Proton Responsive and Hydrogen Bonding Ligands in Organometallic Chemistry

This chapter focuses on the use of proton responsive and hydrogen bonding ligands in organometallic catalysis. These ligands typically can donate or accept protons via NH and OH groups. The factors that lead to catalytic rate enhancements are described including the acidity of the proton, the rigidity of the ligand scaffold, and the geometric constraints that hold these protons near the metal center. These ligands are typically bidentate, tridentate facial, or tridentate meridional when bound to transition metals. The Shvo type ligands are also discussed which incorporate an OH group on a cyclopentadienyl ring. These various protic (or readily protonated) ligands are typically coordinated to ruthenium, iridium, manganese, iron, cobalt, nickel, copper, and other transition metals. Many of these metal complexes are active catalysts for (de)hydrogenation of both organic substrates and CO2, and frequently these are among the most active catalysts in the literature. The reasons why hydrogenation is especially well suited for a metal ligand bifunctional catalysis approach are described. Protic organometallic complexes have also been used for photocatalytic and electrocatalytic redox reactions, and frequently modification of the electronic properties of the catalyst can be beneficial so long as the catalyst does not become a sacrificial electron donor leading to catalyst decomposition. Ultimately, there is great promise to improve and optimize catalyst longevity and catalytic rates using proton responsive catalysts in reactions that have been dominated by aprotic organometallic catalyst analogs.