Group 4 metallocene catalysed full dehydrogenation of hydrazine borane

Article abstract of DOI:10.1039/c3dt51370j

A study of the full dehydrogenation of hydrazine borane (H ₂N-NH₂·BH₃) to give H₂ and N₂ as gaseous products catalysed by a variety of group 4 metallocene alkyne complexes of the type Cp′2M(L)(η²-Me ₃SiC₂SiMe₃) (Cp′ = substituted or unsubstituted η⁵-cyclopentadienyl; M = Ti, no L; M = Zr, L = pyridine) and group 4 metallocene hydrides is presented. Volumetric data show that the amount of hydrogen released is strongly dependent on both, the metal and the cyclopentadienyl ligand.

Full text of DOI:10.1039/c3dt51370j

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Group 4 metallocene catalysed full dehydrogenation of
hydrazine borane†
Cite this: Dalton Trans., 2013, 42, 14668
Received 27th May 2013,
Accepted 9th July 2013
Johannes Thomas, Marcus Klahn, Anke Spannenberg and Torsten Beweries*
DOI: 10.1039/c3dt51370j
www.rsc.org/dalton
A study of the full dehydrogenation of hydrazine borane (H
2
N–
NH ·BH ) to give H and N as gaseous products catalysed by a
2
3
2
2
variety of group 4 metallocene alkyne complexes of the type
2
5
Cp’
2
M(L)(η -Me
3
SiC
2
SiMe
3
) (Cp’ = substituted or unsubstituted η -
Scheme 1 Synthesis of HB.
cyclopentadienyl; M = Ti, no L; M = Zr, L = pyridine) and group
4
metallocene hydrides is presented. Volumetric data show that
the amount of hydrogen released is strongly dependent on both,
the metal and the cyclopentadienyl ligand.
on the synthesis and full characterisation of HB was published
very recently by Demirci et al.
6
The thermal decomposition of hydrazine borane was first
5
reported by Goubeau and Ricker. Recently, Lentz et al. have
Introduction
described a method of improving the hydrogen storage capa-
bilities of HB by adding equimolar amounts of LiH. The
7
The problem of hydrogen storage is one main aspect of
current research in connection with the so-called hydrogen
authors point out that the mechanism of decomposition of
this mixture is complicated and rather difficult to investigate
since the reaction products are insoluble in common organic
solvents. However, theoretical studies by Nguyen and Dixon
suggest a dehydrogenation mechanism very similar to that of
the well-known AB dehydrogenation with the terminal NH
group acting as a substituent for the [H N·BH ] unit, making
the HB system structurally very similar to AB. Additionally, as
another method of hydrogen generation from HB, hydrolysis
in the presence of heterogeneous catalysts was reported by
1
economy. Among all the approaches for storing the hydrogen
cleanly and efficiently, the use of chemical hydrides for this
purpose appears to be promising, mostly due to their high
gravimetric hydrogen capacities (e.g. 19.6 wt% in ammonia
2
borane (AB) and 15.4 wt% in hydrazine borane (HB)).
2
Methods for the generation of the energy carrier from these
compounds include hydrolysis in aqueous solutions, catalytic
dehydrocoupling in suitable solvents as well as controlled
thermal decomposition of the solid species. Significant efforts
regarding regeneration of the spent fuels were made very
2
3
8
9
Demirci and Xu and co-workers (Ni–Pt nanoparticles). In this
report, H selectivities as high as 93% were achieved. Most
2
interestingly, along with hydrolysis of the BH unit in this case
3
recently; however, for practical applications of AB and structu-
4
rally similar chemical hydrides this is still a major problem.
3
HB and its parent, thermally labile and explosive com-
pound hydrazine bisborane (16.9 wt%, HBB), can easily be syn-
thesised by reacting the corresponding hydrazine sulfates (i.e.
dihydrazine sulfate and hydrazine sulfate, respectively) with
also decomposition of the N H moiety was observed to yield
2
4
H
2
and N
2
. The nature of the decomposition pathway and thus
of the ratio H /N in the gas phase was found to be strongly
2
2
dependent on the composition of the Ni1−xPt nanoparticles.
x
5
sodium borohydride in dioxane (Scheme 1). A detailed study
Additionally, Özkar et al. presented a study of the hydrolysis of
HB catalysed by Rh(0), which was generated from a RhCl pre-
3
1
0
cursor. However, to the best of our knowledge, a transition
metal catalysed homogeneous process for the dehydrogenation
of HB has not been described to date. Compared to other
methods described before, this would allow for a better under-
standing of the mechanism of dehydrogenation. Moreover,
reaction temperatures should be much lower than in thermo-
lysis studies.
Leibniz-Insitut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a,
1
8059 Rostock, Germany. E-mail: torsten.beweries@catalysis.de;
Fax: +49 381 1281 104; Tel: +49 381 1281 51104
Electronic supplementary information (ESI) available: NMR spectra, XRD analy-
†
sis of the dehydrogenation product, volumetric data and details referring to the
regeneration experiments as well as crystallographic details in cif format. CCDC
936330. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt51370j
1
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Scheme 2 Hypothetical full dehydrogenation of HB with group 4 metallocene
alkyne complexes as catalysts.
Very recently, our group has reported on the dehydrogena-
tion of dimethylamine borane (3.4 wt% H
2
, DMAB) with tita-
nocene and zirconocene alkyne complexes of the type Cp′ M(L)-
2
2
5
(
3 2 3
η -Me SiC SiMe ) (Cp′ = substituted or unsubstituted η -cyclo-
pentadienyl; M = Ti, Zr; L = pyridine, THF). In these com-
1
1
plexes the highly reactive 14 electron metallocene fragment
[
Cp′
2
M] is stabilised by the bis(trimethylsilyl)acetylene ligand
Fig. 1 Gas evolution curve recorded during dehydrogenation of HB in THF
which is eliminated under reaction conditions and thus liber-
2
with 2 mol% (iPrC
red line).
5
H
4
)
2
Ti(η -Me
3
SiC
2
SiMe
3
) (1f) at 25 °C (blue line) and 50 °C
1
2
ates the free reactive metallocene species. Significant differ-
ences in catalytic activity were observed when changing the Cp
ligand as well as the metal centre. In this contribution, we
extend our previous studies to the catalytic dehydrogenation of
HB using group 4 metallocene alkyne complexes (Scheme 2) as
well as group 4 metallocene hydrides.
(
Table 1 Group 4 metallocene bis(trimethylsilyl)acetylene complex catalysed
dehydrogenation of HB
[
2 2
n(H ) + n(N )]/
Catalyst
T (°C)
t (h)
n[HB]
Results and discussion
Catalytic experiments
2
Cp
2
Ti(η -Me
3
SiC
2
SiMe
3
) 1a
) 1b
25
87
46
24
24
45
47
70
64
177
47
79
32
65
10
93
30
75
30
3.9
4.0
0.0
0.0
0.3
3.9
4.0
3.9
3.9
4.0
3.9
4.0
0.9
0.5
4.0
1.8
4.0
1.8
50
*
2
2
Cp
Ti(η -Me
3
SiC
2
SiMe
3
25
50
In a typical catalytic experiment, the metallocene catalyst was
dissolved in a double-walled thermostatically controlled reac-
2
15
rac-(ebthi)Ti(η -Me
3
SiC
2
SiMe
3
)
1c
25
50
2
tion vessel, followed by the addition of the HB solution to the (MeC H ) Ti(η -Me SiC SiMe ) 1d
25
5
4 2
3
2
3
50
stirred solution. In this context it should be noted that for our
investigations we were restricted to THF as the solvent because
2
(
EtC
5
H
)
4 2
Ti(η -Me
3
SiC
2
SiMe
3
) 1e
25
50
2
HB is only sparingly soluble in non-polar solvents such as (iPrC H ) Ti(η -Me SiC SiMe ) 1f
25
5
4 2
3
2
3
50
toluene or n-hexane. More polar solvents than THF or even
protic solvents were not suited for dehydrogenation experi-
2
Cp
2
Zr(py)(η -Me
3
2
SiC SiMe
3
) 2a
25
50
*
2
2
ments due to the limited stability of the catalyst complexes in Cp Zr(η -Me SiC SiMe ) 2b
25
3
2
3
1
3
50
these media. Gas evolutions were measured using an auto-
matic gas buret; details of the experimental setup were pub-
2
rac-(ebthi)Zr(η -Me
3
SiC
2
SiMe
3
) 2c
25
50
1
1b
lished by us elsewhere. An example of a gas evolution curve
is shown in Fig. 1. Analysis of the gas phase after the reaction
showed the presence of hydrogen along with significant
amounts of nitrogen.
2
(η -Me
SiC
SiMe
) were tested. The results of the catalytic tests
3
2
3
Interestingly, in all cases the gas evolution curves show a are compiled in Table 1. In all cases, reaction times of more
significant kink, indicating that the release of the gaseous pro- than 24 hours were required for full consumption of the sub-
ducts from HB does not take place in a linear fashion. Instead, strate. It became evident that the iPr substituted complex 1f is
the formation of a to some extent stable intermediate is likely. the most active catalyst for HB dehydrogenation. The higher
Such non-linear gas evolution curves were also observed by activity of this catalyst compared to other metallocene bis(tri-
1
1b
Demirci and co-workers, who assigned this to the stepwise methylsilyl)acetylene complexes was reported by us earlier;
decomposition with release of H from the BH unit in a first however, a rationalisation for this reaction behavior is still very
2
3
stage followed by concomitant formation of H
decomposition of the N H fragment.
2 4
2
and N
2
by difficult. Most surprisingly, the permethylated titanocene
complex 1b was found to be totally inactive. It should be noted
1
4
HB dehydrogenation experiments were performed at 25 °C that control experiments without any catalyst in THF gave no
and at 50 °C (Fig. S4–S7†). In order to evaluate the influence of gas evolution at all, thus indicating that HB is stable under
cyclopentadienyl substituents and metal centres, different reaction conditions and the described dehydrogenation reac-
group 4 metallocene alkyne complexes of the type Cp′ M(L)- tions are solely catalysed by the metallocene complexes.
2
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The reactivity of zirconocene complexes 2a–2c turned out to
be more diverse. Whereas 2a did not show full consumption of
HB, most surprisingly the fully methylated complex 2b as well
as the bridged complex 2c gave almost quantitative conversion
at 25 °C. A rationalisation for this behaviour can be found in
the interaction of the solvent THF with the metallocene
1
6
fragments in 2a, which was described earlier. Due to the
increased steric demand of Cp* ligands and the bridged rac-
(ebthi) ligand, respectively, this coordination is not preferred
in 2b and 2c.
As mentioned above, in all experiments using group 4 metallo-
cene alkyne complexes as dehydrogenation catalysts, a kink
Fig. 2 Gas evolution curve recorded during dehydrogenation of HB with in situ
was observed in the hydrogen evolution curves. This motivated generated titanocene hydride systems 3a–c and the isolated titanocene(III)
hydride 5 at 50 °C in THF.
us to investigate whether this is also present if other metallo-
cene complexes are used as catalysts. In previous studies of the
dehydrogenation of other amine borane compounds, metal
5
22
hydrides were assumed to be present as intermediates in the (η -cyclopentadienyltitanium).
The application of the
1
7
catalytic cycle.
Mechanistic studies using computational dimeric zirconocene dihydride complex 4 gave smooth and
methods as well as the synthesis of well-defined intermediate complete dehydrogenation of HB; interestingly, in this case
complexes further supported the role of metal hydride frag- the gas evolution curve did not show the above mentioned
1
8
ments. This prompted us to evaluate if group 4 metal kink. Unfortunately, the reaction behaviour of in situ generated
hydride complexes can also be employed for the catalytic dehy- titanocene hydrides was found to be very different. Volumetric
drogenation of HB. Thus, we used an in situ system consisting curves recorded clearly differ in shape, and catalytic activity is
*
of Cp′
(
2
TiF
2
and iBu
2
AlH [Cp′
2
= Cp
2
(3a), Cp
2
(3b), rac-(ebthi) strongly dependent on the nature of the Cp′ ligand (Fig. 2).
*
3c)] to generate titanocene hydrides as well as the literature- Whereas the mixture of Cp
2 2 2
TiF and iBu AlH (3b) fully con-
known dimeric zirconocene dihydride complex [rac-(ebthi)ZrH- verts the HB, dehydrogenation ceases at a conversion of 65
1
9a
(
μ-H)]
of the corresponding zirconocene difluoride with iBu
Titanocene(III) hydrides are well precedented in the litera- assumed to be the limited stability of the corresponding tita-
ture; however, almost no monomeric species with isolated nocene hydrides. It should be noted that both, iBu AlH and
hydride functionalities are known. To the best of our knowl- the titanocene fluoride complexes, alone only produce trace
2
(4) (Table 2), which can also be obtained by reaction and 55% for Cp
2
TiF
2 2 2 2
/iBu AlH (3a) and rac-(ebthi)TiF /iBu AlH
1
9b
AlH.
(3c), respectively (T = 50 °C). A reason for this behaviour is
2
2
*
2
edge, Cp TiH (5), which was first described by Andersen and amounts of H
. The synergistic effect of both species generates
co-workers, is the only example for this class of compounds. an active and stable titanocene(III) hydride complex.
In order to evaluate the role of this structural motif in the
Thus, we assume that formation of a catalytically active
2
2
0
dehydrogenation of HB, we synthesised the mononuclear hydride intermediate takes place during dehydrogenation
hydride complex 5 via hydrogenolysis of the corresponding of HB with metallocene alkyne complexes. Also, it became
*
2
methyl complex Cp
TiMe. Catalytic tests revealed that this evident that titanocene(III) species play an important role in
complex is also moderately active for the dehydrogenation of the catalytic dehydrogenation of HB, an assumption that was
HB.
made before for other dehydrocoupling reactions using group
It should be noted that many titanocene hydrides are stabil- 4 metallocenes.
ised by interaction with other metal centres, thus yielding
Fortunately, we were able to isolate crystals from the reac-
1
7d,23
*
*21
*
dinuclear complexes such as Cp
2
Ti(μ-H)
2
Mg(μ-H)
2
TiCp
2
or tion of Cp TiF and iBu AlH in toluene, which were suitable
2 2 2
5
5
the dimeric titanocene μ-(η :η -fulvalene)-di-(μ-hydride)-bis- for X-ray analysis. The molecular structure (Fig. 3) of the
formed hitherto unknown complex 6 shows the decamethyl-
titanocene fragment and the aluminium dialkyl unit bridged
by two hydrides.
Table 2 Group 4 metallocene hydride catalysed dehydrogenation of HB
Bond lengths of the central Ti–H–Al–H unit of Ti1–H1 1.88(2)
and Al1–H1 1.66(2) Å were obtained, thus reflecting the
overlap of the resonance forms shown in Fig. 4. Hence, com-
pound 6 is best referred to as an adduct between decamethyl-
Catalyst
T (°C)
t (h)
2 2
[n(H ) + n(N )]/n[HB]
Cp TiF
2 2
/iBu
2
AlH 3a
AlH 3b
25
110
110
110
108
110
110
43
44
132
120
1.1
2.6
1.1
4.0
1.6
2.2
3.9
4.0
1.1
1.5
50
*
2
Cp
TiF
2
/iBu
2
25
titanocene(III) monohydride and the alane iBu AlH. Similar
2
50
complexes were described earlier, e.g. Bulychev and co-workers
rac-(ebthi)TiF
2 2
/iBu AlH 3c
25
2
4
50
reported on the parent compound Cp Ti(μ-H) AlCl ·OEt .
2
2
2
2
[rac-(ebthi)ZrH(μ-H)]
2
4
25
It should be noted that catalytic experiments using the
isolated complex 6 gave similar results compared to the in situ
system 3b.
50
*
2
Cp
TiH 5
25
50
1
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Fig. 4 Resonance forms for complex 6.
Fig. 3 Molecular structure of 6. Thermal ellipsoids correspond to 30% prob-
i
ability. Hydrogen atoms (except bridging hydrides) are omitted for clarity. Sym-
metry operator used to generate equivalent atoms: −x + 2, y, −z + 0.5.
Analysis of the dehydrogenation products
In order to evaluate the mechanism of HB dehydrogenation,
1
1
we aimed to perform in situ NMR experiments. However, in
NMR only a decrease of the starting material was detected
Fig. S2†). Resonances due to the dehydrogenation product or
B
Fig. 5 Comparison of the IR spectra of HB (red) and the dehydrogenation
product (blue).
(
intermediates were not observed, owing to the complete in-
solubility of the formed polyaminoborane polymers.
Thermal decomposition of HB gave a polymeric structure
Previous studies have shown that the full characterisation which – from IR spectroscopy and XRD – appears to be very
of HB dehydrogenation products is very challenging, mainly similar to the material isolated from the herein described
7
due to their insolubility in common polar solvents. Thus, we metallocene catalysed dehydrogenation reaction. As for the
performed XRD studies on the isolated material (Fig. S3†). constitution of the solid decomposition product, the authors
Different samples were obtained by filtration of the reaction suggest several likely structural units, most of them involving
6
solution, washing with cold THF and drying under vacuum. cyclic [H N–NH–BH ] units.
2
2
However, despite repeated XRD runs, no accurate information
Additionally, we attempted to regenerate the spent HB fuel
on the composition of the dehydrogenated material could by reaction of the latter with a hydrogen-rich reducing agent.
be obtained. Broad signals at 2θ = 26 and 43° suggest the We chose hydrazine, an approach which was reported earlier
formation of polyborazylenes and different boron nitride by Sutton and Gordon and co-workers for the regeneration of a
2
5,26
3b
species.
6° was tentatively assigned to higher boranes (e.g. B13
ICDD 00-085-0336).
Results from elemental analysis (ICP-OES) of a material NMR (quartet at −17.65 ppm, Fig. S8†) after heating at 50 °C
that showed the release of 4 equivalents of (H + N ) gave a for 24 h. Further optimisations of the regeneration protocol
The very intense reflection at approximately 2θ = spent AB material. Indeed, addition of an excess of hydrazine
1
19
H ,
to the HB dehydrogenation product dispersed in THF resulted
in the highly selective formation of HB as evidenced by
1
1
B
2
2
B/N ratio of 1 : 1.03, thus suggesting that half a mole of dinitro- are currently under investigation in our group.
gen was present in the evolved gas mixture.
A comparison of the IR spectra of pure HB and the solid
dehydrogenation product is depicted in Fig. 5. Whereas the IR
spectrum of the starting material shows very intense bands in
Conclusions
the N–H as well as in the B–H stretch regions at We have demonstrated the group 4 metallocene alkyne
650–3350 cm− and 2250–2400 cm , respectively, these complex catalysed dehydrogenation of hydrazine borane. Sig-
1
−1
2
signals were not found in the spectrum of the dehydrogena- nificant differences in the catalytic performance of the catalyst
tion product. Moreover, strong bands in the N–H asymmetric complexes became evident with the iPr substituted complex 1f
−1
bending region (1588 and 1620 cm ) were absent in the spec- being the most active. Additionally, isolated and in situ gener-
trum of the dehydrogenated material. Instead, a sharp signal ated group 4 metallocene hydride complexes were tested for
−1
at 3214 cm indicates the presence of NH groups. A broad the dehydrogenation reaction. Most interestingly, in this case
signal at 2450 cm− most likely indicates the presence of BH or the dinuclear bridged zirconocene complex [rac-(ebthi)ZrH-
1
BH
2
groups in a polymeric regime.
(μ-H)] was the most active catalyst. Analysis of the insoluble
2
Our results from XRD and IR spectroscopy are well in line reaction products showed that most likely a mixture of cyclic
6
with data reported very recently by Demirci and co-workers.
[H N–NH–BH ] structures and boron nitride species is present
2 2
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after the dehydrogenation reaction. To the best of our knowl- 13 Examples for the decomposition of group metallocene
edge, this is the first example for a transition metal catalysed
homogeneous process for the dehydrogenation of hydrazine
borane. Significant improvements in catalytic activity are still
needed; however, the potential of hydrazine borane and
related compounds as a chemical hydrogen storage material
appears to be considerable.
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Acknowledgements
We would like to thank our technical and analytical staff for 14 C. Cakanyildirim, E. Petit, U. B. Demirci, R. Moury,
support and Dr Matthias Schneider (LIKAT) for performing the
XRD experiments. Financial support from the DFG (grant code
J.-F. Petit, Q. Xu and P. Miele, Int. J. Hydrogen Energy, 2012,
37, 15983.
BE 4370/2-1) is gratefully acknowledged. T.B. thanks Prof. Uwe 15 ebthi = ethylene-1,2-bis(4,5,6,7-tetrahydro-1-indenyl).
Rosenthal (LIKAT) for useful discussions and support.
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