A Tale of Two Complexes: Electro-Assisted Oxidation of Thioanisole by an "O2 Activator/Oxidizing Species" Tandem System of Non-Heme Iron Complexes.

We report two new FeIII complexes [L1 FeIII (H2 O)](OTf)2 and [L2 FeIII (OTf)], obtained by replacing pyridines by phenolates in a known non-heme aminopyridine iron complex. While the original, starting aminopyridine [(L5 2 )FeII (MeCN)](PF6 ) complex is stable in air, the potentials of the new FeIII/II couples decrease to the point that [L2 FeII ] spontaneously reduces O2 to superoxide. We used it as an O2 activator in an electrochemical setup, as its presence allows to generate superoxide at a much more accessible potential (>500 mV gain). Our aim was to achieve substrate oxidation via the reductive activation of O2 . While L2 FeIII (OTf) proved to be a good O2 activator but a poor oxidation system, its association with another complex (TPEN)FeII (PF6 )2 generates a complementary tandem couple for electro-assisted oxidation of substrates, working at a very accessible potential: upon reduction, L2 FeIII (OTf) activates O2 to superoxide and transfers it to (TPEN)FeII (PF6 )2 leading in fine to the oxidation of thioanisole.

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 O2 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 O2 or H2O2, thus ensuring a heterolytic O-O bond cleavage. Herein, we report the synthesis and study of a new non-heme FeII complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H2O2 generates the classical FeIIIOOH species at high H2O2 loading. But at low H2O2 concentrations, an FeIVO 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 FeII → FeIVO conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the FeII/H2O2 adduct leading to a heterolytic O-O bond cleavage.