Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.