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

The coordinating ability of the ligands 3,4-toluenediamine-N,N,N',N'-tetraa cetate (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 degreesC, I=0.5 mol dm(-3) in NaClO4) and Cd-113 NMR spectroscopy. The analysis of the potenti ometric data showed the formation of complexes with the ligand:metal ratio 2:1 (identified for the first time for these ligands with any metal), [CdH3 L2](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 Cd-113 NMR. X-Ray diffraction structural analysis of the c omplex [(H2O)Cd(mu -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 sam e ligand and one water molecule, Cd2 to six carboxylate-oxygen atoms from f our different ligands and to a water molecule. The selective uptake of cadm ium(II) was analyzed by means of chemical speciation diagrams as well as so -called conditional or effective formation constants K-Cd(eff). The results indicate that, in competition with other ligands that are strong complexin g agents for cadmium(II), the present ligands are better sequestering agent s in acidic media.