Synthesis, characterization, and crystal structures of new 3,5-diaryl-1H-pyrazoles

Article abstract of DOI:10.1016/j.molstruc.2009.10.001

Seven new 3,5-diaryl-1H-pyrazoles were synthesized by the condensation of 1,3-diketones with hydrazine in ethanol, and characterized. The crystal structures for 3(5)-(4-tert-butylphenyl)-5(3)-(4-fluorphenyl)-1H-pyrazole (5) and 3-(4-tert-butylphenyl)-5-(6

Full text of DOI:10.1016/j.molstruc.2009.10.001

Journal of Molecular Structure 938 (2009) 311–315
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, characterization, and crystal structures of new 3,5-diaryl-1H-pyrazoles
*
Dun-Jia Wang , Chun-Yang Zheng, Ling Fan
Department of Chemistry and Environmental Engineering, Hubei Normal University, No. 82 Cihu Road, Huangshi 435002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven new 3,5-diaryl-1H-pyrazoles were synthesized by the condensation of 1,3-diketones with hydra-
zine in ethanol, and characterized. The crystal structures for 3(5)-(4-tert-butylphenyl)-5(3)-(4-fluorphe-
nyl)-1H-pyrazole (5) and 3-(4-tert-butylphenyl)-5-(6-methoxy-naphthalen-2-yl)-1H-pyrazole (7) have
been determined by X-ray crystal structure analysis. The N–H. . .N intermolecular hydrogen bonds join
the molecules into dimers. The NH proton of 5 is disordered, and its dimer is formed by the tautomers
5a5b or 5b5a. Compound 7 only exists as a 7a tautomer, and its dimer consists of the tautomers 7a7a.
Ó 2009 Elsevier B.V. All rights reserved.
Received 13 August 2009
Received in revised form 1 October 2009
Accepted 2 October 2009
Available online 12 October 2009
Keywords:
3,5-Diaryl-1H-pyrazoles
1,3-Diketones
Synthesis
Crystal structure
Hydrogen bonds
1. Introduction
measured on a Varian Mercury-Plus 400 NMR nuclear magnetic res-
onance instrument in CDCl3 solution with TMS as internal standard.
The pyrazole derivatives are well known for their wide range of
biological and pharmacological activities, such as antibacterial,
fungicidal, herbicidal, insecticidal, and other biological activities
[1–3]. They are also useful intermediates for many industrial prod-
ucts [4,5]. On the other hand, the crystal structures of N-unsubsti-
tuted pyrazoles have received much attention due to their variety
of packing forms, cyclic dimers, trimers, tetramers and linear
chains, all of which involve N–H. . .N hydrogen bonding [6–8]. In
this paper, some new 3,5-diaryl-1H-pyrazoles were prepared by
the reaction of 1,3-diketones with hydrazine monohydrate,
and characterized by elemental analysis, FTIR, 1H NMR and
ESI-MS. The X-ray diffraction structures of the compounds 3(5)-
(4-tert-butylphenyl)-5(3)-(4-fluorphenyl)-1H-pyrazole (5) and 3-
(4-tert-butylphenyl)-5-(6-methoxy-naphthalen-2-yl)-1H-pyrazole
(7) were determined. In the solid state, the molecules are con-
nected by N–H. . .N intermolecular hydrogen bonds to form cyclic
dimers.
Infrared spectra (4000–400 cmÀ1) were recorded on a Nicolet FTIR
5700 spectrophotometer with KBr pellets. Low-resolution electro-
spray ionization mass spectra (ESI-MS) were obtained on a Finnigan
LCQ ADVANTAGE MAX spectrometer. Melting points were deter-
mined using X-4 digital melting point apparatus and uncorrected.
2.2. Synthesis
The synthesis of compounds 1–7 was conducted as outlined in
Scheme 1. Double condensation of 1,3-diketone compounds with
hydrazine monohydrate in ethanol afforded 3,5-dissubstituted-
1H-pyrazoles in moderate to good yields, as shown in Table 1. All
products were characterized by element analysis, FTIR, 1H NMR
and ESI-MS.
2.2.1. General synthesis for 3,5-diaryl-1H-pyrazoles (1–7)
The required 1,3-diketone (0.01 mol) was dissolved in hot eth-
anol (30 mL). To this solution hydrazine monohydrate (0.01 mol)
in ethanol was added and the mixture was refluxed under stirring
for 3 h. After completion of the reaction (detected by thin layer
chromatography, TLC), the solvent was removed by evaporation
under reduced pressure to obtain the solid mixture, which was
recrystallized from ethanol solution to give the corresponding
3,5-disubstituted-1H-pyrazole.
2. Experimental
2.1. Materials and methods
The required 1,3-diketone compounds were prepared by our
group[9]. Other reagents used were of analytical grade and without
further purification. Elemental analysis (C, H, N) was performed
using a Perkin-Elmer 2400 elemental analyzer. 1H NMR spectra were
2.2.2. 3-(Furan-2-yl)-5-(naphthalen-2-yl)-1H-pyrazole (1)
White solid, yield 72%, mp 167–168 °C; IR: (KBr, cmÀ1) 3132(b,
s), 3056(s), 2970(s), 2919(s), 1582(m), 1561(s), 1513(s), 1452(m),
* Corresponding author.
E-mail address: dunjiawang@163.com (D.-J. Wang).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.10.001

312
D.-J. Wang et al. / Journal of Molecular Structure 938 (2009) 311–315
794(s), 736(m); 1H NMR (CDCl3, 400 Hz): d = 1.34(s, 9H, C(CH3)3),
2
5
6.78(s, 1H, pyrazolyl C–H), 7.04–7.09(m, 2H, Ar–H), 7.44(d, 2H,
Ar–H, J = 8.4 Hz), 7.64(d, 2H, Ar–H, J = 8.4 Hz), 7.71–7.75(m, 2H,
Ar–H) ppm, pyrazolyl N–H not found; ESI-MS: m/z 295.1
[M + 1]+; Anal. Calcd. for C19H19FN2: C, 77.52; H, 6.51; N, 9.52;
Found C, 77.46; H, 6.43; N, 9.45.
2
2
1
2
2
1
Scheme 1. Reaction of 1,3-diketones with hydrazine.
2.2.7. 3-(4-Tert-butylphenyl)-5-(naphthalen-2-yl)-1H-pyrazole (6)
White solid, yield 70%, mp 201 °C; IR: (KBr, cmÀ1) 3229(b,s),
3051(s), 2962(s), 2865(m), 1582(m), 1500(s), 1448(s), 1362(m),
1267(s), 1175(m), 1051(s), 966(s), 835(s), 793(s), 750(s); 1H NMR
(CDCl3, 400 Hz): d = 1.34(s, 9H, C(CH3)3), 5.20(bs, 1H, N–H),
6.98(s, 1H, pyrazolyl C–H), 7.44–7.78(m, 4H, Ar–H), 7.71(d, 2H,
Ar–H, J = 8.4 Hz), 7.70–7.89(m, 4H, Ar–H), 8.21(s, 1H, Ar–H) ppm;
ESI-MS: m/z 652.9 [2 M]+, 327.2 [M + 1]+; Anal. Calcd. for
C23H22N2: C, 84.63; H, 6.79; N, 8.58; Found C, 84.29; H, 6.70; N,
8.42.
Table 1
Preparation of 3,5-disubstituted-1H-pyrazoles.
1H-Pyrazole R1
R2
Mp (°C)
Yield (%)
1
2
3
4
5
6
7
C4H3O–
2–C10H7–
167–168 72
C4H3O–
C6H4–
C6H4–
6–CH3O–2–C10H6– 188–189 73
2–C10H7– 119–120 76
6–CH3O–2–C10H6– 212–213 71
4–C(CH3)3–C6H4– 4–F–C6H4–
4–C(CH3)3–C6H4– 2–C10H7–
4–C(CH3)3–C6H4– 6–CH3O–2–C10H6– 244
209–210 56
201
70
67
2.2.8. 3-(4-Tert-butylphenyl)-5-(6-methoxynaphthalen-2-yl)-1H-
1222(m), 1169(m), 1061(m), 1012(s), 972(s), 896(s), 787(s), 748(s),
727(s); 1H NMR (CDCl3, 400 Hz): d = 6.47–6.48 (m, 1H, furanyl C–
H), 6.68 (bs, 1H, N–H), 6.82(d, 1H, furanyl C–H, J = 2.8 Hz), 6.94(s,
1H, pyrazolyl C–H), 7.42(s, 1H, furanyl C–H), 7.48–7.51(m, 2H,
Ar–H), 7.81–7.87(m, 4H, Ar–H), 8.23(s, 1H, Ar–H) ppm; ESI-MS:
m/z 261.1 [M + 1]+; Anal. Calcd. for C17H12N2O: C, 78.44; H, 4.65;
N, 10.76; Found C, 78.62; H, 4.56; N, 10.64.
pyrazole (7)
Colorless needles, yield 67%, mp 244 °C; IR: (KBr, cmÀ1
)
3215(b,s), 3056(m), 2963(s), 2868(m), 1614(s), 1585(s), 1507(s),
1445(s), 1393(s), 1261(s), 1168(s), 1029(s), 963(s), 859(s), 794(s);
1H NMR (CDCl3, 400 Hz): d = 1.34(s, 9H, C(CH3)3), 3.92(s, 3H,
CH3O), 5.60(bs, 1H, N–H), 6.94(s, 1H, pyrazolyl C–H), 7.00(s, 1H,
Ar–H), 7.10–7.13(m, 1H, Ar–H), 7.45(d, 2H, Ar–H, J = 8.4 Hz),
7.60(d, 1H, Ar–H, J = 8.4 Hz), 7.69(d, 2H, Ar–H, J = 8.8 Hz), 7.76(d,
2H, Ar–H, J = 8.4 Hz), 8.10(s, 1H, Ar–H) ppm; ESI-MS: m/z 712.9
[2 M]+, 357.2 [M + 1]+; Anal. Calcd. for C24H24N2O: C, 80.87; H,
6.79; N,7.86; Found C, 80.64; H, 6.65; N, 7.92.
2.2.3. 3-(Furan-2-yl)-5-(6-methoxynaphthalen-2-yl)-1H-pyrazole (2)
Colorless needles, yield 73%, mp 188–189 °C; IR: (KBr, cmÀ1
)
3215(b,s), 3054(m), 2958(m), 2853(m), 1610(m), 1565(s),
1507(m), 1486(m), 1452(s), 1391(s), 1261(s), 1173(s), 1028(s),
973(s), 907(s), 851(s), 793(s), 729(s); 1H NMR (CDCl3, 400 Hz):
d = 3.94(s, 3H, CH3O), 6.48(s, 1H, furanyl C–H), 6.78(s, 1H, furanyl
C–H), 6.88(s, 1H, pyrazolyl C–H), 7.14(d, 1H, Ar–H, J = 3.2 Hz),
7.16(d, 1H, Ar–H, J = 2.4 Hz), 7.43(s, 1H, furanyl C–H), 7.72–
7.81(m, 3H, Ar–H), 8.11(s, 1H, Ar–H) ppm, pyrazolyl N–H not
found; ESI-MS: m/z 580.8 [2 M]+, 291.2 [M + 1]+; Anal. Calcd. for
C18H14N2O2: C, 74.47; H, 4.86; N, 9.65; Found C, 74.61; H, 4.82;
N, 9.55.
2.3. X-ray crystal structure determination
Suitable crystals of compounds 5 and 7 for X-ray diffraction
experiments were grown by slow evaporation from CH2Cl2–EtOH
Table 2
Crystal data and structure refinements for 5 and 7.
Compound
5
7
2.2.4. 5-(Naphthalen-2-yl)-3-phenyl-1H-pyrazole (3)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C19H19FN2
294.36
298(2)
Monoclinic
C2/c
19.9638(7)
17.7081(6)
9.7166(3)
90
110.4120(10)
90
3219.33(19)
8
1.215
0.080
C24H24N2O
356.45
298(2)
Monoclinic
C2/c
34.9071(10)
6.15110(10)
21.7645(4)
90
123.620(2)
90
3891.51(15)
4
1.217
0.075
White solid, yield 76%, mp 119–120 °C; IR: (KBr, cmÀ1
)
3228(b,s), 3036(m), 3002(m), 1596(m), 1503(s), 1456(s), 1268(s),
1170(m), 1054(s), 965(s), 830(s), 792(s), 754(s), 695(s); 1H NMR
(CDCl3, 400 Hz): d = 6.35(bs, 1H, N–H), 7.01(s, 1H, pyrazolyl C–H),
7.34–7.49(m, 5H, Ar–H), 7.78–7.84(m, 6H, Ar–H), 8.21(s, 1H, Ar–
H) ppm; ESI-MS: m/z 271.2 [M + 1]+; Anal. Calcd. for C19H14N2: C,
84.42; H, 5.22; N, 10.36; Found C,83.92; H, 5.16; N, 10.18.
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
2.2.5. 5-(6-Methoxynaphthalen-2-yl)-3-phenyl-1H-pyrazole (4)
Volume (Å3)
White solid, yield 71%, mp 212–213 °C; IR: (KBr, cmÀ1
)
Z
Density (calc.) (mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
3202(b,s), 3060(m), 3001(m), 2937(m), 1608(m), 1566(s),
1498(s), 1460(s), 1390(s), 1260(s), 1222(s), 1174(s), 1028(s),
963(s), 862(s), 795(s), 769(s), 700(s); 1H NMR (CDCl3, 400 Hz):
d = 3.93(s, 3H, CH3O), 5.80(bs, 1H, N–H), 6.97(s, 1H, pyrazolyl C–
H), 7.01(s, 1H, Ar–H), 7.12–7.14(m, 1H, Ar–H), 7.62(d, 2H, Ar–H,
J = 8.8 Hz), 7.70(d, 2H, Ar–H, J = 8.8 Hz), 7.74(d, 2H, Ar–H,
J = 8.8 Hz), 7.82(d, 2H, Ar–H, J = 7.2 Hz), 8.10(s, 1H, Ar–H) ppm;
ESI-MS: m/z 600.9 [2 M]+, 301.4 [M + 1]+; Anal. Calcd. for
C20H16N2O: C, 79.98; H, 5.37; N, 9.33; Found C, 80.13; H, 5.28; N,
9.27.
)
1248
1520
Crystal size (mm3)
Theta range for data
0.20 Â 0.10 Â 0.10
0.13 Â 0.10 Â 0.10
1.58 to 28.29
1.40 to 26.00
Collection (°)
h/k/l
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
À26.26/À22.23/À12.8 À42.40/À7.7/À26.26
11163 19456
3796[R(int) = 0.0858] 3819[R(int) = 0.0932]
3796/2/210
0.923
3819/0/248
1.028
Final R indices [I > 2
r
(I)]
R
1 = 0.0575
R
1 = 0.0588
wR2 = 0.1403
wR2 = 0.1611
R indices (all data)
R
1 = 0.0985
R
1 = 0.0774
2.2.6. 3(5)-(4-Tert-butylphenyl)-5(3)-(4-fluorphenyl)-1H-pyrazole (5)
wR2 = 0.1606
wR2 = 0.1713
Colorless needles, yield 56%, mp 209–210 °C; IR: (KBr, cmÀ1
)
Largest diff. peak and
hole (eÅÀ3
0.174 and À0.205
0.308 and À0.239
3214(b,s), 3055(m), 2964(s), 2868(m), 1608(m), 1502(s), 1447(s),
1364(m), 1263(m), 1217(s), 1158(s), 1049(s), 971(s), 838(s),
)

D.-J. Wang et al. / Journal of Molecular Structure 938 (2009) 311–315
313
3.1. Spectral characterization
In IR spectra, compounds 1–7 showed broad bands around
3300–3100 cmÀ1
, strong absorption bands in the regions of
1
2
1
2
1618–1582 cmÀ1 and 1513–1498 cmÀ1 and Medium bands from
1061 to 1028 cmÀ1 indicating the presence of the characteristic
peaks for 1H-pyrazole compounds [13,14].
a
b
Scheme 2. Tautomerism of 3,5-disubstituted-1H-pyrazole.
In 1H NMR (CDCl3), the title compounds showed the aromatic
protons at d = 6.47–8.23 ppm and a typical proton chemical shift
of 3,5-disubstituted-1H-pyrazole (4-H) at d = 6.78–7.01 ppm. In
addition, the N–H resonance of the pyrazole ring of compounds
1, 3, 4, 6, 7 appeared as a broad and flat signal at d = 5.20–
6.68 ppm, but for compounds 2 and 5 the signal corresponding to
the N–H resonance was not observed. Apparently, there is a fast
tautomerism of the 1H-pyrazoles in CDCl3 solution [15,16]
(Scheme 2). This observation is in accordance with the crystal
structures of compound 5.
The mass spectroscopy by ESI-MS showed the molecular ions
([M + 1]+) of all 3,5-disubstituted pyrazoles. However, compounds
2, 4, 6, 7 also displayed their double molecular ions (2 M+) attrib-
utable to the dimers of 3,5-disubstituted-1H-pyrazoles. This result
is in agreement with the X-ray crystal structures of compound 7.
(1:2) solutions. Single crystals were selected and mounted on the
top of the glass fibres. X-ray single-crystal diffraction measure-
ment was carried out at 298(2) K on a Bruker Smart 1000 CCD area
diffractometer equipped with graphite monochromatic MoKa radi-
ation (k = 0.71073 Å) for data collection. The unit cell dimensions
were obtained with the least-squares refinements and the struc-
tures were solved by direct methods with SHELXTL-97 program
[10]. The final refinement was performed by full-matrix least-
squares techniques with anisotropic thermal parameters for the
non-hydrogen atoms on F2. All the H atoms were placed in the cal-
culated positions and constrained to ride on their parent atoms.
Multi-scan absorption correction was applied by using the SADABS
program [11]. The crystallographic data and refinement informa-
tion for 5 and 7 are summarized in Table 2.
3. Results and discussion
3.2. Crystal structures
Compounds 1–7 were synthesized from the corresponding 1,3-
diketone compounds according to the published procedures [1,12].
Nucleophilic attack of hydrazine monohydrate with the required
1,3-diketone precursor in hot ethanol in each case led to formation
of the required 3,5-disubstituted-1H-pyrazole, with isolation of the
product involving a solvent evaporation step followed by recrystal-
lization from ethanol solution. The yields were 56–76%.
Compounds 5 and 7 crystallize in the same space group, C2/c
(Table 2). Their molecular structures with the atom-numbering
scheme and unit cells packing figures are shown in Figs. 1–4, and
selected bond lengths and angles are given in Table 3.
The molecules 5 and 7 consist of three ring systems, a pyrazole
ring and two aryl rings. The dihedral angles formed by two ben-
zene rings (C1–C6) and (C10–C15) with the pyrazole ring are both
Fig. 1. View of 5 with the atom-labeling scheme and 50% probability displacement ellipsoids.
Fig. 2. View of 7 with the atom-labeling scheme and 50% probability displacement ellipsoids.

314
D.-J. Wang et al. / Journal of Molecular Structure 938 (2009) 311–315
Table 3
Selected bond lengths (Å) and angles (°) for 5 and 7.
Compound
Bond lengths
(Å)
Angles
(°)
5
N1–N2
C9–N2
C8–C9
C7–C8
C7–N1
1.351(2)
1.3339(19)
1.391(2)
1.383(2)
1.339(2)
C7–N1–N2
C9–N2–N1
N2–C9–C8
C9–C8–C7
C8–C7–N1
108.93(14)
109.18(14)
107.58(15)
106.58(15)
107.73(15)
7
N1–N2
1.3549(18)
1.3471(19)
1.367(2)
1.392(2)
1.339(2)
C12–N1–N2
C14–N2–N1
N2–C14–C13
C14–C13–C12
C13–C12–N1
112.91(13)
104.63(12)
110.08(14)
107.00(14)
105.37(14)
C12–N1
C13–C12
C14–C13
C14–N2
ers 5a5b or 5b5a. The measured N1. . .N1 distance is 2.874(3) Å,
showing strong intermolecular N–H. . .N bonding interactions.
The C7–N1 and C9–N2 bond lengths are 1.339(2) Å and
1.3339(19) Å, respectively, displaying double-bond character. This
provides additional evidence to the disordered proton of the pyra-
zole ring.
However, the molecular structure for compound 7 only exists as
the 7a tautomer, and its dimer is formed by the tautomers 7a7a
(Figs. 2 and 4). The N–H bond of the pyrazole system resides at
the nitrogen atom N1, adjacent to the naphthyl group. The ob-
served N. . .N distance of 2.871(2) Å is typical for N–H. . .N hydro-
gen bond. The bond lengths of C12–N1 and C14–N2 are
1.3471(19) Å and 1.339(2) Å, respectively.
Fig. 3. The crystal structure of 5, showing the packing mode of the dimers.
Hydrogen atoms are omitted for clarity.
In conclusion, the analysis of the X-ray crystal structure data of
compounds 5 and 7 shows that their molecules form dimmers as a
25.71° for 5, but two benzene rings are not planar, those of two aryl
rings (C2–C11) and (C15–C20) with the pyrazole ring are 9.07° and
15.01° for 7, respectively. The molecules are linked together by
pairs of intermolecular N–H. . .N hydrogen bonds to form the di-
mers [16] (Table 4). The crystals packing are stabilized by van
der Waals forces (Figs. 3 and 4).
Table 4
Hydrogen-bonding geometry for 5 and 7.
D–H. . .A
D–H (Å)
H. . .A (Å)
D. . .A (Å)
D–H. . .A (°)
5
7
N1–H1A. . .N1i
N1–H1. . .N2ii
0.85(2)
0.86(3)
2.17(2)
2.15(2)
2.874(3)
2.871(2)
140(3)
141(2)
In compound 5, the N–H hydrogen linkage of the pyrazole ring
is disordered (Figs. 1 and 3), and its dimer is formed by the tautom-
Symmetry codes: (i) 1 À x, y, Àz + 3/2; (ii) 1 À x + 1/2, Ày + 1/2, Àz + 1.
Fig. 4. The crystal structure of 7, showing the packing mode of the dimers. Hydrogen atoms are omitted for clarity.

D.-J. Wang et al. / Journal of Molecular Structure 938 (2009) 311–315
315
result of intermolecular N–H. . .N hydrogen bonding. In compound
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Probably, the different tautomerism in compounds 5 and 7 owe
to the influence of fluoride atom on the benzene ring.
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4. Supplementary material
Crystallographic data for structures 5 and 7 have been depos-
ited at the Cambridge Crystallographic Data Center, CCDC Nos.
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this information may be obtained free of charge from the Director,
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336033; email: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
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Acknowledgement
This work was supported by the important Foundation of the
Educational Commission of Hubei Province, PR China (No.
Q200822002).
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