Control of Ligand p<i>K</i>_a Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex

Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand p<i>K</i>_a values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis­(pyrazolyl)­pyridine ligand, ^iPrPz₂P, reacts with 2 equiv of AlCl₃ to afford [(^iPrPz₂P)­AlCl₂(THF)]­[AlCl₄] (<b>1</b>). A reaction involving two-electron reduction and single-ligand protonation of <b>1</b> affords [(^iPrHPz₂P^–)­AlCl₂] (<b>2</b>), where each of the electron- and proton-transfer events is ligand-centered. Protonation of <b>2</b> would formally close a catalytic cycle for dihydrogen production. At −1.26 V versus SCE, in a 0.3 M Bu₄NPF₆/tetrahydrofuran solution with salicylic acid or (HNEt₃)⁺ as the source of H⁺, <b>1</b> produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis­(imino)­pyridine system (^PhI₂P) reveals that the p<i>K</i>_a values of the N-donor atoms in ^iPrPz₂P are lower, which facilitates reduction before ligand protonation. In contrast, the ^PhI₂P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen