Nickel-Catalyzed Cross-Coupling Reaction of Aryl Bromides/Nitriles with Imidazolium Salts Involving Inert C-N Bond Cleavage

We herein present a nickel-catalyzed cross-coupling reaction of aryl halides and nitriles with imidazolium salts. A series of 2-arylated imidazoles could be obtained in moderate to good yields through inert C-N bond cleavage. The imidazolium salt in this reaction acts as both a coupling partner and N-heterocyclic carbene (NHC) ligand precursor. Mechanistic studies reveal that consecutive steps of migratory insertion of the NHC into the aryl C-Ni bond and β-C elimination might be involved in the proposed reaction mechanism.

Poly(imidazolyliden-yl)borato Complexes of Tungsten: Mapping Steric vs. Electronic Features of Facially Coordinating Ligands

A convenient synthesis of [HB(HImMe)3](PF6)2 (ImMe = N-methylimidazolyl) is decribed. This salt serves in situ as a precursor to the tris(imidazolylidenyl)borate Li[HB(ImMe)3] pro-ligand upon deprotonation with nBuLi. Reaction with [W(≡CC6H4Me-4)(CO)2(pic)2(Br)] (pic = 4-picoline) affords the carbyne complex [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}]. Interrogation of experimental and computational data for this compound allow a ranking of familiar tripodal and facially coordinating ligands according to steric (percentage buried volume) and electronic (νCO) properties. The reaction of [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}] with [AuCl(SMe2)] affords the heterobimetallic semi-bridging carbyne complex [WAu(μ-CC6H4Me-4)(CO)2(Cl){HB(ImMe)3}].

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.

An NHC-Mediated Metal-Free Approach towards an NHC-Coordinated Endocyclic Disilene

A convenient metal‐free approach towards an N‐heterocyclic carbene (NHC)‐coordinated disilene 2 is described. Compound 2, featuring the disilene incorporated in cyclopolysilane framework, was obtained in good yield and characterized using NMR spectroscopy and X‐ray crystallography. Density functional theory (DFT) calculations of the reaction mechanism provide a rationale for the observed reactivity and give detailed information on the bonding situation of the base‐stabilized disilene. Compound 2 undergoes thermal or light‐ induced (λ=456 nm) NHC loss, and a dimerization process to give a corresponding dimer with a Si₁₀ skeleton. In order to shed light on the dimerization mechanism, DFT calculations were performed. Moreover, the reactivity of 2 was examined with selected examples of transition metal carbonyl compounds.

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 Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

A study on the reactivity of the N-heterocyclic silylene Dipp2 NHSi (1,3-bis(diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-yliden) with the transition metal complexes [Ni(CO)4 ], [M(CO)6 ] (M=Cr, Mo, W), [Mn(CO)5 (Br)] and [(η5 -C5 H5 )Fe(CO)2 (I)] is reported. We demonstrate that N-heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi-bridged complex [{Ni(CO)2 (μ-Dipp2 NHSi)}2 ] (2), complexes [M(CO)5 (Dipp2 NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)3 (Dipp2 NHSi)2 (Br)] (9) and [(η5 -C5 H5 )Fe(CO)2 (Dipp2 NHSi-I)] (10). DFT calculations on several model systems [Ni(L)], [Ni(CO)3 (L)], and [W(CO)5 (L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal-ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ- and π-interaction depends significantly on the system under investigation. The σ-interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π-interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ-donor ligand, and contributions of π-symmetry play only a minor role for the NHC and NHSi co-ligands.

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.

Dimerisation of Dipiperidinoacetylene: Convenient Access to Tetraamino-1,3-Cyclobutadiene and Tetraamino-1,2-Cyclobutadiene Metal Complexes

The reaction of 1,2-dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2 ⋅dioxane afforded the 1,2,3,4-tetrapiperidino-1,3-cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2 , which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4-tetrapiperidino-3-buten-1-yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2 ⋅dioxane furnished the 1,3,4,4-tetrapiperidino-1,2-cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition-metal complexes containing this novel four-membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2 ]2 , and [(Me3 N)W(CO)5 ] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2 ] (6), and [(CBA)W(CO)5 ] (7). The molecular structures of all compounds 2-7 were determined by X-ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Interconversions between manganese silylene and silene complexes are reported, including those involving the first spectroscopically observed silene complexes with an SiH substituent, and their involvement in ethylene hydrosilylation is discussed.

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR(NHC)}] (8a–d: R = Ph or ⁿBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

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.

Nickel-catalysed direct C2-arylation of N-heterocyclic carbenes

Direct C2-arylation of NHCs to C2-arylated imidazolium salts (III) is achieved using Ni-catalysis. The dinuclear Ni(i) species I undergoes oxidative addition with ArX to give the Ni(ii) intermediate II. Reductive elimination delivers the arylation product III and regenerates the catalyst I.