Reductive N–N coupling of NO molecules on transition metal complexes leading to N2O

Nitric oxide reductase (NOR) type reactions (2NO + 2e− + 2H+ → N2O + H2O) on transition metal complexes not involving NO disproportionation (3NO → N2O + NO2) are reviewed. The former has little reported, although the latter is very common reaction. The formation of N2O indicates that N–N coupling of two NO molecules is an essential step. A few examples of N–N coupling on transition metal complexes have been structurally characterized, including several examples of hyponitrite (O–NN–O)2− complexes and only one diruthenium complex bearing neutral (ON–NO) binding mode. Protonation or heating their complexes led to elimination of N2O. In the examination of the NOR-type reaction, only a few functional model complexes for the active site of the metalloenzyme have been developed. These complexes also showed NOR activity. Finally, an NO reduction cycle in the diruthenium system is described.

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► NOR-type reactions (2NO + 2e− + 2H+ → N2O + H2O) on transition metal complexes.
► Key step is N–N coupling of 2 NO molecules on transition metal complexes.
► Several structurally characterized hyponitrite (O–NN–O)2− complexes.
► Only one diruthenium complex bearing neutral (ON–NO) binding mode.
► A few biomimetic model complexes showed the NOR-type reaction.