Three-Coordinate Iron(IV) Bisimido Complexes with Aminocarbene Ligation: Synthesis, Structure, and Reactivity

Iron Corrole-Catalyzed Intramolecular Amination Reactions of Alkyl Azides. Spectroscopic Characterization and Reactivity of [FeV(Cor)(NAd)]

As nitrogen analogues of iron-oxo species, high-valent iron-imido species have attracted great interest in the past decades. FeV-alkylimido species are generally considered to be key reaction intermediates in Fe(III)-catalyzed C(sp3)─H bond aminations of alkyl azides but remain underexplored. Here, it is reported that iron-corrole (Cor) complexes can catalyze a wide range of intramolecular C─H amination reactions of alkyl azides to afford a variety of 5-, 6- and 7-membered N-heterocycles, including alkaloids and natural product derivatives, with up to 3880 turnover numbers (TONs) and excellent diastereoselectivity (>99:1 d.r.). Mechanistic studies including density functional theory (DFT) calculations and intermolecular hydrogen atom abstraction (HAA) reactions reveal key reactive FeV-alkylimido intermediates. The [FeV(Cor)(NAd)] (Ad = adamantyl) complex is independently prepared and characterized through electron paramagnetic resonance (EPR), resonance Raman (rR) measurement, and X-ray photoelectron spectroscopy (XPS). This complex is reactive toward HAA reactions with kinetic isotope effects (KIEs) similar to [Fe(Cor)]-catalyzed intramolecular C─H amination of alkyl azides.

Disparate reactivity of a chiral iron(II) tetracarbene complex with organic azides

A chiral tetra-NHC iron(II) complex and its disparate reactivity with multiple organic azides is reported. Both aryl and alkyl azides react with the iron(II) complex yielding three distinct products: an iron(IV) imide, an iron(IV) tetrazene, and a surprising and unprecedented double imide insertion complex.

[Fe(NCMe)6](BF4)2 is a bifunctional catalyst for styrene aziridination by nitrene transfer and heterocycle expansion by subsequent dipolar insertion

The solvated iron(II) salt [Fe(NCMe)6](BF4)2 (Me = methyl) is shown to be a bifunctional catalyst with respect to aziridination of styrene. The salt serves as an active catalyst for nitrene transfer from PhINTs to styrene to form 2-phenyl-N-tosylaziridine (Ph = phenyl; Ts = tosyl, -S{O}2-p-C6H4Me). The iron(II) salt also acts as a Lewis acid in non-coordinating CH2Cl2 solution, to catalyze heterolytic CN bond cleavage of the aziridine and insertion of dipolarophiles. The 1,3-zwitterionic intermediate is presumably supported by interaction of the metal dication with the anion, and by resonance stabilization of the carbocation. Nucleophilic dipolarophiles then insert to give a five-membered heterocyclic ring. The result is a two-step cycloaddition, formally [2 + 1 + 2], that is typically regiospecific, but not stereospecific. This reaction mechanism was confirmed by conducting a series of one-step, [3 + 2] additions of unsaturated molecules into pre-formed 2-phenyl-N-tosylaziridine, also catalyzed by [Fe(NCMe)6](BF4)2. Relevant substrates include styrenes, carbonyl compounds and alkynes. These yield five-membered heterocylic rings, including pyrrolidines, oxazolidines and dihydropyrroles, respectively. The reaction scope appears limited only by the barrier to formation of the dipolar intermediate, and by the nucleophilicity of the captured dipolarophile. The bifunctionality of an inexpensive, earth-abundant and non-toxic catalyst suggests a general strategy for one-pot construction of heterocyclic rings, as demonstrated specifically for pyrrolidine ring formation.

The synthesis and characterization of an iron(VII) nitrido complex

Complexes of iron in high oxidation states are captivating research subjects due to their pivotal role as active intermediates in numerous catalytic processes. Structural and spectroscopic studies of well-defined model complexes often provide evidence of these intermediates. In addition to the fundamental molecular and electronic structure insights gained by these complexes, their reactivity also affects our understanding of catalytic reaction mechanisms for small molecule and bond-activation chemistry. Here, we report the synthesis, structural and spectroscopic characterization of a stable, octahedral Fe(VI) nitrido complex and an authenticated, unique Fe(VII) species, prepared by one-electron oxidation. The super-oxidized Fe(VII) nitride rearranges to an Fe(V) imide through an intramolecular amination mechanism and ligand exchange, which is characterized spectroscopically and computationally. This enables combined reactivity and stability studies on a single molecular system of a rare high-valent complex redox pair. Quantum chemical calculations complement the spectroscopic parameters and provide evidence for a diamagnetic (S = 0) d 2 Fe(VI) and a genuine S = 1/2, d 1 Fe(VII) configuration of these super-oxidized nitrido complexes.

Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N2

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi2 group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(ii) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe-N bonds. The reactivity of these hydrazido(2-) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(iv) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi2 group to form a new N-C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η1 and η2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe-C bond homolyzes to give a three-coordinate hydrazido(2-) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(iv) centers to form new N-C bonds directly to N2, though product selectivity is highly dependent on the identity of the migrating group.

Synthesis and Reactivity of Base-Stabilized and Base-Free Silaimidoyl Bromides

The reactivity of the base-free bromosilylene dtbpCbzSiBr (dtbpCbz = 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl) toward carbodiimides and azides was studied in order to generate base-stabilized and base-free silaimidoyl bromides, respectively. The steric bulk of carbodiimides and azides allows control over the reactivity. While with small substituents such as tert-butyl or adamantyl, the reactions cannot be stopped at the Si═N stage, with large substituents, they lead to C-H activation in the product. The Dipp substituent (Dipp = 2,6-diisopropylphenyl) allowed the isolation of the silaimidoyl bromide dtbpCbzSi(Br)NDipp and its CNDipp-coordinated analogue. The reactivity of the Si═N double bond species was studied with respect to cycloaddition and donor exchange reactions.

Deciphering Iron-Catalyzed C-H Amination with Organic Azides: N2 Cleavage from a Stable Organoazide Complex

Catalytic C-N bond formation by direct activation of C-H bonds offers wide synthetic potential. En route to C-H amination, complexes with organic azides are critical precursors towards the reactive nitrene intermediate. Despite their relevance, α-N coordinated organoazide complexes are scarce in general, and elusive with iron, although iron complexes are by far the most active catalysts for C-H amination with organoazides. Herein, we report the synthesis of a stable iron α-N coordinated organoazide complex from [Fe(N(SiMe3 )2 )2 ] and AdN3 (Ad=1-adamantyl) and its crystallographic, IR, NMR and zero-field 57 Fe Mössbauer spectroscopic characterization. These analyses revealed that the organoazide is in fast equilibrium between the free and coordinated state (Keq =62). Photo-crystallography experiments showed gradual dissociation of N2 , which imparted an Fe-N bond shortening and correspond to structural snapshots of the formation of an iron imido/nitrene complex. Reactivity of the organoazide complex in solution showed complete loss of N2 , and subsequent formation of a C-H aminated product via nitrene insertion into a C-H bond of the N(SiMe3 )2 ligand. Monitoring this reaction by 1 H NMR spectroscopy indicates the transient formation of the imido/nitrene intermediate, which was supported by Mössbauer spectroscopy in frozen solution.

Discovery of a simple iron catalyst reveals the intimate steps of C-H amination to form C-N bonds

Formation of ubiquitous C-N bonds traditionally uses prefunctionalized carbon precursors. Recently, metal-catalyzed amination of unfunctionalized C-H bonds with azides has become an attractive and atom-economic strategy for C-N bond formation, though all catalysts contain sophisticated ligands. Here, we report Fe(HMDS)2 (HMDS = N(SiMe3)2 -) as an easy-to-prepare catalyst for intramolecular C-H amination. The catalyst shows unprecedented turnover frequencies (110 h-1 vs. 70 h-1 reported to date) and requires no additives. Amination is successful for benzylic and aliphatic C-H bonds (>80% yield) and occurs even at room temperature. The simplicity of the catalyst enabled for the first time comprehensive mechanistic investigations. Kinetic, stoichiometric, and computational studies unveiled the intimate steps of the C-H amination process, including the resting state of the catalyst and turnover-limiting N2 loss of the coordinated azide. The high reactivity of the iron imido intermediate is rationalized by its complex spin system revealing imidyl and nitrene character.

Spin polarization assisted facile C-H activation by an S = 1 iron(iv)-bisimido complex: a comprehensive spectroscopic and theoretical investigation

High valent iron terminal imido species (Fe[double bond, length as m-dash]NR) have been shown to be key reactive intermediates in C-H functionalization. However, the detailed structure-reactivity relationship in Fe[double bond, length as m-dash]NR species derived from studies of structurally well-characterized high-valent Fe[double bond, length as m-dash]NR complexes are still scarce, and the impact of imido N-substituents (electron-donating vs. electron-withdrawing) on their electronic structures and reactivities has not been thoroughly explored. In this study, we report spectroscopic and computational studies on a rare S = 1 iron(iv)-bisimido complex featuring trifluoromethyl groups on the imido N-substituents, [(IPr)Fe(NC(CF3)2Ph)2] (2), and two closely related S = 0 congeners bearing alkyl and aryl substituents, [(IPr)Fe(NC(CMe3)2Ph)2] (3) and [(IPr)Fe(NDipp)2] (1), respectively. Compared with 1 and 3, 2 exhibits a decreased Fe[double bond, length as m-dash]NR bond covalency due to the electron-withdrawing and the steric effect of the N-substituents, which further leads to a pseudo doubly degenerate ground electronic structure and spin polarization induced β spin density on the imido nitrogens. This unique electronic structure, which differs from those of the well-studied Fe(iv)-oxido complexes and many previously reported Fe(iv)-imido complexes, provides both kinetic and thermodynamic advantages for facile C-H activation, compared to the S = 0 counterparts.

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

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 (TIMMN^Mes) 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 [(TIMMN^Mes)Fe^I(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 [(TIMMN^Mes)Fe(NAd)]^0,1+,2+,3+ (2^Ad-5^Ad) in oxidation states II to V. The Fe(V) imide [(TIMMN^Mes)Fe(NAd)]³⁺ (5^Ad) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMN^Mes)Fe^V(N)]²⁺ (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMN^Mes)Fe^IV(N)]⁺ (7) with an ethyl group using the triethyloxonium salt Et₃OPF₆. This gives access to the analogous series of ethyl imides [(TIMMN^Mes)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.

Guanidinates as Alternative Ligands for Organometallic Complexes

For decades, ligands such as phosphanes or cyclopentadienyl ring derivatives have dominated Coordination and Organometallic Chemistry. At the same time, alternative compounds have emerged that could compete either for a more practical and accessible synthesis or for greater control of steric and electronic properties. Guanidines, nitrogen-rich compounds, appear as one such potential alternatives as ligands or proligands. In addition to occurring in a plethora of natural compounds, and thus in compounds of pharmacological use, guanidines allow a wide variety of coordination modes to different metal centers along the periodic table, with their monoanionic chelate derivatives being the most common. In this review, we focused on the organometallic chemistry of guanidinato compounds, discussing selected examples of coordination modes, reactivity and uses in catalysis or materials science. We believe that these amazing ligands offer a new promise in Organometallic Chemistry.

Ene Reactivity of an Fe═NR Bond Enables the Catalytic α-Deuteration of Nitriles and Alkynes

Herein, we report the reactions of an Fe(II) imido complex [Ph₂B(^tBuIm)₂Fe═NDipp]^- (1) with internal alkynes and isobutyronitrile, affording the Fe amido allenyl complexes [Ph₂B(^tBuIm)₂Fe(NHDipp)((R₁)C═C═C(R₂)(H))]^- (R₁ = Et or ⁿPr; R₂ = Me or Et, 2-5) and the Fe amido keteniminate complex [Ph₂B(^tBuIm)₂Fe(NHDipp)(N═C═CMe₂)K(THF)]_n (8-K), respectively. These transformations represent the previously unknown ene-like reactivity of a metal-ligand multiple bond. Stoichiometric reactions of 2 and 8-K with DippNH₂ lead to the regeneration of 3-hexyne and isobutyronitrile, respectively, with concomitant formation of the bis(anilido) complex [Ph₂B(^tBuIm)₂Fe(NHDipp)₂]^- (9). These results provide the platform for 1 as an efficient catalyst for the selective α-deuteration of nitriles and alkynes by RND₂. These results demonstrate a new reaction mode for metal imido complexes and suggest new avenues for using the imido ligand in catalysis.

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

Treatment of Mn(N(SiMe₃)₂)₂(THF)₂ with bulky chelating bis(alkoxide) ligand [1,1′:4′,1′′-terphenyl]-2,2′′-diylbis(diphenylmethanol) (H₂[O-terphenyl-O]^Ph) formed a seesaw manganese(II) complex Mn[O-terphenyl-O]^Ph(THF)₂, characterized by structural, spectroscopic, magnetic, and analytical methods. The reactivity of Mn[O-terphenyl-O]^Ph(THF)₂ with various nitrene precursors was investigated. No reaction was observed between Mn[O-terphenyl-O]^Ph(THF)₂ and aryl azides. In contrast, the treatment of Mn[O-terphenyl-O]^Ph(THF)₂ with iminoiodinane PhINTs (Ts = p-toluenesulfonyl) was consistent with the formation of a metal-nitrene complex. In the presence of styrene, the reaction led to the formation of aziridine. Combining varying ratios of styrene and PhINTs in different solvents with 10 mol% of Mn[O-terphenyl-O]^Ph(THF)₂ at room temperature produced 2-phenylaziridine in up to a 79% yield. Exploration of the reactivity of Mn[O-terphenyl-O]^Ph(THF)₂ with various olefins revealed (1) moderate aziridination yields for p-substituted styrenes, irrespective of the electronic nature of the substituent; (2) moderate yield for 1,1′-disubstituted α-methylstyrene; (3) no aziridination for aliphatic α-olefins; (4) complex product mixtures for the β-substituted styrenes. DFT calculations suggest that iminoiodinane is oxidatively added upon binding to Mn, and the resulting formal imido intermediate has a high-spin Mn(III) center antiferromagnetically coupled to an imidyl radical. This imidyl radical reacts with styrene to form a sextet intermediate that readily reductively eliminates the formation of a sextet Mn(II) aziridine complex.

Synthesis of FeIII and FeIV Cyanide Complexes Using Hypervalent Iodine Reagents as Cyano-Transfer One-Electron Oxidants

We disclose a new reactivity mode for electrophilic cyano λ3 -iodanes as group transfer one-electron oxidants to synthesize FeIII and FeIV cyanide complexes. The inherent thermal instability of high-valent FeIV compounds without π-donor ligands (such as oxido (O2- ), imido (RN2- ) or nitrido (N3- )) makes their isolation and structural characterization a very challenging task. We report the synthesis of an FeIV cyanide complex [(N3 N')FeCN] (4) by two consecutive single electron transfer (SET) processes from FeII precursor [(N3 N')FeLi(THF)] (1) with cyanobenziodoxolone (CBX). The FeIV complex can also be prepared by reaction of [(N3 N')FeIII ] (3) with CBX. In contrast, the oxidation of FeII with 1-cyano-3,3-dimethyl-3-(1H)-1,2-benziodoxole (CDBX) enables the preparation of FeIII cyanide complex [(N3 N')FeIII (CN)(Li)(THF)3 ] (2-LiTHF ). Complexes 4 and 2-LiTHF have been structurally characterized by single crystal X-ray diffraction and their electronic structure has been examined by Mössbauer, EPR spectroscopy, and computational analyses.

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant"

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the “oxidant” as well as “amino” group donor. It activates the high-spin Fe(II) (S_t = 2) catalyst [Fe(acac)₂(H₂O)₂] (1) to generate a high-spin (S_t = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with S_t = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)₂-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N–O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = −524 cm^–1) to an iminyl radical, [Fe(III)(acac)₂-NH·], giving S_t = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N–O bond cleavage to generate the active iron–nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe–N bond which, together with the unpaired electron density along the Fe–N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis.

Stabilization of a high-spin three-coordinate Fe(III) imidyl complex by radical delocalization

High-spin, late transition metal imido complexes have attracted significant interest due to their group transfer reactivity and catalytic C−H activation of organic substrates. Reaction of a new two-coordinate iron complex,...

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

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.

A pair of trigonal imido iron complexes ([Fe(NMes)L₂]^0,−) in two oxidation states is reported. The anionic complex K{crypt.222}[Fe(NMes)L₂] is best described as an iron(ii) imide.

Redox-Controlled and Reversible N-N Bond Forming and Splitting with an IronIV Terminal Imido Ligand

Oxidation of the low-spin Fe^IV imido complex [{(^tBupyrr)₂py}Fe═NAd] (1) ((^tBupyrr)₂py^2- = 2,6-bis(3,5-di-tert-butyl-pyrrolyl)pyridine, Ad = 1-adamantyl) with AgOAc or AgNO₃ promotes reductive N-N bond coupling of the former imido nitrogen with a pyrrole nitrogen to form the respective ferric hydrazido-like pincer complexes [{(^tBupyrrNAd)(^tBupyrr)py}Fe(κ²-X)] (X = OAc^-, 2^OAc; NO₃^-, 2^NO₃). Reduction of 2^OAc with KC₈ cleaves the N-N bond to reform the Fe^IV imido ligand in 1, whereas acid-mediated demetalation of 2^OAc or 2^NO₃ yields the free hydrazine ligand [(^tBupyrrNHAd)(^tBupyrrH)py] (3), the latter of which can be used as a direct entry to the iron imido complex when treated with [Fe{N(SiMe₃)₂}₂]. In addition to characterizing these Fe systems, we show how this nitrene transfer strategy can be expanded to Co for the one-step synthesis of Co{(^tBu-NHAdpyrr)(^tBupyrr)py}] (4) ((^tBu-NHAdpyrr)(^tBupyrr)py^2- = 2-(3-tBu-5-(1-adamantylmethyl-2-methylpropane-2-yl)-pyrrol-2-yl)-6-(3,5-tBu₂-pyrrol-2-yl)-pyridine).

Homoleptic quasilinear metal(i/ii) silylamides of Cr-Co with phenyl and allyl functions - impact of the oxidation state on secondary ligand interactions

Herein we describe the synthesis and characterization of a variety of new quasilinear metal(i/ii) silylamides of the type [M(N(Dipp)SiR₃)₂]^0,- (M = Cr-Co) with different silyl substituents (SiR₃ = SiPh_3-nMe_n (n = 1-3), SiMe₂(allyl)). By comparison of the solid state structures we show that in the case of phenyl substituents secondary metal-ligand interactions are suppressed upon reduction of the metal. Introduction of an allyl substituted silylamide gives divalent complexes with additional metal-π-alkene interactions with only weak activation of the C[double bond, length as m-dash]C bond but substantial bending of the principal N-M-N axis. 1e^--reduction makes cobalt a more strongly bound alkene substituent, whereas for chromium, reduction and intermolecular dimerisation of the allyl unit are observed. It thus indicates that the general view of low-coordinate 3d-metal ions as electron deficient seems not to apply to anionic metal(i) complexes. Additionally, the obtained cobalt(i) complexes are reacted with an aryl azide giving trigonal imido metal complexes. These can be regarded as rare examples of high-spin imido cobalt compounds from their structural and solution magnetic features.

A Pair of Cobalt(III/IV) Terminal Imido Complexes

The reaction of the cobalt(I) complex [(TIMMNmes )CoI ](BPh4 ) (2) (TIMMNmes =tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMNmes )CoIII (NAd)](BPh4 ) (3) with concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine, which results from repulsive electrostatic interaction with the filled d(z2 )-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups. One-electron oxidation of 3 with [FeCp2 ](OTf) provides access to the rare, high-valent cobalt(IV) imido complex [(TIMMNmes )CoIV (NAd)](OTf)2 (4). Despite a half-life of less than 1 h at room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal X-ray diffractometry and EPR spectroscopy corroborate the molecular structure and the d5 low-spin, S= 1 / 2 , electron configuration. A computational analysis of 4 suggests high covalency within the CoIV =NAd bond with non-negligible spin density located at the imido moiety, which translates into substantial triplet nitrene character.

Synthesis and Reactivity of Acyclic Germanimines: Silyl Rearrangement and Cycloadditions

We report the synthesis of aromatic germanimines [(HMDS)₂Ge═NAr] (Ar = Ph, Mes, Dipp; Mes = 2,4,6-Me₃C₆H₂, Dipp = 2,6-iPr₂C₆H₃) and an investigation into their associated reactivity. [(HMDS)₂Ge═NPh] decomposes above -30 °C, while [(HMDS)₂Ge═NDipp] engages in an intramolecular reaction at 60 °C. [(HMDS)₂Ge═NMes] was shown to rearrange via a 1,3-silyl migration to give [(HMDS){(SiMe₃)(Mes)N}Ge(NSiMe₃)] in a 1:7 equilibrium mixture at room temperature. These latter germanimines react with unsaturated polar substrates such as CO₂, ketones, and arylisocyanate via a [2 + 2] cycloaddition pathway.

Catalytic Carbodiimide Guanylation by a Nucleophilic, High Spin Iron(II) Imido Complex

Reduction of the three-coordinate iron(III) imido [Ph₂B(^tBuIm)₂Fe═NDipp] (1) affords [Ph₂B(^tBuIm)₂Fe═NDipp][K(18-C-6)THF₂] (2), a rare example of a high-spin (S = 2) iron(II) imido complex. Unusually for a late metal imido complex, the imido ligand in 2 has nucleophilic character, as demonstrated by the reaction with DippNH₂, which establishes an equilibrium with the bis(anilido) complex [Ph₂B(^tBuIm)₂Fe(NHDipp)₂][K(18-C-6)THF₂] (3). In an unusual transformation, formal insertion of ⁱPrN═C═NⁱPr into the Fe═N(imido) bond yields the guanidinate [Ph₂B(^tBuIm)₂Fe(ⁱPrN)₂CNDipp][K(18-C-6)THF₂] (4). Reaction of 4 with excess DippNH₂ provides 3, along with the guanidine (ⁱPrNH)₂C═NDipp. As suggested by these stoichiometric reactions, 2 is an efficient catalyst for the guanylation of carbodiimides, converting a wide range of aniline substrates under mild conditions.

Structural and spectroscopic characterization of an Fe(VI) bis(imido) complex

High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (3) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (2). Single-crystal x-ray diffraction of 2 and 3 revealed four-coordinate Fe centers with an unusual "seesaw" geometry. ⁵⁷Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (S = 1/2) d³ Fe(V) configuration in 2 and a diamagnetic (S = 0) d² Fe(VI) configuration in 3 Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.

C-H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C-H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt-imido bond. It can cleanly cleave strong C-H bonds with a bond dissociation energy of up to 92 kcal mol-1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)2 (NHt Bu)]- . Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.

Stable group 8 metal porphyrin mono- and bis(dialkylcarbene) complexes: synthesis, characterization, and catalytic activity

Alkyl-substituted carbene (CHR or CR2, R = alkyl) complexes have been extensively studied for alkylcarbene (CHR) ligands coordinated with high-valent early transition metal ions (a.k.a. Schrock carbenes or alkylidenes), yet dialkylcarbene (CR2) complexes remain less developed with bis(dialkylcarbene) species being little (if at all) explored. Herein, several group 8 metal porphyrin dialkylcarbene complexes, including Fe- and Ru-mono(dialkylcarbene) complexes [M(Por)(Ad)] (1a,b, M = Fe, Por = porphyrinato dianion, Ad = 2-adamantylidene; 2a,b, M = Ru) and Os-bis(dialkylcarbene) complexes [Os(Por)(Ad)2] (3a-c), are synthesized and crystallographically characterized. Detailed investigations into their electronic structures reveal that these complexes are formally low-valent M(ii)-carbene in nature. These complexes display remarkable thermal stability and chemical inertness, which are rationalized by a synergistic effect of strong metal-carbene covalency, hyperconjugation, and a rigid diamondoid carbene skeleton. Various spectroscopic techniques and DFT calculations suggest that the dialkylcarbene Ad ligand is unique compared to other common carbene ligands as it acts as both a potent σ-donor and π-acceptor; its unique electronic and structural features, together with the steric effect of the porphyrin macrocycle, make its Fe porphyrin complex 1a an active and robust catalyst for intermolecular diarylcarbene transfer reactions including cyclopropanation (up to 90% yield) and X-H (X = S, N, O, C) insertion (up to 99% yield) reactions.

Three-Coordinate Formal Cobalt(0), Iron(0), and Manganese(0) Complexes with Persistent Carbene and Alkene Ligation

Low-coordinate transition-metal species, i.e., metal species with coordination numbers of less than 4, represent a category of ubiquitous reactive intermediates in metal-catalyzed reactions that take place in solution, in metalloenzymes, on supported nanomaterials and single-atom catalysts, and so on. While reactive intermediates are usually transient and hard to isolate, which makes detailed investigation challenging, molecular representatives of low-coordinate transition-metal intermediates can be synthesized by the judicious use of supporting ligands, allowing detailed study of their inherent chemical and physical properties. By the use of bulky nitrogen- and oxygen-based anionic ligands, plenty of three- and two-coordinate group 4-10 metal complexes with the oxidation states of the metal centers being +3, + 2, and +1 have been prepared and subjected to extensive study. Much less known are low-coordinate zero-valent metal complexes, and knowledge about them had been restricted to group 10 metal complexes until very recently. In this Account, we summarize the studies of the synthesis, spectroscopic features, electronic structures, and reactivities of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes with persistent carbene and alkene ligation. The introduction of the π-accepting alkene ligands 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (dvtms) and vinyltrimethylsilane (vtms) into the reduction reactions of MCl₂ (M = Co, Fe, Mn) with persistent carbenes and alkaline metals effectively suppresses ligand C-H bond activation reactions, leading to the successful preparation of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes LM(η²:η²-dvtms), LM(η²-vtms)₂, and L₂M(η²-vtms) (M = Co, Fe, Mn; L = N-heterocyclic carbene (NHC), cyclic (alkyl)(amino)carbene (cAAC)). These three-coordinate metal complexes feature pronounced back-donation from the filled metal 3d orbitals to the alkene π* orbital(s), resulting in electronic configurations of (d_xy+π_alkene^*)²(d_x²-y²+π'_alkene^*)²(d_z²,d_xz,d_yz)ⁿ (the coordination plane was chosen as the xy plane; n = 5, 4, and 3 for Co, Fe, and Mn, respectively) for the bis(alkene) complexes LM(η²:η²-dvtms) and LM(η²-vtms)₂. The alkene ligands in the low-coordinate formal zero-valent metal complexes are amenable to undergo ligand-exchange reactions with better π-accepting ligands. In reactions with organic azides, hydrosilanes, nitrosoarenes, alkynes, etc., the alkene ligands dissociate from the metal coordination sphere, and three-coordinate formal zero-valent metal complexes function as synthons of L_nM⁰ (L = NHC, cAAC; n = 1, 2; M = Co, Fe, Mn) to perform redox reactions with these substrates, affording divalent and tetravalent cobalt, iron, or manganese complexes. These electronic structure and reactivity features hint at the potential of low-coordinate zero-valent group 7-9 metal complexes for the development of new 3d metal catalysts and magnetic materials.

Hydrosilylation of an Iron(IV) Nitride Complex

The nitride ligand in iron(IV) complex PhB(MesIm)₃Fe≡N reacts with excess H₃SiPh to afford PhB(MesIm)₃Fe(μ-H)₃(SiHPh) as the major product, which has been structurally and spectroscopically characterized. Bulkier silane H_aSiPh₂ provides iron(II) amido complex PhB(MesIm)₃FeN(H)(SiHPh₂) as the initial product of the reaction, with excess H₂SiPh₂ affording diamagnetic PhB(MesIm)₃Fe(μ-H)₃(SiPh₂) as the major product. Unobserved iron(II) hydride PhB(MesIm)₃Fe-H is implicated as an intermediate in this reaction, as suggested by the results of the reaction between iron(II) amido PhB(MesIm)₃FeN(H)^tBu and H₃SiPh, which provides PhB(MesIm)₃Fe(H)(μ-H)₂(Si(NH^tBu)Ph) as the sole product.

Proton-promoted disproportionation of iron(v)-imido TAML to iron(v)-imido TAML cation radical and iron(iv) TAML

An iron(v)-imido TAML complex is disproportionated to give an iron(v)-imido TAML cation radical and an iron(iv) TAML upon addition of acids.

Iron-catalysed 1,2-aryl migration of tertiary azides

1,2-Carbon to nitrogen aryl migration of α,α-diaryl tertiary azides was realized by using FeCl₂ and N-heterocyclic carbene SIPr·HCl as a catalyst.

Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]₂ (1-M, M = Fe, Co; Cp' = η⁵-1,2,4-tri-tert-butylcyclopentadienyl) with [Im^DippNLi]₂ (Im^DippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NIm^Dipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe₃)₂}] (3-Co) and Im^DippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by ⁵⁷Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Octahedral iron(iv)-tosylimido complexes exhibiting single electron-oxidation reactivity

High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron(iv)-tosylimido complexes [FeIV(NTs)(MePy2tacn)](OTf)2 (1(IV)[double bond, length as m-dash]NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2 (2(IV)[double bond, length as m-dash]NTs), (MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me2(CHPy2)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are rare examples of octahedral iron(iv)-imido complexes and are isoelectronic analogues of the recently described iron(iv)-oxo complexes [FeIV(O)(L)]2+ (L = MePy2tacn and Me2(CHPy2)tacn, respectively). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [FeIII(HNTs)(L)]2+, 1(III)-NHTs (L = MePy2tacn) and 2(III)-NHTs (L = Me2(CHPy2)tacn) have been isolated after the decay of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs in solution, spectroscopically characterized, and the molecular structure of [FeIII(HNTs)(MePy2tacn)](SbF6)2 determined by single crystal X-ray diffraction. Reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with different p-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with hydrocarbons containing weak C-H bonds results in the formation of 1(III)-NHTs and 2(III)-NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.

Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C-H Amination Efficacy

We report the catalytic C-H amination mediated by an isolable Co^III imido complex (^TrL)Co(NR) supported by a sterically demanding dipyrromethene ligand (^TrL = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (^TrL)Li with CoCl₂ in THF afforded a high-spin (S = 3/2) three-coordinate complex (^TrL)CoCl. Chemical reduction of (^TrL)CoCl with potassium graphite yielded the high-spin (S = 1) Co^I synthon (^TrL)Co which is stabilized through an intramolecular η⁶-arene interaction. Treatment of (^TrL)Co with a stoichiometric amount of 1-azidoadamantane (AdN₃) furnished a three-coordinate, diamagnetic Co^III imide (^TrL)Co(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co-N_imido-C angle 145.0(3)°. Exposure of 1-10 mol % of (^TrL)Co to linear alkyl azides (RN₃) resulted in catalytic formation of substituted N-heterocycles via intramolecular C-H amination of a range of C-H bonds, including primary C-H bonds. The mechanism of the C-N bond formation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [k_H/k_D = 38.4(1)], suggesting a stepwise H-atom abstraction followed by radical recombination. In contrast to the previously reported C-H amination mediated by (^ArL)Co(NR) (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin), (^TrL)Co(NR) displays enhanced yields and rates of C-H amination without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine inhibition effect on the rate of C-H amination. Furthermore, (^TrL)Co(NAd) exhibits an extremely low one-electron reduction potential (E°_red = -1.98 V vs [Cp₂Fe]^+/0) indicating that the highly basic terminal imido unit contributes to the driving force for H-atom abstraction.

Unprecedented Five-Coordinate Iron(IV) Imides Generate Divergent Spin States Based on the Imide R-Groups

Three five-coordinate iron(IV) imide complexes have been synthesized and characterized. These novel structures have disparate spin states on the iron as a function of the R-group attached to the imide, with alkyl groups leading to low-spin diamagnetic (S=0) complexes and an aryl group leading to an intermediate-spin (S=1) complex. The different spin states lead to significant differences in the bonding about the iron center as well as the spectroscopic properties of these complexes. Mössbauer spectroscopy confirmed that all three imide complexes are in the iron(IV) oxidation state. The combination of diamagnetism and 15 N labeling allowed for the first 15 N NMR resonance recorded on an iron imide. Multi-reference calculations corroborate the experimental structural findings and suggest how the bonding is distinctly different on the imide ligand between the two spin states.