Synthesis and crystal structure of 3,5-diacetyl-4-methylpyrazole

Article abstract of DOI:10.1002/jhet.411

(Chemical Equation Presented) The 3,5-diacetyl-4-methylpyrazole diketone has been synthesized and its crystal structure has been determined. This diketone reacts with hydroxylamine hydrochloride to give the dioxime derivative. This reaction, conducted in

Full text of DOI:10.1002/jhet.411

September 2010 Synthesis and Crystal Structure of 3,5-Diacetyl-4-methylpyrazole
1025
a
´ ´
Moha Outirite, Fouad Bentiss, Eric Buisine, Frederic Capet, and
b
a
a
a
´
Michel Lagrenee *
a
´
Unite de Catalyse et de Chimie du Solide, CNRS UMR 8181, ENSCL, B.P. 90108,
F-59652 Villeneuve d’Ascq Cedex, France
Laboratoire de Chimie de Coordination et d’Analytique, Faculte des Sciences,
b
´
Universite Chouaib Doukkali, B.P. 20, M-24000 El Jadida, Morocco
´
*E-mail: michel.lagrenee@ensc-lille.fr
Received July 9, 2009
DOI 10.1002/jhet.411
Published online 13 July 2010 in Wiley Online Library (wileyonlinelibrary.com).
The 3,5-diacetyl-4-methylpyrazole diketone has been synthesized and its crystal structure has been
determined. This diketone reacts with hydroxylamine hydrochloride to give the dioxime derivative. This
reaction, conducted in presence of copper II ions, leads to the formation of L2M2 copper II complexes.
J. Heterocyclic Chem., 47, 1025 (2010).
INTRODUCTION
ing modes as shown in Scheme 1 (type 2). In type 2
complexes, the metal atom can bind either to the N
atom, either to the deprotonated O-atom of the oxime
moiety thus forming a six-membered ring. In this pur-
pose, we have already employed pyridazine and pyrazo-
late units, such as 3,6-diformylpyridazines [10,11], 3,6-
dibenzoylpyridazines [10], and 3,5-diacetyl-4-methylpyr-
azole [12] and related dioxime species. The synthesis,
crystal structure, and magnetic properties of two binu-
clear copper (II) complexes of the 3,5-diacetyl-4-methyl-
pyrazole dioxime (dampdoH3), involving the two bridg-
ing modes of the oxime function has been also reported
[13]. In the present study, we describe the synthesis and
crystal structure of the 3,5-diacetyl-4-methylpyrazole
ligand used in a new one-step preparation of L2M2 cop-
per II complexes.
Research on transition metal complexes is a flourish-
ing field with major interests focused on biological
mimicry and cooperative phenomena in catalysis and
magnetism. In this respect, pyrazol-based ligands have
received a great deal of attention over the last decades
due to their interesting coordination chemistry [1]. The
pyrazole entity appears indeed particularly suited owing
to its ability to bind simultaneously two metal centers in
close proximity providing moreover an intramolecular
pathway for spin-exchange interactions [2–9]. Further
control of the metal–metal separation as well as the
steric and electronic properties of the individual metal
ions can be achieved by appropriates chelating side
arms. Attached to the 3 and 5 positions of the hetero-
cycle, these side arms are able to afford bis(l-pyrazo-
lato) bridged species L2M2.
In most cases, e.g. in type 1 complexes, the chelated
metal atoms form only five-membered rings involving
the four nitrogen atoms of the two depronated pyrazole
rings and the four chelating heteroatoms bore by the
3,5-substituted side arms as shown in Scheme 1 [4–9].
Interestingly, the use of functional groups such as
oximes as chelating side arms has a singular attractive-
ness. Oximes are indeed not only generally easy to pre-
pare but can also take advantage of two different bridg-
RESULTS AND DISCUSSION
The 3,5-diacetyl-4-methylpyrazole is prepared by the
method of Wolff [12]. Reduced by Zn/H2SO4, the isoni-
trosoacetylacetone (2) gives the aminoacetylacetone (3).
This compound is then diazoted by sodium nitrite in
acid medium (Scheme 2), affording the crude diazoace-
tylacetone (4) as an oily product which was in fact
expected to be the 4-acetyl-5-methyl-oxadiazole (40).
C
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M. Outirite, F. Bentiss, E. Buisine, F. Capet, and M. Lagrenee
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Scheme 1. L2M2 dinuclear complexes of pyrazolate anion.
To provide insight into the thermodynamics and
kinetics of the possible internal cyclization of diazoace-
tylacetone (4) affording 4-acetyl-5-methyl-1,2,3-oxadia-
zole (40), theoretical calculations were carried out at the
B3LYP/6-31þg(d) DFT level. A thorough inspection of
the frontier orbitals (HOMO and LUMO) of diazoacety-
Scheme 2. Synthesis of 3,5-diacetyl-4-methylpyrazole.
Figure 1. Isodensity surface plots of the HOMO (a) and LUMO (b)
for the lowest-energy gas-phase geometry of diazoacetylacetone (4).
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
lacetone (4) was first performed. The HOMO and
LUMO for its most stable geometry in gas-phase show
energy values of ꢀ7.5 and ꢀ2.7 eV, respectively. As
depicted in Figure 1, the two frontier orbitals are distrib-
uted in the molecular plane. Although HOMO is mainly
composed by lone pairs of the two carbonyl oxygen
atoms, LUMO is an antibonding-p orbital located exclu-
sively around the NBN triple bond. This orbital distri-
bution suggests that the unique pathway for the internal
cyclization of diazoacetylacetone (4) should reside in
the attack of one of carbonyl oxygen atom lone pairs at
the p-orbital located on the terminal nitrogen atom of
the diazonium moiety.
Figure 2. Perspective view of dampH structure. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

September 2010
Synthesis and Crystal Structure of 3,5-Diacetyl-4-methylpyrazole
1027
Scheme 3. Synthesis of dampdoH3.
Figure 3. Projection view of dampH structure. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
The optimized geometry for transition state joining
compounds (4) and (40) on the potential energy surface
has a single imaginary frequency (mi ¼ 163 cmꢀ1) corre-
sponding to the stretching vibration of the forming
NAO bond. The distance of the NAO forming bond in
The layered crystal structure is also stabilized by an
intermolecular hydrogen bond network forming dimeric
ligand associations. The hydrogen bond interactions
between two coplanar and adjacent 3,5-diacetyl-4-meth-
ylpyrazole ligands are mediated by a water molecule.
The oxygen atom O3 of this water molecule gives an
hydrogen bond with the hydrogen atom H1 bound to the
nitrogen atom of pyrazole ring in the first ligand
˚
transition state geometry is 1.902 A. The reaction from
diazoacetylacetone (4) leading to the formation of oxa-
diazole (40) is predicted slightly endothermic (þ7.3 kcal/
mol in gas-phase and 8.5 kcal/mol in water). Diazoace-
tylacetone (4) is then thermodynamically more stable
than 4-acetyl-5-methyl-1,2,3-oxadiazole (40).
˚
(O3AH1 ¼ 1.9571 A). Furthermore, one of the two
hydrogen atoms H1w of the water molecule is also
involved in a second hydrogen bond interaction with the
oxygen atom O2 of the ketonic function of the second
Furthermore, the barrier height calculated from diazoace-
tylacetone (4) is 8.0 kcal/mol (9.3 kcal/mol in water) indi-
cating that formation of 4-acetyl-5-methyl-1,2,3-oxadiazole
(40) is not kinetically favored, at least at ambient tempera-
ture. This product is thus very unstable and must react im-
mediately with carbanion of the 2,4-pentanedione as previ-
ously described [14]. This condensation and the subsequent
heterocyclization give a pyrazoline (6), which was not iso-
lated. In aqueous acidic medium, this compound gives the
pyrazole (7) after elimination of acetic acid as described in
Scheme 2. Slow evaporation of an ethanolic solution of 7
gives finally single crystals suitable for X-ray diffraction.
The main features of the crystal structure of com-
pound dampH (7) result from the planarity of the whole
entity (Figs. 2 and 3).
˚
ligand molecule (H1wAO1 ¼ 1.9207 A) (Fig. 4).
This diketone (7) reacts with hydroxylamine hydro-
chloride to give the diacetyl dioxime derivative (damp-
doH3) as described in Scheme 3. On reaction with cop-
per II cations dampdoH3 leads to the bis(l-pyrazolato)
bridged L2M2 species [13] as described in Scheme 4,
with L ¼ dampdoH.
From dampdoH3 (H2L) and CuCl2ꢁ6H2O, two struc-
turally isomeric complexes [Cu2(HL)2Cl2]6H2O, which
differ only from the hydrogen bonding, were previously
obtained [10]. One of these complexes can be obtained
directly from 7 by a one-step reaction with copper II
ions in the presence of hydroxylamine hydrochloride.
This reaction using CuCl2, CuClO4, Cu(NO3)2, or
CuSO4, leads, after several days, to the formation of
dark green crystals of the copper II complexes previ-
ously described [13]. In this complex, the axial ligand is
a chloride ion even in presence of perchlorate, nitrate,
or sulphate ions, indicating that chloride ions are better
complexing species.
The crystal structure is formed by layers of planar
dampH molecules stacked along the axis c direction.
Scheme 4. Dinuclear complex of Cu2(HL)22þ
.
Figure 4. View of dampH crystal structure with its stabilizing hydro-
gen bond network (dashed lines). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Journal of Heterocyclic Chemistry
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M. Outirite, F. Bentiss, E. Buisine, F. Capet, and M. Lagrenee
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Table 1
Summary of the crystal data and structure refinement for dampH.
Table 2
ꢂ
˚
Bond distances (A) and angles ( ) for dampH.
Formula
C8H12N2O3
184.2
Bond distances
Formula weight (g/mol)
Crystal system
Space group
N1AN2
N2AC5
N1AC2
C1AC7
C1AC2
C2AC3
1.327(2)
1.351(2)
1.363(2)
1.498(3)
1.468(2)
1.395(2)
C3AC4
C5AC3
C5AC6
C6AO2
C6AC8
O1AC1
1.492(2)
Orthorhombic
Pbcm
1.407(2)
1.476(2)
1.225(2)
1.490(3)
1.209(2)
˚
a (A)
a ¼ 7.7798 (10)
b ¼ 18.337 (3)
c ¼ 6.7735 (11)
90
˚
b (A)
˚
c (A)
a ¼ b ¼ c (ꢂ)
Angles
3
˚
V(A )
966.3 (2)
4
1.266
N2AN1AC2
N1AN2AC5
O1AC1AC2
O1AC1AC7
C2AC1AC7
N1ACAC1
C3AC2AC1
N1AC2AC3
C2AC3AC5
113.0(1)
104.6(1)
120.1(1)
121.8(2)
118.1(2)
122.1(1)
131.1(1)
106.8(1)
103.5(1)
C2AC3AC4
C5AC3AC4
N2AC5AC3
N2AC5AC6
C3AC5AC6
O2AC6AC5
C5AC6AC8
O2AC6AC8
128.0(1)
128.5(1)
112.1(1)
118.4(1)
129.5(1)
120.4(2)
117.8(1)
121.8(2)
Z
Dcal (mg mꢀ3
T (K)
F(000)
)
296
392
Crystal size (mm)
)
0.180 ꢃ 0.254 ꢃ 0.628
l (mmꢀ1
Range of indices
0.098
h, ꢀ11 to 11;
k, ꢀ26 to 23;
l, ꢀ9 to 9
13460
Reflections measured
Rint
(Rall; R2r
(wRall; wR2r
Number of parameters
2.64
dampH. A single crystal of compound 7 was mounted on a
)
7.43; 4.89
15.96; 14.08
88
Brucker AXS SMART three-circle diffractometer using graph-
˚
0.71073 A),
)
ite monochromated MoKa radiation (k
¼
equipped with a CCD 4K two-dimensional detector [19]. A
total of 1653 independent reflections were collected in the
range of 2.22 < y < 31.03 with Rint ¼ 2.64. Data were cor-
rected for Lorentz and polarization using program SaintPlus
(Table 1). Atomic positions were determined using SHELXS
[20] and refined using full matrix least squares [21]. Hydrogen
positions were calculated and included in the final cycles of
refinement in constrained positions and with fixed isotropic
ꢀ3
˚
˚
Maximum peak in final DF map (eAꢀ3
)
0.28
ꢀ0.15
Minimum peak in final DF map (eA
)
EXPERIMENTAL
Melting points were determined on an IA 9000 series Elec-
trothermal apparatus and are uncorrected. Elemental analyses
of C, H, N, and S were performed at the Elemental Analysis
service of CNRS, Vernaison, France. H and 13C NMR spectra
1
Table 3
were recorded on a Bruker F.T. AC 300 spectrometer (300
MHz for 1H NMR and 75 MHz for 13C NMR) using chloro-
form (CDCl3) as solvent. Matrix-assisted laser desorption ioni-
zation (MALDI) and time-of-flight mass spectrometry (TOF-
MS) are used to record the mass spectra of the correspondent
compounds. All starting materials were of reagent grade and
were used as purchased.
Atomic coordinates and equivalent displacement for dampH.
2
U (A )
˚
Atom
x/a
y/b
z/c
N1
N2
0.6307(2)
0.5408(2)
0.9273(2)
0.8038(2)
0.8288(2)
0.9949(2)
0.6606(2)
0.6021(2)
0.8583(3)
0.4134(3)
0.0799(2)
0.07067(2)
0.3918(2)
0.5845
0.15617(7)
0.21765(7)
0.10700(9)
0.16751(8)
0.24286(9)
0.28354(9)
0.27101(8)
0.34760(9)
0.03073(9)
0.36152(9)
0.11936(9)
0.39751(6)
0.04499(7)
0.1136
1/4
1/4
0.0547(4)
0.0557(4)
0.0610(5)
0.0521(4)
0.0501(4)
0.0599(5)
0.0507(4)
0.0570(5)
0.0774(7)
0.0828(7)
0.1014(7)
0.0772(5)
0.0941(7)
0.0660
C1
C2
1/4
1/4
All theoretical calculations were performed using the Gaus-
sian 03 suite of programs [15]. The gas-phase geometries of
diazoacetylacetone (4), 4-acetyl-5-methyl-1,2,3-oxadiazole (40),
and related transition state were optimized using hybrid den-
sity functional theory (B3LYP) [16] and the 6-31þg(d) basis
set. Frequency calculations were used to verify the nature of
the computed stationary points. Geometries obtained for 4 and
40 were real minima with all real vibrational frequencies while
that of transition state was indeed a first-order saddle point
with a single imaginary frequency. To ensure that the transi-
tion state joins the two compounds (4) and (40) on the potential
energy surface, an intrinsic reaction coordinate (IRC) [17]
analysis was performed. Frequency calculations also allowed
us to calculate the zero-point energy (ZPE) corrections, which
were finally added to electronic energies. The energies com-
puted in water include the ZPE-corrected electronic energy
plus the solvation energy as obtained from the polarizable con-
tinuum model (PCM) [18].
C3
C4
1/4
1/4
C5
C6
1/4
1/4
C7
C8
O1
O2
1/4
1/4
1/4
1/4
1/4
1/4
0.1660
0.2020
0.3820
0.1725
0.1945
0.3830
0.3780
0.2191
0.1528
1/4
O3
HN1
HC4
HC4
HC4
HC1
HC1
HC1
HC6
HC6
HC6
Hw1
Hw2
0.9853
0.0848
0.3255
0.2524
0.0900
0.0900
0.0215
0.3566
0.2989
0.3249
0.0900
0.1240
0.3910
0.3710
0.4087
0.3599
0.1240
0.1240
0.8124
0.9492
0.0195
ꢀ0.0028
0.1160
0.1160
0.7689
0.363(3)
0.294(4)
0.0266
ꢀ0.002(2)
0.067(2)
0.1160
0.089(8)
0.101(9)
Single crystals of 3,5-diacetyl-4-methylpyrazole hydrated
(dampH) were obtained in the form of brown needles, by slow
evaporation at room temperature in aqueous solution of
1/4
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

September 2010
Synthesis and Crystal Structure of 3,5-Diacetyl-4-methylpyrazole
1029
Table 4
Anisotropic displacement parameters for dampH.
U11
U22
U33
U12
U13
U23
N1
N2
C1
C2
C3
C4
C5
C6
C7
C8
O1
O2
O3
0.0412(7)
0.0408(6)
0.0485(9)
0.0418(8)
0.0388(7)
0.0405(8)
0.0407(7)
0.0481(8)
0.0737(13)
0.0491(10)
0.0444(7)
0.0588(8)
0.0448(7)
0.0322(6)
0.0358(6)
0.0408(8)
0.0353(7)
0.0364(7)
0.0453(9)
0.0330(7)
0.0353(8)
0.0366(9)
0.0503(10)
0.0057(8)
0.0342(6)
0.0333(7)
0.0907(11)
0.0905(11)
0.0935(14)
0.0794(12)
0.0752(11)
0.0939(13)
0.0786(11)
0.0876(13)
0.1218(19)
0.0149(2)
0.0204(2)
0.1385(14)
0.0204(2)
0.0037(5)
0.0018(5)
0.0056(7)
0.0010(6)
0.0033(6)
0.0089(6)
0.0024(5)
0.0012(6)
0.0058(8)
0.0116(8)
0.0107(6)
0.0035(5)
0.0008(5)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
thermal parameters. Absorption corrections were not made due
to the small value of the absorption coefficients (Table 1).
Extinction was refined for all the three structures but was min-
imal. Figure 2 shows a perspective view of this compound
with the numbering scheme, while in Table 2 bond distances
and angles in the molecule (dampH) are reported. Atomic
coordinates and equivalent displacement for dampH are
reported in Table 3; anisotropic displacement parameters for
dampH are reported in Table 4.
General procedure for the synthesis of compounds 1, 4,
and 7. Starting material acetylacetone (1) was commercialized.
The formula of the parent compounds with corresponding
numbers to carbons scheme is given below.
crude powder was used for the next step for the synthesis
without further purification.
Synthesis of 3,5-diacetyl-4-methylpyrazole (7). A mixture
of the acetylacetonate diazonium (4) (2 g, 0.016 mol), acetyl-
acetone (1) (1.6 g, 0.015 mol), and sodium hydroxide (0.6 g,
0.015 mol) in water (30 mL) was heated at 50ꢂC for 90 min.
The resulting solution was concentrated under vacuum using a
rotatory evaporator. After cooling, the obtained solution was
neutralized by slow addition of aqueous sulfuric acid solution
(0.5M). The obtained solid was filtered and recrystallized from
ethanol: mp 112ꢂC; yield 2 g, 75 %; 1H NMR (CDCl3) d
(ppm) 10.78 (s, 1H); 2.51 (s, 6H); 2.46 (s, 3H); 13C NMR
(CDCl3)
d (ppm) 192.96; 144.94; 121.58; 28.57; 10.21.
MALDI-TOFMS: m/z 167 (M þ1). Anal. Calcd for
C8H10N2O2: C, 57.82; H, 6.08; N, 16.86. Found: C, 58.02; H,
5.98; N, 16.75.
Synthesis of 3,5-diacetyl-4-methylpyrazole dioxime. A so-
lution of hydroxylamine hydrochloride (1.4 g, 20 mmol) and
NaOH (0.8 g, 20 mmol) in methanol/water solution (in ratio
50:50) was added to a solution of 3,5-diacetyl-4-methylpyra-
zole (1.5 g, 9 mmol) in water (20 mL) and heated under reflux
for 2 h. After cooling, the solid product was filtered and
recrystalized from ethanol: mp 218ꢂC; yield 81%; 1H NMR
(DMSO-d6) d (ppm) 12.96 (s, H, NH); 11.33 (s, 1H, OH);
10.97 (s, 1H, OH); 2.27 (s, 3H); 2.14 (s, 6H); 13C NMR
(DMSO-d6) d (ppm) 150.39; 141.51; 134.62; 111.88; 11.70.
Synthesis of isonitroacetylacetone (2). A suspension of ac-
etylacetone (1) (50 g, 0.5 mol) in 7% sulphuric acid (500 mL)
was stirred until it was completely dissolved. Sodium nitrite
(35 g, 0.5 mol) in water (150 mL) was added, and the stirring
was continued for 90 min. The reaction mixture was extracted
with ether, dried (magnesium sulfate), filtered, and evaporated.
The solid was recrystallized from ethanol: mp 75ꢂC, yield 50
g, 77.5%.
MALDI-TOFMS: m/z 197 (M
C8H12N4O2: C, 48.97; H, 6.16; N, 28,56. Found: C, 49.02; H,
5.98; N, 28.78.
þ
1). Anal. Calcd for
Preparation of complexes. A solution of copper II chlo-
ride dehydrate (85 mg, 0.5 mmol) and hydroxylamine hydro-
chloride (69,5 mg, 1 mmol) in water (20 mL) was added to a
solution of dampH (92 mg, 0.5 mmol) in ethanol (10 mL)
and heated under reflux a few minutes. The mixture was fil-
tered to remove any precipitated material. Slow evaporation
at room temperature during 3 days affords dark green crys-
tals which were filtered, washed with water and dried in
vacuo. The crystals were found to have the formula
[Cu2(dampdoH)2Cl2)2H2O] yield 110 mg; 70%. Anal. Calcd:
C, 30.75; H, 4.20; Cl, 11.35; Cu, 20.35; N, 17.95; O, 15.40
Found: C, 30.69; H, 4.21; Cl, 11.37; Cu, 20.29; N, 18.00; O,
15.44.
Synthesis
of
the
acetylacetonate
diazonium
(4). Isonitrosoacetylacetone 2 (10 g, 0.07 mol) dissolved in
30% sulphuric acid (100 mL) was cooled under 0ꢂC. Zinc
powder (15 g, 0.23 mol) was added, and the reaction mixture
was allowed for 15 min. The limpid solution was filtered and
diluted with water. A solution of sodium nitrite (6 g, 0.085
mol) in water (30 mL) was added and cooled. The reaction
mixture was extracted with ether, and treated with 5% sodium
carbonate. The residue was dried (magnesium sulfate), filtered,
and evaporated to give an oil product: yield 4 g, 45%. The
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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M. Outirite, F. Bentiss, E. Buisine, F. Capet, and M. Lagrenee
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´
[11] Abraham, F.; Lagrenee, M.; Sueur, S.; Mernari, B.; Bre-
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