The Kinetics of the acid-catalysed dissociation of the macrocyclic complexes [Ni( trans-Me 6[18]dieneN 4)] 2+ [Cu( trans-Me 6[18]dieneN 4] 2+ and [Cu( cis-Me 6[18]dieneN 4)] 2+ and the kinetics of the base-catalysed dissociation of the copper(II) complexes

Abstract

The acid-catalysed dissociation of the copper(II) and nickel(II) complexes of trans-Me 6[18]dieneN 4 (2,4,4,11,13,13-hexamethyltetra-azacyclo-octadeca-1,10-diene) and the copper(II) complex of cis-Me 6[18]dieneN 4 (2,4,4,11,11,13-hexamethyltetra-azacyclo-octadeca-1,13-diene) have been studied kinetically. For the complexes of trans-Me 6[18]dieneN 4, the observed rate constant at constant hydrogen ion concentration (k obs) is given by the expression (i), where k o represents the solvolytic dissociation path way ▪ The acid catalysed reactions of the trans-diene complexes involve a rapid pre-equilibrium protonation step, followed by a slow loss of the metal ion from the protonated complex. Activation parameters for the k and k o pathways have been determined. The acid-catalysed dissociation of the copper(II) complex of the cis-diene, which is known to have pseudo-tetrahedral stereochemistry, is very much faster than that of the trans-diene which is essentially planar. This reaction shows no solvolytic pathway, but both general and specific acid catalysed pathways. Thus in the presence of acetic acid (HA) the rate expression at constant pH is given by ▪ The kinetics of dissociation of the two copper(II) complexes in basic solution have also been studied in detail. For 1 X 10 −2 to 5 X 10 −2 M hydroxide, the reactions show a second order dependence on the hydroxide ion concentration and the dissociation can be represented by the equations. ▪ At high base concentrations the reactions show a first order dependence on [OH −] (as all CuL 2+ is converted to CuLOH +). Values of K and k have been determined. The equilibrium CuL 2+ + N − 3 ⇌ [CuLN 3] + has also been studied for the trans diene, the equilibrium constant is 7.3 M −1 compared with 0.27 M −1 for the reaction with hydroxide ion at 25 °C.