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.