Monodisperse Iridium Clusters Protected by Phenylacetylene

Mar 1, 2017 - This finding suggests a drastic change in the binding sites of PA ... sites of PA are shifted dramatically from on-top sites to bridged ...
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Monodisperse Iridium Clusters Protected by Phenylacetylene: Implication for Size-Dependent Evolution of Binding Sites Hiroki Yamamoto, Prasenjit Maity, Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, Wataru Kurashige, Yuichi Negishi, and Tatsuya Tsukuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12121 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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The Journal of Physical Chemistry

Monodisperse Iridium Clusters Protected by Phenylacetylene: Implication for Size-Dependent Evolution of Binding Sites Hiroki Yamamoto,1 Prasenjit Maity,1# Ryo Takahata,1 Seiji Yamazoe,1,2 Kiichirou Koyasu,1,2 Wataru Kurashige,3 Yuichi Negishi,3,4 and Tatsuya Tsukuda1,2* 1

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan. 3 Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. 2

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

#Present address: Institute of Research and Development, Gujarat Forensic Sciences University, Gandhinagar 382007, Gujarat, India.

ABSTRACT: Terminal alkynes form a variety of interfacial structures with the clusters and nanoparticles of Au, Ag, Cu, Ru, Pd, and Pt. In order to extend the scope of modification by alkynes, we herein report the synthesis and structure characterization of Ir clusters protected by phenylacetylene (PA). Small, monodisperse PA-protected Ir clusters (1.3 ± 0.2 nm) were obtained by biphasic ligand exchange from ethylene glycol-stabilized Ir clusters (1.5 ± 0.2 nm). High-resolution transmission electron microscopy, extended X-ray absorption fine structure analysis, and powder X-ray diffraction analysis indicated that Ir clusters exhibit a facecentered cubic structure. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated Ir atoms are almost in the zero valence state, which is in sharp contrast to the partial oxidation observed for Ir clusters stabilized by polymers. Absence of the terminal hydrogen of PA and red-shift of the C ≡ C stretching mode upon ligation observed by Fourier-transform infrared spectroscopy showed that the PA ligands are bound to Ir clusters via Ir–C bonds. Mass spectrometry revealed that the number of the Ir atoms in Irn(PA)m has a much narrower distribution (n = 46–53) than that of the PA ligands (m = 19–33). This finding suggests a drastic change in the binding sites of PA ligands even with a small change of the size of Ir clusters. Surprisingly, the number of PA ligands (m) decreased with increase in the size of Ir clusters (n). Within the framework of the upright binding model of PA, this counterintuitive correlation between n and m values suggests that binding sites of PA are shifted dramatically from on-top sites to bridged sites with increase in the cluster size. and its alloys with Ag.1–7 Hence, it is desirable to extend the scope of applications of alkynyl protection to other metal clusters. In this study, the synthesis and characterization of iridium clusters protected by a typical alkyne, phenylacetylene (PA), was reported. Recently, nanoparticles and clusters of Ir have attracted significant attention as active, selective catalysts for various reactions, e.g., CO2 fixation, hydrogenation, and aerobic oxidation.14–23 In these studies, Ir nanoparticles and clusters were synthesized using ligands (thiols, phosphines), polymers (polyvinylpyrrolidone (PVP), dendrimers), surfactants (ethylene glycol (EG)), as stabilizers or zeolites as support. Examples of atomically precise synthesis are limited to Ir4, Ir6 and Ir9.23–25 Motivated by the successful synthesis of Au analogs, e.g., Au34(PA)16 and Au54(PA)26,1–3 PA-protected Ir clusters (Ir:PA) were synthesized via the ligation of PA to the preformed Ir clusters stabilized by EG. The structures of Ir:PA clusters thus prepared, including core size, atom packing structure, and interfacial structure, were investigated by means of a wide variety of spectroscopic methods. It was found that the PA ligands are bound to Ir clusters with fcc structures via

1. INTRODUCTION Surface modification by alkynes to metal clusters1–7 and nanoparticles8–11 can impart unique properties different from that by conventional ligands such as thiolates and phosphines. This is because alkynes can form a variety of interfacial structures between the clusters and nanoparticles of metal (M = Au, Ag, Cu, Ru, Pd, Pt) such as metal–carbene (M=C),8 metal–acetylide (M–C≡),8,9,11 metal–vinylidene (M=C=C),8,10,11 and PhC≡C–M–C≡CPh oligomers.4–6,12 In contrast, thiolates preferentially form Au(I)–SR oligomers and phosphines are exclusively bonded through the Au–P bonds.13 Electronic charge is delocalized over the functional moieties of ligands and the metallic core through the M–C bonds, leading to novel optical and electronic properties.7,8 For example, it was demonstrated that the photophysical properties of the Au core were tuned by the electronic perturbation of the π-conjugated system induced by protonation of the pyridine moiety.7 Thus, the decoration of well-defined metal clusters with M–C bonds is promising for the development of novel functional nanomaterials. However, thus far, the atomically precise synthesis of alkynyl-protected metal clusters is limited to Au

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Ir–C bonds. Mass analysis showed that the number of the Ir atoms in Irn(PA)m has a narrower distribution (n = 46–53) than that of the PA ligands (m = 19–33) and that the m values decreased with increase in the n values. These counterintuitive correlations between n and m suggested that the binding sites of the PAs on Ir clusters change dramatically even with a small change in the cluster size. 2. METHODS 2.1. Synthesis. The reduction of K2IrCl6 by NaBH4 in the presence of PA afforded black precipitates, which could not be redispersed. Thus, biphasic ligand exchange was employed for synthesis of Ir:PA as in the case of Au:PA clusters.1–3 Ir clusters dispersed in EG (Ir:EG) were extracted by PA dissolved in chloroform: PVP-stabilized Ir clusters were not extracted probably because of the partial oxidation of their surfaces.18,21 Ir:EG clusters were prepared as follows. First, 242 mg (0.5 mmol) of K2IrCl6 was dissolved in 10.0 mL of EG containing 40 mg of NaOH and the solution was stirred for 1 h at 160 °C under Ar.26 Second, 1.0 mL of the Ir:EG dispersion was diluted with 14.0 mL of EG, and it was placed on 10.0 mL of a chloroform solution containing 0–770 µL of PA (0–140 equivalent of Ir). Next, the biphasic solutions were stirred at 60 °C under ambient condition for 1–5 h. In this process, a part of the brownish color was transferred to the organic phase, indicating that Ir clusters were extracted by ligand exchange from EG to PA. However, the clusters could not be completely extracted to the chloroform phase: the EG phase was colored and black flocculates were produced under all experimental conditions employed. To estimate phase-transfer efficiency, these experiments were conducted using volumetric instruments and the optical spectrum of the chloroform layer was recorded after dilution to 50%. Finally, the chloroform layer was collected and evaporated to near dryness. The Ir:PA clusters contained in the solid residue were purified by centrifugal precipitation (2000 rpm, 5 min) with methanol (10 mL) twice and with hexane (10 mL) twice. Hexane solution formed during the washing was colored in yellow (Figure S1a) and contained the polymers of PA identified by mass spectrometry (Figure S1b) and vibrational spectroscopy (Figure S1c). The Ir:PA clusters stored in a chloroform solution containing PA (50 mM) at −20 °C were used for characterization. 2.2. Optical spectroscopy. Ultraviolet–visible–near infrared (UV–Vis–NIR) absorption spectra of Ir:EG dispersed in EG and Ir:PA dispersed in chloroform were recorded on a V-770 spectrophotometer (JASCO). 2.3. Fourier-transformed infrared (FTIR) spectroscopy. FTIR spectra of Ir:PA were measured in the transmission mode using an FTIR-4000 instrument (JASCO). The specimen was prepared by making a pellet from the ground mixture of Ir:PA and KBr at a mass ratio of 1:100. 2.4. High-resolution transmission electron microscopy (HRTEM). HRTEM images were recorded on an ARM-200F microscope operated at 120 kV and a HF-2000 microscope operated at 200 kV (HITACHI). The specimens were prepared by drop-casting the EG dispersion of Ir:EG or the chloroform dispersion of Ir:PA on a carbon film (Okenshoji Co., Ltd.) and drying under vacuum for 1 h. The size distribution was estimated by analyzing ~300 particles. 2.5. Matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectrometry. MALDI-TOF

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mass spectra were recorded using an AXIMA-CFR mass spectrometer (Shimadzu). Trans-2-[3-(4-tert-butylphenyl)-2methyl-2-propenylidene]-malononitrile (DCTB) was used as the matrix. The specimens were prepared as follows: 1 mg of Ir:PA and 1 mg of DCTB were dissolved in chloroform with the volume of 250 and 50 µL, respectively. The mixture of the above chloroform solutions with the same volume was dropcast on the sample plate and dried in vacuum for 5 min. The spectra were measured in the negative-ion mode. 2.6. Electrospray ionization Fourier-transform ion cyclotron resonance (ESI-FT-ICR) mass spectrometry. The dispersion of Ir:PA in a toluene–acetonitrile mixture was electrosprayed and subjected to mass analysis using an FT-ICR mass spectrometer (SolariX, Bruker Daltonics Co., Ltd.). 2.7. X-ray photoelectron spectroscopy (XPS). XPS measurements were carried out on a PHI-5000 VersaProbe (ULVAC-PHI) instrument at an energy resolution of 0.5 eV. The specimens were prepared by drop-casting the chloroform dispersion of Ir:PA on a carbon tape and drying under vacuum for 10 h. Binding energy was calibrated referring to the that of carbon 1s bands (284.5 eV). 2.8. Powder X-ray diffraction (PXRD). PXRD profiles were recorded on a SmartLab 3 diffractometer (Rigaku Co.) with Cu Kα (1.5405 Å) radiation operated at 60 kV and 60 mA. The dried samples were placed on a silicon zero background holder (zero diffraction plate) by drop-casting the chloroform dispersion. 2.9. X-ray absorption spectroscopy (XAS). Ir L3-edge X-ray absorption fine structure (XAFS) spectra were measured using BL-9A at Photon Factory and BL01B1 at SPring-8. The XAFS spectra of Ir bulk and K2IrCl6 were recorded in the transmission mode using ionization chambers as the I0 and I detectors. XAFS spectra for Ir:PA in chloroform solution at room temperature and in solid form at 10 K were recorded in fluorescence mode using ionization chambers and 19-element SSD as the I0 and I detectors respectively. Electron energy was calibrated using a Cu foil. Data analysis was conducted using the REX2000 Ver.2.5.9 program (Rigaku Co.). Data obtained for X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were analyzed by the subtraction of the atomic absorption background from the χ spectra using a cubic spline and normalization to the edge height. The k3-weighted χ spectra in the range of 3.0–14.7 Å–1 were Fourier-transformed into the r space. Curve-fitting analysis was performed for the Ir–Ir and Ir–C bonds in the r range of 1.8–2.9 Å. The phase shift and backscattering amplitude functions of the Ir–Ir and Ir–C bonds were extracted from Ir (space group: Fm3m, ICSD#41524) and IrC (space group: P6m2, ICSD#169404) by calculation using FEFF8.27 3. RESULTS AND DISCUSSION 3.1. Formation of Ir:PA via ligand exchange. While mixing the biphasic system composed of EG layer containing Ir:EG and chloroform phase layer containing PA, we observed gradual and partial transfer of brown color from the EG layer to the chloroform layer and the concurrent formation of black flocculates at the interfacial layer (Figure 1a). The optical spectrum of the as-prepared Ir:EG exhibited an exponentiallike smooth profile (red line of Figure 1b). In contrast, the optical spectrum of the chloroform phase recorded after normalization of the volume exhibited more intense absorbance in the range of m, it was found that few compositions give molecular weights comparable to those for peaks a–i. Table S1 lists all possible chemical compositions of Irn(PA)m for peaks a–i. The most probable compositions were determined by considering the coverage and isotope patterns (Figure 5b). For example, the composition of the most intense peak c was determined to be Ir50(PA)25: this composition was comparable to that of Au54(PA)26.2,3 The chemical compositions of Irn(PA)m are plotted in Figure 5c. It is notable that the numbers of the PA ligands are distributed over a much wider range m = 19–33 than those of the Ir atoms (n = 46–53). The compositions observed for Irn(PA)m are compared with those of the model structures in which PA ligands are bonded on-top or bridged sites of the Ir cuboctahedral core. The blue line in Figure 5c connects the compositions of on-top adsorption models Ir13(PA)12 and Ir55(PA)42, whereas the green one connects those of bridged adsorption models Ir13(PA)6 and Ir55(PA)21. The compositions of Irn(PA)m are distributed between the gap of the two lines, suggesting that the PA ligands can be bound to a variety of bonding sites of Ir clusters depending on their size. The (n, m) values behave counterintuitively: m values decrease with increase in n values (inset of Figure 5c). This correlation indicates that PA ligands on larger Ir clusters prefer bonding with lower coverage.

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The Journal of Physical Chemistry the Ir cluster via Ir–C bonds although an atomistic model of the interfacial structure between PA and Ir clusters cannot be provided. The CN value of the Ir–C bonds (0.8 ± 0.5) is comparable to those expected for the ligation of PA in upright configuration onto cuboctahedral Ir55 clusters: 0.76 for Ir55(PA)42 and 0.38 for Ir55(PA)21. Within the framework of the upright configuration model, the correlation of the (n, m) values shown in Figure 5c indicates that binding sites of PA changes very sensitively against the size of the underlying Ir clusters: from on-top sites on Ir46 to bridged sites on Ir53.

Figure 6. FTIR spectra of (a) PA and (b) Ir:PA. The peaks * are due to the PA polymers.

Figure 5. (a) MALDI-TOF (red) and ESI-FT-ICR (blue) mass spectra of Ir:PA. The numbers on the peaks represent their charge states. (b) Expanded view of the smoothed spectrum of ESI-FTICR mass spectrum in panel (a). Simulated spectra for the candidates are shown below. (c) Plot of (n, m) of Irn(PA)m observed in this study. Blue and green lines represent the compositions expected for the ligation of PAs on-top and bridged sites of cuboctahedral Ir clusters, respectively.

To gain insight into the interfacial structure of Ir:PA, FTIR and Ir L3-edge EXAFS spectra were recorded. Figure 6 shows the IR spectra of free PA and Ir:PA in the range of 400–4000 cm–1. In Ir:PA, the peak corresponding to the C–H stretching mode of the terminal alkynyl group of PA (3290 cm–1) disappeared. The –C≡C‒ stretching mode of PA (2110 cm–1) was significantly red-shifted to 2016 cm–1 in Ir:PA. Red-shift of the –C≡C– stretching mode of PA was also reported for Au:PA clusters (2017 cm–1)1 and 2+ –1 6 [Au19(C≡CPh)9(PPh2NHPPh2)3] (2030 cm ). The red-shift suggested that the back-donation of electrons from the Ir cluster to PA occurs, leading to the weakening of the –C≡C– bond. Partial electron transfer from Ir clusters to PA was supported by XANES and XPS (Figure 4). The peaks due to the C–H stretching mode of the phenyl ring of PA (3020−3100 cm–1) were also red shifted to 2850−2960 cm–1 in Ir:PA, as in the case of Au:PA.1 These results indicated that PA is bound to

4. CONCLUSIONS Phenylacetylenyl-protected Ir clusters (1.3 ± 0.2 nm) were synthesized by ligand exchange from preformed Ir cluster (1.5 ± 0.2 nm). Characterization by various spectroscopic methods indicated that PA ligands are bound to the fcc Ir(0) clusters via C–Ir bonds while simultaneously withdrawing electronic charge from the Ir core. Mass spectrometry revealed that Ir50(PA)25 is contained as the major species and that the number of the Ir atoms in Irn(PA)m has a narrower distribution (n = 46–53) than that of the PA ligands (m = 19–33). More interestingly, m values decrease with increase in n values. These correlations between n and m values indicate that the binding sites of the PAs on Ir clusters evolve dramatically with the size. This study illustrates the first example showing that the binding sites of alkynes are strongly dependent on the size of metal cluster.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Characterization data of polymers of PA, TEM image of Ir:EG remained in chloroform after extraction, MALDI mass spectrum and PXRD pattern of Ir:PA, possible mass assignment, analytic result of XPS data (PDF)

AUTHOR INFORMATION Corresponding Author *E–mail: [email protected]–tokyo.ac.jp

ACKNOWLEDGMENT This study was financially supported by the Elements Strategy Initiative for Catalysis and Batteries (ESICB), and by the Nanotechnology Platform (No. 12024046) and a Grant-in-Aid for Scientific Research (no. 26248003) from the Ministry of

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Education, Culture, Sports, Science, and Technology (MEXT) of Japan, and CREST, Japan Science and Technology. This study was partly supported by ICR-JURC, Kyoto University (grant # 2016-101). The synchrotron radiation experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2015G140) and the Japan Synchrotron Radiation Research Institute (JASRI) as 2016B1493 and 2016A1436.

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