N-H Activation of Ammonia by [{M(μ-OMe)(cod)}<inf>2</inf>] (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 MIII 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. (Chemical Equation Presented). © 2015 American Chemical Society