Normal-to-Abnormal Rearrangement and NHC Activation in Three-Coordinate Iron(II) Carbene Complexes

Divergent Interaction of (Iso)nitriles with a Linear Iron(I) Silylamide─A Combined Structural, Spectroscopic, and Computational Study

Nitriles and isonitriles are important σ-donor ligands in coordination chemistry. Isonitriles also function in low-valent complexes as π-acceptor ligands similar to CO. Herein we present the unusual behavior of the highly reducing, high-spin iron(I) complex [Fe(hmds)2]- toward these compound classes. Rare examples of side-on coordination of nitriles to the metal center are observed. Insights gained by 57Fe Mössbauer spectroscopy as well as DFT and CASSCF calculations give an interplay between the resonance structures of not only an iron(I) π-complex and an iron(III) metallacycle but also point to the importance of an iron(II) nitrile radical anion. For an aromatic isonitrile end-on coordination is observed, which is best described as an iron(I) complex with only minor unpaired spin transfer onto the isonitrile. For aliphatic isonitriles, the selective R-CN bond cleavage occurs and yields stoichiometric mixtures of alkyl iron(II) and cyanido iron(II) complexes. Attempts to isolate presumed (iso)nitrile radical anions void of 3d-metal coordination give for the reaction of an aromatic isonitrile with KC8 facile reductive coupling to the corresponding diamido acetylene.

Unlocking the Metalation Applications of TMP-powered Fe and Co(II) bis(amides): Synthesis, Structure and Mechanistic Insights

Typified by LiTMP and TMPMgCl.LiCl, (TMP=2,2,6,6-tetramethylpiperidide), s-block metal amides have found widespread applications in arene deprotonative metalation. On the contrary, transition metal amides lack sufficient basicity to activate these substrates. Breaking new ground in this field, here we present the synthesis and full characterisation of earth-abundant transition metals M(TMP)2 (M=Fe, Co). Uncovering a new reactivity profile towards fluoroarenes, these amide complexes can promote direct M-H exchange processes regioselectively using one or two of their basic amide arms. Remarkably, even when using a perfluorinated substrate, selective C-H metalation occurs leaving C-F bonds intact. Their kinetic basicity can be boosted by LiCl or NBu4Cl additives which enables formation of kinetically activated ate species. Combining spectroscopic and structural studies with DFT calculations, mechanistic insights have been gained on how these low polarity metalation processes take place. M(TMP)2 can also be used to access ferrocene and cobaltocene by direct deprotonation of cyclopentadiene and undergo efficient CO2 insertion of both amide groups under mild reaction conditions.

Slow magnetic relaxation in a trigonal-planar mononuclear Fe(II) complex

Based on a β-diketiminate ligand, an iron(III) tetrahedral high-spin complex, [LFeIII(Cl)2] (1), and an iron(II) high-spin triangular planar complex, [LFeIICl] (2), have been synthesized and structurally characterized. Also, complex 1...

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co)

⁷Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe₃)₂}₃] (M²⁺ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe₃)₂}₃]^-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe₃)₂ and [M{N(SiMe₃)₂}₂] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe₃)₂}₃]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ₂ nitrogen bridges reveal strong ⁷Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the ⁷Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe₃)₂}₃] and [Li(15-crown-5)][Mn{N(SiMe₃)₂}₃], display almost pure and ideal spin-only paramagnetic behavior of a 3d⁵ high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe₃)₂}₃]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s^-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s^-1.

Lateral Metallation and Redistribution Reactions of Sodium Ferrates Containing Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilide Ligands

Alkali‐metal ferrates containing amide groups have emerged as regioselective bases capable of promoting Fe−H exchanges of aromatic substrates. Advancing this area of heterobimetallic chemistry, a new series of sodium ferrates is introduced incorporating the bulky arylsilyl amido ligand N(SiMe₃)(Dipp) (Dipp=2,6‐iPr₂‐C₆H₃). Influenced by the large steric demands imposed by this amide, transamination of [NaFe(HMDS)₃] (HMDS=N(SiMe₃)₂) with an excess of HN(SiMe₃)(Dipp) led to the isolation of heteroleptic [Na(HMDS)₂Fe{N(SiMe₃)Dipp}]_∞ (1) resulting from the exchange of just one HMDS group. An alternative co‐complexation approach, combining the homometallic metal amides [NaN(SiMe₃)Dipp] and [Fe{N(SiMe₃)Dipp}₂] induces lateral metallation of one Me arm from the SiMe₃ group in the iron amide furnishing tetrameric [NaFe{N(SiCH₂Me₂)Dipp}{N(SiMe₃)Dipp}]₄ (2). Reactivity studies support that this deprotonation is driven by the steric incompatibility of the single metal amides rather than the basic capability of the sodium reagent. Displaying synergistic reactivity, heteroleptic sodium ferrate 1 can selectively promote ferration of pentafluorobenzene using one of its HMDS arms to give heterotrileptic [Na{N(SiMe₃)Dipp}(HMDS)Fe(C₆F₅)]_∞ (4). Attempts to deprotonate less activated pyridine led to the isolation of NaHMDS and heteroleptic Fe(II) amide [(py)Fe{N(SiMe₃)Dipp}(HMDS)] (5), resulting from an alternative redistribution process which is favoured by the Lewis donor ability of this substrate.

Uncommon carbene-to-azole ligand rearrangement of N-heterocyclic carbenes in a ruthenium system

The study of non-innocent behavior of NHCs (NHCs = N-heterocyclic carbenes) has great implications for NHC-involved catalysis. Herein, we report a new type of NHC-to-azole rearrangement, during which process the carbene backbone and the substituent are both non-innocent. To the best of our knowledge, this work also presents the first example of NHC-to-azole rearrangements for aryl-substituted NHCs.

Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics

Twisted boron-based biradicals featuring unsaturated C2 R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C2 R2 -bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.

Unexpected Vulnerability of DPEphos to C-O Activation in the Presence of Nucleophilic Metal Hydrides

C-O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)2 (PPh3 )2 H2 ] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt2 Me2 , C-O activation is accompanied by C-N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C-O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C-O bond activation also occurs upon heating cis-[Ru(DPEphos)2 H2 ]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C-O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal-ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Double donation in trigonal planar iron-carbodiphosphorane complexes - a concise study on their spectroscopic and electronic properties

We present the syntheses of trigonal planar coordinated Fe(ii) carbodiphosphorane (CDPR) complexes, starting from iron(ii)-bis(trimethylsilylamide) [Fe{N(SiMe3)2}2] and hexaphenyl-(CDPPh) and sym-dimethyltetraphenyl-carbodiphosphoranes (CDPMe), respectively. Both complexes [CDPPh-Fe{N(SiMe3)2}2] (1) and [CDPMe-Fe{N(SiMe3)2}2] (2) were examined in solution and in the solid state. 1 shows a dissociation equilibrium in solution which we monitored by variable temperature 1H-NMR spectroscopy. Magnetic measurements of 1 and 2 yielded a high spin configuration (S = 2) for both complexes. Quantum chemical calculations were performed to analyze the bonding situation in compound 1.

Iron N-heterocyclic carbene complexes in homogeneous catalysis

This review article summarizes recent development of homogeneous iron N-heterocyclic carbene catalysts.

Intramolecular Nitrene Insertion into Saturated C-H-Bond-Mediated C-N Bond Cleavage of a Coordinated NHC Ligand

Ruthenium(II) complexes bearing a tridentate bis(N-heterocyclic carbene) ligand reacted with iminoiodanes (PhI=NR) resulting in the formation of isolable ruthenium(III)-amido intermediates, which underwent cleavage of a C-N bond of the tridentate ligand and formation of an N-substituted imine group. The Ru^III -amido intermediates have been characterized by ¹ H NMR, UV/Vis, ESI-MS, and X-ray crystallography. DFT calculations were performed to provide insight into the reaction mechanism.

Influence of N-Substitution on the Formation and Oxidation of NHC-CAAC-Derived Triazaalkenes

We have studied the effect of N-substitution on the course of the reaction of imidazolium triflate. The reaction of N-heterocyclic carbene with N-tBu-substituted pyrrolinium triflate afforded 2-(pyrrolidin-2-yl)-imidazolium triflate, 3^R. Treatment of 3^R with potassium bis(trimethylsilyl)amide (KHMDS) leads to either the dealkylation product 4 or the deprotonation product, triazaalkene 5, depending on the N-substitution at the imidazolium moiety. Density functional studies using the B3LYP/TZVP setup have been employed to explore various pathways for the dealkylation reaction and the calculated energies support the dealkylation by a large energy margin compared to the deprotonatation process. Theoretical calculations revealed that dealkylation reaction is thermodynamically more favorable than deprotonation. The triazaalkene 5 could be oxidized by AgOTf to the corresponding radical cation 6 and dication 7 in-situ. While 6 and 7 could not be isolated, the formation of the former is inferred by electron paramagnetic resonance spectroscopy and its abstraction of a H-atom to afford 3^Me. Similarly, the formation of the dication 7 is inferred by its ready elimination of isobutylene affording 8.

A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NAr^Trip)] (3; IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene; Ar^Trip = 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide Ar^TripN₃. Spectroscopic investigations in combination with DFT calculations established a high-spin S = 2 ground spin state for 3, consistent with its long Fe–N multiple bond of 1.715(2) Å revealed by X-ray diffraction analysis. Complex 3 exhibits unusual activity of nitrene transfer and C–H bond activation in comparison to the reported iron imido complexes. Specifically, the reactions of 3 with CH₂=CHAr^CF3, an electron-deficient alkene, and CO, a strong π acid, readily afford nitrene transfer products, Ar^CF3CH=CHNHAr^Trip and Ar^TripNCO, respectively, yet no similar reaction occurs when 3 is treated with electron-rich alkenes and PMe₃. Moreover, 3 is inert toward the weak C(sp³)–H bonds in 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave the stronger C(sp)–H bond in p-trifluoromethylphenylacetylene to form an iron(II) amido alkynyl complex. Interestingly, intramolecular C(sp³)–H bond functionalization was observed by adding (p-Tol)₂CN₂ to 3. The unique reactivity of 3 is attributed to its low-coordinate nature and the high negative charge population on the imido N atom, which render its iron–imido unit nucleophilic in nature.

The two-coordinate iron(II) imido complex (IPr)Fe(NAr^Trip) (3) exhibits a high-spin ground state (S = 2) and was found to be reactive toward electron-deficient alkene, diazo compounds, terminal alkyne, et al., in which diversified reactivities of nitrene transfer, C−H bond activation, and C−N bond formation have been observed. The reactivity pattern reflects the nucleophilic nature of the imido moiety of the high-spin iron(II) complex.

Molecular Manipulations of a Utility Nitrogen-Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium-magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR₂ (R=CH₂ SiMe₃ ) affords [(THF)₃ Na(μ-IPr^- )MgR₂ (THF)] (2) [IPr^- =:C{[N(2,6-iPr₂ C₆ H₃ )]₂ CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR₃ ] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr₂ Na₂ MgR₄ ] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPr^Me MgR₂ }₂ ] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR₂ and aIPr^Me with the latter isomerizing to the olefinic NHC IPr=CH₂ . The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to Au^I metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr^- )MgR(THF)₂ ] (4) and [aIPr^Me AuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)₃ }⁺ cation.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Activation of C-H bonds by rare-earth metallocene-butyl complexes

The stable metallocene-butyl complexes [(Cp^Me)₂M(ⁿBu)]₂ (M = Y, Dy) were synthesized and their reactivity towards to ferrocene and bulky N-heterocyclic carbenes investigated. Selective mono-deprotonation of ferrocene and a benzylic methyl group of IMes were observed, whereas a control reaction of (Cp^Me)₃M with IMes resulted in a normal-to-abnormal NHC rearrangement.

Expedient Synthesis of Highly Functionalized Abnormal Carbenegold(I) Complexes

The reaction of 4-substituted imidazol-2-ylidenes with various electrophiles produces a series of 2,4-functionalized imidazolium salts. Subsequent metalation of these precursors using AuCl(SMe₂) provides the first examples of highly functionalized abnormal carbenegold(I) complexes. The present protocol introduces a new strategy for the synthesis of metallic abnormal carbenes featuring diverse functional groups.

Unusual rearrangement of an N-heterocyclic carbeneviaa ring-opening and ring-closing process

The reaction of a pentadentate NHC ligand precursor with Ni(OAc)₂·4H₂O or Pd(OAc)₂in the presence of a base yields four-coordinate square-planar Ni(ii) and Pd(ii) complexes with an unusual ligand generatedin situ.

Constructing reactive Fe and Co complexes from isolated picolyl-functionalized N-heterocyclic carbenes

The Fe(ii) and Co(ii) complexes prepared from the isolated free carbene form of the picolyl-NHC ligands display excellent catalytic activity towards the hydrosilylation of ketones.

Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C-H Functionalizations

Merging two topical themes in main-group chemistry, namely, cooperative bimetallics and frustrated-Lewis-pair (FLP) activity, this Forum Article focuses on the cooperativity-induced outcomes observed when the tris(alkyl)gallium compound GaR₃ (R = CH₂SiMe₃) is paired with the lithium amide LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) or the sterically hindered N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (I^tBu). When some previously published work are drawn together with new results, unique tandem reactivities are presented that are driven by the steric mismatch between the individual reagents of these multicomponent reagents. Thus, the LiTMP/GaR₃ combination, which on its own fails to form a cocomplex, functions as a highly regioselective base (LiTMP)/trap (GaR₃) partnership for the metalation of N-heterocycles such as diazines, 1,3-benzoazoles, and 2-picolines in a trans-metal-trapping (TMT) process that stabilizes the emerging sensitive carbanions. Taking advantage of related steric incompatibility, a novel monometallic FLP system pairing GaR₃ with I^tBu has been developed for the activation of carbonyl compounds (via C═O insertion) and other molecules with acidic hydrogen atoms such as phenol and phenylacetylene. Shedding new light on how these non-cocomplexing partnerships operate and showcasing the potential of gallium reagents to engage in metalation reactions or FLP activations, areas where the use of this group 13 metal is scant, this Forum Article aims to stimulate more interest and activity toward the advancement of organogallium chemistry.

Heteroleptic, two-coordinate [M(NHC){N(SiMe3)2}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state

The linear, two-coordinate and isostructural heteroleptic [M(IPr){N(SiMe₃)₂}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally M^I complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe₃)₂}] with KC₈, or [Co(IPr){N(SiMe₃)₂}₂] with mes*PH₂, mes* = 2,4,6-tBu₃C₆H₂. The magnetism of 3 and 4 implies Co^II and Fe^II centres coupled to one ligand-delocalized electron, in line with XPS and XANES data; the ac susceptibility of 4 detected a pronounced frequency dependence due to slow magnetization relaxation. Reduction of [Fe(IPr)Cl{N(SiMe₃)₂}] with excess KC₈ in toluene gave the heteronuclear 'inverse-sandwich' Fe-K complex 7, featuring η⁶-toluene sandwiched between one Fe⁰ and one K⁺ centre.

Normal-to-abnormal rearrangement of an N-heterocyclic carbene with a silylene transition metal complex

The NHC ligand of the complex 3-W undergoes normal-to-abnormal rearrangement on treatment with CsOH and yields the aNHC-complex 6-W, which is found to be 13.5 kcal mol⁻¹ less stable than its normal counterpart.

Improved reactivity of a cyclic 13/15 compound by increased steric demand

The reactivity of a masked flp was investigated for the sterically overloaded group 13/15 ring compound [tBu₂GaP(H)tBu₂Ph]₂, as can be seen from its reaction with phenyl-isocyanate.

Facile routes to abnormal-NHC-cobalt(ii) complexes

Deprotonation of 1 with Co{N(SiMe₃)₂}₂ affords aNHC-Co(ii) complex 2, whereas carbene transfer from 3 to Co{N(SiMe₃)₂}₂ enables access to complex 4.

A comparative study on the N-heterocyclic carbene adducts of Ih-C60, D5h-C70 and Sc3N@Ih-C80

The exclusive formations of the normal nNHC-C_60/70 and abnormal aNHC-Sc₃N@I_h-C₈₀ complexes in two recent experiments were suggested to be thermodynamically and kinetically controlled, respectively.