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 Co^III complex is a mixture of the possible stereoisomers. The base hydrolysis of [COLCl]²⁺ is quite rapid with k_OH= 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 k_aq= 1.66 × 10⁴ s^–1 at 50 °C is quite slow. The Cu^II and octahedral Ni^II 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 =k_H[complex][H ⁺]², indicating the involvement of two protons in the transition state of the reaction. For [CuL]²⁺, k_H= 4.9 × 10^–2 dm⁶ mole^–2 s^–1 and for [NiL]²⁺, k_H= 0.63 dm⁶ mol^–2 s^–1 at 25 °C and I= 0.98 mol dm ^–3. The acid-catalysed dissociation of the Ni^II 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 =k_H[complex][H⁺]³. Possible mechanisms are considered in detail.