Synthesis, structural, vibrational and DFT investigation of new binuclear molecular Pd–Cu and Cu–Cu complexes formed by Schiff base and hexafluoroacetylacetonate building blocks

Article abstract of DOI:10.1016/j.molstruc.2020.128341

In this work, the first binuclear molecular noble metal-contained complex from the Schiff base (SB) and β-diketonate building blocks, viz. [Pd(acacen)Cu(hfac)₂], has been synthesized and investigated in comparison with the homometallic binuclear analog [Cu(acacen)Cu(hfac)₂]. The detailed comparative characterization of the structure of these new compounds as well as their molecular SB components [M(acacen)] was performed using crystallography, vibrational spectroscopy and DFT calculations. Experimental IR bands have been assigned based on DFT calculations. The Hirshfeld surface analysis was used as a tool for the better insight into intermolecular contacts. It was shown that despite both bimetallic complexes had similar molecular structure and packing style, the nature of the closest intermolecular contacts in [M(acacen)Cu(hfac)₂] differed considerably. A topological analysis of the electron density distribution in [M(acacen)] and [M(acacen)Cu(hfac)₂] complexes was carried out and the most probable paths of bond cleavage during decomposition of the compounds were revealed. The binding energies of the molecular components in binuclear complexes were also calculated.

Full text of DOI:10.1016/j.molstruc.2020.128341

Journal of Molecular Structure 1216 (2020) 128341
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, structural, vibrational and DFT investigation of new
binuclear molecular PdeCu and CueCu complexes formed by Schiff
base and hexafluoroacetylacetonate building blocks
,
, c
Evgeniia S. Vikulova a, Nataliya S. Nikolaeva a *, Pavel O. Krasnov b
,
Alexander A. Sukhikh a, Anton I. Smolentsev a, Evgenia A. Kovaleva b,
Natalya B. Morozova a, Tamara V. Basova a
a Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk, 630090, Russia
b Siberian Federal University, Krasnoyarsk, 660041, Russia
c Reshetnev Siberian State University of Science and Technology, Krasnoyarsk, 660049, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the first binuclear molecular noble metal-contained complex from the Schiff base (SB) and
Received 27 November 2019
Received in revised form
22 April 2020
Accepted 24 April 2020
Available online 25 April 2020
b
-diketonate building blocks, viz. [Pd(acacen)Cu(hfac)2], has been synthesized and investigated in
comparison with the homometallic binuclear analog [Cu(acacen)Cu(hfac)2]. The detailed comparative
characterization of the structure of these new compounds as well as their molecular SB components
[M(acacen)] was performed using crystallography, vibrational spectroscopy and DFT calculations.
Experimental IR bands have been assigned based on DFT calculations. The Hirshfeld surface analysis was
used as a tool for the better insight into intermolecular contacts. It was shown that despite both
bimetallic complexes had similar molecular structure and packing style, the nature of the closest
intermolecular contacts in [M(acacen)Cu(hfac)2] differed considerably. A topological analysis of the
electron density distribution in [M(acacen)] and [M(acacen)Cu(hfac)2] complexes was carried out and the
most probable paths of bond cleavage during decomposition of the compounds were revealed. The
binding energies of the molecular components in binuclear complexes were also calculated.
© 2020 Elsevier B.V. All rights reserved.
Keywords:
Bimetallic complexes
Palladium (II) complexes
Copper (II) complexes
Crystal structure
DFT calculations
IR spectra
1. Introduction
to prevent their exchange in the gas phase. Recently the idea of
bimetallic single-source precursors has attracted attention because
1D and 2D nanomaterials of PdeCu bimetallic system are widely
used in different industrial processes. For example, PdxCu(1-x)
nanoparticles are utilized as catalysts for CO2, nitrite and nitrate
hydrogenation [1,2], while palladium-containing thin films are one
of the most effective membranes for the high purity hydrogen
production [3,4]. Chemical vapor deposition methods, in particular
metal-organic chemical vapor deposition (MOCVD), are used to
produce multicomponent materials with specified properties [5].
However, in addition to general requirements (high volatility,
thermal stability, “thermal window” between the vapor sublima-
tion and its decomposition) the MOCVD precursors used for these
purposes must have certain physicochemical properties such as
compatible deposition temperatures, and contain identical ligands
their application enables to avoid the selection of precursor com-
binations, to simplify the experimental procedure and to control
the gas phase composition.
While chemistry of heteropolynuclear complexes is widely
covered in the literature, data on molecular compounds suitable as
precursors in MOCVD processes are sporadic [6]. Different syn-
thetic procedures based on alkoxides [7e9], carboxylates [10,11],
glyoximates [12] etc. have already been described in the literature.
In particular, for PdeСu volatile compounds, there was no infor-
mation about bimetallic palladium-containing complexes with
these types of ligands until 2014 due to their insufficient thermal
stability. Indeed, the first bimetallic PdeCu precursor was formed
due to as a result of specific intermolecular interactions of func-
tionalized metal b-diketonates [13e15]. However, such intermo-
lecular interactions were weak, and the compounds mostly had a
polymer structure. In addition, when heated, dissociation of such
* Corresponding author.
E-mail address: nikolaeva@niic.nsc.ru (N.S. Nikolaeva).
https://doi.org/10.1016/j.molstruc.2020.128341
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
heterometallic compounds can occur. Thus, the design of a new
type of PdeCu bimetallic molecular compounds is of current
importance.
subsequent recrystallizations from hexane. Yield: 1.35 g (60%). m.p.
110 ꢁC. Anal. calc. for C12H20N2O2: C, 64.3; H, 9.0; N, 12.5. Found: C,
64.0; H, 9.1; N, 12.5.
One of the promising approaches is coordination of one metal
atom in a complex containing a Schiff’s tetradentate base (SB) or
[Pd(acacen)]. PdCl2 (0.80 g, palladium content 59.845%,
4.5 mmol) was dissolved in 90 mL of acetonitrile upon heating to
~50 ꢁC, and then the solution of H2acacen (1.00 g, 4.5 mmol) in
30 mL acetonitrile was added. After stirring during 10 min the so-
lution was mixed with the equimolar amount of KOH (0.51 g,
9.1 mmol) dissolved in a minimum amount of water and kept for
10 min. Then water was added in the ratio of 5:1 (vol.). Finally, the
product was filtered and purified by double sublimation in vacuum
(P ¼ 5∙10ꢀ2 Torr, T ¼ 170e200 ꢁC). Yield: 0.92 g (62%) m.p. 226 ꢁC.
Anal. Calc. for C12H18N2O2Pd: C, 43.9; H, 5.5; N, 8.5. Found: C, 44.1;
H, 5.7; N 8.3. 1H NMR (CDCl3): 1.93 s (6H, 2CH3), 1.98 s (6H, 2CH3),
3.39 s (4H, 2CH2), 4.83 s (2H, CH). 13C NMR (CDCl3): 20.64 (CH3),
25.37 (CH3), 55.89 (NCH2), 99.01 (CH), 161.81 (C]N), 177.51 (C]O).
Cu(II) complexes. [Cu(acacen)] and [Cu(hfac)2] were synthesized
according to the method described in the literature [28] by inter-
action of H2acacen (1.00 g, 4.5 mmol) or Hhfac (1.28 ml, 9.0 mmol)
with as-precipitated Cu(OH)2 (0.50 g, 5.1 mmol, 13% excess) in the
presence of acetone (15 mL). Cu(OH)2 was obtained by mixing
aqueous solutions of CuCl2(0.68 g, 5.1 mmol) and KOH(0.58 g,
10.3 mmol). After acetone evaporation, Cu (II) complexes were
extracted with chloroform and purified by double vacuum
sublimation.
similar ligand with another metal atom in the
ronment. For instance, the pioneering work was devoted to the
synthesis and crystal structure of [Cu(salen)Cu(hfac)2] (salen2ꢀ
N,N0-ethylenebis(salicylidenaminato), hfacꢀ
hexa-
fluoroacetylacetonato) [16]. Later syntheses, crystal structures, and
magnetic characterizations of a number of [M(SB)Ln( -diket)3] (M
b-diketonate envi-
e
e
b
e Ni, Cu) compounds were reported by Gleizes, Rayzanov, Kuzmina
and coauthors [17e20], and the obtained results were mainly
presented in the light of comparing the structural features and
magnetic properties of these bimetallic complexes. In the case of
[Ni(SB)La(b-diket)3] thermal behavior has been studied and some
principles of changes in the thermodynamic and thermal stability
in dependence on their crystal structures have been formulated
[17]. This work [17] was the most complete study in the field of
thermochemistry of such compounds. Thus, the attractiveness of
this synthetic approach lies in the possibility of controlling the
thermal properties of complexes by varying the nature of SB or b-
diketonate components in a bimetallic molecule. The ranges of
changes in the volatility of individual palladium and copper che-
lates with SB or b-diketonate ligands demonstrated in the literature
[21e24] provide a promising basis for obtaining effective pre-
cursors of this type.
[Cu(acacen]). Sublimation parameters:
T
¼
110e120 ꢁC,
P ¼ 5$10ꢀ2Torr. Yield: 1.09 g (85%). m.p.136e138 ꢁC. Anal. Calc. for
In this work, we first combine the Schiff base and
b
-diketonate
C12H18N2O2Cu: C, 50.5; H, 6.4; N, 9.8. Found: C, 50.7; H, 6.3; N, 9.7.
building blocks to prepare a heterobimetallic PdeCu compound.
For this purpose, [Pd(acacen)] (acacen2ꢀ e N,N’-(ethylene)-bis(a-
cetylacetoneiminato-) was chosen as an initial SB molecular
component because in contrast to the above mentioned [M(salen)]
[Cu(hfac)2].Sublimation parameters:
T
¼
60e70 ꢁC,
P ¼ 5$10ꢀ2Torr. Yield: 1.38 g (64%). m.p. 131e132 ꢁC. Anal. Calc. for
C
10H2F12O4Cu: C, 25.2; H,0.4; F, 47.8. Found: C, 25.3; H, 0.3; F, 47.8.
[M(acacen)Cu(hfac)2], (M ¼ Pd, Cu). Both bimetallic complexes
it is not prone to
hexafluoroacetylacetonate, [Cu(hfac)2], was used as a
p
-
p
or M-
p
stacking interactions. Copper (II)
-diketonate
were obtained according to the procedure described in the litera-
ture [18]. The chloroform solution (20 mL) of [Pd(acacen)] (0.40 g,
1.2 mmol) or [Cu(acacen)] (0.35 g, 1.2 mmol) was slowly added to
the chloroform solution (20 mL) of [Cu(hfac)2] (0.58 g, 1.2 mmol)
and the mixture was refluxed for 1 h. The brown crystals of the
complexe were grown during solvent evaporation. The yield was
close to 100%.
b
component since it exhibits high ability to coordinate donor mol-
ecules to copper center due to the presence of electron-
withdrawing groups in the ligand [25,26]. Herein, we focus on
the study of the structure of the bimetallic heteronuclear complex
[Pd(acacen)Cu(hfac)2] in comparison with its homonuclear
analogue [Cu(acacen)Cu(hfac)2]. Both the new compounds and the
corresponding mononuclear components were characterized by
the methods of elemental analysis, single-crystal X-ray diffrac-
tometry and vibrational spectroscopy. DFT calculations were
employed for topological analysis of the electron density distribu-
tion and estimation of binding energies of the complex binuclear
molecules.
[Pd(acacen)Cu(hfac)2]. m.p. 172e174 ꢁC. Anal. Calc. for
C22H20N2F12O6CuPd: C, 32.8; H, 3.5; N, 3.5; F, 28.3. Found: C, 32.5;
H, 3.4; N, 3.4; F, 28.1.
[Cu(acacen)Cu(hfac)2]. m.p. 177 ꢁC. Anal. Calc. for
C22H20N2F12O6Cu2: C, 34.6; H, 2.6; N, 3.7; F, 29.9. Found: C, 34.2; H,
2.3; N, 3.7; F, 29.6.
2. Experimental and computational details
2.3. X-ray diffraction studies
2.1. Chemicals and methods
Crystals of [Cu(acacen)], [Pd(acacen)] and bimetallic complexes
were selected under a microscope and then mounted to the tip of a
thin glass fiber with epoxy resin. X-Ray data were collected using a
Bruker DUO single-crystal diffractometer (Mo-anode sealed tube
All the chemicals were analytical grade reagents and were used
without additional purification. The elemental analysis was carried
out using the Carlo-Erba 1106 analyzer. IR-spectra of the complexes
in KBr and polyethylene pellets were recorded on a Vertex 80 FTIR
spectrometer. NMR spectra were recorded on an Avance 500
spectrometer at 500 MHz for 1H NMR and 125 MHz for 13C NMR in
CDCl3 solution. Melting points (m.p.) were determined by visual
method on the Koffler’s table.
with graphite monochromator (
l
¼ 0.71073 Å), APEX II CCD de-
tector) using standard 0.5ꢁ frame width
f
and scans. The frames
u
were collected at 150 K for all compounds except [Cu(acacen)
Cu(hfac)2], while for the latter complex the temperature was held
at 250K because of crystal splitting during cooling. The Apex2
software package (SAINT, SADABS, RLATT) [29] was used for unit
cell determination, integration and absorption correction. The ob-
tained hkl datasets were processed in the Olex2 [30] software using
SHELXT [31] and SHELXL [31] for the structure solution and sub-
sequent refinement. All non-hydrogen atoms were refined aniso-
tropically. ISOR 0.01 0.02 restraints were placed on C8 atom in
[Pd(acacen)Cu(hfac)2].
2.2. Syntheses and characterization
H2acacen was synthesized according to the previously
described method [27] by condensation of ethylenediamine
(0.67 mL, 10 mmol) with acetylacetone (2.04 mL, 20 mmol) with

E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
3
2.4. Computational details
2.81%. Here the crystal structure of the compound [Pd(acacen)] was
determined for the first time. The obtained crystallographic data
along with the selected refinement details are given in Table 1.
According to the single-crystal X-ray analysis, [Pd(acacen)]
crystallizes in a monoclinic P21/c space group with 4 molecules in
the asymmetric unit. The molecular structure of the complex is
shown in Fig. 1, while the most relevant bond distances and angles
related to the Pd coordination sphere are listed in Table 2. The
central Pd atom has nearly square-planar coordination geometry
formed by two N and two O donor atoms belonging to the acacen
ligand. The maximum deviation of Pd atom from the mean N2O2
plane is only 0.009 Å. The PdeO and PdeN bond lengths, as well as
bond distances and angles within the ligand, are similar to those
previously reported for related palladium complexes with aliphatic
Schiff bases [21,52].
Six-membered metallocycles do not show a significant devia-
tion from planarity, which makes the whole molecule almost flat
(the maximal deviation from the mean plane is 0.071 Å). The
methyl groups are slightly displaced from their “parent” rings; the
angle between CeC bridge and the mean plane is in range from
18.33 to 22.89ꢁ. The unit cell volume of [Pd(acacen)] is 4 times
larger compared to that of [Cu(acacen)] with 16 molecules per unit
cell. The asymmetric unit contains 4 molecules arranged in two
pairs which in turn form alternating parquet layers parallel to the
plane (100) (Fig.1, b). The molecules in pairs are almost parallel (the
angles between two molecules are equal to 1.83ꢁ and 1.94ꢁ) and are
rotated 180ꢁ relative to each other. The distance between two
molecules in pairs is ~3.75 Å. The angle between two adjacent pairs
of molecules is 37.36ꢁ.
Quantum-chemical calculations of molecular and electronic
structures, as well as IR-spectra of [M(acacen)] and [M(acacen)
Cu(hfac)2] molecules, where M ¼ Cu, Pd, were carried out by DFT
BLYP-D3/def2-SVP method [32e37] using the ORCA software
package [38,39]. The topological analysis of electron density dis-
tribution functions obtained as a result of these computations was
performed in the framework of Richard Bader “atoms in molecules”
theory [40e42] using the AIMAll software package [43] in order to
identify chemical bonds in these compounds along which the
cleavage more likely to occur.
The geometry optimization of considered compounds was car-
ried out without any symmetry restrictions (e.g. the point group
symmetry was C1). The spin multiplicity in the series of [Cu(aca-
cen)], [Pd(acacen)], [Cu(acacen)Cu(hfac)2] and [Pd(acacen)Cu(h-
fac)2] molecules was equal to 2, 1, 3 and 2, respectively. The spin
states of all molecules were defined with the use of single point
calculations of the structures obtained from the experimental XRD
data without their geometries optimization. The energy gaps be-
tween these spin states and multiplicities 4, 3, 1 and 4 in this series
of molecules are equal to 2.83 eV, 2.62 eV, 0.69 eV and 2.57 eV. It
means that all spin states considered in this work are the most
energetically favorable (see Fig. S1, Supporting Information for the
more details). Therefore, all calculations were performed in the
framework of spin unrestricted Kohn-Sham (UKS) theory. Addi-
tionally, RI approximation [44e49] was used for IR-spectra
calculations.
Hirshfeld surface analysis was used for the visualization of
intermolecular interactions and identification of close contacts
between molecules in the investigated crystals. Fig. 2 shows the
Hirshfeld surfaces [53] obtained using Crystal Explorer [54] for
3. Results and discussion
3.1. Synthesis
[Cu(acacen)] and [Pd(acacen)] complexes, mapped with dnorm
,
New binuclear heteroleptic complexes were synthesized by co-
crystallization of pre-prepared monometallic components [M(aca-
cen)] and [Cu(hfac)2] in the equimolar ratio in chloroform (Scheme
1). Both PdeCu and CueCu complexes are formed as brown crystals
when the solvent evaporates and are stable in air. These com-
pounds can be repeatedly recrystallized from organic solvents such
as chloroform and toluene without changing their characteristics
(IR spectra, melting points, and elemental composition).
curvedness and shape index properties. Since [Pd(acacen)] has 4
individual molecules in the unit cell, separate Hirshfeld surfaces
were generated for each of them. Surfaces mapped with dnorm
property with (ꢀ0.1 ÷ 1.4) scale range show that both complexes
have no close contacts between adjacent molecules. Weak red spots
indicate several C/H, H/H and O/H contacts with interatomic
distances between two atoms ca.0.1 Å less than the sum of their van
der Waals radii. At the same time, in the case of [Cu(acacen)] pairs
of red and blue triangles arranged in the form of an hourglasses on
the surface mapped with the shape index property (Fig. 2, bottom
3.2. Crystal structures of SB molecular components
row) indicate
cent [Cu(acacen)] molecules. This
large flat area on the surface, mapped with the curvedness property
p
-
p
interaction between two metallocycles of adja-
To identify the features of the formation of binuclear com-
pounds, the individual molecular components were first investi-
gated. As for the components of synthesized heteroleptic
complexes, only the structure of [Cu(hfac)2] was studied before
[50]. Crystal structure of [Cu(acacen)] complex has already been
described by Hall et al. [51], however the published structure had a
relatively high R-factor (12%) and hydrogen atoms were not
determined. For this reason, the crystal structure of [Cu(acacen)]
were refined again using single-crystal X-ray diffraction. The
resulting structure is identical to the one previously reported [51],
but has all hydrogen atoms arranged with a much better R-factor of
p-p
interaction also appears as a
(Fig. 2, middle row). In contrast, no
the case of [Pd(acacen)].
p-p interactions are observed in
3.3. Crystal structures of binuclear complexes
Molecular structures and packing diagrams of [Cu(acacen)
Cu(hfac)2] and [Pd(acacen)Cu(hfac)2] are shown in Fig. 3. The ob-
tained crystallographic data together with the selected refinement
Scheme 1. Synthesis of [M(acacen)Cu(hfac)2] complexes, M ¼ Pd, Cu.

4
E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
Table 1
Crystallographic data and selected refinement details.
Compound
Formula
F.W.
[Cu(acacen)]
12H18CuN2O2
285.82
[Pd(acacen)]
C12H18N2O2Pd
328.68
[Cu(acacen)Cu(hfac)2]
C22H20Cu2F12N2O6
763.48
[Pd(acacen)Cu(hfac)2]
C22H20N2O6F12CuPd
806.34
C
Temperature/K
Space group
a/Å
150
P21/c
150
P21/c
250
C2/c
150
C2/c
10.9580(3)
8.9714(2)
12.8174(3)
93.4770(10)
1257.74(5)
4
13.6878(6)
19.5327(12)
21.9344(9)
119.446(3)
5106.8(5)
16
18.6599(9)
19.2046(11)
8.5667(4)
109.706(2)
2890.1(3)
4
17.8001(7)
18.3194(7)
8.6028(3)
92.2600(10)
2803.08(18)
4
b/Å
c/Å
ꢁ
b
/
Volume/Å3
Z
rcalc/g∙cmꢀ3
1.509
1.710
1.755
1.911
F(000)
596
2656
1520
1588
Reflections/Independent reflections
8446/3115
2.33%/3.15%
3115/0/158
1.032
31659/11422
5.59%/8.28%
11422/0/629
0.968
7592/3612
1.87%/2.95%
3612/0/202
1.027
11663/3501
2.68%/2.41%
3501/6/202
1.079
R
int/Rsigma
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I ꢂ 2
s
(I)]/%
R1 ¼ 2.81 wR2 ¼ 6.77
R1 ¼ 3.62 wR2 ¼ 7.10
1961113
R1 ¼ 4.57 wR2 ¼ 7.48
R1 ¼ 9.46 wR2 ¼ 9.17
1961114
R1 ¼ 4.89 wR2 ¼ 13.10%
R1 ¼ 7.19 wR2 ¼ 14.90
1961115
R1 ¼ 5.08 wR2 ¼ 12.08
R1 ¼ 6.18 wR2 ¼ 13.21
1961116
Final R indexes [all data]/%
ꢁ
CCDCN
Fig. 1. Molecular structure of [Pd(acacen)] complex (a) and packing diagram (b, view along a axis).
Table 2
Bond lengths and angles of Cu(acacen) and Pd(acacen) fragments as individual complexes and in bimetallic complexes. DFT results are presented as italic in brackets.
Parameter
[Cu(acacen)]
Cu(acacen) in [Cu(acacen)Cu(hfac)2]
[Pd(acacen)]
Pd(acacen) in [Pd(acacen)Cu(hfac)2]
MeO/Å
<MeO>/Å
MeN/Å
<MeN>/Å
:NeMeO/ꢁ
:<NeMeO>/ꢁ
:(CeC/“mean plane”)/ꢁ
1.919e1.925 (1.965-1.966)
1.922 (1.966)
1.938e1.941 (1.981-1.982)
1.940 (1.982)
93.18e93.64 (93.40-93.47)
93.41 (93.43)
21.57 (21.90)
1.925 (1.967)
1.925 (1.967)
1.920 (1.964)
1.920 (1.964)
95.14 (94.21)
95.14 (94.21)
15.32 (21.35)
1.997e2.006 (2.066)
2.002 (2.066)
1.956e1.976 (2.022-2.023)
1.965 (2.023)
94.55e94.88 (93.56)
94.71 (93.56)
18.33e22.89 (22.27)
2.004 (2.053)
2.004 (2.053)
1.953 (2.006)
1.953 (2.006)
95.96 (94.77)
95.96 (94.77)
19.58 (21.86)
details are given in Table 1. The distance between two metal atoms
is 3.191 Å in [Cu(acacen)Cu(hfac)2] and 3.264 Å in [Pd(acacen)
Cu(hfac)2].
fluorine) deviating from the mean squared plane by no more than
0.08 Å, whereas the Cu(hfac)2 fragment in the bimetallic complexes
is not planar, because the coordination of Cu changes from square
to distorted octahedral. OeCueO angles vary from 72.37ꢁ to
102.34ꢁ in [Cu(acacen)Cu(hfac)2] and from 74.00ꢁ to 101.56ꢁ in
[Pd(acacen)Cu(hfac)2]. In both complexes the OeCueO angle that
deviates most from 90ꢁ is formed by two oxygen atoms belonging
to the M(acacen) fragment.
While hfac-ligands remain flat, Cu atoms deviate significantly
from the mean planes of both ligands. As a consequence, the bend
angles of chelate metallocycle along the line between (OO) donor
atoms are equal to 156.13ꢁ in [Cu(acacen)Cu(hfac)2] and 162.59ꢁ in
Bond distances and angles related to the M(acacen) fragments
are given in Table 2. It should be noted that no significant changes
in bond lengths are observed in M(acacen) fragment, and these
molecules retain their flat structure when coordinated. At the same
time, NeCueO and NePdeO angles become larger, as the M(aca-
cen) fragment becomes more elongated towards the Cu atom of
Cu(hfac)2. On the other hand, Cu(hfac)2 fragment changes its ge-
ometry dramatically (Table 3). The individual complex [Cu(hfac)2]
is characterized by a planar geometry [50] with all atoms (except

E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
5
Fig. 2. Hirshfeld surfaces for [Cu(acacen)] and [Pd(acacen)] complexes, mapped with dnorm (top row), curvedness (middle row) and shape index (bottom row).
Fig. 3. Molecular structure (a) and packing diagram (b) of [Cu(acacen)Cu(hfac)2] complex and molecular structure (c) and packing diagram (d) of [Pd(acacen)Cu(hfac)2].
[Pd(acacen)Cu(hfac)2], whereas in Cu(hfac)2 they are equal to
177.46ꢁ. The chelate bond lengths also change significantly. While
in the individual [Cu(hfac)2] complex they vary slightly from 1.885
to 1.930 Å, in bimetallic [M(acacen)Cu(hfac)2] molecules CueO
bonds attain the 4 þ 2 configuration: four long CueO bonds lie in
the plane of M(acacen), and two short CueO bonds are perpen-
dicular to the M(acacen) plane. This asymmetric geometry can be
attributed to multiple ligands with low-symmetric structures.

6
E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
Table 3
Bond lengths and angles for [Cu(hfac)2] molecule and Cu(hfac)2 fragments in binuclear complexes. DFT results are presented as italic in brackets.
Parameter
[Cu(hfac)2]
[50]
Cu(hfac)2 in [Cu(salen)Cu(hfac)2] Cu(hfac)2 in [Cu(acacen)Cu(hfac)2] Cu(hfac)2 in [Pd(acacen)Cu(hfac)2]
[16]
[this work]
[this work]
CueO/Å
1.885e1.930 1.937e2.288
1.912 2.043
92.20e93.68 86.35e89.06
1.934e2.136 (1.976-2.163)
2.035 (2.070)
87.48 (88.51-90.31)
87.48 (89.41)
156.13ꢁ (158.29ꢁ)
1.936e2.114 (1.962-2.141)
2.025 (2.052)
90.05 (89.69-91.62)
90.05 (90.79)
162.59ꢁ (166.66ꢁ)
<CueO>/Å
:OeCueO/ꢁ
:<OeCueO>/ꢁ
: (metallocycle folding along (O,O) line)/ꢁ 177.46
92.94
87.71
149.34e172.82
Similar distortions to Cu(hfac)2 fragment are also observed in the
homobinuclear complex with salen-type Schiff base [Cu(salen)
Cu(hfac)2] [16], however, in this structure they are noticeably larger.
In particular, the difference between shortest and longest CueO
bonds is 0.35 Å, and the angle of metallocycle bend along the line
between donor atoms can be as low as 149.34ꢁ, which is 28ꢁ
different from the individual [Cu(hfac)2] complex. Another notice-
able difference is the lengths of CueO bridge bonds between Cu
atom of Cu(hfac)2 fragment and two oxygen atoms of M(acacen). In
both [M(acacen)Cu(hfac)2] complexes these bridge bonds are
symmetrically equivalent and equal to 2.179 Å for M ¼ Cu and
2.211 Å for M ¼ Pd, while in [Cu(salen)Cu(hfac)2] the lengths of
these CueO bonds are 2.031 Å/2.375 Å and 1.995 Å/2.493 Å for two
symmetrically independent molecules in the unit cell [16]. Both
[Cu(acacen)Cu(hfac)2] and [Pd(acacen)Cu(hfac)2] compounds have
columnar packing style along the c axis (Fig. 3 (b, d)) with M(aca-
cen) parts stacked parallel to each other and facing “inward” the
column, while the bulky Cu(hfac)2 fragments face “outward”. The
distance between mean planes of two adjacent Pd(acacen) frag-
ments is 3.68 Å, and 3.91 Å between Cu(acacen) fragments. The
stacking angle (the angle between the stacking direction, the c axis
in this case, and the normal to the mean square plane of the stacked
molecule) is 24.25ꢁ for the Cu(acacen) fragment and 31.14ꢁ for the
Pd(acacen) fragment.
Fig. 4 shows Hirshfeld surfaces mapped with the dnorm property
for [Cu(acacen)Cu(hfac)2] and [Pd(acacen)Cu(hfac)2]. Despite the
fact that both compounds have the same molecular structure and
packing style, the nature of their close intermolecular contacts
differs considerably. There are several short (2.85 Å) F/F contacts
between hfac-ligands in the adjacent stacks of [Cu(acacen)Cu(h-
fac)2], while [Pd(acacen)Cu(hfac)2] has weak CeH/O hydrogen
bonds (2.5 Å) between neighbor M(acacen) fragments. These weak
intermolecular interactions apparently have a noticeable effect on
the crystal structure of bimetallic complexes. Despite the fact that
the hkl data set with slightly better quality was obtained for the
[Cu(acacen)Cu(hfac)2] complex with Rint ¼ 1.87% and R1 ¼ 4.89%
(Rint ¼ 2.68% and R1 ¼ 5.08% for [Pd(acacen)Cu(hfac)2]), the Pd(a-
cacen) fragment of [Pd(acacen)Cu(hfac)2] has significantly smaller
thermal ellipsoids (Fig. 3c) than the Cu(acacen) fragment of
[Cu(acacen)Cu(hfac)2] (Fig. 3a), which means that the Pd(acacen)
fragments are packed more rigidly within the stack. At the same
time, hfac-ligands of both bimetallic complexes have large thermal
ellipsoids with weakly localized F atoms, which is typical for -CF3
groups.
complexes are given in Tables S1eS4 (Supporting Information). The
experimentally measured vibrational wavenumbers of the mole-
cules coincide well with DFT theoretical predictions. The RMS dif-
ference between the calculated and experimental wavenumbers
was 20 cmꢀ1
.
In the spectra of all derivatives the group of intense bands in the
range of 1600-1500 cmꢀ1 corresponds mainly to the C]C, C]O,
C]N ring stretching vibrations. The bands in two regions of 1445-
1354 cmꢀ1 and 1040-990 cmꢀ1 are mainly attributed to bending
vibrations of -CH3 and -CH2 groups. The CH out-of-plane vibrations
appear as strong bands at 740-760 cm-l. The bands with the high
contribution of M-O and M-N lie below 700 cmꢀ1. For instance, the
most intense bands corresponding to PdeN and PdeO stretching
vibrations lie at 469, 331, 252 cmꢀ1 in the spectrum of [Pd(acacen)],
while the bands corresponding to CueN and CueO are 458 and
362 cmꢀ1. The experimental IR spectrum of [Cu(hfac)2] has already
been discussed in the literature [55]. In its spectrum, the strong
bands in the range 1100e1300 cmꢀ1 were mainly assigned to CeF
stretching vibrations. CueO stretching vibrations mixed with de-
formations of FeCeC in substituents and ring out-of-plane bending
vibrations appeared at ~530 and 325, while the deformations of
OeCueO and OeCueN are observed in the range from 230 to
260 cmꢀ1. In the IR spectra of bimetallic complexes, the region
above 700 cmꢀ1 comprises the combination of bands correspond-
ing to the vibrations of both counterparts, whereas bands involving
PdeO, CueO stretching and OeCueO deformation vibrations in the
range from 200 to 700 cmꢀ1 are most sensitive to the formation of
new CueO1 bonds (Fig. 5, Tables S3eS4). For example, the bands at
692, 671, 585, 333, 256, and 235 cmꢀ1 in the IR spectrum of
[Pd(acacen)Cu(hfac)2] exhibit noticeable shifts (3-5 cmꢀ1
)
compared to the corresponding bands in spectra of initial [Pd(a-
cacen)] and [Cu(hfac)2] compounds due to the formation of CueO1
bond. Two new bands at 492 and 212 cmꢀ1 with the high contri-
bution of CueO1 stretching and d(OCuO1) vibrations also appear in
the [Pd(acacen)Cu(hfac)2] spectrum. Similar shift of several bands
at 670, 587, 464, 218, 208 and 218 cmꢀ1 and the change of their
intensities compared to the spectra of the initial counterparts are
observed in the spectrum of [Cu(acacen)Cu(hfac)2]; the new bands
at 484 and 292 cmꢀ1 appear in the spectra. All these changes
indicate the formation of new bonds in bimetallic complexes.
3.5. Topological analysis based on DFT calculations
Although studies of [M(SB)M0(
b-diket)2] complexes of 3d metals
have been carried out since the 1980s, they were mainly focused on
structures and magnetic properties [56], and no works have been
found illuminating the binding energy of molecular components in
bimetallic complexes. Currently, this information is necessary both
for fundamental understanding of the coordination ability of the
metals M0 which can form bimetallic compounds of this type, and
for the development of approaches to the molecular design of such
compounds, because it provides a key to determining the stability
of a binuclear molecule. Moreover, the stability factor is crucial for
the application of bimetallic complexes in vapor phase deposition
3.4. Vibrational analysis
The vibrational spectra of the prepared bimetallic complexes
were studied and compared with those of their monometallic
precursor molecules. The experimental IR and far IR spectra of all
complexes are given in Fig. 5. The experimental and calculated IR
spectra of [Pd(acacen)] and [Pd(acacen)Cu(hfac)2] are presented in
Fig. 6 as an example. The assignment and comparison of the most
intense experimental and calculated bands in the IR spectra of

E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
7
Fig. 4. Hirshfeld surfaces mapped with dnorm property (ꢀ0.1 ÷ 1.7 range) and fingerprint plots for [Cu(acacen)Cu(hfac)2] (a) and [Pd(acacen)Cu(hfac)2] (b).
methods with the formation of inorganic films (MOCVD) [57] or
molecular metal-organic magnetic films (physical vapor deposi-
tion) [58]. For related 3d-4f [M(SB)M’(b-diket)3] compounds, it was
shown that the binding energies calculated by the DFT method
were in good agreement with the dissociation enthalpy values
obtained by experimental methods [19]. Therefore, we applied the
DFT method to the theoretical study of the synthesized complexes
[M(acacen)Cu(hfac)2] to reveal the influence of the metal M on the
stability of binuclear complexes.
As for the use of bimetallic complexes in vapor-phase deposi-
tion, it should also be noted that the nature of the bonds stabilizing
the binuclear complex molecule in crystals and in the gas phase
may differ. For this reason, we also performed the topological
analysis as the most appropriate and highly informative tool for
determining most significant intramolecular interactions [40e42].
In addition, this approach allows us to identify the most likely
bonds for cleavage, which gives the first step to understanding the
mechanism of precursor decomposition, which is the key to con-
trolling the MOCVD process [59,60]. This is necessary for moni-
toring the MOCVD process and determining the effect of the
precursor on the composition and characteristics of the resulting
materials obtained. Finally, since mononuclear complexes [M(aca-
cen)] are not only components of binuclear complexes, but also
promising MOCVD precursors [21,22], they were also the objects of
detailed topological study.
agreement (Tables 2 and 3). The calculated metal-oxygen and
metal-nitrogen bond lengths exceed the experimental ones by only
0.04e0.06 Å (Tables 2 and 3), which is less than 2%. The corre-
sponding theoretical valence angles are slightly larger than those
determined experimentally, but the difference in the most cases is
no more than 1-4ꢁ.
As the result of topological analysis of the function of electron
density distribution in the [Cu(acacen)] and [Pd(acacen)] com-
plexes, the bond critical points (3,-1) (or BCP) were established and
their parameters were estimated (Fig. 7, Table 4). These BCP indi-
cate the accumulation of electron density exactly in the space be-
tween two atoms, since this is characterized by the positive
curvature of the electron density along one direction (from one
atom to another) and two negative curvatures along two perpen-
dicular directions. Therefore, the presence of BCP (3,-1) between
atoms is an indicator of the formation of a chemical bond, while the
values of the electron density r
(r) and its Laplacian V2r(r) give
information about the nature of this bond [40e42]. In all cases, the
interaction between atoms in CeC, CeN and CeO chemical bonds
refers to the type of “shared interaction”, and bonds themselves
refer to the covalent type because the r
(r) and V2r(r) values are
sufficiently high, and the last parameters are negative (Table S5).
The chemical bonds between metal atoms and nitrogen and
oxygen atoms are of most interest because the values of their
interatomic interaction energy E, estimated as ½ of potential energy
density according to the previously published data [61], are the
lowest compared to all other chemical bonds (Table 4, Table S5). In
The experimental and theoretical geometries of individual
molecules of mono- and binuclear complexes were in good

8
E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
Fig. 5. Experimental IR (a) and far IR spectra (b): 1 e [Cu(hfac)2], 2 e [Pd(acacen)], 3 e [Cu(acacen)], 4 e [Pd(acacen)Cu(hfac)2], 5 e[Cu(acacen)Cu(hfac)2].
Fig. 6. Experimental and calculated IR spectra of [Pd(acacen)] and [Pd(acacen)Cu(hfac)2].
the case of M-N bonds, the interaction between these two atoms is
of “intermediate” type because of the negative values of he(r) in the
corresponding critical points. However, the interaction between
metal and oxygen atoms refers to the type of “closed-shell inter-
action” realized through electrostatic attraction, because the values
of V2r(r) and he(r) are positive. At the same time, the M-O1 and M-
O2 atom pairs in the case of both molecules possess the lowest
values of interatomic interaction energy; therefore, the cleavage of
these bonds is the most probable. The same conclusion was made
for a series of the related bis-chelated (O^N)-coordinated palla-
dium(II) complexes [62], indicating that the trend to the preferred
cleavage of M-O bonds rather than M-N ones may be common for

E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
9
Fig. 7. Structural formula of [M(acacen)], where M ¼ Cu, Pd (left) and the positions of bond critical points (green balls) in the [Pd(acacen)] structure (right).
Table 4
Topological parameters of the
r
(r) function in the critical points (3,-1) of metal-nitrogen and metal-oxygen bonds of M(acacen) molecules.
Bond
Cu(acacen)
(r), e/Å3
Pd(acacen)
r
V
2r(r), e/Å5
he(r), au
E, eV
r
(r), e/Å3
V
2r(r), e/Å5
he(r), au
E, eV
M-N1
M-N2
M-O1
M-O2
0.592
0.590
0.519
0.519
11.39
11.35
12.56
12.56
ꢀ3.84$10ꢀ3
ꢀ3.77$10ꢀ3
8.59$10ꢀ3
8.59$10ꢀ3
1.71
1.70
1.54
1.54
0.742
0.742
0.568
0.568
11.73
11.73
12.48
12.48
ꢀ1.70$10ꢀ2
ꢀ1.70$10ꢀ2
5.69$10ꢀ3
5.69$10ꢀ3
2.12
2.12
1.61
1.61
the considered metals in the near square-planar coordination
environment.
cleavage of CueO1 and CueO2 bonds is most probable.
The detected values of the electron density and its Laplacian at
the bond critical points between copper or palladium atoms and
oxygen or nitrogen atoms are in good agreement with the results of
other works (Tables 4, 5 and S6). For example, it was shown for
dipeptide Cu(glygly)(OH2)2 complex on the basis of topological
Topological analysis of the electron density distribution function
in the [Cu(acacen)Cu(hfac)2] and [Pd(acacen)Cu(hfac)2] compounds
(Fig. 8) shows that the nature of interaction between atom pairs
such as carbon-carbon, carbon-nitrogen and carbon-oxygen is
identical to that observed in [Cu(acacen)] and [Pd(acacen)] mole-
analysis of experimental data that r(r) in the BCPs in the atomic
cules. Similar situation is also observed for M-O1, M-O2, M-N1
N2 bonds (Table S6).
и
M-
pairs CueO and CueN are in the range 0.3e0.8 e/Å3, while V2r(r)
has a value from 3 to 13 e/Å5 [63]. In the case of other type of
compounds, e.g. Cu(H2O)5 and Cu(H2O)6, where copper atom in-
teracts with oxygen atoms of water molecules, the values of elec-
tron density and its Laplacian in the corresponding bond critical
points lie in the ranges of 0.31e0.57 e/Å3 and 4.92e12.36 e/Å5,
The bridge CueO1 and CueO2 bonds between both parts in
[Cu(acacen)Cu(hfac)2] and [Pd(acacen)Cu(hfac)2] molecules
possess the lower values of electron density in the corresponding
critical points (3,-1) and, as a consequence, the lower values of
interatomic interaction energy (Table 5). For this reason, the
respectively [64,65]. The values of r(r) in the BCPs in the PdeO and
Fig. 8. Structural formula of [M(acacen)Cu(hfac)2], where M ¼ Cu, Pd, (left); the positions of the bond critical points (green balls) in [Cu(acacen)Cu(hfac)2] (right).

10
E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
Table 5
Topological parameters of the
r
(r) function in the critical points (3,-1) of metal-oxygen and hydrogen bonds of [M(acacen)Cu(hfac)2] molecules.
Bond
[Cu(acacen)Cu(hfac)2]
(r), e/Å3 2r(r), e/Å5
[Pd(acacen)Cu(hfac)2]
(r), e/Å3 2r(r), e/Å5
r
V
he(r), au
E, eV
r
V
he(r), au
E, eV
CueO1
CueO2
CueO30
CueO300
CueO40
CueO400
O30-H
0.292
0.292
0.341
0.498
0.341
0.498
0.076
0.076
0.042
0.042
0.042
0.042
5.13
5.13
6.36
11.79
6.36
11.79
0.92
0.92
0.57
0.57
0.59
0.59
2.57$10ꢀ3
2.56$10ꢀ3
4.01$10ꢀ3
9.05$10ꢀ3
4.01$10ꢀ3
9.05$10ꢀ3
8.23$10ꢀ4
8.23$10ꢀ4
8.92$10ꢀ4
8.92$10ꢀ4
8.21$10ꢀ4
8.21$10ꢀ4
0.65
0.65
0.79
1.42
0.79
1.42
0.11
0.11
0.06
0.06
0.06
0.06
0.268
0.268
0.358
0.516
0.358
0.516
0.065
0.065
e
4.48
4.48
6.97
12.41
6.97
12.41
0.81
0.81
e
1.57$10ꢀ3
1.57$10ꢀ3
4.75$10ꢀ3
8.90$10ꢀ3
4.75$10ꢀ3
8.90$10ꢀ3
9.03$10ꢀ4
9.03$10ꢀ4
e
0.59
0.59
0.85
1.51
0.85
1.51
0.09
0.09
e
O40-H
O300-H
O400-H
FeH
e
e
e
e
e
e
e
e
FeH
e
e
e
e
PdeN atomic pairs of the Pd(II)-salen complex are in the ranges of
0.66e0.73 e/Å3 and 0.68e0.85 e/Å3, while the corresponding values
of V2r(r) are in the ranges of 12.68e15.04 e/Å5 and 10.68e13.88 e/
Å5, correspondingly [66].
4. Conclusion
In this work, the binuclear [Pd(acacen)Cu(hfac)2] complex was
synthesized and investigated as the first example of hetero-
Six additional bond critical points are also observed in [Cu(a-
cacen)Cu(hfac)2] (Fig. 8). Four of them are between four hydrogen
atoms of two CH3-groups of Cu(acacen) and four oxygen atoms of
Cu(hfac)2, while the other two points are between two hydrogen
atoms of two CH3-groups of Cu(acacen) and two fluorine atoms of
two CF3-groups. At the same time, there are only two such points
between two hydrogen atoms of two CH3-groups of Pd(acacen) and
two oxygen atoms of Cu(hfac)2 in [Pd(acacen)Cu(hfac)2]. These
bond critical points indicate the existence of hydrogen bonds be-
bimetallic molecular compounds formed by Schiff base and b-
diketonate building blocks with noble metals. The detailed char-
acterization of the structure of this compound in comparison with
the homometallic binuclear analogue [Cu(acacen)Cu(hfac)2] and
the corresponding molecular SB components [M(acacen)] was
performed by single crystal X-ray diffraction, vibrational spectros-
copy and DFT calculations.
It was shown that the structure of SB complex molecules was
almost unchanged after coordination of these molecules to
Сu(hfac)2. Despite both bimetallic complexes had similar molecular
structure and packing style, the nature of the closest intermolecular
contacts in [M(acacen)Cu(hfac)2] differed considerably. In partic-
ular, Hirshfeld surface analysis revealed several short (2.85 Å) F/F
contacts between hfac-ligands in the [Cu(acacen)Cu(hfac)2] crys-
tals, while the weak CeH/O hydrogen bonds (2.5 Å) between
neighbor M(acacen) fragments were found for [Pd(acacen)
Cu(hfac)2].
The topological analysis shown that the strong bridge interac-
tion of CueO is strengthen by the formation of weak hydrogen
bonds between hydrogen atoms of CH3-groups of M(acacen) and
oxygen atoms of Cu(hfac)2 in the both bimetallic complexes.
Additional hydrogen bonds involving CF3-groups were observed in
the case of [Cu(acacen)Cu(hfac)2], which caused slightly stronger
binding between Cu(acacen) and Сu(hfac)2 fragments in this
binuclear complex (1.64 eV) compared to [Pd(acacen)Cu(hfac)2]
(1.44 eV).
tween the mentioned pairs of atoms. The values of r
(r) and V2r(r)
for typical hydrogen bonds are in the range of 0.013e0.236 e/Å3 and
0.578e3.350 e/Å5, correspondingly [42]. The sum of energies of
interatomic interactions between atoms in the bridge CueO bonds
is 5.72 eV in the case of [Cu(acacen)Cu(hfac)2] and 5.90 eV in the
case of [Pd(acacen)Cu(hfac)2]. However, they increase to 6.18 eV
and 6.08 eV, respectively, if hydrogen bonds are taken into account.
Therefore, the binding between Cu(hfac)2 and Cu(acacen) fragments
is stronger than that between Cu(hfac)2 and Pd(acacen). This fact
was also confirmed by the results of calculation of the binding
energy between M(acacen) and Cu(hfac)2 fragments in bimetallic
compounds according to equation (1)
Eb ¼ EM(acacen) þ ECu(hfac)2 e EM(acaсen)Cu(hfac)2
(1)
The binding energy (Eb) was calculated as the difference of the
total energies of the corresponding compound and its initial com-
ponents. It was shown that Eb was equal to 1.64 eV for M ¼ Cu, and
1.44 eV when M ¼ Pd, indicating the higher stability of [Cu(acacen)
Cu(hfac)2] in comparison with [Pd(acacen)Cu(hfac)2]. These values
are little bit higher compared to the binding energy of 1.31 eV
(30.2 kcal/mol) obtained for [Cu(salen)La(ptaс)3] (ptac e pivaloyl-
trifluoroacetonato) [19] (the only data available for such systems).
The binding energy values are rather low and indicate that such
binuclear complexes may dissociate to monometallic precursors in
gas phase upon heating. The experimental investigations of the
behavior of bimetallic complexes in gas phase and their application
as MOCVD precursors are of great interest and will be a subject of
future works. In addition, the binuclear complexes synthesized
here can be used to obtain molecular metal-organic films by
physical vapor deposition [67,68], and due to the proximity of metal
centers the molecules of this type can exhibit interesting magnetic
properties [26,69].
The demonstrated ability of palladium(II) SB complexes to co-
ordinate to fluorinated b-diketonates with acceptor metal centers
opens up a new way for the development of bimetallic Pd-M
complexes for various applications. The approach to the prepara-
tion of binuclear complexes described in this work can be a starting
point for the creation of homo- and heterobimetallic complexes
with other 3d-transition metals.
CRediT authorship contribution statement
Evgeniia S. Vikulova: Conceptualization, Writing - original
draft. Nataliya S. Nikolaeva: Investigation, Writing - original draft.
Pavel O. Krasnov: Investigation, Visualization. Alexander A.
Sukhikh: Investigation, Visualization. Anton I. Smolentsev: Formal
analysis. Evgenia A. Kovaleva: Formal analysis. Natalya B. Moro-
zova: Supervision. Tamara V. Basova: Writing - review & editing.

E.S. Vikulova et al. / Journal of Molecular Structure 1216 (2020) 128341
11
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The authors acknowledge the Russian Foundation for Basic
Research for a financial support (project N 18-33-20128) of the
part of work associated with the investigation of spectral properties
and the Ministry of Science and Education of the Russian Federation
for a financial support of the part of work on structural studies of
compounds.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.128341.
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