Species Absorbing in the 500-nm Region in Pyridoxal Catalysis. V. Divalent Metal Chelates

Abstract

A transient species with an absorption at around 500 nm was formed on addition of Mn (II), Cu (II), Co (II), Zn (II), and Ni (II) ions to a methanol solution of the aldimine of of pyridoxal N-methochloride and ethyl alaninate. It is concluded that the species is the metal chelate of a quinoid intermediate in the metal ion mediated isomerization of the aldimine and the ketimine. The formation of the quinoid metal chelate was completed within a few seconds after the chelation of the aldimine and its rate was not greatly dependent on the metal ions. The rate of the disappearance of the quinoid metal chelate was affected by the properties and the concentrations of the metal ions and, in the case of Cu (II), by the presence of the other chelating ligand. The life time of the 1 : 1 Cuquinoid chelate was several seconds, whereas that of the 1 : 2 chelate and of the ternary complex formed from Cu (II), the quinoid, and a bidentate or tridentate ligand were longer than several hours. The phenomenon was explained on the assumption that the latter complexes are distorted octahedral and the weak coordination between Cu (II) and the carbonyl oxygen retards the protonation of the quinoid.