Reactions of Pd-PEPPSI complexes with protic acids

Article abstract of DOI:10.1007/s11172-018-2201-9

The Pd-PEPPSI complexes widely used to catalyze numerous reactions eliminate the pyridine ligand on treatment with protic acids to give binuclear complexes [Pd(NHC)X₂]₂ with the Pd–X–Pd (X = Cl, Br, I) bridging bonds. The reaction proceeds with high yields (78–98%) and can be regarded as a preparative approach to binuclear complexes. A prolonged heat treatment of either Pd-PEPPSI complexes or binuclear [Pd(NHC)X₂]₂ complexes in the presence of strong protic acids results in the Pd–NHC bond cleavage to give azolium salts (proligands) and palladium salts.

Full text of DOI:10.1007/s11172-018-2201-9

Russian Chemical Bulletin, International Edition, Vol. 67, No. 7, pp. 1196—1201, July, 2018
1196
Reactions of Pd-PEPPSI complexes with protic acids
A. Yu. Chernenko,a D. V. Pasyukov,a A. V. Astakhov,a V. A. Tafeenko,b and V. M. Chernysheva
aPlatov South-Russian State Polytechnic University (NPI),
132 ul. Prosveshcheniya, 346428 Novocherkassk, Russian Federation.
E-mail: chern13@yandex.ru
bLomonosov Moscow State University,
1 Leninskie Gory, 119991 Moscow, Russian Federation
The Pd-PEPPSI complexes widely used to catalyze numerous reactions eliminate the
pyridine ligand on treatment with protic acids to give binuclear complexes [Pd(NHC)X2]2 with
the Pd—X—Pd (X = Cl, Br, I) bridging bonds. The reaction proceeds with high yields (78—98%)
and can be regarded as a preparative approach to binuclear complexes. A prolonged heat treat-
ment of either Pd-PEPPSI complexes or binuclear [Pd(NHC)X2]2 complexes in the presence
of strong protic acids results in the Pd—NHC bond cleavage to give azolium salts (proligands)
and palladium salts.
Key words: palladium, coordination compounds, N-heterocyclic carbenes, PEPPSI, ca-
talysis.
In coordination chemistry, heterocyclic carbenes
(NHC) attract special interest among ligands of other
type.1,2 N-Heterocyclic carbene ligands are capable of
forming strong bonds with the most transition metals and
their spatial and electronic properties can be widely var-
ied.1—3 These form a basis for extensive catalytic applica-
tion4—9 of the metal-NHC complexes and, mostly,
Pd-NHC complexes.10—14 A group of Pd-NHC complexes
with the general formula Pd(NHC)X2L (L is pyridine) also
often defined as Pd-PEPPSI complexes (PEPPSI is pyridine-
enhanced precatalyst preparation, stabilization, and ini-
tiation) has found a widespread use in catalysis.15—20
activity of the catalytic system due to deactivation of the
M-NHC molecular sites but, on the other hand, this
process can be the source of the NHC-free palladium
species that are often more catalytically active in a number
of catalytic reactions.22,23
The main routs of the decomposition of the NHC
complexes involving the M—NHC bond breaking are the
R-NHC- and H-NHC-coupling reactions.24—29 These
reactions are characteristic of the products of oxidative
addition of RX partner to the Pd0—NHC complexes and
PdII—NHC hydride complexes that are the typical inter-
mediates formed upon catalytic processes.22—29 Another
important process that can result in the M—NHC bond
cleavage is protolysis giving rise to azolium cations (pro-
ligands).30—34 Protolysis can significantly affect the
Pd-NHC catalytic systems especially in acidic media
typical of a number of important chemical reactions, e.g.,
C—H bond activation reactions.35,36 Dissociation of the
Pd-PEPPSI complexes could produce relatively strong
bases, namely, pyridines (pKa 3.3 in DMSO)37 and free
NHC ligands (pKa ≈ 12—24 in DMSO).38—40 Apparently,
protonation of these ligands in acidic medium could, in
turn, facilitates the displacement of the hypothetic equi-
librium of the complex dissociation to the dissociated
species. It should be noted that resistance of the Pd-
PEPPSI complexes to acids and their protolytic transfor-
mations were not studied to date.
R1 = H, Alk, Cl; R2, R3 = Alk, Ar; X = Cl, Br, I
High catalytic activity of Pd-PEPPSI complexes is
mainly attributed to the rapid elimination of the pyridine
coligand and high stability of the Pd—NHC bond.15—20
However, during catalytic processes cleavage of the
M—NHC bond (M is metal) of the Pd-NHC complex
may occur to produce the "cocktail"-type systems21 com-
prising various active forms of the catalyst.22,23 On the
one hand, the Pd—NHC bond cleavage can decrease the
In the present work, we studied transformations of the
Pd-PEPPSI complexes in the presence of acids and dem-
onstrated the possibility to apply the protolytic transforma-
tions of these complexes for the synthesis of binuclear
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1196—1201, July, 2018.
1066-5285/18/6707-1196 © 2018 Springer Science+Business Media, Inc.

Reactions of Pd-PEPPSIs with protic acids
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 7, July, 2018
1197
Scheme 1
[Pd(NHC)X2]2 complexes bearing the Pd—X—Pd (X =
= Cl, Br, I) bridging bonds.
We found that complex 1a (Scheme 1) is stable upon
heating at 100 C in 1,4-dioxane in the presence of water
and acetic acid (Table 1). However, stronger acids, H3PO4,
CF3COOH, HBF4, HCl, HNO3, HClO4, and H2SO4,
cause transformation of compound 1a into complex 2a in
the yields depending on the reaction conditions and in-
creasing with an increase in the acid strength41 (see Table 1
and Scheme 1). The yield of compound 2a in the reaction
of complex 1a with concentrated sulfuric acid in dioxane
at a molar ratio 1a : H2SO4 = 1 : 2 reaches 78% at room
temperature within 1 h and 98% at 100 C within
10 min (see Table 1, entries 11 and 12). It should be noted
that prolongation of the heating time decreases the yield
of compound 2a due apparently to the further protolysis
of binuclear complex occurring with the Pd—NHC bond
cleavage. Thus, heating of compound 1a with sulfuric acid
at a 1 : 20 molar ratio at 100 C for 48 h gives compound
2a in 20% yield (entry 13). The main products of this
transformation also were 1,3-dimethylbenzimidazolium
hydrogen sulfate (3) (yield of 35%) and PdI2 (19%) (see
Scheme 1). Similar result (yields of compounds 3 and PdI2
of 39 and 40%, respectively) was obtained by the reaction
of compound 2a with sulfuric acid under analogous con-
ditions (see Scheme 1).
Reagents and conditions: i. see Table 1; ii. molar ratio 1a : H2SO4 =
= 20 : 1, 1,4-dioxane, 100 C, 48 h; iii. molar ratio 2a : H2SO4 =
= 20 : 0.5, 1,4-dioxane, 100 C, 48 h.
is first eliminated and then slow cleavage of the Pd—NHC
bond occurs.
Since the Pd-PEPPSI complexes are readily avail-
able,13,15—20 protolytic transformations of these com-
pounds can be used for the preparative synthesis of bi-
In general, the reaction of compound 1a with strong
acids follows the same route as the reactions of mono-
nuclear Pd-NHC complexes with acetyl acetonate32 and
allylic42,43 coligands. The good leaving pyridine coligand
Table 1. Yields of compound 2a obtained by the reaction of complex 1a with HYa
Entry
HY
Molar ratio 1a : HY
t
T/С
Yield of 2a (%)
1
2
H2O
1 : 20
1 : 20
24 h
24 h
100
100
0b
Trace
amountsb
94
AcOH
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CF3COOH
CF3COOH
H3PO4
H3PO4
HNO3
HBF4
HClO4
HCl
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
1 : 20
1 : 2
1 : 20
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 20
1 : 2
1 : 2
1 : 1
1 : 2
1 h
1 h
100
100
100
100
100
100
100
100
20
100
100
100
100
100
100
78
1 h
93
1 h
43
1 h
74
1 h
95
1 h
98
1 h
69
1 h
78
1 h
98
48 h
10 h
10 min
10 min
3 min
20c
90
99
93
95
a Concentration of the acids in the reaction mixtures was 0.05 mol L–1 at a molar ratio 1a : HY = 1 : 2 and 0.5 mol L–1 at a molar
ratio 1a : HY = 1 : 20.
b After completion of the reaction, 95—98% of the starting compounds 1a were recovered.
c The reaction also produces compound 3 (isolated yield of 35%) and PdI2 (isolated yield of 19%).

1198
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 7, July, 2018
Chernenko et al.
Scheme 2
nuclear [Pd(NHC)X2]2 complexes. Thus, complexes 2a—g
are formed in 78—98% yields upon heating complexes
1a—g with sulfuric acid (a molar ratio of 1a—g : H2SO4
= 1 : 2) at 100 C in dioxane for 10 min (Scheme 2). This
approach not only provides higher yields of compounds 2
than a number of one-step procedures utilizing azolium
and palladium salts44—46 but is also preparatively simpler
than syntheses involving transmetallation of Ag-NHC47
and W-NHC48 complexes.
Structures of compounds 2a—g and 3 were established
by 1H and 13C NMR spectroscopy and confirmed by X-ray
diffraction analysis of complexes 2e and 2g (Fig. 1).
Spectral properties of compounds 2a—g and structural
parameters of compounds 2e and 2g are in agreement with
those of the structurally relative analogs.44—48
In summary, the chemical behavior of the Pd-PEPPSI
complexes in the presence of strong protic acids is similar
to that of other mononuclear Pd-NHC complexes bearing
good leaving coligands.32 In acidic medium, the
Pd-PEPPSI complexes readily eliminate pyridinium li-
gands to give binuclear [Pd(NHC)X2]2 complexes with the
Pd atoms bridged by the Pd—halogen—Pd bonds. On
prolonged heating in the presence of an excess of strong
acids, binuclear complexes undergo relatively slow pro-
tolysis with the Pd—NHC bond cleavage. Protolysis of the
Pd-PEPPSI complexes is found to be a convenient pre-
parative approach to binuclear [Pd(NHC)X2]2 complexes.
Compounds 1, 2
R1
Me
R2
Me
X
I
a
b
c
d
e
f
Bun
Bun
Br
Br
Cl
Br
Br
Br
Pri
Pri
2,6-Pr2iC6H3
Bun
2,6-Pr2iC6H3
Bun
Experimental
Bun
Bun
1H and 13C NMR spectra were run on a Bruker DRX-500
instrument (working frequencies of 500 and 125 MHz, respec-
tively) in CDCl3, CD3CN, and D2O. The chemical shifts are
g
But
Bun
Reagents and conditions: i. H2SO4, 1,4-dioxane, 100 C, 10 min.
C(11)
C(18)
C(20)
C(19)
C(10)
C(10)
C(17)
C(18)
C(9)
C(8)
C(17)
C(9)
N(6)
C(16)
C(7)
N(3)
N(5)
Br(4)
C(8)
Br(1)
C(11)
N(2)
C(1)
C(14)
N(4)
C(15)
Br(3)
Br(1)
Pd(1)
C(2)
C(12)
N(2)
Pd(2)
Br(2)
C(13)
Pd(1)
N(4)
Br(3)
C(12)
N(1)
C(3)
C(2)
C(22)
C(1)
C(4)
C(13)
C(3)
N(3)
N(1)
Pd(2)
C(19)
C(20)
C(14)
C(16)
Br(2)
Br(4)
C(15)
C(5)
C(7)
C(4)
C(6)
C(21)
C(5)
2e
C(6)
2g
Fig. 1. X-ray structures of compounds 2e and 2g.

Reactions of Pd-PEPPSIs with protic acids
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 7, July, 2018
1199
given in the  scale relative to Me4Si (an internal standard).
Melting points were measured in capillaries using a PTP appa-
ratus. Elemental analysis was carried out with a Perkin Elmer
2400 analyzer. Contents of iodine and palladium were determined
by energy dispersive X-ray fluorescence (XRF) spectroscopy on
an ARL QUANT´X spectrometer.
for 10 min under stirring. After cooling to 20 C, the precipitate
formed was collected by filtration and successively washed with
water (20 mL) and acetone (5 mL), and dried at 60 C.
Bis(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)(di-μ-
iodo)diiododipalladium (2a). Yield 0.074 g (98%).
Di-μ-bromo(dibromo)bis(1,3-dibutyl-2,3-dihydro-1H-benz-
imidazol-2-yl)dipalladium (2b). Yield 0.060 g (80%), yellow
crystals, m.p. 231—233 C (CHCl3). 1H NMR (CD3CN), : 1.02
(t, 12 H, 4 CH3, J = 7.4 Hz); 1.45—1.54 (m, 8 H, 4 CH2);
2.09—2.16 (m, 8 H, 4 CH2); 4.69—4.73 (m, 8 H, 4 CH2);
7.33—7.35 (m, 4 H, Ar); 7.55—7.57 (m, 4 H, Ar). 13C NMR
(CD3CN), : 14.1; 20.8; 31.9; 49.2; 111.8; 124.3; 135.1; 159.9.
Found (%): C, 36.27; H, 4.53; N, 5.81. C30H44Br4N4Pd2.
Calculated (%): C, 36.28; H, 4.47; N, 5.64.
Di-μ-bromo(dibromo)bis[1,3-di(propan-2-yl)-2,3-dihydro-
1H-benzimidazol-2-yl]dipalladium (2c). Yield 0.063 g (90%),
orange crystals, m.p. 315—317 C (CHCl3). 1H NMR (CD3CN),
: 1.70 (d, 24 H, 8 CH3, J = 7.1 Hz); 6.08—6.19 (m, 4 H, 4 CH);
7.28—7.31 (m, 4 H, Ar); 7.71—7.74 (m, 4 H, Ar). 13C NMR
(CD3CN), : 20.5; 55.8; 113.9; 123.9; 133.8; 158.4. Found (%):
C, 33.19; H, 3.79; N, 5.87. C26H36Br4N4Pd2. Calculated (%):
C, 33.33; H, 3.87; N, 5.98.
The PEPPSI complexes 1a—g were synthesized by known
procedures;13,22 other reagents are commercially available.
Studies of the reaction of compounds 1a with protic acids. To
a solution of complex 1a (88 mg, 0.15 mmol) in 1,4-dioxane
(5 mL), a solution of the corresponding acid (0.3 mmol for a molar
ratio 1a : HY = 2 : 1 or 3 mmol for a molar ratio 1a : HY = 20 : 1)
in 1,4-dioxane was added and the mixture was stirred at the re-
quired temperature for the time period specified in Table 1. Then
the reaction mixture was cooled to 3—5 C, neutralized with
saturated aqueous NaHCO3 to pH 5—6 and concentrated
in vacuo maintaining temperature below 50 C. The residue was
diluted with water (10 mL) and the resulting suspension was
stirred at room temperature for 30 min. The undissolved solid
was filtered off, dried at 60 C until constant weight was reached,
and the yield of compound 2a was determined by 1H NMR
spectroscopy (see Table 1).
Bis{1,3-bis[2,6-di(propan-2-yl)phenyl]-2,3-dihydro-1H-
imidazol-2-yl}(di-μ-chloro)dichlorodipalladium (2d). Yield
0.068 g (80%), orange crystals, m.p. 293—295 C (CHCl3—hex-
ane). 1H and 13C NMR spectral data of compound 2d are in good
agreement with those published earlier.49
Di-μ-bromo(dibromo)bis(1,3-dibutyl-2,3-dihydro-1H-imid-
azol-2-yl)dipalladium (2e). Yield 0.056 g (84%), yellow crystals,
m.p. 150—152 C (CHCl3—hexane, 1 : 3). 1H NMR (CD3CN),
: 0.96—1.00 (m, 12 H, 4 CH3); 1.36—1.45 (m, 8 H, 4 CH2);
1.97—2.08 (m, 8 H, 4 CH2); 4.36 (t, 4 H, 2 CH2, J = 7.5 Hz);
4.42 (t, 4 H, 2 CH2, J = 7.5 Hz); 7.03 (s, 2 H, CH of imidazole);
7.09 (s, 2 H, CH of imidazole). 13C NMR (CD3CN), : 14.0;
20.5; 20.8; 32.8; 33.6; 51.3; 51.6; 122.1; 123.4; 169.5. Found (%):
C, 29.37; H, 4.42; N, 6.15. C22H40Br4N4Pd2. Calculated (%):
C, 29.59; H, 4.51; N, 6.27.
Di-μ-bromo(dibromo)bis(1,4-dibutyl-4,5-dihydro-1H-1,2,4-
triazol-5-yl)dipalladium (2f). Yield 0.062 g (92%), yellowish
crystals, m.p. 244—246 C (CHCl3—hexane, 1 : 3). 1H NMR
(CD3CN), : 0.96—0.99 (m, 12 H, 4 CH3); 1.35—1.43 (m, 8 H,
4 CH2); 2.00—2.05 (m, 4 H, 2 CH2); 2.06—2.11 (m, 4 H, 2 CH2);
4.39 (t, 4 H, 2 CH2, J = 7.4 Hz); 4.52 (t, 4 H, 2 CH2, J =
= 7.3 Hz); 8.23 (s, 2 H, CH of triazol). 13C NMR (CD3CN),
: 13.9; 14.0; 20.3; 20.4; 31.9; 32.4; 144.8; 152.0. Found (%):
C, 26.95; H, 4.23; N, 9.46. C20H38Br4N6Pd2. Calculated (%):
C, 26.84; H, 4.28; N, 9.39.
Di-μ-bromo(dibromo)bis(4-butyl-1-tert-butyl-4,5-dihydro-
1H-1,2,4-triazol-5-yl)dipalladium (2g). Yield 0.052 g (78%),
orange crystals, m.p. 218—220 C (CHCl3—hexane, 1 : 3).
1H NMR (CD3CN), : 0.98 (t, 6 H, 2 CH3, J = 7.4 Hz); 1.37—
1.43 (m, 4 H, 2 CH2); 1.92—1.95 (m, 18 H, 6 CH3); 2.06—2.11
(m, 4 H, 2 CH2); 4.54—4.57 (m, 4 H, 2 CH2); 8.28 (s, 2 H, CH
of triazole). 13C NMR (CD3CN), : 13.9; 20.4; 31.3; 31.9; 50.6;
63.2; 143.7; 149.3. Found (%): C, 26.74; H, 4.23; N, 9.27.
C20H38Br4N6Pd2. Calculated (%): C, 26.84; H, 4.28; N, 9.39.
X-ray diffraction studies. Single crystals of compounds 2e and
2g were grown by slow evaporation of their solutions in CHCl3
under saturated vapor of n-hexane. The unit cell parameters of
the crystals of compounds 2e and 2g were measured with a STOE
STADIVARI Pilatus 100K diffractometer using CuK radiation
Bis(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)(di-μ-
iodo)diiododipalladium (2a), bright orange crystals, m.p. >340 C
1
(CHCl3). H NMR (CD3CN), : 4.03 (s, 12 H, 4 CH3); 7.34—
7.36 (m, 4 H, Ar); 7.51—7.53 (m, 4 H, Ar). 13C NMR (CD3CN),
: 36.5; 67.7; 111.2; 124.2; 159.8. Found (%): C, 21.27; H, 2.01;
N, 5.66. C18H20I4N4Pd2. Calculated (%): C, 21.35; H, 1.99;
N, 5.53.
To isolate compound 3 (see Table 1, entry 13), the filtrate
was concentrated in vacuo, the residue was extracted with reflux-
ing ethanol (4×5 mL). The combined organic phases were con-
centrated in vacuo to dryness, the residue was crystallized from
10% aqueous H2SO4, dried in vacuo at 100 C, and analyzed.
1,3-Dimethyl-1H-1,3-benzimidazolium hydrogen sulfate (3).
White crystals, m.p. 118—120 C (H2O). 1H NMR (D2O), : 4.03
(s, 6 H, 2 CH3); 7.61—7.63 (m, 2 H, Ar); 7.75—7.77 (m, 2 H,
Ar); 9.11 (s, 1 H, CH). 13C NMR (D2O), : 39.6; 119.5; 133.5;
138.6; 148.6. Found (%): C, 44.16; H, 4.83; N, 11.38. C9H12N2O4S.
Calculated (%): C, 44.25; H, 4.95; N, 11.47.
To isolate PdI2 (see Table 1, entry 13), the precipitate in-
soluble in water was multiply extracted with hot acetone to remove
compound 2a and impurities of other organic compounds,
heated in water (3 mL) at 90—95 C for 5 min under stirring,
and cooled to room temperature. Palladium iodide was centri-
fuged, dried at 100 C, and analyzed by XRF.
Palladium(II) iodide (PdI2). Found (%): I, 70.18; Pd, 29.67.
PdI2. Calculated (%): I, 70.46; Pd, 29.54.
Studies of the protolysis of compound 2a. To a suspension of
compound 2a (0.253 g, 0.25 mmol) in 1,4-dioxane (18 mL), a
solution of 98% H2SO4 (0.51 g, 5 mmol) in 1,4-dioxane (2 mL)
was added. The resulting mixture was heated at 100 C for 48 h
under stirring, cooled down, and neutralized with saturated
aqueous NaHCO3 to pH 5—6. Compounds 2a, 3, and PdI2 were
separated as described above to obtain 63 mg of the starting
compound 2a, 47 mg (39%) of compound 3, and 72 mg (40%)
of PdI2.
Synthesis of complexes 2a—g (general procedure). To a solu-
tion or suspension of complex 1a—g (0.15 mmol) in 1,4-dioxane
(5 mL), a solution of 98% H2SO4 (30 mg, 0.3 mmol) in 1,4-di-
oxane (1 mL) was added and the mixture was heated at 100 C

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Chernenko et al.
Table 2. Crystallographic characteristics, X-ray data collection, and structure refinement statistics for com-
pounds 2e and 2g
Parameter
2e
2g
Molecular formula
C
22H40Br4N6Pd2
893.0
C20H38Br4N6Pd2
895.0
0.26×0.2×0.18
Monoclinic
C2/c
27.9011(6)
11.2500(3)
22.1174(5)
112.947(5)
6393.04(12)
8
Molecular weight
Size of single crystal/mm
0.28×0.23×0.17
Monoclinic
P21/c
Crystal system
Space group
a/Å
13.6933(3)
21.6929(5)
11.6202(2)
112.321(3)
3193.11(11)
4
b/Å
c/Å
/deg
V/Å3
Z
dcalc/g cm–3
1.858
1728
1.860
3546
F(000)
/mm–1
15.069
15.074
2max/deg
144
144
Number of measured reflections
Number of independent reflections
Number of reflections with I > 2(I)
Number of refined parameters
R1/wR2 (I > 2(I))
84034
87829
6278
6339
4445
3381
295
297
0.07/0.18
0.972
0.11/0.34
1.269
GOF
Residual electron density (max/min)/e Å–3
0.85/–2.56
1.59/–1.58
provided by a GeniX3D HF generator equipped with a microfocus
X-ray tube and combined with a Xenocs FOX3D HF multi-layer
thin-film ellipsoidal monochromator. Data collection, determi-
nation and refinement of the unit cell parameters were preformed
using STOE X-Area software package (STOE & Cie GmbH,
Germany). Intensity data were scaled with LANA (part of
X-Area) in order to minimize differences in intensities of the
symmetry-equivalent reflections (multi-scan method). Taking
into account a large absorption coefficient ( of about 15 for both
crystals), no less than 14 equivalents were determined (Redundancy
= 14). The structures were solved by the direct method using
SHELXS-97 software.50,51 Positions and thermal parameters of
non-hydrogen atoms were refined in the anisotropic full-matrix
approximation. Hydrogen atoms were placed geometrically and
refined isotropically using a riding model with fixed displacement
parameters (Uiso(H) = 1.2Uequiv(C)). The atomic coordinates,
bond lengths, bond and torsion angles, and anisotropic displace-
ment parameters of complexes 2e and 2g were deposited with the
Cambridge Crystallographic Data Center (CCDC 1585382 (2e)
and CCDC 1585381 (2g)) and available free of charge via
www.ccdc.cam.ac.uk.
were performed using the equipment acquired at the ex-
pense of the development program of M. V. Lomonosov
Moscow State University.
This work was financially supported by the Russian
Science Foundation (Project No. 14-23-00078).
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The authors are grateful to Corresponding Member of
the Russian Academy of Sciences, Professor V. P. Ananikov
for the fruitful discussion and valuable comments on this
project. The authors are also grateful to the co-workers of
the Center for Collective Use "Nanotechnology" of M. I.
Platov South-Russian State Polytechnic University and
Department of Structural Studies of N. D. Zelinsky
Institute of the Russian Academy of Sciences for perform-
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Received December 6, 2017;
in revised form February 26, 2018;
accepted March 21, 2018