Geometrical Structures and Dissociation Channels of CuP2n + (n = 2-11): Studied by Mass Spectrometry and Theoretical Calculations.

Transition metal phosphorus cluster cations CuP2n + (2 ≤ n ≤ 11) were studied by laser ablation mass spectrometry and collision-induced dissociation (CID). The magic-numbered cluster ion of CuP8 + was identified experimentally, and cluster ions of CuP14 + and CuP18 + were also found to be generated with high abundance. CID results show that the dissociation channels of CuP2n + (n = 4 and 6-10) are all characterized by the loss of the P4 unit. Theoretical calculations combining global minima searching with the basin-hopping method and density functional theory (DFT) optimizations were performed for these clusters. Among them, the magic-numbered cluster CuP8 + was characterized by a D2d symmetry, with the Cu atom bridging two P4 units. The most stable isomer of CuP14 + was found to be characterized by a C2v symmetry. Calculations also reflect that the dissociation channels of the loss of the P4 unit are more energetically favorable than those of the loss of the P2 unit for CuP2n + (n = 4 and 6-10), which are in good consistent with the experimental results.

Introduction

Due to their remarkable structural characteristics and possible applications, binary cluster ions composed of transition metal and non-metal elements have attracted much attention.[1,2] Among them, related phosphorus clusters doped with transition metals have been studied by laser ablation mass spectrometry for a long time.[3−10] For example, Han et al. have detected chromium/phosphorus binary cluster ions with time-of-flight (TOF) mass spectrometry by laser ablation on a tablet of well-mixed chromium and red phosphorus powder.[7] Among the observed ions, the peaks of CrP4+ and CrP8+ were especially prominent, and the odd–even oscillation in the intensity of the CrP+ series was observed. They have also generated binary cluster ions composed of Fe/P, Co/P, and Ni/P by laser ablation of the mixtures of metal powders and red phosphorus.[8] Abundant peaks of MP2+, M2P4+, and MP8+ (M = Fe, Co, Ni, n = 2–4) were observed. The odd–even oscillation rule was observed too. Further photodissociation experiments indicated that, for those clusters, the main channels are the loss of the units of P2 or P4.[7] For Ti/P and Mn/P binary cluster ions, similar results have been observed.[6]

On the other hand, to better understand these results, theoretical calculations have been also applied.[11−17] The geometrical structures and dissociation channels of MP2+ (M = Fe, Co, Ni, n = 1–4) have been studied by Kuang et al.(12) It has been found that the lowest energy structures of these cluster ions are constructed by a two- or four-fold M atom with P4 or P2 units. For clusters of same sizes but with different metals, their most stable structures are still different in many cases. Other clusters, including MnP+ (m = 2–8) and TiP+ (m = 2–4, 6), have been also studied.[11,13,14]

However, for some reason, the binary cluster ions composed of Cu/P have not been studied by now. For theoretical studies, due to their structural diversity and complexity, larger-sized transition metal phosphorus clusters MP+ (m > 8) have been rarely studied.[17] Thus, in the present work, we investigate the formation of CuP2+ with the method of laser ablation mass spectrometry using a Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer. The tandem mass spectrometric method has been also applied to study the dissociation channels of selected cluster ions. The global structural minima of these clusters and relative dissociation pathways have been theoretically studied too.

Results and Discussion

Figure a and Figure b show the laser ablation mass spectra of the samples of CuCl/P and CuCl2/P, respectively. Both spectra are quite similar in the distributions of the identified species. Besides the strong signals of P2+ (5 ≤ n ≤ 12) that have been previously observed in the laser ablation mass spectrum of red phosphorus,[3,18] new cluster ions of CuP2+ (n = 4–10), Cu2ClP2+ (n = 4–8), and Cu3Cl2P2+ (n = 2, 4, 5, and 7) were clearly identified in both spectra. For the observed species of CuP2+ in both cases, the ion of CuP8+ has much higher intensities than its adjacent ions and thus can be considered as a magic-numbered cluster. Clusters of CuP14+ and CuP18+ also have relatively strong signal intensities in both experiments, although they are less prominent than the ion of CuP8+.

Figure 1

Laser ablation mass spectra of (a) CuCl/P and (b) CuCl2/P.

The collision-induced dissociation (CID) mass spectra of these ions (except the weak ion of CuP10+) were performed with the SORI method. The results are shown in Figure . Clearly, all the precursor ions CuP8+, CuP12+, CuP14+, CuP16+, CuP18+, and CuP20+ are characterized by the loss of the P4 unit. No product ions from other dissociation pathways were observed, even under different experimental collision conditions. The results are quite similar to the previously reported CID results of phosphorus cluster cations of P2+ (m = 6–11) but are different from those of P2–.[18,19]

Figure 2

SORI CID mass spectra of (a) CuP8+, (b) CuP12+, (c) CuP14+, (d) CuP16+, (e) CuP18+, and (f) CuP20+.

To better understand the results, theoretical calculations on CuP2+ (n = 2–11) have been performed. The three most energetically favorable structures of CuP2+ (n = 2–6) are shown in Figure (the other top isomers are shown in Figure S1). For CuP4+, the most stable isomer CuP4+-I has a C2v symmetry, with the two-coordinate copper atom connected with a folded P4 unit. The structure is very similar to the previously calculated most stable isomers of MnP4+ and CrP4+ but is quite different from those of FeP4+, CoP4+, NiP4+, and TiP4+. The energies of the second and third stable isomers of CuP4+-II (D2d) and CuP4+-III (C3v) are found to be 42.8 and 50.5 kJ/mol higher, respectively. The linear structure and pyramidal structures, which exists in the most stable isomer of FeP4+ and CoP4+, are considered too, and their relative energies are found to be 94.4 and 183.4 kJ/mol higher, respectively (Figure S1). The three most stable structures of CuP6+ can be derived from CuP4+-I by adding two phosphorus atoms on the other side of the copper atom while keeping the C2v symmetry. Interestingly, the structures of CuP6+-I, CuP6+-II, and CuP6+-III are found to be like those of the most stable isomers of MnP6+, CrP6+, CoP6+, NiP6+, TiP6+, and FeP6+.[11−14,17]

Figure 3

Three most stable structures of CuP2+ (n = 2–6) optimized at the level of B3LYP/6-311+G(d). Symmetry groups and relative energies in kJ/mol are shown under the structures.

For the magic cluster ion of CuP8+, the most stable isomer CuP8+-I is characterized by a D2d symmetry, with a four-coordinate Cu atom shared between two folded P4 units. The structure is very similar to those of MnP8+, CrP8+, and CoP8+ but is different from those of FeP8+ and NiP8+.[11,12] The second most stable isomer of CuP8+-II has a two-coordinate Cu atom shared between two pyramidal P4 units but with a relative energy of 61.1 kJ/mol higher. This structure is like the most stable structure of FeP8+ and the second most stable structure of NiP8+.[12] All the three structures of CuP8+ were further studied with the method of TPSS/def2-TZVP, and similar results have been obtained. The structure of CuP10+-I has a C3v symmetry and is similar to the third stable isomer of CuP8+-III. Other structures with close energies (2.2 and 8.0 kJ/mol higher) were also found. For CuP12+-I, a folded P4 unit and a P8 cuneane unit are connected by a four-coordinate Cu atom. The structure is also characterized by a C2v symmetry, which is 12.7 and 24.5 kJ/mol more stable than the asymmetry structures of CuP12+-II and CuP12+-III, respectively.

For larger clusters with n ≥ 7, the calculations are more difficult and time-consuming. Previous theoretical studies for such kinds of clusters are also insufficient. By increasing the numbers of the randomly built initial structures and program runs, the structures of CuP2+ (n = 7–11) were also studied. Figure shows the three most energetically favorable structures of these clusters (and Figure S1 also shows the other top isomers). Except for the most stable isomer of CuP14+, all these top three stable structures shown in Figure have no symmetry. The structure of CuP14+-I is characterized by its C2v symmetry, with energies of 1.5 and 22.5 kJ/mol more favorable than those of the second and the third most stable structures, respectively. If the magic number property of this ion shown in Figure is considered, then the suggested highly symmetrical structure of CuP14+-I is quite reasonable and easy to be accepted. Although more complicated structures have been observed for CuP2+ with larger sizes, the most stable isomers of CuP16+ and CuP18+ and the second most stable isomer of CuP20+ are still characterized by the same building block of the P4 unit. However, for CuP22+, a curved structure is more energy-favorable.

Figure 4

Three most stable structures of CuP2+ (n = 7–11) optimized at the level of B3LYP/6-311+G(d). Symmetry groups and relative energies in kJ/mol are shown under the structures.

For understanding the localization of the charge in the clusters, natural bond orbital (NBO) charge distribution analysis[20,21] was applied for the top three structures of CuP8+ and CuP18+. As shown in Figure S2, although the positive charge is mainly localized in the Cu atom in all cases, the P atoms have quite different charge distributions in different structures, reflecting the possible multiple roles of phosphorus atoms in the clusters.

Dissociation channels characterized by the loss of the P2 or P4 unit, which have been previously suggested for MP+ (M = Fe, Co, Ni) clusters,[12] were also theoretically considered here

To study which dissociation channel is more thermodynamically favored, the changes in enthalpy (ΔH) and free energy (ΔG) of both pathways were investigated for the ions of CuP2+ (4 ≤ n ≤ 11). For the P4 unit, the most stable tetrahedral isomer was applied here.[19] For these structures shown in Figures and 4, changes in enthalpies (ΔH) and Gibbs free energies (ΔG) for both dissociation pathways are shown in Figure a and Figure b, respectively. The ΔH values for both channels of clusters CuP8+, CuP14+, CuP16+, CuP18+, and CuP20+ are positive, indicating that these reaction processes are endothermic. The results shown in Figure also reflect the instabilities of the cluster ions of CuP10+ and CuP22+, which are in good consistent with the experimental results. Except for that of CuP10+, the P4 loss channels of all clusters have the lower values of both ΔH and ΔG. The priority of this dissociation channel is in good agreement with the experimental results. The differences between ΔH and ΔG of the two channels for CuP12+ are so large that they highlight the stability of the dissociation product—the magic-numbered cluster cation of CuP8+.

Figure 5

Changes in (a) enthalpy (ΔH) and (b) free energy (ΔG) of the two dissociation pathways for the ions of CuP2+ (4 ≤ n ≤ 11). The structures of corresponding ions used in calculation are shown in Figure .

Conclusions

The Cu/P binary cluster ions were generated by laser ablation on the mixed solid samples of red phosphorus and copper salts. Signals of CuP2+ (4 ≤ n ≤ 10) were observed for both salts of CuCl and CuCl2. Among them, the intensity of CuP8+ is much higher than those of its adjacent ions and thus can be identified as a magic-numbered cluster. Cluster ions of CuP14+ and CuP18+ were also generated with higher abundance than other ions. For cluster ions of CuP2+ (n = 4 and 6–10), their dissociation pathways are all clearly characterized by the only channel of the loss of the P4 unit. The most stable isomers of CuP2+ (2 ≤ n ≤ 11) were systematically searched with the self-developed basin-hopping program NKCS followed by the density functional theory (DFT) optimization and calculation on the level of B3LYP/6-311+G(d) for the selected structures. It is found that the magic-numbered cluster ion of CuP8+ is characterized by a D2d symmetry, with the Cu atom bridging two P4 units. The most stable isomers of CuP2+ (n = 2, 3, 6, and 7) are all characterized by a C2v symmetry and that of CuP10+ by a C3v symmetry. However, as the increase in the cluster size, the most stable isomers of CuP2+ with large sizes (n = 8–11) show no symmetry. It is also found that the dissociation channels with the loss of the P4 unit are more energetically favorable than those with the loss of the P2 unit for most of these observed cluster ions, especially the cluster CuP12+, which are in good consistent with the experimental results.

Methods

Experimental Section

Experiments were performed on a Bruker SolariX XR 7.0 T FT ICR mass spectrometer equipped with the commercial MALDI source without any modification. The red phosphorus sample was purchased from Alfa Aesar, and the samples of CuCl and CuCl2 were purchased from the Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). The fresh suspension of red phosphorus (2 mg/mL) was prepared in a mixed solution of methanol and water (1:1, v/v). Both aqueous solutions of CuCl and CuCl2 were prepared with a concentration of 20 mM. Then, 1 μL of red phosphorus and 1 μL of CuCl/CuCl2 were deposited and mixed onto a metal target and dried at room temperature before analysis. Mass spectra reported here were all measured in positive ion mode. Typically, mass spectra were acquired in the range of m/z 100–800. In the collision-induced dissociation (CID) experiments, the precursor ions of interest were pre-selected before entering the cell and reselected in the FT ICR cell. Sustained off-resonance irradiation (SORI) excitation of the selected isotopic peak was performed here.[22] In the processes, the target ions were excited by a waveform with a frequency that is typically 100–500 Hz off-resonance with their corresponding cyclotron frequency and were caused to undergo many acceleration/deceleration cycles and multiple collisions. In the experiments reported here, the SORI was set with a relative power of 0.8–1.4%.

Computations

To find the minima in their potential energy surfaces, a self-developed program named NKCS, which has been previously reported in detail, was applied here.[23] Briefly, initial structures of CuP2+ (1 ≤ n ≤ 11) were randomly generated and optimized with the geometry, frequency, noncovalent, extended tight binding (GFN-xTB) method[24] and then basin-hopping optimization was applied to search for the global structural minima. The initial population size for clusters with N atoms (after the reasonability and similarity check) is set as N2.3–2.8. The 50 most stable structures after basin-hopping were picked out for further optimization with DFT calculation at the level of B3LYP/6-31G(d) using the Gaussian 09 program.[25−28] To make sure of the results, the program has been run three and five times for clusters of CuP2+ with sizes satisfied 3 ≤ n ≤ 6 and 7 ≤ n ≤ 11, respectively. On the other hand, to prevent missing some important structures, some highly symmetrical structures were built manually followed by DFT optimization at the same level and comparison with the previously selected structures. Our previous studies on phosphorus clusters show that the method of B3LYP/6-311+G(d) can provide quite reasonable results.[18,19,23] Thus, the top 10 isomers for each cluster were picked out and finally optimized at this level. These structures were further verified by vibrational frequency analysis. For all structures, their electronic energies were calculated at 0 K with zero-point energy (ZPE) corrected, while their free energies were calculated at 298 K. For all the top three isomers of each cluster, both singlets and triplets were considered. It has been found that, in all cases, the singlets have the lower energies than their triplets. Thus, all the structures discussed below are only shown in their singlets. For further comparison, the DFT method of TPSS/def2-TZVP that has been previously suggested for the transition-metal complex geometrical study[29,30] has been also applied here for some of the clusters with small sizes.