Scientific article
English

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

Published inDalton transactions, vol. 45, no. 4, p. 1737-1745
Publication date2016
First online date2015-12-14
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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JOLIAT, E. et al. Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction. In: Dalton transactions, 2016, vol. 45, n° 4, p. 1737–1745. doi: 10.1039/C5DT04426J
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Additional URL for this publicationhttp://xlink.rsc.org/?DOI=C5DT04426J
Journal ISSN1477-9226
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