Metal ion complexes of ethylene-bis-N,N′-(2,6-dicarboxy)piperidine

Abstract

The preparation of a new hexadentate ligand related to ethylenediaminetetracetic acid, ethylene-bis-N,N′-(2,6-dicarboxy)piperidine, is reported and the stability constants for some representative metal ion complexes determined. At 25.0°C and μ = 0.10, NaNO 3, the acid dissociation constants of the ligand are: p K 1, 2.0; p K 2, 2.64; p K 3, 6.98; p K 4, 10.30. Under the same conditions, the log stability constants for the metal ion complexes are: Mg 2+, 6.36: Ca 2+, 7.28; Sr 2+, 4.94; Ba 2+, 4.14; Mn 2+, 11.20; Ni 2+, 17.43; Cu 2+, 18.24; Zn 2+, 15.08; Cd 2+, 13.56,Pb 2+, 16.05. Comparison of the equilibrium constants to the corresponding ethylenediaminetetracetic acid constants shows that the acid constants are nearly identical while the metal ion stability constants are lower by 0.5 to 3.7 log K units. These trends are interpreted in terms of steric effects originating from the piperidine ring structure of the ligand.