Au Bond Length Research Articles

Supramolecular Template-Assisted Catalytic [2+2] Photocycloaddition in Homogeneous Solution

Supramolecular Template-Assisted Catalytic [2+2] Photocycloaddition in Homogeneous Solution

Elucidating the stabilities and properties of the thiolate-protected Au nanoclusters with detaching the staple motifs.

Thiolate-protected Au nanoclusters (AuNCs) have been widely studied in areas of catalysis, biosensors, and bioengineering. In real applications, e.g., catalytic reactions, the thiolate groups are normally partially detached. However, which of the thiolate groups are easily detached and how the detachment of the ligands affects the geometries and electronic structures of the Au nanoclusters have been rarely studied. In this work, we employed the density functional theory calculations as well as the molecular orbital analysis to explore the detachment effect of the ligands using nine thiolate-protected AuNCs as examples. Our results showed that there existed a nearly linear relationship between the averaged detachment energies and the numbers of Au atoms in the motifs. Detaching longer motifs normally required more energies owing to the stronger aurophilic effects. For detaching a full motif, based on the structure decomposition via the grand unified model, analysis on the inner Au core indicated that the change in Au-Au bond length was more sensitive for the inter-block compared to the intra-block. The detachment of the -SH fragment generally needs less energy and brings less structural deformations when compared to the removal of a full motif. Molecular orbital analysis showed that the relative energies of the HOMO orbitals were elevated, which led to the narrow down of the HOMO-LUMO gap. This work provides a primary description of the correlation of the ligands' detachment with the relative stabilities and structures of the AuNCs, which would be beneficial for establishing the structure-property relationship of AuNCs in real applications.

Bisbiphenyl Phosphines: Structure and Synthesis of Gold(I) Alkene π-Complexes with Variable Phosphine Donicity and Enhanced Stability

This paper describes the synthesis and characterization of RP(o-biphenyl)2 phosphine ligands (where R = PhO, Ph, and t-Bu), corresponding gold(I) chloride precatalysts, gold(I) triflate catalysts, and gold(I) π-complexes. All ligands and gold chlorides and three π-complexes were characterized in the solid state. The most significant differences between complexes in the solid state were varying P–Au bond lengths, a consistent reflection of the different electronic character of each phosphine. NMR spectroscopic data on enol ether π-complexes is consistent with increasing cationic character in the alkene fragment, from R = t-Bu to R = Ph and OPh. The intermolecular alkene exchange process for [LAu(methoxypropene)]SbF6 (L = PhO(o-biphenyl)2P) is faster than that of the analogous complex with t-Bu2(o-biphenyl)P, presumably due to the higher electrophilicity of the former complex. Examination of a series of vinyl silane π-complexes with triaryl phosphine ligands reveals increasing stability and ease of preparation (Ph3P < Ph2P(o-biphenyl) < PhP(o-biphenyl)2). These results reveal that incorporation of a second biphenyl substituent into ligand architectures allows preparation of a series of gold π-complexes with increased stability over a range of phosphine donicity.

Structural evolution and ligand effects of (Au1L2)n, (Au2L3)n, and (Au3L4)n [n = 1–3, L = SCH3,P(CH3)2,PH2,Cl] clusters

ABSTRACTIn this paper, we present a density-functional theory with a particle swarm optimisation algorithm to research the progress of the ligand effects of the gold (Au) structures and growth process of the [Au1L2]n, [Au2L3]n and [Au3L4]n (n = 1–3, L = SCH3, PH2, P(CH3)2,Cl) nanoclusters. Changing of ligand atoms induces minor changes in the Au and Au-L bond lengths. When L = Cl, with increasing n, the size of the Au cores tends to increase, while when L = PH2, with increasing n, the size of Au cores decreases and the length of the ligand protection increases. We give a detailed comparison of the average binding energy, stability, and aromaticity of these clusters and further explain the diverse role of ligand substitution. The order of stability for each ligand, Cl > PH2> SCH3>P(CH3)2, is determined by the average binding energy and the difference between the atomic energies. In addition, the influence of strong Au-Au interactions will weaken the aromaticity of the structure. New nanoclusters that can be widely applied in other fields, and designed by leveraging the ligand effects of the Au structural evolution characteristics.

From the Terrace Contraction to the Hexameric Sulfur Phase in the Au(100) Surface: A Combined Density Functional Theory and Scanning Tunneling Microscopy Study

The lifting of the Au(100) quasi-hexagonal reconstruction by atoms or molecules leaves a rough surface with islands, terraces, and vacancies where different patterns of adsorbates can appear. Although the roughness of the (1 × 1)-Au(100) surface has been well characterized, to our knowledge, there are no theoretical studies considering its effect on the formation of adsorbate patterns, for instance, in the case of sulfur adsorption. In this study, we have combined DFT calculations and STM measurements to explore the structural effects induced on the terrace when the sulfur adsorption takes place on the island and vacancy. Models predict a decrease of the Au–Au bond length in the terrace if the terrace has an appropriate size when the simultaneous adsorption of the sulfur structure (2×2) occurs on the island and vacancy. According to our predictions, this process of gold bond length contraction on the terrace can affect the formation of the sulfur structure on it. STM images reveal regions where the (2×2) ...

Interaction of Cysteine with Aun (n=8, 10, 12) Even Neutral Clusters: A Theoretical Study

AbstractThe intermolecular interactions between gold nanoparticles Aun (n=8, 10, 12) with (neutral (o), cationic (+) and anionic (‐)) cysteine amino acid is investigated by means of density functional theory DFT –B3LYP/LANL2DZ:6‐31+G** mixed basis sets. The studies have been carried out on the structural, electronic and bonding properties of the major bonding factors: (a) the anchoring S−Au, O−Au and N−Au and (b) non‐conventional N–H⋅⋅⋅Au, O–H⋅⋅⋅Au bonds. Bader topological AIM analysis was carried out to interpret the nature of bonds to the complexation behavior. The NBO and frontier molecular orbital analysis indicated that the lone pair electrons of nitrogen, oxygen and sulphur atoms are transferred to the antibonding orbitals of gold for the complex with anchoring bonds, while the lone pair electrons of gold are transferred to the antibonding orbitals of O−H⋅⋅⋅Au bonds. Our results demonstrated that cysteine‐Aun form stable complexes. Among the three (neutral, cationic and anionic) forms of cysteine, cationic cysteine complexes interact more stronger with the gold metal clusters. The bonding strength between gold clusters and cysteine is positively correlated with the S–Au overlap‐weighted bond order and negatively correlated to the S–Au bond length.

Microscopic Structure of Naked Au Nanoparticles Synthesized in Typical Ionic Liquids by Sputter Deposition

Au nanoparticles (AuNPs) of varied sizes were synthesized in typical ionic liquids using the sputter deposition technique. Because AuNPs are dispersed freely in each ionic liquid without bonding to particular stabilizing agents, they are sometimes called “naked AuNPs”. To characterize the structure of naked AuNPs, we obtained a size distribution curve from small-angle X-ray scattering (SAXS) experiments and extracted the value of the peak position as the size of the most abundant AuNPs in each ionic liquid. The size was controlled by the type of ionic liquid and preparation temperature. To elucidate the local structure of the AuNPs in relation to their particle size, we measured the X-ray absorption fine structures (XAFS) at the Au L3-edges. The analyses of the extended XAFS revealed that the Au–Au bond lengths for AuNPs ∼1 nm in size are 2.76 to 2.81 A, in contrast with the 2.88 A bond lengths for bulk Au. Comparing our SAXS and XAFS results with previously reported theoretical studies, we conclude that ...

Theoretical Investigation on the Optical Properties of Diphosphine-protected Au8 Cluster Complexes

Abstract Herein, we have theoretically investigated the optical properties of the two Au8 complexes with different shapes of the Au8 cluster core. From the results of TD-DFT and DFT calculations, it was found that the shape of the Au8 clusters in the two Au8 complexes greatly influenced their optical properties, such as absorption and photoluminescence spectra. In particular, the intensities of their photoluminescence spectra are greatly decreased with an increase of the Au–Au bond length variation of the Au core cluster in the S1 states of the Au8 complexes.

Unidirectional Thermal Diffusion in Bimetallic Cu@Au Nanoparticles

Understanding the atomic diffusions at the nanoscale is important for controlling the synthesis and utilization of nanomaterials. Here, using in situ X-ray absorption spectroscopy coupled with theoretical calculations, we demonstrate a so far unexplored unidirectional diffusion from the Au shell to the Cu core in thermally alloying Cu@Au core@shell architecture of ca. 7.1 nm. The initial diffusion step at 423 K is found to be characterized by the formation of a diffusion layer composed of a Au-dilute substitutional CuAu-like intermetallic compound with short Cu-Au bond length (2.61 Å). The diffusion further happens by the migration of the Au atoms with large disorder into the interior Cu matrix at higher temperatures (453 and 553 K). These results suggest that the structural preference of a CuAu-like compound, along with the nanosized effect, plays a critical role in determining the atomic diffusion dynamics.

Au(111) surface structures induced by adsorption: LEED I(E) analysis of (1 × 1) and (5 × 5) Au(111)–S phases

We report on the various phases formed by sulfur adsorbed on Au(111), at less than 1 monolayer (ML) coverage, as monitored by low-energy electron diffraction (LEED). The phases transform from one to another via coexistence regions. The observation of ordered phases was matched to the coverage information obtained from Auger measurements, using as a calibration point the S coverage of 0.28 ML, corresponding to the sharpest (5×5) LEED pattern observed. The influence of emitting filaments (the Auger beam, ion gauge, or LEED electron beam) upon the adsorption itself is discussed. LEED structural investigations were performed on two particular S–Au phases. The analysis of the Au(1×1)–S structure provided the values for the first four gold interlayer spacings, showing a 1% expansion of the top layer, interpreted as a remnant of the expansion of the Au(111)–(22×√3) reconstructed phase. The analysis of the Au(5×5)–7S phase confirmed the fcc adsorption site of the S atoms reported previously, and resulted in an average S–Au distance of 1.57±0.10Å, and an S–Au bond length of 2.29±0.07Å.

Toward an Understanding of Ligand Selectivity in Nanocluster Synthesis

We performed scalar relativistic density functional theory (DFT) calculations using the projector augmented wave scheme (PAW) to examine the reactivity and selectivity of diphosphine ligands LM, with the formula PH2(CH2)MPH2 (spacer M = 3, 5), toward small-sized cationic Aun (n = 7–11) nanoclusters. By isolating the ligand-induced contribution to the stability condition, we show that such interaction selectively stabilizes the cationic Au11 cluster. Furthermore, we find that L5 with the longer spacer is more capable than L3 of relieving the strain imposed on the spacer by bidentate binding to gold clusters, which have relatively small Au–Au bond lengths. Thus L5 can interact effectively with gold clusters of various sizes, but L3 can do so only with a selected few. This result demonstrates the size-selecting power of L3 toward small gold clusters such as Au113+. To further test the validity of our results we have extended the calculation to a larger cluster, Au13, and also considered the case of a ligand ...

A density functional theory study on the H 2S molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation on Au n H 2S ( n = 1–13) clusters has been performed by using density functional theory with the generalized gradient approximation at PW91 level. The small gold cluster would like to bond with sulfur in the same plane and the H 2S molecule prefers to occupy the on-top and single fold coordination site in the cluster. The Au n structures and H 2S molecule in all Au n H 2S clusters are only slightly perturbed and still maintain their structural integrity. After adsorption, the S–H, H–H bond-lengths and most Au–Au bond-lengths are elongated, only a few Au–Au bond-lengths far from H 2S molecule are shortened. The reactivity enhancement of H 2S molecule is obvious and the strong gold–sulfur bond is observed expectedly. The most favorable adsorption takes place in the case that the H 2S molecule is adsorbed by an even-numbered Au n cluster and becomes Au n H 2S cluster with even number of valence electrons. It is believed that the strong scalar relativistic effect is favorable to H 2S molecule adsorption onto small gold clusters and is also one of the important reasons for the strong gold–sulfur bond.

Au–Au bond length expansion in CTAB stabilized gold nanoparticles

The modifications of the electronic structure and the structural order with decreasing particle size are of great interest for the explanation of catalytic properties of gold nanoparticles. One of the issues is the predicted Au–Au bond length contraction with particle size decrease. For the purpose of the study, X-ray absorption spectroscopy (XAFS) has been chosen as a probe, since it provides information about both, the local coordination and the electronic structure of the absorbing element. We present an X-ray absorption spectroscopy study of gold nanoparticles of different sizes. The XAFS spectra of gold nanoparticles of 1.7, 5.7, 6.4 and 13.8 nm in radius, dispersed in liquid cetyl trimethylammonium bromide (CTAB) solution have been measured at the Au L 3-edge. The extended X-ray absorption fine structure (EXAFS) functions of the measured spectra were analyzed employing ab initio calculations. It was found that the average Au–Au distance is larger by 4.2(3) pm than this was found in bulk gold and there is no size-dependent trend recognizable concerning this value for the different particle radii examined.

EXAFS in total reflection (reflEXAFS) for the study of organometallic Pd(II) thiol complexes based self-assembled monolayers on gold

The molecular structure and organization of self-assembled monolayers (SAMs) and multilayer films grafted onto Au/Si(1 1 1) surfaces of mononuclear transition metal dialkynyl bridged Pd(II) complexes trans-[HS-Pd(PBu 3) 2-SH] ( 1), trans-[HS-Pd(PBu 3) 2(–C C–C 6H 5)] ( 2) and of the binuclear complex trans, trans-[HS-Pd(PBu 3) 2(–C C–C 6H 4–C 6H 4–C C–Pd(PBu 3) 2-SH] ( 3), have been investigated by extended X-ray absorption fine structure spectroscopy in total reflection conditions (reflEXAFS). ReflEXAFS analysis of the data lead to determine the local structure around Pd atoms, assessing the square-planar geometry around the transition metal in the multilayers case, preserved in the monolayer regime. The investigation on the SAMs also provided the assessment of the S–Au bond length and Pd–S–Au bond angle and on the molecular orientation on the gold substrate of the complexes, confirmed by quantum chemical calculations.

The local adsorption site of methylthiolate on Au(1 1 1): Bridge or atop?

Measurements of the local adsorption geometry of the S head-group atom in the Au(1 1 1)(√3 × √3) R30°–CH 3S surface have been made using normal incidence X-ray standing waves (NIXSW) and S 1s scanned-energy mode photoelectron diffraction on the same surface preparations. The results confirm that the local adsorption site is atop an Au atom in a bulk-continuation site with a S–Au bondlength of 2.42 ± 0.02 Å, and that there can be no significant fraction of coadsorbed bridging species as recently proposed in a combined molecular dynamics/experimental study by Mazzarello et al. [R. Mazzarello, A. Cossaro, A. Verdini, R. Rousseau, L. Casalis, M.F. Danisman, L. Floreano, S. Scandolo, A. Morgante, G. Scoles, Phys. Rev. Lett. 98 (2007) 016102]. The results do not, however, clearly distinguish the different local reconstruction (adatom) models proposed for this surface.

Theoretical study of the novel sandwich compound [Au3Cl3Tr2]2+

A theoretical study of a sandwich compound with a metal monolayer sheet between two aromatic ligands is presented. A full geometry optimization of the [Au(3)Cl(3)Tr(2)](2+) (1) compound, which is a triangular gold(I) monolayer sheet capped by chlorines and bounded to two cycloheptatrienyl (Tr) ligands was carried out using perturbation theory at the MP2 computational level and DFT. Compound (1) is in agreement with the 18-electron rule, the bonding nature in the complex may be interpreted from the donation interaction coming from the Tr rings to the Au array, and from the back-donation from the latter to the former. NICS calculations show a strong aromatic character in the gold monolayer sheet and Tr ligands; calculations done with HOMA, also report the same aromatic behavior on the cycloheptatrienyl fragments giving us an insight on the stability of (1). The Au -Au bond lengths indicate that an intramolecular aurophilic interaction among the Au(I) cations plays an important role in the bonding of the central metal sheet.

Effects of Hydrogen on the Reactivity of O2 toward Gold Nanoparticles and Surfaces

Density functional theory (DFT) was used to study the coadsorption of O2 and H2 or H on Au(111) and Au(100) and on free pyramidal clusters (Au25 and Au29) observed on titania- and ceria-based catalysts. For the adsorption of isolated O2, no significant interaction was obtained between the flat metal surfaces and the oxygen molecule. Gold clusters (Au14, Au25, and Au29) showed a higher reactivity, but this reactivity depended strongly on the type of uncoordinated sites exposed, ensemble effects, and the fluxionality of the metal nanoparticles. On the Au14 and Au29 clusters, a superoxo species and a peroxo moiety were formed, respectively. In contrast, no interaction between O2 and Au25 was seen because of the high coordination number of the exposed metal sites. In general, H2 dissociates much easier on the gold clusters than O2, but again Au25 is less reactive than Au29 or Au14. The pyramidal Au25 and Au29 clusters illustrate the importance of geometry in the chemistry of gold nanoparticles: A smaller particle size does not necessarily imply a bigger chemical reactivity. With the presence of predissociated hydrogen on a gold system, the adsorption energy of oxygen is much higher in absolute value than for the O2/Au systems, accompanied by the formation of a hydroperoxo species. The origin of the synergistic effect observed for the coadsorption of hydrogen and O2 is the formation of O−H bonds that enhance the O↔Au interactions, reducing (0.15−0.30 Å) the O−Au bond lengths. The reaction H(a) + OOH(a) → H2O2(a) was found to be highly exothermic on the Au clusters. An interesting case is when the oxygen orientation allows for simultaneous interaction with two H adatoms: The formation of hydrogen peroxide is readily achieved with an energy release of more than 30 kcal/mol. This reaction is only hindered by the need of a concerted approach of O2 to the H adatoms.

The effect of gold particle size on Au [sbnd]Au bond length and reactivity toward oxygen in supported catalysts

Au catalysts with different metallic particle sizes and supported on silica, alumina, titania, zirconia, ceria, and niobia were prepared, and the reduced catalysts were characterized by EXAFS spectroscopy. As the Au Au coordination number decreased, the interatomic bond length decreased. The Au Au bond length contraction appears to be independent of the support type. A correlation between the dispersion of Pt catalysts determined by hydrogen chemisorption and the EXAFS Pt Pt coordination number was established and used to determine the dispersion of fully reduced Au catalysts. In addition, the Au particle size was estimated using a literature correlation of the EXAFS coordination number. For particles larger than about 40 Å, there was little change in the metallic bond length, whereas in catalysts with gold particles smaller than 30 Å, the Au Au distance decreased with decreasing particle size, with a maximum contraction of about 0.15 Å. Decreasing particle size also brought a decrease in the intensity of the white line of the XANES spectrum. Both the decrease in bond distance and white line intensity were consistent with an increase in the d-electron density of Au atoms in very small particles. Au particles smaller than about 30 Å were also reactive to air, leading to oxidation of up to 15% of the atoms of the gold particles, depending on the size; larger particles were not oxidized. These oxidized Au atoms in small particles are suggested to be active for CO oxidation.

(2-Methyldithiobenzoato-κS)(triphenylphosphine-κO)gold(I)

The asymmetric unit of the title compound, o-CH3(C6H4)C(S)S–Au–P(C6H5)3 or [Au(C8H7S2)(C18H15P)], consists of two quasi-linear gold complexes. The S—Au—P bond angles [173.83 (2), 175.03 (2)°], S—Au bond lengths [2.3240 (6), 2.3329 (6) Å] and Au—P bond lengths [2.2568 (6), 2.2632 (6) Å] are consistent with previously reported values. The major difference in the two molecules is that the o-methylphenyl rings are twisted with respect to the dithiocarboxyl planes by 73.0 (3)° and 46.6 (3)°, as measured by S—C—C—C torsion angles.

Pseudomorphic growth of Pd monolayer on Au(111) electrode surface

The surface structure of a Pd layer electrochemically deposited onto Au(111) from PdCl 2− 4 solution has been studied by surface X-ray diffraction. By combining the measurements of the specular and non-specular rod profiles, both in-plane and out-of-plane structures have been determined. On the Au(111) electrode surface, Pd occupies the cubic closest packing site and forms a pseudomorphic smooth monolayer. The formation of the stable Pd overlayer is explained by the lifting of the surface reconstruction of the Au substrate. The PdAu bond length at the Pd/Au(111) interface has been found to be 2.82 Å, which is close to the sum of the atomic radii of Pd and Au in each bulk.