Rational synthesis of normal, abnormal and anionic NHC-gallium alkyl complexes: structural, stability and isomerization insights

Frustrated Lewis Pair-Type Reactivity of Intermolecular Rare-Earth Aryloxide and N-Heterocyclic Carbene/Olefin Combinations

This work reports the cooperative reactivity of rare-earth aryloxide complexes with N-heterocyclic carbene (NHC) or N-heterocyclic olefin (NHO), showcasing their synergistic effect on the activation of H2 and diverse organic substrates. Reactions of RE(OAr)3 (RE=La, Sm, and Y; Ar=2,6-tBu2-C6H3) with unsaturated NHC ItBu (:C[N(R)CH]2, R=tBu) isolated abnormally bound RE metal NHC complexes RE/aNHC. In contrast, no metal-NHO adducts were formed when RE(OAr)3 were treated with NHO (R2C=C[N(R)C(R)]2, R=CH3). Both RE/aNHC and RE/NHO Lewis pairs enabled cooperative H2 activation. Furthermore, RE(OAr)3 were found to catalyze the hydrogenation of the exocyclic C=C double bond of NHO under mild conditions. Moreover, treatment of the La/aNHC complex with benzaldehyde produced a La/C4 1,2-addition product. The La/NHO Lewis pair could react with (trimethylsilyl)diazomethane and α, β-conjugated imine, affording an isocyanotrimethylsilyl lanthanum amide complex and a La/C 1,4-addition product, respectively.

Adaptive carbonyl umpolung involving a carbanionic carbene Breslow intermediate: an alternative mechanism for NHC-mediated organocatalysis

Herein, we propose a novel mechanistic model for NHC-mediated carbonyl umpolung which involves the formation of a carbanionic carbene Breslow intermediate (CCBI). We have demonstrated theoretically that this reactive intermediate can be formed by inserting an aldehyde into the C4-H position of an N-aryl-substituted imidazolium-derived NHC via the generation of an H-bonded ditopic carbanionic NHC (dcNHC). Our DFT study on benzoin condensation has revealed that the mechanism of polarity inversion proceeding through the CCBI may be more energetically favorable than the classical mechanism of umpolung that uses the C2 carbene position in NHC. The potential existence of the CCBI highlights the dynamic and adaptive nature of NHC-mediated organocatalysis, particularly in relation to carbonyl umpolung. This finding also sheds light on new pathways in organocatalytic transformations employing the ambident reactivity of NHC, which may be particularly attractive for reactions involving furanic aldehydes and sterically encumbered N-aryl-substituted carbenes.

Synthesis of MAuAg (M = Ni, Pd, or Pt) and NiAuCu Heterotrimetallic Complexes Ligated by a Tritopic Carbanionic N-Heterocyclic Carbene

Heterotrimetallic complexes (NiAuAg, PdAuAg, PtAuAg, and NiAuCu) containing a tritopic N-heterocyclic carbene (NHC) have been synthesized for the first time through the deprotonation and metalation of heterodimetallic complexes and were structurally characterized by single-crystal X-ray diffraction. The carbene character of the donor groups in the tritopic NHC complexes was established on the basis of structural and NMR analyses.

Progressing the Frustrated Lewis Pair Abilities of N-Heterocyclic Carbene/GaR3 Combinations for Catalytic Hydroboration of Aldehydes and Ketones

Exploiting the steric incompatibility of the tris(alkyl)gallium GaR₃ (R = CH₂SiMe₃) and the bulky N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), here we report the B-H bond activation of pinacolborane (HBPin), which has led to the isolation and structural authentication of a novel ion pair, [{ItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (2). Contrastingly, neither ItBu or GaR₃ was able to react with HBPin under the conditions of this study. Combining an NHC-stabilized borenium cation, [{ItBu-BPin}⁺], with an anionic dinuclear gallate, [{GaR₃(μ-H)GaR₃}^-], 2 proved to be unstable in solution at room temperature, evolving to the abnormal NHC-Ga complex [BPinC{{N(tBu)]₂CHCGa(R)₃}] (3). Interestingly, the structural isomer of 2, with the borenium cation residing at the C4 position of the carbene, [{aItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (4), was obtained when the abnormal NHC complex [aItBu·GaR₃] (1) was heated to 70 °C with HBPin, demonstrating that, under these forced conditions, it is possible to induce thermal frustration of the Lewis base/Lewis acid components of 1, enabling the activation of HBPin. Building on these stoichiometric studies, the frustrated Lewis pair (FLP) reactivity observed for the GaR₃/ItBu combination with HBPin could then be upgraded to catalytic regimes, allowing the efficient hydroboration of a range of aldehydes and ketones under mild reaction conditions. Mechanistic insights into the possible reaction pathway involved in this process have been gained by combining kinetic investigations with a comparative study of the catalytic capabilities of several gallium and borenium species related to 2. Disclosing a new cooperative partnership, reactions are proposed to occur via the formation of a highly reactive monomeric hydride gallate, [{ItBu-BPin}⁺{GaR₃(H)}^-] (I). Each anionic and cationic component of I plays a key role for success of the hydroboration, with the nucleophilic monomeric gallate anion favoring the transfer of its hydride to the C═O bond of the organic substate, which in turn is activated by coordination to the borenium cation.

Distannabarrelenes with Three Coordinated SnII Atoms

Crystalline 1,4-distannabarrelene compounds [(ADCAr )3 Sn2 ]SnCl3 (3-Ar) (ADCAr ={ArC(NDipp)2 CC}; Dipp=2,6-iPr2 C6 H3 , Ar=Ph or DMP; DMP=4-Me2 NC6 H4 ) derived from anionic dicarbenes Li(ADCAr ) (2-Ar) (Ar=Ph or DMP) have been reported. The cationic moiety of 3-Ar features a barrelene framework with three coordinated SnII atoms at the 1,4-positions, whereas the anionic unit SnCl3 is formally derived from SnCl2 and chloride ion. The all carbon substituted bis-stannylenes 3-Ar have been characterized by NMR spectroscopy and X-ray diffraction. DFT calculations reveal that the HOMO of 3-Ph (ϵ=-6.40 eV) is mainly the lone-pair orbital at the SnII atoms of the barrelene unit. 3-Ar readily react with sulfur and selenium to afford the mixed-valence SnII /SnIV compounds [(ADCAr )3 SnSn(E)](SnCl6 )0.5 (E=S 4-Ar, Ar=Ph or DMP; E=Se 5-Ph).

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of gallium hydrides, gallium chlorides and gallium hydrochlorides

The synthesis and characterization of N-heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) gallane and chlorogallane adducts of the type (NHC)·GaH3 (NHC = Me2ImMe1, iPr2Im 2, iPr2ImMe3 and Dipp2ImH4; Me2ImMe = 1,3,4,5-tetra-methyl-imidazolin-2-ylidene; R2Im = 1,3-di-organyl-imidazolin-2-ylidene; Dipp = 2,6-diisopropylphenyl; Dipp2ImH = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene), (NHC)·GaH2Cl (NHC = iPr2ImMe9, Dipp2Im 10 and Dipp2ImH11; iPr2ImMe = 1,3 diisopropyl-4,5-dimethyl-imidazolin-2-ylidene), (NHC)·GaHCl2 (NHC = iPr2ImMe12, Dipp2Im 13 and Dipp2ImH14) and (cAACMe)·GaHCl215 is reported. Compounds 1-3 and 9-11 are unstable in solution as heating to the boiling temperature of toluene (110 °C) leads to decomposition into elemental gallium and the corresponding dihydroaminal NHC-H2. The reaction of the mono-NHC adducts with a second equivalent of NHC also afforded decomposition and formation of NHC-H2, whereas the reaction of the NHC-stabilized gallanes with one equivalent cAACMe leads to an insertion of the cAACMe carbene carbon atom into the Ga-H bond. The synthesis and characterization of (NHC)·GaH2(cAACMeH) (NHC = Me2ImMe5, iPr2ImMe6 and Dipp2Im 7), the products of an oxidative addition of (NHC)·GaH3 to cAACMe, are presented. The adduct (Dipp2ImH)·GaH34 reacts with two equivalents of cAACMe under the release of Dipp2ImH and insertion of two cAACMe molecules into the Ga-H bond to give the bisalkylgallane (cAACMeH)2GaH 8. If one or two hydrogen atoms of the NHC gallane adducts are replaced with an electron-withdrawing chloride substituent, a selective insertion of cAACMe into the Ga-H bond occurs only for the adducts of sterically less demanding NHCs such as iPr2ImMe. The reaction of cAACMe with (iPr2ImMe)·GaH2Cl 9 and (iPr2ImMe)·GaHCl212 afforded (iPr2ImMe)·GaHCl(cAACMeH) 16 and (iPr2ImMe)·GaCl2(cAACMeH) 17. The reaction of cAACMe with (NHC)·GaHCl2 and (NHC)·GaH2Cl (NHC = Dipp2Im, Dipp2ImH), adducts of the sterically more demanding NHCs, leads to an extrusion of the NHC to (cAACMe)·GaHCl215 and (cAACMeH)2GaCl 18.

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.

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.

A crystalline C5-protonated 1,3-imidazol-4-ylidene

The first C5-protonated 1,3-imidazole-based mesoionic carbene (iMIC^Bp) has been isolated and characterized by single-crystal X-ray diffraction.

Normal to abnormal ItBu·AlH3 isomerization in solution and in the solid state

The first example of solid state normal to abnormal NHC complex isomerization is reported.

"Abnormal" Addition of NHC to a Conjugate Acid of CAAC: Formation of N-Alkyl-Substituted CAAC

The addition reactions of N-heterocyclic carbenes (NHCs) are mostly known to occur through the carbenic centre (C2), which leads to a "normal" adduct. Herein, we report the "abnormal" addition of NHC^Dip 1 (1,3-(2,6-iPr₂ C₆ H₃ )-imidazole-2-ylidene) to a conjugate acid of cyclic (alkyl)(amino)carbene 2 (CAAC^iPr =1-iPr-3,3,5,5-Me₄ -pyrrolinium triflate). Mechanistic study revealed that this reaction proceeded through the in situ formation of 1,3-(2,6-iPr₂ C₆ H₃ )-imidazolium cation 4 and N-iPr-substituted CAAC 5 followed by the oxidative addition of compound 5 across the C4-H bond (alias backbone C-H) of compound 4. The in situ formation of compound 5 was also proven by the oxidative addition of it to the N-H group of iPrNH₂ . DFT calculations also supported the mechanistic findings. A different methodology for the in situ generation of compound 5 by using TMPLi is also described.

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.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

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.

Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

The sterically bulky Ga(III) and In(III) (IPr*)MMe3 adducts (1 and 2) and (SItBu)MMe3 adducts (3 and 4) (M = Ga, In; IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}-1,3-dihydro- imidazol-2-ylidene; SItBu = 1,3-bis(1,1-dimethylethyl)-imidazolidin-2-ylidene) were prepared and structurally characterized, allowing an estimation of the steric hindrance of such Lewis pairs (yields in 1–4: 92%, 90%, 73%, and 42%, respectively). While the IPr* adducts 1 and 2 are robust species, the more severely congested SItBu adducts 3 and 4 are more reactive and exhibit a limited stability in solution. Adduct (SItBu)GaMe3 (3) reacts quickly with H2 at room temperature to afford the corresponding aminal product, 1,3-di-tert-butylimidazolidine (5), along with free GaMe3. Such Frustrated Lewis Pair (FLP) reactivity constitutes the first instance of a H2 activation involving a simple trialkyl GaR3 species. Adduct 3 also mediates the ring-opening polymerization (ROP) of rac-lactide at room temperature to afford cyclic polylactide (PLA).

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.

Polar organometallic strategies for regioselective C–H metallation of N-heterocyclic carbenes

This Feature Article focuses on the recent emergence of s-block metal-mediated N-heterocyclic carbene metallations and the new opportunities this methodology offers to access unique anionic NHC fragments.

A tritopic carbanionic N-heterocyclic dicarbene and its homo- and heterometallic coinage metal complexes

Homo (Au₃)- and heterotrinuclear coinage metal complexes (Au₂Ag and Au₂Cu) ligated by the first tritopic dicarbanionic NHC have been prepared by deprotonation of ditopic NHC digold complexes.

NormalandabnormalNHC coordination in cationic hydride iodide complexes of aluminium

Sterically demanding NHC aluminium hydride iodide complexes react with one equivalent of NHC to cationic mixednormal–abnormalNHC Al^IIIcomplexes.

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.

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.

Aryl-NHC-group 13 trimethyl complexes: structural, stability and bonding insights

Treatment of aromatic N-substituted N-heterocyclic carbenes (NHCs) with trimethylgallium and -indium yielded the new Lewis acid–base adducts (as shown in the Figure). The steric and electronic factors affecting the stability of these complexes were quantified using percent buried volume, topographic steric maps and theoretical studies.

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.

NHC adducts of tantalum amidohalides: the first example of NHC abnormally coordinated to an early transition metal

The first abnormally coordinated NHC complex of an early transition metal, [(aIMes)TaCl₂(NMe₂)₃], is prepared.

Post-coordination backbone functionalization of an imidazol-2-ylidene and its application to synthesize heteropolymetallic complexes incorporating the ambidentate IMesCO2−ligand

The C4-carboxylation of the archetypal IMes ligand was achieved directly on its complexed form for the first time, and the resulting ambidentate IMes^CO2−ligand was exploited for the formation of heteropolymetallic complexes.