English

• In the past two decades, the design and construction of polynuclear Ln(Ⅲ)-based clusters have aroused great attention from inorganic chemists and materials scientists due to their interesting structures and functional properties^[1-2]. Recently, polynuclear Ln(Ⅲ)-based clusters possess potential applications, such as single-molecule magnets (SMMs), magnetic refrigerants, catalysis, and fluorescence probe, and attract increasing attention from inter-discipline^[3-6]. For magnetic studies, due to their potential for applications in information storage, the Ln(Ⅲ)-based clusters displaying SMMs behaviors are of particular interest^[7]. With the development of crystallography and magnetic chemistry, lots of Ln(Ⅲ)-based SMMs have been reported^[8-9]. Tang and Tong′s groups have done well work in the Ln(Ⅲ)-based SMM field^[10-11]; therewith, Cui′s group also did lots of work in the design and construction of Ln(Ⅲ)-based SMMs^[12-16]. To a certain extent, these studies promote the rapid development of polynuclear Ln(Ⅲ)-based clusters in the SMMs field.

  For catalytic properties, polynuclear lanthanide clusters used as catalysts for cycloaddition reaction of CO₂ and epoxides have attracted great interest^[17-18]. The design and construction of catalysts showing high selectivity and high catalytic activity under mild conditions to lower the costs of production and energy consumption are significantly pressing^[19]. Up to now, a lot of catalyst systems have been explored to catalyze these reactions. For example, alkali metal salts^[20-21], organocatalysts^[22-23], ionic liquids^[24-25], and transition-metal complexes^[26-27]. However, some limitations need more attention, such as even at high CO₂ pressure and high-temperature conditions^[28-30]. Recently, some polynuclear lanthanide clusters or Ln(Ⅲ)-based MOFs showing high catalytic activity for the cycloaddition reaction of CO₂ with various epoxides have been reported^[31]. In 2017, Zhao′s group synthesized two multifunctional MOFs with Tb₄ clusters as nodes for the chemical fixation of CO₂^[32]. The result shows that two MOFs can effectively catalyze the cycloaddition reaction of CO₂ and epoxides under mild conditions. In particular, they can be reused at least three times. In 2020, Liu′s group constructed a series of asymmetry 3d-4f helicate clusters of Zn₃LnL₄ which shows high catalytic activity in transforming CO₂ into cyclic carbonates with turnover frequency (TOF) up to 38 000 h^-1 under mild conditions and significant selectivity of 99%^[33]. The research findings investigate that the high catalytic activity of these catalysts can be attributed to several reasons: (1) controllable structures of clusters would modulate metal atoms located at suitable positions facilitating efficient bonding of substrates and catalysts; (2) the weak binding ability of coordinated solvent molecules provides potential coordinated sites allowing reversible binding of epoxides; (3) the strong bonding of ligands and lanthanide ions provides a stable catalytic platform for recyclability.

  Based on the above studies, in this work, a novel Ho₄ complex [Ho₄(NO₃)₂(acac)₄(L)₂(CH₃OH)₂]·2CH₃CN (1), where H₄L= (E)-2-(hydroxymethyl)-2-(((2-hydroxynaphthalen-yl)methylene)amino)propane-1,3-diol and acac=acetylacetone, is designed and synthesized. The X-ray diffraction analysis reveals that complex 1 shows a central symmetric tetranuclear defect di-cubane topology. The magnetic study reveals that complex 1 exhibits slow magnetic relaxation behavior. The Ho(Ⅲ) ions in this complex serve as Lewis-acid sites to facilitate catalytic reactivity. The catalytic properties of complex 1 have been systematically investigated, and complex 1 showed excellent catalytic activity for CO₂ conversion with epoxides under mild conditions.

  1.   Experimental

  1.1   Materials and measurements

  2-Hydroxy-1-naphthaldehyde, ammonium hydroxide, acetylacetone, tris(hydroxymethyl)methyl aminomethane, HNO₃ and Ho(NO₃)₃·6H₂O were bought at Energy Chemical Co., Ltd., the common solvents (CH₃OH, C₂H₅OH, and CH₃CN) were purchased from Komeo Reagent Co., Ltd. Ho(acac)₃·2H₂O was prepared according to the method reported in the literature^[34].

  Elemental analysis for complex 1 was performed on a Perkin-Elmer 2400 analyzer. IR data for complex 1 were measured with a Bruker TENOR 27 spectrophotometer. Powder X-ray diffraction (PXRD) data were collected using a Rigaku Ultima Ⅳ instrument with Cu Kα radiation (λ=0.154 056 nm) in a 2θ range from 5° to 50°. The operating voltage and current were 40 kV and 25 mA respectively. Thermogravimetric (TG) analysis data were recorded using a NETZSCHTG 409 PC thermal analyzer. The UV-Vis absorbance spectra of Schiff-base ligand (H₄L) and complex 1 were measured on a TU-1950 UV-Vis spectrophotometer. Inductively coupled plasma (ICP) measurements were performed on an ICP-9000(N+M). The dc (direct current) and ac (alternating current) magnetic properties data of complex 1 were performed on polycrystalline samples using a Quantum Design MPMS (SQUID)-XL magnetometer and PPMS-9T system. ¹H NMR spectra were performed on a 400 MHz Bruker 400 spectrometer in CDCl₃.

  1.2   Synthesis of polydentate Schiff base ligand (H₄L) and complex 1

  Polydentate Schiff base ligand (H₄L): At room temperature, 2-hydroxy-1-naphthaldehyde (1.721 8 g, 10 mmol) and tris(hydroxymethyl)methyl aminomethane (1.211 4 g, 10 mmol) were dissolved in methanol (10 mL) and stirred for 4 h. Then the suspension liquid was vacuum filtrated. The yellow solid was obtained after drying under a vacuum for 24 h. The yield of the H₄L ligand was 63%. The synthesis method is shown in Scheme 1. Elemental analysis Calcd. for C₁₅H₁₇NO₄ (%): C: 65.38; H: 6.18; N: 5.09; Found(%): C: 65.42; H: 6.22; N: 5.08. IR (KBr, cm^-1): 3 325(s), 2 927(m), 2 881(m), 1 636(s), 1 543(s), 1 519(m), 1 490(m), 1 398 (w), 1 359(s), 1 299(w), 1 215(m), 1 041(s), 835(m), 750 (m), 630(w) (Fig.S1, Supporting information).

  Scheme 1

  

  Scheme 1.  Synthesis route of the H₄L ligand

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  Complex 1: Ho(acac)₃·2H₂O (0.1 mmol, 0.050 1 g), H₄L (0.05 mmol, 0.013 8 g) and 100 µL HNO₃ were added to a mixed solution of acetonitrile (5 mL) and methanol (5 mL) and stirred for 30 min. Then the suspension liquid was filtered and sealed in a 20 mL glass vial. After being heated at 70 ℃ for 24 h, yellow block-shaped crystals were obtained with the temperature slowly cooled down to room temperature. Yield: 32% based on Ho(acac)₃·2H₂O. Elemental analysis Calcd. for C₅₆H₇₀Ho₄N₆O₂₄(%): C: 35.92; H: 3.74; N: 4.49; Found(%): C: 35.89; H: 3.77; N: 4.44. IR (KBr, cm^-1): 3 452(s), 3 060(m) 2 877(m), 1 600(s), 1 523(s), 1 450(m), 1 376(s), 1 336(m), 1 286(m), 1 188(w), 1 143 (w), 1 018(s), 925(m), 831(m), 750(m), 657(m), 580(w) (Fig.S1).

  1.3   Cycloaddition experiment of CO₂ and epoxides

  Complex 1 (x=0.5%), substrate (2 mmol), tetrabutylammonium bromide (TBAB, x=3%), and abundant CO₂ were sealed in a 50 mL Schlenk tube, stirred for 12 h at different temperatures. After reacting completely, CH₂Cl₂ (2 mL) was added to dissolve the mixture, and the products were purified by column chromatography. The yield was characterized by ¹H NMR spectroscopy with TMS as the internal standard.

  1.4   X-ray crystallography

  A suitable single crystal was selected for the X-ray diffraction analysis. The data were collected through a Bruker Apex Ⅱ CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.071 073 nm) at 150 K by using the φ-ω scan technique. The structure for 1 was directly solved using SHELXL and Olex 2 programs. All non-hydrogen atoms were refined by full-matrix least-squares methods on F². Due to the disorder of free molecules, the SQUEEZE procedure in the PLATON software was used for 1. Crystallographic parameters of complex 1 are given in Table 1. The selected bond lengths and bond angles are listed in Table S1.

  Table 1

  

  Table 1.  Crystallographic data and structure refinements for complex 1

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  Parameter	1
  Formula	C56H70Ho4N6O24
  Formula weight	1 870.90
  T / K	150.0
  Cryst system	Triclinic
  Space group	P1
  a / nm	1.048 33(5)
  b / nm	1.227 66(6)
  c / nm	1.374 76(5)
  α/(°)	73.936 7(16)
  β/(°)	81.906 9(17)
  γ/(°)	70.314 4(19)
  V / nm3	1.598 64(13)
  Z	1
  Cryst size / mm	0.35×0.21×0.16
  Dc / (g·cm-3)	1.943
  μ / mm-1	4.978
  Limiting indices	-13 ≤ h ≤ 13,
  	-15 ≤ k ≤ 15,
  	-17 ≤ l ≤ 17
  Reflection collected	43 379
  Number of parameters	416
  Rint	0.053 8
  GOF on F2	1.031
  R1, wR2 [I > 2σ(I)]	0.027 9, 0.067 4
  R1, wR2 (all data)	0.033 7, 0.071 3

  2.   Results and discussion

  2.1   Crystal structure of complex 1

  X-ray single-crystal analysis indicates that complex 1 shows a central symmetric tetranuclear structure crystallizing in the triclinic system space and P1 space group with Z=1. As shown in Fig. 1a, the asymmetric unit mainly consists of four Ho(Ⅲ) ions, two L^4- ions, four acac^- ions, two NO₃^- ions, and two methanol molecules. As depicted in Fig. 1c, the central Ho1(Ⅲ) is surounded by seven oxygen atoms and one nitrogen atom, including O7 and O8 from acac^-, O1, O3, O2a, O3a, and N1 originating from L^4- and O12 from a methanol molecule. The Ho2(Ⅲ) is eight-coordinated with O1, O2, and O3a from L^4-, O5, O6, and O8a from acac^-, O9 and O10 from NO₃^-. Located in the same plane, four Ho(Ⅲ) ions are connected by O1, O1a, O2, and O2a from two L^4- ions, O8 and O8a from acac^-. Two µ₃-O of O3 and O3a are located on both side of the Ho₄ plane, constructing a defect di-cubane topology (Fig. 1b). The coordination mode of L^4- and acac^- are shown in Fig. 2, which show the ways of multidentate chelated coordination modes. The distances of Ho1…Ho2, Ho1…Ho2a, and Ho1…Ho1a are 0.373 05(3), 0.354 11(3), and 0. 371 08(4) nm, respectively. The bond lengths of Ho—O and Ho—N are in a range of 0.223 6(3)-0.254 0(3) nm and 0.246 6(4)-0.289 3(4) nm, respectively. The coordination polyhedrons of Ho1(Ⅲ) and Ho2(Ⅲ) in 1 are shown in Fig. 3. By the SHAPE 2.0 software, the geo-metrical configurations are obtained precisely. As shown in Table S2, both Ho1(Ⅲ) and Ho2(Ⅲ) display triangular dodecahedron (D_2d) coordination geometry. In addition, the strong Lewis acid of Ho(Ⅲ) serves as a potential catalytic site that facilitated the activation of epoxy substrates. Therefore, complex 1 can be used as a catalyst for converting CO₂ into cyclic carbonates.

  Figure 1

  

  Figure 1.  (a) Molecular structure for complex 1; (b) A defect di-cubane topology for Ho₄ structure; (c) Coordination environment of Ho(Ⅲ) in complex 1

  Hydrogen atoms are omitted for clarity; Symmetry code: a: -x+1, -y, -z+1

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  Figure 2

  

  Figure 2.  (a) Coordination mode of L^4- in complex 1; (b) Coordination modes of acac^- in complex 1

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  Figure 3

  

  Figure 3.  Coordination polyhedrons for Ho1(Ⅲ) and Ho2(Ⅲ) ions in 1

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  2.2   PXRD and TG analyses

  PXRD was carried out for complex 1 at room temperature. As shown in Fig. S2, the diffraction data agreed with the theoretical values calculated by single crystal structure simulation, indicating the high phase purity. In addition, the solvent stability of complex 1 was investigated (Fig. 4). The crystal samples were immersed in common organic solvents (H₂O, methanol, ethanol, dichloromethane, acetonitrile, DMF, and DMA) for about 10 h at room temperature and then dried naturally. The PXRD results showed that complex 1 possesses excellent solvent stability.

  Figure 4

  

  Figure 4.  Simulated and experimental PXRD patterns of complex 1 after being immersed in different solvents for 10 h

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  The TG analysis for complex 1 was conducted to verify the thermal stability (Fig.S3). The weight loss of 7.98% (Calcd. 7.92%) in a range of 30-230 ℃ is attributed to the loss of free acetonitrile and coordinated methanol molecule. Then, the weight descended rapidly with the increase in temperature due to the collapse of the crystal structure. Complex 1 could maintain structural stability before 230 ℃ at least.

  2.3   UV-Vis spectra

  At room temperature, the UV-Vis spectra for complex 1, Ho(acac)₃·2H₂O, and H₄L were performed (Fig.S4). The absorption peaks of Ho(acac)₃·2H₂O were observed at 204 and 290 nm because of π→ π^* transition of C=O in acac^-. Four absorption peaks for H₄L at 232, 308, 398, and 418 nm result from n→ π^* transition of naphthalene and π → π^* transition of C=N. Complex 1 showed the absorption peaks at 238, 396, and 418 nm resulting from the H₄L ligand and the absorption peaks at 206 and 296 nm attributed to acac^-. Compared with H₄L, the peak at 306 nm disappeared, which may result from the coordination effects among H₄L ligand and Ho(Ⅲ) ions.

  2.4   Magnetic properties

  To investigate the magnetic properties for 1, dc magnetic susceptibility data were collected from 2.0 to 300.0 K under 1 000 Oe applied field. As shown in Fig. 5, the value of χ_MT was 56.25 cm³·mol^-1·K at room temperature, which was consistent with the theoretical value of 56.28 cm³·mol^-1·K for four uncoupled Ho(Ⅲ) (⁵I₈, g=5/4) ion. The value of χ_MT slightly decreased in a range of 300.0 to 50.0 K. Then with the temperature further decreasing, χ_MT rapidly descended and the minimum value was obtained at 2.0 K for 30.92 cm³·mol^-1·K, which can be attributed to the antiferromagnetic coupling between the Ho(Ⅲ) ions in 1 and/or the progressive excited depopulation of stark sublevel^[35-38]. In addition, the magnetic susceptibilities of 1 were fitted by the Curie-Weiss law to estimate the magnetic interactions between Ho(Ⅲ) in 1 (Fig. 5b). The best-fitted parameters were obtained with C=57.44 cm³·mol^-1·K and θ =-4.09 K. The low and negative θ values also demonstrate the existence of weak antiferromagnetic interactions between Ho(Ⅲ) in 1.

  Figure 5

  

  Figure 5.  (a) Temperature dependence of the χ_MT under 1 000 Oe for complex 1 in a temperature range of 300.0-2.0 K; (b) Temperature dependence of χ_M^-1 for complex 1

  Solid line resulted from fitting the Curie-Weiss law

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  The ac magnetic susceptibility was measured in a zero dc field from 11 to 1 910 Hz and 2.0-15.0 K to study the magnetic dynamics of complex 1. As shown in Fig. 6, strong frequency-dependence of both in-phase (χ′) and out-of-phase (χ″) components was observed, which indicates a slow relaxation of the magnetization of SMM behavior occurred in 1^[39-43]. However, no obvious out-of-phase (χ″) peaks were observed until T=2.0 K due to the fast quantum tunneling of the magnetization^[44-46]. To our knowledge, under H_dc=0 Oe filed, complex 1 displaying slow magnetic relaxation behavior is rarely reported in the literature.

  Figure 6

  

  Figure 6.  Temperature dependence of the in-phase (χ′) (a) and out-of-phase (χ″) (b) components for 1 in a zero-dc field with an oscillation of 3.0 Oe

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  2.5   Catalytic activity

  The catalytic performance for the reaction of CO₂ with epoxides was evaluated systematically. Epibromohydrin was chosen as a model substrate to study the catalytic properties. At first, the catalytic activity of complex 1 was explored in solvent-free conditions. Complex 1 (x=0.5%) as a catalyst and TBAB (x=3%) as a co-catalyst were used to catalyze cycloaddition reaction at different temperatures (30-90 ℃) and 100 kPa CO₂ pressure for 12 h. As shown in Table 2 (Entry 1-8), the yield increased with the rise in temperature (20-70 ℃), and complex 1 showed high catalytic activity with a yield of 92% at 70 ℃ (Table 2, Entry 6). However, the yield at 80 and 90 ℃ decreased to some extent (Table 2, Entry 7 and 8), which might be ascribed to the partial decomposition of TBAB at a high temperature. Complex 1 (3%, Table 2, Entry 10) alone and TBAB (3%, Table 2, Entry 9) alone could not effectively catalyze the cycloaddition reaction of CO₂ and epoxybromopropane with desired yields (< 1%, 23% respectively), which indicates that this binary catalytic system of the complex 1 and TBAB presents a synergistic catalytic effect in the reaction. When the equivalent amount of Ho(NO₃)₃·6H₂O or HoCl₃·6H₂O replaced complex 1 as a catalyst under the same conditions (Table 2, Entry 11 and 12), the yields were 5% and 9%, respectively, which illustrates that salt cannot effectively catalyze the reaction and complex 1 can largely improve the yield of cycloaddition reactions.

  Table 2

  

  Table 2.  CO₂ reacting with epibromohydrin under different conditions^a

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  Entry	Catalyst	xTBAB / %	T / ℃	Yield / %b
  1	1	3	20	45
  2	1	3	30	52
  3	1	3	40	65
  4	1	3	50	79
  5	1	3	60	87
  6	1	3	70	92
  7	1	3	80	89
  8	1	3	90	85
  9	—	3	70	23
  10	1	—	70	< 1
  11	Ho(NO3)3·6H2O	3	70	5
  12	HoCl3·6H2O	3	70	9
  a Reaction conditions: epoxybromopropane (273 mg, 2.0 mmol), complex 1 (0.5%), Ho(NO3)3·6H2O (0.5%), HoCl3·6H2O (0.5%), TBAB (3%), CO2 (100 kPa), 12 h, solvent-free; b Determined by 1H NMR spectroscopy, 1,3,5-trimethoxybenzene as an internal standard.

  To study the wider applicability of this catalyst system, a series of terminal epoxides were tested under optimal conditions. With the increase of substituted chain sizes, a descent in the yield was observed (Table 3, Entry 1-4, 7-9). On the other hand, epichlorohydrin and glycidol exhibited high activity (91% and 92%) originating from the electron-withdrawing of the chlo-rine atom and oxygen atom, which make the carbon atom of epoxy substrate exhibiting further positively charged to accelerate Br^- nucleophilic attacking to ring opening. Moreover, the cycloaddition of CO₂ to isobutyl-ene oxide or tert-butyl glycidyl ether (Table 3, Entry 2 and 9) had a slightly lower yield (73% and 74%) owing to the presence of steric hindrance. These results show that complex 1 can catalyze the cycloaddition reaction with relatively extensive epoxides.

  Table 3

  

  Table 3.  Various carbonates from CO₂ reacted with different epoxides catalyzed by complex 1 under optimized conditions^a

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  Entry	Substrate	Product	Yield / %b
  1			85
  2			73
  3			81
  4			75
  5			91
  6			92
  7			86
  8			80
  9			74
  a Reaction conditions: epoxides (2.0 mmol), complex 1 (0.5%), TBAB (3%), CO2 (100 kPa), 70 ℃, 12 h, solvent-free; b Determined by 1H NMR spectroscopy, 1,3,5-trimethoxybenzene as an internal standard.

  Based on the results recently reported^[47-51], the possible catalytic mechanism for this catalytic system was proposed (Fig. 7). Because of the weak binding ability, the coordinated solvent molecules can be replaced by the oxygen atoms of the epoxides, which is activated by the coordinating with the central metal ions as Lewis acid sites. The Br^- ion of the co-catalyst TBAB nucleophilicity attacks carbon atoms of epoxides with less-hindered resulting in the opening of the ternary ring. Then CO₂ interacts with the ring-opened oxygen anion to form alkyl carbonate intermediates. Finally, the target product is obtained through the intramolecular ring closing. TBAB and catalyst enter into the next cycle.

  Figure 7

  

  Figure 7.  Tentative mechanism for the cycloaddition reaction catalyzed by complex 1 and TBAB

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  3.   Conclusions

  A novel Ho₄ complex showing a central symmetric tetranuclear defect di-cubane topology has been obtained. The magnetic study reveals that complex 1 exhibits slow relaxation of the magnetization behavior. The metal centers of Ho(Ⅲ) ions in 1 act as Lewis acid activators that could effectively catalyze the cycloaddition reaction with a high yield of up to 92% under mild conditions. In addition, complex 1 exhibits high catalytic activity with a wide substrate scope. These results provide a new method for designing and constructing bifunctional polynuclear lanthanide complexes.

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

  
  
• 1. [1]

     Zheng X Y, Kong X J, Zheng Z, Long L S, Zheng L S. High-nuclearity lanthanide-containing clusters as potential molecular magnetic coolers[J]. Acc. Chem. Res., 2018, 51:  517-525. doi: 10.1021/acs.accounts.7b00579

  2. [2]

     Zheng X Y, Xie J, Kong X J, Long L S, Zheng L S. Recent advances in the assembly of high-nuclearity lanthanide clusters[J]. Coord. Chem. Rev., 2019, 378:  222-236. doi: 10.1016/j.ccr.2017.10.023

  3. [3]

     Ding Y S, Chilton N F, Winpenny R E P. On approaching the limit of molecular magnetic anisotropy: A near-perfect pentagonal bipyramidal dysprosium(Ⅲ) single-molecule magnet[J]. Angew. Chem. Int. Ed., 2016, 55:  16071-16074. doi: 10.1002/anie.201609685

  4. [4]

     Peng J B, Kong X J, Zhang Q C, Orendáč M, Prokleška J, Ren Y P, Long L S, Zheng Z, Zheng L S. Beauty, symmetry, and magnetocaloric effect-four-shell keplerates with 104 lanthanide atoms[J]. J. Am. Chem. Soc., 2014, 136:  17938-17941. doi: 10.1021/ja5107749

  5. [5]

     Dong J, Cui P, Shi P F, Cheng P, Zhao B. Ultrastrong alkali-resisting lanthanide-zeolites assembled by[Ln₆₀] nanocages[J]. J. Am. Chem. Soc., 2015, 137:  15988-15991. doi: 10.1021/jacs.5b10000

  6. [6]

     Zhu Z H, Peng J M, Wang H L, Zou H H, Liang F P. Assembly mechanism and heavy metal ion sensing of cage-shaped lanthanide nanoclusters[J]. Cell Rep. Phys. Sci., 2020, 1(8):  100165. doi: 10.1016/j.xcrp.2020.100165

  7. [7]

     Liu J, Chen Y C, Liu J L. A stable pentagonal bipyramidal Dy(Ⅲ) single-ion magnet with a record magnetization reversal barrier over 1000 K[J]. J. Am. Chem. Soc., 2016, 138:  5441-5450. doi: 10.1021/jacs.6b02638

  8. [8]

     Tang J K, Hewitt I, Madhu N T, Chastanet G, Wernsdorfer W, Anson C E, Benell C, Sessoli R, Powell A K. Dysprosium triangles showing single-molecule magnet behavior of thermally excited spin states[J]. Angew. Chem. Int. Ed., 2006, 45:  1729-1733. doi: 10.1002/anie.200503564

  9. [9]

     Hewitt I J, Tang J K, Madhu N T, Anson C E, Lan Y H, Luzon J, Etienne M, Sessoli R, Powell A K. Coupling Dy₃ triangles enhances their slow magnetic relaxation[J]. Angew. Chem. Int. Ed., 2010, 49:  6352-6356. doi: 10.1002/anie.201002691

  10. [10]

      Guo Y N, Xu G F, Gamez P, Zhao L, Lin S Y, Deng R P, Tang J K, Zhang H J. Two-step relaxation in a linear tetranuclear dysprosium(Ⅲ) aggregate showing single-molecule magnet behavior[J]. J. Am. Chem. Soc., 2010, 132:  8538-8539. doi: 10.1021/ja103018m

  11. [11]

      Liu J L, Guo F S, Meng Z S, Zheng Y Z, Leng J D, Tong M L, Ungur L, Chibotaru L F, Heroux K J, Hendrickson D N. Symmetry related[Dy^Ⅲ₆Mn^Ⅲ₁₂] cores with different magnetic anisotropies[J]. Chem. Sci., 2011, 2:  1268-1272. doi: 10.1039/c1sc00166c

  12. [12]

      Wang W M, Wu Z L, Cui J Z. Molecular assemblies from linear-shaped Ln₄ clusters to Ln₈ clusters using different β-diketonates: Disparate magnetocaloric effects and single-molecule magnet behaviors[J]. Dalton Trans., 2021, 50:  12931-12943. doi: 10.1039/D1DT01344K

  13. [13]

      Wang W M, He L Y, Wang X X, Shi Y, Wu Z L, Cui J Z. Linear-shaped Ln^Ⅲ₄ and Ln^Ⅲ₆ clusters constructed by a polydentate Schiff base ligand and a β-diketone co-ligand: Structures, fluorescence properties, magnetic refrigeration and single-molecule magnet behavior[J]. Dalton Trans., 2019, 48:  16744-16755. doi: 10.1039/C9DT03478A

  14. [14]

      Wang W M, Kang X M, Shen H Y, Wu Z L, Gao H L, Cui J Z. Modulating single-molecule magnet behavior towards multiple magnetic relaxation processes through structural variation in Dy₄ complexes[J]. Inorg. Chem. Front., 2018, 5:  1876-1885. doi: 10.1039/C8QI00214B

  15. [15]

      Wang W M, Wu Z L, Zhang Y X, Wei H Y, Gao H L, Cui J Z. Self-assembly of tetra-nuclear lanthanide complexes via atmospheric CO₂ fixation: Interesting solvent-induced structures and magnetic relaxation conversions[J]. Inorg. Chem. Front., 2018, 5:  2346-2354. doi: 10.1039/C8QI00573G

  16. [16]

      Wang W M, Yue R X, Gao Y, Wang M J, Hao S S, Shi X Y, Kang X M, Wu Z L. Large magnetocaloric effect and remarkable single-molecule-magnet behavior in triangle-assembled Ln^Ⅲ₆ complexes[J]. New J. Chem., 2019, 43:  16639-16646. doi: 10.1039/C9NJ03921J

  17. [17]

      Qiao N, Xin X Y, Guan X F, Zhang C X, Wang W M. Self-assembly bifunctional tetranuclear Ln₂Ni₂ clusters: Magnetic behavior and highly efficient converting CO₂ under mild conditions[J]. Inorg. Chem., 2022, 61:  15098-15107. doi: 10.1021/acs.inorgchem.2c02180

  18. [18]

      Wang W M, Xin X Y, Qiao N, Wu Z L, Li L, Zou J Y. Self-assembly of octanuclear Ln(Ⅲ)-based clusters: Their large magnetocaloric effects and highly efficient conversion of CO₂[J]. Dalton Trans., 2022, 51:  13957-13969. doi: 10.1039/D2DT01892F

  19. [19]

      Wang L, Zhang G, Kodama K. An efficient metal- and solvent-free organocatalytic system for chemical fixation of CO₂ into cyclic carbonates under mild conditions[J]. Green Chem., 2016, 18:  1229-1233. doi: 10.1039/C5GC02697K

  20. [20]

      Castro-Osma J A, North M, Wu X. Development of a halide-free aluminum-based catalyst for the synthesis of cyclic carbonates from epoxides and carbon dioxide[J]. Chem.-Eur. J., 2014, 20:  15005-15008. doi: 10.1002/chem.201404117

  21. [21]

      Wang W M, Qiao N, Xin X Y, Wu Z L, Cui J Z. Octanuclear Ln(Ⅲ)-based clusters assembled by a polydentate Schiff base ligand and a β-diketone co-Ligand: efficient conversion of CO₂ to cyclic carbonates and large magnetocaloric effect[J]. Cryst. Growth Des., 2023, 23:  87-95. doi: 10.1021/acs.cgd.2c00746

  22. [22]

      Gu A L, Zhang Y X, Wu Z L, Cui H Y, Hu T D, Zhao B. Highly efficient conversion of propargylic alcohols and propargylic amines with CO₂ activated by noble-metal-free catalyst Cu₂O@ZIF-8[J]. Angew. Chem. Int. Ed., 2022, 61:  e202114817.

  23. [23]

      Tamura M, Honda M, Nakagawa Y. Direct conversion of CO₂ with diols, amino alcohols and diamines to cyclic carbonates, cyclic carbamates and cyclic ureas using heterogeneous catalysts[J]. J. Chem. Technol. Biotechnol., 2014, 89:  19-33. doi: 10.1002/jctb.4209

  24. [24]

      Xiao L F, Li F W, Peng J J. Immobilized ionic liquid/zinc chloride: Heterogeneous catalyst for synthesis of cyclic carbonates from carbon dioxide and epoxides[J]. J. Mol. Catal. A: Chem., 2006, 253:  265-269. doi: 10.1016/j.molcata.2006.03.047

  25. [25]

      Wang M Y, Song Q W, Ma R. Efficient conversion of carbon dioxide at atmospheric pressure to 2-oxazolidinones promoted by bifunctional Cu(Ⅱ)-substituted polyoxometalate-based ionic liquids[J]. Green Chem., 2016, 18:  282-287. doi: 10.1039/C5GC02311D

  26. [26]

      Hu J, Ma J, Zhu Q. Zinc(Ⅱ)-catalyzed reactions of carbon dioxide and propargylic alcohols to carbonates at room temperature[J]. Green Chem., 2016, 18:  382-385. doi: 10.1039/C5GC01870F

  27. [27]

      Ma R, He L N, Zhou Y B. An efficient and recyclable tetra oxo-coordinatedZinc catalyst for the cycloaddition of epoxides with carbon dioxide at atmospheric pressure[J]. Green Chem., 2016, 18:  226-231. doi: 10.1039/C5GC01826A

  28. [28]

      Ema T, Miyazaki Y, Taniguchi T. Robust porphyrin catalysts immobilized on biogenous iron oxide for the repetitive conversions of epoxides and CO₂ into cyclic carbonates[J]. Green Chem., 2013, 15:  2485-2492. doi: 10.1039/c3gc41055b

  29. [29]

      Sun J, Fujita S I, Zhao F. Synthesis of styrene carbonate from styrene oxide and carbon dioxide in the presence of zinc bromide and ionic liquid under mild conditions[J]. Green Chem., 2004, 6:  613-616. doi: 10.1039/b413229g

  30. [30]

      Whiteoak C J, Kielland N, Laserna V. A powerful aluminum catalyst for the synthesis of highly functional organic carbonates[J]. J. Am. Chem. Soc., 2013, 135:  1228-1231. doi: 10.1021/ja311053h

  31. [31]

      Song T Q, Dong J, Yang A F, Che X J, Gao H L, Cui J Z, Zhao B. Wheel-like Ln₁₈ cluster organic frameworks for magnetic refrigeration and conversion of CO₂[J]. Inorg. Chem., 2018, 57:  3144-3150. doi: 10.1021/acs.inorgchem.7b03142

  32. [32]

      Dong J, Xu H, Hou S L, Wu Z L, Zhao B. Metal-organic frameworks with Tb₄ clusters as nodes: Luminescent detection of chromium(Ⅵ) and chemical fixation of CO₂[J]. Inorg. Chem., 2017, 56:  6244-6250. doi: 10.1021/acs.inorgchem.7b00323

  33. [33]

      Yang H, Gao G S, Chen W M. Self-assembly of tetranuclear 3d-4f helicates as highly efficient catalysts for CO₂ cycloaddition reactions under mild conditions[J]. Dalton Trans., 2020, 49:  10270-10277. doi: 10.1039/D0DT01743D

  34. [34]

      Katagiri S, Tsukahara Y, Hasegawa Y, Wada Y. Energy-transfer mechanism in photoluminescent terbium(Ⅲ) complexes causing their temperature-dependence[J]. Bull. Chem. Soc. Jpn., 2007, 80:  1492-1503. doi: 10.1246/bcsj.80.1492

  35. [35]

      Wang W M, Wang M J, Hao S S, Shen Q Y, Wang M L, Liu Q L, Guan X F, Zhang X T, Wu Z L. 'Windmill'-shaped Ln^Ⅲ₄ (Ln^Ⅲ =Gd and Dy) clusters: Magnetocaloric effect and single-molecule-magnet behavior[J]. New J. Chem., 2020, 44:  4631-4638. doi: 10.1039/C9NJ05317D

  36. [36]

      Wang W M, Zhang H X, Wang S Y, Shen H Y, Gao H L, Cui J Z, Zhao B. Ligand field affected single-molecule magnet behavior of lanthanide(Ⅲ) dinuclear complexes with an 8-hydroxyquinoline Schiff base derivative as bridging ligand[J]. Inorg. Chem., 2015, 54:  10610-10622. doi: 10.1021/acs.inorgchem.5b01404

  37. [37]

      Wang W M, Zhang L, Li X Z, He L Y, Wang X X, Shi Y, Wang J, Dong J, Wu Z L. Structures, fluorescence properties and magnetic properties of a series of rhombus-shaped Ln^Ⅲ₄ clusters: Magnetocaloric effect and single-molecule-magnet behavior[J]. New J. Chem., 2019, 43:  12941-12949. doi: 10.1039/C9NJ02872B

  38. [38]

      Wang W M, Gao Y, Yue R X, Qiao N, Wang D T, Shi Y, Zhang H, Cui J Z. Construction of a family of Ln₃ clusters using multidentate Schiff base and β-diketonate ligands: Fluorescence properties, magnetocaloric effect and slow magnetic relaxation[J]. New J. Chem., 2020, 44:  9230-9237. doi: 10.1039/D0NJ01172J

  39. [39]

      Wang W M, Li X Z, Zhang L, Chen J L, Wang J H, Wu Z L, Cui J Z. A series of[2×2] square grid Ln^Ⅲ₄ clusters: A large magnetocaloric effect and single-molecule-magnet behavior[J]. New J. Chem., 2019, 43:  7419-7426. doi: 10.1039/C8NJ04454F

  40. [40]

      Yao M X, Cai L Z, Deng X W, Zhang W, Liu S J, Cai X M. Self-assembly of rare octanuclear quad(double-stranded) cluster helicates showing slow magnetic relaxation and the magnetocaloric Effect[J]. New J. Chem., 2018, 42:  17652-17658. doi: 10.1039/C8NJ04169E

  41. [41]

      Ma X F, Wang H L, Zhu Z H, Li B, Mo K Q, Zou H H, Liang F P. Formation of nanocomplex {Dy₁₂} containing Dy-exclusive vertex-sharing[Dy₄(μ₃-OH)₄] cubanes via simultaneous multi-template guided and step-by-step assembly[J]. Dalton Trans., 2019, 48:  11338-11344. doi: 10.1039/C9DT01454C

  42. [42]

      Dinca A S, Mindru A, Dragancea D, Tiseanu C, Shova S, Cornia S, Carrella L M, Rentschler E, Affronte M, Andruh M. Aggregation of[Ln^Ⅲ₁₂] clusters by the dianion of 3-formyl salicylic acid, synthesis, crystal structures, magnetic and luminescence properties[J]. Dalton Trans., 2019, 48:  1700-1708. doi: 10.1039/C8DT04602F

  43. [43]

      Wang H L, Ma X F, Peng J M, Zhu Z H, Li B, Zou H H, Liang F P. Tracking the stepwise formation of the dysprosium cluster (Dy₁₀) with multiple relaxation behavior[J]. Inorg. Chem., 2019, 58:  9169-9174. doi: 10.1021/acs.inorgchem.9b00760

  44. [44]

      Lu J J, Montigaud V, Cador O, Wu J F, Zhao L, Li X L, Guo M, Guennic B L, Tang J K. Lanthanide(Ⅲ) hexanuclear circular helicates: Slow magnetic relaxation, toroidal arrangement of magnetic moments, and magnetocaloric effects[J]. Inorg. Chem., 2019, 58:  11903-11911. doi: 10.1021/acs.inorgchem.9b01068

  45. [45]

      Wu J F, Li X L, Zhao L, Guo M, Tang J K. Enhancement of magnetocaloric effect through fixation of carbon dioxide: Molecular assembly from Ln₄ to Ln₄ cluster pairs[J]. Inorg. Chem., 2017, 56:  4104-4111. doi: 10.1021/acs.inorgchem.7b00094

  46. [46]

      Li L F, Kuang W W, Li Y M, Zhu L L, Xu Y, Yang P P. A series of new octanuclear Ln₈ clusters: Magnetic studies reveal a significant cryogenic magnetocaloric effect and slow magnetic relaxation[J]. New J. Chem., 2019, 43:  1617-1625. doi: 10.1039/C8NJ04231D

  47. [47]

      Shi Y, Tang B, Jiang X L, Jiao Y E, Xu H, Zhao B. Highly effective CS₂ conversion with aziridines catalyzed by novel [Dy₂₄] nano-cages in MOFs under mild conditions[J]. J. Mater. Chem. A, 2022, 10:  4889-4894. doi: 10.1039/D1TA10522A

  48. [48]

      Hou W, Wang G, Wu X J, Sun S Y, Zhao C Y, Liu W S, Pan F X. Lanthanide clusters as highly efficient catalysts regarding carbon dioxide activation[J]. New J. Chem., 2020, 44:  5019-5022. doi: 10.1039/C9NJ05831A

  49. [49]

      Zhang R L, Wang L, Xu C, Yang H, Chen W M, Gao G S, Liu W S. New lanthanide(Ⅲ) coordination polymers: Synthesis, structural features, and catalytic activity in CO₂ fixations[J]. Dalton Trans., 2018, 47:  7159-7165. doi: 10.1039/C8DT01292J

  50. [50]

      Xu C, Liu Y, Wang L, Ma J X, Yang L Z, Pan F X, Kirillov A M, Liu W S. New lanthanide(Ⅲ) coordination polymers: Synthesis, structural features, and catalytic activity in CO₂ fixation[J]. Dalton Trans., 2017, 46:  16426-16431. doi: 10.1039/C7DT03574H

  51. [51]

      Wang L, Xu C, Han Q X, Tang X L, Zhou P P, Zhang R L, Gao G S, Xu B H, Qin W W, Liu W S. Ambient chemical fixation of CO₂ using a highly efficient heterometallic helicate catalyst system[J]. Chem. Commun., 2018, 54:  2212-2215. doi: 10.1039/C7CC09092G

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  Scheme 1  Synthesis route of the H₄L ligand

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  Figure 1  (a) Molecular structure for complex 1; (b) A defect di-cubane topology for Ho₄ structure; (c) Coordination environment of Ho(Ⅲ) in complex 1

  Hydrogen atoms are omitted for clarity; Symmetry code: a: -x+1, -y, -z+1

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  Figure 2  (a) Coordination mode of L^4- in complex 1; (b) Coordination modes of acac^- in complex 1

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  Figure 3  Coordination polyhedrons for Ho1(Ⅲ) and Ho2(Ⅲ) ions in 1

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  Figure 4  Simulated and experimental PXRD patterns of complex 1 after being immersed in different solvents for 10 h

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  Figure 5  (a) Temperature dependence of the χ_MT under 1 000 Oe for complex 1 in a temperature range of 300.0-2.0 K; (b) Temperature dependence of χ_M^-1 for complex 1

  Solid line resulted from fitting the Curie-Weiss law

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  Figure 6  Temperature dependence of the in-phase (χ′) (a) and out-of-phase (χ″) (b) components for 1 in a zero-dc field with an oscillation of 3.0 Oe

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  Figure 7  Tentative mechanism for the cycloaddition reaction catalyzed by complex 1 and TBAB

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  Table 1.  Crystallographic data and structure refinements for complex 1

  Parameter	1
  Formula	C56H70Ho4N6O24
  Formula weight	1 870.90
  T / K	150.0
  Cryst system	Triclinic
  Space group	P1
  a / nm	1.048 33(5)
  b / nm	1.227 66(6)
  c / nm	1.374 76(5)
  α/(°)	73.936 7(16)
  β/(°)	81.906 9(17)
  γ/(°)	70.314 4(19)
  V / nm3	1.598 64(13)
  Z	1
  Cryst size / mm	0.35×0.21×0.16
  Dc / (g·cm-3)	1.943
  μ / mm-1	4.978
  Limiting indices	-13 ≤ h ≤ 13,
  	-15 ≤ k ≤ 15,
  	-17 ≤ l ≤ 17
  Reflection collected	43 379
  Number of parameters	416
  Rint	0.053 8
  GOF on F2	1.031
  R1, wR2 [I > 2σ(I)]	0.027 9, 0.067 4
  R1, wR2 (all data)	0.033 7, 0.071 3

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  Table 2.  CO₂ reacting with epibromohydrin under different conditions^a

  Entry	Catalyst	xTBAB / %	T / ℃	Yield / %b
  1	1	3	20	45
  2	1	3	30	52
  3	1	3	40	65
  4	1	3	50	79
  5	1	3	60	87
  6	1	3	70	92
  7	1	3	80	89
  8	1	3	90	85
  9	—	3	70	23
  10	1	—	70	< 1
  11	Ho(NO3)3·6H2O	3	70	5
  12	HoCl3·6H2O	3	70	9
  a Reaction conditions: epoxybromopropane (273 mg, 2.0 mmol), complex 1 (0.5%), Ho(NO3)3·6H2O (0.5%), HoCl3·6H2O (0.5%), TBAB (3%), CO2 (100 kPa), 12 h, solvent-free; b Determined by 1H NMR spectroscopy, 1,3,5-trimethoxybenzene as an internal standard.

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  Table 3.  Various carbonates from CO₂ reacted with different epoxides catalyzed by complex 1 under optimized conditions^a

  Entry	Substrate	Product	Yield / %b
  1			85
  2			73
  3			81
  4			75
  5			91
  6			92
  7			86
  8			80
  9			74
  a Reaction conditions: epoxides (2.0 mmol), complex 1 (0.5%), TBAB (3%), CO2 (100 kPa), 70 ℃, 12 h, solvent-free; b Determined by 1H NMR spectroscopy, 1,3,5-trimethoxybenzene as an internal standard.

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