An enantiomerically pure siderophore type ligand for the diastereoselective 1 : 1 complexation of lanthanide(III) ions

A facile synthesis of a highly preorganized tripodal enterobactine-type ligand 1a-H₃ consisting of a chiral C₃-symmetric macrocyclic peptide and three tridentate 2-amido-8-hydroxyquinoline coordinating units is presented. Complex formation with various metal ions (Al(³+), Ga(³+), Fe(³+), La(³+) and Eu(³+)) was investigated by spectrophotometric methods. Only in the case of La(³+) and Eu(³+) were well defined 1 : 1 complexes formed. On the basis of CD spectroscopy and DFT calculations the configuration at the metal centre of the La(³+) complex was determined to show Lambda helicity. The coordination compounds [(1a)Ln] presented should be prototypes for further lanthanide(III) complexes with an enterobactine analogue binding situation.