4-(1-Alkylbenzimidazol-2-ylazo)-2-pyrazolin-5-ones: Specific features of prototropic tautomerism

Article abstract of DOI:10.1007/s11172-008-0194-5

Quantum chemical calculations, ¹H and ¹³C NMR, and X-ray studies showed that, in contrast to 4-arylazo-2-pyrazolin-5-ones, 4-(1-alkylbenzimidazol-2-ylazo)-2-pyrazoline-5-ones mainly exist in the condensed phase as unusual ketoazine t

Full text of DOI:10.1007/s11172-008-0194-5

Russian Chemical Bulletin, International Edition, Vol. 57, No. 7, pp. 1496—1507, July, 2008
1496
4ꢀ(1ꢀAlkylbenzimidazolꢀ2ꢀylazo)ꢀ2ꢀpyrazolinꢀ5ꢀones:
specific features of prototropic tautomerism
A. S. Morkovnik,а L. N. Divaeva,а A. I. Uraev,а K. A. Lyssenko,b R. K. Mamin,а
I. G. Borodkina,а G. S. Borodkin,а A. S. Burlov,а and A. D. Garnovskiiа
аInstitute of Organic and Physical Chemistry at South Federal University,
194/2 prosp. Stachki, 344090 RostovꢀonꢀDon, Russian Federation.
Fax: + 7 (863) 243 4028. Eꢀmail: asmork2@ipoc.rsu.ru
bA. N. Nesmeynov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: + 7 (495) 135 6549. Eꢀmail: Kostya@xray.ineos.ac.ru
Quantum chemical calculations, 1H and 13C NMR, and Xꢀray studies showed that, in contrast to
4ꢀarylazoꢀ2ꢀpyrazolinꢀ5ꢀones, 4ꢀ(1ꢀalkylbenzimidazolꢀ2ꢀylazo)ꢀ2ꢀpyrazolineꢀ5ꢀones mainly exꢀ
ist in the condensed phase as unusual ketoazine tautomers of high polarity, while the ketohydrazone
tautomer stabilized by intramolecular hydrogen bond apparently predominates in the gas phase.
According to calculations, various types of tautomerism are possible for 4ꢀ(benzimidazolꢀ2ꢀylazo)ꢀ
2ꢀpyrazolinꢀ5ꢀones, including monoꢀ and bimolecular 1,3ꢀ, 1,5ꢀ, and 1,7ꢀprototropic migrations
proceeding by the singleꢀ and doubleꢀproton transfer mechanism with low activation energies
(ΔE≠ ≈ 2—14 kcal mol–1).
Key words: 4ꢀ(alkylbenzimidazolꢀ2ꢀylazo)ꢀ2ꢀpyrazolinꢀ5ꢀones, 4ꢀ(benzimidazolꢀ2ꢀylazo)ꢀ2ꢀ
pyrazolinꢀ5ꢀone hydrates, NMR spectroscopy, Xꢀray analysis, hydrogenꢀbonded dimers, tautoꢀ
merism, doubleꢀproton mechanism of tautomerism, ab initio quantum chemical calculations, 1H and
13C chemical shifts, quantum chemical calculations, transition states, transformation of hydrogenꢀ
bonded rings.
Scheme 1
oꢀHydroxyazo derivatives mainly exist as ketohydraꢀ
zone (A) or hydroxyazo (B) tautomers.1a—4 Most often,
the former predominates for hydroxyhetarylazo derivaꢀ
tives,1,2—7 whereas the latter for aromatic azo comꢀ
pounds.2—4 The ketohydrazone form is also characterꢀ
istic of compounds formed by azocoupling of aryldiazoniꢀ
um salts with various acyclic CHꢀacids (see, e.g.,
Refs 8—11).
4ꢀArylazoꢀ5ꢀhydroxypyrazoles can exist not only in
the forms A and B, but also С and D, which correspond to
hydroxypyrazole/pyrazolinone tautomerism in the hyꢀ
droxyazo form B. These compounds exist almost excluꢀ
sively in the tautomeric form A both in solutions and in
the crystalline state.4,5,12—23 The same set of tautomers is
considered for 4ꢀhetarylazoꢀ5ꢀhydroxypyrazoles.2 Howꢀ
ever, there should be an exception, namely, the comꢀ
pounds whose molecules contain the azo group bonded
to the C=N group of the hetarene ring. In this case
the fifth, ketoazine, tautomeric form E corresponding to
migration of a hydrogen atom to the nitrogen atom of this
cyclic group (Scheme 1) is possible.
The existence of typeꢀE tautomeric structures can also
be assumed for hetarylazophenols (see, e.g., Ref. 2), but
as far as we know information on their formation is unꢀ
available.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1467—1478, July, 2008.
1066ꢀ5285/08/5707ꢀ1496 © 2008 Springer Science+Business Media, Inc.

Prototropic tautomerism
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1497
In the present work we have studied tautomerism of
new hetarylazoꢀ2ꢀpyrazolinꢀ5ꢀone derivatives including
4ꢀ(1ꢀRꢀbenzimidazolꢀ2ꢀylazo)ꢀ3ꢀmethylꢀ1ꢀphenylꢀ2ꢀ
pyrazolinꢀ5ꢀones (R = Me (1), Bu (2)) obtained by azoꢀ
coupling of the corresponding benzimidazolꢀ2ꢀyldiazoꢀ
nium salts with 3ꢀmethylꢀ1ꢀphenylꢀ2ꢀpyrazolinꢀ5ꢀone.
It was found that the unusual (cf. Refs 1, 4, 5) ketoazine
tautomeric form E is quite important for these bishetaryꢀ
lazo derivatives.
much lower stabilities of other geometric isomers of
molecule 1E.
Restricted Hartree—Fock (RHF/6ꢀ31G*) calculaꢀ
tions revealed planar carbonꢀheteroatom skeletons
(Cs symmetry) for the structures 1B´, 1B″, and 1E. Other
structures calculated by this method, which ignores
the electron correlation, are nonplanar. Only the tauꢀ
tomer 1A and conformer 1B´ are stabilized by hydrogen
bonds. In the conformer 1B″ and isomer (E,E)ꢀ1E the
H—O—N(11) and N(3)—H—N(11) angles (Table 1) are
inappropriate for the formation of the IHB. According to
calculations, the stabilities of structures 1A—E decrease
in the order 1E ≈ 1A >> 1B´ ≈ 1D ≈ 1C > 1B″. The relative
energies of tautomers 1A—E are listed in Table 1. Thus,
the most stable tautomeric forms are 1E and 1A, whereas
structures 1B—D are much less stable and, as in the case
of structurally simpler 2ꢀpyrazolinꢀ5ꢀones (see Ref. 10),
have close energies. Most probably, structures 1B—D do
not fall on the conditional thermodynamic scale of tauꢀ
tomerism, according to which the free energy difference
between tautomers should be at most 20 kJ mol–1
(see Refs 5, 24).
Results and Discussion
Tautomerism of 4ꢀ(benzimidazolꢀ2ꢀylazo)ꢀ2ꢀpyrazolinꢀ
5ꢀones: quantum chemical and Xꢀray diffraction studies.
We have studied pyrazolinone 1 and its structural analog
3 devoid of Nꢀphenyl group.
We calculated the geometric parameters and the total
energies of the tautomers of compound 1 (Scheme 2,
structures 1A—E), which were chosen to be potentially
stabilizable through, e.g., existence of steric prerequiꢀ
sites for the formation of intramolecular hydrogen bond
(IHB), planar geometry of the conjugated fragment of the
molecule, and the Eꢀconfiguration with respect to the
N=N bond. For the hydroxy tautomer 1B we have studꢀ
ied two conformers, 1B´ and 1B″, because formally an
IHB can be formed in both systems. The E,Eꢀ1E isomer
relative to the exocyclic double bonds was chosen based
on the results of preliminary calculations, which predict
The high stabilities of tautomers 1A and 1Е seem to be
first of all determined by the energetically favorable toꢀ
pology of the bonds at the C(4) and C(5) atoms of the
pyrazole ring. Namely, these atoms form exocyclic douꢀ
ble bonds, thus, probably, minimizing angular strain,
which is usually rather high in fiveꢀmembered rings.
Scheme 2
R = Me, R´ = Ph (1); R = Bu, R´ = Ph (2); R = Me, R´ = H (3)

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Morkovnik et al.
Table 1. The total (Etot) and relative (ΔE) energies, dipole moments (μ), and parameters of hydrogen bonds calculated for different forms
of compound 1, monohydrates of compound 3, dimers 4—8, and transition states of tautomerisma
Structure
–Etot/au
ΔEb/kcal mol–1
μcalc/D
R(H...A)c/Å
ω(D—H...A)c/deg
1A
1092.46334d
1098.61894
(1098.55515)e
1092.44464
1092.43409
1092.44070
1092.44265
1092.46689d
1098.61594
(1098.55760)e
944.13504
0
0
0
4.8
4.6
2.11
1.94
127.4
133.4
1B´
1B″
1C
1D
1E
14.0
20.6
16.4
15.2
–2.2
1.9
2.9
4.8
4.3
3.1
7.8
8.6
2.00
2.48
—
—
2.37
2.36
133.5
107.8
—
—
92.0
93.6
(–1.5)f
—
(E,E)ꢀ3B•H2O
(E,E)ꢀ3E•H2O
(Z,Z)ꢀ3A•H2O
(Z,Z)ꢀ3E•H2O
(Z,Z)ꢀ3B•H2O
2.4
5.5
4.0
5.2
2.8
1.57 (O...HO)
1.94 (N...HO)
1.88 (O...HO)
1.75 (O...HN)
2.04 (N...HO)
1.84 (O...HN)
1.96 (O...HO)
1.77 (O...HN)
1.69 (O...HO)
1.92 (N...HO)
—
173.1
170.4
167.0
176.9
136.4
150.2
156.6
155.9
153.3
167.2
—
944.14404
944.13129
944.13416
944.12790
—
—
—
—
TS1g
TS2
TS3
TS4
(E,Z)ꢀ3A
(E,Z)ꢀ3B
4
5
6
944.12771
944.11813
944.11201
867.71512
867.72864
867.71838
1725.08906
1725.07206
1725.07494
13.9
8.4
10.2
8.5
—
—
0
10.7
8.9
3.4
1.8
3.6
3.5
2.9
4.0
1.4
6.8
7.6
—
—
—
1.86
1.81
—
1.93
—
—
—
138.7
142.4
—
138.6
157.9
159.4
149.1
168.2
152.4
1.79 (N(3)H...O´)
2.06 (N(3´)H...N(10))
2.05
1.88 (N(3)H...N(3´))
2.07 (N(10´)H...N(10))
7
8
1725.05360
1725.06511
22.3
15.0
2.6
3.7
a Tautomeric forms 1A,E, monohydrates of pyrazolone 3 and its dimers were calculated by the B3LYP/6ꢀ31G*, B3LYP/6ꢀ31G**, and
RHF/6ꢀ31G methods, respectively; structures 1B—D were calculated by the RHF/6ꢀ31G* method.
b The energies of the tautomers of compound 1 are given relative to that of the ketohydrazone form A, those of the dimeric structures are
given relative to that of dimer 4, and those of transition states are given relative to those of noninteracting reactants.
c "D" and "A" denote the hydrogen bond donor and acceptor, respectively.
d Obtained from RHF/6ꢀ31G* calculations.
e The Gtot value in MeOH obtained from B3LYP/6ꢀ31G* calculations using the PCM model is given in parentheses.
f The ΔG° value corresponding to the Gibbs free energy difference between the forms E and A in MeOH is given in parentheses.
g The imaginary vibrational frequencies of TS1—TS4 are 841, 911, 755, and 1127 cm–1, respectively (according to B3LYP/6ꢀ31G**
calculations).
From this point of view, structures 1B—D are characterꢀ
ized by less favorable bonding topology; in addition,
some of them are destabilized by the absence of IHB
(1B″, 1C, 1D) and (or) by partial or complete violation
of the conjugation chain (1С and 1D) .
Further refinement of the energies of the two lowestꢀ
lying tautomers was carried out by the B3LYP/6ꢀ31G*
method with partial inclusion of the electron correlation
energy. It was found that this effect better stabilizes the
ketohydrazone tautomer 1A through its flattening and
strengthening of the IHB. As a result, the form 1A adopts
a symmetrical Сsꢀstructure and becomes 1.9 kcal mol–1
more stable than (E,E)ꢀ1E in the gas phase (see Table 1).
Here the crucial role is probably played by the fact that
structure 1A is stabilized by the IHB while the tautomer
(E,E)ꢀ1E is not. These results show that the 2ꢀbenzimidꢀ
azolylamino group of the ketohydrazone tautomers A
is prone to quite readily undergo a transition to the imiꢀ
no form.
The Nꢀphenyl group in molecule 1С is considerably rotated about
the С—N bond and deviates from the pyrazole ring plane.

Prototropic tautomerism
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1499
Table 2. Comparison of geometric characteristics (bond lengths (d) and bond angles (ω)) of the ketoazine tautomer (E,E)ꢀ1E obtained
experimentally (Xꢀray analysis data) and theoretically (from B3LYP/6ꢀ31G* calculations)
c
c
Bond
d/Å
Δ /Å
Angle
ω/deg
Δ /deg
Experimenta Calculationsb
Experimenta Calculationsb
С(2)—N(10)
N(3)—H
1.341
0.91(3)
1.344
1.312
1.439
1.456
1.306
1.417
1.371
1.242
2.750
1.789
2.882
1.980
1.327
1.012
1.352
1.299
1.459
1.493
1.302
1.397
1.399
1.223
—
–0.014
0.108d
0.008
–0.013
0.020
0.037
–0.004
–0.020
0.028
–0.019
—
N(3)—C(2)—N(10)
C(2)—N(10)—N(11)
C(12)—C(16)—N(15)
C(13)—N(14)—N(15)
N(14)—N(15)—C(16)
N(14)—N(15)—C(19)
C(20)—C(19)—C(24)
C(19)—C(20)—C(21)
O(17)...H—O(H2O)
N(3)—H...O(H2O)
129.0
107.2
104.1
107.0
112.0
119.0
120.3
119.4
160.7
174.6
128.5
108.9
103.1
109.0
112.1
118.7
119.8
119.4
—
–0.5
1.7
–1.0
2.0
N(10)—N(11)
N(11)—C(12)
C(12)—C(13)
C(12)—C(16)
C(13)—N(14)
N(14)—N(15)
N(15)—C(16)
C(16)—O(17)
O(17)—O(H2O)
O(17)...H(H2O)
N(3)—O(H2O)
H(N(3))...O(H2O)
0.1
–0.3
–0.5
0.0
—
—
—
—
—
—
—
—
—
a According to Xꢀray analysis data for 1•1/2H2O.
b Gasꢀphase data.
c Deviation of the calculated value of this parameter for compound 1from the experimental value obtained for the hemihydrate of this compound.
d The lengths of the bonds formed by the hydrogen atom are determined by Xꢀray analysis with the largest error and reveal systematic
underestimation by ~0.1 Å as compared to the results of neutron diffraction and microwave spectroscopy measurements.
From the results obtained for structures 1A—E it folꢀ
lows that the ketoazine tautomers E are highly polar
systems (see Table 1, μса1с(1E) = 8.6 D) , which differs
them from other tautomers (prototropic isomers). Thereꢀ
fore, in condensed media the ratio of the two main forms of
compounds 1—3 should change in favor of the form E, which
is most efficiently solvated or stabilized by the crystal
lattice. With initially small energy difference between the
tautomers A and E, one can expect that the tautomers Е of
pyrazolinones 1—3 will dominate in the condensed phase.
Indeed, B3LYP/6ꢀ31G* calculations using the PCM
model showed that the Gibbs free energy (G°tot) of (E,E)ꢀ1E
in MeOH solution should be 1.5 kcal mol–1 lower than
that of the tautomer 1A (see Table 1).
The correctness of these conclusions, as applied to the
solid phase, was substantiated in our experiments taking
a hemihydrate of compound 1 as an example. According
to Xꢀray analysis data, in the crystalline state this comꢀ
pound exists as tautomer 1E with the E,Eꢀconfiguration
of substituents at exocyclic double bonds. In this hemihyꢀ
drate each water molecule is bonded to two neighboring
molecules 1E by four hydrogen bonds in such a manner
that two hydrogenꢀcontaining tenꢀmembered macrocyꢀ
cles are closed and the entire trimolecular system has a С2
symmetry (Fig. 1).
The experimental geometric characteristics of the
hemihydrate are in reasonable agreement with the results
of DFT calculations of structure (E,E)ꢀ1E (Table 2).
Molecules 1—3 contain the benzimidazolylazo fragꢀ
ment. As a result, isomerization of these compounds
occurs in a much more complex manner than that of the
simplest 2ꢀpyrazolinꢀ5ꢀones. In particular, a quantum
chemical study predicts the ability of the forms A, B, E to
undergo ready interconversions without both protonatꢀ
ing and deprotonating catalytic additives by the interꢀ or
High polarity of the tautomeric form E is due to intramolecular
charge transfer from the iminobenzimidazoline fragment to the
pyrazolone fragment, as indicated by corresponding spatial orientaꢀ
tion of the dipole moment vector in molecule 1E. In terms of
the valence bond method, this effect can be interpreted as a result of
a large contribution of the bipolar canonical structure shown in
Scheme 2 to the wave function of the form Е.

1500
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Morkovnik et al.
N(3A)
H(26A)
O(17A)
O(H2O)
H(26)
C(4)
C(9)
C(21)
C(5)
O(17)
C(16)
C(20)
C(22)
C(6)
N(3)
C(2)
C(19)
N(15)
N(11)
C(8)
C(23)
C(12)
C(7)
N(1)
C(24)
N(10)
N(14)
C(13)
C(25)
Fig. 1. Molecular structure of a fragment of crystal packing corresponding to hemihydrate 1•1/2Н2О (according to Xꢀray analysis data).
C(18)
intramolecular mechanism, which involves transfer of
a proton and transformation of conjugated cyclic system
of hydrogen bonds. The most versatile is the intermolecꢀ
ular mechanism of tautomerism (isomerization) with the
key step involving doubleꢀproton transfer (DPT) in douꢀ
bly bonded hydrogen associates of benzimidazolylazopyrꢀ
azolinones. This type of reactions is also often efficient in
those cases where monomolecular tautomerism is hamꢀ
pered by high activation barriers.5
In pyrazolinones 1—3, doubleꢀproton transfer proꢀ
vides, in particular, interconversion of the tautomers A,
B, and E, which involves intermediate formation and
tautomerism of hydrogenꢀbonded solvates with hydrꢀ
oxylꢀcontaining solvents, which act as catalysts.
The tendency of benzimidazolylazopyrazolinones to
formation of this type of solvates is substantiated by not
only Xꢀray analysis data, but also the results of quantum
chemical calculations of isomeric monohydrates of comꢀ
pound 3. The calculations predict the lowest energy for
the monohydrate (E,E)ꢀ3E•H2O containing a tenꢀmemꢀ
bered Hꢀbonded macrocycle similar to that of the hemiꢀ
hydrate (E,E)ꢀ1E•1/2H2O (Scheme 3).
and 1,7ꢀprototropic migration of two protons (NHꢀ or
OHꢀproton of the pyrazolinone and one proton of water
molecule) along the perimeter of the sixꢀ, eightꢀ, or tenꢀ
membered Hꢀbonded ring (see Scheme 3). Both migratꢀ
ing protons move in the ring either clockwise or counterꢀ
clockwise. These transformations belong to a more genꢀ
eral class of doubleꢀproton prototropic processes in
monosolvates (selfꢀassociates), which involve the forꢀ
mation of cyclic transition states (TS). Among
them, 1,3ꢀprototropic doubleꢀproton tautomerism5,25—28
has been best studied. It plays an important role
in biochemistry, being, in particular, responible for the
formation of energetically unfavorable tautomers in hyꢀ
drogenꢀbonded pairs of DNA nucleic bases, a key step in
the mechanism of DNA point mutation (see, e.g.,
Ref. 25). The simplest systems appropriate for modeling
the key step of this important biochemical reactions
are hydrogenꢀbonded dimers of amides and amidines,
free nucleic bases, and their structural heterocyclic
analogs.5,25—28
Doubleꢀproton tautomerism in doubly bonded hyꢀ
drogen associates usually proceeds in a concerted fashion
and readily even in the solid phase at rate constants of up
An interesting feature of catalytic tautomerism of
the forms 3A,B,E is the variety of possible mechanisms
of the process, which include concerted 1,3ꢀ, 1,5ꢀ,
to 109—1010 s–1 29—32
.
Here, the key role may be played by
proton tunneling,33—36 whose contribution rapidly inꢀ

Prototropic tautomerism
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1501
Scheme 3
Note. Numbers in parentheses denote the energies of unsolvated monohydrates (in kcal mol–1) calculated relative to that of the most
stable structure (E,E)ꢀ3E•H2O; for transition states shown are the calculated activation energies (ΔE≠) for the direct and reverse (in
brackets) reactions.
creases as the temperature decreases and becomes preꢀ
dominant at low temperatures.37 Stepwise migration of
two protons in symmetrical and many unsymmetrical
unsolvated hydrogen associates is unlikely because transꢀ
fer of the first proton resulting in charge separation is
energetically unfavorable. However, in polar media the
energy difference between the concerted and nonꢀconꢀ
certed reaction pathways becomes much smaller.26,38 For
unsymmetrical associates formed by sufficiently strong
6ꢀ31G** method, tautomerism of pyrazolinone monoꢀ
hydrates 1—3 is of concerted character. Here, the 1,3ꢀ,
1,5ꢀ, and 1,7ꢀprototropic mechanism of the transformaꢀ
tions under study is confirmed by the geometry of these
TS (see Fig. 2). The Hꢀbonded rings in which DPT ocꢀ
curs (including the tenꢀmembered ring in TS3) are alꢀ
most planar with small rootꢀmeanꢀsquare deviation of
atoms from the plane (0.035, 0.089, and 0.01 Å for TS1,
TS2, and TS3, respectively). Concerted character of DPT
in monohydrates of compound 3 follows from the charꢀ
acteristics of transformation of the reacting systems movꢀ
ing along the reaction coordinate. The energies and some
other characteristics of TS1—TS3 are summarized
in Table 1. Note that the activation energies for reactions
(ΔE≠calc) are low and depend only slightly on the size
of the atomic matrix used to describe proton migration
/
Bronsted acids and bases, or for substrates characterized
by intramolecular DPT and having structureꢀstabilized
zwitterionic intermediates of singleꢀproton transfer (SPT)
a twoꢀstep DPT mechanism involving two successive SPT
acts is possible.38—42
According to the results of studies of the transition
states TS1—TS3 of compound 3, located by the B3LYP/

1502
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Morkovnik et al.
0.97
1.11
1.11
1.09
1.40
1.43
1.39
1.40
1.23
1.38
1.41
1.46
1.35
1.38
1.09
1.41
1.09
1.37
1.37
1.01
1.39
1.49
1.31
1.32
1.40
1.45
1.39
1.46
1.09
1.31
1.49
1.09
1.09
1.15
1.10
1.10
TS1
1.10
0.97
1.12
1.14
1.09
1.27
1.39
1.39
1.40
1.38
1.42
1.27
1.35
1.32
1.09
1.41
1.09
1.39
1.38
1.47
1.01
1.39
1.29
1.34
1.39
1.39
1.46
1.40
1.31
1.45
1.09
1.09
1.09
1.49
1.09
1.10
1.09
TS2
1.10
0.99
1.14
1.27
1.31
1.35
1.01
1.40
1.11
1.45
1.09
1.32
1.45
1.34
1.40
1.39
1.40
1.38
1.42
1.31
1.45
1.09
1.41
1.09
1.30
1.38
1.49
1.09
1.39
1.10
1.37
1.10
1.39
1.45
1.09
1.09
TS3
1.10
1.09
Fig. 2. B3LYP/6ꢀ31G** calculated geometries of transition states TS1—TS3. Arrows show the directions of proton motion for direct and
inverse DPT. The bond lengths are given in Å.

Prototropic tautomerism
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1503
between its outermost elements (see Table 1, ΔE≠
=
the formation of tenꢀmembered Hꢀbonded ring. These
8—14 kcal mol–1).
features differ the solventꢀpromoted tautomerism of pyrꢀ
azolinones from degenerate 1,3ꢀprototropic tautomerꢀ
ism of the formic acid43 and formamidine monosolꢀ
vates,34,44—46 which occurs exclusively with conservation
of the size and atomic composition of the Hꢀbonded
rings. Clearly, the last reaction in Scheme 3, which inꢀ
volves the lowestꢀlying tautomer (E,E)ꢀ3E, is the most
practically important.
An important role in intermolecular mechanism of
tautomerism of pyrazolinones 1—3 should also be played
by the selfꢀassociates of solvates (cyclic hydrogenꢀbondꢀ
ed dimers). The aptitude of the tautomeric forms A, B,
and E for dimerization is confirmed by the results of
RHF/6ꢀ31G quantum chemical calculations of pyrazoliꢀ
none 3, according to which this compound can form
dimers 4—8 (Scheme 4, Table 1). Dimer 4 (С2 symmeꢀ
try) is the most stable, it is formed by the lowestꢀlying
tautomer (E,E)ꢀ3E and includes a sixteenꢀmembered
Hꢀbonded macrocycle. This is due to two intermolecular
hydrogen bonds (IMHB) N(3)—H...O=C. Dimers 5 and
6 are respectively formed by the Z,Zꢀ and E,Zꢀforms of
A characteristic feature of tautomerism of the monoꢀ
hydrates of pyrazolinone 3 is that in two of the three
cases it involves transformation of Hꢀbonded rings. No
transformation occurs only in the third reaction (TS3)
proceeding with conservation of the tenꢀmembered
Hꢀbonded ring on going from the reactant to the TS and
then to the reaction product. In the other two reacꢀ
tions the (n+3)ꢀmembered Hꢀbonded ring necessary for
1,nꢀprototropic doubleꢀproton tautomerism is formed by
narrowing or expanding the Hꢀbonded ring in the initial
monohydrate. In particular, in the first reaction 1,3ꢀproꢀ
totropic tautomerism of monohydrate (Z,Z)ꢀ3E•H2O
involves transformation of its tenꢀmembered Hꢀbonded
macrocycle to the sixꢀmembered Hꢀbonded ring TS1,
which is also included in the reaction product. In the
second reaction the eightꢀmembered Hꢀbonded ring
TS2 is formed by transformation of the sixꢀmembered
Hꢀbonded ring of the initial reactant, monohydrate
(Z,Z)ꢀ3A•H2O, while the formation of the final product,
that is, monohydrate (Z,Z)ꢀ3B•H2O, is accompanied by
Scheme 4
Note. Proton migration directions are arrowed.

1504
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Morkovnik et al.
tautomer 3E. The former is structurally similar to
dimer 4, whereas the latter (next in stability) is asymmetꢀ
rical and linked by two IMHBs, N(3)—H...O=C and
N(3)—H...N(10). Structures 7 and 8 belong to amidineꢀ
type dimers, being formed by the tautomer 3A and tauꢀ
tomers 3A and 3B, respectively. Since dimerization of
benzimidazolylazopyrazolinone 3 is sterically hindered,
all dimers have essentially nonplanar geometries; dimer
6 (Fig. 3) is characterized by almost perpendicular arꢀ
rangement of the planes of two monomers relative to
each other.
be due to computational difficulties associated with a large
number of atoms in the dimers and with a complex topolꢀ
ogy of the potential energy surface (PES) in the vicinity of
the TS of DPT. In this connection mention may be made
that the transition state of concerted DPT could be locatꢀ
ed in our recent RHF/6ꢀ31G study of 1,2,4ꢀtriazinoꢀ
[2,3ꢀa]benzimidazolꢀ5(4)Hꢀ3ꢀone homodimers containꢀ
ing a much smaller number of atoms.47
The forms A and B can also undergo an interconverꢀ
sion with ease due to intramolecular singleꢀproton transꢀ
fer within the IHB. Unlike the TS of 1,2ꢀ and 1,3ꢀproꢀ
totropism studied earlier,45,48—51 the cyclic state TS4 of
As in the monohydrates of pyrazolinone 3 considered
above, DPT should occur in dimers 4—8 (see Scheme 4),
resulting in tautomeric forms that can act as intermediꢀ
ates of the following transformations:
the reaction (E,Z)ꢀ3A
(E,Z)ꢀ3B located by the
B3LYP/6ꢀ31G** method (see Scheme 4) is characterꢀ
ized by small strain and low energy. The ΔE≠calc values for
the direct and reverse reactions (see Table 1, 8.5 and
2.0 kcal mol–1, respectively) point to high rates of the
tautomerism proceeding by this mechanism.
A
E
B,
2 E
A + B,
B + A.
NMR study of pyrazolinones 1 and 2. The largest proporꢀ
tion of the ketoazine tautomeric form E in the solutions of
pyrazolinones 1 and 2 was confirmed in a study of the 1H and
13С NMR spectra of these compounds in CDCl3 (this
solvent is characterized by relatively low polarity and
weak solvation effect) and in polar DMSOꢀd6 (Table 3).
A + B
However, we failed to locate the TS of double proton
transfer in the dimers of pyrazolinone 3, which seems to
H
H
H
H
H
C
H
H
C
N
N
C
1.291
C
122.3
N
H
N
C
1.474
1.341
1.317
C
C
C
C
C
114.8
1.343
110.4
122.4
132.7
132.7
H
C
1.240
120.0
N
C
123.7
H
O
N
1.003
H
2.064
H
H
N
C
N
1.785
H
H
H
C
H
O
H
C
N
N
C
C
N
C
H
C
N
C
H
H
C
C
C
H
H
H
Fig. 3. Molecular structure of asymmetrical homodimer 6 according to RHF/6ꢀ31G calculations. Shown are selected bond lengths (in Å) and
bond angles (in degrees).

Prototropic tautomerism
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1505
These NMR spectra exhibit conventional patterns (in parꢀ
ticular, they show no broadened signals, except for the
NH group signal), which suggests a significant shift of the
tautomeric equilibrium toward the predominant tautoꢀ
meric form whose nature could not be determined by
analyzing the NMR spectra. To solve this problem, we have
carried out additional DFT (PW86/IGLOꢀII//B3LYP/
Table 3. Calculated (δcalc) and experimental (δexp) chemical shifts in
the 1Н and 13С NMR spectra of compounds 1 and 2
1
6ꢀ31G*) calculations of the H and 13С chemical shifts
for the tautomers 1A and 1E. They gave quite realistic δcalc
values (see Table 3) and showed that the calculated chemꢀ
ical shifts obtained for tautomer 1E rather than 1A are
much closer to the experimental chemical shifts of pyrꢀ
azolinone 1. The rootꢀmeanꢀsquare deviations (R) of
the calculated 1H and 13С chemical shifts from the
corresponding experimental values were 0.5 and 1.8 ppm
(1H) and 1.7 and 6.3 ppm (13С) for 1E and 1A, respecꢀ
tively.
Compounds 1 and 2 are also characterized by close
values of the chemical shifts of the oꢀphenylene protons
H(4)—H(7), which manifest themselves as a multiplet in
the NMR spectra. This is rarely observed for benzimidꢀ
azole derivatives. This feature of pyrazolinones 1 and 2
also points to their existence in solution in the form E,
because calculations gave the corresponding result for
only one of the two tautomers, namely, 1E (see Table 3).
Thus, 4ꢀ(1ꢀalkylbenzimidazolꢀ2ꢀylazo)ꢀ2ꢀpyrazolinꢀ5ꢀ
ones belong to heteroaromatic oꢀhydroxyazo compounds,
which are prone to exist in the ketoazine tautomeric form
in the condensed phase.
a
b
Atom
δ
δ
exp
calc
(E,E)ꢀ1A
(E,E)ꢀ1E
1H
1c
2
С(4)H
С(5)H
С(6)H
С(7)H
NCH3
(NCH2)
С(18)H
C(20)H
C(21)H
C(22)H
C(23)H
C(24)H
NH
7.97
7.44
7.81
7.48
4.35
7.54
7.56
7.56
7.51
3.63
7.22—7.34d
7.22—7.34
7.22—7.34
7.22—7.34
3.80
7.22—7.32
7.22—7.32
7.22—7.32
7.22—7.32
4.22
2.31
8.92
7.60
7.35
7.53
8.00
13.70
2.19
8.96
7.42
7.18
7.57
7.89
9.20
2.60
7.95
7.38
7.11
7.38
7.95
7.69
2.56
7.95
7.40
7.12
7.40
7.95
7.73
13C
С(2)
С(4)
С(5)
С(6)
С(7)
С(8)
С(9)
C(12)
C(13)
C(16)
С(18)
С(19)
C(20)
C(21)
C(22)
C(23)
C(24)
СNMe
145.52
123.61
126.20
126.77
109.61
140.11
144.02
130.48
147.82
157.62
14.80
142.24
118.41
130.99
127.17
131.31
118.29
32.51
155.76
111.00
126.67
125.62
109.86
135.13
131.36
156.34
112.00
123.61
122.83
110.17
135.26
132.21
157.6
111.6
123.8
122.9
109.3
135.8
131.4
142.4 (к)
129.7
163.4
17.3
138.8
118.5
128.7
124.2
128.7
118.5
42.6
Summing up, it should be emphasized that the results
obtained are of crucial importance for coordination
chemistry of azoꢀligand systems to which researchers pay
incessant interest.4,5,7,52
.
143.49 141.77 (128.3)
140.32 139.19 (148.3)
162.02 162.06 (157.6)
Experimental
20.66
18.05
142.79 139.19 (137.9)
117.15 117.45 (118.3)
130.39 128.67 (128.8)
125.25 123.61 (124.9)
IR spectra of compounds 1 and 2 were measured on a Nicolet
1
Impacsꢀ400 instrument (Nujol mull). H and 13С NMR spectra
were recorded on a Varian Unityꢀ300 spectrometer (300 MHz) in
CDCl3 and DMSOꢀd6 (see Table 3) with internal stabilization
130.57
117.65
28.47
128.67
117.45
28.83
2
the H resonance line of the deuterated solvent. The energies of
tautomers were calculated using the GAMESS (US) program53
(PC Gamess version).54,55 The stationary points on the PES of the
molecules and reaction systems under study were identified by
calculating the corresponding force constant matrices with the same
basis set as that used in the final geometry optimization. The correꢀ
spondence between a given TS and the particular reaction was
substantiated by monitoring the behavior of the reaction system
when moving along the reaction coordinate from the transition state
toward both reactants and products. The energies of reactants, TS,
and products were calculated ignoring the zeroꢀpoint vibrational
energy (ZPE) correction. Ab initio calculations of the 1H and
13С chemical shifts (vs. Me4Si) were carried out using the DeMon
program.56
(СNCH
)
2
Note. The rootꢀmeanꢀsquare deviations (R) of the calculated chemꢀ
ical shifts from corresponding experimental values were 0.5 and
1.8 ppm (1H) and 1.7 and 6.3 ppm (13С) for 1E and 1A, respectively
(averaging was performed over the types of magnetically nonequivꢀ
alent 13С or 1H nuclei).
a Calculated by the PW86/IGLOꢀII//B3LYP/6ꢀ31G* method for
the lowestꢀlying tautomers of compound 1; the chemical shifts are
given relative to Me4Si.
b 13С NMR spectrum of pyrazolinone 1 was recorded in DMSOꢀd6,
other NMR spectra were recorded in CDCl3.
c For comparison, the chemical shifts in the 13С NMR spectra of
3ꢀmethylꢀ1ꢀphenylꢀ4ꢀphenylazoꢀ2ꢀpyrazolinꢀ5ꢀone2 are given in
parentheses.
3ꢀMethylꢀ4ꢀ(1ꢀmethylbenzimidazolꢀ2ꢀylazo)ꢀ1ꢀphenylꢀ2ꢀ
pyrazolinꢀ5ꢀone (1) hemihydrate. 2ꢀAminoꢀ1ꢀmethylbenzimidaꢀ
zole (1.47 g, 10 mmol) was dissolved in 87% phosphoric acid
(15 mL) with stirring at 50 °С, the solution was cooled to –10 °С,
and finely ground sodium nitrite (0.76 g, 11 mmol) was added in
d The R value for the protons of this multiplet was calculated using
an average value of 7.28 ppm.

1506
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Morkovnik et al.
small portions at such a rate that no reddish brown NOx vapor was
observed. The reaction mixture was kept for 5 min at –5 °С, then the
temperature was raised to 15 °С over a period of 30 min and urea
(0.2 g) was added. After the gas liberation ceased, the cooled
solution of the diazo compound was slowly added at 5—7 °C to
a stirred solution of 3ꢀmethylꢀ1ꢀphenylꢀ2ꢀpyrazolinꢀ5ꢀone (1.8 g,
10 mmol) in an AcOH—EtOH mixture (30 mL, 1 : 1 v/v). Then
water (20 mL) was added to the reaction mass in small portions with
stirring over a period of 30 min, the temperature was gradually
raised to room temperature, and the mixture was left for 6 h at this
temperature. The precipitate (bright red protonated form of the azo
compound) was filtered off, washed with water, acetone, and thorꢀ
oughly triturated with 10% NH4OH to maintain a weakly basic
reaction in order to obtain a base. The yield of the hemihydrate of
pyrazolinone 1 was 2.8 g (82%), orange crystals, m.p. 262—263 °С
(from AcOEt) (with decomp.). Found (%): С, 63.15; Н, 4.92;
N, 24.44. 2(C18H16N6O)•H2O. Calculated (%): С, 63.33; Н, 5.02;
N, 24.62. IR, ν/cm–1: 3328 (NH), 1667 (С=О).
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4ꢀ(1ꢀButylbenzimidazolꢀ2ꢀylazo)ꢀ3ꢀmethylꢀ1ꢀphenylꢀ2ꢀpyrꢀ
azolinꢀ5ꢀone (2) was obtained analogously from 2ꢀaminoꢀ1ꢀbuꢀ
tylbenzimidazole and 3ꢀmethylꢀ1ꢀphenylꢀ2ꢀpyrazolinꢀ5ꢀone.
Yield 79%, bright red crystals, m.p. 203—205 °С (from AcOEt).
Found (%): С, 67.36; Н, 5.92; N, 22.45. С21H22N6O. Calculatꢀ
ed (%): С, 67.48; Н, 6.02; N, 22.35. IR, ν/cm–1: 3433 (NH),
1667 (С=О).
Xꢀray diffraction study of [(E,E)ꢀ1]2•H2O (C36H36N12O3) was
carried out at 120 K on a Smart 1000 CCD automated threeꢀcircle
diffractometer (МоꢀКαradiation, graphite monochromator, ωꢀscan,
2θ < 54°). At 120 K crystals are orthorhombic: a = 14.4304(17) Å,
3
b = 7.4263(9) Å, c = 30.572(4) Å, V = 3276.3(7) Å , space group
Рbcn, Z = 8, М = 341.38, dcalc = 1.384 g cm–3, μ = 0.94 cm–1
,
F(000) = 1432. From a total of 20219 measured reflections
(Rint = 0.0873), 3583 independent reflections were used in further
calculations and refinement. The structure of [(E,E)ꢀ1]2•H2O was
solved by the direct method and refined by the least squares method
in the fullꢀmatrix anisotropic approximation. Hydrogen atoms were
located from the difference Fourier syntheses of the electron density
and refined anisotropically. The final Rꢀfactors were R = 0.0528
over 1691 reflections with I > 2σ(I), wR2 = 0.1262, and GOF = 1.002
over all measured reflections. All calculations were carried out using
the SHELXTL PLUS 5 program package.
This work was financially supported by the Council
on Grants at the President of the Russian Federation
(under Programs for State Support of Leading Scientific
Schools in the Russian Federation and Young Candidates
of Science and Their Supervisors, Grants NShꢀ363.208.3,
MKꢀ3351.2007.3 and MKꢀ3534.2007.3), the Russian
Foundation for Basic Research (Project No. 07ꢀ03ꢀ00710),
the Ministry of Education and Science of the Russian
Federation (Program 2.2.2.3.10010), and the Civilian
Research and Development Foundation (CRDF Grant
H4ꢀC04ꢀ02).
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Kluwer Academic Publishers, Boston, 2003, 2, 587.
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Received June 26, 2007;
in revised form January 23, 2008