Copper Complexes Relevant to the Catalytic Cycle of Copper Nitrite Reductase: Electrochemical Detection of NO(g) Evolution and Flipping of NO2 Binding Mode upon CuII → CuI Reduction

Copper complexes of the deprotonated tridentate ligand, N-2-methylthiophenyl-2′-pyridinecarboxamide (HL1), were synthesized and characterized as part of our investigation into the reduction of copper(II) o-nitrito complexes into the related copper nitric oxide complexes and subsequent evolution of NO(g) such as occurs in the enzyme copper nitrite reductase. Our studies afforded the complexes [(L1)CuIICl] n (1), [(L1)CuII(ONO)] (2), [(L1)CuII(H2O)](ClO4)·H2O (3·H 2 O), [(L1)CuII(CH3OH)](ClO4) (4), [(L1)CuII(CH3CO2)]·H2O (5·H 2 O), and [Co(Cp)2][(L1)CuI(NO2)(CH3CN)] (6). X-ray crystal structure determinations revealed distorted square-pyramidal coordination geometry around CuII ion in 1–5. Substitution of the H2O of 3 by nitrite quantitatively forms 2, featuring the κ2-O,O binding mode of NO2 – to CuII. Reduction of 2 generates two CuI species, one with κ1-O and other with the κ1-N bonded NO2 – group. The CuI analogue of 2, compound 6, was synthesized. The FTIR spectrum of 6 reveals the presence of κ1-N bonded NO2 –. Constant potential electrolysis corresponding to CuII → CuI reduction of a CH3CN solution of 2 followed by reaction with acids, CH3CO2H or HClO4 generates 5 or 3, and NO(g), identified electrochemically. The isolated CuI complex 6 independently evolves one equivalent of NO(g) upon reaction with acids. Production of NO(g) was confirmed by forming [Co(TPP)NO] in CH2Cl2 (λmax in CH2Cl2: 414 and 536 nm, νNO = 1693 cm–1).