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

Abstract

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.

1. Redox Processes of [(N4Py)Fe(II)(CH3CN)]2+ , where L = N₄Py, with the Main Ligand Exchange Reactions and Reported Potentials for the Fe­(III)/Fe­(IV) Redox Couple

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Electrochemical oxidation of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{O}\mathrm{H}}_2)]}^{2+}$ from the Fe­(II) to Fe­(III) and then the Fe­(IV)O state occurs at the electrode with the same distance dependence as surface enhancement of Raman scattering (right). Fe­(IV)O diffusing away from the electrode comproportionates with Fe­(II)­OH₂ in the depletion layer.

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Cyclic voltammogram of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{N})]}^{2+}$ (0.5 mM) in H₂O at pH 4–5 at a smooth Au electrode (blue), a smooth Au electrode to a more positive potential (red), an electrochemically roughened Au electrode (black), and a control at a roughened Au electrode in the absence of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{N})]}^{2+}$ (black, dotted). In 0.1 M KNO₃ (aq) at pH 4–5 (adjusted with HNO₃). Scan rate 0.1 V s^–1, RE: Ag/AgCl, and CE: Pt.

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Raman spectra (top) SERS (red) and nonresonant Raman (black) spectrum of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{N})]}^{2+}$ (2 mM) in 0.1 M KNO₃ (aq) in solution at λ_exc 785 nm, (bottom) SERS (λ_exc 785 nm, red), resonance Raman spectrum (λ_exc 473 nm at pH 5, blue) in H₂O, and solid-state Raman spectrum (λ_exc 785 nm, black) of the sample prepared by drop-cast deposition.

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SERS spectra of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{N})]}^{2+}$ (2 mM) recorded during cyclic voltammetry. The potential and time are indicated by color (0.1 V to 1.1 V vs SCE) and indicated on the left and right, respectively. 0.1 M KNO₃ (aq), WE: gold bead, RE: Ag/AgCl, CE: Pt, and scan rate: 0.1 V s^–1, SERS spectra recorded with 0.5 s acquisitions at λ_exc 785 nm. Spectra were normalized to the NO₃ ^– band at 1049 cm^–1, indicated with a dotted black line.

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(A) SERS spectra of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{O}\mathrm{H}}_2)]}^{2+}$ in H₂O recorded with the electrode held at the potentials indicated; fitted surface-enhanced Raman intensity from Fe­(III)­O–Fe­(III) and Fe­(IV)O vs (B) time and (C) voltage, respectively. (D) Fitted SERS intensities of three forward scans from Fe­(III)–O and Fe­(IV)O plotted as ln­(Fe^IV = O/Fe^III) vs potential. See the Supporting Information for further discussion of fitting. Conditions: 2 mM [(N₄Py)­Fe­(II)­(CH₃CN)]­(OTf)₂, 0.1 M KNO₃ (aq). SERS spectra recorded by accumulation of five 5 s acquisitions at 785 nm, scan rate 0.1 V s^–1, WE: gold bead, RE: Ag/AgCl, and CE: Pt. Spectra were normalized on the NO₃ ^– band at 1049 cm^–1.

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Raman spectrum of [(N₄Py)­Fe­(IV)O]²⁺ (black) compared to a SERS spectrum of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{O}\mathrm{H}}_2)]}^{2+}$ at 0.9 V vs SCE in H₂O (red) and compared to a SERS spectrum of ${[({\mathrm{N}}_4\mathrm{P}\mathrm{y})\mathrm{F}\mathrm{e}(\mathrm{I}\mathrm{I})({\mathrm{O}\mathrm{H}}_2)]}^{2+}$ at 1.0 V vs SCE in H₂ ¹⁸O (blue). SERS and Raman spectra were recorded at λ_exc 785 nm, with, respectively, 5 s acquisition and 5 accumulation and 50 s acquisition and 20 accumulations.

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Optimized geometries, Fe–O vibrational mode shifts, Fe–O bond lengths, and electron density transferred for the free [(N₄Py)­Fe­(IV)O]²⁺ complex (PBE/TZ2P) and the complex adsorbed on a gold nanoparticle (PBE/TZ2P, 55 atoms, icosahedral geometry, see the Experimental Details section).