Redetermination of 4-nitro­stilbene

Moreno-Fuquen, R.; Aguirre, L.; Kennedy, A.R.

doi:10.1107/S1600536808035459

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 64| Part 12| December 2008| Page o2259

doi:10.1107/S1600536808035459

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Redetermination of 4-nitrostilbene

Rodolfo Moreno-Fuquen,^a ^* Lina Aguirre ^a and Alan R. Kennedy ^b

^aDepartamento de Química, Facultad de Ciencias, Universidad del Valle, Apartado 25360, Santiago de Cali, Colombia, and ^bDepartment of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland
^*Correspondence e-mail: rodimo26@yahoo.es

(Received 20 October 2008; accepted 29 October 2008; online 8 November 2008)

In the title compound, C₁₄H₁₁NO₂, the benzene rings are inclined to each other with a dihedral angle between their mean planes of 8.42 (6)°. The nitro group is almost coplanar with the attached benzene ring but is rotated about the C—N bond by 5.84 (12)°. This redetermination results in a crystal structure with significantly higher precision than the original determination [Hertel & Romer (1931 ). Z. Kristallogr. 76, 467–469], and the intermolecular interactions have been established. In the crystal structure, molecules are linked by C—H⋯O hydrogen bonds to generate C(5), C(13) and edge-fused R₃³(28) rings.

Related literature

For a previous study of the title compound, see: Hertel & Romer (1931). For background information on photonic materials, see: Luo et al. (2003 ); Vidal et al. (2008 ); Park et al. (2004 ). For general background, see: Allen et al. (1987 ); Etter (1990 ); Nardelli (1995 ).

Experimental

Crystal data

C₁₄H₁₁NO₂
M_r = 225.24
Orthorhombic, P b c a
a = 10.0839 (3) Å
b = 7.6849 (2) Å
c = 28.1176 (8) Å
V = 2178.94 (11) Å³
Z = 8
Mo Kα radiation
μ = 0.09 mm⁻¹
T = 123 (2) K
0.40 × 0.40 × 0.18 mm

Data collection

Oxford Xcalibur-S diffractometer
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2008 $[Oxford Diffraction (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction, Wrocław, Poland.]$ ) T_min = 0.965, T_max = 0.985
13263 measured reflections
3173 independent reflections
2202 reflections with I > 2σ(I)
R_int = 0.027

Refinement

R[F² > 2σ(F²)] = 0.048
wR(F²) = 0.135
S = 1.09
3173 reflections
154 parameters
H-atom parameters constrained
Δρ_max = 0.32 e Å⁻³
Δρ_min = −0.18 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2⋯O1ⁱ	0.95	2.55	3.3762 (17)	146
C12—H12⋯O1ⁱⁱ	0.95	2.66	3.4139 (16)	137
C12—H12⋯O2ⁱⁱⁱ	0.95	2.74	3.4046 (17)	128
C11—H11⋯O2ⁱⁱⁱ	0.95	2.90	3.4820 (17)	121
Symmetry codes: (i) $[x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+2]$ ; (ii) $[-x+{\script{3\over 2}}, -y, z-{\script{1\over 2}}]$ ; (iii) $[x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Data collection: CrysAlis CCD (Oxford Diffraction, 2008 $[Oxford Diffraction (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction, Wrocław, Poland.]$ ); cell refinement: CrysAlis RED (Oxford Diffraction, 2008 $[Oxford Diffraction (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction, Wrocław, Poland.]$ ); data reduction: CrysAlis RED; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ); software used to prepare material for publication: PARST95 (Nardelli, 1995).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808035459/hg2430sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808035459/hg2430Isup2.hkl

checkCIF report

Comment top

A great interest in the design of materials with potential applications in photonic technology has been developed in recent years (Luo et al., 2003; Vidal et al., 2008). Significant efforts have been focused on studying of design and the synthesis of organic molecules with potential nonlinear optical response (NLO), improved optical transparency and thermal stability (Park et al., 2004). A specific type of these molecules consists of electron donor and acceptor end groups interacting through a conjugating segment. In a first stage of work in our group, the synthesis of a stilbene molecule with nitro group with electron-withdrawing capacity as a substituent in para position, is considered. In order to obtain detailed structural information on the molecular conformation, its NLO responses, its hydrogen bonded interactions and its supramolecular arrangement, the crystal structure of p-nitrostilbene (I) was undertaken.

Perspective view of the title molecule, showing the atomic numbering scheme, is given in Fig. 1. The benzene rings are twisted out of the ethylene plane, as defined by the torsion angles C3—C4—C7—C8 and C7—C8—C9—C14 therefore the benzene rings are inclined to each other showing a dihedral angle between their mean planes of 8.42 (6)°. The nitro group is almost coplanar with the benzene ring but it is rotated about the C—N bond with an angle of rotation of 5.84 (12)°. If compared with the C7—C8 bond length to the expected value for a localized double bond [1.317 (13) Å, Allen et al., 1987], the title distance shows some lengthening that is indicative of some π conjugation of the two benzene rings through the central ethene bridge. The torsion angle between the benzene rings [C4—C7═C8—C9 = 178.48 (12)°] indicates a trans geometry between them. The crystal structure of (I) is stabilized by weak C—H···O hydrogen-bonding interactions [Nardelli, 1995, Table 1]. The formation of the framework can be explained in terms of three-one substructures. In the first substructure atom C2 in the molecule at (x, y, z) acts as a hydrogen bond donor to nitro atom O1 in the molecule at (-1/2 + x, 1/2 - y, 2 - z) so generating, by 2₁ screw axes, C(5) chains which are running along [100] (Fig. 2). In the second substructure, atom C12 in the molecule at (x, y, z) acts as hydrogen bond donor to nitro atom O2 in the molecule at (x, 1/2 - y, -1/2 + z) so generating C(13) chains along [001] (Fig. 3). In the third-one dimensional substructure atom C12 in the molecule at (x, y, z) acts simultaneously as hydrogen bond donor to atoms O1 in the molecule at (x, 1/2 - y, -1/2 + z) and atom O1 in the molecule at (3/2 - x, -y, -1/2 + z) so generating a chain of edge-fused with graph motif R₃³(28) rings along [001] (Etter, 1990), [Fig. 4].

Related literature top

For a previous study of the title compound, see: Hertel & Romer (1931). For background information on photonics materials, see: Luo et al. (2003); Vidal et al. (2008); Park et al. (2004). For general background, see: Allen et al. (1987); Etter (1990); Nardelli (1995). [Please check rephrasing]

Experimental top

The synthesis of (I) was prepared by taking equimolar quantities of benzyltriphenylphosphonium bromide (0.9600 g, 2.20 mmol) and 4-nitrobenzaldehyde (0.3355 g, 2.20 mmol). The mixture was stirred and it was taken to reflux in dry THF in a nitrogen atmosphere at 273 K. 3.3 mmol of potassium tert-butoxide was dissolved in 5 ml of t-butanol and this solution was added drop to drop to the phosphonium mixture obtaining a change in the color of the reaction mixture and completion of the reaction after two hours. Single crystals suitable for X-ray analysis were obtained by evaporation at room temperature using ethyl acetate as solvent.

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: PARST95 (Nardelli, 1995).

Figures top

	Fig. 1. An ORTEP-3 (Farrugia, 1997) plot of the (I) compound, with the atomic labelling scheme. The shapes of the ellipsoids correspond to 50% probability contours of atomic displacement and, for the sake of clarity, H atoms are shown as spheres of arbitrary radius.
	Fig. 2. (Part of the crystal structure of (I) showing the formation of C(5) chains which are running parallel to the [100] direction. [Symmetry codes: (i) 1/2 + x, 1/2 - y, 2 - z; (ii) 1 + x, y, z; (iii) -1/2 + x, 1/2 - y, 2 - z; (iv) -1 + x, y, z].
	Fig. 3. Part of the crystal structure of (I) showing the formation of C(13) chains along [001]. [Symmetry codes: (i) x, 1/2 - y, 1/2 + z; (ii) x, 1/2 - y, -1/2 + z;
	Fig. 4. Part of the crystal structure of (I) showing the formation of edge-fused R₃³(28) rings along [001]. [Symmetry codes: (i) 3/2 - x, -y, 1/2 + z; (ii) x, 1/2 - y, 1/2 + z; (iii) 3/2 - x, 1/2 + y, z; (iv) 3/2 - x, -y, -1/2 + z]; (v) x, 1/2 - y, -1/2 + z; (vi) x, y, -1 + z; (vii) 3/2 - x, y, -1 + z.

4-nitrostilbene top

Crystal data top

C₁₄H₁₁NO₂	D_x = 1.373 Mg m⁻³
M_r = 225.24	Melting point: 421(1) K
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 5152 reflections
a = 10.0839 (3) Å	θ = 2.7–30.7°
b = 7.6849 (2) Å	µ = 0.09 mm⁻¹
c = 28.1176 (8) Å	T = 123 K
V = 2178.94 (11) Å³	Cut lathe, light yellow
Z = 8	0.40 × 0.40 × 0.18 mm
F(000) = 944

Data collection top

Oxford Xcalibur-S diffractometer	3173 independent reflections
Radiation source: fine-focus sealed tube	2202 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.027
ω scans	θ_max = 30.0°, θ_min = 2.9°
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2008)	h = −14→12
T_min = 0.965, T_max = 0.985	k = −10→10
13263 measured reflections	l = −37→39

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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.135	H-atom parameters constrained
S = 1.09	w = 1/[σ²(F_o²) + (0.0501P)² + 0.491P] where P = (F_o² + 2F_c²)/3
3173 reflections	(Δ/σ)_max < 0.001
154 parameters	Δρ_max = 0.32 e Å⁻³
0 restraints	Δρ_min = −0.18 e Å⁻³

Crystal data top

C₁₄H₁₁NO₂	V = 2178.94 (11) Å³
M_r = 225.24	Z = 8
Orthorhombic, Pbca	Mo Kα radiation
a = 10.0839 (3) Å	µ = 0.09 mm⁻¹
b = 7.6849 (2) Å	T = 123 K
c = 28.1176 (8) Å	0.40 × 0.40 × 0.18 mm

Data collection top

Oxford Xcalibur-S diffractometer	3173 independent reflections
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2008)	2202 reflections with I > 2σ(I)
T_min = 0.965, T_max = 0.985	R_int = 0.027
13263 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.048	0 restraints
wR(F²) = 0.135	H-atom parameters constrained
S = 1.09	Δρ_max = 0.32 e Å⁻³
3173 reflections	Δρ_min = −0.18 e Å⁻³
154 parameters

Special details top

Experimental. Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.

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.

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

	x	y	z	U_iso*/U_eq
O1	0.69853 (10)	0.14689 (14)	1.05469 (3)	0.0360 (3)
O2	0.51679 (11)	0.28695 (16)	1.04197 (4)	0.0453 (3)
N1	0.60967 (11)	0.19752 (15)	1.02843 (4)	0.0267 (3)
C1	0.61582 (12)	0.15108 (16)	0.97771 (4)	0.0209 (3)
C2	0.51199 (13)	0.19912 (17)	0.94826 (5)	0.0232 (3)
H2	0.4364	0.2572	0.9607	0.028*
C3	0.52059 (13)	0.16074 (16)	0.90021 (5)	0.0239 (3)
H3	0.4496	0.1910	0.8796	0.029*
C4	0.63344 (13)	0.07749 (15)	0.88166 (4)	0.0227 (3)
C5	0.73329 (13)	0.02692 (17)	0.91315 (5)	0.0247 (3)
H5	0.8083	−0.0340	0.9013	0.030*
C6	0.72573 (13)	0.06320 (16)	0.96111 (5)	0.0244 (3)
H6	0.7945	0.0285	0.9822	0.029*
C7	0.65267 (13)	0.04480 (16)	0.83078 (5)	0.0245 (3)
H7	0.7231	−0.0309	0.8221	0.029*
C8	0.58014 (13)	0.11181 (16)	0.79561 (4)	0.0236 (3)
H8	0.5084	0.1851	0.8044	0.028*
C9	0.60077 (12)	0.08280 (15)	0.74429 (4)	0.0218 (3)
C10	0.50863 (13)	0.15248 (16)	0.71267 (4)	0.0229 (3)
H10	0.4342	0.2142	0.7247	0.027*
C11	0.52360 (13)	0.13330 (17)	0.66384 (5)	0.0253 (3)
H11	0.4599	0.1819	0.6428	0.030*
C12	0.63151 (13)	0.04317 (16)	0.64595 (5)	0.0261 (3)
H12	0.6422	0.0302	0.6126	0.031*
C13	0.72366 (13)	−0.02793 (17)	0.67678 (5)	0.0263 (3)
H13	0.7973	−0.0904	0.6645	0.032*
C14	0.70911 (13)	−0.00852 (16)	0.72570 (5)	0.0246 (3)
H14	0.7730	−0.0575	0.7466	0.030*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0325 (6)	0.0503 (6)	0.0254 (5)	−0.0005 (5)	−0.0072 (5)	0.0044 (5)
O2	0.0382 (6)	0.0687 (8)	0.0291 (5)	0.0153 (6)	−0.0009 (5)	−0.0153 (5)
N1	0.0250 (6)	0.0335 (6)	0.0217 (5)	−0.0050 (5)	−0.0026 (5)	0.0008 (5)
C1	0.0213 (6)	0.0227 (6)	0.0188 (6)	−0.0036 (5)	−0.0003 (5)	0.0011 (5)
C2	0.0191 (6)	0.0269 (6)	0.0236 (6)	0.0000 (5)	−0.0005 (5)	−0.0002 (5)
C3	0.0225 (6)	0.0264 (6)	0.0226 (6)	−0.0018 (5)	−0.0041 (5)	0.0024 (5)
C4	0.0263 (6)	0.0196 (5)	0.0220 (6)	−0.0040 (5)	0.0010 (5)	0.0004 (5)
C5	0.0232 (6)	0.0242 (6)	0.0267 (7)	0.0028 (5)	0.0023 (5)	0.0001 (5)
C6	0.0231 (6)	0.0231 (6)	0.0271 (7)	0.0013 (5)	−0.0022 (5)	0.0039 (5)
C7	0.0244 (6)	0.0237 (6)	0.0253 (6)	0.0008 (5)	0.0014 (5)	−0.0023 (5)
C8	0.0243 (6)	0.0223 (6)	0.0242 (6)	−0.0003 (5)	0.0012 (5)	−0.0004 (5)
C9	0.0254 (6)	0.0177 (5)	0.0222 (6)	−0.0030 (5)	0.0010 (5)	−0.0002 (5)
C10	0.0222 (6)	0.0218 (6)	0.0247 (6)	0.0007 (5)	0.0017 (5)	−0.0005 (5)
C11	0.0282 (7)	0.0238 (6)	0.0240 (6)	−0.0002 (5)	−0.0028 (5)	0.0013 (5)
C12	0.0343 (7)	0.0225 (6)	0.0215 (6)	−0.0026 (6)	0.0027 (6)	−0.0023 (5)
C13	0.0257 (7)	0.0222 (6)	0.0309 (7)	0.0017 (5)	0.0057 (6)	−0.0039 (5)
C14	0.0240 (7)	0.0213 (6)	0.0285 (7)	0.0000 (5)	−0.0038 (5)	0.0012 (5)

Geometric parameters (Å, º) top

O1—N1	1.2246 (14)	C7—H7	0.9500
O2—N1	1.2225 (15)	C8—C9	1.4750 (17)
N1—C1	1.4712 (16)	C8—H8	0.9500
C1—C6	1.3793 (17)	C9—C10	1.3931 (17)
C1—C2	1.3851 (18)	C9—C14	1.3998 (17)
C2—C3	1.3855 (17)	C10—C11	1.3890 (18)
C2—H2	0.9500	C10—H10	0.9500
C3—C4	1.4058 (18)	C11—C12	1.3845 (18)
C3—H3	0.9500	C11—H11	0.9500
C4—C5	1.3960 (18)	C12—C13	1.3833 (19)
C4—C7	1.4653 (17)	C12—H12	0.9500
C5—C6	1.3789 (18)	C13—C14	1.3914 (18)
C5—H5	0.9500	C13—H13	0.9500
C6—H6	0.9500	C14—H14	0.9500
C7—C8	1.3334 (18)

O2—N1—O1	123.46 (11)	C4—C7—H7	117.1
O2—N1—C1	118.07 (11)	C7—C8—C9	126.15 (12)
O1—N1—C1	118.46 (12)	C7—C8—H8	116.9
C6—C1—C2	122.37 (12)	C9—C8—H8	116.9
C6—C1—N1	118.74 (11)	C10—C9—C14	118.36 (12)
C2—C1—N1	118.88 (11)	C10—C9—C8	118.19 (11)
C1—C2—C3	118.62 (12)	C14—C9—C8	123.45 (12)
C1—C2—H2	120.7	C11—C10—C9	121.17 (12)
C3—C2—H2	120.7	C11—C10—H10	119.4
C2—C3—C4	120.62 (12)	C9—C10—H10	119.4
C2—C3—H3	119.7	C12—C11—C10	119.85 (12)
C4—C3—H3	119.7	C12—C11—H11	120.1
C5—C4—C3	118.37 (11)	C10—C11—H11	120.1
C5—C4—C7	118.43 (12)	C13—C12—C11	119.86 (12)
C3—C4—C7	123.18 (11)	C13—C12—H12	120.1
C6—C5—C4	121.61 (12)	C11—C12—H12	120.1
C6—C5—H5	119.2	C12—C13—C14	120.42 (12)
C4—C5—H5	119.2	C12—C13—H13	119.8
C5—C6—C1	118.32 (12)	C14—C13—H13	119.8
C5—C6—H6	120.8	C13—C14—C9	120.34 (11)
C1—C6—H6	120.8	C13—C14—H14	119.8
C8—C7—C4	125.82 (12)	C9—C14—H14	119.8
C8—C7—H7	117.1

O2—N1—C1—C6	174.44 (12)	C5—C4—C7—C8	−167.03 (12)
O1—N1—C1—C6	−4.53 (17)	C3—C4—C7—C8	11.53 (19)
O2—N1—C1—C2	−4.71 (18)	C4—C7—C8—C9	178.48 (12)
O1—N1—C1—C2	176.32 (11)	C7—C8—C9—C10	174.99 (12)
C6—C1—C2—C3	−1.49 (19)	C7—C8—C9—C14	−6.1 (2)
N1—C1—C2—C3	177.63 (11)	C14—C9—C10—C11	−0.39 (18)
C1—C2—C3—C4	−1.10 (18)	C8—C9—C10—C11	178.59 (11)
C2—C3—C4—C5	3.13 (18)	C9—C10—C11—C12	0.17 (18)
C2—C3—C4—C7	−175.43 (12)	C10—C11—C12—C13	0.24 (19)
C3—C4—C5—C6	−2.73 (18)	C11—C12—C13—C14	−0.42 (19)
C7—C4—C5—C6	175.91 (11)	C12—C13—C14—C9	0.19 (18)
C4—C5—C6—C1	0.27 (19)	C10—C9—C14—C13	0.21 (17)
C2—C1—C6—C5	1.90 (19)	C8—C9—C14—C13	−178.71 (11)
N1—C1—C6—C5	−177.22 (11)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···O1ⁱ	0.95	2.55	3.3762 (17)	146
C12—H12···O1ⁱⁱ	0.95	2.66	3.4139 (16)	137
C12—H12···O2ⁱⁱⁱ	0.95	2.74	3.4046 (17)	128
C11—H11···O2ⁱⁱⁱ	0.95	2.90	3.4820 (17)	121

Symmetry codes: (i) x−1/2, −y+1/2, −z+2; (ii) −x+3/2, −y, z−1/2; (iii) x, −y+1/2, z−1/2.

Experimental details

Crystal data
Chemical formula	C₁₄H₁₁NO₂
M_r	225.24
Crystal system, space group	Orthorhombic, Pbca
Temperature (K)	123
a, b, c (Å)	10.0839 (3), 7.6849 (2), 28.1176 (8)
V (Å³)	2178.94 (11)
Z	8
Radiation type	Mo Kα
µ (mm⁻¹)	0.09
Crystal size (mm)	0.40 × 0.40 × 0.18

Data collection
Diffractometer	Oxford Xcalibur-S diffractometer
Absorption correction	Multi-scan (CrysAlis RED; Oxford Diffraction, 2008)
T_min, T_max	0.965, 0.985
No. of measured, independent and observed [I > 2σ(I)] reflections	13263, 3173, 2202
R_int	0.027
(sin θ/λ)_max (Å⁻¹)	0.703

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.048, 0.135, 1.09
No. of reflections	3173
No. of parameters	154
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.32, −0.18

Computer programs: CrysAlis CCD (Oxford Diffraction, 2008), CrysAlis RED (Oxford Diffraction, 2008), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997), PARST95 (Nardelli, 1995).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···O1ⁱ	0.95	2.55	3.3762 (17)	145.9
C12—H12···O1ⁱⁱ	0.95	2.66	3.4139 (16)	136.6
C12—H12···O2ⁱⁱⁱ	0.95	2.74	3.4046 (17)	127.5
C11—H11···O2ⁱⁱⁱ	0.95	2.90	3.4820 (17)	120.5

Symmetry codes: (i) x−1/2, −y+1/2, −z+2; (ii) −x+3/2, −y, z−1/2; (iii) x, −y+1/2, z−1/2.

Acknowledgements

RMF is grateful to the Spanish Research Council (CSIC) for the use of a free-of-charge licence to the Cambridge Structural Database (Allen, 2002 ). RMF also thanks the Universidad del Valle, Colombia, for partial financial support.

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