pH-metric and NMR studies of the complexation of Zn2+, Cd2+, and Pb2+ with diazacrown ethers having dangling phosphonate groups

Abstract

The complexation of 4,10-bis(phosphonomethyl)-1,7-dioxa-4,10-diazacyclododecane 1, of 7,13-bis(phosphonomethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane 2, and 7,16-bis(phosphonomethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane 3 with Zn²⁺, Cd²⁺, and Pb²⁺ was studied by potentiometric pH titrations and ¹H and ³¹P NMR techniques using a fully automated pH–NMR titration set-up and selected single spectra. The potentiometry allowed one to identify the species and to determine their stability constants. All three metal ions form a series of more or less strongly protonated 1∶1 complexes (MLH_n, n = 3, 2, 1 or 0) as well as 2∶1 species (M₂LH_m, m = 1, 0, −1 or −2). The stabilities of ML follow the order Pb²⁺ > Cd²⁺ > Zn²⁺ for 2 and 3, whereas the smaller ring 1 shows a different sequence (Cd²⁺ > Pb²⁺ > Zn²⁺), being selective for Cd²⁺. The same order is also found for coordination of a second metal ion to give M₂L. The NMR studies showed in all cases, except for Cd²⁺ with 2, only one ³¹P resonance for the phosphonate groups, which clearly interact with the metal ion since the signals show satellites with metal isotopes having I = ½. Two resonances were observed for Cd²⁺ with 2. Both have satellites indicating that both are coordinated, but not equivalent. In the ¹H NMR spectra one observes signals for both the ligand and the complexes in slow exchange. At some pH values the resonances are broad and indicate that dynamic processes are taking place.