Reactivity of Nitric Oxide and Nitrosonium Ion with Copper(II/I) Schiff Base Complexes: Mechanistic Aspects of Imine CN Bond Cleavage and Oxidation of Pyridine-2-aldehyde to Pyridine-2-carboxylic Acid

Four Schiff base ligands of the general formulas [6-(R)-2-pyridyl-N-(2′-methylthiophenyl)­methylenimine] (RL1) and 6-p-chlorophenyl-2-pyridyl-N-(2′-phenylthiophenyl)­methylenimine (RL2), where R = H, Me, p-ClPh, and their bis-ligand copper­(II) and copper­(I) complexes, 1–4 and 1′–4′, respectively, were synthesized and characterized. The reactivities of 1–4 with nitric oxide (NO) gas and of 1′–4′ with solid NOBF4 (NO+) were examined in dry acetonitrile in the presence and absence of water (H2O). The results revealed that, in the absence of H2O, complexes 1–4 (or 1′–4′) reacts with NO (or NOBF4), leading to imine CN bond cleavage of both (or one) Schiff base(s) that generates 2 (or 1) equiv of 2-(methyl/phenyl)­thiobenzenediazonium perchlorates (5/6) and the corresponding picolaldehyde (RPial) via a copper nitrosyl of a {CuNO}10-type intermediate. In the presence of H 2 O, the in situ formed RPial get oxidized to the corresponding picolinic acid (RPicH) via an in situ formed LCuIOH intermediate (LCuI + HO-NO → LCuIOH + NO+; L = RL1/RL2/RPic– and νO–H of CuIOH = 3650 cm–1) and subsequently produces, with the aid of NO+ oxidant, the picolinate-ligated copper­(II) complexes (i) [(HPic)2Cu] (7), [(MePic)4Cu3(NO3)2] n ·H2O (8·H2O), or [(ClPhPic)2Cu] (9) when NO reacts with 1–4 or (ii) [(RPic)­CuII(RL1/RL2)]+ when NO+ reacts with 1′–4′. The CuII to CuI reduction of [(RPic)­CuII(RL1/RL2)]+ is essential for CN cleavage of the remaining RL1/RL2 Schiff base; excess NO can do it. The X-ray structures (1, 1′, 3′, 5, 7, and 8) and spectroscopic results revealed the role of CuII/I, NO, NO+, and H2O, shedding light on the mechanism of CN bond cleavage and the oxidation of pyridine-2-aldehyde to pyridine-2-carboxylic acid. The reaction of 1 with 15NO revealed that the terminal N of the N2 + group of 5 originates from 15NO [ν 14N14N– = 2248 cm–1 and ν 15N14N– = 2212 cm–1].