Mechanistic study of Schiff base release from bis(N-phenyl-salicylaldiminato)nickel(II) in acetonitrile

Abstract

The reaction of bis(N-phenylsalicylaldiminato)nickel(II) and ammonium or ammonium-like ions has been studied kinetically in acetonitrile solution. The general features of the release of N-phenylsalicylaldimine were investigated. Attempts have been made to assign the observed reaction rates to different species existing in solution. The results show that the protonable groups of the Schiff base are involved in an acid–base equilibrium with ammonium ions, in a complexes stepwise reaction mechanism. The equilibria also involve stereoisomeric nickel complexes, the free metal ion and the free ligand. The elimination of the first Schiff base shows a rate increase with increasing acidity of the ammonium ions, which can be correlated with their pK_a values. The reaction with NHEt₃⁺ is approximately three-fold slower than with NH₃Et⁺, presumably because of steric hindrance. The formation of diphenyl(N-phenylsalicylaldiminato)boron, from the nickel complex and anilinium tetraphenylborate is also briefly discussed.