Four 2D Ln–Cd heterometal–organic coordination polymers based on tetranuclear Ln–Cd oxo-cluster with highly selective luminescent sensing of organic molecules and metal cations

Abstract

Two series of novel lanthanide–cadmium (Ln–Cd) heterometal–organic coordination polymers, i.e., [Ln₂Cd₂(DTPA)₂(H₂O)₄] (H₂O)₄ (I: Ln = Eu 1a; Gd 1b) and [Ln₂Cd₃(DTPA)₂Cl₂(H₂O)₆] (II: Ln = Eu 2a; Sm 2b) (H₅DTPA = diethylenetriamine pentaacetic acid), have been synthesized using a hydrothermal method. Compounds I and II both possess 2D grid-layer structure based on distinct tetranuclear Ln–Cd oxo-cluster units as secondary building units (SBUs). Two enantiomeric forms, i.e., α and β, exist in [Cd(DTPA)]³⁻ in compounds I. Interestingly, compound 1a displays not only a highly sensitive sensing of small molecular organic solvents, especially methanol, triethylamine and ethanol amine, but also the sensing of cations, especially Cr³⁺ and Fe³⁺ ions. Moreover, Cr³⁺, Fe³⁺, Co²⁺, Ni³⁺, Cu²⁺, Nd³⁺ cations quenched the emissions of the Eu³⁺ ions of compound 2a, which indicates the potential of 1a and 2a for sensing of metal cations.

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Introduction

Lanthanide–organic coordination polymers are attracting great interest because of their unique luminescent and magnetic properties that are derived from the electronic configurations [Xe]4fⁿ (n = 0–14) of lanthanide elements.¹ Varieties of lanthanide–organic coordination polymers with fascinating structural diversities and potential applications have been synthesized and researched. In order to enrich the structures and promote the properties of lanthanide–organic coordination polymers, heterometallic ions, especially d-block ions, were introduced into these compounds.^1f–h,2 The introduced heterometallic ions not only make the structures more plentiful but also cause the energy levels of these materials to be more controllable.³ To the best of our knowledge, the vast majority of these lanthanide-transition(f–d) heterometal–organic coordination polymers are focused on Ln–M (M = Cu,^2d,2h,4 Co,^2j,5 Ni,^2j,5b,6 Fe,^2j,5f,7 Ag,⁸ Mn^2b,9), and these transitional metal ions contain interesting magnetic properties. Relatively few studies are focused on the luminescent properties of the compounds that we are interested in. Most lanthanide ions emit line-like and strong lights under stimulus. However, we all know that the f–f transitions of lanthanide ions (Ln³⁺ ions) are parity-forbidden, which results in very low absorption coefficients. Organic ligands with intense absorption properties are coordinated to lanthanide ions as energy donors to induce characteristic emissions of lanthanide ions; i.e., the energy is first absorbed by the organic ligands, and then it will be sent to lanthanide ions by intramolecular energy transfer, which induces emissions of lanthanide ions. This method is known as the antenna effect.¹⁰ There has been significant interest in development of luminescent lanthanide–organic coordination polymers for applications such as optical materials^3a and organic light-emitting diodes (OLEDs).¹¹ The application of luminescent sensors is a promising research field.^1d,12 The intensities of the emission spectra of lanthanide ions could be enhanced or quenched by some organic molecules such as acetone and acetylacetone or cations such as Co²⁺ and Cu²⁺. Thus, lanthanide–organic coordination polymers are potential candidate materials for chemical sensing.

In contrast to other lanthanide-transition(f–d) heterometal–organic coordination polymers, the studies of lanthanide–cadmium (Ln–Cd) heterometal–organic coordination polymers are very few. Cd²⁺ ion itself not only emits luminescence by ligand–metal interactions, but also acts as a bridge of energy transfer between organic ligands and Ln³⁺ ions. Because of their novel luminescent properties, Ln–Cd heterometal–organic coordination polymers will be useful tools as chemical sensors. To the best of our knowledge, the reported Ln–Cd heterometal–organic coordination polymers are mainly zero-dimensional (0D),^1c,2j,13 one-dimensional (1D)¹⁴ and three-dimensional (3D)^1d,15 coordination structures. Our group reported the first 2D Ln–Cd heterometal-layered structure.^12e According to the soft-hard acid-base theory, Ln³⁺ ions belong to hard acids and have strong affinities to coordinate to hard bases like the O atom, whereas the Cd²⁺ ion is a soft acid and prefers to bond to soft bases such as S and N atoms. The competitions between Ln³⁺ and Cd²⁺ in the reaction system make it difficult to synthesize Ln–Cd heterometal–organic coordination polymers. Thus, the ligands with N- and O-donors can be elaborately selected and employed to make heterometal–organic coordination polymers. Diethylenetriamine pentaacetic acid (H₅DTPA) with N- and O-donors has been used to prepare metal–organic coordination polymers with single transition metal (TM) or Ln ions;¹⁶ however, this assembly in the Ln–TM–organic coordination polymers remains unexplored. We selected H₅DTPA as a candidate for assembling heterometal polymers based on the following considerations: (1) it has five carboxylate groups as hard bases and three coordinated N donors as soft bases, which makes it possible to bond to both Ln³⁺ and Cd²⁺ ions simultaneously in the reaction system. (2) It is a chelating agent, which has strong affinities with the Ln³⁺ and Cd²⁺ ions. (3) The carboxylic acid groups and N donors of the ligand can adjust the pH value of the reaction system.

On the basis of the aforementioned points, our aim is to prepare novel Ln–Cd heterometal–organic coordination polymers with luminescent sensing. Herein, we report the syntheses, structures and luminescent sensing of two series of 2D Ln–Cd heterometal–organic coordination polymers: [Ln₂Cd₂(DTPA)₂(H₂O)₄] (H₂O)₄ (I: Ln = Eu 1a; Gd 1b) and [Ln₂Cd₃(DTPA)₂Cl₂(H₂O)₆] (II: Ln = Eu 2a; Sm 2b), which are 2D layers based on different tetranuclear [Ln₂Cd₂] oxo-clusters with highly selective luminescent sensing.

Experimental section

Materials and physical measurements

Commercially available solvents and chemicals were used without further purification. The elemental analyses for C, H and N were performed with an Elementar Vario EL III elemental analyzer. IR spectra were measured as KBr pellets on a Perkin-Elmer Spectrum 2000 FT-IR in the range of 400–4000 cm⁻¹. Thermogravimetric data were collected on a Mettler Toledo TGA/SDTA 851^e analyzer in flowing nitrogen at a heating rate of 10 °C min⁻¹. Luminescence measurements were conducted with an Edinburgh Instrument FS920 TCSPC luminescence spectrometer on the powder crystal material of the compounds. The 1a solution was prepared by introducing 1a powder (5 mg) into methanol, acetonitrile, acetone, n-butanol, ethanol, ammonia water, H₂O, DMF, iso-propanol, triethylamine, pyridine, ethanediamine, 1,2-propane diamine, and ethanol amine (5.00 mL) at room temperature. Similarly, the powder of 1a and 2a was immersed in an aqueous solution containing different M(NO₃)_x (M = Ca²⁺, Sr²⁺, Li⁺, K⁺, Na⁺, Mg²⁺, La³⁺, Ho³⁺, Ce³⁺, Tb³⁺, Yb³⁺, Zn²⁺, Cd²⁺, Pb²⁺, Al³⁺, Sm³⁺, Gd³⁺, Nd³⁺, Pr³⁺, Dy³⁺, Cu²⁺, Co²⁺, Cr³⁺, Fe³⁺, Ni²⁺ and Er³⁺) and different concentrations of Co(NO₃)₂. Samples were prepared by introducing powder (5 mg) of 1a and 2a into an aqueous solution (5.00 mL) at room temperature. Before luminescence measurements, the suspensions were sonicated for 10 minutes using ultrasonic waves to ensure uniform dispersion.

Synthesis of compounds

[Ln₂Cd₂(DTPA)₂(H₂O)₄](H₂O)₄ (I: Ln = Eu 1a; Gd 1b). A mixture of H₅DTPA (0.3931 g, 0.999 mmol), Cd(Ac)₂·2H₂O (0.5330 g, 2.000 mmol), Ln₂O₃ (Eu₂O₃, 0.1768 g, 0.502 mmol for 1a; Gd₂O₃, 0.1846 g, 0.509 mmol for 1b), and H₂O (10 mL) was placed in 23 mL Teflon-lined stainless steel vessels, heated to 170 °C for 7 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 0.6390 g (88% based on Eu(III)) for 1a, 0.5173 g (69.5% based on Gd(III)) for 1b. Anal. calc. for C₂₈N₆O₂₈H₅₂Cd₂Eu₂ 1a: C, 23.18%; H, 3.59%; N, 5.80%. Found: C, 23.20%; H, 3.57%; N, 5.95%; for C₂₈N₆O₂₈H₅₂Cd₂Gd₂ 1b: C, 23.01%; H, 3.56%; N, 5.75%. Found: C, 23.15%; H, 3.65%; N, 5.60%. Selected IR peaks (cm⁻¹): 3190(br), 1595(s), 1442(m), 1403(m), 1321(m), 1243(m), 1100(s), 989(m), 924(s), 849(m), 745(m), 719(m), 643(m), 595(w), 520(w), 448(w) (Fig. S1†).

[Ln₂Cd₃(DTPA)₂Cl₂(H₂O)₆] (II: Ln = Eu 2a; Sm 2b). A mixture of H₅DTPA (0.3939 g, 1.001 mmol), Cd(Ac)₂·2H₂O (0.2670 g, 1.002 mmol), Ln₂O₃ (Eu₂O₃, 0.3510 g, 0.997 mmol for 2a; Sm₂O₃ 0.3475 g, 0.997 mmol for 2b), and H₂O (10 mL) was placed in a 23 mL Teflon-lined stainless steel vessel. The pH was adjusted to 2 by 0.1 M HCl, and then the solution was heated to 170 °C for 7 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 0.1665 g (10.5% based on Eu(III)) for 2a, 0.1187 g (7.5% based on Sm(III)) for 2b. Anal. calc. for C₂₈N₆O₂₆H₄₈Cd₃Eu₂ 2a: C, 21.04%; H, 3.01%; N, 5.26%. Found: C, 21.20%; H, 3.17%; N, 5.15%; for C₂₈N₆O₂₆H₄₈Cd₃Sm₂ 2b: C, 21.09%; H, 3.01%; N, 5.27%. Found: C, 21.15%; H, 3.15%; N, 5.40%. Selected IR peaks (cm⁻¹): 3295(s), 1580(s), 1445(m), 1406(s), 1325(m), 1270(m), 1083(s), 996(w), 937(s), 865(w), 729(m), 647(m), 582(w), 500(w), 445(w) (Fig. S1†).

X-ray crystallography

Suitable single crystals were selected and mounted on a glass fiber. All data of 1a, 1b and 2b were obtained on a Rigaku R-AXIS RAPID diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. Moreover, all data of 2a were obtained on a Rigaku Saturn 724 CCD diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. The structures were solved by direct methods and refined by the full-matrix least-squares method on F² using the SHELXTL-97 program package.¹⁷ Some free water molecules were refined isotropically. Hydrogen atoms bonding O and C were generated geometrically (C–H = 0.93 Å, O–H = 0.85 Å) and refined with fixed isotropic displacement parameters. Note that hydrogen atoms of some free water molecules were not generated. The selected crystal parameters, data collection, and refinements are summarized in Table 1.

Table 1 Crystal data and structural refinement parameters for 1a, 1b, 2a and 2b

	1a	1b	2a	2b
a R1 = ∑||F0| − |Fc||/∑|F0|.b wR2 = {∑[w(F0^2 − Fc)^2]/∑[w(F0^2)^2]}^1/2.
Formula	C28N6O28H52Cd2Eu2	C28N6O28H52Cd2Gd2	C28N6O26H48Cl2Cd3Eu2	C28N6O26H48Cl2Cd3Sm2
fw	1449.48	1460.06	1596.74	1593.52
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄
a (Å)	9.804(3)	9.779(2)	8.6545(17)	8.6521(17)
b (Å)	10.912(4)	10.868(2)	9.0700(18)	9.0758(18)
c (Å)	20.446(6)	20.395(4)	15.407(3)	15.422(3)
α (°)	100.683(12)	100.77(3)	98.47(3)	98.43(3)
β (°)	92.766(9)	92.74(3)	101.37(3)	101.51(3)
γ (°)	91.801(11)	91.79(3)	103.33(3)	103.40(3)
V (Å^3)	2145.1(12)	2125.2(7)	1129.9(4)	1130.5(4)
Z	2	2	1	1
Dc (g cm^−3)	2.244	2.282	2.347	2.341
Rint	0.0317	0.0630	0.0310	0.0198
θ range/°	2.99–25.00	2.99–27.47	3.19–27.49	3.13–27.47
F(000)	1416	1420	770	768
GOOF on F^2	1.01	0.95	1.11	1.22
R1^a, wR2^b (I > 2σ(I))	0.0269, 0.0666	0.0433, 0.1109	0.0298, 0.0661	0.0243, 0.0798
R1, wR2 (all data)	0.0317, 0.0685	0.0630, 0.1237	0.0317, 0.0671	0.0277, 0.0864

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Results and discussion

Description of crystal structures

As shown in Fig. S2,† the PXRD patterns of compounds I and II agree with those calculated from the structures, indicating that complexes 1a and 1b, 2a and 2b are isostructural.

[Ln₂Cd₂(DTPA)₂(H₂O)₄](H₂O)₄ (I: Ln = Eu 1a; Gd 1b). According to single-crystal X-ray diffraction analyses and PXRD patterns, compounds 1a and 1b are isostructural, which both crystallize in triclinic space groups of P1̄. Thus, only the structure of 1a will be described in detail here. In the asymmetrical unit of 1a, there are two unique Cd²⁺ ions, two Eu³⁺ ions, two DTPA⁵⁻ anions and four coordinated water molecules, respectively (Fig. 1). The coordination geometries of the two seven-coordinated Cd²⁺ ions can be viewed as slightly distorted pentagonal bipyramids: four O_COO– atoms and three N atoms from one DTPA⁵⁻ anion (Fig. S3† and 2). Moreover, the two Eu³⁺ ions are both nine-coordinated with geometries of tricapped trigonal prisms: seven O_COO– atoms from four DTPA⁵⁻ anions and two coordinated water molecules (Fig. S4†). Cd–O and Cd–N bond lengths are in the range of 2.344(3) Å–2.417(3) Å and 2.358(3) Å–2.461(3) Å, respectively. Moreover, Eu–O bond lengths are 2.392(3) Å–2.568(3) Å, which are comparable to the reported bond distances in a previous study.¹⁵ The two DTPA⁵⁻ anions in the asymmetrical unit both chelate Cd²⁺ ions to form pentagonal bipyramids. Interestingly, in the [Cd(DTPA)]³⁻ units, the absolute configurations around Cd²⁺ ion, have two enantiomeric forms, i.e. α and β (Fig. 2 and S5†). Two μ₂-O atoms from one DTPA⁵⁻ ligands bridge one Cd²⁺ and one Eu³⁺, thus forming an edge-sharing heterometal binuclear [EuCd(μ₂-O)₂] unit with an Eu⋯Cd distance of 3.985(7) Å. Two binuclear [EuCd (μ₂-O)₂] units are connected each other by μ₂-O from DTPA⁵⁻ ligands between Eu³⁺ and Cd²⁺ ions to generate a cradle-like tetranuclear [Eu₂Cd₂(μ₂-O)₆] cluster unit (Fig. 3a and b). Each tetranuclear [Eu₂Cd₂(μ₂-O)₆] cluster unit acts as a four-connected node and links four other [Eu₂Cd₂(μ₂-O)₆] cluster units by DTPA⁵⁻ anions to form a (4,4) grid layer in the [101] plane (Fig. 3c). The 2-D layers stack in –AAA– mode along the a-axis, and free water molecules are located between the layers. Moreover, the O–H⋯O hydrogen bonds between free water molecules and O_COO– atoms of DTPA⁵⁻ anions link the layers into a 3D supramolecular structure (Fig. 4a). From a topological point of view, the 3-D supramolecular framework of 1a is an eight-connected net (Fig. 4b). Because only [Eu₂Cd₂(μ₂-O)₆] units act as eight-connected nodes, the overall framework can be represented as a hex-topology with the point symbol of (3⁶.4¹⁸.5³.6), and the extended point symbol is [3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.5₂.5₂.5₂.6₆] (Fig. 4b).

Fig. 1 The coordination environments of Eu³⁺ and Cd²⁺ in 1a. Atoms having A, B or C in their labels are symmetrically generated. A: 1 − x, 1 − y, 1 − z; B: 1 − x, −y, 1 − z; C: −1 + x, y, 1 + z. Hydrogen atoms are omitted for clarity. Color codes: Eu, yellow; Cd, cyan; O, red; N, blue; C, grey.

Fig. 2 Two enantiomeric forms in [Cd(DTPA)]³⁻ units of type I. (a) α, (b) β.

Fig. 3 (a) Ball-stick presentation of a cradle-like tetranuclear [Eu₂Cd₂(μ₂-O)₆] cluster unit. (b) Polyhedron presentation of tetranuclear [Eu₂Cd₂(μ₂-O)₆] in 1a. (c) The 2D grid layer of compound 1a viewed along the [201] direction.

Fig. 4 (a) The 3D supramolecular framework via hydrogen bonds of O–H⋯O for 1a viewed along b axis. The green dotted lines for hydrogen bonds of O–H⋯O. (b) The supramolecular net with hex-type topology after simplification. Key: orange, [Eu₂Cd₂(μ₂-O)₆] units, the purple and green lines represent DTPA⁵⁻ anions and hydrogen bonds, respectively.

[Ln₂Cd₃(DTPA)₂Cl₂(H₂O)₆] (II: Ln = Eu 2a; Sm 2b). Compounds II were synthesized from the same raw material as I but with a different pH value. HCl solution was added to adjust pH = 2 in the syntheses of compounds II. Interestingly, in compounds II, Cl⁻ anions are coordinated to Cd²⁺ ions; thus, the structures of II are different from I. Note that compounds 2a and 2b are isostructural and both belong to the triclinic space groups of P1̄. In the asymmetrical unit of 2a, there are one and a half unique Cd²⁺ ions, one Eu³⁺ ion, one DTPA⁵⁻ anion, three coordinated water molecules and one Cl⁻ anion (Fig. 5). The coordination geometries for Cd1 and Cd2 are both octahedrons (Fig. S6†). Cd1 lying in a symmetry inversion coordinates to two O_COO– atoms and four coordinated water molecules, whereas Cd2 coordinates to five O_COO– atoms from three DTPA⁵⁻ anions and one Cl⁻ anion. Cd–O bond lengths are in the range of 2.212(3) Å–2.256(3) Å, which is shorter than that in compound 1a. Cd–Cl bond length is 2.463(9) Å, and this is comparable to the reported Cd–Cl bond distances in a previous study.¹⁸ The Eu³⁺ ion in the structure is chelated by five O_COO– atoms, three N atoms from one DTPA⁵⁻ anion, and one coordinated water molecule to form a tricapped trigonal prism (Fig. S7†). Eu–O and Eu–N bond lengths are in the range of 2.346(3) Å–2.450(3) Å and 2.615(3) Å–2.752(3) Å, respectively. The DTPA⁵⁻ anion effectively cups the Eu³⁺ ion in a hydrophilic cleft, leaving the ninth coordination site available to coordinate water. Four carboxylate μ₂-O atoms alternately connect Eu³⁺ ions and Cd(2) atoms to form a chair-like eight-membered tetranuclear [Eu₂Cd₂(μ₂-O)₄] cluster unit (Fig. 6a and b). The shortest Cd⋯Eu distance is 4.793(6) Å, which is longer than that in compound 1a. Each [Eu₂Cd₂(μ₂-O)₄] cluster unit is connected with two such neighboring cluster units by [Cd(1)O₆] octahedrons, generating an interesting 1D necklace-like chain that runs along the [001] direction (Fig. 6). The interconnections of the Eu³⁺ and Cd²⁺ ions from two neighboring chains via carboxylate groups of DTPA⁵⁻ ligands result in a 2-D grid layer in the bc-plane (Fig. 6c and S8†). Each [Eu₂Cd₂(μ₂-O)₄] cluster unit serves as a four-node, which links four other [Eu₂Cd₂(μ₂-O)₄] cluster units; thus, the 2D layer is a (4,4) net with sql-topology. The layers are connected by hydrogen bonds between coordinated water molecules and O_COO– atoms of DTPA⁵⁻ anions to form a 3D supramolecular framework (Fig. 7a). From the topological point of view, the 3-D supramolecular framework of 2a is a (4,6)-connected net (Fig. 7b). [Eu₂Cd₂(μ₂-O)₄] units act as six-connected nodes and [Cd(1)O₆] octahedra as four-connected nodes, and the overall framework can be represented as a fsc-topology with the point symbol of (4⁴.6¹⁰.8)(4⁴.6²), and the extended point symbol is [4.4.4.4.6₂.6₂.6₅.6₅.6₅.6₅.6₅.6₅.6₅.6₅.8₂₀] for [Eu₂Cd₂O₄] units and [4.4.4.4.6₄.6₄] for [Cd(1)O₆] octahedrons.

Fig. 5 The coordination environments of Eu³⁺ and Cd²⁺ in 2a. Atoms having A, B or C in their labels are symmetrically generated. A: −x, −y, 1 − z; B: −x, −y, −z; C: −x, 1 − y, −z. Hydrogen atoms are omitted for clarity. Color codes: Eu, yellow; Cd, cyan; O, red; N, blue; C, grey.

Fig. 6 (a) Ball-stick presentation of tetranuclear [Eu₂Cd₂(μ₂-O)₄] unit. (b) Polyhedron presentation of [Eu₂Cd₂(μ₂-O)₄] unit in 2a. (c) The 2D grid layer of compound 2a viewed along a-axis. Color codes: Eu, yellow; Cd1, purple; Cd2, cyan; O, red; N, blue; C, grey.

Fig. 7 (a) The 3D supramolecular framework via hydrogen bonds of O–H⋯O for 2a viewed along b axis. The green dotted lines are for hydrogen bonds of O–H⋯O. (b) The supramolecular net with fsc-type topology after simplification. Key: orange, [Eu₂Cd₂(μ₂-O)₄] units; cyan, [Cd(1)O₆] units. The purple and green lines represent DTPA⁵⁻ anions and hydrogen bonds, respectively.

IR spectra

As shown in Fig. S1,† the similarities between the IR spectra of 1a and 1b, 2a and 2b suggests that 1a and 1b, 2a and 2b are iso-structural, respectively. The broad bands at 3700–2800 cm⁻¹ for 1a and 1b and those at 3700–2900 cm⁻¹ for 2a and 2b correspond to the stretching bands of O–H of water molecules. The bands at 1595 cm⁻¹ for 1a and 1b and those at 1580 cm⁻¹ for 2a and 2b correspond to the stretching bands of C=O of COO⁻.¹⁵ The red shifts of the bands of C=O indicate the coordination bonds between the metal cations and carboxylate groups of DTPA⁵⁻.

Thermogravimetric analyses

The thermal stabilities of 1a and 1b, 2a and 2b were examined by the TGA analyses in a dry-air atmosphere (Fig. S9†). In the TGA curves of 1a and 1b, the weight losses of 9.30% (calcd: 9.93%) for 1a and 9.30% (calcd: 9.86%) for 1b were observed in the temperature range 30–310 °C, corresponding to the successive release of all coordinated and free water molecules. The decomposition of H₅DTPA was observed from 310 °C to 800 °C. The residue obtained might be Eu₂O₃·2CdO (calcd/found: 42.00%/43.47%) for 1a or Gd₂O₃·2CdO (calcd/found: 42.42%/42.87%) for 1b. Similarly, in the TGA curves of 2a and 2b, the weight losses of 6.73% (calcd: 6.76%) for 2a and 6.33 (calcd: 6.78%) for 2b in the temperature range 50–255 °C can be assigned to the successive release of six coordinated water molecules. The decomposition of H₅DTPA and the removal of HCl were observed from 255 °C to 800 °C. Thus, the residue obtained may be Eu₂O₃·3CdO (calcd/found: 46.17%/46.60%) for 2a or Sm₂O₃·3CdO (calcd/found: 46.06%/48.42%) for 2b (Fig. S9†).

Luminescent properties

Solid-state emission spectra. The room-temperature luminescent properties of the compounds 1a, 2a and 2b in the solid state were measured. Compounds 1a and 2a show characteristic emissions of the Eu³⁺ ion, in which bands at ∼580, ∼592, ∼617, ∼650, ∼700 nm correspond to ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, ⁵D₀ → ⁷F₄ transitions, respectively^19a (Fig. 8a and b). The intensities of the ⁵D₀ → ⁷F₂ transitions (electric dipole) are stronger than that of the ⁵D₀ → ⁷F₁ transitions (magnetic dipole) of compounds 1a and 2a, which indicates the low symmetrical coordination environment of Eu³⁺ ions, and these results agree with the crystallographic analyses. In the emission spectra of compound 2b, when 2b is excited at 403 nm, it exhibits three bands at 563, 598 and 645 nm, corresponding to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 transitions of Sm³⁺ ions^19b (Fig. 8c).

Fig. 8 Emission spectra of 1a (λ_ex = 395 nm) (a), 2a (λ_ex = 395 nm) (b), 2b (λ_ex = 403 nm) (c).

Luminescent sensing

In order to investigate 1a for the sensing of small molecules, its suspension-state luminescences were obtained. The powder of 1a was dispersed into methanol, acetonitrile, acetone, n-butanol, ethanol, ammonia water, H₂O, DMF, iso-propanol, triethylamine, pyridine, ethanediamine, 1,2-propane diamine and ethanol amine. As shown in Fig. 9 and S10,† its luminescent spectra are largely dependent on the solvent molecules. Methanol can significantly improve luminescent intensity of 1a with remarkable enhancement effects. As for acetonitrile, acetone, n-butanol, ethanol, ammonia water, H₂O, DMF, and iso-propanol, the luminescence spectra do not show significant differences from each other. However, for solvents like triethylamine, pyridine, ethanediamine, 1,2-propane diamine, and ethanol amine, the emission spectra of 1a were covered by emissions of these nitrogenous compounds (Fig. S10†). Particularly, in triethylamine and ethanol amine, the emissions of Eu³⁺ exhibited complete quenching of the luminescent intensity. These results suggest that 1a could be a promising luminescent probe for methanol, triethylamine and ethanol amine. This may be due to the fact that these nitrogenous solvents absorb the excitation light of Eu³⁺ ions (λ_ex = 395 nm) (the excitation spectra of Eu³⁺ ions fall in the UV-vis absorption spectra of these nitrogenous solvents). As for the complete quenching of triethylamine and ethanolamine, this may be attributed to the complete UV-vis absorption of the excitation light of the Eu³⁺ ions.^12e

Fig. 9 (a) Emission spectra of 1a (λ_ex = 395 nm) and (b) the ⁵D₀ → ⁷F₂ transition intensities of 1a dispersed into different solvents.

Similarly, the powder samples of 1a and 2a were dispersed into an aqueous solution containing different 0.1 M M(NO₃)_x (M = Ca²⁺, Sr²⁺, Li⁺, K⁺, Na⁺, Mg²⁺, La³⁺, Ho³⁺, Ce³⁺, Tb³⁺, Yb³⁺, Zn²⁺, Cd²⁺, Pb²⁺, Al³⁺, Sm³⁺, Gd³⁺, Nd³⁺, Pr³⁺, Dy³⁺, Cu²⁺, Co²⁺, Cr³⁺, Fe³⁺, Ni²⁺ and Er³⁺) and the emissions of Eu³⁺ ions were measured (Fig. 10 and S13†). For 1a, most cations show an enhancement effect of the luminescent intensities. However, for the Cr³⁺ and Fe³⁺ ions, particularly the Fe³⁺ ion, the intensity showed significant quenching effects. Moreover, the Cr³⁺, Fe³⁺, Co²⁺, Ni³⁺, Cu²⁺, Nd³⁺ ions quenched the emissions of the Eu³⁺ ion of 2a. As illustrated in Fig. S14,† the luminescent intensity of 2a is almost completely quenched in the solution of Co(NO₃)₂ at a concentration of 10⁻² mol L⁻¹. These results indicate the potential of 1a and 2a for the sensing of metal cations. We assumed that these coloured aqueous solution containing cations, such as Cr³⁺, Fe³⁺, Co²⁺, Ni³⁺, Cu²⁺, and Nd³⁺, had absorbed more or less the excitation light of Eu³⁺ ions (λ_ex = 395 nm), which resulted in the fluorescence quenching.

Fig. 10 (a) Emission spectra of 1a (λ_ex = 395 nm) and (b) the ⁵D₀ → ⁷F₂ transition intensities of 1a dispersed into different aqueous solutions containing different 0.1 M M(NO₃)_x.

Conclusions

Two series of Ln–Cd heterometal organic coordination polymers, i.e., Ln₂Cd₂(DTPA)₂(H₂O)₄] (H₂O)₄ (Ln = Eu 1a; Gd 1b) and [Ln₂Cd₃(DTPA)₂Cl₂(H₂O)₆] (Ln = Eu 2a; Sm 2b), have been synthesized by the hydrothermal reactions of Ln₂O₃, Cd(Ac)₂·2H₂O and H₅DTPA. Although the reaction temperatures and main reactants are the same, the different ratios of the reactants and the introduction of the Cl⁻ anions changed the coordination environments of Ln³⁺ and Cd²⁺ ions, resulting in the formations of different tetranuclear [Ln₂Cd₂] oxo-cluster units. Compound 1a possesses 2D-layered grid networks based on [Eu₂Cd₂(μ₂-O)₆] cluster units. Two enantiomeric forms, i.e., α and β, exist in [Cd(DTPA)]³⁻. These 2D layers are linked via O–H⋯O hydrogen bonds to generate a 3D supramolecular architecture with a hex-type topology. In 2a, [Eu₂Cd₂(μ₂-O)₄] units are connected by DTPA³⁻ and [Cd(1)O₆] octahedra to form a 2D lattice layer, which are further held together by hydrogen bonds, resulting in a 3D supramolecular framework with fsc-topology net. Compounds 1a and 2a emit characteristic emissions of Eu³⁺ ion, and compound 2b emits those of the Sm³⁺ ion. More interestingly, organic molecules such as triethylamine and ethanol amine and cations such as Fe³⁺ ion quenched the emission of 1a, and Cr³⁺, Fe³⁺, Co²⁺, Ni³⁺, Cu²⁺, Nd³⁺ ions quenched that of 2a. Compounds 1a and 2a may be used as an important role in the applications of the luminescent sensing for the organic solvents and metal cations.

Acknowledgements

This work was supported by the NNSF of China (no. 21003020), the Natural Science Fund of Fujian Province (no. 2013J01041) and the Foundation of State Key Laboratory of Structural Chemistry (project no. 20130012).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, table, supplementary 14 plots including structures, luminescent spectra, XPRD, TG and FT-IR. CCDC 975527 (for 1a), 975528 (for 1b), 975529 (for 2a), and 975530 (for 2b). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02953d

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