From Divalent to Pentavalent Iron Imido Complexes and an Fe(V) Nitride via N–C Bond Cleavage

Abstract

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMNMes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMNMes)FeI(N2)]+ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMNMes)Fe(NAd)]0,1+,2+,3+ (2 Ad –5 Ad ) in oxidation states II to V. The Fe(V) imide [(TIMMNMes)Fe(NAd)]3+ (5 Ad ) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMNMes)FeV(N)]2+ (6) via N–C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMNMes)FeIV(N)]+ (7) with an ethyl group using the triethyloxonium salt Et3OPF6. This gives access to the analogous series of ethyl imides [(TIMMNMes)Fe(NEt)]0,1+,2+,3+ (2 Et –5 Et ), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as 57Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

Synthesis and Reactivity of a Cobalt-Supported Singlet Nitrene

“…The average Co–C bond length of 1.992(2) Å in 2 is similar to the one determined in [(TIMMN mes )­Co IV (NAd)]­(OTf) 2 (2.004(2) Å) but significantly longer than those for precursor 1 (1.925(2) Å) and the Co III imidyl radical [(TIMMN mes )­Co­( • NMes)]­(PF 6 ) 2 (1.954(3) Å). This difference may be due to enhanced π-backbonding in the case of 1 . , A similar phenomenon has also been observed for the isoelectronic Fe II and Fe III adamantyl imido complexes supported by the same TIMMN mes ligand . Again, this elongation (0.067 Å) is reminiscent of the [(TIMMN mes )­Co III (NAd)]­(BPh 4 )/[(TIMMN mes )­Co IV (NAd)]­(OTf) 2 pair, where the one-electron oxidation of the Co III complex also induced elongation of the Co–C bond by 0.068 Å, from 1.936(1) to 2.004(2) Å.…”

“…The pre-edge of 1 is characterized by a single and relatively intense peak at 7709.9 eV, which upon one-electron oxidation to 2 becomes a broad, two-featured, and less intense peak with its maximum at 7709.4 eV. The shift to lower energy is apparently inconsistent with a metal-based oxidation, but it has been observed also for the Fe II and Fe III adamantyl imido pair with the same supporting ligand . These are isoelectronic analogues of 1 and 2 , respectively, and a similar rationale applies: The oxidation of the d 6 species is followed by weakening of the ligand field indicated by lengthening of the Co–N­(Anisole) and Co–C bonds.…”

“…The shift to lower energy is apparently inconsistent with a metal-based oxidation, but it has been observed also for the Fe II and Fe III adamantyl imido pair with the same supporting ligand. 45 These are isoelectronic analogues of 1 and 2, respectively, and a similar rationale applies: The oxidation of the d 6 species is followed by weakening of the ligand field indicated by lengthening of the Co−N(Anisole) and Co−C bonds. The stabilized SOMO give rise to an intense low-energy 1s to 3d transition.…”

Iron‐Catalyzed Intermolecular C−H Amination Assisted by an Isolated Iron‐Imido Radical Intermediate

“…[52] The S = 1 room temperature spin state was corroborated in the solid-state by DC SQUID magnetometry on two independently synthesized samples of 1-imido (Figure 1, B) and shows a gradual, temperature-dependent increase in the magnetic moment from an averaged μ eff of 0.72 μ B at 2 K to 2.49 μ B at 300 K. Zero-field 57 Fe-Mössbauer spectroscopy, recorded in the solid-state at 77 K (Figure 1, A) on two independently synthesized samples of 1-imido, reveals a sharp quadrupole doublet with a δ = 0.01 mm s À 1 and ΔE Q = 3.02 mm s À 1 . These parameters are typical values for Fe IV imido complexes, [66][67][68][69] including the related S = 0 Fe IV imido complex [( tBu pyrpyrr 2 )Fe=NAd], [52] and are well reproduced by the BP86 functional (Table S7). [70] Quantum chemical calculations were performed on 1imido using density functional theory (DFT; PBE, r 2 SCAN-3c, TPSSh, PBE0) and ab initio methods (CASSCF).…”

Ruthenium(v) terminal arylimido corroles: isolation, spectroscopic characterization and reactivity

“…3a) is similar to that of its structurally similar complex 3b (g and A values for the simulated spectrum: same as those of 3b), and the signal pattern is assigned to the superhyperne coupling to the 14 N nucleus of the imido ligand, similar to that of Fe(V)-imido species. 12 For the 50% 15 N-enriched sample, 14,15 N-3c (prepared from 15 N-TippN 3 ), the corresponding EPR spectrum (Fig. 3b) shows a signal which can be reasonably tted by summing the simulated spectra of 3c and 15 N-3c, with a 1 : 1 ratio (Fig.…”