N-heterocyclic carbenes in gold catalysis

The appealing properties of N-heterocyclic carbenes (NHC) as ancillary ligands and the high potential of gold as an organometallic catalyst have made their encounter inevitable. Still in its infancy, NHC-gold catalysis is nevertheless growing rapidly. In this tutorial review, catalytic transformations involving NHC-containing gold(i) and gold(iii) complexes are presented. Particular attention is drawn to the versatility and selectivity of these catalysts.     

Inclusion Complexes of Gold(I)-Dithiocarbamates with β-Cyclodextrin: A Journey from Drug Repurposing towards Drug Discovery

The gold(I)-dithiocarbamate (dtc) complex [Au(N,N-diethyl)dtc]₂ was identified as the active cytotoxic agent in the combination treatment of sodium aurothiomalate and disulfiram on a panel of cancer cell lines. In addition to demonstrating pronounced differential cytotoxicity to these cell lines, the gold complex showed no cross-resistance in therapy-surviving cancer cells. In the course of a medicinal chemistry campaign on this class of poorly soluble gold(I)-dtc complexes, >35 derivatives were synthesized and X-ray crystallography was used to examine structural aspects of the dtc moiety. A group of hydroxy-substituted complexes has an improved solubility profile, and it was found that these complexes form 2 : 1 host–guest inclusion complexes with β-cyclodextrin (CD), exhibiting a rarely observed “tail-to-tail” arrangement of the CD cones. Formulation of a hydroxy-substituted gold(I)-dtc complex with excess sulfobutylether-β-CD prevents the induction of mitochondrial reactive oxygen species, which is a major burden in the development of metallodrugs.     

Reactions and structural characterization of gold(III) complexes with amino acids, peptides and proteins

The present review article highlights recent findings in the field of gold(III) complexes with amino acids, peptides and proteins. The first section of this article provides an overview of the gold(III) reactions with amino acids, such as glycine, alanine, histidine, cysteine and methionine. The second part of the review is mainly focused on the results achieved in the mechanistic studies of the reactions between gold(III) and different peptides and structural characterization of gold(III)-peptide complexes as the final products in these reactions. The last section of this article deals with the reactions of gold(III) complexes with proteins as primary targets for cytotoxic gold compounds. Systematic summaries of these results contribute to the future development of gold(III) complexes as potential antitumor agents and also have importance in relation to the severe toxicity of gold-based drugs.     

Bioinorganic and medicinal chemistry: aspects of gold(I)-protein complexes

Gold(I)-based drugs have been used successfully for the treatment of rheumatoid arthritis (RA) for several years. Although the exact mechanism of action of these gold(I) drugs for RA has not been clearly established, the interaction of these compounds with mammalian enzymes has been extensively studied. In this paper, we describe the interaction of therapeutic gold(I) compounds with mammalian proteins that contain cysteine (Cys) and selenocysteine (Sec) residues. Owing to the higher affinity of gold(I) towards sulfur and selenium, gold(I) drugs rapidly react with the activated cysteine or selenocysteine residues of the enzymes to form protein-gold(I)-thiolate or protein-gold(I)-selenolate complexes. The formation of stable gold(I)-thiolate/selenolate complexes generally lead to inhibition of the enzyme activity. The gold-thiolate/selenolate complexes undergo extensive ligand exchange reactions with other nucleophiles and such ligand exchange reactions alter the inhibitory effects of gold(i) complexes. Therefore, the effect of gold(I) compounds on the enzymatic activity of cysteine- or selenocysteine-containing proteins may play important roles in RA. The interaction of gold(I) compounds with different enzymes and the biochemical mechanism underlying the inhibition of enzymatic activities may have broad medicinal implications for the treatment of RA.     

Dinuclear gold(I) complexes of bridging bidentate carbene ligands: synthesis, structure and spectroscopic characterisation

Eight dinuclear Au(i)-carbene complexes have been synthesized from various imidazolium-linked cyclophanes and related acyclic bis(imidazolium) salts, by treatment of the imidazolium salts with [Au(i)(SMe(2))Cl] in the presence of a carboxylate base. Single crystal structural studies showed that the Au(i)-carbene compounds contain dinuclear (AuL)(2) cations in which a pair of gold(i) centres are linked by a pair of bridging dicarbenoid ligands. Interestingly, the structural studies revealed short AuAu contacts of 3.0485(3)[Angstrom] and 3.5425(6)[Angstrom] in two of these complexes. NMR studies showed that the (AuL)(2) cations constructed from the cyclophane-based ligands retain a relatively rigid structure in solution, whilst those of the non-cyclophane ligand systems are fluxional in solution. The electronic absorption and emission spectra of the complexes in solution at room temperature were recorded and the complex with the shortest AuAu contact was found to emit intensely at 400 nm and more weakly at 780 nm upon excitation at 260 nm. The compounds with longer AuAu separations were not emissive under these conditions.     

X-ray-excited optical luminescence (XEOL) and X-ray absorption fine structures (XAFS) studies of gold(I) complexes with diphosphine and bipyridine ligands

Synchrotron techniques, X-ray-excited optical luminescence (XEOL) combined with X-ray absorption fine structures (XAFS), have been used to study the electronic structure and optical properties of a series of luminescent gold(I) complexes with diphosphine and bipyridine ligands using tunable X-rays (in the regions of the C and P K-edges and the Au L3-edge) and UV from synchrotron light sources. The effects of gold-ligand and aurophilic interactions on the luminescence from these gold(I) complexes have been investigated. It is found that the luminescence from these complexes is phosphorescence, primarily due to the decay of the Au (5d) --> PR3 (pi*), metal to ligand charge transfer (MLCT) excitation as well as contributions from the conjugated pi-system in the bipyridine ligands via the gold-nitrogen bond. The large Au 5d spin-orbit coupling enhances the intersystem crossing. The elongation of the hydrocarbon chain of the diphosphine ligand does not greatly affect the spectral features of the luminescence from the gold(I) complexes. However, the intensity of the luminescence was reduced significantly when the bipyridine ligand was replaced with 1,2-bis(4-pyridylamido)benzene. The aurophilic interaction, as investigated by EXAFS at the Au L3-edge, is shown to be only one of the factors that contribute to the luminescence of the complexes.     

Mononuclear aggregation-induced emission (AIE)-active gold(Ⅰ)-isocyanide phosphors: Contrasting phosphorescent mechanochromisms and effect of halogen substitutions on room-temperature phosphorescence nature

Developing phosphors with long-lifetime(millisecond scale or even longer) solid state room temperature phosphorescence(RTP) feature has attracted considerable attention. However, to date, stimuli-responsive phosphors with RTP nature are still rare due to the absence of effective guidelines for the exploitation of luminophors synchronously possessing stimuli-responsive and RTP characteristics. In this work,a series of mononuclear gold(Ⅰ) complexes are reported. All these complexes exhibit various...     

Cationic,Two‐Coordinate Gold π Complexes

Cationic, two‐coordinate gold π complexes that contain a phosphine or N‐heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold‐catalyzed functionalization of C? C multiple bonds. Although neutral two‐coordinate gold π complexes have been known for over 40 years, examples of the cationic two‐coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well‐defined examples of two‐coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.     

Formation of nonclassical carbonyls of Au3+ in zeolite NaY: characterization by infrared spectroscopy

Adsorption of CO on gold supported in zeolite NaY at 85 K led to the formation of (i) various carbonyls and isocarbonyls typical of the zeolite and (ii) carbonyls formed at cationic gold sites (observed in the 2186-2171 cm(-1) region). Analysis of the behavior of the bands allows their assignment to carbonyls of Au(3+) ions. At temperatures higher than 220 K, CO adsorption led to the formation of a new type of Au(3+)-CO species (2207 cm(-1)). Once formed, these complexes could be transformed into the dicarbonyls Au(3+)(CO)(2) when the sample was cooled to 85 K in the presence of CO. The results are explained by migration of Au(3+) ions to more accessible positions within the zeolite at increasing temperatures. When a CO molecule is already adsorbed, it stabilizes the Au(3+) ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species. These results are the first evidence of Au(3+)(CO)(2) complexes.     

When Gold Cations Meet Polyoxometalates

Merging gold(I) cations with polyoxometalate anions results in various interclusters and complexes. Herein, the synthesis of these newly emerging gold(I)/polyoxometalate materials is reviewed. The applications of these promising hybrids in organic catalysis are also summarized and evaluated in terms of the advantages and limitations of the catalysts including efficiency, synergistic effects and recyclability.     

High Efficiency Sky-Blue Gold(III)-TADF Emitters**

Highly efficient sky-blue luminescent gold(III) complexes with emission quantum yields up to 82 %, lifetimes down to 0.67 μs and emission peak maxima at 470–484 nm were prepared through a consideration of pincer gold(III) donor–acceptor complexes. Photophysical studies and time-dependent density functional theory (TDDFT) calculations revealed that the emission nature of these gold(III) complexes is most consistent with TADF. Solution-processed OLEDs with these gold(III) complexes as dopants afforded electroluminescence maxima at 465–473 nm with FWHM of 64–67 nm and maximum external quantum efficiencies (EQEs) of up to 15.25 %. This research demonstrates the first example of gold(III)-OLEDs showing electroluminescence maxima at smaller than 470 nm, and highlights the potential of using gold(III)-TADF emitters in the development of high efficiency blue OLEDs and blue emissive dopant in WOLEDs.     

Gold(I) complexes with selenones and triphenylphosphine as ligands

A number of mixed ligand complexes of gold(I) with various selenones and Ph₃P, [Ph₃PAuSe=C<]Cl have been prepared and characterized by elemental analyses, i.r. and n.m.r. methods. A decrease in the i.r. frequency of the C=Se mode of selenones upon complexation is indicative of gold(I) binding viaa selenone group. An upfield shift in the ¹³ C-n.m.r. for the C=Se resonance of selenones and downfield shifts in ³¹P-n.m.r. for the Ph₃P moiety are consistent with the selenium coordination to gold(I). Available data in the literature suggest that P–Au–Se type complexes are usually linear.     

The chemistry of binuclear palladium(II) and platinum(II) complexes

The chemistry of binuclear palladium(II) and platinum(II) complexes has been reviewed. This review deals with complexes derived from various classes of ligands and covers various aspects, viz. synthesis, spectroscopic and structural features and chemical reactivity, of these complexes. Applications of these complexes are briefly described in the respected sections.     

Tunable Light-Emission Properties of Solution-Processable N-Heterocyclic Carbene Cyclometalated Gold(III) Complexes for Organic Light-Emitting Diodes

N-Heterocyclic carbene (NHC) cyclometalated gold(III) complexes remain very scarce and therefore their photophysical properties remain currently underexplored. Moreover, gold(III) complexes emitting in the blue region of the electromagnetic spectrum are rare. In this work, a series of four phosphorescent gold(III) complexes was investigated bearing four different NHC monocyclometalated (C^C*)-type ligands and a dianionic (N^N)-type ancillary ligand ((N^N)=5,5’-(propane-2,2-diyl)bis(3-(trifluoromethyl)-1 H-pyrazole) (mepzH₂)). The complexes exhibit strong phosphorescence when doped in poly(methyl methacrylate) (PMMA) at room temperature, which were systematically tuned from sky-blue [λ_PL=456 nm, CIE coordinates: (0.20, 034)] to green [λ_PL=516 nm, CIE coordinates: (0.31, 0.54)] by varying the monocyclometalated (C^C*) ligand framework. The complexes revealed high quantum efficiencies (ϕ_PL) of up to 43 % and excited-state lifetimes (τ₀) between 15–266 μs. The radiative rate constant values found for these complexes (k_r=10³–10⁴ s⁻¹) are the highest found in comparison to previously known best-performing monocyclometalated gold(III) complexes. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these complexes further lend support to the excited-state nature of these complexes. The calculations showed a significant contribution of the gold(III) metal center in the lowest unoccupied molecular orbitals (LUMOs) of up to 18 %, which was found to be unique for this class of cyclometalated gold(III) complexes. Additionally, organic light-emitting diodes (OLEDs) were fabricated by using a solution process to provide the first insight into the electroluminescent (EL) properties of this new class of gold(III) complexes.     

Complexation of (trimethylphosphine)gold(I) with Selenones

Mixed ligand complexes of gold(I) with various selenones and Me₃P, [Me₃PAuSe=C<]Cl, have been prepared and characterized by elemental analyses, i.r. and n.m.r. methods. A decrease in the i.r. frequency of the >C=Se mode of selenones upon complexation is indicative of selenone binding to gold(I) via a selenone group. An upfield shift in ¹³C-n.m.r. for the >C=Se resonance of selenones and downfield shifts in ³¹P-n.m.r. for Me₃P moiety are consistent with the selenium coordination to gold(I). The steric effect as well as the basicity of Me₃PAu⁺ plays a significant role in bonding with Se-containing ligands compared to the Et₃PAu⁺ and Ph₃PAu⁺ complexes.     

Photochromic Benzo[b]phosphole Alkynylgold(I) Complexes with Mechanochromic Property to Serve as Multistimuli‐Responsive Materials

A series of novel benzo[b]phosphole alkynylgold(I) complexes has been demonstrated to display photochromic and mechanochromic properties upon applying the respective stimuli of light and mechanical force. Promising multistimuli‐responsive properties of this series of gold(I) complexes have been successfully achieved through judicious molecular design, which involves incorporation of the photochromic dithienylethene‐containing benzo[b]phosphole into the triphenylamine‐containing arylethynyl ligand that is susceptible to mechanical force‐induced color changes via gold(I) complexation. With excellent thermal irreversibility and robust fatigue resistance of this series of gold(I) complexes, multicolor states controlled by the photochromism and mechanochromism have been realized. Repeatable photochromic and mechanochromic cycles without apparent loss of reactivity have also been observed under ambient conditions. The present work provides important insight and an alternative strategy for the molecular design of multistimuli‐responsive materials, paving the way for further development of the underexplored photoresponsive gold(I) complexes and the multistate photocontrolled system.     

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-MeOC₆H₄)]), 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.     

The Stabilizing Effects in Gold Carbene Complexes

Bonding and stabilizing effects in gold carbene complexes are investigated by using Kohn–Sham density functional theory (DFT) and the intrinsic bond orbital (IBO) approach. The π‐stabilizing effects of organic substituents at the carbene carbon atom coordinated to the gold atom are evaluated for a series of recently isolated and characterized complexes, as well as intermediates of prototypical 1,6‐enyne cyclization reactions. The results indicate that these effects are of particular importance for gold complexes especially because of the low π‐backbonding contribution from the gold atom.     

Towards the rational design of platinum(II) and gold(III) complexes as antitumour agents

Metal complexes afford an opportunity for the discovery of new antitumour drugs with truly novel mechanisms of action. Various tactics and some new concepts have been employed to improve the physico-chemical and biological properties of metal complexes. Recent advances in this area demonstrate a bright prospect for the utilization of metal complexes in cancer chemotherapy. The theme of this article focuses on the approaches towards the rational design of platinum(II) and gold(III) complexes with antitumour properties based on the updated understanding of the mechanism of action of these compounds. The complexes summarized in this work include monofunctional platinum(II) complexes, multinuclear platinum(II) complexes, hybrid and targeted platinum(II) complexes, and gold(III) complexes. Most of them violate the established structure-activity relationships and demonstrate different reactivities from cisplatin and thereby show some potential for the prevention of detoxification.     

Reactivity of Gold Complexes towards Elementary Organometallic Reactions

For a while, the reactivity of gold complexes was largely dominated by their Lewis acid behavior. In contrast to the other transition metals, the elementary steps of organometallic chemistry—oxidative addition, reductive elimination, transmetallation, migratory insertion—have scarcely been studied in the case of gold or even remained unprecedented until recently. However, within the last few years, the ability of gold complexes to undergo these fundamental reactions has been unambiguously demonstrated, and the reactivity of gold complexes was shown to extend well beyond π‐activation. In this Review, the main achievements described in this area are presented in a historical context. Particular emphasis is set on mechanistic studies and structure determination of key intermediates. The electronic and structural parameters delineating the reactivity of gold complexes are discussed, as well as the remaining challenges.