The reaction of HOR' (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR')(THF) (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR')(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR')(ArNNNNAr) complexes contain high-spin ( S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR')(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR')(ArNNNNAr) with a different azide (NAr') produces all three possible products ArNNAr, ArNNAr', and Ar'NNAr'. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.
An iron complex with a new chelating bis(alkoxide) ligand leads to an active nitrene dimerization catalyst for a variety of para- and meta-substituted azide precursors.
A mononuclear magnesium complex Mg(OCtBu2Ph)2(THF)2 catalyzes the active polymerization of cyclic esters and alternating co-polymerization of epoxides with cyclic anhydrides.
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