herein, we present a pyridine diamine ethylene glycol macrocycle, which can form Zn(II)-or Fe(III)-complex with the NNN site, while allowing redox-inactive cations to bind to the ethylene glycol moiety. The addition of alkali, alkali earth, and lanthanum ions resulted in positive shifts to the Fe(III/II) redox potential. Calculation of dissociation constants showed the tightest binding with a Ba 2+ ion. Density functional theory calculations were used to elucidate the effects of redox inactive cations toward the electronic structures of Fe complexes. Although the Fe−NNN complexes, both in the absence and presence of cations, can catalyze C−H oxidation of 9,10-dihydroanthracene, to give anthracene [hydrogen atom transfer (HAT) product], anthrone, and anthraquinone [oxygen atom transfer (OAT) products], highest overall activity and OAT/HAT product ratios were obtained in the presence of dications, that is, Ba 2+ and Mg 2+ , respectively.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

“…One of the more broadly studied effects of Lewis acids on electron transfer involves the conjugation of redox-active metal complexes to crown-ethers (Figure ). − Extensive studies on mono- ( 1 ) and bimetallic ( 2 ) manganese Schiff-base complexes first appeared in the 1990s that aimed to elucidate the properties of manganese-dependent enzymes (including the OEC). − While salen-type related ligands have been shown to be useful for binding or sensing of a wide variety of redox-inactive metals via coordination to O atom donors (e.g., phenoxide), inclusion of crown-ether motifs enables stronger binding of Lewis acidic ions with stoichiometric control. Exposing complex 1 to Li + , K + , Ca 2+ , or Ba 2+ leads to incorporation of a single redox-inactive metal into the crown-ether moiety, resulting in a positive shift in reduction potential for the Mn III /Mn II redox couple .…”