Copper(II), nickel(II) and zinc(II) complexes of N, N′, N″, N″'-tetrakis(2-hydroxypropyl)-1,4,8,11-tetra-azacyclotetradecane. Rates of formation and dissociation of the copper(II) complex

Abstract

The preparation of the pendant arm macrocycle N, N′, N″, N″'-tetrakis(2-hydroxypropyl)-1,4,8,11-tetra-azacyclotetradecane (THPC = L) by reaction of propylene oxide with 1,4,8,11-tetra-azacyclotetradecane (cyclam) is described. Copper(II), nickel(II) and zinc(II) complexes of the ligand have been prepared and characterized. The reaction of L with copper(II) in basic solution has been studied by stopped flow techniques L+[Cu(OH) 3] −→[CuL] 2+; k 1 L+[Cu(OH) 4] 2−→[CuL] 2+; k 1 . At 25°C and I = 1.0 mol dm −3, k 1 = 3.7 × 10 5 and k 2 = 2.1 × 10 4 dm 3 mol −1 s −1. These values may be compared with the constants obtained for cyclam, k 1 = 2.7 × 10 6 and k 2 = 3.8 × 10 4 dm 3 mol −1 s −1 at 25°C. The mechanism of the reaction is discussed. The copper(II) complex [CuL] 2+ dissociates readily in acidic solution. The kinetics of dissociation have been studied by stopped flow methods in the acidity range 0.05–1.0 mol dm −3 HClO 4. Two reactions could be detected, the first reaction was complete within 20–30 ms at 25°C and was attributed to a pre-equilibrium protonation step. The second reaction, associated with an absorbance decrease was assigned to the acid dissociation step and was acid dependent at low acidities and acid independent at high acidities. In the acidity range used the dissociation can be fitted to the kinetic scheme [CuL] 2++H +⇌[CuLH] 3+; K [CuLH] 3+→Cu 2++LH +; k and k obs = kK[H +]/(1 + K[H +]), with k = 7.1 ± 0.4 s −1 and K = 1.3 ± 0.1 dm 3 mol −1 at I = 1.0 mol dm −3 and 25°C. The p K values of THPC determined by potentiometric titration are 9.08, 8.30, < 2 and < 2 at 25°C. Studies of the solution equilibria of the ligand with copper(II) and nickel(II) by potentiometric methods indicate that with copper(II) the titration data can be fitted to the single equilibrium Cu 2+ + L ← CuL 2+ with log β 110 = 15.54(3). In the case of nickel(II) deprotonation of the complex occurs with log β 110 = 7.32(3), log β 11-1 = −1.71(3) and log β 11-2 = −12.62(4). Deprotonation of the pendant hydroxyl groups in the nickel(II) complex has been confirmed by visible spectral measurements.