Double-Decker Coordination Cages

Bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate (L) possessing one internal and two terminal pyridine moieties displayed differential coordination ability when combined with suitable Pd-II components. The compound L acted as a bidentate chelating ligand to form mononuclear complexes when combined with cis-[Pd(tmeda)(NO3)(2)] or Pd(NO3)(2) in calculated ratios. The combination of Pd(NO3)(2) with L in a ratio of 3:4, however, afforded the trinuclear double-decker cage [(NO3)(2)subset of Pd-3(L)(4)](NO3)(4), in which L acts as a nonchelating tridentate ligand and the counter anion (i.e., NO3-) acts as template. The encapsulated NO3- can be replaced by F-, Cl-, or Br- but not by I-. The F--encapsulated cage could not be isolated due to its reactivity, whereas the Cl- or Br- encapsulated cages could be isolated. Although anionic guests such as NO3-, Cl-, or Br- stabilized the cages, the presence of excess Cl-or Br-(not NO3-) facilitated decomplexation reactions releasing the ligand. The complexation of Pd(Y)(2) (Y = BF4-, PF6-, CF3SO3-, or ClO4-) with L afforded the corresponding mononuclear complexes under appropriate conditions. However, these counter anions could not act as templates for the construction of double-decker cages.