Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals

Shoving Protons Around with Proton Relays

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Abstract

Conspectus Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands PR2NR′2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(PCy2NR′2)2]2+, CpC6F5Fe(PtBu2NBn2)H, and MnH(PPh2NBn2)(bppm)(CO) [bppm = (PArF2)2CH2]. In the oxidation of H2 catalyzed by [Ni(PCy2NR′2)2]2+, the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(PPh2NBn2)(bppm)(CO)]+ mediates the rapid (>104 s-1 at -95°C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.

Original languageEnglish
Pages (from-to)2017-2026
Number of pages10
JournalAccounts of Chemical Research
Volume48
Issue number7
DOIs
Publication statusPublished - Jul 21 2015

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Electrocatalysts
Amines
Protons
Hydrogen
Metals
Earth (planet)
Oxidation
Deprotonation
Ligands
Hydrogenase
Proton transfer
Carbon Monoxide
Hydrides
Iron
Catalysts
Electrochemical oxidation
Chemical bonds
Alkalinity
Precious metals
Platinum

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

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title = "Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays",
abstract = "Conspectus Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands PR2NR′2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(PCy2NR′2)2]2+, CpC6F5Fe(PtBu2NBn2)H, and MnH(PPh2NBn2)(bppm)(CO) [bppm = (PArF2)2CH2]. In the oxidation of H2 catalyzed by [Ni(PCy2NR′2)2]2+, the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(PPh2NBn2)(bppm)(CO)]+ mediates the rapid (>104 s-1 at -95°C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.",
author = "Bullock, {R Morris} and Monte Helm",
year = "2015",
month = "7",
day = "21",
doi = "10.1021/acs.accounts.5b00069",
language = "English",
volume = "48",
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journal = "Accounts of Chemical Research",
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publisher = "American Chemical Society",
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TY - JOUR

T1 - Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals

T2 - Shoving Protons Around with Proton Relays

AU - Bullock, R Morris

AU - Helm, Monte

PY - 2015/7/21

Y1 - 2015/7/21

N2 - Conspectus Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands PR2NR′2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(PCy2NR′2)2]2+, CpC6F5Fe(PtBu2NBn2)H, and MnH(PPh2NBn2)(bppm)(CO) [bppm = (PArF2)2CH2]. In the oxidation of H2 catalyzed by [Ni(PCy2NR′2)2]2+, the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(PPh2NBn2)(bppm)(CO)]+ mediates the rapid (>104 s-1 at -95°C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.

AB - Conspectus Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands PR2NR′2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(PCy2NR′2)2]2+, CpC6F5Fe(PtBu2NBn2)H, and MnH(PPh2NBn2)(bppm)(CO) [bppm = (PArF2)2CH2]. In the oxidation of H2 catalyzed by [Ni(PCy2NR′2)2]2+, the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(PPh2NBn2)(bppm)(CO)]+ mediates the rapid (>104 s-1 at -95°C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.

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JO - Accounts of Chemical Research

JF - Accounts of Chemical Research

SN - 0001-4842

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