Dissecting the Temperature Dependence of Electron–Proton Transfer Reactivity

Abstract

The rate of elementary reactions usually rises with increasing temperature. In rare cases, however, a slowdown is observed instead. One example of this is hydrogen-atom abstraction from the iron(II)-tris[2,2′-bi(tetrahydropyrimidine)] complex, [FeII(H2bip)3]2+, by the TEMPO radical. So far ascribed to a strongly temperature-dependent equilibrium constant Keq, this description does not fully account for the observed rate deceleration. In this work, we dissect the activation barriers of four electron–proton transfers including this exceptional case, by employing the concept of asynchronicity, derived as an extension to Marcus theory, together with the classical Bell–Evans–Polanyi effect. By also accounting for tunneling and statistics, the presented theoretical model yields near-quantitative accuracy. Based on chemically well-defined quantities, this method offers a detailed insight into temperature-dependent kinetics of hydrogen-atom abstraction reactions and may serve as an alternative to the established Eyring plot analysis.