Synthesis and electrochemical study of complexes of 2-methylthio-5- (pyridylmethylidene)-3,5-dihydro-4H-imidazol-4-ones with transition metals (Co, Ni, and Cu)

Article abstract of DOI:10.1007/s11172-006-0190-6

Itemtitle: Synthesis and electrochemical study of complexes of 2-methylthio-5- (pyridylmethylidene)-3,5-dihydro-4H-imidazol-4-ones with transition metals (Co, Ni, and Cu). Molecular structures of Cu^IIL¹Cl₂ (L¹ i

Full text of DOI:10.1007/s11172-006-0190-6

Russian Chemical Bulletin, International Edition, Vol. 54, No. 12, pp. 2771—2782, December, 2005
2771
Synthesis and electrochemical study of complexes of
2ꢀmethylthioꢀ5ꢀ(pyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones
with transition metals (Co, Ni, and Cu). Molecular structures of
CuIIL1Cl2 (L1 is (5Z )ꢀ2ꢀmethylthioꢀ3ꢀphenylꢀ5ꢀ(αꢀpyridylmethylidene)ꢀ
3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀone) and CoIIL2Cl2 (L2 is (5Z )ꢀ3ꢀmethylꢀ
2ꢀmethylthioꢀ5ꢀ(αꢀpyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀone)
E. K. Beloglazkina,ꢀ S. Z. Vatsadze, A. G. Majouga, N. A. Frolova, R. B. Romashkina,
N. V. Zyk, A. A. Moiseeva, and K. P. Butin†
Department of Chemistry, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Eꢀmail: bel@org.chem.msu.su
New transition metal (NiII, CoII, and CuII) complexes with 2ꢀmethylthioꢀ5ꢀ(pyridylꢀ
methylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones were synthesized by the reaction of 5ꢀ(αꢀ, βꢀ,
or γꢀ)pyridylꢀsubstituted 2ꢀmethylthioꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones with MCl2•nH2O. In
the complexes with αꢀPyꢀsubstituted ligands, the metal atom has a tetrahedral coordination
environment and is coordinated by the nitrogen atoms of the pyridine and thiohydantoin rings
and two chloride anions. The results of electronic spectroscopy and magnetic susceptibility
measurements suggest that the complexes with the βꢀ and γꢀPyꢀsubstituted ligands have polyꢀ
meric structures, and the metal atoms in these complexes are in an octahedral environment.
The molecular and crystal structures of CuIIL1Cl2 (L1 is (5Z)ꢀ2ꢀmethylthioꢀ3ꢀphenylꢀ5ꢀ
(αꢀpyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀone) and CoIIL2Cl2 (L2 is (5Z )ꢀ3ꢀmeꢀ
thylꢀ2ꢀmethylthioꢀ5ꢀ(αꢀpyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀone) were estabꢀ
lished by Xꢀray diffraction. Semiempirical quantumꢀchemical calculations were performed for
2ꢀmethylthioꢀ5ꢀ(pyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones. The complexes, startꢀ
ing ligands, and metal chlorides were studied electrochemically. The mechanism of electroꢀ
oxidation and electroreduction of 2ꢀmethylthioꢀ5ꢀ(pyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀ
imidazolꢀ4ꢀones and their cobalt, nickel, and copper complexes was proposed.
Key words: 2ꢀalkylthioꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones, nickel(II), cobalt(II), copper(II),
transition metal complexes, electrochemistry, molecular structure.
In recent years, 2ꢀthiohydantoins (4ꢀoxoimidazolꢀ
idineꢀ2ꢀthiones) and their Sꢀalkylated derivatives (2ꢀalkylꢀ
thioꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones) have been extenꢀ
sively studied because these compounds have a broad specꢀ
trum of biological activities. The hydantoin and thioꢀ
hydantoin fragments in organic molecules are responsible
for antiarrhythmic1 and antihypertensive2,3 activities. In
addition, thiohydantoins possess fungicidal and herbicidal
properties.4 5ꢀSubstituted 3ꢀarylꢀ and 3ꢀhetarylthioꢀ
hydantoins have considerable or moderate activity against
herpes viruses,5 HIV,6,7 and leukemia cells,8 exhibit antiꢀ
convulsant9,10 and antithrombotic11 activities, and have
moderate antitumor activity.7 SꢀAlkylated 5ꢀarylideneꢀ
thiohydantoins12 possess antidepressant and analgetic
properties. SꢀAlkylation products of thiohydantoins were
examined also for antiviral and antitumor activities.13
2ꢀThiohydantoins and their Sꢀalkylated derivatives
containing donor N and S atoms can be coordinated to
metals as neutral molecules or (for example, in the case of
thiohydantoins containing the NH—C=S fragment)
monoanions, which are formed upon deprotonation of
the N atom.14—18 These compounds are of interest as
ligands for the preparation of chelate complexes because
coordination to transition metal ions often increases antiꢀ
viral and antitumor activities of drugs.19 The introduction
of substituents containing electronꢀdonating atoms at poꢀ
sition 5 of the thiohydantoin ring enhances the coordinaꢀ
tion ability of these classes of compounds.20—22 However,
the known complexes of hydantoins and thiohydantoins
† Deceased.
with the CuII,15,16,20,22 FeIII 15,16 CoII,15,16 NiII,15,16,22
,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2679—2689, December, 2005.
1066ꢀ5285/05/5412ꢀ2771 © 2005 Springer Science+Business Media, Inc.

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Beloglazkina et al.
MnII,16 CsI,14 TlIII 21 ZnII, CdII, and HgII,17 VIII, VIV
,
,
Scheme 1
and VV ions18 are few in number, whereas data on comꢀ
plexes of Sꢀalkylated thiohydantoins are, to our knowlꢀ
edge, lacking.
The aim of the present study was to synthesize comꢀ
plexes of divalent Co, Ni, and Cu ions with isomeric
αꢀ, βꢀ, and γꢀpyridylꢀ(5Z )ꢀ2ꢀmethylthioꢀ3ꢀphenylꢀ5ꢀ
(pyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones
and the Co complex with (5Z )ꢀ3ꢀmethylꢀ2ꢀmethylthioꢀ
5ꢀ(αꢀpyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀ
one and investigate these complexes by electrochemical
methods. Recently, we have developed a convenient proꢀ
cedure for the synthesis of such complexes.23—25
Results and Discussion
Complexes of 3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones 1—4
with Co (a), Ni (b) and Cu (c) salts were synthesized
using two methods depending on the solubility of the
ligand in organic solvents.
5ꢀ(pyridylmethylidene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones
2 and 3, were synthesized by prolonged refluxing in a
CH2Cl2—MeCN mixture. The complexes were isolated
as powdered amorphous compounds. The compositions
of the complexes are given in Table 1. Analysis of the
results (see Table 1) revealed some characteristic features
of the complexes based on the ligands of the series under
consideration. In the case of αꢀPyꢀsubstituted ligands 1
and 4, chelate tetrahedral complexes of composition
L•MCl2 were obtained. The structures of cobalt derivaꢀ
tives 1a 24 and 4a and copper complex 1c were established
by Xꢀray diffraction in the present study and in the earlier
publication.24
Apparently, the coordination environment of the cenꢀ
tral metal ion in the complexes of composition 2L•MCl2
with βꢀ and γꢀPyꢀsubstituted ligands 2 and 3 differs from
that in the aboveꢀdescribed complexes. The measured
specific magnetic susceptibility χr of cobalt complex 3a is
13.36•10–6 cm3 g–1 at 293.8 K. The effective magnetic
moment of complex 3a calculated from this value is
5.09 µB, which agrees well with the published data26 for
the octahedral CoII complexes (4.7—5.2 µB).
In the case of readily soluble (5Z )ꢀ2ꢀmethylthioꢀ3ꢀ
phenylꢀ(or ꢀ3ꢀmethyl)ꢀ5ꢀ(αꢀpyridylmethylidene)ꢀ3,5ꢀ
dihydroꢀ4Hꢀimidazolꢀ4ꢀones 1 and 4, it is most conveꢀ
nient to use slow diffusion of a solution of the ligand in
CH2Cl2 into a solution of MCl2•6H2O (M = Co, Ni, or
Cu) in MeCN (Scheme 1). In the reactions with the
cobalt and copper salts, crystals of the target complexes
deposited on the walls of the reaction flask after 6—7 days.
In the reaction with nickel chloride, the crystalline comꢀ
plex was not isolated. However, the formation of the latꢀ
ter in a solution of a mixture of 1 and NiCl2•6H2O in
MeCN was detected electrochemically (see below).
Complexes with poorly soluble compounds, viz.,
βꢀ and γꢀPyꢀsubstituted (5Z )ꢀ2ꢀmethylthioꢀ3ꢀphenylꢀ
Earlier,25 we have studied the complex of ligand 3
with AgNO3 by Xꢀray diffraction and demonstrated that
this complex exists as a polymer in the crystalline state,
the donor sulfur and nitrogen atoms of the pyridine ring
being involved in complexation.
Apparently, the complexes with ligands 2 and 3 with
the empirical formula 2L•CoCl2 also have polymeric
structures, as evidenced by their magnetic moments. Unꢀ

Complexes of Sꢀmethylated thiohydantoins
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2773
Table 1. Compositions, color, and melting points of the complexes with ligands 1—4
Ligand L
Metal (М)
Complex
Color of the complex
M.p./°C
Composition
of the complex
1
Со
Ni
Cu
Со
Ni
Cu
Со
Со
1a
1b
1c
2a
2b
2c
3a
4a
Dark green
*
Dark cherry
Grayꢀgreen
Yellow
Green
Pale green
Dark green
228
L•СoCl2•1/2MeCN
276
281
L•СuCl2•2H2О
2L•CoCl2•H2O
2L•NiCl2
2L•CuCl2
2L•CoCl2•CH2Cl2
L•СoCl2•2H2О
2
295 (decomp.)
270
263
246
3
4
* The complex was not isolated; its formation was detected electrochemically.
Cl(2)
C(16)
Cl(1)
S
Cu
N(3)
N(1)
C(1)
C(10)
C(9)
C(15)
C(14)
N(2)
C(3)
C(4)
C(8)
C(7)
C(2)
C(13)
C(11)
C(5)
C(6)
C(12)
O(1)
Fig. 1. Molecular structure of compound 1c. Selected bond
lengths/Å: Cu—N(1), 1.967(5); Cu—N(3), 2.032(5); Cu—Cl(1),
2.228(2); Cu—Cl(2), 2.202(2). Selected bond angles/deg:
N(1)—Cu—N(3), 93.5(3); N(1)—Cu—Cl(2), 141.39(17);
N(3)—Cu—Cl(2), 99.8(2); N(1)—Cu—Cl(1), 96.89(16);
N(3)—Cu—Cl(1), 132.02(18); Cl(2)—Cu—Cl(1), 100.46(9).
like the metal atom in the aboveꢀconsidered complex, the
central metal ion in the latter complexes is in an octaꢀ
hedral environment. The structure of complex 3a is shown
below as an example.
per and cobalt atoms have a distorted tetrahedral ligand
environment and are coordinated by two nitrogen atoms
of the pyridine and thiohydantoin rings and two chloride
anions. The thiohydantoin and pyridine rings in molꢀ
ecules 1c and 4a are planar and are virtually coplanar. In
molecule 1c, the carbon atoms of the benzene ring at the
N(2) atom deviate from the plane of the fiveꢀmembered
ring of the ligand; the angle of deviation is ~90°.
In the crystal structures of the complexes with ligands
1 and 4, the tetrahedral geometry of the coordination
environment about the cobalt atom, which has a rather
small valence shell consisting of 15 electrons, is strongly
distorted. The coordination polyhedron can be approxiꢀ
mately described as a trigonal bipyramid with one axial
ligand removed. As a result, dissolution of these comꢀ
plexes can, apparently, lead to coordination by an addiꢀ
tional donor molecule (for example, by a solvent molꢀ
ecule or molecular oxygen) to form a compound, in which
the Co atom has a 17ꢀelectron valence shell. In coordiꢀ
nating solvents, coordination of the second solvent molꢀ
ecule giving rise to a sixꢀcoordinate 19ꢀelectron complex
is probable.
The polymeric structures of the complexes with ligands
2 and 3 are consistent with their very poor solubility in
most organic solvents.
Molecular structures of compounds 1c and 4a. The
structures of complexes 1c and 4a were established by
Xꢀray diffraction (Figs 1 and 2, respectively). The crystalꢀ
lographic data, details of Xꢀray diffraction study, and charꢀ
acteristics of the structure refinement for compounds 1c
and 4a are given in Table 2. In both complexes, the copꢀ

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Beloglazkina et al.
80% yield. It should be noted that neither ligand 1 nor
C(10a)
C(9a)
CoCl2•6H2O exhibited catalytic activity.
S(1a)
N(3a)
Scheme 2
O(1a)
C(11a)
Cl(2a)
C(8a)
C(7a)
N(2a)
Co(1a)
C(6a)
C(5a)
Cl(1a)
According to the aboveꢀconsidered data, complex 1a
can reversibly absorb molecular oxygen. We are now exꢀ
amining the possibility of the use of this complex for
catalysis of various types of oxidation reactions, in parꢀ
ticular, of alkene epoxidation.
N(1a)
C(1a)
C(4a)
C(3a)
C(2a)
Electronic spectroscopy. Ligands 1, 2, and 4 and the
complexes synthesized with the use of these ligands were
studied by UV—Vis spectroscopy. The spectroscopic data
are given in Table 3. The UV—Vis spectra of ligand 1, its
cobalt complex 1a, and the CoCl2•6H2O compound are
shown in Fig. 3. Ligand 1 is characterized only by absorpꢀ
tion bands in the UV region corresponding to p—p*
and n—p* transitions in the organic molecule. The
CoCl2•6H2O compound is characterized by three meꢀ
diumꢀintensity d—dꢀtransition bands in the visible region
(550—700 nm). Complexation leads to small shifts and a
decrease in the intensity of the absorption bands of the
ligand in the UV region and shifts of the d—dꢀtransition
bands of the metal atom to lower wavelengths.
Fig. 2. Molecular structure of compound 4a (one of three crysꢀ
tallographically independent molecules is shown). Selected bond
lengths/Å: Co—N(1), 2.031(3) [2.041(3), 2.024(4)]; Co—N(2),
2.010(3) [2.005(3), 2.005(3)]; Co—Cl(1), 2.2413(14)
[2.2353(15), 2.2311(14)]; Co—Cl(2), 2.2462(15) [2.2381(15),
2.2339(15)]. Selected bond angles/deg: N(2)—Co—N(1),
96.80(14) [97.56(14), 96.59(14)]; N(2)—Co—Cl(1), 115.29(11)
[114.21(11), 118.60(12)]; N(1)—Co—Cl(1), 106.24(11)
[109.35(11), 104.28(11)]; N(2)—Co—Cl(2), 114.44(11)
[113.95(12), 112.48(11)]; N(1)—Co—Cl(2), 105.98(11)
[103.85(11), 106.31(11)]; Cl(2)—Co—Cl(1), 115.41(6)
[115.61(6), 115.44(6)]. Data for two other crystallographically
independent molecules are given in square brackets.
The electronic spectra of the complexes with βꢀ and
γꢀPyꢀsubstituted ligands 2 and 3 differ substantially from
the aboveꢀdescribed spectra. These spectra show an abꢀ
sorption band at ~370 nm (its frequency is equal to that
In recent years, monoꢀ (see Refs 26 and 27) and
dinuclear28,29 CoII complexes with chelate nitrogenꢀconꢀ
taining ligands have been proposed as molecular oxygen
carriers. We examined the possibility of absorption of O2
with cobaltꢀcontaining complex 1a. During bubbling of
air through a 0.1 M solution of 1a in MeCN for several
hours, the solution gradually turned colorless, which was
accompanied by the corresponding changes in the elecꢀ
tronic spectrum; more particularly, the intensities of the
d—d transition bands at λ ~600 nm decreased (see beꢀ
low). Analogous changes in the spectrum were observed
upon the addition of an excess of H2O2. It should be
noted that the colorless solution of 1a obtained after purgꢀ
ing with air instantaneously returned to the initial green
color upon the addition of an equal volume of oxygenꢀ
free MeCN.
A
A
3.0
3
0.10
0.05
0
2.5
2.0
1.5
1.0
0.5
0
2
1
1
–0.05
550
650
λ/nm
λ/nm
2
3
In addition, it was found that complex 1a exhibits
catalytic activity in alkene epoxidation with iodobenzene
(Scheme 2). The addition of 5—10 mol. % of this comꢀ
plex to a solution of an equimolar mixture of alkene and
PhIO in CH2Cl2 afforded the corresponding epoxide in
400
600
800
Fig. 3. Electronic spectra of ligand 1 (1), complex 1a (2), and
CoCl2•6H2O (3) (MeCN, 10–3 mol L–1). An enlarged fragment
is shown in the inset.

Complexes of Sꢀmethylated thiohydantoins
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2775
Table 2. Crystallographic data, details of Xꢀray diffraction study, and characteristics of the structure refinement for
compounds 1c and 4a
Parameter
1c
4a
Molecular formula
Molecular weight
C16H17Cl2CuN3O3S
465.83
C11H11Cl2CoN3OS
363.12
Color, crystal shape
Crystal dimensions/mm
Crystal system
Space group
a/Å
Dark cherry platelets
0.45×0.18×0.10
Monoclinic
P2(1)/с
Dark green prisms
0.38×0.14×0.14
Monoclinic
P2(1)/n
10.668(2)
27.135(5)
6.850(1)
90
91.28(3)
90
1982.4(6)
4
1.561
15.038(3)
10.360(2)
27.451(5)
90
94.57(4)
90
4274.0(14)
12
1.693
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
ρ
calc/g cm–3
Linear absorption coefficient/cm–1
F(000)
1.497
948
1.719
2196
θ Angle range/deg
Ranges of reflection indices
2.05—24.98
–12 < h < 12
0 < k < 32
0 < l < 8
3775
1.52—22.97
0 < h < 16
–9 < k < 0
–30 < l < 30
5185
Number of measured reflections
Number of independent reflections
Number of reflections with I > 2σ(I )
3469 (Rint = 0.1012)
755
4968 (Rint = 0.0455)
2856
Number of parameters in refinement
Goodnessꢀofꢀfit on F 2
233
0.650
646
0.772
R Factors based on reflections with I > 2σ(I )
R1
wR2
0.0492
0.0847
0.0264
0.0601
R Factors based on all data
R1
wR2
0.3300
0.0963
0.545/–0.307
0.0848
0.0646
0.258/–0.288
Residual electron density (min/max)/e Å–3
* The crystal under examination slowly decomposed in the course of Xꢀray diffraction study.
Table 3. Electronic spectra of the ligands, the starting inorganic
salts, and the complexes (~2•10–3 mol L–1) in solutions in
MеCN or DMF
observed for the free ligand but the intensity of the band is
higher) and two very weak d—dꢀtransition bands at λ =
600—700 nm (see Table 3). This agrees well with the
proposed octahedral structures of the complexes with
ligands 2 and 3.
Compound
Solvent
λ/nm
(ε•10–4/L mol–1 cm–1
)
Electrochemistry. Free ligands 1—4 and their comꢀ
plexes with MCl2 were studied also by cyclic voltammetry
(CV) and rotating disk electrode (RDE) voltammetry in
anhydrous DMF in the presence of 0.05 M Bu4NClO4 as
the supporting electrolyte. The complexes, which were
synthesized and characterized, as well as the complexes,
which prepared in situ in an electrochemical cell by mixꢀ
ing ligand 1 with an equimolar amount of a metal salt,
were studied. The reaction of ligand 1 with MCl2 in DMF
is fast, which is evidenced by an instantaneous change in
the color of the solution observed upon mixing of the
reagents. As a rule, the electrochemical characteristics of
the complexes prepared in situ differ only slightly from
1
2
МеCN
DMF
277 (1.59), 353 (1.99), 369 (2.00)
369 (2.00)
4
1a
МеCN
МеCN
273 (2.58), 361 (1.70)
280 (1.59), 388 (1.60), 400 (1.26),
562 (0.32), 616 (0.32), 636 (0.40)
367 (2.94), 608 (0.02), 679 (0.03)
297 (1.49), 369 (3.02), 607 (0.03),
665 (0.03)
2a
3a
DMF
DMF
4a
МеCN
268 (3.51), 378 (3.31), 560 (0.37),
629 (0.16)
CoCl2•6H2O МеCN
CuCl2•2H2O МеCN
570 (0.33), 613 (0.32), 682 (0.35)
329 (0.14), 426 (0.01)

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Beloglazkina et al.
those of the presynthesized complexes. The electrochemiꢀ
cal results and the experimental conditions are given in
Table 4.
corresponds to oxidation of one of the ligands, for exꢀ
ample, of the water molecule
[Cl2CoIII—OH2]+ – e → 1/2 [Cl2CoIII—O]2 + 2 H+
Cobalt, nickel, and copper chlorides. The CoCl2•6H2O
compound undergoes twoꢀelectron reduction on a rotatꢀ
ing disk electrode to give metallic Co0, which is oxidized
at +0.35 V during the reverse scan to the anodic region
with the formation of a triangular (without a diffusion
tail) steepleꢀlike peak characteristic of oxidative desorpꢀ
tion from the electrode surface. At the same time, two
cathodic peaks are observed in the CV curve (see Table 4).
Anodic oxidation on a rotating disk electrode gives two
oneꢀelectron waves. One of them is, apparently, associꢀ
ated with the CoII → CoIII transition, and another wave
or chloride ligands
3 [CoCl2]+ – 2 e– → 3 Co+ + 2 Cl3
.
–
The NiCl2•6H2O compound undergoes oneꢀstep twoꢀ
electron reduction under cathodic polarization with an
anodic desorption peak of Ni0 in the reverse scan of the
CV curve at +0.16 V. Under anodic polarization, oxidaꢀ
tion of this compound also occurs as a oneꢀstep twoꢀ
electron process.
Table 4. Electrochemical reduction potentials (ERed) and oxidation potentials (EOx) of the ligands and complexes measured relative to
Ag|AgCl|KCl(sat.) by the CV (Ep is the peak potential) and RDE (E1/2 is the halfꢀwave potential) methods at a glassyꢀcarbon
electrodea
Red
Red
Ox
Ox
Red
Red
Ox
Ox
Comꢀ
pound
Ep
E1/2
Ep
E1/2
Comꢀ
pound
Ep
E1/2
Ep
E1/2
V
V
1b
–1.29/–1.23c –1.28(1) 1.66
1.66(2)
2b
2c
3
–1.26
–1.96
–2.30
+0.45/+0.51c
–1.32/–1.26 –1.28(1)h 1.21(2)
–1.94/–1.84 –1.96(1) 1.63
–1.16/–1.10c –1.17(1) 0.64
–1.64
–2.32
–1.14
–1.29
–1.46
–2.02
–2.36
–0.68
–1.30
–1.84
–1.22(1) 1.14
1.16(1)
1.60(1)
–1.77
–1.99
–1.77(1) 2.16
–2.47(1)
1a
–0.63
–1.24
–1.79
–0.58(1) 1.76
–1.19(1) 2.10
–1.65(1)
1.72(2)
2.03(>2)
0.56/0.45c 0.44
1.13(1)
1.60
0.77
1.16i
–1.99
1bd
1c
–0.64
–1.35
–1.79
–0.61(1) 1.37
–1.27(1) 1.96
–2.13(1)
1.31(2)
1.52(2)
–1.70(1) 1.13
–2.26
–1.00(1) 1.24
–1.20(1) 1.62
–1.86(1)
3а
4
1.14(2)
1.52(2)
–2.49
+0.45/+0.52c,e
–0.51g
–1.32
0.58d
1.32
0.50(1) f
1.35(1)
–2.04(1)
–2.40(1)
–1.28(1)h
–1.96(1)
–1.94
4а
–0.64j
–1.14
1.33
1.27(2)
2
–1.28/–1.23c –1.33(1) 1.64
1.58(2)
–1.89
–2.46
CoCl2•6H2O –0.86
–1.21/+0.35c
NiCl2•6H2O –1.32/+0.16c –1.30(2) 1.20
CuCl2•2H2O +0.45/+0.58c,e +0.50(1)h
–0.51/–0.16c
–1.30
1.53
1.70
1.50(1)
1.70(1)
1.00(2)
2a
–1.29
–1.97
–1.24(1) 1.25
–2.03(1) 1.60
1.16(2)
1.61(2)
a Experimental conditions: DMF, 0.05 M Bu4NClO4; CV, 200 mV s–1; RDE, 20 mV s–1; 2800 min–1 (the number of electrons
transferred in the step, which was determined on a rotating disk electrode by comparing with the oneꢀelectron oxidation wave of
ferrocene, is given in parentheses).
b In MeCN.
c The peak potentials in the reverse scans of the CV curves.
d The complex was prepared in situ in DMF from equimolar amounts of the ligand and MCl2•nH2O.
e The initial potential was +0.7 V.
f The resulting complex on Pt.
g The peak at –0.51 V is poorly pronounced and is not always reproduced.
h The initial potential was 0 V.
i The height of the second wave is half as large as that of the first wave.
j The height of the peak corresponds to the transfer of 0.5—0.7 electrons.

Complexes of Sꢀmethylated thiohydantoins
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2777
Reduction of CuCl2•2H2O is characterized by two caꢀ
thodic peaks in the CV curve at +0.45 and –0.51 V, which
correspond to two oneꢀelectron steps (CuII → CuI → Cu0).
Each peaks corresponds to a reoxidation peak in the reꢀ
verse scan. However, only the first oneꢀelectron peak
(+0.50 V) is observed for a stirred solution by the RDE
method during a slow cathodic potential scan from +0.7 V,
whereas the second peak does not appear. Apparently,
this is associated with the formation of a nonconducting
film of an insoluble oneꢀelectron reduction product of
copper on the electrode surface, which blocks electron
transfer. No peaks or oxidation waves of CuCl2•2H2O,
which could correspond to the formation of trivalent copꢀ
per, were observed in the anodic region up to +1.7 V.
Ligands. Ligands 1—4 are successively reduced in sevꢀ
eral oneꢀelectron steps and are oxidized to give a twoꢀ
electron wave. The exception is ligand 3, for which a large
prewave was observed at potentials from +0.64 to +0.77 V
by both the CV and RDE methods (see Table 4).
We performed semiempirical quantumꢀchemical calꢀ
culations by the PM3 method.31 These calculations
demonstrated that both the highest occupied molecular
orbitals (HOMO) and the lowest unoccupied molecuꢀ
lar orbitals (LUMO) in molecules 1—4 have a π symꢀ
metry and are localized predominantly on the thioꢀ
hydantoin fragment (Fig. 4). The orbital of the sulfur lone
pair makes the major contribution to HOMO, whereas
the crossꢀconjugated methylidenehydantoin πꢀsystem
O=CC(=C)N=C makes the major contribution to
LUMO. These results suggest that both oxidation and
reduction occur primarily at the thiohydantoin moiety of
the ligands.
the planes of the hydantoin and 2ꢀpyridine substituents
(Py/Hyd), and the torsion angles between the hydantoin
and Nꢀphenyl substituents (Ph/Hyd) for ligand 1 are
also presented. The results of calculations are consisꢀ
tent with the Xꢀray diffraction data on stability of the
(Z )ꢀsyn form.24
In all conformations of compounds 1—3, the torsion
angles between the planes of the thiohydantoin ring and
the Nꢀphenyl group are close to 90°, whereas the torsion
angle between the planes of the thiohydantoin and pyriꢀ
dine rings is strongly related to the geometric isomerism
(see above) and, to a lesser extent, to the isomerism of
the pyridyl fragment (for the (E)ꢀsyn conformations:
1 and 4, 87°; 3, 75°; and 2, 61°). Therefore, the orbitals of
the Nꢀphenyl group are almost not involved in the redox
orbitals of the ligand, whereas the orbitals of the Py groups
have a particular effect on the redox orbitals, the degree of
their influence depending on the geometry of the ligand.
The identification of oxidation and reduction prodꢀ
ucts of the ligands by preparative electrolysis was beyond
the scope of the present study. Nevertheless, taking into
account the types of orbitals (see Fig. 4), it can be hypoꢀ
thesized that oxidation occurs predominantly at the sulfur
atoms.
The probable fourꢀstep (see Table 4) mechanism of
electroreduction of these compounds is presented in
Scheme 3. Four oneꢀelectron waves will be observed, for
example, if the primary radical anion accepts an electron
followed by protonation of the dianion with water at the
imine double bond. Then stepwise reduction occurs at the
1,2 and 1,4 positions of the Michael system O=C—C=C
of thiohydantoin.
The degree of involvement of the Nꢀphenyl and Py
groups in the redox orbitals depends on the degree of their
coplanarity with the thiohydantoin ring. The geometry
optimization of the ligands by the PM3 method demonꢀ
strated that at least four stereoisomers can exist. Their
heats of formation differ by 3.7 kcal mol–1. These isoꢀ
mers, which differ in the configuration of the substituents
at the double bond (E, Z ) and the orientation of the
dipole of the MeS group relative to the C=N bond of the
thiohydantoin ring (syn or anti), are shown below. The
calculated heats of formation, the torsion angles between
Complexes. The cyclic voltammograms of the comꢀ
plexes, the free ligands, and Ni and Co chlorides (see
above) are shown in Fig. 5. As compared to free ligand 1,
tetrahedral cobalt complex 1a is characterized by an addiꢀ
tional cathodic peak at less negative potentials (see
Fig. 5, a) than the first peak of the free ligand. This peak
appears rather early for CoII complexes (–0.63 V) and
corresponds, more likely, to the CoIII → CoII transition
rather than to the CoII → CoI transition.
These results can be explained as follows. The strucꢀ
tures of compounds 1a—c and 4a (based on their empiriꢀ
∆Hf/kcal mol–1
Py/Hyd Angle/deg
67.94
87
70.61
88
66.91
38
69.35
35
Ph/Hyd Angle/deg
86
87
87
87

2778
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Beloglazkina et al.
H
H
H
C
H
C
C
H
C
C
H
C
a
e
H
C
C
H
C
C
C
C
H
H
N
H
H
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
C
C
N
N
C
C
H
H
H
H
1
1
H
H
H
H
C
C
C
H
H
C
C
b
f
C
C
C
C
C
H
H
H
C
H
C
N
C
S
H
H
H
C
H
H
C
C
C
C
H
H
N
N
H
H
N
H
H
H
H
2
2
H
H
H
H
c
C
g
C
C
C
H
C
H
C
N
S
C
C
H
C
H
H
H
C
N
C
N
C
C
H
C
C
C
H
H
H
H
H
H
3
3
H
C
H
H
H
C
d
h
H
H
N
C
H
H
C
C
N
N
C
C
H
H
H
C
H
H
H
C
C
H
C
H
C
C
C
H
H
H
H
H
4
4
Fig. 4. Frontier (redox) orbitals of ligands 1—4 in (E)ꢀsyn conformations calculated by the PM3 method: HOMO (a—d) and
LUMO (e—h).
cal formula LMCl2) can be represented either as an MII
complex with the uncharged organic ligand (A) or as an
MIII complex with the radicalꢀanionic ligand (B). Conseꢀ
quently, complexes with ligands 1 and 4 should be asꢀ
signed to metal complexes with nonꢀinnocent ligands,30
for which neither the oxidation state of the central metal
ion nor the bond orders in the ligand cannot a priori be
determined. The fact that complex 1a is reduced 580 mV

Complexes of Sꢀmethylated thiohydantoins
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2779
Scheme 3
pared to CoCl2•6H2O. It is unlikely that this peak correꢀ
sponds to the CoII → CoIII transition because complexꢀ
ation with donor ligands should facilitate oxidation if it
occurs at the metal atom. The shift of the potential to the
anodic region upon complexation of the ligand with Lewis
acids signifies that oxidation occurs most likely at the
ligand.
The structure B for compound 1a is additionally conꢀ
firmed by the structural data.24,25 The N(1)—C(1) bond
in the thiohydantoin ring of this compound is substanꢀ
tially longer than that in the free ligand, i.e., this formally
double bond in the complex better corresponds to a single
bond. The exocyclic C(1)—S bond length in complex 1a
is virtually equal to that in the free ligand and corresponds
to the single C—S bond.24 In the βꢀPyꢀcontaining comꢀ
plex of ligand 2 with AgNO3, the N(1)—C(1) and C(1)—S
bond lengths are similar to the corresponding bond lengths
in the free ligand.25
Therefore, the electrochemical and structural data sugꢀ
gest that complex 1a can be represented as a CoIII comꢀ
plex with the radicalꢀanionic ligand, i.e., the structure of
the ligand in the complex differs from that of the free
ligand, and ligand 1 should be considered as nonꢀinꢀ
nocent.30
A similar situation is observed for nickel complex 1b,
which was prepared in situ by mixing 2•10–5
M
NiCl2•6H2O solutions in DMF (see Fig. 5, b). The first
reduction potential is 680 mV less negative than the reꢀ
duction potential of NiCl2•6H2O, whereas the first oxiꢀ
dation potential is 170 mV more positive than the oxidaꢀ
tion potential of the salt. The potential of the first reducꢀ
tion step of the complex is also unusually low (–0.64 V).
Consequently, compound 1b can also be represented as a
NiIII complex with the radicalꢀanionic ligand.
The possible mechanism of reduction of the nonꢀinꢀ
nocent complexes (taking into account the fact that reꢀ
duction of the complexes, unlike reduction of the salts, is
not accompanied by metal deposition on an electrode) is
presented in Scheme 4.
more easily than CoCl2•6H2O (see Table 4), even though
coordination by donor ligands should hinder reduction of
metal ions, suggests that the true structure of complex 1a
(and, probably, of 4a) is similar to B.
Therefore, the first reduction step occurs, presumꢀ
ably, at the trivalent metal atom (and then at the ligand).
Copper complex 1c undergoes a reversible redox tranꢀ
sition at the same potential as CuCl2•2H2O (+0.45 V;
CuII → CuI). However, unlike the salt, the copper comꢀ
plex gives the poorly pronounced and sometimes irreproꢀ
ducible second reduction peak at –0.50 V (CuI → Cu0)
(see Fig. 5, c). Suppression of the peak at –0.5 V may be a
consequence of the coverage of the electrode surface by
the adsorbed electrolysis product. During the anodic poꢀ
tential scan from the initial value of 0 V (at which Cu
exists in a monovalent state), the first oxidation peak is
observed at +0.58 V (CuI → CuII), whereas no oxidation
peaks are observed for CuCl2•2H2O in DMF at potenꢀ
tials up to +1.7 V. The potential of the second oxidation
peak of complex 1c is 340 mV more positive than the
It should also be noted that the first peak in the anodic
branch of the polarization curves of complexes 1a and
4a is shifted to more positive potentials by 100 mV
compared to that of the ligand and by 230 mV comꢀ

2780
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Beloglazkina et al.
a
b
d
f
1
5 µA
3 µA
2
1
3
2
2
1
0
–1
–2 E/V
1
0
–1
–2
E/V
1
3 µA
c
5 µA
2
1
0
–1
–2
E/V
1
0
–1
–2
E/V
1
5 µA
2
e
3 µA
1
2
2
1
0
–1
–2
E/V
1
0
–1
–2
E/V
Fig. 5. Cyclic voltammograms: a, 1 (1), 1a (2), CoCl2•6H2O (3); b, 1b (1), 2b (2); c, 1c; d, 2 (1), 2a (2); e, 3 (1), 3a (2);
f, NiCl2•6H2O (1), a background curve (2).
potential of the first oxidation peak of ligand 1 (see
Table 4).
reduction of both at the ligand and metal. Like the aboveꢀ
considered complex, copper complex 2c, is characterized
by the presence of a peak at +0.45 V, which coincides
with the peak for CuCl2•2H2O (CuII → CuI), whereas the
peak at –0.50 V, which is observed for a CuCl2•2H2O
solution (CuI → Cu0), is absent. The first oxidation poꢀ
tentials of complexes 2a,b and 3a are less positive than the
oxidation potentials of the ligand, i.e., oxidation of the
complexes occurs at the metal atom. In addition, the
The polarization curves for complexes 2a and 2b are
shown in Figs 5, b,d, and the corresponding curve for 1a
is presented in Fig. 5, a. In the cathodic region, the first
reduction peaks of 2a,b and 3a are very similar to the first
reduction peaks of the ligands and are only slightly shifted
to more negative potentials compared to CoCl2 and NiCl2
hexahydrates (see Table 4). This may be indicative of

Complexes of Sꢀmethylated thiohydantoins
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005 2781
Scheme 4
tended by including the parameters for all firstꢀrow transition
metals and selected secondꢀ and thirdꢀrow transition metals.
This extended method (PM3(tm)) is implemented in the
HyperChem program package (HyperCube Inc., FL, USA).
Geometry optimization of the molecules was carried out with a
gradient of no higher than 10 kal Å–1 mol–1 as the convergence
criterion.
3ꢀMethylꢀ2ꢀmethylthioꢀ5ꢀ((Z )ꢀ2ꢀpyridylmethylidene)imidꢀ
azolꢀ4ꢀone (4). A 15% KOH solution (1 mL) was added to a
solution of 2ꢀthioxoꢀ5ꢀ((Z )ꢀ2ꢀpyridylmethylidene)imidazolꢀ4ꢀ
one (0.5 g, 2 mmol) in a 1 : 1 MeOH—water mixture (20 mL).
After complete dissolution of 2ꢀthioxoꢀ5ꢀ((Z )ꢀ2ꢀpyridylmethylꢀ
idene)imidazolꢀ4ꢀone, MeI (0.7 g, 4 mmol) was added, and the
reaction mixture was stirred for 1 h. The precipitate that formed
was filtered off, washed with water, and dried in air. The yield of
compound 4 was 0.3 g (70%), m.p. 206 °C. Found (%): C, 56.48;
H, 4.23; N, 18.07. C11H11N3OS. Calculated (%): C, 56.65;
1
H, 4.72; N, 18.03. H NMR (CDCl3), δ: 8.69 (d, 1 H, HαꢀPy,
J = 8.3 Hz); 8.53 (d, 1 H, Hβ´ꢀPy, J = 4.4 Hz); 8.03 (t, 1 H,
HβꢀPy, J = 4.4 Hz); 7.46 (d, HγꢀPy, J = 4.2 Hz); 6.88 (s, 1 H,
CH=); 3.18 (s, 3 H, NMe); 2.70 (s, 3 H, SMe). IR, ν/cm–1
:
1740 (C=O); 1670 (C=N); 1600 (C=C).
Synthesis of complexes with 2ꢀmethylthioꢀ5ꢀ(pyridylmethylꢀ
idene)ꢀ3,5ꢀdihydroꢀ4Hꢀimidazolꢀ4ꢀones. A (for complexes with
ligands 1 and 4). Acetonitrile (3 mL) was added to a solution of
the ligand (0.034 mmol) in CH2Cl2 (2—3 mL) until stratificaꢀ
tion occurred. Then a solution of the salt CoCl2•6H2O or
CuCl2•6H2O (0.034 mmol) in EtOH (2—3 mL) was slowly
added. The reaction mixture was tightly closed and kept until
crystals formed. The yield was 75—80%.
oxidation potentials of complexes 2a,b are less positive
than the oxidation potentials of MCl2•6H2O, which is
also indicative of oxidation at metal.
Experimental
Ligands 1—3 were synthesized according to a known proceꢀ
dure.24 The IR spectra were measured on a URꢀ20 instrument in
Nujol mulls. The UVꢀVis spectra were recorded on Specord
Mꢀ40 (200—900 nm) and Nicolet Heliosꢀα (200—1100 nm)
B (for complexes with ligands 2 and 3). The ligand
(0.678 mmol) and the salt CoCl2•6H2O, CuCl2•6H2O, or
NiCl2•6H2O (0.678 mmol) were dissolved in a mixture of
CH2Cl2 and MeCN (10—15 mL). The reaction mixture was
refluxed with vigorous stirring for 2 h (until a precipitate was
obtained). The precipitate was filtered off and washed with diꢀ
ethyl ether. The yield was 80—90%.
Complex CoII(1)Cl2•0.5MeCN (1a). Found (%): C, 46.58;
H, 3.57; N, 9.26. (C16H13OSN3•CoCl2)2•CH3CN. Calcuꢀ
lated (%): C, 45.79; H, 3.25; N, 9.54. IR, ν/cm–1: 1756, 1746,
1644, 1594.
1
instruments in 0.1ꢀcm quartz cells at 20—22 °C. The H NMR
spectra were measured on a VarianꢀVXRꢀ400 instrument at
400 MHz. The EI mass spectra were obtained on a JMSꢀD300
GLCꢀmass spectrometer equipped with a JMAꢀ2000 computer
and an HPꢀ5890 chromatograph. The magnetic susceptibility
was measured by the Faraday method at 293.8 K.
Xꢀray diffraction data sets were collected on a Syntex P21
diffractometer (graphite monochromator, λ(MoꢀKα)
=
Complex CuII(1)Cl2•2H2O (1c). Found (%): C, 41.53;
H, 3.50; N, 9.20. C16H13OSN3•CuCl2•2H2O. Calculated (%):
C, 41.25; H, 3.68; N, 9.02.
Complex CoII(2)2Cl2•H2O (2a). Found (%): C, 52.51;
H, 3.45; N, 11.01. (C16H13OSN3)2•CoCl2•H2O. Calcuꢀ
lated (%): C, 52.04; H, 3.82; N, 11.38. IR, ν/cm–1: 1745,
1650, 1609.
Complex NiII(2)2Cl2 (2b). Found (%): C, 53.00; H, 3.95;
N, 12.43. (C16H13OSN3)2•NiCl2. Calculated (%): C, 53.36;
H, 3.61; N, 11,67.
Complex CuII(2)2Cl2 (2c). Found (%): C, 52.50; H, 3.43;
N, 11.05. (C16H13OSN3)2•CuCl2. Calculated (%): C, 53.00;
H, 3.61; N, 11,59.
Complex CoII(3)2Cl2•CH2Cl2 (3a). Found (%): C, 49.02;
H, 3.26; N, 9.42. (C16H13OSN3)2•CoCl2•CH2Cl2. Calcuꢀ
lated (%): C, 49.19; H, 3.74; N, 9.94. IR, ν/cm–1: 1725,
1620, 1610.
Complex CoII(4)Cl2•2H2O (4a). Found (%): C, 33.40;
H, 3.58; N, 10.13. C11H11OSN3•CoCl2•2H2O. Calculated (%):
C, 33.10; H, 3.79; N, 10.53.
0.71073 Å, ω scanning technique) at 293 K. Absorption correcꢀ
tions were applied based on the intensities of equivalent reflecꢀ
tions (Tmin/Tmax). The structures were solved by direct methods
(SHELXSꢀ97)31 and refined by the fullꢀmatrix leastꢀsquares
method against F2 with anisotropic displacement parameters for
all nonhydrogen atoms (SHELXLꢀ97).32 All hydrogen atoms
were located from electron density maps and refined isotropically.
Electrochemical studies were carried out on a PIꢀ50ꢀ1.1
potentiostat. A glassyꢀcarbon (GC) disk (2 mm in diameter)
was used as the working electrode, a 0.05 M Bu4NClO4 soꢀ
lution in DMF served as the supporting electrolyte, and
Ag/AgCl/KCl(satur.) was used as the reference electrode. All
measurements were carried out under argon. The samples were
dissolved in the preꢀdeaerated solvent. Dimethylformamide
(highꢀpurity grade) was purified by stirring over freshly calcined
K2CO3 for 4 days followed by successive vacuum distillation
over P2O5 and anhydrous CuSO4.
Quantumꢀchemical calculations were performed with the
use of the semiempirical SCF PM3 method,33 which was exꢀ

2782
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 12, December, 2005
Beloglazkina et al.
Synthesis of exoꢀepoxynorbornene. Compound 1a (0.0425 g,
0.1 mmol) was added to a solution of norbornene (0.47 g,
5 mmol) and iodobenzene (0.2 g, 1 mmol) in MeCN (5 mL).
The reaction mixture was stirred at room temperature for 5 h,
passed through a silica gel filter column (h = 3 cm), and concenꢀ
trated. exoꢀEpoxynorbornene was obtained in a yield of 0.09 g
(80%). 1H NMR (CDCl3), δ: 3.03 (br.s, 2 H, HC(2), HC(3));
2.42 (br.s, 2 H, HC(1), HC(4)); 1.46 (d, 1 H, synꢀHC(7), J =
9.0 Hz); 1.30 (ddd, 2 H, HC(5), endoꢀHC(6), J1 = 2.1 Hz, J2 =
4.5 Hz, J3 = 8.0 Hz); 1.19 (ddd, 2 H, HC(5), exoꢀHC(6), J1 =
2.2 Hz, J2 = 4.5 Hz, J3 = 8.0 Hz); 0.68 (d, 1 H, antiꢀHC(7), J =
9.0 Hz). MS, m/z: 110 [M]+.
14. M. Arca, F. Demartin, F. A. Devillanova, A. Garau, F. Ifaia,
V. Lippolis, and G. Verani, Inorg. Chem., 1998, 37, 4164.
15. A. M. A. Hassaan, Sulfur Lett., 1991, 13, 1.
16. R. S. Srivastava, R. R. Srivastava, and H. N. Bhargava, Bull.
Soc. Chim. Fr., 1991, 128, 671.
17. S. Sharma, M. Kumar, and T. Sharma, J. Ind. Chem. Soc.,
1985, 62, 781.
18. M. Kumar, A. Chaudhary, and S. Sharma, Ind. J. Chem.,
Sect. A, 1986, 25A, 281.
19. J. A. Grim and H. G. Petring, Cancer Res., 1967, 27, 1278.
20. M. M. Chowdhry, D. M. Mingos, A. J. White, and D. J.
Williams, J. Chem. Soc., Perkin Trans. 1, 2001, 20, 3495.
21. J. S. Casas, E. E. Castellano, A. Macfas, N. Playa,
A. Sanchez, J. Sordo, J. M. Varela, and J. Zukermanꢀ
Schpector, Inorg. Chim. Acta, 1995, 238, 129.
22. M. M. Chowdhry, A. Burrows, D. M. Mingos, A. J. White,
and D. J. Williams, J. Chem. Soc., Chem. Commun.,
1995, 1521.
This study was financially supported by the Russian
Foundation for Basic Research (Projects Nos 03ꢀ03ꢀ32401
and 04ꢀ03ꢀ32845), the Program "Russian Universities"
(Grant 05.03.046), and the Presidium of the Russian
Academy of Sciences (Program "Theoretical and Experiꢀ
mental Studies of the Nature of Chemical Bonding and
Mechanisms of Chemical Reactions and Processes").
23. A. G. Majouga, E. K. Beloglazkina, S. Z. Vatsadze, A. A.
Moiseeva, K. P. Butin, and N. V. Zyk, Mendeleev Commun.,
2004, 14, 115.
24. A. G. Majouga, E. K. Beloglazkina, S. Z. Vatsadze, N. A.
Frolova, and N. V. Zyk, Izv. Akad. Nauk, Ser. Khim., 2004,
2734 [Russ. Chem. Bull., Int. Ed., 2004, 53, 2850].
25. A. G. Majouga, Ph. D. (Chem.) Thesis, M. V. Lomonosov
Moscow State University, Moscow, 2005, 137 pp. (in
Russian).
26. R. D. Jones, D. A. Summervile, and F. Barsolo, Chem. Rev.,
1979, 79, 139.
27. S. Park, V. K. Mathur, and R. P. Planalp, Polyhedron, 1998,
17, 325.
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Received February 28, 2005