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

Three new heteroaxial cobalt oxime catalysts, namely [Co-III(prdioxH)((4tBu)py)(Cl)]PF6 (1), [Co-III(prdioxH)((4Pyr)py)(Cl)]PF6 (2), and [Co-III(prdioxH)((4Bz)py)(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 N-4-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 3d(6) Co-III, 3d(7) Co-II, and 3d(8) Co-I oxidation states of the metal, and to the catalytic proton reduction with an evaluation of the pathways for the generation of H-2 via 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 Co-LS(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)Cl-2] species, whereas 2 shows a single event. The reduction of these Co-III complexes yields Co-LS(II) and Co-LS(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.