Catalytic Two-Electron Reduction of Dioxygen by Ferrocene Derivatives with Manganese(V) Corroles

Abstract

Electron transfer from octamethylferrocene (Me(8)Fc) to the manganese(V) imidocorrole complex (tpfc)Mn-V(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl2C6H3] proceeds efficiently to give an octamethylferrocenium ion (Me(8)Fc(+)) and [(tpfc)Mn-IV(NAr)](-) in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn-IV(NAr)](-) by Me(8)Fc gives (tpfc)Mn-III and ArNH2 in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn-V(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn-V(OH)](+) in deaerated MeCN. [(tpfc)Mn-V(OH)](+) is rapidly reduced by 2 equiv of Me(8)Fc in the presence of TFA to give (tpfc)Mn-III in deaerated MeCN. In the presence of dioxygen (O-2), (tpfc)Mn-III catalyzes the two-electron reduction of O-2 by Me(8)Fc with TFA in MeCN to produce H2O2 and Me(8)Fc(+). The rate of formation of Me(8)Fc(+) in the catalytic reduction of O-2 follows zeroth-order kinetics with respect to the concentrations of Me8Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn-V(NAr) and O-2. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn-III to O-2, followed by further proton-coupled electron transfer from Me(8)Fc to produce H2O2 and [(tpfc)Mn-IV](+) Rapid electron transfer from Me(8)Fc to [(tpfc)Mn-IV](+)regenerates (tpfc)Mn-III, completing the catalytic cycle. Thus, catalytic two-electron reduction of O-2 by Me(8)Fc with (tpfc)(MnNAr)-N-V) as a catalyst precursor proceeds via a Mn-III/Mn-IV redox cycle.