Tripodal (N-alkylated) CMP(O) and malonamide ligands: synthesis, extraction of metal ions, and potentiometric studies

Abstract

Tripodal ligands build on the C-pivot (9b–e, 13b–d, and 17a–d) and trialkylbenzene platforms (10a,b, 11, 12, 14a,b, and 18a,b) bearing (N-alkylated) carbamoylmethylphosphine oxide (CMPO), carbamoylmethylphosphonate (CMP), and malonamide moieties were synthesized. Extraction studies with Am³⁺ and Am³⁺ show that in general there is a positive influence of the N-alkyl substituents in C-pivot CMP(O) ligands on the D (distribution) coefficients. The trialkylbenzene CMPO ligands 10a,b, 11, and 12 have considerably larger D coefficients than the corresponding C-pivot analogues 9a–e, although hardly having any selectivity, while N-alkylation gives rise to smaller D coefficients. Although less effective the extraction behavior of the C-pivot CMP analogues 13b–d shows more or less the same trend as the corresponding CMPO ligands 9b–e upon substitution of the carboxamide N-atom with different alkyl chains. The different malonamide ligands 17a–d and 18a,b are bad extractants, while N-alkylation makes them even worse. Potentiometric studies of CMP(O) and malonamide ligands in polymeric membranes on Pb²⁺, Cu²⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺ salts revealed that N-alkyl substituents increase the stability constants of ion–ionophore complexes compared to unsubstituted ligands. In polymeric membrane electrodes the ligands induce a selectivity pattern that differs significantly from the so-called Hofmeister series, giving the highest selectivity coefficients for UO₂²⁺ among all examined cations (Pb²⁺, Cu²⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺).