Cobalt Catalysts for Alkene Hydrosilylation under Aerobic Conditions without Dry Solvents or Additives

Article abstract of DOI:10.1002/ejic.201801068

Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. Here, we report a simple method for the use of air-stable cobalt catalysts for anti-Markovnikov alkene hydrosilylation that can be used under aerobic conditions without dry solvents or additives. These catalysts can be generated from low-cost commercially available materials. In addition, these catalysts possess good catalytic ability for both hydrosilanes and hydroalkoxysilanes. Finally, a mechanistic study demonstrates that the silane and the catalyst generate a Co–H species in the course of the reaction, which has been observed by in situ Raman spectroscopy.

Full text of DOI:10.1002/ejic.201801068

A Journal of
Accepted Article
Title: Cobalt catalysts for alkene hydrosilylation under aerobic
conditions without dry solvents or additives
Authors: Silvia Gutiérrez-Tarriño, Patricia Concepción, and Pascual
Oña-Burgos
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801068
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
FULL PAPER
Cobalt catalysts for alkene hydrosilylation under aerobic
conditions without dry solvents or additives.
Silvia Gutiérrez-Tarriño[a], Patricia Concepción[a] and Pascual Oña-Burgos*[a]
Abstract: Alkene hydrosilylation is typically performed with Pt
catalysts, but inexpensive base-metal catalysts would be preferred.
Here, we report a simple method for the use of air stable cobalt
catalysts for anti-Markovnikov alkene hydrosilylation that can be
used under aerobic conditions without dry solvents or additives.
These catalysts can be generated from low-cost commercially
available materials. In addition, these catalysts possess good
catalytic ability for both hydrosilanes and hydroalcoxysilanes. Finally,
a mechanistic study demonstrates the silane and the catalyst
generates a Co-H species in the course of the reaction, which has
been observed by in-situ Raman spectroscopy.
implies three main disadvantages: 1) these activators are also
moisture sensitive; 2) these activators are stronger nucleophiles,
which could potentially provide the cleave the Si−O bonds in the
alkoxysilanes and siloxanes; 3) the Li, Mg, or B metal from the
activator could be incorporated in the final product. All of these
are potential risk, which are more remarkable in the case of the
synthesis of silicone polymers. In summary, a desirable catalyst
should be air and moisture stable and operate under aerobic
conditions without requiring co-catalysts.
Although, Nagashima[19a] and Chirik[19b] groups have
reported the employment of air-stable cobalt catalysts in this
process they have never used aerobic conditions and non-dry
solvents. Herein the catalytic hydrosilylation of alkenes under
aerobic conditions and without dry solvents or additives is shown,
where the development of air stable cobalt-aquo complexes is
pivotal. The developed catalysts are based on terpyridine (tpy)
ligand and commercial cobalt precursors. Cobalt(II) carboxylates
have been selected as cobalt precursors because these are
stable and inexpensive sources of cobalt and offer structural
diversity.
Introduction
Hydrosilylation of alkenes catalyzed by transition metals
is a well-known reaction to provide organosilanes due to two
main advantages: straightforward and atom-economic. In the
industry, organosilanes are employed as precursor of alcohols
or as building blocks for the production of silicon-based
polymers.[1] Catalysts based on platinum, due to their
effectiveness and high selectivity, are the highlighted ones for
hydrosilylation of alkenes.[2] However, the main disadvantage of
these catalysts is its high and volatile price, which is directly
correlated with its shortage in the Earth´s crust. In addition,
Results and Discussion
We used cobalt(II) acetate tetrahydrate (1a), cobalt(II)
acetylacetonate hydrate (2a), cobalt(II) naphthenate (10% Co)
(3a) and cobalt(II) octanate (8% Co) (4a) as precursors. For
catalyst screening, 4-vinylcyclohexene was reacted with PhSiH3
as the hydrosilane at room temperature under optimized
conditions: 0.1% mol cobalt precursor and 0.1 % mol of tpy
ligand in THF (see Table S1 in SI). In all cases, the anti-
platinum extraction has associated
a high environmental
footprint.[3] All these points have inspired the discovery of a new
generation of catalysts based on earth-abundant transition
metals.
Then, catalysts based on abundant first-road transition
metals such as Fe,[4-11] Co,[4,6,12−14] and Ni[15] have been
developed for the catalytic hydrosilylation of alkenes with a good
catalytic ability. However, these catalysts are limited to silanes
such as PhSiH3 or Ph2SiH2; in fact, only a few studies have
achieved alkene hydrosilylation with silanes containing polar
Markovnikov
product
was
exclusively
observed.
Table 1. Evaluation of cobalt precatalysts for the Hydrosilylation of 4-
[a]
vinylcyclohexene with PhSiH3.
functional
groups
such
as
hydroalkoxysilanes
and
Ph
SiH2
0.1 mol% [Co]/tpy
THF, rt
hydrosiloxanes. In this sense, catalysts which are useful for both
PhSiH3 and Ph2SiH2 are often inactive or less efficient for
hydrosilylation using hydrosiloxanes. Moreover, another
limitation of these catalysts is their high sensitivity toward air and
moisture, which implies a careful preparation and usage. Thus,
the active species can be generated in situ in order to face this
disadvantage, by treating stable well-defined Fe(II) or Co(II)
coordination compounds with an organometallic co-catalyst like
EtMgBr, BuLi, NaEt3BH or NaOtBu.[16-18] However, this strategy
+
PhSiH3
Co(II)
precursor
Time
(min)
Conversion
Entry
TOF(min-1)
(%)[b]
1
2
3
4
5
1a
2a
240
240
180
240
240
72
99
99
99
92
7.00[c]
21.66[d]
22.33[d]
12.10[d]
8.33[c]
3a
4a
1a[e]
6
2a[e]
240
90
7.88[c]
[a] All reactions were performed on 0.89 mmol scale using a 1:1 silane-
olefin mixture under aerobic conditions, where the mixture Co-
precursors/tpy was previously prepared. [b] Determined by 1H NMR
analysis of the crude reaction mixture. [c] Calculated after 60 min from
the beginning of the reaction. [d] TOF calculated after 30 min from the
beginning of the reaction. [e] Employing isolated species. The reaction
without the presence of the tpy ligand does NOT take place.
[a]
Miss. S. Gutiérrez Tarriño, Dr. P. Concepción and Dr. P. Oña-
Burgos
Instituto de Tecnología Química, Universitat Politècnica de
València-Consejo Superior de Investigaciones Científicas (UPV-
CSIC), Avda. de los Naranjos s/n, 46022 Valencia, Spain.
http://itq.upv-csic.es/
Supporting information for this article is given via a link at the end of
the document.
For the evaluation of cobalt precursors, we monitored the of
the catalysis by H NMR (Figure 2). A catalyst loading of 0.1%
1
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
FULL PAPER
mol in THF was used and the reactions were monitored at
several times during four hours. This allowed us to calculate
TOF after 30 min. Also, this reaction was studied by NMR in situ
(Figure S1 and S2), where both results are consistent. In
general, all precursors have shown high TOFs values (entries 1-
4, table 1), but precursor3a (entry 3) is the most active for this
reaction (TOF = 22.33 min-1).
Moreover, EPR spectroscopy at rt and 77 K was
employed to characterize the reaction with precursors 1a and 2a
(see Figure S13), where both reactions show a unique signal
with S=1/2. The observed g value (g =2.09) is consistent with a
low spin homoleptic complex [Co(tpy)2]2+,[22] but not with the
heteroleptic complexes,1b or 2b which are expected to be S=3/2
species. However, Co (II) species with S=3/2 could be EPR
silent.[23] Then, quantitative EPR[24] was employing in order to
measure the formation of homoleptic species with both
precursors. The reaction with precursor 1a provided a signal
with an intensity corresponding to only 1.3% of cobalt this
implying that the homoleptic species is not preferred. On the
other hand, this experiment with precursor 2a showed 9% of the
S=1/2 signal, which is consistent with the exclusive formation of
homoleptic complex. In fact, solution magnetic moment
measurements (method of Evans[25] in MeOD-d4, 23 °C, Figure
S14) on 1b and 2c established an average effective magnetic
susceptibility (μeff) of 3.7 and 1.7, respectively, consistent with 3
and 1 unpaired electrons, respectively. Therefore 1b is a high-
spin Co(II) electronic configuration, while 2c is a low-spin Co(II)
electronic configuration.
Crystallization of 1b revealed a structure consistent with
the molecular formula, [Co(OAc)(H2O)2(tpy)]OAc, (by single
crystal XRD) whereby the THF ligands are replaced with
water.[26a-b] In addition, the homoleptic complex 2c was
confirmed by single crystal x-ray diffraction.[26a,c] Thus, under
non-inert atmosphere, the acetylacetonate anions from the
reaction forming 2c are replaced by chloride anions (derived
from CHCl3). The X-ray study revealed that both 1b and 2c
complexes are six-coordinate cobalt complexes. The first one,
[Co(OAc)(H2O)2(tpy)]OAc, shows that carboxylate ligand adopts
κ1binding mode with two water molecules coordinated to the
metal centre. While in the second one, [Co(tpy)2]Cl2·6H2O, the
six-nitrogens of the two ligands are coordinated to the metal
centre, fact which could make this complex less active as
catalyst since the metal centre is less accessible than for
complex 1b. In addition, it is observed that the central nitrogen
of the tpy ligand in 1b is closer to the Co(II) compared to the
other two nitrogens of this ligand, as observed in the case of
complex 2c (see tables S4 and S5).
Figure 1. Kinetic study of 4-vinylciclohexene
hydrosilylation with PhSiH3 for the evaluation of cobalt
precatalysts. The numbers in the figure correspond to entries
in Table 1.
In order to elucidate the structure of the active species
the reaction between the four cobalt precursors 1a-4a and the
tpy ligand was studied, where the formation of the complexes
1b-4b is expected (Scheme 1) based on the employed
stoichiometry metal to ligand.
Scheme 1. General illustration of the synthesis of cobalt (II) complexes 1b-4b.
RO-
+
THF
N
+
tpy
Co(OR)2
N
N
1a-4a
Co
THF
THF
1b-4b
OR
The reaction between cobalt precursor 1a and tpy
yielded mostly the complex 1b with a small amount of the
homoleptic complex, which was determined by NMR despite its
paramagnetism (see Figure S3 in the SI).[20] On the other hand,
the reaction with cobalt precursor 2a affords exclusively the
homoleptic complex 2c,(2bwas not observed). The NMR spectra
obtained with from the reaction of tpy with precursors 3a and 4a
show also the formation of complexes 3b and 4b, but the
presence of homoleptic complex is also detected. These data
suggest that acetylacetonate is more labile than carboxylates
under these conditions. These data obtained by NMR
spectroscopy were consistent with UV-Vis studies (see Figures
S8-S12) where the band at 350 cm-1 clearly supports the
coordination of the tpy ligand to the metal. [21] In this sense, the
differences between the proposed structure and homoleptic
ones are associated to the molar absorbtivity, which will be
bigger for the homoleptic complexes. The amount of precursor
employed for 2a was a 25% respect to the others three.
Figure 2. Molecular structures of complexes [Co(OAc)(H2O)2(tpy)]OAc, 1b
(left) and [Co(tpy)2]Cl2·6H2O, 2c (right). Thermal ellipsoids shown at 30%
probability. Hydrogen atoms and solvent molecules are omitted for clarity.
Gray = C, blue = N, red = O, brown = Co. Chloride anions of complex 2c are
also omitted for clarity.
In addition, the isolated homoleptic complex 2c (entry 6)
has also shown activity for this reaction, although its TOF value
is smaller than the freshly prepared (entry 2). This fact supports
that one tpy ligand is enough to activate cobalt precursors, and
that the formation of homoleptic species have a negative
influence in the process and reaction rate. Finally, respect to the
catalytic activity for precatalyst 1b, freshly and isolated (both
lightly soluble in the solvent, which is a technical limitation), the
TOF values are in the same order but the efficiency is higher for
the isolated one. The discovery of highly active selective and
On the other hand, the reactions with precursors 3a and
4a cannot be studied using ESI-MS and X-ray techniques since
the naphthenate and octanate precursors are a mixture of
several isomers. In addition, these are provided in solution in
mineral oils, but at the same time both represent the main cobalt
species industrially used, for instance in alkane oxidation. In this
sense, ESI-MS analysis for the reactions with 1a and 2a are also
consistent with the proposed structures by the other techniques
(see figures S4-S7).
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
FULL PAPER
bench-stable cobalt precatalysts enabled exploration of the
scope of the alkene substrates in the hydrosilylation reaction.
As presented in Table 2, freshly prepared cobalt
precursors 2a, 3a and 4a with tpy were effective for the
hydrosilylation of a range of terminal alkenes bearing functional
groups using PhSiH3. High to excellent conversions were
obtained between 0.5 and 8h of reaction time at 23 °C using
0.1% mol of the cobalt precatalyst.
Table 3. Functionalized hydrosilylation products from Ph2SiH2 using the
cobalt precursors 2a, 3a and 4a with tpy.[a,b]
0.1 mol% [Co]/tpy
SiHPh2
+
Ph
2SiH2
R
R
THF, rt
SiHPh2
SiHPh2
SiHPh2
H13
C6
6a
6c
6b
Table 2. Functionalized hydrosilylation products from PhSiH3 using the cobalt
precursors 2a, 3a and 4a with tpy.[a,b]
2b. 60% (6h)
3b. >99% (3h)
4b. >99% (3h)
2b. >99%(3h)
3b. 97% (3h)
4b. 91% (3h)
2b. >99% (16h)
3b. >99% (16h)
4b. 40% (16h)
R'
R'
0.1 mol% [Co]/tpy
THF, rt
Ph
SiH2
H
X = C, O
+
PhSiH3
R
X
R
X
O
N
SiHPh2
N
SiHPh2
SiHPh2
O
6d
6e
Ph
SiH2
6f
Ph
SiH2
N
Ph
SiH2
H13
C6
5c
5a
5b
2b. 70% (1h)
3b. 90% (1h)
4b. 80% (1h)
2b. >99% (8h)
3b.90% (8h)
4b. 82% (8h)
<1% (16h)
2b. 93% (6h)
3b. > 99% (3h)
4b. > 99% (20h)
> 99% (3h)[c]
> 99% (3h)
SiHPh2
H
N
O
N
Ph
SiH2
Ph
SiH2
6g
Ph
SiH2
O
N
5d
2b. <1% (8h)
3b. > 99% (8h)
4b. 89% (8h)
5f
5e
[a]All reactions were performed on 0.89 mmol scale using a 1:1 silane-olefin
mixture under aerobic conditions. [b]Conversions were determined by 1H
NMR analysis of the crude reaction mixture.
2b. 97% (1h)
3b. >99% (1h)
4b. 95% (1h)
2b. 99% (8h)
3b. 99% (8h)
4b. 96% (8h)
2b. <1% (16h)
3b. >99% (3h)
4b. >99% (3h)
Ph
SiH2
O
O
Ph
SiH2
Cl
On the other hand, catalysts useful for PhSiH3 or
Ph2SiH2 often are inactive or less efficient for hydrosilylation
using hydroalkoxysilanes or hydrosiloxanes. Therefore, we
evaluated these precatalysts in the activation of this type of
silanes. In this way, we have used (EtO)3SiH to evaluate the
activity of precursors 2a, 3a and 4a with tpy (Table 4). The
reactions were performed at 50°C to activate the (EtO)3SiH. In
this case, cobalt naphthenate has shown the best results for the
desired products for the selected alkenes. However, this
reaction with this catalyst has been more sensitive to the
presence of some functional groups in the alkenes such as
esters and alcohols. The addition of thrietoxysilane to 1-octene
was conducted on 10g scale as the resulting noctylsilane product
is manufactured annually on >6000 ton scale and finds
commercial applications in coatings such as weatherproofing for
masonry products, metal oxides and glass.[19b] The conversion
for this 10g scale reaction was >99% in three hours.
Ph
SiH2
O
5g
5h
5i
2b. 25% (0.5h)[c]
3b. >99% (0.5h)[c]
4b. 88% (0.5h)[c]
< 1% (16h)
<1 % (16h)[d]
Ph
SiH2
Ph
O
SiH
H
5j
5k
<1% (16h)[d]
>99% (3h)[e]
[a]All reactions were performed on 0.89 mmol scale using a 1:1 silane-olefin
mixture under aerobic conditions. [b]Conversions were determined by 1H
NMR analysis of the crude reaction mixture. [c]Addition of silane at 0°C.
[d]Reaction performed at 50°C. [e]Reaction performed using a 2:1 silane-olefin
mixture.
Notable findings include tolerance of tertiary and
Table 4. Functionalized hydrosilylation products from (EtO)3SiH using the
cobalt precursors 2a, 3a and 4a with tpy.[a,b]
secondary amine functional groups (5c, 5d). Styrene
a
challenging substrate for regioselective hydrosilylation[27] yielded
exclusively the anti-Markovnikov product (5b). On the other
hand, oxygen-based functional groups such as esters (5e, 5h)
were also well-tolerated by the cobalt precatalyst. In addition,
these precatalysts are chemoselective: they are not reactive with
carbonyl groups such as aldehydes (5k), ketones (5j), or internal
alkenes. Finally, a controlled reaction provides mono- or bi-
functionalized products 5b or 5l when 1 or 2 equivalents of
styrene are used, respectively.
0.1 mol% [Co]/tpy
Si(OEt)3
+
SiH
(EtO)3
R
R
THF, 50ºC
Si(OEt)3
Si(OEt)3
Si(OEt)3
H13
C6
7a
7b
7c
2b. >99% (1h)
3b. >99% (10 min)
4b. >99% (1h)
2b. 66% (3h)
3b. >99% (3h)
4b. 94% (3h)
H
2b. 79% (6h)
3b. >99% (6h)
4b. 85% (6h)
N
Si(OEt)3
Reaction of these olefins with the secondary silane
Ph2SiH2 was also performed using 2a, 3a and 4a cobalt
precursors with tpy in order to give, after a few hours, moderate
to high conversions with an excellent selectivity at room
temperature (Table 3). In this study with Ph2SiH2 the regio- and
chemo-selectivity has also been studied with successful results
for the several alkenes used, although the reaction rates are
slower than in the case of PhSiH3.
N
Si(OEt)3
7d
7e
>99% (6h)
>99% (6h)
[a]All reactions were performed on 0.89 mmol scale using a 1:1 silane-olefin
mixture under aerobic conditions. [b]Conversions were determined by 1H
NMR analysis of the crude reaction mixture.
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
FULL PAPER
as well (Figure S16). Finally, the initial rate depends linearly on
the catalyst loading when this is altered in the range of 0.5 to
0.05 mol % (see Figure 3). This first order dependence on the
concentration of the catalyst implies that no induction time is
required and the cobalt center is not aggregate with this range of
concentration.
In addition, pentamethyldisiloxane has also been used to
evaluate the pre-catalysts mentioned (Table 5). In this case
80°C were required to activate this hydrosiloxane to provide
product 8a with excellent yield in 8 hours. This reaction is the
first step in the formation of silicone fluid crosslinking.
Table 5. Functionalized hydrosilylation product from
(CH3)3SiOSi(CH3)2H using the cobalt precursors 2a, 3a and 4a
with tpy.[a,b]
1.2
1
0.1 mol% [Co]/tpy
Si(CH3 2OSi(CH
THF, 80ºC
0.8
)
)
3 3
+
SiOSiH(CH3 2
(CH3)3
R
R
)
0.6
Si(CH3 2OSi(CH3 3
)
)
y = 0.973x + 0.028
H13
C6
R² = 0.995
0.4
8a
0.2
0
>99% (8h)
[a]All reactions were performed on 0.89 mmol scale using a 1:1
silane-olefin mixture under aerobic conditions.[b]Conversions
were determined by 1H NMR analysis of the crude reaction
mixture.
0
0.2
0.4
0.6
0.8
1
1.2
log([Co]/[Co]0)
Figure 3. Initial rate depending on the catalyst loading.
The cross-linking of silicone fluids by platinum-catalyzed
hydrosilylation produces silicone products, which have
applications in release coatings. Due to the high viscosity of the
final product, the platinum catalyst becomes trapped in the final
silicone and is not recovered, accounting for approximately 30%
of the final silicone cost.[19b] Commercial standards require this
curing process to proceed rapidly with low catalyst loadings and
without leaving the product coloured in any way.[21] The cross-
linking activity of 2a, 3a and 4a precursors with tpy was
examined at different catalyst loadings (Scheme 2). It can be
observed a gel formation in each case, and an effective silicon
cross-linking. The catalyst loading used related to the silane was
0.0125% mol, while Chirik and coworkers used 0.0625% mol
related to the silane when they said “1ppm of cobalt” in their
study.[19b] Therefore, this result is in the range of Chirik´s system.
In-situ Raman spectroscopy was employed to study the
intermediate species formed during the reaction between the
catalyst and the two reagents. First, the three isolated
components catalyst, PhSiH3 and 4-vinylcyclohexene were
studied. Then, catalyst and silane mixture was studied where
bubbles are observed that may be due to hydrogen formation.
Indeed, the catalytic reaction is veryfast with PhSiH3 (Table 2),
so when the catalyst:silane ratio is 1:2, the process is more
exothermic, in addition to heating due to irradiation from the
Raman laser. This high temperature can promote other process,
so therefore this is not the best method to elucidate the
mechanism. In order to make the process less exothermic and
slower the PhSiH3 were substituted by (EtO)3SiH and the same
procedure was employed. In this case, the reaction with this
silane needed higher temperatures (Table 4). Under these
conditions, a strong band at 2251 cm-1 was detected (Figure 4 in
black), and this band was clearly shifted respect to the Si-H
bond of the silane (Figure 4 in green).This band was observed
for a long period of time and disappeared when the alkene was
added (Figure 4 in red). Based on this behaviour, this signal was
assigned to a Co-H bond.[29] Moreover, an additional band of Si-
O bond was identified at 465 cm-1.[30]
Scheme 2. Crosslinking of Silicone Fluids.[a]
CH3 CH3
[Co]/tpy 0.012%
Crosslinked Silicone
Gel
+
2
Si
H
H
Si O Si
CH3 CH3
80ºC, 4h
[a]Top: Cure reaction performed on 1.78 mmol scale using 1:2
silane-olefin mixture to obtain cross-linked silicone gel.
Bottom: Isolated gels from cross-linking reactions with cobalt
precursors 2a, 3a and 4a.
Mechanistic study. The first approach was focused on
the rate law of the reaction. In order to face it, we varied the
concentration of each component of the reaction, following a
similar methodology than the one described by Bleith and
Gade.[28] For the hydrosilylation of 4-vinylcyclohexene with
PhSiH3 with catalyst 3b, we found that the initial rate of the
reaction was not appreciably affected as a function of 4-
vinylcyclohexene concentration, indicating
a zeroth order
dependence on alkene concentration (Figure S15). The same
effect was observed when the concentration of PhSiH3 is varied,
indicating a zeroth order dependence on silane concentration,
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
FULL PAPER
Si-H
2196
Si-O
465
Co-H
2251
2000
2100
2200
2300
2400
150
300
450
-1)
600
-1)
(cm
Raman shift
Raman shift
(
cm
300
600
900
1200
1500
-1
1800
2100
2400
Raman shift (cm )
Figure 4. Spectrum of the in-situ Raman experiment: in blue the catalyst 3b, in green (EtO)3SiH, in black the reaction mixture between 3b and (EtO)3SiH, and in
red after addition of the alkene.
Scheme 3. Mechanistic proposal for the cobalt-catalyzed hydrosilylation of
alkenes.
Conclusions
In summary, we have developed new families of
cobalt(II) complexes which are air- and moisture-stable, and
work as catalysts for the hydrosilylation of alkenes with several
silane precursors, avoiding the use of external activators. In
addition, it is the first case where these reactions are carried out
under aerobic conditions with first row transition metal
complexes. Finally, a mechanistic study was performed where
the reaction between the silane and the catalyst provides the
Co-H species which has been observed by in-situ Raman
spectroscopy. This is a main insight into the mechanism of this
process. Based on that,
a new catalyst design for the
hydrosilylation of alkenes under aerobic conditions is ongoing.
Experimental Section
Moreover, based on the in-situ Raman and kinetic
experiments, a plausible mechanism has been proposed in the
scheme 3. First, the cobalt pre-catalysts Co-OR dissociation
takes places to provide [Co]+ and [OR]-, which would have to be
the rate-determining step since the reaction rate respect to the
silane is zero-order as has been observed in the kinetic study.
Then, [Co]+ specie reacts with the corresponding silane to
provide the Co-H intermediate I. This is the first order reaction,
which is based on σ-bond metathesis. In addition, the species I
are in equilibrium with species II. Then species II react with the
alkene to form the alkyl complex III, which is converted to
hydride complex I upon the reaction with silane, accompanied by
the formation of the linear alkylsilane.
Instrumentation information and crystallographic details (CCDC
1817714 and CCDC 1817709) are given in the Supporting
Information.
New hydrosilylation products obtained were characterized by
GC-MS, 1H- and 13C-NMR. When available, characterization
given in the literature was used for comparison. Isolated
catalysts were characterized by elemental analysis, 1H-NMR,
monocrystal X-Ray Diffraction, Electron Paramagnetic
Resonance (EPR) and HR-MS. Freshly prepared catalysts were
characterized by EPR, UV/Vis absorption and HR-MS.
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10.1002/ejic.201801068
European Journal of Inorganic Chemistry
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(0.428 mmol) were dissolved in 50 mL of a mixture of
acetonitrile:chloroform (1:1) with some drops of methanol. An
inmediate color change to orange was observed. Single crystals
were obtained by slow evaporation of the solvent. Anal. For
C30H26Cl2CoN6O6: calc. = C, 51.745; H, 3.764; N, 12.069. Found
= C, 51.899; H, 3.876; N, 11.698.
Chemicals
All reagents and solvents were purchased from commercial
suppliers and used without further purification.
Synthesis
B. General Procedure for Catalytic Hydrosilylation
Reactions
A. Preparation of Cobalt Complexes.
Reaction of tpy ligand with Co(OAc)2 precursor (1b). 5.5 mg
of terpyridine (0.022 mmol) and 5.2 mg of Cobalt (II) acetate
tetrahydrate (0.022 mmol) was dissolved in 5 mL of THF and
stirred for 1 hour. In addition, we prepared the same mixture in
CD3CN:CDCl3 containing a few drops of CD3OD in an NMR tube
and the 1H-NMR spectra shown in Figure S1 (up) shows that we
form mainly the 1:1 paramagnetic complex 1b. 1H NMR (300
MHz): δ = 169.42 (bs), 105.75 (bs), 83.27 (bs), 42.50 (bs),
36.92(bs) and 12.72 (bs).1 In addition, a small amount the
homoleptic complex 1c is also obtained. (300 MHz): δ= 97.05
(bs), 55.75 (bs), 45.80 (bs), 33.91 (bs), 21.98 (bs), and 9.22 (bs).
Method A: 0.2 mL of freshly catalyst solution (0.001 equiv.)
were taken and added to a vial equipped with a stir bar, followed
by olefin (1 equiv.) and Silane (1 equiv.) resulting in formation of
a dark reaction mixture. The vial was sealed with a cap and
stirred at pertinent temperature. N-Hexane was added and the
catalyst was filtered. Then, n-hexane was rotaevaporated and
an aliquot was analyzed by 1H NMR in CDCl3.
Method B: 0.2 mL of freshly catalyst solution (0.001 equiv.)
were taken and added to a vial equipped with a stir bar, followed
by olefin (2 equiv.) and Silane (1 equiv.) resulting in formation of
a dark reaction mixture. The vial was sealed with a cap and
stirred at pertinent temperature. n-Hexane was added and the
catalyst was filtered. Then, n-hexane was rotaevaporated and
an aliquot was analyzed by 1H NMR in CDCl3.
Reaction of tpy ligand with Co(acac)2 precursor (2b). 5.2 mg
of terpyridine (0.022 mmol) and 5.7 mg of Cobalt (II)
acetylacetonate hydrate (0.022 mmol) was dissolved in 5 mL of
THF and stirred for 5 minutes. In the same way, we prepared the
same mixture in CD3CN:CDCl3 containing a few drops of CD3OD
1
in an NMR tube and the H-NMR spectra shown in Figure S1
C. Kinetic study for hydrosilylation of 4-vinylcyclohexene
with PhSiH3
(bottom) shows that we only form the homoleptic complex 2c. 1H
NMR (300 MHz): δ= 96.07 (bs), 55.57 (bs), 45.73 (bs), 33.78
(bs), 21.87 (bs), and 9.48 (bs).
Method A: The isolated catalyst (0.001 equiv.) was dissolved in
0.2 mL of THF and added to a vial equipped with a stir bar,
followed by 4-vinylcyclohexene (96.3 mg, 0.89 mmol) and
phenylsilane (96.3 mg, 0.89 mmol) and mesitylene (107 mg,
0.89 mmol) resulting in formation of a dark reaction mixture. The
vial was sealed with a cap and stirred at room temperature. An
aliquot of crude reaction was taken each 30 minutes and
analized by 1H NMR in CDCl3.
Reaction of tpy ligand with Co(naphth)2 precursor (3b). 5.2
mg of terpyridine (0.022 mmol) and 13.11 mg of Cobalt (II)
naphtenate (10% Co, 0.022 mmol) was dissolved in 5 mL of
THF and stirred for 5 minutes.
Reaction of tpy ligand with Co(oct)2 precursor (4b). 5.2 mg of
terpyridine (0.022 mmol) and 16.4 mg of Cobalt (II) octanate (8%
Co, 0.022 mmol) was dissolved in 5 mL of THF and stirred for 5
minutes.
Method B: 0.2 mL of freshly catalyst solution (0.001 equiv.)
were taken and added to a vial equipped with a stir bar, followed
by 4-vinylcyclohexene (96.3 mg, 0.89 mmol) and phenylsilane
(96.3 mg, 0.89 mmol) and mesitylene (107 mg, 0.89 mmol)
resulting in formation of a dark reaction mixture. The vial was
sealed with a cap and stirred at room temperature. An aliquot of
Preparation of [Co(OAc)(H2O)2(tpy)]OAc (1b’). 100 mg of
terpyridine (0.428 mmol) and 106.6 mg of Co(II) acetate
tetrahydrate (0.428 mmol) were dissolved in 50 mL of a mixture
of acetonitrile:chloroform (1:1) with some drops of methanol. An
immediate color change to orange was observed. Single crystals
were obtained by slow evaporation of the solvent. Anal. For
C19H21CoN3O6: calc. = C, 51.131; H, 4.743; N, 9.415. Found = C,
51.048; H, 4.667; N, 9.543.
1
crude reaction was taken each 30 minutes and analized by H
NMR in CDCl3.
Method C: To a NMR tube were added 0.2 mL of a catalyst’s
solution in THF-d8 followed by 4-vinylcyclohexene (96.3 mg,
0.89 mmol), phenylsilane (96.3 mg, 0.89 mmol) and mesitylene
(107 mg, 0.89 mmol) resulting in formation of dark reaction
mixture. The reaction was carried out inside the NMR
Spectrometer.
Preparation of [Co(tpy)2]Cl2·6H2O (2c). 100 mg of terpyridine
(0.428 mmol) and 110 mg of Cobalt (II) acetylacetonate hydrate
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European Journal of Inorganic Chemistry
FULL PAPER
Catal. 2016, 6, 290−300. (f) C. Wang, W. J. Teo, S. Ge. ACS Catal.
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(EtO)3SiH.
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20 mL of freshly catalyst solution (0.001 equiv.) were taken and
added to a 100 mL flask equipped with a stir bar, followed by 1-
octene (10 g, 89 mmol) and triethoxysilane (14.62 g, 89 mmol).
The flask was sealed with a cap and stirred for 3 hours at 50 ºC,
resulting in formation of a dark reaction mixture. In order to
calculate the isolated yield, first, THF was rotaevaporated. When
n-hexane was added, the precatalyst precipitated and this
allowed us to filter it. Finally, the solvent was evaporated (Yield:
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Conflict of interest
The authors declare no competing financial interest.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
FULL PAPER
Silvia Gutiérrez-Tarriño, Patricia
Concepción and Pascual Oña-Burgos*
Page No. – Page No.
Cobalt
catalysts
for
alkene
hydrosilylation
under
aerobic
conditions without dry solvents or
additives.
Text for Table of Contents
Alkene hydrosilylation catalyzed by cobalt (II) complexes under aerobic conditions without dry
solvent or external activators. In addition, a mechanistic study based on kinetics experiments
and in-situ Raman spectroscopy has been carried out, where a Co-H specie has been identified.
Keywords: Hydrosilylation catalysis, cobalt complexes and mechanistic study
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