Effect of Ligand Modification on the Mechanism of Electrocatalytic Hydrogen Production by Ni(pyridinethiolate)₃^– Derivatives

The effects of ligand modification on the catalytic mechanism of hydrogen production by Ni­(PyS)₃^– derivatives, made with electron-withdrawing and -donating substitutions to the pyridinethiolate (PyS)⁻ ligands, are studied experimentally and computationally using density functional theory. Thermodynamic data, spin density maps, and frontier molecular orbital diagrams were generated for reaction intermediates. Comparison of computed values for <i>E</i>⁰ and p<i>K</i>_a with experimental values supports the proposed mechanisms. The rate of electrochemical hydrogen production is correlated with the effect of ligand modification. Notably, the presence of an electron-donating substituent favors an alternative mechanism for hydrogen production. Computationally it was determined that the electron-donating substituent causes deviation from the original chemical–electrochemical–chemical–electrochemical (CECE) mechanism of Ni­(PyS)₃^– to a CCEE mechanism, while the CECE mechanism is maintained for all catalysts substituted with electron-withdrawing groups