NO Coupling at Copper to cis-Hyponitrite: N2O Formation via Protonation and H-Atom Transfer

Copper nitrite reductases (CuNIRs) convert NO2- to NO as well as NO to N2O under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N-N coupling from NO that results in symmetric trans-hyponitrite [Cu-II]- ONNO-[Cu-II] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp*2M (M = Co, Fe) to give the first copper cis-hyponitrites [Cp*M-2]{[Cu-II](K2- O2N2)[Cu-I]}. Importantly, the K-1-N-bound [Cu-I] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[Cu-II](K-2-O2N2)}-. The addition of electrophiles such as H+ to these anionic copper(II) cis-hyponitrites leads to N2O generation with the formation of the dicopper(II)-bis-mu-hydroxide [Cu-II](2)(mu- OH)(2). One-electron oxidation of the {[Cu-II](K-2-O2N2)}(-) core turns on H-atom transfer reactivity, enabling the oxidation of 9,10-dihydroanthracene to anthracene with concomitant formation of N2O and [Cu-II](2)(mu-OH)(2). These studies illustrate both the reductive coupling of NO at a single copper center and a way to harness the strong oxidizing power of nitric oxide via the neutral cis-hyponitrite [Cu](K-2-O2N2).

Automated summary

Copper nitrite reductases convert nitrite ions to nitric oxide and nitrous oxide at high levels of nitrogen oxide flux. The assembly of nitric oxide at a single copper site to form copper cis-hyponitrites has been demonstrated in the presence of an external reductant. Furthermore, the removal of a fraction of the structure in the copper cis-hyponitrite complex can be achieved by adding specific reagents, resulting in the formation of a spectroscopically similar anion. These studies provide insights into the reductive coupling of nitrogen oxides at a single copper center and the utilization of the strong oxidizing power of nitrite.