Preparation and characterization of mu-nitrido diiron phthalocyanines with electron-withdrawing substituents: application for catalytic aromatic oxidation

mu-Nitrido-bis [tetra-(hexyl-sulfonyl)phthalocyaninatoiron] (3a) and mu-nitrido-bis [tetra-(tert-butylsulfonyl)phthalocyaninatoiron] (3b) complexes have been prepared and fully characterized by electrospray ionization mass spectrometry, UV-Vis, FTIR, EPR, Mossbauer techniques as well as by X-ray photoelectron and Fe K-edge X-ray absorption spectroscopies. Small changes at the periphery of the phthalocyanine ligand introduce a difference in the iron oxidation state. While 3b with tert-butyl substituents is a neutral complex with a mixed-valence Fe-3.5-N-Fe-3.5 structural unit, 3a having n-hexyl substituents is an oxidized cationic Fe-IV-N-Fe-IV complex. The structural parameters of N-bridged diiron phthalocyanine with a Fe-3.5-N-Fe-3.5 unit were determined for the first time. Iron atoms in 3b are displaced out of plane by 0.24 angstrom and the Fe-N bond distance of the linear Fe-N-Fe fragment is equal to 1.67 angstrom. Both complexes selectively catalyze benzylic oxidation of alkyl aromatic compounds by (BuOOH)-Bu-t. Toluene was oxidized to benzoic acid with 80% selectivity, and the total turnover number was as high as 197. p-Toluic acid was the principal product of p-xylene oxidation. In this case the turnover number achieved 587 substrate molecules per molecule of catalyst. The described catalytic system is complementary to the recently reported system based on mu-nitrido diiron tetrabutylphthalocyanine-H2O2 which effectively oxidizes the benzene ring.