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 Schiffbase ligands of the general formulas [6-(R)-2-pyridyl-N-(2 '-methylthiophenyl)methylenimine] (RL1) and6-p-chlorophenyl-2-pyridyl-N-(2 '-phenylthiophenyl)methylenimine (RL2), where R = H, Me,p-ClPh, and their bis-ligandcopper(II) and copper(I) complexes,1-4and1 '-4 ', respectively, were synthesized and characterized. The reactivities of1-4withnitric oxide (NO) gas and of1 '-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, complexes1-4(or1 '-4 ') reacts with NO (or NOBF4), leading to imineC???N bond cleavage of both (or one) Schiffbase(s) that generates 2 (or 1) equiv of 2-(methyl/phenyl)thiobenzenediazoniumperchlorates (5/6) and the corresponding picolaldehyde (RPial) via a copper nitrosyl of a {CuNO}10-type intermediate.In thepresence of H2O, the in situ formedRPial get oxidized to the corresponding picolinic acid (RPicH) via an in situ formed LCuIOHintermediate (LCuI+ HO-NO -> LCuIOH + NO+;L=RL1/RL2/RPic-and nu O-Hof CuIOH = 3650 cm-1) and subsequentlyproduces, with the aid of NO+oxidant, the picolinate-ligated copper(II) complexes (i) [(HPic)2Cu] (7), [(MePic)4Cu3(NO3)2]nmiddotH2O(8middotH2O), or [(ClPhPic)2Cu] (9) when NO reacts with1-4or (ii) [(RPic)CuII(RL1/RL2)]+when NO+reacts with1 '-4 '. The CuIItoCuIreduction of [(RPic)CuII(RL1/RL2)]+is essential for C???N cleavage of the remainingRL1/RL2 Schiffbase; excess NO can do it.The X-ray structures (1,1 ',3 ',5,7, and8) and spectroscopic results revealed the role of CuII/I, NO, NO+, and H2O, shedding lighton the mechanism of C???N bond cleavage and the oxidation of pyridine-2-aldehyde to pyridine-2-carboxylic acid. The reaction of1with15NO revealed that the terminal N of the N2+group of5originates from15NO [nu 14N14N-= 2248 cm-1and nu 15N14N-= 2212 cm-1]