Poly-Hydride [AuI7(PPh3)7H5](SbF6)2 cluster complex: Structure,Transformation, and Electrocatalytic CO2 Reduction Properties

Hydride Au^I bonds are labile due to the mismatch in electric potential of an oxidizing metal and reducing ligand, and therefore the structure and structure–activity relationships of nanoclusters that contain them are seldom studied. Herein, we report the synthesis and characterization of [Au₇(PPh₃)₇H₅](SbF₆)₂ (abbrev. Au₇H₅ ²⁺ ), an Au cluster complex containing five hydride ligands, which decomposed to give [Au₈(PPh₃)₇]²⁺ (abbrev. Au₈ ²⁺ ) upon exposure to light (300 to 450 nm). The valence state of Au^I and H⁻ was verified by density functional theory (DFT) calculations, NMR, UV/Vis and XPS. The two nanoclusters behaved differently in the electrocatalytic CO₂ reduction reaction (CO₂RR): Au₇H₅ ²⁺ exhibited 98.2 % selectivity for H₂, whereas Au₈ ²⁺ was selective for CO (73.5 %). Further DFT calculations showed that the H⁻ ligand inhibited the CO₂RR process compared with the electron-donor H.     

Nature of mercury inclusion in intermediate 6-valence electron [M@Au8Hgx(PPh3)8]n+ (M = Au,Pd, Pt; x = 0-2) protected gold superatoms: Insights from relativistic density functional theory calculations

Superatomic clusters offer useful templates displaying distinctive physical and chemical characteristics. Here, we explore the [M@Au₈(PPh₃)₈]ⁿ⁺ (M = Au, n = 3; Pd, Pt, n = 2) robust framework to gain an understanding of the nature of the inclusion of mercury atoms at Au₄ faces, leading to [M@Au₈Hg_x(PPh₃)₈]ⁿ⁺ (x = 1, 2). Our results show a weak interaction of about 25 kcal mol⁻¹ per Hg atom, which is mainly of electrostatic character, followed by orbital and London dispersion-type interactions. This weak interaction can be understood as the formation of host-guest species, for which the inherent electronic and optical properties of the [M@Au₈(PPh₃)₈] cluster along the series do not vary to a large extent. This demonstrates that, in [M@Au₈Hg_x(PPh₃)₈], each Hg can be considered an inclusion atom rather than a dopant element, where the parent cluster is able to act as a Lewis acid host. Furthermore, the viable formation of such species can serve as useful examples to stimulate future experimental characterization of inclusion complexes involving related superatomic structures with available open faces.     

Atom Exchange Versus Reconstruction: (GexAs4−x)x− (x=2, 3) as Building Blocks for the Supertetrahedral Zintl Cluster [Au6(Ge3As)(Ge2As2)3]3−

The Zintl anion (Ge₂As₂)²⁻ represents an isostructural and isoelectronic binary counterpart of yellow arsenic, yet without being studied with the same intensity so far. Upon introducing [(PPh₃)AuMe] into the 1,2-diaminoethane (en) solution of (Ge₂As₂)²⁻, the heterometallic cluster anion [Au₆(Ge₃As)(Ge₂As₂)₃]³⁻ is obtained as its salt [K(crypt-222)]₃[Au₆(Ge₃As)(Ge₂As₂)₃]⋅en⋅2 tol ( 1 ). The anion represents a rare example of a superpolyhedral Zintl cluster, and it comprises the largest number of Au atoms relative to main group (semi)metal atoms in such clusters. The overall supertetrahedral structure is based on a (non-bonding) octahedron of six Au atoms that is face-capped by four (Ge_xAs_4−x)^x− (x=2, 3) units. The Au atoms bind to four main group atoms in a rectangular manner, and this way hold the four units together to form this unprecedented architecture. The presence of one (Ge₃As)³⁻ unit besides three (Ge₂As₂)²⁻ units as a consequence of an exchange reaction in solution was verified by detailed quantum chemical (DFT) calculations, which ruled out all other compositions besides [Au₆(Ge₃As)(Ge₂As₂)₃]³⁻. Reactions of the heavier homologues (Tt₂Pn₂)²⁻ (Tt=Sn, Pb; Pn=Sb, Bi) did not yield clusters corresponding to that in 1 , but dimers of ternary nine-vertex clusters, {[AuTt₅Pn₃]₂}⁴⁻ (in 2 – 4 ; Tt/Pn=Sn/Sb, Sn/Bi, Pb/Sb), since the underlying pseudo-tetrahedral units comprising heavier atoms do not tend to undergo the said exchange reactions as readily as (Ge₂As₂)²⁻, according to the DFT calculations.     

Synthesis, single crystal X-ray analysis, and TEM for a single-sized Au11 cluster stabilized by SR ligands: The interface between molecules and particles

An SR-modified Au cluster with a sub-nanometer size, Au₁₁(S-4-NC₅H₄)₃(PPh₃)₇ (1), has been synthesized by NaBH₄ reduction of Au(S-py)(PPh₃) or by reacting [Au₉(PPh₃)₈](NO₃)₃ with HS-4-py in good yield. Its molecular structure has been elucidated by single crystal X-ray diffraction, and TEM observation has been achieved for the first time for this size of SR-modified Au clusters. The core structure is best described in terms of an incomplete icosahedron. CV measurements in CH₂Cl₂ have suggested that the cluster does not coagulate in solution with significant concentration.     

Two gold-ruthenium clusters derived from Ru3(μ-H)3(μ3-CBr)(CO)9

The reaction between AuMe(PPh₃) and Ru₃(μ-H)₃(μ₃-CBr)(CO)₉ (1) affords the novel heptanuclear cluster Au₄Ru₃(μ₃-CMe)(Br)(CO)₉(PPh₃)₃ (2), containing an Au/Ru₃/Au trigonal pyramidal cluster face-capped by two Au(PPh₃) groups and a CMe ligand, together with Au₂Ru₃(μ-H)(μ₃-CMe)(CO)₉(PPh₃)₂ (3), formed by isolobal replacement of two of the three μ-H atoms in 1 by Au(PPh₃) groups. The latter co-crystallises with the analogous μ₃-CH complex, as also shown spectroscopically.     

Cluster Chemistry: XXVIII. Utility of fast atom bombardment mass spectrometry for the characterisation of high molecular weight complexes containing metal clusters

Fast atom bombardment (FAB) mass spectrometry has been used to obtain spectra of HRu₃Au(μ₃-S)(CO)₉(PPh₃), Ru₃Au₂(μ₃-S)(CO)₉(PPh₃)₂ and Ru₃Au₃(μ₃-C₁₂H₁₅)(CO)₈(PPh₃)₃, none of which give metal-containing ions in conventional EI mass spectrometry. All the compounds give molecular (M⁺) or quasimolecular ([M + 2H]⁺) ions which fragment by conventional routes.     

Synthesis, Stability, and Photoluminescence Properties of PdAu10(PPh3)8Cl2 Clusters

We report on the synthesis, stability, and photoluminescence (PL) properties of triphenylphosphine (PPh₃)-stabilized PdAu₁₀(PPh₃)₈Cl₂ cluster, which is a mono-Pd-doped cluster of the well-studied Au₁₁(PPh₃)₈Cl₂ cluster. The PdAu₁₀(PPh₃)₈Cl₂ cluster was synthesized by simultaneously reducing two different metal complexes; AuCl(PPh₃) and Pd(PPh₃)₄. Experimental evaluation of the stability showed that PdAu₁₀(PPh₃)₈Cl₂ is more stable against degradation in solution than the monometal Au₁₁(PPh₃)₈Cl₂ cluster. PL measurements revealed that PdAu₁₀(PPh₃)₈Cl₂ exhibits PL at 950?nm with a quantum yield of 1.5?×?10^?3, which has not been observed for the monometal Au₁₁(PPh₃)₈Cl₂ cluster. The results indicate that Pd doping is a powerful method to produce clusters with higher stability and different physical properties than the monometal Au:PPh₃ clusters.     

Bis­[bis­(di­phenyl­phosphino­methyl)­phenyl­phosphine]­di­chloro­tetra­gold(I) bis­[chloro­tris­(penta­fluoro­phenyl)­aurate(III)]

The cation of the title compound, [Au₄(PPh₂CH₂PPhCH₂PPh₂)₂Cl₂][Au(C₆F₅)₃Cl]₂ or [Au₄Cl₂(C₃₂H₂₉P₃)₂][AuCl(C₆F₅)₃]₂, displays a rhomboidal geometry for the Au atoms, with short Au?Au distances of 3.104 (2) and 3.185 (1) Å; the linear coordination at the Au^I atoms is distorted: P—Au—P 164.7 (2)° and P—Au—Cl 170.67 (11)°. The anion shows the expected square‐planar geometry at Au^III, with the Au atom 0.022 (5) Å out of the plane of the four donor atoms.     

The thermal motion of gold atoms in metal cluster compounds as determined by Mössbauer and specific heat

¹⁹⁷Au Mössbauer effect spectroscopy and specific heat measurements have been performed as a function of temperature on three gold polynuclear cluster compounds, [Au₉(PPh₃)₈](NO₃)₃, Au₁₁(PPh₃)₇(SCN)₃, and Au₅₅(PPh₃)₁₂Cl₆. The Mössbauer data yield information on the vibrational motions of the various distinguishable Au sites, as well as on the motion of the clusters as a whole. The Mössbauer and the specific heat data are successfully described by a superposition of inter- and intra-cluster vibrations. The latter are determined by calculating numerically the normal modes of vibration of the metal cores.     

[cis‐{η2‐(Ph3PAu)2}Ph3PCr(CO)4]·THF,featuring the shortest known separation between two Au metal centres in a heteronuclear Au2M cluster

The title compound, tetra­carbonyl‐1κ⁴C‐tris­(tri­phenyl­phos­phino)‐1κP,2κP,3κP‐triangulo‐chromiumdigold(Au—Au)(2 Cr—Au) tetra­hydro­furan solvate, [Au₂Cr(C₁₈H₁₅P)₃(CO)₄]·C₄H₈O, is a stable isolobal analogue of the extremely labile [(η²‐H₂)CrL_n–1] molecular hydrogen complex (n = 6; L is a neutral ligand, e.g. CO or PPh₃), and features the shortest known separation [2.6937 (2) Å] between two Au atoms in a triangular heteronuclear metal‐cluster framework.     

Near-Infrared Spectrum of the First Excited State of Au2+

Au₂⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂⁺ ground state (X²Σ_g⁺) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A²Σ_u⁺). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar-tagged Au₂⁺ cations. This exceptionally low-lying and well isolated A²Σ_(u)⁺←X²Σ_(g)⁺ transition occurs in the near-infrared range. The observed band origin (5738 cm⁻¹, 1742.9 nm, 0.711 eV) and harmonic Au−Au and Au−Ar stretch frequencies (201 and 133 cm⁻¹) agree surprisingly well with those predicted by standard time-dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au₂⁺, because the Au₂⁺⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au−Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σ_u^* orbital (HOMO-1) into the bonding σ_g orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).     

The kinetics and thermodynamics of a novel efficient mixed water–1,4-dioxan solvent for the effective complexation of a neutral ruthenium complex with carbon monoxide at atmospheric pressure

Kinetic and thermodynamic investigations were performed for a mixed aqueous-organic, 1:1 (v/v) water–1,4-dioxane medium, which was found to be an efficient solvent for the interaction of a neutral dichlorotris(triphenylphosphine) ruthenium(II), RuCl₂(PPh₃)₃ complex with carbon monoxide at atmospheric pressure. During the interaction, RuCl₂(PPh₃)₃ dissociates to a neutral complex dichlorobis(triphenylphosphine) ruthenium(II), RuCl₂(PPh₃)₂, by losing a coordinated PPh₃ ligand and RuCl₂(PPh₃)₂ coordinates with CO to form an in situ carbonyl complex RuCl₂(CO)(PPh₃)₂. The in situ formed carbonyl complex RuCl₂(CO)(PPh₃)₂ was thoroughly characterized by equilibrium, spectrophotometric, IR, and electrochemical techniques. Under equilibrium conditions, the rate and dissociation constants for the dissociation of PPh₃ from RuCl₂(PPh₃)₃ were found to be favorable for the formation of the carbonyl complex RuCl₂(CO)(PPh₃)₂. The rates of complexation for the formation of RuCl₂(CO)(PPh₃)₂ were found to follow an overall second-order kinetics being first order in terms of the concentrations of both carbon monoxide and RuCl₂(PPh₃)₂. The determined activation parameters corresponding to the rate constant (ΔH^# = 35.9 ± 2.5 kJ mol⁻¹ and ΔS^# = −122 ± 6 J K⁻¹ mol⁻¹) and thermodynamic parameters corresponding to the formation constant (ΔH° = −33.5 ± 4.5 kJ mol⁻¹, ΔS° = −25 ± 8 J K⁻¹ mol⁻¹, and ΔG° = −25.7 ± 2.0 kJ mol⁻¹) were found to be highly favorable for the formation of the complex RuCl2(CO)(PPh3)2. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 359–369, 2008     

Ag Doped Au44 Nanoclusters for Electrocatalytic Conversion of CO2 to CO

A novel Ag doped Au₄₄(C₁₀H₉)₂₈ nanocluster (C₁₀H₁₀=1-ethynyl-2,4-dimethylbenzene) was synthesized, that is, Ag_4+xAu_40-x(C₁₀H₉)₂₈ (x≤6), where four Ag positions located in the surface staple of the cluster have been determined, while six Au/Ag co-occupying positions have been found in the metal core of the cluster. The electronic configuration of Au₄₄(C₁₀H₉)₂₈ cluster is significantly disturbed by doping Ag atoms, hence promoting the electron transport capability. For the two-electron conversion reaction of CO₂ to CO in electrochemical reduction of CO₂, Ag doped Ag_4+xAu_40-x(C₁₀H₉)₂₈ catalyst exhibited higher effective activity and long-term stability than its counterpart Au₄₄(C₁₀H₉)₂₈ catalyst.     

Photocatalytic Oxidative Coupling of Methane over Au1Ag Single-Atom Alloy Modified ZnO with Oxygen and Water Vapor: Synergy of Gold and Silver

C−H dissociation and C−C coupling are two key steps in converting CH₄ into multi-carbon compounds. Here we report a synergy of Au and Ag to greatly promote C₂H₆ formation over Au₁Ag single-atom alloy nanoparticles (Au₁Ag NPs)-modified ZnO catalyst via photocatalytic oxidative coupling of methane (POCM) with O₂ and H₂O. Atomically dispersed Au in Au₁Ag NPs effectively promotes the dissociation of O₂ and H₂O into *OOH, promoting C−H activation of CH₄ on the photogenerated O⁻ to form *CH₃. Electron-deficient Au single atoms, as hopping ladders, also facilitate the migration of electron donor *CH₃ from ZnO to Au₁Ag NPs. Finally, *CH₃ coupling can readily occur on Ag atoms of Au₁Ag NPs. An excellent C₂H₆ yield of 14.0 mmol g⁻¹ h⁻¹ with a selectivity of 79 % and an apparent quantum yield of 14.6 % at 350 nm is obtained via POCM with O₂ and H₂O, which is at least two times that of the photocatalytic system. The bimetallic synergistic strategy offers guidance for future catalyst design for POCM with O₂ and H₂O.     

Cluster chemistry: XXXIX. Gold—ruthenium clusters with sulphur ligands: Synthesis and structure of HRu3Au(μ3-S)(CO)9(PPh3), Ru3Au(μ3-SBut)(CO)9(PPh3) and Ru3Au2(μ3-S)(CO)9(PPh3)2

H₂Ru₃(μ₃-S)(CO)₉ is deprotonated by K[HBBu^s₃] to give cluster anions which react with [O{Au(PPh₃)}₃]⁺ or with AuCl(PPh₃)/T1⁺ to give HRu₃Au(μ₃-S)(CO)₉(PPh₃) (1) and Ru₃Au₂(μ₃-S)(CO)₉(PPh₃)₂ (3). A similar sequence with HRu₃(μ₃-SBu^t)(CO)₉ leads to Ru₃Au(μ₃-SBu^t)(CO)₉(PPh₃) (2) as the main product although some 1 also forms, indicating SC cleavage competes with deprotonation of HRu₃(μ₃-SBu^t)(CO)₉ by [HBBu^s₃]^?. The X-ray crystal structures of 1, 2 and 3 are described; (1) and (2) have “butterfly” AuRu₃ cores with markedly different hinge angles of 119 and 148° respectively, while 3 has a trigonal-bipyramidal Au₂Ru₃ skeleton. All three clusters have the sulphur atom symmetrically bridging the Ru₃ triangular face.     

Cluster chemistry: XXIII. Mono-, di- and tri-auration of H4Ru4(CO)12 with [{Au(PPh3)}3O][BF4]: X-ray crystal structure of HRu4Au3(CO)12(PPh3)3

Reactions between [H₃Ru₄(CO)₁₂]^? and [{Au(PPh₃)}₃O]⁺ afford H₃Ru₄-Au(CO)₁₂(PPh₃), H₂Ru₄Au₂(CO)₁₂(PPh₃)₂ and HRu₄Au₃(CO)₁₂(PPh₃)₃. The X-ray structure of the latter shows that it has the unusual bicapped trigonal bipyramidal metal core, in which two Ru₂Au faces of the Ru₄Au fragment are capped by the other two Au atoms. The central Au atom is asymmetrically attached to the Ru₃ face as a result of the interaction of a phenyl ring of the PPh₃ ligand with two of the CO groups. Metal-metal separations are: two Au-Au, 2.837(1) Å; Ru-Ru, six between 2.805–3.004(3) Å; Au-Ru, seven between 2.821–3.007(2) Å. HRu₄Au₃(CO)₁₂(PPh₃)₃ is monoclinic, space group P2₁/n, with a 18.754(3), b 18.459(5), c 22.317(4) Å, β 113.06(2)°; 2852 data [I > 2.5σ(I)] were refined to R, R_w 0.038, 0.038.     

Core Charge Density Dominated Size-Conversion from Au6P8 to Au8P8Cl2

The stimulus-response of metal nanoclusters is crucial to their applications in catalysis and bio-clinics, etc. However, its mechanistic origin has not been well studied. Herein, the mechanism of the Au^IPPh₃Cl-induced size-conversion from [Au₆(DPPP)₄]²⁺ to [Au₈(DPPP)₄Cl₂]²⁺ (DPPP is short for 1,3-bis(diphenylphosphino)propane) is theoretically investigated with density functional theory (DFT) calculations. The optimal size-growth pathway, and the key structural parameters were elucidated. The Au−P bond dissociation steps are key to the size-growth, the easiness of which was determined by the charge density of the metallic core of the cluster precursors (i.e., “core charge density”). This study sheds light on the inherent structure–reactivity relationships during the size-conversion, and will benefit the deep understanding on the kinetics of more complex systems.     

Reactions of Au+ and Au− with benzene and fluorine-substituted benzenes

Reactions of gold anions and cations generated by laser desorption/ionization were studied in the FTICR spectrometer. Au⁻ associated with C₆F₆ to give the novel Au⁻(C₆F₆) complex, whose binding energy was estimated to be 24 ± 4 kcal mol⁻¹ from analysis of the radiative association (RA) kinetics. Au⁺ associated with C₆F₅H to give Au⁺(C₆F₅H), with binding energy estimated to be 31 kcal mol⁻¹. Au⁺ reacted with C₆H₆ to form the well known Au⁺(C₆H₆) and Au⁺(C₆H₆)₂ complexes. The observation of rapid charge transfer from Au⁺(C₆H₆) to C₆H₆ was interpreted as showing that benzene binds more strongly to neutral Au than to Au⁺. The neutral Au–C₆H₆ bond is accordingly concluded to be stronger than about 70 kcal mol⁻¹.     

[N(PPh3)2]2[{Au3Ag2(C2Ph)6} {Au3Cu2(Cu2Ph)6}]: Co-crystallized pentanuclear bimetallic gold-silver and gold-copper clusters

The X-ray structural study of the reaction product of equimolar amounts of [Au₃Cu₂(C₂Ph)₆]. [{Au(C₂Ph)} _n ], and [Ag(C₂Ph)} _n ] revealed two bimetallic anionic [N(PPh₃)₂] + [Au₃Ag₂(C₂Ph)₆]^– and [N(PPh₃)₂]⁺[Au₃Cu₂ (C₂ Pg)₆] — clusters co-crystallized in one asymmetric unit. Each cluster has trigonal bipyramidal geometry with three gold atoms occupying equatorial planes and two silver or copper atoms in the apical positions. Our earlier conclusion based upon spectroscopic characterization describing the product of be above reaction as trimetallic cluster containing three coinage-metals with an overall composition [Au₃CuAg(C₂Ph)₆]^–, was erroneous.Presented at the 210th ACS Meeting, August 19–24, 1995, Chicago, Illinois.