Dissociation kinetics of metal complexes in acid. Copper complexes of triazamacrocycles

Abstract

The acid dissociation kinetics of the mono-copper complexes of 1,4,7-triazacyclononane, znn; 1,4,7-triazacyclodecane, zdn; 1,4,8-triazacycloundecane, zud; 1,5,9-triazacyclododecane, zdd; 2,2,4-trimethyl-1,5,9-triazacyclododecane, tmzdd; 1,5,9-triazacyclotridecane, ztd; and cyclohexane- r-1,c-3,c-5 triamine, ccha, were studied in aqueous solution over a range of acid concentrations (0.025-0.5 mol dm^-3), I 1.0 (NaN0₃). A variety of kinetic behaviour is observed. Cu(znn)²⁺, Cu(zdn)²⁺ and Cu(zud)²⁺ display a first-order dependence upon [H⁺] with k_H (298 K) 51 dm³ mol^-1 s^-1 (znn), 17 dm³ mol^-1 s^-1 (zdn), and 5.6 dm³ mol^-1 s^-1 (zud). Cu(zdd)²⁺, Cu(ztd)²⁺ and Cu(ccha)²⁺ show a dependence on [H⁺] at low acid concentrations but become acid-independent at high concentrations. The acid-independent rate constants are k₁ (298 K) 2.2 s^-1 (zdd), 15.4 s^-1 (ztd) and k₁ (283 K) 75 s^-1 (ccha). Cu(tmzdd)²⁺ shows a rate law of the form

rate = k+k_H[H⁺]

with k (298 K) 1.8×10 s^-1 and k_H (298 K) 2.0×10^-3 dm³ mol^-1 s^-1. Activation parameters have been determined in all cases except Cu(ccha)²⁺ which was studied at 10ºC. The results are compared with other macrocyclic complex systems, and a general mechanism for these reactions is discussed.