Second-sphere effects on HO activation by non-heme Fe complexes: role of a phenol group in the [HO]-dependent accumulation of FeO FeOOH.

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O or HO as oxidants and release harmless side-products like water. Their oxidation selectivity is intrinsically linked to the control of the oxidizing species generated during the catalytic cycle. To do so, a second coordination sphere is used in order to create a pull effect during the activation of O or HO, thus ensuring a heterolytic O-O bond cleavage. Herein, we report the synthesis and study of a new non-heme Fe complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with HO generates the classical FeOOH species at high HO loading. But at low HO concentrations, an FeO species is generated instead. The formation of the latter is directly related to the presence of the 2nd sphere phenol group. Kinetic, variable temperature and labelling studies support the involvement of the attached phenol as a second coordination sphere moiety (weak acid) during HO activation. Our results suggest a direct Fe → FeO conversion directed by the 2nd sphere phenol the protonation of the distal O atom of the Fe/HO adduct leading to a heterolytic O-O bond cleavage.