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

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 Co^II complexes [Co^II(OH₂)₂(pyr)], [Co^IIBr(HOMe)(pyr-bu)], [Co^IIBr₂(cis-pyr-bu₂)] and [Co^IIBr₂(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 {Co^II(pyr)} macrocyclic scaffold. The axial bromides in [Co^IIBr(HOMe)(pyr-bu)], [Co^IIBr₂(cis-pyr-bu₂)] and [Co^IIBr₂(trans-pyr-bu₂)] are weakly bound and dissociate upon dissolution in water. While the alkylated complexes are paramagnetic and feature Co^II d⁷ high spin configurations, the unsubstituted complex [Co^II(OH₂)₂(pyr)] displays a rare Co^II low spin configuration. The electronic ground states of [Co^IIBr₂(cis-pyr-bu₂)] and [Co^IIBr₂(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.