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

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-Schrock cycle for a molybdenum complex with such a calix[6]tren ligand. We find that the first protonation step and several reduction steps become thermodynamically less favored compared to the original Yandulov Schrock catalyst, indicating that even stronger acids and reductants than lutidinium and decamethylchromocene, respectively, might be needed. Also, multiple side reactions can occur that are characterized by moderate to high barriers which can reduce the turnover frequency or even prevent catalytic behavior altogether. Strong acidic conditions are, however, found to induce ether cleavage of methoxy substituents in the calix[6]tren ligand. Upon reduction of a protonated methoxy group, a methyl residue is transferred onto the distal nitrogen atom of the coordinated dinitrogen ligand. It is therefore advantageous to avoid alkoxy substituents at the chelate ligand.