Evaluation of the coordination preferences and catalytic pathways of heteroaxial cobalt oximes towards hydrogen generation

Abstract

Three new heteroaxial cobalt oxime catalysts, namely [Co^III(prdioxH)(^4tBupy)(Cl)]PF₆ (1), [Co^III(prdioxH)(^4Pyrpy)(Cl)]PF₆ (2), and [Co^III(prdioxH)(^4Bzpy)(Cl)]PF₆ (3) have been studied. These species contain chloro and substituted tert-butyl/pyrrolidine/benzoyl-pyridino ligands axially coordinated to a trivalent cobalt ion bound to the N₄-oxime macrocycle (2E,2′E,3E,3′E)-3,3′-(propane-1,3-diylbis(azanylylidene))bis(butan-2-one)dioxime, abbreviated (prdioxH)⁻ in its monoprotonated form. Emphasis was given to the spectroscopic investigation of the coordination preferences and spin configurations among the different 3d⁶ Co^III, 3d⁷ Co^II, and 3d⁸ Co^I oxidation states of the metal, and to the catalytic proton reduction with an evaluation of the pathways for the generation of H₂via Co^III–H⁻ or Co^II–H⁻ intermediates by mono and bimetallic routes. The strong field imposed by the (prdioxH)⁻ ligand precludes the existence of high-spin configurations, and 6-coordinate geometry is favored by the ^LSCo^III species. Species 1 and 3 show a split Co^III/Co^II electrochemical wave associated with partial chemical conversion to a [Co^III(prdioxH)Cl₂] species, whereas 2 shows a single event. The reduction of these Co^III complexes yields ^LSCo^II and ^LSCo^I species in which the pyridine acts as the dominant axial ligand. In the presence of protons, the catalytically active Co^I species generates a Co^III–H⁻ hydride species that reacts heterolytically with another proton to generate dihydrogen. The intermediacy of a trifluoroacetate-bound Co^III/Co^II couple in the catalytic mechanism is proposed. These results allow for a generalization of the behavior of heteroaxial cobalt macrocycles and serve as guidelines for the development of new catalysts based on macrocyclic frameworks.