Pincer Cobalt Hydride Catalyzed Distinct Selective Hydrosilylation of Aryl Alkene and Alkyl Alkene

Article abstract of DOI:10.1021/acs.organomet.0c00251

The reactions of unsymmetrical N-heterocyclic carbene (NHC) [CNC]-pincer preligands with CoMe(PMe3)4 gave rise to NHC [CNC]-pincer cobalt(III) hydrides, [(CcarbeneNaminoCnaphthyl)Co(H)(PMe3)2] (3a) and (3b), via Csp2-H activation and the unexpected trans-bischelate [Ccarbene, Namino] cobalt(II) complexes 4a and 4b via a disproportionation reaction, respectively. It was found that both 3a and 3b are efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as substrates, 3a has high Markovnikov selectivity in excellent yields, while 3a is an efficient anti-Markovnikov catalyst in good yields with alkyl alkenes as substrates. The catalytic process could be promoted with pyridine N-oxide as an initiator. The catalytic mechanisms for the two different selectivities were proposed. Complexes 3a, 3b, 4a, and 4b were characterized by spectroscopic methods, and the molecular structures of 3b, 4a, and 4b were determined by single crystal X-ray diffraction.

Full text of DOI:10.1021/acs.organomet.0c00251

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Article
[CNC]-Pincer Cobalt Hydride Catalyzed Distinct Selective
Hydrosilylation of Aryl Alkene and Alkyl Alkene
Shangqing Xie, Xiaoyan Li,* Hongjian Sun,* Olaf Fuhr, and Dieter Fenske
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ABSTRACT: The reactions of unsymmetrical N-heterocyclic carbene (NHC)
[CNC]-pincer preligands with CoMe(PMe3)4 gave rise to NHC [CNC]-pincer
cobalt(III) hydrides, [(CcarbeneNaminoCnaphthyl)Co(H)(PMe3)2] (3a) and (3b), via
Csp2−H activation and the unexpected trans-bischelate [Ccarbene, Namino] cobalt(II)
complexes 4a and 4b via a disproportionation reaction, respectively. It was found that
both 3a and 3b are efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as
substrates, 3a has high Markovnikov selectivity in excellent yields, while 3a is an
efficient anti-Markovnikov catalyst in good yields with alkyl alkenes as substrates. The
catalytic process could be promoted with pyridine N-oxide as an initiator. The catalytic
mechanisms for the two different selectivities were proposed. Complexes 3a, 3b, 4a,
and 4b were characterized by spectroscopic methods, and the molecular structures of
3b, 4a, and 4b were determined by single crystal X-ray diffraction.
Markovnikov product in high yield.6 In 2018, Deng and co-
workers disclosed that (IMes)2CoCl was an efficient catalyst
for the hydrosilylation of alkenes. When aryl-substituted
alkenes were employed as substrates, (IMes)2CoCl generated
the Markovnikov hydrosilylation products in most cases.
Meanwhile, the anti-Markovnikov hydrosilylation products
could be obtained when aliphatic alkenes were studied as
substrates.7 The case of the substrate-dependent regioselectiv-
ity can also be found in Trovitch’s work, and the difference of
the stability of the alkyl cobalt intermediates generated in the
reaction may be responsible for different selectivities.27 Unlike
Deng’s and Huang’s work, Lu and co-workers reported a cobalt
catalyst for alkene hydrosilylation in 2017. This catalytic
system afforded the same regioselectivity for styrenes and
aliphatic alkenes using the same cobalt catalyst and showed
stronger selectivity.14 In addition, in the transition metal
mediated hydrosilylation of alkenes, metal hydride is
considered a key intermediate,15 but there are rare examples
to study the hydrosilylation of alkenes with metal hydride as a
catalyst.
1. INTRODUCTION
Organosilicon compounds have been widely used in
electronics, building materials, textiles, the light industry,
medical treatments, transportation, and plastic rubber and
other industries and gradually penetrated into people’s daily
lives.1,2 Transition metal catalyzed hydrosilylation of alkenes is
an important method for the synthesis of organosilicon
compounds. At present, the catalysts successfully used in
industrial production are platinum complexes.3 However,
platinum complexes as catalysts have some disadvantages.
On the one hand, the platinum catalyst as a precious metal
compound in the hydrosilylation of alkenes cannot be
regenerated. On the other hand, the toxicity of residual
platinum in the final silicon-containing products to a certain
extent limits the application of silicon materials. In recent
years, with people’s increasing attention to environmental
protection and the pursuit of sustainable development, we
gradually began to seek the replacement of precious metal
catalysts in the field of catalytic chemistry. The environmental-
friendly base metal cobalt as the congeneric element of
precious metals has attracted the attention of chemists. Since
Chalk and Harrod found that Co2(CO)8 could catalyze the
hydrosilylation of terminal alkenes with anti-Markovnikov
selectivity,4 some cobalt complexes have been synthesized to
catalyze the hydrosilylation of alkenes.5−14 However, there are
few reports on cobalt-catalyzed hydrosilylation of alkenes with
Markovnikov selectivity.14 In 2016, Huang and co-workers
prepared a series of [PNN]-pincer metal complexes to catalyze
the hydrosilylation of alkenes and found the related iron
complexes were selective for anti-Markovnikov hydrosilyla-
tions, while the related cobalt complexes provided the
In this paper, two [CNC]-pincer cobalt(III) hydrides were
synthesized. It was found that the cobalt hydrides as catalysts
are highly selective for Markovnikov hydrosilylation of aryl
alkenes and for anti-Markovnikov hydrosilylation of alkyl
Received: April 10, 2020
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Scheme 1. Synthesis of [CNC]-Pincer Cobalt Hydrides 3a and 3b
alkenes. The catalytic process could be promoted with pyridine
N-oxide as an initiator. The proposed mechanisms of the
catalytic hydrosilylation of alkenes were discussed and partially
experimentally verified. The key step in the catalytic cycle is
the insertion of alkene into the Co−H bond. For this
hydrosilylation reaction, the alkyl cobalt(III) intermediate
was proposed as the key intermediate.
2. RESULTS AND DISCUSSION
2.1. Synthesis of [CNC]-Cobalt Hydrides 3. The
unsymmetrical N-heterocyclic carbene (NHC) ligand pre-
cursors 2, imidazolium salts, were synthesized according to the
reported method (Scheme 1).16 After the lithiation of 2 with
nBuLi, CoMe(PMe3)4 was added to give rise to a mixture of
the expected [CNC]-pincer cobalt hydride 3 and unexpected
trans-bischelate cobalt(II) complex 4. The extraction of the
crude mixture with n-pentane and recrystallization from the
concentrated n-pentane solution afforded yellow crystals of 3a
and 3b. After that, the extraction of the residue with diethyl
ether and recrystallization from the concentrated diethyl ether
solution provided orange crystals of 4a and 4b.
Figure 1. Molecular structure of 3b at the 50% probability level.
Hydrogen atoms except for the hydrido atom are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Co1−H1 1.34(4), Co1−
P1 2.165(4), Co1−P2 2.208(4), Co1−N3 2.001(11), Co1−C2
1.906(13), Co1−C13 1.941(13); P1−Co1−P2 167.081(16), N3−
Co1−P1 96.11(3), N3−Co1−P2 95.41(3), C2−Co1−P1 94.82(4),
C2−Co1−P2 90.69(4), C2−Co1−N3 91.62(5), C2−Co1−C13
174.09(5), C13−Co1−P1 90.06(4), C13−Co1−P2 85.22(4), C13−
Co1−N3 84.53(5), P1−Co1−H1 82.3(15), P2−Co1−H1 85.6(15),
N3−Co1−H1 172.9(15), C2−Co1−H1 95.4(15), C13−Co1−H1
88.6(15).
Complexes 3 and 4 are sensitive to moisture and air. In the
infrared spectra of complexes 3a and 3b, the typical ν(Co−H)
stretching vibrations were found at 1904 cm−1 (3a) and 1920
cm−1 (3b), respectively. The NMR analyses of complexes 3a
and 3b are consistent with their compositions. In the 1H NMR
spectra, the hydrido signal was shifted significantly upfield to
−21.21 ppm (3a) and −22.14 ppm (3b), respectively. The
signal split into a triplet peak, owing to the coupling with the
coordinated P atoms (2JH,P = 69 Hz). The 31P NMR
resonances of complexes 3a and 3b appeared at 17.18 and
17.82 ppm as a singlet, respectively, suggesting the presence of
two equivalent PMe3 ligands.
The solid state molecular structure of complex 3b was
confirmed by single crystal X-ray diffraction analysis (Figure
1). The central cobalt atom of complex 3b is situated in a
distorted octahedral geometry surrounded by a mer-[CNC]-
pincer ligand and two PMe3 ligands disposed trans to each
other (P1−Co1−P2 = 167.08°) as well as a hydrido ligand. In
complex 3b, the Co−C(carbene) distance (Co1−C2 = 1.906(13)
Å) is shorter than the Co1−C13 distance (1.941(13) Å) due to
the double bond component in the Co−C(carbene) bond and is
in the range of those of other NHC cobalt complexes.17 The
Co1−H1 bond length (1.34(4) Å) is slightly shorter than the
normal bond length of Co−H bonds (1.40−1.50 Å).18
Complexes 4a and 4b are different from the other bischelate
NHCs metal complexes.19 Paramagnetic complexes 4a and 4b
are soluble in diethyl ether and insoluble in n-pentane. The
molecular structures of 4a and 4b were also confirmed by
single crystal X-ray diffraction analysis (Figures 2 and 3). The
structures of 4a and 4b feature a distorted tetrahedral Co(II)
center with two trans-positioned [C,N]-chelate ligands bearing
the naphthalene rings. The averaged Co−CNHC and Co−Namino
bond lengths are 1.998(5) and 1.971(3) Å, respectively, close
to those of the analogous complexes.20
B
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of styrene with Ph2SiH2 at 70 °C for 10 h (Scheme 3). The
experimental results are summarized in Table 1. From Table 1,
we know that the reaction has a conversion of 98% in excellent
selectivity (b/l = 94:6) with 3a as a catalyst without solvent
(entry 1, Table 1). The reaction selectivity in common solvents
is not as good as that in solvent-free reactions (entries 1 and
2−6, Table 1). Complex 3a has a little bit better catalytic
activity and selectivity than complex 3b (entries 1 and 7, Table
1). When the reaction temperature dropped from 70 to 50 °C
and 25 °C, the conversion decreased from 90% to 87% and
56%, respectively, but the selectivity is still good (entries 1 and
8−9, Table 1). If the catalyst loading decreased from 1 mol %
to 0.5% and 0.1%, the conversion decreased from 98% to 91%
and 69%, respectively (entries 1 and 10−11, Table 1). When
the reaction time was shortened to 6 h, the conversion was
reduced to 83% (entry 12, Table 1). A longer reaction time
(12 h) resulted in only a slight increase in conversion (entry
13, Table 1). In addition, when various silanes were used for
the reaction, Ph2SiH2 had the best catalytic result (entries 1
and 14−18, Table 1). It was also found that the addition of
pyridine N-oxide at 25 °C increased the conversion from 78%
to 95% within 24 h (entries 19−20) and with better selectivity
(b/l = 96:4). This suggests that pyridine N-oxide can
significantly promote the reaction. It is considered that the
addition of pyridine N-oxide facilitates the dissociation of
PMe3 ligand of complex 3 by the formation of stable O
PMe3. This result indicates that the formation of an active
unsaturated coordination intermediate produced via dissocia-
tion of PMe3 is the first step in the catalytic cycle. The vacant
coordination site is essential for the coordination of styrene
with the cobalt center. Therefore, the optimized reaction
conditions are as follows: 1 mol % 3a, Ph2SiH2, solvent-free, 25
°C, 24 h, and pyridine N-oxide.
Figure 2. Molecular structure of 4a at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Co1−N1 1.971(3), Co1−N4 1.940(4), Co1−C1
71.997(5), Co1−C40 1.998(5); N1−Co1−C17 91.78(16), N1−
Co1−C40 124.52(16), N4−Co1−N1 119.76(15), N4−Co1−C17
120.06(17), N4−Co1−C40 92.98(16), C17−Co1−C40 109.85(18).
Under the optimized conditions, we explored the substrate
scope of alkenes with [CNC]-pincer cobalt hydride 3a as a
catalyst (Table 2). High Markovnikov regioselectivities were
observed in the hydrosilylation of the aryl alkenes bearing
halogen (5e−5f), methoxy (5i), methyl (5b−5d), tert-butyl
(5h), trifluoromethyl (5k and 5l), and phenyl (5m) groups.
The synthesis of 5e was only mentioned in one patent while
there are a few reports on the synthesis of the other reaction
products.21 The results in Table 2 indicate that the electronic
properties of the aryl group have a significant effect on the
selectivity of the hydrosilylation of the alkenes. The aryl
alkenes with the electron-withdrawing substituents gave a
lower selectivity compared to the aryl alkenes with the
electron-donating substituents. The steric hindrance of the
substituents has little influence upon the catalytic reaction
compared to the electronic property of the substrate. As the
phenyl alkenes, the naphthyl alkenes could also undergo
hydrosilylation to afford the corresponding product in modest
to good yields with high regioselectivities (5n and 5o). For the
diphenyl substituted alkene, 1,1-diphenyl-ethylene, the cobalt
complex 3a had no catalytic activity due to the strong steric
effect of the two phenyl groups (5p). Surprisingly, alkyl alkenes
exhibited reverse regioselectivity to aryl alkenes toward anti-
Markovnikov addition (5q−5t). This indicates that the
aromatic rings in the aryl alkenes might play an important
role for Markovnikov selectivity. In this catalytic system, allyl
benzene also afforded an anti-Markovnikov addition product
(5r) with excellent selectivity.
Figure 3. Molecular structure of 4b at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Co1−N3 1.909(5), Co1−N6 1.944(4), Co1−C2
1.987(5), Co1−C22 1.990(5); N3−Co1−N6 121.7(3), N3−Co1−
C2 91.8(2), N3−Co1−C22 114.1(2), N6−Co1−C2 122.81(18),
N6−Co1−C22 91.47(17), C2−Co1−C22 117.15(19).
The formation mechanisms of 3 and 4 are proposed in
Scheme 2. At first, the imidazolium halide salt reacts with
nBuLi to afford NHC ligand A. We deduce that the reaction of
A with CoMe(PMe3)4 affords intermediate B via N−H bond
activation and, in this process, cobalt(I) is oxidized to
cobalt(III) center. Intermediate C is formed from B through
reductive elimination with the release of a methane molecule.
Complex 3, a cobalt(III) hydride, is formed by the Csp2−H
bond activation of the naphthyl group. However, the
disproportionation of two molecules of C delivers stable
trans-bischelate cobalt(II) complex 4 in higher yields with
Co(PMe)4 as a byproduct. Co(PMe)4 can be proved by an IR
spectrum of the mother solution.
2.2. Hydrosilylation of Alkenes with [CNC]-Pincer
Cobalt(III) Hydrides 3 as Catalysts. With 3 as catalysts, we
initially evaluated their catalytic activity for the hydrosilylation
2.3. Mechanistic Study. The reaction mechanisms of the
hydrosilylation of alkenes catalyzed by metal complexes have
C
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Scheme 2. Suggested Formation Mechanisms of 3 and 4
styrene for 30 min before adding Ph2SiH2. If the catalyst 3a
reacted with Ph2SiH2 for 30 min before adding styrene, the
conversion was reduced from 98% to 55%, and the selectivity
was also greatly decreased from 94:6 to 72:28. This experiment
implies that precatalyst 3a can react with silane to give another
complex having a detrimental effect and also indicates that the
reaction of 3a with the alkene is the first step in the catalytic
cycle. The stoichiometric reactions between 3a and styrene as
well as between 3a and Ph2SiH2 were carried out.
Unfortunately, all attempts to isolate the intermediates failed.
However, some experiments provide important information to
the mechanism. The reaction of 4-tert-butylstyrene (10 equiv)
with 3a (1 equiv) was monitored by in situ NMR. As the
reaction progressed, both the hydrido resonance (−22.22
ppm) of 3a in the 1H NMR spectra and the phosphorus signal
Scheme 3. Template Reaction
been extensively studied in recent years.22,23 Usually, these
reaction mechanisms can be divided into two types, the
Chalk−Harrod mechanism and the modified Chalk−Harrod
mechanism. They have different intermediates.24,25
In order to better understand the mechanism of the catalytic
reaction, we studied the effect of the addition sequence of the
reactants (Table S3). The experiments show that we could get
the same results as in Table 1 if the catalyst 3a reacted with
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
loading (mol %)
solvent
silane
Ph2SiH2
temp (°C)
time (h)
conv. (%)
ratio (b/l)
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3b
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
1
1
1
1
1
1
1
1
neat
70
70
70
70
70
70
70
50
25
70
70
70
50
70
70
70
70
70
25
25
10
10
10
10
10
10
10
10
10
10
10
6
12
10
10
10
10
10
24
24
98
64
93
89
67
29
97
87
56
91
69
83
99
0
53
45
18
35
78
95
94:6
THF
toluene
dioxane
DME
MeCN
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PhSiH3
Ph3SiH
(EtO)3SiH
Et3SiH
(EtO)2SiHMe
Ph2SiH2
Ph2SiH2
65:35
85:15
70:30
56:44
69:31
92:8
95:5
95:5
93:7
93:7
9
1
10
11
12
13
14
15
16
17
18
19
0.5
0.1
1
1
1
1
1
1
1
94:6
94:6
1
1
95:5
96:4
c
20
neat
a
Catalytic reaction conditions: styrene (1.0 mmol), silane (1.2 mmol), solvent (1 mL), conversions and product ratios were determined by GC
b
c
with n-dodecane as an internal standard. b/l: brached/linear. With pyridine N-oxide (1.0 mmol).
D
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a
Table 2. Scope of Alkenes for Cocatalyzed Hydrosilylation
a
Catalytic reaction conditions: alkene (1.0 mmol), Ph2SiH2 (1.2 mmol), pyridine N-oxides (1 mol %), and 3a (1 mol %) were stirred in neat
b
conditions at 25 °C for 24 h. Product ratios and yields were determined by GC with n-dodecane as the internal standard. 5 mol % 3a was
c
d
provided. Alkene (1.0 mmol), Ph2SiH2 (1.2 mmol), and 3a (1 mol %) were stirred in neat conditions at 70 °C for 12 h without additive. Isolated
yields.
(17.18 ppm) of 3a in the 31P NMR spectra gradually
disappeared. At the same time, an increasingly strong
phosphorus signal at 30.79 ppm appeared (Figures S1 and
S2). We speculate that the signal at 30.79 ppm in the 31P NMR
spectra might belong to intermediate F or I, the alkyl cobalt
intermediate in Scheme 4. Additionally, a trace amount of
byproduct 3-octene could be detected from the reaction of 1-
octene with Ph2SiH2 (Figure S3). This alkene isomerization is
attributed to the β-H elimination after the insertion of the C
C bond into the Co−H bond.26
Despite some experimental attempts to prove the mecha-
nism, there is still no way to truly exclude the possibility of
other mechanisms. On the basis of the current experimental
results and literature reports, we propose a possible catalytic
cycle for hydrosilylation of alkene catalyzed by cobalt hydride
(Scheme 4).27−29 At the beginning of the reaction, the
introduction of pyridine N-oxide accelerates the dissociation of
the PMe3 ligand and provides intermediate D with an empty
coordinate site, which can be occupied by alkene to form E or
H. The insertion of alkene into the Co−H bond occurs
subsequently to afford intermediate F or I, an alkyl cobalt
intermediate. This is considered as an important foundation
for the hydrofunctionalization of alkenes.24 It is noteworthy
that the structures of F and I are also speculative, and the
present experimental results cannot confirm their structures.
This step determines the regioselectivity of the reaction. For
aryl alkenes, the migration insertion is more biased toward the
2,1-insertion to form a thermodynamically preferred secondary
benzyl cobalt species F. In addition, the π−π interaction
between the phenyl of aryl alkene and naphthalene ring of the
catalyst and the weak coordination effect between the cobalt
and phenyl of aryl alkene may be also responsible for the 2,1-
insertion mode.30 Contrary to aryl alkene, alkyl alkene is more
suitable for 1,2-insertion to generate a primary alkyl cobalt
E
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Scheme 4. Proposed Mechanisms
methods and freshly distilled before use. CoMe(PMe3)4 was prepared
according to the reported procedure.31 The other chemicals were
purchased from commercial sources and used as received. Infrared
spectra (4000−400 cm−1) were recorded on a Bruker ALPHA FT-IR
instrument by using Nujol mulls between KBr disks. The 1H, 13C, and
31P NMR spectra were recorded with a Bruker 300 spectrometer.
Elemental analyses were carried out on an Elementar Vario ELIII
instrument. Gas chromatography was performed with n-dodecane as
an internal standard.
species I because intermediate I is more reactive to σ-bond
metathesis.24 H2SiPh2 coordinates to the Co center in F or I to
form a four-membered ring transition state G or J. The final
silane product is generated through σ-bond metathesis with the
regeneration of real catalyst D. For this hydrosilylation
reaction, alkyl cobalt(III) intermediate F or I is proposed as
the key intermediate. This is different from the mechanisms
proposed by the groups of Huang and Deng.6,7 They consider
that silyl metal complexes are the key intermediates of the
reactions.
4.2. Synthesis of 1. A Schlenk flask was charged with 2-(1H-
imidazol-1-yl)aniline (0.79 g, 5 mmol), 1-bromonaphthalene (1.035 g,
5
mmol), Pd(OAc)2 (0.05612 g, 0.25 mmol), 1,1′-bis-
(diphenylphosphino)-ferocene (dppf) (0.2772 g, 0.5 mmol), tBuONa
(0.624 g, 6.5 mmol), and toluene (30 mL) under a nitrogen
atmosphere. The reaction mixture was then heated for 48 h at 110 °C.
After cooling to room temperature, the precipitate was filtered and
washed with CH2Cl2. The combined organic fractions were then
evaporated under vacuum, and the crude product was purified by
column chromatography on silica gel (dichloromethane/methanol
(200:1)) to obtain an orange solid. Yield: 0.65 g, 46%. Mp: 150.9−
152.2 °C. 1H NMR (300 MHz, CDCl3, 298 K, ppm): 5.61 (s, 1H, N−
H), 6.83−7.80 (m, 14H, Ar−H). 13C NMR (75 MHz, CDCl3, 298 K,
ppm): 115.3, 118.2, 118.6, 120.9, 123.9, 124.8, 125.1, 125.3, 126.2,
127.7, 128.7, 133.6, 135.7, 140.1 (NCN).
3. CONCLUSION
Two novel NHC [CNC]-pincer cobalt(III) hydrides 3a and
3b were obtained from the reactions of preligands with
CoMe(PMe3)4. It was found that 3a is an efficient catalyst for
the hydrosilylation of alkenes. The experimental results
indicate that Markovnikov products were formed with aryl
alkenes as substrates while anti-Markovnikov silanes could be
produced with alkyl alkenes as substrates. The addition of
pyridine N-oxide promoted the catalytic process significantly.
The catalytic conditions are milder with excellent regioselec-
tivity compared with the results from the literature.7 This
catalytic system is tolerant of a broad range of the substrates
with different substituents in moderate to good yields. The
catalytic mechanisms are proposed on the basis of the
experimental information and literature reports. For this
possible hydrosilylation mechanism, the alkyl cobalt(III)
intermediate is proposed as the key intermediate.
4.3. Synthesis of 2a. To a solution of 1 (1.29 g, 5 mmol) in
acetonitrile (50 mL) was added n-C4H9Br (1.37 g, 10 mmol). The
reaction mixture was refluxed for 24 h. After cooling to room
temperature, the precipitate was filtered and washed with CH3OH
(20 mL). The combined organic fractions were then evaporated
under vacuum, and the crude product was recrystallized from
CH3OH/Et2O to give 2a (1.94 g) as an orange solid in 92% yield.
Mp: 197.7−198.5 °C. Calcd for C23H24BrN3 (422.37 g/mol): C,
65.41; H, 5.73; N, 9.95. Found: C, 65.17; H, 5.40; N, 9.75. 1H NMR
(300 MHz, CDCl3, 298 K, ppm): 0.69−0.73 (t, J = 6 Hz, 3H, CH3),
0.96−1.09 (m, 2H, CH2), 1.45−1.55 (m, 2H, CH2), 4.00−4.05 (t, J =
4. EXPERIMENTAL SECTION
4.1. General Procedures. All experiments and manipulations
were carried out under nitrogen atmosphere using standard Schlenk
techniques, unless otherwise noted. All solvents were dried by general
15 Hz, 2H, CH2), 6.88−8.10 (m, 14H, Harom), 9.71 (s, 1H, NH). 13
C
NMR (75 MHz, CDCl3, 298 K, ppm): 13.4 (Caliphatic), 19.4 (Caliphatic),
F
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31.4 (Caliphatic), 50.0 (Caliphatic), 116.9 (Caromatic), 122.0 (Caromatic),
122.7 (Caromatic), 123.4 (Caromatic), 125.8 (Caromatic), 126.1 (Caromatic),
126.4 (Caromatic), 127.3 (Caromatic), 127.4 (Caromatic), 128.3 (Caromatic),
129.9 (Caromatic), 131.5 (Caromatic), 134.5 (Caromatic), 137.3 (Caromatic),
138.7 (Caromatic), 139.2 (Caromatic), 140.0 (Caromatic), 140.5 (NCN).
4.4. Synthesis of 2b. To a solution of 1 (1.29 g, 5 mmol) in
acetonitrile (50 mL) was added MeI (1.41 g, 10 mmol). The reaction
mixture was refluxed for 24 h. After cooling to room temperature, the
precipitate was filtered and washed with CH3OH (20 mL). The
combined organic fractions were then evaporated under vacuum, and
the crude product was recrystallized from CH3OH/Et2O to give 2b
(1.88 g) as a yellow solid in 88% yield. Mp: 200.4−201.0 °C. Calcd
for C20H18IN3 (427.29 g/mol): C, 56.22; H, 4.25; N, 9.83. Found: C,
55.97; H, 4.17; N, 9.69. 1H NMR (300 MHz, DMSO-d, 298 K, ppm):
3.81 (s, 3H, CH3), 6.62−6.85 (dd, J = 8.2 Hz, 1.0 Hz, 1H, Harom),
7.04−7.09 (m, 2H, Harom), 7.35−7.44 (m, 2H, Harom), 7.50−7.55 (m,
3H, Harom), 7.66−7.69 (d, J = 9 Hz, 1H, Harom), 7.77−7.78 (t, J = 3
Hz, Harom), 7.92−8.09 (m, 4H, Harom), 9.56 (s, 1H, NH). 13C NMR
(75 MHz, DMSO-d, 298 K, ppm): 36.3 (Caliphatic), 118.7 (Caromatic),
119.8 (Caromatic), 120.9 (Caromatic), 124.1 (Caromatic), 124.3 (Caromatic),
126.2 (Caromatic), 126.5 (Caromatic), 126.7 (Caromatic), 128.7 (Caromatic),
131.8 (Caromatic), 134.6 (Caromatic), 138.3 (Caromatic), 138.6 (Caromatic),
141.6 (NCN).
concentrated n-pentane solution. Yield: 67 mg, 14%. Mp: 172.0−
172.5 °C. Calcd for C26H34CoN3P2 (509.50 g/mol): C, 61.30; H,
6.73; N, 8.25. Found: C, 61.51; H, 6.79; N, 8.11. IR (Nujol mull, KBr,
1
cm−1): 1920 ν(Co−H), 943 ρ(PCH3). H NMR (300 MHz, C6D6,
298 K, ppm): −22.14 (t, J = 69 Hz, 1H, CoH), 0.62 (t′, J = 6 Hz,
18H, PCH3), 3.16 (s, 3H, CH3), 6.20 (d, J = 3 Hz, 1H, Harom), 6.61−
6.66 (t, J = 6 Hz, 1H, Harom), 7.02−7.05 (dd, J = 9 Hz, 3 Hz, 1H,
H
arom), 7.09−7.15 (td, J = 6 Hz, 3 Hz, 2H, Harom), 7.37−7.40 (d, J = 9
Hz, 1H, Harom), 7.49−7.58 (m, 3H, Harom), 7.68−7.71 (m, 1H, Harom),
7.97−8.00 (d, J = 9 Hz, 1H, Harom), 8.92−8.95 (dd, J = 9 Hz, 1.1 Hz,
1H, Harom). 31P NMR (121 MHz, C6D6, 298 K, ppm): 17.82 (s,
PCH3). 13C NMR (75 MHz, C6D6, 298 K, ppm): 15.5 (t, JP,C
=
2
12.75, PCH3), 35.9 (Caliphatic), 110.1 (Caliphatic), 112.2 (Caliphatic), 113.9
(Caliphatic), 116.6 (Caliphatic), 116.9 (Caliphatic), 118.8 (Caliphatic), 119.5
(Caliphatic), 119.9 (Caliphatic), 124.3 (Caliphatic), 124.5 (Caliphatic), 124.6
(Caliphatic), 125.6 (Caliphatic), 133.9 (Caliphatic), 134.5 (Caliphatic), 145.0
(Caliphatic), 146.6 (Caliphatic), 158.4 (t, J = 1.7 Hz, NCN).
4.7. General Procedure for Cobalt-Catalyzed Hydrosilyla-
tion Reactions. Under a N2 atmosphere, 1 mol % cobalt hydride 3
was added to a 20 mL Schlenk tube containing a magnetic stirrer
without any solvent. Then, the alkene (1.00 mmol), n-dodecane (170
mg, 1.00 mmol), pyridine N-oxides (1 mol %, 0.9 mg, 0.01 mmol),
and Ph2SiH2 (1.2 equiv, 221 mg, 1.2 mmol) were added in order. The
reaction mixture was stirred at room temperature for 24 h. The
resulting solution was quenched with ethyl acetate. The combined
organic fractions were concentrated in vacuum, and the crude product
was purified by column chromatography on silica gel with petrol ether
as eluent. The pure product was characterized by NMR analysis.
4.8. X-ray Crystal Structure Determination. Intensity data
were collected on a Stoe Stadi Vari diffractometer with graphite-
monochromated Ga Kα radiation (λ = 0.71073 Å). The structure was
solved using the charge-flipping algorithm, as implemented in the
program SUPERFLIP,32 and refined by full-matrix least-squares
techniques against F2 (SHELXL)33 through the OLEX interface.34 All
non-hydrogen atoms were refined anisotropically, and all hydrogen
atoms except for those of the disordered solvent molecules were
placed using AFIX instructions. Appropriate restraints or constraints
were applied to the geometry and the atomic displacement parameters
of the atoms. 1876054 (3b), 1883487 (4a), and 1894294 (4b)
contain the supplementary crystallographic data for this paper.
4.5. Synthesis of 3a and 4a. A 0.63 mL sample of n-BuLi (2.5 M
in hexane, 1.58 mmol) was added slowly to 30 mL of a THF solution
of ligand 2a (0.6080 g, 1.44 mmol) at −78 °C, and the solution
turned red. When the temperature returned to room temperature, the
mixture was stirred for 30 min. After that, a solution of CoMe(PMe3)4
(0.5451 g, 1.44 mmol) in THF (30 mL) was added slowly to the
resulting solution at −78 °C. The reaction mixture was allowed to stir
for 48 h at room temperature to provide a deep yellow solution. After
the reaction, the solvents were removed under vacuum, and the
residue was extracted with 100 mL of n-pentane and 50 mL of diethyl
ether, respectively. The dark-orange crystals of 4a were obtained at
−10 °C from the concentrated diethyl ether solutions. Yield: 289 mg,
27%. Mp: 189.8−190.3 °C. Calcd for C46H44CoN6 (739.83 g/mol):
C, 74.68; H, 5.99; N, 11.36. Found: C, 74.77; H, 5.80; N, 11.55. The
light yellow crystals of 3a were obtained at −20 °C from the
concentrated n-pentane solutions. Yield: 72 mg, 9%. Mp: 180.5−
181.1 °C. Calcd for C29H40CoN3P2 (551.54 g/mol): C, 63.15; H,
7.31; N, 7.62. Found: C, 62.89; H, 7.48; N, 7.74. IR (Nujol mull, KBr,
1
cm−1): 1904 ν(Co−H), 944 ρ(PCH3). H NMR (300 MHz, C6D6,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00251.
■
298 K, ppm): −21.21 (t, J = 66 Hz, 1H, CoH), 0.65 (t′, J = 6 Hz,
18H, PCH3), 0.90 (t, J = 6 Hz, 3H, CH3), 1.20 (m, 2H, CH2), 1.46
(m, 2H, CH2), 3.87 (t, J = 6 Hz, 2H, CH2), 6.34 (d, J = 3 Hz, 1H,
sı
H
arom), 6.65 (t, J = 6 Hz, 1H, Harom), 6.99−7.17 (m, 3H, Harom),
7.38−7.40 (d, J = 6 Hz, 1H, Harom), 7.51−7.69 (m, 4H, Harom), 7.94−
X-ray crystallographic data and IR, NMR, and GC
spectra (PDF)
7.97 (d, J = 9 Hz, 1H, Harom), 8.91−8.94 (d, J = 9 Hz, 1H, Harom). 31
P
NMR (121 MHz, C6D6, 298 K, ppm): 17.18 (s, PCH3). 13C NMR
2
Accession Codes
(75 MHz, C6D6, 298 K, ppm): 12.5 (Caliphatic), 15.4 (t, JP,C = 13.5,
CCDC 1876054, 1883487, and 1894294 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
PCH3), 18.8 (Caliphatic), 32.0 (Caliphatic), 48.9 (Caliphatic), 110.5
(Caromatic), 112.2 (Caromatic), 114.1 (Caromatic), 116.4 (Caromatic), 116.9
(Caromatic), 118.6 (Caromatic), 120.3 (Caromatic), 124.6 (Caromatic), 133.9
(Caromatic), 134.3 (Caromatic), 145.3 (Caromatic), 147.0 (Caromatic), 158.3
(NCN).
4.6. Synthesis of 3b and 4b. A 0.41 mL sample of n-BuLi (2.5
M in hexane, 1.03 mmol) was added slowly to 30 mL of a THF
solution of 2b (400 mg, 0.94 mmol) at −78 °C and the solution
turned red. When the temperature returned to room temperature, the
mixture was stirred for 30 min. After that, a solution of CoMe(PMe3)4
(0.356 g, 0.94 mmol) in THF (30 mL) was added slowly to the
resulting solution at −78 °C. The reaction mixture was allowed to stir
for 48 h at room temperature to provide a deep yellow solution. After
the reaction, the solvents were removed under vacuum, and the
residue was extracted with 100 mL of n-pentane and 50 mL diethyl
ether, respectively. The dark-orange crystals of 4b were obtained at
−10 °C from the concentrated diethyl ether solutions. Yield: 197 mg,
32%. Mp: 195.4−196.5 °C. Calcd for C40H32CoN6 (655.67 g/mol):
C, 73.27; H, 4.92; N, 12.82. Found: C, 73.36; H, 5.06; N, 12.67. The
light yellow crystals of 3b were obtained at −20 °C from the
AUTHOR INFORMATION
Corresponding Authors
■
Hongjian Sun − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan, Shandong
250100, People’s Republic of China; orcid.org/0000-0003-
1237-3771; Email: hjsun@sdu.edu.cn
Xiaoyan Li − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan, Shandong
G
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Organometallics
pubs.acs.org/Organometallics
Article
(9) Mo, Z.; Liu, Y.; Deng, L. Anchoring of Silyl Donors on a
Nheterocyclic Carbene through the Cobalt-Mediated Silylation of
Benzylic C-H Bonds. Angew. Chem., Int. Ed. 2013, 52, 10845−10849.
(10) Wang, C.; Teo, W. J.; Ge, S. Cobalt-Catalyzed Regiodivergent
Hydrosilylation of Vinylarenes and Aliphatic Alkenes: Ligand- and
Silane-Dependent Regioselectivities. ACS Catal. 2017, 7, 855−863.
(11) Lee, K. L. (Aminomethyl)pyridine Complexes for the Cobalt-
Catalyzed Anti-Markovnikov Hydrosilylation of Alkoxy- or Siloxy-
(vinyl)silanes with Alkoxy- or Siloxyhydrosilanes. Angew. Chem., Int.
Ed. 2017, 56, 3665−3669.
250100, People’s Republic of China; orcid.org/0000-0003-
0997-0380; Email: xli63@sdu.edu.cn
Authors
Shangqing Xie − School of Chemistry and Chemical
Engineering, Key Laboratory of Special Functional Aggregated
Materials, Ministry of Education, Shandong University, Jinan,
Shandong 250100, People’s Republic of China
Olaf Fuhr − Institut fu
Nano-Micro-Facility (KNMF), Karlsruher Institut fu
Technologie (KIT), Eggenstein-Leopoldshafen 76344, Germany
Dieter Fenske − Institut fur Nanotechnologie (INT) und
Karlsruher Nano-Micro-Facility (KNMF), Karlsruher Institut
fur Technologie (KIT), Eggenstein-Leopoldshafen 76344,
̈
r Nanotechnologie (INT) und Karlsruher
̈
r
(12) Sang, H.; Yu, S.; Ge, S. Cobalt-Catalyzed Regioselective
Stereoconvergent Markovnikov 1,2-Hydrosilylation of Conjugated
Dienes. Chem. Sci. 2018, 9, 973−978.
̈
(13) Chen, C.; Hecht, M. B.; Kavara, A.; Brennessel, W. W.;
Mercado, B. Q.; Weix, D. J.; Holland, P. L. Rapid, Regioconvergent,
Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst. J. Am.
Chem. Soc. 2015, 137, 13244−13247.
̈
Germany
Complete contact information is available at:
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(14) (a) Cheng, B.; Lu, P.; Zhang, H.; Cheng, X.; Lu, Z. Highly
Enantioselective Cobalt-Catalyzed Hydrosilylation of Alkenes. J. Am.
Chem. Soc. 2017, 139, 9439−9442. (b) Chen, J.; Guo, J.; Lu, Z.
Recent Advances in Hydrometallation of Alkenes and Alkynes via the
First Row Transition Metal Catalysis. Chin. J. Chem. 2018, 36, 1075−
1109. (c) Chen, J.; Lu, Z. Asymmetric Hydrofunctionalization of
Minimally Functionalized Alkenes via Earth Abundant Transition
Metal Catalysis. Org. Chem. Front. 2018, 5, 260−272. (d) Zaranek,
M.; Pawluc, P. Markovnikov Hydrosilylation of Alkenes: How an
Oddity Becomes the Goal. ACS Catal. 2018, 8, 9865−9876.
(15) Pappas, I.; Treacy, S.; Chirik, P. J. Alkene Hydrosilylation Using
Tertiary Silanes with α-Diimine Nickel Catalysts. Redox-Active
Ligands Promote a Distinct Mechanistic Pathway from Platinum
Catalysts. ACS Catal. 2016, 6, 4105−4109.
(16) Sun, Y.; Li, X.; Sun, H. [CNN]-pincer nickel(II) complexes of
N-heterocyclic carbene (NHC): synthesis and catalysis of the
Kumada reaction of unactivated C−Cl bonds. Dalton Trans. 2014,
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(17) Lubitz, K.; Radius, U. The Coupling of N-Heterocyclic
Carbenes to Terminal Alkynes at Half Sandwich Cobalt NHC
Complexes. Organometallics 2019, 38, 2558−2572.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF China 21572119/21971151
and NSF Shandong ZR2019ZD46.
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