Mononuclear Molybdenum and Tungsten Phosphine Complexes for Catalytic Ammonia Synthesis: Development of the pentaPod...

The pentaPod (P5) concept, combining tridentate and tripodal ligand fragments, is developed to obtain chemocatalytic Chatt-type complexes with greater stability than classical molybdenum and tungsten systems. In these pentaPod complexes, side reactions that usually inhibit catalysis in classic Chatt complexes are effectively suppressed. Using the original pentaPod ligand P5^Me, molybdenum and tungsten dinitrogen complexes [M(N₂)(P5^Me)] (M = Mo and W) are synthesized. Indeed, [Mo(N₂)(P5^Me)] generates 26 equivalents of ammonia with the PCET (proton coupled electron transfer) reagent SmI₂(THF)₂/H₂O as electron and proton source, whereas [W(N₂)(P5^Me)] affords 3 equivalents of ammonia, but primarily catalyzes the hydrogen evolution reaction (HER). Despite their different reactivities, both complexes exhibit similar redox potentials, and DFT calculations of the mechanisms of N₂-to-NH₃ reduction and HER show no differences between [Mo(N₂)(P5^Me)] and [W(N₂)(P5^Me)]. To improve the catalytic activity of the pentaPod complexes, the modified pentaPod ligand P5^Pln, containing two phospholane groups, is developed. The corresponding [M(N₂)(P5^Pln)] complexes (M = Mo and W) produce 22 (Mo) and 7 (W) equivalents of NH₃, respectively, rendering the latter the first tungsten complex to chemocatalytically generate ammonia. Surprisingly, spectroscopic and electrochemical data indicate lower Brønsted basicities of the tungsten dinitrogen complexes compared to their molybdenum analogs.