Synthesis of Fe-H/Si-H and Fe-H/Ge-H bifunctional complexes and their catalytic hydrogenation reactions toward nonpolar unsaturated organic molecules

Article abstract of DOI:10.1021/om5000562

The iron hydride complexes bearing a Si-H or a Ge-H functional group in a ligand [2,5-SiPh₃-3,4-butylene-(η⁵-C ₄COEEt₂H)]Fe(CO)₂H (E = Si (4), Ge (5)) and [2,5-SiPh₃-3,4-butylene-(η⁵-C₄COSiMe ₂OSiMe₂H)]Fe(CO)₂H (6), were synthesized in the reaction of the iron acetonitrile complex [2,5-SiPh₃-3,4-butylene- (η⁴-C₄CO)]Fe(CO)₂(NCCH₃) (3) with Et₂EH₂ or HSiMe₂OSiMe₂H. These complexes did not hydrogenate a polar unsaturated bond in either ketones or aldehydes but did hydrogenate a nonpolar unsaturated bond in alkynes and alkenes to give alkenes and alkanes selectively. These complexes also catalyzed a transfer hydrogenation reaction from isopropyl alcohol (IPA) to alkynes and alkenes.

Full text of DOI:10.1021/om5000562

Communication
pubs.acs.org/Organometallics
Synthesis of Fe−H/Si−H and Fe−H/Ge−H Bifunctional Complexes
and Their Catalytic Hydrogenation Reactions toward Nonpolar
Unsaturated Organic Molecules
Masahiro Kamitani, Yoshinori Nishiguchi, Ryosuke Tada, Masumi Itazaki, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
S
* Supporting Information
ABSTRACT: The iron hydride complexes bearing a Si−H or a Ge−H
functional group in a ligand [2,5-SiPh3-3,4-butylene-(η5-
C4COEEt2H)]Fe(CO)2H (E = Si (4), Ge (5)) and [2,5-SiPh3-3,4-
butylene-(η5-C4COSiMe2OSiMe2H)]Fe(CO)2H (6), were synthesized
in the reaction of the iron acetonitrile complex [2,5-SiPh3-3,4-
butylene-(η4-C4CO)]Fe(CO)2(NCCH3) (3) with Et2EH2 or HSi-
Me2OSiMe2H. These complexes did not hydrogenate a polar unsaturated bond in either ketones or aldehydes but did
hydrogenate a nonpolar unsaturated bond in alkynes and alkenes to give alkenes and alkanes selectively. These complexes also
catalyzed a transfer hydrogenation reaction from isopropyl alcohol (IPA) to alkynes and alkenes.
he hydrogenation reaction of unsaturated bonds promoted
by a transition-metal catalyst is one of the most
atoms: one bonded to a metal as a hydride and the other bonded
to an oxygen or a nitrogen atom in its ligand as an acidic proton.
Both of the hydrogen atoms cooperatively interact with a polar
unsaturated bond in an organic molecule such as a ketone or an
aldehyde via an outer-sphere mechanism (Figure 1b).6 These
properties make effective hydrogenation of organic molecules
with a polar unsaturated bond possible. Several examples
involving the Shvo complex catalyze hydrogenation of even a
nonpolar unsaturated bond in organic molecules such as alkenes
and alkynes.7 In these cases, alkene/alkyne insertion into an M−
H bond has been proposed, whereas an acidic proton on a ligand
does not serve as a hydrogen source and remains intact during
the course of the hydrogenation reaction. Milstein and co-
workers reported new type ligand−metal bifunctional Ru
catalysts with an active C−H bond on the pincer PNP or PNN
ligands which transfer two hydrogen atoms from alcohols to
imines.8,9
T
fundamental transformations in organic synthesis. Most hydro-
genation reactions promoted by a transition-metal catalyst
proceed via insertion of an unsaturated bond (R2CX, where X
stands for CR′2, NR′, O) into a metal−hydrogen bond to give
M−X−CHR2 followed by a coupling of the X−CHR2 moiety and
a hydride on M which is produced in the reaction of M with H2
(Figure 1a). A metal−ligand bifunctional catalyst is another type
In iron cases, several bifunctional complexes with Fe−H and
O−H functional groups similar to the Shvo complex (Ru
complex) have been reported and preferential activity has been
exhibited toward a polar unsaturated bond but not toward a
nonpolar bond.10,11 For example, the Knolker complex, being the
̈
first bifunctional iron complex, showed catalytic activity for a
hydrogenation or transfer hydrogenation reaction (hydro-
genation reaction using IPA as a hydrogen source) toward
ketones or aldehydes.12 However, the complex showed no
hydrogenation activity toward alkynes and alkenes. This
preferential reactivity toward polar double bonds is considered
to stem from the nature of the polar transition state in an outer-
sphere mechanism. If a new type of bifunctional complex with
nonpolar (nonacidic) active hydrogen atoms such as a Si−H
functional group on its ligand is used, preferential hydrogenation
Figure 1. Hydrogenation mechanism of unsaturated organic com-
pounds.
of hydrogenation catalyst and is effective for catalytic hydro-
genation of unsaturated organic compounds in the presence of
H2 and for a transfer hydrogenation reaction from HCOOH or
isopropyl alcohol (IPA) as alternative hydrogen sources. The
Shvo complex was the first example of a metal−ligand
bifunctional catalyst that has Ru−H and O−H functional
groups.1 Since its discovery in 1986, various types of metal−
ligand bifunctional complexes have been developed.2−5 These
complexes are characterized by two kinds of active hydrogen
Received: January 28, 2014
Published: April 1, 2014
© 2014 American Chemical Society
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Organometallics
Communication
toward nonpolar unsaturated bonds is expected. However, a
bifunctional catalyst with such a functional group has not been
reported to date.
Scheme 3
In this communication, we report the preparation and isolation
of new iron hydride complexes bearing a Si−H or a Ge−H bond
tethered to the Cp derivative ligand and their hydrogen transfer
activity toward unsaturated organic compounds.
The iron tricarbonyl complex [2,5-bis(triphenylsilyl)-3,4-
butylene-(η4-C4CO)]Fe(CO)3 (2) was synthesized by the
reaction of 1,8-bis(triphenylsilyl)-1,7-octadiyne (1) with Fe-
(CO)5 through [2 + 2 + 1] cycloaddition (Scheme 1).13 To
Scheme 1
[ 2 , 5 - b i s ( t r i p h e n y l s i l y l ) - 3 , 4 - b u t y l e n e - ( η 5 -
C4COSiMe2(OSiMe2)nH)]Fe(CO)2H. Complex 6 (n = 1) was
obtained in 80% yield in the same way as for of 4 and 5. However,
the reactions with a longer tether (n = 2, 3) caused formation of a
mixture of the desired products (60−70% yield) and byproducts
1
introduce a hydride ligand onto Fe and a silyl group onto the
cyclopentadienone ligand in 2, reactions of 2 with hydrosilanes
were carried out, as Casey and co-workers reported the synthesis
of a ruthenium hydride complex with a silyl group on the
cyclopentadienyl ligand in the reaction of the Shvo complex (Ru
complex) with triethylsilane.14 However, in our case, no reaction
took place under thermal conditions, and several hydride
complexes were formed under the photoirradiation conditions.
These results indicate that not even one CO ligand in 2
dissociates from the iron center under thermal conditions but
that more than one CO dissociation followed by Si−H oxidative
addition toward the Fe center takes place under photoirradiation
to give several hydride complexes. We sought another method to
produce a 16e species without photoirradiation and found that
[2,5-bis(triphenylsilyl)-3,4-butylene-(η4-C4CO)]Fe-
(CO)2(NCCH3) (3), having one acetonitrile in place of one CO,
was a good starting complex.
which could not be identified, although their H NMR spectra
indicated the formation of iron hydride complexes. These
byproducts could not be removed because their solubility was
similar to that of the desired complex.
The identification of 1−6 was carried out on the basis of their
spectroscopic data and elemental analyses. Molecular structures
of 2−4 and 6 were confirmed by X-ray diffraction studies, and
their ORTEP drawings are depicted in Figure 2. Complexes 2
and 3 have typical three-legged piano-stool geometries with one
η4-2,5-bis(triphenylsilyl)-3,4-butylenecyclopentadienone ligand
and three carbonyl ligands for 2 and two carbonyl ligands and
one η1-acetonitrile ligand for 3. These molecular structures are
similar to those of the trimethylsilyl analogues reported by
Knolker and co-workers [2,5-bis(trimethylsilyl)-3,4-butylene-
̈
16
(η4-C4CO)]Fe(CO)3 and [2,5-bis(trimethylsilyl)-3,4-buty-
lene-(η4-C4CO)]Fe(CO)(NCCH3)2.15 Complexes 4 and 6
have one hydride ligand, one η5-2,5-bis(triphenylsilyl)-3,4-
butylene-1-diethylsiloxycyclopentadienyl (or η5-2,5-bis-
(triphenylsilyl)-3,4-butylene-1-tetramethyldisiloxyl) ligand, and
two carbonyl ligands.
The photoreaction of 2 in the presence of an excess amount of
acetonitrile at 5 °C showed the formation of monoacetonitrile
complex 3, but diacetonitrile complex 3′ was also produced in the
first stage of the reaction (Scheme 2). Stirring the reaction
Although the main frameworks of 4 and 6 are the same as those
of 2 and 3, the phenyl rings in the triphenylsilyl moieties are
twisted to avoid steric hindrance with the diethylsilyl and
tetramethyldisiloxyl moieties. The C3−O3 bond distance is
1.355(3) Å for both 4 and 6. This is clearly longer than those for
2 and 3, indicating that the C3−O3 has single-bond character.
The Fe−H bond distances are 1.43(3) Å for 4 and 1.44(3) Å for
6, and the Si−H distances are 1.38(5) Å for 4 and 1.50(3) Å for 6.
These are similar to the bond distances of the relevant iron
hydride complexes17 and hydrosilanes.18 The bond distances
(6.27(5) Å for 4 and 7.66(5) Å for 6) between the two hydrogen
atoms on the Fe and Si show no interaction between them.
As bifunctional iron complexes 4−6 were obtained, reactions
of these complexes with various unsaturated organic compounds
were conducted. Conventional bifunctional complexes which
possess O−H on the ligand and M−H bonds have been reported
to react predominantly with ketone or aldehyde rather than with
alkyne or alkene.6,11 In stark contrast, 4 did not react with methyl
phenyl ketone even under thermal reaction conditions (up to 80
°C), but 4 reacted with a stoichiometric amount of p-
tolylacetylene to give the corresponding alkene and alkane in
18 and 9% yields, respectively (Scheme 4). The reactivity of 4 is
considered to be attributable to the polarity of the Si−H bond
being opposite to that of an O−H or N−H bond.
Scheme 2
mixture at ambient temperature for 24 h without photolysis
caused the conversion of 3′ to 3, and 3 could be isolated in 85%
yield. A similar AN/CO exchange reaction has been reported by
Knolker and co-workers.15
̈
The reaction of Et2EH2 (E = Si, Ge) with 3 gave the
corresponding iron hydride complexes bearing an Si−H or Ge−
H functional group on the ligand, [2,5-bis(triphenylsilyl)-3,4-
butylene-(η5-C4COEEt2H)]Fe(CO)2H (E = Si (4; 87% yield),
Ge (5; 70% yield)) (Scheme 3). Characteristic Fe−H signals of
C6D6 solutions of 4 and 5 in the 1H NMR spectra were recorded
at −10.89 and −11.04 ppm, respectively. The Si−H and Ge−H
signals were also observed at 4.06 and 5.04 ppm, respectively.
Reactions of 3 with HSiMe2(OSiMe2)nH (n = 1−3) were also
carried out to obtain a series of iron complexes bearing an Si−H
moiety connected to the Cp carbon through a (SiMe2O)n chain,
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Organometallics
Communication
Table 1. Transfer Hydrogenation Reaction from IPA to p-
a
Tolylacetylene by 4−6
b
yield (%)
entry
complex
time (h)
product 7
product 8
1
2
3
4
5
6
4
5
6
6
2
3
24
24
24
48
24
24
8
7
trace
trace
8
27
30
0
14
0
0
0
a
The reaction was carried out with 2-propanol (320 μmol), alkyne (63
μmol), and catalyst (6.3 μmol (10 mol % based on alkyne)) in toluene
b
(20 mL) at 75 °C for 24 h, except for entry 4. Determined by GC-MS
measurements.
necessary to have a contact of the Si−H group with the Fe center.
The nonfunctionalized complexes 2 and 3 did not show activity
toward hydrogen transfer reaction (entries 5 and 6).19 This is the
first example of iron promoting a transfer hydrogenation reaction
from an alcohol to an alkyne or an alkene in one pot. Such
catalytic transfer hydrogenation reactions have been reported for
Ru, Rh, Ir, and Pd,20 and most of them require addition of a base
such as K2CO3, Na2CO3, or KOH.
Figure 2. ORTEP drawings of 2 (upper left), 3 (upper right), 4 (lower
left), and 6 (lower right) with 50% thermal ellipsoid plots. One of the
independent molecules in each crystal structure of 2 and 3 is shown. The
crystallization solvent and hydrogen atoms except for Fe−H and Si−H
are omitted for clarify. Selected bond lengths (Å) are as follows. For 2:
Fe1−C1, 1.828(4); Fe1−C2, 1.787(4); Fe1−C3, 1.788(4); C1−O1,
1.132(5); C2−O2, 1.143(5); C3−O3, 1.139(5); C5−Si1, 1.877(3);
C8−Si2, 1.888(3); C4−O4, 1.238(4). For 3: Fe1−N1, 1.942(3); Fe1−
C1, 1.779(4); Fe1−C2, 1.774(4); C1−O1, 1.143(5); C2−O2,
1.140(6); C6−Si1, 1.878(4); C9−Si2, 1.878(4); C5−O3, 1.244(4);
N1−C3, 1.133(5); C3−C4, 1.458(5). For 4: Fe1−C1, 1.765(3); Fe1−
C2, 1.745(3); Fe1−H1Fe, 1.43(3); C1−O1, 1.150(4); C2−O2,
1.143(4); C4−Si2, 1.878(3); C7−Si3, 1.881(2); C3−O3, 1.355(3);
O3−Si3, 1.703(3); Si1−H1Si, 1.38(5); H1Fe−H1Si, 6.27(5). For 6:
Fe1−C1, 1.749(3); Fe1−C2, 1.747(3); Fe1−H1Fe, 1.44(3); C1−O1,
1.145(3); C2−O2, 1.150(3); C3−O3, 1.355(3); C4−Si3, 1.876(2);
C11−Si4, 1.879(2); Si2−H2Si, 1.50(3), H1Fe−H2Si, 7.66(5).
To obtain insight into the reaction mechanism of the
hydrogenation reaction of alkyne or alkene catalyzed by 6, the
stoichiometric reaction of 6 with p-tolylacetylene in C6D6 was
1
monitored using H NMR spectrometry. After the reaction
mixture of 6 was heated with p-tolylacetylene at 75 °C for 1 h,
new signals assignable to a vinyl group were observed at 5.82 and
6.45 ppm. In addition, the appearance of a new signal at 3.69 ppm
(assignable to a Si−H bond) appeared and the Si−H and Fe−H
signals of the starting materials disappeared. These results
suggest the formation of a vinyl iron complex, [2,5-SiPh3-3,4-
butylene-(η5-C4COSiMe2OSiMe2H)]Fe(CO)2{CHCH(p-
tolyl)}. In addition, two hydrogen atoms on Fe−H and Si−H are
not cooperatively introduced into alkyne/alkene but are
introduced stepwise into the substrates.
A plausible catalytic cycle is shown in Scheme 5. CO
dissociation of the starting complex (4−6) forms the 16e species
A. The alkyne coordinates to A and inserts into the Fe−H bond
Scheme 4
Scheme 5
Next, this stoichiometric reaction was extended to a catalytic
reaction. Treatment of 4 with 10 equiv of p-tolylacetylene and 5
equiv of IPA as a hydrogen source at 75 °C resulted in the
formation of only 8% of 4-methylstyrene (7) and a trace amount
of 4-ethylmethylbenzene (8) (Table 1, entry 1). Complex 5 also
showed poor reactivity similar to that of 4 under the same
reaction conditions (entry 2). However, 6 exhibited catalytic
activity and gave the corresponding 7 and 8 in 27% and 8% yields,
respectively (entry 3).
Slightly better conversion was obtained by using prolonged
reaction time (entry 4). The low catalytic activities of 4 and 5 in
comparison with that of 6 indicate that a sufficient length of the
chain between the Cp ring and the Si−H functional group is
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Organometallics
Communication
Johnson, J. B. J. Am. Chem. Soc. 2008, 130, 2285. (f) Casey, C. P.; Vos, T.
E.; Singer, S. W.; Guzei, I. A. Organometallics 2002, 21, 5038.
(4) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97.
to give B. Intramolecular oxidative addition of the Si−H bond in
the Cp tether takes place to give C. The low activity of 4 and 5
may be ascribed to the ring strain in C. Reductive elimination of
the H and vinyl group on Fe yields alkene and D. Finally, the
starting hydride complex A is reproduced by the abstraction of
two hydrogen atoms from IPA to form Si−H and Fe−H bonds.
IPA is simultaneously oxidized to acetone. A similar catalytic
cycle for alkene in place of alkyne is conceivable to form the
alkane. [2,5-SiPh3-3,4-butylene-(η5-C4COSiMe2OSiMe2H)]Fe-
(CO)2{CHCH(p-tolyl)} produced in the stoichiometric
reaction mentioned above corresponds to the iron complex
which is formed by CO coordination to B. An alternative
pathway involving ring slippage of η5-Cp to η3-Cp in place of the
CO dissociation cannot be ruled out because the slippage
pathway was reported in a reaction of a ruthenium complex with
an alkyne by Shvo and co-workers.7 Further mechanistic studies
on the dehydrogenation step (D to A) will be reported
elsewhere.
(5) (a) Kashiwame, Y.; Kuwata, S.; Ikariya, T. Organometallics 2012,
31, 8444. (b) Ikariya, T. Bull. Chem. Soc. Jpn. 2011, 84, 1. (c) Ikariya, T.;
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Wang, H.; Watanabe, M.; Gridnev, I. D.; Ikariya, T. Tetrahedron Lett.
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Soc. Jpn. 2012, 85, 316. (g) Watanabe, M.; Kashiwame, Y.; Kuwata, S.;
Ikariya, T. Eur. J. Inorg. Chem. 2012, 504. (h) Touge, T.; Hakamata, T.;
Nara, H.; Kobayashi, T.; Sayo, N.; Saito, T.; Kayaki, Y.; Ikariya, T. J. Am.
Chem. Soc. 2011, 133, 14960. (i) Ito, M.; Ootsuka, T.; Watari, R.;
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́
(6) (a) Comas-Vives, A.; Ujaque, G.; Lledos, A. Organometallics 2007,
In summary, a new series of bifunctional piano-stool iron
complexes with an Fe−H and a Si−H/Ge−H bond tethered to
the Cp ligand were prepared. The investigation of the activity of
these complexes with organic compounds with an unsaturated
bond revealed that they did not transfer the two hydrogens to a
polar unsaturated bond in ketones and aldehydes but could
transfer them to a nonpolar unsaturated bond in alkynes and
alkenes when the length of the Si−H tether is appropriate. In
addition, these iron complexes served as a catalyst in the reaction
of alkynes with IPA. This is the first example of an iron-catalyzed
transfer hydrogenation reaction from alcohols to alkyne/alkene.
26, 4135. (b) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem.
2001, 66, 7931. (c) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc.
2000, 122, 1466. (d) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68,
1998.
(7) Shvo, Y.; Goldberg, I.; Czerkie, D.; Reshef, D.; Sterin, Z.
Organometallics 1997, 16, 133.
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2011, 44, 588.
(9) For selected papers see: (a) Zhang, J.; Leitus, J.; Ben-David, Y.;
Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840. (b) Pingen, D.; Muller,
C.; Vogt, D. Angew. Chem., Int. Ed. 2010, 49, 8130.
(10) (a) Thorson, M. K.; Klinkel, K. L.; Wang, J.; Williams, T. J. Eur. J.
Inorg. Chem. 2009, 295. (b) Bianchini, C.; Farnetti, E.; Graziani, M.;
Peruzzini, M.; Polo, A. Organometallics 1993, 12, 3753.
(11) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2009, 131, 2499.
(12) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816.
(13) (a) Schrauzer, G. N. Chem. Ind. 1958, 1403. (b) Schrauzer, G. N. J.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving detailed experimental
procedures and the characterization data for the products and
crystallographic data for 2−4 and 6. This material is available free
of charge via the Internet at http://pubs.acs.org.
Am. Chem. Soc. 1959, 81, 5307. (c) Knolker, H.-J.; Heber, J.; Mahler, C.
̈
H. Synlett 1992, 12, 1002.
(14) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J. Am. Chem. Soc. 2001, 123, 1090.
AUTHOR INFORMATION
Corresponding Author
*E-mail for H.N.: nakazawa@sci.osaka-cu.ac.jp.
Notes
■
(15) Knolker, H.-J.; Goesmann, H.; Klauss, R. Angew. Chem. 1999, 111,
̈
727; Angew. Chem., Int. Ed. 1999, 38, 702.
(16) (a) Knolker, H.-J.; Heber, J. Synlett 1993, 924. (b) Knolker, H.-J.;
̈
̈
Baum, E.; Heber, J. Tetrahedron Lett. 1995, 36, 7647.
The authors declare no competing financial interest.
(17) Knolker, H.-J.; Baum, E.; Goesmann, H.; Klauss, R. Angew. Chem.,
̈
Int. Ed. 1999, 38, 2064.
ACKNOWLEDGMENTS
(18) (a) Sutton, L. E., In Table of Interatomic Distances and
Configuration in Molecules and Ions, 1958, No.11. (b) Mitchel, A. D.;
Somerfield, A. E.; Cross, L. C. Table of Interatomic Distances and
Configuration in Molecules and Ions; The Chemical Society: London,
1965; No. 18.
■
This work was supported by a Challenging Exploratory Research
grant (No. 25620048) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
(19) For further optimization of reaction conditions and the reactivity
toward other unsaturated compounds, see the Supporting Information.
(20) For selected papers see: (a) Nixon, T. D.; Whittlesey, M. K.;
Williams, J. M. J. Dalton Trans. 2009, 753. (b) Grigg, R.; Mitchell, T. R.
B.; Sutthivaiyakit, S.; Tongpenyai, N. Tetrahedron Lett. 1981, 22, 4107.
(c) Kwon, M. S.; Kim, K.; Seo, S. H.; Park, I. S.; Cheedrala, R. K.; Park, J.
Angew. Chem., Int. Ed. 2005, 44, 6913.
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