New Multidentate Ligands. XXVI.N,N' -Bis(2-Hydroxybenzyl)ethylenediamine-N,n' -Bis(methylenephosphonic Acid Monomethyl Ester), Andn,n' -Bis(2-Hydroxybenzyl) Ethylenediamine-N,n' -Bis (Methylenephosphonic Acid Monoethyl Ester): New Chelating Ligands for Trivalent Metal Ions

The synthesis and equilibrium study of a new type of sexadentate ligand, N,N ' -bis(2-hydroxybenzyl)ethylenediamine-N,N ' -bis(methylenephosphonic acid monomethyl ester) (HBEDPOMe) and of the corresponding ethyl ester, are described. Their proton and metal ion affinities are found to be nearly identical. The ligands have two phenolate donors which impart specificity for trivalent metal ions such as Fe(III), In(III) and Ga(III). Potentiometric and spectrophotometric studies of the protonation constants of the ligands and stability constants of their Cu(II), Ni(II), Co(II), Zn(II), Fe(III), In(III) and Ga(III) chelates are compared with those of analogous ligands. The binding of the ligand donor groups to the metal ions occurs in several cases in a stepwise fashion, with the more basic phenolate groups requiring higher pH for activation. The order in which each phenolate group becomes coordinated to metal ions, and the structures of the intermediate complexes, are deduced by correlating the electronic absorption spectra with potentiometric data.