Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

The conversion of solar energy into fuels involves coupled multiproton-multielectron reactions. Because of different length scales for electron transfer and proton transfer, efficient catalysts must couple proton hops to redox events. We have developed a hangman motif where placing a pendant acid-base group over the face of a redox macrocycle ensures coupling of the electron to the proton. We now show that the presence of the pendant acid relay of a Ni hangman porphyrin facilitates proton-coupled electron transfer (PCET) during turnover of the hydrogen evolution reaction (HER). By comparing HER activity of the Ni hangman catalyst to its Co analog, a unified HER mechanism begins to emerge for complexes that use a secondary coordination sphere to manage PCET.