Kinetics and mechanism of dissociation of copper(II) complexes of biguanide and someN ¹-substituted biguanides in aqueous acetate buffer media

Summary

The kinetics fo dissociation of thebis complexes [Cu(LH)₂]²⁺ formed by Cu^II with biguanide andN ¹-substituted methyl, phenyl, dimethyl and diethyl biguanides into the mono biguanide complexes in aqueous NaOAc-HOAc buffer media have been studied by stopped-flow spectrophotometry. The results, under pseudo-first-order conditions, indicate k_obs=k_o+k_H[H⁺]. For the different complexes k_o values are comparable, but k_H values differ appreciably; log k_H versus log K ^H_d is linear withca. unit slope K ^H_d being the equilibrium constant for the process:

$$[{\text{Cu(LH)}}_{\text{2}} ]^{2 + } + 2{\text{H}}_{\text{2}} {\text{O}} + {\text{H}}^{\text{ + }} \rightleftharpoons [{\text{Cu(LH)(H}}_{\text{2}} {\text{O)}}_{\text{2}} ]^{2 + } + {\text{LH}}_{\text{2}}^{\text{ + }}$$

Based on these observations and from consideration of ΔH^‡ and ΔS^‡ values, we propose and associative mechanism, with significant bond formation by the incoming H₂O in the K_o path, and a dissociative mechanism for the k_H path.