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From Racemic Metal Nanoparticles to Optically Pure Enanti-omers in One Pot Huayan Yang, Juanzhu Yan, Yu Wang, Guocheng Deng, Haifeng Su, Xiaojing Zhao, Chaofa Xu, Boon K. Teo, and Nanfeng Zheng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10448 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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Huayan Yang,† Juanzhu Yan,† Yu Wang, Guocheng Deng, Haifeng Su, Xiaojing Zhao, Chaofa Xu, Boon K. Teo,* Nanfeng Zheng* State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Supporting Information ABSTRACT: A general strategy, using mixed ligands, is utilized to synthesize atomically precise, intrinsically chiral nanocluster [Ag78(DPPP)6(SR)42] (Ag78) where DPPP is the achiral 1,3-bis(diphenyphosphino)propane and SR=SPhCF3. Ag78 crystallizes as racemates in a centric space group. Using chiral diphosphines BDPP=2,4bis(diphenylphosphino)pentane, the enantiomeric pair [Ag78(R/S-BDPP)6(SR)42] can be prepared with 100% optical purity. The chiral diphosphines gives separately rise to asymmetric surface coordination motifs composed of tetrahedral R3PAg(SR)3 moieties. The flexible nature of C–C –C angles between the two phosphorous atoms restricts the relative orientation of the tetrahedral R3PAg(SR)3 moieties, thereby resulting in the enantiomeric selection of the intrinsic chiral metal core. This proof-of-concept strategy raises the prospect of enantioselectively synthesizing optically pure, atomically precise chiral noble metal nanoclusters for specific applications.
Chirality plays many important roles in nature.1-7 The determination of structures of chiral molecules can shed light on the origin of the chirality, thereby allowing control of their related physical, chemical, or biological properties.2,8-12 Recently, the chirality of metal nanoparticles or nanoclusters has attracted much interest in nano-research. A wide range of prospective applications in catalysis, pharmaceutics, sensors, liquidcrystals, optoelectronics, nanoelectronics, etc., have been proposed or demonstrated.13-22
nanoparticles passivated with chiral surface ligands have been known for a long time,29,31-38 the solution-based synthesis of metal nanoclusters with well-defined structures remains relatively scarce. The problem here is not whether it is possible to synthesize chiral nanoparticles but rather it is often challenging to carry out such synthesis enantioselectively to produce optically pure nanoparticles. We report herein the synthesis of intrinsically chiral metal nanoparticles (NP) as racemates using a mixed ligand system, in the present case, a combination of thiolates and diphosphines. Subsequent employment of chiral diphosphine gave rise to optically pure enantiomers in one pot. We note that many chiral nanoclusters, as prepared, are obtained as racemates.39 Though great efforts have been made to enantio-separate these chiral nanoparticles/nanoclusters by HPLC, chiral ion-pairing, or other methodologies, they are generally tedious and plagued with low efficiency.40-43 The methodology advocated herein allows the enantioselective synthesis of the chiral nanoclusters with high optical purity and high yield.
A prerequisite to understanding chirality of metal nanoparticles is to devise a general synthetic method for creating them. Structural characterization of atomically precise, nano-sized intrinsic chiral metal clusters provide excellent opportunities for probing their chirality as well as allowing modification of their properties tailored for specific applications. Generally speaking, chiral metal NPs can be broadly classified as three categories: (1) intrinsically chiral metal core; (2) a chiral arrangement of achiral surface structure; (3) extrinsic chirality induced by homochiral ligands.20,23-30 Despite the fact that
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Figure 1. (a) Photograph of the [Ag78(SPhCF3)42(DPPP)6] crystal. (b) Molecular structure of [Ag78(SPhCF3)42(DPPP)6] (Ag78). (c) Space-filling model of Ag78 which can be described as a twisted trigonal prism. (d) The Ag66 unit encapsulated in 4a shell containing three [Ag4(DPPP)2(SR)8] units and eighteen SR . Color legend: green and orange sphere, Ag; yellow sphere, S; pink sphere, P; gray sphere/stick, C. All hydrogen and fluorine atoms are omitted for clarity.
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edges of the chiral Ag66 core is capped by a complex 4+ [Ag4(DPPP)2(SPhCF3)8] unit. Color legend: green and dark green sphere, Ag; yellow sphere, S; pink sphere, P.
The molecular architecture of Ag78 can be described as a core-shell structure Ag@Ag21@Ag44@[Ag12(DPPP)6(SPhCF3)24(SPhCF3)18] (Figure 2). The Ag@Ag21 kernel displays D3 symmetry and can be described as three mutually interpenetrating icosahedra. The threefold axis and three twofold axes pass through the center Ag atoms (Figures 2a and S3). The coordination number of the central Ag atom is 9 instead of 12. As shown in Figure 2b, Ag@Ag21 kernel is encapsulated in an Ag44 shell whose structure can be rationalized as follows. The Ag@Ag21 core has a total of 38 triangular Ag3 faces, 12 of which form six concave Ag4 “butterflies” (two triangles sharing an edge). The remaining 26 convex triangular faces are each capped by an Ag atom. Due to steric hindrance, each of the six Ag4 “butterflies” is capped by only one Ag atom. Twelve (12) atopcapping Ag atoms complete the 44-atom shell (26 + 6 + 34). Thus, the Ag@Ag21 kernel and 44-Ag shell comprise the Ag66 core. The Ag66 core can also be described as a chiral tessellated polyhedron (Figure 2c) The Ag-Ag distances in the core of the cluster range from 2.760 to 3.078 Å with an average of 2.869 Å , which is close to the Ag-Ag distance (2.889 Å ) of bulk silver.
Specifically, this paper reports the syntheses and structures of three closely related intrinsic metal nanoparticles. The first is the racemic nanocluster [Ag78(SPhCF3)42(DPPP)6] (hereafter denoted as Ag78). It was synthesized by reducing a mixture of AgBF4, 4(trifluoromethyl)thiophenol (HSPhCF3), 1,3bis(diphenyphosphino)propane (DPPP, an achiral diphosphine), and trimethylamine with NaBH4 at 0 °C (see Methods for details). Centimeter-sized single crystals suitable for X-ray diffraction were grown by layering nhexane or toluene into a CH2Cl2 solution at 4 C (Figure 1a). Single-crystal structure determination revealed that Ag78 sits on a crystallographic twofold axis of an orthorhombic unit cell under centrosymmetric space group Ccc2 (see Table S1). As shown in Figure 1b-d, the overall molecular shape of Ag78 can be described as a twisted trigonal prism, with an Ag66 core unit capped by three [Ag4(DPPP)2(SPhCF3)8]4- units and eighteen SPhCF3ligands (Figures 1b, c, S2 and S3). The structure of Ag78 conforms to an idealized molecular symmetry of D3 point group，with one threefold axis and three twofold axes passing through the central silver atom of the Ag66 unit (Figures 1c and S3). There are two pairs (Z=4) of enantiomers (as racemates) of Ag78 per unit cell (Figure S4).
As depicted in Figure 1d, the Ag66 unit is encapsulated by an Ag-PR-SR complex shell and shaped as a twisted trigonal prism. The diphosphine ligands decorate the three vertical edges of the trigonal prism while the thiolates are situated on the five faces (three rectangular faces and two triangular faces) of the trigonal prism. Specifically, each of the three vertical sides is capped by a complex [Ag4(DPPP)2(SPhCF3)8]4- unit (Figures 2d and S5) which comprises a pair of diphosphine-linked vertexsharing-bitetrahedral [PAgS3]2 moieties connected by two doubly-bridging thiolates. Each of the three rectangular faces of trigonal prism is capped by four [SPhCF3], forming a rhombus framework. Finally, the two triangular faces of the trigonal prism are each capped by three [SPhCF3] moieties. The overall structure conforms to idealized D3 symmetry (Figure S6). It turns out that the three [Ag4(DPPP)2(SPhCF3)8]4units on the three vertical edges of the trigonal prismatic framework can twist in either left- or right-handed orientation (Figure S5). As noted earlier, in each [Ag4(DPPP)2(SPhCF3)8]4- unit, four PAgS3 tetrahedral moieties are linked by two DPPP ligands and four shared S atoms: the two “outer” tetrahedra are vertex-S-sharing and the two “inner” tetrahedrons are edge-S-sharing. The crucial factor here is that the C-C-C bond angle constraint of the DPPP ligands forced two PAgS3 tetrahedral moieties to have the same orientation. An opposite orientation of the remaining two PAgS3 tetrahedrons amplifies the twisting of the tetrahedral units. In this fashion, the surface ligand twisting “transfers” the steric constraints from the surface [Ag4(DPPP)2(SPhCF3)8]4- units to the Ag66 core, thereby “locking in” the respective
Figure 2. (a) The Ag@Ag21 kernel comprises three interpenetrating icosahedra. (b) The Ag@Ag21 unit is encapsulated in an Ag44 shell. (c) The 6 pair-capping, 26 facecapping, 12 vertex-capping and the 21 Ag atoms of the Ag@Ag21 unit form 26 tetrahedrons and 6 hexahedrons, conforming to D3 symmetry. (d) Each of the three vertical 2
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enantiomeric form of the latter (Figure S6). Since DPPP is achiral, the left- and right-handed twists are equally probable, giving rise to the racemic pairs of Ag 78 (Figure S5).
R/S-Ag78 exhibit nearly perfect mirror-images signals from 250 to 800 nm. And the identified peaks are in good agreement with their absorption spectra. For R/S-Ag78 eight clear signals are observed at 253, 301,355, 412, 455, 524, 568 and 679 nm. The intensity of CD spectra depends on the concentration of the sample, and the concentration-independent anisotropy factors g = ΔA/A = θ[mdeg] / (32980 × A) were calculated over the spectral range. Similar to Au38(2-PET)24,26 the anisotropy factors increase with increasing wavelength and the maximum anisotropy factor is up to 2 x 10-3 at 678 nm (Figure 4d). The high anisotropy factor indicates that intrinsic chirality core contributes significantly to the net optical activity. The Ag78 clusters can be described as superatomic clusters with 78-42=36 Jellium electrons and the Jelliumatic electronic configuration of 1S21P61D102S21F142P2. If we define the principal C3 symmetry axis as the z axis, the superatomic HOMO would be 2Pz2 with the degenerate 2Px2Py as the corresponding superatomic LUMOs.
Figure 3. Structural anatomy of the two enantiomers of Ag78, S-Ag78 and R-Ag78, from core to surface. The clusters display idealized D3 symmetry and perfect mirror symmetry to each other. Color legend: green and dark green sphere, Ag; yellow sphere, S; pink sphere, P; gray sphere/stick, C.
Based on this analysis, it occurs to us that enantioselective synthesis of optically pure intrinsic chiral metal nanoparticles Ag78 should be possible by controlling the orientation of PAgS3 tetrahedra of the [Ag4(DPPP)2(SPhCF3)8]4- units. The strategy adopted here is to use chiral ligands (2s,4s)-2,4-bis(diphenylphosphino) pentane (2s,4s-BDPP) and (2r,4r)-2,4bis(diphenylphosphino)pentane (2r,4r-BDPP) (Figure 4a) in place of achiral DPPP. The steric hindrance of two chiral C atoms on the propyl group of (2s,4s)-BDPP or (2r,4r)-BDPP should restrict the relative rotations of the alkyl group and thus the absolute configuration of the respective [Ag4(BDPP)2(SPhCF3)8]4- units (Figure S7). Furthermore, the strong Ag–S interactions between surface units and the metal core facilitate the “transfer” of the chirality from the surface to the metal core, thereby “locking in” the particular enantiomer and exerting the enantioselective control. The resulting metal nanoparticles were structurally characterized as [Ag78(SPhCF3)42(Chiral-BDPP)6] (abbr. R/S-Ag78). As shown in Figure 3 (also Tables S2 and S3 and Figures S8 and S9), the two enantiomers have exactly the same structure and exhibit perfect mirror symmetry. To the best of our knowledge, this is the first report of crystal structures of an enantiomeric pair of intrinsic chiral nanoparticles of 100% optical purity, using a chiral ligand pair.
Figure 4. (a) Chemical structures of different diphosphine ligands. (b) UV–Vis spectra of R–Ag78, S–Ag78 and Rac–Ag78. (c) Circular dichroism (CD) spectra of enantiomers of chiral Ag78. (d) Corresponding anisotropy factors of L– and S–Ag78 –3 enantiomers (g = ∆A/A of up to 2.6 × 10 ).
In conclusion, a general methodology, using mixed ligand as surface capping ligands，namely, diphosphines and thiolates, is used to synthesize a series of atomically precise racemic intrinsically chiral Ag78 nanoclusters. The C-C-C bond angles of diphosphines constrains the rotational direction of the tetrahedral R3PAg(SR)3 moieties, thereby dictating the chirality of the nanoparticles. Using chiral diphosphines in place of achiral diphosphines, it was demonstrated that enantioselective synthesis of chiral nanoclusters, namely,
The UV-vis spectra of pure crystals of Ag78 in CH2Cl2 were measured. As shown in Figure 4b, racemic Ag78 and the two enantiomers R/S-Ag78 exhibit three strong bands at 365, 429 and 528 nm, and two weak shoulder peaks at 654 and 733 nm. As shown in Figure 4c and d, the circular dichroism (CD) spectra of the optically pure solution of 3
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the enantiomeric pair R/S-Ag78, can be accomplished with 100% optical purity and high yield in a one-pot synthesis. The crystal structures of the enantiomer pair of R/S-Ag78 prove that the absolute handedness of nanoparticles can be controlled by surface chiral diphosphine ligands. This proof-of-concept strategy raises the prospect of enantioselectively synthesizing atomically precise chiral metal nanoclusters for specific applications. It is noted that, until now, even the most advanced HRTEM faces real challenges in characterizing and tailoring the composition of chiral nanoparticles precisely. Finally, the high stability and the large crystal size of these chiral nanoclusters show promise in applications such as catalysis, nonlinear optics, biomedicine, etc.
Supporting Information. Experimental details, detailed crystallographic structure and data including the CIF file, symmetry and chirality analysis, total structure, packing structure. This information is available free of charge via the internet at http://pubs.acs.org, AUTHOR INFORMATION
[email protected],
[email protected] †
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H. Y. Yang and J. Z. Yan contributed equally to this work.
We thank the National Key R&D Program of China (2017YFA0207302) and the NSF of China (21731005, 21420102001, 21333008, 21390390) for financial support. The financial support (to BKT) from iChEM, Xiamen University is gratefully acknowledged.
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