Styrene Aziridination by Iron(IV) Nitrides

An Open-Shell FeIV Nitrido

We report the photogeneration and characterization of an open-shell, terminal iron nitrido (EmL)Fe(N) using a sterically encumbered dipyrrin ligand environment. The Fe-N distance in the solid-state, zero-field 57Fe Mössbauer spectrum, and computational analysis are consistent with a triplet electronic ground state of the iron nitrido. Notably, the attenuation of Fe-N multiple bond character through occupying π*Fe-N enables (i) primary C(sp3)-H amination, (ii) H2 cleavage, (iii) aromatic C-C cleavage, and (iv) photocatalytic N-atom transfer reactivity. These modes of reactivity have not previously been observed in low-spin Fe(N) analogues.

C=C Dissociative Imination of Styrenes by a Photogenerated Metallonitrene

Photolysis of a platinum(II) azide complex in the presence of styrenes enables C=C double bond cleavage upon dissociative olefin imination to aldimido (PtII-N=CHPh) and formimido (PtII-N=CH2) complexes as the main products. Spectroscopic and quantum chemical examinations support a mechanism that commences with the decay of the metallonitrene photoproduct (PtII-N) via bimolecular coupling and nitrogen loss as N2. The resulting platinum(I) complex initiates a radical chain mechanism via a dinuclear radical-bridged species (PtII-CH2CHPhN•-PtII) as a direct precursor to C-C scission. The preference for the PtI mediated route over styrene aziridination is attributed to the distinct nucleophilicity of the triplet metallonitrene.

Design of Highly Electrophilic and Stable Metal Nitrido Complexes

ConspectusMetal oxo (M═O) and nitrido (M≡N) complexes are two important classes of high-valent transition metal complexes. The use of M═O as oxidants in chemical and biological systems has been extensively investigated. Nature makes use of M═O in enzymes such as cytochrome P450 to oxidize a variety of substrates. Highly oxidizing oxo species have also been synthesized and they have been shown to oxidize organic and inorganic substrates via one-electron oxidation, O atom transfer, and H atom abstraction pathways. In contrast, the oxidation chemistry of M≡N is much less investigated. Although a variety of nitrido complexes are known, most of them are inert and do not show appreciable oxidizing properties, which is not unexpected since the N3- ligand is much more electron-donating than the O2- ligand. In principle, highly electrophilic/oxidizing nitrido complexes may be designed by using weakly coordinating ancillary ligands and/or by increasing the oxidation state of the metal centers. A number of such species have been generated in solution at low temperatures. However, attempts to isolate them are often hampered by their ease of decomposition via bimolecular N···N coupling to generate N2. In some cases, decomposition occurs by intramolecular nitrogenation of the ancillary ligand.In this account, we describe our recent efforts into the design of nitrido complexes that are highly oxidizing but stable enough so they can be isolated and characterized, and their reactivity toward organic substrates can be readily investigated.We have successfully isolated and determined the structure of the first stable manganese(VI) nitrido complex bearing an oxidation-resistant macrocyclic tetraamido TAML ligand, [MnVI(N)(TAML)]- (H4TAML = 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-benzo[e][1,4,7,10] tetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone). This complex readily undergoes direct aziridination of alkenes; it also abstracts hydrides from NADH analogues via a Separated CPET mechanism. Coupling of the nitrido ligands to give dinitrogen is a major decomposition pathway for electrophilic nitrido complexes. In order to shut down this pathway, we made use of a bulky trianionic corrole ligand TTPPC (H3TTPPC = 5,10,15-tris(2,4,6-triphenylphenyl)corrole) to prepare manganese nitrido complexes. Remarkably, we were able to isolate and determine the structures of [MnV(N)(TTPPC)]- and its one- and two-electron ligand-oxidized products, [MnV(N)(TTPPC+•)] and [MnV(N)(TTPPC2+)]+ ("TTPPC" has a 3- charge, 'TTPPC+•' has an overall 2- charge and 'TTPPC2+' has an overall 1- charge). Although [MnV(N)(TTPPC2+)]+ is formally a manganese(V) complex, it was found to be the most electrophilic among isolated metal nitrido complexes. The use of the bulky corrole ligand effectively prevents the decomposition of Mn≡N by N···N coupling.A number of luminescent M═O species that possess highly oxidizing excited states are known. We have also developed a strongly luminescent osmium(VI) nitrido complex, [OsVI(N)(L)(CN)3]- (OsN, HL = 2-(2-hydroxy-5-nitrophenyl)benzoxazole), that absorbs visible light to generate a highly oxidizing/electrophilic excited state. The excited state readily reacts with a wide variety of organic and inorganic substrates, many of these reactions are unprecedented. Notably, it reacts with cyclohexane to give an osmium(IV) cyclohexyliminato product, and with benzene to give an osmium(IV) p-benzoquinone iminato species.

Divalent Titanium via Reductive N-C Coupling of a TiIV Nitrido with π-Acids

The nitrido-ate complex [(PN)2Ti(N){μ2-K(OEt2)}]2 (1) (PN-=(N-(2-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form rare, five-coordinate TiII complexes having a linear cumulene motif, [K(L)][(PN)2Ti(NCE)] (E=O, L=Kryptofix222, (2); E=NAd, L=3 DME, (3); E=NtBu, L=3 DME, (4); E=NAd, L=Kryptofix222, (5)). Oxidation of 2-5 with [Fc][OTf] afforded an isostructural TiIII center containing a neutral cumulene, [(PN)2Ti(NCE)] (E=O, (6); E=NAd (7), NtBu (8)) and characterization by CW X-band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e- reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete TiII complexes 2 and 5, which were further characterized by solution magnetization measurements and high-frequency and -field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C6F5)4] resulted in a ligand disproportionated TiIV complex having transoid carbodiimides, [(PN)2Ti(NCNAd)2] (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as TiII→TiIII→TiIV and increasing order of π-donation as TiII→TiIII→TiIV, thus displaying more covalency in TiIII species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N-atom to CO and CNAd.

A Mechanistic Study on Iron-Based Styrene Aziridination: Understanding Epoxidation via Nitrene Hydrolysis

Transition-metal-catalyzed nitrene transfer reactions are typically performed in organic solvents under inert and anhydrous conditions due to the involved air and water-sensitive nature of reactive intermediates. Overall, this study provides insights into the iron-based ([FeII(PBI)3](CF3SO3)2 (1), where PBI = 2-(2-pyridyl)benzimidazole), catalytic and stoichiometric aziridination of styrenes using PhINTs ([(N-tosylimino)iodo]benzene), highlighting the importance of reaction conditions including the effects of the solvent, co-ligands (para-substituted pyridines), and substrate substituents on the product yields, selectivity, and reaction kinetics. The aziridination reactions with 1/PhINTs showed higher conversion than epoxidation with 1/PhIO (iodosobenzene). However, the reaction with PhINTs was less selective and yielded more products, including styrene oxide, benzaldehyde, and 2-phenyl-1-tosylaziridine. Therefore, the main aim of this study was to investigate the potential role of water in the formation of oxygen-containing by-products during radical-type nitrene transfer catalysis. During the catalytic tests, a lower yield was obtained in a protic solvent (trifluoroethanol) than in acetonitrile. In the case of the catalytic oxidation of para-substituted styrenes containing electron-donating groups, higher yield, TON, and TOF were achieved than those with electron-withdrawing groups. Pseudo-first-order kinetics were observed for the stoichiometric oxidation, and the second-order rate constants (k2 = 7.16 × 10-3 M-1 s-1 in MeCN, 2.58 × 10-3 M-1 s-1 in CF3CH2OH) of the reaction were determined. The linear free energy relationships between the relative reaction rates (logkrel) and the total substituent effect (TE, 4R-PhCHCH2) parameters with slopes of 1.48 (MeCN) and 1.89 (CF3CH2OH) suggest that the stoichiometric aziridination of styrenes can be described through the formation of a radical intermediate in the rate-determining step. Styrene oxide formation during aqueous styrene aziridination most likely results from oxygen atom transfer via in situ iron oxo/oxyl radical complexes, which are formed through the hydrolysis of [FeIII(N•Ts)] under experimental conditions.

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.

Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

We detail the relative role of ancillary ligand electron-donating ability in comparison to the locus of oxidation (either metal or ligand) on the electrophilic reactivity of a series of oxidized Mn salen nitride complexes. The electron-donating ability of the ancillary salen ligand was tuned via the para-phenolate substituent (R = CF3, H, tBu, OiPr, NMe2, NEt2) in order to have minimal effect on the geometry at the metal center. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we have demonstrated that metal-based oxidation to [MnVI(SalR)N]+ occurs for R = CF3, H, tBu, OiPr, while ligand radical formation to [MnV(SalR)N]+˙ occurs with the more electron-donating substituents R = NMe2, NEt2. We next investigated the reactivity of the electrophilic nitride with triarylphosphines to form a MnIV phosphoraneiminato adduct and determined that the rate of reaction decreases as the electron-donating ability of the salen para-phenolate substituent is increased. Using a Hammett plot, we find a break in the Hammett relation between R = OiPr and R = NMe2, without a change in mechanism, consistent with the locus of oxidation exhibiting a dominant effect on nitride reactivity, and not the overall donating ability of the ancillary salen ligand. This work differentiates between the subtle and interconnected effects of ancillary ligand electron-donating ability, and locus of oxidation, on electrophilic nitride reactivity.

Enabling Nucleophilic Reactivity in High-Spin Fe(II) Imido Complexes: From Elementary Steps to Cooperative Catalysis

ConspectusTransition metal complexes featuring an M═NR bond have received great attention as critical intermediates in the synthesis of nitrogen-containing compounds. In general, the properties of the imido ligand in these complexes are dependent on the nature of the metal center. Thus, the imido ligand tends to be nucleophilic in early transition metal complexes and electrophilic in late transition metal complexes. Nonetheless, the supporting ligand can have a dramatic effect on its reactivity. For example, there are sporadic examples of nucleophilic late transition metal imido complexes, often based on strongly donating supporting ligands. Building on these earlier works, in this Article, we show that the imido ligand in a low-coordinate high-spin bis(carbene)borate Fe(II) complex is able to access previously unknown reaction pathways, ultimately leading to new catalytic transformations. We first focus on the synthesis, characterization, and stoichiometric reactivity of a highly nucleophilic Fe(II) imido complex. The entry point for this system is the intermediate-spin three-coordinate Fe(III) imido complex, which is generated from the reaction of an Fe(I) synthon with an organic azide. Alkali metal reduction leads to a series of M+ (M = Li, Na, K) coordinated and charge-separated (M = K(18-C-6)) high-spin Fe(II) imido complexes, all of which have been isolated and fully characterized. Combined with the electronic structure calculations, these results reveal that the alkali ions moderately polarize the Fe═N bond according to K+ ≈ Na+ < Li+. As a result, the basicity of the imido ligand increases from the charged separated complex to K+, Na+, and Li+ coordinated complexes, as validated by intermolecular proton transfer equilibria. The impact of the counterion on imido ligand reactivity is demonstrated through protonation, alkylation, and hydrogen atom abstraction reactions. The counterion also directs the outcome of [2 + 2] reactions with benzophenone, where alkali coordination facilitates double bond metathesis. Building from here, we describe how the unusual nucleophilicity of the high-spin Fe(II) imido complex revealed in stoichiometric reactions can be extended to new catalytic transformations. For example, a [2 + 2] cycloaddition reaction serves as the basis for the catalytic guanylation of carbodiimides under mild conditions. More interestingly, this complex also exhibits the first ene-like reactivity of an M═NR bond in reactions with alkynes, nitriles, and alkenes. These transformations form the basis of catalytic alkyne and nitrile α-deuteration and pKa-dictated alkene transposition reactions, respectively. Mechanistic studies reveal the critical role of metal-ligand cooperativity in facilitating these catalytic transformations and suggest the new avenues for transition metal imido complexes in catalysis that extend beyond classical nitrene transfer chemistry.

Hydrogen atom abstraction by a high spin [FeIII=S] complex

Iron sulfur clusters are critical to a plethora of biological processes; however, little is known about the elementary unit of these clusters, namely the [Fe=S]n+ fragment. Here, we report the synthesis and characterization of a terminal iron sulfido complex. Despite its high spin (S = 5/2) ground state, structural, spectroscopic, and computational characterization provide evidence for iron sulfur multiple bond character. Intriguingly, the complex reacts with additional sulfur to afford an S = 3/2 iron(III) disulfido (S2 2-) complex. Preliminary studies reveal that the sulfido complex reacts with dihydroanthracene to afford an iron(II) hydrosulfido complex, akin to the reactivity of iron oxo complexes. By contrast, there is no reaction with the disulfido complex. These results provide important insight into the nature of the iron sulfide unit.

Cationic Divalent Metal Sites (M = Mn, Fe, Co) Operating as Both Nitrene-Transfer Agents and Lewis Acids toward Mediating the Synthesis of Three- and Five-Membered N-Heterocycles

The tripodal compounds [(TMG3trphen)MII-solv](PF6)2 (M = Mn, Fe, Co; solv = MeCN, DMF) and bipodal analogues [(TMG2biphen)MII(NCMe)x](PF6)2 (x = 3 for Mn, Fe; x = 2 for Co) and [(TMG2biphen)MIICl2] have been synthesized with ligands that feature a triaryl- or diarylmethyl-amine framework and superbasic tetramethylguanidinyl residues (TMG). The dicationic M(II) sites mediate catalytic nitrene-transfer reactions between the imidoiodinane PhI═NTs (Ts = tosyl) and a panel of styrenes in MeCN to afford aziridines and low yields of imidazolines (upon MeCN insertion) with an order of productivity that favors the bipodal over the tripodal reagents and a metal preference of Fe > Co ≥ Mn. In CH2Cl2, the more acidic Fe(II) sites favor formation of 2,4-diaryl-N-tosylpyrrolidines by means of an in situ (3 + 2) cycloaddition of the initially generated 2-aryl-N-tosylaziridine with residual styrene. In the presence of ketone, 1,3-oxazolidines can be formed in practicable yields, involving a single-pot cycloaddition reaction of alkene, nitrene, and ketone (2 + 1 + 2). Mechanistic studies indicate that the most productive bipodal Fe(II) site mediates stepwise addition of nitrene to olefins to generate aziridines with good retention of stereochemistry and further enables aziridine ring opening to unmask a 1,3-zwitterion that can undergo cycloaddition with dipolarophiles (MeCN, alkene, ketone) to afford five-membered N-heterocycles.

Prospects and challenges for nitrogen-atom transfer catalysis

Conversion of C-H bonds to C-N bonds via C-H amination promises to streamline the synthesis of nitrogen-containing compounds. Nitrogen-group transfer (NGT) from metal nitrenes ([M]-NR complexes) has been the focus of intense research and development. By contrast, potentially complementary nitrogen-atom transfer (NAT) chemistry, in which a terminal metal nitride (an [M]-N complex) engages with a C-H bond, is underdeveloped. Although the earliest examples of stoichiometric NAT chemistry were reported 25 years ago, catalytic protocols are only now beginning to emerge. Here, we summarize the current state of the art in NAT chemistry and discuss opportunities and challenges for its development. We highlight the synthetic complementarity of NGT and NAT and discuss critical aspects of nitride electronic structure that dictate the philicity of the metal-supported nitrogen atom. We also examine the characteristic reactivity of metal nitrides and present emerging strategies and remaining obstacles to harnessing NAT for selective, catalytic nitrogenation of unfunctionalized organic small molecules.

Facile Addition of B-H and B-B Bonds to an Iron(IV) Nitride Complex

The nitride ligand in the iron(IV) complex PhB(ⁱPr₂Im)₃Fe≡N reacts with boron hydrides to afford PhB(ⁱPr₂Im)₃FeN(B)H (B = 9-BBN (1), Bpin (2)) and with (Bpin)₂ to afford PhB(ⁱPr₂Im)₃FeN(Bpin)₂ (3). The iron(II) borylamido products have all been structurally and spectroscopically characterized, demonstrating facile insertion into B-H and B-B bonds by PhB(ⁱPr₂Im)₃Fe≡N. Density functional theory (DFT) calculations reveal that the quintet state (S = 2) is significantly lower in energy than the singlet (S = 0) and triplet (S = 1) states for all products. Stoichiometric reaction with (Bpin)₂ does not produce the mono-borylated iron imido species PhB(ⁱPr₂Im)₃FeN(Bpin). DFT calculations suggest that this is because PhB(ⁱPr₂Im)₃FeN(Bpin) is unstable toward disproportionation to the starting iron(IV) nitride and PhB(ⁱPr₂Im)₃FeN(Bpin)₂. Attempts at B-C bond insertion using phenyl- and benzyl-pinacol borane were unsuccessful, which we attribute to unfavorable kinetics.

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.

Oxidation of Hypophosphorous Acid by a Ruthenium(VI) Nitrido Complex in Aqueous Acidic Solution. Evidence for a Proton-Coupled N-Atom Transfer Mechanism

The oxidation of hypophosphorous acid (H₃PO₂) by a ruthenium(VI) nitrido complex, [(L)Ru^VI(N)(OH₂)]⁺ (Ru^VIN; L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), has been studied in aqueous acidic solutions at pH 0-2.50. The reaction has the following stoichiometry: 2[(L)Ru^VI(N)(OH₂)]⁺ + 3H₃PO₂ + H₂O → 2[(L)Ru^III(NH₂P(OH)₂)(OH₂)]⁺ + H₃PO₃. The pseudo-first-order rate constant, k_obs, depends linearly on [H₃PO₂], and the second-order rate constant k₂ depends on [H⁺] according to the relationship k₂ = k[H⁺]/([H⁺] + K_a), where k is the rate constant for the oxidation of H₃PO₂ molecule and K_a is the dissociation constant of H₃PO₂. At 298.0 K and I = 1.0 M, k = (2.04 ± 0.19) × 10^-2 M^-1 s^-1 and K_a = (6.38 ± 0.63) × 10^-2 M. A kinetic isotope effect (KIE) of 2.9 ± 0.1 was obtained when kinetic studies were carried out with D₃PO₂ at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in D₂O, an inverse KIE of 0.21 ± 0.03 (H₃PO₂ in H₂O vs H₃PO₂ in D₂O) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of H₂P(O)(OH) to HP(OH)₂; the latter molecule is the reacting species which reacts with Ru^VIN via a proton-coupled N-atom transfer pathway.

Nitrogen Atom Transfer Catalysis by Metallonitrene C-H Insertion: Photocatalytic Amidation of Aldehydes

C-H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C-H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd-N) with a diradical nitrogen ligand that is singly bonded to PdII . Despite the subvalent nitrene character, selective C-H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N3 SiMe3 . Based on these results, a photocatalytic protocol for aldehyde C-H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C-H nitrogen atom transfer offers facile access to primary amides after deprotection.

Visible light-induced oxidative N-dealkylation of alkylamines by a luminescent osmium(vi) nitrido complex

N-Dealkylation of amines by metal oxo intermediates (M Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O) is related to drug detoxification and DNA repair in biological systems. In this study, we report the first example of N-dealkylation of various alkylamines by a luminescent osmium(vi) nitrido complex induced by visible light.

The visible light-induced N-dealkylation of various alkylamines by a luminescent osmium(vi) nitrido complex has been investigated. We provide definitive evidence that these reactions occur via an ET/PT mechanism.

Structure and Reactivity of a Manganese(VI) Nitrido Complex Bearing a Tetraamido Macrocyclic Ligand

Manganese complexes in +6 oxidation state are rare. Although a number of Mn(VI) nitrido complexes have been generated in solution via one-electron oxidation of the corresponding Mn(V) nitrido species, they are too unstable to isolate. Herein we report the isolation and the X-ray structure of a Mn(VI) nitrido complex, [Mn^VI(N)(TAML)]^- (2), which was obtained by one-electron oxidation of [Mn^V(N)(TAML)]^2- (1). 2 undergoes N atom transfer to PPh₃ and styrenes to give Ph₃P═NH and aziridines, respectively. A Hammett study for various p-substituted styrenes gives a V-shaped plot; this is rationalized by the ability of 2 to function as either an electrophile or a nucleophile. 2 also undergoes hydride transfer reactions with NADH analogues, such as 10-methyl-9,10-dihydroacridine (AcrH₂) and 1-benzyl-1,4-dihydronicotinamide (BNAH). A kinetic isotope effect of 7.3 was obtained when kinetic studies were carried out with AcrH₂ and AcrD₂. The reaction of 2 with NADH analogues results in the formation of [Mn^V(N)(TAML-H⁺)]^- (3), which was characterized by ESI/MS, IR spectroscopy, and X-ray crystallography. These results indicate that this reaction occurs via an initial "separated CPET" (separated concerted proton-electron transfer) mechanism; that is, there is a concerted transfer of 1 e^- + 1 H⁺ from AcrH₂ (or BNAH) to 2, in which the electron is transferred to the Mn^VI center, while the proton is transferred to a carbonyl oxygen of TAML rather than to the nitrido ligand.

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co(II) sites (S = 3/2) feature a minimal [N3N] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative L4Co catalyst (-COCMe3 arm) is operating faster than the L8Co analogue (-COCF3 arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C b atom of styrene, followed by product-determining ring-closure), the L4Co catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [L4Co]NTs and [L8Co]NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for L4Co (due to the polarizability of the tert-Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its Fe^III -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol^-1 could be activated, and the second-order rate constants (k₂ ) are on the order of 10² -10⁴  s^-1  m^-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

Ligand Tailoring Toward an Air-Stable Iron(V) Nitrido Complex

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(^tBuIm)₃FeN]^0/+. On the basis of ⁵⁷Fe Mössbauer, 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.

One-pot construction of functionalized aziridines and maleimides via a novel pseudo-Knoevenagel cascade reaction

An Ugi, novel pseudo-Knoevenagel, ring expansion cascade reaction was discovered and utilized for the synthesis of aziridinyl succinimides in one-pot. Subsequently, densely functionalized aziridines and maleimides have been designed and synthesized through similar cascade reactions. The target compounds were prepared by means of a mild reaction and a simple operation procedure, which could be applicable to a broad scope of starting materials. This series of novel cascade reactions generates opportunities for the tailored synthesis of a wide range of biologically active scaffolds through tuneable Ugi inputs. Discovery of compound 8i with comparable potency to sorafenib in liver cancer cell lines could provide a new avenue for liver cancer drug discovery.

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.

Ruthenium porphyrin catalysed intermolecular amino-oxyarylation of alkenes to give primary amines via a ruthenium nitrido intermediate

Ruthenium porphyrin catalysed direct intermolecular amino-oxyarylation of alkenes including styrenes and 1,3-dienes to give primary amines with O-(2,4-dinitrophenyl)hydroxylamine as the amine source was achieved in moderate to good yields under mild reaction conditions. Spectroscopic analyses revealed that a ruthenium nitrido complex was the key reaction intermediate for the amino-oxyarylation reaction.

Elucidation of the Reaction Mechanism of C2 + N1 Aziridination from Tetracarbene Iron Catalysts

A combined computational and experimental study was undertaken to elucidate the mechanism of catalytic C₂ + N₁ aziridination supported by tetracarbene iron complexes. Three specific aspects of the catalytic cycle were addressed. First, how do organic azides react with different iron catalysts and why are alkyl azides ineffective for some catalysts? Computation of the catalytic pathway using density functional theory (DFT) revealed that an alkyl azide needs to overcome a higher activation barrier than an aryl azide to form an iron imide, and the activation barrier with the first-generation catalyst is higher than the activation barrier with the second-generation variant. Second, does the aziridination from the imide complex proceed through an open-chain radical intermediate that can change stereochemistry or, instead, via an azametallacyclobutane intermediate that retains stereochemistry? DFT calculations show that the formation of aziridine proceeds via the open-chain radical intermediate, which qualitatively explains the formation of both aziridine diastereomers as seen in experiments. Third, how can the formation of the side product, a metallotetrazene, be prevented, which would improve the yield of aziridine at lower alkene loading? DFT and experimental results demonstrate that sterically bulky organic azides prohibit formation of the metallotetrazene and, thus, allow lower alkene loading for effective catalysis. These multiple insights of different aspects of the catalytic cycle are critical for developing improved catalysts for C₂ + N₁ aziridination.

Mechanistic study of styrene aziridination by iron(iv) nitrides

A combined experimental and computational investigation was undertaken to investigate the mechanism of aziridination of styrene by the tris(carbene)borate iron(iv) nitride complex, PhB( t BuIm)3Fe[triple bond, length as m-dash]N. While mechanistic investigations suggest that aziridination occurs via a reversible, stepwise pathway, it was not possible to confirm the mechanism using only experimental techniques. Density functional theory calculations support a stepwise radical addition mechanism, but suggest that a low-lying triplet (S = 1) state provides the lowest energy path for C-N bond formation (24.6 kcal mol-1) and not the singlet ground (S = 0) state. A second spin flip may take place in order to facilitate ring closure and the formation of the quintet (S = 2) aziridino product. A Hammett analysis shows that electron-withdrawing groups increase the rate of reaction σ p (ρ = 1.2 ± 0.2). This finding is supported by the computational results, which show that the rate-determining step drops from 24.6 kcal mol-1 to 18.3 kcal mol-1 when (p-NO2C6H4)CH[double bond, length as m-dash]CH2 is used and slightly increases to 25.5 kcal mol-1 using (p-NMe2C6H4)CH[double bond, length as m-dash]CH2 as the substrate.

High-Oxidation-State 3d Metal (Ti-Cu) Complexes with N-Heterocyclic Carbene Ligation

High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

Reduction of RuVI≡N to RuIII-NH3 by Cysteine in Aqueous Solution

The reduction of metal nitride to ammonia is a key step in biological and chemical nitrogen fixation. We report herein the facile reduction of a ruthenium(VI) nitrido complex [(L)Ru^VI(N)(OH₂)]⁺ (1, L = N, N'-bis(salicylidene)- o-cyclohexyldiamine dianion) to [(L)Ru^III(NH₃)(OH₂)]⁺ by l-cysteine (Cys), an ubiquitous biological reductant, in aqueous solution. At pH 1.0-5.3, the reaction has the following stoichiometry: [(L)Ru^VI(N)(OH₂)]⁺ + 3HSCH₂CH(NH₃)CO₂ → [(L)Ru^III(NH₃)(OH₂)]⁺ + 1.5(SCH₂CH(NH₃)CO₂)₂. Kinetic studies show that at pH 1 the reaction consists of two phases, while at pH 5 there are three distinct phases. For all phases the rate law is rate = k₂[1][Cys]. Studies on the effects of acidity indicate that both HSCH₂CH(NH₃⁺)CO₂^- and ^-SCH₂CH(NH₃⁺)CO₂^- are kinetically active species. At pH 1, the reaction is proposed to go through [(L)Ru^IV(NHSCH₂CHNH₃CO₂H)(OH₂)]²⁺ (2a), [(L)Ru^III(NH₂SCH₂CHNH₃CO₂H)(OH₂)]²⁺ (3), and [(L)Ru^IV(NH₂)(OH₂)]⁺ (4) intermediates. On the other hand, at pH around 5, the proposed intermediates are [(L)Ru^IV(NHSCH₂CHNH₃CO₂)(OH₂)]⁺ (2b) and [(L)Ru^IV(NH₂)(OH₂)]⁺ (4). The intermediate ruthenium(IV) sulfilamido species, [(L)Ru^IV(NHSCH₂CHNH₃CO₂H)(OH₂)]²⁺ (2a) and the final ruthenium(III) ammine species, [(L)Ru^III(NH₃)(MeOH)]⁺ (5) (where H₂O was replaced by MeOH) have been isolated and characterized by various spectroscopic methods.

C-H activation and nucleophilic substitution in a photochemically generated high valent iron complex

The (photo) chemical oxidation of a (TAML)Fe^III complex using outer-sphere oxidants results in valence tautomerisation and C–H activation governed by exogenous anions.

The photochemical oxidation of a (TAML)Fe^III complex 1 using visible light generated Ru(bpy)₃³⁺ produces valence tautomers (TAML)Fe^IV (1⁺) and (TAML˙⁺)Fe^III (1-TAML˙⁺), depending on the exogenous anions. The presence of labile Cl^– or Br^– results in a ligand-based oxidation and stabilisation of a radical-cationic (TAML˙⁺)Fe^III complex, which subsequently leads to unprecedented C–H activation followed by nucleophilic substitution on the TAML aryl ring. In contrast, exogenous cyanide culminates in metal-based oxidation, yielding the first example of a crystallographically characterised S = 1 [(TAML)Fe^IV(CN)₂]^2– species. This is a rare report of an anion-dependent valence tautomerisation in photochemically accessed high valent (TAML)Fe systems with potential applications in the oxidation of pollutants, hydrocarbons, and water. Furthermore, the nucleophilic aromatic halogenation reaction mediated by (TAML˙⁺)Fe^III represents a novel domain for high-valent metal reactivity and highlights the possible intramolecular ligand or substrate modification pathways under highly oxidising conditions. Our findings therefore shine light on high-valent metal oxidants based on TAMLs and other potential non-innocent ligands and open new avenues for oxidation catalyst design.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Nitrogen atom transfer mediated by a new PN3P-pincer nickel core via a putative nitrido nickel intermediate

A synthetic cycle for a complete nitrogen atom transfer reaction is achieved by irradiating the (PN³P)Ni(N₃)/RNC mixture and subsequent treatment of the resultant products with alkyl halides.

Cyanide Ligand Assembly by Carbon Atom Transfer to an Iron Nitride

The new iron(IV) nitride complex PhB(ⁱPr₂Im)₃Fe≡N reacts with 2 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) to provide PhB(ⁱPr₂Im)₃Fe(CN)(N₂)(BAC). This unusual example of a four-electron reaction involves carbon atom transfer from BAC to create a cyanide ligand along with the alkyne ⁱPr₂N-C≡C-NⁱPr₂. The iron complex is in equilibrium with an N₂-free species. Further reaction with CO leads to formation of a CO analogue, which can be independently prepared using NaCN as the cyanide source, while reaction with B(C₆F₅)₃ provides the cyanoborane derivative.

Migratory insertion and hydrogenation of a bridging azide in a thiolate-bridged dicobalt reaction platform

A novel well-defined thiolate-bridged dicobalt azido complex is converted to a rare sulfilimide-bridged dicobalt complex via nitrogen atom migratory insertion into the Co-S bond upon thermolysis. Intriguingly, the homolytic cleavage of hydrogen is achieved by this azide under mild conditions to furnish a partially hydrogenated azido complex.

High-Spin Iron Imido Complexes Competent for C-H Bond Amination

Reduction of previously reported (ArL)FeCl with potassium graphite furnished a low-spin (S = 1/2) iron complex (ArL)Fe which features an intramolecular η6-arene interaction and can be utilized as an FeI synthon (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin). Treatment of (ArL)Fe with adamantyl azide or mesityl azide led to the formation of the high-spin (S = 5/2), three-coordinate imidos (ArL)Fe(NAd) and (ArL)Fe(NMes), respectively, as determined by EPR, zero-field 57Fe Mössbauer, magnetometry, and single crystal X-ray diffraction. The high-spin iron imidos are reactive with a variety of substrates: (ArL)Fe(NAd) reacts with azide yielding a ferrous tetrazido (ArL)Fe(κ2-N4Ad2), undergoes intermolecular nitrene transfer to phosphine, abstracts H atoms from weak C-H bonds (1,4-cyclohexadiene, 2,4,6-tBu3C6H2OH) to afford ferrous amido product (ArL)Fe(NHAd), and can mediate intermolecular C-H amination of toluene [PhCH3/PhCD3 kH/kD: 15.5(3); PhCH2D kH/kD: 11(1)]. The C-H bond functionalization reactivity is rationalized from a two-step mechanism wherein each step occurs via maximal energy and orbital overlap between the imido fragment and the C-H bond containing substrate.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im^R)₃}FeN], R = ^tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im^R)₃}Fe(NR')]⁺, R = Mes, R' = 1-adamantyl (3), ^tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN^Ar)FeN]⁺ (TIMEN^Ar = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by ⁵⁷Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d_z² (a₁) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

Kinetics and Mechanism of the Reaction of a Ruthenium(VI) Nitrido Complex with HSO3 (-) and SO3 (2-) in Aqueous Solution

The kinetics and mechanism of the reaction of S(IV) (SO3 (2-) +HSO3 (-) ) with a ruthenium(VI) nitrido complex, [(L)Ru(VI) (N)(OH2 )](+) (Ru(VI) N, L=N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), in aqueous acidic solutions are reported. The kinetic results are consistent with parallel pathways involving oxidation of HSO3 (-) and SO3 (2-) by Ru(VI) N. A deuterium isotope effect of 4.7 is observed in the HSO3 (-) pathway. Based on experimental results and DFT calculations the proposed mechanism involves concerted N-S bond formation (partial N-atom transfer) between Ru(VI) N and HSO3 (-) and H(+) transfer from HSO3 (-) to a H2 O molecule.