1,4-Dihy­droxy-2,3-di­nitro-9,10-anthra­quinone

Furukawa, W.; Takehara, M.; Inoue, Y.; Kitamura, C.

doi:10.1107/S2414314616009068

organic compounds

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ISSN: 2414-3146

Volume 1| Part 6| June 2016| x160906

doi:10.1107/S2414314616009068

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1,4-Dihydroxy-2,3-dinitro-9,10-anthraquinone

Wataru Furukawa,^a Munenori Takehara,^a Yoshinori Inoue ^a and Chitoshi Kitamura ^a ^*

^aDepartment of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone, Shiga 522-8533, Japan
^*Correspondence e-mail: kitamura.c@mat.usp.ac.jp

Edited by J. Simpson, University of Otago, New Zealand (Received 31 May 2016; accepted 5 June 2016; online 14 June 2016)

In the title compound, C₁₄H₆N₂O₈, the anthraquinone unit is essentially planar [maximum deviation = 0.0645 (10) Å], and there are two intramolecular O–H⋯O hydrogen bonds forming S(6) motifs. The planes of the two nitro substituents make dihedral angles of 54.77 (8) and 55.60 (3)° with the anthraquinone ring system. In the crystal, molecules are linked by short intermolecular O⋯O contacts, leading to a three-dimensional network structure.

Keywords: crystal structure; anthraquinone; hydrogen bond; short O⋯O contacts.

CCDC reference: 1483502

Structure description

Various kinds of anthraquinone derivatives are manufactured as dyes and pigments. Among them, for example, hydroxyanthraquinones are used as mordant dyes for dyeing cotton. Recently, we have been interested in the effect of substitution of the aromatic ring on optical properties both in solution and in the solid state. We have previously reported several alkoxy-substituted anthraquinones (Kitamura et al. 2015a ,b ; Ohta et al. 2012a ,b ). We have also investigated the preparation of anthraquinone derivatives with hydroxyl substituents (Furukawa et al. 2014 ; Ohira et al. 2016 ). In this paper we report the treatment of 1,4-dihydroxy-9,10-anthraquinone with fuming HNO₃ to give a nitro compound. However, we failed to characterize this product by NMR techniques due to its low solubility. Thus to elucidate the structure of the title compound, an X-ray crystallographic study was carried out.

The molecular structure of the title compound is shown in Fig. 1. The anthraquinone ring is essentially planar [maximum deviation = 0.0645 (10) Å for C13], and there are two intramolecular O—H⋯O hydrogen bonds, each forming an S(6) ring motif (Table 1 and Fig. 1). The nitro group planes make dihedral angles of 54.77 (8) (N1/O5/O6) and 55.60 (3)° (N2/O7/O8) with the anthraquinone ring system.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯O4	0.854 (19)	1.788 (18)	2.5691 (11)	151.0 (17)
O2—H2⋯O3	0.963 (18)	1.686 (19)	2.5480 (11)	146.9 (16)

Figure 1
The molecular structure of the title compound, showing the atom-numbering and 50% probability displacement ellipsoids. The intramolecular hydrogen bonds are drawn as dashed lines.

There are not only short intramolecular O⋯O contacts [O1⋯O5 = 2.7960 (12), O2⋯O8 = 2.7833 (12) and O6⋯O7 = 2.9097 (12) Å] but also short intermolecular O⋯O contacts [O1⋯O3ⁱ = 2.8305 (11), O5⋯O2ⁱ = 2.8369 (11), and O6⋯O4ⁱⁱ = 2.9796 (11) Å; symmetry codes: (i) x, −y + $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (ii) −x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ] in the crystal (Fig. 2). The latter O⋯O contacts form a three-dimensional network structure. No π–π stacking interactions are observed.

Figure 2
The crystal packing of the title compound. Short O⋯O contacts are shown as blue lines.

Synthesis and crystallization

A mixture of 1,4-dihydroxy-9,10-anthraquinone (502 mg, 2.09 mmol) and fuming HNO₃ (5 ml) was stirred at room temperature for 25 h (Fig. 3). Water (100 ml) was added to the reaction mixture, then the resulting solid was filtered off and dried under vacuum. After chromatography on silica gel with an eluent of dichloromethane-hexane (2:1), recrystallization from ethyl acetate (4 ml) afforded the title compound (35 mg, 5% yield) as red single crystals suitable for X-ray diffraction.

Figure 3
The reaction scheme for the synthesis of the title compound.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C₁₄H₆N₂O₈
M_r	330.21
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	200
a, b, c (Å)	11.7590 (11), 7.2852 (6), 15.9670 (16)
β (°)	105.420 (3)
V (Å³)	1318.6 (2)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.14
Crystal size (mm)	0.6 × 0.35 × 0.2

Data collection
Diffractometer	Rigaku R-Axis Rapid
Absorption correction	Numerical (NUMABS; Higashi, 1999 )
T_min, T_max	0.938, 0.972
No. of measured, independent and observed [I > 2σ(I)] reflections	12217, 3003, 2637
R_int	0.017
(sin θ/λ)_max (Å⁻¹)	0.649

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.039, 0.114, 1.06
No. of reflections	3003
No. of parameters	225
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.22, −0.38
Computer programs: PROCESS-AUTO (Rigaku, 1998 ), SIR2004 (Burla et al., 2005 ), SHELXL2014 (Sheldrick, 2015 ), Mercury (Macrae et al., 2008 ) and WinGX (Farrugia, 2012 ).

Structural data

CCDC reference: 1483502

Crystal structure: contains datablocks global, I. DOI: 10.1107/S2414314616009068/sj4045sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S2414314616009068/sj4045Isup2.hkl

Supporting information file. DOI: 10.1107/S2414314616009068/sj4045Isup3.cml

checkCIF report

Computing details top

Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: PROCESS-AUTO (Rigaku, 1998); program(s) used to solve structure: SIR2004 (Burla et al., 2005); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: WinGX (Farrugia, 2012).

1,4-Dihydroxy-2,3-dinitro-9,10-anthraquinone top

Crystal data top

C₁₄H₆N₂O₈	F(000) = 672
M_r = 330.21	D_x = 1.663 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 10510 reflections
a = 11.7590 (11) Å	θ = 3.1–27.5°
b = 7.2852 (6) Å	µ = 0.14 mm⁻¹
c = 15.9670 (16) Å	T = 200 K
β = 105.420 (3)°	Prism, red
V = 1318.6 (2) Å³	0.6 × 0.35 × 0.2 mm
Z = 4

Data collection top

Rigaku R-Axis Rapid diffractometer	3003 independent reflections
Radiation source: fine-focus sealed x-ray tube	2637 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.017
Detector resolution: 10 pixels mm^-1	θ_max = 27.5°, θ_min = 3.1°
ω scans	h = −15→15
Absorption correction: numerical (NUMABS; Higashi, 1999)	k = −9→9
T_min = 0.938, T_max = 0.972	l = −20→20
12217 measured reflections

Refinement top

Refinement on F²	0 constraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F² > 2σ(F²)] = 0.039	Hydrogen site location: mixed
wR(F²) = 0.114	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ²(F_o²) + (0.079P)² + 0.1104P] where P = (F_o² + 2F_c²)/3
3003 reflections	(Δ/σ)_max < 0.001
225 parameters	Δρ_max = 0.22 e Å⁻³
0 restraints	Δρ_min = −0.38 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. All the H atoms except for the OH groups were positioned geometrically and refined using a riding model. The H atoms of the OH groups were located in a difference Fourier map and freely refined [O1—H1 = 0.854 (19) Å; O2—H2 = 0.963 (18) Å].

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

	x	y	z	U_iso*/U_eq
C1	0.43626 (9)	0.09726 (13)	0.18654 (6)	0.0228 (2)
C2	0.55559 (8)	0.11243 (13)	0.18766 (6)	0.0226 (2)
C3	0.59008 (8)	0.17506 (13)	0.11682 (6)	0.0222 (2)
C4	0.50778 (9)	0.23341 (13)	0.04100 (6)	0.0238 (2)
C5	0.38757 (8)	0.22601 (13)	0.03995 (6)	0.0226 (2)
C6	0.29867 (9)	0.29396 (14)	−0.03798 (6)	0.0269 (2)
C7	0.17279 (9)	0.28732 (14)	−0.03954 (7)	0.0272 (2)
C8	0.08861 (10)	0.34966 (17)	−0.11327 (7)	0.0361 (3)
H8	0.1123	0.3943	−0.162	0.043*
C9	−0.03031 (10)	0.34580 (17)	−0.11475 (8)	0.0406 (3)
H9	−0.0879	0.3876	−0.1648	0.049*
C10	−0.06520 (10)	0.28146 (16)	−0.04380 (8)	0.0372 (3)
H10	−0.1465	0.2814	−0.0452	0.045*
C11	0.01757 (9)	0.21716 (14)	0.02919 (8)	0.0315 (2)
H11	−0.007	0.1719	0.0774	0.038*
C12	0.13735 (9)	0.21906 (13)	0.03167 (7)	0.0256 (2)
C13	0.22486 (9)	0.14497 (13)	0.10835 (6)	0.0241 (2)
C14	0.35224 (8)	0.15643 (12)	0.11118 (6)	0.0218 (2)
N1	0.64537 (7)	0.05360 (13)	0.26604 (6)	0.0291 (2)
N2	0.71694 (7)	0.19212 (11)	0.12268 (5)	0.0252 (2)
O1	0.40976 (7)	0.02703 (11)	0.25620 (5)	0.0314 (2)
O2	0.54693 (7)	0.29723 (12)	−0.02477 (5)	0.0335 (2)
O3	0.33025 (7)	0.35632 (13)	−0.09998 (5)	0.0401 (2)
O4	0.19430 (6)	0.07395 (11)	0.16893 (5)	0.0322 (2)
O5	0.64416 (8)	0.12276 (16)	0.33468 (5)	0.0491 (3)
O6	0.71535 (7)	−0.06315 (12)	0.25598 (6)	0.0403 (2)
O7	0.77730 (7)	0.27811 (11)	0.18428 (5)	0.0354 (2)
O8	0.75222 (7)	0.11905 (13)	0.06575 (6)	0.0408 (2)
H2	0.4783 (17)	0.333 (3)	−0.0697 (12)	0.072 (5)*
H1	0.3344 (17)	0.027 (3)	0.2432 (11)	0.067 (5)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0236 (4)	0.0244 (4)	0.0221 (5)	0.0001 (4)	0.0091 (4)	0.0012 (3)
C2	0.0217 (4)	0.0251 (4)	0.0204 (4)	0.0012 (4)	0.0046 (3)	0.0006 (3)
C3	0.0193 (4)	0.0245 (4)	0.0238 (5)	−0.0006 (4)	0.0076 (3)	−0.0024 (3)
C4	0.0251 (5)	0.0279 (4)	0.0198 (4)	−0.0022 (4)	0.0086 (4)	−0.0011 (3)
C5	0.0220 (5)	0.0252 (4)	0.0206 (4)	−0.0011 (4)	0.0059 (4)	−0.0014 (3)
C6	0.0271 (5)	0.0308 (5)	0.0222 (5)	−0.0010 (4)	0.0053 (4)	0.0003 (4)
C7	0.0237 (5)	0.0280 (5)	0.0272 (5)	−0.0006 (4)	0.0020 (4)	−0.0014 (4)
C8	0.0302 (5)	0.0407 (6)	0.0324 (6)	0.0003 (5)	−0.0003 (4)	0.0032 (5)
C9	0.0280 (5)	0.0421 (6)	0.0429 (6)	0.0021 (5)	−0.0061 (5)	0.0005 (5)
C10	0.0223 (5)	0.0353 (6)	0.0501 (7)	−0.0008 (4)	0.0027 (5)	−0.0047 (5)
C11	0.0232 (5)	0.0294 (5)	0.0414 (6)	−0.0021 (4)	0.0075 (4)	−0.0035 (4)
C12	0.0218 (5)	0.0238 (4)	0.0300 (5)	−0.0014 (4)	0.0046 (4)	−0.0036 (4)
C13	0.0222 (4)	0.0235 (4)	0.0273 (5)	−0.0020 (4)	0.0077 (4)	−0.0028 (4)
C14	0.0217 (4)	0.0220 (4)	0.0224 (5)	−0.0009 (4)	0.0072 (4)	−0.0012 (3)
N1	0.0230 (4)	0.0370 (5)	0.0262 (4)	−0.0006 (4)	0.0048 (3)	0.0075 (3)
N2	0.0214 (4)	0.0270 (4)	0.0281 (4)	−0.0009 (3)	0.0085 (3)	0.0011 (3)
O1	0.0264 (4)	0.0437 (4)	0.0261 (4)	0.0018 (3)	0.0105 (3)	0.0114 (3)
O2	0.0285 (4)	0.0506 (5)	0.0240 (4)	−0.0017 (3)	0.0116 (3)	0.0074 (3)
O3	0.0338 (4)	0.0614 (6)	0.0245 (4)	0.0004 (4)	0.0066 (3)	0.0125 (4)
O4	0.0262 (4)	0.0392 (4)	0.0332 (4)	−0.0044 (3)	0.0114 (3)	0.0049 (3)
O5	0.0399 (5)	0.0836 (7)	0.0214 (4)	0.0060 (5)	0.0040 (3)	−0.0016 (4)
O6	0.0324 (4)	0.0363 (4)	0.0489 (5)	0.0093 (3)	0.0052 (4)	0.0093 (4)
O7	0.0254 (4)	0.0412 (4)	0.0371 (4)	−0.0070 (3)	0.0041 (3)	−0.0054 (3)
O8	0.0313 (4)	0.0539 (5)	0.0433 (5)	0.0014 (4)	0.0205 (4)	−0.0104 (4)

Geometric parameters (Å, º) top

C1—O1	1.3343 (12)	C8—H8	0.95
C1—C2	1.4028 (13)	C9—C10	1.3851 (18)
C1—C14	1.4062 (13)	C9—H9	0.95
C2—C3	1.3770 (13)	C10—C11	1.3863 (16)
C2—N1	1.4703 (12)	C10—H10	0.95
C3—C4	1.4000 (13)	C11—C12	1.3984 (14)
C3—N2	1.4749 (12)	C11—H11	0.95
C4—O2	1.3372 (12)	C12—C13	1.4763 (14)
C4—C5	1.4102 (14)	C13—O4	1.2318 (12)
C5—C14	1.4052 (13)	C13—C14	1.4887 (13)
C5—C6	1.4815 (14)	N1—O5	1.2098 (12)
C6—O3	1.2328 (13)	N1—O6	1.2233 (13)
C6—C7	1.4745 (15)	N2—O8	1.2176 (11)
C7—C8	1.3973 (15)	N2—O7	1.2216 (11)
C7—C12	1.4022 (15)	O1—H1	0.854 (19)
C8—C9	1.3925 (17)	O2—H2	0.963 (18)

O1—C1—C2	118.30 (8)	C10—C9—H9	119.8
O1—C1—C14	124.34 (9)	C8—C9—H9	119.8
C2—C1—C14	117.37 (8)	C9—C10—C11	120.50 (11)
C3—C2—C1	121.85 (8)	C9—C10—H10	119.8
C3—C2—N1	119.60 (8)	C11—C10—H10	119.8
C1—C2—N1	118.51 (8)	C10—C11—C12	119.81 (11)
C2—C3—C4	121.61 (8)	C10—C11—H11	120.1
C2—C3—N2	119.32 (8)	C12—C11—H11	120.1
C4—C3—N2	118.96 (8)	C11—C12—C7	119.66 (10)
O2—C4—C3	118.79 (9)	C11—C12—C13	119.65 (10)
O2—C4—C5	123.87 (9)	C7—C12—C13	120.68 (9)
C3—C4—C5	117.32 (9)	O4—C13—C12	121.34 (9)
C14—C5—C4	121.02 (9)	O4—C13—C14	120.06 (9)
C14—C5—C6	120.39 (9)	C12—C13—C14	118.59 (9)
C4—C5—C6	118.58 (9)	C5—C14—C1	120.76 (9)
O3—C6—C7	120.92 (9)	C5—C14—C13	120.48 (9)
O3—C6—C5	120.11 (9)	C1—C14—C13	118.76 (8)
C7—C6—C5	118.96 (9)	O5—N1—O6	125.37 (9)
C8—C7—C12	120.12 (10)	O5—N1—C2	118.10 (9)
C8—C7—C6	119.12 (10)	O6—N1—C2	116.54 (9)
C12—C7—C6	120.76 (9)	O8—N2—O7	125.80 (9)
C9—C8—C7	119.40 (11)	O8—N2—C3	117.09 (8)
C9—C8—H8	120.3	O7—N2—C3	117.10 (8)
C7—C8—H8	120.3	C1—O1—H1	104.7 (12)
C10—C9—C8	120.50 (11)	C4—O2—H2	106.5 (11)

O1—C1—C2—C3	176.79 (9)	C10—C11—C12—C13	−178.04 (9)
C14—C1—C2—C3	−2.76 (14)	C8—C7—C12—C11	−1.28 (16)
O1—C1—C2—N1	−1.16 (14)	C6—C7—C12—C11	178.90 (9)
C14—C1—C2—N1	179.29 (8)	C8—C7—C12—C13	177.20 (9)
C1—C2—C3—C4	2.42 (15)	C6—C7—C12—C13	−2.62 (15)
N1—C2—C3—C4	−179.65 (9)	C11—C12—C13—O4	3.16 (15)
C1—C2—C3—N2	178.55 (8)	C7—C12—C13—O4	−175.32 (9)
N1—C2—C3—N2	−3.52 (14)	C11—C12—C13—C14	−177.22 (9)
C2—C3—C4—O2	178.29 (9)	C7—C12—C13—C14	4.30 (14)
N2—C3—C4—O2	2.15 (14)	C4—C5—C14—C1	1.90 (14)
C2—C3—C4—C5	0.15 (14)	C6—C5—C14—C1	−177.89 (8)
N2—C3—C4—C5	−176.00 (8)	C4—C5—C14—C13	−178.44 (8)
O2—C4—C5—C14	179.70 (9)	C6—C5—C14—C13	1.78 (14)
C3—C4—C5—C14	−2.26 (14)	O1—C1—C14—C5	−178.90 (9)
O2—C4—C5—C6	−0.51 (15)	C2—C1—C14—C5	0.61 (14)
C3—C4—C5—C6	177.53 (8)	O1—C1—C14—C13	1.42 (14)
C14—C5—C6—O3	179.38 (9)	C2—C1—C14—C13	−179.06 (8)
C4—C5—C6—O3	−0.42 (15)	O4—C13—C14—C5	175.75 (8)
C14—C5—C6—C7	−0.01 (14)	C12—C13—C14—C5	−3.87 (14)
C4—C5—C6—C7	−179.80 (8)	O4—C13—C14—C1	−4.57 (14)
O3—C6—C7—C8	1.22 (16)	C12—C13—C14—C1	175.80 (8)
C5—C6—C7—C8	−179.39 (9)	C3—C2—N1—O5	125.97 (11)
O3—C6—C7—C12	−178.95 (10)	C1—C2—N1—O5	−56.03 (13)
C5—C6—C7—C12	0.43 (15)	C3—C2—N1—O6	−54.34 (13)
C12—C7—C8—C9	0.94 (17)	C1—C2—N1—O6	123.66 (10)
C6—C7—C8—C9	−179.24 (10)	C2—C3—N2—O8	128.07 (10)
C7—C8—C9—C10	0.23 (19)	C4—C3—N2—O8	−55.70 (12)
C8—C9—C10—C11	−1.06 (18)	C2—C3—N2—O7	−51.84 (12)
C9—C10—C11—C12	0.71 (17)	C4—C3—N2—O7	124.39 (10)
C10—C11—C12—C7	0.46 (16)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O4	0.854 (19)	1.788 (18)	2.5691 (11)	151.0 (17)
O2—H2···O3	0.963 (18)	1.686 (19)	2.5480 (11)	146.9 (16)

Acknowledgements

This work was supported financially by JSPS KAKENHI Grant No. 15K05482.

References

Burla, M. C., Caliandro, R., Camalli, M., Carrozzini, B., Cascarano, G. L., De Caro, L., Giacovazzo, C., Polidori, G. & Spagna, R. (2005). J. Appl. Cryst. 38, 381–388. Web of Science CrossRef CAS IUCr Journals Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Furukawa, W., Takehara, M., Inoue, Y. & Kitamura, C. (2014). Acta Cryst. E70, o1130. CSD CrossRef IUCr Journals Google Scholar
Higashi, T. (1999). NUMABS. Rigaku Corporation, Tokyo, Japan. Google Scholar
Kitamura, C., Li, S., Takehara, M., Inoue, Y., Ono, K. & Kawase, T. (2015a). Acta Cryst. E71, o504–o505. Web of Science CSD CrossRef IUCr Journals Google Scholar
Kitamura, C., Li, S., Takehara, M., Inoue, Y., Ono, K., Kawase, T. & Fujimoto, K. J. (2015b). Bull. Chem. Soc. Jpn, 88, 713–715. Web of Science CSD CrossRef CAS Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Ohira, N., Takehara, M., Inoue, Y. & Kitamura, C. (2016). IUCrData 1, x160753. Google Scholar
Ohta, A., Hattori, K., Kobayashi, T., Naito, H., Kawase, T. & Kitamura, C. (2012a). Acta Cryst. E68, o2843. CSD CrossRef IUCr Journals Google Scholar
Ohta, A., Hattori, K., Kusumoto, Y., Kawase, T., Kobayashi, T., Naito, H. & Kitamura, C. (2012b). Chem. Lett. 41, 674–676. Web of Science CSD CrossRef CAS Google Scholar
Rigaku (1998). PROCESS-AUTO. Rigaku Corporation, Tokyo, Japan. Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar

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