Structural systematic design of organic templated samarium sulfates and their luminescence property

Abstract

Four organic amine templated samarium sulfates[(CH₃)₂NH₂]₉[Sm₅(SO₄)₁₂] 1, [NH₃(CH₂)₂NH₃]₄[Sm₂(SO₄)₇] 2, [H₃O]₂[(CH₃)₂NH₂][Sm(SO₄)₃] 3 and [(CH₃)₂NH₂]₃[Sm(SO₄)₃] 4 have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Compound 1 exhibits a 3D porous structure containing intersecting extra-large 20-membered ring channels; compound 2 is a 2D corrugated layered sulfate constructed by two different zigzag chains and 12-membered rings; compound 3 exhibits a 1D anionic chain structure compensated by protonated dimethylamine and water cations occluded in the helical [Sm–O–S]_n chains, and compound 4 is a novel 1D sulfate constructed by tetrameric [Sm₂(SO₄)₂O₁₀] unit and 4-membered rings. The syntheses of compounds 3 and 4 demonstrate that large organic amine (1,3,5-triazine-2,4,6-triamine or 4,4′-diaminobiphenyl) may be used as second structural directing agent (SDA) to prevent the formation of high dimensional inorganic framework as well as induce crystal growth of 1D chain structural samarium sulfate. The luminescent properties of compounds 1–4 were investigated. The strong luminescence of 3 in the orange light (598 nm ⁴G_5/2→⁶H_7/2) region indicates it is an excellent candidate for orange fluorescent materials.

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Introduction

Over the past two decades, the design and synthesis of novel inorganic frameworks templated by organic amines receive great current interest owing to not only their intriguing varieties of architectures but also their potential applications such as precursors for oxides and catalysis,¹ ion exchange,² magnetic properties³ and luminescence properties.⁴ The work mainly focuses on silicates and phosphates,⁵ but persistent research over the years has established that other tetrahedral anions such as the arsenates, sulfates, and selenates.⁶ Lanthanide compounds play a critical important role in material science because of their efficient fluorescent properties.⁷ Since the first organically templated cadmium sulfate was synthesized by Rao, studies focusing on the organic templated lanthanide sulfates have been an important advancement in the field of solid state materials.^8–9 Compared with 3d transition metals, such as Fe and Zn, the rare-earth elements can adopt a large range of coordination numbers from 8 to 12 and flexible Ln–O bond lengths to allow the formation of lanthanide sulfates with new inorganic topological frameworks.^9–16 Up to now, successful examples including organic amine templated lanthanide sulfates materials have been reported. For example, [C₄H₁₆N₃]₆[Sm₄F₂(SO₄)₁₄]^12d consists of a novel isolated large Z-type heteropolyanion [Sm₄F₂(SO₄)₁₄]¹⁸⁻ and full protonated organic amine cations; [C₂N₂H₁₀]_1.5[Eu(SO₄)₃(H₂O)]·2H₂O¹⁴ is a 2D europium sulfate which contains interesting corrugated layers in the inorganic framework; [Sm(en)(μ₃–OH)(μ₃–SO₄)]_∞¹⁶ is a novel samarium hydroxyl sulfate constructed by tetranuclear cubane-like [Sm₄(en)₄(μ₃–OH)₄]⁸⁺clusters and η³–SO₄²⁻ ion bridges.

Although samarium is an important rare earth element due to its luminescence property, only one isolated example of [C₄H₁₆N₃]₆[Sm₄F₂(SO₄)₁₄]^12d was prepared. While 1D, 2D and 3D organic templated samarium sulfate has been no reported up to now. Therefore, it is vital to design novel samarium sulfates with intriguing varieties of architectures in order to explore their optical properties as well as understand the formation mechanism. In this work, we used different amine as SDA (structure directing agent) to synthesize four samarium sulfates with different topological structures. [(CH₃)₂NH₂]₉[Sm₅(SO₄)₁₂] 1 is a 3D intersecting extra-large 20MR channels organic templated samarium sulfate by use of dimethylamine as SDA; [NH₃(CH₂)₂NH₃]₄[Sm₂(SO₄)₇] 2 is an interesting corrugated layered samarium sulfate which constructed by two different zigzag chains and 12MR channels; [H₃O]₂[(CH₃)₂NH₂][Sm(SO₄)₃] 3 is a 1D samarium sulfate with helical [Sm–O–S]_n chains with a large amine (1,3,5-triazine-2,4,6-triamine) as a second SDA to prevent the formation of 3D Ln–O–S frameworks, and [(CH₃)₂NH₂]₃[Sm(SO₄)₃] 4 is a novel 1D sulfate constructed by tetrameric [Sm₂(SO₄)₂O₁₀] unit and 4-membered rings via another large amine (4,4′-diaminobiphenyl) as a second SDA to prevent the formation of 3D Ln–O–S frameworks.

Experimental

Materials and apparatus

Compounds 1–4 were prepared from a mixture of rare earth oxide (99.9%), concentrated sulfuric acid (95–98 wt%), dimethylamine (dma, 40%, aq.), ethylenediamine (en, 99.8%), N,N-dimethylformamide (DMF, 99.5%), N,N-dimethylacetamide (DMAC, 99.5%), 1,3,5-triazine-2,4,6-triamine (99.7%) and 4,4′-diaminobiphenyl (99.7%) under solvothermal conditions. All chemicals purchased with reagent grade were used without further purification. The element analyses were performed on a Perkin-Elmer 2400 element analyzer and the inductively coupled plasma (ICP) analysis was performed on a Perkin-Elmer optima 3300 DV ICP spectrometer. The infrared (IR) spectrum was recorded within the 400–4000 cm⁻¹ region on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. A NETZSCH STA 449C unit was applied to carry out the TGA analyses under nitrogen atmosphere with a heating rate of 10 °C min⁻¹.

Preparation

In a typical synthesis of compound 1, a solution was prepared by dissolving 0.1094 g (0.313 mmol) of Sm₂O₃ into 7.0178 g (95.5 mmol) DMF, 0.1245 g (1.10 mmol) dma and 1.5048 g (15.0 mmol) sulfuric acid under constant stirring for an hour. The resulting mixture was transferred into a 25 mL Teflon-lined stainless-steel autoclave and heated at 453 K for 6 days. After cooling to room temperature, the product was washed with ethanol and dried in air for one day. Finally, the block crystals (yield 47%, with respect to Sm) were obtained. The element analysis shows that the C, H and N contents are 9.47, 3.60 and 4.74%, respectively (calculated: C, 9.31; H, 3.10; N, 5.43%). IR (KBr pellet, cm⁻¹): 3422 (b), 2781 (m), 1631 (w), 1467 (m), 1412 (w), 1225 (m), 1100 (m), 658 (w), 599 (w).

The block crystal of compound 2 was prepared by mixing 0.1021 g (0.292 mmol) of Sm₂O₃, 6.9964 g (79.9 mmol) DMAC, 0.3020 g (5.01 mmol) en, 1.4846 g (14.8 mmol) sulfuric acid under constant stirring for an hour, the final pH was 1.0. The resulting mixture was heated at 423 K for 4 days. The final yield was 63% (with respect to Sm). The element analysis shows that the C, H and N contents are 7.69, 3.43 and 9.31%, respectively (calculated: C, 7.86; H, 3.27; N, 9.17%). IR (KBr pellet, cm⁻¹): 3156 (b), 1632 (m), 1518 (m), 1384 (w), 1071 (vs), 652 (w), 606 (m). The block crystal of compound 3 was prepared by mixing 0.1136 g (0.325 mmol) Sm₂O₃, 8 mL (98.6 mmol) DMF, 1.5118 g (15.1 mmol) sulfuric acid and 0.1021 g (0.807 mmol) 1,3,5-triazine-2,4,6-triamine under constant stirring for an hour. The resulting mixture was heated at 433 K for 6 days. The final yield was 42% (with respect to Sm). The element analysis shows that the C, H and N contents are 4.33, 3.08 and 3.31%, respectively (calculated: C, 4.59; H, 2.68; N, 2.68%). IR (KBr pellet, cm⁻¹): 3450 (b), 3237 (vs), 1430 (m), 1124 (b), 652 (m), 619 (m). The block crystal of compound 4 was prepared by mixing 0.1066 g (0.305 mmol) Sm₂O₃, 8 mL (98.6 mmol) DMF, 1.3007 g (13.0 mmol) sulfuric acid and 0.1072 g (0.580 mmol) 4,4′-diaminobiphenyl under constant stirring for an hour. The resulting mixture was heated at 433 K for 6 days. The final yield was 34% (with respect to Sm). The element analysis shows that the C, H and N contents are 12.13, 4.23 and 7.54%, respectively (calculated: C, 12.48; H, 4.16; N, 7.28%). IR (KBr pellet, cm⁻¹): 3435 (b), 3023 (vs), 2839 (m), 2780 (m), 1635 (m), 1470 (w), 1118 (b), 645 (m), 614 (m).

X-Ray crystallography

The single crystals of compounds 1–4 were chosen onto a thin glass fiber by epoxy glue in air for data collection. The diffraction data were collected on a Bruker Apex 2 CCD with Mo-Kα radiation (λ = 0.71073 Å) at 296 K using ω-2θ scan method. An empirical absorption correction was applied. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of organic molecule were refined in calculated positions, assigned isotropic thermal parameters, and allowed to ride on their parent atoms, while the H atoms of protonated water in 3 are located from different map. All calculations were performed using the SHELX97 program package.¹⁷ Further details of the X-ray structural analyses for compounds 1–4 are given in Table 1 and selected bond lengths and angles are listed in Table S1.†

Table 1 Crystal data and structure refinement for 1–4.

	1	2	3	4
Empirical formula	C18H72Sm5N9O48S12	C8H40Sm2N8O28S7	C2H14SmNO14S3	C6H24SmN3O12S3
Formula weight	2319.32	1221.60	522.67	576.81
T/K	296(2)	296(2)	296(2)	296(2)
Wavelength/Å	0.71073	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Orthorhombic	Triclinic	Triclinic
Space group	C2/c	Pca21	P1̄	P1̄
Unit cell dimensions	a = 20.6494(13) Å	a = 21.761(4) Å	a = 9.237(3) Å	a = 9.618(3) Å
	b = 35.261(2) Å	b = 14.877(2) Å	b = 9.364(3) Å	b = 10.496(4) Å
	c = 10.1318(6) Å	c = 10.4238(17) Å	c = 9.557(3) Å	c = 10.535(4) Å
	α = 90°	α = 90°	α = 70.491(2)°	α = 71.826(4)°
	β = 115.0040(10)°	β = 90°	β = 78.215(3)°	β = 73.584(4)°
	γ = 90°	γ = 90°	γ = 79.378(3)°	γ = 76.454(4)°
Volume/Å^3	6685.7(7)	3374.6(9)	756.7(4)	956.7(6)
Z	4	4	2	2
Calculated density/Mg m^−3	2.304	2.404	2.294	2.022
Absorption coefficient/Mg m^−3	4.809	3.996	4.362	3.445
F(000)	4516	2416	510	574
Crystal size/mm	0.12 × 0.11 × 0.10	0.10 × 0.10 × 0.09	0.14 × 0.13 × 0.12	0.15 × 0.13 × 0.11
θ range for data collection	1.15 to 25.50°	1.66 to 25.50°	2.27 to 26.00°	2.07 to 25.00°
Limiting indices	−25 ≤ h ≤ 24, −42 ≤ k ≤ 42, −11 ≤ l ≤ 12	−26 ≤ h ≤ 24, −18 ≤ k ≤ 17, −12 ≤ l ≤ 12	−11 ≤ h ≤ 11, −11 ≤ k ≤ 11, −11 ≤ l ≤ 11	−11 ≤ h ≤ 11, −12 ≤ k ≤ 12, −12 ≤ l ≤ 12
Reflections collected/unique	25302/6822	23746/6240	5648/2859	6592/3317
	[Rint = 0.0312]	[Rint = 0.0522]	[Rint = 0.0188]	[Rint = 0.0358]
Completeness to θ= 25.49	99.6%	99.7%	95.9%	98.1%
Max. and min. transmission	0.6449 and 0.5961	0.7150 and 0.6907	0.6226 and 0.5803	0.7024 and 0.6252
Data/restraints/parameters	6206/12/431	6240/1/478	2859/12/208	3317/16/235
Goodness-of-fit on F^2	1.159	1.019	1.066	1.011
Final R indices [I > 2σ(I)]	R 1 = 0.0237	R 1 = 0.0325	R 1 = 0.0232	R 1 = 0.0424
	wR2 = 0.0700	wR2 = 0.0654	wR2 = 0.0596	wR2 = 0.1088
R indices (all data)	R 1 = 0.0302	R 1 = 0.0429	R 1 = 0.0251	R 1 = 0.0494
	wR2 = 0.0833	wR2 = 0.0688	wR2 = 0.0605	wR2 = 0.1140
Largest diff. peak and hole	0.849 and −0.757 e/Å^−3	1.849 and −1.287 e/Å^−3	0.683 and −0.597 e/Å^−3	2.052 and −1.880 e/Å^−3

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Results and discussion

Synthesis

The solvothermal synthesis method has been demonstrated to be an important approach in building new types of inorganic solid-state materials. During the solvothermal synthesis, many factors including reaction temperature, pH values, solvent and time affect the nucleation and the crystal growth of the final products. We used dimethylamine as SDA (structure directing agent) to give an organic templated 3D samarium sulfate 1 with intersecting extra-large 20MR channels, but used ethylenediamine as SDA to give an interesting 2D corrugated layered samarium sulfate 2. In the syntheses of compounds 3 and 4, DMF not only acted as solvent but also provided the SDA (dimethylamine) after its decomposition and we used two large amines (1,3,5-triazine-2,4,6-triamine and 4,4′-diaminobiphenyl) as the second SDA to prevent the formation of 3D framework and give two 1D chain samarium sulfates.

Description of the structure

Crystal structure of 1. Crystal structural analysis indicates that the open-framework 1 is a porous samarium sulfate with extra-large 20MR channels. The asymmetric unit of 1 contains three samarium atoms, six sulfate groups and five protonated dimethylamine cations (Fig. S1). The metal atoms Sm(1) and Sm(2) are eight-coordinated by O atoms, while Sm(3) is nine-coordinated by O atoms. The average bond distances of Sm–O is 2.484 Å, whereas the angles of O–Sm–O are between 54.95(11) and 165.50(11)°. Six crystallographic independent S atoms can be divided into four groups: S(4) and S(6) are bonded by one μ₃–O, two μ₂–O and one Ot (terminal O atoms) to form four S–O–Sm linkages; S(1) is bonded by three μ₂–O atoms and one Ot to generate three S–O–Sm linkages; S(2) and S(3) are coordinated by four μ₂–O to form four S–O–Sm linkages; while S(5) is bonded by two μ₂–O atoms and two Ot to generate two S–O–Sm linkages. The S–O distances are from 1.455(4) to 1.509(3) Å, which is similar to those of the previous reports about lanthanide sulfates.^8–16

The structure of 1 can be described as two types of secondary building units (SBUs). Two [Sm₃(SO₄)₆]³⁻ fragments connect with each other by sharing Sm(1) to form the SBU-1 [Sm₅(SO₄)₁₂]⁹⁻. Two adjacent SBU-1s are linked by bridging [SO₄] tetrahedra to generate an interesting 20MR structure (Fig. 1). The SBU-2 is 10MR. Adjacent 10 and 20MRs are connected by bridging SO₄²⁻groups to form an open framework with a 3D channel system (Fig. 2a). Each 20MR channel is surrounding by six 10MRs, and they are alternately surrounded by three 20MRs and three 10MRs. The approximate window sizes for 20MR and 10MR are 5.4 × 9.6 Ų and 3.8 × 3.8 Ų, respectively. Interestingly, there are other rectangular 20MR-2 channels along the crystallographic [101] direction (Fig. 2b) and the pore size of rectangular 20MR is about 2.9 × 12.9 Ų. We used the PLATON software to calculate the percent void volume in the 3D network of 1, and the result is 48.3%. Although the structure of 1 is similar to our previous reported 3D terbium and europium sulfate,^13,15 the guest molecules are different. In our previous work, there are two free water molecules, but in 1 the free water was not included. Also compound 1 is the first 3D organic amine templated samarium sulfate. As shown in Fig. 3, the protonated dimethylamine molecules are located in two kinds of 20MR and 10MR channels of the inorganic framework by the strong hydrogen bonding interactions between N atoms of organic amine molecules and O atoms of the open-framework of 1. Hydrogen bonds for compound 1 are given in Table S2.

Fig. 1 Two SBU-1 of [Sm₅(SO₄)₁₂]⁹⁻ in 1 consist of the structure of 20MR.

Fig. 2 (a) The open framework of 1 including 10MRs and 20MRs; (b) the rectangle 20MR-2 channels along the crystallographic [101] direction.

Fig. 3 The protonated dimethylamine molecules are located in the 20MR-1(a), 10MR(b) and 20MR-r(c).

Crystal structure of 2. Compound 2 crystallizes in orthorhombic space groupPca2₁. The structural analysis reveals that compound 2 is an ethylenediamine templated samarium sulfate, which consists of inorganic 2D anionic [Sm₂(SO₄)₇]_n⁸ⁿ⁻ layer, charge compensated by the protonated ethylenediamine cations. The asymmetric unit of 2 contains two samarium atoms, seven sulfate groups and four protonated ethylenediamine cations (Fig. 4). In the inorganic layer of 2, Sm(1) is nine-coordinated, while Sm(2) is eight-coordinated by O atoms and the average bond distances of Sm–O is 2.482 Å. The angles of O–Sm–O are between 53.02(5) and 154.89(17)°. All the S atoms are tetrahedrally coordinated by four O atoms with the S–O distances of 1.437(5)∼1.502(5) Å, which is similar to the reported lanthanide sulfates.^8–16

Fig. 4 ORTEP view of the [NH₃(CH₂)₂NH₃]₄[Sm₂(SO₄)₇] 2 structure showing the atom labeling scheme.

For recent years, there are some successful 2D layered examples including organic amine templated lanthanide sulfates reported, such as La₂(H₂O)₂(C₂H₁₀N₂)₃(SO₄)₆·4H₂O,^9d La₂(H₂O)(dabco)₂(SO₄)₅·5H₂O,^9c [C₂N₂H₁₀][La₂(H₂O)₄(SO₄)₄]·2H₂O,^12a [C₂N₂H₁₀]_1.5[Eu(SO₄)₃(H₂O)]·2H₂O,^14a [C₂N₂H₁₀]_1.5[Nd(SO₄)₃(H₂O)]·2H₂O.^14b But no organic amine templated samarium sulfates were reported. Compared with the above reported 2D layered lanthanide sulfates, the structure of the inorganic layer of 2 is very different, and constructed by two different zigzag chains. [S(7)O₄] and [Sm(1)O₉] polyhedra connected each other through four μ₂–O atoms to form a zigzag chain A, whereas [S(4)O₄] and [Sm(2)O₈] polyhedra connected each other through three μ₂–O atoms to form a zigzag chain B. Chain A and B cross-linked each other by three [SO₄] tetrahedra (S1, S2 and S3) to form an interesting corrugated layer (Fig. 5). S(6) makes two S–O–Sm linkages through two μ₂–O atoms attached on chain A and S(5) forms only one S–O–Sm linkage through one μ₂–O atom attached on chain B to make them more stable. On the other hand, the inorganic layer of 2 can also be described as connections of 12MRs (Fig. 6). Three SmO₈ and three SmO₉ polyhedra are linked by six SO₄ tetrahedra to generate building unit of 12 MR (Sm₆S₆), while adjacent 12MRs are connected by bridging SO₄²⁻groups to form an open framework with a 2D layer system and all of them are arranged in an ordered close packing structure. Each 12MR is surrounding by six 12MRs. 4 MRs, 8 MRs and 10 MRs are often observed, while 12 MRs are much bigger, and not found in the reported 2D lanthanide sulfates. The approximate window size for it is 6.65 × 5.69 Ų. As shown in Fig. 7, four protonated ethylenediamine cations can be divided into two groups: N1H₃–C1H₂–C2H₂–N2H₃ is located in 12MR and the remaining three protonated ethylenediamine cations are located between the inorganic layers by the strong hydrogen bonding interactions between N or C atoms of organic amine molecules and O atoms of the open-framework of 2. Hydrogen bonds for compound 2 are given in Table S3.

Fig. 5 Two different zigzag chains A and B connected by [SO₄] tetrahedral bridge to form an interesting corrugated layer.

Fig. 6 Zigzag chains connected each other by [SO₄] tetrahedra bridge to forming a 2D layer.

Fig. 7 (a) One protonated ethylenediamine cation located in the 12MR as an organic template; (b) the remaining three protonated ethylenediamine cations are located between the inorganic layers.

Crystal structure of 3. Compound 3 crystallizes in triclinic space groupP1̄. The structure of 3 consists of inorganic 1D anionic [Sm(SO₄)₃]_n³ⁿ⁻ chains, charge compensated by the protonated dimethylamine and water cations. The asymmetric unit of 3 contains one samarium atom, three sulfate groups, one protonated dimethylamine and two H₃O⁺ cations (Fig. S2). In the inorganic chain of 3, Sm is eight-coordinated by O atoms and the average bond distances of Sm–O is 2.433 Å. The angles of O–Sm–O are between 56.51(7) and 153.41(7)°. All the S atoms are tetrahedrally coordinated by four O atoms with the S–O distances of 1.430(3)∼1.504(2) Å, which is similar to the reported lanthanide sulfates.^8–16

The structure of inorganic chain of 3 can be described as double helical chains with terminal [SO₄] tetrahedra attached to them. Sm atoms were connected by the bridging sulfates (S(1), S(2) and their crystallographic partners) to form a single helical [–Sm–O–S–O–]_n chain and other helical chains were further linked by O–Sm–O linkages to generate intertwined {–Sm–O–S–O–} double helices of the same handedness (Fig. 8). These kinds of double helices are particularly rare in inorganic materials, however, one germanate and two phosphates were reported, formulated as Ge₇O₁₂(OH)₄(C₄N₃H₁₃)_0.5(H₂O)₅,¹⁸[(CH₃)₂NH₂]K₄[V₁₀O₁₀(H₂O)₂(OH)₄(PO₄)₇]¹⁹ and [Zn₂(HPO₄)₄][Co(dien)₂]H₂O.²⁰ There are two crystallographically independent protonated water (H₃O⁺) cations. Interestingly, O2w makes four O–H…O hydrogen bonds with the O atoms from double helices to generate a flat layer along a axis (Fig. 9a). The remaining protonated water cation (O1w) connects adjacent layers by three hydrogen bonds to generate 3D soft open framework along b axis (Fig. 9b). Hydrogen bonds for compound 3 are given in Table S4. The protonated dimethylamine cations are located in the channels and involved hydrogen bonding interactions with the soft framework and make it more stable (Fig. 10). Although the structure of 3 is similar to our previous reported 1D terbium and europium sulfate,^13,24 it is vital to study the structure and its excellent luminescent property.

Fig. 8 (a) Single helical [–Sm–O–S–O–]_n chain; (b) An intertwined –Sm–O–S–O– double helices of the same handedness.

Fig. 9 (a) O2w cations make a flat layer by four hydrogen bonds; (b) O1w cations connect adjacent layers by three hydrogen bonds to make 3D soft open framework.

Fig. 10 The guest cations are located in the channels.

Crystal structure of 4. Although the structure of 4 is 1D samarium sulfate, the guest molecules and inorganic framework are different from 3. The asymmetric unit of 4 contains one samarium atom, three sulfate groups and three protonated dimethylamine cations (Fig. 11). In the inorganic chain of 4, Sm is eight-coordinated by O atoms and the average bond distances of Sm–O is 2.496 Å. The angles of O–Sm–O are between 53.07(15) and 152.05(17)°. All the S atoms are tetrahedrally coordinated by four O atoms with the average S–O distances is 1.468 Å, which is similar to the reported lanthanide sulfates.^8–16

Fig. 11 ORTEP view of the [(CH₃)₂NH₂]₃[Sm(SO₄)₃] 4 structure showing the atom labeling scheme.

There are two different building units: [Sm₂(SO₄)₂O₁₀] and 4MRs in the structure of 4. The tetrameric [Sm₂(SO₄)₂O₁₀] unit is composed of two edge-sharing [SmO₉] polyhedra and two edge-sharing [S(1)O₄] tetrahedra, and S1 forms four S–O–Sm linkages through one μ₃–O and two μ₂–O atoms. The 4-membered ring is constructed by two [SmO₉] polyhedra and two [S(2)O₄] tetrahedral. S2 makes three S–O–Sm linkages through three μ₂–O atoms. Each tetrameric [Sm₂(SO₄)₂O₁₀] unit connects two 4MR units as well as each 4MR connects two tetrameric [Sm₂(SO₄)₂O₁₀] units to form a novel 1D chain (Fig. 12). S(3) forms two S–O–Sm linkages through two μ₂–O attached on the chain to make it more stable. Compare to compound 3, there is a μ₃–O in the 1D chain. The μ₃–O has shortened the distance between the two adjacent Sm atoms to generated the tetrameric [Sm₂(SO₄)₂O₁₀] unit (Fig. 12). The protonated dimethylamine cations are located between the chains and involved hydrogen bonding interactions with the 1D framework and make it more stable (Fig. 13). Hydrogen bonds for compound 4 are given in Table S5.

Fig. 12 Two different building units connected each other to form a novel 1D chain.

Fig. 13 The guest cations are located between the chains along a axis.

For compounds 3 and 4, it is somewhat curious that the compounds include fully protonated dimethylamine instead of 1,3,5-triazine-2,4,6-triamine or 4,4′-diaminobiphenyl. It must be noted that dimethylamine, along with formic acid, would be the product of decomposition of DMF.²¹ The organic amine 1,3,5-triazine-2,4,6-triamine or 4,4′-diaminobiphenyl was not included in the compounds, but it is very important during the synthesis process, they can act as the second SDA. Without 1,3,5-triazine-2,4,6-triamine or 4,4′-diaminobiphenyl, the product would be 3D porous compound 1 rather than the 1D compound 3 or 4. Through experiments, we demonstrate that adding or removing the second SDA the 1D and 3D structure would be interconvertible. For example, compound 3 can be converted to a 3D structure in DMF and H₂SO₄ (98%) solution. The single-crystal data is same as compound 1. Meanwhile compound 1 can also be converted to 1D structure in 1,3,5-triazine-2,4,6-triamine, DMF and H₂SO₄ (98%) solution. So the second SDA is like a cutting machine to prevent the formation of 3D porous structure.

Luminescence property

Luminescent properties of compounds 1–4 were investigated because of existence of Sm³⁺. The solid-state emission spectra of the four compounds at room temperature were measured under excitation at 400 nm and their emission spectra are similar to each other. We take compound 3 as an example to explain their luminescent properties. As shown in Fig. 14, compound 3 shows three emission bands at 561 nm (⁴G_5/2→⁶H_5/2), 597 nm (⁴G_5/2→⁶H_7/2) and 643 nm (⁴G_5/2→⁶H_9/2). Comparatively, compound 3 shows a sharp emission band for Sm³⁺, ⁴G_5/2→⁶H_7/2, which is in agreement with the reported Sm³⁺ compounds.^22,23 The spectra for compounds 1, 2 and 4 are given in Fig. S3–S5. The luminescence intensity of compound 3 is much stronger than those in 1, 2 and 4 (we measured solid state emission spectra under same conditions). The enhanced fluorescence efficiency of 3 is attributed to the fact that the protonated organic amine and (H₃O)⁺ cations link the Sm–O–S chains by using strong hydrogen bonds, which increases the rigidity of the Sm atoms effectively and reduce the loss of energy by thermal vibrations.

Fig. 14 The photoluminescence spectra of compound 3. The excitation wavelength is 400 nm, and spectrum is taken at room temperature.

Thermal behavior

The thermogravimetric analyses (TGA) of compounds 1–4 were performed in a N₂ atmosphere when heated to 1000 °C at a rate of 10 °C min⁻¹. For compound 1, the total weight loss is 58.9%, which is in agreement with the calculated value 59.2% (Fig. S6). The weight loss of 16.7% in the range of 40–300 °C corresponds to the removal of dimethylamine (cal. 17.8%). The last step loss of 42.2% in the range of 300–1080 °C can be attributed to loss of SO₃ (cal. 41.4%). The final product is Sm₂O₃. For compound 2, the total weight loss is 64.6%, which is in agreement with the calculated value 66.1% (Fig. S7). The weight loss of 36.1% in the range of 25–440 °C corresponds to the removal of all the ethylenediamine molecules and some of SO₃ molecules. The second step loss of 28.5% in the range of 440–1000 °C can be attributed to removal of all the SO₃ molecules. The final product is Sm₂O₃. For compound 3, the total weight loss is 62.9%, which is in agreement with the calculated value 65.0% (Fig. S8). The weight loss of 32.6% in the range of 40–400 °C corresponds to the removal of two free water molecules, dimethylamine and some of SO₃. The last step loss of 30.3% in the range of 400–1000 °C can be attributed to loss of SO₃. The final product is Sm₂O₃. For compound 4, the total weight loss is 62.3%, which is in agreement with the calculated value 65.5% (Fig. S9). The weight loss of 36.1% in the range of 25–380 °C corresponds to the removal of all the dimethylamine molecules and some of SO₃ molecules. The second step loss of 26.2% in the range of 380–1000 °C can be attributed to removal of all the SO₃ molecules. The final product is Sm₂O₃.

Conclusions

In summary, a series of open-framework samarium sulfates templated by amines and possessing 3D channel, 2D layer or 1D chain structures has been successfully synthesized by using DMF or DMAC as solvent. The 3D structure of [(CH₃)₂NH₂]₉[Sm₅(SO₄)₁₂] 1 is an intersecting extra large 20MR ring channel, while the 2D layered [NH₃(CH₂)₂NH₃]₄[Sm₂(SO₄)₇] 2 contains an interesting corrugated layer constructed by two different zigzag chains. In the syntheses of compounds [H₃O]₂[(CH₃)₂NH₂][Sm(SO₄)₃] 3 and [(CH₃)₂NH₂]₃[Sm(SO₄)₃] 4, DMF decomposed into dimethylamine and formic acid and the dimethylamine acts as an organic template. The 1,3,5-triazine-2,4,6-triamine and 4,4′-diaminobiphenyl act as a second SDA to prevent the formation of 3D structure. The formation of the compounds 3 and 4 demonstrated that the solvent and the second SDA (1,3,5-triazine-2,4,6-triamine and 4,4′-diaminobiphenyl) play a critical important role, and new lanthanide sulfates can be designed and prepared by applying solvothermal method. The luminescent spectra of the compounds reveal that the rare-earth organic template samarium may become promising photoluminescence materials.

Acknowledgements

We thank the National Natural Science Foundation of China (20971068 and 21171093) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC reference numbers 826954–826957. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00749a

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