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-()-2-pyridyl--(2'-methylthiophenyl)methylenimine] (L1) and 6--chlorophenyl-2-pyridyl--(2'-phenylthiophenyl)methylenimine (L2), where R = H, Me, -ClPh, and their bis-ligand copper(II) and copper(I) complexes, - and -, respectively, were synthesized and characterized. The reactivities of - with nitric oxide (NO) gas and of - with solid NOBF (NO) were examined in dry acetonitrile in the presence and absence of water (HO). The results revealed that, , complexes - (or -) reacts with NO (or NOBF), 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 (/) and the corresponding picolaldehyde (Pial) via a copper nitrosyl of a {CuNO}-type intermediate. , the in situ formed Pial get oxidized to the corresponding picolinic acid (PicH) via an in situ formed LCuOH intermediate (LCu + HO-NO → LCuOH + NO; L = L1/L2/Pic and ν of CuOH = 3650 cm) and subsequently produces, with the aid of NO oxidant, the picolinate-ligated copper(II) complexes (i) [(Pic)Cu] (), [(Pic)Cu(NO)]·HO (·HO), or [(Pic)Cu] () when NO reacts with - or (ii) [(Pic)Cu(L1/L2)] when NO reacts with -. The Cu to Cu reduction of [(Pic)Cu(L1/L2)] is essential for C═N cleavage of the remaining L1/L2 Schiff base; excess NO can do it. The X-ray structures (, , , , , and ) and spectroscopic results revealed the role of Cu, NO, NO, and HO, 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 with NO revealed that the terminal N of the N group of originates from NO [ν = 2248 cm and ν = 2212 cm].