English

• 0.   Introduction

  In recent years, polynuclear Ln(Ⅲ)-based complexes have attracted growing attention from researchers due to their unique physical properties and remarkable application potential in some fields such as catalysis^[1-3], magnetism^[4-7], gas storage and separation^[8-9]. With the depth of research on these complexes, the design and synthesis of such complexes have gradually emerged as a research hotspot in this field^[10-12]. Owing to the high coordination numbers, variable coordination environments, and distinctive coordination kinetic characteristics of lanthanide elements, polynuclear Ln(Ⅲ)-based complexes tend to form abundant and intriguing molecular topological structures. These exceptional structural features, coupled with their versatile functional prospects, have positioned this class of complexes as a topic of intense interest^[13-14]. Meanwhile, given the central role of DNA as the carrier of genetic information in organisms, investigating the interaction between Ln(Ⅲ)-based complexes and DNA has emerged as a pivotal research direction for the development of anticancer, antiviral, and antibacterial agents^[15-17]. In-depth elucidation of the underlying interaction mechanisms is expected to facilitate the creation of a new generation of clinical therapeutic drugs, thereby providing novel strategies for addressing major diseases such as cancer and viral infections^[18-19].

  Over the past few years, the excessive emission of CO₂ has triggered a series of severe environmental issues. Converting CO₂ into cyclic carbonates constitutes a carbon reduction technology with both environmental and economic benefits^[20-21]. However, this reaction requires the synergistic catalysis of multiple active sites, and traditional catalysts are difficult to meet the requirements of green and sustainable development^[22]. Therefore, the development of catalytic systems with high catalytic activity and excellent recyclability under mild conditions is imperative^[23-25]. Based on the established catalytic mechanisms, Lewis acid sites are the key to driving CO₂ activation, a characteristic that renders polynuclear Ln(Ⅲ)-based complexes promising catalytic materials^[26]. In addition, polynuclear Ln(Ⅲ)-based complexes also exhibit excellent catalytic performance in the Knoevenagel condensation reaction, which is attributed to the synergistic effect between their highly unsaturated Lewis-acidic rare-earth metal ion centers and Lewis-basic pyridyl groups^[27-28]. Meanwhile, the Knoevenagel condensation reaction has been extensively applied in diverse fields, including pharmaceutical synthesis, the essence and flavor industry, as well as materials science^[29-30]. In summary, the research on the catalytic performance of Ln(Ⅲ)-based complexes opens up new avenues for the development of highly efficient and green catalytic reaction systems.

  With the continuous development and application of polynuclear Ln(Ⅲ)-based complexes, their advantages in various fields have gradually become apparent. Based on previous studies on the synthesis of rare earth metal ion complexes, we successfully synthesized complex 1 using Schiff base ligands HL (Scheme 1). The general formula is [Ho₄(L)₂(dbm)₆(CH₃O)₄] (1). Subsequent studies analyzed its structure, DNA interaction, and catalytic property.

  Scheme 1

  

  Scheme 1.  Structure of HL

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  1.   Experimental

  1.1   Synthesis of complex 1

  Ho(dbm)₃·6H₂O (0.05 mmol), HL (0.05 mmol), NaOH (0.05 mmol), CH₃OH (0.015 mL), and CH₂Cl₂ (5 mL) were placed in a 20 mL vial, stirred at room temperature for 30 min, the vial was then sealed and heated at 70 ℃ for 48 h, followed by cooling to room temperature. The precipitated solid was washed with ethanol three times and dried to obtain yellow block crystals. The synthetic routes of HL and complex 1 are shown in Scheme S1 and S2 (Supporting information). The details of the materials and testing methods are in the Supporting information. The experimental details for the CO₂ cycloaddition with epoxides and the Knoevenagel condensation reaction are also provided in the Supporting information.

  Yield: 50% (based on Ho). Anal. Calcd. for C₁₂₄H₁₀₆Ho₄N₆O₂₄(%): C, 54.67; H, 3.92; N, 3.09. Found(%): C, 54.61; H, 3.87; N, 3.08. IR (cm^-1, Fig.S1): 3 059(w), 1 598(s), 1 554(s), 1 516(s), 1 476(s), 1 458(m), 1 401(s), 1 310(m), 1 234(m), 1 180(w), 1 070(m), 1 026(w), 941(w), 848(w), 810(m), 747(m), 725(m), 688(m), 648(s), 610(s), 522(s), 409(s).

  1.2   X-ray crystallography

  Single-crystal X-ray diffraction data for complex 1 were collected using a computer-controlled Rigaku Saturn CCD area detector diffractometer equipped with confocal monochromated Mo Kα radiation, employing the ω-φ scan technique (λ=0.071 073 nm). The structure was solved and refined via the SHELXS-2016 and SHELXL-2016 programs using a full-matrix least-squares method based on F ², with all non-hydrogen atoms acquiring anisotropic thermal parameters. Due to immediate measurement after isolation from the solution, the crystals contained some disordered solvent molecules. Table 1 covers crystal parameters, data collection, and refinement details for complex 1.

  Table 1

  

  Table 1.  Crystallographic data and structure refinements for complex 1

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  Parameter	1		Parameter	1
  Formula	C124H106Ho4N6O24		Dc / (g·cm-3)	1.643
  Formula weight	2 723.86		μ / mm-1	2.920
  T / K	150.0		θ range / (°)	2.127-26.439
  Crystal system	Monoclinic		F(000)	2704.0
  Space group	P21/c		Reflection collected	190 817
  a / nm	1.504 55(6)		Unique reflection	11 299
  b / nm	1.480 57(6)		Rint	0.111 1
  c / nm	2.475 02(7)		GOF (F 2)	1.060
  β / (°)	92.992 8(13)		R1, wR2 [I>2σ(I)]	0.059 2, 0.150 3
  V / nm3	5.505 8(3)		R1, wR2 (all data)	0.074 7, 0.164 1
  Z	2

  2.   Results and discussion

  2.1   Crystal structure of complex 1

  Complex 1 crystallizes in the monoclinic system P2₁/c space group with Z=2. Complex 1, which is described in Fig.1, contains four independent Ho³⁺ ions, two L^- ligands, six dbm^- ions, and four CH₃O^- ions. The coordination environment of Ho³⁺ is shown in Fig.S2. The central Ho³⁺ (Ho1) ion is eight-coordinated (Fig.S3), coordinated by two nitrogen atoms (N1 and N2) from two L^- ligands [Ho1—N1 0.248 3(7) nm, Ho1—N2 0.268 1(7) nm] and six oxygen atoms from one L^- ligand (O2), two dbm^- ions (O5, O6, O7, O8), and one CH₃O^- ion (O11), with the Ho—O distances ranging from 0.224 1(5) to 0.239 2(5) nm. The central Ho³⁺ (Ho2) ion is also in an eight-coordinated configuration (Fig.S3), which is coordinated by eight oxygen atoms from one dbm^- ion (O9 and O10), two L^- ligands (O3, O3a, O2, O4a), and two CH₃O^- ions (O11 and O12). In the same way as Ho1 and Ho2, Ho1a and Ho2a also adopt an eight-coordinated structure, in which the distance of Ho—O varies from 0.227 3(5) to 0.246 8(5) nm. The coordination mode of L^- ligand and dbm^- ion in complex 1 is shown in Fig.S4. Each L^- ligand is associated with three Ho³⁺ ions [Ho1—N1 0.268 1(7) nm, Ho1—N2 0.248 3(7) nm, Ho1—O2 0.239 2(5) nm, Ho2—O2 0.235 5(5) nm, Ho2—O3 0.230 0(4) nm, Ho2a—O3 0.234 0(4) nm, Ho2a—O4 0.250 6(6) nm, N1—Ho1—N2 63.6(2)°, O2—Ho1—N2 63.33(19)°, Ho1—O2—Ho2 105.90(19)°, O2—Ho2—O3 76.10(18)°, Ho2—O3—Ho2a 109.9(2)°). In addition, the dbm^- ion adopts a bidentate chelation mode to connect each central Ho³⁺ ion. These bond distances and bond angles are comparable to those of the already reported Ln₄ complexes^[31-33]. The important bond lengths and bond angles are listed in Table S1.

  Figure 1

  

  Figure 1.  Molecular structure for complex 1 with ellipsoids being drawn at a 30% probability level

  Hydrogen atoms have been omitted for clarity.

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  2.2   Powder X-ray diffraction and thermogravimetric analyses

  To determine the phase purity of complex 1, powder X-ray diffraction (PXRD) analysis was performed on a single-crystal sample of complex 1 at room temperature. As shown in Fig.S5, the PXRD data from experimental tests were carefully compared with the theoretical values calculated by simulating the single-crystal structures. The analysis showed that the positions of the main peaks in the experimental data set were basically consistent with those of the simulated PXRD pattern, indicating that complex 1 has high phase purity.

  The thermogravimetric analysis (TGA) of complex 1 within the temperature range of 25 to 800 ℃ is shown in Fig.S6. The TGA curve of complex 1 remained essentially unchanged at temperatures from 25 to 260 ℃. A weight loss of 43.7% was observed in the temperature range of 260 to 610 ℃, compared with a theoretical weight loss of 51.1%, which indicates the loss of two L^- ligands and four CH₃O^- ions. Subsequently, the molecular structure decomposed gradually with the increase in temperature. The excellent thermal stability provides a solid foundation for its subsequent applications, including DNA interaction, catalytic conversion of CO₂, and catalysis of the Knoevenagel condensation.

  2.3   Interaction between complex 1 and DNA

  2.3.1   UV spectrometric analysis

  UV spectroscopy is a simple and universal method for studying the interaction mechanism between a compound and DNA. The conjugated double bonds formed by pyrimidine and purine bases in DNA molecules endow it with strong UV absorption in a range of 200-290 nm^[34]. Therefore, changes in the peak position before and after the compound binds to DNA can reveal the binding mode of the compound to DNA. As shown in Fig.2, after the interaction between complex 1 (or HL) with CT-DNA, the UV absorption spectra of the complex-CT-DNA systems and the ligand-CT-DNA systems exhibited redshifts compared to that of pure CT-DNA. As the concentration of CT-DNA increased, the redshift magnitude became more pronounced; notably, the redshift intensity was 1>HL. Additionally, a hypochromic effect was observed. This is because complex 1 intercalates between DNA base pairs through intercalation, leading to local extension of the DNA double helix structure and disruption or reorganization of the π-π* conjugated system of base pairs^[35]. At this point, the electron cloud distribution of the conjugated double bonds in the bases is altered, reducing the energy required for electron transitions and shifting the absorption peak to longer wavelengths (red shift). The above results indicate that both HL and complex 1 can bind to DNA through intercalation, with complex 1 exhibiting a stronger binding affinity to DNA than HL.

  Figure 2

  

  Figure 2.  UV spectra for the interaction between 1 (a)/HL (b) and CT-DNA

  c₁=c_HL=1.7×10^-2 mmol·L^-1.

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  2.3.2   Fluorescence spectrometry analysis

  Ethidium bromide (EB) exhibits fluorescent emission upon binding to DNA. Utilizing this characteristic, the interaction mechanism between a compound and DNA can be investigated. Through fluorescence spectroscopy experiments, the binding constant between the two substances can be calculated, which in turn enables the determination of the magnitude of the interaction force between them. The relationship between the fluorescence intensities of the EB-DNA system is described by the Stern-Volmer equation, which is formulated as: I₀/I=1+K_svr. I₀ corresponds to the original fluorescence intensity of the EB-DNA mixture before any extra compound is added, and I stands for the fluorescence intensity that is obtained after introducing the compound with diverse concentrations. The concentration ratio of the compound to DNA is represented by the variable r. The linear Stern-Volmer quenching constant, denoted as K_sv, can be derived from the slope of the linear graph plotted between I₀/I and r. This constant quantifies the binding capability of the compound to DNA, where a greater K_sv value corresponds to a more robust binding affinity^[36-37].

  In accordance with the aforementioned theoretical analysis, EB-DNA fluorescence experiments were carried out to investigate the binding capacity of complex 1 (or HL) with DNA. As shown in Fig.3a and 3b, the uppermost curve represented the fluorescence emission of the EB-DNA composite in the absence of complex 1 and HL. As the concentrations of complex 1 and HL gradually increased, the fluorescence intensity decreased stepwise. The foregoing results implied that EB, which was intercalated between DNA base pairs, was substituted by complex 1 and HL, resulting in the generation of a new system composed of CT-DNA and the compound. Additionally, by analyzing the data in Fig.3c and 3d, the K_sv values of complex 1 and HL were determined to be 4.86 and 2.97, respectively. These values clearly indicate that the binding strength follows the order of complex 1>HL. This data demonstrates that the interaction strength between complex 1 and DNA is higher than that of ligand HL. Compared with the analogous Ln(Ⅲ)-based complexes reported in the literature, the K_sv of 1 in this study was higher, and its value exceeded the K_sv range of some reported Ln(Ⅲ)-based complexes (Table S2), which indicates that 1 has a stronger binding affinity with DNA. This result is consistent with the UV spectroscopy result, indicating that synergistic effects occur between the rare earth metals and the ligands after the formation of the complex^[38].

  Figure 3

  

  Figure 3.  Fluorescence spectra of EB-DNA (c_DNA=4.2 μmol·L^-1) system in the presence of 1 and HL with different concentrations (a, b) and corresponding Stern-Volmer plots (c, d)

  r=c_compound/c_DNA; from a to j: r=0, 0.013, 0.026, 0.039, 0.052, 0.065, 0.078, 0.091, 0.104, respectively.

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  2.3.3   Cyclic voltammetry analysis

  Cyclic voltammetry (CV) analysis is a powerful electrochemical technique that enables the exploration of interactions between a compound and biological molecules^[39]. As shown in Fig.4, the anodic peak potential (E_pa) and the cathodic peak potential (E_pc) of complex 1 were 0.346 0 and 0.240 0 V, respectively. Correspondingly, the equation potential of complex 1 was 0.293 0 V. After adding DNA, the E_pa and the E_pc of complex 1 were 0.368 0 and 0.230 0 V. The equation potential was 0.299 0 V. Similarly, the equation potential of HL was 0.300 5 V before adding DNA. After adding DNA, the equation potential of HL shifted to 0.302 5 V. The equation potential of complex 1 and HL has shifted positively after the addition of DNA, indicating that complex 1 (HL) has undergone intercalation with DNA^[40-41]. Notably, the interaction between complex 1 and DNA is stronger than that between HL and DNA. The above results also indicate a synergistic effect between the rare-earth metals and the ligands, which strengthens the interaction between complex 1 and DNA. Moreover, the conclusions derived from the CV showed a high degree of consistency with UV and fluorescence spectroscopy data. Significantly, the three methodologies not only mutually corroborate one another but also provide cross-validation for the proposed binding mechanism, thereby reinforcing the scientific rigor and reliability of the overall findings.

  Figure 4

  

  Figure 4.  CV curves for the interaction between 1 (1.7×10^-2 mmol·L^-1)/HL (1.7×10^-2 mmol·L^-1) and CT-DNA

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  2.3.4   Cycloaddition reaction of CO₂ and epoxides

  The cycloaddition reaction of epoxides with CO₂ can be utilized to prepare cyclic carbonates, and the products can be used as polycarbonate monomers, secondary batteries, and chemical power source electrolytes^[42-43]. Therefore, we explored the catalytic performance of complex 1 in the cycloaddition reaction of epoxides with CO₂.

  As shown in Table S3, to determine the optimal reaction conditions, the effects of parameters such as reaction temperature, duration, and co-catalyst dosage on the catalytic performance of complex 1 were systematically investigated. First of all, the reaction temperature is one of the important factors that cannot be ignored. Therefore, we explored the role of temperature in this catalytic activity and executed catalytic cycloaddition reactions at different temperatures each in turn (Table S3, entries 1-6). As the reaction temperature was raised from 30 to 70 ℃, the yield of the target product increased from 32% to 95%. It follows that the reaction temperature will enhance the catalytic performance. This is because elevated temperature accelerates the interaction between catalytically active sites and reactants, thus boosting catalytic performance. However, upon further elevating the temperature to 80 ℃, the yield stabilized at 95%. It can be seen from this that further heating is of no benefit to the improvement of the catalytic effect. Meanwhile, given that the energy consumption required for heating will increase accordingly, 60 ℃ was ultimately determined as the optimal temperature for this reaction. What′s more, reaction time is also a key influencing factor. When the reaction proceeded for a duration of 8-10 h, the product yield exhibited an elevation, varying in a range of 95%-96%. Consequently, a reaction duration of 10 h was selected as the optimum (Table S3, entries 7-8). Last but not least, the dosage of the catalyst constitutes another pivotal factor. With the amounts of co-catalyst tetrabutylammonium bromide (TBAB) set at 0.5, 1, 2, and 3 mmol, the yields of the intended target product stood at 53%, 82%, 96%, and 96% in turn (Table S3, entries 8-11). Drawing on the current data, 2 mmol stood as the optimum dosage for the co-catalyst. A synergistic catalytic effect between complex 1 and the co-catalyst is evident from the fact that their individual use as catalysts yields less than 1% and 25% of the target product, respectively (Table S3, entries 12-13). In contrast, under the optimal conditions, equivalent dosages of Ho(NO₃)₃·6H₂O and HoCl₃·6H₂O as catalysts resulted in target product yields of 39% and 40%, respectively (Table S3, entries 14-15).

  Subsequently, the generality of complex 1 toward other epoxides was systematically investigated. Under the optimized reaction conditions, complex 1 was utilized to mediate the cycloaddition reactions between CO₂ and nine distinct epoxides; the associated experimental findings are detailed in Table 2. As documented in previous studies, both a reduced steric hindrance and an electron-withdrawing property are conducive to enhancing catalytic activity. Notably, the experimental findings of the present work have further confirmed these conclusions^[44-45]. As indicated in Table 2 (entries 1-3), the yield of the target product showed a gradual decline as the steric hindrance of the substituted chain increased. Likewise, 1 exhibited considerable catalytic activity for epoxy bromopropane and 2-oxiranemethanol (Table 2, entries 4 and 5), which can be attributed to the strong electron-withdrawing effects of the Br and O atoms. 1 exhibited relatively low catalytic activity for 2-phenyloxirane (Table 2, entry 6) due to the presence of an electron-donating group. Specifically, the high electronegativity of the oxygen atom causes the carbon atoms in the epoxide ring to become more positively charged, thereby promoting ring-opening (Table 2, entries 7-8). As demonstrated in entry 9, the yield was remarkably low with the steric hindrance on the carbon atoms of the oxirane ring restricting the nucleophilic attack of Br^-, leading to a significant reduction in the ring-opening step. Notably, the catalytic conversion efficiency of complex 1 was comparable to the levels reported to date, and the reaction conditions required for it were mild^[46].

  Table 2

  

  Table 2.  Various epoxides participate in the cycloaddition reaction with CO₂ under optimized conditions*

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  Entry	Substrate	Product	Yield / %
  1			89
  2			87
  3			81
  4			91
  5			89
  6			86
  7			88
  8			86
  9			30
  *The reaction involved 5 mmol of epoxides and 100 kPa of CO2; complex 1 was added at a dosage of 0.5% (molar fraction), along with 2 mmol of TBAB and a reaction time of 10 h; significantly, it was conducted without any solvent; the product was separated by column chromatography.

  Based on the research findings regarding the cycloaddition reaction between CO₂ and epoxides, it is evident that the ring-opening stage of epoxides constitutes the rate-controlling step of the cycloaddition reaction^[47]. The metal center of complex 1 exhibits a propensity to function as Lewis acidic sites, enabling it to form M—O bonds with the oxygen atom. Thereby, a metal-epoxide is generated, facilitating the activation of the epoxide. Subsequently, the Br^- anion attacks the carbon atom, which has relatively less steric hindrance on the epoxide, inducing the ring-opening of the epoxide. Thereafter, CO₂ inserts into the Ho—O bond formed between the Ho³⁺ ion and the epoxide. Subsequently, the negatively charged O atom assaults the C—Br bond, initiating the cyclization process. Concurrently, the catalyst undergoes regeneration. The possible mechanism of the cycloaddition reaction is shown in Fig.S7.

  2.3.5   Knoevenagel condensation reaction

  Based on recent literature^[48] and the unique structural features of complex 1 in this study, it is expected to be an excellent catalytic candidate for the Knoevenagel condensation reaction, which is about its abundant Lewis acid active sites on the surface, with specific results in Table S4. When it comes to building carbon-carbon double bonds during organic synthesis, the Knoevenagel condensation stands out as an important method. Experiments concerning 1 dosage showed that the yield rose sharply from 1% to 98% with increasing catalyst loading, verifying that 1 dosage is a key determinant of the product yield (Table S4, entries 1-6). When the reaction time was extended from 4 to 10 h, the yield increased from 74% to 92%, representing an 18% improvement in yield (Table S4, entries 7-10). Temperature studies revealed the yield increased from 72% to 99% as the temperature rose from 40 to 60 ℃, with no further improvement at 70 ℃, identifying 60 ℃ as optimal (Table S4, entries 11-13). Next, we tested the catalytic efficiencies of 1 in different solvents. The results showed that when the solvents were DMSO, DMF, MeOH, and EtOH, the yields of the target product ranged from 91% to 99% (Table S4, entries 13-16). Consequently, EtOH was recognized as the top-performing solvent. Comparative experiments showed that lower solvent polarity reduced yield. Compared to equal amounts of Ho(NO₃)₃·6H₂O and HoCl₃·6H₂O (Yield: 29% and 31%, respectively; Table S4, entries 17-18), complex 1 demonstrated superior catalysis. The optimal reaction conditions were determined as 10 mmol benzaldehyde, 20 mmol malononitrile, 0.5% (n/n) complex 1, 60 ℃, and 6 h.

  To systematically investigate the catalytic activity and substrate selectivity of complex 1 in the Knoevenagel condensation, the study explored its universality through substrate expansion experiments. These experiments used a series of aldehyde derivatives as substrates under optimized reaction conditions. When the hydrogen atoms on the benzaldehyde ring were replaced by electron-withdrawing groups (—F, —Br, and —NO₂), the reaction system achieved nearly complete conversion under the optimized reaction conditions. This result indicates that electron-withdrawing groups such as —F, —Br, and —NO₂ can significantly enhance the reactivity of the reaction system through electronic effects and accelerate the reaction process (Table 3, entries 1-3). When the substrates (aldehyde derivatives) bear electron-donating groups, the reaction performance showed significant steric hindrance dependence: the yield of the target product generally decreases as the molecular size of the substituent increases. Among them, small-sized electron-donating substituents such as methyl have little impact on the reaction. However, due to the steric hindrance effect, large-sized electron-donating substituents hinder the effective contact and interaction between reactants, resulting in the reduction of reaction efficiency (Table 3, entries 4-7).

  Table 3

  

  Table 3.  Knoevenagel condensation of aldehyde derivatives and malononitrile under optimized conditions*

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  Entry	Substrate	Product	Yield / %
  1			99
  2			99
  3			98
  4			97
  5			95
  6			89
  7			75
  *The reaction, which involved 0.5% of complex 1, 20 mmol of benzaldehyde, 10 mmol of malononitrile, and 5 mL of ethanol, was carried out at 60 ℃ for 6 h; the product was separated by column chromatography.

  Complex 1 exhibits excellent performance in the Knoevenagel condensation, and its catalytic mechanism can be summarized as shown in Fig.S8^[49]. At the initial stage of the reaction, the formyl oxygen atom interacts with the Lewis acidic Ho³⁺ ion through weak van der Waals forces, driving the polarization of the aldehyde molecule and its adsorption on the active site of the metal. Meanwhile, the hydrogen atom on the methylene carbon in the malononitrile molecule contacts the Lewis acidic active site, thereby generating an imine intermediate. Finally, through an intramolecular rearrangement reaction, the target product 2-benzylidenemalononitrile is generated, with the simultaneous release of one water molecule and the complex 1 catalyst, thus completing the catalytic cycle.

  In addition to possessing high catalytic activity, the recyclable catalytic performance of heterogeneous catalysts is also an indispensable key factor for practical applications. As shown in Fig.S9a, the yield of the product decreased only slightly after three catalytic cycling experiments. Furthermore, PXRD tests were carried out on the newly synthesized catalyst and the samples after three cycles, respectively. As shown in Fig.S9b, comparative analysis of the patterns revealed that the PXRD pattern of complex 1 after three cycles was nearly identical to that of the original sample in terms of peak positions, confirming that complex 1 still maintained the integrity of its framework after three catalytic cycles. The filtrate after the reaction was collected for inductively coupled plasma mass spectrometry (ICP-MS) analysis, and as shown in Table S5, the leakage amount of Ho³⁺ after catalysis was only 0.18%, which further proved that complex 1 could maintain good stability after the catalytic reaction. All the above experimental results confirm that complex 1 exhibits excellent recyclability.

  3.   Conclusions

  In brief, a rare-earth complex 1, namely [Ho₄(L)₂(dbm)₆(CH₃O)₄], was synthesized. X-ray structural analysis revealed that complex 1 is tetranuclear. A systematic investigation into the structures of complex 1 and its interactions with DNA strongly indicated that the complex binds to CT-DNA via an intercalation mechanism, which provides valuable insights into the study of complex-DNA interactions. Additionally, the complex exhibits excellent catalytic activity: results show that complex 1, as a heterogeneous catalyst, can catalyze the cycloaddition reaction of epoxides with CO₂ and the Knoevenagel condensation. This serves as a useful reference for the rational design of multifunctional polynuclear Ln(Ⅲ)-based complexes.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Scheme 1  Structure of HL

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  Figure 1  Molecular structure for complex 1 with ellipsoids being drawn at a 30% probability level

  Hydrogen atoms have been omitted for clarity.

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  Figure 2  UV spectra for the interaction between 1 (a)/HL (b) and CT-DNA

  c₁=c_HL=1.7×10^-2 mmol·L^-1.

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  Figure 3  Fluorescence spectra of EB-DNA (c_DNA=4.2 μmol·L^-1) system in the presence of 1 and HL with different concentrations (a, b) and corresponding Stern-Volmer plots (c, d)

  r=c_compound/c_DNA; from a to j: r=0, 0.013, 0.026, 0.039, 0.052, 0.065, 0.078, 0.091, 0.104, respectively.

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  Figure 4  CV curves for the interaction between 1 (1.7×10^-2 mmol·L^-1)/HL (1.7×10^-2 mmol·L^-1) and CT-DNA

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  Table 1.  Crystallographic data and structure refinements for complex 1

  Parameter	1		Parameter	1
  Formula	C124H106Ho4N6O24		Dc / (g·cm-3)	1.643
  Formula weight	2 723.86		μ / mm-1	2.920
  T / K	150.0		θ range / (°)	2.127-26.439
  Crystal system	Monoclinic		F(000)	2704.0
  Space group	P21/c		Reflection collected	190 817
  a / nm	1.504 55(6)		Unique reflection	11 299
  b / nm	1.480 57(6)		Rint	0.111 1
  c / nm	2.475 02(7)		GOF (F 2)	1.060
  β / (°)	92.992 8(13)		R1, wR2 [I>2σ(I)]	0.059 2, 0.150 3
  V / nm3	5.505 8(3)		R1, wR2 (all data)	0.074 7, 0.164 1
  Z	2

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  Table 2.  Various epoxides participate in the cycloaddition reaction with CO₂ under optimized conditions*

  Entry	Substrate	Product	Yield / %
  1			89
  2			87
  3			81
  4			91
  5			89
  6			86
  7			88
  8			86
  9			30
  *The reaction involved 5 mmol of epoxides and 100 kPa of CO2; complex 1 was added at a dosage of 0.5% (molar fraction), along with 2 mmol of TBAB and a reaction time of 10 h; significantly, it was conducted without any solvent; the product was separated by column chromatography.

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  Table 3.  Knoevenagel condensation of aldehyde derivatives and malononitrile under optimized conditions*

  Entry	Substrate	Product	Yield / %
  1			99
  2			99
  3			98
  4			97
  5			95
  6			89
  7			75
  *The reaction, which involved 0.5% of complex 1, 20 mmol of benzaldehyde, 10 mmol of malononitrile, and 5 mL of ethanol, was carried out at 60 ℃ for 6 h; the product was separated by column chromatography.

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•