organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

rac-2-(2-Amino-4-oxo-4,5-di­hydro-1,3-thia­zol-5-yl)-2-hy­droxy­indane-1,3-dione

aDepartment of Pharmaceutical Sciences, College of Pharmacy, University of Kentucky, Lexington, KY 40536, USA, and bDepartment of Chemistry, University of Kentucky, Lexington, KY 40506, USA
*Correspondence e-mail: pcrooks@email.uky.edu

(Received 25 June 2009; accepted 1 July 2009; online 18 July 2009)

In the crystal of the title compound, C12H8N2O4S, mol­ecules are linked into chains by a series of inter­molecular O—H⋯O, N—H⋯O and N—H⋯N hydrogen bonds. The ninhydrin and amino­thia­zolidine units make a dihedral angle of 66.41 (3)°. The crystal structure indicates the presence of equimolar R and S enanti­omers in the crystal lattice, due to the presence of a chiral centre in the title compound.

Related literature

The NADPH-dependent oxidase activity of 2-indol-3-yl- methyl­enequinuclidin-3-ols has been reported by Sekhar et al. (2003[Sekhar, K. R., Crooks, P. A., Sonar, V. N., Friedman, D. B., Chan, J. Y., Meredith, M. J., Stames, J. H., Kelton, K. R., Summar, S. R., Sasi, S. & Freeman, M. L. (2003). Cancer Res. 63, 5636-5645.]) and novel substituted (Z)-2-(N-benzyl­indol-3-ylmethyl­ene) quinuclidin-3-one and (Z)-(±)-2-(N-benzyl­indol-3-ylmethyl­ene) quinuclidin-3-ol derivatives have been identified as potent thermal sensitizing agents (Sonar et al., 2007[Sonar, V. N., Reddy, Y. T., Sekhar, K. R., Sowmya, S., Freeman, M. L. & Crooks, P. A. (2007). Bioorg. Med. Chem. Lett. 17, 6821-6824.]). The crystal structure and bond-length data for ninhydrin have been described by Medrud (1969[Medrud, R. C. (1969). Acta Cryst. B25, 213-220.]) and Fun et al. (2009[Fun, H.-K., Quah, C. K., Parveen, M., Ghalib, R. M. & Mehdi, S. H. (2009). Acta Cryst. E65, o1209.]).

[Scheme 1]

Experimental

Crystal data
  • C12H8N2O4S

  • Mr = 276.26

  • Monoclinic, P 21 /n

  • a = 14.1702 (2) Å

  • b = 5.6713 (1) Å

  • c = 14.8296 (3) Å

  • β = 114.0171 (9)°

  • V = 1088.58 (3) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.31 mm−1

  • T = 90 K

  • 0.25 × 0.12 × 0.05 mm

Data collection
  • Nonius KappaCCD diffractometer

  • Absorption correction: multi-scan (SCALEPACK; Otwinowski & Minor, 1997[Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307-326. New York: Academic Press.]) Tmin = 0.927, Tmax = 0.985

  • 23811 measured reflections

  • 2498 independent reflections

  • 2268 reflections with I > 2σ(I)

  • Rint = 0.035

Refinement
  • R[F2 > 2σ(F2)] = 0.029

  • wR(F2) = 0.076

  • S = 1.06

  • 2498 reflections

  • 173 parameters

  • H-atom parameters constrained

  • Δρmax = 0.38 e Å−3

  • Δρmin = −0.26 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O2—H2⋯O3i 0.84 1.99 2.8225 (13) 170
N2—H2A⋯N1ii 0.88 2.07 2.9372 (15) 168
N2—H2B⋯O2iii 0.88 2.14 2.9629 (14) 155
Symmetry codes: (i) x, y+1, z; (ii) -x+1, -y, -z; (iii) [-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{1\over 2}}].

Data collection: COLLECT (Nonius, 1998[Nonius (1998). COLLECT and DENZO. Nonius BV, Delft, The Netherlands.]); cell refinement: SCALEPACK (Otwinowski & Minor, 1997[Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307-326. New York: Academic Press.]); data reduction: DENZO-SMN (Otwinowski & Minor, 1997[Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307-326. New York: Academic Press.]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: XP in SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); software used to prepare material for publication: SHELXL97 and local procedures.

Supporting information


Comment top

In continuing studies on the design and synthesis of novel radiosensitizers such as (Z)-2-(N-benzylindol-3-ylmethylene)quinuclidin-3-one and (Z)-(±)-2-(N-benzylindol-3-ylmethylene) quinuclidin-3-ol derivatives (Sekhar et al., 2003; Sonar et al., 2007), we have undertaken the design, synthesis and structural analysis of a series of ninhydrin analogs with a variety of active methylene compounds. The X-ray analysis of the title compound was carried out to confirm stereochemistry and to obtain detailed information on the structural conformation of the molecule, that might be useful in structure-activity relationship (SAR) studies. The title compound was prepared by the condensation of ninhydrin with 2-aminothiazol-4(5H)-one in ethanol at reflux temperature. The compound was crystallized from ethanol. The molecular structure and the atom-numbering scheme are shown in Fig.1. The ninhydrin ring is planar (r.m.s. deviation = 0.0255 (10) Å) with bond distances and angles comparable with those previously reported for ninhydrin (Medrud, 1969 and Fun et al. (2009). The title compound has a chiral centre at C10 and the X-ray data indicate that the compound is racemic (Fig. 2). The ninhydrin and 2-aminothiazol- 4(5H)-one moieties make a dihedral angle of 66.41 (3)°. Intermolecular O—H···O, N—H···O and N—H···N hydrogen bonds stabilize the crystal structure, and form a three-dimensional network.

Related literature top

The NADPH-dependent oxidase activity of 2-indol-3-yl- methylenequinuclidin-3-ols has been reported by Sekhar et al. (2003) and novel substituted (Z)-2-(N-benzylindol-3-ylmethylene) quinuclidin-3-one and (Z)-(±)-2-(N-benzylindol-3-ylmethylene) quinuclidin-3-ol derivatives have been identified as potent thermal sensitizing agents (Sonar et al., 2007). The crystal structure and bond-length data for ninhydrin has been described by Medrud (1969) and Fun et al. (2009).

Experimental top

A mixture of ninhydrin (1 mmol) and 2-aminothiazol-4(5H)-one (1.1 mmol) was stirred under reflux in ethanol for 6 hrs. After the reaction was complete, the reaction mixture was cooled to room temperature. The precipitate thus obtained was collected by filtration, washed with cold ethanol and dried, to afford the crude product. Crystallization from ethanol afforded a light yellow crystalline product of 2-(2-amino-4-oxo-4,5-dihydrothiazol-5-yl)-2-hydroxy -1H-indene-1,3(2H)-dione that was suitable for X-ray analysis. 1H NMR (DMSO-d6): δ 5.01 (s, 1H, CH), 7.10 (s, 1H, OH), 7.93–8.03 (m, 4H, Ar—H), 8.94–9.06 (bd, 2H, NH2), p.p.m.; 13C NMR (DMSO-d6): δ 62.15, 73.62, 123.47, 123.57, 136.67, 137.26, 140.68, 141.09, 182.49, 185.12, 197.48, 198.25 p.p.m..

Refinement top

H atoms were found in difference Fourier maps and subsequently placed in idealized positions with constrained distances of 1.00 Å (R~3~CH), 0.95 Å (CÃr~H), 0.84 Å (O—H), 0.88 Å (N—H), and with Uĩso~(H) values set to either 1.2U~eq~ or 1.5U~eq~(OH) of the attached atom.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and local procedures.

Figures top
[Figure 1] Fig. 1. A view of the molecule with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
[Figure 2] Fig. 2. Crystal packing of the molecule viewed along the a axis. H atoms have been omitted for clarity.
rac-2-(2-amino-4-oxo-4,5-dihydro-1,3-thiazol-5-yl)-2-hydroxyindane- 1,3-dione top
Crystal data top
C12H8N2O4SF(000) = 568
Mr = 276.26Dx = 1.686 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 2750 reflections
a = 14.1702 (2) Åθ = 1.0–27.5°
b = 5.6713 (1) ŵ = 0.31 mm1
c = 14.8296 (3) ÅT = 90 K
β = 114.0171 (9)°Tablet, pale yellow
V = 1088.58 (3) Å30.25 × 0.12 × 0.05 mm
Z = 4
Data collection top
Nonius KappaCCD
diffractometer
2498 independent reflections
Radiation source: fine-focus sealed tube2268 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.035
Detector resolution: 9.1 pixels mm-1θmax = 27.5°, θmin = 1.7°
ω scans at fixed χ = 55°h = 1818
Absorption correction: multi-scan
(SCALEPACK; Otwinowski & Minor, 1997)
k = 77
Tmin = 0.927, Tmax = 0.985l = 1919
23811 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.029Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.076H-atom parameters constrained
S = 1.06 w = 1/[σ2(Fo2) + (0.0342P)2 + 0.6796P],
where P = (Fo2 + 2Fc2)/3
2498 reflections(Δ/σ)max < 0.001
173 parametersΔρmax = 0.38 e Å3
0 restraintsΔρmin = 0.26 e Å3
Crystal data top
C12H8N2O4SV = 1088.58 (3) Å3
Mr = 276.26Z = 4
Monoclinic, P21/nMo Kα radiation
a = 14.1702 (2) ŵ = 0.31 mm1
b = 5.6713 (1) ÅT = 90 K
c = 14.8296 (3) Å0.25 × 0.12 × 0.05 mm
β = 114.0171 (9)°
Data collection top
Nonius KappaCCD
diffractometer
2498 independent reflections
Absorption correction: multi-scan
(SCALEPACK; Otwinowski & Minor, 1997)
2268 reflections with I > 2σ(I)
Tmin = 0.927, Tmax = 0.985Rint = 0.035
23811 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0290 restraints
wR(F2) = 0.076H-atom parameters constrained
S = 1.06Δρmax = 0.38 e Å3
2498 reflectionsΔρmin = 0.26 e Å3
173 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 F^2^ against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F^2^, conventional R-factors R are based on F, with F set to zero for negative F^2^. The threshold expression of F^2^ > 2σ(F^2^) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F^2^ 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
xyzUiso*/Ueq
S10.66382 (2)0.49412 (5)0.17138 (2)0.01284 (10)
O10.42723 (7)0.96668 (16)0.23512 (7)0.0153 (2)
N10.49352 (8)0.24974 (19)0.06881 (8)0.0125 (2)
C10.45157 (9)0.7729 (2)0.27136 (9)0.0120 (2)
O20.63896 (6)0.76852 (16)0.33559 (7)0.01299 (19)
H20.62500.91290.33350.019*
N20.64515 (8)0.0977 (2)0.06834 (8)0.0143 (2)
H2A0.61120.02000.03030.017*
H2B0.71250.10840.08780.017*
C20.54859 (9)0.6411 (2)0.27535 (9)0.0111 (2)
O30.61330 (7)0.25829 (17)0.35097 (7)0.0167 (2)
C30.54791 (9)0.4118 (2)0.33094 (9)0.0120 (2)
O40.37082 (7)0.45803 (17)0.10054 (7)0.0162 (2)
C40.45890 (10)0.4216 (2)0.35841 (9)0.0127 (2)
C50.42889 (10)0.2577 (2)0.41132 (9)0.0153 (3)
H50.46540.11350.43260.018*
C60.34424 (10)0.3100 (3)0.43217 (9)0.0178 (3)
H60.32200.19960.46760.021*
C70.29109 (10)0.5234 (3)0.40164 (10)0.0180 (3)
H70.23480.55780.41860.022*
C80.31936 (10)0.6860 (3)0.34679 (9)0.0158 (3)
H80.28230.82930.32480.019*
C90.40390 (9)0.6314 (2)0.32529 (9)0.0130 (2)
C100.54450 (9)0.6061 (2)0.17129 (9)0.0116 (2)
H100.52960.76200.13680.014*
C110.45938 (9)0.4304 (2)0.10930 (9)0.0121 (2)
C120.59513 (9)0.2577 (2)0.09614 (9)0.0121 (2)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
S10.01022 (15)0.01467 (17)0.01409 (16)0.00198 (11)0.00543 (12)0.00411 (11)
O10.0149 (4)0.0140 (5)0.0158 (4)0.0023 (4)0.0051 (4)0.0012 (4)
N10.0116 (5)0.0137 (5)0.0120 (5)0.0008 (4)0.0046 (4)0.0010 (4)
C10.0101 (5)0.0138 (6)0.0107 (5)0.0008 (5)0.0027 (4)0.0026 (5)
O20.0108 (4)0.0104 (4)0.0154 (4)0.0007 (3)0.0030 (3)0.0023 (3)
N20.0109 (5)0.0154 (5)0.0162 (5)0.0014 (4)0.0051 (4)0.0049 (4)
C20.0098 (5)0.0113 (6)0.0117 (5)0.0005 (4)0.0038 (4)0.0009 (5)
O30.0153 (4)0.0131 (5)0.0209 (5)0.0027 (4)0.0065 (4)0.0013 (4)
C30.0123 (6)0.0118 (6)0.0101 (5)0.0012 (5)0.0029 (5)0.0015 (5)
O40.0112 (4)0.0197 (5)0.0176 (4)0.0001 (4)0.0056 (4)0.0015 (4)
C40.0133 (6)0.0139 (6)0.0108 (6)0.0017 (5)0.0047 (5)0.0029 (5)
C50.0180 (6)0.0150 (6)0.0125 (6)0.0023 (5)0.0056 (5)0.0005 (5)
C60.0193 (6)0.0234 (7)0.0113 (6)0.0065 (5)0.0071 (5)0.0021 (5)
C70.0131 (6)0.0278 (8)0.0136 (6)0.0039 (5)0.0060 (5)0.0046 (5)
C80.0122 (6)0.0199 (7)0.0140 (6)0.0001 (5)0.0041 (5)0.0026 (5)
C90.0123 (6)0.0146 (6)0.0109 (5)0.0022 (5)0.0035 (5)0.0017 (5)
C100.0109 (5)0.0125 (6)0.0119 (5)0.0002 (4)0.0051 (4)0.0006 (5)
C110.0126 (6)0.0133 (6)0.0095 (5)0.0006 (5)0.0035 (5)0.0009 (5)
C120.0137 (6)0.0128 (6)0.0096 (5)0.0017 (5)0.0046 (4)0.0004 (5)
Geometric parameters (Å, º) top
S1—C121.7623 (13)C3—C41.4759 (17)
S1—C101.8056 (12)O4—C111.2180 (16)
O1—C11.2101 (16)C4—C51.3903 (18)
N1—C121.3283 (16)C4—C91.3973 (18)
N1—C111.3715 (17)C5—C61.3873 (19)
C1—C91.4764 (17)C5—H50.9500
C1—C21.5449 (17)C6—C71.400 (2)
O2—C21.4245 (14)C6—H60.9500
O2—H20.8400C7—C81.3922 (19)
N2—C121.3166 (16)C7—H70.9500
N2—H2A0.8800C8—C91.3942 (18)
N2—H2B0.8800C8—H80.9500
C2—C101.5334 (17)C10—C111.5460 (17)
C2—C31.5418 (17)C10—H101.0000
O3—C31.2167 (16)
C12—S1—C1089.51 (6)C5—C6—C7120.88 (12)
C12—N1—C11111.88 (11)C5—C6—H6119.6
O1—C1—C9128.58 (12)C7—C6—H6119.6
O1—C1—C2123.02 (11)C8—C7—C6121.13 (12)
C9—C1—C2108.20 (10)C8—C7—H7119.4
C2—O2—H2109.5C6—C7—H7119.4
C12—N2—H2A120.0C7—C8—C9117.69 (13)
C12—N2—H2B120.0C7—C8—H8121.2
H2A—N2—H2B120.0C9—C8—H8121.2
O2—C2—C10110.66 (10)C8—C9—C4121.14 (12)
O2—C2—C3107.00 (9)C8—C9—C1128.92 (12)
C10—C2—C3115.04 (10)C4—C9—C1109.91 (11)
O2—C2—C1109.71 (10)C2—C10—C11112.48 (10)
C10—C2—C1110.82 (10)C2—C10—S1113.13 (8)
C3—C2—C1103.28 (10)C11—C10—S1106.11 (8)
O3—C3—C4127.68 (12)C2—C10—H10108.3
O3—C3—C2124.33 (11)C11—C10—H10108.3
C4—C3—C2107.91 (10)S1—C10—H10108.3
C5—C4—C9120.86 (12)O4—C11—N1125.52 (12)
C5—C4—C3128.50 (12)O4—C11—C10120.08 (11)
C9—C4—C3110.63 (11)N1—C11—C10114.40 (10)
C6—C5—C4118.26 (12)N2—C12—N1122.39 (12)
C6—C5—H5120.9N2—C12—S1119.58 (9)
C4—C5—H5120.9N1—C12—S1118.02 (10)
O1—C1—C2—O264.08 (15)C5—C4—C9—C1179.91 (11)
C9—C1—C2—O2111.21 (11)C3—C4—C9—C11.32 (14)
O1—C1—C2—C1058.41 (15)O1—C1—C9—C80.5 (2)
C9—C1—C2—C10126.30 (11)C2—C1—C9—C8175.40 (12)
O1—C1—C2—C3177.88 (11)O1—C1—C9—C4177.45 (12)
C9—C1—C2—C32.59 (12)C2—C1—C9—C42.51 (14)
O2—C2—C3—O363.22 (15)O2—C2—C10—C11169.35 (10)
C10—C2—C3—O360.14 (16)C3—C2—C10—C1147.95 (14)
C1—C2—C3—O3178.97 (12)C1—C2—C10—C1168.72 (13)
O2—C2—C3—C4113.91 (10)O2—C2—C10—S149.13 (12)
C10—C2—C3—C4122.72 (11)C3—C2—C10—S172.27 (12)
C1—C2—C3—C41.84 (12)C1—C2—C10—S1171.06 (8)
O3—C3—C4—C51.2 (2)C12—S1—C10—C2124.72 (9)
C2—C3—C4—C5178.24 (12)C12—S1—C10—C110.93 (9)
O3—C3—C4—C9177.42 (12)C12—N1—C11—O4176.05 (12)
C2—C3—C4—C90.41 (14)C12—N1—C11—C103.27 (15)
C9—C4—C5—C61.35 (18)C2—C10—C11—O452.60 (16)
C3—C4—C5—C6177.19 (12)S1—C10—C11—O4176.80 (10)
C4—C5—C6—C70.65 (19)C2—C10—C11—N1126.76 (11)
C5—C6—C7—C82.1 (2)S1—C10—C11—N12.56 (13)
C6—C7—C8—C91.44 (19)C11—N1—C12—N2178.04 (12)
C7—C8—C9—C40.56 (19)C11—N1—C12—S12.57 (14)
C7—C8—C9—C1178.27 (12)C10—S1—C12—N2179.74 (11)
C5—C4—C9—C81.99 (19)C10—S1—C12—N10.85 (10)
C3—C4—C9—C8176.79 (11)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O3i0.841.992.8225 (13)170
N2—H2A···N1ii0.882.072.9372 (15)168
N2—H2B···O2iii0.882.142.9629 (14)155
Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z; (iii) x+3/2, y1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaC12H8N2O4S
Mr276.26
Crystal system, space groupMonoclinic, P21/n
Temperature (K)90
a, b, c (Å)14.1702 (2), 5.6713 (1), 14.8296 (3)
β (°) 114.0171 (9)
V3)1088.58 (3)
Z4
Radiation typeMo Kα
µ (mm1)0.31
Crystal size (mm)0.25 × 0.12 × 0.05
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
(SCALEPACK; Otwinowski & Minor, 1997)
Tmin, Tmax0.927, 0.985
No. of measured, independent and
observed [I > 2σ(I)] reflections
23811, 2498, 2268
Rint0.035
(sin θ/λ)max1)0.649
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.029, 0.076, 1.06
No. of reflections2498
No. of parameters173
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.38, 0.26

Computer programs: COLLECT (Nonius, 1998), SCALEPACK (Otwinowski & Minor, 1997), DENZO-SMN (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 2008), XP in SHELXTL (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and local procedures.

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O3i0.841.992.8225 (13)170.0
N2—H2A···N1ii0.882.072.9372 (15)168.4
N2—H2B···O2iii0.882.142.9629 (14)154.6
Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z; (iii) x+3/2, y1/2, z+1/2.
 

Acknowledgements

This investigation was supported by NIH/National Cancer Institute grant PO1 CA104457 (PAC) and by NSF MRI grant CHE 0319176 (SP).

References

First citationFun, H.-K., Quah, C. K., Parveen, M., Ghalib, R. M. & Mehdi, S. H. (2009). Acta Cryst. E65, o1209.  Web of Science CSD CrossRef IUCr Journals Google Scholar
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