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Structure-Activity and Stability Relationships for Cobalt Polypyridyl-Based Hydrogen-Evolving Catalysts in Water


Schnidrig, Stephan; Bachmann, Cyril; Müller, Peter; Weder, Nicola; Spingler, Bernhard; Joliat-Wick, Evelyne; Mosberger, Mathias; Windisch, Johannes; Alberto, Roger; Probst, Benjamin (2017). Structure-Activity and Stability Relationships for Cobalt Polypyridyl-Based Hydrogen-Evolving Catalysts in Water. ChemSusChem, 10(22):4570-4580.

Abstract

A series of eight new and three known cobalt polypyridyl-based hydrogen-evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene-bridged bis-bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol-bridged bispyridyl–bipyridyl complex [CoIIBr(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton-coupled electron transfer, assigned to formation of CoIII−H, is found upon reduction of CoII, in line with a pKa(CoIII−H) of approximately 7.25. Subsequent reduction (−0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pKa(CoIII−H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts.

Abstract

A series of eight new and three known cobalt polypyridyl-based hydrogen-evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene-bridged bis-bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol-bridged bispyridyl–bipyridyl complex [CoIIBr(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton-coupled electron transfer, assigned to formation of CoIII−H, is found upon reduction of CoII, in line with a pKa(CoIII−H) of approximately 7.25. Subsequent reduction (−0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pKa(CoIII−H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
08 University Research Priority Programs > Solar Light to Chemical Energy Conversion
Dewey Decimal Classification:540 Chemistry
Language:English
Date:2017
Deposited On:09 Feb 2018 08:00
Last Modified:18 Apr 2018 11:49
Publisher:Wiley-VCH Verlag
ISSN:1864-5631
Funders:SNF Sinergia project CRSII2 160801/2
OA Status:Closed
Publisher DOI:https://doi.org/10.1002/cssc.201701511
Project Information:
  • : FunderSNSF
  • : Grant ID
  • : Project TitleSNF Sinergia project CRSII2 160801/2

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