Anionic N-heterocyclic Carbene Research Articles

Anionic N-Heterocyclic Carbenes from Mesoionic Imidazolium-4-pyrrolides: The Influence of Substituents, Solvents, and Charge on their 77Se NMR Chemical Shifts.

Mesoionic compounds are the starting material for the synthesis of unique anionic N-heterocyclic carbenes. Herein, mesoionic imidazolium pyrrolides synthesized from pyrrole-2-carbaldehyde via various N-alkyl-4-pyrroyl-imidazoles are described. These were converted into nine new 4-(pyrrol-2-yl)-substituted imidazolium salts and transformed into the mesoionic title compounds using an anion exchange resin. The DFT-calculated (B3LYP/6-311++G**) CREF values indicate a great potential for the formation of anionic N-heterocyclic carbenes by deprotonation, which were generated and reacted with selenium to obtain selenoureas. The 77Se NMR shifts investigated under systematic variation of conditions are dependent on the substitution pattern (ΔδSe = 133 ppm) and the steric demand of the substituents. Solvent dependencies of the 77Se NMR shifts were investigated applying toluene-d8, THF-d8, CDCl3, CD2Cl2, pyridine-d5, acetone-d6, DMSO-d6, CD3CN, AcOD, and MeOD. The influences of the referencing method on the 77Se shifts using external or internal Me2Se or Ph2Se2 and solvent can add up to ΔδSe = ca. 80 ppm. In addition, we observed a temperature dependence of both the selenoureas and the reference reagent Ph2Se2 as well as a 77Se shift difference of the analyte caused by interaction with internally added Ph2Se2. The negative charge of deprotonated selenoureas shifts the values by an additional -20 ppm.

Imidazolium Dicyanomethylides as N‐Ylide Precursors of Anionic N‐Heterocyclic Carbenes

AbstractIn the past, the use of nitrogen ylides focused primarily on cycloadditions, where the 1,3‐dipole was used as a building block for the formation of heterocycles such as pyrroles or pyrrolinones. In this work, we present a new perspective on imidazolium dicyanomethylides with their synthesis and utilization as anionic N‐heterocyclic carbenes (NHCs). The values of relative carbene formation energies (CREF) for a wide range of substituted imidazolium ylides unknown in the literature showed promising properties. In situ trapping reactions of these anionic NHCs with selenium led to anionic, water‐soluble compounds that exhibited remarkable coordination behavior according to X‐ray structure analyses. In addition, the 77Se shifts of the investigated compounds were measured to draw conclusions about their π‐acceptance.

Flexible NHC-aryloxido aluminum complex and its zwitterionic imidazolium aluminate precursor in ring-opening polymerization of ε-caprolactone.

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe2 complex [Al(L)Me2] (2), following a stepwise synthesis from the proligand [HO-4,6-tBu2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH2Br; Ar = 2,6-iPr2-C6H3 (Dipp)] and AlMe3via the zwitterionic intermediate [Al(LH)Me2Br] (1). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT-1H NMR spectroscopic analysis. The ∠O-Al-C (ca. 100.5°) bite angle is also wider than the ∠O-Ti-C (ca. 80.6°) as seen in our recently reported Ti complex [Ti(L)(NMe2)2Br]. DFT analysis showed that the CNHC-Al bond is significantly ionic, as is the CNHC-Ti bond. Both 1 and 2 are active in the ring-opening polymerization (ROP) of ε-caprolactone (CL). 2, similar to [Ti(L)(NMe2)2Br], exhibits bifunctional MLC-type monomer activation, but only at an elevated temperature. However, the 2/BnOH combination is catalytically active at room temperature, likely through a zwitterionic [Al(LH)Me2(OBn)]. The 1/BnOH combination follows a similar mechanism but surprisingly at a faster rate.

Ruthenium Metathesis Catalysts Bearing Anionic N-Heterocyclic Carbenes: A Computational Study on Failed Approaches to Their Synthesis

Despite recent efforts, the synthesis of ruthenium metathesis catalysts bearing anionic N-heterocyclic carbenes has not been successful. Using a computational density functional theory approach, we show that previously synthesized anionic N-heterocyclic carbenes are, with one exception, too weak Lewis bases to replace a chloride ion in the structure of common ruthenium metathesis catalysts. On the other hand, the transmetalation from silver or copper complexes is not feasible because of the very high affinity of anionic N-heterocyclic carbenes to these transition metals in comparison to ruthenium. We also show that the heterobimetallic Ag/Ru and Cu/Ru complexes obtained during such transmetalation are unlikely to catalyze olefin metathesis but may be good candidates for other catalysts as a result of facile dissociation of the phosphine moiety to produce an active Ru complex.

Anionic N-heterocyclic carbenes featuring weakly coordinating perfluoroalkylphosphorane moieties.

The anionic 1-methyl-3-(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate 3 and the 1,3-bis(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate dianion 4 were obtained in high yield by deprotonation of {(C2F5)3PF2}-methylimidazole 1 and the {(C2F5)3PF2}2-imidazolate anion 2. Carbenes 3 and 4 are first examples for a novel class of NHCs carrying weakly coordinating anions (WCA-NHCs). First reactions of these new ligands with elemental selenium and chloro(phosphine)gold(I) complexes to result in an anionic selenium adduct (5) and WCA-NHC gold complexes (6 and 7) have been undertaken. The structural and spectroscopic properties of these NHC derivatives in conjunction with data from quantum chemical calculations provide an insight into the electronic and steric properties of the WCA-NHCs 3 and 4. Especially the combination of properties such as weakly coordinating periphery combined with the coordinative carbene center, negative charge, large buried volume (%Vbur), and strong σ-donor as well as efficient π-acceptor ability render NHCs 3 and 4 unique and promising new ligands.

Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties.

The 1‐methyl‐3‐(tricyanoborane)imidazolin‐2‐ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)3‐methylimidazole adduct 1. Regarding charge and stereo‐electronic properties, anion 2 closes the gap between well‐known neutral NHCs and the ditopic dianionic NHC, the 1,3‐bis(tricyanoborane)imidazolin‐2‐ylidenate dianion (IIb). The influence of the number of N‐bonded tricyanoborane moieties on the σ‐donating and π‐accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3‐dimethylimidazolin‐2‐ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)4] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V bur) and σ‐donor and π‐acceptor abilities of NHCs can be effectively fine‐tuned via the number of tricyanoborane substituents.

Pyrazoles in the Intersection of Mesomeric Betaines and N-Heterocyclic Carbenes: Formation of NHC Selenium Adducts of Pyrazolium-4-aminides

AbstractStarting from 4-nitropyrazole, eight mesoionic pyrazolium-4-aminides were prepared by a six-step reaction sequence. The deprotonation of 1,2-disubstituted 4-amido-1H-pyrazolium salts by an anion exchange resin in its hydroxide form is the final step of the synthesis. A tautomeric equilibrium between the mesoionic compounds (pyrazolium-4-aminides) and N-heterocyclic carbenes (pyrazol-3-ylidenes) can be formulated; however, the NHC tautomers were not detected by means of NMR spectroscopy in polar aprotic solvents such as DMSO-d 6 or MeCN-d 3. Apart from tautomerism, anionic N-heterocyclic carbenes can be formulated as a result of a deprotonation of the mesoionic compounds­. Trapping reactions were performed with selenium, which resulted in the formation of pyrazole-3-selenones. Methylation at the selenium atom gave the corresponding 3-(methylselanyl)-4-amido-1H-pyrazolium salts, which were deprotonated to give new mesomeric betaines, 3-(methylselanyl)-1H-pyrazolium-4-aminides as unique compounds­. DFT-calculations as well as 77Se NMR spectroscopic measurements were carried out.

Cover Feature: Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N‐Heterocyclic Carbenes (Chem. Eur. J. 4/2022)

Anionic N-heterocyclic carbenes that bear a weakly coordinating fluoroborate moiety have been used to prepare a new class of anionic imidazolin-2-thione and -selone ligands; their coordination chemistry towards rhodium and iridium was explored to reveal their ability to develop secondary arene–metal interactions with the “wings” of the carbene ligands. More information about these new additions to the family of chalcogenolate ligands and their potential future application in coordination chemistry, bioinorganic chemistry and homogeneous catalysis can be found in the Research Article by M. Tamm et al. (DOI: 10.1002/chem.202104139).

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(η 2‐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)(η 6‐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.

Sydnone Methides-A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N-Heterocyclic Carbenes.

Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π‐electron rich anionic N‐heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C−C coupling reactions with 1‐fluoro‐4‐iodobenzene under Pd(PPh3)4 and CuBr catalysis. 77Se NMR resonance frequencies and 1 J C4‐Se as well as 1 J C4‐H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N‐heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13C NMR spectroscopy at −40 °C which is extremely shifted upfield in comparison to classical N‐heterocyclic carbenes.

Determination of the π-Accepting Properties of Borate-, Aluminate-, and Gallate-Functionalized N-Heterocyclic Carbenes by 77Se NMR Spectroscopy.

Anionic N-heterocyclic carbenes with weakly coordinating borate, aluminate, and gallate moieties of the type [(F5C6)3E-NHC]- (E = B, Al, Ga) were isolated as lithium salts by the lithiation of 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp) followed by the addition of E(C6F5)3 (E = B, Al, Ga). Treatment with elemental selenium afforded the lithium salts of the corresponding anionic selenourea derivatives [{(F5C6)3E-NHC}Se]- (NHC = IMes, E = B; NHC = IDipp, E = B, Al, Ga), which were examined, among other things, by means of 77Se{1H} NMR spectroscopy to assess the π-accepting properties of the WCA-NHC ligands in comparison with their neutral NHC congeners.

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

The bismuth dichloride complex (WCA-IDipp)BiCl2, which bears an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA = B(C6F5)3, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene), was prepared by salt metathesis reaction between BiCl3 and the lithium salt (WCA-IDipp)Li·toluene. Subsequent two-electron reduction with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine afforded the dibismuthene (WCA-IDipp)2Bi2, which displays a bismuth-bismuth double bond.

Halogen Complexes of Anionic N-Heterocyclic Carbenes.

The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA‐NHC) reacted with iodine, bromine, or CCl4 to afford the zwitterionic 2‐halogenoimidazolium borates (WCA‐NHC)X (X=I, Br, Cl; WCA=B(C6F5)3, B{3,5‐C6H3(CF3)2}3; NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene, or NHC=IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene). The iodine derivative (WCA‐IDipp)I (WCA=B(C6F5)3) formed several complexes of the type (WCA‐IDipp)I⋅L (L=C6H5Cl, C6H5Me, CH3CN, THF, ONMe3), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA‐IDipp)I(NHC)] and [PPh4][(WCA‐IDipp)I(WCA‐NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh4][(WCA‐IDipp)2Br] was isolated as a rare example of a hypervalent (10‐Br‐2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

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

Mercury(II) complexes (Me-maloNHCDipp)HgCl (1b), (t-Bu-maloNHCDipp)HgCl (2b) and (t-Bu-maloNHCDipp)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-maloNHCDipp)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-maloNHCDipp)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.

Group 13 Element Trihalide Complexes of Anionic N-Heterocyclic Carbenes.

A series of group 13 complexes of the general type [{(WCA‐IDipp)EX3}Li(solv)] (E=B, Al, Ga, In; X=Cl, Br) that bear an anionic N‐heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA‐IDipp, WCA=B(C6F5)3 and IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were prepared by the reaction of the respective group 13 trihalides (EX3) with the lithium salt [(WCA‐IDipp)Li ⋅ toluene]. The molecular structures of the BBr3, AlCl3, AlBr3, GaCl3 and InCl3 adducts were established by X‐ray diffraction analyses, revealing the formation of coordination polymers linked by halide‐lithium interactions, except for the indium derivative, which consists of isolated [Li(THF)4]+ and [(WCA‐IDipp)InCl3]− ions in the solid state.

Betaine–N‐Heterocyclic Carbene Interconversions of Quinazolin‐4‐One Imidazolium Mesomeric Betaines. Sulfur, Selenium, and Borane Adduct Formation

Reaction of N‐alkylated imidazoles with 2‐chloro‐4‐quinazolinone gave mesomeric betaines, 2‐(1‐alkyl‐1H‐imidazolium‐3‐yl)quinazolin‐4‐olates, for which three tautomeric forms of N‐heterocyclic carbenes (NHCs) can be formulated, in addition to an anionic NHC after deprotonation. The NHC tautomers were trapped with sulfur, selenium, triethylborane, and triphenylborane as thiones, selenones and borane adducts, respectively. We obtained two isomers of the cyclic borane adducts, diazaboroloquinazolinones with [1,5‐a] and [5,1‐b]‐type fusion between the quinazolinone and the diazaborole rings. They correspond to two different NHC tautomers and to the anionic NHC derived thereof. The third NHC tautomer was trapped as a non‐cyclic adduct with tris(pentafluorophenyl)borane by coordination to the quinazoline oxygen atom. 2D 1H‐15N HMBC experiments of 15N‐labeled quinazolinone fragments, quantitative measurements of long‐range 1H‐15N coupling constants (JHN), and five X‐ray single crystal analyses have been carried out for the structure elucidations and to gain insight into the NMR spectroscopic properties of these compounds.

Chalcogen complexes of anionic N-heterocyclic carbenes.

Several group 16 adducts of the type [(WCA-IDipp)E]Li(solv.) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA = B(C6F5)3, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, E = O, S, Se, Te) were prepared. This was achieved by deprotonation of the corresponding ketone (IDipp)O or thione (IDipp)S with n-BuLi and subsequent reaction with B(C6F5)3 or by direct reaction of [WCA-IDipp]Li(toluene) with elemental Se or Te. The oxygen, sulfur and selenium adducts can be protonated to give derivatives (WCA-IDipp)EH (E = O, S, Se), whereas the oxidation of the sulfur and selenium adducts yielded neutral disulfide and diselenide species (WCA-IDipp)2E2 (E = S, Se).

XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands.

(Arylimido)vanadium(V) dichloride complexes containing anionic N-heterocyclic carbene (NHC) ligands that contain weakly coordinating B(C6F5)3 moieties (WCA-NHC) of the type [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3, Ar′ = 2,6-iPr2C6H3) showed significant catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and AliBu3); the activity by the 5–MAO catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min–1) is the highest among those reported using the other (imido)vanadium(V) complexes in the presence of MAO, and the 5–AliBu3 catalyst showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200 min–1). The V K-edge X-ray absorption near-edge structure (XANES) analyses (in toluene) strongly suggest the formation of certain vanadium(III) species by reduction with AliBu3 accompanying structural changes; the EXAFS analysis suggests the presence of the arylimido ligand and one V–Cl bond (2.34 ± 0.04 Å), which is longer than those [2.1901(8)–2.2462(8) Å] in the reported (imido)vanadium(V) complexes. The XANES analysis of [V(NAr)Cl2(OAr)] strongly suggests the formation of the other vanadium(III) species by reduction with Me2AlCl or Et2AlCl, and the EXAFS analysis suggests the presence of the arylimido ligand and two V–Cl bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant changes in both the pre-edge peak(s) and the edge peak when these complexes were treated with MAO, suggesting that the basic geometry and the high oxidation state were preserved under these conditions.

Synthesis of Half-Titanocenes Containing Anionic N-Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety, Cp′TiX2(WCA-NHC), and Their Use as Catalysts for Ethylene (Co)polymerization

Synthesis and structural analysis of half-titanocenes containing anionic N-heterocyclic carbenes with a weakly coordinating borate [B(C6F5)3] moiety (WCA-NHC) of the type, [Cp′TiX2(WCA-NHC)] [Cp′ =...

Cycloadditions of anionic N-heterocyclic carbenes of sydnone imines

Sydnone imines were deprotonated with lithium bis(trimethylsilyl)amide at the C4 position to give the corresponding sydnone imine anions as lithium adducts. These can be represented as lithium stabilized anionic N-heterocyclic carbenes. Treatment with diisopropyl azodicarboxylate (DIAD) gave the corresponding C4 adducts, i.e. 4-hydrazinyl-sydnone imines, which form tautomers in solution. Reductive 1,3-dipolar cycloadditions of the sydnone imine anions with tetracyanoethylene (TCNE) resulted in the formation of pyrazoles, the mechanism of formation of which differs from known reactions. Reaction of the anion derived from the 2-methoxyphenyl sydnone imine with N,N-diisopropylcarbodiimide gave a ring-cleaved bisiminonitrile. Structure elucidations were accomplished by NMR spectroscopy and by four single crystal X-ray analyses.