A single-crystal 3D Zn-tetrathiolate connected metal–organic framework

Abstract

Due to thiolate's fast and robust coordination, crystallographically resolved thiolate-ligated metal–organic frameworks (MOFs) are hard to obtain. Following the previous success of the masked synthetic strategy to achieve single-crystal Pb-thiolate connected MOFs, thioester-protected hexathiotriphenylene (HVaTT) was employed in this work to assemble with a Zn(II) centre to yield the first example of a first-row transition-metal based crystallographically resolved metal-tetrathiolate connected MOF, HTT-Zn. HTT-Zn features a 3D anionic framework connected by tetrahedral [ZnS₄] nodes with 3-fold interpenetration and windmill-like topology. Thanks to the anionic nature, HTT-Zn demonstrates a fast colorimetric response (from yellow to black) to the positively charged toxic pollutant, paraquat, and potentially serves as a naked-eye sensor for paraquat. Isolation of HTT-Zn proves masked synthesis as a general approach and adds a new dimension to explore metal-thiolate-based MOFs.

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Thiol metal–organic frameworks (S-MOFs), a subcategory of MOFs, are composed of thiol-containing organic linkers and metal nodes. Owing to the redox activeness and intense interaction with heavy/transition metal centres of electronically diffused thiolate, S-MOFs usually feature rich activity, decent conductivity, etc. These properties allow S-MOFs to excel in a wide range of applications including conductivity,¹ photocatalysis,² electrocatalysis,³ energy storage,⁴ sensing,⁵ heavy metal adsorption,⁶etc.

Generally, there are two types of S-MOF: (1) frameworks composed of metal ions and linkers bearing carboxyl and thiol groups (aka Mercarb linker);⁷ (2) planar π-conjugated two-dimensional (2D) networks built from soft metal ions and linkers equipped with three dithiolate ends (e.g. 2,3,6,7,10,11-hexakis(thiolate)triphenylene, HTT⁶⁻).⁸ Carboxyl groups in the Mercarb linker withdraw the electron density of thiol and slow down the formation of the M–S bond on the one hand, while they coordinate the metal centre reversibly on the other hand to facilitate assembly of crystalline S-MOF, suitable for resolving single-crystal structure.⁹ However, strongly coordinating HTT⁶⁻ chelate metal centres so fast that the S-MOFs obtained are usually poor in crystallinity, let alone single-crystal structure determination. This type of S-MOF typically adopts a 2D planar network connected by metal-bis(dithiolene) nodes and is prepared by solvothermal or biphasic interfacial diffusion methods.¹⁰ To expose the metal-bis(dithiolene) active sites and explore structure–property correlation, obtaining three-dimensional (3D) crystallographically resolved thiolate-ligated S-MOFs is insightful. Yet, limited by the fast and strong coordination of thiolate, only two single-crystal 3D analogues of this type of S-MOF have been reported by our group.¹¹

To slow down the coordination for the framework of higher crystallinity and even obtain crystals suitable for single-crystal determination, we proposed a masked strategy to functionalise thiol as thioester, which is not just bench-stable (as thiol is more susceptible to oxidation under air) but can be easily deprotected during assembly for an extended coordination process. Specifically, thiol-masked linker, 2,3,6,7,10,11-hexakis(pentanoylthio)triphenylene (HVaTT), was prepared and allowed to assemble with Pb(II) salt in basic medium (for deprotection) to give two examples of single-crystal Pb-based S-MOFs.¹¹ Utilising the same strategy, we successfully extend the scope to first-row transition metals and herein report a crystallographically resolved 3-fold interpenetrated Zn-based S-MOF.

HVaTT (Fig. S1 and S2 (ESI†)), prepared from a modified procedure of our work,^11b was allowed to react solvothermally with Zn(OAc)₂⋅2H₂O in methanol (MeOH) in the presence of ethylenediamine (EDA, as modulator to temporarily occupy Zn(II) ions) and NaOH (for in situ hydrolysis of HVaTT to HTT⁶⁻) at 120 °C for 48 h to obtain HTT-Zn as yellow block-shaped crystals (Fig. 2b and Fig. S3, ESI†). Noteworthily, unprotected HTT could only give HTT-Zn of far worse crystallinity upon assembly with Zn(II) ions, highlighting the masked strategy as an effective tactic to obtain highly crystalline S-MOFs.

A block-shaped HTT-Zn single crystal was analysed by single-crystal X-ray diffraction (SCXRD). The crystal data indicate that HTT-Zn adopts a hexagonal crystal system with a space group of P6₃/m and lattice parameters of a = b = 30.4115 Å, c = 56.222 Å, α = β = 90°, γ = 120°. Each Zn(II) centre is chelated by two sets of dithiolate (Fig. 1a, note: Zn–S bond ∼2.36 Å, comparable to single Zn–S bond reported in other molecular systems¹²) from two HTT⁶⁻ linkers to give a tetrahedral four-coordinated [ZnS₄] unit (Fig. 1b) rather than the well-known planar metal-bis(dithiolene), [MS₄], site as a secondary building unit (SBU). Each triangular HTT⁶⁻ is trigonally connected to three SBUs, forming an infinitely expanding 3D structure. Viewed along the c-axis (Fig. 1c), taking HTT⁶⁻ and Zn²⁺ as a triangle and stick, the framework can be simplified to a windmill-like topology. The windmill-like 3D network can be described as a new 2,2,2,3,3,3-c net calculated by ToposPro software. The dihedral angle between adjacent HTT⁶⁻ planes surrounding the windmill disc is about 85° whereas that between adjacent HTT⁶⁻ planes forming the windmill blades is about 90°. Note that HTT-Zn exhibits 3-fold interpenetration where the disc of one windmill alternately intertwines with the blades of two other windmills (Fig. 1d and e). HTT-Zn features an anionic framework bearing a formula unit of [Zn₃(C₁₈H₆S₆)₂]⁶⁻, with the negative charge being balanced by the Na⁺ ions in the channel. The existence of EDA was also verified by ¹H NMR spectroscopy (Fig. S4, ESI†). Scanning electronic microscopy (SEM) reveals the HTT-Zn blocks to be elongated hexagonal-based polyhedra. Energy dispersive X-ray spectroscopy (EDS) analysis indicates the presence and uniform distribution of C, N, S, Zn and Na (Fig. S5, ESI†). Together with the elemental analysis (EA) data, the formula of HTT-Zn could be fitted as Zn₃(C₁₈H₆S₆)₂(Na)₆(C₂H₈N₂)_3.6(H₂O)₉. The satisfactory Rietveld refinement result (R_p = 3.96% and R_wp = 2.49%) suggests good phase purity of HTT-Zn (Fig. 2a). The Fourier-transform infrared (FT-IR) spectrum shows that vanishing C=O and C–H stretching signals of the thioester chain of HVaTT at 1702 and 2850–3000 cm⁻¹ echoes the complete alkaline hydrolysis of the protective thioester chain and coordination of S⁻ to Zn²⁺ centres.

Fig. 1 (a) Self-assembly of HTT⁶⁻ and Zn(II) ions; (b) coordination environment of the Zn(II) centre; (c) 3D framework and topological network of HTT-Zn; representation of the 3-fold interpenetration network of HTT-Zn along the (d) c-axis and (e) a-axis directions.

Fig. 2 (a) Rietveld refinement of the experimental PXRD pattern of HTT-Zn (R_wp = 3.96%; R_p = 2.49%). (b) SEM images of HTT-Zn. (c) FT-IR spectra of HVaTT, HTT and HTT-Zn. High-resolution XPS spectra of (d) Zn 2p, and (e) S 2p of HTT-Zn.

The residual peak at 2850–3000 cm⁻¹ in HTT-Zn originates from the –CH₂ stretching vibration of the ethylenediamine molecule. The disappearing S–H stretching peak at 2512 cm⁻¹ of HTT after coordination with Zn²⁺ indicates no free HTT in HTT-Zn (Fig. 2c). It is worth mentioning that a significant red-shift was observed from the HTT linker to HTT-Zn as demonstrated by the UV-vis absorption profiles (Fig. S6, ESI†).

X-ray photoelectron spectroscopy (XPS) analysis was employed to study HTT-Zn (Fig. S7, ESI†) to probe the chemical environment of the elements. As demonstrated in the Zn 2p spectrum (Fig. 2d), the peaks at 1045.5 and 1022.3 eV are assigned to Zn^II 2p_1/2 and 2p_3/2, respectively.¹³ Deconvolution of the S 2p signals generates three doublets (Fig. 2e). The doublets at 162.27 and 163.48 eV correspond to the Zn–S coordination. Also, the doublets at 163.86 and 165.12 eV can be ascribed to the S–C on HTT,^10b while those at 166.67 to 167.97 eV indicate the presence of oxidised sulphur, probably in the form of a thiyl radical or S=O.^10a On the other hand, thermogravimetric (TG) curves indicate HTT-Zn to be thermally stable up to around 200 °C in air or a N₂ atmosphere (Fig. S8, ESI†). N₂ sorption data at 77 K reveal that HTT-Zn adopts a type I isotherm (Fig. S9a, ESI†), with the specific surface area calculated to be 20.416 m² g⁻¹ by the Brunauer–Emmett–Teller (BET) equation (Fig. S9b, ESI†). The occupancy of guest ions/molecules and interpenetration can explain the extremely low surface area.

Bearing an anionic framework, HTT-Zn was taken as a host to absorb the cationic lethal toxic pollutant, paraquat, a widely used herbicide. Since there are few reports on the colorimetric detection of paraquat,¹⁴ we speculate the big clear yellow HTT-Zn block to be a naked-eye sensor of paraquat. Specifically, a few grains of HTT-Zn crystals from the MeOH stock solution were taken in a Petri dish (diameter: 35 mm; depth: 10 mm). After soaking MeOH with tissue or filter paper, a few drops of paraquat diiodide (PDI) aqueous solution were added to the crystals. By microscopic or naked-eye observation, HTT-Zn crystals were found to darken significantly (Fig. 3a and b), and the darkening intensifies with higher PDI concentrations (Fig. S10, ESI†). The ¹H NMR spectrum obtained from the digestion of PDI-treated HTT-Zn in DCl/DMSO-d₆ shows paraquat signals, revealing the paraquat absorption by HTT-Zn (Fig. S11, ESI†). Significant red shift of optical absorption from HTT-Zn to PDI-treated HTT-Zn demonstrates the darkening of HTT-Zn upon exposure to PDI spectroscopically (Fig. 3c). Presumably, a strong narrow-gap donor–acceptor junction establishes between positive paraquat (acceptor) and negative electron-rich HTT-Zn (donor) and exhibits absorption covering the whole visible spectral region (i.e. dark crystals of PDI-treated HTT-Zn). Consistent PXRD patterns and FT-IR spectra of HTT-Zn before and after PDI treatment indicate the structural integrity of HTT-Zn throughout the sensing (Fig. S12 and S13, ESI†).

Fig. 3 (a) Schematic diagram showing PDI sensing by HTT-Zn; (b) optical photograph of the PDI-treated HTT-Zn crystals ([PDI] = 1000 ppm) under a microscope; (c) UV-vis absorption spectra of HTT-Zn before and after PDI treatment.

In short, utilising the masked strategy, we successfully extend the members of the thiolate-coordinated S-MOFs from heavy metals (Pb^II) to first-row transition metals (Zn^II this time), highlighting the generality of this strategy to assemble S-MOFs. Success in preparing single crystals, resolving the structure of HTT-Zn and facile response to positively charged pollutants by HTT-Zn undoubtedly fuels up fundamental and application study of metal-thiolate-connected S-MOFs, which was once tricky because of the challenging preparation of structurally well-defined S-MOFs.

This work was supported by the National Natural Science Foundation of China (22371054, 22301045), the Foundation of Basic and Applied Basic Research of Guangdong Province (2024A1515012801), the Science and Technology Planning Project of Guangdong Province (2023A0505050164), and the Postdoctoral Fellowship Program of CPSF (GZC20240311). We also thank the Instrumental Analysis Centre of Guangdong University of Technology for collecting UV-vis absorption spectra, SEM images, and XPS measurements.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for HTT-Zn has been deposited at Cambridge Crystallographic Data Centre as CCDC 2407349,† accessible from https://www.ccdc.cam.ac.uk/structures.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Preparation details of HVaTT and HTT-Zn, spectroscopic data, and additional plots/figures. Contains the supplementary crystallographic data for HTT-Zn. CCDC 2407349. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc06478j

‡ These authors contributed equally.

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