Syntheses, crystal structures, and single small molecule encapsulation properties of cavitand-porphyrins

New capsule-shaped hosts "cavitand-linked porphyrin" metal complexes (Mcp: M =H ₂, Ni, Zn, and Pd) have been synthesized to mimic the substrate binding functions of metalloenzymes such as cytochrome P-450 _cam. Crystal structures of cavitand 1·MeOH·CHCl ₃, [Zntpp(MeOH)], [Nicp]·MeOH·2CHCl ₃· 3H ₂O, [Zncp(MeOH)]·2CHCl ₃·3H ₂O, and [Pdcp]·MeOH·2CHCl ₃·3H ₂O have been determined. One methanol molecule, originating from crystallization solvent is encapsulated in each host cavity, and coordinates to Zn in Zncp but not in Ni- and Pdcp. Encapsulation of various small hydrocarbon molecules in CDCl ₃ solutions of Mcps have been evaluated by determining binding constants and thermodynamic parameters obtained from ¹HNMR titrations. All Mcps encapsulate hydrocarbons smaller than propane under atmospheric pressure. The guest size selectivity is primarily influenced by cavity size, and partly by metal insertion. The metal ion radius does not affect guest size selectivity. Encapsulation of coordinating guest molecules (MeOH, EtOH, MeCN, and H ₂O) in Mcps has also been investigated. Only Zncp favors coordination of non-hydrocarbon guests such as MeOH. We concluded that accommodation of different size guests by Mcps depends upon guest sizes and coordination of functional groups depends upon both the identity of the porphyrin's metal ion and guest sizes.