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Chirality at the metal center is a well‐established concept, exemplified by numerous four‐ and six‐coordinated complexes. Here, a pioneering example of linear chirality‐at‐metal is described: [Ag(NHC)2]+ (NHC=N‐heterocyclic carbene). In solution, stabilizing π‐π interactions preserve the chiral information of the solid state. Consequently, a novel class of atropisomers is identified, whose stereogenicity arises from hindered rotation around a C−Ag−C axis, instead of a typical C−C bond. [ABSTRACT FROM AUTHOR]

The synthesis of sulfoxide‐functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6‐p‐cymene)(NHC‐SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X‐ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2‐C,S) coordination mode of the NHC‐SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β‐alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC‐functionalised S‐Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β‐ol‐selectivities. [ABSTRACT FROM AUTHOR]

Two chiral imidazolium salts and one chiral triazolium salt were synthesized containing a chiral cyclohexane bridge based on the salen ligand framework of the Jacobsen-Katsuki epoxidation catalysts. They are promising NHC and aNHC ligand precursors for asymmetric epoxidation or C–H oxidation catalyzed by organometallic compounds. The ligand framework offers plenty of opportunities for modification.

Gold(I) complexes with N‐heterocyclic carbene ligands functionalized with a pendant phosphonium moiety were synthesized by simple procedures. In particular, the simple addition of LiBr salt in the reaction media allows the formation of the NHC gold(I) mononuclear complexes, whilst in the absence of excess bromide ions the deprotonation of the methylene group in alpha position to the phosphonium group occurs, allowing the isolation of the dinuclear complexes with two C,C‐bridging NHC‐phosphonium ylide ligands. The complexes were characterized in solution with NMR spectroscopy and ESI‐MS spectrometry, as well as in the solid state by means of single crystal X‐ray diffraction analysis. Mononuclear gold(III) species were also isolated by Br2 oxidative addition to the mononuclear gold(I) species and fully characterized. Preliminary results of the biological effects on MCF7 cancer cells are also reported. [ABSTRACT FROM AUTHOR]

The cycloaddition reactions of eleven substituted anthracenes with nosylated quinone imines provides a convenient route to the respective triptycenes. Following re‐aromatization, selective O‐butylation and cleavage of the nosyl‐group the respective triptycene anilines are obtained, which are converted into the respective imidazolium salts according to established procedures. Deprotonation of the imidazolium salts provide new triptycene‐NHC‐metal complexes (M=AuCl, RhCl(cod), IrCl(cod), RhCl(CO)2, IrCl(CO)2, PdI2(py), PtCl2(py), Pd(allyl)Cl) with unusual ligand sterics. [ABSTRACT FROM AUTHOR]

• Computational study of azide-alkyne cycloaddition with Au(I) and Ag(I). • DFT shows copper catalysts are more efficient than silver and gold. • NCI and RDG analyses reveal noncovalent interactions in metal catalysts. • ELF and LOL analyses explain lower efficiency of Ag(I) and Au(I) catalysts. • Proposes ligand modifications for improved Ag(I) and Au(I) catalysts. This study evaluated the catalytic efficiency of Au(I), Ag(I), and Cu(I) complexes in the azide‒alkyne cycloaddition (AAC) reaction through density functional theory (DFT) calculations. Cu(I) complexes exhibit superior catalytic performance, with lower energy barriers (8.8 kcal/mol) and a favorable Gibbs free energy of -0.9 kcal/mol in the key cycloaddition step, significantly outperforming Ag and Au complexes. Structural analysis revealed that shorter M−C bond lengths in the Cu complex contributed to increased stability. Additionally, the copper complex has a more negative Gibbs free energy for the formed metallacycle, indicating a thermodynamically favorable reaction pathway. Noncovalent interaction (NCI) and reduced density gradient (RDG) analyses of the Cu, Ag, and Au systems highlighted distinct interaction patterns influencing the reactivity. Furthermore, electron localization function (ELF) and localized orbital locator (LOL) analyses revealed bonding characteristics in those complexes. This study offers valuable insights into the mechanistic differences among Au(I), Ag(I), and Cu(I) complexes, paving the way for future research on enhancing the catalytic activity of copper, silver and gold complexes through ligand modification. [Display omitted] [ABSTRACT FROM AUTHOR]

Four new deep‐blue‐emitting iridium(III) NHC complexes containing sterically demanding ligands are synthesized. The four complexes show bright, deep‐blue emission, with emission maxima between 420 and 427 nm in both acetonitrile solution and 30 wt% doped films in TSPO1; the two meridional isomers showing photoluminescence quantum yields, ΦPL, in doped films of 80% and 89%. The two meridional isomers are used to assess the impact of emitters containing bulky, sterically demanding ligands on the performance of organic light‐emitting diodes (OLEDs). OLEDs employing a stepped doping profile with mer‐Ir(tfpi_tmBn)3 as the emitter produce the highest performing devices in this study, with these devices exhibiting deep‐blue [λEL = 429 nm, CIE = (0.16, 0.08)] emission and a maximum external quantum efficiency (EQEmax) of 14.9%, which decreases to 11.7% at 100 cd m−2. The performance of the OLEDs shows very good efficiencies and moderate efficiency roll‐offs in comparison to reported phosphorescent deep‐blue OLEDs with CIEy ≤ 0.08, as required for commercial displays. The promising results suggest that the design strategy of adding steric bulk to blue emitting iridium complexes containing NHC ligands is a useful strategy for reducing intermolecular interactions between emitters in OLEDs. [ABSTRACT FROM AUTHOR]

An efficient method for the C(2)−H arylation of (benz)imidazoles and (benz)oxazoles with aryl chlorides and aryl bromides under Ni/NHC catalysis has been developed. The main benefit of the method is the in situ generation of active Ni/NHC complexes from the air‐tolerant bench‐stable precursors NiCl2Py2, IMes⋅HCl, and potassium tert‐butoxide, which plays a dual role as base and Ni(II) to Ni(0) reductant. The approach represents a user‐friendly alternative for procedures relying on the use of toxic phosphine ligands or unstable air‐sensitive Ni(cod)2. The concept highlighted in the present study shows that mapping a competitive picture of catalyst dynamics and revealing the competitive processes towards the destruction and stabilization of catalytically active species enables a highly efficient catalytic system to be built under simple conditions. [ABSTRACT FROM AUTHOR]

A new imidazolium salt 3⋅HI with 8‐Bodipy as one N‐aryl substituent was synthesized from 8‐chloro‐Bodipy and imidazole followed by alkylation with iPrI. NHC‐metal complexes [CuCl(3)], [AuCl(3)], [Pd(allyl)Cl(3)] and [MCl(cod)(3)] and [MCl(CO)2(3)] (M=Rh, Ir) were synthesized and the X‐ray crystal structure of [IrCl(CO)2(3)] determined. The donation of NHC 3 was determined via IR (ν(CO) and cyclic voltammetry (Ir(I/II) redox potential). The photosensitizing properties of the complexes for the generation of 1O2 were quantified observing singlet oxygen quantum yield of up to Φs.o.=0.63. [IrCl(cod)(3)] displays poor Φs.o.=0.09, but, following a simple ligand exchange reaction of cod by two CO [IrCl(CO)2(3)], a pronounced increase to Φs.o.=0.63 is observed. [ABSTRACT FROM AUTHOR]

A new imidazolium salt 3⋅HI with 8‐Bodipy as one N‐aryl substituent was synthesized from 8‐chloro‐Bodipy and imidazole followed by alkylation with iPrI. NHC‐metal complexes [CuCl(3)], [AuCl(3)], [Pd(allyl)Cl(3)] and [MCl(cod)(3)] and [MCl(CO)2(3)] (M=Rh, Ir) were synthesized and the X‐ray crystal structure of [IrCl(CO)2(3)] determined. The donation of NHC 3 was determined via IR (ν(CO) and cyclic voltammetry (Ir(I/II) redox potential). The photosensitizing properties of the complexes for the generation of 1O2 were quantified observing singlet oxygen quantum yield of up to Φs.o.=0.63. [IrCl(cod)(3)] displays poor Φs.o.=0.09, but, following a simple ligand exchange reaction of cod by two CO [IrCl(CO)2(3)], a pronounced increase to Φs.o.=0.63 is observed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The masked carbenes have been used to transfer the NHC ligands to metal salts. • 1,3-Dimethylimidazolium-2-carboxylate react with metallic gold to generate Au-NHC. • There is great interest in environmentally friendly extraction of precious metals. 1,3-Dimethylimidazolium-2-carboxylate is a convenient precursor for the in situ generation of N -heterocyclic carbene 1,3-dimethylimidazolylidene (MeNHC) in aprotic solvents and aerobic atmosphere. The MeNHC carbene readily reacts with [Au(tht)Cl], [Au(PPh 3)Cl], and K[AuCl 4 ], which are common starting compounds in gold organometallic chemistry, to generate [Au(MeNHC)Cl], [Au(MeNHC) 2 ]+, and [Au(MeNHC) 2 Cl 2 ]+ complexes, respectively. Interestingly, 1,3-dimethylimidazolium-2-carboxylate can react with metallic gold powder in the presence of CuI to generate [Au(MeNHC) 2 ]+ cation. This methodology avoids the use of aggressive oxidizing agents, toxic chelating agents, or strongly acid or basic media commonly used in gold leaching processes. This reaction suggests the potential for developing more environmentally friendly methods for gold extraction in the future. [ABSTRACT FROM AUTHOR]

The use of alkynyl gold(I) complexes covers different research fields, such as bioinorganic chemistry, catalysis, and material science, considering the luminescent properties of the complexes. Regarding this last application, we report here the synthesis of three novel dinuclear gold(I) complexes of the general formula [(diNHC)(Au-C≡CPh)2]: two Au-C≡CPh units are connected by a bridging di(N-heterocyclic carbene) ligand, which should favor the establishment of semi-supported aurophilic interactions. The complexes can be easily synthesized through mechanochemistry upon reacting the pristine dibromido complexes [(diNHC)(AuBr)2] with phenylacetylene and KOH. Interestingly, we were also able to isolate the monosubstituted complex [(diNHC)(Au-C≡CPh)(AuBr)]. The gold(I) species were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. The emission properties were also evaluated, and the salient data are comparable to those of analogous compounds reported in the literature. [ABSTRACT FROM AUTHOR]

Although Schleyer's computations in 1979 predicted that the ground state of the parent diborirane features a planar‐tetracoordinate carbon atom (anti van't Hoff–Le Bel geometry), this work demonstrates that substitution of C coupled with N‐heterocyclic carbene (NHC) coordination provides access to isolable diborirane derivatives 3 and 4 with van't Hoff–Le Bel geometry. Species 3 and 4 are isoelectronic with cyclopropane and the highly strained B2C rings feature 2c–2e bent σ bonds. Consequently, the B−B and B−C bonds in 3 are cleaved in the presence of hydride, proton, and chalcogens. The former two reactions gave NHC‐coordinated fluorenyldihydridoborane 5 and dichlorofluorenylborane 6, respectively, whereas the latter transformations afforded novel thiaborirane 8 and selenaborirane 9. In addition, transfer hydrogenation of 3 with ammonia borane (H3N⋅BH3) led to the formation of (μ‐hydrido)diborane 7 via selective cleavage of the B−B bond. These reactivities show potential for their future application in organic synthesis. [ABSTRACT FROM AUTHOR]

A series of C,O-bidentate chelating mesoionic carbene nickel(ii) complexes [Ni(NHC^PhO)2] (NHC = imidazolylidene or triazolylidene) were applied for hydrosilylation of carbonyl groups. The catalytic system is selective towards aldehyde reduction and tolerant to electron-donating and -withdrawing group substituents. Stoichiometric experiments in the presence of different silanes lends support to a metal–ligand cooperative activation of the Si–H bond. Catalytic performance of the nickel complexes is dependent on the triazolylidene substituents. Butyl-substituted triazolylidene ligands impart turnover numbers up to 7,400 and turnover frequencies of almost 30,000 h-1, identifying this complex as one of the best-performing nickel catalysts for hydrosilylation and demonstrating the outstanding potential of O-functionalised NHC ligands in combination with first-row transition metals.

We present an experimental method, which provides information on the steric and electronic properties of ligands in metal complexes. This approach is based on the equilibrium of ion‐pairing of a cationic [(NHC)Ir(cod)]+ and a fluorescent bodipy‐sulfonate (bdpSO3−). The close ion‐pair of [(NHC)Ir(bdpSO3)(cod)] in toluene solution is weakly fluorescent, but dissociates into solvent‐separated ion pairs upon exposure to slightly more polar solvents. This spatial separation leads to a very pronounced increase of the fluorescence. The separation into distinct ions depends on the polarity of the solvent, but more importantly also on the stereoelectronic properties of the NHC ligand. 26 different NHC ligands with varying steric and electronic properties were probed in 1,2‐dichloroethane solvent. The electronic properties of the ligands were examined via the established descriptors (redox potential and ν(CO)). The systematic variation of NHC donation enables the deconvolution of electronic and steric contributions of the respective ligand. [ABSTRACT FROM AUTHOR]

The transition‐metal‐catalyzed C−N cross‐coupling has revolutionized the construction of amines. Despite the innovations of multiple generations of ligands to modulate the reactivity of the metal center, ligands for the low‐temperature enantioselective amination of aryl halides remain a coveted target of catalyst engineering. Designs that promote one elementary reaction often create bottlenecks at other steps. We here report an unprecedented low‐temperature (as low as −50 °C), enantioselective Ni‐catalyzed C−N cross‐coupling of aryl chlorides with sterically hindered secondary amines via a kinetic resolution process (s factor up to >300). A bulky yet flexible chiral N‐heterocyclic carbene (NHC) ligand is leveraged to drive both oxidative addition and reductive elimination with low barriers and control the enantioselectivity. Computational studies indicate that the rotations of multiple σ‐bonds on the C2‐symmetric chiral ligand adapt to the changing needs of catalytic processes. We expect this design would be widely applicable to diverse transition states to achieve other challenging metal‐catalyzed asymmetric cross‐coupling reactions. [ABSTRACT FROM AUTHOR]

The 7‐exo‐ and 8‐endo‐dig‐selective gold‐catalyzed cycloisomerizations of 2‐propargylamino biphenyl derivatives were developed. The reaction of terminal alkynes gave dibenzo[b,d]azepines by 7‐exo‐dig cycloisomerization. In contrast, when internal alkynes were subjected to the reaction, 8‐endo‐dig cycloisomerization proceeded to provide dibenzo[b,d]azocines. The nucleophilicity at the reaction site and the electron‐withdrawing effect of a tosyl group were important for the present selective transformation. This protocol could be used for ynamide substrates and a silver‐catalyzed reaction gave 7‐exo‐dig products selectively. [ABSTRACT FROM AUTHOR]

A copper‐based complex that contains a sulfonate N‐heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto‐boryl additions to alkenes, boryl and silyl substitutions, hydride‐allyl additions to alkenyl boronates, and additions of boron‐containing allyl moieties to N‐H ketimines. In this review article, we detail the shortcomings in the state‐of‐the‐art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC‐Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC‐Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio‐ and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals. [ABSTRACT FROM AUTHOR]

A mild stereo‐ and regioselective Cu‐catalyzed hydroboration method for the synthesis of (Z)‐seleno‐alkenyl boronates and (Z)‐thio‐alkenylboronates from internal alkynes in the presence of commercially available B2pin2 is presented. This highly selective transformation relies on the use of N‐heterocyclic carbene (NHC) complex IPrCuCl as the active catalytic species. We also explore the functionalization of the alkenylboronates obtained via oxidation to give α‐chalcogeno ketones, useful building blocks for the synthesis of more complex chalcogen‐containing molecules. [ABSTRACT FROM AUTHOR]

The isolation of simultaneously low‐coordinate and low‐valent compounds is a timeless challenge for preparative chemists. This work showcases the preparation and full characterization of tri‐coordinate rhodium(‐I) and rhodium(0) complexes as well as a rare rhodium(I) complex. Reduction of [{Rh(μ‐Cl)(IPr)(dvtms)}2] (1, IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolyl‐2‐ylidene; dvtms=divinyltetramethyldisiloxane) with KC8 gave the trigonal complexes K[Rh(IPr)(dvtms)] and [Rh(IPr)(dvtms)], whereas the cation [Rh(IPr)(dvtms)]+ results from their oxidation or by abstraction of chloride from 1 with silver salts. The paramagnetic Rh0 complex is a unique fully metal‐centered radical with the unpaired electron in the dz2 orbital. The Rh(‐I) complex reacts with PPh3 with replacement of the NHC ligand, and behaves as a nucleophile, which upon reaction with [AuCl(PPh3)] generates the trigonal pyramidal complex [(IPr)(dvtms)Rh‐Au(PPh3)] with a metal–metal bond between two d10 metal centers. [ABSTRACT FROM AUTHOR]

Mechanochemical synthesis of nine contemporary ruthenium catalysts used for olefin metathesis is described, being the first reported example of formation of Ru carbene organometallic complexes in solid state. Three key organometallic transformations commonly used in the synthesis of the second and third generations of Ru catalysts in solution—phosphine ligand (PCy3) exchange with in situ formed N‐heterocyclic carbene (NHC) ligand, PCy3 to pyridine ligand replacement, and benzylidene ligands interchange—were proved to work under mechanochemical conditions, affording the targets in high purity. Mechanochemical approach not only requires less amounts of organic solvent (null for synthesis, only for purification) and is scalable, but also allows for transformations that were reported impossible in the solution phase. [ABSTRACT FROM AUTHOR]

The Ni(NHC) catalyzed rearrangement of 1‐acyl‐2‐vinylcyclopropanes to the corresponding 4‐acyl‐cyclopent‐1‐enes is highly promising for the synthesis of keto‐functionalized annelated bi‐ and tricyclic subunits of natural products. Therefore, we investigated the catalytic activity of Ni(NHC) complexes in the rearrangement of 1‐acyl‐2‐vinylcyclopropanes with different ring sizes and substitution patterns. Surprising effects regarding substrate scope and stereoselectivity of the Ni(NHC) catalyzed vinylcyclopropane‐cyclopentene rearrangement were observed. Only vinylcyclopropanes with 1‐methyl, 1‐phenyl, 1,2‐dialkyl or 2‐phenyl‐substitution at the vinyl moiety could be rearranged successfully. Moreover, an endo‐configuration on the cyclopropane ring was required for successful rearrangement. By treatment of the vinylcyclopropanes with Rh catalysts or Lewis acids, the involvement of Lewis acid catalysis could be ruled out. In order to understand these experimental results and to rationalize the reactivity of the Ni(NHC) complexes computational studies were performed, which provided insights into mechanistic details. [ABSTRACT FROM AUTHOR]

The reaction of Collman's reagent Na2[Fe(CO)4]·2thf with one equivalent of Au(NHC)Cl (NHC = IMes, IPr; IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2) in dmso resulted in the [Fe(CO)4(AuNHC)]– (NHC = IMes, 1; IPr, 2) mono‐anions. 1–2 further reacted with Au(NHC)Cl or Au(PPh3)Cl affording the neutral complexes Fe(CO)4(AuNHC)2 (NHC = IMes, 3; IPr, 4), Fe(CO)4(AuIMes)(AuIPr) (5) and Fe(CO)4(AuNHC)(AuPPh3) (NHC = IMes, 6; IPr, 7). 1–7 have been spectroscopically characterized by IR, 1H, 13C{1H} and 31P{1H} NMR techniques. Moreover, the molecular structures of 1, 2, 4, 6 and 7 have been determined through single‐crystal X‐ray diffraction as their [NMe4][Fe(CO)4(AuIMes)], [NEt4][Fe(CO)4(AuIMes)], [NEt4][Fe(CO)4(AuIPr)], Fe(CO)4(AuIPr)2·1.5toluene, Fe(CO)4(AuIPr)(AuPPh3), Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2 salts and solvates. The nature of the bonds in 1 and 2 was elucidated on the basis of atoms‐in molecules (AIM) analyses on the DFT‐optimized structures and compared with the corresponding compounds 3 and 4. 1–7 contained strong Fe‐CO, Fe‐Au, Au‐P and Au–NHC bonds as well as weak Au···Au interactions. The different stability and reactivity of IMes‐derivatives vs. IPr‐ones was rationalized on the basis of steric effects. [ABSTRACT FROM AUTHOR]

Palladium complexes containing various N‐heterocyclic carbene (NHC) ligands have been successfully synthesized by the reaction with various β‐diketonate complexes. Treatments of Pd(acac)2 with NHC salts such as 1,3‐di(2,6‐diisopropylphenyl)imidazolium chloride (IPr·HCl) or 1,3‐di(mesityl)imidazolium chloride (IMes·HCl) resulted in [Pd(acac)(NHC)Cl] complexes and these complexes were characterized by NMR. Moreover, these palladium complexes showed moderate to good catalytic activities towards addition polymerization of various functional norbornenes. The effects of various reaction parameters onto the catalytic activity towards polymerization of functional norbornenes have been thoroughly investigated. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and photophysical properties of luminescent PtIV complexes that combine cyclometalated 1,2,3‐triazolylidene and bi‐ or terdentate 2,6‐diarylpyridine ligands are reported. The targeted complexes represent the first examples of PtIV species with a cyclometalated mesoionic aryl‐NHC ligand. They exhibit moderate or weak emissions in fluid solution at 298 K arising from 3LC states, which become very intense in poly(methyl methacrylate) (PMMA) matrices at 298 K. DFT and TD‐DFT calculations confirm that the chromophoric ligand is the cyclometalated 2,6‐diarylpyridine and show that the aryl‐NHC ligand exerts a beneficial effect on the emission efficiencies of these derivatives by increasing the energy of deactivating LMCT excited states with respect to comparable PtIV complexes with cyclometalated 2‐arylpyridine ligands. [ABSTRACT FROM AUTHOR]

A new heteroleptic cationic copper(I) complex bearing two N-heterocyclic carbene (NHC) ligands has been prepared. In situ, a Cu–O- bond can be generated which enables the complex to catalytically activate H2. The resulting complex shows activity in catalytic chemo- and stereoselective alkyne semihydrogenations as well as conjugate reductions of enones. [ABSTRACT FROM AUTHOR]

A set of nitro-activated ruthenium-based Hoveyda-Grubbs type olefin metathesis catalysts bearing sterically modified N-hetero-cyclic carbene (NHC) ligands have been obtained, characterised and studied in a set of model metathesis reactions. It was found that catalysts bearing standard SIMes and SIPr ligands (4a and 4b) gave the best results in metathesis of substrates with more accessible C–C double bonds. At the same time, catalysts bearing engineered naphthyl-substituted NHC ligands (4d–e) exhibited high activity towards formation of tetrasubstituted C–C double bonds, the reaction which was traditionally Achilles’ heel of the nitro-activated Hoveyda–Grubbs catalyst.

Complexes of the type (η3-allyl)Pd(L)(Cl) and (η3-indenyl)Pd(L)(Cl) are highly active precatalysts for the Suzuki–Miyaura reaction. Even though allyl and indenyl ligands are similar to cyclopentadienyl (Cp) ligands, there have been no detailed comparative studies exploring the activity of precatalysts of the type (η5-Cp)Pd(L)(Cl) for Suzuki–Miyaura reactions. Here, we compare the catalytic activity of (η5-Cp)Pd(IPr)(Cl) (IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, Cp) with two commercially available catalysts (η3-cinnamyl)Pd(IPr)(Cl) (Cin) and (η3-1-t-Bu-indenyl)Pd(IPr)(Cl) (tBuInd). We show that Cp gives slightly better catalytic activity than Cin, but significantly inferior activity than tBuInd. This order of activity is rationalized by comparing the rates at which the precatalysts are activated to the monoligated Pd(0) active species along with the tendency of the starting precatalysts to comproportionate with monoligated Pd(0) to form inactive Pd(I) dimers. As part of this work the Cp supported Pd(I) dimer (μ-Cp)(μ-Cl)Pd2(IPr)2 (CpDim) was synthesized and crystallographically characterized. It does not readily disproportionate to form monoligated Pd(0) and consequently CpDim is a poor catalyst for the Suzuki–Miyaura reaction.

The molecular structures of the chiral title compounds, [Pd(C19H28N2)2I2], (I), and [Pd(C19H28N2)Cl2(C18H15P)], (II), show a distorted square-planar coordination around the PdII atoms with two halogenide (Hal) ligands each and two N-heterocyclic carbene (NHC) ligands in (I) or one NHC and one triphenylphosphane ligand in (II). The deviations of the PdII atoms from the L2Hal2 best plane (L = NHC or triphenylphosphane ligand) are 0.206 (1) Å for (I) and 0.052 (1) Å for (II). The crystal packings exhibit intermolecular C—H...Hal hydrogen bonds.

Multicomponent reactions including Biginelli reaction and A3 coupling are useful synthetic methodologies as they can provide valuable intermediates and building blocks for the synthesis of bioactive natural compounds. The heterocyclic products of these transformations represent antibacterial, antitumor or anti‐inflammatory properties. In presented study, copper‐mediated protocols for Biginelli reaction and A3 coupling in sustainable solvents were reported. At first, charged NHC copper complexes were synthesized in few steps and characterized using spectroscopic methods followed by preliminary activity tests in sustainable media. Later in this study, Biginelli reaction and A3 coupling were proceeded using prepared copper catalytic systems under mild conditions. Desired nitrogen‐bearing products were obtained with moderate to very high yields. Some attempts to recycle one of the received NHC copper initiators were also investigated. [ABSTRACT FROM AUTHOR]

Cancer is one of the main public health problems globally, there is a public demand for better drugs. Rational strategies or approaches are used to improve the success of drug discovery. Our strategy was to the repurposing of well-known antifungal agents as potential anticancer drugs, such as Clotrimazole (CTZ) and Ketoconazole (KTZ). We prepared the respective iodide imidazolium salt L1 : (CTZ-Me)I and L2 : (KTZ-Me)I to be the intermediates toward the synthesis of its respective NHC ligand and achieve the respective silver(I)-monoNHC and silver(I)-bisNHC derivatives: [Ag(L1)I] (1), [AgI(L2)] (2) [Ag(L1) 2 ]I. (3), [Ag(L2) 2 ]I. (4), as well as their corresponding coordination compounds [Ag(CTZ) 2 ]NO 3 (5) and [Ag(KTZ) 2 ]NO 3 (6) where these ligands (CTZ and KTZ) coordinate to silver through the N-imidazole atom. These compounds (L1 , L2 and complexes 1 – 6) exhibited significant activity against the tested cancer cell lines (B16-F1, murine melanoma strains and CT26WT, murine colon carcinoma). The silver(I) complexes were more active than the free ligands, complexes 2 and 4 being the most selective in B16-F1 cancer cell line. Two possibles biological targets such as DNA and albumin were examined for the observed anticancer activity. Results show that DNA is not the main target, however, the interactions with albumin suggest it can transport/delivery the metal complexes. The iodide imidazolium salt of clotrimazole (CTZ) and ketoconazole (CTZ), as precursors of the respective N-Heterocyclic carbenes (NHC) ligands and their silver(I)-monoNHC and silver(I)-bisNHC derivatives, as well as their corresponding coordination compounds [Ag(L) 2 ]NO 3 (L = CTZ or KTZ) were synthesized. The NHC-KTZ silver(I) complexes were the most selective against B16-F1 cells. [Display omitted] • Four Ag(I)-NHC complexes and two Ag(I) coordination compounds were synthesized. • They were evaluated against two cancer cell lines. • The silver(I) complexes were more active than the free ligands. • DNA is not the main target. Their interactions with albumin were very promising. [ABSTRACT FROM AUTHOR]

The synthesis of heterobimetallic complexes remains a synthetic challenge in the field of organometallic chemistry. A possible approach in this regard might be the use of a bidentate heteroditopic bis(carbene) ligand that combines an imidazol-2-ylidene (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by an organic spacer. The optimized strategy to heterobimetallic complexes with this type of ligand involves a 3-step procedure: (i) Coordination of the nNHC, functionalized with a 1,2,3-triazole ring, to a metal center; (ii) formation of the triazolium ring by alkylation of the triazole N-3; (iii) deprotonation of the tzNHC precursor and coordination of the second metal center. Following this procedure, a novel Au(I)-Ag(I) dinuclear complex was isolated and its properties were compared to the analogous homobimetallic Ag(I)-Ag(I) and Au(I)-Au(I) complexes. The study was completed by the determination of the molecular structures of some synthetic intermediates.

A copper(I)-catalyzed semihydrogenation of internal alkynes has been developed. A variety of oxygen- and nitrogen-tethered N-heterocyclic carbene (NHC) complexes have been investigated, leading to a highly Z-selective transformation. The catalyst is generated from inexpensive copper(I) chloride in situ and allows catalytic semihydrogenation down to 10 bar H2. [ABSTRACT FROM AUTHOR]

A series of imidazolinium-based CCC pro-ligands featuring N -Mes, Dipp, i Pr and t Bu substituents ( 1 ·2HCl) have been prepared. The corresponding free carbenes are readily generated through deprotonation by strong bases and, in addition to being characterised in situ by 1 H and 13 C NMR spectroscopy, were trapped through reaction with CuCl. Iridium pincer compounds of the N -Mes ( 5a ) and N -Dipp ( 5b ) substituted ligands, viz . [Ir( 1 )HCl(NCMe)], were obtained through reaction between the respective pro-ligand, [Ir(COE) 2 Cl] 2 , and Et 3 N in acetonitrile at ca. 80 °C. Under similar conditions the N - i Pr and N - t Bu analogues were not formed. The new iridium pincer complexes 5a and 5b were fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state by X-ray diffraction. Under relatively forcing reaction conditions neither 5a nor 5b , in combination with KO t Bu, show any significant catalytic activity for the transfer dehydrogenation of cyclooctane to cyclooctene using tert -butylethylene as the sacrificial hydrogen acceptor (ca. 2 TONs, 150 °C, 24 h). [ABSTRACT FROM AUTHOR]

The synthesis of Rh(III)Cp*–NHC and Ir(III)Cp*–NHC complexes from amino acid ligand precursors is reported. These complexes were tested as catalysts in TH with good conversions in 2 h and TON values greater than 100. [Display omitted] Heteroleptic [RhIIICp*(NHC)] and [IrIIICp*(NHC)] complexes bearing a N -amino acid functionality derived from imidazolium salts were synthesized and characterized by NMR, IR, HR-MS, and single-crystal X-ray diffraction. A discussion of the structure of the [RhCp*{κ2(C,O)–NHC}Cl] complex 3f as a pair of diastereomers is presented. Catalysts were assessed for the transfer hydrogenation (TH) of ketones in basic propan-2-ol, obtaining good yields for the reduction products. A chromatographic method applied for measurements of compounds in TH as analytes was validated. The Rh and Ir complexes described here offer an original approach in the development of catalysts based in classic NHC precursors. [ABSTRACT FROM AUTHOR]

Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal–ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO2. In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.

The synthesis and characterization of the σ -mesityl NHC-complex (SIMes)CuMes ( 1 ) and the dicopper(I) oxalato complex [(SIMes)Cu] 2 (C 2 O 4 ) ( 2 ; SIMes = 1,3-dimesitylimidazolin-2-ylidene) are described. 1 , a new example of a σ -organocopper complex bearing an imidazolin-2-ylidene ligand, is obtained from the stoichiometric reaction of mesitylcopper with SIMes. The single crystal X-ray structure determination of 1 reveals a linearly coordinated copper ion. Clean reaction of 1 with 0.5 equiv. of oxalic acid affords the dinuclear copper(I) oxalato complex 2 with concomitant elimination of mesitylene. As confirmed by X-ray crystallography, the cuprous ions are bridged by the oxalate in a μ -1,2,3,4 mode and are each coordinated by one NHC-capping ligand, thus both copper(I) exhibit trigonal coordination geometries, though with slight distortions. 2 represents the first example of a copper(I) oxalato complex stabilized by two N -heterocyclic carbene ligands. Whereas 1 is only moderately air-stable in the solid state, 2 resists to non-inert conditions at least for several days. The thermal decomposition behavior of 2 was investigated by thermal gravimetric analysis/differential scanning calorimetry coupled with mass spectrometry (TGA/DSC-MS) and revealed the elimination of HCN, CO and CO 2 in the temperature range 210–350 °C to finally give elemental copper in a controlled degradation process. [ABSTRACT FROM AUTHOR]

A direct correlation of the NHC ligand structure and the induced enantioselectivity has been uncovered for a series of new chiral gold(I) complexes. The Au(I)Cl complexes were identified by 1 H, 13 C NMR spectroscopy, and X-ray structure analysis. The corresponding activated Au(I) catalysts with a phenyl nitrile dynamic ligand are prepared and used in the catalysis of 1,6-enyne cyclization reactions. High chemical yields and good enantioselectivity have been obtained with 6% mol of the chiral NHC–Au(I) catalysts. The products in these reactions reach up to 75% ee. The structure of the chiral NHC ligand, where an ortho -biphenyl moiety is connected to the nitrogen atoms, the 3,5-dialkyl distal phenyl group plays a determining role in enantioselectivity. [ABSTRACT FROM AUTHOR]

To achieve high selectivity in directed C-H activation, an NHC ligand was introduced to a palladium catalyst. A range of Pd-PEPPSI complexes were applied in the direct acetoxylation of 2-phenylpyridine. The best catalyst was found to be the one based on a diisopropylphenyl-substituted NHC ligand, and this was successfully used for the functionalization of sp2 as well as more challenging sp3 bonds for a broad variety of substrates. The explored method showed a highly improved selectivity compared to previously reported results with up to 96 % yield for the monoacetoxylated product. Kinetic studies show that sterics of the catalyst is less important in dictating conversion and selectivity; despite an induction period, it was shown that the catalyst is molecular in nature. [ABSTRACT FROM AUTHOR]

The coordination chemistry of chiral NHC-oxazoline bidentate ligands was investigated with silver(I) and gold(I), yielding stable dinuclear cationic complexes in good yields via N -coordination of the oxazoline heterocycle. After full characterization of these complexes, the reactivity in homogeneous catalysis and under oxidative conditions was studied. [Display omitted] [ABSTRACT FROM AUTHOR]

The catalytic properties of olefin metathesis ruthenium complexes bearing N-heterocyclic carbene ligands with stereogenic centers on the backbone are described. Differences in catalytic behavior depending on the backbone configurations of symmetrical and unsymmetrical NHCs are discussed. In addition, an overview on asymmetric olefin metathesis promoted by chiral catalysts bearing C2-symmetric and C1-symmetric NHCs is provided. [ABSTRACT FROM AUTHOR]

In this account the authors' latest results in C-C coupling catalysis are reviewed. First, an efficient catalytic system for the Kumada-Tamao-Corriu coupling reaction based on NHC-phosphine (NHC = N-heterocyclic carbene) nickel complexes is presented. Then the use of palladium complexes of chiral ferrocenyl NHC-phosphines in the asymmetric Suzuki-Miyaura coupling reaction is reported. High catalytic activities and moderate enantioselectivities (ee up to 46%) were obtained. Chiral ferrocenyl phosphine-ethers were also tested in the asymmetric Suzuki-Miyaura reaction yielding good activities and moderate enantioselectivities (ee up to 37%). Finally, the original synthesis of a ferrocenyl rhodium(III) complex and its successful use as catalyst for a C-C coupling reaction via C-H activation of 2-phenylpyridine is presented. [ABSTRACT FROM AUTHOR]

Silver(I) and copper(I) complexes containing neutral bis(N-heterocyclic carbene) (NHC) ligands and coordinated or non-coordinated chloride, bromide, iodide, or tetrafluoroborate anions, were synthesised. The nature of the anions impacts deeply the structural features of the complexes in the solid-state and neutral cubane-, neutral coordination polymer-, or dicationic bridged-type architectures have been characterised. The structures of (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)disilver(I) dichloride ( 2a ), bis(μ-1,3-bis(3′-butylimidazol-2′-ylidene)benzene-κ-C)tetra-μ 3 -bromotetrasilver(I) ( 2b ), bis(1,3-bis(3′-butylimidazol-2′-ylidene)benzene)disilver(I) tetrafluoroborate ( 2d ) in 2d ·CH 2 Cl 2 , (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)dicopper(I) dichloride ( 3a ) and (1,3-bis(3′-butylimidazol-2′-ylidene)benzene)dicopper(I) dibromide ( 3b ) were established by X-ray diffraction. [ABSTRACT FROM AUTHOR]

A series of new chiral [(NHC)Au(I)Cl] are synthesized. The chiral NHC ligands feature pairs of ortho-biphenyl groups. The distal phenyl groups of the biphenyl substituents are varied from simple phenyl, 3,5-dimethylphenyl, to 3,5-di-t-butylphenyl. X-Ray structure analysis were performed for two such [(NHC)Au(I)Cl] complexes. High percent buried volumes were calculated for two novel chiral [(NHC)Au(I)Cl]complexes whose structure were solved by an X-ray analysis. The rotational motion in the biphenyl group is slow on the NMR time scale. Rotational barriers were determined using WINDNMR for two of the chiral gold complexes. [ABSTRACT FROM AUTHOR]