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.

A series of eight new and three known cobalt polypyridyl‐based hydrogen‐evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene‐bridged bis‐bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol‐bridged bispyridyl–bipyridyl complex [CoIIBr(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton‐coupled electron transfer, assigned to formation of CoIII−H, is found upon reduction of CoII, in line with a pKa(CoIII−H) of approximately 7.25. Subsequent reduction (−0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pKa(CoIII−H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts.