Toward Total Synthesis of Thiolate-Protected Metal Nanoclusters

Article Cite This: Acc. Chem. Res. 2018, 51, 1338−1348

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Toward Total Synthesis of Thiolate-Protected Metal Nanoclusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Qiaofeng Yao,† Tiankai Chen,† Xun Yuan,‡ and Jianping Xie*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 College of Materials Science and Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Shibei District, Qingdao, Shandong Province, China 266042

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‡

CONSPECTUS: Total synthesis, where desired organic- and/or biomolecules could be produced from simple precursors at atomic precision and with known step-by-step reactions, has prompted centuries-lasting bloom of organic chemistry since its conceptualization in 1828 (Wöhler synthesis of urea). Such expressive science is also highly desirable in nanoscience, since it represents a decisive step toward atom-by-atom customization of nanomaterials for basic and applied research. Although total synthesis chemistry is less established in nanoscience, recent years have witnessed seminal advances and increasing research efforts devoted into this field. In this Account, we discuss recent progress on introducing and developing total synthesis routes and mechanisms for atomically precise metal nanoclusters (NCs). Due to their molecular-like formula and properties (e.g., HOMO−LUMO transition, strong luminescence and stereochemical activity), atomically precise metal NCs could be regarded as “molecular metals”, holding potential applications in various practical sectors such as biomedicine, energy, catalysis, and many others. More importantly, the molecular-like properties of metal NCs are sensitively dictated by their size and composition, suggesting total synthesis of them as an indispensable basis for reliably realizing their practical applications. Atomically precise thiolate-protected Au, Ag and their alloy NCs are employed as model NCs to exemplify design strategies and governing principles in total synthesis of inorganic nanoparticles. This Account starts with a brief summary of total synthesis methodologies of atomically precise metal NCs. Following the methodological summary is a detailed discussion on the mechanisms governing these synthetic strategies, which is the main focus of this Account. Based on unprecedented precision (at atomic resolution) and ease (ensured by size-dependent properties) of tracking clusters’ size/structure changes, mechanisms driving growth (e.g., reduction growth and seeded growth) and functionalization (e.g., alloying reaction and ligand exchange) of metal NCs have been explored at molecular level. With def initive step-by-step reaction routes, two-electron (2 e−) reduction (driving the growth reactions) and surface motif exchange (SME, prompting alloying and ligand exchange reactions) are discussed in depth and details. In addition to those sub- and/or individual-cluster level understandings, the self-assembly chemistry delivering high orderliness and enhanced materials performance in NC assemblies/supercrystals is also deciphered. This Account is then concluded with our perspectives toward potential development of cluster chemistry. Advances in total synthesis chemistry of metal NCs could not only serve as guidelines for future synthetic practice of NCs, but also provide molecular-level clues for many pending fundamental puzzles in nanochemistry, including nucleation growth, alloying chemistry, surface engineering and evolution of metamaterials.

Received: February 7, 2018 Published: May 24, 2018 © 2018 American Chemical Society

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DOI: 10.1021/acs.accounts.8b00065 Acc. Chem. Res. 2018, 51, 1338−1348

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Accounts of Chemical Research

Figure 1. Illustrative map of total synthesis routes of metal NCs: (a → b) formation of M(I)-SR complexes, (b → c) reduction growth, (c → d) seeded growth, (c → f) alloying reaction, (c → g) ligand-exchange process, (c → h) self-assembly of metal NCs, and (d → e) evolution from metal NCs to metal nanocrystals.

structure are unique molecular-like properties of metal NCs, such as HOMO−LOMO transition, quantized charging, strong luminescence, and intrinsic chirality, which exhibit strong size-/ structure-dependence.3,4,7−10 Such size-/structure-dependence not only provides a good means to diversify cluster properties for applications in many practical sectors (e.g., biomedicine,11 energy,12 and catalysis13,14), but also represents a facile means to monitor the size and structure change of NCs. Therefore, taking the molecular-like structure and property together, metal NCs could be regarded as “molecular metals”. Moreover, the atomic precision in synthesis and ease of tracking the size and structure evolution suggest these metal NCs as an ideal model platform for pursuing total synthesis chemistry of inorganic NPs. In this Account, we discuss recent achievements on introducing total synthesis routes into Au, Ag, and AuAg alloy NCs, as a prototype attempt toward establishing total synthesis chemistry for inorganic NPs. The discussion starts with a brief summary of state-of-the-art development of synthetic strategies that deliver atomically precise modulability over size and structure of metal NCs. Following is a concise and in-depth discussion on the molecular-level mechanisms governing synthetic strategies (Figure 1), including reduction growth (a → c), seeded growth (c → d), alloying reaction (c → f) and ligand-exchange process (c → g). Based on the as-revealed knowledge on the formation and functionalization fundamentals, promising utilization of high quality metal NCs as functional building blocks for construction of novel metamaterials is also explored (Figure 1c → h).

1. INTRODUCTION Total synthesis is a milestone discovery in continuous efforts of mankind toward demystification of nature. It refers to an ideal synthetic practice that generates desired organic- and/or biomolecules at atomic precision from simple and easily accessible precursors. More importantly, it features known chemistry of stepby-step reaction along the synthetic route of target molecules. Since the conceptualization of total synthesis in 1820s,1 it has prompted centuries-lasting bloom of organic chemistry, and shaped the latter into not only an expressive science but also a fine art. Such delicate reaction routes are also highly desirable in nanoscience. Given that almost all useful physicochemical properties of nanomaterials are dictated by their composition, size, and morphology, atom-by-atom customization of these structural attributes has become one of the most ambitious dreams of materials scientists.2 Such dream should root in total synthesis (i.e., synthesis at atomic precision and with known stepby-step reaction route) exploration of nanoparticles (NPs). Nevertheless, due to inherent difficulties in synthesis and characterization of conventional NPs at atomic precision, total synthesis exploration of NPs has long tramped in synthetic methodology development phase, leaving the governing mechanisms especially those at molecular level (i.e., step-by-step reaction route) largely unrevealed. The discovery of thiolate-protected atomically precise metal nanoclusters (NCs) in the 1990s opens a new avenue for advancing total synthesis research of NPs.3 Atomically precise metal NCs are a family of ultrasmall particles with 3 nm) of metal materials. By monitoring the growth pathway of [Au44(SR)26]2− 1346

DOI: 10.1021/acs.accounts.8b00065 Acc. Chem. Res. 2018, 51, 1338−1348

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Accounts of Chemical Research NCs all the way to crystalline Au NPs (Figure 1d → e), we might be able to reveal the fundamentals driving the formation (crystallization) of nanocrystals as well as the emerging of surface plasmon resonances (SPR) in crystalline metal NPs. Besides size growth exploration, fundamental interest also arises from the composition engineering (made possible by alloying and ligandexchange reactions) of metal NCs. It should be pointed out that current alloy and/or heteroligand-protected NCs are mostly produced in a distribution of metal atoms and/or ligands. Although a size-focusing process could eliminate the size heterogeneity and it has been well-investigated, a similar metal- and/or ligand-focusing process has been rarely explored. Such metal- and ligand-focusing exploration of NCs could lead to crucial implications to the alloying chemistry and surface engineering which are widely used but not well-understood in modulation of functional NPs. In addition to total synthesis chemistry at the sub- or individualcluster level, self-assembly chemistry at the intercluster level also deserves intense attention. Future efforts in this field may be placed on the shape- and crystallinity-controlled crystallization of metal NCs, which is a rising topic in metamaterials construction.51,52 It is entirely expected that continuous research efforts devoted to the aforementioned aspects of total synthesis study could fuel rapid growth of cluster chemistry, spurring crucial implications to many pending fundamental challenges (e.g., nucleation growth, alloying chemistry, surface engineering) that have puzzled the nanoscience community for decades.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tiankai Chen: 0000-0003-0730-3373 Jianping Xie: 0000-0002-3254-5799 Notes

The authors declare no competing financial interest. Biographies Qiaofeng Yao is working as a Research Fellow with Prof. Jianping Xie in National University of Singapore (NUS). His research interest is total synthesis of metal nanoclusters. Tiankai Chen is a Ph.D. candidate in NUS under supervision of Prof. Jianping Xie. His current research interest is mechanism exploration in the synthesis of metal nanoclusters. Xun Yuan received his Ph.D. from NUS under the supervision of Prof. Jianping Xie, and he is currently a Professor in Qingdao University of Science and Technology (QUST). His research focuses on the synthesis and applications of metal nanoclusters. Jianping Xie is an Associate Professor in NUS and his research interest is engineering metal nanoclusters for biomedical and catalytic applications.

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ACKNOWLEDGMENTS We acknowledge the financial support from Ministry of Education, Singapore (under Research Grant R279-000-481-112). REFERENCES

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