Syntheses, Crystal Structures, and Photoluminescent Properties of a Series of M(II) Coordination Polymers Containing M-X2-M Bridges: From 1-D Chains to 2-D Networks Kai-Ju Wei,† Yong-Shu Xie,*,† Jia Ni,‡ Min Zhang,† and Qing-Liang Liu*,†
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1341-1350
Department of Chemistry, UniVersity of Science and Technology of China, Hefei, 230026, People’s Republic of China, and Center Lab, Shantou UniVersity, Shantou, People’s Republic of China ReceiVed NoVember 3, 2005; ReVised Manuscript ReceiVed April 14, 2006
ABSTRACT: The novel blue-purple luminescent bridging ligand N,N,N′,N′-tetrakis(2-pyridyl)-2,6-pyridinediamine (L) has been synthesized. The coordination of this ligand with a series of Zn(II), Cd(II), and Hg(II) salts has been investigated. Seven new complexes (1-7) were obtained and characterized by elemental analyses and single-crystal X-ray analyses. In the compounds Zn2LCl4 (1), Hg2LCl4 (5), Hg2LBr4 (6), and Hg2LI4 (7), each ligand is coordinated to two metal ions, forming dinuclear moieties, which are further connected via weak intermolecular M‚‚‚X interactions, affording one-dimensional chains. The complexes {[Cd2L(µCl)2]Cl2}n‚nH2O (2) and {[Cd2L(µ-Br)2]Br2}n‚nH2O (3) form pseudo-helix 1-D coordination polymers through Cd-X2-Cd (X ) Cl, Br) bridges. The compound {[Cd2L(µ-I)(µ‚‚‚I)]I2}n (4) forms a zigzag 1-D motif with two kinds of Cd-I-Cd bridges. Furthermore, the 1-D chains in all these complexes are connected by face-to-face π-π stacking interactions, affording 2-D networks with various structural geometries. All of these complexes are luminescent in the solid state, with the emission maxima varying in the visible light region within the range of 440-520 nm. Introduction Recently, the construction of metal-organic coordination polymers has attracted intense attention not only because of their intriguing structural diversity,1 such as helixes2 and nanotubes,3 but also because of their unique chemical and physical properties and their potential applications as optoelectronic, magnetic, and porous materials.4 One of the current interesting topics is to rationally design and synthesize inorganic-organic polymers and supramolecular assemblies by coordination bonds or noncovalent contacts such as hydrogen bonding and π-π interactions. Bridging ligands play an important role in controlling the structures of the products. In this respect, rodlike bridging ligands were the most extensively studied.5 In contrast, angular ligands are relatively less used as building blocks for highdimensional coordination polymers. However, the bent conformations may facilitate the formation of novel oligomers or lowdimensional polymers, such as helixes,1b-g cages,6 macrocycles,7 and other beautiful and novel supramolecular structures. On the other hand, a number of reports have shown that 2,2′dipyridylamine and its derivatives and metal complexes exhibit excellent luminescent properties,8 which can be used not only as efficient blue emitters in electroluminescent devices but also as chemical sensors for specific organic molecules.9 Furthermore, fine tuning of the luminescent properties can also be realized by modifying the ligand subunits.10 On the basis of these considerations, we herein report the synthesis of a new ligand with angular and flexible spacers based on dipyridylamine, N,N,N′,N′-tetrakis(2-pyridyl)-2,6-pyridinediamine (L), and used it as a bent bridging ligand to obtain a series of novel complexes with MX2 salts (M ) Zn(II), Cd(II), Hg(II); X ) Cl, Br, I): Zn2LCl4 (1), {[Cd2L(µ-Cl)2]Cl2}n‚nH2O (2), {[Cd2L(µ-Br)2]Br2}n‚nH2O (3) {[Cd2L(µ-I)(µ‚‚‚I)]I2}n (4), Hg2LCl4 (5), Hg2LBr4 (6), and Hg2LI4 (7). Generally, the study * To whom correspondence should be addressed. Fax: +86-5513603388. Tel: +86-551-3603214. E-mail:
[email protected] (Y.-S.X.)
[email protected] (Q.-L.L.). † University of Science and Technology of China. ‡ Shantou University.
of bent ligands has been mainly focused on dipyridyl, diimidazolyl, or triazolyl units.1b-g,6,11 The use of a pentapyridyl ligand as a bent building block is rather rare. In contrast to the pyridylethene, pyridylacetylene, pyridylbenzene, imidazolyl, and triazolyl groups,1b-g,6,11 dipyridylamino groups are more flexible, which will enhance the structural diversity. In complexes 1 and 5-7, one-dimensional chains are formed by connecting the [M2LX4] moieties via weak intermolecular M‚‚‚X interactions. While 2 and 3 form two pseudo-helix chain 1-D structures through Cd-X2-Cd bridges, compound 4 forms a zigzag chain 1-D motif with two kinds of Cd-I-Cd bridges. Furthermore, the 1-D chains in all these complexes are connected by faceto-face π-π stacking interactions, affording 2-D networks. In this work, we also report the luminescence properties of the ligand and complexes 1-7 in the solid state. Experimental Procedures Materials and Methods. All the starting chemicals were of reagent grade and were obtained from commercial sources. All reagents and solvents were dried and purified by the usual methods. 1H NMR spectra were recorded on a Bruker 300 Ultrashield spectrometer. The FT-IR spectra were recorded in the region 400-4000 cm-1 on a Bruker EQUINOX 55 VECTOR22 spectrophotometer. Elemental analyses were carried out with an Elmentar Vario EL-III analyzer. Fluorescence measurements were made on a JOBIN YVON Analytical Instrument FLUOROLOG-3-TAU at room temperature. Thermogravimetric analyses (TGA) were performed with a TGA-50H thermoanalyzer under N2 (20-800 °C) at a heating rate of 10 °C/min. Synthesis of L. 2,2′-Dipyridylamine (2.565 g, 15.0 mmol), 2,6dibromopyridine (1.185 g, 5.00 mmol), anhydrous potassium carbonate (3.45 g, 25.0 mmol), and cupric sulfate (0.19 g) were placed in a 50 mL flask, heated to 200 °C, and stirred for 8 h under N2. The reaction was monitored by TLC. After the reaction mixture was cooled to ambient temperature, dichloromethane and water were added to dissolve the solid. The aqueous phase was discarded, and the organic phase was washed with distilled water to neutral pH and then dried with MgSO4. The product was purified by a silica gel column with ethyl acetate as the eluent to obtain a colorless solid of N,N,N′,N′-tetrakis(2-pyridyl)-2,6-pyridinediamine (L; 2.7 g, 65%). 1H NMR (300 MHz, CDCl3, ppm): δ 8.30 (d, 4H), 7.50 (t, 5H), 7.07 (d, 4H), 6.95 (t, 4H), 6.68 (d, 2H). IR (cm-1, KBr): 1585 (s), 1566 (s), 1467 (ms), 1426 (vs).
10.1021/cg0505822 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/17/2006
CCDC formula formula wt T/K λ(Mo KR)/Å cryst syst cryst size/mm3 space group a/Å b/Å c/Å β/deg V/Å3 Dc/Mg m-3 Z F(000) µ/mm-1 no. of rflns collected no. of unique rflns R(int) no. of data/restraints/params final R indices (I > 2σ(I)) R1 wR2 R indices (all data) R1 wR2 GOF on F2 0.0313 0.0725 0.0357 0.0748 1.025
0.0492 0.0992
0.0838 0.1115 1.005
2 287153 C12.5H10.5Cl2CdN3.5O0.5 401.16 293(2) 0.710 73 monoclinic 0.26 × 0.08 × 0.07 C2/c 17.0876(17) 14.9487(15) 12.0009(12) 109.473(2) 2890.1(5) 1.841 8 1564 1.874 8785 3252 0.0197 3252/0/178
287152 C25H19Cl4N7Zn2 690.01 293(2) 0.710 73 monoclinic 0.21 × 0.18 × 0.15 C2/c 15.875(2) 15.131(2) 12.1758(16) 110.960(2) 2731.1(6) 1.678 4 1384 2.178 8359 3079 0.0445 3079/0/173
1
0.0451 0.0930 1.040
0.0364 0.0887
286339 C12.5H10.5Br2CdN3.5O0.5 489.96 293(2) 0.710 73 monoclinic 0.18 × 0.17 × 0.15 C2/c 17.180(3) 15.269(3) 12.1374(19) 108.606(3) 3017.6(8) 2.157 8 1856 6.738 9244 3438 0.0205 3438/0/178
3
0.0570 0.1083 1.005
0.0425 0.1005
286340 C25H19I4Cd2N7 1149.87 293(2) 0.710 73 orthorhombic 0.35 × 0.20 × 0.12 C2/c 17.360(9) 15.178(8) 24.457(13) 90 6444(6) 2.370 8 4208 5.179 37 943 7347 0.0425 7347/0/343
4
5
0.0499 0.0789 1.035
0.0312 0.0676
286342 C25H19Cl4Hg2N7 960.45 293(2) 0.710 73 orthorhombic 0.33 × 0.26 × 0.33 Pbca 16.7768(12) 14.4667(10) 23.5340(16) 90 5711.8(7) 2.234 8 3568 11.142 62 225 6761 0.0556 6761/0/343
Table 1. Crystallographic Data and Structure Refinement Summary for 1-7 6
0.0759 0.1071 1.009
0.0447 0.0954
286341 C25H19Br4Hg2N7 1138.29 293(2) 0.710 73 orthorhombic 0.32 × 0.26 × 0.23 Pbca 17.1651(8) 14.7379(7) 23.6550(11) 90 5984.2(5) 2.527 8 4144 15.619 35 325 6971 0.0682 6971/0/343
7
0.0652 0.0943 0.987
0.0384 0.0838
286343 C25H19Hg2I4N7 1326.25 293(2) 0.710 73 orthorhombic 0.35 × 0.21 × 0.12 Pbca 17.755(3) 15.139(3) 24.304(5) 90 6533(2) 2.697 8 4720 13.196 52 986 7428 0.0800 7428/0/343
1342 Crystal Growth & Design, Vol. 6, No. 6, 2006 Wei et al.
M(II) Polymers Containing M-X2-M Bridges
Crystal Growth & Design, Vol. 6, No. 6, 2006 1343
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for 1-7 Zn(1)-Cl(1) Zn(1)-N(1) N(1)-Zn(1)-Cl(1) N(1)-Zn(1)-Cl(2) Cd(1)-Cl(1) Cd(1A)-Cl(1) N(3)-Cd(1)-N(1) N(3)-Cd(1)-Cl(1A) N(1)-Cd(1)-Cl(2) Cd(1)-Br(1A) Cd(1)-N(2) N(1)-Cd(1)-N(2) N(2)-Cd(1)-Br(2) N(2)-Cd(1)-Br(1A) Cd(1)-I(1) Cd(1)-N(5) Cd(1)-I(3) N(4)-Cd(1)-N(5) N(5)-Cd(1)-I(1) N(5)-Cd(1)-I(2) N(4)-Cd(1)-I(3) I(1)-Cd(1)-I(3) Hg(1)-Cl(1) Hg(2)-Cl(3) Hg(1)-N(7) Cl(2)-Hg(1)-Cl(1) Cl(1)-Hg(1)-N(7) Cl(1)-Hg(1)-N(6) Hg(1)-Br(1) Hg(2)-Br(3) Hg(1)-N(1) N(5)-Hg(2)-N(7) N(7)-Hg(2)-Br(3) N(7)-Hg(2)-Br(4) Hg(1)-I(1) Hg(2)-I(3) Hg(1)-N(1) N(3)-Hg(1)-N(1) I(1)-Hg(1)-N(3) I(2)-Hg(1)-N(3)
2.205(10) 2.047(3)
Zn(1)-Cl(2) Zn(1)-N(3)
122.09(8) 110.99(9)
N(3)-Zn(1)-Cl(1) N(1)-Zn(1)-N(3)
2.7373(9) 2.4779(8)
Cd(1)-Cl(2) Cd(1)-N(1)
78.72(8) 126.88(6) 99.32(6)
N(1)-Cd(1)-Cl(1) Cl(2)-Cd(1)-Cl(1A) Cd(1)-Cl(1)-Cd(1A)
2.5982(6) 2.399(3)
Cd(1)-N(1)
78.41(12) 99.66(8) 91.61(8)
N(1)-Cd(1)-Br(1) Br(2)-Cd(1)-Br(1) Cd(1)-Br(1)-Cd(1A)
2.7228(13) 2.394(5) 3.2167(12) 80.78(17) 94.87(12) 99.15(12) 83.19(13) 91.09(3) 2.4061(15) 2.3663(14) 2.439(4) 136.42(6) 104.98(12) 96.60(10) 2.4810(8) 2.4839(8) 2.433(6) 77.97(19) 106.90(14) 94.03(14) 2.6272(7) 2.6721(7) 2.408(6) 75.9(2) 107.71(16) 97.05(15)
Cd(2)-I(4) Cd(2)-N(6) Cd(1)-N(4) I(4)-Cd(2)-I(3) N(7)-Cd(2)-I(4) N(7)-Cd(2)-I(3) Cd(2)-I(3)-Cd(1)
Hg(1)-Cl(2) Hg(2)-Cl(4) Hg(1)-N(6) Cl(3)-Hg(2)-Cl(4) Cl(4)-Hg(2)-N(1) Cl(4)-Hg(2)-N(3) Hg(1)-Br(2) Hg(2)-Br(4) Hg(1)-N(3) N(1)-Hg(1)-Br(1) Br(1)-Hg(1)-N(3) Br(1)-Hg(1)-Br(2) Hg(1)-I(2) Hg(2)-I(4) Hg(1)-N(3) N(7)-Hg(2)-N(5) I(4)-Hg(2)-N(5) I(3)-Hg(2)-N(5)
Complex 1 2.1935(12) Zn(1)‚‚‚Cl(1A) 2.119(3) 101.20(8) 86.53(11)
N(3)-Zn(1)-Cl(2)
Complex 2 2.4166(9) Cd(1)-Cl(1A) 2.402(2) Cd(1A)-Cl(1A) 157.15(6) 117.40(3) 93.00(3)
N(3)-Cd(1)-Cl(2) N(3)-Cd(1)-Cl(1) N(1)-Cd(1)-Cl(1A)
3.568 107.28(9)
2.4779(8) 2.7373(9) 115.73(6) 83.11(6) 92.87(6)
Zn(1A)‚‚‚Cl(1) Cl(2)-Zn(1)-Cl(1)
Cd(1)-N(3) Cl(2)-Cd(1)-Cl(1) Cl(1)-Cd(1)-Cl(1A) Cd(1)-Cl(1A)-Cl(1A)
3.568 120.44(5)
2.282(2) 101.00(4) 87.00(3) 93.00(3)
Complex 3 2.289(3) Cd(1)-Br(1)
2.8812(7)
83.19(8) 100.24(2) 90.73(2)
115.38(9) 128.04(8) 116.56(2)
N(2)-Cd(1)-Br(1A) Br(1)-Cd(1)-Br(1A) Cd(1)-Br(1A)-Cd(1A)
Complex 4 2.6941(12) Cd(1)-I(2) 2.376(5) Cd(2)-I(3) 2.313(5)
2.7623(11) 2.7067(12)
Cd(2)-N(7) Cd(2)‚‚‚I(2)
2.269(5) 3.525
128.32(4) 115.86(12) 114.57(12) 95.09(4)
128.25(12) 106.26(13) 125.22(3) 163.20(12)
I(2)-Cd(1)-I(3) N(7)-Cd(2)-N(6) N(6)-Cd(2)-I(4) N(6)-Cd(2)-I(3)
90.13(4) 80.54(17) 95.61(13) 102.98(14)
2.373(4)
Hg(2)-N(3) Hg(2′′)‚‚‚Cl(1)
2.462(4) 3.333
117.41(12) 100.58(11) 79.19(15)
Cl(3)-Hg(2)-N(1) Cl(3)-Hg(2)-N(3) N(1)-Hg(2)-N(3)
107.35(10) 106.33(11) 78.45(14)
N(1)-Cd(1)-Br(2) N(1)-Cd(1)-Br(1A) Br(2)-Cd(2)-Br(1A)
N(4)-Cd(1)-I(1) N(4)-Cd(1)-I(2) I(1)-Cd(1)-I(2) N(5)-Cd(1)-I(3)
Complex 5 2.3560(15) Hg(2)-N(1) 2.3689(17) Hg(1)‚‚‚Cl(3′′) 2.511(4) 141.67(6) 108.85(11) 92.89(11)
Cl(2)-Hg(1)-N(7) Cl(2)-Hg(1)-N(6) N(7)-Hg(1)-N(6)
Complex 6 2.5255(8) Hg(2)-N(5) 2.4883(9) Hg(1)‚‚‚Br(3′′) 2.513(5) 118.63(15) 100.26(13) 136.22(3)
N(5)-Hg(2)-Br(3) N(5)-Hg(2)-Br(4) Br(3)-Hg(2)-Br(4)
Complex 7 2.6518(8) Hg(2)-N(7) 2.6437(7) Hg(1)‚‚‚I(3′′) 2.461(6) 77.7(2) 98.28(14) 101.75(14)
Synthesis of Zn2LCl4 (1). A methanol solution of L (20 mL, 0.0417 g, 0.100 mmol) was slowly added to a methanol solution of ZnCl2 (20 mL, 0.0136 g, 0.100 mmol). The mixture was stirred at room temperature for 8 h in a 100 mL flask, concentrated in vacuo, filtered, and left in the dark for slow diffusion of ether vapor. Crystals suitable for X-ray structure analysis were obtained several days later. Yield: 0.0190 g (55%). Anal. Calcd for C25H19Cl4N7Zn2: C, 43.20; H, 2.775; N, 14.21. Found: C, 43.19; H, 2.801; N, 14.23. Synthesis of {[Cd2L(µ-Cl)2]Cl2}n‚nH2O (2). Compound 2 was prepared by a procedure similar to that for 1 using CdCl2‚2.5H2O (0.0224 g, 0.100 mmol) in place of ZnCl2. Colorless prismatic crystals of 2 suitable for X-ray structure determination were obtained. Yield: 0.0205 g (51%). Anal. Calcd for C25H21Cl4Cd2N7O: C, 37.43; H, 2.639; N, 12.22. Found: C, 37.44; H, 2.611; N, 12.30. Synthesis of {[Cd2L(µ-Br)2]Br2}n‚nH2O (3). Compound 3 was prepared by a procedure similar to that for 1 using CdBr2 (0.0272 g, 0.100 mmol) in place of ZnCl2. Colorless prismatic crystals of 3 suitable for X-ray structure determination were obtained. Yield: 0.0196 g (40%).
I(1)-Hg(1)-N(1) I(2)-Hg(1)-N(1) I(1)-Hg(1)-I(2)
2.385(5) 3.263 107.61(12) 108.88(13) 140.90(3) 2.429(6) 3.995 108.87(13) 106.30(13) 140.82(3)
Cd(1)-Br(2)
Hg(2)-N(7) Hg(2′′)‚‚‚Br(2) N(3)-Hg(1)-N(1) N(1)-Hg(1)-Br(2) N(3)-Hg(1)-Br(2) Hg(2)-N(5) Hg(2′′)‚‚‚I(1) I(4)-Hg(2)-N(7) I(3)-Hg(2)-N(7) I(4)-Hg(2)-I(3)
2.5272(7) 157.26(8) 89.27(2) 90.73(2)
2.454(6) 3.546 79.11(19) 103.82(15) 97.58(13) 2.514(6) 3.571 118.56(16) 101.16(15) 138.46(2)
Anal. Calcd for C25H21Br4Cd2N7O: C, 30.64; H, 2.160; N, 10.01. Found: C, 30.72; H, 2.147; N, 10.05. Synthesis of {[Cd2L(µ-I)(µ‚‚‚I)]I2}n (4). A methanol solution of L (30 mL, 0.0417 g, 0.100 mmol) was slowly added to a methanol solution of CdI2 (20 mL, 0.0336 g, 0.100 mmol). The mixture was stirred at room temperature for 10 h in a 100 mL flask, concentrated, filtered, and left in the dark. Colorless blocklike crystals of 4 suitable for X-ray structure determination were obtained 2 weeks later. Yield: 0.0362 g (63%). Anal. Calcd for C25H19I4Cd2N7: C, 26.11; H, 1.665; N, 8.527. Found: C, 26.09; H, 1.664; N, 8.521. Synthesis of Hg2LCl4 (5). Compound 5 was prepared by a procedure similar to that for 4 using HgCl2 (0.0272 g, 0.1 mmol) instead of CdI2. Straw yellow blocklike crystals of 5 suitable for X-ray structure determination were obtained. Yield: 0.0211 g (44%). Anal. Calcd for C25H19Cl4Hg2N7: C, 31.26; H, 1.994; N, 10.21. Found: C, 31.21; H, 1.990; N, 10.30. Synthesis of Hg2LBr4 (6). Compound 6 was prepared by a procedure similar to that for 4 using HgBr2 (0.0360 g, 0.100 mmol) instead of
1344 Crystal Growth & Design, Vol. 6, No. 6, 2006 Scheme 1. Preparation of the Ligand
CdI2. Straw yellow blocklike crystals of 6 suitable for X-ray structure determination were obtained. Yield: 0.0319 g (56%). Anal. Calcd for C25H19Br4Hg2N7: C, 26.38; H, 1.682; N, 8.614. Found: C, 26.40; H, 1.673; N, 8.602. Synthesis of Hg2LI4 (7). Compound 7 was prepared by a procedure similar to that for 4 using HgI2 (0.0454 g, 0.100 mmol) instead of CdI2. Straw yellow blocklike crystals of 7 suitable for X-ray structure determination were obtained. Yield: 0.0398 g (60%). Anal. Calcd for C25H19Hg2I4N7: C, 22.64; H, 1.444; N, 7.393. Found: C, 22.63; H, 1.448; N, 7.393. X-ray Crystallography. X-ray diffraction data were collected on a Bruker-AXS SMART CCD area detector diffractometer at 293 K using ω rotation scans with a scan width of 0.3° and Mo KR radiation (λ ) 0.710 73 Å). The structures were solved by direct methods and refined with full-matrix least squares techniques using SHELXTL.12 Anisotropic thermal parameters were applied to all non-hydrogen atoms. All of the hydrogen atoms in these structures are located from the differential electron density map and constrained to ideal positions in the refinement procedure. The crystallographic calculations were conducted using the SHELXL-97 programs. Crystal data and experimental details for the crystals of 1-7 are summarized in Table 1, and selected bond distances and bond angles are given in Table 2.
Results and Discussion Syntheses and Characterization. The ligand L was prepared in moderate yield using the Ullmann condensation method, where copper(II) and K2CO3 were used as the catalyst and base, respectively (Scheme 1). Generally, the significant factors in the Ullmann reaction are the reaction time and temperature. In this reaction, we select 200 °C and 8 h as the optimized conditions.
Wei et al.
The ligand reacts readily with a variety of Zn(II), Cd(II), and Hg(II) salts (Scheme 2) to form complexes 1-7, which have been characterized by elemental and single-crystal X-ray diffraction analyses. Although reactions of MX2 with the ligand L in a molar ratio of 1:1 were carried out under various conditions, only the 2:1 M:L products were obtained. Crystal Structures. The solid-state structure of compound 1 is shown in Figure 1. Two Zn(II) ions are related by an inversion center and bridged by a L ligand. Each Zn(II) is additionally coordinated by two terminal chlorides, resulting in a severely distorted tetrahedral environment, with the angles around the Zn center ranging from 86.53 to 122.09°. The ZnN(1), Zn-N(3), Zn-Cl(1), and Zn-Cl(2) bond lengths are 2.047(3), 2.119(3), 2.205(10), and 2.1936(12) Å, respectively, within the normal range of Zn-N and Zn-Cl lengths in similar complexes.9,13 Interestingly, the adjacent [Zn2LCl4] monomers are connected through two Zn‚‚‚Cl bridging interactions, which extend the monomers into 1-D pseudo-helix chains. However, the Zn‚‚‚Cl bond distance is 3.568 Å, indicating a very weak interaction. The central nitrogen atom N(2) is coplanar with the three carbon atoms to which it is bonded. (the sum of bond angles around N(2) is 360°). Similar results are also observed for compounds 2-7 and in previous reports.8b-g The pyridyl ring containing N(4) is nearly coplanar with the central NC3 plane with a dihedral angle of 1.9°, while the remaining two pyridyl rings have dihedral angles of 66.8 and 55.4°, respectively, with the NC3 unit. In compound 1, there are some face-to-face π-stacking interactions between the pyridyl rings of the ligands. The centroid-centroid distance between the parallel intramolecular pyridyl rings is 3.97 Å, which is above the optimal distance for π-stacking14 (The corresponding dihedral angles and centroid-centroid distances between the intramolecular pyridyl rings for 2-7 are given in the Supporting Information.). Meanwhile, the N(3) pyridyl rings are involved in a weak faceto-face π-stacking interaction with its symmetry-related equiva-
Scheme 2. Summary of the Reactions between L and Transition-Metal Salts
M(II) Polymers Containing M-X2-M Bridges
Crystal Growth & Design, Vol. 6, No. 6, 2006 1345
Figure 1. ORTEP diagram showing the structure of compound 1 with thermal ellipsoids at the 30% probability level and the atom-labeling scheme. Hydrogen atoms have been omitted for clarity in this and in following figures.
Figure 2. 2D network viewed along the ac plane in compound 1, showing π-π stacking interactions between the pyridine rings and weak Cl‚‚‚Zn interactions: Zn‚‚‚Cl ) 3.568 Å.
lents, with a centroid-centroid distance of 3.98 Å and an interplane angle of 0°. The interaction is inclined to be optimal because more than half their widths of the rings are overlapped. Finally, 2-D interlaced networks are arranged in the ac plane through dual interactions of the face-to-face π-π stacking and weak Zn‚‚‚Cl interactions (Figure 2). The structures of compounds 2 and 3 are shown in Figure 3. They also possess crystallographically imposed inversion centers. Although the metal ion is different from that of 1, chelation by the L ligands in a similar manner was observed. Unlike the case for complex 1, the anions in 2 and 3 display strong bridging, resulting in 1-D pseudo-helix chains (Figure 4). The coordination sphere around each Cd center is composed of two N atoms of the dipyridylamino group and three X- (X ) Cl, Br) anions. The Cd-N bond lengths for 2 (Cd(1)-N(1) ) 2.402(2) Å; Cd(1)-N(3) ) 2.282(2) Å) are appreciably smaller than the upper limit for the covalent Cd-N distance (2.54(2) Å)15 and are close to those in complex 3 (Cd(1)-N(2) ) 2.399(3) Å; Cd(1)-N(1) ) 2.289(3) Å). Similar Cd-N bond lengths have been previously observed in the cadmium complexes of other nitrogen-containing ligands.15,16 Among the three Xanions, two of them are bridging, whereas the third one is terminal. The bridging Cd(1)-X(1) bond distances are some-
Figure 3. Coordination units of complexes 2 and 3 (X ) Cl (2), Br (3)) showing the atomic numbering scheme with thermal ellipsoids at the 30% probability level. Water molecules have been omitted for clarity. Labeling schemes are as follows. 2: XA, Cl2A; XB, Cl1A; XC, Cl1A; XD, Cl2A; XE, Cl2; XF, Cl1A; XG, Cl1; XH, Cl2A; NA, N1A; NB, N3A; NC, N3; ND, N1; NE, N1A; NF, N3A; CdA, Cd1A; CdB, Cd1A; CdC, Cd1; CdD, Cd1A. 3: XA, Br2; XB, Br1; XC, Br1A; XD, Br2A; XE, Br2A; XF, Br1A; XG, Br1A; XH, Br2A; NA, N2; NB, N1; NC, N1A; ND, N2A; NE, N2A; NF, N1A; CdA, Cd1A; CdB, Cd1; CdC, Cd1A; CdD, Cd1A.
what longer (2.7373(9) and 2.8812(7) Å, for 2 and 3, respectively) but are significantly shorter than the sum of the van der
1346 Crystal Growth & Design, Vol. 6, No. 6, 2006
Wei et al.
Figure 4. (a) Space-filling model of 2 and 3 (X ) Cl (2), Br (3)): blue, N; gray, C; orange, Cd; green, X. H atoms and water molecules have been omitted for clarity. (b) Schematic presentation of pseudohelix chains showing ...P,M,P,M... helicities: orange ball, Cd; green bar, X (Cl, Br); dashed and solid black lines, ligand (L).
Figure 6. ORTEP drawing of complex 4 showing the atomic numbering scheme with thermal ellipsoids at the 30% probability level. Table 3. Hydrogen Bonds for 2 and 3 (Å and deg)
Figure 5. 2D network viewed along the ac plane in compounds 2 and 3 (X ) Cl (2), Br (3)) showing the π-π stacking interactions between the pyridine ring and X‚‚‚Hwater hydrogen-bond interactions: red ball, Owater. Hwater atoms have been omitted for clarity.
Waals radii for Cd and X (3.33 and 3.43 Å, for 2 and 3, respectively).17 Thus, the final coordination geometry around the metal center is irregular, with bond angles ranging from 78.72 to 126.88° for 2 and from 78.41 to 128.04° for 3, respectively. Analogously, these 1-D pseudo-helix chains arrange in the ac plane to form a wavy 2-D network by intermolecular faceto-face π-π stacking interactions between the lateral pyridyl rings of the dipyridylamino group with the same centroidcentroid distance of 3.95 Å for 2 and 3 (Figure 5). Interestingly, the water molecules are located between the helical chains. Furthermore, the H(water)‚‚‚X distances (2.591 and 2.633 Å for 2 and 3, respectively) are significantly shorter than the sum of the van der Waals radii for H and X (ca. 1.2 Å for H, 1.75 Å for Cl, and 1.85 Å for Br),17 which indicates a typical intermolecular hydrogen bond between water molecules and the terminal X- anion (Table 3). Therefore, the guest molecule is present not only to fill the void space in the lattice but also to contribute to the packing of the complexes. The water molecules, in compound 2, do not escape from the crystal lattice at ambient temperature, as evidenced by the stability of the crystals over
D-H‚‚‚A
d(D-H)
d(H‚‚‚A)
d(D‚‚‚A)
∠(DHA)
O-H‚‚‚Cl O-H‚‚‚Br
0.850 0.820
2.591 2.633
3.238 3.369
133.70 150.15
several weeks at ambient temperature, which is attributed to their complete encapsulation by molecules of 2. However, in compound 3, it is found that the water molecules could obviously escape from the crystal lattice (see Thermogravimetric Analysis), potentially due to the weaker intermolecular hydrogen bond in the frameworks. Two-dimensional layers in 2 and 3 stack in an ...ABAB... sequence, and similar stacking fashions have been observed by others.18 Although the anion is different in compound 4, chelation by the L ligand in the same manner as in 2 and 3 was observed and the four Cd-N bond lengths are comparable to those of 2 and 3, ranging from 2.269 to 2.394 Å. However, unlike the case for the Cl and Br anions, the I anions in 4 do not display the same bridging modes (Figure 6). Actually, two kinds of Cd(II) centers coexist in compound 4: one is four-coordinate with a distorted-tetrahedral geometry, and the other is five-coordinate with an irregular geometry. The coordination sphere around the five-coordinate Cd(1) cation is composed of two N atoms and three I- anions, similar to the case for 2 and 3. The bond lengths of Cd(1)-I(1) and Cd(1)-I(2) are within the typical Cd-I bond lengths previously reported.19 The I(3)-Cd(1) bond distance is somewhat longer (3.217 Å) but is significantly shorter than the corresponding sum of the van der Waals radii for Cd and I (3.56 Å),17 which indicates a weak coordination. The coordination sphere around the four-coordinate Cd(2) cations contains only two I- anions, similar to the case for compound 1. However, the distance Cd(2)‚‚‚I(2) is slightly shorter (3.525 Å) than the sum of the van der Waals radii, which also indicates a very weak interaction. As a result, compound 4 forms an interesting
M(II) Polymers Containing M-X2-M Bridges
Crystal Growth & Design, Vol. 6, No. 6, 2006 1347
Figure 7. (a) Two kinds of space-filling models in compound 4: blue, N; gray, C; orange, Cd; purple, I. (b) Schematic presentation of 1-D chains: orange ball, Cd; purple bar, I; dashed and solid black lines, ligand (L).
Figure 8. 2D network viewed along the ac plane in compound 4, showing the π-π stacking interactions between the pyridine ring, I-Cd coordination bonding, and weak I‚‚‚Cd interactions: blue, N; gray, C; orange, Cd; purple, I.
Figure 9. 2D network viewed along the ac plane in compounds 5-7, showing the π-π stacking interactions between the pyridine ring and weak X‚‚‚Hg interactions: blue, N; gray, C; yellow, Hg; green, X (X ) Cl (5), Br (6), I (7)).
“zigzag” 1-D coordination polymer, different from the case for compounds 2 and 3 (Figure 7), based on two different Ibridging interactions. Obviously, this structural diversity is generated by the nature of the anions. Similar to the case for 2 and 3, the laterally parallel pyridyl rings from adjacent chains are paired to furnish weak π-π stacking interactions with a centroid-centroid distance of 4.06 Å, which extend the 1-D chains into 2-D supramolecular networks along the ac plane (Figure 8). These 2-D layers are also stacked in an alternating ...ABAB... mode. By using an Hg(II) ion as the coordination center, the dinuclear complexes 5-7 were obtained. The crystal structures of compounds 5-7 are essentially identical, with similar unit cell parameters (see the Supporting Information). The L ligand in 5-7 also coordinates to two M(II) metal centers in a manner similar to that in 1. Moreover, the same coordination sphere of the Hg(II) ion with a severely distorted tetrahedral geometry was observed in 5-7, which is composed of two N atoms and two X- anions. In contrast to compounds 2-4, the corresponding MII-N bonds are slightly longer in compounds 5-7, which is attributed to the large radius of Hg(II), but all Hg-N lengths are well below the upper limit of a typical Hg-N covalent bond (2.75(2) Å).15 Analogously, the average HgII-X bond lengths
are 2.3743, 2.4815, and 2.6487 Å for 5-7, respectively, which are longer than those of CdII-X in 2-4 but are also within the typical Hg-X bond lengths previously reported.15,20 The supramolecular architectures also consist of a onedimensional “zigzag” chain based on [Hg2LX4] units via Hg‚ ‚‚X interactions similar to those in 4. With the exception of Hg(1)‚‚‚I(3′′), the distances of Hg‚‚‚Cl, Hg‚‚‚Br, and Hg(2′)‚‚ ‚I(1) are well below the sum of the van der Waals radii for mercury (rvdw )1.75 Å)21 and X (rvdw ) 1.75, 1.85, 1.98 Å for Cl, Br, and I, respectively).17 Furthermore, the connectivity between the one-dimensional “zigzag” chains is such that the pyridyl rings orient alternately up and down, thereby generating a two-dimensional network along the ac plane (Figure 9), different from that of complex 1. The centroid-centroid distances between the two neighboring pyridine rings are 3.91, 3.93, and 4.02 Å for compounds 5-7, respectively, which also indicate weak π-π stacking interactions. Herein, we report seven exceptional examples of 1-D f 2-D assemblies generated by face-to-face π‚‚‚π interactions between 1-D chains. The results indicated that the counteranions and bridging ligands are two greatly important factors in determining the structure and geometry of the self-assemblies. Generally, in the MX2 (M ) Zn(II), Cd(II), Hg(II); X ) Cl, Br, I)
1348 Crystal Growth & Design, Vol. 6, No. 6, 2006
Figure 10. Thermogravimetric analysis of compounds 2 and 3.
coordination system, the formation of M-X-M bridging is universal,20,22 but the formation of helical architectures by polyhalide bridges, to our knowledge, is rather rare. Furthermore, similar results have not been obtained from the rigid and linear bridging ligands N,N,N′,N′-tetrakis(2-pyridyl)-1,4-phenylenediamine and N,N,N′,N′-tetrakis(2-pyridyl)biphenyl-4,4′-diamine by the same procedure.23 These results reveal that properties of the bridging ligands, such as configuration, flexibility, π-conjugated spacers, and relative orientation of the donor groups, play important roles in controlling the structural topologies of their metal-organic supramolecular architectures. Furthermore, π‚‚‚π stacking interactions between pyridyl rings are responsible for the fact that complexes 1-7 have 2-D networks. Thermogravimetric Analysis. Thermogravimetric analysis of compounds 2 and 3 was performed by heating the complexes from 20 to 800 °C under N2. The TGA curve for complex 2 showed that the first major weight loss of the lattice water occurred between 60 and 110 °C by 1.85% weight (calcd 2.04%). The TGA curve for complex 3 showed that the first major lattice water molecule weight loss occurred between 60 and 90 °C by 0.91% (calcd 1.8%), which indicates that water molecules could easily escape from the crystal lattice of 3. As shown in Figure 10, further thermogravimetric data show a high stability up to about 320 °C for 2 and 304 °C for 3, respectively. These values are quite high as compared to those of the supramolecules based on related rigid and linear ligands.24 The complexes gradually decomposed at higher temperatures. Luminescent Properties. Complexes 1-7 and free ligand exhibit photoluminescence in the solid state at room temperature. The emission spectra of free ligand and complexes 1-4 are shown in Figure 11. The very strong emission of the free ligand with wavelength from 375 to 450 nm (λmax ) 397 nm) upon excitation at 270 nm is observed. When the ligand is coordinated to zinc and cadmium centers, blue and blue-green luminescence are observed. In contrast to the case for the free ligand, complex 1 in the solid state has a slightly broader emission band (bandwidth at half-height 110 nm) with a λmax value of 440 nm and is red-shifted by ∼40 nm. A similar red shift as a consequence of Zn(II) coordination are observed for Zn(II) tris(2-pyridyl)amine and 1,3,5-tris(p-(2,2′-dipyridylamino)phenyl)benzene complexes8g,9 where the Zn(II) ion is chelated by two 2-pyridyl groups in a fashion similar to that in 1. The emission spectra of 2 show a broad emission band centered at 433 nm and a medium-intensity emission band occurring at 465 nm.
Wei et al.
Figure 11. Emission spectra in the solid state of the free ligand L (O) and compounds 1 (0), 2 (4), 3 (s), and 4 (|) at room temperature.
Figure 12. Emission spectra in the solid state of compounds 5 (O), 6 (0), and 7 (s) at room temperature.
Compounds 3 and 4 in the solid state each have a very broad emission band (bandwidth at half-height 120-140 nm) with λmax at 445 and 490 nm, respectively. It is clear that a significant red shift of the emission occurs in 4, compared with compounds 2 and 3, which is probably due to the differences of anions and coordination environment around the central metal ions, because photoluminescent behavior is closely associated with the local environments around metal ions.25 Furthermore, in light of previous studies26 and the red shift in emission energy on going from the chloro or bromo to the iodo complex, this emission can be tentatively assigned to a ligand-to-metal charge-transfer (LMCT) transition, although the possibility of intraligand transition mixing with LMCT cannot be ruled out. Similar to the case for complex 2, complexes 5 and 6 (Figure 12) also show interesting dual emissions: a similar high-energy emission band centered at 430 nm and different low-energy emission bands occurring at 503 and 487 nm, respectively. Different from the cases for complexes 5 and 6, complex 7 only shows one emission band, with an emission maximum of 517 nm. Analogously, such a low-energy emission band also might come from ligand-to-metal charge transfer. Furthermore, the fluorescent intensity of compounds 2-7 is significantly weaker than those of the ligand and compound 1 in the solid state, which
M(II) Polymers Containing M-X2-M Bridges
can be attributed to the heavy-atom effect8b,27 due to the coordination of the ligand to a heavy M(II) center. Conclusions. In summary, seven novel 2-D supramolecular networks based on a pentadentate angular bridging ligand and MX2M bridges (M ) Zn, Cd, Hg, X ) Cl-, Br-, I-) have been demonstrated, whose structures depend greatly on the cooperative effects of the metals, anions, and ligands. These new networks have proper cavities, but water molecules were determined by crystallography to exist only in the networks of complexes 2 and 3. The complexes 1-7 all exhibit a blue or green luminescence at room temperature when excited by UV light. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China (No. 30270321). Y.-S.X. also wishes to acknowledge the Distinguished Team Fund of the National Science Foundation of China (NSFC, No. 20321101). Supporting Information Available: CIF files giving crystallographic data for complexes 1-7, tables giving dihedral angles and centroid-centroid distances between the intramolecular pyridyl rings for 1-7, and figures giving crystal structures of compounds 5-7 and crystal-packing diagrams for 2-4. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG0505822
