Dicarbene Complexes Research Articles

Synthesis of N-heterocyclic carbene gold(I) complexes from the marine betaine 1,3-dimethylimidazolium-4-carboxylate.

The marine natural product norzooanemonin (1,3-dimethylimidazolium-4-carboxylate) has been used to prepare a series of carboxyl- or carboxylate-functionalized N-heterocyclic carbene (NHC) gold(I) complexes from [(Me2S)AuCl] in the presence of potassium carbonate. The potential of the resulting mono- and dicarbene complexes to act as cytotoxic or antibacterial drugs was investigated.

Putting a new spin on imido chemistry with an FeII dicarbene complex

Studies by Smith et al. published in 2021 and 2022 describe the intriguing reactivity of a high-spin three-coordinate FeII anion containing borate-bridged dicarbene and nucleophilic imido ligands. Acting as both a Lewis and Brønsted base, the imido fragment facilitates the catalytic formation of guanidines from carbodiimides and the selective α-deuteration of nitriles or alkynes.

Luminescent “Chugaev-type” cyclometalated iridium(III) complexes synthesized by nucleophilic addition of hydrazine

In this work, we describe a series of six neutral Chugaev-type chelating dicarbene iridium complexes with different cyclometalating ligands. The effects of different cyclometalating ligands on the emission properties of dicarbene iridium complexes are evaluated. The triazole- and NHC-derived cyclometalating ligands combine with the strong σ–donating Chugaev-type dicarbene ligand, resulting in phosphorescence that is blue-shifted compared to previously described analogues. The complexes are synthesized by the nucleophilic addition of hydrazine to cationic bis-isocyanide iridium complexes, characterized by 1H, 13C{1H}, and 19F NMR spectroscopy as well as high-resolution mass spectrometry. Four of the six molecular structures of the dicarbene complexes were determined by X-ray crystallography. Electrochemical properties of the compounds were evaluated by cyclic voltammetry analysis, which provides evidence that the HOMO and LUMO energies are dependent on the cyclometalating ligands employed. All the Chugaev-type iridium complexes exhibit blue to blue-green phosphorescence in solution at 77 K and all but one in poly(methyl methacrylate) (PMMA) thin films at room temperature. Although the quantum yields for all the complexes are modest, we succeeded in blue-shifting the emission wavelength of these dicarbene-type iridium complexes.

Synthesis and biological evaluation of fluoro-substituted cationic and neutral antibiotic NHC* silver derivatives of SBC3

Two fluoro-substituted neutral carbene silver acetates and thirteen cationic dicarbene silver complexes were synthesised from the appropriate imidazolium precursors in good to very good yields using batch and flow synthesis. Six of the derivatives were characterised by XRD and showed expected structural aspects in the solid state. Most of these fifteen new silver-based antimicrobial drug candidates showed improved stability as well as comparable biological activity against E. coli and Methicillin-resistant S. aureus when compared against the lead compound 1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene silver(I) acetate (SBC3). Fluoro-substitution as well as the use of dicarbene complexes have the potential to develop the field of silver-based antimicrobials further.

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.

Synthesis and coordination chemistry of silver(I), gold(I) and gold(III) complexes with picoline-functionalized benzimidazolin-2-ylidene ligands

Abstract The reactions of N-alkyl-N′-picolyl-benzimidazolium bromides or N,N′-dipicolyl-benzimidazolium bromide with silver oxide yielded the silver dicarbene complexes of the type [Ag(NHC)2][AgBr2] 1–4 (NHC = picoline-functionalized benzimidazolin-2-ylidene). The silver complexes 1–4 have been used in carbene transfer reactions to yield the gold(I) complexes of the type [AuCl(NHC)] 5–8 in good yields. A halide exchange at the metal center of complexes 5–8 with lithium bromide yielded the gold bromide complexes 9–12. Finally, the oxidation of the gold(I) centers in complexes 9–12 with elemental bromine gave the gold(III) complexes of the type [AuBr3(NHC)] 13–16. Molecular structures of selected Au(I) and Au(III) complexes have been determined by X-ray diffraction studies.

Rational Design of Penta-Coordinated Nickel(II) Dicarbene Complexes

A series of penta-coordinated nickel(II) complexes bearing tetradentate pyridine-functionalized diNHC ligands with flexible propylene (1a–4a) and ortho-xylylene linkers (1b–4b) have been synthesize...

The first mesomorphic benzimidazolium-based silver(I)-N-heterocyclic dicarbene complexes: Synthesis, characterization and phase properties

The first mesomorphic benzimidazolium based mononuclear silver (I) N-heterocyclic dicarbene complexes, Ag(I)-NHC were prepared via in situ deprotonation of 1-octadecyl-3-alkyl/arylbenzimidazolium salts with silver oxide (Ag2O). Both series of Ag (I)-NHC complexes contained a C18 alkyl chain on one N atom and differed in term of the substituent (alkyl and substituted benzyl moiety) on the other N atom in the benzimidazolium ring. All the synthesized compounds were characterized by elemental analysis, Fourier-transform infrared, 1H and 13C NMR spectroscopy. Cation geometry, anion and alkyl chain length were found to be essential parameters for mesophase formation in the salts and Ag(I)-NHC complexes in this study. All dialkylated salts except the one with the shortest chain, n = 10 were smectogenic. On the other hand, a large width-to-length ratio of the cationic core in the benzyl analogues had a negative effect on the formation and stability of mesophase. No mesophases were observed for these compounds. Wider mesophase range was seen with decrease in anion size and elongation of alkyl chains. A metastable SmA phase was observed in two Ag(I)-NHC complexes. The minimum chain length required for compounds with different anions to exhibit liquid crystal (LC) properties was addressed. Mesophase was observed for bromide salts with C12 onwards while C14 is the minimum chain length required for the hexaflurophosphate containing Ag(I)-NHC complexes to display LC properties. All Ag(I)-NHC complexes except the one bearing a cyano group could be considered ionic liquids with melting/clearing temperatures below 100 °C.

Di(N-heterocyclic carbene) gold(III) imidate complexes obtained by oxidative addition of N-halosuccinimides

The reaction between the gold(I) dicarbene complexes [Au2(μ2-MeIm(CH2)nImMe)2](PF6)2 (Im = imidazol-2-ylidene, n = 1, 2, 3) and N-bromosuccinimide affords the gold(III) complexes [{AuBr(N-suc)}2(μ2-MeIm(CH2)nImMe)2](PF6)2, in which both gold centres have a bromide and a N-succinimidate anion in the coordination sphere. In the case of the CH2CH2 bridge, two different conformers of the gold(III) complex are formed; the structure of the major species has been clarified by single crystal X-ray diffraction analysis, while the nature and the properties of the minor one have been investigated by means of DFT calculations. Compared to N-bromosuccinimide, oxidative additions are slower with the N-chloro- and faster with the N-iodosuccinimides. In both cases the reactions are scarcely selective and the products distribution markedly depends on halide size and reaction conditions.

Bis-Cyclometalated Iridium Complexes with Chelating Dicarbene Ancillary Ligands

In this work, we report that covalent postsynthetic modification can be used for the preparation of a class of bis-cyclometalated iridium complexes featuring Chugaev-type chelating dicarbene ligands. Bis-cyclometalated iridium complexes with electron-deficient aryl isocyanide ancillary ligands react with hydrazine to form neutral dicarbene complexes. The neutral iridium carbene complexes have a basic site that can be protonated by strong acid, permitting access to complexes in two protonation states and allowing an additional layer of control over the key properties. These new Chugaev-type iridium complexes exhibit blue phosphorescence at both room temperature and 77 K. Compared to their bis-isocyanide precursors, the electrochemical and photophysical properties of these new complexes are substantially perturbed, demonstrating the concept that the electronic structure and excited state dynamics can be controlled by ancillary ligand modification. Furthermore, the emission spectra and excited-state dynamics...

Synthesis and structure of palladium(II) complexes supported by bis-NHC pincer ligands for the electrochemical activation of CO2

A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These pyridyl-spaced dicarbene complexes ([(PDCR)Pd(MeCN)](PF6)2) were synthesized with substituents of varying steric bulk at the wingtip positions, which include R=methyl, ethyl, isopropyl, cyclohexyl, mesityl and 2,6-diisopropylphenyl. The synthesis of this library of complexes was accomplished either by direct metallation of the prerequisite pyridyl-spaced bis-imidazolium proligands with Pd(OAc)2 or via treatment with Ag2O to afford the corresponding silver carbenes, which were then transmetallated onto palladium. Solid-state structures for each of the [(PDCR)Pd(MeCN)](PF6)2 derivatives were obtained via X-ray crystallography and allowed for the steric properties of each PDCR ligand to be evaluated by two methods. These analyses, which included calculation of the percent buried volume (%VBur) and solid angles of the PDCR ligands, served to characterize the steric environment around the palladium center in each of the complexes that was prepared. Finally, voltammetry and controlled potential electrolysis studies were performed to characterize the redox chemistry of the [(PDCR)Pd(MeCN)](PF6)2 derivatives and assess if they could electrocatalyze the reduction of CO2. The influence of the steric properties of the PDCR ligand on the electrochemistry of the resulting complexes [(PDCR)Pd(MeCN)](PF6)2 is also discussed.

Insights into the Halogen Oxidative Addition Reaction to Dinuclear Gold(I) Di(NHC) Complexes.

Gold(I) dicarbene complexes [Au2 (MeIm-Y-ImMe)2 ](PF6 )2 (Y=CH2 (1), (CH2 )2 (2), (CH2 )4 (4), MeIm=1-methylimidazol-2-ylidene) react with iodine to give the mixed-valence complex [Au(MeIm-CH2 -ImMe)2 AuI2 ](PF6 )2 (1 a(I) ) and the gold(III) complexes [Au2 I4 (MeIm-Y-ImMe)2 ](PF6 )2 (2 c(I) and 4 c(I) ). Reaction of complexes 1 and 2 with an excess of ICl allows the isolation of the tetrachloro gold(III) complexes [Au2 Cl4 (MeIm-CH2 -ImMe)2 ](PF6 )2 (1 c(Cl) ) and [Au2 Cl4 (MeIm-(CH2 )2 -ImMe)2 ](Cl)2 (2 c(Cl) -Cl) (as main product); remarkably in the case of complex 2, the X-ray molecular structure of the crystals also shows the presence of I-Au-Cl mixed-sphere coordination. The same type of coordination has been observed in the main product of the reaction of complexes 3 or 4 with ICl. The study of the reactivity towards the oxidative addition of halogens to a large series of dinuclear bis(dicarbene) gold(I) complexes has been extended and reviewed. The complexes react with Cl2 , Br2 and I2 to give the successive formation of the mixed-valence gold(I)/gold(III) n a(X) and gold(III) n c(X) (excluding compound 1 c(I) ) complexes. However, complex 3 affords with Cl2 and Br2 the gold(II) complex 3 b(X) [Au2 X2 (MeIm-(CH2 )3 -ImMe)2 ](PF6 )2 (X=Cl, Br), which is the predominant species over compound 3 c(X) even in the presence of free halogen. The observed different relative stabilities of the oxidised complexes of compounds 1 and 3 have also been confirmed by DFT calculations.

Tandem Suzuki–Miyaura/transfer hydrogenation reaction catalyzed by a Pd–Ru complex bearing an anionic dicarbene

A hetero-bimetallic Pd–Ru N-Heterocyclic Dicarbene (NHDC) complex was synthesized from a Ag–Ru complex by transmetalation. The Pd–Ru system displays high catalytic activity in tandem Suzuki–Miyaura cross-coupling/transfer hydrogenation reaction of bromoaryl ketones with boronic acids in toluene and 2-propanol in the presence of KOH.

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

Reaction of [Ru(OAc)2(PPh3)2] with a P-functionalized imidazolium bromide easily affords a cationic abnormal carbene Ru system. Metalation with Ag2O yields a Ru-Ag complex containing an anionic dicarbene ligand, while subsequent transmetalation with Au(tht)Cl leads to the corresponding Ru-Au system. The bimetallic complexes were characterized by single crystal X-ray diffraction and are the first examples of complexes bearing anionic dicarbene ligands connecting two different d-block elements.

Synthesis and Biological Activity of Gold(I) N‐Heterocyclic Carbene Complexes with Long Aliphatic Side Chains

AbstractSeven imidazolium salts have been synthesized from octadecylimidazole (Im18). These salts differ in the length of the alkyl chain length bound to the second nitrogen atom of the imidazolium ring [R = Me, Et, iPr, Pr, Bu, decyl (Dec), octadecyl] and were used as synthetic precursors to obtain two series of gold(I) carbene complexes (AuNHC). The first series contains one labile ligand at the second coordinative position {monocarbene, [AuCl(NHC)]}, and the second one comprises dicarbene complexes. Their biological activity has been evaluated with respect to two different cell lines, and thioredoxin reductase (TrxR) inhibition has also been evaluated for selected examples. Distinct effects have been observed for the imidazolium salts and monocarbene derivatives.

Synthesis and Reactivity of Heteroditopic Dicarbene Rhodium(I) and Iridium(I) Complexes Bearing Chelating 1,2,3-Triazolylidene–Imidazolylidene Ligands

1,2,3-Triazol-5-ylidenes (tzNHC) have become a popular class of NHC ligands in homogeneous catalysis. Herein, we introduce chelate monovalent Rh- and Ir(cod) complexes bearing bidentate ligands that combine this tzNHC and an Arduengo-type NHC motif. The reactivity of these complexes with H2 and CO gas has been investigated, leading to an interesting octahedral [Ir(tzNHC-CH2-NHC)(CO)2(H)2]OTf complex and [M(tzNHC-CH2-NHC)(CO)2]OTf complexes. The carbonyl stretching frequencies of the latter indicate that the ligand has stronger electron-donating properties than classic di-NHC ligands. The square planar rhodium and iridium NHC-tzNHC complexes have been applied in transfer hydrogenation employing isopropyl alcohol as the hydrogen donor, in which they show moderate activity (Ir > Rh) toward a range of ketones as well as for an aldehyde, an imine, and a diene. The new dicarbene complexes proved to be more active for this reaction than the analogues in which the triazolyl moiety coordinates through a nitrogen donor.

Template Synthesis of Tetracarbene Platinum(II) Complexes with Five- and Six-Membered-Ring N-Heterocyclic Carbenes

Reaction of 2 equiv of 2-azidoethyl isocyanide and the tetraphosphine platinum(II) complexes [1a–c](X)2 (a, PR3 = PMe3; b, PR3 = PMe2Ph; c, PR3 = PMePh2) yielded via phosphine substitution and template-controlled cyclization of the isocyanide ligands the trans-di(NH,NH-NHC) complexes [2a–c](X)2. Treatment of [2c](PF6)2 with an additional 2 equiv of 2-azidoethyl isocyanide gave the tetracarbene complex [3](PF6)2, bearing four protic NH,NH-NHC ligands derived from a five-membered heterocycle. The same template synthesis utilizing 3-azidopropyl isocyanide and complex [1c](PF6)2 gave the dicarbene complex [5](PF6)2 and, after addition of 2 equiv more of 3-azidopropyl isocyanide, the tetracarbene complex [6](PF6)2 with four protic NH,NH-NHC ligands derived from a six-membered heterocycle. The intermediate trans-di(NH,NH-NHC) platinum(II) complex [5](PF6)2 also reacts with 2 equiv of 2-azidoethyl isocyanide to give the tetracarbene complex [7](PF6)2, bearing two five-membered-ring NHCs and two six-membered-ring NHCs.

Synthesis and Reactivity of Cationic Bis(N‐Heterocyclic Dicarbene) Ruthenium(II) Complexes

AbstractFour ruthenium(II) N‐heterocyclic dicarbene complexes of general formula [Ru{MeIm(CH2)nImMe}2(RCN)2][PF6]2 (1: n = 1, R = Me; 2: n = 1, R = Ph; 3: n = 2, R = Me; 4: n = 2, R = Ph; MeIm = 3‐methylimidazol‐2‐yliden‐1‐yl) have been synthesised by transmetalation of the dicarbene ligand from a dicarbene silver complex with [RuCl2(COD)]x (COD = 1,5‐cyclooctadiene) or [RuCl2(p‐cymene)]2 as ruthenium precursors. The relative position of the coordinated nitrile molecules (cis or trans) depends on the group that links the two carbene moieties: the dicarbene with the methylene bridge affords the trans isomer, whereas the cis complex is obtained with the ethylene bridge. The lability of coordinated nitriles towards entering nucleophiles, in particular, cyclohexyl and tert‐butyl isocyanide, has been carefully checked, and the corresponding substitution complexes [Ru{MeIm(CH2)nImMe}2(RNC)2][PF6]2 (6: n = 1, R = Cy; 7: n = 2, R = Cy; 8: n = 2, R = tBu) have been synthesised. The trans complexes are more inert than the cis ones, and benzonitrile is a better leaving group than acetonitrile; these characteristics allow a proper tuning of the coordination set and the stereoelectronic properties of these dicarbene ruthenium complexes. The X‐ray crystal structures of 1 and 7 have also been determined.

Synthesis of di(Imdazolium) and di(Pyrazolium) Salts as Precursors for N-heterocyclic Dicarbene Complexes

Alpha,omega-bis(pyrazol-1-yl)alkanes and alpha,omega-bis(imidazol-1-yl)alkanes with spacers consisting of four to ten methylene groups have been prepared from pyrazole, 3,5-dimethylpyrazole or imidazole and corresponding dibromoalkanes in a superbasic medium KOH-DMSO. The proposed method of synthesis allowed the preparation of new flexible bidentate ligands without the need to use toxic solvents and tedious workup procedures. Bis(pyrazol-1-yl)alkanes were further functionalized for their use as precursors for “non-classical” mesoionic N-heterocyclic carbene ligands. One the first step, iodine atoms were introduced to positions 4 of pyrazole rings by oxidative iodination using I2-HIO3 system. On the next step, nitrogen atoms in positions 2 of pyrazole rings were alkylated using several agents. Reaction with methyliodide unexpectedly led to the formation of only mono-alkylated products even after 7 days of refluxing in a neat alkyliodide. Methylation by trimethyloxonium tetrafluoroborate or methyltriflate led to dimethylated products in high yields. Bis(imidazol-1-yl)alkanes were easily alkylated by methyliodide to give di(imidazolium) salts – precursors to “classic” N-heterocyclic dicarbenes.

Ligand Exchange at a Gold(I) Carbene Complex

Abstract Gold complex [AuCl(NHC)] (NHC=N,N0-dipropylbenzimidazolin-2-ylidene) 1 undergoes facile substitution reactions at the gold(I) center. Treatment of 1 with anionic phenylacetylide or thiophenolate led to the neutral gold complexes 2 and 3, respectively. The cationic gold complexes [Au(NHC)(pyridine)](BF4) [4]BF4 and [Au2(NHC)2(4,4'-bipyridine)](BF4)2 [5](BF4)2 were obtained via abstraction of the chloro ligand from 1 and reaction with the appropriate amine. Reaction of 1 with AgBF4 in the presence of PPh3 instead of an amine led to an inseparable product mixture of the mixed NHC=PPh3 complex [6]BF4, the dicarbene complex [Au(NHC)2]BF4, [7]BF4, and [Au(PPh3)2]BF4, [8]BF4. Crystals of 2 and [6]BF4 were obtained, and X-ray diffraction structure analyses revealed that the gold(I) atoms are coordinated in a linear fashion by the NHC and the co-ligand