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β-Octafluorocorroles

The one-pot corrole synthesis first reported by the Gross and Paolesse groups appears to have evolved into a remarkably general and predictable self-assembly based synthetic reaction. Gross's solvent-free procedure (refs 8 and 9) has proven particularly effective in our hands and, in fact, more general than originally claimed. In earlier work (ref 17), we showed that the reaction works for a variety of aromatic aldehyde starting materials and was not limited to relatively electron-deficient aldehydes, as reported by Gross and co-workers. Here, we show that the pyrrole component is also variable in that 3,4-difluoropyrrole undergoes oxidative condensation with four different p-X-substituted benzaldehydes to yield the corresponding β -octafluoro-meso-tris(para-X-phenyl)corroles (X = CF3, H, CH 3, and OCH3). Further, we have prepared the Cu and FeCl derivatives of the β-octafluorocorrole ligands. The XPS nitrogen 1s ionization potentials of these fluorinated ligands are some 0.7 eV higher than those of the corresponding β-unfluorinated ligands. The oxidation half-wave potentials of the Cu and FeCl complexes of the fluorinated corroles are also positively shifted by 300-400 mV relative to their β -unsubstituted analogues, demonstrating the strongly electron-deficient character of the fluorinated ligands. 1H NMR spectroscopy suggests that like their β-unfluorinated counterparts, the new β -octafluorinated triarylcorroles act as substantially noninnocent ligands, i.e., exhibit corrole π-cation radical character, in the FeCl complexes. Quantitatively, however, NMR spectroscopy and DFT calculations indicate that the β-octafluorinated corroles are somewhat less noninnocent (i.e., carry less radical character) than their β-unfluorinated counterparts in the FeCl complexes. Temperature-dependent 19F NMR spectroscopy suggests that the Cu octafluorocorroles have a thermally accessible paramagnetic excited state, which we assign as a Cu(II) corrole π-cation radical. We have previously reported that the electronic absorption spectra, particularly the Soret absorption maxima, of high-valent transition metal triarylcorroles are very sensitive to the nature of the substituents in the meso positions. In contrast, the Soret absorption maxima of free-base triarylcorroles are not particularly sensitive to the nature of the meso substituents. This scenario also holds for the fluorinated corroles described here. Thus, although the four free-base fluorinated triarylcorroles exhibit practically identical Soret absorption maxima, the Soret bands of the Cu derivatives of the same corroles red-shift by approximately 35 nm on going from the p-CF3 to the p-OCH3 derivative.