Potentiometry of the dioxa-triaza macrocyclic complexes as receptors for first-row transition and lanthanide metals

The protonation constants and the complex stability constants of 1,7-dioxa-4,10,13-triazacyclopentadecane-4,10,13-tripropionis acid (N-pr(3)[15]ane N3O2 with some first-row transition metals (Co2+, Ni2+, Cu2+, and Zn2+) and lanthanide metals (Ce3+, Eu3+, Gd3+, and Yb3+) have been determined at 25 degrees C in aqueous 0.1 M NaClO4 solution by a potentiometric method. The results obtained are compared to those obtained for a similar ligand, 1,7-dioxa-4,10,13-triazacyclopentadecane-4,10,13-triacetic acid (N-ac(3)[15]ane N3O2, which has been previously reported. The stability constants of lanthanide complexes of N-pr(3)[15]ane N3O2) increase with the decreasing ionic radii of Ln(3+). The order of metal binding strengths with N-pr(3)[15]ant: N3O2 for the first-row transition metals was Ni2+ > Zn2+ > Co2+ > Cu2+, which is obviously reversed from that with N-ac(3)[15]ane N3O2, Cu2+ > Co2+ = Zn2+ > Ni2+. The tendency to the reversal of this trend is rationalized and discussed in terms of the matching of the ligand properties with metal ion characteristics by the use of molecular mechanic calculations. These facts suggest that the structure of a polyoxa-polyaza macrocyclic compound can be preorgnnized by modifying the pendant arms to achieve unique selectivity for a particular metal cation. (C) 1996 Academic Press, Inc.