Thermochemical and mechanistic studies of electrocatalytic hydrogen production by cobalt complexes containing pendant amines

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Abstract

Two cobalt(tetraphosphine) complexes [Co(PnC-PPh2 2NPh 2)(CH3CN)](BF4) 2 with a tetradentate phosphine ligand (PnC-PPh2 2NPh 2 = 1,5-diphenyl-3,7- bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H]+:DMF. A turnover frequency (TOF) of 980 s-1 with an overpotential at Ecat/2 of 1210 mV was measured for [Co II(L2)(CH3CN)]2+, and a TOF of 980 s -1 with an overpotential at Ecat/2 of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the TOF of [CoII(L2)(CH3CN)]2+ to 18,000 s-1. The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCoIII complex. Comprehensive thermochemical studies of [CoII(L2)(CH 3CN)]2+ and [CoII(L3)(CH3CN)] 2+ and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using density functional theory (DFT). Notably, HCoI(L2) and HCoI(L3) were found to be remarkably strong hydride donors, with HCoI(L2) being a better hydride donor than BH4 -. Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCoII intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the nonproductive exoisomer to the productive endo isomer.

Original languageEnglish
Pages (from-to)14391-14403
Number of pages13
JournalInorganic Chemistry
Volume52
Issue number24
DOIs
Publication statusPublished - Dec 16 2013

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phosphine
hydrogen production
Hydrogen production
Cobalt
Hydrides
Amines
Protons
amines
cobalt
hydrides
Protonation
Isomerization
Isomers
Catalysis
Density functional theory
protons
Ligands
phosphines
Hydrogen
isomerization

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

@article{27cf77ea545e4c94848c984073bf05ec,
title = "Thermochemical and mechanistic studies of electrocatalytic hydrogen production by cobalt complexes containing pendant amines",
abstract = "Two cobalt(tetraphosphine) complexes [Co(PnC-PPh2 2NPh 2)(CH3CN)](BF4) 2 with a tetradentate phosphine ligand (PnC-PPh2 2NPh 2 = 1,5-diphenyl-3,7- bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H]+:DMF. A turnover frequency (TOF) of 980 s-1 with an overpotential at Ecat/2 of 1210 mV was measured for [Co II(L2)(CH3CN)]2+, and a TOF of 980 s -1 with an overpotential at Ecat/2 of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the TOF of [CoII(L2)(CH3CN)]2+ to 18,000 s-1. The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCoIII complex. Comprehensive thermochemical studies of [CoII(L2)(CH 3CN)]2+ and [CoII(L3)(CH3CN)] 2+ and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using density functional theory (DFT). Notably, HCoI(L2) and HCoI(L3) were found to be remarkably strong hydride donors, with HCoI(L2) being a better hydride donor than BH4 -. Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCoII intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the nonproductive exoisomer to the productive endo isomer.",
author = "Eric Wiedner and Aaron Appel and DuBois, {Daniel L} and Bullock, {R Morris}",
year = "2013",
month = "12",
day = "16",
doi = "10.1021/ic4025475",
language = "English",
volume = "52",
pages = "14391--14403",
journal = "Inorganic Chemistry",
issn = "0020-1669",
publisher = "American Chemical Society",
number = "24",

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TY - JOUR

T1 - Thermochemical and mechanistic studies of electrocatalytic hydrogen production by cobalt complexes containing pendant amines

AU - Wiedner, Eric

AU - Appel, Aaron

AU - DuBois, Daniel L

AU - Bullock, R Morris

PY - 2013/12/16

Y1 - 2013/12/16

N2 - Two cobalt(tetraphosphine) complexes [Co(PnC-PPh2 2NPh 2)(CH3CN)](BF4) 2 with a tetradentate phosphine ligand (PnC-PPh2 2NPh 2 = 1,5-diphenyl-3,7- bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H]+:DMF. A turnover frequency (TOF) of 980 s-1 with an overpotential at Ecat/2 of 1210 mV was measured for [Co II(L2)(CH3CN)]2+, and a TOF of 980 s -1 with an overpotential at Ecat/2 of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the TOF of [CoII(L2)(CH3CN)]2+ to 18,000 s-1. The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCoIII complex. Comprehensive thermochemical studies of [CoII(L2)(CH 3CN)]2+ and [CoII(L3)(CH3CN)] 2+ and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using density functional theory (DFT). Notably, HCoI(L2) and HCoI(L3) were found to be remarkably strong hydride donors, with HCoI(L2) being a better hydride donor than BH4 -. Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCoII intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the nonproductive exoisomer to the productive endo isomer.

AB - Two cobalt(tetraphosphine) complexes [Co(PnC-PPh2 2NPh 2)(CH3CN)](BF4) 2 with a tetradentate phosphine ligand (PnC-PPh2 2NPh 2 = 1,5-diphenyl-3,7- bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H]+:DMF. A turnover frequency (TOF) of 980 s-1 with an overpotential at Ecat/2 of 1210 mV was measured for [Co II(L2)(CH3CN)]2+, and a TOF of 980 s -1 with an overpotential at Ecat/2 of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the TOF of [CoII(L2)(CH3CN)]2+ to 18,000 s-1. The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCoIII complex. Comprehensive thermochemical studies of [CoII(L2)(CH 3CN)]2+ and [CoII(L3)(CH3CN)] 2+ and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using density functional theory (DFT). Notably, HCoI(L2) and HCoI(L3) were found to be remarkably strong hydride donors, with HCoI(L2) being a better hydride donor than BH4 -. Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCoII intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the nonproductive exoisomer to the productive endo isomer.

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