Halogen as Template to Modulate the Structures of the Nanocage-Based Silver(I)-Thiolate Coordination Polymers

Abstract

By the reaction of AgNO₃, 2-methyl-2-propanethiol (HStBu), with various-sized halogen ions as templates, three multi-nuclear silver-thiolate cluster-based chain-like coordination polymers, [Ag₆(μ-SBu)₆]_n (USC-CP-2), [Ag₆(μ-StBu)₅Br]_n (USC-CP-4) and [Ag₁₄(μ-StBu)₁₂I₂]_n (USC-CP-3) constructed by different Ag(I)-nanocages, have been synthesized and characterized by X-ray diffraction analyses. With F⁻, Cl⁻ or without template, USC-CP-2 exhibits a one-dimensional structure composed of detached Ag₆-cages, absent of fluoride or chloridion. While with Br⁻ and I⁻, USC-CP-4 and USC-CP-3, two distinct halogen-templating multi-sliver cages-based chain-like polymeric structures have been observed, which are a mono-Br⁻ encapsulated Ag₈-cage, or a dual-I⁻ embedded Ag₁₆-cage, respectively. In these three compounds, the multi-Ag(I) cages were self-assembled by Ag-S bonds through bridged μ₂-StBu ligands, and stabilized argentophilic interactions between neighboring silver atoms. This study demonstrates that the halide anions of varying sizes play a critical role in inducing the nucleation and structural evolution of the silver-thiolate clusters.

1. Introduction

Silver(I)-thiolate clusters coordination complexes, as an important class of organometallic compounds, have garnered significant research interest due to their diverse structural architectures [1]. Crucially, in contrast to conventional nanoparticles, atomically precise Ag(I)-thiolate clusters can be used as well-defined building blocks to construct coordination compounds with tailored functionalities for promising applications, such as multifunctional biomaterials, semiconductor devices, and luminescent materials [2,3,4,5,6,7,8]. The hierarchical structural patterns are constructed through the self-assembly of Ag(I) nodes and thiolate-based organic linkers, ranging from the discrete cluster to high-dimensional structures, including 1-D chains, 2-D sheets, and 3-D frameworks [3,4,5,6,9]. The structural diversity of Ag(I)-thiolate coordination complexes was collectively dictated by many factors, beyond conventional metal-ligand coordination, also including the synergistic π-π stacking, argentophilic Ag⋯Ag interactions, and dynamic anion bridging. The influencing factors are detailed as follows: (i) Ag(I)-to-ligand ratios, (ii) pH values to protonate/deprotonate thiolates, (iii) steric bulk of ancillary side substituent groups, (iv) solvent polarity and Lewis basicity, and (v) nature of counter-anions (hard vs. soft, mono- vs. poly-dentate) [4,5,6]. Thus, their self-assembly often becomes complex and is associated with uncontrolled processes, making for rational design and targeted functionalization difficult and challenging.

To address this limitation, the introduction of templating anion has emerged as a powerful strategy for directing the formation of high-dimensional Ag(I)-thiolate coordination structures [10,11,12,13,14]. The use of anion templates offers several advantages: (i) precise control of cluster nuclearity and geometry, (ii) enhancing structural stability through its charge balance to cationic metal centers, and (iii) incorporating unique physical properties of functional anions. To date, a wide range of anionic templates have been employed, ranging from simple spherical (S²⁻ and halides X⁻ = F⁻, Cl⁻, Br⁻ and I⁻), planar trigonal (NO₃⁻ and CO₃²⁻), to tetrahedral species moieties (PS₄³⁻ and VO₄³⁻) [15,16,17,18,19].

In our previous research, we reported on the incorporation of iodide to induce the formation of the Ag₁₆-cage, which resulted in the corresponding chain-type Ag(I)-thiolate coordination polymer [16]. Herein, this work systematically investigated the inducing template effects of halide anions on the structural control of self-assembly of chain-like Ag(I)-thiolate coordination polymers. Through employing variation in the halide template (F⁻, Cl⁻, Br⁻ and I⁻), three distinct structures were obtained: [Ag₆(μ-StBu)₆]_n (USC-CP-2), [Ag₆(μ-StBu)₅Br]_n (USC-CP-4) and [Ag₁₄(μ-StBu)₁₂I₂]_n (USC-CP-3, where USC-CP stands for University of South China coordination polymer). Without or with smaller F⁻/Cl⁻ ions as template, the formation of USC-CP-2 has been obtained, which is a one-dimensional bead-like polymer composed of the detached Ag_6-cages. While larger Br⁻/I⁻ ions were introduced, USC-CP-4 and USC-CP-3 exhibit one-dimensional structures based on edge-shared Ag_8- or Ag₁₆-cages that encapsulate mono-Br⁻ or dual-I⁻ anions, respectively. The structural comparison indicated that halides have a significant impact on the structures of multi-silver cages. More specifically, using halide templates of different sizes resulted in a progressive increase in the nucleation of silver clusters formed, from six to sixteen, thereby altering the corresponding chain structures. This study provides fundamental insights into the anion-directed assembly of Ag(I)-thiolate coordination polymers and the potential to guide the establishment of structure-property relationships for the future design of the cognate functional materials.

2. Experimental Section

2.1. Materials and Methods

All reagents and solvents used were received from commercial suppliers without further purification.

The elemental analyses (C, H, and N) were performed with a Vario Micro CHNOS Elemental Analyzer (manufactured by Elementar Analysensysteme GmbH, Langenselbold, Hessen, Germany). The powder X-ray diffraction (PXRD) data were collected on a DMAX-2500 diffractometer with Cu Kα (λ = 1.5418 Å) (produced by Rigaku Corporation, Tokyo, Japan). The thermogravimetric analysis was performed on NETZSCH TG 209F3 (manufactured by NETZSCH-Gerätebau GmbH, Selb, Bavaria, Germany) in an N₂ atmosphere.

2.2. Synthetic Procedures

2.2.1. Preparation of [Ag₆(μ-StBu)₆]_n (USC-CP-2)

F⁻ ion as template. Sodium ethylate (0.028 g, 0.4 mmol) and 2-methyl-2-propanethiol (0.043 mL, 0.4 mmol) were dissolved in 6 mL of ethanol. After stirring for half an hour, AgNO₃ (0.034 g, 0.2 mmol) dissolved in 1 mL water and 2 mL ethanol was added in slowly to the solution. The solution immediately turned milky white and more flocs were formed. Then add 2 mL of acetone and 1 mL ethanol solution of NaF (0.008 g, 0.2 mmol). The turbid liquid was sealed in a 20 mL Teflon-lined autoclave and heated at 130 °C for 33 h. After the autoclave was cooled to room temperature, strip-shaped colorless crystals of USC-CP-2 were separated, with a yield of 51% (based on Ag). Anal. Calcd for Ag₆S₆C₂₄H₅₄: C 24.38, H 4.60; found: C 24.32, H 4.72%.

Cl⁻ ion as template. The same synthesis procedure was used, replacing NaF with NaCl (0.012 g, 0.2 mmol). Yield: 50%

No template. A similar synthesis procedure was used, without adding a template agent. Yield: 51%.

2.2.2. Preparation of [Ag₆(μ-StBu)₅Br]_n (USC-CP-4)

Sodium ethylate (0.028 g, 0.4 mmol) and 2-methyl-2-propanethiol (0.043 mL, 0.4 mmol) were dissolved in 6 mL of ethanol. After stirring for half an hour, AgNO₃ (0.034 g, 0.2 mmol) dissolved in 1 mL water and 2 mL ethanol was added in slowly. The solution immediately turned milky white and more flocs were formed. Then add 2 mL of acetone and 1 mL ethanol solution of tetraphenylphosphine bromide (0.025 g, 0.06 mmol). The turbid liquid was sealed in a 20 mL Teflon-lined autoclave and heated at 130 °C for 33 h. After the autoclave was cooled to room temperature, massive colorless crystals of USC-CP-4 were separated with a yield of 10% (based on Ag). Anal. Calcd for Ag₆S₅C₂₀H₄₅Br: C 20.48, H 3.87; found: C 20.65, H 3.65%.

2.2.3. Preparation of [Ag₁₄(μ-StBu)₁₂I₂]_n (USC-CP-3)

Sodium ethylate (0.028 g, 0.4 mmol) and 2-methyl-2-propanethiol (0.043 mL, 0.4 mmol) were dissolved in 6 mL of ethanol. After stirring for half an hour, AgNO₃ (0.034 g, 0.2 mmol) dissolved in 1 mL water and 2 mL ethanol was added in slowly. The solution immediately turned milky white and more flocs were formed. Then add 2 mL acetone and 1 mL ethanol solution of potassium iodide (0.033 g, 0.2 mmol). The turbid liquid was sealed in a 20 mL Teflon-lined autoclave and heated at 130 °C for 33 h. After the autoclave was cooled to room temperature, massive light yellow crystals of 3 were separated yield: 19% (based on Ag). Anal. Calcd for Ag₁₄S₁₂C₄₈H₁₀₈I₂: C 20.34, H 3.84; found: C 20.22, H 3.93%.

2.3. X-Ray Crystallographic Analysis

All data were collected on a Rigaku SCXmini CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation source (produced by Rigaku Corporation, Tokyo, Japan) (λ = 0.71073 Å) by ω scan mode. The structures were solved by the direct method using the SHELXTL Version 5 package of crystallographic software, and refined with a full-matrix least-squares refinement on F². Metal atoms were located from the E-maps and refined anisotropically. The other non-hydrogen atoms were located by the difference Fourier maps based on these atomic positions and refined anisotropically. Hydrogen atoms were added according to the theoretical models. The crystallographic data have been deposited into the Cambridge Crystallographic Data Centre, CCDC no. 2435678, for USC-CP-4. Copies of this information may be obtained free of charge from the Director, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk. Crystal data collection and refinement parameters are summarized in Table 1, and their selected bond lengths and angles are provided in Tables S1–S3, respectively.

Table 1. Crystallographic data for USC-CP-4.

Parameter	USC-CP-4
Formula	C20H45Ag6S5Br
Mr	1172.99
Cryst system	Tetragonal
Space group	$I\stackrel{\_}{4}2d$
a/Å	12.208(3)
b/Å	12.208(3)
c/Å	44.948(2)
α/°	90
β/°	90
γ/°	90
V/Å3	6699(3)
Z	8
Dc/g cm−3	2.326
μ/mm−1	4.954
F(000)	4496
R(int)	0.0572
Total reflections	28,639
Unique reflections	3917
I > 2σ(I)	3781
R1	0.0721
wR2	0.2466
S	1.086

3. Results and Discussion

3.1. Synthesis

Polynuclear silver(I)-thiolate clusters are of special interest not only due to the central role they play in multifunctional biomaterials but also due to interest in the establishment of luminescent structural correlations. A variety of multi-silver(I) complexes with different clusters have been reported. When KI was added into the reaction system of AgNO₃ and HStBu, the dual-I⁻ embedded Ag₁₆-cage based structure was isolated in our previous work [16], in which the strong interaction of Ag⋯I (the shortest distance of 2.99 Å) was observed. This effect should also be exhibited by other halide and silver ions. Thus, we attempted to systematically investigate the impact of other halogens (F⁻, Cl⁻ and Br⁻) on the structure of silver(I)-thiolate clusters. However, when the solvent polarity, temperature and reactant ratio of the reactants in reaction systems with F⁻ or Cl⁻ as templates were extensively regulated, Ag₆-cage based USC-CP-2 was isolated as the sole pure-phase product. Interesting, it could also be obtained without the addition of templates. In USC-CP-2, neither the F⁻ nor Cl⁻ ions are encapsulated within the multi-silver cages. In contrast, analogous reactions with Br⁻ or I⁻ anions yielded USC-CP-4 and USC-CP-3, which are based on Ag₈- and Ag₁₆-cages that encapsulate mono-Br⁻ and dual-I⁻ anions, respectively. This stark difference can be attributed to the high solvation energy, small ionic radii of F⁻/Cl⁻, as well as the weaker interaction with Ag(I) than that of the Br⁻/I⁻; thus, F⁻/Cl⁻ ultimately did not serve as a template for regulating the putative multi-silver(I) cages [20].

It is important to note that this may be due to the strong binding affinity between silver and thiol groups, coupled with the high stability of compound USC-CP-2. It has been observed that the synthesis of USC-CP-3 and USC-CP-4 is accompanied by the partial formation of USC-CP-2, which consequently reduces the yield of the target products. In addition, despite endeavors to optimize the procedure through adjustments to the sequence and amount of bromide and iodide addition, no substantial enhancement in yield was observed.

3.2. Crystal Structure Description

Even though the structures of USC-CP-2 and USC-CP-3 have been reported in our previous work [16], it is merited to indicate the detailed dissimilarities induced by the effects of templates.

3.2.1. Molecular Structure of [Ag₆(μ-StBu)₆]_n (USC-CP-2)

Single-crystal X-ray diffraction analysis indicates that USC-CP-2 crystallizes in the triclinic P$\stackrel{\_}{1}$ space group. The asymmetric unit consists of six independent Ag⁺ atoms and six deprotonated μ-StBu⁻ (Figure 1a). All silver atoms are linearly coordinated by two sulfur atoms from a μ₂-bridging StBu⁻ ligand. The Ag-S bond distances are in the range of 2.369(2)-2.400(1) Å, with a mean value of 2.385 Å. The distances of Ag⋯Ag range from 3.059(6) to 3.336(7) Å, which are longer than the Ag-Ag distance in metallic silver (2.889 Å) but shorter than the sum of the van der Waals radii (3.44 Å), indicating the presence of significant argentophilic interactions [21]. Thus, it can be considered to form a thiol-bridging Ag₆-cage. These discretely adjacent Ag₆-cages are linked by two bridging μ₂-StBu⁻ into a one-dimensional bead-like coordination chain, and further assembled into a three-dimensional supramolecular structure via weak intermolecular interactions of methyl groups from μ-StBu⁻ ligands [22,23,24,25,26].

Figure 1. (a) ORTEP view of the asymmetric unit in USC-CP-2 with 50% probability ellipsoids; (b) polyhedral hexanuclear silver cage supported by μ₂-StBu; (c) the 1-D beaded-like chain; (d) off-set packing of the chains in USC-CP-2 (Ag turquoise, S golden, C gray, H light gray).

3.2.2. Molecular Structure of [Ag₆(μ-StBu)₅Br]_n (USC-CP-4)

USC-CP-4 crystallizes in the tetragonal system with the $I\stackrel{\_}{4}2d$ space group. The asymmetric unit comprises three independent silver atoms, two and a half deprotonated StBu⁻ ligands and half a bromide anion (Figure 2a). And as depicted in Figure 2b, it can be expanded as a central bromide anion encapsulating eight Ag atoms formed Ag₈-cage as secondary building units. Of the eight Ag atoms in the cage display different coordination geometries: four in V-shaped AgS₂, two in distorted trigonal pyramidal AgS₃ and two in a nearly triangular AgS₂Br geometry, respectively. The StBu⁻ adopted two coordination modes: the μ₃-bridging three Ag(I) atoms and μ₂-bridging two. The Ag-S bond lengths range from 2.371(4) to 2.589(8) Å, with an average value of 2.436 Å. The Br⁻ anion acts as a V-shaped configuration to bridge two Ag⁺ ions with an Ag-Br distance of 2.871(2) Å, which is substantially shorter than the sum of the relevant van der Waals radii (3.57 Å) [27]. Similarly to USC-CP-2, there are argentophilic interactions in the Ag₈-cage, with the Ag-Ag distances ranging from 3.178(3) to 3.220(3) Å. Each Ag₈-cage connects two neighboring cages through edge-sharing to form a one-dimensional wave chain, and is further formed into a three-dimensional supramolecular structure via weak intermolecular interactions of methyl groups from StBu⁻ ligands.

Figure 2. (a) ORTEP view of the asymmetric unit in USC-CP-4 with 50% probability ellipsoids; (b) polyhedral octanuclear silver cage supported by μ₂-StBu and Br⁻ template; (c) the 1-D wave-chain; (d) interlaced packing of the chains in USC-CP-4 (Ag turquoise, S golden, Br olive green, C gray, H light gray).

3.2.3. Molecular Structure of Ag₁₄(μ-StBu)₁₂I₂]_n (USC-CP-3)

The USC-CP-3 crystallizes in the monoclinic P2₁/c space group. The asymmetric unit contains seven independent Ag(I) atoms, six deprotonated StBu⁻ ligands, and one iodide (I⁻) anion (Figure 3a), which can be expanded as a dual-iodide enclosed Ag₁₆-cage. Similarly to USC-CP-4, there are two coordination modes StBu⁻ in USC-CP-3: the μ₃- and μ₂-bridging fashions. The Ag-S bond lengths range from 2.269(6) to 2.695(2) Å, with an average one of 2.444 Å. The I⁻ weakly coordinates with three silver atoms in a trigonal pyramidal fashion. The Ag-I distances are 2.991(2), 3.210(2), and 3.256(1) Å, respectively. The short Ag-Ag distances, ranging from 2.980(7) to 3.344(2) Å, also confirmed the presence of weak metal-metal interactions. Within the Ag₁₆-cage, six Ag(I) atoms are coordinated in a distorted tetrahedral AgS₃I geometry, while the other ten Ag(I) atoms possess the V-shaped AgS₂ coordination environment. Every Ag₁₆-cage is linked to two adjacent cages by edge-sharing to generate a one-dimensional linear chain. The neighboring chains pack together via weak intermolecular forces of methyl groups from StBu⁻ ligands to form light yellow crystals.

Figure 3. (a) ORTEP view of the asymmetric unit in USC-CP-3 with 50% probability ellipsoids; (b) polyhedral sixteen-nuclear silver cage supported by μ₂-StBu; (c) the 1-D linear chain; (d) offset packing of the chains in USC-CP-3 (Ag turquoise, S golden, I purplish red, C gray, H light gray).

3.3. Structural Comparison

All three coordination polymers exhibit nano-cages based on one-dimensional beaded-like chain structures constructed by Ag-S coordination bonds and sustained by argentophilic interactions. However, the introduction of halide anions as structure-directing templates results in significant differences in the supra-molecular building units (SBU) and their overall chain architectures. Using F⁻ or Cl⁻ as the template agent, a neutral one-dimensional chain is constructed from Ag₆-cage SBUs in the absence of F⁻ and Cl⁻, where all the silver ions adopt an approximately linear coordination geometry. When bromide anions are used as templates, USC-CP-4 features mono-Br⁻ inside edging-sharing Ag₈-cage SBUs, where the silver ions display linear, trigonal, and trigonal pyramidal coordination environments. When iodide anions replace bromide, USC-CP-3 is assembled based on dual-I⁻ inside edging-sharing Ag₁₆-cage SBUs, with silver ions exhibiting tetrahedral and V-shaped geometries.

These variations in the bonding configurations and coordination numbers of the central atoms result in distinct Ag-S bond lengths and Ag⋯Ag distances in the three structures. In detail, the Ag-S bond distances range from 2.369(2) to 2.400(1) Å in USC-CP-2, 2.371(4) to 2.589(8) Å in USC-CP-4, and 2.269(6) to 2.695(2) Å in USC-CP-3, respectively. Meanwhile, the Ag⋯Ag separations span 3.059(6)-3.336(7) Å in USC-CP-2, 3.178(3)-3.220(3) Å in USC-CP-4, and 2.980(7)–3.344(2) Å in USC-CP-3, respectively, which are consistent with literature values [28,29,30,31,32,33]. The data indicate that the use of bromide and iodide templates leads to more complex crystal structures and a broader distribution of bond lengths. This outcome may be attributed to the tendency of silver ions, when coordinating with μ-StBu, to encapsulate halide anions within silver cages while directing the bulky tert-butyl groups outward. The larger ionic radii of bromide and iodide can effectively support and stabilize the resulting architectures, whereas the smaller fluoride and chloride ions fail to stabilize larger clusters, leading only to the more compact Ag-S framework observed in USC-CP-2.

3.4. Stability Studies

The crystals of the three CPs mentioned above can remain undamaged after being stored in air for three months or immersed in water for half a month. As depicted in Figure 4, their experimental powder X-ray diffraction patterns perfectly match the simulated one in peak positions. Thus confirm their stability as well as the phase purity of the three CPs. Simultaneously, a comparison of the PXRD pattern of USC-CP-4 and silver iodide confirmed the absence of AgI impurities in USC-CP-4 (Figure S4). In addition, thermal gravimetric analysis of coordination polymers USC-CP-2, USC-CP-4 and USC-CP-3 was performed under a nitrogen atmosphere to evaluate their thermal stability (Figure 4). The thermograms reveal a consistent thermal event initiating around 250 °C, which is attributed to the sublimation of the coordination polymers. Following this initial weight loss, the frameworks undergo progressive decomposition. The pyrolysis process culminates in the formation of stable inorganic residues at 600 °C, with residual masses of 63.19%, 69.36%, and 66.53% for USC-CP-2, USC-CP-4 and USC-CP-3, respectively. These residual masses correspond closely to the theoretical values calculated for the formation of silver sulfide (Ag₂S) from the respective complexes (calcd.: 62.88%, 68.85%, 66.86 wt%), strongly indicating that the final decomposition product is Ag₂S.

Figure 4. Powder X-ray diffraction patterns (top) and thermal gravimetric analyses (bottom) of USC-CP-2, USC-CP-4 and USC-CP-3.

4. Conclusions

In summary, three one-dimensional chain-like coordination polymers constructed from different multinuclear Ag(I)-thiolate nanocages have been successfully synthesized via the reactions of AgNO₃, HStBu, and halide anions as templates, and characterized by single crystal X-ray diffraction analyses. With smaller halogen (F⁻/Cl⁻) or without template agents, the Ag₆-cages form a one-dimensional beaded-like chain structure, which is free of fluoride or chloridion. While with larger size Br⁻ as additives, the mono-Br embedded Ag₈-cages based wave-chain structure is obtained. The largest halogen, I⁻, employed as a template, leads to the formation of the dual-I⁻ encapsulated Ag16-cage-based linear coordination chain. There are argentophilic interactions between neighboring silver atoms, which stabilize these diverse multi-Ag(I) cages. The size of the halide anion is shown to be critical, directly influencing the nucleation process and guiding the structural evolution of the multi-nuclear Ag(I)-thiolate cages and their corresponding chain-like coordination polymers. This study clearly demonstrates that larger halide anions act as effective templates to promote the formation of higher-nuclearity silver-thiolate nano-cages. Future work will extend this methodology to other thiolate ligands, thereby investigating the generality and scope of this templating effect.