Copper(II)-Assisted Ammonia Borane Dehydrogenation: An Insight

Article abstract of DOI:10.1002/ejic.202100628

Amine boranes are in limelight in recent times as they have been considered as one of the potential candidates for on-board hydrogen storage. Among various amine boranes, Ammonia borane (AB, 19.6 wt % H) with hydridic (B?H) and protic (N?H) hydrogens on a

Full text of DOI:10.1002/ejic.202100628

Full Papers
doi.org/10.1002/ejic.202100628
Copper(II)-Assisted Ammonia Borane Dehydrogenation: An
Insight
Siripurapu Mounika,[a] Valle Krishnaveni,[a] Marilyn Esclance Dmello,[b] and
Suresh Babu Kalidindi*[a]
Amine boranes are in limelight in recent times as they have
been considered as one of the potential candidates for on-
board hydrogen storage. Among various amine boranes,
Ammonia borane (AB, 19.6 wt% H) with hydridic (BÀ H) and
protic (NÀ H) hydrogens on adjacent atoms is the most studied
molecule. From the past decade, as the chemists try to develop
effective catalysts for the release of hydrogen from AB,
simultaneously new interesting chemistry of AB has been
evolved. These studies increased the scope of AB for other
applications such as in the synthesis of nanoparticles, as transfer
hydrogenation agent, in organic synthesis, etc. Here we focus
on Cu2+-AB system in water and organic solvents (THF/diglyme)
to unveil new chemistry. CuCl2 readily reacted with AB in both
media to generate H2 and Cu2+ was reduced to Cu(0) when AB
is present in excess. We got an insight into these reactions by
carrying out reactions for various Cu2+/AB mole ratios and
characterizing the reaction mixtures with 11B NMR spectroscopy.
The copper products were identified by powder X-ray diffrac-
tion technique. We noticed the formation of CuH and CuCl
intermediates upon BÀ H bond scission by Cu2+
.
Introduction
plexes (Ti, Re, Ru, Ir, Rh, and Ni based metal complexes) as
catalysts under relatively low temperatures.[14] Few heteroge-
neous catalysts such as Pt, Ru and Pd nanoparticles also have
been employed in this direction.[15] Whereas, hydrogen gen-
eration from AB in protic solvents with faster kinetics was
realized at room temperature using several heterogeneous
catalysts (both precious and first-row transition metal-based
nanocatalysts) such as Pt/Al2O3, Pt/C, CoÀ NiÀ B etc.[16–25]
Ammonia borane (AB, H3N·BH3) is a classic example of Lewis
acid-base adduct compound. AB contains both hydridic (BÀ H)
and protic (NÀ H) hydrogens on adjacent atoms which makes its
chemistry interesting.[1–5] From the past few years, the material
has been widely studied as a chemical hydrogen storage
medium given that it contains 19.6 wt% hydrogen in its
structure. Controlled release of pure H2 from AB remains one of
the intense areas of research although the regeneration of AB
First-row transition metal-based catalysts are attractive for
the dehydrogenation of AB due to their cost effectiveness.
Among various first transition metals, Cu-based catalysts are
one of the widely explored systems. Various copper based
from its dehydrogenated products remains
a matter of
concern.[6,7]
Although dehydrogenation of solid AB can be achieved via
nanocatalysts
Cu/γ-Al2O3
catalyst,[26]
Cu@Cu2Ocore-shell
°
direct thermolysis at moderate temperatures (>100 C), the
nanoparticles,[21] flower-like mesoporous Cu nanostructures,[27]
Cu NPs supported on h-BN,[28] Zeolite confined Cu[18] and Cu/
Co3O4 nanoparticles[29] were found to be effective catalysts for
the generation of hydrogen through hydrolysis/ methanolysis
route.[30]
direct hydrogen release rates are too slow for practical
applications.[8,9] Also, impurities such as borazine and diborane
are found to be liberated along with hydrogen during
thermolysis of AB in solid-state. It has been shown that
chemical additives can increase the kinetics of hydrogen release
from AB in solid-state and suppresses the release of aforemen-
tioned gas impurities.[10–13]
In our previous studies, Cu2+ has been employed for
dehydrogenation of AB through hydrolysis route and in solid-
state.[13,25] In water, among various first-row transition metal
chlorides, CuCl2 readily reacted with AB to generate three
equivalents of hydrogen whereas CoCl2 and NiCl2 showed
induction periods of around 60 and 120 min, respectively.[25]
Interestingly, conversion of Cu2+ into Cu(0) was also noticed
during the hydrolysis. CuCl2 was also found to destabilize AB in
solid state and can trigger the release of hydrogen from AB
even at room temperature.[13] Also, in this case, the conversion
of Cu2+ into Cu(0) was noticed. This property has been
exploited for the synthesis of air-stable copper nanoparticles. Q.
Xu’s group synthesized Cu@Fe, Cu@Co and Cu@Ni core-shell
nanoparticles by using AB as a reducing agent.[31] The method-
ology exploits AB reduction ability to first reduce Cu2+ to form
the core and successively reduce Co2+ or Fe2+ or Ni2+. The
preferential reduction of Cu2+ can be assigned to its lower
Dehydrogenation of AB can be carried out in protic solvents
as well as aprotic organic solvents in the presence of a catalyst.
Hydrogen release from AB has been achieved in non-aqueous
medium using mostly homogeneous transition metal com-
[a] S. Mounika, V. Krishnaveni, Prof. Dr. S. B. Kalidindi
Department of Inorganic and Analytical Chemistry,
School of Chemistry, Andhra University,
Visakhapatnam 530003, India
E-mail: ksureshu@gmail.com
[b] M. E. Dmello
Materials Science and Catalysis Division,
Poornaprajna Institute of Scientific Research,
Bidalur, Devanahalli, Bengaluru, 562 164, India
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100628
Eur. J. Inorg. Chem. 2021, 1–8
1
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
Results and Discussion
Cu2+- assisted AB hydrolysis
When CuCl2 ·2H2O was mixed with AB aqueous solution, the
hydrogen gas evolved almost instantaneously. For Cu2+/AB=
0.3, the hydrogen evolution was rapid initially and slowed
down after ~4 min. The hydrogen evolution profile (Figure 1a)
indicates the presence of at least two types of catalytically
active species in the reaction. To probe the reaction further, the
pH of the reaction was monitored during the hydrolysis of AB. It
is apparent from Figure 1b that large quantities of H+
production took place soon after the addition of Cu2+ and this
explains the initial rapid kinetics of the reaction. It is well
documented in the literature that acids trigger very rapid
hydrogen release from aqueous AB solutions.[33] After about
5 min of reaction time, the hydrogen evolution slowed down,
and the reaction was catalysed by in-situ generated Cu particles
(black powders). In our earlier work, some of us have shown
that copper nanoparticles synthesized using solvated metal
atom dispersion method were catalytically active for AB
hydrolysis.[21] From these observations one can conclude that in
case of Cu2+/AB=0.3 the initial period of reaction was
homogeneous (catalysed by H+) and final was heterogeneous
(catalysed by in-situ generated Cu) in nature. But it is unclear
what goes on during the initial period of reaction which
involves reduction of Cu2+ to Cu(0) and the generation of H+.
The simple and competent way to isolate intermediates in a
homogeneous catalytic reaction is to carry out reactions at
stochiometric ratios. For Cu2+/AB=0.3, the colour of the
reaction mixture changed from blue to orange brown and
finally to reddish black (Cu). For Cu2+/AB=0.5 and 1, the colour
change stopped at orange brown colour. For Cu2+/AB=0.5, the
hydrogen evolution was rapid and ~2.7 moles of hydrogen
were released in 3 min. The purity of H2 was confirmed using
gas chromatography using GS- GasPro column and thermal
conductivity detector(TCD) (SI, Figure S1). In case of Cu2+/AB=
1, ~2 eqivalents of hydrogen were evolved in 3 min. We also
noticed a sudden drop in pH after 2 min of reaction time. After
this time, for Cu2+/AB=0.3 and 0.5, the pH of the reaction
mixture raised to above 6, due to the consumption of in-situ
produced H+. However, for Cu2+/AB=1, the pH of the reaction
dropped to 1.7 and remained at that value even after the
completion of reaction. Noteworthy, such change in pH is not
noted when CoCl2 and NiCl2 (metal ions with higher reduction
potential) were added to aqueous solution of AB. The pH
remained in the range of 6 and 7 for these two metal ions. So,
pH changes observed in case of Cu2+ assisted AB hydrolysis are
unique and are specially related to Cu2+-AB system. Attempts
to isolate the reaction intermediates by using low temperatures
Scheme 1. Hydrolysis and thermolysis products of AB in presence of Cu2+
.
Figure 1. a) H2 evolution profiles for the hydrolysis of ammonia borane for
various Cu2+/AB ratios; b) pH change profiles for the hydrolysis of ammonia
borane for various Cu2+/AB ratios and c) powder X-ray diffraction pattern of
isolated solids after hydrolysis of AB for various AB: CuCl2 ratios.
reduction potential compared to other metal ions. However,
Cu2+-AB chemistry that has led to these interesting observa-
tions remains unexplored. In this study the efforts are devoted
to get an insight into the reaction between Cu2+ and AB in
both aqueous and non-aqueous media (Scheme 1). It is
observed that in both media the reaction is initiated by
breaking of BÀ H bond of AB to give H3N·BH2Cl species which
undergoes further reaction to liberate H2.
°
(5 C) were unsuccessful. The final powders obtained after AB
hydrolysis were characterized using PXRD technique in three
cases. From Figure 1c the formation of Cu(0), Cu(0) & CuH and
CuH & CuCl for Cu2+/AB=0.3, 0.5 and 1, respectively were
observed.
The following mechanism (Scheme 2) was postulated in
order to account for the formation of CuH, CuCl, H+ and H2. In
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
2
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
Scheme 2. The operating mechanism for the hydrolysis of AB in presence of
1 equivalent of CuCl2.
the first step, hydridic BÀ H hydrogens of AB would interact with
Lewis acidic [Cu(H2O)6]2+Cl2 to form copper hydrochloride
([CuH]+Cl) and boronium cation(Cl[H2BNH3]). Formation of
[CuH]+ unstable intermediate species has been previously
reported in the literature to explain the reaction between H2
[34–36]
°
and CuCl2 at 145 C.
It is noteworthy that the hydrolysis of
AB over metal nanocatalysts (Pt, Ni..etc) proceeds via the
oxidative cleavage of the HÀ OH bond in water. It is proposed
that in a rate determining step protic H of water interacts with
hydridic H of AB over catalyst surface and results in the
generation of H2. In our case, Cu2+ being strongly electron
deficient readily react with BÀ H bond of AB.[37–40] In subsequent
step, [CuH]+Cl can undergo disproportionation reaction to give
CuH, CuCl and HCl. As Cl[H2BNH3] is not a stable species in
water, it readily undergoes hydrolysis to B(OH)3 and H2. This
mechanism is consistent with quantitative formation of H+ and
H2. As anticipated by the postulated mechanism, 2 eqivalents of
H2 was noted for the Cu2+/AB=1 reaction. We also accounted
for the in-situ generated H+, wherein the obtained pH=1.7
corresponds to 0.020 M of H+ concentration which matches
quite well with an expected value of 0.025 M (as we started the
reaction with 0.025 M AB solution).
Figure 2. Reaction between CuCl2 and AB in diglyme at room temperature,
Cu2+/AB=1:1. a) 11B NMR spectrum and b) powder X-ray diffraction pattern.
surprisingly we can observe H3N·BH2Cl (the predicted inter-
mediate in the hydrolysis reaction) by 11B NMR spectroscopy
(Figure 2a). The chemical shift (δ –8.6) and the coupling
constant (triplet, J (B, H)=121 Hz) of H3N·BH2Cl species is in
conformity with what has been reported previously by Hu and
Geanangel.[41,42] These experimental observations points out to
the mechanism observed similar to the one in aqueous
medium. However, the formation of CuCl as a product suggests
a reaction given by Eq 1.
(1)
Dehydrogenation of AB with Cu2+ in non-aqueous medium
An observed release of close to 1 equivalent hydrogen is
consistent with Eq 1. Further, if Eq 1 is operative for Cu2+/AB=
0.5, we should observe around 0.5 equivalents of hydrogen
along with significant amount of CuH. In line with this
argument, we noted 0.44 equivalent of hydrogen and PXRD
showed the presence of a good amount of CuH along with
CuCl (Figure S2). It may be noted that CuH reactivity varies
widely in aqueous and non-aqueous medium. CuH is stable in
aqueous solutions even at pH=1. In fact, in most of the
literature reports CuH was synthesized in aqueous solutions at
pH close 1.[43,44] Even in this case, we attained a pH of 1.7 during
hydrolysis. At higher pH, CuH degrades to Cu(0) in aqueous
solutions. Further, unlike in aqueous medium the species
Dehydrogenation of AB in non-aqueous medium in the
presence of Cu2+ is explored for the first-time in this work.
Reactions were carried out in diglyme/ THF medium. When the
reaction was carried out between CuCl2 (dehydrated) and AB in
1:1 molar ratio in diglyme at room temperature, we observed a
liberation of around 0.9 equivalents of H2 and the formation of
a light green solid. The evolved H2 gas was found to be highly
pure using GC-TCD technique (SI, Figure S3). The solid products
of the reactions were isolated by centrifugation, washed with
dry petroleum ether, and dried under vacuum. The PXRD
pattern (Figure 2b) of the light green solid showed the presence
of CuCl (JCPDF- 822114) along with small quantities of CuH. The
supernatant was characterized using 11B NMR spectroscopy and
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
3
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
H3N·BH2Cl is relatively more stable in diglyme, therefore it did
J(B,H)=124 Hz (Figure S4). We ascribe this signal to the
[H3N·BH2(THF)][Cl] species. All these experiments suggest that
irrespective of the medium (aqueous or non-aqueous) the first
step involves activation of BÀ H bond, which results in the
formation of H3N·BH2Cl.
not undergo any further reaction at room temperature.[42]
The observed scission of BÀ H bond by Lewis acidic Cu2+ is
quite interesting. Earlier, activation of the BÀ H σ-bonds
(hydridic) in amine boranes is achieved only by highly electro-
philic complexes or strong Lewis acids or Brønsted acids. In the
late 1970s, Hu and Geanangel demonstrated that strong Lewis
acids such as BCl3 can activate the BÀ H bond of AB in diethyl
ether to afford products such as H3N·BH2Cl, H3N·BHCl2,
H3N·BCl3, (C2H5)2O·BH2Cl, (C2H5)2O·BHCl2 and so on, depending
on the concentration of BCl3 used.[41] Baker and co-workers
studied the acid-initiated AB dehydrogenation using various
strong Lewis or Brønsted acids such as B(C6F5)3, HOSO2CF3 and
HCl and observed the formation of H3N·BH2Cl species.[45] Herein,
we employed economical and user-friendly CuCl2 as a Lewis
acid to activate the BÀ H bond in various amine boranes. We
extended the scope of this reaction to various alkyl amine
boranes such as, t-BuH2N·BH3, Me2HN·BH3, and Me3N·BH3. In
the case of the mono- and the disubstituted alkyl amine
boranes, the formation of the boronium cation took place at
room temperature. However, the boronium ion formation in the
Further, to study the implications of BÀ H bond scission
towards hydrogen generation, the dehydrogenation reactions
were carried out with Cu2+/AB=0.3 in diglyme at 60 C. The
°
boronium cation can react further with AB at high temperature
to generate hydrogen. Figure S5 shows the hydrogen evolution
profiles of AB dehydrogenation. For Cu2+/AB=0.3, about 1.5
equivalents of H2 were liberated in 1.6 h. On the other hand,
Co2+ and Ni2+ under same conditions gave 0.4 and 0.1
equivalents of H2 (Figure S6). Thus, the use of Cu2+ for the
dehydrogenation of AB can be considered to be economical
and an effective initiator which is compounded by the fact that
the quantity of H2 generated is much more compared to that
generated using a strong acid such as triflic acid (1.1 equiv. at
[45]
60 C in 18 h). We also followed this reaction (Cu2+/AB=0.3
°
at 60 C) using 11B NMR spectroscopy by collecting aliquots at
°
different time intervals (Figure 4). To start with a quartet at δ
À 21.6 that corresponds to the AB present. After 2 min of time
we noted a signal at δ À 8.2 (J(B,H)=118 Hz) along with
reduced signal for AB. This can be assigned to H3N·BH2Cl. After
15 min, we observed a broad signal at δ À 12.9 along with two
former peaks. As the reaction progressed, both the signals
corresponding to H3N·BH2Cl and AB disappeared but the broad
signal at δ À 12.9 was intact. This broad signal resolved into a
°
case of Me3N·BH3 was observed only at 60 C. This could be
ascribed to stronger donating effect of Me3N which increases
electron density on boron and thereby decreases hydridic
nature of BÀ H bond. Figure 3 displays the 11B NMR spectra of t-
BuH2N·BH2Cl, HMe2N·BH2Cl, and Me3N·BH2Cl species formed
via the BÀ H bond activation of various amine boranes brought
about by CuCl2.
°
The reaction of AB with CuCl2 was carried out in various
solvents such as diglyme, THF, and 1,4-dioxane. We found that
the reaction proceeded rapidly in THF followed by diglyme and
1,4-dioxane. This solvent dependence is in accordance with the
ability with which the boronium cation forms an adduct of the
type [H3N·BH2(solvent)][Cl] with the solvent: THF being a good
donor promotes the formation of this adduct quite rapidly
compared to the others.[42] In addition to the triplet signal at δ
À 8.3 due to H3N·BH2Cl species, we also noted another broad
resonance at δ À 0.4 in the 11B NMR spectrum. Heating the
well-defined triplet with J(B, H)=118 Hz at 60 C (Figure S7).
Based on the chemical shift, feature of the signal, in combina-
tion with the coupling constant, we assign this resonance to
the cation of diammoniate of diborane (DADB) [(NH3)2BH2]+.[46]
The coupling constant is in close agreement with that seen for
DADB in liquid ammonia (120 Hz). The resolution of the broad
signal into a well-defined triplet is a direct consequence of
increased solubility of (NH3)2BH2Cl at high temperatures in
diglyme. Autrey and co-workers proposed the formation of
DADB in the nucleation phase of the thermal dehydrogenation
of AB.[47] Notably, the rather unstable and reactive DADB reacts
°
sample to 50 C in a pressure stable NMR tube inside the probe
resulted in the resolution of the broad signal into a triplet with
Figure 3. 11B NMR spectra of amine boranes solutions after reacting AB with
CuCl2 in diglyme solvent for 5 min a) t-BuH2N·BH2Cl, b) Me2HN·BH2Cl, and c)
Me3N·BH2Cl. Except for Me3N·BH2Cl (at 60 C), all other reactions were
Figure 4. 11B NMR spectral stack plot showing the progress of the reaction of
Cu2+ with AB (Cu2+/AB=0.3) in diglyme at 333 K.
°
carried out at room temperature.
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
4
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
with AB resulting in the formation of cyclic H2N-BH2 dimers and
Synthesis of phase pure CuH from CuCl and AB
[H3N(BH2NH2)xNH3][BH4] accompanied by the evolution of H2.
Whereas, in our case the [(NH3)2BH2]Cl is much more stable
than the DADB which is formed via the abstraction of NH3 from
AB by H3N·BH2Cl (Scheme 3a). We noted no further reaction of
the [(NH3)2BH2]Cl with AB which was apparent by the
persistence of the signal at δ À 12.9. No hydrogen evolution
took place in this reaction. In these reactions, we realized a
stable cation of DADB, namely [(NH3)2BH2]Cl. Since this species
has much longer existence compared to the fleeting existence
of DADB, it is possible to study its chemistry and thereby obtain
better insight into the reaction mechanism leading to the poly-
aminoborane growth process in the thermal dehydrogenation
of AB. Studies in this direction are in progress in our
laboratories.
It is also likely that the H3N·BH2Cl formed via the BÀ H bond
activation will react with the unreacted AB to form more
quantity of H3N·BH2Cl via the ejection of H2 and H2N–BH2
reactive species (Scheme 3b). In order to test this hypothesis,
we carried out the dehydrogenation of AB in the presence of 2
equivalents of cyclohexene. The 11B NMR spectrum after 3 h of
reaction time showed a broad peak at δ 48 assignable to
Cy2BNH2 along with a broad peak at δ À 12.9 (Figure S8).[48] The
H2N-BH2 species that is ejected has been trapped in the form of
Cy2B-NH2. This Cy2B-NH2 is much more stable and hence was
observed by 11B NMR. Further, this trapping experiment
Copper hydride is a widely used reducing agent in organic
synthesis. It is routinely synthesized by the reduction of CuSO4
with hypophosphorous acid (H3PO2) in water. However, this
methodology often yields small quantities of metallic Cu(0)
along with CuH.[44,50–52] Improvisation of this method for
increased CuH purity has been recently reported using
CuCO3 ·Cu(OH)2 precursor.[53] It was also observed that the
properties of CuH depends on the purity as well as its method
of preparation (solvents used). In literature, CuH solid was
synthesized in protic solvents such as water /alcohols. In order
to utilize the outcomes of this study for the preparation of pure
CuH in non-protic medium, we reacted CuCl with AB in 1:1 and
1:2 ratios in THF. After 15 min of reaction time, the reaction
mixture was centrifuged, and the obtained dark precipitate was
characterized by PXRD. The PXRD data (Figure 5) suggests the
formation of phase pure CuH for 1:1 ratio and Cu(0) for 1:2
ratio. The obtained powder XRD data is consistent with Wurtzite
structure of copper hydride (JCPDF: 08-0256). We have calcu-
lated the crystallite size from PXRD data using Scherrer
equation and the value is 6.96 nm. The new methodology
reported here for the synthesis of CuH is attractive as it is
simple and avoids the usage of corrosive acids.
supports our proposition of the formation of H2N-BH2 species as Conclusion
a reactive intermediate in this reaction as schematically shown
in Scheme S1. Along with these two peaks, a broad unassign-
able shoulder was observed at δ 53, which could be due to
[Cy2BNH3]+ type of products. Consistent with the formation of
NH2-BH2 and release of more than 1 equivalent of H2, we noted
the formation of BNHx type of compounds with B=N bonds
using the 11B NMR spectroscopy (Scheme S1). The reaction
mixture showed broad signals at around δ 27 and 29 (Figure S9)
which could be assigned to cross-linked borazine polymers.[49]
Lewis acidic copper (II) interacted with hydridic BÀ H bond of AB
to form boronium cation(H3N·BH2Cl) in both aqueous and non-
aqueous media. Because of the BÀ H bond scission at the initial
stages of AB hydrolysis, HCl was produced in-situ which
explains rapid release of H2 in the first 2 min of AB hydrolysis.
CuCl2 underwent step wise reduction to Cu(0) during AB
hydrolysis. We could isolate the intermediates CuCl and CuH by
carrying out reactions in stoichiometric amounts (Cu2+/AB=0.5,
Scheme 3. Possible pathways through which H3N·BH2Cl can react with
ammonia borane. a) shows the formation of cation of diammoniate of
diborane b) generation of hydrogen through linear compound formation.
Figure 5. Powder XRD pattern of the product of the reaction between CuCl
and AB at room temperature.
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
5
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
twice and dried under vacuum for 5 min. The obtained powders
were characterized immediately with powder XRD in air.
and 1). In non-aqueous medium, we also noted the BÀ H bond
scission in other amine boranes such as t-BuH2N·BH3,
HMe2N·BH3, and Me3N·BH3 along with AB. The observed
products are respective boronium cations, CuH and CuCl.
Implications of BÀ H bond activation towards hydrogen gener-
Synthesis of CuH from CuCl and AB in THF: Ammonia borane
(1 mmol, 32 mg) was dissolved in 5 mL of THF in a Schlenk flask,
followed by the addition of CuCl (1 mmol, 99 mg) under stirring.
After 5 min, the reaction was stopped, and the mixture was
centrifuged. Further, the solid was washed with n-hexane and
dried. The freshly prepared solid was characterized using PXRD.
°
ation were studied at 60 C, wherein, 1.5 equivalents of hydro-
gen were noted in 1.5 h (for Cu2+/AB=0/3). At this temperature
the boronium cation, H3N·BH2Cl reacts with the unreacted AB
present in the solution in three separate pathways, one
involving linear product formation, a second resulting in the
formation of a stable cation of diammoniate of diborane, Acknowledgements
[(NH3)2BH2]Cl, and a third involving the precipitation of cross-
linked borazine polymers. Further, some of these observations
allowed us to prepare, phase pure CuH from CuCl and AB.
Overall, the studies demonstrated herein, shed a light on the
dehydrogenation chemistry of Cu2+-AB system as well as
reduction chemistry of AB.
SBK and SM thank the Department of Science and Technology
(DST), Govt. of India, for funding under INSPIRE faculty award
(IFA-14/CH-166). Authors sincerely acknowledge the help of Prof.
Balaji. R. Jagirdar, Department of inorganic and physical
chemistry, Indian institute of science(IISc), Bangalore in carrying
out this work.
Experimental Section
Conflict of Interest
Cu2+ -assisted AB Hydrolysis: Ammonia borane (0.5 mmol, 16 mg)
was dissolved in 20 mL of water in a Schlenk flask. A fixed mole
percent of the metal ion salt (CuCl2 ·6H2O) was added to this AB
solution under stirring. Hydrogen evolution was monitored using a
gas burette, which was connected to a schlenk flask through a
water trap containing 50 mL of water. Time taken for the evolution
of 1 mL of hydrogen was noted down for each millilitre. When
hydrogen evolution ceased, the water levels were adjusted to equal
height by moving the reservoir, and then the final reading was
corrected for the water vapor pressure. Reactions were carried out
for various Cu2+/AB ratios, i.e., 0.3, 0.5, and 1. The experiment setup
used for the measurement of H2 gas was shown in supporting
information (Figure S10) The purity of the evolved H2 was
monitored by using Agilent 7890B Gas chromatography system
equipped with GS- GasPro column. The standard for H2 gas was
taken from 99.9% purity H2 cylinder for the comparison. After the
reaction, the copper containing solids were separated by centrifu-
gation, washed with methanol and dried under vacuum for 2 min.
The obtained powders were characterized immediately with
powder XRD in air.
The authors declare no conflict of interest.
Keywords: Boranes · Copper · Hydrogen · Dehydrogenation ·
Hydride
[1] E. Fakiogˇlu, Y. Yürüm, T. N. Vezirogˇlu, Int. J. Hydrogen Energy 2004, 29,
1371–1376.
[2] C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 2009,
38, 279–293.
[3] T. B. Marder, Angew. Chem. Int. Ed. 2007, 46, 8116–8118; Angew. Chem.
2007, 119, 8262–8264.
[4] B. Peng, J. Chen, Energy Environ. Sci. 2008, 1, 479–483.
[5] F. H. Stephens, V. Pons, R. Tom Baker, J. Chem. Soc. Dalton Trans. 2007,
2, 2613–2626.
[6] B. L. Davis, D. A. Dixon, E. B. Garner, J. C. Gordon, M. H. Matus, B. Scott,
F. H. Stephens, Angew. Chem. Int. Ed. 2009, 121, 6944–6948.
[7] A. D. Sutton, A. K. Burrell, D. A. Dixon, E. B. Garner, J. C. Gordon, T.
Nakagawa, K. C. Ott, J. P. Robinson, M. Vasiliu, Science 2011, 331, 1426–
1429.
[8] F. Baitalow, J. Baumann, G. Wolf, K. Jaenicke-Roöβler, G. Leitner,
Thermochim. Acta 2002, 391, 159–168.
[9] G. Wolf, J. Baumann, F. Baitalow, F. P. Hoffmann, Thermochim. Acta
2000, 343, 19–25.
[10] Z. Xiong, C. K. Yong, G. Wu, P. Chen, W. Shaw, A. Karkamkar, T. Autrey,
M. O. Jones, S. R. Johnson, P. P. Edwards, W. I. F. David, Nat. Mater. 2008,
7, 138–141.
[11] D. J. Heldebrant, A. Karkamkar, N. J. Hess, M. Bowden, S. Rassat, Z. Feng,
K. Rappe, T. Autrey, Chem. Mater. 2008, 20, 5332–5336.
[12] Z. Xiong, G. Wu, Y. S. Chua, J. Hu, T. He, W. Xu, P. Chen, Energy Environ.
Sci. 2008, 1, 360–363.
Activation of the BÀ H bond in amine boranes using CuCl2 in
diglyme: Copper chloride (67.5 mg, 0.5 mmol) and AB (31 mg,
1 mmol) were stirred in 5 mL of diglyme at room temperature in a
Schlenk tube for 5 min. The supernatant was separated by
centrifugation and characterized using 11B NMR spectroscopy.
Activation of the BÀ H bond in tBuH2N·BH3, HMe2N·BH3, and
Me3N·BH3 was carried out using similar procedure except that the
amine borane to CuCl2 ratio was maintained at 1 and in the case of
°
Me3N·BH3 the reaction was carried out at 60 C.
CuCl2 induced hydrogen generation from AB in diglyme:
Ammonia borane (31 mg, 1 mmol) and diglyme (5 mL) were placed
in a Schlenk tube which was connected to a bubbler containing
distilled water and further to a gas burette filled with water. The
[13] S. B. Kalidindi, J. Joseph, B. R. Jagirdar, Energy Environ. Sci. 2009, 2,
1274–1276.
[14] S. Bhunya, T. Malakar, G. Ganguly, A. Paul, ACS Catal. 2016, 6, 7907–
7934.
°
solution was stirred at 60 C for 5 min and a fixed quantity (0.3, 0.5
[15] R. P. Shrestha, H. V. K. Diyabalanage, T. A. Semelsberger, K. C. Ott, A. K.
Burrell, Int. J. Hydrogen Energy 2009, 34, 2616–2621.
[16] M. Chandra, Q. Xu, J. Power Sources 2007, 168, 135–142.
[17] F. Cheng, H. Ma, Y. Li, J. Chen, Inorg. Chem. 2007, 46, 788–794.
[18] M. Zahmakiran, F. Durap, S. Özkar, Int. J. Hydrogen Energy 2010, 35, 187–
197.
[19] T. J. Clark, G. R. Whittell, I. Manners, Inorg. Chem. 2007, 46, 7522–7527.
[20] S. B. Kalidindi, M. Nethaji, B. R. Jagirdar, Int. J. Hydrogen Energy 2010, 35,
10819–10825.
and 1 mmol) of CuCl2 was added. Hydrogen evolution started
almost instantaneously, and the solution turned orange brown in
colour initially and finally to dark brown. Hydrogen evolution was
monitored for 4 h by noting down the gas burette readings at
different time intervals. After the reaction, the copper containing
solids were separated by centrifugation, washed with 10 mL THF
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
6
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejic.202100628
[21] S. B. Kalidindi, U. Sanyal, B. R. Jagirdar, Phys. Chem. Chem. Phys. 2008, 10,
[41] M. G. Hu, R. A. Geanangel, Inorg. Chem. 1979, 18, 3297–3301.
[42] H. K. Lingam, C. Wang, J. C. Gallucci, X. Chen, S. G. Shore, Inorg. Chem.
2012, 51, 13430–13436.
5870–5874.
[22] S. B. Kalidindi, A. A. Vernekar, B. R. Jagirdar, Phys. Chem. Chem. Phys.
2009, 11, 770–775.
[43] J. C. Warf, J. Inorg. Nucl. Chem. 1961, 19, 304–313.
[44] N. P. Fitzsilmons, W. Jones, P. J. Herley, J. Chem. Soc. Faraday Trans.
1995, 91, 713–718.
[45] F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant, D. A. Dixon, Angew.
Chem. Int. Ed. 2007, 46, 746–749; Angew. Chem. 2007, 119, 760–763.
[46] D. W. Himmelberger, W. Y. Chang, M. E. Bluhm, P. J. Carroll, L. G.
Sneddon, J. Am. Chem. Soc. 2009, 131, 14101–14110.
[23] M. Rakap, S. Özkar, Int. J. Hydrogen Energy 2010, 35, 1305–1312.
[24] C. Wang, J. Tuninetti, Z. Wang, C. Zhang, R. Ciganda, L. Salmon, S. Moya,
J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2017, 139, 11610–11615.
[25] S. B. Kalidindi, M. Indrani, B. R. Jagirdar, Inorg. Chem. 2008, 47, 7424–
7429.
[26] Q. Xu, M. Chandra, J. Power Sources 2006, 163, 364–370.
[27] Q. Yao, M. Huang, Z. H. Lu, Y. Yang, Y. Zhang, X. Chen, Z. Yang, Dalton
Trans. 2014, 44, 1070–1076.
[47] A. C. Stowe, W. J. Shaw, J. C. Linehan, B. Schmid, T. Autrey, Phys. Chem.
Chem. Phys. 2007, 9, 1831–1836.
[28] X. Qiu, X. Wu, Y. Wu, Q. Liu, C. Huang, RSC Adv. 2016, 6, 106211–106217.
[29] Y. Yamada, K. Yano, S. Fukuzumi, Energy Environ. Sci. 2012, 5, 5356–
5363.
[30] Y. W. Yang, Z. H. Lu, X. S. Chen, Mater. Technol. 2015, 30, A89–A93.
[31] H. L. Jiang, T. Akita, Q. Xu, Chem. Commun. 2011, 47, 10999–11001.
[32] P. V. Ramachandran, P. D. Gagare, Inorg. Chem. 2007, 46, 7810–7817.
[33] H. C. Kelly, V. B. Marriott, Inorg. Chem. 1979, 18, 2875–2878.
[34] J. Halpern, E. R. Macgregor, E. Peters, J. Phys. Chem. 1956, 60, 1455–
1456.
[35] A. J. Chalk, J. Halpern, J. Am. Chem. Soc. 1959, 81, 5852–5854.
[36] J. Halpern, J. Phys. Chem. 1959, 63, 398–403.
[37] X. Ren, H. Lv, S. Yang, Y. Wang, J. Li, R. Wei, D. Xu, B. Liu, J. Phys. Chem.
Lett. 2019, 10, 7374–7382.
[48] V. Pons, R. T. Baker, N. K. Szymczak, D. J. Heldebrant, J. C. Linehan, M. H.
Matus, D. J. Grant, D. A. Dixon, Chem. Commun. 2008, 6597–6599.
[49] M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, L. G. Sneddon, ACS
Nano 2006, 232, 7748–7749.
[50] J. C. Warf, W. Feitknecht, Helv. Chim. Acta 1950, 33, 613–639.
[51] P. A. Korzhavyi, I. L. Soroka, E. I. Isaev, C. Lilja, B. Johansson, Proc. Natl.
Acad. Sci. USA 2012, 109, 686–689.
[52] N. P. Fitzsimons, W. Jones, P. J. Herley, Catal. Lett. 1992, 15, 83–94.
[53] C. M. Lousada, R. M. F. Fernandes, N. V. Tarakina, I. L. Soroka, Dalton
Trans. 2017, 46, 6533–6543.
[38] H. Lv, R. Wei, X. Guo, L. Sun, B. Liu, J. Phys. Chem. Lett. 2021, 12, 696–
703.
[39] J. Li, X. Ren, H. Lv, Y. Wang, Y. Li, B. Liu, J. Hazard. Mater. 2020, 391,
122199.
[40] C. Wang, J. Tuninetti, Z. Wang, C. Zhang, R. Ciganda, L. Salmon, S. Moya,
J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2017, 139, 11610–11615.
Manuscript received: July 21, 2021
Revised manuscript received: August 10, 2021
Accepted manuscript online: August 12, 2021
Eur. J. Inorg. Chem. 2021, 1–8
www.eurjic.org
7
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

FULL PAPERS
S. Mounika, V. Krishnaveni, M. E.
Dmello, Prof. Dr. S. B. Kalidindi*
1 – 8
Copper(II)-Assisted Ammonia
Borane Dehydrogenation: An
Insight
Lewis acidic copper (II) cleaves
hydridic BÀ H bond of ammonia
borane to form boronium cation
(H3N·BH2Cl) in both aqueous and
non-aqueous media. Copper(II) is
initially reduced to Cu(I) species CuH/
CuCl, and later to Cu(0). The inter-
mediates generated undergo further
reactions depending on the reaction
medium to generate hydrogen from
ammonia borane.