Kinetically stable Ln ^III complexes comprising a trinuclear core sandwiched between two thiacalix[4]arene ligands self-assembled in water (Ln ^III = Nd ^III, Yb ^III)

In aqueous solutions, thiacalixarene (TCAS) and Ln ^III (= Nd ^III, Yb ^III) self-assembled over a long period of time (1 d) to form 3:2 complexes Ln ^III ₃·TCAS ₂ in which the Ln ^III ₃ core is sandwiched between two TCAS ligands, which results in a high kinetic stability. This was not expected from the exo-coordination fashion of TCAS with the O,S,O donors found in the 1:1 complex Ln ^III·TCAS, and thus this feature was attributed to the multiple bonds between the Ln ^III ₃ core and TCAS. Our observations open an exciting possibility of designing kinetically stable Ln ^III complexes by using exo-type ligands as scaffolds to sandwich a multi-Ln ^III core rather than endo-type ones to encapsulate a Ln ^III core. Lanthanides(III) (Ln = Nd, Yb) and thiacalix[4]arene (TCAS) self-assembled in water to form Ln ₃·TCAS ₂, over a period of 1 d. The high kinetic stability of Ln ₃·TCAS ₂ revealed by HPLC was unexpected from the tridentate coordination fashion of TCAS in Ln·TCAS. This suggests a new way to design kinetically stable complexes through the formation of clusters.