Carboxylate engineering for manipulating the optical and assembly properties of copper clusters

Abstract

Surface ligands are critical in the construction and stabilization of atomically precise metal nanoclusters (NCs) with diverse structures, and ligand engineering remains one of the most effective ways to tailor their properties. In this work, we report the synthesis, structure and surface engineering of novel copper nanoclusters co-protected by carboxylic and thioate ligands. The two clusters share the same formula Cu₁₄(RCOO)₆(AdmS)₈ (RCOOH is benzoic acid or 2-[(2,6-dichlorophenyl) amino] benzeneacetic acid, and AdmSH is 1-adamantanethiol) and a similar molecular structure. What is surprising to us, however, is that the optical properties, stability and assembly structures of the two clusters are significantly different, thus strongly indicating the potential of engineering carboxylates for manipulating the physicochemical properties of atomically precise copper NCs. This work not only provides model clusters stabilized by carboxylic ligands for further study of the structure–property relationship, but also outlines the big picture of carboxylic-stabilized metal nanoclusters that will flourish in the near future.

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Introduction

Owing to ultra-small sizes, well-defined structures, and unique electronic structures, atomically precise metal nanoclusters (NCs) have attracted widespread interest across the fields of bio-imaging, chemical sensing, catalysis, and nanomedicine.^1–7 The clusters with precise structures are model platforms to explore the relationship between structures and properties at the atomic level, and thus direct the synthesis of functional nanomaterials for target applications in a reasonable way.^8–16 In particular, the properties of metal NCs heavily rely on their compositions, charges and structures, in which the metal–ligand interface is crucial. It has been well documented that electronic structures, local coordination environments, and physicochemical properties are susceptible to delicate changes of ligand–metal interfacial structures.^17–25 Therefore, it is of significance to deepen the understanding of the correlation between interfacial structures and physicochemical properties of metal NCs.^19,22,23,26 In the past decades, great effort was made in engineering ligands for the construction and stabilization of atomically precise metal NCs with diverse interfacial structures, which are represented by metal-phosphine, metal-thiol, metal-alkynyl, metal-halide, and metal-N-heterocyclic carbene.^{6,11,15,19–21,23,24,27–45}

Close examination of the literature reveals that although significant advancements have been made in illustrating the interfaces of metal NCs by introducing diverse ligands, much less explored in the community are carboxylates. Due to their great affordability, availability, and variety, carboxylate ligands have evolved to be some of the most common compounds that find wide application in the fields of organometallics, materials, catalysis, etc.^18,46–59 Of note, the presence of carboxylate ligands on the surface of a handful of metal NCs is a pioneering implication for tailoring physicochemical properties of metal NCs by engineering carboxylate groups.^60–62 Recently, several important studies have clearly demonstrated that carboxylates have emerged as a new class of ligands in the field of NCs. It has even been claimed from the viewpoint of theoretical calculations that carboxylates would enable great tunability of geometric structures, hence making them attractive ligands for endowing NCs with interesting catalytic, photocatalytic, and optical properties.⁶³ Experimentally, the reports by Liu and co-workers for determining the crystal structure of all-carboxylate protected superatomic Ag₈ for the first time, and by Wang and co-workers for preparing amino carboxylate-stabilized Ag₄₇ with homochirality have clearly illustrated that carboxylate-stabilized NCs feature interesting optical properties, thus providing us a bright prospect for further functionalizations.^64,65

Although carboxylate ligands have been introduced in the protection of metal NCs in previous reports, engineering carboxylate ligands for manipulating the optical and assembly properties of metal NCs such as copper, to the best of our knowledge, has not been claimed. Herein, we report the synthesis and structural determinations of two novel copper clusters co-stabilized by carboxylate and thiol ligands: [Cu₁₄(BEN)₆(AdmS)₈] (Cu₁₄-1) and [Cu₁₄(diclofenac)₆(AdmS)₈] (Cu₁₄-2) (BEN is benzoic acid, diclofenac is 2-[(2,6-dichlorophenyl) amino] benzeneacetic acid, and AdmSH is 1-adamantanethiol). Although the two Cu₁₄ clusters share similar formulas, charges and structures, the use of different carboxylate ligands induces significant changes in their optical, stability and assembly properties, which therefore indicates the bright future of engineering carboxylate ligands for regulating the properties of NCs.

Results and discussion

The two Cu₁₄ clusters were prepared by the hydrothermal reduction method reported by Melosh et al.⁶⁶ Cu(RCOO)₂ salts were reduced by ethylene glycol under thermal treatment in the presence of thiols to afford raw products (see the Experimental section for details). Bright yellow crystals suitable for single crystal X-ray diffraction analysis were obtained by vapor diffusion of ether into the cluster solution (Fig. S1 and S2†).

Structural analysis revealed that Cu₁₄-1 crystallized in a triclinic system with a space group of P1̄, and Cu₁₄-2 in a monoclinic unit cell in the P2₁c space group (Tables S1 and S2†). Due to the structural similarity of these two nanoclusters, we here present only the detailed structure of Cu₁₄-1. The asymmetric unit in the structure of Cu₁₄-1 contains 156 crystallographically independent sites, i.e., fourteen Cu, eight AdmSH ligands and six BEN ligands. In addition, no counterion was observed in the lattice, which indicates that it is a neutral molecule. Thus, the empirical formula of Cu₁₄-1 was proposed to be [Cu₁₄(BEN)₆(Adms)₈] (Fig. S3 and S4†). On the basis of its formula, all the 14 copper atoms are in the +1 oxidation state, which suggests the effective reduction of Cu²⁺ by ethylene glycol under heating conditions.

The metal skeleton of Cu₁₄-1 was simultaneously protected by different types of donors of hard Lewis groups of O and the soft Lewis ones of S (Fig. 1a). The structure features a Cu₁₂S₈ core which could be viewed as two S atoms tetrahedrally embedded in the Cu₁₂ framework (Fig. 1b). Considering the monovalent valence state of copper ions, the metal framework of Cu₁₄-1 has an excessively positive charge. In order to make the structure stable, the core of Cu₁₄-1 was further passivated by six BEN ligands that are less sterically demanding to form isolated neutral clusters. The metal framework of the cluster can be regarded as a malformed prismatic structure consisting of four prongs, and the thiol ligands act as bridging units to connect the four prongs to each other (Fig. 1c). Besides, the two Cu atoms at the end positions of each prong are capped by BEN ligands with Cu–O bond lengths of 1.863–2.044 Å (Fig. 1d), which is consistent with the literature reports.⁶⁷

Fig. 1 (a) The total structure of the [Cu₁₄(BEN)₆(AdmS)₈] cluster. (b) The tetrahedral S₄ unit in the core. (c and d) The binding pattern between the Cu atoms and ligands in the Cu₁₄ metal core (one of them is highlighted). (e) Cu₄ plane at the center of the cluster. (f) Triangular conical Cu₃S (orange) and tetragonal conical Cu₄S (blue) fragment units. Color codes for atoms: blue spheres, Cu; orange spheres, O; yellow spheres, S; grey spheres, C. All hydrogen atoms are omitted for clarity.

Compared with previously reported copper clusters capped by thiolate ligands, the introduction of BEN ligands not only makes the entire structure of Cu₁₄-1 distinct, but also brings great significance for the construction of new connection patterns within the clusters. For example, the structure of Cu₁₄-1 is highly different from that of Cu₁₄(C₂B₁₀H₁₀S₂)₆(CH₃CN)₈ (C₂B₁₀H₁₀S₂ is 1,2-dithiolate-o-carborane) which features a cubic Cu₁₄ core.⁶⁸ Furthermore, in sharp contrast to [NEt₄][Cu(AdmS)₂] and [NEt₄][Cu₅(AdmS)₆], the copper atoms in the center of Cu₁₄-1 are interconnected to form a planar quadrilateral structure of Cu₄ (Fig. 1e).⁵⁶ Notably, Cu–Cu bonds (green) with 2.94 Å are found in the Cu₄ plane (Fig. 2a). The value is slightly larger than the average Cu–Cu bond lengths (Table S3†). The Cu–S–Cu bond lengths are of a wide range as well, with values between 2.138 and 2.647 Å, which is related to the two bridging modes (μ₃-S, μ₄-S) within the cluster in triangular conical Cu₃S (orange) and tetragonal conical Cu₄S (blue) fragment units, respectively (Fig. 1f). Furthermore, the Cu–S–Cu bond angles thus are in a relatively large range of 66.99–129.11°.

Fig. 2 Structural comparisons of Cu₁₄-1 and Cu₁₄-2 clusters stabilized by different acids. The angles of the Cu₄ plane are slightly different due to the use of different acids. Color codes for atoms: blue spheres, Cu; orange spheres, O; yellow spheres, S; grey spheres, C. All hydrogen atoms are omitted for clarity.

To confirm the formula determined by X-ray crystallographic analysis, high-resolution electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS) of Cu₁₄-1 was then conducted in the positive mode. As shown in Fig. S5,† the ESI-MS of Cu₁₄-1 in the positive mode shows one prominent peak at ∼2470 and a small peak ∼3010 m/z. The two peaks can be assigned to be [Cu₁₂(BEN)₃(AdmS)₈]⁺ and [Cu₁₅(BEN)₆(AdmS)₈]⁺, respectively. The excellent match of the experimental and simulated isotope patterns verified the composition proposed by single crystal diffraction analysis (Fig. S5,† inset).

With Cu₁₄-1 in hand, we thus in the following section wonder whether its surface structure can be regulated by introducing different carboxylic ligands. 2-[(2,6-Dichlorophenyl) amino] benzeneacetic acid (diclofenac) was selected here considering its large steric hindrance and wide use in the area of medicine (Fig. 2). [Cu₁₄(diclofenac)₆(AdmS)₈] (Cu₁₄-2) can be readily obtained by a similar synthetic approach to that of Cu₁₄-1 by using diclofenac instead of BEN (Fig. S6 and S7†). It is noteworthy that Cu₁₄-2, to the best of our knowledge, represents the first copper cluster stabilized by diclofenac. Structural comparisons reveal that the metal core and coordination mode of Cu₁₄-2 are similar to those of Cu₁₄-1, with the Cu–Cu bond lengths ranging from 2.609 to 2.842 Å, Cu–O from 1.876 to 2.026 Å and Cu–S from 2.149 to 2.573 Å, respectively. Careful comparison reveals that the average Cu–Cu bond lengths in Cu₁₄-2 are slightly shorter than those of Cu₁₄-1 (Table S3†). Notably, the angles of the Cu₄ plane are highly different for the two clusters. The angle of quadrilateral in Cu₁₄-1 is 77.1°, while it is only 75.4° for Cu₁₄-2 (Fig. 2).

Despite the small structural deviations induced by using different carboxylic ligands, the physicochemical properties of the two Cu₁₄ clusters are highly different. As shown in Fig. 3 (red trace), the room temperature ultraviolet-visible spectrum (UV-vis) of Cu₁₄-1 manifests a unique electronic structure, with characteristic peaks at ∼235, ∼275 and ∼330 nm, respectively. To our surprise, the UV-Vis spectrum of Cu₁₄-2 is extremely distinct from that of Cu₁₄-1, which displays several prominent absorption bands at around 270, 350 and 500 nm, respectively (Fig. 3, green trace). We note that the heavy dependence of the optical properties on the surface ligands of Cu₁₄ may facilitate their application in optics down to the nanometer scale.

Fig. 3 Experimental UV-Vis spectra of Cu₁₄-1 and Cu₁₄-2 clusters in dichloromethane. The inset shows the simulated absorbance spectra of Cu₁₄-1 and Cu₁₄-2 clusters.

To understand why the two clusters exhibit significantly different spectral properties, we have thus performed density functional theory (DFT) calculation to investigate their electronic structures. The experimental structures of the clusters were directly employed as the moieties for calculation (see technical details in the Experimental section). The simulated spectra of the two clusters, as shown in Fig. 3 (inset), exhibit similar profiles to the experimental ones. The calculated UV-Vis spectrum of Cu₁₄-1 mainly displays three absorption band wavelengths at 364, 307 and 230 nm, respectively, which are attributed to the transition of α (HOMO to LUMO transition), β (HOMO−2 to LUMO) and γ (HOMO−5 to LUMO+2), respectively (Fig. 4a). As a comparison, the simulated UV spectrum of Cu₁₄-2 exhibits an electronic transition from HOMO−2 to LUMO+2 with the minimum wavelength at 268 nm (Fig. 4b). It suggests that the energy levels for the electronic transitions of the two clusters are different, therefore rationalizing their distinct optical properties.

Fig. 4 Orbitals involved in the α, β and γ transitions of Cu₁₄-1 (a) and Cu₁₄-2 (b).

Not only absorbance features of the two clusters have been tailored significantly by using different types of carboxylates, but also their stability and self-assembly modes. As suggested by the time-dependent UV-Vis spectra (Fig. S8†), while Cu₁₄-1 exhibits high stability in the solution form, Cu₁₄-2 is prone to decompose under similar conditions. Moreover, the two clusters are in practically different space groups (P1̄ for Cu₁₄-1 and P2₁c for Cu₁₄-2). Shown in Fig. 5 is the comparison of the assembly structures of the two clusters. The packing structure of Cu₁₄-1 is loosely arranged in the linear mode along the c-axis (Fig. 5a), while that of Cu₁₄-2 is in a twisted style by closely touching with each other (Fig. 5b). The distinction observed above strongly indicates the efficiency of employing different carboxylates as protective shells for regulating assembly chemistry of metal clusters.

Fig. 5 Assembly structures of Cu₁₄-1 (a) and Cu₁₄-2 (b) clusters along the c-axis. Color codes for atoms: blue spheres, Cu; magenta and orange spheres, O; yellow spheres, S. All other atoms are omitted for clarity.

Conclusions

In summary, we report the synthesis and structural determination of two novel copper nanoclusters stabilized by carboxylate ligands, which motivates the wide exploration of carboxylates as the next generation of surface ligands for the protection of metal nanoclusters. We have also introduced, for the first time, diclofenac as a ligand for the stabilization of copper clusters, implying their potential applications in medicine. Moreover, it has been demonstrated that the metal skeleton, surface structure, optical properties, stability, and assembly structures of carboxylic-protected metal nanoclusters can be tuned by employing different carboxylic groups. These Cu₁₄ clusters reported herein not only have application prospects in the fields of optics and catalysis, but also can be regarded as a model system to study the fundamental issue of structure–activity relationship. More studies on preparing carboxylate-functionalized metal nanoclusters are ongoing in our laboratories to deepen the understanding of metal–carboxylic interfaces and the relationship between their structures and performances.

Experimental

Materials

Copper sulfate (CuSO₄, 99%), 2-[(2,6-dichlorophenyl) amino] benzeneacetic acid (diclofenac, 98%), benzoic acid (C₆H₅COOH, 98%) and 1-adamantanethiol (AdmSH, 98%) were purchased from Bidepharm (Shanghai, China). Dichloromethane (CH₂Cl₂, A.R.), ethylene glycol ((CH₂OH)₂, A.R.), toluene (C₇H₈, A.R.) and ether (C₄H₁₀O, A.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Water used in all experiments was ultrapure. All other reagents were used as received without further purification.

Synthesis of Cu(C₆H₅COO)₂ and Cu(diclofenac)₂

Cu(C₆H₅COO)₂ was synthesized by dissolving 28.8 mg of C₆H₅COONa and 16 mg of CuSO₄ in 10 mL of deionized water. After stirring for 2 h at room temperature, the reaction mixture was filtered, and the green precipitate was washed with water and dried in air. The resulting copper salts were obtained in good yield. Cu(diclofenac)₂ was prepared using a similar procedure except for the use of 32 mg of Na(diclofenac) and 16 mg of CuSO₄ as precursors.

Synthesis of Cu₁₄

Cu₁₄-1 was prepared by a modified procedure of copper-based metal–organic chalcogenides. Typically, Cu(C₆H₅COO)₂ and AdmSH at a molar ratio of 1 : 1 were dissolved in a mixed solvent of ethylene glycol and toluene. The mixture was heated at 80 °C over a period of 48 h, resulting in a yellow solution. The solution was then carefully filtered and dried up. To the solid, dichloromethane was added, affording a yellow solution. Yellow block crystals were obtained by vapor diffusion of ether into the solution. Cu₁₄-2 was prepared by a similar procedure of Cu₁₄-1, except for the use of Cu(diclofenac)₂ instead of Cu(C₆H₅COO)₂.

Characterization studies

UV-Vis spectra. UV-Vis spectra were collected using a Carry-5000 Spectrophotometer using a quartz cuvette of 1 cm path length. The scanning speed was 1000 nm min⁻¹. The spectra were recorded in diluted solutions of dichloromethane and the signal of the blank solvent was subtracted.

ESI-MS. Electrospray ionization mass spectra (ESI-MS) were recorded using an Agilent 6224 time-of-flight mass spectrometer in the positive mode. The samples dissolved in dichloromethane were filtered before the measurements. Then the sample was directly infused at a flow rate of 1.2 mL h⁻¹ using a syringe pump. Typical parameters used for the measurements were as follows: capillary voltage: 4.0 kV; drying gas temp.: 150 °C; drying gas flow: 4 L min⁻¹; nebulizer pressure: 20 psi.

Crystallography. X-ray single-crystal analysis: the diffraction data of the single crystals of compounds Cu₁₄-1 and Cu₁₄-2 were collected using a Rigaku Oxford Diffraction system X-ray single-crystal diffractometer using Cu Kα (λ = 1.54184 Å) at 293 K. The data were processed using CrysAlis^Pro. The structure was solved and refined using full-matrix least-squares based on F₂ using ShelXT,⁶⁹ ShelXL⁷⁰ in Olex2,⁷¹ and Shelxle.⁷² The thermal ellipsoids of the ORTEP diagram were generated at 50% probability. Detailed crystal data and structural refinements for the two compounds are given in Tables S1 and S2.† CCDC 2216708 and 2216705† contain the supplementary crystallographic data for this paper.

Computational details

As for the optical-absorption spectrum and orbital information, the density functional theory (DFT) calculations were implemented in the quantum chemistry program Turbomole V4.2.⁷³ Geometry optimization of the two Cu₁₄ clusters was carried out by DFT calculations with the TPSS (Tao, Perdew, Staroverov, and Scuseria) functional for electron exchange and correlation and def2-SV(P) for orbital and auxiliary basis sets.⁷⁴ Of note, the def2-SV(P) basis sets were used for S, N, O, Cl, C and H, while effective core potentials which include scalar relativistic corrections were considered for Cu.^75,76 Then, the time-dependent DFT computation of the optical absorption spectrum was performed at the PBE level with the def2-SV(P) basis sets. All transitions together with their oscillator strengths were then convoluted with a Gaussian line shape of 0.5 eV broadening to generate the optical-absorption spectrum.

Author contributions

The manuscript was written through contributions of all authors. H. S., Q. T. and Chaolumen supervised the project. J. S., F. S., J. T. and X. T. prepared the cluster samples and conducted the characterization studies. Q. W., R. H., A. H., Sachurilatu and X. S. were responsible for analyzing the data. All authors contributed to composing the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support provided by the NSFC (2022161035), the Inner Mongolia Autonomous Region Fund for Natural Science (2021MS02001), and the Education Department of Inner Mongolia Autonomous Region (NJYT22089).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2216708 and 2216705. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00409k

‡ These authors contributed equally to this work.

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