Pt2Cl82– Dimer Formation of [Bmim]2PtCl4 Ionic Liquid When

Article pubs.acs.org/JPCC

Pt2Cl82− Dimer Formation of [Bmim]2PtCl4 Ionic Liquid When Confined in Silica Nanopores Cheng Li, Yaxing Wang, Xiaojing Guo, Zheng Jiang, Fangling Jiang, Wenli Zhang, Wenfa Zhang, Haiying Fu, Hongjie Xu, and Guozhong Wu* Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China S Supporting Information *

ABSTRACT: [Bmim]2PtCl4 (Bmim: 1-butyl-3-methylimidazolium) ionic liquid was confined in porous SiO2 nanoparticles ([Bmim]2PtCl4@SiO2) with an average pore size of 7.1 nm. The structure of [Bmim]2PtCl4@SiO2 was investigated through X-ray diffraction and X-ray absorption fine structure (XAFS). The average coordination numbers of Pt species in [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2 were 4.0 and 5.0, respectively. Parts of Pt2+ in [Bmim]2PtCl4@SiO2 were oxidized to form high-valence Pt4+. The possible forms and structures of Pt in the ionic liquid were analyzed according to the coordination number. XAFS analysis and density functional theory calculations imply the favorable formation of dimer (i.e., Pt2Cl82−) in [Bmim]2PtCl4@SiO2, in which two bridge Cl atoms were connected to two Pt atoms. In addition, Pt2Cl82− contained mixed-valence platinum (Pt2+ and Pt4+). The results of differential scanning calorimetry also showed that the melting point of this nanoconfined ionic liquid increased. X-ray absorption fine structure (XAFS) analysis is a sensitive tool used in probing the components and structure of specific species.17,18 Zou et al.18 have employed XAFS to study the component and structure of ChCl-ZnCl2 ionic liquids at different x(ZnCl2). Jensen et al.17 have investigated the coordination environment of Sr atom during extraction through RTIL. In our previous work, we have found that IL nanoconfinement leads to a decrease in the distance between anion and cation and that the compression of ILs in nanopores of silica particles leads to an electron transfer from anion to cation through XAFS.19 The metal ion-containing ionic liquid [Bmim]2PtCl4 has been found as an active agent in catalysis.20 The confined [Bmim]2PtCl4 ILs also have some interesting characteristics. Knowledge of the actual structure of confined [Bmim]2PtCl4 ILs is of importance to understand their appealing characteristics. In this study, we employed XAFS to investigate the components and structure of [Bmim]2PtCl4 confined in porous SiO2 nanoparticles. [Bmim]2PtCl4 is a good probe molecule for the study of confined ILs because the XAFS spectra of Pt can provide sensitive atom-specific information regarding the surrounding electronic structure; moreover, its crystal structure has previously been measured.20 Without destroying the confined [Bmim]2PtCl4 ILs during the measurement, XAFS detects the actual structure of confined [Bmim]2PtCl4 ILs. In addition to XAFS, conventional methods,

1. INTRODUCTION The study of the behavior of confined molecules is of great importance because of their relevance in enhanced catalysis,1 ordered water structure,2 molecular mobility in cells and membranes,3 and controlled drug release.4 The properties and behavior of nanoconfined substances differ from those of bulk systems. Many interesting phenomena have been reported on the effects of nanoconfinement on changes in the melting point,5 crystal structure,6 and chemical reactivity7 of confined species. Ionic liquids (ILs) have received much attention because of their importance in a broad range of applications.8−12 Compared with evaporable water and organic molecules, ILs have an extremely low vapor pressure and relatively large molecular weights, enabling their intentional confinement in nanopores of silica under vacuum conditions. Ferretti et al.13 reported variation in the melting points of [BMI][TFSI] confined in silica-based ionogels and methylmodified ionogels. Kim et al.14 reported the melting point enhancement of imidazolium-based ILs confined in GMLs (graphene multilayers). The confined ILs have many appealing characteristics, such as excellent catalytic activities15 and high heat resistance.16Knowledge of the intrinsic structure of confined ILs is of paramount importance from the perspectives of both theory and practice. Up to now, the actual structure of confined ILs is barely understood as a result of the inherent experimental difficulties involved in the study of such. Further experimental exploration is expected to provide more insights into the structure of confined liquids. © 2014 American Chemical Society

Received: November 26, 2013 Revised: January 21, 2014 Published: January 27, 2014 3140

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including transmission electron microscopy and X-ray diffraction (XRD), were applied to measure the properties of ILs confined in porous silica nanoparticles.

2. EXPERIMENTAL SECTION Materials. The mesoporous silica (MCU-H, 99%) used in our investigation was purchased from Sigma. The average pore diameter of the sample was 7.1 nm. The [Bmim]2PtCl4 sample was prepared following the methods of Zhong et al.20 Experimental Details. In a manner similar to our previous procedure for a typical filling experiment,19 100 mg of SiO2 was placed in a two-necked flask (one of the necks was sealed by a rubber stopper, and the other was connected to a high-vacuum line). The flask was boiled on a gas burner for 4 h at 185 °C under ultrahigh vacuum conditions (1 × 10−5 Pa) to draw out the gas inside the SiO2. Then, 100 mg of [Bmim]2PtCl4 dissolved in acetonitrile was transferred into the flask through a syringe, and the mixture was ultrasonically vibrated for 6 h at 185 °C to fill the porous SiO2 with IL. The resulting mixture was cooled for 3 h at room temperature. The filled samples were separated from the mixture by centrifugation and were further purified through three cycles of washing with acetonitrile and filtration to completely remove the [Bmim]2PtCl4 adsorbed on the SiO2 surface. The as-obtained IL-filled samples are hereinafter referred to as [Bmim]2PtCl4@ SiO2. STEM Measurement. STEM observations were conducted using a JEOL JEM 2100F/HR (accelerating at 200 kV) microscope. The microscope was equipped with a superatmospheric thin-window X-ray detector. XRD Measurement. The XRD patterns were recorded using a Philips X-ray diffractometer (PW-1710) with a Cu Kα radiation ranging from 5 to 50 2θ. The XRD measurement was recorded at room temperature. XAFS Measurement. The X-ray absorption data of the Pt L3-edge (11.564 keV) of the samples were recorded at room temperature in transmission mode using ion chambers at the beamline BL14W1 of Shanghai Synchrotron Radiation Facility. The station was operated using a Si(111) double crystal monochromator. During the measurement, the synchrotron was operated at an energy of 3.5 GeV and a current between 150 and 210 mA. The photon energy was calibrated using the first inflection point of the Pt L3-edge in platinum metal foil. The data were analyzed using IFEFFIT.21 The fitting was performed in the R-space of the first shell of platinum (1.45− 2.9 Å) using FEFFIT. The k-space range was set from 2.93 to 10.6 Å−1. The theoretical backscattering phase and amplitude functions for the fitting were calculated using the FEFF 8.2 code.22

Figure 1. HAADF-STEM images and the corresponding EDX spectra of mesoporous SiO2 (A, C) and [Bmim]2PtCl4@SiO2 (B, D). The scale bar in (A) applies to both images.

spots in Figure 1B), and virtually no ionic liquid is seen on the outer surface of the SiO2 nanoparticle. From the energy dispersive X-ray(EDX) spectrum taken for the [Bmim]2PtCl4@ SiO2 (Figure 1D), the peaks at 2.62 and 9.44 keV can be assigned to Cl Kα and Pt Lα, respectively, confirming the existence of [Bmim]2PtCl4 within the SiO2 cavity. Structural Analysis of Confined IL. Figure 2 shows the XRD patterns of bulk [Bmim]2PtCl4 ionic liquids and

Figure 2. XRD patterns of [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2.

[Bmim]2PtCl4@SiO2 samples. Characteristic peaks are observed for both the pure [Bmim]2PtCl4 ionic liquid and the [Bmim]2PtCl4@SiO2 samples. As shown in Figure 2, the relative intensities of some of the peaks of the [Bmim]2PtCl4@ SiO2 sample are obviously different from the relative intensities of those of the neat ionic liquid. For example, the relative peak intensities of [Bmim]2PtCl4 are dramatically different at 2θ: 15.6°, 16.2°, 18.6°, 28.8°, 31.5°, 32.2°, and 36.8°. The XRD analysis indicates a critical change in the conformation or stacking of ionic liquid molecules after confinement in porous SiO2 nanoparticles. Figure 3 shows the XAFS spectra of Pt species in [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2. The XAFS spectra

3. RESULTS AND DISCUSSION Characterization of Confined IL. As previously reported, the air present in the cavities of SiO2 nanoparticles can be completely removed through evacuation under ultrahigh vacuum conditions, so that filling the cavities with IL becomes relatively easy.23 The [Bmim]2PtCl4 in the nanocomposites are observable in the atomic resolution high-angle annular darkfield (HAADF) images of the scanning electron probe STEM mode because of the existence of heavy-atom platinum in [Bmim]2PtCl4. As shown in Figure 1, the confined IL in the [Bmim]2PtCl4@SiO2 sample clearly reveals that some IL is encapsulated in the channel of SiO2(indicated by the white 3141

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Figure 3. Pt L3-edge XAFS spectra of Pt species.

of the two samples seem to be similar, indicating similar coordination shells and atoms around the Pt atom. Figure 4

Figure 5. Fitting results in R-space of Pt species in [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2.

electrostatic forces and geometric factors.25 Thus, nanoconfinement changes the structure of [Bmim]2PtCl4. This result is in good agreement with the XRD result. In [Bmim]2PtCl4, the coordination number of Pt species is 4.0. This result is in good agreement with that of Zhong et al.,20 who have found that the PtCl42− anion in [Bmim]2PtCl4 has a square-planar geometry and that the coordination number of Pt species is 4.0. In [Bmim]2PtCl4@SiO2, the coordination number of Pt species is 5.0. Nanoconfinement substantially influences the coordination number of Pt species. We presume that the anion form changed. The DSC result also indicated a change in the melting point. As shown in Figure 6, the [Bmim]2PtCl4 shows two melting peaks at 177 and 185.3 °C, whereas that of [Bmim]2PtCl4@SiO2 shows only one melting peak at 186.5 °C. This result is similar to that of Zou et al.,18 who have found that the melting point changes when the main Zn species changes. Estager et al.26 have investigated the structure of [C8mim]Cl-ZnCl2 ionic liquid at different χ(ZnCl2). At χZnCl2 > 0.33, they found that [ZnCl4]2− connects with ZnCl2 via the cleavage of the Zn−Cl bridging bond. As a result, [Zn2Cl6]2− dimer forms. Moreover, in the BMIC/FeCl3 system, for χFeCl3 > 0.5, [Fe2Cl7]2− with one Fe−Cl bridging bond can be detected.27 The assumption is that the dimer in [Bmim]2PtCl4@SiO2 forms based on the coordination number. The molar ratio of Pt:Cl is 1:4 in [Bmim]2PtCl4@SiO2. We speculate that the new Pt species is the Pt2Cl8x−, in which two bridge Cl atoms connect with two Pt atoms, so that the coordination number is 5.0. Unlike the [C8mim]Cl-ZnCl2 ionic liquid, the dimer between PtCl2 and [PtCl4]2− cannot form in a similar manner as in Lewis acidic metals (Zn, Cu, and Al) because PtCl2 is absent, as in our case. In nanoconfinement, the mechanism of the formation of the Pt2Cl8x− dimer is much different. The change in the white line peak intensity of confined IL can be interpreted as the variations in the electron density of the Pt atom in [Bmim]2PtCl4. The increase in the white line peak intensity is attributed to the loss of an electron in Pt2+.28 As shown in Figure 5, the white line peak intensity of Pt2+in [Bmim]2PtCl4@SiO2 increases as a result of the loss of an electron in Pt2+. Thus, parts of Pt2+ are oxidized to form high-valence Pt. (The valence is most likely +4 as a result of the

Figure 4. Pt L3-edge XANES spectra of Pt species in [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2.

shows the XANES spectra of Pt species in [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2 and provides detailed information on the changes of the Pt inner coordination sphere. At the Pt L3-edge, a resonance peak (white line) caused by the transition from 2p3/2 to vacant 5d states of the Pt atom is observed.24 The white line peak of [Bmim]2PtCl4@SiO2 is clearly higher than that of [Bmim]2PtCl4, indicating an obvious change in the coordination environment of Pt species in [Bmim]2PtCl4@ SiO2. In Figure 5, the fitting curves of inverse Fourier transform in R-space indicate that the fittings are acceptable. Table 1 shows the local structural parameters of [Bmim]2PtCl4 and [Bmim]2PtCl4@SiO2. In [Bmim]2PtCl4 and [Bmim]2PtCl4@ SiO2, the first shell Pt−Cl bond lengths are 2.33 and 2.31 Å, respectively. The much shorter Pt−Cl bond length in [Bmim]2PtCl4@SiO2 indicates that the IL is compressed when nanoconfined in silica nanoparticles. In our previous work, we have also found the compression of IL in nanoconfinement and the reduction of the distance between anion and cation.19 Indeed, nanoconfinement changes some of the geometric factors of the encapsulated [Bmim]2PtCl4. The structure of ILs results from a balance between long-range 3142

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Table 1. Local Structural Parameters for Pt Samples sample

shell

N

R (Å)

σ2

Rfactor

[Bmim]2PtCl4 [Bmim]2PtCl4@SiO2

Pt−Cl Pt−Cl

4.0 ± 0.4 5.0 ± 0.3

2.33 ± 0.01 2.31 ± 0.01

0.003 ± 0.001 0.001

0.0007 0.0003

4. CONCLUSIONS XAFS analysis was performed to investigate the species and local structures of Pt in [Bmim]2PtCl4@SiO2. By analyzing the coordination environment of Pt, we characterized the forms and structural parameters of the Pt species in [Bmim]2PtCl4@ SiO2 as Pt2Cl82−, in which two bridge Cl atom connects two Pt atoms.

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ASSOCIATED CONTENT

S Supporting Information *

Computational method and DFT calculations for Pt2Cl8x− (x = 2, 3, 4). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. DSC heating curves for [Bmim]2PtCl4.

stable Pt4+.) Dell’Amico et al.29 have prepared Pt2Cl102− from PtCl4 and PtCl62− at room temperature, indicating that the electron-rich Cl− easily forms acoordination bond with Pt4+. In nanoconfinement, the high-valence Ptx+(i.e., Pt4+) forms coordination bonds with the electron-rich Cl− in the adjacent PtCl42−. As a result, Pt2Cl8x− forms. To further understand the structure of Pt2Cl8x−, density functional theory (DFT) calculations were employed (see Supporting Information). Our DFT calculations found the lowest energy structure, C1 symmetry for Pt2Cl82−, as seen in Figure 7. Moreover, Pt2Cl83− and Pt2Cl84− were also calculated,

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China No. 11079007, 11005148, 20973192, and 10705046.

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REFERENCES

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Figure 7. Structures of the Pt species in [Bmim]2PtCl4@SiO2.

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