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

Abstract

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 (Fe III (H2bim)3) 3+ causes broadening of the 1 H NMR resonances of the ferrous analogue, (Fe II (H2bim)3) 2+ . This indicates electron self-exchange with ke - ) (1.7 ( 0.2) 10 4 M -1 s -1 at 298 K in MeCN-d3 (I ) 0.1 M). Similar broadening is observed when the deprotonated ferric complex (Fe III (Hbim)(H2bim)2) 2+ is added to (Fe II (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) 10 3 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, (Fe III (Hbim)(H2bim)2) 2+ and (Fe III (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 IH ¥ > Ie - > IH + is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.