Supramolecular organization of the crystals of allylsulfide clathrochelate: Influence of the nature of solvate molecules

Article abstract of DOI:10.1023/B:RUCB.0000046246.04868.88

The influence of the nature of solvate molecules on the supramolecular organization of the crystals of α-benzyldioximate FeBd ₂((AllylS)₂Gm)(BF)₂ clathrochelate containing the allylsulfide substituents was studied by X-ray

Full text of DOI:10.1023/B:RUCB.0000046246.04868.88

1496
Russian Chemical Bulletin, International Edition, Vol. 53, No. 7, pp. 1496—1502, July, 2004
Supramolecular organization of the crystals of allylsulfide clathrochelate:
influence of the nature of solvate molecules
c
b
b
Ya. Z. Voloshin,a,b O. A. Varzatskii, Z. A. Starikova, M. Yu. Antipin,
ꢀ
A. Yu. Lebedev,a and A. S. Belova
aL. Ya. Karpov Institute of Physical Chemistry,
10 ul. Vorontsovo Pole, 105064 Moscow, Russian Federation.
Fax: +7 (095) 975 2450. Eꢀmail: voloshin@cc.nifhi.ac.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085. Eꢀmail: star@xray.ineos.ac.ru
cV. I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,
32/34 ul. Palladina, 03680 Kiev, Ukraine.
Fax: +38 (044) 424 3070
The influence of the nature of solvate molecules on the supramolecular organization of the
crystals of αꢀbenzyldioximate FeBd2((AllylS)2Gm)(BF)2 clathrochelate containing the
allylsulfide substituents was studied by Xꢀray diffraction analysis.
Key words: supramolecular chemistry, clathrochelates, Xꢀray diffraction analysis.
The preparation of single crystals of complexes with
an encapsulated metal ion (in particular, clathrochelates)
suitable for Xꢀray diffraction analysis presents considerꢀ
able difficulties.1 In many cases, the use of solvents of
different nature does not give the desired results. In an
effort to elucidate the influence of various solvents
on the crystal packing, we studied the allylsulfide
FeBd2[(AllylS)2Gm](BF)2 clathrochelate, where Bd2– is
the αꢀbenzyldioxime dianion and Gm is the glyoxime
residue. The molecular structure of this complex is shown
in Fig. 1. The clathrochelate molecule has a diskꢀlike
shape and its periphery is formed by the nonpolar Ph and
Me groups. Only crossꢀlinking fragments of the clathroꢀ
chelate framework containing the oxygen and fluorine
atoms are polar. One would expect that the solvent molꢀ
ecules would be involved in specific dipoleꢀdipole interꢀ
actions with these (although insufficiently "open") moiꢀ
eties of the molecule.
O(3)
O(5)
O(4)
O(6)
Fe(1)
Experimental
Fig. 1. Molecular structure of the FeBd2[(AllylS)2Gm](BF)2
clathrochelate.
(1,8ꢀBis(2ꢀfluorobora)ꢀ2,7,9,14,15,20ꢀhexaoxaꢀ
3,6,10,13,16,19ꢀhexaazaꢀ4,5,11,12ꢀtetraphenylꢀ17,18ꢀ
diallylmercaptylbicyclo[6.6.6]eicosaꢀ3,5,10,12,16,18ꢀ
hexaene(2–)iron(II), FeBd2[(AllylS)2Gm](BF)2. A solution of
allylthiol (0.25 mL, 3.14 mmol) and Et3N (0.44 mL, 3.16 mmol)
in CH2Cl2 (10 mL) was added with stirring to a solution of the
FeBd2(Cl2Gm)(BF)2 complex2 (1.07 g, 1.43 mmol) in CH2Cl2
(30 mL) under argon. The reaction mixture was stirred for 2 h
and concentrated to dryness. The residue was washed with
methanol (15 mL) and dissolved in a small amount of CH2Cl2.
The solution was filtered through a layer of silica gel SPHꢀ300
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1439—1444, July, 2004.
1066ꢀ5285/04/5307ꢀ1496 © 2004 Springer Science+Business Media, Inc.

Supramolecular organization of clathrochelate
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 7, July, 2004
1497
(Chemapol) and precipitated with an excess of hexane. The
precipitate was filtered off, washed with hexane, and dried in
vacuo. The yield was 0.79 g (67%). Found (%): C, 52.69; H, 3.65;
N, 10.11; Fe, 6.55. C36H30N6O6B2S2F2Fe. Calculated (%):
methods and refined by fullꢀmatrix leastꢀsquares against F2 with
anisotropic thermal parameters for nonhydrogen atoms. The
positions of the hydrogen atoms were revealed from the differꢀ
ence Fourier synthesis and refined using the riding model with
Uiso(H) = nUeq(C), where n = 1.5 for the methyl groups and
1.2 for all other groups, and Ueq(C) are the equivalent isotropic
factors of the corresponding pivot carbon atoms. All calculaꢀ
tions were carried out using the SHELXTLꢀ97 program packꢀ
age.5 The atomic coordinates were deposited with the Camꢀ
bridge Structural Database. The crystallographic parameters and
details of structure refinement of these solvates are given in
Table 1.
C, 52.58; H, 3.65; N, 10.22; Fe, 6.79. MS, m/z 822 [M]+•
.
1H NMR (CDCl3), δ: 3.96 (d, 4 H, SCH2); 5.02 (m, 4 H,
=CH2); 5.81 (m, 2 H, =CH—); 7.32 (m, 20 H, Ph). 13C NMR
{1H} (CDCl3), δ: 36.7 (SCH2); 119.0 (=CH2); 127.9, 129.0,
130.1, and 130.6 (all Ph); 132.3 (=CH—); 148.4 (S—C=N);
156.6 (Ph—C=N).
Isothermal crystallization of the FeBd2((AllylS)2Gm)(BF)2
clathrochelate was carried out at room temperature using the
chloroform—heptane (solvate 1), acetonitrile—isooctane (solꢀ
vate 2), and dichloromethane—nꢀhexane (solvate 3) systems. As
a result, we obtained crystals of the compound with polar chloꢀ
roform solvate molecules characterized by a low donor numꢀ
ber (1), the crystals with polar and donor acetonitrile solvate
molecules (2), and the solvate with nonpolar nꢀhexane (3).
Lowꢀtemperature Xꢀray diffraction experiments were
carried out on automated Bruker SMART and Syntex P21
diffractometers. Reflection intensities were integrated using the
SAINT Plus software and corrected for absorption using the
SADABS program.3,4 The structures were solved by direct
Results and Discussion
The crystal structures of solvates 1—3 contain the isoꢀ
lated clathrochelate molecules and disordered solvate
molecules located in the cavities between the clathroꢀ
chelate molecules. It should be noted that crystals 1 and 2
with polar solvate molecules (CHCl3 and MeCN, respecꢀ
tively) are isostructural, whereas a different molecuꢀ
Table 1. Crystallographic parameters and details of structure refinement of solvates 1—3
Parameter
FeBd2[(AllylS)2Gm](BF)2• FeBd2[(AllylS)2Gm](BF)2• FeBd2[(AllylS)2Gm](BF)2•
0.5 CHCl3 (1)
0.75 MeCN (2)
0.5 C6H14 (3)
Molecular formula
Molecular weight
Space group
C36.5H30.5B2Cl1F2FeN6O6S2 C37.5H32.25B2F2FeN6O6S2
C39H37B2F2FeN6O6S2
865.34
881.93
P21/n
853.04
P21/n
P21/n
Temperature/K
120(2)
130(2)
110(2)
a/Å
b/Å
c/Å
12.933(3)
15.505(3)
19.210(4)
102.578(5)
3760(1)
12.944(2)
15.550(3)
19.133(3)
102.549(4)
3759(1)
13.892(4)
17.541(4)
16.615(4)
95.34(2)
β/deg
V/Å3
4031(2)
Z
4
4
4
dcalc/g cm–3
1.558
1.507
1.426
Color, crystal habit
Dimensions/mm
Diffractometer
Radiation
Red needle
0.30×0.35×0.50
SMART Bruker
Red plate
0.35×0.30×0.15
SMART Bruker
MoꢀKα (λ = 0.71073 Å)
5.80
Red needle
0.45×0.35×0.30
Syntex P21
µ/cm–1
6.85
SADABS
0.472/0.862
φ/ω
5.41
Absorption correction
Tmin/Tmax
Scan mode
2θmax/deg
SADABS
0.564/0.862
φ/ω
q/2q
52.74
56.56
58.00
Total number of reflections
27439
30506
8648
Number of independent reflections (Rint
)
9291 (0.1077)
9970 (0.0817)
0.0603 (on 5168)
0.1294
8109 (0.0144)
0.0535 (on 6107)
0.0895
R1 (based on F for reflections with I >2σ(I )) 0.0558 (on 3757)
wR2 (based on F2 for all reflections)
0.1548
Number of refinement parameters
Weighting scheme
523
516
568
2
2
w–1 = σ (Fo2) + (aP)2 + bP, P = 1/3(Fo2 + 2Fc
)
a
b
0.0554
0.1000
0.0869
3.1221
GOOF
F(000)
emax/emin (eÅ–3
0.777
1804
0.688/–0.343
0.859
1754
0.794/–0.374
0.973
1788
1.050/–0.485
)

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Russ.Chem.Bull., Int.Ed., Vol. 53, No. 7, July, 2004
Voloshin et al.
F(2)
B(2)
F(1)
C(15)
C(14)
C(16)
C(17)
C(21)
C(27)
C(28)
B(1)
C(17)
C(7)
C(22)
O(1)
O(2)
O(5)
O(2)
C(20)
O(3)
C(8)
C(10)
S(2)
O(3)
C(18)
C(18)
C(3)
C(14)
C(9)
C(26)
C(29)
C(25)
C(13)
N(1)
N(3)
S(1)
N(3)
N(2)
C(12)
N(5)
N(2)
C(23)
C(11)
C(19)
C(2)
C(1)
C(2)
C(1)
C(4)
C(24)
C(4)
C(5)
C(19)
Fe(1)
C(3)
C(5)
Fe(1)
N(4)
C(6)
C(20)
N(6)
C(26)
C(27)
C(24)
C(23)
S(1)
N(1)
S(2)
C(6)
O(4)
O(6)
C(32)
C(33)
C(28)
C(7´)
C(8)
N(6)
C(36)
N(5)
C(31)
C(11)
C(31)
C(32)
O(4)
C(7)
O(1)
C(12)
C(36)
O(5)
C(9´)
B(1)
C(35)
O(6)
C(10)
B(2)
C(8´)
C(9)
C(35)
C(34)
F(1)
F(2)
C(33)
C(34)
1
3
Fig. 2. Clathrochelate molecular structures in solvates 1 and 3.
lar packing was revealed in crystals 3 with nonpolar
nꢀhexane solvate molecules. All these crystals belong to
the monoclinic system and have the same space group
(P21/n, Z = 4).
The clathrochelate ligand (Fig. 2) is formed by three
planar dioximate fragments O—N=C—C=N—O (two
fragments contain two Ph groups each, whereas the third
fragment contains two allylsulfide substituents) crossꢀ
linked with boron atoms. In solvate 3, the carbon atoms
of one of the two allyl fragments are disordered over two
positions with occupancies of 0.5 (see Fig. 2). The diꢀ
hedral angles between the planes of these three fragments
are close to 120°. Therefore, the clathrochelate frameꢀ
work of the molecule has a threefold symmetry pseudoaxis.
Selected geometric parameters of the clathrochelate molꢀ
ecules in crystals 1—3 are given in Tables 2 and 3. These
Table 2. Selected bond lengths (Å) in clathrochelate molecules 1—3
Bond
1
2
3
Fe—N
1.905—1.919
(σ 0.003 Å)
1.902—1.918
(σ 0.003 Å)
1.906—1.915
(σ 0.002 Å)
B—F
B—O
1.355(5), 1.364(5)
1.474—1.489
1.359(4), 1.366(4)
1.472—1.487
1.355(3), 1.361(4)
1.473—1.497
(σ 0.005 Å)
1.303—1.324
(σ 0.004—0.005 Å)
1.312—1.317
(σ 0.003—0.004 Å)
1.304—1.312
C=N
(σ 0.004 Å)
1.364—1.374
(σ 0.004 Å)
1.361—1.375
(σ 0.003—0.004 Å)
1.369—1.377
N—O
(σ 0.003—0.004 Å)
1.461, 1.447, 1.450
(σ 0.005 Å)
(σ 0.003 Å)
(σ 0.003 Å)
C—C(ring)
1.450, 1.458, 1.455
(σ 0.004—0.005 Å)
1.755(3), 1.741(4)
1.819(5), 1.830(4)
1.449(7), 1.470(6)
1.282(7), 1.285(6)
1.482, 1.468, 1.478, 1.472
(σ 0.004—0.005 Å)
1.370—1.397
1.460, 1.466, 1.462
(σ 0.003—0.004 Å)
1.753(3), 1.744(3)
1.891(8)*, 1.844(4)
1.52(1)*,1.481(6)
1.32(1)*, 1.296(7)
1.483, 1.478, 1.477, 1.481
(σ 0.004 Å)
C(ring)—S
S—CH2
CH2—CH
CH=CH2
1.752(4), 1.751(4)
1.818(5), 1.831(4)
1.467(6), 1.493(6)
1.310(6), 1.293(5)
1.485, 1.475, 1.470, 1.475
(σ 0.005 Å)
C(ring)—C(Ph)
C(Ph)—C(Ph)
1.362—1.394
1.357—1.403
* Data for the disordered allylsulfide substituent.

Supramolecular organization of clathrochelate
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 7, July, 2004
1499
Table 3. Main conformational characteristics (angles in deg, distances in Å) of the clathrochelate molecules in
crystals 1—3
Parameter
1
2
3
Dihedral angle between
planar chelate ringsa
I/III 119.1
I/II 112.4
I/III 118.9
I/II 112.4
I/III 122.7
I/II 118.9
II/III 114.0
II/III 111.8
II/III 110.5
Torsion angle
F—B—O—N
Fe B distance
173.3, 174.2, 170.6
171.0, 172.5, 172.8
2.968, 2.978
171.4, 173.2, 172.2
171.6, 172.4, 173.0
2.975, 2.982
170.6, 173.2, 170.9
172.2, 171.2, 172.9
2.976, 2.983
...
Dihedral angle between
phenyl substituents
Torsion angles
51.6, 55.3
52.4, 56.2
52.8, 59.7
C(ring)—S—CH2—CH
S—CH2—CH=CH2
127.4, 118.1
53.0, 74.9
124.8, 116.1
54.3, 74.3
122.5, 106.5
75.2, 58.7
(77.9, 76.0)b
a The chelate rings of fragments I and II containing the Ph groups and the chelate ring of the fragment III
containing the allylsulfide groups.
b Data for the disordered allylsulfide substituent.
Table 4. Selected characteristics of the molecular packing of
compounds 1—3
molecules have the identical Fe—N, B—O, C=N, N—O,
S—C, and S—CH2 bond lengths (average values are 1.912,
1.482, 1.313, 1.370, 1,749, and 1.824 Å, respectively),
which are close to standard values.6 The conformational
parameters of the clathrochelate molecules are also simiꢀ
lar (see Table 3). In the structures of solvates 1—3, the
phenyl and allylsulfide substituents, which are rather laꢀ
bile (rotation of the Ph groups about the C(oxime)—C(Ph)
bond and rotations about the S—CH2 and CH2—CH
bonds in the allylsulfide fragments), have nearly identical
orientations.
Comꢀ
pound
Orientation*/deg
V/Å3
Cavity size
/Å
a
b
c
1
2
3
88.2 127.6 141.2
87.8 127.5 141.4
80.9 94.1 170.4 103.76
41.40
39.37
10.07×9.16×7.09
10.12×8.70×7.20
16.87×9.60×7.73
...
* The orientation of the threefold pseudoaxis (В1 В2) relative
to the crystallographic axes a, b, c.
Therefore, the nature of the solvate molecules
has no effect on the molecular structure of the
FeBd2((AllylS)2Gm)(BF)2 clathrochelate. The crystal
structures of 1 and 2 are characterized by the same packꢀ
ing of the clathrochelate molecules, which differs subꢀ
stantially from that in crystal 3 (Fig. 3).
In all structures, the solvate molecules are located in
the centrosymmetric cavities and are disordered about the
corresponding center of symmetry. The structures of 1
and 2 differ substantially from the structure of 3 in that
the clathrochelate molecules are shifted along the c axis.
Besides, the molecules differ in the orientation of the
threefold pseudoaxis with respect to the crystallographic
a, b, and c axes. In crystals 1 and 2, this pseudoaxis is
perpendicular to the a axis and is inclined to the b
and c axes by ~45°. By contrast, this axis in the crystal of 3
is perpendicular to the a and b axes and parallel to the
c axis.
sizes of the cavities are also shown. The shortest intermoꢀ
lecular contacts between the molecules of the clathroꢀ
chelate "host" and the solvate "guest" are given in Table 4.
In crystals 1 and 2, there are Cl...C and N...C contacts
smaller than the sums of the van der Waals radii of the
corresponding atoms (3.60 and 3.20 Å).7 These contacts
can be considered as weak specific dipoleꢀdipole interacꢀ
tions. In crystal 3, such interactions are absent and all
intermolecular contacts are larger than the sum of the van
der Waals radii C...C (3.42 Å).7
In molecule 1, the centrosymmetrical cavity is formed
by atoms of four phenyl groups and two allyl groups of
two clathrochelate molecules (see Fig. 4). The oxygen
atoms (O(1) and O(1a)) of the clathrochelate framework
also form the shortest contacts with the chlorine atom.
The C(19)...C(19b) atoms of the Ph rings form the longꢀ
est diagonal (10.07 Å). Two other diagonals are formed by
the C(9a)—C(9c) atoms (9.16 Å) of one of the allyl fragꢀ
ments and the C(18)...C(18b) atoms (7.09 Å) of the
Ph rings. Taking into account the van der Waals radius
of the carbon atoms, the approximate volume of this
cavity is 41.3 Å3. The CHCl3 molecules, which are
In crystals 1—3, the centrosymmetrical cavities, which
are occupied by the solvate molecules, are formed by the
atoms of the Ph and allylsulfide substituents and have an
ellipsoid shape. Figures 4 and 5 schematically show the
structures of these cavities (solvate molecules are denoted
by circles in the centers); the contacts determining the

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Voloshin et al.
a
a
0
c
c
0
b
b
1
3
Fig. 3. Crystal packing of solvates 1 and 3 (structure projected onto the 0yz plane, the y axis is vertical).
1
2
Fig. 4. Schematic representation of the location of the solvate molecules in the cavities of the crystal structures of 1 and 2.
disordered over two positions, form the dimeric
Cl—CHꢀ(µ2ꢀCl2)—CH—Cl fragment, in which the disꢀ
tance between two most remote chlorine atoms is 4.94 Å,
and the distance between the µ2ꢀCl atoms is 2.90 Å. This
fragment can be approximated by an ellipsoid with the
long and short axes of 4.0 and 2.9 Å, respectively. Taking
into account the van der Waals radius of the chlorine
atom (1.9 Å), the volume of this ellipsoid is ~29 Å3.

Supramolecular organization of clathrochelate
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 7, July, 2004
1501
Table 5. Shortest intermolecular contacts (d) in the crystal strucꢀ
tures of compounds 1—3
Bond
d/Å
Bond
Structure 3
d/Å
Structure 1
C(l1s)...O(1b)
C(l1s)...C(8а)
C(l1s)...C(9а)
C(l1s)...C(19b)
C(l1s)...C(24b)
C(l1s)...H(18c)
C(l1s)...H(8aa)
C(l1s)...H(9ba)
3.311
3.418
3.500
3.386
3.293
2.76
С(5s)...C(16b)
C(5s)...C(26c)
С(5s)...C(36c)
C(4s)...C(7´)
C(4s)...C(8´)
C(4s)...C(34c)
C(3s)...C(9´)
C(3s)...C(30c)
C(3s)...C(33c)
C(3s)...C(34c)
C(2s)...C(8)
3.789
3.803
3.734
3.802
3.838
3.574
3.762
3.744
3.565
3.627
3.378
3.251
3.624
2.76
2.96
Structure 2
N(1sa)...C(8c)
N(1sa)...C(9c)
N(1sa)...C(13а)
N(1sa)...C(18a)
3.322
3.463
3.360
3.151
C(2s)...C(9´)
C(2s)...C(33a)
ellipsoid (C(36c)...C(36d), 17.77 Å) is 14.17 Å, and two
other axes are 4.9 and 3.7 Å. The length of the solvate
fragment is 11.26 Å (C(5s)...C(5sa) distance), whereas
the length of the hexane chain is ~7.5 Å.
3
A comparative analysis of the volume of the solvate
molecule and the elliptical cavity occupied by the solvate
molecules (taking into account the van der Waals radius
of the carbon atom) shows (see Table 4) that these caviꢀ
ties in all structures studied are accessible for the solvate
molecules (particularly, in the structure of 2). As a result,
the solvate molecules are disordered about the center of
symmetry.
Fig. 5. Schematic representation of the location of the nꢀhexane
solvate molecules in the cavities of the crystal structure of 3.
Therefore, the cavity size in the crystal structure is comꢀ
parable to the volume of the solvate molecule, whereas
the volume of the CHCl3 molecule is substantially smaller.
Presumably, that is the reason why the CHCl3 solvate
molecules are disordered in the crystal.
In the crystal sructure of 2, the cavity is formed by the
atoms of six Ph rings and two allyl groups of two clathroꢀ
chelate molecules. Unlike the structure of 1, the long axis
of the ellipsoid (10.12 Å) is formed by the carbon atoms
of the allylsulfide substituents (C(13) and C(13a)), and
two other axes (7.02 and 8.71 Å) are formed by the atoms
of the Ph rings. The volume of the cavity is 37.6 Å3,
whereas the volume of the CH3CN molecule approxiꢀ
mates 22 Å3. As a result, the CH3CN molecule is disorꢀ
dered over four positions.
In the structure of 3, the cavity is more elongated (see
Fig. 5). This cavity is formed by six "host" molecules, viz.,
the carbon atoms of twelve Ph and four allylsulfide subꢀ
stituents (two of them are disordered). No shortened conꢀ
tacts between the atoms of the solvent and the clathroꢀ
chelate framework were revealed. The hexane molecule is
also disordered about the center of symmetry, which coꢀ
incides with the midpoint of the C(1s)—C(1sa) bond, i.e.,
the solvate fragment is a tenꢀatom planar aliphatic chain,
which is extended along the long axis of the cavity and
oriented so that its plane is parallel to the plane of the
C(31)—C(36) and C(31a)—C(36a) rings. Taking into acꢀ
count the van der Waals radius of the carbon atoms, the
volume of the cavity is 75.8 Å3. The long axis of the
The character of packing of the bulky clathrochelate
molecules and smaller solvate molecules in crystals 1—3
is determined, apparently, not only by the volume and
shape of the solvate molecule but also by its polarity. The
chlorine and nitrogen atoms are involved in specific diꢀ
poleꢀdipole interactions between atoms of the "host" and
"guest" molecules. This is, in particular, reflected in
the densities of the crystal structures. Thus, dcalc for
crystals 1 and 2 are substantially higher than that for crysꢀ
tal 3; the larger value of dcalc for solvate 1 compared to
dcalc for solvate 2 also correlates with the larger number
of shortened intermolecular contacts in crystal 1 (see
Table 4).
Therefore, weak intermolecular interactions between
the clathrochelate "host" molecules provide the possibilꢀ
ity of using solvate "guest" molecules for the desired
changes in the supramolecular organization of the crystals
of these complexes. The hiearhical architecture of the
crystals is determined by the electronic properties of the
solvate molecule rather than by its geometric parameters.
This conclusion can be extended to the polymorphism of
molecules containing polar groups: even weak intermoꢀ
lecular contacts involving these groups might play a key
role in the crystal packing of these molecules.

1502
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 7, July, 2004
Voloshin et al.
4. G. M. Sheldrick, SADABS: А Program for Exploiting the
Redundancy of Areaꢀdetector Xꢀray Data, University of
Göttingen, Göttingen, Germany, 1999.
5. G. M. Sheldrick, SHELXTLꢀ97, Program for Solution and
Refinement of Crystal Structure, Bruker AXS Inc., Madison,
WIꢀ53719, USA, 1997.
This study was financially supported by the Russian
Foundation for Basic Research (Project Nos. 03ꢀ03ꢀ32531
and 03ꢀ03ꢀ32241).
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Received December 23, 2003;
in revised form June 5, 2004