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)]PF 6 (1), [Co III (prdioxH)( 4Pyr py)(Cl)]PF 6 (2), and [Co III (prdioxH)( 4Bz py)(Cl)]PF 6 (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 (2 E ,2' E ,3 E ,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 LS Co 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 LS Co II and LS Co 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. Finally, 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.