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

Reduction of nitrite (NO 2 − ) 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 ( i Pr 3 ‐tren)( trans ‐ κ 1 ‐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 ‐ κ 1 ‐O binding copper(II) nitrite complex, detailed kinetic studies in company with theoretical calculations have been performed for oxidation of triphenylphosphine (PPh 3 ), which shows that isomerization of trans ‐ κ 1 ‐O to κ 1 ‐N binding mode is necessary to exert electrophilic OAT. The better reactivity of κ 1 ‐N binding mode is attributed to a fine orbital mixing of Cu‐d z 2 with highest occupied molecular orbital (HOMO) of NO 2 − , thereby imposing much larger electron density on NO 2 − moiety. Thus, it is suggested that the reactivity of the copper(II) nitrite complex is conjunctly related to the binding mode of nitrite.