Electrocatalytic H₂ Evolution by Proton-Gated Hangman Iron Porphyrins

The ability to control proton translocation is essential for optimizing electrocatalytic reductions in acidic solutions. We have synthesized a series of new hangman iron porphyrins with hanging groups of differing proton-donating abilities and evaluated their electrocatalytic hydrogen-evolving ability using foot-of-the-wave analysis. In the presence of excess triphenylphosphine, iron porphyrins initiate proton reduction electrocatalysis upon reduction to Fe^I. By changing the proton-donating ability of the hanging group, we can affect the rate of catalysis by nearly 3 orders of magnitude. The presence of an acid/base moiety in the second coordination sphere results in a marked increase in turnover frequency when extrapolated to zero overpotential

Chromium(IV) Siloxide

The reaction of Na­(OSi^tBu₂Me) with CrCl₃ yields solid [Cr­(OSi^tBu₂Me)₃]_<i>n</i> (<b>1</b>), which can be crystallized in the presence of excess Na­(OSi^tBu₂Me) to yield [Na­(THF)]­[Cr­(OSi^tBu₂Me)₄] (<b>2</b>). This complex is oxidized to yield Cr­(OSi^tBu₂Me)₄ (<b>3</b>), a crystalline chromium­(IV) siloxide complex that is air- and moisture-stable. Electronic spectroscopic analysis of the absorption spectrum of <b>3</b> indicates a particularly weak ligand field (Δ_T = 7940 cm^–1) and covalent Cr–O bonding. <b>3</b> provides the first structural and spectroscopic characterization of a homoleptic chromium­(IV) siloxide complex and provides a benchmark for tetrahedral chromium­(IV) ions residing in solid oxide lattices

Spectroscopic Studies of Nanoparticulate Thin Films of a Cobalt-Based Oxygen Evolution Catalyst

Nanoparticle (NP) cobalt–phosphate (Co-P_i) water oxidation catalysts are prepared as thin films by anodic electrodeposition from solutions of Co²⁺ dissolved in proton-accepting electrolytes. Compositional and structural insight into the nature of the catalyst film is provided from advanced spectroscopy. Infrared spectra demonstrate that counteranions incorporate into the Co-P_i thin films and that the phosphate ion, among various anion electrolytes, exhibits the highest binding affinity to the cobalt centers. Atomic force microscopy images show a highly porous morphology of the thin film that is composed of Co-P_i NPs. Whereas conventional X-ray powder diffraction technique shows catalyst films to be amorphous, synchrotron-based X-ray grazing incidence diffraction reveals well-defined diffraction patterns that are indicative of long-range ordering within the film. Azimuthal scans imply that as-prepared films possess a highly preferred orientation and texture on the electrode surface

Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes

Selective reduction of oxygen is mediated by a series of monometallic rhodium­(III) hydride complexes. Oxidative addition of HCl to <i>trans</i>-Rh^ICl­(L)­(PEt₃)₂ (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide (CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces the corresponding Rh^III hydride complex <i>cis</i>-<i>trans</i>-Rh^IIICl₂H­(L)­(PEt₃)₂ (<b>2a</b>–<b>c</b>). The measured equilibrium constants for the HCl-addition reactions show a pronounced dependence on the identity of the “L” ligand. The hydride complexes effect the reduction of O₂ to water in the presence of HCl, generating <i>trans</i>-Rh^IIICl₃(L)­(PEt₃)₂ (<b>3a</b>–<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic evidence for an intermediate species. For <b>2b/c</b>, an aqua intermediate, <i>cis</i>-<i>trans</i>-[Rh^III(OH₂)­Cl₂(L)­(PEt₃)₂]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently prepared by treating peroxo complexes <i>trans</i>-Rh^IIICl­(L)­(η²-O₂)­(PEt₃)₂ (<b>4b/c</b>) with HCl to rapidly produce a mixture of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the peroxo species demonstrates that they are plausible intermediates in the O₂-reduction chemistry of hydride complexes <b>2a</b>–<b>c</b>. These results together show that monometallic rhodium hydride complexes are capable of promoting selective reduction of oxygen to water and that this reaction may be controlled with systematic alteration of the ancillary ligand set

Chromium(IV) Siloxide

The reaction of Na­(OSi^tBu₂Me) with CrCl₃ yields solid [Cr­(OSi^tBu₂Me)₃]_<i>n</i> (<b>1</b>), which can be crystallized in the presence of excess Na­(OSi^tBu₂Me) to yield [Na­(THF)]­[Cr­(OSi^tBu₂Me)₄] (<b>2</b>). This complex is oxidized to yield Cr­(OSi^tBu₂Me)₄ (<b>3</b>), a crystalline chromium­(IV) siloxide complex that is air- and moisture-stable. Electronic spectroscopic analysis of the absorption spectrum of <b>3</b> indicates a particularly weak ligand field (Δ_T = 7940 cm^–1) and covalent Cr–O bonding. <b>3</b> provides the first structural and spectroscopic characterization of a homoleptic chromium­(IV) siloxide complex and provides a benchmark for tetrahedral chromium­(IV) ions residing in solid oxide lattices

Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes

Selective reduction of oxygen is mediated by a series of monometallic rhodium­(III) hydride complexes. Oxidative addition of HCl to <i>trans</i>-Rh^ICl­(L)­(PEt₃)₂ (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide (CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces the corresponding Rh^III hydride complex <i>cis</i>-<i>trans</i>-Rh^IIICl₂H­(L)­(PEt₃)₂ (<b>2a</b>–<b>c</b>). The measured equilibrium constants for the HCl-addition reactions show a pronounced dependence on the identity of the “L” ligand. The hydride complexes effect the reduction of O₂ to water in the presence of HCl, generating <i>trans</i>-Rh^IIICl₃(L)­(PEt₃)₂ (<b>3a</b>–<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic evidence for an intermediate species. For <b>2b/c</b>, an aqua intermediate, <i>cis</i>-<i>trans</i>-[Rh^III(OH₂)­Cl₂(L)­(PEt₃)₂]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently prepared by treating peroxo complexes <i>trans</i>-Rh^IIICl­(L)­(η²-O₂)­(PEt₃)₂ (<b>4b/c</b>) with HCl to rapidly produce a mixture of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the peroxo species demonstrates that they are plausible intermediates in the O₂-reduction chemistry of hydride complexes <b>2a</b>–<b>c</b>. These results together show that monometallic rhodium hydride complexes are capable of promoting selective reduction of oxygen to water and that this reaction may be controlled with systematic alteration of the ancillary ligand set

A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid

First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn³⁺ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process

Cobalt in a Bis-β-diketiminate Environment

The reaction of Co₂(mesityl)₄ with acetonitrile leads to the formation of a planar, low spin, bis-β-diketiminate cobalt­(II) complex, (1-mesitylbutane-1,3-diimine)₂Co (<b>1</b>). EPR spectroscopy, magnetic studies, and DFT calculations reveal the Co­(II) ion to reside in a tetragonal ligand field with a ²B₂(d_<i>yz</i>)¹ ground state electronic configuration. Oxidation of <b>1</b> with ferrocenium hexafluorophosphate furnishes (1-mesitylbutane-1,3-diimine)₂Co­(THF)₂PF₆ (<b>2</b>). The absence of significant changes in the metal–ligand bond metrics of the X-ray crystal structures of <b>1</b> and <b>2</b> supports ligand participation in the oxidation event. Moreover, no significant changes in C–C or C–N bond lengths are observed by X-ray crystallography upon oxidation of a β-diketiminate ligand, in contrast to typical redox noninnocent ligand platforms

Mechanistic Studies of the Oxygen Evolution Reaction Mediated by a Nickel–Borate Thin Film Electrocatalyst

A critical determinant of solar-driven water splitting efficiency is the kinetic profile of the O₂ evolving catalyst (OEC). We now report the kinetic profiles of water splitting by a self-assembled nickel–borate (NiB_i) OEC. Mechanistic studies of anodized films of NiB_i exhibit the low Tafel slope of 2.3 × <i>RT</i>/2<i>F</i> (30 mV/decade at 25 °C). This Tafel slope together with an inverse third order rate dependence on H⁺ activity establishes NiB_i as an ideal catalyst to be used in the construction of photoelectrochemical devices for water splitting. In contrast, nonanodized NiB_i films display significantly poorer activity relative to their anodized congeners that we attribute to a more sluggish electron transfer from the catalyst resting state. Borate is shown to play two ostensibly antagonistic roles in OEC activity: as a promulgator of catalyst activity by enabling proton-coupled electron transfer (PCET) and as an inhibitor in its role as an adsorbate of active sites. By defining the nature of the PCET pre-equilibrium that occurs during turnover, trends in catalyst activity may be completely reversed at intermediate pH as compared to those at pH extremes. These results highlight the critical role of PCET pre-equilibria in catalyst self-assembly and turnover, and accordingly suggest a reassessment in how OEC activities of different catalysts are compared and rationalized

Oxidative Degradation of Multi-Carbon Substrates by an Oxidic Cobalt Phosphate Catalyst

The development of heterogeneous catalysts to affect the activation of recalcitrant biomolecules has applications for biomass processing, biomass fuel cells, and wastewater remediation. We demonstrate that a cobalt oxygen evolution catalyst (Co-OEC) can catalyze the oxidation of carbon feedstocks completely to CO₂. A quantitative analysis of the product distribution from the oxidative degradation of the C₂ compound, ethylene glycol, is elaborated and a reaction sequence is proposed. The Co-OEC is also found to be competent for oxidatively degrading C₂₊ compounds, including glucose and lignin, to carbon dioxide at consequential Faradaic efficiencies