The Design of Selective Chelating Agents: 1,3,5‐Trideoxy‐1,3,5‐tris‐(dimethylamino)‐ cis ‐inositol, a Powerful Ligand for Hard and Highly Charged Metal Ions

A series of compounds containing the mononuclear complexes [M(tdci) 2 ] 3+ (tdci = 1,3,5‐trideoxy‐1,3,5‐tris(dimethylamino)‐ cis ‐inositol, M = Al, Fe, Ga, In) and [M(tdci) 2 ] 4+ (M = Ti, Ge, Sn) was prepared and characterized by elemental analysis, NMR spectroscopy, and FAB mass spectrometry. Characteristic fragmentation reactions in the mass spectra were elucidated. X‐ray analysis of the Al III , Fe III , Ga III , and In III complexes revealed that two neutral, zwitterionic tdci ligands coordinate to the metal cation exclusively through deprotonated alkoxo groups. The six coordinated oxygen donors and the six NH protons form a hydrophilic pocket, whereas the two cyclohexane rings and the twelve methyl groups form two hydrophobic shells. The hydrophilic pocket is filled with twelve water molecules, which are arranged as a second and a third coordination sphere around the metal cation. The reactivity in aqueous solution was investigated by potentiometric measurements. The bis complexes proved to be stable at pH 7. The evaluated formation constants show an increase of stability in the order Al III III III III . The measurements established that tdci is one of the most effective tridentate ligands for small ( r ⩽ 0.8Å) and highly charged cations. The different chelating properties of tdci and of the unmethylated 1,3,5‐triamino‐ 1,3,5‐trideoxy‐ cis ‐inositol are discussed in terms of different steric requirements and different types of solvation of the corresponding complexes in aqueous solution.