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. 2025 Sep 18;64(38):19227–19237. doi: 10.1021/acs.inorgchem.5c02558

Lanthanide Tetrabromoterephthalates: Promotion of Cycloaddition Reactions of Carbon Dioxide without Preactivation, Solvent, and Co-Catalyst at Ambient Pressure through Structural Design

Malee Sinchow †,‡, Thammanoon Chuasaard †,‡, Natthiti Chiangraeng †,‡, Stephanie A Bird §, Piyarat Nimmanpipug ‡, Nobuto Yoshinari ∥, Apinpus Rujiwatra ‡,*
PMCID: PMC12486200  PMID: 40966046

Abstract

To catalyze the cycloaddition reactions of carbon dioxide and epoxides without additional solvent or cocatalyst under ambient pressure, [LnIII 2(tbta)2(COO)2(DMF)4] (LnIII = SmIII (I Sm), EuIII (I Eu), and GdIII (I Gd); H2tbta = tetrabromoterephthalic acid; DMF = dimethylformamide) were synthesized and characterized. Their crystal structures were elucidated. LnIII were selected because of their hard acidity and tendency to function without preactivation. H2tbta was chosen because of its abundant bromide substituents and its potential acidic and basic sites. [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu) and [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd) were additionally prepared through the transformation of I Eu and I Gd. The catalytic activities of I Sm, I Eu, and I Gd were explored using a range of epoxides under ambient pressure without additional solvent and cocatalyst. They showed selective activities toward epichlorohydrin. The best performances, in terms of turnover number and turnover frequency, were obtained at 90 °C and 12 h (I Sm: 307 and 26 h–1; I Eu: 327 and 27 h–1; I Gd: 340 and 28 h–1). Unfortunately, they were unstable after catalysis due to the loss of bromide, which is their essential limitation. However, they demonstrated the possibility of using an organic linker to activate the epoxide and the potential for solid cocatalysts. Computational studies of structural transformation and the feasibility of substituting DMF with ECH were also conducted.

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1. Introduction

As current technologies to mitigate carbon dioxide (CO2) emission are still far from sufficient and natural carbon sinks are struggling to keep up, the rise in atmospheric CO2 concentration persists. , Additional strategies, specifically carbon capture and storage (CCS) and carbon capture and utilization (CCU), have therefore been introduced and have gained significant attention. − The use of CO2 as a carbon source to produce economically important chemicals is one of the most sustainable CCU possibilities, and it is being vigorously explored. − Along this line, the cycloaddition reactions of CO2 with epoxides to produce cyclic carbonates have attracted significant attention. The key challenge lies in developing heterogeneous catalysts that are highly efficient at room temperature and atmospheric CO2 pressure without the need for a solvent or a cocatalyst, which would complicate the purification process. −

Coordination polymers (CPs), along with their porous subgroup, namely metal-organic frameworks (MOFs), are coordination compounds with one- (1D), two- (2D), or three-dimensional (3D) framework structures composed primarily of inorganic coordination entities as nodes and organic ligands as linkers. The key characteristics of these materials are their capability to be tailor-made and their high crystallinity. Through careful selection of nodes and linkers, their potential applications, such as in sensing − and catalysis, − have been demonstrated. With respect to the cycloaddition reactions of CO2 with epoxides, research advancements on CPs-based catalysts have been substrantial since the first report just over a decade ago. A few reported CPs, particularly those built from the lanthanides (LnIII) or so-called lanthanide coordination polymers (LnIII-CPs), have provided promising results with performance comparable to homogeneous catalysts. − However, the preparation of LnIII-CPs remains challenging due to the large and flexible coordination chemistry of LnIII as well as their extreme hydrophilicity. Their coordination behavior is also highly dependent on the molecular structure of ligands and the choice of solvent, both of which can complicate the synthesis.

In addition to the common requirement of high pressure and temperature for the catalysts to perform, another critical issue of nearly all heterogeneous catalysts is the necessity of using a cocatalyst to initiate the nucleophilic attack on epoxide. − The most common cocatalyst is the homogeneous tetrabutylammonium bromide (TBABr), which is difficult to remove and thus compromises the process heterogeneity. This challenge has driven vigorous efforts to develop new heterogeneous catalysts, which are effective even in the absence of a cocatalyst. Several strategies have been pursued, such as compositing CPs with ionic liquids, e.g., [HeMOEim]­[Br]/MIL-53­(Al) ([HeMOEim]­[Br]) = 1-(2-hydroxyl-ethyl)-3-methoxyethyl-imidazolium bromide), MIL101-2AMP-DBrP (2AMP-DBrP = 1,5-bis­(2-aminopyrimidinium)­pentane bromide), MIL101-2AMP-DBrEE (2AMP-DBrEE = 1,5-bis­(2-aminopyrimidinium)­ethyl ether bromide, and UiO-66-ILs­(PCA-EtBr). Postsynthesis modification has also been demonstrated to be viable, e.g., NU-1000­(M = 4-PyCOOH/CH3I), UiO66­(Hf)-DCCBr, FJI-C10, and MOF-I. The preparation of the composites and the postsynthesis modification require several steps, and complications may occur. There are nonetheless few examples of CP-based catalysts that could be facilely synthesized, i.e., {[M6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O} n (M = Co, Ni, Zn and H3TATAB = 4,4′,4″-s-triazine-1,3,5-triyl-trip-aminobenzoic acid), , (Me2NH2)2·[Zn8(Ad)4(DABA)6O]·7DMF} n (Ad = adeninate, DABA = 2,2′-dimethyl-4,4′-azodibenzoate), and the bromide-containing UiO67-[TBD]+Br– ([TBD]+Br– = 1,5,7-triazabicyclo[4.4.0]­dec-5-ene-biphenyl-4,4′-dicarboxylic acid). Achieving high-performance catalysts with practical turnover frequency (TOF), which is the standard parameter used to evaluate and compare catalyst performance remains however a significant challenge.

In pursuit of new catalysts that can perform efficiently in the absence of solvent and TBABr at ambient CO2 pressure, tetrabromoterephthalic acid (H2tbta) was selected as a ligand to synthesize new LnIII-CPs. H2tbta is a simple ligand, commercially available, and abundant in bromide substituents. While bromide may serve as the nucleophile, its π electron-rich phenyl ring and carboxylates may act as Lewis bases in catalysis. LnIII ions were chosen owing to their hard acidity, capability to function without activation, and tendency to provide high catalytic performance. This resulted in a new series of 2D [LnIII 2(tbta)2(COO)2(DMF)4] of which Ln = I Sm, I Eu, and I Gd (Scheme ). The other new 3D [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu) and 1D [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd) frameworks were additionally prepared through transformation from the corresponding I Eu and I Gd. Their single-crystal structures and structural transformations are presented. Catalytic activities of the as-synthesized I Sm, I Eu, and I Gd were evaluated, and their catalytic potential in the absence of solvent and TBABr has been illustrated. Computational insights into structural transformations and active sites related to the catalytic activities are also included.

1. Illustration showing the structure of H2tbta and the correlation between the synthesized I Sm, I Eu, and I Gd and the transformation to II Eu and III Gd, as well as the catalytic studies.

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2. Experimental Section

Details on materials, instruments, and characterization methods are provided in the Supporting Information.

2.1. Synthesis and Crystal Growth

To synthesize [SmIII 2(tbta)2(COO)2(DMF)4] (I Sm, 80% yield based on SmIII), SmIII(NO3)3·6H2O (0.0222 g, 0.0499 mmol) was dissolved in 5.00 mL of deionized water. Separately, H2tbta (0.0361 g, 0.0749 mmol) and triethylamine (10.0 μL, 0.0717 mmol) were dissolved in 5.00 mL of dimethylformamide (DMF). The two solutions were then mixed in a 12 mL glass vial and kept at 60 °C for 4 days. [EuIII 2(tbta)2(COO)2(DMF)4] (I Eu, 81% yield based on EuIII) and [GdIII 2(tbta)2(COO)2(DMF)4] (I Gd, 79% yield based on GdIII) were synthesized following the same procedure, but using the corresponding EuIII(NO3)3·6H2O (0.0223 g, 0.0500 mmol) and GdIII(NO3)3·6H2O (0.0225 g, 0.0498 mmol). The yielded crystals were stable in the mother liquor but somewhat air-sensitive and gradually lost their single-crystal quality under ambient atmosphere.

The crystals of [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu) and [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd) were prepared simply through the dissolution of the corresponding I Eu (0.150 g, 0.0912 mmol) and I Gd (0.150 g, 0.0907 mmol) in 5 mL of deionized water, followed by slow evaporation at 60 °C.

2.2. Single Crystal Structure Determination

Single crystal data of I Sm, I Eu, II Eu, and III Gd were collected at 293(2) K using a Rigaku XtaLAB SuperNova diffractometer equipped with a single microfocus sealed X-ray tube of Mo Kα radiation (λ = 0.71073 Å) and a direct photon-counting HyPix3000 detector. Data collection and reduction were performed using CrysAlisPro 1.171.39.46. Empirical absorption corrections were applied to all the data sets, implemented in the SCALE3 ABSPACK scaling algorithm. The structures were solved by an intrinsic phasing method provided in the SHELXT program and refined on F 2 by the full-matrix least-squares technique using the SHELXL program via the Olex2 interface. The crystallographic and refinement data are summarized in Table S1. The extended asymmetric units of the refined structures are shown in Figures S1–S4. The analysis of the hydrogen bonding and halogen interactions is provided in Tables S2-S5. The crystals of I Gd were too small and inappropriate for full data collection.

2.3. Catalytic Activity Evaluation of I Sm, I Eu, and I Gd

Typically, the finely ground crystals (0.030 mmol) were weighed into a 10 mL two-neck round-bottom flask, into which epichlorohydrin (ECH; 1.6 mL, 20 mmol) as a model epoxide, was subsequently added. The flask was then sealed and equipped with a CO2 balloon before the reaction was carried out with continuous stirring and regular refilling of the CO2 balloon. The reaction temperature and time were varied (60–90 °C and 3–24 h). To stop the reaction, the flask was cooled to room temperature using an ice bath, and the remaining CO2 was slowly released. The progress of the reactions was monitored by 1H NMR spectroscopy, from which the percentage conversion (%conversion) and selectivity (%selectivity) were calculated. The percentage yields (%yield) were also determined using mesitylene as an internal standard. Based on %yield, the turnover number (TON) and turnover frequency (TOF in h–1) were calculated. In addition to ECH, a range of epoxides was also tested using I Eu, and the reactions were conducted at 90 °C for 12 h. Triplicate results were produced for every experiment unless specified otherwise. Attempts to use the H2tbta pro-ligand, and the corresponding nitrate salts of LnIII, namely SmIII(NO3)36H2O, EuIII(NO3)36H2O and GdIII(NO3)36H2O, to catalyze the reaction were also conducted as references. The recyclability and robustness of the catalysts were studied by using I Eu as a representative. The catalyst was recovered from the reaction by filtration and then washed with chloroform several times before drying. The retention of the framework structure was monitored by powder X-ray diffraction (PXRD).

2.4. Computational Study

With reference to the single-crystal structures of I Eu and II Eu, the models were constructed and optimized. All the carboxylates were capped with hydrogen atoms, and the ligands were constrained during the calculation to maintain the stability of the frameworks. Density functional theory (DFT) was used to investigate the possible interactions. All the calculations were carried out using the Gaussian 16 program. The hybrid density functional B3PW91 was utilized for geometry optimization. The Stuttgart effective core potential (ECP) was applied to the EuIII atoms in the model, while the 6-31G­(d,p) basis set was used for the other atoms.

To study the feasibility of substituting DMF with water and, consequently, the transformation between I Eu and II Eu, the binding energies (ΔE bind) between different chemical species were calculated:

ΔEbind=ECP···LABILE MOLECULE−(ECP+ELABILEMOLECULE) 1

E CP···LABILE MOLECULE, E CP, and E LABILE MOLECULE are the DFT energies in kcal·mol–1 of the optimized structures of the tested CP (I Eu or II Eu) complexed with the labile molecules, CP (I Eu or II Eu), and labile molecules (DMF and H2O), respectively. The larger negative value of ΔE bind indicates a more favorable interaction.

To explore the possible substitution of DMF by ECH in I Eu, the interaction between ECH and I Eu was investigated. The stability of the complexes of I Eu and ECH, in terms of the binding energy of I Eu and ECH ΔEbind:IEu···ECH , at various possible interacting sites, was calculated and compared:

Ebind:IEu···ECH=EIEu···ECH−(EIEu+EECH) 2

EIEu···ECH , EIEu , and E ECH are the calculated energies of the optimized structures for the complex, I Eu, and ECH, respectively.

Effective molecular sizes of the epoxides were also calculated by using the same aforementioned level of calculations.

3. Results and Discussion

3.1. Synthesis and Chemical Stability

I Sm, I Eu, and I Gd could be prepared in pure forms based on the PXRD experiments (Figure S5). The elemental analyses showed consistent results (Table S6) and the FT-IR spectra (Figure S6) displayed all the characteristic vibrational bands of the expected functional groups. Notably, trace impurities were frequently found with the I Gd crystals. These impurities existed as fine powder and were visible in the PXRD pattern if present (Figure S5). However, they could not be characterized based on only one diffraction peak. Nonetheless, the crystals of I Gd were separated from the powder under an optical microscope for further characterization and experiments.

As a representative of all the samples, the stability of I Eu was studied using common solvents, including water, methanol, ethanol, acetonitrile, acetone, ethyl acetate, THF, dichloromethane, chloroform, benzene, toluene, and cyclohexane. Corresponding to the PXRD experiments (Figure S7), I Eu was practically stable in nonpolar solvents even after being soaked in the solvents for 24 h at room temperature. In polar solvents, I Eu had a tendency to decompose and transform into other crystalline phases. These results suggested interactions between I Eu and polar solvents and, therefore, its polar nature. In the cases of highly polar solvents, namely methanol, ethanol, and water, I Eu transformed into a new phase identified later as [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu). Based on these results, the growth of II Eu crystals was attempted using water.

By dissolving I Eu in water followed by solvent evaporation, the crystals of II Eu were prepared (Table S1 and Figure S8). The absence of DMF in II Eu was confirmed by the disappearance of the characteristic ν­(C–N) band in the FT-IR spectrum (Figure S6). The same procedure was applied to I Sm and I Gd. In the case of I Sm, a mixture of numerous phases was obtained (Figure S9). There were also tiny crystals, which were unfortunately inappropriate for full data collection. Preliminary checks of their unit cell parameters, which could not be matched with any crystal data in the CSD database, suggest that other new phases were formed. Regarding I Gd, the formation of [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd) was concluded from single-crystal X-ray diffraction. The PXRD pattern of the as-synthesized sample indicates, however, that III Gd existed as a trace product mixed with other unknown phases (Figure S10). Therefore, only II Eu could be prepared in pure form.

3.2. Single Crystal Structure Description

3.2.1. [LnIII 2(tbta)2(COO)2(DMF)4] (I)

The structure of I Eu features a 2D coordination framework constructed based on two coordination motifs (Figures a and S11), i.e., the 9-fold distorted tricapped trigonal prismatic TPRS-{EuIIIO9} and the 8-fold distorted square antiprismatic SAPRS-{EuIIIO8}. The coordination environment of the two coordination entities is closely similar, made up of three tbta2–, two formate ligands, and two DMF molecules. The three tbta2– of both motifs exhibit the same coordination modes (Figure S12), including two bridging μ4-η1:η1:η1:η1 and one chelating-bridging μ2-η1:η1:η1:η1. Contrarily, the formate ligands of the two motifs show different modes of coordination, which lead to dissimilarity in their coordination numbers and geometry. Whereas the formate ligands of TPRS-{EuIIIO9} adopt the same chelating-bridging μ2-η2:η1, the formate ligands of SAPRS-{EuIIIO8} exhibit two different modes of coordination, i.e., the chelating-bridging μ2-η2:η1 and the bridging μ2-η1:η1. The distance between one of the O atoms of the bridging μ2-η1:η1 formate is too far from EuIII to bond.

1.

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Views of (a) the [EuIII 2(tbta)2(COO)2(DMF)4] layer made up of the corner-shared dimer in I Eu, (b) the simplified sq1 net of the established layer, (c) the hydrogen bonding, and (d) the halogen interactions.

The TPRS-{EuIIIO9} and SAPRS-{EuIIIO8} motifs share corners to form a {EuIII 2O15} dimer (Figures a and S11). Each dimer is connected to four equivalent dimers through tbta2– leading to the formation of the [EuIII 2(tbta)2(COO)2(DMF)4] layer in the ab plane (Figure a). Using the dimer as a node and tbta2– as a linker, the layered framework can be simplified to a 4-connected uninodal sq1 net with a 44.62 point symbol (Figure b). The established layers are stacked in the direction of c with an interlayer distance of approximately 8.26 Å (Figure c,d), which corresponds to the d-spacing calculated from the (002) diffraction peak in the PXRD pattern (Figure S5). Their arrangement is governed by hydrogen bonding (Figure c) and halogen interactions (Figure d). The change in the interlayer distance after the sample was soaked in various solvents can, however, be inferred from the splitting of this peak. This implies that the interlayer interactions could be affected by these solvents and, therefore, the interactions between them. Based on the N2 adsorption isotherm collected at 77 K (Figure S13), the BET surface area of the sample was revealed to be 1.1395 m2·g–1. As calculations using PLATON suggested negligible interlayer void, this value should primarily represent the external surface area. The structures of I Sm and I Gd are equivalent to I Eu, so they will not be discussed in detail.

3.2.2. [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu)

The structure of II Eu features a 3D framework (Figure a) and is built up of two independent capped square antiprismatic CSA-{EuIIIO9} motifs one of which is more distorted than the other (Figure S14a). The distortion is introduced through the disordering of the coordinating organic ligands (Figure S3). Disregarding the difference in the distortion, the nine O atoms of both CSA-{EuIIIO9} motifs are from two chelating-bridging μ2:η1η1η1η1 tbta2–, two bridging μ2:η1η1 formate ligands, and three water molecules. The lack of the bridging μ4:η1η1η1η1 mode of tbta2– should be noted (Figure S12). Additionally, the spatial arrangements of the two carboxylates of formate ligands in II Eu are syn-anti bridging, which differs from the syn–syn bridging and the chelating-bridging in I Eu. The change in coordination modes of these linkers correlates with the framework transformation, particularly the syn-anti conformation of the formate ligands, which should account for the existence of CSA-{EuIIIO9} as a discrete motif.

2.

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Views of (a) the 3D framework of [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O in II Eu, (b) the simplified sqc187 net topology, (c) the hydrogen bonding, and (d) the halogen interactions.

The two independent CSA-{EuIIIO9} motifs are linked to their equivalents in the b direction through tbta2– leading to two independent 1D chains (Figures S14b and S15). Intriguingly, the phenyl rings of the two tbta2– molecules coordinating to the same EuIII are almost perpendicular to each other. Similar arrangements are observed between the phenyl rings of the two tbta2– ions in adjacent chains. These differ from the arrangement of tbta2– in I Eu of which the two tbta2– molecules anchoring to the same dimer at opposite sites are almost coplanar. Each established chain is further connected by formate ligands to two other neighboring chains in an alternating fashion and in such a way that each CSA-{EuIIIO9} is connected to four other CSA-{EuIIIO9} motifs in the ac plane (Figure a). These linkages lead to the formation of the 3D framework.

The framework contains 1D channels with an opening of approximately 3.9 × 4.6 Å2 running along the b direction, which accounts for approximately 14.5% void, calculated using PLATON. Its inner surface is defined by the bromide of tbta2–. Within the channels, the crystallizing water molecules are accommodated and transfixed to the framework by hydrogen bonding (Figure c) and halogen interactions (Figure d). Using the two independent coordination motifs as nodes, the framework can be simplified to a 4-connected, two-nodal sqc187 net with {66}­{65.8} point symbol (Figure b).

3.2.3. [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd)

The crystal structure of III Gd is constructed based on the 1D ladder of [GdIII(tbta)1.5(DMF)­(H2O)4] (Figure a). The basic building motif of the framework is a distorted square-antiprismatic SAPRS-{GdIII–O8} defined by eight O atoms from two bridging μ4:η1η1η1η1 tbta2–, one monodentate μ1:η1η0η0η0 tbta2–, one DMF, and four water molecules (Figure S4). Every two SAPRS-{GdIII–O8} motifs are fastened through two carboxyl bridges to form a {O6GdIII(OCO)2GdIIIO6} dimer. Each dimer is further linked to other equivalents by tbta − linkers along the c direction. These chains are noticeably enveloped by monodentate tbta2–, coordinating DMF, and water molecules, all of which facilitate hydrogen bonding (Figure c) and halogen interactions (Figure d) leading to their nonporous assembly (Figure b). The crystallizing water molecules residing between these chains amplify hydrogen bonding. In comparison with I Gd, the change in framework dimensionality from 2D (I Gd) to 1D (III Gd) should be noted because it is rarely observed, with only a few previously reported cases. ,,

3.

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Views of (a) the [GdIII(tbta)1.5(DMF)­(H2O)4] chain in III Gd, (b) the packing of these chains regulated by (c) hydrogen bonding, and (d) halogen interactions.

3.3. Solvent-Driven Framework Transformation

As the transformation of I Eu (2D) to II Eu (3D) and I Gd (2D) to III Gd (1D) occurred through dissolution in water and could not be prepared by direct synthesis, the replacement of DMF by water molecules can be assumed. This assumption also corresponds with their chemical formulas: [EuIII 2(tbta)2(COO)2(DMF)4] (I Eu) compared with [EuIII 2(tbta)2(COO)2(H2O)6]·5H2O (II Eu), and [GdIII 2(tbta)2(COO)2(DMF)4] (I Gd) compared with [GdIII(tbta)1.5(DMF)­(H2O)4]·H2O (III Gd). Considering the modes of coordination adopted by the organic linkers (Figure S12), the rearrangement of the organic ligands after the ligand substitution is also certain.

Concerning II Eu, which was obtained in pure form, its reverse transformation to I Eu was further investigated. The use of a water/DMF mixed solvent (1-to-1 by volume) could provid I Eu in pure form (Figure S8). The transformation between I Eu and II Eu is therefore completely reversible (I Eu ⇌ II Eu) promoted by the appropriate solvent. It should be noted, nonetheless, that the transformation of I Eu to II Eu happened more rapidly than the reverse process, from which the substitution of DMF by water can be assumed to be more rapid than the reverse. This reversibility can be confirmed by both the PXRD (Figure S8) and FT-IR experiments (Figure S16).

3.4. Air Sensitivity and Thermal Robustness of I Eu and II Eu

The air sensitivity of I Eu (2D) and II Eu (3D) under ambient atmosphere was studied using PXRD experiments (Figure S17). I Eu was stable for up to a week, whereas II Eu could last for at least a month. The better stability of II Eu under atmospheric air may be rationalized by its higher-dimensional framework structure (3D) and the abundance of water molecules within the voids. As the structural change of I Eu under ambient atmosphere was gradual, its thermal robustness was evaluated in comparison to II Eu. Based on the thermogravimetric/differential thermal analysis (Figure S18), their behavior corresponds well with the crystal structures, displaying two major weight losses in good accordance with the liberation of the solvent molecules and the decomposition of the organic linkers. The ready and gradual loss of DMF in I Eu under the flow of N2 gas even at temperatures lower than 100 °C, agrees with its labile nature and weak bonding to EuIII. This is contrary to II Eu of which the onset of dehydration occurred at ca. 100 °C, followed by a plateau extending to approximately 300 °C.

I Eu and II Eu were treated at 80 and 150 °C for 30 min after which their PXRD patterns were collected (Figure S19). To our surprise, I Eu could withstand the treatment at both temperatures, whereas II Eu showed changes in its PXRD pattern starting from the treatment at 80 °C. These results indicate that I Eu is more thermally robust than II Eu, despite being unstable in water. Regarding II Eu, the essential presence of water molecules is conclusive.

3.5. Catalytic Potential of I Sm, I Eu, and I Gd

Through the careful selection of LnIII as inorganic coordination nodes, DMF as a coordinating solvent, and tbta2– as an organic linker, several acidic and basic sites with the potential to catalyze the CO2 cycloaddition with epoxides have been identified (Figure S20). LnIII ions are hard Lewis acids in nature, and the inclusion of labile DMF allows substitution with ECH to take place and permits catalysis to occur without prior treatment to generate vacant coordination sites. The carboxylate O atoms and bromides can also function as Lewis bases. In addition, bromide may interact with the chloride of ECH through halogen interactions and promote the ring opening of ECH. Formate may help activate both epoxide substrates and CO2 through hydrogen bonding interactions.

The temperature-programmed desorption profiles of NH3 (NH3-TPD) and CO2 (CO2-TPD) (Figure S21) support the hypothesis. The profiling, carried out over the temperature range of 50–200 °C, revealed the release of NH3 to onset at approximately 80 °C and CO2 at approximately 140 °C. The requirement of high temperatures to activate the release of these gases indicates their noteworthy interactions with the framework. In addition to the CO2-TPD profiling, the interaction between I Eu (taken as a representative) and CO2 was inferred from the CO2 sorption/desorption isotherm (Figure S13). Although the CO2 uptake capacity was insignificant, i.e., 1.54 mL·g–1 at 195 K, its retention during desorption was undeniable. Such retention ensures interactions between CO2 and the framework. The ability of I Eu to attract CO2 underlines the CCU concept. However, I Sm, I Eu, I Gd, and II Eu did not show any catalytic activity in the absence of solvent and TBABr at room temperature (31–33 °C) and ambient CO2 pressure. Experiments at elevated temperatures were thus decided upon. This limited the evaluation to only I Sm, I Eu, and I Gd because II Eu was revealed to be thermally unstable by the thermogravimetric/differential thermal analysis (Figure S19). As references, the catalytic activities of the proligand (H2tbta) and the nitrate salts of LnIII (SmIII(NO3)2·6H2O, EuIII(NO3)2·6H2O, and GdIII(NO3)2·6H2O) were also assessed (Table ). Their incapability to catalyze the reaction was confirmed.

1. Catalytic Performances of the Non-Activated Catalysts in CO2 Cycloaddition with ECH under Ambient CO2 Pressure without Additional Solvent and TBABr .

3.5.

Entry Catalyst (mol. %) Temp. (°C) Time (h) %Conversion %Selectivity %Yield TON TOF (h–1)
1 H2tbta 90 12 trace trace trace - -
2 SmIII(NO3)•6H2O 90 12 trace trace trace - -
3 EuIII(NO3)3•6H2O 90 12 trace trace trace - -
3 GdIII(NO3)3•6H2O 90 12 trace trace trace - -
4 I Eu (0.15 mol. %) 60 12 trace trace trace - -
5   70 12 4.0 50 2.0 13 1.1
6   80 12 12 80 8.1 54 4.5
7   90 12 52(±6) 89(±1) 49(±2) 327 27
8 I Eu (0.15 mol. %) 90 3 6(±1) 50(±0) 3(±0) 20 7
9   90 6 15(±4) 83(±3) 15(±4) 100 17
10   90 8 29(±2) 90(±1) 28(±1) 187 23
11   90 12 52(±6) 89(±1) 49(±2) 327 27
12   90 24 90(±2) 91(±2) 84(±6) 560 23
13 I Eu (0.30 mol. %) 90 12 83(±2) 89(±1) 81(±3) 270 22
14   90 24 97(±1) 90(±1) 94(±3) 313 13
15 I Sm (0.15 mol. %) 90 12 50(±4) 90(±1) 46(±4) 307 26
16 I Gd (0.15 mol. %) 90 12 54(±3) 90(±2) 51(±2) 340 28
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a

Calculated from triplicate results unless specified otherwise.

b

Results from one repeat.

c

Using mesitylene as an internal standard.

d

Calculated based on % yield.

Taking I Eu as a representative, its activities at 60–90 °C were evaluated using 20 mmol of ECH as a model epoxide and 0.030 mmol (0.15 mol %) of the catalyst (Table ). The reactions were conducted under ambient CO2 pressure without an additional solvent and TBABr. After 12 h of reaction time, the direct dependence of %conversion, %selectivity, and %yield on temperature was explicit. The highest %conversion of 52 ± 6%, %selectivity of 89 ± 1%, and %yield of 49 ± 2% were obtained from the reaction conducted at 90 °C. The corresponding TON and TOF values calculated from %yield are 327 and 27 h–1, respectively.

At 90 °C, the reaction time was varied from 3 to 24 h using the same mole ratio of ECH and I Eu (0.15 mol %, Table ). Similarly, a direct dependence of the catalytic performance on the reaction time was observed. The best performance was obtained at 24 h, with %conversion, %selectivity, and %yield of 90(±2), 91(±2), and 84(±6), respectively. The corresponding TON and TOF were 560 and 23 h–1. Apparently, the prolongation of the reaction time improved %conversion and %yield more substantially than %selectivity. Keeping the reaction temperature at 90 °C, an increase in the mole ratio of I Eu from 0.15 mol % (0.030 mmol) to 0.30 mol % (0.060 mmol) was also attempted using 12 and 24 h of reaction time (Table ). The doubled amount of catalyst did not improve %selectivity but significantly enhanced the conversion and, therefore, the yield. The highest %yield of 94(±3) was achieved from a 24-h reaction. It should be emphasized that the catalyst was employed in all the experiments without prior activation.

Based on the conditions under which the highest TOF of I Eu was obtained, i.e., 0.15 mol % of the catalyst, 90 °C, and 12 h of reaction time , the catalytic performances of I Sm and I Gd were evaluated (Table ). Intriguingly, I Sm performed poorer than I Eu and I Gd, respectively. The best %conversion, % selectivity, and %yield were obtained from I Gd, i.e., 54(±3), 90(±2), and 51(±2), respectively. The corresponding TON and TOF values were 340 and 28 h–1. This tendency agrees with the increasing trend of LnIII acidity from SmIII to GdIII and reflects the involvement of LnIII in the catalysis. Regarding the incomplete selectivity, the byproduct was identified as 1-bromo-3-chloro-2-propanol based on 1H NMR spectroscopy.

Compared with the other CP-based catalysts (Figure and Table S7), the performances of I Sm, I Eu, and I Gd are among the frontier group. − The yielded TOFs are superior to those of other CP-based catalysts tested under similar conditions (0.15 mol % catalyst, 90 °C). There are a few CPs showing better activities, namely [Tb4(tcbpp)2(μ3–OH)4(H2O)9]­Cl8·21H2O of which chloride was exchanged for iodide (tcbpp = 2,4,6-tris­(1-(4-carboxylatobenzyl)­pyridinium-4-yl)­pyridine) and {[M6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O}n (M = Co, Ni, H3TATAB = 4,4′,4″-striazine-1,3,5-triyl-tri-p-aminobenzoic acid). Nonetheless, they needed to be activated before use, whereas I Sm, I Eu, and I Gd did not require any activation.

4.

4

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Comparison of the highest TOF values obtained from I Sm, I Eu, and I Gd with other CP-based catalysts effective under similar conditions using ECH as a model epoxide. −

The catalysts were unfortunately unstable, as shown by the PXRD experiments of I Eu (Figure S22). The framework structure apparently started to decompose after the first cycle of catalysis and was completely demolished after the second cycle. Correspondingly, their catalytic performances declined (Figure S23). The EDS analyses (Table S8) and FT-IR experiments (Figure S24) suggest the loss of bromide from the framework during catalysis. This is likely the reason for the collapse of the framework structure and the decline in catalytic activity. The release of bromide was thus qualitatively tested through a gravimetric analysis. The addition of silver nitrate solution into the yellow filtrate separated from the catalytic reaction led to the formation of a yellowish precipitate (Figure S25). The precipitate was reactive under ambient light and rapidly turned black color. This is characteristic of silver bromide. Although the instability of the catalyst is unfortunate, the results reported in this work provide the very first experimental evidence for the involvement of bromide from the organic linker, i.e., tbta–, in the activation of epoxide and therefore the possibility of using solid cocatalysts.

3.6. Selective Catalysis toward ECH

In addition to ECH, a range of epoxides, including 1,2-epoxyoctane, butyl glycidyl ether, benzyl glycidyl ether, 1,2-epoxy-3-phenoxypropane, styrene oxide, and allyl glycidyl ether, was tested using I Eu. The reactions were performed at 90 °C for 12 h using 20 mmol of the epoxide substrate and 0.15 mol. % (0.030 mmol) of the representative I Eu (Table S9). Practically, I Eu could not catalyze any reaction with the other tested epoxides besides ECH. It is therefore conclusive that I Eu was selective toward ECH. A few factors may be responsible for this selectivity. The first factor could be the effective molecular sizes of the epoxides and the corresponding products (Table S9) relative to the interlayer distance, as well as their structural hindrance. If the catalysis occurred inside the interlayer space, only ECH is hypothetically small enough to access this space. The second factor could be polarity. The longer the chain attached to the epoxide ring, the less polar it becomes. As the activation of the epoxide occurs through the polar part of the molecule and the framework, the less polar molecule should result in less effective activation. This factor should influence both the framework surface and the interlayer space. Based on these results, the catalysis by I Eu should happen mostly on the framework surface, although the possibility of occurrence inside the interlayer space cannot be ignored.

3.7. Computational Study

3.7.1. Solvent-Driven Transformation of I Eu to II Eu

Based on the optimized asymmetric models (Figure S26), the I Eu ⇌ II Eu transformation was studied. The substitution of the coordinating DMF (in I Eu) by water molecules (to yield II Eu) in the forward reaction was assumed, and the change in the coordinate covalent bonds from EuIII···DMF (Figure S26b) to EuIII···H2O (Figure S26c) was anticipated. From the DFT calculation, the average ΔE bind values for EuIII···DMF (−9.86 kcal·mol–1) and EuIII···H2O (−20.78 kcal·mol–1) were negative, indicating that both coordination types are stable. As the value of EuIII···H2O was substantially more negative than that of EuIII···DMF, the EuIII···H2O interaction, and thus the forward reaction, are more favorable. This is consistent with the experimental results showing the rapid transformation of I Eu (EuIII···DMF) to II Eu (EuIII···H2O) and the slower reverse process.

3.7.2. Binding Energy and Ligand Substitution of DMF by ECH

Founded on the single crystal structure of I Eu containing four coordinating DMF molecules, four structural candidates where the ECH substitution could occur were assumed and labeled by the O atoms of DMF, i.e., O1, O2, O3, and O4 (Figure a). The model was constructed and successfully optimized (Figure S27). After the optimization, the DMF molecule was removed from each possible site and subsequently reoptimized to prepare the opened coordination site for ECH substitution (Figure b). Then, the ECH was positioned at each site, and the complex structure was reoptimized, leading to four different I Eu···ECH models, as exemplified in Figure c. The calculations provided negative binding energy E bind:IEu ... ECH for all the possibilities, i.e., −7.67, −5.79, −7.04, and −7.67 kcal·mol–1 for O1, O2, O3, and O4, respectively. These results imply that substitution can occur at every possible site. It should be noted, nonetheless, that the substitution has been assumed because the catalysts were used without pretreatment to free the coordination site. Among all the possibilities, the substitution at O2 provided the less negative value, suggesting it is somewhat less favorable than the other sites. On the other hand, the preferable ECH substitution sites can be O1 or O4 coordinating DMF positions, as they provided the largest negative values.

5.

5

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(a) View of the I Eu model after optimization with four potential sites for ECH substitution, and the representative presentations of (b) the optimized I Eu with one DMF removed from the O1 site, (c) the optimized I Eu after the insertion of ECH at the O1 site, (d) and (d,(e) the corresponding ESP surfaces of (b) and (c), respectively. The yellow arrow indicates the interacting site of ECH on the EuIII.

In addition, the electrostatic potential (ESP) surfaces of both the I Eu and I Eu···ECH models were calculated and visualized (Figure d,e). Prior to the ECH insertion, the EuIII site was opened and surrounded by the positive surfaces (blue region), indicating the distribution of positive charges and the Lewis acid site introducing the I Eu···ECH complex. Combined with the calculated binding energy, the ECH is likely to interact well with the metal site, as shown by the negative value, implying that this step might be the first step of the CO2 cycloaddition reaction with ECH using I Eu as a catalyst in the absence of the cocatalyst.

4. Conclusions

In summary, the use of H2tbta resulted in a new series of 2D frameworks of I Eu, I Sm, and I Gd. Through dissolution in an appropriate solvent, the framework transformations, including the reversible I Eu ⇌ II Eu, have been revealed. Taking I Eu and II Eu as representatives, computational studies were conducted, and negative ΔE bind values for both EuIII···DMF and EuIII···H2O were obtained. The negative values indicate stable interactions between EuIII and both solvent molecules, although the more negative value of EuIII···H2O suggests a more favorable interaction. This corresponds well with experimental results showing a more rapid transformation of I Eu to II Eu. In the cases of I Sm and I Gd, the transformation resulted in multiphasic products, most of which were unknown. III Gd was nonetheless identified. The chemical stability of I Eu in solvents of different polarity and the thermal stability of I Eu and II Eu were studied. Apparently, I Eu was not stable in highly polar solvents, including water, and was sensitive to air humidity. However, it was more thermally stable than II Eu that collapsed upon the loss of occluded water molecules.

All of the as-synthesized I Eu, I Sm, and I Gd could catalyze the cycloaddition reactions of CO2 with ECH without any additional solvent and TBABr under atmospheric CO2 pressure. Elevated temperature, however, was necessary. The performances of the catalysts were shown to directly depend on the reaction temperature and time and to be selective for ECH. The best performance of I Eu (as a representative of all the catalysts), reflected through a %conversion of 97(±1), %selectivity of 90(±1), and %yield of 94(±3), was achieved from the reaction using 0.30 mol % of the catalyst conducted at 90 °C for 24 h. Under the same conditions (0.15 mol % of the catalyst, 90 °C, and 12 h), I Gd performed better than I Eu and I Sm, successively. The highest TON and TOF values obtained from I Gd were 340 and 28 h–1. The robustness of the catalyst (I Eu) upon recycling was a limiting issue. The decomposition of the catalyst was evidently due to the loss of bromide. The computational studies were consistent with the experimental results.

Supplementary Material

ic5c02558_si_001.pdf (1.5MB, pdf)

Acknowledgments

This research was cofunded by the National Research Council of Thailand (NRCT, Contract Number N42A670317) and Chiang Mai University. The authors are grateful to the Office of the Permanent Secretary of the Ministry of Higher Education, Science, Research and Innovation through the Reinventing University project and the Networked Exchange, United Strength for Stronger Partnerships between Japan and ASEAN (NEXUS) program for their support in collaboration. M. Sinchow, T. Chuasaard, and N. Chiangraeng thank Chiang Mai University for the support provided under the CMU Proactive Researcher Program.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c02558.

  • Details on materials and methods, crystallographic data and other results including additional crystallographic presentations, PXRD patterns, FT-IR spectra and band assignments, thermogravimetric/differential thermal analysis results, exemplified 1H NMR spectra, lists of hydrogen bonding and halogen interactions, elemental analysis, SEM images and EDS analysis, N2 and CO2 sorption/desorption isotherms, NH3-TPD and CO2-TPD profiles, as well as additional results of the catalytic experiments including the exemplified 1H NMR spectra (PDF)

The authors declare no competing financial interest.

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