Electron-Withdrawing Effects in Cobalt Porphyrin Catalysts Boost Homogeneous Photocatalytic Hydrogen Evolution in Neutral Aqueous Solutions

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)] ^<i>n</i>-, [M(MeP)] ^<i>n</i>-) and electron-withdrawing ([M(F8P)] ^<i>n</i>-, [M(F16P)] ^<i>n</i>-) 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. H₂ 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.