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May 16, 2017 - La2PtC90 show that La2Pt@C2(99915)-C90 is the most stable isomer. Unlike other reported endohedral metal atoms, the entrapped Pt atom i...
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Encapsulation of Platinum in Fullerenes: Is That Possible? Lei Mu,† Shumei Yang,† Ruxia Feng,† and Xianglei Kong*,†,‡ †

The State Key Laboratory of Elemento-organic Chemistry and ‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China S Supporting Information *

novel EMFs. Our previous results showed that graphene can be used as a useful precursor to generate fullerene and EMF ions.29−33 Herein, we report the successful observation of EMF ions including Pt atoms in the laser ablation MS of graphene. Further comprehensive theoretical calculations are also performed to deduce the structure of La2Pt@C90 (La = lanthanum). These results indicate that the generation of heterotrimetallofullerenes might be more performable than that of mono- or homomultimetallofullerenes and may shed some light on the stability of some new EMF species. The EMF ions were produced by laser ablation of a mixture of graphene/LaCl3/PtCl4 on a metal plate and characterized by a Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer. Besides the abundant fullerene ions of C2n+ (n = 50−105), EMF ions of LamC2n+ (m = 1−3; n = 50−64) were also observed. Interestingly, the ions of LamPtC2n+ (m = 2−3; n = 43− 63) were also identified, as shown in Figure 1. Also, portions of

ABSTRACT: Whether transition metals can be entrapped inside fullerenes has remained unclear for a long time. Here mass spectrometric proof of entrapment of the group VIII transition-metal platinum (Pt) in fullerenes is first reported. Theoretical calculations on the example of La2PtC90 show that La2Pt@C2(99915)-C90 is the most stable isomer. Unlike other reported endohedral metal atoms, the entrapped Pt atom is negatively charged. This work provides valuable clues for the synthesis of some important missing endohedral metallofullerenes.

T

he existence of endohedral metallofullerenes (EMFs) was demonstrated by mass spectrometry (MS) soon after the discovery of C60 by Smalley et al.1 The macroscopic synthesis of EMFs was also achieved soon after the large-scale production of C60. Since then, great effort has been devoted to encaging many kinds of metal atoms and metallic clusters into fullerenes.2−17 The research for encapsulating atoms and molecules inside carbon (C) cages is driven by both the curiosity of scientists and their different properties and application potentials in materials and fundamental study. For example, some EMFs exhibit unique electronic properties that are induced by electron transfer from the encapsulated species to the fullerenes.18−20 By now, EMFs containing rare-earth metals and nearby elements (groups I−IV) have been widely studied, and many of them have been synthesized with the method of arc discharge.6−11 However, it is still unclear whether their corresponding EMFs can be formed for a large number of dblack transition metals, except some group IIIB and IVB ones. Recently, a group VB transition metal, vanadium (V), was successfully entrapped in a fullerene cage of C80 to form the novel V-containing EMFs of VxSc3−xN@C80 by Wei et al.21 However, entrapping transition metals in fullerene cages is still a great challenge. For example, platinum (Pt) is proven to be a versatile element with fascinating properties and has become of great importance in very diverse areas.22−24 So, a simple but quite challenging question is, can we encapsulate Pt atoms in fullerenes? This question begs us to recall the history of EMFs. It is wellknown that laser ablation MS played a very important role in the discovery of fullerenes and EMFs.24−28 However, this method has been neglected to a great extent in the synthesis of fullerenes and EMFs because of its low output and high cost. On the one hand, it is still unique because of its advantages in the research of the growth mechanism of fullerenes and EMFs.29−36 On the other hand, because EMFs can be readily characterized by MS, it surely can be applied as an effective screening method to find © 2017 American Chemical Society

Figure 1. Laser ablation MS spectrum of the mixture of graphene/ LaCl3/PtCl4.

such a MS spectrum are presented in Figure S2, in which the peaks of LamPtC2n+ were mingled with those of LamC2n+ but could be identified clearly according to their isotropic distributions. The MS spectrum in the low mass region (m/z 700−1400) was also obtained experimentally. As shown in Figure S1, no EMF ions of LamPtC2n+ with sizes smaller than 2n < 86 were observed. It should also be noted that the existence of LaCl3 in the mixture is important. If only PtCl4 was used with Received: February 19, 2017 Published: May 16, 2017 6035

DOI: 10.1021/acs.inorgchem.7b00161 Inorg. Chem. 2017, 56, 6035−6038

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Inorganic Chemistry

tures.43,44 However, in our experiment here, only fullerenes with even-numbered C atoms were observed, indicating that it was unlikely that these ions were of heterofullerene structure. In order to further verify this, the Pt atom was also inserted in several places of the EMF cage of La2@C2(99915)-C90, but optimization of all structures resulted in the isomer of La2Pt@ C2(99915)-C90 at last. To investigate the thermodynamic stability of the EMFs, the entropy effect and the relative concentrations of isomers based on equilibrium statistical analysis were performed. The temperature-relative concentrations of the calculated La2PtC90 isomers shown in Table 1 were carried out and are presented in Figure 2.

graphene, no metallofullerene ions were observed in the MS spectrum. Experimentally, the endohedral structures of these ions in the gas phase can be verified by a collision-activated dissociation or a gas-phase reaction test. To verify the endohedral structures of the observed ions here, oxygen gas was introduced to the FT ICR cell as the reaction gas after isolation of the corresponding peaks. Also, no reaction was observed between the metallofullerene ions and oxygen gas, indicating their endohedral structures (Figure S3). However, there are still two possible structural styles for LamPtC2n: metal carbide EMFs of LamPtC2@C2n−2 or classical trimetallofullerene of LamPt@C2n. In fact, considering strong repulsion among the metal cations in the cage, usually the chance for carbide cluser EMFs is thought to be larger than that for classical tri-EMFs, as shown in the example of [email protected] However, there are still some reported results about true triEMFs, such as Sm3@C80,38 which was verified experimentally by X-ray. In order to answer the question of which kind of isomer is more stable, La2PtC90 was selected here as an example to be investigated by theoretical calculations.34−42 Systematic calculations by means of density functional theory (DFT) methods were performed here. The relative energies and highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps of the most stable isomers of La2PtC90 obtained on the level of B3LYP/6-311g(d)∼LANL2DZ are listed in Table 1. Among them, the isomer with the lowest energy Table 1. Relative Energies and HOMO−LUMO Gaps (eV) of La2PtC90 Isomers Obtained with the Method of B3LYP/6311g(d)∼LANL2DZ spiral ID

IPR ID

PA

symmetry

ΔE(kcal/mol)

gap

C90-99915 C90-99916 C90-99912 C90-99913 C90-99914 C90-99054 C88-81738 C88-81735 C88-81720 C88-81729 C88-81736 C88-80982

43 44 40 41 42

0 0 0 0 0 1 0 0 0 0 0 1

C2 C2 C1 C2 C1 C1 D2 Cs Cs C1 C2 C1

0 8.13 8.46 12.10 18.85 22.08 89.45 97.54 100.39 106.32 116.82 110.40

1.28 1.13 1.45 1.17 1.12 1.04 1.43 1.01 0.85 1.14 1.06 0.91

35 32 17 26 33

Figure 2. Relative concentrations of La2PtC90 isomers.

At low temperature, the relative concentration of La2Pt@ C2(99915)-C90 is significantly higher than other isomers. With increasing temperature, it descends but remains the most important species. In the whole range of 0−5000 K, trimetallofullerenes of La2Pt@C90 dominate, and the contribution of metal carbide EMFs can be neglected. This result also means that if the EMFs can be generated by the typical electric arc method, the main product for La2PtC90 should be La2Pt@ C2(99915)-C90, and some product of La2Pt@C2(99916)-C90 may also be separated. The geometry structures of the two lowest-energy isomers of La2PtC90 are presented in Figure 3. The distance between two La atoms located on both sides of Pt is about 4.01 Å, and the La−Pt distance is ∼2.73 Å, indicating the existence of metal−metal

is La2Pt@C2(99915)-C90, with a HOMO−LUMO gap of 1.28 eV. Another isomer of La2Pt@C2(99916)-C90 has an energy 8.1 kcal/mol higher than that of the former. The most stable trimetallofullerene with a non-IPR cage is found to have an energy 22 kcal/mol higher than that of the most stable isomer of La2Pt@C2(99915)-C90. The energies of metal carbide EMFs are generally much higher than those of La2Pt@C90. The lowestenergy isomer of all of them, La2PtC2@D2(81738)-C88, has an energy 89.45 kcal/mol higher than that of La2Pt@C2(99915)C90. In order to verify the results, two different DFT methods of M06-2X39,40 and TPSSH41,42 were further applied to the top two isomers of LamPt@C90 and the most stable isomer of metal carbide EMFs of La2PtC2@C88, and the energy orders were the same (Table S2). The energies of their corresponding cations were also calculated and compared, and their energy orders were same (Table S1). Pt-containing heterofullerene cages were also considered here.43−45 Balch et al. observed ions of C59Pt+ by laser ablation MS and identified their metal-cage fullerene struc-

Figure 3. DFT-optimized molecular structures: (a) La2Pt@C2(99915)C90; (b) La2Pt@C2(99916)-C90. The C atoms of the fullerene cages are shown in gray, the La atoms in orange, and the Pt atoms in yellow. 6036

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bonding between La and Pt. In the geometry of La2Pt@ C2(99915)-C90, the La atoms stay in a hexagon with a La−C distance of 2.67−2.68 Å. Also, the Pt−C distance in the Ptcoordinated hexagon spans the range of 3.06 Å. For La2Pt@ C2(99916)-C90, the distance ranges of La−C and Pt−C are 2.65−2.69 and 3.40 Å, respectively. Compared with the corresponding empty cages, the cage C2(99915)-C90 is slightly elongated but the cage C2(99916)-C90 does not change much after encapsulation of the cluster (Table S3). Mulliken charge distributions of La2Pt@C2(99915)-C90 and La2Pt@C2(99916)-C90 are presented in Figure S4. For both structures, the C cages are negatively charged and the La atoms present a positive state. For La2Pt@C2(99915)-C90, the charges of the two La atoms is 1.815+ and 1.809+, respectively. Very interestingly, the Pt atom is negatively charged (with a charge of −0.635). This phenomenon is also discovered in other isomers of La2Pt@C90 (Table S3). As far as we know, this is the first example that the entrapped metal atoms can exhibit negative charge in cages. Also, this also can be used to explain why the trimetallofullerene is so stable because the Coulomb interaction can greatly enhance the metal−metal interactions and lower the charge state of the fullerene cages. The main frontier molecular orbitals of La2Pt@C2(99915)-C90 and La2Pt@C2(99916)-C90 and two empty cages with their HOMOs and LUMOs are calculated and shown in Figure S5. For both structures, their HOMO−LUMO gaps are enlarged with encapsulation of the La2Pt clusters. To better understand the intramolecular bonding, electron localization function analysis was done. Figure S8 shows the isosurface graph (left) and color-filled map (right) of La2Pt@ C2(99915)-C90 and La2Pt@C2(99916)-C90. It can be found that there are strong interaction between Pt and two La atoms and weak interaction between the La and C cages. Mayer bond order analysis also supports this conclusion; the bond order between Pt and La is about 0.92−0.93, and that of La and a nearby C atom on the cage is about 0.05−0.1. The bond order between two adjacent C atoms is about 1.18−1.47, while the ideal value of the double bonds is 2.0. Simulated IR and 13C NMR spectra of the main La2PtC90 isomers were also carried out and are shown in Figures S9 and S10. In summary, the heretofore unknown Pt-containing EMFs, LamPt@C2n (m = 2, 3), have been successfully observed in the MS spectrum, opening access to group VIII transition-metalentrapped fullerenes for the first time. A systematic theoretical investigation was performed on the EMF La2PtC90, and both trimetallofullerenes and carbide cluster fullerene structures have been taken into consideration. The isomer of La2Pt@C2(99915)C90 possesses the lowest relative energy, and a large HOMO− LUMO gap of 1.28 eV thermodynamically prevails at elevated temperature. Also, for metal carbide EMFs of La2PtC2@C88, their energies are much higher. Mulliken charge distribution revealed that, for the trimetallofullerenes of La2Pt@C90, La atoms are positively charged, while the Pt atom is negatively charged. The Coulomb interaction greatly lowers the negative charge state of the fullerene cages and stabilizes the whole EMFs. Considering the recent success in the preparation, separation, and characterization of very large EMFs, such as La2@D5(450)-C100,7 the preparation of such Pt-including EMFs is also performable in the future. On the other hand, the capture of an alloy cluster inside fullerene cages may open a door for the successful synthesis of other missing EMFs with transition-metal atoms inside.

Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00161. Materials, experirmental section, computational details, Figures S1−S10, and Tables S1−S9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.K.). ORCID

Xianglei Kong: 0000-0002-8736-6018 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 21475065 and 21627801) is gratefully acknowledged.

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REFERENCES

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