Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes

Abstract

Hydrogen atom transfer (HAT) reactions of metal–oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes are yet to be studied in comparison with those of the corresponding metal-hydroxo complexes. In this study, a series of Mn(III)-aqua complexes, [(dpaq^5R)Mn^III(OH₂)]²⁺ (dpaq^5R = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate with a substituent at the 5-position; R = NO₂, Cl, H, Me and OMe) were synthesized by protonating the corresponding Mn(III)-hydroxo complexes, [(dpaq^5R)Mn^III(OH)]⁺, with trifluoromethanesulfonic acid (HOTf). X-ray crystal structures of both [(dpaq^5R)Mn^III(OH)]⁺ and [(dpaq^5R)Mn^III(OH₂)]²⁺ complexes showed a mononuclear Mn^III center in a distorted octahedral environment. The electronic substitution effect of the dpaq ligand on the HAT reactions of [(dpaq^5R)Mn^III(OH)]⁺ and [(dpaq^5R)Mn^III(OH₂)]²⁺ was then examined by determining the rate constants (k₂) of the HAT reactions from 4-methoxy-2,6-di-tert-butylphenol to the Mn(III)-hydroxo and -aqua complexes. Hammett plots indicate that HAT from 4-methoxy-2,6-di-tert-butylphenol to [(dpaq^5R)Mn^III(OH)]⁺ proceeds via an oxidative asynchronous coupled proton–electron transfer (CPET), whereas HAT from 4-methoxy-2,6-di-tert-butylphenol to [(dpaq^5R)Mn^III(OH₂)]²⁺ proceeds mainly via a basic asynchronous CPET. The contribution of a stepwise electron transfer (ET)/proton transfer (PT) pathway is discussed by comparing the electronic substitution effect on the rate constants of outer-sphere electron transfer and HAT reactions. This study has provided valuable insights into the asynchronous CPET mechanisms of HAT reactions in which the oxidative vs. basic pathways are determined to exhibit reversed electronic substitution effects on HAT reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes.