Catalytic deuteration of silanes mediated by N-heterocyclic carbene-Ir(iii) complexes

Article abstract of DOI:10.1039/c1cc13492b

The catalytic activity of a series of coordinatively unsaturated NHC-M(iii) (M = Rh, Ir; NHC = N-heterocyclic carbene) complexes was tested in the deuteration of secondary and tertiary silanes. Among these, [IrCl(I ^tBu′)₂] provides t

Full text of DOI:10.1039/c1cc13492b

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COMMUNICATION
Catalytic deuteration of silanes mediated by N-heterocyclic
carbene-Ir(III) complexesw
a
b
c
a
George C. Fortman, Heiko Jacobsen, Luigi Cavallo* and Steven P. Nolan*
Received 13th June 2011, Accepted 1st July 2011
DOI: 10.1039/c1cc13492b
t
activation of I Bu ligand is reversible. Under H , 2 can be
The catalytic activity of a series of coordinatively unsaturated
NHC-M(III) (M = Rh, Ir; NHC = N-heterocyclic carbene)
complexes was tested in the deuteration of secondary and
2
converted back to 1.
Additionally, complex 3 has been reported to evolve into
t
0
t
[Ir(I Bu)
tertiary silanes. Among these, [IrCl(I Bu )
2
] provides the highest
2
(H)
2
][PF
6
] when placed under one atmosphere of
8
hydrogen. The reversible nature of the C–H bond activation
conversions to the deuterated species. Mechanistic studies high-
t
of the I Bu ligand in tandem with the electronically unsaturated
light the reversible nature of the ortho-metalation reaction.
nature of the metal center in these complexes prompted us to
examine 2 and 3 as potential catalysts in s-bond metathesis
reactions.
Isotopically labeled compounds play an ever increasingly
1
important role in chemistry. Be it as a mechanistic probe or
2
in medicinal uses, compounds containing an unnatural
9
,10
abundance of rare isotopes are highly desirable. Few reports
in the literature have been published which deal with the
catalytic formation of deuterosilanes; despite the fact that
3
silanes play such an important role in organic synthesis.
Initial reports utilized Au and Cu heterogeneous catalysts
4
for isotopic exchange of the Si–H/D bond. More generally,
t
Scheme 1 [MHCl(I Bu )(I Bu)]
0
t
t
0
2
]
(1), (2)
[MCl(I Bu )
and
deuterosilanes have been prepared by reacting either KSiR3
t
M(I Bu )
0
[
2
][PF
6
]
(3) (M = Rh(a), Ir (b); coe = cyclooctene)
2 4
with D O or reacting the protio-silane with LiAlD , however
complexes.
both methods involve the use of potentially dangerous inorganic
5
hydrides in stoichiometric amounts. Alternative methodologies,
Isotopic exchange of silanes was identified as a promising
reaction for just such an undertaking. Herein, we report
results of an investigation of the catalytic H/D exchange of
protio-silanes utilizing ambient temperature and as little as
especially one involving a catalytic transformation would be
6
highly desirable for this transformation.
In 2004, our group reported the synthesis of a series of
coordinatively unsaturated complexes, which were subsequently
0
.5 atm of deuterium gas using several of these NHC-M(III)
7
shown to have the ability to reversibly add H . The complexes
(
M = Rh, Ir) complexes.
2
2 2
Reactions were carried out in a Schlenk tube in CD Cl as
are prepared (Scheme 1) by simple substitution reaction of
shown in eqn (1).
[
equivalents of I Bu (I Bu = 1,3-bis[(tert-butyl)]-2H-imidazol-
MCl(coe)2]2 (M = Ir or Rh; coe = cyclooctene) with 4
t
t
t
-ylidene). Complex 1 ([MHCl(I Bu )(I Bu)] M = Ir and Rh)
0
t
2
is formed within 20 h and exhibits an ortho-metalation of one
ð1Þ
t
t
I Bu ligand. Ortho-metalation of the second I Bu is much
slower and occurs over a period of 5 days and is aided by
repeated removal of liberated hydrogen using vacuum. The
Triphenylsilane was selected as the model substrate in initial
screening studies. Conversion of this silane was expected to occur
slowly in view of the crowded environment about the Si–H
moiety. We hoped this reaction to be slow enough to permit
unambiguous catalyst optimization studies. Reactions were
cationic complex is formed quickly when reacted with AgPF
ꢀ
6
ꢀ
.
Complexes 2 and 3 are unusual in being stable 16e and 14e
structures, as previously shown. Also, the intramolecular C-H
1
29
monitored by H and Si NMR spectroscopies as well as by
FTIR spectrometry (by monitoring the nSi-H/D). The catalytic
performances of both Rh and Ir systems were examined. Results
of the catalyst screening are shown in Table 1. Reactions in the
absence of a catalyst showed no sign of H/D exchange. Both Rh
catalysts proved wanting in comparison to their Ir congeners.
a
EaStCHEM School of Chemistry, University of St Andrews,
St Andrews, KY16 9ST, UK. E-mail: snolan@st-andrews.ac.uk
KemKom, 1215 Ursulines Ave., New Orleans, Louisiana, 70116,
USA
Department of Chemistry and Biology, University of Salerno,
via ponte don Melillo, 84084, Fisciano, Italy.
E-mail: lcavallo@unisa.it
b
c
t
0
[IrCl(I Bu ) ] (2b) exhibited the highest activity amongst catalysts
tested, with a conversion of 93% to DSiPh after 3 h.
w Electronic supplementary information (ESI) available: Experimental
details and NMR spectra. See DOI: 10.1039/c1cc13492b
2
3
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9723–9725 9723

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a
Table 1 Catalyst screening for isotopic exchange of silanes
b
Catalyst
Conversion (%)
—
0
87
64
91
93
87
t
RhCl(I Bu )
0
2
] (2a)
[
[
[
[
[
t
Rh(I Bu )
0
2
][PF
6
] (3a)
t
IrHCl(I Bu)(I Bu )] (1b)
t
0
t
0
IrCl(I Bu )
Ir(I Bu )
2
] (2b)
0
2
][PF ] (3b)
t
6
a
3
Reaction conditions: 0.22 mmol of HSiPh , 1 mol% catalyst, 0.5 atm
b
1
D
2
, 0.8 mL CD
2
Cl
2
, 25 1C. 3 h. Conversion determined by H NMR.
ꢀ
Surprisingly, the formally 14 e cationic complexes 3 were
less active than the neutral 2. This may be the result of either
t
a more stable [M(I Bu) (H) ] containing species for the
ꢀ1
Fig. 1 Free energy profile (kcal mol ) in CH
1b and 2b with H
2
Cl
2
of the reactivity of
2
2
8
cationic species or possibly the result of a more stable
2
.
t
2
[
neutral complexes were found to be more efficient catalysts
2 3
M(I Bu) (H)(Z -H-SiR )] entity vide infra. Regardless, the
The intermediacy of such an unstable hydride species may
very well be at the origin of the H/D exchange behavior
displayed by 2b.
in this exchange reaction.
Ten silanes were examined for H/D exchange and catalytic
results are presented in Table 2. Alkyl, aryl and alkoxy
silanes were readily amenable to isotopic exchange. Sterically
unencumbered silanes proved relatively easy to deuterate
1
2
DFT calculations resulted in the reaction profile depicted
in Fig. 1. In agreement with the experiments, 1b is the more
stable species. More interesting is the evolution of the reaction
as indicated by the DFT calculations. While coordination of
H2 favorably occurs in the vacant coordination site of 2b
(2b/H cis in Fig. 1) the free energy barrier connecting 2b/H cis
(
Table 2; entries 1 and 4). Catalyst loadings for the H/D
exchange involving HSiEt3 could be as low as 0.01% with
complete conversion within 3 h. (See SI, Fig. S1).
The more sterically demanding substrate, HSi(SiMe3)3
2
2
ꢀ
1
to 1b via transition state 2b/H cis-1b is high (35.5 kcal mol ).
2
(
Table 2, entry 10) required elevated temperature and longer
Much lower, and consistent with the experimental results, is
ꢀ
1
the free energy barrier (only 16.5 kcal mol ) when the
reaction time to reach appreciable levels of deuteration. The need
for longer reaction times and more elevated temperatures for the
more sterically encumbered substrates suggest the transition state
of the reaction is the subject of steric constraints.
addition of H proceeds from 2b/H trans, in which the two
2
2
1
Ir–C bonds are trans to each other. Isomer 2b/H trans, at
3
2
ꢀ
1
15.3 kcal mol above 2b plus free H , is quite unstable and is
2
Turning our attention to mechanistic aspects of the reaction,
we examined first the behavior of the Ir complexes in the presence
of H2 only. Placing red solutions of 2b under 1 atm of H2
converts the complex to an orange solution of 1b within minutes
a transient intermediate, since the barrier preventing it to
precipitate into 1b via the 2b/H trans-1b transition state is
2
only 1.2 kcal/mol. The preference for 2b/H trans-1b is
2
explained by the preference of forming the Ir–H bond trans
0
t
to the softer Cl ligand, rather than to the remaining Ir–C I Bu
(See SI, Fig. S2.) Increasing the pressure to as much as 20 atm of
H reverses the ortho-metalation of the I Bu arm and generates
t
0
2
s-bond. Once 1b has been reached, its transformation into 4
can again occur at least via two different H2 coordinated
isomers, namely 1b/H cis and 1b/H trans (cis and trans
t
[
Ir(H)
2 2 2
(I Bu) Cl] (4). Releasing the pressure of H from the high-
pressure reactor cell rapidly converts 4 back into 1b. Unfortunately
2
2
1
all attempts to isolate 4 proved unsuccessful. This reactivity
1
indicate here the relative coordination position of H2 and
the Cl ligands). Again, the transition state presenting the
forming Ir–H bond trans to the softer Cl ligand, 1b/H trans-4
is in sharp contrast to that displayed by 3b which reacts
8
with H
Table 2 H/D isotopic exchange of silanes using [IrCl(I Bu )
Entry Product Time (h) Loading (mol%) Conversion (%)
2
and leads to an isolable and stable dihydride complex.
2
ꢀ
1
at 25.1 kcal mol above 1b, is remarkably lower in energy than
t
0
a
2
](2)
transition state 1b/H cis-4, which presents the forming Ir–H
2
14
bond trans to the harder H ligand. The high activation
energy barrier in the reaction going from 1b to 4 is in stark
contrast to those found for the analogous transformation from
1
2
3
4
5
6
7
8
9
1
1
DSiEt
3
0.5
3
3
3
3
0.5
1
3
3
16
3
1.0
0.01
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
99
94
90
99
95
96
98
97
98
93
48
DSiEt
3
t
t
0
t
15
i
DSi( Pr)
b
[IrH(I Bu)(I Bu )][PF ] to [Ir(H) (I Bu) ][PF ]. The added steric
6 2 2 6
3
t
bulk of the Cl ligand in 1b within the metal coordination appears
DSiMe
SiPh
DSiMe
DSiMePh
DSiPh
2
Bu
D
2
2
to cause significant differences in the reactivity between
Ph
t
t
0
t
t
0
2
[
IrH(I Bu)(I Bu )][PF
The DFT reaction profile is consistent with the experimental
results observed in the reaction of 1b and 2b with H . As
previously mentioned solutions of 2b placed under 1 atm of H
] and [IrHCl(I Bu)(I Bu )] with H .
6
2
2
3
DSi(OEt)
DSi(SiMe
3
2
c
0
1
3
)
3
2
i
DSiCl( Pr)
2
were readily converted into 1b, while in the preparation
of 2b through 1b, H must be periodically removed from the
a
Reactions carried out in CD
Conversion based on integration of H NMR spectra. Conversion
2
Cl
2
1
at 25 1C with 0.5 atm D
2
.
2
b
reaction mixture. The second ortho-metalation is calculated to
be endoergonic, which is in agreement with the large H2
overpressure required to form small amounts of 4 in solution.
29
c
determined using Si NMR data. Reaction conducted at 50 1C with
conversion based on nSi-H/D FTIR data.
9
724 Chem. Commun., 2011, 47, 9723–9725
This journal is c The Royal Society of Chemistry 2011

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synergistic experimental and computational work allowed us
to clarify the mechanism of interconversion between 2b, 1b
and 4 in the presence of D . The same work allowed to exclude
2
a series of plausible mechanisms for the Si–H/D exchange,
which suggests a more complex scenario, which is not unusual
1
6
when Ir-hydride species are involved. Clarification of the
complete mechanism as well as further investigations on the
nature of the interaction between Si-H bonds and metal
centers in the context of these fascinating ortho-metalatated
Scheme 2 Model systems used in the DFT calculations.
Moving to mechanistic aspects when silanes are present, no
17
complexes are currently underway.
reaction was observed when either 2b or 3b was reacted with
HSiEt at room temperature over the course of two days, even
in the presence of a ten-fold excess of silane. Stoichiometric
The ERC (Advanced Investigator Award FUNCAT to
SPN) and the EPSRC are gratefully acknowledged for support.
H.J. is indebted to the Louisiana Optical Network Initiative
3
reactions between DSiEt
mixtures of HSiEt and DSiEt
b and DSiEt were reacted for 4 h, a 32% D/H exchange
3
and the mono-hydride 1b led to
(
LONI) for access to its computational facilities. LC thanks
3
3
. When equimolar amounts of
ENEA (www.enea.it) and the HPC team for support and for
using ENEA-GRID and the HPC facilities CRESCO (www.
cresco.enea.it) Portici (Naples), Italy. SPN is a Royal Society-
Wolfson Research Merit Award holder.
1
3
conversion had occurred and this slowly progressed to 70%
after 5 days (see SI, Fig. S3). This suggests a possible involvement
t
of the pendant tert-butyl groups of the I Bu in the H/D
exchange process. Examination of 1b in the catalytic isotopic
exchange of silanes revealed it to have similar activity to that
of 2b (see Table 1). This implies that reversible formal silane
addition/coordination to either 2b or 1b with simultaneous
Notes and references
1
L. Melander Jr. and S. W. H., Reaction Rates of Isotopic Molecules,
Wiley, New York, 2nd edn, 1980.
(a) L. Shao and M. C. Hewitt, Drug News Perspect., 2010, 23,
t
2
de-ortho-metalation of a pendant I Bu arm is not a viable
398–404; (b) C. S. Elmore and E. M. John, in Annual Reports in
Medicinal Chemistry, Academic Press, 2009, vol. 44, pp. 515–534..
3 (a) B. Marciniec, Coord. Chem. Rev., 2005, 249, 2374;
(b) B.-H. Kim, M.-S. Cho and H.-G. Woo, Synlett, 2004, 761.
mechanism. Further, it also suggests that
(
a s-CAM
s-complex assisted metathesis) type reaction between the
Ir–H bond of 1b and the silane is at least very slow under
the catalytic regime. These conclusions suggest that these
mechanisms are probably not operative during the H/D
exchange of hydrosilanes reported in Table 1. DFT calculations
are again in agreement with the experiments, since much larger
4
(a) D. I. Bradshaw, R. B. Moyes and P. B. Wells, Chem. Commun.,
1
1
975, 137; (b) M. Bartok and A. J. Molnar, J. Organomet. Chem.,
982, 235, 161.
5 (a) R. J. P. Corriu, C. Guerin and B. Kolani, Bull. Soc. Chim. Fr.,
985, 973; (b) J. C. Gilbert and D. H. Giamalva, J. Org. Chem.,
1985, 50, 2586.
J. Campos, A. C. Esqueda, J. Lo
1
ꢀ
1
free energy barriers (35.5 and 43.3 kcal mol for 2b and 1b,
respectively) are indeed predicted when Me SiH is reacted with
6
´
pez-Serrano, L. Sa
´
nchez,
´
3
F. P. Cossio, A. de Cozar, E. Alvarez, C. Maya and
E. J. Carmona, J. Am. Chem. Soc., 2010, 132, 16765.
(a) R. Dorta, E. D. Stevens and S. P. Nolan, J. Am. Chem. Soc., 2004,
2
b and 1b via coordination followed by de-ortho-metalation
addition (see SI, Fig. S4), and the products of Me SiH
7
3
126, 5054; (b) N. M. Scott, R. Dorta, E. D. Stevens, A. Correa,
L. Cavallo and S. P. Nolan, J. Am. Chem. Soc., 2005, 127, 3516.
8 N. M. Scott, V. Pons, E. D. Stevens, D. M. Heinekey and
S. P. Nolan, Angew. Chem., Int. Ed., 2005, 44, 2512.
addition to 2b and 1b are very high in free energy (21.3 and
ꢀ
1
2
3.4 kcal mol , respectively). An even higher free energy
ꢀ1
barrier, 63.2 kcal mol , is predicted for the s-CAM reaction
between 1b and Me SiH.
To highlight if the unfavorable reactivity of Me
and 1b via coordination/addition or s-CAM depends on steric
or electronic factors, we investigated the reactivity of Me SiH
9
For a recent review on the s-CAM mechanism see: R. N. Perutz
and S. Sabo-Etienne, Angew. Chem., Int. Ed., 2007, 46, 2578.
3
3
SiH with 2b
1
0 For recent reviews on s-bond methathesis see: (a) J.-M. Basset,
C. Coperet, D. Soulivong, M. Taoufik and C. J. Thivolle, Acc. Chem.
Res., 2010, 43, 323; (b) Z. Lin, Coord. Chem. Rev., 2007, 251, 2280.
1 NMR samples of 2b left under 50 atm H in DCM for 4 days
´
3
1
2
with model systems 2b-xs and 1b-xs, which are model systems
that minimize steric stress on the reacting atoms.
showed the presence of 1b and a small amount of another hydride
peak attributed to 4.
De-ortho-metalation of 2b-xs by Me SiH is calculated to
3
12 The DFT calculations were performed with the BP86 GGA functional
using the small quasi-relativistic SDD effective core potential on Ir and
the TZVP basis set on all main group atoms. All structures were
characterized as minima or transition state via vibrational analysis. The
reported energies correspond to free energies in CH Cl , and have been
ꢀ
1
give an addition product that is 5.5 kcal mol more stable
than 2b-xs+free Me SiH, indicating that the poor reactivity
3
of 2b with silanes is probably driven by steric factors.
Differently, the transition state for the s-CAM reactivity
2
2
2 2
obtained by adding solvent effects (CH Cl ) to the gas-phase free
ꢀ
1
energies, via single point calculations on gas phase geometries.
3 The closing of the Ir–C bonds in 2b from trans to cis is an almost
between 1b-xs and Me
which is quite lower than that calculated for the s-CAM
reactivity between 1b and Me SiH, but still extremely high in
3
SiH is calculated at 49.7 kcal mol ,
1
ꢀ
1
barrierless process, below 2 kcal mol
14 A slightly more stable isomer of 4, presenting a highly elongated
molecule coordinated trans to the Cl ligand is calculated at
.
3
H
2
energy, which indicates that the poor s-CAM reactivity
between 1b and silanes is probably driven by electronic factors.
In conclusion, we have highlighted the catalytic activity of
complexes 1 and 2 in the H/D exchange of hydrosilanes.
Reactions were found to proceed with good conversion to
the corresponding deuterio-silane at ambient temperatures
with catalytic loadings as low as 0.01 mol%. Preliminary
ꢀ1
4
.6 kcal mol in free energy above 2b + 2 free H molecules.
2
1
5 L. Cavallo, S. P. Nolan and H. Jacobsen, Can. J. Chem., 2009,
87, 1362.
16 (a) O. G. Shirobokov, L. G. Kuzmina and G. I. Nikonov, J. Am.
Chem. Soc., 2011, 133, 6487; (b) G. E. Dobereiner, A. Nova,
N. D. Schley, N. Hazari, S. J. Miller, O. Eisenstein and
R. H. Crabtree, J. Am. Chem. Soc., 2011, 133, 7547.
17 B.-J. Li and Z.-J. Shi, Chem. Sci., 2011, 2, 488.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9723–9725 9725