Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Water-Soluble Mixed-Phosphine-Protected Gold Clusters: Can a Single Axially Chiral Ligand Lead to Large Chiroptical Responses? Hiroshi Yao,*,† Shuhei Tsubota,‡ and Rena Nobukawa‡ †
Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan ‡ Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan S Supporting Information *
ABSTRACT: We present isolation and exploration of chiroptical activity of water-soluble achiral/chiral mixed-phosphine-protected Au9 and Au11 clusters. The mixed-phosphine we use is triphenylphosphine monosulfonate (TPPS)/ tetrasulfonated-(R)-BINAP in a molar equivalent ratio of 3/1 with respect to the P atom, where BINAP represents 2,2′-bis(diphenylphosphino)-1,1′binaphthyl. In both Au9 and Au11 magic-number clusters, (R)-BINAP protection is partial (single-molecule ligation in each cluster), and thus the obtained chiroptical responses are rather small. Quantum-chemical calculations for model nanogold (Au9) cluster compounds are performed to evaluate the optical and chiroptical responses. On this basis, we find that (i) single coordination of axially chiral diphosphine ligand onto the Au9 cluster surface can give small CD responses only in the high-energy region (≥ ∼3 eV) and (ii) if the axially chiral diphosphine ligand contains π-electron systems, then it can additionally enhance the CD intensity in the low-energy region (∼2.0 to 2.5 eV). The effect of the number of chiral diphosphine ligands in Au9 clusters is also examined, and an effective modulation in the CD intensity is observed as a function of its number.
1. INTRODUCTION Gold clusters with defined nuclearity and geometry have been a topic of considerable interest because of their interesting structure- or size-dependent optical/electronic properties.1,2 In most cases, such magic-number Au clusters have been synthesized with protective thiol (or thiolate) ligands, which have important and multiple roles as stabilizers.1−4 Thiolate ligands can bridge gold centers through strong S−Au interactions, lifting the gold atoms out of the cluster surface to form staple or oligomer motifs (RS−(Au−SR)n).1−4 On the contrary, phosphines constitute another important class of ligands to successfully prepare small Au clusters, and various clusters with phosphine passivation have been examined for catalysis, imaging, drug delivery, and targeting agents.5−7 Unidentate phosphines, typically triphenylphosphine (PPh3) and its derivatives, have been utilized as protective ligands in Au cluster formation in organic phases5−9 and are known to prefer directly to coordinate to the outermost gold atoms, yielding a simple Au−P interface.10,11 However, the Au−P bonds are generally weaker than Au−S bonds, so the phosphine protection sometimes has a stability issue on the polyhedral skeletal Au clusters. In an attempt to increase the stability of such clusters, some synthetic strategies are applied; for instance, the use of a multidentate phosphine leads to a stable Au cluster with core chirality [Au20(PP3)4]Cl4, where PP3 = tris[2(diphenylphosphino)-ethyl]phosphine.12,13 A bidentate phosphine is sometimes found to favor clusters with unusual © XXXX American Chemical Society
geometry with extra gold atoms located outside the polyhedral core such as [Au6(bis(diphenylphosphino)propane) 4]2+), which has a tetrahedral Au4 core and two Au atoms bridged at opposite edges of the tetrahedron.14 BINAP, 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, is a bidentate phosphine with axial chirality and has also been used to prepare small Au clusters, resulting in the formation of optically active Au clusters.15−17 It should be noted that these clusters are all hydrophobic and thus are only organically soluble. To obtain water-soluble Au clusters with phosphine protection, ligand (place)-exchange reactions have been often applied; for example, the ligand-exchange reaction of Au55(PPh3)12Cl6 by Ph2P(Ph-SO3−) (triphenylphosphine monosulfonate; TPPS) affords small, atomically precise, water-soluble clusters.6 The reaction of [Au9(PPh3)8](NO3)3 using TPPS is also reported for examining the cellular response or apoptosis.18 Recently, direct synthesis and isolation of atomically precise, water-soluble phosphine-protected Au clusters have been reported by Simons’ group as well as our group,19,20 which can reduce the overall time and cost of the cluster syntheses and thus may be useful for further diagnostic applications.18 Received: August 26, 2017 Revised: December 16, 2017 Published: December 18, 2017 A
DOI: 10.1021/acs.jpcc.7b08528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
atmosphere. Then, the reaction was quenched by pouring icecooled water (50 mL) and the product was neutralized by aqueous potassium hydroxide solution until pH 7 was reached. The solution was mostly evaporated and then methanol (100 mL) was added, yielding precipitates mostly consisting of potassium sulfate. After evaporating the supernatant methanol solution, the product was obtained, which was confirmed by mass spectrometry with positive-ion mode: [BINAP(SO3−)4 4K+Na+] (m/z = 1117, main peak), [BINAP(SO3−)4 5K+] (m/ z = 1133), [BINAP(SO3−)4 3K+2Na+] (m/z = 1101), and [BINAP(SO3−)4 3K+2Na+] (m/z = 1085). Synthesis of TPPS/R-sulfo-BINAP mixed-phosphine-protected Au clusters was carried out as follows: Typically, mixtures of HAuCl4 (0.5 mmol), TPPS sodium salt (0.375 mmol), and R-sulfo-BINAP potassium salt (0.0625 mmol) were at first mixed in methanol (100 mL) under an argon atmosphere, followed by the rapid addition of a freshly prepared ice-cooled 0.2 M aqueous NaBH4 solution (20 mL) under vigorous stirring. Note here that R-sulfo-BINAP is a bidentate ligand, so the molar equivalent ratio between TPPS and R-sulfo-BINAP is 3/1 with respect to the P atom. After further stirring of ∼2 h, the solution was stored overnight. Most of the solvent was evaporated under vacuum below 30 °C; then, the addition of 2-propanol yielded a brown precipitate. The precipitate was thoroughly washed with 2-propanol through redispersion−centrifugation processes. Finally, a powdery sample was obtained by a vacuum-drying procedure. The as-prepared product was separated using polyacrylamide gel electrophoresis (PAGE) because the cluster surface is negatively charged with −SO3− groups in the TPPS ligands.25 The separating gel concentration was 28% (pH 8.8). To extract the Au cluster compound into water, a part of the gel containing the fraction was cut out, followed by the addition of distilled water. Then, the gel lumps were removed by a syringe filter with 0.2 mm pores. It is of note that we tried to synthesize TPPS/R-sulfo-BINAP mixed-phosphine-protected Au clusters with different TPPS/Rsulfo-BINAP ratios (1/1 and 0/1 mol equiv ratio with respect to the P atom) in similar procedures, but unfortunately, we could not obtain small Au clusters with low nuclearity such as Au9 and Au11. See the Supporting Information for details. 2.3. Instrumentation. UV−vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Circular dichroism (CD) spectra were recorded with a JASCO J-820 spectropolarimeter. Elemental analysis was carried out by energy-dispersive X-ray (EDX) spectroscopy excited by an electron beam at 9.0 kV with an EDAX Genesis-2000 system attached to the S-4800 electron microscope. The negative-ion ESI mass analysis of methanol/water solution of the sizeseparated clusters was conducted using a mass spectrometer (JMS-T1000LC, JEOL). 2.4. Computations. Quantum-chemical calculations for model phosphine-protected nanogold (Au9) clusters were carried out within the framework of density functional theory (DFT) using the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzerhof (PBE) parametrization for the exchange and correlation interactions26,27 on the basis of the Gaussian 09 program.28 It is well known that a typical nanogold (Au9) cluster has a butterfly-like skeletal structure with D2h core symmetry,29,30 so we adopted the structure as a starting point. Geometry optimizations and excitation calculations were performed on several isomers of the undecagold clusters. The LanL2DZ basis set for Au and the 6-31G* basis
In the present study, we make an attempt to directly synthesize optically active, water-soluble Au clusters protected by phosphines.20,21 A water-soluble, chiral bidentate BINAP derivative, tetrasulfonated (R)-BINAP, is used for passivating the cluster surface. The use of pure tetrasulfonated (R)-BINAP does not give magic-number small Au clusters, but mixed phosphines with TPPS successfully produce Au9 and Au11 clusters with chirality. Because phosphine-protected Au clusters with chirality are currently limited to those protected by mostly (R)-/(S)-BINAP, MeO-BIPHEP, and DIOP (BIPHEP = 2,2′bis(diphenylphosphino)-1,1′-biphenyl and DIOP = 1,4-bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol),15−17,22,23 which are only organically soluble, our synthesis methodology will offer a different strategy for the purpose. The present work experimentally demonstrates the degree of ligandinduced asymmetry in the phosphine-protected cluster system. Furthermore, the effects of the number (or fraction) of chiral bidentate phosphine (such as BINAP derivatives) on the chiroptical activity of Au (typically Au9) clusters are also examined from a theoretical viewpoint. We then find that single-molecule ligation of chiral bidentate phosphine (even if it has a π-electron moiety in close vicinity of the Au core) induces a rather small optical activity in the clusters.22 Quantumchemical calculations also predict that an increase in the number of chiral ligands enhances the cluster’s chiroptical responses accordingly, but its enhancement is not proportional to their number (or fraction).
2. EXPERIMENTAL SECTION 2.1. Materials. HAuCl4·4H2O (99%), sodium borohydride (NaBH4, > 90%), methanol (GR grade), ethanol, 2-propanol (GR grade), and sulfuric acid (GR grade) were received from Wako Pure Chemical and used as received. Sodium triphenylphosphine monosulfonate (Ph2P(Ph-SO3−)Na+, abbreviated as TPPS (chemical structure is shown in Figure 1)
Figure 1. (a) Photograph of PAGE separation for mixed-phosphineprotected Au clusters. Fractions 1 and 2 were primarily evaluated. Chemical structures of protective phosphines (TPPS and R-sulfoBINAP) are also shown. (b) Typical EDX spectra of the fractioned Au cluster compounds 1 and 2.
and (R)-BINAP are received from Tokyo Chemical Industry. All gel electrophoresis reagents were received from Nacalai Tesque. Pure water was obtained by a water-distillation supplier (Advantec GS-200). 2.2. Synthesis and Isolation. Water-soluble chiral diphosphine ligand, tetrasulfonated (R)-BINAP (abbreviated here as R-sulfo-BINAP; see also in Figure 1), was synthesized according to a literature procedure.24 In brief, (R)-BINAP (500 mg) was first dissolved in concentrated sulfuric acid (1.75 mL) with subsequent dropwise addition of fuming sulfuric acid (7.5 mL), followed by stirring for about 3 days under an argon B
DOI: 10.1021/acs.jpcc.7b08528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C set for P, C, and H were used. Time-dependent DFT (TDDFT), as also implemented in Gaussian 09, was used to calculate optical and chiroptical responses.
3. RESULTS AND DISCUSSION 3.1. Mixed-Phosphine-Protected Au9 and Au11 Clusters with Chirality. 3.1.1. Isolation. The as-prepared Au cluster sample could be separated using polyacrylamide gel electrophoresis (PAGE). A photograph of typical PAGE separation is shown in Figure 1a. Typically, under normal illumination, we could observe four discrete bands in the separation gel; the separable compounds are referred to as 1−4, with 1 being the most mobile species. Species in band 3 or 4 exhibited a localized surface plasmonic resonance peak in its absorption spectrum, suggesting the existence of nanoparticles larger than ∼2 nm in diameter (N.B. The chiroptical responses of compounds 3 and 4 were extremely small. See the Supporting Information), so we focus on the fractions 1 and 2 (small-sized cluster species). Elemental analysis based on EDX spectroscopy gives us information on the chemical nature of the fractioned clusters 1 and 2. In both cluster compounds, peaks of Au, P, S, and Na were obvious (Figure 1b). It is known that phosphine-protected Au clusters can be generally formulated as [Aun(PR3)mXs]z+ (n > z), where tertiary phosphines (PR3) serve as primary protecting ligands and additional ligands (X; typically halide anions such as Cl−) sometimes coordinate with the surface metals as subligands.10 According to the EDX spectra, the peak of Cl (2.621 keV) was detected solely in compound 2, although the intensity was very weak, suggesting the minor presence of Cl in this compound. 3.1.2. Absorption and CD Spectroscopy. UV−vis absorption spectroscopy is one of the useful methods for characterizing the phosphine-protected clusters because the difference between the peak position and spectral shape is sensitive to the nuclearity of the Au core and ligand environment but far less dependent on the phosphine structures; therefore, the optical responses can serve as a fingerprint for the magic-number clusters.31 Figure 2a shows UV−vis absorption spectra of compounds 1 and 2 in aqueous solution, and the inset exhibits their CD spectra. Note that absorption and CD spectra of pure R-sulfo-BINAP in water (Figure 2b) have an absorption onset at ∼350 nm with a large negative Cotton effect. The absorption spectrum of compound 1 showed a rather structured line shape; three distinct peaks at 447, 350, and 314 nm, along with two shoulders at 525 and 380 nm, indicate molecule-like electronic structures. Remarkably, the spectral shape is almost identical with that of [Au9(TPPS)8] in aqueous solution.19,30 The fact assures the validity that compound 1 can be formulated as [Au9(P)8], where P denotes phosphine with ligation. The absorption spectrum of compound 2 is also structured and exhibits broad absorption peaks (or shoulders) due to the characteristic molecule-like transitions at ∼310 and 417 nm, which are quite similar to organically soluble phosphine-protected Au11 (undecagold) clusters.32 Among the phosphine-protected Au11 clusters, some discrepancies are present in the spectral features,32 strongly indicating that the fine structure is sensitive to the core structure and ligand composition. In this regard, PPh3-stabilized undecagolds having Cl atoms, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]+Cl−, have been successfully isolated recently.32 Absorption spectra of these purified clusters show distinct peaks and shoulders characteristic of undecagold clusters,33 but some small differences are detected between these clusters. The differences
Figure 2. (a) Absorption and CD (inset) spectra of the aqueous Au cluster compounds 1 and 2. (b) Absorption and CD spectra of R-sulfoBINAP in ethanol.
between two forms likely result from changes in the slight core geometry required to accommodate the phosphine or chloride in Au11 clusters. In the present compound 2, the observed peaks nearly agree with those of [Au11(PPh3)8Cl2]+. In consideration with the possible inclusion of chlorine atoms in compound 2 from EDX spectroscopy, it would be formulated as [Au11(P)8Cl2]. The CD spectra of compounds 1 and 2 (inset in Figure 2a) showed appreciable chiroptical responses in the metal-coreinvolved electronic transitions (
