Computational design of cooperatively acting molecular catalyst systems: carbene based tungsten- or molybdenum-catalysts with rhodium- or iridium-complexes for the ionic hydrogenation of N2 to NH3

Abstract

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N₂ with H₂, leading to NH₃ formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M¹(H)₂] (M¹ = W/Mo) were chosen to bind N₂ at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M²{2-(2-pyridyl)phenyl}(CH₃CN)]⁺ (Cp* = C₅(CH₃)₅ and M² = Rh/Ir), are employed as cooperative coordination partners for heterolytic H₂ splitting. The stepwise transfer of protons and hydrides to the bound N₂ and intermediate N_xH_y units results in the formation of NH₃. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol⁻¹ depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol⁻¹ demonstrating significant potential for molecular catalysts for the N₂/H₂ reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected.