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

摘要

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 [Co-II(OH2)(2)(pyr)], [(CoBr)-Br-II(HOMe)(pyr-bu)], [(CoBr2)-Br-II(cis-pyr-bu(2))] and (CoBr2)-Br-II-(trans-pyr-bu(2))] 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 [(CoBr)-Br-II(HOMe)(pyr-bu)], [(CoBr2)-Br-II(cis-pyr-bu(2))] and [(CoBr2)-Br-II(trans-pyr-bu(2))] are weakly bound and dissociate upon dissolution in water. While the alkylated complexes are paramagnetic and feature Co-II d(7) high spin configurations, the unsubstituted complex [Co-II(OH2)(2)(pyr)] displays a rare Co-II low spin configuration. The electronic ground states of [(CoBr2)-Br-II(cis-pyr-bu(2))] and [(CoBr2)-Br-II(trans-pyr-bu(2))] are similar, as evident from the almost identical UV/vis spectra. Electro-chemical analyses show redox-non-innocent ligand frameworks. All complexes are highly robust and efficient H+ reducing catalysts. In the presence of [Ru(bpy)(3)] Cl-2 as a photosensitizer and TCEP/NaHasc as a sacrificial electron donor and shuttle, turnover numbers (TONs, H-2/Co) up to 22 000 were achieved.