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 K^eff_Cd. 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.