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

Abstract

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°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°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°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.