New dioxygen-inert triphenylantimony(v) catecholate complexes based on o-quinones with electron-withdrawing groups

Article abstract of DOI:10.1007/s11172-009-0052-0

New triphenylantimony(v) catecholate complexes were synthesized by oxidative addition of sterically hindered o-benzoquinones containing electron-withdrawing substituents in different positions of the carbon ring to triphenylantimony. The complexes were ch

Full text of DOI:10.1007/s11172-009-0052-0

Russian Chemical Bulletin, International Edition, Vol. 58, No. 3, pp. 532—537, March, 2009
532
New dioxygenꢀinert triphenylantimony(v) catecholate complexes
based on oꢀquinones with electronꢀwithdrawing groups*
A. I. Poddel´sky,a I. V. Smolyaninov,b Yu. A. Kurskii,a
N. T. Berberova,b V. K. Cherkasov,a and G. A. Abakumova
aG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 462 7497. Eꢀmail: aip@iomc.ras.ru
bSouthern Scientific Center, Russian Academy of Sciences,
41 ul. Chekhova, 344006 RostovꢀonꢀDon, Russian Federation.
Fax: +7 (851 2) 25 0923
New triphenylantimony(v) catecholate complexes were synthesized by oxidative addition
of sterically hindered oꢀbenzoquinones containing electronꢀwithdrawing substituents in different
positions of the carbon ring to triphenylantimony. The complexes were characterized using
IR spectroscopy, NMR spectroscopy, and cyclic voltammetry. The oxygenꢀinertness of the
complexes is shown by NMR spectroscopy and electrochemical studies. The introduction of
electronꢀwithdrawing substituents to the catecholate ligand shifts the first oxidation potential
of the complexes to the electropositive region and thus deactivates the triphenylantimony(v)
catecholate complexes in the reaction with molecular oxygen.
Key words: antimony(v), oꢀbenzoquinone, catecholate, NMR spectroscopy, cyclic
voltammetry.
Scheme 1
The chemistry of transition and nontransition metal
complexes with sterically hindered oꢀsemiquinone and
catecholate ligands is one of the actively developing
directions of modern organoelement chemistry.
The main results in this area of chemistry were
obtained for the transition metal compounds.1—4 Much
less information is available about molecular structures
and physical and chemical properties of individual nonꢀ
transition metal compounds5—8 compared to those of tranꢀ
sition metals. This is valid for the antimony derivatives.
Only few scanty data on the individual catecholate9—12
and oꢀamidophenolate13,14 organic antimony derivatives
have been known to the recent time, whereas no data on
their sterically hindered analogs were available. The first
examples of the sterically hindered catecholate and
oꢀamidophenolate triphenylantimony(v) complexes are
described in Refs 15—19.
The reversible binding of molecular oxygen by the
nontransition metal complexes has first been observed
when oꢀiminobenzoquinone was used as the ligand in the
triphenylantimony(v) complex. It was found that the
antimony oꢀamidophenolate complexes can reversibly add
and eliminate molecular oxygen under the mild condiꢀ
tions (Scheme 1).17,19
R = Me, Pri
Contrary to this fact, the antimony catecholate comꢀ
plexes based on 3,6ꢀdiꢀtertꢀbutylꢀоꢀbenzoquinone and
related to oꢀaminophenolate complexes do not react with
molecular oxygen under similar conditions.15
The mechanism proposed17 for the process assumes
that one of the key steps is the oneꢀelectron oxidation of
the dianionic ligand to the radicalꢀanion one. The redox
potential of the transformation of catecholate into semiꢀ
quinolate (oꢀaminophenolate into oꢀiminobenzosemiꢀ
quinolate) should play a critical role, allowing or forbidꢀ
ding the process itself to occur. Antimony complexes 1
and 2 with 4ꢀmethoxyꢀ3,6ꢀdiꢀtertꢀbutylꢀ and 4,5ꢀdiꢀ
methoxyꢀ3,6ꢀdiꢀtertꢀbutylcatecholate ligands,18 whose
redox potentials are lower than those of 3,6ꢀdiꢀtertꢀ
* Dedicated to Academician I. I. Moiseev on his 80th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 520—525, March, 2009.
1066ꢀ5285/09/5803ꢀ0532 © 2009 Springer Science+Business Media, Inc.

New triphenylantimony(V) catecholates
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
533
butylcatecholate,15 were synthesized and their reactivity
was studied in the reaction with molecular oxygen. These
complexes were shown to be able to add molecular oxyꢀ
gen with the formation of cyclic endoperoxide complexes
also containing the fiveꢀmembered trioxastibolane cycle
(Scheme 2).
Results and Discussion
Five substituted catecholate complexes of triphenylꢀ
antimony(v) were synthesized: (3,6ꢀdiꢀtertꢀbutylꢀ4ꢀchloroꢀ
catecholate)triphenylantimony(v) (3), (3,5ꢀdiꢀtertꢀbutylꢀ
6ꢀchlorocatecholate)triphenylantimony(v) (4), (3,5ꢀdiꢀ
tertꢀbutylꢀ6ꢀchlorocatecholate)(methanol)triphenylꢀ
antimony(v) (4•MeOH), (3,6ꢀdiꢀtertꢀbutylꢀ4,5ꢀdifluoroꢀ
catecholate)triphenylantimony(v) (5), and (3,6ꢀdiꢀtertꢀ
butylꢀ4ꢀnitrocatecholate)triphenylantimony(v) (6).
The triphenylantimony complexes were synthesized
by oxidative addition to the corresponding oꢀquinones at
room temperature (Scheme 3). Compounds 3—6 and
4•MeOH were characterized by the data of IR spectroꢀ
Scheme 2
1
scopy, H NMR spectroscopy, elemental analysis, and
electrochemical studies.
The IR spectra of compounds 3—6 and 4•MeOH
contain a set of bands in the region 1100—1300 cm–1
R = H, OMe
,
The deactivation of the triphenylantimony catecholate
complexes in the reaction with molecular oxygen should
be expected when the redox potential of the catecholate
complexes is shifted to the electropositive potential region.
To confirm this fact, in the present work we have syntheꢀ
sized the triphenylantimony catecholate complexes
containing the electronꢀwithdrawing substituents in the
ring of the catecholate ligand and studied them by elecꢀ
trochemical methods.
which is characteristic of catecholate dianions, and
also bands of vibrations of the functional groups of the
complexes. Particularly, skeletal vibrations of the SbPh3
fragment lie in the region from 680 to 750 cm–1
stretching vibrations of the Sb—O bonds are observed at
580—650 cm–1, and stretching vibrations of the Sb—CPh
lie at 430—480 cm–1. The vibration band of the C—F
bond in complex 5 appears at 964 cm–1, being very
,
Scheme 3

534
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
Poddel´sky et al.
Table 1. Electrochemical oxidation potentials of the triphenylꢀ
antimony(v) catecholate complexes according to the CV method
intense. The stretching vibrations of the nitro group
in complex 6 manifest themselves as bands at 1515 and
1354 cm–1
.
1
Comꢀ
pound
I
II
The H NMR spectra of the compounds synthesized
were measured at room temperature in a CDCl3 solution.
Е11/2/V
Iс/Iа
Е2p/V
Ic/Iа
1
Tetramethylsilane was used as standard. The H NMR
1
7
3
4
0.70
0.89
0.98
1.00
1.07
1.16
0.90
0.82
0.81
0.52
0.56
0.66
1.14a
1.40b
1.37b
1.60b
1.60a
1.62a
—
0.50
0.60
0.50
—
spectra confirm the proposed structures of the syntheꢀ
1
sized complexes. The H NMR spectra of chloroꢀ and
difluoroꢀcontaining catecholates 3 and 5, respectively,
are shown in Fig. 1. Since catecholate 3 is unsymmetric,
the protons of the tertꢀbutyl groups are nonequivalent
and appear in the NMR spectrum as two singlets, whereas
the NMR spectrum of symmetric difluoroꢀcontaining
catecholate 5 contains one singlet from two equivalent
tertꢀbutyl groups. The 1H NMR spectra of catecholates 4,
4•MeOH, and 6 are analogous to the 1H NMR spectrum
of complex 3 and, hence, are not presented. The 1Н NMR
spectrum of complex 4•MeOH additionally exhibits a
singlet with δ = 3.49 from protons of the methyl group and
a broad singlet at δ = 0.93 from the proton of the hydroxy
group of methanol coordinated to the antimony atom.
Catecholate complexes 3, 4, and 4•MeOH containꢀ
ing chlorine in positions 4 or 6 of the phenyl ring of the
catecholate ligand are inert toward air oxygen in both the
solid state and solution. A similar situation is observed for
complexes 5 (contains fluorine atoms in positions 4 and 5
of the phenyl ring of the catecholate ligand) and 6 (conꢀ
4•MeOH
6
—
Note: GC electrode, CH2Cl2, V = 0.2 V s–1, 0.1 М NBu4ClO4,
С = 3•10–3 mol L–1, Ar, vs Ag/AgCl/KCl (sat.).
I and II are the first and second anodic stages; Е1 is the halfꢀ
1/2
wave potential of the first anodic process; Iс/Iа is the ratio of
currents of the inverse cathodic and direct anodic peaks; Е2p is
the potential of the second oxidation peak; the number of elecꢀ
trons of the first anodic stage relative to ferrocene as standard
is n = 1.
a Irreversible peak.
b Quasiꢀreversible oxidation process.
tains the nitro group in position 4 of the phenyl ring of the
catecholate ligand).
The electrochemical properties of synthesized antiꢀ
mony complexes 3, 4, 4•MeOH, and 6 were studied by
cyclic voltammetry (CV). The complexes are oxidized at
the glassyꢀcarbon electrode in dichloromethane in two
stages (Table 1). For complex 3 and 3,6ꢀdiꢀtertꢀbutylꢀ
catecholate)triphenylantimony (7), the first redox proꢀ
cess is quasiꢀreversible oneꢀelectron (Scheme 4, Fig. 2).
The reversibility coefficients show that the radicalꢀanion
form of the coordinated ligand that formed is rather stable.
However, in the both cases, the inverse branch of the CV
curve contains peaks of the fragmentation products of the
cationic complexes (Еp1 = 0.03 V; Еp2 = –0.38 V), and
a
1
1
1´
I/mA
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
2
3
1
2
b
2´
–0.01
–0.02
–0.03
–0.4 –0.2
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
E/V
Fig. 2. Cyclic voltammograms of the oxidation of complex 3
δ
8.0
7.5
7.0
6.5
1.5
(CH2Cl2, GC anode, Ag/AgCl/KCl, 0.1
M NBu4ClO4,
Fig. 1. 1Н NMR spectra of catecholates 3 (a) and 5 (b) (CDCl3,
~20 °C). The groups are designated in Scheme 1.
C = 3•10–3 mol L–1, argon): 1, potential sweep to 1.3 V;
2, potential sweep to 1.78 V.

New triphenylantimony(V) catecholates
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
535
Scheme 4
dinated complex 4. Coordination of a methanol molecule
shifts the halfꢀwave oxidation potential of complex
4•MeOH compared to that of 4 to the anodic region by
0.07 V. The second anodic peaks for the both compounds
are similar. The observed effects of shifting the oxidation
potentials suggests that the redox properties of these comꢀ
pounds can be controlled due to coordination of solvent
molecules of different nature.
For complex 6 containing the nitro group, according
to the acceptor influence of the latter, the oxidation
potentials are shifted to the anodic region (see Table 1).
Rather stable cationic complex is formed upon the priꢀ
mary oxidation process (Fig. 4), and no decomposition
products are observed in the inverse branch of the CV
curve. The stability of this complex can be due to the
ability of the nitro group to participate in the delocalizaꢀ
tion of the unpaired electron of the oꢀbenzosemiquinone
form of the oxidized complex (Scheme 5).
the second cathodic process corresponds to the reduction
of the free oꢀquinoid ligand. The extension of the potenꢀ
tial sweep range to the anodic region allows one to detect
the second irreversible oxidation process corresponding
to the transition of the semiquinone form of the ligand
into the quinoid one with the formation of the dicationic
complex.
The values of the cathodic peaks of the products of
complex decomposition increase, indicating the decoorꢀ
dination of neutral oꢀquinone. The acceptor character of
the chlorine atom affects the value of only the first redox
transition, while the second anodic peaks are almost idenꢀ
tical for the both complexes.
The change in the mutual arrangement of substituents
(chlorine atom and tertꢀbutyl groups) in the carbon ring
of complex 4 (with respect to 3) exerts no considerable
effect on the first oxidation potential. However, the Iс/Iа
values (ratio of currents of the inverse cathodic and anꢀ
odic peaks) show that the stability of the intermediates
formed decreases (see Table 1). Complex 4 is also characꢀ
terized by the appearance of the decomposition products
during backward scan. The reversibility factor for the
hexacoordinated analog of 4•MeOH, which contains
solvated methanol in the coordination sphere of the antiꢀ
mony atom, at the first oxidation stage (Fig. 3) differs
insignificantly from the data obtained for pentacoorꢀ
Scheme 5
I/mA
0.06
I/mA
0.07
2
0.05
0.04
0.06
0.05
0.04
0.03
1
0.03
0.02
0.01
0
0.02
0.01
0
–
0.01
–0.01
–0.02
–
0.2
E/V
0.02
0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.2
0.4
0.6
0.8
1.0
1.2
E/V
Fig. 3. Cyclic voltammograms of the oxidation of comꢀ
plex 4•MeOH (CH2Cl2, GC anode, Ag/AgCl/KCl, 0.1 M
NBu4ClO4, C = 3•10–3 mol L–1, argon): 1, potential sweep
to 1.3 V; 2, potential sweep to 1.78 V.
Fig. 4. Cyclic voltammogram of the oxidation of complex 6
(CH2Cl2, GC anode, Ag/AgCl/KCl, 0.1
M NBu4ClO4,
C = 3•10–3 mol L–1, argon).

536
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
Poddel´sky et al.
It has previously18 been found that (4ꢀmethoxyꢀ
(with decomp.). Found (%): C, 63.36; H, 5.97; Sb, 18.91;
Cl, 5.87. C32H34ClO2Sb. Calculated (%): C, 63.23; H, 5.64;
Sb, 20.03; Cl, 5.83. IR (Nujol), ν/cm–1: 1478 w, 1432 w, 1387 m,
1367 m, 1332 w, 1294 m, 1244 s, 1202 w, 1076 m, 1069 m, 1056 m,
1025 w, 996 m, 985 s, 950 s, 854 w, 843 s, 813 m, 766 s, 736 s,
693 s, 673 w, 637 w, 615 w, 598 w, 481 w, 469 m, 449 s. 1H NMR
(CDCl3), δ: 1.39 and 1.61 (both s, 9 H each, But), 6.67 (s, 1 H,
C6H), 7.40—7.53 (m, 9 H, SbPh3), 7.70—7.79 (m, 6 H, SbPh3).
(3,5ꢀDiꢀtertꢀbutylꢀ6ꢀchlorocatecholate)triphenylantimony(v)
(4). Complex 4 was synthesized by the reaction of triphenylꢀ
antimony (0.251 g, 0.71 mmol) and 3,5ꢀdiꢀtertꢀbutylꢀ6ꢀchloroꢀ
oꢀbenzoquinone (0.181 g, 0.71 mmol) using a method similar to
that for complex 3. The complex isolated from hexane represents
light yellow fine crystals. M.p. 145—147 °С (decomposes at
t > 180 °С). Found (%): C, 63.52; H, 5.49; Sb, 19.20; Cl, 5.74.
C32H34ClO2Sb. Calculated (%): C, 63.23; H, 5.64; Sb, 20.03;
Cl, 5.83. IR (Nujol), ν/cm–1: 1478 w, 1437 w, 1416 m, 1360 m,
1310 w, 1270 m, 1258 m, 1243 m, 1178 w, 1157 w, 1070 m,
1024 w, 990 s, 955 m, 871 s, 856 w, 757 w, 738 s, 730 s, 692 s,
3,6ꢀdiꢀtertꢀbutylcatecholate)triphenylantimony (1) can
reversibly bind molecular oxygen. The voltammetric data
show that the introduction of an electronꢀdonor substiꢀ
tuent considerably decreases the potential of the first
anodic process, which assumes the reaction with molecuꢀ
lar oxygen to occur. The complex is electrochemically
oxidized in oneꢀelectron reversible stage to form the stable
cation (see Table 1). The voltammogram contains no
secondary peaks of fragmentation product reduction. The
second redox process is irreversible.
Thus, compounds 3, 4, 4•MeOH, and 6 undergo twoꢀ
stage electrochemical oxidation. The first electron transfer
is the quasiꢀreversible oneꢀelectron process, due to which
the catecholate ligand is transformed into the oꢀsemiꢀ
quinone form. The subsequent oxidation of the complexes
is irreversible and results in the decoordination of neutral
oꢀquinone. The electrochemical potentials obtained for
the compounds with electronꢀwithdrawing substituents
differ considerably from a threshold value of +0.7 V typiꢀ
cal of (4ꢀmethoxyꢀ3,6ꢀdiꢀtertꢀbutylcatecholate)triphenylꢀ
antimony(v).
1
642 w, 634 w, 615 w, 593 w, 453 s. H NMR (CDCl3), δ: 1.42
and 1.46 (both s, 9 H each, But), 6.73 (s, 1 H, C6H), 7.40—7.56
(m, 9 H, SbPh3), 7.74—7.88 (m, 6 H, SbPh3).
Solvate with methanol (3,5ꢀdiꢀtertꢀbutylꢀ6ꢀchloroꢀ
catecholate)triphenylantimony(v), (4•MeOH). A finely crysꢀ
talline sample of complex 4•MeOH was isolated by the reꢀ
crystallization of complex 4 from methanol. M.p. 122—126 °С.
Found (%): C, 61.46; H, 5.63; Sb, 18.70; Cl, 6.06. C33H38ClO3Sb.
Calculated (%): C, 61.94; H, 5.99; Sb, 19.03; Cl, 5.54.
IR (Nujol), ν/cm–1: 3330 s, 1480 w, 1432 s, 1409 s, 1359 s,
1312 m, 1279 w, 1262 s, 1243 s, 1184 m, 1174 w, 1158 w, 1110 w,
1076 m, 1064 m, 1024 w, 997 s, 985 s, 953 s, 868 s, 858 w, 754 m,
738 s, 730 s, 695 s, 660 m, 634 w, 618 w, 588 w, 456 s. 1H NMR
(CDCl3), δ: 0.93 (br.s, 1 H, OH) 1.42 and 1.46 (both s, 9 H
each, But), 3.49 (s, 3 H, CH3 of methanol), 6.73 (s, 1 H, C6H),
7.40—7.60 (m, 9 H, SbPh3), 7.76—7.86 (m, 6 H, SbPh3).
(3,6ꢀDiꢀtertꢀbutylꢀ4,5ꢀdifluorocatecholate)triphenylꢀ
antimony(v) (5). The complex was synthesized by the reaction of
triphenylantimony (0.353 g, 1.0 mmol) and 3,6ꢀdiꢀtertꢀbutylꢀ
4,5ꢀdifluoroꢀоꢀbenzoquinone (0.256 g, 1.0 mmol) using a method
similar to that for the synthesis of complex 3. The comꢀ
plex isolated from toluene represents yellowꢀorange crystals.
Found (%): C, 63.20; H, 5.70; Sb, 19.67. C32H33F2O2Sb.
Calculated (%): C, 63.07; H, 5.46; Sb, 19.98. IR (Nujol),
ν/cm–1: 1477 s, 1432 s, 1410 s, 1357 s, 1333 w, 1305 w, 1280 s,
1246 m, 1203 m, 1181 w, 1158 w, 1075 m, 1070 m, 1059 m,
1046 s, 1023 w, 997 m, 964 s, 929 w, 891 m, 849 w, 800 w, 773 w,
737 s, 732 s, 692 s, 673 w, 666 w, 656 w, 624 m, 582 w, 523 m,
447 s. 1H NMR (CDCl3), δ: 1.50 (s, 18 H, 2 But), 7.40—7.56
(m, 9 H, Ph), 7.68—7.76 (m, 6 H, Ph).
Experimental
All experiments on synthesis and investigation of the comꢀ
plexes were carried out in evacuated ampules without oxygen
and water. In all cases, the yields of the target products were
higher than 90%. The solvents used were purified and dehydrated
according to standard procedures.20 oꢀBenzoquinones, viz.,
3,6ꢀdiꢀtertꢀbutylꢀ4ꢀchloroꢀoꢀbenzoquinone, 3,5ꢀdiꢀtertꢀbutylꢀ
6ꢀchloroꢀоꢀbenzoquinone, 3,6ꢀdiꢀtertꢀbutylꢀ4,5ꢀdifluoroꢀ
оꢀbenzoquinone, and 3,6ꢀdiꢀtertꢀbutylꢀ4ꢀnitroꢀоꢀbenzoquinone,
were synthesized earlier at the Laboratory of Chemistry of
Organoelement Compounds of the G. A. Razuvaev Institute of
Organometallic Chemistry of the Russian Academy of Sciences.
IR spectra were recorded on an FSM 1201 FTIR spectroꢀ
meter in Nujol. 1Н NMR spectra were measured on a Bruker
AVANCE DPXꢀ200 instrument using tetramethylsilane as
internal standard and CDCl3 as solvent. The oxidation potentials
were measured by cyclic voltammetry in a threeꢀelectrode cell
with an IPCꢀpro potentiostat in argon. The working electrode
was a stationary glassyꢀcarbon (GC) electrode with a diameter
of 2 mm, and a platinum wire (S = 18 mm2) served as the
auxiliary electrode. The reference electrode (Ag/AgCl/KCl) was
equipped with a waterꢀproof membrane. The potential sweep
rate was 0.2 V s–1. The supporting electrolyte 0.1 М Bu4NClO4
(99%, Acros) was two times recrystallized from aqueous EtOH
and dried in vacuo (48 h at 50 °С). Dichloromethane was
dried and purified by known procedures.21 The concentration of
(3,6ꢀDiꢀtertꢀbutylꢀ4ꢀnitrocatecholate)triphenylantimony(v)
(6). Complex 6 was synthesized by the reaction of triphenylꢀ
antimony (0.296 g, 0.837 mmol) and 3,6ꢀdiꢀtertꢀbutylꢀ4ꢀnitroꢀ
оꢀbenzoquinone (0.222 g, 0.837 mmol) using a method analogous
to that for the synthesis complex 3. The complex isolated
the antimony complexes was 0.003 mol L–1
.
from hexane represents fine yellow crystals. IR (Nujol), ν/cm–1
:
(3,6ꢀDiꢀtertꢀbutylꢀ4ꢀchlorocatecholate)triphenylantimony(v)
(3). A solution of 3,6ꢀdiꢀtertꢀbutylꢀ4ꢀchloroꢀоꢀbenzoquinone
(0.255 g, 1 mmol) in toluene was added with stirring to a solution
of triphenylantimony (0.353 g, 1 mmol) in toluene. The color of
the solution changed from green to orange. After hexane was
added, a finely dispersed light yellow precipitate of product 3
was formed, filtered off, and dried in vacuo. M.p. 160—164 °С
1515 m, 1438 m, 1371 m, 1354 s, 1255 s, 1217 m, 1160 w,
1070 w, 1059 m, 1038 w, 984 s, 890 w, 855 w, 812 m, 788 w,
774 w, 737 s, 790 m, 619 w, 529 w, 456 m, 447 s. 1H NMR
(CDCl3), δ: 1.38 and 1.47 (both s, 9 H each, 2 But), 6.66 (s,
1 H, arom. C6H), 7.40—7.60 (m, 9 H, Ph), 7.66—7.82 (m,
6 H, Ph).

New triphenylantimony(V) catecholates
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
537
11. M. Hall, D. B. Sowerby, J. Am. Chem. Soc., 1980, 102/2,
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J. Organomet. Chem., 2005, 690, 1273.
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Cherkasov, G. K. Fukin, Yu. A. Kurskii, L. G. Abakumova,
Angew. Chem., Int. Ed., 2005, 44, 2767.
18. G. A. Abakumov, V. K. Cherkasov, E. V. Grunova, A. I.
Poddel´sky, L. G. Abakumova, Yu. A. Kurskii, G. K. Fukin,
E. V. Baranov, Dokl. Akad. Nauk, 2005, 405, 199 [Dokl.
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This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 07ꢀ03ꢀ00819,
06ꢀ03ꢀ32442, and 07ꢀ03ꢀ12101), the Council on Grants
at the President of the Russian Federation (Program for
State Support of Leading Scientific Schools of the Rusꢀ
sian Federation (Grant NShꢀ4182.2008.3) and Young
Candidates of Science (Grant MKꢀ3523.2007.3)), and
the Russian Science Support Foundation.
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Received March 13, 2008;
in revised form July 3, 2008