English

• Polynuclear Ln(Ⅲ)-based complexes have attracted great interest and attention from inorganic chemists and materials chemists due to their wide and potential applications in the fields of magnetic refrigeration^[1-4], single-molecule magnets (SMMs)^[5-11], luminescence probe^[12-13], catalysis^[14-16], biological activities^[17-19], and so on^[20]. For the above-applied research, the study of the magnetic refrigeration property of polynuclear Ln(Ⅲ)-based complexes is an important research hotspot in recent years^[21-22]. In general, the Gd(Ⅲ) ion is an ideal candidate for designing and constructing Ln(Ⅲ)-based magnetic refrigeration materials due to its isotropic and a large spin ground state with weak magnetic interaction in complexes^[23-24]. Based on this, lots of polynuclear or high-nuclear Gd^Ⅲ-based complexes displaying remarkable magnetocaloric effects (MCEs) have been constructed in the past two decades^[25], such as the Gd₂₄ complex^[26], Gd₆₀ complex^[27], Gd₁₀₄ complex^[28], and Gd₁₄₀ complex^[29]. To some extent, these studies stimulate and promote the development of molecular-based magnetic refrigeration materials.

  To develop the versatility of rare earth compounds, we found that such compounds also have potential applications in biomedicine. Since the discovery of the antibacterial and anti-inflammatory effects of Ln(Ⅲ) in the 1960s, research on the application of Ln(Ⅲ) in the medical field has received increasing attention and has achieved numerous research results. Researchers have used the nuclear magnetic properties of the rare earth element gadolinium as a nuclear magnetic resonance imaging contrast agent to enhance imaging contrast. For example, gadolinium diethylenetriamine penta- acetic acid has been officially used in clinical practice. This is also the first time that a rare earth compound has been officially used as an injectable drug^[30]. The new drug Sm-153-EDTMF (injection) developed by Merck and launched in 1998 contains rare earth samarium, which can effectively relieve bone pain and metastasis in cancer patients. In recent years, people have also conducted a lot of research on the interaction between rare earth complexes and tumor cells. Jiang et al.^[31] reported that lanthanum trichloride can inhibit the growth of CBRH-7919 cells by changing the protein expression of rat liver cancer P16 and P21; Yang et al.^[32] showed that the injection of a citric acid complex of lanthanum has a significant inhibitory and breaking effect on genetic materials such as DNA and RNA, and is currently preparing to conduct clinical trials for treating lung cancer. Ma et al.^[33] used Schiff base, 1, 10-phenanthroline, Ln(Ⅲ) ions (Ce³⁺, Sm³⁺, Eu³⁺, Y³⁺, Gd³⁺) to synthesize five Ln(Ⅲ) ternary complexes. The results showed that this type of complex has a significant inhibitory effect on leukemia tumor cells. Wang et al.^[34] synthesized 12 Ln(Ⅲ)-based complexes of aminothioxamic acid (HL) with the general formula of REL₃ (RE=La, Ge, Pr, Nd, Sm, Eu, Gd, Tb, Er, Tm, Yb, Y), studied their pharmacological activities and found that they have broad-spectrum antibacterial activity.

  Consider the above, by selecting and employing a polydentate diacyl hydrazone organic ligand H₄L (Scheme 1), two novel Gd₂ complexes [Gd₂(HL)₂]·2CH₃OH·CH₃CN (1) and [Gd₂(dbm)₂(H₂L)₂]·6CH₃CN (2) with novel structures were obtained, where Hdbm=dibenzoylmethane. The crystal structures, magnetic characteristics, and biological activities of 1 and 2 have all been thoroughly studied in this work. Our work aims to serve as a reference for exploring and investigating the multifunctional properties of Ln(Ⅲ)-based complexes. It will be beneficial and significant for the directional design and synthesis of multifunctional Ln(Ⅲ)-based materials in the future.

  Scheme 1

  

  Scheme 1.  Molecular structure of the polydentate diacylhydrazone organic ligand (H₄L)

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  1.   Experimental

  1.1   Starting materials and methods

  All solvents and chemicals used in our work were of reagent grade and used without further purification. Ammonium hydroxide, triethylamine, 1, 10-phenanthroline, and salicyl hydrazide were obtained from Energy Chemical Co., Ltd. Gd(NO₃)₃·6H₂O (99.99%) was purchased from Aladdin Co., Ltd. Hdbm was obtained from Adamas Co., Ltd. Solvents including acetonitrile, dichloromethane, methanol, and ethanol were also acquired from Energy Chemical Co., Ltd. Bipolaris sorokiniana (B. sorokiniana), Exserohilum tucicum (E. tucicum), and Pseudomonas syringae (P. syringae) were supplied by Mall Beina Chuanglian Biotechnology Co., Ltd. We purchased nutritious agar medium and PDA medium biochemical reagents (AR) from Mall Beina Chuanglian Biotechnology Co., Ltd. Other biochemical reagents Trans2k DNA Marker, Calf Thymus DNA (CTDNA), pBR322 plasmid DNA, 6X Glycerol Gel Loading Buffer, and GelRed nucleic acid dye were obtained from Sangon Biotech (Shanghai) Co., Ltd. The β-diketonate salt Gd(dbm)₃·2H₂O and the polydentate diacyl hydrazone organic ligand H₄L were produced using the procedure described in the literature^[35-37].

  Physical measurements, biological activity test, and interaction between complexes 1-2 and DNA can be found in the Supporting information.

  1.2   Synthesis of ligand H₄L and complexes 1 and 2

  1.2.1   Synthesis of organic ligand H₄L

  The detailed synthesis pathway for diacyl hydrazone organic ligand H₄L is shown in Scheme 2. The salicyl hydrazide (20 mmol, 3.04 g) was dissolved in ethanol (50 mL), and soon afterward, 1, 10-phenan-throline-2, 9-dicarbaldehyde (20 mmol, 4.72 g) was slowly added to the above solution. The mixed solution was refluxed at 80 ℃ for about 8 h, and a red solid (H₄L) with a yield of 80% (based on salicyl hydrazide) was produced after cooling and filtering. Anal. Calcd. for C₂₈H₂₀N₆O₄(%): C, 66.66; H, 4.00; N, 16.66. Found(%): C, 66.63; H, 3.98; N, 16.68. IR (cm^-1): 3 699(m), 3 230(w), 3 110(w), 3 054(w), 2 391(w), 2 368(w), 1 649(m), 1 626(m), 1 603(m), 1 580(m), 1 547(m), 1 510(s), 1 491(s), 1 463(w), 1 412(s), 1 357(s), 1 329(m), 1 306(s), 1 269(s), 1 222(m), 1 191(s), 1 148(s), 1 014(s), 894(s), 856(s), 838(w), 800(w), 754(s), 708(m), 661(w), 574(w), 531(m) (Fig.S1, Supporting information).

  Scheme 2

  

  Scheme 2.  Synthesis of ligand H₄L

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  1.2.2   Synthesis of complexes 1 and 2

  Gd(NO₃)₃·6H₂O (0.05 mmol), H₄L (0.025 mmol), and triethylamine (45 μL) were dissolved in the mixed solvents of CH₃OH (6.0 mL) and CH₃CN (2.0 mL). Whereafter, the above mixture was filled into a glass sample vase (15 mL) and sealed, then it was autogenously pressure-heated for 48 h at 80 ℃. After cooling to room temperature, the block-shaped yellow crystals of complex 1 were obtained, and the crystal structure of 1 was measured using the single-crystal X-ray diffractometer. Yield: 47% (based on Gd(NO₃)₃·6H₂O). Anal. Calcd. for C₆₀H₄₅Gd₂N₁₃O₁₀(%): C, 50.61; H, 3.16; N, 12.79. Found(%): C, 50.60; H, 3.17; N, 12.80. IR (cm^-1): 3 411(w), 3 049(w), 2 391(w), 2 368(w), 1 598(m), 1 570(m), 1 557(m), 1 506(s), 1 478(w), 1 459(w), 1 399(s), 1 338(w), 1 288(s), 1 260(s), 1 204(s), 1 143(s), 1 074(s), 1 046(s), 1 009(m), 935(m), 889(m), 856(s), 792(s), 759(s), 736(w), 689(w), 754(s), 625(m), 587(m), 531(m)(Fig.S1).

  Using a similar synthetic method, hereon, we used the β-diketonate salt Gd(dbm)₃·2H₂O instead of Gd(NO₃)₃·6H₂O, the yellow crystals of 2 were obtained. Yield: 43% (based on Gd(dbm)₃·2H₂O). Anal. Calcd. for C₉₈H₇₈Gd₂N₁₈O₁₂(%): C, 58.38; H, 3.87; N, 12.51. Found(%): C, 58.38; H, 3.88; N, 12.49. IR (cm^-1): 3 425(w), 2 364(w), 1 594(s), 1 570(s), 1 533(m), 1 519(m), 1 483(w), 1 380(s), 1 315(w), 1 274(w), 1 158(s), 1 116(m), 1 069(m), 1 005(s), 945(m), 894(s), 856(s), 787(w), 759(w), 531(m) (Fig.S1).

  1.3   Crystallographic analysis

  The solid single crystal samples of complexes 1 and 2 with regular shape and moderate size were selected, and the solid single crystal samples with sizes of 0.39 mm×0.36 mm×0.31 mm (1) and 0.37 mm×0.26 mm×0.13 mm (2) were tested on CCD single crystal diffractometer and the crystal diffraction data were collected. The single crystals of complexes 1 and 2 were tested using Cu Kα ray (λ=0.154 18 nm) as the incident light source, and the diffraction points were collected by φ-ω scanning at 150.0 K. The structures of 1 and 2 were solved by direct methods and refined with a full-matrix least-squares technique based on F² using the SHELXL programs^[38]. All non-hydrogen atoms were assigned isotropic thermal characteristics. Table 1 provides the crystallographic information and structural refinement parameters for 1 and 2. Table S1 and S2 present a selection of bond lengths and angles.

  Table 1

  

  Table 1.  Crystallographic data and structure refinements for complexes 1 and 2

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  Parameter	1	2
  Formula	C60H45Gd2N13O10	C98H78Gd2N18O12
  Formula weight	1 422.59	2 014.28
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	1.241 24(6)	1.263 77(7)
  b / nm	1.511 55(8)	1.303 94(7)
  c / nm	1.556 43(10)	1.471 01(7)
  α / (°)	90.865(4)	69.736(3)
  β / (°)	100.747(4)	68.547(4)
  γ / (°)	111.792(3)	83.655(4)
  V / nm3	2.652 5(3)	2.116 1(2)
  Z	2	1
  Dc / (g·cm-3)	1.781	1.581
  μ / mm-1	16.642	10.662
  Rint	0.111 5	0.080 1
  Limiting indices	-15 ≤ h ≤ 13, 18 ≤ k ≤ 18, -18 ≤ l ≤ 18	-15 ≤ h ≤ 15, -16 ≤ k ≤ 16, -18 ≤ l ≤ 16
  Reflection collected	22 156	20 418
  Reflection used	1 931	3 158
  Number of parameters	771	590
  GOF on F2	1.005	1.004
  R1, wR2 [I > 2σ(I)]	0.066 2, 0.144 9	0.064 6, 0.160 7
  R1, wR2 (all data)	0.128 6, 0.172 1	0.081 8, 0.171 6

  2.   Results and discussion

  2.1   Crystal structures of complexes 1 and 2

  Both complexes 1 and 2 crystallize in triclinic space group P1, however, they possess completely different molecular structures. As shown in Fig. 1, complex 1 consists of two nine-coordinated Gd(Ⅲ) ions, one deprotonated multidentate diacylhydrazone L^4-, and one partially deprotonated multidentate diacylhydrazone H₂L^2-. Complex 1 accommodates two Gd(Ⅲ) ions in a normal coordination manner, but in the synthesis, there are enough ligand raw materials, so that after the first ligand chelates the ion, all other coordination numbers of the ion are supplemented by the second ligand, and the planes formed by the two ligands are perpendicular to each other. Careful analysis reveals that four nitrogen atoms (N8, N9, N10, and N11) and five oxygen atoms (O1, O2, O4, O6, and O7) are coordinated to the Gd1 ion; among these coordinated atoms, N8, N9, N10, N11, O6 and O7 are from the L^4- ligand, and O1, O2 as well as O4 come from H₂L^2- ligand. While another four nitrogen atoms (N2, N3, N4, and N5) and one oxygen atom (O2) which come from the H₂L^2- ligand, and four oxygen atoms (O5, O6, O7, and O8) which are from the L^4- ligand are connected to the Gd2 ion (Fig. 2a). Both the two Gd(Ⅲ) ions show a distorted hula-hoop configuration (Fig. 2b), it also can be calculated by the SHAPE 2.0 program (Table S3)^[39]. In addition, the coordination modes of L^4- and H₂L^2- are shown in Fig.S2, and they adopt two different multidentate mastiff models to pocket two Gd(Ⅲ) ions. The Gd1 and Gd2 ions are bridged by three μ₂-O atoms forming a trigonal bipyramid-shaped [Gd₂O₃] core (Fig.S3). The distance of Gd1…Gd2 in 1 is 0.399 84(9) nm. Furthermore, Gd—O and Gd—N bond lengths in complex 1 are in the ranges of 0.221 1(7)-0.253 9(6) nm and 0.260 3(8)-0.275 7(9) nm, respectively. The range of bond angles for O/N—Gd—O/N is 56.6(3)°-178.5 (3)°. Compared with recently reported Ln₂ complexes^[40-43], these bond lengths and angles in complex 1 fall in the normal range.

  Figure 1

  

  Figure 1.  (a) Molecules structure of complex 1; (b) Molecules structure of complex 2

  All hydrogen atoms and free CH₃OH, CH₃CN molecular are omitted for clarity; Symmetry code: a: 1-x, -y, 1-z.

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  Figure 2

  

  Figure 2.  Coordination environments of Gd(Ⅲ) ions in complexes 1 (a) and 2 (c); Geometric polyhedron of Gd₂ ions observed in 1 (b) and 2 (d)

  Symmetry code: a: 1-x, -y, 1-z.

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  Complex 2 shows a different structure compared with complex 1 (Fig. 1b). It mostly comprises two H₂L^2- ligands, two dbm^- ligands, and two Gd(Ⅲ) ions. Although complex 2 also has two main ligands, it is only coordinated by the six-toothed chelating environment, and the four-toothed environment was not coordinated in a normal form, but by rotating the benzene ring and using a phenol-oxygen to bridge the other Gd(Ⅲ) ion. In addition, the two centrosymmetric Gd1 and Gd1a ions in 2 are also nine-coordinated and have a [N₄O₅] coordination environment around the Gd(Ⅲ) ions (Fig. 2c). The polyhedron of the Gd1 ion in 2 shows a deformed muffin geometrical configuration (Fig. 2d). It is different from that in 1, which also can be calculated by the SHAPE 2.0 software (Table S3). As shown in Fig. 3, the coordination mode of H₂L^2- in 2 is also different from that in complex 1. It adopts a multidentate mastiff model to pocket only one Gd(Ⅲ) ion in 1, while it pockets two Gd(Ⅲ) ions in 2. The dbm^- and H₂L^2- connect the Gd(Ⅲ) ions by using a bidentate mastiff model. The whole molecular structure of 2 resembles a pinwheel. The distance of two Gd(Ⅲ) ions in 2 is 0.682 896 nm, which is much larger than that in 1. The Gd—O/N bond lengths are between 0.233 2(4) and 0.267 3(5) nm. The range of bond angles for O/N—Gd—O/N is 60.20(15)°-167.40(16)°. These bond lengths and angles of 2 also compare to those of 1 and reported Gd₂ complexes based on Schiff-base organic ligand^[44-47].

  Figure 3

  

  Figure 3.  Coordination modes for H₂L^2- (a) and dbm^- (b) in 2

  Symmetry code: a: 1-x, -y, 1-z.

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  2.2   Powder X-ray diffraction

  The powder X-ray diffraction analysis of complexes 1 and 2 has been carried out at room temperature. With the use of the CIF files of complexes 1 and 2 and the Mercury program, the simulated patterns of 1 and 2 can be obtained. As shown in Fig.S4, the positions of the experimental peaks followed the simulated ones, which indicates a high phase purity of the crystal samples.

  2.3   Magnetic properties of complexes 1 and 2

  The DC magnetic susceptibility measurements for the two binuclear Gd(Ⅲ)-based complexes 1 and 2 have been studied in the temperature interval 300 to 2 K under 0.1 T DC field (Fig. 4). The χ_MT values were 15.79 cm³·mol^-1·K for 1 and 15.74 cm³·mol^-1·K for 2 at 300 K, respectively; they were close to the theoretical value 15.76 cm³·mol^-1·K for two isolated Gd(Ⅲ) ions (⁸F_7/2, g=2). The χ_MT values of 1 and 2 almost remained unchanged with declining temperature (300-20 K), therewith, they dropped rapidly to the minimum values of 13.10 cm³·mol^-1·K (1) and 13.97 cm³·mol^-1·K (2) at 2 K. The downtrend illustrates that the antiferromagnetic interaction exists for the adjacent Gd(Ⅲ) ions in 1 and 2^[48]. To further deeply understand the magnetic interaction in complexes 1 and 2, the magnetic curves (χ_MT vs T) of 1 and 2 have been analyzed through the derivation Eq.1 based on the isotropic spin Hamiltonian equation: Ĥ=-2JŜ₁·Ŝ₂. Hereon, J is the exchange coupling parameter, and Ŝ₁=Ŝ₂=7/2, which are the spin operators of the local spins^[49-51]. Eq.1 is shown as follows^[52-53]:

  $ \chi_{\mathrm{M}} T=\left[2 N g^2 \mu_{\mathrm{B}}^2 /(k T)\right]\left[\left(140+91 \mathrm{e}^{7 x}+55 \mathrm{e}^{13 x}+30 \mathrm{e}^{18 x}+14 \mathrm{e}^{22 x}+5 \mathrm{e}^{25 x}+\mathrm{e}^{27 x}\right) /\left(15+13 \mathrm{e}^{7 x}+11 \mathrm{e}^{13 x}+7 \mathrm{e}^{18 x}+5 \mathrm{e}^{22 x}+3 \mathrm{e}^{25 x}+\mathrm{e}^{28 x}\right)\right] $

  (1)

  Figure 4

  

  Figure 4.  χ_MT vs T plots in a temperature range of 2 to 300 K at 0.1 T for complexes 1 (a) and 2 (b)

  The red solid lines are the best fit for the experimental data.

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  where k is the Boltzmann constant, N is Avogadro′s number, g is the Landé constant, and μ_B is Bohr magneton, and x=-J/(kT). The two key parameters [J=-0.089 cm^-1 (1) and-0.047 cm^-1 (2); g=1.98 (1) and 2.01 (2)] have been obtained. The negative and low J values for 1 and 2 imply that there are weak antiferromagnetic interactions between the adjacent Gd(Ⅲ) centers. The |J| value of 1 was larger than that of 2, suggesting the antiferromagnetic interactions in 1 is stronger than that in 2. In addition, we also used the Curie-Weiss law (χ_M^-1=T/C-θ/C) to fit the χ_M^-1 vs T plots of the two Gd₂ complexes 1 and 2 (Fig.S5), and the two important parameters (C=15.91 and 15.79 cm³·mol^-1·K for 1 and 2; θ=-0.86 and -0.24 K for 1 and 2) were obtained. The negative θ values further prove that there is an antiferromagnetic exchange between the adjacent Gd(Ⅲ) ions of 1 and 2^[54].

  Under various temperatures (T=2.0-10.0 K), the magnetization (M) of complexes 1 and 2 was measured in the magnetic field (H) interval of 0-7.0 T (Fig.S6). The M values displayed a steady increase and reached complete saturation of 14.06Nβ and 13.99Nβ for 1 and 2 at T=2.0 K and ΔH=7.0 T, respectively; they were in agree with the theoretical value (14Nβ) for two free Gd(Ⅲ) ions (g=2, S=7/2). The magnetic entropy change (-ΔS_m) of the two Gd₂ complexes was obtained following the Maxwell equation: ΔS_m(T)=$\int[\partial M(T, H) / \partial T]_H \mathrm{~d} H$^[55]. The -ΔS_m vs T plots of 1 and 2 are shown in Fig. 5. When T=2.0 K and ΔH=70 kOe, the maximal -ΔS_m values were 23.35 and 15.09 J·kg^-1·K^-1 for 1 and 2, respectively. Based on the equation -ΔS_m=nRln(2S_Gd(Ⅲ)+1) (S_Gd(Ⅲ)=7/2, and R=8.314 5 J·K^-1·mol^-1), the theoretical values of 1 and 2 were 24.31 and 17.17 J·kg^-1·K^-1, respectively. The significant divergence between the theoretical and experimental values mainly originates from the antiferromagnetic interaction in 1 and 2^[56]. The maximum -ΔS_m value of 1 was much larger than that of 2, which could be ascribed to the smaller M_w/N_Gd ratio (711.30) in 1. Compared with the reported Gd₂ complex by Cheng Peng′s group^[57], complexes 1 and 2 have comparatively smaller -ΔS_m values.

  Figure 5

  

  Figure 5.  -ΔS_m vs T plots for complexes 1 (a) and 2 (b)

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  2.4   Antibacterial activities

  Using the plate method, we assessed the inhibitory effects of complexes 1 and 2, organic ligand H₄L, Gd(NO₃)₃·6H₂O, and Gd(dbm)₃·2H₂O on the growth of E. tucicum and B. sorokiniana. Fig. 6 displays the inhibitory rates of 1, 2, H₄L, Gd(NO₃)₃·6H₂O, and Gd(dbm)₃·2H₂O against E. tucicum and B. sorokiniana on the fifth day (Table S4 and S5). These results demonstrated that the two Gd₂ complexes 1 and 2 exhibited better antifungal activity against E. tucicum and B. sorokiniana.

  Figure 6

  

  Figure 6.  Antibacterial activity against E. tucicum (a) and B. sorokiniana (b) by using Gd₂ complexes 1 and 2, rare-earth salts (Gd(NO₃)₃·6H₂O and Gd(dbm)₃·2H₂O) and organic ligand H₄L on the fifth day at corresponding concentrations

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  The antibacterial activities of complexes 1 and 2 have been further assessed using the zone of inhibition test and the minimum inhibitory concentration test, which can refer to the methodology in the previously reported literature^[58-59]. As shown in Fig.S7, DMF had almost no bacteriostatic influence on P. syringae (a common gram-negative bacteria) with a bacteriostatic zone of less than 7.0 mm. Complexes 1 and 2, organic ligand H₄L, and Gd(dbm)₃·2H₂O exhibited stronger antibacterial action against P. syringae. It′s worth noting that 1 and 2 showed greater antibacterial activities compared with the rare-earth salts Gd(NO₃)₃·6H₂O, Gd(dbm)₃·2H₂O, and organic ligand H₄L.

  2.5   Interaction between the two Gd₂ complexes with DNA

  2.5.1   Cyclic voltammetry analysis

  By comparing the electrochemical properties of complexes 1 and 2 before and after the introduction of DNA, it is possible to determine the interaction between complexes and DNA. The diffusion coefficient of the complexes decreased when they were inserted into DNA, which caused a drop in the peak current and a forward shift in their cyclic voltammetric curve potential. When the complexes were electrostatically linked with the negatively charged phosphate groups in DNA, the cyclic voltammetric curve potential of the complexes changed negatively^[60-61]. As shown in Fig.S8, after 1 and 2 were bound to DNA, the redox current of the complex decreased, and the front potential moved, which reflects the insertion binding between the complex and DNA.

  2.5.2   Gel electrophoresis analysis

  When Ln(Ⅲ)-based complexes contact with superhelical DNA, the covalently DNA supercoiling will unwind, and the DNA molecules will split into several smaller molecules. Hence, gel electrophoresis analysis has been used to evaluate the interaction of complexes with DNA and analyze the changes in the molecular structure of DNA. We selected the two Gd₂ complexes 1 and 2 as examples to examine their interactions with pBR322DNA. As shown in Fig.S9, the brightness of the superhelical DNA bands was darker than that of blank pBR322DNA when different concentrations of complex 1 were added to pBR322DNA, indicating that the superhelical DNA is cut into several DNA bands with lower molecule weight. As a result, complex 1 has a strong interaction with DNA and a severe cleavage effect on DNA.

  2.5.3   Fluorescence spectrometry analysis

  GelRed molecules can emit fluorescence after inserting to DNA. Complexes would compete for the binding location between DNA and GelRed if they also preferred to insert into DNA. Consequently, the fluorescence of the GelRed-DNA combination was muted. The strength of these complexes′ insertion with DNA can be inferred from the degree of fluorescence quenching of the GelRed-DNA. This method is frequently employed to examine the manner of interaction between complexes and DNA. Moreover, according to the Stern-Volmer equation: I₀/I=1+K_sqr^[62], the fluorescence quenching constant (K_sq) of the GelRed-DNA system can be calculated, which can further indicate the interaction intensity between the complexes and DNA. As shown in Fig. 7, when complexes 1 and 2 were added, it could be observed that the fluorescence intensity of GelRed-DNA decreased gradually with the increase of the ratio of complexes to GelRed-DNA. The result implied that complexes 1 and 2 are inserted into the DNA. The K_sq values of complexes 1 and 2, and H₄L were 0.83, 0.88, and 0.74, respectively, which could be fitted by the Stern-Volmer equation. It indicates that complexes 1 and 2 possess greater interaction with DNA than organic ligand H₄L.

  Figure 7

  

  Figure 7.  Fluorescence quenching of GelRed-DNA complex system by two Gd₂ complexes 1, 2 and organic ligand H₄L

  Inset: plot of I₀/I vs r, where r=c_complex/c_DNA; From ⅰ to ⅸ: r=0.06, 0.13, 0.27, 0.41, 0.54, 0.76, 0.92, 1.26, respectively.

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  3.   Conclusions

  In summary, two novel Gd₂ complexes 1 and 2 have been designed and synthesized in this work by using a large conjugated diacylhydrazone organic ligand (H₄L) via a solvothermal method. The single crystal structures, magnetocaloric effect, and biological activities of complexes 1 and 2 were systematically studied. The two Gd₂ complexes 1 and 2 display different molecule structures and different magnetic refrigeration properties. In addition, bacteriostatic activities study suggests that the two Gd₂ complexes have greater antibacterial activity than two Gd(Ⅲ) salts (Gd(NO₃)₃·6H₂O and Gd(dbm)₃·2H₂O) and H₄L. Moreover, the interaction between the Gd₂ complexes and calf thymus DNA was also studied. Our work reports a new way to design multifunctional Ln(Ⅲ)-based complexes by using a large conjugated diacyl hydrazone organic ligand. It also reveals the potential application of Ln(Ⅲ)-based complexes in magnetism and biological activity. Research on other functional polynuclear Ln(Ⅲ)-based complexes is ongoing in our laboratory.

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

  Scheme 1  Molecular structure of the polydentate diacylhydrazone organic ligand (H₄L)

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  Scheme 2  Synthesis of ligand H₄L

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  Figure 1  (a) Molecules structure of complex 1; (b) Molecules structure of complex 2

  All hydrogen atoms and free CH₃OH, CH₃CN molecular are omitted for clarity; Symmetry code: a: 1-x, -y, 1-z.

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  Figure 2  Coordination environments of Gd(Ⅲ) ions in complexes 1 (a) and 2 (c); Geometric polyhedron of Gd₂ ions observed in 1 (b) and 2 (d)

  Symmetry code: a: 1-x, -y, 1-z.

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  Figure 3  Coordination modes for H₂L^2- (a) and dbm^- (b) in 2

  Symmetry code: a: 1-x, -y, 1-z.

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  Figure 4  χ_MT vs T plots in a temperature range of 2 to 300 K at 0.1 T for complexes 1 (a) and 2 (b)

  The red solid lines are the best fit for the experimental data.

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  Figure 5  -ΔS_m vs T plots for complexes 1 (a) and 2 (b)

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  Figure 6  Antibacterial activity against E. tucicum (a) and B. sorokiniana (b) by using Gd₂ complexes 1 and 2, rare-earth salts (Gd(NO₃)₃·6H₂O and Gd(dbm)₃·2H₂O) and organic ligand H₄L on the fifth day at corresponding concentrations

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  Figure 7  Fluorescence quenching of GelRed-DNA complex system by two Gd₂ complexes 1, 2 and organic ligand H₄L

  Inset: plot of I₀/I vs r, where r=c_complex/c_DNA; From ⅰ to ⅸ: r=0.06, 0.13, 0.27, 0.41, 0.54, 0.76, 0.92, 1.26, respectively.

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  Table 1.  Crystallographic data and structure refinements for complexes 1 and 2

  Parameter	1	2
  Formula	C60H45Gd2N13O10	C98H78Gd2N18O12
  Formula weight	1 422.59	2 014.28
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	1.241 24(6)	1.263 77(7)
  b / nm	1.511 55(8)	1.303 94(7)
  c / nm	1.556 43(10)	1.471 01(7)
  α / (°)	90.865(4)	69.736(3)
  β / (°)	100.747(4)	68.547(4)
  γ / (°)	111.792(3)	83.655(4)
  V / nm3	2.652 5(3)	2.116 1(2)
  Z	2	1
  Dc / (g·cm-3)	1.781	1.581
  μ / mm-1	16.642	10.662
  Rint	0.111 5	0.080 1
  Limiting indices	-15 ≤ h ≤ 13, 18 ≤ k ≤ 18, -18 ≤ l ≤ 18	-15 ≤ h ≤ 15, -16 ≤ k ≤ 16, -18 ≤ l ≤ 16
  Reflection collected	22 156	20 418
  Reflection used	1 931	3 158
  Number of parameters	771	590
  GOF on F2	1.005	1.004
  R1, wR2 [I > 2σ(I)]	0.066 2, 0.144 9	0.064 6, 0.160 7
  R1, wR2 (all data)	0.128 6, 0.172 1	0.081 8, 0.171 6

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