Heterometallic Group 4-Lanthanide Oxo-alkoxide Precursors for Synthesis of Binary Oxide Nanomaterials

Article abstract of DOI:10.1021/acs.inorgchem.0c02478

In this study, an efficient procedure for the synthesis of uncommon group 4-lanthanide oxo-alkoxide derivatives was developed. Heterometallic clusters with the structures [La2Ti4(μ4-O)2(μ3-OEt)2(μ-OEt)8(OEt)6(Cl)2(HOEt)2] (1), [La2Zr2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n (2), [La2Hf2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n (3), [Nd2Ti4(μ4-O)2(μ3-OEt)2(μ-OEt)8(OEt)6(HOEt)2(Cl)2] (4), [Nd4Zr4(μ3-O)2(μ-OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (5), and [Nd4Hf4(μ3-O)2(μ-OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (6) were synthesized via the reaction of a metallocene dichloride, Cp2M′Cl2 (where M′ = Ti, Zr, and Hf), and metallic lanthanum or neodymium in the presence of excess ethanol. This procedure gave crystalline precursors with molecular stoichiometries suitable for obtaining group 4-lanthanide oxide materials. Compounds 1-6 were examined by analytical and spectroscopic techniques and single-crystal X-ray diffraction. The magnetic properties of 5 and 6 were investigated by using direct and alternating current (dc and ac) susceptibility measurements. The results indicated weak antiferromagnetic interactions between NdIII ions and a field-supported slow magnetic relaxation. Lanthanum-titanium compound 1 decomposed at 950 °C to give the perovskite compound La0.66TiO3 and small amounts of rutile TiO2. Under the same conditions, 4 decomposed to give a mixture of Nd4Ti9O24 and Nd0.66TiO3. When 4 was calcined at 1300 °C, decomposition of Nd4Ti9O24 to Nd0.66TiO3 and TiO2 was observed. Calcination of 2, 3, 5, and 6 at 950-1500 °C led to the selective formation of heterometallic La2Zr2O7, La2Hf2O7, Nd2Zr2O7, and Nd2Hf2O7 phases, respectively.

Full text of DOI:10.1021/acs.inorgchem.0c02478

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Heterometallic Group 4−Lanthanide Oxo-alkoxide Precursors for
Synthesis of Binary Oxide Nanomaterials
́
́
Rafał Petrus,* Katarzyna Chomiak, Jozef Utko, Alina Bienko, Tadeusz Lis, and Piotr Sobota*
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ABSTRACT: In this study, an efficient procedure for the synthesis of uncommon group 4−lanthanide oxo-alkoxide derivatives was
developed. Heterometallic clusters with the structures [La2Ti4(μ4-O)2(μ3-OEt)2(μ-OEt)8(OEt)6(Cl)2(HOEt)2] (1), [La2Zr2(μ3-
O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n (2), [La2Hf2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n (3), [Nd2Ti4(μ4-O)2(μ3-
OEt)2(μ-OEt)8(OEt)6(HOEt)2(Cl)2] (4), [Nd4Zr4(μ3-O)2(μ-OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (5), and [Nd4Hf4(μ3-O)2(μ-
OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (6) were synthesized via the reaction of a metallocene dichloride, Cp2M′Cl2 (where M′ = Ti,
Zr, and Hf), and metallic lanthanum or neodymium in the presence of excess ethanol. This procedure gave crystalline precursors
with molecular stoichiometries suitable for obtaining group 4−lanthanide oxide materials. Compounds 1−6 were examined by
analytical and spectroscopic techniques and single-crystal X-ray diffraction. The magnetic properties of 5 and 6 were investigated by
using direct and alternating current (dc and ac) susceptibility measurements. The results indicated weak antiferromagnetic
interactions between NdIII ions and a field-supported slow magnetic relaxation. Lanthanum−titanium compound 1 decomposed at
950 °C to give the perovskite compound La0.66TiO3 and small amounts of rutile TiO2. Under the same conditions, 4 decomposed to
give a mixture of Nd4Ti9O24 and Nd0.66TiO3. When 4 was calcined at 1300 °C, decomposition of Nd4Ti9O24 to Nd0.66TiO3 and TiO2
was observed. Calcination of 2, 3, 5, and 6 at 950−1500 °C led to the selective formation of heterometallic La2Zr2O7, La2Hf2O7,
Nd2Zr2O7, and Nd2Hf2O7 phases, respectively.
[Ln2Ti4(O)2(OEt)16(HOEt)2(Cl)2] (Ln = Ce, Eu, and
Nd),20−22 [LnTi8(O)7(OEt)21(HOEt)x] (Ln = Ce and Er and
x = 0 and 1),22,23 [PrTi29 (O)3 8(OEt)4 0(Br)],24
[LnTi28(O)36(OEt)40(OH)2(Cl)] (Ln = Ce and La),20 and
[LnTi11(O)16(NO3)2(OiPr)17] (Ln = Sm, Eu, and Gd).25 Of
these heterometallic clusters, 15 are zirconium, and 16 are
titanium derivatives. There are no known hafnium−lanthanide-
based compounds. The significant number of zirconium
derivatives reflects the availability of a simple and efficient
INTRODUCTION
■
Over the last four decades, structurally authenticated group 4
heterometallic alkoxides have been intensively investigated
because of their numerous practical applications in catalysis, as
well as material and polymer chemistry. Various combinations of
titanium group metals with alkali,1−4 alkaline earth,5−8 and
transition metal elements9−13 have been reported. Most of these
compounds involve alkaline earth derivatives. Heterometallic
alkoxides composed of group 4 (M′4+) and lanthanide (Ln2+/
Ln3+) ions are uncommon and have not been widely studied.
Until now, the structures of only 31 such compounds have been
deposited in the Cambridge Structural Database.14 Most of
them (29) are isopropoxide or ethoxide derivatives of formula
[Ln4 Ti(O)(Oi Pr)1 4 ] (Ln = Sm and Tb),1 5 , 1 6
[Ln2Zr4(OiPr)18(Cl)4] (Ln = Nd, Ho, and Ce),
[CpLnZr2(OiPr)9] (Ln = Yb and Sm),17 [LnZr2(OiPr)9(X)2]
(Ln = Tm, Er, and Yb; X = Cl and I),18 [La2Zr3(O)(OiPr)16],19
Received: August 19, 2020
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Scheme 1. Synthesis of 1−3
method for their synthesis, i.e., the substitution of the halogen
ions in LnX3 by [Zr2(OiPr)9]−. Heterometallic group 4−
lanthanide carboxylates form a much larger family of
coordination compounds than metal alkoxides. For example,
[LnTi6O3(OiPr)9(salicylate)6] for Ln = La, Ce, Pr, Nd, Sm, Eu,
Gd, Tb, Dy, Ho, and Er were investigated as a platform for the
in-depth study of the NMR behavior of paramagnetic Ln3+
ions.26 The ion-size-dependent formation of mixed titanium−
lanthanide oxo clusters [LnTi4(O)3(OiPr)2(OMc)11] (Ln = La
and Ce; OMc = methacrylate), [Ln2Ti6O6(OMc)18(HOiPr)]
(Ln = La, Ce, Nd, and Sm) and [Ln2Ti4O4(OMc)14(HOMc)2]
(Ln = Sm, Eu, Gd, and Ho) was observed during the reaction of
titanium isopropoxide, lanthanide acetate, and methacrylic
acid.27 Titanium group−lanthanide compounds are of particular
interest for use as catalysts in various photochemical reactions28
and as magnetic and luminescent molecular materials.29−31
They are also attractive single-source molecular precursors for
advanced multicomponent oxide ceramics.
struction of metal−ferroelectric−insulator semiconductor struc-
tures.36 La2Ti2O7 has been intensively investigated as a
photocatalyst for water splitting, decomposition of organic
waste,37 CO2 reduction, and H2 production. Lanthanide
zirconates have been mainly investigated as materials for
thermal barrier coating (TBC) of turbine components for
thermal insulation and protection against hot and corrosive gas
streams.38 La2Zr2O7 is particularly useful in this context and has
many advantages compared to traditional TBC systems based
on 8 wt % yttria-stabilized zirconia (Y2O3/ZrO2), such as no
phase transformation from room temperature to its melting
temperature, high sintering resistance, low thermal conductivity,
and low oxygen ion diffusivity.39 Because of its high density
(7.84 g/cm3), large effective atomic number, and high stopping
power for X- and γ-rays, La2Hf2O7 is technologically important
in radiation-resistant materials.40 The development of efficient
and inexpensive methods for the synthesis of group 4−
lanthanide oxide ceramics is therefore important for increasing
their range of applications.
The compounds Ln2M′2O7 are a family of heterometallic
materials with a wide range of chemical and physical properties,
which depend on oxygen vacancies, chemical composition, and
ordering−disordering of the Ln3+ and M′4+ ions. These oxides
can exist in two closely related structures, i.e., cubic pyrochlore
or disordered fluorite. The fluorite structure is favored when the
ionic radii of the Ln3+ and M′4+ ions are comparable [r(Ln3+)/
r(M′4+) < 1.46]. The ordered pyrochlore structure is not stable
and can be transformed into the fluorite structure by the actions
of temperature or pressure and by chemical doping. Ln2M′2O7
oxides are of considerable interest because of their excellent
thermal stabilities, high radiation stabilities, high melting points,
high thermal expansion coefficients, low thermal conductivities,
and chemical resistance. Because of their high electrical
conductivities, they are promising candidates for use as high-
temperature solid electrolytes in solid-oxide fuel cells.32 They
are also excellent host lattices for lanthanide-doped (Er3+, Tb3+,
Pr3+, and Eu3+) phosphor materials for use in light-emitting
devices, fiber amplifiers, lasers, color displays, brand anti-
counterfeiting, and temperature sensors.33−35 Lanthanide
titanates are promising ferroelectric materials for the con-
Several of our previous studies showed that heterometallic
alkoxides are attractive starting materials for the preparation of
high-purity group 4−alkaline earth oxides.41 The molecular
precursors were synthesized by protonolysis of the cyclo-
pentadienyl ligand in Cp2M′Cl2 in the presence of an alcohol (2-
methoxyethanol or ethanol) and M(OR)2 (M = Mg, Ca, Sr, and
Ba).42,43 The current study aimed to use this approach for the
synthesis of group 4−lanthanide alkoxides and their conversion
to binary metal oxide materials.
RESULTS AND DISCUSSION
■
Syntheses and Structural Study. Treatment of Cp2TiCl2
with lanthanum powder (1 equiv) in an ethanol/toluene mixture
(3:1) at ambient temperature (Scheme 1) gave crystalline
cyclopentadienyl-free [La2Ti4(μ4-O)2(μ3-OEt)2(μ-OE-
t)8(OEt)6(Cl)2(HOEt)2] (1, 36%). When the same reaction
was performed with zirconocene or hafnocene dichloride, the
isostructural compounds [La2M′2(μ3-O)(μ-OEt)5(μ-Cl)-
(OEt)2(HOEt)4(Cl)4]n, where M′ = Zr (2, 42%) or Hf (3,
33%), were obtained. In these reactions, Cp2M′Cl2 (M′ = Ti, Zr,
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and Hf) undergoes protonolysis of the cyclopentadienyl ligands,
which leads to the release of CpH and formation of group 4
ethoxide or chloro-ethoxide species. This process is accom-
panied by the elimination of Cl− from Cp2M′Cl2, which results
in the generation of chloro or dichloro lanthanum ethoxide
derivatives.
These species were involved in the formation of hetero-
metallic clusters 1−3. Analytical methods, infrared (IR)
spectroscopy, and single-crystal X-ray diffraction (XRD) were
used to characterize 1−3 (Figures S1−S6). These compounds
were isolated as crystalline materials. They are insoluble in most
conventional solvents. The crystallographic data in Figure 1
Figure 2. Molecular structures of [La2M′2(μ3-O)(μ-OEt)5(μ-Cl)-
(OEt)2(HOEt)4(Cl)4]n for M′ = Zr (2) and Hf (3). Displacement
ellipsoids are drawn at the 30% probability level. Hydrogen atoms of
alkyl groups are omitted for clarity [symmetry code: (i) x, −y + 0.5, z +
0.5; (ii) x, −y + 0.5, z − 0.5].
The continuous shape measure (CShM) method was used to
investigate the geometric distortions around the metal atoms in
[La2M′2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n. These
calculations showed similar deformations of the coordination
environments around La atoms, with metric parameters
S(TDD-8) equal to 1.99 and 1.36 for 2 and 2.08 and 1.38 for
3. For the Zr and Hf atoms, the values of S(Oh) vary from 3.57
to 3.73. The bond lengths La−O (2.471(6)−2.580(6) Å), La−
Cl (2.890(2)−2.967(2) Å), M′−O (1.901(8)−2.137(6) Å),
and M′−Cl (2.603(3)−2.636(3) Å) in 2 and 3 are similar and lie
in the expected ranges for lanthanum and group 4 chloro-alkoxo
derivatives.47,48 Note that 3 is the first reported example of a
heterometallic lanthanide−hafnium alkoxide.
When Cp2M′Cl2 was reacted with Nd (1 equiv) and excess
ethanol, heterometallic alkoxide clusters were obtained, i.e.,
[Nd2Ti4(μ4-O)2(μ3-OEt)2(μ-OEt)8(OEt)6(HOEt)2(Cl)2] (4,
3 2 % ) a n d [ N d 4 M ′ 4 ( μ 3 - O ) 2 ( μ - O E t ) 1 0 ( μ -
Cl)4(OEt)8(HOEt)10(Cl)2], M′ = Zr (5, 51%) and M′ = Hf
(6, 42%; Scheme 2).
Compound 4 is isostructural with 1 and had been previously
obtained from the reaction of NdCl3 with Ti(OEt)4.20
Compounds 5 and 6 are octanuclear clusters formed by two
{Nd2M′2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)4(HOEt)5(Cl)} units
bridged by two μ-Cl groups (Figure 3). These structural units
are similar to those observed in 2 and 3, but with the M′ atoms
bonded to OEt anions rather than to Cl ligands. The
coordination of additional ethanol solvent molecules to the
peripheral Nd atoms prevents the formation of polymeric
structures.
The Nd ions in 5 and 6 surrounded by O5Cl3 or O6Cl2 donor
atoms adopt a triangular dodecahedral geometry, and the M′
atoms coordinated with five ethoxo and one oxo groups form
distorted octahedrons. CShM calculations showed similar
distortions in the neodymium coordination geometries in 5
and 6, with measurement parameters from 0.973 to 1.174. For
the M′ atoms, a slightly larger deformation of the octahedral
geometry was observed around the zirconium atoms in 5, as
shown by the S(Oh) parameters of 1.921 and 2.041 for Zr ions
and 1.741 and 1.800 for Hf ions. The Nd−O and Nd−Cl bond
Figure 1. Molecular structure of [La2Ti4(μ4-O)2(μ3-OEt)2(μ-OE-
t)8(OEt)6(Cl)2(HOEt)2] (1). Displacement ellipsoids are drawn at the
30% probability level. Solvent molecules and hydrogen atoms of alkyl
groups are omitted for clarity [symmetry code: (i) −x + 1, −y + 1, −z +
1].
show that 1 is a centrosymmetric hexanuclear cluster in which
lanthanum centers surrounded by O7Cl donors adopt a square
antiprismatic geometry, and titanium atoms coordinated with
O6 donors form distorted octahedrons. The La−O, La−Cl, and
Ti−O bond lengths for 1, i.e., 2.445(2)−2.641(2), 2.803(2),
and 1.774(2)−2.182(2) Å, respectively, are similar to those
r e p o r t e d f o r [ L a 5 ( O ) ( O i P r ) 1 3 ( H O i P r ) 2 ] , 4 4
[La2Cl6(HOiPr)2],45 and [Ti4(OH)(OEt)15].21 The structure
of 1 corresponds well to that previously reported for titanium−
l a n t h a n i d e
a l k o x i d e s
o f
f o r m u l a
[Ln2Ti4(O)2(OEt)16(HOEt)2(Cl)2]20−23 (Ln = Ce, Eu, and
Nd) or [Er2Ti4(O)2(OEt)18(HOEt)2].46 The central core
structure of these compounds contains the double oxo-bridged
dilanthanide unit {Ln2(O)2X} (X = Cl and OEt) capped by four
Ti(OEt)4 molecules through two μ4-O oxo atoms. The metric
parameters of 1 and the four other hexanuclear clusters are very
similar with the bond lengths of 2.368−2.495 Å for Ln−O,
2.412−2.653 Å for Ln−OEt, 2.684−2.803 Å for Ln−Cl, 1.905−
2.183 Å for Ti−OEt(bridging), 1.774−1.876 Å for Ti−OEt(terminal)
,
and 1.938−2.012 Å for Ti−O. The XRD study (Figure 2) shows
that 2 and 3 are linear 1D coordination polymers based on
t e t r a n u c l e a r [ L a 2 M ′ 2 ( μ 3 - O ) ( μ - O E t ) 5 ( μ - C l ) -
(OEt)2(HOEt)4(Cl)4] units. The lanthanum atoms are
surrounded by one oxo, three chloro, and four ethoxo ligands
and adopt a triangular dodecahedral geometry; the M′ atoms
coordinated by one chloro, one oxo, and four ethoxo groups
form distorted octahedrons.
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Scheme 2. Synthesis of 4−6
Figure 3. Molecular structures of [Nd4M′4(μ3-O)2(μ-OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (5, M′ = Zr) and (6, M′ = Hf). Displacement ellipsoids
are drawn at the 30% probability level. Hydrogen atoms of alkyl groups are omitted for clarity [symmetry code: (i) −x + 1, −y + 1, −z + 1].
lengths of 2.387(4)−2.559(4) and 2.827(3)−2.885(3) Å, are
similar to those reported for [Nd5(O)(OiPr)13(HOiPr)x]49,50 (x
= 0 and 2) and [Nd2(Cl)6(HOiPr)2].51 The geometric
parameters for the M′ atoms correlate well with those for 2
and 3. In 2/3 and 5/6 the stoichiometry of metal elements
Ln:M′ = 1:1 differs from those usually observed for group 4−
lanthanide alkoxides with Ln:M′ ratio of 1:2. The double chloro-
bridged Ln atoms found in 2/3 and 5/6 have been before
observed only in the clusters [Ln2Zr4(OiPr)18(Cl)4] (Ln = Nd,
Ho, and Ce).17 The comparison of bond lengths of lanthanide−
zirconium alkoxides 2 and 5 with the data available for
compounds [Ln2Zr4(OiPr)18(Cl)4] revealed the expected values
for this type of coordination compounds with Ln−OR distances
of 2.330−2.520 Å, Ln−Cl(bridging) distances of 2.729−2.967 Å,
and Zr−OR distances of 1.908−2.169 and 2.068−2.302 Å for
terminal and bridging alkoxide groups.
products of the reaction of Cp2ZrCl2 with metallic Nd were
recrystallized from ethanol/tetrahydrofuran solution. However,
separation of the two compounds in quantitative yield was
impossible, and only a few crystals of 7 for XRD studies were
isolated manually. The presence in 7 of two bridging μ3-OH
groups, which arise from moisture present in the solution, shows
that the use of an ethanol/H2O mixture as the solvent for these
reactions could provide a promising method for the synthesis of
a new group of coordination compounds based on group 4 and
lanthanide ions (Figure 4). The zirconium atoms in 7 are
octahedrally coordinated with oxygen atoms from six ethoxy or
five ethoxy and one oxo ligands. Neodymium ions coordinated
with O7Cl or O8 donor atoms adopt a triangular dodecahedral
(Nd1) or biaugmented trigonal prismatic (Nd2−Nd5)
geometry. The presence of four peripheral Zr(OEt)4 units in 7
suggests that it is formed by the coordination of Zr(OEt)4
molecules with a pentanuclear neodymium unit, i.e.,
[Nd5(O)2(OH)2(OEt)6(Cl)3(HOEt)6], which prevents further
aggregation in the solid state. A similar nonanuclear central core
In addition to compound 5, we isolated crystals of a
nonanuclear cluster [Nd5Zr4(μ4-O)2(μ4-Cl)(μ3−OH)2(μ3-
OEt)2(μ-OEt)10(μ-Cl)2(OEt)10(HOEt)6] (7, 2%) when the
D
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Figure 4. Molecular structure of [Nd5Zr4(μ4-O)2(μ4-Cl)(μ3−OH)2(μ3-OEt)2(μ-OEt)10(μ-Cl)2(OEt)10(HOEt)6] (7). Displacement ellipsoids are
drawn at the 20% probability level. The second disordered counterpart of alkoxy group and hydrogen atoms of alkyl groups are omitted for clarity.
structure was previously reported for [Nd4Na5(L)4(OH)-
(OMe)4(dme)3(HOMe)] (LH3 = 2-oxidopropane-1,3-diylbis-
(nitrilomethylylidene)diphenol, dme = 1,2-dimethoxy-
ethane).52
several percent. At temperatures above 80 °C, traces of the
ethanol ligands were eliminated from the lanthanum or
neodymium coordination sphere. The ethanol ligands were
weakly coordinated to the Ln atoms and were removed from the
samples during vacuum drying. At 120−310 °C, ethoxide
ligands were removed as C2H4, H2O, and CO2. In the range of
400−950 °C, only CO2 and HCl were removed. The presence in
the DSC curve of single endothermic peaks at 858 and 758 °C,
which correspond to the melting points of LaCl3 and NdCl3,
respectively, together with the overall mass losses of 25.1% for 2,
22.9% for 3, 32.5% for 5, and 27.2% for 6 suggest that in a N2
atmosphere these compounds decompose to a mixture of M′O2,
LnCl3, and Ln2O3.
When TG/DTA measurements were performed under N2 in
the temperature range of 20−1500 °C, LaCl3 and NdCl3
vaporized above 1000 and 1500 °C, respectively, and were
removed from the sample. When thermal degradation was
performed in air, LnCl3 was oxidized to LnOCl and then to
Ln2O3. These species together with M′O2 are involved in the
formation of heterobimetallic phases.
Synthesis and Powder XRD (PXRD) Characterization of
Heterometallic Oxide Materials. A combination of thermog-
ravimetry and differential scanning calorimetry/differential
thermal analysis (TG-DSC/DTA) coupled with quadrupole
mass spectrometry, and PXRD was used to study the thermal
decomposition of 1−6 to identify appropriate temperatures for
removing organic ligands and formation of heterometallic oxide
phases. Calcination of 1−6 from 20 to 1300 °C in a N2
atmosphere was investigated. Representative thermograms for
the isostructural compounds 2, 3, 5, and 6 are shown in Figure 5.
The results indicate that the heterometallic compounds
underwent multistep decomposition. All the compounds were
thermally stable up to 78 °C with mass losses not exceeding
Oxides were synthesized by heating precursors 1−6 at 950 °C
and between 1300 and 1500 °C for 3 h in air. The phase purities
of the resulting materials were determined by PXRD; the data
are shown in Figures 6−11. Treatment of 1 at 950 and 1300 °C
resulted in the formation of La0.66TiO3 as the main product, but
a small amount of TiO2 was also detected because of the
stoichiometry of 1 (Figure 6). The PXRD patterns of the oxides
synthesized by heating 2 at 950 and 1500 °C (Figure 7) clearly
show the formation of only one pure phase, namely, La2Zr2O7.
Calcination of 3 at 950 °C gave La2Hf2O7 as the main phase, but
small diffraction peaks from La2O3 and HfO2 were also detected.
When the calcination temperature was increased to 1500 °C,
only La2Hf2O7 was formed (Figure 8). Figure 9 shows that
calcination of 4 at 950 °C gave a mixture of Nd4Ti9O24 and
Nd0.66TiO3. When 4 was heated at 1300 °C, the intensities of the
Nd4Ti9O24 phase peaks decreased with simultaneous increases
in the Nd0.66TiO3 and rutile TiO2 peak intensities. The PXRD
patterns of the oxides prepared by heating 5 and 6 at 950 and
Figure 5. TGA curves (dashed lines) for 2, 3, 5, and 6 recorded at a
heating rate of 10 °C min−1 in nitrogen atmosphere over a temperature
range 25−1300 °C.
E
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Figure 6. PXRD patterns of oxide materials prepared by calcination of 1
at 950 °C [blue] (a) and 1300 °C [red] (b), La0.66TiO3 [Powder
Diffraction File 4 (PDF-4) no. 04−019−9846 (2018) green] (c), and
TiO2 reference pattern [01−072−4815, brown] (d).
Figure 9. PXRD patterns of oxide materials prepared by calcination of 4
at 950 °C [blue] (a) and 1300 °C [red] (b), Nd4Ti9O24 [PDF-4+ 04−
010−7646, 2018; green] (c), Nd0.66TiO3 [04−013−3633, brown] (d),
and TiO2 reference pattern [01−079−6031, cyan] (e).
Figure 7. PXRD patterns of oxide materials prepared by calcination of 2
at 950 °C [blue] (a) and 1500 °C [red] (b), and La2Zr2O7 reference
pattern [PDF-4+ no. 04−006−2370, 2018; green] (c).
Figure 10. PXRD patterns of oxide materials prepared by calcination of
5 at 950 °C [blue] (a) and 1500 °C [red] (b), and Nd2Zr2O7 reference
pattern [PDF-4+ 04−001−9352, 2018; green] (c).
Figure 8. PXRD patterns of oxide materials prepared by calcination of 3
at 950 °C [blue] (a) and 1500 °C [red] (b), La2Hf2O7 [PDF-4+ 04−
016−6623, 2018; green] (c), HfO2 [04−002−5353, brown] (d), and
La2O3 reference pattern [04−014−8421, cyan] (e).
Figure 11. PXRD patterns of oxide materials prepared by calcination of
6 at 950 °C [blue] (a) and 1500 °C [red] (b), and Nd2Hf2O7 reference
pattern [PDF-4+ 04−006−1654, 2018; green] (c).
1500 °C showed selective crystallization of the Nd2Zr2O7 or
Nd2Hf2O7 phases (Figures 10 and 11).
Figures 12 and 13 show transmission electron microscopy
(TEM) photographs of materials prepared by thermal
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Figure 12. TEM images of oxide materials prepared by calcination of 1 (a), 2 (b), and 3 (c) at 950 °C.
decomposition of molecular precursors 1−6 at 950 °C. The
crystallites prepared from 1 had a regular morphology and a
particle size of 50−100 nm (Figure 12a). The La2Zr2O7 particles
obtained from precursor 2 had irregularly shaped particles that
varied significantly in size, with a mean particle size varying from
30 to 200 nm (Figure 12b). Similar shape and size properties to
those of the product obtained from 2 were observed for
La2Hf2O7, which was derived from 3 (Figure 12c). The obtained
Nd4Ti9O24 had an interesting morphology; the material
consisted of round and elongated crystallites of diameter 50−
200 nm (Figure 13a). Nd2Zr2O7 was obtained as regular
crystallites of diameter 10−30 nm (Figure 13b). Nd2Hf2O7 was
obtained as highly aggregated irregularly shaped particles
(Figure 13c).
expected for formula units consisting of four noninteracting
NdIII species (4f3, S = 3/2, J = 9/2, L = 6, 4I9/2; χMT = 6.59 cm3
mol−1 K), possibly because of the effect of strong spin−orbit
coupling. The product function χMT (Figure 14) decreased
gradually at 300−150 K, decreased greatly below 150 K, and
reached values of 3.33 and 3.01 cm3 mol−1 K for compounds 5
and 6, respectively, at 2 K. The magnetic features of both these
complexes indicate antiferromagnetic exchange interactions
together with thermal depopulation of the Stark levels of the
NdIII ion.
Fitting of the magnetic data of 5 and 6 were performed using
the PHI program,53 which enables simultaneous fitting of the
χT(T) and M(μ0H) dependences, by taking account of the two
different exchange pathways, i.e., via a double symmetric
chloride bridge (J1) and a double, heterogeneous chlorido-
oxide bridge (J2). The intermolecular interactions between NdIII
ions in neighboring molecules are omitted because of the long
Nd···Nd distances (the shortest one is 10.303 Å in 5, and then
10.421 Å in 6). In the fitting procedure, we also considered the
terms of the spin−orbit constant λ of paramagnetic NdIII ions.
The least-squares fit of the experimental data by this expression
leads to the following results: g = 2.07, J1 = −0.87 cm−1, J1 =
−14.18 cm−1 for 5; g = 2.03, J1 = −0.81 cm−1, J1 = −13.82 cm−1
for 6. The discrepancy factors are 3.48 × 10−7 and 2.83 × 10−7
for 5 and 6, respectively. The calculated curve matches the
Direct and Alternating Current Susceptibilities. The
molar magnetic susceptibility was converted to the χMT product
(or effective magnetic moment). The temperature dependence
of χMT is shown in Figure 14 (left). The field dependence of the
magnetization per formula unit, M1 = Mmol/NAμB, at constant
temperature is shown in Figure 14 (right).
The octanuclear NdIII4ZrIV (5) and NdIII4HfIV (6)
4
4
complexes behaved magnetically as homonuclear tetramers
with SNd = 3/2 because ZrIV and HfIV ions are diamagnetic. The
values of χMT at room temperature, i.e., 7.99 cm3 mol−1 K for 5
and 7.10 cm3 mol−1 K for 6, are slightly higher than those
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Figure 13. TEM images of oxide materials prepared by calcination of 4 (a), 5 (b), and 6 (c) at 950 °C.
○
Figure 14. Direct current (dc) magnetic data for 5 and 6. Left: temperature dependences of χM ( , black and blue, respectively) and χMT (◑, black and
blue, respectively). Right: field dependence of magnetization per formula unit. Solid lines (in both graphs) were calculated using the HDVV spin
Hamiltonian and PHI program.
magnetic data well (solid lines in Figure 14). The obtained J
values confirm antiferromagnetic properties of similar magni-
tudes for both complexes and show good agreement with
magnetostructural correlations parameters, e.g., the large
bridging angles of Nd−Cl−Nd (107.9° for 5 and 108.31° for
6) and Nd−O−Nd (126.5° for 5 and 126.6° for 6), and Nd···Nd
distances (4.27 or 4.60 Å for 5 and 4.28 or 4.63 Å for 6).
Isothermal magnetization curves, M(H), were recorded at 2 and
5 K (Figure 14, left). Both curves show a sharp increase in
magnetization at the lower field before reaching μB values of 6.14
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for 5 and 6.51 for 6 at 5.0 T and 2 K. The reduced curves are not
perfectly superimposed, which suggests contributions from
excited levels.
= Zr and Hf) at 950−1500 °C gave the heterobimetallic oxide
nanomaterials La2Zr2O7, La2Hf2O7, Nd2Zr2O7, and Nd2Hf2O7.
The results of this study show that group 4−lanthanide
ethoxides are attractive starting materials for the production of
numerous advanced multicomponent oxide ceramics with a
large variety of chemical and physical properties. These
properties will enable further applications of these compounds
in energy storage and conversion, photocatalysis, electronic
devices, electrochemical components, thermal coatings, and
surface protection.
Although no hysteresis was observed for 5 and 6, the slow
magnetization relaxation dynamics were probed by performing
alternating current (ac) susceptibility measurements. The first
result obtained at T = 2.0 K for a set of representative
frequencies of the alternating field (f = 1.0, 11, 111, and 1116
Hz) by ramping the magnetic field from zero to Bdc = 1.0 T
(working amplitude Bac = 0.3 mT) showed that there was no
absorption signal for either complex (out-of-phase susceptibility
component χ′′) at zero field because of fast magnetic tunneling.
For 6, this component increased with increasing external field
passed through a maximum and then attenuated. This behavior
confirms that the 6 shows field-supported slow magnetic
relaxation. The position of the maximum depends on the
frequency f of the oscillating field (Figure S7). Subsequent
experiments for 6 were done for a fixed external magnetic field
Bdc = 0.4 T (at which there is a maximum of the high-frequency
signal) by changing frequency in the range of f = 0.1−1500 Hz
for a set of temperatures ranging between T = 1.8 and 7 K
(Figure S8). The in-phase (χM′) and out-of-phase (χM″)
components exhibit small-frequency dependences with the
application of an external field of 0.4 T, indicative the possibility
of slow relaxation of magnetization, although the maxima in χ″
are missing. The absence of the maxima precludes the direct
extraction of the energy barrier but it can be estimated if the
relaxation process is dominated by a single relaxation time and
the additive constant to the real part χ′ is vanishing. Further
precise ac measurements and analysis based on theoretical
calculations are needed to explain the effects of diamagnetic ions
on the presence and dynamics of relaxation processes and will be
performed in the nearest future.
EXPERIMENTAL SECTION
■
All reactions were performed under N2 and using standard Schlenk and
vacuum-line techniques. All reagents were obtained from commercial
sources: Ethanol, hexane, tetrahydrofuran, toluene, HgCl2, lanthanum,
and neodymium powder were obtained from Sigma-Aldrich, and
Cp2M′Cl2 (M′ = Ti, Zr, and Hf) was obtained from Alfa Aesar. Solvents
were purified using standard methods and distilled under N2 before use.
Hexane and toluene were purified by refluxing over sodium.
Tetrahydrofuran was distilled from sodium/benzophenone, and
ethanol was distilled over metallic magnesium. FTIR-ATR spectrosco-
py was performed with an FT-IR Nicolet 6700 (Thermo Scientific)
spectrometer. Elemental analysis was performed with a PerkinElmer
2400 CHN elemental analyzer. Chlorine analysis was performed by the
̈
Schoninger method using an Elementar Micro K analyzer. Metal ion
concentrations were determined by inductively coupled plasma optical
emission spectrometry (-iCAP 7400 Duo spectrometer, Thermo
Scientific). The observed slight differences between the theoretical
and found values of the elemental analysis for the received compounds
are due to the presence of EtOH ligands, which can be easily removed
during vacuum drying. TG-DSC/DTA was performed with a Netzsch
STA 449 F1 Jupiter system coupled with Bruker FTIR Tensor 27 and
QMS Aeolos spectrometers in an N2 atmosphere at a heating rate of 10
°C min−1. Thermal decomposition of the metal alkoxide precursors was
performed with an NT 1313 furnace (Neotherm) equipped with a
KXP4 thermostat in atmospheric air. The resulting oxide materials were
investigated by powder XRD (Empyrean, PANalytical); the powder
diffraction database PDF-4+ was used. The morphologies of the
resulting oxide materials were examined with an FEI Tecnai G2 20 X-
Twin TEM microscope equipped with a field-emission gun and
integrated energy-dispersive spectrometer (EDAX). For the TEM
observations, 200-mesh copper grids with lacey carbon films were used.
Signals from both copper and carbon (from the grids) were observed in
the TEM-EDX spectra. Variable-temperature (2−300 K) DC magnetic
susceptibility measurements under applied fields of 0.1 (T < 20 K) and
1.0 (T ≥ 20 K) kG and variable-field (0−5 T) magnetization
measurements at low temperatures (2 and 5 K) were performed with a
Quantum Design SQUID magnetometer. Raw magnetic susceptibility
data were corrected for the effects of underlying diamagnetism and the
sample holder.54 Magnetic measurements were performed by crushing
the crystals and restraining the sample to prevent displacement caused
by magnetic anisotropy. Single-crystal XRD data were collected using
an Xcalibur or Xcalibur Ruby diffractometer at 100 K for 1−5, 190 K for
6, and 150 K for 7.55 The experimental details and crystal data are
summarized in Tables S1 and S2. The structures were solved by direct
methods and refined by the full-matrix least-squares method on F2,
using the SHELXTL package.56 Non-hydrogen atoms were refined with
anisotropic thermal parameters. All hydrogen atoms were positioned
geometrically and added to the structure factor calculations, but were
not refined. Molecular graphics for the resulting structures were created
using Diamond (version 3.1e).57
CONCLUSIONS
■
We developed a simple and efficient strategy for the synthesis of
heterometallic oxide nanomaterials based on lanthanide and
group 4 elements. The nanomaterials were synthesized by
thermal decomposition of single-source molecular precursors
derived from the reaction of Cp2M′Cl2 (M′ = Ti, Zr, and Hf)
with metallic lanthanum or neodymium in the presence of
ethanol as a protonation agent. The six obtained compounds are
new examples of heterometallic group 4−lanthanide alkoxides,
31 examples of which have been previously reported.
Compounds 3 and 6 are the first reported heterometallic
alkoxide clusters of hafnium and lanthanide ions. The dc
susceptibility measurements on 5 and 6 showed the presence of
diamagnetic ZrIV and HfIV ions and weak antiferromagnetic
interactions between neighboring paramagnetic NdIII ions in the
Nd4O8Cl4 core together with thermal depopulation of the Stark
levels. A preliminary ac study indicated field-supported slow
magnetic relaxation. The obtained heterometallic compounds,
which have three different central core coordination arrange-
ments, were used as molecular models for investigation of solid-
state structural effects on the phase compositions, homoge-
neities, and morphologies of the synthesized nanomaterials.
Thermal decomposition of the isostructural compounds
[Ln2Ti4(μ4-O)2(μ3-OEt)2(μ-OEt)8(OEt)6(Cl)2(HOEt)2] (Ln
= La and Nd) at 950 °C gave La0.66TiO3 or a mixture of
Nd4Ti9O24 and Nd0.66TiO3. We showed that above 950 °C,
Nd4Ti9O24 decomposes to Nd0.66TiO3 and TiO2. Calcination of
[La2M′2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n and
[Nd4M′4(μ3-O)2(μ-OEt)10(μ-Cl)4(OEt)8(HOEt)10(Cl)2] (M′
S y n t h e s i s o f [ L a 2 T i 4 ( μ 4 - O ) 2 ( μ 3 - O E t ) 2 ( μ - O E -
t)8(OEt)6(Cl)2(HOEt)2] (1). A 150 mL Schlenk flask equipped with a
stir bar was charged with Cp2TiCl2 (0.75 g, 3.01 mmol), La powder (0.4
g; 2.9 mmol), HgCl2 (0.008 g; 0.029 mmol), ethanol (70 mL), and
toluene (20 mL). Then the reaction mixture was stirred under reflux for
35 h. During the reaction progress, the dark red solution slowly changed
color to green and finally light yellow. Then the resulting solution was
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filtered, and the filtrate was concentrated under vacuum to 20 mL.
Hexane (15 mL) was added and formed a layer over the solution.
Colorless crystals of 1 were grown at room temperature after 8 weeks.
The crystals were filtered off, washed with hexane (3 × 5 mL), and dried
under vacuum. Overall yield: 0.38 g (0.27 mmol, 36%). Anal. Calcd for
C36H92O20Cl2La2Ti4: C, 31.21; H, 6.69; Cl, 5.12; La, 20.05; Ti, 13.82.
Found: C, 32.07; H, 6.75; Cl, 5.16; La, 21.12; Ti, 13.59. IR (cm−1, ATR
diamond): 3331 (s), 2972 (s), 2929 (m), 2904 (m), 2871 (m), 1634
(w), 1476 (w), 1456 (w), 1380 (w), 1260 (m), 1095 (s), 1050 (vs), 915
(s), 881 (s), 799 (vs), 541 (s). Compound 1 is not soluble in most
conventional solvents or crystallizes immediately upon dissolution and
therefore was not investigated by NMR spectroscopy.
Synthesis of [La2Zr2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n
(2). A 150 mL Schlenk flask was charged with Cp2ZrCl2 (0.75 g, 2.57
mmol), ethanol (70 mL), and toluene (20 mL). The mixture was stirred
at room temperature within 1 h, and La powder (0.4 g; 2.9 mmol) and
HgCl2 (0.008 g; 0.029 mmol) were introduced. Then the reaction
mixture was stirred under reflux for an additional 36 h and was filtered
off. The yellow filtrate was concentrated under vacuum to 20 mL. Then
hexane (10 mL) was added, forming a layer over the solution. Colorless
blocklike crystals of 2 were grown at room temperature after 6−7
weeks. The crystals were filtered off, washed with hexane (3 × 5 mL),
and dried under vacuum. Overall yield: 0.62 g (0.54 mmol, 42%); Anal.
Calcd for C22H59O12Cl5La2Zr2: C, 22.91; H, 5.16; Cl, 15.37; La, 24.09;
Zr, 15.82. Found: C, 23.14; H, 5.21; Cl, 15.49; La, 24.99; Zr, 14.97. IR
(cm−1, ATR diamond): 3324 (vs), 2976 (w), 1609 (s), 1447 (vw),
1398 (vw), 1264 (vw), 1084 (vw), 1035 (w), 872 (vw), 802 (w), 653
(vw).
Synthesis of [La2Hf2(μ3-O)(μ-OEt)5(μ-Cl)(OEt)2(HOEt)4(Cl)4]n
(3). A 150 mL Schlenk flask was charged with Cp2HfCl2 (0.82 g, 2.16
mmol), ethanol (70 mL), and toluene (20 mL). The mixture was stirred
at room temperature within 1 h, and La powder (0.37 g; 2.66 mmol)
and HgCl2 (0.007 g; 0.025 mmol) were introduced. Then the reaction
mixture was stirred under reflux for an additional 48 h and was filtered
off. The light yellow filtrate was concentrated under vacuum to 30 mL,
and 10 mL of hexane was slowly added to the solution. Colorless
blocklike crystals of 3 were grown at room temperature after 4 weeks.
The crystals were filtered off, washed with hexane (3 × 5 mL), and dried
under vacuum. Overall yield: 0.47 g (0.35 mmol, 33%). Anal. Calcd for
C22H59O12Cl5La2Hf2: C, 19.90; H, 4.48; Cl, 13.35; La, 20.92; Hf, 26.89.
Found: C, 20.15; H, 4.56; Cl, 13.49; La, 21.12; Hf, 27.54. IR (cm−1,
ATR diamond): 3570 (m), 3555 (m), 3344 (vs), 1623 (m), 1262 (w),
1091 (w), 1017 (vw), 800 (w), 700 (m), 529 (w), 456 (vw). Crystalline
3 was not soluble in most conventional solvents and therefore it was not
possible to carry out an NMR study.
temperature and filtered. The filtrate was concentrated to 15−20 mL,
and 5 mL of hexane was added. Crystals of 6 were formed after 12 weeks
of crystallization at room temperature. Overall yield: 0.81 g (0.33 mmol,
51%). Anal. Calcd for C56H150O30Cl6Nd4Zr4: C, 27.36; H, 6.15; Cl,
8.65; Nd, 23.47; Zr, 14.84. Found: C, 27.54; H, 6.27; Cl, 8.73; Nd,
23.54; Zr, 14.76. IR (cm−1, ATR diamond): 3326 (vs), 2965 (m), 1614
(m), 1447 (w), 1402 (w), 1381 (w), 1260 (m), 1148 (w), 1071 (m),
1038 (m), 873 (w), 795 (m), 655 (w). Compound 5 is not soluble in
most conventional solvents or crystallizes immediately upon dis-
solution and therefore was not investigated by NMR spectroscopy.
S y n t h e s i s o f [ N d 4 H f 4 ( μ 3 - O ) 2 ( μ - O E t ) 1 0 ( μ -
Cl)4(OEt)8(HOEt)10(Cl)2] (6). A 150 mL Schlenk flask equipped with
a stir bar was charged with Cp2HfCl2 (0.82 g, 2.16 mmol), ethanol (70
mL), and toluene (20 mL). The mixture was stirred at room
temperature within 0.5 h, and Nd powder (0.38 g; 2.66 mmol) and
HgCl2 (0.007 g; 0.025 mmol) was added. Then the reaction was stirred
under reflux for an additional 45 h and then was filtered off. The yellow
filtrate was concentrated under vacuum to 30 mL, and 10 mL of hexane
was slowly added to the solution. Colorless crystals of 6 were grown at
room temperature after 6 weeks. The crystals were filtered off, washed
with hexane (3 × 5 mL), and dried under vacuum. Overall yield: 0.63 g
(0.22 mmol, 42%). Anal. Calcd for C56H150O30Cl6Nd4Hf4: C, 23.96; H,
5.39; Cl, 7.58; Nd, 20.55; Hf, 25.43. Found: C, 24.22; H, 5.43; Cl, 7.71;
Nd, 20.95; Hf, 25.13. IR (cm−1, ATR diamond): 3333 (m), 2969 (m),
2930 (w), 2904 (w), 2873 (w), 1614 (w), 1476 (vw), 1445 (w), 1383
(w), 1260 (m), 1154 (w), 1080 (m), 1037 (vs), 874 (w), 797 (m).
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02478.
IR spectroscopic and crystallographic data for 1−7 and
field dependence of the ac susceptibility for 5 and 6
(PDF)
Accession Codes
CCDC 2014396−2014401 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
S y n t h e s i s o f [ N d 2 T i 4 ( μ 4 - O ) 2 ( μ 3 - O E t ) 2 ( μ - O E -
t)8(OEt)6(HOEt)2(Cl)2] (4). A 150 mL Schlenk flask equipped with a
stir bar was charged with Cp2TiCl2 (0.75 g, 3.01 mmol), Nd powder
(0.42 g; 2.9 mmol), HgCl2 (0.008 g; 0.029 mmol), ethanol (70 mL),
and toluene (20 mL). Then the reaction mixture was stirred under
reflux for 28 h. During the reaction progress, the dark red solution
slowly changed color to light yellow. Then the resulting solution was
filtered, and the filtrate was concentrated under vacuum to 20 mL.
Hexane (10 mL) was added and formed a layer over the solution.
Colorless crystals of 4 were grown at room temperature after several
weeks. The crystals were filtered off, washed with hexane (3 × 5 mL),
and dried under vacuum. Overall yield: 0.34 g (0.24 mmol, 32%). Anal.
Calcd for C36H92O20Cl2Nd2Ti4: C, 30.97; H, 6.64; Cl, 5.08; Nd, 20.67;
Ti, 13.72. Found: C, 31.17; H, 6.69; Cl, 5.19; Nd, 21.23; Ti, 13.66. IR
(cm−1, ATR diamond): 3568 (s), 3548 (s), 3366 (vs), 2974 (s), 2933
(m), 1634 (m), 1508 (w), 1489 (w), 1475 (w), 1459 (vw), 1380 (w),
1260 (w), 1094 (m), 1048 (s), 943 (m), 881 (m), 799 (m), 720 (s), 556
(vs). Compound 4 for the same reason as 1 was not investigated by
NMR spectroscopy.
AUTHOR INFORMATION
Corresponding Authors
■
Rafał Petrus − Faculty of Chemistry, Wrocław University of
Science and Technology, 50-370 Wrocław, Poland; orcid.org/
0000-0001-7572-3252; Email: rafal.petrus@pwr.edu.pl
Piotr Sobota − Łukasiewicz Research Network − PORT Polish
Center For Technology Development, 54-066 Wrocław, Poland;
orcid.org/0000-0002-4364-2957; Email: piotr.sobota@
port.org.pl
Authors
Katarzyna Chomiak − Łukasiewicz Research Network − PORT
Polish Center For Technology Development, 54-066 Wrocław,
Poland
́
Jozef Utko − Faculty of Chemistry, University of Wrocław, 50-383
Wrocław, Poland
́
Alina Bienko − Faculty of Chemistry, University of Wrocław, 50-
S y n t h e s i s o f [ N d 4 Z r 4 ( μ 3 - O ) 2 ( μ - O E t ) 1 0 ( μ -
Cl)4(OEt)8(HOEt)10(Cl)2] (5). A Schlenk flask was charged with
Cp2ZrCl2 (0.75 g, 2.57 mmol), ethanol (70 mL), and toluene (20
mL) and stirred by 0.5 h. Then Nd powder (0.38 g; 2.66 mmol) and
HgCl2 (0.008 g; 0.025 mmol) were added, and the mixture was stirred
under reflux for 56 h. The resulting cloudy solution was cooled to room
383 Wrocław, Poland
Tadeusz Lis − Faculty of Chemistry, University of Wrocław, 50-
383 Wrocław, Poland
Complete contact information is available at:
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Author Contributions
The manuscript was written through the contributions of all
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support by the Polish
National Science Center (grant numbers 2017/26/D/ST5/
01123 and 2018/29/B/ST5/00341).
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