[{Au(IPr)}2(μ-OH)]X complexes: synthetic, structural and catalytic studies

Broadening the catalytic region from the cavity to windows by M6L12 nanospheres in cyclizations

Supramolecular cages have received tremendous attention as they can contain catalysts that exhibit confinement effects in the cavity, leading to excellent performances. Herein, we report an example wherein the catalytic region is extended from the cage cavity to the windows, and investigate its confinement effect by utilizing the Pd6LAu12 cage that contains rigidly fixed and isolated gold complexes at the windows. Pd6LAu12 exhibit three features of particular interest while assessing their properties in gold-catalyzed cyclization reactions. First, the catalysts experience a cage effect as they display higher reactivity and selectivity compared to the monomeric analogue, as a result of substrate pre-organization at the windows. Second, the metal complexes are physically separated by the cage structure, preventing the formation of less active dinuclear gold complexes making it more stable under hydrous conditions. Third, the cage windows present the characteristics of enzymatic catalysis via Michaelis-Menten-type mechanism analysis. This contribution presents an alternative way to engineer supramolecular catalysts through extending the catalytic region.

Hydration Reactions Catalyzed by Transition Metal-NHC (NHC = N-Heterocyclic Carbene) Complexes

The catalytic addition of water to unsaturated C-C or C-N π bonds represent one of the most important and environmentally sustainable methods to form C-O bonds for the production of synthetic intermediates, medicinal agents and natural products. The traditional acid-catalyzed hydration of unsaturated compounds typically requires strong acids or toxic mercury salts, which limits practical applications and presents safety and environmental concerns. Today, transition-metal-catalyzed hydration supported by NHC (NHC = N-heterocyclic carbene) ligands has attracted major attention. By rational design of ligands, choice of metals and counterions as well as mechanistic studies and the development of heterogeneous systems, major progress has been achieved for a broad range of hydration processes. In particular, the combination of NHC ligands with gold shows excellent reactivity compared with other catalytic systems; however, other systems based on silver, ruthenium, osmium, platinum, rhodium and nickel have also been discovered. Ancillary NHC ligands provide stabilization of transition metals and ensure high catalytic activity in hydration owing to their unique electronic and steric properties. NHC-Au(I) complexes are particularly favored for hydration of unsaturated hydrocarbons due to soft and carbophilic properties of gold. In this review, we present a comprehensive overview of hydration reactions catalyzed by transition metal-NHC complexes and their applications in catalytic hydration of different classes of π-substrates with a focus on the role of NHC ligands, types of metals and counterions.

Synthesis, Reactivity, and Bonding of Gold(I) Fluorido-Phosphine Complexes

Gold(I) fluorido complexes with phosphine ligands have been synthesized from their respective iodido precursors. The bonding situation in comparison between complexes bearing phosphines and N-heterocyclic carbenes (NHCs) was explored quantum-chemically, obtaining similar results for both. Calculations of the ¹⁹F NMR chemical shifts match the experimental values well, including the approximately 40 ppm low-field shifts for the phosphine complexes compared to the NHC complexes, in spite of similar negative charges on fluorine. The reactivity of the highly water-sensitive gold(I) fluorido complexes was studied, resulting in substitution at the metal using trimethylsilyl reagents. The compounds studied were characterized using NMR as well as X-ray diffraction methods.

Ligand-based control of nuclearity in (NHC)gold(I) sulfides

N-Heterocyclic carbene (NHC) ligands support gold(I) sulfide complexes of varying nuclearity and charge. For sterically undemanding ligands, gold(I) chlorides react with sulfide to form trigold μ₃-sulfido cations as the first observed products. The ligand IMes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] supports a monomeric cation, whereas the ICy-(1,3-dicyclohexylimidazol-2-ylidene-) supported cation crystallises as a dimer linked through an aurophilic interaction. The more sterically demanding IDipp [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] supports a terminal hydrosulfide, a (μ-hydrosulfido)digold cation, and a μ₃-sulfido cation. Use of the expanded-ring NHC 7Dipp [1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene] allows the isolation of a neutral digold sulfide.

Gold N-Heterocyclic Carbene Catalysts for the Hydrofluorination of Alkynes Using Hydrofluoric Acid: Reaction Scope, Mechanistic Studies and the Tracking of Elusive Intermediates

An efficient and chemoselective methodology deploying gold-N-heterocyclic carbene (NHC) complexes as catalysts in the hydrofluorination of terminal alkynes using aqueous HF has been developed. Mechanistic studies shed light on an in situ generated catalyst, formed by the reaction of Brønsted basic gold pre-catalysts with HF in water, which exhibits the highest reactivity and chemoselectivity. The catalytic system has a wide alkyl substituted-substrate scope, and stoichiometric as well as catalytic reactions with tailor-designed gold pre-catalysts enable the identification of various gold species involved along the catalytic cycle. Computational studies aid in understanding the chemoselectivity observed through examination of key mechanistic steps for phosphine- and NHC-coordinated gold species bearing the triflate counterion and the elusive key complex bearing a bifluoride counterion.

Isolation and reactivity of a gold(I) hydroxytrifluoroborate complex stabilized by anion-π+ interactions

A 9,9-dimethylxanthene-based ligand substituted at the 4- and 5-positions by a phosphine and a xanthylium unit, respectively, has been prepared and converted into an AuCl complex, the structure of which reveals an intramolecular Au-Cl⋯π⁺ interaction. This new ligand platform was also found to support the formation of an unprecedented hydroxytrifluoroborate derivative featuring a "hard/soft" mismatched Au-μ(OH)-BF₃ motif. Despite its surprising stability, this gold hydroxytrifluoroborate complex is a remarkably potent carbophilic catalyst which readily activates alkynes, without activator.

Synthesis and Reactivity of Multinuclear Gold Complexes with (Diphenylphosphanyl)ferrocene and Oxygen Ligands

Au^I complexes combining hard oxygen and soft (diphenylphosphanyl)ferrocene (L) ligands in their molecules were synthesized, viz. the gold hydroxides [Au(OH)(L-κP)] (5) and [{Au(L-κP)}₂ (μ-OH)][BF₄ ] (4), and the oxonium cluster [{Au(L-κP)}₃ (μ₃ -O)][BF₄ ] (1). In-situ auration of 1 produced [{Au(L-κP)}₄ (μ₄ -O)][BF₄ ]₂ (2), which spontaneously converted into a dimeric tetragold complex featuring bridging phosphanylferrocenyl groups geminally diaurated in position 2 of the ferrocene scaffold. The same complex and its isomer incorporating ferrocene-1,1'-diyl bridges resulted similarly from 4. Upon crystallization, compound 5 underwent a redox reaction, producing a structurally unique, crown-like, mixed-valent Au⁰ /Au^I cluster, [Au₇ (L-κP)₆ ]OH. Compounds 1 and 5 were used to prepare the analogous, N-bridged complexes, [{Au(L-κP)}₃ (μ₃ -NFc)][BF₄ ] (Fc=ferrocenyl) and [{Au(L-κP)}₄ (μ₄ -N)][BF₄ ]. The compounds were structurally characterized and further studied by DFT calculations.

Synthesis of N-heterocyclic carbene gold(I) complexes

N-heterocyclic carbene gold(I) chloride and hydroxide complexes are regularly used as synthons to access various oxygen-, nitrogen- or carbon-bound gold complexes. They are also widely employed as efficient catalysts in addition reactions of hydroelements to unsaturated bonds and in several rearrangement and decarboxylation protocols. Here we describe the multigram synthesis of the most common mononuclear N-heterocyclic carbene gold(I) chloride complexes bearing the N,N'-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N'-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (IPr*) ligands. Their synthesis is achieved through the straightforward and practical weak base approach in a total time of 4-5 h. This straightforward methodology is conducted under air and possesses considerable advantages over alternative routes, such as the use of a sustainable reaction solvent, minimal amounts of a mild base and commercially available or easily obtained starting materials. Additionally, we describe the synthesis of the mononuclear gold(I) hydroxide complex bearing the IPr ligand, using the state-of-the-art method requiring 24 h. Finally, the improved synthesis of the dinuclear gold(I) hydroxide complex [{Au(IPr)}₂(μ-OH)][BF₄] is described (~3 h). All procedures can be performed by researchers with standard training and lead to high yields (76-99%) of microanalytically pure bench-stable materials.

Gold(I)-NHC-catalysed synthesis of benzofurans via migratory cyclization of 2-alkynylaryl ethers

Homogeneous cationic gold(i) catalysis emerged as a preferred avenue for the activation of alkenes and alkynes towards reactions with weak nucleophiles, especially in cyclization reactions. Here we report an intramolecular carboalkoxylation reaction of electron-rich benzyl ethers of 2-ethynylaryl phenols catalysed by a digold(i)-NHC complex. The reaction proceeds efficiently with low catalyst loading and the resulting 2,3-disubstituted benzofurans form in moderate to good yields. Based on the results of a cross-over experiment, spectroscopic data, and DFT calculations, we propose a mechanism that accounts for the observed chemo- and regioselectivity.

Simple Synthetic Routes to N-Heterocyclic Carbene Gold(I)-Aryl Complexes: Expanded Scope and Reactivity

The discovery of sustainable and scalable synthetic protocols leading to gold-aryl compounds bearing N-heterocyclic carbene (NHC) ligands sparked an investigation of their reactivity and potential utility as organometallic synthons. The use of a mild base and green solvents provide access to these compounds, starting from widely available boronic acids and various [Au(NHC)Cl] complexes, with reactions taking place under air, at room temperature and leading to high yields with unprecedented ease. One compound, (N,N'-bis[2,6-(di-isopropyl)phenyl]imidazol-2-ylidene)(4-methoxyphenyl)gold, ([Au(IPr)(4-MeOC₆ H₄ )]), was synthesized on a multigram scale and used to gauge the reactivity of this class of compounds towards C-H/N-H bonds and with various acids, revealing simple pathways to gold-based species that possess attractive properties as materials, reagents and/or catalysts.

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.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Gold-Catalyzed Hydrofluorination of Internal Alkynes Using Aqueous HF

The gold-catalyzed hydrofluorination reaction of internal alkynes using hydrofluoric acid is reported. Notably, those conditions use one of the most economical sources of HF and are free of additional additives. Both symmetrical and unsymmetrical internal alkynes can be utilized, and the use of alkynes bearing a fluorinated group at the propargylic position as substrates allowed for a regioselective hydrofluorination reaction.

Solvent-tailored Pd3P0.95 nano catalyst for amide-nitrile inter-conversion, the hydration of nitriles and transfer hydrogenation of the C[double bond, length as m-dash]O bond

For the first time, a one pot thermolysis of [Pd(PPh3)2Cl2] prepared by reacting Ph3P with PdCl2 in a 2 : 1 molar ratio in MeOH at 280 °C in a trioctylphosphine (TOP) and oleylamine(OA)-octadecane(ODE) mixture (1 : 1) was used to prepare quantum dots (QDs; size ∼2-3 nm) and nanoparticles (NPs; size ∼13-14 nm), respectively, of composition Pd3P0.95. TEM, SEM-EDX, powder-XRD and XPS (for QDs only) were used to authenticate the two nanophases. 31P{1H}NMR experiments performed to monitor the progress of thermolysis reactions revealed that the phosphorus present in the Pd3P0.95 QDs had come from TOP, whereas in Pd3P0.95 NPs, its source is triphenylphosphine. The nature of the solvent did not affect the chemical composition of the nano-phase but controlled its size. Probably, solvent dependent, unique, single source precursors (SSPs) of palladium were generated in situ, and controlled the size. The catalytic activity of both Pd3P0.95 QDs and NPs was explored. The QDs were found to be efficient as a catalyst for the amide-nitrile interconversion at room temperature (yield up to 92% in 4 h), hydration of nitriles and transfer hydrogenation (TH) of carbonyl compounds with yields up to 96% in 3-4 h. The yields and reaction rates of amide-nitrile inter-conversion and TH when catalyzed by Pd3P0.95 QDs were found to be higher compared to the ones observed with the Pd/C catalyst. The binding energy of Pd(3d) in the X-ray photoelectron spectrum (XPS) of Pd3P0.95 indicated an electron transfer from the metal to phosphorus, resulting in electron deficient palladium, which facilitates the coordination of a substrate to Pd and drives the reaction. The reusability of Pd3P0.95 QDs for the interconversion was found to be up to 4-times, while for the transfer hydrogenation of carbonyl compounds it was up to 6-times, but with a diminished yield. Pd3P0.95 NPs were found to be less active (yield up to 36% in optimized reaction conditions) in comparison to Pd3P0.95 QDs. The mercury poisoning test suggested that the catalysis predominantly proceeded heterogeneously on the surface of the QDs. The PXRD and XPS results did not suggest a significant variation in the phase of QDs after the third catalytic cycle. The bleeding of Pd during catalysis (determined by flame AAS) and the agglomeration of QDs as supported by the SEM-EDX and TEM results are probably responsible for the reduction in the catalytic activity of QDs after reusing three times.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

New insights in Au-NHCs complexes as anticancer agents

Within the research field of antitumor metal-based agents alternative to platinum drugs, gold(I/III) coordination complexes have always been in the forefront due mainly to the familiarity of medicinal chemists with gold compounds, whose application in medicine goes back in the ancient times, and to the rich chemistry shown by this metal. In the last decade, N-heterocyclic carbene ligands (NHC), a class of ligands that largely resembles the chemical properties of phosphines, became of interest for gold(I) medicinal applications, and since then, the research on NHC-gold(I/III) coordination complexes as potential antiproliferative agents boosted dramatically. Different classes of gold(I/III)-NHC complexes often showed an outstanding in vitro antiproliferative activity, however up to now very few in vivo data have been reported to corroborate the in vitro results. This review summarizes all achievements in the field of gold (I/III) complexes comprising NHC ligands proposed as potential antiproliferative agents in the period 2004-2016, and critically analyses biological data (mainly IC₅₀ values) in relation to the chemical structures of Au compounds. The state of art of the in vivo studies so far described is also reported.

Monomeric nickel hydroxide stabilized by a sterically demanding phosphorus–nitrogen PN3P-pincer ligand: synthesis, reactivity and catalysis

Various new nickel complexes and amides are synthesized using monomeric nickel hydroxide (PN³P)Ni(OH) (3) as the precursor or catalyst.

The role of the metal in the dual-metal catalysed hydrophenoxylation of diphenylacetylene

Computational studies on homo- and heterobimetallic group 11 metal-NHC complexes were carried out, providing insights into the catalysed-hydrophenoxylation of alkynes.

Au(i)-Catalyzed hydroarylation of alkenes with N,N-dialkylanilines: a dual gold catalysis concept

In the present work, we report the efficiency of the commercially available [(PPh₃)AuCl] complex in combination with a set of potassium salts to catalyze the hydroarylation of a broad variety of alkenes using N,N-dialkylanilines, leading to full conversion and complete selectivity toward the Markovnikov products.

The preference for dual-gold(i) catalysis in the hydro(alkoxylation vs. phenoxylation) of alkynes

Dinuclear gold complexes and their use in catalysis have received significant recent attention, but there are few critical comparisons of mono- versus dual gold-catalysed pathways.

A Switchable Gold Catalyst by Encapsulation in a Self-Assembled Cage

Dinuclear gold complexes have the ability to interact with one or more substrates in a dual-activation mode, leading to different reactivity and selectivity than their mononuclear relatives. In this contribution, this difference was used to control the catalytic properties of a gold-based catalytic system by site-isolation of mononuclear gold complexes by selective encapsulation. The typical dual-activation mode is prohibited by this catalyst encapsulation, leading to typical behavior as a result of mononuclear activation. This strategy can be used as a switch (on/off) for a catalytic reaction and also permits reversible control over the product distribution during the course of a reaction.

Mechanism of the Transmetalation of Organosilanes to Gold

Density functional theory (DFT) calculations were carried out to study the reaction mechanism of the first transmetalation of organosilanes to gold as a cheap fluoride‐free process. The versatile gold(I) complex [Au(OH)(IPr)] permits very straightforward access to a series of aryl‐, vinyl‐, and alkylgold silanolates by reaction with the appropriate silane reagent. These silanolate compounds are key intermediates in a fluoride‐free process that results in the net transmetalation of organosilanes to gold, rather than the classic activation of silanes as silicates using external fluoride sources. However, here we propose that the gold silanolate is not the active species (as proposed during experimental studies) but is, in fact, a resting state during the transmetalation process, as a concerted step is preferred.

Influence of bulky yet flexible N-heterocyclic carbene ligands in gold catalysis

Three new Au(I) complexes of the formula [Au(NHC)(NTf₂)] (NHC = N-heterocyclic carbene) bearing bulky and flexible ligands have been synthesised. The ligands studied are IPent, IHept and INon which belong to the ‘ITent’ (‘Tent’ for ‘tentacular’) family of NHC derivatives. The effect of these ligands in gold-promoted transformations has been investigated.

Competitive Gold-Promoted Meyer-Schuster and oxy-Cope Rearrangements of 3-Acyloxy-1,5-enynes: Selective Catalysis for the Synthesis of (+)-(S)-γ-Ionone and (-)-(2S,6 R)-cis-γ-Irone

We report a simple, highly stereoselective synthesis of (+)-(S)-γ-ionone and (-)-(2S,6R)-cis-γ-irone, two characteristic and precious odorants; the latter compound is a constituent of the essential oil obtained from iris rhizomes. Of general interest in this approach are the photoisomerization of an endo trisubstituted cyclohexene double bond to an exo vinyl group and the installation of the enone side chain through a [(NHC)Au(I)]-catalyzed Meyer-Schuster-like rearrangement. This required a careful investigation of the mechanism of the gold-catalyzed reaction and a judicious selection of reaction conditions. In fact, it was found that the Meyer-Schuster reaction may compete with the oxy-Cope rearrangement. Gold-based catalytic systems can promote either reaction selectively. In the present system, the mononuclear gold complex [Au(IPr)Cl], in combination with the silver salt AgSbF6 in 100:1 butan-2-one/H2O, proved to efficiently promote the Meyer-Schuster rearrangement of propargylic benzoates, whereas the digold catalyst [{Au(IPr)}2(μ-OH)][BF4] in anhydrous dichloromethane selectively promoted the oxy-Cope rearrangement of propargylic alcohols.

Stereoselective Gold(I)-Catalyzed Intermolecular Hydroalkoxlation of Alkynes

We report the use of cationic gold complexes [Au(NHC)(CH3CN)][BF4] and [{Au(NHC)}2(μ-OH)][BF4] (NHC = N-heterocyclic carbene) as highly active catalysts in the solvent-free hydroalkoxylation of internal alkynes using primary and secondary alcohols. Using this simple protocol, a broad range of (Z)-vinyl ethers were obtained in excellent yields and high stereoselectivities. The methodology allows for the use of catalyst loadings as low as 200 ppm for the addition of primary alcohols to internal alkynes (TON = 35 000, TOF = 2188 h-1).

Trapping atmospheric CO2 with gold

The ability of gold-hydroxides to fix CO2 is reported. [Au(IPr)(OH)] and [{Au(IPr)}2(μ-OH)][BF4] react with atmospheric CO2 to form the trigold carbonate complex [{Au(IPr)}3(μ(3)-CO3)][BF4]. Reactivity studies revealed that this complex behaves as two basic and one cationic Au centres, and that it is catalytically active. DFT calculations and kinetic experiments have been carried out.