Dimethyl 2,6-dimethyl-4-(2-nitro­phen­yl)pyridine-3,5-dicarboxyl­ate

Zheng, J.; Wang, X.; Pang, D.; Sun, Y.; Su, W.

doi:10.1107/S1600536811039626

organic compounds

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ISSN: 2056-9890

Volume 67| Part 11| November 2011| Page o2820

doi:10.1107/S1600536811039626

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Dimethyl 2,6-dimethyl-4-(2-nitrophenyl)pyridine-3,5-dicarboxylate

Juanjuan Zheng,^a Xueyuan Wang,^b Dongying Pang,^b Yan Sun ^c and Wei Su ^c ^*

^aDepartment of Chemistry, School of Science, Tianjin University, Tianjin 300072, People's Republic of China, ^bTianJin Centralpharm Limited Company, Tianjin 300072, People's Republic of China, and ^cHigh Pressure Adsorption Laboratory, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China
^*Correspondence e-mail: suweihb@tju.edu.cn

(Received 21 September 2011; accepted 27 September 2011; online 5 October 2011)

The title compound, C₁₇H₁₆N₂O₆, is a decomposition product of the hypertension drug nifedipine [systematic name: dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate]. The dihedral angle between the nitrosophenyl ring and the pyridine ring is 67.1 (5)°.

Related literature

For the calcium antagonistic activity of compounds of the 1,4-dihydropyridine class, which inhibit the influx of Ca²⁺ ions through plasma membrane channels, see: Núnez-Vergara et al. (1994 ) and for their current use in the treatment of a variety of cardiovascular disorders such as angina and hypertension, see: Triggle et al. (1989 ); Hurwitz et al. (1991 ). For general background to derivatives of the dihydropyridine calcium channel blockers nifedipine [3,5-dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate] and nisoldpine [isobutyl methyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate], see: Chen et al. (2010 ); Rowan & Holt (1996 , 1997a ,b ); Schultheiss et al. (2010 ). For standard bond lengths, see: Allen et al. (1987 ).

Experimental

Crystal data

C₁₇H₁₆N₂O₆
M_r = 344.32
Triclinic, $[P \overline 1]$
a = 7.578 (4) Å
b = 8.141 (4) Å
c = 14.235 (9) Å
α = 103.32 (2)°
β = 93.75 (5)°
γ = 105.39 (3)°
V = 816.4 (8) Å³
Z = 2
Mo Kα radiation
μ = 0.11 mm⁻¹
T = 298 K
0.20 × 0.18 × 0.12 mm

Data collection

Rigaku Saturn724 CCD diffractometer
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005 ) T_min = 0.979, T_max = 0.987
8658 measured reflections
3843 independent reflections
2247 reflections with I > 2σ(I)
R_int = 0.047

Refinement

R[F² > 2σ(F²)] = 0.038
wR(F²) = 0.097
S = 1.03
3843 reflections
230 parameters
H-atom parameters constrained
Δρ_max = 0.21 e Å⁻³
Δρ_min = −0.29 e Å⁻³

Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear; data reduction: CrystalClear; 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: CrystalStructure (Rigaku, 2005).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536811039626/qm2031sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811039626/qm2031Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536811039626/qm2031Isup3.cml

checkCIF report

Comment top

Compounds of the 1,4-dihydropyridine class exhibit calcium antagonistic activity, as they inhibit the influx of Ca²⁺ ions through plasma membrane channels (Núnez-Vergara, Sunkel & Squella, 1994). Compounds of this class are currently being used in the treatment of a variety of cardiovascular disorders, such as angina and hypertension (Triggle et al., 1989; Hurwitz et al., 1991). Nifedipine [dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate], is the best known member of this class. The molecular structure of (I) is shown in Fig. 1. The bond lengths and angles are within normal ranges (Allen et al., 1987). The dihedral angle between the nitrosophenyl ring and the pyridine ring is 67.1°.

Related literature top

For the calcium antagonistic activity of 1,4-dihydropyridines, see: Núnez-Vergara et al. (1994) and for their use in the treatment of a variety of cardiovascular disorders, such as angina and hypertension, see: Triggle et al. (1989); Hurwitz et al. (1991). For standard bond lengths, see: Allen et al. (1987). For related literature, see: Chen et al. (2010); Rowan & Holt (1996, 1997a,b); Schultheiss et al. (2010).

Experimental top

The title compound was prepared by adding following steps. 1: Add 1 g nifedipine and 10 g (NH₄)₂S₂O₈ to the 100 ml acetone solution(50%). 2: Stir for 12 h at 30 °C.3:Regulate the solution to pH=8 with Na₂CO₃. The resulting solution was extracted with methylene chloride. The organic layer was dried over MgSO₄ and evaporated under reduced pressure. Following washing the extract with water, crystals of suitable size for single-crystal analysis were recrystallized from methanol.

Computing details top

Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear (Rigaku, 2005); data reduction: CrystalClear (Rigaku, 2005); 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: CrystalStructure (Rigaku, 2005).

Figures top

Fig. 1. [3,5-dimethyl 2,6-dimethyl-4-(2-nitrophenyl)pyridine-3,5-dicarboxylate]

Dimethyl 2,6-dimethyl-4-(2-nitrophenyl)pyridine-3,5-dicarboxylate top

Crystal data top

C₁₇H₁₆N₂O₆	Z = 2
M_r = 344.32	F(000) = 360
Triclinic, P1	D_x = 1.401 Mg m⁻³
a = 7.578 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.141 (4) Å	Cell parameters from 2919 reflections
c = 14.235 (9) Å	θ = 1.5–28.0°
α = 103.32 (2)°	µ = 0.11 mm⁻¹
β = 93.75 (5)°	T = 298 K
γ = 105.39 (3)°	Prism, yellow
V = 816.4 (8) Å³	0.20 × 0.18 × 0.12 mm

Data collection top

Rigaku Saturn724 CCD diffractometer	3843 independent reflections
Radiation source: rotating anode	2247 reflections with I > 2σ(I)
Multilayer monochromator	R_int = 0.047
Detector resolution: 14.22 pixels mm^-1	θ_max = 27.9°, θ_min = 1.5°
ω and ϕ scans	h = −9→9
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005)	k = −10→10
T_min = 0.979, T_max = 0.987	l = −18→17
8658 measured reflections

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)] = 0.038	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.097	H-atom parameters constrained
S = 1.03	w = 1/[σ²(F_o²) + (0.028P)²] where P = (F_o² + 2F_c²)/3
3843 reflections	(Δ/σ)_max = 0.001
230 parameters	Δρ_max = 0.21 e Å⁻³
0 restraints	Δρ_min = −0.29 e Å⁻³

Crystal data top

C₁₇H₁₆N₂O₆	γ = 105.39 (3)°
M_r = 344.32	V = 816.4 (8) Å³
Triclinic, P1	Z = 2
a = 7.578 (4) Å	Mo Kα radiation
b = 8.141 (4) Å	µ = 0.11 mm⁻¹
c = 14.235 (9) Å	T = 298 K
α = 103.32 (2)°	0.20 × 0.18 × 0.12 mm
β = 93.75 (5)°

Data collection top

Rigaku Saturn724 CCD diffractometer	3843 independent reflections
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005)	2247 reflections with I > 2σ(I)
T_min = 0.979, T_max = 0.987	R_int = 0.047
8658 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.038	0 restraints
wR(F²) = 0.097	H-atom parameters constrained
S = 1.03	Δρ_max = 0.21 e Å⁻³
3843 reflections	Δρ_min = −0.29 e Å⁻³
230 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
O1	0.76803 (15)	0.93198 (15)	0.92024 (8)	0.0333 (3)
O2	0.84798 (13)	0.68996 (14)	0.85050 (7)	0.0240 (3)
O3	0.40716 (15)	0.78829 (15)	0.70787 (8)	0.0344 (3)
O4	0.63979 (19)	0.91761 (16)	0.64507 (10)	0.0520 (4)
O5	0.13443 (14)	0.30250 (15)	0.61801 (7)	0.0279 (3)
O6	0.18628 (14)	0.11367 (14)	0.70271 (7)	0.0259 (3)
N1	0.26875 (17)	0.55143 (17)	0.95102 (9)	0.0218 (3)
N2	0.54570 (19)	0.78817 (18)	0.66853 (9)	0.0283 (3)
C1	0.5124 (2)	0.7905 (2)	1.05399 (10)	0.0256 (4)
H1A	0.4799	0.8996	1.0549	0.038*
H1B	0.6471	0.8165	1.0651	0.038*
H1C	0.4582	0.7401	1.1054	0.038*
C2	0.4386 (2)	0.6606 (2)	0.95644 (10)	0.0200 (3)
C3	0.53855 (19)	0.65051 (19)	0.87679 (10)	0.0184 (3)
C4	0.45649 (19)	0.52477 (19)	0.78913 (10)	0.0175 (3)
C5	0.2806 (2)	0.4125 (2)	0.78490 (10)	0.0188 (3)
C6	0.1892 (2)	0.4295 (2)	0.86757 (11)	0.0200 (3)
C7	−0.0011 (2)	0.3140 (2)	0.86810 (11)	0.0278 (4)
H7A	0.0077	0.2160	0.8954	0.042*
H7B	−0.0653	0.2675	0.8013	0.042*
H7C	−0.0701	0.3832	0.9080	0.042*
C8	0.7278 (2)	0.7743 (2)	0.88633 (11)	0.0214 (3)
C9	1.0316 (2)	0.8018 (2)	0.84997 (12)	0.0297 (4)
H9A	1.0228	0.8948	0.8183	0.045*
H9B	1.1038	0.7310	0.8141	0.045*
H9C	1.0925	0.8554	0.9171	0.045*
C10	0.55534 (18)	0.50220 (19)	0.70104 (10)	0.0176 (3)
C11	0.6085 (2)	0.3487 (2)	0.67264 (10)	0.0226 (4)
H11	0.5774	0.2612	0.7077	0.027*
C12	0.7059 (2)	0.3213 (2)	0.59432 (11)	0.0267 (4)
H12	0.7401	0.2154	0.5759	0.032*
C13	0.7538 (2)	0.4482 (2)	0.54255 (11)	0.0263 (4)
H13	0.8218	0.4296	0.4893	0.032*
C14	0.7024 (2)	0.6014 (2)	0.56854 (11)	0.0237 (4)
H14	0.7342	0.6887	0.5334	0.028*
C15	0.60384 (19)	0.6254 (2)	0.64663 (10)	0.0202 (3)
C16	0.19127 (19)	0.2747 (2)	0.69257 (11)	0.0200 (3)
C17	0.1122 (2)	−0.0294 (2)	0.61593 (12)	0.0364 (4)
H17A	−0.0139	−0.0318	0.5937	0.055*
H17B	0.1105	−0.1415	0.6307	0.055*
H17C	0.1898	−0.0116	0.5646	0.055*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0287 (7)	0.0182 (6)	0.0483 (8)	0.0042 (5)	0.0079 (5)	0.0014 (6)
O2	0.0200 (6)	0.0216 (6)	0.0293 (6)	0.0058 (5)	0.0056 (5)	0.0041 (5)
O3	0.0382 (7)	0.0388 (8)	0.0381 (7)	0.0221 (6)	0.0167 (6)	0.0170 (6)
O4	0.0680 (10)	0.0273 (8)	0.0727 (10)	0.0148 (7)	0.0355 (8)	0.0278 (8)
O5	0.0279 (6)	0.0343 (7)	0.0211 (6)	0.0059 (5)	0.0023 (5)	0.0102 (5)
O6	0.0320 (6)	0.0194 (6)	0.0223 (6)	0.0061 (5)	−0.0008 (5)	0.0004 (5)
N1	0.0245 (7)	0.0220 (7)	0.0214 (7)	0.0086 (6)	0.0074 (5)	0.0072 (6)
N2	0.0376 (9)	0.0251 (8)	0.0255 (8)	0.0098 (7)	0.0069 (6)	0.0111 (7)
C1	0.0318 (9)	0.0260 (9)	0.0212 (8)	0.0119 (8)	0.0040 (7)	0.0058 (7)
C2	0.0248 (8)	0.0195 (8)	0.0188 (8)	0.0109 (7)	0.0036 (6)	0.0057 (7)
C3	0.0200 (8)	0.0168 (8)	0.0211 (8)	0.0080 (7)	0.0040 (6)	0.0065 (7)
C4	0.0205 (8)	0.0173 (8)	0.0188 (8)	0.0094 (7)	0.0062 (6)	0.0072 (7)
C5	0.0210 (8)	0.0190 (8)	0.0188 (8)	0.0077 (7)	0.0052 (6)	0.0066 (7)
C6	0.0222 (8)	0.0190 (8)	0.0217 (8)	0.0082 (7)	0.0065 (6)	0.0072 (7)
C7	0.0247 (9)	0.0281 (10)	0.0287 (9)	0.0048 (8)	0.0119 (7)	0.0045 (8)
C8	0.0248 (8)	0.0223 (9)	0.0176 (8)	0.0074 (7)	0.0039 (6)	0.0051 (7)
C9	0.0192 (8)	0.0312 (10)	0.0363 (10)	0.0029 (7)	0.0059 (7)	0.0085 (8)
C10	0.0131 (7)	0.0195 (8)	0.0170 (8)	0.0008 (6)	0.0016 (6)	0.0034 (6)
C11	0.0219 (8)	0.0213 (9)	0.0241 (9)	0.0054 (7)	0.0041 (7)	0.0055 (7)
C12	0.0244 (9)	0.0258 (9)	0.0285 (9)	0.0101 (8)	0.0054 (7)	0.0005 (8)
C13	0.0201 (8)	0.0334 (10)	0.0210 (9)	0.0047 (8)	0.0067 (6)	0.0012 (8)
C14	0.0207 (8)	0.0276 (9)	0.0194 (8)	0.0001 (7)	0.0039 (6)	0.0071 (7)
C15	0.0185 (8)	0.0193 (8)	0.0207 (8)	0.0034 (7)	0.0023 (6)	0.0036 (7)
C16	0.0142 (7)	0.0234 (9)	0.0231 (9)	0.0041 (7)	0.0080 (6)	0.0073 (7)
C17	0.0424 (11)	0.0262 (10)	0.0303 (10)	0.0072 (9)	−0.0033 (8)	−0.0074 (8)

Geometric parameters (Å, º) top

O1—C8	1.2086 (18)	C5—C16	1.498 (2)
O2—C8	1.3392 (18)	C6—C7	1.500 (2)
O2—C9	1.4485 (18)	C7—H7A	0.9800
O3—N2	1.2218 (16)	C7—H7B	0.9800
O4—N2	1.2349 (16)	C7—H7C	0.9800
O5—C16	1.2107 (18)	C9—H9A	0.9800
O6—C16	1.3427 (19)	C9—H9B	0.9800
O6—C17	1.4481 (19)	C9—H9C	0.9800
N1—C2	1.342 (2)	C10—C15	1.393 (2)
N1—C6	1.344 (2)	C10—C11	1.394 (2)
N2—C15	1.478 (2)	C11—C12	1.385 (2)
C1—C2	1.506 (2)	C11—H11	0.9500
C1—H1A	0.9800	C12—C13	1.388 (2)
C1—H1B	0.9800	C12—H12	0.9500
C1—H1C	0.9800	C13—C14	1.381 (2)
C2—C3	1.403 (2)	C13—H13	0.9500
C3—C4	1.401 (2)	C14—C15	1.385 (2)
C3—C8	1.495 (2)	C14—H14	0.9500
C4—C5	1.390 (2)	C17—H17A	0.9800
C4—C10	1.502 (2)	C17—H17B	0.9800
C5—C6	1.402 (2)	C17—H17C	0.9800

C8—O2—C9	115.40 (13)	O2—C8—C3	111.74 (14)
C16—O6—C17	115.28 (12)	O2—C9—H9A	109.5
C2—N1—C6	120.11 (13)	O2—C9—H9B	109.5
O3—N2—O4	123.19 (15)	H9A—C9—H9B	109.5
O3—N2—C15	118.96 (13)	O2—C9—H9C	109.5
O4—N2—C15	117.85 (14)	H9A—C9—H9C	109.5
C2—C1—H1A	109.5	H9B—C9—H9C	109.5
C2—C1—H1B	109.5	C15—C10—C11	116.86 (13)
H1A—C1—H1B	109.5	C15—C10—C4	125.17 (14)
C2—C1—H1C	109.5	C11—C10—C4	117.94 (13)
H1A—C1—H1C	109.5	C12—C11—C10	121.28 (15)
H1B—C1—H1C	109.5	C12—C11—H11	119.4
N1—C2—C3	121.69 (15)	C10—C11—H11	119.4
N1—C2—C1	114.91 (13)	C11—C12—C13	120.22 (15)
C3—C2—C1	123.39 (14)	C11—C12—H12	119.9
C4—C3—C2	118.72 (14)	C13—C12—H12	119.9
C4—C3—C8	121.45 (13)	C14—C13—C12	119.93 (14)
C2—C3—C8	119.83 (14)	C14—C13—H13	120.0
C5—C4—C3	118.81 (13)	C12—C13—H13	120.0
C5—C4—C10	118.94 (14)	C13—C14—C15	118.90 (15)
C3—C4—C10	122.20 (13)	C13—C14—H14	120.5
C4—C5—C6	119.36 (14)	C15—C14—H14	120.5
C4—C5—C16	119.84 (13)	C14—C15—C10	122.80 (15)
C6—C5—C16	120.80 (14)	C14—C15—N2	116.56 (14)
N1—C6—C5	121.30 (14)	C10—C15—N2	120.61 (13)
N1—C6—C7	116.40 (13)	O5—C16—O6	123.94 (15)
C5—C6—C7	122.30 (14)	O5—C16—C5	125.59 (15)
C6—C7—H7A	109.5	O6—C16—C5	110.45 (13)
C6—C7—H7B	109.5	O6—C17—H17A	109.5
H7A—C7—H7B	109.5	O6—C17—H17B	109.5
C6—C7—H7C	109.5	H17A—C17—H17B	109.5
H7A—C7—H7C	109.5	O6—C17—H17C	109.5
H7B—C7—H7C	109.5	H17A—C17—H17C	109.5
O1—C8—O2	123.61 (15)	H17B—C17—H17C	109.5
O1—C8—C3	124.63 (14)

C6—N1—C2—C3	−0.6 (2)	C5—C4—C10—C15	115.28 (17)
C6—N1—C2—C1	−179.75 (12)	C3—C4—C10—C15	−67.4 (2)
N1—C2—C3—C4	1.1 (2)	C5—C4—C10—C11	−66.86 (18)
C1—C2—C3—C4	−179.85 (13)	C3—C4—C10—C11	110.45 (17)
N1—C2—C3—C8	−179.70 (13)	C15—C10—C11—C12	0.4 (2)
C1—C2—C3—C8	−0.7 (2)	C4—C10—C11—C12	−177.67 (14)
C2—C3—C4—C5	−1.2 (2)	C10—C11—C12—C13	0.4 (2)
C8—C3—C4—C5	179.60 (13)	C11—C12—C13—C14	−0.7 (2)
C2—C3—C4—C10	−178.56 (13)	C12—C13—C14—C15	0.2 (2)
C8—C3—C4—C10	2.3 (2)	C13—C14—C15—C10	0.6 (2)
C3—C4—C5—C6	0.9 (2)	C13—C14—C15—N2	−177.47 (13)
C10—C4—C5—C6	178.33 (13)	C11—C10—C15—C14	−0.9 (2)
C3—C4—C5—C16	−178.88 (13)	C4—C10—C15—C14	176.99 (14)
C10—C4—C5—C16	−1.5 (2)	C11—C10—C15—N2	177.12 (13)
C2—N1—C6—C5	0.3 (2)	C4—C10—C15—N2	−5.0 (2)
C2—N1—C6—C7	−179.65 (13)	O3—N2—C15—C14	152.38 (14)
C4—C5—C6—N1	−0.5 (2)	O4—N2—C15—C14	−27.0 (2)
C16—C5—C6—N1	179.35 (13)	O3—N2—C15—C10	−25.7 (2)
C4—C5—C6—C7	179.50 (13)	O4—N2—C15—C10	154.89 (15)
C16—C5—C6—C7	−0.7 (2)	C17—O6—C16—O5	1.9 (2)
C9—O2—C8—O1	−3.1 (2)	C17—O6—C16—C5	−176.59 (11)
C9—O2—C8—C3	175.45 (12)	C4—C5—C16—O5	−70.6 (2)
C4—C3—C8—O1	131.12 (17)	C6—C5—C16—O5	109.63 (18)
C2—C3—C8—O1	−48.0 (2)	C4—C5—C16—O6	107.95 (15)
C4—C3—C8—O2	−47.44 (18)	C6—C5—C16—O6	−71.86 (17)
C2—C3—C8—O2	133.40 (14)

Experimental details

Crystal data
Chemical formula	C₁₇H₁₆N₂O₆
M_r	344.32
Crystal system, space group	Triclinic, P1
Temperature (K)	298
a, b, c (Å)	7.578 (4), 8.141 (4), 14.235 (9)
α, β, γ (°)	103.32 (2), 93.75 (5), 105.39 (3)
V (Å³)	816.4 (8)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.11
Crystal size (mm)	0.20 × 0.18 × 0.12

Data collection
Diffractometer	Rigaku Saturn724 CCD diffractometer
Absorption correction	Multi-scan (CrystalClear; Rigaku, 2005)
T_min, T_max	0.979, 0.987
No. of measured, independent and observed [I > 2σ(I)] reflections	8658, 3843, 2247
R_int	0.047
(sin θ/λ)_max (Å⁻¹)	0.659

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.038, 0.097, 1.03
No. of reflections	3843
No. of parameters	230
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.21, −0.29

Computer programs: CrystalClear (Rigaku, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008), CrystalStructure (Rigaku, 2005).

Acknowledgements

This study was supported by the Tianjin Natural Science Foundation (10JCZDJC23900).

References

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