Differential substituent effects of β-halogens in water-soluble porphyrins

Abstract

The first water-soluble β-octafluorinated porphyrins, 5,10,15,20-tetrakis(4-sulfonatophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrin, 1, and 5,10,15,20-tetrakis(2,6-difluoro-3-sulfonatophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrin, 2, have been prepared and their aqueous aggregation, acid–base, and optical properties have been characterized. The porphyrins are tetraanionic at neutral pH (at pH = 3–11 for 1 and pH = 0–9 for 2). Semiempirical (AM1) calculations provide evidence that somewhat unusual acidity characteristics of the fluorinated compounds (with respect to similar brominated porphyrins) can be rationalized solely on the basis of chemical hardness and electronegativity arguments. These results indicate that the large conformational differences seen in the structures of brominated and fluorinated water-soluble porphyrins have little impact upon N–H acidity. Metalation of 1 and 2 with ZnCl₂ yielded the zinc complexes, which were characterized by optical spectroscopy and electrochemistry.