Synthesis of ditopic ligands containing bis(1H-pyrazol-1-yl)-methane fragments

Article abstract of DOI:10.1134/S1070428007110188

New approaches have been proposed for the synthesis of compounds containing two bis(1H-pyrazol-1-yl)methane fragments. Nucleophilic replacement of the halogen atoms in appropriate tetrabromo derivatives by pyrazoles in the superbasic system KOH-DMSO gave

Full text of DOI:10.1134/S1070428007110188

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 11, pp. 1698–1702. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © E.A. Nudnova, A.S. Potapov, A.I. Khlebnikov, V.D. Ogorodnikov, 2007, published in Zhurnal Organicheskoi Khimii, 2007, Vol. 43,
No. 11, pp. 1698–1702.
Synthesis of Ditopic Ligands Containing Bis(1H-pyrazol-1-yl)-
methane Fragments
E. A. Nudnovaa, A. S. Potapova, A. I. Khlebnikova, and V. D. Ogorodnikovb
a Polzunov Altai State Technical University, pr. Lenina 46, Barnaul, 656038 Russia
e-mail: aikhl@nm.ru
b Institute of Petroleum Chemistry, Siberian Division, Russian Academy of Sciences,
Akademicheskii pr. 3, Tomsk, 634021 Russia
Received January 5, 2007
Abstract—New approaches have been proposed for the synthesis of compounds containing two bis(1H-pyr-
azol-1-yl)methane fragments. Nucleophilic replacement of the halogen atoms in appropriate tetrabromo deriv-
atives by pyrazoles in the superbasic system KOH–DMSO gave ditopic chelating ligands: 1,1,2,2-tetrakis(1H-
pyrazol-1-yl)ethane, 1,4-bis[bis(1H-pyrazol-1-yl)methyl]benzene, and 1,4-bis[bis(3,5-dimethyl-1H-pyrazol-
1-yl)methyl]benzene. 1,4-Bis[bis(1H-pyrazol-1-yl)methyl]benzene was also synthesized by reaction of 1H-pyr-
azole with terephthalaldehyde in the presence of thionyl chloride. 1,1,2,2-Tetrakis(1H-pyrazol-1-yl)ethane was
converted into the corresponding tetraiodo and tetranitro derivatives.
DOI: 10.1134/S1070428007110188
Bis(1H-pyrazol-1-yl)alkanes are used as chelating
ligands in coordination compounds with various metals
[1]. Among these, specific interest is attracted by com-
pounds whose molecules contain several bis(1H-pyra-
zol-1-yl)methane fragments (Fig. 1) connected through
various linkers; such derivatives are multitopic chelat-
ing ligands. Ditopic ligands could give rise to homo-
and heterobimetallic complexes exhibiting high cata-
lytic and biological activity, as well as to coordination
polymers [2, 3]. Supramolecular structure of such
polymers may be varied by changing the linker nature;
as a result, new materials, ion exchangers, molecular
sieves, and sensors may be obtained [4–6].
drated solvents [4–6]. One of these is based on reaction
of appropriate heterocyclic systems with dialdehyde
acetals in the presence of a catalytic amount of p-tolu-
enesulfonic acid. For example, prolonged heating of
a mixture of malonaldehyde tetramethyl acetal and
pyrazole in acid medium with continuous removal of
the liberated methanol by distillation gave 1,1,3,3-tetra-
(1H-pyrazol-1-yl)propane. Compounds with a longer
linker, 1,1,4,4-tetra(1H-pyrazol-1-yl)butane and
1,1,5,5-tetra(1H-pyrazol-1-yl)pentane, were synthe-
sized in a similar way [5]. Disadvantages of this proce-
dure are low stability of the initial acetals and long
reaction time. Compounds containing two bis(1H-pyr-
azol-1-yl)methane fragments are also formed by reac-
tions of carbonyl- and sulfinyldipyrazoles (prelimi-
narily prepared from pyrazole potassium salt and phos-
gene or thionyl chloride) with various aldehydes in the
presence of anhydrous cobalt(II) chloride [2, 6]. This
procedure ensures high yields of the target ligands but
involves some experimental difficulties related to the
necessity of using alkali metals and phosgene. We can
conclude that search for new methods of synthesis of
compounds having two bis(1H-pyrazol-1-yl)methane
fragments is quite important.
The known synthetic approaches to ditopic pyra-
zole-containing ligands involve the use of unstable
compounds, high temperatures, and thoroughly dehy-
R
R
N
N
N
N
N
N
Linker
N
N
R
R
In the present article we propose two new synthetic
approaches to the above compounds. One of these is
based on the reaction of the corresponding pyrazoles
Fig. 1. Schematic representation of ditopic ligands contain-
ing bis(1H-pyrazol-1-yl)methane fragments.
1698

SYNTHESIS OF DITOPIC LIGANDS
1699
Scheme 1.
CHO
N
N
N
N
N
N
N
N
SOCl2, PhH
N
4
+
N
H
CHO
Ia
with terephthalaldehyde in the presence of thionyl
chloride; it was used to obtain ditopic pyrazole-con-
taining ligands with an aromatic linker. As reported in
[7], benzotriazole is capable of reacting with benzalde-
hyde at a ratio of 2:1 in the presence of thionyl
chloride. A procedure for the preparation of (1-chloro-
alkyl)pyrazoles by reaction of equimolar amounts of
the corresponding aldehyde and pyrazole in the pres-
ence of thionyl chloride was described in patent [8];
however, no data were given on the synthesis of bis-
(pyrazol-1-yl)alkanes at a pyrazole-to-aldehyde molar
ratio of 2:1.
the first time 1,1,2,2-tetrakis(1H-pyrazol-1-yl)ethane
(IIa) by reaction of 1,1,2,2-tetrabromoethane with
pyrazole in the system KOH–DMSO (Scheme 2). Our
attempt to obtain in a similar way 1,1,2,2-tetrakis(3,5-
dimethyl-1H-pyrazol-1-yl)ethane (IIb) resulted in
isolation of only initial 3,5-dimethylpyrazole; obvious-
ly, the reason is increased steric hindrances as com-
pared to unsubstituted pyrazole. Pyrazole and 3,5-di-
methylpyrazole reacted with 1,4-bis(dibromomethyl)-
benzene in the system KOH–DMSO to give compound
Ia and previously unknown derivative Ib, respectively
(Scheme 2).
To attain high yields of the target products, it is
necessary to use 1.5-fold excess of the halogen deriv-
ative, for superbasic reaction medium facilitates side
processes with their participation.
The other method involves reaction of aliphatic
halogen derivatives with azoles in the superbasic sys-
tem KOH–DMSO; this system was successfully used
previously to effect alkylation of phenoxazine, pheno-
thiazine [9], and aromatic amines [10], as well as
double alkylation of pyrazoles with formation of bis-
(1H-pyrazol-1-yl)methanes [11].
To estimate the role of steric factor related to
increase in the number of substituents in the pyrazole
molecule, we also performed syntheses with
3(5)-methylpyrazole as nucleophile. According to the
1H NMR data, the product obtained by reaction of tere-
phthalaldehyde with 3(5)-methylpyrazole in the pres-
ence of SOCl2, was a mixture of isomers (obviously,
derivatives of 3- and 5-methylpyrazoles). The product
was very unstable, and it decomposed on storage to
form terephthalaldehyde. Likewise, isomer mixtures
were formed in reactions of 3(5)-methylpyrazole with
1,1,2,2-tetrabromoethane and 1,4-bis(dibromomethyl)-
benzene in superbasic medium.
By reaction of pyrazole with terephthalaldehyde in
the presence of thionyl chloride we obtained 1,4-bis-
[bis(1H-pyrazol-1-yl)methyl]benzene (Ia) (Scheme 1).
The reaction was characterized by a short time (2 h)
and good yield (71%). However, we failed to obtain
1,4-bis[bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl]ben-
zene (Ib) from 3,5-dimethylpyrazole as nucleophile;
instead, 3,5-dimethylpyrazole hydrochloride was iso-
lated. Presumably, in going from unsubstituted pyra-
zole to 3,5-dimethylpyrazole the stability of the prod-
uct toward acids sharply decreases for steric reasons.
Following the second procedure, we synthesized for
As follows from the signal intensity ratio for pro-
tons in positions 3 and 5 of the pyrazole rings (δ 7.44–
Scheme 2.
R
R
R
R
R
R
N
N
N
N
Br
Br
Br
Br
N
N
N
N
KOH–DMSO
+
4
X
X
N
R
N
H
R
R
R
Ia, Ib, IIa, IIb
R = H (a), Me (b); I, X = p-C6H4; II, X = bond.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 11 2007

NUDNOVA et al.
(b)
1700
(a)
system DMSO–KOH makes it possible to synthesize
0.055
–0.220
0.258
–0.369
derivatives of pyrazoles having one or two methyl
groups in the ring, while the reaction with terephthal-
aldehyde in the presence of thionyl chloride is success-
ful only with unsubstituted pyrazole and 3(5)-methyl-
pyrazole (no reaction occurs with 3,5-dimethylpyra-
zole). Thus the procedure for the synthesis of com-
pounds containing two bis(1H-pyrazol-1-yl)methane
fragment in superbasic medium is more general and
simple; the products are readily isolated by filtration or
extraction, and the procedure is more benign from the
ecological viewpoint.
–0.400 N
N
–0.436
Fig. 2. (a) Charge distribution and (b) HOMO structure of
3(5)-methylpyrazolate ion according to the DFT B3LYP
6-31G** calculations.
1
7.39 and 7.34–7.36 ppm, respectively) in the H NMR
spectra of the products obtained by reaction of
3(5)-methylpyrazole with terephthalaldehyde, about
71% of the methyl groups occupy position 3. The
reaction in superbasic medium is characterized by
a lower regioselectivity. The fraction of 3-methyl-sub-
stituted pyrazole rings in the products formed by reac-
tion of 3(5)-methylpyrazole with 1,4-bis(dibromo-
methyl)benzene is 51%; in the products obtained from
1,1,2,2-tetrabromomethane, about 64% of methyl
groups are attached to C3 of the pyrazole rings.
Coordination properties of the obtained ligands
may be varied over a wide range via introduction of
different substituents into the pyrazole rings. For this
purpose, we were the first to synthesize tetraiodo and
tetranitro derivatives of compound IIa (Scheme 3). It
is known that N-alkylpyrazoles [12] and bis(1H-pyra-
zol-1-yl)methanes [13] are readily iodinated with the
system I2–HIO3–H2SO4 in acetic acid. The oxidative
iodination of IIa with I2–HIO3–H2SO4 in acetic acid at
80°C in 16 h afforded 73% of tetraiodo derivative III.
According to the NMR data, electrophilic replacement
of hydrogen in the pyrazole ring by iodine occurred
with complete regioselectivity at the C4 atom of the
pyrazole rings, which possesses the largest electron
density. The nitration of compound IIa was performed
using a mixture of nitric and sulfuric acids at a molar
ratio of 1:5.5, by analogy with the procedure for
nitration of bis(1H-pyrazol-1-yl)methanes [14]. As in
the iodination, the nitro group entered exclusively the
4-position of the pyrazole rings. The obtained iodo and
nitro derivatives III and IV are characterized by poor
solubility in most organic solvents and enhanced
thermal stability (according to the data of thermo-
gravimetric analysis, they melt with decomposition at
349 and 388°C, respectively).
The observed regioselectivity is consistent with the
results of quantum-chemical calculations of the anion
derived from 3(5)-methylpyrazole. Figure 2a shows
charge distribution in 3(5)-methylpyrazolate ion. It is
seen that the largest negative charge is localized on the
N2 atom; on the other hand, the highest occupied mo-
lecular orbital (HOMO) of the anion, which determines
its nucleophilicity, is contributed mainly by the N1
atom (Fig. 2b). These data led us to conclude that the
reaction with terephthalaldehyde in benzene is orbital-
controlled; therefore, it yields mainly 3-methyl-substi-
tuted derivatives. In going from nonpolar benzene to
polar DMSO, charge control of the process becomes
more significant, and the fraction of 5-methylpyrazolyl
increases.
Steric factor also favors formation of 3-methylpyra-
zolyl derivatives. The obtained isomer ratios suggest
that steric effects in superbasic medium are not so
significant as in the reaction with terephthalaldehyde
in the presence of thionyl chloride. Therefore, the
EXPERIMENTAL
The NMR spectra were recorded on a Bruker AV-
300 spectrometer. The IR spectra were measured on
a Specord 71IR spectrometer. Mass spectrometric and
Scheme 3.
N
N
N
N
N
N
I
I
I
I
O2N
O2N
NO2
NO2
N
N
N
N
N
N
I2–HIO3, H2SO4, AcOH
HNO3, H2SO4
N
N
N
N
N
N
N
N
N
N
N
N
III
IIa
IV
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 11 2007

SYNTHESIS OF DITOPIC LIGANDS
1701
thermogravimetric analyses of compounds III and IV
were performed on TRACE DSQ (Thermo Electron
Corporation, USA) and SDT Q600 instruments (TA
Instruments, USA) at the Analytical Center of the
Tomsk Polytechnic Universitety. The progress of reac-
tions and the purity of products were monitored by
TLC on Silufol plates using benzene–acetone (3:2) as
eluent; development with iodine vapor.
The spectral parameters of samples of Ia obtained as
described in a and b were identical.
1,4-Bis[bis(3,5-dimethyl-1H-pyrazol-1-yl)meth-
yl]benzene (Ib) was synthesized as described above
for compound Ia (method a) from 2 g (20.8 mmol) of
3,5-dimethylpyrazole, 2.33 g (41.7 mmol) of KOH,
and 2.22 g 5.21 mmol) of 1,4-bis(dibromomethyl)ben-
zene; reaction time 24 h. Yield 1.49 g (60%), light
yellow needles, mp 224.5–226°C (from benzene). IR
spectrum, ν, cm–1: 1540, 1500, 1330, 1010 (pyrazole).
1H NMR spectrum (DMSO-d6), δ, ppm: 2.09 s (12H,
3-CH3), 2.13 s (12H, 5-CH3), 5.90 s (4H, 4-H), 6.99 s
(4H, Harom), 7.70 s (2H, CH). Found, %: C 70.01;
H 7.10; N 22.90. C28H34N8. Calculated, %: C 69.68;
H 7.10; N 23.22.
1,4-Bis(dibromomethyl)benzene was synthesized
according to the procedure described in [15]. Dimethyl
sulfoxide was kept for 24 h over pelleted KOH and
distilled under reduced pressure. Thionyl chloride was
distilled just before use. The other reagents were com-
mercially available products which were used without
additional purification.
1,1,2,2-Tetrakis(1H-pyrazol-1-yl)ethane (IIa).
A mixture of 1 g (14.7 mmol) of finely powdered
potassium hydroxide and 10 ml of DMSO was heated
to 80°C under vigorous stirring. After 30 min, a solu-
tion of 1.29 g (3.68 mmol) of 1,1,2,2-tetrabromoethane
in 10 ml of DMSO was added dropwise to the result-
ing suspension, the mixture was heated for 5 h at 80°C
and diluted with 200 ml of water, and the precipitate
was filtered off and dried (0.15 g). The filtrate was
treated with chloroform (5×15 ml), the extract was
washed with water (4×10 ml) and dried over calcium
chloride, and the solvent was distilled off to obtain an
additional portion (0.468 g) of compound IIa. Overall
yield 0.618 g (57%), colorless crystals, mp 271–
273°C. IR spectrum, ν, cm–1: 1521, 1437, 1310, 1051
(pyrazole). 1H NMR spectrum (CDCl3), δ, ppm: 6.13 t
(4H, 4-H, J = 2 Hz), 7.47 d (4H, 3-H, J = 2 Hz), 7.65 d
(4H, 5-H, J = 2 Hz), 7.83 s (2H, NCHN). Found, %:
C 56.84; H 4.48; N 38.07. C14H14N8. Calculated, %:
C 57.13; H 4.79; N 38.29.
Nonempirical calculations were performed using
HyperChem for Windows [16]. Initial structures for
optimization of geometric parameters were simulated
by the PM3 method. Geometry optimization was per-
formed using 6-31G* basis set for isolated ions by the
conjugate gradient method until a mean-square gradi-
ent of less than 10 cal Å–1 mol–1 was reached. The op-
timized structures were calculated in terms of the DFT
UHF approximation (6-31G** basis set, B3LYP com-
bination potential).
1,4-Bis[bis(1H-pyrazol-1-yl)methyl]benzene (Ia).
a. A mixture of 2 g (29.4 mmol) of pyrazole, 3.29 g
(58.8 mmol) of finely powdered potassium hydroxide,
and 10 ml of DMSO was stirred for 2 h at 80°C. A so-
lution of 3.13 g (7.35 mmol) of 1,4-bis(dibromo-
methyl)benzene in 15 ml of DMSO was added drop-
wise, the mixture was stirred for 21 h at 80°C, diluted
with 200 ml of water, and neutralized with hydro-
chloric acid, and the precipitate was filtered off. Yield
1.439 g (53%), light yellow crystals, mp 168–169°C
(from EtOH); published data [2]: mp 165–166°C. IR
spectrum, ν, cm–1: 1500, 1330, 1020 (pyrazole); 1610
Reaction of 3(5)-methylpyrazole with terephthal-
aldehyde. Thionyl chloride, 3.55 g (29.84 mmol), was
added dropwise on cooling to a solution of 1.223 g
(14.92 mmol) of 3(5)-methylpyrazole and 0.5 g
(3.37 mmol) of terephthalaldehyde in 5 ml of benzene.
The mixture was heated for 2 h under reflux, excess
thionyl chloride was removed by repeated distillation
each time with addition of toluene (5×2 ml). The
remaining solution was diluted with hexane, and the
precipitate was filtered off, washed with 20% aqueous
ammonia, and dried in a vacuum desiccator over KOH.
Yield 1.16 g (81%) (mixture of 3- and 5-methyl deriv-
1
(C=Carom). H NMR spectrum (CDCl3), δ, ppm:
6.33 d.d (4H, 4-H, J = 0.3, 2.3 Hz), 7.05 s (4H, Harom),
7.54 d (4H, 3-H, J1 = 2.3, J2 = 0.6 Hz), 7.62 d (4H,
5-H, J = 1.7 Hz), 7.72 s (2H, NCHN). Found, %:
C 64.46; H 4.62; N 30.45. C20H18N8. Calculated, %:
C 64.85; H 4.90; N 30.25.
b. Thionyl chloride, 3.55 g (29.8 mmol), was
slowly added on cooling with ice to a mixture of 1 g
(14.9 mmol) of pyrazole, 0.5 g (3.73 mmol) of tere-
phthalaldehyde, and 5 ml of benzene. The mixture was
heated for 2 h under reflux and poured into 150 ml of
hexane, and the precipitate was filtered off and dried
under reduced pressure (20 mm). Yield 0.973 g (71%).
1
atives). H NMR spectrum (CDCl3), δ, ppm: 2.27–
2.38 (12H, 3-CH3, 5-CH3), 6.96–6.99 (4H, Harom), 7.34–
7.36 (2.84H, 5-H), 7.44–7.49 (1.16H, 3-H), 7.53–
7.57 (2H, CH).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 11 2007

NUDNOVA et al.
1702
[M – Pz(NO2)H]+, 251 (13) [M – 2Pz(NO2)]+, 237
(100) [M – (Pz(NO2))2CH]+, 191 (18) [M – 2Pz(NO2) –
NO2 – H]+, 113 (38) [Pz(NO2)H]+. Found, %: C 35.72;
H 2.30; N 35.85. C14H10N12O8. Calculated, %: C 35.45;
H 2.13; N 35.44.
Reaction of 3(5)-methylpyrazole with 1,1,2,2-
tetrabromoethane. The reaction of 1 g (12.20 mmol)
of 3(5)-methylpyrazole with 1.067 g (3.05 mmol) of
1,1,2,2-tetrabromoethane was carried out as described
above for the synthesis of compound IIa; the reaction
mixture was heated for 91 h at 80°C. Yield 0.51 g
(57%) (mixture of 3- and 5-methyl derivatives).
1H NMR spectrum (CDCl3), δ, ppm: 2.14–2.38 (12H,
3-CH3, 5-CH3), 7.37 (1.45H, 3-H), 7.51–7.53 (2H,
CH), 7.57–7.72 (2.55H, 5-H).
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1
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3-H), 7.53–7.57 (2H, CH).
1,1,2,2-Tetrakis(4-iodo-1H-pyrazol-1-yl)ethane
(III). A suspension prepared from 0.082 g
(0.279 mmol) of compound IIa, 0.113 g (0.446 mol) of
iodine, 0.039 g (0.223 mmol) of iodic acid, 0.1 ml of
30% sulfuric acid, and 1 ml of acetic acid was stirred
for 6 h at 80°C (until the iodine color disappeared).
The mixture was diluted with 20 ml of water, and
the precipitate was filtered off, repeatedly washed with
a solution of KI and with water, and dried under
reduced pressure (20 mm). Yield 0.163 g (73%), color-
less crystals, mp 349°C (decomp., from DMF).
1H NMR spectrum (DMF-d7), δ, ppm: 8.05 s (4H,
3-H), 8.44 s (4H, 5-H), 8.47 s (2H, CH). Mass spec-
trum, m/z (Irel, %): 798 (1) [M]+, 413 (24) [M –
2Pz(I)]+, 399 (100) [M – (PzI)2CH]+, 273 (93) [M –
(PzI)2CH – I]+, 194 (33) [Pz(I)H]+. Found, %: C 20.74;
H 1.31; N 13.90. C14H10I4N8. Calculated, %: C 21.07;
H 1.26; N 14.04.
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1,1,2,2-Tetrakis(4-nitro-1H-pyrazol-1-yl)ethane
(IV). A mixture of 0.29 g (1.00 mmol) of compound
IIa, 1.3 ml of 68% nitric acid (20.0 mmol of HNO3),
and 6 ml of 98% sulfuric acid was kept for 24 h at
room temperature. The mixture was then diluted with
an ice–water mixture and neutralized with sodium car-
bonate. The precipitate was filtered off, washed with
water, and dried under reduced pressure (20 mm).
Yield 0.33 g (81%), colorless crystals, mp 388°C
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1
(decomp.; from DMF–water, 5:2). H NMR spectrum
(DMF-d7), δ, ppm: 8.46 s (4H, 3-H), 8.95 s (2H, CH),
9.38 s (4H, 5-H). Mass spectrum, m/z (Irel, %): 361 (4)
16. HyperChem Computational Chemistry: Molecular
Visualization and Simulation (release 7 for Windows),
Hypercube, 2002.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 11 2007