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-bound nitrite (NO) to nitric oxide (NO + 2H + e → NO(g) + HO), 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(HO)Cl] (1), [(L2)Cu(ONO)] (2), [(L2)Cu(CHCO)] (3), and [Co(Cp)][(L2)Cu(NO)(CHCN] (4), where HL2 = N-[2-(methylthio)ethyl]-2'-pyridinecarboxamide. Complex 1 readily reacts with a NO anion to form the nitrito-O-bound copper(II) complex 2. Electrochemical reduction of Cu → Cu indicates coordination isomerization from asymmetric nitrito-κ-O,O to nitro-κ-N. Isolation and spectroscopic characterization of 4 support this notion of nitrite coordination isomerization (ν ∼ 460 cm). 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)(CHCN)] of the {CuNO} type is evident from low-temperature UV-vis absorption (λ = 722 nm) and electron paramagnetic resonance spectroscopy. A density functional theory (DFT)-optimized model of [(L2)Cu(NO)(CHCN)] 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-NO. A spin-state change that triggers NO release is observed. Considering singlet- and triplet-state electronic configurations of this model, DFT-calculated ν values of 1802 and 1904 cm, 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).