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

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 kinetics of the cis-to- planar interconversion of cis-[Ni(isocyclam)(H 2O) 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 = k OH[OH −][ cis-[Ni(isocyclam)(H 2O) 2] 2+], where k OH = 3.84 × 10 3 dm 3 mol −1 s −1 at 25.0 ± 0.1 °C with I = 0.10 mol dm −3 (NaClO 4). The large Δ H ≠, 61.7 ± 3.2 kJ mol −1, and the large positive Δ S ≠, 30.2 ± 10.8 J K −1 mol −1, strongly support a free-base-catalyzed mechanism for the reaction.