Synthesis, structure and near-infrared luminescence of a new 2D praseodymium(III) coordination polymer

JOURNAL OF RARE EARTHS, Vol. 29, No. 11, Nov. 2011, P. 1100

Synthesis, structure and near-infrared luminescence of a new 2D praseodymium(III) coordination polymer LIU Guangxiang (߬‫)⼹ܝ‬1, ZHOU Hong (਼ ᅣ)1, REN Xiaoming (ӏᇣᯢ)2 (1. Department of Chemistry, Nanjing Xiaozhuang University, Nanjing 210017, China; 2. College of Science, Nanjing University of Technology, Nanjing 210009, China) Received 17 May 2011; revised 3 June 2011

Abstract: A novel coordination polymer, [Pr2(BIPA)3(H2O)2]·2H2O (1) (H2BIPA=5-bromoisophthalic acid), was prepared by hydrothermal synthesis and characterized by IR spectroscopy, elemental analysis and single-crystal X-ray diffraction. The crystal was of monoclinic system, space group C2/c, with a=1.98037(14), b=1.44189(14), c=2.15281(18) nm, ȕ=95.220(2)°, V=6.1218(9) nm3, C24H17Br3O16Pr2, Mr=1082.93, Dc= 2.350 g/cm3, F(000)=4096, ȝ=7.136 mm–1 and Z=8. The final R1=0.0608 and wR2=0.1371 for 5624 observed reflections (I>2ı(I)). Complex 1 featured an interesting 2D layer containing {Pr2(CO2)3}n right-handed and left-handed helical chains. Furthermore, hydrogen bonds linked the adjacent 2D layers to form a 3D supramolecular framework. Moreover, the near-infrared luminescent properties of 1 were also investigated in the solid state. Keywords: Pr(III) coordination polymer; 5-bromoisophthalic acid; crystal structure; near-infrared luminescence; rare earths

There is currently a great deal of interest in the design and construction of metal-organic frameworks (MOFs) created by joining the metal centers with organic linkers because of their intriguing structural topologies and their versatile applications in the areas of redox catalysis, ion exchange, adsorption, separation, sensors, magnetism and photoluminescence[1–8]. The pivotal point for the design and synthesis of desirable structures with functional properties heavily depends on the judicious choice and assembly of metal centers with the organic bridging ligands[9–11]. The lanthanide ions are extensively employed as building blocks to construct MOFs which exhibit fascinating architectures due to their high and variable coordination numbers and flexible coordination geometry[12–15]. More importantly, lanthanide ions often endow the MOFs with unique magnetic and luminescent properties. With respect to luminescence, the emissions from near infrared (NIR) radiation to blue light of different lanthanide ions make them of interest for a range of applications. The NIR luminescence from Nd-containing system, for example, has been regarded as the most popular infrared luminescence materials for application in laser systems[16]. However, direct excitation of the 4f-4f transitions is difficult because of poor absorption abilities of lanthanides. Efficient excitation of the metal-centred luminescence has to rely on energy transfer from the surroundings of the lanthanide ion[17]. Thus, the selection of a suitable ligand is crucial in building of lanthanide coordination polymers. Isophthalic acid possesses excellent characteristics as follows: it has two carboxyl groups with four oxygen atoms which can be used for coordination; the skew coordination orientation of the carboxyl groups should

favor the formation of a helical structure[18,19] and its phenyl group having high structuring effect could act as efficient sensitizer for lanthanide emissions[17]. For this purpose, we chose 5-bromoisophthalic acid (H2BIPA) as the organic ligand, while Pr(III) ion as metal centers. Compared with isophthalate, the -Br substituent of the isophthalate may result in different electronic effect and the steric hindrance to change the coordination modes of carboxylate groups in the assembly process. Herein, we presented the synthesis, structure and near-infrared luminescent properties of a new two-dimensional coordination polymer [Pr2(BIPA)3 (H2O)2]·2H2O (1). 1 Experimental 1.1 Materials and physical measurements All commercially available chemicals and solvents are of reagent grade and were used as received without further purification. 5Bromoisophthalic acid was purchased from Tokyo Chemical Industry Co., Ltd. Elemental analyses (C, H and N) were performed on a Vario EL III elemental analyzer. Infrared spectra were performed on a Nicolet AVATAR-360 spectrophotometer with KBr pellets in the 400–4000 cm–1 region. The NIR luminescence spectra were obtained at room temperature on a BIO-RAD PL9000 Photoluminescence System (UK) with an Ar ion laser and the Ge detector worked at liquid nitrogen temperature. Thermogravimetric analyses were performed on a simultaneous SDT 2960 thermal analyzer under nitrogen flow with a heating rate of 10 ºC/min. 1.2 Synthesis A mixture containing Pr(NO3)3·6H2O (0.1 mmol, 45 mg) and H2BIPA (0.2 mmol, 49 mg), dissolved in

Foundation item: Project supported by the National Natural Science Foundation of China (20971004) and the Key Project of Chinese Ministry of Education (210102) Corresponding author: LIU Guangxiang (E-mail: [email protected]; Tel.: +86-556-5500690) DOI: 10.1016/S1002-0721(10)60606-0

LIU Guangxiang et al., Synthesis, structure and near-infrared luminescence of a new 2D praseodymium(III)…

15 ml H2O/ethanol was sealed in a 16 ml Teflon lined stainless steel container and heated at 180 °C for 5 d. Pale green block crystals of 1 were washed by water and ethanol several times and collected by filtration with a field of 52%. Anal. calcd for C24H17Br3O16Pr2: C, 26.62; H, 1.58%. Found: C, 26.52; H, 1.63%. IR (KBr, cm–1): 3459 (br), 3126 (w), 3051 (w), 2912 (w), 1609 (s), 1579 (s), 1555 (m), 1449 (s), 1409 (s), 1389 (s), 1273 (m), 1186 (m), 1122 (w), 1100 (m), 1049 (m), 993 (w), 852 (w), 773 (m), 669 (w), 523 (w). 1.3 X-ray crystallography The crystallographic data collections for 1 were carried out on a Bruker Smart Apex II CCD with graphite-monochromated Mo-KĮ radiation (Ȝ= 0.071073 nm) at 293(2) K using the Ȧ-scan technique. The data were integrated by using the SAINT program[20], which also did the intensities corrected for Lorentz and polarization effect. An empirical absorption correction was applied using the SADABS program[21]. The structures were solved by direct methods using the program SHELXS-97 and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXL97 crystallographic software package[22]. The hydrogen atoms of water molecules were located with difference Fourier map and the other hydrogen atoms were generated geometrically. All calculations were performed on a personal computer with the SHELXL-97 crystallographic software package[23]. The details of the crystal parameters, data collection and refinement for 1 are summarized in Table 1. Selected bond lengths and bond angles for 1 are listed in Table 2. CCDC No. 802907 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. Table 1 Crystal data and details of the structure determination for 1 Complex

1

Empirical formula

C24H17Br3O16Pr2

Formula weight

1082.93

Temperature/K

293(2)

Crystal system

Monoclinic

Space group

C2/c

a/nm

1.98037(14)

b/nm

1.44189(14)

c/nm

2.15281(18)

ȕ/(º)

95.220(2)

Volume/nm3

6.1218(9)

Z

8

Calculated density/(mg/m3) –1

Absorption coefficient/mm

2.350 7.136

Crystal size/mm3

0.24×0.20×0.16

F(000)

4.96

ș range for data collection/(º)

1.95 to 25.50

Reflections collected/unique

15475/5624

Goodness-of-fit on F2

1.096

Final R indices [I>2ı(I)]

R1=0.0608, wR2=0.1371

R indices (all data)

R1=0.0695, wR2=0.1432

ǻȡ/(e/nm3)

1559, –1386

1101

Table 2 Selected bond distances and angles for 1 Bond distances/(10–1 nm) Pr(1)–O(3)#1

2.364(7)

Pr(1)–O(1)

2.950(6)

Pr(1)–O(7)#2

2.416(6)

Pr(2)–O(8)#4

2.303(6)

Pr(1)–O(11)#3

2.425(7)

Pr(2)–O(12)#3

2.363(6)

Pr(1)–O(5)

2.442(6)

Pr(2)–O(4)#5

2.405(6)

Pr(1)–O(2)

2.452(6)

Pr(2)–O(6)

2.423(6)

Pr(1)–O(10)

2.483(6)

Pr(2)–O(9)

2.499(6)

Pr(1)–O(1W)

2.612(11)

Pr(2)–O(1)#1

2.511(7)

Pr(1)–O(9)

2.895(6)

Pr(2)–O(2W)

2.517(8)

80.4(2)

O(7)#2–Pr(1)–O(1)

78.2(2)

71.9(2)

O(11)#3–Pr(1)–O(1)

75.7(2)

Bond angles/(º) O(3)#1–Pr(1)–O(7)#2 #1

#3

O(3) –Pr(1)–O(11)

O(7)#2–Pr(1)–O(11)#3

146.8(2)

O(5)–Pr(1)–O(1)

128.83(19)

O(3)#1–Pr(1)–O(5)

140.0(2)

O(2)–Pr(1)–O(1)

46.8(2)

O(7)#2–Pr(1)–O(5)

133.1(2)

O(10)–Pr(1)–O(1)

151.31(19)

O(11)#3–Pr(1)–O(5)

79.8(2)

O(1W)–Pr(1)–O(1)

101.4(2)

O(3)#1–Pr(1)–O(2)

114.8(2)

O(9)–Pr(1)–O(1)

#2

#4

143.50(18) #3

O(7) –Pr(1)–O(2)

100.3(3)

O(8) –Pr(2)–O(12)

O(11)#3–Pr(1)–O(2)

76.0(3)

O(8)#4–Pr(2)–O(4)#5

81.2(3)

O(5) –Pr(1)–O(2)

84.0(2)

O(12)#3–Pr(2)–O(4)#5

168.7(3)

O(3)#1–Pr(1)–O(10)

94.1(2)

O(8)#4–Pr(2)–O(6)

132.7(2)

#2

#3

93.4(3)

O(7) –Pr(1)–O(10)

75.3(2)

O(12) –Pr(2)–O(6)

102.4(2)

O(11)#3–Pr(1)–O(10)

123.7(2)

O(4)#5–Pr(2)–O(6)

74.8(2)

O(5)–Pr(1)–O(10)

78.4(2)

O(8)#4–Pr(2)–O(9)

155.2(2)

#3

O(2)–Pr(1)–O(10)

149.9(2)

O(12) –Pr(2)–O(9)

O(3)#1–Pr(1)–O(1W)

147.5(3)

O(4)#5–Pr(2)–O(9)

106.0(3)

O(7)#2–Pr(1)–O(1W)

67.1(3)

O(6)–Pr(2)–O(9)

71.7(2)

O(11)#3–Pr(1)–O(1W)

138.2(3)

O(8)#4–Pr(2)–O(1)#1

75.4(2)

O(5)–Pr(1)–O(1W)

69.9(3)

O(12)#3–Pr(2)–O(1)#1

87.2(2)

O(2)–Pr(1)–O(1W)

72.9(3)

O(4)#5–Pr(2)–O(1)#1

100.9(2)

O(10)–Pr(1)–O(1W)

78.1(3)

O(6)–Pr(2)–O(1)#1

148.5(2)

O(3)#1–Pr(1)–O(9)

78.5(2)

O(9)–Pr(2)–O(1)#1

80.0(2)

O(7)#2–Pr(1)–O(9)

116.0(2)

O(8)#4–Pr(2)–O(2W)

72.4(3)

#3

#3

82.9(2)

O(11) –Pr(1)–O(9)

76.4(2)

O(12) –Pr(2)–O(2W)

70.9(3)

O(5)–Pr(1)–O(9)

67.54(19)

O(4)#5–Pr(2)–O(2W)

98.0(3)

O(2)–Pr(1)–O(9)

143.2(2)

O(6)–Pr(2)–O(2W)

71.4(2)

O(10)–Pr(1)–O(9)

47.33(18)

O(9)–Pr(2)–O(2W)

128.2(2)

O(1W)–Pr(1)–O(9)

115.1(2)

O(1)#1–Pr(2)–O(2W)

139.3(2)

O(3)#1–Pr(1)–O(1)

70.7(2)

* Syn. Codes: #1 –x+1/2, y+1/2, –z+3/2; #2 –x+1/2, –y+3/2, –z+1; #3 –x+1/2, y–1/2, –z+3/2; #4 x, –y+2, z+1/2; #5 x, y+1, z

2 Results and discussion 2.1 Structural description Single-crystal X-ray structure analysis shows that 1 crystallizes in the C2/c space group and has a 2D layer structure. As illustrated in Fig. 1, the asymmetric unit of 1 contains two unique Pr(III) ions, three unique BIPA ligands, two coordinated water molecules and two lattice water molecules. Pr1 ion is nine-coordinated with eight oxygen atoms from six different BIPA ligands and one coordinated water molecule. Pr2 ion is surrounded by seven carboxylate oxygen atoms from six different BIPA ligands and one coordinated water molecule. The coordination environment is a capped square antiprism coordination geometry

1102

JOURNAL OF RARE EARTHS, Vol. 29, No. 11, Nov. 2011

for Pr1 center and a distorted pentagonal-bipyramidal geometry for Pr2 center, respectively. The Pr1–O bond distances range from 0.2364(7) to 0.2950(6) nm, and the Pr2–O bond distances range from 0.2303(6) to 0.2517(8) nm (Table 2), which are similar to those observed in the reported complexes[24,25]. The O–Pr1–O and O–Pr2–O bond angles are in the range of 46.8(2)º to 151.31(19)º and 70.9(3)º to 168.7(3)º, respectively. The BIPA ligands display two types of coordination modes (Scheme 1). In mode (a), each BIPA ligand connects four adjacent Pr(III) ions in ȝ2-Ș1:Ș1-bridging fashion, while in mode (b), each BIPA ligand links to four adjacent Pr(III) ions through ȝ2-Ș1:Ș1- and ȝ2-Ș2:Ș1-bridging fashion. Interestingly, the neighboring Pr(III) ions are linked by two carboxyl moieties in ȝ2-Ș1:Ș1-bridging fashion and one carboxyl group in ȝ2-Ș2:Ș1-bridging fashion to form alternative righthanded and left-handed helical chains viewed along the b-axis with a pitch of 1.4419 nm, the Pr···Pr distance of 0.4602 nm and Pr–Pr–Pr angle of 120.49º (Fig. 2). BIPA ligands link the right-handed and left-handed helical chains alternatively to construct 2D layer structure (Fig. 3), So there is not chiral information in 1. There exist strong O–H···O hydrogen bonding interactions among the oxygen atoms of coordinated water molecules, lattice water molecules and the carboxylate oxygen atoms. The hydrogen bonds link the adjacent 2D layers to form a 3D supramolecular framework

Fig. 2 Two kinds of helical chains constructed by carboxylate groups and Pr(III) ions, left-handed helix (a) and right-handed helix (b)

Fig. 3 A 2D layer constructed by {Pr2(CO2)3}n helical chains and BIPA ligands in 1

(Fig. 4). The hydrogen bonding interactions could enhance the stability of the complex. 2.2 FT-IR spectroscopy The IR spectra of 1 indicate the complete protonation due to the absence of the absorption

Fig. 1 ORTEP drawing of 1 showing Pr(III) coordination environment with thermal ellipsoids at 30% probability

Scheme 1 Coordination modes of BIPA ligand in 1

Fig. 4 Hydrogen-bonded 3D supramolecular framework of 1 viewed along the b-axis

LIU Guangxiang et al., Synthesis, structure and near-infrared luminescence of a new 2D praseodymium(III)…

peak at 1700 cm–1. Furthermore, the absorption peaks of 1555 and 1389 cm–1 show the typical antisymmetric and symmetric stretching bands of carboxylate groups. The respective values of (Ȟasym(COO–)-Ȟsym(COO–)) suggest that the carboxylate group adopts three coordination modes, which are in agreement with the observed X-ray structure[26]. In addition, a strong and broad band has been observed at 3459 cm–1, and assigned to Ȟ(OH) absorption to water molecules. 2.3 Thermogravimetric analyses Thermogravimetric analyses (TGA) were carried out for the synthesized complex, and the results of the TGA are shown in Fig. 5. A mass loss of 6.78% below 150 ºC (calcd 6.60%) was observed, due to the loss of coordinated and solvated water molecules. Further mass loss was found from ca. 300 ºC, due to the decomposition of the framework. 2.4 Near-infrared photoluminescent property As depicted in Fig. 6, the UV-Vis-NIR diffusion reflection spectrum in the solid state of 1 exhibits the f-f electronic transitions of Pr(III) ions from the 4H3 ground level to 3P2 (446 nm), 3P1 (471 nm), 1D2 (593 nm), 1G4 (996 nm), 3F4 (1409 nm) and 3 F3 (1513 nm) levels according to the energy diagram of trivalent lanthanide elements[27]. The strong absorption below 360 nm can be assigned to the organic linker’s ʌ-ʌ transitions[28].

Fig. 5 TG curve of 1

Fig. 6 Diffuse reflectance spectrum of 1 in the solid state

3 Conclusions A new Pr(III) coordination polymer was successfully synthesized under hydrothermal condition. Complex 1 featured

1103

an interesting 2D layer containing {Pr2(CO2)3}n right-handed and left-handed helical chains. Furthermore, hydrogen bonds linked the adjacent 2D layers to form a 3D supramolecular framework. Complex 1 was highly thermally stable and displayed the characteristic emissions of Pr(III) ions in NIR region. Therefore, they appeared to be good candidates for novel hybrid inorganic-organic photoactive materials.

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