Electrocatalytic Hydrogen Generation by Ni-PN³P Pincer Complexes: Role of Phosphorus Substituents in Tuning the Reactivity

Electrocatalytic hydrogen evolution reaction (eHER) is crucial in addressing the growing global energy demand. Although nickel-pincer-based molecular complexes, varying in donor atoms, were studied previously for eHER, the impact of variations in the substituents attached to the donor atoms was not investigated. Herein, three air-stable ^R1PN³P^R2-based Ni^II–pincer complexes [R1=R2=Ph₂ (7); R1=R2=^tBu₂ (9); R1=^tBu₂, R2=Ph₂ (10)], varying solely in P-substituents, were studied in acetonitrile. While the redox potentials for Ni^II/I and Ni^I/0 couples underwent anodic shifts by ~100 mV upon progressively substituting tert-butyl by phenyl groups on each P-atom, the corresponding eHER reactivity with organic acids (acetic acid, p-toluenesulfonic acid and trifluoromethanesulfonic acid) of different strengths followed different trends; likely influenced by the pK_a of intermediate metal-hydride (M–H) species [pKa(M–H₉)>pKa(M–H₁₀)>pKa(M–H₇)]. Depending on the acid strength, different oxidation states of the metal were activated to promote eHER. The catalytic rates for 9, 10, and 7 were calculated to be 85 s⁻¹, 77 s⁻¹ and 95 s⁻¹ with Faradaic efficiencies of 88.5±2 %, 66.1±1.4 %, and 91.7±1.5 % respectively, in acetic acid. Electrochemical data supported by theoretical results reinforce a significant electronic influence of the anchoring P-substituents on the activity of these complexes.