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

Abstract

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(N-4-methyl-3-thiosemicarbazone) and zinc diacetyl-bis(N-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.

Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation with a Ni(PS)2 Complex

“…No catalytic current was observed. This excludes the possibility of catalyst degradation or film formation on the electrode surface that is responsible for catalytic activity , …”

Molecular Electrocatalysts for the Hydrogen Evolution Reaction: Input from Quantum Chemistry

“…Ar edox-active ligand,s uch as thiosemicarbazone, could play this role. [48] The absence of ar ole for the metal is mainly supported by the activity of the ligand alone. ChemSusChem 2019, 12,4905 -4915 www.chemsuschem.org relax the requirement of having am etal complex with open coordination,w hich should also have access to multiple redox states.…”

Homogeneous ligand-centered hydrogen evolution and hydrogen oxidation : exploiting redox non-innocence to drive catalysis.

“…71,76 As discussed previously in Chapter Four, we reported the first examples of HER/HOR activity with the non-innocent bis(thiosemicarbazone) ligand framework. 76 The zinc complex of diacetyl-bis(N-4-methyl-3-thiosemicarbazone), 194 In their studies, electrochemical characterizations were performed in an array of conditions, including under slightly acidic reducing conditions (22 mM maximum). In that report, the authors state that they observe the production of hydrogen, however, no complete analysis of these observations was provided.…”

“…The work described in Chapter Four was supported in part by the National Science 192 has expanded the realm of HER electrocatalysts beyond traditional transition metalhydrides to include non-transition metal, 76,99 and metal-free catalysts 76 for HER invoking ligand-directed radical coupling of H-atoms 77,193 or protonation of ligand-centered hydrides. 71,76 As discussed previously in Chapter Four, we reported the first examples of HER/HOR activity with the non-innocent bis(thiosemicarbazone) ligand framework.…”

“…76 Further, these systems overcome the limitations noted above as they: 1) incorporate sustainable first-row metals; 2) the ligand syntheses require no specialized skills and can be performed on the benchtop in water and/or alcohol using inexpensive synthons; 3) catalysis occurs in water/alcohol or acetonitrile; 4) the framework is easily derivatized and very stable to hydrolysis; 5) the structures are small with metals that are well calibrated from a computational perspective.…”