Electrocatalytic Nitrogen Reduction on a Molybdenum Complex Bearing a PNP Pincer Ligand

Electrocatalytic nitrogen reduction (N2R)
mediated by
well-defined molecular catalysts is poorly developed by comparison
with other reductive electrocatalytic transformations. Herein, we
explore the viability of electrocatalytic N2R mediated
by a molecular Mo-PNP complex. A careful choice of acid, electrode
material, and electrolyte mitigates electrode-mediated HER under direct
electrolysis and affords up to 11.7 equiv of NH3 (Faradaic
efficiency < 43%) at −1.89 V versus Fc+/Fc. The
addition of a proton-coupled electron transfer (PCET) mediator has
no effect. The data presented are rationalized by an initial electron
transfer (ET) that sets the applied bias needed and further reveal
an important impact of [Mo] concentration, thereby pointing to potential
bimolecular steps (e.g., N2 splitting) as previously proposed
during chemically driven N2R catalysis. Finally, facile
reductive protonation of [Mo(N)Br(HPNP)] with pyridinium
acids is demonstrated.