pH-Dependent Reactivity of a Water-Soluble Nickel Complex: Hydrogen Evolution vs Selective Electrochemical Hydride Generation

Mechanistic details for the aqueous hydrogen evolution reaction (HER) electrocatalyst [Ni(DHMPE)(2)](2+) (DHMPE = 2-bis(bis(hydroxymethyl)phosphino)ethane) are reported. The proposed 2e(-) reduced catalytic intermediate [Ni(DHMPE)(2)] was added to solutions between pH 1 and 5. The two sequential protonation events leading to H-2 evolution were monitored by P-31{H-1} NMR spectroscopy. The initial protonation event to generate the metal hydride [HNi-(DHMPE)(2)](+) proceeds to completion within 1.5 min. In contrast, significant variation in the pH-dependent rate and reaction progress of the second protonation event to form the H-H bond is observed. The differences are discussed in the context of the free energy of each protonation event, which also defines the pH conditions under which H2 evolution is exergonic. The analysis provides useful information on the functional pH range of HER electrocatalysts that proceed through metal hydride intermediates. Reductive electrolysis of [Ni(DHMPE)(2)](2+) under conditions in which H-2 evolution is endergonic (pH 7) leads to selective generation of the corresponding metal hydride [HNi(DHMPE)(2)](+). The electrolytic generation of a kinetically competent reducing metal hydride intermediate with minimal H-2 evolution illustrates a route for accessing selective reduction of non-proton substrates.