Unprecedented reactivity of an aluminium hydride complex with ArNH 2BH3: Nucleophilic substitution versus deprotonation

Article abstract of DOI:10.1039/c1cc14689k

Reaction of DIPPnacnacAlH₂ with DIPPNH₂BH₃ did not give the anticipated deprotonation but nucleophilic substitution at B was observed instead. The product DIPPnacnacAl(BH₄)₂ was isolated and structura

Full text of DOI:10.1039/c1cc14689k

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COMMUNICATION
Unprecedented reactivity of an aluminium hydride complex with
ArNH2BH3: nucleophilic substitution versus deprotonationw
Sjoerd Harder*a and Jan Spielmannb
Received 29th July 2011, Accepted 21st September 2011
DOI: 10.1039/c1cc14689k
Reaction of DIPPnacnacAlH2 with DIPPNH2BH3 did not give
the anticipated deprotonation but nucleophilic substitution at B
was observed instead. The product DIPPnacnacAl(BH4)2 was
isolated and structurally characterized. Nucleophilic displace-
ment at B might play a role in mechanistic pathways related to
metal amidoborane complexes.
Early main group metal amidoboranes have been identified as
advantageous hydrogen storage materials that, in contrast
to ammonia-borane, release hydrogen at significantly lower
temperatures without foaming and borazine contamination.1–3
In addition, the near thermoneutrality of hydrogen release is
promising in the search for possible reversibility.
Scheme 1 Thermal decomposition of b-diketiminate metal amido-
borane model compounds.
Although this work is very much the domain of solid-state
chemists we recently demonstrated that molecular chemists
can contribute to this field by using molecular model systems
that allow for isolation of intermediates.4–9 The latter are
based on complexes that contain the bulky, strongly bidentate
coordinating, b-diketiminate ligand DIPPnacnac; DIPPnacnac =
CH{(CMe)(2,6-iPr2C6H3N)}2. These complexes are soluble
in most organic solvents and form a platform for solution
NMR studies and single crystal structure investigations. Thus,
we revealed the identity of new species that are useful in
mechanistic considerations for the hydrogen release process
in solid metal amidoborane materials10,11 (Scheme 1).
From previous work on the use of AlH3/NH3BH3 mixtures
for the synthesis of ceramic Al/B/N materials, it is suspected
that aluminium amidoboranes could be quite unstable towards
H2 loss.12 Likewise, Hill’s recent investigations on catalytic
dehydrocoupling of HNMe2BH3 by group 3 metal compounds
like Y[(N(SiMe3)2]3 and Sc[N(SiHMe2)2]3 underscore the
high instability of amidoborane complexes with significantly
Lewis-acidic metals like Y3+ and Sc3+ 13
Rapid b-H elimi-
.
nation and formation of highly reactive Me2N Q BH2 led to
formation of the anion: Me2N–BH2–NMe2–BH3À (Scheme 2).
A similar species was also found in Mg-catalyzed reactions.14,15
This is proposedly an intermediate on the way to our earlier
Part of our investigation deals with the influence and role
of the metal. Hitherto we have not been able to isolate a
b-diketiminate zinc amidoborane complex: all attempts led
to formation of a zinc hydride complex (Scheme 1) which
is presumably formed by the rapid decomposition of a
zinc amidoborane intermediate through b-H elimination.7 In
the follow-up work we now address the possibility to prepare
a b-diketiminate aluminium amidoborane complex. Such
a complex could provide information on the stability and
reactivity of amidoborane complexes with a highly Lewis-
acidic metal center.
2À
reported RN–BH–NH–BH3 Àanion (Scheme 1).4 As the
anion Me2N–BH2–NMe2–BH3 lacks protic N–H units for
further dehydrogenation it could be trapped as a product.
Recent publications by Wright et al. corroborate the high
instability of Al amidoborane complexes: reaction of
Al(NMe2)3 with two equivalents of HNMe2BH3 gave the
complex [H2B(NMe2)2]2AlH.16 Further evidence for instability
a Stratingh Institute, University of Groningen, Nijenborgh 4, 9747AG,
Groningen, The Netherlands. E-mail: s.harder@rug.nl;
Fax: +31 50 3634296; Tel: +31 50 3634322
b Fachbereich Chemie, Universitatsstraße 5, 45117 Essen, Germany
¨
w Electronic supplementary information (ESI) available: Details of
syntheses and crystallographic work. CCDC 837323. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc14689k
Scheme 2 Tentative mechanism for the catalytic dehydrocoupling of
HNR2BH3.
c
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of amidoboranes with Lewis-acidic metals like Y3+ is reported
by Grochala et al.: under ambient conditions Y(NH2BH3)3/
(LiCl)3 spontaneously decomposes within days.17
The bidentate chelation of these borate ligands is typical
for Al and also found in simple aluminium borates like
Al(BH4)3 and Me2Al(BH4).23 The average AlÁ Á ÁB distance
in DIPPnacnacAl(BH4)2 of 2.219(5) A is in between those
of the latter two aluminium borate complexes (2.12(2) A and
2.411(6) A, respectively).
Despite indications for the high instability of Al amido-
borane complexes, LiAl(NH2BH3)4 is exemplarily described
in a recent patent.18 We investigated the possibility to obtain a
b-diketiminate aluminium amidoborane complex. Steric bulky
From the catalytic conversion, 2DIPPNH2BH3
-
ligands could prevent B–H agostic interactions and block the
19
HB[N(H)DIPP]2 + BH3 + 2H2, with the Mg catalyst
DIPPnacnacMgN(SiMe3)2 we isolated the borate complex
[DIPPnacnacMg(BH4)]2 as the residual metal species.5 However,
a similar reaction scheme cannot be applied to the here reported
formation of DIPPnacnacÀAl(BH4)2. 11B NMR spectra only
show signals for the BH4 ion. Monitoring the reaction by
1H NMR shows no sign of H2 evolution but rapid formation
of DIPPNH2. We therefore propose a mechanism in which
DIPPNH2BH3 precoordinates to the Lewis-acidic Al3+ center,
thus activating the sp3-hybridized B for nucleophilic substitution
(Scheme 3).
b-H elimination pathway. As [DIPPnacnacMgH]2
was
found to be an excellent precursor for the syntheses of a series
of Mg amidoborane complexes in near quantitative yields,8,20
we pursued this deprotonation route with the highly potent
reagent DIPPnacnacAlH2.21 Stoichiometric reaction with
several ammonia-boranes like NH3BH3, MeNH2BH3 and
iPrNH2BH3 proceeded with gas evolution but led to compli-
cated reaction mixtures from which no defined products could
be isolated. In contrast, the reaction with DIPPNH2BH3
proceeded smoothly already at room temperature but no gas
evolution was visible (Scheme 3). Monitoring the reaction with
11B NMR showed that the consumption of DIPPNH2BH3
(broad signal at À15.1 ppm) is complete within one hour. The
clean formation of a product with a characteristic quintet at
À36.6 ppm (1JBH = 85.3 Hz) is indicative of formation of the
Although there are only few reports on systematic studies of
nucleophilic displacement reactions at tetracoordinated B, its
isolobal relationship with C atoms in methane-type compounds
suggests that similar principles could apply. Indeed, experi-
mental as well as theoretical evidence confirm the existence of
SN1-B and SN2-B reactions.24–26 The identity reaction, NH3 +
BH3NH3 - NH3BH3 + NH3, preferably proceeds via a
second-order SN2 mechanism.25 Although activation energies
for SN2-B are substantially higher than for similar SN2 reactions
on C,26 the calculated value of 13.8 kcal molÀ1 (MP3/6-31G*)25
is low enough to consider this reaction in mechanistic investi-
gations. As the activation energy for SN1-B type reactions equals
À
BH4 ion. Crystallization from cold hexane and subsequent
analysis by X-ray diffraction revealed the complex
DIPPnacnacAl(BH4)2 (Fig. 1).
The crystal structure shows a monomeric complex in which
the Al center is chelated by the bulky DIPPnacnac ligand (with
Al–N distances similar toÀthose in DIPPnacnacAlH2)21,22
and binds to two k2-BH4 ions, one nearly in the plane
of the DIPPnacnac ligand and the other outside this plane.
the B–N bond dissociation energy (exp. 31.1 kcal molÀ1 27
33.1 kcal molÀ1 25
MP3/6-31G*) the SN2-B mechanism is likely
,
calc.
,
favoured. It should be noted, however, that temperature and
substituents on N and/or B influence the energy difference
between SN2-B and SN1-B to such an extent that both can be
operative simultaneously.24,26
Nucleophilic displacement is a common reaction for masked
boranes, like BH3(THF) or BH3(Me2S): AlH3 reacts with
BH3(THF) to a range of AlH3Àn(BH4)n species.28 However,
ammonia-boranes with protic N–H units generally react with
Al hydrides by proton abstraction.12,16 Deprotonation of the
amine is highly favoured on account of the acidifying effect of
BH3 coordination. Whereas the more ionic Mg and Ca hydride
complexes rapidly deprotonate any ammonia-borane,4,8 the
basicity of DIPPnacnacAlH2 is likely tempered by the high
Lewis-acidity of Al3+. The same Lewis-acidity determines the
course of the reaction by activation of DIPPNH2BH3 for
nucleophilic substitution (Scheme 3).
Scheme 3 Reaction of DIPPnacnacAlH2 with DIPPNH2BH3.
It is not clear whether the observed hydride-amine replace-
ment is a special case for aryl-substituted ammonia-boranes.
On account of the high pKb values for aryl amines (pKb aniline
9.40) the B–N bond should be weaker than that in NH3BH3
(pKb NH3 4.77). The low calculated activation energy for NH3
substitution in ammonia-boranes,25 however, suggests that
nucleophilic displacement could play a role in reactions of
metal species with any ammonia-borane (it is suspected that
Lewis-acid activation, as sketched in Scheme 3, even lowers
this transition state). It should be noticed that thermal decom-
position of NH3BH3 proceeds through a key intermediate
Fig. 1 Crystal structure of DIPPnacnacAl(BH4)2. Selected bond
distances (A): Al–N1 1.898(3), Al–N2 1.898(3), AlÁ Á ÁB1 2.207(4),
AlÁ Á ÁB2 2.231(6), Al–H1 1.78(4), Al–H2 1.74(5), Al–H5 1.76(4),
Al–H6 1.81(5).
c
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DIPPNH2BH3 and lower basicity of the Al–H compared to
early main group metal hydride compounds. Substitution
at B is a pathway that should be considered in mechanistic
evaluations.
Notes and references
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Scheme 4 Alternative catalytic scheme for dehydrocoupling of
HNR2BH3 (based on nucleophilic substitution).
[H2B(NH3)2+][BH4À] (DADB) which is proposed to be
formed by substitution reactions on B centers.29
In the light of these results, alternative mechanisms could
be operative in catalytic ammonia-borane dehydrogenation.
E.g. the recently reported dehydrogenation, 2HNMe2BH3 -
(H2BNMe2)2 + 2H2, by Al(NMe2)3 alternatively could proceed
through the coupled cycles shown in the general Scheme 4. The
difference with the earlier proposed mechanism (Scheme 2)14–16 is
that the [M]-NR2BH3 is not formed by deprotonation but by
substitution. Reaction of the leaving group HNR2 with [M]-H
would regenerate the metal amide catalyst [M]-NR2. Such species
could be formed in the reaction from any nucleophilic metal
complex with ammonia-borane. The importance of nucleophilic
substitution is naturally dependent on the nature of the sub-
stituents R and the Lewis-acidity of the metal but could be an
alternative to be taken into account in a discussion of reaction
mechanisms.
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As aluminium amidoborane complexes are apparently
not formed by attempted deprotonation of DIPPNH2BH3,
we also investigated the salt metathesis route. Reaction of
DIPPnacnacAlCl2 with two equivalents of KNR(H)BH3 (R =
iPr or DIPP) in THF gave in good yields DIPPnacnacAlH2.
This is likely formed by double b-hydrogen elimination in the
expected product. This not only demonstrates the instability of
aluminium amidoborane complexes but especially underscores
the usefulness of amidoboranes in the preparation of metal
hydride complexes (see ESIw for this alternative synthetic
route to DIPPnacnacAlH2).
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reaction with DIPPnacnacAlH2 proceeds not through a
deprotonation protocol but nucleophilic substitution at B is
observed instead. This reaction pathway is likely favoured on
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11945–11947 11947