Imide Complexes Research Articles

Multielectron Reduction of Nitrosoarene via Aluminylene-Silylene Cooperation.

The cooperative effects of main-group elements pave the way for novel chemical transformations. However, the potential of bimetallic complexes featuring the most abundant aluminum and silicon elements remains largely unexplored. In this study, we present the synthesis and characterization of bis(silylene)-stabilized aluminylene 2. The cooperation between aluminylene and silylene allows for the facile cleavage of the N-O bond in nitrosoarenes, producing an aluminum imide complex 4 and tetracyclic oxazasilaalanes 5 and 6, and also promotes the dearomatization of 2-methylquinoline, yielding a silylalane 7. In addition, 2 is an effective precatalyst for the reductive coupling of nitrosoarenes to azoxyarenes. These results outline an approach for orchestrating aluminum and silicon cooperation to facilitate chemical bond activation.

Multinuclear beryllium amide and imide complexes: structure, properties and bonding.

The beryllium amide and imide complexes [Be(HNMes)2]3, [(py)2Be(HNMes)2], [Be(HNDipp)2]2, [Be(NPh2)(μ2-HNDipp)]2 and [Be(NCPh2)2]3 have been prepared and characterised with NMR and IR spectroscopy as well as single crystal X-ray diffraction. Analysis of the localised molecular orbitals (LMOs) and intrinsic atomic orbital (IAO) atomic charges in the framework of the intrinsic bond orbital (IBO) localization method revealed a covalent bonding network consisting of 2-electron-2-centre and 2-electron-3-centre σ bonds, in which one electron pair of the anionic N-donor ligands is involved. The electron deficiency at the beryllium atoms is partially compensated through additional electron donation from the lone pair at the nitrogen atoms.

Terminal dysprosium and holmium organoimides.

Terminal rare-earth-metal imide complexes TptBu,MeLn(NC6H3iPr2-2,6)(dmap) of the mid-late rare-earth elements dysprosium and holmium were synthesized via double methane elimination of Lewis acid stabilized dialkyl precursors TptBu,MeLnMe(GaMe4) with primary aniline derivative H2NC6H3iPr2-2,6 (H2NAriPr). Exploiting the weaker Ln-CH3⋯[GaMe3] interaction compared to the aluminium congener, addition of the aniline derivative leads to the mixed methyl/anilido species TptBu,MeLnMe(HNAriPr) which readily eliminate methane after being exposed to the Lewis base DMAP ([double bond, length as m-dash]N,N-dimethyl-4-aminopyridine). Under the same conditions, [AlMe3]-stabilized dimethyl rare-earth-metal complexes transform immediately to Lewis acid bridged imides TptBu,MeLn(μ2-NC6H3Me2-2,6)(μ2-Me)AlMe2 (Ln = Dy, Ho). DMAP/THF donor exchange is accomplished by treatment of TptBu,MeLn(NC6H3iPr2-2,6)(dmap) with 9-BBN in THF while the terminal imides readily insert carbon dioxide to afford carbamate complexes.

Catalytic production of ammonia from dinitrogen employing molybdenum complexes bearing N-heterocyclic carbene-based PCP-type pincer ligands

Mechanistic insight into the catalytic production of ammonia from dinitrogen is needed to improve the synthesis of this vital molecule. Here we study the use of samarium diiodide (SmI2) and water in the presence of molybdenum complexes that bear PCP-type pincer ligands to synthesize ammonia. The proton-coupled electron transfer during the formation of a N–H bond on the molybdenum imide complex was found to be the rate-determining step at high catalyst concentrations. Additionally, the dimerization step of the catalyst became the rate-determining step at low catalyst concentrations. We designed PCP-type pincer ligands with various substituents at the 5- and 6-positions and observed that electron-withdrawing groups promoted the reaction rate, as predicted by density functional theory calculations. A molybdenum trichloride complex that bears a trifluoromethyl group functioned as the most effective catalyst and produced up to 60,000 equiv. ammonia based on the molybdenum atom of the catalyst, with a molybdenum turnover frequency of up to 800 equiv. min−1. The findings reported here can contribute to the development of an environmentally friendly next-generation nitrogen-fixation system.

Reactions of Low-Valent Gallium Species with Organic Azides: Formation of Imido-, Azoimido-, and Tetrazene Complexes.

The reactivity of two α-diimine-ligated digallanes, [L2-Ga-GaL2-] (La = [(2,6-iPr2C6H3)NC(CH3)]2, dpp-dad, 1; Lb = 1,2-[(2,6-iPr2C6H3)NC]2C10H6, dpp-bian, 2), and a gallylene, [(La)2-GaNa(THF)3] (3), toward organic azides was studied. Reaction of digallane 1 or 2 with trimethylsilyl azide (Me3SiN3), 2-azido-benzonitrile (2-CNC6H4N3), or tosylazide (TosN3) results in imido-bridged complexes, [(La)·-Ga(μ-NSiMe3)2Ga(La)·-] (4) [(Lb)·-Ga(μ-NSiMe3)2Ga(Lb)·-] (5), [(Lb)·-Ga(μ-2-CNC6H4N)2Ga(Lb)·-] (6), and [(Lb)·-Ga(μ-NTos)2Ga(Lb)·-] (7), with elimination of dinitrogen. Treatment of 1 or 2 with 1-adamantyl azide (1-AdN3), on the other hand, affords the unsymmetrical dinuclear complexes [(La)·-Ga(NAd)(N3Ad)Ga(La)·-] (8) and [(Lb)·-Ga(NAd)(N3Ad)Ga(Lb)·-] (9), which contain both imido and triazene bridges. Different from the Ga(II) complexes 1 and 2, the reactions of Ga(I) species 3 with benzylazide or trimethylsilyl azide result in the tetrazene complex {Na(THF)}2[(La)2-Ga(benzyl-N4-benzyl)]2 (10) and amide complex {Na(THF)4}[(La)2-Ga(NHSiMe3)(benzyl)] (11). It is likely that these latter transformations proceed via the transient formation of the corresponding Ga═N imide complex, which undergoes either cycloaddition with a second azide (to form 10) or activation of the C-H bond of methyl in one solvent toluene molecule (to yield 11).

An Isolable Azide Adduct of Titanium(II) Follows Bifurcated Deazotation Pathways to an Imide

AdN3 (Ad = 1-adamantyl) reacts with the tetrahedral TiII complex [(TptBu,Me)TiCl] (TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate) to generate a mixture of an imide complex, [(TptBu,Me)TiCl(NAd)] (4), and an unusual and kinetically stable azide adduct of the group 4 metal, namely, [(TptBu,Me)TiCl(γ-N3Ad)] (3). In these conversions, the product distribution is determined by the relative concentration of reactants. In contrast, the azide adduct 3 forms selectively when a masked TiII complex (N2 or AdNC adduct) reacts with AdN3. Upon heating, 3 extrudes dinitrogen in a unimolecular process proceeding through a titanatriazete intermediate to form the imide complex 4, but the observed thermal stability of the azide adduct (t1/2 = 61 days at 25 °C) is at odds with the large fraction of imide complex formed directly in reactions between AdN3 and [(TptBu,Me)TiCl] at room temperature (∼50% imide with a 1:1 stoichiometry). A combination of theoretical and experimental studies identified an additional deazotation pathway, proceeding through a bimetallic complex bridged by a single azide ligand. The electronic origin of this deazotation mechanism lies in the ability of azide adduct 3 to serve as a π-backbonding metallaligand toward free [(TptBu,Me)TiCl]. These findings unveil a new class of azide-to-imide conversions for transition metals, highlighting that the mechanisms underlying this common synthetic methodology may be more complex than conventionally assumed, given the concentration dependence in the conversion of an azide into an imide complex. Lastly, we show how significantly different AdN3 reacts when treated with [(TptBu,Me)VCl].

N═N Bond Cleavage by Tantalum Hydride Complexes: Mechanistic Insights and Reactivity.

The reaction of [TaCpRX4] (CpR = η5-C5Me5, η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) with SiH3Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCpRX2)2(μ-H)2], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCpRX2(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η5-C5Me5)X2}2(μ-H)2] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η5-C5Me5)X2}2(μ-NC6H4C6H4N)] along with the release of molecular hydrogen. When the compounds [(TaCpRX2)2(μ-H)2] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCpRX)2{μ-(η2,η2-NC6H4C6H4N)}] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) as intermediates in the N=N bond cleavage process. DFT calculations provide insights into the N=N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N=N bond at two different metal–metal bond splitting stages.

Cover Feature: Manganese‐Catalyzed Ammonia Oxidation into Dinitrogen under Chemical or Electrochemical Conditions (ChemPlusChem 11/2021)

The Cover Feature shows manganese-catalyzed ammonia oxidation, which is currently attracting attention because of its potential applications to direct ammonia fuel cells. Catalytic reactions involving a manganese salen complex occur under chemical or electrochemical conditions. Furthermore, mechanistic studies, including density functional theory (DFT) calculations, reveal that nucleophilic attack of ammonia on manganese imide complexes occurs to form a nitrogen–nitrogen bond leading to dinitrogen. More information can be found in the Communication by K. Sakata, Y. Nishibayashi, and co-workers.

Two, Three, or Not to Be? Elucidating Multiple Bonding in d6 Pseudotetrahedral Oxo and Imide Complexes.

Late-transition-metal oxo and imide complexes play an important role in the catalytic functionalization and activation of small molecules. An emerging theme in this area over the past few decades has been the use of lower coordination numbers, and pseudotetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d6 oxo and imide complexes in this geometry is ambiguous. These species are typically depicted with a triple bond; however, recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded complexes with lower d counts. Here we use a suite of computational orbital localization methods and electron density analyses to probe the bonding structure of isoelectronic d6 CoIII oxo and imide complexes. These analyses suggest that a triple-bond description is inaccurate because of a dramatically weakened σ interaction. While the exact bond order in these cases is necessarily dependent on the model used, several metrics suggest that the strength of the metal-O/N bond is most similar to that of other formally doubly bonded complexes.

Platinum(II) Complexes of Tridentate ‐Coordinating Ligands Based on Imides, Amides, and Hydrazides: Synthesis and Luminescence Properties

AbstractFive Pt(II) complexes are described in which the metal ion is bound to anionic ‐coordinating ligands. The central, deprotonated N atom is derived from an imide Ar−C(=O)−NH−C(=O)−Ar {PtL1–2Cl; Ar=pyridine or pyrimidine}, an amide py−C(=O)−NH−CH2−py {PtL3Cl}, or a hydrazide py−C(=O)−NH−N=CH−py {PtL4Cl}. The imide complexes PtL1–2Cl show no significant emission in solution but are modestly bright green/yellow phosphors in the solid state. PtL3Cl is weakly phosphorescent. PtL4Cl is formed as a mixture of isomers, bound through either the amido or imino nitrogen, the latter converting to the former upon absorption of light. Remarkably, the imino form displays fluorescence in solution, λ0,0=535 nm, whereas the amido shows phosphorescence, λ0,0=624 nm, τ=440 ns. It is highly unusual for two isomeric compounds to display emission from states of different spin multiplicity. The amido‐bound PtL4Cl can act as a bidentate ‐coordinating ligand, demonstrated by the formation of bimetallic complexes with iridium(III) or ruthenium(II).

Carbon Monoxide Activation by a Molecular Aluminium Imide: C-O Bond Cleavage and C-C Bond Formation.

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K2 [(NON)Al(NDipp)]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al-N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al-N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C-C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al-O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

AbstractAnionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K2[(NON)Al(NDipp)]2 (NON=4,5‐bis(2,6‐diisopropylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethyl‐xanthene; Dipp=2,6‐diisopropylphenyl) structural characterization by X‐ray crystallography reveals a short Al−N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al−N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C−C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al−O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium‐bound isocyanate fragment.

C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride.

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe3)2 ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H2. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)3][((Me3Si)2N)2U(μ-N)(μ-κ2:CN̵-CH2SiMe2NSiMe3)U(N(SiMe3)2)2] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt3BPh3 yielded the cationic U(IV)/U(IV) nitride complex, [{((Me3Si)2N)2U(THF)}2(μ-N)][BPh4] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe3)2 ligand and the uranium-methylene bond. Reduction of 3 with KC8 afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me3Si)2N)2U(THF)}2(μ-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe3)2 ligand across the uranium-nitride moiety to give the U(III)/U(IV) imide cyclometalate complex, [((Me3Si)2N)2(THF)U(μ-NH)(μ-κ2:C,N̵-CH2SiMe2NSiMe3)U(N(SiMe3)2))(THF)], 5. Complex 4 also reacts with toluene at -80 °C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me3Si)2N)2U(THF)}2(μ-N)][{((Me3Si)2N)3U(μ-NH)U(N(SiMe3)2)}2(C7H8)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me3Si)2N)2U(THF)}2(μ-N)][((Me3Si)2N)2U(η1-CCPh)(μ2-NH)(μ2-η2:η1-CCPh)U(N(SiMe3)2)2], 7. Complex 4 also reacts with H2 to produce an imide hydride U(III)/U(IV) complex, [{((Me3Si)2N)2U(THF)}2(μ-NH)(μ-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes.

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds (Mesdmx, tBudmx: dimethylxanthine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes (Rdmx)Cu2(μ2-NAr) (R: Mes, tBu; Ar: 4-MeOC6H4, 3,5-(F3C)2C6H3) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of (Mesdmx)Cu2(μ2-N(C6H4OMe)) with potassium graphite initiates an intramolecular, benzylic C-H amination at room temperature, chemical reduction of (tBudmx)Cu2(μ2-NAr) leads to isolable [(tBudmx)Cu2(μ2-NAr)]- product salts. The electronic structures of the thermally robust [(tBudmx)Cu2(μ2-NAr)]0/- complexes were assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L2,3/K-edge, N K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence complexes of the type [(Rdmx)Cu2(μ2-NAr)]-feature significant NAr-localized spin following reduction from electronic population of the [Cu2(μ2-NAr)] π* manifold, contrasting previous methods for engendering iminyl character through chemical oxidation. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g., nitrene transfer and H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N-radical character afforded from different aryl substituents.

Synthesis and reactivity of a terminal aluminium-imide bond.

The reaction of the potassium aluminyl K[Al(NONDipp)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) with an organic azide generates the aluminium imide complex, K[Al(NONDipp)(NMes)] (Mes = mesityl = 2,4,6-Me3C6H2). DFT calculations indicate the Al-Nimide interaction is polarized but has appreciable multiple-bond character. This is demonstrated experimentally by the reaction with carbon dioxide, giving a rare example of a main group carbamate dianion via a [2+2] cycloaddition reaction.

Coinage Metal (Bisfluorosulfonyl)imide Complexes: Preparation, Characterization, and Catalytic Applications

Triflate (–OTf) and triflimide (–NTf2) represent two most widely used weakly coordinating counteranions in transition metal catalysis, yet their high price hinders large‐scale application. Herein, we report the preparation, characterization, and catalytic applications of silver(I), gold(I), and copper(II) (bisfluorosulfonyl)imide (–FSI) complexes, showing –FSI as a low cost alternative of –OTf and –NTf2. These complexes, including AgFSI·2MeCN (1), AgFSI·MeCN (2), AgFSI·H2O (3), (AgFSI)6·(H2O)4 (4), AgFSI (5), LAuFSI (6a–6e), and CuFSI2·4H2O (7), are prepared conveniently starting from KFSI, an inexpensive chemical, and shown interesting structural features and some unprecedented coordination modes. In comparison with the corresponding coinage metal triflimides, the FSI complexes have exhibited comparable or better catalytic performance in a series of the model chemical transformations.

One macrocyclic ligand, four oxidation states: A 16-atom ringed dianionic tetra-NHC macrocycle and its Cr(II) through Cr(V) complexes.

Despite chromium being among the first transition metals ever reported to bind to an NHC, chromium NHC complexes, especially in mid and high oxidation states, have received scant attention. Herein, the synthesis, characterization, and reactivity of a series of Cr(II) to Cr(V) complexes bearing a 16-atom ringed dianionic tetra-NHC macrocycle are reported. The Cr(II) dimer is diamagnetic and displays a very short Cr-Cr quadruple bond, unprecedented for Cr-NHC complexes to date. Oxidative cleavage of the Cr-Cr bond leads to the formation of a highly stable diamagnetic Cr(IV) oxo complex. Similar reactions with organic azides lead to paramagnetic Cr(IV) imide complexes. Notably, the Cr(IV) oxo can be oxidized in a reversible reaction to yield a Cr(V) cationic oxo complex, which is a very rare high oxidation state Cr-NHC-compound. This Cr(V) oxo undergoes stoichiometric oxygen atom transfer. Similar reactions were attempted with molybdenum and tungsten to form macrocyclic NHC complexes, but only a molybdenum dimer could be isolated.

Electrolyte Solvent Mixtures for a Symmetric, Non-Aqueous Redox Flow Battery Based on [Fe(bpy)3][Fsi

Redox flow batteries (RFBs) store energy in liquid redox-active electrolytes which are charged and discharged while flowing through an electrochemical cell. This separation of storage and reaction sites enables flexible scalability of power and energy density. [1] Research suggests that non-aqueous RFB systems offer a promising combination of cost and performance for stationary applications, such as storage of excess renewable energy from domestic- to grid-scale [2]. In this work, we investigate a RFB based on the iron(II)-tris(2,2’-bipyridine) bis(fluorosulfonyl)imide complex, which shows one metal-centred oxidation process and up to three ligand-centred reduction processes, providing 2.4 V cell potential [3]. Using the same active material on both positive and negative sides of the cell mitigates the detrimental effects of cross-over through the separator membrane. These redox processes are solvent-dependent [3] and their electrochemistry was studied in a range of aprotic single and mixed solvents, some already used in lithium-ion batteries [4]. Since energy density is determined by the cell potential and the concentration of the active material, the solubility of the reduced and oxidated states of the complex was examined by UV/vis spectroscopy. Furthermore, conductivity and viscosity were measured over a range of temperatures. In RFBs, the dynamic viscosity of the electrolyte influences not just the conductivity but also affects the power required to pump the electrolyte. Cost, safety and environmental impact were considered as well.

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.

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.