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The title inorganic-organic hybrid complex, [CdBr
2(C
10H
8N
2)]
n, features two-dimensional [CdBr
2(4,4'-bipy)]
n (4,4'-bipy is 4,4'-bipyridine) neutral networks, based on the octahedral Cd atom coordinated by four
2-Br and two
2-4,4'-bipy at
trans positions, yielding a CdBr
4N
2 octahedron. It crystallizes in the orthorhombic system (
Cmmm). All the crystallographically independent atoms are on special positions, namely Cd on
mmm, Br on
mm, N on
mm2, and C on sites of symmetry
m or
mm2. Optical absorption spectroscopy reveals the presence of an optical gap of 3.76 eV, indicating that the complex is a wide-gap semiconductor. Photoluminescence investigation reveals that the complex displays strong colour-tunable emissions, which might originate from a ligand-to-ligand charge-transfer (LLCT) transition. Thermogravimetric differential thermal analysis shows that the complex is thermally stable up to 493 K.
Supporting information
CCDC reference: 627931
CdBr2·4H2O (0.3 mmol, 103 mg), 4,4'-bipyridine (0.2 mmol, 31 mg) and
distilled water (3 ml) were loaded into a Teflon-lined stainless steel
autoclave (25 ml) and kept at 473 K for 3 d. After being slowly cooled to room
temperature at a rate of 8 K h-1, colourless crystals of (I) suitable for
X-ray crystallographic analysis were obtained (yield 31%, based on Cd).
All H atoms were positioned geometrically and refined using a riding model, with
C—H = 0.93 Å and with Uiso(H) = 1.2Ueq(C).
Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2; data reduction: APEX2; program(s) used to solve structure: SHELXTL (Sheldrick, 1998); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 2004); software used to prepare material for publication: SHELXTL.
poly[di-µ
2-bromido-µ
2-4,4'-bipyridine-cadmium(II)]
top
Crystal data top
[CdBr2(C10H8N2)] | F(000) = 400 |
Mr = 428.40 | Dx = 2.450 Mg m−3 |
Orthorhombic, Cmmm | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2 2 | Cell parameters from 853 reflections |
a = 11.787 (3) Å | θ = 4.7–27.5° |
b = 12.554 (3) Å | µ = 8.73 mm−1 |
c = 3.9242 (7) Å | T = 293 K |
V = 580.7 (2) Å3 | Block, colourless |
Z = 2 | 0.10 × 0.09 × 0.06 mm |
Data collection top
Bruker APEXII CCD area-detector diffractometer | 337 independent reflections |
Radiation source: rotating-anode generator | 330 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.016 |
ϕ/ω scans | θmax = 25.4°, θmin = 4.7° |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | h = −14→14 |
Tmin = 0.820, Tmax = 1.000 | k = −15→14 |
1912 measured reflections | l = −4→4 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.017 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.042 | H-atom parameters constrained |
S = 1.10 | w = 1/[σ2(Fo2) + (0.0206P)2 + 1.6288P] where P = (Fo2 + 2Fc2)/3 |
337 reflections | (Δ/σ)max < 0.001 |
28 parameters | Δρmax = 0.96 e Å−3 |
0 restraints | Δρmin = −0.31 e Å−3 |
Crystal data top
[CdBr2(C10H8N2)] | V = 580.7 (2) Å3 |
Mr = 428.40 | Z = 2 |
Orthorhombic, Cmmm | Mo Kα radiation |
a = 11.787 (3) Å | µ = 8.73 mm−1 |
b = 12.554 (3) Å | T = 293 K |
c = 3.9242 (7) Å | 0.10 × 0.09 × 0.06 mm |
Data collection top
Bruker APEXII CCD area-detector diffractometer | 337 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 330 reflections with I > 2σ(I) |
Tmin = 0.820, Tmax = 1.000 | Rint = 0.016 |
1912 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.017 | 0 restraints |
wR(F2) = 0.042 | H-atom parameters constrained |
S = 1.10 | Δρmax = 0.96 e Å−3 |
337 reflections | Δρmin = −0.31 e Å−3 |
28 parameters | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Cd1 | 0.0000 | 0.0000 | 0.0000 | 0.02255 (17) | |
Br1 | 0.0000 | 0.15746 (4) | −0.5000 | 0.02806 (18) | |
N1 | 0.2000 (3) | 0.0000 | 0.0000 | 0.0277 (9) | |
C1 | 0.2587 (3) | 0.0879 (3) | 0.0000 | 0.0675 (19) | |
H1A | 0.2194 | 0.1521 | 0.0000 | 0.081* | |
C2 | 0.3752 (3) | 0.0913 (3) | 0.0000 | 0.0688 (19) | |
H2A | 0.4121 | 0.1568 | 0.0000 | 0.083* | |
C3 | 0.4371 (4) | 0.0000 | 0.0000 | 0.0250 (10) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Cd1 | 0.0159 (3) | 0.0254 (3) | 0.0263 (3) | 0.000 | 0.000 | 0.000 |
Br1 | 0.0336 (3) | 0.0237 (3) | 0.0268 (3) | 0.000 | 0.000 | 0.000 |
N1 | 0.0191 (19) | 0.026 (2) | 0.038 (2) | 0.000 | 0.000 | 0.000 |
C1 | 0.0204 (19) | 0.025 (2) | 0.157 (6) | 0.0021 (18) | 0.000 | 0.000 |
C2 | 0.0210 (19) | 0.026 (2) | 0.160 (6) | −0.0058 (17) | 0.000 | 0.000 |
C3 | 0.020 (2) | 0.029 (2) | 0.026 (3) | 0.000 | 0.000 | 0.000 |
Geometric parameters (Å, º) top
Cd1—N1 | 2.358 (4) | N1—C1v | 1.302 (4) |
Cd1—N1i | 2.358 (4) | C1—C2 | 1.375 (6) |
Cd1—Br1ii | 2.7852 (5) | C1—H1A | 0.9300 |
Cd1—Br1iii | 2.7852 (5) | C2—C3 | 1.358 (5) |
Cd1—Br1 | 2.7852 (5) | C2—H2A | 0.9300 |
Cd1—Br1i | 2.7852 (5) | C3—C2v | 1.358 (5) |
Br1—Cd1iv | 2.7852 (5) | C3—C3vi | 1.484 (9) |
N1—C1 | 1.302 (4) | | |
| | | |
N1—Cd1—N1i | 180.0 | Br1—Cd1—Br1i | 180.0 |
N1—Cd1—Br1ii | 90.0 | Cd1iv—Br1—Cd1 | 89.57 (2) |
N1i—Cd1—Br1ii | 90.0 | C1—N1—C1v | 115.8 (5) |
N1—Cd1—Br1iii | 90.0 | C1—N1—Cd1 | 122.1 (2) |
N1i—Cd1—Br1iii | 90.0 | C1v—N1—Cd1 | 122.1 (2) |
Br1ii—Cd1—Br1iii | 180.0 | N1—C1—C2 | 123.9 (4) |
N1—Cd1—Br1 | 90.0 | N1—C1—H1A | 118.1 |
N1i—Cd1—Br1 | 90.0 | C2—C1—H1A | 118.1 |
Br1ii—Cd1—Br1 | 89.57 (2) | C3—C2—C1 | 120.6 (4) |
Br1iii—Cd1—Br1 | 90.43 (2) | C3—C2—H2A | 119.7 |
N1—Cd1—Br1i | 90.0 | C1—C2—H2A | 119.7 |
N1i—Cd1—Br1i | 90.0 | C2v—C3—C2 | 115.1 (5) |
Br1ii—Cd1—Br1i | 90.43 (2) | C2v—C3—C3vi | 122.4 (2) |
Br1iii—Cd1—Br1i | 89.57 (2) | C2—C3—C3vi | 122.4 (2) |
Symmetry codes: (i) −x, −y, −z; (ii) x, y, z+1; (iii) −x, −y, −z−1; (iv) x, y, z−1; (v) x, −y, z; (vi) −x+1, −y, −z. |
Experimental details
Crystal data |
Chemical formula | [CdBr2(C10H8N2)] |
Mr | 428.40 |
Crystal system, space group | Orthorhombic, Cmmm |
Temperature (K) | 293 |
a, b, c (Å) | 11.787 (3), 12.554 (3), 3.9242 (7) |
V (Å3) | 580.7 (2) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 8.73 |
Crystal size (mm) | 0.10 × 0.09 × 0.06 |
|
Data collection |
Diffractometer | Bruker APEXII CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.820, 1.000 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1912, 337, 330 |
Rint | 0.016 |
(sin θ/λ)max (Å−1) | 0.602 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.017, 0.042, 1.10 |
No. of reflections | 337 |
No. of parameters | 28 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.96, −0.31 |
Selected geometric parameters (Å, º) topCd1—N1 | 2.358 (4) | Cd1—Br1iii | 2.7852 (5) |
Cd1—N1i | 2.358 (4) | Cd1—Br1 | 2.7852 (5) |
Cd1—Br1ii | 2.7852 (5) | Cd1—Br1i | 2.7852 (5) |
| | | |
Br1ii—Cd1—Br1 | 89.57 (2) | | |
Symmetry codes: (i) −x, −y, −z; (ii) x, y, z+1; (iii) −x, −y, −z−1. |
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Inorganic–organic hybrid materials have been of great interest owing to their intriguing structural features and their potential in various applications, such as electrical conductivity, photochemistry, ion exchange, catalysis, biochemistry and nonlinear optical behaviour (Stupp & Braun, 1997; Finn & Zubieta, 2000). A large variety of ligands containing bridging functionalities such as carboxylates, phosphonates and 4,4'-bipyridine have been exploited in order to prepare novel inorganic–organic hybrid materials. Being an important class of such materials, metal halide–bipy (bipy = 4,4'-bipyridine) systems have attracted increasing attention in recent years, not only due to their intrinsic aesthetic appeal, but also for their various potential applications.
The ability of bifunctional bipy to act as a rigid rod-like organic building block in the self-assembly of coordination frameworks is well known, such as acting as a charge-compensating cation (Zapf et al., 1997; Lu et al., 2005), a pillar bonding to an inorganic skeletal backbone (Chen et al., 2003; Wang, Liao et al., 2005 or Wang, Zhou et al., 2005?), an uncoordinated guest molecule and organic template (Gillon et al., 1999, 2000; Biradha & Mahata, 2005), or a ligand linking a metal and an inorganic framework (Yaghi & Li, 1996; Shi et al., 2000; Wang, Liao et al., 2005 or Wang, Zhou et al., 2005?). In addition, bipy has attracted increasing attention due to the delocalized π-electrons in its pyridyl rings, which makes it an excellent candidate for preparing light-emitting complexes with potential in various technical applications, such as sensitisers in solar energy conversion (Hagfeldt & Gratzel, 2000; Balzani & Juris, 2001) and emitting materials for organic light-emitting diodes (Baldo et al., 1998; Gao & Bard, 2000).
Very large-scale structures of metal halide–bipy materials have been reported (Figgis et al., 1983; Lu et al., 1998; Hu et al., 2003), but among these group 12 (IIB) metal halide–bipy materials are relatively rare. In fact, complexes 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, fluorescent properties, the widespread applications of IIB compounds, the essential role of zinc in biological systems, and so on. Our recent efforts in synthesizing novel IIB-based complexes have focused largely on systems containing bifunctional ligands, such as 4,4'-bipy. Here, we describe the synthesis and characterization of the title complex, [CdBr2(4,4'-bipy)]n, (I).
X-ray diffraction analysis reveals that complex (I) features two-dimensional layered [CdBr2(4,4'-bipy)]n neutral networks, as shown in Fig. 2. The divalent metal centres have a slightly distorted octahedral coordination with four µ2-Br and two bridging 4,4'-bipy in trans positions, yielding edge-shared CdBr4N2 octahedra. The CdBr4N2 octahedra interconnect with each other via two µ2-Br atoms, forming a linear inorganic mono-chain running along the c direction. The mono-chains in (I) are different from the double-chains reported by Lu et al. (1999). These mono-chains are bridged by µ2-4,4'-bipy ligands to form an inorganic–organic hybrid two-dimensional layer (Fig. 2), which is similar to that reported by Chen et al., 2006), but different from other known two-dimensional metal-4,4'-bipy structures, e.g. diamondoid (MacGillivray et al., 1994), brick wall (Gable et al.,1990), ladder (Fujita et al., 1995), square network (Li et al., 1997; Noro et al., 2002), and so forth. These layers stack in an ···ABAB··· mode along the b axis to yield a three-dimensional structure (Fig. 3).
In complex (I), π–π stacking interactions in the layer give rise to 'perfect facial alignment', as shown by the angle of the centroids of three consecutive rings (180°). The two pyridyl rings of the 4,4'-bipy ligand are coplanar, with a dihedral angle of 0°, which is remarkably different from the cases found in many other complexes containing 4,4'-bipy ligands in which the two pyridyl rings are far from coplanar (Chen et al., 1996; Marinescu et al., 2005). In each layer, neighbouring µ2-4,4'-bipy ligands interact through weak π–π contacts with a distance equal to the c axis length, 3.924 Å.
Optical absorption spectroscopy reveals an optical gap of 3.76 eV (Fig. 4). This suggests that complex (I) is a wide-gap semiconductor, which is consistent with the colour of the crystal, as observed for cases in the literature (Chondroudis et al., 1997; Chondroudis & Kanatzidis, 1998; Aitken et al., 2000; Choi & Kanatzidis, 2000). The gradual slope of the optical absorption edge is indicative of the existence of indirect transitions (Huang et al., 2001). The optical absorption of (I) likely originates from charge-transfer excitations, mainly from the valence band of the Br atoms to the conduction band of the Cd centre.
The solid-state emission spectrum of the title complex was investigated at room temperature and is given in Fig. 5. The fluorescent spectrum study shows that (I) exhibits a broad and strong emission with a maximum wavelength of 491 nm upon intensive photo-excitation at 391 nm (Fig. 5), which is red-shifted by 53 nm compared with that of the pure 4,4'-bipy ligand (Chen et al., 2006). Interestingly, complex (I) exhibits a broad and slightly weaker emission with a maximum wavelength of 527 nm upon weak photo-excitation at 315 nm. During the measurement process in a dark room, we opened a small slit of the sample chamber and found that the title complex emitted blue–white light upon intensive photo-excitation at 391 nm, while it emitted greenish–yellow light upon weak photo-excitation at 315 nm. According to the results reported by Chen et al., 2006, the emission of (I) is probably assigned to the ligand-to-ligand charge-transfer (LLCT) transition (from the highest occupied molecular orbital of the Br atom to the lowest unoccupied molecular orbital of the 4,4'-bipy moiety). Thus, this complex may be a potential colour-tunable organic luminescent material upon different photo-excitation.
Thermogravimetric differential thermal analysis (TG-DTA) shows that complex (I) is thermally stable up to 493 K and undergoes a two-step decomposition process. The TG-DTA displays an initial mass loss of 35.74% (calculated 36.41%) with an onset temperature of about 493 K, corresponding to the loss of a 4,4'-bipy molecule. A second weight loss of 64.26% (calculated 63.49%), with two small endothermic peaks centred at 446 and 825 K, is found in the range 546–923 K, corresponding to the loss of cadmium bromide. These results are consistent with the composition of the X-ray crystal structure.
In brief, using the hydrothermal reaction of CdBr2 with 4,4'-bipy, an inorganic–organic hybrid complex with a novel two-dimensional layered structure was obtained. The optical absorption spectrum shows that the title complex may be a candidate as a potential photoelectric material. This complex exhibits broad and strong fluorescent emission bands; and it might have use as a potential colour-tunable organic luminescent material. The scope for the syntheses of new metal halide–bipy complexes with novel structures and properties appears to be very large, and further systematic investigations of this system are in progress.