Calixarene-based metalloporphyrins: molecular tweezers for complexation of DABCO

We investigated the formation of host–guest complexes between zinc porphyrins covalently attached to calixarenes via amidic bonds and a small bidentate ligand bearing two nitrogen atoms. Depending upon the calixarene structure (S vs CH2 bridges), the ligand 1,4-diazabicyclo[2.2.2]octane (DABCO) is complexed by metalloporphyrin units by two different ways. While the thiacalix[4]arene prefers an intramolecularly closed cavity with a binding constant of (1.0±0.1)×107 M−1 in CHCl3 at 294 K (stoichiometry 1:1), the classical calix[4]arene forms a complex by ligation of both porphyrin units separately (stoichiometry 2:1). The differences observed can be rationalized in terms of cavity size and the preorganization due to intramolecular hydrogen bonding of the calixarene lower rim.