Mixed-anion and mixed-cation borohydride KZn(BH4)Cl2: Synthesis, structure and thermal decomposition

Article abstract of DOI:10.1002/ejic.201000119

KZn(BH₄)Cl₂, synthesized for the first time, contains a heteroleptic complex anion [Zn(BH₄)Cl₂]^-, extending the structural diversity of metal borohydrides. In-situ synchrotron powder diffraction, NMR and Raman spectroscopy were used to characterize KZn(BH₄)Cl₂ and to evaluate the mechanism, for its thermal decomposition. The title compound decomposes at a significantly lower temperature than KBH₄ and may be used for inspiration for the design of novel hydrogen storage materials. Combining different ligands in modified metal borohydrides is proposed as a way to adjust stability with respect to hydrogen, desorption.

Full text of DOI:10.1002/ejic.201000119

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000119
Mixed-Anion and Mixed-Cation Borohydride KZn(BH4)Cl2: Synthesis,
Structure and Thermal Decomposition
Dorthe B. Ravnsbæk,[a] Lise H. Sørensen,[a] Yaroslav Filinchuk,[a,b] Daniel Reed,[c]
David Book,[c] Hans J. Jakobsen,[a] Flemming Besenbacher,[d] Jørgen Skibsted,[a] and
Torben R. Jensen*[a]
Keywords: Hydrides / X-ray diffraction / Solid-state structures / Solid-phase synthesis / Transition metals
KZn(BH4)Cl2, synthesized for the first time, contains a hetero- significantly lower temperature than KBH4 and may be used
leptic complex anion [Zn(BH4)Cl2]–, extending the structural
diversity of metal borohydrides. In-situ synchrotron powder
diffraction, NMR and Raman spectroscopy were used to char-
acterize KZn(BH4)Cl2 and to evaluate the mechanism for its
thermal decomposition. The title compound decomposes at a
for inspiration for the design of novel hydrogen storage mate-
rials. Combining different ligands in modified metal borohy-
drides is proposed as a way to adjust stability with respect to
hydrogen desorption.
dynamic stability comparable to the average of the two
compounds, LiBH4 and KBH4.[6] Other examples of mixed-
cation borohydrides are MZn2(BH4)5 and MSc(BH4)4 (M
= Li or Na), which consist of isolated [Zn2(BH4)5]– and
[Sc(BH4)4]– units, respectively, counterbalanced by M+ cat-
ions.[7–10] Partial anion substitution was recently demon-
strated by the preparation of Li(BH4)1–xClx.[11,12]
Here, we present the synthesis, crystal structure, MAS
NMR spectroscopic data and an investigation of the ther-
mal decomposition for a novel compound, KZn(BH4)Cl2
(1), which is the first report on a mixed-anion and mixed-
cation borohydride. The decomposition temperature for
KZn(BH4)Cl2 is found to be significantly lower than that
for KBH4, hence this study provides inspiration for the de-
sign and preparation of novel materials, which may be suit-
able for hydrogen storage applications. Furthermore, two
other new borohydrides, 2 and 3, are identified.
Introduction
The increasing demand for clean energy and the negative
environmental influence caused by the present carbon-
based fossil fuels make the development and utilization of
clean and sustainable energy a compelling challenge for the
future. Hydrogen has been suggested as a future carrier of
renewable energy; however, a safe, compact and efficient hy-
drogen-storage method still remains to be identified.[1]
Borohydride-based materials currently receive great interest
as potential hydrogen-storage systems due to their high
gravimetric hydrogen densities. Unfortunately, many of the
well-known borohydrides, such as LiBH4, NaBH4 and
KBH4, exhibit poor thermodynamic and kinetic properties,
which hamper their utilization in technological applica-
tions.[2,3] Thermodynamic and kinetic properties have suc-
cessfully been improved by the formation of reactive hy-
dride composites, such as 2LiBH4/MgH2, whose enthalpy
of decomposition is reduced, whilst they are still capable
of storing hydrogen reversibly.[4,5] Recently, a mixed-cation
borohydride, LiK(BH4)2, was prepared and characterized.
It structurally resembles LiBH4 and possesses high thermo-
Results and Discussion
Compound 1 was synthesized by a mechano-chemical
method, that is, ball milling, which is a commonly used
[a] Interdisciplinary Nanoscience Center (iNANO) and Depart- method for the preparation of borohydride-based hydrogen-
ment of Chemistry, Aarhus University,
storage materials. Usually, metathesis reactions occur in
Langelandsgade 140, 8000 Aarhus C, Denmark
mixtures of metal borohydride and metal halide, but this
study reveals a less common addition reaction [Equa-
tion (1)].
Fax: +45-8619-6199
E-mail: trj@chem.au.dk
[b] Swiss-Norwegian Beam Lines at ESRF,
B. P. 220, 38043 Grenoble, France
[c] School of Metallurgy and Materials, The University of Bir-
mingham,
KBH4 + ZnCl2 Ǟ KZn(BH4)Cl2
(1)
Edgbaston, Birmingham, B15 2TT, UK
[d] Interdisciplinary Nanoscience Center (iNANO) and Depart-
ment of Physics and Astronomy, Aarhus University,
8000 Aarhus C, Denmark
The high degree of crystallinity of the synthesized com-
pound allows detailed diffraction studies of the crystal
structure and of the chemical reactions occurring during
synthesis and decomposition.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000119.
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Mixed-Anion and Mixed-Cation Borohydride KZn(BH4)Cl2
The monoclinic structure (P21/m) of KZn(BH4)Cl2 was
solved and refined from synchrotron radiation powder X-
ray diffraction (SR-PXD) data measured on an almost pure
sample of the title compound (Figure 1). The new structure
of 1 consists of isolated [Zn(BH4)Cl2]– anions and octacoor-
dinated K+ cations, associated with the anions by weak K–
Cl and K–BH4 interactions (Figure 2). One Zn2+, one K+,
two Cl– and one BH4– ion are all located on the mirror plane.
The Zn2+ ion has a trigonal-planar environment by two Cl–
and one BH4– ion, the latter acting as an η2 ligand. The low
coordination number for Zn explains the short Zn–B and
Zn–H bond lengths [Zn–B 2.162(11), Zn–H 1.78(3) Å;
Table 1]. The K+ ion has a somewhat unusual coordination,
forming a distorted square antiprism by five Cl– and three
BH4 ions, which are acting as η2 ligands. The K–Cl
–
Figure 2. Crystal structure of KZn(BH4)Cl2. The complex
[Zn(BH4)Cl2]– anions and eightfold coordination polyhedron for
the K atom are highlighted.
[3.159(14)–3.66(2) Å] and K–B [3.274(13)–3.781(10) Å] dis-
tances in 1 are slightly longer than in KCl [3.130(2) Å[13]] and
KBH4 [3.364(11) Å[2]], where the K+ ion has an octahedral
–
environment by six BH4 ions. Significantly different bond
Table 1. Selected interatomic distances in KZn(BH4)Cl2 at about
82 °C (for further details see Supporting Information, Table S2).
lengths and coordination numbers for the Zn and K atoms
allow us to describe 1 as K[Zn(BH4)Cl2].
Atom 1
Atom 2
Distance [Å]
Zn
Zn
Zn
K
K
K
H
B
Cl
H
B
Cl
H
1.78(3)
2.162(11)
2.219(11)–2.338(13)
2.74(2)–2.84(2)
3.274(13)–3.781(10)
3.159(14)–3.665(16)
1.117(12)–1.13(3)
B
is determined here by the coordination via the short Zn–B
and the long K–B contacts. Interestingly, the volume of the
KZn(BH4)Cl2 formula unit (V/Z = 149.5 Å3) is nearly equal
to the sum of formula volumes for the reactants ZnCl2 (V/
Z = 74.3 Å3)[17] and KBH4 (V/Z = 76.2 Å3).[2]
In the Raman spectrum for KZn(BH4)Cl2 measured at
room temp. (Figure 3, b) the bidentate B–Hterminal and B–
Hbridge stretching modes are observed at 2420, 2398 and
2144 cm–1, respectively. Weaker bands are observed at 1401
and 1190 cm–1, corresponding to the bidentate bridge
stretching and BH2 bending modes of the bridging hydro-
gen atoms, respectively.[18] From 400 to 100 cm–1, several
Figure 1. Rietveld refinement profile for KZn(BH4)Cl2 from SR-
PXD data measured at BM01A, ESRF (λ = 0.703511 Å) at 82 °C
on the ball-milled KBH4/ZnCl2 (1:1) sample.
The two Cl– anions have different coordination: one is sharp bands are observed, suggesting well-defined lattice
trigonal-planar with one Zn2+ and two K+ ions, the other is modes, which underlines the high degree of crystallinity of
tetrahedral with one Zn2+ and three K+ ions in the nearest KZn(BH4)Cl2. The Raman spectroscopic data measured at
coordination sphere. The BH4– ion has a saddle-like coordi- –196 °C (Figure 3, a) in general exhibits the same features
nation consisting of one Zn2+ and three K+ ions. The two as the data measured at room temp., suggesting that no
nearest neighbours (Zn2+ and K+) define an almost linear phase transition has occurred upon cooling. However, low-
Zn–BH4–K coordination with an angle of 177.2(5)°, similar ering of the temperature results in a decrease of the line
to the Mg–BH4–Mg angle in the Mg(BH4)2 structures.[14–16] widths of the Raman bands, hence several additional Ra-
Hydrogen positions can be accurately obtained from X-ray man shifts are resolved around the shift frequencies ob-
powder diffraction for the light-element systems only, ex- served in the data measured at room temp. The Raman
cept some particular cases, e.g. when heavy atoms are in spectrum measured at 200 °C (Figure 3, c) shows no bands
special positions.[16] In the presence of Zn, Cl and K atoms, from vibration due to BH4, suggesting that KZn(BH4)Cl2
H atoms in KZn(BH4)Cl2 are determined only tentatively. is fully decomposed, which is in accordance with the TGA/
The BH4– ion is coordinated to Zn2+ via its tetrahedral edge DSC and in situ PXD measurements discussed later. The
(η2 ligand), while the opposite edge is pointing towards K+. strong band at 290 cm–1 might be related to a Zn–Cl vi-
–
Coordination of the BH4 anion via the edge is typical for bration of the tetrahedral [ZnCl4]2– ion in K2ZnCl4, which
metal borohydrides.[2] Thus, the orientation of the BH4– ion is the main decomposition product.[19]
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T. R. Jensen et al.
SHORT COMMUNICATION
lated to experimental spinning sideband intensities[20] pro-
vides the isotropic chemical shift, δiso(11B) = –43.6(2) ppm,
and the quadrupole coupling parameters, CQ
=
0.648(15) MHz, ηQ = 0.51(2). The excellent agreement be-
tween the simulated and experimental 11B MAS NMR
spectra (Figure 4) demonstrates that KZn(BH4)Cl2 is pres-
ent as a highly ordered phase. The isotropic chemical shift
is observed at a frequency about 5 ppm lower than the cor-
responding resonance of KBH4 [δiso(11B) = –37.8(2) ppm],
which may reflect the presence of Zn2+ ions in near coordi-
nation to the boron atoms in 1.
The decomposition of KZn(BH4)Cl2 was investigated by
in-situ SR-PXD (Figure 5). Diffraction from KZn(BH4)Cl2
is visible in the powder patterns up to about 110 °C, where
the compound decomposes into an intermediate phase (de-
noted 2), metallic Zn and K2ZnCl4. Phase 2 exists in a nar-
row temperature interval and also decomposes to zinc and
K2ZnCl4 at about 130 °C. Hence the decomposition tem-
peratures for 1 and 2 are significantly lower than that of
KBH4 (Tdec = 500 °C).[21] The intermediate phase 2 was
indexed from the SR-PXD pattern, obtained at about
112 °C, in a monoclinic unit cell with cell parameters: a =
8.88, b = 23.74, c = 8.91 Å and β = 102.46°.
Figure 3. Raman spectrum for KZn(BH4)Cl2 measured at (a)
–196 °C (liquid nitrogen), (b) room temp. and (c) 200 °C.
The 11B MAS NMR spectrum of the central and satellite
transitions for 1 (Figure 4, a) reveals a single centreband
resonance and the corresponding manifold of spinning side-
bands from the satellite transitions for the unique BH4– site
in this compound. The least-squares optimization of simu-
Figure 5. In-situ SR-PXD for the sample containing KZn(BH4)Cl2
measured at BM01A, ESRF (λ = 0.701135 Å) from room temp. to
200 °C, ∆T/∆t = 1 °C/min. Symbols: black squares: Zn; open
squares: K2ZnCl4; triangles: unidentified phase; unmarked reflec-
tions at room temp. to about 110 °C are due to KZn(BH4)Cl2. At
temperatures between approximately 110 and 130 °C, diffraction
from the intermediate is observed.
Thermogravimetric analysis (TGA) confirms that 1 and
2 decompose in the temperature range from 110 to 160 °C
(Supporting Information, Figure S1), giving an observed
mass loss of 6.5 wt.-%. Because hydrogen represents only
2.1 wt.-% of KZn(BH4)Cl2, this indicates that some (BH3)n
gases are also lost during the decomposition. The calculated
mass loss according to the reaction according to Equa-
tion (2) is 7.8 wt.-%.
Figure 4. (a) 11B MAS NMR spectrum (7.05 T) of the central (in-
set) and satellite transitions for KZn(BH4)Cl2 obtained with a spin-
ning speed of 10.0 kHz and high-power H decoupling. (b) Opti-
mized simulation of the satellite transitions in (a), resulting in the
isotropic chemical shift, δiso(11B) = –43.6 ppm, and the quadrupole
coupling parameters, CQ = 0.648 MHz, ηQ = 0.51, for the unique
11B site in KZn(BH4)Cl2.
1
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Mixed-Anion and Mixed-Cation Borohydride KZn(BH4)Cl2
2 KZn(BH4)Cl2 Ǟ K2ZnCl4 + Zn + 2/n (BH3)n + 2 H2
(2) tor, a calibrated wavelength of λ = 0.701135 Å and 0.5 mm glass
capillaries as sample holders. The 2D SR-PXD data were inte-
grated into 1D powder patterns. Diffraction peaks were indexed by
Dicvol[23] in a primitive monoclinic cell. The structure was solved
in the space group P21 by global optimization in direct space with
the program FOX,[24] varying positions of one Zn2+, one K+, two
The loss of (BH3)n gases indicates an irreversible charac-
ter of the hydrogen release from 1 and 2. Furthermore, we
mention that ball-milled samples of KBH4 and ZnCl2 in
higher molar ratios, i.e. 2:1, 3:1 and 4:1, have also been
prepared and investigated. SR-PXD data for these samples
reveal a diffraction pattern from another novel material (3),
which is different from that of KZn(BH4)Cl2 and 2 (Sup-
–
Cl– and one BH4 ion and using antibump restraints of 1.6–2.1 Å.
Examination of the resulting structure by Platon[25] revealed higher
crystallographic symmetry, e.g. space group P21/m. The final refine-
ment was performed by the Rietveld method in P21/m with the
porting Information, Figure S3). The powder diffraction program Fullprof.[26] The background was described by linear in-
terpolation between selected points. Because all atoms occupy spe-
cial positions on the mirror plane, the refinement involved only few
refined parameters. At room temperature the unit cell parameters
are a = 7.6257(9), b = 5.7375(6), c = 6.8786(9) Å, β = 97.794(15)°,
Z = 2. The highest-quality data set was recorded at 82 °C, for which
the refined structure is reported. The agreement factors are: Rwp
(not corrected for background) = 6.60%, Rp (corrected for back-
ground) = 16.8%, RBragg = 11.2%.
pattern of 3 differs from the one reported for the previously
suggested K2Zn3(BH4)8 composition,[22] which has not been
observed in this study. The samples milled at different
KBH4/ZnCl2 ratios have also been investigated by 11B MAS
NMR spectroscopy, which reveals two boron-containing
phases: KBH4 [δiso(11B) = –37.8 ppm], and 3 [δiso(11B) =
–42.0 ppm]. The shift towards lower frequency for 3 sug-
–
gests that the BH4 ions are coordinated by Zn2+ ions.
Raman spectroscopic study was performed using a Renishaw InVia
Reflex, using a 488 nm excitation laser, with a power of about
2 mW on the sample. An Instec HCS621V cell was used for inert
sample transfer and measurement.
However, we have so far not been successful in solving the
structure of 3. Compound 3 appears to decompose in two
steps between 120 and 350 °C, which is significantly lower
than for KBH4. This indicates that 3 may be another new
zinc-based borohydride.
Solid-state 11B MAS NMR spectra were recorded with Varian
Unity-INOVA-300 (7.05 T) and -400 (9.39 T) spectrometers by
using homebuilt X-[1H] double-resonance MAS NMR probes for
5 mm o.d. rotors. The NMR experiments were performed at ambi-
ent temperatures by using air-tight end-capped zirconia rotors
packed with the sample in an argon-filled glove box. The 11B iso-
tropic chemical shifts are in ppm relative to neat F3B·O(CH2CH3)2.
Simulations of the MAS NMR spectra were performed by using
the STARS software package.[20]
Conclusions
The first mixed-anion and mixed-cation borohydride,
KZn(BH4)Cl2, has been synthesized and characterized
along with two other new zinc-based borohydrides. The sig-
nificant structural diversity of homoleptic (homoligand)
complex anions, such as [Sc(BH4)4]–, [Zn2(BH4)5]–, and
[Zn(BH4)3]nn–, recently found in the mixed alkali and d-
block metal borohydrides,[5,6,8,10] is complemented now by
the first heteroleptic complex anion [Zn(BH4)Cl2]–. Various
modifications of the coordination sphere for the transition
element is an additional factor enabling adjustment of the
stability of similar heteroleptic structures. Thus, the ad-
dition of metal halides to borohydrides may open a route
for the design and preparation of new hydrogen storage ma-
terials, potentially with improved thermodynamic and ki-
netic properties for hydrogen release and uptake.
Simultaneous thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were performed, and the decomposi-
tion temperatures were measured as the onset temperatures ob-
served by DSC [heating rate 10 °C/min, room temp. to 500 °C, he-
lium flow: 50 mL/min (protective flow: 25 mL/min), corundum cru-
cibles].
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, crystallographic information, Raman
spectrum, details on the thermal expansion of 1, TGA/DSC curve
and XRD pattern for the compound 3.
Acknowledgments
Experimental Section
The authors are grateful to the Swiss-Norwegian Beam Lines for
the provision of in-house beam time. We are also grateful to Dr.
Hans Hagemann, Department of Physical Chemistry, University of
Geneva, for helpful discussions. We thank the Danish Council for
Strategic Research via Centre for Energy Materials, CEM for fund-
ing, the Danish Natural Science Research Councils for funding to
the Instrument Centre for Solid-State NMR Spectroscopy and
DanScatt.
The samples were prepared from KBH4 and ZnCl2 mixed in the
molar ratios 1:1, 2:1, 3:1 and 4:1. All samples were ball-milled un-
der argon for 120 min by using a sample/ball ratio of approximately
1:35. The chemicals used were KBH4 (Ն90%, Aldrich) and ZnCl2
(Ն98%, Aldrich). All handling and manipulation of the chemicals
were performed in an argon-filled glove box with a circulation puri-
fier, p(O2, H2O) Ͻ 0.1 ppm.
All samples were initially investigated by powder X-ray diffraction
with a Stoe diffractometer and a curved position sensitive detector
[Ge(111) monochromator, Cu-Kα1, λ = 1.54060 Å and 0.4 mm glass
capillaries sealed with glue used as sample holders]. Subsequently,
synchrotron radiation powder X-ray diffraction (SR-PXD) data
were collected at beamline BM01A at the European Synchrotron
Radiation Facility, Grenoble, France by using an MAR345 detec-
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Published Online: March 5, 2010
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