Two Novel Dinuclear Cobalt Polypyridyl Complexes in Electro- and Photocatalysis for Hydrogen Production:...

Syntheses and mechanisms of two dinuclear Co-polypyridyl catalysts for the H₂ 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 [HNEt₃ ](BF₄ ) by simulation, foot of the wave analysis, and linear sweep voltammetry (LSV) with in-line detection of H₂ . All methods agreed well with an (E)ECEC mechanism and the first protonation being rate limiting (≈10⁴  m^-1  s^-1 ). The second reduction was more anodic than the first one. pK_a values of around 10 and 7.5 were found for the two protonations. LSV analysis with H₂ detection for all catalysts and acids with different pK_a values [HBF₄ , pK_a (DMF)≈3.4], intermediate {[HNEt₃ ](BF₄ ), pK_a (DMF)≈9.2} to weak [AcOH, pK_a (DMF)≈13.5] confirmed electrochemical H₂ production, distinctly dependent on the pK_a values. Only HBF₄ protonated Co^I intermediates. The two metals in the dualcore C2 cooperated with an increase in rate to a competitive 10⁵  m^-1  s^-1 with [HNEt₃ ](BF₄ ). 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.