Curtis Berlinguette

Light-driven decomposition of Ir(acac)3 spin-cast on a conducting glass substrate produces a thin conformal film of amorphous iridium oxide, a-IrOx. The decomposition process, which was carried out under an ambient atmosphere at room temperature and tracked by Fourier transform infrared (FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal charge transfer (LMCT) process. The amorphous nature of the films is based on the lack of any observable Bragg reflections by powder X-ray… Show more

Light-driven decomposition of Ir(acac)3 spin-cast on a conducting glass substrate produces a thin conformal film of amorphous iridium oxide, a-IrOx. The decomposition process, which was carried out under an ambient atmosphere at room temperature and tracked by Fourier transform infrared (FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal charge transfer (LMCT) process. The amorphous nature of the films is based on the lack of any observable Bragg reflections by powder X-ray diffraction techniques; the elemental composition was corroborated by X-ray photoelectron spectroscopy (XPS) measurements. The films are found to be excellent electrocatalysts for mediating the oxygen evolution reaction (OER) in acidic media, as evidenced by the onset of catalysis at 130 mV and a Tafel slope of 34 mV dec–1. These parameters enable current densities of 1 and 10 mA cm–2 to be reached at 190 and 220 mV, respectively. Exposing the films to higher temperatures (500 °C) renders a film of crystalline iridium oxide, c-IrOx, which displays a Tafel slope of 60 mV dec–1, thus requiring an additional 50 mV to reach a current density of 1 mA cm–2. The film of a-IrOx reported here is among the best OER electrocatalysts reported to date. Show less

Photochemical metal–organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of… Show more

Photochemical metal–organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations. Show less

The proton-coupled electron transfer (PCET) chemistry associated with the [Co-OH]2+/[Co-OH2]2+ redox couple for [Co(PY5)(OH2)]2+ (1; PY5 = 2,6-(bis(bis-2-pyridyl)-methoxymethane)-pyridine) and [CoII(pz4depy)(OH2)]2+ (2; pz4depy = 2,6-bis(1,1-di(1H-pyrazol-1-yl)ethyl)pyridine) is reported. It is found that the couple is acutely sensitive to the geometry of the axially ligated group in addition to the electronic-donating/-withdrawing character of the ligand. Interrogation of the electron-transfer… Show more

The proton-coupled electron transfer (PCET) chemistry associated with the [Co-OH]2+/[Co-OH2]2+ redox couple for [Co(PY5)(OH2)]2+ (1; PY5 = 2,6-(bis(bis-2-pyridyl)-methoxymethane)-pyridine) and [CoII(pz4depy)(OH2)]2+ (2; pz4depy = 2,6-bis(1,1-di(1H-pyrazol-1-yl)ethyl)pyridine) is reported. It is found that the couple is acutely sensitive to the geometry of the axially ligated group in addition to the electronic-donating/-withdrawing character of the ligand. Interrogation of the electron-transfer kinetics by electrochemical methods also shows for the first time that the interconversion of [CoIII-OH]2+ and [CoII-OH2]2+ proceeds through a concerted pathway in favour of energetically unfavourable stepwise electron-transfer or proton-transfer reaction steps. Show less

The recent recognition that a single metal site is capable of mediating the multiple electron and proton transfer events associated with water oxidation represents a pivotal discovery for the field. This finding has led to a remarkable expansion of known synthetic water oxidation catalysts, and has provided the means to gain unprecedented insight into the reaction steps involved with O–O bond formation. This perspective reflects on the key studies that have advanced our understanding of water… Show more

The recent recognition that a single metal site is capable of mediating the multiple electron and proton transfer events associated with water oxidation represents a pivotal discovery for the field. This finding has led to a remarkable expansion of known synthetic water oxidation catalysts, and has provided the means to gain unprecedented insight into the reaction steps involved with O–O bond formation. This perspective reflects on the key studies that have advanced our understanding of water oxidation catalysis while summarizing molecular features that are integral to negotiating this complicated reaction pathway with the goal of helping identify new frontiers of discovery for the field. Show less

Examination of the aqueous electrochemistry of a Co(II) complex bearing a pentadentate ligand suggests that the catalytic current corresponding to water oxidation is molecular in origin, and does not emanate exclusively from Co-oxide phases formed in situ.

The oxidation of water catalyzed by [Ru(tpy)(bpy)(OH2)](ClO4)2 (1; tpy = 2,2′;6′′,2′′-terpyridine; bpy = 2,2′-bipyridine) is evaluated in different acidic media at variable oxidant concentrations. The observed rate of dioxygen evolution catalyzed by 1 is found to be highly dependent on pH and the identity of the acid; e.g., d[O2]/dt is progressively faster in H2SO4, CF3SO3H (HOTf), HClO4, and HNO3, respectively. This trend does not track with thermodynamic driving force of the electron-transfer… Show more

The oxidation of water catalyzed by [Ru(tpy)(bpy)(OH2)](ClO4)2 (1; tpy = 2,2′;6′′,2′′-terpyridine; bpy = 2,2′-bipyridine) is evaluated in different acidic media at variable oxidant concentrations. The observed rate of dioxygen evolution catalyzed by 1 is found to be highly dependent on pH and the identity of the acid; e.g., d[O2]/dt is progressively faster in H2SO4, CF3SO3H (HOTf), HClO4, and HNO3, respectively. This trend does not track with thermodynamic driving force of the electron-transfer reactions between the terminal oxidant, (NH4)2[Ce(NO3)6] (CAN), and Ru catalyst in each of the acids. The particularly high reactivity in HNO3 is attributed to the NO3− anion: (i) enabling relatively fast electron-transfer steps; (ii) participating in a base-assisted concerted atom-proton transfer process that circumvents the formation of high energy intermediates during the O−O bond formation process; and (iii) accelerating the liberation of dioxygen from the catalyst. Consequently, the position of the rate-determining step within the catalytic cycle can be affected by the acid medium. These factors collectively contribute to the position of the rate-determining step within the catalytic cycle being affected by the acid medium. This offering also outlines how other experimental issues (e.g., spontaneous decay of the Ce(IV) species in acidic media; CAN/catalyst molar ratio; types of catalytic probes) can affect the Ce(IV)-driven oxidation of water catalyzed by homogeneous molecular complexes. Show less

The pH-dependent electrochemical behavior for a Co(II) complex, [Co(Py5)(OH2)](ClO4)2 (1; Py5 = 2,6-(bis(bis-2-pyridyl)methoxymethane)pyridine), indicates consecutive (proton-coupled) oxidation steps furnish a CoIV species that catalyzes the oxidation of water in basic media.

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as [Ru(tpy)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 1; 4,4′-dimethoxy-2,2′-bipyridine (bpy-OMe), 2; 4,4′-dicarboxy-2,2′-bipyridine (bpy-CO2H), 3; tpy = 2,2′;6′′,2′′-terpyridine] is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the [RuV(tpy)(L)O]3+ ([RuV=O]3+) motif; the… Show more

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as [Ru(tpy)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 1; 4,4′-dimethoxy-2,2′-bipyridine (bpy-OMe), 2; 4,4′-dicarboxy-2,2′-bipyridine (bpy-CO2H), 3; tpy = 2,2′;6′′,2′′-terpyridine] is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the [RuV(tpy)(L)O]3+ ([RuV=O]3+) motif; the relative positions of each of these redox couples reflects the nature of the electron-donating or withdrawing character of the substituents on the bpy ligands. The addition of one (or more) equivalents of the terminal electron-acceptor, (NH4)2[Ce(NO3)6] (CAN), to the [RuIV(tpy)(L)O]2+ ([RuIV=O]2+) forms of each of the catalysts, however, leads to divergent reaction pathways. The addition of 1 eq of CAN to the [RuIV=O]2+ form of 2 generates [RuV=O]3+ (k3 = 3.7 M−1 s−1), which, in turn, undergoes slow O−O bond formation with the substrate (kO−O = 3 × 10−5 s−1). The rate-determining step is assigned as the liberation of dioxygen from the [RuIV−OO]2+ level under catalytic conditions for each complex. Complex 2, however, passes through the [RuV−OO]3+ level prior to the rapid loss of dioxygen. Evidence for a competing reaction pathway is provided for 3, where the [RuV=O]3+ and [RuIII−OH]2+ redox levels can be generated by disproportionation of the [RuIV=O]2+ form of the catalyst (kd = 1.2 M−1 s−1). An auxiliary reaction pathway involving the abstraction of an O-atom from CAN is also implicated during catalysis. The variability of reactivity for 1-3, including the position of the RDS and potential for O-atom transfer from the terminal oxidant, is confirmed to be intimately sensitive to electron density at the metal site through extensive kinetic and isotopic labeling experiments. Show less

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as [Ru(tpy)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 1; 4,4′-dimethoxy-2,2′-bipyridine (bpy-OMe), 2; 4,4′-dicarboxy-2,2′-bipyridine (bpy-CO2H), 3; tpy = 2,2′;6′′,2′′-terpyridine] is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the [RuV(tpy)(L)O]3+ ([RuV=O]3+) motif; the… Show more

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as [Ru(tpy)(L)(OH2)]2+ [L = 2,2′-bipyridine (bpy), 1; 4,4′-dimethoxy-2,2′-bipyridine (bpy-OMe), 2; 4,4′-dicarboxy-2,2′-bipyridine (bpy-CO2H), 3; tpy = 2,2′;6′′,2′′-terpyridine] is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the [RuV(tpy)(L)O]3+ ([RuV=O]3+) motif; the relative positions of each of these redox couples reflects the nature of the electron-donating or withdrawing character of the substituents on the bpy ligands. The addition of one (or more) equivalents of the terminal electron-acceptor, (NH4)2[Ce(NO3)6] (CAN), to the [RuIV(tpy)(L)O]2+ ([RuIV=O]2+) forms of each of the catalysts, however, leads to divergent reaction pathways. The addition of 1 eq of CAN to the [RuIV=O]2+ form of 2 generates [RuV=O]3+ (k3 = 3.7 M−1 s−1), which, in turn, undergoes slow O−O bond formation with the substrate (kO−O = 3 × 10−5 s−1). The rate-determining step is assigned as the liberation of dioxygen from the [RuIV−OO]2+ level under catalytic conditions for each complex. Complex 2, however, passes through the [RuV−OO]3+ level prior to the rapid loss of dioxygen. Evidence for a competing reaction pathway is provided for 3, where the [RuV=O]3+ and [RuIII−OH]2+ redox levels can be generated by disproportionation of the [RuIV=O]2+ form of the catalyst (kd = 1.2 M−1 s−1). An auxiliary reaction pathway involving the abstraction of an O-atom from CAN is also implicated during catalysis. The variability of reactivity for 1-3, including the position of the RDS and potential for O-atom transfer from the terminal oxidant, is confirmed to be intimately sensitive to electron density at the metal site through extensive kinetic and isotopic labeling experiments. Show less

A family of compounds based on the mononuclear coordination complex [Ru(tpy)(bpy)(OH2)]2+ (1b; tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine) are shown to be competent catalysts in the Ce(IV)-driven oxidation of water in acidic media. The systematic installation of electron-withdrawing (e.g., −Cl, −COOH) and −donating (e.g., −OMe) groups at various positions about the periphery of the polypyridyl framework offers insight into how electronic parameters affect the properties of water… Show more

A family of compounds based on the mononuclear coordination complex [Ru(tpy)(bpy)(OH2)]2+ (1b; tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine) are shown to be competent catalysts in the Ce(IV)-driven oxidation of water in acidic media. The systematic installation of electron-withdrawing (e.g., −Cl, −COOH) and −donating (e.g., −OMe) groups at various positions about the periphery of the polypyridyl framework offers insight into how electronic parameters affect the properties of water oxidation catalysts. It is observed, in general, that electron-withdrawing groups (EWGs) on the bpy ligands suppress catalytic activity (kobs) and enhance catalytic turnover numbers (TONs); conversely, the presence of electron-donating groups (EDGs) accelerate catalytic rates while decreasing catalyst stability. We found that 2,2′-bipyridine N,N′-dioxide is produced when 1b is subject to excess Ce(IV) in acidic media, which suggests that dissociation of the bpy ligand is a source of catalyst deactivation and/or decomposition. Density functional theory (DFT) calculations corroborate these findings by showing that the Ru−Nbpy bond trans to the O atom is weakened at higher oxidation levels while the other Ru−N bonds are affected to a lesser extent. We also show that the Ru−Cl bond is not robust in aqueous media, which has implications in studying the catalytic behavior of systems of this type. Show less