Topological isomers in lanthanide–chloranilate coordination polymers: synthesis, structure and CO₂ adsorption properties

Abstract

The effect of lanthanide contraction on the self-assembly of two-dimensional (2D) topological isomers in lanthanide coordination polymers (LnCPs) is presented in this work. Hydrothermal reactions involving LnCl₃ and chloranilic acid (H₂ca) afforded good yields of ten LnCPs with the general formula [Ln(ca)_1.5(H₂O)₃]·xH₂O (1Ln; Ln = Pr, Nd; 2Ln; Ln = Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb; x = 3.5–5). The isostructural compounds crystallize in the centrosymmetric triclinic P1̄ space group and display 2D layered networks featuring topologically distinct isomers, specifically honeycomb for 1Ln and brick-wall for 2Ln. The lattice water molecules are located in interlayered spaces with the number of water molecules per formula unit increasing as the ionic radii of Ln³⁺ decrease. The incorporation of the semi-rigid rod 4,4′-bipyridine (4,4′-bipy) into brick-wall 2D networks aims to establish a porous 3D pillared framework, leading to the creation of an intercalation network comprising the guest 4,4′-bipy and lattice water molecules, [Ln(ca)_1.5(H₂O)₂]·2H₂O·4,4′-bipy (3Ln; Ln = Tm, Yb). The physicochemical properties of the isomers 1Ln and 2Ln were clearly illustrated, highlighting their unique structural features, thermal stability, and adsorption characteristics. At both low pressure (1 bar) and elevated pressure (up to 20 bar), the thermally activated brick-wall network 2Yb demonstrated better absorption of CO₂ relative to the honeycomb network 1Pr. This study illustrates that minor variations in the size of lanthanide metal ions significantly influence the architecture and application of innovative materials through structural topological isomers.

---

Introduction

In recent decades, the advancement of lanthanide coordination polymers (LnCPs) has emerged as a rapidly expanding area within crystal engineering, structural chemistry, and chemical crystallography.¹ These polymeric organic–inorganic hybrids demonstrate a diverse array of intriguing capabilities such as luminescence sensing, optical characteristics, magnetism, and adsorption, owing to their extraordinary structural diversity.² Nonetheless, the rational design and construction of LnCPs with predicted geometries is more challenging than that of d-block metal-based CPs. This is due to the fact that harder Ln³⁺ ions possess larger coordination spheres and exhibit more flexible coordination geometries.³ Moreover, several parameters such as the metal-to-ligand ratios, solvent, temperature, time, and concentration might affect the self-assembly crystallization of the final structures.⁴ Consequently, systematic investigation on these parameters is significant for understanding the structure–property relationship in these materials. In crystal engineering, the rational selection of suitable organic linkers with beneficial characteristics has been demonstrated as a successful strategy for constructing LnCPs with expected structural topologies and desirable attributes.⁵ It is widely recognized that Ln³⁺ ions show a high affinity and a strong tendency to coordinate with hard donor atoms. Therefore, multidentate organic ligands featuring oxygen donors, including aliphatic and aromatic polycarboxylic acids, are extensively utilized as foundational components in the synthesis of new LnCPs.⁶ Diprotic organic acids including squaric acid (H₂sa), 2,5-dihydroxy-1,4-benzoquinone (H₂dhbq), and chloranilic acid (H₂ca) have been widely used as ligands in the development of functional coordination chemistry involving Ln³⁺ metals.⁷ The deprotonation of two hydroxy groups led to a dianion molecule featuring a highly delocalized π-conjugated electron system, which originated from the parent quinone.⁸ The stability and reactivity of molecules could possibly be enhanced by these features. LnCPs usually exhibit 3D structures and contain coordinated solvents at the metal centers due to the high coordination numbers of Lns. The removal of solvents creates active open metal sites, thereby enhancing adsorption properties. In the meantime, 2D CPs offer a range of distinctive advantages mainly due to their layered structure, greater flexibility, and excellent processability. With these features such materials can be easily utilized in a wide range of applications including gas storage and separation, catalysis, and electronic devices.⁹

This work presents the synthesis, topological isomers, and CO₂ adsorption properties of 2D LnCPs. Hydrothermal reactions utilizing LnCl₃ and H₂ca produced good yields of chloranilate-based LnCPs with excellent repeatability. The compounds are classified into two groups and can be chemically formulated as [Ln(ca)_1.5(H₂O)₃]·xH₂O (1Ln; Ln = Pr, Nd; 2Ln; Ln = Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb; x = 3.5–5), based on results from single crystal X-ray diffraction, elemental analysis, infrared (IR) spectroscopy, and thermogravimetric (TG) analysis. The relationship between structures-physicochemical–adsorption properties of these two structural isomers was examined and discussed. Additionally, the semi-rigid rod 4,4′-bipyridine (4,4′-bipy) was incorporated into the brick-wall 2D networks under hydrothermal conditions, with the aim of potentially constructing a porous 3D pillared framework. This results in the creation a 4,4′-bipy-intercalated network, as illustrated by the Tm- and Yb-containing compounds formulated as [Ln(ca)_1.5(H₂O)₂]·2H₂O·4,4′-bipy (3Ln; Ln = Tm, Yb). A detailed analysis of their supramolecular structures is provided comprehensively.

Experimental

Materials and methods

All commercially available chemicals and solvents were of reagent grade and were used as received without any further purification. Analyses of carbon, hydrogen, and nitrogen were performed using an LECO CHNS 932 elemental analyzer. IR spectra were acquired with a Bruker INVENIO R spectrometer in ATR mode, in the range of 400–4000 cm⁻¹. TG analysis was conducted using a TGA55 TA instrument over a temperature range of 30–800 °C, with a heating rate of 10 °C min⁻¹ in a N₂ atmosphere. Powder X-ray diffraction (PXRD) studies were performed utilizing a Rigaku SmartLab2 X-ray diffractometer equipped with graphite monochromatized Cu-Kα radiation (λ = 1.54056 Å) at 30 kV and 10 mA. The simulated XRD patterns were generated from single-crystal X-ray diffraction data and processed using the Mercury software (version 2024.3.1) from the Cambridge Crystallographic Data Centre.¹⁰ The adsorption–desorption isotherms of CO₂ were determined using a Quantachrome Autosorb iQ volumetric analyzer. High-pressure volumetric sorption isotherms for CO₂ were obtained in a pressure range of 0.1–20 bar and at temperature of 318 K using a Quantachrome iSorb HP1 analyzer. All experiments employed CO₂ with a purity of 99.995%.

Synthesis and crystallization

Compounds of the series 1Ln, [Ln(ca)_1.5(H₂O)₃]·5H₂O (Ln = Pr, Nd) and 2Ln [Ln(ca)_1.5(H₂O)₃]·3.5H₂O (Ln = Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb) were synthesized under hydrothermal conditions. Typically, a mixture of LnCl₃·6H₂O (0.1 mmol) and H₂ca (0.1 mmol) in distilled H₂O (4 mL) were placed in a 20 mL vial and stirred at ambient temperature for 10 min. The mixture was sealed tightly, placed in an oven, and then heated to 85 °C for a duration of 12 h. The reaction mixture was allowed to cool, subsequently filtered, washed with water, and dried at ambient temperature before further characterisation.

1Pr. Brown hexagonal crystals with yields of 68% based on the Pr³⁺ source. Anal. calc. for C₉H₁₈Cl₃O₁₅Pr: C 17.62, H: 2.96, N 0.00%; found: C 17.34, H 3.12, N 0.00%. IR (ATR, ν/cm⁻¹, s for strong, m medium, w weak): 3401 (m), 1777 (w), 1762 (w), 1613 (m), 1596 (s), 1509 (s), 1384 (s), 1358 (m), 1315 (s), 1299 (w), 1058 (w), 1033 (w), 1000 (w), 845 (m), 800 (m).

1Nd. Brown hexagonal crystals with yields of 72% based on the Nd³⁺ source. Anal. calc. for C₉H₁₈Cl₃NdO₁₅: C 17.53, H 2.94, N 0.00%; found: C 17.69, H 2.88, N 0.00%. IR (ATR, ν/cm⁻¹): 3401 (m), 1760 (w), 1613 (m), 1595 (s), 1510 (s), 1381 (s), 1359 (m), 1313 (s), 1294 (w), 1058 (w), 1033 (w), 1001 (w), 846 (m), 798 (m).

2Sm. Brown hexagonal crystals with yields of 79% based on the Sm³⁺ source. Anal. calc. for C₉H₁₃Cl₃O_12.5Sm: C 18.56, H 2.34, N 0.00%; found: C 18.85, H 2.40, N 0.00%. IR (ATR, ν/cm⁻¹): 3402 (m), 1762 (w), 1613 (m), 1481 (s), 1381 (s), 1258 (w), 1059 (w), 1033 (w), 1004 (m), 846 (s).

2Eu. Brown hexagonal crystals with yields of 75% based on the Eu³⁺ source. Anal. calc. for C₉H₁₃Cl₃EuO_12.5: C 18.65, H 2.26, N 0.00%; found: C 18.73, H 2.09, N 0.00%. IR (ATR, ν/cm⁻¹): 3401 (m), 1762 (w), 1613 (m), 1471 (s), 1379 (s), 1255 (w), 1063 (w), 1033 (w), 1004 (m), 846 (s).

2Gd. Brown hexagonal-like shaped crystals with yields of 84% based on the Gd³⁺ source. Anal. calc. for C₉H₁₃Cl₃GdO_12.5: C 18.49, H 2.24, N: 0.00%; found: C 18.55, H 2.37, N: 0.00%. IR (ATR, ν/cm⁻¹): 3041 (m), 1762 (w), 1613 (m), 1469 (s), 1379 (s), 1256 (w), 1060 (w), 1033 (w), 1005 (m), 846 (s).

2Tb. Brown hexagonal-like shaped crystals with yields of 77% based on the Tb³⁺ source. Anal. calc. for C₉H₁₃Cl₃O_12.5Tb: C C: 18.43, H 2.23, N: 0.00%; found: C 18.51, H 2.37, N: 0.00%. IR (ATR, ν/cm⁻¹): 3401 (m), 1761 (w), 1613 (m), 1503 (w), 1456 (s), 1379 (s), 1257 (w), 1059 (w), 1033 (w), 1005 (m), 847 (s).

2Dy. Brown hexagonal-like shaped crystals with yields of 84% based on the Dy³⁺ source. Anal. calc. for C₉H₁₃Cl₃DyO_12.5: C 18.21, H, 2.28, N: 0.00%; found: C 18.48, H, 2.15, N: 0.00%. IR (ATR, ν/cm⁻¹): 3400 (m), 1762 (w), 1613 (m), 1502 (w), 1456 (s), 1379 (s), 1300 (w), 1256 (w), 1059 (w), 1033 (w), 1006 (m), 847 (s).

2Er. Brown hexagonal-like shaped crystals with yields of 88% based on the Er³⁺ source. Anal. calc. for C₉H₁₃Cl₃ErO_12.5: C 18.17, H 2.20, N 0.00%; found: C 18.26, H 2.34, N 0.00%. IR (ATR, ν/cm⁻¹): 3401 (m), 1762 (w), 1613 (m), 1502 (w), 1456 (s), 1379 (s), 1257 (w), 1060 (w), 1033 (w), 1006 (m), 847 (s).

2Tm. Brown hexagonal-like shaped crystals with yields of 82% based on the Tm³⁺ source. Anal. calc. for C₉H₁₃Cl₃O_12.5Tm: C 18.12, H 2.20, N 0.00%; found: C 18.01, H 2.31, N 0.00%. IR (ATR, ν/cm⁻¹): 3406 (m), 1762 (w), 1613 (m), 1462 (w), 1380 (s), 1257 (w), 1059 (w), 1033 (w), 1006 (m), 847 (s).

2Yb. Brown hexagonal-like shaped crystals with yields of 90% based on the Yb³⁺ source. Anal. calc. for C₉H₁₄Cl₃O₁₃Yb: C 18.00, H 2.18, N 0.00%; found: C 18.09, H 2.22, N 0.00%. IR (ATR, ν/cm⁻¹): 3401 (m), 1762 (w), 1613 (m), 1456 (s), 1379 (s), 1249 (w), 1059 (w), 1033 (w), 1007 (m), 847 (s).

Compounds of the series 3Ln, [Ln(ca)_1.5(H₂O)₂]·2H₂O·4,4′-bipy (Ln = Tm, Yb), were prepared by adding one equivalent of 4,4′-bipy into a crystalline sample of 2Ln, following the identical synthetic procedures as 1Ln and 2Ln.

3Tm. Dark purple block shaped crystals with yields of 62% based on the Tm³⁺ source. Anal. calc. for C₁₉H₁₆Cl₃N₂O₁₀Tm: C 32.25, H 2.28, N: 3.96%; found C 32.31, H 2.17, N: 3.86%. IR (ATR, ν/cm⁻¹): 3614 (m) 3531 (m), 1699 (m), 1623 (m), 1600 (m), 1561 (m), 1484 (s), 1432 (s), 1370 (s), 1295 (m), 1095 (m), 1061 (w), 1001 (m), 848 (s), 808 (s), 738 (m), 680 (m).

3Yb. Dark purple block shaped crystals with yields of 51% based on the Yb³⁺ source. Anal. calc. for C₁₉H₁₆Cl₃N₂O₁₀Yb: C 32.06, H: 2.27, N: 3.94%; found C 32.11, H 2.29, N 3.95%. IR (ATR, ν/cm⁻¹): 3531 (m), 1670 (m), 1623 (m), 1600 (m), 1560 (m), 1483 (s), 1432 (s), 1369 (s), 1295 (m), 1095 (m), 1059 (w), 1002 (m), 848 (s), 738 (m), 680 (m).

X-ray crystallography

Single crystals of all compounds were carefully mounted on MiTeGen micromounts using paratone oil. X-ray diffraction data were obtained utilizing a Bruker D8 QUEST CMOS PHOTON II at a temperature of 296(2) K. The data were collected using Mo-K_α radiation (λ = 0.71073 Å) through omega and phi scans. The APEX6 software performed strategic calculations to calculate the overall number of runs and images, and SAINT refined unit cell indexing.¹¹ SAINT was used for data reduction, and SADABS for absorption correction. The integrity of the symmetry was verified using PLATON.¹² The structures were determined using the algorithm implemented in SHELXT¹³ via intrinsic phasing, followed by refinement through least squares on F² using SHELXL¹⁴ within the Olex2-1.5 graphical user interface.¹⁵ All non-hydrogen atoms were found in the different Fourier maps and were refined anisotropically in the final refining cycles. The disordered water oxygen atoms were purified to an occupancy of one, and their hydrogen atoms were positioned based on calculations and refined utilizing a riding model. Table 1 shows the crystallographic information for the compounds in the series 1Ln–3Ln, which have been deposited at the Cambridge Crystallographic Data Centre under CCDC numbers (2532672–2532683).

Table 1 Crystallographic data for all compounds in the series 1Ln–3Ln

Compound (CCDC)	1Pr (2532672)	1Nd (2532673)	2Sm (2532674)	2Eu (2532675)	2Gd (2532676)	2Tb (2532677)
Formula	C9H18Cl3O15Pr	C9H18Cl3NdO15	C9H13Cl3O12.5Sm	C9H13Cl3EuO12.5	C9H13Cl3GdO12.5	C9H13Cl3TbO12.5
Formula weight	613.49	616.82	582.40	579.50	584.79	586.46
Temperature (K)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄	P1̄	P1̄
a (Å)	9.9173(8)	9.8881(4)	9.3868(3)	9.3530(3)	9.3410(3)	9.2918(3)
b (Å)	10.0358(9)	10.0028(4)	10.2842(3)	10.2576(3)	10.2493(3)	10.2132(3)
c (Å)	11.1647(10)	11.1361(4)	10.8837(4)	10.8835(4)	10.9033(4)	10.8882(4)
α (°)	88.905(3)	89.059(2)	64.594(1)	64.711(1)	64.837(1)	64.990(1)
β (°)	84.580(3)	84.379(2)	69.763(1)	69.823(1)	69.948(1)	70.009(1)
γ (°)	67.775(3)	67.8480(10)	82.487(1)	82.333(1)	82.218(1)	82.027(1)
V (Å)	1023.88(16)	1015.00(7)	890.28(5)	886.03(5)	887.47(5)	879.95(5)
Z	2	2	2	2	2	2
ρcalcd (g cm^−3)	1.990	2.018	2.173	2.172	2.202	2.213
λ (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
R[F^2 > 2σ(F^2)], wR(F^2)	0.0294, 0.0720	0.0330, 0.0748	0.0217, 0.0397	0.0306, 0.0531	0.0336, 0.0669	0.0335, 0.0510
GOF on F^2, S	1.06	1.08	1.09	1.08	1.07	1.05
Δρmax, Δρmin	0.50, −0.77	1.02, −0.78	1.24, −0.58	1.06, −0.94	1.69, −1.28	1.51, −1.45

---

Compound	2Dy (2532678)	2Er (2532679)	2Tm (2532680)	2Yb (2532681)	3Tm (2532682)	3Yb (2532683)
Formula	C9H13Cl3DyO12.5	C9H13Cl3ErO12.5	C9H13Cl3TmO12.5	C9H13Cl3YbO12.5	C19H16Cl3N2O10Tm	C19H16Cl3N2O10Yb
Formula weight	590.04	594.80	596.47	600.58	707.62	711.73
Temperature (K)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	P1̄	P1̄	P21/c	P21/c
a (Å)	9.2734(3)	9.2039(5)	9.1844(3)	9.3530(3)	13.003(3)	12.9769(11)
b (Å)	10.2087(3)	10.1659(5)	10.1584(3)	10.2576(3)	13.505(2)	13.5156(12)
c (Å)	10.9033(4)	10.9042(5)	10.8980(4)	10.8835(4)	14.591(3)	14.5543(13)
α (°)	65.088(1)	65.318(2)	65.412(1)	64.711(1)	90	90
β (°)	70.104(1)	70.294(2)	70.356(1)	69.823(1)	107.155(7)	107.308(3)
γ (°)	81.891(1)	81.578(2)	81.469(1)	82.333(1)	90	90
V (Å)	880.29(5)	872.74(8)	870.72(5)	886.03(5)	2448.3(8)	2437.1(4)
Z	2	2	2	2	4	4
ρcalcd (g cm^−3)	2.226	2.263	2.275	2.251	1.920	1.940
λ (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
R[F^2 > 2σ(F^2)], wR(F^2)	0.0316, 0.0519	0.0230, 0.0444	0.0279, 0.0476	0.0235, 0.0431	0.0218, 0.0473	0.0235, 0.0431
GOF on F^2, S	1.09	1.08	1.12	1.11	1.08	1.11
Δρmax, Δρmin	1.31, −0.94	1.48, −1.19	2.33, −1.41	1.81, −1.05	0.70, −0.66	0.92, −0.86

---

Results and discussion

Structural description

The single crystal X-ray diffraction study demonstrates that all compounds in series 1Ln and 2Ln are isostructural, crystallizing in the triclinic space group P1̄, consistent with the previously reported crystal structures by Robson et al.^7a The asymmetric unit comprises one crystallographically distinct Ln³⁺ ion, three half-occupied ca²⁻ anions situated at crystallographic inversion centres, three coordinated water molecules, and five lattice water molecules. As seen in Fig. 1a, the Ln³⁺ ion in both series 1Ln and 2Ln shows similar nine-coordinate arrangement, however, an analysis conducted with SHAPE software¹⁶ reveals that the coordination geometry around the core metal center is distinct (Table S1). Accordingly, a capped cube (JCCU-9, C_4v) for 1Ln and a Johnson triangular cupola J3 (JTC-9, C_3v) for 2Ln, Fig. 1b, represent the best described coordination polyhedral of the metal center. The analysis of Ln–O bond lengths in different compounds reveals a decrease in distances corresponding to the decreasing ionic radius of the Ln³⁺ ions (Table S2), from Pr³⁺ to Yb³⁺, consistent with the lanthanide contraction effect.¹⁷

Fig. 1 (a) The coordination environment of the Ln³⁺ ion in the representative compounds of the series. The coordination polyhedron of a central Ln³⁺ ion for (b) 1Ln and (c) 2Ln.

The ca²⁻ dianion in a series of compounds behaves as a bidentate bis-chelating ligand, connecting two Ln³⁺ centres via a µ₂-κ²O,O′:κ²O″,O‴ coordination mode. Consequently, two intriguing geometric motifs of the (6³) topological networks are established, consisting of alternating three-connected Ln³⁺ nodes and the ca²⁻ ligands acting as linear linkers that form six-membered rings. As shown in Fig. 2(a), the geometric motif of 1Ln is identified as honeycomb (hcb), distinguished by nearly planar hexagonal symmetry with a deviation of approximately 6° from the mean plane, attributed to the incorporation of semi-rigid ca²⁻ ligands. In hexagons, the angles around the three-connected metal nodes are roughly 105°, 115°, and 135°, which deviates from the ideal angle (120°). The dimensions within the hexagon determined through interatomic distance (Ln⋯Ln) are approximately 9.0 × 16.5 Ų. Along the crystallographic b axis, the hcb networks of 1Ln are arranged in a −ABAB− sequence, as depicted in Fig. 2b. This arrangement is likely influenced by the synergistic effects of strong O–H⋯O hydrogen bonding among interlayer water molecules or between water molecules and the oxygen atoms of ca²⁻ ligands (O⋯O = 2.99–3.01 Å), along with Cl⋯Cl and Cl⋯O halogen bonding¹⁸ among the ca²⁻ ligands (Cl⋯Cl/O = 3.21–3.45 Å).

Fig. 2 (a) The 2D hcb networks, (b) a schematic representation of the 6³ topological networks arranged in a –ABAB– stacking sequence, with metal nodes depicted as spheres and ligands as linear rods, and (c) the crystal packing and solvent-accessible surface, analyzed with a probe radius of 1.2 Å using Mercury software for 1Ln.

There are lattice water molecules at an interlayer spacing of about 6.1 Å (Ln⋯Ln separation). These water molecules establish hydrogen bonds with each other and with coordinated waters as well as ca²⁻ ligands (O⋯O = 2.83–3.04 Å). In addition, Mercury software¹⁹ was utilized to identify void spaces with a probe radius of 1.2 Å, revealing that 1Ln possesses a potential void space of 41–45% per unit cell after the complete removal of water molecules, as seen in Fig. 2(c). The significant volume of voids suggests that the material may be suitable for gas adsorption applications.

The change in angles surrounding the metal center from hexagonal hcb results in a change of the geometric pattern into a brick-wall rectangular grid motif present in the network of 2Ln, Fig. 3(a). The angles in hexameric [Ln₆(ca)₆(H₂O)₁₈] units are about 76°, 96°, and 170°, deviating from the ideal angles of 90° and 180° that define a perfect rectangular grid. The dimensions of the grid are approximately 8.1 × 17.6 Ų. The distances between the Ln atoms along the ca²⁻ bridging ligand vary from 8.6 to 8.9 Å, while the shortest diagonal distance between Ln atoms in the rectangular grid is about 7.5 Å. As shown in Fig. 3(b), the crystal packing of 2Ln exhibits parallel layers of brick-wall patterns arranged in a −ABAB− pattern similar to that seen in 1Ln, with a comparable interlayer spacing of around 6.3. Å. The brick-wall interlayers are sustained by O–H⋯O and O–H⋯Cl hydrogen bonding interactions with water molecules or water–ca²⁻ ligands (O⋯O = 2.92–3.08 Å; O⋯Cl = 2.99–3.30 Å) lead to the formation of a 3D supramolecular architecture. No Cl··Cl halogen bonding was observed in the crystal packing of 2Ln, with contacts shorter than the sum of Bondi van der Waals radii (3.5 Å).²⁰ This lack could account for the presence of topological isomers between 1Ln and 2Ln. Additionally, the brick-wall networks of 2Ln exhibit an estimated guest-accessible volume of 24–28%, as analyzed using Mercury, Fig. 3(c). The void spaces in this series of compounds are approximately 45% smaller than those found in 1Ln.

Fig. 3 (a) The 2D brick-wall networks, (b) a schematic illustration of the 6³ topological networks organized in a –ABAB– stacking sequence, with metal nodes represented as spheres and ligands as linear rods, and (c) the crystal packing and solvent-accessible surface, examined with a probe radius of 1.2 Å utilizing Mercury software for 2Ln.

Given that aqua ligands in the aforementioned compounds are orientated vertically within the 2D layers, the addition of the rod-like semi-rigid 4,4′-bipy might replace these aqua ligands and potentially lead to the formation of 3D pillared frameworks with porous features. Indeed, appropriate single crystals were obtained from hydrothermal reactions, and single crystal X-ray diffraction analysis clearly demonstrated that the 4,4′-bipy molecules are situated between the 2D layers of 3Ln, forming hydrogen bonds with the aqua ligands. This phenomenon is probably a result of stabilizing interactions formed by strength and directional O–H⋯N hydrogen bonding (O⋯N = 2.99–3.01 Å), which preserves the integrity of the multilayer structure. This interaction may hinder the generation of a fully porous 3D pillared framework, as the existence of 4,4′-bipy molecules may be insufficient to enable the desired structural transformation. Notably, the shortened analogue of compounds like pyrimidine has been employed, nevertheless, crystallization has not been achieved, rendering structural analysis unfeasible.

The layered coordination networks of 3Ln crystallize in the monoclinic system with the P2₁/c space group. The asymmetric unit of the compounds in series 3Ln is similar to that of 1Ln and 2Ln, consisting of one Ln³⁺ ion and three half-occupied ca²⁻ anions located at the inversion centers, except for the presence of the 4,4′-bipy molecule and the different number of water molecules. As shown in Fig. 4(a), the eight-coordinated Ln³⁺ ion is surrounded by six oxygen atoms from three different ca²⁻ ligands. According to SHAPE software analysis, the coordination geometry of the core Ln³⁺ ion is a heptagonal pyramid (HPY-8, C_7v), depicted in Fig. 4b. The Ln–O bond lengths and the O–Ln–O bond angles of 3Ln compounds (Table S3) exhibit typical values.^17c In contrast to the Tm- and Yb-containing compounds within the 2Ln brick-wall network, the compounds in the 3Ln series exhibit a distorted hcb network. The diameter of the hexagon in the hcb network is about 13.7 Å, which corresponds closely with the length of the 4,4′-bipy molecules, allowing it to interact with the terminal aqua ligands through classical hydrogen bonding, Fig. 4(b). The hcb networks are arranged along the b axis in a –ABAB– manner, creating a 3D supramolecular network through O–H⋯O hydrogen bonding between lattice water molecules and aqua or ca²⁻ ligands (O⋯O = 2.88–3.19 Å), C–H⋯π (C⋯C_g = 3.02–3.24 Å) interactions among 4,4′-bipy molecules, and Cl⋯π (Cl⋯C_g = 3.15–3.27 Å) interactions that involve the ca²⁻ ligands. No aromatic π⋯π interactions are found between centroids 3.3 to 3.8. Å.²¹

Fig. 4 (a) The coordination environment and (b) the coordination polyhedron of a central Ln³⁺ ion for 3Ln. (c) The 2D structure of 3Ln, with the light blue solid lines representing the ca²⁻ ligand that illustrates the hexagon in the hcb network.

Physicochemical properties

The powder X-ray diffraction analysis clearly showed distinct patterns among the compounds in the 1Ln and 2Ln series, as illustrated in Fig. 5(a). The diffractograms for each series of compounds match closely with the simulated patterns derived from single crystal structural determination, indicating that the compounds possess a well-defined crystalline phase without any significant impurities or defects. The phase purity of the compounds in the 3Ln is verified by a perfect match between their simulated and experimental diffractogram patterns (Fig. S1). Thus, the synthesis procedures used have shown reliable, allowing for further uses of these compounds. The IR spectra of series 1Ln and 2Ln display notable discrepancies in comparison to that of 3Ln (Fig. S2). However, all compounds in these series display absorption bands at 1625, 1380, 975, and 845 cm⁻¹, signifying the existence of ca²⁻ ligands. The presence of broad bands in the 1500–1450 cm⁻¹ range suggests that deprotonated H₂ca increases the aromaticity of ca²⁻ molecules, as evidenced by the C–C stretching vibrations of aromatic rings.²² The O–H stretching of H₂O molecules is represented by a broad vibration that occurs between 3400 and 3100 cm⁻¹. The characteristic bands of 4,4′-bipyridine molecules in 3Ln are represented by aromatic C–H (3000–3100 cm⁻¹) and C=C/C=N (1590–1615 cm⁻¹) stretching vibrations.

Fig. 5 Comparison of (a) simulated instances with as-synthesized PXRD patterns and (b) TG curves for the compounds in series 1Ln and 2Ln.

The TG analysis of compounds in the series 1Ln and 2Ln reveals two distinct sets of thermograms, as illustrated in Fig. 5(b), when subjected to heating in a N₂ atmosphere at a rate of 10 °C min⁻¹. For 1Ln, the first three stages between room temperature and 225 °C can be attributed to the liberation of five lattice water molecules (found 14.9–15.2%, calc 14.59–14.67%). At about 290 °C, the fourth stage of weight loss took place, probably signifying the loss of coordinated water and ca²⁻ components, accompanied by the collapse of the network. In the case of 2Ln, the initial two stages of weight loss occurring up to 200 °C can be ascribed to the release of five lattice water molecules (found 19.2–20.2%, calc 17.72–18.40%). The third weight loss observed between 300–400 °C is likely due to the loss of coordinated water and ca²⁻ components, leading to the collapse of the network. The two isomers exhibit distinct decomposition behaviors, influenced by their temperature ranges and mass loss profiles. This highlights how differences in the topological structures of isomeric compounds could impact their thermal stability characteristics.

Carbon dioxide adsorption

Previous studies indicate that 2D layered networks can adsorb CO₂ and exhibit gate-opening behavior under specific pressure and temperature thresholds, thus providing significant insights for their application in carbon capture technology.²³ The hcb network 1Pr and brick-wall network 2Yb were examined as series examples for their CO₂ adsorption capacities at 1 bar over a temperature range of 273 to 303 K as well as at high pressures up to 20 bar at 298 K. Prior to conducting gas adsorption–desorption tests, the crystalline samples 1Pr and 2Yb were activated and dried in a vacuum oven at 100 °C for 12 h, and kept under vacuum overnight. The activated samples were subjected to a 12 h degassing process at 80 °C utilizing the outgas function of the adsorption analyzer. As shown in Fig. 6a,b, neither 1Pr nor 2Yb exhibit the gate-opening pressure step for CO₂ sorption at a pressure of up to 1 bar. At temperatures of 273, 298, and 308 K, 1Pr showed negligible uptakes of 11.42, 5.21, and 3.07 cm³ g⁻¹, respectively. In contrast, 2Yb exhibited an increase in uptake compared to 1Pr, with volumetric values of 30.91, 19.90, and 15.78 cm³ g⁻¹ at the same temperatures. The desorption branches for 1Pr exhibited more significant hysteresis than those for 2Yb. This may result from the differing molecular interactions and structural characteristics of the two isomers.

Fig. 6 The adsorption (darker coloured spheres)–desorption (lighter coloured spheres) isotherms for CO₂ at 273, 298, and 308 K in (a) 1Pr and (b) 2Yb. (c) High-pressure CO₂ sorption isotherms of 1Pr and 2Yb at 298 K.

The CO₂ sorption isotherms for 1Pr and 2Yb at 298 K and pressures up to 20 bar, exhibiting a quasi–linear isotherm with the absence of gate opening, as shown in Fig. 6c. 1Pr displays a maximum CO₂ uptake of 7.05 cm³ g⁻¹, which is significantly lower than the 34.79 cm³ g⁻¹ observed for 2Yb. The higher adsorption capacity of 2Yb relative to 1Pr can be attributed to the dimensions and shape the pores, along with variations in the surface areas of their activated samples. These variables contribute to a more extensive and accessible pore network, thereby increasing contact with adsorbates. Notably, the lack of a gate-opening effect in both cases is likely attributable to the structural rigidity of the materials, which limits substantial expansion and conformational changes, hence diminishing their efficacy in situations requiring gate-switching capabilities.

Additionally, PXRD was performed at ambient temperature to assess the stability of the network or the structural phase transition after thermal activation and gas sorption analyses for instances 1Pr and 2Yb. The diffractograms of the synthesized, activated, and after CO₂ sorption experiments exhibit notable differences (Fig. S3), highlighting the distinctive phases and structural features inherent in each sample. A notable change in the main peak positions to higher 2θ values was seen in the diffractograms for 1Pr, especially in the range of 8.11 to 22.31°. In a similar manner, the diffractogram of the synthesized 2Yb sample reveals a shift in the main peak positions at 2θ of 10.12, 11.67, and 13.07°, accompanied by the absence of a peak at 15.78°. It was observed that the crystallinity of both 1Pr and 2Yb is preserved after CO₂ sorption experiments performed at high pressures relative to their activated counterparts. Nonetheless, the PXRD profiles for 2Yb show consistency, while discrepancies are observed in 1Pr. Moreover, efforts to ascertain the crystal structures of the activated or post-sorption study materials have been futile due to the inadequate diffraction quality of the crystalline samples. This complicates the understanding of the relationships between material structures and CO₂ molecules. Nonetheless, the PXRD findings suggest that 2Yb retains robust structural integrity under varying pressure conditions, while 1Pr may undergo more denser structures that could affect its gas sorption efficiency. This difference highlights the importance of materials with different topological isomers in gas adsorption.

Conclusions

In summary, distinct types of lanthanide coordination polymers incorporating the ca²⁻ ligand were successfully synthesized and characterized. The lanthanide contraction influences their two-dimensional structures, leading to the emergence of two unique topological isomers, namely honeycomb 1Ln and brick-wall 2Ln arrangements. The incorporation of the semi-rigid rod 4,4′-bipy into brick-wall 2D networks, intended to construct a porous 3D pillared framework, leads to the formation of an intercalation honeycomb network 3Ln comprising the guest 4,4′-bipy and lattice water molecules. The physicochemical characteristics of the isomers 1Ln and 2Ln were distinctly illustrated, emphasizing their unique thermal stability and structural arrangements. The brick-wall network of instance 2Yb shows better CO₂ adsorption capacities under both ambient and high-pressure conditions compared to 1Pr, possibly due to its larger pore size and arrangement which leads to a more extensive and accessible pore network. The findings of the present study highlight the complex interplay between structural topological isomers and their properties, demonstrating that even minor differences among lanthanide metals can result in major changes to the structure and functionality of novel materials.

Author contributions

K. K., C. S., P. J., K. C., and S. L.: data collection, formal analysis, visualization, and writing the original draft. B. R.: gas adsorption. M. K.: supervisor. K. C.: conceptualization, formal analysis, writing – review & editing, and funding acquisition. All authors analyzed the results and provided commentary on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2532672–2532683 1Ln–3Ln contain the supplementary crystallographic data for this paper.^24a–l

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: lanthanide geometry analysis by SHAPE software, selected bond lengths, IR spectra, and PXRD pattern. See DOI: https://doi.org/10.1039/d6ra02049f.

Acknowledgements

This research received funding from the Faculty of Science and Technology, Thammasat University (SciGR 6/2565), Thammasat Postdoctoral Fellowship (to C. S.), and the National Research Council of Thailand (NRCT): High-Potential Research Team Grant Program (No. 42A690190). K. C. expresses gratitude for the support provided by the Hub of Talent: Sustainable Materials for Circular Economy, NRCT.

References

1. (a) K. Bernot, C. Daiguebonne, G. Calvez, Y. Suffren and O. Guillou, Acc. Chem. Res., 2021, 54, 427–440; (b) Y. Hasegawa and Y. Kitagawa, J. Photochem. Photobiol., A, 2022, 51, 100485; (c) A. Hauser, L. Münzfeld, C. Uhlmann, S. Lebedkin, S. Schlittenhardt, T.-T. Ruan, M. M. Kappes, M. Ruben and P. W. Roesky, Chem. Sci., 2024, 15, 1338–1347.
2. (a) T. Gorai, W. Schmitt and T. Gunnlaugsson, Dalton Trans., 2021, 50, 770–784; (b) C. Alexander, Z. Guo, P. B. Glover, S. Faulkner and Z. Pikramenou, Chem. Rev., 2025, 125, 2269–2370; (c) M. L. P. Reddy and K. S. Bejoymohandas, Dalton Trans., 2025, 54, 9441–9452; (d) X. Shao, Y. Yang, X. Dong, S. Wang, Y. Liang, Z. Liu, H. Min, Y.-X. Wang and P. Cheng, Chem. Commun., 2025, 61, 16862–16865; (e) Z. W. Jiang, X. Gong, P. Zhang and Y. Wang, Trends Anal. Chem., 2025, 184, 118133.
3. (a) K. Bernot, C. Daiguebonne, G. Calvez, Y. Suffren and O. Guillou, Acc. Chem. Res., 2021, 54, 427–440; (b) S. Li, S. Jansone-Popova and D.-e. Jiang, Sci. Rep., 2024, 14, 11301; (c) J. Zhou, X. Hu, C. Liu, Y. Liu, N. Tian, F. Wu, W. Li, J. Lei and Z. Dai, Coord. Chem. Rev., 2025, 534, 216574; (d) H. Yang, S. Zhao, W. Song, X. Zhao, B. Zhang, R. He and Y. Wang, Inorg. Chem., 2026, 65, 2980–2996.
4. (a) S. Jiajaroen, W. Dungkaew, F. Kielar, M. Sukwattanasinitt, S. Sahasithiwat, H. Zenno, S. Hayami, M. Azam, S. I. Al-Resayesh and K. Chainok, Dalton Trans., 2022, 51, 7420–7435; (b) B. P. Carpenter, A. R. Talosig, B. Rose, G. D. Palma and J. P. Patterson, Chem. Soc. Rev., 2023, 52, 6918–6937.
5. (a) N. Dannenbauer, P. R. Matthes, T. P. Scheller, J. Nitsch, S. H. Zottnick, M. S. Gernert, A. Steffen, C. Lambert and K. Müller-Buschbaum, Inorg. Chem., 2016, 55, 7396–7406; (b) K. Chainok, N. Ponjan, C. Theppitak, P. Khemthong, F. Kielar, W. Dungkaew, Y. Zhou and S. R. Batten, CrystEngComm, 2018, 20, 7446–7457; (c) M. Boone, F. Artizzu, J. Goura, D. Mara, R. V. Deun and M. D'hooghe, Coord. Chem. Rev., 2024, 501, 215525; (d) A. K. K. Arachchige, B. Lottes, D. K. Unruh, S. M. Greer, B. W. Stein and K. P. Carter, Dalton Trans., 2026, 55, 748–760.
6. (a) W.-J. Yu, X. Chen, J. Li, B. Li, T.-l. Zhang and J. Tao, CrystEngComm, 2013, 15, 7732–7739; (b) W. Lu, Z. Wei, Z.-Y. Gu, T.-F. Liu, J. Park, J. Park, J. Tian, M. Zhang, Q. Zhang, T. Gentle III, M. Bosch and H.-C. Zhou, Chem. Soc. Rev., 2014, 43, 5561–5593; (c) E. R. Engel and J. L. Scott, Green Chem., 2020, 22, 3693–3715; (d) J. Chen, H. Lu and J. Lin, Chem. Commun., 2025, 61, 13066–13079; (e) K. Patra and H. Pal, RSC Sustain., 2025, 3, 629–660.
7. (a) B. F. Abrahams, J. Coleiro, K. Ha, B. F. Hoskins, S. D. Orcharda and R. Robson, J. Chem. Soc., Dalton Trans., 2002, 1586–1594; (b) C. Hua, H. M. Tay, Q. He and T. D. Harris, Aust. J. Chem., 2019, 72, 778–785; (c) S. Konar, A. Mondal and S. Roy, Chem. - Eur. J., 2020, 26, 8774–8783; (d) S. Benmansour, I. Pérez-Herráez, C. Cerezo-Navarrete, G. López-Martínez, C. M. Hernández and C. J. Gómez-García, Dalton Trans., 2018, 47, 6729–6741; (e) S. Benmansour and C. J. Gomez-Garcia, Magnetochemistry, 2020, 6, 71; (f) S. Benmansour, C. Pintado-Zaldo, J. Martínez-Ponce, A. Hernández-Paredes, A. Valero-Martínez, M. Gómez-Benmansour and C. J. Gómez-García, Cryst. Growth Des., 2023, 23, 1269–1280; (g) S. Benmansour, C. Pintado-Zaldo, S. H. Casal-Garcia, J. Martinez-Ponce and C. J. Gomez-Garcia, Chem. - Eur. J., 2024, 30, e2024004.
8. (a) J.-F. Petit, A. Gleizes and J.-C. Trombe, Inorg. Chim. Acta, 1990, 167, 51–68; (b) B. F. Abrahams, T. A. Hudson, L. J. McCormick and R. Robson, Cryst. Growth Des., 2011, 11, 2717–2720; (c) K. V. Nielson, L. Zhang, Q. Zhang and T. L. Liu, Inorg. Chem., 2019, 58, 10756–10760; (d) K. Kon, K. Uchida, K. Fuku, S. Yamanaka, B. Wu, D. Yamazui, H. Iguchi, H. Kobayashi, Y. Gambe, I. Honma and S. Takaishi, ACS Appl. Mater. Interfaces, 2021, 13, 38188–38193; (e) D. Yamazui, K. Uchida, S. Koyama, B. Wu, H. Iguchi, W. Kosaka, H. Miyasaka and S. Takaishi, ACS Omega, 2022, 7, 18259–18266; (f) X.-Z. Wang, Y. Chen, X.-M. Cao, R.-Y. Li, W.-Y. Chen, Y. Li and D.-S. Guo, Angew. Chem., Int. Ed., 2025, 137, e202413115.
9. (a) Y. Hijikata, S. Horike, M. Sugimoto, M. Inukai, T. Fukushima and S. Kitagawa, Inorg. Chem., 2013, 52, 3634–3642; (b) S. Hiraide, Y. Sakanaka, H. Kajiro, S. Kawaguchi, M. T. Miyahara and H. Tanaka, Nat. Commun., 2020, 11, 3867; (c) D. Pandey, T. Samarth, V. K. Verma, C. Patel, L. Ponvijayakanthan, N. K. Jaiswal, S. Mukherjee and A. Raghuvanshi, J. Mater. Chem. A, 2025, 13, 11416–11424.
10. C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E. Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler and P. A. Wood, J. Appl. Crystallogr., 2020, 53, 226–235.
11. Bruker, APEX6, SAINT, and SADABS, Bruker AXS Inc., Madison, Wisconsin, USA, 2023.
12. A. L. Spek, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 9–18.
13. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2015, 71, 3–8.
14. G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.
15. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
16. M. Llunell, D. Casanova, J. Cirera, J. M. Bofill, P. Alemany, S. Alvarez, M. Pinsky and D. Avnir, SHAPE, version 2.1, Universitat de Barcelona and Hebrew University of Jerusalem, 2013.
17. (a) S. Sairenji, S. Akine and T. Nabeshima, Dalton Trans., 2016, 45, 14902–14906; (b) J. A. Peters, K. Djanashvili, C. F. G. C. Geraldes and C. Platas-Iglesias, Coord. Chem. Rev., 2020, 406, 213146; (c) S. C. Bart, Inorg. Chem., 2023, 62, 3713–3714.
18. (a) G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, G. Resnati and G. Terraneo, Chem. Rev., 2016, 116, 2478–2601; (b) L. Brammer, A. Peuronena and T. M. Rosevearea, Acta Crystallogr., Sect. C: Struct. Chem., 2023, 79, 204–216; (c) S. An and P. Xing, Chem. Soc. Rev., 2026, 55, 1460–1493.
19. C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E. Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler and P. A. Wood, J. Appl. Crystallogr., 2020, 53, 226–235.
20. A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
21. C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885–3896.
22. (a) R. Ishikawa, S. Michiwaki, T. Noda, K. Katoh, M. Yamashita, K. Matsubara and S. Kawata, Inorganics, 2017, 6, 7; (b) P. Zhang, M. Perfetti, M. Kern, P. P. Hallmen, L. Ungur, S. Lenz, M. R. Ringenberg, W. Frey, H. Stoll, G. Rauhut and J. v. Slageren, Chem. Sci., 2018, 9, 1221–1230; (c) R. Ishikawa, S. Michiwaki, T. Noda, K. Katoh, M. Yamashita and S. Kawata, Magnetochem, 2019, 5, 30.
23. (a) D. Li and K. Kaneko, Chem. Phys. Lett., 2001, 335, 50–56; (b) Y. Ying, Z. Zhang, S. B. Peh, A. Karmakar, Y. Cheng, J. Zhang, L. Xi, C. Boothroyd, Y. M. Lam, C. Zhong and D. Zhao, Angew. Chem., Int. Ed., 2021, 60, 11318–11325; (c) S. R. Acharya, A. Elias, K. Tan, S. Jensen, R.-B. Lin, B. Chen, M. D. Gross and T. Thonhauser, Inorg. Chem., 2022, 61, 5025–5032; (d) S.-Q. Wang, V. Bon, S. Darwish, S.-M. Wang, Q.-Y. Yang, Z. Xu, S. Kaskel and M. J. Zaworotko, ACS Mater. Lett., 2024, 6, 666–673; (e) M.-Y. Gao, L. Liu, C. Deng, V. Bon, B.-Q. Song, S. Yang, M. Schröder, S. Kaskel and M. J. Zaworotko, Angew. Chem., Int. Ed., 2024, 63, e202404084.
24. (a) CCDC 2532672: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g3c; (b) CCDC 2532673: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g4d; (c) CCDC 2532674: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g5f; (d) CCDC 2532675: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g6g; (e) CCDC 2532676: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g7h; (f) CCDC 2532677: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g8j; (g) CCDC 2532678: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0g9k; (h) CCDC 2532679: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0gbl; (i) CCDC 2532680: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0gcm; (j) CCDC 2532681: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0gdn; (k) CCDC 2532682: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0gfp; (l) CCDC 2532683: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r0ggq.

---