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Rules and trends of metal cation driven hydride-transfer mechanisms
in metal amidoboranesw
Dong Young Kim, Han Myoung Lee, Jongcheol Seo, Seung Koo Shin and
Kwang S. Kim*
Received 1st December 2009, Accepted 3rd March 2010
First published as an Advance Article on the web 6th April 2010
DOI: 10.1039/b925235e
Group I and II metal amidoboranes have been identified as one of the promising families of
materials for efficient H2 storage. However, the underlying mechanism of the dehydrogenation
of these materials is not well understood. Thus, the mechanisms and kinetics of H2 release in
metal amidoboranes are investigated using high level ab initio calculations and kinetic simulations.
The metal plays the role of catalyst for the hydride transfer with formation of a metal hydride
intermediate towards the dehydrogenation. In this process, with increasing ionic character of the
metal hydride bond in the intermediate, the stability of the intermediate decreases, while the
dehydrogenation process involving ionic recombination of the hydridic H with the protic
H proceeds with a reduced barrier. Such correlations lead directly to a U-shaped relationship
between the activation energy barrier for H2 elimination and the ionicity of metal hydride bond.
Oligomerized intermediates are formed by the chain reaction of the size-driven catalytic effects
of metals, competing with the non-oligomerization pathway. The kinetic rates at low temperatures
are determined by the maximum barrier height in the pathway (a L-shaped relation), while those
at moderately high temperatures are determined by most of multiple-barriers. This requires
kinetic simulations. At the operating temperatures of proton exchange membrane fuel cells,
the metal amidoboranes with lithium and sodium release H2 along both oligomerization and
non-oligomerization paths. The sodium amidoboranes show the most accelerated rates, while
others release H2 at similar rates. In addition, we predict that the novel metal amidoborane-based
adducts and mixtures would release H2 with accelerated rates as well as with enhanced
reversibility. This comprehensive study is useful for further developments of active metal-based
better hydrogen storage materials.
1. Introduction
‘‘Towards a Hydrogen Economy’’1 is one of the major
worldwide projects to solve the global demand for clean
energy. Developing a safe, reliable, compact, and cost-effective
hydrogen-storage medium is a technically challenging issue.2
One of the main approaches is developing solid-state storage
media using complex chemical hydrides,3 which include
Group I and II salts of [AlH4], [NH2], and [BH4], or a
combination thereof, or with other light metal hydrides such
as LiH or MgH2. As an alternative approach, ammonia
borane (NH3BH3)4 has recently received great interest due to
its high potential H2 capacity of 19.6 wt% and low releasing
temperatures of H2. Solid NH3BH3 releases one molar equivalent
of H2 at temperatures of B380 K, and a second equivalent
at B420 K. Thus, NH3BH3-based materials have been further
developed in combination with metal catalysts,5 acid catalysts,6
Center for Superfunctional Materials, Department of Chemistry,
Pohang University of Science and Technology, San 31, Hyojadong,
Namgu, Pohang 790-784, Korea. E-mail: [email protected];
Fax: +82-84-279-8137; Tel: +82-54-279-2110
w Electronic supplementary information (ESI) available: Detailed
pathways; kinetic schemes; microcanonical rate-energy curve; rate
constants; relative energies. See DOI: 10.1039/b925235e
5446 | Phys. Chem. Chem. Phys., 2010, 12, 5446–5453
ionic liquids,7 and nanoscaffolds,8 etc. Although these
materials exhibit some promising properties for application
in H2 storage, they have one or more drawbacks such as
low H2 density, high H2-release temperatures (kinetics),
irreversibility (thermodynamics), and impurities. In the case
of NH3BH3, irreversibility and an unwanted byproduct of
borazine are the major limitations.
More recently, new families of metal amidoboranes9 of
I
M (NH2BH3) (M = K, Na, Li) and MII(NH2BH3)
(M = Ca, Mg) have been developed by replacing one H in
NH3BH3 by an alkali or alkaline-earth element. These are
easily synthesized. For example, NaNH2BH3 has been
prepared by mechanically milling a mixture of NH3BH3 and
NaH.9a Several metal amidoboranes such as LiNH2BH3,
NaNH2BH3, and Ca(NH2BH3)2 are highlighted as potential
materials satisfying many of the criteria demanded by H2-storage
media. They not only meet the ultimate goal of 7.5 wt% set by
the US Department of Energy (DOE)10 but also release H2
at B363 K, which is the operating temperature of proton
exchange membrane (PEM) fuel cells.11 Furthermore, they are
stable and environmentally harmless solids at room temperature
under normal pressure. The thermodynamic property of
M(NH2BH3) is less exothermic than NH3BH3,9a,e which facilitates
the search for regeneration routes. However, to be suitable for
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an on-board H2-storage application, the irreversibility and
byproduct issues need to be overcome. Burrell and co-workers
recently reported at a DOE progress meeting that the
H2-release process from KNH2BH3-based compounds was
less exothermic than other M(NH2BH3) compounds, and H2
releases in one step with no impurities.10 This positive result
further justifies pursuit of a deeper understanding of these new
materials.
Basically, the dehydrogenation reactions of M(NH2BH3)
compounds are based on the local combination of Hd and
Hd+.12 Wu et al. suggested that the improved H2-loss kinetics
of M(NH2BH3) over NH3BH3 is attributable to the ionic
character of [NH2BH3].9d Recently, we studied the H2-release
mechanism in the specific case of LiNH2BH3 and then
identified the catalytic roles of Li+ in the H2-release from
LiNH2BH3. We also provided a theoretical proof of the
formation of a [NHBHNHBH3]2 chain as an intermediate
of these reactions, which is also experimentally found by
Autrey et al.9c and Harder et al.9f However, currently, the
H2-release mechanism in M(NH2BH3) is not well understood.
The purpose of this study is to gain a deep understanding of
the kinetic mechanism of the H2 release from M(NH2BH3), to
suggest a methodological scheme which properly describes
these reactions, and to tune the reactions to improve the
properties for application in H2 storage. Thus, we investigate
the reaction pathways of the H2 release from the M(NH2BH3)
systems and examine the catalytic effects of various alkali or
alkaline-earth elements, M, using ab initio calculations on the
reaction pathways. Potential energy surfaces on the reaction
channels describe the heights of the activation energy barriers
and the thermodynamic stabilities, but cannot explain the
detailed kinetics.13 Many H2-release processes are multiplebarrier pathways.14 To properly describe these kinetic models,
we fully simulate the kinetic sequence based on the potential
energy surfaces.15 Finally, we report salient findings related to
general rules and trends on the underlying H2-release mechanisms
of the N–B systems with alkali and alkaline-earth metal
catalysts. We evaluate the thermal rate constants of the
H2-release on competing multiple-barrier pathways for each
M(NH2BH3). Subsequently, we determine the most effective
metal catalyst for the best accelerating rate. Furthermore, we
suggest new adduct and mixture materials which would
enhance the reversibility with facile H2 release.
using Møller–Plesset second-order perturbation theory (MP2)
with the 6-311++G** basis set. To identify transition state
structures, intrinsic reaction coordinate (IRC)17 calculations
were performed at the MP2/6-311++G** level. The
frequency analysis was made at the same level of theory.
The zero point energy (ZPE) corrected MP2/6-311++G**
relative energies are reported. In our previous work,9i we
treated the specific case of LiNH2BH3. As with the work on
LiNH2BH3, we first investigated three possible pathways:
(1) the direct pathway which occurs through the redox reaction
of B–Hd and N–Hd+ with the subsequent release of H2,
(2) the cleavage pathway which forms NH2 and BH3 by N–B
bond cleavage, and (3) the metal mediated pathway, forming
metal hydride, followed by dehydrogenation through the
M–Hd and N–Hd+ dihydrogen bond. For all M(NH2BH3),
the metal mediated pathway is dominant.
Kinetic simulations
For each barrier on the multiple-energy-barrier reaction
pathways, the microcanonical rate constant is evaluated using
Rice–Ramsperger–Kassel–Marcus (RRKM)18 theory given by
eqn (1).
kuni ðEÞ ¼
Ab initio calculations
We investigated the reaction pathways for H2 elimination
from the monomeric/dimeric M(NH2BH3) systems. While
the mechanistic pathways for the release of H2 from monomeric
compounds give some insight into the H2-loss mechanism,
the investigation on the dimeric compounds, including
oligomerization, is useful as a model study for the solid
compounds.14a,b We performed ab initio calculations using
the Gaussian 03 suite of programs.16 The geometries of
stationary points were initially optimized using density
functional theory with the hybrid B3LYP functional and the
6-311++G** basis set. Further optimization was carried out
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ð1Þ
where s is the symmetry number. The transition state’s sum of
states (N) and the reactant’s density of states (r) were computed
using the direct count Beyer–Swinehart algorithm19 with the
grain size of 1 cm1. The ZPE-corrected MP2/6-311++G**
energies (E0) were used. Each vibrational mode was treated as a
harmonic oscillator. MP2/6-311++G** vibrational frequencies
were scaled by a factor 0.9608.20 Because the molecular
rotation is very much restricted in solid samples, rotational
energies of the reactant and transition states are adiabatically
treated. Then, the microcanonical rate constants of the
reactions in steady state are calculated. The microcanonical
rate constant is converted to the thermal rate constant with the
following eqn (2)–(3).
PðE; TÞ ¼ R 1
0
k(T) =
2. Calculation methods
sNðE E0 Þ
hrðEÞ
rðEÞeE=kB T
rðEÞeE=kB T
¼
E=k
T
B
QðTÞ
rðEÞe
dE
RN
0 kuni(E)P(E,T) dE
ð2Þ
(3)
P(T) and Q(T) are the Boltzmann distribution function and
the vibrational partition function at a given temperature T,
respectively.
3. Results and discussion
Reaction pathways for the H2 release from metal amidoboranes
The mechanistic pathways for the loss of H2 from the
monomeric/dimeric M(NH2BH3) are described in Fig. 1.
Monomeric conformers are denoted by a prime ( 0 ). Tn is a
transition state at a minimum point n. As illustrated in a
schematic view for the monomer/dimer (Fig. 1c–f), there are
three steps involved in the H2-release process:
Phys. Chem. Chem. Phys., 2010, 12, 5446–5453 | 5447
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Fig. 1 Reaction pathways for one molar equivalent H2 release from the monomeric systems [MI]+[NH2BH3] (MI = K, Na, Li) and
[Y–MII]+[NH2BH3] (MII = Ca, Mg, Y: H) (a–c) and from the dimeric systems ([MI]+[NH2BH3])2 and [MII]2+([NH2BH3])2 (d–f). The
schematic illustration (c, f) shows how H2 releases; after the hydride transfer from boron to metal cations M (M: K, Na, Li, Ca, Mg) to form M–H
(M-step), the redox reaction of the M–Hd Hd+–N dihydrogen bond (H-step) takes place. See Fig. 2 for the chemical reactions and also see ESI,
S1w). An N atom is given in black, H in small circle, B in medium solid circle, and M in large circle. (O-pathway: oligomerization pathway,
D-pathway: direct, non-oligomerization pathway).
5448 | Phys. Chem. Chem. Phys., 2010, 12, 5446–5453
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(i) O-Step: Oligomerization occurs with the formation of
intermolecular N–B bond between Nd in one moiety and Bd+
in the other moiety, noted by superscript O as in T2O;
(ii) M-Step: Metal hydride [MI]+H or [Y–MII]+H
(Y = H/N) is formed as an intermediate by hydride transfer
from boron to metal cation, noted by subscript M as in TnM
and TnOM (n = 1, 1 0 , 3);
(iii) H-Step: Dehydrogenation takes place via an
M–Hd Hd+–N intermediate, denoted by subscript H as in
TnH and TnOH (n = 2, 2 0 , 4).
In the H2-release process of the M(NH2BH3), the metal M
mediates hydride transfer from boron to amino proton to
produce H2 by forming an intermediate of metal hydride
M–H. In that process, M catalyzes oligomerization with the
formation of an intermolecular N–B bond. This oligomerization
pathway (O-pathway) competes with the direct H2 release
pathway (D-pathway, i.e., non-oligomerization pathway)
and this competition plays the key role in determining
intermediates and products.
Conformation 1 is the lowest energy structure, while M
bound to an anion can easily move to the region around the
anionic [NH2BH3] at moderate high temperatures such as
300–400 K.9i The transfer of Hd to M through T1M leads to
the formation of M–H and NH2BH2 in 2M. Both [NH2BH3]
and NH2BH2 are bound to M through electrostatic interactions between M and negative charged nitrogen atoms.
2M splits into two reaction pathways. One is through T2O
along the O-pathway, while the other is through T2H along the
D-pathway.
In the O-pathway, the intermolecular N–B bond in 2O
forms between [NH2BH3] and NH2BH2 through T2O where
M bound to [NH2BH3] moves to [NH2BH2NH2BH3]. In
solids, the produced intermediate [NH2BH2NH2BH3] will
generate an intermediate in a subsequent step, M(NH2BH3) +
nNH2BH2 - M(NH2BH2NH2BH3) + (n 1)NH2BH2 M((NH2BH2)nNH2BH3). Thus, this oligomerization is a chain
reaction and an alkali or alkaline-earth metal acts as a chain
carrier. Subsequently, one molar equivalent of H2 is released
by the ionic recombination of Hd with Hd+ through T2OH
forming 3O. After desorption of H2, M is again bound to a
negative nitrogen atom. A second H2 also occurs by the
formation of M–H, T3OM - 4OM, followed by the ionic
recombination of the M–Hd Hd+–N dihydrogen bond,
T4OH - 5O. In the D-pathway, two molar equivalents of H2
release without oligomerization by the redox reaction of Hd
and Hd+ in T2H - 3 and T4H - 5 with the formation of
M–H in T3M - 4M.
The energy profiles of the above reaction pathways are
presented in Fig. 3. In our previous work9i, we thoroughly studied
the mechanistic pathways for the specific case of the Li complex.
Thus, here we analyze each M-, H-, and O-steps with several
alkali and alkaline-earth metal catalysts and summarize
important trends and rules which generally govern these systems.
A Covalent character-type M-step vs. ionic character-type
H-step. Hd in the intermediate M–H is a strong Lewis base
due to the electropositive metal. The first ionization of
alkali/alkaline-earth metal increases in the order, K o Li o
Na o Ca o Mg. As the ionization energy increases, the ionic
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Fig. 2 Dehydrogenation reactions in (a) ([MI]+[NH2BH3])2 and
(b) [MII]2+([NH2BH3])2.
character of M–H decreases; hence, Hd becomes less basic
towards a Lewis acid Hd+. In the case of hydride transfer from
boron to the metal cation in monomeric amidoborane, potassium
shows the highest activation energy in T1 0 M, whereas magnesium
shows the lowest activation energy, as illustrated in Fig. 3a.
This trend persists in the dimeric system. For dimers, Fig. 4
presents plots of activation energy of M-/H-step vs. first
ionization energy of alkali and alkaline-earth metals. As the
ionization energy increases (K o Li o Na o Ca o Mg), the
activation barrier for the M-step decreases, whereas that for
the H-step increases. Thus, metal amidoborane complexes
with Na and Li having intermediate ionization energies exhibit
low energy barriers for the release of H2 (Fig. 5).
B Size-dependent metal-catalyst effect on the O-step. At
T2O, [NH2BH3] and NH2BH2 are combined together to
form [NH2BH2NH2BH3] through the intermolecular bond
between N1d of [N1H2B1H3] and B2d+ of N2H2B2H2
(Fig. 6). In this process, the metal M bound to [N1H2B1H3]
moves to N2d of [N2H2B2H2N1H2B1H3] by charge transfer
from the M–N1 bond to the weakly-bound N1 B2 state,
from the N2QB2 bond to the weakly-bound M N2 state.
Here, a smaller size metal drives Bd+ in NH2BH2 closer to Nd
in [NH2BH3] with stronger interaction energy, thereby
further lowering the activation barrier. Thus, the smallest
sized alkali and alkaline-earth metals such as Li and Mg are
considered to be good catalysts for the O-step. The activation
energy barrier of T2O is 3.5, 5.5, and 8.2 kcal mol1 for Li, Na,
and K, respectively, and 1.4 and 3.1 kcal mol1 for Mg and
Ca, respectively. This alkali or alkaline-earth metal catalystdriven oligomerization forms a unique chain structure
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Fig. 4 Activation barriers for the formation of metal hydride
(M-step) and the dehydrogenation (H-step) depending on the first
ionization energy (IE1) of metal: (a) schematic, and (b–h) activation
energy vs. IE1 (K: 4.34, Na: 5.14, Li: 5.39, Ca: 6.11, Mg: 7.64) for each
transition state of T1M/T3M/T3OM (b/c/d) or T2H/T2OH/T4H/T4OH
(e/f/g/h).
Fig. 5 Activation energy barriers from conformation 1 of the lowest
pathways for first and second H2 releases in dimeric M(NH2BH3)
depending on the first ionization energy of metal. The activation
energy barrier and the ionization energy show a U-shaped relation.
Na and Li complexes having intermediate ionization energies exhibit
low energy barriers.
C Effects of the dative N-B bond on both NH and BH.
The dative N - B bond makes hydrogen on NH more acidic
but that on BH more basic, which leads to the facile H2 release
from the N–B system by the redox reaction of a dihydrogen
bond. Among H-steps, either T2H or T4H leads to the lowest
Fig. 3 Energy profiles of H2-loss pathways in (a) monomeric and
(b–f) dimeric alkali and alkaline-earth metal amidoborane complexes.
For the specific case of Li, see our previous work.9i The monomeric
systems for K, Na, Li, Ca, and Mg release H2 with the highest barrier
of 41, 34, 33, 36, and 37 kcal mol1, respectively. Along the
D-pathway, the dimeric systems for K, Na, Li, Ca, and Mg release
the first H2 with the highest barriers of 36, 34, 38, 36, and 36 kcal mol1,
and the second H2 with the highest barriers of 48, 44, 45, 47, and
46 kcal mol1, respectively. Along the O-pathway, the dimeric systems
for K, Na, Li, Ca, and Mg release the first and second H2 with the
highest barriers of 39, 32, 34, 43, and 47 kcal mol1, respectively.
[NH2BH2NH2BH3], which is experimentally identified as
an intermediate in the dehydrogenation process of some
amidoborane materials.
5450 | Phys. Chem. Chem. Phys., 2010, 12, 5446–5453
Fig. 6 Size dependency of the alkali/alkaline-earth metal catalysts in
the oligomerization between NH2BH2 and [NH2BH3]: (a) Schematic.
(b–f) Metal cation M (M: Mg/Ca/Li/Na/K; b/c/d/e/f) acts as a linker
of NH2BH2 and [NH2BH3] towards oligomerization. Smaller sized
M such as Li and Mg effectively drives the B atom of NH2BH2 to be
closer to the N atom of [NH2BH3].
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activation barrier, while T2OH leads to the highest barrier.
This is because the amino hydrogen in the NQB double bond
formed in T2H or T4H is more acidic than that in the resonance
N–B hybrid bond in T4OH which is more positive than that in
the single N–B bond in T2OH (see the ESI, S2w).
D Resonance-stabilized chain intermediates. In the second
dehydrogenation process, the intermediates and product
(T3OM - 5O) in the O-pathway are more stable than those
(T3M - 5) in the D-pathway. This is mainly due to the
stabilization by the resonance (M–N–BQN M 2
M NQB–N–M) hybrid bond in the chain structures of
([NH2BHQNHBH3] 2 [NH2QBHNHBH3]) in T3OM and
4OM and ([NHBHQNHBH3]2 2 [NHQBHNHBH3]2) in
T4OH and 5O (see the ESI, S3w).
Full kinetic simulations
Referring back to Fig. 1–3, the O/D-pathway with multipleenergy-barriers can be expressed with the following kinetic
scheme.
ð4Þ
For each barrier of k1/k1/k2/k2/k3/k4/k4/k5/k2 0 /k3 0 /k3 0 /k4 0 ,
the microcanonical rate constant is evaluated using
Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Applying
the steady-state approximation (ESI, S4w), the microcanonical
rate constant of kcat1/kcat1 0 /kcat2/kcat2 0 is calculated.
ð5Þ
The microcanonical rate constant is converted to the thermal
rate constant. The molecules in higher energy decompose faster
than those in lower energy, thus the population distribution of 1
at steady-state might not follow the Boltzmann distribution.
However, provided that the rate constant of kcat1/kcat10 is
sufficiently smaller than thermal energy exchange, the
Boltzmann distribution can be applied to 1 at steady-state.
Therefore, the thermal rate constant of kcat1/kcat10 at the given
temperature can be evaluated. Similarly, the thermal rate
constant of kcat2 0 can also be evaluated because kcat20 is
sufficiently smaller than kcat1 0 as well as thermal energy
exchange (ESI, S5w). However, the thermal rate constant of
kcat2 cannot be defined. The population of 3O cannot be
expressed by a well-defined Boltzmann distribution because
the second H2 desorption occurs instantaneously after the first
H2 desorption; kcat2 is much faster than kcat1 (ESI, S5w).
Fig. 7 shows the overall rates of kcat1 and kcat1 0 at 200–500 K
depending on the first ionization energy of metals. In the case
of the Na/Li complex, the O-pathway is dominant below
300 K, whereas the D-pathway is dominant above 400 K.
The rates of both O- and D-pathways are comparable at
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Fig. 7 Rate constants of kcat1 and kcat1 0 on the O- and D-pathways at
200–500 K depending on the first ionization energy of metals. At low
temperatures, a L-shaped relation between the rates of dominating
paths and IE1 is directly from the U-shaped relation between energy
barriers and IE1 in Fig. 5. At high temperatures, the rates to loss H2
are almost identical.
300–400 K. Below 300 K, the activation energy barrier of
T2O (5.5 and 3.5 kcal mol1 for Na and Li, respectively),
which is lower than that of T2H (7.9 and 11.1 kcal mol1,
respectively), drives the reactions to the O-pathway. Above
400 K, the formation of the stable oligomerized intermediate
2O as a detour of the O-pathway drives the reaction to the
D-pathway. At low temperatures, the barrier height in a minimum
energy pathway determines the reaction pathway, while at high
temperatures, it is also important to consider the number of steps.
In the case of the K and Ca complexes, the first H2 releases mostly
along the D-pathway, which leads to the lower activation energy
barrier of T2H (2.6 and 1.1 kcal mol1, respectively) than T2O
(8.2 and 3.1 kcal mol1, respectively) with no oligomerization.
In the case of Mg complex, though Mg is a good catalyst for the
O-step of T2O because of the very high activation barrier for T2OH
in the N–B single bond formation, the D-pathway becomes
dominant.
Metal amidoboranes with Na and Li having intermediate
ionization energies lead to a lowering of energy barriers for
these reactions compared to those with K and Mg, due to the
covalent character-type intermediate formation and the ionic
character-type dehydrogenation. At lower temperatures, these
trends are more obvious than at higher temperatures (Fig. 7).
At 300–400 K, which is related to the operating temperatures
of PEM fuel cells, all of complexes release H2 at almost
identical rates (Table 1), while the Na complex shows the
most accelerated rates. The overall rates of the Na complex at
300 K and 400 K are B1.4 1012 s1 and B1.1 106 s1,
respectively.
Metal amidoborane-based adducts and mixtures
In the cases of alkaline-earth metal complexes, the product 5O
of the D-pathway is higher in energy than the product 5 of the
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Table 1 Rate Constants of kcat1 and kcat1 0 at 300 K and at 400 K for
the first H2 release on the O/D-pathway
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300 K
400 K
kcat1 0
kcat1
K
Na
Li
Ca
Mg
1.4
1.4
2.7
5.0
3.9
17
10
1012
1013
1021
1022
6.9
4.9
3.3
6.7
5.5
kcat1 0
kcat1
14
10
1013
1014
1014
1014
2.4
8.8
4.2
3.4
8.4
10
10
107
107
1013
1014
3.8
1.1
4.1
3.4
3.0
107
106
107
107
107
Rate constants are in s1. The steady-state approximation was used
for the reaction channels from 1 to 3O and 3. Numbers in bold
character indicate a larger value between the O-pathway (1 - 3O)
and D-pathway (1 - 3).
O-pathway due to the unfavorable coordination of the
alkaline-earth metal on the two nitrogen atoms (such as the
N–Ca–N bond angle of 671 in 5O). In solids, however, this
kind of unstable bonding structure might be stabilized by the
coordination of more anionic molecules around the metals. In
the case of alkali metal complexes, tuning the reactions to
follow the D-pathway will lead the H2-release process to be
significantly less exothermic than that of the O-pathway,
resulting in improved reversibility. In addition, the H2-release
rates of amidoborane complexes with metal catalysts of low
ionization energy would be accelerated by adding metals of
high ionization energy, which reduces the activation barrier
for the dehydrogenation.
For examples, the rates of the H2 release in LiNH2BH3 are
likely to be improved by methyl substitution for amidoborane
that blocks a detour oligomerization step, which would
enhance the irreversibility of the system.21 Also, based on
the understanding of this study, the rates of the H2 release in
LiNH2BH3 would be accelerated by a potassium catalyst22
forming a mixture of potassium–lithium amidoboranes. For
the desorption of the first H2 at 400 K, our full kinetic
simulations based on the potential energy surfaces predict that
(LiNH2BH2CH3)2 shows an overall rate of k = B5 105 s1
and KLi(NH2BH3)2 of k = B8 107 s1, as compared with
(LiNH2BH3)2 of k = B4 107 s1 (ESI, S6–7w).
4. Conclusions
By investigating the molecular model study of the H2-loss
mechanism in amidoboranes with various alkali and alkalineearth metal catalysts, we find important general rules and
trends related to the N–B materials with metal catalysts. These
are well described in Scheme 1. The metal catalysts bring a
hydride (via an intermediate of metal hydride) towards the
dehydrogenation. When the alkali or alkaline-earth metal
interacts with a hydridic H bonded to a boron atom in the
intermediate step, it forms the metal hydride bond of the
covalent nature. On the other hand, when the metal hydride
bond dissociates towards the dehydrogenation step, it shows
the ionic character. Thus, the metal amidoboranes, having
intermediate ionization energies, exhibit low energy barriers,
showing a U-shaped relation between the activation barrier
and the ionicity of the metal hydrogen bond. In the H2-release
process, oligomerized intermediates can be formed by the
5452 | Phys. Chem. Chem. Phys., 2010, 12, 5446–5453
Scheme 1 Metal cation driven hydride-transfer mechanism: For the
H2 release from alkali and alkaline-earth metal amidoboranes, the
stability of the intermediate decreases with increasing ionic character
of the metal hydride bond in the intermediate, while the dehydrogenation
process involving ionic recombination of hydridic H with protic H
proceeds with a reduced barrier. An oligomerized intermediate can be
formed by the size-driven metal-catalyst effect. This process competes
with the non-oligomerization process. After desorption of H2, the
metal ion is again bound to a nitrogen atom that prepares a
subsequent H2-release process.
chain reactions through the size-driven catalytic effects of
alkali and alkaline-earth metals. The oligomerization pathway
competes with the direct H2 release pathway.
Thus, via two competing pathways, the oligomerization
pathway vs. the direct pathway (non-oligomerization pathway),
metal amidoboranes should overcome multiple-barriers to
release H2. We performed fully kinetic simulations to evaluate
thermal rate constants. At low temperatures, the barrier height
in a minimum energy pathway determines the dominant pathway.
However, at high temperatures, not only the energy barrier
height but also the number of steps is important to determine
the pathways. This result indicates why we need to not only
investigate the potential energy surfaces on the reaction pathways,
but also perform the kinetic simulations. At 300–400 K, all of
dimeric alkali and alkaline-earth metal amidoboranes release
H2 at similar rates, while sodium amidoboranes show the most
accelerated rates. The oligomerization pathway and the direct
pathway are comparable for lithium and sodium amidoboranes, while the direct pathway is dominant for potassium,
calcium, and magnesium amidoboranes. The present understanding is highly beneficial to further developments of active
metal-based hydrogen and energy storage materials.23 In
addition, the theoretical prediction that the novel metal
amidoborane-based adducts and mixtures would release H2
with more accelerated rates than pure amidoboranes with
enhancement of the reversibility, should strongly motivate
researchers to perform the associated experiments.
Acknowledgements
This work was supported by GRL (KICOS), KOSEF(WCU:
R32-2008-000-10180-0, EPB Center: R11-2009-0063312),
BK21 (KRF), and KISTI (KSC-2008-K08-0002).
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