Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, prepared: Via a one-pot solvent-mediated method

Article abstract of DOI:10.1039/c9dt01967g

Two new derivatives of scandium borohydride, MSc(BH₄)₄, M = Rb, Cs, were prepared via two different synthetic methodologies-mechanochemical and solvent-mediated. The latter led to products free from the commonly present halide contamination, as evidenced by powder X-ray diffraction, FTIR spectroscopy and TGA/DSC/MS. The rubidium derivative crystallizes in an orthorhombic unit cell of the Pbcm space group in the structure which can be derived from ht-CrVO₄, while CsSc(BH₄)₄ adopts a monoclinic (P2₁/c) unit cell which has monazite (CePO₄) as a structural aristotype. Thermal decomposition of the samples obtained using the two methods was compared, evidencing the influence of lithium chloride on the decomposition reactions as well as chemical identity of the decomposition products. Uncontaminated MSc(BH₄)₄ salts decompose thermally yielding nearly pure hydrogen with the maximum decomposition rate at 230 °C and 235 °C, for M = Rb and Cs, respectively. Among the by-products of the solvent-mediated synthesis, a new cubic crystalline phase of M₃ScCl₆, M = Rb, Cs, has been detected.

Full text of DOI:10.1039/c9dt01967g

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DOI: 10.1039/C9DT01967G
ARTICLE
The two new derivatives of scandium borohydride, MSc(BH4)4,
M = Rb, Cs, prepared via a one-pot solvent-mediated method†
Agnieszka Starobrat,*a,b Tomasz Jaroń*b and Wojciech Grochalab
Received 00th January 20xx,
Accepted 00th January 20xx
Two new derivatives of scandium borohydride: MSc(BH4)4, M = Rb, Cs, were prepared via two different synthetic
methodologies – mechanochemical and solvent-mediated. The latter led to the products free from the commonly present
halide contamination, as evidenced by powder X-ray diffraction, FTIR spectroscopy and TGA/DSC/MS. Rubidium derivative
crystallizes in an orthorhombic unit cell of Pbcm space group in the structure which can be derived from ht-CrVO4, while
CsSc(BH4)4 adopts a monoclinic (P21/c) unit cell which has monazite (CePO4) as structural aristotype. Thermal decomposition
of the samples obtained using the two methods was compared, evidencing the influence of lithium chloride on the
decomposition reactions as well as chemical identity of the decomposition products. Uncontaminated MSc(BH4)4 salts
decompose thermally evolving nearly pure hydrogen with the maximum decomposition rate at 230 °C and 235 °C, for M =
Rb and Cs, respectively. Among the by-products of the solvent-mediated synthesis, the new cubic crystalline phase of
M3ScCl6, M = Rb, Cs has been detected.
DOI: 10.1039/x0xx00000x
hydrogen release, such as e.g. temperature and purity, are
significant. Taking all the parameters into account, a lot of
material aspects and features still have to be improved to make
chemical storage of hydrogen a true and competitive
alternative for the durable and possibly lightweight tanks
containing the highly-compressed gas.
Introduction
Lately, the subject of clean and reusable energy sources is
receiving much attention. A lot of effort is made to damp
carbon-based fuels and decrease the amount of CO2 released to
the atmosphere. Therefore, numerous alternative means of
energy production and storage are being developed, among
which those utilizing hydrogen as an energy carrier seem
particularly promising. Although pure H2 reveals impressively
high gravimetric energy density (heat of combustion of 120 MJ
kg–3 as compared to ca. 44 MJ kg-3 for gasoline), the energy
stored in a volume unit is unacceptably low under normal
conditions, and even for the liquefied hydrogen fuel it is ca. 3.5
times smaller than for gasoline1. Therefore, one of the
challenges that still needs to be overcome is creating an
effective storage system that can be used in the on-board
applications such as vehicle tanks. Although compression of
hydrogen seems to be the simplest solution, the alternative
techniques of storage are scrutinized in order to improve the
achievable parameters, which includes storage of hydrogen in
the hydrogen-rich chemical compounds 2,3. However, a high
hydrogen content alone is not enough for a material to fulfill all
the economically-based requirements listed by The U.S.
Department of Energy4. Along with the required gravimetric
(6.5 wt%) and volumetric (50 g/L) capacity, the parameters of
For about the two last decades a large number of hydrogen-
rich substances have been synthesized and widely characterized
in the context of their potential application as hydrogen storage
materials. Consequently, the inorganic borohydrides became
one of the most intensely investigated groups of materials5.
Interestingly, although many monometallic borohydrides of the
formula M(BH4)x are known, plain Sc(BH4)3 is still elusive. The
dry mechanochemical methods targeting this compound
resulted instead in bimetallic borohydrides, MSc(BH4)4, where
M = Li–K 6–9. Their rubidium or cesium analogues have not yet
been reported. Lately, the new polymorph of LiSc(BH4)4, namely
β-LiSc(BH4)4, has been detected among the products of room-
temperature decomposition of NH4Sc(BH4)4 in the presence of
LiCl 10.
One serious difficulty often encountered during preparation
of bimetallic scandium borohydrides, especially via dry
mechanochemical approach, is the purity of prepared samples.
The mechanochemical synthetic methods used so far 6–9 led to
the products contaminated with a significant amount of metal
chlorides, and it is uncertain how their presence affects the
thermal decomposition profiles of these samples. Even the
convenient synthetic method towards pure bimetallic
borohydrides from Jaroń et al. 11–13 has resulted in the product
which still contained up to 12 wt.% of LiCl when tested for
LiSc(BH4)4 14. Although this is a substantial improvement over
the mechanochemical method (53.3 wt.% of LiCl), the level of
impurities remains unsatisfactorily high especially when
a. College of Inter-Faculty Individual Studies in Mathematics and Natural Sciences
(MISMaP), University of Warsaw, Banacha 2c, 02-097 Warsaw, Poland.
b. Centre of New Technologies, University of Warsaw, Banacha 2c, 02-097 Warsaw,
Poland.
E-mail: a.starobrat@cent.uw.edu.pl , t.jaron@cent.uw.edu.pl
† This manuscript is dedicated to prof. Grzegorz Chałasiński at his 70th birthday.
Electronic Supplementary Information (ESI) available: The additional PXRD, FTIR,
EGA data, structural details and Rietveld plots. See DOI: 10.1039/x0xx00000x
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compared to the other bimetallic borohydrides synthesized in most important samples prepared in this work were
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the same way 14.
summarized in Table 1.
DOI: 10.1039/C9DT01967G
This motivated us for testing a different solvent-mediated
method with a dimethyl sulfide (DMS) applied as a solvent.
DMS, being a soft base (in Pearson sense), does not form the
extremely stable adducts with the hard metal cations, therefore
it often can be removed under vacuum at the temperatures
below those initiating thermal decomposition of the
borohydrides. DMS or its derivatives (such as S(CH3)2BH3) have
been recently used for preparation of several bimetallic
Table 1. The samples investigated in this work.
Reagents
Products (A)
Products (B)
ScCl3+3.15LiBH4+RbBH4; milling
ScCl3+3.05RbBH4; DMS
RbSc(BH4)4 + 3LiCl +0.15LiBH4*
RbSc(BH4)4
#
0.39Rb3ScCl6 + RbBH4
CsSc(BH4)4 + 3LiCl +0.15LiBH4*
ScCl3+3.15LiBH4+CsBH4; milling
ScCl3+3.05CsBH4; DMS
#
CsSc(BH4)4
0.38Cs3ScCl6 + CsBH4
*nominal composition; #composition from Rietveld analysis
borohydrides, e.g. by Zanella et al. 15, Filinchuk et al. 16, Richter
17
et al.
or Ley et al. 18,19, resulting in several adducts:
Mg(BH4)2·½S(CH3)2, Mn(BH4)2·½S(CH3)2 and RE(BH4)3·nS(CH3)2,
RE-rare earth metals, respectively, which can be transformed to
the corresponding solvent-free borohydrides during prolonged
heating to 75–140 oC. Recently, a series of numerous
borohydrides containing mono-, di- and tri-valent metals has
been prepared in reactions of their hydrides with BH3·S(CH3)2
20,21
complex
which further supported usefulness of the DMS-
mediated method for synthesis of thermally stable single-metal
borohydrides of high purity.
In the current work, we have tested a similar synthetic
approach resulting in the two bimetallic scandium borohydrides
– RbSc(BH4)4 and CsSc(BH4)4 of high purity. It appeared that
these borohydrides can be readily obtained without a need of
desolvation using only a simplified setup for synthesis.
Mechanochemical synthesis approach was used for
comparison. The newly prepared borohydrides were
characterized using FTIR spectroscopy, powder X-ray
diffraction, DFT calculations and thermal analysis
(TGA/DSC/MS).
Figure 1. The H-shaped reaction vessel used for the DMS-mediated synthesis.
FTIR spectroscopy
The Fourier Transform Infrared (FTIR) spectra of the samples
were investigated using Vertex 80v FTIR spectrometer (Bruker).
200 mg of dry KBr has been used as a pellet material. The
spectra were collected in 400-4000cm-1 range as an average of
16 scans.
PXRD and structure solution
Powder X-ray diffraction (PXRD) patterns were collected using
Bruker D8 Discover diffractometer (Cu Kα1 and Kα2 radiation in
ca. 2:1 intensity ratio). The samples were placed in quartz
capillaries of 0.3–0.5 mm diameter, and all the measurements
were performed at room temperature.
Experimental
Synthesis
The samples were prepared, manipulated and stored in an inert
gas (Ar) atmosphere of the MBRAUN Labmaster DP glovebox
(O2 < 1ppm, H2O < 1ppm) or under vacuum if required in the
particular step of synthesis. The high energy milling has been
performed using a stainless-steel vessel. The overall milling time
was 60 min, divided into the 5 min periods, between which the
vessel was cooled with liquid nitrogen. The anhydrous fine
quality ScCl3 (ABCR), LiBH4 (Sigma-Aldrich) and MBH4, M = Rb,
Cs (Katchem) were used as the reagents and milled in the ca.
1:3.15:1 ratio, respectively. The solvent mediated syntheses
have been performed in the H-shaped glass vessels, with a frit
filter between the arms, using a vacuum-argon line, Fig. 1. MBH4
and ScCl3 in the ratio of ca. 3.05:1 were suspended in a few
milliliters of dimethyl sulfide (Sigma-Aldrich) on one side of the
vessel (B). The so obtained mixture was stirred for a few days at
room temperature, then decanted through the filter to the side
(A) of the H-vessel. After the filtration the solvent has been
distilled to the side (B) using liquid nitrogen and these
extraction-distillation processes were repeated several times.
Finally, the DMS has been removed from the H-vessel and the
obtained powder was dried under vacuum (<1·10-3 mbar). The
The diffraction patterns of the new phases were indexed
22
using X-Cell and then the structures were solved in the real
space using FOX 23. The initial structural models obtained this
way were refined (Rietveld method) in Jana2006 24. Several
restraints concerning hydrogen atoms were defined including
fixing of the BH4 groups as the hydrogen tetrahedra around the
boron atoms (with H–B–H angles of 109.47(1)° and B–H
distances of 1.15(1) Å). The BH4 tetrahedra were enforced to
coordinate Sc atoms with three equidistant hydrogen atoms (12
equal Sc–H distances with s.u. 0.01 Å). The atomic displacement
parameters (ADP) of the H atoms were restrained as 1.2 times
larger than those of the respective B atoms, according to the
riding model. In the case of CsSc(BH4)4 a few more restrictions
were needed e.g. the Sc–B distances (2.30(1) Å) or B–Sc–B
angles (109.47(1)°).
DFT calculations
The DFT calculations have been performed in the CASTEP code
37. We have used ultrasoft pseudopotentials for approximation
of the core electrons and generalized gradient approximation of
Perdew Burke and Ernzerhof functional for solids (PBEsol) 38. A
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plane-wave basis set with 800 eV cutoff energy was applied, that it binds additional borohydride ligands prVeieswenArtticilenOntlhinee
DOI: 10.1039/C9DT01967G
while the Brillouin zone has been sampled with the k-point reaction pot.
separation of 0.07 Å-1. The SCF convergence criterion has been On the side B the mixed-cation chlorides, M3ScCl6, together with
set to 5·10-7 eV/atom. The structures were optimized using the unreacted MBH4 were detected, Fig. 2(b) and S11(b). The
BFGS algorithm with unit cell parameters fixed at the identified products implicate the following equation of the
experimental values. During the structure optimization, the reactions performed in DMS, written according to the used ratio
following convergence criteria have been used: the maximum of reactants:
ionic displacement tolerance of 5·10-4 Å, the maximum ionic
force of 0.01 eV Å-1 and the maximum energy change of 5·10-6 The molar ratio of the corresponding products calculated during
ScCl3 + 3MBH4 → 0.5MSc(BH4)4 + MBH4 + 0.5M3ScCl6.
(1b)
eV/atom.
Rietveld refinement slightly deviates from the value expected
TGA/DSC/MS analyses
on the basis of equation (1b), i.e. RbBH4:Rb3ScCl6 = 1:0.39 and
Thermal decomposition of RbSc(BH4)4 and CsSc(BH4)4
synthesized using both methods was investigated in the 20–
650 °C temperature range using a combined thermogravimetric
analyzer (TGA) and differential scanning calorimeter (DSC)
(Netzsch STA 409). The samples placed in Al2O3 capped crucibles
were heated at the rate of 5 °C min–1 at Ar (99.999 %) flow of 80
ml min–1. The gases evolved during decomposition were
analyzed using a quadrupole mass spectrometer QMS 403 C
(Pfeiffer Vacuum). The solid decomposition products were
characterized by heating the samples to a particular
temperature and subsequent cooling down freely to the RT.
Figure 2. PXRD patterns of the RbSc(BH4)4 samples obtained via solvent-mediated (a)
and mechanochemical (c) synthesis and the by-products of solvent-mediated
synthesis (b); * - LiCl , ^ - RbBH4, unmarked - RbSc(BH4)4 or Rb3ScCl6, respectively. The
background has been removed for clarity.
Results and discussion
CsBH4:Cs3ScCl6 = 1:0.38, which could be caused by only partial
crystallinity of the M3ScCl6 phases (the FWHM of the halide
phases vary depending on the sample, indicating sample-
dependent size of crystallites). The quantitative phase analysis
Synthesis and analysis of the products
We have tested the two different synthetic approaches towards
the borohydrides containing scandium – a solvent-mediated
and a mechanochemical. Initially we aimed at the monometallic
Sc(BH4)3 in the DMS-mediated syntheses. Therefore, we have
used the respective ratio of precursors, according to the
equation (1a) describing the assumed reaction:
–
could also be influenced by partial substitution between BH4
and Cl– anions, a process observed in similar systems 25. The
equation (1b) indicates that the reactions proceed according to
the de facto 1ScCl3:2MBH4 stoichiometry, as in equation (1):
ScCl3 + 3MBH4 → Sc(BH4)3 + 3MCl,
(1a)
ScCl3 + 2MBH4 → 0.5MSc(BH4)4 + 0.5M3ScCl6.
(1)
where M = Rb, Cs. The post-reaction suspensions have been
filtered, and the solid residues have been washed several times
with pure DMS. Subsequently, the solvent has been evaporated
at room temperature under vacuum. This procedure allowed to
divide the mixture of the products to those easily soluble (side
A) and insoluble in DMS (side B). However, the products
identified on a basis of the PXRD (Fig. 2 and S11) and the FTIR
results (Fig. S3 and S4 in ESI) appeared different than those
expected from the eq. (1a). On the side A the solvent-free
mixed-cation borohydrides, MSc(BH4)4, have been detected as
the only crystalline phases. Contrary to the previously reported
Similar observations have been reported for mechanochemical
synthesis of NaSc(BH4)4 and KSc(BH4)4 from ScCl3 and
corresponding MBH4, where also the M3ScCl6 were detected as
the only side products 8,9. Such mixed-metal halides are formed
in addition reactions instantly occurring between the initially-
formed LiCl and remaining ScCl3, as it has been discussed before
26
.
For comparison we have performed the mechanochemically
induced reactions according to the following equation:
ScCl3 + MBH4 + 3LiBH4 → MSc(BH4)4 + 3LiCl.
(2)
As it can be seen in the PXRD patterns in Fig. 2 and Fig. S11, the
samples prepared using both synthetic methodologies contain
the same crystalline phases of MSc(BH4)4, however a significant
amount of LiCl is present in the mechanochemically prepared
samples. Formation of LiCl as the only side product instead of
mixed-metal halides resembles the situation observed in
mechanochemical synthesis of LiSc(BH4)4 via reactions of ScCl3
and LiBH4 in various ratios 6,7. Due to larger crystallites present
in the samples prepared in the solvent-mediated reactions,
manifested in significantly narrower diffraction peaks, these
PXRD patterns were selected for the Rietveld refinements. The
crystal structures of the new crystalline phases solved from the
DMS-mediated synthetic protocols towards the single-cation
15–19
borohydrides
,
the inconvenient high-temperature
desolvation appeared unnecessary for preparation of
MSc(BH4)4. Clearly, the hard Sc3+ center is well shielded in the
tetrahedral complex anion, [Sc(BH4)4]–, and binding the soft
sulfur donor present in the solvent molecules is not
energetically favorable. Such behavior of scandium borohydride
complex remains in the strong contrast with its heavier yttrium
analogue, as Y(BH4)3·DMS – here, a solvate of monometallic
borohydride forms instead of RbY(BH4)4 in reaction of YCl3 with
the three equivalents of RbBH4, Fig. S14 and S15. Clearly, Lewis
acidity of a hypothetical in situ formed Sc(BH4)3 is so substantial,
This journal is © The Royal Society of Chemistry 20xx
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PXRD data, i.e. MSc(BH4)4 and M3ScCl6, M = Rb, Cs, will be six Rb atoms, similarly to the case of NaSc(BH4)4. However,
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discussed in a separate section (3c).
FTIR spectra
these ScRb6 trigonal prisms are significanDtlOyIi:r1r0e.1g0u3l9a/rC,9tDhTe0re19f6o7rGe
not all the centers of the BH4 groups sit inside them, in contrary
–
The results of FTIR measurements are compatible with those
expected for the targeted stoichiometries and revealing
virtually identical features for the samples prepared using both
methods of synthesis, Fig. S3 and S4 in ESI. The observed
absorption bands very well agree with those reported for the
other MSc(BH4)4, M = Li, Na, K 27. In the range of δH–B–H bands
(1000–1300 cm-1) two specific bands can be observed at ca.
1111 and 1185 cm-1 while in the νB–H region (2100–2600 cm-1) a
,
Figure 3. The unit cells of CsSc(BH4)4 (left) and RbSc(BH4)4 (right).
characteristic set of bands with the two particularly strong at ca.
2494 and 2235 cm-1 appeared. It is also worth noticing that no
significant signals of impurities can be seen in the spectra
(besides the background compensation artifacts at 1385 cm-1)
what is another prerequisite for purity of solvent-mediated
synthesis products.
to the previously known mixed-cation borohydrides of
scandium in which the [Sc(BH4)4]– anions can be found inside
the polyhedra composed of cations 5 (Fig. 4).
At the same time, in the cesium derivative each ion, either Cs+
or [Sc(BH4)4]–, has the three counterions as the nearest
neighbors, with the Cs–Sc distance of 4.788(11)–4.993(7) Å, all
four atoms laying within the same plane. The cations form
rather irregular octahedra with three further neighbors (5.685–
5.787(8) Å) around anions and vice versa. Again, not all the
Crystal structures‡
RbSc(BH4)4 adopts an orthorhombic unit cell of Pbcm space
group, while CsSc(BH4)4 – a monoclinic P21/c, Fig. 3, for the
Rietveld plots see ESI, Fig. S1 and S2. All the structural details of
the known inorganic mixed-cation scandium borohydrides are
presented in Table 2. The molecular volumes in the obtained
structures follow the monotonic trend observed before for
MSc(BH4)4, namely V/Z(Na) < V/Z(K) < V/Z(NH4) < V/Z(Rb) <
V/Z(Cs). Interestingly, the first known polymorph of lithium
derivative, α-LiSc(BH4)4, containing separate layers of ions along
the c-axis, does not precisely fit into this series due to its
anomalously large molecular volume, and contrary to the more
dense β-LiSc(BH4)4, recently detected during thermal
decomposition of NH4Sc(BH4)4 in the presence of LiCl and LiBH4
–
centers of the BH4 groups are contained within the cationic
octahedra.
To support our experimental structures, we have performed
DFT optimizations of both MSc(BH4)4 with their unit cells fixed,
cf. ESI for the calculated structural parameters. While the
computed positions of heavy atoms differ only slightly from the
experimental results, the hydrogen atoms are shifted in more
pronounced way, especially for CsSc(BH4)4, Fig. S16 and S17.
The interatomic distances, however, diverge within only a few
percent, cf. Table S1.
10
.
The ternary scandium halides, M3ScCl6, M = Rb, Cs, detected
as the side products of the DMS-mediated reaction, eq. (1),
crystallize in a cubic, Fm-3m space group, in the elpasolite-type
structure, Fig. S10. While a number of fluorides containing +3
metal ions adopt such structure, which include Na3AlF6 or
K3AlF6, it is less-common for the chlorides. The elpasolite-type
structure has been reported as the high-temperature phase for
few ternary chlorides of rare-earth metals and rubidium or
cesium, e.g. Rb3YCl6 or Cs3YCl6, above 405 and 390 °C,
respectively 31. Considering the ternary halides containing
scandium and rubidium or cesium adopting a general formula
of M3ScX6, only one compound has been reported for M = Cs
and X = Cl (measured at 100 K), which is isostructural to the
room-temperature C2/c polymorph of Rb3YCl6 32. Both the
monoclinic and the elpasolite structures of M3ScCl6 are built of
the [ScCl6]3– octahedra, however while the monoclinic
polymorph contains the two independent Sc positions, only one
symmetry independent Sc atom is present in the cubic form. We
have not applied any restrains during the refinement of these
structures. Therefore, while the Sc–Cl distance of 2.46(2) Å in
Rb3ScCl6 is rather typical for the chlorides of scandium, the
precise position of the Cl– ions in Cs3ScCl6 are slightly biased by
weaker diffraction data due to stronger absorption, resulting in
the Sc–Cl distance of 2.65(11) Å.
As it has been observed for all the known homoleptic
borohydride compounds of scandium, both structures are
composed of the alkali metal cations and the complex
[Sc(BH4)4]– anions, Fig. 3 and 4. Despite some similarities
between the Na, K and Rb derivatives, and NH4 and Cs
compounds, respectively, significant differences in their
cationic and anionic sublattices rationalize that those structures
cannot be considered as fully isostructural. However, on more
general level of considerations, the structures of NaSc(BH4)4 and
RbSc(BH4)4 share the topology of ht-CrVO4, which can be
treated as the structural aristotype of both of them. On the
other hand, monazite, CePO4, is the structural aristotype of
CsSc(BH4)4. It is worth to mention that monazite remains closely
related to scheelite, CaWO4, which is the structural aristotype
28
of both CsY(BH4)4
and ht-CsAl(BH4)4 29. A systematic
classification of the structures of borohydrides on the basis of
topology and structural prototypes can be found in the review
by Černý and Schouwink 30.
In the case of RbSc(BH4)4 the Rb+ cations and the [Sc(BH4)4]–
anions (with the Sc atom being taken as anion center) create
zigzag lines of alternate ions along the c-axis with the two Rb–
Sc distances of 4.487(4) Å (Fig. S5). Considering also the slightly
further Rb+ cations, with the Rb–Sc distances of 5.374–5.512(4)
Å, the Sc atoms lay in the center of trigonal prisms composed of
Thermal decomposition
4 | J. Name., 2012, 00, 1-3
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The samples of MSc(BH4)4, M = Rb and Cs, were investigated both pure, as well as the halide-containing samples of both
View Article Online
DOI: 10.1039/C9DT01967G
using thermal methods (DSC/TGA/EGA) in the temperature RbSc(BH4)4 and CsSc(BH4)4 to compare the influence of common
range from RT to 650 °C. Thermal decomposition of the impurities on the decomposition routes.
scandium based mixed-cation borohydrides has been
Pure RbSc(BH4)4, as prepared in the DMS-mediated
investigated so far only for the samples obtained directly in synthesis according to eq. (1), contains 8.5 wt% of hydrogen,
mechanochemical synthesis, which were significantly while the sample prepared mechanochemically according to eq.
contaminated with the metal halides and contained only ca. 18– (2) is contaminated with ca. 40 wt% of LiCl. Taking these values
40 wt% of MSc(BH4)4 , M = Li–K. Therefore we have investigated into account, it can be deduced that in the latter case nearly all
Table 2. Comparison of unit cell parameters and selected interatomic distances of yet known scandium borohydride derivatives of monovalent cations, MSc(BH4)4, M = α-Li, β-Li,
Na, K, NH4, Rb, Cs. All the measurements were taken at room temperature, except M = NH4 which was measured at 240 K.
MSc(BH4)4
Reference
Space group
Z
α-Li
7
β-Li
10
Na
8
K
9
NH4
10
Rb
this work
Pbcm
Cs
this work
P21/c
P-42c
2
I4/m
8
Cmcm
4
Pnma
4
P21/c
4
4
4
a [Å]
6.075929(55)
6.075929(55)
12.03384(14)
90
14.2842(4)
14.2842(4)
7.3758(2)
90
8.170(2)
11.875(3)
9.018(2)
90
11.8558(47)
7.7998(34)
10.1258(63)
90
7.8857(10)
11.8920(16)
12.8350(16)
127.202(4)
958.7(2)
239.675
7.6514(10)
11.1821(14)
11.2443(14)
90
9.587(2)
10.727(3)
12.228(3)
126.351(3)
1012.8(5)
253.2
b [Å]
c [Å]
β [°]
V [Å3]
V/Z [Å3]
444.25
1504.96(8)
188.12
874.92
218.73
936.36
962.1(2)
240.525
222.125
234.09
1.91452(0)-
2.55363(0)
2.10(3)-
2.133(10)
2.153(17)-
2.174(17)
2.03(12)-
2.44(13)
2.213(12)-
2.22(3)
Sc–H [Å]
2.255(8)
2.202(7)
4.788(11)-
4.993(7);
5.685-5.787(8)
3.796(13)-
4.702(19)
4.768(8)-
4.978(4)
4.91(2)-
5.548(13)
4.487(4);
5.374-5.512(4)
Cat–Sc [Å]
4.644(8)
4.69(3)- 5.75(3)
2.199(5)-
2.221(10)
2.250(16)-
2.270(17)
2.307(13)-
2.322(18)
Sc–B [Å]
2.28024(0)
2.27(2)- 2.38(4)
3.51(5)- 3.95(3)
2.360(8)
2.300(3)
2.540(4)-
3.987(16)
2.41(3)-
2.837(11)
2.935(13)-
3.217(12)
3.36(3)-
3.725(17)
3.494(7)-
3.806(6)
3.630(8)-
4.165(11)
Cat–B [Å]
Figure 4. M+ coordination polyhedra around Sc(BH4)4– anions in MSc(BH4)4, M = (a) Li (with simplified Li positions) - square prisms, (b) Na - triangular prisms, (c) K - mono-capped
triangular prisms, (d) NH4 - irregular nonahedra, (e) Rb – trigonal prisms and (f) Cs – irregular octahedra. M atoms in purple, Sc - pink, B - green, H - cream.
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hydrogen has been evolved during a single step of
decomposition, around 220 oC (8.37 wt% mass loss as
normalized to the pure RbSc(BH4)4). Only the metal halides, i.e.
RbLiCl2 and LiCl were detected using PXRD in this sample heated
up to 650 °C and slowly cooled down. Kim et al. 33 presented a
theoretical route of LiSc(BH4)4 decomposition to LiH, ScB2 and
ScB12, while Černý et al. proposed decomposition routes of its
View Article Online
DOI: 10.1039/C9DT01967G
8,9
Na and K analogues resulting in the amorphous ScBx and B
among the final decomposition products. However, the first of
the reported decomposition paths did not include chloride
contamination as an affecting factor. Metal borides (mainly di-
or tetraborides) were detected among the solid decomposition
products of several rare earth borohydrides 34. Considering ours
and the previously reported findings for the similar systems, we
propose the following idealized equation describing thermal
decomposition of the sample containing LiCl:
Figure 5. TGA/DSC plots of RbSc(BH4)4 samples obtained via solvent-mediated (blue)
and mechanochemical (red) synthesis. Grayed area at low temperatures is artifactual.
The decomposition route of RbSc(BH4)4 terminated before the
last event can be described with the following idealized
equation:
RbSc(BH4)4 → RbBH4 + 6H2 + B + ScB2
(4a).
RbSc(BH4)4 + 3LiCl → RbLiCl2 + LiCl + 15/2 H2 + ScBx + (4-x)B + LiH
(3).
In this case the mass loss assumed according to eq. (4a) is 6.4
wt%, what compared with observed drop of 7.7 wt% suggests
coexisting minor route, which could be expressed as:
The DSC curve reveals the three significant endothermic events
(Fig. 5). As the first event at about 159 °C associated with
virtually no mass loss, it can be assigned either to melting or a
phase transition in the solid state. The main decomposition step
is related to a double peak with the maxima at 232 °C and
238 °C. These two DSC events occur with a significant amount
of released gas, which, according to the time-resolved MS,
consists mainly of hydrogen but some amount of boranes can
also be detected (the MS signals albeit of at least one order of
magnitude smaller intensity as compared to H2, see Fig. S6 in
ESI). The last DSC peak at ca. 286 °C may be assigned to
formation of the final decomposition products as no clear mass
loss step is visible, while only barely detectable signals of
boranes are observed in the mass spectra of the evolved gas.
Thermal decomposition of pure RbSc(BH4)4 reveals
somewhat different features, Fig. 5. The first DSC event
observed for the mechanochemically-prepared sample is now
preceded by a shoulder at ca. 147 °C, which corresponds to
evaporation of the traces of DMS remaining in the sample
probably as an occlusion (Fig. S7 in ESI). Again, the main
decomposition step at 237 °C results in emission of H2 only
slightly contaminated with BxHy, however this process is related
to a combination of the exo- and endothermic DSC events. The
third DSC event observed at ca. 286 °C in the sample containing
LiCl is now missing, which indicates that it should be attributed
to the reactions related to the metal chlorides, like formation of
RbLiCl2 (eq.3). The chloride-free sample heated to 450 °C
contains crystalline RbBH4, which apparently decomposes
above 500 °C, with the accompanying mass loss extending
above the measured temperature range. Besides emission of
hydrogen during decomposition of RbBH4 also metallic
rubidium gradually evaporates from the sample (its boiling
point is 688 oC), as indicated by a relatively large mass loss and
drop of the signals related to the impurities of the carrier gas –
mainly those from H2O and O2. It is worth noticing that the
decomposition temperature of RbBH4 is much lower than
600 °C reported previously on the basis of Mikheeva and
Arkhipev’s results 2,35, possibly because the B-containing
products act here as catalyzing agents.
RbSc(BH4)4 →RbBH4 + 9/2 H2 + ½ B2H6 + ScB2
in which the theoretical mass loss is significantly higher (12.1
wt%).
(4b)
Figure 6. TGA/DSC plots of CsSc(BH4)4 samples obtained via solvent-mediated (blue)
and mechanochemical (red) synthesis. Grayed area at low temperatures is artifactual.
Many similarities can be seen comparing both samples of
CsSc(BH4)4 with their rubidium analogues, Fig. 6. The sample
containing LiCl loses 4.21 wt% up to 550 °C, while having 4.15
wt% of hydrogen. PXRD results reveals presence of CsLi2Cl3 as
the only crystalline product in the sample heated to this
temperature. As the MS spectra (Fig. S8 in ESI) show mainly
hydrogen as gaseous product, the following decomposition
equation can be written:
CsSc(BH4)4 + 3LiCl → CsLi2Cl3 + 15/2H2 + ScBx + (4-x)B + LiH (5)
which is very similar to the eq. (3). Three main endothermic DSC
events can be observed here. The first of them, with the
maximum at 207 °C, is either melting of the sample or a
polymorphic phase transition of CsSc(BH4)4. The second one,
with maximum at ca. 238 °C, is strictly connected with the main
stage of hydrogen release, resulting in the major mass loss.
Immediately after the main decomposition event, a small peak
can be seen at DSC curve at 259 °C, being probably the effect of
the ternary chloride formation.
Thermal decomposition of the chloride-free sample of
CsSc(BH4)4 obtained via the DMS-mediated synthesis proceeds
similarly to the previously described samples. The first steps of
decomposition look alike those of the LiCl-contaminated
6 | J. Name., 2012, 00, 1-3
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CsSc(BH4)4. However, only these two events can be seen below presence and products of decomposition differed fVrioewmArtthicele oOnnlienes
DOI: 10.1039/C9DT01967G
300 °C, among which the second one is connected with mass of uncontaminated samples. Interestingly, while the DMS-
loss of 5.46 %. At the same time, the time-resolved mass mediated method of synthesis can be used for preparation of
spectroscopy (Fig. S9 in ESI) reveals traces of DMS and some pure and solvent-free bimetallic borohydrides containing Sc3+,
BxHy compounds, although these signals are two orders of in case of significantly larger Y3+ it leads to the solvate of a
magnitude weaker than those from H2. CsBH4 is the only simple borohydride, Y(BH4)3·DMS, due to the difference in Lewis
crystalline product of this step that can be detected. The main acidity of parent monometallic borohydrides.
decomposition route may be thus written as:
CsSc(BH4)4 → CsBH4 + 6H2 + B + ScB2
(6a)
Conflicts of interest
There are no conflicts to declare.
However, as the observed mass loss is slightly larger than the
expected on the basis of eq. (6a), a parallel minor route is highly
possible, just as for the case of Rb compound:
CsSc(BH4)4 → CsBH4 + 9/2H2 + ½B2H6 + ScB2
(6b)
as additionally confirmed by the observed B2H6 emission. At
higher temperature (422 °C) a significant mass loss can be seen,
extending above the examined range of temperature, which is
the result of CsBH4 decomposition to Cs, B and H2. As in the case
of its rubidium analogue, the temperature of CsBH4
decomposition is significantly lower than 660 °C reported
previously for the pure compounds 2,35 as being catalyzed with
the boron-containing compounds. The elemental Cs gradually
evaporates from the sample (its boiling point is 671 oC), as it is
indicated by a relatively large mass loss and drop of the signals
related to the impurities of the carrier gas, as in the case of the
sample containing rubidium.
Acknowledgements
The authors would like to acknowledge financial support from
National Science Center, Poland, grant OPUS no. UMO-
2014/15/B/ST5/05012.
Notes and references
‡ The supplementary crystallographic data can be obtained from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/structures on quoting the CCDC deposition
numbers: 1914477 for RbSc(BH4)4, 1914478 for CsSc(BH4)4,
1914482 for Cs3ScCl6 and 1914489 for Rb3ScCl6.
Comparing thermal decomposition of the all known
MSc(BH4)4, some similarities as well as differences can be
observed. First of all, the main mass loss for M = Rb and Cs takes
place at temperatures slightly higher (ca. 20-30 °C) than for
their lighter siblings, with M = Li–K 8,9. This signifies increased
thermal and thermodynamic stability, which stems from more
Lewis basic nature of MBH4 for M=Rb and Cs, than for M=Li–K 2.
At the same time, melting temperature of the cesium derivative
significantly differs from others being ca. 50 °C higher. Among
similarities, a wide shoulder of DSC signal above the
characteristic temperature of the main decomposition event
can be seen in all the samples in which chlorides were present.
This feature cannot be seen in the chloride-free samples (Fig. 5
and 6), thus, it can be assigned the impact of impurities into the
decomposition route. Similar features were observed in a series
1
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View Article Online
DOI: 10.1039/C9DT01967G
The new derivatives of scandium borohydride have been prepared in a pure form using facile
synthetic method and physicochemically characterized.