Ruthenium-catalyzed dimethylamineborane dehydrogenation: Stepwise metal-centered dehydrocyclization

Article abstract of DOI:10.1002/chem.200901372

Amine-borane dehydrogenation: Ruthenium PNP amido pincer complex I catalyzes the dehydrocoupling of dimethylamineborane to cyclic dimer (Me ₂NBH₂)₂. The results are in agreement with a mechanism including initial alternati

Full text of DOI:10.1002/chem.200901372

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DOI: 10.1002/chem.200901372
Ruthenium-Catalyzed Dimethylamineborane Dehydrogenation: Stepwise
Metal-Centered Dehydrocyclization
[
a]
Anja Friedrich, Markus Drees, and Sven Schneider*
Ammonia–borane (H NBH , A), has recently attracted
netic isotope effects would be in agreement with concerted
3
3
considerable interest as material for chemical hydrogen stor-
hydrogen transfer, but the small rate of H loss from amino
2
[1]
H
H
age, owing to its high hydrogen content (19.6 w%). Fur-
thermore, amine–boranes have been utilized as hydrogen
complex
trans-[Ru(H)
A
H
U
G
R
N
U
G
ACHTUNGERTNNUNG( PNP )]
(2,
PNP =
2
3
HN(CH CH PiPr ) ) suggest a different mechanism, at least
2
2
[4m]
2 2
[2]
source in hydrogenation reactions, and polymeric products
of amine–borane dehydrocoupling provide access to novel
for this step.
In this contribution we present our results
on B dehydrocoupling with catalysts 1 and 2, which support
metal-centered BÀN coupling as a viable step for this
[3]
inorganic materials. Since thermal dehydrogenation exhib-
its high kinetic barriers, in the last years transition-metal
system.
[4]
catalysts have been developed for this reaction. Most re-
cently, ruthenium–amido catalysts were introduced by
Fagnou and co-workers and our group, which currently ex-
hibit the highest reported activities in the dehydrocoupling
Addition of 1 to a solution of B in THF (0.54m) at room
[7]
temperature results in rapid H evolution [Eq. (1)]. How-
2
ever, volumetric monitoring reveals, that the reaction con-
siderably slows down prior to complete conversion
(Figure 1). In contrast to parent A, for which rapid dehydro-
[4l,m]
of A.
Mechanistic information for metal catalyzed amine–
borane (R HNBH ) dehydrogenation remains scarce. All
2
3
computational studies have suggested initial proton and hy-
dride transfer from the same substrate molecule to the cata-
[
5]
lyst, resulting in the release of aminoborane R NBH ,
2
2
[6]
which undergoes rapid oligomerization.
In fact, for
[
TiCp ]-mediated dehydrocoupling of Me HNBH (B) to
2
2
3
aminoborane dimer (Me NBH ) (C) a mechanism, with
2
2 2
stepwise, metal-centered BÀN coupling via diborazane
[5a]
Me HNBH NMe BH (D) was dismissed.
Likewise, for
2
2
2
3
Ru–amide catalysts a mechanism was proposed with release
+
À
of free aminoborane by concerted H /H transfer to the
RuÀN moiety and rate-determining H elimination from the
2
[4l]
resulting Ru(H)ÀN(H) hydrido amine species.
To our
knowledge, only for an iridium catalyst has metal-centered
BÀN coupling been confirmed by aminoborane trapping ex-
Figure 1. Volumetric monitoring of B dehydrocoupling with catalysts 1
and 2.
[4j]
periments with olefin hydroboration.
For A dehydrogenation with Ru–amido pincer complex
Ru(H)(PMe )(PNP)] (1, PNP=N(CH CH PiPr ) ), H/D ki-
[
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
3
2
2
2 2
genation and turnover numbers (TONs) of up to 8300 were
[
4m]
[
a] Dipl.-Chem. A. Friedrich, Dr. M. Drees, Dr. S. Schneider
Department Chemie
Technische Universitꢀt Mꢁnchen
Lichtenbergstr. 4, 85748 Garching (Germany)
Fax : (+49)89-289-13473
E-mail: sven.schneider@ch.tum.de
observed with catalyst 1,
for B only up to 70% H are
2
quickly released with catalyst loadings of 2 mol%. However,
slow completion of the reaction is observed over 28 h by
B NMR spectroscopy (2 mol% 1) ultimately giving quanti-
1
1
tative conversion to C. Therefore, the high initial rates
À1
(
turnover frequency (TOF)ꢀ3600 h ) contrast with a TOF
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901372.
À1
of 1.5 h after slowdown, suggesting two catalytic regimes
Chem. Eur. J. 2009, 15, 10339 – 10342
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10339

owing to catalyst deactivation in the course of the reaction.
1
1
Monitoring the reaction by B NMR spectroscopy (Figure 2
and Supporting Information) confirms almost quantitative
Scheme 1. Thermochemistry of B dehydrocoupling with free enthalpies in
À1
kcalmol
.
D if kinetically feasible. However, a solution of B and C in
THF without catalyst 1 displayed no reaction over one week
at room temperature. As the monomer–dimer equilibrium
1
1
Figure 2. B NMR spectrum of catalytic B (0.54m in THF) dehydrocou-
pling with 1 (2 mol%) after 27 min.
[8]
of C and F is known to be rapid, these results suggest, that
[16]
the uncatalyzed Path B exhibits a high kinetic barrier, and
D must be formed by metal-centered dehydrocoupling. To
account for intermediate aminoborane F the reaction was
formation of dehydrocoupling product (Me NBH ) (C, d=
.4 ppm) and trace amounts of (Me N) BH (E, d=
2 2
2
2 2
4
27.9 ppm). The concentration of intermediate monomeric
performed at different substrate loadings [B] . Under identi-
0
aminoborane Me NBH (F, d=36.8 ppm) quickly drops
cal conditions, [B] has a strong effect on the relative con-
2
2
8]
0
[
owing to dimerization. Furthermore, an intermediate at
centrations of intermediates F and D, with higher [F]/[D]
ratios at lower substrate loadings (e.g., after 3 min, 2 mol%
1, [B] =0.54m: [F]/[D]=0.44; [B] =0.1m: [F]/[D]=4.03).
1
d=1.2 ppm (t,
J ACHTUNGTRENNUNG( B,H)=109 Hz) is observed. This signal
was previously tentatively assigned to trimer (Me NBH ) in
2
2
3
0
0
+
dehydrocoupling of B with [TiCp ], [Rh
A
H
U
G
E
N
N
(PiBu ) ] , and a
This result suggests that dehydrogenation of one B and re-
lease of F versus metal-centered dehydrocoupling of two B
towards D, which should be favored at higher B concentra-
tions, are two parallel pathways. Furthermore, F was ob-
served as an intermediate in catalytic dehydrogenation of di-
2
3 2
I
[4c,e,i,9]
Cu –carbene complex.
(
However, this peak correlates
1
1:1 integral ratio) with a signal at d=À14.0 ppm (q, J-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(B,H)=94 Hz) superimposed with substrate
B
(d=
1
À14.4 ppm, q, J
ACHTUNGTRENNUNG( B,H)=98 Hz). From comparison with an
original sample, the signals at d=1.2 and À14.0 ppm can be
borazane D ([D] =0.54m, 2 mol% 1; Path C). Accordingly,
0
[
4b,10,11]
assigned to diborazane D and not to (Me NBH ) .
for the hypothetical linear dehydrogenation product of D,
2
2 3
Similarly, Manners and co-workers reported, that
Me NH BMe NBH , no minimum was found in calculations
2
2
2
2
[12]
Ph HPBH3 dehydrogenation gives Ph HPBH Ph PBH ,
(MP2/6-31G* or G3MP2). Instead geometry optimizations
starting from an anti conformation of the N-B-N-B chain re-
sulted in BÀN bond scission towards 2F, while the gauche
2
2
2
2
3
and D was previously proposed as an intermediate in rhodi-
[4b,13,14]
um-catalyzed and thermal dehydrocoupling of B.
conformation cyclized to dimer C. A recent computational
study suggested the formation of NH BH from dehydro-
2
2
[17]
genation of parent H NH BH NBH , as well.
3
2
2
3
As for the dehydrogenation of A, 2 is the only ruthenium
3
1
species initially detected by P NMR spectroscopy. Further-
more, dehydrocoupling of B catalyzed by 2 (0.1 mol%),
gives virtually the same results as compared with
1
Two pathways seem viable for the formation of D: Metal-
centered dehydrogenative BÀN coupling of B (Scheme 1,
(Figure 1), suggesting that 2 represents the catalyst resting
state in the rapid dehydrogenation regime. However, a new
31
Path A), or catalytic dehydrogenation and release of F fol-
lowed by uncatalyzed rearrangement of B and F (Path B).
Thermodynamic calculations (G3MP2) showed that dehy-
drocoupling of 2B to C is strongly exergonic (DG=
ruthenium species quickly evolves, as observed by P and
1
1
B NMR spectroscopy, which could be isolated in 70%
yield upon reaction of 1 with 2.3 equivalents of B [Eq. (2)].
BH2NMe2
This compound was assigned to [Ru(H)
A
H
U
G
R
N
U
G
)]
3
À1
À27.4 kcalmol ). Dehydrocoupling to D is thermodynami-
(3a) with a rare M-N-B-H four-membered borametallacy-
[
18]
cally slightly more favorable compared with dehydrogena-
cle.
The hydrogen atoms bound to boron could be as-
À1
11
1
tion to aminoborane F (DDG=À2.3 kcalmol ) and dimeri-
signed by B– H HETCOR NMR spectroscopy and feature
a strongly reduced coupling constant for the bridging ( J-
(H,B)=50 Hz) compared with the terminal hydride ( J-
À1 [15]
1
zation of 2F to C is exergonic by DG=À6.6 kcalmol .
1
Therefore, rearrangement of 2B and cyclic dimer C towards
A
H
U
G
R
N
U
G
linear dimer D would be almost thermoneutral (DG=
ACHTUNGTRENNUN(G H,B)=113 Hz), as derived by selectively proton decoupled
B NMR spectra. Furthermore, the terminal hydride bound
À1
11
2.0 kcalmol ), and B and C should be in equilibrium with
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Chem. Eur. J. 2009, 15, 10339 – 10342

Dehydrogenation
COMMUNICATION
to ruthenium is considerably shifted to higher field com-
drogenation of B, coupling to D competes with release of F,
which cyclizes towards C. Catalytic dehydrogenation of D
results in BÀN bond scission and ultimately cyclization of F,
pared with 2 (Dd([D ]benzene)=4.9 ppm), indicating the re-
6
duced trans-influence of the bridging hydride. At higher
temperatures, both the PNP phosphorous atoms and the BÀ
H hydrides coalesce. However, the terminal RuÀH hydride
as well. Alternatively, direct metal-centered dehydrocycliza-
tion of D towards C is likely to compete with the latter
pathway. Unfortunately, quantitative kinetic analysis to
answer this question was hampered by underlying catalyst
deactivation. Hence, our conclusions are:
is not broadened, suggesting exchange of BÀH
and BÀ
bridge
H
terminal
by borane rotation around the BÀN
bond without
PNP
BÀN
rupture. Activation parameters for this process were
PNP
31
°
estimated by
P NMR line shape analysis (DH =
À1
°
À1 À1
7
0.9 kJmol ; DS =À2.6 kJmol K ). Furthermore, the re-
1) All dehydrogenation and BÀN coupling steps, except for
aminoborane cyclodimerization, are metal-centered.
2) Head-to-tail dehydrocoupling to linear dimer D is a
viable pathway. This had been proposed earlier for ther-
action of 1 with A (3.2 equiv) gives parent complex [Ru(H)-
BH2NH2
31
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PMe )(PNP
)] (3b) in high yield, as observed by P,
3
1
11
H, and B NMR spectroscopy. Analytically pure 3a was
tested for dehydrogenation of B. Quantitative dehydrocou-
mal and Rh-catalyzed dehydrogenation and is most
+
pling to C is observed (2 mol% 3a), with catalytic activities
likely operative with [TiCp ], [Rh
A
H
U
G
R
N
N
(PiBu ) ] , and Cu–car-
2
3 2
À1
(
TOF 1.5 h ) as found for the slow dehydrogenation regime
bene catalysts, but was dismissed in a computational
[5a]
with precatalyst 1. Likewise, 3a slowly dehydrogenates A.
No conversion of 3a to 3b by exchange of Me NBH versus
study. If this result can be generalized to other amine–
boranes, it provides a mechanistic rationale for the for-
mation of linear aminoborane polymers instead of poly-
2
2
H NBH was observed, confirming the inertness of the BÀ
2
2
[3a,4j]
N
bond. The possible formation of 3a/3b was examined
borazine.
PNP
by DFT methods. Relatively low barriers were found for
3) For Ru catalyst 1, deactivation proceeds via formation of
a rare, four-membered Ru-N-B-H metallacycle, provid-
ing valuable information for further catalyst design. Its
high thermal stability and the dehydrogenation of A with
3a suggest, that F does not dissociate from 3a during the
reaction.
catalyst deactivation by trapping of 1 with aminoboranes
°
À1
°
R NBH2 (R=H: DG =12.8 kcalmol ; R=Me: DG =
2
À1
1
5.7 kcalmol ). While Me NBH (F) was observed by
B NMR spectroscopy, parent H NBH is a highly reactive
molecule rapidly forming polymeric, insoluble products.
2
2
1
1
2
2
[6]
Consequently, higher steady state concentration of F, com-
pared with H NBH , could explain the considerably higher
Further studies will focus on the catalytic cycle to unfold
the full mechanistic picture of extraordinarily fast amine–
borane dehydrogenation with Ru-amide complexes.
2
2
TONs obtained for A with respect to B in the rapid dehy-
drocoupling regime. Furthermore, the higher TONs for B
dehydrogenation from the fast catalytic regime at higher
substrate loadings (Figure 1) are in agreement with the ki-
netics of intermediate F formation (vide supra).
Acknowledgements
We are grateful to the DFG (Emmy-Noether Programm, SCHN950/2-1)
and the Elitenetzwerk Bayern (graduate fellowship for A.F.) for funding.
We want to thank one of the referees for valuable suggestions.
Keywords: BÀN coupling · dehydrogenation · homogeneous
catalysis · hydrogen storage · ruthenium
From these results a mechanism for B dehydrocyclization
is proposed (Scheme 2). Upon initial metal-catalyzed dehy-
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Chem. Eur. J. 2009, 15, 10339 – 10342
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Received: May 22, 2009
Published online: September 10, 2009
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Chem. Eur. J. 2009, 15, 10339 – 10342