Crystal structure of the 3-D complex [(H2O)Cd(μ-3,4-TDTA)Cd(H2O)]. Potentiometric and 113Cd NMR studies in aqueous solution (3,4-TDTA = 3,4-toluenediamine-N,N,N ′,N ′-tetraacetate)

Abstract

The coordinating ability of the ligands 3,4-toluenediamine-N,N,N′,N′-tetraacetate (3,4-TDTA), ortho-phenylenediamine-N,N,N′,N′-tetraacetate (o-PhDTA) and 4-chloro-1,2-phenylenediamine-N,N,N′,N′-tetraacetate (4-Cl-o-PhDTA) (L4−) towards cadmium(II) was studied by potentiometry (25 °C, I = 0.5 mol dm−3 in NaClO4) and 113Cd NMR spectroscopy. The analysis of the potentiometric data showed the formation of complexes with the ligand ∶ metal ratio 2 ∶ 1 (identified for the first time for these ligands with any metal), [CdH3L2]3−, [CdH2L2]4−, [CdHL2]5− and [CdL2]6−, with ratio 1 ∶ 1, [Cd(H2L)], [Cd(HL)]− and [CdL]2−, and with ratio 1 ∶ 2, [Cd2H2L]2+, and [Cd2L]. The stability constants were determined. The formation of these complexes was confirmed by 113Cd NMR. X-Ray diffraction structural analysis of the complex [(H2O)Cd(μ-3,4-TDTA)Cd(H2O)] 1 revealed a polymeric 3-D structure in which two types of cadmium environments are produced, Cd1 and Cd2. The 3-D structure is built-up by the self-assembly of [Cd1(3,4-TDTA)]2− units sharing Cd2(II) ions giving interlocked infinite chains that run along the 3 directions. Cd1 is coordinated to two N atoms and four O atoms of the same ligand and one water molecule, Cd2 to six carboxylate-oxygen atoms from four different ligands and to a water molecule. The selective uptake of cadmium(II) was analyzed by means of chemical speciation diagrams as well as so-called conditional or effective formation constants KeffCd. The results indicate that, in competition with other ligands that are strong complexing agents for cadmium(II), the present ligands are better sequestering agents in acidic media.