Pseudohalide-Bipy Systems: Syntheses, Structures

Investigations of Group 12 (IIB) Metal Halide/Pseudohalide-Bipy Systems: Syntheses, Structures, Properties, and TDDFT Calculations (Bipy ) 2,2′-bipyridine or 4,4′-bipyridine) Wen-Tong Chen,†,‡ Ming-Sheng Wang,† Xi Liu,† Guo-Cong Guo,*,† and Jin-Shun Huang†

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2289-2300

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed March 16, 2006; ReVised Manuscript ReceiVed August 2, 2006

ABSTRACT: Ten compounds of the IIB metal halide/pseudohalide-bipy system, β-[ZnBr2(4,4′-bipy)]n (1), [Cd2Br5(4,4′-bipyH)2](CdBr3) (2), [Cd3Br6(4,4′-bipy)]n (3), [CdI2(4,4′-bipy)]n (4), (HgI2)2(4,4′-bipy) (5), [Hg(CN)2(4,4′-bipy)(H2O)2]n (6), HgCl2(2,2′bipy) (7), [HgCl2(2,2′-bipy)]‚2HgCl2 (8), [Hg3Br6(2,2′-bipy)]‚HgBr2 (9), and [HgI2(2,2′-bipy)]‚HgI2 (10), have been synthesized via hydro(solvo)thermal and solid-state reactions; except for 4, all the other compounds are reported for the first time. Compound 1 features a 1-D zigzag chain, based on tetrahedral zinc atoms bridged by 4,4′-bipy molecules and terminally coordinated by two bromine atoms. The chains are linked by hydrogen bonds and π‚‚‚π interactions to form a 2-D sheet. Compound 2 possesses dinuclear [Cd2Br5(4,4′-bipyH)2]+ cations and CdBr3- anions. Compound 3 has a 2-D layered structure, constructed from novel triple chains and bridging 4,4′-bipy ligands. The triple chains contain edge-shared CdBr6 and CdBr5N octahedra. In 4, the layers are formed by monochains and bridging 4,4′-bipy molecules. The monochains contain edge-shared CdI4N2 octahedra. Compound 5 features a discrete structure with a 4,4′-bipy ligand bridging two HgI2 moieties. Compound 6 has a 1-D zigzag chain, containing octahedral mercury atoms and bridging 4,4′-bipy molecules. In 7 and 8, mercury atoms are tetrahedrally coordinated to two chlorine atoms and two nitrogen atoms. In 9, 2-, 3-, 4-, and 5-coordinated mercury atoms coexist, whereas in 10, the mercury atoms are only 2- and 4-coordinated. Photoluminescence investigations reveal that the compounds display strong emissions in the blue/green/yellow regions, which, in combination with the molecular orbital (MO) calculations of 1, 5, and 7, leads us to conclude that the emissions originate from a ligand-to-ligand charge-transfer (LLCT) transition. The solid-state diffuse reflectance spectra, IR, and TG-DTA are also presented. Introduction Inorganic-organic hybrid materials have been of great interest because of their intriguing structural features and potential in various applications, such as electrical conductivity, photochemistry, ion exchange, catalysis, biochemistry, and nonlinear optical behavior.1 A large variety of ligands containing bridging functionalities such as carboxylates, phosphonates, 4,4′bipy, and 2,2′-bipy have been exploited to prepare novel inorganic-organic hybrid materials. Being an important class of inorganic-organic hybrid materials, metal halide-bipy (bipy ) 2,2′-bipyridine or 4,4′-bipyridine) systems have attracted more and more attention in recent years not only for their intrinsic aesthetic appeals but also for their various potential applications, as well as the special coordination modes of bipy. The ability of bifunctional 4,4′-bipyridine to act as a rigid, rodlike organic building block in the self-assembly of coordination frameworks is well-known, such as acting as a chargecompensating cation,2 a pillar bonding to inorganic skeletal backbone,3 an uncoordinated guest molecule and organic template,4 a bridge connecting two metal complex moieties,5 or a ligand linking a metal and an inorganic framework.6 In comparison with 4,4′-bipy, the 2,2′-bipy has fewer functions because the two functional nitrogen atoms of 2,2′-bipy are close and thus usually chelate to the same metal center. This coordination mode prevents 2,2′-bipy from being exploited as a bridging ligand to link two different metal centers; as a result, 2,2′-bipy is relatively poorly investigated compared with 4,4′* To whom correspondence should be addressed. Fax: 86 591 83714946. E-mail: [email protected]. † State Key Laboratory of Structural Chemistry. ‡ Graduate School of Chinese Academy of Sciences.

bipy. However, both 4,4′-bipy and 2,2′-bipy have gained increased attention in recent years because of their common character delocalized π-electrons of the pyridyl rings, which makes 4,4′-bipy and 2,2′-bipy excellent candidates for preparing light-emitting compounds with potential in various technical applications, such as chemical sensors,7 sensitizers in solar energy conversion,8 and emitting materials for organic lightemitting diodes.9 As expected, some of the compounds containing 4,4′-bipy or 2,2′-bipy are known to exhibit strong emission in solution or in solid-state at room temperature.10,12-14 Recently, huge structures of metal halide-bipy materials were reported,11 showing various architectures with discrete one(1D), two- (2D), and three-dimensional (3D) connections between inorganic and organic species. However, among the known metal halide-bipy materials, IIB metal halide-bipy materials are relatively rare. In fact, compounds containing IIB elements are particularly attractive for many reasons: the variety of coordination numbers and geometries provided by the d10 configuration of the IIB metal ions, photoelectric properties, fluorescence properties, widespread applications of IIB compounds, the essential role in biological systems of zinc, and so on. Fluorescence materials, particularly blue fluorescence materials, have been of intense interest because blue fluorescence is one of the key color components required for fullcolor EL displays and blue fluorescence materials are still rare. Nowadays, many IIB metal compounds possessing fluorescence properties have been reported, including some IIB metal halidebipy materials. For most of these compounds, the luminescence mechanism is usually metal-to-ligand charge-transfer (MLCT) transition12 or ligand-to-metal charge-transfer (LMCT) transition,13 whereas ligand-to-ligand charge-transfer (LLCT) transition14 is rarely documented in metal halide-bipy systems.

10.1021/cg060146v CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006

2290 Crystal Growth & Design, Vol. 6, No. 10, 2006

Our recent efforts in synthesizing novel IIB-based compounds have focused largely on the systems containing bifunctional ligands, such as 4,4′-bipy and 2,2′-bipy. Herein, we describe the syntheses and characterizations of a new family of IIB metal halide/pseudohalide-bipy compounds: β-[ZnBr2(4,4′-bipy)]n (1), [Cd2Br5(4,4′-bipyH)2](CdBr3) (2), [Cd3Br6(4,4′-bipy)]n (3), [CdI2(4,4′-bipy)]n (4), (HgI2)2(4,4′-bipy) (5), [Hg(CN)2(4,4′-bipy)(H2O)2]n (6), HgCl2(2,2′-bipy) (7), [HgCl2(2,2′-bipy)]‚2HgCl2 (8), [Hg3Br6(2,2′-bipy)]‚HgBr2 (9), and [HgI2(2,2′-bipy)]‚HgI2 (10), which were obtained from the hydro/solvothermal and solid-state reactions of IIB metal halide/pseudohalide with 4,4′bipyridine or 2,2′-bipyridine. These compounds display a wide range of novel structure types from 0D and 1D to 2D and exhibit interesting properties. The solid-state diffuse reflectance spectra, IR, and TG-DTA of the title compounds are reported and the electronic transitions in the photoluminescence process of 1, 5, and 7 have been studied by means of time-dependent density functional theory (TDDFT) calculations. Experimental Section Measurements. Elemental analyses of carbon, hydrogen, and nitrogen were carried out with an Elementar Vario EL III microanalyzer. The infrared spectra were recorded on a PE Spectrum-One FT-IR spectrophotometer over the frequency range 4000-400 cm-1 using the KBr pellet technique. The UV-vis spectra were recorded at room temperature on a computer-controlled PE Lambda 35 UV-vis spectrometer equipped with an integrating sphere in the wavelength range 190-1100 nm. BaSO4 plate was used as a reference (100% reflectance), on which the finely ground powder of the samples were coated. The absorption spectra were calculated from reflection spectra by the Kubelka-Munk function:15 R/S ) (1 - R)2/2R, where R is the absorption coefficient, S is the scattering coefficient, which is practically wavelength independent when the particle size is larger than 5 µm, and R is the reflectance. Thermogravimetry-differential thermal analyses (TG-DTA) were performed on a NETZSCH STA 449C analyzer. The solid-state fluorescence studies were conducted at room temperature on an Edinburgh FLS920 or JY Fluorolog322 fluorescence spectroscopy instrument. Calculation Details. Time-dependent density functional theory (TDDFT) calculations were performed, employing the Gaussian03 suite of programs,16 at the B3LYP level. Calculations on the electronic ground states of compounds 1 and 5 were carried out using B3LYP density functional theory. “Doubleζ” quality basis sets were employed for the C, H, and N (631G) and the Zn, Br, Hg, and I (LANL2DZ). For compound 7, “Double-ζ” quality basis sets were employed for the C, H, and N (6-311G), the Cl [6-311++G (3df, 3pd)], and the Hg (LANL2DZ). The electron density diagrams of molecular orbitals were obtained with the ChemOffice Ultra 7.0 graphics program. Syntheses. All reactants of A. R. grade were obtained commercially and used without further purification. β-[ZnBr2(4,4′-bipy)]n (1). ZnBr2 (0.2 mmol, 45.0 mg), 4,4′bipy (0.2 mmol, 31.2 mg), and distilled water (3 mL) were loaded into a Teflon-lined stainless steel autoclave (25 mL) and kept at 373 K for 3 days. After being slowly cooled to room temperature at a rate of 6 K/h, colorless crystals suitable for X-ray analysis were obtained. Yield: 70% (based on zinc). Anal. Calcd for C10H8Br2N2Zn: C, 31.47; H, 2.10; N, 7.34. Found: C, 32.66; H, 2.27; N, 7.56. IR peaks (cm-1): 2960(m), 1611(s), 1535(w), 1415(m), 1262(s), 1214(w), 1100(s), 1071(vs), 1014(s), 805(vs), 724(w), 641(s), 477(s).

Chen et al.

[Cd2Br5(4,4′-bipyH)2](CdBr3) (2). This compound was prepared by the procedure described for 1 using CdBr2‚4H2O (0.3 mmol, 103.2 mg) instead of ZnBr2. Yield: 55% (based on cadmium). Anal. Calcd for C20H18Br8Cd3N4: C, 18.59; H, 1.39; N, 4.34. Found: C, 18.82; H, 1.34; N, 4.54. IR peaks (KBr, cm-1): 2926(m), 1642(m), 1615(vs), 1538(w), 1452(m), 1416(vs), 1384 (s), 1257(w), 1217(w), 1166(m), 1069(s), 1019(m), 817(vs), 714(w), 666(m), 506(m). [Cd3Br6(4,4′-bipy)]n (3). This compound was prepared from the reaction of CdBr2‚4H2O (0.3 mmol, 103.2 mg) and 4,4′bipy (0.2 mmol, 31.2 mg). The starting materials were ground into fine powders in an agate mortar before they were pressed into a pellet 1 cm in diameter. The pellet was then loaded into a silica tube. The tube was flame-sealed under a 1 × 10-3 Torr atmosphere and subsequently placed into a furnace. The tube then was heated to 573 K in 12 h from room temperature and kept for 10 days, followed by cooling to 373 K at a rate of 6 K/h to promote crystal growth; the power was then turned off. Yield: 51% (based on cadmium). Anal. Calcd for C10H8Br6Cd3N2: C, 12.34; H, 0.82; N, 2.88. Found: C, 12.36; H, 0.81; N, 2.87. IR peaks (KBr, cm-1): 2913(w), 1631(s), 1495(w), 1415(w), 1192(s), 1118(s), 1074(vs), 985(m), 804(m), 641(m), 610(s). [CdI2(4,4′-bipy)]n (4). CdI2 (0.2 mmol, 73.2 mg), 4,4′-bipy (0.2 mmol, 31.2 mg), and methanol (1 mL) were loaded into a silica tube and frozen in liquid nitrogen for 5 min. The tube was flame-sealed under a 1 × 10-3 Torr atmosphere and subsequently placed into a computer-controlled furnace. The tube then was heated to 623 K in 12 h from room temperature and kept there for 10 days, followed by cooling to 373 K at a rate of 6 K/h to promote crystal growth; the power was then turned off. Yield: 48% (based on cadmium). Anal. Calcd for C10H8CdI2N2: C, 22.97; H, 1.53; N, 5.36. Found: C, 21.84; H, 1.65; N, 5.12. IR peaks (KBr, cm-1): 2920(w), 1650(m), 1598(vs), 1531(m), 1490(m), 1411(s), 1323(m), 1221(m), 1098(m), 1074(m), 1043 (m), 994 (s), 810 (vs) and 615(vs). (HgI2)2(4,4′-bipy) (5). Compound 5 was prepared by the procedure described for 3 using HgI2 (0.4 mmol, 181.6 mg) instead of CdBr2‚4H2O, and the highest heating temperature was 383 K instead of 623 K. Yield: 45% (based on mercury). Anal. Calcd for C10H8Hg2I4N2: C, 11.27; H, 0.75; N, 2.63. Found: C, 11.25; H, 0.74; N, 2.63. IR peaks (KBr, cm-1): 2913(m), 2851(w), 1632(s), 1597(vs), 1532(m), 1481(m), 1405(s), 1317 (w), 1215(s), 1058(s), 996(m), 810(vs), 724(m), 621(vs). [Hg(CN)2(4,4′-bipy)(H2O)2]n (6). This compound was prepared by the procedure described for 1 using Hg(CN)2 (0.2 mmol, 50.4 mg) instead of ZnBr2. Yield: 63% (based on mercury). Anal. Calcd for C12H8HgN4: C, 32.36; H, 2.70; N, 12.58. Found: C, 33.67; H, 2.48; N, 13.20. IR peaks (KBr, cm-1): 2959(w), 2925(w), 2193(s), 1632(s), 1599(vs), 1535(w), 1486(w), 1412(s), 1262 (s), 1218(m), 1099(s), 1065(vs), 1033(s), 996(m), 806(vs), 726(w), 621(s), 442(s). HgCl2(2,2′-bipy) (7). Compound 7 was prepared by the procedure described for 3 using HgCl2 (0.2 mmol, 54.2 mg) and 2,2′-bipy (0.2 mmol, 31.2 mg) instead of CdBr2‚4H2O and 4,4′-bipy, and the highest heating temperature was 403 K instead of 623 K. Yield: 73% (based on mercury). Anal. Calcd for C10H8Cl2HgN2: C, 28.06; H, 1.87; N, 6.55. Found: C, 28.01; H, 1.88; N, 6.56. IR peaks (KBr, cm-1): 3097(w), 3063(w), 1633(m), 1590(vs), 1486(m), 1472(s), 1438(vs), 1314 (s), 1245(m), 1172(w), 1154(m), 1096(w), 1056(w), 1012(vs), 967(w), 898(w), 766(vs), 731(s), 648(s), 410(s). [HgCl2(2,2′-bipy)]‚2HgCl2 (8). This compound was prepared by the procedure described for 3 using HgCl2 (0.6 mmol, 162.6

Group 12 (IIB) Metal Halide/Pseudohalide-Bipy Systems

mg) and 2,2′-bipy (0.2 mmol, 31.2 mg) instead of CdBr2‚4H2O and 4,4′-bipy, and the highest heating temperature was 403 K instead of 623 K. Yield: 75% (based on mercury). Anal. Calcd for C10H8Cl6Hg3N2: C, 12.36; H, 0.82; N, 2.88. Found: C, 12.38; H, 0.82; N, 2.87. IR peaks (KBr, cm-1): 2919(m), 2850(w), 1638(s), 1587(m), 1470(m), 1434(s), 1308 (m), 1245(m), 1151(m), 1011(s), 763(vs), 646(m), 410(s). [Hg3Br6(2,2′-bipy)]‚HgBr2 (9). Compound 9 was prepared by the procedure described for 3 using HgBr2 (0.8 mmol, 288.0 mg) and 2,2′-bipy (0.2 mmol, 31.2 mg) instead of CdBr2‚4H2O and 4,4′-bipy, and the highest heating temperature was 473 K instead of 623 K. Yield: 56% (based on mercury). Anal. Calcd for C10H8Br8Hg4N2: C, 7.51; H, 0.50; N, 1.75. Found: C, 7.52; H, 0.51; N, 1.74. IR peaks (KBr, cm-1): 2957(w), 2923(w), 1613(m), 1438(m), 1384 (w), 1260(m), 1192(vs), 1117(vs), 1072(vs), 1015(m), 983(m), 800(w), 765(s), 640(m), 610(s). [HgI2(2,2′-bipy)]‚HgI2 (10). This compound was prepared by the procedure described for 3 using HgI2 (0.4 mmol, 181.6 mg) and 2,2′-bipy (0.2 mmol, 31.2 mg) instead of CdBr2‚4H2O and 4,4′-bipy, and the highest heating temperature was 473 K instead of 623 K. Yield: 61% (based on mercury). Anal. Calcd for C10H8Hg2I4N2: C, 11.27; H, 0.75; N, 2.63. Found: C, 11.24; H, 0.76; N, 2.64. IR peaks (KBr, cm-1): 2963(m), 2923(w), 1631(m), 1590(s), 1468(m), 1436(s), 1383 (m), 1310(w), 126(vs), 1096(vs), 1023(vs), 1015(vs), 802(vs), 765(s), 647(w), 607(m). X-ray Crystallographic Studies. The intensity data sets were collected on Rigaku Mercury CCD (1, 3, 4, 6, and 8-10), Rigaku AFC-7R (2, 5), and Siemens SMART CCD (7) X-ray diffractometers with graphite-monochromated Mo-KR radiation (λ ) 0.71073 Å) using the ω scan technique. CrystalClear (1, 3, 4, 6, and 8-10), CrystalStructure (2, 5), and Siemens SAINT (7) software programs were used for data reduction and empirical absorption corrections.17 The structures were solved by direct methods using the Siemens SHELXTL version 5 package of crystallographic software.18 The difference Fourier maps based on these atomic positions yield the other nonhydrogen atoms. The hydrogen atom positions were generated symmetrically, allowed to ride on their respective parent atoms, and included in the structure factor calculations with assigned isotropic thermal parameters. The structures were refined using a full-matrix least-squares refinement on F2. All atoms except for hydrogen atoms were refined anisotropically. X-ray powder diffraction (XRPD) patterns were measured on a Rigaku DMAX2500 powder diffractometer at 40 kV and 100 mA using Cu-KR (λ ) 1.54056 Å) with a scan speed of 0.375 s/step and a step size of 0.05°. The simulated powder patterns were calculated using single-crystal X-ray diffraction data and processed by the free Mercury version 1.4 program provided by the Cambridge Crystallographic Data Centre, as shown in Figure S1 of the Supporting Information. Results and Discussion Syntheses and General Characterization. Hydrothermal synthesis has recently been proven to be a useful technique in the preparation of solid-state inorganic-organic hybrid materials; this method is well-known for its effectiveness in promoting crystal growth.19 When superheated, water behaves very differently from what is observed under ambient conditions. The significantly lowered viscosity, for example, increases the solubility as well as the diffusion rate of the solid reagents, therefore enhancing the crystal growth. Recently, we have initiated a program focusing on the single-crystal growth of transition-metal dihalide bipy inorganic-organic hybrid materi-

Crystal Growth & Design, Vol. 6, No. 10, 2006 2291

als. In most of the reactions of metal halide with bipy, the products are normally obtained as powdered materials through conventional synthetic methods, so their structures are difficult to characterize. To gain suitable single crystals for X-ray diffraction, we have explored the possibility of growing single crystals of metal dichloride bipy inorganic-organic hybrid materials by hydrothermal reactions. During the hydrothermal syntheses of metal halide-bipy compounds, the Teflon-lined stainless steel autoclaves could not be heated to 523 K. Additionally, the water used as solvent in hydrothermal reactions could affect the final products, for example, to protonate the product, as was the case in 2, or to hydrate the product, as was the case in 6. To overcome the effects of water, we used fused-silica tubes and methanol as vessels and solvent, respectively, to substitute autoclaves and water. Compound 4 was prepared via this method. To entirely exclude the effects of the solvents, we exploited the solid-state reaction method and compounds 3, 5, and 7-10 were successfully synthesized through this method. In a word, by improving the synthetic methods, a total of ten IIB metal halide/pseudohalide-bipy compounds in the crystalline form have been successfully obtained. The IR spectra of 1-10 show the bands in the range of 5001700 cm-1 that are attributed to bipy. For 7-10, the bands at around 770 cm-1 provide evidence that the 2,2′-bipy is coordinated.20 The IR spectrum of 6 shows a sharp band in the range of 2100-2200 cm-1, which is attributed to the C≡N stretching modes, indicating the existence of the cyanide group. Description of Structures. The summary of crystallographic data and structure analyses for 1-10 is listed in Table 1. Selected bond lengths and bond angles are listed in Table S1 of the Supporting Information. β-[ZnBr2(4,4′-bipy)]n (1). Hu et al. have reported the R-form that exhibits a zigzag chain with a tetrahedral arrangement of zinc center,21 and they also reported a Cl analogue very recently.22 X-ray diffraction analysis reveals that compound 1 features a zigzag chain structure, which is similar to its R-form. Compound 1 belongs to the orthorhombic system with Pnma space group, whereas the R-form is monoclinic C2/c. The tetrahedral zinc atom is coordinated by two bridging 4,4′-bipy molecules and two terminal bromine atoms with the bond lengths of Zn1-N1, Zn1-N2(x - 1/2, y, -z - 1/2), and Zn1Br1 being 2.054(4), 2.060(4), and 2.320(1) Å, respectively. The Zn-N distances are normal and comparable with those in the literature.23 The two µ2-4,4′-bipy ligands around a zinc(II) ion are nearly perpendicular to each other, with the N1-Zn1-N2 (x - 1/2, y, -z - 1/2) bond angle being 106.5(2)°, which is smaller than the 107.17° of the R-form.21 The zinc(II) ions, separated at ca. 11.186 Å (11.192 Å in the R-form), are bridged by µ2-4,4′-bipy ligands to form the inorganic-organic hybrid polymeric chains running along the a direction (Figure 1). The hybrid chains connect to each other through the C-H‚‚‚Br hydrogen bonds (C‚‚‚Br ) 3.522(5) Å, H‚‚‚Br ) 3.466 Å, and C-H‚‚‚Br ) 85.76 °) and weak π‚‚‚π stacking interactions (centroid-to-centroid distance is ca. 3.921 Å), forming a layer parallel to the ac plane (Figure S2 of the Supporting Information). These layers stack in an ABAB-fashion along the b axis to complete the 3D structure (Figure S3 of the Supporting Information). The rings of the µ2-4,4′-bipy ligand involved in the π‚‚‚π stacking interactions are arranged in such a way that the six atoms of the ring do not completely eclipse those of the other ring, meaning that the interaction is not “perfect face alignment” but “offset or slipped stacking” (distances between the corre-


Chen et al.

Table 1. Crystal Parameters of 1-10

formula fw color cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) V (Å3) Z 2θmax (deg) no. of reflns collected no. of independent, observed reflns (Rint) dcalcd (g/cm3) µ (mm-1) T (K) F(000) R1, wR2 S largest and mean ∆/σ ∆F(max/min) (e/Å3)

1

2

3

C10H8Br2N2Zn 381.37 colorless 0.45 × 0.15 × 0.10 orthorhombic Pnma 17.032(3) 12.774(6) 5.395(2) 1173.8(7) 4 50 6945 1075, 629 (0.0370) 2.158 8.867 293(2) 728 0.0577, 0.1491 1.008 0.001, 0 0.788/-0.848

C20H18Br8Cd3N4 1290.86 yellow 0.07 × 0.07 × 0.05 orthorhombic Pnma 14.845(5) 30.493(8) 8.876(2) 4018(2) 4 50 3700 3566, 1479 (0.0269) 2.134 9.543 293(2) 2360 0.0684, 0.0752 0.971 0.001, 0 0.951/-0.997

C10H8Br6Cd3N2 972.84 yellow 0.45 × 0.04 × 0.04 orthorhombic P21212 13.343(2) 17.950(3) 3.9550(5) 947.2(2) 2 50 5874 1678, 1549 (0.0387) 3.411 15.983 293(2) 872 0.0352, 0.0723 1.068 0.005, 0.001 0.865/-1.243

4 formula fw color cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z 2θmax (deg) no. of reflns collected no. of independent, observed reflns (Rint) dcalcd (g/cm3) µ (mm-1) T (K) F(000) R1, wR2 S largest and mean ∆/σ ∆F(max/min) (e/Å3)

formula fw color cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z 2θmax (deg) no. of reflns collected no. of independent, observed reflns (Rint) dcalcd (g/cm3) µ (mm-1) T (K) F(000) R1, wR2 S largest and mean ∆/σ ∆F(max/min) (e/Å3)

C10H8CdI2N2 522.38 colorless 0.20 0.10 0.08 orthorhombic Cmmm 11.837(2) 13.129(3) 4.1515(8) 645.2(2) 2 50 2107 359, 353 (0.0196) 2.689 6.448 293(2) 472 0.0165, 0.0415 1.006 0, 0 1.524/-0.636

5

6

C10H8Hg2I4N2 1064.96 colorless 0.08 0.03 0.03 monoclinic C2/c 25.760(4) 4.425(5) 18.225(8) 120.44(3) 1791.1(2) 4 50 1679 1580, 1129 (0.0150) 3.950 24.004 293(2) 1816 0.0391, 0.0643 1.007 0.001, 0 0.940/-0.858

C12H12HgN4O2 444.85 colorless 0.20 0.20 0.15 monoclinic C2/c 17.218(3) 4.949(8) 16.279(3) 116.0(1) 1247(2) 4 50 4117 1087, 875 (0.0914) 2.370 12.347 293(2) 832 0.0570, 0.1362 1.002 0.001, 0 2.503/-1.587

7

8

9

10

C10H8Cl2HgN2 427.67 colorless 0.48 0.14 0.10 monoclinic C2/c 16.994(4) 8.635(2) 7.719(2) 101.970(4) 1108.0(4) 4 50 1514 952, 803 (0.0457) 2.564 14.337 293(2) 784 0.0564, 0.1397 1.002 Row700.001, 0 1.623/-1.694

C10H8Cl6Hg3N2 970.65 colorless 0.23 0.08 0.04 monoclinic C2/c 12.350(2) 20.674(3) 7.174(2) 100.621(9) 1800.4(6) 4 50 5893 1589, 1322 (0.0462) 3.581 26.401 293(2) 1696 0.0323, 0.0662 1.007 Row700.002, 0 1.364/-1.193

C10H8Br8Hg4N2 1597.82 colorless 0.24 0.10 0.05 monoclinic P21 4.1325(6) 12.926(2) 22.610(5) 93.138(8) 1206.0(4) 2 50 7960 4016, 2718 (0.0501) 4.400 38.633 293(2) 1364 0.0552, 0.0951 1.003 Row700.003, 0 1.300/-1.456

C10H8Hg2I4N2 1064.96 yellow 0.40 0.05 0.05 monoclinic P21/c 4.339(8) 13.517(4) 15.208(8) 102.58(1) 871(2) 2 50 5244 1520, 747 (0.1084) 4.063 24.692 293(2) 908 0.0634, 0.1317 1.006 Row700.001, 0 1.264/-1.786

sponding atoms of the two rings are 4.006, 3.929, 3.923, 3.985, 3.923, and 3.929 Å, respectively). For clarity, we define the

two pyridyl rings of the µ2-4,4′-bipy ligand as R1 and R2, containing N1 and N2 atoms, respectively. The R1 ring in one


Figure 1. ORTEP drawing of 1 with 30% probability level thermal ellipsoids. Hydrogen atoms were omitted for clarity.

Figure 2. ORTEP drawing of 2 with 30% probability level thermal ellipsoids.

chain has a π‚‚‚π contact with a R2′ ring in one adjacent chain; the R2 ring in the same chain connects to a R1′ ring in another adjacent chain via π‚‚‚π interaction (see Figure S2 of the Supporting Information). The Br atoms link to two carbon atoms of the R1 ring via C-H‚‚‚Br hydrogen bonds. For each µ24,4′-bipy ligand, rings R1 and R2 are nearly coplanar; they have a very small dihedral angle of 0.31°, which is similar to the case in the R-form (0.049°) and remarkably different from the cases found in many other compounds containing 4,4′-bipy ligands in which the two pyridyl rings are far from coplanar.2c,24 This small dihedral angle may be due to the following: (1) two zinc atoms link to a µ2-4,4′-bipy ligand; (2) the C-H‚‚‚Br hydrogen bonds; and (3) the π‚‚‚π interaction between the rings, which impede the rotation of the rings. [Cd2Br5(4,4′-bipyH)2](CdBr3) (2). An ORTEP drawing of 2 is shown in Figure 2. The structure of 2 consists of [Cd2Br5(4,4′-bipyH)2]+ cations and CdBr3- anions. The Cd1 atom has an approximately right trigonal geometry, coordinating with three bromine atoms with bond lengths of 2.689(2), 2.709(2), and 2.688(2) Å for Cd1-Br1, Cd1-Br2, and Cd1-Br3, respectively, and bond angles of 118.67(8), 121.12(8), and 120.20(8)° for Br1-Cd1-Br2, Br1-Cd1-Br3, and Br2-Cd1Br3, respectively. The Cd2 atom is in a slightly distorted tetrahedron, with three bromine atoms and one nitrogen atom coordinating to it. The Br4 and Br5 atoms are terminally bonded to Cd2 with approximately equivalent bond lengths of 2.646(2) and 2.657(2) Å, which are slightly smaller than those in the reference.11c The Br6 atom is a µ2-bridge, bridging two Cd2 atoms with a Cd2‚‚‚Cd2′ distance of ca. 5.884 Å. The bond length of Cd2-Br6 (2.942(1) Å) is obviously longer than those of Cd2-Br4 and Cd2-Br5. The bond angle of N1-Cd2-Br6 is 87.1(2)°, close to a right angle. The bond angle of Cd2-


Br6-Cd2′(-x, 1 - y, 2 - z) is 180.0°. For the requirement of charge balance, the noncoordinated nitrogen atom of the 4,4′bipy ligand must be protonated, as is the case in many other compounds.2 A zigzag dinuclear [Cd2Br5(4,4′-bipyH)2]+ cation is constructed from two CdBr2(4,4′-bipyH) moieties bridged by a µ2-Br6 atom. The electrostatic interactions between the dinuclear [Cd2Br5(4,4′-bipyH)2]+ cations and the CdBr3- anions contribute to the stabilization of the crystal packing of 2 (Figure S4 of the Supporting Information). It is noteworthy that a search of the Cambridge Structural Database (CSD) for cadmium-4,4′-bipy systems shows that the distances of the Cd-N4,4′-bipy range between 2.24925 and 2.49526 Å; these values are obviously shorter than the value of 2.527(7) Å of the Cd-N4,4′-bipy distance in compound 2. Two reasons may be responsible for this discrepancy: (1) the attraction of the proton in the protonated nitrogen atom reduces the electron density on the nitrogen atom coordinated to the cadmium atom, similar to the cases found in the literature;27 and (2) the bromine atoms coordinated to the cadmium atom increase the electron density of the cadmium, as in the cases found in the references.28 Both aspects may weaken and lengthen the Cd-N4,4′-bipy bond of 2. [Cd3Br6(4,4′-bipy)]n (3). The structure of 3 comprises polymeric [Cd3Br6(4,4′-bipy)]n neutral layers. The Cd1 atom is coordinated by six bromine atoms, whereas the Cd2 atom is bound by five bromine atoms and one nitrogen atom from the bridging 4,4′-bipy ligand, forming CdBr6 and CdBr5N octahedra, respectively. The CdBr6 octahedra are edge-shared to each other via two µ3-Br atoms to form a straight chain with the bond lengths of Cd1-Br1, Cd1-Br3, and Cd1-Br3(x, y, z + 1) being 2.7708(5), 2.7700(5), and 2.7685(5) Å, respectively, which are comparable to those in the literature.29 Similarly, the CdBr5N octahedra are also edge-shared to each other via one µ2-Br and one µ3-Br atom, forming a linear chain with bond lengths of 2.334(2), 2.7023(5), 2.6921(5), 2.8265(5), 2.8581(5) and 2.9107(5) Å for Cd2-N1, Cd2-Br2, Cd2-Br2(x, y, z - 1), Cd2Br1(-x - 1, -y + 1, z), Cd2-Br1(-x - 1, -y + 1, z - 1), and Cd2-Br3, respectively. The Cd1 octahedral chain condenses with two Cd2 octahedral chains through edge-sharing to construct a triple inorganic chain extending along the c direction, which, to the best of our knowledge, is the first example among the metal-bipy systems. The triple chains are bridged by 4,4′bipy ligands along the b direction to form an inorganic-organic hybrid 2D layer (Figure 3), which is different from the known 2D metal-4,4′-bipy structures: baluster network,30 herringbone architecture,31 diamondoid,32 brick wall,33 ladder,34 square network,35 and so forth. A 3D structure is constructed from the stack of the layers in an ABAB mode along the a axis (Figure S5 of the Supporting Information). The two pyridyl rings of the 4,4′-bipy ligand are twisted with a large dihedral angle of ca. 35.93°, which is comparable to those previously documented.10b,36 There exist weak π‚‚‚π contacts (ca. 3.955 Å) between the µ2-4,4′-bipy ligands in the layer. The rings of the 4,4′-bipy involved in the π‚‚‚π interactions are arranged in such a way that the six atoms of the ring completely eclipse those of the other ring, meaning that the interaction is “perfect face alignment” (all the distances between the corresponding atoms of the two rings are of ca. 3.955 Å). The angle formed by the centroids of the three consecutive rings is 180.0°, which also suggests a complete interaction. [CdI2(4,4′-bipy)]n (4). The structure of 4 has been reported with a less-detailed structural description,11c and the β- and γ-forms have also been documented.37 Similar to 3, the crystal structure of 4 consists of 2D [CdI2(4,4′-bipy)]n neutral networks,


Chen et al.

Figure 3. Two-dimensional layered structure of 3 with hydrogen atoms being omitted for clarity. The green dashed lines represent the π-π stacking interactions.

Figure 4. Layered structure of 4 with hydrogen atoms omitted for clarity. The dashed lines represent the π-π stacking interactions.

as shown in Figure 4. The divalent metal centers have a slightly distorted octahedral coordination, with four µ2-I and two bridging 4,4′-bipy at trans positions, yielding a CdI4N2 octahedron. The bond length of Cd1-N1 is 2.364(4) Å, which is in the normal range and obviously shorter than that of 2. The bond length of Cd1-I1 is 2.9757(5) Å, which is comparable with those reported.38 The bond angles of N1-Cd1-N1(-x + 1, -y + 1, -z + 1), N1-Cd1-I1, I1-Cd1-I(-x + 1, -y + 1, -z + 1), I1(-x + 1, -y + 1, -z + 1)-Cd1-I1(-x + 1, -y + 1, -z), and I1-Cd1-I1(-x + 1, -y + 1, -z) are 180.0, 90.0, 180.0, 88.46(2), and 91.54(2)°, respectively. The CdI4N2 octahedra interconnect to each other via two µ2-I atoms, forming a linear inorganic chain running along the c direction. These chains are bridged by µ2-4,4′-bipy ligands to form an inorganicorganic hybrid 2D layer parallel to the ac plane (Figure 4). These layers stack in an ABAB mode along the b axis to yield a 3D structure (see Figure S6 of the Supporting Information). In this case, the π‚‚‚π stacking interactions in the layer also gives rise to perfect facial alignment, as shown by the angle of the centroids of three consecutive rings (180.0 °). The two pyridyl rings of the 4,4′-bipy ligand are slightly twisted with a small dihedral angle of ca. 2.23°, which is obviously different from that in 3 but similar to that in 1 and other literature.11b In each layer, the neighboring µ2-4,4′-bipy ligands interact through weak π‚‚‚π contacts (ca. 4.152 Å). Both compounds 3 and 4 feature an inorganic-organic hybrid 2D layered structure, which stacks in an ABAB mode to construct a 3D structure. However, compound 3 has a wavy layer formed by alternately interconnecting Cd-Br octahedral triple chains and 4,4′-bipy ligands, whereas compound 4 has a


planar slab constructed from Cd-I octahedral monochains bridged by 4,4′-bipy ligands (see Figures S5 and S6 of the Supporting Information). The triple chains and monochains in 3 and 4 are different from the double chains reported by Lu et al.39 (HgI2)2(4,4′-bipy) (5). The structure of 5 consists of discrete (HgI2)2(4,4′-bipy) molecules as shown in Figure 5. The Hg1 atom coordinates to two terminal iodine atoms and one nitrogen atom from 4,4′-bipy ligand in a trigonal environment with bond lengths of 2.615(1), 2.635(1) and 2.507(3) Å for Hg1-I1, Hg1I2, and Hg1-N1, respectively, which are comparable with those in the literature.40 Two HgI2 entities are bridged by one 4,4′bipy ligand to construct an isolated (HgI2)2(4,4′-bipy) molecule. The distance between the centroids of two neighboring 4,4′bipy ligands is ca. 4.425 Å, indicating no π‚‚‚π stacking interactions. Therefore, the (HgI2)2(4,4′-bipy) molecules are stacked together via van de Waals’ force (see Figure S7 of the Supporting Information). Generally, 4,4′-bipy acting as a bridging ligand favors linking metal atoms into an extended


Figure 6. ORTEP drawing of 6 with 30% probability level thermal ellipsoids. Hydrogen atoms were omitted for clarity.

structure, whereas discrete structures containing bridging 4,4′bipy ligands are relatively rare in metal halides.41 The discrete group 12 metal halides containing bridging 4,4′-bipy ligands are rare, although a similar structure of [Hg2I4(Pyp)] consisting of two HgI2 units connected by a pyrazine molecule has been reported by Nockemann et al.42 [Hg(CN)2(4,4′-bipy)(H2O)2]n (6). Compound 6 features an infinite zigzag [Hg(CN)2(4,4′-bipy)(H2O)2]n chain structure, as shown in Figure 6. The octahedral Hg1 atom is bridged by two 4,4′-bipy molecules and terminally coordinated by two water molecules and two cyanide groups with bond lengths of 2.449(5), 1.953(3) and 1.997(5) Å for Hg1-N1, Hg1-O1W, and Hg1-C11, respectively. The two µ2-4,4′-bipy ligands around a mercury ion are nearly perpendicular to each other, with a N1-Hg1-N1(-x + 1/2, -y - 1/2, -z) bond angle of 97.8(2)°. The mercury ions are bridged by µ2-4,4′-bipy ligands to form a zigzag hybrid chain running along the [1 0 1] direction, with a Hg1‚‚‚Hg1 separation of ca. 12.125 Å. Similar to what we did for 1, for convenience, we define the two pyridyl rings of the µ2-4,4′-bipy ligand as R1 and R2, respectively. The R1 ring in one chain has a π‚‚‚π contact (centroid-to-centroid distance is ca. 3.719 Å) with a R2′ ring in one adjacent chain; the R2 ring in the same chain connects to a R1′ ring in another adjacent chain via π‚‚‚π interaction (centroid-to centroid distance is ca. 3.719 Å). There are Owater-H‚‚‚π-electron interactions between the coordination water molecules and the pyridyl rings, with the distance from the centroid of the pyridyl ring to the oxygen atom of the water molecule being ca. 2.011 Å. The chains link together via the π‚‚‚π and Owater-H‚‚‚π-electron interactions, forming a layer parallel to the crystallographic ab plane (see Figure S8 of the Supporting Information), which further stack on top of each other in an ABAB mode to complete a three-dimensional structure (see Figure S9 of the Supporting Information). To the best of our knowledge, compound 6 is the first chainlike structure of metal cyanide bridged by 4,4′-bipy, although a 2D sheet of metal cyanide bridged by 4,4′-bipy has been reported.43 The Owater-H‚‚‚π-electron interactions in metalbipy systems are not common, although many compounds with O-H‚‚‚π-electron interactions in other systems have been reported.44 HgCl2(2,2′-bipy) (7). An ORTEP drawing of 7 together with the atomic numbering scheme is shown in Figure 7. The structure of 7 consists of isolated HgCl2(2,2′-bipy) molecules. The Hg(II) ion has a distorted tetrahedral geometry, coordinated by two terminal chlorine atoms and two nitrogen atoms from one 2,2′-bipy ligand. The bond lengths of Hg1-Cl1 and Hg1-




N1 are 2.415(1) and 2.392(4) Å, respectively, which are comparable with the counterparts found in the literature.45 The 2,2′-bipy ligand is in a classical coordination mode with its two nitrogen atoms chelating to one metal center. The bite angle of N1-Hg1-N1(-x, y, -z - 1/2) is 69.3(2)°, slightly larger than the corresponding value in the reference.45 The two pyridyl rings of the 2,2′-bipy ligand are approximately coplanar, with a small dihedral angle of 1.65°, which is similar to the case found in the literature.46 The HgCl2(2,2′-bipy) molecules arrange in a head-to-tail mode along the b axis, with the angle of the centroids of the three consecutive rings being around 175.4°. There are weak π‚‚‚π stacking interactions between the adjacent molecules, with the centroid-to-centroid distance of the 2,2′bipy ligands being ca. 3.863 Å. The HgCl2(2,2′-bipy) molecules pack in an ABAB mode along the c axis, as shown in Figure S10 of the Supporting Information. In an analogous compound HgI2(2,2′-bipy),47 the HgI2(2,2′-bipy) molecules stack in an AAAA mode along the c axis, although they also arrange in a head-to-tail mode along the b axis. [HgCl2(2,2′-bipy)]‚2HgCl2 (8). Figure 8shows a perspective view of 8. The structure of 8 comprises neutral isolated HgCl2(2,2′-bipy) and HgCl2 moieties. The Hg1 atom is coordinated by two chlorine atoms and two nitrogen atoms from one 2,2′bipy, yielding a distorted tetrahedron. The Hg2 atom is 2-fold coordinated with two chlorine atoms in an approximately linear arrangement (Cl2-Hg2-Cl3 ) 173.39(5)°, which is comparable with that of isolated HgCl2 moieties in the literature48). The bond lengths of Hg1-Cl1 and Hg1-N1 are 2.511(1) and 2.296(4) Å, which are about 0.1 Å longer and shorter,


Chen et al.

Figure 9. ORTEP drawing of 9 with 30% probability level thermal ellipsoids. The occupancies of Hg3 and Hg5 are equal to 0.5 (symmetry code: a: -1 + x, y, z).

respectively, than their counterparts in 7. The bond lengths of Hg2-Cl2 and Hg2-Cl3 are 2.290(2) and 2.273(1) Å, respectively, obviously shorter than that of Hg1-Cl1. This is due to the fact that the formation of the coordinative Hg1-N bonds weakens the Hg1-Cl1 bond. The two nitrogen atoms of the 2,2′-bipy ligand chelate to the metal center with a N1-Hg1N1(-x + 1, y, -z - 1/2) bite angle of 71.7(2)°, slightly larger than those in 7 and the literature.47 The dihedral angle between the two pyridyl rings of the 2,2′-bipy ligand is 3.56°. In this structure, no π‚‚‚π stacking interactions were established between the 2,2′-bipy ligands. This is different from the case of 7. Therefore, the van de Waals’ force should be the only contribution to the stability of the crystal packing (Figure S11 of the Supporting Information). It is noteworth that the HgCl2(2,2′-bipy) moieties in 8 array in a head-to-head mode along the b axis, which is different from the head-to-tail mode of 7. Another feature of 8 is the coexistence of the HgCl2 moieties widening the distances among the HgCl2(2,2′-bipy) moieties in the a and b directions. [Hg3Br6(2,2′-bipy)]‚HgBr2 (9). The structure of 9 consists of neutral Hg3Br6(2,2′-bipy) moieties and discrete HgBr2 molecules (Figure 9). There are four couples of indistinguishable carbon and nitrogen atoms of the 2,2′-bipy occupying the same sites with equivalent occupancies, i.e., C9-N1, C10-N2, C11N3, and C12-N4 atom pairs, respectively. The mercury atoms are in four different coordination environments. The Hg1 atom is coordinated by two terminal bromine atoms in a linear mode (Br1-Hg1-Br2 ) 179.46(3)°) with bond lengths of 2.4381(6) and 2.4305(6) Å for Hg1-Br1 and Hg1-Br2, respectively, which are comparable with those in the discrete HgBr2 molecules.49 The Hg2 and Hg4 atoms are coordinated by two terminal (Br3 and Br4 for Hg2, Br6 and Br7 for Hg4) and one bridged (Br5 for Hg2, Br8 for Hg4) bromine atoms in a trigonal environment. The Hg2-Br5 and Hg4-Br8 bond distances of ca. 3.0 Å are comparable with those of Hg-Brbridged in the literature.49b,50 It should be noted that the occupancies of Hg3 and Hg5 atoms must be set to 0.5 to get rational structure model and thermal displacement parameters. The Hg3 atom is fourcoordinated in a distorted tetrahedral environment with two bridged bromine atoms (Br5 and Br5A(-1 + x, y, z)) and two nitrogen atoms (N1/C9 and N2/C10) from the 2,2′-bipy ligand, with bond lengths of 2.3938(8), 2.5039(8), 2.256(5) and 2.375(4) Å for Hg3-Br5, Hg3-Br5A, Hg3-N1, and Hg3-N2, respectively. The Hg5 atom is five-coordinated by three bromine atoms and two nitrogen atoms (N3/C11 and N4/C12) from the 2,2′-bipy ligand, with bond lengths of 3.1125(9), 2.4156(8), 2.7157(8), 2.252(5) and 2.574(6) Å for Hg5-Br6A, Hg5-Br8, Hg5-Br8A, Hg5-N3, and Hg5-N4, respectively. Although the two-, three-, four-, and five-coordination geometry of mercury have been reported in separate cases, to the best of our knowledge, this is the first time these four kinds of coordination geometry have been found coexisting in one mercury compound.

Figure 10. ORTEP drawing of 10 with 30% probability level thermal ellipsoids. The occupancies of Hg1 and Hg2 are equal to 0.5. Table 2. Comparison of the Energy Band Gaps of 1-10 compd

molecular formula

energy band gap Eg (eV)

1 2 3 4 5 6 7 8 9 10

β-[ZnBr2(4,4′-bipy)]n 1-D [Cd2Br5(4,4′-bipyH)2](CdBr3) 0-D [Cd3Br6(4,4′-bipy)]n 2-D [CdI2(4,4′-bipy)]n 2-D (HgI2)2(4,4′-bipy) 0-D [Hg(CN)2(4,4′-bipy)(H2O)2]n 1-D HgCl2(2,2′-bipy) 0-D [HgCl2(2,2′-bipy)]‚2HgCl2 0-D [Hg3Br6(2,2′-bipy)]‚HgBr2 0-D [HgI2(2,2′-bipy)]‚HgI2 0-D

4.74 4.69 4.63 4.60 2.84 3.05 3.38 3.79 3.04 2.10

For the structure of 9, a Flack x parameter of 0.00(6) was calculated, indicating a correct absolute structure.51 The bite angles of N1-Hg3-N2 and N3-Hg5-N4 are 73.0(2) and 72.0(2)°, slightly larger than those in 7, 8, and the reference.47 The two pyridyl rings of the 2,2′-bipy ligand have a small dihedral angle of 3.62°, close to the value in 8. In this case, the π‚‚‚π stacking interactions also gives rise to perfect facial alignment, as shown by the angle formed by the centroids of the three consecutive rings (ca. 179.98 °). The neighboring 2,2′-bipy ligand interact through weak π‚‚‚π contacts (ca. 4.133 Å), as shown in Figure S12 of the Supporting Information. [HgI2(2,2′-bipy)]‚HgI2 (10). The discrete HgI2(2,2′-bipy) and HgI2 moieties construct the crystal structure of 10, as shown in Figure 10. The Hg1 atom is coordinated by two terminal iodine atoms in a linear mode (I1-Hg1-I1A(-x, -y, -z) ) 180.00(2)°), with the bond length of Hg1-I1 being 2.557(2) Å, which is comparable with that in the isolated HgI2 moiety.52 C3-N1 and C5-N2 atom pairs are indistinguishable and occupy the same sites with equivalent occupancies. The Hg2 atom is fourcoordinated in a distorted tetrahedral environment with two bridging iodine atoms, one carbon atom, and one nitrogen atom from the 2,2′-bipy ligand with bond lengths of 2.477(3), 2.535(4), 2.388(5) and 2.501(4) Å for Hg2-I2, Hg2-I2A(x + 1, y, z), Hg2-N1, and Hg2-N2, respectively. The bite angle of N1-Hg2-N2 is 68.1(2)°, which is smaller than those in 7-9 and the literature.47 The bond angle of I2-Hg2-I2A(x + 1, y, z) is 120.0(1)°. The two pyridyl rings of the 2,2′-bipy ligand are entirely parallel, with a dihedral angle of zero. In this structure, no π‚‚‚π stacking interaction is found among the 2,2′bipy ligands (Figure S13 of the Supporting Information), which is different from the cases of 7 and 9 but similar to that of 8. The nitrogen atoms of the 2,2′-bipy ligands in both 9 and 10 are disordered, as in the case found in the literature.53 However, for the same 2,2′-bipy-containing compounds 7 and 8, no disordered nitrogen atoms are found. This discrepancy should be related to three possible factors: the reaction temperatures (403 K for 7 and 8; 473 K for 9 and 10), the ratios of the



Figure 11. Solid-state emission spectra for 1-10. Inset: excitation bands.

reactants, and the halogen atoms. Both compounds 7 and 8 are prepared with HgCl2 and 2,2′-bipy in different ratios under the same reaction temperatures, but no disordered 2,2′-bipy are found, which suggests that the ratios of the reactants are not the dominating factor for the disorder. Therefore, the dominating factors may be the reaction temperatures and the halogen atoms. Further investigations on this system are in progress. Thermal Analysis. To examine the thermal stability of 1-10, thermal gravimetric-differential thermal analyses (TG-DTA) were carried out (see Figures S14-S23 of the Supporting Information). The TG-DTA shows that all the compounds except for 2 are thermally stable up to 100 °C (see Table S2 of the Supporting Information). The differences in thermal stability are ascribable to the different interactions that exist in 1-10, such as hydrogen bonds and π‚‚‚π stacking interactions. For example, compound 7 has π‚‚‚π stacking interactions, whereas the structurally similar compound 8 does not have π‚‚‚π interactions; therefore, compound 7 is more thermally stable

than 8 (see Table S2 of the Supporting Information). For another example, compound 9 is more thermally stable than 10, because the former has π‚‚‚π stacking interactions, whereas the latter does not. UV-Vis Spectroscopy. The band gap energy value was determined by extrapolation from the linear portion of the absorption edge in a (R/S) versus energy plot.54 The energy band gaps of the title compounds are 4.74, 4.69, 4.63, 4.60, 2.84, 3.05, 3.38, 3.79, 3.04, and 2.10 eV for 1-10, respectively (Table 2 and Figures S24-S30). The gradual slope of the optical absorption edges for 1-10 is indicative of the existence of indirect transitions.55 The band gaps of compounds are related to the ratio of the covalent/ionic bonding and the dimensionality or complexity of the structures. From Table 2, two trends of band gaps for the present compounds are observed: (a) the band gaps of the compounds decrease according to the order of Zn > Cd > Hg or Cl > Br > I, which originate from the higher ratio of the covalent/ionic bonding, as in the case found in the


Chen et al.

Figure 12. Electron-density distribution of HOMO (left) and LUMO (right) calculated for (a) 1 and (c) 7, and (b) top, LUMO and bottom, HOMO, HOMO-1, HOMO-2 calculated for 5. The isosurfaces correspond to electronic density differences of -0.015 e Å-3 (blue) and +0.015 e Å-3 (red).

literature;56 (b) the band gaps of the compounds decrease with increasing dimensionality or complexity of the structures, as pointed out by Kanatzidis57 and Papavassiliou.58 The structure of 3 is more complex than that of 4, which results in the band gap of 3 being smaller than that of 4, whereas the lower ratio of covalent/ionic bonding of 3 makes the band gap of 3 larger than that of 4, so the larger band gap of 3 than 4 from the UVvis measurement implies that the band gap is dominated by the ratio of covalent/ionic bonding. Fluorescence Properties. The emission spectra of 1-10 in the solid state at room temperature are investigated. It can be observed that a broad emission with a maximum wavelength of 512 nm upon photoexcitation at 328 nm for 1, which is redshifted by 74 nm compared with that of pure 4,4′-bipy ligand (Figures 11a and S31). Compound 2 has a strong emission band centered at 396 nm with an excitation band at 336 nm (Figure 11b). The emission spectrum of 3 features an overlapping band formed by two bands centered at 520 and 550 nm upon photoexcitation at 293 nm (Figure 11c). Compound 4 exhibits one strong emission maximum at 425 nm (λexc ) 351 nm) (Figure 11d). An intense emission band of 5 is found at 441 nm with an excitation wavelength of 394 nm (Figure 11e). Upon

excitation at 355 nm, an intense emission spectrum of 6 is observed in the blue region at 433 nm (Figure 11f). When excited by 365 nm light, compound 7 exhibits an emission spectrum centered at 430 nm, which is blue-shifted by 100 nm compared with that of pure 2,2′-bipy ligand, as in the case found in the reference59 in which the value of blue-shift is 95 nm (Figures 11g and S32). The emission spectrum of 8 shows an emission band similar to that of 7 with λmax ) 422 nm upon photoexcitation at 360 nm (Figure 11h). This similarity may correspond to the similar structural features between 7 and 8. When excited at 380 nm, compound 9 exhibits an emission spectrum in the blue region at 448 nm (Figure 11i). Compound 10 has an intense emission band centered at 441 nm upon photoexcitation at 367 nm (Figure 11j). To understand the nature of the fluorescence emissions of 1-10, we performed theoretical computation on 1, 5, and 7. To avoid the complexity, we truncated compound 1 into a segment of the 1D [ZnBr2(4,4′-bipy)]n chain, containing two ZnBr2 entities and three 4,4′-bipy ligands, whereas compounds 5 and 7 were used as the entire molecules without any truncation. The ground-state geometries were adapted from the truncated (1) or un-truncated (5 and 7) X-ray data. On the basis


of these geometries, time-dependent DFT (TDDFT) calculation using the B3LYP functional was performed.60 Figure 12a depicts the features of the lowest unoccupied (LUMO) and the highest occupied (HOMO) frontier orbitals of 1. Apparently, the electron densities of the singlet state for the HOMO are located on the bromine atoms, whereas that of the LUMO is distributed on the 4,4′-bipy moiety; this suggests that the emission band of 1 is attributed to the ligand-to-ligand charge transfer (LLCT). Figure 12b shows the characters of the LUMO and the three highest occupied (HOMO, HOMO-1, and HOMO-2, whose energy levels are close to the values of -0.259, -0.259, and -0.260 Hartrees, respectively) frontier orbitals of 5. Obviously, the electron densities of the singlet state for the HOMO, HOMO1, and HOMO-2 are located on the iodine atoms, whereas those of the LUMO are distributed on the 4,4′-bipy moiety, indicating that the lowest electronic transition is also dominated by LLCT character (from HOMO, HOMO-1, and HOMO-2 of the iodine atom to the LUMO of the 4,4′-bipy moiety). For 7, the singlet state for the HOMO is located on the chlorine atoms, whereas that of the LUMO is distributed on the 2,2′-bipy moiety, also suggesting the LLCT mechanism of the lowest electronic transition (Figure 12c). In a word, the emissions of these compounds originate from LLCT transitions. We propose that the emission bands of other compounds (2-4, 6, and 8-10) may also be attributed to LLCT because they have components that are similar to those of 1, 5, and 7. Conclusion A family of metal halide/pseudohalide bipy (4,4′-bipy and 2,2′-bipy) compounds have been synthesized and characterized. These compounds show different and interesting structural modes. Photoluminescence investigations reveal that they display strong emissions in the blue/green/yellow regions, which, in combination with the molecular orbital (MO) calculations of 1, 5, and 7, leads us to conclude that the emissions originate from a ligand-to-ligand charge transfer (LLCT) transition. The scope for the syntheses of new metal halide/pseudohalide-bipy compounds with novel structures and properties appears to be very large, and further systematically experimental and theoretical investigations on this system are in progress. Acknowledgment. We gratefully acknowledge the financial support of the NSF of China (20521101), the NSF for Distinguished Young Scientists of China (20425104) and the NSF of Fujian Province (2004J039). Supporting Information Available: X-ray crystallographic files in CIF format for 1-10, packing diagrams, solid-state diffuse reflectance spectra, TGA curves, XRPD figures for 1, 2, 4, and 6, and solidstate emission spectra of free 2,2′-bipy and 4,4′-bipy ligands. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) See for example: (a) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (b) Finn, R.; Zubieta, J. Chem. Commun. 2000, 1321. (2) For example: (a) Zapf, P. J.; Haushalter, R. C.; Zubieta, J. Chem. Commun. 1997, 321. (b) Lu, Y.; Li, Y. G.; Wang, E. B.; Lu¨, J.; Xu, L.; Cle´rac, R. Eur. J. Inorg. Chem. 2005, 1239. (c) Lu, Y.; Xu, Y.; Wang, E. B.; Lu¨, J.; Hu, C. W.; Xu, L. Cryst. Growth Des. 2005, 5, 257. (3) For example: (a) Chen, C. Y.; Lii, K. H.; Jacobson, A. J. J. Solid State Chem. 2003, 172, 252. (b) Wang, C. M.; Liao, C. H.; Kao H. M.; Lii, K. H. Inorg. Chem. 2005, 44, 6294. (4) For example: (a) Prabusankar, G.; Murugavel, R.; Butcher, R. J. Organometallics 2004, 23, 2305. (b) Biradha, K.; Mahata, G. Cryst. Growth Des. 2005, 5, 61.

Crystal Growth & Design, Vol. 6, No. 10, 2006 2299 (5) For example: (a) Jude, H.; Bauer, J. A. K.; Connick, W. B. Inorg. Chem. 2005, 44, 1211. (b) Yang, B. P.; Mao, J. G. Inorg. Chem. 2005, 44, 566. (6) For example: (a) Yaghi, O. M.; Li, H. J. Am. Chem. Soc. 1996, 118, 295. (b) Shi, Z.; Feng, S.; Zhang, L.; Yang, G.; Hua, J. Chem. Mater. 2000, 12, 2930. (c) Wang, R. H.; Zhou, Y. F.; Sun, Y. Q.; Yuan, D. Q.; Han, L.; Lou, B. Y.; Wu, B. L.; Hong, M. C. Cryst. Growth Des. 2005, 5, 251. (7) (a) Shen, Y.; Sullivan, B. P. J. Chem. Educ. 1997, 74, 685. (b) Vogler, A.; Kunkely, H. Top. Curr. Chem. 2001, 213, 143. (c) Sun, S. S.; Lees, A. J. Organometallics 2002, 21, 39. (8) (a) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (b) Balzani, V.; Juris, A. Coord. Chem. ReV. 2001, 211, 97. (9) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426. (10) For example: (a) Rajendran, T.; Manimaran, B.; Lee, F. Y.; Lee, G. H.; Peng, S. M.; Wang, C. M.; Lu, K. L. Inorg. Chem. 2000, 39, 2016. (b) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y. Organometallics 2002, 21, 685. (11) For example: (a) Figgis, B. N.; Patrick, J. M.; Reynolds, P. A.; Skelton, B. W.; White, A. H.; Healy, P. C. Aust. J. Chem. 1983, 36, 2043. (b) Lu, J.; Yu, C.; Niu, T. Y.; Paliwala, T.; Crisci, G.; Somosa, F.; Jacobson, A. J. Inorg. Chem. 1998, 37, 4637. (c) Hu, C. H.; Li, Q.; Englert, U. CrystEngComm 2003, 5, 519. (12) For example: (a) Rajendran, T.; Manimaran, B.; Lee, F. Y.; Lee, G. H.; Peng, S. M.; Wang, C. M.; Lu, K. L. Inorg. Chem. 2000, 39, 2016. (b) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y. Organometallics 2002, 21, 685. (13) For example: (a) Dai, J. C.; Hu, S. M.; Wu, X. T.; Fu, Z. Y.; Du, W. X.; Zhang, H. H.; Sun, R. Q. New J. Chem. 2003, 27, 914. (b) Xu, Y. Q.; Yuan, D. Q.; Wu, B. L.; Jiang, F. L.; Zhou, Y. F.; Hong, M. C. Inorg. Chem. Commun. 2005, 8, 651. (14) For example: (a) Yang, L.; Ren, A. M.; Feng, J. K.; Liu, X. D.; Ma, Y. G.; Zhang, H. X. Inorg. Chem. 2004, 43, 5961. (b) Yang, L.; Ren, A. M.; Feng, J. K.; Liu, X. J.; Ma, Y. G.; Zhang, M.; Liu, X. D.; Shen, J. C.; Zhang, H. X. J. Phys. Chem. A 2004, 108, 6797. (15) (a) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Interscience Publishers: New York, 1966. (b) Kortu¨m, G. Reflectance Spectroscopy; Springer-Verlag: New York, 1969. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennuci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Oritz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. GAUSSIAN03, revision C.02; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) (a) CrystalClear, version 1.3.5; Rigaku Corp.: Shibuya-Ku, Japan, 2002. (b) CrystalStructure, version 3.6.0; Rigaku Corp.: ShibuyaKu, Japan, 2002. (c) SAINT Software Reference Manual; Siemens Energy & Automation Inc.: Madison, WI, 1994. (18) SHELXTL Reference Manual, version 5; Siemens Energy & Automation Inc.: Madison, WI, 1994. (19) Li, J.; Chen, Z.; Wang, R.-J.; Proserpio, D. M. Coord. Chem. ReV. 1999, 190-192, 707. (20) Rodrı´guez, A.; Sousa-Pedrares, A.; Garcia-Va´zquez, J. A.; Romero, J.; Sousa, A. Eur. J. Inorg. Chem. 2005, 2242. (21) Hu, C. H.; Englert, U. CrystEngComm 2001, 3, 91. (22) Hu, C. H.; Englert, U. Angew. Chem., Int. Ed. 2005, 44, 2281. (23) (a) Sampanthar, J. T.; Vittal, J. J. J. Chem. Soc., Dalton Trans. 1999, 1993. (b) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. J. Chem. Soc., Dalton Trans. 1997, 1801. (24) For example: (a) Chen, C. M.; Tong, M. L.; Luo, Y. J.; Chen, Z. N. Aust. J. Chem. 1996, 49, 835. (b) Marinescu, G.; Andruh, M.; Julve, M.; Lloret, F.; Llusar, R.; Uriel, S.; Vaissermann, J. Cryst. Growth Des. 2005, 5, 261. (25) Wang, R. H.; Han, L.; Jiang, F. L.; Zhou, Y. F.; Yuan, D. Q.; Hong, M. C. Cryst. Growth. Des. 2005, 5, 129. (26) Abrahams, B. F.; Hoskins, B. F.; Winter, G. Aust. J. Chem. 1990, 43, 1759.

2300 Crystal Growth & Design, Vol. 6, No. 10, 2006 (27) (a) Huang, L.-H.; Kao, H.-M.; Lii, K.-H. Inorg. Chem. 2002, 41, 2936. (b) Wirth, A.; Lange, I.; Henschel, D.; Moers, O.; Blaschette, A.; Jones, P. G. Z. Anorg. Allg. Chem. 1998, 624, 1308. (28) (a) Xu, H.; Ji, J. Y.; Wu, Z. Z.; Zou, J. Z.; Xu, Z.; You, X. Z. Polyhedron 1993, 12, 2261. (b) Mueller-Westerhoff, U.; Rheingold, A. L.; Allen, M. B. Private communication, 2006. (29) (a) Paul, G.; Choudhury, A.; Rao, C. N. R. J. Chem. Soc., Dalton Trans. 2002, 3859. (b) Hassen, R. B.; Salah, A. B.; Kallel, A.; Daoud, A.; Jaud, J. J. Chem. Cryst. 2002, 32, 427. (30) Xu, Y. Q.; Yuan, D. Q.; Wu, B. L.; Jiang, F. L.; Zhou, Y. F.; Hong, M. C. Inorg. Chem. Commun. 2005, 8, 651. (31) Zheng, X.-J.; Jin, L.-P.; Gao, S.; Lu, S.-Z. New J. Chem. 2005, 29, 978. (32) MacGillivray, L. R.; Subramanian, S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1994, 1325. (33) Gable, R. W.; Hoskins, B. F.; Robson, R. J. Chem. Soc., Chem. Commun. 1990, 1677. (34) Fujita, M.; Kwon, Y. J.; Sasaki, Y. O.; Yamaguchi, K.; Ogura, K. J. Am. Chem. Soc. 1995, 117, 7287. (35) (a) Li, J. M.; Zeng, H. Q.; Chen, J. H.; Wang, Q. M.; Wu, X. T. Chem. Commun. 1997, 1213. (b) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 2568. (36) (a) Batsanov, A. S.; Begley, M. J.; Hubberstey, P.; Stroud, J. J. Chem. Soc., Dalton Trans. 1996, 1947. (b) Zheng, X.-J.; Jin, L.-P.; Gao, S.; Lu, S.-Z. New J. Chem. 2005, 29, 798. (37) (a) Zhu, H. G.; Hu, H. M.; Yu, Z.; Cai, H.; You, X.-Z. Private communication, 2004. (b) Niu, Y.; Hou, H.; Zhu, Y. Private communication, 2005. (38) (a) Sharma, C. V. K.; Broker, G. A.; Huddleston, J. G.; Baldwin, J. W.; Metzger, R. M.; Rogers, R. D. J. Am. Chem. Soc. 1999, 121, 1137. (b) Biradha, K.; Fujita, M. J. Inclusion Phenom. Macrocyclic Chem. 2001, 41, 201. (39) Lu, J. Y.; Cabrera, B. R.; Wang, R.-J.; Li, J. Inorg. Chem. 1999, 38, 4608. (40) Niu, Y. Y.; Zheng, H. G.; Fun, H. K.; Razak, I. A.; Chantrapromma, S.; Xin, X. Q. Synth. React. Inorg. Met.-Org. Chem. 2003, 33, 1. (41) For example: (a) Belanger, S.; Hupp, J. T.; Stern, C. L.; Slone, R. V.; Watson, D. F.; Carrell, T. G. J. Am. Chem. Soc. 1999, 121, 557. (b) Kinnunen, T. J. J.; Haukka, M.; Pesonen, E.; Pakkanen, T. A. J. Organomet. Chem. 2002, 655, 31. (42) Nockemann, P.; Meyer, G. Acta Crystallogr., Sect. E 2004, 60, m749.

Chen et al. (43) Zhang, X. M.; Hao, Z. M.; Wu, H. S. Inorg. Chem. 2005, 44, 7301. (44) For example: (a) Galand, N.; Wipff, G. New J. Chem. 2003, 27, 1319. (b) Aoki, K.; Shimizu, C.; Fujisawa, I. Bull. Chem. Soc. Jpn. 2005, 78, 718. (45) Schoberl, U.; Magnera, T. F.; Harrison, R. M.; Fleischer, F.; Pflug, J. L.; Schwab, P. F. H.; Meng, X.; Lipiak, D.; Noll, B. C.; Allured, V. S.; Rudalevige, T.; Lee, S.; Michl, J. J. Am. Chem. Soc. 1997, 119, 3907. (46) Xie, J.-Y.; Mao, J.-G. Inorg. Chem. Commun. 2005, 8, 375. (47) Freire, E.; Baggio, S.; Baggio, R.; Suescun, L. J. Chem. Cryst. 1999, 29, 825. (48) Drabowicz, J.; Dudzinski, B.; Mikolajczyk, M.; Wang, F.; Dehlavi, A.; Goring, J.; Park, M.; Rizzo, C. J.; Polavarapu, P. L.; Biscarini, P.; Wieczorek, M. W.; Majzner, W. R. J. Org. Chem. 2001, 66, 1122. (49) (a) Falvello, L. R.; Fornies, J.; Martin, A.; Navarro, R.; Sicilia, V.; Villarroya, P. Inorg. Chem. 1997, 36, 6166. (b) Zhang, X. J.; Yu, W. T.; Xie, Y.; Zhao, Q. R.; Tian, Y. P. Inorg. Chem. Commun. 2003, 6, 1338. (50) Hezel, F.; Fenske, D.; Eisenmann, J.; Wetzel, T. Z. Anorg. Allg. Chem. 2000, 626, 290. (51) Bernardinelli, G.; Flack, H. D. Acta Crystallogr., Sect. A 1985, A41, 500. (52) Dong, Y.-B.; Smith, M. D.; zur Loye, H.-C. Angew. Chem., Int. Ed. 2000, 39, 4271. (53) Hazell, A. C.; Hazell, R. G. Acta Crystallogr., Sect. C 1984, 40, 806. (54) Schevciw, O.; White, W. B. Mater. Res. Bull. 1983, 18, 1059. (55) Huang, F. Q.; Mitchell, K.; Ibers, J. A. Inorg. Chem. 2001, 40, 5123. (56) Axtell, E. A.; Liao, J.-H.; Pikramenou, Z.; Kanatzidis, M. G. Chem.s Eur. J. 1996, 2, 656. (57) Axtell, E. A.; Park, Y.; Chondroudis, K.; Kanatzidis, M. G. J. Am. Chem. Soc. 1998, 120, 124. (58) Papavassiliou, G. C. Prog. Solid State Chem. 1997, 25, 125. (59) Zhou, Y. F.; Zhao, Y. J.; Sun, D. F.; Weng, J. B.; Cao, R.; Hong, M. C. Polyhedron 2003, 22, 1231. (60) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Wang, C. C.; Yang, C. H.; Tseng, S. M.; Lin, S. Y.; Wu, T. Y.; Fuh, M. R.; Lee, G. H.; Wong, K. T.; Chen, R. T.; Cheng, Y. M.; Chou, P. T. Inorg. Chem. 2004, 43, 4781.

CG060146V

Pseudohalide-Bipy Systems: Syntheses, Structures

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