title: Control of Ligand pKaValues Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex.
abstract: Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pKavalues 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 AlCl3to afford [(iPrPz2P)AlCl2(THF)][AlCl4] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(iPrHPz2P-)AlCl2] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 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+, 1 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 pKavalues of the N-donor atoms iniPrPz2P are lower, which facilitates reduction before ligand protonation. In contrast, thePhI2P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.
authors.affiliation: Department of Chemistry, University of California , Davis, California 95616, United States.
authors.forename: Tobias J
authors.initials: TJ
authors.lastname: Sherbow
authors.affiliation: Department of Chemistry, University of California , Davis, California 95616, United States.
authors.forename: James C
authors.initials: JC
authors.lastname: Fettinger
authors.affiliation: Department of Chemistry, University of California , Davis, California 95616, United States.
authors.forename: Louise A
authors.identifier: http://orcid.org/0000-0001-6461-1829
authors.initials: LA
authors.lastname: Berben