The synthesis, characterization, and ligand-binding properties of a porphyrin with sterically protected pockets on both faces is described. 5,10,15,20-Tetrakis(2,4,6-triphenylphenyl)porphyrin, or the “bis-pocket” porphyrin, is synthesized by the condensation of 2,4,6-triphenylbenzaldehyde with pyrrole in refluxing propionic acid. Metalation of this porphyrin with Fe(CO)5/I2 in toluene and reduction with (CH3)4NBH4 yields the four-coordinate Fe(II) complex. Upon addition of 1,2-dimethylimidazole, the five-coordinate adduct is produced which is capable of completely reversible oxygenation. The equilibrium constant for imidazole binding indicates that the protected pockets do not sterically interfere with axial ligation, while still providing the spacing necessary to prevent bimolecular irreversible oxidation of the Fe(II) complex. In contrast, the O2 affinity shown by this porphyrin complex is dramatically reduced compared to other synthetic analogues or to most heme proteins. This is attributed in part to the completely nonpolar binding site present in this bis-pocket porphyrin, compared to the relatively polar and hydrogen-bonding environment of other systems. Confirmation of this interpretation comes from the effect of solvent polarity on dioxygen affinities: as the solvent is changed from mesitylene to toluene to chlorobenzene to o-dichlorobenzene, the P1/2 for O2 binding decreased from 640 torr to 508 to 299 to 227 torr, in good correlation with empirical solvolytic scales.
ASJC Scopus subject areas
- Colloid and Surface Chemistry