Aquation

About: Aquation is a research topic. Over the lifetime, 1443 publications have been published within this topic receiving 17507 citations.

Synthetic, kinetic, and stereochemical studies on the copper(II), nickel(II), and cobalt(III) complexes of 1,4,7,10,13-penta-aza-cyclopentadecane

Abstract: The preparation of the title quinquedentate macrocyclic ligand, L, is described. The complexes [NiL(OH2)][ClO4]2, [CuL][ClO4]2, and [CoLCl][ClO4]2 have been prepared and characterised. The complexes can have three possible configurations as a result of the two chiral nitrogen centres; there are two N-meso diastereoisomers and one N-racemic diastereoisomer. Carbon-l3 n.m.r. measurements establish that the CoIII complex is a mixture of the possible stereoisomers. The base hydrolysis of [COLCl]2+ is quite rapid with kOH= 2.45 × 104 dm3 mol–1 s–1 at 30 °C and I= 0.1 mol dm–3, which can be rationalised in terms of the mer-N3 donor set. Acid aquation with kaq= 1.66 × 104 s–1 at 50 °C is quite slow. The CuII and octahedral NiII complexes are labile in acidic solution, unlike the complexes of 1,4,8,11-tetra-azacyclotetradecane. The kinetics of the acid-catalysed dissociation have been studied in detail. For both complexes rate =kH[complex][H +]2, indicating the involvement of two protons in the transition state of the reaction. For [CuL]2+, kH= 4.9 × 10–2 dm6 mole–2 s–1 and for [NiL]2+, kH= 0.63 dm6 mol–2 s–1 at 25 °C and I= 0.98 mol dm –3. The acid-catalysed dissociation of the NiII complex of 1,4,7,10,13,16-hexa-azacyclo-octadecane has also been studied in order to define the mechanism of these reactions. In this case, rate =kH[complex][H+]3. Possible mechanisms are considered in detail.