Ligands for Dinitrogen Fixation at Schrock-Type Catalysts

Catalytic dinitrogen reduction with the Schrock complex is still hampered by low turn-over numbers that are likely to result from a degradation of the chelate ligand. In this work, we investigate modifications of the original HIPTN3N ligand applied by Schrock and co-workers in catalytic reduction of dinitrogen with density functional methods. We focus on ligands that are substituted in the para position of the central phenyl ring of the terphenyl moieties and on a ligand where the bridging nitrogen is exchanged by phosphorus. In addition, results for tris(pyrrolyl-α-methyl)amine, tris(pyrrolyl-α-ethyl)amine, and tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine are reported. For this study, we take into account the full ligands without approximating them by model systems. Reaction energies for the various derivatives of HIPTN3N are found to be similar to those of the unchanged parent system. However, the most promising results for catalysis are obtained for the [{tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine}Mo](N2) complex. Feasibility of the exchange of NH3 by N2 is found to be the pivotal question whether a complex can become a potential catalyst or not. A structure−reactivity relationship is derived which allows for the convenient estimation of the reaction energy for the NH3/N2 exchange reaction solely from the wavenumber of the NN stretching vibration. This relationship may guide experiments as soon as a dinitrogen Mo complex is formed.