Polyboramines for Hydrogen Release: Polymers Containing Lewis Pairs in their Backbone

Article abstract of DOI:10.1002/anie.201508395

The one-step polycondensation of diamines and diboranes triggered by the in situ deprotonation of the diammonium salts and concomitant reduction of bisboronic acids leads to the assembly of polymer chains through multiple Lewis pairing in their backbone. These new polyboramines are dihydrogen reservoirs that can be used for the hydrogenation of imines and carbonyl compounds. They also display a unique dihydrogen thermal release profile that is a direct consequence of the insertion of the amine-borane linkages in the polymeric backbone.

Full text of DOI:10.1002/anie.201508395

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International Edition: DOI: 10.1002/anie.201508395
German Edition: DOI: 10.1002/ange.201508395
Polymers Very Important Paper
Polyboramines for Hydrogen Release: Polymers Containing Lewis
Pairs in their Backbone
Audrey Ledoux, Paolo Larini, Christophe Boisson, Vincent Monteil, Jean Raynaud,* and
Emmanuel Lacôte*
Abstract: The one-step polycondensation of diamines and
diboranes triggered by the in situ deprotonation of the
diammonium salts and concomitant reduction of bisboronic
acids leads to the assembly of polymer chains through multiple
Lewis pairing in their backbone. These new polyboramines are
dihydrogen reservoirs that can be used for the hydrogenation
of imines and carbonyl compounds. They also display a unique
dihydrogen thermal release profile that is a direct consequence
of the insertion of the amine–borane linkages in the polymeric
backbone.
monomers assembled through Lewis pairs. We selected N and
B hydrogen-disubstituted amine boranes (ABs) in poly-Lewis
paired polymers as a starting point (see Figure 1). There are
very few reports on the synthesis of poly(amine-borane)s or
polyboramines, and examples are limited to the use of
B
oron-based polymers have been the focus of growing
attention because they exhibit outstanding properties such as
photoluminescence, electroluminescence, nonlinear optical
properties, and n-type semiconductivity that make them well-
suited as materials for organic electronics, imaging, or ion and
molecule sensing.[1] A vast majority of these polymers are
derived from boron-containing monomers polymerized by
free-radical additions, or organometallic coupling reac-
tions;[1,2] that is, their main chain contains only covalent
bonds. In addition, polymeric boron nitrides are obtained
from the dehydrogenation of ammonia borane (NH3·BH3),
and are thus crucial materials for dihydrogen-storage pur-
poses.[3,4] Indeed, the storage and controlled release of
dihydrogen has become an essential area of research to
address the ever-growing energetic demand. While ammonia
borane was identified early on as an ideal candidate to
constitute a H2 reservoir because of its maximal storage
capacity (19.6 wt%. H2),[3–5] its shortcomings such as poor
processability, foaming, corrosive or toxic emissions, as well as
troublesome material recycling have been identified. As part
of our program devoted to the use of Lewis bases to access
new reactivities at boron,[6] we set out to investigate the
properties of new long-chain boron polymers built from
Figure 1. General strategy for the synthesis of H2-releasing polybor-
amines.
bipyridines or pyrazines with aryl/aliphatic bisborane mono-
mers.[7] The target application of these systems is essentially
the elaboration of dynamic polymers[7] that have potentially
interesting fluorescent properties.[1,2f] Most of these polymers
are synthesized by precipitation methods and have poor
stability in solution. Therefore, none of them was extensively
characterized. Moreover, substituted ABs were employed in
those cases.[7] Thus, none of the few materials obtained could
be engaged in further reactions. A report by Manners et al.
mentions the Ir-catalyzed synthesis of polyaminoboranes
which are covalently bonded polymers, that is, analogues of
polyolefins, but high temperatures are required to achieve the
dehydrogenation of polyaminoboranes.[8]
Herein we report a new type of polyboramines, compris-
ing bis-hydrogenated N and B atoms, obtained in one step
from commercially available simple monomers by using
unusual heterogeneous conditions (Figure 1). The polymer
is soluble and stable in THF and DMSO. This new type of
boron-based polymer was evaluated as a reactive material in
hydrogen-transfer reactions based on ammonia borane
chemistry. We also investigated the potential of these poly-
boramines as dihydrogen-storage materials, in particular the
influence of the polymer chains on the dehydrogenation
profile (Figure 1).
[*] A. Ledoux, Dr. C. Boisson, Dr. V. Monteil, Dr. J. Raynaud, Dr. E. Lacôte
UniversitØ de Lyon, Institut de Chimie de Lyon, C2P2, UMR 5265
CNRS-CPE Lyon-UCBL
43 Bd du 11 Novembre 1918, 69616 Villeurbanne (France)
E-mail: jean.raynaud@univ-lyon1.fr
emmanuel.lacote@univ-lyon1.fr
Homepage: http://c2p2-cpe.com/index.php
Dr. P. Larini[+]
UniversitØ de Lyon, Institut de Chimie de Lyon, ICBMS, UMR 5246
CNRS-CPE Lyon-UCBL
To access the polymers, an AA/BB type polycondensation
strategy was selected (Figure 1), using simple bisboronic acids
or esters and diammonium chlorides as difunctional mono-
mers, starting from a heterogenous suspension of insoluble
starting materials in THF.[9] This approach is versatile and
43 Bd du 11 Novembre 1918, 69616 Villeurbanne (France)
[+] Correspondence concerning computational studies should be
addressed to paolo.larini@univ-lyon1.fr.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508395.
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uses readily accessible building blocks.[10] However it requires
a precise stoichiometry to achieve high molar masses.
Aliphatic diamines were chosen based on electronic consid-
erations. As good Lewis bases, they provide increased
stability for the amine–borane bond. Bisboronic species
with a para-phenylene motif were selected to avoid potential
oxidative deborylation and provide rigidity to the polymer.
The polymerization relies on the formation of the key
Lewis pairs. We chose to develop a simple, ideally one-step,
synthetic route.[9] The reaction had to be 100% selective and
quantitative for an efficient polycondensation. The acid/base
reaction of borohydrides and ammonium salts releases
dihydrogen gas as a side-product. We hoped to harness this
favorable entropic driving force toward the formation of the
polymer.
fraction by filtration followed by several washes. Part of the
product remained trapped in the filter cake, thus decreasing
the yield of isolated material. A white powder was obtained
after evaporation of the volatile components. However, even
after 2 days under high vacuum (10À5 mbar), the material still
retained some THF. TGA analyses indicated that the vast
majority of the lithium and aluminum salts remained in the
insoluble fraction (less than 4% residues, after heating the
polymer to 9008C). In contrast, when para-phenylenebisbor-
onic pinacol ester was used as a precursor, a large amount of
salts was detected (up to 30%), together with residual
pinacol. In the latter case, AB oligomers were isolated as
a white solid.
We synthesized four polymers by using our method.
Polymers 1a, 1b, and 1c have respectively zero, one, and two
oxygen atoms in the aliphatic fragment. They were obtained
in 65%, 52%, and 48% yields, respectively. Starting from
a tertiary diammonium salt, polymer 1d was isolated in 20%
yield. We believe this behavior is due to the increased steric
hindrance on the nitrogen atom, which lowers the stability of
the Lewis pair, and thus of the polymer. The presence of
diborylaryl chromophores in the polymer main chain means
that the products are fluorescent white powders (see the
Supporting Information for 1a and 1b).[1] Compounds 1a and
1b both exhibit a glass transition temperature (Tg) of about
608C (638C and 608C, respectively, see Figures S43, S44, and
S52). These average values might be a consequence of rigidity
induced by the aromatic linker and flexibility by the aliphatic
one. However, after solvent removal, the crude 1a becomes
poorly soluble in THF. The extra oxygen atoms in 1b
significantly enhanced the solubility of the material at low
temperatures in THF. This behavior led us to develop
conditions for SEC characterization. We used THF with
1 wt% tetra n-butyl ammonium bromide (TBAB) as eluent.
TBAB is a salt that shields the chains from intermolecular
dipolar interactions, thus breaking up the aggregates and
enabling total elution of the polymers. The molar mass could
thus be estimated (> 105 gmolÀ1, based on polystyrene stand-
ards).
We first attempted to generate the desired borohydrides
in a separate flask from the bisboronic acid (or pinacol ester)
precursor and to mix the resulting crude product with the
diammonium salt. This procedure, however, led to only
minute amounts of polymers (reaction 1, Scheme 1). Various
side-reactions occurred, among which significant deboryla-
Scheme 1. Synthesis of the polyboramines. [a] Yield of isolated product
after precipitation and filtration.
The 11B NMR spectra of polymers 1a–d display a broad
À
singlet from d = À3 to around À12.0 ppm. The B H bonds
1
tion, which is likely due to the prolonged exposure of the
boron-containing molecules to the hydride, leading to a stoi-
chiometric imbalance that is detrimental to the polyconden-
sation. We therefore decided to trigger the reduction of the
diboronic acid precursor with LiAlH4 in THF in the presence
of the diammonium salt by a one-pot procedure (reactions 2–
3, Scheme 1). Indeed, we surmised that the direct deproto-
nation of the ammonium by the basic hydride (LiAlH4) would
take place first to generate an amine–alane in situ,[11] which
would lead to the ABs by reduction of the boronic acids. This
step also generates insoluble aluminum oxides. When using
this procedure, deborylation was largely avoided, but the
bisboronic pinacol ester precursors gave only short-chain
oligomers (reaction 2, Scheme 1).
were identified by H NMR (broad singlet at d = 2.36 ppm)
and IR spectroscopy (n(B–H) = 2313 cmÀ1, see pp. S19–23
and S46 in the Supporting Information).
We next investigated the thermal behavior of the boron-
containing materials, and particularly their dehydrogenation
profile. Polyboramines 1a and 1b can liberate H2 at 968C and
958C, which was evidenced by temperature-programmed
desorption (TPD) measurements, on an apparatus specifically
set up to evidence H2 desorption from materials. This result
was further supported by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC, Figure 2, and
pp. S35–S42 in the Supporting Information). These H2 release
temperatures are lower than that of any of the ABs reported
to date for H2 storage,[4] and is also much lower than the
dehydrogenation temperature of the isolated molecular
analogues (see pp. S44–S45 in the Supporting Information),
without catalysis.[12] The polyaminoboranes[8] generated after
dehydrogenation retain a partial solubility in THF. They
By switching to the para-phenylene-bisboronic acid as
bisborane precursor, we were able to generate longer polymer
chains (reaction 3, Scheme 1, Mn > 105 gmolÀ1, see Figure 2).
The polymer was isolated from the insoluble precipitated
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Figure 2. Dehydrogenation profile of polyboramine 1b with multiple characterization techniques: TGA, DSC, and TPD analyses (left), VTIR (top-
right corner), and 11B NMR and SEC analyses associated with H2 release (2 equivalents, bottom-right corner).
undergo a second dehydrogenation to form the fully dehy-
drogenated polyborimines at around 1608C. The latter
products are soluble in LiBr-containing DMF (0.5 wt%) at
608C to prevent significant aggregation. SEC analysis showed
that much higher molar masses than those of the starting
polyboramines were present (> 105 gmolÀ1; Figure 2). The
dehydrogenation appears to increase the molar masses rather
than induce chain scissions. This behavior is likely due to the
intermolecular interactions between the borimines.[13]
The DSC profiles of polyboramines 1a and 1b showed an
endothermic peak below 1008C. This phenomenon has been
attributed to dehydrogenation (as evidenced by comparison
of DSC, TGA, TPD, in situ variable-temperature IR data and
recrystallization studies of 1b, see Figure 2 (left and top-right)
and Figure S49) rather than melting as observed in 2a
(Figures S48, S50, S51). The H2-release profiles of the
polyboramines are endothermic (Figure S43–S44), and are
exothermic for the molecular analogues (Figure S45). This
feature will be discussed below.
genation of imines, ketones, and aldehydes. For this purpose,
we selected the most soluble polymer 1b (Table 1).
The polymer was first mixed with diphenylimine at 408C
in THF (Table 1, entry 1). This led to a conversion of 52%
into the desired amine. The selectivity (against tetrachloro-
ethane as internal standard) was 100%, thus suggesting that
no decomposition occurred.
By increasing the stoichiometry of 1b, the reduction
proceeded smoothly and quantitatively at room temperature
(Table 1, entry 2). Use of different N-alkyl imines resulted in
equally high yields; however, in these cases, the temperature
had to be slightly increased (408C, entries 5–8). In contrast,
the more activated N-phenylsulfonyl phenylimine reacted
very efficiently at lower stoichiometry and at room temper-
ature (entry 9). The polymer was also able to hydrogenate
benzaldehyde (entry 11) and acetophenone (entry 12) in
equally high yields at the lower polymer stoichiometry. As
expected, the ketone reacted less rapidly and required
heating, however, its use resulted in the direct formation of
alcohols as reduction products. We next carried out control
experiments. Almost no imine reduction took place with
borazane NH3·BH3 at 408C (9% conv., entry 4).[14] Benzalde-
hyde was consumed, but no free alcohol was observed, thus
suggesting the product remains bound to boron in this case
(entry 12). No reactivity difference was observed when the
The combined analyses show that the insertion of ABs
into polymeric chains completely changes their behavior and
generates properties that are distinct from those of their
molecular analogues.[12] In order to establish further the scope
of this reactivity change, we examined the transfer hydro-
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Table 1: Transfer hydrogenation from polyboramine 1b as H2 donor.
Entry[a]
X
R
T [8C]
Conv. [%][b]
Yield [%][b]
1
–NPh
–NPh
–NPh
–NPh
–NMe
–NMe
–NtBu
–NtBu
–NSO2Ph
O
H
H
H
H
H
H
H
H
H
Me
H
H
40
RT
40
40
40
40
40
40
RT
40
RT
RT
52
>99
79
9
77
52
>99
39
9
77
2[c]
3[d]
4[e]
5
6[c]
7
>99
65
>99
63
8[c]
9
>99
>99
>99
>99
80
>99
>99
>99
>99
0
10
11
12[e]
O
O
[a] Conditions: 1b (0.04 mmol/unit), imine (0.08 mmol), [D8]THF
(0.6 mL), 12 h; [b] determined by 1H NMR with C2H2Cl4 as internal
standard (4.7 mL, 0.045 mmol); [c] 1b (0.08 mmol/unit); [d] 2b was used
(0.04 mmol); [e] borazane was used (0.08 mmol).
reactions were performed in the presence of LiCl as additive
(see the Supporting Information). Thus, the residual salts in
polyboramines do not influence the reactivity.
With a molecular equivalent of the polymer (2b, see
Scheme 1), the reduction was faster than with borazane or 1b
(79% conversion; Table 1, entry 3), but it was accompanied
by a great deal of degradation. Finally, the polymer resulting
from the reduction was characterized by NMR spectroscopy,
which showed that only the first equivalent of H2 was
transferred to the substrates.[15]
Figure 3. Comparison of the energy profiles leading to H2 release.
Selected intermediates of the two lower energetic pathways (black and
blue) are represented. Only H atoms linked to N (blue) and B (red)
are shown for clarity.
barrier heights, these two pathways are thus unlikely. There-
À
The polyboramines are thus more than the sum of their
monomeric equivalents. The polymer chains seem to have
a specific effect that is especially marked for the thermal
dehydrogenation. To understand better the phenomena
linked to H2 release, we carried out theoretical modeling.
The mechanisms of the thermal dehydrogenation of AB
systems involve multiple pathways.[16–18] A selection of four
fore, the plausible pathway involves the cleavage of a N B
bond (TS-I; see the Supporting Information), thus leading to
key intermediate I (Figure 3), which contains both a three-
À À
À
center two–electron B H B bond and an NH N hydrogen
bond. From there one can envisage two pathways, which start
from conformers I and I’, evidenced by Dixon et al. as the two
reactive conformers that lead to dehydrogenation.[17] From I,
À
energetically favored pathways for [PhBH2 NH2(nPr)] was
TS-II features a six-membered transition state (DH =
thus computed by using DFT calculations (Figure 3). This
model fragment was chosen because it was closest to our real
system. We used the B97D3 functional, which has been
employed by Grimme et al. to compute frustrated Lewis
pairs.[19] to best depict the Lewis base/acid adducts.
30.2 kcalmolÀ1), which leads to H2 release and formation of
aminoborane II (see the Supporting Information). Intermedi-
ate II promptly dimerizes through TS-II-dim, leading to the
thermodynamic product II-dim. Alternatively, conformer I’
produces an ionic diammoniate of diborane ion-pair species
(DADB) through TS-II’ (DH = 34.8 kcalmolÀ1; see the Sup-
porting Information). From DADB, H2 release is subse-
quently achieved by TS-III’ (DH = 25.3 kcalmolÀ1; see the
Supporting Information).
Our starting point is the AB dimer dim involving
dihydrogen-bonding interactions (DHB) between the N H
and B H moieties (Figure 3), since it is lower in energy than
À
À
the monomeric form.[16–18]
An intramolecular reaction leads to molecular hydrogen
formation via TS-dim (DH = 40.6 kcalmolÀ1). Alternatively,
the loss of DHB interactions to form monomeric AB-mon will
lead to TS-uni (see the Supporting Information) for H2
release (DH = 50.6 kcalmolÀ1). Taking into account the
It is crucial to use a functional such as B97D3 that
properly takes in account dispersion contributions (D3BJ).[19]
This correction led us to obtain energetic barriers for the H2
release (DH = 30–34 kcalmolÀ1) which are consistent with the
experimental observations. DSC and TPD analyses of the
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molecular AB showed that the polymer has a higher propen-
sity to dehydrogenate compared to its molecular analogue
(see above). We believe that the polymer chains favor the
À
initial cleavage of the N B bond because of entropic effects.
Then, it is unlikely that ion pairing can occur in the polymer,
since this would require substantial, energetically unfavorable
chain rearrangements. Thus, it is likely that TS-II is substan-
tially lower in energy in the polymers. Conversely, a cooper-
ative ziplike behavior arising from the intermolecular poly-
mer chain entanglement, which preorganizes the AB pairs
might also be at play. In that case, direct hydrogen release via
a neutral intermediate would be again favored. This specific-
ity of the polymer chains is the reason for the lower
dehydrogenation temperature. Similarly, the final pseudodi-
merization leading to II-dim is likely to be strongly affected
by the polyboramine macromolecular nature (long chains,
entanglement), which conceivably would lower the associated
energetic gain. This would result in an overall endothermic
process, with II-dim higher in energy, in contrast to the
molecular case. The entropic factor for the polymer is also
likely to disfavor dimerization.[20]
To conclude, we have synthesized polyboramine polymers
by simple treatment of bisboronic building blocks and
bisammonium salts with LiAlH4. This procedure allowed us
to access Lewis pair assembled H2-storing polymers with
molar masses high enough for entanglement. The polymers
retained a solubility in polar organic solvents and are
amorphous (Tg ꢀ 608C), potentially enabling their process-
ability. They hydrogenate imines, aldehydes, and ketones
under mild conditions, and perform better than their molec-
ular analogues. Furthermore, the polyboramines displayed
unique thermal behavior. The thermal dehydrogenation
occurs below 1008C and is endothermic, in stark contrast to
the regular ABs used for this purpose. This is a direct
consequence of the incorporation of the ABs into polymeric
chains. We believe that this effect should be of interest for
other areas in boron, polymer, and dihydrogen-storage
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Acknowledgements
We thank the CNRS, CPE Lyon, UCBL, and ANR (Grant
NHCX) for funding. We acknowledge Olivier Boyron and
Manel Taam for their help on SEC and DSC characterization.
CCIR-ICBMS (UCBL) is gratefully acknowledged for the
allocation of computational resources, as is the UCBL for
a fellowship (A.L.).
Keywords: amines · borane · hydrogen release · Lewis pairs ·
polymerization
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15744–15749
Angew. Chem. 2015, 127, 15970–15975
[13] The polymers may crosslink, see some proposed structures in the
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Angewandte
Chemie
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Received: September 8, 2015
Revised: October 20, 2015
Published online: November 13, 2015
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