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

Herein, we describe the synthesis of a copper(II) nitrite complex which possesses unusual trans-κ ¹-O binding mode in trigonal bipyramidal geometry, and characterization through various physicochemical methods. The reduction of NO₂ ⁻ to release NO• was examined through PCET and OAT by using ferrocene and PPh₃, respectively. DFT calculations revealed that the isomerization of trans-κ ¹-O to κ ¹-N binding mode is the key step for the OAT. The superiority of κ ¹-N binding mode in the OAT was further rationalized by molecular orbital analysis.

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( ⁱ Pr₃-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 (PPh₃), 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_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.

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