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This review considers phosphine and N-heterocyclic carbene complexes of gold(I) that are used as (pre)catalysts for a range of reactions in organic synthesis. These are divided according to the structure of the ligand, with the narrative focusing on studies that offer a quantitative comparison between the ligands and readily available or widely used existing systems.

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., The generous financial support from MINECO/FEDER/AGE (CTQ2017‐83421‐P, C.T.), Gobierno de Aragón/FEDER (GA/FEDER, Inorganic Molecular Architecture Group E08_17R; C.T.) and the Netherlands Organization for Scientific Research (NWO) (TOP Grant 716.015.001, BdB) is gratefully acknowledged. V.V.‐I. thanks MINECO/FEDER for a FPI fellowship.

Resumen del trabajo presentado a la XXXVIII Reunión Bienal de la Real Sociedad Española de Química, celebrada en el Palacio de Congresos de Granada, del 27 de junio al 30 de junio de 2022.

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.

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.

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&ndash, C double bonds. At the same time, catalysts bearing engineered naphthyl-substituted NHC ligands (4d&ndash, e) exhibited high activity towards formation of tetrasubstituted C&ndash, C double bonds, the reaction which was traditionally Achilles&rsquo, heel of the nitro-activated Hoveyda&ndash, Grubbs catalyst.

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.

Copper(I) species exhibit various coordination modes as a function of surrounding ligands. Complexes bearing N-heterocyclic carbenes as ligands are usually monomeric species with the copper adopting a linear geometry. Contrarily to the parent phosphine complexes, in which the presence of species of higher nuclearity is well established, fewer examples of such copper(I)–NHC species are reported and, interestingly, their number is growing. Moreover, these are no longer a mere curiosity from a coordination perspective since they have been shown successful in catalysis. The aim of this review is to present a summary of the synthesis and structural properties of dinuclear copper(I)–NHC complexes. It is our hope that by centralizing all information available on such species, future efforts aimed at exploiting their full potential can be facilitated.

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.

Two gold(I) complexes of an amido-N functionalized 1,2,4-triazole derived N-heterocyclic carbene ligands have been synthesized and structurally characterized. In particular, the {[1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene](2)Au}Cl-+(-) [R = Et (1c) and i-Pr (2c)] complexes were conveniently synthesized from their respective silver(I) analogs (1-2)b by the direct reaction with (SMe2)AuCl in CH3CN at room temperature. The molecular structure determination of the gold(I) (1-2)c complexes using single crystal X-ray diffraction study revealed a syn-disposition of two N-heterocyclic carbene (NHC) ligands bound across the Au(I) center in a linear fashion with it's two amido-N functionalized sidearms interacting with a non-coordination Cl anion through two intramolecular H-bonds.

The reaction of the Collman’s reagent Na2Fe(CO)4 with two equivalents of Au(NHC)Cl (NHC = IMes, IPr, IBu) in thf results in the bimetallic Fe(CO)4(AuNHC)2 (NHC = IMes, 2; IPr, 3; IBu, 4; IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H 3 i Pr2)2; IBu = C3N2H2(CMe3)2) clusters in good yields. Heating 2 in dmf at 100 °C results in the higher nuclearity cluster [Au3Fe2(CO)8(IMes)2]− (5). 2–5 have been fully characterized via IR, 1H and 13C NMR spectroscopies and their structures determined by means of single crystal X-ray crystallography. Gas-phase DFT calculations were carried out on 2–5 and the model compound cis-Fe(CO)4(AuIDM)2 (6) (IDM = C3N2H2Me2), in order to better understand the metal–metal and metal–ligand interactions in these compounds without the influence of packing forces.

As petrochemical resources become increasingly scarce and expensive, much attention has been focused on renewable resources from biomass as alternative options for producing basic building blocks for chemical manufacturing. Catalytic olefin metathesis represents a powerful tool to transform biosourced structural motifs in valuable commodity, fine, and specialty chemicals. In that respect, the appropriate choice of the catalyst is the key issue of each metathesis transformation. The current study examines the influence of different N-heterocyclic carbene (NHC) ligands containing one or two N-alkyl substituents on the efficiency of Hoveyda&ndash, Grubbs-type catalysts in the cross-metathesis reaction of ethyl oleate with cis-1,4-diacetoxy-2-butene and cross-metathesis of eugenol acetate with cis-1,4-dichloro-2-butene. Interestingly, the introduction of alkyl N-substituents in the NHC ligand was revealed as beneficial for catalytic performances in the examined cross-metathesis (CM) reactions, leading to higher activity and/or selectivity than those observed in the presence of the classical, commercially available Hoveyda&ndash, Grubbs second generation catalyst (HGII).

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, 13C1H and 31P1H 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.

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.

The authors are grateful to the University of St Andrews and EPSRC financial support (grants EP/P010482/1 and EP/M02105X/1). A.K.P thanks the Leverhulme Trust Early Career Fellowship (ECF-2017-326). I.D.W.S. acknowledges a Royal Society Wolfson Research Merit award. High‐efficiency pure blue phosphorescent organic light‐emitting diodes (OLEDs) remain one of the grand challenges, principally because the emissive complexes employed either do not possess sufficiently high photoluminescence quantum yields or exhibit unsatisfactory Commission International de l'Éclairage (CIE) coordinates. Here two deep‐blue‐emitting homoleptic iridium(III) complexes are reported and OLEDs are demonstrated with CIE coordinates of (0.15, 0.05) and maximum external quantum efficiency of 13.4%, which decreases slightly to 12.5% at 100 cd m−2. They represent examples of the most efficient OLEDs surpassing the CIEy requirement of the National Television System Committee (NTSC) and the European Broadcasting Union (EBU). Emitter orientation contributes to the excellent device performance. Postprint

The chapter one introduces the reported examples of rare earth metal (RE) complexes with different oxidation states. It also reviews the synthesis and reactivity study of N-heterocyclic carbene (NHC) supported transition metal and RE metal complexes. Chapter two focusses on the synthesis and characterisation of a series of tetraaryloxide Ce and Pr complexes. With the reaction of bulky tetraphenol proligand H4LR(R = P, PT, M) with four equivalents of KN"(N" = N(SiMe3)2), a dimerised complex of [K4LP]2(thf)11 was synthesised and characterised. The salt metathesis reactions of this complex with RECl3(thf)2 afford bimetallic aryloxide complexes of K2L2RE2(thf)11 (RE = Ce, Pr), which display divergent structures under different conditions. Reactions of the CeIII complex of K2L2Ce2(thf)11 with a variety of oxidants(I2, CuCl2 and O2, etc.) lead to the oxidation of CeIII to CeIV, affording purple ceric dimer of L2Ce2. The reaction of the PrIII complexes with I2 under 60 °C affords a mixture from which PrIII iodide (LPr2I2) has been isolated and characterised. This chapter also discusses the reactivity of the bimetallic aryloxide complexes towards different substrates, such as MeLi, KC8 and KBn (Bn = benzyl). Bimetallic complexes of L(REX)2(py)8 (RE = Ce, Pr; X = Cl, BH4) are synthesised and characterised. The preliminary study on the copolymerization of cyclohexene oxide (CHO) and CO2 is conducted for CeIII and PrIII complexes. Chapter three details the work on two different types of NHC ligand. The first ligand is the β-ketoimidazolinium salts H2LBr {L = RC(O)CH2{CH[NCH2CH2NMes], R = tBu, naphth} which reacts with MHBEt3 (M = Na, K) to form carbene-borane adducts RC(O)CH2{C(BEt3[NCH2CH2NMes]}. This type of reactivity differs from the previous work on imidazole derivatives. The possible mechanism of these reactions is provided and discussed. The other ligand is p-aryloxy-tethered imidazolinium salt H2LX (L = N-3,5-di-tert-butyl-4-hydrooxyphenyl-N’-mesityl-imidazolinium, X = Cl, Br, PF6 ), which have been synthesised and characterised. The reactions of these salts with MN"(M = Na, K) enabled the characterisation of polymerised complexes of [NaL]n and [KL(thf)2]n. The yttrium complex YL3 is synthesised and its reactivity towards small molecules such as boranes, CO2 and CS2 is discussed. Chapter four presents the primary results on the study of macrocyclic NHC based cyclophane ligand H6LPF6 (L = calix[4]imidazolylidene[2]pyrazolato). Investigations on the reactivity of the ligand towards different bases (NaN", KN", KBn etc.) are examined and subsequent metathesis reactions with RE complexes are explored. Chapter five concludes the work presented in this thesis. Chapter six contains all experimental and characterisation details.

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.

International audience; 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-μ3-bromotetrasilver(I) (2b), bis(1,3-bis(3′-butylimidazol-2′-ylidene)benzene)disilver(I) tetrafluoroborate (2d) in 2d·CH2Cl2, (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.

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.

The PdII atoms in two N-heterocyclic carbene(NHC)/halogenide complexes show distorted square-planar coordination environments. In one complex, two NHC ligands are present, and the second complex contains an auxiliary tri­phenyl­phosphane ligand., 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 L 2Hal2 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.

Transition metal-nanoparticles (MNPs) have received a great deal of attention in fields such as organometallic drugs, materials, and catalysis. The choice of an appropriate stabilizer (e.g. polymers, surfactants or ligands) is a crucial aspect to consider for the synthesis of MNPs in order to finely control their size, shape and surface state. Due to their strong coordination properties, N-heterocyclic carbenes (NHC) are excellent stabilizers for MNPs, able to tune their surface properties and consequently their catalytic behavior. NHC-stabilized MNPs have been obtained by using organometallic complexes as precursors [1] or by a ligand-exchange procedure. We report herein a new route to the synthesis of a long chained NHC-stabilized Pd NPs (NHC-Pd NPs) by exploiting the metal vapor synthesis (MVS) technique (Figure 1). This synthetic approach provides access to various NHC-M NPs with tailored size and composition (heterometallic M NPs). The NHC-Pd NPs were characterized by TEM (dm = 2.5 nm), NMR and TGA and further immobilized onto a carbonaceous support (Vulcan XC-72). The catalytic efficiency of carbon-supported NHC-Pd NPs was evaluated in the hydrogenation of R-(+)-limonene under mild conditions (P(H2) = 5 Bar, T = 30°C). The NHC surface modification of Pd NPs resulted in significant differences in reaction selectivity compared to analogous carbon-supported Pd NPs synthesized without the ligand, affording high yield toward (+)-p-1-menthene product.

International audience; 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 TiO 2. 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.

Various ways of preparing enantiomerically pure 2-amino[6]helicene derivatives were explored. Ni(0) mediated cyclotrimerization of enantiopure triynes provided (M)- and (P)-7,8-bis(p-tolyl)hexahelicene-2-amine in >99% ee as well as its benzoderivative in >99% ee. The stereocontrol was found to be inefficient for a 2- aminobenzo[6]helicene congener with an embedded five-membered ring. Helically chiral imidazolium salts bearing one or two helicene moieties have been synthesized and applied in enantioselective [2+2+2] cyclotrimerization catalyzed by an in situ formed Ni(0)-NHC complex. The synthesis of the first helically chiral Pd- and Ru- NHC complexes and their application in enantioselective catalysis was demonstrated. The latter shows promising results in enantioselective olefin metathesis reactions. A mechanistic proposal for asymmetric ring closing metathesis is provided.

Michael addition reactions of cyclic beta-oxo ester and alpha-methyl cyano ester substrates with acceptor activated olefins were catalyzed by iron N-heterocyclic carbene (NHC) complexes of the type, {Cp[ 1benzyl- 3-alkyl-benzimidazol-2-ylidene] Fe(CO)(2)} I, where [alkyl = i-Pr (1) and Et (2)] in the presence of L-proline as chiral auxiliary under ambient conditions. In case of the cyclic b-oxo ester substrates good yields of up to 68% and enantioselectivities of up to 18% were observed while for the a-methyl cyano ester substrates similar yields of up to 82% and enantioselectivities of up to 8% were observed. Notably, for the cyclic b-oxo ester substrates, the five membered ethyl-2-oxocyclopentane-1-carboxylate gave superior yields and enantioselectivities over the six membered ethyl-2-oxocyclohexane-1-carboxylate. In case of alpha-methyl cyano ester substrates, methyl vinyl ketone gave higher yields as compared to the other acceptor activated olefins namely, ethyl vinyl ketone and acrylonitrile. Based on the mass spectrometry studies, that detected the presence of L-proline adduct species (1-2) A, L-proline and substrate adduct species (1-2) B and L-proline and product adduct species (1-2) C in the catalysis reaction mixture, a mechanistic cycle has been proposed for the Fe-NHC mediated Michael addition reaction. (C) 2018 Elsevier B.V. All rights reserved.

Neutral bimetallic silver(I) and gold(I) complexes of N- heterocyclic carbene ligands derived from racemic biphenyl and axially chiral binapthyl frameworks have been synthesized and structurally characterized. In particular, the neutral bimetallic silver(I) complexes of the type {L(NHC)(2)M-2} (M=Ag, Au) [L=2,2-dioxo-biphenyl, NHC=1-methyl-3-(-2-phenyl-acetamido)-imidazol-2-ylidene (1f); L=2,2'-dioxo-binapthyl, NHC=1-methyl-3-(-2-phenyl-acetamido)-imidazol-2-ylidene (2e)] are synthesized from the corresponding imidazolium chloride salts (1-2)d by the reaction with Ag2O in quantitative yields. The gold(I) complexes are synthesized by the transmetallation of their corresponding {L(NHC)(2)}Ag-2 complexes. Quite interestingly, a weaker argentophilic interaction was observed in the solid state X-ray crystalllographic structure of binapthyl derivative 2e, as Ag center dot center dot center dot Ag contact of 3.3011 angstrom while in case of biphenyl derivative 1f there is no such aurophilic interaction exhibited a significant Au center dot center dot center dot Au contact.

S.P.N. thanks the ERC (Advanced Investigator Award ‘FUNCAT’), the EPSRC and Syngenta for funding. S.P.N. is a Royal Society Wolfson Merit Award Holder. D.J.N. thanks the University of Strathclyde for a Chancellor's Fellowship. A.P. thanks the Spanish MINECO for a Ramón y Cajal contract (RYC-2009-05226) and the European Commission for a Career Integration Grant (CIG09-GA-2011-293900). The electronic nature of the interaction of NHCs with metal centres is of interest when exploring their properties, how these properties influence those of metal complexes, and how these properties might depend on ligand structure. Selenourea and phosphinidene complexes have been proposed to allow the measurement of the π-accepting ability of NHCs, independent of their σ-donating ability, via the collection of 77Se or 31P NMR spectra, respectively. Herein, the synthesis and characterisation of selenoureas derived from a range of imidazol-2-ylidenes, 4,5-dihydroimidazol-2-ylidenes and triazol-2-ylidenes are documented. Computational studies are used to explore the link between the shielding of the selenium centre and the electronic properties of the NHCs. Results show that δSe is correlated to the energy gap between a filled lone pair orbital on Se and the empty π* orbital corresponding to the Se–NHC bond. Bond energy decomposition analysis indicated no correlation between the orbital σ-contribution to bonding and the chemical shielding, while a good correlation was found between the π-contribution to bonding and the chemical shielding, confirming that this technique is indeed able to quantify the ability of NHCs to accept π-electron density. Calculations conducted on phosphinidene adducts yielded similar results. With the link between δSe and δP and π-back bonding ability clearly established, these compounds represent useful ways in which to fully understand and quantify this aspect of the electronic properties of NHCs. Publisher PDF

A new type of tricyclic triazolooxazine derived N-heterocyclic carbene precursors were developed by the alkylation reaction of a tricyclic triazolooxazine framework. In particular, the reaction of 5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazine with methyl iodide and ethyl iodide yielded the tricyclic triazolooxazine derived N-heterocyclic carbene precursors, (1-2)a, in 67 -84% yield. The tricyclic triazolooxazinium iodide salts, (1-2)a, underwent metallation in a straight forward manner upon treatment with PdCl2 in the presence of K2CO3 in pyridine to give the trans-{3-(R)-5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazin-4-ylidene} PdI2(pyridine) [R = Me (1b), Et (2b)] complexes in 23 -25% yield. The new tricyclic triazolooxazine derived N-heterocyclic carbene moiety, as stabilized upon binding to palladium in the (1-2)b complexes, was structurally characterized by the X-ray single crystal diffraction studies.

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed.

International audience; Homoleptic and cationic [Ag(IPr)2]PF6 [IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazolylidene] is an efficient catalyst for the A3‐coupling reaction. Propargylamines are synthesized in short reaction times with low catalyst loadings under microwave irradiation or continuous flow. This approach is user friendly, versatile, and compatible with both aliphatic and aromatic aldehydes and alkynes.

A series of gold(I) pioneer complexes bearing N-heterocyclic carbenes and steroid derivatives (ethynylestradiol and ethisterone) with the generic formula [Au(R-imidazol-2-ylidene)(steroid)] (where R=CH or CHCHOCH) were synthesized, and the X-ray structure of a rare of gold(I)–estradiol derivative is discussed. Toxicity studies reveal notable antibacterial activity of the gold-based compounds, which is significantly increased in vivo by the presence of the estradiol unit. Toxicity profiling was estimated in vitro versus Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, and in vivo on Galleria mellonella larvae against E. coli., This work was supported by the University of Zaragoza (UZ2015-CIE-09). A.V.thanks the AragonGovernment for apredoctoral fel-lowship.

The 1 : 1 molar reactions of niobium and tantalum pentahalides with the monodentate NHC ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Ipr), in toluene (or benzene) at ca. 80 °C, afforded the complexes NbX5(Ipr) (X = F, 2; Br, 3) and TaX5(Ipr) (X = F, 4; Cl, 5; Br, 6), in generally good yields. Complexes 2–6 represent uncommon cases of stable NHC adducts of metal halides with the metal in an oxidation state higher than +4, and also rare examples of Nb–NHC and Ta–NHC bonding systems. In particular, the X-ray molecular structure determined for 6 provides the unprecedented crystallographic characterization of a tantalum compound with a monodentate NHC ligand. DFT results indicate that the metal–carbon bond in 2–6 is a purely σ one. According to NMR studies (1H, 13C, 93Nb), the formation of 3, 5, 6, as well as the previously communicated NbCl5(Ipr), 1, proceeded with the intermediacy of [MX6]− salts, presumably due to steric reasons. On the other hand, the intermediate formation of MF6− in the pathways to 2 and 4 was not observed, according to 19F (and 93Nb in the case of 2) NMR. DFT calculations were carried out in order to shed light on structural and mechanistic aspects, and allowed to trace possible reaction routes.

The direct functionalization of aromatic C-H bonds by carbene insertion from diazocompounds catalyzed by gold complexes with N-heterocyclic ligands is described. The reaction is completely selective toward the Csp2-H bonds, other Csp3-H bonds remaining unreacted. A study with a several NHC ligands in Au(I) and Au(III) complexes has been performed. A potential application in profen derivatives has also been developed, DGI (CTQ2008-00042BQU and Consolider Ingenio 2010, Grant CSD2006-0003) and Junta de Andalucía (P07-FQM-02870)

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.

Bring on the big cats: New, C2-symmetric bulky N-heterocyclic carbene ligands bring major improvements in the palladium-catalyzed asymmetric intramolecular α-arylation of amides to give oxindoles (see picture, dba=trans,trans-dibenzylideneacetone), which are formed in high yield and excellent enantiomeric purity.

International audience; A series of palladium complexes of the abnormal N-heterocyclic carbene ligands of the type (a-NHC)PdI2(L) [L = NC5H5(1–3)b and PPh3(1–3)c] effectively catalyzed the Hiyama coupling of aryl bromides and iodides with PhSi(OMe)3 under the highly desired fluoride-free conditions. Interestingly enough, the pyridine based trans-(1–3)b complexes and a PPh3 derived cis-3c complex exhibited higher yields than the related PPh3 derived trans-(1–2)c complexes. The superior performances of the pyridine based trans-(1–3)b complexes and the PPh3 derived cis-3c complex have been correlated to a tighter binding of the a-NHC ligand to the palladium center in these complexes, leading to a greater (a-NHC) ligand influence on the metal center partaking in the catalysis

A series of iron(II) complexes of N-heterocyclic carbene ligands was synthesized and fully structurally characterized. Specifically, the benzimidazole based {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO)(2)}I [R = Et (1b), i-Pr (2b) and n-Bu (3b)] and the imidazole based {Cp[1-benzyl-3-R-imidazol-2-ylidene] Fe(CO)(2)}PF6 [R - Me (4b) and Et (5b)] type of complexes were synthesized from their respective benzimidazolium iodide (1-3)a and their imidazolium hexafluorophosphate (4-5)a salts by the reaction with CpFe(CO)(2)I in the presence of MN(SiMe3)(2) (M = Li or K) as a base. The molecular structures of the (1-5)b complexes reveal that the metal center display a conventional piano stool structure. More importantly, the (1-5)b complexes, when irradiated with visible light, effectively catalyzed the hydrosilylation reaction of carbonyl compounds namely, of the aldehyde and ketone substrates, using organosilane reagents. Specifically, the (1-5)b complexes performed the hydrosilylation of a representative benzaldehyde substrate using phenylsilane in ambient conditions at 30 degrees C while that of the representative acetophenone substrate at a more elevated temperature of 70 degrees C. The benzimidazole derived complexes (1-3)b displayed superior activity than the imidazole derived (4-5)b complexes. (C) 2014 Elsevier B. V. All rights reserved.

International audience; A series of new piano-stool iron(II) complexes comprising N-heterocyclic carbene ligands [Fe(Cp)(CO)2(NHC)]I (NHC = 1,3-disubstituted imidazolidin-2-ylidene) have been synthesized and analyzed by 1H NMR, 13C NMR, IR, elemental analysis and mass spectrometric techniques. These compounds were easily prepared from the reaction of disubstituted imidazolidin-2-ylidene with [FeI(Cp)(CO)2] in toluene at room temperature. These complexes were tested in the catalytic hydrosilylation reaction of aldehydes and ketones with phenylsilane in solvent-free conditions. After a basic hydrolysis step, the corresponding alcohols were obtained in good yields. Copyright © 2013 John Wiley & Sons, Ltd.

The photophysical and photochemical properties of the new tricarbonyl rhenium(I) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(I) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(I) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands.

We have studied the ion pair structure of [NHC-Au(eta(2)-3-hezyne)] [BF4] (NHC = nitrogen-heterocyclic carbene) by solution NOE NMR spectroscopy and relativistic DFT calculation. The neutral complexes [NHC-AuCl] have been synthesized through an improved, silver-free one-pot synthesis, by reaction (in air and using solvents and substrates without any previous purification) of the appropriate [NHC(H)]Cl, gold precursor, and KHCO3. Ion pairs were generated in situ in NMR tubes. In our previous work, two main ion pair orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the alkyne (B). Here we focus on the effect of the carbene backbone on the ion pair structure, comparing the unsaturated NHC (1BF(4)) with two different variants: a saturated NHC (2BF(4)) and a polycyclic ligand with an extended aromatic system (3BF(4)). For 2BF(4), the A:B ratio remains almost the same as for 1BF(4), while the ion pair structure of 3BF(4) becomes mainly nonspecific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, which shows an attractive region on the backbone of 2BF(4) and a flat weak potential around the whole 3BF(4).

This thesis describes the catalysed addition of X-H bonds (X = S, O and N) to alkynes using a range of novel rhodium(I) and iridium(I) complexes containing hybrid bidentate phosphine-pyrazolyl, phosphine-imidazolyl and phosphine-N heterocyclic carbene (NHC) donor ligands. The synthesis of novel bidentate phosphine-pyrazolyl, phosphine-imidazolyl (P-N) and phosphine-NHC (PC) donor ligands and their cationic and neutral rhodium(I) and iridium(I) complexes [M(P N)(COD)]BPh4, [M(PC)(COD)]BPh4, [Ir(P-N)(CO)2]BPh4 and [M(P-N)(CO)Cl] were successfully performed. An unusual five coordinate iridium complex with phosphine-NHC ligands [Ir(PC)(COD)(CO)]BPh4 was also obtained. Seventeen single crystal X-ray structures of these new complexes were determined. A range of these novel rhodium and iridium complexes were effective as catalysts for the addition of thiophenol to a variety of alkynes. Iridium complexes were more effective than rhodium analogues. Cationic complexes were more effective than neutral complexes. Complexes with hybrid phosphine-nitrogen donor were more effective than complexes containing bidentate nitrogen donor ligands. An atom-economical, efficient method for the synthesis of cyclic acetals and bicyclic O,O-acetals was successfully developed based on the catalysed hydroalkoxylation. Readily prepared terminal and non-terminal alkyne diols were cyclised into bicyclic O,O-acetals in quantitative conversions in most cases. The efficiency of a range of rhodium and iridium complexes containing bidentate P-N and PC donor ligands as catalysts for the cyclisation of 4-pentyn-1-amine to 2-methyl-1-pyrroline varied significantly. The cationic iridium complexes with the bidentate phosphine-pyrazolyl ligands, [Ir(R2PyP)(COD)]BPh4 (2.39-2.42) were extremely efficient as catalysts for this transformation. Increasing the size of the substituent on or adjacent to the donor led to improvement in catalytic activity of the corresponding metal complexes. The mechanism of the catalysed hydroalkoxylation was proposed to proceed by the initial activation of the alkyne via π coordination to the metal centre. The π binding of both aliphatic and aromatic alkynes to [Ir(PyP)(CO)2]BPh4 (2.44) was observed by low temperature NMR and no reaction between 2.44 and alcohols was observed. In contrast, the facility in which thiol and amine oxidatively added to 2.44 led the proposal that in the hydrothiolation and hydroamination reaction, the catalytic cycle commences with the activation of the X-H bond (X = S, N) by an oxidative addition process.

The coordination of 2-pyridyltetrazolate and ortho-xylene-linked bis(NHC)cyclophane to Pt(II) yielded a novel complex characterised by enhanced pure deep-blue emission, whose intensity can be modulated via methylation of the tetrazole ring.