Catalytic hydrosilylation of alkenes by iron complexes containing terpyridine derivatives as ancillary ligands

Article abstract of DOI:10.1021/om300279t

Iron complexes formulated as Fe(terpy)X₂ (terpy = 2,2′:6′,2 -terpyridine derivatives; X = Cl, Br) were prepared and their catalytic activities for hydrosilylation of olefin with hydrosilane were examined. Although Fe(terpy)X₂ did not

Full text of DOI:10.1021/om300279t

Communication
pubs.acs.org/Organometallics
Catalytic Hydrosilylation of Alkenes by Iron Complexes Containing
Terpyridine Derivatives as Ancillary Ligands
Kouji Kamata, Atsuko Suzuki, Yuta Nakai, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
S
* Supporting Information
ABSTRACT: Iron complexes formulated as Fe(terpy)X2
(terpy = 2,2′:6′,2″-terpyridine derivatives; X = Cl, Br) were
prepared and their catalytic activities for hydrosilylation of
olefin with hydrosilane were examined. Although Fe(terpy)X2
did not show catalytic activity, the treatment of Fe(terpy)X2
with NaHBEt3 caused catalytic activity. The influence of
substituents in terpy on the catalytic activity was examined,
and it was found that some complexes with an unsymmetri-
cally disubstituted terpy selectively produced 1-silylalkane with quite high catalytic activity. In the reaction of 1-octene with
PhSiH3, the double-hydrosilylation product Ph(1-octyl)2SiH was selectively obtained.
rganosilicon compounds have now become indispensable
to our daily life. As organosilicons consisting mainly of
silyl).5 CpFe(CO)2Me (Cp = η5-C5H5) was reported to
catalyze the reaction of 1,3-divinyldisiloxane with hydrosilane
to give the compound produced by dehydrogenative silylation
at one vinyl group and by hydrogenation at the other vinyl
group.6 A trinuclear iron carbonyl complex, Fe3(CO)12, showed
catalytic activity in the reaction of styrene and its derivatives
with Et3SiH to give dehydrogenative silylation products,
whereas the reaction of 1-hexene with Et3SiH produced both
dehydrogenative silylation and hydrogenation products.7 In
both cases, Fe3(CO)12 did not produce the hydrosilylation
product. A heterotrinuclear complex containing Fe and Pt,
FePt2(CO)5(PPh3)2(PhC2Ph), also exhibited catalytic activity
toward hydrosilylation of alkynes;8 however, the role of Fe in
the catalytic system was not clear. The reaction of CpFe-
(CO)2(SiMe3) (Cp = η5-C5H5) with CH2CHR (R = SiMe3,
Ph, H) stoichiometrically gave dehydrogenative silylation
products, and the Fe(CO)5 + Et3SiH system catalyzed the
reaction of (CH2CH)SiMe2OSiMe2(CHCH2) with
Et3SiH to give both the dehydrogenative silylation and
hydrogenation products.9 Recently, iron bis(imino)pyridine
complexes were reported by Chirik and co-workers to serve as
excellent catalysts for alkene hydrosilylation.10 Their impressive
report prompted us to investigate the catalytic activity of iron
complexes with terpyridine and its derivatives as a ligand, since
an N3-type tridentate ligand such as bis(imino)pyridine seemed
to endow iron complexes, which have been considered to have
low selectivity and poor activity, with high selectivity and good
catalytic activity. We herein report the synthesis of mono-
nuclear iron complexes with a 2,2′:6′,2″-terpyridine ligand and
its derivatives having two substituents, either symmetrically on
the 6- and 6″-positions or unsymmetrically on the 6- and 6″-
O
Si−C bonds do not exist naturally, these compounds have been
synthesized artificially. One of the most convenient and widely
used methods of forming Si−C bonds is the hydrosilylation
reaction, which involves the addition of the Si−H bond in
hydrosilane across an unsaturated hydrocarbon such as an
alkene or alkyne. A transition-metal catalyst is required for
hydrosilylation.1
Among the many transition-metal catalysts reported to date,
platinum catalysts such as Speier’s catalyst and Karstedt’s
catalyst are known to be very powerful and are often used
commercially.2 However, we now know that the use of precious
metals in catalysis is not always required. The relatively high
cost, environmental concerns, and uncertainty of the long-term
supply of precious metals have inspired the search for
equivalent or superior base-metal alternatives. Iron is an
attractive precious metal surrogate because of its high natural
abundance, low cost, and precedent in numerous biocatalytic
reactions.
One of the most important issues with hydrosilylation of
alkenes is selectivity; side reactions such as dehydrogenative
silylation, giving a vinylsilane, and hydrogenation, providing an
alkane, often accompany the desired reaction.1 Therefore,
creation of new catalysts with high selectivity as well as high
activity is very important.
Several iron complexes have been reported for use in
catalytic hydrosilylation of unsaturated hydrocarbons. To the
best of our knowledge, the first such example was reported by
Nesmeyanov et al. in 1962,3 who achieved the thermal reaction
of R3SiH with alkenes in the presence of a catalytic amount of
Fe(CO)5 to give a hydrosilylation product and a dehydrogen-
ative silylation product. Photoreaction of R3SiH with alkenes
using the Fe(CO)5 catalyst was also reported.4 A mechanism
including insertion of a CC double bond into an Fe−Si bond
was proposed for Cp*Fe(CO)2R (Cp* = η5-C5Me5, R = alkyl,
Received: April 6, 2012
Published: May 8, 2012
© 2012 American Chemical Society
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Organometallics
Communication
positions or on the 6- and 5″-positions (see Chart 1), and their
catalytic activities for the hydrosilylation of olefins.11
Table 1. Hydrosilylation Reaction of 1-Hexene with
Hydrosilane Catalyzed by Iron Complexes
Chart 1. Terpyridine and the Atom Numbering
entry
Fe cat.
R1R2SiH2
TON
1
1-FeCl2
1-FeBr2
2-FeCl2
2-FeBr2
3-FeBr2
4-FeCl2
4-FeBr2
4-FeCl2
4-FeBr2
4-FeBr2
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
Ph2SiH2
Ph2SiH2
MePhSiH2
0
0
2
3
4
4
19
4
5
6,6″-Symmetrically disubstituted terpyridines were prepared
from 6,6″-dibromoterpyridine and the corresponding arylbor-
onic acid using a Suzuki−Miyaura coupling reaction.12,13 These
terpyridine derivatives, together with nonsubstituted terpyr-
idine, were treated with FeX2 (X = Cl, Br) to give Fe(terpy)X2
in good yields (eq 1) (terpy stands for terpyridine and its
6
15
70
8
7
8
9
71
90
10
precursors than the corresponding dichloroiron complexes. It
has been reported that the bond energy of Fe−Cl in FeCl2
(95.6 kcal/mol) is greater than that of Fe−Br in FeBr2 (81.2
kcal/mol).16 If this tendency is applied to Fe(terpy)X2, our
observation that Fe(terpy)Br2 is a better precursor than
Fe(terpy)Cl2 would indicate that Fe−X (X = Cl, Br) bond
cleavage is involved in the formation of a catalytically active
species. We expected the reaction of Fe(terpy)X2 with
NaBHEt3 to give Fe(terpy)(H)2, and the reaction mixture
was monitored by 1H NMR. However, no species containing an
Fe−H portion was observed. We tentatively think that
Fe(terpy) is a real catalyst which is formed via Fe(terpy)(H)2
being too reactive to be detected by 1H NMR or by a concerted
reaction of Fe(terpy)X2 with NaBHEt3 without Fe(terpy)(H)2
formation.
Comparison of entries 4, 5, and 7 revealed that Fe(terpy)Br2
with mesityl substituents showed better catalytic activity than
that with phenyl substituents, which showed better catalytic
activity than that with o-tolyl substituents. Although this trend
cannot be explained simply by steric factors, the mesityl groups
in the terpy ligand seemed to be good substituents. 4-Br2 also
showed good catalytic activity for secondary silanes (Ph2SiH2
and MePhSiH2, entries 9 and 10). The iron complexes 1-
FeX2−4-FeX2 did not show catalytic activity toward hydro-
silylation of tertiary silanes or internal olefins, and the starting
materials were recovered.
derivatives). These complexes showed good stability in the
solid state and in solution. The reaction of nonsubstituted terpy
with FeCl2 has already been reported to give [Fe(terpy)2]-
[FeCl4]2.12 Our results, on the basis of the elemental analysis
data, showed that the reaction with FeBr2 produced Fe(terpy)-
Br2 but not [Fe(terpy)2][FeBr4]2.
The catalytic activity of these iron complexes was then
examined. The reactions of PhSiH3 with 1-hexene were
examined in the presence of a catalytic amount of the iron
complexes, but no reaction was observed at 50 °C. Adding
NaBHEt3 to the reaction system, however, led to the formation
of the hydrosilylated product. Hydrosilane and 1-hexene in a
1:32 molar ratio and 0.3 mol % (based on the concentration of
the hydrosilane) of the iron complex were placed in a Schlenk
tube. To the suspension was added 1.1 mol % of NaBHEt3 at
room temperature, which resulted in the formation of a
homogeneous solution that turned dark brown within several
minutes. The solution was heated at 50 °C for 24 h. After the
reaction mixture was cooled to room temperature, the
hydrosilylated product was measured by HPLC.14 The results
are summarized in Table 1. Iron complexes with unsubstituted
terpy showed no catalytic activity (entries 1 and 2), whereas
those with 6,6″ disubstituted terpy did show catalytic activity
(entries 3−10). In all cases, the product was 1-silylhexane and
no 2-silylhexane was formed, showing that selective anti-
Markovnikov addition took place. These results are consistent
with the fact that hydrosilylation to olefins with aliphatic
substituent(s) occurs with anti-Markovnikov regioselectivity.15
In addition, neither the corresponding dehydrogenative
silylation products nor hydrogenated products were formed.
Comparisons of entries 3 and 4, entries 6 and 7, and entries 8
and 9 revealed that the dibromoiron complexes were better
Next, we examined the effect of changing the olefin from 1-
hexene to 1-octene on the catalytic activity of 4-FeBr2 (Table
2). Under the same reaction conditions (1/32 (mol) PhSiH3/
olefin, 50 °C, 24 h, 0.3 mol % 4-FeBr2 based on PhSiH3), the
TON (turnover number) decreased from 70 to 33 on going
from 1-hexene to 1-octene. When 1-octene was used, a higher
reaction temperature could be used (100 °C), which resulted in
Table 2. Hydrosilylation Reaction Catalyzed by 4-FeBr2
cat. Fe
1
R R2SiH2 + olefin ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ hydrosilylated product
cat. NaHBEt3
entry Fe cat. (amt (mol %)) R1R2SiH2
olefin
temp (°C) TON
1
2
3
4
5
6
4-FeBr2 (0.3)
4-FeBr2 (0.3)
4-FeBr2 (0.3)
4-FeBr2 (0.1)
4-FeBr2 (0.3)
4-FeBr2 (0.1)
PhSiH3
PhSiH3
PhSiH3
PhSiH3
Ph2SiH2
Ph2SiH2
1-hexene
1 -octene
1 -octene
1 -octene
1-hexene
1 -octene
50
50
70
33
100
100
50
56
34
71
100
372
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Communication
a
Scheme 1. Synthetic Routes of Iron Complexes with Unsymmetrically Disubstituted Terpyridine
a
Conditions: (i) anhydrous CuCl2, HCl, Et2O, at −80 °C (ca. 50% yield); (ii) n-BuLi, Bu3SnCl, in THF, at −78 °C (over 95% yield); (iii)
Pd(PPh3)4, in toluene, at reflux temperature (ca. 45% yield); (iv) Pd(PPh3)4, arylboronic acid, Na3PO4·12H2O, H2O/DME, at reflux temperature
(75−95% yield).
Table 3. Hydrosilylation of 1-Octene with R1R2SiH2
Catalyzed by 4-FeBr2−10-FeBr2
a better TON (56 at 100 °C versus 33 at 50 °C). Moreover, the
amount of precatalyst 4-FeBr2 could be reduced to 0.1 mol %,
although the TON did decrease slightly. When Ph2SiH2 was
used, the hydrosilylation of 1-octene at 100 °C resulted in a
better TON (372) than that of 1-hexene at 50 °C (TON = 71).
The reaction of 6,6″-disubstituted terpy with FeCl2 has been
reported to give Fe(terpy)Cl2; in contrast, nonsubstituted terpy
reacted with FeCl2 to give [Fe(terpy)2][FeCl4]2.12 This may
provide insight as to why there was no catalytic activity of the
iron complexes with nonsubstituted terpy. Bulky substituents at
the 6- and 6″-positions in terpy presumably suppress the
conversion of Fe(terpy) into the inactive Fe(terpy)2. In order
to improve the catalytic activity of Fe(terpy), we attempted to
modify the terpy substituents. Having a substituent at the 6-
position seems to be required to prevent the formation of
Fe(terpy)2 species. On the other hand, the central iron should
require an appropriate space in its coordination sphere to
conduct hydrosilylation of the olefin. Therefore, we attempted
the preparation of unsymmetrically disubstituted terpy ligands
and their complexation with FeBr2 that satisfied both
requirements.
As unsymmetrically disubstituted terpy species have not been
reported, our synthetic routes are shown briefly in Scheme 1.
6,6′-Dibromo-2,2′-bipyridine (11) was prepared using an
Ullmann coupling of 2,6-dibromopyridine. 14 (15) was
synthesized from 11 and 12 (13) using Stille coupling. Finally,
14 and 15 were converted into 5−7 and 8−10, respectively,
using Suzuki−Miyaura coupling. The unsymmetrical terpy
compounds thus formed were treated with FeBr2 to prepare the
corresponding Fe complexes (5-FeBr2−10-FeBr2). Those
products were identified as Fe(terpy)Br2 according to
spectroscopic and elemental analysis data; 9-FeBr2 was also
characterized by X-ray diffraction analysis.
a
b
entry
Fe cat. (amt (mol %))
R1R2SiH2
TON
1
4-FeBr2 (0.1)
5-FeBr2 (0.1)
6-FeBr2 (0.1)
7-FeBr2 (0.05)
8-FeBr2 (0.1)
9-FeBr2 (0.05)
10-FeBr2 (0.05)
4-FeBr2 (0.1)
7-FeBr2 (0.1)
9-FeBr2 (0.1)
10-FeBr2 (0.1)
4-FeBr2 (0.1)
7-FeBr2 (0.1)
9-FeBr2 (0.1)
PhSiH3
34
0
2
PhSiH3
3
PhSiH3
0
4
PhSiH3
1458
0
5
PhSiH3
6
PhSiH3
1533
898
372
377
444
300
66
7
PhSiH3
8
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
MePhSiH2
MePhSiH2
MePhSiH2
MePhSiH2
9
10
11
12
13
14
15
222
153
124
10-FeBr2 (0.1)
a
b
At 100 °C for 24 h. Molar ratios: entries 1−3 and 5, 32/1 1-octene/
R1R2SiH2; entries 4, 6, and 7, 2/1 1-octene/R1R2SiH2; entries 8−15,
1/1 1-octene/R1R2SiH2.
substituent (Me) at the 5″- or 6″-position. According to these
criteria, 9-FeBr2 had the best combination of the two
substituents. Complexes 7-FeBr2, 9-FeBr2, and 10-FeBr2
showed good catalytic activity for Ph2SiH2 and MePhSiH2 as
compared with 4-FeBr2. It should be noted that the catalytic
activity of 4-FeBr2 was relatively and unexpectedly higher for
Ph2SiH2 than for PhSiH3 and for MePhSiH2; the reason for this
is yet unclear.
The hydrosilylation reactions of 1-octene with hydrosilanes
in the presence of a catalytic amount of Fe(terpy)Br2 bearing
unsymmetrical terpy derivatives at 100 °C were examined. The
results are summarized in Table 3, together with those for 4-
FeBr2 with a symmetric terpy. Complexes 5-FeBr2, 6-FeBr2,
and 8-FeBr2 showed no catalytic activities (entries 2, 3, and 5).
In contrast, 7-FeBr2, 9-FeBr2, and 10-FeBr2 exhibited much
better catalytic activities (entries 4, 6, and 7) than 4-FeBr2
(entry 1). Therefore, we surmised that catalytic activity is
favored by the central iron atom containing a terpy derivative
with a bulky aryl substituent at the 6-position and with a small
Another interesting point in this work is that the product in
the reaction of 1-octene with PhSiH3 in 2:1 molar ratio
depends on the amount used of 7-FeBr2 and 9-FeBr as catalyst
precursors (Scheme 2). When 0.05−0.3 mol % of the iron
complex was used at 100 °C, the product was a mixture of
Ph(octyl)SiH2 and Ph(octyl)2SiH. When the amount was more
than 0.3 mol %, PhSiH3 was completely converted into
Ph(octyl)2SiH. In contrast, when the amount was less than 0.05
mol %, the product was only Ph(octyl)SiH2. To the best of our
knowledge, selective single/double alkylation of hydrosilane via
hydrosilylation using a transition-metal catalyst has not been
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Organometallics
Communication
Scheme 2. Hydrosilylation Reaction of 1-Octene with
PhSiH3 Catalyzed by 0.3 and 0.05 mol % of an Fe Complex
ACKNOWLEDGMENTS
■
This work was supported by a Challenging Exploratory
Research grant (No. 23655056) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan
and by the Innovation Promotion Program in 2010 from
NEDO.
REFERENCES
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PhSiH3 at 50 °C catalyzed by 7-FeBr2 produced Ph(hexyl)SiH2
and that by 9-FeBr2 produced a mixture of Ph(hexyl)SiH2 and
Ph(hexyl)2SiH, and no selectivity was observed even by
changing the amount of 9-FeBr2 (Table 4). Suitable selection
of the kind and the amount of the catalyst precursor and high
reaction temperature are required to achieve the selective
single/double alkylation.
(2) (a) Speier, J. L. Adv. Organomet. Chem. 1979, 17, 407.
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24 h
(5) Randolph, C. L.; Wrighton, M. S. J. Am. Chem. Soc. 1986, 108,
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Fe cat. (amt
(mol %))
temp
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(11) During our investigation, Chirik et al. submitted patents
concerning hydrosilylation iron catalysts with a terpyridine ligand.
(a) Delis, J. G. P.; Chirik, P. J.; Tondreau, A. M. US 2011/0009565A1.
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substituents at the 6,6″- and 6,5″-positions exhibited noticeable
catalytic activities, we report our results here.
entry
1
olefin
(°C)
7-FeBr2
(0.3)
1-hexene
50 Ph(hexyl)
SiH2 (225)
Ph(hexyl)2SiH
(0)
2
3
4
9-FeBr2
(0.3)
1-hexene
1-hexene
1-hexene
50 Ph(hexyl)
SiH2 (179)
Ph(hexyl)2SiH
(128)
7-FeBr2
(0.05)
50 Ph(hexyl)
SiH2 (601)
Ph(hexyl)2SiH
(0)
9-FeBr2
(0.05)
50 Ph(hexyl)
SiH2
Ph(hexyl)2SiH
(139)
(1207)
5
6
7
7-FeBr2
(0.3)
1 -octene 100 Ph(octyl)
SiH2 (0)
Ph(octyl)2SiH
(333)
9-FeBr2
(0.3)
1 -octene 100 Ph(octyl)
SiH2 (0)
Ph(octyl)2SiH
(333)
7-FeBr2
(0.05)
1 -octene 100 Ph(octyl)
Ph(octyl)2SiH
(0)
SiH2
(1458)
8
9-FeBr2
(0.05)
1 -octene 100 Ph(octyl)
Ph(octyl)2SiH
(0)
SiH2
(1533)
In conclusion, we found that iron complexes bearing newly
prepared 2,2′:6′,2″-terpy derivatives with substituents at the 6-
and 5″-/6″-positions, especially those bearing unsymmetrically
disubstituted terpy, exhibit high catalytic activity for hydro-
silylation of olefins with hydrosilanes. In the reaction of 1-
octene with PhSiH3 at 100 °C selective single/double
hydrosilylation was achieved by changing the amount of the
iron catalyst.
(12) Nakayama, Y.; Baba, Y.; Yasuda, H.; Kawakita, K.; Ueyama, N.
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1458.
(14) The molar ratio of hydrosilane and 1-hexane was found to be
important in this reaction system. The reaction with a ratio of 1:2
produced some unidentified byproducts. Changing the ratio from 1:2
to 1:8 and 1:16 caused a decrease in the amount of the byproduct, and
when a ratio of 1:32 was used, no byproducts were observed.
(15) For example, see: (a) Takahashi, T.; Hasegawa, M.; Suzuki, N.;
Saburi, M.; Rousset, C. J.; Fanwick, P. E.; Negishi, E. J. Am. Chem. Soc.
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Organometallics 2008, 27, 3131. (d) Konkol, M.; Kondracka, M.; Voth,
P.; Spaniol, T. P.; Okuda, J. Organometallics 2008, 27, 3774.
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(16) Huheey, J. E. In Inorganic Chemistry, 3rd ed.; Harper & Row:
New York, 1983.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and CIF files giving detailed experimental
procedures, characterization data for the products, and
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nakazawa@sci.osaka-cu.ac.jp.
■
Notes
The authors declare no competing financial interest.
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