Two Novel Dinuclear Cobalt Polypyridyl Complexes in Electro‐ and Photocatalysis for Hydrogen Production: Cooperativity Increases Performance

Syntheses and mechanisms of two dinuclear Co‐polypyridyl catalysts for the H 2 evolution reaction (HER) were reported and compared to their mononuclear analogue ( R1 ). In both catalysts, two di‐(2,2’‐bipyridin‐6‐yl)‐methanone units were linked by either 2,2’‐bipyridin‐6,6’‐yl or pyrazin‐2,5‐yl. Complexation with Co II gave dinuclear compounds bridged by pyrazine ( C2 ) or bipyridine ( C1 ). Photocatalytic HER gave turnover numbers (TONs) of up to 20000 ( C2 ) and 7000 ( C1 ) in water. Electrochemically, C1 was similar to the R1 , whereas C2 showed electronic coupling between the two Co centers. The E (Co II/I ) split by 360 mV into two separate waves. Proton reduction in DMF was investigated for R1 with [HNEt 3 ](BF 4 ) by simulation, foot of the wave analysis, and linear sweep voltammetry (LSV) with in‐line detection of H 2 . All methods agreed well with an (E)ECEC mechanism and the first protonation being rate limiting (≈10 4   m −1  s −1 ). The second reduction was more anodic than the first one. p K a values of around 10 and 7.5 were found for the two protonations. LSV analysis with H 2 detection for all catalysts and acids with different p K a values [HBF 4 , p K a (DMF)≈3.4], intermediate {[HNEt 3 ](BF 4 ), p K a (DMF)≈9.2} to weak [AcOH, p K a (DMF)≈13.5] confirmed electrochemical H 2 production, distinctly dependent on the p K a values. Only HBF 4 protonated Co I intermediates. The two metals in the dualcore C2 cooperated with an increase in rate to a competitive 10 5   m −1  s −1 with [HNEt 3 ](BF 4 ). The overpotential decreased compared to R1 by 100 mV. Chronoamperometry established high stabilities for all catalysts with TON lim of 100 for R1 and 320 for C1 and C2 .