A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13Cu2, into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13Cu2(DPPP)3(SPy)6] + was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13Cu2((2r,4r)/(2s,4s)-BDPP)3(SPy)6] + was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images from 250-500nm with maximum anisotropy factors of 1.2 × 10. Density Functional Theory (DFT) calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70°C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis. Chirality is ubiquitous in nature 2] and plays a key role in enantioselective catalysis, pharmaceutical sciences, optical devices, etc. In the past few decades, people have established lots of chemical means for enantioselective synthesis of chiral molecules for specific applications. To date, many chiral nanoclusters have been discovered or synthesized. Atomically precise nanoclusters are particularly good models to probe the origin of chirality. Many efforts have been dedicated to unraveling the chirality in cluster science and several plausible mechanisms have been proposed such as the dissymmetric field effect and chiral footprint model. However, unlike chiral organic compounds or other small molecules, asymmetric synthesis of large chiral structures such as nanoparticles or nanoclusters remains a real challenge, which hampers the progress of unraveling chirality of metal nanoclusters. Generally speaking, chiral nanoclusters can be classified into three broad categories by different origins of the chirality: (1) chiral metal core; (2) asymmetry arrangement of achiral surface structure (such as RS-Au-SR unit); (3) homochiral ligands induced extrinsic chirality. Some chiral clusters may belong to two of the three categories, e.g., with chirality originating from both staple and kernel. 23] Symmetry breaking is a universal phenomenon in nature and the concept of symmetry breaking in the present context includes spontaneous resolution, chirality amplification, chiral induction, asymmetric catalysis, crystal growth, or combinations thereof. 25] Since most metal clusters have an achiral, highly symmetrical metal core (often possessing mirror or inversion symmetries), the simplest method to synthesize a chiral metal cluster is to lower the overall symmetry with the ligands. However, symmetry breaking by asymmetry arrangement of achiral ligands is often difficult to control. Furthermore, it inevitably leads to racemic mixtures. Many efforts have been made to enantioseparate those racemic mixtures by using chiral HPLC columns and phase transfer method using a chiral ammonium salt. The problem here is that it is often challenging to carry out such separations to produce optically pure nanoparticles. Moreover, racemization can exist in chiral metal cluster systems. For example, racemization studies on thiolate-protected gold cluster have been done by Bu ̈rgi’s group. They found the gold−thiolate interface is flexible and racemization phenomenon have been observed at 30°C. 31] Herein we report the syntheses of three closely related, extrinsically chiral metal nanoclusters via these two strategies. The first is a racemate, characterized as [Au13Cu2(DPPP)3(SPy)6] (2), which has an achiral metal core, six Spy, and three achiral diphosphine ligands DPPP in asymmetric arrangements (where Spy= pyridine-2-thiol and DPPP=1,3bis(diphenyphosphino)propane). Using chiral (2r,4r)/(2s,4s)-2,4bis(diphenylphosphino)pentane (BDPP) (Figure S1) in place of achiral diphosphine, two optically pure enantiomers [Au13Cu2(2r,4r-BDPP)3(SPy)6] (3-R) and [Au13Cu2(2s,4sBDPP)3(SPy)6] (3-S), were obtained in one pot. All the three clusters were synthesized by reducing a mixture of Au, Cu, pyridinethiol and diphosphine with NaBH4 in an ice bath (see Supporting Information (SI) for more details). Single crystals suitable for X-ray diffraction were grown by slow solvent evaporation after ten days. Single-crystal structure determination revealed that 2 is a racemate crystallized in the centrosymmetric space group P21/c. The unit cell comprises two pairs (Z = 4) of enantiomers (Table S1 and Figure S2). As shown in Figure 1a, the molecular architecture of 2 is similar to the previously reported [Au13Cu2(PPh3)6(SPy)6] (1, depicted in Figure S3). As shown in Figure 1b and c, 2 has an icosahedral Au13 core and two Cu atoms capping two Au3 triangles of the icosahedral Au13 core oriented along one of the 3-fold axes of the icosahedral Au13. The core of 2 is achiral and displays D3d symmetry. The six pyridine2-thiol ligands bind to the two Cu atoms and six gold (one each) atoms from the two Au3 triangles capped by Cu as bidentate ligands (Figure 1a). The pyridine-2-thiol ligands in two sides of the metal core rotate some degree (Figure 1d and Table 4). The pyridine-2-thiol ligands are in asymmetry arrangement and displays C3 symmetry. So the symmetry of this cluster is effectively lowered here. Three DPPP ligands bind to six Au atoms at the ‘equatorial’ positions of the icosahedral core. As shown in Figures 1e, the three DPPP ligands cap on the cluster [a] G. C. Deng, J. Z. Yan, Y. Z. Han, P. Yuan, Dr. C. W. Zhao, X. T. Yuan, Dr. S.C. Lin, Prof. Z. C. Tang, Prof. B. K. Teo, Prof. N. F. Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University Xiamen 361005, China E-mail: nfzheng@xmu.edu.cn boonkteo@xmu.edu.cn [b] Dr. S. Malola, Prof. H. Häkkinen Departments of Physics and Chemistry Nanoscience Center, University of Jyväskylä Jyväskylä 40014, Finland Supporting information for this article is available on the WWW under http:// 10.1002/anie.201800327 A cc ep te d M an us cr ip t Angewandte Chemie International Edition This article is protected by copyright. All rights reserved.

中文翻译：

---

从对称性破缺到揭示金属纳米团簇的手性

设计了一种使用混合配体（此处为二膦和硫醇盐）的通用方法，通过破坏（降低）与配体的整体分子对称性，将非手性金属簇 Au13Cu2 转变为对映体对。使用非手性二膦，制备了在中心对称空间群中结晶的外消旋 [Au13Cu2(DPPP)3(SPy)6] + 。使用手性二膦，在一个锅中实现了光学纯对映体对 [Au13Cu2((2r,4r)/(2s,4s)-BDPP)3(SPy)6] + 的对映选择性合成。它们的圆二色性 (CD) 光谱给出了 250-500nm 的完美镜像，最大各向异性因子为 1.2 × 10。密度泛函理论 (DFT) 计算提供了与观察到的对映体 CD 光谱的良好相关性，更重要的是，揭示了起源的手性。外消旋化研究表明，这些手性纳米团簇具有高稳定性（在 70°C 下无外消旋化），在不对称催化等应用中具有广阔的前景。手性在自然界中无处不在 2] 并且在对映选择性催化、药物科学、光学器件等方面起着关键作用。在过去的几十年里，人们已经建立了许多化学手段来对特定应用的手性分子进行对映选择性合成。迄今为止，已经发现或合成了许多手性纳米团簇。原子精确的纳米团簇是探测手性起源的特别好模型。许多努力致力于解开集群科学中的手性，并提出了几种可能的机制，例如不对称场效应和手性足迹模型。然而，与手性有机化合物或其他小分子不同，大型手性结构（如纳米颗粒或纳米团簇）的不对称合成仍然是一个真正的挑战，这阻碍了揭示金属纳米团簇手性的进展。一般来说，手性纳米团簇可以根据手性来源的不同分为三大类：（1）手性金属核；(2)非手性表面结构的不对称排列（如RS-Au-SR单元）；(3)同手性配体诱导外在手性。一些手性簇可能属于三个类别中的两个，例如，手性来自主食和籽粒。23] 对称性破缺是自然界的普遍现象，目前对称性破缺的概念包括自发分解、手性放大、手性诱导、不对称催化、晶体生长或其组合。25] 由于大多数金属簇具有非手性的、高度对称的金属核（通常具有镜像对称性或反转对称性），因此合成手性金属簇的最简单方法是降低配体的整体对称性。然而，非手性配体的不对称排列破坏对称性通常难以控制。此外，它不可避免地导致外消旋混合物。已经通过使用手性 HPLC 柱和使用手性铵盐的相转移方法对这些外消旋混合物进行了对映分离。这里的问题是进行这种分离以生产光学纯纳米粒子通常具有挑战性。此外，手性金属簇体系中可能存在外消旋化。例如，Bu rgi 的小组已经对硫醇盐保护的金簇进行了外消旋化研究。他们发现金-硫醇盐界面是灵活的，并且在 30°C 下观察到外消旋化现象。31] 在此，我们报告了通过这两种策略合成三种密切相关的外在手性金属纳米团簇。第一种是外消旋体，表征为 [Au13Cu2(DPPP)3(SPy)6] (2)，它具有非手性金属核、六个 Spy 和三个非手性二膦配体 DPPP，以不对称排列（其中 Spy=pyridine-2-硫醇和 DPPP=1,3 双（二苯膦基）丙烷）。使用手性 (2r,4r)/(2s,4s)-2,4bis(diphenylphosphino)pentane (BDPP)（图 S1）代替非手性二膦，两种光学纯对映体 [Au13Cu2(2r,4r-BDPP)3(SPy) )6] (3-R) 和 [Au13Cu2(2s,4sBDPP)3(SPy)6] (3-S)，在一锅中获得。所有三个簇都是通过在冰浴中用 NaBH4 还原 Au、Cu、吡啶硫醇和二膦的混合物来合成的（有关更多详细信息，请参见支持信息 (SI)）。十天后通过缓慢的溶剂蒸发生长适用于 X 射线衍射的单晶。单晶结构测定表明2是在中心对称空间群P21/c中结晶的外消旋体。晶胞包含两对 (Z = 4) 对映异构体（表 S1 和图 S2）。如图 1a 所示，2 的分子结构类似于之前报道的 [Au13Cu2(PPh3)6(SPy)6]（1，如图 S3 所示）。如图 1b 和 c 所示，2 具有一个二十面体 Au13 核心和两个 Cu 原子，覆盖了沿二十面体 Au13 的 3 重轴之一定向的二十面体 Au13 核心的两个 Au3 三角形。2 的核心是非手性的，并显示 D3d 对称性。六个吡啶 2-硫醇配体与两个 Cu 原子和六个金（每个一个）原子结合，来自两个 Au3 三角形，由 Cu 作为双齿配体封盖（图 1a）。金属核两侧的吡啶-2-硫醇配体在一定程度上旋转（图 1d 和表 4）。吡啶-2-硫醇配体呈不对称排列并显示 C3 对称性。所以这个簇的对称性在这里被有效地降低了。三个 DPPP 配体在二十面体核心的“赤道”位置与六个 Au 原子结合。如图 1e 所示，三个 DPPP 配体覆盖簇 [a] GC Deng, JZ Yan, YZ Han, P. Yuan, Dr. CW Zhao, XT Yuan, Dr. SC Lin, Prof. ZC Tang, Prof. BK Teo, Prof. NF Zheng 固体表面物理化学国家重点实验室，厦门大学化学化工学院化学与能源材料协同创新中心厦门361005 E-mail: nfzheng@xmu.edu.cn boonkteo@xmu.edu.cn [b] Dr. S. Malola, Prof. H. Häkkinen 物理和化学系纳米科学中心，Jyväskylä 大学 Jyväskylä 40014，芬兰 支持本文的信息可在 WWW 上获得，网址为 http://10.1002/anie.201800327 A cc ep te d M an us cr ip t Angewandte Chemie International Edition 本文受版权保护。版权所有。Häkkinen 物理和化学系纳米科学中心，Jyväskylä 大学 Jyväskylä 40014，芬兰 这篇文章的支持信息可在 WWW 上获得，网址为 http://10.1002/anie.201800327 A cc ep te d M an us cr ip t International Angewandte版本 本文受版权保护。版权所有。Häkkinen 物理和化学系纳米科学中心，Jyväskylä 大学 Jyväskylä 40014，芬兰 这篇文章的支持信息可在 WWW 下获得：http://10.1002/anie.201800327 A cc ep te d M an us cr ip t Chemieandte版本 本文受版权保护。版权所有。