Article pubs.acs.org/Langmuir
Water-Soluble Phosphine-Protected Au11 Clusters: Synthesis, Electronic Structure, and Chiral Phase Transfer in a Synergistic Fashion Hiroshi Yao* and Mana Iwatsu Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan S Supporting Information *
ABSTRACT: Synthesis of atomically precise, water-soluble phosphine-protected gold clusters is still currently limited probably due to a stability issue. We here present the synthesis, magic-number isolation, and exploration of the electronic structures as well as the asymmetric conversion of triphenylphosphine monosulfonate (TPPS)-protected gold clusters. Electrospray ionization mass spectrometry and elemental analysis result in the primary formation of Au11(TPPS)9Cl undecagold cluster compound. Magnetic circular dichroism (MCD) spectroscopy clarifies that extremely weak transitions are present in the low-energy region unresolved in the UV−vis absorption, which can be due to the Faraday B-terms based on the magnetically allowed transitions in the cluster. Asymmetric conversion without changing the nuclearity is remarkable by the chiral phase transfer in a synergistic fashion, which yields a rather small anisotropy factor (g-factor) of at most (2.5−7.0) × 10−5. Quantum chemical calculations for model undecagold cluster compounds are then used to evaluate the optical and chiroptical responses induced by the chiral phase transfer. On this basis, we find that the Au core distortion is ignorable, and the chiral ion-pairing causes a slight increase in the CD response of the Au11 cluster.
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INTRODUCTION Ligand-protected gold clusters with defined nuclearity and geometry have been a topic of considerable interest because of their structure- or size-dependent optical/electronic properties.1,2 In most cases, such atomically precise gold clusters’ syntheses have been developed with protecting ligands of thiols, which have important and multiple roles as stabilizers and etching agents.1−4 Interestingly, thiolate ligands can bridge gold centers with the formation of staple or oligomer motifs (RS− (Au−SR)n).1−4 On the other hand, phosphines have been also used in the preparation of small gold clusters, and various clusters with phosphine passivation have been examined for catalysis, imaging, drug delivery platforms, and targeting agents.5−7 Triphenylphosphine (PPh3) and related compounds are typically utilized as protective ligands in gold cluster formation in organic phases.5−9 In general, Au−P bonds are weaker than Au−S bonds, so the phosphine protection sometimes has a stability issue though it may be useful for applications in catalysis.10,11 In an attempt to increase the stability of the clusters, some synthetic strategies are applied: for example, the use of pyridyl phosphines © XXXX American Chemical Society
since the N atoms make the phosphines available for bridging and protecting two gold metal centers12 or the use of multidentate (tetradentate) phosphine which leads to a gold cluster with core chirality [Au20(PP3)4]Cl4, where PP3 = tris[2(diphenylphosphino)ethyl]phosphine.13,14 Unlike thiolates with a negative charge, phosphines are neutral and can be terminal ligands. The structures of phosphineprotected gold clusters generally involve centered polyhedral geometries of metal because the center-to-peripheral interactions effectively enhance the stability of the cluster skeleton, and the outermost gold atoms bind directly to P ligation in the phosphine ligands.15,16 Syntheses of phosphine-protected gold clusters in solution phases commonly involve the reduction of an ionic Au precursor (typically AuClPPh3) in the presence of PPh3, and some synthetic parameters such as the reaction rate through changes in temperature, reducing agent, and so forth, are Received: February 12, 2016 Revised: March 15, 2016
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DOI: 10.1021/acs.langmuir.6b00539 Langmuir XXXX, XXX, XXX−XXX
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Langmuir controlled.17,18 Ligand (place) exchange reactions have been often applied for their synthesis,19 and then water-soluble phosphine-protected gold clusters are exclusively prepared by using this methodology; for example, the exchange of Ph2P(PhSO3−) (triphenylphosphine monosulfonate; TPPS) onto Au55(PPh3)12Cl6 affords small, atomically precise, water-soluble clusters.6 The ligand exchange reaction of [Au9(PPh3)8](NO3)3 using TPPS is also reported for examining the cellular response or apoptosis;20 however, it often changes nuclearity, that is, the Au9(PPh3)8 cluster changed its nuclearity to Au8 by a two-phase ligand exchange reaction.21 Hence a direct synthesis and isolation of atomically precise, water-soluble phosphine-protected Au clusters are still a synthetic challenge for decreasing the overall time and cost of the cluster syntheses and thus may solve the stability issue for the clusters. Meanwhile, water-soluble, ionic metal clusters can have advantages in many versatile reactions based on the electrostatic interactions between ions. Among them, one of the significant topics in chemistry includes effective asymmetric (or chiral) conversions using chiral ion-pairing,22 in which the strong interionic attraction exits between chiral/achiral molecules.23 According to this strategy, we have reported an efficient asymmetric induction of optically inactive thiolate-protected Au clusters by a chiral phase-transfer reaction.24 Unlike the thiolate ligation that formsRS−(Au−SR)nsurface oligomers, phosphines form direct Au−P bonding, thereby causing different chiral interaction between the ligands of thiolates and phosphines. In the present study, we succeed in the direct synthesis of water-soluble Au clusters protected by anionic phosphine. The clusters are size-selected by gel electrophoresis, yielding magic-number gold clusters possessing 11 Au atoms (undecagold clusters). In addition, chiral phase transfer from water to chloroform is elaborately controlled via hydrophobization of the anionic Au cluster surface with chiral ephedrinium cations, and we find two kinds of chiral phasetransfer reagents are required (synergistic effect). Since phosphine-protected Au clusters with chirality are currently limited to those protected by (R)-/(S)-BINAP,25,26 the proposed methodology will be a new strategy for that purpose. In comparison to the asymmetric conversion for thiolate-protected Au clusters performed in a similar process, the induced chiroptical activity is rather small, suggesting the importance (or less importance) of thiolate−metal (or phosphine−metal) core interactions in the chiral transfer from ligand to metal, respectively.27−29 Some theoretical results that predict a geometrical structure of the undecagold clusters are presented, and optical/chiroptical properties as well as the electronic structures are discussed on this basis.
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Synthesis and Fractionation of TPPS-Protected Au Clusters. Synthesis of water-soluble TPPS-protected Au clusters was successfully conducted as described below: Typically, mixtures of HAuCl4 (0.5 mmol) and TPPS sodium salt (0.75 mmol) were at first mixed in methanol (100 mL) under an inert argon atmosphere, followed by rapid addition of a freshly prepared ice-cooled 0.2 M aqueous NaBH4 solution (25 mL) under vigorous stirring. After further stirring (∼2 h), the solution was stored overnight. Most of the solvent was evaporated under vacuum below 30 °C; then addition of ethanol yielded an oily precipitate. After removal of the oily material, the supernatant was centrifuged to obtain a brown powdery precipitate. The precipitate was thoroughly washed with ethanol/2-propanol (1:10) through redispersion−centrifugation processes. Finally, a powdery sample was obtained by a vacuum-drying procedure. The as-prepared product was then separated using polyacrylamide gel electrophoresis (PAGE) since the cluster surface is negatively charged with -SO3− groups in the TPPS ligands.27 The separating gel concentration used was 28% (pH 8.8). The sample solution was loaded onto a gel top and eluted for ∼6 h at a constant voltage (150 V) to achieve separation. To extract the metallic 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 μm pores. Asymmetric Conversion by a Synergistic Chiral Phase Transfer. Chiral ion association and subsequent phase transfer of the Au cluster from water to chloroform were achieved. Importantly, the sole use of DME-Br (in organic phase) did not induce the phase transfer, and further addition of its methyl analogue MME-Br into water phase was indispensable (synergistic effect): When 6.0 mL of an aqueous solution containing the fractioned TPPS-protected Au clusters and MME-Br (5.0 × 10−3 M) and 6.0 mL of chloroform containing DME-Br (5.0 × 10−3 M) were mixed and shaken vigorously, a sample color was almost completely transferred into an organic phase. After overnight storage, we started spectroscopic measurements for the extracts. 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. Magnetic circular dichroism (MCD) measurements were made with the aforementioned spectropolarimeter equipped with a JASCO permanent magnet (PM-491LB) with parallel (+1.6 T) and antiparallel (−1.6 T) fields. Elemental analysis was carried out by energy dispersive X-ray (EDX) spectroscopy excited by an electron beam at 6.0 kV with an EDAX Genesis-2000 system attached to the S-4800 electron microscope. The mean core diameter of the Au cluster sample was determined by a solution-phase small-angle X-ray scattering (SAXS) technique.27 The negative-ion ESI mass analysis of methanol−aqueous alkaline (pH 8.8) solution (∼5:1, v/v) of the size-separated clusters was conducted using a mass spectrometer (JMS-T100LC, JEOL).30,31 Computations. Quantum chemical calculations for model phosphine-protected undecagold (Au11) clusters were carried out within the framework of density functional theory (DFT) using the generalized gradient approximation (GGA) and the PBE parametrization for the exchange and correlation interactions32,33 on the basis of the Gaussian 09 program.34 It is well-known that a typical undecagold cluster has one central and 10 peripheral gold atoms,7,26 which are located at the center of the vertices of an icosahedron with one triangular face replaced by a single vertex, so we adopted the structures 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 set for P, S, O, C, and H were used. Time-dependent DFT (TD-DFT), as also implemented in Gaussian 09, was used to calculate optical and chiroptical responses.33,34
EXPERIMENTAL SECTION
Materials. HAuCl4·4H2O (99%), sodium borohydride (NaBH4, > 90%), methanol (GR grade), ethanol, and 2-propanol (GR grade) were received from Wako Pure Chemical and used as received. Diphenylphosphinobenzene-3-sulfonic acid sodium salt (Ph2P(PhSO3−)Na+, triphenylphosphine monosulfonate; abbreviated as TPPS; chemical structure is shown in Figure 1) was received from Tokyo Chemical Industry. (1R,2S)-N-dodecyl-N-methylephedrinium bromide (abbreviated as DME-Br) and (1R,2S)-N,N-dimethylephedrinium bromide (abbreviated as MME-Br) were purchased from Aldrich. The chemical structure with stereochemistry of DME-Br or MME-Br is shown in Figure 4. All gel electrophoresis reagents were received from Nacalai Tesque. Pure water was obtained by a water-distillation supplier (Advantec GS-200).
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RESULTS AND DISCUSSION Formation of TPPS-Protected Undecagold (Au11) Clusters. The as-prepared TPPS-protected Au cluster sample could be separated using polyacrylamide gel electrophoresis (PAGE). A photograph of a typical PAGE separation is shown in Figure 1a. Typically, under normal illumination, we could observe two discrete bands in the separation gel, but the amount B
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Figure 1. (a) Photograph of PAGE separation for TPPS-protected Au clusters. The fraction marked with “#” in the image was isolated and evaluated. The chemical structure of TPPS is also shown. (b) Typical EDX spectra of the fractioned TPPS-protected Au cluster compound.
of species with the highest mobility was very small. In the lowmobility region of the gel, Au clusters were continuously distributed, suggesting that relatively large-sized clusters were polydisperse or in low “magic” quality. Hence we focus on the fraction with the next-highest mobility in the gel (displayed as “#” in Figure 1a). Elemental analysis based on EDX spectroscopy gives us information on the chemical nature of the fractioned cluster. In particular, an inclusion of Cl (chloride) can be a good indication for the cluster composition/structure, because 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) sometimes coordinate with the surface metals as subligands.15,35 According to the EDX spectrum of the fractioned Au cluster compound (Figure 1b), the peak of Cl was detected although the intensity was considerably weak. The fractioned cluster compound was characterized by mass spectrometry with the negative-ion mode. The ESI-MS spectrum of the isolated cluster species is shown in Figure 2a, presenting the existence of undecagold (Au11) clusters consisting of primarily [Au11(TPPS)9Cl]7− (with H+; m/z = 879) and [Au11(TPPS)7Cl3]7− (m/z = 666). The peak assignments including fragments are also displayed in the figure. [Au(TPPS)Cl]− (m/z = 573) was the most prominent fragment of the product. Note here that the presence of one chlorine atom in the [Au(TPPS)Cl]− fragment can be reasonably recognized by two distinct isotope peaks (m/z = 573 and 575) differing by 2 amu with an intensity ratio of ∼3:1. Moreover, we found that [Au10(TPPS)8Cl]7− could be derived by the removal of Au(TPPS) from [Au11(TPPS)9Cl]7− and [Au11(TPPS)6Cl3] by the removal of TPPS from [Au11(TPPS)7Cl3]7−, indicating that phosphine ligands can be readily dissociated from the compounds.36 The mean core size of the fractioned cluster compound was determined by a solution-phase SAXS measurement. Figure 2b displays the core diameter distribution determined by the SAXS profile analysis, and the average diameter is 0.78 nm. Note that the SAXS signal originates from the difference between the electron density of the sample and the solvent, so the dominant scattering should come from the Au (core) atoms with very high electron density. In addition, the present analysis is based on the perfectly spherical clusters with a size distribution of Γ-
Figure 2. (a) Negative-ion ESI mass spectrum of the cluster compound. Note that signals marked with “*” are due to the ions from TRIS.73 (b) Obtained cluster size distribution of the Au cluster compound. Typical small-angle X-ray scattering (SAXS) intensity profile along with the simulated curve are shown by dots and curves, respectively.
distribution function,27 so the appearance of a size distribution may be possible even in an actual monodisperse cluster. Taking into account that the undecagold clusters have a core of 0.8 nm in diameter,37−40 the cluster size is in good agreement with that derived from the mass spectrometric data. Mingos, Pyykkö, and Häkkinen have independently demonstrated that the electronic structure determines the stability of small phosphine-protected Au clusters.41−43 If individual phosphine or halide is assumed to respectively take zero or one valence electron from a gold cluster, the total number of valence electrons (N*) of [Aun(PR3)mXs]z+ can be expressed as N* = n − s − z. The values of N* agree with those expected from the superatom concept in which Au clusters with total numbers of valence electrons (N*) of 2, 8, 18, 34, 58, ... are stable because they have closed electronic structures.41−43 In the present case, not taking into account the anionic contributions from sulfonate groups, N* = 8, a magic number indicating closed electronic structures. Absorption and Magnetic Circular Dichroism Spectroscopy. UV−vis absorption spectroscopy is also a useful method for characterizing the clusters because the difference C
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conducted. MCD is the differential absorption of left and right circularly polarized light, induced in a sample by an external magnetic field oriented parallel to the direction of light propagation. The MCD features must correspond to electronic transitions in the absorption spectrum, so MCD provides information on the magnetic properties of the system as well as new insights into the assignment of the corresponding absorption spectrum.47 The MCD spectrum of the present Au11 cluster compound at the magnetic field of −1.6 T is shown in Figure 3b. The MCD showed overall positive features in the energy region of 15000−45000 cm−1 (∼700−220 nm in wavelength), suggesting that the MCD exhibits only Faraday B-terms and therefore no degeneracies.48 The B-terms arise because of the mixing between the ground or excited state and an energetically close intermediate state induced by the magnetic field. The absence of degenerate ground and excited states excludes the MCD A-terms showing a derivative line shape with respect to the absorption peak, since the molecule should have a 3-fold or higher rotational or improper axis in its point group. In addition, the C-terms are absent due to the diamagnetic ground state.48 Note that the sign of the MCD signal is completely reversed when the field is switched (+1.6 T), confirming that signatures should originate from real MCD responses. A spectral deconvolution analysis of both the absorption and MCD data was performed to quantitatively estimate accurate transition energies of the Au11 cluster’s electronic transitions. The Gaussian fits of MCD as well as electronic absorption spectra for the cluster compound are also shown in Figure 3a,b. For deconvoluting the experimental data, we assumed that the analysis is constrained by the requirement that a “single set” of Gaussian components be used for the fitting of both the absorption and the MCD spectra. The fitting parameters obtained are listed in Table 1. To obtain excellent (satisfactory)
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.44 Figure 3a shows the
Figure 3. (a) Absorption spectra of the undecagold cluster compound. (b) MCD spectra of the undecagold cluster compound. Applied magnetic fields are +1.6 T. Black curves indicate the experimental absorption and MCD spectra, and red curves show the sum of the deconvoluted spectra. Gaussian band fits using seven components are also shown (blue curves).
Table 1. Results of the Gaussian Fit Analysis of the Gold Cluster Compound
UV−vis absorption spectrum of the fractioned TPPS-protected Au11 compound. Note that pure TPPS has a broad absorption feature in the wavelength region shorter than ∼310 nm (see the Supporting Information). The spectrum is somewhat structured and exhibits broad absorption peaks (or shoulders) due to the characteristic molecule-like transitions at ∼265 nm (∼38000 cm−1), 320 nm (∼31000 cm−1), and 490 nm (∼20500 cm−1), which are similar but slightly red-shifted to organically soluble phosphine-protected Au11 (undecagold) clusters.45 Among the phosphine-protected Au11 clusters, some discrepancies are present in the spectral features,45 strongly indicating that the fine structure is sensitive to the core structure and ligand composition. In this regard, structurally similar TPP-stabilized undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, have been isolated recently.45 Absorption spectra of these purified clusters show distinct peaks and shoulders characteristic of undecagold clusters,46 but some peak shifts were detected between these clusters. Hence mixtures of the two clusters can bring about blurred absorption, rather than the distinct peaks found in the purified clusters. According to the mass spectrometric data for our fractioned cluster, the compound is probably a mixture of [Au11(TPPS)9Cl]7− (predominant species) and [Au11(TPPS)7Cl3]7−, which should influence its absorption property. The differences in transition wavelengths for the two forms likely result from changes in the slight core geometry required to accommodate the phosphine or chloride in Au11 clusters. To obtain detailed information on the cluster’s electronic structures, magnetic circular dichroism spectroscopy was
agreement between the measured and calculated spectra, at least seven Gaussian components (1−7) were necessary. We found no derivative line shapes appeared in the MCD spectrum, corroborating the absence of the Faraday A-terms for the Au11 clusters. Importantly, MCD responses are better resolved and show more distinct spectroscopic features than absorption spectra; that is, the MCD spectrum exclusively had the deconvoluted components of 1 and 3. D
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Figure 4. Absorption spectrum of the (a) original undecagold cluster compound in aqueous phase (Au11(w)) and (b) its phase-transferred sample in chloroform (Au11(o)). (c) Photograph of sample vials with the biphasic mixture in the presence of (i) MME, (iii) DME, or (iii) MME and DME. The existence of both MME and DME brings about the phase transfer of the TPPS-protected Au11 cluster compound. Additionally, chemical structures of (1R,2S)-N-dodecyl-N-methylephedrinium bromide (DME·Br) and (1R,2S)-N,N-dimethylephedrinium bromide (MME·Br) are also shown.
Figure 5. Circular dichroism (CD) spectra of (a) phase-transferred TPPS (pure ligand; TPPS(o)) and (b) the TPPS-protected undecagold cluster compound in chloroform (Au11(o))).
so far for the water-soluble phosphine-protected Au clusters, so we tried to achieve asymmetric conversion via surface hydrophobization of the anionic Au cluster with chiral ephedrinium cations. Chiral phase-transfer reactions were carried out using catalysts in combination both with DME and MME having the same stereostructure in a water/chloroform binary phase system. The lack of either DME or MME never led to the achievement of the phase transfer (synergistic ef fect; see Figure 4c). Vigorous shaking of the biphasic mixture containing the aqueous Au11 clusters and MME (upper phase) and DME in chloroform (lower phase) resulted in the almost complete transfer of the clusters into the organic phase. The behavior is demonstrated in Figure 4c that shows a photograph of sample vials with a biphasic mixture after the unsuccessful/successful phase transfer. The color transfer from the upper to the lower phase directly indicates the cluster transfer itself. The synergistic effect is probably due to the fact that the sulfonate ion is very highly hydrophilic, so the high degree of hydrophobization is required for the ionic screening of the substituents. Panels a and b of Figure 4 show optical absorption spectra of the Au11 cluster compound in water (Au11(w); before the phase transfer) and in chloroform (Au11(o); after the phase transfer). Note that pure TPPS transferred into chloroform using the chiral
An MCD spectrum of triphenylphosphine-protected Au9 cluster [Au9(PPh3)8]3+ has been reported in the UV−vis energy range by Mason and co-worker.48 Similarly to the case of our Au11 cluster system, the MCD had better-resolved spectroscopic features than optical absorption. The spectrum has been essentially interpreted in terms of molecular orbitals that are approximated by 6s orbitals on the Au atoms (that is, core-based transitions). In addition, the MCD signal, with weak transitions in the low-energy region that were unresolved in the absorption, has been logically assigned to transitions to spin−orbit states of predominantly triplet parentage.48 In a later section, we will revisit this issue. Asymmetric Conversion: Synergistic Effect. Chirality has been found in a variety of Au clusters, but most of them are in thiolate-protected Au cluster species.29,49−55 In the case of phosphine-protected Au clusters, we only found a few reports on chiral (R)-/(S)-BINAP-stabilized clusters dispersed in organic media.25,26 On the other hand, versatile asymmetric conversion of achiral Au cluster compounds still continues to be one of the challenging topics in gold cluster chemistry, and in the present case of water-soluble undecagold clusters, chiral phase transfer should offer great advantages in their conversion procedure.24,56,57 To our knowledge, no attempts have been made E
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Figure 6. Optimized structures, theoretical absorption spectra, and Kohn−Sham orbitals (HOMO and LUMO) of model undecagold cluster compounds NC1 and NC2. The formulas of the clusters are also indicated.
CD signals only in the wavelength region shorter than ∼275 nm (see the inset in Figure 5a). These findings demonstrate the versatility of asymmetric conversion of optically inactive phosphine-protected metal clusters by ion association. Here, the appreciable induced optical activity was limited in a relatively high-energy region (≤∼420 nm), and this will be discussed in the next section through model quantum chemical calculations. Quantum Chemical Calculations. Some crystal structures of phosphine-protected undecagold clusters have been determined; for example, Au11(PPh3)7X3 (X = thiolate or halide) compounds tend to have incomplete icosahedral skeletons with idealized C3v symmetry,58,59 the Au11(PPh3)8Cl2 cluster has a similar incomplete icosahedral shell, and the two halide ions are attached to two opposite gold atoms on a 5-fold ring.60 Overall, these structures suggest that the 10 surface gold atoms in undecagold compounds are relatively fluxional.45,61 Meanwhile, theoretical studies have examined the electronic structures of many kinds of gold−phosphine cluster systems on the basis of the DFT approaches.62−64 Here, to examine how the surface phosphine ligands influence the core geometry and the resultant optical/chiroptical responses in the Au11 undecagold clusters,
DME (that is, TPPS-DME ion-pair adduct) has no absorption in the wavelength region longer than ∼310 nm (see the Supporting Information). Significantly, the spectral shape of the Au11 cluster compound transferred into chloroform was identical with that in water, suggesting that “solvent effect” is marginal, and the gold core rearrangement and/or nuclearity change is unlikely to take place upon the phase transfer. The circular dichroism spectrum of the phase-transferred Au11 cluster in chloroform (Au11(o)) is shown in Figure 5b along with that of the similarly transferred pure TPPS in chloroform (Figure 5a). Au11(w) did not show any CD signals (namely, optically inactive) reasonably. In contrast, the extracted ion-pair species Au11(o) showed small but appreciable chiroptical responses in the metal-core-involved electronic transitions (
