Second-sphere effects on H2O2 activation by non-heme FeII complexes: role of a phenol group in the [H2O2]-dependent accumulation of FeIVO vs. FeIIIOOH

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O-2 or H2O2 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-2 or H2O2, thus ensuring a heterolytic O-O bond cleavage. Herein, we report the synthesis and study of a new non-heme Fe-II complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H2O2 generates the classical (FeOOH)-O-III species at high H2O2 loading. But at low H2O2 concentrations, an (FeO)-O-IV 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 H2O2 activation. Our results suggest a direct Fe-II -> (FeO)-O-IV conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the Fe-II/H2O2 adduct leading to a heterolytic O-O bond cleavage.

Automated summary

Redox metalloenzymes achieve selective oxidation reactions using O-2 or H2O2 as oxidants and release harmless side-products like water. A new non-heme Fe-II complex with a pentaazadentate first coordination sphere and a pendant phenol group was synthesized and studied. The involvement of the attached phenol as a second coordination sphere moiety during H2O2 activation was supported by kinetic, variable temperature, and labelling studies. The results suggest a Fe-II -> (FeO)-O-IV conversion directed by the 2nd sphere phenol via protonation, leading to heterolytic O-O bond cleavage.