Synthesis and complexation properties of ditopic ligands built from a macrocyclic and an open chain chelator moiety

Abstract

Two heteroditopic ligands 5 and 6, containing the macrocyclic unit cyclam and the open chain ligand 2,2′,2″-triaminotriethylamine (tren), as well as the pendant macrocycle 1-(2-methylaminoethyl)-1,4,8,11-tetraazacyclotetradecane 9, were synthesized. Their complexation potential towards Cu²⁺ and Ni²⁺ was studied using potentiometric pH-measurement and spectrophotometric titrations, from which the stability constants and thus the existence range of the species were determined. Both ligands 5 and 6 form a series of mononuclear MLH_n (n = 3, 2, 1, 0) and dinuclear M₂LH_p (p = 0, −1, −2) species, and in some cases also dimeric complexes (ML)₂H_m (m = 2, 1, 0). Using 9 for comparison purposes, the structures of the species observed with ligands 5 and 6 can be deduced from their spectral properties. Thus in the complexes MLH_n, the metal ion is always bound by the macrocyclic ring, whereas in the species M₂LH_p one metal is coordinated by the macrocycle and the other by the tren unit. Kinetic measurements show that the metal ion first binds to the more reactive open chain unit but then it is transferred to the cyclam moiety, which gives the thermodynamically most stable compound. In the heterodinuclear species the sequence of addition of the metal ions determines where they are bound. The first metal ion is always coordinated by the macrocycle, whereas the second one is bound by the tren moiety, so that it is possible to selectively prepare the two isomers.