Isolation of a Ruthenium Pyrazole-Amido Intermediate in Catalytic Ammonia Oxidation by a Ruthenium Complex Bearing a Bipyrazole-Bipyridine Ligand

Metal nitrido/imido/amido (M≡N/M=NH/M-NH₂) species are generally proposed as active intermediates for N-N bond formation in catalytic ammonia oxidation (AO). We report herein electrochemical and chemical AO to N₂ catalyzed by ruthenium complexes with tetradentate bipyrazole-bipyridine ligands that bear alkyl groups at the pyrazole rings, [Ru(H₂^t-BuL)(pic)₂]²⁺ (1, H₂^t-BuL = 6,6'-bis(5-(tert-butyl)-1H-pyrazol-3-yl)-2,2'-bipyridine) and [Ru(H₂^MeL)(pic)₂]²⁺ (2, H₂^MeL = 6,6'-bis(5-methyl-1H-pyrazol-3-yl)-2,2'-bipyridine). In the presence of excess NH₃, the cyclic voltammogram (CV) of 1 exhibits a catalytic current with an onset potential at 0.05 V (vs Fc^+/0). Controlled potential electrolysis at 0.70 V produced N₂ with a Faradaic efficiency of up to 97%. In chemical AO by 1 using (4-BrPh)₃N^•+ as an oxidant, an unprecedented ruthenium(IV) pyrazole-amido intermediate (Ru-NH-N(pyz), 3') was isolated and characterized; its chloro derivative (4) was also isolated and structurally characterized with a Ru-NH bond length of 1.9037(108) Å. 3' is formed by insertion of NH derived from NH₃ into the Ru-N(pyz) bond. 3' is readily deprotonated by excess NH₃ to give a ruthenium(IV) pyrazole-imido species (Ru=N-N(pyz), 3). 3 undergoes N-N bond formation via nucleophilic attack by NH₃. Density functional theory (DFT) studies showed that the ruthenium(IV) pyrazole-imido intermediate is much more stable than terminal Ru≡N/Ru=NH intermediates. Similar stable intermediates may also occur in other ruthenium AO catalysts with ligands that have pyrazole- or pyridine-type end groups.