POTENTIOMETRIC AND SPECTROPHOTOMETRIC DETERMINATION OF THE STABILITIES OF IN(III), GA(III) AND FE(III) COMPLEXES OF N,N',N''-TRIS(3,5-DIMETHYL-2-HYDROXYBENZYL)-1,4,7-TRIAZACYCLONONANE

The protonation constants of potentially six-coordinate N,N',N"-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (TX-TACN) were determined by spectrophotometric and potentiometric methods in 75% ethanol-25% water (mu = 0.100 M (KCl), t = 25.0-degrees-C), and were found to be (log values) 14.2, 13.6, 12.9, 8.04 and 2.15. The stability constants of the complexes of In(III), Ga(III) and Fe(III) with TX-TACN were determined in the same medium by spectrophotometric methods, except for In(III)-TX-TACN, which was determined directly by potentiometric titration. The log stability constant for Fe(III)-TX-TACN (K(ML) = [ML]/[L3-][M3+]), determined by competition with EDTA in 75% ethanol-25% water is 51.3. The log stability constant for Fe(III)-EDTA (K(ML) = [ML-]/[L4-][M3+]) was found to be 26.3 in the same medium. The stability constants for the Ga(III) and In(III) complexes with TX-TACN are 10(44.2) and 10(33.99), respectively. The high stabilities of the Ga(III) and Fe(III) complexes with TX-TACN are rationalized on the basis of the close fit of these hexacoordinate metal ions in the ligand cavity formed by the pseudo-octahedral arrangement of ligand donor atoms: the basic nitrogen atoms of the macrocycle and the very basic phenolate oxygen atoms.