Kinetic studies on copper(II) complexes with 1,5-diazacyclooctane (= daco): Uncatalyzed and base-catalyzed acid dissociation of Cu(daco) 22+ and ligand replacement by EDTA

Abstract

Stopped-flow spectrophotometry was used to study the acid dissociation of the complexes Cu- (daco) 2 2+ and Cu(daco) 2+ (daco = 1,5-diazacyclooctane) with several acids HA at 20 °C and I = 0.1 M (NaClO 4). The rate of acid dissociation is independent of [H +] in the pH range 1.5–6.5 and independent of [HA]. It is base-catalyzed and follows a two-term rate law, rate = ( k o + k A[A −]) × [Cu- (daco) 2 2+] with k o = 0.173 ± 0.003 s −1 and k A = 0 (A − = ClO 4 −, Cl −, I −, NO 3 −, sulfonate), 16.3 ± 1.3 (acetate), 4.6 ± 0.2 (formiate), 3.1 ± 0.3 (chloro- acetate), 1.02 ± 3 (H 2PO 4 −), 139 ± 12 (HPO 4 2−) M −1 s −1 Acid dissociation of the species Cu(daco) 2+ is by a factor of 7 slower than that of the species Cu(daco) 2 2+. The ratio k o(Cu(daco) 2 2+): k o(Ni- (daco) 2 2+)is 5 × 10 4:1. There is a LFE relationship between log k A and the p K a of the corresponding acids HA. Ligand replacement in Cu(daco) 2 2+ by EDTA as studied at pH 8 follows the rate law: rate = ( k o + k L[L])[Cu(daco) 2 2+] with k o = 0.15 ± 0.01 s −1 and k L = 27.4 ± 0.8 M −1 s −1 (L ≙ HEDTA 3−). Rate constant k L also fits reasonably well into the LFE relationship between log k A and p K a. A unifying mechanistic interpretation is presented for both acid dissociation and ligand replacement.