Poly[[aqua­(μ2-oxalato)(μ2-2-oxido­pyridinium-3-carboxylato)holmium(III)] monohydrate]

Zhu, H.-L.; Lai, H.-L.; Han, L.; Luo, Y.-F.; Zeng, R.-H.

doi:10.1107/S1600536809036344

metal-organic compounds

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ISSN: 2056-9890

Volume 65| Part 10| October 2009| Pages m1217-m1218

doi:10.1107/S1600536809036344

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Poly[[aqua(μ₂-oxalato)(μ₂-2-oxidopyridinium-3-carboxylato)holmium(III)] monohydrate]

Hui-Lan Zhu,^a Hui-Ling Lai,^a Lu Han,^a Yi-Fan Luo ^a ^* and Rong-Hua Zeng ^a,^b

^aSchool of Chemistry and Environment, South China Normal University, Guangzhou 510006, People's Republic of China, and ^bKey Laboratory of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, South China Normal University, Guangzhou 510006, People's Republic of China
^*Correspondence e-mail: luoyf2004@yahoo.com.cn

(Received 25 August 2009; accepted 8 September 2009; online 16 September 2009)

In the title complex, {[Ho(C₂O₄)(C₆H₄NO₃)(H₂O)]·(H₂O)}_n, the Ho^III ion is coordinated by three O atoms from two 2-oxidopyridinium-3-carboxylate ligands, four O atoms from two oxalate ligands and one water molecule in a distorted bicapped trigonal-prismatic geometry. The 2-oxidopyridinium-3-carboxylate and oxalate ligands link the Ho^III ions into a layer in (100). These layers are further connected by intermolecular O—H⋯O hydrogen bonds involving the coordinated water molecules to assemble a three-dimensional supramolecular network. The uncoordinated water molecule is involved in N—H⋯O and O—H⋯O hydrogen bonds within the layer.

Related literature

For general background to molecular self-assembly of supramolecular architectures, see: Deng et al. (2008 ); Zeng et al. (2007 ). For 2-oxidopyridinium-3-carboxylic acid and oxalic acid as building blocks, see: Huang et al. (2009 ).

Experimental

Crystal data

[Ho(C₂O₄)(C₆H₄NO₃)(H₂O)]·H₂O
M_r = 427.08
Triclinic, $[P \overline 1]$
a = 6.5391 (11) Å
b = 9.5305 (16) Å
c = 9.7391 (16) Å
α = 71.810 (2)°
β = 78.862 (2)°
γ = 80.359 (2)°
V = 562.01 (16) Å³
Z = 2
Mo Kα radiation
μ = 7.09 mm⁻¹
T = 296 K
0.20 × 0.18 × 0.17 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.332, T_max = 0.379
2884 measured reflections
1983 independent reflections
1862 reflections with I > 2σ(I)
R_int = 0.018

Refinement

R[F² > 2σ(F²)] = 0.034
wR(F²) = 0.090
S = 1.06
1983 reflections
184 parameters
6 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 2.75 e Å⁻³
Δρ_min = −2.19 e Å⁻³

Table 1
Selected bond lengths (Å)

Ho1—O1	2.354 (5)
Ho1—O2ⁱ	2.348 (5)
Ho1—O3	2.279 (5)
Ho1—O4	2.406 (4)
Ho1—O5ⁱⁱ	2.364 (5)
Ho1—O6	2.394 (5)
Ho1—O7ⁱⁱⁱ	2.392 (5)
Ho1—O1W	2.348 (5)
Symmetry codes: (i) -x, -y, -z+1; (ii) -x, -y+1, -z; (iii) -x, -y+1, -z+1.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1W—H1W⋯O2^iv	0.84 (6)	2.02 (6)	2.737 (7)	142 (8)
O1W—H2W⋯O4^v	0.85 (7)	1.96 (4)	2.749 (7)	154 (8)
O2W—H3W⋯O6	0.84 (7)	2.36 (11)	2.882 (10)	121 (8)
O2W—H4W⋯O1	0.86 (7)	2.27 (3)	3.082 (8)	157 (9)
N1—H1⋯O2W^vi	0.86	1.99	2.781 (10)	154
Symmetry codes: (iv) -x+1, -y, -z+1; (v) x+1, y, z; (vi) x, y, z-1.

Data collection: APEX2 (Bruker, 2007 ); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999 ); software used to prepare material for publication: SHELXL97.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809036344/hy2226sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809036344/hy2226Isup2.hkl

checkCIF report

Comment top

Molecular self-assembly of supramolecular architectures has received much attention during recent decades (Deng et al., 2008; Zeng et al., 2007). The structures and properties of such systems depend on the coordination and geometric preferences of both the central metal ions and the bridging building blocks, as well as the influence of weaker non-covalent interactions, such as hydrogen bonds and π–π stacking interactions. As a building block, 2-oxynicotinic acid and oxalic acid are good ligands with multifunctional coordination sites providing a high likelihood for the generation of structures with high dimensions (Huang et al., 2009). Recently, we obtained the title coordination polymer, which was synthesized under hydrothermal conditions.

In the title compound (Fig. 1), the Ho^III centre is eight-coordinated by seven O atoms from two 2-oxynicotinate ligands and two oxalate ligands, and by one water molecule in a distorted bicapped trigonal prismatic geometry, with Ho—O distances and O—Ho—O angles ranging from 2.279 (5) to 2.406 (4) Å and 67.30 (17) to 148.26 (17)°, respectively (Table 1). The carboxylate groups of the 2-oxynicotinates and oxalates act as bridging ligands, linking the Ho centres into a layer parallel to the (100) plane (Fig. 2). The layers are further connected by intermolecular O—H···O hydrogen bonds involving the coordinated water molecules into a three-dimensional supramolecular network (Table 2). The uncoordinated water molecule is involved in N—H···O and O—H···O hydrogen bonds within the layer (Table 2).

Related literature top

For general background to molecular self-assembly of supramolecular architectures, see: Deng et al. (2008); Zeng et al. (2007). For 2-oxidopyridinium-3-carboxylic acid and oxalic acid as building blocks, see: Huang et al. (2009);

Experimental top

A mixture of Ho₂O₃ (0.375 g, 1 mmol), 2-oxynicotinic acid (0.127 g, 1 mmol), oxalic acid (0.09 g, 1 mmol), water (10 ml) in the presence of HNO₃ (0.024 g, 0.385 mmol) was stirred vigorously for 20 min and then sealed in a Teflon-lined stainless-steel autoclave (20 ml capacity). The autoclave was heated and maintained at 446 K for 2 d, and then cooled to room temperature at 5 K h^-1, giving colourless block crystals.

Computing details top

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top

	Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (i) -x, -y, 1 - z; (ii) -x, 1 - y, -z; (iii) -x, 1 - y, 1 - z.]
	Fig. 2. View of a layered network of the title compound.

Poly[[aqua(µ₂-oxalato)(µ₂-2-oxidopyridinium-3-carboxylato)holmium(III)] monohydrate] top

Crystal data top

[Ho(C₂O₄)(C₆H₄NO₃)(H₂O)]·H₂O	Z = 2
M_r = 427.08	F(000) = 404
Triclinic, P1	D_x = 2.524 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.5391 (11) Å	Cell parameters from 2285 reflections
b = 9.5305 (16) Å	θ = 2.2–28.1°
c = 9.7391 (16) Å	µ = 7.09 mm⁻¹
α = 71.810 (2)°	T = 296 K
β = 78.862 (2)°	Block, colourless
γ = 80.359 (2)°	0.20 × 0.18 × 0.17 mm
V = 562.01 (16) Å³

Data collection top

Bruker APEXII CCD diffractometer	1983 independent reflections
Radiation source: fine-focus sealed tube	1862 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.018
ϕ and ω scans	θ_max = 25.2°, θ_min = 2.2°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −7→6
T_min = 0.332, T_max = 0.379	k = −11→11
2884 measured reflections	l = −11→10

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.090	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ²(F_o²) + (0.0695P)²] where P = (F_o² + 2F_c²)/3
1983 reflections	(Δ/σ)_max < 0.001
184 parameters	Δρ_max = 2.75 e Å⁻³
6 restraints	Δρ_min = −2.19 e Å⁻³

Crystal data top

[Ho(C₂O₄)(C₆H₄NO₃)(H₂O)]·H₂O	γ = 80.359 (2)°
M_r = 427.08	V = 562.01 (16) Å³
Triclinic, P1	Z = 2
a = 6.5391 (11) Å	Mo Kα radiation
b = 9.5305 (16) Å	µ = 7.09 mm⁻¹
c = 9.7391 (16) Å	T = 296 K
α = 71.810 (2)°	0.20 × 0.18 × 0.17 mm
β = 78.862 (2)°

Data collection top

Bruker APEXII CCD diffractometer	1983 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	1862 reflections with I > 2σ(I)
T_min = 0.332, T_max = 0.379	R_int = 0.018
2884 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.034	6 restraints
wR(F²) = 0.090	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	Δρ_max = 2.75 e Å⁻³
1983 reflections	Δρ_min = −2.19 e Å⁻³
184 parameters

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

	x	y	z	U_iso*/U_eq
Ho1	0.09083 (4)	0.29059 (3)	0.30045 (3)	0.01784 (15)
O4	−0.1655 (7)	0.3863 (5)	0.1370 (5)	0.0219 (10)
O3	0.1549 (8)	0.1295 (6)	0.1626 (5)	0.0311 (11)
O6	0.1351 (9)	0.3228 (5)	0.5267 (6)	0.0338 (12)
O1	0.2127 (8)	0.0581 (5)	0.4520 (5)	0.0251 (10)
O5	−0.2292 (7)	0.5458 (5)	−0.0767 (5)	0.0258 (10)
N1	0.2729 (11)	−0.0557 (8)	0.0581 (7)	0.0366 (15)
H1	0.2407	0.0054	−0.0227	0.044*
C6	0.2307 (12)	−0.0067 (8)	0.1802 (8)	0.0266 (15)
C8	0.0606 (11)	0.4422 (8)	0.5578 (8)	0.0261 (15)
C5	0.3624 (14)	−0.1948 (10)	0.0574 (10)	0.044 (2)
H5	0.3862	−0.2213	−0.0290	0.052*
C4	0.4170 (14)	−0.2947 (9)	0.1807 (10)	0.048 (3)
H4	0.4851	−0.3882	0.1795	0.057*
C3	0.3695 (13)	−0.2550 (9)	0.3094 (10)	0.0396 (19)
H3	0.3999	−0.3250	0.3960	0.048*
C2	0.2776 (10)	−0.1133 (7)	0.3126 (7)	0.0251 (15)
C1	0.2306 (10)	−0.0747 (7)	0.4529 (7)	0.0236 (14)
C7	−0.1147 (10)	0.4807 (7)	0.0161 (7)	0.0190 (13)
O2W	0.2278 (13)	0.0635 (9)	0.7646 (8)	0.0621 (19)
O2	0.2125 (7)	−0.1813 (5)	0.5703 (5)	0.0262 (10)
O7	0.0730 (9)	0.4748 (6)	0.6691 (5)	0.0358 (13)
O1W	0.4526 (8)	0.2970 (7)	0.2822 (6)	0.0427 (15)
H3W	0.164 (16)	0.149 (7)	0.758 (10)	0.064*
H4W	0.203 (16)	0.041 (10)	0.691 (6)	0.064*
H1W	0.508 (12)	0.263 (11)	0.359 (5)	0.064*
H2W	0.549 (10)	0.332 (11)	0.215 (6)	0.064*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ho1	0.0166 (2)	0.0197 (2)	0.0158 (2)	−0.00221 (12)	−0.00168 (12)	−0.00363 (13)
O4	0.018 (2)	0.027 (2)	0.017 (2)	−0.0088 (18)	−0.0019 (18)	0.0028 (19)
O3	0.039 (3)	0.031 (3)	0.020 (2)	0.002 (2)	−0.005 (2)	−0.007 (2)
O6	0.046 (3)	0.023 (3)	0.034 (3)	0.010 (2)	−0.013 (2)	−0.013 (2)
O1	0.027 (3)	0.024 (2)	0.022 (2)	−0.0029 (19)	−0.0024 (19)	−0.0050 (19)
O5	0.016 (2)	0.035 (3)	0.023 (2)	−0.0035 (19)	−0.0071 (19)	−0.001 (2)
N1	0.040 (4)	0.043 (4)	0.028 (3)	−0.009 (3)	0.003 (3)	−0.016 (3)
C6	0.025 (4)	0.027 (4)	0.029 (4)	−0.007 (3)	−0.001 (3)	−0.010 (3)
C8	0.024 (4)	0.026 (4)	0.028 (4)	−0.005 (3)	−0.003 (3)	−0.007 (3)
C5	0.048 (5)	0.046 (5)	0.042 (5)	−0.011 (4)	0.014 (4)	−0.028 (4)
C4	0.055 (6)	0.036 (5)	0.050 (6)	−0.008 (4)	0.013 (5)	−0.021 (5)
C3	0.038 (5)	0.032 (4)	0.047 (5)	0.000 (3)	0.001 (4)	−0.015 (4)
C2	0.018 (3)	0.025 (3)	0.029 (4)	−0.008 (3)	0.007 (3)	−0.006 (3)
C1	0.012 (3)	0.026 (3)	0.028 (4)	0.000 (3)	−0.003 (3)	−0.003 (3)
C7	0.018 (3)	0.020 (3)	0.020 (3)	−0.006 (2)	0.001 (3)	−0.007 (3)
O2W	0.069 (5)	0.078 (5)	0.041 (4)	−0.022 (4)	−0.020 (4)	−0.005 (4)
O2	0.019 (2)	0.027 (3)	0.027 (3)	−0.0043 (19)	−0.0037 (19)	0.002 (2)
O7	0.056 (4)	0.031 (3)	0.024 (3)	0.008 (2)	−0.018 (2)	−0.013 (2)
O1W	0.021 (3)	0.062 (4)	0.029 (3)	−0.012 (3)	−0.008 (2)	0.015 (3)

Geometric parameters (Å, º) top

Ho1—O1	2.354 (5)	C6—C2	1.421 (10)
Ho1—O2ⁱ	2.348 (5)	C8—O7	1.237 (9)
Ho1—O3	2.279 (5)	C8—C8ⁱⁱⁱ	1.546 (15)
Ho1—O4	2.406 (4)	C5—C4	1.347 (13)
Ho1—O5ⁱⁱ	2.364 (5)	C5—H5	0.9300
Ho1—O6	2.394 (5)	C4—C3	1.384 (11)
Ho1—O7ⁱⁱⁱ	2.392 (5)	C4—H4	0.9300
Ho1—O1W	2.348 (5)	C3—C2	1.390 (10)
O4—C7	1.262 (8)	C3—H3	0.9300
O3—C6	1.279 (9)	C2—C1	1.487 (10)
O6—C8	1.263 (9)	C1—O2	1.271 (8)
O1—C1	1.249 (8)	C7—C7ⁱⁱ	1.554 (12)
O5—C7	1.232 (8)	O2W—H3W	0.84 (7)
O5—Ho1ⁱⁱ	2.364 (4)	O2W—H4W	0.86 (7)
N1—C5	1.358 (11)	O1W—H1W	0.84 (6)
N1—C6	1.373 (10)	O1W—H2W	0.85 (7)
N1—H1	0.8600

O3—Ho1—O1W	90.4 (2)	C5—N1—C6	123.9 (7)
O3—Ho1—O2ⁱ	90.37 (18)	C5—N1—H1	118.0
O1W—Ho1—O2ⁱ	148.26 (17)	C6—N1—H1	118.0
O3—Ho1—O1	73.45 (17)	O3—C6—N1	116.8 (7)
O1W—Ho1—O1	75.15 (18)	O3—C6—C2	127.2 (7)
O2ⁱ—Ho1—O1	74.69 (16)	N1—C6—C2	116.1 (7)
O3—Ho1—O5ⁱⁱ	81.80 (17)	O7—C8—O6	127.4 (7)
O1W—Ho1—O5ⁱⁱ	67.83 (16)	O7—C8—C8ⁱⁱⁱ	117.7 (8)
O2ⁱ—Ho1—O5ⁱⁱ	143.49 (16)	O6—C8—C8ⁱⁱⁱ	114.9 (8)
O1—Ho1—O5ⁱⁱ	134.91 (16)	C4—C5—N1	120.6 (8)
O3—Ho1—O7ⁱⁱⁱ	148.06 (18)	C4—C5—H5	119.7
O1W—Ho1—O7ⁱⁱⁱ	105.7 (2)	N1—C5—H5	119.7
O2ⁱ—Ho1—O7ⁱⁱⁱ	89.77 (18)	C5—C4—C3	118.5 (8)
O1—Ho1—O7ⁱⁱⁱ	136.83 (16)	C5—C4—H4	120.7
O5ⁱⁱ—Ho1—O7ⁱⁱⁱ	79.25 (18)	C3—C4—H4	120.7
O3—Ho1—O6	144.55 (17)	C4—C3—C2	121.7 (8)
O1W—Ho1—O6	75.0 (2)	C4—C3—H3	119.1
O2ⁱ—Ho1—O6	86.35 (18)	C2—C3—H3	119.1
O1—Ho1—O6	71.64 (16)	C3—C2—C6	119.1 (7)
O5ⁱⁱ—Ho1—O6	119.81 (18)	C3—C2—C1	120.1 (7)
O7ⁱⁱⁱ—Ho1—O6	67.30 (17)	C6—C2—C1	120.8 (6)
O3—Ho1—O4	77.17 (17)	O1—C1—O2	122.8 (6)
O1W—Ho1—O4	135.57 (16)	O1—C1—C2	119.9 (6)
O2ⁱ—Ho1—O4	75.25 (15)	O2—C1—C2	117.3 (6)
O1—Ho1—O4	137.32 (16)	O5—C7—O4	126.5 (6)
O5ⁱⁱ—Ho1—O4	68.24 (14)	O5—C7—C7ⁱⁱ	117.4 (7)
O7ⁱⁱⁱ—Ho1—O4	72.00 (17)	O4—C7—C7ⁱⁱ	116.0 (7)
O6—Ho1—O4	135.14 (16)	H3W—O2W—H4W	105 (9)
C7—O4—Ho1	118.2 (4)	C1—O2—Ho1ⁱ	128.4 (4)
C6—O3—Ho1	136.0 (5)	C8—O7—Ho1ⁱⁱⁱ	119.8 (5)
C8—O6—Ho1	120.3 (5)	Ho1—O1W—H1W	119 (6)
C1—O1—Ho1	136.8 (4)	Ho1—O1W—H2W	136 (6)
C7—O5—Ho1ⁱⁱ	119.9 (4)	H1W—O1W—H2W	106 (3)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1W···O2^iv	0.84 (6)	2.02 (6)	2.737 (7)	142 (8)
O1W—H2W···O4^v	0.85 (7)	1.96 (4)	2.749 (7)	154 (8)
O2W—H3W···O6	0.84 (7)	2.36 (11)	2.882 (10)	121 (8)
O2W—H4W···O1	0.86 (7)	2.27 (3)	3.082 (8)	157 (9)
N1—H1···O2W^vi	0.86	1.99	2.781 (10)	154

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

Experimental details

Crystal data
Chemical formula	[Ho(C₂O₄)(C₆H₄NO₃)(H₂O)]·H₂O
M_r	427.08
Crystal system, space group	Triclinic, P1
Temperature (K)	296
a, b, c (Å)	6.5391 (11), 9.5305 (16), 9.7391 (16)
α, β, γ (°)	71.810 (2), 78.862 (2), 80.359 (2)
V (Å³)	562.01 (16)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	7.09
Crystal size (mm)	0.20 × 0.18 × 0.17

Data collection
Diffractometer	Bruker APEXII CCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.332, 0.379
No. of measured, independent and observed [I > 2σ(I)] reflections	2884, 1983, 1862
R_int	0.018
(sin θ/λ)_max (Å⁻¹)	0.599

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.034, 0.090, 1.06
No. of reflections	1983
No. of parameters	184
No. of restraints	6
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	2.75, −2.19

Computer programs: APEX2 (Bruker, 2007), SAINT (Bruker, 2007), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999).

Selected bond lengths (Å) top

Ho1—O1	2.354 (5)	Ho1—O5ⁱⁱ	2.364 (5)
Ho1—O2ⁱ	2.348 (5)	Ho1—O6	2.394 (5)
Ho1—O3	2.279 (5)	Ho1—O7ⁱⁱⁱ	2.392 (5)
Ho1—O4	2.406 (4)	Ho1—O1W	2.348 (5)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1W···O2^iv	0.84 (6)	2.02 (6)	2.737 (7)	142 (8)
O1W—H2W···O4^v	0.85 (7)	1.96 (4)	2.749 (7)	154 (8)
O2W—H3W···O6	0.84 (7)	2.36 (11)	2.882 (10)	121 (8)
O2W—H4W···O1	0.86 (7)	2.27 (3)	3.082 (8)	157 (9)
N1—H1···O2W^vi	0.86	1.99	2.781 (10)	153.5

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

Acknowledgements

The authors acknowledge the Chan Xue Yan Cooperative Special Project of Guangdong Province and the Ministry of Science and Technology of China (project No. 2007A090302046), the Project of Science and Technology of Guangdong Province (project No. 2007A020200002-4) and the Natural Science Foundation of Guangdong Province (No. 9151063101000037) for supporting this work.

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