Triphenylphosphine-gold(0)/gold(I) compounds

Catalysis-free synthesis of thiazolidine-thiourea ligands for metal coordination (Au and Ag) and preliminary cytotoxic studies

The reaction of propargylamines with isothiocyanates results in the selective formation of iminothiazolidines, aminothiazolines or mixed thiazolidine-thiourea compounds under mild conditions. It has been observed that secondary propargylamines lead to the selective formation of cyclic 2-amino-2-thiazoline derivatives, while primary propargylamines form iminothiazoline species. In addition, these cyclic thiazoline derivatives can further react with an excess of isothiocyanate to give rise to thiazolidine-thiourea compounds. These species can also be achieved by reaction of propargylamines with isothiocynates in a molar ratio of 1 : 2. Coordination studies of these heterocyclic species towards silver and gold with different stoichiometries have been carried out and complexes of the type [ML(PPh₃)]OTf, [ML₂]OTf (M = Ag, Au) or [Au(C₆F₅)L] have been synthesised. Preliminary studies of the cytotoxic activity in lung cancer cells have also been performed in both ligands and complexes, showing that although the ligands do not exhibit anticancer activity, their coordination to metals, especially silver, greatly enhances the cytotoxic activity.

Gold Clusters: From the Dispute on a Gold Chair to the Golden Future of Nanostructures

The present work opens with an acknowledgement to the research activity performed by Luciana Naldini while affiliated at the Università degli Studi di Sassari (Italy), in particular towards gold complexes and clusters, as a tribute to her outstanding figure in a time and a society where being a woman in science was rather difficult, hoping her achievements could be of inspiration to young female chemists in pursuing their careers against the many hurdles they may encounter. Naldini’s findings will be a key to introduce the most recent results in this field, showing how the chemistry of gold compounds has changed throughout the years, to reach levels of complexity and elegance that were once unimagined. The study of gold complexes and clusters with various phosphine ligands was Naldini’s main field of research because of the potential application of these species in diverse research areas including electronics, catalysis, and medicine. As the conclusion of a vital period of study, here we report Naldini’s last results on a hexanuclear cationic gold cluster, [(PPh₃)₆Au₆(OH)₂]²⁺, having a chair conformation, and on the assumption, supported by experimental data, that it comprises two hydroxyl groups. This contribution, within the fascinating field of inorganic chemistry, provides the intuition of how a simple electron counting may lead to predictable species of yet unknown molecular architectures and formulation, nowadays suggesting interesting opportunities to tune the electronic structures of similar and higher nuclearity species thanks to new spectroscopic and analytical approaches and software facilities. After several decades since Naldini’s exceptional work, the chemistry of the gold cluster has reached a considerable degree of complexity, dealing with new, single-atom precise, materials possessing interesting physico-chemical properties, such as luminescence, chirality, or paramagnetic behavior. Here we will describe some of the most significant contributions.

A mono-copper doped undeca-gold cluster with up-converted and anti-stokes emissions of fluorescence and phosphorescence

We have synthesized single crystals of a highly stable Cu-doped undeca-gold cluster protected by both triphenylphosphine (PPh3) and 2-pyridinethiol (-SPy) ligands, formulated as [Au11Cu1(PPh3)7(SPy)3]+. This cluster (Au11Cu1 NCs for short) has a metallic core of C3v Au@Au10 with the Cu atom capped on one of the nine triangular facets and it is triply-coordinated to three N atoms of the SPy ligands of which the sulfur atom simultaneously binds to three adjacent Au atoms via singly-coordinated S-Au bonds, respectively. The other seven gold atoms form a crown structure by a link of three orthogons with common sides and are protected by seven PPh3 ligands. Besides the well-organized coordination, this Au11Cu1 nanocluster is demonstrated to exhibit superatom stability of the metallic core within 8 valence electrons (assuming that the 3 electrophilic-SPy ligands capture 3 electrons from the metal center). More interestingly, this Au11Cu1 nanocluster shows interesting emissions in both ultraviolet visible (UV-Vis) and near infrared (NIR) regions, and the emissions display novel anti-Stokes up-conversion lasing characteristics. TD-DFT calculated UV-vis and emission spectra well reproduce the experimental results, shedding light on the nature of excitation states and underlying mechanism of electronic transitions between diverse energy levels of such a monolayer-protected bimetallic cluster.

High Precision Electronic Spectroscopy of Ligand-Protected Gold Nanoclusters: Effects of Composition, Environment, and Ligand Chemistry

Atomically precise gold nanoclusters (AuNCs) are a class of nanomaterials valued for their electronic properties and diverse structural features. While the advent of X-ray crystallography of AuNCs has revealed their geometric structures with high precision, detailed electronic structure analysis is challenged by environmental, compositional, and thermal averaging effects present in electronic spectra of typical samples. To circumvent these challenges, we have adapted mass spectrometer-based electronic absorption spectroscopy techniques to acquire high-resolution electronic spectra of atomically precisely defined nanoclusters separated from a synthetic mixture. Here we discuss recent results using this approach to link the surface chemistry of triphenylphosphine-protected AuNCs to their electronic structure and expand on key elements of the experiment and the link between these gas-phase measurements and solution-phase behavior of AuNCs. Chemically derivatized Au₈(P(p-X-Ph)₃)₇²⁺ and Au₉(P(p-X-Ph)₃)₈³⁺ clusters, where X = -H, -CH₃, or -OCH₃, are used to derive systematic trends in the response of the electronic spectrum to the electron-donating character of the ligand shell. We find a linear relationship between the substituent Hammett parameter σ_p and the transition energy between both sets of clusters' highest occupied and lowest unoccupied molecular orbitals, a transition that is localized in the metal core within the limits of the superatomic model. The similarity of the mass-selective and solution-phase UV/vis spectra of Au₉(PPh₃)₈³⁺ indicates that the interpretation of these experiments is transferable to the condensed phase. He and N₂ environments are introduced to a series of isovalent clusters as a subtle probe of discrete environmental effects over electronic structure. Strikingly, select bands in the UV/vis spectrum respond strongly to the identity of the environment, which we interpret as a state-selective indicator of interfacially relevant electronic transitions. Physically predictable trends such as these will aid in building molecular design principles necessary for the development of novel materials based on nanoclusters.

Bioactive and luminescent indole and isatin based gold(i) derivatives

Combination of bioactive indole and isatin derivatives with Au(i) affords highly cytotoxic metallic species even for cisplatin resistant leukemia cells (Jurkat-shBak).

Direct self-focusing synthesis of monodisperse [Au8(PPh3)7]2+ nanoclusters

Direct synthesis of atomically monodisperse Au₈ nanoclusters via the self-focusing process during NaBH₄ reduction of Au(PPh₃)₂Cl.

Re(I) derivatives functionalised with thioether crowns containing the 1,10-phenanthroline subunit as a new class of chemosensors

A series of luminescent fac-[Re(CO)3(L)(NN)](+) complexes, where L is a pyridine or an imidazole and NN is the 1,10-phenanthroline subunit of mixed donor pentadentate thioether crowns have been synthesised and their luminescence properties have been analysed. Then, heterometallic Re(i)/Au(i) complexes, with the Au(i) fragment bonded directly to the imidazole ligand, and heterometallic Re(i)/Ag(i) complexes, with the silver fragment coordinating the S-donor thioether linker of the rings have also been prepared. Analysis of their luminescence properties showed a considerable blue shift of the emission maxima for the Re(i)/Ag(i) derivatives, upon coordination of the silver centre to the S-donor atoms of the aliphatic chain of the macrocyclic units.

Water-Soluble Phosphine Capable of Dissolving Elemental Gold: The Missing Link between 1,3,5-Triaza-7-phosphaadamantane (PTA) and Verkade's Ephemeral Ligand

We herein describe a tricyclic phosphine with previously unreported tris(homoadamantane) cage architecture. That water-soluble, air- and thermally stable ligand, 1,4,7-triaza-9-phosphatricyclo[5.3.2.1(4,9)]tridecane (hereinafter referred to as CAP) exhibits unusual chemical behavior toward gold and gold compounds: it readily reduces Au(III) to Au(0), promotes oxidative dissolution of nanocrystalline gold(0) with the formation of water-soluble trigonal CAP-Au(I) complexes, and displaces cyanide from [Au(CN)2](-) affording triangular [Au(CAP)3](+) cation. From the stereochemical point of view, CAP can be regarded as an intermediate between 1,3,5-triaza-7-phosphaadamantane (PTA) and very unstable aminophosphine synthesized by Verkade's group: hexahydro-2a,4a,6a-triaza-6b-phosphacyclopenta[cd]pentalene. The chemical properties of CAP are likely related to its anomalous stereoelectronic profile: combination of strong electron-donating power (Tolman's electronic parameter 2056.8 cm(-1)) with the low steric demand (cone angle of 109°). CAP can be considered as macrocyclic counterpart of PTA with the electron-donating power approaching that of strongest known phosphine electron donors such as P(t-Bu)3 and PCy3. Therefore, CAP as sterically undemanding and electron-rich ligand populates the empty field on the stereoelectronic map of phosphine ligands: the niche between the classic tertiary phosphines and the sterically undemanding aminophosphines.

Structurally similar triphenylphosphine-stabilized undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, exhibit distinct ligand exchange pathways with glutathione

Ligand exchange is frequently used to introduce new functional groups on the surface of inorganic nanoparticles or clusters while preserving the core size. For one of the smallest clusters, triphenylphosphine (TPP)-stabilized undecagold, there are conflicting reports in the literature regarding whether core size is retained or significant growth occurs during exchange with thiol ligands. During an investigation of these differences in reactivity, two distinct forms of undecagold were isolated. The X-ray structures of the two forms, Au₁₁(PPh₃)₇Cl₃ and [Au₁₁(PPh₃)₈Cl₂]Cl, differ only in the number of TPP ligands bound to the core. Syntheses were developed to produce each of the two forms, and their spectroscopic features correlated with the structures. Ligand exchange on [Au₁₁(PPh₃)₈Cl₂]Cl yields only small clusters, whereas exchange on Au₁₁(PPh₃)₇Cl₃ (or mixtures of the two forms) yields the larger Au₂₅ cluster. The distinctive features in the optical spectra of the two forms made it possible to evaluate which of the cluster forms were used in the previously published papers and clarify the origin of the differences in reactivity that had been reported. The results confirm that reactions of clusters and nanoparticles may be influenced by small variations in the arrangement of ligands and suggest that the role of the ligand shell in stabilizing intermediates during ligand exchange may be essential to preventing particle growth or coalescence.

Insertion of the Ga(I) Bis-imidinate Ga(DDP) into the Metal Halogen Bonds of Rh(I) Complexes. How Electrophilic Are Coordinated Ga(DDP) Fragments?

The reactivity of Ga(DDP) (DDP = 2-((2,6-diisopropylphenyl)amino-4-((2,6-diisopropylphenyl)imino)-2-pentene) towards the rhodium-chloride bonds of [RhCl(PPh3)3] and [RhCl(COE)2]2 (COE = cyclooctene) is investigated. Reaction of the first complex leads to [(Ph3P)2Rh{Ga(DDP)}(mu-Cl)] (1), exhibiting a chloride bridging the gallium and the rhodium atoms, whereas the second complex leads to a full insertion of the Ga(DDP) ligand into the Rh-Cl bond giving [(COE)(benzene)Rh{(DDP)GaCl}] (2) on coordination of the solvent C6H6. Compounds 1 und 2 readily react with the halide abstracting reagent Tl[BArF] (BArF = B[3,5-(CF3)2C6H3]4), yet the products could not be isolated and characterized because of their lability. The Au(I) complex [{(DDP)Ga}Au{Ga(DDP)}Cl] reacts with Na[BArF] giving the linear, symmetric cationic complex [{(DDP)Ga.THF}2Au][BArF] (3.2THF), exhibiting two THF molecules coordinated to the Ga(DDP) moieties.

A new 19-metal-atom cluster [(Me2PhP)10Au12Ag7(NO3)9] with a nearly staggered–staggered M5 ring configuration

New mixed metal clusters with M19 metal frameworks have been synthesized by NaBH4 reduction of Au(NO3)(PMe2Ph) together with AgNO3 in ethanol. Single crystal X-ray diffraction has revealed Au12Ag7 and Au17Ag2 metal skeletons for these clusters, which are best described in terms of bicapped pentagonal antiprismatic cages with a staggered-staggered M(5) ring configuration. These clusters connect the missing link between M13 icosahedral and M25 biicosahedral clusters providing a view of the cluster growth process. A TEM image of this cluster has been observed, which has clearly demonstrated single-sized nano-particles of less than 1.0 nm.

First examples of Au(I)-X-Ag(I) halonium cations (X = Cl and Br)

Novel {[(mu-PAnP)(AuX)2]2Ag}+SbF6- halonium ions (X = Cl, Br; PAnP = 9,10-bis(diphenylphosphino)anthracene) were synthesized from the reactions between (mu-PAnP)(AuX)2 and 1/2 mol equiv of AgSbF6. The compounds feature an unprecedented distorted Au4X4 dodecahedron which encapsulates a silver(I) ion at its center. The halonium ions are stabilized by collective actions of metallophilic Au-Ag, aromatic pi-pi, and Ag-X interactions.

Synthesis and study of the cluster [Pd(AuPPh3)6(HgNO3)](NO3) by reversed-phase high-performance liquid chromatography, thermoanalysis, infrared, nuclear magnetic resonance, and fast atom bombardment mass spectrometry

The addition reaction of Hg0 and Hg(2)2+ to the bimetallic cluster [(PPh3)Pd(AuPPh3)6](NO3)2 (I) has been studied. The reaction, monitored by HPLC, showed the time elapsed to obtain the pure cluster [Pd(AuPPh3)6(HgNO3)](NO3) (II). The use of 31P[1H] NMR, IR spectra, FABMS and thermoanalysis data was very useful for confirming the results. The reaction was monitored by taking five samples after 3, 6, 17, 21 and 24 h. The process was finalised after 24 h and the pure trimetallic cluster II, free of the intermediate, appeared in the chromatogram as a large, unique peak.