Self-Assembly of a Trilanthanide(III) Core Sandwiched between Two Thiacalix[4]arene Ligands

Thiacalix[4]arene‐p‐tetrasulfonate (TCAS) incorporated a trilanthanide(III) core to form a 3:2 Ln3TCAS2 complex in aqueous solution. The self‐assembly process was monitored at pH 9.5 and 7.4 by HPLC for all lanthanides except PmIII. Self‐assembly took place in three steps. First, TCAS and LnIII formed a 1:1 Ln1TCAS1 complex. Second, two molecules of Ln1TCAS1 condensed to form Ln2TCAS2. Finally, Ln2TCAS2 reacted with additional LnIII to form Ln3TCAS2. This self‐assembly exhibited extremely slow kinetics and required 1–4 d to reach completion. Small amounts of minor species, including Ln1TCAS1, Ln2TCAS2, and Ln4TCAS2, were observed depending on the LnIII species and pH. The generation of these species is controlled by factors such as the stability of the Ln‐TCAS species, effect of coordinated water, and size of LnIII. XAFS spectra of aqueous Gd3TCAS2 solutions established the solution structure of the tri‐LnIII cluster sandwiched by two TCAS ligands. Thiacalix[4]arene‐p‐tetrasulfonate (TCAS) and lanthanide(III) ions self‐assemble to form a 3:2 Ln3TCAS2 complex, in which a tri‐LnIII cluster is sandwiched between two TCAS ligands. The reaction attains equilibrium in a stepwise manner over a very long time (1–4 d). Certain amounts of minor products, including Ln1TCAS1, Ln2TCAS2, and Ln4TCAS2, are found in the final product.