Stability and kinetics of the acid-promoted decomposition of tertiary binuclear CuII2LXz+ complexes (L = octaaza cryptand, X = SCN−, N3− and 2 NH3): the ancillary ligand modulates the kinetics of dissociation of the cryptand

Abstract

Solution studies indicate that the Cu₂L(OH)³⁺ complex, where L is a large octaaza cryptand, reacts with NH₃, KSCN and NaN₃ to form the binuclear tertiary complexes Cu₂L(NH₃)₂⁴⁺, Cu₂L(SCN)³⁺ and Cu₂L(N₃)³⁺. The equilibrium constants show a special stabilisation of the complexes with ligands able to bridge the Cu^II centres, with a maximum stabilisation for azide. Upon addition of an excess of acid, all the complexes decompose with release of Cu²⁺ and the protonated ligands. The kinetics of the acid-promoted decomposition have been studied under pseudo-first order conditions of excess acid. The decomposition of the OH⁻, SCN⁻ and NH₃ complexes is first order with respect to H⁺, although with a clear tendency to saturation, and the values of the rate constants depend on the nature of the ancillary ligand. In contrast, Cu₂L(N₃)³⁺ decomposes with a second order dependence on the acid concentration. These results are interpreted in terms of modifications induced by the ancillary ligand in the kinetics of the acid-assisted dissociation of Cu–N(cryptate) bonds. Depending on the steric requirements of the ancillary ligand, the Cu–N(cryptate) bonds are strained at different degrees and the more distorted complexes decompose faster. In the case of azide, there is an optimum fit between the ancillary ligand and the Cu₂L⁴⁺ site that causes a change in the rate-determining step from dissociation of the first Cu–N bond to the second.