Amplification of the Optical Activity of Gold Clusters by the Proximity of BINAP

Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au₁₁(<i>R</i>/<i>S</i>-DIOP)₄Cl₂]⁺ (DIOP = 1,4-bis­(diphenylphosphino)-2,3-<i>o</i>-isopropylidene-2,3-butanediol) and [Au₈(<i>R</i>/<i>S</i>-BINAP)₃(PPh₃)₂]²⁺ (BINAP = 2,2′-bis­(diphenylphosphino)-1,1′-binaphthyl). [Au₈(BINAP)₃(PPh₃)₂]²⁺ showed stronger rotatory strengths than [Au₁₁(DIOP)₄Cl₂]⁺ in the visible region, while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than that of the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)­benzoic acid (4-MEBA) gave Au₇₆(4-MEBA)₄₄ clusters that exhibited a strong (3 × 10⁵ M^–1 cm^–1) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction

Hydride-Doped Gold Superatom (Au₉H)²⁺: Synthesis, Structure, and Transformation

Doping of a hydride (H^–) into an oblate-shaped gold cluster [Au₉(PPh₃)₈]³⁺ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au₉H­(PPh₃)₈]²⁺ demonstrated that the (Au₉H)²⁺ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au₉H)²⁺ was successfully converted to the well-known superatom Au₁₁³⁺, providing a new atomically precise synthesis of Au clusters via a bottom-up approach

Binding Motif of Terminal Alkynes on Gold Clusters

Gold clusters protected by terminal alkynes (1-octyne (OC-H), phenylacetylene (PA-H) and 9-ethynyl-phenanthrene (EPT-H)) were prepared by the ligand exchange of small (diameter <2 nm) Au clusters stabilized by polyvinylpyrrolidone. The bonding motif of these alkynes on Au clusters was investigated using various spectroscopic methods. FTIR and Raman spectroscopy revealed that terminal hydrogen is lost during the ligand exchange and that the CC bond of the alkynyl group is weakened upon attachment to the Au clusters. Acidification of the water phase after the ligand exchange indicated that the ligation of alkynyl groups to the Au clusters proceeds via deprotonation of the alkynes. A series of precisely defined Au clusters, Au₃₄(PA)₁₆, Au₅₄(PA)₂₆, Au₃₀(EPT)₁₃, Au₃₅(EPT)₁₈, and Au_41–43(EPT)_21–23, were synthesized and characterized in detail to obtain further insight into the interfacial structures. Careful mass analysis confirmed the ligation of the alkynes in the dehydrogenated form. An upright configuration of the alkynes on Au clusters was suggested from the Au to alkyne ratios and photoluminescence from the excimer of the EPT ligands. EXAFS analysis implied that the alkynyl carbon is bound to bridged or hollow sites on the cluster surface

Collision-Induced Fission of Oblate Gold Superatom in [Au₉(PPh₃)₈]³⁺: Deformation-Mediated Mechanism

Collision-induced dissociation (CID) patterns of the phosphine-protected Au-based clusters [PdAu8(PPh3)8]2+ (PdAu8) and [Au9(PPh3)8]3+ (Au9), featuring crown-shaped M@Au8 (M = Pd, Au) cores, were investigated. For PdAu8, ordinary sequential PPh3 losses (PdAu8 → [PdAu8(PPh3)m]2+ + (8 – m)PPh3 (m = 7, 6, 5)) were observed. In contrast, Au9 underwent cluster-core fission (Au9 → [Au6(PPh3)6]2+ (Au6) + [Au3(PPh3)2]+ (Au3)) upon sufficiently high energy collision, associated with splitting the number of valence electrons in the superatomic orbitals from 6e (Au9) into 4e (Au6) and 2e (Au3). Density functional theory calculations revealed oblate and prolate cores of Au9 and Au6 with semiclosed superatomic electron configurations of (1S)2(1Px)2(1Py)2 and (1S)2(1Pz)2, respectively. This result indicated a significant deformation of the cluster-core motif during the CID process. We attribute the clear difference between PdAu8 and Au9 to the softer Au–Au bond in Au9 and propose that the collision-induced structural deformation plays a critical role in the fission

Collision-Induced Fission of Oblate Gold Superatom in [Au₉(PPh₃)₈]³⁺: Deformation-Mediated Mechanism

Collision-induced dissociation (CID) patterns of the phosphine-protected Au-based clusters [PdAu8(PPh3)8]2+ (PdAu8) and [Au9(PPh3)8]3+ (Au9), featuring crown-shaped M@Au8 (M = Pd, Au) cores, were investigated. For PdAu8, ordinary sequential PPh3 losses (PdAu8 → [PdAu8(PPh3)m]2+ + (8 – m)PPh3 (m = 7, 6, 5)) were observed. In contrast, Au9 underwent cluster-core fission (Au9 → [Au6(PPh3)6]2+ (Au6) + [Au3(PPh3)2]+ (Au3)) upon sufficiently high energy collision, associated with splitting the number of valence electrons in the superatomic orbitals from 6e (Au9) into 4e (Au6) and 2e (Au3). Density functional theory calculations revealed oblate and prolate cores of Au9 and Au6 with semiclosed superatomic electron configurations of (1S)2(1Px)2(1Py)2 and (1S)2(1Pz)2, respectively. This result indicated a significant deformation of the cluster-core motif during the CID process. We attribute the clear difference between PdAu8 and Au9 to the softer Au–Au bond in Au9 and propose that the collision-induced structural deformation plays a critical role in the fission

Synergistically Activated Pd Atom in Polymer-Stabilized Au₂₃Pd₁ Cluster

Single Pd atom doped Au23Pd1 clusters stabilized by polyvinylpyrrolidone (Au23Pd1:PVP) were selectively synthesized by kinetically controlled coreduction of the Au and Pd precursor ions. The geometric structure of Au23Pd1:PVP was investigated by density functional theory calculation, aberration-corrected transmission electron microscopy, extended X-ray absorption fine structure analysis, Fourier transform infrared spectroscopy of adsorbed CO, and hydrogenation catalysis. These results showed that Au23Pd1:PVP takes polydisperse but the same atomic arrangements as undoped Au24:PVP while exposing all the atoms including the Pd atom on the surface. Au23Pd1:PVP exhibited a significantly higher catalytic activity than Au24:PVP for the aerobic oxidation of p-substituted benzyl alcohols. The kinetic studies showed that the rate-determining step was the hydride abstraction from the α-carbon of the alkoxides for both systems. The activation energy for hydride abstraction by Au23Pd1:PVP was lower than that by Au24:PVP, indicating that the doped Pd atom acts as the active center

Collision-Induced Dissociation of Undecagold Clusters Protected by Mixed Ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh)

We herein investigated collision-induced dissociation (CID) processes of undecagold clusters protected by mixed ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh) using mass spectrometry and density functional theory calculations. The results showed that the CID produced fragment ions [Au_<i>x</i>(PPh₃)_<i>y</i>X_<i>z</i>]⁺ with a formal electron count of eight via sequential loss of PPh₃ ligands and AuX­(PPh₃) units in a competitive manner, indicating that the CID channels are governed by the electronic stability of the fragments. Interestingly, the branching fraction of the loss of the AuX­(PPh₃) units was significantly smaller for X = CCPh than that for X = Cl. We ascribed the effect of X on the branching fractions of dissociations of PPh₃ and AuX­(PPh₃) to the steric difference

Suppressing Isomerization of Phosphine-Protected Au₉ Cluster by Bond Stiffening Induced by a Single Pd Atom Substitution

The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au₉(PPh₃)₈]³⁺ with a crown motif (<b>Au</b>_<b>9</b>(C)) and that with a butterfly motif (<b>Au</b>_<b>9</b>(B)) induced by association and dissociation with compact counteranions (NO₃^–, Cl^–). However, structural isomerization was suppressed by substitution of the central Au atom of the Au₉ core in [Au₉(PPh₃)₈]³⁺ with a Pd atom: [PdAu₈­(PPh₃)₈]²⁺ with a crown motif (<b>PdAu</b>_<b>8</b>(C)) did not isomerize to that with a butterfly motif (<b>PdAu</b>_<b>8</b>(B)) upon association with the counteranions. Density functional theory calculation showed that the energy difference between <b>PdAu</b>_<b>8</b>(C) and <b>PdAu</b>_<b>8</b>(B) is comparable to that between <b>Au</b>_<b>9</b>(C) and <b>Au</b>_<b>9</b>(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye–Waller factors obtained by X-ray absorption fine-structure analysis revealed that the intracluster bonds of <b>PdAu</b>_<b>8</b>(C) were stiffer than the corresponding bonds in <b>Au</b>_<b>9</b>(C). Natural bond orbital analysis suggested that the radial Pd–Au and lateral Au–Au bonds in <b>PdAu</b>_<b>8</b>(C) are stiffened due to the increase in the ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal–metal bonds by Pd atom doping inhibits the isomerization from <b>PdAu</b>_<b>8</b>(C) to <b>PdAu</b>_<b>8</b>(B)

An Au₂₅(SR)₁₈ Cluster with a Face-Centered Cubic Core

A representative thiolate (RS)-protected gold cluster, Au₂₅(SR)₁₈, shows a fingerprint-like characteristic spectral profile regardless of the R-groups, reflecting the common motif of the structural backbone made of Au and S: an icosahedral Au₁₃ core fully protected by six staple units of Au₂(SR)₃. On the other hand, we reported in 2006 that an Au₂₅(SPG)₁₈ cluster (PGSH = <i>N</i>-(2-mercaptopropionyl)­glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au₂₅(SR)₁₈, suggesting the formation of a nonicosahedral Au core. Here, we investigated the structure of Au₂₅(SPG)₁₈ by UV–vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au₂₅(SPG)₁₈ has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au₁₅(SPG)₄[Au₂(SPG)₃]₂[Au₃(SPG)₄]₂ in which an Au₁₅(SPG)₄ core with an FCC motif is protected by two types of staples with different lengths, Au₂(SPG)₃ and Au₃(SPG)₄. The formation of an FCC-based Au core is attributed to bulkiness around the α-carbon of the PGS ligand