N–H Activation of Ammonia by [{M(μ-OMe)(cod)}₂] (M = Ir, Rh) Complexes: A DFT Study

In this work, a computational study at the DFT level is carried out to determine the reaction mechanism for the N–H bond activation of ammonia by dinuclear [{M­(μ-OMe)­(cod)}₂] complexes (M = Ir, Rh) to yield amido species [{M­(μ-NH₂)­(cod)}₂] reported experimentally by Mena et al. (Angew. Chem., Int. Ed. 2011, 50, 11735–11738). A stepwise mechanism is proposed for the replacement of μ-OMe bridging ligands considering associative or dissociative approaches for NH₃ coordination to the metal. Reaction pathways for the homolytic and heterolytic N–H σ-bond cleavage of ammonia, such as oxidative addition through M^III species or hydrogen transfer to the ligand, are investigated. The energetically preferred mechanism involves the participation of both metallic centers through the formation of and intermediate bearing M₁-NH₃ and M₂-OMe moieties followed by heterolytic hydrogen transfer of the amino ligand to the methoxo ligand. A bonding analysis on the metallacycle [M₂X₂] core (M = Ir, Rh; X = μ-OMe, μ-NH₂) is performed, showing that the amido bridging complex is stabilized due to the presence of metal–metal bonding interactions.