A Copper(II) Nitrite That Exhibits Change of Nitrite Binding Mode and Formation of Copper(II) Nitrosyl Prior to Nitric Oxide Evolution

The proton-coupled reduction of Cu^II-bound nitrite (NO₂^–) to nitric oxide (NO₂^– + 2H⁺ + e^– → NO­(g) + H₂O), such as occurs in the enzyme copper nitrite reductase, is investigated in this work. Our studies focus on the copper­(II/I) model complexes [(L2)­Cu­(H₂O)­Cl] (1), [(L2)­Cu­(ONO)] (2), [(L2)­Cu­(CH₃CO₂)] (3), and [Co­(Cp)₂]­[(L2)­Cu­(NO₂)­(CH₃CN] (4), where HL2 = N-[2-(methylthio)­ethyl]-2′-pyridinecarboxamide. Complex 1 readily reacts with a NO₂^– anion to form the nitrito-O-bound copper­(II) complex 2. Electrochemical reduction of Cu^II → Cu^I indicates coordination isomerization from asymmetric nitrito-κ²-O,O to nitro-κ¹-N. Isolation and spectroscopic characterization of 4 support this notion of nitrite coordination isomerization (ν_Cu–N ∼ 460 cm^–1). A reduction of 2, followed by reaction with acetic acid, causes evolution of stoichiometric NO via the transient copper­(II) nitrosyl species and subsequent formation of the acetate-bound complex 3. The probable copper nitrosyl intermediate [(L2)­Cu­(NO)­(CH₃CN)]⁺ of the {CuNO}¹⁰ type is evident from low-temperature UV–vis absorption (λ_max = 722 nm) and electron paramagnetic resonance spectroscopy. A density functional theory (DFT)-optimized model of [(L2)­Cu­(NO)­(CH₃CN)]⁺ shows end-on NO binding to Cu with Cu–N­(NO) and N–O distances of 1.989 and 1.140 Å, respectively, and a Cu–N–O angle of 119.25°, consistent with the formulation of Cu^II-NO^•. A spin-state change that triggers NO release is observed. Considering singlet- and triplet-state electronic configurations of this model, DFT-calculated ν_NO values of 1802 and 1904 cm^–1, respectively, are obtained. We present here important mechanistic aspects of the copper-mediated nitrite reduction pathway with the use of model complexes employing the ligand HL2 and an analogous phenyl-based ligand, N-[2-(methylthio)­phenyl]-2′-pyridinecarboxamide (HL1).