Selective Electrocatalytic Reduction of NO to NH3 by Iron Porphyrins at Physiologically Relevant Potentials

Nitric oxide (NO), a toxic gas, is a key intermediate in the global nitrogen cycle that may be reduced to produce NH3. However, there are few molecular catalysts which can catalyze this transformation. The heme nitrosyls, which are NO adducts of ferric or ferrous heme, are intermediates in denitrification, dissimilatory nitrite reduction, and assimilatory ammonification which are abundant in nature, and they need to have diverse reactivity. A particularly curious case of such divergent reactivity is presented by the ferrous nitrosyl ({FeNO}(7)) species in heme c containing cytochrome c nitrite reductase (CcNiR) and siroheme containing CSNiR, both of which catalyze the 6e(-)/8H(+) reduction of NO2 (-) to NH4 (+). In CcNiR the {FeNO}(7) species is generally considered to be a kinetically inert thermodynamic sink. CSNiR, on the other hand, comfortably uses this {FeNO}(7) species as an intermediate in the 6e(-)/8H(+) reduction of NO2 (-) to NH4 (+). A clear demarcation between the heme cofactors heme c and siroheme in CcNiR and CSNiR is the presence of four -CH2-COOH substituents in the latter. Past efforts to reduce NO to NH4 (+) using iron porphyrins required very high cathodic potentials (-1 V vs NHE) and were mostly not selective and produced N2O and NH2OH as major products. In this work, iron porphyrins with different numbers of electron-withdrawing -COOEt groups are used to understand any role that the inductive effect of substituents on the porphyrin ring may play in the reduction of NO. The crystallographic structure of the ferrous heme nitrosyl {FeNO}(7) species is found to be not very sensitive to the number of -COOEt substituents. However, the potential for electrochemical NO reduction in these porphyrins is shifted by 500 mV and 700 mV more positive relative to iron porphyrins without -COOEt groups when two and four -COOEt groups are introduced, respectively, raising it into the physiologically relevant potential range. In situ spectroelectrochemistry with labeled (NO)-N-15 shows that the reduction of {FeNO}(7) species follows a PCET pathway to produce {FeHNO}(8) species in the presence of water (a weak acid, pK a similar to 25). Under these conditions NO is reduced to N2O via a hyponitrite intermediate, but in the presence of strong acid (pK a 8.5 in MeCN) the {FeHNO}(8) species is rapidly reduced to NH4 (+), and FY yields >90% are obtained with the iron porphyrin with two -COOEt groups. In these iron porphyrins, the {FeHNO}(8) species appears to be a branching point in the reduction of NO, and either N2O or NH4 (+) can be obtained selectivity by controlling the acidity of the proton source.

Automated summary

Nitric oxide (NO) is a key intermediate in the nitrogen cycle, but there are limited molecular catalysts for its transformation. Heme nitrosyls, particularly the ferrous nitrosyl species, play important roles in the reduction of NO2 (-) to NH4 (+). This study investigates the effect of substituents on iron porphyrins on the reduction of NO, and demonstrates that the presence of -COOEt groups can shift the potential for electrochemical NO reduction, making it more physiologically relevant.