Article pubs.acs.org/IC
Suppressing Isomerization of Phosphine-Protected Au9 Cluster by Bond Stiffening Induced by a Single Pd Atom Substitution Seiji Yamazoe,†,‡,§ Shota Matsuo,† Satoru Muramatsu,† Shinjiro Takano,† Kiyofumi Nitta,∥ and Tatsuya Tsukuda*,†,‡ †
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 § CREST, JST, K's Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ∥ Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Koto, Sayo, Hyogo 679-5198, Japan ‡
S Supporting Information *
ABSTRACT: The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au9(PPh3)8]3+ with a crown motif (Au9(C)) and that with a butterfly motif (Au9(B)) induced by association and dissociation with compact counteranions (NO3−, Cl−). However, structural isomerization was suppressed by substitution of the central Au atom of the Au 9 core in [Au9(PPh3)8]3+ with a Pd atom: [PdAu8(PPh3)8]2+ with a crown motif (PdAu8(C)) did not isomerize to that with a butterfly motif (PdAu8(B)) upon association with the counteranions. Density functional theory calculation showed that the energy difference between PdAu8(C) and PdAu8(B) is comparable to that between Au9(C) and Au9(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye−Waller factors obtained by X-ray absorption fine-structure analysis revealed that the intracluster bonds of PdAu8(C) were stiffer than the corresponding bonds in Au9(C). Natural bond orbital analysis suggested that the radial Pd−Au and lateral Au−Au bonds in PdAu8(C) are stiffened due to the increase in the ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal−metal bonds by Pd atom doping inhibits the isomerization from PdAu8(C) to PdAu8(B).
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INTRODUCTION Metal clusters protected by organic ligands are potential candidates for key building units of novel functional materials because of their unique physicochemical properties that are significantly different from those of bulk metal.1 The recent development of atomically precise synthesis of gold clusters has demonstrated that they exhibit specific catalysis, photoluminescence, magnetism, circular dichroism, and redox properties.2−9 Major interest is focused on the correlation between these properties and static structures determined by single-crystal X-ray diffraction. However, these properties are significantly affected by the structural flexibility of metal clusters arising from a large surface-to-volume ratio due to strong coupling between electronic and geometric structures.10,11 Structural isomers of gold clusters were isolated through the proper choice of protecting ligands, such as [Au11(Ph2P(CH2)2PPh2)6]3+/[Au11(Ph2P(CH2)3PPh2)5]3+,12 Au 1 8 (glutathione) 1 4 /Au 1 8 (N-(2-mercaptopropionyl)glycine) 1 4 , 1 3 Au 2 4 (SCH 2 Ph- t Bu) 2 0 /Au 2 4 (SePh) 2 0 , 1 4 Au 2 5 (glutathione) 1 8 /Au 2 5 (N-(2-mercaptopropionyl)glycine)18,13 Au30S(S-tBu)18/Au30(S-Adm)18,15,16 and Au144(pMBA)60/Au144(SC6H13)60.17 Thermal-induced isomerization from a metastable form to a stable form having a biicosahedral Au23 core was reported for Au38(SC2H4Ph)24.18 Racemization © 2017 American Chemical Society
of a chiral isomer of Au38(SC2H4Ph)24 was observed at modest temperatures,19 and the activation energy for the inversion reaction was reduced in Pd2Au36(SC2H4Ph)24.20 Electrochemical isomerization between [Au8(Ph2P(CH2)3PPh2)4]2+ and [Au8(Ph2P(CH2)3PPh2)4]4+ was found.21 Reversible isomerization was observed in thiolate-protected Au28 clusters during the ligand exchange22 and in phosphine-protected Au9 clusters during evaporation to dryness and redispersion.23−26 [Au9(PPh3)8]3+ with a crown motif (Au9(C), Figure S1) retained its structure by solidification in the presence of [PMo12O40]3−, while it was transformed into [Au9(PPh3)8]3+ with a butterfly motif (Au9(B), Figure S1) by evaporation in the presence of Cl− or NO3− (Scheme 1a). This observation illustrates that the fluxional Au9 core is deformed by counteranion packing.27 The present work was motivated by our observation that [PdAu8(PPh3)8]2+ with a crown motif (PdAu8(C))28,29 did not isomerize to that with a butterfly motif (PdAu8(B)) upon drying (Scheme 1b) in sharp contrast to the case of the monometallic Au9(C) analogue. To gain insight into the mechanism of Pd dopant-induced suppression of the isomerReceived: April 20, 2017 Published: June 23, 2017 8319
DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
Article
Inorganic Chemistry Scheme 1. Isomerization Behavior of (a) [Au9(PPh3)8]3+ and (b) [PdAu8(PPh3)8]2+a
a
retained in the solidification. This conclusion was supported by the powder XRD pattern of [Au9(PPh3)8](PMo12O40) (Figure S2).27 In contrast, the DR UV−vis-NIR spectrum of the [Au9(PPh3)8](NO3)3 solid (Figure 1a) obtained in the presence of compact anion NO3− exhibited a profile with peaks at 435, 478, and 703 nm, which is in good agreement with that of Au9(B).26 This indicates that the Au9(C) structure was transformed into Au9(B) upon solidification. We confirmed that Au9(B) was retransformed into Au9(C) by redispersion of the [Au9(PPh3)8](NO3)3 solid in ethanol. In contrast, the peak positions in the DR UV−vis−NIR spectrum of [PdAu8(PPh3)8]Cl2 obtained by solidification in the presence of Cl− agree with those in the absorption spectrum of [PdAu8(PPh3)8]2+ disperse in ethanol (Figure 1b).29 Broadening of the DR spectrum is not associated with the irreversible decomposition as supported by the fact that the optical spectrum of PdAu8(C) was recovered by redispersion of the [PdAu8(PPh3)8]Cl2 solid in ethanol. These results indicate that PdAu8(C) retains the motif even upon solidification in the presence of compact anion Cl− or NO3− (ref 28) (Scheme 1b). Stability and Structures of Isomers in [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+. One of the driving forces of isomerization is the relative stability of the isomers. Relative stabilities of the crown and butterfly motifs for [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+ were studied by DFT calculation. Figure 2
Phenyl groups are omitted for simplicity.
ization, the relative stabilities of the isomers were studied by density functional theory (DFT) calculations. The stiffness of metal−metal bonds within Au9(C) and PdAu8(C) was compared using X-ray absorption spectroscopy (XAS), which is a powerful tool to determine the local structures of ligandprotected metal clusters.30,31 Analysis of Debye−Waller (DW) factors for each bond as a function of temperature31 revealed that all the metal−metal bonds are stiffened by a Pd atom substitution. On the basis of these results, we discuss the mechanism of the suppression of isomerization of Au9 clusters by a single Pd atom doping at the center.
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RESULTS AND DISCUSSION Suppression of Isomerization in [PdAu8(PPh3)8]2+. Isomerization of [Au 9 (PPh 3 ) 8 ] 3+ reported previously 26 (Scheme 1a) was confirmed by optical spectroscopy. Figure 1a shows the optical absorption spectrum of [Au9(PPh3)8]3+
Figure 2. Optimized structures of (a) Au9(C), (b) Au9(B), (c) PdAu8(C), and (d) PdAu8(B). Color code: P (pink), Au (gold), and Pd (red). Phenyl rings are shown by gray wire frames. The relative energies with respect to the crown motifs are shown. The numbers indicate NBO charges.
Figure 1. (a) UV−vis−NIR spectrum of ethanol dispersion of [Au9(PPh3)8]3+ and DR UV−vis−NIR spectra of solid forms of [Au9(PPh3)8](PMo12O40) and [Au9(PPh3)8](NO3)3. (b) UV−vis− NIR spectrum of ethanol dispersion of [PdAu8(PPh3)8]2+ and DR UV−vis−NIR spectrum of [PdAu8(PPh3)8]Cl2 solid.
shows the optimized structures of Au9(C), Au9(B), PdAu8(C), and PdAu8(B). The structures of Au9(C), Au9(B), and PdAu8(C) agree with those determined by single-crystal XRD. 27,29 The theoretical calculations predicted that PdAu8(B) can be formed as a stable entity. Au9(C) was calculated to be more stable than Au9(B) by 0.21 eV, whereas PdAu8(C) was more stable than PdAu8(B) by 0.18 eV. Namely, the crown motifs are more stable than the butterfly motifs by a similar extent both in [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+.
dispersed in ethanol. Previous single-crystal X-ray diffraction (XRD) studies showed that absorption peaks at 350, 380, and 442 nm are characteristics of Au9(C).5,26 The diffuse reflectance (DR) UV−vis−NIR spectrum of the [Au9(PPh3)8](PMo12O40) solid (Figure 1a) obtained in the presence of bulky anion [PMo12O40]3− agreed well with the absorption spectrum of Au9(C) in ethanol, indicating that the core structure was 8320
DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
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Thermal properties of the radial and lateral Au−Au(Pd) bonds were investigated to evaluate their bond stiffness. Figure 4a shows the Au L3-edge FT-EXAFS spectra of Au9(C)
Both the PdAu8 and Au9 cores can be viewed as oblate superatoms with six valence electrons.32 The energy levels and shapes of the superatomic orbitals of the four clusters are shown in Figure S3. As for Au9(C), the highest occupied molecular orbitals (HOMOs) are constructed by doubly degenerate 1Px and 1Py orbitals, and the lowest unoccupied molecular orbital (LUMO) corresponds to the 1Pz orbital. Au9(C) has a closed electron configuration of (1S)2(1P)4, consistent with the subshell closure for an oblate superatom.32 The optical gap is assigned to the electron transition from HOMO(1Px,y) to LUMO+1(Dx2‑y2) based on time-dependent DFT (TDDFT) calculations. In Au9(B), on the other hand, one of the 1P orbitals (1Py) constitutes a HOMO and the 1Dx2‑y2 orbital becomes a LUMO. The characteristic peak at 703 nm observed in Au9(B) in Figure 1a is assigned to the transition from HOMO (1Py) to LUMO (1Dx2‑y2). The nature of the frontier orbitals of PdAu8(C) and PdAu8(B) looks similar to that of Au9(C) and Au9(B), respectively (Figure S3). Stiffness of Metal−Metal Bonds in [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+. Another key for the isomerization is the stiffness of metal−metal bonds within the clusters. This dynamic nature is crucial because clusters must overcome the barriers for isomerization by elongating and/or dissociating certain bonds. Stiffened bonds will make the clusters less fluxional and prohibit isomerization. The stiffness of the bonds in Au9(C), Au9(B), and PdAu8(C) was estimated by analyzing X-ray absorption fine structure (XAFS) data recorded at various temperatures. Single-crystal XRD studies showed that there are two types of metal−metal bonds: short and long bonds located at radial and lateral positions, respectively (Figure S4). Figure 3
Figure 4. (a) Temperature dependence of Au L3-edge FT-EXAFS of Au9(C). (b) Temperature dependence of DW factors for radial (red) and lateral (blue) Au−Au(Pd) bonds of Au9(C) and PdAu8(C).
measured at temperatures in the range of 10−300 K. The peak intensity in the range of 2.2−3.0 Å due to the Au−Au bonds gradually decreased with increasing temperature because of thermal-induced oscillation. Similar results were observed for Au9(B) and PdAu8(C). The thermal-induced damping is reflected in the increase of the DW factors for the metal− metal bonds with the increase in temperature. Figure 4b shows the DW factors for the radial and lateral Au−Au(Pd) bonds as a function of temperature for Au9(C) and PdAu8(C). The increase in DW factors with increasing temperature occurred more rapidly for the lateral Au−Au bonds compared to the radial Au−Au(Pd) bonds. The DW factors for the lateral Au− Au bonds were significantly reduced by single Pd atom substitution, whereas those for the radial Au−Au(Pd) bonds were only slightly reduced. The Einstein temperature (θE), which is a measure of the bond stiffness, was estimated by fitting the plots in Figures 4b and S6 (for Au9(B)) using the Einstein equation.31 The θE values thus estimated for all the clusters are summarized in Figure 5 (Table S1). It is found that
Figure 3. (a) Au L3-edge FT-EXAFS spectra of Au9(B), Au9(C), and PdAu8(C) and (b) Pd K-edge FT-EXAFS spectrum of PdAu8(C) measured at 10 K.
Figure 5. Einstein temperatures for radial and lateral Au−Au(Pd) bonds in Au9(B), Au9(C), and PdAu8(C). The θE values for the radial Au−Pd bonds obtained from Au L3-edge and Pd K-edge XAFS data are shown in red and purple, respectively.
shows the Au L3- and Pd K-edge Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra obtained from the extended X-ray absorption fine structure (EXAFS) oscillations recorded at 10 K (Figure S5). The curve fitting analysis of Au L3- and Pd K-edges FT-EXAFS spectra (Figure 3) revealed that the coordination numbers (CNs) and bond lengths of the short and long Au−Au(Pd) bonds were in good accordance with those of the radial and lateral bonds determined by single-crystal XRD as shown in Tables S1 and S2. This coincidence indicates that the short and long metal− metal bonds correspond to the radial and lateral bonds, respectively.
the radial Au−Au(Pd) bonds were stiffer than the lateral Au− Au bonds in all the clusters. This tendency is in good agreement with the previous work on Aun(SR)m with (n, m) = (25, 18), (38, 24), and (144, 60).31 The radial and lateral Au− Au bonds of Au9(C) and Au9(B) were stiffer and slightly softer than the Au−Au bonds of bulk Au (θE = 135 K),31 respectively. The radial Au−Au bonds of Au9(B) were softer than those of 8321
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probably due to poorer packing geometry27 and delocalization of the negative charge over the larger space. The structures of PdAu8(C) and PdAu8(B) are similar to those of Au9(C) and Au9(B), respectively. In addition, the relative stabilities of the two isomers are comparable (Figure 2). Under these conditions, we can expect the isomerization of PdAu8(C) to PdAu8(B) by the interaction of small counteranions. However, the results in Figure 1b indicate that this is not the case. We propose that the suppression of isomerization is associated with the enhancement of the barrier between the isomers. The stiffness of the bond is related to the steepness of the potential energy along the reaction coordinate for isomerization. Large amplitude motion can be induced for soft bonds, which is required for the large structural deformation and isomerization. In contrast, the potential energy surface associated with the stiffer bonds is steeper, and the amplitude of motion of stiffer bonds is confined to the area near the equilibrium structure (Figure 6b). Our DW analysis revealed that all the intracluster bonds in PdAu8(C) are stiffer than those of Au9(C). This bond stiffening due to the Pd atom substitution reduces the vibrational entropy, which affects the structural stability,35 hinders core deformation, and suppresses the isomerization from PdAu8(C) to PdAu8(B). In addition to the stiffening of the intracluster bonds in PdAu8(C), weaker steric stress on PdAu8(C) from the counteranions than that on Au9(C) due to the smaller net charge (+2 for PdAu8(C) and +3 for Au9(C)) may also contribute to the suppression of isomerization upon solidification.
Au9(C) due to their elongation (Figure S4), whereas the stiffness of the lateral Au−Au bonds in Au9(B) was comparable to those of Au9(C). The single Pd atom substitution significantly stiffened the radial Pd−Au and lateral Au−Au bonds. Interestingly, the radial Pd−Au bonds were stiffer than the Au−Au bonds in the bulk Au31 and the Pd−Pd bonds in the bulk Pd (θE = 204 K).33 Ortigoza et al. predicted by DFT calculation that Fe−Pt bonds of the core−shell Fe7−Pt27 cluster are stiffer than those of bulk metal.34 To the best of our knowledge, the results in Figure 5 are the first experimental demonstration of anomalously stiff bonds by the heterometal substitution. In addition, the lateral Au−Au bonds are stiffened by the Pd atom substitution and became stiffer than those in the bulk Au.31 The Pd-induced stiffening of the intracluster bonds suggests that the nature of the metal−metal bonds is changed by the Pd doping. The natural bond orbitals (NBOs) charges of the Au and Pd atoms of Au9(C) and PdAu8(C) are shown in Figure 2. The radial Au−Pd bonds of PdAu8(C) are more ionic in nature than the radial Au−Au bonds of Au9(C) due to the large negativity of the Pd atom. The stiffening of the Au−Pd bonds of PdAu8(C) can be a qualitatively explained in terms of an increase in the ionic nature. In contrast, the NBO analysis revealed that the surface Au atoms in PdAu8(C) become less cationic by the Pd doping. The reduction in electrostatic repulsion between the less cationic Au atoms enhanced the stiffness of the lateral Au− Au bonds in PdAu8(C). Mechanism of Suppression of Isomerization in [PdAu8(PPh3)8]2+. We first discuss the mechanism of isomerization observed between Au9(C) and Au9(B) (Figure 1a). The DFT calculations and UV−vis spectroscopy indicated that Au9(C) is more stable than Au9(B) in solution. The isomerization of Au9(C) into Au9(B) induced by association with compact counteranions indicates that Au9(B) is more stabilized by the small anions compared to Au9(C). This is probably because electrostatic interaction with the compact anions is greater in Au9(B) than in Au9(C) (Figure 6a) due to
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CONCLUSION In summary, we showed that PdAu8(C) did not isomerize upon drying, whereas Au9(C) was converted to Au9(B) by drying in the presence of small counterions. DFT calculation indicates that the structural stability of PdAu8(C) is not a direct reason for the suppression of isomerization because the energy difference between PdAu8(C) and PdAu8(B) is comparable to that between Au9(C) and Au9(B). Analysis of temperaturedependent DW factors revealed that both the radial Pd−Au and lateral Au−Au bonds are stiffened by the single Pd atom substitution compared to the corresponding Au−Au bonds in Au9(C). NBO analysis based on DFT calculations suggests that the stiffness of the Pd−Au and Au−Au bonds arises from the increase in the ionic nature and the decrease in the electrostatic repulsion between the surface Au atoms, respectively. The formation of stiffer intracluster metal−metal bonds by Pd atom doping inhibits the isomerization from PdAu 8 (C) to PdAu8(B).
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Figure 6. Schematic potential energy curves for (a) counteranioninduced isomerization for Au9 and (b) suppression of isomerization in PdAu8.
EXPERIMENTAL SECTION
Synthesis. [PdAu8(PPh3)8]Cl2 was synthesized by the reported method.29 First, AuCl(PPh3) (142 mg, 0.29 mmol) and Pd(PPh3)4 (133 mg, 0.12 mmol) were dispersed in ethanol (12 mL). A solid form of NaBH4 (15.2 mg, 0.40 mmol) was added slowly over 10 min to this yellow dispersion. After being stirred for 1 h, the reaction mixture was poured into hexane (200 mL) to form a precipitate. The precipitate was collected by filtration with a membrane filter (pore diameter = 0.2 μm), washed with hexane and pure water, and extracted with ethanol. The solid product (∼130 mg) was obtained by evaporating the solvent. [Au9(PPh3)8](NO3)3 was synthesized by a method similar to those in the literature.24,25 NaBH4 (0.48 mmol) dissolved in ethanol (23 mL) was added dropwise into Au(PPh3)(NO3) (1.0 g, 1.9 mmol, prepared according to the literature36) dispersed in ethanol (40 mL). After 2 h of stirring at room temperature, the reaction mixture was
preferable crystal packing.27 The steric stress during solidification induces the isomerization. Owing to the larger stabilization of Au9(B) than Au9(C), the barrier height of the isomerization is lowered and, as a result, Au9(C) is transformed into Au9(B) during the evaporation of solvent containing small anions. In contrast, no isomerization of Au9(C) was observed by the association with bulky counteranions suggesting that Au9(C) is still more stable than Au9(B) after association with large counteranions. Less stabilization by bulky anions is 8322
DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
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Inorganic Chemistry filtrated. The filtrate was evaporated and then dissolved in dichloromethane (5 mL). After filtration and evaporation, the obtained solid was washed with THF and hexane. Finally, 700 mg of green powder was obtained by drying in a vacuum. [(n-Bu)4N]3(PMo12O40) was synthesized according to the reported procedure with slight modification. 27,37,38 (n-Oct) 4 NBr (395 mg) was added to H3PMo12O40·xH2O (566 mg) dissolved in 50 mL of water. The precipitate was collected by filtration and then washed with water. After recrystallization in acetone and hexane, [(n-Bu)4N]3(PMo12O40) was obtained as a yellow solid. [Au9(PPh3)8](PMo12O40) was synthesized by a method similar to those in the literature.27,37 [Au9(PPh3)8](NO3)3 (41 mg, 0.010 mmol) was dissolved in acetonitrile (40 mL) and then filtrated. To this filtrate was added [(n-Bu) 4 N] 3 (PMo 12 O40 ) (41 mg, 0.016 mmol) dissolved in acetonitrile (20 mL) dropwise over 3 min. After being stirred at room temperature for 30 min, the precipitate was collected by centrifugation (2300 rpm) and then washed with acetonitrile. Drying in a vacuum gave 43 mg of a red-orange solid. Characterization. Ultraviolet−visible−near-infrared (UV−vis− NIR) spectra of the cluster dispersions and solids were recorded on a V-670 spectrophotometer (Jasco) using transmission and diffusereflectance (DR) modes, respectively. The structure of [Au9(PPh3)8](PMo12O40) was confirmed by X-ray diffractometry using a SmartLab III (Rigaku). X-ray Absorption Fine-Structure (XAFS) Measurements and Analysis. Au L3- and Pd K-edge XAFS measurements were carried out at beamline BL01B1 at the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (Proposal Nos. 2016A1436, 2015B1308, 2015A1590, and 2012B1986). The incident X-ray beam was monochromatized by a Si(311) double crystal monochromator. A solid sample of each cluster diluted with boron nitride powder was pressed into a pellet and mounted on a copper holder attached to the cryostat. Au L3-edge XAFS spectra were measured in the transmission mode using ionization chambers at 10−300 K. In the case of Pd Kedge XAFS measurements, the fluorescence mode was applied using a 19-element Ge solid state detector. The X-ray energy was calibrated using Cu foil for the Au L3 edge and Pd foil for the Pd K edge. The EXAFS spectra were analyzed using the REX2000 Ver. 2.5.9 program (Rigaku Co.) as follows. The χ spectra were extracted by subtracting the atomic absorption background by cubic spline interpolation and were normalized to the edge height. The k3-weighted χ spectra within the k range of 3.0−21.0 Å−1 for the Au L3 edge and 3.0−17.5 Å−1 for the Pd K edge were Fourier transformed (FT) into r-space for structural analysis. The curve fitting analysis was conducted for Au−P and Au−Au(Pd) bonds over the r range of 1.5−3.0 Å in the Au L3edge FT-EXAFS spectra. The r range of 2.0−3.0 Å was analyzed for Pd−Au bonds by curve fitting analysis in the Pd K-edge FT-EXAFS. In the curve fitting analysis, the phase shifts and backscattering amplitude functions of Au−P, Au−Au, Au−Pd, and Pd−Au were extracted from Au2P3 (ICSD#8058), Au metal (ICSD#44362), and PdAu24(SCH3)18 clusters,39 respectively, using the FEFF8 program40 by setting σ2 = 0.0036 (σ: DW factor). This value did not significantly affect the phase shift and backscattering amplitude functions. Curve Fitting Analysis. Local structural analysis was performed to obtain the structural parameters of the clusters using the curve fitting analysis of the FT-EXAFS spectra measured at 10 K. In the case of the curve fitting analysis for Au L3-edge FT-EXAFS, the peaks appearing in the range of 1.5−3.0 Å were fitted using Au−P, short Au−Au(Pd), and long Au−Au bonds for all samples. The results are shown in Table S1. The coordination number (CN) and bond length (r) for Au−P, short Au−Au(Pd), and long Au−Au bonds of all clusters were in good accordance with the average (CN, r) values of Au−P, radial Au− Au(Pd), and lateral Au−Au bonds estimated from single-crystal XRD analysis, as shown in Table S1.26,27,29 In the case of the curve fitting analysis of the Pd K-edge FT-EXAFS for PdAu8(C), only the Pd−Au bonds were observed, which supported that the Pd atom is surrounded exclusively by Au atoms. The CN and r of the Pd−Au bonds agreed with those of the crystal structure,29 shown in Table S2. Evaluation of DW. The DW values were evaluated from the FTEXAFS at each temperature obtained from the temperature-dependent
EXAFS data (3.0 ≤ k ≤ 16.0) according to the previous work.31 The values of r, DW, and third cumulant at each temperature were determined by least-squares fit analysis while keeping the CN values the same as those obtained from the curve fitting analysis at 10 K using the analytical EXAFS range of 3.0−21.0 Å−1 for the Au L3 edge and 3.0−17.5 Å−1 for the Pd K edge. Evaluation of Einstein Temperatures. The DW factor (σ2) is given as the sum of dynamic DW (σD2) and static DW (σs2) factors, which arise from temperature-dependent atomic oscillation and temperature-independent structural disorder, respectively.16 According to the Einstein model that assumes three independent quantum oscillators with different Einstein temperatures (θE) for Au−P, and short and long metal−metal bonds, respectively, σD2 is expressed as follows:31 σD2 =
θ h2 coth E 2T 8π μkBθE 2
where h, kB, μ, and T represent the Planck constant, Boltzmann constant, reduced mass of adjacent atoms, and temperature, respectively. The θE values were determined by fitting the temperature dependence of the DW factors for each bond. Density Functional Theory (DFT) Calculation. Electronic and geometric structures of [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+ were studied by DFT calculations using the B3LYP functional. Basis sets used were LanL2dz for Au and Pd atoms and 6-31G(d) for P, C, and H. Structural optimization was carried out to obtain four structures: PdAu8(B), PdAu8(C), Au9(B), and Au9(C). At our calculation levels, optimization with small phosphine ligands such as PH3 did not give butterfly-type structures, implying the importance of ligand conformation41,42 for stability of whole structures. Frequency calculations were conducted to confirm that the optimized structure was located at potential minima. Note that for a few cases, low imaginary frequencies were identified as rotational contributions, which hardly affected the whole structures and total energies. Atomic charges were evaluated by natural population analysis based on NBOs. Excitation energies were calculated by time-dependent (TD) DFT method solving 40 singlet states, to reproduce the optical spectra. All calculations were conducted using the Gaussian 09 program.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00973. Details of analytical results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shinjiro Takano: 0000-0001-9262-5283 Tatsuya Tsukuda: 0000-0002-0190-6379 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was financially supported by the Elements Strategy Initiative for Catalysts and Batteries (ESICB), a Grant-in-Aid for Scientific Research (No. 26248003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. Theoretical calculations were partly performed on the supercomputers of the Research Center for Computational Science, Okazaki, Japan. We are grateful for a research fellowship of the Institute for Quantum Chemical Exploration for young scientists. The synchrotron radiation experiments 8323
DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
Article
Inorganic Chemistry
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were performed under the approval of the Japan Synchrotron Radiation Research Institute (JASRI) as 2016A1436, 2015B1308, 2015A1590, and 2012B1986.
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ABBREVIATIONS XAS, X-ray absorption spectroscopy; EXAFS, extended X-ray absorption fine structure; FT-EXAFS, Fourier transformed extended X-ray absorption fine structure; XRD, X-ray diffraction; SCXRD, single-crystal X-ray diffraction; UV−vis− NIR, ultraviolet−visible−near-infrared; DR UV−vis−NIR, diffuse-reflectance ultraviolet−visible-near-infrared; DFT, density functional theory; TDDFT, time-dependent density functional theory; DW, Debye−Waller; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital
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DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
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DOI: 10.1021/acs.inorgchem.7b00973 Inorg. Chem. 2017, 56, 8319−8325
