CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1221-1226
PerspectiVe Synthesis, Structure, Luminescence, and Water Induced Reversible Crystal-to-Amorphous Transformation Properties of Lanthanide(III) Benzene-1,4-dioxylacetates with a Three-Dimensional Framework Xian-Lan Hong,† Yi-Zhi Li,† Huaimin Hu,‡ Yi Pan,† Junfeng Bai,*,† and Xiao-Zeng You† State Key Laboratory of Coordination Chemistry and School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P.R. China, and Department of Chemistry, Northwest UniVersity, Xi’An 710069, P.R. China ReceiVed NoVember 30, 2005; ReVised Manuscript ReceiVed March 21, 2006
ABSTRACT: Three crystalline lanthanide(III) coordination polymers and their high quality single crystals with empirical formula [M2(BDOA)3(H2O)4]‚6H2O [M ) Tb (1), Gd (2), and Sm (3); BDOA ) benzene-1,4-dioxylacetate] were obtained via the agar gel diffusion method. Their compositions and structures were determined and analyzed in detail by infrared spectroscopy, elemental analysis, and the single-crystal X-ray diffraction method. The ligand has two crystallographically independent coordination modes, La and Lb (L denotes BDOA); the metal centers are connected with La to form a double helix; these act as secondary building units (SBUs), which are linked by Lb resulting in a plane topology geometry with 54-membered macrorings; and each metal ion is saturated by water molecules to form a nine-coordinated sphere. These planes are further constructed into three-dimensional networks by a complicated H-bond system with an inversion center among the water molecules (coordinated and guest), the carboxylate group, and the phenolic O atoms. Investigations of their luminescent properties demonstrate that compound 1 has characteristic emissions of Tb(III) corresponding to electronic transitions from the excited state 5D4 to the multiplets 7F6, 7F5, 7F4, and 7F3, respectively; however, 2 and 3 have no detectable luminescence under the same conditions. Thermal decomposition and powder X-ray diffraction results indicate that the transformation from the crystal form, [M2(BDOA)3(H2O)4]‚6H2O, to the amorphous powder, M2(BDOA)3(H2O)2, is reversible, indicating that the amorphous form, M2(BDOA)3(H2O)2, may be utilized as an absorbing agent for water and water vapor. Introduction Metal polycarboxylate coordination polymers have attracted considerable attention because of their fascinating architectures and promising applications as new materials in catalysis, absorption, and host-guest chemistry, for example.1-6 Two kinds of these ligands have been generally used in this field, either flexible ligands, such as succinic and glutaric acids and such analogous compounds,7-12 or rigid ligands, such as benzenedicarboxylate, benzenetricarboxylate, and so on.13-16 However, only a limited amount of work has been reported using polycarboxylate ligands combining the characteristics of both flexibility and rigidity, which will be quite important in the development of so-called third generation coordination polymers with dynamic frameworks and striking functions.2 We are interested in utilization of polycarboxylate ligands with characteristics of both flexibility and rigidity, such as benzene-1,4-dioxylacetate and benzene-1,3-dioxylacetate,17 to * To whom correspondence
[email protected]. † Nanjing University. ‡ Northwest University.
should
be
addressed.
E-mail:
construct coordination polymers. The -OCH2- group makes these ligands more flexible in comparison with the corresponding benzenedicarboxylate, while the existence of the benzene ring provides a rigid element. Moreover, the phenolic oxygen atom may function as electron donor on binding to the metal center or can form H-bonds. Up to now, no structural characterization of the lanthanide complexes of benzene-1,4-dioxylacetic acid has been reported although there are several examples of lanthanide carboxylates in the literature.18-21 We herein reported a series of lanthanide(III) benzene-1,4-dioxylacetate (BDOA) compounds with threedimensional networks arising from the linking of twodimensional layers through complicated H-bonding interactions, and their luminescent and water induced crystal-to-amorphous transformation properties were also investigated. Experimental Section Materials and Methods. Samarium(III), gadolinium(III), and terbium(III) nitrates were obtained commercially and were used without further purification. Microelemental analyses were carried out with a PE-2400CHN analyzer. The IR spectra were recorded on a Bruker VECTOR22 FT-IR spectrometer using the KBr pellet technique.
10.1021/cg0506323 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006
1222 Crystal Growth & Design, Vol. 6, No. 6, 2006 Fluorescence measurements were performed on an AMINCO-BOWMAN Series AB2 luminescence spectrometer. The as-synthesized samples were characterized by thermogravimetric analysis (TGA) on a thermoflex analyzer (Rigaku) up to 1073 K using a heating rate of 20 K min-1 for ∼10 mg samples open to air. Powder X-ray diffraction data were recorded on an automated diffractometer. XRD measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu KR radiation (0.154 18 nm), and the X-ray tube was operated at 35 kV and 20 mA. Simulated XRD patterns were calculated with the SHELXTL-XPOW program using the single-crystal data. Preparation of Benzene-1,4-dioxylacetic Acid (BDOA). This ligand was synthesized using an improved method of that described elsewhere.22 To 50 mL of water suspension of 1,4-dihydroxybenzene (0.1 mol) was added a 50 mL solution of NaOH (0.2 mol) under N2 at ambient temperature. After the reaction mixture became clear, a 50 mL solution of ClCH2COOH (0.25 mol) was added dropwise at 6070 °C with vigorous stirring, and then the pH of the mixture was adjusted to 11 with 4 M NaOH solution. The temperature was quickly elevated to 120 °C and maintained for 3 h, during which time the pH of the reaction solution was adjusted to 8-9 with NaOH. After continuing the reaction for another 2 h, the resulting mixture was acidified with HCl to pH 1-2 and a gray product precipitated, which was filtered off, washed with 2 × 30 mL of CH3CH2OH and 3 × 30 mL of H2O, respectively, and air-dried. Yield 79%. Anal. Calcd: C, 53.10; H, 4.24. Found: C, 52.86; H, 4.41. Mp: 250-251 °C (ref 250 °C). Main IR frequencies: 3423, 1738, 1506, 1426, 1288, 1224, 1091, 906, 824, 800 cm-1. Preparation of [Tb2(BDOA)3(H2O)4]‚6H2O. The same procedure was employed to prepare all three compounds, and that for the terbium compound is described here in detail. BDOA suspension (10 mL, 0.1 mmol) was slowly added to a 1 M NaOH solution with vigorous stirring until it became clear, and the pH of the resulting solution was maintained at about 6.0. Then Tb(NO3)3‚5H2O (5 mL, 0.1 mmol) was slowly diffused into this sodium benzene-1,4-dioxylacetate solution through a 2.5 cm thick layer of 1% (by weight) agar gel in a diffusion device. One month later, well-shaped grayish-white prismatic crystals were formed, washed thoroughly with deionized water, and air-dried to give the product with yield 72%. Anal. Calcd: C, 30.78; H, 3.79. Found: C, 31.46; H, 4.11. Main IR frequencies: 3459, 1641, 1576, 1509, 1420, 1363, 1331, 1231, 1078, 811, 715 cm-1. For [Gd2(BDOA)3(H2O)4]‚6H2O. Yield: 67%. Anal. Calcd: C, 30.87; H, 3.80. Found: C, 31.30; H, 3.99. Main IR frequencies: 3459, 1640, 1576, 1508, 1420, 1362, 1331, 1231, 1078, 811, 714 cm-1. For [Sm2(BDOA)3(H2O)4]‚6H2O. Yield: 75%. Anal. Calcd: C, 31.24; H, 3.85. Found: C, 31.51; H, 4.12. Main IR frequencies: 3459, 1639, 1572, 1508, 1420, 1361, 1318, 1230, 1079, 814, 715 cm-1. X-ray Data Collection and Structure Determination. Suitable single crystals of 1-3 were mounted on a Pyrex fiber with epoxy and affixed to a brass pin for X-ray data collection. The intensity data were collected at room temperature on a Bruker Smart Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The data collection covered over a hemisphere of reciprocal space by a combination of three sets of exposures; each set had a different φ angle (0°, 88°, and 180°) for the crystal, and each exposure of 30 s covered 0.3 in ω. The crystal to detector distance was 4 cm, and the detector swing angle was -35° to give a data set over 99% complete. The 30 initial frames were recollected at the end of data collection to monitor crystal decay via analyzing the duplicate reflections, and no significant decay was observed. The raw data were reduced and corrected for Lorentz and polarization effects using the SAINT program and for absorption using the SADABS program. The structures were solved by direct methods using SHELXS 97, and all non-hydrogen atoms were refined anisotropically by full-matrix least-squares based on F2 values.23 The largest residual density peak is close to the lanthanide atom. Hydrogen atoms were added at calculated positions and were not refined. CCDC285625 (for 1), 285626 (for 2), and 285627 (for 3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.
Results and Discussion Syntheses. All the title compounds as well-shaped, high purity, and high quality single crystals suitable for X-ray
Perspective
Figure 1. Distorted tricapped trigonal prismatic coordination environment of complex 1. Lattice water molecules and all hydrogen atoms are omitted for clarity. The added blue dot represents the inversion center. Symmetry labels: (#1) x, y, -1 + z; (#2) 1 - x, -y, 2 - z; (#3) 1 - x, -y, 1 - z.
Scheme 1. Coordination Modes of the BDOA Ligand in Compounds 1, 2, and 3
diffraction were obtained by the direct reaction of the corresponding lanthanide(III) nitrate with deprotonated benzene-1,4dioxylacetatic acid in a agar gel diffusion device. These crystalline solids are stable in air and insoluble in water or common organic solvents such as methanol and ethanol. The IR spectra of the three compounds show strong absorption bands between 1361 and 1640 cm-1 that can be assigned to coordinated carboxylate groups,24 and the absence of the strong carboxyl absorption band at 1738 cm-1 of benzene-1,4dioxylacetic acid indicates the complete deprotonation. Description of the Crystal Structures. Single-crystal X-ray diffraction analyses revealed that the three compounds are isostructural, and the structure of [Tb2(BDOA)3(H2O)4]‚6H2O is representatively described in detail here. The BDOA anion has two crystallographically independent coordination modes La and Lb as shown in Scheme 1, with Lb located on an inversion center. The asymmetric unit of the compound comprises one Tb3+ ion, one La, a half Lb, two coordinated water molecules, and three guest water molecules. The coordination environment around the Tb3+ ion can be clearly seen from a pair of Tb(III) with the distance of 3.892 Å between two Tb atoms and an inversion center as shown in Figure 1. Tb(III) ion is nine coordinated, and the coordination geometry around Tb1 may be described as a distorted tricapped trigonal prism, with O1, O6#3, OW1, O5#1, O2#2, and O8 filling the vertexes and O2, OW2, and O7 capping the rectangular faces. The same situation was observed in the compound [Er2(PDA)3(H2O)]‚2H2O (here PDA ) 1,4-phenylendiacetato ligand).16 The Tb-O (carboxyl-
Perspective
Crystal Growth & Design, Vol. 6, No. 6, 2006 1223
Table 1. Crystal Data and Structure Refinement for 1, 2, and 3
empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) Z V (Å3) T (K) dcalcd (g/cm3) λ (Mo KR) (Å) µ (cm-1) final R indices [I > 2σ(I)] R1a wR2b R indices (all data) R1a wR2b
1
2
3
C15H22O14Tb 585.25 triclinic P1h 8.5935(18) 9.675(2) 12.697(3) 102.633(4) 95.234(4) 106.584(3) 2 973.8(4) 293(2) 1.996 0.71073 3.705
C15H22O14Gd 583.58 triclinic P1h 8.593(2) 9.693(3) 12.704(3) 102.751(4) 95.233(4) 106.532(4) 2 975.9(4) 293(2) 1.986 0.71073 3.472
0.0453 0.1201
0.0599 0.1308
C15H22O14Sm 576.68 triclinic P1h 8.6227(17) 9.7572(19) 12.738(3) 102.899(3) 95.136(3) 106.539(4) 2 987.9(4) 293(2) 1.939 0.71073 3.045 1 0.0343 0.0963
0.0462 0.1212
0.0700 0.1329
0.0350 0.0970
a Based on F’s. b Based on F2 with w ) 1/(σ2(F )2 + (aP)2 + bP) and o P ) (max(Fo2,o) + 2Fc2)/3. Values of a and b: 0.09 and 1.22 for 1; 0.08 and 1.99 for 2; 0.07 and 1.8 for 3.
ate) bond distances range from 2.370(4) to 2.550(4) Å, and those of the Tb-OW1 and Tb-OW2 bonds are 2.408(4) and 2.329(5) Å, respectively, all of which are within the range of those observed for other nine-coordinate Tb(III) complexes with oxygen donor ligands.11,25 A summary of the crystallographic data and structure refinement is given in Table 1, and the selected bond distances and angles for 1, 2, and 3 are listed in Table 2. The average lengths of the M-O(carboxylate) bonds, for 3, 2, and 1, respectively, are 2.487, 2.461, and 2.448 Å, and those of M-O (water) are 2.414, 2.376, and 2.368 Å, in accordance with the decrease of the metal radius. In the polymeric structure of [Tb2(BDOA)3(H2O)4]‚6H2O, a pair of Tb(III) ions are interconnected through -COO- groups of two ligands of mode La as in a knot. These are in an antiorientation parallel to each other to form a double helix that
extends in the crystallographic c direction (Figure 2). The distance between two knots along the double helix is 12.697 Å, which is equal to the length of ligand of mode La, as well as the unit length of the c axis. These one-dimensional infinite helixes may be viewed as supramolecular SBUs (secondary building units) that are further cross-linked via ligand of mode Lb through COO- groups to form neutral layers. The distance corresponding to ligand of mode Lb is 15.454 Å, which is longer than that of the ligand of mode La. It is worth noting that there is a 54-member macroring containing six Tb ions (Tb1 and Tb#1 to Tb#5), as shown in Figure 2. The longest length is between Tb#1 and Tb1#5, and it is 29.085 Å. Interestingly, the view of the layer down the b axis displays a triple helix. These triple helixes are further connected by the complicated H-bonding system between them to form the threedimensional structure (Figure 3). The geometry of the hydrogen bonding is shown in Table 3. The two coordinated water molecules on each Tb atom have different numbers of hydrogen bonds, with one two-coordinate (OW1) and the other threecoordinate (OW2). The guest waters can be divided into two kinds in terms of forming H-bonds. One is two-coordinate (OW3 or OW5) and the other fully coordinate (OW4). The O atoms of carboxylate groups of ligand mode La (O8) and mode Lb (O6#3), as well as the phenolic oxygen atom (O3), take part in forming H-bonds. In addition, the H-bonding system has an inversion center (not shown in Figure 3). Luminescent Properties. It is well documented that the Ln(III), especially Eu(III) and Tb(III), can absorb ultraviolet radiation efficiently through an allowed electronic transition to convert to excited state 5D4, and these excited states are deactivated to the multiplet 7Fn states radiatively via emission of visible radiation named as luminescence. Furthermore, the intensity of Ln(III) ion luminescence emission can be greatly enhanced by an efficient energy transfer from organic ligands to the metal ion.18,26 In recent decades, the luminescent properties of lanthanide(III) complexes with aromatic acids have greatly interested chemists due to their potential application in many fields.11,12,27-29 In this paper we also investigated the luminescence properties of compound 1 (2 and 3 have no
Table 2. Selected Bonds Lengths (Å) and Bond Angles (deg) for 1, 2, and 3a Complex 1 Tb1-O1 Tb1-O8 Tb1-O6#2 O2-Tb1-O1 OW2-Tb1-OW1 O2#3-Tb1-O8 O2#3-Tb1-O1
2.550(4) 2.424(4) 2.378(4) 51.72(12) 70.66(16) 85.63(14) 122.36(13)
Tb1-O2 Tb1-O2#3 Tb1-OW1
2.503(4) 2.412(4) 2.408(4)
O8-Tb1-O7 O5#1-Tb1-O2#3 OW1-Tb1-O1 O5#1-Tb1-O6#2
52.76(14) 71.94(14) 77.63(15) 138.95(14)
Tb1-O7 Tb1-O5#1 Tb1-OW2 O6#2-Tb1-O1 O5#1-Tb1-O8 OW2-Tb1-O5#1 OW2-Tb1-O6#2
2.501(4) 2.370(4) 2.329(5) 71.41(15) 74.68(15) 69.92(15) 140.46(16)
Complex 2 Gd1-O1 Gd1-O8 Gd1-O6#2 O2-Gd1-O1 OW2-Gd1-OW1 O2#3-Gd1-O8 O2#3-Gd1-O1
2.567(6) 2.439(6) 2.374(6) 51.40(17) 70.7(2) 85.22(19) 122.53(19)
Gd1-O2 Gd1-O2#3 Gd1-OW1
2.535(5) 2.413(5) 2.422(5)
O8-Gd1-O7 O5#1-Gd1-O2#3 OW1-Gd1-O1 O5#1-Gd1-O6#2
52.3(2) 71.78(19) 78.2(2) 138.66(18)
Gd1-O7 Gd1-O5#1 Gd1-OW2
2.518(5) 2.387(5) 2.329(7)
O6#2-Gd1-O1 O5#1-Gd1-O8 OW2-Gd1-O5#1 OW2-Gd1-O6#2
71.1(2) 75.2(2) 69.8(2) 140.4(2)
Complex 3 Sm1-O1 Sm1-O8 Sm1-O6#2 O2-Sm1-O1 OW2-Sm1-OW1 O2#3-Sm1-O8 O2#3-Sm1-O1 a
2.587(3) 2.475(3) 2.408(3) 50.77(10) 70.10(13) 84.94(11) 121.87(11)
Sm1-O2 Sm1-O2#3 Sm1-OW1 O8-Sm1-O7 O5#1-Sm1-O2#3 OW1-Sm1-O1 O5#1-Sm1-O6#2
2.542(3) 2.456(3) 2.449(3) 52.08(11) 71.51(11) 78.63(12) 138.12(11)
Sm1-O7 Sm1-O5#1 Sm1-OW2 O6#2-Sm1-O1 O5#1-Sm1-O8 OW2-Sm1-O5#1 OW2-Sm1-O6#2
2.537(3) 2.406(3) 2.379(4) 71.47(13) 75.39(12) 70.34(13) 140.80(13)
Symmetry transformations used to generate equivalent atoms: (#1) x, y, z - 1; (#2) -x + 1, -y, -z + 2; (#3) -x + 1, -y, -z + 1.
1224 Crystal Growth & Design, Vol. 6, No. 6, 2006
Perspective
Figure 2. Packing diagram down the a axis showing the 54-member macroring enclosed by Tb1 and Tb#1 to Tb#5 (Tb, light gray; O, red; C, gray). H-atoms have been omitted for clarity. Symmetry labels: (#1) 1 - x, 1 - y, -z; (#2) x, 1 + y, -1 + z; (#3) 1 - x, 1 - y, 1 - z; (#4) x, y, 1 + z; (#5) 1 - x, -y, 2 - z.
Figure 3. Packing diagram down the b axis showing the triple helixes and H-bonds (green dotted lines) between layers (Tb, light red; O, red; C, gray). H-atoms have been omitted for clarity. Symmetry labels: (#1) 2 - x, -y, 1 - z; (#2) 1 + x, y, z; (#3) x, 1 + y, -1 - z.
Perspective
Crystal Growth & Design, Vol. 6, No. 6, 2006 1225 Table 3. Hydrogen Bonding Geometry
D-H‚‚‚Aa
d(D‚‚‚A)
d(H‚‚‚A)
∠(DHA)
OW4-H4A‚‚‚O8 OW1-H1A‚‚‚OW4#1 OW2-H2A‚‚‚OW4#1 OW2-H2B‚‚‚OW3#1 OW3-H3A‚‚‚O6#5 OW4-H4B‚‚‚OW5#2 OW5#3-H5A#3‚‚‚O3
2.723(7) 2.748(7) 2.719(7) 2.660(7) 3.198(8) 2.952(9) 3.030(8)
2.00 1.90 1.94 2.12 2.40 2.11 2.34
142.4 176.5 151.5 120.9 157.6 173.2 139.1
a Symmetry codes: (#1) 2 - x, 1 - y, 1 - z; (#2) 1 + x, y, z; (#3) 1 x, 1 - y, 1 - z; (#5) x, 1 + y, -1 + z.
Figure 5. TGA and DTG (inserted) curves of compound 1.
Figure 4. Solid-state excitation-emission spectrum for compound 1.
detectable luminescence under the same conditions). The excitation and emission spectra for 1 are shown in Figure 4. The excitation bands at 353, 369, and 378 nm could be assigned to n,π* transitions of the three coordination mode OCO groups according to ref 29; this is because the benzene-1,4-dioxylacetic acid is deprotonated completely and thus the OCO groups are all coordinated to the terbium ions, a π-conjugation system is formed in the OCO group, and the energy of the n,π* excited state could be transferred to terbium ions efficiently. In the emission spectra (λex ) 340 nm), the narrow and strong bands at 490, 544, 584, and 621 nm are attributed to the characteristic emissions of Tb(III) corresponding to electronic transitions from the excited state 5D4 to the multiplets 7F6, 7F5, 7F4, and 7F3, respectively, whereas the wide band centered approximately at 410 nm can be assigned to the transition emission from the T1π,π* and T1n,π* states to the ground state of the -OC6H4Ogroup.29 It is noteworthy that the energy of the T1π,π* and T1n,π* excited states of the -OC6H4O- group cannot be transferred to terbium ions through the nonconjugated flexible chain of -O-CH2- efficiently. Conclusively, the excited state which transfers the energy to Tb3+ and generates luminescence of terbium ions is the n,π* excited state of the OCO group and not the T1π,π* and T1n,π* excited states of the -OC6H4Ogroup. Water Induced Reversible Crystal-to-Amorphous Transformation Properties. The three compounds contain both guest and coordinated water. To get more insight into the properties relative to these water molecules, their dehydration behavior has been investigated using thermogravimetric analysis and X-ray power diffraction. The TGA and DTG curves of complex 1 are shown in Figure 5. The first step (55-110 °C) corresponds to the loss of eight water molecules per formula unit, two of which are coordinated ones. The second step between 120 and
Figure 6. XRD patterns: (a) simulated from single-crystal X-ray data; (b) as-synthesized [Tb2(BDOA)3(H2O)4]‚6H2O; (c) restored [Tb2(BDOA)3(H2O)4]‚6H2O from Tb2(BDOA)3(H2O)2; (d) Tb2(BDOA)3(H2O)2.
220 °C corresponds to the loss of the last two coordinated water molecules. Decomposition occurs beyond 330 °C, but no attempt was made to identify the final residue. After the as-synthesized compound 1 was heated to 120 °C in air and maintained at this temperature for 12 h, a gray powder was obtained. The elemental analysis and powder X-ray diffraction results indicate that the chemical composition of the power is Tb2(BDOA)3(H2O)2 in an amorphous form. This result is in accordance with that of the thermal decomposition experiment. Interestingly, the amorphous powder of Tb2(BDOA)3(H2O)2 can be restored to the crystalline form of [Tb2(BDOA)3(H2O)4]‚6H2O after being soaked in water for 24 h, as shown by the elemental analysis experiment (Anal. Calcd: C, 30.78; H, 3.79. Found: C, 30.77; H, 3.98) and the XRD measurement results. The restored solid has the same XRD pattern as the simulated one from singlecrystal X-ray diffraction data of 1 (see Figure 6). This indicates that the amorphous Tb2(BDOA)3(H2O)2 could be a new absorbing agent for small molecules such as water and the absorptive capacity for water is about 12.3% (w/w). In fact, the [Tb2(BDOA)3(H2O)4]‚6H2O has a dynamic structure resulting from the special BDOA ligand which features a combination of flexibility and rigidity and the water adsorption behavior of its evacuated solid is typical of a third-generation porous solid
1226 Crystal Growth & Design, Vol. 6, No. 6, 2006
following the Kitagawa classification with the “guest-induced crystal-to-amorphous transformation (CAT)” framework.2,30 When the water molecules (guest and coordinated) are removed, the framework collapses due to the destruction of the hydrogen bond system, and the close-packing force is effective. However, the 2-D sheet structure cannot be destroyed in this process, and the framework of the crystalline [Tb2(BDOA)3(H2O)4]‚6H2O is restored with the reintroduction of water molecules between 2-D sheets and the re-formation of the H-binding system. The analogous situation was reported on the compound [{Cu(BF4)2(4,4′-bipy)(H2O)2}‚4,4′-bipy]n, in which the 1-D linear chains are linked by hydrogen bonds between metal-free 4,4′-bipy molecules and coordinated H2O molecules to form 2-D noninterpenetrated sheets. This compound can absorb N2, Ar, and CO2 above a certain pressure, named the gate pressure, and this unique adsorption phenomenon is attributed to its hydrogen bond-regulated microporous nature.31,32 Further investigations are in progress. Conclusion In summary, a series of novel lanthanide(III) coordination polymers based on BDOA, a polycarboxylate ligand with characteristics of both flexibility and rigidity, have been successfully prepared and structurally characterized. They are isostructural with the ligand displaying two crystallographically independent coordination modes, La and Lb (L denotes BDOA). The metal centers are connected with La to form a double helix, and these act as secondary building units (SBUs), which are linked by Lb, resulting in a planar topology with a 54-membered macroring. These planes are further constructed to give threedimensional networks by a complicated H-bonding system with an inversion center among the water molecules. Thermal decomposition, elemental analysis, and powder X-ray diffraction results indicate that the transformation from the crystal form of [M2(BDOA)3(H2O)4]‚6H2O to the amorphous powder of M2(BDOA)3(H2O)2 is reversible, and the dried amorphous powder of M2(BDOA)3(H2O)2 may be applied to the absorbance of water and water vapor. The investigation of luminescent properties demonstrates that compound 1 has characteristic emissions of Tb(III) corresponding to electronic transitions from the excited state 5D4 to the multiplets 7F6, 7F5, 7F4, and 7F3, and the excited state which transfers the energy to Tb3+ and generates luminescence of terbium ions is the n,π* excited state of the OCO group and not the T1π,π* and T1n,π* excited states of the -OC6H4O- group. Acknowledgment. We acknowledge support for this work from Jiangsu Planned Projects for Postdoctoral Research Funds; Talent Development Foundation of Nanjing University; Twentyone Century Talent Foundation of the Ministry of Education; Foundation for the Returnee of the Ministry of Education; and Measurement Foundation of Nanjing University and National Natural Science Foundation of China (No. 20301010). We thank Prof. Annie K. Powell (Institut fu¨r Anorganische Chemie, Universita¨t Karlsruhe, Engesserstr, Germany) for his many helpful discussions.
Perspective
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