Ion aggregation of methylpyrazinium iodide and its derivatives in crystals and in solutions of nonpolar solvents

Article abstract of DOI:10.1007/s11172-012-0047-0

Aggregation of methylpyrazinium iodide and its derivatives in chloroform and dichloromethane solutions was studied by EAS and ¹H NMR spectroscopy. The isopentylpyrazinium iodide ion quadruple formation constants in solutions were evaluated. For

Full text of DOI:10.1007/s11172-012-0047-0

Russian Chemical Bulletin, International Edition, Vol. 61, No. 2, pp. 343—350, February, 2012
343
Ion aggregation of methylpyrazinium iodide and its derivatives
in crystals and in solutions of nonpolar solvents
K. P. Birin,a D. S. Dunin,b V. Yu. Kotov,b and Yu. V. Nelyubinac
aA. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119071 Moscow, Russian Federation.
Fax: +7 (495) 952 4874. Eꢀmail: kirill.birin@gmail.com
bMoscow City Teacher Training University, Institute of Natural Sciences,
1 ul. Chechulina, 105568 Moscow, Russian Federation.
Fax: +7 (499) 308 9546. Eꢀmail: kotovv@ins.mgpu.ru
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: unelya@xrlab.ineos.ac.ru
Aggregation of methylpyrazinium iodide and its derivatives in chloroform and dichloroꢀ
methane solutions was studied by EAS and 1H NMR spectroscopy. The isopentylpyrazinium
iodide ion quadruple formation constants in solutions were evaluated. Formal extinction coefꢀ
ficient in the outerꢀsphere charge transfer band maxima does not depend on the degree of
aggregation in the system. A model suggested for the formation of ion quadruples in solutions
was discussed in terms of the density functional theory and using the Xꢀray diffraction data for
crystalline methylpyrazinium iodides and its derivatives.
Key words: methylpyrazinium iodide, ion aggregation, ion pairs, ion quadruples, EAS,
1H NMR, density functional theory, the B97D functional.
In solutions 1 : 1 of electrolytes in chloroform and other
solvents with low permittivity, various processes of associꢀ
ation of ions into ion pairs, ion triples, ion quadruples,
and more complex ion aggregates can take place.1—4 Asꢀ
sociation is mainly caused by electrostatic attraction of
ions with unlike charges. In addition, such interactions as
hydrogen bonding, stacking interactions, etc., can conꢀ
tribute into aggregation of ions in solution. In the last
years, NMR became the most important method for the
study of such systems. Intramolecular nuclear Overhauser
effect and diffusion NMR experiments allow one to obtain
information on relative orientation of species in aggreꢀ
gates and of their sizes.
Iodides of Nꢀheterocyclic cations are the most imporꢀ
tant objects of spectrophotometric studies of ion associaꢀ
tion and solvation in solutions, as well as of charge transꢀ
fer in the ground and excited states of ion pairs.5—10 Acꢀ
cording to the work,11 solutions of Nꢀmethylpyrazinium
iodide (RPz+) absorb in the visible region of the spectrum
and, therefore, are convenient objects for the study. In the
present work, we obtained and structurally characterized
Nꢀmethylpyrazinium iodide and its analogs, studied soluꢀ
1
tions of these compounds by H NMR and EAS in the
solvents with low permittivity in order to gather informaꢀ
tion on similarities and distinctions of ion aggregation in
the solid compound and in solutions.
The growth of aggregates is usually accompanied by
changes in chemical shifts of the protons, indicating existꢀ
ence of noncovalent interactions between ions. Accordꢀ
ing to the literature data,1—4 when the concentrations
of electrolytes in chloroform are 10–5—10–1 mol L–1
they exist in the solution mainly in the form of ion
pairs and ion quadruples. Theoretical approaches allow
one to evaluate the most favorable forms of such ion asꢀ
sociates.1 Comparison of calculated data with results
of Xꢀray diffraction studies enables revealing specific
features of the electrolyte structures in solution and the
solid phase.1
Studies of solutions by 1H NMR
The obtained compounds 1—6 were characterized by
1H NMR in methanol and were found to be the individual
compounds. The solubility of these compounds in chloroꢀ
form is low and increases as the R chain in the cations
RPz+ grows or if the aromatic protons in RPz+ are reꢀ
placed with methyl groups. Therefore, compounds 3, 5,
and 6a were used to study dependences of spectral characꢀ
1
teristics from concentrations. The H NMR spectra of
compound 5 in CDCl3 and CD2Cl2 resemble its spectrum
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 341—348, February, 2012.
1066ꢀ5285/12/6102ꢀ343 © 2012 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
Birin et al.
was neglected, assuming that their concentration is signifꢀ
icantly lower than that of ion pairs and quadruples:13
2(RPz+, I–) = (RPz+, I–)2.
(1)
The constant of formation of quadruple in the reaction
(1) does not depend on the ionic strength of the solution
and can be represented in the form
Comꢀ
pound
1a
1b
2a
2b
3
4
5
6a
6b
R1
R2
R3
R5
R6
X
Me
Me
Me
Me
Me
Me
iꢀC5H11
Pr
H
H
H
Me
Me
Me
H
H
H
Me
H
Me
Me
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
I–
К = /2C0(1 – )2,
(2)
2–
I4
I–
I–
I–
I–
I–
I–
where C0 is the concentration of compound 5,  is the
degree of its aggregation in the reaction (1).
A chemical shift  = 9.18, observed for the solutions of
5 in methanol or chloroform with 5 vol.% of methanol
and corresponding to compound 5 completely unassociꢀ
ated into ion quadruples, was chosen as a boundary shift.
The boundary value, corresponding to compound 5 comꢀ
pletely associated into ion quadruples, was selected in such
a way that the coefficients of the linear correlation beꢀ
tween the calculated values of /2(1 — )2 and C0 would
be the maximum for CDCl3 and CD2Cl2 (Fig. 1). The
resulted value was  = 9.69. The ion quadruples formation
constants for compound 5 calculated by Eq. (2) are 180 6 in
chloroform and 16.6 0.8 L mol–1 in dichloromethane.
Noticeable downꢀfield shifts of signals for the protons
in positions 2 and 6 were also observed for the solutions of
compounds 3 and 6a in CDCl3, that indicates a general
character of the processes taking place upon concentraꢀ
tion of the solutions. However, quantitative studies of these
systems are complicated by insufficient solubility of comꢀ
pound 3 in chloroform and formation of two immiscible
liquids in the system 6a—chloroform.
H
H
H
H
–
Pr
H
I3
in methanol. However, an increase in the compound
concentration causes a downꢀfield shift of the resonance
signals for the pyrazine ring protons (Table 1). The most
dramatic changes were observed in chloroform for the proꢀ
tons in positions 2 and 6. The resonance signals for the
protons in positions 2, 6 and 3, 5 were reliably assigned
based on the NOESY experiment, which showed the inꢀ
teraction between the protons in positions 2 and 6 and the
protons of the ꢀCH2 group in the aliphatic substituent.
Earlier,12 it has been noted that for the solutions containꢀ
ing alkylpyridinium iodides, the protons in positions 2 and
6 are more susceptible to the solvent nature than the proꢀ
tons in positions 3 and 5.
1
In the H NMR spectrum of compound 5 in CDCl3
solution of high concentration (0.216 mol L–1), we obꢀ
served considerable changes in positions of the signals for
the terminal methyl protons of the chain, indicating the
micelle formation in the system. Therefore, the systems
with the maximum concentration of 0.072 mol L–1, which
is lower than the critical concentration of the micelle forꢀ
mation, were used for further treatment.
NMR diffusion spectroscopy (DOSY) was additionalꢀ
ly used for the study of association processes of molecules
of compound 5. In the present time, this method is widely
used in the studies of compositions of complex multiꢀ
component mixtures, supramolecular association, and
To quantitatively describe phenomena observed in soꢀ
lutions of compound 5 in CDCl3 and CD2Cl2, a model for
the formation of ion quadruples from two ion pairs was
suggested, in which formation of ion triples in the system
/2(1 — )2
14
12
1
10
Table 1. Dependences of chemical shifts for the protons of the
pyrazine ring from the concentration of compound 5 in chloroꢀ
form and dichloromethane
8
6
4
C5/mol L–1
CDCl3
CD2Cl2




3,5
2,6
3,5
2,6
2
2
0.0036
0.0072
0.0144
0.036
9.42
9.48
9.54
9.58
9.61
9.49
9.50
9.51
9.52
9.53
9.34
9.36
9.41
9.45
9.52
9.52
9.52
9.53
9.54
9.55
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 C0/M
Fig. 1. Reduced conversion /2(1 — )2 of ion pairs to ion
quadruples in solutions of compound 5 versus its concentration
in the solvent: CDCl3 (1), CD2Cl2 (2).
0.072

Ion aggregation of methylpyrarinium iodide
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
345
selfꢀassembling processes.14,15 A twoꢀdimensional specꢀ
trum obtained in the DOSY experiment allows one to deꢀ
termine diffusion coefficients for all the components of
a sample under study, whereas the Stocks—Einstein relaꢀ
tion (D = kbT/6r) allows to compare sizes of the associꢀ
ates formed.
species: rCHCl /rMeOH = 2.48. It should be noted that in
3
both cases, the spectra exhibited signals for the protons of
the solvents, characterized by the same diffusion coeffiꢀ
cient as the compound studied. This fact indicates a sigꢀ
nificant solvation of ions and involvement of the solvent
into the formation of ion aggregates in the system, that, in
turn, influences the effective size of the species in soluꢀ
tion. Solvation effect, incomplete association of the moleꢀ
cules of compound 5 in methanol, and significant nonꢀ
sphericity of cations RPz+ allow us to explain the deviaꢀ
tion of the ratio of species sizes from that expected for the
formation of ion quadruples.
The concentration of compound 5 in CD3OD and
CDCl3 in the samples used for the DOSY studies was
0.072 mol L–1. In this case, we assumed that in methanol
this compound exists predominantly in the monomeric
form (as individual ions and ion pairs), whereas formation
of more complex aggregates occurs at this concentration
in chloroform. Experiments were carried out at 30 C to
minimize the influence of convection on the diffusion coꢀ
efficients. The diffusion coefficients obtained for comꢀ
Spectrophotometric studies of solutions
pound 5 were DMeOH = 1.29•10–5 cm2 s–1 and DCHCl
=
3
Solutions of methylpyrazinium iodides and its analogs
in chloroform and dichloromethane are characterized by
absorption bands of the outerꢀsphere charge transfer in
ion pairs in the UVꢀvisible region of electron absorption
spectra:
= 5.16•10–6 cm2 s–1 (Fig. 2). More than a twofold change
in the diffusion coefficient, with the viscosity () of methꢀ
anol and chloroform being close at this temperature
(0.510 mPa s–1 and 0.514 mPa s–1, respectively16), indiꢀ
cates an increase in effective size of the species in solution
as methanol is exchanged for chloroform as the solvent.
The Stocks—Einstein equation was used to determine
the ratio of the corresponding hydrodynamic radii of the
RPz+ + I– = (RPz+, I–),
(3)
(4)
(RPz+, I–) + h = (RPz •, I•).
Positions of the longꢀwave maxima () in chloroform
and dichloromethane virtually do not depend on the solꢀ
vent nature (Table 2) and depend differently on the conꢀ
centration of dissolved compounds. Thus, for compound 1a
the absorption maxima significantly shift to the shortꢀwave
region (with respect to 0) as the concentration grows. The
initial segment of the curve can be approximated by a straight
line. For it the ratio /C is 1.54•106 cm–1 L mol–1, that
is comparable with the value 0.59•106 cm–1 L mol–1 calꢀ
culated from the data5 for the concentration shift of the
absorption maxima observed earlier for 4ꢀcarboxymethylꢀ
1ꢀmethylpyridinium iodide in chloroform. The effect
becomes significantly smaller as chloroform is exchanged
D/cm2 s–1
10–4
CD3OD
10–5
10–6
9.5 8.5 7.5 6.5 4.5 3.5 2.5 1.5 0.5

Table 2. Spectral characteristics of the outerꢀsphere charge transꢀ
fer bands in compounds 1—6 in chloroform and dichloromethane
D/cm2 s–1
10–4
Comꢀ  (CH2Cl2)  (CHCl3) (CH2Cl2) (CHCl3) /С
0
0
CD3Cl
pound
(CHCl3),
cm–1
L mol–1 cm–1
rel. units
1a
2a
2b
3
4
5
21580
22180
22240
23190
23760
—
21520
22080
—
22970
23660
21430
1653
1800
1074
911
976
—
1822 1 (0.09)*
1710 0.45
10–5
—
—
948
0.27
1084 0.05
1742 0.54
10–6
9.5 8.5 7.5 6.5 4.5 3.5 2.5 1.5 0.5

6a
—
21500
—
1485 0.54
Fig. 2. The DOSY spectra of solutions of compound 5 in methaꢀ
nol and chloroform with the concentration of 0.072 mol L–1
.
* The value for CH2Cl2 is given in parentheses.

346
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
Birin et al.
for dichloromethane, that also agrees with the previꢀ
ous results.5
contacts I... exist in the ion quadruple because of the
interaction between an anion of one ion pair and a cation
of the other with the formation of the contacts C—H...I.
Extinction coefficient in the maximum of the outerꢀ
sphere charge transfer band, like for the pyridinium derivꢀ
atives,5,6 significantly changes on introduction of substitꢀ
uents in positions 2 and 6 and is weakly susceptible to the
nature of both the substituents in positions 3 and 5 and the
solvent. This allows us to draw a conclusion that upon
formation of ion pairs, an anion is localized in the vicinity
of the quaternary N atom.
Addition of substituents onto the aromatic ring signifiꢀ
cantly decreases the shortꢀwave displacements of the
absorption maxima (see Table 2), that indicates involveꢀ
ment of the CH groups into aggregation. The absence of
the concentration shift for the absorption maximum in
solutions of 1,2,6ꢀtrimethylpyridinium iodide and the
shift observed in the work5 were attributed by the authors6
to the formation of triiodide ions in the chloroform soꢀ
lutions.
According to our preliminary data, the concentration
displacements of the absorption maxima of ion pairs for
the chloroform solutions of Nꢀheterocyclic cation iodides
and bromides are virtually the same. Therefore, like the
authors17 we explain the changes occurred by the formaꢀ
tion of ion aggregates in the concentrated solutions of
compounds 1—6. For the optical density (A) in the absorpꢀ
tion maxima of the chloroform and dichloromethane soꢀ
lutions of compounds 1—6, the Bouguer—Lambert—Beer
law (A = Cl) ideally operates within a wide range of conꢀ
centrations (C = 0.00004—0.01 mol L–1).
To sum up, the formal extinction coefficient in the
maximum of the outerꢀsphere charge transfer band does
not depend on the degree of aggregation in the system,
with ion aggregates absorbing in the shortꢀwave region of
the spectrum. This allows us to suggest that in the ion
pairs there is an efficient overlap of the halide anion orbital
and the ꢀsystem of the aromatic cation, whereas no new
Calculations of ion pairs and ion quadruples
Supramolecular organization18 in the crystal of comꢀ
pound 1b suggests that four fragments in the cation anionꢀ
ic associates can be used as the ion pair models in soluꢀ
tion, as well as two fragments from two cations and two
anions as the ion quadruple models (Fig. 3). Results of
quantum chemical calculations for the chosen models of
ion pairs and quadruples without optimization of their
geometry showed that the ion pair, in which the iodide
anion is in the contact with the ꢀsystem of the pyrazine
ring, possesses the minimum energy (see Fig. 3, b). Forꢀ
mation of such an ion pair in solution should be accompaꢀ
nied by the appearance of a strong charge transfer band in
the electron absorption spectrum, which was indeed obꢀ
served in the experiment.
a
b
c
d
I(1AA)
I(1A)
C(2)
C(7)
N(1)
C(6)
C(2)
C(3)
C(5)
I(1A)
C(7)
C(3)
N(4)
C(3)
C(2)
N(4)
C(7)
N(1)
C(2)
C(6)
C(5)
N(4)
C(3)
N(4)
N(1)
C(6)
C(7)
I(1)
N(1)
C(5)
C(5)
C(6)
e
f
I(1)
C(7)
N(4A)
I(1A)
C(6)
N(1)
C(3A)
C(5)
C(3A)
C(2A)
N(1)
C(2)
N(4A)
C(5A)
C(6A)
C(7)
C(2A)
C(2)
C(6)
C(5)
C(7A)
N(4)
N(1A)
C(3)
C(3)
C(5A)
C(6A)
N(1A)
C(7A)
I(1)
N(4)
I(1A)
Fig. 3. Fragments of supramolecular associates in the crystal of 1b. The distances C...I (H...I), Å: 4.053(1) (3.15) (a), 3.629(1) (b),
3.924(1) (3.02) (c), 3.685(1) (d).

Ion aggregation of methylpyrarinium iodide
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
347
Formation of ion quadruples from two ion pairs proꢀ
ceeds with considerable gain in energy (on average
25 kcal mol–1), with the energies of two such quadruples
(see Fig. 3, e and f) differing by 4 kcal mol–1. The model
ion quadruples in solution do not contain new contacts
I..., which explains the unchanged values of extinction
coefficients. However, the quadruple structures do not corꢀ
respond to the changes in the NMR spectra of solutions
observed in the process of aggregation. Thus, in both cases
one should expect the strongest displacement of the sigꢀ
nals for the alkyl group: the upꢀfield in the first case (see
Fig. 3, e) and the downꢀfield in the second (see Fig. 3, f),
which contradicts the experiment, according to which only
insignificant downꢀfield shift was observed for the alkyl
protons during aggregation.
Proceeding from the stated above, the following strucꢀ
ture for quadruples in solution can be suggested. Two ion
pairs (see Fig. 3, b) can bond to a quadruple (Fig. 4) due to
the weak contacts C—H...I involving aromatic and aliꢀ
phatic protons (see Fig. 3, a). Formation of an aggregate
with the structure suggested (see Fig. 4) strictly correꢀ
sponds to the changes observed in the NMR spectra durꢀ
ing its formation. In this case, the protons in positions 2
and 6 of the pyrazine ring are placed in the shielding reꢀ
gion of the aromatic system of the neighboring molecule,
which explains their downꢀfield shift. Other protons of the
aggregate are quite at a distance from its aromatic systems
and are, therefore, less affected by their anisotropy.
At the same time, the iodide anions in such an aggreꢀ
gate would form the strongest hydrogen bonds with the
protons in positions 2 and 6. However, in the structure 1b
the ion pairs (see Fig. 3, a and b) are combined by the
contacts C—H...I with the formation of infinite double
chains (C...I 3.924(1)—4.020(1), H...I 2.02—3.09 Å) and
a threeꢀdimensional cage (C...I 3.924(1)—4.053(1) Å,
H...I 2.02—3.15 Å), whereas no analog of the model quaꢀ
druple (see Fig. 4) is present in the structure. Apparently,
arising of additional contacts C—H...I in the solid state
explains the differences in the processes of ion aggregation
in the solid compound and in solutions.
is displaced aside and located over the C(2)—C(3) bond.
In the crystal 6a with four independent formula units,
the quadruple in question can also be found, though slightꢀ
ly distorted. It is formed by the weak contacts I...
(C...I 3.761(1)—3.923(1) Å) and C—H...I (C...I
3.836(1)—4.032(2), H...I 2.93—3.37 Å) and additionally
stabilized by the contacts C—H... and H...H between the
aliphatic groups and the aromatic rings. In this case, the
"quadruples", in which the aromatic rings of the neighborꢀ
ing cations are not coplanar and in some case are even
perpendicular, are also effected in the crystal 6a.
It should be noted that the ion pairs of the three types
found for compound 1b are present in the structure 3 (see
Fig. 3): a (C...I 3.873(2) and 4.200(2), H...I 2.94 and
3.27 Å), b (the shortest distance C...I 3.626(2) Å), and c
(C...I 3.998(2), H...I 3.07 Å). However, instead of formaꢀ
tion of an ion pair of the type d, it is possible that the Me
group in the orthoꢀposition to the methyl group bonded to
the ring N atom interacts with the bromide anion (the
distance C...I 4.008(2) Å).
In the crystal 6a, only ion pairs of the type a (C...I
3.836(2)—4.186(2), H...I 2.93—3.41 Å) and, possibly,
of the type b (the shortest distances N...I and C...I
3.560(3)—3.718(3) and 3.699(3)—3.857(3) Å) can be sinꢀ
gled out. Similarly, ion pairs of the type c are exclusively
present in the crystal 1a (C...I 3.958(3)—3.987(3), H...I
3.02—3.05 Å), in the crystal 2a, of the type a (C...I 3.851(1)
and 4.174(1), H...I 2.92 and 3.25 Å) and c (C...I 3.944(1),
H...I 3.12 Å), whereas in the crystal 4, ion pairs of the type
a (C...I 3.849(4) and 4.319(4), H...I 2.90 and 3.42 Å) and
b (the shortest distance C...I 3.586(4) Å). In the case of
salt 6b, in which the species I3– serves as a counterion, ion
pairs can be described by none of the association types
indicated previously for compound 1b.
Comparison of the ion aggregate states in the solid and
dissolved electrolytes
We undertook in detail analysis of supramolecular orꢀ
ganization in the crystals 1a, 2a, 3, 4, 6a, and 6b in order
to clarify the reasons responsible for the absence of the
fragment corresponding to a quadruple in most Nꢀmethylꢀ
pyrazinium iodides in the crystalline state. Thus, in the
crystal 1a, instead of the expected contacts I..., the interꢀ
actions of the type N... are effected between cations (the
shortest distance N...C 3.182(4) Å), leading to the formaꢀ
tion of cationꢀcationic tetramers, which combine between
each other due to the described above contacts C—H...I
and are additionally stabilized by the weaker contacts
C—H...I (C...I 3.970(3), H...I 3.23 Å). It should be noted
that instead of ion association by the type d, the CH3
group interacts with a ꢀsystem of the neighboring cation
(the shortest distances C...C and H...C 3.497(3) and
2.92 Å), leading to the formation of analogous cationꢀ
cationic tetramers.
According to the Xꢀray diffraction data for the crystals
1a, 2a, 3, 4, 6a, and 6b, the fragment corresponding to the
quadruple 5 can be singled out only in two cases. Thus, in
the crystal 3 the mutual arrangement of the ions correꢀ
sponds to the model quadruple, however, the iodide anion
Fig. 4. Suggested structure of the quadruple 5 in solution.

348
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
Birin et al.
The crystals 2a have no expected contacts I... either.
on the central C atom of the propyl group. Formation of
a threeꢀdimensional cage in the crystal 6a is finished by
the contacts C—H...I formed by the aromatic C atoms
(C...I 4.144(1), H...I 3.18 Å).
In conclusion, the model of formation of the ion quaꢀ
druples in solutions, suggested based on the studies of agꢀ
gregation processes by EAS and 1H NMR, has no unamꢀ
biguous analogy for the crystalline structures of methylꢀ
pyrazinium iodides and its derivatives. Instead of the conꢀ
tacts I... effected in solutions during formation of ion
pairs and ion quadruples, interactions of the types N...,
..., and C—H... are present in most crystalline strucꢀ
tures studied.
Interactions of the type ... between cations are observed
instead (the shortest distance C...C 3.381(2) Å), leading
to the formation of cationꢀcationic dimers, which are
additionally stabilized by the contacts C—H...I (C...I
3.851(1)—4.140(1), H...I 2.92—3.17 Å). The CH3 group
also forms the contact C—H... (the shortest distances
C...C and H...C 3.693(2) and 2.83 Å), which, together
with the contact C—H...N (C...N 3.472(2), H...N 2.70 Å),
leads to the formation of a double cationꢀcationic chain.
In the crystal 4, instead of formation of the quadruple
in question, ion pairs of the type a combine with each
other by the contact I... into infinite layers, in which the
iodide anion placed over the ꢀsystem of one of the catꢀ
ions is located in the same plane with the other (as in the
desired cation). However, in this case the shortest disꢀ
tances C...I and H...I with the two methyl groups in orthoꢀ
and metaꢀpositions with respect to the CH3 group bonded
to the ring N atom are 3.926—4.284(4) and 3.47—3.82 Å.
The crystal have also the contacts C—H...N (C...N
3.568(7) and H...N 2.62 Å) between cations, combining
them into centrosymmetric dimers. The CH3 group, inꢀ
stead of interactions by the type d, is apparently involved
into the formation of the contact H...H with the symmetꢀ
rically equivalent group of the neighboring cations (the
distance H...H 2.4 Å).
In the crystal 6b, instead of association of the type
I..., the triiodide anion weakly interacts with the ꢀsysꢀ
tem of the cation (I...C 3.648(7) Å), in which case the ion
I3– is not positioned over the ring, rather it is significantly
displaced with respect to it. The ions in the crystal are also
combined with each other through the multitude of conꢀ
tacts C—H...I with involvement of the aromatic C atoms
(C...I 3.706(7)—4.096(7), H....I 3.01—3.17 Å), as well as
the contacts N... between cations (the shortest distance
N...C 3.238(13) Å), leading to the formation of cationꢀ
cationic chains.
In the crystal 6a, in contrast to the contacts C—H...I
described above, leading to the formation of ion pairs of
the type a and combining the I...ꢀbonded associates into
the quadruple of interest, no cꢀ and dꢀtype associations
are effected. Instead, the C3H7 group of one of indepenꢀ
dent molecules is involved into the formation of the
contacts C—H...N (C...N 3.361(3), H...N 2.74 Å) and
C—H... (the shortest distances C...C and H...C 3.638(3)
and 2.83 Å) with the symmetrically equivalent cation. The
C3H7 group of another independent molecule forms the
contacts C—H... (the shortest distances C...C and H...C
3.769(3) and 2.86 Å) and H...H (2.32 Å), whereas the
C3H7 group of a third molecule, the contacts C—H...N
(C...N 3.496(3), H...N 2.67 Å) and H...H (2.32 Å). In this
case, the C3H7 group of the latter independent molecule
forms the contacts C—H...N (C...N 3.401(3), H...N 2.50 Å)
and H...H (2.39—2.79 Å), as well as C—H...I (C...I
4.144(1), H...I 3.18 Å) with involvement of the H atom
Experimental
Samples for NMR studies were prepared by dissolution of
1
compounds in MeOD, CD2Cl2, or CDCl3. H NMR spectra of
methylpyrazinium iodides and its derivatives in methanolꢀd4 were
recorded on Bruker DRX 500 and Bruker AM 300 spectrometers
(500.13 and 300.13 MHz at 300 K, respectively) (N. D. Zelinsky
Institute of Organic Chemistry, Russian Academy of Sciences).
Chemical shift of the resonance signals were measured relative
to the residual protons of the solvent. 1H NMR spectra in CD2Cl2
and CDCl3, as well as DOSY diffusion spectra in CDCl3 and
MeOD were recorded on a Bruker AvanceꢀIII spectrometer
(600.31 MHz at 303 K) in the Center of NMR studies in the
A. N. Frumkin Institute of Physical Chemistry and Electroꢀ
chemistry, Russian Academy of Sciences.
Electron absorption spectra were recorded on a Specord
50 PC spectrophotometer in quartz cuvettes with the path length
of 0.2, 1, and 3 cm in the range of wavelengths 300—550 nm.
From the spectra obtained, the fragment of the absorption
band longꢀwave slope was cut off and approximated with the
Gaussian curve.
Xꢀray diffraction studies of compounds 1a, 2a, 3, 4, 6a, and
6b were performed on a SMART APEX II CCD diffractometer
(MoꢀK irradiation, graphite monochromator, ꢀscan techꢀ
nique). The structures were solved by direct method and refined
in terms of the least squares method in anisotropic fullꢀmatrix
approximation on F2hkl. Positions of H atoms were calculated
geometrically and refined in isotopic approximation using the
riding model. The main crystallographic data and parameters of
refinement are given in Table 3. All the calculations were perꢀ
formed using the SHELXTL PLUS program package.19
Quantum chemical calculations of individual model dimers
and tetramers without optimization of geometry were performed
using the Gaussian09 program package20 and the B97D funcꢀ
tional21 in the POL basis set22 for the I atom and in the 6ꢀ311+G
basis set** for all other atoms.
Methylpyrazinium iodide (1a). Pyrazine (25 g, Aldrich, 99+%)
was dissolved in 95% EtOH (180 mL), followed by addition of
MeI (200 mL, Merck, 99+%). Large yellow crystals were formed
within 5 days from this solution left to stand in the vessel isolated
from air. After crystallization from hot EtOH, the yield was
47.8 g (68.9%), that is significantly higher than that obtained in
the procedure23 which does not use EtOH. The solvent from the
mother liquor completely evaporated on standing in air within
two weeks, leaving additionally formed on the glass walls and

Ion aggregation of methylpyrarinium iodide
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 2, February, 2012
349
Table 3. The main crystallographic data and parameters of refinement for compounds 1a, 2a, 3, 4, 6a, and 6b
Parameter
1a
2a
3
4
6a
6b
Molecular formula
Molecular weight
T/K
C5H7IN2
222.03
100
C6H9IN2
236.05
100
C7H11IN2
250.08
C8H9IN2
260.07
100
C7H11IN2
250.08
100
C7H11I3N2
503.88
100
100
Crystal system
Space group
Z
Tetragonal
P4/ncc
16
Orthorhombic
Pbca
Monoclinic
P21/n
Orthorhombic
Cmca
Monoclinic
Pna21
16
Orthorhombic
P21/c
8
4
8
4
a/Å
b/Å
c/Å
10.4004(9)
10.4004(9)
26.484(3)
—
2864.8(5)
2.059
11.1316(2)
10.9948(2)
13.6567(3)
—
1671.44(5)
1.876
6.7331(6)
11.8495(10)
11.4321(10)
90.4150(10)
912.07(14)
1.821
7.1048(4)
11.1081(7)
26.2568(16)
—
2072.2(2)
1.667
14.869(5)
12.570(4)
20.065(6)
—
3750(2)
1.772
15.7620(14)
12.1988(11)
6.4919(6)
95.850(2)
1241.75(19)
2.695
/deg
V/Å3
dcalc/g cm–3
/cm–1
43.74
37.54
34.46
30.37
33.52
75.11
F(000)
1664
896
480
992
1920
904
2max/deg
57
60
58
58
60
58
Number of measured
reflections
Number of independent
reflections
Number of reflections
with I > 2(I)
Number of
refined parameters
R1
17871
96790
8781
11687
30833
14668
1903
1120
77
2441
2400
85
2408
2102
94
1453
1426
67
10513
10224
367
3280
2942
114
0.0230
0.1401
0.0147
0.0552
0.0181
0.0430
0.0422
0.1338
0.0191
0.0414
0.0543
0.2481
wR2
GOOF
1.001
1.004
1.003
1.072
1.004
1.057
Residual electron
density, (dmax/dmin)/e Å–3
0.545/–1.020
0.699/–0.509
1.222/–0.532
1.723/–0.903
0.724/–0.442
2.184/–1.343
bottom large dark brown crystals of methylpyrazinium tetraꢀ
iodide (1b). The yield was 3.5 g (3.2%). 1H NMR of compound
1a (CD3OD), : 4.56 (s, 3 H, N—CH3); 9.11 (d, 2 H, Pz,
3J = 2.3 Hz); 9.52 (m, 2 H, Pz).
1,2,3,5ꢀTetramethylpyrazinium iodide (4) was obtained similarꢀ
ly from 2,3,5ꢀtrimethylpyrazine (25.2 g) as yellow crystals. The
1
yield was 43.2 g (76.7%). H NMR of compound 4 (CD3OD),
: 2.64 (s, 3 H, C—CH3); 2.75 (s, 6 H, C—CH3); 4.26 (s, 3 H,
N—CH3); 8.63 (s, 1 H, Pz).
1,3ꢀDimethylpyrazinium (2a) and 1,2ꢀdimethylpyrazinium
iodides (2b) were obtained by the reaction of 2ꢀmethylpyrazine
(Aldrich, 99+%) with MeI under the same conditions as a yelꢀ
lowish green crystalline substance, which was a mixture of isoꢀ
mers. To separate the isomers, the larger in the size crystals (on
dissolution in water give colorless solutions) were separated from
the smaller in size crystals (on dissolution in water give crimson
solutions,  = 17000 and 18400 cm–1). The yields of pure prodꢀ
ucts after crystallization from hot ethanol were 20.7 g (33.0%)
and 6.2 g (9.9%), respectively. 1H NMR for compound 2a
(CD3OD), : 2.79 (s, 3 H, C—CH3); 4.40 (s, 3 H, N—CH3);
8.77 (m, 1 H, Pz); 8.95 (s, 1 H, Pz); 9.24 (s, 1 H, Pz). 1H NMR
for compound 2b (CD3OD), : 2.87 (3 H, C—CH3); 4.33 (3 H,
N—CH3); 8.90 (s, 1 H, Pz); 9.16 (s, 1 H, Pz); 9.32 (s, 1 H, Pz).
According to the 1H NMR data, each of the isolated isomers
contains 3—5% of the other isomer.
Isopentylpyrazinium iodide (5) is formed as yellowish orange
scaly crystals upon a slow concentration of the reaction mixture
(obtained after 5 days of standing of a solution of pyrazine (2.5 g)
in EtOH (18 mL) and isopentyl iodide (12.1 g, Aldrich, 99+%))
in air at 4 C). 1H NMR of compound 5 (CD3OD), : 1.08 (d, 6 H,
Me, 3J = 6.6 Hz); 1.80 (sept, 1 H, CH, 3J = 6.7 Hz); 2.02 (dt, 2 H,
C—CH2, 3J = 8.8 Hz, 3J = 7.1 Hz); 4.77 (t, 2 H, N—CH2,
3J = 8.0 Hz); 9.18 (br.s, 2 H, Pz); 9.51 (br.s, 2 H, Pz).
Propylpyrazinium iodide (6a) and propylpyrazinium triiodide
(6b) are formed under identical conditions as a mixture of large
orange and fine black needleꢀlike crystals upon the reaction of
1
pyrazine with PrI (Aldrich, 99+%). H NMR of compound 6a
(CD3OD), : 1.07 (t, 3 H, Me, 3J = 7.4 Hz); 2.14 (tq, 2 H,
C—CH2, 3J = 7.4 Hz, 7.5 Hz); 4.73 (t, 2 H, N—CH2, 3J = 7.5 Hz);
9.19 (d, 2 H, Pz, 3J = 2.9 Hz); 9.51 (br.s, 2 H, Pz).
1,2,3ꢀTrimethylpyrazinium iodide (3) was obtained as yelꢀ
lowish green crystals by the reaction of a solution of 2,3ꢀdimethylꢀ
pyrazine (24.4 g, Aldrich, 99+%) in EtOH (60 mL) with MeI
(165 mL). The yield was 46.5 g (79.3%). 1H NMR of compound 3
(CD3OD), : 2.81 (s, 6 H, C—CH3); 4.31 (s, 3 H, N—CH3);
8.70 (m, 1 H, Pz); 8.95 (s, 1 H, Pz).
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Received October 6, 2011;
in revised form December 15, 2011