The Influence of C(sp3)H–Selenium Interactions on the 77Se NMR Quantification of the π-Accepting Properties of Carbenes

Selenium NMR has become a standard tool for scaling the π-accepting character of carbenes. Herein, we highlight that non-classical hydrogen bonding (NCHB), likely resulting from hyperconjugation, can play a significant role in the carbene–selenium ⁷⁷Se NMR chemical shift, thus triggering a non-linear behavior of the Se-Scale.     

Synthetically Versatile Nitrogen Acyclic Carbene Stabilized Gold Nanoparticles

N-heterocyclic carbenes (NHCs) have received significant attention as gold nanoparticle stabilizers due to their strong binding affinity towards gold. However, their tunability is limited by the difficulty in obtaining nonsymmetric NHCs. In this regard, N-acyclic carbenes (NACs) are attractive alternatives due to their high synthetic versatility, allowing easy tuning of their steric and electronic properties towards specific applications. This work reports the first series of stable and monodisperse NAC-functionalized gold nanoparticles. These particles with sizes ranging 3.8 to 11.6 nm were characterized using NMR, UV/Vis and TEM. The nanoparticles display good stability at elevated temperatures and for extended periods both dried or dispersed in a medium, as well as in the presence of exogenous thiols. Importantly, these NAC-stabilized gold nanoparticles offer a promising and versatile alternative to NHC-stabilized gold nanoparticles.     

Ambident Reactivity of Imidazolium Cations as Evidence of the Dynamic Nature of N-Heterocyclic Carbene-Mediated Organocatalysis

This work reveals ambident nucleophilic reactivity of imidazolium cations towards carbonyl compounds at the C2 or C4 carbene centers depending on the steric properties of the substrates and reaction conditions. Such an adaptive behavior indicates the dynamic nature of organocatalysis proceeding via a covalent interaction of imidazolium carbenes with carbonyl substrates and can be explained by generation of the H-bonded ditopic carbanionic carbenes.     

Carbene-Stabilized Exceptional Silicon Halides

The isolation of stable carbenes, such as N-heterocyclic carbenes, cyclic alkyl amino carbenes, and abnormal N-heterocyclic carbenes, has encouraged synthetic chemists to use them as neutral ligands for stabilizing unusual compounds with main group elements. In this Minireview, an overview is provided of the recent developments in the chemistry of carbene-stabilized silicon halides, in which the silicon atoms are in low oxidation state. The structural diversities emanating from the differences in the HOMO–LUMO gaps of various carbenes will also be discussed.     

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a – h were prepared by the reaction of readily available N,N′-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a – h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC-gold(I) complexes 3 – 10 . These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded-ring architecture.     

Gold Carbene or Carbenoid: Is There a Difference?

By reviewing the recent progress on the elucidation of the structure of gold carbenes and the definitions of metal carbenes and carbenoids, we recommend to use the term gold carbene to describe gold carbene‐like intermediates, regardless of whether the carbene or carbocation extreme resonance dominates. Gold carbenes, because of the weak metal‐to‐carbene π‐back‐donation and their strongly electrophilic reactivity, could be classified into the broader family of Fischer carbenes, although their behavior and properties are very specific.     

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ‐donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus‐based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon‐based ligands (which are not NHCs), and which feature even stronger σ‐donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.     

An Imidazolylidene-Based Mesoionic Carbene–Mn(I) Complex and Its Catalytic Potential in N-Heteroarene Hydrogenation

Herein we report the first mesoionic carbene (MIC)-Mn(I) complex Mn-bim-MIC^imz derived from imidazolylidene motif. Structurally the octahedral Mn(I) complex Mn-bim-MIC^imz was assembled with an anionic benzimidazolato-anchored imidazolylidene MIC-based bidentate ligand ( bim-MIC^imz ) and four CO ligands, as supported by detailed characterization using NMR and FTIR spectroscopy, mass spectrometry, and single crystal X-ray diffraction study. We reckoned that the bim-MIC^imz ligand would provide a robust and stable bonding with the Mn(I) centre, and also enhance electron density at the Mn(I) centre through its stronger σ-donating/weaker π-accepting property. These structural and electronic attributes triggered to exploit Mn-bim-MIC^imz in catalytic hydrogenation of N-heteroarenes, where efficient hydride (Mn−H) delivery is a key step.     

Investigations on Novel 1,3-Diazetidine Based Four-Membered N-Heterocyclic Carbenes

Attempts towards the synthesis of two novel four-membered 1,3-diazetidine based N-heterocyclic carbenes (NHCs) containing an organic backbone with a carbonyl functionality were undertaken. These carbenes cannot be isolated but the respective carbene dimers are obtained in quantitative yield which undergo a degradation and rearrangement sequence upon thermal exposure. Some of the species involved in these thermal reactions could be isolated and characterized, others were observed by mass spectrometric experiments. Ab initio and density functional theory (DFT) calculations provide a mechanistic rationale for the experimental observations. Since dimerization is strongly favored, classic carbene trapping reactions remain a goal to achieve.     

Frontispiece: BIAN-NHC Ligands in Transition-Metal-Catalysis: A Perfect Union of Sterically Encumbered,Electronically Tunable N-Heterocyclic Carbenes?

The discovery of NHCs (NHC = N-heterocyclic carbenes) as ancillary ligands in transition-metal-catalysis ranks as one of the most important developments in synthesis and catalysis. It is now well-recognized that the strong σ-donating properties of NHCs along with the ease of scaffold modification and a steric shielding of the N-wingtip substituents around the metal center enable dramatic improvements in catalytic processes, including the discovery of reactions that are not possible using other ancillary ligands. In this context, although the classical NHCs based on imidazolylidene and imidazolinylidene ring systems are now well-established, recently tremendous progress has been made in the development and catalytic applications of BIAN-NHC (BIAN = bis(imino)acenaphthene) class of ligands. The enhanced reactivity of BIAN-NHCs is a direct result of the combination of electronic and steric properties that collectively allow for a major expansion of the scope of catalytic processes that can be accomplished using NHCs. BIAN-NHC ligands take advantage of (1) the stronger σ-donation, (2) lower lying LUMO orbitals, (3) the presence of an extended π-system, (4) the rigid backbone that pushes the N-wingtip substituents closer to the metal center by buttressing effect, thus resulting in a significantly improved control of the catalytic center and enhanced air-stability of BIAN-NHC-metal complexes at low oxidation state. Acenaphthoquinone as a precursor enables facile scaffold modification, including for the first time the high yielding synthesis of unsymmetrical NHCs with unique catalytic properties. Overall, this results in a highly attractive, easily accessible class of ligands that bring major advances and emerge as a leading practical alternative to classical NHCs in various aspects of catalysis, cross-coupling and C−H activation endeavors.     

Indol-2-ylidene (IdY): Ambiphilic N-Heterocyclic Carbene Derived from Indole**

The synthesis of ambiphilic N-heterocyclic carbene ligand, indol-2-ylidene (IdY, A ), is described. A series of indolenium precursors ( 2 a – f ) were prepared on a gram scale in good yields. Trapping experiments with elemental selenium, [RhCl(cod)]₂ and CuCl provided the expected carbene adducts. Further computational and spectroscopic studies supported the ambiphilicity of IdY, which lies between cyclic (alkyl)(amino)carbenes (CAAC-5) and cyclic (amino)(aryl)carbene (CAArC). The copper complexes ( 6 ) show high percent buried volume (% V_bur = 58.1) and allow for carboboration of terminal alkynes within 30 minutes in a demonstration of synthetic utility with good yields and high regioselectivity.     

Normal‐to‐Abnormal NHC Rearrangement of AlIII,GaIII, and InIII Trialkyl Complexes: Scope,Mechanism, Reactivity Studies,and H2 Activation

The present contribution reports experimental and theoretical mechanistic investigations on a normal‐to‐abnormal (C2‐to‐C4‐bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe₃ (nItBu=1,3‐di‐tert‐butylimidazol‐2‐ylidene; M=Al, Ga, In; 1 a – c ) readily rearrange to quantitatively afford the corresponding C4‐bonded complexes (aItBu)MMe₃ ( 4 a – c ), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC‐to‐aNHC process converting the Al‐nNHC ( 1 a ) to its aNHC analogue 4 a . A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF–AlMe₃. The reaction of benzophenone with (nItBu)AlMe₃ afforded the zwitterionic species (aItBu)(CPh₂‐O‐AlMe₃) ( 6 ), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)₃ Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H₂ under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.     

Electronic and ligating properties of carbocyclic carbenes: A theoretical investigation

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength–related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.     

N‐Heterocyclic Carbenes and Imidazole‐2‐thiones as Ligands for the Gold(I)‐Catalysed Hydroamination of Phenylacetylene

Gold(I) complexes of 1‐[1‐(2,6‐dimethylphenylimino)alkyl]‐3‐(mesityl)imidazol‐2‐ylidene (C^Imine_R), 1,3‐dimesitylimidazol‐2‐ylidene (IMes) and of the corresponding thione derivatives (S^Imine_R and IMesS) were prepared and structurally characterised. The solid‐state structure of the C^Imine_R and S^Imine_R gold(I) complexes showed monodentate coordination of the ligand and a dangling imine group that could bind reversibly to the metal centre to stabilise otherwise unstable catalytic intermediates. Interestingly, reaction of C^Imine_tBu with [AuCl(SMe₂)] led to the formation of [(C^Imine_tBu)AuCl], which rearranges upon crystallisation into the unusual complex cation [(C^Imine_tBu)₂Au]⁺, with AuCl₂^? as the counterion. The activity of the gold complexes in the hydroamination of phenylacetylene with substituted anilines was tested and compared to control catalyst systems. The best catalytic performance was obtained with [(C^Imine_tBu)AuCl], with the exclusive formation of the Markovnikov addition product in excellent yield (>95 %) regardless of the substituents on aniline.     

Rhodium(I)-catalyzed [3+2+2]-cyclization of alkenyl Fischer carbene complexes with methyl buta-2,3-dienoate

A variety of alkenyl chromium Fischer carbene complexes react with methyl buta-2,3-dienoate in the presence of a rhodium catalyst to afford substituted cycloheptene derivatives 7, along with minor amounts of cyclopentene derivatives 6. Through this [3+2+2]-cyclization two allene units and one alkenyl carbene ligand are assembled in a selective manner.     

Bonding Analysis of N‐Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition‐Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal–ligand bonding in transition‐metal complexes that contain imidazole (IMID), imidazol‐2‐ylidene (nNHC), or imidazol‐4‐ylidene (aNHC). The calculated complexes are [Cl₄TM(L)] (TM=Ti, Zr, Hf), [(CO)₅TM(L)] (TM=Cr, Mo, W), [(CO)₄TM(L)] (TM=Fe, Ru, Os), and [ClTM(L)] (TM=Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID<nNHC<aNHC. The energy levels of the carbon σ lone‐pair orbitals suggest the trend aNHC>nNHC>IMID for the donor strength, which is in agreement with the progression of the metal–ligand bond‐dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand–metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)₅TM(L)] (L=nNHC, aNHC, IMID) with phosphine complexes (L=PMe₃ and PCl₃) shows that the phosphine ligands are weaker σ donors and better π acceptors than the NHC tautomers nNHC, aNHC, and IMID.