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

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.