n+ (n = 1, 2): Synthesis and Geometric and Electronic Structures

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Article 17

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[Pt (CO) (PPh)] (n = 1, 2): Synthesis and Geometric/Electronic Structures Lakshmi V. Nair, Sakiat Hossain, Shota Wakayama, Shunjiro Takagi, Mahiro Yoshioka, Juri Maekawa, Atsuya Harasawa, Bharat Kumar, Yoshiki Niihori, Wataru Kurashige, and Yuichi Negishi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00978 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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The Journal of Physical Chemistry

[Pt17(CO)12(PPh3)8]n+ (n = 1, 2): Synthesis and Geometric/Electronic Structures Lakshmi V. Nair,a Sakiat Hossain,b Shota Wakayama,a Shunjiro Takagi,a Mahiro Yoshioka,a Juri Maekawa,a Atsuya Harasawa,a Bharat Kumar,a Yoshiki Niihori,a Wataru Kurashige,a and Yuichi Negishia,b,* a

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162−8601, Japan.

b

Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278−8510, Japan.

1 Environment ACS Paragon Plus


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ABSTRACT. Recently, platinum (Pt) clusters have attracted attention as miniaturized fuel-cell redox catalysts. Although Pt clusters can be synthesized with atomic accuracy using carbon monoxide (CO) and phosphine as ligands, few studies have examined their electronic structure. We obtained experimental information about the electronic structure of these Pt clusters. We precisely synthesized the cationic Pt17 cluster, [Pt17(CO)12(PPh3)8]n+ (n = 1, 2), protected by CO and triphenylphosphine (PPh3) by a simple method and studied its geometric and electronic structure by single-crystal X-ray structure analysis, X-ray photoelectron spectroscopy, optical absorption spectroscopy, differential pulse voltammetry, and photoluminescence spectroscopy. The results indicated that cationic [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) has a geometric structure similar to that of previously reported neutral Pt17(CO)12(PEt3)8. The Pt17 skeleton of Pt17(CO)12(PPh3)8 depended on the charge state of the cluster ([Pt17(CO)12(PPh3)8]+ or [Pt17(CO)12(PPh3)8]2+). [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) possessed a discretized electronic structure, similar to that of fine gold clusters, and exhibited photoluminescence in the near-infrared region. This research will aid fundamental and applied research on Pt clusters.


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1. INTRODUCTION In recent years, it has become possible to synthesize gold clusters,1−8 silver clusters,9−11 and alloy clusters12−14 with atomic precision using thiolate or phosphine (PR3) as a ligand. The electronic/geometric structures and size-specific physical/chemical properties of these metal clusters have also been investigated extensively.15−18 Applied research on these clusters has been conducted in a wide variety of fields such as sensing,19 imaging,20 cancer radiation therapy,21 catalysis,22 photocatalysis,23 solar cells,24 photosensitization,25 and single-electron devices26.27,28 Similar to these metal clusters, platinum (Pt) clusters have also attracted much interest. An attractive feature of Pt clusters is their high catalytic activity in a variety of reactions.29−31 In particular, Pt clusters have recently attracted considerable attention from the viewpoint of the miniaturization of redox catalysts used in fuel cells.30,31 In the precise synthesis of these Pt clusters, carbon monoxide (CO) or PR3 is used as the main ligand.32−39 Since the late 1970s, a number of Pt clusters with the structure Ptn(CO)m(PR3)l have been synthesized, including [Pt15(CO)19]4−, [Pt19(CO)22]4−, [Pt38(CO)44]2−, and Pt17(CO)12(PEt3)8, and their geometric structures have been determined by single-crystal X-ray structural analysis.32−39 However, little information has been obtained on the electronic structure and physical/chemical properties of Ptn(CO)m(PR3)l clusters to date. In this research, the final objective is to obtain experimental information about the largely unknown electronic structure of Ptn(CO)m(PR3)l clusters. To this end, we precisely synthesized a Pt17 cluster ([Pt17(CO)12(PPh3)8]n+; n = 1, 2) protected by CO and triphenylphosphine (PPh3) by a simple method and studied its geometric and electronic structure. Mass spectrometry, elemental analysis, and single-crystal X-ray structural analysis of the product revealed that the obtained Pt17(CO)12(PPh3)8 comprises positively charged [Pt17(CO)12(PPh3)8]+ and [Pt17(CO)12(PPh3)8]2+,



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having a geometric structure similar to that36 of neutral Pt17(CO)12(PEt3)8. The optical absorption spectroscopy and electrochemical measurements of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) demonstrated that [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) has a discrete electronic structure. Furthermore, the emission spectroscopy revealed that [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) exhibits photoluminescence in the near-infrared region.

Scheme 1. Comparison of the synthesis methods of (a) neutral Pt17(CO)12(PEt3)8 (ref. 36) and (b) cationic [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) (this work). Each method requires a purification process. 2. RESULTS AND DISCUSSION 2.1. Synthesis. In previous work, neutral Pt17(CO)12(PEt3)8 was synthesized in the following three steps36: 1) a Pt salt (Na2PtCl4) was reacted with CO to produce a [Pt(CO)2]n oligomer, 2) the [Pt(CO)2]n oligomer was reacted with PEt3 to give Pt5(CO)6(PEt3)4, and 3) Pt17(CO)12(PEt3)8 was synthesized by thermal decomposition of Pt5(CO)6(PEt3)4 (Scheme 1(a)). In this study, we synthesized cationic [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) in quite a different way (Scheme 1(b)). Specifically, an ethylene glycol solution containing H2PtCl6 and NaOH was heated at 120 °C, which induced the reduction of Pt ions40,41 and the generation of CO by oxidation of ethylene


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glycol.41 Then, PPh3 was added to the solution to prepare Ptn(CO)m(PR3)l clusters containing both CO and PPh3 (Experimental Section).

Figure 1. Positive-ion MALDI mass spectra of Ptn(CO)m(PPh3)l clusters (a) before and (b) after purification. The possible assignments of peaks in (b) are shown in (c).

Figure 1(a) shows the matrix-assisted laser desorption ionization (MALDI) mass spectrum of the obtained clusters (Figure S1). In the mass spectrum, broad peaks appear around m/z = 3200, 4200, and 7000, indicating that the obtained clusters have a mass distribution (Figure S2). The main product (m/z ~ 4200) was separated from this mixture by the difference in solubility of the clusters in a mixture of acetone/toluene (1:1). Figure 1(b) shows the MALDI mass spectrum of



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Figure 2. Positive-ion ESI mass spectra of the main products of the experiments using (a) PPh3 and (b) PPh2(p-tol). Insets show the isotope distributions of +2 peak groups together with calculated patterns. The chemical composition of the peak indicated by an asterisk (*) in (b) is assigned in Figure S3.

the major product after separation. In this mass spectrum, a peak appears only in the vicinity of m/z ~ 4200, indicating that a product with a mass of m/z ~ 4200 was separated with high purity. Thus, when using MALDI mass spectrometry, almost all clusters are ionized. Therefore, it is possible to obtain information about the mass distribution of a product. However, laser irradiation of the cluster induces desorption of CO and dissociation/elimination of PR3 (Figure 1(c)). Consequently, it is difficult to determine the exact chemical composition of the product by MALDI mass spectrometry (Figure 1(c)). Then, the electrospray ionization (ESI) mass spectrum of the obtained major product (Figure 1(b); simply called the “product” hereafter) was collected to determine its chemical composition. Figure 2(a) shows the positive-ion ESI mass spectrum of the product. Strong peak groups appear in the mass spectrum around m/z = 2875 and 5750. In the former peak group, isotopic distribution was observed at an interval of m/z = 0.5 (inset of Figure 2(a)), whereas the interval


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Figure 3. FT-IR spectra of [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) over the regions of (a) 500−4000 cm−1 and (b) 1200−2200 cm−1. Assignments are listed in Table S1. was m/z = 1.0 in the latter one (Figure S3). These isotopic distributions agreed well with those of [Pt17(CO)12(PPh3)8]2+ and [Pt17(CO)12(PPh3)8]+, respectively. No peaks were observed in the spectrum obtained in negative-ion mode (Figure S4). These results indicate that: 1) CO was generated and subsequently coordinated41, and thereby a Pt cluster containing both CO and PPh3 as ligands was synthesized by our simple method, and 2) the product has a chemical composition of Pt17(CO)12(PPh3)8. For Pt clusters with both CO and PR3 ligands, few mass spectra that do not contain dissociated components have been reported.42 We conducted two further experiments to confirm the abovementioned interpretation. First, to confirm that CO was actually contained in the product, a Fourier transform infrared (FT-IR) spectrum of the product was obtained using the attenuated total reflectance (ATR) method (Figure 3). In addition to a peak attributed to PPh3, the spectrum contained peaks assigned to



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terminal CO, terminal CO, and μ2-CO at 1974, 1941, and 1742 cm−1, respectively (Table S1).43 These three peak positions are in good agreement with those derived from CO in the spectrum of Pt17(CO)12(PEt3)8 (1963, 1944, and 1720 cm−1).36 These results indicate that the product contains both CO and PPh3. Next, we synthesized the clusters by using diphenyl(p-tolyl)phosphine (PPh2(p-tol)) instead of PPh3 to confirm the chemical composition of the products. Figure 2(b) shows the positive-ion ESI mass spectrum of the product obtained from this experiment (Figure S5). The mass spectrum contains strong groups of peaks near m/z = 2931 and 5862. The isotopic distributions of these peaks confirmed that the former was divalent (inset of Figure 2(b)) and the latter monovalent. Compared with the spectrum in Figure 2(a), the mass of the product increased by 112 Da. The mass difference between PPh3 and PPh2(p-tol) is 14.0 Da. Thus, a deviation of 112 Da (= 14.0 Da × 8) indicates that each cluster contains eight PR3 units. These results clearly show that clusters with chemical compositions of Pt17(CO)12(PPh3)8 and Pt17(CO)12(PPh2(p-tol))8 were produced in this study. We also examined the charge states of the clusters by inductively coupled plasma mass spectrometry (ICP-MS). Based on the results of ESI mass spectrometry, it was anticipated that the synthesized clusters would be positively charged. Furthermore, considering the synthesis conditions, the counter anion in the product was expected to be a chloride ion (Cl−). However, because it is difficult to detect Cl− by ICP-MS, the product was mixed with NaSbF6 to exchange the counter anion from Cl− to SbF6−. No change in the peak distribution in the ESI mass spectrum was observed after this exchange (Figure S6). ICP-MS analysis of the product after exchange revealed that it contained Pt and Sb with a ratio of 17.0:1.2. This result indicates that the synthesized Pt17(CO)12(PPh3)8 was actually positively charged and the main product (~80%) was


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Figure 4. Geometric structures of (a) [Pt17(CO)12(PPh3)8][SbF6] and (b) [Pt17(CO)12(PPh3)8][(SbF6)2]. H and SbF6 were removed for clarity. (c) Comparison of the Pt17 framework structure of [Pt17(CO)12(PPh3)8][SbF6] (left) and [Pt17(CO)12(PPh3)8][(SbF6)2] (right).

[Pt17(CO)12(PPh3)8]+. This interpretation is also consistent with the results of the single-crystal Xray structural analysis of the clusters described next. It is considered that the observation of [Pt17(CO)12(PPh3)8]2+ with higher intensity in the ESI mass spectrum is related to the ionization of a part of [Pt17(CO)12(PPh3)8]+ into [Pt17(CO)12(PPh3)8]2+ during ESI process. Similar phenomenon has often seen in the ESI mass spectrometry of the thiolate-protected gold clusters.44−46 2.2. Geometric Structure. The geometric structure of Pt17(CO)12(PPh3)8 obtained by anion substitution was determined by single-crystal X-ray structural analysis. Single crystals were grown by the vapor diffusion method (Experimental Section). Black single crystals with two shapes (needle and block; Figure S7) were obtained after 3–4 days at room temperature. Based on the structural analysis of each type of single crystal, we concluded that the needle- and block-



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Figure 5. Pt 4f7/2 spectrum of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) together with the peak positions of Pt(0)51 and Pt(II)52 (gray lines). Orange curve shows the results of the curve fittings and green curve shows the base line. like crystals were single crystals of [Pt17(CO)12(PPh3)8][SbF6] and [Pt17(CO)12(PPh3)8][(SbF6)2], respectively. The most (~80%) crystals were of needle shape. This means that most of the products were mono-cationic [Pt17(CO)12(PPh3)8]+, consisting with the result of ICP-MS analysis. Figure 4(a) and (b) depict the geometric structures of [Pt17(CO)12(PPh3)8]+ and [Pt17(CO)12(PPh3)8]2+, respectively. Both clusters have a geometric structure in which the icosahedral Pt13 core is surrounded by the following four units:47 (i) CO, ii) μ2-CO, iii) PPh3, and iv) capping Pt2(µ2-CO)2(PPh3)2. These structures are very similar to that of neutral Pt17(CO)12(PEt3)8.36 Figure 4(c) compares the Pt17 skeletons of [Pt17(CO)12(PPh3)8]+ and [Pt17(CO)12(PPh3)8]2+. Both clusters have a Pt17 skeleton with an ellipsoid-like structure. However, a Pt17 skeleton in [Pt17(CO)12(PPh3)8]2+ is more elongated than that in [Pt17(CO)12(PPh3)8]+. This means that the Pt17 skeleton of Pt17(CO)12(PPh3)8 changes slightly depending on the charge state of the cluster. This phenomenon is in good agreement with the characteristics of the phenylethanethiolate (SC2H4Ph)-protected Au25 cluster Au25(SC2H4Ph)18; Jin et al.48,49 and Ackerson et al.50 demonstrated that the Au13 skeleton is slightly different depending on the charge state of the cluster ([Au25(SC2H4Ph)18]−, [Au25(SC2H4Ph)18]0 or [Au25(SC2H4Ph)18]+).


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Figure 6. Optical absorption spectra of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) in dichloromethane solution (blue) and solid [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) (green). In (a) and (b), spectra are displayed with a horizontal axis of wavelength and photon energy, respectively.

2.3. Electronic Structure. Four type of measurements were performed to investigate the electronic structure of the synthesized mixture of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2). Figure 5 shows the Pt 4f7/2 X-ray photoelectron spectrum of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2). In the spectrum, a peak appears around 71.7 eV. This peak position is close to that of bulk Pt (71.0 eV)51, which is quite different from that of Pt(II) (73.4 eV)52. This indicates that the Pt atoms in each cluster are mostly zero-valent Pt(0). Importantly, the peak position is shifted to slightly higher energy compared with that of bulk Pt. This peak shift is considered to be caused by the overlap of three effects: charge transfer between Pt and CO (electron donation and back-



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donation),53 charge transfer between Pt and PPh3 (electron donation and back-donation),53 and the average charge of +1.2 of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2). Figure 6(a) depicts optical absorption spectra of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2). The spectrum contains peaks at 283, 320, 382, 410, 445, 565, 664, and 916 nm. These peak structures are observed more clearly in the spectra obtained by transforming the horizontal axis from wavelength to photon energy (Figure 6(b)). For Ptn clusters without protective ligands, experimental54 and theoretical55 studies have revealed that discretization of the electronic levels occurs at small n. Furthermore, a theoretical calculation predicted that [Pt13(CO)12]2− and [Pt13(CO)24]2− also possess discrete electronic structures.56 Although the absorption at shorter wavelength might include peaks originating from charge transfer between Pt and the CO or PPh3 ligands,57 the peak structures observed in the visible to near-infrared range appear to be caused by the discretization of the electronic structure of the Pt core. It is also expected that the electronic structure of fine metal clusters depends on their charge state.48,49,58 However, the present sample is expected to contain a high proportion (~80%) of [Pt17(CO)12(PPh3)8][SbF6]. This could be the reason why the peak structure was clearly observed. Figure 7 shows the differential pulse voltammetry (DPV) curve of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2). Clear peak structures appear in the voltammogram. The intervals between the peaks observed in this measurement were not identical. Therefore, the observed peaks were not caused by the Coulomb blockade but rather by discretization of the electronic structure of the cluster.59 Considering the charge state of the major product ([Pt17(CO)12(PPh3)8][SbF6]) and the position of the open-circuit potential, the peaks at 0.864, 0.04, −0.335, −1.24, and −1.69 V can be attributed to redox reactions between the +2/+1, +1/0, 0/−1, −1/−2, and −2/−3 states, respectively (Figure


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Figure 7. DPV curve of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) in dichloromethane solution. The vertical arrow indicates the open-circuit potential. Tentative assignments of the peaks in this curve are denoted, where +2/+1 represents the redox reactions between the charge states of +2 and +1. The peak indicated by an asterisk (*) is presumed to be caused by impurities.

Figure 8. Photoluminescence spectrum (red) of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) in dichloromethane solution together with optical absorption (gray) and excitation (blue) spectra.

7). Figure 7 reveals that Pt17(CO)12(PPh3)8 remains stable in the charge states between +2 and −3 over the time scale of the DPV measurement. Figure 8 shows the photoluminescence spectrum of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) excited at 672 nm. [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) exhibits photoluminescence in the nearinfrared region (~940 nm). The quantum yield of this photoluminescence was too low (probably below 1 × 10−3) to determine exactly in this experiment.



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3. CONCLUSIONS The cationic [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) clusters were synthesized and their geometric and electronic structures were examined. The results revealed that [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) have a geometric structure similar to that of neutral Pt17(CO)12(PEt3)8. The Pt17 skeleton of Pt17(CO)12(PPh3)8 changed slightly depending on the charge state of the cluster. The electronic structure of [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) was discrete, similar to that of fine Au clusters such as Au25(SC2H4Ph)18. The method used to synthesize [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) is extremely simple compared with those reported for other Pt clusters such as Pt17(CO)12(PEt3)8. Furthermore, we demonstrated that the chemical composition of Ptn(CO)m(PR3)l clusters can be estimated by ESI mass spectrometry. Because Pt17(CO)12(PPh3)8 contains PPh3 as a ligand, it could be used as a precursor of a supported Pt clusters.1 Based on the results of this research, it is expected that more fundamental and applied research will be conducted on platinum clusters in the future.

4. EXPERIMENTAL SECTION 4.1. Chemicals. All chemicals were commercially obtained and used without further purification. Hydrogen hexachloroplatinate hexahydrate (H2PtCl6·6H2O) was purchased from Tanaka Kikinzoku. Sodium hydroxide (NaOH), ethylene glycol, triphenylphosphine (PPh3), dichloromethane, dry dichloromethane, diphenyl(p-tolyl)phosphine (PPh2(p-tol)), barium sulfate, platinum (Pt) standard solution (1000 mg/L), antimony (Sb) standard solution (1000 mg/L), and bismuth (Bi) standard solution (1000 mg/L) were obtained from Wako Pure Chemical Industries. Acetone, methanol, diethyl ether, and toluene were sourced from Kanto Chemical Co., Inc. Sodium hexafluoroantimonate (NaSbF6) was purchased from Aldrich. Tetrabutylammonium


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perchlorate (TBAP) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) were purchased from Tokyo Chemical Industry. Pure Milli-Q water (18.2 MΩ·cm) was generated using a Merck Millipore Direct 3 UV system. 4.2. Synthesis of [Pt17(CO)12(PPh3)8]n+ (n = 1, 2). H2PtCl6 (0.1 mmol) and NaOH (~2 mmol) were dissolved in ethylene glycol (25 mL). NaOH was used to control the pH of the solution and thereby suppress the particle size obtained by the polyol reduction40,41,60 and accelerate the oxidation of ethylene glycol.41 The mixture was heated at 120 °C for 10 min (Figure S8) to reduce Pt ions and produce CO catalyzed by Pt ions.41 The color of the solution changed from yellow to dark brown. After cooling to room temperature (25 °C), acetone (10 mL) containing PPh3 (0.5245 g, 2 mmol) was added to this solution at once. After several minutes, toluene (~20 mL) and water (~20 mL) were added to the reaction solution. The Pt clusters including Pt17(CO)12(PPh3)8 were transferred into the organic phase. Then, the organic phase was separated from the water phase and dried with a rotary evaporator. The dried product was washed with water and then methanol to eliminate ethylene glycol and excess PPh3. At this stage, the product was still a mixture of clusters of several sizes, as shown in Figure 1(a) and S2. The product was dried and then washed with a mixture of acetone/toluene (1:1). Only ~1 mg of Pt17(CO)12(PPh3)8 was obtained in this study because of the rigorously washing for obtaining the high-purity cluster. As-synthesized Pt17(CO)12(PPh3)8 was soluble in both dichloromethane and toluene. It was stable for 1-2 days in solution under atmospheric condition. However, it became a blackish precipitate when it was further left in solution. Thus, as-synthesized Pt17(CO)12(PPh3)8 was not very stable in the solution phase, although it was stable in the powder form. This low stability in the solution phase was overcome to some degree by exchanging the counter anions to SbF6−. As shown in Figure

S9,

a

negligible

degradation

was

observed


during

one

week

for


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[Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) in solution. It has been revealed that CO works as reducing agent in the synthesis of gold clusters.61,62 However, Figure S9 shows that the ligated CO does not further reduce the cluster and instead highly stabilizes the clusters unlike the case of gold clusters. 4.3. Exchange of Counter Anions. The counter anion of [Pt17(CO)12(PPh3)8]n+ (n = 1, 2) was exchanged to both monitor the counter ion by ICP-MS and facilitate the crystallization of the cluster. Specifically, the synthesized Pt17(CO)12(PPh3)8 (1.73 × 10−4 mmol) was stirred with excess NaSbF6 (1.73 × 10−2 mmol) in toluene (10 mL) at room temperature for 1 h. The exchanged

clusters

precipitated

in

toluene

because

[Pt17(CO)12(PPh3)8][SbF6]

and

[Pt17(CO)12(PPh3)8][(SbF6)2] are less soluble in toluene in contrast to the as-synthesized [Pt17(CO)12(PPh3)8]n+ (n = 1, 2). Most of the products precipitated and the solution became almost colorless, implying that most Pt17(CO)12(PPh3)8 clusters in the product were cationic [Pt17(CO)12(PPh3)8]+ or [Pt17(CO)12(PPh3)8]2+. After the supernatant was removed from the solution, the precipitate was washed with toluene. 4.4.

Crystallization.

[Pt17(CO)12(PPh3)8][SbF6]

and

[Pt17(CO)12(PPh3)8][(SbF6)2]

were

crystallized by vapor diffusion at room temperature. To grow crystals, the anion-exchanged Pt17(CO)12(PPh3)8 (1 mg) was dissolved in dichloromethane (500 μL) in an inner vial and diethyl ether was used as the anti solvent in the outer vial. Needle-type (Figure S6(b)) and block-type (Figure S6(a)) crystals were obtained in 3–4 days. 4.5. Stability Experiment. [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) (1.6 mg) was dissolved in dichloromethane

(1

mL)

and

left

at

room

temperature.

The

degradation

of

[Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) in solution was monitored by observing the optical absorption spectrum of the solution over time (Figure S9).


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4.6. Characterization. MALDI mass spectra were collected by a spiral time-of-flight mass spectrometer (JEOL, JMS-S3000) with a semiconductor laser. DCTB63 was used as the MALDI matrix. To minimize dissociation of the cluster caused by laser irradiation, we used a cluster-tomatrix ratio of 1:1000. ESI mass spectrometry was performed using a reflectron time-of-flight mass spectrometer (Bruker, microTOF II). In these measurements, a cluster solution with a concentration of ~10 μg/mL in dichloromethane was electrosprayed at a flow rate of 180 μL/h. FT-IR ATR spectra were recorded in the region between 400 and 4000 cm−1 using a JASCO FT/IR−4600−ATR−PRO ONE spectrometer equipped with a DLATGS detector as the average of 50 scans at 4-cm−1 resolution. ICP-MS was conducted using an Agilent 7500c spectrometer (Agilent Technologies, Tokyo, Japan). ICP-MS analysis was used to estimate the ratio between Pt and Sb. Bi was used as an internal standard. Pt and Sb standard aqueous solutions were used for calibration. Diffraction data for the crystal samples were collected on a SMART APEX 2 Ultra equipped with an Apex II CCD diffractometer. From the collected data, unit cell, integration, absorption correction (multi-scan), and space group (P-1 based on intensity statistics and systematic absences) were determined using the Bruker APEX 3 software package.64 Crystal structures were solved by the intrinsic phasing method in APEX 3.65 Final refinements were performed by SHELXL-2014/766 using the Olex 2 platform.67 X-ray photoelectron spectroscopy data were collected using an electron spectrometer (JEOL, JPS-9010MC) equipped with a chamber at a base pressure of ~2×10−8 Torr. X-rays from the MgKa line (1253.6 eV) were used for excitation.



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Optical absorption spectra of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) were acquired in dichloromethane solution or the solid state at ambient temperature with a spectrometer (JASCO, V-630 or V-670). Wavelength-dependent optical data, I(w), were converted to energy-dependent data, I(E), using the following equation, which conserved the integrated spectral areas.

! " =

$ % &' &(

∝ ! * *+

DPV was performed at room temperature using an electrochemical analyzer (BAS, ALS610D). Cluster solutions were prepared by adding [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) (~5 mg) into dry dichloromethane (6 mL) containing TBAP (200 mg). The electrochemical cell contained a glassy carbon working electrode and Pt wire counter electrode. Measurements were conducted under an argon atmosphere. After obtaining DPV data for the clusters, ferrocene (~5 mg) was added to the solution and the potential of the observed peaks was used as a reference. Photoluminescence spectra of [Pt17(CO)12(PPh3)8][(SbF6)n] (n = 1, 2) were recorded in dichloromethane at ambient temperature using a spectrofluorometer (Shimadzu, NIR-PL system). Measurements were performed at room temperature without prior degassing of the sample solutions.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Tel.: +81−3−5228−9145


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Notes The authors declare no competing financial interest.

Supporting Information. ESI mass spectra of the products, photographs of the crystals, condition dependence of the products, and crystal data for [Pt17(CO)12(PPh3)8][SbF6] and [Pt17(CO)12(PPh3)8][SbF6]2.

ACKNOWLEDGMENTS We thank Associate Prof. Michito Yoshizawa (Tokyo Institute of Technology) for use of their ESI and single-crystal XRD apparatus, and Dr. Yoshihisa Sei (Tokyo Institute of Technology), Dr. Kohei Yazaki (Tokyo Institute of Technology), Mr. Yoshihiro Kikuchi (Tokyo University of Science) and Mr. Shun Yoshino (Tokyo University of Science) for technical assistance. This work was supported by JSPS KAKENHI Grants (numbers JP15H00763, JP15H00883, JP16H04099, 16K17480, and 16K21402). Funding from the Nippon Sheet Foundation for Materials Science and Engineering, the Sumitomo Foundation, the Takahashi Industrial and Economic Research Foundation, the Tanaka Kikinzoku Memorial Foundation, and the Futaba Electronics Memorial Foundation is also gratefully acknowledged.

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n+ (n = 1, 2): Synthesis and Geometric and Electronic Structures

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