Double‐Decker Coordination Cages

TLDR: 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], but these counter anions could not act as templates for the construction of double-decker cages.

Abstract: 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 PdII 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.