CHEN Ru-Ru YANG Wei-Guang CAO Min-Na② LI Hong-Fang②

a (College of Chemistry, Fuzhou University, Fuzhou 350002, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT A new supramolecular self-assembly [Li(H2O)2]2[(H2O@Me10CB[5])][PtCl6]·7H2O (1, Me10CB[5] = decamethylcucurbit[5]uril), has been successfully constructed with Me10CB[5] and [PtCl6]2- anion in the presence of lithium cation (Li+). Single-crystal X-ray diffraction study reveals that compound 1 crystallizes in monoclinic space group P21/n with a = 11.1681(18), b = 28.5425(4), c = 18.9342(3) ?, β = 99.0143(15)°, V = 5961.02(16) ?3, Z = 4, F(000) = 3224, μ = 2.714 mm-1, R = 0.0470 and wR = 0.1076 (I ＞ 2σ(I)). In the supramolecular self-assembly, two of the portal carbonyl oxygen atoms of Me10CB[5] are coordinated to Li+ cation, yielding a “half-open” molecular capsule. Then adjacent molecular capsules are connected with each other through hydrogen-bonding to form a one-dimensional (1D) supramolecular chain structure. The [PtCl6]2- anions are fixed on one side of the 1D supramolecular chain through supramolecular interactions. The thermal stability, electronic valence and morphology of compound 1 are also investigated.

Keywords: decamethylcucurbit[5]uril, noble metal, supramolecular self-assembly, crystal structure; DOI: 10.14102/j.cnki.0254-5861.2011-2440

1 INTRODUCTION

Cucurbit[n]urils are a family of molecular con- tainer hosts bearing a rigid hydrophobic cavity and two identical carbonyl fringed portals. They have attracted much attention in supramolecular chemi- stry because of their superior molecular recognition properties in aqueous media[1,2]. Decamethylcu- curbit[5]uril (Me10CB[5]) is one of the smallest members of cucurbit[n]urils family. Previous researches reveal that Me10CB[5] is environmentally benign with reasonable solubility in water, excellent chemical and thermal stability[3,4]. In addition, the smaller portal size of Me10CB[5] offers concentrated carbonyl oxygens, which have strong coordination ability with metal atoms asσdonor[5]. The increased electron-donating effect as a result of alkyl substi- tuents also contributes to the coordination of Me10CB[5] with metal ions[6].

Recently, a lot of Me10CB[5]-based single metal complexes have been successfully synthesized[7-9]. The first reported crystal structure is a Me10CB[5]-based “molecular bowl” with portal of carbonyl oxygen atomscovered by alkaline earth cation Ba2+[3]. Subsequently, a series of Me10CB[5]-based complexes with closed molecular capsules have also been obtained through the coor- dination between carbonyl oxygen atoms to alkali cation (Na+, K+, Rb+, Cs+)[10]. However, there is rare report about the supramolecular assemblies con- taining bimetal cations in the crystal structure based on Me10CB[5]. As we known, the self-assembly process is sensitive to the synthetic conditions, such as used solvent, the pH value and the ratio between metal and ligand[11-13]. Introducing the second metal cation into the synthetic procedure will greatly influence the coordination models. Therefore, it is interesting to design and synthesize specially engineered architectures with bimetal cations.

It is well known that noble metal species have been applied as highly efficient catalysts in a wide range of fields. However, the drawbacks including high cost and scarcity have limited their practical applications[14-17]. Therefore, it’s important to minimize the usage of noble metals and maximize the atom utilization efficiency[18,19]. Designing noble metals supramolecular self-assembly can regulate the distribution of noble metals from atoms level, which will increase their utilization efficiency[20]. However, there is rare report on the synthesis and application of supramolecular self-assembly con- structed with Me10CB[5] and noble metals. In 2013, our group synthesized five different supramolecular crystals using Me10CB[5] as organic ligands, while [PdCl4]2-anions and alkali metal cations as metal precursors, respectively[21]. But there is no report on the supramolecular assembly constructed by pla- tinum (Pt) with Me10CB[5] ligand. Pt metal complexes have many potential applications in catalysis, medicine and material design[14,22,23]. Therefore, the design and synthesis of Pt-Me10CB[5] crystalline hybrid materials has practical and academic significance.

Herein, a Pt-containing supramolecular assembly [Li(H2O)2]2[(H2O@Me10CB[5])][PtCl6]·7H2O has been successfully self-assembled from Me10CB[5] and the [PtCl6]2-anions in the presence of the Li+anions. X-ray crystal structural analyses show that the carbonyl oxygen atoms at the Me10CB[5] portals coordinate to Li+cations into “half-open” molecular capsules, which further connected through hydro- gen-bonding to form a 1D supramolecular chain structure. The [PtCl6]2-anions are fixed on one side of the 1D supramolecular chain through supra- molecular interactions. To the best of our knowledge, this is the first supramolecular assembly containing Pt and alkaline metal based on Me10CB[5].

2 EXPERIMENTAL

2. 1 Materials

Lithium chloride (LiCl) was bought from Sino- pharm Chemical Reagent. Chloroplatinic acid hydrate (H2PtCl6·6H2O) (99.9%）was bought from J&K Chemical Reagent. All chemicals were used directly without further purification. Me10CB[5] was synthesized according to the procedures reported in the literature[24]. Ultrapure water (18 M?) used in the experiments was supplied by a Millipore System.

2. 2 Synthesis of compound 1

The supramolecular self-assembly has been syn- thesized through a simple diffusion method. In a typical way, 66 mg (0.13 mmol) of H2PtCl6·6H2O was dissolved in 15 mL ultrapure water to obtain a clear aqueous solution I. Me10CB[5] (75 mg, 0.075 mmol) and lithium chloride (6.4 mg, 0.15 mmol) were dissolved into the ultrapure water (15 mL) under ultrasonic treatment to form transparent solution II. Then, solution I and II were carefully transferred to each side of an H-tube, respectively. Orange crystal was obtained after three days in the H-tube by slow diffusion with a yield of 41%. Elemental Analysis Calcd. (%) for the C40H74N20O22Cl6Li2Pt (Mr= 1608.81): C, 29.86; H, 4.64; N, 17.41%. Found: C, 30.15; H, 4.49; N, 18.53%.

2. 3 Characterization

Powder X-ray diffraction (PXPD) pattern was performed with a Rigaku Miniflex 600 diffrac- tometer with a Cu/Kαradiation source (λ= 1.54184 ?) at a low scanning speed of 1 °·min-1. Thermogra- vimetric analysis (TGA) was carried out under a flow of nitrogen (30 mL·min-1) using a TA SDT-Q600 instrument. The thermal measurement was conducted from 20 to 900 °C at constant heating rate of 10 °C·min-1. X-ray photoelectron spec- troscopy (XPS) measurement was performed by an ESCALAB 250 Xi XPS system. Elemental analy (EA) was carried on an Elementar Vario EL III analyzer. Transmission electron microscopy (TEM) measurement was performed using a FEI Tecnai G2 F20 electron microscope.

2. 4 Structural refinements

Single-crystal X-ray diffraction data of Compound 1 were collected on a a SuperNova CCD diffractometer equipped with graphite-monochroma- tized Mo-Kαradiation (λ= 0.71073 ?). Empirical absorption corrections were applied to the data using the Crystal Clear program[9]. Crystal structure was solved by direct methods and refined onF2by full-matrix least-squares with the Superflip and ShelXL2015 program package[25]. All non-hydrogen atoms, except some water oxygen atoms, were refined anisotropically. The hydrogen atoms of organic molecules were generated geometrically. Selected bond lengths and bond angles for compound 1 are shown in Tables 1 and 2.

Table 1. Selected Bond Lengths (?) for Compound 1

Table 2. Selected Bond Angles (°) for Compound 1

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compound 1

3 RESULTS AND DISCUSSION

3. 1 Description of the crystal structure

In compound 1, each Me10CB[5] molecule coor- dinates to two Li+(Li(1) and thus Li(2)), forming a “half-open” molecular capsule {Li2(H2O@Me10CB[5])} due to the smaller atomic radius of Li+(0.84 ?) compared with the portal size (2.5 ?) of Me10CB[5]. Meanwhile, one uncoordina- ted water molecule (O(1W)) is included in the inner cavity of the Me10CB[5] molecule (Fig. 1). The tetrahedral coordination sphere of Li+cation is defined by two aqua ligands (O(6) and O(7) for Li(1), O(13) and O(14) for Li(2)) and two portal carbonyl oxygen atoms (O(1) and O(2) for Li(1), O(10) and O(11) for Li(2)) of Me10CB[5] unit. The bond lengths of Li-Owaterare 1.954(8) and 1.930(8) ?, while the distances of Li-Ocarbonylare 1.965(9) and 1.946(8) ?, respectively. Adjacent {Li2(H2O@Me10CB[5])} molecular capsules are connected through hydrogen bonds between the aqua ligands on lithium centers and the portal carbonyl oxygen atoms (O(7)-O(8) and O(3)-O(13)), with the distances of 2.7938(1) and 2.7624(1) ?, resulting in a one-dimensional (1D) supramolecular chain structure (Fig. 2). [PtCl6]2-anion is connected to Me10CB[5] macrocycles through supramolecular interactions, which is typical and similar to the most supramolecular structure composed of CB[n][12]. The formula of compound 1 [Li(H2O)2]2[(H2O@Me10CB[5])] [PtCl6]·7H2O was confirmed by the combination results of single- crystal X-ray diffraction, TGA and EA.

Fig. 1. Coordination patterns of Me10CB[5] with Li+ into the half-open molecular capsule

Fig. 2. View of the one-dimensional chain in compound 1

3. 2 PXRD, XPS and TGA analysis

Powder X-ray diffraction (PXRD) was employed to verify the phase purity and crystallinity of compound 1. As shown in Fig. 3, the experimental PXRD pattern for 1 is in good agreement with the simulated one from single-crystal X-ray diffraction data, indicating the good phase purity and hom- ogeneity of bulky product[26]. No typical diffraction peaks of Pt0can be found from the PXRD pattern.

Fig. 3. Experimental and simulated PXRD patterns of compound 1

TGA curve of compound 1 is shown in Fig. 4, exhibiting two major steps of weight loss. The first one (13.0%) from 20 to 260 °C corresponds to the departure of uncoordinated and coordinated molecu- les[27], which is identical to the calculated value of 13.4%. The numbers of solvent molecules have been confirmed by combining EA and X-ray single- crystal diffraction results. There is no obvious mass loss between 260 and 400 °C. The dramatic weight loss from 400 to 900 °C is attributed to the decomposition of Me10CB[5]. The result of TGA indicates good heat stability of the obtained supramolecular self-assembly.

Fig. 4. TGA curve of compound 1

X-ray Photoelectron Spectroscopy (XPS) was employed to examine the valence of platinum. The corresponding high resolution XPS spectra of Pt 4f(Fig. 5) can be deconvoluted into two component peaks at 72.54 and 74.95 eV. The two peaks at 74.95 and 78.18 eV are attributed to the 4f7/2and 4f5/2orbitals of Pt(IV) species. Meanwhile, there are two low peaks at binding energy values of 72.54 and 75.5 eV assigned to the 4f7/2and 4f5/2orbitals of Pt(II) species, which may be originated from the slight reduction of Pt(IV) into Pt(II) under the illumination of high energy electron beam during the test process[28]. There is no Pt(0) in the compound.

Fig. 5. High-resolution Pt 4f XPS spectrum obtained from compound 1

Transmission electron microscopy (TEM) was further conducted to investigate the morphology of compound 1. As shown in Fig. 6, no visible metallic Pt0particles were observed in the TEM image for the supramolecular assembly, indicating the existence of ionic state of Pt species in 1, which is consistent with the results of PXRD and XPS.

Fig. 6. TEM image of compound 1

4 CONCLUSION

A new noble metal-containing supramolecular self- assembly [Li(H2O)2]2[(H2O@Me10CB[5])][PtCl6]· 7H2O was successfully synthesizedviathe reaction of Me10CB[5] with [PtCl6]2-anion in the presence of Li+. This compound exhibits a 1D supramolecular chain structure constructed by “half-open” molecular capsule. The [PtCl6]2-anions are fixed on one side of the 1D supramole- cular chain through supramolecular interactions. Such supramolecular assembly is a potential new catalyst in many applications because of its high thermal stability combined with the homogeneous distribution of Pt species at the atomic level, which will maximize the use efficiency of noble metals.