Elucidating Two Distinct Pathways for Electrocatalytic Hydrogen Production Using Co Pincer Complexes.

Hydrogen gas is a sustainable energy source with water as the sole combustion product. As a result, efforts to catalyze H production are pertinent and widespread. The electrocatalytic H generating capabilities of two Co complexes, [Co(κ -2,6-{Ph PNR} (NC H ))Br ] with R=H (I) or R=Me (II), were presented for a variety of proton sources including trifluoroacetic acid (TFA), acetic acid (AA), and trifluoroethanol (TFE). Cyclic voltammetry and controlled potential coulometry demonstrated that electrocatalysis from I and II occurred at two different potentials and are associated with different reduction processes. Density functional theory analysis provided insight into the identities of the catalyst and supported two distinct reaction pathways for electrocatalytic proton reduction. Specifically, stronger acids (e. g., AA, TFA) proceeded at -1.31 to -1.45 V through a M /M pathway while sources with higher pK values (e. g., TFE, H O) generated hydrogen at -2.4 V via M /M ligand-assisted metal-centered reduction.