From 1D zigzag chains to 3D chiral frameworks: synthesis and properties of praseodymium(III) and neodymium(III) coordination polymers

Abstract

Reaction of H₃TDA with Pr^III/Nd^III salts resulted in two coordination polymers, [Ln(H₂O)₄(HTDA)(H₂TDA)]·H₂O (Ln = Pr(1) and Nd(2), H₃TDA = 1H-[1,2,3]-triazole-4,5-dicarboxylic acid), crystallizing in the monoclinic P2₁/c space group with a one-dimensional zigzag chain structure. The number of ligands and water molecules coordinated to the Ln^III ions can be reduced by a hydrothermal method and the products transformed into {[Ln(TDA)(H₂O)]·2.5H₂O}_n (Ln = Pr(3) and Nd(4)), crystallizing in the trigonal P3₁21 space group with a three-dimensional chiral framework structure. The compounds were characterized by elemental analysis, IR, PXRD, circular dichroism spectra and single crystal X-ray diffraction. The variable temperature magnetic susceptibility studies indicated that there are antiferromagnetic interactions between the Ln^III ions in 1–4. Gas sorption and separation were studied as well.

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Introduction

Metal–Organic Frameworks (MOFs) have attracted chemists' interest recently for not only the fascinating structures but also the interesting properties such as in catalysis,¹ magnetism,² gas storage and separation,³ luminescent sensing⁴ and drug delivery.⁵ The structures of coordination polymers can be fine-tuned by the metal nodes and organic linkers, which is beneficial to the systematic engineering of both chemical and physical properties. Induction of chirality into coordination polymers was expected to gain specific properties such as enantioselective catalysts and chiral separator, and hence show promising applications in the pharmaceutical engineering and petrochemical industry.⁶ Chiral MOFs can be synthesized by using achiral or chiral bridging ligands. However, during the synthetic process, the chiral features usually disappear as racemic products; even the individual single crystal with small Flack parameter is chiral, the bulk sample may be a racemic mixture of 50% left-handed and 50% right-handed crystals.⁷ The current challenge for chemists to the chiral applications mentioned above is to obtain bulk sample of expected chirality with enantiomeric excess as high as possible. Though it is a common view that using chiral ligands to synthesize homochiral coordination polymers is rational, using achiral ligands to obtain homochiral product is difficult but still in progress for not only the better understanding of the chiral assemble process but also the cheaper and easy-obtained raw materials compared to the enantiomeric pure ones.

Strategies to the design and synthesis of aimed MOFs have been realized as crystal engineering or isoreticular chemistry.⁸ Considering that nodes of metal ions (limited in the periodic table) and organic bridging ligands are two of the most important components of MOF, judiciously selection of organic ligands is the key factor. The coordination modes related with specific symmetries of the bridging ligands play an important role to the final structures and properties. Our group has proposed a symmetry approach to the construction of porous coordination polymers. For instance, porous Ln- and 4f–3d coordination polymers with ligands of C₂-like symmetry, such as pyridine-2,6-dicarboxylic acid,⁹ thiophene-2,5-dicarboxylic acid¹⁰ and furan-2,5-dicarboxylic acid (H₂FDA),¹¹ have been demonstrated to exhibit interesting properties including luminescent sensing, radical adsorption, magnetic properties and gas sorption and separation properties. As a multi-dentate achiral ligand containing bi-functional groups, 1H-[1,2,3]-triazole-4,5-dicarboxylic acid (H₃TDA) was given considerable attention for both the variation of the coordination modes and the symmetry (C_s for H₂TDA⁻ and HTDA²⁻, C_2v for TDA³⁻) which have been studied by us¹² and other groups.¹³ As a continuous work, we reported herein four lanthanide coordination polymers [Ln(H₂O)₄(HTDA)(H₂TDA)]·H₂O (Ln = Pr(1) and Nd (2)) and {[Ln(TDA)(H₂O)]·2.5H₂O}_n (Ln = Pr(3) and Nd(4)). 1 and 2 show one-dimensional zigzag chain structure. Interestingly, the number of the H₃TDA ligand and water molecule coordinated to the Ln^III ions can be reduced by hydrothermal method to the raw materials or 1 and 2, and the products transformed into three-dimensional homochiral samples of 3 and 4, which were fully characterized by single crystal X-ray diffraction and solid state circular dichroism spectroscopy. The magnetic properties of 1–4 and gas sorption and separation of the porous coordination polymers 3 and 4 were studied in detail.

Results and discussion

Synthesis

The synthetic route of 1–4 is shown in Scheme 1. By using the reflux method with the raw material, 1D zigzag chain compounds 1 and 2 were obtained. Considering that high temperature can reduce the number of the coordinated ligands and enhance the possibility of obtained high-dimensional frameworks without the aid of any auxiliary ligands, hydrothermal method was applied to both the raw materials of 3 and 4, and the one-dimensional chain compounds of 1 and 2. Interestingly, the two synthetic routes both resulted in the same homochiral coordination polymers of 3 and 4, which indicate that the three-dimensional chiral frameworks of 3 and 4 are thermodynamics stable in the reaction condition. It is noted that the crystal structures of isostructural coordination polymers to 3 and 4 crystallized in enantiotopic P3₂21 space group have been well studied by Su et al. during our studies.¹⁴ So this work mainly focuses on the synthetic strategy, spectra, magnetic and gas sorption properties of the title compounds. The phase purity of the bulk samples of 1–4 was well checked by PXRD (Fig. S1†).

Scheme 1 The synthesis route of 1–4.

Crystal structures of 1–4

X-ray crystallography reveals that 1 and 2 both crystallize in monoclinic space group P2₁/c. They are isostructural so that the presentation and discussion was restricted to 1. Each Pr^III ion lies in a monocapped square-antiprism coordination environment and is surrounded by one nitrogen atom and four oxygen atoms from three individual H₃TDA ligands, and four water molecules, as shown in Fig. S2.† The Pr–N distance is 2.699(4) Å and the Pr–O bonds range from 2.397(3) to 2.621(4) Å. The O–Pr–O bond angles range from 66.80(16) to 144.07(14)°, and the O–Pr–N bond angles range from 61.68(10) to 133.36(11)°, respectively. There are two types of H₃TDA exhibiting as bridged (HTDA²⁻) or terminal (H₂TDA⁻) ligands. As shown in Fig. 1, the μ₂-HTDA²⁻ dianions link two crystallographically equivalent Pr^III centers to form the infinite 1D zigzag chain structure. The neighboring Pr⋯Pr distance is 6.776 Å. For each H₂TDA⁻ ligand, one carboxyl oxygen atom doesn't participate in coordination (Scheme 2a). It forms O–H⋯O=C hydrogen bonding interactions (2.978, 3.023 and 3.054 Å) between the chains, which generate a 3D supramolecular architecture with the nearest interchain Pr⋯Pr distance of 7.434 Å (Fig. S3†).

Fig. 1 1D zigzag chain structure of 1. O, red; C, gray; N, blue; Pr, green.

Scheme 2 Coordination modes of H₂TDA⁻ (a) and TDA³⁻ (b) in 1–4.

3 and 4 crystallize in trigonal space group P3₁21, with an absolute structure parameter of −0.02 and 0.00, respectively. Although isostructural coordination polymers crystallized in enantiotopic P3₂21 space group were known, the aim of structural description here is only for better understanding the properties. Since 3 and 4 are also isostructural, the structure of 3 will be described as an example. As shown in Fig. S4,† the asymmetrical unit contains one Pr^III, one μ₄-TDA^3- and one μ₂-water molecule. Each Pr^III ion is nona-coordinated by four carboxylate oxygen atoms and three nitrogen atoms from four individual TDA³⁻ ligands, and two water molecules, forming a distorted square antiprism geometry. The Pr–N distances are 2.648(4) and 2.665(3) Å, and the Pr–O bonds range from 2.384(2) to 2.666(2) Å. These distances are comparable to the reported values. Each μ₄-TDA³⁻ anion is quasi-planar and connects four Pr^III ions in a symmetrical coordination mode (Scheme 2b) with C₂ symmetry. The Pr⋯Pr distances around TDA³⁻ ligand are 4.774 Å (Pr1⋯Pr1A and Pr1B⋯Pr1A) and 6.928 Å (Pr1⋯Pr1C and Pr1B⋯Pr1C), respectively. It is noteworthy that the μ₂-water molecules alternately bridge the Pr^III cations to form a 1D right-handed helical infinite chain along c axis (Fig. 2). The 3₁ axis of the 1D helical chain and the interchain helical coordination bonds spread the chirality to transfer uniformly along the ab plane, and further leading to the formation of a 3D chiral framework (Fig. 3).

Fig. 2 The homochirality transfer among the neighboring helical chains through the interchain coordination bonds. O, red; C, gray; N, blue; Pr, green.

Fig. 3 3D chiral framework of 3 viewed down c axis. O, red; C, gray; N, blue; Pr, green.

IR and UV-vis spectra

The FT-IR spectra of 1–4 all present strong broad bands of ν(H₂O), centered at ca. 3450 cm⁻¹ for 1 and 2, ca. 3400 cm⁻¹ for 3 and 4, respectively. The spectra of 1 and 2 are similar because of the same backbones. For 1, a very strong peak of ca. 1663 cm⁻¹ is attributed to the antisymmetry stretching of carboxylate group, and the symmetrical carboxylate stretching bands appear at ca. 1459 and 1411 cm⁻¹. The difference between the ν_as and ν_s is 252 and 204 cm⁻¹, which indicates both the bridge and mono-dentate modes of the carboxylic groups,¹⁵ and is in accord with the X-ray crystal analysis. The spectra of 3 and 4 are also similar. For 3, the ν_as of carboxylate group is shifted to higher frequency at 1638 cm⁻¹ compared to 1 and the ν_s of carboxylate groups appears at 1428 cm⁻¹. Detailed IR peaks of 1–4 were summarized in experimental section.

The UV-vis spectra of 1–4 were measured in solid state. The spectra of 1 and 2 are similar, both exhibit strong peaks at ca. 266 nm, which are assigned to the intraligand π–π* transition. The hypersensitive transition bands of the Pr^III and Nd^III ions were not observed. For 3, the strong peak at ca. 218 nm with a shoulder peak at ca. 269 nm is due to the intraligand transitions. The bands at 450, 469, 482 and 595 nm are correspond to ³H₄ → ³P₂, ³H₄ → ³P₁, ³H₄ → ³P₀ and ³H₄ → ¹D₂ transitions, as shown in Fig. S5.† For 4, the strong peak at ca. 212 nm with a shoulder peak at 270 nm is also due to the intraligand transitions. The hypersensitive transition bands are observed at ca. 519 and 581 nm which can be assigned to the ⁴I_9/2 → ⁴G_7/2 and ⁴I_9/2 → ⁴G_5/2, ²G_7/2 transitions, respectively. The bands at 429, 468, 683, 743 nm are correspond to the ⁴I_9/2 → ²P_1/2, ⁴I_9/2 → ²G_9/2, ⁴I_9/2 → ⁴F_9/2 and ⁴I_9/2 → ⁴F_7/2 transitions, respectively.¹⁶

Circular dichroism spectra

The results of solid-state circular dichroism measurement of 3 and 4 for bulk crystals indicated that both of the 3D coordination polymers display dichroic signals corresponding to the absorptions of the UV spectra (Fig. 4). On the basis of the absolute configurations, the results can be inferred that the crystallizations of 3 and 4 are enantiomeric excess rather than racemic. This could be explained by the fact that the initial crystals formed may seed the handedness of the bulk product, and the stochastic generated chirality can be preserved and further propagated to the bulk product. However, the detailed mechanism of such homochiral crystallization with achiral ligand is not clear and difficult to achieve in current time, and it is a great challenge for chemists to study further.

Fig. 4 The solid-state circular dichroism spectra of 3 (a) and 4 (b) for the bulk samples.

Magnetic properties

For lanthanide compounds, the lanthanide ions always possess the first-order orbital angular momentum, which prevents the use of spin-only hamiltonian for quantitative study of the magnetic interactions. And the nature of the magnetic interactions between lanthanide ions is still unclear because the 4f electrons in the inner orbitals always lead to weak exchange interactions, which is still a research interest for magneto chemists and deserve to be investigated.¹⁷

Magnetic properties of 1–4. The temperature dependence of magnetic susceptibilities of 1–4 are shown in Fig. 5. The χ_MT values of 1–4 at 300 K are 1.61, 1.61, 1.58 and 1.55 cm³ K mol⁻¹, respectively, which are in accord with the expected 1.60 (for Pr^III) and 1.63 (for Nd^III) cm³ K mol⁻¹, per free Ln^III ion in the ground state ^2S+1L_J. (³H₄, g = 4/5 for Pr^III ion and ⁴I_9/2, g = 8/11 for Nd^III ion). The decrease of the χ_MT values when lowering the temperature may arise from the depopulation of Stark sublevels and/or antiferromagnetic interactions between Ln^III ions. The Curie–Weiss fits [χ_M = C (T⁻¹ − θ)] of 1–4 in the temperature range from 50 to 300 K give Curie constants of 1.77, 1.75, 1.69, and 1.79 cm³ K mol⁻¹ and Weiss constants of −31.52, −40.87, −17.84 and −51.25 K, respectively. The negative θ values suggest the existence of antiferromagnetic interactions between the Ln^III ions and/or crystal field effects. To quantitative analysis the magnetic interactions in such system need the magnetic susceptibility expressions as the function of temperature, which is difficult to derive in such 1D/3D systems with unquenched orbital angular momentum. To obtain a rough quantitative estimation of the magnetic interaction parameters between Ln^III ions, the Pr^III (or Nd^III) ions may be assumed to be populated in the splitting of the m_J energy levels (Ĥ = ΔĴ_z²) in a axial crystal field.¹⁸ Then χ_Pr and χ_Nd can be described as eqn (1) and (2), respectively.

Fig. 5 Temperature dependence of χ_MT(○) for 1 (a), 2 (b), 3(c) and 4(d). The solid lines represent the theoretical curves based on the corresponding equations.

In the expressions, Δ is the zero-field splitting parameter and the Zeeman splitting was treated isotropically for the sake of simplicity.¹⁹ The magnetic interactions transmitted by the TDA³⁻ ligand were considered as the zJ′ parameter based on the molecular field approximation.²⁰ For 3 and 4 with 3D structure, the zJ′ parameters denote the average values of the magnetic interactions among the Ln^III ions. Attempt to fit the magnetic susceptibility data for 2 and 4 with the above equations in the whole temperature range was failed, which may be due to the unconsidered energy levels in the approximate equations. The least-squares fit to the data leads to Δ = 0.13 cm⁻¹, zJ′ = −0.39 cm⁻¹, g = 0.74, TIP = 980 × 10⁻⁶ cm³ mol⁻¹, R = ∑(χT_obsd − χ′T_calcd)²/∑(χ_obsd)² = 1.00 × 10⁻³ for 1 in the range of 2–300 K. For 2, the best fit gives Δ = 2.00 cm⁻¹, zJ′ = −0.50 cm⁻¹, g = 0.72, TIP = 700 × 10⁻⁶ cm³ mol⁻¹, R = 3.05 × 10⁻⁴ in the range of 50–300 K. For 3, the parameters are Δ = 1.62 cm⁻¹, zJ′ = −0.33 cm⁻¹, g = 0.83, TIP = 0 cm³ mol⁻¹, R = 7.11 × 10⁻⁵ in the range of 2–300 K. And for 4, Δ = 1.21 cm⁻¹, zJ′ = −0.96 cm⁻¹, g = 0.70, TIP = 820 × 10⁻⁶ cm³ mol⁻¹, R = 4.39 × 10⁻⁵ in the range of 50–300 K. The failure to fit the magnetic data of 2 and 4 at low temperature is probably due to the neglecting of transverse component of the crystal fields, which mixes up the m_J energy levels. Nevertheless, the results show that the model is much better to be applied for Pr^III rather than Nd^III. The negative zJ′ values suggested the antiferromagnetic interactions between the Ln^III ions in these complexes. It should be noted that the use of isotropic exchange coupling constant herein to describe the magnetic interactions between lanthanide ions is only for a rough estimation. Ab initio calculations are more helpful when more precise analysis are needed, but such analysis is beyond the scope of this work.

Gas adsorption and separation. Before gas adsorption study, thermogravimetric analyses (TGA) and powder X-ray diffraction (PXRD) patterns were studied to evaluate the stabilities of the frameworks under solvent-removed condition. As shown in Fig. S6 and S7,† the porous frameworks are retained after the removal of the guest molecules, suggesting the stabilities of the structures and providing the feasible open metal sites.

N₂ adsorption. 3 and 4 are isostructural, and only 4 was measured for gas sorption. The as-synthesized samples of 4 were immersed in methanol and dichloromethane alternately for three days, respectively. Thermal activation of the solvent-exchanged samples were heated at 235 °C for 12 h under vacuum. The N₂ sorption isotherms measured at 273 K and 298 K show that the activated 4a could adsorb only a little amount of N₂ (Fig. 6a). It reveals that the adsorption happens only on the external surface of the material due to the small channels according to the crystal structure.

Fig. 6 (a) Adsorption isotherms of 4a for CO₂ and N₂ at 273 and 298 K, respectively. Adsorption and desorption branches are shown with closed and open symbols, respectively. (b) IAST calculated selectivity from a CO₂ : N₂ of 10 : 90 gas mixture based upon the experimentally observed adsorption isotherms of the pure gases. (c) Isosteric heat of adsorption for CO₂ in 4a.

CO₂ adsorption. The CO₂ sorption measurements for 4a at 273 K show typical type I isotherms with uptake of 33.3 cm³ g⁻¹ for 4a at 1 bar, while the adsorption maximum at 298 K is 15.7 cm³ g⁻¹ (Fig. 6a). For 4a, desorption branches are consistent with that of adsorption of CO₂, which reveals the weak interaction between CO₂ and the frameworks. The uptake of CO₂ for 4a at 273 K/1 bar is about 48 times higher than the uptake of N₂ (0.7 cm³ g⁻¹).

The N₂ and CO₂ sorption strongly suggested good CO₂/N₂ separation properties of the coordination polymer. The selective sorption ability of gas mixtures in porous materials can be calculated from single-component adsorption isotherms. Ideal adsorbed solution theory (IAST) calculations indicate binary gas adsorption selectivity which based on the experimental CO₂ and N₂ isotherms at 273 K, as shown in Fig. 6b.²¹ The CO₂/N₂ selectivity of 4a is approximately 80 : 1 at 273 K and zero-loading, and shows a little increase throughout the entire pressure range measured. To the best of our knowledge, such high CO₂/N₂ selective ability has only been reported for a few MOFs for which the selectivity are obtained from experimental data.²² These results reveal that the selective ability of 4a maybe possible for applications in the separation of CO₂ from CO₂/N₂ mixtures.

The CO₂ adsorption isotherms at 273 and 298 K are used to calculate the isosteric heat of adsorption with virial method. Based on CO₂ adsorption isotherms at 273 K, the Langmuir surface areas are estimated to be 318 m² g⁻¹.²³ At zero-loading, 4a has heat of sorption (Q_st) of 36.2 kJ mol⁻¹ (Fig. 6c), which is among the highest values of MOFs.²⁴

Conclusions

In summary, three-dimensional porous chiral framework of lanthanide coordination polymers (3 and 4) have been successfully synthesized by reduced the number of the ligand and water molecule coordinated to the Ln(III) ions of the one-dimensional coordination polymers (1 and 2) under hydrothermal condition. The solid-state circular dichroism measurement of 3 and 4 for bulk crystals indicated that the crystallizations of 3 and 4 are enantiomeric excess rather than racemic. The variable temperature magnetic susceptibility studies indicate that there are antiferromagnetic interactions between Ln(III) ions in 1–4. The desolvated 4a reveal adsorption capabilities of CO₂, as well as high selective sorption of CO₂ over N_2. This work illustrates a rational synthetic strategy for the construction of high-dimensional lanthanide coordination polymer via controlling the number of coordinated water molecules by temperature, as well as obtaining chirality based on achiral ligand.

Experimental section

Crystal structure determination

Crystals of 1–4 were mounted on glass fibers. Determination of the unit cell and data collection were performed with Mo-Kα radiation (λ = 0.71073 Å) on a Rigaku 007 diffractometer and equipped with a CCD camera. The ω–φ scan technique was employed. The structures were solved primarily by direct method and second by Fourier difference techniques and refined by the full-matrix least-squares method. The computations were performed with the SHELXL-97 program.²⁵ Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter. The crystal parameters, data collection, and refinement results for 1–4 are summarized in Table 1.

Table 1 Crystal data and structure refinements for 1–4

	1	2	3	4
Formula	C8H10N6O13Pr	C8H10N6NdO13	C4H7N3O7.5Pr	C4H7N3NdO7.5
fw	539.13	545.48	358.04	361.37
T (K)	293(2)	294(2)	293(2)	293(2)
Cryst syst	Monoclinic	Monoclinic	Trigonal	Trigonal
Space group	P21/c	P21/c	P3121	P3121
a (Å)	16.477(3)	16.414(2)	8.826(1)	8.756(1)
b (Å)	6.852(1)	6.8343(7)	8.826(1)	8.756(1)
c (Å)	14.560(3)	14.543(2)	9.910(2)	9.855(2)
α (deg)	90	90	90	90
β (deg)	101.79(3)	101.808(2)	90	90
γ (deg)	90	90	120	120
V (Å^3)	1609.2(6)	1596.9(3)	668.6(2)	654.4(2)
Z	4	4	3	3
ρ (g cm^−3)	2.225	2.269	2.668	2.751
μ (mm^−1)	3.117	3.341	5.498	5.984
θ (deg)	2.53–25.01	2.54–26.40	3.37–27.48	3.39–27.48
Index ranges	−18 ≤ h ≤ 19	−20 ≤ h ≤ 20	−11 ≤ h ≤ 11	−11 ≤ h ≤ 11
	−8 ≤ k ≤ 6	−7 ≤ k ≤ 8	−11 ≤ k ≤ 11	−11 ≤ k ≤ 11
	−17 ≤ l ≤ 15	−17 ≤ l ≤ 18	−12 ≤ l ≤ 12	−12 ≤ l ≤ 12
Reflns collected/independent	7769	8664	6804	6814
	2835	3257	1028	1008
Reflns	Rint = 0.0454	Rint = 0.0313	Rint = 0.0200	Rint = 0.0377
Data/restraints/parameters	2835/15/283	3257/18/297	1028/78/112	1008/78/116
GOF on F^2	1.053	1.037	1.200	1.077
R1, ωR2[I > 2σ(I)]	0.0291, 0.0703	0.0212, 0.0500	0.0150, 0.0372	0.0180, 0.0452
R1, ωR2 (all data)	0.0351, 0.0744	0.0261, 0.0521	0.0151, 0.0373	0.0186, 0.0455
GOF on F^2	1.053	1.037	1.200	1.077

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Materials and physical techniques

Elemental analysis for C, H and N were performed on a Perkin-Elmer 240 elemental analyzer. Powder X-ray diffraction measurements were recorded on a D/Max-2500 X-ray diffractometer using Cu-Kα radiation. The FT-IR spectra were measured with a Bruker Tensor 27 Spectrometer on KBr disks. The UV-vis spectra were measured on a JASCO V–570 Spectrophotometer. Solid state circular dichroism spectra were measured using a JASCOJ-810 spectropolarimeter. Spectra were collected on powder samples embedded in KBr pellets. Magnetic susceptibility measurements were performed on a Quantum Design SQUID MPMS XL-7 magnetometer. Diamagnetic corrections were made with Pascal's constants for all the constituent atoms. The ligand was prepared according to literature method.²⁶

Preparations

{[Pr(H₂O)₄(HTDA)(H₂TDA)]·H₂O}_n (1). A mixture of H₃TDA (0.039 g, 0.25 mmol), Mn(ClO₄)₂·6H₂O (0.091 g, 0.25 mmol), Pr(ClO₄)₃·6H₂O (0.109 g, 0.25 mmol) and H₂O (15 mL) was refluxed for 2 h and filtrated. Laurel-green needle-like crystals were obtained by slow evaporation of the mother solution at room temperature for ca. one month. The product was collected after washing with water (3 × 5 mL). Yield: 43% (based on Pr). Anal. calcd for C₈H₁₃N₆O₁₃Pr (542.15): C, 17.72; H, 2.42; N, 15.50. Found: C, 17.35; H, 2.53; N, 14.96. IR (KBr, cm⁻¹): 3505 (vs), 3406 (vs), 1663 (s), 1572 (s), 1459 (m), 1411 (m), 1376 (s), 1270 (w), 1125 (w), 1013 (w), 830 (w), 810 (m).

{[Nd(H₂O)₄(HTDA)(H₂TDA)]·H₂O}_n (2). Lilac crystals were obtained following the similar procedure for 1 except that Pr(ClO₄)₃·6H₂O was substituted with Nd(ClO₄)₃·6H₂O. Yield: 39% (based on Nd). Anal. calcd for C₈H₁₃N₆O₁₃Nd (545.48): C, 17.62; H, 2.40; N, 15.41. Found: C, 17.28; H, 2.30; N, 15.13. IR (KBr, cm⁻¹): 3500 (vs), 3409 (vs), 1662 (s), 1573 (s), 1459 (m), 1412 (m), 1377 (s), 1271 (w), 1124 (w), 1014 (w), 791 (m).

{[Pr(TDA)(H₂O)]·2.5H₂O}_n (3).
Method a. A mixture of H₃TDA (0.159 g, 1.0 mmol), Mn(ClO₄)₂·6H₂O (0.091 g, 0.25 mmol), PrCl₃·6H₂O (0.089 g, 0.25 mmol) and H₂O (15 mL) was put in a 23 mL acid digestion bomb and heated at 180 °C for 3 days. Laurel-green diamond-like crystals were collected after washing with water (3 × 5 mL). Yield: 56% (based on Pr).
Method b. A mixture of 1 (0.010 g, 0.018 mmol) and H₂O (15 mL) was put in a 23 mL acid digestion bomb and heated at 180 °C for 3 days. Laurel-green diamond-like crystals were collected after washing with water (3 × 5 mL). Yield: 78% (based on Pr). Anal. calcd for C₄H₇N₃O_7.5Pr (358.02): C, 13.42; H, 1.97; N, 11.74. Found: C, 12.81; H, 1.72; N, 11.12. IR (KBr, cm⁻¹): 3407 (br), 1638 (s), 1562 (s), 1428 (s), 1288 (w), 1203 (m), 1160 (m), 829 (w), 796 (m).

{[Nd(TDA)(H₂O)]·2.5H₂O}_n (4). Lilac diamond-like crystals can be obtained following the similar procedure for 3 with both methods a and b. Anal. calcd for C₄H₇N₃O_7.5Nd (361.36): C, 13.30; H, 1.95; N, 11.63. Found: C, 13.03; H, 1.83; N, 11.57. IR (KBr, cm⁻¹): 3386 (br), 1639 (s), 1562 (s), 1429 (s), 1289 (w), 1204 (m), 1161 (m), 830 (w), 795 (m).

Low-pressure sorption experiments

All gas-sorption experiments were performed on a Quantachrome Autosorb-IQ₂ automatic volumetric instrument. All gases used are of the purity of 99.999%. The CO₂ and N₂ sorption isotherms at 273 K and 298 K were measured in a temperature-controlled water bath in the pressure range from 10⁻³ to 1 bar.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: PXRD spectra, TG spectra and additional figures. CCDC 287163, 287165, 789544 and 789545. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06629d

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