Atom-precise Modification of Silver(I) Thiolate Cluster by Shell Ligand

Atom-precise Modification of Silver(I) Thiolate Cluster by. Shell Ligand Substitution: A New Approach to Generation of. Cluster Functionality and Chir...
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Atom-precise Modification of Silver(I) Thiolate Cluster by Shell Ligand Substitution: A New Approach to Generation of Cluster Functionality and Chirality Si Li, Xiang-Sha Du, Bing Li, Jia-Yin Wang, Guo-Ping Li, Guang-Gang Gao, and Shuang-Quan Zang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12136 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Journal of the American Chemical Society

Atom-precise Modification of Silver(I) Thiolate Cluster by Shell Ligand Substitution: A New Approach to Generation of Cluster Functionality and Chirality Si Li,a Xiang-Sha Du,a Bing Li,a Jia-Yin Wang,a Guo-Ping Li,a Guang-Gang Gao*b and ShuangQuan Zang*a a

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

b

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

Supporting Information Placeholder ABSTRACT: To realize the molecular design of new functional silver(I) clusters, a new synthetic approach has been proposed in this paper, by which the weakly-coordinating – ligands NO3 in a Ag20 thiolate cluster precursor can be substituted by carboxylic ligands while keeping its inner core intact. By rational design, novel atom-precise carboxylic or amino acid protected 20-core Ag(I)-thiolate clusters have been demonstrated for the first time. The fluorescence and electrochemical activity of the post-modified Ag20 clusters can be modulated by alrestatin or ferrocenecarboxylic acid substitution. More strikingly, when chiral amino acids were used as post-modified ligands, CD-activity was observed for the Ag20 clusters, unveiling an efficient way to obtain atomprecise chiral silver(I) clusters.

Noble metal clusters have been shown to be very promising materials in applications such as catalysis, sensors, bio1-3 labeling and medical therapy. To date, a number of atomprecise Au(I)- and Ag(I)- clusters have been synthesized, and the size, shape, and composition of which can be modulated 4,5 by metal-ligand and metal-metal bonding capabilities. However, most of these atom-precise noble metal clusters are usually formed by an uncontrollable self-assembly process, which hampers their post-modification and molecular functionalization. As a special family of noble metal clusters, structure-defined silver(I) clusters can be synthesized by selfassembly through dissolving silver(I) alkynyl or thiolate in a 6 concentrated silver(I) salt solution. Current study on these silver(I) clusters mainly focus on their diverse supramolecular structures and superficial physicochemical functions, while the rational design and functional targeting synthesis of atom-precise silver(I) clusters remain elusive. Moreover, chiral noble metal clusters have attracted considerable attention in recent years, and most of which with atom-precise structures focused on shell ligand protection or 7 intrinsic gold complexes. However, chirality of silver(I) clus8 ter was seldom observed. Till very recently, only two ingeniously designed examples of atom-precise chiral silver clusters have been reported by Zheng’s group, in which cluster chirality is induced by a chiral ammonium cation or diphos-

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phine ligand. Thus, it remains a challenge to develop a universal method for the rational design of chiral silver(I) clusters. The alkynyl or thiolate ligand possesses strong coordination ability to the silver(I) ion by diverse Ag−C or Ag−S bonds, thus making it difficult for a silver(I) cluster to realize additional coordination with chiral ligands. On the other hand, unlike the Au(I) ion in gold clusters, the light-sensitive Ag(I) ion possesses higher oxidation potential and can be easily reduced by reductive ligands in solution chemistry. The formation of Ag(0) usually hinders the construction of crystalline Ag(I) clusters. Thereafter, if the chiral ligand contains a reactive coordination group, the formation of its pro10,11 tected alkynyl or thiolate silver(I) cluster is unfavorable. In addition, in the synthesis of such clusters, trifluoroacetic acid (TFA) or a solvent molecule (e.g. methanol and acetonitrile) usually acts as a small coordinated ligand to stabilize the naked Ag(I) ion. The existence of such a smaller organic ligand also impedes the coordination of post-modified chiral 12 ligands. As a result, post-modification of a structure-defined silver(I) cluster by value-adding functional ligands to realize designable functions constitutes a core issue in current re13 search of silver(I) clusters.

Figure 1. Structural evolution (functional ligand substitution) of the Ag20 cluster core.

Several years ago, Su and Sun groups reported silver(I) thiolate clusters in which the silver(I) ions self-assemble around template anions to form very similar thermodynamically t stable 20-core clusters with tert-butyl thiolate (S Bu) shell 12c,12d ligands, Inspired by this finding, we presume that an Ag20 core bearing weakly-coordinating ligands may be a versatile matrix to be modified by strong coordination shell

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ligands, and hence put forward a new synthetic approach for the rational design of new silver(I) thiolate clusters by postmodification. In this work, a 20-core thiolate Ag(I) cluster that is similar to the reported complex t (CO3)@Ag20(S Bu)10(NO3)8(DMF)6 (DMF = N,N-dimethyl12d formamide) by Sun’s group has been synthesized as a pre− cursor containing the weakly-coordinating ligands NO3 and N,N-dimethylacetamide (DMAc), here expressed as t (CO3)@Ag20-(S Bu)10(NO3)8(DMAc)4 (1). It is envisaged that the twelve weakly coordinated shell ligands of 1 facilitate their substitution by other ligands (Figure 1).

Figure 2. Ball-and-stick representation of complexes 2 (a), 3 (b) and 4 (c). H atoms; the tert-butyl groups and co-crystallized solvent molecules are omitted for clarity. Color code: Ag, green; Fe, orange; S, yellow; C, gray; O, red; N, blue. −

Despite the frequent use of CF3COO for synthesizing silver(I) clusters, there are very rare examples using carboxylic ligands for such purpose. Herein, by the reaction of three selective carboxylic ligands (benzoic acid, C6H5COOH; 2-(1,3dioxobenzo[de]isoquinolin-2-yl)acetic acid, common name alrestatin, C12H6O2NCH2COOH; and ferrocenecarboxylic acid (Fc), CpFeC5H4COOH) with complex 1 in acetonitrile-DMAc medium, three cluster complexes t [(CO3)@Ag20(S Bu)10(C6H5COO)8(DMAc)2]·(CH3CN)2 (2), t (CO3)@Ag20(S Bu)10(C12H6O2NCH2COO)8(CH3CN)4 (3) and t [(CO3)@Ag20(S Bu)10(CpFeC5H4COO)8(CH3CN)4]·(CH3CN)(H 2O)2 (4) have been obtained for the first time. The phase purity of 1-4 is proved by their X-ray powder diffraction (PXRD) patterns, which are consistent with simulated ones based on single-crystal X-ray diffraction data (Figures S1-4). X-ray crystallographic analysis revealed that complexes 2-4 all belong to triclinic space group P-1 (No. 2) and possess a Ag20 core similar to that of 1 (Figure 2). The twenty core silver(I) ions t − are bridged by ten S Bu ligands, eight RCOO auxiliary ligands (R = C6H5- for 2; C12H6O2NCH2- for 3; and CpFeC5H4for 4), two DMAc molecules for 2, and four acetonitrile mol1 ecules for 3 and 5. H NMR spectra recorded in CDCl3 further revealed the clusters remained unchanged and rigid in solu2− tion (Figures S5-7). The existence of one CO3 anion in each cluster center was further confirmed by a Fourier transform −1 infrared (FT-IR) band around 1455 cm (Figure S8). Ten – t 1 1 1 1 S Bu ligands adopt the μ4-η , η , η , η ligation mode to link four silver(I) ions to build a Ag4S square pyramid, which is made up of a Ag4 quadrangle and four Ag−S−Ag triangle faces (Figure S9). Ten Ag4S square pyramids are arranged in the

corner-sharing mode to construct the Ag20S10 core features a sandwich-like structure, where the top and bottom layers are two planes each composed of an Ag5S5 moiety that resembles a pentagram, and the central layer consists of a planar Ag10 ring. The three layers are parallel to another. The Ag−S (2.344(7)-2.655(15) Å) and Ag−Ag (2.48(3)-3.375(10) Å) bond lengths are very similar to those values in 1, and transmission electron microscopy (TEM) images of 2-4 revealed a diameter dispersion of about 2 nm (Figure S10), which all indicate that these complexes possess the same Ag20 core. The fluorescent properties of 1 to 3 have been investigated to unveil the effect of post-modified ligand on the central Ag20 cores. Neither complex 1 nor 2 is fluorescent at room temperature (Figure 3a, black and red curves). While 2 only shows a red emission at ca. 617 nm at low temperature (Figures S11, 12), which can be assigned to the ligand-to-metal12d charge-transfer (LMCT) from the S 3p to Ag 5s transition . To further realize room-temperature fluorescence in the synthetic design, we selected alrestatin as a postmodification ligand to obtain complex 3. To our delight, complex 3 shows green emission around 513 nm with the high quantum yield (QY) of 6.36% at room temperature (Figure 3a). The nanosecond lifetime (τem = 19.29 ns) of 3 indicates that the excited state responsible for the luminescence is of singlet origin (Figure S13). Nevertheless, the large difference between the photophysical parameters of cluster 3 (λem = 513 nm, τem = 19.29 ns) and the pure alrestatin ligand (λem = 450 nm, τem = 6.46 ns) (Figure S14, 15) indicates their distinct emissive nature, which is plausible because grafting alrestatin on the silver cluster resulted in a metal-to-ligand (MLCT) transition with some mixing of ligand-to-ligand 12a,14 (LLCT) character, and inter-cluster π-π stacking interactions between chromophores (Figure S16) may also contribute to the bright green luminescence of solid-state cluster 3. Furthermore, it should be noted that the fluorescence intensity of 3 can be enhanced by cooling. As shown in Figure S17, when the temperature drops to 93 K, the emission peak at 513 nm is enhanced with a ten-fold increase in intensity. This phenomenon is similar to our recent findings for the temper6d,6e ature-dependent fluorescent thiolate silver(I) clusters.

Figure 3. (a). Fluorescence spectra of 1 (black curve), 2 (red curve) and 3 (blue curve) in the solid state at room temperature, excited at 372 nm. Inset is the fluorescence image of 3 under 365 nm UV light. (b). Cyclic voltammogram of Fc (black curve) and 4 (red curve) in 0.1 M dichloromethane solution of nBu4NPF6. The scan rate is 100 mV·s-1. Inset: schematic of complex 4, showing electrochemical activity shell (Fc) enclosing an inert center (Ag20S10).

In addition, the electrochemical reactivity of complex 4 was investigated by cyclic voltammetry (CV) in a 0.1 M dichloromethane solution of tetra-n-butylammonium hexn afluorophosphate ( Bu4NPF6). The CV curve of 4 (Figure 3b, red curve) shows an oxidation peak at 0.37 V and a reduction -1 peak at 0.23 V (vs. Ag/AgCl; scan rate of 100 mV·s ), which

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Journal of the American Chemical Society can be assigned to one-electron oxidation/reduction of the iron(II) centers of the Fc moieties (black curve), while complexes 1-3 have no electrochemical activity under the same measured conditions (Figure S18). The shape of the CV peaks suggests that the silver(I) core is electrochemically inert and II not affected by the linkage of the Fe centers. This result also indicates that the Ag20 cluster core in complex 4 is stable and 15 precludes any redox reaction at the above setting potentials. Based on the above successful post-modification of 1 by carboxylic ligands, we tried to decorate the Ag20S10 core by different amino acid ligands to realize chirality, which is a difficult issue in silver(I) cluster chemistry. As compared to rigid carboxylic ligands, it is difficult to obtain silver(I) clusters containing amino acids due to their flexible structures and diverse coordination modes. Moreover, unlike the crystallization of racemic complexes, it is much harder to crystallize enantiopure complexes due to their lack of reflection and inversion symmetry in the molecular packing process. To our delight, we successfully achieved a high yield of several amino acid-substituted (alanine, C2H4NH2COOH; valine, C4H8NH2COOH; and proline, C4H7NHCOOH) Ag20S10 cluster structures in this work, namely t [(CO3)@Ag22(S Bu)10(L1)4(NO3)6(CH3CN)2]2·(H2O) (5a, L1 = L− − C2H4NH2COO ; 5b, L1 = D-C2H4NH2COO ), t (CO3)@Ag23(S Bu)10(L2)6(NO3)4(OH)(CH3CN) (6a, L2 = L− − C4H8NH2COO ; 6b, L2 = D-C4H8NH2COO ) and t [(CO3)@Ag24(S Bu)10(L3)8(NO3)4(H2O)]·(CH3CN)4(H2O)3 (7a, − − L3 = L-C4H7NHCOO ; 7b, L3 = D-C4H7NHCOO ), in which additional AgNO3 has been introduced into the reaction system that is different to the synthetic procedures of 2-4. X-ray crystallographic analysis revealed that complexes 5-7 all belong to chiral space group P21 (NO. 4). The Flack parameters of 0.030(12) for 6a, 0.054(14) for 6b, 0.046(12) for 7a and 0.030(11) for 7b indicate a homochiral molecular packing in these crystals. In addition, the Flack parameters of 0.113(12) for 5a and 0.106(11) for 5b are slightly higher than those of other complexes, which is mainly due to the structural disorder of the alanine molecules (Figure S19). Complexes 5a-7b possess the same characteristic Ag20S10 core structure of 1, and parts of the weakly-coordinating ligands in 1 have been replaced by amino acid ligands which were characterized by PXRD, FT-IR and TEM as well (Figures S8, S20-26). As shown in Figure 4, two, three or four pairs of inverted amino acid ligands all taking the μ2-O,O’ mode are attached to the Ag10 ring through Ag–O bonds in the range 2.228(10)–2.543(11) Å. In addition, control complexes 5’, 6’ and 7’ were prepared from the reaction solution of complexes 5, 6, and 7 with no addition of AgNO3. The consistent NMR responses between 5’ and 5, 6’ and 6, as well as 7’ and 7 (Figures S27-29, Figures S30-32, Table S1) suggest that the substitution reaction with amino acids also goes to completion after adding triethylamine, but 5-7 cannot crystallize without additional AgNO3, + showing that Ag plays a unique role in the crystallization of these chiral clusters. It is notable that the amine groups from two adjacent amino acid ligands are coordinated with the same silver ion to form a rigid structure which possibly plays a key role in crystal growth. UV-vis absorption spectra indicated that colorless complexes 5a-7b absorb the UV band with wavelengths under 400 nm (Figure S33). Therefore, the ethanol solutions of 5 and 7 and the acetonitrile solution of 6 were employed for circular dichroism (CD) measurements in the range of 200-

400 nm. As shown in Figure 4, the CD spectra of the enantiomers of 6a and 6b, 7a and 7b all display an intense Cotton effect and an excellent mirror image relationship in the 200400 nm range, whereas the same concentrated solution of 5a and 5b only shows a weak CD response. The CD spectrum of 5a exhibits three signals of 215 (+), 237 (−), and 291 (−) nm, respectively, while 6a shows five intense CD signals at 241 (+), 270 (−), 297 (+), 316 (−) and 348 (+) nm, respectively. The most intense signals at 232 (+), 276 (−), 316 (+) and 350 (+) nm are observed in the CD spectrum of 7a. Taking into account the fact that the CD spectra of amino acid ligands present intense signals at 204 nm, 213 nm and 203 nm, respectively (Figure S34), the Ag20S10 core is an achiral structure, the CD responses of 5a-7b possibly arise from ligand-to-silver(I) core based electronic transitions. The incorporation of a chiral carbon center in the amino acid can indeed strongly induce chiral character in the electronic transitions of Ag20S10. These results indicate that the presented approach has the potential to be a universal method to introduce the chirality of post-modified ligand into the silver(I) cluster, thereby leading to an efficient route to atomprecise chiral silver(I) clusters.

Figure 4. On the left: ball-and-stick representation of chiral silver(I) clusters of 5, 6 and 7, and H atoms; the tert-butyl groups and cocrystallized solvent molecules are omitted for clarity. Color code: Ag, green; Fe, orange; S, yellow; C, gray; O, red; N, blue. On the right: CD spectra of 5 and 7 in ethanol and 6 in acetonitrile.

In summary, a universal post-modification synthetic approach has been firstly proposed for the syntheses of new atom-precise silver(I) clusters. By modification of a versatile Ag20 precursor with elaborately chosen functional ligands, the fluorescence or electrochemical activity has been demonstrated for newly-formed Ag20 clusters. Most strikingly, when enantiomeric amino acids were used as post-modified ligands, CD-activity has been observed for these artificial silver(I) clusters. It should be noted that such complexes not only demonstrate a new synthetic approach to the rational design of functional silver(I) clusters, but may also be potentially useful as photoelectrical materials, chiroptical devices, nanocluster based chiral catalysts, and biological medicines. By the proposed post-modification approach, designed synthesis of other chiral silver(I) or gold(I) clusters is underway in our group.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xxxxxxx. Details of the chemicals, instrumentation, synthesis and characterization (PDF). X-ray crystal details for 1-7b (CIF).

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21671175, 21371153, 21271034), the Program for Science & Technology Innovation Talents in Universities of Henan Province (164100510005) and Zhengzhou University, and the Grant-in-aid for Youth Innovation Fund from the University of Jinan.

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