Schiff Base Macrocycles: Reliable Templates for Multinuclear Metallocavitands

Recent studies of polydentate Schiff base macrocycles indicate larger diameter macrocycles often deviate from planarity upon metal complexation, taking on a concave conformation. These bowl shaped complexes are reminiscent of organic cavitands, however, their dependence on metal coordination rather than C-C bonds for curvature has led us to dub them metallocavitands. In particular, we have prepared a family of soluble [3 + 3] Schiff base macrocycles from the condensation of 3,6-diformylcatechol with 1,2-dialkoxy-4,5-diaminobenzenes and investigated their ability to template the formation of metallocavitands. When reacted with seven equivalents of Zn(OAc)2 or Cd(OAc)2, hepta-nuclear metallocavitands are formed. Dimerization of these complexes into capsules has been observed and the thermodynamics of self-association has been studied. Tuning the cavity dimensions and strength of supramolecular self-association is easily achieved by switching metals. Isolation of a tetra-zinc intermediate has provided insight into the template effect our macrocycles exhibit. These results highlight the reliability of Schiff base macrocycles as scaffolds for metallocavitand formation.