Amide Synthesis from Alcohols and Amines Catalyzed by Ruthenium N‐Heterocyclic Carbene Complexes

The direct synthesis of amides from alcohols and amines is described with the simultaneous liberation of dihydrogen. The reaction does not require any stoichiometric additives or hydrogen acceptors and is catalyzed by ruthenium N‐heterocyclic carbene complexes. Three different catalyst systems are presented that all employ 1,3‐diisopropylimidazol‐2‐ylidene (IiPr) as the carbene ligand. In addition, potassium tert‐butoxide and a tricycloalkylphosphine are required for the amidation to proceed. In the first system, the active catalyst is generated in situ from [RuCl₂(cod)] (cod=1,5‐cyclooctadiene), 1,3‐diisopropylimidazolium chloride, tricyclopentylphosphonium tetrafluoroborate, and base. The second system uses the complex [RuCl₂(IiPr)(p‐cymene)] together with tricyclohexylphosphine and base, whereas the third system employs the Hoveyda–Grubbs 1st‐generation metathesis catalyst together with 1,3‐diisopropylimidazolium chloride and base. A range of different primary alcohols and amines have been coupled in the presence of the three catalyst systems to afford the corresponding amides in moderate to excellent yields. The best results are obtained with sterically unhindered alcohols and amines. The three catalyst systems do not show any significant differences in reactivity, which indicates that the same catalytically active species is operating. The reaction is believed to proceed by initial dehydrogenation of the primary alcohol to the aldehyde that stays coordinated to ruthenium and is not released into the reaction mixture. Addition of the amine forms the hemiaminal that undergoes dehydrogenation to the amide. A catalytic cycle is proposed with the {(IiPr)Ru^II} species as the catalytically active components.     

Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N‐Heterocyclic Carbene Linker

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co‐planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L₄Ru‐C,N‐bbi‐C,N‐RuL₄] were obtained by a transmetalation methodology (C,N‐bbi‐C,N=benzobis(N‐pyridyl‐N′‐methyl‐imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L₄=[(cym)Cl]). Ligand exchange of the Cl^?/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal‐centered oxidation processes that are separated by 134 mV (L₄=[(bpy)₂]; bpy=2,2′‐bipyridine) and 244 mV (L₄=[(MeCN)₄]), respectively. Hush analysis of the intervalence charge‐transfer bands in the mixed‐valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed‐valent Ru^II/Ru^III species and the fully oxidized Ru^III/Ru^III complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.     

Proton‐Transfer Polymerization (HTP): Converting Methacrylates to Polyesters by an N‐Heterocyclic Carbene

A new polymerization termed proton (H)‐transfer polymerization (HTP) has been developed to convert dimethacrylates to unsaturated polyesters. HTP is catalyzed by a selective N‐heterocyclic carbene capable of promoting intermolecular Umpolung condensation through proton transfer and proceeds through the step‐growth propagation cycles via enamine intermediates. The role of the added suitable phenol, which is critical for achieving an effective HTP, is twofold: shutting down the radically induced chain‐growth addition polymerization under HTP conditions (typically at 80–120 °C) and facilitating proton transfer after each monomer enchainment. The resulting unsaturated polyesters have a high thermal stability and can be readily cross‐linked to robust polyester materials.     

Fusing N‐Heterocyclic Carbenes with Carborane Anions

Here we describe the fusion of two families of unusual carbon‐containing molecules that readily disregard the tendency of carbon to form four chemical bonds, namely N‐heterocyclic carbenes (NHCs) and carborane anions. Deprotonation of an anionic imidazolium salt with lithium diisopropylamide at room temperature leads to a mixture of lithium complexes of C‐2 and C‐5 dianionic NHC constitutional isomers as well as a trianionic (C‐2, C‐5) adduct. Judicious choice of the base and reaction conditions allows the selective formation of all three stable polyanionic carbenes. In solution, the so‐called abnormal C‐5 NHC lithium complex slowly isomerizes to the normal C‐2 NHC, and the process can be proton‐catalyzed by the addition of the anionic imidazolium salt. These results indicate that the combination of two unusual forms of carbon atoms can lead to unexpected chemical behavior, and that this strategy paves the way for the development of a broad new generation of NHC ligands for catalysis.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

New Route to Stabilize Ruthenium Nanoparticles with Non‐Isolable Chiral N‐Heterocyclic Carbenes

Ru nanoparticles (RuNPs) stabilized by non‐isolable chiral N‐heterocyclic carbenes (NHCs), namely SIDPhNp ((4S,5S)‐1,3‐di(naphthalen‐1‐yl)‐4,5‐diphenylimidazolidine) and SIPhOH ((S)‐3‐((1S,2R)‐2‐hydroxy‐1,2‐diphenylethyl)‐1‐((R)‐2‐hydroxy‐1,2‐diphenylethyl)‐4,5‐dihydro‐3H‐imidazoline), have been synthesized through a new procedure that does not require isolation of the free carbenes. The obtained RuNPs have been characterized by state‐of‐the‐art techniques and their surface chemistry has been investigated by FTIR and solid‐state MAS NMR upon the coordination of CO, which indicated the presence of free and reactive Ru sites. Their catalytic activity has been tested in various hydrogenation reactions involving competition between different sites, whereby interesting differences in selectivity were observed, but no enantioselectivity.     

Well‐Defined N‐Heterocyclic Carbene‐Palladium Complexes as Efficient Catalysts for Domino Sonogashira Coupling/Cyclization Reaction and C‐H bond Arylation of Benzothiazole

Well‐defined and air‐stable PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed palladium bis‐N‐heterocyclic carbene complexes have been developed for the domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with a variety of terminal alkynes and C‐H bond arylation of benzothiazole with aryl iodides. The PEPPSI themed palladium complexes, 2a and 2b were synthesized in good yields from the reaction of corresponding imidazolium salts with PdCl₂ and K₂CO₃ in pyridine. The new air‐stable palladium‐NHC complexes were characterized by NMR spectroscopy, X‐ray crystallography, elemental analysis, and mass spectroscopy studies. The PEPPSI themed palladium(II) bis‐N‐heterocyclic carbene complexes 2a and 2b exhibited excellent catalytic activities for domino Sonogashira coupling/cyclization reaction of 2‐iodophenol with terminal alkynes yielding benzofuran derivatives. In addition, the palladium complexes, 2a and 2b successfully catalyzed the direct C‐H bond arylation of benzothiazole with aryl iodides as coupling partners in presence of CuI as co‐catalyst.     

N‐Heterocyclic Carbene‐Catalysed Direct Synthesis of Cyano Esters via Cyanation‐Esterification Reaction of α‐Keto Esters

The cyanation‐esterification reaction of α‐keto esters catalysed by N‐heterocyclic carbenes (NHCs) is developed. Under the catalysis of 10 mol% 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene, aromatic and aliphatic α‐keto esters reacted with ethyl cyanoformate or acetyl cyanide to produce the corresponding cyano esters with a tetrasubstituted carbon center in high yields.     

Synthesis and Structures of RhI and IrI Complexes Supported by N‐Heterocyclic Carbene‐Phosphinidene Adducts

N‐Heterocyclic carbene‐phosphinidene adducts of the type (IDipp)PR [R = Ph ( 5 ), SiMe₃ ( 6 ); IDipp = 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] were used as ligands for the preparation of rhodium(I) and iridium(I) complexes. Treatment of (IDipp)PPh ( 5 ) with the dimeric complexes [M(μ‐Cl)(COD)]₂ (M = Rh, Ir; COD = 1,5‐cyclcooctadiene) afforded the corresponding metal(I) complexes [M(COD)Cl{(IDipp)PPh}] [M = Rh ( 7 ) or Ir ( 8 )] in moderate to good yields. The reaction of (IDipp)PSiMe₃ ( 6 ) with [Ir(μ‐Cl)(COD)]₂ did not yield trimethylsilyl chloride elimination product, but furnished the 1:1 complex, [Ir(COD)Cl{(IDipp)PSiMe₃}] ( 9 ). Additionally, the rhodium‐COD complex 7 was converted into the corresponding rhodium‐carbonyl complex [Rh(CO)₂Cl{(IDipp)PPh}] ( 10 ) by reaction with an excess of carbon monoxide gas. All complexes were fully characterized by NMR spectroscopy, microanalyses, and single‐crystal X‐ray diffraction studies.     

N‐Heterocyclic Carbene–Phosphinidyne Transition Metal Complexes

The N‐heterocyclic carbene–phosphinidene adduct IPr?PSiMe₃ is introduced as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPr?P)ML_n] (ML_n=(η⁶‐p‐cymene)RuCl) and (η⁵‐C₅Me₅)RhCl). Their spectroscopic and structural characteristics, namely low‐field ³¹P NMR chemical shifts and short metal–phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPr?P ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi‐ and trimetallic RuAu, RhAu, Rh₂, and Rh₂Au complexes.     

Photofunctional Platinum Complexes Featuring N‐heterocyclic Carbene‐Based Pincer Ligands

Photoactive platinum complexes of stoichiometry [Pt(^RCCC^R)L]^0/+ (R=Me, nBu and L=? CN, ? C≡CPh, ? N≡CCH₃, ? Py, ? CO) featuring pincer‐type bis N‐heterocyclic carbene (NHC) ligands (^RCCC^R) were synthesized. Organometallic syntheses of these complexes are facile and achievable through standard laboratory procedures. Control of intermolecular Pt???Pt interaction, π–π stacking, and emission tuning is achieved through suitable choice of the NHC‐wingtip substituent (R) and the auxiliary ligand (L). Exposure to specific volatile organic compounds (VOCs) or mechanical grinding triggers changes in emission colors, which render these complexes photofunctional. Solid‐state structures and photoluminescence results are described herein.     

Silver–N‐Heterocyclic Carbene Complexes: Synthesis,Characterization, and Antimicrobial Properties

Six new silver complexes containing symmetrical N ‐heterocyclic carbene (NHC ) ligands were synthesized by the reaction of azolium salts with Ag₂O in CH₂Cl₂ . These complexes were tested against Gram‐negative bacterial strains (Escherichia coli and Pseudomonas aeruginosa ), Gram‐positive bacterial strains (Enterococcus faecalis and Staphylococcus aureus ), and fungal strains (Candida albicans and Candida tropicalis ), and all tested complexes showed good activity against the different microorganisms.     

Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

By means of a combined experimental and theoretical approach, the electronic features and chemical behavior of metalla‐N‐heterocyclic carbenes (MNHCs, N‐heterocyclic carbenes containing a metal atom within the heterocyclic skeleton) have been established and compared with those of classical NHCs. MNHCs are strongly basic (proton affinity and pK_a values around 290 kcal mol^?1 and 36, respectively) with a narrow singlet–triplet gap (around 23 kcal mol^?1). MNHCs can be generated from the corresponding metalla‐imidazolium salts and trapped by addition of transition‐metal complexes affording the corresponding heterodimetallic dicarbene derivatives, which can serve as carbene transfer agents.     

Sulfonated N‐heterocyclic carbine‐silver (I) complexes: Synthesis,characterisation and biological evaluation

The synthesis, characterisation and biological activity of water‐soluble Ag(I)‐NHC complexes, general formula Na[(NHC)AgCl] where NHC is a sulfonated and sterically hindered N‐heterocyclic carbene, is reported. The Ag‐NHC complexes (2a–e) were synthesised by reacting the corresponding sulfonated NHC ligands with Ag₂O in the presence of NaCl or NaBr in methanol/water (1:1) solution. Synthesised silver (I)‐N‐heterocyclic carbene complexes have been characterised by NMR, micro‐analysis and HRMS spectroscopic methods. The IC₅₀ values of these complexes were determined by a proliferation BrdU enzyme‐linked immunosorbent assay (ELISA) against HeLa (human cervix carcinoma), HT29 (human adenocarcinoma) and L929 (mouse fibroblast) cell lines. These complexes have been highlighted as promising and original platforms for building new types of metalodrug. All new water‐soluble Ag(I) complexes demonstrated remarkable cytotoxic activity against HeLa, HT29 and L929 cell lines.     

Color‐Tunable Electrogenerated Chemiluminescence of Ruthenium N‐Heterocyclic Carbene Complexes

Ru(II) complexes 1 – 3 bearing various N‐heterocyclic carbene (NHC) ligands were synthesized, and their photophysical, electrochemical, and electrogenerated chemiluminescence (ECL) properties were discussed to evaluate a potential of their use as multicolor ECL labels. Interestingly, they exhibited ECL emission ranging from greenish‐yellow to red both in nonaqueous and mixed aqueous solutions, which might show the potential of the Ru(II) complexes as multicolor ECL labels.     

Synthesis and Characterization of New N‐Heterocyclic Silylazides

Three novel pyridine functionalized N‐heterocyclic silanes, bearing chloride and azide moieties, were synthesized and characterized by NMR spectroscopy (¹H, ¹³C, ²⁹Si), mass spectrometry, elemental analysis, and single‐crystal XRD. The molecular structures show a comparably strong dative interaction of the pyridine‐N with the Si center, formally inducing a penta‐coordination arrangement at the silicon(IV). Under appropriate conditions, the silylazides, presented in this work, might be able to thermo‐ or photolytically liberate gaseous nitrogen giving rise to a promising synthetic option to access a variety of new transition metal silylene complexes with potential applications in various catalytic reactions.     

Synthesis,Characterization and Catalytic Application of Pyridine‐Bridged N‐Heterocyclic Carbene–Ruthenium Complexes in the Hydrogenation of Carbonates

A series of bulky pyridine‐bridged NHC–Ru complexes have been rationally designed and synthesized; these exhibited very high catalytic activity in the hydrogenation of cyclic and linear carbonates under mild reaction conditions. In the presence of catalytic amounts of a weak base, a broad range of substrates with different ring size and steric bulk were well tolerated, providing methanol and the corresponding diols in excellent yields with a catalyst loading as low as 0.5 mol %.     

Synthesis and antimicrobial activity of Ag(I)‐N‐heterocyclic carbene complexes derived from benzimidazol‐2‐ylidene

Novel benzimidazol‐2‐ylidene carbene complexes of Ag(I) were prepared by interaction of the corresponding benzimidazolium salt with Ag₂O in dichloromethane. Their structures were characterized by elemental analyses, ¹H‐NMR, ¹³C‐NMR and IR spectroscopy techniques. All compounds studied in this work were screened for their in vitro antimicrobial activities against the standard strains: Enterococcus faecalis (ATCC 29212), Staphylococcus aureus (ATCC 29213), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and the fungi Candida albicans and Candida tropicalis. The new complexes were found to be effective antimicrobial activity against a series of bacteria and fungi. Copyright © 2010 John Wiley & Sons, Ltd.     

Palladacycles bearing COOH‐/ester‐functionalized N‐heterocyclic carbenes: Divergent syntheses and catalytic applications

Two new [C^N]‐type palladacyclic dinuclear complexes bearing carboxylate‐containing N‐heterocyclic carbenes (NHCs) were synthesized, and in both cases the carboxylato‐NHC ligand adopts a bridging mode. Both complexes proved to be suitable precursors, which can be used to divergently access palladacycles bearing ester‐ or COOH‐functionalized NHCs upon esterification or acidolysis. In the esterification reactions, alkyl halides are found to selectively react with the carboxylato moieties, and the palladacycle scaffold is retained even when excess haloalkane is employed. In the acidolysis reactions, the desired COOH‐tethered complexes can only be obtained when stoichiometric acid (with respect to Pd) is used, while excess acid destroys the metallacycle scaffold. Finally, a preliminary catalytic study reveals the good performances of all newly synthesized complexes in direct aromatic C─H functionalization reactions with alkynes. Poisoning experiments indicate that these hydroarylation reactions are likely to be homogeneously catalyzed.