selective functionalization of unactivated c-H bonds and ammonia production are extremely important industrial processes. a range of metalloenyzmes achieve these challenging tasks in biology by activating dioxygen and dinitrogen using cheap and abundant transition metals, such as iron, copper and manganese. High-valent iron-oxo and -nitrido complexes act as active intermediates in many of these processes. the generation of well-described model compounds can provide vital insights into the mechanism of such enzymatic reactions. advances in the chemistry of model high-valent iron-oxo and -nitrido systems can be related to our understanding of the biological systems.H igh-valent oxoiron(IV) and formally oxoiron(V) species have been spectroscopically identified as active intermediates in the catalytic cycles of a number of enzymatic systems 1-10 . Haem and non-haem proteins use these reactive intermediates to couple the activation of dioxygen to the oxidation of their substrates. In most cases, an oxygen atom is inserted into an unactivated C-H bond of the substrate; for example, in hydroxylation reactions [1][2][3][4][5][6][7][8][9][10] . However, many other reactions, including halogenation, desaturation, cyclization, epoxidation and decarboxylation, are also known to involve oxoiron species 1,3 . Superoxidized iron complexes with (valence) isoelectronic imido and nitrido ligands, as well as 'surface nitrides' , have also been implicated as key intermediates in the nitrogen atom transfer reactions 11 , the biological synthesis of ammonia by the nitrogenase enzyme 12-16 and the industrial Haber-Bosch process 17 .The generation of well-described model compounds can provide vital insights into the mechanism of such enzymatic reactions. Consequently, considerable effort has been made by synthetic chemists to prepare viable models for the putative reaction intermediates in the catalytic cycles of O 2 and N 2 activating enzymes. In this review, we provide an overview of all high-valent oxoiron and nitridoiron species that have been either identified or proposed as reactive intermediates in biology. Subsequently, we summarize some of the recent advances in bioinorganic chemistry that have led to the identification and isolation of iron complexes in unusually high formal oxidation states, containing iron-oxygen or iron-nitrogen multiple bonds. The spectroscopic characterization and the reactivity studies of these model complexes provide vital insights into the mechanism that nature uses to induce the reductive cleavage of dioxygen or dinitrogen in carrying out a number of important biochemical oxidative transformations. Moreover, the comparative review of the electronic structures of the isoelectronic oxoiron and nitridoiron functionalities reveals that the Fe-N bonds are intrinsically more covalent than the Fe-O bonds.

Reaction of [fc(NH2)2]RuCl2(PPh3)2 (fc = 1,1'-ferrocenylene) with 2 equiv of KO(t)Bu led to the formation of a diamido ruthenium complex, [fc(NH)2]Ru(PPh3)2, whose solid-state molecular structure revealed a short Fe-Ru distance. A metal-to-metal charge transfer band was observed in the electronic absorption spectrum of [fc(NH)2]Ru(PPh3)2. The Fe-Ru interaction was characterized by resonance Raman spectroscopy for the first time and also by (1)H NMR, UV-vis, NIR, Mössbauer spectroscopy, and X-ray crystallography. Density functional theory (DFT) calculations including natural bond order analysis, Bader's atom in molecules method, and time-dependent DFT (TDDFT) provided further support that the iron-ruthenium bond is a weak donor-acceptor interaction with iron acting as the Lewis base.

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.

Facile pseudohalide activation occurs in the reaction of SCN–, SeCN– and N3– with the iron half-sandwich [Cp′Fe(μ-I)]2.

Salt metathesis of [Cp′FeI]2 [1, Cp′ = η5‐1,2,4‐(Me3C)3C5H2] and NaBH4 (2 equiv.) in tetrahydrofuran yields the low‐spin iron complex [Cp′Fe(η3‐BH4)] (2), which is thermally unstable and degrades slowly to the trimeric complex [Cp′FeBH2]3 (3) and H2. Both molecules have been fully characterized by various spectroscopic techniques. An X‐ray structure analysis of 3 confirms its nature and reveals rather long B–B distances of 1.883(9) Å. DFT methods in combination with three complementary approaches [Wiberg bond indices (WBI), atoms in molecules (AIM) and compliance constants] have been used to explain the bonding in these iron(II) complexes. All methods predict a weak bonding interaction between the boron atoms in 3.

Two novel tripodal ligands, (BIMPN(Mes,Ad,Me))(-) and (MIMPN(Mes,Ad,Me))(2-), combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex's reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by (1)H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field (57)Fe Mössbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1-in which a pyridine molecule is situated next to the Mn-Cl bond-and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMEN(R) ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.

The half-sandwich complex [Cp'Fe(μ-I)] (1; Cp' = η-1,2,4-(MeC)CH) is cleaved when heated in toluene to form a cation-anion pair [{Cp'Fe(η-toluene)}{Cp'FeI}] (2), in which the two Fe(ii) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with CHI, the thermally stable 15VE species [Cp'FeI] (3) can be isolated, in which the Fe(iii) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CHSiMe) to form the unprecedented Fe(iii) (S = 3/2) bis(alkyl) complex [Cp'Fe(CHSiMe)] (4). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(ii) alkyl species [Cp'FeCH(SiMe)], which resists the reaction with H as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H (8 bar) in cyclohexane to yield iron hydrides [{Cp'Fe}(μ-H)] (5) and [Cp'Fe(μ-H)] (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp'Fe(η-CH)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp'Fe)(μ-η:η-CH)] (7). Further evidence for the intermediacy of [Cp'Fe(η-CH)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF proceeded via C-C bond cleavage instead of metal oxidation to form [Cp'Fe(CH)][SbF] (8).

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