Synthesis, Structures, and Photoluminescence of Zinc(II), Cadmium(II

DOI: 10.1021/cg901182c

Synthesis, Structures, and Photoluminescence of Zinc(II), Cadmium(II), and Mercury(II) Coordination Polymers Constructed from Two Novel Tetrapyridyl Ligands

2010, Vol. 10 1611–1622

Fanhua Zeng,† Jia Ni,‡ Quanguo Wang,† Yubin Ding,† Seik Weng Ng,§ Weihong Zhu,† and Yongshu Xie*,† †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, P. R. China, ‡Center Lab, Shantou University, Shantou, 515063, P. R. China, and §Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Received September 25, 2009; Revised Manuscript Received January 30, 2010

ABSTRACT: Self-assembly of N,N,N0 ,N0 -tetra(4-pyridyl)-1,4-phenylenediamine (L1) and N,N-di(2-pyridyl)-N0 ,N0 -di(4-pyridyl)1,4-phenylenediamine (L2) with MX2 (M = Zn, Cd, and Hg; X = Cl, Br, and I) generated novel supramolecular structures with a rich structural diversity. In the crystal of L1, all pyridyl rings are involved in intermolecular π 3 3 3 π stacking and T-shaped C-H 3 3 3 π interactions to form a two-dimensional (2D) network. [CdL1Cl2]n 3 nH2O (4), [CdL1I2]n (6), [HgL1Cl2]n (7), [HgL1Br2]n (8), [Cd4(L2)4Cl8]n 3 2nDMF (13), [CdL2Br2]n (14), [CdL2I2]n (15), and [HgL2Cl2]n 3 0.5nDMF (16) are coordination polymers. In complex 4, L1 utilizes all its pyridyl nitrogens to coordinate with Cd(II) centers to afford an unprecedented three-dimensional (3D) binodal (3,4)-connected network with the Schl€ afli symbol of (8.102)2(84.102). Complexes 6, 7, 8, 14, and 15 have zigzag or centipedelike 1D coordination structures. For complex 13, each Cd has an octahedral coordination environment, and the Cd centers are linked by dichloro-bridges to form interesting infinite (CdCl2)¥ chains, which are further bridged by L2 ligands to form a 2D coordination network. Crystal of 16 has a one-dimensional (1D) chain structure. The chains are arranged in an alternate up-down-up mode, with all the pyridyl rings involved in π 3 3 3 π stacking interactions to afford a 3D structure which consists of hexagonal channels along the c-axis. [Hg2(L2)2Br4] 3 H2O (17) and [Hg2(L2)2I4] 3 H2O (18) have binuclear coordination moieties, which are linked with water molecules by hydrogen bonds to form 1D structures. From these results, it is demonstrated that the structures of the complexes and the coordination modes of L1 and L2 are strongly dependent on the metal cations and the anions. The Hg(II) atoms in these complexes have tetrahedral coordination environments, whereas the Cd(II) centers have octahedral coordination geometries when Cl- is used as the anion, affording 3D and 2D coordination networks. The photoluminescence of L1, L2, and the complexes measured in the solid state at room temperature indicated that the emission colors vary from violet to yellow, and the emission intensity varies to a large extent, which can be rationalized by the contribution of a conflicting coordination effect and the heavy atom effect.

*To whom correspondence should be addressed. E-mail: yshxie@ecust. edu.cn.

decades, many coordination compounds of 2,20 -dpa and its derivatives have been prepared for generating new supramolecular architectures.11 In more recent years, 4,40 -dipyridylamine (4,40 -dpa) has also been investigated as a bridging ligand to construct various amazing coordination assemblies with onedimensional (1D), two-dimensional (2D), or three-dimensional (3D) structures.12 Regarding the metal cations, closed-shell d10 metal ions, such as Zn(II), Cd(II), and Hg(II), have attracted intense attention not only because of well documented examples of their coordination polymers,13 arising from their coordination flexibility, but also because of the interesting luminescent properties14 exhibited by their complexes. With these backgrounds in mind, in this work we designed and synthesized two novel ligands L1 and L2 (Scheme 1) containing 2,20 -dpa and 4,40 -dpa as coordination moieties, and used them to obtain a series of novel complexes with MX2 salts (M = Zn(II), Cd(II), and Hg(II); X = Cl, Br, and I), in view of the following structural characteristics: (1) L1 has two 4,40 -dpa units connected by a 1,4-phenylene unit. Therefore, it may be possible to bridge four metal centers, affording novel structures totally different from those obtained from 4,40 -dpa, which can bridge only two metal centers; (2) L2 has one 4,40 dpa moiety and one 2,20 -dpa moiety connected by a 1,4phenylene unit. Thus, the chelating character of 2,20 - dpa

r 2010 American Chemical Society

Published on Web 02/19/2010

Introduction The significant contemporary interest in coordination polymers containing transition metal ions and organic ligands has rapidly developed in recent years, not only because of their interesting structural diversity but also because of their potential applications in optical,1 electronic,2 magnetic,3 molecular sensing,4 catalysis,5 and porous gas adsorption materials.6 One of the current interesting topics is to rationally design and synthesize coordination polymers and supramolecular assemblies by coordination bonds or noncovalent contacts such as metal-halogen bonds, metal-metal interactions, hydrogen bonding, and π-π interactions.7 The key steps in building coordination polymers are to use suitable organic ligands, metal cations, and counteranions. Regarding the organic ligands, both bridging and chelating pyridyl ligands have been intensely investigated in this respect due to the strong coordination ability and structural diversity.8,9 For example, 2,20 -bipyridine (2,20 -bpy) and 4,40 -bipyridine (4, 40 -bpy) are very well-known and widely used classical bidentate chelating and bridging ligands, respectively.10 Compared with 2,20 -bpy, 2,20 -dipyridylamine (2,20 -dpa) is a more flexible bidentate chelating ligand. In the past few

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Scheme 1. Structures of L1, L2, and Related Pyridyl Ligands

and the bridging character of 4,40 -dpa may be combined. Therefore, it is promising in chelating and bridging metal centers to afford novel coordination polymers and supramolecular assemblies; (3) Both L1 and L2 contain four pyridyl rings, which may be involved in π-π stacking interactions to form novel supramolecular assemblies. On the basis of these structural characteristics, attractive structures and luminescent properties can be expected from the Zn(II), Cd(II), and Hg(II) complexes of L1 and L2. Thus, coordination of L1 and L2 with d10 metal salts afforded 18 coordination compounds. Ten of the complexes, in addition to L1, have been characterized by X-ray diffraction analyses, which exhibit a rich variety of novel structures, including centipede-like 1D chains, 2D network containing unprecedented infinite (CdCl2)¥ chains, and a 3D structure consisting of hexagonal channels, which are composed of pyridyl columns formed by π 3 3 3 π stacking interactions between 1D coordination chains. The effects of metal ions and counteranions on the coordination modes of ligands and the coordination geometries are also discussed in detail. Furthermore, the photoluminescence of the complexes has also been measured in the solid state, which indicates that the emission intensity and wavelength can be well tuned by both the cations and the anions. Experimental Section Materials. All chemicals were of reagent grade quality and were used as received from commercial sources. Physical Measurements. 1H NMR spectra were recorded on a Bruker AVANCE spectrometer (400 MHz). FT-IR spectra were recorded in the region of 400-4000 cm-1 on a Thermo Electron Avatar 380 FT-IR instrument (KBr Discs). Elemental analyses were carried out with an Elmentar Vario EL-III analyzer. Fluorescence measurements were made on a Varian Cary Eclipse fluorescence spectrophotometer. Synthesis. L1. 4,40 -Dipyridylamine12 (4.28 g, 25 mmol), 1,4dibromobenzene (2.36 g, 10 mmol), anhydrous potassium carbonate (4.8 g, 34.4 mmol), cupric sulfate (994 mg, 6.2 mmol), 18crown-6 (220 mg, 0.83 mmol), and diphenyl ether (35 mL) were added to a three-necked flask and heated at 200 C under N2 for 3 days, then additional 1,4-dibromobenzene (7 g, 29.7 mmol) was added. The reaction mixture was stirred at 200 C for another day. After the reaction was cooled, dichloromethane and water were added to dissolve the solid, and the organic phase was washed with distilled water to neutral pH and then dried with Na2SO4. After removal of the solvent, the residue was purified by a silica gel column to afford colorless solids of the following two compounds: (4-bromophenyl)-di(4-pyridyl)amine (2.92 g, yield 36% based on 4,40 -dipyridylamine). 1H NMR (CDCl3, 400 MHz): δ=8.45 (d, J= 8.0 Hz, 4H, pyridyl-H), 7.56 (d, J = 11 Hz, 2H, phenylene-H), 7.08 (d, J=11 Hz, 2H, phenylene-H), 6.96 (d, J=8.0 Hz, 4H, pyridyl-H); N,N,N0 ,N0 -tetra(4-pyridyl)-1,4-phenylenediamine (L1; 2.1 g, 40.8% based on 4,40 -dipyridylamine). 1H NMR (CDCl3, 500 MHz): δ = 8.49 (d, J = 6.0 Hz, 8H, pyridyl-H), 7.23 (s, 4H, phenylene-H), 7.01 (d, J = 6.0 Hz, 8H, pyridyl-H). Anal. Calcd (%) for C26H20N6: C,

74.98; H, 4.84; N, 20.18. Found: C, 74.57; H, 4.71; N, 20.06. IR (KBr pellet, cm-1): 1592(s), 1577 (vs), 1498(s), 1488(s), 130(m), 1304(s), 1277(vs), 1217(m), 993(m), 813(m), 740(w), 624(m), 542(m). L2. A mixture of 2,20 -dipyridylamine (5.27 g, 30.76 mmol), (4-bromophenyl)-di(4-pyridyl)amine (2.50 g, 7.69 mmol), anhydrous potassium carbonate (2.66 g, 19.23 mmol), bronze powder (4.93 g, 77.64 mmol), 18-crown-6 (200 mg, 0.76 mmol), and DMF (125 mL) was heated at 145 C under N2 for 30 h. The reaction was cooled and worked up by a procedure similar to that for L1. A yellowish solid was collected from a silica gel column, and was further purified by recrystallization from dichloromethane and petroleum ether, to afford a white solid of N,N-di(2-pyridyl)-N0 , N0 -di(4-pyridyl)-1,4-phenylenediamine (L2, 1.32 g, yield: 41%). 1H NMR (CDCl3, Bruker 400 MHz): 8.43 (d, J = 4.4 Hz, 4H, pyridylH), 8.37 (d, J = 3.6 Hz, 2H, pyridyl-H), 7.60-7.65 (m, 2H, pyridylH), 7.21 (d, J = 8.8 Hz, 2H, phenylene-H), 7.14 (d, J = 8.8 Hz, 2H, phenylene-H), 6.98-7.09 (m, 8H, pyridyl-H). Anal. Calcd (%) for C26H20N6: C, 74.98; H, 4.84; N, 20.18. Found: C, 74.53; H, 4.72; N, 19.89. IR (KBr pellet, cm-1): 1575(vs), 1505(s), 1490(s), 1467(s), 1431(vs), 1324(s), 1305.13(s), 1276(s), 1218(m), 1163(w), 1103(w), 991(m), 811(m), 777(m), 737(w), 654(w), 622(m), 530(m). Zn2L1Cl4 (1). A mixture of L1 (20.8 mg, 0.05 mmol) and ZnCl2 (14 mg, 0.1 mmol) in MeOH (50 mL) was refluxed for 3 h. After cooling to room temperature, the yellowish microcrystalline precipitate was collected, washed with MeOH, and dried. Yield: 14 mg, 42%. Anal. (%) calcd. for C26H20Cl4N6Zn2: C, 45.32; H, 2.93; N, 12.20. Found: C, 45.39; H, 3.06; N, 12.09. IR (KBr pellet, cm-1): 3424(br), 2921(w), 1613(s), 1583(s), 1498(s), 1367(w), 1351(m), 1258(w), 1221(m), 1066(s), 1021(vs), 819(w), 646(w), 545(m), 466(m). Zn2L1Br4 3 MeOH (2). Compound 2 was prepared by a procedure similar to that for 1, using ZnBr2 (23 mg, 0.1 mmol) in place of ZnCl2. A yellowish microcrystalline product was obtained. Yield: 22 mg, 49%. Anal. (%) Calcd. for C27H24Br4N6OZn2: C, 36.08; H, 2.69; N, 9.35. Found: C, 35.86; H, 2.77; N, 9.05. IR (KBr pellet, cm-1): 3423(br), 3047(w), 1616(s), 1582(vs), 1545(w), 1498(vs), 1448(w), 1346(m), 1317(m), 1216(w), 1068(m), 1020(m), 825(m), 635(m), 547(m), 472(w). Zn2L1I4 (3). Compound 3 was prepared by a procedure similar to that for 1, using ZnI2 (32 mg, 0.1 mmol) in place of ZnCl2. Yellowish microcrystalline solid was obtained. Yield: 25 mg, 34%. Anal. (%) calcd. for C26H20I4N6Zn2: C, 29.60; H, 1.91; N, 7.97. Found: C, 29.54; H, 2.16; N, 7.86. IR (KBr pellet, cm-1): 3445(br), 1616(s), 1597(vs), 1560(w), 1545(w), 1497(vs), 1439(w), 1349(m), 1337(m), 1311(m), 1284(m), 1213(s), 1061(m), 1025(s), 823(m), 734(w), 646(m), 539(m), 458(w). [CdL1Cl2]n 3 nH2O (4). A mixture of L1 (4.2 mg, 0.01 mmol), Cd(NO3)2 3 4H2O (6.2 mg, 0.02 mmol), and NaCl (2.4 mg, 0.04 mmol) in DMSO, DMF, H2O, and MeOH (8: 8: 16: 1) was sealed in a small vial, which was heated to 120 C for 90 h. After cooling to room temperature, colorless block crystals of 4 suitable for X-ray structure determination were obtained. Yield: 4 mg, 65%. Anal. (%) calcd. for C26H22CdCl2N6O: C, 50.55; H, 3.59; N, 13.60%. Found: C, 50.49; H, 4.03; N, 13.59. IR (KBr pellet, cm-1): 3427(br, H2O), 3048(w), 2970(w), 2924(w), 2355(w), 1673(m), 1605(m), 1586(s), 1562(w), 1548(w), 1493(s), 1338(w), 1308(m), 1280(m), 1215(m), 1089(w), 1049(w), 1006(m), 935(w), 824(w), 793(w), 702(w), 634(m), 546(m). [CdL1Br2] 3 7.5H2O (5). Compound 5 was prepared by a procedure similar to that for 1, using KBr (24 mg, 0.2 mmol) and Cd(NO3)2 3 4H2O (31 mg, 0.1 mmol) in place of ZnCl2. A yellowish

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Table 1. Crystallographic Data and Structure Refinements Summary for L1 and the Metal Complexes compounds

L1

4

6

7

8

empirical formula fw crystal System space group T (K) a (A˚) b (A˚) c (A˚) β (deg) V (A˚ 3) Z Dcalcd (Mg/m3) μ (mm-1) no. of reflns (I > 2σ(I)) final R1a [I > 2σ(I)] wR2b (all data) goodness of fit

C26H20N6 416.48 monoclinic P21/c 293(2) 5.8975(6) 14.1068(15) 12.6935 96.270(2) 1049.72(19) 2 1.318 0.082 1344 0.0420 0.1171 1.046

C26H22CdCl2N6O 617.8 monoclinic C2/c 273 (2) 21.043(1) 9.0757(4) 17.1751(8) 116.037(1) 2947.2(2) 4 3.872 7.234 2552 0.0439 0.1403 1.065

C26H20CdI2N6 782.68 monoclinic C2/c 100(2) 16.7908(8) 7.1429(4) 22.1775(11) 94.599(1) 2651.3(2) 4 1.961 3.179 2882 0.0201 0.0554 1.078

C26H20Cl2HgN6 687.97 monoclinic C2/c 273(2) 16.828(3) 7.0758(14) 21.274(4) 91.943(2) 2531.8(8) 4 1.805 6.317 2441 0.0370 0.0907 0.987

C26H20Br2HgN6 776.89 monoclinic C2/c 293(2) 16.884(1) 7.1365(5) 21.510(1) 92.446(1) 2589.4(3) 4 1.993 9.057 2008 0.0258 0.0611 1.068

complexes

13

14

15

16

17

18

empirical formula fw crystal system space group T (K) a (A˚) b (A˚) c (A˚) R β (deg) γ V (A˚3) Z Dcalcd (Mg/m3) μ (mm-1) no. of reflns (I > 2σ(I)) final R1a [I > 2σ(I)] wR2b (all data) goodness of fit

C110H94Cd4Cl8N26O2 2545.31 triclinic P1 293(2) 14.2164(2) 17.2630(2) 24.4238(3) 109.2510(10) 98.1200(10) 91.8650(10) 5582.02(12) 2 1.514 1.005 16926 0.1145 0.2890 1.133

C26H20Br2CdN6 688.7 triclinic P1 100(2) 9.3576(2) 12.1165(2) 13.7787(2) 112.2070(10) 105.395(1) 108.3550(10) 1234.39(4) 2 1.853 4.149 4947 0.0239 0.0664 1.078

C26H20I2CdN6 782.68 triclinic P1 100(2) 9.3358(1) 12.2736(2) 12.9081(2) 108.7510(10) 97.874(1) 108.0860(10) 1284.47(3) 2 2.024 3.281 5377 0.0227 0.0611 1.092

C55H47Cl4Hg2N13O 1449.04 monoclinic C2/c 298(2) 11.0741(12) 29.584(3) 8.8100(10) 90.00 94.166(1) 90.00 2878.7(5) 2 1.672 5.562 2037 0.0831 0.2290 1.109

C52H42Br4Hg2N12O 1571.77 monoclinic P2/c 100(2) 12.3973(3) 8.7623(2) 25.0546(5) 90.00 113.070(1) 90.00 2503.99(10) 2 2.083 9.368 4423 0.0764 0.2440 1.702

C52H42Hg2I4N12O 1759.76 monoclinic P2/n 273(2) 12.6460(9) 9.1140(6) 23.362(2) 90.00 94.459(1) 90.00 2684.4(3) 2 2.177 8.062 3676 0.0336 0.0811 1.032

a

R = Σ||Fo| - |Fc||/Σ|Fo|. b Rw = Σ||Fo| - |Fc||w1/2/Σ|Fo|w1/2.

microcrystalline product was obtained. Yield: 25 mg, 64%. Anal. (%) calcd. for C26H35Br2CdN6O7.5: C, 37.91; H, 4.28; N, 10.20%. Found: C, 38.13; H, 4.63; N, 9.74. IR (KBr pellet, cm-1): 3426(m, br), 3038(w), 2922(w), 1607(s), 1583(vs), 1494(vs), 1434(w), 1384(vs), 1342(s), 1312(s), 1279(m), 1218(s), 1013(s), 944(w), 819(m), 735(w), 703(w), 635(m), 541(m), 473(w). [CdL1I2]n (6). Compound 6 was prepared by a procedure similar to that for 4, using KI (6.6 mg, 0.04 mmol) in place of NaCl. Colorless block crystals of 6 suitable for X-ray structure determination were obtained. Yield: 4 mg, 54%. Anal. (%) calcd. for C26H20CdI2N6: C, 39.90; H, 2.58; N, 10.74. Found: C, 40.16; H, 2.72; N, 10.67. IR (KBr pellet, cm-1): 3423(m, br), 3050(w), 2969.49(w), 1610(vs), 1581(vs), 1564(s), 1544(m), 1496(vs), 1444(m), 1411(w), 1345(vs), 1274(w), 1255(w), 1226(s), 1104(w), 1067(m), 1013(s), 993(w), 943(w), 835(m), 823(vs), 740(w), 704(w), 634(vs), 544(s), 438(w). [HgL1Cl2]n (7). A mixture of L1 (4.16 mg, 0.01 mmol) and HgCl2 (5.4 mg, 0.02 mmol) in DMSO, DMF, and H2O (70: 70: 10) was refluxed for 3 h in air, then transferred in a small vial Teflon stainless container, which was heated to 120 C for 90 h. After cooling to room temperature, colorless block crystals of 7 suitable for X-ray structure determination were obtained. Yield: 5 mg, 72%. Anal. (%) calcd. for C26H20Cl2HgN6: C, 45.39; H, 2.93; N, 12.22. Found: C, 45.42; H, 3.28; N, 12.31. IR (KBr pellet, cm-1): 3440(br), 1605(s), 1580(vs), 1546(w), 1493(s), 1439(w), 1343(s), 1316(m), 1275(w), 1217(m), 1065(w), 1006(m), 834(w), 823(m), 702(w), 631(m), 542(m). [HgL1Br2]n (8). Compound 8 was prepared by a procedure similar to that for 4, using KBr (6.6 mg, 0.04 mmol) and Hg(NO3)2 3 1/2H2O (6.7 mg, 0.02 mmol) in place of NaCl and Cd(NO3)2 3 4H2O, respectively. Colorless block crystals of 8 suitable for X-ray structure determination were obtained. Yield: 4 mg, 52%. Anal. (%) calcd. for C26H20Br2HgN6: C, 40.20; H, 2.59; N, 10.82. Found: C,

40.12; H, 2.71; N, 10.81. IR (KBr pellet, cm-1): 3417(s), 1607(m), 1580(vs), 1505(m), 1492(m), 1439(w), 1399(m), 1384(m), 1343(m), 1316(w), 1225(w), 1145(w), 1066(w), 1005(s), 993(s), 834(w), 822(m), 702(w), 631(m), 543(m). [HgL1I2]n (9). Compound 9 was prepared by a procedure similar to that for 1, using HgI2 (45.5 mg, 0.1 mmol) in place of ZnCl2. Yellowish microcrystalline solid was obtained. Yield: 26 mg, 60%. Anal. (%) calcd. for C26H20I2HgN6: C, 35.86; H, 2.31; N, 9.65. Found: C, 35.76; H, 2.34; N, 9.57. IR (KBr pellet, cm-1): 3441(br), 3052(w), 1605(s), 1580.08(vs), 1545(m), 1492(vs), 1439(w), 1341(s), 1315(s), 1282(w), 1223(m), 1105(w), 1006(s), 834(m), 821(m), 739(w), 702(w), 632(m), 542(m). ZnL2Cl2 (10). A mixture of L2 (20.8 mg, 0.05 mmol), ZnCl2 (14 mg, 0.1 mmol) in MeOH (50 mL) was refluxed for 3 h in air. After slow cooling to the room temperature, a yellowish microcrystalline solid was obtained. Yield: 15 mg, 54%. Anal. (%) calcd. for C26H20Cl2N6Zn: C, 56.49; H, 3.65; N, 15.20. Found: C, 56.27; H, 3.75; N, 15.10. IR (KBr pellet, cm-1): 3440(br), 1598(vs), 1497(s), 1466(m), 1430(s), 1341(m), 1217(m), 1064(w), 1024(m), 825(w), 772(w), 740(w), 655(w), 537(w). ZnL2Br2 (11). Compound 11 was prepared by a procedure similar to that for 10, using ZnBr2 (23 mg, 0.1 mmol) in place of ZnCl2. A yellowish microcrystalline solid was obtained. Yield: 16 mg, 50%. Anal. (%) calcd. for C26H20Br2N6Zn: C, 48.67; H, 3.14; N, 13.10. Found: C, 48.44; H, 3.57; N, 13.06. IR (KBr pellet, cm-1): 3441(br), 1622(m), 1599(vs), 1496(s), 1464(m), 1430(s), 1340(m), 1216(m), 1065(w), 1025(m), 825(w), 775(w), 739(w), 654(w), 538(w), 469(w). ZnL2I2 (12). Compound 12 was prepared by a procedure similar to that for 10, using ZnI2 (32 mg, 0.1 mmol) in place of ZnCl2. Yellowish microcrystalline solid was obtained. Yield: 21 mg, 56%. Anal. (%) calcd. for C26H20I2ZnN6: C, 42.45; H, 2.74; N, 11.42. Found: C, 42.16; H, 3.19; N, 11.37. IR (KBr pellet, cm-1): 3445(br),

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Table 2. Selected Bond Lengths (A˚) and Angles () for the Metal Complexes Complex 4 Cd(1)-N(3)#2 2.434(3) Cd(1)-Cl(1) 2.5406(8) N(3)#2-Cd(1)-N(1) 95.02(12) N(1)-Cd(1)-N(1)#4 180.00(17) N(1)#4-Cd(1)-Cl(1) 90.73(8) N(1)#4-Cd(1)-Cl(1)#4 89.27(8)

Cd(1)-N(3)#3 2.434(3) Cd(1)-Cl(1)#4 2.5406(8) N(3)#3-Cd(1)-N(1) 84.98(12) N(3)#2-Cd(1)-Cl(1) 88.69(8) N(3)#2-Cd(1)-Cl(1)#4 91.31(8) Cl(1)-Cd(1)-Cl(1)#4 180.00(4)

Cd(1)-N(1) 2.460(3) Cd(1)-N(1)#4 2.460(3) N(3)-Cd(1)#5 2.434(3) N(3)#2-Cd(1)-N(3)#3 180.00(17) N(3)#2-Cd(1)-N(1)#4 84.98(12) N(3)#3-Cd(1)-N(1)#4 95.02(12) N(3)#3-Cd(1)-Cl(1) 91.31(8) N(1)-Cd(1)-Cl(1) 89.27(8) N(3)#3-Cd(1)-Cl(1)#4 88.69(8) N(1)-Cd(1)-Cl(1)#4 90.73(8)

Complex 6 2.7048(2) I(1)-Cd(1) 2.7048(2) Cd(1)-N(1)#1 2.2793(19) Cd(1)-N(1) 2.2793(19) Cd(1)-I(1)#1 N(1)#1-Cd(1)-N(1) 91.42(10) N(1)#1-Cd(1)-I(1)#1 105.03(5) N(1)-Cd(1)-I(1)#1 105.86(5) N(1)#1-Cd(1)-I(1) 105.85(5) N(1)-Cd(1)-I(1) 105.03(5) I(1)#1-Cd(1)-I(1) 135.174(12) Complex 7 Hg(1)-Cl(3) 2.3579(16) Hg(1)-Cl(3)#1 2.3579(16) Hg(1)-N(1) 2.417(4) Hg(1)-N(1)#1 2.417(4) Cl(3)-Hg(1)-Cl(3)#1 150.52(11) Cl(3)-Hg(1)-N(1) 105.09(11) Cl(3)#1-Hg(1)-N(1) 97.23(10) Cl(3)-Hg(1)-N(1)#1 97.23(10) Cl(3)#1-Hg(1)-N(1)#1 105.09(11) N(1)-Hg(1)-N(1)#1 81.26(19) Complex 8 Hg(1)-N(1)#1 2.4078(18) Hg(1)-N(1) 2.4078(18) Hg(1)-Br(1) 2.4839(5) Hg(1)-Br(1)#1 2.4839(5) N(1)#1-Hg(1)-N(1) 83.03(9) N(1)#1-Hg(1)-Br(1) 99.70(4) N(1)-Hg(1)-Br(1) 104.73(4) N(1)#1-Hg(1)-Br(1)#1 104.73(4) N(1)-Hg(1)-Br(1)#1 99.70(4) Br(1)-Hg(1)-Br(1)#1 147.20(3) Complex 13 Cd(1)-N(3) Cd(1)-Cl(2) Cd(2)-Cl(2) Cd(3)-N(16) Cd(3)-Cl(5) Cd(4)-Cl(8)#4

2.334(10) 2.669(3) 2.598(3) 2.322(10) 2.687(3) 2.614(3)

Cd(1)-N(9) Cd(1)-Cl(3) Cd(2)-Cl(4) Cd(3)-N(10)#3 Cd(3)-Cl(7)#3 Cd(4)-Cl(8)

2.337(10) 2.698(3) 2.619(3) 2.358(10) 2.727(3) 2.626(3)

Cd(1)-Cl(1) Cd(2)-N(21) Cd(2)-Cl(4)#2 Cd(3)-Cl(7) Cd(4)-N(22) Cd(4)-Cl(5)

2.595(3) 2.366(10) 2.643(3) 2.586(3) 2.392(10) 2.632(3)

Cd(1)-Cl(3)#1 Cd(2)-N(15) Cd(2)-Cl(1) Cd(3)-Cl(6) Cd(4)-N(4) Cd(4)-Cl(6)

2.605(3) 2.393(10) 2.683(3) 2.608(3) 2.471(11) 2.645(3)

Complex 14 Cd(1)-N(1) N(1)-Cd(1)-N(4)#1 N(4)#1-Cd(1)-Br(1)

2.241(2) 117.09(9) 100.99(6)

Cd(1)-N(4)#1 N(1)-Cd(1)-Br(2) Br(2)-Cd(1)-Br(1)

2.272(2) 109.42(6) 122.208(13)

Cd(1)-Br(2) N(4)#1-Cd(1)-Br(2)

2.5377(4) 107.19(6)

Cd(1)-Br(1) N(1)-Cd(1)-Br(1)

2.5693(3) 100.34(6)

Cd(1)-N(4)#1 N(1)-Cd(1)-I(1)

2.290(2) 100.21(6)

Complex 15 I(1)-Cd(1) N(1)-Cd(1)-N(4)#1 N(4)#1-Cd(1)-I(1)

2.7492(3) 119.18(9) 102.84(6)

I(2)-Cd(1) N(1)-Cd(1)-I(2) I(2)-Cd(1)-I(1)

2.7185(3) 107.66(6) 119.968(10)

Cd(1)-N(1) N(4)#1-Cd(1)-I(2)

2.256(2) 107.62(6)

Complex 16 Hg(1)-N(3) 2.364(10) Hg(1)-N(3)#1 2.364(10) Hg(1)-Cl(1) 2.369(3) Hg(1)-Cl(1)#1 2.369(3) N(3)-Hg(1)-N(3)#1 107.5(5) N(3)-Hg(1)-Cl(1) 98.9(3) N(3)#1-Hg(1)-Cl(1) 103.7(3) N(3)-Hg(1)-Cl(1)#1 103.7(3) N(3)#1-Hg(1)-Cl(1)#1 98.9(3) Cl(1)-Hg(1)-Cl(1)#1 141.36(17) Complex 17 Hg(1)-N(1) N(3)-Hg(1)#1 N(1)-Hg(1)-I(2)

2.457(5) 2.460(5) 99.6(3)

Hg(1)-N(3)#1 N(1)-Hg(1)-N(3)#1 N(3)#1-Hg(1)-I(2)

2.430(13) 84.48(17) 104.15(11)

Hg(1)-I(3) N(1)-Hg(1)-I(3) I(3)-Hg(1)-I(2)

2.6410(6) 107.3(3) 142.52(5)

Hg(1)-I(2) N(3)#1-Hg(1)-I(3)

2.4805(16) 106.19(13) 141.152(18)

Hg(1)-Br(2) N(3)#1-Hg(1)-Br(1)

2.4961(16) 104.99(11)

Complex 18 Hg(1)-N(1) N(3)-Hg(1)#1 N(1)-Hg(1)-Br(2)

2.400(12) 2.430(13) 101.66(13)

Hg(1)-N(3)#1 N(1)-Hg(1)-N(3)#1 N(3)#1-Hg(1)-Br(2)

2.460(5) 86.2(4) 102.8(3)

1616(m), 1597(vs), 1497(s), 1439(s), 1337(m), 1311(m), 1284(m), 1213(s), 1061(m), 1025(s), 823(m), 734(w), 646(w), 539(w), 458(w). [Cd4(L2)4Cl8]n 3 2nDMF (13). Compound 13 was prepared by a procedure similar to that for 4, using L2 (4.16 mg, 0.01 mmol) in place of L1. Colorless block crystals of 13 suitable for X-ray structure determination were obtained. Yield: 3 mg, 70%. Anal. Calcd for C110H94Cd4Cl8N26O2: C, 51.91; H, 3.72; N, 14.31. Found: C, 51.69; H, 4.11; N, 14.31. IR (KBr pellet, cm-1): 3421(br), 3060(w), 2922(w), 1670(m), 1591(vs), 1564(m), 1545(s), 1468(s), 1429(vs), 1386(w), 1326(m), 1272(m), 1221(s), 1151(w), 1095(w),

Hg(1)-Br(1) N(1)-Hg(1)-Br(1) Br(1)-Hg(1)-Br(2)

2.6499(6) 104.5(3)

1064(w), 1015(m), 820(m), 778(m), 738(m), 657(w), 628(m), 535(m), 458(w). [CdL2Br2]n (14). A mixture of L2 (4.16 mg, 0.01 mmol), Cd(NO3)2 3 4H2O (6.6 mg, 0.02 mmol), and KBr (4.8 mg, 0.04 mmol) in DMSO, DMF, and H2O (100:100:10) was sealed in a small vial, which was heated to 120 C for 90 h. After cooling to room temperature, colorless block crystals of 14 suitable for X-ray structure determination were obtained. Yield: 4 mg, 55%. Anal. (%) calcd. for C26H20Br2CdN6: C, 45.34; H, 2.93; N, 12.20. Found: C, 44.93; H, 2.92; N, 12.16. IR (KBr pellet, cm-1): 3441(br),

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3064(w), 1614(s), 1581(vs), 1541(w), 1504(vs), 1482(vs), 1458(s), 1434(vs), 1347(s), 1313(vs), 1269(s), 1252(m), 1216(s), 1154(m), 1112(w), 1061(m), 1016(vs), 992(w), 935(w), 835(w), 818(s), 781(m), 741(m), 708(w), 657(w), 643(m), 634(m), 622(m), 532(m), 465(m). [CdL2I2]n (15). Compound 15 was prepared by a procedure similar to that for 14, using KI (6.6 mg, 0.04 mmol) in place of KBr. Colorless block crystals of 15 suitable for X-ray structure determination were obtained. Yield: 4 mg, 51%. Anal. (%) calcd. for C26H20I2CdN6: C, 39.90; H, 2.58; N, 10.74. Found: C, 39.99; H, 2.59; N, 10.70. IR (KBr pellet, cm-1): 3441(br), 3061(w), 1615(s), 1603(s), 1581(vs), 1541(w), 1500(vs), 1483(vs), 1460(s), 1434(vs), 1349(s), 1314(vs), 1269(s), 1252(m), 1216(s), 1156(m), 1062(m), 1018(vs), 992(w), 839(w), 818(s), 783(m), 741(m), 708(w), 656(w), 644(m), 635(m), 622(m), 533(m), 466(m). [HgL2Cl2]n 3 0.5nDMF (16). Compound 16 was prepared by a procedure similar to that for 7, using L2 (4.16 mg, 0.01 mmol) in place of L1. Colorless block crystals of 16 suitable for X-ray structure determination were obtained. Yield: 5 mg, 71%. Anal. (%) calcd. for C55H47Cl4Hg2N13O: C, 45.59; H, 3.27; N, 12.57. Found: C, 45.39; H, 3.49; N, 12.27. IR (KBr pellet, cm-1): 3445(br), 2920(w), 1591(s), 1577(vs), 1549(m), 1498(s), 1417(w), 1342(m), 1304(s), 1277(s), 1217(m), 993(m), 935(w), 848(w), 813(m), 740(w), 693(w), 624(m), 542(m), 528(w). [Hg2(L2)2Br4] 3 H2O (17). Compound 17 was prepared by a procedure similar to that for 14, using Hg(NO3)2 3 0.5H2O (6.7 mg, 0.02 mmol) in place of Cd(NO3)2 3 4H2O. Colorless block crystals of 17 suitable for X-ray structure determinations were obtained. Yield: Anal. (%) calcd. for C52H42Br4Hg2N12O: C, 39.74; H, 2.69; N, 10.69. Found: C, 39.49; H, 2.89; N, 10.48. IR (KBr pellet, cm-1): 3424(br), 3047(w), 1586(vs), 1543(w), 1504(s), 1467(s), 1428(s), 1324(m), 1312(m), 1265(m), 1212(m), 1151(w), 1055(w), 1010(m), 826(w), 775(m), 737(m), 709(w), 642(m), 625(w), 536(m). [Hg2(L2)2I4] 3 H2O (18). Compound 18 was prepared by a procedure similar to that for 14, using HgI2 (9.1 mg, 0.02 mmol) in place of Cd(NO3)2 3 4H2O and KBr. Colorless block crystals of 18 suitable for X-ray structure determination were obtained. Yield: 4 mg, 47%. Anal. (%) calcd. for C52H42Hg2I4N12O: C, 35.49; H, 2.41; N, 9.55. Found: C, 35.13; H, 2.50; N, 9.39. IR (KBr pellet, cm-1): 3424(br), 3045(w), 2921(w), 2361(w), 1586(vs), 1543(w), 1505(s), 1467(s), 1428(s), 1325(m), 1312(m), 1266(m), 1212(m), 1149(w), 1058(w), 1007(m), 825(w), 774(m), (m), 709(w), 642(m), 624(w), 535(m), 472(w). X-ray Crystallography. X-ray diffraction data were collected on a Bruker-AXS APEX or an Oxford Diffraction Ge- mini S Ultra diffractometer utilizing MoKR radiation (λ = 0.71073 A˚). The structures were solved by direct methods and refined with fullmatrix least-squares technique. 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 the ideal positions in the refinement procedure. All calculations were performed using the SHELX-97 software package.15 Crystal data and experimental details for the crystals are summarized in Table 1, and selected bond lengths and bond angles are given in Table 2. Topology analysis was performed using Olex.16 Powder X-ray diffraction (PXRD) data were collected on a DMAX2500 diffractometer using Cu KR radiation. The PXRD patterns for complexes 1-6 are shown in Figures S3 and S4, Supporting Information. The measured and simulated PXRD patterns of complex 16 are shown in Figure S5, Supporting Information as a representative example to indicate the phase purity of the samples for photoluminescence measurements.

Scheme 2. Preparation of Ligands L1 and L2

Results and Discussion Synthesis and Characterization. The ligand L1 was prepared in a moderate yield from an Ullmann condensation reaction of 4,40 -dipyridylamine with 1,4-dibromobenzene at 200 C in diphenyl ether, using cupric sulfate, 18-crown-6 and K2CO3 as catalysts and the base, respectively (Scheme 1). To obtain a reasonable yield of L1, in the initial stage of the reaction, 4,40 -dipyridylamine was used in excess. The reaction was monitored by thin layer chromotography (TLC), and after 3 days it was found that the reaction mixture still

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contains some 4,40 -dpa, but the reaction is not proceeding further. To further convert 4,40 -dpa and to obtain a reasonable yield of (4-bromophenyl)-di(4-pyridyl)amine, excessive 1,4-dibromobenzene was added. After reacting for another day, no 4,40 -dipyridylamine could be detected by TLC. The reaction mixture was worked up to afford moderate yields of (4-bromophenyl)-di(4-pyridyl)amine and L1. L2 was prepared in a moderate yield from an Ullmann condensation reaction of (4-bromophenyl)-di(4-pyridyl)amine with excessive 2,20 -dpa. When the reaction was performed under a condition similar to that for L1, a very complicated mixture was obtained and practically no product could be separated, so we tried DMF as the solvent, and bronze powder as the catalyst. Thus, the reaction was performed at 145 C for 30 h to afford a moderate yield of L2. The ligands coordinate readily with Zn(II), Cd(II), and Hg(II) salts in MeOH or MeOH/DMF to afford microcrystalline solids of complexes 1-18, which were characterized by elemental analyses and IR spectra. Single crystals of L1 were obtained by slow evaporation of the MeOH solution, and the single crystals of the complexes were generally obtained by heating the ligand and the corresponding metal salts in a mixed solvent of DMF, DMSO and water at 90-120 C. Crystals gradually appeared during heating or after cooling to room temperature. It is noteworthy that for the syntheses of complexes [HgL1Cl2]n (7) and [HgL1Br2]n (8) in such a way, only crystals of the ligand could be obtained, which may be ascribed to the suppression of coordination of L1 or L2 caused by the strongly coordinating DMF and DMSO solvents. To obtain the single crystals, these two complexes were initially synthesized in MeOH, and then collected and put in small vials, to which were added suitable volumes of DMF, DMSO, and H2O. The resulting suspension was heated to gradually dissolve the solid and then single crystals appeared after cooling to ambient temperature. Thus, L1 and 10 of the metal complexes are characterized by single-crystal X-ray diffraction analyses. Crystal Structure of L1. The molecule of ligand L1 in its crystal is centrosymmetric (Figure 1A), and it is deviated from a planar structure due to steric hindrance. The dihedral angle between the neighboring pyridyl rings is 64.7 (1), and the angles between the central benzene and the pyridyl rings are 59.8 (1) and 71.1(1), respectively. It is noteworthy that all the pyridyl rings are involved in π 3 3 3 π stacking and T-shaped C-H 3 3 3 π interactions. The former is slipped parallel, associated with the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.632(3) and 3.98(1) A˚, respectively. These values lie above the optimal distances,17 and the latter is associated with an interplane angle of 54.2(1),

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Figure 1. (A) Molecular structure of ligand L1 with thermal ellipsoids at the 50% probability level and the atom-labeling scheme. (B) 2-D packing of L1 showing intermolecular interactions. Hydrogen atoms are omitted for clarity.

and the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.580(2), and 4.62(1) A˚, respectively. Thus, the molecules are linked to form a 2-D network consisting of pyridyl tetramers, as shown by the same colors in Figure 1B. Crystal Structure of [CdL1Cl2]n 3 nH2O (4). Crystal structure of compound 4 is shown in Figure 2. L1 utilizes all its pyridyl nitrogens to coordinate with four Cd(II) atoms (Scheme 3a). Each Cd(II) has a distorted octahedral coordination geometry, with two chlorides coordinated at the axial positions and four pyridyl nitrogens coordinated in the equatorial plane. The Cd-N bond lengths are 2.433(3) and 2.461(3) A˚, and the Cd-Cl distances are 2.5407(9) A˚. By the linkage of four-connecting ligand L1 and four-connecting Cd(II), a 3D network is formed (Figure 2B). Topologically, one Cd atom could be viewed as a squareplanar four-connected node and two central N atoms in the ligand as two trigonal three-connected nodes, hence generating an unprecedented 3D binodal (3,4)-connected network with the Schl€ afli symbol of (8.102)2(84.102) and the Vertex symbol of (82.105.103)(82.105.103)(8.8.8.8.102.104). Crystal Structures of [CdL1I2]n (6), [HgL1Cl2]n (7), and [HgL1Br2]n (8). In the crystal of complex 6 (Figure S6, Supporting Information), each Cd(II) coordinates with two iodides in addition to two nitrogens from two L1 ligands, affording a distorted tetrahedral geometry, which is in contrast to the octahedral geometry observed for complex 4. Cd(1)-N distances are 2.2793(19) A˚, which is significantly shorter than those of 2.433(3) and 2.461(3) A˚ in complex 4, which may be caused by the difference in coordination geometry. Cd(1)-I distances are 2.2793(19) and 2.7048(2) A˚.

Figure 2. (A) Molecular structure of compound 4 with thermal ellipsoids at the 50% probability level. Hydrogen atoms and water have been omitted for clarity. (B) 3D coordination network of 4 with chlorides omitted for clarity. (C) Schematic representation of the 3D binodal (3,4)-connected network of 4.

Scheme 3. Various Coordination Modes of L1

The N(1)-Cd(1)-N(1), I(1)-Cd(1)-I(1), and N(1)Cd(1)-I(1) angles are 91.42(10), 135.174(12), and 105.85(5), respectively. In contrast to the coordination of all four pyridyl groups of L1 in complex 4, each L1 ligand in complex 6 utilizes only two pyridyl nitrogens from two dpa units to coordinate (Scheme 3b). Thus, L1 and Cd(II) are linked

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Figure 3. 2D network approximately along the ac plane in compound 6, linked by π-π stacking interactions, indicated by the orange-colored pyridyl rings. Hydrogen atoms have been omitted for clarity.

alternately to form a one-dimensional (1D) zigzag chain approximately along the a axis (Figure S6, Supporting Information). Parallel pyridyl rings in the adjacent chains are π-π stacked in an edge-to-edge fashion, with the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.366(1) and 4.993(2) A˚, respectively. Thus, the 1D chains are assembled into 2D supramolecular networks approximately along the ac plane as shown in Figure 3. Crystals of 7 and 8 are isostructural to 6 (Figures S7 and S8, Supporting Information). The Hg(II)-N distances are 2.417(4) and 2.4078(18) A˚, respectively. Hg(II)-Cl and Hg(II)-Br distances are 2.3579(16) and 2.4839(5) A˚, respectively. These values lie in the normal range observed for tetrahedral Hg(II) complexes. Crystal Structures of [Cd4(L2)4Cl8]n 3 2nDMF (13). The asymmetric unit of crystal 13 consists of four crystallographically independent Cd(II) atoms (Figure 4A). Each Cd has an octahedral coordination environment donated by four chloro ligands and two nitrogens from the 4,40 -dpa units of two L2 ligands, with the 2,20 -dpa units left noncoordinated (Scheme 4a,b). The nitrogens coordinate at transpositions of Cd1 and Cd3, and cis-positions of Cd2 and Cd4. Cd-Cl and Cd-N bond lengths lie in the ranges of 2.5857(2)2.7264(3) A˚ and 2.3211(2)-2.4693(3) A˚, respectively. The Cd centers are linked by dichloro-bridges to form interesting infinite (CdCl2)¥ chains, which are further bridged by the 4,40 -dpa moieties to form a 2D network (Figure 4B). Complexes containing similar (CdCl2)¥ chains have been reported.18 Crystal Structures of [CdL2Br2]n (14) and [CdL2I2]n (15). The coordination sphere of Cd(II) in complex 14 is tetrahedral, consisting of two terminal Br atoms and two pyridyl N atoms (Figure 5A), with the coordination angles ranging from 100.34(6) to 122.11(1). The bond lengths of Cd(1)-N(1), Cd(1)-N(4), Cd(1)-Br(1), and Cd(1)-Br(2) are 2.241(2), 2.272(2), 2.5693(3), and 2.5377(4) A˚, respectively. Unexpectedly, the 2,20 -dpa unit does not coordinate in a commonly observed chelating mode. Instead, each L2 ligand utilizes one N from the 2,20 -dpa unit and another N from the 4,40 -dpa unit to coordinate with Cd(II) (Scheme 4c). Thus, Cd(II) atoms and L2 ligands are linked alternately to afford a 1D centipede-like chain structure (Figure 5B),

Figure 4. (A) The coordination environments of the Cd(II) centers in 13. Symmetry operations, i: 2 - x, 2 - y, -z; ii: 1 - x, 2 - y, -1 - z; iii: 2 - x, 2 - y, -1 - z. (B) 2D coordination network of 13 approximately along the ac plane. Hydrogen atoms, the phenylene and 2,20 -dpa moieties are omitted for clarity.

wherein the L1 ligands look like a segmented trunk and the Br- anions look like pairs of legs. It is noteworthy that there are intrachain π 3 3 3 π stacking interactions between a coordinated 4-pyridyl ring and a noncoordinated 2-pyridyl ring from a neighboring ligand, with the dihedral angles of 7.98(1), and the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.455(1) A˚ and 3.596(1) A˚, respectively. In addition, there are two classes of interchain slipped parallel π 3 3 3 π stacking interactions (Figure S9, Supporting Information). One occurs between 4-pyridyl rings, associated with the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.475(1) A˚ and 4.252(1) A˚, respectively. The other occurs between 2-pyridyl rings with the closest interplane C 3 3 3 C and centroid 3 3 3 centroid distances of 3.570(1) A˚, and 4.190(1) A˚, respectively. These values lie above the optimal distances for π-stacking.17 By the linkage of these interchain π-π interactions, a 2D network is formed (Figure S9, Supporting Information). Crystal of 15 is isostructural to 14 (Figure S10, Supporting Information). The Cd(II) atom also is located in a tetrahedral coordination environment, with coordination angles ranging from 100.21(6) to 119.968(1). The bond lengths of

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Scheme 4. Various Coordination Modes of L2

Figure 5. (A) Structure of compound 14 with thermal ellipsoids at the 30% probability level. (B) A one-dimensional centipede-like chain structure of 14.

Cd(1)-N(1), Cd(1)-N(4), Cd(1)-I(1), and Cd(1)-I(2) are 2.256(2), 2.290(2), 2.7492(3), and 2.7185(3) A˚, respectively. Similar intra- and interchain π-π-stacking interactions also afford a 2D network (Figure S11, Supporting Information). Crystal Structures of [HgL2Cl2]n 3 0.5nDMF (16). In the crystal of 16, the Hg(II) atom locates in a tetrahedral coordination environment, which is similar to the Cd(II) atoms in compounds 14 and 15, whereas the L2 ligand utilizes two 4,40 -dpa nitrogens for coordination (Scheme 4d), which is in sharp contrast to the coordination of two nitrogens from one 2,20 -dpa moiety and one 4,40 -dpa moiety in 14 and 15. The coordination angles range from 98.9(3) to 141.36(17), indicating a serious distortion of the tetrahedral geometry. Hg(II) atoms and L2 ligands are also alternately linked to form a 1D chain structure (Figure 6A). The chains are arranged in an alternate mode, with all the pyridyl rings involved in two kinds of π 3 3 3 π stacking interactions. All interplane angles are 6.23(1), and the closest interplane

C 3 3 3 C distances are 3.494(2), and 3.679(3) A˚, respectively. The centroid 3 3 3 centroid distances are 4.241(3) and 4.588(3) A˚, respectively. Thus, columns consisting of alternately stacked 4-pyridyl and 2-pyridyl rings are formed (Figure 6B). Finally, the alternately arranged chains afford a 3D structure consisting of hexagonal channels (Figure 6C) along the c-axis, which are filled with DMF solvents. From the calculation using PLATON softwares,19 the percentage of the unit cell volume occupied by the solvent was determined to be 18%. The 3D structure formed by the π 3 3 3 π stacking interactions is not very stable. Upon heating, continuous weight loss was observed (Figure S12, Supporting Information), indicating that the 3D framework collapsed when the guest solvent molecules were removed. Crystal Structures of [Hg2(L2)2Br4] 3 H2O (17) and [Hg2(L2)2I4] 3 H2O (18). In complex 17, Hg(II) atom locates in a tetrahedral coordination environment, consisting of two terminal I atoms and two pyridyl N atoms, with the coordination angles ranging from 84.48(17) to 141.152(18). The bond lengths of Hg(1)-N(1), Hg(1)-N(3), Hg(1)-Br(1), and Hg(1)-Br(2) are 2.457(5), 2.460(5), 2.6410(6), and 2.6499(6) A˚, respectively. The L2 ligand utilizes two 4-pyridyl nitrogens for coordination, which is similar to that in complex 16, but in complex 17, four nitrogens from two L2 ligands coordinate to two Hg(II) centers (Scheme 4e), affording a binuclear structure containing a 20-membered M2L2 macrocycle, which is totally different from the chain structure containing alternately linked L2 ligands and Hg(II) centers observed in complex 16. Hydrogen bonds are observed between the N atoms of the 2,20 -dpa moieties and the H atoms of lattice waters, with the H 3 3 3 N distances of 2.55 A˚, and the O-H 3 3 3 N angles of 125. Thus, the binuclear moieties are linked to afford a 1D structure (Figure 7). Complex 18 is isostructural to 17 (Figure S13, Supporting Information). Hg(II) atoms have tetrahedral coordination geometries, with the coordination angles ranging from 86.2(4) to 142.52(5). The bond lengths of Hg(1)-N(1),

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Figure 7. A one-dimensional structure of 17 formed by the O-H 3 3 3 N hydrogen bonds between H atoms of lattice waters and the N atoms of the 2,20 -dpa moieties. H attached to carbons are omitted for clarity.

Figure 8. The photograph of L1, L2, and compounds 1-18 taken under UV light (λ = 365 nm).

Figure 9. Emission spectra of L1 and compounds 1-9 in the solid state at room temperature.

Figure 6. (A) A chain structure of 16. (B) Arrangement of the chains showing interchain π 3 3 3 π interactions. (C) Space filling model showing the channel along the c axis. DMF molecules are omitted for clarity. Blue, N; gray, C; magenta, Hg; green, Br.

Hg(1)-N(3), Hg(1)-I(1), and Hg(1)-I(2) are 2.400(12), 2.430(13), 2.4805(16), and 2.4961(16) A˚, respectively. The O-H 3 3 3 N hydrogen bonds are associated with the H 3 3 3 N distances of 2.21 A˚, and the O-H 3 3 3 N angles of 156. The binuclear moieties are also linked by intermolecular hydrogen bonds to afford a 1D structure. From the above structural results, it is demonstrated that the structures of the metal complexes are strongly dependent on the metal cations and the counteranions. The Hg(II) atoms in all complexes have tetrahedral coordination environments, whereas the Cd(II) centers have octahedral coordination geometries when Cl- is used as the anion, affording 3D and 2D coordination networks as observed for complexes 4 and 13, respectively. In other Cd(II) complexes, Cd(II) have tetrahedral coordination geometries. These observations are consistent with the tendency of Cd(II) to form octahedral configuration and bridged polymeric 2D and 3D

networks, especially when Cl- is used for coordination, as have been observed in a variety of complexes.20 Photoluminescence. Photoluminescence properties of d10 metal coordination compounds have been extensively studied due to their potential applications as luminescent materials.21 Ligands L1, L2, and complexes 1-18 exhibit photoluminescence in the solid state at room temperature (Figure 8 (under UV light) and Figure S14 (shows the photograph taken under ambient light). The emission intensity and color strongly depend on the metal centers and the anions. The colors vary from violet (for 8 and 16) to yellow (for 1). To more clearly understand the luminescent properties, the emission spectra of the compounds were measured in the solid state. The ligand L1 exhibits a medium-intensity emission centered at 411 nm (Figure 9) upon excitation at 315 nm. For complexes 1, 2, 4, and 5, intense emission bands are observed at 529, 493, 435, and 471 nm, respectively, upon excitation at 320, 325, 310, and 310 nm, respectively. The origin of the broad and red-shifted emission bands may be tentatively attributed to the contribution of ligand-to-metal charge transfer (LMCT), which results in the rearrangement of energy levels.14a,22 The differences in the band positions might also be related to the differences in the metal centers and the local coordination environments.23 The significantly stronger fluorescence of these complexes compared with ligand L1 may be ascribed to the coordination effect, which effectively reduces the loss of energy through a nonradiative

1620


Figure 10. Emission spectra of L2 and compounds 10-18 in the solid state at room temperature.

pathway by suppression of the rotation of the pyridyl rings and increasing the rigidity of the ligand. On the other hand, complexes 7, 8, and 9 exhibit emission bands entered at 388, 414, and 373 nm, respectively, upon excitation at 310, 310, and 305 nm, respectively. The emission of these Hg(II) complexes are significantly weaker than that of L1, which may be ascribed to the heavy-atom effect. In contrast, medium-intensity emission bands are observed at 494 and 410 nm for complexes 3 and 6, respectively, upon excitation at 320, and 330 nm, respectively. The emission intensities are similar to that of L1, which may be attributed to the balance of fluorescence increasing and decreasing effects caused by the coordination effect and the heavy atom effect, respectively. Similar to L1, the ligand L2 exhibits a medium-intensity emission centered at 388 nm (Figure 10) upon excitation at 320 nm. Complexes 10, 11, and 13 exhibit intense emission bands at 456, 448, and 480 nm, respectively, upon excitation at 360, 360, and 345 nm, respectively. All other complexes of L2 exhibit rather weak emission at shorter wavelengths. The variation in emission intensities can also be rationalized in terms of the coordination effect and the heavy atom effect. It is highly demanding to develop compounds that exhibit photoluminescence in the solid state due to the potential applications in various optoelectronic devices.24 Ligands L1, L2, and complexes 1-18 exhibited solid state emissions, with the colors varying from violet to yellow, and the maxima wavelengths varying in a large range of 373-529 nm. In addition, the emission intensities also vary to a large extent. These complexes are good examples of photoluminescent compounds with emission wavelengths and intensities modulated by the change of the ligands, the central atoms, and the anions, although part of the complexes cannot be characterized by single crystal X-ray analyses due to the difficulties encountered in crystal growth. Conclusions Two novel tetrapyridyl ligands L1 and L2 containing 2,20 dpa and 4,40 -dpa moieties were prepared and used to coordinate with Zn(II), Cd(II), and Hg(II) halides to afford a rich variety of coordination structures such as various 1D chains, binuclear moieties containing a 20-membered M2L2 macrocycle, 2D coordination network consisting of infinite (CdCl2)¥ chains, and a 3D coordination network. Furthermore, the coordination chains are assembled by π 3 3 3 π interactions to afford 2D networks and a 3D network with hexagonal channels; the binuclear complex is linked by hydrogen bonds with water molecules to form a chain assembly.

Zeng et al.

In these complexes, the metal centers adopt octahedral or tetrahedral coordination geometries, and the ligands can adopt various coordination modes. Thus, L1 utilizes two or four pyridyl nitrogens to coordinate to metal centers, whereas L2 utilizes two nitrogens, both from 4,40 -dpa moieties, or one from 4,40 -dpa moiety and another from 2,20 -dpa moiety, to coordinate with metal atoms. Unexpectedly, the chelation mode of the 2,20 -dpa moiety has not been observed in these complexes. This study leads to further insight into d10 metal coordination polymers constructed by the selection of polypyridyl ligands, metal centers, and various anions. The photoluminescence measurements illustrated that L1, 2 L , and complexes 1-18 exhibited emissions, with the colors varying from violet to yellow. The emission intensities and colors are strongly dependent on the metal centers, the anions, and the structures of the ligands. The variation in emission intensities can be rationalized in terms of the coordination effect and the heavy atom effect. It can be expected that design and utilization of similar tetrapyridyl ligands with various sizes of extended aromatic systems and various HOMOLUMO gaps are a practical approach to tuning the fluorescence wavelength and intensity and to designing luminescent compounds with desired emissions at tunable wavelengths, which is underway in our lab. Acknowledgment. This work was financially supported by the Shanghai Pujiang Program (08PJ14037), the Program for New Century Excellent Talents in University (NCET), NSFC/China, SRF for ROCS, and SEM. We thank Shunze Zhan and Mian Li from Shantou University for topology analysis. The authors greatly appreciate the kind help and valuable suggestions from Prof. He Tian. Supporting Information Available: Crystallographic information files (CIF format), NMR spectra of the ligands, TGA data, PXRD data, part of the crystal figures, and photographs of all compounds taken under ambient light are available free of charge via the Internet at http://pubs.acs.org.

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