A readily accessible ruthenium catalyst for the solvolytic dehydrogenation of amine-borane adducts

Article abstract of DOI:10.1039/c4dt01216j

The use of the readily available complex [Ru(p-Cym)(bipy)Cl]Cl as an efficient and robust precatalyst for homogeneously catalysed solvolysis of amine-borane adducts to liberate the hydrogen content of the borane almost quantitatively is being presented. The reactions can be carried out in tap water, and in aqueous mixtures with non-deoxygenated solvents. The system is also efficient for the dehydrocoupling of dimethylamine-borane under solvent-free conditions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01216j

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PAPER
A readily accessible ruthenium catalyst for the
solvolytic dehydrogenation of amine–borane
adducts†
Cite this: Dalton Trans., 2014, 43,
11404
Martín Muñoz-Olasagasti,a Ainara Telleria,a Jorge Pérez-Miqueo,a María A. Garraldaa
and Zoraida Freixa*a,b
The use of the readily available complex [Ru(p-Cym)(bipy)Cl]Cl as an efficient and robust precatalyst for
homogeneously catalysed solvolysis of amine–borane adducts to liberate the hydrogen content of the
borane almost quantitatively is being presented. The reactions can be carried out in tap water, and in
aqueous mixtures with non-deoxygenated solvents. The system is also efficient for the dehydrocoupling
of dimethylamine–borane under solvent-free conditions.
Received 25th April 2014,
Accepted 28th May 2014
DOI: 10.1039/c4dt01216j
www.rsc.org/dalton
vage of the adduct.22 Consequently, it renders up to 3 equiva-
lents of H2 per mole of AB, half of it coming from the H2O or
Introduction
Nowadays, hydrogen is considered as a competent energy alcohols present in the solvent mixture (eqn (2)).
vector capable of replacing fossil fuels in the near future. Its
safe storage and controlled liberation are some of the most
nH3NꢀBH3 ! ½NH2BH2ꢁn þ nH2
ð1Þ
ð2Þ
challenging problems that need to be solved for its consolida-
tion. Several materials are being considered for solid hydrogen
storage. Among them B–N based compounds are the most
H3NꢀBH3 þ 2H2O ! NH4þ þ BO2ꢂ þ 3H2
Several metallic nanoparticles efficiently catalyse this
reaction.4,5,22–24 To the best of our knowledge there are only
four reported homogeneous catalytic systems for the hydrolytic
cleavage of AB, intended for H2 production. In 2010 Garralda
and coworkers reported on the use of an Ir(III) hydrido-β-di-
ketone complex able to generate up to three equivalents of H2
per mole of the substrate operating in mixtures of THF–
H2O.25,26 Nearly concomitant was the publication of an in situ
formed ruthenium catalyst species generated from dicarbonyl-
ruthenacyclic complexes under catalytic conditions.27 More
recently two other very effective systems based on iridium(III)
have been reported: a bis-ortho-metalated Ir(III) NHC28,29 and
an Ir(III) P–N–P pincer,30 which showed exceptional activities
and robustness. Based on these few examples it seems that
iridium complexes modified with sophisticated ligands (P–N–P
pincers, cyclometalated-NHC or phosphane-β-diketones) are
the most effective catalysts for this reaction. Lower price of
ruthenium compared to iridium would make it a priori a more
adequate metal of choice, nevertheless its low activity
(Ru loading ∼10% reported for Djukic’s system)27 does not
compensate for this advantage.
promising ones due to their high gravimetric hydrogen storage
capacity. Ammonia–borane (H3N·BH3, AB), with a hydrogen
content of 19.6 wt% and high stability, is one of the most
plausible candidates for this purpose.1–5
The use of catalytic amounts of metals to induce a con-
trolled release of H2 from AB is an appealing strategy, and it is
being intensively pursued in recent years. Two different cata-
lytic pathways are known for a controlled hydrogen release from
AB: dehydrogenation and solvolysis. The former, promoted by
several metallic nanoparticles or clusters6–8 and organometallic
complexes (i.e. based on Rh,9,10 Ru,11–13 Ir,14,15 Os,16 Fe8 or
Pd17), generates one equivalent of H2 per mole of substrate and
the AB ends up in the form of [H2B–NH2]n oligomeric or poly-
meric materials (eqn (1)). Only some Ru nanoclusters13 and two
homogeneous catalysts based on Ru18–20 and Ni21 are able to
further dehydrogenate the formed polyaminoboranes generat-
ing up to 2.8 equiv. of H2. The solvolytic route is generally
accepted to proceed by a fast hydrolysis/alcoholysis of the BH3
liberated in the aqueous or alcoholic media after catalytic clea-
aDepartment of Applied Chemistry, Faculty of Chemistry, University of the Basque
Country (UPV-EHU), San Sebastián, Spain. E-mail: Zoraida_freixa@ehu.es;
We present here a readily accessible [Ru(p-Cym)(bipy)Cl]Cl
(p-Cym = para-cymene, bipy = 2,2′-bipyridine) water-soluble
complex as an efficient catalyst precursor for solvolytic
dehydrogenation of amine–boranes. It operates in air, tap
water and aqueous mixtures with non-deoxygenated solvents,
Tel: +34943018848
bIKERBASQUE, Basque Foundation for Science, Bilbao, Spain
†Electronic supplementary information (ESI) available: Reaction profiles, NMR
analysis and crystallographic data are included. See DOI: 10.1039/c4dt01216j
11404 | Dalton Trans., 2014, 43, 11404–11409
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with high activities and stability. This catalytic system is also species, which is consistent with a tetrahedral, negatively
active for an unprecedented dehydrocoupling of dimethylam- charged boron centre with four B–O sigma bonds.29,32 Two
monia–borane in the absence of a solvent, forming the cyclic blank experiments (one without the catalyst and the other with
[Me2NBH2]2 as the only reaction product.
0.5% (Ru) of the dimeric complex [Ru(p-Cym)Cl2]2 used to syn-
thesize the catalyst precursor) showed only residual H2 pro-
duction (less than 0.1 equivalents for the former and 0.4
equivalents for the latter) when the reactions were conducted
for 2 hours in THF–H2O = 1/3 at rt. To test the homogeneous
nature of the active catalyst, ∼1000 equivalents of metallic Hg
have been added at 33% conversion, which did not affect the
Results and discussion
Catalytic experiments
In a standard experiment the precatalyst, [Ru(p-Cym)(bipy)Cl]-
Cl (0.5 mol%), was mixed with the substrate for 30 min. This
pretreatment guarantees the reproducibility of the results.
After this period, the solvent was added and the gas evolution
was measured in a water-filled inverted gas burette.
reaction profile, supporting the hypothesis of
geneously catalysed reaction.6
a homo-
In general, 97% pure AB has been used for these exper-
iments, but the use of technical grade AB or tap water did not
affect the activity of the system.
Several solvent mixtures have been assayed to optimise the
reaction conditions (see Table 1, and Fig. S1†). These results
showed that the hydrogen generation is moderate in pure alco-
hols and undetectable in freshly distilled dry THF. This obser-
vation points to a hydrolytic mechanism rather than a
dehydrogenation being responsible for the hydrogen liber-
ation. Accordingly, increasing the H2O proportion in the
solvent mixture has a beneficial effect on the activity of the
system. Surprisingly, in pure water, in spite of the complete
solubility of both the catalyst and substrate, the overall activi-
ties are similar to those observed for the mixture THF–H2O =
1/1. For this reason THF–H2O = 1/3 and iPrOH–H2O = 1/1 were
selected as standard reaction conditions.
Using these solvent mixtures ∼2.8 equivalents of H2 per AB
have been released within 18 minutes at room temperature
(2 eq. were evolved in ∼10 min). When these reactions were
conducted at 40 °C, 2.8 equivalents of H2 were evolved in less
than 10 minutes. 11B-NMR analysis at the end of the reaction
(using an internal D2O capillary) showed only a broad singlet
between 8 and 19 ppm attributed to the different borates in
equilibrium in solution.24,27,29,31 After evaporation of the
solvent, 11B-NMR of the reaction residue in DMSO-d6 showed a
sharp singlet at 1.54 ppm as the only boron-containing
To extend the scope of this system, the same catalytic con-
ditions were assayed on different substrates (see Fig. 1). The
lack of reactivity of trisubstituted amine–boranes (triethyl-
amine–borane TEAB and trimethylamine–borane TMAB),
already observed by Garralda and coworkers25 for iridium-
based catalysts, suggests the need for at least one N–H func-
tionality on the substrate for the reaction to proceed. It is often
claimed that a loosely bound coordination of the ammonia–
borane to the metal centre is responsible for the activation of
AB for either dehydrocoupling or hydrolytic dehydrogenations.
Except for bifunctional catalysts,33 the initial interaction of the
AB with the metal centre is commonly accepted to proceed
through the hydric hydrogens of the borane or the B–H
bonds.8,27,34–40 In fact, an Ir(III) complex containing co-
ordinated BH3 through an elongated B–H σ bond is postulated
as dormant state species in AB dehydrogenation reactions
Table 1 The measured TOF (h−1) for the solvolytic dehydrogenation of
AB using different solvents
TOFa (h−1
)
b
Total H2
1.0 eq. H2 1.5 eq. H2 2.0 eq. H2 time (min)
THF
0
277
538
766
530
909
66
0
225
451
775
455
856
69
0
182
383
774
342
830
68
0
THF–H2O = 3/1
THF–H2O = 1/1
THF–H2O = 1/3
H2O
iPrOH–H2O = 1/1
iPrOH
2.7(100)
2.7(45)
2.8(18)
2.7(50)
2.9(17)
2.7(235)
2.7(85)
1.4(40)
MeOH–H2O = 1/1 323
MeOH 211
288
232
—
122c
Fig. 1 Reaction profile (gas evolution) using different substrates.
Unreactive substrates (TEAB and TMAB have not been represented since
no gas evolution was detected). Reaction conditions: 1.38 mmol sub-
strate, 0.5 mol% [Ru(p-Cym)(bipy)Cl]Cl, 3 mL THF–H2O 1/3, rt (iPrOH–
H2O 1/1 in ESI†).
Reaction conditions: 1.38 mmol AB, 0.5 mol% [Ru(p-Cym)(bipy)Cl]Cl,
3 mL solvent, rt. a TOF reported at 1.0, 1.5 and 2.0 equivalents of H2
liberated with respect to AB. Calculated based on the consumption of
AB. b The total number of equivalents of H2 liberated with respect to
AB at the end of the reaction. c 1.4 eq.
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when using Ir(tBuPOCOP)H2 as a catalyst.41 According to our lysts showed a faster reaction rate for DMAB compared to
observations, it could be suggested that the presence of a N–H TBAB.25,26 In contrast, with our system similar reaction rates
bond in the substrate, obviously necessary for dehydro- have been measured for both substrates. In fact, the best activi-
coupling reactions, also played an important role in solvolytic ties were measured for TBAB in iPrOH–H2O 1/1, for which a
dehydrogenations, lowering the energetic barrier that prevents TOF of 4152 h−1 was measured at 33% conversion.
the cleavage of the amine–borane adduct and subsequent
borane solvolysis.
To test the recyclability and robustness of the catalyst,
several successive additions of 200 equivalents of AB in 0.5 mL
When dimethylamine–borane (DMAB) was used as the sub- of the reaction solvent per mole of the catalyst have been con-
strate, during the pre-catalysis mixing period some gas release ducted (Fig. 2 and S5 in ESI†). The system showed that,
was observed. To gain some insight into the nature of the although there is a gradual reduction of the reaction rate
process responsible for this gas evolution, DMAB was mixed (which can be partially attributed to the change in the reaction
with 0.5 mol% of [Ru(p-Cym)(bipy)Cl]Cl and the generation of medium: incremental dilution and precipitation of large quan-
gas was measured in a water-filled inverted gas burette (see tities of borates which hamper the homogeneity of the solu-
Fig. 1, series DMAB solvent free). After 25–30 min the volume tion), the catalyst remained active for at least 6 consecutive
corresponding to a half equivalent of hydrogen per mol of cycles, liberating a total of 3360 equivalents of H2 per mole
DMAB was detected. The gas production stopped after liber- of Ru.
ation of 1 equivalent of gas (3 h) per mole of substrate. During
To gain some insight into the nature of the active species,
1
the course of the reaction, the mixture of solids turned dark we performed H-NMR analysis of the precatalyst, [Ru(p-Cym)-
and liquefied, and a colourless crystalline material sublimed (bipy)Cl]Cl, in D2O. As stated before for related compounds
and condensed on the inner wall of the reaction vial. 11B-NMR the spectrum showed that in aqueous solutions it exists as an
analysis results of this crystalline sublimed material and of the equilibrium mixture of [Ru(p-Cym)(bipy)Cl]+ and the bis-
reaction residue in DMSO-d6 were identical.‡ They showed the cationic aqua complex [Ru(p-Cym)(bipy)(H2O)]2+.44,45 This
complete consumption of the substrate, DMAB (δ −13.8 ppm, equilibrium can be shifted back and forth by addition of chlor-
quartet, JH–B = 114 Hz) and the appearance of a clean triplet at ide salts (NaCl) or chloride abstractors (AgOTf) respectively
5.3 ppm, JH–B = 112 Hz. This chemical shift is in agreement (see S9, ESI†). To test whether the aquo complex was a better
with the one reported for the cyclic [Me2NBH2]2.17,42,43 The suited precatalyst compared to the [Ru(p-Cym)(bipy)Cl]+, we
molecular structure of this compound was confirmed by X-ray performed catalytic experiments in which an excess of NaCl or
diffraction of the sublimed crystals (Fig. S12†), and it is iso- AgOTf was added to the catalytic mixture. The results obtained
structural to the ones previously published by Manners for the showed that, as expected, the addition of NaCl produces a
same compound.7,42 These results suggest that the system also slight decrease in the catalytic activity. Surprisingly, addition
acts as an effective catalyst for the dehydrocoupling of DMAB of silver salts caused a dramatic deactivation of the catalytic
in the absence of a solvent. This dehydrocoupling reaction system. To corroborate whether it was due to some interference
occurring during the mixing period in the case of DMAB gen- from the silver salt in the process, we synthesized the [Ru(p-
erates a mixture of substrates (DMAB and [Me2NBH2]2) that Cym)(bipy)(H2O)](OTf)2 complex and tested it as a precatalyst.
creates an artefact in the reaction profile for this substrate (see This compound showed identical activities compared to the
Fig. 1, series DMAB), which eventually liberates less than 2 eq. parent chloride compound in the mixture THF–H2O = 1/3.
of H2. To avoid this, this reaction was also conducted without
a premixing period to ensure that all the substrate was in the
form of DMAB (Fig. 1, series DMAB no pretreatment) and also
after liberation of 1 equivalent of H2, when it was in fact fully
converted to [Me2NBH2]2 (Fig. 1, series [Me2NBH2]2). The
results obtained showed that the cyclic [Me2NBH2]2 was a
rather bad substrate liberating only 0.5 equivalents of H2
(1 hour). In contrast, when DMAB§ or tert-butylamine-borane
(TBAB) was employed as the substrate a total hydrogen volume
corresponding to more than 2.8 equivalents of hydrogen per
mol of substrate was measured at the end of the reaction
(5–15 min), which has been considered as evidence that the
reaction proceeded through a solvolytic pathway.25,26,29 When
these substrates were used, a much faster hydrogen liberation
was observed compared to AB. Previous reports on hydrolytic
dehydrogenation of these substrates with iridium-based cata-
Fig. 2 Reaction profile (equivalents of H2 per mole of Ru evolved vs.
‡DMSO-d6 was used as a solvent for solubility reasons, and to avoid further
hydrolysis in aqueous solvents.
time) in 6 successive cycles. Reaction conditions (first cycle): 1.38 mmol
AB, 0.5 mol% [Ru(p-Cym)(bipy)Cl]Cl, 3 mL THF–H2O 1/3, rt. Successive
additions of 1.38 mmol AB dissolved in 0.5 mL of THF–H2O 1/3.
§Without a pre-mixing period.
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In situ NMR studies
In situ 1H-NMR and 11B-NMR experiments were performed
using THF-d8–H2O = 3/1 to observe the process at different
stages. This solvent mixture was chosen due to the relatively
slow reaction rate in this media. 11B-NMR spectra have been
recorded at 5 minutes intervals using the exact catalytic con-
ditions. These spectra showed the progressive consumption of
AB (quartet at −22.8 ppm, JH–B = 93.8 Hz) and the formation of
a species that presents a broad singlet at 19.7 ppm (Fig. S6,
ESI†). This singlet was assigned to the borates formed in solu-
tion in comparison with the data published elsewhere for
other AB hydrolysis catalytic systems.25,27,29 11B-NMR in
DMSO-d6 of the residue after evaporation of this reaction
mixture showed a singlet at 1.54 ppm, as already observed for-
merly in catalytic runs. The low catalyst loadings prevented a
direct observation of the catalyst resting state under these cata-
lytic conditions. Additional NMR experiments have been per-
formed using 10 times higher catalyst concentrations and a
10/1 AB/Ru ratio in THF-d8–H2O 3/1 at 15 °C. 11B-NMR (Fig. 3)
1
and H-NMR (Fig. S7 in ESI†) spectra were acquired at 5 min
intervals. 11B-NMR spectra show that more than 60% of the AB
was already hydrolyzed in the first spectrum (in the delay
necessary to introduce the sample into the spectrometer), and
it was fully consumed within the first 45 minutes.
Fig. 4 1H-NMR spectra, aromatic region, (500 MHz) in THF-d8–H2O =
3/1. (a) In situ NMR experiment with 60 min of reaction at 15 °C. (b)
In situ NMR experiment after venting the tube. (c) Ru(p-Cym)(bipy)Cl]Cl.
(d) Ru(p-Cym)(bipy)Cl]Cl + excess of AgOTf.
1H-NMR spectra showed that in the presence of AB, there is
one main set of signals in the aromatic region that corres-
ponds to coordinated p-cymene and bipyridine ligands. A
clear hydride singlet is observed at −5.71 ppm. This spectro-
scopic pattern indicates that most of the Ru present under
catalytic conditions is in the form of [Ru(p-Cym)(bipy)H]+, in
comparison with the reported spectroscopic data for this
ruthenium hydride.46 Whether this Ru(II)-hydride is the resting
state of the catalyst or a dormant state formed in the presence
of H2 (observed as a singlet at 4.5 ppm) is uncertain. When
the NMR tube was vented, a new set of signals appeared in
the 1H-NMR spectra (Fig. 4, and Fig. S7 in ESI†). It shows
the characteristic pattern of a Ru(II) complex of the type
[Ru(p-Cym)(bipy)L]n+. The aromatic signals of this new species
are downfield shifted compared to [Ru(p-Cym)(bipy)H]+ and
[Ru(p-Cym)(bipy)Cl]+ (which was confirmed by adding more
precatalysts into the vented NMR tube).
In an effort to identify this species, we studied by NMR
spectroscopy the reaction of [Ru(p-Cym)(bipy)Cl]Cl with one
equivalent of AB in THF-d8–H2O 3/1. The 1H-NMR spectrum
showed 3 sets of signals, corresponding to [Ru(p-Cym)(bipy)-
Cl]+, the putative hydride [Ru(p-Cym)(bipy)H]+, and the new
species observed at the end of the catalytic runs (see Fig. S11,
ESI†). The exact nature of this compound is still uncertain. It
might be either an ammonia or a borane complex which
would trap the catalyst in the absence of hydrogen, which
reverts into the active species when a new batch of substrates
is added.
Conclusions
The use of the readily available complex [Ru(p-Cym)(bipy)Cl]Cl
as an efficient and versatile precatalyst for the homogeneously
catalysed solvolytic dehydrogenation of ammonia–borane and
amine–borane adducts has been presented. It is shown to be a
robust catalyst even in tap water and in aqueous mixtures with
non-deoxygenated solvents. In situ NMR experiments showed
that the [Ru(p-Cym)(bipy)H]+ species is the main ruthenium
component present under catalytic conditions, though an un-
identified compound of the type [Ru(p-Cym)(bipy)L]n+ seems
Fig. 3 11B-NMR spectra at different reaction times. Reaction conditions:
0.14 mmol AB, 0.014 mmol [Ru(p-Cym)(bipy)Cl]Cl, 1 mL THF-d8–H2O
3/1, 15 °C.
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to act as a dormant state of the catalyst after venting the reac- In situ NMR experiments
tion solutions. [Ru(p-Cym)(bipy)Cl]Cl also catalyses the unpre-
Catalytic conditions. 14.2 mg of ammonia–borane
(0.46 mmol) and 1.1 mg of [Ru(p-Cym)(bipy)Cl]Cl
(0.0023 mmol) were stirred for 30 minutes and dissolved in
1 mL of THF–H2O mixture (3 : 1) using an internal D2O capil-
lary. After addition of the solvent, the release of hydrogen was
immediate. The NMR tube was introduced as fast as possible
into the spectrometer and 11B-NMR spectra were acquired at
regular time intervals.
cedented dehydrogenation of DMAB in the solid-state, forming
the cyclic [Me2NBH2]2 as the only reaction product.
Further studies to unravel the exact nature of the catalyst
and to extend the scope of this catalytic system are currently
under progress in our laboratories.
Ru concentrated conditions. 4.2 mg of ammonia–borane
(0.14 mmol) and 6.3 mg of [Ru(p-Cym)(bipy)Cl]Cl
(0.014 mmol) were stirred for 30 minutes and dissolved in
Experimental
Materials and methods
Substrates were obtained from commercial sources and 1 mL of THF-d8–H2O mixture (3 : 1). After addition of the
were used without further purification. The precatalysts, solvent, the release of hydrogen was immediate. The NMR
[Ru(p-Cym)(bipy)Cl]Cl and [Ru(p-Cym)(bipy)(H2O)](OTf)2, were tube was introduced as fast as possible into the spectrometer
prepared according to published methodologies.46 Unless and 1H-NMR and 11B-NMR spectra were acquired at regular
otherwise stated, NMR spectra were recorded on a Bruker 500 time intervals.
AVANCE spectrometer equipped with a z gradient BBO probe.
When aqueous solvent mixtures were used, 1H-NMR spectra
were recorded using a watergate methodology47 for solvent
suppression using a time domain of 32k, and a spectral width
Acknowledgements
This work was supported by MICINN (CTQ2011-23333),
SAIOTEK (S-PE13UN020), UPV-EHU (GIU13/06) and Basque
Government Grants to J.P-M (BFI-2012-147MEC) and M.M-O
(COL_2013_1_351). The UPV-EHU Research Support Unit
(SGIker) is acknowledged for technical assistance, and
Dr H. Gulyás and Dr I. Rivilla for fruitful discussions.
of 20 kHz; the number of scans is 4, the interpulse delay is 1 s,
and the acquisition time is 3 s.
11B-NMR spectra were recorded at 160.46 MHz, using a time
domain of 32k, and a spectral width of 16 kHz. The number of
scans is 128. The interpulse delay is 0.2 s. The acquisition
time is 0.3 s.
Catalytic experiments
Notes and references
Solvolytic dehydrogenation of ammonia–borane and amine–
boranes. Catalytic reactions were carried out in a septum-
closed vial connected to a water-filled inverted gas-burette
used to follow the hydrogen release. 1.38 mmol of the sub-
strate (42.5 mg of ammonia–borane) and 0.0069 mmol of the
precatalyst (3.2 mg of [Ru(p-Cym)(bipy)Cl]Cl, 0.5%) were
mixed in the vial and stirred for 30 minutes. After that premix-
ing period, 3 mL of the solvent was added and the progression
of the reaction was followed until no further gas liberation was
observed. All these procedures were carried out under air. For
the reaction at 40 °C the reaction vial was immersed in a thermo-
stated water bath.
Consecutive solvolytic dehydrogenation of AB. After an
initial run (performed following the afore-mentioned pro-
cedure and quantities), the water in the burette was reset to
zero by disconnecting it from the reaction vial. Then, a solu-
tion of 1.38 mmol of substrate in 0.5 mL of the solvent was
added to the former reaction residue and the gas evolution
was measured in the burette.
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