Acid dissociation kinetics of the copper(II) Ccomplex of 1,5,8,12-tetra-azacyclo-octadecane ([18]aneN4) and the nickel(II) complex of 1,5,8,12-tetra-azacycloheptadecane ([17]aneN4)

The kinetics of the dissociation of [Cu([18]aneN49)]2+ and [Ni([17]aneN48)]2+ in acidic solution have been studied in detail. The dissociation of [Cu([18]aneN48)]2+ displays saturation kinetics beyond 0.4 mol dm−3 HC1O4 with values of kobs becoming independent of [HClO4]. The kinetic behaviour can be rationalised in terms of the scheme, $$\begin{gathered} CuL^{2 + } + H^ + \mathop \rightleftharpoons \limits^K CuLH^{3 + } \hfill \\ CuLH^{3 + } \mathop \to \limits^k Cu^{2 + } + protonated ligand \hfill \\ \end{gathered} $$ with K = 64 dm3mol−1 and k=0.625 s−1 at 25 °C. Saturation kinetics are not observed in the dissociation of [Ni([17]aneN48)]2+ and significant amounts of the protonated complex do not occur even in 0.5 mol dm−3 HClO4. In this case protonation of the complex may be the rate-determining step. Dissociation of [Ni([17]aneN48)]2+ is 1010-fold faster than that of [Ni(cyclam)]2+ at 25 °C.