Copper(II) and nickel(II) complexes of hexacyclen (1,4,7,10,13,16-hexa-azacyclo-octadecane) and the kinetics of dissociation of the copper(II) and nickel(II) complexes in acidic solution

The copper(II) and nickel(II) complexes of hexacyclen (1,4,7,10,13,16-hexaazacyclo-octadecane, [18]aneN(6)) have been characterized as their perchlorate salts, [ML](ClO4)(2) . H2O. Analytical, conductivity, spectral and magnetic measurements are reported. The kinetics of dissociation of the copper(II) and nickel(II) complexes in acidic solution have been studied in detail, For the copper complex at high acidities ([H+] = 0.075-0.275 M) a single kinetic process is observed which displays a first order dependence on the hydrogen ion concentration with rate = k(H)[CuL(2+)][H+] and k(H) = 73 M(-1) s(-1) at 25 degrees C and I = 1.0 M. The temperature dependence of k(H) gives Delta H-double dagger = 30.5 kJ mol(-1) and Delta S-298(double dagger) = -107 JK(-1) mol(-1). Cleavage of the first equatorial Cu-N bond is slow with all of the subsequent steps being rapid. At lower acidities ([H+] cn 1 x 10(-3) M) only the monoprotonated complex CuLH(3+) is expected to occur in solution. The reaction displays a second order dependence on the hydrogen ion concentration with rate = k'(H)[CuLH(3+)][H+](2) and k'(H) = 8.5 x 10(3) M(-2) S-1 at 25 degrees C and I = 1.0 M. The equilibrium constant for the second protonation equilibrium CuLH(3+) + H+ reversible arrow CuLH(2)(4+) is given by K = k'(H)/k(H) = 116 M(-1) at 25 degrees C. Dissociation of CuL(2+) in 1 M acid is some 10-fold slower than Cu-N cleavage in [Cu(2,2,2-tet)](2+) (2,2,2-tet = 1,8-diamino-3,7-diazoctane). The acid hydrolysis of [NiL](2+) is biphasic at both low and high acidities. At high acidities (0.05-0.4 M [H+]) both reactions display saturation kinetics and it was possible to determine the equilibrium constant K-2 for the second protonation equilibrium NiLH(3+) + H+ reversible arrow NiLH(2)(4+) as 2.5 M(-1) at 25 degrees C and I = 1.0 M. For the dissociation of NiLH(2)(4+) the rate constant is 59 s(-1), while for the reaction NiLH(2)(4+) + H+ --> Ni2+ + LH(6)(6+) the rate constant is 6.4 M(-1) s(-1) at 25 degrees C and I = 0.1 M. At low acidities the two reactions display a second order dependence on the hydrogen ion concentration and the rate constants are 95 +/- 3 M(-2) s(-1) and 24 +/- 2 M(-2) s(-1) at 25 degrees C. The temperature dependence of the rate constant for the slower reaction gives Delta H-double dagger = 44.6 +/- 2 kJ mol(-1) and Delta S-298(double dagger) = -70 +/- 5 JK(-1) mol(-1).