Cyclodextrin Rotaxanes of Pt Complexes and Their Conversion to Pt Nanoparticles

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Article Cyclodextrin Rotaxanes of Pt Complexes and Their Conversion to Pt Nanoparticles

Yuji Suzaki 1, Yuhei Fujii 1 and Kohtaro Osakada 1,2,* 1 Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan; [email protected] (Y.S.); [email protected] (Y.F.) 2 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan * Correspondence: [email protected]; Tel.: +81-45-924-5224

Received: 12 November 2020; Accepted: 23 November 2020; Published: 29 November 2020

Abstract: 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.

Keywords: platinum; cyclodextrin; rotaxane; nanoparticles

1. 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 H2PtCl6 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 H2PtCl6 to an isooctane solution of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) formed a colloidal suspension containing Pt salt. An NaBH4 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 H2PtCl6 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 H2PtCl6 solution and PVP to

Molecules 2020, 25, 5617; doi:10.3390/molecules25235617 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 14 Molecules 2020, 25, 5617 2 of 14 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 methanol, followed by heating the mixture, caused the formation of Pt particles with a diameter of with a diameter of 2.9 nm. The Pt particles supported by silica were used as the catalyst for the 2.9 nm. The Pt particles supported by silica were used as the catalyst for the selective synthesis of selective synthesis of heterocyclic compounds. heterocyclic compounds. Rotaxanes are molecules consisting of more than one component, i.e., macrocyclic and linear 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 compounds, with the former encircling the latter. Major research interest in rotaxanes pertains to their their applicability as molecular machines, molecular devices [11–20], and supramolecular catalysts applicability as molecular machines, molecular devices [11–20], and supramolecular catalysts [21–25]. [21–25]. Cyclodextrins have tube-like molecular structures and form rotaxanes with linear-shaped Cyclodextrins have tube-like molecular structures and form rotaxanes with linear-shaped organic and organic and polymer molecules. Cyclodextrins and transition-metal complexes having alkyl or polymer molecules. Cyclodextrins and transition-metal complexes having alkyl or alkylene ligands have alkylene ligands have been investigated for several decades [26–30]. Studies on the cyclodextrin been investigated for several decades [26–30]. Studies on the cyclodextrin rotaxanes of hydrophobic rotaxanes of hydrophobic organic polymers have revealed their aggregation in the solid state caused organic polymers have revealed their aggregation in the solid state caused by the intermolecular by the intermolecular interaction of cyclodextrin components [31–40]. Previously, we prepared interaction of cyclodextrin components [31–40]. Previously, we prepared pseudo-rotaxanes composed pseudo-rotaxanes composed of 4-alkyl-4,4′-bipyridinium, with a bulky 3,5-dimethylphenyl end of 4-alkyl-4,4 -bipyridinium, with a bulky 3,5-dimethylphenyl end (linear component) and cyclodextrins (linear component)0 and cyclodextrins (macrocyclic component) [41–44]. The addition of Pd and Pt (macrocyclic component) [41–44]. The addition of Pd and Pt complexes to the mixture of cyclodextrins complexes to the mixture of cyclodextrins and ω-(3,5-dimethylphenyl)-4-alkyl-4,4′-bipyridine and ω-(3,5-dimethylphenyl)-4-alkyl-4,4 -bipyridine yielded rotaxanes containing the metal complex of yielded rotaxanes containing the metal0 complex of the bipyridinium ligand as the axle component. the bipyridinium ligand as the axle component. The rotaxanes and pseudorotaxanes form micelles in The rotaxanes and pseudorotaxanes form micelles in aqueous media because of their tendency to aggregate.aqueous media Our becausegoal is to of use their the tendency aggregated to aggregate. rotaxanes Our as goala precursor is to use to the metal aggregated nanoparticles. rotaxanes The as reductiona precursor of to micellar metal nanoparticles. aggregates of The transition reduction me oftal micellar complexes aggregates has been of transition reported metal as an complexes efficient procedurehas been reported for preparing as an e metalﬃcient nanoparticles procedure for under preparing mild metalconditions. nanoparticles Lee reported under that mild a conditions.mixture of Lee reported that a mixture of an organic surfactant and H2PtCl6 was reduced by dodecanol to form an organic surfactant and H2PtCl6 was reduced by dodecanol to form Pt nanoparticles with a diameter Pt nanoparticles with a diameter of 1.7 0.5 nm [45]. The reduction of a mixture of HAuCl4 and of 1.7 ± 0.5 nm [45]. The reduction of a mixture± of HAuCl4 and a cationic surfactant dispersed in water wasa cationic found surfactantto produce dispersed Au nanoparticles in water with was a found diameter to produce of 1.9 ± Au0.3 nm nanoparticles [46]. Chaudhary with aemployed diameter of 1.9 0.3 nm [46]. Chaudhary employed metallomicelles of a Pd(II) complex with dodecylamine metallomicelles± of a Pd(II) complex with dodecylamine ligands as the precursor to Pd nanoparticles, andligands obtained as the a precursor product towith Pd a nanoparticles, diameter of 3.4–4.0 and obtained nm [47]. a product with a diameter of 3.4–4.0 nm [47]. Scheme 11 outlinesoutlines thethe formationformation ofof PtPt nanoparticlesnanoparticles inin thisthis work.work. MixingMixing cyclodextrin,cyclodextrin, aa Pt(II)Pt(II) N complex with N-alkylbipyridinium ligand ligand (A), and nitrate-platinum complex (B) in aqueous media forms pseudorotaxane of cyclodextrin and the dissociated ligand (C), as well as Pt-containingPt-containing rotaxane (D). (D). Both Both Pt Pt complexes complexes (A) (A) and and (D) (D) are are aggregated aggregated in micellar in micellar form form in aqueous in aqueous media, media, and theyand theyare converted are converted into intoPt nanoparticles Pt nanoparticles upon upon redu reduction.ction. We planned We planned to use to several use several mixtures mixtures of (A) of (A)and andexcess excess amounts amounts of (B) of and (B) and cyclodextrins cyclodextrins as a as prec a precursorursor to Pt to nanoparticles. Pt nanoparticles. Different Diﬀerent amounts amounts of theof the starting starting compounds compounds shift shift the the equilibrium equilibrium to to depress depress the the formation formation of of the the CD-free CD-free complex complex (A) (A) and pseudorotaxane (C). Thus, the reduction of th thee mixture would yield Pt nanoparticles from the rotaxane (D)(D) ratherrather than than a a CD-free CD-free complex complex (A). (A). The The reduction reduction of complexof complex (B) also(B) also forms forms Pt, but Pt, in but bulk, in bulk,because because of the dissolutionof the dissolution of the complex of the complex in aqueous in aqueous media. In media. this paper, In this we paper, report thewe conversionreport the of a Pt complex with a rotaxane structure into Pt nanoparticles by NaBH reduction, as well as the conversion of a Pt complex with a rotaxane structure into Pt nanoparticles4 by NaBH4 reduction, as wellcharacterization as the characterization of the rotaxanes of the in rotaxanes micellar aggregatesin micellar andaggregates metal nanoparticles. and metal nanoparticles.

Scheme 1. FormationFormation of of Pt-containing Pt-containing rotaxanes rotaxanes and Pt nanoparticles in this study.

Molecules 2020, 25, 5617 3 of 14

2. Results and Discussion

2.1. Preparation and Characterization of the Cyclodextrin Rotaxanes of the Pt Complex

A reaction of 4,40-bipyridinium derivative having a ω-(3,5-methoxyphenoxy)dodecyl substituent + + at a pyridine nitrogen (NC6H4-C6H4N-(CH2)12-O(C6H3-3,5-(OMe)2) ([1] ) with a Pt complex having a + + 1,1,4,7,7-pentamethyl-1,4,7-triaza-heptane (Me5dien) ligand (Pt(NO3)(Me5dien)) ([2] ) in the presence of AgNO3 aﬀords a complex containing the amphiphilic ligand (Pt(Me5dien)(1)](NO3)3 ([3](NO3)). The addition of α-cyclodextrin to a mixture of [3](NO3)3 and [2](NO3) (molar ratio of 2:1:2), followed by heating the resulting aqueous solution for 102 h, caused the formation of [2]rotaxane ((3)(α-CD)](NO3)3 as shown in Scheme2(i). 1H-nuclear magnetic resonance (NMR) spectrum of the reaction mixture suggested the complete complexation of the bipyridinium ligand to the Pt center and the absence of bare alkylbipyridinium or its organic pseudorotaxane with α-CD ([1]+ and [(1)(α-CD)]+). The reaction without the addition of extra [2] was very slow and did not reach completion even after prolonged + heating. An excess amount of [2] in the mixture enhanced the formation of [(3)(α-CD)](NO3)3 via the reversible dissociation and association of the bipyridinium ligand. Complexes [3]3+ and [2]+ undergo a mutual exchange of the bipyridinium ligand, and rapidly form a rotaxane via the dissociation of the ligand of [3]3+, formation of pseudorotaxane [(1)(α-CD)]+, and re-coordination of the pseudorotaxane to the Pt center of [2]+. Molecules 2020, 25, x 4 of 14 The individual reaction of β-cyclodextrin and γ-cyclodextrin with a mixture of [3](NO3)3 and [2](NO3) (molar ratio: 2:1:1 for the former and 1:2:4 for the latter) aﬀorded the respective cationic of the formation of [(3)(β-CD)](NO3)3 and [(3)2(γ-CD)2](NO3)6 by using 1H-NMR spectroscopy in rotaxanes [(3)(β-CD)](NO ) and [(3) (γ-CD) ](NO ) within 5 min (r.t.) (Scheme2(ii), (iii)). The rapid order to obtain further insight3 3 of the2 reaction.2 3 6 formation of the rotaxanes is ascribed to the large ring size of β- and γ-CD, which enables slipping Figure 1 presents the results of titration using β-cyclodextrin. The aromatic hydrogen signals (e, synthesis of the rotaxanes. Isolation of the resulting rotaxanes was not successful because of the f) of [3]3+ (K) are shifted to lower magnetic field positions upon the increase of β-cyclodextrin reversible formation and degradation of the rotaxanes in the solution. We conducted a titration study concentration (the total concentration of [3]3+ and β-CD are 10.0 mM). Job’s plots based on the change of the formation of [(3)(β-CD)](NO ) and [(3) (γ-CD) ](NO ) by using 1H-NMR spectroscopy in of the 1H-NMR signal due to aromatic3 3 hydrogen2 e suggest2 the3 aggregation6 of the complex and β-CD order to obtain further insight of the reaction. close to 1:1 stoichiometry (inset of Figure 1).

Figure 1. 1H-nuclearH-nuclear magnetic resonance (NMR) spectra of the mixtures ofof [3][3]33++ and β-cyclodextrin.-cyclodextrin.

(D2O, r.t., 500500 MHz).MHz). [3][3/]/([([3]3]+ +[ β[β-CD])-CD])= =( A(A)) 0, 0, (B ()B 0.3,) 0.3, (C ()C 0.4,) 0.4, (D ()D 0.45,) 0.45, (E) ( 0.5,E) 0.5, (F) 0.55,(F) 0.55, (G) 0.6,(G) (0.6,H) 0.67,(H) (0.67,I) 0.7, (I) ( J0.) 0.8,7, (J ()K 0.8,) 1.0. (K Signals) 1.0. Signals a-f are a-f assigned are assigned to the hydrogensto the hydrogens of complex of complex3, in Scheme 3, in Scheme2. A signal 2. A withsignal asterisk with asterisk is dueto is HDOdue to in HD theO solvent. in the solvent. Job’s plots Job’s based plots on based the positions on the ofpositions the signal of (e)the aresignal shown (e) arein the shown inset. in the inset.

Complex [3]NO3 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 1H NMR signals of complex [3]NO3, 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 [(3) (γ-CD)]3+ in the solution.

Molecules 2020, 25, x 3 of 14

2. Results and Discussion

2.1. Preparation and Characterization of the Cyclodextrin Rotaxanes of the Pt Complex A reaction of 4,4′-bipyridinium derivative having a ω-(3,5-methoxyphenoxy)dodecyl substituent at a pyridine nitrogen (NC6H4-C6H4N-(CH2)12-O(C6H3-3,5-(OMe)2)+ ([1]+) with a Pt complex having a 1,1,4,7,7-pentamethyl-1,4,7-triaza-heptane (Me5dien) ligand (Pt(NO3)(Me5dien))+ ([2]+) in the presence of AgNO3 affords a complex containing the amphiphilic ligand (Pt(Me5dien)(1)](NO3)3 ([3](NO3)). The addition of α-cyclodextrin to a mixture of [3](NO3)3 and [2](NO3) (molar ratio of 2:1:2), followed by heating the resulting aqueous solution for 102 h, caused the formation of [2]rotaxane ((3)(α-CD)](NO3)3 as shown in Scheme 2(i). 1H-nuclear magnetic resonance (NMR) spectrum of the reaction mixture suggested the complete complexation of the bipyridinium ligand to the Pt center and the absence of bare alkylbipyridinium or its organic pseudorotaxane with α-CD ([1]+ and [(1)(α-CD)]+). The reaction without the addition of extra [2] was very slow and did not reach completion even after prolonged heating. An excess amount of [2]+ in the mixture enhanced the formation of [(3)(α-CD)](NO3)3 via the reversible dissociation and association of the bipyridinium ligand. Complexes [3]3+ and [2]+ undergo a mutual exchange of the Moleculesbipyridinium2020, 25 ligand,, 5617 and rapidly form a rotaxane via the dissociation of the ligand of [3]3+, formation4 of 14 of pseudorotaxane [(1)(α-CD)]+, and re-coordination of the pseudorotaxane to the Pt center of [2]+.

OMe d e c N (CH ) O f a b 2 12 N N OMe + N 3(NO ) N Pt NO3 NO Pt 3 3 N N N 3 2

OMe N (CH2)12 O -CD N OMe (i) N + 2 Pt H2O N 3(NO ) reflux, 102 h N 3· -CD 3

OMe -CD N (CH2)12 O N OMe + 2 (ii) H O N 2 N Pt r.t., 5 min N 3· -CD 3(NO3 )

N N Pt N N OMe -CD N (CH2)12 O OMe + 2 (iii) OMe H O 2 N (CH2)12 O r.t., 5 min N OMe N Pt N (3)2· -CD N 6(NO3 ) and/or N N Pt N N OMe N (CH2)12 O OMe (3)· -CD

3(NO3 ) Scheme 2.2.Preparation Preparation of cyclodextrinof cyclodextrin rotaxanes. rotaxanes. Symbols Symbols a-f in complexa-f in complex3 denote aromatic3 denote hydrogens aromatic athydrogens the respective at the respective positions. positions. They correspond They corresp to theond NMR to the spectra NMR spectra in Figure in 1Figures(K) and 1(K) Figure and2 (N).2(N). α β γ Molecules(i)–(iii)(i)-(iii) 2020 show show, 25, x the the rotaxane rotaxane or or pseudorotaxane obtained from α-, β-,-, and and γ-cyclodextrins,-cyclodextrins, respectively. respectively. 5 of 14

The individual reaction of β-cyclodextrin and γ-cyclodextrin with a mixture of [3](NO3)3 and [2](NO3) (molar ratio: 2:1:1 for the former and 1:2:4 for the latter) afforded the respective cationic rotaxanes [(3)(β-CD)](NO3)3 and [(3)2(γ-CD)2](NO3)6 within 5 min (r.t.) (Scheme 2(ii), (iii)). The rapid formation of the rotaxanes is ascribed to the large ring size of β- and γ-CD, which enables slipping synthesis of the rotaxanes. Isolation of the resulting rotaxanes was not successful because of the reversible formation and degradation of the rotaxanes in the solution. We conducted a titration study

1 1 3+ 3+ Figure 2. 2. H-NMRH-NMR spectra spectra of mixtures of mixtures of [3] of and [3] γ-cyclodextrin.and γ-cyclodextrin. (D2O, r.t., 500 (D2 MHz).O, r.t., [3 500]/([3 MHz).] + [γ- CD])[3]/([3] = (+A[)γ 0,-CD]) (B) 0.3,= ( (AC)) 0,0.4, (B ()D 0.3,) 0.45, (C) (E 0.4,) 0.5, (D ()F 0.45,) 0.55, (E (G) 0.5,) 0.6, (F ()H 0.55,) 0.65, (G (I)) 0.6,0.67, (H (J)) 0.65,0.7, (K (I)) 0.75, 0.67, ( (LJ) 0.8, 0.7, (KM)) 0.75,0.9, ( (NL)) 0.8,1.0. (SignalsM) 0.9, a-f (N )are 1.0. assigned Signals a-fto the are assignedhydrogens to of the complex hydrogens 3, in of Scheme complex 2. 3A, in signal Scheme with2. Aasterisk signal is with due asterisk to HDO is in due the to solvent. HDOin Job’s the plots solvent. based Job’s on plots the positions based on of the the positions signal (e) of are the shown signal (e)in theare showninset. in the inset.

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.

Figure 3. 1H-NMR spectra with the rotating-frame Overhauser effect (ROE) difference mode. (a) [3]3+ (6.0 mM) + [2]+ (12.0 mM). (b) [3]3+ (6.0 mM) + [2]+ (12.0 mM) + γ-CD (30.0 mM).

More information on the relative positions of the axle molecules and cyclodextrins was obtained by two-dimensional rotating frame Overhauser effect spectroscopy (ROESY) NMR spectroscopy experiments; the results are shown in Figures 4 and 5.

Molecules 2020, 25, 5617 5 of 14

Figure1 presents the results of titration using β-cyclodextrin. The aromatic hydrogen signals (e, f) of [3]3+ (K) are shifted to lower magnetic field positions upon the increase of β-cyclodextrin concentration (the total concentration of [3]3+ and β-CD are 10.0 mM). Job’s plots based on the change of the 1H-NMR signal due to aromatic hydrogen e suggest the aggregation of the complex and β-CD close to 1:1 stoichiometry (inset of Figure1). 6+ Complex [3]NO3 and γ-cyclodextrin could form not only [3]rotaxane [(3)2(γ-CD)] 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, 1 whose results are shown in Figure2. The H NMR signals of complex [3]NO3, 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 Figure2). 3+ 6+ Thus, complex [3] forms majorly [3]rotaxane [(3)2(γ-CD)] , and a negligible amount of [2]rotaxane [(3)(γ-CD)]3+ in the solution. The positions of the aromatic hydrogen signals are influenced by the ring size of the cyclodextrins. 6+ + Figure3 compares the spectrum of the mixture of [( 3)2(γ-CD)2] 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). ThisArticle suggests that the 3,5-dimethoxyphenyl group of the Pt complex is at a proximity to γ-CD.

FigureFigure 3. 1 H-NMR3. 1H-NMR spectra spectra with with the the rotating-frame rotating-frame Overhauser effect eﬀect (ROE (ROE)) difference diﬀerence mode. mode. (a) [3 (]a3+) [3]3+ (6.0 mM)(6.0 mM)+ [2] ++ [2(12.0]+ (12.0 mM). mM). ( b(b)) [3] [3]33++ (6.0(6.0 mM) mM) + +[2][2]+ (12.0+ (12.0 mM) mM) + γ-CD+ γ (30.0-CD mM) (30.0. mM).

More information on the relative positions of the axle molecules and cyclodextrins was obtained by two-dimensional rotating frame Overhauser eﬀect spectroscopy (ROESY) NMR spectroscopy experiments; the results are shown in Figures4 and5.

Figure 4. ROEROESYSY NMR NMR spectra of [( [(33))·αα-CD]-CD]33++ (D(D2O,O, 25 25 °C,C, 500 MHz). Total Total spectrum spectrum (a)) and · 2 ◦ expanded spectrum ( b).).

Molecules 2020, 25, x; doi: www.mdpi.com/journal/molecules

Figure 5. ROESY NMR spectra of [(3)2·γ-CD]6+ (D2O, 25 °C, 500 MHz). Total spectrum (a) and expanded spectrum (b).

Figure 4(a), (b) present the cross peaks between the signals due to the CH2 group hydrogens and the signals assigned to the interior hydrogens of α-CD (3.8 and 4.0 ppm) with high intensity. The NCH2 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 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.

Molecules 2020, 25, x; doi: www.mdpi.com/journal/molecules

3+ MoleculesFigure2020, 25 4., 5617ROESY NMR spectra of [(3)·α-CD] (D2O, 25 °C, 500 MHz). Total spectrum (a) and6 of 14 expanded spectrum (b).

FigureFigure 5. 5.ROESY ROESY NMR NMR spectra spectra of of [( 3 [()3)γ2·-CD]γ-CD]6+6+(D (D2O,O, 25 25 C,°C, 500 500 MHz). MHz). Total Total spectrum spectrum ( a ()a) and and 2· 2 ◦ expandedexpanded spectrum spectrum (b ().b).

FigureFigure4 (a),4(a) (b), (b) present present the the cross cross peakspeaks betweenbetween the signals signals due due to to the the CH CH2 group2 group hydrogens hydrogens and andthe the signals signals assigned assigned to the to the interior interior hydrogens hydrogens of α of-CDα-CD (3.8 (3.8and and 4.0 4.0ppm) ppm) with with high high intensity. intensity. The TheNCH NCH2 hydrogens2 hydrogens show show significant signiﬁcant correlation correlation with with the the signals signals of αα-CD.-CD. Considerably Considerably low-intensity low-intensity cross-peakscross-peaks are are observed observed at the at the positions positions between between the aromatic the aromatic hydrogen hydrogen signals signals of the terminalof the terminal group ofgroup the ligand of the (e, ligand f) and (e, the f) and interior the interior hydrogens hydrogens of α-CD. of Thus,α-CD. the Thus, rotaxane the rotaxane prefers prefers the structure the structure with thewith macrocyclic the macrocyclic component component (α-CD) at ( proximityα-CD) at to prox theimity CH2 hydrogens to the CH adjacent2 hydrogens to the adjacent bipyridinium to the group.bipyridinium Another group. structure Another with thestructure terminal with 3,5-dimethylphenyl the terminal 3,5-dimethylphenyl group and α-CD group is also and observed α-CD is in also a muchobserved smaller in a amount. much smaller Figure amount5 shows. theFigure spectra 5 shows of a the mixture spectra containing of a mixture the rotaxanecontaining composed the rotaxane of thecomposed Pt complex of and the γPt-CD. complex Clear correlation and γ-CD. peaks Clear are correlation observed, peaks indicative are observed, of the interaction indicative between of the theinteraction cyclodextrin between hydrogen the cyclodextrin and 3,5-dimethoxyphenyl hydrogen and 3,5 hydrogens-dimethoxyphenyl (e, f) and no hydrogens cross-peak (e, between f) and no thecross cyclodextrin-peak between and the the CH cyclodextrin2 hydrogens. and The the CHROESY2 hydrogens. NMR spectra The ROESY of the rotaxane NMR spectra containing of the βrotaxane-cyclodextrin containing also shows β-cyclodextrin the cross-peak, also indicative shows the of cross the interaction-peak, indicative between of thethe aromatic interaction hydrogens between ofthe the aromatic terminal hydrogens group and of the the hydrogens terminal group of cyclodextrin. and the hydrogens of cyclodextrin. Thus, we propose a main conformation of the rotaxanes with α-, β-, and γ-cyclodextrins, as shown 3+ in Figure6. [( 3)(α-CD)] contains the rotaxane having interaction between the CH2 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 andMolecules cyclodextrins. 2020, 25, x; doi: www.mdpi.com/journal/molecules 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 (Figure7a) shows an inﬂection point at [3] = 1.2 mM, as shown in Figure7b. 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. Figure8 shows the TEM and AFM images obtained from the solutions of [3](NO 3), [(3)(α-CD)](NO3)3, [(3)(β-CD)](NO3)3, and [(3)2(γ-CD)](NO3)6 in aqueous media ([Pt] = 6.0 mM). The spots in the TEM images correspond to the Pt atoms in the complexes and rotaxanes. The average diameters of the spots are estimated as follows: [3](NO ) = 4.0 1.1 nm, 3 ± [(3)(α-CD)](NO ) = 79.5 15.1 nm, and [[3](β-CD)](NO ) = 6.0 1.3 nm. The rotaxane 3 3 ± 3 3 ± [(3) (γ-CD)](NO ) provided elongated spots with a size of 125,800 108 nm, which were assigned to 2 3 6 × the rod micelles formed in the solution. The spots observed in the TEM image of [(3)(α-CD)](NO3)3 are Molecules 2020, 25, 5617 7 of 14

much larger than the aggregates of [3](NO3) and [(3)(β-CD)](NO3)3. Many of the spots show contrast betweenMolecules 2020 a dark, 25, particlex edge and less-dark central region. This suggests the formation of vesicles7 of by 14 aggregated rotaxanes. Since the solutions contain both rotaxanes and complex [2]+ added to enhance the formation of the rotaxane, the aggregates, observedH5 H3 in the TEM images, also contain the rotaxane N OMe and the cationic complex without bipyridiniumN ligands. The sizeO and shape of the aggregates observed N H H H H OMe Pt + by the micrograph diﬀer dependingN on the size3(NO of3 ) CDs, in spite of the co-existence of complex [2] . N The formed aggregates have a regulated size[( and3)( -CD)] di3+ﬀerent sizes and shapes depending on the CD involved in the rotaxane molecules, despite the presence ofH3 such impurities. Molecules 2020, 25, x H5 7 of 14

HH e OMe N O f N H H OMe N H3 Pt 3(NOH53 ) N OMe N N N O N H H H H 3+ OMe Pt [(3)( -CD)] N 3(NO3 ) N [(3)( -CD)]3+ N H3 N H5 Pt N H3 N H5 e OMe N O f HH e OMeOMe 6(NO3 )N O f OMe N N H H OMe N O PtN 3(NO3 ) OMe N N Pt NN N 3+ 6+ [(3[()(3)-CD)]2( -CD)] Figure 6. Conformation of polyrotaxanes based on ROESY NMR spectra measurement. Symbols e N H3 N H5 and f are due to the aromatic hydrogPt ens at the positions. They correspond to the structures in Scheme N e OMe 2. N O f N OMe 6(NO3 ) OMe N O 3+ N OMe Complex [3] with the cationic metalN center and a long alkylene group in the ligand is expected Pt N to show an amphiphilic nature and toN be aggregat[(3) (ed-CD)] in6+ aqueous solution. The absorption spectra of 2 the complex in aqueous solutions containing nile red showed absorption at 577 nm. Plots of the Figure 6. Conformation of polyrotaxanes based on ROESY NMR spectra measurement. Symbols e absorbanceFigure 6. dependingConformation on of the polyrotaxanes concentration based of on complex ROESY NMR 3 (Figure spectra 7(a)) measurement. shows anSymbols inflection e and point at and f are due to the aromatic hydrogens at the positions. They correspond to the structures in Scheme [3] =f are1.2 duemM, to as the shown aromatic in hydrogensFigure 7(b). at It the suggests positions. fo Theyrmation correspond of micelles to the above structures the concentration, in Scheme2. and 2. solubilization of the dye molecules in the micelles above the critical micelle concentration. 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 7(a)) shows an inflection point at [3] = 1.2 mM, as shown in Figure 7(b). It suggests formation of micelles above the concentration, and solubilization of the dye molecules in the micelles above the critical micelle concentration.

Figure 7. (a) Absorption spectra of nile red depending on coexisting complex 3 in aqueous solutions Figure 7. (a) Absorption spectra of nile red depending on coexisting complex 3 in aqueous solutions ([3] = 0.08–5.68 mM, [nile red] = 10 µM). (b) Plots of absorbance at 577 nm vs. the concentration of 3. ([3] = 0.08–5.68 mM, [nile red] = 10 µM). (b) Plots of absorbance at 577 nm vs. the concentration of 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 the TEM and AFM images obtained from the solutions of [3](NO3), [(3)(α- Figure 7. (a) Absorption spectra of nile red depending on coexisting complex 3 in aqueous solutions CD)](NO3)3, [(3)(β-CD)](NO3)3, and [(3)2(γ-CD)](NO3)6 in aqueous media ([Pt] = 6.0 mM). The spots ([3] = 0.08–5.68 mM, [nile red] = 10 µM). (b) Plots of absorbance at 577 nm vs. the concentration of 3. in the TEM images correspond to the Pt atoms in the complexes and rotaxanes. The average diameters of theWe spots prepared are estimated the samples as follows: for transmission [3](NO3) = 4.0 electron ± 1.1 nm, microscopy [(3)(α-CD)](NO (TEM)3)3 = and79.5 ±atomic 15.1 nm, force and microscopy[[3](β-CD)](NO (AFM)3)3 measurements= 6.0 ± 1.3 nm. by The dropping rotaxane the [( 3solution)2(γ-CD)](NO on carbon-coated3)6 provided Cu elongated grids and spots removing with a thesize water of 125,800 at room × 108temperature. nm, which were assigned to the rod micelles formed in the solution. The spots observed in the TEM image of [(3)(α-CD)](NO3)3 are much larger than the aggregates of [3](NO3) and Figure 8 shows the TEM and AFM images obtained from the solutions of [3](NO3), [(3)(α- [(3)(β-CD)](NO3)3. Many of the spots show contrast between a dark particle edge and less-dark CD)](NO3)3, [(3)(β-CD)](NO3)3, and [(3)2(γ-CD)](NO3)6 in aqueous media ([Pt] = 6.0 mM). The spots in the TEM images correspond to the Pt atoms in the complexes and rotaxanes. The average diameters of the spots are estimated as follows: [3](NO3) = 4.0 ± 1.1 nm, [(3)(α-CD)](NO3)3 = 79.5 ± 15.1 nm, and [[3](β-CD)](NO3)3 = 6.0 ± 1.3 nm. The rotaxane [(3)2(γ-CD)](NO3)6 provided elongated spots with a size of 125,800 × 108 nm, which were assigned to the rod micelles formed in the solution. The spots observed in the TEM image of [(3)(α-CD)](NO3)3 are much larger than the aggregates of [3](NO3) and [(3)(β-CD)](NO3)3. Many of the spots show contrast between a dark particle edge and less-dark

Molecules 2020, 25, x 8 of 14 central region. This suggests the formation of vesicles by aggregated rotaxanes. Since the solutions 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 Moleculeshave a regulated2020, 25, 5617 size and different sizes and shapes depending on the CD involved in the rotaxane8 of 14 molecules, despite the presence of such impurities.

Figure 8. Transmission electron microscopy (TEM) images (upper) and AFM image (lower) for each Figure 8. Transmission electron microscopy (TEM) images (upper) and AFM image (lower) for each sample. Measurement was conducted after dropping the solution on the surface of the carbon-coated sample. Measurement was conducted after dropping the solution on the surface of the carbon-coated Cu grids. (a) [3]3+ = 6.0 mM, [2]+ = 12.0 mM. (b) [(3) α-CD]3+ = 3.4 mM, [3]3+ = 2.6 mM, [2]+ = 12.0 mM, Cu grids. (a) [3]3+ = 6.0 mM, [2]+ = 12.0 mM. (b) [(3)·α-CD] 3+ = 3.4 mM, [3] 3+ = 2.6 mM, [2] + = 12.0 mM, 3+ + · 6+ [α-CD] = 8.6 mM. (c) [(3) β-CD] = 6.0 mM, [2] = 12.0 mM. (d) [(3)2 γ-CD] = 3.0 mM, [2] = 12.0 mM. [α-CD] = 8.6 mM. (c) [(3·)·β-CD] 3+ = 6.0 mM, [2] + = 12.0 mM. (d) [(3)2··γ-CD] 6+ = 3.0 mM, [2] = 12.0 mM. 2.2. Preparation of Pt Nanoparticles 2.2. Preparation of Pt Nanoparticles The addition of NaBH4 to a solution of the Pt complex with and without CDs afforded Pt nanoparticlesThe addition via reductionof NaBH4 ofto thea solution Pt(II) compound. of the Pt complex The original with aqueousand without solution CDs wasafforded yellow; Pt nanoparticles via reduction of the Pt(II) compound. The original aqueous solution was yellow; upon upon the addition of NaBH4 (30 equiv./Pt) dispersed in H2O (1.0 mL), the color changed to blue at first, andthe addition subsequently of NaBH to brown.4 (30 equiv./Pt) The precipitation dispersed in of H a2O solid (1.0 materialmL), the wascolor not changed observed. to blue The at resultingfirst, and aqueoussubsequently solution to brown. was dropped The precipitation onto the Cu ofgrid a solid and material was analyzed was not by observed. TEM after The removing resulting the aqueous water. solutionFigure was9 presentsdropped theonto TEM the Cu images grid and of the was Pt analyzed particles by obtained TEM after by removing the reduction the water. of [3] 3+ and 3+ its rotaxanesFigure 9 withpresentsα, β ,theγ-CDs TEM at images 3.0 mM of Pt.the InPt all particles cases, theobtained measurements by the reduction revealed of the [3] presence and its , , ofrotaxanes sphere-shaped with α spotsβ γ-CDs with at similar 3.0 mM sizes. Pt. In The all cases, aggregation the measurements of the Pt complex revealed and the its presence rotaxanes, of assphere-shaped micelles, vesicles, spots andwith rod-micelles, similar sizes. is The not aggreg directlyation related of the to thePt complex shape of and the Ptits nanoparticlesrotaxanes, as obtainedmicelles, byvesicles, the reduction. and rod-micelles, The average is radii not ofdirectly the nanoparticles related to obtainedthe shape from of [3]the3+ Ptincrease nanoparticles slightly 3+ withobtained an increase by the reduction. in the concentration, The average 1.4 radii0.4 of the nm nanoparticles (0.05 mM), 1.8 obtained0.5 nm from (0.75 [3 mM),] increase 1.9 slightly0.4 nm ± ± ± (1.5with mM), an increase2.0 0.4 in nmthe (3.0concentration, mM), 3.1 1.40.8 ± nm0.4 nm (6.0 (0.05 mM). mM), The average1.8 ± 0.5 nm size (0.75 of the mM), formed 1.9 ± Pt 0.4 particles nm (1.5 ± ± aremM), as 2.0 follows: ± 0.4 nm1.3 (3.00.4 mM), nm (0.05 3.1 ± mM),0.8 nm 1.8 (6.0 0.5mM). nm The (0.75 average mM), 1.9size of0.4 the nm formed (1.5 mM), Pt particles 2.0 0.4 are nm as ± ± ± ± (3.0follows: mM), 1.32.3 ± 0.40.8 nm nm (0.05(6.0 mM)mM), for 1.8 [( ±3 0.5)(α -CD)](NOnm (0.75 mM),) , 1.7 1.9 ±0.4 0.4 nm nm (0.05 (1.5 mM), mM), 2.0 2.0 ±0.5 0.4 nm nm (0.75 (3.0 mM), ± 3 3 ± ± 2.12.3 ± 0.80.3 nm nm (6.0 (1.5 mM) mM), for2.6 [(3)(α0.4-CD)](NO nm (3.03 mM),)3, 1.7 ± and 0.4 2.9nm (0.050.5 mM), nm (6.0 2.0 mM)± 0.5 nm for [[3]((0.75β mM),-CD)](NO 2.1 ± 0.3) , ± ± ± 3 3 andnm (1.5 1.9 mM),0.4 nm2.6 ± (0.05 0.4 nm mM), (3.0 2.2 mM),0.5 and nm 2.9 (0.75 ± 0.5 mM), nm (6.0 2.5 mM)0.3 for nm [[ (1.53](β-CD)](NO mM), 2.7 3)30.5, and nm 1.9 (3.0 ± 0.4 mM), nm (0.05 mM),± 2.2 ± 0.5 nm (0.75 mM),± 2.5 ± 0.3 nm (1.5 mM), ±2.7 ± 0.5 nm (3.0 mM), and± 2.8 ± +0.5 nm (6.0 and 2.8 0.5 nm (6.0 mM) for [(3)2(γ-CD)](NO3)3. The treatment of a mixture of ligand ([1] , 6.0 mM), mM) for± [(3)2(γ+-CD)](NO3)3. The treatment of a mixture of ligand ([1]+, 6.0 mM), Pt complex ([2]+, 18 Pt complex ([2] , 18 mM), and α-CD (12 mM) in an aqueous solution with NaBH4 did not form the nanoparticles,mM), and α-CD but (12 caused mM) the in depositionan aqueous of solution a black solidwith composedNaBH4 did of not metallic form Pt.the nanoparticles, but caused the deposition of a black solid composed of metallic Pt. The reduction of [(3)(β-CD)](NO3)3 by NaBH4 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 [48–50]. 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)](NO ) , 2.6 0.4 nm to 2.5 0.5 nm for [(3)(β-CD)](NO ) , ± ± 3 3 ± ± 3 3 and 2.8 0.6 nm to 2.8 0.6 nm for [(3) (γ-CD)](NO ) . ± ± 2 3 3 Molecules 2020, 25, x 9 of 14

Molecules 2020, 25, 5617 9 of 14 Molecules 2020, 25, x 9 of 14

Figure 9. TEM images of the Pt nanoparticles obtained from the reduction of the complex or rotaxane in an aqueous solution (3.0 mM). Measurement was conducted after dropping the solution on the surface of carbon-coated Cu grids. (a) [3](NO3), (b) [[3](α-CD)](NO3)3, (c) [[3](β-CD)](NO3)3, (d) [[3]2(γ-CD)](NO3)6.

The reduction of [(3)(β-CD)](NO3)3 by NaBH4 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 10(a)-(c)). TCPP and the Pt complex competeFigureFigure for 9. 9. coordinationTEMTEM images images of of with the the Pt Ptβ nanoparticles-CD nanoparticles because obtained both obtained can fr fromformom the the rotaxane reduction reduction or of of pseudorotaxane the the complex complex or or rotaxane rotaxane with β -CD. The inhighin an an aqueousstability aqueous solutionof solution the Pt (3.0 (3.0nanoparticles mM). mM). Measurement Measurement from the wa wasrotaxaness conducted conducted in after this after droppingstudy dropping is partlythe thesolution attributed solution on the on to the surface of carbon-coated Cu grids. (a) [3](NO3), (b) [[3](α-CD)](NO3)3, (c) [[3](β-CD)](NO3)3, (d) protectionthe surface of the of cyclodextrins carbon-coated on Cu the grids. surface (a) [3](NO of the3 Pt), (particles.b) [[3](α-CD)](NO Stable metal3)3,(c nanoparticles) [[3](β-CD)](NO protected3)3, [[3]2(γ-CD)](NO3)6. by functionalized(d) [[3]2(γ-CD)](NO cyclodextrins3)6. were synthesized and used as efficient catalysts for synthetic organic reactions [48–50]. The reduction of [(3)(β-CD)](NO3)3 by NaBH4 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 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 [48–50].

Figure 10.9. TEMTEM images images of of the the Pt nanoparticles obtained byby NaBHNaBH44 reductionreduction ofof [([(33)()(ββ-CD)](NO-CD)](NO33))33 inin an aqueousaqueous solution solution (3.0 mM):(3.0 ( a)mM): as prepared, (a) (asb) afterprepared, addition ( ofb) tetrakis(4-carboxyphenyl)porphyrin after addition of tetrakis(4- (TCPP)carboxyphenyl)porphyrin (0.5 eq to Pt), (c) after (TCPP) further (0.5 addition eq to Pt), of ( TCPPc) after (1.0 further eq to addition Pt). of TCPP (1.0 eq to Pt).

3. MaterialsThermal and stability Methods 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 3.1. General 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 Allof the the Pt chemicals particles wereobtained commercially from the availablerotaxanes, from however, Tokyo did Kasei not Chemicals change even Co. after1H- heat and 13 1 treatments:C{FigureH}-NMR 9. 1.9 TEM spectra± 0.4 images nm were toof 2.0the acquired ±Pt 0.4 nanoparticles nm onfor [( a3 Bruker)(obtainedα-CD)](NO AV-400M by NaBH3)3, 2.64 (400reduction ± 0.4 MHz) nm of to [( and2.53)(β ±-CD)](NOa 0.5 JEOL nm 3for JNM-500)3 in [( 3)(β- (500CD)](NOan MHz). aqueous3)3, and The 2.8 chemicalsolution ± 0.6 nm (3.0 shiftsto 2.8mM): were± 0.6 nm referenced(a) foras [(3prepared,)2(γ with-CD)](NO respect (b) 3)3after. to CHCl addition3 (δ 7.26),of tetrakis(4- HDO (δ 4.79) 1 13 for carboxyphenyl)porphyrinH, and CDCl3 (δ 77.0), (TCPP) DSS (sodium (0.5 eq to 3-(trimethylsilyl)-1-propanesulfonate)Pt), (c) after further addition of TCPP (1.0 eq (δ to0.0) Pt). for C as internal3. Materials standards. and Methods High resolution electrospray ionization mass spectrometry (HR-ESI-MS) was recordedThermal on astability Bruker micrOTOFof Pt nanoparticles II. The TEM obtained was performed by the above at 100 procedures kV using Hitachiwas compared. H-7650 ZeroThe + particleswith3.1. General collodion obtained /carbon-support from the reduction ﬁlm grids, of complex COL-C15 [3] STEM are stable Cu150P at room (Okenshoji, temperature; Japan). however, The AFM their was 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 performedAll the using chemicals Oxford were Instruments commercially Cypher available S using from OLYMPUS Tokyo Kasei AC160TS. Chemicals Co. 1H- and 13C{1H}- size of the Pt particles obtained from the rotaxanes, however, did not change even after heat NMR spectra were acquired on a Bruker AV-400M (400 MHz) and+ a JEOL JNM-500 (500 MHz). The treatments:3.2. Preparation 1.9 ± of 0.4 [4,4 nm0-bpy-N-(CH to 2.0 ± 0.42) 12nmOC for6H 3[(-3,5-(OMe)3)(α-CD)](NO2][Br]3)3 ([1], 2.6 Br± 0.4−) andnm to 2.5 ± 0.5 nm for [(3)(β- chemical shifts were referenced with respect to +CHCl3 (δ 7.26), HDO (δ 4.79) for 1H, and CDCl3 (δ CD)](NO[4,40-bpy-N-(CH3)3, and2 2.8)12 OC± 0.66H nm3-3,5-(OMe) to 2.8 ± 0.62][NO nm3 for] ([1] [(3)(NO2(γ-CD)](NO3) −) 3)3. 77.0), DSS (sodium 3-(trimethylsilyl)-1-propanesulfonate) (δ 0.0) for 13C as internal standards. High 1 The title compound with Br− was prepared according to the literature. H-NMR (400 MHz, 3. Materials and Methods D2O, r.t.): δ 0.88–1.26 (m, 16H, CH2), 1.39 (quin, 2H, OCH2CH2, J = 6.6 Hz), 1.95 (quin, 2H, NCH2CH2, J = 6.6 Hz), 3.54 (t, 2H, OCH , J = 7.0 Hz), 3.56 (s, 6H, OCH ), 4.67 (t, 2H, NCH , J = 6.8 Hz), 3.1. General 2 3 2 5.73 (s, 2H, C6H3), 5.90 (s, 1H, C6H3), 7.83 (d, 2H, C10H8N2, J = 6.3 Hz), 8.36 (d, 2H, C10H8N2, J = 13 1 1 13 1 6.6 Hz),All the 8.75 chemicals (d, 2H, C 10wereH8N commercially2, J = 6.2 Hz), available 8.94 (d, 2H, from C10 TokyoH8N2 ,KaseiJ = 6.8 Chemicals Hz). C{ Co.H}-NMR H- and (125 C{ MHz,H}- NMRCDCl spectra3, r.t.): δ were26.1 (CHacquired2), 26.3 on (CH a Bruker2), 29.2 AV-400M (CH2), 29.3 (400 (CH MHz)2), 29.4 and (CH a JEOL2), 29.5 JNM-500 (CH2), (500 29.6 MHz). (CH2), The 29.7 chemical shifts were referenced with respect to CHCl3 (δ 7.26), HDO (δ 4.79) for 1H, and CDCl3 (δ 77.0), DSS (sodium 3-(trimethylsilyl)-1-propanesulfonate) (δ 0.0) for 13C as internal standards. High

Molecules 2020, 25, 5617 10 of 14

(CH2), 29.9 (CH2), 30.0 (CH2), 55.5 (OCH2), 62.1 (NCH2), 68.2 (OCH3), 92.9 (C6H3), 93.5 (C6H3), 121.6 (C10H8N2), 125.9 (C10H8N2), 141.1 (C10H8N2), 145.8 (C10H8N2), 151.7 (C10H8N2), 154.0 (C10H8N2), 161.2 (C6H3), 161.6 (C6H3). 1 Compound with NO3 anion. H-NMR (500 MHz, D2O, r.t.): δ 0.84–1.23 (m, 16H, CH2), 1.34 (quin, 2H, OCH2CH2, J = 6.4 Hz), 1.95 (quin, 2H, NCH2CH2, J = 6.9 Hz), 3.49 (s, 6H, OCH3), 3.49 (brs, 2H, OCH2), 4.70 (t, 2H, NCH2, J = 7.2 Hz), 5.68 (s, 2H, C6H3), 5.81 (s, 1H, C6H3), 7.84 (d, 2H, C10H8N2, J = 5.5 Hz), 8.38 (d, 2H, C10H8N2, J = 6.5 Hz), 8.72 (d, 2H, C10H8N2, J = 5.0 Hz), 9.00 (d, 2H, C10H8N2, 1 J = 6.5 Hz). H-NMR (400 MHz, CDCl3, r.t.): δ 1.17–1.48 (m, 16H, CH2), 1.72 (brs, 2H, OCH2CH2), 2.01 (brs, 2H, NCH2CH2), 3.74 (s, 6H, OCH3), 3.88 (t, 2H, OCH2, J = 7.9 Hz), 4.76 (t, 2H, NCH2, J = 6.2 Hz), 6.05 (s, 2H, C6H3), 6.06 (s, 1H, C6H3), 7.67 (d, 2H, C10H8N2, J = 4.6 Hz), 8.28 (d, 2H, C10H8N2, J = 5.5 Hz), 8.85 (brs, 2H, C10H8N2, J = 5.0 Hz), 9.19 (d, 2H, C10H8N2, J = 5.8 Hz).

3.3. Preparation of [Pt(Me5dien)Cl]Cl ([2]Cl) The complex was prepared by slight modification of the literature method. MeOH (30 mL) was added to Me5dien (2.0 mL, 9.5 mmol) to form a solution. Further addition of cis-PtCl2(dmso)2 (4.0 g, 9.5 mmol), and addition of MeOH (20 mL) was followed by heating the mixture for 6 h under reflux. Cooling the solution at room temperature and evaporation of the solvent left a pale brown solid. Recrystallization of the solid product by acetone/CH2Cl2 yielded off-white crystals of [Pt(Me5dien)Cl]Cl 1 ([2]Cl) (4.1 g, 9.3 mmol, 98%). H-NMR (500 MHz, D2O, r.t.): δ 2.79 (s, 6H, CH3, JPt-H = 10.1 Hz), 2.93 (t, 2H, CH2, J = 3.2 Hz), 2.96 (t, 2H, CH2, J = 3.7 Hz), 2.98 (s, 6H, CH3, JPt-H = 12.5 Hz), 3.11 (s, 3H, CH3, 13 1 JPt-H = 21.0 Hz), 3.50 (td, 2H, CH2, J = 14.1, 3.5 Hz), 3.72 (td, 2H, CH2, J = 13.3, 3.9 Hz). C{ H}-NMR (125 MHz, D2O, r.t.): δ 48.4 (CH3), 54.4 (CH3), 55.7 (CH3), 64.9 (CH2), 70.7 (CH2). Treatment of the product by AgNO3 yielded [Pt(Me5dien)NO3]NO3 ([2](NO3)) in high yield 1 (168.0 mg, 0.33 mmol, 85%). H-NMR (400 MHz, D2O, r.t.): δ 2.79 (s, 6H, CH3, JPt-H = 10.3 Hz), 2.94 (t, 2H, CH2, J = 3.8 Hz), 2.96 (s, 6H, CH3), 2.97 (t, 2H, CH2, J = 3.5 Hz), 3.09 (s, 3H, CH3, JPt-H = 22.5 Hz), 13 1 3.52 (td, 2H, CH2, J = 14.0, 3.8 Hz), 3.70 (td, 2H, CH2, J = 13.3, 4.2 Hz). C{ H}-NMR (125 MHz, D2O, r.t.): δ 48.4 (CH3), 54.4 (CH3), 55.7 (CH3), 64.9 (CH2), 70.7 (CH2).

3.4. Preparation of [Pt(Me5dien)(4,40-bpy-N-(CH2)12OC6H3-3,5-(OMe)2)][NO3]3 ([3](NO3))

A suspension of [1](NO3) (372 mg, 0.67 mmol) in water (100 mL) was stirred, and [2]Cl (879 mg, 2.0 mmol) and AgNO3 (792 mg, 4.67 mmol) were added to the mixture, and mixed with water (50 mL). Water (50 mL) was added to the mixture, and the resulting suspension was heated for 66 h at 80 ◦C. After cooling to room temperature, the reaction mixture was ﬁltered to remove AgCl. Evaporation of the solvent from the ﬁltrate produced a brown solid (1.40 g). 1H- and 13C{1H}-NMR spectra of the solid showed that it contained [Pt(Me5dien)(4,40-bpy-N-(CH2)12OC6H3-3,5-(OMe)2)][NO3]3 (3) and 2 in 1:2 1 molar ratio. H-NMR (500 MHz, D2O, r.t.): δ 1.00–1.40 (m, 16H, CH2), 1.57 (quin, 2H, OCH2CH2, J = 6.7 Hz), 2.01 (quin, 2H, NCH2CH2, J = 6.5 Hz), 2.50–3.20 (19H, NCH3, NCH2), 3.43–3.80 (4H, NCH2), 3.54 (s, 6H, OCH3), 3.73 (brs, 2H, OCH2), 4.64 (t, 2H, NCH2, J = 7.2 Hz), 5.96 (s, 2H, C6H3), 6.02 (s, 1H, C6H3), 8.22 (d, 2H, C10H8N2, 4.5), 8.47 (d, 2H, C10H8N2, J = 7.0 Hz), 9.01 (d, 2H, C10H8N2, J = 6.5 Hz), 13 1 9.17 (d, 1H, C10H8N2, J = 5.5 Hz), 9.41 (d, 1H, C10H8N2, J = 6.0 Hz). C{ H}-NMR (100 MHz, D2O, r.t.): δ 30.5 (CH2), 31.3 (CH2), 31.3 (CH2),31.6 (OCH2CH2), 31.7 (CH2), 31.8 (CH2), 31.8 (2CH2) 33.2 (NCH2CH2), 47.6 (NCH2), 53.4 (NCH3), 55.2 (NCH3), 63.9 (NCH2), 64.0 (NCH2), 69.9 (NCH2), 70.3 (OCH2), 94.7 (C6H3), 95.6 (C6H3), 128.4 (C10H8N2), 128.6 (C10H8N2), 147.3 (C10H8N2), 147.9 (C10H8N2), 153.2 (C10H8N2), 155.6 (C10H8N2), 156.1 (C10H8N2), 163.0 (C6H3), 163.5 (C6H3). ESI-MS (eluent:H2O): 2+ 2+ calcd for [C39H64N5O3Pt] 422.73, found 422.73 (M ).

3.5. Rotaxane of α-CD with [Pt(Me5dien)(4,40-bpy-N-(CH2)12OC6H3-3,5-(OMe)2)][NO3]3) A mixture of [3]+ and [2]+ in 1:2 molar ratio (266.2 mg, 3: 0.13 mmol, 2: 0.26 mmol) was dissolved in water (12.0 mL). To the solution was added α-CD (256.8 mg, 0.26 mmol) and water (10.0 mL). The solution was heated for 102 h under reﬂux. The solution was cooled to room temperature, and the Molecules 2020, 25, 5617 11 of 14 resulting solution was analyzed by NMR spectroscopy and TEM. NMR data were obtained from the 1 equilibrated mixture of the rotaxane and starting materials. H-NMR (400 MHz, D2O, r.t.): δ 1.11–1.77 (m, 16H, CH2), 1.70 (quin, 2H, OCH2CH2, J = 5.2 Hz), 2.16 (quin, 2H, NCH2CH2, J = 6.1 Hz), 2.58–3.14 (19H, NCH3, NCH2), 3.46–3.80 (4H, NCH2), 3.56–3.74 (m, 12H, C36H72O36-H4, -H2), 3.74–3.96 (m, 12H, C36H72O36-H6, -H5), 3.82 (s, 6H, OCH3), 3.96–4.07 (m, 6H, C36H72O36-H3), 4.04 (brs, 2H, OCH2), 4.75 (t, 2H, NCH2, J = 8.4 Hz), 5.05–5.11 (m, 6H, C36H72O36-H1), 6.23 (s, 2H, C6H3), 6.30 (s, 1H, C6H3), 8.26 (d, 2H, C10H8N2, J = 5.8 Hz), 8.48 (d, 2H, C10H8N2, J = 6.3 Hz), 9.08 (d, 2H, C10H8N2, J = 6.4 Hz), 9.19 (d, 1H, C10H8N2, J = 6.1 Hz), 9.42 (d, 1H, C10H8N2, J = 5.5 Hz). ESI-MS (eluent:H2O): calcd for 2+ 2+ [C75H124N5O33Pt] 606.26, found 606.26 (M ).

3.6. Rotaxane of β-CD with [Pt(Me5dien)(4,40-bpy-N-(CH2)12OC6H3-3,5-(OMe)2)][NO3]3) A mixture of [3]+ and [2]+ in 1:2 molar ratio (12.1 mg, 3: 0.006 mmol, 2: 0.012 mmol) was dissolved in water (12.0 mL). To the solution was added β-CD (6.8 mg, 0.006 mmol) and water (1.0 mL). Stirring the solution for 5 min yielded a yellow solution. The solution was analyzed by NMR spectroscopy and TEM. NMR data were obtained from the equilibrated mixture of the rotaxane and starting materials. 1 H-NMR (500 MHz, D2O, r.t.): δ 1.24–1.49 (m, 16H, CH2), 1.69 (m, 2H, OCH2CH2), 2.12 (m, 2H, NCH2CH2), 2.58–3.18 (19H, NCH3, NCH2), 3.46–3.80 (4H, NCH2), 3.59–3.73 (m, 14H, C42H84O42-H4, -H2), 3.73–3.96 (m, 12H, C42H84O42-H6, -H5), 3.83 (s, 6H, OCH3), 3.96–3.90 (m, 6H, C42H84O42-H3), 4.01 (brs, 2H, OCH2), 4.72 (t, 2H, NCH2, J = 7.6 Hz), 5.05–5.11 (d, 6H, C36H72O36-H1, J = 3.6 Hz), 6.17 (s, 2H, C6H3), 6.28 (s, 1H, C6H3), 8.25 (brs, 2H, C10H8N2, J = 5.8 Hz), 8.48 (d, 2H, C10H8N2, J = 6.0 Hz), 9.08 (d, 2H, C10H8N2, J = 5.4 Hz), 9.18 (d, 1H, C10H8N2, J = 6.3 Hz), 9.41 (d, 1H, C10H8N2, J = 6.1 Hz).

3.7. Rotaxane of γ-CD with [Pt(Me5dien)(4,40-bpy-N-(CH2)12OC6H3-3,5-(OMe)2)][NO3]3 Preparation was conducted, similar to the rotaxane with β-CD. NMR data were obtained from the 1 equilibrated mixture of the rotaxane and starting materials. H-NMR (500 MHz, D2O, r.t.): δ 1.02-1.39 (m, 16H, CH2), 1.63 (m, 2H, OCH2CH2), 2.04 (m, 2H, NCH2CH2, J = 7.4 Hz), 2.58–3.22 (m, 19H, NCH3, NCH2), 3.59–3.70 (m, 14H, C48H96O48-H4, -H2), 3.66 (td, 2H, NCH2, J = 13.6, 4.1 Hz), 3.70–3.77 (m, 12H, C48H96O48-H6, -H5), 3.86 (td, 2H, NCH2, J = 12.7, 3.0 Hz), 3.81 (s, 6H, OCH3), 3.77–3.84 (m, 6H, C48H96O48-H3), 3.89 (brs, 2H, OCH2), 4.67 (t, 2H, NCH2, J = 8.3 Hz), 5.09 (d, 6H, C48H96O48-H1, J = 3.7 Hz), 5.97 (s, 2H, C6H3), 5.06 (s, 1H, C6H3), 8.24 (brs, 2H, C10H8N2), 8.49 (d, 2H, C10H8N2, J = 6.6 Hz), 9.04 (d, 2H, C10H8N2, J = 6.6 Hz), 9.18 (d, 1H, C10H8N2, J = 6.4 Hz), 9.42 (d, 1H, C10H8N2, J = 6.2 Hz).

3.8. Reduction of Pt Complex and Its Rotaxanes

A solution of [3](NO3) (0.1–12 µmol) in water ([3] = 0.1–12.0 mM, 1.0 mL) was prepared. It was treated with a solution of NaBH4 (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.

4. 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 NaBH4 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. Molecules 2020, 25, 5617 12 of 14

Author Contributions: Individual contributions of the authors are as follows. Conceptualization, Y.S. and Y.F.; investigation, Y.S., Y.F., and K.O.; writing—original draft preparation and editing, K.O.; funding acquisition, Y.S. and K.O. All authors have read and agreed to the published version of the manuscript. 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. Acknowledgments: We appreciate our colleagues in the Materials Analysis Division, Technical Department, Tokyo Institute of Technology. Conﬂicts of Interest: The authors have no conﬂict of interest.

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