trans-Tetra­aqua­bis(nicotinamide-κN)cadmium(II) biphenyl-4,4′-disulfonate

Li, C.; Chen, M.; Shao, C.

doi:10.1107/S1600536808002390

metal-organic compounds

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ISSN: 2056-9890

Volume 64| Part 2| February 2008| Page m424

doi:10.1107/S1600536808002390

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trans-Tetraaquabis(nicotinamide-κN)cadmium(II) biphenyl-4,4′-disulfonate

Chunyuan Li,^a ^* Min Chen ^b and Changlun Shao ^c

^aDepartment of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, People's Republic of China, ^bCentre of Experimental Teaching of Common Basic Courses, South China Agricultural University, Guangzhou 510642, People's Republic of China, and ^cSchool of Medicine and Pharmacy, Ocean University of China, Qingdao 266003, People's Republic of China
^*Correspondence e-mail: chunyuan-li@163.com

(Received 11 January 2008; accepted 22 January 2008; online 25 January 2008)

In the title compound, [Cd(C₆H₆N₂O)₂(H₂O)₄](C₁₀H₈O₆S₂), the Cd^II ion is located on a crystallographic inversion centre. An octahedral coordination geometry is defined by four water molecules in one plane, and two trans N-atom donors of the nicotinamide ligands. The biphenyl-4,4′-disulfonate anion also lies on a crystallographic inversion centre. In the crystal structure, the complex cations are connected to the counter-anions via N—H⋯O and O—H⋯O hydrogen bonds, forming a three-dimensional network.

Related literature

For related literature, see: Beatty (2001 ); Christer et al. (2004 ); Holman et al. (2001 ); Lian & Li (2007a ,b ,c ,d ).

Experimental

Crystal data

[Cd(C₆H₆N₂O)₂(H₂O)₄](C₁₀H₈O₆S₂)
M_r = 741.02
Monoclinic, P 2/c
a = 14.742 (8) Å
b = 6.899 (4) Å
c = 15.292 (8) Å
β = 110.980 (9)°
V = 1452.2 (13) Å³
Z = 2
Mo Kα radiation
μ = 0.96 mm⁻¹
T = 298 (2) K
0.40 × 0.36 × 0.31 mm

Data collection

Bruker SMART APEX CCD diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.682, T_max = 0.741
7656 measured reflections
2842 independent reflections
2477 reflections with I > 2σ(I)
R_int = 0.034

Refinement

R[F² > 2σ(F²)] = 0.030
wR(F²) = 0.092
S = 1.07
2842 reflections
197 parameters
H-atom parameters constrained
Δρ_max = 0.57 e Å⁻³
Δρ_min = −1.19 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5—H5A⋯O3ⁱ	0.81	2.03	2.832 (3)	171
O5—H5B⋯O4ⁱⁱ	0.86	1.83	2.676 (2)	172
O6—H6A⋯O3	0.85	1.93	2.777 (3)	178
O6—H6B⋯O2ⁱⁱⁱ	0.83	1.89	2.716 (3)	171
N2—H2A⋯O1^iv	0.86	2.08	2.932 (2)	171
N2—H2B⋯O2^v	0.86	2.17	3.025 (3)	172
Symmetry codes: (i) x, y-1, z; (ii) $[-x+1, y-1, -z+{\script{1\over 2}}]$ ; (iii) $[-x+1, y, -z+{\script{1\over 2}}]$ ; (iv) -x, -y, -z+1; (v) $[x, -y, z+{\script{1\over 2}}]$ .

Data collection: SMART (Bruker, 1997 ); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808002390/fj2097sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808002390/fj2097Isup2.hkl

checkCIF report

Comment top

The strong and directional nature of hydrogen bonds is exploited in the organized self-assembly of molecules in the solid state. Amides are commonly used functional groups in crystal engineering owing to the inherent coordination and hydrogen bonding donor/acceptor functionalities, and were used to construct extended frameworks sustained both by hydrogen bonds and coordination bonds (Beatty 2001; Christer et al., 2004). On the other hand, the sulfonate group is a suitable hydrogen-bond acceptor, and has been used to build extended frameworks. (Holman et al., 2001; Lian & Li, 2007a,b,c,d) In this paper, we report the synthesis and crystal structure of the title compound.

In the title compound, the metal centre, located on a crystallographic inversion centre, is in an octahedral geometry defined by four O atoms from four aqua ligands,respectively, and two trans-positioned N-atom donors of the nicotinamide ligands, as shown in Fig 1. The amide substituents are twisted away from the conformation in the pyridine ring plane by 37.6 (6)°. The biphenyl-4,4'-disulfonate anion also lies on a crystallographic inversion centre. The planes through phenyl ring in biphenyl-4,4'-disulfonate are twisted by 28.5 (7)°. In the crystal structure, all the amide groups are involved in amide-amide hydrogen bonded linkage in head to head fashion leading to infinite chains of the cations. These chains are linked by hydrogen bonds formed by the remaining N—H protons on the nicotinamide and sulfonate oxygen atoms, and coordinated water molecules and sulfonate oxygen atoms into three-dimensional networks as shown in Fig 2.

Related literature top

For related literature, see: Beatty (2001); Christer et al. (2004); Holman et al. (2001); Lian & Li (2007a,b,c,d).

Experimental top

Nicotinamide (0.050 g, 0.4 mmol) was added with constant stirring to an aqueous solution (10 mL) of Cd(CH₃COO)₂.2H₂O (0.058 g, 0.2 mmol). The solution was then treated with disodium biphenyl-4,4'-disulfonate (0.070 g, 0.2 mmol). Colourless crystals of the title complex were collected by slow evaporation at room temperature after 7 days, (80% yield based on Cd).

Computing details top

Data collection: SMART (Bruker, 1997); cell refinement: SMART (Bruker, 1997); data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. The molecular structure of (1), with the atom labelling scheme. Displacement ellipsoids are drawn at the 40% probability level. H atoms are represented as spheres of arbitrary radii. Hydrogen bonds are shown as dashed lines. Symmetry codes:(i) -x, y, -z + 1/2; (ii): -x + 1, -y, -z + 1.
	Fig. 2. The three dimensional H-bonded network viewed from b axis direction. Hydrogen bonds are shown as dashed lines.

trans-Tetraaquabis(nicotinamide-κN)cadmium(II) biphenyl-4,4'-disulfonate top

Crystal data top

[Cd(C₆H₆N₂O)₂(H₂O)₄](C₁₀H₈O₆S₂)	F(000) = 752
M_r = 741.02	D_x = 1.695 Mg m⁻³
Monoclinic, P2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yc	Cell parameters from 7656 reflections
a = 14.742 (8) Å	θ = 12–18°
b = 6.899 (4) Å	µ = 0.97 mm⁻¹
c = 15.292 (8) Å	T = 298 K
β = 110.980 (9)°	Block, colourless
V = 1452.2 (13) Å³	0.40 × 0.36 × 0.31 mm
Z = 2

Data collection top

Bruker SMART APEX CCD diffractometer	2842 independent reflections
Radiation source: fine-focus sealed tube	2477 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.034
ϕ and ω scans	θ_max = 26.0°, θ_min = 3.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −18→12
T_min = 0.682, T_max = 0.741	k = −8→7
7656 measured reflections	l = −13→18

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.030	H-atom parameters constrained
wR(F²) = 0.092	w = 1/[σ²(F_o²) + (0.0578P)² + 0.3728P] where P = (F_o² + 2F_c²)/3
S = 1.07	(Δ/σ)_max < 0.001
2842 reflections	Δρ_max = 0.58 e Å⁻³
197 parameters	Δρ_min = −1.19 e Å⁻³
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.052 (2)

Crystal data top

[Cd(C₆H₆N₂O)₂(H₂O)₄](C₁₀H₈O₆S₂)	V = 1452.2 (13) Å³
M_r = 741.02	Z = 2
Monoclinic, P2/c	Mo Kα radiation
a = 14.742 (8) Å	µ = 0.97 mm⁻¹
b = 6.899 (4) Å	T = 298 K
c = 15.292 (8) Å	0.40 × 0.36 × 0.31 mm
β = 110.980 (9)°

Data collection top

Bruker SMART APEX CCD diffractometer	2842 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	2477 reflections with I > 2σ(I)
T_min = 0.682, T_max = 0.741	R_int = 0.034
7656 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.030	0 restraints
wR(F²) = 0.092	H-atom parameters constrained
S = 1.07	Δρ_max = 0.58 e Å⁻³
2842 reflections	Δρ_min = −1.19 e Å⁻³
197 parameters

Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Cd1	0.5000	0.0000	0.5000	0.02772 (14)
S1	0.31842 (5)	0.50038 (6)	0.18505 (5)	0.03230 (18)
O1	0.03394 (9)	0.0836 (3)	0.40310 (11)	0.0485 (4)
O2	0.30985 (11)	0.3259 (3)	0.12698 (14)	0.0544 (5)
O3	0.39387 (14)	0.4851 (2)	0.27876 (16)	0.0462 (5)
O4	0.32299 (12)	0.6768 (3)	0.13379 (15)	0.0582 (5)
O5	0.51977 (12)	−0.2553 (3)	0.41016 (16)	0.0738 (7)
H5A	0.4803	−0.3193	0.3698	0.089*
H5B	0.5698	−0.2876	0.3976	0.089*
O6	0.51991 (12)	0.2033 (3)	0.38506 (14)	0.0657 (6)
H6A	0.4802	0.2889	0.3531	0.079*
H6B	0.5701	0.2527	0.3817	0.079*
N1	0.34204 (14)	−0.00860 (19)	0.41343 (14)	0.0296 (4)
N2	0.12317 (10)	−0.1123 (3)	0.52234 (10)	0.0370 (4)
H2A	0.0802	−0.1148	0.5484	0.044*
H2B	0.1766	−0.1754	0.5469	0.044*
C1	0.27511 (17)	−0.0072 (2)	0.45320 (16)	0.0279 (5)
H1	0.2932	−0.0044	0.5180	0.033*
C2	0.18103 (17)	−0.0100 (2)	0.39791 (17)	0.0277 (5)
C3	0.15424 (17)	−0.0111 (2)	0.29767 (16)	0.0316 (5)
H3	0.0888	−0.0066	0.2600	0.038*
C4	0.22217 (18)	−0.0186 (3)	0.25694 (16)	0.0334 (5)
H4	0.2057	−0.0249	0.1923	0.040*
C5	0.31554 (17)	−0.0164 (2)	0.31683 (16)	0.0320 (5)
H5	0.3645	−0.0202	0.2919	0.038*
C6	0.10639 (16)	−0.0082 (2)	0.44191 (16)	0.0302 (5)
C7	0.21328 (17)	0.5140 (2)	0.20881 (19)	0.0303 (5)
C8	0.21293 (18)	0.4744 (3)	0.30073 (19)	0.0348 (5)
H8	0.2707	0.4455	0.3494	0.042*
C9	0.12884 (18)	0.4791 (3)	0.31652 (19)	0.0349 (5)
H9	0.1271	0.4538	0.3756	0.042*
C10	0.04503 (16)	0.5228 (2)	0.24163 (18)	0.0311 (5)
C11	0.04791 (14)	0.5662 (3)	0.15037 (15)	0.0353 (4)
H11	−0.0095	0.5976	0.1017	0.042*
C12	0.13142 (14)	0.5621 (3)	0.13407 (14)	0.0342 (4)
H12	0.1338	0.5904	0.0754	0.041*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cd1	0.01281 (17)	0.03481 (19)	0.03689 (19)	−0.00051 (5)	0.01054 (11)	−0.00146 (6)
S1	0.0174 (3)	0.0352 (3)	0.0500 (4)	−0.00152 (14)	0.0190 (3)	−0.00351 (17)
O1	0.0210 (7)	0.0697 (11)	0.0582 (9)	0.0136 (8)	0.0181 (6)	0.0197 (9)
O2	0.0275 (8)	0.0627 (11)	0.0810 (11)	−0.0076 (8)	0.0290 (8)	−0.0316 (10)
O3	0.0196 (9)	0.0565 (11)	0.0619 (12)	−0.0001 (5)	0.0136 (8)	−0.0056 (6)
O4	0.0369 (9)	0.0594 (11)	0.0942 (13)	0.0054 (8)	0.0427 (9)	0.0243 (10)
O5	0.0318 (8)	0.0794 (13)	0.1183 (16)	−0.0128 (9)	0.0366 (10)	−0.0596 (13)
O6	0.0289 (8)	0.0791 (13)	0.0921 (13)	0.0042 (8)	0.0254 (8)	0.0459 (11)
N1	0.0152 (9)	0.0393 (10)	0.0338 (9)	−0.0004 (5)	0.0083 (7)	−0.0008 (6)
N2	0.0186 (7)	0.0542 (11)	0.0399 (9)	0.0035 (7)	0.0125 (6)	0.0069 (8)
C1	0.0161 (11)	0.0381 (12)	0.0295 (10)	−0.0004 (6)	0.0082 (8)	0.0001 (6)
C2	0.0189 (11)	0.0288 (10)	0.0353 (12)	0.0000 (6)	0.0095 (9)	−0.0004 (6)
C3	0.0184 (11)	0.0377 (12)	0.0340 (12)	0.0001 (6)	0.0036 (9)	0.0009 (7)
C4	0.0283 (12)	0.0421 (12)	0.0277 (10)	−0.0005 (7)	0.0075 (9)	−0.0006 (7)
C5	0.0228 (11)	0.0398 (11)	0.0362 (11)	0.0007 (7)	0.0139 (9)	−0.0008 (7)
C6	0.0154 (10)	0.0387 (12)	0.0352 (11)	−0.0019 (6)	0.0074 (8)	−0.0013 (7)
C7	0.0183 (11)	0.0288 (10)	0.0499 (13)	−0.0005 (6)	0.0197 (10)	−0.0014 (7)
C8	0.0212 (10)	0.0388 (10)	0.0483 (13)	0.0048 (7)	0.0172 (9)	0.0074 (8)
C9	0.0254 (11)	0.0388 (11)	0.0469 (12)	0.0054 (7)	0.0209 (10)	0.0098 (8)
C10	0.0200 (11)	0.0287 (9)	0.0508 (12)	0.0002 (7)	0.0202 (9)	−0.0007 (8)
C11	0.0195 (9)	0.0427 (11)	0.0454 (11)	0.0000 (9)	0.0139 (8)	−0.0004 (10)
C12	0.0237 (9)	0.0419 (10)	0.0412 (10)	−0.0029 (9)	0.0169 (8)	−0.0012 (10)

Geometric parameters (Å, º) top

Cd1—N1ⁱ	2.230 (2)	C1—C2	1.341 (3)
Cd1—N1	2.230 (2)	C1—H1	0.9300
Cd1—O5ⁱ	2.316 (2)	C2—C3	1.439 (3)
Cd1—O5	2.316 (2)	C2—C6	1.481 (3)
Cd1—O6ⁱ	2.3479 (19)	C3—C4	1.357 (4)
Cd1—O6	2.3479 (19)	C3—H3	0.9300
S1—O4	1.463 (2)	C4—C5	1.353 (3)
S1—O3	1.470 (2)	C4—H4	0.9300
S1—O2	1.4742 (19)	C5—H5	0.9300
S1—C7	1.717 (2)	C7—C12	1.373 (3)
O1—C6	1.200 (3)	C7—C8	1.434 (4)
O5—H5A	0.8108	C8—C9	1.346 (3)
O5—H5B	0.8551	C8—H8	0.9300
O6—H6A	0.8517	C9—C10	1.384 (4)
O6—H6B	0.8328	C9—H9	0.9300
N1—C1	1.332 (3)	C10—C10ⁱⁱ	1.439 (4)
N1—C5	1.387 (3)	C10—C11	1.443 (3)
N2—C6	1.368 (3)	C11—C12	1.341 (3)
N2—H2A	0.8600	C11—H11	0.9300
N2—H2B	0.8600	C12—H12	0.9300

N1ⁱ—Cd1—N1	180.00 (6)	C2—C1—H1	120.7
N1ⁱ—Cd1—O5ⁱ	87.37 (7)	C1—C2—C3	119.9 (2)
N1—Cd1—O5ⁱ	92.63 (7)	C1—C2—C6	118.8 (2)
N1ⁱ—Cd1—O5	92.63 (6)	C3—C2—C6	121.2 (2)
N1—Cd1—O5	87.37 (7)	C4—C3—C2	121.5 (2)
O5ⁱ—Cd1—O5	180.0	C4—C3—H3	119.2
N1ⁱ—Cd1—O6ⁱ	87.44 (7)	C2—C3—H3	119.2
N1—Cd1—O6ⁱ	92.56 (7)	C5—C4—C3	115.4 (2)
O5ⁱ—Cd1—O6ⁱ	86.23 (10)	C5—C4—H4	122.3
O5—Cd1—O6ⁱ	93.77 (10)	C3—C4—H4	122.3
N1ⁱ—Cd1—O6	92.56 (7)	C4—C5—N1	123.5 (2)
N1—Cd1—O6	87.44 (7)	C4—C5—H5	118.3
O5ⁱ—Cd1—O6	93.77 (10)	N1—C5—H5	118.3
O5—Cd1—O6	86.23 (10)	O1—C6—N2	124.3 (2)
O6ⁱ—Cd1—O6	180.0	O1—C6—C2	117.0 (2)
O4—S1—O3	114.71 (11)	N2—C6—C2	118.65 (18)
O4—S1—O2	111.55 (16)	C12—C7—C8	123.5 (2)
O3—S1—O2	113.62 (11)	C12—C7—S1	115.34 (19)
O4—S1—C7	106.64 (9)	C8—C7—S1	121.17 (18)
O3—S1—C7	102.88 (13)	C9—C8—C7	119.9 (2)
O2—S1—C7	106.44 (9)	C9—C8—H8	120.1
Cd1—O5—H5A	131.1	C7—C8—H8	120.1
Cd1—O5—H5B	129.3	C8—C9—C10	117.6 (2)
H5A—O5—H5B	97.4	C8—C9—H9	121.2
Cd1—O6—H6A	126.9	C10—C9—H9	121.2
Cd1—O6—H6B	130.0	C9—C10—C10ⁱⁱ	117.4 (3)
H6A—O6—H6B	97.1	C9—C10—C11	121.3 (2)
C1—N1—C5	120.96 (19)	C10ⁱⁱ—C10—C11	121.4 (3)
C1—N1—Cd1	121.05 (16)	C12—C11—C10	121.50 (19)
C5—N1—Cd1	117.99 (15)	C12—C11—H11	119.3
C6—N2—H2A	120.0	C10—C11—H11	119.3
C6—N2—H2B	120.0	C11—C12—C7	116.2 (2)
H2A—N2—H2B	120.0	C11—C12—H12	121.9
N1—C1—C2	118.7 (2)	C7—C12—H12	121.9
N1—C1—H1	120.7

Symmetry codes: (i) −x+1, −y, −z+1; (ii) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5A···O3ⁱⁱⁱ	0.81	2.03	2.832 (3)	171
O5—H5B···O4^iv	0.86	1.83	2.676 (2)	172
O6—H6A···O3	0.85	1.93	2.777 (3)	178
O6—H6B···O2^v	0.83	1.89	2.716 (3)	171
N2—H2A···O1^vi	0.86	2.08	2.932 (2)	171
N2—H2B···O2^vii	0.86	2.17	3.025 (3)	172

Symmetry codes: (iii) x, y−1, z; (iv) −x+1, y−1, −z+1/2; (v) −x+1, y, −z+1/2; (vi) −x, −y, −z+1; (vii) x, −y, z+1/2.

Experimental details

Crystal data
Chemical formula	[Cd(C₆H₆N₂O)₂(H₂O)₄](C₁₀H₈O₆S₂)
M_r	741.02
Crystal system, space group	Monoclinic, P2/c
Temperature (K)	298
a, b, c (Å)	14.742 (8), 6.899 (4), 15.292 (8)
β (°)	110.980 (9)
V (Å³)	1452.2 (13)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.97
Crystal size (mm)	0.40 × 0.36 × 0.31

Data collection
Diffractometer	Bruker SMART APEX CCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.682, 0.741
No. of measured, independent and observed [I > 2σ(I)] reflections	7656, 2842, 2477
R_int	0.034
(sin θ/λ)_max (Å⁻¹)	0.617

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.030, 0.092, 1.07
No. of reflections	2842
No. of parameters	197
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.58, −1.19

Computer programs: SMART (Bruker, 1997), SAINT (Bruker, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5A···O3ⁱ	0.81	2.03	2.832 (3)	171
O5—H5B···O4ⁱⁱ	0.86	1.83	2.676 (2)	172
O6—H6A···O3	0.85	1.93	2.777 (3)	178
O6—H6B···O2ⁱⁱⁱ	0.83	1.89	2.716 (3)	171
N2—H2A···O1^iv	0.86	2.08	2.932 (2)	171
N2—H2B···O2^v	0.86	2.17	3.025 (3)	172

Symmetry codes: (i) x, y−1, z; (ii) −x+1, y−1, −z+1/2; (iii) −x+1, y, −z+1/2; (iv) −x, −y, −z+1; (v) x, −y, z+1/2.

Acknowledgements

The authors thank the President's Foundation of South China Agricultural University (Nos. 2006X013, 2007Y006 and 2007 K031) for financial support.

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