Iron Catalyzed CO2 Activation with Organosilanes

Article abstract of DOI:10.1007/s10562-018-2305-8

Abstract: Iron nanoparticles generated in situ from [Fe₃(CO)₁₂] catalyzed CO₂ reduction in the presence of Et₃SiH as a reductant and tetrabutylammonium fluoride as a promoter to yield silyl formate (1s) under relatively mild reaction conditions. Additionally, when CO₂ hydrosilylation was carried out in water, the product of CO₂ reduction was formic acid. Additionally, a similar reaction using [Fe₃(CO)₁₂] as a catalytic precursor, PhSiH₃ as a reductant, and CO₂ in the presence of amines allowed the immediate formation of ureas at room temperature. Here, CO₂ acted as a C1 building block for value-added products.

Full text of DOI:10.1007/s10562-018-2305-8

Catalysis Letters
https://doi.org/10.1007/s10562-018-2305-8
Iron Catalyzed CO Activation with Organosilanes
2
Tamara Jurado‑Vázquez · Juventino J. García1
1
Received: 9 November 2017 / Accepted: 7 January 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Iron nanoparticles generated in situ from [Fe (CO) ] catalyzed CO reduction in the presence of Et SiH as a reductant and
3
12
2
3
tetrabutylammonium fluoride as a promoter to yield silyl formate (1s) under relatively mild reaction conditions. Additionally,
when CO hydrosilylation was carried out in water, the product of CO reduction was formic acid. Additionally, a similar
2
2
reaction using [Fe (CO) ] as a catalytic precursor, PhSiH as a reductant, and CO in the presence of amines allowed the
3
12
3
2
immediate formation of ureas at room temperature. Here, CO acted as a C1 building block for value-added products.
2
Graphical Abstract
O
O
Et SiH
3
Et Si
O
H
3
HO
H
H
2
O
CO2
RT
Fe NPs
O
NH
2
N
H
N
H
Keywords Carbon dioxide · Reduction · Silane · Formate · Iron · Nanoparticles · Urea · Catalysis
1
Introduction
demand for energy and, consequently, CO emissions are not
2
yet under control [1]. That is why efforts are focused on the
Nowadays, one of the most biggest pollution problems
related to the greenhouse effect is the increasing concentra-
development of new and efficient methods that use CO as a
2
building block for organic molecules, and its hydrogenation
to produce formate or methanol is badly needed [2].
Despite many reports using a variety of transition metals
tion of atmospheric CO . This is associated with the global
2
acting as catalysts for the activation of CO , the most active
2
Dedicated to Professor Peter M. Maitlis on occasion of his 85th
birthday.
systems often require low-abundance, expensive, and toxic
metals, such as ruthenium, rhodium, palladium, or iridium
[
3–7], which are used for the synthesis of the most active
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2305-8) contains
supplementary material, which is available to authorized users.
catalytic systems. Thus, carrying out the activation of CO
at industrial levels is still not profitable.
2
Sustainable catalysis aims to eliminate the above prob-
lems in the field through the development of novel, envi-
ronmentally benign, cost-effective catalytic pathways. This
approach involves the use of metal complexes based on
*
Juventino J. García
juvent@unam.mx
1
Facultad de Química, Universidad Nacional Autónoma de
México, 04510 Mexico City, Mexico
Vol.:(0
1
123456789)
3

T. Jurado-Vázquez, J. J. García
inexpensive, earth-abundant, environmentally friendly ele-
ments, such as magnesium, iron, copper and zinc [8].
In this context, there are relatively few reports of hydro-
promoter (entry 1) the yield was low and the main reduction
product was triethylmethoxysilane (2s) with a 7% yield. The
use of tetrabutylammonium fluoride (TBAF) as a reaction
promoter (entry 4) allowed a higher conversion and pro-
duced silyl formate 1s (17% yield). Compound 1s is a valu-
able product since it is a precursor of a variety of organic
compounds [2, 22, 23].
genation of CO with iron catalysts [9–16]. It is worth men-
2
tioning that reports by Beller in 2010 [13], Milstein in 2011
[
15], and Gonsalvi in 2016, produced formates. In all of these
cases, the CO reduction was carried out in the presence of
2
hydrogen gas (H ). Due to the inherent risks related to han-
Considering these results, it was decided to use TBAF as
a reaction promoter; therefore, a variety of reaction condi-
2
dling H , a safe alternative is to replace this reactive gas with
2
alternative sources of hydrogen such as silanes; however, pro-
tions for CO hydrosilylation were assessed (see Table S1
2
ducing residues from the corresponding silanes. In the area
and 2S). It was found that the optimum reaction conditions
of hydrosilylation of CO , a study using iron was reported
were 60 °C and 60 psi after 24 h.
2
by Cantat and colleages [17], in 2014 using [Fe(acac) ] as
Under these conditions, the amount of reaction promoter
(TBAF) was further optimized (Table 2). It was found that
2
a catalyst precursor and PP as ancillary ligand. Recently
3
the photo- and electrocatalytic reduction of CO to produce
the use of 5 mol % of TBAF:Et SiH gave the best conversion
2
3
methane, HCOOH and carbon monoxide, using an iron tet-
raphenylporphyrin catalyst has been disclosed [18].
In 2013, Feng and colleagues [19] reported the complete
to produce 1s. Most likely, TBAF had an important role in
the hydrosilylation of CO due to the formation of a very
2
−
strong Si–F bond, which released a very reactive H .
reduction of CO to produce methane at 200 °C in 75 h.
Likewise, the amount of catalytic precursor [Fe (CO) ]
2
3
12
In 2016, Kang and colleagues [20] reported the complete
amount was then optimized (Table 3), and optimal amount
reduction of CO to produce methane using UV radiation
found to be 0.5 mol % versus Et SiH.
2
3
and iron nanoparticles (Fe–NPs) as a catalyst.
As shown in Table 2, there was a strong dependence on
Our group recently explored CO activation in the pres-
the amount of TBAF and CO hydrosilylation. Consequently,
2
2
ence of a commercially available catalytic precursor, such
it was necessary to carry out a control test without the use of
the iron precursor (Table 3, entry 4). Only a small yield of
product 1s was observed; therefore, the iron catalytic precur-
sor was necessary for an efficient reaction.
as [Ru (CO) ] [21]. With the aim to carry out the CO
3
12
2
transformation in an efficient way with low costs, we report
the catalytic activation of this polluting gas with iron using
[
Fe (CO) ] as a readily available and inexpensive precursor
Additionally, a mercury drop test allowed us to iden-
tify the decomposition of the used catalytic precursor that
formed Fe–NPs [24] (Table 3, entry 5) [25]. The reac-
3
12
of Fe–NPs.
tion yield decreased significantly from 50 to 10% for CO
2
2
Results and Discussion
hydrosilylation.
To confirm the formation of Fe–NPs in the reaction mix-
ture, centrifugation of the mother liquid of the catalytic mix-
ture led to a residue which was characterized by transmission
electron microscopy (TEM) (Fig. 1). As observed in Fig. 1,
The reduction of CO with [Fe (CO) ] as a catalytic precur-
2
3
12
sor and Et SiH as a reductant was initially assessed with a
3
variety of different promoters (Table 1). Without the use of a
Table 1 Promoter screening
O
1
mol%
0°C / 48 h
[
Fe ]
3
(CO)12
8
O
O
3
Et SiH + CO2
Et Si O
3
H
+
Et Si
3
Me
+ Et Si
SiEt3
3
THF 10mL
100psi
1s
2s
3s
Yield (%)a
Entry
Promoter (10 mol%)
1
s
2s
3s
Total
1
2
3
4
None
NaB(Ph)4
n.d.
n.d.
n.d.
17
7
n.d.
5
10
n.d.
8
17
0
13
94
Et B
3
TBAF
n.d.
77
a
All yields were determined by GC-MS, using Et SiOCOH as internal std
3
1
3

Iron Catalyzed CO Activation with Organosilanes
2
Table 2 TBAF optimization
1
mol %
TBAF X mol %
[Fe
3
(CO)12
]
O
O
Et SiH + CO2
Et Si
SiEt3
Et Si F
3
Et Si
O
H
3
3
3
6
0 °C / 24 h
3s
4s
6
0psi
1s
THF
Yield (%)a
Entry
TBAF (mol %)
1
s
3s
4s
Total
1
2
3
4
10
5
2.5
0
33
40
0
57
43
37
2
2
2
n.d.
92
85
39
n.d.
n.d.
n.d.
a
All yields were determined by GC-MS, using Et SiOCOH as internal std
3
Table 3 [Fe (CO) ] amount optimized
3
12
X mol %
TBAF 5 mol %
0 °C / 24 h
THF
[Fe
3
(CO)12
]
O
O
Et SiH + CO2
Et Si
SiEt3
Et Si F
3
Et Si O
H
3
3
3
6
3s
4s
6
0psi
1s
Yield (%)a
Entry
[Fe (CO) ] (mol %)
3 12
1
s
3s
4s
Total
1
2
3
4
5
1
40
50
13
9
43
47
56
58
42
2
2
3
4
4
85
99
72
71
56
0.5
0.3
0.0
0.5
b
10
a
All yields were determined by GC-MS, using Et SiOCOH as internal std
Mercury drop test Hg(0):Fe(0) 5:1
3
b
the formation of iron nanoparticles was confirmed in the
reaction medium. These nanoparticles had an average size
of 5 nm (Graphic 1).
as a reducing agent (Table 4). With this methodology,
1,3-bis(benzyl)urea (1a) was obtained as the main product.
The best conversion was obtained using PhSiH (entry 2). A
3
These Fe–NPs could be recycled several times
control experiment (entry 3) demonstrated that the reaction
requires the use of the corresponding reducing agent.
This reaction was optimized in a variety of reaction con-
ditions (see SI, Tables S3 and S4). The best yields were
(
Graphic 2). However, the conversion to product 1s sub-
stantially decreased after the first recycling, yielding a final
TON=232. This was probably due to agglomeration of the
Fe–NPs.
obtained at 60 °C and 20 mol % of PhSiH at atmospheric
3
In an effort to substitute the use of a solvent (THF) for a
green solvent, the reaction was carried out in water using a
pressure (Scheme 2) with a urea yield of 46%.
Considering the previous results, we explored the same
reaction without a solvent (THF) (i.e., under neat con-
ditions) (Table 5). A good conversion was obtained in
entry 1 for the preparation of 1a. However, we noticed
the immediate formation of a white solid soon after all
reagents mixed (entry 2), resulting in the immediate
formation of 1a at room temperature at a 46% yield. In
entry 3, the reaction was performed without the use of
type of silane not prone to hydrolysis (Et SiH) (Scheme 1).
3
With this method, formic acid was obtained (38% isolated
yield).
With the aim of using CO as a building block for organic
2
molecules [26–28], the reactivity of CO in the presence
2
of benzylamine was studied. Here, [Fe (CO) ] was used
3
12
as a catalytic precursor, along with two different silanes
1
3

T. Jurado-Vázquez, J. J. García
Fig. 1 TEM of final reaction mix
50
45
40
35
30
25
20
15
10
5
0
50
45
4
3
3
2
2
1
0
5
0
5
0
5
ꢀ
ꢁ
ꢂꢃ
ꢂꢂ
10
5
ꢄꢅ
0
Fresh
1°
2°
3°
Cycle
1
2
3
4
5
6
7
8
9
NPs size (nm)
Graphic 2 Recycling of Fe–NPs
Graphic 1 Size distribution of Fe–NPs
[
Fe
3
(CO)12] 0.5%mol
TBAF 5%mol
O
[
Fe (CO) ], suggesting that the use of such a catalytic
Et SiH + CO
3
2
(60 psi)
3
12
HO
H
H O / 60°C / 24h
2
precursor is mandatory in order to obtain the correspond-
ing urea.
3
8%
Scheme 1 Reduction reaction in water
3
Conclusions
was carried out in water to produce formic acid under rela-
tively mild conditions. Additionally, CO was obtained in
2
The catalytic reduction of CO was carried out using iron
2
the presence of benzylamine, generating 1,3-bis(benzyl)
nanoparticles in a catalytic amount of 0.5 mol%. TBAF
urea and PhSiH using the same catalytic precursor at
3
was a necessary reaction promotor at 5 mol%, and Et SiH
3
room temperature.
was a reductant, yielding silyl formate. The CO reduction
2
1
3

Iron Catalyzed CO Activation with Organosilanes
2
Table 4 CO activation to yield ureas
2
O
N
H
N
H
[
3
(CO)12
Fe ]
O
NH2
0.5mol%
+
CO2
+
R SiH
1a
(60 psi)
3
THF / 24h
N
N
H
C
O
2a
3a
Yield (%)a
Entry
R SiH
3
1a
2a
3a
Total
1
2
3
Et SiH
PhSiH3
None
0
78
0
n.d.
6
n.d.
n.d.
4
n.d.
0
88
0
3
aAll yields were determined by GC-MS, using N-benzylmethylamine as internal std
O
[
Fe
.5mol%
PhSiH
20 mol%
3
3
(CO)12
]
0
N
N
H
N
NH2
C
+
CO2
H
(1atm)
THF / 24h
O
6
0 °C
46%
2%
2a
1a
Scheme 2 CO activation to produce urea
2
Table 5 Urea preparation using neat conditions
O
[
Fe
0.5mol%
PhSiH 20 mol%
3
(CO)12
]
N
N
H
N
NH2
+
CO2
H
C
(1atm)
3
O
1a
2a
Yield (%)a
Entry
Conditions
1
a
2a
Total
1
2
3
17 h/60 °C
T=0/R.T.
T=0/R.T.
76
46
n.d.
3
n.d.
n.d.
79
46
0
aAll yields were determined by GC-MS, using N-benzylmethylamine as internal std
4
Experimental Section
an argon atmosphere (Praxair 99.998) or by using high-
vacuum and standard Schlenk techniques under an argon
atmosphere. Triethylsilane (99% purity, Aldrich), carbon
dioxide (99.998% purity, supplied by Praxair), phenylsilane
(97% purity, Aldrich), triethylborane (95% purity, Aldrich),
triiron dodecacarbonyl (99% Aldrich), TBAF (98% Aldrich),
4.1 General Considerations
Unless otherwise stated, all processes were performed
using an MBraun glovebox (<1 ppm H O and O ) under
2
2
1
3

T. Jurado-Vázquez, J. J. García
benzylamine (80% Aldrich), THF (J. T. Baker, reagent
grade) were dried and distilled from sodium/benzophenone.
All reagents for the catalytic reactions were loaded in the
glove box. The GC-MS determinations were made using
an Agilent 5975C system equipped with a 30 m DB-5MS
capillary column (0.25 mm i.d.; 0.25 µm).
heated at 60 °C for 24 h at 60 psi of CO , and the reaction
2
mixtures were analyzed by GC/MS.
4.6 Hg Drop Test
This reaction was performed as in the previously described
reaction, but an additional 25.8 mg (0.129 mmol) of Hg and
4
.2 Reactivity of CO with Et SiH Catalyzed
CO at 60 psi was used. This was then heated at 60 °C for
2
3
2
by [Fe (CO) ] in Presence of Different
24 h. The reaction mixture was filtered on Celite and ana-
3
12
Promoters
lyzed by GC/MS.
All reactions were performed in a 100 mL stainless steel Parr
autoclave. The reactor was loaded with 8.6 mg (0.017 mmol)
of [Fe (CO) ], 200.0 mg (1.724 mmol) of Et SiH, and
4
.7 Nanoparticles Isolation and Characterization
3
12
3
This was carried out in a reaction under optimized condi-
tions (60 °C, 60 psi of CO , 24 h, 200 mg of Et SiH, 22.5 mg
0
.172 mmol (16.8 mg) of Et B, 58 mg of Na(BPh) , 44.9 mg of
3 4
2
3
TBAF, and 10 mL of THF. This sample was then pressurized
of TBAF, 4.3 mg of [Fe (CO) ], and 10 mL of THF). After
3
12
with CO . All the reagents were loaded in a glove box except
2
the reaction time, the autoclave was opened under an argon
atmosphere, and all reactions mixtures were transferred to
a 50 mL Schlenk flask equipped with a Rotaflo valve. Then,
the reaction mixture was evaporated to dryness using a high
vacuum for 4 h. TEM was used for further characterization.
CO . CO was added by using a transfer hose at 100 psi. The
2
2
mixture was heated at 80 °C in a silicon oil bath for 48 h. The
final reaction mixtures were analyzed by GC/MS.
4.3 Optimized Reaction Conditions
4
.8 Iron Nanoparticles Recycling
Using a 100 mL stainless steel Parr autoclave, the reac-
tor was loaded with 8.6 mg (0.017 mmol) of [Fe (CO) ],
3
12
This reaction was carried out under optimized conditions
60 °C, 60 psi of CO , 24 h, 200 mg of Et SiH, 22.5 mg of
2
00 mg (1.724 mmol) of Et SiH, 44.9 mg (0.172 mmol) of
3
(
2
3
TBAF, 10 mL of THF, and CO at a desired pressure. The
2
TBAF, 4.3 mg of [Fe (CO) ], and 10 mL of THF). After
3
12
reactor was heated at different temperatures and reaction
times. After this, the reactor was cooled to room temperature
and depressurized in a hood. The final reaction mixture was
analyzed by GC/MS.
the reaction, the autoclave was opened under an argon
atmosphere. The reaction mixtures were analyzed by GC/
MS, and all reaction mixtures were transferred to a 50 mL
Schlenk flask equipped with a Rotaflo valve. Then, the reac-
tion mixture was evaporated to dryness using a high vacuum
for 4 h. After that, the dirty nanoparticles were used in the
4.4 Load of TBAF
next reaction (cycle 1) with 200 mg of Et SiH and 22.5 mg
3
All reactions were performed in a 100 mL stainless steel Parr
autoclave. The reactor was loaded with 8.6 mg (0.017 mmol)
of TBAF. The reaction was heated at 60 °C for 24 h at 60 psi
of CO , and the reaction mixture was analyzed by GC/MS.
2
of [Fe (CO) ], 200 mg (1.724 mmol) of Et SiH, and 44.9,
3
12
3
This procedure was repeated two more times (for second
2
2.5, and 11.2 mg (0.172, 0.086, 0.043 mmol, respectively)
and thirty cycle).
of TBAF. This represented a catalysis load of 10, 5, and
2
.5 mol%, respectively, in 10 mL of THF. All reactions were
4.9 CO Reduction Using Water as a Solvent
2
heated at 60 °C for 24 h at 60 psi of CO . The reaction mix-
2
tures were analyzed by GC/MS.
This was performed using optimized conditions (60 °C,
6
4
0 psi of CO , 24 h, 200 mg of Et SiH, 22.5 mg of TBAF,
2 3
.3 mg of [Fe (CO) ], and 10 mL of H O). After the reac-
3
12
2
4
.5 Load of [Fe (CO) ]
3 12
tion time, KOH was added to the reaction mixture to obtain
potassium formate.
All reactions were performed in a 100 mL stainless steel Parr
autoclave. The reactor was loaded with 22.5 mg (0.86 mmol)
4.10 Reactivity of CO with Benzylamine
2
of TBAF, 200 mg (1.724 mmol) of Et SiH, and 8.6, 4.3,
3
and 2.5 mg (0.0172, 0.0086, 0.0051 mmol, respectively) of
This was catalyzed by [Fe (CO) ] in the presence of differ-
3
12
[
Fe (CO) ]. This represented a catalysis load of 1, 0.5, and
3
12
ent reductants. All reactions were performed in a 100 mL
0
.3 mol%, respectively, in 10 mL of THF. All reactions were
1
3

Iron Catalyzed CO Activation with Organosilanes
2
stainless steel Parr autoclave, and the reactor was loaded
with 2.3 mg (0.0045 mmol) of [Fe (CO) ], 0.9259 mmol
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3
12
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.12 Synthesis of Urea at Atmospheric Pressure
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1
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Acknowledgements We thank CONACYT 178265 and PAPIIT-
DGAPA-UNAM 202516 for their financial support of this work. T.
J-V. thanks CONACYT (583731) for graduate studies grant. We also
thank Dr. Alma Arévalo for her technical assistance.
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