Iron–nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark−Feltham introduced the {Fe–NO} n notation. In this work, we succeeded in characterizing a series of {Fe–NO}6–9 complexes, including a reactive {Fe–NO}10 intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris­[2-(3-mesitylimidazol-2-ylidene)­ethyl]­amine (TIMENMes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENMes)­Fe]2+ resulted in formation of the {Fe–NO}6 compound [(TIMENMes)­Fe­(NO)­(CH3CN)]­(BF4)3 (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe–NO}7,8,9 (2–4). Reduction of {Fe–NO}9 with Cs electride finally yields the highly reduced {Fe–NO}10 intermediate, key to formation of [Cs­(crypt-222)]­[(TIMENMes)­Fe­(NO)], (5) featuring a metalacyclic [Fe−(NO−NHC)3−] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe–NO}8 complex was found via oxygenation of the nitride complex [(TIMENMes)­Fe­(N)]­(BF4). Surprisingly, the resulting {Fe–NO}8 species is electronically and structural similar to the [(TIMENMes)­Fe­(N)]+ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark−Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)­Fe­(N)]+ as a {Fe–N}8 species.

A new supporting ligand, tris-[2-(3-mesityl-imidazol-2-ylidene)methyl]amine (TIMMN Mes ), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (FeN) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)N/Fe(V)N redox pair, namely, [PhB-( tBu Im) 3 FeN] 0/+ . On the basis of 57 Fe Mossbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn−Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.

The synthesis of N-heterocycles is of paramount importance for the pharmaceutical industry. They are often synthesized through atom economic and environmentally unfriendly methods, generating significant waste. A less explored, but greener, alternative is the synthesis through the direct intramolecular C–H amination utilizing organic azides. Few examples exist by using this method, but many are limited due to the required use of stoichiometric amounts of Boc2O. Herein, we report a homoleptic C,O-chelating mesoionic carbene–iron complex, which is the first iron-based complex that does not require the addition of any protecting groups for this transformation and that is active also in strong donor solvents such as THF or even DMSO. The achieved turnover number is an order of magnitude higher than any other reported catalytic system. A variety of C–H bonds were activated, including benzylic, primary, secondary, and tertiary. By following the reaction over time, we determined the presence of an initiation period. Kinetic studies showed a first-order dependence on substrate concentration and half-order dependence on catalyst concentration. Intermolecular competition reactions with deuterated substrate showed no KIE, while separate reactions with deuterium-labeled substrate resulted in a KIE of 2.0. Moreover, utilizing deuterated substrate significantly decreased the initiation period of the catalysis. Preliminary mechanistic studies suggest a unique mechanism involving a dimeric iron species as the catalyst resting state.

A comparison of P4 activations mediated by low‐valent β‐diketiminato (L) cobalt complexes is presented. The formal Co0 source [K2(L3Co)2(μ2:η1,η1‐N2)] (1) reacts with P4 to form a mixture of the monoanionic complexes [K(thf)6][(L3Co)2(μ2:η4,η4‐P4)] (2) and [K(thf)6][(L3Co)2(μ2:η3,η3‐P3)] (3). The analogue CoI precursor [L3Co(tol)] (4 a), however, selectively yields the corresponding neutral derivative [(L3Co)2(μ2:η4,η4‐P4)] (5 a). Compound 5 a undergoes thermal P atom loss to form the unprecedented complex [(L3Co)2(μ2:η3,η3‐P3)] (6). The products 2 and 3 can be obtained selectively by an one‐electron reduction of their neutral precursors 5 a and 6, respectively. The electrochemical behaviour of 2, 3, 5 a, and 6 is monitored by cyclic voltammetry and their magnetism is examined by SQUID measurements and the Evans method. The initial CoI‐mediated P4 activation is not influenced by applying the structurally different ligands L1 and L2, which is proven by the formation of the isostructural products [(LCo)2(μ2:η4,η4‐P4)] [L=L3 (5 a), L1 (5 b), L2 (5 c)].

The versatile coordination behavior of the P butterfly complex [{Cp'''Fe(CO) } (μ,η -P )] (1, Cp'''=η -C H Bu ) towards different iron(II) compounds is presented. The reaction of 1 with [FeBr ⋅dme] (dme=dimethoxyethane) leads to the chelate complex [{Cp'''Fe(CO) } (μ ,η -P ){FeBr }] (2), whereas, in the reaction with [Fe(CH CN) ][PF ] , an unprecedented rearrangement of the P butterfly structural motif leads to the cyclo-P moiety in {(Cp'''Fe(CO) ) (μ ,η -P )} Fe][PF ] (3). Complex 3 represents the first fully characterized "carbon-free" sandwich complex containing cyclo-P R ligands in a homoleptic-like iron-phosphorus-containing molecule. Alternatively, 2 can be transformed into 3 by halogen abstraction and subsequent coordination of 1. The additional isolated side products, [{Cp'''Fe(CO) } (μ ,η -P ){Cp'''Fe(CO)}][PF ] (4) and [{Cp'''Fe(CO) } (μ ,η -P ){Cp'''Fe}][PF ] (5), give insight into the stepwise activation of the P butterfly moiety in 1.