Dehydrogenation of amine-boranes with a frustrated Lewis pair

Article abstract of DOI:10.1039/b925659h

Bulky tertiary phosphine/borane Lewis pairs P^tBu ₃/B(C₆F₅)₃ react with amine-boranes to afford dehydrocoupling products and phosphonium borohydride salts.

Full text of DOI:10.1039/b925659h

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Dehydrogenation of amine–boranes with a frustrated Lewis pairw
Alexander J. M. Miller* and John E. Bercaw
Received (in Berkeley, CA, USA) 7th December 2009, Accepted 16th January 2010
First published as an Advance Article on the web 2nd February 2010
DOI: 10.1039/b925659h
Bulky tertiary phosphine/borane Lewis pairs PtBu3/B(C6F5)3
react with amine–boranes to afford dehydrocoupling products
and phosphonium borohydride salts.
literature procedures in C6D5Cl,22 and no discernable reaction
was observed by NMR. The pre-formed FLP was added to a
C6D5Cl solution of Me2NHÁBH3 at 25 1C, giving a clear
colorless solution.30 1H, 31P (Fig. 1), 19F, and 11B (Fig. 2) NMR
experiments confirmed that 495% of the FLP-derived pro-
duct was [tBu3PH][HB(C6F5)3].22 The major dehydrocoupling
product was dimeric (Me2NBH2)2, assigned by a diagnostic
11B NMR resonance at d 5.2 (t, JBH = 112 Hz), and by signals
in the 1H NMR spectrum at d 2.28 (s, Me2N) and d 2.83
(1 : 1 : 1 : 1 q, JBH = 112 Hz, BH2). Minor side products,
Amine–boranes are the subject of intense interest due to their
potential application in hydrogen storage schemes.1,2
A
number of fast and efficient transition metal dehydrocoupling
catalysts have been discovered,3–7 and various coordination
compounds relevant to the catalytic pathways have been
isolated.8–10 Progress has also been made in the challenging
area of regeneration of spent ammonia–borane fuel.11–13
Theoretical studies14,15 of one amine–borane dehydrocoupling
system employing N-heterocyclic carbenes (NHCs) as ligands3
suggested that the free NHC could heterolytically dehydrogenate
NH3ÁBH3, yielding (NH2BH2)n and NHC–H2, similar to the
H2 cleavage reactivity of Bertrand’s carbenes.16 Because the
reactivity of these NHC species has been compared to that of
bulky phosphine/borane ‘‘frustrated Lewis pairs’’ (FLPs), it
occurred to us that FLPs might also be able to dehydrogenate
amine–boranes.
including monomeric Me2NQBH2 (11B d 37.6, t, JHB
=
127 Hz) and HB(NMe2)2 (11B d 28.5, d, JHB = 124 Hz),
dissipated over time, leaving B97% dimeric (Me2NBH2)2 along
with traces of (BH2)2NMe2(m-H) (A) and H3BÁNMe2BH2Á
NHMe2 (B).31 This product distribution is similar to that observed
in metal-catalyzed dehydrocoupling of Me2NHÁBH3;32,33 thermo-
lysis of a Me2NHÁBH3 melt at 150 1C also affords (Me2NBH2)2,
along with trace impurities including HB(NMe2)2.33,34
The order of reagent addition is important in the dehydro-
coupling reaction. If B(C6F5)3 and Me2NHÁBH3 are dissolved
FLPs—combinations of particularly bulky Lewis acids and
bases that are unable to form traditional Lewis acid/base
pairs17—exhibit a number of unusual reactions,18–20 in particular
the heterolytic cleavage of H2,21–23 which has been utilized for
metal-free catalytic hydrogenation of bulky imines.24,25 Amine
activation reactions have also been reported.26,27 To our
knowledge, however, no dehydrogenation reactions have been
reported using these systems,28 although calculations suggest
such reactions should be possible.29 Herein we report that the
simple frustrated Lewis pair PtBu3/B(C6F5)3 rapidly and cleanly
dehydrogenates amine–boranes at room temperature, with con-
comitant formation of the phosphonium borohydride salt
[tBu3PH][HB(C6F5)3] (Scheme 1).
Initial experiments were carried out with Me2NHÁBH3, a
common model for NH3ÁBH3 which has desirable solubility
properties and generally releases only 1 equivalent of H2. The
frustrated Lewis pair PtBu3/B(C6F5)3 was formed according to
Fig. 1 1H NMR spectrum after FLP-mediated dehydrocoupling of
Me2NHÁBH3. Left inset is a blow-up of the borohydride 1H NMR
resonances; right inset is the 31P NMR spectrum.
Scheme 1
Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, CA, USA.
E-mail: ajmiller@caltech.edu; Fax: +1 626 585 0147;
Tel: +1 626 395 6576
w Electronic supplementary information (ESI) available: Full
experimental details and characterization data, with accompanying
NMR spectra. See DOI: 10.1039/b925659h
Fig. 2 11B NMR spectrum shortly after FLP-mediated dehydrocoupling
of Me2NHÁBH3. The underlying broad feature arises from borosilicate
glass in the probe construction.
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in C6D5Cl a few minutes prior to addition of PtBu3, several
products including only B50% (Me2NBH2)2 are produced. On
the other hand, combining PtBu3 and Me2NHÁBH3 in C6D5Cl,
followed by addition of B(C6F5)3 led to near-quantitative
formation of [tBu3PH][HB(C6F5)3] and (Me2NBH2)2.35
To explore the possibility of H2 release and amine–borane
regeneration, the mixture of [tBu3PH][HB(C6F5)3] and
(Me2NBH2)2 was heated between 90 and 130 1C. Complete
decomposition of the dimer to a variety of species was
observed; only BH3ÁPtBu3 and [HB(C6F5)3]À were readily
identified by 11B NMR,36 leaving this experiment inconclusive.
In isolation, (Me2NBH2)2 is stable up to 450 1C in the gas
phase33 and 130 1C in a melt.34 Similarly, it has been reported
that [tBu3PH][HB(C6F5)3] is stable to H2 loss up to 150 1C.22
The FLP system can dehydrogenate NH3ÁBH3 as well
(Scheme 1). Treatment of NH3ÁBH3 with PtBu3/B(C6F5)3
afforded once again [tBu3PH][HB(C6F5)3] as a principal
product (85%, 31P NMR; B80%, 19F NMR). The major
dehydrocoupling product is consistent with branched-chain
polyaminoborane ((NH2BH2)n, broad 11B NMR, d À7.7,
À14.4, À27.2) as observed upon thermolysis of NH3ÁBH3 in
ionic liquids.37,38 The other 11B signals correspond to
[HB(C6F5)3]À and two smaller peaks which could be Lewis
adducts between PtBu3 and boranes; two broad resonances in
the 31P NMR spectrum are consistent with this formulation.
Apparently dehydrocoupling is more favorable than forming
stable Lewis pairs, although in this case some of that competing
pathway is observed.39 Some insoluble colorless material was
extracted with pyridine and shown to be unreacted NH3ÁBH3,
consistent with some amount of deactivation of the FLP
before quantitative dehydrocoupling could take place.
Neither the insoluble cyclic pentamer (NH2BH2)54,10 nor long
linear polymers40 were detected, in contrast to a number of
metal-catalyzed reactions. Treatment of NH3ÁBH3 with
excess FLP did not lead to detectable amounts of borazine,
suggesting that only 1 equiv. of H2 could be released using this
method.
Scheme 2 Proposed mechanism for FLP-mediated dehydrocoupling.
We have shown that frustrated Lewis pairs consisting of
bulky tertiary phosphines and B(C6F5)3 are capable of rapidly
dehydrocoupling Me2NHÁBH3 and NH3ÁBH3. While the
current FLP systems could serve as H2 storage compounds
themselves,21 their weight % capacity is far from ideal. Using
FLPs as a H2 shuttle with lighter, non-frustrated amine–
boranes as the terminal H2 storage medium is perhaps more
attractive. Recent advances in H2 release from FLPs23,45 may
open the door to catalytic dehydrocoupling, and indeed
perhaps dehydrogenation of a wider variety of substrates.
The authors gratefully acknowledge BP (MC2 program) and
the Moore Foundation for funding. Dr Jay Labinger provided
insightful discussion.
Notes and references
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B(C6F5)3 by itself can catalytically dehydrogenate NH3ÁBH3,41
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product are observed under the present conditions without
PtBu3. Importantly, B(C6F5)3 by itself is unable to dehydro-
couple Me2NHÁBH3,33 leading us to conclude that the FLP is
essential for rapid, ambient-condition dehydrocoupling.
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not been fully elucidated, although a number of theoretical
studies have been performed.18,42–44 DFT calculations suggest
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proximity; a molecule of H2 approaches the pair and is cleaved
in a concerted fashion. Since the heterolytic dehydrogenation
of amine–boranes constitutes a net transfer of H2 and gives a
similar product, one could invoke a similar mechanism. We
prefer an alternative, stepwise mechanism in which B(C6F5)3
first abstracts a hydride from Me2NHÁBH3, followed by fast
deprotonation of the resulting [Me2NHBH2]+, affording the
Me2NQBH2 unit that dimerizes to the final product
(Scheme 2). In the absence of PtBu3, oligomerization of the
cationic intermediate might take place (as suggested by Baker
for the reaction of acid with NH3ÁBH341) accounting for the
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boranes. FLPs have been noted to have unusual colors in some
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phosphine or borane cannot be ruled out.
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1
absence of H NMR resonances consistent with Me2NHÁBH3, and
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