English

• INTRODUNTION

  Ligand-protected atomically precise metal clusters have attracted more and more attention due to their unusual physiochemical properties associated with their unique electronic structures, ^{[1, 2]} which have various potential applications in catalysis, ^[3-5] photoluminescence, ^[6-8] chirality, ^[9-11] magnetism, ^[12-14] bio-imaging, ^[15-17] chemical sensing, ^{[18, 19]} nanomedicine, ^{[20, 21]} energy and environment.^{[22, 23]} Due to the feature of well-defined structure and molecular composition, metal clusters have become ideal models to explore and comprehensively understand the surface science, structure-property correlations, growth mechanism and to realize rational design of complex nanomaterials at the atomic/molecular level.^{[1, 24-26]} As a molecular characterization technique, especially due to the characteristics of soft ionization, mass spectrometry (MS) with electrospray ionization (ESI) source^{[27, 28]} and matrix assisted laser desorption ionization (MALDI) source, ^{[29, 30]} which can ionize of clusters directly from solution and from solid state respectively, has shown the significant effectiveness to characterize the composition^{[31, 32]} and gives insights into their chemical behaviors of metal clusters, such as growth mechanism, ^{[25, 33, 34]} ligand exchange, ^{[35, 36]} fragmentation behaviors, ^[37] alloying and intercluster reactions.^{[38, 39]} However, because of the weak noncovalent interactions, such as coordination bonds and metal-metal bonds, metal clusters are easily decomposed during the ionization processes in MS analysis, and it is challenging to obtain intact molecular ion peaks of the clusters.^{[40, 41]} In 2008, Dass et al.^[42] reported the first MALDI spectra of intact ligand-protected Au nanoclusters (Au₂₅(SCH₂CH₂Ph)₁₈) by using DCTB as a matrix which assists ionization through electron transfer, and successfully suppressed the extensive fragmentation in previous experiments to detect intact molecular ions. A different approach was explored by Su and co-workers in 2016, ^[43] who applied two types of ESI-MS analyzers to compare the MS behaviors of metal clusters, and demonstrated that time-of-flight (TOF) analyzer is easier to gain the intact cluster information than ion trap (IT) analyzer, but the latter could also have a chance to keep whole clusters intact without fragmentation by carefully optimizing the buffer gas and radiofrequency amplitude. However, there are relatively few studies on the stability of metal clusters in mass spectrometry environments.

  In this contribution, a positive tetravalent bimetallic cluster [(C)Au₆Ag₂(L)₆]⁴⁺ (L = 2-(diphenylphosphino)-5-pyridinecarbox-aldehyde, with four "BF₄^-" as counter anions) is applied to compare the MS fragmentation behaviors by high resolution (HR)-ESI-TOF-MS, especially the molecular ion peaks of the cluster under different solvent environments. We optimized the crystal structure of the cluster by ab initio calculation based on the published crystallographic information file (cif).^[44] As shown in Figure 1, the cluster has a bicapped octahedral configuration

  Figure 1

  

  Figure 1.  Representative crystal structure of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster. (a) The full picture of the cluster structure, in which hydrogens are not displayed, and carbons and oxygens are displayed as a framework; (b) Structure of the (C)Au₆Ag₂ core; (c) Coordination bonds between the (C)Au₆Ag₂ core and one ligand L.

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  which could be viewed as the fusion of two Au₃Ag moieties sharing a carbon center (C). In a Au₃Ag moiety, each Au atom is connected to a L ligand by forming Au-P coordination bond, and the Ag atom is wrapped by three pyridyl groups of L ligands with Ag-N coordination bonds. Here we report a combined MS and theoretical work, finding that under given MS conditions, the fragmentation behaviors of this metal cluster are directly related to the relative strength of various chemical bonds.

  RESULTS AND DISCUSSION

  MS Behaviors of the Cluster in CH₃CN and CH₃CN+CH₃OH Solvents. Figure 2a is the HR-ESI-TOF MS signal of the cluster crystal dissolved in CH₃CN. There shows a group of dominant peaks at m/z around 1470.6, together with a low intensity but still visible peak at m/z around 1016.4, which can be well assigned to [(C)Au₆(L)₆]²⁺ and [(C)Au₆Ag(L)₆]³⁺, respectively. These two observed ions are the fragments of the parent ion [(C)Au₆Ag₂(L)₆]⁴⁺ via losing two or one Ag⁺. The intact cluster ion ([(C)Au₆Ag₂(L)₆]⁴⁺, m/z around 789.0) has not been detected in the spectrum. In contrast, as reported by Su et al., ^[43] the similar metal clusters can remain intact in the ESI-TOF-MS experiment using the solvent of CH₃OH. It should be noted that the clusters we studied here cannot dissolve well in CH₃OH. Although TOF is believed to be a "soft" analyzer, the results indicate the solvent also has a great relationship with the stability of the cluster under TOF-MS conditions.

  Figure 2

  

  Figure 2.  Mass spectra obtained by HR-ESI-TOF-MS of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster dissolved in (a) CH₃CN; (b) Mixed solvent of CH₃CN and CH₃OH in equal volume. (c) Enlarged details of (b) in the m/z range of 820 to 840, which show both experimental (black) and simulated (red) isotopic patterns of the molecular ion peaks of [(C)Au₆Ag₂(L)_m(L_α)_n]⁴⁺.

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  The above results show that the cluster dissolved in acetonitrile (CH₃CN) is prone to dissociate in the MS condition, leading the intact cluster ions to be not detected. However, when using a mixed solvent of CH₃CN and CH₃OH with equal volume, we obtained an obviously different mass spectrum, as shown in Figure 2b. Two sets of prominent ion peaks are clearly shown in the m/z range of 1000 to 1600. The set with a smaller m/z contains five positive trivalent ion peaks, and the interval between each adjacent peak is 10.7. Similarly, the set with a larger m/z involves six positive divalent ion peaks, with the interval to be 16.0. The above two sets of peaks are composed of two series of peaks with exactly the same mass difference of 32.0, corresponding to the molecular weight of CH₃OH. Thus, we believe the six L ligands containing aldehyde groups (L = RCHO, R = PPh₂C₅H₃N, as illustrated in Figure 2a) in the original cluster underwent varying degrees of hemiacetal reaction with the solvent molecule CH₃OH, and transformed to hemiacetal ligands L_α (L_α = RHC(OH)(OCH₃), as illustrated in Figure 2b). This structure was further confirmed by the infrared (IR) spectroscopy (see Figure S1). The IR peak at 1712 cm^-1 which represents the vibrational mode of C=O group almost disappears when CH₃OH is applied as the mixed solvent, implying the occurrence of hemiacetal reaction. Therefore, the two sets of peaks mentioned above can be assigned to [(C)Au₆Ag(L)_4~0(L_α)_2~6]³⁺ and [(C)Au₆(L)_5~0(L_α)_1~6]²⁺, respectively. One can also see in Figure 2b that, unlike the MS behavior using pure CH₃CN solvent, the MS peak intensity of trivalent ions increased significantly when using a mixed solvent of CH₃CN and CH₃OH, even slightly exceeding the peak intensity of divalent ions. More importantly, the intact cluster ion peaks have been observed in Figure 2b at m/z around 820 to 840. The enlarged details of the peaks are shown in Figure 2c, together with the well matched simulation results of these isotopic patterns. With the adding of CH₃OH into CH₃CN, the probability of Ag⁺ dissociation from the clusters has been greatly reduced under MS conditions. Although the intensity of these peaks is relatively low, the ability to detect complete molecular ion peaks is of great significance in MS characterizations.

  Hydrogen Bond Effect on the MS Behavior of Clusters. Whether Ag⁺ in the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster tends to dissociate under MS conditions depends largely on the type of solvent. As the commonly used solvent in cluster characterization, CH₃CN aggravates the dissociation of Ag⁺ in the cluster. However, with the participation of CH₃OH and formation of hydrogen bond with CH₃CN, the dissociation of Ag⁺ is greatly suppressed. A detailed mechanism will be discussed in the next section. Based on the above facts, here we propose a strategy to control the MS stability of metal clusters. Feasibility of this strategy has been further validated by another four solvents, by mixing CH₃CN with water (H₂O), ethylene glycol (C₂H₆O₂), acetone (C₃H₆O), and toluene (C₇H₈), respectively. Only the first two mixed solvent molecules can form hydrogen bonds with CH₃CN. Since ethylene glycol has a high viscosity, a mixing ratio of V(C₂H₆O₂): V(CH₃CN) = 1:4 is adopted in order to achieve a smooth MS testing. For other three types of mixed solvents, the two components are prepared in an equal volume. The mass spectra obtained under the same detection conditions are displayed in Figure 3. As we expected, after adding H₂O (Figure 3a) or C₂H₆O₂ (Figure 3b), the molecular ion peaks containing intact cluster structure "(C)Au₆Ag₂" can be successfully detected. Due to the occurrence of hemiacetal reactions, the original six L ligands were converted into the hemiacetal products L_β or L_γ to a certain extent (shown in the Figure for the molecular structures). In contrast, no molecular ion peaks were detected when C₃H₆O (Figure 3c) or C₇H₈ (Figure 3d) was added. These results further illustrate that the important influence of hydrogen bonds on the coordination bonds of metal clusters will lead to significant changes in their dissociation behaviors in MS.

  Figure 3

  

  Figure 3.  Mass spectra obtained by HR-ESI-TOF-MS of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster dissolved in mixed solvents of CH₃CN with (a) H₂O, (b) C₂H₆O₂, (c) C₃H₆O, and (d) C₇H₈ in equal volumes (except for C₂H₆O₂ with V(C₂H₆O₂): V(CH₃CN) = 1:4), respectively.

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  Analysis and Discussion. According to Figure 2, the apparent differences in dissociation behavior of the cluster in MS under two different solvents prompt us to reconsider the influence of the solvent molecules on the cluster structure. As shown in Figure 1b, the metal core of the cluster is a regular octahedron composed of six Au atoms, together with two Ag atoms on two opposite surfaces. Each Au atom is coordinated to the P atom of a ligand, and each Ag atom is coordinated with the N atoms on the three ligands. Here the strength of these chemical bonds was evaluated based on the relative level of molecular orbital energies. Figure 4a shows the strongest molecular orbital between Au and P atoms, which is bonded from $ {\text{5}\text{d}}_{{\text{z}}^{\text{2}}}\text{} $orbital of the Au atoms and 3s/3p orbitals of the P atoms. Figure 4b depicts the strongest molecular orbital between Ag and N atoms, formed through the 4d orbital of the Ag atom and the π orbital on the N-containing heterocyclic ring. Figure 4c shows a discrete orbital containing the bonding properties between the $ {\text{4}\text{d}}_{{\text{z}}^{\text{2}}} $ orbital of the Ag atom and 5d orbitals of the three Au atoms (See Figure S2 for the enlarged part of Ag and Au). The energy levels of these molecular orbitals are –0.481, –0.214 and –0.182 a.u., respectively, relative to the HOMO. It is obvious that the Au atoms are tightly bonded to the central C and surrounding P atoms, while Ag atoms are weakly bonded to the "(C)Au₆" core as well as the ligands. This explains why the dominate signals in Figure 2 correspond to cluster fragments with all six ligands, and only one or two Ag atoms are dissociated from the cluster. The acetonitrile molecules can promote the dissociation of Ag from the cluster because the coordination strength with Ag atoms is stronger than that between N and Ag in the cluster, which leads to the dissolution of Ag atoms in acetonitrile. Take the simplest case where only one CH₃CN molecule reacts with the cluster, a complex ion like {(C)Au₆Ag(L)₆[(N, N)-Ag-NCCH₃]}⁴⁺ may be generated, in which "(N, N)-Ag" represents that only two N atoms from two L ligands form coordination bonds with the Ag atom. In this complex, one of the Ag–N bonds between Ag and one ligand in the original cluster is broken and Ag–N bond between Ag and the attacking CH₃CN molecule is newly formed. The computed overall enthalpy for the formation of this complex is +0.48 kcal/mol (T_l = 298 K) in the acetonitrile solution. Although slightly endothermic, we can calculate from the Maxwell-Boltzmann rule that about 30% of the original clusters will exist in such a complex form in the thermodynamic equilibrium acetonitrile solution. In contrast, the above reaction has an enthalpy of –15.84 kcal/mol in the gas phase (T_g = 423 K), indicating a very favorable thermodynamic driving force in the MS condition. Furthermore, the complex is unstable in the mass spectrometer due to strong electric field, and will further loose the "(Ag-NCCH₃)⁺" fraction via Coulomb explosion, leaving the structurally stable [(C)Au₆Ag(L)₆]³⁺ component to be detected. Similarly, the other Ag⁺ can also be lost, resulting in the detected [(C)Au₆(L)₆]²⁺ signal as shown in Figure 2a. In fact, we successfully captured the fragment ion [Ag(L)(CH₃CN)]⁺ in MS experiments (see Figure S3), which should come from the dissociation of {(C)Au₆Ag(L)₆[(N, N)-Ag-NCCH₃]}⁴⁺ with binding one ligand from surrounding environment.

  Figure 4

  

  Figure 4.  Molecular orbitals and corresponding energy levels of the chemical bond between (a) Au–P, (b) Ag–N, and (c) Au–Ag. Hydrogens and phenyls are not displayed for clarity.

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  Hydrogen bond interactions between solvent molecules are believed to have a significant impact on the intact cluster information by MS. According to Figure 2b, after adding methanol solvent, the fragmentation tendency of the cluster was obviously suppressed, indicating that methanol molecules inhibited the coordination interaction between acetonitrile molecules and the cluster. The hydrogen bond strength between CH₃CN and CH₃OH is 5.02 kcal/mol from theoretical computations. Considering the CH₃CN…CH₃OH hydrogen bond breaking and the {(C)Au₆Ag(L)₆[(N, N)-Ag-NCCH₃]}⁴⁺ complex forming processes together, the chance of complex formation is reduced, and the intact cluster is more likely to be retained under the subsequent MS conditions.

  CONCLUSION

  In this work, we found that the metal cluster [(C)Au₆Ag₂(L)₆]⁴⁺ easily dissociated two Ag⁺ into [(C)Au₆(L)₆]²⁺ fragments during ESI-TOF-MS analysis, which has been proved by molecular orbital studies of the cluster finding the weak Au–Ag and Ag–N bonding interactions in the cluster. However, we can adjust the solvent composition, and further interfere the interaction between metal clusters and solvent molecules, to weaken the dissociation tendency of Ag ions from the cluster and to obtain the intact molecular ions in mass spectrometry. These results have contributed greatly to our deeper understanding into the structure and bonding of the noncovalent metal cluster system. Mass spectrometry shows powerful capabilities in studying metal clusters and their reactions.

  EXPERIMENTAL

  Materials and Chemicals. All reagents used in this work were commercially available and used without further purification. HPLC-grade acetonitrile and methanol were purchased from Spectrum Co. (New Brunswick, NJ, USA). Ethylene glycol (AR), acetone (AR), and toluene (AR) were bought from Sinopharm Chemical Reagent Co. (Shanghai, China). Ultrapure water was produced by laboratory water purification system.

  Sample Preparation. The metal cluster [(C)Au₆Ag₂(L)₆]⁴⁺ was prepared in a synthetic procedure according to the literature.^[44] The cluster was dissolved in CH₃CN to obtain a solution at 1×10^-5 mol/L (0.035 mg samples in 1 mL CH₃CN). As for mixed solvents, except for the 1:4 volume ratio of C₂H₆O₂ and CH₃CN, the others (CH₃OH, H₂O, C₃H₆O, C₇H₈) are mixed in equal volumes.

  Mass Spectrometric Analysis. The mass spectrometric analysis was performed on a high-resolution electrospray ionization time-of-flight mass spectrometer (6224, Agilent, Santa Clara, CA, USA) in the positive-ion mode. The instrument was calibrated with an ESI tuning mix (Agilent) before MS analysis. The sample was introduced by a syringe pump (KDS-100, KD Scientific, Holliston, MA, USA) at a rate of 20 μL/min. Typical parameters used for the measurements were as follows. Capillary voltage: 4.0 kV; Drying gas temp: 150 ℃; Drying gas flow: 4 L/min; Nebulizer pressure: 20 psig.

  Computational Method. The hydrogen bond energies, enthalpy change of the cluster dissociation process, and the molecular orbital levels of the cluster were computed using the Gaussian 09 Rev. D01 software.^[45] All computations were performed with the doubly hybrid density functional theory of XYGJ-OS.^[46] The LANL2DZ basis set for Au, Ag and 6-31G(d) basis set for O, C, H, P and N were used in geometry optimizations and harmonic frequencies, and def2-TZVP together with 6-311+G(d) basis set was used for transition metals and main group elements, respectively, in single point energy calculations.

  
  ACKNOWLEDGEMENTS: This work was supported by the National Natural Science Foundation of China (Nos. 91961107, 21827801, 21805231). We also thank Dr. Yang Yang (School of Chemistry and Materials Science, Jiangsu Normal University) for his assistance in sample preparation and Dr. Jun Chen (Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) for discussion in theoretical computations. The authors declare no competing interests.
  COMPETING INTERESTS
  http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0032
  For submission: https://mc03.manuscriptcentral.com/cjsc
  Supplementary information is available for this paper at
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  Figure 1  Representative crystal structure of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster. (a) The full picture of the cluster structure, in which hydrogens are not displayed, and carbons and oxygens are displayed as a framework; (b) Structure of the (C)Au₆Ag₂ core; (c) Coordination bonds between the (C)Au₆Ag₂ core and one ligand L.

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  Figure 2  Mass spectra obtained by HR-ESI-TOF-MS of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster dissolved in (a) CH₃CN; (b) Mixed solvent of CH₃CN and CH₃OH in equal volume. (c) Enlarged details of (b) in the m/z range of 820 to 840, which show both experimental (black) and simulated (red) isotopic patterns of the molecular ion peaks of [(C)Au₆Ag₂(L)_m(L_α)_n]⁴⁺.

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  Figure 3  Mass spectra obtained by HR-ESI-TOF-MS of the [(C)Au₆Ag₂(L)₆]⁴⁺ cluster dissolved in mixed solvents of CH₃CN with (a) H₂O, (b) C₂H₆O₂, (c) C₃H₆O, and (d) C₇H₈ in equal volumes (except for C₂H₆O₂ with V(C₂H₆O₂): V(CH₃CN) = 1:4), respectively.

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  Figure 4  Molecular orbitals and corresponding energy levels of the chemical bond between (a) Au–P, (b) Ag–N, and (c) Au–Ag. Hydrogens and phenyls are not displayed for clarity.

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