Seeing an Unobservable Fe(III)/Fe(IV) Redox Process of the Nonheme Iron N4Py Complex by High-Speed Surface-Enhanced...

High-valent iron oxido species, central to many enzymatic and biomimetic catalyzed organic oxidative transformations, can be generated by direct electrochemical oxidation, circumventing high-energy O atom donor reagents. Electrochemical generation necessitates knowledge of the redox potentials involved, which is hindered by the lack of well-defined Fe(III)/Fe(IV) redox waves in the voltammetry of many iron-based catalysts. Hence, other approaches including chemical oxidation and bulk spectro(electro)chemical methods need to be taken. In the case of the well-studied oxidation catalyst, ${\left[\left(N_{4}Py\right)Fe\left(II\right){OH}_2\right]}^{2+}$, where N₄Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine, estimates of the Fe(III/IV) redox potentials range from 0.4 to 1.3 V vs SCE. Here, we show that electrochemical surface-enhanced Raman scattering spectroscopy reveals "hidden" redox waves, and hence redox potentials, when coupled with cyclic voltammetry. Rapid spectral acquisition (>2 Hz) of surface-enhanced Raman spectra at electrochemically roughened gold electrodes enables real-time spectral acquisition during cyclic voltammetry. We show that the Fe(III)/Fe(IV) redox potential of ${\left[\left(N_{4}Py\right)Fe\left(II\right){OH}_2\right]}^{2+}$ is close to that determined earlier by chemical redox titrations (0.85 V vs SCE). Furthermore, comproportionation and adsorption processes are shown to impact the rates of electron transfer observed, which rationalizes the absence of a distinct Fe(III)/Fe(IV) redox wave in its cyclic voltammetry.