Mechanistic Insights on the Functionalization of CO2 with Amines and Hydrosilanes Catalyzed by a Zwitterionic Iridium Carboxylate-Functionalized Bis-NHC Catalyst

Article abstract of DOI:10.1002/cctc.201901687

The zwitterionic complex [Cp*IrCl{(MeIm)₂CHCOO}] (1) efficiently catalyzes the selective hydrosilylation of CO₂ to afford the corresponding silylformate. The best reaction performance has been achieved in acetonitrile at 348 K using HSiMe₂Ph. The 1-catalyzed reaction of pyrrolidine with CO₂ and HSiMe₂Ph strongly depends on the CO₂ pressure. At low concentration of CO₂ (1 bar) formation of the corresponding silylcarbamate, by insertion of CO₂ into the Si?N bond of the in situ generated silylamine was observed, while at higher pressure (3 bar) the formamide derivative was obtained as major reaction product. The unexpected formation of pyrrolidin-1-ium formate as intermediate of the reaction the 1-catalyzed of CO₂ with pyrrolidine and HSiMe₂Ph has been observed, and its role in the formation of 1-formylpyrrolidine rationalized. Moreover, a mechanism for the reaction of CO₂ with hydrosilanes, in the presence and in the absence of amines, based on theoretical calculations has been proposed.

Full text of DOI:10.1002/cctc.201901687

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DOI: 10.1002/cctc.201901687
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Mechanistic Insights on the Functionalization of CO2 with
Amines and Hydrosilanes Catalyzed by a Zwitterionic
Iridium Carboxylate-Functionalized Bis-NHC Catalyst
Ana I. Ojeda-Amador,[a] Julen Munarriz,[b] Pablo Alamán-Valtierra,[a] Víctor Polo,[b]
Raquel Puerta-Oteo,[a] M. Victoria Jiménez,[a] Francisco J. Fernández-Alvarez,*[a] and
Jesús J. Pérez-Torrente*[a]
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The zwitterionic complex [Cp*IrCl{(MeIm)2CHCOO}] (1) effi-
ciently catalyzes the selective hydrosilylation of CO2 to afford
the corresponding silylformate. The best reaction performance
has been achieved in acetonitrile at 348 K using HSiMe2Ph. The
1-catalyzed reaction of pyrrolidine with CO2 and HSiMe2Ph
strongly depends on the CO2 pressure. At low concentration of
CO2 (1 bar) formation of the corresponding silylcarbamate, by
insertion of CO2 into the SiÀ N bond of the in situ generated
silylamine was observed, while at higher pressure (3 bar) the
formamide derivative was obtained as major reaction product.
The unexpected formation of pyrrolidin-1-ium formate as
intermediate of the reaction the 1-catalyzed of CO2 with
pyrrolidine and HSiMe2Ph has been observed, and its role in the
formation of 1-formylpyrrolidine rationalized. Moreover,
a
mechanism for the reaction of CO2 with hydrosilanes, in the
presence and in the absence of amines, based on theoretical
calculations has been proposed.
1. Introduction
Nowadays, there is an increasing interest in developing
chemical processes that allow using carbon dioxide as a C1
carbon source for the industrial preparation of value-added
chemicals.[1] The reasons, beyond the well-known problem of
the increase in the concentration of CO2 in the Earth
atmosphere, are related to its low toxicity, abundance and
availability.[1a,b]
The catalytic reaction of CO2 with hydrosilanes has emerged
as a thermodynamically favored procedure that allows for the
reduction of CO2 under mild reaction conditions.[2] Nonetheless,
this methodology faces lack of selectivity, as silylformates are
usually obtained among other products, such as bis(silyl)acetals,
methoxysilanes and methane, in variable relative amounts
(Scheme 1).[3] In this sense, it should be noted that during the
last few years, several examples of effective catalytic systems for
the selective reduction of CO2 with hydrosilanes to give
Scheme 1. Stepwise reduction of CO2 to methane with hydrosilanes.
silylformates[4–10]
bis(silyl)acetals,[11]
methoxysilanes[12]
or
methane[13] have been reported. Moreover, some catalysts have
also been employed for the formylation and/or subsequent
methylation of N–H amine bonds by their reaction with CO2
and hydrosilanes.[14] This methodology has also been recently
employed to prepare unsymmetrical aminals[15] and
spiroindolepyrrolidines.[16]
N-heterocyclic carbenes (NHCs) are now ubiquitous in
organometallic chemistry and catalysis because of their strong
coordination ability and tunable character, which facilitates the
control of the steric and electronic properties at the metal
center.[17,18] The chelate effect derived from coordination of bis-
NHC ligands results in a stable metal-ligand platform with easily
modulable properties.[19] In this context, it is worth noting that
iridium(III) bis-NHC complexes have been reported as active and
selective catalysts for the hydrosilylation of alkynes.[20,21] How-
ever, it draws attention that examples of CO2 hydrosilylation
catalysts based on iridium(III) bis-NHC species have not been so
far reported.
[a] Dr. A. I. Ojeda-Amador, P. Alamán-Valtierra, Dr. R. Puerta-Oteo,
Prof. M. V. Jiménez, Prof. F. J. Fernández-Alvarez, Prof. J. J. Pérez-Torrente
Departamento de Química Inorgánica
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH)
Facultad de Ciencias
Universidad de Zaragoza
Zaragoza 50009 (Spain)
E-mail: paco@unizar.es
perez@unizar.es
[b] Dr. J. Munarriz, Prof. V. Polo
Departamento de Química Física
Instituto de Biocomputación y Física de Sistemas complejos (BIFI)
Facultad de Ciencias
Universidad de Zaragoza
Zaragoza 50009 (Spain)
We have recently focused our attention on the potential of
a carboxylate-functionalized methylene-bridged bis-NHC ligand
for the construction of a versatile metal-ligand platform with
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201901687
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application in catalysis.[22] In this regard, we have shown that
the carboxylate group at the linker provides hemilabile proper-
ties to the ligand, allowing for the stabilization of catalytic
intermediates through the k3-C,C’,O-tridentate coordination
mode.[23] However, in most complexes the bis-NHC ligand
studies. Next, a temperature screening was performed for
experiments containing 1 (5 mol% with respect to the hydro-
silane) in acetonitrile-d3, along with HSiEt3, CO2 (3 bar) and
hexamethylbenzene as internal standard. The reactions were
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monitored by H NMR at 298 K and 348 K, showing the best
exhibits
a
k2-C,C bidentate coordination mode with an
catalytic performance in acetonitrile-d3 at 348 K. 1H NMR
measurements revealed the formation of the respective
silylformate, HC(O)OSiEt3 (2a), reaching almost full HSiEt3
conversion (95%) within 7.5 h at 348 K. This result was
evidenced by a resonance observed at δ 8.12 ppm at the time
that the signal at δ 3.63 ppm corresponding to the Si–H proton
uncoordinated functional group in the skeleton that has shown
to be a reactive site.[22]
As a continuation of our studies on the catalytic applications
of functionalized bis-NHC metal complexes, we have found that
the zwitterionic complex, [Cp*IrCl{(MeIm)2CHCOO}] (1) (MeIm=
3-methylimidazol-2-yliden-1-yl),[22] which features a carboxylate
bridge-functionalized bis-NHC ligand, efficiently catalyzes the
reduction of CO2 with hydrosilanes to selectively afford the
corresponding silylformates. The investigation of the reaction
mechanism has disclosed the key role of the uncoordinated
carboxylate fragment on the catalytic performance. In addition,
the catalytic activity in the reductive functionalization of CO2
with amines and hydrosilanes, which combines both CO2
reduction and CÀ N bond formation, has also been investigated.
Moreover, the reaction mechanism for the reduction of CO2
with hydrosilanes, both in the presence and in the absence of
amines, has been studied by theoretical calculations.
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of the hydrosilane disappears in the H NMR spectrum. More-
over, the influence of the catalyst loading on the activity of this
catalytic system was also investigated. These studies revealed
that when using catalyst loadings of 1.0 and 2.0 mol% under
the same conditions, full HSiEt3 conversion required 72 h and
16 h, respectively.
The success of this reaction prompted us to explore its
generality. Thus, the influence of the hydrosilane nature on the
activity of 1 as CO2 hydrosilylation catalyst was studied. The 1-
catalyzed (1.0 mol%) reactions of CO2 (3 bar) with HSiEt3,
HSiMe2Ph, HSiMePh2 and HSiMe(OSiMe3)2 in acetonitrile-d3 at
348 K with hexamethylbenzene as internal standard were
1
monitored by H NMR spectroscopy (Scheme 2). The results of
these experiments are summarized in Table 1 and the reaction
profiles shown in Figure 1. A first view of the Figure Shows a
significant influence of the hydrosilane nature on the catalytic
activity. Using HSiMe2Ph, full conversion of the starting hydro-
silane to selectively afford the corresponding silylformate, HC
(O)OSiMe2Ph (2b), was accomplished in less than two hours.
However, when HSiMePh2 was used as the reducing agent, a
notable activity decrease was observed, probably due to the
steric hindrance around the Si–H bond. Indeed, under the same
reaction conditions, more than 8 hours were required to fulfill
the conversion of HSiMePh2 into HC(O)OSiMePh2 (2c). In
contrast, HSiEt3 was comparatively much less reactive reaching
a 24% conversion in 10 h. The 1-catalyzed reactions of CO2 with
HSiMe2Ph and HSiMePh2 were highly selective to the formation
of the corresponding silylformate. Conversely, in the case of
HSiEt3 both parameters activity and selectivity decreased, and
the resulting 2a (88%) was always accompanied by CH3OSiEt3
and O(SiEt3)2, which were identified by 1H and 29Si{1H} NMR
2. Results and Discussion
Catalytic hydrosilylation of carbon dioxide. The zwitterionic
iridium(III) derivative 1 has proven to be an active catalyst
precursor for the selective reduction of CO2 with hydrosilanes
to afford the corresponding silylformate (Scheme 2).
The 1-catalyzed reaction of CO2 (3 bar) with HSiEt3 was
investigated as a test bench to explore the viability of using 1
as a catalyst for the hydrosilylation of CO2. A preliminary study
of the catalytic activity of 1 was performed in different
deuterated solvents. Acetonitrile-d3 was found to be the most
suitable solvent for the hydrosilylation of CO2 using catalyst 1,
this addressing solubility of the catalyst and inhibition of
competitive reactions. Conversely, benzene-d6 and chlorinated
solvents such as dichlorometane-d2 and chloroform-d1 did not
show to be a good option for different reasons. In the case of
benzene-d6, the poor solubility of the catalyst precursor
prevented its use, while chlorinated solvents yielded mixtures
of reaction products, including ClSiEt3, as confirmed by NMR
Table 1. [Cp*IrCl{(MeIm)2CHCOO}] (1) catalyzed hydrosilylation of CO2 with
different hydrosilanes.[a]
Entry 1[b]
[mol
Hydrosilane
Time
[h]
Conversion[c] Silylformate[c]
[%]
[%]
%]
1
2
3
4
5
5
1
1
1
1
HSiEt3
HSiEt3
HSiMe2Ph
HSiMePh2
HSiMe
7.5
10
2
8
10
95
24
95
95
–
96
88
100
100
–
(OSiMe3)2
[a] Reaction conditions: hydrosilane (0.153 mmol), CO2 (3 bar), acetonitrile-
d3 (0.50 mL), 348 K, [b] 1.0 mol% or 5.0 mol% with respect to the
hydrosilane, [c] Determined by 1H NMR integration using hexameth-
ylbenzene as internal standard.
Scheme 2. CO2 hydrosilylation catalyzed by the zwitterionic iridium(III)
compound [Cp*IrCl{(MeIm)2CHCOO}] (1) (SiR3 =SiEt3, 2a; SiMe2Ph, 2b;
SiMePh2, 2c; SiMe(OSiMe3)2, 2d).
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Figure 2. Time dependence of the hydrosilylation of CO2 (3 bar) with 1,4-bis
(dimethylsilyl)benzene catalyzed by 1 (1.0 mol%) monitored by 1H NMR in
acetonitrile-d3 at 348 K.
Figure 1. Hydrosilane conversion versus time for the 1-catalyzed (1.0 mol%)
reaction of CO2 (3 bar) with HSiEt3 (2a), HSiMe2Ph (2b) and HSiMePh2 (2c)
monitored by 1H NMR in acetonitrile-d3 at 348 K.
spectroscopy. On the other hand, when HSiMe(OSiMe3)2 was
used as reductant, no reaction was observed during the first
24 hours, and only 12% of the corresponding silylformate HC
(O)OSiMe(OSiMe3)2 (2d) was produced after 40 hours.
To ascertain the role of the uncoordinated carboxylate
fragment on the catalytic activity of 1, the performance of the
related cationic carboxylate-free iridium(III) species, [Cp*IrCl
{(MeIm)2CH2)}][PF6] (5),[25] as catalyst for the hydrosilylation of
CO2 (3 bar) with HSiMe2Ph has also been investigated. This
experiment evidenced that 5 also promotes the formation of
silylformate 2b but showed to be a less active catalyst
precursor than 1 (Figure 3). Indeed, under the same reaction
conditions but using 5 (1.0 mol%) as precatalyst only 39% of
2b was produced after 6 hours (Figure 3). This result demon-
strates that the uncoordinated carboxylate moiety at the linker
of the bis-NHC ligand in 1 plays a key role on the mechanism,
speeding up the CO2 hydrosilylation process.
Theoretical calculations on the 1-catalyzed CO2 hydro-
silylation. To gain insight into the operating mechanism in the
hydrosilylation of CO2 catalyzed by 1, a computational study
using Density Functional Theory (DFT) at the B3LYPÀ D3BJ/def2-
TZVP//def2-SVP (PCM, acetonitrile) level of theory was per-
formed. Within the calculations, we have considered the full
structure of precatalyst 1, as the use of model catalyst systems
for this type of processes where the metallic center bears non-
The higher activity found for HSiMe2Ph in comparison with
HSiEt3 and HSiMePh2, which has been attributed to a balance of
steric and electronic factors, agrees with the behavior reported
for Ir-NSiN[24] and Ir-NSi[12d] catalysts. However, draws attention
the results obtained when using HSiMe(OSiMe3)2. The low
activity found for the catalytic system 1/HSiMe(OSiMe3)2 con-
trast with the activity observed for systems based on Ir-NSiN
and Ir-NSi catalysts, for which the best catalytic performance
was attained using HsiMe(OsiMe3)2. This different behavior is
attributed to steric effects.
Moreover, the 1-catalyzed (1.0 mol%) reaction of 1,4-bis
(dimethylsilyl)benzene with CO2 (3 bar) in acetonitrile-d3 at
1
348 K has been studied. Monitoring of the reaction by H NMR
evidenced the formation of 1-(dimethylsilylformate)-4-(dimeth-
ylsilyl)benzene (3) and 1,4-bis(dimethylsilylformate)benzene (4)
(Scheme 3). At the early stage of the reaction, a continuous
growth of the resonances corresponding to compounds 3 and
4 was observed. However, after 40 minutes the concentration of
4 increased to detriment of that of 1,4-bis(dimethylsilyl)benzene
and the mono-silylformate derivative 3 (Figure 2). Notice that a
98% conversion was attained after 90 minutes to give a mixture
of 4 (72%) and 3 (26%).
Scheme 3. Reaction of CO2 with 1,4-bis(dimethylsilyl)benzene in acetonitrile-
d3 at 348 K catalyzed by the zwitterionic iridium species [Cp*IrCl
{(MeIm)2CHCOO}] (1) (1.0 mol%).
Figure 3. Hydrosilylation of CO2 (3 bar) with HSiMe2Ph to HC(O)OSiMe2Ph
(2b) catalyzed by [Cp*IrCl{(MeIm)2CHCOO}] (1) and [Cp*IrCl{(MeIm)2CH2)}]
[PF6] (5) (1.0 mol%) monitored by 1H NMR in acetonitrile-d3 at 348 K.
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innocent ligands has been severely discouraged.[26] The most
active reductant, HSiMe2Ph, was used as a model for the
hydrosilane. For the sake of clarity, the different reaction
intermediates and transition structures will be referred to by
capital letters, starting by precatalyst 1, which corresponds to
structure A. The proposed reaction mechanism is provided in
Figure 4.
of 39.6 kcalmolÀ 1, which is not at all attainable at the reaction
temperature (348 K). The result of the activation step is C, which
constitutes the active species for the subsequent catalytic
cycles. On balance, the effective energy span for the catalyst
activation is 25.4 kcalmolÀ 1, according to the model proposed
by Kozuch et al.,[30] which corresponds to the ligand exchange
between the hydrosilane and the chloro ligand.
1
2
3
4
5
6
7
The theoretical calculations revealed the presence of a pre-
activation step (see left part of Figure 4) that is required to form
the catalytic active species (C). First, a ligand exchange between
the chloro ligand and a hydrosilane takes place, via TSAB.
Notice that within the transition structure, the Cp* ligand
changes its coordination mode from η5 to η1, which is possible
thanks to its coordination modes flexibility.[27,28] The change in
Cp* hapticity has also been proposed by other authors in
similar catalytic processes.[29] This step requires to surmount an
energy barrier of 25.4 kcalmolÀ 1, which is affordable under the
reaction conditions. As a result, the chloro ligand is released,
upon formation of intermediate B. This species is only
1.6 kcalmolÀ 1 less stable than the starting structure (A).
The next step involves the activation of a hydrosilane
molecule by means of a ligand assisted Si–H bond cleavage.[24]
This process is possible because of the high oxophilicity of
silicon atoms in conjunction with the presence of an available
oxygen atom in the uncoordinated carboxylate group at the
bis-NHC ligand linker. For this step to proceed, an energy
barrier of 18.7 kcalmolÀ 1 must be overcome, dictated by
transition structure TSBC. Therefore, a hydride species, inter-
mediate C, is produced with concomitant formation of a silyl-
carboxylate moiety at the linker of the bis-NHC ligand. Notice
that this step is very favorable from a thermodynamic point of
view, as C has a relative energy of À 19.3 kcalmolÀ 1. Moreover,
this step is kinetically irreversible, as the opposite process (the
recovering of B) would require surmounting an energy barrier
The computed reaction profile for the catalytic cycle is
presented in the right part of Figure 4. The first reaction step
consists on the carbon dioxide activation by means of the
hydride migration from the metallic center to the CO2
molecule.[31] The process requires to surmount an energy barrier
of 23.4 kcalmolÀ 1, dictated by transition structure TSCD. As a
result, the CO2 molecule transforms into a formate ligand, which
is k1-O-coordinated to the iridium center, yielding intermediate
D. The silylformate product is generated by means of a σ-
complex assisted metathesis (σ-CAM) between the IrÀ O bond of
the metallic complex and the Si–H bond of the hydrosilane, as
previously proposed for other processes of hydrosilanes
activation by related complexes.[32,33,34] Interestingly, within the
transition state, the Cp* ligand coordinates in a η1 manner,
leaving a coordination vacancy. Consequently, the presence of
a coordinating solvent, such as acetonitrile, able to stabilize the
compound is essential for the reaction to proceed. Then, upon
acetonitrile coordination, intermediate E is produced, which can
undertake the σ-CAM process via TSEC. Remarkably, the
effective energy barrier for this process is only 15.2 kcalmolÀ 1,
being determined by the energy difference between transition
structure TSEC and intermediate D. As a result, the final reaction
product is yielded, and the catalytic active species C is
regenerated. Notice that, contrarily to the previous reaction
steps, this one is kinetically irreversible, as the inverse process
would have to overcome an energy barrier of 33.3 kcalmolÀ 1 to
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Figure 4. DFT calculated Gibbs free energy profile at 348 K (in kcal·molÀ 1) relative to A and the isolated molecules.
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take place, which is not affordable under the experimental
reaction conditions (348 K).
conjunction with proper catalysts could be further reduced to
the corresponding methylamines (Scheme 5).[14,34] In view of the
catalytic activity of 1 as CO2 hydrosilylation catalyst, we decided
to explore its potential as catalyst for the reductive functional-
ization of CO2 with secondary amines using HSiMe2Ph as
reductant.
1
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5
6
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Overall, the thermodynamics for the catalytic cycle is highly
favorable, with ΔG equals to À 19.2 kcalmolÀ 1 (energy differ-
ence between C and C+products). The effective activation
energy is 23.4 kcal molÀ 1 and corresponds to the CO2 fixation
step. This value agrees with the experimental observations as
the reaction proceeds in good yields at a temperature of 348 K.
On the other hand, the effect of the solvent is crucial as
contributes to the stabilization of the transition state involved
in the σ-CAM process. The catalytic cycle proposed in Scheme 4
summarizes the mechanistic information obtained from exper-
imental and theoretical studies.
Preliminary studies of the 1-catalyzed reaction of pyrroli-
dine, c-H(NC4H8), with CO2 (1 bar) and one equivalent of
HSiMe2Ph in acetonitrile-d3 (0.5 mL) at 348 K showed the
consumption of the starting amine to yield roughly an
equimolar mixture of the corresponding silylamine, Me2PhSi
(NC4H8) (6a), and silylcarbamate, Me2PhSiOC(O)(NC4H8) (7a)
(Table 2, entry 1). Further addition of CO2 (up to 3 bar) to this
mixture (Figures SI61 and SI62) resulted in the full trans-
formation of silylamine 6a into the corresponding silylcarba-
mate 7a, which was quantitatively obtained under this reaction
conditions (Scheme 6). This outcome suggests that the 1-
catalyzed dehydrogenative silylation of pyrrolidine could be the
driving force behind the formation of silylcarbamate 7a.[35]
Prompted by these promising results, we decided to study
the 1-catalyzed (1.0 mol%) reactions of HSiMePh2 with various
secondary amines, HNR2 (NR2 =c-(NC4H8), c-(NC5H10), c-(NC4H8O),
NEt2, NMe(c-hex) and NiPr2) in acetonitrile-d3 at 348 K. All of
them, except HNiPr2, were quantitatively transformed into the
corresponding silylamine derivatives (Scheme 7). The reaction
profiles, determined by 1H NMR, displaying the hydrosilane
conversion versus time are shown in Figure 5.
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1H NMR studies of the reaction of 1 with 4 equiv. of
HSiMe2Ph at 348 K in acetonitrile-d3, in absence and in presence
of CO2 (3 bar), showed a tiny resonance at δ-16.28 ppm
corresponding to traces of an Ir-hydride species which could
not be characterized.
1-Catalyzed reaction of secondary amines with carbon
dioxide and HSiMe2Ph: Synthesis of silylcarbamates. The
catalytic reaction of secondary and primary amines with CO2
using hydrosilanes as reductants has emerged as a useful
methodology for the preparation of formamides, which in
A first view of Figure 5 shows that amines of similar nature
exhibit comparable profiles. The use of secondary amines
Table 2. Effect of the CO2 pressure in the 1-catalyzed reaction of
pyrrolidine with CO2 and HSiMe2Ph.[a][b]
Entry
CO2
[bar]
6a
[%]
7a
[%]
8
[%]
9
[%]
f.a.[c]
[%]
1
2
3
4
1
1.5
2
45
13
–
46
53
61
52
–
–
–
15
18
9
21
–
13
24
30
3
–
–
[a] Conditions: HSiMe2Ph (0.5 mmol), c-H(NC4H8) (0.5 mmol) in acetonitrile-
d3 (0.50 mL) at 348 K for 15 h. [b] 1.0 mol% of 1 with respect to the
hydrosilane. (%) Determined by 1H NMR integration using hexameth-
ylbenzene as internal standard. [c] f.a.=free amine.
Scheme 4. Proposed mechanism for the 1-catalyzed hydrosilylation of CO2.
Scheme 5. Catalytic N-formylation and N-methylation of amines with CO2
and hydrosilanes.
Scheme 6. Functionalization of CO2 with pyrrolidine and HSiMe2Ph catalyzed
by 1 (1.0 mol%).
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1
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Scheme 8. Synthesis of silylcarbamates by insertion of CO2 into the SiÀ N
bond of silylamines.
8
9
Scheme 7. Dehydrogenative silylation of amines with HSiMe2Ph catalyzed by
1 (1.0 mol%).
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addition, the 29Si{1H} NMR spectra has a resonance around δ
10.0–11.4 ppm also down-field shifted compared to that of the
corresponding silylamines.
N-formylation of amines: influence of the CO2 pressure
and amine concentration. The catalytic N-formylation of
amines by their reaction with CO2 and hydrosilanes is a subject
of great interest.[36,39] The reaction proceeds by a series of fast
and complex chemical equilibria, involving carbamate salts,[37]
silylformates[38a–e] or silylcarbamates[39] as intermediates.
For the case of the 1-catalyzed reaction of pyrrolidine with
CO2 and HSiMe2Ph, it has been observed that the CO2 pressure
and amine concentration strongly influence the reaction out-
come (Tables 2 and 3). To shed light on the reasons behind this
behavior a series of NMR experiments were carried out.
Figure 5. Hydrosilane conversion versus time for the 1-catalyzed (1.0 mol%)
reaction of HSiMe2Ph with HNR2 (NR2 =c-(NC4H8), c-(NC5H10), c-(NC4H8O), NEt2
and NMe(c-hex)) monitored by 1H NMR in acetonitrile-d3 at 348 K.
1
As we have just commented, H NMR evidenced that under
low CO2 pressure (1 bar), silylamine, Me2PhSi(NC4H8) (6a) (45%),
and silylcarbamate, Me2PhSiOC(O)(NC4H8) (7a) (46%), are
obtained as products (Table 2, entry 1).
containing cyclic or acyclic aliphatic substituents such as c-H
(NC4H8), c-H(NC5H10) c-H(NC4H8O), HNEt2 and HNMe(c-hex), gave
conversions close to 95% into the corresponding silylamines
Me2PhSiNR2 (6a–6e) within 10 hours of reaction. However,
there are some significant differences between them, the cyclic
amines c-H(NC4H8) and c-H(NC5H10) require a preactivation step
(induction time) of around 1 hour, whilst the reaction with c-H
(NC4H8O) starts immediately. In this sense, a 73% conversion
was attained for c-H(NC4H8O) in 5 hours whereas c-H(NC4H8) and
c-H(NC5H10) afforded only 42% conversion at the same reaction
time. HNEt2, HNMe(c-hex) and c-H(NC4H8O) present a similar
profile being the most active among the studied derivatives. No
reaction was observed with bulkier amines such as HNiPr2 after
10 h under the same conditions. The silylamines 6a–6e were
characterized by multinuclear NMR spectroscopy (see Exper-
imental Section and Supporting Information).
As the pressure of CO2 is increased to 1.5 bar, a mixture of
products, silylamine 6a, silylcarbamate 7a, formamide HC(O)
(NC4H8) (8) and free amine was obtained (Figure SI63 and
Table 2, entry 2). When the pressure of CO2 was successively
increased to 2 and 3 bar, silylcarbamate 7a and formamide 8
along with carbamic acid derivative HOC(O)(NC4H8) (9), were
observed (Table 2, entries 3 and 4). The decrease in the amount
of silylamine 6a and the increase in that of silylcarbamate 7a at
1.5 bar of CO2 agrees with the reactivity studies of Schemes 6–8
that have shown that 7a is formed by CO2 insertion into 6a
(Table 2, entry 2). In this context, it is necessary to consider that
an equilibrium between free amine and carbamic acid is
stablished under reaction conditions (see Figures SI65, SI66).[40]
Table 3. Effect of the concentration of amine and hydrosilane in the 1-
catalyzed reaction of pyrrolidine with CO2 and HSiMe2Ph.[a,b]
In agreement with our previous observations, the addition
of CO2 (3 bar) to the solutions containing the freshly prepared
silylamines Me2PhSiNR2 (6a–6e) at 298 K allows for the
quantitative formation of the corresponding silylcarbamates
Me2PhSiOC(O)NR2 (7a–7e) (Scheme 8). These products, 7a–7e,
Entry
Amine
[mmol]
HSiMe2Ph [mmol]
7a
[%]
8
[%]
9
[%]
10
[%]
1
were fully characterized by H, 29Si and 13C{1H} NMR spectro-
scopy, presenting distinctive signals that demonstrate the
formation of silylcarbamates. Specifically, 1H NMR spectra of the
species Me2PhSiOC(O)NR2 show a signal at δ 0.55 ppm for the
methyl groups of the silyl moiety, which appears down-field
shifted with respect to the parent silylamines (δ 0.30–
0.36 ppm). Moreover, 13C{1H} NMR spectra exhibit the typical
signal for the CO2 insertion products at around δ 155 ppm. In
1
0.50
0.15
0.15
0.30
0.50
0.15
0.30
0.15
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30
74
68
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18
–
–
–
20
–
–
2
3[c]
4
50
[a] Reaction conditions: CO2 (3 bar), in acetonitrile-d3 (0.50 mL) at 348 K for
15 h, [b] 1.0 mol% of 1 with respect to the reagent in lower concentration.
(%) Determined by 1H NMR integration using hexamethylbenzene as
internal standard, [c] 3% of unidentified products.
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Thus, formation of silylcarbamate 7a could also result from the
reaction of carbamic acid with HSiMe2Ph (pathway a, Scheme 9).
In agreement with this possibility, it is worth mentioning that it
has been proven that, as other iridium complexes,[41] 1 catalyzes
the dehydrogenative silylation of formic acid with HSiMe2Ph to
give the silylformate 2b and H2 (Figure SI64). On the other
hand, the equilibrium between free amine and carbamic acid is
displaced to the formation of 9 as the CO2 pressure is increased
(Figure SI65) which agrees with the presence of carbamic acid 9
in the reaction medium (Table 2, entries 3 and 4).
Under 3 bar of CO2 the selectivity in formamide product of
the 1-catalyzed reaction of pyrrolidine with the stoichiometric
amount of HSiMe2Ph is surprisingly low, reaching a maximum
of 30% with silylcarbamate as the major reaction product
(Table 2, entry 4). In this regard, it is worth mentioning that no
reaction of silylcarbamate 7a with HSiMe2Ph has been observed
at 348 K. Therefore, it is reasonable to propose that in this case
formamide 8 should exclusively be generated by reaction of
silylformate 2b intermediate (not observed) with pyrrolidine
(pathway b, Scheme 9).
To unravel the reasons behind the low selectivity in the
formamide product, we have carried out a study on the
influence of the relative hydrosilane and amine concentration.
Experiments performed at constant CO2 pressure (3 bar)
evidenced that the reaction outcome is strongly dependent on
the relative concentration of the amine and the silane (Table 3).
Thus, reducing the amine and hydrosilane concentration from
0.50 mmol to 0.15 mmol resulted in the formation of formamide
8 (74%) and a 20% of the formate salt, [H2(NC4H8)][HCO2] (10)
(Table 3, entry 2).[42] The formation of 10 could only be
explained by reaction of formic acid with pyrrolidine (Fig-
ure SI67). In this regard, although the presence of silanol was
not detected, it is known that silanols react with silylformate in
presence of amine to generate formic acid and siloxane.[36b] The
decrease in the amine concentration results in a decrease in the
amount of carbamic acid 9, as a consequence of the shift of the
amine/carbamic acid equilibrium, and in turn of that of the
silylcarbamate 7a (6%). This outcome supports the participa-
tion of silylformate 2b as a reaction intermediate, which further
reacts with pyrrolidine through pathway b (Scheme 9) to give
formamide 8 and silanol. Thus, under this reaction conditions,
CO2 hydrosilylation to silylformate 2b is favored over silylative
dehydrogenation of carbamic acid to 7a. Therefore, a significant
increase on the amount of produced formamide is observed.
Interestingly, against our initial expectations, the catalyst-
free reaction of freshly prepared silylformate 2b, which was
synthesized according to a catalyst-free methodology,[43] with
one equivalent of pyrrolidine quantitatively yielded a 1:1
mixture of formamide 8 and the formate salt 10 (Scheme 10).
This result further supports that at least a 20% of the
formamide obtained from the 1-catalyzed reaction of pyrroli-
dine with CO2 (3 bar) and HSiMe2Ph is formed via pathway b of
Scheme 9 (Table 3, entry 2).
The increase of the relative hydrosilane concentration
resulted in the disappearance of the formate salt 10 and an
increase of that of silylcarbamate 7a (29%) (Table 3, entry 3).
This result suggests that 10 further reacts with HSiMe2Ph to
give silylformate 2b and H2. In addition, we have found that the
reaction of the formate salt 10 (independently prepared) with
HSiMe2Ph in acetonitrile-d3 at 298 K affords 1-formylpyrrolidine
8 and O(SiMe2Ph)2 with hydrogen evolution (NMR evidence,
Figure SI68). On the other hand, the shift of the amine/carbamic
acid equilibrium when increasing the relative pyrrolidine
concentration affords a large amount of carbamic acid 9 (50%)
and silylcarbamate 7a (30%) and, therefore, a significant
decrease of formamide 8 (20%) (Table 3, entry 4). Interestingly,
although the formate salt is not observed at the end of the
reaction, formation of 10 has been detected by 1H NMR
spectroscopy along the reaction. This result indicates that under
the reaction conditions pathway b (Scheme 9) is also an active
route for the formation of 8 (Figure SI69).
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The set of reactions that take place in the hydrosilylation of
CO2 with HSiMe2Ph in the presence of pyrrolidine are shown in
Scheme 11. Silylformate 2b reacts with pyrrolidine to give 1-
formylpyrrolidine 8 and silanol, HOSiMe2Ph. The later reacts
with 2b to afford formic acid and siloxane O(SiMe2Ph)2. In
presence of pyrrolidine formic acid is neutralized to yield the
pyrrolidin-1-ium formate salt 10. Salt 10 reacts with HSiMe2Ph
to produce hydrogen, 2b and pyrrolidine. Under these con-
ditions, 2b and pyrrolidine reacts to produce 1-formylpyrroli-
dine and silanol, restarting the cycle (Scheme 11).
Proposed Mechanistic Pathways for the 1-catalyzed Reactions
of Pyrrolidine with CO2 and Hydrosilanes
To further understand the reaction mechanism that operates in
the amine silylation process, a computational study at the same
level of theory as in the formylation process was performed. We
have compared the energy profile for the carboxylate silylation
(catalyst 1 preactivation, steps B to C in Figure 4) with the 1-
catalyzed dehydrogenative amine silylation (B to C’ in Figure 6).
Scheme 9. Proposed mechanistic pathways for the 1-catalyzed dehydrogen-
ative silylation of pyrrolidine-1-carboxylic acid versus 1-catalyzed N-formyla-
tion of pyrrolidine with CO2 and HSiMe2Ph.
Scheme 10. Reaction of pyrrolidine with HC(O)OSiMe2Ph (2b).
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energy barrier for the formylation process is too high to be
overcome, and thus the CO2 insertion in the silylamine, the
dehydrogenative silylation product, takes place. It should be
mentioned that the insertion of CO2 into the SiÀ N of silylamines
is a thermodynamically and kinetically favored process.[35,44]
Therefore, accordingly with the experimental observations,
under low CO2 pressure silylamine and silylcarbamate should be
the only expected reaction products (Table 2, entry 1). However,
increasing the CO2 pressure should favor the CO2 activation
pathway via TSCD’ (Figure 6), enabling the formation of
silylformate, which evolves according to Scheme 11. Therefore,
the observed change in selectivity when increasing the CO2
pressure (Table 2) is a consequence of the direct involvement of
CO2 in the formation of the transition state TSCD’.
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1H NMR studies of the 5-catalyzed reaction of pyrrolidine
with HSiMe2Ph under low CO2 pressure (1 bar) showed a
comparable product selectivity to that of 1, which is agreement
with the DFT calculations (Figure 6), suggesting that the
carboxylate moiety at the bis-NHC ligand in 1 plays a minor role
in the 1-catalyzed reactions of pyrrolidine with CO2 and
hydrosilanes.
Scheme 11. Reactions involved in the 1-catalyzed hydrosilylation of CO2 with
HSiMe2Ph in the presence of pyrrolidine
In particular, we have explored the direct amine silylation
through a concerted process (via TSBC’), as previously proposed
for similar processes.[35] As is shown in Figure 6, the amine
silylation is more energetically favorable than the carboxylate 3. Conclusions
silylation process with transition structure TSBC’ being
21.6 kcalmolÀ 1 lower in energy than TSBC.
The zwitterionic complex [Cp*IrCl{(MeIm)2CHCOO}] (1) catalyzes
From C’ there are two possible pathways: the first one
consists in the CO2 insertion into the Ir–H bond to generate a
formate intermediate D’; which would result into the silylfor-
mate accordingly with Figure 4. This process requires to
surmount an energy barrier of 24.6 kcalmolÀ 1, dictated by
TSCD’. The second pathway involves the elimination of the
silylamine and formation of an iridium-dihydrogen intermediate
F, the energy barrier for this step is TSC’F (9.9 kcalmolÀ 1). These
findings allow to rationalize the reaction selectivity. At the
reaction temperature (348 K) and lower CO2 pressure, the
the selective hydrosilylation of CO2 with HSiMe2Ph, HSiMePh2,
HSiEt3, HSiMe(OSiMe3)2 and 1,4-bis(dimethylsilyl)benzene to the
corresponding silylformate. The best reaction performance has
been achieved in acetonitrile at 348 K using HSiMe2Ph as
reductant. It has been proven that the carboxylate moiety at
the bis-NHC ligand in 1 plays a relevant role on the catalytic
mechanism. Indeed, the related carboxylate-free iridium cati-
onic species [Cp*IrCl{(MeIm)2CH2)}]+ (5) shows a poor activity as
CO2 hydrosilylation catalyst compared to 1. In agreement with
this experimental finding, theoretical calculations have revealed
Figure 6. DFT calculated Gibbs free energy profile at 348 K (in kcal·molÀ 1) relative to A and the isolated molecules for the amine silylation and posterior CO2
activation.
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(pyrrolidine, piperidine, morpholine, Et2NH, Me(c-hexyl)NH and
iPr2NH) and silane reagents (HSiMe2Ph, HSiMePh2, HSiEt3 and 1,4-
(HMe2Si)2(C6H4)) used in the catalytic experiments were purchased
from commercial sources and dried over 4 Å molecular sieves prior
to use.
that the carboxylate moiety is involved in the precatalyst
activation step leading to the formation of an iridium(III)
hydrido active species having a silyl-carboxylate moiety at the
bis-NHC linker.
1
2
3
4
5
6
7
8
9
Complex 1 promotes the reaction of pyrrolidine with CO2 in
the presence of HSiMe2Ph as reductant although in a non-
selective way. The catalyst performance has proven to be highly
dependent on both the CO2 pressure and the relative concen-
tration of pyrrolidine and HSiMe2Ph. Thus, under a pressure of
CO2 of 3 bar and both reagents in low concentration the
corresponding formamide was obtained as the major product,
while at lower CO2 pressure (1 bar) a mixture of silylamine and
silylcarbamate was formed. Moreover, the 1-catalyzed dehydro-
genative silylation of in situ generated carbamic acid leading to
silylcarbamate is also operative when increasing the pressure of
CO2. In full agreement with these observations, compound 1
efficiently catalyzes the dehydrogenative silylation of a range of
secondary amines with HSiMe2Ph to afford the corresponding
silylamine derivatives which further react with CO2 (3 bar) to
quantitatively afford the corresponding silylcarbamates, formed
by CO2 insertion into the SiÀ N bond. Theoretical calculations
confirm that the dehydrogenative silylation of amines is
preferred to the CO2 hydrosilylation at low CO2 pressure.
However, increasing the CO2 concentration favors the CO2
hydrosilylation and therefore the formation of formamide.
Interestingly, an amine-promoted pre-activation pathway lead-
ing to an iridium(III) hydrido active species, without the
participation of carboxylate moiety at the bis-NHC linker, has
been identified.
Reductive functionalization studies have shown that the
formation of 1-formylpyrrolidine from silylformate and pyrroli-
dine might not be as simple as it had previously been reported.
In fact, the catalyst-free reaction of silylformate with pyrrolidine
in acetonitrile afforded an equimolar mixture of 1-formylpyrro-
lidine and the pyrrolidin-1-ium formate salt. This salt is
presumably formed from the formic acid generated by reaction
of the produced silanol with silylformate in presence of
pyrrolidine. Interestingly, the pyrrolidin-1-ium formate salt
reacts with excess of hydrosilane to give 1-formylpyrrolidine
and O(SiMe2Ph)2 with hydrogen evolution.
General procedure for the NMR studies of the 1-catalyzed
reactions of CO2 with HSiR3. A Young NMR tube was charged with
0.153 mmol of the corresponding HSiR3 (SiR3 =SiMe2Ph, SiMePh2 or
SiEt3, complex 1 (0.89 mg, 0.00153 mmol, 1 mol%; for experiment
with 5 mol%, 4.45 mg, 0.00765 mmol), CD3CN (0.5 mL) and
hexamethylbenzene (0.016 mmol, as internal standard). The tube
was pressurized with CO2 (3 bar) and the reaction was monitored
by 1H NMR at 348 K.
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1
Data for HC(O)OSiEt3 (2a). H NMR (300 MHz, CD3CN, 348 K): δ 8.14
(s, 1H, HC(O)O), 1.02 (t, 9H, CH3), 0.84 (q, 6H, CH2). 1H NMR
(300 MHz, CD3CN, 298 K): δ 8.12 (s, 1H, HC(O)O), 0.99 (t, 3H, À CH3),
1
0.80 (m, 2H, À CH2À ). H-29Si HMBC (80 MHz, CD3CN, 298 K): δ 26.64
(s).
1
Data for HC(O)OSiMe2Ph (2b). H NMR (300 MHz, CD3CN, 348 K): δ
8.15 (s, 1H, HC(O)O), 7.67 (m, 2H, o-Ph), 7.46 (m, 3H, m, p-Ph), 0.60
1
(s, 6H, SiMe2). H NMR (300 MHz, CD3CN, 298 K): δ 8.13 (s, 1H, HC(O)
1
O), 7.67 (m, 2H, o-Ph), 7.45 (m, 3H, m, p-Ph), 0.58 (s, 6H, Me). H-29Si
HMBC (80 MHz, CD3CN, 298 K): δ 13.78 (s).
1
Data for HC(O)OSiMePh2 (2c) H NMR (300 MHz, CD3CN, 348 K): δ
8.25 (s, 1H, HC(O)O), 7.68 (m, 4H, o-Ph), 7.46 (m, 6H, m, p-Ph), 0.90
1
(s, 6H, SiMe2). H NMR (300 MHz, CD3CN, 298 K): δ 8.23 (s, 1H, HC(O)
O), 7.66 (m, 4H, o-Ph), 7.45 (m, 6H, m, p-Ph), 0.89 (s, 6H, Me). 29Si{1H}
NMR (80 MHz, CD3CN, 298 K): δ 1.68 (s).
1-Catalyzed reaction of CO2 with 1,4-bis(dimethylsilyl)benzene. A
Young NMR tube was charged with 0.0765 mmol of 1,4-bis(dimeth-
ylsilyl)benzene, complex 1 (0.89 mg, 0.00153 mmol), CD3CN (0.5 mL)
and hexamethylbenzene (0.016 mmol, as internal standard). The
tube was pressurized with CO2 (3 bar) and the reaction was
monitored by 1H NMR at 348 K.
Data for {HC(O)OSiMe2)(C6H4)(SiHMe2)} (3). 1H NMR (300 MHz,
CD3CN, 348 K): δ 8.14 (s, 1H, HC(O)O), 7.65 (d, 2H, o-Ph), 7.57 (d, 2H,
m-Ph), 4.42 (m, 1H, SiÀ H), 0.36 (d, 6H, SiMe2), 0.59 (s, 6H, SiMe2). 29Si
{1H} NMR plus 1H-29Si HMBC (80 MHz, CD3CN, 298 K): δ 13.71 (s,
HCO2Si), À 0.79 (s, SiÀ H).
Data for {HC(O)OSiMe2)}2(C6H4) (4). 1H NMR (300 MHz, CD3CN,
348 K): δ 8.14 (s, 2H, HC(O)O), 7.72 (s, 4H, o-, m-Ph), 0.60 (s, 12H,
1
SiMe2). H NMR plus HSQC (300 MHz, CD3CN, 298 K): δ 8.12 (s, 2H,
HC(O)O), 7.70 (d, 4H, m-, p-Ph), 0.58 (s, 12H, SiMe2). 29Si{1H} NMR
plus 1H-29Si HMBC (80 MHz, CD3CN, 298 K): δ 13.71 (s).
General procedure for the NMR studies of the 1-catalyzed
reaction of secondary amines with HSiMe2Ph. A Young NMR tube
was charged with 0.5 mmol of HSiMe2Ph, 0.5 mmol of the
corresponding secondary amine HNR2 (NR2 =c-(NC4H8), c-(NC5H10),
Experimental Section
General procedures. All reactions and manipulations were per-
formed under an argon atmosphere using Schlenk-type techniques
or in a glovebox (MBraun UNIlab). Organic solvents were dried by
standard procedures and distilled under an argon atmosphere prior
c-(NC4H8O), NEt2, NMe(c-hexyl), NiPr2), complex
1
(2.9 mg,
0.005 mmol, 1 mol%), CD3CN (0.5 mL) and hexamethylbenzene
(0.016 mmol, as internal standard) and the reaction monitored by
1H NMR at 348 K.
to use or obtained oxygen- and water-free from
a solvent
purification system (Innovative Technologies). Acetonitrile-d3 was
1
1
dried with CaH2 and degassed under argon prior to use. H NMR,
Data for Me2PhSi(NC4H8) (6a). H NMR (300 MHz, CD3CN, 298 K): δ
13C-APT NMR, and 29Si NMR spectra were recorded on a Bruker AV-
300 and AV-400 spectrometer using hexamethylbenzene as internal
reference. All chemical shifts (δ) are reported in ppm and coupling
constants (J) are reported in Hz and refer to apparent peak
7.55 (m, 2H, o-Ph), 7.36 (m, 3H, m,p-Ph), 2.98 (t, 4H, NÀ CH2À ), 1.71
(q, 4H, À CH2À ), 0.33 (s, 6H, À CH3). 1H-29Si HMBC NMR (80 MHz,
CD3CN, 298 K): δ-4.68 (s). 13C{1H} NMR (100.6 MHz, CD3CN, 298 K): δ
139.8 (Ph Cipso), 133.6 (Ph Co), 128.9 (Ph Cp), 127.7 (Ph Cm), 46.8
(NÀ CH2À ), 26.7 (À CH2À ), 3.1 (À CH3).
1
1
1
multiplicities. H/13C-HSQC, H/13C-HMBC and H/29Si-HMBC sequen-
ces were used to assign the H and 13C{1H} spectra. The iridium(III)
1
1
complexes [Cp*IrCl{(MeIm)2CHCOO}] (1)[22] and [Cp*IrCl{(MeIm)2CH2}]
[PF6] (5)[25] were prepared following the reported procedure. Amine
Data for Me2PhSi(NC5H10) (6b). H NMR (300 MHz, CD3CN, 298 K): δ
7.57 (m, 2H, o-Ph), 7.36 (m, 3H, m,p-Ph), 2.86 (t, 4H, NÀ CH2À ), 1.58
1
(q, 2H, À CH2À ), 1.39 (q, 4H, À CH2À ), 0.30 (s, 6H, À CH3). H-29Si HMBC
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NMR (80 MHz, CD3CN, 298 K): δ-2.84 (s). 13C{1H} NMR (100.6 MHz,
CD3CN, 298 K): δ 140.0 (Ph Cipso), 133.7 (Ph Co), 128.9 (Ph Cp), 127.8
(Ph Cm), 46.5 (NÀ CH2À ), 27.7 (À CH2À ), 25.4 (À CH2À ), À 2.7 (À CH3).
(100.6 MHz, CD3CN, 298 K): δ 155.45 (À CO2À ), 137.9 (Ph Cipso), 134.4
(Ph Co), 130.8 (Ph Cp), 128.8 (Ph Cm), 57.0, 55.6 (NMe), 31.3, 30.7
(À CH2À ), 29.5, 28.9 (À CHÀ ), 26.6 (À CH2À ), 26.2 (À CH2À ), À 0.9 (À CH3).
1
2
3
4
5
6
7
8
9
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Data for Me2PhSi(NC4H8O) (6c). H NMR (300 MHz, CD3CN, 298 K): δ
7.58 (m, 2H, o-Ph), 7.38 (m, 3H, m,p-Ph), 3.49 (t, 4H, NÀ CH2À ), 2.86 (t,
General procedure for the NMR Studies of the 1-catalyzed
Reaction of Secondary Amines with CO2 and HSiMe2Ph.
1
2H, OCH2À ), 0.33 (s, 6H, À CH3). H-29Si HMBC NMR (80 MHz, CD3CN,
298 K): δ À 1.37 (s). 13C{1H} NMR (100.6 MHz, CD3CN, 298 K): δ 140.0
(Ph Cipso), 134.8 (Ph Co), 130.1 (Ph Cp), 128.8 (Ph Cm), 69.1 (NÀ CH2À ),
46.6 (À OCH2À ), À 2.2 (À CH3).
Procedure a: A Young NMR tube was charged with 0.5 mmol of
HSiMe2Ph, 0.5 mmol of c-H(NC4H8), CO2 (1, 2 or 3 bar), complex 1
(2.9 mg, 0.005 mmol, 1 mol%), CD3CN (0.5 mL) and hexameth-
ylbenzene (0.016 mmol, internal standard), and the reaction
monitored by 1H NMR at 348 K.
1
Data for Me2PhSiNEt2 (6d). H NMR (300 MHz, CD3CN, 298 K): δ 7.59
(m, 2H, o-Ph), 7.36 (m, 3H, m,p-Ph), 2.87 (q, 4H, À CH2À ), 1.00 (t, 6H,
À CH3), 0.33 (s, 6H, À CH3). 1H-29Si HMBC NMR (80 MHz, CD3CN,
298 K): δ 2.33 (s) 13C{1H} NMR (100.6 MHz, CD3CN, 298 K): δ 141.9
(Ph Cipso), 135.0 (Ph Co), 130.0 (Ph Cp), 128.9 (Ph Cm), 41.5 (NÀ CH2À ),
16.6 (À CH3), 0.73 (À CH3).
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Data for HC(O)(NC4H8) (8). H NMR (300 MHz, CD3CN, 348 K): δ 8.19
(s, 1H, HCO), 3.46 (t, 2H, À NCH2À ), 3.32 (t, 2H, À NCH2À ), 1.86 (m, 4H,
À CH2À )
Data for HOC(O)(NC4H8) (9). 1H NMR (300 MHz, CD3CN, 348 K): δ 11.8
(s, 1H, COOH), 3.22 (t, 4H, À NCH2À ), 1.82 (m, 4H, À CH2À ).
1
Data for Me2PhSiNMe(c-hex) (6e). H NMR (300 MHz, CD3CN, 298 K):
δ 7.58 (m, 2H, o-Ph), 7.34 (m, 3H, m,p-Ph), 2.76 (m, 1H, NÀ CHÀ ), 2.41
(s, 3H, NMe), 1.71 (m, 2H, À CH2À ), 1.57 (m, 4H, À CH2À ), 1.23 (m, 2H,
Procedure b: A Young NMR tube was charged with 0.153 mmol of
1
À CH2À ) 1.04 (m, 2H, À CH2À ), 0.31 (s, 6H, À CH3). H-29Si HMBC NMR
HSiMe2Ph, 0.153 mmol of c-H(NC4H8), CO2 (3 bar), complex
1
(80 MHz, CD3CN, 298 K): δ À 2.21 (s). 13C{1H} NMR (100.6 MHz,
CD3CN, 298 K): δ 142.0 (Ph Cipso), 134.9 (Ph Co), 130.0 (Ph Cp), 128.8
(Ph Cm), 57.2 (NMe), 33.2 (À CH2À ), 29.2 (À CHÀ ), 27.5 (À CH2À ), 27.0
(À CH2À ), 0.9 (À CH3).
(0.89 mg, 0.00153 mmol, 1 mol%), CD3CN (0.5 mL) and hexameth-
ylbenzene (0.016 mmol, as internal standard) and the reaction
monitored by 1H NMR at 348 K.
1
Data for [H2(NC4H8)][HCO2] (10). H NMR (300 MHz, CD3CN, 348 K): δ
General procedure for the NMR studies of the 1-catalyzed
reaction of silylamines with CO2 (3bar). A Young NMR tube
containing the corresponding silylamine (6a–6g; prepared from
the reaction of hydrosilane Me2PhSiH and the corresponding
secondary amine HNR2) was pressurized with CO2 (2 or 3 bar) and
the reaction monitored by 1H NMR at 298 K.
10.44 (s, 2H, H2N+À ), 8.49 (s, 1H, HCOOÀ ), 3.15 (m, 4H, NCH2À ), 1.82
(m, 4H, À CH2À CH2À ).
Computational Methods. DFT calculations were performed using
the Gaussian09 software package, D.01revision.[45] The B3LYP
exchange-correlation functional,[46] in conjunction with the D3BJ
dispersion correction scheme[47] was applied in energies and
gradient calculations, in conjunction with the “ultrafine” grid. The
df2-SVP basis set was used for all the geometry optimization, and
energies were further refined by means of single point calculations
using the def2-TZVP basis set.[48] Solvent corrections were included
in all the reported calculations through the PCM model, as
implemented in the G09 suite.[49] The reported Gibbs energies were
calculated at 348 K, removing the translational entropy contribution
from dissolved species as indicated by Morokuma et al.[50] The
nature of the stationary points has been confirmed by analytical
frequency analysis, and transition states were characterized by
calculation of reaction paths following the intrinsic reaction
coordinate.
Data for Me2PhSiOC(O)(NC4H8) (7a). 1H NMR (300 MHz, CD3CN,
298 K): δ 7.70 (m, 2H, o-Ph), 7.42 (m, 3H, m,p-Ph), 3.36 (m, 2H,
NÀ CH2À ), 3.27 (m, 2H, NÀ CH2À ) 1.83 (m, 4H, À CH2À ), 0.55 (s, 6H,
1
À CH3). H-29Si HMBC NMR (80 MHz, CD3CN, 298 K): δ 9.74 (s). 13C{1H}
NMR (100.6 MHz, CD3CN, 298 K): δ 154.3 (À CO2À ), 137.9 (Ph Cipso),
134.5 (Ph Co), 130.9 (Ph Cp), 128.8 (Ph Cm), 47.4, 46.7 (NÀ CH2À ), 26.4,
25.9 (À CH2À ), À 0.9 (À CH3).
Data for Me2PhSiOC(O)(NC5H10) (7b). 1H NMR (300 MHz, CD3CN,
298 K): δ 7.69 (m, 2H, o-Ph), 7.43 (m, 3H, m,p-Ph), 3.40 (m, 4H,
NÀ CH2À ), 1.56 (m, 2H, À CH2À ), 1.51 (m, 4H, À CH2À ), 0.55 (s, 6H,
1
À CH3). H-29Si HMBC NMR (80 MHz, CD3CN, 298 K): δ 10.39 (s). 13C
{1H} NMR (100.6 MHz, CD3CN, 298 K): δ 154.63 (À CO2À ), 137.81 (Ph
C
ipso), 134.5 (Ph Co), 130.9 (Ph Cp), 128.8 (Ph Cm), 46.5, 45.1 (NÀ CH2À ),
26.7, 26.4 (-CH2-), 25.0 (À CH2À ), À 0.99 (À CH3).
Data for Me2PhSiOC(O)(NC4H8O) (7c). 1H NMR (300 MHz, CD3CN,
298 K): δ 7.69 (m, 2H, o-Ph), 7.43 (m, 3H, m,p-Ph), 3.58 (t, 4H,
OÀ CH2À ), 3.43 (dt, 4H, NÀ CH2À ), 0.56 (s, 6H, À CH3). 1H-29Si HMBC
NMR (80 MHz, CD3CN, 298 K): δ 11.41 (s). 13C{1H} NMR (100.6 MHz,
CD3CN, 298 K): δ 154.8 (À CO2À ), 137.5 (Ph Cipso), 134.5 (Ph Co), 131.0
(Ph Cp), 129.0 (Ph Cm), 67.2 (OÀ CH2À ), 46.0, 44.5 (NÀ CH2À ), À 1.1
(À CH3).
Acknowledgements
The authors express their appreciation for the financial support
from MICINN/FEDER projects PGC2018-099383-B-100 and
CTQ2016-75884-P, and the Regional Government of Aragón/
FEDER 2014-2020 “Building Europe from Aragón” (group E42_
17R). J.M. acknowledges the financial support provided by the
Spanish “Ministerio de Ciencia, Innovación y Universidades”
(FPU14/06003). In addition, the resources from the supercomputer
“Memento”, technical expertise and assistance provided by
BIFIZCAM (Universidad de Zaragoza) are acknowledged.
1
Data for Me2PhSiOC(O)NEt2 (7d). H NMR (300 MHz, CD3CN, 298 K):
δ 7.68 (m, 2H, o-Ph), 7.43 (m, 3H, m,p-Ph), 3.27 (m, 4H, À CH2À ), 1.12
(m, 6H, À CH3), 0.55 (s, 6H, À CH3). 1H-29Si HMBC NMR (80 MHz,
CD3CN, 298 K): δ 10.07 (s). 13C{1H} NMR (100.6 MHz, CD3CN, 298 K): δ
155.2 (À CO2À ), 137.9 (Ph Cipso), 134.4 (Ph Co), 130.9 (Ph Cp), 128.8 (Ph
Cm), 42.8, 42.2 (À CH2À ), 14.6, 13.8 (À CH3), À 1.0 (À CH3).
1
Data for Me2PhSiOC(O)NMe(c-hex) (7e). H NMR (300 MHz, CD3CN,
348 K): δ 7.70 (m, 2H, o-Ph), 7.45 (m, 3H, m,p-Ph), 3.89 (m, 1H,
À CHÀ ), 2.80 (bs, 3H, -NMe), 1.82 (m, 2H,À CH2À ), 1.66 (m, 3H, À CH2À ),
Conflict of Interest
1
1.41 (m, 4H, À CH2À ), 1.15 (m, 1H, À CH2À ), 0.57 (s, 6H, À CH3). H-29Si
The authors declare no conflict of interest.
HMBC NMR (80 MHz, CD3CN, 298 K): δ 10.07 (s). 13C{1H} NMR
ChemCatChem 2019, 11, 1–13
www.chemcatchem.org
10
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!

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Keywords: CO2 · hydrosilylation · formylation · iridium · NHC
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ChemCatChem 2019, 11, 1–13
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[42] Salt 10 has been characterized by comparison of the 1H and 13C{1H}
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1.85 (m, 4H, À CH2À ).
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Manuscript received: September 5, 2019
Accepted manuscript online: September 10, 2019
Version of record online: ■■■, ■■■■
ChemCatChem 2019, 11, 1–13
www.chemcatchem.org
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Understanding CO2 reduction: The
zwitterionic iridium complex
Dr. A. I. Ojeda-Amador, Dr. J. Munarriz,
P. Alamán-Valtierra, Prof. V. Polo,
Dr. R. Puerta-Oteo, Prof. M. V. Jiménez,
Prof. F. J. Fernández-Alvarez*, Prof. J. J.
Pérez-Torrente*
catalyzes the reduction of CO2 with
hydrosilanes to selectively afford the
corresponding silylformates with the
carboxylate fragment playing a key
role. The mechanism of the reductive
functionalization of CO2 with amines
and hydrosilanes has been investi-
gated both experimentally and by
theoretical calculations
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Mechanistic Insights on the Func-
tionalization of CO2 with Amines
and Hydrosilanes Catalyzed by a
Zwitterionic Iridium Carboxylate-
Functionalized Bis-NHC Catalyst