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

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)(H2O)2]2+ (isocyclam = 1,4,7,11-tetraazacyclotetradecane) has been examined spectrophotometrically. All kinetic data have been satisfactorily fitted by the rate law, R = (k1KOH[OH−]2 + k2[OH−])(1 + KOH[OH−])−1(cis-[Ni(isocyclam)(H2O)2]2+ + [Ni(isocyclam)(OH)]+), where k2 = (3.40 ± 0.12) × 103 dm3 mol−1 s−1 is almost equal to kOH determined in buffer solution (lowly basic media), KOH = 22.7 ± 1.4 dm3 mol−1 at I (ionic strength) = 0.10 mol dm−3 (NaClO4 + NaOH) and 25.0 °C. Rate constants, k2 and KOH, 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.