Mononuclear Gold Research Articles

Mononuclear gold(iii) complexes with l-histidine-containing dipeptides: tuning the structural and biological properties by variation of the N-terminal amino acid and counter anion.

Gold(iii) complexes with different l-histidine-containing dipeptides, [Au(Gly-l-His-NA,NP,N3)Cl]Cl·3H2O (1a), [Au(Gly-l-His-NA,NP,N3)Cl]NO3·1.25H2O (1b), [Au(l-Ala-l-His-NA,NP,N3)Cl][AuCl4]·H2O (2a), [Au(l-Ala-l-His-NA,NP,N3)Cl]NO3·2.5H2O (2b), [Au(l-Val-l-His-NA,NP,N3)Cl]Cl·2H2O (3), [Au(l-Leu-l-His-NA,NP,N3)Cl]Cl (4a) and [Au(l-Leu-l-His-NA,NP,N3)Cl][AuCl4]·H2O (4b), have been synthesized and structurally characterized by spectroscopic (1H NMR, IR and UV-vis) and single-crystal X-ray diffraction techniques. The antimicrobial efficiency of these gold(iii) complexes, along with K[AuCl4] and the corresponding dipeptides, was evaluated against the broad panel of Gram-positive and Gram-negative bacteria and fungi, displaying their moderate inhibiting activity. Moreover, the cytotoxic properties of the investigated complexes were assessed against the normal human lung fibroblast cell line (MRC5) and two human cancer, cervix (HeLa) and lung (A549) cell lines. None of the complexes exerted significant cytotoxic activity; nevertheless complexes that did show selectivity in terms of cancer vs. normal cell lines (2a/b and 4a/b) have been evaluated using zebrafish (Danio rerio) embryos for toxicity and antiangiogenic potential. Although the gold(iii) complexes achieved an antiangiogenic effect comparable to the known angiogenic inhibitors auranofin and sunitinib malate at 30-fold higher concentrations, they had no cardiovascular side effects, which commonly accompany auranofin and sunitinib malate treatment. Finally, binding of the gold(iii) complexes to the active sites of both human and bacterial (Escherichia coli) thioredoxin reductases (TrxRs) was demonstrated by conducting a molecular docking study, suggesting that the mechanism of biological action of these complexes can be associated with their interaction with the TrxR active site.

Mono- and Digold(I) Complexes with Mesoionic Carbenes: Structural Characterization and Use in Catalytic Silver-Free Oxazoline Formation

Triazolylidenes are a prominent class of mesoionic carbenes that have found use as supporting ligands in homogeneous catalysis in recent years. We present here the syntheses of three new mononuclear gold(I) chlorido and two new dinuclear gold(I) chlorido complexes. The ligands in the aforementioned complexes are derived from either the corresponding monotriazolium or the bitriazolium salts. All complexes have been characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Structural characterization delivers a delocalized bonding situation within the triazolylidene ligands and a linear coordination at the gold(I) centers. The gold(I) centers in all cases are bound to one triazolylidene-C donor and a chlorido ligand. Additionally, for the digold(I) complexes large Au–Au distances were observed, ruling out the existence of aurophilic interactions in these digold complexes in the solid state. All of the gold(I) complexes were tested as (pre)catalysts fo...

Group 11 Metal Complexes with Unsymmetrical Bifunctional Ferrocene Ligands

The reactions of two unsymmetrical P^N (L1) and P^P (L2) ferrocene (Fc) ligands towards group 11 metal complexes were studied. Ligand L1 shows different behavior with the three metals. Gold forms mononuclear derivatives with the metal center coordinated to the phosphorus atom, whereas the reactions with silver(I) or copper(I) complexes afford mononuclear or dinuclear compounds in which both donor atoms, phosphorus and nitrogen, are involved in the coordination to the metal center. The reactions of the diphosphine ligand [Fc(CH2PPh2)(PPh2)] (L2) with gold(I), gold(III), silver(I), or copper(I) complexes afford mono‐, di‐, or trinuclear derivatives. Some homo‐ and heterodinuclear compounds were obtained through the reactions of the mononuclear gold(III) complex [Au(C6F5)3{Fc(CH2PPh2)(PPh2)}] (6), in which one of the phosphorus atoms is free, with gold(I), gold(III), and silver(I) deriiivatives. Some of these derivatives were characterized by X‐ray diffraction analysis.

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.

Fluorene-based mononuclear gold(i) complexes: the effect of alkyl chain, aggregation-induced emission (AIE) and mechanochromism characteristics

Three gold(i) complexes were synthesized, and these gold(i) complexes exhibit AIE or alkyl chain-dependent mechanochromism characteristics.

Structural characterization of dinuclear gold(I) diphosphine complexes with anion-triggered luminescence

Dinuclear and mononuclear gold(I) complexes containing the xantphos ligand (xantphos = 9,9′-dimethyl-4,5-bis(diphenylphosphino)-xanthene), [Au2(xantphos)2](X)2 with X = BF4 (1), PF6 (2) and SbF6 (3 and 4) and Au(xantphos)(SCN) (5), were characterized by X-ray structural analysis. Two type of colourless crystals (3 and 4) crystallized together from the same solution of [Au2(xantphos)2](SbF6)2. The [Au2(xantphos)2]2+ cations have almost the same molecular structure with two xantphos ligands coordinated to two gold(I) centres with short aurophilic interaction of 2.803 (1), 2.825 (2), 2.817 (3) and 2.837 A (4), respectively. The [Au2(xantphos)2]2+ cation is in a somewhat distorted figure-eight conformation in 1–3, whereas in 4, it has and ideal figure-eight conformation with a twofold axis passing through the Au···Au bond. The molecular packing is primarily governed by C–H···F hydrogen bonding interactions between the [Au2(xantphos)2]2+ cations and fluorinated BF4 −, PF6 − and SbF6 − anions. In 5, the gold(I) centre is in a trigonal-planar geometry, and it is coordinated to two phosphorous atom of the xantphos ligand and one sulphur atom of the SCN− anion. Different anions cause some conformational changes and alter the molecular packing of these crystalline structures. These variations in the solid-state structures alter the luminescent properties of the dinuclear and mononuclear gold(I) xantphos complexes. Dinuclear complexes 1, 2 and 4 exhibit intense yellow luminescence, while compound 3 and mononuclear 5 produces green emission when irradiated with a 365-nm UV lamp at room temperature.

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.

Catalytic role of dinuclear σ,π-acetylide gold(I) complexes in the hydroamination of terminal alkynes: theoretical insights.

A detailed density functional theory (DFT) investigation of the hydroamination of 1-octyne with aniline mediated by a σ,π-digold(I) bulky phosphine-based complex was undertaken in order to shed light on the mechanistic aspects of such processes. With the purpose to probe whether the performance that the cationic digold complexes exhibit is superior to those of mononuclear complexes, the same hydroamination reaction was explored by considering separately the reaction of aniline with both the monogold(I) complexes formed by π- and σ-coordination of 1-octyne to the dialkylbiarylphosphine Au(I) precatalyst. The outcomes of the computational analysis presented here show that, when the σ,π-digold alkynide complex can be formed, the reaction is not necessarily assisted by such a complex, as the computed energy barrier is almost equal to that found when the π-coordinated alkyne mononuclear gold complex is involved in the hydroamination process. The catalytic assistance of the Au(I)-σ-alkynyl complex, instead, can be surely excluded as the hydroamination product is formed by overcoming an energy barrier significantly higher than that computed when both σ,π-digold and π-coordinated alkyne monogold complexes assist the reaction. Moreover, regardless of the implicated gold(I) species, the investigated mechanism accounts for the Markovnikov selectivity of the reaction, confirming the experimental evidence. The proposed mechanisms for the conversion of Au(I) π-coordinated alkyne complexes into the corresponding σ,π-digold alkynide complexes were also explored.

A novel carbazole-based gold(I) complex with interesting solid-state, multistimuli-responsive characteristics.

A new carbazole-based mononuclear gold(i) complex was designed and synthesized. The novel luminogen shows significative solid-state reversible mechanochromism, dual-responsive thermochromism and sensitive thin-film vapochromism properties. In addition, we obtained two kinds of crystals of the luminogen that may explain these interesting characteristics.

Catalytically active Au-O(OH)x-species stabilized by alkali ions on zeolites and mesoporous oxides.

We report that the addition of alkali ions (sodium or potassium) to gold on KLTL-zeolite and mesoporous MCM-41 silica stabilizes mononuclear gold in Au-O(OH)x-(Na or K) ensembles. This single-site gold species is active for the low-temperature (<200°C) water-gas shift (WGS) reaction. Unexpectedly, gold is thus similar to platinum in creating -O linkages with more than eight alkali ions and establishing an active site on various supports. The intrinsic activity of the single-site gold species is the same on irreducible supports as on reducible ceria, iron oxide, and titania supports, apparently all sharing a common, similarly structured gold active site. This finding paves the way for using earth-abundant supports to disperse and stabilize precious metal atoms with alkali additives for the WGS and potentially other fuel-processing reactions.

A Drastic Difference in Photoluminescent Behavior between Cysteinato and Penicillaminato Gold(III) Complexes with a Phenylpyridinato Ligand

Abstract The reactions of [Au(ppy)Cl2] (Hppy: 2-phenylpyridine) with d-penicillamine (d-H2pen) and l-cysteine (l-H2cys) in a 1:1 ratio gave mononuclear gold(III) complexes, [Au(ppy)(d-pen-N,S)] and [Au(ppy)(l-cys-N,S)], respectively, which were both structurally characterized to have a square-planar cis(N)-CN2S geometry. While the l-cys complex was nonemissive, the d-pen complex was found to exhibit orange emission in the solid state.

ChemInform Abstract: Enantioselective Gold(I) Catalysis with Chiral Monodentate Ligands

AbstractReview: gold‐catalyzed asymmetric cycloaddition/cycloisomerization reactions; 37 refs.+ subrefs.

ChemInform Abstract: Dual Gold Catalysis

AbstractReview: 71 refs.

Mononuclear gold catalysts for the asymmetric intramolecular hydroamination of alkenes

The intramolecular gold catalysed asymmetric hydroamination of alkenes was studied screening a series of mononuclear gold(I) and (III) complexes in combination with silver salts. Among the various chiral mono-phosphine and diaminocarbene ligands tried, the best catalysts arose from mononuclear gold(I) complexes synthesized from BINOL based phosphoramidite ligands. The latest were improved by addition of bulky substituents at specific positions of the BINOL scaffold. The resulting gold(I) complexes were combined with selected silver salts to afford efficient catalysts for intramolecular hydroamination of alkenes at mild temperatures, with good conversions and average enantioselectivities.

Dual Gold Catalysis

For more than a decade the innovative field of homogeneous catalysis by gold was dominated by the interaction of the substrate molecule with one gold center, in most cases in mononuclear gold complexes. The initial interaction was typically a π-coordination of a carbon-carbon double bond to the gold, an activation of the unsaturated substrate molecule by a π-acidic metal center. Only recently clear evidence for reactions that involve the activation of organic substrates by two gold centers was obtained. In that new class of gold-catalyzed reactions the two gold centers interact with the substrate in a very different way. One gold complex is σ-bonded to a terminal alkynyl group in the substrate, the other one is π-coordinated. Only in a few cases, a combination π-coordination and σ-coordination to the same alkyne, which is the energetically preferred mode of interaction with two gold centers, initiates the reaction. In most of the cases, the reaction proceeds through an intermediate with one alkyne σ-bonded to one gold complex and a different alkyne π-coordinated to the second gold complex. Experimental and computational results for many new reactions provide a clear picture of the overall sequence of elemental steps of these conversions; some of the steps are unprecedented in organometallic catalysis and chemistry. For example, the reaction of diynes can involve gold vinylidene-like species as very reactive substructures formed by a 5-exo-dig cyclization. NBO analysis indicates no gold-carbon double bond character in these "vinylidenes". In other reactions, a 6-endo-dig cyclization is energetically preferred; after that gold aryne complexes are not local minima but transition states of a 1,2-shift of gold. Computational studies showed a good correlation of the cyclization mode with the aromaticity of the intermediate. For both the 5-exo-dig and the 6-endo-dig cyclization modes, the intermediates are able to react even with unactivated alkyl-C,H bonds, in low yields even in intermolecular reactions. The final step of the catalytic cycles is also remarkable, because the protodeauration has to occur with the next alkyne substrate molecule. Only then the next gold acetylide is formed directly and a loss of selectivity can be avoided. A computational study suggests that two gold complexes are on the substrate throughout the catalytic cycle. The most recent results indicate that analogous intermediates can be accessible by the reaction of other electrophiles with gold acetylides. With regard to organic synthesis, the overall catalytic conversions open up a universe of new possibilities. Selective C,H-activations now allow to one use usually innocent alkyl side chains for additional anellation reactions by an sp(3)-C,H activation. The C,H activation can even be combined with halogen transfer reactions, directly providing vinyl iodides as versatile building blocks. Short and efficient routes to different carbo- and heterocycles including benzocyclobutenes, fulvenes, and pentalenes demonstrate the synthetic potential not only for total synthesis but also for material science.

Synthesis, characterization, photophysics, and anion binding properties of gold(i) acetylide complexes with amide groups

A mononuclear gold(i) acetylide complex with an amide group, 3a, shows a dramatic color change upon addition of F− in DMSO.

Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes forming mononuclear gold(iii) oxidative addition complexes and reversible chemical reductive elimination products

Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes were prepared to serve as precursors for subsequent oxidative addition (OA) chemistry following reaction with mild oxidant iodine, I2. The new OA products circumvented the formation of the expected dinuclear Au(ii) complexes, but instead formed novel chelating mononuclear square-planar gold(iii) products of the type [AuI2{E2PR2}] (R = (CH2)2Ph; E = S, 2; E = Se, 3) and [AuI2{Se2P(OR)2}] (R = Et, 4; (i)Pr, 5) directly. We further demonstrate that this process is chemically reversible as all the Au(iii) complexes undergo chemical reductive elimination to the starting dinuclear Au(i) complexes in the presence of SnI2 as determined by (119)Sn and (31)P NMR. The complexes (2-5) were all prepared from the reaction between I2 and the corresponding dinuclear gold(i) precursor material. All new complexes were characterized by (1)H, (31)P, (77)Se (for E = Se) NMR, infrared spectroscopy, elemental analysis and single crystal X-ray diffraction.

Gold chloride clusters with Au(iii) and Au(i) probed by FT-ICR mass spectrometry and MP2 theory

Microsolvated clusters of gold chloride are probed by electrospray ionization mass spectrometry (ESI-MS) and scalar relativistic electronic structure calculations. Electrospray ionization of aqueous AuCl3 leads to mononuclear clusters of types [AuCl2](+)(H2O)n (n = 0-4), [AuOHCl](+)(H2O)n (n = 0-1) and [AuCl2](+)(HCl)2(H2O)n (n = 0-4). In addition, strong ion signals due to dinuclear [Au2Cl5-xOHx](+)(H2O)n (x = 0-1) are present in ESI mass spectra of aqueous AuCl3, with the abundance of individual dinuclear species controlled by the concentration-dependent variation of the precursor complexes [AuCl2-xOHx](+)(H2O)n and AuCl3. Equilibrium structures, energies and thermodynamic properties of mono- and dinuclear gold clusters have been predicted using MP2 and CCSD(T) theory, and these data have been applied to examine the influence of microsolvation on cluster stability. Specifically, results from CCSD(T) calculations indicate that non-covalently bound ion-neutral complexes Au(+)(Cl2)(H2O)n, with formal Au(I), are the dominant forms of mononuclear gold with n = 0-2, while higher hydrates (n > 2) are covalently bound [AuCl2](+)(H2O)n complexes in which gold exists as Au(III). MP2 calculations show that the lowest energy structure of dinuclear gold is an ion-molecule cluster [Au2Cl(Cl2)2](+) consisting of a single-bridged digold-chloronium ion bound end-on to two dichlorine ligands, with two higher energy isomers, single-bridged [Au2Cl3(Cl2)](+) and double-bridged [Au2Cl5](+) clusters. Finally, AuAu interactions in the singly-bridged clusters [Au2Cl(Cl2)2](+)(H2O)n and [Au2Cl3(Cl2)](+)(H2O)n are examined employing a wide range of computational tools, including natural bond order (NBO) analysis and localized orbital locator (LOL) profiles.

Asymmetric Intramolecular Hydroamination of Allenes using Mononuclear Gold Catalysts

The intramolecular gold-catalyzed asymmetric hydroamination of allenes was studied by screening a series of mononuclear gold(I) and -(III) complexes in combination with silver salts. Among the various chiral monophosphine and diaminocarbene ligands tried, the best catalysts arose from mononuclear gold(I) complexes synthesized from BINOL-based phosphoramidite ligands. The latest were improved by addition of bulky substituents at specific positions of the BINOL scaffold. The resulting gold(I) complexes were combined with selected silver salts to afford efficient catalysts for intramolecular hydroamination of allenes at room temperature or below, with good conversions and enantioselectivities.

Enantioselective Gold(I) Catalysis with Chiral Monodentate Ligands

AbstractEnantioselective gold(I) catalysis is receiving a growing level of credit in the field of asymmetric synthesis. After a pioneering work in the mid‐1980s the area remained almost silent for several years, until densely substituted diphosphine chiral ligands were disclosed as competent chiral units for gold‐catalyzed stereoselective manipulation of unactivated unsaturated hydrocarbons. The chemistry of alkynes, allenes and most recently also alkenes received significant benefits from the latter developments. More recently, ad hoc designed chiral monodentate ligands have risen to prominence in producing robust, highly tunable (electronic and steric) and efficient chiral cationic mononuclear gold species for asymmetric transformations. In this scenario, electron‐deficient phosphorus‐based (i.e., phosphoramidites, phosphites) but also axially chiral electron‐rich carbene ligands deserve particular mention and the corresponding applications will be summarized herein.