Dissecting the Temperature Dependence of Electron–Proton Transfer Reactivity

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.