Structure and extractive ability of 1-alkyl- and 3-methyl-1-phenyl-2- pyrazolin-5-ones

Article abstract of DOI:10.1007/s11176-005-0217-y

Extractive ability of 1-substituted 3-methylpyrazol-5-ones (LH) is studied. From acidic chloride complexes, ionic thallium(III) associates (LH ₂)[TlCl₄] are extracted; from trichloroacetate, coordination scandium complexes Sc(LH)₄(CCl₃COO) ₃; and from ammine, copper(II) complexes CuL₂. The extractive ability decreases in the order R = C₇H₁₅ > C₆H₁₃ > C₅H₁₁ > C ₆H₅ > C₄H₉. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0217-y

Russian Journal of General Chemistry, Vol. 75, No. 2, 2005, pp. 298 302. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005,
pp. 326 330.
Original Russian Text Copyright
2005 by Lesnov, Sazonova, Pavlov.
Structure and Extractive Ability of 1-Alkyl-
and 3-Methyl-1-phenyl-2-pyrazolin-5-ones
A. E. Lesnov, E. A. Sazonova, and P. T. Pavlov
Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
Perm State University, Perm, Russia
Received July 1, 2003
Abstract Extractive ability of 1-substituted 3-methylpyrazol-5-ones (LH) is studied. From acidic chloride
complexes, ionic thallium(III) associates (LH2)[TlCl4] are extracted; from trichloroacetate, coordination
scandium complexes Sc(LH)4(CCl3COO)3; and from ammine, copper(II) complexes CuL2. The extractive
ability decreases in the order R = C7H15 > C6H13 > C5H11 > C6H5 > C4H9.
1
Design of new highly effective extractants requires
knowledge of their extractive ability structure correla-
tions. From this viewpoint, of considerable interest
are pyrazolone derivatives, since, depending on the
composition of the aqueous phase, different extraction
mechanisms can be realized: anion-exchange, coordi-
nation, and cation-exchange.
Actually, by H and IR spectroscopy we revealed
in chloroform tautomers B and C. The IR spectrum
of compound III contains bands at 3280 and
1
1660 cm , corresponding to hydroxy and carbonyl
groups. The H NMR spectrum shows signals of the
1
OH proton at
12.12 ppm and 2-pyrazoline CH2
protons at 3.15 ppm. Apparently, there is an inter-
molecular hydrogen bond between these forms. From
the calculated dipole moments of the tautomers it
follows that form A is more polar than forms B and
C. Thus, form A can exist in polar solvents. Accord-
ing to [2], compound V in CHCl3 exists in form B.
Increasing chain length of the 1-alkyl group has
almost no effect on the electron density on oxygen
atoms that are the most probable coordination centers
for metal ions. Phenyl substitution decreases the
We undertook an attempt to predict, on the basis
of quantum-chemical calculations, the effect of re-
placement of the phenyl radical in the 1 position of
the pyrazoline ring with an aliphatic radical. The key
factors that affect the extractive ability of organic
reagents are the electron density of the coordination
center and the relative solubility in aqueous and
organic phases of the reagents and their metal
complexes.
Table 1. Heats of formation ( Hf), dipole moments ( ),
charges (q) on potential coordination centers, and distribu-
tion constants (D) of various tautomeric forms of extra-
gents I V
1,3-Substituted pyrazol-5-ones I V can exist in
three tautomeric forms [1]: A (NH form), B (CH
form), and C (OH form).
CH3
CH3
CH3
NH
q
Reagent
(tautomer) kJ mol
Hf,
N
N
HO
, D
logD
1
O
N
R
B
O
N
R
A
N
R
O
N2
C
I(B)
122.94 2.278
145.63 2.248
168.35 2.260
138.76 1.927
132.40 3.759
191.07 2.246
84.50 2.504
109.60 2.046
114.89 3.733
0.316
0.316
0.316
0.205
0.329
0.316
0.310
0.202
0.323
0.086
0.086
0.086
0.245
0.049
0.086
0.094
0.252
0.041
0.74
1.15
1.57
2.95
II(B)
III(B)
III(C)
III(A)
IV(B)
V(B)
V(C)
V(A)
I V
R = C4H9 (I), C5H11 (II), C6H13 (III), C7H15 (IV), C6H5
(V).
1.99
1.16
2.54
0.71
According to quantum-chemical calculations, the
heats of formation of the tautomers of molecules III
and V decrease in the order A > C > B (Table 1), i.e.
tautomer B should be the most stable.
1070-3632/05/7502-0298 2005 Pleiades Publishing, Inc.

STRUCTURE AND EXTRACTIVE ABILITY OF 1-ALKYL- AND 3-METHYL-...
299
electron density on the coordination center, which
should render the resulting complexes less stable. In
view of the close electronic structures of reagents with
80
various-length alkyl substituents, one can suggest that
they would behave similarly with respect to metals,
and differences in the extraction behavior would be
associated with the intrinsic hydrophobilicity of the
1
2
60
reagents and resulting complexes. The hydrophobicity
can be measured by organic water distribution con-
40
20
0
stants (logD) [3]. Increasing alkyl chain length in-
creases the hydrophobicity of the molecule (Table 1),
which should improve extractive ability. Compound
V is close in hydrophobility to compound II. Thus,
from the data in Table 1 we can suggest that com-
pounds III and IV should be better extractants than
compound V.
3
4
0.05
0.10
0.15 0.20 0.25
Extraction by the coordination mechanism was
considered using the example of the extraction of
scandium ions from trichloroacetate solutions.
c(H2SO4), M
Fig. 1. Effect of acidity on the extraction of scan-
The effect of the aliphatic chain length on the ex-
tractive ability of the reagents is shown in Fig. 1. The
most effective is compound IV.
dium(III) (0.01 M) by 0.1 M solutions of compounds
I IV in CHCl from a 0.5 M solution of CCl COONa.
3
3
Compound: (1) IV, (2) III, (3) II, and (4) I.
The process can be described by a two-equation
system [4].
The distribution of the reagents between hydro-
chloric acid and chloroform and their protonation in
the organic phase were studied. Increasing concentra-
tion of the acid in the extraction system gives rise to
a sharp decrease in D (Fig. 2). The highest D values
are observed with compound IV. The reagent is
mostly present in the extract is salt (the protonation
degree for 2 M HCl in the aqueous phase is 85%).
Compound V scarcely extracts the acid.
Sc3+ + 3CCl3COO + 4LH(o)
H+ + CCl3COO + LH(o)
Sc(LH)4(CCl COO) (o)
,
3
3
LH2
.
CCl3COO(o)
Here LH(o) is the extraction reagent in the organic
phase.
Studying the extraction of trichloroacetic acid
showed that, starting from the acid concentration of
0.5 M, the reagents are almost completely protonated
in the organic phase. At higher acid concentrations,
overstoichiometric extraction takes place.
The protonation position is unknown: It can occur
both by heteroring nitrogen and by carbonyl oxygen.
Moreover, the process is complicated with the pre-
sence of various tautomers. Organic molecules are
most frequently protonated by an atom bearing the
largest negative charge. This is the carbonyl oxygen
atom in tautomers A and B and the pyrazole C4 and
N2 atoms in tautomer C. The protonation by C4
provides a structure that is identical to that formed by
the protonation of tautomer B by the carbonyl oxygen
atom. Since the charges on N2 and N4 in tautomer C
of compound V are close to each other ( 0.233 and
0.236, respectively), it is expedient to consider the
structure of the cation resulting from the protonation
by N2. In this case, a structure indentical to that
formed by the protonation of tautomer A by the
oxygen atom is formed. In view of the aforesaid, we
calculated structures D F.
The water chloroform distribution constants of
reagents I IV increase with increasing acidity. The
highest D values are characteristic of compound IV:
144 [c(CCl3COOH) 0.1 M] and 14 [c(CCl3COOH)
3 M].
The extraction of scandium(III) from trichloro-
acetate solutions with a 0.1 M solution of compound
V in a 10:1 chloroform 1-butanol mixture has been
studied in [5]. In view of the fact that scandium(III)
ions are partially extracted from trichloroacetate solu-
tions with aliphatic alcohols, it is safe to state that
here we deal with a synergistic effect.
The anion-exchange mechanism of metal extraction
with pyrazolones was studied using the example of
the extraction of tallium(III) from acidic chloride
solutions.
It follows from the data in Table 2 that the most
energetically favorable structure is cation E irrespec-
tive of the nature of the 1-substituent in the pyrazoline
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

300
LESNOV et al.
Table 2. Heats of formation ( Hf) of cations D F
Table 3. Heats of formation of tautomers C [ Hf(LH)] of
compounds I V and their anions [( Hf(L )], and deprotona-
tion energies (Ed)
1
Hf, kJ mol
Comp. no.
D
E
F
Hf(LH)
kJ mol
Hf(L )
kJ mol
Ed
kJ mol
Comp. no.
1
1
1
I
573.7
550.8
527.9
505.0
770.3
586.9
564.0
541.1
518.3
779.5
567.3
544.4
521.4
498.6
746.8
I
93.34
116.06
138.74
161.50
109.62
236.98
260.16
283.13
306.06
60.50
1336.36
1335.9
1335.61
1335.44
1309.88
II
III
IV
V
II
III
IV
V
~
Kex,H = [LH2X](o)/([H+][X ][LH](o)),
H3C
H3C
H3C
~
2
.
.
Kex,H = Kex,H
/
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. +..
. .
.
. +..
. .
.
.
. +..
. .
.
N
HN
HN
Here [LH2X](o), [LH](o), [H+], and [X ] are the equi-
librium concentrations of the protonated and basic
forms of the reagent in the organic phase and of the
hydrogen and halide ions in the aqueous phase; and
, mean ionic activity coefficient of the hydrohalic
acid [7].
N
N
N
O
OH
O
H
R
E
R
F
R
D
R = Alk, Ar.
ring. The resulting data are consistent with the calcu-
lated electronic structure of 1-methyl-2-pyrazolin-5-
one [6].
The extraction of hydrochloric acid is enhanced as
the length of the aliphatic radical increases. The
apparent extraction rate constants increase from 2.3
0.2 for compound I to 5.6 0.3 for compound IV. The
extraction constants of hydrobromic acid are higher
(the concentration and apparent extraction rate con-
stants for compound III are 4.1 0.9 and 8.3 0.9,
respectively), which is nicely consistent with the
hydration energy of the bromide ion. The acids are
not extracted by a chloroform solution of compound
V.
The processes that occur in the extraction system
can be described by a two-equation system.
LH(o) + H+ + X
LH2X(o)
LH2X.
,
LH2X(o)
From the extraction equation of the acid we can
~
write equations of the concentration (Kex,H) and
apparent (Kex,H) extraction constants of the acid.
Data on the extraction of thallium(III) from HCl
solutions with 0.1 M chloroform solutions of reagents
with various-length aliphatic radicals are presented in
Fig. 3. The most effective are compounds III and IV
and the least effective, compound V.
2
log D
1
1
Deprotonation of pyrazolones gives rise to an anion.
The deprotonation energies (Ed) were calculated for
the reaction LH
2
L + H+ by the following equa-
tion [6] (Table 3).
3
0
Ed
=
Hf(L ) + Hf(H+)
Hf(LH).
4
Here Hf(L ) is the formation enthalpy of the anion;
Hf(H+), ionization energy of the hydrogen atom
(1480 kJ mol ); and Hf(LH), formation enthalpy of
5
1
1
0
0.4
0.8
1.2
1.6
c(HCl), M
Fig. 2. Distribution of reagents I V in the H O CHCl
2.0
the neutral molecule.
The strongest acid is compound V. The length of
the hydrocarbon radical in alkylpyrazolones has
almost no effect on the acidity of the compounds.
2
3
HCl system. Compound: (1) IV, (2) III, (3) V, (4) II,
and (5) I.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

STRUCTURE AND EXTRACTIVE ABILITY OF 1-ALKYL- AND 3-METHYL-...
301
2
100
80
log D
1
1
60
3
1
2
0
1
2
40
3
20
0
4
5
5
4
2
4
6
0
0.4
0.8
1.2
1.6
2.0
c(HCl), M
c(NH3), M
Fig. 3. Extraction of thallium(III) (0.01 M) from HCl
solutions with 0.1 M solutions of compounds I V in
chloroform. Compound: (1) IV, (2) III, (3) V, (4) II,
and (5) I.
Fig. 4. Distribution of reagents I V in the H O CHCl
2
3
NH system. Compound: (1) IV, (2) III, (3) II, (4) V,
3
and (5) I.
The hydroxy forms of pyrazolones can extract
elements by the cation-exchange mechanism. In view
of the weak acidity of these compounds, extraction of
metal ions is only possible in an alkaline medium. To
prevent formation of sparingly soluble metal hydr-
oxides, ligands that favor formation of cationic com-
plexes should be present. The required conditions are
provided by ammine complexes.
Copper(II) is extracted by the cation-exchange
mechanism.
[Cu(NH3)4]2+ + 2LH(o)
CuL2(o) + 2NH+4 + 2NH3.
Figure 5 presents data on the extraction of cop-
per(II) from ammonia solutions with pyrazolone with
various alkyl chain length. Usually electronegative
substituents in acidic extractants enhance the strength
of the acid and thus increase its extractive ability, but
here we deal with the opposite trend. This result
suggests that the hydrophobicity of the substituent
exerts a decisive effect of the extractive ability.
We studied the distribution of compounds I IV
between water and chloroform at various concentra-
tions of ammonia in the extraction system (Fig. 4).
As seen, D sharply decreases with decreasing alkali-
nity of the medium. Noteworthy are the higher D
values for compound IV.
EXPERIMENTAL
The extraction of metal ions capable of forming
ammoniates showed that appreciably recovered are
copper(II) and silver(I) only. The compositions of the
complexes of copper(II) with compounds III and IV
were determined by the equilibrium-shift method and
saturation and chemical analysis of the isolated
complexes. All methods gave evidence for the 1:2
[Cu(II)]:[LH] ratio. According to the chemical
analysis, the complex contains no ammonia.
1
The IR and H NMR spectra were recorded on a
UR-20 spectrophotomer (solvent CHCl3) and Bruker
WP-80SY spectrometer (working frequency 80 MHz,
solvent CDCl3, internal reference HMDS), respec-
tively.
Quantum-chemical calculations of the geometry
and electronic structure of various tautomers and
conformers, as well as their protonated and ionized
forms were performed by the PM3 method with full
geometry optimization using the MOPAC program
package. The logD values were calculated by a
program incorporated into the ChemDraw package.
The thermoanalytical curves of the copper(II) al-
kylpyrazolone complexes lack the endothermic peak
of melting. The content of copper(II) oxide in the
complex with complex III, found from the TG curve,
was 18.3% (calculated for the complex Cu(C10H17
N2O)2 18.63%). For the complex Cu(C11H19N2O)2:
calculated 17.52%, found 17.4%.
Compound V was prepared by condensation of
acetoacetic ester with phenylhydrazine and purified
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

302
LESNOV et al.
repeated twice. The combined extracts were placed
into a beaker, the solvent was let to evaporate, and the
residue was heated at 105 110 C to constant weight.
100
80
2
Distribution of reagents between phases in the
ammonium system was studied by the same procedure,
except that the extract was not titrated with NaOH.
4
1
Distribution of metals (2 10
mol) between
60
phases was performed in separatory funnels using
0.1 M reagent solutions in chloroform over the course
of 5 min at equal volumes of the aqueous and organic
phases. Analysis of the phases for metal ions was
performed by complexometry.
40
20
0
5
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3
4
0.4
0.8
1.2
1.6
2.0
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c(NH3), M
Fig. 5. Extraction of copper(II) (0.01 M) from ammonia
solutions with 0.1 M solutions of compounds I V in
chloroform. Compound: (1) IV, (2) III, (3) II, (4) I, and
(5) V.
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20 ml of chloroform was shaken for 5 min in a sepa-
ratory funnel with 20 ml of solutions of the corres-
ponding acid of various concentrations. An aliquot of
the extract was filtered and placed in a titration flask,
diluted with 50 ml of water, and titrated with 0.1 M
NaOH with indicator (Bromocresol Green) to blue
color. Concurrently, blank experiment was performed.
The aqueous layer was neutralized to pH 7 8, diluted
with 10 ml of chloroform, and shaken for 5 min in a
separatory funnel. The extraction procedure was
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005