Reactivity Modes of an Iron Bis(alkoxide) Complex with Aryl Azides: Catalytic Nitrene Coupling vs Formation of Iron(III) Imido Dimers

Abstract

The iron bis(alkoxide) complex Fe(OR)2(THF)2 (R = CtBu2Ph), 1, was found to have strikingly different reactivity with various aryl azides, ArN3. Azides with methyl or ethyl groups in the ortho positions of the phenyl ring react catalytically via nitrene coupling to give azoarenes, ArNNAr. Catalyst loading as low as 1 mol % yields clean, quantitative conversion of aryl azides to azoarenes at room temperature in as little as 4 h. A combination of two different aryl azides leads to the catalytic formation of all three possible azoarenes, including the asymmetric one. In contrast, reactions with aryl azides lacking ortho substituents yield stable dimeric iron imido complexes of the form (RO)(THF)Fe(μ-NAr)2Fe(THF)(OR) (Ar = 4-(trifluoromethyl)phenyl, 5; Ar = phenyl, 6; Ar = 3,5-dimethylphenyl, 7), which do not undergo catalytic nitrene coupling. The isocyanide adduct Fe(OR)2(CNR)2 (4, R = 2,6-dimethylphenyl) was obtained from the reaction of Fe(OR)2(THF)2 with two equivalents of isocyanide. No C–N bond formation was observed in the reaction of compound 4 with azides or in the reaction of compounds 5–7 with isocyanides.