Second-sphere effects on H 2 O 2 activation by non-heme Fe II complexes: role of a phenol group in the [H 2 O 2 ]-dependent accumulation of Fe IV O vs. Fe III OOH.

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O 2 or H 2 O 2 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 H 2 O 2 , 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 H 2 O 2 generates the classical Fe III OOH species at high H 2 O 2 loading. But at low H 2 O 2 concentrations, an Fe IV O 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 H 2 O 2 activation. Our results suggest a direct Fe II → Fe IV O conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the Fe II /H 2 O 2 adduct leading to a heterolytic O-O bond cleavage.