Catalytic Defluorination of Perfluorinated Aromatics under Oxidative Conditions Using N-Bridged Diiron Phthalocyanine
Résumé
Carbon fluorine bonds are the strongest single bonds in organic chemistry, making activation and cleavage usually associated with organometallic and reductive approaches particularly difficult. We describe here an efficient defluorination of poly- and perfluorinated aromatics under oxidative conditions catalyzed by the mu-nitrido diiron phthalocyanine complex [(Pc)Fe-III(mu-N)Fe-IV(Pc)] under mild conditions (hydrogen peroxide as the oxidant, near-ambient temperatures). The reaction proceeds via the formation of a high-valent diiron phthalocyanine radical cation complex with fluoride axial ligands, [(Pc)(F)Fe-IV(mu-N)Fe-IV(F)(Pc+center dot)], which was isolated and characterized by UV-vis, EPR, F-19 NMR, Fe K-edge EXAFS, XANES, and K beta X-ray emission spectroscopy, ESI-MS, and electrochemical techniques. A wide range of per- and polyfluorinated aromatics (21 examples), including C6F6, C6F5CF3, C6F5CN, and C6F5NO2, were defluorinated with high conversions and high turnover numbers. [(Pc)Fe-III(mu-N)Fe-IV(Pc)] immobilized on a carbon support showed increased catalytic activity in heterogeneous defluorination in water, providing up to 4825 C-F cleavages per catalyst molecule. The mu-nitrido diiron structure is essential for the oxidative defluorination. Intramolecular competitive reactions using C6F3Cl3 and C6F3H3 probes indicated preferential transformation of C-F bonds with respect to C-Cl and C-H bonds. On the basis of the available data, mechanistic issues of this unusual reactivity are discussed and a tentative mechanism of defluorination under oxidative conditions is proposed.