Ligated Gold Clusters Research Articles

Synthesis and Stability of Mixed-Diphosphine Ligated Gold Clusters

Sub-nanometer gold clusters are promising size- and composition-tunable materials that may be used for advanced technological applications such as catalysis, energy generation, and microelectronics. Synthesis and characterization of phosphine ligated gold clusters containing different ligands provide insight into how steric and electronic effects resulting from changes in chemical functionality influence cluster size, stability, and formation in solution. Herein, we demonstrate that synthesizing gold clusters using two different diphosphines in solution at the same time results in a broad distribution of novel mixed-ligand clusters. In comparison, adding a second diphosphine to a solution of gold clusters presynthesized with another diphosphine does not result in extensive formation of mixed-ligand species. Utilizing high-mass resolution electrospray ionization mass spectrometry, we determined novel cluster compositions and observed size-dependent trends in gold clusters that undergo ligand exchange forming mixed diphosphine species. Adjacent peaks in the mass spectra, separated by characteristic mass-to-charge ratios, provide evidence for multiple 1,3-bis(diphenylphosphino)propane (L3) and 1,5-bis(diphenylphosphino)pentane (L5) ligands on cationic clusters containing 8, 10, 11, and 22 gold atoms. Energy-resolved collision-induced dissociation experiments provide qualitative insight into how different diphosphine ligands affect the relative stability of specific size gold clusters. Our results indicate that mixed-ligand clusters containing both L3 and L5 are generally more stable than their single ligand counterparts containing either L3 or L5. These molecular-level insights will facilitate the rational and scalable synthesis of gold clusters for targeted applications.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis.

Atomically precise gold clusters are highly desirable due to their well‐defined structure which allows the study of structure–property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal–ligand interaction and type of ligands. This critical feature renders gold–phosphine clusters unique and distinct from other ligand‐protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold–phosphorous (Au–P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold–phosphine clusters and to present an in‐depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold–phosphine clusters. Advanced characterization techniques, including synchrotron‐based spectroscopy, have unraveled substantial effects of Au–P interaction on the composition‐, structure‐, and size‐dependent properties. State‐of‐the‐art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold–phosphine clusters in catalysis is presented.

Au101-rGO nanocomposite: immobilization of phosphine-protected gold nanoclusters on reduced graphene oxide without aggregation.

Graphene supported transition metal clusters are of great interest for potential applications, such as catalysis, due to their unique properties. In this work, a simple approach to deposit Au101(PPh3)21Cl5 (Au101NC) on reduced graphene oxide (rGO) via an ex situ method is presented. Reduction of graphene oxide at native pH (pH ≈ 2) to rGO was performed under aqueous hydrothermal conditions. Decoration of rGO sheets with controlled content of 5 wt% Au was accomplished using only pre-synthesised Au101NC and rGO as precursors and methanol as solvent. High resolution scanning transmission electron microscopy indicated that the cluster size did not change upon deposition with an average diameter of 1.4 ± 0.4 nm. It was determined that the rGO reduction method was crucial to avoid agglomeration, with rGO reduced at pH ≈ 11 resulting in agglomeration. X-ray photoelectron spectroscopy was used to confirm the deposition of Au101NCs and show the presence of triphenyl phosphine ligands, which together with attenuated total reflectance Fourier transform infrared spectroscopy, advocates that the deposition of Au101NCs onto the surface of rGO was facilitated via non-covalent interactions with the phenyl groups of the ligands. Inductively coupled plasma mass spectrometry and thermogravimetric analysis were used to determine the gold loading and both agree with a gold loading of ca. 4.8–5 wt%. The presented simple and mild strategy demonstrates that good compatibility between size-specific phosphine protected gold clusters and rGO can prevent aggregation of the metal clusters. This work contributes towards producing an agglomeration-free synthesis of size-specific ligated gold clusters on rGO that could have wide range of applications.

Light Exposure Promotes Degradation of Intermediates and Growth of Phosphine-Ligated Gold Clusters

Subnanometer clusters exhibit unique properties not observed at the bulk or nanometer scale. Many applications, however, require clusters to be monodisperse in size and composition. While substantial progress has been made empirically on producing specific-sized gold clusters using different ligands, the mechanisms of cluster formation remain insufficiently understood. Using high mass-resolution electrospray ionization mass spectrometry, we characterized the formation of cationic triphenylphosphine-ligated gold clusters [Aux(PPh3)yz+] in solution. Our data provides evidence that small hydrogen-containing intermediate “building blocks” (i.e., Au2(PPh3)2H+, Au4(PPh3)4H+, and Au7(PPh3)7H52+) are responsible for the growth of larger gold clusters (i.e., Au6(PPh3)62+ and Au8(PPh3)72+) over several hours. In addition, intermediate degradation and cluster growth are shown to be dependent on irradiation of the reacting solution with light. Specifically, the rate of growth of large clusters is demonstrated to scale with the intensity of light exposure. Complementary theoretical calculations were also performed that provide molecular-level insight into the nature of the bonding between the hydrogens and gold atoms in the intermediate Au-phosphine clusters. Collectively, our joint experimental and theoretical findings advance the understanding of the factors at play in the size-selective synthesis of ligated gold clusters in solution.

Influence of Interligand Interactions and Core-Charge Distribution on Gold Cluster Stability: Enthalpy Versus Entropy

Understanding the factors underlying the formation, reactivity, and stability of ligated gold clusters is necessary for directing the synthesis of atomically precise clusters with tailored properti...

Au3(μ3-S)(0e) elementary block: new insights into ligated gold clusters with μ3-sulfido motifs.

An understanding of the structural stability and formation mechanism of ligated gold nanoclusters with triply coordinated μ3-sulfido (μ3-S) motifs is important not only for gold chemistry but also for the design of more effective catalysts or drug carriers for various applications. In this article, a new elementary block Au3(μ3-S) with zero valence electrons [referred to as Au3(μ3-S)(0e)] has been identified, which describes all crystallized ligated gold clusters with μ3-S motifs, in conjunction with the previously reported Au3(2e) and Au4(2e) elementary blocks, in a grand unified model (GUM). This Au3(μ3-S)(0e) elementary block has a tripod structure with the S atom bonding to three Au atoms in a μ3 bridging mode, and can be considered as a μ3-S atom balancing out the 2e valence electrons of the Au3(2e) block. Using Au3(μ3-S) as a building block, a special group of quasi-fullerene hollow-cage [Au3n(μ3-S)2n]n- gold(i) μ3-S clusters are designed, which exhibit high stabilities. In addition, a series of theoretical structures are predicted to be increasingly stable after introducing μ3-S atoms, based on the crystallized clusters. Overall, the introduction of a Au3(μ3-S)(0e) elementary block can help with the understanding of diverse structures of ligated gold clusters with μ3-S motifs, thereby assisting the rational design of new forms of gold nanoclusters.

Ligand induced structural isomerism in phosphine coordinated gold clusters revealed by ion mobility mass spectrometry.

Structural isomerism in ligated gold clusters is revealed using electrospray ionization ion mobility spectrometry mass spectrometry. Phosphine ligated Au8 clusters are shown to adopt more extended type structures with increasing exchange of methyldiphenylphosphine (MePPh2) for triphenylphosphine (PPh3). These ligand-dependant structure-property relationships are critical to theoretical modeling of clusters as well as their applications in catalysis and photovoltaics.

Charge Redistribution Effects on the UV–Vis Spectra of Small Ligated Gold Clusters: a Computational Study

We analyze and compare the UV–visible absorption profiles, computed at the TD-DFT level, of several thiolate, chloride, and phosphine ligand protected gold clusters with Au13, Au25, and Au28 metallic cores. We show by investigating several model clusters and experimentally reported ones that the charge redistribution between the gold core and the ligand shell can be controlled by the electron acceptor or electron donor character of the ligands and their spatial distribution around the metallic core. A correlation between the increase of electronic charge in the gold core and a shift of the optical absorption band toward lower transition energies is highlighted. The identified correlation between the partial charge on the metallic gold core and the position of the lowest electronic transition is robust and remains valid even for small clusters with little structural or electronic similarity.

Cationic gold clusters ligated with differently substituted phosphines: effect of substitution on ligand reactivity and binding.

We present a systematic study of the effect of the number of methyl (Me) and cyclohexyl (Cy) functional groups in monodentate phosphine ligands on the solution-phase synthesis of ligated sub-nanometer gold clusters and their gas-phase fragmentation pathways. Small mixed ligand cationic gold clusters were synthesized using reactions between pre-formed triphenylphosphine ligated (PPh3) gold clusters and monodentate Me- and Cy-substituted phosphine ligands in solution and characterized using electrospray ionization mass spectrometry (ESI-MS) and collision-induced dissociation (CID) experiments. Under the same experimental conditions, larger gold-PPh3 clusters undergo efficient exchange of unsubstituted PPh3 ligands for singly Me- and Cy-substituted PPh2Me and PPh2Cy ligands. The efficiency of reaction decreases with an increasing number of Me or Cy groups in the substituted phosphine ligands. CID experiments performed for a series of mixed-ligand gold clusters indicate that loss of a neutral Me-substituted ligand is preferred over loss of a neutral PPh3 ligand while the opposite trend is observed for Cy-substituted ligands. The branching ratio of the competing ligand loss channels is strongly correlated with the electron donating ability of the phosphorous lone pair as determined by the relative proton affinity of the ligand. The results indicate that the relative ligand binding energies increase in the order PMe3 < PPhMe2 < PPh2Me < PPh3 < PPh2Cy < PPhCy2 < PCy3. Furthermore, the difference in relative ligand binding energies increases with the number of substituted PPh(3-m)Me(m) or PPh(3-m)Cy(m) ligands (L) on each cluster. This study provides the first experimental determination of the relative binding energies of ligated gold clusters containing differently substituted monophosphine ligands, which are important to controlling their synthesis and reactivity in solution. The results also indicate that ligand substitution is an important parameter that must be considered in theoretical modeling of these complex systems.

Soft landing of mass-selected gold clusters: Influence of ion and ligand on charge retention and reactivity

We employ a combination of reduction synthesis in solution, soft landing of mass-selected precursor and product ions, and in situ time-of-flight secondary ion mass spectrometry (TOF-SIMS) to examine the influence of ion and the length of diphosphine ligands on the charge retention and reactivity of ligated gold clusters deposited onto self-assembled monolayer surfaces (SAMs). Product ions (Au10L42+, (10,4)2+, L=1,3-bis(diphenylphosphino) propane, DPPP) were prepared through in-source collision induced dissociation (CID) and precursor ions [(8,4)2+, L=1,6-bis(diphenylphosphino) hexane, DPPH] were synthesized in solution for comparison to (11,5)3+ precursor ions ligated with DPPP investigated previously (ACSNano 2012, 6, 573 and J.Phys. Chem. C. 2012, 116, 24, 977). Similar to (11,5)3+ precursor ions, the (10,4)2+ product ions are shown to retain charge on 1H,1H,2H,2H-perfluorodecanethiol monolayers (FSAMs). Additional abundant peaks at higher m/z indicative of reactivity are observed in the TOF-SIMS spectrum of (10,4)2+ product ions that are not seen for (11,5)3+ precursor ions. The abundance of (10,4)2+ on 16-mercaptohexadecanoic acid (COOHSAMs) is demonstrated to be lower than on FSAMs, consistent with partial reduction of charge. The (10,4)2+ product ion on 1-dodecanethiol (HSAMs) exhibits peaks similar to those seen on the COOHSAM. On the HSAM, higher m/z peaks indicative of reactivity are observed similar to those on the FSAM. The (8,4)2+ DPPH precursor ions are shown to retain charge on FSAMs similar to (11,5)3+ precursor ions ligated with DPPP. An additional peak corresponding to attachment of one gold atom to (8,4)2+ is observed at higher m/z for DPPH-ligated clusters. On the COOHSAM, (8,4)2+ is less abundant than on the FSAM consistent with partial neutralization. The results indicate that although retention of charge by product ions generated by in-source CID is similar to precursor ions their reactivity during analysis with SIMS is different resulting in the formation of peaks corresponding to reaction products. The length of the diphosphine ligand exerts only a minor influence on the charge retention and reactivity of gold clusters. Based on the observed reactivity of (10,4)2+ product ions it is anticipated that in-source CID will be increasingly applied for the preparation of a distribution of ions, including undercoordinated and reactive species, for soft landing onto surfaces.

Synthesis and gas-phase uni- and bi-molecular reactivity of bisphosphine ligated gold clusters, [AuxLy]n+

The condensed phase synthesis of bis-phosphine Ph2P(CH2)nPPh2 (Ln) (n=3–6) protected gold clusters containing 6–11 gold core atoms was monitored via electrospray ionisation mass spectrometry (ESI-MS) and UV–vis spectroscopy. The size of the methylene spacer, n, which influences the ligand conformational flexibility, not only dictates Au cluster size, Au:L stoichiometry, and heterogeneity of the cluster mixture formed, but also influences the kinetics of cluster growth. Thus for n=3, the cluster growth is relatively slow and [Au11L5n]3+ is the main cluster formed after equilibrium in ca. 7days. For n=4, the cluster growth rate is intermediate between n=3 and n=5 or 6 and [Au11L5n]3+, [Au10L4n]2+, [Au8L4n]2+ and [Au6L3n]2+ are the main clusters formed after equilibrium in ca. 24h. For n=5 or 6, the cluster growth rate is relatively fast and [Au10L4n]2+, [Au8L4n]2+ and [Au6L3n]2+ are the main clusters formed after equilibrium in less than 1h.Low-energy collision induced dissociation (CID) of each of the gold cluster cations was carried out in an ion trap mass spectrometer with the aim of “synthesising” a range of novel gas-phase [AuxLyn]z+ (x=3, 5–11; y=1–5 and z=1–3) clusters via ligand loss and core fission fragmentation pathways. The reactivity of mass selected gold cluster cations was studied via ion-molecule reactions using various neutral reagents in order to determine possible reactive sites. The observed reactivity of some of these clusters appears to relate to previously determined X-ray structures. Thus the lack of reactivity of [Au11L5n]3+ and [Au8L4n]2+ towards a range of neutrals can be attributed to the fact that all their surface gold atoms are capped by a phosphine group.

Gold‐Mediated CI Bond Activation of Iodobenzene

Controversy resolved! A combination of gas-phase ion-molecule reactions and theoretical studies confirm bisligated mononuclear Au(I) complexes are unable to undergo oxidative addition of iodobenzene for Sonogashira coupling, but that the ligated gold clusters [Au(3)L(n)](+) (L=Ph(2)P(CH(2))(n)PPh(2); n=3-6) activate the C-I bond. DFT calculations on the transition states show that the linker size n tunes the cluster reactivity.

Predictive Gold Nanocluster Formation Controlled by Metal‐Ligand Complexes

The formation of ligand-protected gold nanoclusters during size-selective syntheses is seemingly driven by the inherent properties of the protecting ligands, but a general description of the product formation has not been presented. This study uses diphosphine-protected Au clusters as a model system to examine i) control of metal-ligand complex distributions in methanol-chloroform solutions, ii) role of solution perturbations, e.g., oxidation, and iii) nanocluster formation through reduction of characterized complex distributions. By selectively reducing complexes and monitoring cluster formation with electrospray ionization mass spectrometry and UV-vis, data show the distribution of complexes can be controlled through ligand exchange, and the reduction of specific complexes produce characteristic ligated gold clusters based on ligand class. Specifically, 1,n-bis(diphenylphosphino)n-alkane ligands, L(n), where n = 1 through 6, are classified into two distinct sets. The classes represent ligands that either form mainly [AuL(n)(2)](+) (Class I, n = 1-3) or bridged [Au(2)L(n)(2)](2+) (Class II, n = 4-6) complexes after complete ligand exchange with AuClPPh(3). Selectively reducing gold-phosphine ligand complexes allows mapping of product formation, resulting collectively in a predictive tool for ligated gold cluster production by simply monitoring the initial complex distribution prior to reduction.

Relativistic density-functional studies of naked and ligated gold clusters

Electronic structure investigations on a broad range of gold compounds, including naked and ligated gold clusters, are reviewed. The calculations have been carried out with a recently introduced relativistic variant of the linear combination of Gaussian-type orbitals density-functional (LCGTO-DF) method which affords all-electron investigations for very large systems. The accuracy of the method will be evaluated for the gold dimer. Then the electronic structure of the naked cluster Au{sub 55} is studied, both in I{sub h} and O{sub h} symmetry. Nonrelativistic and relativistic results obtained by the present method are compared to those of the much simpler jellium model. Since triphenylphosphine is among the most common ligands in gold chemistry a series of mononuclear gold phosphine compounds MeAuPR{sub 3} with increasingly complex ligands PR{sub 3}(R = H, Ch{sub 3}, C{sub 5}H{sub 6}) is discussed. The calculations reveal the success and the limitations of simpler phosphines often employed as model ligands in theoretical studies. Some aspects of the phosphine gold interaction in these simpler compounds carry over to the main group element centered gold clusters. Thereby one arrives at a rationalization of the particularly high stability of the carbon-centered octahedral cluster cation [(R{sub 3}PAu){sub 6}C]{sup 2+} as compared to the neighboringmore » isoelectronic boron and nitrogen-centered clusters.« less