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, spe ctral and magnetic measurements are reported. The kinetics of dissocia tion of the copper(II) and nickel(II) complexes in acidic solution hav e been studied in detail, For the copper complex at high acidities ([H +] = 0.075-0.275 M) a single kinetic process is observed which display s 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 sol ution. 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 const ant for the second protonation equilibrium CuLH(3+) + H+ reversible ar row 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-diazocta ne). 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+ reve rsible arrow NiLH(2)(4+) as 2.5 M(-1) at 25 degrees C and I = 1.0 M. F or the dissociation of NiLH(2)(4+) the rate constant is 59 s(-1), whil e for the reaction NiLH(2)(4+) + H+ --> Ni2+ + LH(6)(6+) the rate cons tant is 6.4 M(-1) s(-1) at 25 degrees C and I = 0.1 M. At low aciditie s 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 th e 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).