Kinetic study of base-catalyzed asymmetric nitrogen conversion of cis-folded macrocyclic nickel(II) complex of 1,4,7,11-tetraazacyclotetradecane

Abstract

In order to examine the effects of coordinated hydroxide ion and free hydroxide ion in configurational conversion of a tetraamine macrocyclic ligand complex, the kinetic of the cis-to- planar interconversion of cis-[Ni(isocyclam)(H 2O) 2] 2+ (isocyclam = 1,4,7,11-tetraazacyclotetradecane) has been examined spectrophotometrically. All kinetic data have been satisfactorily fitted by the rate law, R = ( k 1 K OH[OH −] 2 + k 2[OH −])(1 + K OH[OH −]) −1( cis-[Ni(isocyclam)(H 2O) 2] 2+ + [Ni(isocyclam)(OH)] +), where k 2 = (3.40 ± 0.12) × 10 3 dm 3 mol −1 s −1 is almost equal to k OH determined in buffer solution (lowly basic media), K OH = 22.7 ± 1.4 dm 3 mol −1 at I (ionic strength) = 0.10 mol dm −3 (NaClO 4 + NaOH) and 25.0 °C. Rate constants, k 2 and K OH, are functions of ionic strength, giving a good evidence for an intermolecular pathway. The reaction follows a free-base-catalyzed mechanism where nitrogen inversion, solvation and ring conformational changes are occurred.