The role of cadmium(II) bridges in the self-assembly with lanthanide(III) and thiacalix[4]arene (TCAS) to selectively form a luminescent ternary CdII2·TbIII2·TCAS2 complex

The ability of metal ions to self-assemble to form luminescent ternary complexes with TbIII and thiacalix[4]arene-p-tetrasulfonate (TCAS) was investigated, as exemplified by AgI in AgI2·TbIII2·TCAS2 and AgI2·TbIII·TCAS2 at pH 6.5 and 10, respectively. Among FeIII, NiII, ZnII, PdII, CdII, HgII, TlI, and PbII metals, only CdII resulted in more intense luminescence than a TbIII–TCAS binary system, which occurs at pH values of ∼6.5 and suggests the formation of a ternary complex. Determination of the molar ratio and electrospray-ionization mass spectrometry results revealed that the species formed at pH 6.5 was CdII2·TbIII2·TCAS2. Analogous to the solid-state structure of AgI2·TbIII2·TCAS2, CdII2·TbIII2·TCAS2 should also adopt a double-cone structure with the cyclic tetrametal CdII–CdII–TbIII–TbIII core sandwiched by two cone-shaped TCAS ligands that are connected by two S–CdII–S linkages at the two adjacent S atoms in TCAS. However, the CdII system did not form a ternary complex like AgI2·TbIII·TCAS2 at higher pH values; this is because the S–CdII–S linkages are shorter than S–AgI–S, which brings the two TCAS ligands closer together and ejects TbIII. In aqueous solutions CdII2·TbIII2·TCAS2 exhibited the following photophysical properties: a luminescence quantum yield (Φ) of 0.15 and lifetime (τ) of 1.12 ms; these values are very close to those obtained for AgI2·TbIII2·TCAS2. The number of water molecules coordinated to TbIII was determined to be 2.2, which is identical to the number coordinated to TbIII in AgI2·TbIII2·TCAS2. Together, these results elucidate the structural similarities of CdII2·TbIII2·TCAS2 and AgI2·TbIII2·TCAS2. Other LnIII species (i.e., NdIII and YbIII) also afforded ternary complexes with the same stoichiometry, which shows that it is the CdII bridges that restrict the self-assembly to CdII2·LnIII2·TCAS2.