Size Effect on Aurophilic Interaction in Gold-Chloride Cluster Anions of Au n Cl n+1 - (2 ≤ n ≤ 7).

Aurophilic interaction plays a very important role in gold-related clusters. Here, we investigate the Au n Cl n+1 - (n = 1-7) cluster ions using Fourier transform ion cyclotron resonance mass spectrometry in combination with theoretical calculations. Three cluster ions of Au2Cl3 -, Au3Cl4 -, and Au4Cl5 - show their remarkable intensities in the mass spectrum. Geometric structure optimizations for Au n Cl n+1 - (n = 1-7) were performed on the MP2 level. The results show that the most stable structures of Au n Cl n+1 - (n = 2-7) are all characterized by a zigzag structure. Furthermore, it can be found that the aurophilic interactions containing terminal gold atoms strengthen with the increase of total gold atoms and progressively stabilize for large clusters of Au6Cl7 - and Au7Cl8 -, whereas the aurophilic interactions between nonterminal adjacent gold atoms stabilize at n = 5.

Introduction

Gold-containing compounds and clusters play an important role in the fields of catalysis,[1−4] supramolecular chemistry,[5] and material science.[6,7] The chemical properties of gold clusters are affected by their structures as well as the interactions among Au atoms very much. In 1988, Schmidbaur first observed that the gold(I) molecule showed a short Au···Au distance in the crystalline state.[8,9] Furthermore, the term “aurophilicity” was formally proposed.[10] Aurophilic interaction has an energy between those of van der Waals force and covalent bonding and plays an important role in stabilizing Au(I) compounds.[11,12] Pyykkö and co-workers theoretically investigated the aurophilic interactions between closed shell Au(I) cationic centers over the past years.[13−16] They suggested that the nature of aurophilic interactions can be understood as a dispersion effect.

Among these related species, gold halides are found to be very useful in gold-catalyzed reactions.[1,17−20] Thus, understanding their structures and properties have attracted a great amount of research in recent years.[21−27] For example, Lemke investigated the microsolvated clusters of gold chloride by experiment and theoretical calculations.[22] The results show that microsolvation enhances electron localization between Au centers. Li et al. systematically investigated the periodicity and electronic structures of gold tetrahalides [AuX4]− (X = F, Cl, Br, I, At, or Uus).[23] Their results show that the Au(I) tends to be preferred for heavy halides. Wang et al. observed two types of isomers of Au2I3– with acute and obtuse Au–I–Au angles and named them bond-bending isomers (BBIs).[25]

Although the aurophilic interactions have been widely studied, investigation of the effect of cluster sizes on aurophilic interactions for gold-related clusters is rare. Because the sizes of clusters have a great impact on their structures, properties, and catalytic activities, this kind of research will be very important and valuable. Here, we investigate clusters of AuCl– (n = 2–7) through Fourier transform ion cyclotron resonance (FT ICR) mass spectrometry combined with theoretical calculations. The present study focuses on the relationship between the size of gold-chloride cluster anions and the aurophilic interaction inside these clusters.

Results and Discussion

A typical laser ablation mass spectrum of AuCl4H is shown in Figure . Cluster ions of AuCl– (n = 1–4), AuCl– (n = 2–4), and AuClH– (n = 2–4) can be clearly identified in the mass spectrum. Among them, anions of Au2Cl3–, Au3Cl4– and Au4Cl5– show their remarkable intensities.

Figure 1

Laser ablation mass spectrum of AuCl4H obtained in the negative ion mode.

The structures of AuCl– (n = 1–7) cluster ions were examined through theoretical calculations. Figure shows some structures and the relative energies (ΔE) of AuCl– (n = 1–4) cluster ions on the MP2 level. As shown in the figure, the lowest energy geometry of AuCl2– has a linear form with a C2 symmetry, in which Au atom is in the middle of two Cl atoms. For Au2Cl3–, the most stable geometric structure is similar to the obtuse isomer of Au2I3– found by Wang et al.,[25] and the Au–Au distance reported here is 3.69 Å. Interestingly, the most stable geometric structures of Au3Cl4– and Au4Cl5– are very similar. They are all characterized by a zigzag structure. The zigzag structures are found to be similar to those previously suggested or reported for some gold-related species.[28−32] For example, previous study showed that the structure of AuCl crystals prepared by a vapor-transport method consisted of zigzag chains of Au and Cl with Au–Cl–Au bond angles of 92°.[33,34] Other structures that also have some kinds of symmetry are found to be 10–70 kcal/mol higher in energies than the zigzag structures. All calculated vibrational frequencies for the most stable structures of AuCl– (n = 1–7) are shown in Table S1.

Figure 2

Some structures and their relative energies (ΔE, in kcal mol–1) of AuCl– (n = 1–4) cluster ions on the MP2 level.

Table provides the main geometric parameters of the most stable structures. It can be found that the distances between adjacent gold atoms in Au2Cl3– and Au3Cl4– are 3.69, and 3.15 Å, respectively. For Au4Cl5–, the nonterminal and terminal Au–Au distances are 3.07 and 3.11 Å, respectively. Here, the terminal Au–Au distance is defined as the distance between the terminal gold atom and its adjacent gold atom; and the nonterminal Au–Au distance is defined as the distance between two inner adjacent gold atoms. These distances in Au3Cl4– and Au4Cl5– are shorter than the sum of the van der Waals radii of two Au atoms (3.32 Å), indicating the aurophilic attraction among Au atoms in the cluster ions. A comparison with these distances in AuCl– (n = 2–4) cluster ions shows that with the increase of Au atoms, the distances between adjacent gold atoms become shorter. In other words, the aurophilic interactions between adjacent gold atoms get stronger with the increase in the cluster size of AuCl– from n = 2–4.

Table 1

Equilibrium MP2 Structural Parameters (with the Sets of ECP60MDF for Au and aug-cc-pVTZ for Cl) of AuCl– (n = 2–7); Bond Lengths are in Å and Angles are in Degreesa,b

MP2	<Au Cl AuT	<Au Cl AuNT	Au–AuT	Au–AuNT	average dihedral angle
Au2Cl3–	105.39		3.69
Au3Cl4–	85.51 ± 0		3.15		167.45
Au4Cl5–	84.14 ± 0.01	83.01	3.11 ± 0	3.07	160.36 ± 0.01

The variations of these values are also listed.

T represents terminal and NT represents nonterminal.

To better understand the relationship between the size of gold-chloride cluster ions and the aurophilic interaction, theoretical calculations were also performed for anions of Au5Cl6–, Au6Cl7–, and Au7Cl8– on the same level. As shown in Figure , the most stable structures of Au5Cl6–, Au6Cl7–, and Au7Cl8– are very similar in their structures. Remarkably, all of them are characterized by the zigzag structure. The terminal Au–Au distances in Au5Cl6–, Au6Cl7–, and Au8Cl7– are 3.08, 3.07, and 3.06 Å, respectively, which are slightly shorter than that in Au4Cl5– (3.11 Å). All nonterminal Au–Au distances in Au5Cl6–, Au6Cl7–, and Au8Cl7– stabilized at 3.06 Å. These results are also listed in Table . From this, it can be found that the more gold atoms in the clusters, the stronger the aurophilic interactions existed. This trend is prominent for small clusters (n ≤ 4) but inconspicuous for large ones (n > 4). For the aurophilic interaction between two nonterminal adjacent Au atoms, it stabilized more quickly than that between those including one terminal Au atom; but both of them stabilized at a distance of 3.06 Å at last.

Figure 3

Most stable structures of (a) Au5Cl6–, (b) Au6Cl7–, and (c) Au7Cl8– found on the MP2 level.

Relativistic effects in chemistry are based on the electrons that move at high speed in the atom.[13] One of the most obvious elements of the relativistic effects is gold.[35] For exploring the influence of relativistic effects on the distances of adjacent Au atoms, nonrelativistic ECP60MHF pseudopotentials[26] were applied for Au atoms, and the results are given in Table S2. For instance, the nonrelativistic Au···Au distance in Au3Cl4– is 0.91 Å longer than that obtained based on the relativistic pseudopotentials ECP60MDF. For other cluster anions, similar results were observed. All results strongly illustrate that relativistic effects play a very important role in the aurophilic interactions.

To gain information on the natural charges and electron configurations of Au and Cl in these anions, natural bond orbital (NBO) analyses were performed for AuCl– (n = 1–7) on the same level. The results are listed in Table S3. Compared with the Au atom in AuCl2– (without aurophilic interaction), the Au atoms in AuCl– (n ≥ 2) with aurophilic interactions have higher charges. Besides, the charges of two terminal Au atoms are basically the same but lower than those of nonterminal Au atoms. For Au2Cl3–, the electron configuration of the Au atom is 6s0.635d9.796p0.12, the charges on two Au atoms are both +0.463, and the electron transfer from Cl to Au is about 0.537e. Meanwhile, the Au 6s orbital population increases, whereas the 5d orbital is depopulated, which are due to the strong relativistic effects. This trend maintains with the increase in the number of Au atoms, and the charge on Au atoms is almost unaffected for these clusters.

Furthermore, chemical bonding analyses were examined employing electron localization function (ELF) in Multiwfn program.[36]Figure provides typical ELF profiles for AuCl– (n = 2–7). For Au2Cl3–, there is nearly no electron-pair density between two Au atoms. As the number of Au atoms increased, the regions between the terminal gold atom and its adjacent gold atom change from dark to light blue, indicating the enhanced electron-pair densities, and the regions between nonterminal adjacent Au atoms do not change much for AuCl– (n ≥ 4). The results illustrate progressively increased aurophilic interactions among these cluster anions and the difference between terminal and nonterminal Au atoms.

Figure 4

ELFs calculated for AuCl– (n = 2–7) cluster ions.

There are two types of isomers of Au2I3– observed with acute (72.0°) and obtuse (100.7°) Au–I–Au angles.[25] Very interestingly, these isomers are called BBIs, which are quite different from other type of isomers. Inspired by this work, we wonder if the BBIs existed in AuCl– (n = 2–4) cluster ions. The geometries were fully searched based on the structures shown in Figures and 3, but no such BBIs were found for AuCl– on the MP2 level. However, we found some geometries with different dihedral angles in Au3Cl4– and Au4Cl5– clusters. For example, a planar structure of Au3Cl4– is shown in Figure . As shown in the figure, the Aua···Aub distance is 0.11 Å shorter than that in the most stable structure of 3a-1, and the energy difference between the two structures is only 2.96 kcal/mol. Similarly, Au4Cl5– also has a planar structure (Figure S1). In the structure, the Aua···Aub distance is 3.02 Å, and the angle between Aua and Aub is 78.28°. The energy difference between the two structures of Au4Cl5– increases to 3.98 kcal/mol. The softness of the ligand is significant for the aurophilic attraction.[16] Previous research has shown that the aurophilic attraction gets stronger as the ligand goes from hard (X = Cl) to soft (X = I).[13] Thus, it is suggested here that the aurophilic attraction would be stronger in gold-iodine clusters, AuI–. Considering the previously found BBIs of Au2I3–, it is reasonable to suggest that the isomers and geometries of gold-iodine clusters AuI– should be more interesting and diverse and need to be further investigated by both experimental methods and calculations.

Figure 5

Two geometries of Au3Cl4– and their relative energies are given in kcal mol–1.

To further evaluate the stability of AuCl– (n = 2–7) cluster anions, we have calculated their binding energies. The binding energies are calculated based on the formula, AuCl– + AuCl → AuCl–. As seen, Figure provides the binding energies of AuCl– (n = 2–7) cluster ions. The result shows that Au2Cl3– and Au3Cl4– have higher binding energies than others, which is consistent with the abundance of Au2Cl3– and Au3Cl4– observed in the mass spectrum. The difference between the binding energies of Au5Cl6–, Au6Cl7–, and Au7Cl8– decreases, indicating that the clusters have convergent structures and properties. Although only AuCl– (n ≤ 4) cluster ions were observed in our mass spectrum, it is believed that the larger cluster anions of Au5Cl6–, Au6Cl7–, and Au7Cl8– with zigzag structures can also be prepared under some appropriate experimental conditions, considering the strong aurophilic interactions.

Figure 6

Binding energy of AuCl– (n = 2–7) cluster ions.

Conclusions

In summary, cluster ions of AuCl– (n = 1–7) were examined by laser ablation mass spectrometry combined with theoretical calculations. Three major cluster ions of Au2Cl3–, Au3Cl4–, and Au4Cl5– were observed in the mass spectrum with high abundance. Theoretical calculations of AuCl– (n = 1–7) were performed to investigate their geometries on the MP2 level. The results show that most stable isomers for clusters of AuCl– (n = 2–7) have similar structures. All of them are characterized by a zigzag structure. Besides, the average Au(I)···Au(I) aurophilic interaction increases with the number of Au atoms and progressively stabilizes for large-sized cluster ions. The size effect on the aurophilic interactions between nonterminal gold atom pairs is different from the pairs including one terminal gold atom. For Au7Cl8–, all adjacent Au···Au distances (including terminal and nonterminal) stabilize at 3.06 Å, much shorter than the sum of the van der Waals radii of two Au atoms at 3.32 Å. According to the NBO analyses, strong relativistic effects lead to an increase in the 6s orbital population and a decrease in the 5d population for these clusters, and the progressively increased aurophilic interactions containing terminal gold atoms are also supported by the ELF calculations.

Methods

Experimental Section

All experiments were performed on a 7.0T FT ICR mass spectrometer (IonSpec, Varian Inc., Lake Forest, CA, USA) equipped with a ProMALDI ion source in the negative ion mode. A 355 nm Nd:YAG laser (Orion, New Wave) was used for the experiments. Typical laser energy is 3 mJ/pulse. The sample of AuCl4H was purchased from Energy Chemical and prepared in water at a concentration of 20 mg/mL just before the experiment. The sample of graphene was purchased from Timesnano Company and dispersed in acetone with a concentration of 1 mg/mL by sonication for 10 min prior to usage. Then, the two samples were separately applied onto the stain-less target spot using a two-layer method. In the method, 1 μL dispersion of graphene was applied on the sample spot first; after it dried up, 1 μL solution of AuCl4H was added on the same spot. The target was then dried in air and placed into the source region of the FT ICR mass spectrometer.

Computational

Geometry optimizations of AuCl– (n = 1–7) were carried out with the program Gaussian 09.[37] In consideration of the strong relativistic effects and electron correlations, theoretical calculations of AuCl– (n = 1–7) were performed on the MP2 level[38,39] with the Stuttgart energy-consistent relativistic pseudopotentials ECP60MDF for Au[40] and aug-cc-pVTZ for Cl.[41] All optimized structures were confirmed to be local minima structures by vibrational frequency analysis on the same level. The reported electronic energies were calculated at 0 K with zero-point energy corrections. To obtain the atomic charge distributions and electron configurations of Au and Cl in AuCl– (n = 1–7), NBO analyses were also performed with the program of Gaussian 09. Besides, ELF[42] was carried out to examine the chemical bonding using Multiwfn program.[36]