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 Ln₃TCAS₂complex in aqueous solution. The self-assembly process was monitored at pH 9.5 and 7.4 by HPLC for all lanthanides except Pm^III. Self-assembly took place in three steps. First, TCAS and Ln^IIIformed a 1:1 Ln₁TCAS₁complex. Second, two molecules of Ln₁TCAS₁condensed to form Ln₂TCAS₂. Finally, Ln₂TCAS₂reacted with additional Ln^IIIto 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 Ln^IIIspecies 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 Ln^III. XAFS spectra of aqueous Gd₃TCAS₂solutions established the solution structure of the tri-Ln^IIIcluster sandwiched by two TCAS ligands.