Syntheses,structures and properties of two one-dimensional coordination polymers constructed by phenol-sulfonate ligands and rare earth metal ions

Abstract

Two new isomorphic and heterogeneous coordination polymers (CPs), namely {[RE (L) (H₂O) ₂] · (Hbipy) · H₂O} _n (RE = Yb(1), Y(2), Na₂H₂L = 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, bipy = 2,2^′-bipyridine) have been synthesized by hydrothermal methods and structurally characterized by elemental analyses, power X-ray diffraction (PXRD) and single crystal X-ray diffraction. Compounds 1 and 2 feature one-dimensional (1D) wavy ladder-like chain structures, which are further connected by hydrogen bonds from water molecules to result in two-dimensional (2D) layered structures. The 1H-protonated bipy molecules act as uncoordinated cations, leading to valences of zero in compounds 1 and 2. Compounds 1 and 2 emit luminescence at 382 nm and 372 nm in the ultraviolet regions, respectively. The luminescent emissions originate from the typical ligand-centered (n–π^* and π–π^*) transitions. In addition, compound 1 has characteristic near-infrared emission of ytterbium ion at 964 nm.

Keywords

Coordination polymer rare earth metal one dimensional luminescence

1 Introduction

The chemistry of coordination polymers (CPs) involving rare earth metal ions has been an active research area in recent years owing to their potential applications in the areas of catalysis, ion exchange, proton conductivity, intercalation chemistry and material chemistry [1 –5]. Assembly of CPs allows for a wide choice of various parameters including versatile functions of aromatic ligands as well as diverse electronic properties and coordination preference of the rare earth metal ions [6 –8]. Nowadays, there is a growing interest in employment of aromatic sulfonate ligands attached by various functional groups, such as amine, hydroxyl, and carboxylate groups, for construction of CPs with varying topologies [9 –11]. Furthermore, the structure-dependent properties have been further expanded through the multifunctional ligands, such as sulfonate-phenol ligands [12]. The synergistic combination of the unique characteristics of the functional groups can expand the number of possible geometrical combination between the O-donors and allow for bridging the rare earth metal ions with new coordination modes, so as to improve the characteristic coordination behaviors of ligands and provide the CPs with unusual structures and intriguing features [13 –15].

We are interested in the sulfonate-phenol ligands because they can exhibit discriminative coordination abilities [16, 17]. Na₂H₂L is an important kind of sulfonate-phenol ligands. This is based on the following considerations: (i) it is a rigid aromatic ligand and plays the role of a bridging rod; (ii) owing to the presence of two kinds of functional groups, a variety of binding modes can offer new building blocks for the construction of diverse topological structures; (iii) the functional groups are the potential interaction sites of O–H⋯O hydrogen bonding for the construction of specific supramolecular networks. However, the reports on metal sulfonate-phenol CPs are still rare [12].

In this work, we use a bisulfonate molecule attached by two phenol groups as ligand, Na₂H₂L. The sulfonate groups possess weak coordination ability to rare earth metal ions, and the introduction of the phenol groups could improve its characteristic coordination feature and crystallinity of its coordination polymer, in addition to forming hydrogen bonds. This publication describes the syntheses, crystal structures, thermal stability, and luminescence of two new 1D phenol-sulfonate CPs.

2 Experimental

2.1 Materials and general measurements

All reagents commercially available were of reagent grade and used without further purification. Elemental analyses were performed on a Perkin-Elmer 240C instrument. Powder X-ray diffraction patterns were performed on a RIGAKU DMAX2500PC diffractometer. Thermogravimetric analyses were carried out on a NETZSCH STA 449C analyzer with the temperature range of 25C–900C, at a heating rate of 10C min^–1 under a nitrogen atmosphere. Luminescence spectra for the solid samples were recorded at room temperature. Fluorescence spectra were obtained using a Perkin Elemer LS55 fluorescence spectrometer and an Edinburgh Instruments FLS980 in the NIR range.

2.2 Synthesis of compound 1

A mixture of Yb(NO₃)₂·6H₂O (0.049 g, 0.2 mmol), Na₂H₂L (0.056 g, 0.2 mmol), and bipy (0.079 g, 0.4 mmol) in H₂O (10 ml) and ethanol (10 ml) was put into a Teflon-lined stainless-steel reactor (25 mL) and heated at 140C for 5 d. After cooling to room temperature, the yellow blocked crystals were collected. The single crystals were obtained in ca. 60% yield based on Yb. Elemental Analysis Calculated (%) for C₁₆H₁₇N₂O₁₁S₂Yb (1): C 29.54, H 2.61, N 4.31. Found (%): C 29.61, H 2.88, N 4.46.

2.3 Synthesis of compound 2

Compound 2 was synthesized by a method similar to that used for compound 1. Y(NO₃)₂·6H₂O was used instead of Yb(NO₃)₂·6H₂O. The yellow blocked crystals of compound 2 were recovered in 57% based on Y. Elemental Analysis Calculated (%) for C₁₆H₁₇N₂O₁₁S₂Y (2): C 33.93, H 3.00, N 4.95. Found (%): C 33.97, H 3.10, N 5.01.

2.4 X-ray crystallographic measurement

Single crystals of compounds 1 and 2 were mounted on a Bruker Apex-II diffractometer equipped with a graphite-monochromated Mo-K_α radiation (λ = 0.71073Å). The data were collected at 296 K. Data reduction was performed using SAINT and corrected for Lorentz and polarization effects. Adsorption corrections were applied using the SADABS routine. Both structures were solved by direct methods and refined by full-matrix least-squares fitting on F² by SHELX-2014 [18, 19]. All non-hydrogen atoms are refined with anisotropic thermal parameters. Hydrogen atoms were located at geometrically calculated positions. The details of the crystal parameters, data collection, and refinement for compounds 1 and 2 are summarized in Table 1, and the selected bond lengths and bond angles are summarized in Table 2. Hydrogen bond lengths and bond angles for compounds 1 and 2 are listed in Table 3.

Table 1
Crystal data and structure refinement for compounds 1 and 2

Identification code 1 2

Chemical formula C₁₆H₁₇N₂O₁₁S₂Yb C₁₆H₁₇N₂O₁₁S₂Y

M _r 650.47 566.34

Crystal system, space group Triclinic, P-1 Triclinic, P-1

Temperature (K) 296 296

a, b, c (Å) 8.4575(6), 9.0424(7), 13.7894(10) 8.4761(4), 9.0355(4), 13.8009(7)

α, β, γ (°) 72.281(2), 81.841(1), 75.143(2) 72.1247(14), 81.6114(15), 75.2667(14)

V (Å³) 968.61 (12) 970.21 (8)

Z 2 2

μ (mm^–1) 5.11 3.29

T_min, T_max 0.545, 0.735 0.556, 0.746

No. of measured, independent and 5809, 3360, 3278 7010, 3407, 3182

observed [I > 2σ(I)] reflections R_int 0.02 0.026

(sin θ/λ)_max (Å^–1) 0.595 0.595

R [F² > 2σ (F²)], wR (F²), S 0.019, 0.048, 1.05 0.026, 0.062, 1.06

No. of reflections 3360 3407

No. of parameters 289 289

No. of restraints 0 2

Δρ_max, Δρ_min (e Å ^–3) 1.41, –1.36 0.60, –0.80

CCDC 1974121 1972684

Identification code	1	2
Chemical formula	C₁₆H₁₇N₂O₁₁S₂Yb	C₁₆H₁₇N₂O₁₁S₂Y
M _r	650.47	566.34
Crystal system, space group	Triclinic, P-1	Triclinic, P-1
Temperature (K)	296	296
a, b, c (Å)	8.4575(6), 9.0424(7), 13.7894(10)	8.4761(4), 9.0355(4), 13.8009(7)
α, β, γ (°)	72.281(2), 81.841(1), 75.143(2)	72.1247(14), 81.6114(15), 75.2667(14)
V (Å³)	968.61 (12)	970.21 (8)
Z	2	2
μ (mm^–1)	5.11	3.29
T_min, T_max	0.545, 0.735	0.556, 0.746
No. of measured, independent and	5809, 3360, 3278	7010, 3407, 3182
observed [I > 2σ(I)] reflections R_int	0.02	0.026
(sin θ/λ)_max (Å^–1)	0.595	0.595
R [F² > 2σ (F²)], wR (F²), S	0.019, 0.048, 1.05	0.026, 0.062, 1.06
No. of reflections	3360	3407
No. of parameters	289	289
No. of restraints	0	2
Δρ_max, Δρ_min (e Å ^–3)	1.41, –1.36	0.60, –0.80
CCDC	1974121	1972684

Table 2

Selected bond lengths (Å) and bond angles (°) for compounds 1 and 2

1				2
O2—Yb1ⁱ	2.267(2)	O8—Yb1ⁱⁱ	2.331(2)	O2—Y1	2.4564(16)	O8—Y1	2.2919(17)
O5—Yb1	2.430(2)	O9—Yb1	2.309(2)	O3—Y1	2.5401(17)	O11—Y1	2.3036(17)
O6—Yb1	2.546(2)	O10—Yb1	2.320(2)	O4—Y1	2.2664(17)	O11ⁱ—Y1	2.3526(17)
O7—Yb1ⁱⁱ	2.233(2)	Yb1—Yb1ⁱⁱ	3.8142(3)	O5—Y1	2.3349(17)	Y1—Y1ⁱ	3.8519(5)
O8—Yb1	2.278(2)			O6—Y1	2.3500(18)
O7ⁱⁱ—Yb1—O2ⁱⁱⁱ	83.82(8)	O10—Yb1—O8ⁱⁱ	143.34(8)	O4—Y1—O8	84.05(6)	O6—Y1—O11ⁱ	142.83(6)
O7ⁱⁱ—Yb1—O8	139.64(8)	O7ⁱⁱ—Yb1—O5	137.48(8)	O4—Y1—O11	138.98(6)	O4—Y1—O2	138.10(6)
O2ⁱⁱⁱ—Yb1—O8	94.41(8)	O2ⁱⁱⁱ—Yb1—O5	71.16(8)	O8—Y1—O11	93.67(6)	O8—Y1—O2	71.07(6)
O7ⁱⁱ—Yb1—O9	80.24(8)	O8—Yb1—O5	77.70(8)	O4—Y1—O5	80.13(6)	O11—Y1—O2	77.39(6)
O2ⁱⁱⁱ—Yb1—O9	159.49(9)	O9—Yb1—O5	129.29(8)	O8—Y1—O5	159.44(6)	O5—Y1—O2	129.38(6)
O8—Yb1—O9	89.48(8)	O10—Yb1—O5	77.87(8)	O11—Y1—O5	89.76(6)	O6—Y1—O2	78.35(6)
O7ⁱⁱ—Yb1—O10	72.41(8)	O8ⁱⁱ—Yb1—O5	135.52(8)	O4—Y1—O6	72.61(6)	O11ⁱ—Y1—O2	135.50(6)
O2ⁱⁱⁱ—Yb1—O10	98.55(8)	O7ⁱⁱ—Yb1—O6	136.49(8)	O8—Y1—O6	98.67(7)	O4—Y1—O3	137.44(6)
O8—Yb1—O10	146.84(8)	O2ⁱⁱⁱ—Yb1—O6	127.78(8)	O11—Y1—O6	147.43(6)	O8—Y1—O3	127.39(6)
O9—Yb1—O10	88.90(8)	O8—Yb1—O6	74.26(7)	O5—Y1—O6	89.07(6)	O11—Y1—O3	74.01(6)
O7ⁱⁱ—Yb1—O8ⁱⁱ	71.44(8)	O9—Yb1—O6	72.64(8)	O4—Y1—O11ⁱ	70.69(6)	O5—Y1—O3	73.00(6)
O2ⁱⁱⁱ—Yb1—O8ⁱⁱ	83.45(8)	O10—Yb1—O6	73.67(8)	O8—Y1—O11ⁱ	83.38(6)	O6—Y1—O3	74.54(6)
O8—Yb1—O8ⁱⁱ	68.31(9)	O8ⁱⁱ—Yb1—O6	132.86 (7)	O11—Y1—O11ⁱ	68.36(6)	O11ⁱ—Y1—O3	132.66(6)
O9—Yb1—O8ⁱⁱ	79.29(8)	O5—Yb1—O6	56.65(7)	O5—Y1—O11ⁱ	79.07(6)	O2—Y1—O3	56.38(5)

Symmetry codes for compound 1: (i) –x, –y, –z + 1; (ii) x, y + 1, z; (iii) x, y–1, z. Symmetry code for compound 2: (i) –x, –y + 1, –z + 1.

Table 3

Hydrogen bond lengths (Å) and bond angles (°) for compounds 1 and 2

D–H⋯A	d(D-H)	d(H⋯A)	d(D⋯A)	∠DHA	Symmetry transformations
1
O11—H11D⋯O4ⁱ	0.810	2.540	3.316(3)	159.00	–x+1, –y+1, –z+1
O11—H11B⋯O7ⁱⁱ	0.740	2.180	2.905(3)	166.00	–x, –y+1, –z+1
O10—H10B⋯O11ⁱⁱⁱ	0.800	1.900	2.686(3)	168.00	–x+1, –y, –z+1
O10—H10A⋯O1^iv	0.790	2.070	2.842(3)	166.00	x+1, y–1, z
O9—H9A⋯O6ⁱⁱⁱ	0.810	1.960	2.747(3)	161.00	–x, –y+1, –z+1
O9—H9B⋯O1ⁱⁱ	0.860	1.940	2.751(3)	157.00	–x+1, –y, –z+1
N1—H1⋯O4^v	0.860	2.010	2.786(4)	149.00	–x+1, –y+1, –z
2
O10—H10D⋯O1ⁱ	0.850	2.470	3.310(3)	171.00	x, y–1, z
O10—H10B⋯O4ⁱⁱ	0.830	2.070	2.895(2)	174.00	–x+1, –y, –z+1
O6—H6A⋯O10	0.880	1.810	2.670(3)	165.00
O6—H6B⋯O9ⁱⁱⁱ	0.890	1.930	2.821(2)	179.00	x+1, y, z
O5—H5A⋯O3^iv	0.780	1.990	2.750(2)	166.00	–x+1, –y+1, –z+1
O5—H5B⋯O9^v	0.810	1.950	2.753(3)	170.00	–x, –y+1, –z+1
N2—H2⋯O1^vi	0.860	2.010	2.783(3)	149.00	x, y, z + 1

3 Results and discussion

3.1 Structure descriptoin of compounds 1 and 2

Compounds 1 and 2 are heterogeneous and isomorphic, and their structures are similar. Compound 1 has been selected as a representative example to describe their structures in detail. Single crystal X-ray diffraction indicates that compound 1 crystallized in triclinic, P-1 space group (Table 1). The asymmetric unit consists of one ytterbium ion, one L^4– ligand, one 1H-protonated bipy, two coordinated and one free water molecules, as shown in Fig. 1a. The ytterbium atom in compound 1 is eight-coordinated by three sulfonate and three phenol oxygen atoms as well as two water molecules, showing the dodecahedral coordination configuration (Fig. 1b). The Yb-O distances are between 2.233(2) Å and 2.546(2) Å, and the bond angles of O-Yb-O are in the range of 56.65(7)–159.49(9) (Table 2), which are comparable to those reported in other ytterbium complexes [20, 21]. The result shows that 1H-protonated bipy molecule does not take part in coordination with ytterbium atom, and only balances the charge as anion in compound 1, whereas in other complexes reported, the bipy molecules only act as the ligands chelated the central metal atoms [22, 23].

Fig.1

(a) The molecular structure of compound 1. The asymmetric structure unit of compound 1 with atomic labeling scheme. All hydrogen atoms, but H1, are omitted for clarity; (b) Yb ion exhibits a dodecahedral coordination configuration and (c) the coordination mode of L^4– ligand.

Each L^4– ligand adopts one coordination mode: (i) sulfonate groups of L^4– ligand connect two different ytterbium ions via bidentate-chelating and monodentate modes, respectively, to form a μ₂-bridge; (ii) phenol groups of L^4– ligand bind with two different ytterbium ions via bridging and monodentate modes, respectively, to form a μ₂-bridge. That is to say, the L⁴^– ligand acts as a pentadentate mode to combine three ytterbium atoms (Fig. 1c). This coordination mode is different with those of the reported L^4– complexes [24, 25]. The L^4– is a versatile ligand and is able to adopt many types of coordination modes. The two sulfonate groups are coordinated to terbium in different forms; one sulfonate group adopts a bidentate coordination mode, the other is in unidentate form. The dimers are bridged by a unidentate sulfonate oxygen atom to form a chain structure [24]. The biphenol groups are easy to coordinate with alkaline earth metal ions. The biphenol groups can chelate to one barium center. The sulfonate group coordinates to two different Ba^2 + ions via a μ²η²-coordination mode. The other sulfonate adopts a μ³η³-coordination mode [25].

Two adjacent ytterbium atoms are linked by two bridging phenol groups of L^4– ligands, with the Yb⋯Yb distance of 3.8142 Å. The Yb⋯Yb distances, when bridged by phenol groups only, are slightly shorter than those, when bridged by phenol groups and bidentate carboxylate groups. The interconnection of ytterbium ions by L⁴^– ligands results into a 1D chain along the b axis (Fig. 2a). Each 1D chain is interlinked with neighboring chains via hydrogen bonds involving of coordinated and lattice water molecules (O11—H11D⋯O4ⁱ, O11—H11B⋯O7ⁱⁱ, O10—H10B⋯O11ⁱⁱⁱ, O9—H9A⋯O6ⁱⁱⁱ and O9—H9B⋯O1ⁱⁱ). There exist hydrogen bonds that interlink coordinated water molecules with lattice water molecules, and link water molecules with sulfonate oxygen and phenol oxygen atoms in L^4– ligands, with the hydrogen bond distances in the range of 2.686(3) Å–3.316(3) Å (Table 3). These hydrogen bonding interactions can extend the 1D chains into a 2D supramolecular layered structure (Fig. 3).

Fig.2

Schematic representation of (a) 1D chain structure; (b) the three-connected Yb node (bule); (c) the L^4– ligand-based three-connected node (violet); (d) (3, 3)-connected topological wavy ladder-like chain.

Fig.3

1D chains connected by hydrogen bonds involving of water molecules to form a layered structure (a) along c axis and (b) b axis.

Many metal complexes with various topologies based carboxylate or sulfonate ligands, especially multifunctional ligands, are known [5 , 26–28]. However, for rare earth metal ions, they are easy to form isostructural complexes [29 –31]. Rare earth metal coordination complexes are currently of great interest for the syntheses of novel molecular materials with luminescent properties. Two isostructural complexes with the formula [Ln₂(DMF)₂(OAc)₂(HL)₂]_n (Ln = Dy, Eu H₃L = N^′-(2-hydroxybenzylidene)pyridine N-oxide-carbohydrazide) were solvothermally synthesized. Two centrosymmetric Ln ions are connected by a pair of μ²-η²:η¹-CH₃COO^– anions, leading to a centrosymmetric {Ln₂} subunits. Furthermore, these separate {Ln₂} subunits are periodically extended by HL^2– connectors, generating a two-dimensional layer structure [30]. A successful synthetic approach to rare earth metal complexes is the employment of bridging multifunctional ligands and chelating N-donor ligands. The reaction of several Ln(NO₃)₃·6H₂O salts with 9-anthracenecarboxylic acid (9-HAC) and bipy in a mixture of CH₃OH/H₂O has allowed the isolation of the dinuclear complexes, such as [Ln₂(9-AC)₄(9-AC)₂(bpy)₂] (Ln = Nd, Eu, Gd, Tb, Er, Yb) [31].

From the topological point of view, each ytterbium ion is surrounded by three L^4– ligands, thus, the ytterbium centers define as 3-connected nodes (Fig. 2b). Likewise, the L^4– ligand serves as a linker to bridge with three neighboring ytterbium ions. In this way, the L^4– ligands act as 3-connected nodes (Fig. 2c). Therefore, compound 1 is a wavy trinodal ladder-like chain structure, as shown in Fig. 2d.

3.2 Powder X-ray diffraction

As shown in Fig. 4, the peak positions of simulated and experimental PXRD pattern are in agreement with each other, which suggests the good phase purity of compounds 1 and 2 [32].

Fig.4

(a) The simulated and measured PXRD patterns for compound 1 and (b) for compound 2.

3.3 Thermogravimetric analyses

Thermal gravimetric analyses(TGA) of compounds 1 and 2 under the nitrogen atmosphere from 25C to 900C at a heating rate of 10C min^–1 are shown in Fig. 5. The TGA curves indicate that compounds 1 and 2 remain stable up to 162C and 167C respectively. The first weight losses in the ranges of 162C–214C for compound 1 and 167C–230C for compound 2 correspond to the release of all water molecules, with observed weight losses of 8.96% (calculated 8.30%) and 9.13% (calculated 9.53%), respectively. Upon further heating, the successive weight loss steps in the ranges of 214C–468C and 230C–509C can be ascribed to the decomposition of Hbipy cations (found 24.61%; calcd 24.16% for compound 1, found 26.70%; calcd 27.72% for compound 2), respectively. The subsequent weight losses are probably caused by the decomposition of L^4– ligands, resulting in the collapse of chain structures. The chain structures successively decompose to 900C without stopping. The residues likely continue to decompose until the final products (the mixture of metal oxides and sulfides) [33].

Fig.5

TGA curves for compounds 1 and 2.

3.4 Luminescent properties

The solid state luminescence of compounds 1, 2 and free Na₂H₂L ligand is examined at room temperature. The emission spectra of them are depicted in Fig. 6. The free Na₂H₂L ligand shows photoluminescence with an emission maximum at 443 nm upon excitation at 350 nm. Compound 1 shows the emission band at 382 nm, which is probably due to n–π^* and π–π^* intraligand fluorescence [34]. The emission of compound 1 occurs at shorter wavelength than that of the free Na₂H₂L ligand. The blue shifted band is due to metal-to-ligand charge transfer transitions [35]. Compound 2 exhibits a broad emission band with maxima at 372 nm upon excitation with wavelength of 350 nm. The emission wavelength is also shifted to blue region in compound 2 due to the same reason as mentioned for compound 1. Another interest in the investigation of the rare earth metal complexes with the aromatic ligands is their ability to sensitize ytterbium luminescence, so infrared luminescence upon through-ligand excitation of ytterbium complex was studied. Upon excitation at 350 nm, compound 1 exhibits the characteristic emission band around 964 nm from the transitions of electronic dipole, which is attributed to the ²F_7/2⟶²F_5/2 transitions of ytterbium ion [33].

Fig.6

(a) Solid state emission spectra of free Na₂H₂L, compounds 1 and 2 at room temperature; (b) Infrared luminescence of compound 1 in power.

4 Conclusion

In summary, two new CPs, {[RE(L)(H₂O)₂]·(Hbipy)·H₂O}_n (RE = Yb, Y), were synthesized by hydrothermal methods. The phenol groups possess bridging and monodentate coordination modes, and the sulfonate groups adopt μ₁ and μ₂ coordination modes, respectively. The whole L^4– anions act as the pentadentate ligands to form wavy ladder-like chain structures. The hydrogen bonds involving of water molecules can interlinker the adjacent 1D chains to generate 2D layered structres. The fluorescence of compounds 1 and 2 is attributed to the ligand-centered n–π^* and π–π^* transitions. Additionally, compound 1 exhibits the characteristic emission band at 964 nm, which is attributed to the ²F_7/2⟶²F_5/2 transitions of ytterbium ion.

Appendix A. Supplementary material

CCDC 1974121 and 1972684 contain the supplementary crystallographic data for compounds 1 and 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

Funding

This work was supported by Scientific research fund project from The Educational Department of Liaoning Province (No. L2019010).

Footnotes

Acknowledgments

We thank The Educational Department of Liaoning Province (No. L2019010) for financial support.

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