N-heterocyclic Carbene Moieties Research Articles

Gold(I) N-heterocyclic carbene complexes with naphthalimide ligands as combined thioredoxin reductase inhibitors and DNA intercalators.

Organometallic conjugates consisting of a gold(I) N-heterocyclic carbene (NHC) moiety and a naphthalimide were prepared and investigated as cytotoxic agents that interact with both DNA and the disulfide reductase enzyme thioredoxin reductase (TrxR). The complexes were potent DNA intercalators related to their naphthalimide partial structure, inhibited TrxR as a consequence of the incorporation of the gold(I) moiety, and triggered efficient cytotoxic effects in MCF-7 breast and HT-29 colon adenocarcinoma cells. Strong effects on tumor cell metabolism were noted for the most cytotoxic complex, chlorido[1-(3'-(4''-ethylthio-1'',8''-naphthalimid-N''-yl))-propyl-3-methyl-imidazol-2-ylidene]gold(I) (4 d). In conclusion, the conjugation of naphthalimides with gold(I) NHC moieties provided a useful strategy for the design of bioorganometallic anticancer agents with multiple modes of action.

Synthesis and Structure of Imine–N-Heterocyclic Carbene Palladium Complexes and Their Catalytic Behavior in Norbornene Polymerization

On the basis of the steric effects of ligand, a series of imine–N-heterocyclic carbene (NHC) ligands and their corresponding five-membered palladium complexes with bulky substituents on both the imine and the NHC moieties were synthesized and characterized. Transpalladation of silver carbene complexes with (COD)PdCl2 and (COD)PdMeCl afforded the palladium dichloride and methylpalladium complexes, respectively. Bulky cationic palladium complexes were further obtained by treatment of the methylpalladium complexes with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) in CH3CN. Well-defined cationic palladium complexes were confirmed by X-ray crystal diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was proven to proceed through a coordination–insertion mechanism by NMR studies. Analysis of oligomers obtained by polymerizing the monomer in the presence of H2 reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.

Neutral and Cationic N‐Heterocyclic Carbene Zinc Adducts and the BnOH/Zn(C6F5)2 Binary Mixture – Characterization and Use in the Ring‐Opening Polymerization of β‐Butyrolactone, Lactide, and Trimethylene Carbonate

AbstractA variety of N‐heterocyclic carbene (NHC) zinc adducts of the type NHC–ZnMe2 [2a, NHC = 1,3‐dimesitylimidazol‐2‐ylidene; 2b, NHC = 1,3‐(2,6‐iPr2Ph)2‐imidazol‐2‐ylidene; 2c, NHC = 1,3‐di‐tert‐butylimidazol‐2‐ylidene] and NHC–Zn(C6F5)2 [4a, NHC = 1,3‐dimesitylimidazol‐2‐ylidene; 4b, NHC = 1,3‐(2,6‐iPr2Ph)2‐imidazol‐2‐ylidene; 4c, NHC = 1,3‐di‐tert‐butylimidazol‐2‐ylidene] were synthesized by reaction of the appropriate NHC ligand with ZnMe2 or (toluene)Zn(C6F5)2. The X‐ray molecular structures of derivatives 2b, 4b, and 4c confirmed their adduct nature, and their bonding and geometrical parameters are influenced by the nature of the Zn–X group. The ionization reaction of the dimethyl Zn species 2a and 2b with the methide abstracting reagent B(C6F5)3 in the presence of tetrahydrofuran (THF) afforded the NHC–Zn organocations (1,3‐dimesitylimidazol‐2‐ylidene)Zn(Me)(THF)2+ (3a+) and [1,3‐(2,6‐iPr2Ph)2‐imidazol‐2‐ylidene]Zn(Me)(THF)+ (3b+) as dissociated MeB(C6F5)3– salts, as deduced from NMR spectroscopic data. All of these NHC–Zn compounds and BnOH/Zn(C6F5)2 were tested as ring‐opening polymerization (ROP) initiators of β‐butyrolactone (β‐BL), rac‐lactide (rac‐LA), and trimethylene carbonate (TMC). Although cations 3a+ and 3b+ were both inactive ROP initiators, as they appear to decompose prior to any ROP activity, the NHC–Zn(C6F5)2 compounds readily mediate the ROP of β‐BL, rac‐LA, and TMC to afford the corresponding polymeric materials with a moderate level of control, as deduced from NMR, size‐exclusion chromatography (SEC), and mass spectrometric MALDI‐TOF data. In contrast, the use of the alcohol BnOH (instead of a NHC moiety) as a nucleophile for association with (toluene)Zn(C6F5)2 led to efficient, highly controlled, and immortal ROP of TMC and rac‐LA with the formation of controlled chain length and narrowly disperse materials.

ChemInform Abstract: Development of Efficient Catalytic Arylation of Aldehydes with Thioether‐Imidazolinium Carbene Ligands

AbstractReview: 74 refs.

Reactivity of [Ru4(μ-H)4(CO)12] with bidentate ligands containing at least one N-heterocyclic carbene moiety

The tetraruthenium carbonyl cluster complexes [Ru4(μ-H)4(κ2-Ln)(CO)10], which contain a ditopic ligand (Ln) in which an N-heterocyclic carbene moiety is combined with a thioether (L1), a phosphine (L2 and L3), an NHC (L4), or a pyridine (L5 and L6) fragment, have been prepared by treating [Ru4(μ-H)4(CO)12] with the corresponding ligand. In these compounds, the ditopic ligand chelates a ruthenium atom that is also attached to three hydrides. In the reactions involving L1, L2 and L3, an ionic species, identified as [HLn]+ [Ru4(μ-H)3(CO)12]−, has been recognized as a reaction intermediate.

Development of Efficient Catalytic Arylation of Aldehydes with Thioether-Imidazolinium Carbene Ligands

Effective methods of ligand design have been highly sought due to the significant roles of ligands in controlling metal catalyses. In particular, easy-to-handle ligands to realize high reaction efficacy, substrate tolerance, and environmental friendliness are desirable. Novel bidentate ligands containing N-heterocyclic carbene and thioether moieties were developed based on findings of hemilabile coordination, whose precursors were crystalline solids stable enough to handle and store in the air. The thioether-imidazolinium carbene ligand successfully brought out high catalyst performance of palladium in the catalytic arylation of aldehydes with organoboron reagents, which tolerated a diverse range of substrates including poorly reactive, sterically hindered, and heterocyclic compounds. This process was applied to gram-scale synthesis using only water as solvent with high efficiency and also achieved the effective one-pot synthesis of 3-arylphthalides known as useful biologically active agents and important synthetic intermediates for naturally occurring compounds.

New Mixed-Donor Bidentate Ligands Based on N-Heterocyclic Carbene and Thione Donors

A new family of mixed-donor bidentate ligands containing both N-heterocyclic carbene (NHC) and thione functionalities is reported. The imidazolium salt precursors 1,1′-methylene(3-R-imidazol-2-ylidene)(3-R-2H-imidazole-2-thione) [CSRH] halide (where R = methyl, benzyl; halide = bromide, iodide) have been synthesized. Their coordination to copper(I) salts has been explored, providing the complexes [Cu(CSR)X]2 (where R = methyl, benzyl; halide = bromide, iodide). Structural characterization of two of the complexes confirmed the dimeric nature of these complexes in the solid state but revealed different coordination modes for each case (κ1-C, κ1-S, μ-S and κ1-C, κ1-S). Solutions of the copper complexes slowly react with oxygen under aerobic conditions to form uncoordinated 1,1′-methylene(3-R-2H-imidazol-2-one)(3-R-2H-imidazole-2-thione), SOR (where R = methyl, benzyl), where the NHC moiety has been converted into a urea functional group. The complexes were investigated as candidates in copper-catalyzed cross-coupling reactions.

Electronic and geometrical manipulation of the excited state of bis-terdentate homo- and heteroleptic ruthenium complexes

This work describes the synthesis and characterization of two new bis-terdentate Ru(II) complexes. Compound 1 is a homoleptic complex containing two CNC N-heterocyclic carbene (NHC) based ligands, whereas compound 2 bears one CNC ligand and an ancillary terpyridine ligand. The redox and photophysical properties of both compounds have been investigated and their X-ray crystal structures determined. Complex 1 displays a close-to-perfect octahedral coordination geometry and is not luminescent at room temperature while complex 2 features room temperature and 77 K luminescence despite its partially distorted geometry. The presence of the NHC moieties brings a significant amount of electronic density to the metal centre therefore lowering its oxidation potential with respect to that of analogous polypyridyl complexes.

Pt(II) complexes with diimine and chelating 5-ring iminocarbene ligands: synthesis, characterization, and structural and spectroscopic trends

A series of new L2PtMe2 and L2PtPh2 complexes have been prepared, where L2 = chelating diimine or NHC iminocarbene ligands with similar substituent patterns. These have been investigated by 1H- and 13C-NMR spectroscopy and single-crystal X-ray structure determinations to assess differences between the two ligand systems. All the three approaches underscore the strong trans influence of the N-heterocyclic carbene moiety relative to the imine group. This is supported by trends in J(195Pt–H) and J(195Pt–C) coupling constants in the hydrocarbyl NMR resonances as well as in differences in Pt–ligand bond distances trans to the imine and carbene moieties. This is paralleled by a clear trans effect of the carbene ligand in protonolysis; treatment of the Pt(II) dimethyl or diphenyl iminocarbene complexes with acid causes the elimination of methane and benzene, respectively, with selective loss of the methyl or phenyl groups from the trans position relative to the carbene part of the chelating ligand.

Biscarbene Palladium(II) Complexes. Reactivity of Saturated Versus Unsaturated N-Heterocyclic Carbenes

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

New chiral ligands bearing two N-heterocyclic carbene moieties at a dioxolane backbone. Gold, palladium and rhodium complexes as enantioselective catalysts

Biscarbene ligands with two imidazolin-2-ylidene moieties at a chiral dioxolane backbone were used as ligands for gold, rhodium and palladium complexes. All new complexes showed varying degrees of enantioselectivity toward hydrogenation of prochiral alkenes with ees up to 95%.

Heterotopic silver–NHC complexes: from coordination polymers to supramolecular assemblies

New coordination polymers based on different combinations of silver atoms and pyridyl-substituted N-heterocyclic carbene moieties are described. The addition of Zn(ii) templates leads to Ag-Zn supramolecular assemblies via selective ZnN interactions; a process that can be reverted.

Controlling the coordination modes of a new and highly flexible ligand bearing two N-heterocyclic carbene moieties at a bipyridine backbone

A new and highly flexible biscarbene ligand with two imidazolin-2-ylidene moieties in 6,6'-position of a 2,2'-bipyridine was prepared and structurally characterised by X-ray analysis. Various silver complexes were prepared from the imidazolium salts. Depending on the counterion, the coordination mode of the ligand can be mono- or bidentate. Pd complexes are feasible directly from the imidazolium salt using palladium(II)-acetate or via transmetallation from silver carbene complexes with [PdCl(2)(CH(3)CN)(2)] or [PdCl(2)(COD)]. Controlled by the structure of the silver complexes the biscarbenebipyridine ligand is a di- or tetracoordinating ligand forming mono- or dinuclear complexes. The high flexibility of this ligand is confirmed by transmetallation from silver to copper, which results in formation of a dinuclear copper complex with two bridging biscarbene ligands.

Crystals of an Iron Nitride

CHEMISTRY Both industrial and enzymatic nitrogen reduction catalysts rely on iron centers. However, high valent molecular iron nitride complexes (FeN) have stubbornly eluded crystallographic characterization, in contrast to analogous terminal oxo structures (Fe=O). Vogel et al. now find that a tridentate ligand comprising three coordinating N-heterocyclic carbene moieties offers the solution. Reaction of the ligand (bearing either xylyl or mesityl groups for steric protection) with ferrous chloride followed by reduction with sodium amalgam and treatment with trimethylsilylazide yields a cationic Fe-N3 complex that loses dinitrogen under xenon lamp photolysis to afford the terminal iron nitride. Air-stable purple crystals of the compound were characterized by x-ray diffraction, revealing a short FeN bond length of 1.53 . Mssbauer spectroscopy further suggested an iron center more electron-rich than a previously prepared phosphorus-coordinated iron nitride characterized in solution; the authors attribute the difference to -donation from the carbene ligands. JSY Angew. Chem. Int. Ed. 47 , 10.1002/anie.200800600 (2008).

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors.

Unexpected Solvent-Induced Cis/Trans Isomerization and Catalytic Application of a Bis-bidentate Nickel(II) Complex with N-Heterocyclic Carbene and Amido Functionalities

A series of bis-bidentate nickel(II) complexes with bifunctional ligands of N-heterocyclic carbene and amido moieties were prepared from the ligand precursors, [L1H1H2]Cl and [L2H12H2]Cl (H1 = NHCO, H2 = NCHN), in 48−82% yields. The complexes NiL12 and Ni(L2H1)2 are stable in the air and toward moisture. They are characterized by NMR (1D and 2D) and single-crystal X-ray diffraction studies. According to the NMR, crystallographic, and PXRD studies, one of the NiL12 complexes undergoes an intriguing reversible solvent-induced cis/trans transformation process, producing the cis form with DMSO and trans with CHCl3. The exchange process is highly specific and can be explained by the higher polarity of cis-NiL12, which favors the polar DMSO solvent, whereas the less polar trans form due to cancellation of polarity from the trans disposition of ligands favors the less polar CHCl3 solvent. A DFT calculation is in support of the experimental findings. Unlike the previous reported palladium analogues, NiL12 and Ni(L2H1)2, in the presence of PPh3 as cocatalyst, are highly efficient in catalyzing the Suzuki cross-coupling reaction of aryl chlorides with phenylboronic acid. A 3 mol % of NiL12 (Ni:PPh3 = 1:2) was sufficient to mediate the formation of 4-methoxy-1,1‘-biphenyl from 4-chloroanisole in 95% yield in 12 h.

Synthesis and reactivity of zirconium and hafnium complexes incorporating chelating diamido-N-heterocyclic-carbene ligands

Early transition metal complexes employing a diamido N-heterocyclic carbene (NHC) ligand set (denoted [NCN]) render the centrally disposed NHC moiety stable to dissociation. Aminolysis reactions with the mesityl-substituted ligand precursor ( Mes[NCN]H 2) and M(NMe 2) 4 (M = Zr, Hf) provide bis(amido)-NHC-metal complexes that can be further converted to chloro and alkyl derivatives. Activation of Mes[NCN]M(CH 3) 2 with [Ph 3C][B(C 6F 5) 4] yields { Mes[NCN]MCH 3}{B(C 6F 5) 4}, which is surprisingly inactive for the polymerization of 1-hexene. The zirconium cation did, however, show moderate ability to catalytically polymerize ethylene. The hafnium dialkyls are thermally stable with the exception of the diethyl complex, Mes[NCN]Hf(CH 2CH 3) 2, which undergoes β-hydrogen transfer and subsequent C–H bond activation with an ortho-methyl substituent on the mesityl group. The hafnium dialkyl complexes also insert carbon monoxide and substituted isocyanides to yield η 2-acyls and η 2-iminoacyls, respectively. In some circumstances, further C–C bond coupling occurs to yield enediolates and eneamidolate metallocycles. The molecular structures of Mes[NCN]Hf(CH 2CHMe 2) 2, Mes[NCN]Hf(η 2-(2,6-Me 2C 6H 3NCCH 3)(CH 3), Mes[NCN]Hf(η 2-(2,6-Me 2C 6H 3NCCH 3) 2, Mes[NCN]Hf(OC(CH 3) C(CH 3)NXy), and [ Mes[NCN]Hf(OC( i Bu) C( i Bu)O)] 2 are included.

Silver alkoxide and amino N-heterocyclic carbenes; syntheses and crystal structures

Silver(I) complexes of heterobidentate ligands that incorporate one or two N-heterocyclic carbene moieties coupled with an alcohol or amine group have been made by direct deprotonation of ligands of the form [HOCR 1R 2CH 2(1-HC{NCHCHNR})][X], H 2L 1X (X = Br, I), [H 2NR 1CHR 2CHR 2(1-HC{NCHCHNR})][Br] 2 H 3L 2X 2 (X = Cl, Br), and [H 2N{CH 2CH 2(1-HC[NCHCHNMes])} 2][X] 3 H 4L 3X 3 (X = Cl, Br). Silver(I) oxide is sufficiently basic to deprotonate both the imidazolium and the alcohol functional groups of all but one of the L 1 ligand precursors, to afford rare examples of silver alkoxide complexes [Ag(L 1)], stabilised by the soft donor carbene. Another complex of L 1 is characterised as the carbene alcohol adduct [Ag(HL 1) 2I]. The analogous reactions of silver(I) oxide with the amino imidazolium precursors afford silver amino-carbenes [Ag(HL 2)Br] with the potentially bidentate L 2 ligand, and [Ag(HL 3)X] (X = Cl, Br) with the potentially tridentate L 3 ligand. A single crystal X-ray diffraction study of the latter complex confirms that the neutral amine of the potentially tridentate L 3 ligand is unco-ordinated; instead the structure contains discrete chains of T-shaped silver bis(carbene) halide moieties that bridge to form a zig-zag 2-connected polymer. Protonolysis of two of the silver alkoxide and amino adducts, [Ag(L 1a)] and [Ag(HL 2a)Br], affords imidazolium complexes salts [H 2L 1a][AgCl 2] and [Ag(H 2L 2a)Br][AgBr 2] that retain the Ag(I) centre as complex counterions. The single crystal X-ray structures of these salts have been determined and show the silver(I) cations are now incorporated into ladders or chains as silver(I) halo-anions, and a silver amine dative bond is present in the latter complex.

A Novel Class of Ruthenium Catalysts for Olefin Metathesis.

The different nature of carbene ligands is clearly demonstrated by the first ruthenium-based complexes 1, which contain both alkylidene and N-heterocyclic carbene moieties. The latter exhibit a pronounced Lewis base behavior. Moreover, this difference forms the basis of the high catalytic activities of these compounds in olefin metathesis reactions.

A Novel Class of Ruthenium Catalysts for Olefin Metathesis

The different nature of carbene ligands is clearly demonstrated by the first ruthenium-based complexes 1, which contain both alkylidene and N-heterocyclic carbene moieties. The latter exhibit a pronounced Lewis base behavior. Moreover, this difference forms the basis of the high catalytic activities of these compounds in olefin metathesis reactions.