2,3-Bis(2-chloro­benz­yl)naphthalene-1,4-dione

Huang, S.; Huang, Z.-S.; Gu, L.-Q.; Zhou, Z.; Ng, S.W.

doi:10.1107/S1600536807068006

organic compounds

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ISSN: 2056-9890

Volume 64| Part 3| March 2008| Page o593

doi:10.1107/S1600536807068006

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2,3-Bis(2-chlorobenzyl)naphthalene-1,4-dione

Shiliang Huang,^a ^* Zhi-Shu Huang,^a Lian-Quan Gu,^a Zhongyuan Zhou ^b and Seik Weng Ng ^c

^aSchool of Pharmaceutical Science, Sun Yat-Sen University, Guangzhou, People's Republic of China, ^bDepartment of Applied Biology and Chemical Technology, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, and ^cDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
^*Correspondence e-mail: lsshsl@mail.sysu.edu.cn

(Received 21 November 2007; accepted 21 December 2007; online 15 February 2008)

The title disubstituted naphthalene-1,4-dione, C₂₄H₁₆Cl₂O₂, has the two chlorobenzyl substituents related by a non-crystallographic twofold rotation axis, generating a chiral conformation; both enantiomers are present. The two chlorobenzene rings are nearly perpendicular to the fused ring system, making angles of 88.8 (1) and 77.5 (1)° with it.

Related literature

2,3-Bis(2-chlorobenzyl)naphthalene-1,4-dione was the unexpected product in the attempted synthesis of 2-(2-chlorobenzyl)naphthalene-1,4-dione by a free-radical alkylation (see Boehm et al., 1981 ; Chen et al., 2005 ; Tsai et al., 2001 ). Although the title dichloro compound has not been reported, 2,3-dibenzylnaphthalene-1,4-dione has been known for a long time (Baxter & Sanders, 1974 ; Chen et al., 2005; Oettmeier et al., 1986 ; Sharma & Torssell, 1978 ; Yamago et al., 2002 ). This class of compounds exhibits antitumour activity (Driscoll, 1974 ; Driscoll et al., 1974 ).

Experimental

Crystal data

C₂₄H₁₆Cl₂O₂
M_r = 407.27
Monoclinic, P 2₁ /n
a = 9.998 (1) Å
b = 10.272 (1) Å
c = 18.804 (2) Å
β = 99.071 (1)°
V = 1907.1 (3) Å³
Z = 4
Mo Kα radiation
μ = 0.36 mm⁻¹
T = 295 (2) K
0.5 × 0.5 × 0.5 mm

Data collection

Bruker SMART area-detector diffractometer
Absorption correction: none
12440 measured reflections
4366 independent reflections
2921 reflections with I > 2σ(I)
R_int = 0.035

Refinement

R[F² > 2σ(F²)] = 0.044
wR(F²) = 0.131
S = 1.02
4366 reflections
253 parameters
H-atom parameters constrained
Δρ_max = 0.18 e Å⁻³
Δρ_min = −0.42 e Å⁻³

Data collection: SMART (Bruker, 2001 ); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: X-SEED (Barbour, 2001 ); software used to prepare material for publication: publCIF (Westrip, 2008 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536807068006/ob2105sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536807068006/ob2105Isup2.hkl

checkCIF report

Related literature top

2,3-Bis(2-chlorobenzyl)naphthalene-1,4-dione was the unexpected product in the attempted synthesis of 2-(2-chlorobenzyl)naphthalene-1,4-dione by a free-radical alkylation (see Boehm et al., 1981; Chen et al., 2005; Tsai et al., 2001). Although the title dichloro compound has not been reported, 2,3-dibenzylnaphthalene-1,4-dione has been known for a long time (Baxter & Sanders, 1974; Chen et al., 2005; Oettmeier et al., 1986; Sharma & Torssell, 1978; Yamago et al., 2002). This class of compounds exhibits antitumour activity (Driscoll, 1974; Driscoll et al., 1974).

Experimental top

The title compound (I) was synthesized by using a modification of the procedure by Boehm et al. (1981). 1,4-Naphthoquinone (4 g, 25.3 mmol), 2-(2-chlorophenyl)acetic acid (8.60 g, 50.6 mmol), and silver nitrate (2.13 g, 12.6 mmol) in acetonitrile (60 ml) were heated to 353 K. A solution of ammonium peroxydisulfate in 30 ml water was added dropwise within 30 min. The mixture was then refluxed for another 2 h. The solution was concentrated to give a brown solid. The crude product was purified by column chromatography (silica gel, 1:2 dichloromethane:petroleum ether) to give yellow solid in 40% yield. Single crystals of (I) were obtained by slow evaporation of a 1:2 dichloromethane:petroleum ether (50 ml) solution.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2008).

Figures top

Fig. 1. The molecular structure of (I) with 50% probability ellipsoids for the non-H atoms.

2,3-Bis(2-chlorobenzyl)naphthalene-1,4-dione top

Crystal data top

C₂₄H₁₆Cl₂O₂	F(000) = 840
M_r = 407.27	D_x = 1.418 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 6278 reflections
a = 9.998 (1) Å	θ = 2.2–27.5°
b = 10.272 (1) Å	µ = 0.36 mm⁻¹
c = 18.804 (2) Å	T = 295 K
β = 99.071 (1)°	Block, yellow
V = 1907.1 (3) Å³	0.5 × 0.5 × 0.5 mm
Z = 4

Data collection top

Bruker SMART area-detector diffractometer	2921 reflections with I > 2σ(I)
Radiation source: medium-focus sealed tube	R_int = 0.035
Graphite monochromator	θ_max = 27.5°, θ_min = 2.2°
ϕ and ω scans	h = −12→12
12440 measured reflections	k = −13→13
4366 independent reflections	l = −24→11

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)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.131	H-atom parameters constrained
S = 1.02	w = 1/[σ²(F_o²) + (0.0628P)² + 0.3578P] where P = (F_o² + 2F_c²)/3
4366 reflections	(Δ/σ)_max = 0.001
253 parameters	Δρ_max = 0.18 e Å⁻³
0 restraints	Δρ_min = −0.42 e Å⁻³

Crystal data top

C₂₄H₁₆Cl₂O₂	V = 1907.1 (3) Å³
M_r = 407.27	Z = 4
Monoclinic, P2₁/n	Mo Kα radiation
a = 9.998 (1) Å	µ = 0.36 mm⁻¹
b = 10.272 (1) Å	T = 295 K
c = 18.804 (2) Å	0.5 × 0.5 × 0.5 mm
β = 99.071 (1)°

Data collection top

Bruker SMART area-detector diffractometer	2921 reflections with I > 2σ(I)
12440 measured reflections	R_int = 0.035
4366 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.044	0 restraints
wR(F²) = 0.131	H-atom parameters constrained
S = 1.02	Δρ_max = 0.18 e Å⁻³
4366 reflections	Δρ_min = −0.42 e Å⁻³
253 parameters

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

	x	y	z	U_iso*/U_eq
Cl1	0.53007 (6)	0.55702 (6)	0.66580 (4)	0.0779 (2)
Cl2	−0.07641 (7)	0.55387 (6)	0.39047 (4)	0.0856 (2)
O1	0.48173 (15)	1.00324 (15)	0.60292 (8)	0.0693 (4)
O2	0.03492 (16)	1.00716 (16)	0.40921 (9)	0.0763 (5)
C1	0.39509 (19)	0.64277 (17)	0.69086 (10)	0.0508 (5)
C2	0.3610 (3)	0.6185 (2)	0.75798 (11)	0.0707 (7)
H2	0.4108	0.5589	0.7887	0.085*
C3	0.2548 (3)	0.6819 (2)	0.77899 (13)	0.0821 (8)
H3	0.2317	0.6658	0.8242	0.099*
C4	0.1817 (3)	0.7694 (2)	0.73391 (13)	0.0743 (7)
H4	0.1081	0.8118	0.7481	0.089*
C5	0.2169 (2)	0.79478 (19)	0.66737 (10)	0.0550 (5)
H5	0.1667	0.8550	0.6373	0.066*
C6	0.32481 (18)	0.73306 (16)	0.64433 (9)	0.0438 (4)
C7	0.3679 (2)	0.76083 (18)	0.57207 (10)	0.0524 (5)
H7A	0.4658	0.7688	0.5789	0.063*
H7B	0.3431	0.6871	0.5405	0.063*
C8	0.30674 (17)	0.88202 (17)	0.53547 (9)	0.0426 (4)
C9	0.37930 (17)	1.00544 (18)	0.55815 (9)	0.0438 (4)
C10	0.32835 (17)	1.12940 (16)	0.52474 (9)	0.0422 (4)
C11	0.39225 (19)	1.24608 (19)	0.54606 (11)	0.0555 (5)
H11	0.4650	1.2469	0.5836	0.067*
C12	0.3487 (2)	1.3603 (2)	0.51213 (14)	0.0698 (6)
H12	0.3920	1.4382	0.5266	0.084*
C13	0.2408 (2)	1.3596 (2)	0.45657 (15)	0.0745 (7)
H13	0.2131	1.4368	0.4330	0.089*
C14	0.1739 (2)	1.2458 (2)	0.43582 (12)	0.0618 (5)
H14	0.1002	1.2465	0.3988	0.074*
C15	0.21608 (18)	1.12966 (17)	0.47002 (9)	0.0447 (4)
C16	0.14175 (18)	1.00724 (18)	0.45086 (9)	0.0476 (4)
C17	0.19512 (18)	0.88289 (17)	0.48466 (9)	0.0443 (4)
C18	0.1139 (2)	0.76407 (19)	0.45892 (11)	0.0566 (5)
H18A	0.0185	0.7835	0.4574	0.068*
H18B	0.1372	0.6946	0.4936	0.068*
C19	0.13550 (17)	0.71606 (17)	0.38532 (10)	0.0466 (4)
C20	0.23605 (19)	0.76496 (19)	0.35008 (10)	0.0542 (5)
H20	0.2938	0.8290	0.3724	0.065*
C21	0.2529 (2)	0.7209 (2)	0.28236 (12)	0.0675 (6)
H21	0.3211	0.7555	0.2597	0.081*
C22	0.1689 (3)	0.6263 (3)	0.24877 (13)	0.0762 (7)
H22	0.1804	0.5965	0.2034	0.091*
C23	0.0682 (2)	0.5759 (2)	0.28197 (13)	0.0694 (6)
H23	0.0108	0.5121	0.2592	0.083*
C24	0.05248 (19)	0.62039 (18)	0.34967 (11)	0.0553 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0746 (4)	0.0631 (4)	0.0901 (5)	0.0227 (3)	−0.0056 (3)	0.0124 (3)
Cl2	0.0785 (4)	0.0736 (4)	0.1006 (5)	−0.0274 (3)	0.0012 (3)	0.0022 (3)
O1	0.0628 (9)	0.0722 (10)	0.0629 (9)	0.0011 (7)	−0.0211 (7)	0.0104 (7)
O2	0.0696 (9)	0.0745 (10)	0.0715 (10)	0.0136 (8)	−0.0305 (8)	−0.0104 (8)
C1	0.0583 (11)	0.0404 (9)	0.0490 (10)	−0.0050 (8)	−0.0064 (9)	0.0029 (8)
C2	0.1079 (19)	0.0510 (12)	0.0466 (12)	−0.0113 (12)	−0.0080 (12)	0.0110 (10)
C3	0.134 (2)	0.0653 (15)	0.0519 (13)	−0.0148 (16)	0.0309 (14)	0.0074 (11)
C4	0.0997 (18)	0.0636 (14)	0.0682 (14)	−0.0003 (13)	0.0402 (13)	0.0038 (11)
C5	0.0640 (12)	0.0510 (11)	0.0520 (11)	0.0054 (9)	0.0157 (9)	0.0062 (9)
C6	0.0519 (10)	0.0376 (8)	0.0399 (9)	−0.0032 (7)	0.0012 (7)	0.0009 (7)
C7	0.0638 (11)	0.0465 (10)	0.0479 (10)	0.0142 (9)	0.0118 (9)	0.0056 (8)
C8	0.0507 (10)	0.0447 (10)	0.0333 (8)	0.0093 (8)	0.0100 (7)	0.0031 (7)
C9	0.0442 (9)	0.0532 (10)	0.0332 (8)	0.0052 (8)	0.0037 (7)	0.0011 (7)
C10	0.0453 (9)	0.0439 (9)	0.0385 (9)	0.0045 (7)	0.0102 (7)	−0.0013 (7)
C11	0.0516 (10)	0.0549 (11)	0.0603 (12)	−0.0011 (9)	0.0097 (9)	−0.0046 (9)
C12	0.0709 (14)	0.0443 (11)	0.0967 (18)	−0.0021 (10)	0.0214 (13)	−0.0018 (11)
C13	0.0813 (15)	0.0472 (12)	0.0973 (18)	0.0131 (11)	0.0211 (14)	0.0212 (12)
C14	0.0621 (12)	0.0606 (13)	0.0610 (12)	0.0173 (10)	0.0050 (10)	0.0161 (10)
C15	0.0478 (9)	0.0467 (10)	0.0397 (9)	0.0097 (8)	0.0073 (7)	0.0034 (7)
C16	0.0505 (10)	0.0539 (11)	0.0359 (9)	0.0089 (8)	−0.0009 (8)	−0.0028 (8)
C17	0.0509 (10)	0.0465 (10)	0.0365 (9)	0.0017 (8)	0.0097 (7)	−0.0027 (7)
C18	0.0641 (12)	0.0530 (11)	0.0531 (11)	−0.0076 (9)	0.0101 (9)	−0.0038 (9)
C19	0.0468 (9)	0.0418 (9)	0.0476 (10)	0.0059 (8)	−0.0036 (8)	−0.0018 (8)
C20	0.0497 (10)	0.0586 (11)	0.0514 (11)	0.0030 (9)	−0.0007 (9)	−0.0095 (9)
C21	0.0657 (13)	0.0817 (16)	0.0549 (12)	0.0107 (12)	0.0092 (10)	−0.0092 (11)
C22	0.0836 (16)	0.0850 (17)	0.0555 (13)	0.0203 (14)	−0.0027 (12)	−0.0212 (12)
C23	0.0766 (15)	0.0565 (12)	0.0658 (14)	0.0061 (11)	−0.0176 (12)	−0.0163 (11)
C24	0.0538 (11)	0.0436 (10)	0.0626 (12)	0.0045 (8)	−0.0089 (9)	−0.0003 (9)

Geometric parameters (Å, º) top

Cl1—C1	1.738 (2)	C11—H11	0.9300
Cl2—C24	1.740 (2)	C12—C13	1.379 (3)
O1—C9	1.219 (2)	C12—H12	0.9300
O2—C16	1.221 (2)	C13—C14	1.372 (3)
C1—C2	1.381 (3)	C13—H13	0.9300
C1—C6	1.388 (2)	C14—C15	1.389 (3)
C2—C3	1.357 (3)	C14—H14	0.9300
C2—H2	0.9300	C15—C16	1.476 (3)
C3—C4	1.365 (3)	C16—C17	1.488 (2)
C3—H3	0.9300	C17—C18	1.503 (3)
C4—C5	1.377 (3)	C18—C19	1.516 (3)
C4—H4	0.9300	C18—H18A	0.9700
C5—C6	1.379 (3)	C18—H18B	0.9700
C5—H5	0.9300	C19—C20	1.383 (3)
C6—C7	1.516 (2)	C19—C24	1.389 (2)
C7—C8	1.505 (2)	C20—C21	1.386 (3)
C7—H7A	0.9700	C20—H20	0.9300
C7—H7B	0.9700	C21—C22	1.371 (3)
C8—C17	1.350 (2)	C21—H21	0.9300
C8—C9	1.489 (3)	C22—C23	1.367 (3)
C9—C10	1.475 (2)	C22—H22	0.9300
C10—C11	1.387 (3)	C23—C24	1.385 (3)
C10—C15	1.398 (2)	C23—H23	0.9300
C11—C12	1.373 (3)

C2—C1—C6	121.6 (2)	C14—C13—C12	120.6 (2)
C2—C1—Cl1	118.05 (16)	C14—C13—H13	119.7
C6—C1—Cl1	120.36 (15)	C12—C13—H13	119.7
C3—C2—C1	119.8 (2)	C13—C14—C15	120.1 (2)
C3—C2—H2	120.1	C13—C14—H14	120.0
C1—C2—H2	120.1	C15—C14—H14	120.0
C2—C3—C4	120.1 (2)	C14—C15—C10	119.49 (18)
C2—C3—H3	119.9	C14—C15—C16	120.95 (17)
C4—C3—H3	119.9	C10—C15—C16	119.53 (15)
C3—C4—C5	120.0 (2)	O2—C16—C15	120.86 (17)
C3—C4—H4	120.0	O2—C16—C17	119.48 (18)
C5—C4—H4	120.0	C15—C16—C17	119.62 (15)
C4—C5—C6	121.55 (19)	C8—C17—C16	120.59 (16)
C4—C5—H5	119.2	C8—C17—C18	124.53 (16)
C6—C5—H5	119.2	C16—C17—C18	114.86 (16)
C5—C6—C1	116.92 (17)	C17—C18—C19	114.23 (16)
C5—C6—C7	122.83 (16)	C17—C18—H18A	108.7
C1—C6—C7	120.25 (16)	C19—C18—H18A	108.7
C8—C7—C6	114.41 (15)	C17—C18—H18B	108.7
C8—C7—H7A	108.7	C19—C18—H18B	108.7
C6—C7—H7A	108.7	H18A—C18—H18B	107.6
C8—C7—H7B	108.7	C20—C19—C24	116.75 (18)
C6—C7—H7B	108.7	C20—C19—C18	122.56 (16)
H7A—C7—H7B	107.6	C24—C19—C18	120.68 (18)
C17—C8—C9	120.54 (15)	C19—C20—C21	121.61 (19)
C17—C8—C7	124.07 (17)	C19—C20—H20	119.2
C9—C8—C7	115.39 (15)	C21—C20—H20	119.2
O1—C9—C10	120.42 (17)	C22—C21—C20	120.0 (2)
O1—C9—C8	120.02 (17)	C22—C21—H21	120.0
C10—C9—C8	119.54 (15)	C20—C21—H21	120.0
C11—C10—C15	119.39 (16)	C23—C22—C21	120.0 (2)
C11—C10—C9	120.69 (16)	C23—C22—H22	120.0
C15—C10—C9	119.90 (15)	C21—C22—H22	120.0
C12—C11—C10	120.40 (19)	C22—C23—C24	119.5 (2)
C12—C11—H11	119.8	C22—C23—H23	120.2
C10—C11—H11	119.8	C24—C23—H23	120.2
C11—C12—C13	120.0 (2)	C19—C24—C23	122.1 (2)
C11—C12—H12	120.0	C19—C24—Cl2	119.53 (16)
C13—C12—H12	120.0	C23—C24—Cl2	118.37 (16)

C6—C1—C2—C3	1.4 (3)	C9—C10—C15—C14	−176.07 (17)
Cl1—C1—C2—C3	−178.88 (18)	C11—C10—C15—C16	−175.53 (16)
C1—C2—C3—C4	0.0 (4)	C9—C10—C15—C16	5.7 (2)
C2—C3—C4—C5	−0.9 (4)	C14—C15—C16—O2	−6.5 (3)
C3—C4—C5—C6	0.4 (4)	C10—C15—C16—O2	171.68 (18)
C4—C5—C6—C1	0.9 (3)	C14—C15—C16—C17	175.61 (17)
C4—C5—C6—C7	−178.7 (2)	C10—C15—C16—C17	−6.2 (3)
C2—C1—C6—C5	−1.8 (3)	C9—C8—C17—C16	−0.6 (2)
Cl1—C1—C6—C5	178.44 (14)	C7—C8—C17—C16	−179.65 (16)
C2—C1—C6—C7	177.85 (18)	C9—C8—C17—C18	−179.10 (16)
Cl1—C1—C6—C7	−1.9 (2)	C7—C8—C17—C18	1.8 (3)
C5—C6—C7—C8	14.0 (3)	O2—C16—C17—C8	−174.32 (18)
C1—C6—C7—C8	−165.61 (16)	C15—C16—C17—C8	3.6 (3)
C6—C7—C8—C17	−96.4 (2)	O2—C16—C17—C18	4.4 (3)
C6—C7—C8—C9	84.5 (2)	C15—C16—C17—C18	−177.76 (16)
C17—C8—C9—O1	−178.37 (17)	C8—C17—C18—C19	−104.7 (2)
C7—C8—C9—O1	0.8 (2)	C16—C17—C18—C19	76.7 (2)
C17—C8—C9—C10	0.1 (2)	C17—C18—C19—C20	9.4 (3)
C7—C8—C9—C10	179.22 (15)	C17—C18—C19—C24	−169.87 (16)
O1—C9—C10—C11	−3.0 (3)	C24—C19—C20—C21	0.2 (3)
C8—C9—C10—C11	178.54 (16)	C18—C19—C20—C21	−179.10 (18)
O1—C9—C10—C15	175.71 (17)	C19—C20—C21—C22	−0.2 (3)
C8—C9—C10—C15	−2.7 (2)	C20—C21—C22—C23	0.3 (3)
C15—C10—C11—C12	−2.2 (3)	C21—C22—C23—C24	−0.4 (3)
C9—C10—C11—C12	176.55 (18)	C20—C19—C24—C23	−0.3 (3)
C10—C11—C12—C13	0.1 (3)	C18—C19—C24—C23	179.01 (18)
C11—C12—C13—C14	1.6 (4)	C20—C19—C24—Cl2	179.97 (14)
C12—C13—C14—C15	−1.1 (3)	C18—C19—C24—Cl2	−0.7 (2)
C13—C14—C15—C10	−1.1 (3)	C22—C23—C24—C19	0.4 (3)
C13—C14—C15—C16	177.1 (2)	C22—C23—C24—Cl2	−179.87 (17)
C11—C10—C15—C14	2.7 (3)

Experimental details

Crystal data
Chemical formula	C₂₄H₁₆Cl₂O₂
M_r	407.27
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	295
a, b, c (Å)	9.998 (1), 10.272 (1), 18.804 (2)
β (°)	99.071 (1)
V (Å³)	1907.1 (3)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.36
Crystal size (mm)	0.5 × 0.5 × 0.5

Data collection
Diffractometer	Bruker SMART area-detector diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	12440, 4366, 2921
R_int	0.035
(sin θ/λ)_max (Å⁻¹)	0.650

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.044, 0.131, 1.02
No. of reflections	4366
No. of parameters	253
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.18, −0.42

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), X-SEED (Barbour, 2001), publCIF (Westrip, 2008).

Acknowledgements

The authors are indebted to the National Natural Science Foundation of China (grant No. 30701050), the Guangdong Provincial Science Foundation (grant No. 7301303) and the University of Malaya for supporting this study.

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