Intramolecular Hydrogen Bonding Facilitates Electrocatalytic Reduction of Nitrite in Aqueous Solutions

This
work reports a combined experimental and computational mechanistic
investigation into the electrocatalytic reduction of nitrite to ammonia
by a cobalt macrocycle in an aqueous solution. In the presence of
a nitrite substrate, the Co­(III) precatalyst, [Co­(DIM)­(NO2)2]+ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene),
is formed in situ. Cyclic voltammetry and density
functional theory (DFT) calculations show that this complex is reduced
by two electrons, the first of which is coupled with nitrite ligand
loss, to provide the active catalyst. Experimental observations suggest
that the key N–O bond cleavage step is facilitated by intramolecular
proton transfer from an amine group of the macrocycle to a nitro ligand,
as supported by modeling several potential reaction pathways with
DFT. These results provide insights into how the combination of a
redox active ligand and first-row transition metal can facilitate
the multiproton/electron process of nitrite reduction.