1,3-Bis(4-chloro­phen­yl)-4,5-diethoxy­imidazolidine

Wan, Y.; Chen, X.; Zhang, P.; Wu, H.

doi:10.1107/S1600536808031176

organic compounds

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ISSN: 2056-9890

Volume 64| Part 11| November 2008| Page o2158

doi:10.1107/S1600536808031176

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1,3-Bis(4-chlorophenyl)-4,5-diethoxyimidazolidine

Yu Wan,^a Xiu-mei Chen,^a Pu Zhang ^a and Hui Wu ^a ^*

^aSchool of Chemistry and Chemical Engineering, Xuzhou Normal University, Xuzhou, Jiangsu 221116, People's Republic of China
^*Correspondence e-mail: wuhui72@yahoo.com.cn

(Received 22 September 2008; accepted 26 September 2008; online 22 October 2008)

In the molecule of the title compound, C₁₉H₂₂Cl₂N₂O₂, the two benzene rings are oriented at a dihedral angle of 3.70 (3)°. The five-membered ring adopts an envelope conformation.

Related literature

For general background, see: Bunnage & Owen (2008 ); Weinreb (2007 ); Jin (2006 ); Farnia et al. (1997 ); Reed & Schleyer (1988 ). For bond-length data, see: Allen et al. (1987 ).

Experimental

Crystal data

C₁₉H₂₂Cl₂N₂O₂
M_r = 381.29
Monoclinic, P 2₁ /c
a = 10.928 (2) Å
b = 11.123 (2) Å
c = 16.006 (3) Å
β = 94.480 (15)°
V = 1939.6 (6) Å³
Z = 4
Mo Kα radiation
μ = 0.35 mm⁻¹
T = 296 (2) K
0.50 × 0.42 × 0.22 mm

Data collection

Siemens P4 diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.843, T_max = 0.926
4160 measured reflections
3609 independent reflections
1867 reflections with I > 2σ(I)
R_int = 0.010
3 standard reflections every 97 reflections intensity decay: 1.1%

Refinement

R[F² > 2σ(F²)] = 0.039
wR(F²) = 0.072
S = 0.94
3609 reflections
227 parameters
H-atom parameters constrained
Δρ_max = 0.12 e Å⁻³
Δρ_min = −0.13 e Å⁻³

Data collection: XSCANS (Siemens, 1996 ); cell refinement: XSCANS; data reduction: XSCANS; 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/S1600536808031176/hk2538sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808031176/hk2538Isup2.hkl

checkCIF report

Comment top

Imidazoles are commonly utilized substructures within the pharmaceutical industry, as these heterocycles impart unique physical and biological properties to compounds of interest (Bunnage & Owen, 2008; Weinreb, 2007; Jin, 2006). Furthermore, molecules containing anomeric effect in N—C—N system influences many structural and electronic properties (Farnia et al., 1997; Reed & Schleyer, 1988). We report herein the synthesis and crystal structure of the title compound.

In the molecule of the title compound (Fig. 1), the bond lengths (Allen et al., 1987) and angles are within normal ranges. Rings A (C1-C6) and C (C10-C15) are, of course, planar and the dihedral angle between them is 3.70 (3)°. So, they are also nearly coplanar. Ring B (N1/N2/C7-C9) is not planar, and adopts envelope conformation with C8 atom displaced by 0.336 (3) Å from the plane of the other ring atoms.

Related literature top

For general background, see: Bunnage & Owen (2008); Weinreb (2007); Jin (2006); Farnia et al. (1997); Reed & Schleyer (1988). For bond-length data, see: Allen et al. (1987).

Experimental top

The title compound was simply prepared by the reaction of 4-chloro- benzaldehyde (2.0 mmol) with glyoxal(1.0 mmol) and formaldehyde (1.0 mmol) in ethanol (3.0 ml) at 273–278 K without catalyst for 40 h (yield; 81%). Single crystals suitable for X-ray analysis were obtained from an ethanol solution by slow evaporation.

Computing details top

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS (Siemens, 1996); data reduction: XSCANS (Siemens, 1996); 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 the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.

1,3-Bis(4-chlorophenyl)-4,5-diethoxyimidazolidine top

Crystal data top

C₁₉H₂₂Cl₂N₂O₂	F(000) = 800
M_r = 381.29	D_x = 1.306 Mg m⁻³
Monoclinic, P2₁/c	Melting point = 445–446 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 10.928 (2) Å	Cell parameters from 28 reflections
b = 11.123 (2) Å	θ = 4.9–14.4°
c = 16.006 (3) Å	µ = 0.35 mm⁻¹
β = 94.480 (15)°	T = 296 K
V = 1939.6 (6) Å³	Block, colorless
Z = 4	0.50 × 0.42 × 0.22 mm

Data collection top

Siemens P4 diffractometer	1867 reflections with I > 2σ(I)
Radiation source: normal-focus sealed tube	R_int = 0.010
Graphite monochromator	θ_max = 25.5°, θ_min = 1.9°
ω scans	h = 0→13
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	k = 0→13
T_min = 0.843, T_max = 0.926	l = −19→19
4160 measured reflections	3 standard reflections every 97 reflections
3609 independent reflections	intensity decay: 1.1%

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)] = 0.039	H-atom parameters constrained
wR(F²) = 0.072	w = 1/[σ²(F_o²) + (0.0226P)²] where P = (F_o² + 2F_c²)/3
S = 0.94	(Δ/σ)_max = 0.001
3609 reflections	Δρ_max = 0.12 e Å⁻³
227 parameters	Δρ_min = −0.13 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: 0.0056 (4)

Crystal data top

C₁₉H₂₂Cl₂N₂O₂	V = 1939.6 (6) Å³
M_r = 381.29	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 10.928 (2) Å	µ = 0.35 mm⁻¹
b = 11.123 (2) Å	T = 296 K
c = 16.006 (3) Å	0.50 × 0.42 × 0.22 mm
β = 94.480 (15)°

Data collection top

Siemens P4 diffractometer	1867 reflections with I > 2σ(I)
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	R_int = 0.010
T_min = 0.843, T_max = 0.926	3 standard reflections every 97 reflections
4160 measured reflections	intensity decay: 1.1%
3609 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.039	0 restraints
wR(F²) = 0.072	H-atom parameters constrained
S = 0.94	Δρ_max = 0.12 e Å⁻³
3609 reflections	Δρ_min = −0.13 e Å⁻³
227 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
Cl1	0.87517 (6)	−0.07126 (6)	0.61089 (5)	0.0967 (3)
Cl2	0.24717 (6)	0.95415 (6)	0.65343 (4)	0.0981 (3)
O1	0.92261 (12)	0.55731 (13)	0.57515 (9)	0.0596 (4)
O2	0.66798 (11)	0.63533 (12)	0.42946 (8)	0.0586 (4)
N1	0.73769 (14)	0.43816 (15)	0.56395 (10)	0.0544 (5)
N2	0.63188 (15)	0.61147 (15)	0.57001 (10)	0.0547 (5)
C1	0.69410 (19)	0.23950 (19)	0.61290 (13)	0.0576 (6)
H1	0.6210	0.2672	0.6320	0.069*
C2	0.7256 (2)	0.1204 (2)	0.62321 (13)	0.0628 (6)
H2	0.6740	0.0681	0.6493	0.075*
C3	0.8326 (2)	0.07892 (19)	0.59514 (14)	0.0601 (6)
C4	0.9094 (2)	0.1553 (2)	0.55715 (14)	0.0669 (7)
H4	0.9820	0.1264	0.5380	0.080*
C5	0.8794 (2)	0.2745 (2)	0.54722 (13)	0.0605 (6)
H5	0.9325	0.3263	0.5222	0.073*
C6	0.76964 (18)	0.31880 (19)	0.57444 (12)	0.0481 (5)
C7	0.81176 (18)	0.52749 (18)	0.52714 (12)	0.0520 (6)
H7	0.8327	0.4979	0.4724	0.062*
C8	0.72362 (17)	0.63322 (18)	0.51288 (12)	0.0510 (5)
H8	0.7660	0.7093	0.5262	0.061*
C9	0.62711 (17)	0.48661 (18)	0.59411 (12)	0.0517 (6)
H9A	0.6272	0.4782	0.6544	0.062*
H9B	0.5548	0.4474	0.5677	0.062*
C10	0.54049 (18)	0.6921 (2)	0.58676 (12)	0.0482 (5)
C11	0.44177 (18)	0.6556 (2)	0.63022 (13)	0.0587 (6)
H11	0.4354	0.5755	0.6458	0.070*
C12	0.3535 (2)	0.7359 (2)	0.65051 (14)	0.0666 (7)
H12	0.2884	0.7096	0.6798	0.080*
C13	0.3603 (2)	0.8540 (2)	0.62809 (13)	0.0613 (6)
C14	0.4559 (2)	0.8919 (2)	0.58461 (13)	0.0651 (7)
H14	0.4610	0.9723	0.5692	0.078*
C15	0.54485 (19)	0.8117 (2)	0.56344 (13)	0.0615 (6)
H15	0.6087	0.8385	0.5331	0.074*
C16	0.9094 (2)	0.5868 (2)	0.66041 (14)	0.0808 (8)
H16A	0.8870	0.5157	0.6908	0.097*
H16B	0.8450	0.6462	0.6639	0.097*
C17	1.0262 (2)	0.6350 (3)	0.69752 (16)	0.1104 (10)
H17A	1.0183	0.6549	0.7552	0.166*
H17B	1.0473	0.7058	0.6676	0.166*
H17C	1.0894	0.5757	0.6941	0.166*
C18	0.7407 (2)	0.6940 (2)	0.37180 (15)	0.0825 (8)
H18A	0.8159	0.6493	0.3665	0.099*
H18B	0.7624	0.7741	0.3919	0.099*
C19	0.6704 (2)	0.7017 (3)	0.29006 (15)	0.1278 (12)
H19A	0.7187	0.7417	0.2509	0.192*
H19B	0.5962	0.7461	0.2957	0.192*
H19C	0.6504	0.6221	0.2701	0.192*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0971 (5)	0.0588 (4)	0.1362 (6)	0.0063 (4)	0.0216 (5)	−0.0038 (4)
Cl2	0.0926 (5)	0.0975 (6)	0.1076 (6)	0.0270 (4)	0.0303 (4)	−0.0125 (4)
O1	0.0454 (9)	0.0792 (11)	0.0551 (10)	−0.0086 (8)	0.0086 (7)	−0.0004 (9)
O2	0.0550 (9)	0.0737 (10)	0.0472 (9)	−0.0135 (8)	0.0046 (7)	0.0102 (8)
N1	0.0432 (10)	0.0544 (12)	0.0678 (12)	−0.0047 (10)	0.0177 (9)	0.0051 (10)
N2	0.0517 (11)	0.0586 (12)	0.0557 (11)	0.0029 (10)	0.0168 (9)	0.0101 (10)
C1	0.0485 (13)	0.0586 (15)	0.0666 (16)	−0.0042 (12)	0.0103 (12)	−0.0005 (13)
C2	0.0601 (15)	0.0582 (16)	0.0709 (16)	−0.0117 (13)	0.0111 (13)	0.0019 (13)
C3	0.0608 (15)	0.0520 (15)	0.0669 (16)	−0.0025 (13)	0.0020 (13)	−0.0105 (13)
C4	0.0563 (15)	0.0714 (18)	0.0742 (17)	0.0052 (14)	0.0118 (13)	−0.0077 (15)
C5	0.0559 (15)	0.0652 (17)	0.0620 (16)	−0.0039 (13)	0.0150 (12)	0.0014 (13)
C6	0.0417 (12)	0.0543 (15)	0.0484 (13)	−0.0077 (12)	0.0037 (11)	−0.0028 (11)
C7	0.0491 (13)	0.0641 (15)	0.0434 (13)	−0.0069 (12)	0.0065 (11)	−0.0005 (12)
C8	0.0480 (13)	0.0579 (14)	0.0474 (14)	−0.0078 (12)	0.0064 (11)	0.0014 (12)
C9	0.0435 (13)	0.0579 (15)	0.0541 (14)	−0.0041 (11)	0.0057 (11)	0.0031 (12)
C10	0.0454 (13)	0.0590 (15)	0.0399 (13)	−0.0031 (12)	0.0024 (11)	0.0001 (11)
C11	0.0515 (14)	0.0584 (15)	0.0675 (15)	−0.0062 (13)	0.0126 (12)	0.0010 (13)
C12	0.0521 (15)	0.0777 (18)	0.0721 (17)	−0.0063 (14)	0.0176 (13)	−0.0044 (15)
C13	0.0596 (15)	0.0689 (17)	0.0558 (15)	0.0058 (14)	0.0077 (12)	−0.0100 (13)
C14	0.0762 (17)	0.0588 (16)	0.0605 (15)	0.0058 (14)	0.0066 (14)	0.0058 (13)
C15	0.0601 (15)	0.0659 (17)	0.0605 (15)	−0.0007 (13)	0.0167 (12)	0.0082 (13)
C16	0.0719 (17)	0.117 (2)	0.0537 (16)	−0.0122 (16)	0.0074 (14)	−0.0114 (15)
C17	0.086 (2)	0.150 (3)	0.090 (2)	−0.005 (2)	−0.0247 (17)	−0.016 (2)
C18	0.0868 (18)	0.0938 (19)	0.0683 (18)	−0.0140 (16)	0.0153 (16)	0.0235 (15)
C19	0.131 (3)	0.195 (3)	0.0564 (18)	−0.041 (2)	−0.0022 (18)	0.041 (2)

Geometric parameters (Å, º) top

Cl1—C3	1.747 (2)	C9—H9A	0.9700
Cl2—C13	1.736 (2)	C9—H9B	0.9700
O1—C16	1.422 (2)	C10—C15	1.384 (3)
O1—C7	1.422 (2)	C10—C11	1.389 (2)
O2—C18	1.423 (2)	C11—C12	1.372 (3)
O2—C8	1.424 (2)	C11—H11	0.9300
N1—C6	1.380 (2)	C12—C13	1.365 (3)
N1—C7	1.437 (2)	C12—H12	0.9300
N1—C9	1.440 (2)	C13—C14	1.366 (3)
N2—C10	1.384 (2)	C14—C15	1.381 (3)
N2—C8	1.429 (2)	C14—H14	0.9300
N2—C9	1.443 (2)	C15—H15	0.9300
C1—C2	1.376 (3)	C16—C17	1.466 (3)
C1—C6	1.385 (2)	C16—H16A	0.9700
C1—H1	0.9300	C16—H16B	0.9700
C2—C3	1.366 (3)	C17—H17A	0.9600
C2—H2	0.9300	C17—H17B	0.9600
C3—C4	1.369 (3)	C17—H17C	0.9600
C4—C5	1.372 (3)	C18—C19	1.467 (3)
C4—H4	0.9300	C18—H18A	0.9700
C5—C6	1.397 (3)	C18—H18B	0.9700
C5—H5	0.9300	C19—H19A	0.9600
C7—C8	1.526 (3)	C19—H19B	0.9600
C7—H7	0.9800	C19—H19C	0.9600
C8—H8	0.9800

C16—O1—C7	115.22 (15)	H9A—C9—H9B	109.1
C18—O2—C8	113.37 (16)	N2—C10—C15	122.01 (19)
C6—N1—C7	124.76 (17)	N2—C10—C11	120.6 (2)
C6—N1—C9	122.01 (16)	C15—C10—C11	117.4 (2)
C7—N1—C9	113.14 (16)	C12—C11—C10	121.1 (2)
C10—N2—C8	124.56 (17)	C12—C11—H11	119.4
C10—N2—C9	121.99 (17)	C10—C11—H11	119.4
C8—N2—C9	112.07 (16)	C13—C12—C11	120.7 (2)
C2—C1—C6	121.0 (2)	C13—C12—H12	119.7
C2—C1—H1	119.5	C11—C12—H12	119.7
C6—C1—H1	119.5	C12—C13—C14	119.4 (2)
C3—C2—C1	119.9 (2)	C12—C13—Cl2	120.12 (19)
C3—C2—H2	120.1	C14—C13—Cl2	120.5 (2)
C1—C2—H2	120.1	C13—C14—C15	120.4 (2)
C2—C3—C4	120.4 (2)	C13—C14—H14	119.8
C2—C3—Cl1	120.04 (18)	C15—C14—H14	119.8
C4—C3—Cl1	119.48 (18)	C14—C15—C10	121.0 (2)
C3—C4—C5	120.1 (2)	C14—C15—H15	119.5
C3—C4—H4	119.9	C10—C15—H15	119.5
C5—C4—H4	119.9	O1—C16—C17	108.81 (19)
C4—C5—C6	120.5 (2)	O1—C16—H16A	109.9
C4—C5—H5	119.8	C17—C16—H16A	109.9
C6—C5—H5	119.8	O1—C16—H16B	109.9
N1—C6—C1	120.87 (19)	C17—C16—H16B	109.9
N1—C6—C5	121.08 (19)	H16A—C16—H16B	108.3
C1—C6—C5	118.0 (2)	C16—C17—H17A	109.5
O1—C7—N1	115.03 (16)	C16—C17—H17B	109.5
O1—C7—C8	113.71 (17)	H17A—C17—H17B	109.5
N1—C7—C8	103.01 (15)	C16—C17—H17C	109.5
O1—C7—H7	108.3	H17A—C17—H17C	109.5
N1—C7—H7	108.3	H17B—C17—H17C	109.5
C8—C7—H7	108.3	O2—C18—C19	109.0 (2)
O2—C8—N2	109.47 (16)	O2—C18—H18A	109.9
O2—C8—C7	111.79 (16)	C19—C18—H18A	109.9
N2—C8—C7	103.88 (16)	O2—C18—H18B	109.9
O2—C8—H8	110.5	C19—C18—H18B	109.9
N2—C8—H8	110.5	H18A—C18—H18B	108.3
C7—C8—H8	110.5	C18—C19—H19A	109.5
N1—C9—N2	102.80 (16)	C18—C19—H19B	109.5
N1—C9—H9A	111.2	H19A—C19—H19B	109.5
N2—C9—H9A	111.2	C18—C19—H19C	109.5
N1—C9—H9B	111.2	H19A—C19—H19C	109.5
N2—C9—H9B	111.2	H19B—C19—H19C	109.5

C6—C1—C2—C3	−0.1 (3)	C9—N2—C8—C7	−21.7 (2)
C1—C2—C3—C4	0.4 (3)	O1—C7—C8—O2	138.53 (16)
C1—C2—C3—Cl1	178.08 (16)	N1—C7—C8—O2	−96.31 (17)
C2—C3—C4—C5	0.2 (3)	O1—C7—C8—N2	−103.52 (18)
Cl1—C3—C4—C5	−177.52 (17)	N1—C7—C8—N2	21.63 (19)
C3—C4—C5—C6	−1.0 (3)	C6—N1—C9—N2	179.82 (17)
C7—N1—C6—C1	178.06 (18)	C7—N1—C9—N2	3.1 (2)
C9—N1—C6—C1	1.7 (3)	C10—N2—C9—N1	179.41 (16)
C7—N1—C6—C5	−1.4 (3)	C8—N2—C9—N1	12.3 (2)
C9—N1—C6—C5	−177.81 (18)	C8—N2—C10—C15	−14.3 (3)
C2—C1—C6—N1	179.80 (19)	C9—N2—C10—C15	−179.81 (19)
C2—C1—C6—C5	−0.7 (3)	C8—N2—C10—C11	167.52 (18)
C4—C5—C6—N1	−179.23 (19)	C9—N2—C10—C11	2.0 (3)
C4—C5—C6—C1	1.3 (3)	N2—C10—C11—C12	177.02 (19)
C16—O1—C7—N1	−50.9 (2)	C15—C10—C11—C12	−1.2 (3)
C16—O1—C7—C8	67.6 (2)	C10—C11—C12—C13	0.3 (3)
C6—N1—C7—O1	−68.0 (2)	C11—C12—C13—C14	0.4 (3)
C9—N1—C7—O1	108.68 (19)	C11—C12—C13—Cl2	179.42 (17)
C6—N1—C7—C8	167.75 (18)	C12—C13—C14—C15	0.0 (3)
C9—N1—C7—C8	−15.6 (2)	Cl2—C13—C14—C15	−179.05 (16)
C18—O2—C8—N2	160.47 (17)	C13—C14—C15—C10	−1.0 (3)
C18—O2—C8—C7	−85.0 (2)	N2—C10—C15—C14	−176.62 (19)
C10—N2—C8—O2	−68.9 (2)	C11—C10—C15—C14	1.6 (3)
C9—N2—C8—O2	97.87 (19)	C7—O1—C16—C17	−170.4 (2)
C10—N2—C8—C7	171.58 (17)	C8—O2—C18—C19	−174.5 (2)

Experimental details

Crystal data
Chemical formula	C₁₉H₂₂Cl₂N₂O₂
M_r	381.29
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	296
a, b, c (Å)	10.928 (2), 11.123 (2), 16.006 (3)
β (°)	94.480 (15)
V (Å³)	1939.6 (6)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.35
Crystal size (mm)	0.50 × 0.42 × 0.22

Data collection
Diffractometer	Siemens P4 diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.843, 0.926
No. of measured, independent and observed [I > 2σ(I)] reflections	4160, 3609, 1867
R_int	0.010
(sin θ/λ)_max (Å⁻¹)	0.606

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.039, 0.072, 0.94
No. of reflections	3609
No. of parameters	227
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.12, −0.13

Computer programs: XSCANS (Siemens, 1996), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

We are grateful to the National Science Foundation of China (grant No. 20772103), the Natural Science Foundation in Jiangsu Province (grant No. BK 2007028) and the Surpassing Project in Jiangsu Province (grant No. QL 200607) for financial support.

References

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