Facile activation of H-H and Si-H bonds by boranes

Article abstract of DOI:10.1021/ja300365s

The borane B(C₆F₅)₃ is a precatalyst for H/Dexchange between H₂ and deuterium-labeled silanes (D ₃SiPh, D₂SiMePh, DSiMe₂Ph, DSiEt₃). Experimental and DFT studies revea

Full text of DOI:10.1021/ja300365s

Communication
pubs.acs.org/JACS
Facile Activation of H−H and Si−H Bonds by Boranes
,†
,‡
†
Georgii I. Nikonov,* Sergei F. Vyboishchikov,* and Oleg G. Shirobokov
†
Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada
‡
Institut de Química Computacional and Departament de Química, Campus de Montilivi, Universitat de Girona, 17071 Girona, Spain
S Supporting Information
*
judged by the time required for the appearance of the HD and
the extent of broadening of the SiH signal. After 1 day
the extent of proton scrambling into PhSiD , PhMeSiD , and
ABSTRACT: The borane B(C F ) is a precatalyst for H/D
6
5 3
exchange between H2 and deuterium-labeled silanes
D SiPh, D SiMePh, DSiMe Ph, DSiEt ). Experimental
3
2
(
3
2
2
3
PhMe SiD is 80%, 60%, and 30%, respectively. The electro-
2
and DFT studies reveal that B(C F ) itself cannot activate
6
5 3
philicity of borane as a prerequisite for the exchange is
dihydrogen but converts to HB(C F ) under the action of
6
5 2
evidenced in the BPh -catalyzed reaction, which provides
3
hydrosilane. The latter species easily activates H−H and
Si−H bonds by a σ-bond metathesis mechanism, which
was further confirmed by the reactions of BD ·THF with H .
observable H/D exchange only after 1 month.
We were intrigued by the mechanism of this reaction, as
7
3
2
previous reports by other workers as well as our own
experimental and DFT studies demonstrate that B(C F ) does
6
5 3
not form a stable adduct with dihydrogen. Our initial me-
chanistic suggestion for the reaction shown in eq 1 was that
hydrogen activation may proceed on a SiR3+ cation formed
in situ by hydride abstraction from HSiR by B(C F ) (i.e., an
ctivation of hydrogen gas by non-transition-metal systems
17
A
is an area of active research, both in the context of
1
hydrogen storage and for the development of metal-free
3
6 5 3
2
hydrogenation/reduction catalysis. There have been several
ionic mechanism). However, our DFT calculations showed that
such a process is energetically implausible in nonpolar media
such as benzene (Table 1, entry 1). Under these conditions,
silylium ion is not formed, even as a π-complex with benzene,
recent breakthroughs in transition-metal-free H activation: the
2
3
4
irreversible hydrogenations of digermyne and carbene, and
the discovery of H activation by frustrated Lewis pairs. In
5
,6
2
the latter case, the H molecule is believed to be heterolytically
+
18
2
SiEt ·C H (Table 1, entry 2). Stabilization due to a Si−H−Si
3
6
6
split inside an R P···BAr cage to give the phosphonium ion
+
19
3
3
bridge formation to give the [(Et Si) (μ-H)] ion is not
3 2
+
−
7,8
2a,b,d
R PH and borate HBAr . This process can be done both
reversibly and catalytically.
3
3
sufficient either (Table 1, entry 3). Alkyl abstraction by borane
from triethylsilane is even less favorable than hydride abstraction
9
Activation of dihydrogen on a
single boron center has been observed only very recently in the
(
Table 1, entry 4).
During our further quest by ab initio calculations for a
10
special case of irreversible hydrogenation of borole. At room
temperature no reaction takes place between H and tertiary
2
hydrogen-active intermediate, we found a stable adduct of the
boranes, such as the highly electrophilic B(C F ) , according to
6
5 3
electrophilic borane B(C F ) with silane, Et Si−H···B(C F ) ,
1
1
7,12
6
5 3
3
6 5 3
experimental and computational studies.
In contrast,
which displays a short B···H distance of 1.398 Å and a
noticeable elongation of the Si−H bond (1.554 Å in
Et SiH·B(C F ) vs 1.489 Å in Et SiH). Nonetheless, the
calculations of the reaction between H and BH3 were
2
1
3
shown to give a weakly bound BH ·H adduct, and such a
species was indeed generated by reacting boron atoms with
3
2
3
3
6
5 3
3
δ+
δ−
1
4,15
Et
Si −H polarization caused by the adduct formation is not
hydrogen in an argon matrix.
sufficient, and the adduct remains completely inert with respect
to dihydrogen. Keeping in mind the known examples of
dihydrogen activation by means of frustrated Lewis pairs, we
Here we report an H/D exchange between the dihydrogen
and silane catalyzed by boranes and provide evidence for the
activation of H−H and Si−H bonds on a single boron center.
We also provide evidence that the reaction proceeds via a
σ-bond metathesis mechanism rather than by heterolytic bond
cleavage.
6
also examined the possibility of an H−H activation by way of
δ+
δ−
δ+
δ−
an R Si −H ···H −H ···BAr complex, in which the R SiH
3
3
3
and BAr moieties would be kept together by a kind of a long-
3
7
range intermolecular interaction. However, extensive calcu-
Our initial observation was that B(C F ) catalyzes exchange
6
5 3
lations did not reveal any such intermediate.
between gaseous H and deuterium-labeled silanes according to
2
The fact that both R SiH and BAr , as well as their adduct,
3
3
the equation
turned out to be inactive with respect to molecular hydrogen
implies that there must be another species that actually reacts
R Si‐D + H → R Si‐H + HD
(1)
3
2
3
with H . A number of experimental observations suggested the
2
(
where R Si = D PhSi, DMePhSi, Me PhSi, and Et Si),
3 2 2 3
existence of an active intermediate. We noticed that broadening
of the Si−H NMR signals upon addition of borane involves
an induction period depending on the silane. Similarly, an
characterized by the appearance of the HD triplet at 4.54
ppm with a J(H−D) spin−spin coupling constant of 43.2 Hz in
1
16
the H NMR spectrum in C D , significant broadening of the
6
6
Si−H signal, and appearance of the H D Si signal(s) in the case
x
y
of primary and secondary silanes. There is a clear qualitative
trend that bulkier silanes undergo the exchange slower, as
Received: January 12, 2012
Published: March 6, 2012
©
2012 American Chemical Society
5488
dx.doi.org/10.1021/ja300365s | J. Am. Chem. Soc. 2012, 134, 5488−5491

Journal of the American Chemical Society
Communication
Table 1. Relative Energies ΔE at MP2/6-311G** and
e
CCSD/6-311++G**//MP2/6-311G** Levels and ΔG°
2
98
−1
CCSD COSMO (in kcal·mol )
ΔEe
ΔG°2
98
CCSD
a
process
MP2
CCSD COSMO
+
1
2
3
4
5
6
7
8
.
.
.
.
.
Et SiH + B(C F ) → [Et Si] +
106.9
99.8
59.6
50.1
44.2
74.0
−4.4
27.8
6.1
3
6
5
3
3
−
[
HB(C F ) ]
6 5 3
Et SiH + C H + B(C F ) →
73.0
70.5
70.8
3
[
6
6
6 5 3
+
−
Et Si] ·C H + [HB(C F ) ]
3 6 6 6 5 3
+
2Et SiH + B(C F ) → (Et Si) H
63.8
3
6
5
3
3
2
−
+
[HB(C F ) ]
6 5 3
+
Et SiH + B(C F ) → [Et HSi] +
125.6
−17.7
18.3
117.1
−18.8
24.5
3
6
5
3
2
−
[EtB(C F ) ]
6 5 3
Et SiH + B(C F ) →
3
6 5 3
Et SiH·B(C F )
3
6 5 3
Et SiH·B(C F ) → Et Si(C F ) +
Figure 1. Formation of protiosilane in the solution of PhSiD
in C D
3 6 6
3
6
5
3
3
6 5
HB(C F ) ; barrier
under hydrogen atmosphere in the presence of B(C F ) . 100% conversion
6
5
2
6 5 3
.
.
Et SiH + B(C F ) → Et Si(C F )
4.6
4.7
corresponds to the exchange of all three deuteriums of PhSiD for protons.
3
3
6
5
3
3
6 5
+
HB(C F )
6 5 2
H + HB(C F ) → H +
12.2
15.1
23.5
HB(C F ) was used as a catalyst for the H /PhSiD exchange,
6 5 2 2 3
70% exchange was reached without an induction period in
2
6
5
2
2
HB(C F ) (degenerate
exchange); barrier
6
5 2
<
13.5 h (cf. 14.5% after 24 h in Figure 1).
9
1
.
Et SiH + HB(C F ) →
−17.2
−15.6
−1.4
3
6 5 2
According to calculations, HB(C F ) does not form a stable
6
5 2
Et SiH·HB(C F ) ; adduct
3
6 5 2
adduct with H either, but undergoes a direct degenerate 2+2
2
0. Et SiH·HB(C F ) →
1.8
3.4
5.1
3
6 5 2
σ-bond metathesis via a nearly symmetric transition state
Et SiH·HB(C F ) ; adduct
3
6 5 2
2
⧧
(degenerate exchange); barrier
(η -H )B(C F ) (Table 1, Figure 2b) with a moderate Δ G°
3
6
5 2
298
−1
1
1. Et SiH + HB(C F ) → Et Si-
4.6
5.9
8.6
barrier of 23.5 kcal·mol . The feasibility of such metathesis was
3
6
5
2
3
B(C F ) + H ;
further proved experimentally by the direct reaction of BD ·THF
6
5
2
2
3
barrier
7.4
9.1
21.7
in THF/C D with hydrogen at atmospheric pressure, which
6
6
1
1
2. PhH + HB(C F ) → Ph-B(C F )
6 5 2
−6.6
−3.6
−3.7
results in very fast (<5 min) appearance of H−D in the H and
H NMR spectra. The concomitant formation of BH D ·THF
6
5
2
+
H2;
barrier
3. H + BH → H ·BH ; adduct
2
x
y
14.6
−3.8
2.6
21.5
−2.3
5.1
31.3
10.4
18.1
1
11
was confirmed by H− B HSQC NMR experiment.
Quantum mechanical calculations established that dis-
sociation of the BH ·THF adduct proceeds easily (ΔG°
1
1
2
3
2
3
4. H + BH → H + BH (degenerate
2
3
2
3
=
298
3
exchange); barrier
5. BH ·THF → BH + THF
−1
5
.9 kcal·mol ). The subsequent degenerate H /BH exchange
2
3
1
25.4
21.9
5.9
3
3
−1
occurs with a reasonable barrier of 18.1 kcal·mol . For the
a
An estimate at the CCSD/6-311++G**//MP2/6-311G** level
taking into account solvent effects (benzene).
latter process we did find a loosely bound adduct H ·BH
2
3
13
(
Table 1), in accord with the previous calculations.
A similar 2+2 degenerate hydride metathesis (Figure 2c),
induction period was observed for the H /silane exchange
2
20
(
Figure 1). Also, it was shown previously that thermolysis of
HB(C F ) + Et SiH → HB(C F ) + Et SiH
6
5 2
3
6 5 2
3
Et SiH with B(C F ) at 60 °C for 3 days gives a secondary
borane according to the equation
3
6 5 3
requires a low Δ G° barrier of 5.1 kcal·mol−1 (Table 1),
⧧
298
which eventually accounts for the experimentally observed iso-
tope exchange between H and the deuterium atom in Et SiD.
Et SiH + B(C F ) → HB(C F ) + Et Si(C F )
3
6 5 3
6 5 2
3
6 5
2
3
Apart from the hydrogen exchange, we have also looked at
the possibility of productive dehydrogenative B−Si coupling,
We found that this H/C exchange process occurs via
formation of the Et Si−H···B(C F ) adduct followed by a
3
6 5 3
σ-bond metathesis (Figure 2a) with the calculated activation
Et SiH + HB(C F ) → Et Si‐B(C F ) + H
2
3
6 5 2
3
6 5 2
⧧
−1
free energy Δ G°
= 27.8 kcal·mol . In the σ-bond
2
98
metathesis, the cleavage of old σ bonds (e.g., C−B and Si−
H) occurs simultaneously with the formation of new σ bonds
(Figure 2d), that would afford synthetically useful silylbor-
23
anes. According to calculations, the reaction is energetically
−
1
(
e.g., Si−C and B−H) in a concerted four-center transition
uphill (ΔG° = +8.6 kcal·mol ) but still has an affordable
298
2
1
⧧
−1
state. Experimentally, we found that such an H/C exchange is
barrier (Δ G° = 21.7 kcal·mol , Table 1). Such a coupling
298
much more facile for primary (PhSiH ) and secondary silanes
may therefore become feasible if the evolved dihydrogen is
consumed in a thermodynamically favorable process (e.g., in a
reaction with an unsaturated substrate) or is removed from the
reaction media. It is noteworthy that the synthetically
3
(
PhMeSiH ) and is observed already within a few hours at
2
room temperature (9% yield of HB(C F ) overnight in the
6
5 2
case of PhMeSiH , and 5% yield of HB(C F ) overnight in the
2
6 5 2
1
9
22
24
case of PhMe SiH, according to F NMR). These obser-
important dehydrogenative B−C coupling between benzene
2
vations are in line with the ease of the boron-catalyzed
and borane (in this example HB(C F ) ) is thermodynamically
6
5 2
−
1
H /DSiR exchange (vide supra). It is, therefore, reasonable to
feasible (ΔG° = −3.7 kcal·mol ) but proceeds with a large
2
3
298
⧧
−1
assume that HB(C F ) formed in situ is the true catalyst of the
barrier (Δ G° = 31.3 kcal·mol , Table 1). This kinetic
6
5 2
298
hydrogen/silane and silane/silane exchanges. Indeed, when
hurdle is in agreement with the fact that transition-metal
5
489
dx.doi.org/10.1021/ja300365s | J. Am. Chem. Soc. 2012, 134, 5488−5491

Journal of the American Chemical Society
Communication
Figure 2. σ-Bond metathesis reaction on boron centers. The numbers above the arrows are standard Gibbs energy changes (ΔG°298) or barriers
⧧
−1
(
Δ G° ) in kcal·mol calculated at the CCSD level taking into account solvation effects.
298
catalysts are needed for dehydrogenative coupling of arenes Ar−H
ASSOCIATED CONTENT
Supporting Information
■
24,25
and boranes HB(OR) to give arylboranes Ar−B(OR) .
2
2
*
S
In summary, we have demonstrated activation of H−H and
Si−H bonds on a single borane center and unveiled a mechanism
based on a series of previously unrecognized direct σ-bond
metatheses resulting in substituent exchange on the boron center.
General experimental procedures and computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
5
490
dx.doi.org/10.1021/ja300365s | J. Am. Chem. Soc. 2012, 134, 5488−5491

Journal of the American Chemical Society
Communication
(
18) Lambert, J. B.; Zhang, S.; Stren, C. L.; Huffman, J. C. Science
AUTHOR INFORMATION
Corresponding Author
gnikonov@brocku.ca; vyboishchikov@googlemail.com
■
1
(
997, 275, 39.
19) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed, C. A. Chem.
Commun. 2005, 767.
Notes
(20) Parks, D. J.; Piers, W. E.; Yap, G. P. A Organometallics 1998, 17,
5
492.
The authors declare no competing financial interest.
(21) σ-Bond metathesis is a well-established mechanism in transition
metal chemistry, e.g.: Hartwig, J. Organotransition Metal Chemistry.
From Bonding to Catalysis; University Science Books: Mill Valley, CA,
ACKNOWLEDGMENTS
This work was supported by NSERC (DG to G.I.N.) and the
Spanish Ministerio de Ciencia e Innovacion
3077/BQU). We are very grateful to Razvan Semionescu
■
2
(
010.
22) R Si(C F ) was observed by 19F NMR as another coproduct.
́
(grant CTQ2008-
3
6 5
0
(23) The silylboranes are interesting reagents for making
(
Brock University) for help with NMR. We also thank the CFI/
functionalized materials via Si−B bond additions. See, for example:
OIT for a generous equipment grant.
Oshima, K.; Ohmura, T.; Suginome, M. J. Am. Chem. Soc. 2011, 133,
7
(
324.
24) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.;
Hartwig, J. F. Chem. Rev. 2010, 110, 890.
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696. (b) Cho, J.-Y.; Kin Tse, M.; Holmes, D.; Maleczka, R. E. Jr.;
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6
5
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5
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dx.doi.org/10.1021/ja300365s | J. Am. Chem. Soc. 2012, 134, 5488−5491