Mechanistic Consequences of Chelate Ligand Stabilization on Nitrogen Fixation by Yandulov–Schrock-Type Complexes

Abstract

The Yandulov–Schrock catalyst, a mononuclear molybdenum complex with a tetra-coordinate triamidoamine chelate ligand with hexa-iso-propyl-terphenyl groups at the amide nitrogen atoms, catalyzes the reduction of dinitrogen to ammonia. Its turnover number is very low, which may be attributed to the (partial) loss of the chelate ligand. Protonation of an amide nitrogen atom of the ligand and subsequent reduction leads to the formation of a labile amine ligand. We find that this equatorial amine group can detach from the molybdenum center of the Yandulov–Schrock complex with a comparatively small barrier. This decomposition reaction is in direct competition with reactions producing intermediates of the Chatt–Schrock cycle. Clamping the substituents on the amide nitrogen atoms by a calix[6]arene unit (replacing the hexa-iso-propyl-terphenyl groups) successfully suppresses the detachment of a generated equatorial amine group from the molybdenum center. We discuss dinitrogen reduction according to the Chatt–Schr...