posted on 2021-10-04, 18:51authored byEmma N. Cook, Diane A. Dickie, Charles W. Machan
We report a bioinspired non-heme
Fe complex with a tripodal [N3O]− ligand
framework (Fe(PMG)(Cl)2) that is electrocatalytically active
toward dioxygen reduction with
acetic acid as a proton source in acetonitrile solution. Under electrochemical
and chemical conditions, Fe(PMG)(Cl)2 selectively produces
water via a 2+2 mechanism, where H2O2 is generated
as a discrete intermediate species before further reduction to two
equivalents of H2O. Mechanistic studies support a catalytic
cycle for dioxygen reduction where an off-cycle peroxo dimer species
is the resting state of the catalyst. Spectroscopic analysis of the
reduced complex FeII(PMG)Cl shows the stoichiometric formation
of an Fe(III)-hydroxide species following exposure to H2O2; no catalytic activity for H2O2 disproportionation is observed, although the complex is electrochemically
active for H2O2 reduction to H2O.
Electrochemical studies, spectrochemical experiments, and DFT calculations
suggest that the carboxylate moiety of the ligand is sensitive to
hydrogen-bonding interactions with the acetic acid proton donor upon
reduction from Fe(III)/(II), favoring chloride loss trans to the tris-alkyl amine moiety of the ligand framework. These results
offer insight into how mononuclear non-heme Fe active sites in metalloproteins
distribute added charge and poise proton donors during reactions with
dioxygen.