What determines catalyst functionality in molecular water oxidation? dependence on ligands and metal nuclearity in cobalt clusters

Paul F. Smith, Christopher Kaplan, John E. Sheats, David M. Robinson, Nicholas S. McCool, Nicholas Mezle, G Charles Dismukes

Research output: Contribution to journalArticle

49 Citations (Scopus)

Abstract

The metal-oxo M4O4 "cubane" topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co 4O4(OAc)4(py)4 and [Co 4O4(OAc)2(bpy)4]2+, using the common Ru(bpy)3 2+/S2O8 2- photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s-1, respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither "incomplete cubane" trimers [Co3O(OH)3(OAc)2(bpy)3] 2+ and [Co3O(OH)2(OAc)3(py) 5]2+ nor "half cubane" dimers [Co 2(OH)2(OAc)3(bpy)2]+ and [Co2(OH)2(OAc)3(py)4]+ were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O 4]5+ core. The origin of catalytic activity by Co 4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O 4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.

Original languageEnglish
Pages (from-to)2113-2121
Number of pages9
JournalInorganic Chemistry
Volume53
Issue number4
DOIs
Publication statusPublished - Feb 17 2014

Fingerprint

cubane
Cobalt
cobalt
Metals
Ligands
catalysts
Oxidation
ligands
oxidation
Catalysts
Water
metals
water
Dimers
trimers
Deprotonation
Photosynthesis
Electrons
Substrates
dimers

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

What determines catalyst functionality in molecular water oxidation? dependence on ligands and metal nuclearity in cobalt clusters. / Smith, Paul F.; Kaplan, Christopher; Sheats, John E.; Robinson, David M.; McCool, Nicholas S.; Mezle, Nicholas; Dismukes, G Charles.

In: Inorganic Chemistry, Vol. 53, No. 4, 17.02.2014, p. 2113-2121.

Research output: Contribution to journalArticle

Smith, Paul F. ; Kaplan, Christopher ; Sheats, John E. ; Robinson, David M. ; McCool, Nicholas S. ; Mezle, Nicholas ; Dismukes, G Charles. / What determines catalyst functionality in molecular water oxidation? dependence on ligands and metal nuclearity in cobalt clusters. In: Inorganic Chemistry. 2014 ; Vol. 53, No. 4. pp. 2113-2121.
@article{bfbe87f9ccc14d62af1bffff4074f70c,
title = "What determines catalyst functionality in molecular water oxidation? dependence on ligands and metal nuclearity in cobalt clusters",
abstract = "The metal-oxo M4O4 {"}cubane{"} topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co 4O4(OAc)4(py)4 and [Co 4O4(OAc)2(bpy)4]2+, using the common Ru(bpy)3 2+/S2O8 2- photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s-1, respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither {"}incomplete cubane{"} trimers [Co3O(OH)3(OAc)2(bpy)3] 2+ and [Co3O(OH)2(OAc)3(py) 5]2+ nor {"}half cubane{"} dimers [Co 2(OH)2(OAc)3(bpy)2]+ and [Co2(OH)2(OAc)3(py)4]+ were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O 4]5+ core. The origin of catalytic activity by Co 4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O 4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.",
author = "Smith, {Paul F.} and Christopher Kaplan and Sheats, {John E.} and Robinson, {David M.} and McCool, {Nicholas S.} and Nicholas Mezle and Dismukes, {G Charles}",
year = "2014",
month = "2",
day = "17",
doi = "10.1021/ic402720p",
language = "English",
volume = "53",
pages = "2113--2121",
journal = "Inorganic Chemistry",
issn = "0020-1669",
publisher = "American Chemical Society",
number = "4",

}

TY - JOUR

T1 - What determines catalyst functionality in molecular water oxidation? dependence on ligands and metal nuclearity in cobalt clusters

AU - Smith, Paul F.

AU - Kaplan, Christopher

AU - Sheats, John E.

AU - Robinson, David M.

AU - McCool, Nicholas S.

AU - Mezle, Nicholas

AU - Dismukes, G Charles

PY - 2014/2/17

Y1 - 2014/2/17

N2 - The metal-oxo M4O4 "cubane" topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co 4O4(OAc)4(py)4 and [Co 4O4(OAc)2(bpy)4]2+, using the common Ru(bpy)3 2+/S2O8 2- photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s-1, respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither "incomplete cubane" trimers [Co3O(OH)3(OAc)2(bpy)3] 2+ and [Co3O(OH)2(OAc)3(py) 5]2+ nor "half cubane" dimers [Co 2(OH)2(OAc)3(bpy)2]+ and [Co2(OH)2(OAc)3(py)4]+ were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O 4]5+ core. The origin of catalytic activity by Co 4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O 4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.

AB - The metal-oxo M4O4 "cubane" topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co 4O4(OAc)4(py)4 and [Co 4O4(OAc)2(bpy)4]2+, using the common Ru(bpy)3 2+/S2O8 2- photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s-1, respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither "incomplete cubane" trimers [Co3O(OH)3(OAc)2(bpy)3] 2+ and [Co3O(OH)2(OAc)3(py) 5]2+ nor "half cubane" dimers [Co 2(OH)2(OAc)3(bpy)2]+ and [Co2(OH)2(OAc)3(py)4]+ were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O 4]5+ core. The origin of catalytic activity by Co 4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O 4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.

UR - http://www.scopus.com/inward/record.url?scp=84894373103&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84894373103&partnerID=8YFLogxK

U2 - 10.1021/ic402720p

DO - 10.1021/ic402720p

M3 - Article

VL - 53

SP - 2113

EP - 2121

JO - Inorganic Chemistry

JF - Inorganic Chemistry

SN - 0020-1669

IS - 4

ER -