Amplification of the Optical Activity of Gold Clusters by the Proximity of

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Amplification of Optical Activity of Gold Clusters by the Proximity of BINAP Shinjiro Takano, and Tatsuya Tsukuda J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02294 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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The Journal of Physical Chemistry Letters

Amplification of Optical Activity of Gold Clusters by the Proximity of BINAP Shinjiro Takano† and Tatsuya Tsukuda*,†,‡ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113-0033, Japan. ‡

Elements Strategy Initiative for Catalysis and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan.

AUTHOR INFORMATION Corresponding Author [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au11(R/S-DIOP)4Cl2]+ (DIOP = 1,4bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol) and [Au8(R/S-BINAP)3(PPh3)2]2+ (BINAP = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl). [Au8(BINAP)3(PPh3)2]2+ showed stronger rotatory strengths than [Au11(DIOP)4Cl2]+ in the visible region while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core.

TOC GRAPHIC

KEYWORDS Phosphine-protected gold cluster, Chirality, Circular dichroism, X-ray crystallography.


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Induction of chirality on metal clusters and the understanding of its origin are important for the rational development of asymmetric catalysts and chiroptical devices on the nanoscale.1–7 Since the first observation of chiroptical activity in L-glutathionate (GS)-protected gold clusters,8 the induction of chirality on atomically defined Au clusters has been demonstrated extensively, using intrinsically chiral ligands9–13 and chiral counter ions14–16 as “chiral modifiers.” Recent single-crystal X-ray diffraction studies have revealed that chiral Au clusters can be formed even by using achiral ligands.17–26 For example, left- and right-handed enantiomers were found in the single crystals of Au38(SC2H4Ph)24,18 Au102(p-SC6H4CO2H)44,17 and Au133(SPh-p-But)52.22,23 Enantiomeric pairs of Au38(SC2H4Ph)24 separated by HPLC showed an intense Cotton effect in their circular dichroism (CD) spectra.27–29 Theoretical calculations30–33 have shown that the optical activity of the ligand-protected Au clusters originates either from a chiral arrangement of ligands around achiral Au cores or from intrinsically chiral Au cores. In order to gain insight into chemical or physical manifestations of chirality like optical activity, geometrical structures of the ligand-protected Au clusters were quantified using the Hausdorff chirality measure (HCM),34,35 which was originally introduced as a measure of molecular chirality.36 It was claimed that geometrical quantification of chirality using the HCM can explain the known trends in experimental CD measurements. Despite recent progress in the synthesis and characterization of optically active Au clusters, the determining factor for the rotatory strength or anisotropic factor has not yet been clarified. Theoretical studies have suggested the importance of the dissymmetric field created by chiral ligands.31,32 Recently, it was proposed that the rotatory strengths in the low- and highenergy regions are related to the HCM values of the core and ligand moieties, respectively.35 The understanding of correlation between the rotatory strength and structure is hampered, because



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single-crystal data of chiral Au clusters available at present are limited to racemic mixtures17–26 and rigorous optical separation is required to study the optical activity. This paper reports the synthesis and structure determination of enantiomerically pure Au clusters protected by chiral diphosphines: 1,4-bis(diphenylphosphino)-2,3-O-isopropylidene-2,3-butanediol (DIOP) and 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP) (Chart 1). The CD spectra showed that the anisotropy factor of BINAP-protected Au8 clusters is significantly larger than that of DIOPprotected Au11 clusters. This trend could not be explained in terms of the HCM values calculated from the single-crystal X-ray diffraction (SCXRD) analysis. We propose that the rotatory strength is significantly amplified by the presence of binaphthyl moieties near the Au core. The DIOP-protected Au11 clusters were synthesized using a method similar to that for [Au11(PPh3)8Cl2]+.37 Briefly, the (AuCl)2(R-/S-DIOP) complex in dichloromethane was reduced by adding an ethanolic solution of NaBH4 at room temperature. After stirring the mixture for 2 h, the solvent was evaporated to dryness and the residue was extracted with a minimum amount of dichloromethane. After the dispersion was refluxed, clusters 1R/1S were collected using silica gel column chromatography. Clusters 1R/1S were crystallized by layering n-hexane onto a mixed dispersion of dichloromethane and ethanol. Reddish-orange platelet crystals were obtained after 1 week. BINAP-protected Au8 clusters were synthesized by a method similar to that reported by Wang.38 An excess amount of the borane tert-butylamine complex was added to the chloroform solution containing equal amounts of Au(PPh3)(NO3) and R-/S-BINAP. After stirring the mixture for 8 h, the solvent was evaporated to dryness. Byproducts were removed by solvent extraction and the clusters were treated with an excess amount of NaPF6 in ethanol to metathesize the counter anions. Finally, the component dispersible in acetonitrile was obtained as the final 4 ACS Paragon Plus Environment

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product 2R/2S. Black-red crystals of 2R/2S were obtained by adding diethyl ether to the acetonitrile solution. (Chart 1) Electrospray ionization mass spectrometry (Figure S1) and elemental analyses indicated that the clusters 1R/1S and 2R/2S represent [Au11(S-/R-DIOP)4Cl2]Cl and [Au8(S-/RBINAP)3(PPh3)2](PF6)2, respectively. Figure 1a shows the optical absorption spectra of 1R/1S and 2R/2S. The absorption spectra of the enantiomeric pairs of 1 and 2, respectively, exhibit essentially the same profiles. The spectra of 1R/1S show distinct peaks at 302 and 413 nm, similar to that of [Au11(PPh3)8Cl2]+.37,39 The molar absorption coefficient of 1 (3.5×104 M–1cm–1 at 413 nm) is comparable to that of [Au11(PPh3)8Cl2]+ (4.0×104 M–1cm–1 at 421 nm).39 The spectra of 2R/2S exhibit three distinct peaks at 320, 380 and 500 nm. The CD spectra of 1R/1S and 2R/2S are represented in Figure 1b. The CD spectra of the enantiomeric pairs are mirror images of each other. The peak positions of the CD spectra in the range of >350 nm agree well with those of the UV-visible spectra, suggesting that chiroptical activity in the range of 350–500 nm is associated with optical transitions within the Au cores (Figure 1a). Although the absorption coefficients of 1 and 2 are comparable (Figure 1a), there are two remarkable differences between their CD spectra. Firstly, the CD spectra of 2R/2S show more intense Cotton effects than those of 1R/1S. Secondly, the CD spectra of 2R/2S extend to longer wavelengths than those of 1R/1S. Figure 1c quantitatively compares the anisotropy factors (Δε/ε) (Δε: the molar dichroic absorption, ε: the extinction coefficient) of 1R and 2R. Figure 1c also includes the data of relevant systems: [Au11(R-BIPHEP)4Cl2]+ (BIPHEP = 2,2'bis(diphenylphosphino)-1,1'-biphenyl)12 and [Au11(R-BINAP)4Br2]+,10 whose structures have not been crystallographically resolved. The comparison among the Au11 clusters protected by three



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different ligands (DIOP, BIPHEP, and BINAP) demonstrates that the anisotropy factor in the visible region is largest when BINAP is used as the ligand. Furthermore, the Au8 clusters protected by BINAP (2) showed even larger anisotropy factors in the visible region than [Au11(RBINAP)4Br2]+. These results suggest that the ligation by BINAP plays a key role in enhancing the optical activity of Au clusters. This hypothesis is supported by the theoretical study of Aikens,33 ascribing the optical activity of [Au11(dpb)4Cl2]+ (dpb = 1,4-diphosphino-1,3-butadiene) to the chiral arrangement of the π-ligands. In contrast, Garzón suggested that the optical rotatory strength in the visible region is ascribed to the core deformation based on large HCM values of the Au cores.35 In order to elucidate the origin of the intense optical response of BINAPprotected Au clusters, the effect of the chiral structure on the anisotropy factor will be discussed in terms of the HCM values calculated from the SCXRD data of 1 and 2. (Figure 1) Figure 2a shows the crystal structure of 1S. The Au11 core has a deformed icosahedral morphology similar to that of [Au11(PPh2C3H6PPh2)5]3+.40 The formal number of valence electrons is calculated to be 8, in agreement with that required for the closure of the electronic shell of a spherical superatom.41,42 Thus, the Au11 core can formally be viewed as a spherical superatom with closed electron configuration (1S)2(1P)6. The comparison of the structures of 1R and 1S (Figure 2b) indicates that they are enantiomerically pure and have one-handed chirality. The crystal structure of 2R is shown in Figure 3a. The Au8 core has a bi-capped chair structure. The two Au atoms capping the six-membered chair structure are coordinated by two PPh3 ligands and the six Au atoms at the periphery are coordinated by three BINAP ligands. The formal number of valence electrons is calculated to be 6, in agreement with that for the closure of the electronic shell of an oblate superatom.42 Thus, the Au8 core can formally be viewed as an oblate superatom


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with electron configuration (1S)2(1P)4. Cluster 2 has one C2 axis perpendicular to the principal axis (Figure 3b), indicating that 2R and 2S are enantiomerically pure and have one-handed chirality. (Figures 2 and 3) The HCM values of the Au cores, ligand layers, and overall structures were calculated for 1 and 2, using a reported technique.34,35 The results are summarized in Table 1 along with the superimposed structures of the enantiomeric pairs. Non-zero HCM values indicate that both 1 and 2 are intrinsically chiral. In both clusters, the HCM values of the ligand layers are larger than those of the Au cores, indicating that the ligand layer contributes more than the Au core to the chirality of the overall structures, similar to the case of thiolate-protected Au clusters.35 The optical activity in the visible region is considered to originate from the chirality of the Au core since the ligands themselves do not show optical activity in the visible region. A slightly smaller HCM value for the core of 2 (0.034) than that of 1 (0.046) does not account for the larger anisotropy factor of 2, although a quantitative correlation between the HCM values and the anisotropy factor has not been established. This trend suggests that a factor beyond core chirality contributes to the intense optical activity of 2. We propose that the location of chiral binaphthyl groups having a diffuse π-electron in close proximity of the Au core magnifies the optical activity of 2, consistent with a previous theoretical study.33 (Table 1) In summary, we synthesized two enantiopure Au clusters [Au11(R-/S-DIOP)4Cl2]Cl (1R/1S) and [Au8(R-/S-BINAP)3(PPh3)2](PF6)2 (2R/2S). Cluster 2 and Au11 clusters protected by BINAP ligands showed stronger optical rotatory strength than 1. SCXRD analysis of the enantiopure samples of 1R/1S and 2R/2S revealed that both the Au11 and Au8 cores are



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intrinsically chiral and the ligand shells were arranged in a chiral geometry. The HCM value for the Au core of 2 is smaller than that of 1, which does not correspond with the larger anisotropy factor of 2 in the visible region. We propose that the optical response in the Au core-based transitions is enhanced by a chiral arrangement of the π-electron system in the close vicinity of the Au core. The experimental results reported here will stimulate more detailed theoretical studies on the origin of the intense optical response in BINAP-protected Au clusters and will aid in controlling the optical response of Au superatoms by rational design of ligand shell structures. ASSOCIATED CONTENT Supporting Information. Synthetic details, experimental details, crystallographic data, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Prof. Hiroaki Suga (The University of Tokyo) for approval to use their CD apparatus. This research was financially supported by Elements Strategy Initiative for Catalysis & Batteries (ESICB) and by “Nanotechnology Platform” (no. 12024046) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and a Grant-in-Aid for Scientific Research (A) (Grant No. JP26248003) from the Japan Society for the Promotion of Science (JSPS).


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(33) Lopez-Acevado, O.; Tsunoyama, H.; Tsukuda, T.; Häkkinen, H.; Aikens, C. M. Chirality and Electronic Structure of the Thiolate-Protected Au38 Nanocluster. J. Am. Chem. Soc. 2010, 132, 8210–8218. (34) Garzón, I. L.; Beltrán, M. R.; González, G.; Gutíerrez-González, I.; Michaelian, K.; Reyes-Nava, J. A.; Rodríguez-Hernández, J. I. Chirality, Defects, and Disorder in Gold Clusters. Euro. Phys. J. 2003, D24, 105–109. (35) Pelayo J. J.; Whetten, R. L.; Garzón, I. L. Geometric Quantification of Chirality in Ligand-Protected Metal Clusters. J. Phys. Chem. C 2015, 119, 28666−28678. (36) Buda, A. B.; Mislow, K. A Hausdorff Chirality Measure. J. Am. Chem. Soc. 1992, 114, 6006–6012. (37) McKenzie, L. C.; Zaikova, T. O.; Hutchison, J. E. Structurally Similar Triphenylphosphine-Stabilized Undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, Exhibit Distinct Ligand Exchange Pathways with Glutathione. J. Am. Chem. Soc. 2014, 136, 13426–13435. (38) Bertino, M.; Sun, Z.-M.; Zhang, R.; Wang, L.-S. Facile Syntheses of Monodisperse Ultrasmall Au Clusters. J. Phys. Chem. B 2006, 110, 21416–21418. (39) Konishi, K. Phosphine-coordinated Pure-Gold Clusters: Diverse Geometrical Structures and Unique Optical Properties/Responses. Struct. Bonding (Berlin) 2014, 161, 49–86. (40) Smits, J. M. M.; Bour, J. J.; Collenbrock, F. A.; Beurskens, P. T. Preparation and X-ray Structure Determination of [Pentakis{1,3bis(diphenylphosphino)propane}]undecagoldtris(thiocyanate) [Au11{PPh2C3H6PPh2}5](SCN)3. J. Cryst. Spect. Res. 1983, 13, 355–363. (41) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. A Unified View of Ligand-Protected Gold Clusters as Superatom Complexes. Proc. Nat. Acad. Sci. USA 2008, 105, 9157–9162. (42) Mingos, D. M. P. Structural and Bonding Patterns in Gold Clusters. Dalton Trans. 2015, 44, 6680–6695.



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Chart 1. Chiral diphsophine ligands used in this study.


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Figure 1. (a) UV-vis spectra and (b) CD spectra of 1 and 2 in CH3CN at room temperature. In each spectrum, the scale of the vertical axes is the same on the left and right. (c) A comparison of the anisotropy factor of 1R (green), 2R (orange), [Au11(R-BIPHEP)4Cl2]+ (yellow),12 and [Au11(RBINAP)4Br2]+ (black).10 The spectra reported in refs. 10 and 12 are adapted with permission. Copyright 2006 American Chemical Society and The Royal Society of Chemistry 2010.



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Figure 2. (a) Total X-ray crystal structure of 1S and (b) comparison of the geometric structures between 1S and 1R. Yellow, blue, and green balls represent gold, phosphorous, and chlorine atoms, respectively. In (a), the bridging methylene chains of the (S)-DIOP moiety are depicted as gray sticks and other parts of the (S)-DIOP moiety are depicted as gray lines.


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Figure 3. (a) Total X-ray crystal structure of 2R and (b) comparison of the geometric structures between 2R and 2S. Gray spheres represent carbon atoms and the other color codes are the same as in Figure 1. In panel (a), the binaphthyl moiety is highlighted and the phenyl rings of BINAP and PPh3 are depicted as gray lines.



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Table 1. Result of HCM analysis of 1 and 2. Cluster 1

Cluster 2

Structurea

HCM

Structurea

HCM

Core

0.046

0.034

Ligand

0.054

0.083

Core + Ligand

0.066

0.083

a

Color codes of the models: yellow (Au atom of R isomer), silver (Au atoms of S isomer), blue (P atoms of R isomer), red (P atoms of S isomer), green (Cl atoms of R isomer), light blue (Cl atoms of S isomer), white bonds (R isomer), and blue bonds (S isomer).


Amplification of the Optical Activity of Gold Clusters by the Proximity of

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