Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB3H8 (M=K, Rb, and Cs), Mechanisms of Formation, and the Crystal Structure of KB3H8

Article abstract of DOI:10.1002/anie.201812795

A facile synthesis of heavy alkali metal octahydrotriborates (MB₃H₈; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF?BH₃, does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH₄/NaBH₄ with THF?BH₃ have to be carried out under reflux. The high stability and solubility of KB₃H₈ were examined, and a crystal structure thereof was obtained for the first time.

Full text of DOI:10.1002/anie.201812795

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International Edition Chemie
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Accepted Article
Title: Facile Syntheses of Unsolvated Alkali Octohydrotriborate Salts
MB3H8 (M = K, Rb, and Cs), Mechanisms of Formation, and the
Crystal Structure of KB3H8
Authors: Xi-Meng Chen, Nana Ma, Xin-Ran Liu, Changgeng Wei,
Chongchao Cui, Bu-La Cao, Yanhui Guo, qinfen Gu, Lai-
Sheng Wang, and Xuenian Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812795
Angew. Chem. 10.1002/ange.201812795
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Angewandte Chemie International Edition
10.1002/anie.201812795
COMMUNICATION
Facile Syntheses of Unsolvated Alkali Octohydrotriborate Salts
MB H (M = K, Rb, and Cs), Mechanisms of Formation, and the
3
8
Crystal Structure of KB H
3
8
[
a]†
[a]†
[a]
[a]
[a]
[a]
Xi-Meng Chen, Nana Ma, Xin-Ran Liu, Changgeng Wei, Chongchao Cui, Bu-La Cao, Yanhui
[c]
[d]
[e]
[a,b]
Guo,* Lai-Sheng Wang,* Qinfen Gu, and Xuenian Chen*
–
–
Abstract:
octahydrotriborates (MB
simply through reactions of pure alkali metals with THF·BH
A
facile synthetic method of heavy alkali metal
, M = K, Rb, and Cs) has been developed
without
that the nucleophilicity of the B-H bond in BH
an important role. Surprisingly, this convenient synthetic method
did not work for the preparation of KB , probably because of
the poor solubility of KBH in THF solution.
Here we report a facile method for the syntheses of the
4 2 7
and B H play
3 8
H
3
3 8
H
[
19]
using electron carriers or other reaction media, such as mercury,
silca gel or inert salts as done previously, at room temperature with
very high yields. However, no discernable reactions were observed
when pure Li and Na were used. The reaction mechanisms for the
heavy alkali metals were investigated both experimentally and
computationally. It is found that the low sublimation energies of K,
Rb, and Cs are the key factor for the initiation of the reactions. The
syntheses can be carried out at room temperature because all the
elementary reaction steps have low energy barriers, which are
4
–
8
heavier alkali salts of B
3
H
, MB
3
H
8
(M = K, Rb, and Cs), by
in
directly reacting the respective alkali metals with THF·BH
3
THF at room temperature. In the current method, no electron
carriers, such as naphthalene and triphenylboron or other
reaction media, such as mercury, silica gel, and inert salts, were
[
8,12-16,20-22]
used, as done in previous works.
synthetic method allows convenient preparation of the heavier
MB alkali salts in large scales with very high yields. We also
The simplified
completely different from the reactions of Li/NaBH
under reflux. The high stability and solubility of KB
4
with THF·BH
are examined
3
3 8
H
3
H
8
report a detailed study of the mechanisms of formation of KB
3 8
H
and its crystal structure is obtained for the first time.
and its crystal structure for the first time.
In our new synthetic method, pure metals (M = K, Rb, Cs) are
reacted directly with THF·BH to produce MB and MBH at
room temperature according to Eq. 1. It was observed that the
reaction rate depends mainly on the particle-size of the metals.
3
3
H
8
4
–
The octahydrotriborate anion, B
3
H
8
, has attracted widespread
attention since its derivatives can be used as chemical hydrogen
[
1-4]
storage materials (NH
deposition (CVD) precursors [Mg(B
reducing agents for a series of organic reactions.
3
B
3
H
7
and NH
4
B
3
H
8
),
chemical vapor
To prepare KB
pieces of ~ 3 × 3 × 3 mm in size and reacted it with 1 M
THF·BH solution (300 mL, 300 mmol). The reaction was
complete in 12 h and the reactants were converted quantitatively
3 8
H , we cut 5.85 g (150 mmol) potassium to
[5,6]
3
H
8
)
2
and Cr(B
3
[
H
8
)
2
],
and
3
7,8]
However,
3
although its synthetic method and reactivity have been
continually improved and explored, its mechanism of formation
11
to KB
completion of the reaction, the KBH
the THF in the filtrate was removed under dynamic vacuum to
produce a THF-solvated KB product. Addition of toluene to
the THF-solvated product led to the precipitation of unsolvated
KB with an isolated yield of 90% by filtration (Figures 1 and
3
H
8
and KBH
4
on the basis of B NMR (Figure S1). Upon
[
8-18]
has not been fully understood.
reported that MB (M = Li and Na) could be synthesized by
reacting the corresponding borohydrides with THF·BH
Very recently, our group
4
precipitate was filtrated and
3
H
8
3
in THF
3
H
8
[
19a]
–
under reflux.
elucidated, involving
undergoes H elimination via a high barrier.
The reaction mechanism of
B
3
H
8
was
–
a
key
B
3
H
10
intermediate, which
3
H
8
[
19a]
2
We also found
S2), which represents the highest yield obtained so far for the
preparation of alkali metal salts of octahydrotriborate (Table
S1).
[
a]
X.-M. Chen, R.-X. Liu, C. Wei, C. Cui, B.-L. Cao, Dr. N. Ma, Prof. Dr.
X. Chen
2 M + 4 THF·BH
3 3 8 4
→ MB H + MBH + 4THF
School of Chemistry and Chemical Engineering, Henan Key
Laboratory of Boron Chemistry and Advanced Energy Materials,
Henan Normal University
(
M = K, Rb, and Cs)
(1)
Xinxiang, Henan 453007, China
E-mail: xnchen@htu.edu.cn (X.C.)
Prof. Dr. X. Chen
College of Chemistry and Molecular Engineering,
Zhengzhou University
Zhengzhou, Henan 450001, China
Prof. Dr. Y. Guo
Department of Materials Science, Fudan University
Shanghai, 200433, China
E-mail: gyh@fudan.edu.cn (Y.G)
Prof. Dr. L.S. Wang
Department of Chemistry, Brown University,
Providence, Rhode Island 02912, USA
E-mail: Lai-Sheng_Wang@brown.edu (L.S. W.)
Dr. Q. Gu
[b]
[c]
[d]
[e]
11
1
Figure 1. B NMR (a) and H NMR (b) spectra of the unsolvated KB
CD CN.
3 8
H in
3
Similar reactions at room temperature for Rb and Cs
produced unsolvated RbB and CsB with isolated yields of
3
H
8
3 8
H
70% and 73%, respectively (Figures S3-10). The reaction time
was 5 h for Rb and 2 h for Cs, much shorter than that for K.
However, similar procedures using Na or Li could not produce
Australian Synchrotron (ANSTO),
8
00 Blackburn Rd, Clayton, 3168 VIC, Australia
†
These authors contributed equally to this work.
NaB
and 12).
3 8 3 8
H or LiB H even at elevated temperatures (Figures S11
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Angewandte Chemie International Edition
10.1002/anie.201812795
COMMUNICATION
+
2-
9
Figure 2. Energy profiles for the reaction of K + THF·BH
3
 (2K )B
3
H
calculated at the SMD(THF)/M062X/6-311++G(d,p) level.
(
Table 1), which implies that the nucleophilic ability of the B-H
2
–
–
bond in B
B
2
H
6
is similar to that of the B-H bond in BH
4
and
–
–
–
2
H
7
, which reacted with THF·BH
3
to form B
2
H
7
3 10
and B H
with
a
free energy barrier of 15.8 and 10.3 kcal/mol,
[
19]
2–
respectively. The nascent B
3
H
9
in this reaction then cleaves
–
–
to B
Figure 3). Finally, the B
fourth THF·BH to form B
converted to the B
barrier of 3.7 kcal/mol. In another possible pathway, if BH
2
H
5
and BH
4
(TS5) with a free energy of 5.2 kcal/mol
–
(
2
H
5
intermediate reacts with BH
3
in the
(TS6, 22.3 kcal/mol), which is
8
H final product via TS7 with a free energy
–
3
3 8
H
–
3
–
3
–
–
reacts with two THF·BH
3
molecules, a B
3
H
9
([BH
3
BH
2
3
(-H)BH ] )
Table 1.Calculated electronic energy (kcal/mol at 0 K), enthalpies (kcal/mol at
98 K), and free energy (kcal/mol at 298 K).
2
∆
(0K)
E
∆H
(298K)
∆G
(298K)
+
2-
9
Figure 3. Energy profiles for the reaction of (2K )B
3
H
 KB
3
H
8
calculated at
the SMD(THF)/M062X/6-311++G(d,p) level.
(THF•BH
3
)(K)
+ K
0.0
-4.8
0.0
-4.0
0.0
1.9
THF•BH
3
TS1
11.7
0.2
4.2
We further investigated the mechanisms of formation of
octahydrotriborate from the direct reactions of K, Rb, and Cs
+
-
K BH
3
-THF
9.7
-5.4
-4.2
+
-
(
K BH
3
-THF)+K
-13.9
-12.2
-19.0
-34.4
-27.0
-25.7
-31.7
-39.8
-14.9
-12.6
-20.0
-16.8
3
with THF·BH both experimentally and theoretically. Theoretical
TS2
+
2-
calculations were performed using density functional theory
2K BH
3
-THF
+ THF•BH
TS3
+
2-
[
23]
2
K BH
3
3
(
DFT) in the Gaussian 09 program. The proposed mechanism
includes seven steps (Figures 2 and 3). In the first step, bulk K
was solvated by THF·BH and THF followed by a single-electron
donation from the K atom to the BH group in the THF·BH
. Then, the second K atom donates its
-13.1
-79.4
-65.2
-74.5
-65.7
-25.6
-99.0
-90.6
-97.0
-87.2
-4.9
+
2-
6
2
K B
2
+
H
-THF
-74.1
-75.9
-74.9
-61.5
3
2-
2
2 6
K B H
+ THF•BH
TS4
3
3
+
2-
6
2
K B
2
H
3
–
complex to form BH
valence electron to BH
of the two transition states (TS) are 4.2 (TS1) and 2.3 kcal/mol
3
–
2–
+
2-
3
to form BH
3
. The free energy barriers
2K B H -THF
3
+
9
-95.0
-82.6
-76.7
-85.4
-105.5
-97.3
-91.0
-98.3
-81.4
-83.1
-77.9
-85.6
2-
9
2K B
3
H
(
TS2) for the reactions of K (on the basis of K gas state) with
TS5
KBH +KB
KB
2
–
THF·BH
3
in THF solvent (Table 1). In the next step, the BH
3
4
2 5
H
species undergoes a nucleophilic substitution reaction using its
lone pair electrons to replace THF from a second THF·BH to
with an energy barrier of 15.1 kcal/mol (TS3). This
2
H
5
+ THF•BH
3
-69.3
-52.2
-90.6
-68.7
-91.0
32.1
8.1
-89.6
-76.8
-110.2
-96.9
-110.9
9.2
-6.0
10.8
-0.2
-76.4
-63.3
-97.2
-93.5
-98.2
9.3
-6.0
13.5
1.2
TS6
3
KB
2
H
5
- BH
TS7
3
-THF
2
–
form B
step is exothermic by 71.0 kcal/mol (Figure 2). Then, B
attacks the B in a third THF·BH to replace THF using one of its
2 6
H
2–
KB
3 8
H
2
H
6
TS2-1
+
-
3
K B H -THF
2
6
2
9
–
2–
TS3-1
23.5
8.5
B-H bonds to form B
3
H
([BH BH
3 2
3
(-H)BH ] ). The free energy
+
-
9
K B
3
H
-THF
barrier of TS4 is calculated to be 14.4 kcal/mol in THF solvent
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Angewandte Chemie International Edition
10.1002/anie.201812795
COMMUNICATION
2
–
not B
3
H
9
is formed (red part in Scheme S1). This reaction will
started from solid metals. Thus, the sublimation energy of the
alkali metals must be the key factor that determines whether the
reaction happens or not. The high sublimation energies of Li
undergo two transition states TS2-1 and TS3-1 by 13.5 and 19.5
kcal/mol, which are much higher than TS2 and TS3 (Table 1).
2
3
–
[25]
Hence, the reaction via the BH
than the direct conversion form BH
The proposed mechanism is supported by experimental
observations. The reactions of K, Rb, and Cs with THF·BH can
be carried out at room temperature, while the reaction of Li and
Na borohydrides with THF·BH have to take place under reflux
in THF solution. The different temperature-dependence of the
two types of reactions is consistent with the different reaction
mechanisms. In the reaction of K with THF·BH
intermidate is more favorable
(38.0 kcal/mol) and Na (25.6 kcal/mol) (Table S3) prevent the
–
–
3
to B
3
H
9
.
Li or Na atoms from escaping from the metal lattice to take part
in the reaction. This conclusion is supported by the fact that the
–
[12,22]
3
Li/Hg or Na/Hg alloys can react with THF·BH
Further evidence is provided by the fact that the reaction rate
of K with THF·BH is influenced by the size of the metal. When
3 3 8
to form B H .
3
3
[
19]
3
the potassium metal was cut to 5 × 5 × 5 mm , the reaction time
increased to 20 h, which is significantly longer than 12 h for a
2–
3
3
, the B
3
H
9
smaller piece of potassium (3 × 3 × 3 mm ) at room temperature.
intermediate (after TS4 in Figure 3) could directly cleave to
The smaller particle with a larger surface area enhances the
interaction of K with THF·BH , leading to the faster reaction.
3
Both Rb and Cs have lower melting points than K. Under
vigorous stirring, the metal pieces easily deform and seem to
display fluid-like behaviors. Thus, the reactions of Rb and Cs
–
–
–
B
2
H
5
and BH
4
. The B
to form the final B
2
H
5
intermediate then reacted directly
–
with THF·BH
3
3
H
8
product. In all steps, the
highest energy barrier is 22.3 kcal/mol (TS6). Thus, the reaction
can be carried out at room temperature, but not at low
temperatures (Figure S13). The total reaction is exothermic by
3
with THF·BH are much more vigorous and the reaction times
9
8.2 kcal/mol, and is thus irreversible. On the other hand, in the
are much shorter.
In the literature, pure alkali metals reacted with THF·BH to
3
[
19]
–
reaction of LiBH
intermediate was suggested to cleave to B
then dihydrogen interactions between a partially negatively-
4
with THF·BH
3
as reported recently, a B
3
H
first;
10
–
2
H
6
and BH
4
3 8 4
afford MB H and MBH (M = Na, K) at moderate yields only
o
when they were absorbed on silica gel at 160 C, finely
–
–
4
[13,14]
charged H in the BH
group and the partially positively-
molecule lead to the formation of
Subsequently, one molecule was
)BH and the resulting BH group was
product. A high
was found in the dihydrogen
interaction, so that this reaction can only take place under reflux
dispersed in inert salts by ball milling,
or in the form of liquid
K/Na alloy. Compared with the previous methods, the current
synthesis using pure potassium metal with THF·BH has several
+
[15]
2 6
charged bridge H in the B H
–
B
2
H
5
and (H
2
)BH
3
.
H
2
3
eliminated from (H
2
3
3
advantages. First, the reaction is carried out without using
mercury or any other electron carriers, greatly simplifying the
subsequent workup. Furthermore, the quantitative conversion
and short reaction time make the reaction much more efficient.
–
5
–
combined with B
2
H
3 8
to form the final B H
[
19]
energy barrier (31.0 kcal/mol)
with the release of H
2
.
Finally, the solubility difference between KB
these two compounds to be readily separated: KB
in THF while KBH is almost insoluble in THF.
The unsolvated KB is more stable than the unsolvated
NaB in the solid state or in aqueous or THF solution (Figure
3
H
8
and KBH
4
allows
2
–
2–
The BH
3
and B
2
H
6
intermediates were previously monitored
in
THF solution. However, when we monitored the reaction of K
3 8
H
is soluble
1
1
by B NMR in the reaction of potassium naphthalide and B
2
H
6
4
[
24]
3
H
8
1
1
with THF·BH
the starting material THF·BH
S14). The other KBH product is completely insoluble in THF, so
3
by B NMR in the current study, we only observed
3 8
H
[
26]
[12-14]
3
and the product KB (Figure
3
H
8
S16).
Though NaB
3
H
8
is soluble in ethers,
strong
4
solvation makes it difficult to be accurately measured for further
[
2,27]
that its NMR signal was not detected but its formation was
confirmed by XDR (Figure S15). No intermediate signals were
detected in the whole reaction. These observations suggested
use.
3 8
Unsolvated NaB H can be obtained by dissolving the
oily THF-solvated product in diethyl ether and dichloromethane,
which will help to break the coordination between THF and the
+
+
[12-14]
that the transformation from K(s) to solvated [K(THF)
6
] , and at
is the rate-
Na cation to completely remove THF.
coordination bond between the THF molecule and the potassium
cation is so weak that unsolvated KB can be conveniently
prepared. After the reaction of potassium and THF·BH (Eq.1),
the KBH precipitate was filtrated out and the THF in the filtrate
could be removed by dynamic vacuum to produce THF-solvated
KB product. The addition of toluene to the THF-solvated
product leads to the precipitation of unsolvated KB . With pure
unsolvated KB in hand, its structure was characterized by
3 8
For KB H , the
–
2–
the same time, from THF·BH
determining step, consistent with the theoretical results. In the
current condition, THF·BH is always excessive, so that once
are formed they will be rapidly transferred to the
3
to BH
3
and BH
3
3
H
8
3
3
–
2–
BH
3
and BH
3
4
next intermediate all the way to the final product, because the
energy barrieries in all steps are low.
In the current mechanism, the influence of different alkali
metals on the reaction should mainly involve the first two steps,
in which the alkali metals donate their valence electron to the
3
H
8
3 8
H
3
H
8
synchrotron X-ray powder diffraction (XRPD) (see Figure S17 for
the refinement profile). Most of the diffraction peaks can be
2–
BH
3
group to form BH
3
(black part in Figure 2 and Scheme S1),
because the energy profiles in the subsequent steps should be
similar for the different alkali metals (blue part and black part in
Figures 2, 3 and Scheme S1). Thus, we expected the different
chemical activity of the alkali metals to be crucial to explain why
pure Li and Na could not initiate the reaction. However, the
computational results indicate that the free energy barriers of
TS1 and TS2 for Li, Na and K are similar (Table S2). It should
be pointed out that the DFT calculations started from the
1
indexed into a monoclinic unit cell in space group P2 (No 4),
with lattice parameters of a = 5.4804 Å, b = 8.2153 Å, c = 5.4926
Å, β = 95.271º. Details of the structure solution can be found in
the supporting information (Tables S4 and S5). Figure 4 shows
bridging K-H bondings in the range of 2.849 to 3.112 Å. Each K
cation coordinates with six B
3 8 3 8
H anions with an overall K:B H
ratio of 1:1. There are two types of asymmetric K-H bridges
–
3 8
linked through each pair of K cation and B H anion to construct
gaseous alkali metal atoms, i.e. an M(g) atom reacting with
the 3D molecular structure. The obtained K-B distances are in
the range of 3.175 to 3.457 Å, which are slightly longer than the
–
THF·BH
3
to form BH
3
, whereas in the experiment the reaction
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Angewandte Chemie International Edition
10.1002/anie.201812795
COMMUNICATION
Na-B distances of 3.130-3.370 Å in the analogous structure of
NaB H .
3 8
[5]
D. Y. Kim, Y. Yang, J. Y. Abelson, G. S. Girolami, Inorg. Chem. 2007,
[
12]
4
6, 9060-9066.
D. M. Goedde, G. S. Girolami, J. Am. Chem. Soc. 2004, 126, 12230-
2231.
[
[
6]
7]
1
Y. N. Shevchenko, A. V. Fesenko, T. M. Nazarova, O. K. Biryukovich, K.
B. Yatsimirskii, U. M. Ogenko, Y. N. Sleasarenko, S. V. Naprasnaya, E.
I. Larikov and R. A. Svitsyn, Otkrytiya, Izobret., Prom. Obrazrsy,
Tovarnye Zhaki 1984, 31, 78.
[
8]
9]
T. G. Hill, R. A. Godfroid, J. P. White, S. G. Shore, Inorg. Chem. 1991,
30, 2952-2954.
[
[
A. Stock, E. Kuss,Chem. Ber.1923, 56, 789−808.
10] A. Stock, H. Z. Laudenklos, Anorg. Allg. Chem. 1936, 228, 178−192.
Figure 4. Crystal structure of KB
3
H
8
in a unit cell (left), local coordination of K
–
[11] A. Stock, E. Pohland, Chem. Ber. 1926, 59, 2210−2215.
cation with six B
3
H
8
anions (right). K, B, H atoms are shown in purple, green,
[
12] a) Z. Huang, G. King, X. Chen, J. Hoy, T. Yisgedu, H. K. Lingam, S. G.
pink color, respectively.
Shore, P. M. Woodward, J.-C. Zhao, Inorg. Chem. 2010, 49,
8185−8187; b) M. Chong, M. Matsuo, S. Orimo, T. Autrey, C. M.
3 8
In summary, we found that unsolvated MB H (M = K, Rb, and
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3
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3 8
synchrotron X-ray powder diffraction. The mechanisms for the
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4 3
reflux. It was found that the physical properties of alkali metals
control the reactions. The high sublimation energies of Li and Na
3 3 8
prevent them from reacting with THF·BH to produce Li/NaB H .
The low melting point of Rb and Cs leads to more vigrous
reactions and higher reaction rates.
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The authors are grateful for the financial support from the
National Natural Science Foundation of China (Grant Nos.
U1804253 and 21771057), Science and Technology Committee
of Shanghai (17ZR1402500, 16520721800), and the U.S.
National Science Foundation (DMR-1655066 to L.S.W.). Part of
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Conflict of interest
The authors declare no conflict of interest.
Keywords: octahydrotriborate anion • heavy alkali salts • B-H
bond • nucleophilicity • formation mechanism
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COMMUNICATION
[
a]†
[a]†
Xi-Meng Chen, Nana Ma, Xin-Ran
[
a]
[a]
Liu, Changgeng Wei, Chongchao
[
a]
[a]
[c]
Cui, Bu-La Cao, Yanhui Guo,* Lai-
[
d]
[e]
Sheng Wang,* Qinfen Gu, and
[
a,b]
Xuenian Chen*
Page No. – Page No.
Unsolvated MB
respective pure alkali metals with THF·BH
the structure of unsolvated KB was obtained. This reaction is not suitable for
preparing Li/NaB . The reaction mechanisms were proposed on the basis of the
experimental and theoretical results. It was found that the physical properties of
alkali metals play a crucial role on the process of the reaction.
3
H
8
(M = K, Rb, and Cs) can be prepared by reactions of the
Facile Syntheses of Unsolvated Alkali
3
in high yield at room temperature, and
Octohydrotriborate Salts MB
K, Rb, and Cs), Mechanisms of
Formation, and the Crystal Structure
of KB
3 8
H (M =
3
H
8
3
H
8
3
H
8
This article is protected by copyright. All rights reserved.