Conformational Change with Steric Interactions Affects the Inner Sphere Component of Concerted Proton-Electron Transfer in a Pyridyl-Appended Radical Cation System.

Abstract

Proton-coupled electron transfer (PCET) model systems combine one-electron oxidants and bases to generate net hydrogen atom acceptors. We have generated two persistent pyridyl-appended radical cations: 10-(pyrid-2-yl)-10H-phenothiazinium (PPT•+) and 3-(pyrid-2-yl)-10-methyl-10H-phenothiazinium (MPTP•+). EPR spectra and corresponding calculations indicate phenothiazinium radical cations with minimal spin on the pyridine nitrogen. Addition of hindered phenols causes the radical cations to decay, and protonated products and the corresponding phenoxyl radicals to form. The ΔG° values for the formation of intermediates (determined through cyclic voltammetry and pKa measurements) rule out a stepwise mechanism, and kinetic isotope effects support concerted proton–electron transfer (CPET) as the mechanism. Calculations indicate that the reaction of PPT•+ + tBu3PhOH undergoes a significant conformational change with steric interactions on the diabatic surface while maintaining the hydrogen bond; in contrast, MPTP•+ + tBu3PhOH maintains its conformation throughout the reaction. This difference is reflected in both experiment and calculations with ΔG(⧧)MPTP•+ < ΔG(⧧)PPT•+ despite ΔG°MPTP•+ > ΔG°PPT•+. Experimental results with 2,6-di-tert-butyl-4-methoxyphenol are similar. Hence, despite the structural similarity between the compounds, differences in the inner sphere component for CPET affect the kinetics.