Dissociation kinetics of metal complexes in aqueous acid. III. The effects of methyl substitution and chelate ring size on the rates of dissociation of copper(II) triazamacrocyclic complexes

Abstract

The acid dissociation kinetics of mono copper(II) complexes of the ligands 2-methyl-1,4,7-triazacyclononane, mznn ; 1,4,7-triazacycloundecane, zaud ; 1,4,7-triazacyclododecane, zadd ; and 1,5,9-triazacyclotetradecane, zted were studied in aqueous solution over a range of acid concentra- tions (0.025-0.5 mol 1^-1), I1.0 (NaN0₃). Cu(mznn)²⁺ displays a first-order dependence on [H⁺] with k_H (298 K) 26 1. mol^-1 s^-1. The other complexes show an acid dependence at low [H⁺] but become independent of acid at higher concentrations. The acid-independent rate constants k₁ were determined to be Cu(zaud)²⁺, 67 s^-1 (280 K); Cu(zadd)²⁺, 45 s^-1 (298 K); Cu(zted)²⁺ 50 s^-1 (298 K). The results are compared with those obtained with other copper(II) triazamacro- cyclic complexes.