Beyond Metal-Hydrides: Non-Transition-Metal and Metal-Free Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation

A new pathway for homogeneous electrocatalytic H₂ evolution and H₂ oxidation has been developed using a redox active thiosemicarbazone and its zinc complex as seminal metal-free and transition-metal-free examples. Diacetyl-bis­(<i>N</i>-4-methyl-3-thiosemicarbazone) and zinc diacetyl-bis­(<i>N</i>-4-methyl-3-thiosemicarbazide) display the highest reported TOFs of any homogeneous ligand-centered H₂ evolution catalyst, 1320 and 1170 s^–1, respectively, while the zinc complex also displays one of the highest reported TOF values for H₂ oxidation, 72 s^–1, of any homogeneous catalyst. Catalysis proceeds via ligand-centered proton-transfer and electron-transfer events while avoiding traditional metal-hydride intermediates. The unique mechanism is consistent with electrochemical results and is further supported by density functional theory. The results identify a new direction for the design of electrocatalysts for H₂ evolution and H₂ oxidation that are not reliant on metal-hydride intermediates.