Abstract
Molecular catalysts offer an ideal platform for conducting mechanistic studies of the hydrogen evolution reaction (HER) due to their electronic tunability. This study explores a series of anionic M=Co(III)- and M=Zn(II)-porphyrin complexes with electron-donating ([M(OMeP)] (n-), [M(MeP)] (n-)) and electron-withdrawing ([M(F8P)] (n-), [M(F16P)] (n-)) substituents. The activity of these complexes for the HER was analyzed in homogeneous photocatalytic conditions using [Ru(bpy)(3)](2+) as a photosensitizer under blue light (450 nm) irradiation. The substituent-induced electronic effects were found to tightly control the activity and mechanism of the photocatalytic HER. As expected, the electron-rich [Co(OMeP)](3-) catalyst showed higher activity in acidic media (pH 4.1) with a maximum TOF of 7.2 ± 0.4 h(-1) and TON of 175 ± 5 after 39.5 h. DFT calculations were performed to investigate the HER mechanism. H(2) formation was found to initiate following proton-coupled reduction of a Co(III)-H hydride intermediate in such conditions. More surprisingly, however, the electron-poor [Co(F16P)](3-) catalyst was more active at neutral pH (7.0), achieving a maximum TOF of 6.7 ± 0.3 h(-1) and TON of 70 ± 3 after 39.5 h. Instead of forming the Co(III)-H hydride, an additional ligand-based reduction led to a ligand-protonated intermediate. This work demonstrates that electron-poor HER catalysts can outperform electron-rich catalysts near neutral pH conditions.