Molecular Catalysts for Water Oxidation Blakemore, James D; Crabtree, Robert H; Brudvig, Gary W
Chemical reviews,
12/2015, Volume:
115, Issue:
23
Journal Article
A series of Cp*Ir catalysts are the most active known by over an order of magnitude for water oxidation with Ce(IV). DFT calculations support a Cp*IrO complex as an active species.
The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in nonaqueous conditions, commonly ...used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pK a values of the corresponding metal-aqua species, M(H2O)mn+, measured in H2O vs the 31P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d 3-MeCN) and deuterated dichloromethane (CD2Cl2), display tightly colinear relationships, suggesting similar behavior for these ions in water, d 3-MeCN, and CD2Cl2. This colinearity reinforces the utility of the common approach of using the aqueous pK a values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d 3-MeCN suggest a 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous pK a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.
This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the Cp*Rh motif (Cp* = ...η5-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H2 generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H2 generation. Conversely, N, a more electron-withdrawing center, favors H2 generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.
We show that molecular catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. A pyrene-appended bipyridine ligand (P) serves as ...the linker between each complex and the surface. Immobilization of a rhodium proton-reduction catalyst, Cp*Rh(P)ClCl (1), and a rhenium CO2-reduction catalyst, Re(P)(CO)3Cl (2), afford electrocatalytically active assemblies. X-ray photoelectron spectroscopy and electrochemistry confirm catalyst immobilization. Reduction of 1 in the presence of p-toluenesulfonic acid results in catalytic H2 production, while reduction of 2 in the presence of CO2 results in catalytic CO production.
Capture and activation of the water-soluble uranyl dication (UO2 2+) remains a challenging problem, as few rational approaches are available for modulating the reactivity of this species. Here, we ...report the divergent synthesis of heterobimetallic complexes in which UO2 2+ is held in close proximity to a range of redox-inactive metals by a tailored macrocyclic ligand. Crystallographic and spectroscopic studies confirm assembly of homologous UVI(μ-OAr)2M n+ cores with a range of mono-, di-, and trivalent Lewis acids (M n+). Cyclic voltammetry data demonstrate that the UVI/UV reduction potential in these complexes is modulated over a span of 600 mV, depending linearly on the Lewis acidity of the redox-inactive metal with a sensitivity of 61 ± 9 mV/pK a unit. These findings suggest that interactions with Lewis acids could be effectively leveraged for rational tuning of the electronic and thermochemical properties of the 5f elements, reminiscent of strategies more commonly employed with 3d transition metals.
Surfactant-free mixed-metal hydroxide water oxidation nanocatalysts were synthesized by pulsed-laser ablation in liquids. In a series of Ni-Fe-layered double hydroxides with intercalated nitrate and ...water, Ni1–x Fe x (OH)2(NO3) y (OH) x−y ·nH2O, higher activity was observed as the amount of Fe decreased to 22%. Addition of Ti4+ and La3+ ions further enhanced electrocatalysis, with a lowest overpotential of 260 mV at 10 mA cm–2. Electrocatalytic water oxidation activity increased with the relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets.
Surfactant-free, size- and composition-controlled, unsupported, <5-nm, quantum-confined cobalt oxide nanoparticles with high electrocatalytic oxygen-evolution activity were synthesized by pulsed ...laser ablation in liquids. These crystalline Co3O4 nanoparticles have a turnover frequency per cobalt surface site among the highest ever reported for Co3O4 nanoparticle oxygen evolution catalysts in base and overpotentials competitive with the best electrodeposited cobalt oxides, with the advantage that they are suitable for mechanical deposition on photoanode materials and incorporation in integrated solar water-splitting devices.
Iridium half-sandwich complexes of the types Cp*Ir(N−C)X, Cp*Ir(N−N)XX, and CpIr(N−N)XX are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ...ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, Cp*Ir(H2O)3SO4 and (Cp*Ir)2(μ-OH)3OH can show even higher turnover frequencies (up to 20 min−1 at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H2 18O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide.
The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in non-aqueous conditions, commonly ...used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pKa values of the corresponding metal-aqua species, M(H2O)mn+, measured in H2O vs. the 31P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD2Cl2) display tightly co-linear relationships, suggesting similar behavior for these ions in water, d3-MeCN, and CD2Cl2. This collinearity reinforces the utility of the common approach of using the aqueous pKa values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d3-MeCN suggest 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Altogether, these data suggest that both aqueous pKa values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.
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