STABILITY OF COMPLEXES OF THE MACROCYCLES 1,5,9,13-TETRAAZACYCLOHEXADECANE AND 1,4,7,11-TETRAAZACYCLOTETRADECANE WITH METAL-IONS - EFFECT OF LARGE HOLE SIZE AND OF CHELATE RING SEQUENCE

A glass-electrode study of the formation constants of [16]aneN4 (1,5,9,13-tetraazacyclohexadecane) and isocyclam (1,4,7,11-tetraazacyclotetradecane) with several metal ions is reported. The formation constants, log K1, for the complexes of [16]aneN4 at 25-degrees-C and in 0.1 mol dm-3 NaNO3 are: Zn(II), 13.02; Cd(II), 12.65; Pb(II), 9.29. With isocyclam, log K1 at 25-degrees-C in 0.1 mol dm-3 NaNO3 is: Zn(II), 15.44; Cd(II), 11.84; Pb(II), 10.86. A spectroscopic study of the copper(II) complex of isocyclam at 25-degrees-C in 0.5 mol dm-3 NaNO3 gave log K1 as 27.3. The surprising result of the complexes of isocyclam being of almost identical stability to those of cyclam (1,4,8,11-tetraazacyclotetradecane) is discussed, as is the pattern of complex stability of the tetraaza macrocycles as a function of macrocyclic ring size.