STABILITIES OF GALLIUM(III), IRON(III), AND INDIUM(III) CHELATES OF HYDROXYAROMATIC LIGANDS WITH DIFFERENT OVERALL CHARGES

The stability constants of the Ga(III), In(III), and Fe(III) chelates of three new ligands, N-(2-hydroxybenzyl)-N'-(pyridoxyl)-ethylenediamine-N,N'-diacetic acid (HBPLED), N-(2-hydroxy-3,5-dimethylbenzyl)-N'-((3-hydroxy-1,2,5-trimethyl-4-pyridiniumyl)methyl)ethylenediamine-N,N'-diacetic acid (Me4HBPLED), and N,N'-bis((1,2-dimethyl-3-hydroxy-5-(hydroxy-methyl)-4-pyridiniumyl)methyl)ethylenediamine-N,N'-diacetic acid (DMPLED) are reported and compared with those of the parent ligands N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED), N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)-ethylenediamine-N,N'-diacetic acid (TMHBED), and N,N'-bis(pyridoxyl)ethylenediamine-N,N'-diacetic acid (PLED). This comparison shows the effect on chelate stability of the methylation of the pyridine nitrogens, which increases the overall charges of the complex molecules without changing the donor atoms in the coordination sphere. The data show that quaternization of the pyridine nitrogen decreases the stability constants of the Fe(III) and Ga(III) chelates by about 2 orders of magnitude. A perturbation in the trends is seen in the chelates of Me4HBPLED, whereby methylation of the benzene ring tends to increase the metal chelate stabilities. The log K values reported are as follows: For HBPLED: Ga, 31.02; Fe, 31.01; In, 28.97. For Me4HBPLED: Ga, 31.85; Fe, 32.97; In, 27.82. For DMPLED: Ga, 27.27; Fe, 27.20; In, 21.47.