Between imide, imidyl and nitrene - an imido iron complex in two oxidation states

Abstract

Imidyl and nitrene metal species play an important role in the N-functionalisation of unreactive C-H bonds as well as the aziridination of olefines. We report on the synthesis of the trigonal imido iron complexes [Fe(NMes)L₂]^0,- (L = -N{Dipp}SiMe₃); Dipp = 2,6-diisopropyl-phenyl; Mes = (2,4,6-trimethylphenyl) via reaction of mesityl azide (MesN₃) with the linear iron precursors [FeL₂]^0,-. UV-vis-, EPR-, ⁵⁷Fe Mössbauer spectroscopy, magnetometry, and computational methods suggest for the reduced form an electronic structure as a ferromagnetically coupled iron(ii) imidyl radical, whereas oxidation leads to mixed iron(iii) imidyl and electrophilic iron(ii) nitrene character. Reactivity studies show that both complexes are capable of H atom abstraction from C-H bonds. Further, the reduced form [Fe(NMes)L₂]^- reacts nucleophilically with CS₂ by inserting into the imido iron bond, as well as electrophilically with CO under nitrene transfer. The neutral [Fe(NMes)L₂] complex shows enhanced electrophilic behavior as evidenced by nitrene transfer to a phosphine, yet in combination with an overall reduced reactivity.

Fig. 1. Examples of imido metal complex oxidation leading to M–N bond lengthening and imidyl character (Ad = adamantyl, Mes = 2,4,6-trimethylphenyl).

Scheme 1. Synthesis of complexes K{m}[Fe(NMes)L₂], K{m}[1] (m = crypt.222), and [Fe(NMes)L₂], [1], as well as their interconversion by oxidation and reduction.

Fig. 2. (a) Solid state structure of [1]. The H atoms are omitted for clarity. (b) Selected bond metrics for [1]⁻ (green) and [1] (red) [Å].

Fig. 3. (a) Variable-temperature magnetic susceptibility of solid K{m}[1] (green) and [1] (red) in the range of 2–210 K. (b) Zero-field ⁵⁷Fe Mössbauer spectra of solid K{m}[1] (green) and [1] (red) at 80 K. Parameters for K{m}[1]: δ = 0.43 mm s⁻¹, |ΔE_Q| = 4.18 mm s⁻¹ (92%); [1]: δ = 0.21 mm s⁻¹, |ΔE_Q| = 3.45 mm s⁻¹. (c) Cyclic voltammogram of K{m}[1] (500 mV s⁻¹, thf, 0.1 m nBu₄ [PF₆] vs. Fc/Fc⁺).

Fig. 4. Electronic structure of [1]⁻ by CASSCF(11,9). Dipp groups were truncated by methyl groups, hydrogen atoms are omitted for clarity, one set of bonding and antibonding Dipp-centred π-orbitals is omitted for clarity.

Fig. 5. Electronic structure of [1] by CASSCF(10,9). Dipp groups were truncated by methyl groups, hydrogen atoms are omitted for clarity, one set of bonding and antibonding Dipp-centred π-orbitals is omitted for clarity.

Scheme 2. Diverging reactivity of [1] and K{m}[1] towards nucleophiles, electrophiles, and C–H bonds.

Fig. 6. Solid state structure of K{m}[2]. The K{m} cation as well as H atoms are omitted for clarity.