Structure and assembly mechanism of a centipede-shaped high-nuclear Dy₁₄Cu₁₂ heterometallic nanocluster

Abstract

The assembly studies of high-nuclear clusters have received much interest in recent years due to their targeted synthesis for some special usages. A Schiff base ligand 2-[N-(2-hydroxynaphthylidene)amino]-2-(hydroxymethyl)propane-1,3-diol (H₄L) was thus prepared in this work and was allowed to react with [Cu₂(OAc)₄(H₂O)₂] and Dy(NO)₃·6H₂O, giving a high-nuclear 3d–4f heterometallic Cu₁₂Dy₁₄ nanocluster (1) with the formula [(C₂H₅)₃NH]₄[Cu₁₂Dy₁₄O(OH)₁₆(H₂L)₈(HL)₄(HL′)₄(OAc)₁₀](NO₃)₄(OAc)₂·6CH₃CN·5EtOH·1.5H₂O (H₃L′ = 1,1,1-tris-(hydroxymethyl)methanamine). It features a centipede-shaped structure constructed from eight Dy₃Cu cubanes sharing the Dy(III) vertices. Its assembly mechanism was explored through time-dependent high-resolution electrospray ionization mass spectrometry (HRESI-MS). This revealed that the hierarchical assembly of 1 undergoes two possible routes: H₄L → DyCuL → Dy₂CuL → Dy₃Cu₂L₂ → Dy₄Cu₂L₂ → Dy₄Cu₃L₃ → Dy₅Cu₄L₄→ Dy₁₄Cu₁₂L₁₂L′₄ and H₄L → DyCuL → DyCu₂L → Dy₂Cu₂L₂ → Dy₂Cu₃L₄ → Dy₃Cu₄L₄ → Dy₅Cu₄L₄ → Dy₁₄Cu₁₂L₁₂L′₄. These results not only provide solid evidence for understanding the assembly mechanism of high-nuclear 3d–4f clusters, but also help to give some hints for designing and preparing new high-nuclear clusters.

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Introduction

Lanthanide-containing high-nuclear clusters are attracting increasing interest due to their aesthetic nano-scaled structures and their potential applications in fields such as catalysis, information storage, optical devices, and bioactivity.^1–7 However, reasonable design and assembly of these clusters are still a huge challenge due to diverse coordination configurations of lanthanide ions and their variable self-assembly pathways. The common method is to assemble metal ions into the anticipated structures using well-designed organic ligands with a special geometry and size and multiple coordination groups which contribute a significant role in the construction of high-nuclear heterometallic clusters.^8–10 In the past decades, some well-designed hydroxyl- or carboxylato-containing ligands were employed to build high-nuclear 4f and 3d–4f clusters, such as Gd₁₀₄,¹¹ Ln₆₀,¹² Dy₇₂,¹³ La₂₀Ni₃₀,¹⁴ Gd₄₀Ni₄₄,¹⁵ Ni₃₆Gd₁₀₂,¹⁶ Cu₂₄Ln₆,¹⁷ Cu₉Dy₂,¹⁸ Gd₃₀Co₁₂,¹⁹ Cu₃₆Ln₂₄,²⁰ Mn₈Ln₈,²¹ Ln₂₄Zn₄,²² Fe₁₀Ln₁₀,²³ and Fe₁₆Ln_4.²⁴ Most of them are homometallic lanthanide clusters, and heterometallic Ln–M clusters (M = Co, Ni, Mn). However, high-nuclear Cu(II)–Dy(III) clusters were rarely reported. It is well known that the reported 4f and 3d–4f high-nuclear clusters present versatile aesthetic structures such as cage,^25,26 ring,^27,28 linear,^29–32 prism,^33–35 pancake,^36–38 dumbbell^39–42 and sphere motifs.^43–45 However, 3d–4f high-nuclear clusters with sheet-like skeletons are difficult to form probably due to the limitations from stability. We thus take this challenge to develop high-nuclear Cu(II)–Dy(III) clusters with sheet-like skeletons.

In synthesizing high-nuclear clusters, one usually pays attention to the final products, but ignores their assembling processes and mechanisms. Now, more and more high-nuclear clusters were prepared and applied in a wide field, which requires their precise synthesis under control.^46–49 As we all know, the assembling process of high-nuclear clusters is extremely complicated with the assembling mechanism difficult to be clearly detected through routine techniques such as NMR, IR and single crystal X-ray diffraction analysis. Lately, ESI-MS technology has been proved to be an effective method in exploring the assembling mechanism through detecting the intermediates that emerged in the formation process of clusters. However, related studies on the assembly mechanisms of 3d–4f heterometallic complexes are still very limited.^47,50–53 Thus, we aimed to investigate the assembling mechanism of the targeted clusters through detecting the intermediates in their formation processes using high resolution ESI-MS.

One of the important tasks for the construction of high-nuclear clusters is the selection of linking atoms or groups which are responsible for gathering metal ions together to achieve a high-nuclear core, as well as for tuning the intermetallic interactions.^27,54,55 In this sense, the hydroxyl group is a nice selection, which has strong bridging ability for both lanthanide and transition metal ions and can tune intermetallic interactions through different bridging modes and M–O–M bridging angles. Besides, it is necessary to tune the type and strength of the ligand field around metal ions using appropriate coordination atoms, as well as to control the dimensionality of the clusters through the use of some non-coordinating groups. Taking an overall consideration of these factors, we aimed to design polyhydroxyl multidentate bridging ligands containing also N atoms, which can encapsulate both lanthanide and transition metal ions in their multiple coordination pockets. Thus we designed the ligand 2-[N-(2-hydroxynaphthylidene)amino]-2-(hydroxymethyl)propane-1,3-diol (H₄L) to achieve the targeted clusters.

In this study, we successfully obtained a centipede-shaped Dy₁₄Cu₁₂ cluster from the reaction of H₄L with Co(OAc)₂·4H₂O and Dy(NO)₃·6H₂O. Its possible formation mechanism was investigated using a time-dependent HRESI-MS technique. As far as we know, it is the first time to track the formation process of a high-nuclear heterometallic Cu(II)–Dy(III) cluster with a proposed assembly mechanism.

Experimental

Starting materials

All reagents (analytical grade) in this work were used directly without purification. 2-[N-(2-hydroxynaphthylidene)amino]-2-(hydroxymethyl)propane-1,3-diol was prepared by following the literature method.⁵⁶ Details for all materials and characterization such as ESI-MS measurements and single-crystal X-ray crystallography are shown in the ESI.† The synthetic route of cluster 1 is shown in Scheme 1.

Scheme 1 Schematic diagram for the synthesis of cluster 1.

Synthesis of the cluster

Cluster 1 was obtained from the solvothermal reaction of 2-[N-(2-hydroxynaphthylidene)amino]-2-(hydroxymethyl)propane-1,3-diol (0.1 mmol, 0.0105 g) with Dy(NO₃)₃·6H₂O (0.1 mmol, 0.0456 g) and [Cu₂(OAc)₄(H₂O)₂] (0.1 mmol, 0.0398 g) at 80 °C for 3 d in the presence of trimethylamine (40 μL) in an evacuated 20 cm-long sealed Pyrex tube using ethanol (1 mL) and acetonitrile (1 mL) as solvents. Cluster 1 crystallized into green crystals with a yield of 24% [calculated from Dy(III)]. Anal. calcd. for C₂₆₆H₃₇₇Cu₁₂Dy₁₄N₃₀O_119.5: C, 35.72; N, 4.70; H, 4.25%. Found: C, 35.42; N, 4.45; H, 4.44%. IR (KBr, cm⁻¹; Fig. S1†): 3398(s), 1614(s), 1542(s), 1394(s), 1252(m), 1188(w), 1142(m), 1035(m), 829(m), 739(m), 675(w), 579(m), 451(w).

Results and discussion

Crystal structures of the cluster [(C₂H₅)₃NH]₄[Cu₁₂Dy₁₄O(OH)₁₆(H₂L)₈(HL)₄(HL′)₄(OAc)₁₀](NO₃)₄(OAc)₂·6CH₃CN·5EtOH·1.5H₂O

Single-crystal diffraction analysis revealed that the title cluster crystallized in a triclinic crystal system with a space group of P1̄ (Table S1†), which consists of fourteen Dy^III ions, twelve Cu^II ions, one O²⁻, sixteen OH⁻, eight (H₂L)²⁻, four (HL)³⁻, four (HL′)²⁻ and eight OAc⁻ in the coordination unit of 1 (Fig. 1a). The detailed crystallographic parameters, bond length and bond angles for the cluster are given in Tables S8 and S9 in the ESI,† respectively. All Cu^II centers exhibit a planar square geometry completed by three O and one N atom from one Schiff base ligand by ignoring weak Cu⋯O bonds. According to the analysis of SHAPE software (Tables S1–S7†),⁵⁷ the fourteen Dy^III ions present six different coordination configurations as shown in Fig. S5.† Dy1 is nine-coordinated in monocapped square antiprism. Dy5 and Dy7 are seven-coordinated in monocapped trigonal prism and pentagonal bipyramid, respectively. The other Dy^III ions are all eight-coordinated in biaugmented trigonal prism for Dy2, square antiprism for Dy4, and triangular dodecahedron for Dy3 and Dy6. The Dy–O and Cu–O/N bond distances range from 2.2075(4) to 2.8545(4) Å and 1.885(5) to 1.993(3) Å, respectively, which are consistent with the values reported in the literature.^58–60 The angles of O–Dy–O and O–Cu–O/N are in the range of 51.1(17) to 167.9(4)° and 83.9(2) to 175.1(2)°, respectively. As shown in Fig. S6,† the H₄L ligand presents two deprotonated forms (H₂L)²⁻ and (HL)³⁻ with two different bridging modes μ₂-η¹:η³ and μ₃-η¹:η¹:η³ for (H₂L)²⁻ and one bridging mode μ₄-η¹:η¹:η²:η³ for (HL)³⁻. As shown in Fig. 1b, fourteen Dy^III ions and twelve Cu^II ions were linked by forty one O atoms to form a hitherto unknown {Dy₁₄Cu₁₂O₄₁} skeleton. Interestingly, the title cationic cluster [Cu₁₂Dy₁₄O(OH)₁₆(H₂L)₈(HL)₄(HL′)₄(OAc)₈]²⁺ exhibits a vertex-sharing octa-cubane structure with a length of approximately 3.70 nm, a width of around 1.69 nm, and a thickness of about 2.20 nm (Fig. S4†), which looks like a “centipede” in appearance. To the best of our knowledge, 1 is the first nanocluster with a centipede-like structure although some Dy^III–Cu^II heterometallic nanoclusters CuLn,⁶¹ Cu₂Ln₂,⁶² Cu₄Ln,⁶³ Cu₅Ln₂,⁶³ Cu₉Ln₂⁶⁴ and Cu₂₄Ln₆⁶⁵ with different kinds of skeletons were reported.

Fig. 1 The molecular structure (a) and skeleton (b) of 1 with selected atoms labeled. Hydrogen atoms are omitted for clarity.

Assembly mechanism analysis of the cluster

It is usually difficult to track and explore the self-assembly process of high-nuclear clusters. There are only a few heterometallic complexes reported on the assembly mechanism. In the previous work of our group, the assembly mechanisms of a series of clusters Dy₁₀,⁶⁶ Dy₁₂,⁶⁷ Dy₂Co₈⁶⁸ and Dy₄Co₈/Dy₄Ni₈⁶⁹ were clearly studied. Based on these previous research studies, the self-assembly process of the title cluster of Dy₁₄Cu₁₂ was tracked by analyzing the species formed in the reaction at different reaction times through HRESI-MS. The same amount of reaction solution was taken out from parallel experiments at irregular intervals and immediately diluted with chromatographic methanol. Their HRESI-MS spectra were subsequently recorded in anionic and cationic modes and analyzed, which showed us the change of intermediates and their abundance with the reaction time.

As shown in Fig. 2a and S7–S9,† the reaction was monitored for a duration of about 24 h with the HRESI-MS data collected in positive mode at different times. A series of positively charged ion peaks are concentrated in the m/z range of 200–3000, from which ten intermediates (Table S10†) were detected with the maximum m/z value at 2624.00 for Dy₅Cu₄L₄. This revealed that the species in the reaction solution underwent a change from low-nuclear to high-nuclear with the reaction time. The time-dependent change trend of the peak intensity of each intermediate is plotted in Fig. 2b. It showed that the species at the beginning of the reaction (approximately 5 min) are mainly inorganic metal salts (labelled Dy) and organic ligands. Subsequently, the species named DyL′, DyCuL and Dy₂CuL (Table S11†) were detected as the kinetic products which increased with the reaction time and reached their maximum at about 45 min for DyL′ and DyCuL and 90 min for Dy₂CuL. When the reaction was carried out for 60 minutes, DyCu₂L appeared and reached a maximum at about 240 min with a subsequent fast decrease. At a reaction time of 90 min, a new species Dy₃Cu₂L₂ was observed and reached its maximum at about 450 min. The species Dy₂Cu₂L₂ and Dy₄Cu₂L₂ appeared at 180 min and increased with the reaction time to their maxima at about 360 and 600 min, respectively. As the reaction was progressed further, the species Dy₄Cu₃L₃, Dy₃Cu₄L₄ and Dy₂Cu₃L₄ were obviously detected in sequence at about 240, 240, and 360 min, and reached their maximum at 810, 1080 and 600 min, respectively. The species Dy₅Cu₄L₄ was first detected at 600 min and reached maximum at 1080 min. Based on the information on these species and their peak intensity changes with reaction time, two possible assembly routes can presumably be established: H₄L → DyCuL → Dy₂CuL → Dy₃Cu₂L₂ → Dy₄Cu₂L₂ → Dy₄Cu₃L₃ → Dy₅Cu₄L₄→ Dy₁₄Cu₁₂L₁₂L′₄ and H₄L → DyCuL → DyCu₂L → Dy₂Cu₂L₂ → Dy₂Cu₃L₄ → Dy₃Cu₄L₄ → Dy₅Cu₄L₄ → Dy₁₄Cu₁₂L₁₂L′₄ (Fig. 3). There are a few reports on the assembly mechanisms of 3d–4f heterometallic nanoclusters such as Eu₂₄Ti₈,⁷⁰ Ni^II₂Dy^III₂,⁷¹ Dy^III_xCo^II_10−x (x = 2, 4),⁶⁸ Dy^III₄M^II₈ (M = Ni, Co)⁶⁹ and [Dy^III₁₀Mn^III₄Mn^II₂].⁷² However, there are no related studies reported on the assembly mechanism of Dy^III–Cu^II heterometallic nanoclusters. Thus, this is the first case clarifying the assembly mechanism of Dy^III–Cu^II heterometallic nanoclusters through HRESI-MS.

Fig. 2 (a) Time-dependent HRESI-MS for the reaction solution used for tracking the assembly process of 1; (b) HRESI-MS spectral intensity versus time profiles of intermediates during self-assembly of 1.

Fig. 3 Possible assembly mechanism for 1.

Magnetic properties

A polycrystalline sample of 1 was collected and its direct current magnetic susceptibilities were measured from 2 to 300 K under an external field of 1000 Oe as plotted in Fig. 4a. The χ_MT value of 1 at room temperature is 201.71 cm³ K mol⁻¹, close to the corresponding theoretical values for fourteen independent Dy^III centers (198.38 cm³ K mol⁻¹; ⁶H_15/2, S = 5/2, L = 5, J = 15/2, and g = 4/3)^43,73,74 and twelve independent Cu^II centers (4.50 cm³ K mol⁻¹; S = 1/2, g = 2 and C = 0.375 cm³ K mol⁻¹).^53,75 As the temperature is lowered, the χ_MT value of 1 decreases slowly to a value of 187.72 cm³ K mol⁻¹ at 50 K. The further decreasing temperature of the sample led to an abrupt decrease of χ_MT of 1, reaching 126.22 cm³ K mol⁻¹ at 2 K. The χ_MT versus T profile of 1 might be caused by the depopulation of Stark levels and possible antiferromagnetic interactions.

Fig. 4 Plots of χ_M and χ_MT versus T (a), M vs. H T⁻¹ (b) for the cluster.

The field-dependent magnetization of 1 was measured in the temperature range of 2.0–6.0 K and its M vs. HT⁻¹ curves (Fig. 4b) and M vs. H curves are shown in Fig. S10.† The magnetization of 1 has a sharp increase in the low field range with a subsequent much slower increase in the high field range, reaching a value of 27.78 Nμ_B at 70 kOe and 1.8 K.^51,76 The M vs. H T⁻¹ isotherms collected at different temperatures are not superimposed, which revealed possible anisotropy and/or low energy states for 1. The magnetic hysteresis loops at 2 K for 1 were not apparent, which is shown in Fig. S11.†

In order to investigate the presence of slow magnetic relaxation behaviors in 1, the variable-frequency alternating current (ac) susceptibilities were obtained at a temperature range of 2–5 K and an applied dc field of 0 Oe with an oscillating ac field of 2 Oe, as shown in Fig. S12.† This revealed no obvious temperature- and frequency-dependence of its ac susceptibilities (χ′ and χ′′) with the absence of peaks in χ′ and χ′′ vs. v plots and a nearly zero value of χ′′ in 1, as shown in Fig. S13.† Even when a series of different dc fields were applied for the ac susceptibility measurements of 1 at 2 K (Fig. S14†), no frequency-dependence was found in its χ′ and χ′′ vs. v plots with a nearly zero χ′′ value in 1.

Conclusions

In summary, we prepared a heterometallic coordination cluster [(C₂H₅)₃NH]₄[Cu₁₂Dy₁₄O(OH)₁₆(H₂L)₈(HL)₄(HL′)₄(OAc)₁₀](NO₃)₄(OAc)₂·6CH₃CN·5EtOH·1.5H₂O (1) bearing the Schiff base ligand 2-[N-(2-hydroxynaphthylidene)amino]-2-(hydroxymethyl)propane-1,3-diol (H₄L), which presents a centipede-shaped structure bearing eight Dy₃Cu cubanes sharing the Dy(III) vertices. The time-dependent HRESI-MS analysis revealed a possible formation mechanism with two routes H₄L → DyCuL → Dy₂CuL → Dy₃Cu₂L₂ → Dy₄Cu₂L₂ → Dy₄Cu₃L₃ → Dy₅Cu₄L₄ → Dy₁₄Cu₁₂L₁₂L′₄ and H₄L → DyCuL → DyCu₂L → Dy₂Cu₂L₂ → Dy₂Cu₃L₄ → Dy₃Cu₄L₄ → Dy₅Cu₄L₄ → Dy₁₄Cu₁₂L₁₂L′₄. As far as we know, this is one of the rare examples of exploring the formation mechanism of heterometallic clusters, and it is the first study of the assembly mechanism of Dy–Cu clusters. This will provide an inspiration for designing and synthesizing high-nuclear clusters.

Author contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 12064002, 22061004 and 21901050), the Guangxi Natural Science Foundation of China (grant no. 2018GXNSFBA050031, 2020GXNSFAA159132, 2018GXNSFDA281002 and AD20238043), the Guangxi Technology Base and Talent Subject (grant no. GUIKE AD19245002), and the Key Project of Guangxi Normal University (grant no. 2018ZD003).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, crystal data, IR, PXRD, TG, and HRESI-MS data and additional figures for magnetic properties are provided. CCDC 2223068 contains the supplementary crystallographic data for this paper. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ce01291j

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