Cyclodextrin Rotaxanes of Pt Complexes and Their Conversion to Pt Nanoparticles

The cationic Pt complex (Pt(NC6H4-C6H4N-(CH2)10-O(C6H3-3,5-(OMe)2)(MeN-(CH2CH2NMe2)2))+ was prepared by the reaction of alkylbipyridinium ligand with a nitrateplatinum(II) complex. Mixing the complex and α- and β-cyclodextrins in aqueous media produced the corresponding [2]rotaxanes with 1:1 stoichiometry. γ-Cyclodextrin and the Pt complex formed a rotaxane having components in a 1:1 or 2:1 molar ratio. The results of mass and nuclear magnetic resonance (NMR) measurements confirmed the rotaxane structures of the Pt complexes. Transmission electron microscopy (TEM) and atomic force microscope (AFM) analyses revealed the formation of micelles or vesicles. The addition of NaBH4 to the rotaxanes in aqueous media formed Pt nanoparticles with diameters of 1.3–2.8 nm, as characterized by TEM. The aggregated size of the nanoparticles formed from the rotaxane did not change even at 70 °C, and they showed higher thermal stability than those obtained from the reduction of the cyclodextrin-free Pt complex.


Introduction
Pt nanoparticles have been investigated because of their usability in catalysis, fuel cell materials, and sensing materials [1]. A common preparation procedure of Pt nanoparticles involves the preparation of precursors from H 2 PtCl 6 and template compounds, such as surfactants and polymers dispersed in aqueous media. The subsequent reduction of the Pt(IV) species results in the deposition of the Pt nanoparticles under mild conditions. Surfactants have a dual role in the process: coordination with the Pt-containing precursors to keep their colloidal aggregation in the aqueous media, and protection of the formed nanoparticles by preventing their further growth. A typical preparation procedure was reported as follows. The addition of an aqueous solution of H 2 PtCl 6 to an isooctane solution of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) formed a colloidal suspension containing Pt salt. An NaBH 4 reduction of the precursor suspension at 4 • C yielded Pt nanoparticles with an average diameter of 13 nm [2]. Tetradecyltrimethylammonium bromide (TTAB) and sodium citrate were also used as templates for the synthesis of the Pt nanoparticles [3,4]. Polymers such as polyvinylpyrrolidone (PVP) [5,6] and poly(acrylic acid) (PA) [7] function as templates to stabilize metal nanoparticles effectively. Natural polymers, such as silk, were reported to function as the template in the Pt nanoparticle formation [8]. The reduction of the Pt(IV) precursors with diol and alcohol at elevated temperatures also produced the Pt nanoparticles [9]. Typically, Toste et al. heated an ethylene glycol solution of NaOH and H 2 PtCl 6 at 160 • C, and neutralized the mixture by using HCl [10]. Dispersing the resulting mixture in an ethanol solution of PVP caused the formation of Pt nanoparticles with a diameter of 1.5 nm. Conversely, the sequential addition of the aqueous H 2 PtCl 6 solution and PVP to nanoparticles with a diameter of 1.5 nm. Conversely, the sequential addition of the aqueous H2PtCl6 solution and PVP to methanol, followed by heating the mixture, caused the formation of Pt particles with a diameter of 2.9 nm. The Pt particles supported by silica were used as the catalyst for the selective synthesis of heterocyclic compounds.
Rotaxanes are molecules consisting of more than one component, i.e., macrocyclic and linear compounds, with the former encircling the latter. Major research interest in rotaxanes pertains to their applicability as molecular machines, molecular devices [11][12][13][14][15][16][17][18][19][20], and supramolecular catalysts [21][22][23][24][25]. Cyclodextrins have tube-like molecular structures and form rotaxanes with linear-shaped organic and polymer molecules. Cyclodextrins and transition-metal complexes having alkyl or alkylene ligands have been investigated for several decades [26][27][28][29][30]. Studies on the cyclodextrin rotaxanes of hydrophobic organic polymers have revealed their aggregation in the solid state caused by the intermolecular interaction of cyclodextrin components [31][32][33][34][35][36][37][38][39][40]. Previously, we prepared pseudo-rotaxanes composed of 4-alkyl-4,4′-bipyridinium, with a bulky 3,5-dimethylphenyl end (linear component) and cyclodextrins (macrocyclic component) [41][42][43][44]. The addition of Pd and Pt complexes to the mixture of cyclodextrins and ω-(3,5-dimethylphenyl)-4-alkyl-4,4′-bipyridine yielded rotaxanes containing the metal complex of the bipyridinium ligand as the axle component. The rotaxanes and pseudorotaxanes form micelles in aqueous media because of their tendency to aggregate. Our goal is to use the aggregated rotaxanes as a precursor to metal nanoparticles. The reduction of micellar aggregates of transition metal complexes has been reported as an efficient procedure for preparing metal nanoparticles under mild conditions. Lee reported that a mixture of an organic surfactant and H2PtCl6 was reduced by dodecanol to form Pt nanoparticles with a diameter of 1.7 ± 0.5 nm [45]. The reduction of a mixture of HAuCl4 and a cationic surfactant dispersed in water was found to produce Au nanoparticles with a diameter of 1.9 ± 0.3 nm [46]. Chaudhary employed metallomicelles of a Pd(II) complex with dodecylamine ligands as the precursor to Pd nanoparticles, and obtained a product with a diameter of 3.4-4.0 nm [47].
Scheme 1 outlines the formation of Pt nanoparticles in this work. Mixing cyclodextrin, a Pt(II) complex with N-alkylbipyridinium ligand (A), and nitrate-platinum complex (B) in aqueous media forms pseudorotaxane of cyclodextrin and the dissociated ligand (C), as well as Pt-containing rotaxane (D). Both Pt complexes (A) and (D) are aggregated in micellar form in aqueous media, and they are converted into Pt nanoparticles upon reduction. We planned to use several mixtures of (A) and excess amounts of (B) and cyclodextrins as a precursor to Pt nanoparticles. Different amounts of the starting compounds shift the equilibrium to depress the formation of the CD-free complex (A) and pseudorotaxane (C). Thus, the reduction of the mixture would yield Pt nanoparticles from the rotaxane (D) rather than a CD-free complex (A). The reduction of complex (B) also forms Pt, but in bulk, because of the dissolution of the complex in aqueous media. In this paper, we report the conversion of a Pt complex with a rotaxane structure into Pt nanoparticles by NaBH4 reduction, as well as the characterization of the rotaxanes in micellar aggregates and metal nanoparticles.
Complex [3]NO 3 and γ-cyclodextrin could form not only [3]rotaxane [(3) 2 (γ-CD)] 6+ via a 1:2 aggregation of the macrocyclic and axle components, but also [2]rotaxane [(3)(γ-CD)] 3+ by a 1:1 aggregation in the solution. We conducted titration of the two components in the NMR samples, whose results are shown in Figure 2. The 1 H NMR signals of complex [3]NO 3 , particularly those of the aromatic hydrogens, shift towards lower magnetic field positions. Job's plots of the signal positions show a maximum close to 0.65, which is consistent with the 1:2 aggregation (inset of Figure 2). Thus, complex [3] 3+ forms majorly [3]rotaxane [(3) 2 (γ-CD)] 6+ , and a negligible amount of [2]rotaxane The positions of the aromatic hydrogen signals are influenced by the ring size of the cyclodextrins. Figure 3 compares the spectrum of the mixture of [(3) 2 (γ-CD) 2 ] 6+ and [2] + with the rotating-frame Overhauser effect (ROE) mode. The irradiation of the aromatic hydrogen signals of the terminal group enhanced the signals at 3.8 ppm, assigned to the hydrogens of γ-CD (at the interior side of the ring). This suggests that the 3,5-dimethoxyphenyl group of the Pt complex is at a proximity to γ-CD.       Considerably low-intensity cross-peaks are observed at the positions between the aromatic hydrogen signals of the terminal group of the ligand (e, f) and the interior hydrogens of α-CD. Thus, the rotaxane prefers the structure with the macrocyclic component (α-CD) at proximity to the CH2 hydrogens adjacent to the bipyridinium group. Another structure with the terminal 3,5-dimethylphenyl group and α-CD is also observed in a much smaller amount. Figure 5 shows the spectra of a mixture containing the rotaxane composed of the Pt complex and γ-CD. Clear correlation peaks are observed, indicative of the interaction between the cyclodextrin hydrogen and 3,5-dimethoxyphenyl hydrogens (e, f) and no cross-peak between the cyclodextrin and the CH2 hydrogens. The ROESY NMR spectra of the rotaxane containing β-cyclodextrin also shows the cross-peak, indicative of the interaction between the aromatic hydrogens of the terminal group and the hydrogens of cyclodextrin.  The NCH 2 hydrogens show significant correlation with the signals of α-CD. Considerably low-intensity cross-peaks are observed at the positions between the aromatic hydrogen signals of the terminal group of the ligand (e, f) and the interior hydrogens of α-CD. Thus, the rotaxane prefers the structure with the macrocyclic component (α-CD) at proximity to the CH 2 hydrogens adjacent to the bipyridinium group. Another structure with the terminal 3,5-dimethylphenyl group and α-CD is also observed in a much smaller amount. Figure 5 shows the spectra of a mixture containing the rotaxane composed of the Pt complex and γ-CD. Clear correlation peaks are observed, indicative of the interaction between the cyclodextrin hydrogen and 3,5-dimethoxyphenyl hydrogens (e, f) and no cross-peak between the cyclodextrin and the CH 2 hydrogens. The ROESY NMR spectra of the rotaxane containing β-cyclodextrin also shows the cross-peak, indicative of the interaction between the aromatic hydrogens of the terminal group and the hydrogens of cyclodextrin.
Thus, we propose a main conformation of the rotaxanes with α-, β-, and γ-cyclodextrins, as shown in Figure 6. [(3)(α-CD)] 3+ contains the rotaxane having interaction between the CH 2 groups adjacent to the bipyridinium group of the ligand and α-cyclodextrin in the main, although a minor structure with the terminal 3,5-dimethoxyphenyl group and α-CD is also formed. The rotaxanes with β-, and γ-cyclodextrins have structures with an attractive interaction between the terminal aromatic group and cyclodextrins.
Complex [3] 3+ with the cationic metal center and a long alkylene group in the ligand is expected to show an amphiphilic nature and to be aggregated in aqueous solution. The absorption spectra of the complex in aqueous solutions containing nile red showed absorption at 577 nm. Plots of the absorbance depending on the concentration of complex 3 (Figure 7a) shows an inflection point at [3] = 1.2 mM, as shown in Figure 7b. It suggests formation of micelles above the concentration, and solubilization of the dye molecules in the micelles above the critical micelle concentration.
We prepared the samples for transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements by dropping the solution on carbon-coated Cu grids and removing the water at room temperature. Figure 8 shows the TEM and AFM images obtained from the solutions of [3](NO 3   We prepared the samples for transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements by dropping the solution on carbon-coated Cu grids and removing the water at room temperature. Figure 8 shows   We prepared the samples for transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements by dropping the solution on carbon-coated Cu grids and removing the water at room temperature.  contain both rotaxanes and complex [2] + added to enhance the formation of the rotaxane, the aggregates, observed in the TEM images, also contain the rotaxane and the cationic complex without bipyridinium ligands. The size and shape of the aggregates observed by the micrograph differ depending on the size of CDs, in spite of the co-existence of complex [2] + . The formed aggregates have a regulated size and different sizes and shapes depending on the CD involved in the rotaxane molecules, despite the presence of such impurities.

Preparation of Pt Nanoparticles
The addition of NaBH4 to a solution of the Pt complex with and without CDs afforded Pt nanoparticles via reduction of the Pt(II) compound. The original aqueous solution was yellow; upon the addition of NaBH4 (30 equiv./Pt) dispersed in H2O (1.0 mL), the color changed to blue at first, and subsequently to brown. The precipitation of a solid material was not observed. The resulting aqueous solution was dropped onto the Cu grid and was analyzed by TEM after removing the water. Figure 9 presents the TEM images of the Pt particles obtained by the reduction of [3] 3+ and its rotaxanes with α, β , γ-CDs at 3.0 mM Pt. In all cases, the measurements revealed the presence of sphere-shaped spots with similar sizes. The aggregation of the Pt complex and its rotaxanes, as micelles, vesicles, and rod-micelles, is not directly related to the shape of the Pt nanoparticles obtained by the reduction. The average radii of the nanoparticles obtained from [3]

Preparation of Pt Nanoparticles
The addition of NaBH 4 to a solution of the Pt complex with and without CDs afforded Pt nanoparticles via reduction of the Pt(II) compound. The original aqueous solution was yellow; upon the addition of NaBH 4 (30 equiv./Pt) dispersed in H 2 O (1.0 mL), the color changed to blue at first, and subsequently to brown. The precipitation of a solid material was not observed. The resulting aqueous solution was dropped onto the Cu grid and was analyzed by TEM after removing the water. Figure 9 presents the TEM images of the Pt particles obtained by the reduction of [3] 3+ and its rotaxanes with α, β, γ-CDs at 3.0 mM Pt. In all cases, the measurements revealed the presence of sphere-shaped spots with similar sizes. The aggregation of the Pt complex and its rotaxanes, as micelles, vesicles, and rod-micelles, is not directly related to the shape of the Pt nanoparticles obtained by the reduction. The average radii of the nanoparticles obtained from [3]  The reduction of [(3)(β-CD)](NO 3 ) 3 by NaBH 4 in the presence of tetrakis(4-carboxyphenyl)porphyrin (TCPP) formed Pt particles with a much larger particle size than those without addition of TCPP, as shown in the TEM images (Figure 10a-c). TCPP and the Pt complex compete for coordination with β-CD because both can form rotaxane or pseudorotaxane with β-CD. The high stability of the Pt nanoparticles from the rotaxanes in this study is partly attributed to the protection of the cyclodextrins on the surface of the Pt particles. Stable metal nanoparticles protected by functionalized cyclodextrins were synthesized and used as efficient catalysts for synthetic organic reactions [48][49][50].
Thermal stability of Pt nanoparticles obtained by the above procedures was compared. The particles obtained from the reduction of complex [3] + are stable at room temperature; however, their particle size increases on heating the solution at 70 • C for 3 h (from 2.0 ± 0.4 nm to 3.2 ± 0.6 nm). The size of the Pt particles obtained from the rotaxanes, however, did not change even after heat treatments: 1.9 ± 0.4 nm to 2.0 ± 0. The reduction of [(3)(β-CD)](NO3)3 by NaBH4 in the presence of tetrakis(4carboxyphenyl)porphyrin (TCPP) formed Pt particles with a much larger particle size than those without addition of TCPP, as shown in the TEM images (Figure 10(a)-(c)). TCPP and the Pt complex compete for coordination with β-CD because both can form rotaxane or pseudorotaxane with β-CD. The high stability of the Pt nanoparticles from the rotaxanes in this study is partly attributed to the protection of the cyclodextrins on the surface of the Pt particles. Stable metal nanoparticles protected by functionalized cyclodextrins were synthesized and used as efficient catalysts for synthetic organic reactions [48][49][50]. Thermal stability of Pt nanoparticles obtained by the above procedures was compared. The particles obtained from the reduction of complex [3] + are stable at room temperature; however, their particle size increases on heating the solution at 70 °C for 3 h (from 2.0 ± 0.4 nm to 3.2 ± 0.6 nm). The size of the Pt particles obtained from the rotaxanes, however, did not change even after heat treatments: 1.9 ± 0.4 nm to 2.0 ± 0.4 nm for [(3)(α-CD)](NO3)3, 2.6 ± 0.4 nm to 2.5 ± 0.5 nm for [(3)(β-CD)](NO3)3, and 2.8 ± 0.6 nm to 2.8 ± 0.6 nm for [(3)2(γ-CD)](NO3)3.

General
All the chemicals were commercially available from Tokyo Kasei Chemicals Co. 1 H-and 13 C{ 1 H}-NMR spectra were acquired on a Bruker AV-400M (400 MHz) and a JEOL JNM-500 (500 MHz). The chemical shifts were referenced with respect to CHCl3 (δ 7.26), HDO (δ 4.79) for 1 H, and CDCl3 (δ 77.0), DSS (sodium 3-(trimethylsilyl)-1-propanesulfonate) (δ 0.0) for 13 C as internal standards. High  The reduction of [(3)(β-CD)](NO3)3 by NaBH4 in the presence of tetrakis(4carboxyphenyl)porphyrin (TCPP) formed Pt particles with a much larger particle size than those without addition of TCPP, as shown in the TEM images (Figure 10(a)-(c)). TCPP and the Pt complex compete for coordination with β-CD because both can form rotaxane or pseudorotaxane with β-CD. The high stability of the Pt nanoparticles from the rotaxanes in this study is partly attributed to the protection of the cyclodextrins on the surface of the Pt particles. Stable metal nanoparticles protected by functionalized cyclodextrins were synthesized and used as efficient catalysts for synthetic organic reactions [48][49][50]. Thermal stability of Pt nanoparticles obtained by the above procedures was compared. The particles obtained from the reduction of complex [3] + are stable at room temperature; however, their particle size increases on heating the solution at 70 °C for 3 h (from 2.0 ± 0.4 nm to 3.2 ± 0.6 nm). The size of the Pt particles obtained from the rotaxanes, however, did not change even after heat treatments: 1.9 ± 0.4 nm to 2.0 ± 0.4 nm for [(3)(α-CD)](NO3)3, 2.6 ± 0.4 nm to 2.5 ± 0.5 nm for [(3)(β-CD)](NO3)3, and 2.8 ± 0.6 nm to 2.8 ± 0.6 nm for [(3)2(γ-CD)](NO3)3.

Reduction of Pt Complex and Its Rotaxanes
A solution of [3](NO 3 ) (0.1-12 µmol) in water ([3] = 0.1-12.0 mM, 1.0 mL) was prepared. It was treated with a solution of NaBH 4 (30 eq. to [3], 0.1-13.6 mg, 3.0-360 µmol) in water (3.0-360 mM, 1.0 mL). Stirring the mixture for 17 h at room temperature caused formation of a colorless to pale brown solution. A drop of the solution was transferred to the surface of carbon-coated Cu grids, and the resulting product was analyzed by TEM. Reduction of the rotaxanes to Pt nanoparticles was carried out analogously.

Conclusions
The Pt complex with a bipyridinium ligand having a long N-alkyl substituent forms rotaxanes with α-, β-, and γ-cyclodextrins. These rotaxane aggregate in aqueous media to form micelles or vesicles, depending on the size of the cyclodextrins. The reduction of the Pt(II) complex and its rotaxanes by NaBH 4 afforded Pt nanoparticles with regulated and small sizes (diameters of 1.3-2.8 nm). The Pt nanoparticles from the cyclodextrins are stable at 70 • C, while those without CDs tended to aggregate in the solution. The protection of the resulting Pt particles by cyclodextrins and/or alkylbipyridnium functioned effectively to stabilize the nanoparticles. Funding: This research was funded by Japan Society for Promotion of Science, grant number 25810059 and by Dynamic Alliance for Open Innovation Bridging from Ministry of Education, Culture, Sports, Science, and Technology-Japan.