Cationic Au Research Articles

Synthesis of cationic gold(III) complexes using iodine(III)

We report the synthesis and characterization of cationic Au(III) complexes supported by nitrogen-based ligands. The syntheses are achieved by reacting Au(I) complexes [Au(N-Me-imidazole)2]+ and [Au(pyridine)(NHC)]+ with iodine(III) reagents PhI(OTf)(OAc) and [PhI(pyridine)2]2+ yielding a series of cationic gold(III) complexes. In contrast, reactions of phosphine ligated gold(I) complexes with iodine(III) reagents results in the oxidation of the phosphine ligand.

Mineralogy and Geochemistry of Pyrite and Arsenopyrite from the Zaozigou Gold Deposit in West Qinling Orogenic Belt, Central China: Implications for Ore Genesis

AbstractThe Zaozigou gold deposit lies in the West Qinling orogenic belt, Gansu Province, China. It is one of the largest gold deposits, and the orebodies are hosted in fine‐grained slates intercalated with limestone of the Middle‐Triassic Gulangdi Formation and varied dykes. The gold orebodies are strictly controlled by the NE‐, NW‐, and SN‐trending tensional and shearing faults with high dipping angle. The mineralogy and geochemistry of pyrite and arsenopyrite are measured by electron microprobe. Pyrite has up to 0.12 wt.% Au, and arsenopyrite contains up to 0.17 wt.% Au. The antithetic correlation between S and As indicates the substitution of As for S in pyrite, and arsenic occurs in anionic As1− state in the pyrite structure under the reduced conditions. Pyrite has relatively high Co (~364–2248 ppm) but relatively low Ni (~109–497 ppm) contents, with Co/Ni ratios ranging from ~1.63 to 10.50, indicating that the deposit originated from a volcanogenic fluid and remobilized by hydrothermal fluid. Au in arsenopyrite occurs as cationic Au in solid solution, whereas Au in pyrite is in solid solution and metal nanoparticles (Au0). The texture characteristics and trace element geochemistry among cores, transition zones, and rims of pyrites demonstrate that there are at least four pulses of fluid participating in the generation of pyrite in the deposit. The calculated formation temperatures of the Zaozigou deposit vary from 148°C to 304°C, with an average temperature of 213°C based on Au contents in pyrite. The Pb isotopic compositions of pyrite samples suggest that the metallogenic materials of the Zaozigou deposit were derived from the mantle and upper crust. All the characteristics above lead us to draw the conclusion that the Zaozigou gold deposit is classified as an epithermal deposit.

Plasma-promoted Au/TiO2 nanocatalysts for photocatalytic formaldehyde oxidation under visible-light irradiation

Abstract As a promising plasmonic photocatalyst under visible-light irradiation, Au/TiO2 nanocatalyst commonly suffers from risk of Au nanoparticles aggregation and relatively long activation period by using the conventional calcination activation method. We report here that a short-term O2 plasma treatment enables the Au/TiO2 catalyst to exhibit higher visible-light photocatalytic activity for formaldehyde (HCHO) oxidation compared with the calcination activated sample. X-ray photoelectron spectroscopy characterization suggests that Au/TiO2 catalyst activated by O2 plasma achieves metallic Au content of 72.4% and surface oxygen species content of 29.8%, which not only lead to rapid reduction of cationic Au in HCHO oxidation but also facilitate the preliminary oxidation of HCHO to dioxymethylene and formate on catalyst surface. UV–vis diffuse reflectance spectra, photoluminescence spectra and diffuse reflectance infrared Fourier transform spectra measurements indicate that O2 plasma activated Au/TiO2 catalyst presents an enhanced visible-light absorption and surface charge transfer efficiency in HCHO oxidation, which accelerates the formation of superoxide (O2−) and OH radicals via hot electron transfer mechanism under visible-light irradiation, remarkably promoting formate oxidation to carbonate, carbonate decomposition and thus HCHO oxidation.

Integrated Synthesis of Gold Nanoparticles Coated with Polyoxometalate Clusters.

Polyoxometalates (POMs) have been found to be good end-capping ligands for gold nanoparticles (AuNPs). Herein, we introduce a new synthetic method to synthesize gold nanoparticle-POM hybrids by heating a solution of AuNO3(PMe3) in acetonitrile in the presence of appropriate POM species with tetrabutylammonium (TBA) as a countercation at 120 °C in a microwave. This method allowed us to produce POM-capped AuNPs without over-reduction of the solution causing decomposition or reorganization of the POMs. Analysis of the resulting material by transmission electron microscopy showed that the POM's size, charge, and functionality are key factors controlling the resulting POM-AuNP hybrid structure. Additionally, the reaction was monitored by electrospray ionization mass spectrometry (ESI-MS), ultraviolet-visible spectroscopy, and dynamic light scattering. The ESI-MS studies reveal crucial information regarding the nature of the reaction that takes place, showing the cation exchange between Au(I) and TBA cations, followed by self-reduction of the Me3PAu(I)-POM complex.

Sulfur Moiety as a Double-Edged Sword for Realizing Ultrafine Supported Metal Nanoclusters with a Cationic Nature

Heterogeneously and uniformly dispersed metal nanoclusters with high thermal stability and stable nonmetallic nature show outstanding catalytic performance. In this work, we report on the role of sulfur moieties in hydrochlorination catalysis over carbon-supported gold (Au/C). A combination of experimental and theoretical analyses shows that the -SO3H and derived -SO2H sulfur species in high oxidation states at the interface between Au and -SO3H at ≥180 °C give rise to high thermal stability and catalytic activity. By contrast, the grafted thiol group (-SH) and the derived low-valence sulfur species on carbon markedly destabilize the Au nanoclusters, promoting their rapid sintering into large Au nanoparticles and leading to the loss of their cationic nature. Theoretical calculations suggest that -SO3H favorably adsorbs and stabilizes cationic Au species. Compared to Au/C and Au-SH/C with the Auα+/Au0 atomic ratios of 1.02 and 0.24, respectively (α = 1 or 3), the activity and durability of acetylene hydrochlorination are remarkably enhanced by the interaction between the -SO3H moieties and cationic Au species that enables the high oxidation state of Au to be effectively retained (Auα+/Au0 = 3.82). These results clearly demonstrate the double-edged sword effect of sulfur moieties on the catalytic Au component in acetylene hydrochlorination. The double-edged sword effect of sulfur species in the stabilization/destabilization of metal nanoclusters is also applicable to other metals such as Ru, Pd, Pt, and Cu. Overall, this study enriches the general understanding of the stabilization of metal clusters and provides insight into a wet chemistry strategy for stabilizing supported ligand-free nanoclusters.

Formation of Gold Particles via Thiol Groups on Glycoconjugates Comprising the Sheath Skeleton of Leptothrix

Leptothrix, iron-oxidizing bacterium, produces microtubular sheaths that surround the catenulate cells. Organic nanofibrils excreted from the cell surfaces interweave and coalesce to form immature sheaths, which attract aqueous-phase inorganics to eventually form mature organic–inorganic sheaths. Such inorganic encrustation of the sheaths results from interactions between functional groups in the sheath skeleton and inorganics. Based on our previous findings that Leptothrix sheath skeleton sorbed 47 inorganics (Au was one of the most abundant adsorbates), we examined the sorption status of Au cations on cell-enclosing sheaths and their protein-free remnants and found that nano to sub-micron Au particles (AuNPs and AuSMPs, respectively) formed on the sheath-forming polymer consisting of a glycoconjugate (an amphoteric glycan modified with cysteine, glycine, and 3-hydroxypropionic acid). When the purified polymer was incubated in HAuCl4 solution, AuNPs and AuSMPs formed on the polymer surfaces. Both particles formed also on cell-enclosing sheaths and protein-free sheath remnants incubated in HAuCl4 solution. When SH groups in the cell-enclosing sheaths were masked with a fluorescent protein, Au particles did not form after incubation in HAuCl4 solution. Results implicate that SH groups are at least partially involved in the reduction of Au cations to metallic Au and eventual formation of Au particles.

Mechanism and chemoselectivity origins of bioconjugation of cysteine with Au(iii)-aryl reagents

Organometallic reagents, in particular Pd(ii)- and Au(iii)-aryl reagents, have recently emerged as an efficient tool for bioconjugation. However, the detailed mechanism and origins of chemoselectivity are not well established, but are highly desirable from both synthetic and theoretical viewpoints. In this paper, we report that a computational study dealing with the reaction mechanism of Au(iii)-aryl reagents enabled selective cysteine S-arylation of peptides and proteins developed by Maynard and Spokoyny et al. (J. Am. Chem. Soc., 2018, 140, 7065). Our calculation results suggest that the reaction proceeds by a cationic Au(iii)/Au(i) pathway involving elementary steps of (a) binding of the SH residue to the Au(iii) center, (b) deprotonation of the SH residue, and (c) reductive elimination from a key four-coordinate square planar (L)Au(iii)(thiolate)(Ar) (L is a P,N-bidentate ligand) intermediate. Furthermore, the chemoselectivity of S-arylation against arylation of other nucleophilic residues can be rationalized in terms of energy demand of the three elementary steps. For instance, amine N-arylation is more difficult than S-arylation due majorly to the much higher energy required for deprotonation of much more basic N-H bonds than for deprotonation of weakly acidic S-H bonds. Carboxylate O-arylation is challenging due to the high activation energy of reductive elimination from LAu(iii)(carboxylate)(aryl), because carboxylate is much less nucleophilic than thiolate. These results thus identify acidity and nucleophilicity of the residue as two inherent factors for bioconjugation. This study provides a useful and convenient approach for predicting and rationalizing the feasibility and chemoselectivity of related bioconjugation reactions.

Synthesis of Indolo[1,2-a]indole Derivatives by Cationic Au(I)-Catalyzed exo-Selective Cycloisomerization and Their Photophysical Properties

Synthesis of Indolo[1,2-a]indole Derivatives by Cationic Au(I)-Catalyzed exo-Selective Cycloisomerization and Their Photophysical Properties

The π-acidity/basicity of cyclic trinuclear units (CTUs): from a theoretical perspective to potential applications.

Cyclic trinuclear units (CTUs) based on Au(i), Ag(i) and Cu(i) cations, featuring near planar nine-membered coordination rings, represent an important class of metal-organic π-acids/bases with highly adjustable π-acidity/basicity. Their superior π-acidity/basicity coupled with Lewis-acidic and metalmetal bonding sites offers excellent attraction for a wide range of acidic/basic species, and usually followed by noticeable changes of luminescence or charge transfer behaviors. A series of representative cases from the past two decades have been selected herein for such cyclic trinuclear units in both oligomeric and polymeric systems. Their fascinating and profound potential applications related to π-acidity/basicity are highlighted, including molecular absorption and separation, luminescence sensing and detection, organic light-emitting diodes (OLEDs), metal-organic field-effect transistors (MOFETs), molecular wires, and catalysis. The challenges in improving the performance for practical application will also be discussed.

Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions

AbstractDissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine‐4‐thiol (4‐PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4‐PSH to pyridine‐4‐thione (4‐PS), coordination with Au0, and then oxidation of the Au0 thione species to AuI simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a AuI complex bearing two 4‐PS ligands and SO42− as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic Solutions.

Dissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine-4-thiol (4-PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4-PSH to pyridine-4-thione (4-PS), coordination with Au0 , and then oxidation of the Au0 thione species to AuI simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a AuI complex bearing two 4-PS ligands and SO4 2- as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Protamine-gold nanoclusters as peroxidase mimics and the selective enhancement of their activity by mercury ions for highly sensitive colorimetric assay of Hg(II).

We certify that protamine-gold nanoclusters (PRT-AuNCs) synthesized by one-pot method exhibit peroxidase-like activity. The catalytic activity of PRT-AuNCs followed typical Michaelis-Menten kinetics and exhibited higher affinity to 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate compared to that of natural horseradish peroxidase. Meanwhile, we found that Hg(II) could dramatically and selectively enhance the peroxidase-like activity of PRT-AuNCs, and the enhanced mechanism by Hg(II) was demonstrated to be generation ofthe cationic Au species and the partly oxidized Au species (Auδ+) by Hg2+-Au0/Au+ interaction. Based on this finding, quantitative determinations of Hg(II) via visual observation and absorption spectra were achieved. The proposed strategy displays high selectivity that arises from the strong aurophilic interaction of mercury towards gold. Moreover, the developed method is highly sensitive with a wide linear range and low detection limit of 1.16nM. This strategy is not only helpful to develop effective nanomaterials-based artificial enzyme mimics but also irradiative to discover new applications of artificial mimic enzymes in bio-detection, medical diagnostics, and biotechnology. Graphical abstract Protamine-gold nanoclusters (PRT-AuNCs) synthesized by one-pot method exhibit peroxidase-like activity. Hg(II) can stimulate the peroxidase-like activity of PRT-AuNCs selectively, enhancing their ability to catalyze the chromogenic reaction of TMB by H2O2.

The interplay of Au nanoparticles and ZnO nanorods for oxygen-promoted, base-free, complete formaldehyde reforming into H2 and CO2

Au nanoparticles on ZnO nanorods is a sustainable catalyst for oxygen-promoted, base-free, complete formaldehyde reforming into H2 and CO2 at low temperatures. The strong metal-support interaction (SMSI) between Au and ZnO is suggested to be responsible for this catalytic feature partially due to the presence of surface cationic Au+ species formed during high temperature treatment in air. in-situ EPR and resonance Raman analysis show that a two-electron reduction pathway is involved to produce hydrogen peroxide as the active species and CH bond cleavage is the rate-determining step.

Selective Catalytic Hydration of Alkynes in the Presence of Au‐Cavitands: A Study in Structure–Activity Relationships

The effects of the catalytic cavities in gold‐functionalized cavitands in the hydration of internal alkynes have been studied. Variations on cavitand structures revealed the importance of two features that were studied: (1) flanking aromatic rings, and (2) an adjacent P=O moiety. The di‐quinoxaline‐spanned resorcin[4]arene system provides a well‐defined compartment, in which a cationic Au ion activates an internal alkyne for conversion into a ketone by delivery of water that has also been activated, this time by a P=O moiety. We synthesized four variations on our parent cavitand. Variations of the cavitand walls include replacement of quinoxaline components with pyrazine or methylene units. Variation of the P=O center was accomplished with methylene or quinoxaline moieties. All variants displayed lower catalytic activity or selectivity, allowing us to confirm the significance both of an internal cavity and of an activation site for water.

Ultrarapid Cationization of Gold Nanoparticles via a Single-Step Ligand Exchange Reaction.

We propose a novel method for the ultrarapid cationization of gold nanoparticles with diameters ranging from several to a hundred nanometers via a single-step ligand exchange reaction of citrate-protected anionic gold nanoparticles with cationic thiol ligand. This reaction was performed only in an aqueous medium via a single step from citrate to cationic thiol and therefore enables a rapid preparation of cationic Au nanoparticles without a contamination of organic solvent or lipid-soluble molecules. The cationization was successfully completed within 20 min without changes in the core diameter and optical characteristic.

Towards a greener approach for the preparation of highly active gold/carbon catalyst for the hydrochlorination of ethyne

Gold on activated carbon (Au/AC) materials are promising alternative catalysts for ethyne hydrochlorination. The preparation of active, stable Au/AC catalysts without aqua regia for ethyne hydrochlorination remains a significant challenge. A novel catalyst preparation protocol involving impregnation using a H2O2/HCl mixture is established for highly active Au/AC catalysts comprising primarily of single-site cationic Au species, as identified by systematic X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) analyses and transmission electron microscopy (TEM) imaging. In addition, evaluation of the Au-C interface by temperature-programmed desorption (TPD) analyses showed that the oxidation of activated carbon by the H2O2/HCl mixture, which creates surface oxygen-containing functional groups (SOGs), is a crucial step for the formation of active Au/AC catalysts. The structure determination and comprehensive experimental evidence allow density functional theory (DFT) to predict that single-site cationic AuCl species stabilized by SOGs via -O- linkages are efficient active sites for Au-catalyzed ethyne hydrochlorination. In addition, these catalysts can be reused for several times with negligible changes in performance after treatment with the H2O2/HCl mixture. The H2O2/HCl mixture is thus envisioned as a viable, green alternative to toxic aqua regia for the preparation of Au/AC catalysts for ethyne hydrochlorination.

Structures and electronic properties of halogenated Au(III) phthalocyanine AuPcX (X = Cl, Br): A Density Functional Theoretical Study

The structure and electronic property of halogenated Au(III) phthalocyanine AuPcX (X = Cl, Br) have been investigated theoretically with density functional theory (DFT). The calculation results show that the Au cation is coordinated with one halogen atom and four nitrogen atoms of phthalocyanine ligand. In addition, the electro-absorption spectrum of AuPcX (X = Cl, Br) have been calculated by time-dependent density functional theory. The calculated results show that both of the two compounds have strong absorption in the Q-band and Soret-band region, the solvent has a great influence on the absorption spectrum as the two compounds have different transition mechanism in vacuum and solvent environment. In addition, with the increase of solvent polarity, the absorption spectrum especially the Q-band of the two compounds is red-shifted evidently.

Effect of palladium doping on the stability and fragmentation patterns of cationic gold clusters

We analyze in detail how the interplay between electronic structure and cluster geometry determines the stability and the fragmentation channels of single Pd-doped cationic Au clusters, $\mathrm{PdA}{\mathrm{u}}_{N}{}_{\ensuremath{-}1}{}^{+}$ ($N=2\ensuremath{-}20$). For this purpose, a combination of photofragmentation experiments and density functional theory calculations was employed. A remarkable agreement between the experiment and the calculations is obtained. Pd doping is found to modify the structure of the Au clusters, in particular altering the two-dimensional to three-dimensional transition size, with direct consequences on the stability of the clusters. Analysis of the electronic density of states of the clusters shows that depending on cluster size, Pd delocalizes one $4d$ electron, giving an enhanced stability to $\mathrm{PdA}{{\mathrm{u}}_{6}}^{+}$, or remains with all $4{d}^{10}$ electrons localized, closing an electronic shell in $\mathrm{PdA}{{\mathrm{u}}_{9}}^{+}$. Furthermore, it is observed that for most clusters, Au evaporation is the lowest-energy decay channel, although for some sizes Pd evaporation competes. In particular, $\mathrm{PdA}{{\mathrm{u}}_{7}}^{+}$ and $\mathrm{PdA}{{\mathrm{u}}_{9}}^{+}$ decay by Pd evaporation due to the high stability of the $\mathrm{A}{{\mathrm{u}}_{7}}^{+}$ and $\mathrm{A}{{\mathrm{u}}_{9}}^{+}$ fragmentation products.

Cationic Au(I) complexes with aryl-benzothiazoles and their antibacterial activity

Two cationic Au(I) complexes derived from aryl-benzothiazoles, namely [(PPh3)Au(pbt)](OTf) (1) and [(PPh3)Au(qbt)](OTf) (2) (where pbt = 2‑(pyridyl)benzothiazole and qbt = (quinolyl)benzothiazole, and OTf− = trifluoromethanesulfonate anion), have been synthesized and structurally characterized by X-ray crystallography. Both complexes exhibit strong antibacterial effects against Gram-negative bacteria such as Acinetobacter baumannii and Pseudomonas Aeruginosa. Results of examination of the reactions of 1 and 2 indicate that these cationic Au(I) complexes rapidly cross the bacterial membrane and exert drug action by disrupting cellular function(s) through binding of cytosolic thiol-containing peptides (such as glutathione) and proteins to the highly reactive (PPh3)Au+ intermediate formed upon in situ dissociation of pbt or qbt.

Synthesis and characterization of gold-containing oxides of K2NiF4 or Nd2CuO4 structure type

In this work, different materials of Nd2CuO4 and K2NiF4 Ruddlesden-Popper structure have been prepared by solid-state reaction in air and high temperatures. The powders of mixed oxides, of general formula La2Li0.5M0.5 − xAu x O4 (M = Cu, Ni), were prepared from [Au(NH3)4](NO3)3 as gold source. The stoichiometric proportions of the all reagents (including La2O3, NiCO3, CuO) were grounded with a LiOH excess as mineralizer, pressed to form into pellets and heated in static air at temperatures between 750 and 850 °C for several hours. Structural characterization of the synthesized oxides using X-ray diffraction (XRD) are presented. La2Li0.5Au0.5O4, La2Li0.5Cu0.5O4, and La2Li0.5Ni0.5O4, three previously known oxides, were obtained. As expected, La2Li0.5Ni0.5O4 and La2Li0.5Cu0.5O4 consist of an ordered K2NiF4 Ruddlesden-Popper structure, while the La2Li0.5Au0.5O4 compound exhibits a Nd2CuO4 type structure. On the other hand, the preparation of the new proposed group La2Li0.5M0.5 − xAuxO4 (x = 0.0125–0.5) containing gold and nickel/copper simultaneously, under the studied conditions, was not possible. The experimental results revealed that the host structures are intolerant with respect to M (Cu or Ni) and Au cation double substitution. The difficulty to obtain the new phases is discussed regarding the exceptional instability displayed by the trivalent state of gold ion (Au3+) and the soft interactions of this specie with the lithium cation. Finally, the monophasic oxides, more interesting in terms of catalytic activity, were analyzed by temperature-programmed reduction (TPR-H2).