Preparation of the Novel Chelating Agent N-(2-Aminoethyl)-trans-1,2-diaminocyclohexane- N,N‘,N‘‘-pentaacetic Acid (H5CyDTPA), a Preorganized Analogue of Diethylenetriaminepentaacetic Acid (H5DTPA), and the Structures of BiIII(CyDTPA)2- and BiIII(H2DTPA) Complexes

To illustrate the usefulness of conformational preorganization in amine carboxylate ligands, synthesis of the trans-cyclohexylene backbone analogue of diethylenetriaminepentaacetic acid (H5DTPA), N-(2-aminoethyl)-trans-1,2-diaminocyclohexane-N,N,N‘,N‘‘,N‘‘-pentaacetic acid (H5CyDTPA), was effected. Synthesis of [guanidinium]2[Bi(CyDTPA)2-] (complex anion on left) and the uncharged complex of diprotonated DTPA5-, Bi(H2DTPA)·2H2O, were prepared and structurally characterized. Conformational preorganization is responsible for the shorter Bi−O and Bi−N bond lengths of Bi(CyDTPA)2- compared to Bi(DTPA)2-. The shorter bond lengths of Bi(CyDTPA)2- are in accord with a greater stability of an antibody-labeled derivative of Bi(CyDTPA)2- over antibody-labeled Bi(DTPA)2- in vivo. Competition of protons for ligand donor atoms leads to bond length irregularities in the structure of Bi(H2DTPA) when compared to the known structure Bi(DTPA)2-.