Synthesis and catalytic application in hydrosilylation of the complex mer-hydrido(2-mercaptobenzoyl)tris(trimethylphosphine)cobalt(III)

Article abstract of DOI:10.1021/om4005687

The sulfur-coordinated acyl(hydrido)cobalt(III) complex 1 was synthesized by reaction of thiosalicylaldehyde with CoMe(PMe₃)₄. The crystal structure of 1 was determined by X-ray diffraction. Complex 1 is an excellent catalyst for the hydrosilylation of aldehydes and ketones under mild conditions. This might be the first example of hydrosilylation of aldehydes and ketones catalyzed by (hydrido)cobalt complexes.

Full text of DOI:10.1021/om4005687

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pubs.acs.org/Organometallics
Synthesis and Catalytic Application in Hydrosilylation of the
Complex mer-Hydrido(2-
mercaptobenzoyl)tris(trimethylphosphine)cobalt(III)
Qingfen Niu,† Hongjian Sun,† Xiaoyan Li,*,† H.-F. Klein,‡ and Ulrich Florke§
̈
†School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education,
Shandong University, Shanda Nanlu 27, 250199 Jinan, People’s Republic of China
‡Institut fur Anorganische Chemie der Technischen Hochschule Darmstadt, Petersenstrasse 18, 64287 Darmstadt, Federal Republic
̈
of Germany
§Anorganische und Analytische Chemie, Fakultat fur Naturwissenschaften, Universitat Paderborn, Warburgerstrasse 100, 33098
̈
̈
̈
Paderborn, Federal Republic of Germany
S
* Supporting Information
ABSTRACT: The sulfur-coordinated acyl(hydrido)cobalt(III) complex 1 was
synthesized by reaction of thiosalicylaldehyde with CoMe(PMe3)4. The crystal
structure of 1 was determined by X-ray diffraction. Complex 1 is an excellent
catalyst for the hydrosilylation of aldehydes and ketones under mild conditions.
This might be the first example of hydrosilylation of aldehydes and ketones
catalyzed by (hydrido)cobalt complexes.
ransition-metal hydrides are important intermediates in
industrial and chemical processes. Some of them have
silylation catalysts involving well-defined Co(III) precursors are
rare.
T
been widely used as catalysts in different transformations.1
Metal hydrides are also involved in C−H, S−H, O−H, and H−
H bond activations. The cleavage of a M−H bond is an
indispensable step in catalytic and stoichiometric reactions of
transition-metal hydrides. In these reactions the hydrido
hydrogen exhibits versatility and can react as a hydrogen
atom,2 a proton,3 or a hydride.4 The negatively charged
transition-metal hydrides exhibit good hydridic reactivity and
can be rationally utilized in the catalytic cycles by governing
cleavage of the M−H bond. Furthermore, hydrido−metal-
lothiol complexes have been the focus of great attention due to
the particularly interesting structures exhibited by various
coordination modes5 (terminal, edge, face bridging, or
interstitial), bonding, and function of some relevant catalytic
processes.6
In this paper we report the synthesis of the sulfur-
coordinated acyl(hydrido)cobalt(III) complex 1 by reaction
of thiosalicylaldehyde with methyltetrakis(trimethyl-
phosphine)cobalt(I), CoMe(PMe3)4. More importantly, com-
plex 1 has excellent catalytic activity for the hydrosilylation of
aldehydes and ketones under mild conditions. To our
knowledge, this is the first example of the hydrosilylation of
CO bonds of aldehydes and ketones catalyzed by a
(hydrido)cobalt complex.
C−H and S−H bond activation reactions of thiosalicylalde-
hyde with Co(PMe3)4Me afforded the mer-hydrido(2-
mercaptobenzoyl)tris(trimethylphosphine)cobalt(III) complex
1 as brown-yellow crystals in a yield of 84% (eq 1).
The reduction of carbonyl functionalities such as aldehyde
and ketone groups to useful alcohols through hydride transfer
plays an important role in organic synthesis. Metal-catalyzed
hydrosilylation reactions across the CO bond of aldehydes
and ketones constitute a practical protocol in synthetic
chemistry, because the reduction and protection steps are
carried out in a single and atom-efficient fashion.7 In
comparison with the use of stoichiometric quantities of harsh
reducing agents such as lithium aluminum hydride and sodium
borohydride, these reactions could be performed under mild
conditions in the presence of a catalyst. Hydrosilylation
catalysis has been reported for more than 10 years with various
transition-metal complexes such as tungsten,8 molybdenum,9
manganese,10 rhenium,711 rhodium,12 silver,13 ruthenium,14
iron,15 and nickel.16 To our knowledge, analogous hydro-
Product 1 is a hexacoordinate cobalt(III) complex containing
a five-membered chelate ring with [C,S] coordination. In the IR
spectra the characteristic ν(Co−H) band is found at 1908 cm−1
for complex 1, showing an unusually strong absorption peak. In
comparison with the stretch of the Co−H bond of the
hydrido(acyl)enolatocobalt(III) complexes (1870−1898
cm−1),17 a hypochromic shift is observed. This could be
explained by the replacement of the harder O atom by the soft
Received: June 21, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4005687 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Note
S atom because cobalt(III) is a hard acid. The weaker trans
influence of the S atom makes the Co−H bond in complex 1
stronger than those in hydrido(acyl)enolatocobalt(III) com-
plexes.17 The typical band of the PMe3 ligands is recorded at
944 cm−1 for complex 1. The ν(CO) and ν(CC) bands
are shifted from 1690 and 1589 cm−1 for thiosalicylaldehyde18
to 1589 and 1556 cm−1 for complex 1 owing to the
coordination of the acyl O and S atoms to the Co atom and
Table 1. Catalytic Activity of 1 in the Hydrosilylation of
Aldehydes
a
1
the weakening of these bonds. In the H NMR spectra, the
resonance of Co−H is recorded as a triplet at −18.60 ppm for
complex 1. Two 31P NMR signals are observed at 6.9 and 14.3
ppm as singlets in an integral ratio of 1:2. This implies that the
two phosphorus atoms are in an axial orientation and one
phosphorus atom is in the equatorial plane. Spectroscopic data
are in accordance with the expected octahedral molecular
configuration and are comparable with those in the literature.17
Complex 1 is stable for at least 1 week in the air. It is proposed
that the possible mechanism of formation of complex 1 is
similar to that of the hydrido(acyl)enolatocobalt(III) com-
plexes through two steps of oxidative addition and the final
reductive elimination reaction.17
The molecular structure of complex 1 with selected bond
distances and angles is shown in Figure 1. The molecular
a
Reaction conditions: RCHO (1.0 mmol), (EtO)3SiH (1.1 mmol),
Figure 1. Molecular structure of 1. Selected distances (Å) and angles
(deg): Co1−H1 = 1.24(3), Co1−C1 = 2.2645(6), Co1−P2A =
2.1972(4), Co1−P2 = 2.1972(4), Co1−P1 = 2.2645(6), Co1−S1 =
2.2896(6); C1−Co1−P2A = 84.94(2), C1−Co1−P2 = 84.94(2),
P2A−Co1−P2 = 158.93(3), C1−Co1−P1 = 176.16(7), P2A−Co1−
P1 = 95.66(2), P2−Co1−P1 = 95.66(2), C1−Co1−S1 = 88.85(6),
P2A−Co1−S1 = 99.11(2), P2−Co1−S1 = 99.11(2), P1−Co1−S1 =
87.31(3).
and 1 (0.010 mmol) in 2.0 mL of THF; 40 °C; 2 h.
carbonyl compound). During the required reaction time (2 h),
benzaldehyde, 2-furaldehyde, and Br-, OH-, and MeO-
substituted benzaldehydes (entries 1−4, 7, and 8, Table 1)
were fully converted to the hydrosilylation products while the
substrates with electron-withdrawing groups could not be
completely converted to the hydrosilylation products (entries 5,
6, and 9). The conversion of the substrates with electron-
withdrawing groups to the hydrosilylation products within the
same reaction time is in the order 4-fluorobenzaldehyde (entry
9) > 2-chlorobenzaldehyde (entry 5) > 2,6-dichlorobenzalde-
hyde (entry 6) (Table 1).
Under similar reaction conditions, the hydrosilylation of
ketones (eq 3) was also carried out. For ketones these
hydrosilylation reactions are obviously sluggish during the
required reaction time (Table 2) and the conversions are lower
(13−37%) within 2 h. An aliphatic ketone, such as cyclo-
hexanone, has no conversion (entry 5). However, the
heterocyclic aromatic ketones (entries 3 and 4) are more
reactive than the phenyl and aliphatic ketones (entries 1, 2, and
5).
structure of complex 1 confirms the hexacoordinate octahedral
geometry with two axial trans phosphine ligands and the
equatorial plane of the chelate [C,S] coordination. These trans
phosphine ligands are symmetry-related by a crystallographic
mirror plane which is coplanar with the molecule’s equatorial
plane. The third P atom and the hydrido H atom are also in the
equatorial plane. This is consistent with the observations from
the NMR spectra. The strong trans influence of the acyl carbon
atom leads to a longer Co1−P1 distance (2.2645(6) Å) in
comparison with Co1−P2/P2A (2.1972(4) Å). The Co1−S1
distance (2.2896(6) Å) is in the normal range. Co1−C1
(2.2645(6) Å) in complex 1 is significantly longer than the
related bond (Co−C1 = 1.952(3) Å) in the corresponding
hydrido(acylenolato)cobalt(III) complex.17 This suggests that
the replacement of an O atom by an S atom has great influence
upon the molecular structure.
To further expand the scope of the application of this
catalytic system, the silyl ether products of reactions 2 and 3
were hydrolyzed with 10% sodium hydroxide to afford alcohols
(eqs 4 and 5). The experimental results are summarized in
Tables 3 and 4. From the results in Table 3 it is concluded that
all of the expected alcohols were obtained in good to excellent
The hydrosilylation of aldehydes and ketones using
(EtO)3SiH as the silyl reagent was studied with complex 1 as
catalyst (eq 2). The desired hydrosilylation products RCH2OSi-
(OEt)3 were obtained with 1 mol % of 1 in THF at 40 °C in
the presence of (EtO)3SiH (1.1 equiv with respect to the
B
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Note
a
Table 2. Catalytic Activity of 1 in the Hydrosilylation of
Ketones
Table 3. Catalytic Hydrosilylation of Aldehydes with 1
a
a
Reaction conditions: RCOR′ (1.0 mmol), (EtO)3SiH (1.1 mmol),
and 1 (0.010 mmol) in 2.0 mL of THF; 40 °C; 2 h.
yields. The aldehyde substrates with electron-withdrawing
groups (entries, 5, 6, and 9) also reached good yields of the
corresponding alcohols for a much longer period.
In comparison with the results in Table 3 the alcohols from
ketones could be obtained in good yields for a much longer
period (8−26 h) and at higher temperature (55 °C) (Table 4).
It is obvious that the electron-withdrawing atoms (N in
pyridinyl, entry 3; S in thienyl, entry 4) make the
corresponding ketones more reactive than the phenyl ketones
(entries 1 and 2) and the aliphatic ketone (entry 5). During an
8 h timespan, 1-(pyridin-2-yl)ethanone and 1-(thiophen-2-
yl)ethanone could be fully converted to the corresponding
alcohols in moderate yield (entry 1, 88%; entry 2, 85%).
Cyclohexanone (entry 5, 26 h) is the least active among the five
substrates.
a
Reaction conditions: RCHO (1.0 mmol), (EtO)3SiH (1.1 mmol),
and 1 (0.010 mmol) in 2.0 mL of THF, 40 °C. b Yields of alcohols.
The proposed reaction mechanism of the hydrosilylation is
shown in Scheme 1. The first step is the ligand replacement of a
PMe3 molecule by one molecule of the carbonyl compound to
afford the π(CO)-coordinated intermediate a. Nucleophilic
attack of the hydrido hydrogen atom at the carbon atom of the
carbonyl group leads to formation of the intermediate b with a
vacant coordination site. Coordination of triethoxysilane to the
Co center in b gives rise to intermediate c, which delivers the
silyl ether via Si−H bond activation in the presence of PMe3
with the recovery of the catalyst, complex 1. A similar
mechanism has been reported by Nikonov19 and Schreiber.20
In order to verify the reaction mechanism proposed in
Scheme 1, the following two experiments were designed. The
first experiment verified that no reaction between complex 1
and (EtO)3SiH occurred under the catalytic reaction conditions
(eq 6), while the second experiment also under the catalytic
reaction conditions indicated that the combination of complex
1 with phenylaldehyde afforded the tetracoordinate cobalt(I)
complex 2 as a red powder (eq 7), which could be formed
through the C,O-coupling of intermediate b in Scheme 1.
Complex 2 is paramagnetic. Unfortunately, an attempt to
obtain crystals of 2 suitable for X-ray single crystal diffraction
failed. Complex 2 was only characterized through IR spectra
and elemental analysis. In the IR spectrum of 2 the typical
absorption band of the CO group appeared at 1699 cm−1.
In the control experiments, no expected products could be
found with Co(PMe3)4, CoMe(PMe3)4, and mer-hydrido(2-
oxobenzoyl)tris(trimethylphosphine)cobalt17 as catalyst under
these catalytic reaction conditions. This indicates that complex
1 with 2-mercaptobenzoyl as the chelate ligand is a good
catalyst for the hydrosilylation of aldehydes and ketones.
In summary, the sulfur-coordinated acyl(hydrido)cobalt(III)
complex 1 was successfully synthesized by the reaction of
thiosalicylaldehyde with CoMe(PMe3)4. It is important to note
C
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Note
a
Table 4. Catalytic Hydrosilylation of Ketones with 1
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that complex 1 is an excellent catalyst for the hydrosilylation of
aldehydes and ketones under mild conditions. This might be
the first example of hydrosilylation of aldehydes and ketones
catalyzed by hydrido cobalt complexes.
ASSOCIATED CONTENT
* Supporting Information
Text giving experimental details and analytical data and a CIF
file giving crystallographic data for 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail for X.L.: xli63@sdu.edu.cn.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge the support by the NSF of China,
No. 21372143.
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