From Metal Hydrides to Metal Borohydrides

Article abstract of DOI:10.1021/acs.inorgchem.8b01398

Commencing from metal hydrides, versatile synthesis, purification, and desolvation approaches are presented for a wide range of metal borohydrides and their solvates. An optimized and generalized synthesis method is provided for 11 different metal borohydrides, M(BH₄)_n, (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb), providing controlled access to more than 15 different polymorphs and in excess of 20 metal borohydride solvate complexes. Commercially unavailable metal hydrides (MH_n, M = Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure hydrogenation. For synthesis of metal borohydrides, all hydrides are mechanochemically activated prior to reaction with dimethylsulfide borane. A purification process is devised, alongside a complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal borohydrides, α-, α′- β-, γ-Yb(BH₄)₂, α-Nd(BH₄)₃ and new solvates Sr(BH₄)₂·1THF, Sm(BH₄)₂·1THF, Yb(BH₄)₂·xTHF, x = 1 or 2, Nd(BH₄)₃·1Me₂S, Nd(BH₄)₃·1.5THF, Sm(BH₄)₃·1.5THF and Yb(BH₄)₃·xMe₂S ( x = unspecified), are presented here. Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile access to a large number of materials and by performing a scale-up synthesis of LiBH₄.

Full text of DOI:10.1021/acs.inorgchem.8b01398

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
From Metal Hydrides to Metal Borohydrides
,
†
†
†,‡
Mark Paskevicius,†,‡
Bo Richter,* Jakob B. Grinderslev, Kasper T. Møller,
,†
and Torben R. Jensen*
†Center for Materials Crystallography, Interdisciplinary Nanoscience Center and Department of Chemistry, Aarhus University,
Langelandsgade 140, 8000 Aarhus C, Denmark
‡
Department of Physics and Astronomy, Fuels and Energy Technology Institute, Curtin University, Wark Avenue, Bentley, Western
Australia 6102, Australia
*
S Supporting Information
ABSTRACT: Commencing from metal hydrides, versatile syn-
thesis, purification, and desolvation approaches are presented for a
wide range of metal borohydrides and their solvates. An optimized
and generalized synthesis method is provided for 11 different
metal borohydrides, M(BH ) , (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd,
4
n
Sm, Gd, Yb), providing controlled access to more than 15 different
polymorphs and in excess of 20 metal borohydride solvate
complexes. Commercially unavailable metal hydrides (MH , M =
n
Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure
hydrogenation. For synthesis of metal borohydrides, all hydrides
are mechanochemically activated prior to reaction with dimethyl-
sulfide borane. A purification process is devised, alongside a
complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure
metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal
borohydrides, α-, α′- β-, γ-Yb(BH ) , α-Nd(BH ) and new solvates Sr(BH ) ·1THF, Sm(BH ) ·1THF, Yb(BH ) ·xTHF, x = 1
4
2
4 3
4 2
4 2
4 2
or 2, Nd(BH ) ·1Me S, Nd(BH ) ·1.5THF, Sm(BH ) ·1.5THF and Yb(BH ) ·xMe S (“x” = unspecified), are presented here.
4
3
2
4 3
4 3
4 3
2
Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The
reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and
polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile
access to a large number of materials and by performing a scale-up synthesis of LiBH4.
1
. INTRODUCTION
drides involved the reaction of diborane gas with alkali metal
2
9−35
hydrides or alkoxides.
On the basis of this pioneering
The analysis of reaction mechanisms for material synthesis is of
key importance in creating high quality chemicals, possessing
high polymorphic purity, for the investigation of material
properties relying on specific structures. This knowledge is
crucial in order to develop synthetic protocols for novel
materials or precursors for further chemical processing. Metal
borohydrides are recognized as a highly versatile class of
materials with regard to structure, composition, and chemical
and physical properties, and has expanded tremendously
during the past few years. Borohydrides originally served as
reducing agents in organic synthesis. Later they were also
considered for solid state hydrogen storage and ion conductors
for new types of solid state batteries.
applications such as explosives, magnetic materials, gas
adsorption materials with permanent nanoporosity,
polymerization initiators,
preparation of higher boranes were realized, and some
materials were even shown to be luminescent. Metal
borohydrides possess a richly faceted history. By 1940, the
reactions of diborane with metal alkyls afforded Al(BH ) ,
research, several other borohydrides and derivatives were
synthesized and made readily available. For instance, synthesis
of the selective reducing agent NaBH(OCH ) was devised,
3
3
LiBH and KBH were synthesized, and, most importantly, a
4
4
highly effective process for the synthesis of NaBH in large
4
36−38
quantities was established.
Among the most common
synthetic approaches is the metathesis, conducted by solvent
methods or mechanochemically, of a precursor borohydride
(
e.g., NaBH or LiBH ) and an appropriate metal halide salt,
4 4
1
MX . This yields the respective metal borohydride, M(BH ) ,
n
4 n
2
,3
and an essentially undesired sodium or lithium halide
12,39−49
byproduct (reaction 1).
4
−8
More exotic
9
10
n[Na/Li]BH + MX → n[Na/Li]X + M(BH )
(1)
4
n
4 n
11,12
use as
1
3−23
Multiple studies of these metal borohydride−metal halide
and use as precursors for
24
mixtures have been undertaken to establish the properties of
1
2
5
the borohydride component. However, the halide salt is not
Received: May 22, 2018
4
3
2
6−28
Be(BH ) , and LiBH .
Further early work on borohy-
4
2
4
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Overview of Synthesized Metal Borohydrides and Solvent Complexes
a
entry
metal hydride
solv.
nMH (mmol)
nDMSB (mmol)
reaction time (h)
reaction T/°C
product
o-LiBH4
1
1
1
1
2
2
3
3
4
4
4
5
5
5
6
6
7
7
7
8
8
9
9
9
1
1
1
1
1
a
b
c
d
a
b
a
b
a
b
c
a
b
c
a
b
a
b
c
a
b
a
b
c
LiH
LiH
LiH
LiH
Tol.
Tol.
THF
25.0
625.0
13.0
13.0
25.0
10.0
10.0
10.0
10.0
25.0
5.0
10.0
10.0
5.0
10.0
5.0
10.0
10.0
5.0
10.0
5.0
3.75
1.0
1.0
37.5
750.0
16.0
16.0
37.5
12.5
25.0
25.0
25.0
55.0
15.0
25.0
30.0
15.0
25.0
15.0
45.0
45.0
20.0
24
72
24
24
36
24
36
24
24
144
24
24
36
24
36
24
24
120
24
RT
RT
RT
RT
40
60
40
40
40
RT
40
40
40
40
40
40
40
40
40
40
40
RT/40
RT
40
40
40
b
o-LiBH4
c
LiBH ·xTHF
4
d
Et O
LiBH ·1Et O
2
4
2
NaH
NaH
MgH2
MgH2
CaH2
CaH2
CaH2
SrH2
Tol.
Tol.
Tol.
THF
Tol.
Tol.
THF
Tol.
Tol.
THF
Tol.
THF
Tol.
Tol.
THF
Tol.
THF
Tol.
THF
THF
Tol
NaBH4
NaBH4
1
Mg(BH ) · / Me S
4 2 2 2
Mg(BH ) ·xTHF
4 2
α-Ca(BH4)2
α-Ca(BH4)2
Ca(BH ) ·2THF
4
2
Sr(BH4)2
Sr(BH4)2
e
SrH2
SrH2
BaH2
BaH2
YH3
Sr(BH ) ·2THF
4
2
o1-Ba(BH4)2
Ba(BH ) ·2THF
4
2
Y(BH ) ·1Me S
4 3 2
Y(BH ) ·1Me S
4 3 2
Y(BH ) ·3THF
4 3
Nd(BH ) ·1Me S
4 3 2
Nd(BH ) ·1.5THF
4 3
Sm(BH4)2
e
YH3
YH3
f
f
NdH3
NdH3
SmH3
SmH3
SmH3
GdH3
GdH3
YbH3
YbH3
YbH3
45.0
48
20.0
17.0
4.50
4.50
45.0
20.0
24
36
24
24
24
24
g
Sm(BH ) ·1.5THF
4
3
Sm(BH ) ·1THF
4 2
Gd(BH ) ·1Me S
4 3 2
Gd(BH ) ·3THF
4 3
α-Yb(BH4)2
0a
0b
1a
1b
1c
10.0
5.0
10.0
1.0
THF
Tol.
Tol.
THF
f
f
45.0
48
f
40
RT
40
f
h
4.5
72
Yb(BH ) ·xMe S
4
3
2
2.5
10.0
24
Yb(BH ) ·2THF
4 2
a
b
c
5
M for reactions in toluene, 1 M for reactions in THF or Et O; Large scale reaction at RT; Mixture of o-LiBH /LiBH ·xTHF after collection,
2
4
4
d
e
f
g
“
x” undetermined (see SI_Li). o-LiBH collected after drying in vacuo. Hand ground hydride. 4 M DMSB, reaction time extended. 1.5THF
4
h
based on thermal analysis and comparison to Nd(BH ) ·1.5THF (see SI_Sm). “x” undetermined, see SI_Yb.
4
3
1
always an inert byproduct and can react with the metal
borohydride, forming solid solutions or ordered metal
of the Mg(BH ) · / Me S solvate complex. Unfortunately,
4 2 2 2
access to appropriate alkylmetal reagents is limited to, e.g.,
Mg(nBu)2 or nBuLi, thus preventing this method from
evolving into a general synthesis method for other metal
borohydrides. In the present work, a new general approach for
the synthesis of metal borohydrides is developed and
optimized individually for each metal by selecting an
appropriate solvent, adjusting the borane complex concen-
tration, reaction temperature, reaction time, metal hydride
particle size, etc. Hereby, we attempt to minimize the
confusion arising from the large array of more or less
successful synthetic approaches reported in the literature.
Especially, exploiting the differences in coordination strength
of sulfur/nitrogen/oxygen based donors (i.e., Me S·BH /Et N·
BH /THF·BH ) to the metal of the borohydride is advanta-
geous in order to synthesize pure, selected polymorphs of
solvent free metal borohydrides. For instance, removal of Et N
from Mg(BH ) synthesized from MgH and Et N·BH is
tedious and difficult to fulfill completely, while removal of THF
from many more borohydrides has proven otherwise problem-
atic. Experimental details and procedures are rationally
compiled (elemental order) in the Supporting Information as
“SI_M” (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb),
where X-ray diffraction patterns, thermal analysis data,
structural information, tables, illustrations, and relevant
references are grouped as easily available, separate (metals
based) sections for each individual metal borohydride
5
,6,50,51
borohydride halides.
In recent years, increased focus
on the development of new synthesis strategies has resulted in
access to many borohydrides without a halide salt impurity,
allowing for more accurate determination of material proper-
ties. Solvent extraction of the desired metal borohydride
produced by a metathesis reaction is considered as one
12,47
alternative approach to its direct synthesis.
Yet, the
formation of soluble byproducts, such as bimetallic borohy-
drides and/or excess or unreacted LiBH , can render this
4
39
approach inadequate. Other more elaborate methods have
arisen, such as the continuous grinding of LiH while admitting
B2H6 gas, serving the purpose of breaking the metal
borohydride “passivation layer” that forms on the outside of
2
3
3
3
3
5
2
the LiH particles. Staying in the context of synthesis from
metal hydrides, the synthesis of both calcium and magnesium
borohydrides from their hydrides and triethylamine borane
3
4
2
2
3
3
53
(
TEA·BH ) should also be mentioned. Very recently, that
3
same method was successfully appointed to the synthesis of
1
0
europium and barium borohydrides and B-enriched
5
4,55
NaBH4.
An elegant synthesis of Mg(BH ) , utilizing an
4 2
insertion reaction, was demonstrated in 2009 by reacting
dibutylmagnesium, Mg(nBu) , with dimethylsulfide borane
2
1
(
Me S·BH /DMSB) yielding Mg(BH ) · / Me S as a precip-
2
3
4 2
2
2
1
1,56
itate.
This method eliminates the halide impurity that
arises from metathesis reactions, allowing for simple isolation
B
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Solubilities of Metal Borohydrides and Corresponding Formation of Solvent Complexes
borohydride (Mn+)
solvent
solubility (mmol/L)
a
solvent complex
LiBH ·1Et O
reference
70−74
70, 73, this work
73
77
73
73
37
80
81
81
this work
82, 83
84/this work
77, 75
77, 75
this work/31
31
76/this work
this work
this work/47
this work
this work
20/this work
this work
this work/47
this work
this work
this work
Li+
Et O
THF
monoglyme
diglyme
highly soluble
highly soluble
4700.0
4250.0
1270.0
1100.0
soluble
soluble
highly soluble
highly soluble
60.0
soluble
soluble
1690.0/1710.0
450.0
40.0
2
4
2
Li+
LiBH ·1THF/xTHF
4
Li+
LiBH ·monoglyme
4
Li+
LiBH ·diglyme
4
Li+
isopropylamine
isopropylamine
LiBH ·iPrNH2
4
Na+
Na+
Na+
Na+
Na+
Mg2+
Mg2+
Mg2+
Ca2
NaBH ·iPrNH2
4
b
RNH2
NaBH ·R-NH2
4
c
diglyme
NaBH ·diglyme
4
triglyme
DMF
NaBH ·triglyme
4
NaBH ·DMF
4
1
Me2S
Mg(BH ) · / Me S
4
2
2
2
d
Et O
Mg(BH ) ·2Et O
4 2 2
2
e
THF
Mg(BH ) ·3THF/xTHF
4
2
+
THF
diglyme
THF
Ca(BH ) ·2THF
4 2
2
+
Ca
Sr
Sr
Ba
Y3
Ca(BH ) ·diglyme
4
2
2
+
+
Sr(BH ) ·2THF
4
2
2
Et O
insol.
insol. /<25.0
Sr(BH ) ·2THF
4 2
2
2+
f
THF
THF
Me2S
Me2S
THF
THF
THF
Me2S
THF
THF
THF
Me2S
Ba(BH ) ·2THF
4
2
+
40.0
50.0
60.0
35.0
Y(BH ) ·3THF
4 3
Y3+
Y(BH ) ·1Me S
4
3
2
Nd3+
Nd3+
Sm2+
Nd(BH ) ·1Me S
4 3 2
Nd(BH ) ·1.5THF
4
3
g
g
40.0
Sm(BH ) ·1THF
4 2
3
+
+
+
Sm
Gd
Gd
Yb
Yb
Yb
<25.0
<20.0
40.0
25.0
60
Sm(BH ) ·1.5THF
4 3
Gd(BH ) ·1Me S
4 3 2
3
3
Gd(BH ) ·3THF
4
3
2+
Yb(BH ) ·2THF
4 2
3
3
+
+
Yb(BH ) ·3THF
4
3
Yb(BH ) ·xMe S
this work
c
4
3
2
a
d
g
b
Approximate solubility (mmol/L) estimated at RT unless otherwise stated; see references. Soluble in primary amines. Soluble in diglyme.
Mg(BH ) reported as a solution in Et O. Mg(BH ) reported to be readily soluble in THF. Ba(BH ) reported to be insoluble in THF.
e
f
4
2
2
4
2
4 2
Sm(BH ) reported to be moderately soluble in THF, resulting in Sm(BH ) ·2THF (note that we collected Sm(BH ) ·1THF).
4
2
4
2
4 2
synthesized. Increased access to metal borohydrides leaves
promise for future investigations of many potentially
interesting properties of this multifaceted class of materials,
without influence from inherent byproducts or impurities.
Sigma-Aldrich, 99.9%). The synthesized metal hydrides were collected
as brittle pieces or powders and characterized using powder X-ray
diffraction (PXRD). Commercially available metal hydrides, NaH
(
(
dry, 95%, Sigma-Aldrich), LiH (95%, Sigma-Aldrich), MgH2
hydrogen storage grade, Sigma-Aldrich), and CaH (95%, Sigma-
2
Aldrich), were used as supplied. Borane reagents and anhydrous
solvents were purchased from Sigma-Aldrich; toluene (C H CH ,
2
. EXPERIMENTAL SECTION
6
5
3
anh., 99.8%), tetrahydrofuran (C H O, anh., > 99.9%), dimethylsul-
2
.1. A Note on Safety. All reagents and starting materials are
4
8
fide (S(CH ) , anh., ≥ 99.0%), diethyl ether (O(C H ) , anh., ≥
highly air and moisture sensitive, and care should be exercised when
handling these materials outside the protective atmosphere of a
glovebox. Working inert, using protective gases and dry glassware is
mandatory. Any excess reagents or waste (dimethylsulfide borane,
metal hydrides, metal borohydrides) were disposed of under
controlled circumstances and contained in the fume hood until
rendered fully inactive. Hydrolysis using alcoholic solutions
isopropanol) or water was done in fume hoods for small quantities.
2
material or chemical throughout this study was conducted under inert
atmosphere due to the air and moisture sensitivity of metal hydrides,
borane complexes, and metal borohydrides. All powders were handled
in an argon filled glovebox (MBRAUN/LabMaster) maintaining O2
3
2
2
5 2
9
9.7%), borane dimethylsulfide complex (DMSB; BH ·S(CH ) ,
3 3 2
1
0.0−10.2 M in dimethylsulfide), borane tetrahydrofuran complex
solution (THFB; BH ·OC H , 1.0 M in tetrahydrofuran).
3
4
8
2
.3. Hydrogenation of Metals. Formation of commercially
unavailable metal hydrides (SrH , BaH , SmH , YH , NdH , GdH ,
2
2
3
3
3
3
YbH ) was performed on a custom built Sieverts apparatus by
3
exposing the corresponding metal to p(H ) ≈ 135−145 bar at RT,
(
2
followed by heating at 5 °C/min to 200−450 °C and cooling under
.2. Materials and Their Handling. All manipulation of any
57
hydrogen atmosphere. The decrease in hydrogen pressure during
absorption allows for calculation of the gravimetric hydrogen uptake
based on a known system volume. Neodymium(II) hydride, NdH
was also prepared from powdered Nd-metal hydrogenated at RT by
stepwise addition of hydrogen up to a pressure of p(H ) = 60 bar. The
,
2
and H O levels at 0.1 ppm. All liquid chemicals were handled in fume
2
2
hoods using standard inert syringe techniques, Schlenk equipment,
and custom-made flasks with integrated filter discs (Figure S1,
Supporting Information). Commercially available metals were
purchased and used without further purification: strontium (pieces,
Sigma-Aldrich, 99%), barium (pieces, Sigma-Aldrich, 99%), yttrium
chips, Sigma-Aldrich, 99.9%), samarium (chips, Sigma-Aldrich,
9.9%), neodymium (powder, Sigma-Aldrich, 99%, −40 mesh),
gadolinium (ingot, 99.9%, cut to <8 mm), and ytterbium (chips,
metal hydrides were mechanochemically “activated” in a Fritsch
Pulverisette 6 planetary ball mill under inert conditions to maximize
3
powder surface area. A material volume ∼1.5 cm of metal hydride
was loaded into an 80 mL tungsten carbide (WC) vial with 7 tungsten
carbide balls (5.5 g each), resulting in a powder volume/ball mass
ratio of 1:25. Milling speed of 350 rpm and milling time of 9 × 10 min
with 2 min breaks were used for a total milling time of 90 min. For
further details, see Supporting Information.
(
9
C
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
.4. Metal Borohydride Synthesis. General Synthesis Method.
Table 3. Desolvation Protocols and Resulting Polymorphs
for Metal Borohydride Solvent Complexes
A mechanochemically activated metal hydride was suspended in an
anhydrous solvent such as toluene (tol.), tetrahydrofuran (THF), or
diethyl ether (Et O), and a 50% stoichiometric excess of
dimethylsulfide borane complex (DMSB) was added at RT to reach
a concentration between 1 and 5 M. The reaction mixture was heated
to 40 °C and stirred for 24−48 h. After the reaction was complete,
toluene based reaction mixtures were filtered and washed with
toluene, and the solid metal borohydrides were collected and dried in
vacuo at RT. For THF/Et O based reactions, extra solvent was added
before filtration to increase the dissolved amount of metal
borohydride and increase the yield. The dissolved products were
extracted as a solution in THF or Et O, concentrated in vacuo, and
collected as their respective metal borohydride solvate complexes.
Yields are reported as essentially quantitative, since the reactions
proceed with full consumption of the metal hydride. See Table 1 for
details of specific entries and Supporting Information for further
details.
Up-Scaling. Increasing the reaction scale was successfully
attempted, evidenced by synthesis of LiBH (132.5 g). LiH (6.25
mol, 50.0 g, 95% purity, mechanochemically activated) and DMSB (5
M, 7.5 mol, 1.2 equiv) were transferred to a 3 L three neck flask
equipped with a magnetic stirrer and a thermometer. Using double-
needle transfer techniques, 800 mL toluene (anh.) was added to form
a LiH suspension, to which 750 mL DMSB (10 M) was added from
an addition funnel until a concentration of 5 M was reached. To
control the exothermic reaction, DMSB was added in 50 mL aliquots,
and the temperature of the suspension was kept below 10 °C using an
ice bath. After addition of DMSB, the reaction was allowed to heat up
to RT, and stirring was maintained for 3 days, while taking out a daily
sample to monitor reaction progress. Upon reaction completeness,
stirring was discontinued to allow sedimentation of the products, and
then toluene and excess reagent were removed using a double tipped
needle. The products were dried for 16 h in vacuo at 50 °C, yielding
desolvation
2
metal borohydride
solvent complex
temperature
a
(°C)
resulting material
o-LiBH
reference
1
b
LiBH
· /
Et O
2
RT - 60
71, 88, this
work
4
2
4
LiBH ·xTHF
200
o-LiBH4
89, this
work
11
4
2
1
Mg(BH ) · / Me S
75−90
140 Ar → 140/
vac.
γ-Mg(BH4)2
4
2
2
2
1
Mg(BH ) · / Me S
α-Mg(BH4)
2
this work
4
2
2
2
c
2
Mg(BH ) ·2Et O
160−180
200
200−250
α-Mg(BH4)2
90
90
82, this
work
4
2
2
Mg(BH ) ·2Et O
β-Mg(BH4)
4
2
2
2
d
Mg(BH
) ·3THF/
2
amorphous
4
xTHF
Ca(BH ) ·2THF
80
Ca(BH ) ·1THF this work
4 2
4
2
Ca(BH
)
·2THF
180
β-Ca(BH
α-Ca(BH4)2
Sr(BH ) ·1THF
)
87
4
2
4
2
e
Ca(BH ) ·2THF
160 → ice bath
90
170
150
140
175
175
200
this work
this work
this work
this work
47
this work
this work
this work
4
4 2
Sr(BH ) ·2THF
4
2
4 2
Sr(BH ) ·2THF
Sr(BH4)2
4
2
Ba(BH ) ·2THF
o1-Ba(BH4)2
α-Y(BH4)3
4
2
Y(BH ) ·1Me S
4
3
2
d
Y(BH ) ·3THF
amorphous
4
3
Nd(BH ) ·1Me S
Nd(BH ) ·1.5THF
Sm(BH ) ·1.5THF
α-Nd(BH4)3
α-Nd(BH )
4
3
2
4
3
4
3
f
60−170
Sm(BH4)
2
48, this
work
4
3
Gd(BH ) ·1Me S
140
200
90
180
180
140
Gd(BH4)
3
47
this work
·1THF this work
4
3
2
d
Gd(BH ) ·3THF
amorphous
Yb(BH
4
3
Yb(BH
Yb(BH ) ·2THF
) ·2THF
2
)
4
4
2
α-Yb(BH4)2
α-Yb(BH4)2
γ-Yb(BH4)2
this work
this work
this work
4
2
f
1
32.5 g of LiBH (6.06 mol, 97% yield). See Supporting Information
for details.
2
Yb(BH ) ·3THF
4 3
4
f
Yb(BH ) ·xMe S
4
3
2
.5. Extraction of Borohydrides. Purification of metal
a
Unless otherwise stated all entries are desolvated in vacuo.
borohydrides (removal of unreacted starting reagents, impurities, or
byproducts) can be achieved by extracting the metal borohydride with
Et O, THF, or Me S. We found that the metal borohydrides studied
b
c
Formation of “hemietherate” reported at RT. Note that two
d
distinct treatment steps are performed. No diffraction observed.
2
2
e
f
Contains minor quantities of β-Ca(BH ) . Reduction takes place
4
2
were soluble at RT, depending on the individual solvents utilized. The
extraction solvent was added at RT and stirred for several hours, and
then stirring was discontinued and any undissolved material was
allowed to sediment. The solution is separated from the solid residue
by decantation or filtration, then concentrated in vacuo using a rotary
evaporator, and the corresponding solid metal borohydride or metal
borohydride solvate complex was collected. Extra solvent can be
added and the process repeated, to ensure dissolving all metal
borohydride products formed in the reaction. Alternatively a Soxhlet
extractor can be employed to allow for continuous extraction and to
minimize solvent waste. Table 2 contains approximate RT solubilities
for the individual metal borohydrides synthesized.
(
see SI_Sm and SI_Yb).
Norwegian Beamline (SNBL) at the European Synchrotron Radiation
Facilities (ESRF) in Grenoble, France. Experiments were also carried
out at PETRAIII in the Deutsches Elektronen Synchrotron (DESY) in
Hamburg, Germany. At ESRF, samples were mounted in glass
capillaries (o.d. 0.4 mm) and sealed with glue inside an argon-filled
glovebox. Two-dimensional (2D) powder data were recorded using a
PILATUS2M detector at a selected wavelength of λ = 0.6945 Å. For
all measurements, the capillaries were rotated 90° during 10 s X-ray
exposures. For experiments at DESY, samples were mounted in glass
capillaries (o.d. 0.5 mm) and sealed with glue inside an argon-filled
glovebox. The 2D powder data were recorded using a PerkinElmer
XRD1621CN3-EHS 410 mm detector at a selected wavelength of λ =
0.20775 Å. Single PXRD patterns were collected at three different
sample to detector distances 1475, 1675, and 1875 mm.
2
.6. Desolvation of Metal Borohydride Solvates. The
temperatures for desolvation (removal of coordinated solvent) were
initially determined using thermogravimetric analysis (TGA).
Subsequently, heating a 300−500 mg metal borohydride solvate
sample in a Schlenk tube (inner ϕ ≈ 10 mm) at the appropriate
temperature in an argon atmosphere for 3−5 h, followed by
subsequent extraction (in vacuo) of the formed solvent vapor,
effectively removes the coordinated solvent. Detailed conditions are
found in Table 3, and TGA curves are found in the relevant sections
of the Supporting Information.
1
X-ray Data Analysis. Known structure types were identified and
58
refined using the Rietveld method implemented in Fullprof. Unit
cells were indexed in the software program FOX (Free objects for
59
crystallography). The PXRD patterns of the new compounds are
analogous to known compounds, and hence structural refinements
were performed in the same space groups, using equivalent atomic
coordinates. Overall scale factor, unit cell parameters, zero-point, peak
shape, Gaussian profile parameters (U, V, W), and isotropic
displacement (B) were refined.
2.7. X-ray Diffraction. In House Powder X-ray Diffraction.
Analysis was performed using a Rigaku Smart Lab X-ray
diffractometer with a parallel beam multilayer mirror (Cu Kα1
radiation, λ = 1.5406 Å). Data were typically collected at RT between
5
° and 80° 2θ at 5°/min. All samples were mounted in 0.3−0.5 mm
2.8. Thermal Analysis. Thermogravimetric analysis and differ-
ential scanning calorimetry (TGA/DSC) were performed for all
solvate complexes using a PerkinElmer STA 6000 instrument. The
measurements were typically performed with a heating rate of 5 °C/
boron silica glass capillaries in a glovebox and sealed.
Synchrotron Radiation Powder X-ray Diffraction (SR-PXRD).
Powder X-ray diffraction experiments were undertaken at the Swiss
D
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Figure 1. Hydrogenation curves measured by the Sieverts technique as a function of temperature for (A) Sr, (B) Ba, (C) Y, (D) Nd, (E) Gd, (F)
Yb, and (G) Sm (heating rate 5 °C/min). Increasing pressure is caused by thermal hydrogen expansion during heating. The hydrogen uptake is
displayed as wt % in brackets. (H) Schematic representation of the experimental setup.
min in a continuous flow of Ar (40 mL/min), using Al O crucibles
glovebox a single drop was placed on the holey-carbon coated copper
grid, left to dry, and mounted on the sample holder. The sample
holder was transferred from the glovebox to the electron microscope
under a blanket of argon.
2
3
fitted with a lid to prevent exposure to air. Gases produced from
sample decomposition during heating were vented through a small
hole in the lid to avoid pressure build-up. Where relevant, the released
gas was analyzed with a Hiden Analytical HPR-20 quadrupole mass
spectrometer.
2
.10. Infrared Spectroscopy (FTIR). FTIR spectra were
collected by a Nicolet 380 Avatar spectrometer, using attenuated
total reflectance-Fourier transformed infrared spectroscopy (ATR-
FTIR). The sample was placed directly on the diamond crystal and
compressed with a tight screw. The collected wavenumber was 4000−
2
.9. Transmission Electron Microscopy (TEM). TEM images
for mechanochemically activated YH3 were collected on a Talos
F220A electron microscope (FEI systems) at 200 kV (Figure S2,
Supporting Information). The sample was prepared by ultrasonication
−
1
(
10 min) of YH suspended in toluene (anh.). Inside an argon filled
400 cm with 32 scans.
3
E
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Inorganic Chemistry
Article
2
.11. Liquid State 1H NMR Spectroscopy. Samples for 1
H
hydride formed during hydrogenation, which has reacted.
Extraction with THF provided the desired products, in low
yields. Furthermore, the presence of tributoxyborane with the
Sm(BH ) from decomposing THF·BH is unaccounted for in
NMR were prepared by dissolving the sample in CDCl (0.5 mL) in
an NMR tube. The measurements were performed using a Bruker
AVANCE III HD spectrometer at 400 MHz. Chemical shifts (δ) are
reported in ppm relative to CDCl (7.26 ppm).
3
4
2
3
3
that study (see section 3.3, below). The metal borohydride
initially forms a surface layer on the metal hydride particles,
but will also have some, or significant, solubility in the solvent.
Thus, the reaction solvent mediates the reaction and also has a
tendency to dissolve and remove the reaction products from
the surface. This leads to solvent mediated dissolution,
transport, and recrystallization of M(BH ) (s) (Figure 3).
3
. RESULTS AND DISCUSSION
.1. Metal Hydride Formation. Formation of metal
3
hydrides can be observed as a decrease in hydrogen pressure in
the sample cell of the precursor metal as a function of
temperature; see Figure 1. The results reveal that a complete
hydrogenation of the metals may occur within seconds once a
critical temperature is reached. The rate of hydrogenation of
gadolinium (3.75 g) is rapid and exothermic, and caused the
steel sample cell (166 g) to heat up significantly, in this case
well above the furnace set point (Figure 1E). A stepwise
hydrogen absorption was also observed, e.g., for yttrium, where
an interstitial (metallic) YH forms prior to YH (ρ = 3.29 wt
4
n
2
3
m
%
H , Figure 1C). Neodymium also absorbs hydrogen in two
2
steps, and NdH forms already at RT upon hydrogen
2
6
0
exposure. Thus, only 0.42 wt % H was absorbed at T ≈
2
3
20 °C, forming NdH (calculated, 0.69 wt % H , Figure 1D).
3 2
Samarium is fully hydrogenated to SmH in two steps, from
3
1
30 °C and ending at 280 °C (Figure 1G). The compositions
of the metal hydrides were confirmed by calculating the
hydrogen uptake from Δp(H ) (Table S1, Supporting
2
Information) and by PXRD analysis (Figure S3, Supporting
6
1−66
Information).
.2. Reactivity of Metal Hydrides. Finely ground metal
3
hydrides react much faster than coarse particles due to the
larger surface area. TEM images of mechanochemically
Figure 3. Formation, dissolution, and continuous transport of
M(BH4)n driving the reaction of the borane source with the
activated YH reveal agglomerates, <200 nm, consisting of
3
smaller particles, ∼20 nm (Figure S2, Supporting Informa-
tion). In this work, it is shown that hand ground metal
hydrides react less satisfactorily, or at least more slowly, than
their mechanochemically activated counterparts. We suggest
that the solvent mediated reaction between a solid metal
hydride surface and a borane donor complex, e.g.,
dimethylsulfide borane, can be described as a nucleophilic
addition mechanism (Figure 2).
remaining available metal hydride (“3x” − “1x” MH ). The red “x”
n
marks the full consumption of the metal hydride.
We demonstrate this solvent mediated BH3 addition
reaction for a wide range of (partly) ionic hydrides: LiH,
NaH, MgH , CaH , SrH , BaH , YH , NdH , SmH , GdH ,
2
2
2
2
3
3
3
3
and YbH . The reaction described may also be considered as a
3
−
Lewis base type addition of an electron pair donating H ion
to a Lewis acid, BH , to form BH . The solid−gas reactions
−
3
4
between metal hydrides and diborane, mentioned in the
introduction, can be considered as the solvent free analogue to
the methodology presented in this work; the mechanochemical
treatment continuously removes the surface layer of the
formed borohydride from the metal hydride particles, which
Figure 2. Proposed reaction mechanism for the hydrogen transfer
from a metal hydride surface to the Me S·BH complex. The product
52
would otherwise retard the process.
2
3
3.3. Synthesis Conditions. An overview of all syntheses
is M(BH ) .
4
n
performed in this work is found in Table 1. Individually
optimized synthesis conditions for each product entry is
described in detail in the Supporting Information. The overall
process is outlined in Figure 4 and can be summarized as two
integrated cycles: synthesis and purification. Route “A” yields
metal borohydride solvent complexes, coordinated, e.g., with
Me S, THF, or Et O, while route “B” constitutes the process
A similar reaction between NaH and a different borane
donor complex (Et N·BD ) yielding NaBD H indicates that
3
3
3
6
7
same mechanistic pathway to be valid. This reaction
mechanism is only possible for ionic or polar covalent M−H
bonds, via a nucleophilic attack on the electron deficient boron
of the borane complex. This is supported by the fact that the
2
2
forming solvent free, solid metal borohydrides M(BH ) .
4
n
essentially metallic NdH did not react with DMSB, but the
Optimization of Synthesis Conditions. Reactions of LiH
2
ionic NdH readily formed a neodymium(III) borohydride
and SrH with DMSB were employed as model reactions to
3
2
solvate, Nd(BH ) ·1Me S. Synthesis of Sm(BH ) from SmH
converge on a general method. The progress of reaction is
assessed, based on XRD data estimating the amount of metal
hydrides present after a given reaction time. Initially, formation
4
3
2
4 2
2
and THF·BH has in fact been reported, but the yields were
3
6
8
very low. This is likely due to the presence of minute
quantities of samarium(III) hydride in the samarium(II)
of LiBH was obtained in 6−7 days at RT with mechanochemi-
4
F
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Inorganic Chemistry
Article
7
5−77
Table 2 and references therein).
Thus, LiBH ·xTHF,
4 2
4
LiBH ·1Et O, Ca(BH ) ·2THF, Sr(BH ) ·2THF, and Sm-
4
2
4 2
(
BH ) ·1THF (Table 1 entries 1c/1d/4c/5c/9c) are obtained
4 2
after concentration of the respective THF or Et O solutions.
2
Reactions performed in THF and the synthesis of LiBH in
4
Et O were all complete in less than 24 h. NaBH , however, is
2
4
not appreciably soluble in Et O or THF, but synthesis at
2
slightly elevated temperatures in toluene was complete after 24
h (Table 1, 2b). Further details and more examples can be
found in Table 1 and the Supporting Information. Post-
synthetic purification can be performed by extraction with
various solvents (see Table 2).
Reductive Synthesis; Samarium and Ytterbium Borohy-
drides. Reacting samarium(III) hydride, SmH , with DMSB in
3
toluene at RT or 40 °C, directly yields Sm(BH ) (Table 1,
4
2
9
a); i.e., a reduction has occurred in the course of reaction.
However, performing the reaction in THF (9b) reveals a new
samarium(III) borohydride THF solvate, Sm(BH ) ·1.5THF.
Thus, THF may stabilize the higher oxidation state, samarium-
4
3
Figure 4. Simplified schematic overview of the synthesis method. (A)
(
(
III), which is reduced to Sm(BH4)2 upon desolvation
reaction 2). The reduction is observed as a release of
Mechanochemically activated metal hydrides MH react with DMSB
n
providing the corresponding metal borohydrides as solvates (M-
diborane (see SI_Sm).
(
BH ) ·Solv.) or as solvent free metal borohydrides, M(BH ) (B).
4
n
4
n
M(BH ) ·xTHF → M(BH ) + xTHF + 1/2 B H +
4
3
4 2
2
6
cally activated LiH and 2 M DMSB, but the reaction was
complete in 24 h using 5 M DMSB (Table 1, 1a). For SrH2,
the reaction with 2 M DMSB proceeds slowly at RT, not
complete after 6 days, but faster at 40 °C, completing within
1
/2 H2 (M = Sm, Yb/x = 1.5 or 3)
(2)
Similarly, reacting YbH with DMSB in toluene at 40 °C
results in reductive formation of α-Yb(BH ) (11a), whereas
an ytterbium(III) borohydride solvent complex Yb(BH ) ·
4 3
xMe S is formed (“x” is undetermined) at RT (11b). Thermal
analysis and mass spectroscopy reveal diborane release at 50−
140 °C, indicating that ytterbium(III) is reduced to γ-
Yb(BH ) (reaction 2), which is isostructural to γ-Ca(BH )
(see SI_Yb). To sum up, the difficulties in obtaining Yb(III)
and Sm(III) borohydrides were partially overcome by
formation of solvate complexes. However, desolvation leads
to metal reduction, release of diborane, and formation of
divalent M(BH ) . This procedure is similar to synthesis of
3
4
2
2
4 h with 5 M DMSB (Table 1, 5a). Increased DMSB
concentrations, >5 M, cause higher viscosity and mixing
problems, exacerbated by the fact that metal borohydrides have
lower densities than their respective metal hydrides, leaving
thick slurries and difficulties in homogenizing the mixtures to
retard the reaction progress. Accordingly, it was concluded that
mechanically activated metal hydrides in 5 M DMSB at 40 °C
was optimal for a general synthesis method (Figure S5,
Supporting Information).
Up-Scale Synthesis. The scale of synthesis was successfully
increased from “lab scale” (5−10 mmol) to 6.25 mol (× 250),
forming high purity LiBH4 (132.5 g, 97.5% yield) (see
Supporting Information for details). These results demonstrate
that the method is practical, scalable, and can be utilized for
the production of larger quantities of metal borohydride in a
single batch.
2
4
2
4 2
78
4
2
Fe(II) and Co(II) borohydride solutions in Me S at T ≈ −50
2
2
+
°C with stabilization of the M by ammonia, forming
79
[Fe(NH ) ](BH ) and [Co(NH ) ](BH ) .
3
6
4 2
3 6
4 2
3.4. Purification. Mechanical activation of metal hydrides
may introduce contamination to the reaction product, from
either starting materials, balls, or vial. Insufficient activation or
utilization of partially hydrogenated metal samples will result in
metal hydrides or metals in the reaction product. Therefore, we
devised an efficient protocol for borohydride extraction and
purification of reaction products by solvent extraction with,
e.g., THF, Me S, or Et O (Figure 5). Continuous extraction,
utilizing a Soxhlet extraction apparatus (operated under inert
atmosphere), can be employed, being especially advantageous
for compounds with limited solubilities. Only the metal
borohydride component is soluble in THF, Me S, or Et O and
metals, metal hydrides, and oxides are insoluble. Established
and experimentally estimated RT solubilities of the metal
borohydrides studied are listed in Table 2. Filtration of the
resulting solution and concentration in vacuo yields the
purified metal borohydride solvate complex. Photographs of
colorful solutions of metal borohydrides are found in the
Supporting Information (Figure S6).
Dimethylsulfide Borane (DMSB) vs Tetrahydrofuran
Borane (THFB). Potentially, exchanging DMSB for THFB
could serve as a practical alternative. First, the strongly
6
9
unpleasant odor of dimethylsulfide would be avoided.
Second, a THF based reagent would promote the formation
of metal borohydrides soluble in THF and essentially combine
synthesis with purification. Third, the borane concentration is
lower (only 1 M available), leaving a safer procedure.
Ultimately, implementing THFB for DMSB failed, due to the
competing formation of tributoxyborane from the decom-
position of THFB (Figures S6 and S7, Supporting
Information).
2
2
2
2
Other Reaction Solvents. On the basis of the above
findings, and combining knowledge of metal borohydride
solubilities and employing THF (or Et O) as solvent with
2
DMSB, a highly efficient synthesis methodology for certain
metal borohydrides was devised. For instance, LiBH is highly
3.5. Desolvation and Polymorphic Control. Solvents
coordinated to the metal cation can normally be removed by
simultaneously employing dynamic vacuum and heating. The
4
70−74
soluble in THF (and Et O) at RT,
while Ca(BH4)2,
2
Sr(BH ) , and Sm(BH ) are reasonably soluble in THF (see
4
2
4 2
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elevated temperatures, T ≈ 230 °C, α-Yb(BH ) transforms to
4
2
β-Yb(BH ) , with α′-Yb(BH ) and γ-Yb(BH ) as intermedi-
4
2
4 2
4 2
ate compounds, observed by in situ SR-PXD (SI_Yb).
Desolvation of Yb(BH ) ·xMe S at T = 140 °C yields γ-
4
3
2
Yb(BH ) (i.e., a reduction takes place), whereas desolvation
4
2
of Yb(BH ) ·2THF at T = 180 °C provides α-Yb(BH )
4
2
4 2
(
Table 3). Structural details are described in section 3.6. The
purification and transformation protocols presented here form
the basis for transforming a large number of metal borohydride
solvates, into previously unknown metal borohydride solvent
complexes and further into other polymorphs of solvent free
metal borohydrides.
Figure 5. Schematic overview of the purification process. Starting
from the “impure product” (center); “Extraction” provides a
borohydride solution, M(BH ) , leaving solid impurities undissolved;
3.6. Structural Characterization of Synthesized New
Metal Borohydrides. A large number of new metal
borohydrides and solvates were synthesized and structurally
characterized in this study (see Supporting Information). The
majority of these new materials adopt known structure-types
similar to other metal borohydride solvates.
Metal Borohydride Tetrahydrofuran Complexes. LiBH4·
xTHF. An unknown lithium borohydride THF complex was
obtained. No structural refinements were performed due to
4
n
“Filtration” separates solid impurities from the metal borohydride
solution. Concentration in vacuo “Vac./Conc.” yields a solid metal
borohydride solvate complex, M(BH ) ·Solv. (s), and “Desolvation”
4
n
results in the isolation of the purified metal borohydride M(BH ) .
4
n
specific conditions applied for desolvation are crucial to avoid
decomposition of the metal borohydride and to obtain a
specific polymorph hereof. For instance, removal of THF from
Mn(BH ) ·3THF will result in the decomposition of Mn-
limited diffraction data. LiBH ·3THF is reported to form at
4
low temperatures, transforming into LiBH ·1THF at ambient
4
8
9,91
temperature.
Etherates of LiBH have been structurally
4
71,88
4
2
investigated elsewhere.
(
BH ) , caused by the high temperatures needed to release the
4 2
85
Mg(BH ) ·xTHF. A magnesium borohydride THF complex
ligand. In certain cases, a solvated metal borohydride can also
be required before attaining a desired polymorph. For instance,
nanoporous γ-Mg/Mn(BH ) are obtainable solely by gently
4 2
was prepared (Table 1, 3b), but detailed structural analysis was
unsuccessful due to limited quality of the PXRD data.
However, the recorded pattern is clearly different from the
4
2
1
11,12
desolvating Mg/Mn(BH ) · / Me S.
An efficient and
4
2
2
2
9
2
reported pattern of Mg(BH ) ·3THF. Thermal analysis
controllable approach for desolvation is developed in this
work. Initially, the sample is heated to a fixed temperature
under ambient pressure, followed by vacuum extraction of the
generated solvent vapor. Thermal analysis, TGA (see
Supporting Information) is helpful to formulate the conditions
for desolvation, and specific approved conditions to obtain
certain polymorphs is compiled in Table 3.
4 2
suggests less than 3THF in the formula unit (SI_Mg).
Accordingly, we denoted it Mg(BH
M(BH ·nTHF (M = Ca, Sr, Ba, Sm, Yb, n = 1, 2).
Ca(BH ·2THF and Yb(BH ·2THF, were found to be
isostructural to M(BH ·2THF, M = Sr, Ba, and Sm. The
) ·xTHF (Tables 1 and 3).
4 2
)
4 2
)
4
)
4 2
2
20,76
)
4
2
structure is described as an infinite chain of “−M−(BH ) −
4 2
Magnesium Borohydride. Preparation of nanoporous γ-
M−” along the a-axis as shown below (Figure 6). Different
chains are connected by weak interactions between neighbor-
ing apolar THF moieties.
1
1,12
Mg(BH4)2
is conducted in one step (simultaneous vacuum
1
and heating) by desolvating Mg(BH ) · / Me S, which
4
2
2
2
demands strict control of temperature, i.e., T < 90 °C
Table 3 and SI_Mg, Supporting Information). Obtaining α-
Mg(BH ) requires a two-step procedure of heating Mg-
Upon heating, THF is lost from M(BH
Sr, Yb) in two distinct steps, showing that two different,
isolatable solvates exist, M(BH ·2THF and M(BH ·1THF.
Thus, tetrahydrofuran complexes, Sm(BH ·1THF and Yb-
(BH ·1THF, were also discovered. Sm(BH ·1THF was
collected directly after synthesis. Sr(BH ·1THF, Sm(BH
1THF, and Yb(BH ·1THF are isostructural to Ca(BH
4 2
) ·2THF (M = Ca,
(
)
2
)
4 2
4
2
4
1
(
BH ) · / Me S to 140 °C for 3−5 h, followed by evacuation
)
4 2
4
2
2
2
of the resulting Me S vapor. Desolvation of Mg(BH ) ·2Et O
4
)
2
)
4 2
2
4 2
2
or Mg(BH ) ·3THF irreversibly results in the formation of a
)
4
2
4
)
)
2
·
·
4
2
high temperature polymorph, β-Mg(BH ) , at T ≈ 200
)
4 2
4
2
4
2
8
2−84
°C.
In our work, amorphous materials were obtained
1THF, described in the literature as a double-stranded, one-
dimensional polymeric structure (Figure 7). Note that we
93
when heating Mg(BH ) ·xTHF at T > 200 °C (Table 3 and
4
2
SI_Mg).
did not succeed in preparing Sm(BH
1THF (see Table S2).
) ·2THF, nor Ba(BH ) ·
4 2 4 2
Calcium Borohydride. Following the synthetic protocol
described in this work, completely pure α-Ca(BH ) is formed
M(BH ) ·nTHF (M = Nd, Sm, Gd, Yb, Y, n = 1.5, 3). For a
4
2
4 3
directly at RT (Table 1, 4a, 4b, and Figure Ca_VI, Supporting
Information). Nearly pure α-Ca(BH4)2 (with residual β-
Ca(BH ) ) can be obtained by heating Ca(BH ) ·2THF to
number of trivalent metal borohydrides, contrasting observa-
tions to earlier reported findings were experienced. Thermog-
ravimetric data for all metal borohydride solvates investigated
in this work are in accord with the expected structure and
composition, except for the series of M(BH ) ·3THF, M = Y,
4
2
4 2
T = 200 °C, while removing the solvent vapors by evacuation
and abruptly cooling (quenching) the sample to T ≈ 0 °C
4
3
86,87
(
Figure Ca_8, Supporting Information).
β-Ca(BH ) is
Nd, Sm, Gd, and Yb (see also the individual sections in the
Supporting Information).
4
2
readily obtained from Ca(BH ) ·2THF at T > 180 °C, with
4
2
minor quantities of α-Ca(BH ) forming.
Nd(BH ) ·1.5THF. Extraction of Nd(BH ) with THF (Table
4
2
4 3
4 3
Ytterbium Borohydride. Four halide free polymorphs of
2) results in a new monoclinic, P2 , neodymium borohydride
1
divalent Yb(BH ) were discovered in this work and denoted
THF-complex with an apparent stoichiometry Nd(BH4)3·
4
2
in parallel to their isostructural calcium analogues, α-
Yb(BH ) , α′-Yb(BH ) , β-Yb(BH ) , and γ-Yb(BH ) . At
1.5THF (Le-Bail fit). This contrasts the reported composition
94
Nd(BH ) ·3THF obtained from NaBH and NdCl in THF.
4
2
4 2
4 2
4 2
4 3
4
3
H
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
of Nd(BH ) ·1.5THF and the trivalent samarium borohydride
4
3
THF complex indicate similarities in structure and composi-
tion (see Figure Sm_VI in the Supporting Information).
Thermal analysis with mass spectroscopy reveals a liberation of
diborane while desolvating, but before decomposition,
indicative of a reduction of a Sm(III) borohydride to a Sm(II)
borohydride material. The collected (after desolvation/
reduction) dark/purple material is identified as Sm(BH ) by
4
2
XRD (Figure Sm_VIII).
M(BH ) ·3THF (M = Y, Gd, Yb). Yb(BH ) ·3THF was
4
3
4 3
collected after extraction of Yb(BH ) ·xMe S (Table 1, 11b)
4
3
2
with THF and was found to adhere to the structure of
9
5,96
Y(BH ) ·3THF and Gd(BH ) ·3THF.
Thermal analysis
4
3
4 3
fits well with the stoichiometry of 3THF, and the material
releases diborane, being reduced to Yb(BH ) (see SI_Yb). As
4
2
mentioned, Y(BH ) ·3THF and Gd(BH ) ·3THF are both
4
3
4 3
structurally similar to Yb(BH ) ·3THF, yet thermal analysis
4
3
indicates release of only part of their complexed THF. We
speculate that higher boranes may form, strongly complexing
the remaining THF, i.e., M(III)B H ·3-zTHF. We note that
x
y
the THF complexes that undergo this incomplete desolvation
−
2+
3+
appear to be those not reduced by BH , e.g., Mg , Y , and
4
3
+
Gd .
Metal Borohydride Dimethylsulfide Complexes. Yb(BH ) ·
xMe S. A new ytterbium borohydride Me S complex was
4
3
Figure 6. Representative views of crystal structure models of
M(BH ) ·2THF (M = Ca, Sr, Ba, Yb) along the a-axis (A) and b-
2
2
4
2
discovered and collected as a light green solid (Table 1, 11b).
The structure appears to be composed of a large unit cell based
on preliminary analysis of diffraction data (see SI_Yb).
Nd(BH ) ·1Me S. A new monoclinic (P2 c) neodymium(III)
axis (B). Orange spheres − metal cation (M), red − oxygen, light gray
−
carbon, light blue − boron. Transparent tetrahedra − BH groups.
4
Hydrogens are omitted for clarity.
4
3
2
1
borohydride solvate, Nd(BH ) ·1Me S (Table 1, 8a) is
4
3
2
This observation may be explained by loss of 1.5 THF during
sample collection using our methodology. Thermogravimetric
data reveals three distinct, similar mass losses for the new
Nd(BH ) ·1.5THF complex (total observed and calculated
synthesized from a purple dimethylsulfide solution and
crystallizes as a pale purple solid. It is isostructural to the
4
7
well-known M(BH ) ·1Me S (M = Y, Gd), and this
4 3 2
structural resemblance allowed us to perform full structural
characterization (Figure 8).
4
3
mass loss are 38.4 and 36.4 wt %, respectively). Furthermore,
we recover α-Nd(BH ) (see below) after desolvation, clearly
New Solvent Free Metal Borohydrides. Nd(BH
vation of Nd(BH ·1Me S yields α-Nd(BH , which in
analogy to the solvate complex described above is isostructural
) . Desol-
4 3
4
3
emphasizing that all THF is removed (SI_Nd).
4
)
3
2
)
4 3
Sm(BH ) ·1.5THF. Trivalent samarium borohydride com-
4
3
47,97−99
plexed with THF was synthesized (Table 1, 9b). Collection of
an off-white solid with a diffraction pattern clearly different
to M(BH ) (M = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb).
4 3
The material is collected as a cubic (Pa-3), pale purple
substance (see Supporting Information).
2
3
from the reported Sm(BH ) ·3THF and a mass loss
4
3
corresponding to 1.5 THF may suggest a composition of
Yb(BH ) . The four new ytterbium borohydrides crystallize
4
2
Sm(BH ) ·1.5THF. Interestingly, the X-ray diffraction patterns
in the same space groups as α-Ca(BH ) (F2dd), α′-Ca(BH )
4
3
4 2
4 2
Figure 7. Representative views of crystal structure models of M(BH ) ·1THF (M = Ca, Sr, Sm, Yb) along the a-axis (A) and b-axis (B). Dark blue
4
2
spheres − metal cation (M), red − oxygen, light gray − carbon, light blue − boron. Transparent tetrahedra − BH groups. Hydrogens are omitted
4
for clarity.
I
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. Representative views of crystal structure models of α-Nd(BH ) viewed along the a-axis (A), and Nd(BH ) ·1Me S also viewed along the
4
3
4
3
2
a-axis (B). Color code; purple spheres − neodymium, yellow − sulfur, light gray − carbon, light blue − boron. Transparent tetrahedra − BH
4
groups, gray − hydrogen. Hydrogens in BH units are omitted for clarity.
4
(
(
I4
̅
2d), β-Ca(BH ) (P4
̅
), and γ-Ca(BH ) (Pbca), respectively
combination of coordinating solvents. As a result, five new
solvent free metal borohydrides, α-Nd(BH ) , α-Yb(BH ) , β-
4
2
4 2
1
00,101
Figure 9),
completing the full structural analogy
4
3
4 2
Yb(BH ) , γ-Yb(BH ) , α′-Yb(BH ) , and six new metal
4
2
4 2
4 2
borohydride solvate complexes, M(BH ) ·2THF (M = Ca,
4
2
Yb); M(BH ) ·1THF (M = Sr, Sm, Yb) and Nd(BH ) ·1Me S,
4
2
4 3
2
were discovered and characterized structurally, physically, and
chemically. Additionally, three new and unknown THF and
Me S solvate complexes, Nd(BH ) ·1.5THF, Sm(BH ) ·
2
4
3
4 3
1
1
.5THF, and Yb(BH ) ·xMe S, were discovered. This totals
4 3 2
4 new borohydride materials synthesized, alongside the
already known materials also synthesized by this method.
This approach provides metal borohydrides in near quantita-
tive yields with high purity and exhibits a high degree of
polymorphic control. Postreaction transformations to obtain
specific polymorphs are also described. We anticipate that this
general synthesis approach can be readily extended to synthesis
of other metal borohydrides for which ionic or polar covalent
metal hydrides exist. This constitutes a direct pathway to high
purity metal borohydrides, without the formation of halides
and other impurities, thus allowing for proper investigation of
physical properties, e.g., magnetism, photoluminescence, ionic
conductivity, gas adsorption, hydrogen storage, and more. The
metal borohydride products may also prove valuable as
selective organic reduction agents, or serve as starting materials
for synthesis of higher boranes. The results presented here and
details about the reaction mechanism may be inspirational for
further development of supplementary strategies to provide
novel functional materials.
Figure 9. Crystal structure of α-Yb(BH ) viewed along the c-axis, β-
4
2
Yb(BH4)2 along the c-axis, α′-Yb(BH ) along the a-axis, and γ-
4
2
Yb(BH ) along the c-axis. Color code; green spheres − ytterbium,
4
2
blue transparent tetrahedra − BH complexes. Hydrogens in BH
4
4
units are omitted for clarity.
between calcium and ytterbium borohydride (see SI_Ca and
SI_Yb, Supporting Information). Previously reported Cl-
substituted tet- and ortho-Yb(BH ) Cl are isomorphous to
4
2−x
x
102
β-Ca(BH ) and γ-Ca(BH ) .
4
2
4 2
ASSOCIATED CONTENT
■
4
. CONCLUSIONS
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01398.
A new general approach with an integrated purification step,
for the synthesis of a wide variety of metal borohydrides and
their solvates, from metal hydrides and a borane donating
complex is demonstrated. The approach is well documented
and tested, and the results are rationally compiled as a
collection of individual synthesis “recipes” for Li, Na, Ca, Mg,
Sr, Ba, Y, Nd, Gd, Sm, and Yb borohydrides and their solvates.
Commercially unavailable metal hydrides are synthesized
directly from the elements and mechanochemically activated
in order to maximize their reactivity. Only ionic and polar
covalent metal hydrides were found to possess reasonable
reaction kinetics and form metal borohydrides. This finding
correlates well to a nucleophilic addition reaction mechanism.
A general purification routine is devised, utilizing a
1H NMR spectra, thermal analysis data, mass spectros-
copy data, photographs and FTIR spectra (PDF)
Accession Codes
CCDC 1845345−1845355 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
J
DOI: 10.1021/acs.inorgchem.8b01398
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Accompanied by Structural Transition. Appl. Phys. Lett. 2007, 91,
AUTHOR INFORMATION
Corresponding Authors
(B.R.) E-mail: richter@chem.au.dk.
(T.R.J.) E-mail: trj@chem.au.dk.
■
2
(
24103.
9) Paskevicius, M.; Richter, B.; Polans
Jensen, T. R. Sulfurized Metal Borohydrides. Dalton Trans. 2016, 45,
39.
(10) Schouwink, P.; Didelot, E.; Lee, Y. S.; Mazet, T.; Cerny, R.
́
́
ki, M.; Thompson, S. P.;
*
*
6
ORCID
Structural and Magnetocaloric Properties of Novel Gadolinium
Jakob B. Grinderslev: 0000-0001-7645-1383
Kasper T. Møller: 0000-0002-1970-6703
Mark Paskevicius: 0000-0003-2677-3434
Torben R. Jensen: 0000-0002-4278-3221
Borohydrides. J. Alloys Compd. 2016, 664, 378−384.
(11) Filinchuk, Y.; Richter, B.; Jensen, T. R.; Dmitriev, V.;
Chernyshov, D.; Hagemann, H. Porous and Dense Magnesium
Borohydride Frameworks: Synthesis, Stability, and Reversible
Absorption of Guest Species. Angew. Chem., Int. Ed. 2011, 50,
Notes
1
(
1162−11166.
The authors declare no competing financial interest.
12) Richter, B.; Ravnsbæk, D. B.; Tumanov, N.; Filinchuk, Y.;
Jensen, T. R. Manganese Borohydride; Synthesis and Character-
ization. Dalton Trans. 2015, 44, 3988.
(13) Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Nyassi, A.;
ACKNOWLEDGMENTS
■
The Innovation Fund Denmark (Project HyFill-Fast) and the
Danish Research Council for Nature and Universe (Danscatt).
We are grateful to the Carlsberg Foundation. The Fuel Cells
and Hydrogen Joint Technology Initiative under Grant
Agreement No. 303428 BOR4STORE. Danish council for
independent research (HyNanoBorN, DFF-4181-00462).
NordForsk and The Nordic Neutron Science Program is
acknowledged for funding of FunHy (Project No. 81942). The
Australian Research Council for Future Fellowship
FT160100303. The Danish National Research Foundation is
thanked for funding to the Center for Materials Crystallog-
raphy (CMC, DNRF93). We are grateful to the Swiss-
Norwegian Beamline at ESRF, Grenoble and P02 at DESY,
Hamburg. Access to Transmission Electron Microscopy
facilities, funded by the Villum Foundation at the Interdiscipli-
nary Nanoscience Center (iNANO), is duly appreciated.
K.T.M. thanks The Independent Research Fund Denmark for
International Postdoctoral grant 8028-00009B.
Sitzmann, H.; Visseaux, M. Synthesis and X-ray Crystal Structures
of (C
Precursors for Polymerization Catalysts. Eur. J. Inorg. Chem. 2000,
000, 2333−2336.
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Baudry, D. Stereospecific Polymerization of Isoprene with Nd-
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DOI: 10.1021/acs.inorgchem.8b01398
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