Promoting N−N Bond Formation of Ammonia Oxidation by a Dangling Carboxylate Proton Relay

Abstract Inspired by enzymatic proton-coupled electron transfer (PCET) control, introducing proton relays into catalysts' second coordination sphere accelerates proton-transfer reactions, yet this strategy remains underexplored for the catalysis of ammonia oxidation reaction (AOR). Here, we report ammonia oxidation catalyzed by [Ru(pdc-κ-N1O1)(tpy)(NH3)] (Ru(pdc-t)(NH3); H2pdc = 2, 6-pyridinedicarboxylic acid; tpy = terpyridine), featuring a pendant carboxylate group, and its structural analog [Ru(pic)(tpy)(NH3)](PF6) (Ru(pic-t)(NH3); Hpic = picolinic acid). Such a structural design creates a dangling carboxylate group as a proton relay, assisting the proton transfer during N−N bond formation. Ru(pdc-t)(NH3) exhibits improved AOR performance relative to Ru(pic-t)(NH3) and previously reported [Ru(pdc-κ-N1O2)(bpy)(NH3)] (Ru(pdc-b)(NH3); bpy = bipyridine), with a catalytic rate of 209.7 ± 12.2 s−1 (11.1-fold increase) and a turnover number of 319 for N2 (4.2-fold increase). The calculated barrier for N−N bond formation for Ru(pdc-t)(NH3) is +0.8 kcal/mol, which is significantly lower than the +7.8 kcal/mol barrier for Ru(pic-t)(NH3). This work demonstrates the promotional effect of a proton relay on the catalytic oxidation of ammonia to N2.