Anion-templated Self-assembly for the Preparation of Silver-t-butylthiolate Clusters①

JIN Jun-Ling SHEN Yng-Lin MI Li-Wei②XIE Yun-Peng LU Xing

a (Henan Key Laboratory of Functional Salt Materials, Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou 450007, China)

b (College of Materials Engineering, Henan International Joint Laboratory of Rare Earth Composite Materials, Henan University of Engineering, Zhengzhou 451191, China)

c (State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China)

ABSTRACT Silver thiolate polymers are intercepted to form different structural fragments when reacting with variant solubilizing reagents, which usually serve as the starting point for the preparation of clusters. However,such a process is still far from clear. Herein, we report the controlled synthesis of silver-t-butylthiolate clusters from reactions of polymeric [AgtBuS]n and suitable templates in the presence of solubilizing reagents to offer a detailed look at the mechanism of cluster’s formation. As the provided solubilizing reagents have weak coordination ability, such as O- or N-donating ligands, the obtained polymeric compound retains the linear structure pattern that S and Ag atoms are arranged alternately. When extra templates NO3- and CO32- are applied, the disk-like clusters Ag19 and Ag20 are constructed with the same [AgtBuS]5 circles that may directly cyclize from the linear[AgtBuS]n fragments. In contrast, (EtO)2PS2- and (iPrO)2PS2- anions have large size and strong coordination ability rendering the structure of the polymer completely fragmented. Thus extremely short [AgtBuS]n pieces with silver ions and solubilizing ligands assemble around the templates V2O74- and W2O94-, leading to the formation of clusters Ag22 and Ag24.

Keywords: silver cluster, X-ray crystal structure, silver thiolate polymer, self-assembly;

1 INTRODUCTION

Silver(I) thiolate clusters have attracted extensive attention due to structural diversity and potential applications in luminescence, sensors, medicine, and imaging to name a few[1-11]. However, the preparation of silver(I) thiolate clusters still follows a trial-and-error method and the uncontrollable syntheses remain the main barrier to produce specific clusters and further achieve these potentials that are mentioned above[12,13]. Polymeric 1:1 silver thiolate coordination polymers [AgSR]nare often utilized as a precursor in the syntheses of clusters. However, under the action of solubilizing ligands[14], what kind of components such polymers will be cut into is still not clear and there are few related discussions[15,16]. Of note, the template method could be used to capture and fix the polymer fragments in the solution, and the formed clusters provide the atomically precise structure to infer the species of polymer fracture[17,18]. Anion templates such as halogen, sulfide, oxyacid, and polyoxometallate (POM) are applied to assist the assembly of silver thiolate clusters with [AgSR]npolymer fragments and different solubilizing ligands[19,20].

Benefiting from the extraordinary solubleness with the addition of extra silver salts and solubilizing ligands, silvert-butylthiolate polymer [AgtBuS]nhas been widely used as precursors for the synthesis of silver clusters[21]. Many serendipitous silver-t-butylthiolate clusters were obtained,such as (CO3)@Ag20(tBuS)10, (W6O21)@Ag34(tBuS)26,Ag62S13(tBuS)32, and Ag320S130(tBuS)60[22-25]. To deepen the understanding of the self-assembly process of silver thiolate clusters, systematic cluster synthesis and single-crystal structure determination are urgently needed[26].

Herein, polymer [AgtBuS]nwas chosen as the precursor for this systematic study of cluster assemblies. By reacting with N-donating ligands, a similar polymeric compound{[Ag6(tBuS)4]·2BF4}n(1) was obtained. Moreover, the provided template NO3-led to the formation of cluster[(NO3)@Ag19(tBuS)10(CF3CO2)8(4-cp)]·2H2O (4-cp = 4-cyano pyridine; 2). CO32-template was insitugenerated from the fixation of atmospheric carbon dioxide and gave a larger cluster (CO3)@Ag20(tBuS)10(CF3CO2)8(CH3CN)4(3) sharing the same structure pattern [AgtBuS]5circles. In contrast, by introducing bulky and strong ligands (EtO)2PS2-and(iPrO)2PS2-[27-29], clusters (V2O7)@Ag22(tBuS)8[(EtO)2PS2]9·OH·H2O (4) and (W2O9)@Ag24(tBuS)14[(iPrO)2PS2]6·CH3OH(5) with extremely short [AgtBuS]npieces were produced. In this work, we report the synthesis and crystal structures of five well-defined compounds 1～5, while attempt to clarify the possible [AgtBuS]nfragmented species and the actual starting point of cluster assemblies.

2 EXPERIMENTAL

2. 1 Materials and methods

All reagents were purchased from commercial companies and used as received without further purification. [AgtBuS]nprecursor was prepared according to the reported literature[30]. Elemental analyses for C and H were performed with a PerkinElmer 2400 elemental analyzer. Crystal data of 1～5 were collected on a Bruker D8 Quest diffractometer with Mo-Kαradiation. The multi-scan method was used for absorption corrections. The structures were solved by direct methods and refined with SHELXL-2014[31].

2. 2 Synthesis of {[Ag6(tBuS)4]·2BF4}n (1)

[AgtBuS]n(0.039 g, 0.2 mmol), AgBF4(0.016 g, 0.08 mmol) and 1,4-diazabicyclo[2.2.2]octane (0.010 g, 0.09 mmol) were dissolved in 6 mL CH3OH under ultrasonication.A colorless solution was collected by filtration and slow evaporation of this solution afforded the product as colorless crystals. Yield: ca. 12%. Elemental analysis (%) calcd. for Ag6S4C16H36F8B2: C, 16.32; H, 3.06. Found: C 16.14, H 3.17.

2. 3 Synthesis of [(NO3)@Ag19(tBuS)10(CF3CO2)8(4-cp)]·2H2O (2)

[AgtBuS]n(0.039 g, 0.2 mmol), AgNO3(0.017 g, 0.1 mmol), AgCF3CO2(0.022 g, 0.1 mmol) and 4-cyanopyridine(0.010 g, 0.096 mmol) were dissolved in 6 mL CH3OH/toluene (v:v = 1:1) under ultrasonication. A colorless solution was collected by filtration and slow evaporation afforded the product as light yellow crystals. Yield: ca. 21%. Elemental analysis (%) calcd. for Ag19S10C62H98O21N3F24: C,18.40; H, 2.42. Found: C, 18.31; H, 2.54.

2. 4 Synthesis of (CO3)@Ag20(tBuS)10(CF3CO2)8-(CH3CN)4 (3)

[AgtBuS]n(0.039 g, 0.2 mmol) and AgCF3CO2(0.022 g,0.1 mmol) were dissolved in 6 mL CH3CN under ultrasonication. A colorless solution was collected by filtration and slow evaporation of this solution afforded the product as colorless crystals. Yield: ca. 28%. Elemental analysis (%)calcd. for Ag20S10C65H102O19N4F24: C, 18.69; H, 2.44. Found:C, 18.58; H, 2.55.

2. 5 Synthesis of (V2O7)@Ag22(tBuS)8[(EtO)2PS2]9·OH·H2O (4)

[AgtBuS]n(0.039 g, 0.2 mmol) and AgBF4(0.008 g, 0.04 mmol) were dissolved in 6 mL CH3OH under ultrasonication.Then (C2H5O)2PS2NH4(0.004 g, 0.02 mmol) and Et4NVO3(0.003 g, 0.008 mmol) were added under stirring to obtain a clear yellow solution. A yellow solution was collected by filtration and slow evaporation of this solution afforded the product as yellow crystals. Yield: ca. 15%. Elemental analysis (%) calcd. for Ag22S26C68H147O27P9V2: C, 16.39; H, 2.95.Found: C, 16.31; H, 3.02.

2. 6 Synthesis of (W2O9)@Ag24(tBuS)14[(iPrO)2-PS2]6·CH3OH (5)

The synthetic processes of 5 were similar to that of 4,except that Et4NVO3and (C2H5O)2PS2NH4were replaced by Et4NWO4(0.003 g, 0.006 mmol) and (iPrO)2PS2NH4(0.004 g, 0.02 mmol). Yield: ca. 11%. Elemental analysis (%) calcd.for Ag24S26C92H210O21P6W2: C, 19.63; H, 3.73. Found: C,19.56; H, 3.81.

2. 7 Crystal structure determination

Colorless crystals 1 and 3 with yellow crystals 2, 4 and 5 were selected for diffraction data collection on a Bruker Smart Apex3 CCD diffractometer equipped with a graphitemonochromatic MoKαradiation (λ= 0.71073 ?). Complex 1 crystallizes in orthorhombic, space groupIma2 witha=8.0211(1),b= 17.506(3),c= 25.923(4) ?,V= 3640.1(10)?3,Z= 4, C16H36B2F8S4Ag6,Mr= 1177.53,F(000) = 2240,Dc= 2.149 Mg/m3andμ= 3.442 mm-1. A total of 8631 reflections were collected for 1, of which 2895 (Rint= 0.0360)were independent in the range of 2.808≤θ≤25.022o for 1 by using aφ-ωscan mode. Complex 2 is of monoclinic system, space groupP21/nwitha= 28.993(3),b= 13.3666(1),c= 30.602(3) ?,β= 103.405(2)o,V= 11536.4(19) ?3,Z= 4,C62H98F24N3O21S10Ag19,Mr= 4047.56,F(000) = 7712,Dc=2.330 Mg/m3andμ= 3.416 mm-1. A total of 64135 reflections were collected for 2, of which 16493 (Rint= 0.0272)were independent in the range of 2.783≤θ≤23.257o for 2 by using aφ-ωscan mode. Complex 3 crystallizes in space groupP1 witha= 13.5484(2),b= 15.7398(2),c= 16.2419(2)?,α= 118.629(2),β= 103.322(2),γ= 99.445(3)o,V=2801.4(5) ?3,Z= 1, C65H102F24N4O19S10Ag20,Mr= 4177.50,F(000) = 1988,Dc= 2.476 Mg/m3andμ= 3.686 mm-1. A total of 17605 reflections were collected for 3, of which 9633 (Rint= 0.0478) were independent in the range of 2.564≤θ≤25.027o for 3 by using aφ-ωscan mode. Complex 4 crystallizes in space groupP1 witha= 18.149(3),b=18.817(3),c= 26.043(5) ?,α= 68.967(4),β= 79.628(6),γ=76.198(4)o,V= 8018(2) ?3,Z= 2, C73H187O33P9S26V2Ag22,Mr= 5180.52,F(000) = 5040,Dc= 2.146 Mg/m3andμ=3.210 mm-1. A total of 81754 reflections were collected for 4,of which 22912 (Rint= 0.0637) were independent in the range of 2.880≤θ≤23.256o for 4 by using aφ-ωscan mode.Complex 5 crystallizes in hexagonal, space groupP63/mwitha= 18.3286(5),b= 18.3286(5),c= 30.1460(2) ?,γ=120o,V= 8770.4(7) ?3,Z= 2, C92H210O21P6S26W2Ag24,Mr=8770.4(7),F(000) = 5424,Dc= 2.131 Mg/m3andμ= 4.324 mm-1. A total of 44444 reflections were collected for 5, of which 4213 (Rint= 0.0458) were independent in the range of 2.992≤θ≤23.093o for 5 by using aφ-ωscan mode.

The structures of complexes 1～5 were solved by direct methods with SHELXTL XT-2014 program and refined by full-matrix least-squares techniques onF2with SHELXL-2014. The finalR= 0.0419,wR= 0.1033 (w= 1/[σ2(Fo2) +(0.1138P)2+ 4.9551P], whereP= (Fo2+ 2Fc2)/3),Rint=0.0360, (Δ/σ)max= 0.000,S= 1.080, (Δρ)max= 0.0648 and(Δρ)min= -0.941 e/?3for 1. The finalR= 0.0735,wR=0.2067 (w= 1/[σ2(Fo2) + (0.1138P)2+ 4.9551P], whereP=(Fo2+ 2Fc2)/3),Rint= 0.0272, (Δ/σ)max= 0.000,S= 1.056,(Δρ)max= 2.584 and (Δρ)min= -1.425 e/?3for 2. The finalR= 0.0583,wR= 0.1550 (w= 1/[σ2(Fo2) + (0.1138P)2+4.9551P], whereP= (Fo2+ 2Fc2)/3),Rint= 0.0478, (Δ/σ)max= 0.000,S= 1.087, (Δρ)max= 1.855 and (Δρ)min= -2.255 e/?3for 3. The finalR= 0.0704,wR= 0.1701 (w= 1/[σ2(Fo2)+ (0.1138P)2+ 4.9551P], whereP= (Fo2+ 2Fc2)/3),Rint=0.0637, (Δ/σ)max= 0.000,S= 0.993, (Δρ)max= 1.192 and(Δρ)min= -0.815 e/?3for 4. The finalR= 0.0845,wR=0.2050 (w= 1/[σ2(Fo2) + (0.1138P)2+ 4.9551P], whereP=(Fo2+ 2Fc2)/3),Rint= 0.0458, (Δ/σ)max= 0.000,S= 0.958,(Δρ)max= 1.938 and (Δρ)min= -1.702 e/?3for 5.

3 RESULTS AND DISCUSSION

3. 1 Synthesis discussion

The synthetic details are provided in the experimental section. The [AgtBuS]nprecursor is generally insoluble in solvents, and its structure is predicted to be a supramolecular polymer where the monomers are end-to-end connectedviaAg-S bonding and further stabilized by the so-called metallophilic attractions[15,32]. After reacting with different silver salts in the presence of solubilizing ligands, the [AgtBuS]nchain structure is intercepted, and the following rearrangement and self-assembly of [AgtBuS]nfragments are significantly affected by the templates during the synthesis[33].Initially, [AgtBuS]nprecursor reacted with 1,4-diazabicyclo-[2.2.2]octane ligands, forming {[Ag6(tBuS)4]}nthat retained the end-to-end pattern of [AgtBuS]nprecursor. Although the 1,4-diazabicyclo[2.2.2]octane ligand does not appear in the final structure, in the absence of this ligand, the compound{[Ag6(tBuS)4]}ncannot be obtained. Moreover, through intervention of NO3-and CO32-templates, clusters NO3@Ag19and CO3@Ag20were isolated. Both of them exclusively contain [AgtBuS]5structural units. In contrast, bulky and strong ligands (EtO)2PS2-and (iPrO)2PS2-support the generation of V2O7@Ag22and W2O9@Ag24with extremely short[AgtBuS]npieces, which in turn proves that solubilizing ligands affect the fragmentation of [AgtBuS]nchain and change the actual starting point of the assembly.

3. 2 X-ray crystal structure

X-ray crystallography results reveal that compound 1 crystallizes in the orthorhombicIma2 space group, and it was identified as a polymeric species {[Ag6(tBuS)4]·2BF4}n.As displayed in Fig. 1, compound 1 with an exclusiveμ3-η1η1η1coordination oftBuS-bridging three silver ions presents a tubular structure with about 1 nm diameter. The Ag-S bond lengths in 1 range from 2.366 to 2.415 ?. To the best of our knowledge, the structure of the precursor is predicted as chain-like [AgtBuS]nwithμ2-η1η1coordination of thetBuS-(Fig. 1d)[15]. Thus, the formation of 1 may involve the fusion of four [AgtBuS]nchains by generating S-Ag-S bridges while the extra required silver ions are provided by AgBF4salts[34].

Fig. 1. (a, b) X-ray structure of 1 viewed along the a and b axes, (c) Coordination mode of tBuS- ligands, (d) Predicted structure of[AgtBuS]n with the hydrogen atoms omitted for clarity. Color codes: Ag, pink and green; S, yellow; C, grey

By introducing NO3-template, compound 2 was obtained from the mother liquor. This species crystallizes in monoclinic space groupP21/n(Fig. 2b). Compound 2 was determined as[(NO3)@Ag19(tBuS)10(CF3CO2)8(4-cp)]·2H2O. The NO3-template is encapsulated by Ag19(tBuS)10unit, which generates from the assembly of a Ag9circle and two [AgtBuS]5synthons in a sandwich way. The anionic NO3-template builds electrostatic interactions with positively charged silver ions to form the silver-rich entity Ag9, which is further consolidated by the[AgtBuS]5synthons that are assigned to [AgtBuS]nfragment.The Ag-S and Ag···Ag bond lengths are in the range of 2.365～2.643 and 2.919～3.358 ?, respectively.

As the NO3-was replaced by a CO32-molecule, the crystals of 3 were prepared. Single-crystal XRD result reveals a neutral disk-like (CO3)@Ag20(tBuS)10(CF3CO2)8(CH3CN)4structure of 3, and the same skeleton structure has been reported by the Sun and Zhou et al[22,35]. 3 has a core-shell structure containing a CO32-core and a Ag20(tBuS)10-(CF3CO2)8(CH3CN)4shell (Fig. 3). The Ag20silver cage also consists of a Ag10circle and two [AgtBuS]5synthons in a sandwich way, which is almost the same as the structure of NO3@Ag19except that the Ag20silver cage displays a bigger Ag10circle. This phenomenon could be ascribed to the CO32-template bearing more charges than that of NO3-, which is consistent with the conclusion suggested by Liu et al about the preparation of the templated silver alkynyl clusters[36].

Fig. 3. X-ray structure of 3. The hydrogen atoms are omitted for clarity.Color codes: Ag, pink and green; S, yellow; C, grey; O, red; N, blue; F, light green

When Et4NVO3was used as precursors, the template V2O74-was insitugenerated, leading to the formation of 4.It crystallizes in triclinic space groupP1 and is crystallographically identified as a +1 cationic cluster, consisting of 22 silver atoms, exteriorly stabilized by 8tBuS-and 9(EtO)2PS2-ligands, and interiorly supported by the V2O74-template (Fig. 4b). The formula of 4 is described as(V2O7)@Ag22(tBuS)8[(EtO)2PS2]9·OH·H2O. Three of eighttBuS-ligands coordinate silver ions with theμ3-?1?1?1binding mode, and the remaining 5tBuS-ligands adopt theμ4-?1?1?1?1ligation mode. Three of nine (EtO)2PS2-anions adopt theμ3-?1?2mode and the remaining isμ4-?2?2. Various[AgtBuS]nfragments occur on the silver cage, including a linear [AgtBuS]4and [AgtBuS]3, as well as a [AgtBuS] unit.This in turn proves that the addition of (EtO)2PS2-anions with huge size and strong coordination ability makes the structure of the polymer completely fragmented, which is further reorganized into a discrete core-shell cluster assisted by silver ions and V2O74-anions.

Fig. 4. (a, b) Structures of [AgtBuS]n fragments and V2O74- template, (c) X-ray structures of 4.The hydrogen atoms are omitted for clarity. Color codes: Ag, pink and green; S, yellow; C, grey; O, red; P, orange; V, indigo

Similarly, as (EtO)2PS2-and V2O74-were replaced by(iPrO)2PS2-and W2O94-, discrete cluster 5 was produced. As portrayed in Fig. 5, the structure of 5 was revealed by X-ray crystallography with all 24 Ag ions, 14tBuS-ligands, and 6(iPrO)2PS2-anions as well as a W2O94-in the interior. It crystallizes in the hexagonalP63/mspace group. 5 exhibits a barrel-like structure, arising from the silver ions arranged as a Ag3-Ag6-Ag6-Ag6-Ag3five-layer configuration. The external 14tBuS-ligands show diverse coordination modes,6 inμ2-?1?1; 2 inμ3-?1?1?1; 6 inμ4-?1?1?1?1. Interestingly,there is a [AgtBuS]6circle in the middle layer of 5, but the remaining 8tBuS-ligands present an isolated distribution without forming likewise RS-Ag-SR bonding pattern,which is consistent with the finding in the structure of 4.Besides, ligands (iPrO)2PS2-display an exclusiveμ3-?1?2coordination mode with Ag-S bond lengths ranging from 2.434 to 2.492 ?. Besides, the insitugenerated W2O94-resides in the silver cage and interacts with Ag ions by aμ15-?1?1?1?2?2?2?2?2?2mode with Ag-O distances of 2.339～2.514 ?.

Fig. 5. (a, b) Structures of [AgtBuS]n fragments and W2O94- template, (c) X-ray structure of 5.The hydrogen atoms are omitted for clarity. Color codes: Ag, purple and green; S, yellow; C, grey; O, red; P, orange; W, rose

4 CONCLUSION

In summary, we have assembled and structurally determined five silver-t-butylthiolate compounds. Based on these atomically precise structures, we explore the actual starting species of the cluster self-assembly process in which the[AgtBuS]npolymer participates with different solubilizing ligands. As the provided solubilizing reagents have weak coordination ability, the obtained polymeric compound retains the end-to-end pattern of [AgtBuS]nprecursor. Meanwhile, when NO3-and CO32-are applied, [AgtBuS]5circles that may be transferred from soluble [AgtBuS]nfragments are found in NO3@Ag19and CO3@Ag20. In contrast,(EtO)2PS2-and (iPrO)2PS2-anions have large size and strong coordination ability rendering the structure of the polymer completely fragmented. Therefore, extremely short [AtBuS]npieces with silver ions assemble around the templates V2O74-and W2O94-, leading to the formation of clusters V2O7@Ag22and W2O9@Ag24. This work focuses on the actual starting species of the solution self-assembly of classical cluster synthesis, but more research work is needed to clarify this problem.