Electrocatalytic Ammonia Oxidation by Pyridyl-Substituted Ferrocenes.

Abstract

Ammonia (NH₃) is a promising carbon-free fuel when prepared from sustainable resources. First-row transition metal electrocatalysts for ammonia oxidation are an enabling technology for sustainable energy production. We describe electrocatalytic ammonia oxidation using robust molecular complexes based on Earth-abundant iron. Electrochemical studies of ferrocenes with covalently attached pyridine arms reveal facile ammonia oxidation in DMSO (2.4 M NH₃) with modest overpotentials (η = 770-820 mV) and turnover frequencies (125-560 h^-1). Experimental and computational studies indicate that the pendant pyridyl base serves as an H-bond acceptor with an N-H bond of ammonia that transfers a proton to the pyridine following oxidation by the attached ferrocenium moiety in a proton-coupled electron transfer (PCET) step. This generates an amidyl (^•NH₂) radical stabilized via H-bonding to a pendant pyridinium moiety that rapidly dimerizes to hydrazine (H₂N-NH₂), which is easily oxidized to nitrogen (N₂) at the glassy carbon working electrode. This report identifies a general strategy to oxidize ammonia via H-bonding to a base (B:), thereby activating [B···H-NH₂] toward PCET by a proximal oxidant to form [BH···NH₂]^+/• radical cations, which are susceptible to dimerization to form easily oxidized hydrazine.