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 H-1 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) x 10(4) M-1 s(-1) at 298 K in MeCN-d(3) (mu = 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) x 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 lambda(H .) > lambda(e)(-) > lambda(H+) is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.