Dehydrocoupling of dimethylamine-borane catalysed by rhenium complexes and its application in olefin transfer-hydrogenations

Article abstract of DOI:10.1039/b708913a

Re(I) complexes are applied as catalysts for the dehydrocoupling of Me ₂NH·BH₃ and the transfer-hydrogenation of olefins. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708913a

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Dehydrocoupling of dimethylamine-borane catalysed by rhenium
complexes and its application in olefin transfer-hydrogenations{
Yanfeng Jiang and Heinz Berke*
Received (in Cambridge, UK) 13th June 2007, Accepted 17th July 2007
First published as an Advance Article on the web 2nd August 2007
DOI: 10.1039/b708913a
observed. After 4 h a 11B-NMR spectrum indicated that the
Re(I) complexes are applied as catalysts for the dehydro-
conversion to the cyclic aminoborane dimer [Me2N–BH2]2
(d 4.50 ppm, t, JBH 5 112 Hz) was almost complete with a 92%
yield. Significantly, besides the signal of a small amount of starting
material, another signal of low intensity is generated in the 11B-
NMR spectrum at 244.8 ppm which was identified as iPr3P?BH3.11
This result suggested that Me2NH?BH3 not only acts as a
hydrogen donor, but also as a phosphine abstractor. However,
from the NMR data it was not possible to judge whether one or
two phosphine ligands were abstracted during the reaction course.
Besides 1a, the analogous Re(I) complexes 1b, 2a,b, 3a,b are all
effective for dehydrocoupling of Me2NH?BH3 (Table 1). In all
cases trace amounts of the corresponding R3P?BH3 species were
detected. For comparison, the thermally induced dehydrogenation
of Me2NH?BH3 can be initiated only at temperatures above
130 uC.12 A blank reaction carried out at 85 uC for 6 h demon-
strated that Me2NH?BH3 was totally stable at this temperature. It
is worth mentioning at this point that in preliminary experiments
these rhenium complexes showed also activity for dehydrocoupling
of ammonia-borane NH3?BH3 revealing borazine [NH–BH]3 as
the major product.13
coupling of Me2NH?BH3 and the transfer-hydrogenation
of olefins.
Recently there has been increasing interest in the development of
transition-metal-catalysed reactions which allow the generation of
new bonds between main group elements.1 In particular, the
dehydrocoupling of amine-borane adducts catalysed by transition
metal complexes have become more and more important with
respect to hydrogen storage.2 Late transition metal compounds
including those with Rh, Pd, Ru, Ir and Ni centers, as well as such
with early transition metals, such as Ti, have been employed. For
example, the group of Ian Manners has reported the dehydro-
coupling of amine-boranes catalysed by Rh colloids3 and an
early transition metal Ti complex.4 Both, colloidal Rh and the
Ti complex, are even active in the tandem dehydrocoupling–
hydrogenation reactions.5 More active catalysts for hydrogen
release from ammonia–borane are an Ir-pincer6 complex and a
first row transition metal Ni-complex.7
Recent research of our group focuses on the development of
new catalysts for the dehydrogenation–hydrogenation of amine-
boranes. Considerable attention was paid to the non-noble middle
transition metal rhenium, which is known to often show ‘‘non-
catalytic’’ tendencies obeying the 18-electron rule and possessing a
certain reluctance to form coordinatively unsaturated species. This
however can in most cases be overcome using special ligand sphere
effects. So far, rhenium complexes can catalyse C–H activations8
and based on the fact that amine-boranes are isoelectronic with
alkanes, we indeed considered their application to the chemistry of
amine-boranes.
Based on the fact that one equivalent of H2 could be released
from Me2NH?BH3 using the Re(I) complexes as catalysts, we
proceeded to investigate Me2NH?BH3 as a hydrogen source
in olefin transfer-hydrogenations. When equal amounts of
Me2NH?BH3 and 1-octene were mixed in dioxane with 1 mol%
of the Re(I) species 1a, integration of the 1H-NMR signals
indicated a 93% conversion of 1-octene to the hydrogenated
Herein we report the first example of mono-nitrosyl Re(I)
complexes [ReBr2(NO)(PR3)2L] (L 5 H2 1, CH3CN 2 and
ethylene 3) to catalyse dehydrocoupling of dimethylamine-borane
(Me2NH?BH3) and its application in transfer-hydrogenation
reactions. The organometallic chemistry of mono-nitrosyl Re(I)
complexes bearing the phosphines PiPr3 (a) and PCy3 (b) and
dihydrogen, acetonitrile and ethylene as labile ligands was reported
by our group.9 These species can also serve as precursors for the
preparation of rhenium hydride complexes which are catalysts in
hydrogenation and hydrosilylation reactions.10 When a solution
of Me2NH?BH3 in dioxane was treated with a catalytic amount
(1 mol%) of the rhenium dihydrogen complex [ReBr2(NO)(PiPr3)2-
(g2-H2)] (1a) and stirred at 85 uC, evolution of hydrogen gas was
Table 1 Dehydrocoupling of Me2NHBH3 catalyzed by Re(l) at 85 uC
Entry Catalyst
L
R
Dehydro (%).a TOF (h21
)
1
2
3
4
5
6
7
a
none
1a
1b
2a
2b
none
H–H
none
iPr
Cy
0
92
96
99
91
88
91
0
23
24
25
23
22
23
H–H
CH3CN
CH3CN
iPr
Cy
CH2LCH2 iPr
CH2LCH2 Cy
3a
3b
Anorganisch-chemisches Institut, Universita¨t Zu¨rich, Winterthurerstr.
190, CH-8057 Zu¨rich, Switzerland. E-mail: hberke@aci.unizh.ch
{ Electronic supplementary information (ESI) available: experimental
details. See DOI: 10.1039/b708913a
On the basis of the integration of 11B-NMR spectrum (96 MHz) of
the reaction mixture.
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Chem. Commun., 2007, 3571–3573 | 3571

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product octane after 1 h at 85 uC. 11B-NMR provided evidence
that Me2NH?BH3 was completely consumed and that besides the
center could be abstracted by Me2NH?BH3 yielding the R3P?BH3
adducts, we wondered whether the PR3 group would indeed leave
the coordination sphere initiating the catalytic cycle. To investigate
the PR3 effect, the phosphine-free Re(I) complex [ReBr2(NO)-
(CH3CN)3] (4) was used as a catalyst in both reactions.
Dehydrocoupling of Me2NH?BH3 did indeed work in 96% yield
in absence of a phosphine after 4 h at 85 uC using 4 as the catalyst.
This revealed that the PR3 group is not a key player in the
Me2NH?BH3 dehydrogenation. In contrast, however, 4 turned out
to be incapable of catalysing the transfer-hydrogenation of olefins.
A hydrogenated product could not be observed at all and only
38% of the Me2NH?BH3 was dehydrogenated. This indicated that
the presence of rhenium bound phosphine(s) was quite critical in
the olefin activation process. However in contrast to this observa-
tion when an excess of the corresponding phosphine (10 mmol%)
was added, catalytic transfer-hydrogenations of 1, 2 and 3 became
completely blocked, neither a dehydrocoupling nor a hydro-
genated product could be detected after 4 h at 85 uC. This
indicated that transient dissociations of PR3 are essential to the
transfer-hydrogenation process.
i
major product [Me2N–BH2]2, again trace amounts of Pr3P?BH3
and also (Me2NH)2BH (d 28.00 ppm, d, JBH 5 137 Hz) were
present. In contrast to the colloidal rhodium5 catalysed dehydro-
coupling–hydrogenation tandem-reaction with free H2 as an
intermediate, the reaction catalysed by Re(I) is homogeneous
and the conversions remained quantitative regardless whether the
reaction was carried out in open or closed vessels. This suggested
that free H2 does not appear in the reaction course and that it is
indeed transfer-hydrogenation in its true sense instead of a
tandem-reaction. The other Re(I) complexes 1b, 2a,b, 3a,b were
all effective catalysts in such reactions (Table 2). Cyclic substrates
like 1,5-cyclooctene, COD and indene could also be hydrogenated
with Me2NH?BH3 as a hydrogen donor. In the case of substituted
olefins, like a-methylstyrene and 1-methylcyclohexene, the yields
were lower, presumably due to higher steric congestion inhibiting
the required close contacts with the Re center. In addition, NMR
evidence for hydroboration of the olefin was not found.
Subsequently some other details of the mechanisms of the
Re(I)-catalysed processes were studied. The key question was what
ligand dissociation (Br21, L or PR3) might get crucially involved
in the reaction pathway. 1) In the presence of 20 eq. of [NEt4]Br,
both dehydrocoupling and transfer-hydrogenation were unaff-
ected. This suggested that Br21 dissociation would not occur in
both cases of catalysis. 2) Considering the role of the labile ligand
L, it is interesting to note that the rhenium complexes of type 1, 2
and 3 are interchangeable by labile ligand substitution. For
example, in the presence of ethylene gas, the dihydrogen complex 1
and the acetonitrile complex 2 could easily be converted at room
temperature to the ethylene species 3. In the same way, the
acetonitrile complex 2 and the ethylene complex 3 could be
converted to the dihydogen complex 1 applying a H2 atmosphere.
3) Based on our previous observation that PR3 ligand on the Re(I)
A mechanism for the dehydrocoupling and the transfer-
hydrogenation was thus established and is sketched in Scheme 1.
In cycle B first a phosphine ligand is abstracted by BH3
generated by Me2NH?BH3 dissociation creating a vacant site for
occupancy by the olefinic substrate. Secondly, the labile ligand L is
substituted by Me2NH?BH3 leading to the B–H s-complex 9,
which subsequently oxidatively adds to form the Re(III) hydride
10. During this process the other phosphine ligand may also
dissociate. In a relatively fast step the coordinated olefin inserts
into a Re–H bond to generate a Re-alkyl species, thus preventing
H2 complex formation of type 8 of cycle A. The alkyl species is
then stabilized by re-coordination of free phosphine resulting in the
Table 2 Rhenium-catalyzed transfer-hydrogenation reaction of
olefins applying Me2NH?BH3 as a H2 sourcea
Time Dehydro Hydro
(%).c
Entry Substrate
Catalyst (h)
(%).b
1
2
3
4
5
6
7
8
9
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
cis-Cyclooctene
1,5-Cyclooctadiene
a-Methylstyrene
4-Methoxystyrene
Indene
none
1a
1b
2a
2b
3a
3b
1a
1a
1a
1a
1a
4
1
1
1
1
2
2
4
4
4
4
4
4
4
0
99
99
99
99
99
99
99
99
99
99
99
99
99
0
93
94
95
84
99
99
99
99
76
99
99
47
99
10
11
12
13
14
a
1-Methylcyclohexene 1a
4-Methylcyclohexene 1a
Reactions were performed using 0.01 mmol of catalyst, 1.0 mmol
of Me2NHBH3 and appropriated amount of alkene in dioxane
(0.5 mL). On the basis of the integration of 11B-NMR spectrum.
b
On the basis of the integration of 1H-NMR spectrum.
c
Scheme 1
3572 | Chem. Commun., 2007, 3571–3573
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six-coordinated complex 11. Then a b-H shift occurs presumably
accompanied by phosphine elimination and readdition giving the
dehydrogenated borazene and 12. Finally, the alkane is formed via
reductive-elimination, the generated vacant site is re-occupied by
an olefin molecule and Me2NH?BH3 leading to the s-complex 9
and closing of the catalytic cycle. In the case of the catalytic
dehydrocouping reaction (cycle A), the presence of three potential
vacant sites of 5 enable B–H coordination and subsequent
oxidative addition (conversion of 6 into 7). Proton transfer from
the acidic ammonium type function to the hydride ligand furnishes
the dihydrogen–borazene complex 8, from which H2 gas and
borazene are liberated. A reaction path involving primary N–H
acitivation cannot be excluded. N–H reactivity seems however less
preferred for thermodynamic and kinetic reasons, since N–H
bonds are generally stronger (bond dissociation energies: B–H
Notes and references
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83 kcal mol21, N–H 104 kcal mol21 14
and show less propensity
)
for H-coordination. For transfer hydrogenations cycle B is
expected to be faster than cycle A.
In conclusion, complexes of the middle-transition element
rhenium catalysed the dehydrocoupling of Me2NH?BH3 and the
transfer-hydrogenation of olefins. A plausible mechanism has been
put forward, which is based on the availability of three potential
vacant sites and a facile Re(I)/Re(III) redox change so that primary
interaction between the rhenium center, the amine-borane and the
olefins can take place. Essential for the generation of vacant sites is
the permanent presence of the two non-dissociating p-donor
bromide ligands acting as powerful cis-labilizers in the course of
the catalytic reactions. Extension of these studies will target
development of still more active rhenium complexes by ligand
sphere tuning and their applications in related amine-borane
chemistry for hydrogen storage.
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11 When equivalents of Me2NH?BH3 and free PiPr3 were mixed together in
dioxane and stirred at 85 uC for 1 h, the 11B-NMR spectrum indicated
that H3B?PiPr3 was formed (d 244.80 ppm).
12 A. B. Burg and C. L. Randolph, Jr, J. Am. Chem. Soc., 1949, 71, 3451.
13 In the case of 1a (1%), borazine is formed at 85 uC within 1 h in 75%
yield and at 45 uC borazine is formed in 65% yield within 24 h.
14 Y. Nakajima, H. Kameo and H. Suzuki, Angew. Chem., Int. Ed., 2006,
45, 950; J. M. L Martin, J. P. Francois and R. Gijbels, J. Chem. Phys.,
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Support from the Swiss National Science Foundation, the
European COST programme and the Funds of the University of
Zurich are gratefully acknowledged.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3571–3573 | 3573