Experimental Investigation of the Stability of Au_nCl_n+m⁻ (n = 1–5; m = 1, 3, 5, 7) Clusters by Laser Desorption/Ionization Mass Spectrometry

Abstract

The stability of gold chloride clusters is an important topic in catalysis and nanomaterials, but experimental data are missing. Here, fourteen different clusters were obtained simultaneously using laser desorption/ionization mass spectrometry and were identified as Au_nCl_n+m⁻ (n = 1–5; m = 1, 3, 5, 7) or AuCl_n+1⁻, Au₂Cl_2n+1⁻, Au₃Cl_2n+2⁻, Au₄Cl_2n+1⁻ and Au₅Cl_2n+2⁻. Consequently, the effects of laser intensity on their stability were evaluated, considering differences in the AuCl unit or the number of Cl atoms. For the Au_nCl_n+1⁻ and Au_nCl_n+3⁻ groups, the relative intensity of the clusters decreased with each additional AuCl unit as the laser intensity increased. Au_nCl_n+5⁻ clusters showed a different trend in relative intensities: Au₃Cl₈⁻ > Au₂Cl₇⁻ > Au₄Cl₉⁻ > Au₅Cl₁₀⁻. The mononuclear AuCl₄⁻ showed the highest stability, which is consistent with their “superhalogen” character. In the Au₂Cl_2n+1⁻ clusters, Au₂Cl₅⁻ with Au (III)–Au(I) interaction was more stable at lower laser intensities, while Au₂Cl₃ with Au(I)–Au(I) bonds became more dominant at higher intensities. Among the Au₃Cl_2n+2⁻, Au₄Cl_2n+1⁻ clusters, those with purely “aurophilic” interactions became increasingly stable with increasing laser intensity. These results emphasize the importance of bond type and cluster size for the stability of gold chloride clusters at different laser intensities.

1. Introduction

Understanding the chemical bonding and structure of gold chloride compounds and clusters plays an important role in many fields, such as nanoscience [1,2,3,4,5], environmental science, geological studies [6,7,8,9] and homogeneous and heterogeneous catalysis [10,11,12,13,14,15,16]. Barngrover and colleagues investigated the transformation of gold(III) chloride complexes into gold(I) thiolate species—a key step in the synthesis of thiolate-stabilized gold nanoparticles. Their work highlighted that the ligand exchange between chloride and thiolate ligands occurs with a low-activation energy barrier (~0.35 eV), facilitating the formation of gold chloride–thiolate or gold–thiolate clusters or nanoparticles from gold chloride precursors [2]. In a separate study, Davies and co-workers examined the catalytic behavior of gold supported on carbon (Au/C) in the hydrochlorination of acetylene—an important industrial process for the production of vinyl chloride monomers. Their results demonstrated that Au–Cl complexes play an essential role in catalytic activity [11]. Accordingly, additional insight into the stability of various gold chloride clusters can offer valuable information for understanding nanoparticle formation mechanisms and for designing catalysts with tailored properties.

In general, the chemistry of gold chloride differs from that of other related metals. Metal chloride complexes have mainly ionic properties, whereas gold-containing compounds have a pronounced covalent character. These differences are due to the strong relativistic effect that occurs in gold, leading to the stabilization of the outer 6s orbital and destabilization of the 5d orbitals, resulting in a decrease in the 6s–5d energy gap and an increase in s-d hybridization, which is responsible for the presence of covalent bonds between Au and Cl [17,18,19]. Gold chloride clusters also have an interesting structure and properties [20,21,22,23,24]. Theoretical studies have shown that the anions of the Au_nCl_n+1 cluster (n = 2–7), as the most stable isomers, have a planar zigzag structure characterized by an interesting interaction between the gold atoms. At the base of the cluster structure Au_nCl_n+1 for n > 2, there is a terminal Au···Au interaction (interaction between the terminal gold atom and the neighboring gold atom) and the interaction between two inner neighboring gold atoms (so-called non-terminal Au···Au interaction) [22]. These distances between the terminal and non-terminal gold atom are shorter than the sum of the van der Waals radii of two gold atoms, which is unexpected since the gold(I) centers have a closed-shell electronic configuration [5d¹⁰]. Therefore, this Au···Au interaction, whose energy lies between the van der Waals and covalent bonding, has been termed the “aurophilic interaction” [25]. An increase in n in Au_nCl_n+1 clusters leads to a stronger “aurophilic interaction” between neighboring gold atoms. It has also been shown that the “aurophilic interactions” in gold–iodine clusters (Au_nI_n+1) are stronger than in these gold–chlorine clusters [26]. The distances of Au···Au in the clusters of Au_nCl_n+3 (n = 3–7) are somewhat shorter than the corresponding distances in Au_nCl_n+1. The main structural difference between these clusters is the presence of the AuCl₄ unit, i.e., there is an Au (III) atom at the edge connected to four Cl atoms, which is not the case in the Au_nCl_n+1 cluster [21,22].

The relativistic effects can be decisive for the stability of the oxidation states of the 5d elements [27]. While the halogen chemistry of silver and copper is restricted to the oxidation states +I and +II, the oxidation states +I and +III are known for gold, while the divalent Au(II) occurs less frequently [28]. Gold can have different oxidation states in anionic, neutral and cationic complexes such as AuCl₂ (the valence of Au is +I in AuCl₂⁻, the valence of Au is +II for AuCl₂ and the valence of Au is +III for AuCl₂⁺). Since the oxidation states of Au can be increased by adding electronegative ligands such as chlorine (e.g., AuCl, AuCl₂ and AuCl₃), AuCl_n-type gold clusters (n = 1–6) were selected as a good prototype for exploring the maximum possible oxidation states of gold [24,29,30]. Theoretical studies have shown that the participation of d electrons in the bonding increases with the increase in Cl atoms, so +V is the highest possible oxidation state of Au in the AuCl_n clusters. It is also shown that with an increase in the number of Cl atoms above 3 in the AuCl_n clusters (n= 2–6), a delocalization of the additional electron over several Cl atoms occurs. The consequence of this electron delocalization is the fact that AuCl_n clusters (n ≥ 2) have an adiabatic electron affinity (EA) that is greater than the EA of Cl, so these clusters belong to the group of “superhalogens” [31,32,33]. It is interesting to note that the electron affinity of Au_2n clusters increases with the addition of Cl atoms. It should be emphasized that in the group of Au_2nCl clusters (n = 1−4), the electron affinity of Au₂Cl is higher than that of Cl, which is very unusual for a multimetal cluster belonging to a group of “superhalogens” [24].

Experimentally, gold chloride clusters are generally obtained in the gas phase using various types of mass spectrometers. Karataev et al., for example, used a high-resolution time-of-flight mass spectrometer with electron impact ionization (EI-TOF-MS) for their experiment [34]. A sample of HAuCl₄ (weighing 1–2 mg) was placed in a quartz crucible and heated at 100–120 °C in a tantalum furnace. In this way, the following gold clusters were recorded in the positive-mode mass spectrum: Au₂Cl⁺, Au₂Cl₂⁺, Au₂Cl₃⁺, Au₂Cl₄⁺, Au₂Cl₆⁺. Lemke et al. have presented that the mononuclear clusters of the types [AuCl₂]⁺(H₂O)_n (n = 0–4), [AuOHCl]⁺ (H₂O)_n (n = 0–1) and [AuCl₂]⁺(HCl)₂(H₂O)_n (n = 0–4) and the dinuclear [Au₂Cl_5-xOH_x]⁺(H₂O)_n (x = 0–1) can be obtained by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The sample was an aqueous AuCl₃ solution (concentration 5–50 mM). The results showed that with increasing AuCl₃ concentration, the abundance of the dinuclear gold–chloride cluster fraction increases, especially [Au₂Cl₅]⁺(H₂O)_n [35]. Ma et al. used a matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometer (MALDI-FTICR-MS) in their study [23]. The sample was HAuCl₄ at a concentration of 2 mg/mL prepared in water, while the graphene was a matrix at a concentration of 1 mg/mL dispersed in acetone. First, 1 μL of the graphene dispersion was applied to the stainless target spot, and after drying, 1 μL of the HAuCl₄ solution was added to the same spot. The mass spectrum was generated using a Nd:YAG laser with typical laser energy of 3 mJ/pulse and a wavelength of 355 nm. In contrast to the two previous cases, the mass spectrum here was recorded in negative ion mode. In this mass spectrum, three main peaks corresponding to Au₃Cl₂⁻ > Au₄Cl⁻ > Au₂Cl₃⁻ clusters and low-intensity peaks of Au₅⁻, Au₃⁻, Au₇⁻~Au₆Cl⁻, Au₈Cl⁻, Au₅Cl₂⁻ and Au₄Cl₃⁻ were detected. The researchers in the same group have shown that when the HAuCl₄ concentration was increased to 20 mg/mL and the laser energy was increased by 10%, the cluster anions Au₃Cl₄⁻ > Au₂Cl₃⁻ > Au₄Cl₅⁻ could be identified. In this case, the cluster anions Au_nCl_n−1⁻ and Au_nCl_nH⁻ were also found with low intensity [22].

It is worth noting that previous studies reported only selected gold chloride clusters with low intensity and insufficient stability for detailed analysis, which made systematic classification difficult. Although previous work showed the detection of Au_nCl_n+1⁻, Au_nCl_n+3⁻ and Au_nCl_n+5⁻, (n = 2–4) by LDI-TOF-MS without graphene, the focus was mainly on theoretical considerations [21]. In contrast, this study provides the first experimental insight into how cluster size—by the sequential addition of AuCl or Cl units—affects stability. By varying the laser intensity, we systematically investigated the relative stabilities of the simultaneously formed clusters. These results provide valuable insights into the stability and transformation of gold chloride clusters and help researchers evaluate their role in relevant systems. The results were also compared with previous mass spectra to assess the influence of graphene and with theoretical data to better understand the stability of clusters.

2. Results and Discussion

A typical LDI mass spectrum of HAuCl₄ in the positive mode is shown in Figure 1.

For clarity, the representative LDI mass spectrum of HAuCl₄ in the negative mode is divided into four parts—m/z 260–360, m/z 480–700, m/z 700–930 and m/z 930–1500—and shown in Figure 2. The theoretical isotopology of the assumed stoichiometry of the gold chloride clusters is shown in Figure 2 next to the corresponding peak groups.

For an easier comparison of the results obtained in this work with the corresponding results available in the literature,

Table 1. Ions identified in previous work using EI-TOF-MS [34] and MALDI- FTICR-MS (at a concentration of 20 mg/mL and 2 mg/mL) of HAuCl₄ [22,23].

Mass Spectrometry Methods	Ions	Ions	Ions	Ions	Ions
The positive-mode EI-TOF-MS	/	AunCl+ n = 1, 2 AuCl, Au2Cl+	Au2Cln+1+ n = 2, 3, 5 Au2Cl3+ Au2Cl4+ Au2Cl6+	/	/
The negative-mode MALDI- FTICR-MS 20 mg/L	/	/	AunCln+1− n = 1−4 AuCl2− Au2Cl3− Au3Cl4− Au4C5−	AunCln−1− n = 2−4 Au2Cl− Au3Cl2− Au4Cl3−	AunClnH− n = 2−4 Au2Cl2H− Au3Cl3−
The negative-mode MALDI- FTICR-MS 2 mg/mL	Aun− n = 2−9 Au3− Au5− Au7− Au9−	AunCl− n = 2, 4, 6, 8 Au2Cl− Au4Cl− Au6Cl− Au8Cl−	AunCln+1− n = 2 Au2Cl3−	AunCln−1− n = 3, 4 Au3Cl2− Au4Cl3−	/

lists the ions obtained by other authors from the EI-TOF-MS and MALDI-FTICR mass spectra of HAuCl₄. It should be noted that Table 1 does not include the ESI-FTICR-MS [35] and LDI-MS results (at a concentration of 2.0–0.025 mg/mL) for HAuCl₄ [36] because both studies detected hydrated gold chloride clusters, which are not the subject of this study.

In this study, the positive mode of the LDI mass spectrum of HAuCl₄ contains three peaks at m/z 197, 394 and 591, which are identified as Au⁺, Au₂⁺ and Au₃⁺, respectively (Figure 1). In the laser energy range of 1100–2000 a.u., the individual intensities of the Au_n⁺ clusters (n = 1, 2, 3) decreased with increasing laser energy, while the ratio of their relative intensities was Au⁺ > Au₂⁺ > Au₃⁺.

The comparison of the results in Figure 1 with the results from the literature (Table 1) shows that the choice of ionization method influences the type of clusters detected. For example, the LDI-MS method in positive mode favors the formation of Au_n⁺ clusters (n = 2, 3), while EI-TOF-MS favors the formation of Au_nCl⁺ and Au₂Cl_n+1⁺ cations from the same sample [34].

In the negative mode of the LDI mass spectrum of HAuCl₄ (Figure 2), the following clusters were identified: a “superhalogen” mononuclear cluster of type AuCl_n+1⁻ (n = 1, 2, 3) in the first part of Figure 2a (m/z 260–360); a dinuclear cluster of type Au₂Cl_2n+1⁻ in the second part of Figure 2b (m/z 480–700); a trinuclear cluster of type Au₃Cl_2n+2⁻ in the third part of Figure 2c (m/z 700–930); and a tetranuclear cluster of type Au₄Cl_2n+1⁻ and pentanuclear Au₅Cl_2n+2⁻ clusters in the fourth part of Figure 2d (m/z 930–1500). These clusters were detected in the laser intensity range from 1200 to 2600 a.u.

Comparing the available results for the positive and negative modes from Table 1, it can be seen that mononuclear and dinuclear clusters can be obtained in the positive-mode EI-TOF-MS [34], while mononuclear, dinuclear, trinuclear and tetranuclear gold chloride clusters were detected in the negative-mode LDI-TOF-MS. It should also be noted that clusters with two gold atoms and an even number of Cl atoms (Au₂Cl₄⁺, Au₂Cl₆⁺) were detected in the positive EI-TOF-MS mode, while dinuclear gold clusters with an odd number of Cl atoms (Au₂Cl₅⁻, Au₂Cl₇⁻) are stable in the negative LDI-TOF-MS mode. Interestingly, the dinuclear Au₂Cl₃ cluster is stable in both positive and negative modes.

The influence of graphene on the type of the gold chloride clusters can be observed by comparing the results of the LDI and MALDI methods in negative mode (Figure 2 and Table 1). With the MALDI method, clusters of type Au_nCl (n = 2, 4, 6, 8), Au_n (n = 1–9) and Au_nCl_n−1⁻ (n = 3, 4) were detected (the matrix was graphene and the concentration of HAuCl₄ was 2 mg/mL, Table 1). The mentioned clusters were not found in the LDI mass spectrum (without graphene and the concentration of HAuCl₄ was 2.5 mg/mL, Figure 2).

On the other hand, as mentioned above, the anions Au_nCl_n+1, Au_nCl_n+3 and Au_nCl_n+5 were detected in the LDI mass spectrum (without graphene, the concentration of HAuCl₄ was 2.5 mg/mL, Table 1 and Figure 2). However, Au_nCl_n+1⁻, Au_nCl_n−1⁻, Au_nCl_nH⁻ anions (n = 1–4) were detected in the MALDI mass spectrum and at a much higher concentration (the concentration of HAuCl₄ was 20 mg/mL, Table 1) than in the LDI method.

These indicates that in the negative mode, the presence of graphene is an important factor affecting the stability of some clusters, such as Au_n⁻ and Au_nCl⁻, Au_nCl_n⁻ and AuCl_n+1⁻, while graphene suppresses the formation of the anions AuCl_n+1⁻, AuCl_n+3⁻ and AuCl_n+5⁻.

To avoid confusion, it must be emphasized that the clusters identified in Figure 2 can be grouped in two ways: as Au_nCl_n+m⁻ (n = 2–5, m = 1, 3, 5, 7), i.e., as clusters of type AuCl_n+1⁻, AuCl_n+3⁻ and AuCl_n+5⁻ and Au_nCl_n+7⁻ (as shown in previous work, which is consistent with previous theoretical results), or as mononuclear, dinuclear, trinuclear, tetranuclear and pentanuclear gold chloride clusters. In accordance with the above, two issues are discussed below: (1) How the increase in Au_nCl_n+1, Au_nCl_n+3 and Au_nCl_n+5 clusters for the AuCl unit affects its stability in the range of laser intensity from 1200 to 2600 a.u.; (2) How the increase in the number of chlorine atoms affects the stability of AuCl_n+1⁻, Au₂Cl_2n+1⁻, Au₃Cl_2n+2⁻, Au₄Cl_2n+1⁻ and Au₅Cl_2n+2⁻ clusters due to the increase in laser intensity.

Under our experimental conditions, the Au₅Cl₁₀⁻ and Au₅Cl₁₂⁻ (pentanuclear Au₅Cl_2n+2 clusters) were of low intensity (Figure 2d) and could therefore not be included in further investigations. The dependence of the relative intensity of the most abundant isotope vs. the laser intensity (in the range of 1200 do 2600 a.u.) for AuCl_n+1⁻, AuCl_n+3⁻ and AuCl_n+5⁻ clusters is presented in Figure 3a, b and c, respectively.

The relative intensity of the Au_nCl_n+1⁻ (n = 2–4) cluster changes in a similar way with increasing laser intensity (Figure 3a). This can be explained by the fact that all clusters of the group Au_nCl_n+1⁻ have similar zigzag structures with the characteristic chemical bond Au(I)-A(I). In this case, the Au₂Cl₃⁻ cluster shows the highest stability in the observed range of laser intensity, which is consistent with previous experimental results [22]. Results have shown that increasing the Au₂Cl₃ cluster by one AuCl unit leads to a small decrease in stability, and with the addition of another AuCl unit, the stability decreases significantly (Figure 3a). Therefore, the ratio of the relative intensities of Au_nCl_n+1⁻ clusters was Au₂Cl₃⁻ > Au₃Cl₄⁻ > Au₄Cl₅⁻ and does not change with the change in laser intensity. This is consistent with theoretical calculations of their binding energies based on the formula Au_n−1Cl_n⁻ + AuCl = AuCl_n+1, where the binding energies decrease in the series Au₂Cl₃ > Au₃Cl₄ > Au₄Cl₅ [22]. In our experimental conditions, we have shown that the Au₂Cl₃ clusters without “aurophilic” interaction are more stable than Au₃Cl₄ and Au₄Cl₅ clusters with “aurophilic” interaction.

The relative intensity of the Au_nCl_n+3⁻ clusters changes in the same way with increasing laser intensity, as in the previous case (Figure 3b). The most stable cluster in this group is the Au₂Cl₅ cluster, which can be described as a fusion of the structural units of AuCl₄⁻ and AuCl₂⁻ by the elimination of a Cl atom, without “aurophilic” Au(I)-Au(I) interaction [21]. Also in this case, similar to Au_nCl_n+1⁻, the stability of the cluster decreases with an increasing number of AuCl units, so the Au₄Cl₇⁻ cluster has the lowest intensity and could not be detected at laser intensities of 2500 and 2600 a.u. (Figure 3b). In the laser intensity range from 1200 to 2400, the ratio of the relative intensities of the Au_nCl_n+3⁻-type clusters is the same, i.e., Au₂Cl₅⁻ > Au₃Cl₆⁻ > Au₄Cl₇⁻ (Figure 3b).

A comparison of the results in Figure 3a,b shows that increasing the Au_nCl_n+3⁻ cluster by one AuCl unit leads to greater instability than for the Au_nCl_n+1⁻ cluster. It should be noted that for the Au_nCl_n+1⁻ and Au_nCl_n+3⁻ clusters, the “aurophilic” interaction is present when n > 2 [21,22]. Accordingly, the experimental results indicate that the “aurophilic” Au(I)-Au(I) interaction has a more favorable effect on the stability of the Au_nCl_n+1⁻ clusters than the combination of Au(III)-Au(I) and an “aurophilic” Au(I)-Au(I) interaction in the Au_nCl_n+3⁻ cluster.

The Au_nCl_n+5⁻ clusters were detected in a slightly narrower range of laser intensity from 1200 to 2400 (Figure 3c), indicating their lower stability compared to Au_nCl_n+1⁻ and Au_nCl_n+3⁻ (Figure 3a,b). However, it should be noted that the relative intensity of the Au₂Cl₇⁻ and Au₃Cl₈⁻ clusters was more than 60% in the observed laser intensity ranges. It is interesting to note that four clusters (Au₂Cl₇⁻, Au₃Cl₈⁻, Au₄Cl₉⁻ and Au₅Cl₁₀⁻) were identified in the Au_nCl_n+5⁻ cluster group, in contrast to the previous two groups where three clusters were identified (Figure 2d). The Au₅Cl₁₀⁻ cluster was of low intensity; its relative intensity was between 6 and 10%, but like the other Au_nCl_n+5⁻ clusters, it was detected in the same laser range. In addition, the Au₃Cl₈⁻ cluster (the second largest cluster in this group) is more stable than A₂Cl₇⁻ and Au₄Cl₉⁻. However, it should be noted that the Au₂Cl⁻ and Au₃Cl₈⁻ clusters, which differ from AuCl, do not show a significant difference in relative intensity, as is the case for the Au_nCl_n+3⁻ clusters (Figure 3c). Unfortunately, the results of the theoretical studies on the structure of the Au_nCl_n+5⁻ clusters are missing in the literature, so we cannot compare the structure of the Au_nCl_n+5⁻ clusters with the other two groups.

The dependence of the relative intensity of the most abundant isotope of AuCl_n+1⁻, Au₂Cl_2n+1⁻, Au₃Cl_2n+2⁻ and Au₄Cl_2n+1⁻ on the laser intensity is shown in Figure 4a, b, c and d, respectively.

The theoretical studies of Srivastava and Misra have shown that the mononuclear AuCl_n species (n = 2–6) belong to the group of “superhalogens”, i.e., clusters whose electron affinity is higher than that of Cl (3.6 eV); therefore, it was expected that this type of cluster would dominate in the mass spectrum in the negative mode [24]. Under our experimental conditions, three “superhalogens” of AuCl_n+1⁻ (n = 1–3) were detected, but the relative intensity of the AuCl₃⁻ was very low in the observed laser intensity range (Figure 2a). For that reason, the dependence of the relative intensity of the most abundant isotope on the laser intensity for two “superhalogen” (AuCl₄⁻ and AuCl₂⁻) is shown in Figure 4a. In the laser intensity range from 1200 to 2400, the dominant anion is AuCl₄⁻. In this laser intensity range, the relative intensity of AuCl₄⁻ is significantly higher than the relative intensity of AuCl₂⁻. This is consistent with the fact that the electron affinity of AuCl₄ is higher than that of AuCl₂. However, with increasing laser intensity above 2400 a.u., the dominant ion in the mass spectrum is AuCl₂⁻. It should be kept in mind that the intensity of the ions in the negative mode is influenced not only by the electron affinity but also by the dissociation energy of the observed clusters. Theoretical studies also show that the dissociation energies of the AuCl_n anion for the Cl atom and the Cl₂ molecule exhibit a similar trend, decreasing in the order AuCl₂ > AuCl₄ > AuCl₃ > AuCl₅ > AuCl₆. The dissociation energy for the Cl atom and the Cl₂ molecule is low for the AuCl₅, which, despite having the highest EA in the AuCl_n series, was not detected in the mass spectrum, and nor was AuCl₆. The structure of the AuCl₅ and AuCl₆ ions is described as (AuCl₄)Cl_n [24]. Theoretical studies by Xu and others have also shown that the structure of the Au_nCl_n+3 cluster is such that it contains an Au(III) ion surrounded by four Cl atoms. Therefore, the AuCl₄ ion could have been formed by the dissociation of the above clusters. It should be mentioned that the Au_nCl_n+1 clusters have a zigzag structure (without an AuCl₄ unit), which could be one of the reasons why the AuCl₄ ion was not detected in the previous work [22]. Regardless of the origin of the “superhalogens” AuCl₂ and AuCl₄, their stability under these experimental conditions is more significant than the stability of the other detected clusters.

Theoretical studies have shown that there is no “aurophilic” interaction in the most stable isomers of the dinuclear Au₂Cl_2n+1⁻ clusters: Au₂Cl₃⁻ and Au₂Cl₅⁻. The Au(III)-Au(I) interaction (3.99A) in the Au₂Cl₅⁻ cluster ensures that this cluster is more stable than Au₂Cl₃⁻ with Au(I)-Au(I) interaction (3.69A) in the laser intensity range from 1300 to 2000 a.u. (Figure 4b). With the increase in laser intensity above 2000 a.u., the intensities of Au₂Cl₃⁻ and Au₂Cl₅⁻ clusters are very similar, while above 2400 a.u., the Au₂Cl₃⁻ cluster is much more stable than Au₂Cl₅⁻ and Au₂Cl₇⁻. Increasing the number of chlorine atoms in the dinuclear clusters does not significantly affect the stability, so the intensity of the Au₂Cl₇ cluster is higher than that of the Au₂Cl₃⁻ cluster in the range from 1300 to 1900 and at 2300 a.u., but above 2400 a.u., the Au₂Cl₇⁻ cluster is not recorded (Figure 4b).

In the group of Au₃Cl_n+1⁻ clusters, the cluster with the largest number of chlorine atoms, Au₃Cl₈⁻, shows the highest stability in the range of laser intensity from 1200 a.u. to 2400 a.u.; above 2400 a.u., this cluster was not detected (Figure 4c). The calculated geometrical parameters for the most stable structures available in the literature show that the Au₃Cl₆⁻ structure is formed by Au(III)-Au(I) interaction and “aurophilic” Au(I)-Au(I) interaction [21]. This structure is more stable than the structure of the Au₃Cl₄⁻ cluster, which is only formed by “aurophilic” interactions in the laser intensity range from 1300 to 2100 a.u. Above 2400 a.u., the intensity of Au₃Cl₆⁻ decreases, while the Au₃Cl₄⁻ cluster is detected at 2600 a.u.

The tetranuclear Au₄Cl_2n+1⁻ clusters exhibit lower stability than the dinuclear and trinuclear clusters in the observed range of laser intensity (Figure 4d). In the group of tetranuclear gold chloride clusters, the largest cluster, Au₄Cl₉⁻, is the most stable in the laser intensity range from 1200 to 2400 a.u. In this laser intensity range, the Au₄Cl₇⁻ cluster, which contains both Au(III)-Au(I) and thermal and non-thermal “aurophilic” Au(I)-Au(I) bonds, exhibits the lowest stability in the Au₄Cl_2n+1⁻ group. The cluster with only thermal and non-thermal “aurophilic” bonds, Au₄Cl₅⁻, is between Au₄Cl₉⁻ and Au₄Cl₇⁻ in intensity. However, it should be noted that the Au₄Cl₅⁻ cluster is the only one detected above the laser intensity of 2400 a.u., suggesting that the clusters with only “aurophilic” interactions exhibit greater stability than others with increasing laser intensity.

3. Experimental Section

All mass spectra in this work were recorded with a commercially available MALDI TOF mass spectrometer (Voyager-DE PRO Sciex, Foster City, CA, USA). Mass spectra were obtained without the use of conventional or other matrices, i.e., using the laser desorption ionization method. The device was equipped with an ultraviolet N₂ laser (wavelength 337 nm, pulse width 3 ns and repetition rates of 20.00 Hz). The number of laser shots was 300 per mass spectrum. The instrumental parameters were as follows: An acceleration voltage of 20,000 V, a grating voltage of 94% and a delayed extraction time of 100 ns. These mass spectrometers have a laser attenuator and a prism between the laser and the sample. The laser attenuator is a device for controlling the laser intensity, while the prism deflects the laser beam into the ion source. The laser attenuator is controlled by adjusting the laser level. The laser levels are controlled using the sliders on the manual laser control page. The slider has a range of 0–5000 arbitrary units (a.u.); therefore, this relative scale is used below for the laser intensity. The relative intensity of the cluster anions was determined using five measurements from five different positions. The sample was aqueous HAuCl₄ solutions (2.5 g/dm³) (99%, Sigma-Aldrich, St. Louis, MO, USA) prepared in deionized water (Millipore, Burlington, MA, USA) immediately prior to the experiment. An amount of 0.5 μL of the HAuCl₄ solution was applied to a spot on the stainless-steel plate. The sample was dried at room temperature and placed in the source area of the mass spectrometer.

4. Conclusions

Laser desorption/ionization mass spectrometry is a particularly useful method for the study of gold chloride clusters, as 14 clusters were identified in the LDI mass spectrum of HAuCl₄, which can be represented in two ways: as Au_nCl_n+m- (n = 1–5; m = 1, 3, 5, 7) or as AuCl_n+1⁻-, Au₂Cl_2n+1⁻-, Au₃Cl_2n+2⁻-, Au₄Cl_2n+1⁻- and Au₅Cl_2n+2⁻-type clusters. All these clusters were obtained simultaneously, so the effects of increasing the laser intensity on the stability of these clusters could be studied in two cases: when the clusters differed by AuCl unit and when they differed in the number of Cl atoms.

Clusters Au_nCl_n+1⁻ and Au_nCl_n+3⁻ were obtained in the range of laser intensity from 1200 to 2600 a.u., while Au_nCl_n+5⁻ clusters were demonstrated to be less stable than the other two groups of clusters as they were detected in a lower laser intensity range from 1200 to 2400 a.u. With increasing laser intensity, the ratio of the relative intensities of the clusters of groups Au_nCl_n+1⁻ and Au_nCl_n+3⁻ did not change and amounted to Au₂Cl₃⁻ > Au₃Cl₄⁻ > Au₄Cl₅⁻ and Au₂Cl₅⁻ > Au₃Cl₆⁻ > Au₄Cl₇⁻, respectively. This result indicates that increasing the size of clusters Au_nCl_n+1⁻ and Au_nCl_n+3⁻ by one AuCl unit leads to their decreasing stability. Furthermore, increasing the size of the cluster by one AuCl unit has a stronger effect on the stability of the Au_nCl_n+3⁻ cluster than on that of the Au_nCl_n+1⁻ cluster. This indicates that the “aurophilic” Au(I)-Au(I) interaction has a more favorable effect on the stability of the Au_nCl_n+1⁻ cluster than the combination of Au (III)-Au(I) and “aurophilic” Au(I)-Au(I) interaction in the Au_nCl_n+3⁻ cluster. The ratio of the relative intensities of the Au_nCl_n+5⁻ clusters was Au₃Cl₈⁻ > A₂Cl₇⁻ > Au₄Cl₉⁻ > Au₅Cl₁₀⁻, which is different from the previous two cases.

The stability of mononuclear AuCl₄⁻ and AuCl₂⁻ species is more significant than that of the other detected clusters. The result is consistent with the theoretically hypothesized “superhalogen” nature of chemical bonding and the calculated dissociation energy for AuCl₄⁻ and AuCl₂⁻.

If the gold chloride clusters are classified according to the number of gold atoms into di-, tri-, tetra- and pentanuclear clusters, the Au₂Cl₃⁻ and Au₂Cl₅⁻ clusters without “aurophilic” interactions exhibit the greatest stability. Au₂Cl₅⁻, which is stabilized by an Au(III)-Au(I) interaction, is more stable than Au₂Cl₃⁻ up to a laser intensity of 2000 a.u., while Au₂Cl₃⁻ (with Au(I)-Au(I) interaction) becomes more stable above 2400 a.u. Among the trinuclear Au₃Cl_2n+2⁻ clusters, Au₃Cl₈⁻ is the most stable from 1200 to 2400 a.u., while above 2400 a.u., the Au₃Cl₄⁻ with “aurophilic” interactions is the most stable. For the tetranuclear Au₄Cl_2n+1⁻ clusters, Au₄Cl₉⁻ shows the highest stability up to 2400 a.u., but above this intensity, only Au₄Cl₅⁻ (with purely “aurophilic” bonds) is detected. These results show that purely “aurophilic” bonds increase the stability of clusters at high laser intensities.

Increasing the number of chlorine atoms has a smaller effect on the stability of gold chloride clusters than increasing the size of the clusters for the AuCl unit.