Infrared Spectroscopic Observation of Oxo- and Superoxo-Intermediates in the Water Oxidation Cycle of a Molecular Ir Catalyst

Abstract

Molecular Ir catalysts have emerged as an important class of model catalysts for understanding structure−activity relationships in water oxidation, a reaction that is central to renewable fuel synthesis. Prior efforts have mostly focused on controlling and elucidating the emergence of active species from prepared precursors. However, the development of efficient and stable molecular Ir catalysts also necessitates probing of reaction intermediates. To date, relatively little is known about the key intermediates in the cycles of the molecular Ir catalysts. Herein, we probed the catalytic cycle of a homogeneous Ir catalyst ("blue dimer") at a Au electrode/aqueous electrolyte interface by combining surface-enhanced infrared absorption spectroscopy (SEIRAS) with phase-sensitive detection (PSD). Cyclic voltammograms (CVs) from 1.4 to 1.7 V RHE (RHE = reversible hydrogen electrode) give rise to a band at ∼818 cm −1 , whereas CVs from 1.4 to ≥1.85 V RHE generate an additional band at ∼1146 cm −1 . Isotope labeling experiments indicate that the bands at ∼818 and ∼1146 cm −1 are attributable to oxo (Ir V �O) and superoxo (Ir IV − OO • ) moieties, respectively. This study establishes PSD-SEIRAS as a sensitive tool for probing water oxidation cycles at electrode/ electrolyte interfaces and demonstrates that the relative abundance of two key intermediates can be tuned by the thermodynamic driving force of the reaction.

Highly Selective Electrochemical Baeyer–Villiger Oxidation through Oxygen Atom Transfer from Water

“…They are ascribed to the O–O and O–C–O stretching modes within [Fe–O–O–R], respectively (see the SI for detailed discussions). ,,, The reversibility of these signals with changing potentials was further confirmed by filtering out static background absorbance with phase sensitive detection (Figure S27). The successful detection of the [Fe–O–O–R] intermediate lends additional strong support to the proposed mechanisms. It is important to note that the nucleophilic attack by the M-OOH species on the substrate is the key enabling factor for the Baeyer–Villiger oxidation.…”

“…A pseudo-first-order characteristic was thus observed relative to the cyclohexanone concentration. By comparison, at low applied potentials, a relatively slow H 2 O oxidation is expected, implying a relatively low coverage of surface *OOH species . In this case, the overall reaction rate would be largely determined by the availability of surface *>OOH species (Figure c) .…”

“…A subsequent rearrangement promptly releases the product and recovers MO for the next catalytic cycle. To further support the proposed catalytic cycle, we next performed attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS, see the SI for experimental details) in operando . ,, The IR spectra in Figure b clearly show two vibrational bands centered at around 1042 and 1396 cm –1 (see Figure S26 for the full spectra). They are ascribed to the O–O and O–C–O stretching modes within [Fe–O–O–R], respectively (see the SI for detailed discussions). ,,, The reversibility of these signals with changing potentials was further confirmed by filtering out static background absorbance with phase sensitive detection (Figure S27).…”

“…It is last noted that the Baeyer–Villiger oxidation could, in principle, be enabled by M-OO • (superoxo radicals), as well. If so, it would imply that the OAT takes place at a later stage of the H 2 O oxidation reactions. ,, The difference notwithstanding, all discussions presented here would still be valid should this mechanism be the preferred one. However, we consider such a mechanism less likely for Fe 2 O 3 -based catalysts.…”

“…To test this hypothesis, the next set of experiments were designed to prove that the reaction is indeed surface initiated. With the premise that *OOH is a key intermediate for the activation of cyclohexanone, we sought to vary its surface coverage and observe how the Baeyer–Villiger oxidation kinetics change (Figure a) . For this purpose, we chose two electrochemical potentials that correspond to different oxidation kinetics: at 1.42 V, the nominal current density as measured in Figure a was 4.45 mA/cm 2 , while at 1.71 V, the value was 13.17 mA/cm 2 .…”