Nonlinear Acid Promotion of Oxidation Reactions by Mononuclear Nonheme Iron(III)-Aqua Complexes

Abstract

Acids are known to promote oxidation of substrates by metal complexes that have a metal–oxygen bond such as metal-oxo, -hydroxo, -peroxo, -hydroperoxo, -superoxo, and -aqua complexes. Binding more than one acid molecule to metal–oxygen intermediates is expected to enhance the oxidation reactivity, exhibiting a higher-order dependence of the rate constant on the acid concentration than the first order. Such nonlinear acid promotion may open a new frontier of acid-promoted redox reactions as nature has already developed in contrast to artificial systems. Herein, we report that binding of not only one but also two triflic acid (HOTf) molecules to nonheme Fe­(III)-aqua complexes, [(dpaq)­FeIII(OH2)]2+ {1: dpaq = 2-[bis­(pyridin-2-ylmethyl)]­amino-N-(quinolin-8-yl)­acetamidate} and [(tdpaq)­FeIII(OH2)]2+ {2: tdpaq = 2-[benzyl­(pyridin-2-ylmethyl)­amino]-N-(quinolin-8-yl)­acetamidate}, results in enhancement of the oxidation reactivity of the iron­(III)-aqua complexes to exhibit the first- and second-order dependence of the rate constant on concentration of HOTf. The rate constants of electron transfer (ET) and oxygen atom transfer of 1 and 2 exhibit the second-order dependence on [HOTf], whereas those of hydrogen atom transfer of 1 and 2 exhibit both the first- and second-order dependences on [HOTf], accompanied by the change in the deuterium kinetic isotope effects. The dependence of the logarithm of the rate constants of acid-promoted oxidation reactions by 1 and 2 on the ET driving force was well evaluated in light of the Marcus theory of outer-sphere ET to discuss the acid-promoted oxidation mechanisms by 1 and 2.