Reduction of Nitrite at a Copper(II) Trans-κ1-ONO Complex via Proton-Coupled Electron Transfer and Oxygen Atom Transfer.

Abstract

Reduction of nitrite (NO₂ ^-) to nitric oxide (NO) serves important roles in NO-dependent signaling as well as in the broad nitrogen biogeochemical cycle. In biological system, copper-containing nitrite reductases (CuNiRs) are well known to bind a nitrite anion to mediate the nitrite reduction to release NO, of which the mechanism still requires further understanding. Herein, synthetic copper(II) nitrite complex with a rare binding mode, [Cu^II(ⁱPr₃-tren)(trans-κ¹-ONO)]⁺ (2), is characterized physicochemically and examined in proton-coupled electron transfer (PCET) and oxygen atom transfer (OAT) to release NO. For the first time to gain mechanistic insights into the trans-κ¹-O binding copper(II) nitrite complex, detailed kinetic studies in company with theoretical calculations have been performed for oxidation of triphenylphosphine (PPh₃), which shows that isomerization of trans-κ¹-O to κ¹-N binding mode is necessary to exert electrophilic OAT. The better reactivity of κ¹-N binding mode is attributed to a fine orbital mixing of Cu-d_z2 with highest occupied molecular orbital (HOMO) of NO₂ ^-, thereby imposing much larger electron density on NO₂ ^- moiety. Thus, it is suggested that the reactivity of the copper(II) nitrite complex is conjunctly related to the binding mode of nitrite.