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Issue 4, 2016

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

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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.

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

Supplementary files

Article information


Submitted
10 Nov 2015
Accepted
11 Dec 2015
First published
14 Dec 2015

Dalton Trans., 2016,45, 1737-1745
Article type
Paper
Author version available

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

E. Joliat, S. Schnidrig, B. Probst, C. Bachmann, B. Spingler, K. K. Baldridge, F. von Rohr, A. Schilling and R. Alberto, Dalton Trans., 2016, 45, 1737 DOI: 10.1039/C5DT04426J

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