Electrocatalytic Reduction of Nitrogen Oxyanions with a Redox-Active Cobalt Macrocycle Complex

The cobalt complex, [Co(CR)Br₂]⁺, where CR is the redox-active macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene, has been investigated for the electrocatalytic reduction of aqueous NO₂^- and NO₃^-. At neutral pH, the bromide ligands are hydrolyzed, providing [Co(CR)(OH₂)(OH)]²⁺ as the major species in aqueous solution. In the presence of nitrite, [Co(CR)(NO₂)₂]⁺ is formed as the major species in solution and is a precursor to the electrocatalytic reduction of NO₂^-, which is selectively converted to ammonium with high Faradaic efficiency. There is evidence for both homogeneous and heterogeneous electrocatalysis. Although similar NO₃^- binding is not observed, electrocatalytic reduction to ammonium also occurs, albeit with a lower Faradaic efficiency. In this case, NO₂^- is generated as an intermediate product of NO₃^- reduction.