Ammonia Synthesis from a Pincer Ruthenium Nitride via Metal–Ligand Cooperative Proton-Coupled Electron Transfer

The conversion of
metal nitride complexes to ammonia may be essential
to dinitrogen fixation. We report a new reduction pathway that utilizes
ligating acids and metal–ligand cooperation to effect this
conversion without external reductants. Weak acids such as 4-methoxybenzoic
acid and 2-pyridone react with nitride complex [(H-PNP)­RuN]+ (H-PNP = HN­(CH2CH2PtBu2)2) to generate
octahedral ammine complexes that are κ2-chelated
by the conjugate base. Experimental and computational mechanistic
studies reveal the important role of Lewis basic sites proximal to
the acidic proton in facilitating protonation of the nitride. The
subsequent reduction to ammonia is enabled by intramolecular 2H+/2e– proton-coupled electron transfer from
the saturated pincer ligand backbone.