Mechanistic Insights on the Reduction of CO2 to Silylformates Catalyzed by Ir-NSiN Species

Article abstract of DOI:10.1002/chem.201702246

The hydrosilylation of CO₂ with different silanes such as HSiEt₃, HSiMe₂Ph, HSiMePh₂, HSiMe(OSiMe₃)₂, and HSi(OSiMe₃)₃ in the presence of catalytic ammounts of the iridium(III) complex [Ir(H)(CF₃CO₂)(NSiN*)(coe)] (1; NSiN=fac-bis-(4-methylpyridine-2-yloxy); coe=cis-cyclooctene) has been comparatively studied. The activity of the hydrosilylation catalytic system based on 1 depends on the nature of the reducing agent, where HSiMe(OSiMe₃)₂ has proven to be the most active. The aforementioned reactions were found to be highly selective toward the formation of the corresponding silylformate. It has been found that using 1 as catalyst precursor above 328 K decreases the activity through a thermally competitive mechanistic pathway. Indeed, the reduction of the ancillary trifluoroacetate ligand to give the corresponding silylether CF₃CH₂OSiR₃ has been observed. Moreover, mechanistic studies for the 1-catalyzed CO₂-hydrosilylation reaction based on experimental and theoretical studies suggest that 1 prefers an inner-sphere mechanism for the CO₂ reduction, whereas the closely related [Ir(H)(CF₃SO₃)(NSiN)(coe)] catalyst, bearing a triflate instead of trifluoroacetate ligand, follows an outer-sphere mechanism.

Full text of DOI:10.1002/chem.201702246

A Journal of
Accepted Article
Title: Mechanistic Insights on the Reduction of CO2 to Silylformates
Catalyzed by Ir-NSiN Species
Authors: Luis A. Oro, Alejandro Julián, Jefferson Guzmán, E. A.
Jaseer, Francisco Fernández-Alvarez, Raquel Royo, Victor
Polo, María Pilar García Orduña, and Fernando Lahoz
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Mechanistic Insights on the Reduction of CO to Silylformates
2
Catalyzed by Ir-NSiN Species
[
a]
[a]
[b]
[a]
Alejandro Julián, Jefferson Guzmán, E. A. Jaseer, Francisco J. Fernández-Alvarez,* Raquel
[c]
[c]
[a]
[a]
[a,b]
Royo, Víctor Polo,* Pilar García-Orduña, Fernando J. Lahoz, and Luis A. Oro*
Dedication ((optional))
Abstract: The hydrosilylation of CO
HSiEt , HSiMe Ph, HSiMePh , HSiMe(OSiMe
presence of catalytic ammounts of the iridium(III) complex
Ir(H)(CF CO )(NSiN*)(coe)] (1; NSiN* = fac-bis-(4-methylpyridine-2-
2
with different silanes such as
reduction of CO
examples of catalytic systems that allow the selective reduction
of CO with hydrosilanes or hydrosiloxanes to give the
2
with silanes is of great interest. Some
3
2
2
)
3 2
and HSi(OSiMe in
3 3
)
2
[
4-12]
[13]
[
3
2
corresponding
methoxysilane
silylformate,
or methane
bis-silylacetal,
are known. Particularly,
[
14]
[15]
yloxy); coe = cis-cyclooctene) has been comparatively studied. The
activity of the hydrosilylation catalytic system based on 1 depends
silylformates have attracted attention during the last recent years
due to their potential as C1 source in organic synthesis. Indeed,
silylformates have been proposed as intermediates in catalytic
3 2
on the nature of the reducing agent, where HSiMe(OSiMe ) has
proven to be the most active. The aforementioned reactions were
found to be highly selective to the formation of the corresponding
silylformate. It has been found that using 1 as catalyst precursor
above 328 K there is a decrease in the activity. This has been
attributed to consumption of the active species through a thermally
competitive mechanistic pathway. Indeed, the reduction of the
ancillary trifluoroacetate ligand to give the corresponding silylether
reactions of CO
give formamides and methylamines.
2
and secondary amines with hydrosilanes to
[
2a,5,9,16]
In addition, it has
been recently reported that silylformates could also be used as
surrogates of hydrosilanes in aldehydes transfer hydrosilylation
[
17]
processes.
In connection with the aforementioned background, our research
team has developed a series of iridium(III) complexes with
CF
for the 1-catalyzed CO
experimental and theoretical studies suggest that 1 prefers an inner
sphere mechanism for the CO reduction while the closely related
Ir(H)(CF SO )(NSiN)(coe)] catalyst, having a triflate instead of
3
CH
2
OSiR
3
has been observed. Moreover, mechanistic studies
2
-hydrosilylation reaction based on
monoanionic tridentate NSiN and NSiN* (NSiN
=
fac-bis-
fac-bis-(4-methylpyridine-2-
yloxy) ) ligands, which have proven to be active catalysts for
the selective and solvent-free reduction of CO with
HSiMe(OSiMe to afford the corresponding silylformate. The
iridium(III) species [Ir(H)(CF
1) (coe = cis-cyclooctene) shown in Figure 1 has been found to
[
18]
*
(pyridine-2-yloxy),
NSiN
=
[
19]
2
[
3
3
2
trifluoroacetate ligand, follows an outer sphere mechanism.
3 2
)
3
*
3
2
CO ){fac-(κ -Si,N,N)-NSiN }(coe)]
(
be the most active and selective of the above mentioned series
of Ir-NSiN catalyst precursors, allowing the selective preparation
Introduction
2 3 2
of the silylformate HCO SiMe(OSiMe ) (2a) in gram scale after
The design of new processes that allow the large-scale
[19]
2
one hour of reaction at 328 K and 8 bar of CO (Scheme 1).
utilization of CO
one of the challenges for nowadays chemists.
the catalytic reduction of CO with hydrosilanes has emerged as
an useful methodology for the transformation of CO into value-
added chemicals under mild reaction conditions. Conversely,
CO -hydrosilylation processes are commonly unselective and
afford mixtures of reduction products.
development of active and selective catalytic systems for the
2
as chemical feedstock could be considered as
[
1,2]
In this regard,
2
2
[
3]
2
[3a]
Therefore, the
[a]
A. Julián, J. Guzmán, Dr. F. J. Fernández-Alvarez, Dr. P. García-
Orduña, Prof. F. J. Lahoz and Prof. L. A. Oro. Departamento de
Química Inorgánica
– Instituto de Síntesis Química y Catálisis
Homogénea (ISQCH). Universidad de Zaragoza. Facultad de
Ciencias 50009, Zaragoza – Spain.
Figure 1. Representation of the structure of complex 1.
E-mail: paco@unizar.es, oro@unizar.es
[
b]
c]
Dr. E. A. Jaseer and Prof. L. A. Oro (visiting Prof.) Center of
Refining & Petrochemicals, King Fahd University of Petroleum &
Minerals, 31261 Dhahran - Saudi Arabia.
R. Royo and Dr. V. Polo. Departamento de Química Física –
Instituto de Biocomputación y Física de Sistemas complejos (BIFI) –
Universidad de Zaragoza. Facultad de Ciencias 50009, Zaragoza –
Spain. E-mail: vipolo@unizar.es
In this work we have expanded the scope of our earlier work on
[19]
the 1-catalyzed CO
different silanes such as HSiEt
HSi(OSiMe
2
-hydrosilylation with HSiMe(OSiMe
3
)
2
to
[
3
, HSiMe Ph, HSiMePh
2
2
and
3 3
) .
In all the cases the reactions were found to be highly selective to
the formation of the corresponding silylformate (Scheme 1). The
observed activity strongly depends on the nature of the reducing
agent. The reasons behind the decline in activity observed when
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complex 1 was employed at temperatures above 328 K have
been unveiled by means of experimental and theoretical studies.
These studies show the formation of the silyl ether
3 2 3
CF CH OSiR (3) by reaction of the trifluoroacetate ancillary
ligand with the corresponding hydrosilane. This causes the
consumption of the active species, and subsequently a decrease
in the activity and selectivity of the catalytic system based on 1.
Results and Discussion
1
H NMR studies of the 1-catalyzed (10 mol%) reduction of CO
bar, 328 K) with HSiEt HSiMe Ph, HSiMePh
HSiMe(OSiMe and HSi(OSiMe in using
2
(
3
3
,
2
2
,
3
)
2
3
)
3
C
6
D
6
,
Figure 2. Representation of the concentration (%) of silylformate versus time
from the reaction of CO
2
(3 bar, 328 K) with the corresponding hydrosilane in
hexamethylbenzene as internal standard have been performed.
These studies showed that in all cases the reactions were highly
selective to the formation of the corresponding silylformate
6 6
C D
and using complex 1 (10 mol%) as catalyst precursor. The values were
1
obtained from H NMR integration using hexamethylbenzene (0.016 mmol) as
internal standard.
(
Scheme 1). Compounds 2a, 2c, 2d and 2e were identified by
1
13
29
comparison of their H, C and Si NMR spectra with reported
data and 2b was characterized by H, C{ H} and Si{ H} NMR
1
13
1
29
1
The activity of the catalytic system based on
HSiMe(OSiMe decreases above 328 K. In order to explain
that behavior, the reaction of HSiMe(OSiMe with CO (3 bar,
48 K) in C and in presence of 1 (10 mol%) was monitored
by H NMR spectroscopy. These experiments showed that in
consonance with that observed under the solvent-free conditions,
1 and
spectroscopy (See experimental).
3 2
)
3
)
2
2
3
6 6
D
1
after 2 hours of reaction all the starting HSiMe(OSiMe
been transformed into a mixture of 2a, O{SiMe(OSiMe
CH OSiMe(OSiMe together with traces
CH {OSiMe(OSiMe ) } (Figure S.1.1.). However, what catched
3 2
) has
3 2 2
) } ,
3
3
)
2
of
2
3 2
1
our attention from those H NMR spectra was the presence of a
quartet resonance at δ 3.82 ppm ( JH-F = 8.7 Hz). This
Scheme 1. Hydrosilylation of CO
10 mol%) as catalyst precursor.
2
(3 bar) with HSiR
3
in C
6
D
6
using complex 1
3
(
2
resonance corresponds to the protons of the CH moiety of the
silylether species CF CH OSiMe(OSiMe (3). This has been
3
2
3 2
)
corroborated by comparison with NMR spectra of samples of
compound 3 (see experimental and Figures S.1.2 to S.1.5). In
this regard, it should be pointed out that compound 3 could be
easily and quantitatively obtained from the 1-catalyzed
It is noteworthy that HSiMe(OSiMe
active of the aforementioned reducing agents. Conversely,
HSi(OSiMe has showed the poorest activity, being necessary
days to achieve the full conversion of the starting siloxane into
the corresponding silylformate 2b. This could be explained
taking into account the greater steric hindrance around the Si-H
3 2
) has proven to be the most
3 3
)
3
3 2 3 2
dehydrogenative silylation of CF CH OH with HSiMe(OSiMe )
(
Scheme 2). Related examples of Ir-catalyzed dehydrogenative
[
20]
silylations of alcohols with silanes have been reported.
in HSi(OSiMe
found to be slightly less active than HSiMe(OSiMe
than HSiMePh , HSiEt and HSi(OSiMe , respectively (Figure
). These observations suggest that both hindrance and
electronic richness of the Si-H bond play an important role in the
-catalyzed reaction of CO with hydrosilanes and/or
hydrosiloxanes. In this context, it should be pointed out that
besides its comparatively high reactivity, using HSiMe(OSiMe
3
)
3
compared to HSiMe(OSiMe
3
)
2
2
. HSiMe Ph has
3 2
)
but more
2
3
3 3
)
2
1
2
3 2
)
3 2 3 2
Scheme 2. Dehydrogenative silylation of CF CH OH with HSiMe(OSiMe ) in
as reducing agent presents some additional advantages, as for
instance, it is relatively cheap and easily available in comparison
6 6
C D
using complex 1 (1.0 mol%) as catalyst precursor.
2
with organosilanes such as HSiMe Ph.
Therefore, the formation of the silylether 3 during the 1-
catalyzed reaction of CO with HSiMe(OSiMe at 348 K has
2
3 2
)
been unambiguously demonstrated. This behavior could explain
why, in contrast with that observed for the catalytic systems
based on the triflate derivatives [Ir(H)(CF
3
SO
3
)(LSiL)(coe)] (LSiL
[
18]
[19]
=
NSiN
(4a); NSiN* (4b)), the activity of 1 decreases with
the temperature.
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The reaction of complex
1
with four equivalents of
hydrosilylation catalytic system. Thus, the iridium(III) complex
[21]
*
HSiMe(OSiMe
3
)
2
and CO
2
(4 bar) at 328 K was monitored by
[Ir(H){CF
3
(CF
2
)
2
CO
2
}(NSiN )(coe)] (6), with a heptafluorobutyrate
1
H NMR with the aim of obtaining information about any reaction
intermediate. This study evidenced the slow transformation of 1
into a new highly symmetric species, namely compound 5 in
Scheme 3, together with free cyclooctane (see experimental).
ligand instead of trifluoroacetate (Scheme 3 and Figure 3),
affords a catalytic system which at 328 K operates similarly to 1
(Table 1 entries 1 and 2). Complex 6 has been prepared by
[19]
reaction of [Ir(H)(Cl)(NSiN*)(coe)] with one equivalent of silver
heptafluorobutyrate, and fully characterized by elemental
Accordingly with the symmetric structure proposed for
1
1
13
1
19
1
29
1
compound 5, its
H
NMR spectra show
a
set of three
analysis and by H,
C{ H},
F{ H} and
Si{ H} NMR
1
resonances in the aromatic region and one signal due to the
methyl substituents of the N-heterocyclic rings of the NSiN
spectroscopy (see experimental). H NMR spectra of 6 show the
resonance due to the Ir-H moiety as a singlet at δ -14.12 ppm,
ligand (Figure S.1.6.). The presence of two Ir-SiMe(OSiMe
moieties in 5 has been corroborated by means of H, C, and
3
)
2
slightly low-field shifted compared to the value observed for 1 (δ
1
13
[19]
-14.79 ppm)
(Figure S.1.7.). Furthermore, the solid state
2
9
1
Si NMR spectroscopies. Thus, the H NMR spectra of 5 exhibit
the resonance due to the 6 protons of the Ir-SiMe moieties of
3 2
two SiMe(OSiMe ) groups as a singlet resonance at δ 0.55 ppm. metal geometry, with NSiN* ligand facially coordinated to the
1 29
structure of 6 has been analyzed by X-ray diffraction studies
(Figure 3). Complex 6 exhibits a slightly distorted octahedral
This resonance correlates in the H- Si Heteronuclear Multiple-
iridium atom. The pyridine-N atoms are located trans to the
2
Quantum Correlation (HMQC) with a signal at δ -58.6 ppm in the
hydride and to the η -cyclooctene ligands. As it may be expected,
2
9
1
1
13
Si{ H} spectra and in the H- C Heteronuclear Single
stronger structural trans effect of the hydride compared to that of
the olefin induces significant differences in Ir-N bond lengths.
Accordingly, the Ir-N(2) bond length trans to the hydride has
Quantum Correlation (HSQC) with a singlet resonance observed
1
3
1
at δ -1.7 ppm in the C{ H} NMR spectra. Furthermore, the
OSiMe groups appear as singlets at δ 0.10, 1.2 and 12.4 ppm in
3
found to be longer than Ir-N(1). This effect has already been
1
13
1
29
1
[19]
the H,
C{ H} and
Si{ H} NMR spectra, respectively.
observed
[Ir(H)(CF
in compound
SO )(NSiN)(coe)]
1
and in the
related
structural
[
18a]
Conversely, even though variable temperature NMR
experiments were accomplished it was not possible to identify
the signal corresponding to Si-H-Si moiety. This could be
attributable to a fast exchange between 5a and 5b (Scheme
3
3
(4a).
In
fact,
parameters in 6 are very close to those of complex 1. No
statistically significant differences have been found in the
coordination sphere bond lengths and angles of both complexes.
Moreover, both Ir-Si-O-C-N metallacycles, with enantiomorphic
[
22]
3
).
HSiMe(OSiMe
of 5 and 2a and the resulting mixture was pressurized with CO
4 bar) and heated at 328 K the full transformation of the starting
Interestingly, when four additional equivalents of
1
2
3
)
2
were added to the in situ generated solutions
twisted T
2 1
and T conformations, exhibit, in 6, ring puckering
2
parameters (Ir-Si-O(1)-C(2)-N(1): q = 0.130(6) Å, φ = 25(4)°; Ir-
[
23]
(
Si-O(2)-C(8)-N(2): q = 0.146(6) Å, φ = -160(4)°) very close to
those reported for compound and similar to those of
)(NSiN)(coe)]. This feature evidences the rigidity
siloxane into 2a was observed after two hours. Under these
conditions no consumption of compound 5 was observed.
1
[
24]
3 3
[Ir(H)(CF SO
of these monoanionic tridentate ligands, which could be a
property to take into account when designing Ir(III)-NSiN based
catalysts.
Scheme 3. Reactivity of complexes 1 and 6 with HSiMe(OSiMe
of CO
3
)
2
in presence
Figure 3. Molecular structure of complex 6. Selected bond lengths (Å) and
angles (°):Ir-Si, 2.222(3); Ir-O(3), 2.246(7); Ir-N(1), 2.105(9); Ir-N(2), 2.149(8);
Ir-Ct, 2.062(12); Ir-H,1.63(2); Si-Ir-O(3), 166.47(18); Si-Ir-N(1), 80.7(3); Si-Ir-
N(2), 80.8(2); Si-Ir-Ct, 102.5(3); Si-Ir-H, 89(4); O(3)-Ir-N(1), 91.3(3), O(3)-Ir-
N(2), 87.8(3), O(3)-Ir-Ct, 86.7(3), O(3)-Ir-H, 100(4); N(1)-Ir-N(2), 86.2(3); N(1)-
Ir-Ct, 172.8(4); N(1)-Ir-H, 81(4); N(2)-Ir-Ct, 100.6(3); N(2)-Ir-H, 165(4); Ct-Ir-H,
92(4). Ct is the centroid of the olefinic C(14)=C(15) bond.
2
.
It should be mentioned that slight modifications in the ancillary
ligand influence the thermal stability of the resulting CO
2
-
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Regarding the catalytic activity it should be mentioned that at
found to be due to the trifluoroacetate ligand reduction (Scheme
3
48 K the 6-catalyzed (1.0 mol%) solvent-free reaction of CO
2
4, path c)
with HSiMe(OSiMe keeps its catalytic performance (Table 1,
3 2
)
entry 4) while under the same reaction conditions the activity of
the catalytic system based on complex 1 clearly decreases at
that temperature (Table 1 entry 3) (see Figures S.2.1 to S.2.3.).
1
In addition, H NMR studies of the 6-catalyzed (10 mol%)
2 3 2 6 6
reaction of CO with HSiMe(OSiMe ) in C D did not evidence
reduction of the heptafluorobutyrate ligand. Therefore, it is
reasonable to propose that the decrease of the activity and
2
selectivity, observed for the 1-catalyzed CO hydrosilylation with
HSiMe(OSiMe at temperatures above 328 K, could be
3 2
)
attributable to the partial consumption of the active species
during the catalysis (Scheme 2).
Table 1. Solvent-free Ir-NSiN (1.0 mol%) CO
HSiMe(OSiMe
2
-hydrosilylation with
3 2
) .
Scheme 4. Representation of three paths proposed for the 1-catalyzed CO
2
hydrosilylation.
[
a]
[a]
Entry
Catalyst
T (K)
328
328
348
348
358
Time (h)
1.0
Conversion
% of 2
[
19]
1
2
3
4
5
1
99.9
98.9
The theoretical study starts at intermediate A (Figure 4), which is
the model structure of 5a (or b), bearing a hydrosilane molecule
ƞ -(Si-H)-coordinated to the metal. It is reasonable to think that
6
1.0
99.9
98.4
2
[
19]
[b]
1
4.0
88.0
72.7
2
the outer sphere mechanism proposed for the 4a-catalyzed CO -
[
18a]
hydrosilylation,
could also take place in the case of catalyst 1.
6
6
1.0
99.9
94.6
Thus, the trifluoroacetate ligand may promote the Si-H bond
heterolytic cleavage, affording the corresponding Ir-NSiN-
[
b]
2.5
97.1
71.8
1
hydride-(k -O-silylester), which may undergo CO
2
hydrosilylation.
-1
[
a] The yield was determined by GC. [b] GC studies evidence the formation
The energetic barrier for this step has found to be 8.2 kcal mol
see TSAB in Figure 4). The formation of intermediate B is
exergonic (6.2 kcal mol ) and three different reaction paths may
be followed. Two of them correspond to the CO hydrosilylation
of {SiMe(OSiMe O as side reaction product.
3 2 2
) }
(
-1
2
process, which may occur via the concerted outer sphere
mechanism (a) or in a stepwise fashion, replacing the silylester
ligand by CO (b). Alternatively, the hydride may be transferred
2
to the carbon atom of the carboxylic group forming a silylacetal
1
H NMR studies of the reaction of 6 with HSiMe(OSiMe
3
)
2
and
[
21]
CO
2
(4 bar) at 328 K showed the consumption of the starting
siloxane to afford 2a and the symmetric complex 7, which has
been characterized in solution by multinuclear NMR
spectroscopy (Scheme 3). Analogously to that previously
described for species 5, the highly symmetry observed in the
NMR spectra of complex 7 could be consequence of a fast
equilibrium between species 7a and 7b (Scheme 3, Figure
S.1.8.).
intermediate (path c in Scheme 4).
The outer sphere concerted silyl/hydride transfer from B to CO
2
takes place via a-TSBC (Figure 4), which presents an energetic
-1
barrier of 27.1 kcal mol , excessively high at the low
temperature working conditions (328 K).
Computational Study.
Theoretical studies at the DFT level have been carried out in
order to shed light into selectivity and activity dependence with
the temperature that has been experimentally observed for the
2
1-catalyzed CO -hydrosilylation. Trimethylsilane and the
monoanionic tridentate NSiN (NSiN = bis-{pyridine-2-yloxy})
ligand fac-coordinated to iridium have been used as model
systems. The reported energetic data are Gibbs energies
calculated at 328 K and 1M. Two reaction paths are proposed in
Scheme 4 for explaining the 1-catalyzed CO
paths a and b). In addition, the observation of by-products
experimentally detected at higher temperature (>328 K) has
2
-hydrosilylation
(
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The alternative pathway proposes the exchange of the silyl-ester
2
ligand at B by a CO molecule, leading to b-C (see Figure 5) and
-1
increasing the energy in 3.8 kcal mol . A molecule of silane may
be also coordinated instead, yielding b-D which is the most
stable intermediate calculated so far.
The migratory insertion of one of the C=O bonds in b-C into the
Ir-H bond affords the formate intermediate b-E, which takes
place via b-TSCE. Although direct insertion into the Ir-H bond
gives an excessively high energetic barrier, the hydride transfer
may be assisted by a silane molecule present in the reaction
-1
medium reducing the barrier to 23.0 kcal mol relative to b-D
Figure S.3.1.). The formate ligand of intermediate b-E may
(
promote Si-H bond cleavage of a molecule of silane coordinated
to the metal through b-TSEF, with an energetic barrier of 6.3
-1
kcal mol , leading to the silylformate and recovering the hydride.
Finally, releasing of the product and coordination of a CO
molecule closes the catalytic cycle. Comparison of overall
2
-
1
-
energy barriers of paths a (27.1 kcal mol ) and b (23.0 kcal mol
)
1
shows that the latter is preferred for CO
2
hydrosilylation.
Therefore, while an outer sphere mechanism was proposed for
[
18a]
the 4a-catalyzed CO
2
hydrosilylation,
when using species 1
as catalyst precursor the inner sphere mechanism showed in
Figure 5 has found to be the most kinetically favourable below
Figure 4. Outer sphere mechanism (path a). DFT Gibbs energy profile
-
1
(relative to A and isolated molecules, in kcal mol ) for the A catalyzed Si-H
bond activation in a molecule of HSiMe and concerted CO hydrosilylation.
3
2
328 K.
A mechanistic pathway involving the reduction of the ancillary
trifluoroacetate ligand, which has been experimentally observed
above 328 K, begins at B by migratory insertion of carboxylic
group into the Ir-H bond to give c-C (Figure 6). Once again, this
process is facilitated when a silane molecule assists the hydride
transfer through c-TSBC showing a relative energy of 13.8 kcal
-1
mol (Figure S.3.2.). Then, the overall energetic barrier for path
-1
c is 24.0 kcal mol , relative to b-D. As the difference in the
-1
energetic barrier between paths a and c is only 1.0 kcal mol , it
is expected that at higher temperature a larger amount of
molecules will follow c. After the transition state c-TSBC the
formed silylacetal c-C may evolve quickly to give the
intermediate c-D with a bis(silyl)acetal coordinated to the metal.
This step is characterized by the low energy transition state c-
-1
TSCD (3.6 kcal mol ) which corresponds to a four member
sigma bond methatesis involving the H-Si bond of a coordinated
silane molecule and the Ir-O bond. Intermediate c-D is very
stable and the reversible reaction is energetically unaffordable.
Figure 5. Inner sphere mechanism (path b). DFT Gibbs energy profile (relative
-
1
to A and isolated molecules, in kcal mol ) for path b inner sphere CO
2
hydrosilylation.
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Chemistry - A European Journal
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-
1
Figure 6. DFT Gibbs energy profile (relative to A and isolated molecules, in kcal mol ) for the reduction of trifluoroactetate ancillary ligand following path c.
Although the conversion of bis(silyl)acetal to the corresponding
ethers and even methane is catalyzed by Lewis acids (such as
[13b], [15g]
B(C
the reaction media and larger energetic barriers can be
expected. Hence, the silyl ether (SiR O and an aldehyde
6
F
5
)
3
)
, in our case, such species are not present in
3 2
)
coordinated to the metal, c-E, are obtained by direct reductive
elimination via c-TSDE. The energy barrier for this step is 29.8
-1
kcal mol , much higher than the barriers found for path b. Then,
molecules following this path should retard the catalytic process.
As the aldehyde is very instable due to the CF group, the
3
insertion of the C=O bond into the Ir-H bond through c-TSEF
transition state is very favorable, and leads to the metal alkoxy
complex c-F after coordination of another silane molecule at the
vacancy. Finally, another sigma bond methatesis process
between the H-Si/Ir-O bonds releases the experimentally
2
observed silylether 3, and a CO molecule can coordinate to the
hydride complex forming b-C, which is the starting point of the
catalytic cycle previously described in path b. Therefore,
according to experimental data and theoretical calculations, it is
2
reasonable to propose that the 1-catalyzed CO -hydrosilylation
could follow the catalytic cycle summarized in Figure 7. The first
step consists in the catalyst activation to give the active species
A, which reacts with silane via path b to give species B at T <
328 K. Path c could also take place at higher temperatures
generating by-products (Figure 7).
2
Figure 7. Catalytic cycle proposal for the 1-catalyzed CO -hydrosilylation.
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Chemistry - A European Journal
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Conclusions
3 3 2 2 3
HSi(OSiMe ) , HSiMePh , HSiMe Ph and HSiEt were purchased from
commercial sources and used without further purifications. Compounds
[
18a]
[4d,4e]
[5]
[4d,15b,15g]
1
2
a,
2c,
2d, and 2e
were characterized by H NMR,
2
The results from the studies of the 1-catalyzed CO -
1
3
1
29
1
C{ H} NMR and Si{ H} NMR by comparison with reported data.
hydrosilylation
HSiMe(OSiMe
with
HSiEt
3
,
HSiMe
2
Ph,
HSiMePh
2
,
CAUTION, it is important to avoid the presence of water since Ir-NSiN
3
)
2
and HSi(OSiMe
3
)
3
evidenced
a
clear
species have proven to be highly active catalysts for the hydrogen
dependence of the activity with the nature of the reducing agent,
being HSiMe(OSiMe the most active. Therefore, not only
[25]
generation by hydrolysis of hydrosilanes.
3 2
)
hindrance but also electronic factors influence the reactivity of
the Si-H bond. In all the cases, the reactions were found to be
selective to the formation of the corresponding silylformate.
On the other hand, the activity and selectivity of the solvent-free
General procedure for the NMR studies of the 1-catalyzed reactions
of CO2 with HSiR . A Young NMR tube was charged with 0.14 mmol of
3
the corresponding HSiR
SiEt or Si(OSiMe ), complex 1 (10 mg, 0.014 mmol), C
and hexamethylbenzene (0.016 mmol, as internal standard). The tube
3
(SiR
3
= SiMe(OSiMe
3
)
2
, SiMe
2
Ph, SiMePh
2
,
3
3
)
3
6 6
D
(0.5 mL)
CO
3 2
HSiMe(OSiMe )
2
-hydrosilylation catalytic system based on complex 1 and
1
was pressurized with CO
2
(3 bar) and the reaction was monitored by H
strongly depends on the temperature. Thus,
NMR at the desired temperature.
when the reaction mixtures were heated above 328 K a drastic
decrease of both parameters activity and selectivity of the
reaction was observed. NMR studies of this reaction suggest
that this effect could be attributable to the partial consumption of
the catalytic active species during the catalysis. In this regard,
the reduction of the ancillary trifluoroacetate ligand in 1 to afford
the silyl ether 3 has been observed during the catalytic studies.
Accordingly, theoretical calculations show that the energetically
1
Data for HC(O)O{SiMe(OSiMe
3 2
) } (2a). H NMR plus HSQC (400 MHz,
6 6
C D
, 298 K): δ 7.68 (s, 1H, HC(O)O), 0.27 (s, 3 H, SiMe), 0.15 (s, 18 H,
). C{ H} NMR plus HSQC (100 MHz, C D6, 298K) δ 158.8 (s,
3 6
13
1
OSiMe
HC(O)O), 1.45 (s, OSiMe
HMBC (80 MHz,
OSi(OSiMe ).
29
1
1
29
3
), -2.82 (s, SiMe). Si{ H} NMR plus H- Si
298 K): δ 10.6 (s, OSiMe ), -58.3 (s,
3
C D
6 6
,
3 2
)
1
preferred reaction mechanism is the stepwise CO
hydrosilylation path b, which means, migratory insertion of the
CO C=O bond into metal hydride bond followed by H-Si bond
2
-
Data for HC(O)OSi(OSiMe
3
)
3
(2b). H NMR plus HSQC (400 MHz, C
98 K): δ 7.66 (s, 1H, HC(O)O), 0.18 (s, 27 H, OSiMe
plus HSQC (100 MHz, C 6, 298K) δ 158.4 (s, HC(O)O), 1.40 (s,
6
D
6
,
13
1
2
3
). C{ H} NMR
6
D
2
29
1
1
29
OSiMe
3 6 6
). Si{ H} NMR plus H- Si HMBC (80 MHz, C D , 298 K): δ
heterolytic cleavage of a silane molecule by the metal and the
formate group, respectively. These results evidenced that the
ancillary ligand clearly influences the mechanism of Ir-NSiN
based catalytic processes. Indeed, while the catalytic system
based on 4a (with a triflate ligand) undergoes an outer sphere
1
1.8 (s, 3Si, OSiMe
3
3 3
), -104.0 (s, 1Si, OSi(OSiMe ) ).
1
Selected data for HC(O)OSiMe
2
Ph (2c). H NMR plus HSQC (400 MHz,
29
1
C
6
D
6
1
, 298 K): δ 7.74 (s, 1H, HC(O)O), 0.42 (m, 6H, SiMe
2
). Si{ H} NMR
Ph).
29
plus H- Si HMBC (80 MHz, C
6
D
6
, 298 K): δ 13.0 (s, SiMe
2
2
mechanism for the CO activation, the inner sphere (path b) is
1
kinetically favored at (T < 328 K) when using 1 (with a
trifluoroacetate ligand) as catalyst precursor. In addition, path c,
involving the reduction of the trifluoroacetate ligand is
energetically unfavored but accessible at higher temperatures
Selected data for HC(O)OSiMePh (2d). H NMR plus HSQC (300 MHz,
2
29
1
6 6
C D , 298 K): δ 7.72 (s, 1H, HC(O)O), 0.74 (m, 3H, SiMe). Si{ H} NMR
1
29
plus H- Si HMBC (80 MHz, C
D
6 6
2
, 298 K): δ 1.5 (s, SiMe Ph).
≠
-1
1
Selected data for HC(O)OSiEt
3
(2e). H NMR plus HSQC (400 MHz,
, 298 K): δ 7.79 (s, 1H, HC(O)O), 0.90 (m, 9H, CH
). Si{ H} NMR plus H- Si HMBC (80 MHz, C
).
(
∆G = 1 kcal mol ). Along path c, a higher energetic barrier is
C
CH
6
D
6
3
), 0.66 (m, 6H,
D , 298 K): δ 26.2 (s,
found corresponding to the bis(silyl)acetal reductive elimination.
Then, path c can be considered as a thermally competitive
reaction pathway that consumes the catalytically active species.
29
1
1
29
2
6 6
SiEt
3
2
In conclusion, it has been found that the 1-catalyzed CO -
1
-catalyzed reaction of CF
tube, provided with a gas bubbler, was charged with C
CF CH OH (1.0 mmol, 76.0 µL)), HSiMe(OSiMe (281µL, 1.01 mmol),
and 1 (6.47 mg, 0.01 mmol). The reaction mixture was stirred at 298K for
4 h. After that the resulting C solutions was monitored by NMR. Data
for compound 3: H NMR plus HSQC (400 MHz, C , 298 K): δ 3.82 (q,
F-H = 8.7, 2H, CH ), 0.09 (br, 18H, OSiMe ), 0.05 (br, 3H, SiMe). C{ H}
NMR plus HSQC and HMBC (100 MHz, C 6, 298K): δ 124.9 (q, JF-C
78.2, CF ), 60.93 (q, JF-C = 36.1, CH ), 1.46 (s, OSiMe ), -4.09 (s, SiMe).
Si{ H} NMR plus H- Si HMBC (60 MHz, C , 298 K): δ 9.4 (OSiMe ),
, 298 K): δ -76.7 (t, JF-H = 8.7,
3
CH
2
OH with HSiMe(OSiMe
3
)
2
. A Schlenk
hydrosilylation depends on the nature of the silane. Moreover,
the reasons why an increase in the temperature of the reaction
produces a decrease in the activity and selectivity of the catalytic
system have been clarified.
6
D (0.5 mL),
6
3
2
3 2
)
6 6
D
1
6 6
D
13
1
J
2
3
6
D
=
Experimental Section
2
3
2
3
29
1
1
29
D
6 6
3
General Details. All reactions and manipulations were carried out under
an argon atmosphere using Schlenk-type techniques. Organic solvents
were dried by standard procedures and distilled under argon prior to use
or obtained oxygen- and water-free from a Solvent Purification System
19
6 6
-56.6 (SiMe). F NMR (282.23 MHz, C D
2
2H, CH ).
General procedure for the NMR studies of the symmetric
compounds 5 and 7. A Young NMR tube was charged with complexes
1
mL) and then 0.16 mmol of HSiMe(OSiMe , were added. The tube was
pressurized with CO
(
Innovative Technologies). NMR spectra were obtained on a Bruker AV-
00 and AV-400 spectrometer using TMS as the internal reference. All
chemical shifts (δ) are reported in ppm and coupling constants (J) are
3
or 6 (30 mg, 0.014 mmol). The complexes were dissolved in C
6 6
D (0.5
3 2
)
1
1
13
reported in Hz to apparent peak multiplications. H- H-COSY, C-APT,
2
(4 bar) and heated to 328 K.
1
13
1
13
1
29
H/ C HSQC, H/ C HMBC and H/ Si HMBC sequences were used
for help in the assignments of the H and C spectra. Complex 1 was
prepared according to the reported method.
1
13
2
*
1
[
19]
Data for [Ir{Si(OSiMe
NMR plus COSY, HSQC and HMBC (400 MHz, C
3
)
2
}{κ -Si,H-Si(OSiMe
3
)
2
}(CF
3
CO
2
)(NSiN )] (5). H
3 2
HSiMe(OSiMe ) ,
6 6
D , 298 K): δ 7.91, (d,
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Chemistry - A European Journal
10.1002/chem.201702246
FULL PAPER
J
H-H = 5.1, 2H, CH-py), 6.46 (s, 2H, CH-py), 6.32 (d, JH-H = 5.1, 2H, CH-
py), 1.78 (s, 6H, CH -py), 0.55 (s, 6H, SiMe(OSiMe ), 0.22 (s, SiMe-
ligand, overlapped with the SiMe(OSiMe resonances of traces of
SiMe(OSiMe and CH OSiMe(OSiMe
catalysis), 0.10 (s, 36H, OSiMe
CO
2
after each sample withdrawal and the pressure was returned back to
3
3
)
2
2
the desired value bar using CO .
3 2
)
[
2
25]
{
3
)
2 2
} O
3
3
)
formed during the
). C{ H} NMR plus HSQC and HMBC
6, 298K): δ 162.4 (Cipso-O-py), 150.4 (Cipso-Me-py), 147.0
CH-py), 118.9 (CH-py), 113.2 (CH-py), 20.5 (CH -py), 1.2 (OSiMe ), -1.7
), -1.8 (SiMe-ligand). Si{ H} NMR plus H- Si HMBC
298 K): 12.4 (OSiMe ), 7.5 (SiMe), -58.3
). F NMR (282.23 MHz, C , 298 K): δ -76.4 (s, CF ).
GC-MS samples preparation. The collected samples were diluted in dry
tetrahydrofuran (1.0 mL) and analyzed by quantifiable GC-MS. The
product yield was obtained by comparison of the area corresponding to
the peaks assigned to the reaction products with the area of the peak
13
1
3
(
(
(
(
(
6
100 MHz, C D
3
3
29
1
1
29
SiMe(OSiMe
60 MHz,
SiMe(OSiMe
3 2
)
3 2
corresponding to HSiMe(OSiMe ) .
C
6
D
6
,
δ
3
1
9
3
)
2
D
6 6
3
Crystal structure determination for complex 6. X-ray diffraction data
were collected at 100(2) K on a Bruker Smart APEX area detector
*
Synthesis of [Ir(H){CF
3 2 2 2 2 2
(CF ) CO }(NSiN )(coe)] (6). CH Cl (15 mL)
α
diffractometer with graphite-monochromated MoK radiation (λ=0.71073
was added to a light-protected Schlenk tube containing a mixture of
Å) by using narrow ω rotations (0.3°). Data reduction was carried out with
APEX 2 package, intensities were integrated with SAINT+, SADABS
*
[
Ir(H)(Cl)(NSiN )(coe)]
(334
mg,
0.558
mmol)
and
silver
[26]
[27]
heptafluorobutyrate (180 mg, 0.560 mmol). The resulting suspension was
stirred during 90 min at room temperature, after which the reaction
mixture was filtered out through Celite. The solvent was removed and the
residue was washed with pentane to give the product as a white-off solid.
was used to correct the data from absorption effects. The structure was
[
28]
solved by direct methods with SHELXS-2013,
and refined by full-
included in WINGX
2
[29]
matrix least squares on F with SHELXL-2014
[30]
package.
All hydrogen atoms have been observed in Fourier
Yield (0.34g) (78%). Data for 6: Elemental analysis for C25
30 7 2 4
H F IrN O Si;
difference maps. In order to increase the data over parameter ratio, most
of the hydrogen atoms have been included in the model in calculated
positions and refined with a riding model. Hydride position has been
located in Fourier difference maps, and refined with a restraint in Ir-H
bond length. At the end of the refinement residual density peaks are
observed around the metal atom. They have no chemical sense.
Calculated: C 38.70%; H 3.90%; N 3.61%. Found: C 38.47%; H 3.88%; N
1
3
.67%. H NMR plus COSY, HSQC and HMBC (300 MHz, C
6 6
D , 298 K):
δ 8.89 (d, J = 6.1, 1H, CH-py), 8.62 (d, J = 6.3 Hz, 1H, CH-py), 6.43 and
6
6
2
1
.32 (m, 1H, CH-py), 6.11 (dd, J = 6.1, 1.7 Hz, 1H, CH-py), 5.84 (dd, J =
.3, 1.8 Hz, 1H, CH-py), 5.10 (m, 1H, =CH-coe), 3.69 (m, 1H, =CH-coe),
.59 (m, 1H, -CH
.46 (m, 6H, CH-coe), 1.42 (s, 3H, Me-py), 1.12 (m, 4H, -CH
2
-coe), 1.78 (m, 1H, -CH
2
-coe ), 1.52 (s, 3H, Me-py),
-coe), (0.67,
2
Crystal data for complex 6: C25
H
30
F
7
IrN
.070 x 0.140 x 0.150 mm ; triclinic; P-1; a = 9.8017(7), b = 11.4040(8), c
13.7379(10) Å; α = 88.1970(10), β = 72.2710(10), γ = 69.2860(10)°; Z
2 4
O Si; M = 775.80; white prism;
1
3
1
s, 3H, Si-Me), -14.12 (s, 1H, H-Ir). C{ H} NMR plus HSQC and HMBC
75 MHz, C 6, 298 K): δ 166.2 (s, Cipso-O-py), 164.8 (s, Cipso-O-py),
53.7 (s, Cipso-Me-py), 153.0 (s, Cipso-Me-py), 146.3 (s, CH-py), 145.6 (s,
3
0
=
=
(
1
6
D
3
-3
-1
2; V = 1363.10(17) Å ; ρcalc= 1.890 g cm ; μ = 5.025 mm ; min. and
CH-py), 119.1 (s, CH-py), 118.3 (s, CH-py), 112.4 (s, CH-py), 111.4 (s,
CH-py), 68.7 and 65.8 (s, =CH-coe), 33.4, 32.6, 31.4, 28.1, 26.1 and 25.9
max. transmission factors: 0.455 and 0.645; 2θmax = 56.308°; 13120
reflection collected, 6149 unique [Rint 0.0501]; number of
data/restraints/parameters: 6149/7/376; final GoF: 1.102; R = 0.0632
5319 reflections I> 2σ(I)], wR2 = 0.1556 for all data; largest difference
=
29
1
(
s, CH
NMR plus H- Si HMBC (60 MHz, C
282 MHz, Benzene-d ): δ -80.5 (t, J = 8.8 Hz, 3F), -116.30 (m, 2F), -
25.9 (brs, 2F).
2
-coe), 20.2 (s, Me-py), 20.0 (s, Me-py), -2.0 (s, Si-Me). Si{ H}
1
1
29
19
6 6
D , 298 K): δ 25.3 (Ir-Si F NMR
[
(
1
6
-3
peak: 3.005 e Å .
CCDC 1548786 contains the supplementary crystallographic data for this
paper. These data are provided free of charge by the Cambridge
Crystallographic Data Centre.
2
*
Data for [Ir{Si(OSiMe
(
7
2
3
)
2
}{κ -Si,H-Si(OSiMe
3
)
2
}(CF
3
CF
2
CF
2
CO
, 298 K): δ
.89, (d, JH-H = 5.2, 2H, CH-py), 6.44 (s, 2H, CH-py), 6.34 (d, JH-H = 5.2,
H, CH-py), 1.80 (s, 6H, CH -py), 0.52 (s, 6H, SiMe(OSiMe ), 0.20 (s,
2
)(NSiN )]
1
7). H NMR plus COSY, HSQC and HMBC (400 MHz, C D
6 6
3
3 2
)
Computational details. All DFT theoretical calculations were carried out
SiMe-ligand, overlapped with the SiMe(OSiMe
3
)
2
resonances of traces of
[
31]
[32]
[
25]
using the Gaussian program package.
The B3LYP method,
{
SiMe(OSiMe
catalysis), 0.10 (s, 36H, OSiMe
100 MHz, C 6, 298K): δ 162.4 (Cipso-O-py), 150.4 (C
3 2 2 3 3 2
) } O and CH OSiMe(OSiMe ) formed during the
[
33]
13
1
including the D3 dispersion correction scheme developed by Grimme
3
). C{ H} NMR plus HSQC and HMBC
with Becke Johnson damping, has been used for both energies and
gradient calculations. All atoms were treated with the def2-SVP basis
(
6
D
[
34]
set together with the corresponding core potential for Ir for geometry
optimizations. Energies were further refined by single point calculations
using the def2-TZVP basis set. The “ultrafine” grid was employed in all
calculations. All reported energies are Gibbs free energies referred to a 1
ipso-Me-py), 147.0 (CH-py), 118.9 (CH-py), 113.2 (CH-py), 20.5 (CH
3
-py),
), -1.9 (SiMe-ligand). Si{ H} NMR
, 298 K): δ 12.7 (OSiMe ), 7.6 (SiMe), -
). F NMR (282 MHz, C ): δ -81.0 (t, J = 8.6 Hz,
F), -119.4 (q, J = 8.6 Hz, 2F), -126.9 (brs, 2F).
29
1
1
.2 (OSiMe
3
), -1.8 (SiMe(OSiMe
3 2
)
1
29
plus H- Si HMBC (60 MHz, C
D
6 6
3
1
9
58.4 (SiMe(OSiMe
3
)
2
6 6
D
M
standard state at 328.15 K removing the contribution to the
3
[35]
translational entropy, as indicated by Morokuma et al . The nature of
the stationary points was confirmed by analytical frequency analysis, and
transition states were characterized by a single imaginary frequency
corresponding to the expected motion of the atoms.
General procedure for the solvent-free 6-catalyzed reactions of CO
with HSiMe(OSiMe . A 25 mL batch reactor with Teflon lining inside
was charged with dry HSiMe(OSiMe (3 mL, 11.04 mmol). Then the
reactor was closed, purged with CO gas and heated to the
corresponding temperature. After stabilization to the corresponding
temperature and CO pressure, the reactor was opened under argon
2
3 2
)
3 2
)
2
Acknowledgements
2
atmosphere and complex 6 (85.6 mg, 0.11 mmol), which was weighed in
a glove box, was added. After closing the reactor, the system was purged
Financial support from MINECO/FEDER projects: CTQ2013-
using vacuum and CO
bar. The reaction mixture was stirred with mechanical stirrer. The liquid
samples were taken periodically after releasing the CO pressure, without
2 2
gas. Then the pressure of CO was adjusted to 8
42532-P and CTQ2015-67366-P and DGA/FSE group E07) is
gratefully acknowledged. Dr. V. Polo thankfully acknowledges
the resources from the supercomputer “Memento”, and the
technical expertise and assistance provided by BIFI-ZCAM
2
opening the reactor, using a long needle through the sample withdrawal
valve and analyzed by GC-MS. The reactor was purged with vacuum and
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201702246
FULL PAPER
(
Universidad de Zaragoza). Dr. P. García-Orduña acknowledges
Serrano, A. Rodríguez, Chem. Commun. 2016, 52, 2114-2117; e) P.
Rios, A. Rodriguez, J. López-Serrano, ACS Catal. 2016, 6, 5715-5723.
14] a) T. C. Eisenschmid, R. Eisenberg, Organometallics 1989, 8, 1822 –
CSIC, European Social Fund and Ministerio de Economía y
Competitividad of Spain for a PTA contract. Authors would like
to acknowledge the use of Servicio General de Apoyo a la
Investigación-SAI, Universidad de Zaragoza. The support of
KFUPM is also appreciated.
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Keywords: CO
2
-reduction
•
Iridium
•
Hydrosilylation
•
Homogeneous Catalysis • NSiN ligands
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2
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A mechanism for explaining the
Ir-NSiN-catalyzed hydrosilylation
Alejandro Julián, Jefferson Guzmán,
E. A. Jaseer, Francisco J.
of CO
2
has been proposed
Fernández-Alvarez*, Raquel Royo,
Víctor Polo,* Pilar García-Orduña,
Fernando J. Lahoz, and Luis A. Oro*
Page No. – Page No.
Mechanistic Insights on the
2
Hydrosilylation of CO Catalyzed
by Ir-NSiN Species
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