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 (H 2 bim) have been investigated by the dynamic NMR line-broadening technique. Addition of the ferric complex [Fe III (H 2 bim) 3 ] 3+ causes broadening of the 1 H NMR resonances of the ferrous analogue, [Fe II (H 2 bim) 3 ] 2+ . This indicates electron self-exchange with k e − = (1.7 ± 0.2) × 10 4 M - 1 s - 1 at 298 K in MeCN- d 3 (μ = 0.1 M). Similar broadening is observed when the deprotonated ferric complex [Fe III (Hbim)(H 2 bim) 2 ] 2+ is added to [Fe II (H 2 bim) 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 k H • = (5.8 ± 0.6) × 10 3 M - 1 s - 1 . This is a factor of 3 smaller than k e − under the same conditions. The H-atom exchange reaction exhibits a primary kinetic isotope effect k NH / k ND = 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, [Fe III (Hbim)(H 2 bim) 2 ] 2+ and [Fe III (H 2 bim) 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 λ H • > λ e − > λ H + is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.