COPPER(II), NICKEL(II) AND ZINC(II) COMPLEXES OF -HYDROXYPROPYL)-1,4,8,11-TETRA-AZACYCLOTETRADECANE - RATES OF FORMATION AND DISSOCIATION OF THE COPPER(II) COMPLEX

The preparation of the pendant arm macrocycle N,N',N'',N'''-tetrakis(2 -hydroxypropyl)- 1,4,8,11-tetra-azacyclotetradecane (THPC = L) by reac tion of propylene oxide with 1,4,8,11 -tetra-azacyclotetradecane (cycl am) is described. Copper(II), nickel(II) and zinc(II) complexes of the ligand have been prepared and characterized. The reaction of L with c opper(II) in basic solution has been studied by stopped flow technique s L + [Cu(OH)(3)](-) --> [CuL](2+); k(1) L + [Cu(OH)(4)](2-) --> [CuL] (2+); k(2) At 25 degrees C and I = 1.0 mol dm(-3), k(1) = 3.7 x 10(5) and k(2) = 2.1 x 10(4) dm(3) mol(-1) s(-1). These values may be compar ed with the constants obtained for cyclam, k(1) = 2.7 x 10(6) and k(2) = 3.8 x 10(4) dm(3) mol(-1) s(-1) at 25 degrees C. The mechanism of t he reaction is discussed. The copper(II) complex [CuL](2+) dissociates readily in acidic solution. The kinetics of dissociation have been st udied by stopped how methods in the acidity range 0.05-1.0 mol dm(-3) HClO4. Two reactions could be detected, the first reaction was complet e within 20-30 ms at 25 degrees C and was attributed to a pre-equilibr ium protonation step. The second reaction, associated-with an absorban ce decrease was assigned to the acid dissociation step and was acid de pendent at low acidities and acid independent at high acidities. In th e acidity range used the dissociation can be fitted to the kinetic sch eme [CuL](2+) + H+ reversible arrow [CULH](3+); K [CuLH](3+) --> Cu2+ LH+; k and k(Obs) = kK[H+]/(1 + K[H+]), with k = 7.1 +/- 0.4 s(-1) a nd K = 1.3 +/- 0.1 dm(3) mol(-1) at I = 1.0 mol dm(-3) and 25 degrees C. The pK values of THPC determined by potentiometric titration are 9. 08, 8.30, < 2 and < 2 at 25 degrees C. Studies of the solution equilib ria of the ligand with copper(II) and nickel(II) by potentiometric met hods indicate that with copper(II) the titration data can be fitted to the single equilibrium Cu2+ + L reversible arrow CuL2+ with log beta( 110) = 15.54(3). In the case of nickel(II) deprotonation of the comple x occurs with log beta(110) = 7.32(3), log beta(11-1) = - 1.71(3) and log beta(11-2) = - 12.62(4). Deprotonation of the pendant hydroxyl gro ups in the nickel(II) complex has been confirmed by visible spectral m easurements. (C) 1997 Elsevier Science Ltd.