NHC-Stabilized Triorganozincates: Syntheses, Structures, and Transformation to Abnormal Carbene-Zinc Complexes

Mesoionic Janus-Type Dicarbene: Complexes, Adducts, and Catalytic Studies

The preparations of homo- and hetero-bimetallic complexes as well as thiourea and selenourea derivatives of a mesoionic Janus-type N-heterocyclic dicarbene (diNHC) are reported. Analogues of its monocationic intermediate NHC have also been obtained for comparison. Using the main group adducts, the π-acceptor properties of both NHCs were determined using low temperature 77Se NMR spectroscopy completing their stereoelectronic profiling. Moreover, catalytic investigations reveal that the mesoionic dipalladium Janus-diNHC complex can be used in the sequential C2- and C5-arylation of 1-methylpyrrole for the preparation of non-symmetrical 2,5-diarylpyrroles.

Synthesis and Derivatives of Beryllium Triflate

Various pathways for the synthesis of beryllium triflate were investigated. The reaction of triflic acid or trimethylsilyl triflate with beryllium metal in liquid ammonia led to the formation of mono-, di-, and tetra-nuclear beryllium ammine complexes. Utilization of SMe2 as a solvent gave homoleptic Be(OTf)2. Various beryllium triflate complexes with N- and O-donor ligands as well as the complex anions [Be(OTf)4]2- and [Be2(OTf)6]2- were synthesized to evaluate the reactivity and solution properties of beryllium triflate. This showed that OTf- is not a weakly coordinating anion for Be2+ cations and that it exhibits good bridging properties.

Morphological Plasticity of LiCl Clusters Interacting with Grignard Reagent in Tetrahydrofuran

Ab initio molecular dynamics simulations are used to explore tetrahydrofuran (THF) solutions containing pure LiCl and LiCl with CH₃MgCl, as model constituents of the turbo Grignard reagent. LiCl aggregates as Li₄Cl₄, which preferentially assumes compact cubane-like conformations. In particular, an open-edge pseudotetrahedral frame is promoted by solvent-assisted Li-Cl bond cleavage. Among the Grignard species involved in the Schlenk equilibrium, LiCl prefers to coordinate MgCl₂ through μ₂-Cl bridges. Using a 1:1 Li:Mg ratio, the plastic tetranuclear LiCl cluster decomposes to a highly solvated mixed LiCl·MgCl₂ aggregate with prevalent Li-(μ₂-Cl)₂-Mg rings and linear LiCl entities. The MgCl₂-assisted disaggregation of Li₄Cl₄ occurs through transient structures analogous to those detected for pure LiCl in THF, also corresponding to moieties observed in the solid state. This study identifies a synergistic role of LiCl for the determination of the compounds present in turbo Grignard solutions. LiCl shifts the Schlenk equilibrium promoting a higher concentration of dialkylmagnesium, while decomposing into smaller, more soluble, mixed Li:Mg:Cl clusters.

Recent Advancement of the Current Aspects of g-C3 N4 for its Photocatalytic Applications in Sustainable Energy System

Being one of the foremost enticing and intriguing innovations, heterogeneous photocatalysis has also been used to effectively gather, transform, and conserve sustainable sun's radiation for the production of efficient and clean fossil energy as well as a wide range of ecological implications. The generation of solar fuel-based water splitting and CO₂ photoreduction is excellent for generating alternative resources and reducing global warming. Developing an inexpensive photocatalyst can effectively split water into hydrogen (H₂ ), oxygen (O₂ ) sources, and carbon dioxide (CO₂ ) into fuel sources, which is a crucial problem in photocatalysis. The metal-free g-C₃ N₄ photocatalyst has a high solar fuel generation potential. This review covers the most recent advancements in g-C₃ N₄ preparation, including innovative design concepts and new synthesis methods, and novel ideas for expanding the light absorption of pure g-C₃ N₄ for photocatalytic application. Similarly, the main issue concerning research and prospects in photocatalysts based g-C₃ N₄ was also discussed. The current dissertation provides an overview of comprehensive understanding of the exploitation of the extraordinary systemic and characteristics, as well as the fabrication processes and uses of g-C₃ N₄ .

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.

Insights into the past and future of Janus-di-N-heterocyclic carbenes

Janus di-N-heterocyclic carbene (NHC) ligands are a subclass of poly-NHCs that feature coordination to two transition metals in a facially opposed manner. The combination of the structural features of Janus type ligands, with the properties conferred by the NHC ligands, has conferred Janus-di-NHCs with privileged attributes for their use in diverse areas of research, such as homogeneous catalysis, materials chemistry and supramolecular chemistry. In molecular chemistry, Janus di-NHCs constitute one of the most useful chemical platforms for constructing dimetallic structures, and this includes both homo- and hetero-dimetallic compounds. This review aims to cover the most relevant advances in the use of Janus-di-NHCs during the last 15 years, by classifying them according to their specific structural features.

Heterobimetallic Coinage Metal-Ruthenium Complexes Supported by Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC, WCA=B(C₆F₅)₃, NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were used for the preparation of silver(I) or copper(I) WCA‐NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono‐ and dicarbene complexes with solvated lithium counterions [Li(THF)_n]⁺ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA‐NHC)M(PPh₃)] and [(WCA‐NHC)M(η²‐toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA‐NHC)M(μ‐X)₂Ru(PPh₃)(η⁶‐p‐cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA‐NHC transmetalation.

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.

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.

Towards Extended Zinc Ethylsulfinate Networks by Stepwise Insertion of Sulfur Dioxide into Zn-C Bonds

The ability to utilize polluting gases in efficient metal-mediated transformations is one of the most pressing challenges of modern chemistry. Despite numerous studies on the insertion of SO₂ into M-C bonds, the chemical reaction of SO₂ with organozinc compounds remains little explored. To fill this gap, we report here the systematic study of the reaction of Et₂ Zn towards SO₂ as well as the influence of Lewis bases on the reaction course. Whereas the equimolar reaction provided a novel example of a structurally characterized organozinc ethylsulfinate compound of general formula [(EtSO₂ )ZnEt]_n , the utilization of an excess of SO₂ led to the formation of the zinc(II) bis(ethylsulfinate) compound [(EtSO₂ )₂ Zn]_n . Moreover, we have discovered that the presence of N-donor Lewis bases represents an efficient tool for the preparation of extended zinc ethylsulfinates, which in turn led to the formation of 1D [(EtSO₂ ZnEt)₂ (hmta)]_n and 2D [((EtSO₂ )₂ Zn)₂ (DABCO)]_n ⋅solv (in which solv=THF or toluene, hmta= hexamethylenetetramine, and DABCO=1,4-diazabicyclo[2.2.2]octane) coordination polymers, respectively. The results of DFT calculations on the reactivity of SO₂ towards selected Zn-C reactive species as well as the role of an N-donor Lewis base on the stabilization of the transition states complement the discussion.

Synergic Deprotonation Generates Alkali-Metal Salts of Tethered Fluorenide-NHC Ligands Co-Complexed to Alkali-Metal Amides

Synergic combinations of alkali-metal hydrocarbyl/amide reagents were used to synthesise saturated N-heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li2 derivatives displayed a bridging amide between two Li atoms within the fluorenide-NHC pocket, whereas the Na2 and K2 analogues displayed extended solid-state structures with the fluorenide-NHC ligand chelating one alkali metal centre.

Cationic abnormal N-heterocyclic carbene ruthenium complexes as suitable precursors for the synthesis of heterobimetallic compounds

Heterobimetallic dicarbene compounds are easily prepared from cationic imidazolyl-based abnormal N-heterocyclic carbene ruthenium complexes which allow facile metallation.

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.

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.

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.

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.

Abnormal carbene–silicon halide complexes

Reaction of the anionic NHDC ligand, [:C{[N(2,6-Prⁱ₂C₆H₃)]₂CHCLi}]_n (1), with SiCl₄ gives the trichlorosilyl-substituted NHC ligand (7). Abnormal carbene–SiCl₄ complex (8) can be conveniently synthesized by combining 7 with HCl·NEt₃. Meanwhile, 7 may react with CH₂Cl₂ in warm hexane, giving the abnormal carbene-complexed SiCl₃⁺ cation (9). The structure and bonding of 9 have also been probed by DFT computations.

Carbon-based two electron σ-donor ligands beyond classical N-heterocyclic carbenes

Recent advances in N-heterocyclic carbene-derived carbon-based two electron σ-donor ligands are presented in this perspective.

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

Advancing the rational design of main-group N-heterocyclic carbene complexes, this study reports the synthesis, X-ray crystallographic and NMR spectroscopic characterisation of a novel series of Ga complexes containing neutral or anionic NHC ligands using the unsaturated carbene IPr (IPr = 1,3-bis-(2,6-di-isopropylphenyl)imidazol-2-ylidene). Starting from normal adduct GaR3·IPr (1) (R = CH2SiMe3), the addition of polar LiR led to the formation of NHC-stabilised gallate species IPr·LiGaR4 (2), resulting from co-complexation of the single-metal species. Contrastingly, reversing the order of addition of these organometallic reagents, by treating unsaturated free IPr, first with LiR followed by GaR3, furnished novel heteroleptic gallate (THF)2Li[:C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (3), which contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Ga and through its normal C2 position to Li. Electrophilic interception studies of 3 using methyl triflate (MeOTf), methanol and imidazolium salt (IMes·HCl) led to the isolation and structural elucidation of the two novel neutral abnormal NHC (aNHC) complexes [CH3C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (4) and aIPr·GaR3 (5) (aIPr = HC{[N(2,6-iPr2C6H3)]2CHC}). These studies disclose the preference of the anionic IPr ligand present in 3 to react with electrophiles via its C2 position, leaving its Ga-C4 bond intact. Abnormal complex 5 can also be accessed by a thermally induced rearrangement of its normal isomer 1. Combining NMR spectroscopic and kinetic studies with DFT calculations, new light has been shed on this intriguing transformation, which suggests that it occurs via a dissociative mechanism, highlighting the importance of the donor ability of the solvent used in these thermal isomerizations as well as the steric bulk of the substituents on the NHC and the Ga reagent. These findings intimate that relief of the steric hindrance around Ga by forming an abnormal complex is a key driving force behind these rearrangements.

Ru–Ag and Ru–Au dicarbene complexes from an abnormal carbene ruthenium system

The first complexes containing an anionic dicarbene connected to two different d-block elements are reported. The synthetic pathway entails metalation and transmetalation reactions, starting from a cationic abnormal Ru-NHC complex.

Synthesis of unsymmetrical N-carboranyl NHCs: directing effect of the carborane anion

The selective synthesis of unsymmetrically substituted anionic C-2 and dianionic C-2/C-5 N-carboranyl NHCs is reported, revealing a directing effect conveyed by the anionic carborane substituent.

A spiroborate-based anionic bis-N-heterocyclic carbene

A twisted bis-benzimidazolium salt was isolated from the reaction of 5,6-dihydroxyl-benzimidazolium and benzenediboronic acid. Deprotonation of the azolium salt led to the formation of a spiroborate-linked bis-NHC, which was further transformed into the corresponding diborane adduct and the di-Rh complex.