Kinetics of the cis-to-planar interconversion of cis-diaqua(1,4,7,11-tetraazacyclotetradecane)nickel(II) ion

In order to examine the effects of coordinated hydroxide ion and free hydroxide ion in configurational conversion of a tetraamine macrocyclic ligand complex, the kinetics of the cis-to-planar interconversion of cis-[Ni(isocyclam)(H2O)2]2+ (isocyclam, 1,4,7,11-tetraazacyclotetradecane) has been studied spectrophotometrically in basic aqueous solution. The interconversion requires the inversion of one sec-NH center of the folded cis-complex to have the planar species. Kinetic data are satisfactorily fitted by the rate law, R = kOH[OH−][cis-[Ni(isocyclam)(H2O)2]2+], where kOH = 3.84 × 103 dm3 mol−1 s−1 at 25.0 ± 0.1 °C with I = 0.10 mol dm−3 (NaClO4). The large ΔH≠, 61.7 ± 3.2 kJ mol−1, and the large positive ΔS≠, 30.2 ± 10.8 J K−1 mol−1, strongly support a free-base-catalyzed mechanism for the reaction.