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

Main-Group Metallocenophanes

Metallocenes with interlinked cyclopentadienide ligands are commonly referred to as ansa-metallocenes or metallocenophanes. These can have drastically different properties than their unbridged parent compounds. While this concept is best known for transition metals such as iron, it can also be adopted for many main-group elements. This review aims to summarize recent advances in the field of metallocenophanes based on main-group elements of group 2, group 13, group 14 and group 15, focusing on synthesis, structure and properties of these compounds.

Synthesis, characterization, crystal structure, α-glycosidase, and acetylcholinesterase inhibitory properties of 1,3-disubstituted benzimidazolium salts

Chloro-/fluorobenzyl-substituted benzimidazolium salts were synthesized from the reaction of 4-fluorobenzyl/2-chloro-4-fluorobenzyl-substituted benzimidazole and chlorinated aromatic hydrocarbons. They were characterized using various spectroscopic techniques (Fourier-transform infrared and nuclear magnetic resonance) and elemental analysis. In addition, the crystal structures of the complexes 1a -d and 2b were determined by single-crystal X-ray diffraction methods. These compounds were crystallized in the triclinic crystal system with a P-1 space group. The crystal packing of all complexes is dominated by O-H⋯Cl hydrogen bonds, which link the water molecules and chloride anions, forming a chloride-water tetrameric cluster. These synthesized salts were found to be effective inhibitors for α-glycosidase and acetylcholinesterase (AChE), with K_i values ranging from 45.77 ± 6.83 to 102.61 ± 11.56 µM for α-glycosidase and 0.94 ± 0.14 to 10.24 ± 1.58 µM for AChE. AChE converts acetylcholine into choline and acetic acid, thus causing the return of a cholinergic neuron to its resting state. Discovering AChE and α-glycosidase inhibitors is one of the important ways to develop new drugs for the treatment of Alzheimer's disease and diabetes.

Stabilization of a bismuth–bismuth double bond by anionic N-heterocyclic carbenes

Anionic N-heterocyclic carbenes have been employed for the isolation of the first dicarbene–dibismuth complex; the resulting dibismuthene features a trans-bent geometry with a Bi–Bi double bond and short intramolecular Bi–C_ipso contacts.

Reactivity of phospha-Wittig reagents towards NHCs and NHOs

Phospha-Wittig reagents, RPPMe3 (R = Mes* 2,4,6-tBu3-C6H2; MesTer 2,6-(2,4,6-Me3C6H2)-C6H3; DipTer 2,6-(2,6-iPr2C6H3)-C6H3), can be considered as phosphine-stabilized phosphinidenes. In this study we show that PMe3 can be displaced by NHCs or NHOs. Interestingly, phosphinidene-like reactivity results in a subsequent C(sp2)-H activation of the exocyclic CH2 group in NHOs. This concept was further extended to allyl-apended NHOs, which resulted in phosphine-substituted allyl species.

Carbodicarbene: geminal-Bimetallic Coordination in Selective Manner

The reaction of Pd(OAc)₂ with free carbodicarbene (CDC) generates a Pd acetate trinuclear complex 1 via intramolecular C(sp³ )-H bond activation at one of the CDC methyl side arms. The solid structure of 1 reveals the capability of CDC to facilitate a double dative bond with two palladium centers in geminal fashion. This is attributed to the chelating mode of CDC, which can frustrate π-conjugation within the CDC framework. Such effect maybe also amplified by ligand-ligand interaction. The formation of other gem-bimetallic Pd-Pd, Pd-Au, and Ni-Au provides further structural evidence for this proof-of-concept in selective installation. Structural analysis is supported by computational calculations based on state-of-the-art energy decomposition analysis (EDA) in conjunction with natural orbitals for chemical valence (NOCV) method.

Rare antimony(III) imidazole selone complexes: steric controlled structural and bonding aspects

Novel antimony(iii) imidazole selone complexes in a super crowded environment are reported for the first time. The super bulky selone antimony complexes, [{IPr*Se}(SbCl3)2] (1) and [{IPr*Se}(SbBr3)2] (2), were isolated from the reactions between IPr*Se (IPr*Se = [1,3-bis(2,6-diphenylmethylphenyl)imidazole selone]) and suitable antimony(iii) halides. 1 and 2 are dinuclear complexes with a Sb : Se ratio of 1 : 0.5 with an unusual coordination mode of selone. The molecules 1 and 2 consist of both Menshutkin-type Sbπaryl interactions and a Sb-Se coordination bond. However, the reaction between antimony(iii) halides and [(IPaul)Se] ([(IPaul)Se] = [1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole selone]) with a spatially defined steric impact gave the dinuclear complex [{(IPaul)Se}(SbCl3)]2 (3) and the mononuclear complex [{(IPaul)Se}(SbBr3)] (4) without Menshutkin-type interactions. The Sb : Se ratio in 3 and 4 is 1 : 1. Interestingly, the Menshutkin-type interaction was absent in 3 and 4 due to the efficient coordinating ability of the ligand [(IPaul)Se] with the Sb(iii) center compared to that of the super bulky ligand IPr*Se. The thermal property of these antimony selone complexes was also investigated. Density functional theory (DFT) calculations were carried out on the model systems [L(SbCl3)2] (1A), [L(SbCl3)] (1B), [L'(SbCl3)2] (1C), and [L'(SbCl3)] (1D), where L = [1,3-bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L' = [1,3-bis(phenyl)imidazole selone], to understand the nature of orbitals and bonding situations. The computed metrical parameters of 1A are in good agreement with the experimental values. Natural population analysis of the model system reveals that the natural charge and total population of antimony(iii) are comparable. The unequal interaction between selenium and antimony obtained using Wiberg bond indices (WBIs) is fully consistent with the findings of the single-crystal X-ray studies.

Cationic Phosphorus Compounds Based on a Bis(1-piperidinyl)-Substituted Carbodiphosphorane: Syntheses, Structures, and Csp3–H Activation

The use of the bis(1-piperidinyl)-substituted carbodiphosphorane (Ph₂(Pip)P)₂C (1) as an NCN ligand for the stabilization of phosphorus cations was studied. A simple ligand for halide exchange allowed the synthesis and isolation of a series of phosphorus monocations of the type [1-PR₂]⁺ (with R = Cl, Br, I, CyCl, Ph). These cations exhibit characteristic NMR and structural properties which nicely correlate with the charge at the central phosphorus atom and the interaction between the ligand and the PR₂ moiety. Halide abstraction from the monocations does not result in isolable dicationic compounds but in an unexpected intramolecular C_sp³–H activation in the piperidinyl group. DFT studies show that the selective activation of the CH₂ group next to the nitrogen atom instead of a CH group at the phenyl substituents proceeds via an iminium intermediate formed by hydride transfer from the carbon atom to the cationic phosphorus center. This observation clearly demonstrates the pronounced π acidity of the dicationic phosphorus species in comparison to compounds with a further π-donor substituent.

A Prelude to Biogermylene Chemistry*

The biological applications of germylenes remain unrealised owing to their unstable nature. We report the isolation of air-, water-, and culture-medium-stable germylene DPMGeOH (3; DPM=dipyrromethene ligand) and its potential biological application. Compound 3 exhibits antiproliferative effects comparable to that of cisplatin in human cancer cells. The cytotoxicity of compound 3 on normal epithelial cells is minimal and is similar to that of the currently used anticancer drugs. These findings provide a framework for a plethora of biological studies using germylenes and have important implications for low-valent main-group chemistry.

The [(NHC)B(H)C6 F5 ]+ Cations and Their [B](H)-CO Borane Carbonyls

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH₂ C₆ F₅ (NHC: IMes or IMe₄ ) gave the B-H containing [(NHC)B(H)C₆ F₅ ]⁺ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C₆ F₅ )CO]⁺ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

N-Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes

Chloride abstraction from the complexes [(η6 -p-cymene){(IDipp)P}MCl] (2 a, M=Ru; 2 b, M=Os) and [(η5 -C5 Me5 ){(IDipp)P}IrCl] (3 b, IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF ) in the presence of trimethylphosphine (PMe3 ), 1,3,4,5-tetramethylimidazolin-2-ylidene (Me IMe) or carbon monoxide (CO) afforded the complexes [(η6 -p-cymene){(IDipp)P}M(PMe3 )]BArF ] (4 a, M=Ru; 4 b, M=Os), [(η6 -p-cymene){(IDipp)P}Os(Me IMe)]BArF ] (5) and [(η5 -C5 Me5 ){(IDipp)P}IrL][BArF ] (6, L=PMe3 ; 7, L=Me IMe; 8, L=CO). These cationic N-heterocyclic carbene-phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low-field 31 P NMR resonances and short metal-phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal-phosphorus bond can be described in terms of an interaction between a triplet [(IDipp)P]+ cation and a triplet metal complex fragment ligand with highly covalent σ- and π-contributions. Crystals of the C-H activated complex 9 were isolated from solutions containing the PMe3 complex, and its formation can be rationalized by PMe3 dissociation and formation of a putative 16-electron intermediate [(η5 -C5 Me5 )Ir{P(IDipp)}I][BArF ], which undergoes C-H activation at one of the Dipp isopropyl groups and addition along the iridium-phosphorus bond to afford an unusual η3 -benzyl coordination mode.

Preferential N-H⋯:C[double bond splayed right] hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes

Hydrogen bonding plays a critical role in maintaining order and structure in complex biological and synthetic systems. N-heterocyclic carbenes (NHCs) represent one of the most versatile tools in the synthetic chemistry toolbox, yet their potential as neutral carbon hydrogen bond acceptors remains underexplored. This report investigates this capability in a strategic manner, wherein carbene-based hydrogen bonding can be assessed by use of ditopic NH-containing molecules. N-H bonds are unique as there are three established reaction modes with carbenes: non-traditional hydrogen bonding adducts (X-H⋯:C[double bond splayed right]), salts arising from proton transfer ([H-C[double bond splayed right]]⁺[X]^-), or amines from insertion of the carbene into the N-H bond. Yet, there are no established rules to predict product distributions or the strength of these associations. Here we seek to correlate the hydrogen bond strength of symmetric and asymmetric ditopic secondary amines with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, a representative NHC). In symmetric and asymmetric ditopic amine adducts both the solid-state (hydrogen bond lengths, NHC interior angles) and solution-state (¹H Δδ of NH signals, ¹³C signals of carbenic carbon) can be related to the pK _a of the parent amine.

Molecular Complexes Featuring Unsupported Dispersion-Enhanced Aluminum-Copper and Gallium-Copper Bonds

The reaction of the copper(I) β-diketiminate copper complex {(Cu(BDI^Mes))₂(μ-C₆H₆)} (BDI^Mes = N,N'-bis(2,4,6-trimethylphenyl)pentane-2,4-diiminate) with the low-valent group 13 metal β-diketiminates M(BDI^Dip) (M = Al or Ga; BDI^Dip = N,N'-bis(2,6-diisopropylphenyl)pentane-2,4-diiminate) in toluene afforded the complexes {(BDI^Mes)CuAl(BDI^Dip)} and {(BDI^Mes)CuGa(BDI^Dip)}. These feature unsupported copper-aluminum or copper-gallium bonds with short metal-metal distances, Cu-Al = 2.3010(6) Å and Cu-Ga = 2.2916(5) Å. Density functional theory (DFT) calculations showed that approximately half of the calculated association enthalpies can be attributed to London dispersion forces.

Donor-Stabilized Monocarba-Bridged Bis(cyclopentadienyl)alanes

Five monocarba-bridged bis(cyclopentadienyl)aluminum halide NHC and thione complexes and one monocarba-bridged bis(cyclopentadienyl)phosphanylalane NHC complex are reported. The former were synthesized by transmetalation of a C[1]magnesocenophane with the corresponding aluminum(III) chloride and aluminum(III) bromide donor adducts. The phosphanylalane complex was obtained by a subsequent functionalization of the corresponding bromoalane with lithium diphenylphosphide. All complexes were characterized in solution by multinuclear NMR spectroscopy and in the solid state by single crystal X-ray diffraction. Bonding energies of the NHC and thione ligands to the aluminum centres were estimated by DFT calculations.

The Brönsted Basicities of N-Heterocyclic Olefins in DMSO: An Effective Way to Evaluate the Stability of NHO-CO2 Adducts

A Brönsted basicity scale (∼24 pK units) for 85 commonly seen imidazole-, imidazoline-, triazole-, and thiazole-based N-heterocyclic olefins (NHOs) in DMSO was established using a well-examined computational model. The influence of substituents on the Brönsted basicities of these NHOs was investigated through basicity comparisons and rationalized by geometric analyses. The Gibbs energy (ΔG_r) of the reaction between NHO and CO₂ was also calculated, which linearly correlates with the basicity of the corresponding NHO, suggesting that the stability of NHO-CO₂ adducts can be evaluated by the basicity of NHOs and a stronger basicity leads to a more stable NHO-CO₂ adduct.

(N),C,N-Coordinated Heavier Group 13-15 Compounds: Synthesis, Structure and Applications

The aim of this review is to summarize recent achievements in the field of (N),C,N-coordinated group 13-15 compounds not only regarding their synthesis and structure, but mainly focusing on their potential applications. Relevant compounds contain various types of N-coordinating ligands built up on an ortho-(di)substituted phenyl platform. Thus, group 13 and 14 derivatives were used as single-source precursors for the deposition of semiconducting thin films, as building blocks for the preparation of high-molecular polymers with remarkable optical and chemical properties or as compounds with interesting reactivity in hydrometallation processes. Group 15 derivatives function as catalysts in the Mannich reaction, in the allylation of aldehydes or activation of CO₂ . They were used as transmetallation reagents in transition metal catalysed coupling reactions. The univalent species serve as ligands for transition metals, activate alkynes or alkenes and are utilized as catalysts in the transfer hydrogenation of azo-compounds.

Recyclable polyetheretherketone fiber-supported N-heterocyclic carbene catalysts for nucleophilic acylation of fluorobenzenes

We report for the first time a novel support of polyetheretherketone fiber for the synthesis of recyclable N-heterocyclic carbene (NHC) catalysts. The fiber catalysts were verified in nucleophilic acylation of fluorobenzenes with superior catalytic activities, and successfully recycled by a tiny pair of tweezers over 21 cycles with minimal loss of performance.

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

Mercury(II) complexes (Me-maloNHC_Dipp)HgCl (1b), (t-Bu-maloNHC_Dipp)HgCl (2b) and (t-Bu-maloNHC_Dipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl₂ and MeHgI. These molecules have been characterized by ¹H-NMR, ¹³C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHC_Dipp)HgCl}_n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHC_Dipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

1,2,4,5-Tetrakis(tetramethylguanidino)-3,6-diethynyl-benzenes: Fluorescent Probes, Redox-Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para-ethynyl substituents (CCSi(iPr)3 or CCH) to the organic electron-donor 1,2,4,5-tetrakis(tetramethylguanidino)-benzene is evaluated. The redox-properties and redox-state dependent fluorescence are evaluated, and dinuclear CuI and CuII complexes synthesized. The Lewis-acidic B(C6 F5 )3 substitutes the proton of the ethynyl -CCH groups to give new anionic -CCB(C6 F5 )3 - substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two-electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox-active guanidine that is neutral (instead of cationic) in its oxidized form.

Insight into the Decomposition Mechanism of Donor-Acceptor Complexes of EH2 (E = Ge and Sn) and Access to Germanium Thin Films from Solution

Electron-donating N-heterocyclic carbenes (Lewis bases, LB) and electron-accepting Lewis acids (LA) have been used in tandem to yield donor-acceptor complexes of inorganic tetrelenes LB·EH₂·LA (E = Si, Ge, and Sn). Herein, we introduce the new germanium (II) dihydride adducts ImMe₂·GeH₂·BH₃ (ImMe₂ = (HCNMe)₂C:) and ImⁱPr₂Me₂·GeH₂·BH₃ (ImⁱPr₂Me₂ = (MeCNⁱPr)₂C:), with the former complex containing nearly 40 wt % germanium. The thermal release of bulk germanium from ImMe₂·GeH₂·BH₃ (and its deuterated isotopologue ImMe₂·GeD₂·BD₃) was examined in solution, and a combined kinetic and computational investigation was undertaken to probe the mechanism by which Ge is liberated. Moreover, the thermolysis of ImMe₂·GeH₂·BH₃ in solution cleanly affords conformal nanodimensional layers of germanium as thin films of variable thicknesses (20-70 nm) on silicon wafers. We also conducted a computational investigation into potential decomposition pathways for the germanium(II)- and tin(II)-dihydride complexes NHC·EH₂·BH₃ (NHC = [(HCNR)₂C:]; R = 2,6-ⁱPr₂C₆H₃ (Dipp), Me, and H; and E = Ge and Sn). Overall, this study introduces a mild and convenient solution-only protocol for the deposition of thin films of Ge, a widely used semiconductor in materials research and industry.

Probing the potential of metalla-N-heterocyclic carbenes towards activation of enthalpically strong bonds

Density functional theory calculations are employed to explore the reactivity of metalla-N-heterocyclic carbenes (MNHCs) towards activation of a variety of small molecules (H2, NH3, PH3, SiH3Ph and CH4). All the MNHCs considered are found to have a stable singlet ground state and possess suitable electronic properties for their application in small molecule activation. The calculated energy barriers of E-H (E = H, C, N, Si, P) activation for the MNHCs are found to be in agreement with those of the experimentally evaluated cyclic alkyl(amino)carbene (CAAC) and diamidocarbenes (DACs), thereby indicating the activating effect of the incorporation of an ancillary metal center within a cyclic NHC, and highlighting a new, underexplored strategy in achieving difficult bond activations with carbenes.

Syntheses and Reactivity of New Zwitterionic Imidazolium Trihydridoborate and Triphenylborate Species

In this study, four new N-(alkyl/aryl)imidazolium-borates were prepared, and their deprotonation reactions were investigated. Addition of BH₃•THF to N-benzylimidazoles and N-mesitylimidazoles leads to imidazolium-trihydridoborate adducts. Ammonium tetraphenylborate reacts with benzyl- or mesityl-imidazoles with the loss of one of the phenyl groups yielding the corresponding imidazolium-triphenylborates. Their authenticity was confirmed by CHN analysis, ¹H-NMR, ¹³C-NMR, ¹¹B-NMR, FT-IR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). 3-Benzyl-imidazolium-1-yl)trihydridoborate, (HIm^Bn)BH₃, and (3-mesityl-imidazolium-1-yl)trihydridoborate, (HIm^Mes)BH₃, were also characterized by X-ray crystallography. The reactivity of these new compounds as carbene precursors in an effort to obtain borate-NHC complexes was investigated and a new carbene-borate adduct (which dimerizes) was obtained via a microwave-assisted procedure.

Copper-Catalyzed Modular Assembly of Polyheterocycles

Easy operation, readily accessible starting materials, and short syntheses of the privileged scaffold indeno[1,2-c]isoquinolinone were achieved by an multicomponent reaction (MCR)-based protocol via an ammonia–Ugi-four component reaction (4CR)/copper-catalyzed annulation sequence. The optimization and scope and limitations of this short and general sequence are described. The methodology allows an efficient construction of a wide variety of indenoisoquinolinones in just two steps.