Structural tautomerism of 4-acylpyrazolone schiff bases and crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one

Article abstract of DOI:10.1007/s10947-009-0170-4

The Schiff base derivatives prepared from 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one and alkyl amines are shown to remain exclusively in the amine-one(I) tautomeric form in chloroform solutions at room temperature using a combination of ¹

Full text of DOI:10.1007/s10947-009-0170-4

Journal of Structural Chemistry. Vol. 50, No. 6, pp. 1159-1165, 2009
Original Russian Text Copyright © 2009 by A. S. Amarasekara, O. S. Owereh, K. A. Lyssenko, and T. V. Timofeeva
STRUCTURAL TAUTOMERISM OF 4-ACYLPYRAZOLONE
SCHIFF BASES AND CRYSTAL STRUCTURE
OF 5-METHYL-2-PHENYL-4-{1-[(PYRIDIN-2-YLMETHYL)-
AMINO]-ETHYLIDENE}-2,4-DIHYDRO-PYRAZOL-3-ONE
A. S. Amarasekara,1 O. S. Owereh,1 K. A. Lyssenko,2
and T. V. Timofeeva3
UDC 548.737
The Schiff base derivatives prepared from 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one and alkyl
amines are shown to remain exclusively in the amine-one(I) tautomeric form in chloroform solutions at
1
room temperature using a combination of H, 13C, APT, COSY, HMQC, and HMBC NMR spectroscopic
methods. The crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-
dihydro-pyrazol-3-one showed that this 4-acylpyrazolone Schiff base stays in the amine-one(I) form in the
solid state as well, and the solid state structure supports the fact that strong hydrogen bonding between
amine hydrogen and the pyrazolone C3 carbonyl oxygen helps to stabilize the amine-one(I) tautomer.
Keywords: 4-acylpyrazolone, Schiff-base, tautomerism.
INTRODUCTION
4-Acylpyrazolones are a well known class of E-diketone ligands forming complexes with various metal ions [1]. The
lanthanide complexes of 4-acylpyrazolones are renowned for their interesting fluorescence properties [2, 3] and have recently
attracted renewed attention due to useful applications in biochemistry [4], electronics and in spent fuel re-processing
technology. On the other hand the chemistry of the Schiff base derivatives of 4-acylpyrazolones are less explored, even
though these ligands are also known to show appealing complexation properties [5-8]. Furthermore, the Schiff base formation
extend the scope of the classical E-diketone system and provides an opportunity for constructing tridentate and tetradentate
ligands from the initial bidentate 4-acylpyrazolone. There are limited attempts to produce tridentate [9-12] and tetradentate
[7, 8] ligands from 4-acyl pyrazolone Schiff base derivatives and synthesis of their complexes. Jia has reported [10] the
preparation of a tridentate salicylidene hydrazone derivative of 4-acylpyrazolone, which form complexes with trivalent
lanthanides Sm, Eu and Gd, all showing blue fluorescence in the solid state. The synthesis of symmetrical tetradentate ligands
are also reported with 4-acylpyrazolone system using diamines such as 1,2-ethylenediamine and 1,2-phenylenediamine and
these ligands are shown to complex with Cr(III) [7] and Ni(II) [8]. Thiosemicarbazone of 4-acylpyrazolones are another
interesting group of Schiff base derivatives showing photochromism and acidichromism properties [13-15] due to
tautomerism in their molecular structures. The pyrazolones and 4-acylpyrazolones are known to exhibit interesting keto-enol
1Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA;
2
asamarasekara@pvamu.edu. Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia.
3Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701, USA. The text was submitted
by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 50, No. 6, pp. 1213-1218, November-December, 2009. Original
article submitted December 21, 2008.
0022-4766/09/5006-1159 © 2009 Springer Science+Business Media, Inc.
1159

Fig. 1. Structural tautomerism of 4-acylpyrazolone Schiff base 1 with N-alkyl group.
Fig. 2. Synthesis of 4-acylpyrazolone Schiff bases 1a-e.
tautomerism, and in principle Schiff base derivatives of 4-acylpyrazolones can exist in five possible tautomeric forms; imine-
ol, imine-one(I), imine-one(II), amine-one(I) and amine-one(II) forms as shown in Fig. 1. The solution state tautomeric
molecular structures are usually assigned by using NMR methods and this can be often difficult due to simultaneous presence
of several tautomeric forms or observation of average signals due to rapid interconversions. Holzer has recently reported [16]
a spin coupling constant based approach for the assignment of tautomeric forms of 4-acylpyrazolones, excluding the Schiff
base derivatives.
During our studies on multidentate 4-acylpyrazolone Schiff base ligands grafted on silica surfaces and their
complexation with lanthanides, we required the synthesis of a variety of 4-acylpyrazolone Schiff base derivatives.
Identification of the possible tautomeric forms is an important aspect in the understanding of the molecular structures of the
multidentate ligands with selective chelation characteristics, and as far as we are aware the tautomerism has not been studied
in detail for the 4-acylpyrazolone Schiff base systems formed from the reaction between 4-acylpyrazolones and alkylamines.
Therefore we have undertaken to study the structures of these compounds by spectroscopic and X-ray crystallography
methods to explore the tautomerism of these multidente systems. In this publication we describe the synthesis of five
4-acylpyrazolone Schiff base compounds by condensation of 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one with
different alkylamines (Fig. 2) and their solution state structure determination using NMR methods. Furthermore, we report
the solid state structure determination of the multidentate ligand 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-
ethylidene}-2,4-dihydro-pyrazol-3-one using single crystal X-ray crystallography to support the structural data obtained using
spectroscopic methods.
EXPERIMENTAL
1
Instrumentation and measurements. H NMR Spectra were recorded in CDCl3 on a Varian Mercury plus
spectrometer operating at 400 MHz and chemical shifts are given in ppm downfield from TMS (G = 0.00). 13C NMR spectra
in CDCl3 were recorded in the same spectrometer operating at 100 MHz; chemical shifts were measured relative to CDCl3
and converted to G(TMS) using G(CDCl3) = 77.00. FT-IR spectra were recorded on a JASCO-470 PLUS IR spectrometer
1160

using KBr pellets. A Bruker SMART APEX2 CCD diffractometer was used for the single crystal X-ray crystallography.
Elemental analysis were performed at QTI laboratories, New Jersey.
General procedure for the preparation of 4-acylpyrazolone Schiff bases (1a-e). A mixture of 4-acylpyrazolone
[17] 2 (0.432 g, 2.0 mmol) and amine 3a-c (2.0 mmol) in 10 mL of ethanol was refluxed for 30 minutes. Upon cooling of the
solution 4-acylpyrazolone Schiff bases were separated as microcrystalline precipitates after one day at room temperature. The
crude products were recrystallized with ethanol to give pure 4-acylpyrazolone Schiff bases. Compounds 1d,e were prepared
by using excess (20 mmol) of the corresponding amines (3d,e) to prevent the formation of bis-Schiff base products and the
desired products were isolated after removal of excess amine under reduced pressure in the rotavapor.
1a 0.453 g, 74% yield. Found: C, 70.35; H, 6.03; N, 18.20%. Calc. for C18H18N4O: C, 70.57; H, 5.92; N, 18.29%. IR
(KBr) 653 cm–1, 695 cm–1, 752 cm–1, 831 cm–1, 1009 cm–1, 1127 cm–1, 1351 cm–1, 1485 cm–1, 1588 cm–1, 1641 cm–1,
2975 cm–1, 3060 cm–1, and 3406 cm–1.
1b 0.422 g, 82% yield. Found: C, 69.85; H, 7.54; N, 16.15%. Calc. for C15H19N3O: C, 70.01; H, 7.44; N, 16.33%. IR
(KBr) 652 cm–1, 694 cm–1, 760 cm–1, 839 cm–1, 1092 cm–1, 1352 cm–1, 1491 cm–1, 1535 cm–1, 1592 cm–1, 1637 cm–1,
2893 cm–1, and 2910 cm–1.
1c 0.368 g, 71% yield. Found: C, 64.62; H, 6.82; N, 16.00%. Calc. for C14H17N3O2: C, 64.85; H, 6.61; N, 16.20%.
IR (KBr) 652 cm–1, 696 cm–1, 761 cm–1, 839 cm–1, 1085 cm–1, 1353 cm–1, 1491 cm–1, 1535 cm–1, 1592 cm–1, 1630 cm–1,
2893 cm–1, 2919 cm–1, and 3452 cm–1.
1d 0.372 g, 72% yield. Found: C, 64.95; H, 7.11; N, 21.62 %. Calc. for C14H18N4O: C, 65.09; H, 7.02; N, 21.69%.
IR (KBr) 652 cm–1, 694 cm–1, 758 cm–1, 836 cm–1, 1349 cm–1, 1488 cm–1, 1535 cm–1, 1591 cm–1, 1635 cm–1, 2855 cm–1,
2920 cm–1, and 3392 cm–1.
1e 0.370 g, 68% yield. Found: C, 66.27; H, 7.55; N, 20.30 %. Calc. for C15H20N4O: C, 66.15; H, 7.40; N, 20.57%.
IR (KBr) 653 cm–1, 693 cm–1, 758 cm–1, 836 cm–1, 1350 cm–1, 1488 cm–1, 1535 cm–1, 1590 cm–1, 1635 cm–1, 2853 cm–1,
2926 cm–1, and 3389 cm–1.
1H and 13C NMR data of compounds 1a-e are shown in Tables 1 and 2.
Crystal structure determination of compound 1a. The single crystals suitable for X-ray crystallography were
obtained from the slow evaporation of a solution of 1a in ethanol at room temperature. Crystals of 1a C18H18N4O,
M = 306.36) are monoclinic, space group P21/n, at 100 K: a = 11.7945(19) Å, b = 6.7731(11) Å, c = 18.975(3) Å,
E = 97.507(3)q, V = 1502.8(4) Å3, Z = 4 (Zc = 1), dcalc = 1.354 gcm–3, P(MoKD) = 0.88 cm–1, F(000) = 648. Intensities of
13517 reflections were measured with a Bruker SMART APEX2 CCD diffractometer [O(MoKD) = 0.71072 Å, Z-scans,
2T < 58q] and 3609 independent reflections [Rint = 0.0759] were used in further refinement. The structure was solved by
direct method and refined by the full-matrix least-squares technique against F2 in the anisotropic-isotropic approximation.
The H atom of the –NH group was localized in the difference Fourier map and refined isotropically. The other H atoms were
placed in calculated positions and refined within riding model with fixed thermal parameters. For 1a the refinement
converged to wR2 = 0.1513 and GOOF = 1.027 for all independent reflections (R1 = 0.0650 was calculated against F for
2126 observed reflections with I > 2V(I)). Data reduction and further calculations were performed using the Bruker SAINT+
[18] and the SHELXTL NT [19] program packages. CCDC 704439 contains supplementary crystallographic data for this
paper.
RESULTS AND DISCUSSION
Identification of the tautomeric structures of 1a-e. Solution state structure of the compound 1a in CDCl3 was
1
identified as the 1a amine-one(I) form by using a combination of H, 13C, APT, COSY, HMQC, and HMBC NMR
spectroscopic methods. The 1H NMR spectrum of 1a displayed a two proton doublet (J = 6.0 Hz) at G 4.75 corresponding to
the CH2 group attached to the pyridine ring. The single proton broad peak at G 12.00 was assigned to the NH group and the
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TABLE 1. 1H NMR Data of Compounds 1a-e (CDCl3, 23qC)
N–H
N–Ph
N–R
2uCH3
1a
2.39 (s)
2.41 (s)
12.00 (bs)
7.12 (t, J = 7.2 Hz, 1H)
7.37 (t, J = 7.2 Hz, 2H)
7.98 (d, J = 7.2 Hz, 2H)
4.75 (d, J = 6.0 Hz, 2H),
7.23 (d,d, J = 4.8, 7.6 Hz, 1H)
7.29 (d, J = 7.6 Hz, 1H)
7.69 (d,d,d, J = 2.0, 7.6, 7.6 Hz, 1H)
8.60 (d,d, J = 2.0, 4.8 Hz)
1.03 (t, J = 7.6 Hz, 3H)
1.72 (m, 2H)
1b
1c
1d
1e
2.33 (s)
2.38 (s)
11.50 (bs)
11.56 (bs)
11.85 (bs)
11.50 (bs)
7.13 (t, J = 7.2 Hz, 1H)
7.37 (t, J = 7.2 Hz, 2H)
7.97 (d, J = 7.2 Hz, 2H)
7.14 (t, J = 7.2 Hz, 1H)
7.39 (t, J = 7.2 Hz, 2H)
7.96 (d, J = 7.2 Hz, 2H)
7.12 (t, J = 7.2 Hz, 1H)
7.37 (t, J = 7.2 Hz, 2H)
7.98 (d, J = 7.2 Hz, 2H)
7.12 (t, J = 7.2 Hz, 1H)
7.37 (t, J = 7.2 Hz, 2H)
7.97 (d, J = 7.2 Hz, 2H)
3.36 (m, 2H)
2.24 (s)
2.36 (s)
1.68 (bs, 1H)
3.53 (m, 2H)
3.83 (m, 2H)
2.39 (s)
2.41 (s)
0.45 (bs, 1H)
3.04 (m, 2H)
3.49 (m, 2H)
2.39 (s)
2.40 (s)
1.39 (bs, 1H)
1.87 (m, 2H)
2.87 (t, J = 6.8 Hz, 2H)
3.55 (m, 2H)
TABLE 2. 13C NMR Data of Compounds 1a-e (CDCl3, 23qC)
C3
C4
C5
=C–N
99.9
N–Ph
N–R
2uCH3
1a
1b
1c
1d
1e
16.2
17.9
15.8
17.8
16.0
17.8
16.0
17.9
15.8
17.9
165.5
147.1
166.0
119.5, 124.4
128.9, 139.4
119.5, 124.4
128.9, 139.4
119.6, 124.6
129.0, 139.3
119.5, 124.4
128.9, 139.4
119.6, 124.4
128.9, 139.4
48.9, 121.5, 123.1
137.5, 150.1, 155.9
11.6, 23.0, 45.3
165.1
165.7
165.3
165.3
146.9
147.4
146.9
146.9
165.9
166.2
165.9
166.9
99.9
99.3
99.4
99.1
45.9, 61.3
41.5, 46.4
32.9, 39.3, 41.1
1H–1H COSY spectrum of 1a showed a cross peak indicating the coupling of these CH2 and NH protons. The 13C NMR
spectrum exhibited 16 carbons, assigned by APT experiment as six quaternary sp2, one CH2, two CH3 and eight CH carbons.
The HMQC experiment was used to establish that three CH carbon signals at G 119.5, 128.9, and 124.4 corresponds to the
phenyl ring. Furthermore, the carbon resonances at G 121.5, 123.1, 137.5, and 150.1 were assigned to the pyridine ring CH
carbons by using the 1H–13C cross-peaks in the HMQC spectrum. The quaternary signals were assigned by using the HMBC
1162

Fig. 3. Single crystal X-ray structure of 1a, representation of atoms by
thermal ellipsoids (p = 50%).
spectrum; for instance sp2 quaternary carbon signal at G 155.9 showed three distinct cross peaks with proton signals at G 4.75
(CH2), 7.29 (C-3, Py), and 7.69 (C-4, Py) thus confirming as C-2 carbon of the pyridine ring. The sp2 quaternary carbon
signal at 139.4 showed two cross peaks with G 7.98 (C-2, C-6, Ph) and 7.37 (C-3, C-5, Ph) in the HMBC spectrum, therefore
G 139.4 peak was assigned to C-1 in the phenyl ring. The carbon -13 signal at G 99.9 was attributed to the olefinc carbon
attached to NH (=C–NH) as this signal indicated the coupling to G 4.75 (N–CH2) and one of the methyl group signals (G 2.39)
in the HMBC spectrum. The imine carbon in the pyrazolone ring was assigned to G 166.0 signal in the carbon spectrum,
based on the fact that this carbon showed one cross peak to the directly attached methyl group at G 2.41 in the HMBC
spectrum. The only remaining quaternary carbon signal at G 165.5 which did not show any long range couplings in the
HMBC spectrum was assigned to the C-3 carbonyl carbon of the pyrazolone ring.
The tautomeric structures of the related compounds 1b–e were also established as amine-one(I) forms by
1
comparison of the H and 13C NMR data shown in Tables 1 and 2. For instance, the C3, C4 and C5 pyrazolone ring carbon
atoms resonates at G 165.5, 147.5, G 165.5, 147.1, and 166.0 in the compound 1a and these 13C resonances are observed in the
ranges of G 165.1-165.7, 146.9-147.4, and 165.9-166.9 respectively for related compounds 1b-e (Table 2). In addition to this
1H NMR signal of the hydrogen bonded amine N–H of the compound 1a was observed as a broad peak at G 12.00 and
thecorresponding hydrogen signals of compounds 1b-e were found as similar broad peaks in the G 11.50-12.00 region
(Table 1), further supporting the structural similarity of the five Schiff bases studied. It is interesting to note that a possible
weaker hydrogen bonding from the nitrogen (1a, d, e) or oxygen (1c) atoms in the alkyl group of the Schiff base moieties has
little or no effect on the geometry and the tautometric form of these molecules. Furthermore, no NMR signals from other four
possible tautomeric forms were observed for any of the compounds studied, confirming that compounds 1a-e exclusively
remain in the amine-one(I) tautomeric form in the chloroform solution.
Identification of the solid state structure of 1a. The solid state structure of 1a obtained by single crystal X-ray
crystallography is shown in Fig. 3. This structure clearly points out that compound 1a exists as the amine-one(I) tautomeric
form in the solid state, which is the same tautomeric form found in the solution state. Structural data in Table 3 show that the
O(1)–C(3) bond distance in the pyrazolone system is 1.253 Å, which is comparable to the pyrazolone C=O bond distance of
1.254 Å found in a similar pyrazolone thiosemicarbazone [20]. The significantly longer N(3)–C(6) bond distance of 1.328 Å
indicates a C–N single bond rather than C=N imine form, further this bond distance is comparable to reported C–N bond
distances of similar compounds [13]. The double bond character in the C(4)–C(6) bond is indicated by the relatively short
1.400 Å bond distance. The solid state structure of 1a showed an inter-atomic distance between hydrogen
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TABLE 3. Selected Bond Distances and Bond Angles for 1a
Bond length
Å (esd)
Bond length
Å (esd)
Bond length
Å (esd)
O(1)–C(3)
N(1)–C(5)
N(1)–N(2)
N(2)–C(3)
1.253(3)
1.309(3)
1.403(2)
1.393(3)
N(2)–C(13)
N(3)–C(6)
N(3)–C(7)
N(3)–H(3N)
1.417(3)
1.328(3)
1.452(3)
0.91(3)
C(3)–C(4)
C(4)–C(6)
C(4)–C(5)
C(7)–C(8)
1.438(3)
1.400(3)
1.435(3)
1.514(3)
Bond angle
deg (esd)
Bond angle
deg (esd)
Bond angle
deg (esd)
C(5)–N(1)–N(2)
C(3)–N(2)–N(1)
C(3)–N(2)–C(13)
C(6)–N(3)–C(7)
106.28(18)
111.81(18)
128.92(19)
125.2(2)
C(6)–N(3)–H(3N)
C(7)–N(3)–H(3N)
O(1)–C(3)–N(2)
O(1)–C(3)–C(4)
115.4(16)
119.3(16)
125.6(2)
129.9(2)
N(2)–C(3)–C(4)
C(6)–C(4)–C(5)
C(6)–C(4)–C(3)
C(5)–C(4)–C(3)
104.44(18)
132.3(2)
122.1(2)
105.58(19)
TABLE 4. Hydrogen Bonding Distances Å (esd) and Angles q (esd)
D–H…A
d(D–H)
d(H…A)
d(D…A)
‘(DHA)
N(3)–H(3N)…O(1)
N(3)–H(3N)…N(4)
0.90(2)
0.90(2)
1.98(2)
2.39(2)
2.732(2)
2.763(3)
140(2)
105.0(18)
attached to N(3) and O(1) as 1.98 Å, which points towards a strong hydrogen bonding interaction between these atoms. The
interatomic distance of pyridine nitrogen [N(4) in Fig. 3] and N(3) hydrogen atom is 2.39 Å (Table 4) also indicates a
hydrogen bonding interaction. In addition to this data, the total sum of the bond angles; C(6)–N(3)–C(7) = 125.2q, C(6)–
N(3)–H(3N) = 115.4q, and C(7)–N(3)–H(3N) = 119.3q is 359.9q, which is practically 360q. These bond angles around N(3)
confirms that this nitrogen exists in a planer geometry. The solid state structure determination suggests that a strong hydrogen
bonding between N(3)H and O(1) helps to stabilize the amine-one(I) tautameric form of this compound.
CONCLUSION
The Schiff bases produced by the reaction between alkyl amines and 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-
pyrazol-3-one are shown to exist exclusively in the amine-one(I) tautomeric form in chloroform solutions at room
temperature by NMR methods. The crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-
2,4-dihydro-pyrazol-3-one (1a) showed that this 4-acylpyrazolone Schiff base exists in the amine-one(I) form in the solid
state as well. Furthermore, this solid state structure supports the fact that strong hydrogen bonding between amine hydrogen
and the pyrazolone C3 carbonyl oxygen helps to stabilize the structures of these compounds in the amine-one(I) form.
NMR spectrometer used in this study was obtained through a NSF MRI grant CHE-0421290. We are also grateful
for support of crystallography studies via NSF/DMR grant 0420863.
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