Ligand-Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution

Keita Koshiba, Kosei Yamauchi, Ken Sakai

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3 Citations (Scopus)

Abstract

Proton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one-electron reductions (bpy=2,2′-bipyridine; dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).

Original languageEnglish
Pages (from-to)2273-2281
Number of pages9
JournalChemElectroChem
Volume6
Issue number8
DOIs
Publication statusPublished - Apr 15 2019

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Electrocatalysts
Alkalinity
Protons
Hydrogen
1,2-benzenedicarbonitrile
Metals
Ligands
Hydrides
Catalysts
Electrons
Electron injection
Protonation
Nickel
Discrete Fourier transforms

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Electrochemistry

Cite this

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title = "Ligand-Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution",
abstract = "Proton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one-electron reductions (bpy=2,2′-bipyridine; dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).",
author = "Keita Koshiba and Kosei Yamauchi and Ken Sakai",
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T1 - Ligand-Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution

AU - Koshiba, Keita

AU - Yamauchi, Kosei

AU - Sakai, Ken

PY - 2019/4/15

Y1 - 2019/4/15

N2 - Proton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one-electron reductions (bpy=2,2′-bipyridine; dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).

AB - Proton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one-electron reductions (bpy=2,2′-bipyridine; dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).

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