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 CuII-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 CuII → CuI 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 (νCu-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 (λmax = 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 CuII-NO•. A spin-state change that triggers NO release is observed. Considering singlet- and triplet-state electronic configurations of this model, DFT-calculated νNO 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).