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)(CuCl)-Cl-II](n) (1), [(L1)Cu-II(ONO)] (2), [(L1)Cu-II(H2O)](ClO4)center dot H2O (3 center dot H2O), [(L1)Cu-II(CH3OH)](ClO4) (4), [(L1)Cu-II(CH3CO2)]center dot H2O (5 center dot H2O), and [Co(Cp)(2)][(L1)Cu-I(NO2)(CH3CN)] (6). X-ray crystal structure determinations revealed distorted square-pyramidal coordination geometry around Cu-II ion in 1-5. Substitution of the H2O of 3 by nitrite quantitatively forms 2, featuring the kappa(2)-O,O binding mode of NO2- to Cu-II. Reduction of 2 generates two Cu-I species, one with kappa(1)-O and other with the kappa(1)-N bonded NO2- group. The Cu-I analogue of 2, compound 6, was synthesized. The FTIR spectrum of 6 reveals the presence of kappa(1)-N bonded NO2-. Constant potential electrolysis corresponding to Cu-II -> Cu-I 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 Cu-I 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 (lambda(max) in CH2Cl2: 414 and 536 nm, nu(NO) = 1693 cm(-1)).