Influences of Bifunctional PNP-Pincer Ligands on Low Valent Cobalt Complexes Relevant to CO₂ Hydrogenation

Pincer ligated coordination complexes bearing bifunctional sites have been at the center of recent developments in reversible hydrogenation catalysis, especially in cases utilizing base metals. The influence of bifunctional ligands on low valent cobalt complexes is detailed here using comparisons between the PNP-pincer ligands MeN[CH₂CH₂(PR₂)]₂ and HN[CH₂CH₂(PR₂)]₂ (R = ⁱPr, Cy). Comparative catalytic studies of CO₂ hydrogenation show that cobalt(I) precatalysts bearing the tertiary amine ligand dramatically outperform those bearing the secondary amine pincer ligand. Despite strong similarities between the precatalyst ground state structure and the redox potentials of the two systems, ligand bifunctionality was found to be detrimental to catalyst productivity. The enhanced stability imparted by the MeN[CH₂CH₂(PR₂)]₂ ligand also enabled isolation and characterization of a zero-valent cobalt dicarbonyl species, which was used to study the catalytically active oxidation state of cobalt in CO₂ hydrogenation.