4,4′-Bipyridinium tetra­bromo­cadmate(II) with strong fluorescence

The title compound, (C₁₀H₁₀N₂)[CdBr₄], was synthesized via a hydro­thermal reaction. Its structure features discrete 4,4′-bipyridinium cations and tetra­hedral [CdBr₄]²⁻ anions linked into ion pairs by single N—H...Br hydrogen bonds. Photoluminescent investigation reveals that the title compound displays a strong emission in the blue region, which may originate from π → π* charge-transfer inter­actions of the 4,4′-bipyridinium cations.

S1. Comment

The ability of bifunctional 4,4'-bipyridine (bipy) to act as a rigid rod-like organic building block in the self-assembly of coordination frameworks is well known. Bipy is found 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), an uncoordinated guest molecule and organic template (Prabusankar et al., 2004; Biradha & Mahata, 2005), a bridge connecting two metal complex moieties (Jude et al., 2005; Yang & Mao, 2005), or a ligand linking a metal and an inorganic framework (Yaghi & Li, 1996; Shi et al., 2000; Wang, Zhou et al.,2005). Bipy has also attracted attention in recent years because of its common characteristic of delocalized π electrons across the pyridyl rings, which makes it an excellent candidate in preparing light-emitting compounds with potential in various technical applications, such as chemical sensors (Vogler & Kunkely, 2001; Sun & Lees, 2002), sensitizers 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).

Recently, many structures of metal halide–bipy materials have been reported (Lu et al., 1998; Hu et al., 2003). However, among these group 12 (IIB) metal halide–bipy materials are relatively rare. Compounds containing group IIB elements are particularly attractive for many reasons, such as the variety of coordination numbers and geometries provided by the d¹⁰ configuration of the IIB metal ions, their photoelectric and fluorescent properties, the widespread application of IIB compounds, and the essential role of zinc in biological systems. Fluorescent materials, particularly blue fluorescent materials, are of considerable interest because blue fluorescence is one of the key colour components required for full-colour electroluminescent displays and blue fluorescent materials are still rare. While neutral 4,4'-bipy can act as a ligand, the 4,4'-H₂bipy dication usually forms supramolecular frameworks through hydrogen bonding and it cannot act as a ligand. Our recent efforts in synthesizing novel group IIB-based compounds have focused largely on systems containing 4,4'-H₂bipy. Here, we describe the hydrothermal synthesis and characterization of the title compound, (4,4'-H₂bipy)[CdBr₄], (I).

The structure of (I) consists of discrete 4,4'-H₂bipy dications and tetrabromocadmate(II) dianions (Fig. 1). The anion has a reasonably regular tetrahedral geometry (Table 1) with Cd—Br distances in the normal range and comparable with those reported by Chesnut et al. (1999) and Liu et al. (2002). The two pyridyl rings of the cation are slightly twisted, with a small dihedral angle of 5.53 (18)°, which is comparable with that previously documented (Lu et al., 1998). There are no π → π^* stacking interactions between the cations. Single N—H···Br hydrogen bonds link the cations and anions into ion pairs (Table 2 and Fig. 1).

The structures of several analogous salts have already been documented (Gillon et al., 1999, 2000), of which five are isomorphous with the title compound, namely (4,4'-H₂bipy)[ZnCl₄], (II), (4,4'-H₂bipy)[ZnBr₄], (III), (4,4'-H₂bipy)[HgCl₄], (IV), (4,4'-H₂bipy)[CoCl₄] and (4,4'-H₂bipy)[CoBr₄]. Salts (II)–(IV) are group IIB metal halides with 4,4'-H₂bipy. The tetrachloro analogue of the title compound, (4,4'-H₂bipy)[CdCl₄], (V), features a hydrogen-bonded two-dimensional sheet motif in which the tetrachlorometallate anions polymerize to form a one-dimensional chain; this motif is very different from the discrete anions found in (I). This difference may be attributable to the different halide anions and the different number of hydrogen bonds. For the analogues mentioned above, besides the structural motif exhibited by compound (V) (motif A), there are two other motifs, namely B, a hydrogen-bonded polymeric ribbon structure, and C, a hydrogen-bonded dimeric ring motif.

Compound (II) exhibits motif C, in which both bipy N atoms are donors for hydrogen bonds. Although the title compound is essentially isostructural with (II)–(IV), there is only one classical hydrogen bond that links the dication and dianion together, and no hydrogen-bonded dimeric ring motif can be formed because the second hydrogen bond is `missing'. Actually, the missing hydrogen bond is present in principle, but the N2···Br1ⁱ [symmetry code: (i) −1 − x, 1 − y, 1 − z] distance of 3.434 (3) Å is slightly longer than what might be a true interaction, and the N2—H2B···Br1ⁱ angle of 97.7° is smaller than 110°. Therefore, the structural motif of the title compound may be classified as the fourth type, D, if the missing hydrogen bond is not considered as a true hydrogen bond.

The small difference between the structural motifs displayed by (I) and the isosmorphous compounds (II)–(IV) may be caused by the different metal halides. That compounds (I) and (V) are both Cd^II salts yet adopt very different structural motifs indicates that the halide atoms play a vital role in the structural motifs. However, this is not always true, because the isostructural compounds (II) and (III) both contain Zn^II but different halide atoms, which poses the question of why this is the case. We reason that the ability of Cd to adopt an octahedral coordination is larger than that of Zn. Therefore, the numbers of hydrogen bonds, metal centres and halide atoms are three key factors in forming structural motifs.

The solid-state emission spectrum of (I) was investigated at room temperature (Fig. 2). The fluorescence spectrum shows a broad and strong emission with a maximum wavelength of 426 nm upon photo-excitation at 325 nm. The emission should probably be assigned to the π → π^* charge-transfer interaction of the 4,4'-H₂bipy cations. Thus, this compound may be a candidate for blue light luminescent materials and it is believed that more IIB metal halide–bipy compounds with good luminescent properties can be developed.

S2. Experimental

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 373 K for 3 d. After being slowly cooled to room temperature at a rate of 6 K h⁻¹, yellow crystals of (I) suitable for X-ray analysis were obtained (yield 20%, based on Cd).

S3. Refinement

All H atoms were positioned geometrically and refined using a riding model, with C—H distances of 0.93 Å and with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: CrystalClear (Rigaku, 2002); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXTL (Siemens, 1994); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A packing diagram for (I), showing the atom-labelling scheme. Dashed lines represent N—H···Br hydrogen bonds. Displacement ellipsoids are drawn at the 35% probability level and H atoms have been omitted for clarity.
	Fig. 2. Solid-state emission and excitation spectra of (I) at room temperature. The solid line denotes the emission spectrum and the dashed line the excitation spectrum.

4,4'-Bipyridinium tetrabromocadmate(II)

Crystal data

(C10H10N2)[CdBr4]	Dx = 2.510 Mg m^−3
Mr = 590.24	Melting point: not measured K
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.852 (2) Å	Cell parameters from 9983 reflections
b = 20.795 (2) Å	θ = 3.3–25.4°
c = 9.987 (3) Å	µ = 11.61 mm^−1
β = 106.684 (4)°	T = 293 K
V = 1562.0 (6) Å^3	Block, yellow
Z = 4	0.26 × 0.14 × 0.11 mm
F(000) = 1088

Data collection

Rigaku Mercury CCD area-detector diffractometer	2813 independent reflections
Radiation source: rotating-anode generator	1611 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.055
ω scans	θmax = 25.4°, θmin = 3.3°
Absorption correction: multi-scan (SPHERE; Rigaku, 2000)	h = −9→8
Tmin = 0.364, Tmax = 1.000	k = −25→22
9983 measured reflections	l = −10→12

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.051	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.137	H-atom parameters constrained
S = 0.88	w = 1/[σ^2(Fo^2) + (0.0448P)^2] where P = (Fo^2 + 2Fc^2)/3
2813 reflections	(Δ/σ)max = 0.002
154 parameters	Δρmax = 1.10 e Å^−3
0 restraints	Δρmin = −1.11 e Å^−3

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cd1	0.10563 (3)	0.630127 (10)	0.19821 (3)	0.05087 (7)
Br1	0.11617 (7)	0.546299 (19)	0.38321 (5)	0.08902 (18)
Br2	−0.14859 (4)	0.593482 (18)	−0.02245 (3)	0.06183 (12)
Br3	0.41051 (5)	0.639558 (18)	0.14868 (4)	0.07024 (13)
Br4	0.02346 (6)	0.736438 (19)	0.30023 (5)	0.09291 (16)
N1	−0.2633 (4)	0.69653 (15)	0.4674 (3)	0.0822 (12)
H1B	−0.2026	0.7149	0.4190	0.099*
N2	−0.7867 (4)	0.54688 (12)	0.8289 (3)	0.0647 (10)
H2B	−0.8522	0.5291	0.8743	0.078*
C1	−0.2342 (6)	0.6348 (2)	0.4986 (4)	0.0961 (16)
H1A	−0.1464	0.6130	0.4713	0.115*
C2	−0.3321 (5)	0.6033 (2)	0.5704 (4)	0.0850 (14)
H2A	−0.3094	0.5603	0.5950	0.102*
C3	−0.4665 (4)	0.63629 (14)	0.6064 (3)	0.0508 (10)
C4	−0.4863 (5)	0.69847 (16)	0.5707 (4)	0.0817 (14)
H4A	−0.5768	0.7222	0.5902	0.098*
C5	−0.3719 (5)	0.7285 (2)	0.5039 (4)	0.0853 (15)
H5A	−0.3793	0.7726	0.4878	0.102*
C6	−0.5774 (4)	0.60468 (13)	0.6838 (3)	0.0441 (9)
C7	−0.7118 (5)	0.63636 (16)	0.7136 (4)	0.0682 (13)
H7A	−0.7348	0.6789	0.6853	0.082*
C8	−0.8190 (5)	0.60556 (19)	0.7877 (4)	0.0817 (13)
H8A	−0.9125	0.6278	0.8064	0.098*
C9	−0.6570 (5)	0.51307 (15)	0.8043 (4)	0.0747 (14)
H9A	−0.6412	0.4703	0.8326	0.090*
C10	−0.5434 (5)	0.54067 (15)	0.7363 (4)	0.0649 (12)
H10A	−0.4466	0.5178	0.7252	0.078*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.04669 (12)	0.04563 (11)	0.06182 (13)	−0.00350 (10)	0.01803 (10)	−0.00199 (11)
Br1	0.1022 (3)	0.0735 (2)	0.0878 (3)	0.0042 (2)	0.0216 (3)	0.0070 (2)
Br2	0.04817 (18)	0.0847 (2)	0.05245 (18)	−0.01916 (16)	0.01420 (15)	0.00033 (18)
Br3	0.04813 (19)	0.0735 (2)	0.0912 (2)	−0.00440 (16)	0.02337 (18)	−0.0054 (2)
Br4	0.1079 (3)	0.0576 (2)	0.1396 (3)	−0.00504 (19)	0.0775 (2)	−0.0046 (2)
N1	0.0642 (19)	0.110 (2)	0.0721 (19)	−0.0133 (18)	0.0193 (16)	−0.0001 (19)
N2	0.0592 (16)	0.0699 (17)	0.0704 (17)	−0.0177 (13)	0.0272 (14)	0.0012 (14)
C1	0.069 (2)	0.130 (3)	0.106 (3)	0.020 (2)	0.0523 (19)	−0.016 (3)
C2	0.099 (2)	0.076 (2)	0.107 (2)	0.0177 (19)	0.0722 (18)	0.000 (2)
C3	0.0406 (16)	0.0622 (18)	0.0494 (17)	0.0061 (14)	0.0123 (14)	−0.0038 (15)
C4	0.096 (2)	0.0498 (18)	0.118 (3)	0.0159 (18)	0.061 (2)	0.012 (2)
C5	0.077 (2)	0.087 (3)	0.105 (3)	−0.002 (2)	0.048 (2)	0.005 (2)
C6	0.0317 (15)	0.0404 (14)	0.0578 (18)	−0.0017 (12)	0.0088 (14)	−0.0063 (14)
C7	0.076 (2)	0.0553 (19)	0.085 (2)	0.0110 (17)	0.0417 (17)	0.0073 (18)
C8	0.084 (2)	0.084 (2)	0.103 (2)	0.001 (2)	0.0686 (16)	−0.005 (2)
C9	0.093 (3)	0.0377 (16)	0.090 (3)	−0.0240 (17)	0.021 (2)	0.0002 (17)
C10	0.068 (2)	0.0520 (18)	0.080 (2)	−0.0015 (16)	0.0298 (18)	−0.0161 (17)

Geometric parameters (Å, º)

Cd1—Br1	2.5241 (7)	C3—C4	1.339 (4)
Cd1—Br3	2.5871 (8)	C3—C6	1.474 (5)
Cd1—Br4	2.5921 (6)	C4—C5	1.409 (6)
Cd1—Br2	2.6260 (6)	C4—H4A	0.9300
N1—C5	1.216 (6)	C5—H5A	0.9300
N1—C1	1.325 (5)	C6—C7	1.348 (5)
N1—H1B	0.8600	C6—C10	1.427 (4)
N2—C8	1.290 (5)	C7—C8	1.423 (6)
N2—C9	1.318 (5)	C7—H7A	0.9300
N2—H2B	0.8600	C8—H8A	0.9300
C1—C2	1.360 (6)	C9—C10	1.390 (6)
C1—H1A	0.9300	C9—H9A	0.9300
C2—C3	1.390 (5)	C10—H10A	0.9300
C2—H2A	0.9300
Br1—Cd1—Br3	110.769 (18)	C3—C4—C5	120.9 (4)
Br1—Cd1—Br4	105.12 (2)	C3—C4—H4A	119.6
Br3—Cd1—Br4	111.031 (17)	C5—C4—H4A	119.6
Br1—Cd1—Br2	105.73 (2)	N1—C5—C4	119.8 (4)
Br3—Cd1—Br2	112.56 (2)	N1—C5—H5A	120.1
Br4—Cd1—Br2	111.243 (19)	C4—C5—H5A	120.1
C5—N1—C1	123.2 (4)	C7—C6—C10	116.8 (3)
C5—N1—H1B	118.4	C7—C6—C3	121.0 (3)
C1—N1—H1B	118.4	C10—C6—C3	122.2 (3)
C8—N2—C9	122.5 (3)	C6—C7—C8	120.8 (3)
C8—N2—H2B	118.8	C6—C7—H7A	119.6
C9—N2—H2B	118.8	C8—C7—H7A	119.6
N1—C1—C2	120.4 (4)	N2—C8—C7	120.0 (4)
N1—C1—H1A	119.8	N2—C8—H8A	120.0
C2—C1—H1A	119.8	C7—C8—H8A	120.0
C1—C2—C3	119.0 (4)	N2—C9—C10	120.6 (3)
C1—C2—H2A	120.5	N2—C9—H9A	119.7
C3—C2—H2A	120.5	C10—C9—H9A	119.7
C4—C3—C2	116.5 (4)	C9—C10—C6	119.1 (3)
C4—C3—C6	122.0 (3)	C9—C10—H10A	120.5
C2—C3—C6	121.5 (3)	C6—C10—H10A	120.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1B···Br4	0.86	2.44	3.272 (4)	162