Catalytic Ammonia Synthesis via Protonation-Induced Cleavage of a Dinitrogen-Bridged Dimolybdenum Complex with a 𝜋8 Mo2(μ-N2) Core

Bimetallic cleavage of dinitrogen has emerged as a highly promising approach to synthesis using N2, particularly its conversion to NH3. It is generally considered that thermal bimetallic cleavage proceeds only through MNNM units with a delocalized 𝜋10 electronic configuration. We report herein a N2-bridged complex with a 𝜋8 configuration, [(PArNP)MoI]2(μ-1:1-N2) (2; PArNP = Ozerov’s anionic PNP pincer ligand). As expected, 2 displays a high barrier to thermal N2 cleavage which occurs only slowly at 110 °C (k = 1.65 x 10-4 s-1). However, at room temperature 2 catalyzes the conversion of N2 to NH3 by Cp*2Co and collidinium triflate. Experiments in the absence of reductant reveal that cleavage is catalyzed by Brønsted acids. DFT analysis indicates that this proceeds via protonation of the μ-N2 ligand, to give a diazenido bridge; N-N cleavage of this bridge is spin- and symmetry-allowed with a low calculated barrier (G‡ = 20 kcal/mol). The mononuclear product of cleavage of 2, (PArNP)Mo(N)I (1-(N)I), was characterized crystallographically and by EPR spectroscopy. 1-(N)I has a half-filled non-bonding d orbital; as a result, hydrogen-atom transfer or proton-coupled electron transfer to yield the corresponding imide is calculated to be much more thermodynamically favorable than analogous additions to the closed-shell nitrides derived from 𝜋10 complexes. This finding is calculated to be general for 𝜋8 versus 𝜋10 cleavage products, with implications for the design of molecular catalysts for N2 conversion to NH3.