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 kH (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+kH[H+] with k (298 K) 1.8×10 s-1 and kH (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 10�C. The results are compared with other macrocyclic complex systems, and a general mechanism for these reactions is discussed.