Solvent-Responsive Metalloporphyrins: Binding and Catalysis
Reprinted (adapted) with permission from Organometallics 26 (2007): 358, doi:10.1021/om060791z. Copyright 2007 American Chemical Society.
A cholate-functionalized tetraphenylporphyrin (H2CFTPP) was obtained by attaching eight cholate units at the meta positions of the phenyl rings. Zn(CFTPP) favored binding a hydrophilic pyridyl ligand over a hydrophobic analogue in nonpolar solvents such as 20% MeOH/CCl4 but had the reverse selectivity in 95% MeOH/CCl4. Tunability of the ligand binding resulted from the cholates that aggregated intramolecularly to form either unimolecular micelle-like or reversed micelle-like structures, depending on solvent polarity. The micelle-like structures appear to be less well organized than the reversed micelle-like conformations and might be induced by hydrophobic guests. The solvent-dependent intramolecular aggregation of cholates was used to tune the catalytic activity of an iron porphyrin derivative. Epoxidation catalyzed by Fe(CFTPP)Cl gave a selectivity of up to 10:1 for alkene substrates differing by only one or two hydroxyl groups.