Structural Diversity of Five New Lanthanide Coordination Polymers Tuned by Different Salt Anions

Abstract

Five new lanthanide coordination polymers, $[Ln_2(1,4-NDC)_2(1,4-HNDC)_2(phen)_2]_n$ (Ln = Er (1), Yb (2)), and $[Ln_2(1,4-NDC)_3(phen)_2(H_2O)_2]_n$ (Ln = Nd (3), Gd (4), Er (5)) ($1,4-H_2NDC$ = 1,4-naphthalenedicarboxylic acid, phen = 1,10-phenanthroline), have been successfully prepared via the reaction of corresponding trivalent lanthanide salt, $1,4-H_2NDC$ and phen in the presence of NaOH and pyridine under hydrothermal condition. Pyridine plays a key role in the synthesis of these lanthanide coordination polymers. Single-crystal X-ray diffraction analyses indicate that compounds 1-5 all form a 2-D network while different salt anions result in the diversity of crystal structures. These lanthanide coordination polymers showed a considerable thermal stability in TGA analyses.

Introduction

There continues to be significant interests focusing on the synthesis and characterization of lanthanide coordination polymers because of their novel structural features and potential applications in magnetism,1 catalysis,2,3 lumine-scence, 4-7 hydrogen storage,8 molecular recognition and adsorption,9 medical devices,10,11 sensors and displays,12 etc.

As we know, coordination polymers contain two main components, metal (connector) and ligand (linker). It is very crucial to choose reasonable ligands during the synthesis of coordination polymers. Herein we chose 1,4-naphthalenedi-carboxylic acid (1,4-H2NDC) as the main ligand due to following interesting features: (i) It has two carboxylate groups with four oxygen atoms, which have strong binding toward lanthanide ions; (ii) It can form novel frameworks due to the versatile coordination modes of two carboxylate groups; (iii) It can be partially or completely deprotonated to form HNDC− and NDC2− by controlling the solution pH carefully;13 (iv) The naphthalene ring with easily delocalized π electrons enables efficient energy transfer between metal and ligand. Ancillary ligands could be employed to occupy additional coordination sites considering the high coordination number of lanthanide ions. 1,10-Phenanthroline (phen) has been widely used as an ancillary ligand in the synthesis of coordi-nation polymers, which is very helpful to construct supramolecular structures via π-π aromatic interaction.14,15 It can also increase the rigidity and thermal stability of the resulted coordination polymers.

Lanthanide ions, as the metal centers of the coordination polymers, have high coordination number and various coordi-nation geometries which are beneficial to obtain more un-usual network topologies.16-21 Only three lanthanide coordi-nation polymers based on 1,4-H2NDC and phen have been reported so far.21 In this contribution, we chose four different lanthanide elements (Er, Yb, Nd, Gd) to investigate the structure diversity of lanthanide coordination polymers based on 1,4-H2NDC and phen. We used lanthanide nitrates to prepare compounds [Ln2(1,4-NDC)2(1,4-HNDC)2(phen)2]n (Ln = Er (1), Yb (2)) and used lanthanide chlorides to prepare compounds [Ln2(1,4-NDC)3(phen)2(H2O)2]n (Ln = Nd (3), Gd (4), Er (5)) in the presence of pyridine. Pyridine seemed to play an important role in the formation of these coordination polymers. Different salt anions could affect the structures of the lanthanide coordination polymers.

 

Experimental

The Synthesis of Compounds 1-2. A mixture of Ln(NO3)3·5H2O (0.1 mmol, Ln = Er(1), Yb(2)), 1,4-H2NDC (0.15 mmol), phen (0.1 mmol) and NaOH (0.3 mmol) was dissolved in distilled water (10 mL), then added 2 drops of pyridine, stirred 10 minutes, finally sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 140 ℃ for 5 days. After slowly cooling down to room temperature, light yellow crystals 1 and 2 were obtained by filtration and successively washed with H2O, DMF and EtOH for several times. Elem. Anal. Calcd. for 1 (％): C, 55.61; N, 3.60. Found: C, 55.28; N, 4.30. Elem. Anal. Calcd. for 2 (％): C, 55.20; N, 3.58. Found: C, 54.32; N, 4.02. IR spectra were recorded as KBr pellet. The observed IR bands were listed in Table S3, ESI.

The Synthesis of Compounds 3-5. A mixture of LnCl3·6H2O (0.1 mmol, Ln = Nd (3), Gd (4), Er (5)), 1,4-H2NDC (0.15 mmol), phen (0.1 mmol) and NaOH (0.3 mmol) was dissolved in distilled water (10 mL), then added 2 drops of pyridine, stirred for 10 minutes, finally sealed in a 25 mL Teflon-lined stainless steel autoclave and heated at 140 ℃ for 5 days. After slowly cooling down to room temper-ature, light purple crystal 3, colorless crystal 4 and light pink crystal 5 were obtained by filtration and successively wash-ed with H2O, DMF and EtOH for several times. Elem. Anal. Calcd. for 3 (％): C, 54.26; H, 2.86; N, 4.22. Found: C, 54.06; H, 2.61; N, 4.03. Elem. Anal. Calcd. for 4 (％): C, 53.22; H, 2.81; N, 4.14. Found: C, 53.03; H, 2.98; N, 4.01. Elem. Anal. Calcd. for 5 (％): C, 52.58; H, 2.77; N, 4.09. Found: C, 52.25; H, 2.58; N, 3.88. IR spectra were recorded as KBr pellet. The observed IR bands were listed in Table S3, ESI.

Determination of Crystal Structures. A suitable single crystal of each compound was selected and glued to a thin glass fiber. Single crystal diffraction data for the these compounds were collected at 293 K on a Bruker Smart APEXII diffractometer with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) in an ω scan mode. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 using SHELXL-97 program. All non-hydrogen atoms of carboxylic acids and phen molecules were refined with anisotropic displacement parameters. Crystal data and structure refinement parameters for compounds 1-5 were listed in Table 1. Selected bond lengths and bond angles for compounds 1-5 were listed in Table S1, ESI.

Table 1.R = ∑||F0|−|Fc||∑|F0|; w = 1/[σ2 (F02) + (aP)2 + bP], where P = (Max (F02, 0) + 2 Fc2)/3, where a = 0 and b = 45.9844 for 1, a = 0.0172 and b = 25.9838 for 2, a = 0 and b = 6.2700 for 3, a = 0.0105 and b = 4.6900 for 4, a = 0.0672 and b = 0.1000 for 5.

CCDC-952907 (1), CCDC-953086 (2), CCDC-952904 (3), CCDC-952906 (4) and CCDC-978850 (5) contain the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223/336 033; Email: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)

 

Results and Discussion

Description of Crystal Structure.

[Er2(1,4-NDC)2(1,4-HNDC)2(phen)2]n (1), [Yb2(1,4-NDC)2(1,4-HNDC)2(phen)2]n (2): Single crystal X-ray diffraction analyses reveal that compounds 1 and 2 (Fig. S1, ESI.) are isostructural. Therefore we choose the structure of compound 1 to describe in detail. Compound 1 is in a monoclinic system with P21/c space group, which includes two Er3+, two 1,4-NDC2−, two 1,4-HNDC− and two phen molecules. Er1 and Er2 are both eight-coordinate with N2O6 donor set in a dodecahedral environment. Central lanthanide metal ion Er1 is eight-coordinate with two oxygen atoms from two independent 1,4-NDC2− (O6a, O5b), two oxygen atoms from two independent 1,4-HNDC− (O3, O4c), two oxygen atoms (O1, O2) from chelating carboxylate group of one 1,4-NDC2− and two nitrogen atoms from one phen (N1, N2) (Fig. 1). The Er1-O bond lengths vary from 2.292(5) Å to 2.375(6) Å. The Er1-N bond lengths are 2.547(7) Å and 2.537(7) Å. The O-Er1-O bond angles vary from 76.2(2)° to 147.0(2)°. The N2-Er1-N1 bond angle is 64.8(2)°. The bond distances and angles all lie in the normal range of reported Er complexes.22-25 The O−H⋯O intermolecular hydrogen bonds between 1,4-HNDC− ligands result in an octatomic ring composed of four oxygen atoms, two hydrogen atoms and two carbon atoms (Fig. 2).

Figure 1.The coordination environment of Er3+ ion in compound 1 with ball and stick style (hydrogen atoms attached to carbon are omitted for clarity). Symmetry codes: a: x, 0.5 − y, −0.5 + z; b: 2 − x, 0.5 + y, 0.5 − z; c: 2 − x, 1 − y, − z; d: x, 1.5 − y, −0.5 + z; e: 1 − x, −0.5 + y, 1.5 − z; f: 1 − x, 1 − y, 1 − z.

Figure 2.The intermolecular hydrogen bonds of compound 1 (dashed lines indicate the hydrogen-bonding interactions).

Er1 and Er1c (symmetry code c: 2 − x, 1 − y, −z) are linked by four carboxylate groups to form a secondary building unit (SBU). The distance between the two Er3+ is 4.1744(10) Å. 1,4-NDC2− ligands adopt bridging bidentate-chelating mode (O1-Er1-O2, O1c-Er1c-O2c) and syn-syn bridging mode (Er1-O6a-C-O5a-Er1c, Er1-O5b-C-O6b-Er1c) (Fig. 3). These bridging 1,4-NDC2− ligands are linking the neighbour SBUs to form a 2-D square grid network (Fig. 4(a)). The carbox-ylate groups of remaining pair of 1,4-HNDC− ligands adopt a non-bridging syn-syn mode (Er1-O3-C-O4-Er1c and Er1-O4c-C-O3c-Er1c), while the carboxylic acid groups interact to the neighbor carboxylic acid groups by intermolecular hydrogen bonds to form a layered structure (Fig. 4(b)).

Figure 3.The coordination mode of carboxylate groups in compound 1.

Figure 4.(a) The 2-D network along a axis of compound 1, (b) The 2-D layered network along c axis of compound 1.

[Nd2(1,4-NDC)3(phen)2(H2O)2]n (3), [Gd2(1,4-NDC)3-(phen)2(H2O)2]n (4), [Er2(1,4-NDC)3(phen)2(H2O)2]n (5): Single crystal X-ray diffraction analyses reveal that the crystal system of compounds 3, 4 (Fig. S2, ESI.) and 5 (Fig. S3, ESI.) are isostructural which are in a triclinic system with P-1 space group. Only the structure of compound 3 will be discussed. Compound 3 has an asymmetric unit contain-ing two Nd3+, three 1,4-NDC2−, two phen molecules and two water molecules. Central lanthanide ion Nd1 is eight-coordi-nate while Nd2 is nine-coordinate (Fig. 5). The O−H⋯O hydrogen bonds between 1,4-NDC2− and coordination water molecules result in an octatomic ring composed of four hydrogen atoms and four oxygen atoms (Fig. 6). The intermolecular O−H⋯O hydrogen bonding distances are within the usual range of 2.737−2.849 Å (Table S2, ESI.).

Three lanthanide (La, Eu, Ho) coordination polymers constructed from 1,4-NDC2− and phen have been reported.21 Compounds 3-5 possess similar structure to those reported lanthanide coordination polymers, but they could not be prepared in accordance with the literature experimental conditions. Various factors could influence the structure of the lanthanide coordination polymers, such as temperature, solution pH, solvent and the ratio of metal ion to ligand.26-28 We modified the reaction procedures with the addition of 2 drops of pyridine instead of NaAc·3H2O, reducing the amount of solvent from 15 mL to 10 mL, changing the temperature from 160 °C to 140 °C, and extending the reaction time from 75 h to five days. Pyridine seems to play an important role in the formation of these coordination polymers because no crystalline samples were obtained without adding pyridine during the synthesis for compounds 1-5.

Figure 5.The coordination environment of Nd3+ ion in compound 3 with ball and stick style (all hydrogen atoms are omitted for clarity). Symmetry codes: a: −x, −y, 1−z; b: −x, 1−y, 2−z.

Figure 6.The intermolecular hydrogen bonds of compound 3. (Dashed lines indicate hydrogen-bonding interactions.)

There are some literature reports about anions playing an important role in the crystal structure tuning.29-31 Herein we investigated the structures of the lanthanide coordination polymers tuned by two different anions (NO3−, Cl−). We use lanthanide nitrates and lanth-anide chlorides to prepare lanthanide coordination polymers with four different lanthanide elements (Er, Yb, Nd, Gd), respectively. The PXRD patterns of compounds prepared with different anions for each lanthanide were compared in Figure S5. The structures of compounds 1-2 prepared with lanthanide nitrates are com-pletely different from those compounds prepared by corre-sponding lanthanide chlorides, while the structures of compounds 3-4 prepared with lanthanide chlorides are same with those compounds prepared by corresponding lanthanide nitrates. The structure tuning by salt anions was probably due to that different salt anions could affect the deproto-nation process for 1,4-H2NDC. 1,4-H2NDC was partially deprotonated to form HNDC− and NDC2− ligands in nitrate for compound 1 while 1,4-H2NDC was completely deproto-nated to form NDC2− ligands in chloride for compound 5.

Compounds 1-5 all have O−H⋯O intermolecular hydro-gen bonds with an octatomic ring, but the composition atoms of the ring in compounds 1-2 are different from those in compounds 3-5.

X-ray Powder Diffraction (PXRD) Analysis.In order to confirm the phase purity of the obtained crystals, PXRD experiments were carried out on compounds 1-5 (Fig. S4, ESI.). The experimental PXRD patterns were entirely con-sistent with the simulated PXRD patterns generated based on the single-crystal data of compounds 1-5, which proved the purity of each compound. The similar PXRD patterns between compound 1 and compound 2 indicated that they are isostructural to each other. Similar case also happened for compounds 3-5.

Thermogravimetric Analysis (TGA). TGA data of compounds 1-5 were performed in N2 atmosphere at heating rate of 5 °C/min for these five lanthanide coordination polymers. The TGA curves of compounds 1-2 are similar and only the TGA curve of compound 1 is described in detail. The TGA curve of compound 1 presents two main stages of decomposition. The first stage, from 320 °C to 470 °C, is due to the loss of the ligand phen with a weight loss of 26.07％ (calcd. 26.94％), and the second stage, from 470 °C to 520 °C, is due to the decomposition of the other ligand 1,4-NDC2− to give the final inorganic oxide Er2O3 with a weight remaining of 28.62％ (calcd. 28.66％).

The TGA curves of compounds 3-5 are similar. Therefore only TGA plot of compound 4 is described in detail. From the TGA curve of compound 4, we can easily draw a conclusion that the first weight loss of 2.58％ from 140 °C to 300 °C is due to the dissociation of two coordinated water molecules (calcd. 2.66％). When the temperature is close to 500 °C, the organic ligand phen is gradually decomposed leading to the second weight loss of 25.81％ (calcd. 26.60％). It continues to decompose into a final inorganic oxide Gd2O3 with a weight remaining of 25.80％ (calcd. 26.87％).

Luminescent Properties. Most lanthanide ions are well-known to give rise to luminescence and have been widely used in optical and medical devices.32 We have examined the solid-state luminescence properties of our lanthanide coordi-nation polymers at room temperature, but only the ligand-based emissions were shown in the visible light region which is in agreement with the literature reports.17,33

 

Conclusion

Five new lanthanide coordination polymers have been pre-pared with 1,4-H2NDC, phen, NaOH, pyridine and different lanthanide salts under hydrothermal condition. These coordi-nation polymers display 2-D frameworks with different structures. Pyridine plays a key role in the formation of these coordination polymers. Their structural diversities demon-strate that different salt anions play an important role in the formation of these frameworks. Furthermore, TGA data of compounds 1-5 imply a considerable thermal stability of these lanthanide coordination polymers.