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Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction


Joliat, Evelyne; Schnidrig, Stephan; Probst, Benjamin; Bachmann, Cyril; Spingler, Bernhard; Baldridge, Kim K; von Rohr, Fabian; Schilling, Andreas; Alberto, Roger (2016). Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction. Dalton Transactions, 45(4):1737-1745.

Abstract

Complexes with purely pyridine-based macrocycles are rarely studied in photo(electro)catalysis. We synthesized and investigated macrocycles, in which two 2,2′-bipyridine (bpy) units are linked twice by two cyano-methylene groups, to yield the basic tetradentate, bipyridine based ligand framework (pyr). The protons in the bridges were substituted to obtain derivatives with one (pyr-alk) or two (pyr-alk2) alkyl-chains, respectively. We present the crystal structures of the mono-pentylated and the cis-dibutylated ligands. The corresponding CoII complexes [CoII(OH2)2(pyr)], [CoIIBr(HOMe)(pyr-bu)], [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] were prepared, their physico-chemical properties elucidated and their crystal structures determined. X-ray analyses revealed for the latter three complexes distorted octahedral coordination and a fairly planar {CoII(pyr)} macrocyclic scaffold. The axial bromides in [CoIIBr(HOMe)(pyr-bu)], [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] are weakly bound and dissociate upon dissolution in water. While the alkylated complexes are paramagnetic and feature CoII d7 high spin configurations, the unsubstituted complex [CoII(OH2)2(pyr)] displays a rare CoII low spin configuration. The electronic ground states of [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] are similar, as evident from the almost identical UV/vis spectra. Electrochemical analyses show redox-non-innocent ligand frameworks. All complexes are highly robust and efficient H+ reducing catalysts. In the presence of [Ru(bpy)3]Cl2 as a photosensitizer and TCEP/NaHasc as a sacrificial electron donor and shuttle, turnover numbers (TONs, H2/Co) up to 22 000 were achieved.

Abstract

Complexes with purely pyridine-based macrocycles are rarely studied in photo(electro)catalysis. We synthesized and investigated macrocycles, in which two 2,2′-bipyridine (bpy) units are linked twice by two cyano-methylene groups, to yield the basic tetradentate, bipyridine based ligand framework (pyr). The protons in the bridges were substituted to obtain derivatives with one (pyr-alk) or two (pyr-alk2) alkyl-chains, respectively. We present the crystal structures of the mono-pentylated and the cis-dibutylated ligands. The corresponding CoII complexes [CoII(OH2)2(pyr)], [CoIIBr(HOMe)(pyr-bu)], [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] were prepared, their physico-chemical properties elucidated and their crystal structures determined. X-ray analyses revealed for the latter three complexes distorted octahedral coordination and a fairly planar {CoII(pyr)} macrocyclic scaffold. The axial bromides in [CoIIBr(HOMe)(pyr-bu)], [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] are weakly bound and dissociate upon dissolution in water. While the alkylated complexes are paramagnetic and feature CoII d7 high spin configurations, the unsubstituted complex [CoII(OH2)2(pyr)] displays a rare CoII low spin configuration. The electronic ground states of [CoIIBr2(cis-pyr-bu2)] and [CoIIBr2(trans-pyr-bu2)] are similar, as evident from the almost identical UV/vis spectra. Electrochemical analyses show redox-non-innocent ligand frameworks. All complexes are highly robust and efficient H+ reducing catalysts. In the presence of [Ru(bpy)3]Cl2 as a photosensitizer and TCEP/NaHasc as a sacrificial electron donor and shuttle, turnover numbers (TONs, H2/Co) up to 22 000 were achieved.

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3 citations in Web of Science®
3 citations in Scopus®
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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:2016
Deposited On:12 Jan 2016 15:51
Last Modified:05 Apr 2016 19:52
Publisher:Royal Society of Chemistry
ISSN:1477-9226
Publisher DOI:https://doi.org/10.1039/C5DT04426J
PubMed ID:26701123

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