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] ( R L1) and 6- p -chlorophenyl-2-pyridyl- N -(2'-phenylthiophenyl)methylenimine ( R L2), 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 NOBF 4 (NO + ) were examined in dry acetonitrile in the presence and absence of water (H 2 O). The results revealed that, in the absence of H 2 O , complexes 1 - 4 (or 1' - 4' ) reacts with NO (or NOBF 4 ), 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 ( R Pial) via a copper nitrosyl of a {CuNO} 10 -type intermediate. In the presence of H 2 O , the in situ formed R Pial get oxidized to the corresponding picolinic acid ( R PicH) via an in situ formed LCu I OH intermediate (LCu I + HO-NO → LCu I OH + NO + ; L = R L1/ R L2/ R Pic - and ν O-H of Cu I OH = 3650 cm -1 ) and subsequently produces, with the aid of NO + oxidant, the picolinate-ligated copper(II) complexes (i) [( H Pic) 2 Cu] ( 7 ), [( Me Pic) 4 Cu 3 (NO 3 ) 2 ] n ·H 2 O ( 8 ·H 2 O), or [( ClPh Pic) 2 Cu] ( 9 ) when NO reacts with 1 - 4 or (ii) [( R Pic)Cu II ( R L1/ R L2)] + when NO + reacts with 1' - 4' . The Cu II to Cu I reduction of [( R Pic)Cu II ( R L1/ R L2)] + is essential for C═N cleavage of the remaining R L1/ R L2 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 Cu II/I , NO, NO + , and H 2 O, 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 15 NO revealed that the terminal N of the N 2 + group of 5 originates from 15 NO [ν 14 N 14 N- = 2248 cm -1 and ν 15 N 14 N- = 2212 cm -1 ].