Synthesis of Well-Defined N-Heterocyclic Carbene Silver(I) Complexes

Silver Organometallics that are Highly Potent Thioredoxin and Glutathione Reductase Inhibitors: Exploring the Correlations of Solution Chemistry with the Strong Antibacterial Effects

The antibacterial activity of silver species is well-established; however, their mechanism of action has not been adequately explored. Furthermore, issues of low-molecular silver compounds with cytotoxicity, stability, and solubility hamper their progress to drug leads. We have investigated silver N-heterocyclic carbene (NHC) halido complexes [(NHC)AgX, X = Cl, Br, and I] as a promising new type of antibacterial silver organometallics. Spectroscopic studies and conductometry established a higher stability for the complexes with iodide ligands, and nephelometry indicated that the complexes could be administered in solutions with physiological chloride levels. The complexes showed a broad spectrum of strong activity against pathogenic Gram-negative bacteria. However, there was no significant activity against Gram-positive strains. Further studies clarified that tryptone and yeast extract, as components of the culture media, were responsible for this lack of activity. The reduction of biofilm formation and a strong inhibition of both glutathione and thioredoxin reductases with IC50 values in the nanomolar range were confirmed for selected compounds. In addition to their improved physicochemical properties, the compounds with iodide ligands did not display cytotoxic effects, unlike the other silver complexes. In summary, silver NHC complexes with iodide secondary ligands represent a useful scaffold for nontoxic silver organometallics with improved physicochemical properties and a distinct mechanism of action that is based on inhibition of thioredoxin and glutathione reductases.

N-heterocyclic-carbene vs diphosphine auxiliary ligands in thioamidato Cu(I) and Ag(I) complexes towards the development of potent and dual-activity antibacterial and apoptosis-inducing anticancer agents

Group 11 metal complexes exhibit promising antibacterial and anticancer properties which can be further enhanced by appropriate ligands. Herein, a series of mononuclear thioamidato Cu(I) and Ag(I) complexes bearing either a diphosphine (P^P) or a N-heterocyclic carbene (NHC) auxiliary ligand (L) was synthesized, and the impact of the co-ligand L on the in vitro antibacterial and anticancer properties of their complexes was assessed. All complexes effectively inhibited the growth of various bacterial strains, with the NHC-Cu(I) complex found to be particularly effective against the Gram (+) bacteria (IC50 = 1-4 μg mL-1). Cytotoxicity studies against various human cancer cells revealed their high anticancer potency and the superior activity of the NHC-Ag(I) complex (IC50 = 0.95-4.5 μΜ). Flow cytometric analysis on lung and breast cancer cells treated with the NHC-Ag(I) complex suggested an apoptotic cell-death pathway; molecular docking calculations provided mechanistic insights, proving the capacity of the complex to bind on apoptosis-regulating proteins and affect their functionalities.

109Ag NMR chemical shift as a descriptor for Brønsted acidity from molecules to materials

Molecular-level understanding of the acid/base properties of heterogeneous catalysts requires the development of selective spectroscopic probes to establish structure-activity relationships. In this work we show that substituting the surface protons in oxide supports by isolobal N-heterocyclic carbene (NHC) Ag cations and measuring their 109Ag nuclear magnetic resonance (NMR) signatures enables to probe the speciation and to evaluate the corresponding Brønsted acidity of the substituted OH surface sites. Specifically, a series of silver N-heterocyclic carbene (NHC) Ag(i) complexes of general formula [(NHC)AgX] are synthesized and characterized, showing that the 109Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X- (i.e., HX), thus establishing an acidity scale based on 109Ag NMR chemical shift. The methodology is then used to evaluate the Brønsted acidity of the OH sites of representative oxide materials using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy.

%VBur index and steric maps: from predictive catalysis to machine learning

Steric indices are parameters used in chemistry to describe the spatial arrangement of atoms or groups of atoms in molecules. They are important in determining the reactivity, stability, and physical properties of chemical compounds. One commonly used steric index is the steric hindrance, which refers to the obstruction or hindrance of movement in a molecule caused by bulky substituents or functional groups. Steric hindrance can affect the reactivity of a molecule by altering the accessibility of its reactive sites and influencing the geometry of its transition states. Notably, the Tolman cone angle and %VBur are prominent among these indices. Actually, steric effects can also be described using the concept of steric bulk, which refers to the space occupied by a molecule or functional group. Steric bulk can affect the solubility, melting point, boiling point, and viscosity of a substance. Even though electronic indices are more widely used, they have certain drawbacks that might shift preferences towards others. They present a higher computational cost, and often, the weight of electronics in correlation with chemical properties, e.g. binding energies, falls short in comparison to %VBur. However, it is worth noting that this may be because the steric index inherently captures part of the electronic content. Overall, steric indices play an important role in understanding the behaviour of chemical compounds and can be used to predict their reactivity, stability, and physical properties. Predictive chemistry is an approach to chemical research that uses computational methods to anticipate the properties and behaviour of these compounds and reactions, facilitating the design of new compounds and reactivities. Within this domain, predictive catalysis specifically targets the prediction of the performance and behaviour of catalysts. Ultimately, the goal is to identify new catalysts with optimal properties, leading to chemical processes that are both more efficient and sustainable. In this framework, %VBur can be a key metric for deepening our understanding of catalysis, emphasizing predictive catalysis and sustainability. Those latter concepts are needed to direct our efforts toward identifying the optimal catalyst for any reaction, minimizing waste, and reducing experimental efforts while maximizing the efficacy of the computational methods.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

Silver N-heterocyclic carbene complexes are potent uncompetitive inhibitors of the papain-like protease with antiviral activity against SARS-CoV-2

The ongoing SARS-CoV-2 pandemic has caused a high demand for novel innovative antiviral drug candidates. Despite promising results, metal complexes have been relatively unexplored as antiviral agents in general and in particular against SARS-CoV-2. Here we report on silver NHC complexes with chloride or iodide counter ligands that are potent inhibitors of the SARS-CoV-2 papain-like protease (PL^pro) but inactive against 3C-like protease (3CL^pro) as another SARS-CoV-2 protease. Mechanistic studies on a selected complex confirmed zinc removal from a zinc binding domain of PL^pro as relevant factor of their activity. In addition, enzyme kinetic experiments revealed that the complex is an uncompetitive inhibitor and with this rare type of inhibition it offers great pharmacological advantages in terms selectivity. The silver NHC complexes with iodide ligands showed very low or absent host cell toxicity and triggered strong effects on viral replication in cells infected with SARS-CoV-2, making them promising future antiviral drug candidates.

ItOct (ItOctyl) - pushing the limits of ItBu: highly hindered electron-rich N-aliphatic N-heterocyclic carbenes

ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene) represents the most important and most versatile N-alkyl N-heterocyclic carbene available in organic synthesis and catalysis. Herein, we report the synthesis, structural characterization and catalytic activity of ItOct (ItOctyl), C₂-symmetric, higher homologues of ItBu. The new ligand class, including saturated imidazolin-2-ylidene analogues has been commercialized in collaboration with MilliporeSigma: ItOct, 929 298; SItOct, 929 492 to enable broad access of the academic and industrial researchers within the field of organic and inorganic synthesis. We demonstrate that replacement of the t-Bu side chain with t-Oct results in the highest steric volume of N-alkyl N-heterocyclic carbenes reported to date, while retaining the electronic properties inherent to N-aliphatic ligands, such as extremely strong σ-donation crucial to the reactivity of N-alkyl N-heterocyclic carbenes. An efficient large-scale synthesis of imidazolium ItOct and imidazolinium SItOct carbene precursors is presented. Coordination chemistry to Au(i), Cu(i), Ag(i) and Pd(ii) as well as beneficial effects on catalysis using Au(i), Cu(i), Ag(i) and Pd(ii) complexes are described. Considering the tremendous importance of ItBu in catalysis, synthesis and metal stabilization, we anticipate that the new class of ItOct ligands will find wide application in pushing the boundaries of new and existing approaches in organic and inorganic synthesis.

Ag-NHC Complexes in the π-Activation of Alkynes

Silver-NHC (NHC = N-heterocyclic carbene) complexes play a special role in the field of transition-metal complexes due to (1) their prominent biological activity, and (2) their critical role as transfer reagents for the synthesis of metal-NHC complexes by transmetalation. However, the application of silver-NHCs in catalysis is underdeveloped, particularly when compared to their group 11 counterparts, gold-NHCs (Au-NHC) and copper-NHCs (Cu-NHC). In this Special Issue on Featured Reviews in Organometallic Chemistry, we present a comprehensive overview of the application of silver-NHC complexes in the p-activation of alkynes. The functionalization of alkynes is one of the most important processes in chemistry, and it is at the bedrock of organic synthesis. Recent studies show the significant promise of silver-NHC complexes as unique and highly selective catalysts in this class of reactions. The review covers p-activation reactions catalyzed by Ag-NHCs since 2005 (the first example of p-activation in catalysis by Ag-NHCs) through December 2022. The review focuses on the structure of NHC ligands and p-functionalization methods, covering the following broadly defined topics: (1) intramolecular cyclizations; (2) CO₂ fixation; and (3) hydrofunctionalization reactions. By discussing the role of Ag-NHC complexes in the p-functionalization of alkynes, the reader is provided with an overview of this important area of research and the role of Ag-NHCs to promote reactions that are beyond other group 11 metal-NHC complexes.

N-heterocyclic Carbene Gold Complexes Active in Hydroamination and Hydration of Alkynes

Until the year 2000, gold compounds were considered catalytically inert. Subsequently, it was found that they are able to promote the nucleophilic attack on unsaturated substrates by forming an Au–π-system. The main limitation in the use of these catalytic systems is the ease with which they decompose, which is avoided by stabilization with an ancillary ligand. N-heterocyclic carbenes (NHCs), having interesting s-donor capacities, are able to stabilize the gold complexes (Au (I/III) NHC), favoring the exploration of their catalytic activity. This review reports the state of the art (years 2007–2022) in the nucleophilic addition of amines (hydroamination) and water (hydration) to the terminal and internal alkynes catalyzed by N-heterocyclic carbene gold (I/III) complexes. These reactions are particularly interesting both because they are environmentally sustainable and because they lead to the production of important intermediates in the chemical and pharmaceutical industry. In fact, they have an atom economy of 100%, and lead to the formation of imines and enamines, as well as the formation of ketones and enols, all important scaffolds in the synthesis of bioactive molecules, drugs, heterocycles, polymers, and bulk and fine chemicals.

Galactopyranoside-Substituted N-Heterocyclic Carbene Gold(I) Complexes: Synthesis, Stability, and Catalytic Applications to Alkyne Hydration

A series of novel gold(I) complexes bearing galactopyranoside-based N-heterocyclic carbene ligands have been synthesized via transmetalation of the corresponding Ag(I) complex. Gold(I) complexes have been characterized by NMR, Fourier transform infrared (FTIR), and elemental analysis. An exhaustive NMR analysis shows that the complexes are not stable when hydroxyl groups are deprotected. Catalytic studies, using the alkyne hydration as a model reaction, indicate that the synthesized complexes are active and reusable.

The use of DOSY experiments to determine the solution structures of coinage metal pyrazolates: The case of {[3,5-(CF3 )2 Pz]Ag}3

A series of DOSY experiments have been carried out to determine the solution stoichiometry of silver(I) 3,5-bis (trifluoromethyl)pyrazolate species. This compound exists as a trimer in the solid state (n = 3) but in solutions of chlorinated solvents, the DOSY data suggest the presence of a mixture of solvent stabilized monomer (n = 1) and dimer (n = 2) in equilibrium. Different approximations have been used including the Stokes-Einstein and the Stokes-Einstein-Gierer-Wirtz equations. Some methodological problems are discussed.

On the reactivity of Al-group 11 (Cu, Ag, Au) bonds

Reactions of the seven-membered heterocyclic potassium diamidoalumanyl, [K{Al(SiN^Dipp)}]₂ (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-di-isopropylphenyl), with a variety of Cu(I), Ag(I) and Au(I) chloride N-heterocyclic carbene (NHC) adducts are described. The resultant group 11-Al bonded derivatives have been characterised in solution by NMR spectroscopy and, in the case of [{SiN^Dipp}Al-Au(NHC^iPr)] (NHC^iPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), by single crystal X-ray diffraction. Although similar reactions of LAgCl and LAuCl, where L is a more basic cyclic alkyl amino carbene (CAAC), generally resulted in reduction of the group 11 cations to the base metals, X-ray analysis of [(^CyCAAC)AgAl(SiN^Dipp)] (^CyCAAC = 2-[2,6-bis(1-methylethyl)phenyl]-3,3-dimethyl-2-azaspiro[4.5]dec-1-ylidene) provides the first solid-state authentication of an Ag-Al σ bond. The reactivity of the NHC-supported Cu, Ag and Au alumanyl derivatives was assayed with the isoelectronic unsaturated small molecules, N,N'-di-isopropylcarbodiimide and CO₂. While these reactions generally provided products consistent with nucleophilic attack of the group 11 atom at the electrophilic heteroallene carbon centre, treatment of the NHC-supported copper and silver alumanyls with N,N'-di-isopropylcarbodiimide yielded less symmetric Cu-C and Ag-C-bonded isomers. In contrast to the previously described copper and silver alumanyl derivatives, [(NON)Al(O₂C)M(Pt-Bu₃)] (M = Cu or Ag; NON = 4,5-bis(2,6-di-isopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), which were prone to facile CO extrusion and formation of carbonate derivatives, the NHC-supported dioxocarbene species, [(NHC^iPr)M(CO₂)Al(SiN^Dipp)] (M = Cu, Ag, Au), are all stable at room and moderately elevated temperatures. The stabilising role of the NHC co-ligand was, thus, assessed by preparation of the t-Bu₃P adducted copper-alumanyl, [(t-Bu₃P)CuAl(SiN^Dipp)]. Treatment of this latter compound, which was also structurally characterised by X-ray analysis, with both N,N'-di-isopropylcarbodiimide and CO₂ again provided smooth heteroallene insertion and formation of the relevant Cu-C-bonded products. Although both compounds were quite stable at room temperature, heating of [(t-Bu₃P)Cu(CO₂)Al(SiN^Dipp)] at 60 °C induced elimination of CO and formation of the analogous carbonate, [(t-Bu₃P)Cu(OCO₂)Al(SiN^Dipp)], which was identified by ¹³C and ³¹P NMR spectroscopy. Reflective of the more reliable nucleophilic behaviour of the gold centres in these group 11 alumanyls, computational (QTAIM and NBO) analysis highlighted a lower level of covalency of the Al-Au linkage in comparison to the analogous Al-Cu and Al-Ag interactions. Although substitution of the co-ligand significantly perturbs the charge distribution across the Cu-Al bond of [LCuAl(SiN^Dipp)] (L = NHC^iPr or t-Bu₃P), only a negligible difference is observed between the phosphine-coordinated copper systems derived from either the [SiN^Dipp]- or (NON)-based alumanyl ligands. Computational mapping of the reaction profiles arising from treatment of the various group 11 alumanyls with N,N'-di-isopropylcarbodiimide indicates that the observed formation of the Cu-N and Ag-N bound isomers do not provide the thermodynamic reaction outcome. In contrast, examination of the CO₂-derived reactions, and their potential toward CO extrusion and subsequent carbonate formation, implies that the identity of the co-ligand exerts a greater influence on this aspect of reactivity than the architecture of the diamidoalumanyl anion.

Synthesis, characterization, antimicrobial and antibiofilm activity, and molecular docking analysis of NHC precursors and their Ag-NHC complexes

Microorganisms attach to surfaces and interfaces and form biofilms which create a sheltered area for host cell response. Therefore, biofilms provide troubles in fields such as medicine, food, and pharmaceuticals. Inhibition of formation of biofilms through hindering of quorum sensing could be a method for the production of new generation antibiotics. In this study, four new benzimidazole type NHC precursors (1-allyl-3-benzyl-5,6-dimethylbenzimidazolium chloride, 1-allyl-3-(2,4,6-trimethylbenzyl)-5,6-dimethylbenzimidazolium chloride, 1-allyl-3-(2,3,5,6-tetramethylbenzyl)-5,6-dimethylbenzimidazolium chloride, and 1-allyl-3-(2,3,4,5,6-pentamethylbenzyl)-5,6-dimethylbenzimidazolium chloride and Ag-NHC complexes of these molecules were synthesized and characterized by elemental analysis, FT-IR spectroscopy, ¹H, and ¹³C{¹H} NMR spectroscopy, LC-MS, and single crystal crystallography. Antimicrobial and biofilm formation inhibition activities of the molecules were evaluated. In addition, the activities of the molecules were examined in detail by molecular docking analysis. According to the results obtained, higher activity was achieved with the complex molecules when compared with the benzimidazole derivative ligands.

Application of the Redox-Transmetalation Procedure to Access Divalent Lanthanide and Alkaline-Earth NHC Complexes*

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands have been accessed by redox-transmetalation between air-stable NHC-AgI complexes and the corresponding metals. By using the small ligand 1,3-dimethylimidazol-2-ylidene (IMe), two series of isostructural complexes were obtained: the tetra-NHC complexes [LnI2 (IMe)4 ] (Ln=Eu and Sm) and the bis-NHC complexes [MI2 (IMe)2 (THF)2 ] (M=Yb, Ca and Sr). In the former, distortions in the NHC coordination were found to originate from intermolecular repulsions in the solid state. Application of the redox-transmetalation strategy with the bulkier 1,3-dimesitylimidazol-2-ylidene (IMes) ligand yielded [SrI2 (IMes)(THF)3 ], while using a similar procedure with Ca metal led to [CaI2 (THF)4 ] and uncoordinated IMes. DFT calculations were performed to rationalise the selective formation of the bis-NHC adduct in [SrI2 (IMe)2 (THF)2 ] and the tetra-NHC adduct in [SmI2 (IMe)4 ]. Since the results in the gas phase point towards preferential formation of the tetra-NHC complexes for both metal centres, the differences between both arrangements are a result of solid-state effects such as slightly different packing forces.

Acenaphthene-Based N-Heterocyclic Carbene Metal Complexes: Synthesis and Application in Catalysis

N-Heterocyclic carbene (NHC) ligands have become a privileged structural motif in modern homogenous and heterogeneous catalysis. The last two decades have brought a plethora of structurally and electronically diversified carbene ligands, enabling the development of cutting-edge transformations, especially in the area of carbon-carbon bond formation. Although most of these were accomplished with common imidazolylidene and imidazolinylidene ligands, the most challenging ones were only accessible with the acenaphthylene-derived N-heterocyclic carbene ligands bearing a π-extended system. Their superior σ-donor capabilities with simultaneous ease of modification of the rigid backbone enhance the catalytic activity and stability of their transition metal complexes, which makes BIAN-NHC (BIAN—bis(imino)acenaphthene) ligands an attractive tool for the development of challenging reactions. The present review summarizes synthetic efforts towards BIAN-NHC metal complexes bearing acenaphthylene subunits and their applications in modern catalysis, with special emphasis put on recently developed enantioselective processes.

Heterobimetallic Coinage Metal-Ruthenium Complexes Supported by Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC, WCA=B(C₆F₅)₃, NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were used for the preparation of silver(I) or copper(I) WCA‐NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono‐ and dicarbene complexes with solvated lithium counterions [Li(THF)_n]⁺ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA‐NHC)M(PPh₃)] and [(WCA‐NHC)M(η²‐toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA‐NHC)M(μ‐X)₂Ru(PPh₃)(η⁶‐p‐cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA‐NHC transmetalation.

Reversal of Enantioselectivity in the Conjugate Addition Reaction of Cyclic Enones with the CuOTf/Azolium Catalytic System

Hydroxyamide-functionalized azolium salt (NHC•HI 4) was evaluated for dual enantioselective control in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. This investigation was based on our previously reported ACA reaction catalyzed using CuOTf combined with NHC•AgI complex 1. It was revealed that the stereocontrol of the catalytic ACA reaction depended on the order of the addition of the substrates. Additionally, the chiral NHC ligand precursors, substrates, the relationship between the catalyst ee (ee_cat) and product ee (ee_pro), and halogen counter anion were completely evaluated. These results suggested that the catalytic performance of the CuOTf/4 system was comparable with that of the CuOTf/1 system. Furthermore, to gain knowledge of the Cu species generated using CuOTf and NHC ligand precursor, the reaction of CuOTf with 1 was investigated. Although obtaining the corresponding NHC•CuX species failed, the corresponding NHC•AuCl complex 11 could be synthesized by allowing 1 to react with AuCl•SMe₂.

Antibacterial Thiamine inspired silver (I) and gold (I) N-heterocyclic carbene compounds

Three new coinage metal carbene complexes of silver and gold were synthesized from a thiamine inspired proligand. The compounds were characterized by HRMS, NMR spectroscopy (1H, 19F, 31P and 13C), FT-IR and elemental analysis. The coordination environment around the metal centers was correlated to the diffusion coefficients obtained from DOSY-NMR experiments and was in agreement with the nuclearity observed in the solid-state by single crystal X-ray crystallography. The silver and gold carbene compounds were subjected to MIC studies against a panel of pathogenic bacteria, including multidrug resistant strains, with the gold carbene derivative showing the most potent antimicrobial activity against Gram-positive methicillin resistant Staphylococcus aureus (MRSA).

Synthesis, characterization, and coordination chemistry of a phenanthridine-containing N-heterocyclic carbene ligand

An N-heterocyclic carbene ligand precursor bearing a π-extended phenanthridine (3,4-benzoquinoline) unit is presented. The proligand was isolated as the imidazolium salt of chloride (1•HCl), bromide (1•HBr), or hexafluorophosphate (1•HPF6) counterions. These salts can be deprotonated and the carbene installed on silver centres using Ag2O as both a base and a source of metal ion. The resulting Ag(I) complex (1)AgCl can be used in a transmetalation reaction to generate a Pd(II) coordination complex (1)Pd(CH3CN)Cl2. The characterization and photophysical properties of these complexes is presented, along with a demonstration of the utility of (1)Pd(CH3CN)Cl2 in mediating a catalytic C-N cross-coupling reaction for the preparation of the pharmaceutical Piribedil.

Strong metal-metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes

Metallophilicity is defined as the interaction among closed-shell metal centers, the origin of which remains controversial, particularly for the roles of spd orbital hybridization (mixing of the spd atomic orbitals of the metal atom in the molecular orbitals of metal complex) and the relativistic effect. Our studies reveal that at close M-M' distances in the X-ray crystal structures of d8 and d10 organometallic complexes, M-M' closed-shell interactions are repulsive in nature due to strong M-M' Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M-M' Pauli repulsion and repulsive M-M' orbital interaction, and (n + 1)p-nd hybridization suppresses M-M' Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level coupled-cluster singles and doubles with perturbative triples computations and is used to account for the fact that the intermolecular or intramolecular Ag-Ag' distance is shorter than the Au-Au' distance, where a weaker Ag-Ag' Pauli repulsion plays an important role. The experimental studies verify the importance of ligands in intermolecular interactions. Although the M-M' interaction is repulsive in nature, the linear coordination geometry of the d10 metal complex suppresses the L-L' (ligand-ligand) Pauli repulsion while retaining the strength of the attractive L-L' dispersion, leading to a close unsupported M-M' distance that is shorter than the sum of the van der Waals radius (rvdw) of the metal atoms.

Catalytic Arylative Endo Cyclization of Gold Acetylides: Access to 3,4-Diphenyl Isoquinoline, 2,3-Diphenyl Indole, and Mesoionic Normal NHC-Gold Complex

3,4-Diphenyl isoquinoline and 2,3-diphenyl indole are readily accessed by catalytic selective bis-arylative endo cyclization of gold acetylides. The synthetic approach could be also extended to prepare six-membered mesoionic NHC complex, which could further undergo deprotonation/complexation to afford 1,3-N-heterocyclic dicarbene (NHDCs) Au₂ and Au/Ag complexes. The key vicinal diaurated alkene intermediates have been isolated and the studies on their reactivities towards coupling partners reinforce the proposed mechanisms.

The carbene transfer to strong Lewis acids: copper is better than silver

Air-stable copper(i) N-heterocyclic carbene complexes are excellent reagents for carbene transfer to Lewis Acids based on group 13 metals.

Cluster Expansion versus Complex Formation: Coinage Metal Coordination to Silylated [Ge9] Cages

Deltahedral nine-atom tetrel element Zintl clusters are promising building blocks for the straightforward solution-based synthesis of intermetalloid clusters through the reaction with organometallic compounds. Herein we report on novel coordination sites of metal-N-heterocyclic carbene (NHC) complexes to [Ge₉] clusters and unexpected cluster isomerization. We present the synthesis of a series of coinage metal-NHC complexes of silylated [Ge₉] clusters [NHC^iPrCu(η⁴-Ge₉{Si(TMS)₃}₃)] (1; TMS = trimethylsilyl) and [NHC^RM(η⁴-Ge₉{Si(TMS)₃}₂)]^- (2a, M = Cu, R = iPr; 3a, M = Cu, R = Mes; 4a, M = Cu, R = Dipp; 5a, M = Ag, R = Dipp; 6a, M = Au, R = Dipp), in which the coinage metals coordinate to open rectangular cluster faces and act as additional cluster vertex atoms. Besides representing promising intermediates on the way to larger intermetalloid clusters, the formation of compound 1 shows that Cu-NHC fragments also coordinate to the open-square Ge faces of the tris-silylated [Ge₉] clusters, contrasting the typical interactions with triangular faces of tris-silylated [Ge₉] clusters. In compounds 3a and 4a bearing bulky NHC moieties, an unusual silyl group substitution pattern is observed in contrast to 2a, which corresponds to the silyl group arrangement of other metal complexes of bis-silylated [Ge₉] clusters. In this context, potential silyl group migration mechanisms are discussed.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands

The chemistry of coinage metal ions with Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- has been explored using Mes₃P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)₅][SbF₆], [(^Et2CAAC)Au-Fe(CO)₅][SbF₆] and [(Mes₃P)Au-Fe(CO)₅][SbF₆] shows that the ligand donor strength towards Au(i) follows the order Mes₃P > ^Et2CAAC > SIPr. These Fe(CO)₅ complexes show significant blue shifts in [small nu, Greek, macron]_CO bands relative to those observed for free Fe(CO)₅ as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)₅ compared to Ag(i) in (SIPr)Ag-Mn(CO)₅. The structural details of Mes₃PAu-Mn(CO)₅ are also presented. [Fe(CO)₄CN]^- afforded CN bridged coinage metal complexes with (IPr*)Au⁺, (SIPr)Ag⁺ and (SIPr)Cu⁺ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)₅]^- and [Fe(CO)₄CN]^- are stronger donors toward Au(i) than Fe(CO)₅. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

The synthesis and full characterization of new air-stable AgI and CuI complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag2 O afforded a collection of AgI complexes, and their first use as ligand transfer reagents led to novel isostructural CuI or AuI complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new CuI complexes. The solid-state structures feature large N-CNHC -N angles (118-128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%Vbur ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%Vbur =57.5) overcomes to other known erNHC-metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76-93 % product at the 0.5-2.5 mol % catalyst loading, proving the stability of CuI erNHC complexes at elevated temperatures (100 °C).

Organic Azide and Auxiliary-Ligand-Free Complexes of Coinage Metals Supported by N-Heterocyclic Carbenes

Organic azide complexes of copper(I) and silver(I), [(SIPr)CuN(1-Ad)NN][SbF₆], [(SIPr)CuN(2-Ad)NN][SbF₆], [(SIPr)CuN(Cy)NN][SbF₆], and [(SIPr)AgN(1-Ad)NN][SbF₆] have been synthesized by using Ag[SbF₆] and the corresponding organic azides with (SIPr)CuBr and (SIPr)AgCl (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene). The copper and silver organic azide complexes were characterized by various spectroscopic techniques and X-ray crystallography. Group trends of isoleptic Cu(I), Ag(I), and Au(I) organic azide complexes are presented on the basis of experimental data and a detailed computational study. The ν_asym(N₃) values of the metal-bound 1-AdNNN in [(SIPr)MN(1-Ad)NN]⁺ follow the order Ag < Cu < Au. DFT calculations show that gold(I) forms the strongest bond with 1-AdNNN in this series, while silver has the weakest interaction. Furthermore, auxiliary ligand free coinage metal N-heterocyclic carbene complexes, [(SIPr)M][SbF₆], have been synthesized via metathesis reactions of (SIPr)MCl (M = Cu, Ag, Au) with Ag[SbF₆]. X-ray crystal structures of dinuclear [(SIPr)Ag]₂[SbF₆]₂ and [(SIPr)Au]₂[SbF₆]₂ are also reported. They show close metallophilic contacts. [(SIPr)Au]₂[SbF₆]₂ reacts with OEt₂, SMe₂, and CN^tBu to afford [(SIPr)Au(OEt₂)][SbF₆], [(SIPr)Au(SMe₂)][SbF₆], and [(SIPr)Au(CN^tBu)][SbF₆] adducts, respectively.