Terminal Imido Complex Research Articles

Bonding and Reactivity of d0 Transition Metal Imido Complexes Encoded in Their 15N NMR Signatures.

Terminal imido complexes containing metal-nitrogen multiple bonds have been widely used in organometallic chemistry and homogeneous catalysis. The role of terminal imido ligands spans from reactive sites to spectator motifs, largely depending on the nature of the metal center and its specific coordination sphere. Aiming at identifying reactivity descriptors for M-N multiple bonds, we herein explore solid-state 15N NMR spectroscopy (ssNMR) on early transition metal terminal imido complexes augmented by computational studies and show that the asymmetry parameter, κ (skew, 1 ≥ κ ≥ -1), readily available from experiments or calculations, is diagnostic for the reactivity of M-N multiple bonds in imido complexes. While inert imido ligands exhibit skew values (κ) close to 1, highly reactive imido moieties display significantly lower skew values (κ ≪ 1) as found in metallocene or bis-imido complexes. Natural chemical shielding analysis shows that skew values away from 1 are associated with an asymmetric development of π-orbitals around the M-N multiple bond of the imido moiety, with a larger double-bond character for reactive imido. Notably, this descriptor does not directly relate to the M-N-C bond angle, illustrating the shortcoming of evaluating bonding and hybridization from geometrical parameters alone. Overall, this descriptor enables to obtain direct experimental evidence for the π-loading effect seen in bis(imido) and related complexes, thus explaining their bonding/reactivity.

Synthesis and Reactivity of a Cobalt-Supported Singlet Nitrene.

The synthesis, characterization, and reactivity of a series of cobalt terminal imido complexes supported by an N-anchored tripodal tris(carbene) chelate is described, including a Co-supported singlet nitrene. Reaction of the CoI precursor [(TIMMNmes)CoI](PF6) (TIMMNmes = tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with p-methoxyphenyl azide yields a CoIII imide [(TIMMNmes)CoIII(NAnisole)](PF6) (1). Treatment of 1 with 1 equiv of [FeCp2](PF6) at -35 °C affords a formal CoIV imido complex [(TIMMNmes)Co(NAnisole)](PF6)2 (2), which features a bent Co-N(imido)-C(Anisole) linkage. Subsequent one-electron oxidation of 2 with 1 equiv of AgPF6 provides access to the tricationic cobalt imido complex [(TIMMNmes)Co(NAnisole)](PF6)3 (3). All complexes were fully characterized, including single-crystal X-ray diffraction (SC-XRD) analyses, infrared (IR) vibrational, ultraviolet/visible (UV/vis) electronic absorption, multinuclear NMR, X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XAS). Quantum chemical calculations provide additional insight into the electronic structures of all compounds. The dicationic CoIV imido complex 2 exhibits a doublet ground state with considerable imidyl character as a result of covalent Co-NAnisole bonding. At room temperature, 2 readily converts to a CoII amine complex involving intramolecular C-H bond amination. Electronically, tricationic complex 3 can be understood as a singlet nitrene bound to CoIII with significant CoIV imidyl radical character. Verifying the pronounced electrophilicity, nucleophiles such as H2O and tBuNH2 add to 3─analogous to the parent free nitrene─in the para position of the aromatic substituent, thus, clearly corroborating singlet nitrene-type reactivity.

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

Palladium Terminal Imido Complexes with Nitrene Character.

Whereas triplet-nitrene complexes of the late transition metals are isolable and key intermediates in catalysis, singlet-nitrene ligands remain elusive. Herein we communicate three such palladium terminal imido complexes with singlet ground states. UV-vis-NIR electronic spectroscopy with broad bands up to 1400 nm as well as high-level computations (DFT, STEOM-CCSD, CASSCF/NEVPT2, EOS analysis) and reactivity studies suggest significant palladium(0) singlet-nitrene character. Although the aliphatic nitrene complexes proved to be too reactive for isolation in analytically pure form as a result of elimination of isobutylene, the aryl congener could be characterized by SC-XRD, elemental analysis, IR-, NMR spectroscopy, and HRMS. The complexes' distinguished ambiphilicity allows them to activate hexafluorobenzene, triphenylphosphine, and pinacol borane, catalytically dehydrogenate cyclohexene, and aminate ethylene via nitrene transfer at or below room temperature.

Terminal Imido Complexes of the Groups 9–11: Electronic Structure and Developments in the Last Decade

AbstractWe thank the Fonds der Chemischen Industrie (Liebig fellowship for D.M.), the German‐American Fulbright Commission (Fulbright‐Cottrell Award for D.M.) as well as the Bavarian Equal Opportunities Sponsorship – Realization of Equal Opportunities for Women in Research and Teaching (fellowship for A.G.) for financial support. We thank the RRZ Erlangen for computational resources.

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, structure, and C–H bond activation reaction of an iron(IV) terminal imido complex bearing trifluoromethyl groups

Iron terminal imido species featuring electron-withdrawing N-substituents represent an important category of reactive metal-ligand-multiple-bond intermediates in iron-catalyzed reactions. Knowledge of them is very limited, however, as isolable complexes of this type are hard to access. Here, we report an isolable iron(IV) imido complex with trifluoromethyl on nitrogen, namely [(IPr)Fe(NC(CF3)2Ph)2]. We prepared the complex from the reaction of a low-coordinate iron(0) complex with N3C(CF3)2Ph. It has an S = 1 ground spin-state, long Fe-N(imido) bonds, and bent Fe-N(imido)-C(imido) alignments and can undergo intramolecular C–H bond activation reaction to transform into an iron(II) amido complex. Theoretical study suggests that the C–H bond activation reaction involves hydrogen-atom-abstraction mechanism and that the electron-withdrawing nature of trifluoromethyl group renders the imido ligands in [(IPr)Fe(NC(CF3)2Ph)2] and the transition state of the hydrogen-atom-abstraction step structurally close to each other, resulting in a low activation barrier.

A Terminal Imido Complex of an Iron-Sulfur Cluster.

We report the synthesis and characterization of the first terminal imido complex of an Fe-S cluster, (IMes)3 Fe4 S4 =NDipp (2; IMes=1,3-dimesitylimidazol-2-ylidene, Dipp=2,6-diisopropylphenyl), which is generated by oxidative group transfer from DippN3 to the all-ferrous cluster (IMes)3 Fe4 S4 (PPh3 ). This two-electron process is achieved by formal one-electron oxidation of the imido-bound Fe site and one-electron oxidation of two IMes-bound Fe sites. Structural, spectroscopic, and computational studies establish that the Fe-imido site is best described as a high-spin Fe3+ center, which is manifested in its long Fe-N(imido) distance of 1.763(2) Å. Cluster 2 abstracts hydrogen atoms from 1,4-cyclohexadiene to yield the corresponding anilido complex, demonstrating competency for C-H activation.

A Terminal Imido Complex of an Iron–Sulfur Cluster

AbstractWe report the synthesis and characterization of the first terminal imido complex of an Fe–S cluster, (IMes)3Fe4S4=NDipp (2; IMes=1,3‐dimesitylimidazol‐2‐ylidene, Dipp=2,6‐diisopropylphenyl), which is generated by oxidative group transfer from DippN3 to the all‐ferrous cluster (IMes)3Fe4S4(PPh3). This two‐electron process is achieved by formal one‐electron oxidation of the imido‐bound Fe site and one‐electron oxidation of two IMes‐bound Fe sites. Structural, spectroscopic, and computational studies establish that the Fe–imido site is best described as a high‐spin Fe3+ center, which is manifested in its long Fe−N(imido) distance of 1.763(2) Å. Cluster 2 abstracts hydrogen atoms from 1,4‐cyclohexadiene to yield the corresponding anilido complex, demonstrating competency for C−H activation.

Reactions of a Bis(vinyltrimethylsilane)nickel(0) N-Heterocyclic carbene complex with organic azides

The reactions of three-coordinate iron(0) and cobalt(0) alkene complexes (NHC)M(η2-CH2CHSiMe3)2 (NHC = N-heterocyclic carbene, M = Fe, Co) with organic azides proved an effective way for the preparation of iron and cobalt terminal imido complexes. The relevant reactions of nickel(0) alkene complex (NHC)Ni(η2-CH2CHSiMe3)2 with organic azides, however, have remained unexplored. Herein, we report the study on the reactions of [(IPr)Ni(η2-CH2CHSiMe3)2] (IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazole-2-ylidene) with organic azides. The interaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with 2,6-dimesitylphenyl azide (DmpN3) produced the two-coordinate imido complex [(IPr)Ni(NDmp)] (1) in a trace amount with the starting materials being largely remained. The reaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with 2,6-diisopropylphenyl azide (DippN3) gave IPrNNNDipp (2) as the only isolable product. The interaction of [(IPr)Ni(η2-CH2CHSiMe3)2] with PhC(CF3)2N3 produced an azanickelacyclobutane compound [(IPr)Ni(κ-C,N–N(C(CF3)2Ph)CH(SiMe3)CH2)] (3) in high yield. The different products formed in these reactions reflect the influence of the steric and electronic nature of organic azides on reaction outcomes. Further study on the azanickelacyclobutane complex 3 indicated that it can convert into a new azanickelacyclobutane [(IPr)Ni(κ-C,N–N(C(CF3)2Ph)CH2CHSiMe3)] (4). A nickel(II) amido alkyl complex [(IPr)Ni(NHC(CF3)2Ph)(CH2CHNC(CF3)2Ph)] (5) was also isolated from the decomposition reaction of 3.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes.

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

Carbon Dioxide Activation by a Palladium Terminal Imido Complex

We recently reported the first example of a palladium(ii) terminal imido complex. We proposed that this complex features exceptional high nucleophilicity at the nitrogen atom and a peculiar zwitterionic electronic structure with an anti-bonding highest-occupied molecular orbital (HOMO). This complex swiftly activated moderately acidic CH, OH, and NH bonds and also reacted with dihydrogen. However, unambiguous nucleophilic reactivity with substrates not featuring a hydrogen atom could not be observed. Herein, we now show that this nucleophilic complex also reacts with CO2 to give a ring-strained four-membered palladium(ii) carbamate complex. Remarkably, the same product is obtained in the reaction of the related bisamido complex, albeit at a slower reaction rate. Density functional theory calculations indicate that the addition of CO2 does not proceed via initial 1,2-addition across the Pd–N bond, but instead through nucleophilic attack by the imido (amido respectively) nitrogen atom.

An Isolable Terminal Imido Complex of Palladium and Catalytic Implications.

Herein, we report the isolation and a reactivity study of the first example of an elusive palladium(II) terminal imido complex. This scaffold is an alleged key intermediate for various catalytic processes, including the amination of C-H bonds. We demonstrate facile nitrene transfer with H-H, C-H, N-H, and O-H bonds and elucidate its role in catalysis. The high reactivity is due to the population of the antibonding highest occupied molecular orbital (HOMO), which results in unique charge separation within the closed-shell imido functionality. Hence, N atom transfer is not necessarily associated with the high valency of the metal (PdIII , PdIV ) or the open-shell character of a nitrene as commonly inferred.

How to tame a palladium terminal imido

Palladium terminal imido complexes hold promise as catalysts for the amination of CH bonds. This computational study elucidates the electronic structure of palladium(II) and palladium(IV) terminal imido complexes by CASSCF as well as DFT calculations. In search of stable palladium terminal imido complexes, a series of synthetically relevant ancillary ligands are evaluated computationally. The thermodynamic stability of the palladium(II) complexes is correlated with the singlet/triplet gap of the compounds, whereas the palladium(IV) complexes are stabilized by strong electron donor ligands as indicated by the Tolman electronic parameter.

An iridium(iii/iv/v) redox series featuring a terminal imido complex with triplet ground state.

The iridium(iii/iv/v) imido redox series [Ir(NtBu){N(CHCHPtBu2)2}]0/+/2+ was synthesized and examined spectroscopically, magnetically, crystallographically and computationally. The monocationic iridium(iv) imide exhibits an electronic doublet ground state with considerable 'imidyl' character as a result of covalent Ir-NtBu bonding. Reduction gives the neutral imide [Ir(NtBu){N(CHCHPtBu2)2}] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO2) and nucleophiles (PMe3), respectively, is reported.

Closing the Loop on Transition-Metal-Mediated Nitrogen Fixation: Chemoselective Production of HN(SiMe3)2 from N2, Me3SiCl, and X-OH (X = R, R3Si, or Silica Gel).

Treatment of the Mo(IV) terminal imido complex, (η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo(NSiMe3) (3), with a 1:2 mixture of iPrOH and Me3SiCl resulted in the rapid formation of the Mo(IV) dichloride, (η5-C5Me5)[N(Et)C(Ph)N(Et)]MoCl2 (1), and the generation of 1 equiv each of HN(SiMe3)2 and iPrOSiMe3. Similarly, a 1:2 mixture of Me3SiOH and Me3SiCl provided 1, HN(SiMe3)2, and O(SiMe3)2. Finally, silica gel, when coupled with excess equivalents of Me3SiCl, was also effectively used as the X-OH reagent for the generation of 1 and HN(SiMe3)2. A proposed mechanism for the 3 → 1 transformation involves formal addition of HCl across the Mo═N imido bond through initial hydrogen-bonding between X-OH and the N-atom of 3 to form the adduct IIIb, followed by chloride delivery from Me3SiCl to the metal center via a six-membered transition state (IV) that leads to the intermediate, (η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo(Cl)(NHSiMe3) (V), and XOSiMe3 as a co-product. Metathetical exchange of the new Mo-N amido bond of V by a second equivalent of Me3SiCl then generates 1 and HN(SiMe3). These results serve to complete a highly efficient chemical cycle for nitrogen fixation that is mediated by a set of well-characterized transition-metal complexes.

Synthesis and Reactivity of a Scandium Terminal Hydride: H2 Activation by a Scandium Terminal Imido Complex.

Dihydrogen is easily activated by a scandium terminal imido complex containing the weakly coordinated THF. The reaction proceeds through a 1,2-addition mechanism, which is distinct from the σ-bond metathesis mechanism reported to date for rare-earth metal-mediated H2 activation. This reaction yields a scandium terminal hydride, which is structurally well-characterized, being the first one to date. The reactivity of this hydride is reported with unsaturated substrates, further shedding light on the existence of the terminal hydride complex. Interestingly, the H2 activation can be reversible. DFT investigations further eludciate the mechanistic aspects of the reactivity of the scandium anilido-terminal hydride complex with PhNCS but also on the reversible H2 activation process.

Reactivity of Scandium Terminal Imido Complex toward Boranes: C(sp3)–H Bond Borylation and B–O Bond Cleavage

Scandium terminal imido complex [(NNNN)Sc═NDIPP] (2; NNNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2NMe2)]−, DIPP = 2,6-iPr2C6H3) reacts with 9-borabicyclononane (9-BBN) to give scandium borohydride [(NNNN(B)H)Sc(N(H)DIPP)] (3; NNNN(B)H = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2N(Me)CH2(BBN)]2–), and C(sp3)–H bond borylation of the NNNN ligand occurs during this reaction. In contrast, the reaction between complex 2 and catecholborane (CatBH) gives scandium catecholate [(NNNN)Sc(Cat)] (4), and B–O bond cleavage happens during this reaction. Both 3 and 4 have been well-characterized including the single-crystal X-ray diffraction analysis. Reaction of 2 with bis(catecholato)diboron (CatB–BCat) also gives a B–O bond cleavage product.

Synthesis, Structure, and Reactivity of Low-Spin Cobalt(II) Imido Complexes [(Me3P)3Co(NAr)

The reactions of [Co(PMe3)4] with the bulky organic azides, DippN3 and DmpN3 [Dipp, 2,6-diisopropylphenyl; Dmp, 2,6-di(2',4',6'-trimethylphenyl)phenyl], afforded the cobalt(II) terminal imido complexes [(Me3P)3Co(NAr)] (Ar = Dipp, 1; Dmp, 2). The cobalt imido complexes in their solid states show trigonal pyramidal coordination geometry and long Co-N(imido) separations (ca. 1.71 Å). Spectroscopic characterization and theoretical studies indicated their low-spin cobalt(II) nature. Reactivity studies on 1 revealed its nitrene-transfer reactions with PMe3 and CO, the imido/oxo and imido/sulfido exchange reactions with PhCHO and CS2, and the single-electron oxidation reaction by ferrocenium cation to form cobalt(III) imide.

ChemInform Abstract: Lanthanides and Actinides: Annual Survey of Their Organometallic Chemistry Covering the Year 2013

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