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Control of Ligand pK a Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex.
Inorganic Chemistry 2017 August 8
Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pKa 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,iPr Pz2 P, reacts with 2 equiv of AlCl3 to afford [(iPr Pz2 P)AlCl2 (THF)][AlCl4 ] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(iPr HPz2 P- )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 Bu4 NPF6 /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 (Ph I2 P) reveals that the pKa values of the N-donor atoms iniPr Pz2 P are lower, which facilitates reduction before ligand protonation. In contrast, thePh I2 P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.
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