Group 6 dinitrogen complexes supported by diphosphine ligands containing proton relays: Toward the reduction of dinitrogen to ammonia

The reduction of dinitrogen to ammonia from N₂ and H₂ is currently carried out by the Haber-Bosch process, an energy intensive process that requires high pressures and high temperatures and accounts for the production of millions of tons of ammonia per year. The development of a catalytic, energy-efficient process for N₂ reduction is of great interest and remains a formidable challenge. In our approach, we are developing "Chatt-type" ((P-P)₂M(N₂)₂, P-P = diphosphine ligand) molecular electrocatalysts of the Group 6 metals (Cr, Mo, W), supported by diphosphine ligands containing non-coordinating pendant amines (e.g., PNP, (R₂PCH₂)₂N(R′) and P^R₂N^R′₂, substituted 1,5-diaza-3,7-diphosphacyclooctanes) to serve as proton relays. These studies are aimed at answering fundamental questions regarding the role of proton relays in the second coordination sphere of the complexes in their ability to facilitate proton movement from an external acid to metal-bound dinitrogen ligands in the challenging, multi-proton/electron reduction of N₂ to ammonia.