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Journal logoCRYSTALLOGRAPHIC
COMMUNICATIONS
ISSN: 2056-9890

N-(2,4-Di­fluoro­phen­yl)-5-methyl-1,2-oxazole-4-carboxamide hemihydrate

aState Key Laboratory of Materials-Oriented Chemical Engineering, School of Pharmaceutical Sciences, Nanjing University of Technology, Xinmofan Road No. 5 Nanjing, Nanjing 210009, People's Republic of China, and bState Key Laboratory of Materials-Oriented Chemical Engineering, College of Food Science and Light Industry, Nanjing University of Technology, Xinmofan Road No. 5 Nanjing, Nanjing 210009, People's Republic of China
*Correspondence e-mail: dc_wang@hotmail.com

(Received 22 May 2012; accepted 28 June 2012; online 4 July 2012)

In the title compound, C11H8F2N2O2·0.5H2O, the dihedral angle between the benzene and isoxazole rings is 8.08 (3)°. In the crystal, the components are linked by O—H⋯N and N—H⋯O hydrogen bonds, in which the water mol­ecule acts as both a donor and an acceptor, into a tape with an R44(16) graph-set motif along the a axis. The water mol­ecule is located on a twofold rotation axis. The methyl H atoms were treated as disordered groups over two sites with a refined site-occupancy ratio of 0.48 (6):0.52 (6).

Related literature

For applications of leflunomide [systematic name: 5-methyl-N-[4-(trifluoro­meth­yl) phen­yl]-isoxazole-4-carboxamide] in the treatment of rheumatoid arthritis, see: Shaw et al. (2011[Shaw, J. J., Chen, B., Wooley, P., Palfey, B., Lee, A. R., Huang, W. H. & Zeng, D. (2011). Am. J. Biomed. Sci. 3, 218-227.]); Schattenkirchner (2000[Schattenkirchner, M. (2000). Immunopharmacology, 47, 291-298.]); For leflunomide analogs, see: Huang et al. (2003[Huang, W. H., Yang, C. L., LEE, A. R. & Chiu, H. F. (2003). Chem. Pharm. Bull. 51, 313-314.]). For graph-set motifs, see: Bernstein, et al. (1995[Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555-1573.]).

[Scheme 1]

Experimental

Crystal data
  • C11H8F2N2O2·0.5H2O

  • Mr = 247.20

  • Monoclinic, C 2/c

  • a = 15.182 (3) Å

  • b = 13.803 (3) Å

  • c = 12.159 (2) Å

  • β = 120.06 (3)°

  • V = 2205.3 (8) Å3

  • Z = 8

  • Mo Kα radiation

  • μ = 0.13 mm−1

  • T = 293 K

  • 0.30 × 0.20 × 0.10 mm

Data collection
  • Enraf–Nonius CAD-4 diffractometer

  • Absorption correction: multi-scan (North et al., 1968[North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351-359.]) Tmin = 0.962, Tmax = 0.987

  • 2076 measured reflections

  • 1997 independent reflections

  • 1486 reflections with I > 2σ(I)

  • Rint = 0.050

  • 3 standard reflections every 200 reflections intensity decay: 1%

Refinement
  • R[F2 > 2σ(F2)] = 0.050

  • wR(F2) = 0.130

  • S = 1.04

  • 1997 reflections

  • 168 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • Δρmax = 0.28 e Å−3

  • Δρmin = −0.18 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1A⋯O1W 0.88 (3) 2.26 (3) 3.052 (3) 150 (2)
O1W—H1W⋯N2i 0.92 (3) 2.03 (3) 2.934 (2) 167 (3)
Symmetry code: (i) [x, -y+1, z-{\script{1\over 2}}].

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994[Enraf-Nonius (1994). CAD-4 EXPRESS. Enraf-Nonius, Delft, The Netherlands.]); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995[Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany.]); 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: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); software used to prepare material for publication: SHELXTL.

Supporting information


Comment top

Leflunomide is one of the most effective isoxazole-containing disease-modifying drugs for treating rheumatoid arthritis (Shaw, et al., 2011; Schattenkirchner, 2000). Many leflunomide analogs have been synthesized and exhibit potent immunomodulating effect (Huang, et al., 2003). The title compound, N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide monohydrate, was synthesized as a novel and potent immunomodulating leflunomide analog. We report herein its crystal structure.

As illustrated in Fig. 1, the molecular structure of the title compound is not planar and consists of one N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide molecule and one solvate water molecule. The C1-C6 benzene and the C8-C10/N2/O2 isoxazole ring is almost coplannar with each other with the dihedral angle of 8.08 (3) °. The central nitrogen atom (N1) and carbon atom (C7) are nearly coplanar with the benzene ring and the isoxazole rings[N1-C6-C5-C4 torsion angles = -178.8 (2) ° and C7-C8-C10-O2 torsion angles = 177.4 (2) °], respectively. The length of the C9=N2 double bond is 1.296 (3) Å, slightly longer than standard 1.28 Å value of a C=N double bond. The crystal structure is stabilized by O—H···N and N—H···O hydrogen bonds among the solvate water molecules, amide group and isoxazole nitrogen atoms (Table 1). It is noted that the connections of water molecules and N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide molecules generate a tape with R44(16) pattern based on graph set analysis nomenclature (Bernstein, et al., 1995).

Related literature top

For applications of leflunomide [systematic name: 5-methyl-N-[4-(trifluoromethyl) phenyl]-isoxazole-4-carboxamide] in the treatment of rheumatoid arthritis, see: Shaw et al. (2011); Schattenkirchner (2000); For leflunomide analogs, see: Huang et al. (2003). For graph-set motifs, see: Bernstein, et al. (1995).

Experimental top

A solution of 0.005 mole of 5-methylisoxazole-4-carboxylic acid chloride (0.73 g) in 2 ml of acetonitrile was added drop-wise, while stirring, to 0.01 mole of 2,4-difluoroaniline (1.29 g),dissolved in 15 ml of acetonitrile at room temperature. After stirring for 20 minutes, the precipitated 2,4-difluoroaniline hydrochloride was filtered off and washed with 10 ml portions of acetonitrile, and the combined filtrates were concentrated under reduced pressure. 8.7g (73.10% yield) of white crystalline N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide were thus obtained. Crystals of the title compound suitable for X-ray diffraction were obtained by slow evaporation of a toluene solution.

Refinement top

H atoms of the water molecule were located in a difference Fourier map and refined as riding with O—H = 0.85 Å, with Uiso(H) = 1.5 Ueq. Carbon and nitrogen bound H atoms were placed at calculated positions and were treated as riding on the parent C or N atoms with C—H = 0.96 (methyl), 0.93 (methylene) and N—H = 0.88 Å, Uiso(H) = 1.2 or 1.5 Ueq(C, N). The methyl H atoms are treated as disordered groups over two sites with a refined site-occupancy ratio of 0.48: 0.52 (6). The hydrogen atoms involving the hydrogen bonding interaction perform not well when freely refined and it can not provide a reasonable basis for discussion of hydrogen bonding interaction

Structure description top

Leflunomide is one of the most effective isoxazole-containing disease-modifying drugs for treating rheumatoid arthritis (Shaw, et al., 2011; Schattenkirchner, 2000). Many leflunomide analogs have been synthesized and exhibit potent immunomodulating effect (Huang, et al., 2003). The title compound, N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide monohydrate, was synthesized as a novel and potent immunomodulating leflunomide analog. We report herein its crystal structure.

As illustrated in Fig. 1, the molecular structure of the title compound is not planar and consists of one N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide molecule and one solvate water molecule. The C1-C6 benzene and the C8-C10/N2/O2 isoxazole ring is almost coplannar with each other with the dihedral angle of 8.08 (3) °. The central nitrogen atom (N1) and carbon atom (C7) are nearly coplanar with the benzene ring and the isoxazole rings[N1-C6-C5-C4 torsion angles = -178.8 (2) ° and C7-C8-C10-O2 torsion angles = 177.4 (2) °], respectively. The length of the C9=N2 double bond is 1.296 (3) Å, slightly longer than standard 1.28 Å value of a C=N double bond. The crystal structure is stabilized by O—H···N and N—H···O hydrogen bonds among the solvate water molecules, amide group and isoxazole nitrogen atoms (Table 1). It is noted that the connections of water molecules and N-(2,4-difluorophenyl)-5-methylisoxazole-4-carboxamide molecules generate a tape with R44(16) pattern based on graph set analysis nomenclature (Bernstein, et al., 1995).

For applications of leflunomide [systematic name: 5-methyl-N-[4-(trifluoromethyl) phenyl]-isoxazole-4-carboxamide] in the treatment of rheumatoid arthritis, see: Shaw et al. (2011); Schattenkirchner (2000); For leflunomide analogs, see: Huang et al. (2003). For graph-set motifs, see: Bernstein, et al. (1995).

Computing details top

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); 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: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The structure of the title compound, showing the atomic numbering scheme. Non-H atoms are shown with 30% probability displacement ellipsoids.
N-(2,4-Difluorophenyl)-5-methyl-1,2-oxazole-4-carboxamide hemihydrate top
Crystal data top
C11H8F2N2O2·0.5H2OF(000) = 1016
Mr = 247.20Dx = 1.489 Mg m3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 25 reflections
a = 15.182 (3) Åθ = 9–13°
b = 13.803 (3) ŵ = 0.13 mm1
c = 12.159 (2) ÅT = 293 K
β = 120.06 (3)°Block, white
V = 2205.3 (8) Å30.30 × 0.20 × 0.10 mm
Z = 8
Data collection top
Enraf–Nonius CAD-4
diffractometer
1486 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.050
Graphite monochromatorθmax = 25.2°, θmin = 2.1°
ω/2θ scansh = 018
Absorption correction: multi-scan
(North et al., 1968)
k = 016
Tmin = 0.962, Tmax = 0.987l = 1412
2076 measured reflections3 standard reflections every 200 reflections
1997 independent reflections intensity decay: 1%
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.050H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.130 w = 1/[σ2(Fo2) + (0.060P)2 + 1.6P]
where P = (Fo2 + 2Fc2)/3
S = 1.03(Δ/σ)max < 0.001
1997 reflectionsΔρmax = 0.28 e Å3
168 parametersΔρmin = 0.18 e Å3
0 restraintsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.0247 (19)
Crystal data top
C11H8F2N2O2·0.5H2OV = 2205.3 (8) Å3
Mr = 247.20Z = 8
Monoclinic, C2/cMo Kα radiation
a = 15.182 (3) ŵ = 0.13 mm1
b = 13.803 (3) ÅT = 293 K
c = 12.159 (2) Å0.30 × 0.20 × 0.10 mm
β = 120.06 (3)°
Data collection top
Enraf–Nonius CAD-4
diffractometer
1486 reflections with I > 2σ(I)
Absorption correction: multi-scan
(North et al., 1968)
Rint = 0.050
Tmin = 0.962, Tmax = 0.9873 standard reflections every 200 reflections
2076 measured reflections intensity decay: 1%
1997 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0500 restraints
wR(F2) = 0.130H atoms treated by a mixture of independent and constrained refinement
S = 1.03Δρmax = 0.28 e Å3
1997 reflectionsΔρmin = 0.18 e Å3
168 parameters
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. 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 > 2sigma(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
xyzUiso*/UeqOcc. (<1)
O1W0.50000.52321 (17)0.25000.0620 (7)
H1W0.505 (2)0.564 (2)0.193 (3)0.093*
N10.65863 (15)0.36286 (13)0.32726 (18)0.0484 (5)
H1A0.6314 (18)0.4180 (19)0.332 (2)0.058*
O10.66094 (13)0.20060 (11)0.36060 (16)0.0611 (5)
F10.88573 (17)0.40738 (15)0.0910 (2)0.1141 (8)
C10.75004 (19)0.28877 (17)0.2259 (2)0.0584 (6)
H1B0.73360.22630.23810.070*
F20.71615 (12)0.53616 (10)0.28913 (16)0.0739 (5)
O20.49066 (13)0.27282 (12)0.53567 (16)0.0615 (5)
N20.51344 (18)0.37236 (15)0.5487 (2)0.0658 (6)
C20.8072 (2)0.3024 (2)0.1673 (3)0.0720 (8)
H2B0.82930.24950.14030.086*
C30.8304 (2)0.3937 (2)0.1496 (3)0.0720 (8)
C40.80009 (19)0.47406 (19)0.1878 (2)0.0649 (7)
H4A0.81620.53620.17420.078*
C50.74512 (18)0.45851 (16)0.2468 (2)0.0530 (6)
C60.71701 (16)0.36750 (15)0.2668 (2)0.0461 (5)
C70.63608 (16)0.28191 (15)0.37245 (19)0.0454 (5)
C80.57905 (16)0.30012 (15)0.43950 (19)0.0450 (5)
C90.56469 (19)0.38588 (17)0.4907 (2)0.0556 (6)
H9A0.58930.44600.48380.067*
C100.53140 (17)0.23129 (16)0.4714 (2)0.0492 (6)
C110.5167 (2)0.12579 (18)0.4509 (3)0.0674 (7)
H11A0.47390.10280.48270.101*0.48 (6)
H11B0.58150.09380.49500.101*0.48 (6)
H11C0.48500.11210.36170.101*0.48 (6)
H11D0.54230.10490.39700.101*0.52 (6)
H11E0.44540.11090.41120.101*0.52 (6)
H11F0.55260.09290.53110.101*0.52 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O1W0.0925 (18)0.0490 (13)0.0742 (16)0.0000.0640 (15)0.000
N10.0634 (12)0.0366 (9)0.0596 (11)0.0022 (8)0.0415 (10)0.0026 (8)
O10.0854 (12)0.0377 (9)0.0823 (12)0.0037 (8)0.0584 (10)0.0019 (8)
F10.1517 (18)0.1052 (14)0.1635 (19)0.0058 (12)0.1374 (17)0.0148 (13)
C10.0760 (16)0.0463 (12)0.0695 (15)0.0001 (11)0.0489 (14)0.0005 (11)
F20.0977 (11)0.0427 (8)0.1108 (12)0.0031 (7)0.0743 (10)0.0029 (7)
O20.0787 (11)0.0558 (10)0.0700 (11)0.0022 (8)0.0522 (9)0.0004 (8)
N20.0953 (16)0.0509 (12)0.0720 (14)0.0040 (11)0.0574 (14)0.0064 (10)
C20.092 (2)0.0656 (17)0.0854 (18)0.0076 (14)0.0646 (17)0.0003 (14)
C30.0829 (19)0.0760 (18)0.0865 (19)0.0005 (15)0.0644 (17)0.0076 (15)
C40.0736 (17)0.0581 (15)0.0808 (18)0.0062 (13)0.0518 (15)0.0070 (13)
C50.0591 (13)0.0448 (12)0.0642 (14)0.0004 (10)0.0376 (12)0.0015 (10)
C60.0505 (12)0.0457 (12)0.0474 (12)0.0001 (10)0.0283 (11)0.0022 (9)
C70.0525 (13)0.0397 (11)0.0467 (12)0.0004 (9)0.0268 (10)0.0004 (9)
C80.0548 (13)0.0427 (12)0.0415 (11)0.0009 (9)0.0270 (10)0.0009 (9)
C90.0759 (16)0.0490 (13)0.0541 (13)0.0033 (11)0.0418 (13)0.0016 (10)
C100.0572 (13)0.0459 (13)0.0509 (12)0.0001 (10)0.0318 (11)0.0007 (10)
C110.0874 (19)0.0469 (14)0.0852 (19)0.0087 (12)0.0561 (16)0.0020 (12)
Geometric parameters (Å, º) top
O1W—H1W0.92 (3)C3—C41.368 (4)
N1—C71.362 (3)C4—C51.363 (3)
N1—C61.409 (3)C4—H4A0.9300
N1—H1A0.88 (3)C5—C61.387 (3)
O1—C71.215 (2)C7—C81.478 (3)
F1—C31.360 (3)C8—C101.363 (3)
C1—C21.384 (3)C8—C91.404 (3)
C1—C61.389 (3)C9—H9A0.9300
C1—H1B0.9300C10—C111.475 (3)
F2—C51.354 (3)C11—H11A0.9600
O2—C101.344 (3)C11—H11B0.9600
O2—N21.406 (3)C11—H11C0.9600
N2—C91.299 (3)C11—H11D0.9600
C2—C31.355 (4)C11—H11E0.9600
C2—H2B0.9300C11—H11F0.9600
C7—N1—C6126.79 (19)C4—C5—C6123.9 (2)
C7—N1—H1A117.3 (16)C5—C6—C1116.6 (2)
C6—N1—H1A115.8 (16)C5—C6—N1117.51 (19)
C2—C1—C6120.6 (2)C1—C6—N1125.85 (19)
C2—C1—H1B119.7O1—C7—N1123.6 (2)
C6—C1—H1B119.7O1—C7—C8121.66 (19)
C10—O2—N2108.88 (17)N1—C7—C8114.70 (18)
C9—N2—O2105.04 (18)C10—C8—C9103.82 (19)
C3—C2—C1119.3 (2)C10—C8—C7125.4 (2)
C3—C2—H2B120.4C9—C8—C7130.7 (2)
C1—C2—H2B120.4N2—C9—C8112.8 (2)
C2—C3—F1119.4 (3)N2—C9—H9A123.6
C2—C3—C4122.8 (2)C8—C9—H9A123.6
F1—C3—C4117.8 (2)O2—C10—C8109.43 (19)
C5—C4—C3116.7 (2)O2—C10—C11116.39 (19)
C5—C4—H4A121.6C8—C10—C11134.2 (2)
C3—C4—H4A121.6H11D—C11—H11E109.5
F2—C5—C4118.5 (2)H11D—C11—H11F109.5
F2—C5—C6117.62 (19)H11E—C11—H11F109.5
C10—O2—N2—C91.0 (3)C6—N1—C7—O13.4 (4)
C6—C1—C2—C30.2 (4)C6—N1—C7—C8176.0 (2)
C1—C2—C3—F1179.4 (3)O1—C7—C8—C1013.8 (3)
C1—C2—C3—C40.3 (5)N1—C7—C8—C10166.8 (2)
C2—C3—C4—C50.6 (4)O1—C7—C8—C9162.3 (2)
F1—C3—C4—C5179.7 (3)N1—C7—C8—C917.1 (3)
C3—C4—C5—F2178.2 (2)O2—N2—C9—C80.6 (3)
C3—C4—C5—C61.6 (4)C10—C8—C9—N20.0 (3)
F2—C5—C6—C1178.2 (2)C7—C8—C9—N2176.8 (2)
C4—C5—C6—C11.6 (4)N2—O2—C10—C81.0 (3)
F2—C5—C6—N11.3 (3)N2—O2—C10—C11178.9 (2)
C4—C5—C6—N1178.9 (2)C9—C8—C10—O20.6 (2)
C2—C1—C6—C50.7 (4)C7—C8—C10—O2177.6 (2)
C2—C1—C6—N1179.9 (2)C9—C8—C10—C11179.2 (3)
C7—N1—C6—C5169.5 (2)C7—C8—C10—C112.2 (4)
C7—N1—C6—C19.9 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O1W0.88 (3)2.26 (3)3.052 (3)150 (2)
O1W—H1W···N2i0.92 (3)2.03 (3)2.934 (2)167 (3)
Symmetry code: (i) x, y+1, z1/2.

Experimental details

Crystal data
Chemical formulaC11H8F2N2O2·0.5H2O
Mr247.20
Crystal system, space groupMonoclinic, C2/c
Temperature (K)293
a, b, c (Å)15.182 (3), 13.803 (3), 12.159 (2)
β (°) 120.06 (3)
V3)2205.3 (8)
Z8
Radiation typeMo Kα
µ (mm1)0.13
Crystal size (mm)0.30 × 0.20 × 0.10
Data collection
DiffractometerEnraf–Nonius CAD-4
Absorption correctionMulti-scan
(North et al., 1968)
Tmin, Tmax0.962, 0.987
No. of measured, independent and
observed [I > 2σ(I)] reflections
2076, 1997, 1486
Rint0.050
(sin θ/λ)max1)0.599
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.050, 0.130, 1.03
No. of reflections1997
No. of parameters168
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.28, 0.18

Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O1W0.88 (3)2.26 (3)3.052 (3)150 (2)
O1W—H1W···N2i0.92 (3)2.03 (3)2.934 (2)167 (3)
Symmetry code: (i) x, y+1, z1/2.
 

Acknowledgements

The work was supported by the Center for Testing and Analysis, Nanjing University.

References

First citationBernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573.  CrossRef CAS Web of Science Google Scholar
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