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(II)Br(HOMe)(pyr-bu)], [Co(II)Br2(cis-pyr-bu2)] and [Co(II)Br2(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(II)Br(HOMe)(pyr-bu)], [Co(II)Br2(cis-pyr-bu2)] and [Co(II)Br2(trans-pyr-bu2)] 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 [Co(II)Br2(cis-pyr-bu2)] and [Co(II)Br2(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.