Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes

Self-exchange reactions between high-spin iron complexes of 2,2‘-bi-imidazoline (H2bim) have been
investigated by the dynamic NMR line-broadening technique. Addition of the ferric complex [FeIII(H2bim)3]3+
causes broadening of the 1H NMR resonances of the ferrous analogue, [FeII(H2bim)3]2+. This indicates electron
self-exchange with ke− = (1.7 ± 0.2) × 104 M-1 s-1 at 298 K in MeCN-d3 (μ = 0.1 M). Similar broadening
is observed when the deprotonated ferric complex [FeIII(Hbim)(H2bim)2]2+ is added to [FeII(H2bim)3]2+. Because
these reactants differ by a proton and an electron, this is a net hydrogen atom exchange reaction. Kinetic and
thermodynamic results preclude stepwise mechanisms of sequential proton and then electron transfer, or electron
and then proton transfer. Concomitant electron and proton (H•) transfer occurs with bimolecular rate constant
kH• = (5.8 ± 0.6) × 103 M-1 s-1. This is a factor of 3 smaller than ke− under the same conditions. The H-atom
exchange reaction exhibits a primary kinetic isotope effect kNH/kND = 2.3 ± 0.3 at 324 K, whereas no such
effect is detected in the electron exchange reaction. Proton self-exchange between the two ferric complexes,
[FeIII(Hbim)(H2bim)2]2+ and [FeIII(H2bim)3]3+, has also been investigated and is found to be faster than both
the electron and H-atom transfer reactions. From kinetic analyses and the application of simple Marcus theory,
an order of intrinsic reaction barriers λH• > λe− > λH+ is derived. The reorganization energies are discussed in
terms of their inner-sphere and outer-sphere components.