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⊂Pd₃(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.