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*₂M]{[Cu^II](κ²-O₂N₂)[Cu^I]}. Importantly, the κ¹-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](κ²-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-μ-hydroxide [Cu^II]₂(μ-OH)₂. One-electron oxidation of the {[Cu^II](κ²-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(μ-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](κ²-O₂N₂).