A Series of Trigonal Arylimido Iron Complexes and the Influence of Oxidation State and Steric Demand on Reactivity

We describe the reaction between the linear iron(I/II) silylamides [FeL₂]^0,- (L = N{Dipp}SiMe₃) and aryl azides N₃R with different steric demand (R = Ph, 2-Me-C₆H₄ (Tol), 2,4-Me₂-C₆H₃ (Xyl), 2,6-^<i>i</i>Pr₂-C₆H₃ (Dipp), 2,4,6-^<i>i</i>Pr₃-C₆H₂ (Tripp)). Obtained trigonal anionic high-spin iron(II) imidyl complexes exhibit long Fe-N bonds of approximately 1.76 Å. In case of the largest substituent (R = Tripp) a slow SiMe₃-shift from the ancillary ligand to the imido nitrogen is observed. In contrast, for the smallest azide (PhN₃) the tetrazenido complex [Fe(η²-κN:¹ κN⁴-N₄Ph₂)L₂] is obtained. Isolable imido complexes react nucleophilic toward benzaldehyde and mesityl isocyanate under cycloaddition to metallacycles. In case of the neutral precursor [FeL₂] imido iron species were only obtained for R = Tol or Xyl, with Fe-N bonds of approximately 1.74 Å. Based on computational analysis these compounds exhibit isoenergetic quintet and triplet states with shared electronic structures of an iron(IV) imide, iron(III) imidyl and iron(II) nitrene. For the smallest aromatic azide PhN₃, the reaction with FeL₂ results directly in C-H amination of the ancillary ligands. For sterically more encumbered neutral imido complexes, intramolecular C-H amination requires heating or irradiation. The observed absence for intramolecular C-H amination of anionic imido complexes is computationally rationalized.