Homoleptic cobalt(II) phenoxyimine complexes for hydrosilylation of aldehydes and ketones without base activation of cobalt(II)

Article abstract of DOI:10.1021/acs.organomet.1c00151

Air-stable, easy to prepare, homoleptic cobalt(II) complexes bearing pendant-modified phenoxyimine ligands were synthesized and determined. The complexes exhibited high catalytic performance for reducing aldehydes and ketones via catalytic hydrosilylation, where a hydrosilane and a catalytic amount of the cobalt(II) complex were added under base-free conditions. The reaction proceeded even in the presence of excess water, and excellent functional-group tolerance was observed. Subsequent hydrolysis gave the alcohol in high yields. Moreover, H₂O had a critical role in activation of the Co(II) catalyst with hydrosilane. Several additional results also indicated that the cobalt(II) center acts as an active catalyst in the hydrosilylation of aldehydes and ketones.

Full text of DOI:10.1021/acs.organomet.1c00151

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Homoleptic Cobalt(II) Phenoxyimine Complexes for Hydrosilylation
of Aldehydes and Ketones without Base Activation of Cobalt(II)
Kouki Matsubara,* Tomoaki Mitsuyama, Sayaka Shin, Momoko Hori, Ryuta Ishikawa, and Yuji Koga
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ABSTRACT: Air-stable, easy to prepare, homoleptic cobalt(II)
complexes bearing pendant-modified phenoxyimine ligands were
synthesized and determined. The complexes exhibited high
catalytic performance for reducing aldehydes and ketones via
catalytic hydrosilylation, where a hydrosilane and a catalytic
amount of the cobalt(II) complex were added under base-free
conditions. The reaction proceeded even in the presence of excess
water, and excellent functional-group tolerance was observed.
Subsequent hydrolysis gave the alcohol in high yields. Moreover,
H2O had a critical role in activation of the Co(II) catalyst with
hydrosilane. Several additional results also indicated that the
cobalt(II) center acts as an active catalyst in the hydrosilylation of
aldehydes and ketones.
Furthermore, the oxidative addition of a Si−H bond onto a
cobalt(0) complex showed catalytic activity for the aldehyde
hydrosilylation.5h
INTRODUCTION
■
The catalytic hydrosilylation of unsaturated hydrocarbons is an
important reduction protocol in organic chemistry.1 Alkenes,
alkynes, and various carbonyl compounds can be reduced
without using basic hydride reagents or a high-pressure
apparatus. For instance, catalysts using precious metals, such
as Rh, Ir, Ru, and Pt, have been developed. However,
alternative, abundant base metals, such as Fe, Co, and Mn,
have recently attracted attention and have been used as
hydrosilylation catalysts.2 In comparison to that on iron and
manganese catalysts, the research on cobalt catalysts has been
less advanced.3
Significant research has been done on the cobalt-catalyzed
hydrosilylation of alkenes and alkynes,4 but for carbonyl
compounds the studies are limited.5 For unknown reasons, the
cobalt complexes that catalyze the hydrosilylation of alkenes
and alkynes are not always applicable to ketones and
aldehydes.4i Therefore, independent studies are required to
develop catalysts for these reactions. Brookhart and Lyons
reported that the monovalent cobalt complex [CoCp*(H2C
CHSiMe3)2] promoted the hydrosilylation of acetophenone
derivatives using vinylhydrosilane.5a The reaction was followed
by intramolecular hydrogen transfer from a silyl alkyl ether to
the vinyl group of silane to a form silyl enol ether. Cobalt(III)
hydride complexes with cyclometalating C∧X (X = S, N)
ligands are also possible precursors for aldehyde hydro-
silylation, where reductive elimination of a C−H bond may
form a cobalt(I) active species for catalysis.5b,c Cobalt(I)
complexes with pincer-type, tridentate ligands were also
effective.5d−f Divalent cobalt complexes can be used but
require the basic hydride reagent NaBEt3H for activation.5g
These studies have shown that low-valent cobalt(I) or
cobalt(0) complexes are effective for the hydrosilylation of
aldehydes and ketones. However, these low-valent metals are
active during oxidative addition and break undesirable
substrate bonds. The availability of high-valent, active cobalt
catalysts, without reduction protocols, would greatly expand
the functional group tolerance of these hydrosilylations. To the
best of our knowledge, there have been no examples employing
cobalt(II) complexes, without using additives, such as hydride
reagents. Wei et al. conducted theoretical calculations of
catalytic pathways in the hydrosilylation of benzaldehyde using
POCOP-pincer iridium(III) and iron(II) hydride complexes.6a
They concluded that it was not the oxidative addition of the
Si−H bond but the σ-bond metatheses of the M−O (M =
metal) and Si−H bonds that occurred after the carbonyl
inserted into the metal−hydride bond. Moreover, Gade and
Bleith experimentally uncovered a similar catalytic pathway
involving σ-bond metathesis using iron(II) boxmi complexes.
At present, the existence of a high-spin, cobalt(II)-catalyzed
Received: March 11, 2021
Published: April 28, 2021
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process that maintains the metal’s oxidation state is
unknown.6b
ence of this value is dictated by the Curie law, which does not
account for dimer formation in solution. From cyclic
voltammetry, dichloromethane solutions of 1 and 2 showed
similar redox waves, suggesting that they have the same
coordination structure in solution (Figures S7 and S8).
The crystal structures of 1 and 2 were determined using X-
ray diffraction studies, as shown in Figure 1. As expected, these
On this basis, we have designed a heat- and air-stable
homoleptic cobalt complex that bears anionic, tridentate Schiff
base ligands linked to pendant groups, which stabilize the
octahedral, high-spin Co(II) center. Schiff bases, such as N-
substituted phenoxyimine, can be easily synthesized in one step
from commercially available chemicals, such as salicylaldehyde
and amine derivatives.7 In fact, a number of useful cobalt
catalysts bearing bidentate or tridentate phenoxyimine ligands
are known.7 A few examples of similar, pendant-modified
phenoxyimine cobalt complexes have been used for catalysis,8
but there have been no examples of them promoting
hydrofunctionalization reactions.
RESULTS AND DISCUSSION
■
Synthesis and Structures of Cobalt(II) Complexes.
The Schiff-base ligands L1 and L2 were obtained quantita-
tively. The addition of 1 equiv of NaH to THF solutions of L1
and L2, and the subsequent reaction with 0.5 equiv of CoCl2,
gave the corresponding cobalt(II) complexes Co(L1)2 (1) and
Co(L2)2 (2) as crystals in 43% and 59% yields, respectively
(Scheme 1). These crystals are stable and can be stored in air;
however, solutions of 1 and 2 at room temperature gradually
changed color, suggesting air oxidation.
Figure 1. ORTEP drawings of (a) 1 and (b) 2 (50% probability
thermal ellipsoids). Color code: purple, cobalt; red, oxygen; blue,
nitrogen; silver, carbon. All hydrogen atoms and THF molecules (2)
are omitted for clarity.
complexes have distorted-octahedral geometries composed of
two phenoxyimine ligands. The dimethylamino groups of L1 in
complex 1 weakly coordinated to the cobalt atom, resulting in
long Co−N bond distances of 2.410(3) Å (Co(1)−N(2)) and
2.904(3) Å (Co(1)−N(4)) (Figure 2a), whereas the distances
Scheme 1. Synthetic Procedure for Complexes 1 and 2
The cobalt(II) complexes 1 and 2 were revealed to have a
homoleptic octahedral geometry in solution and in the solid
state, and analytical studies showed their paramagnetic high-
1
spin nature. The H NMR spectra for 1 and 2 showed several
broad signals from −20 to +110 ppm, indicating that the cobalt
centers are paramagnetic (Figures S1 and S2). Moreover, the
ligands of each complex were chemically equivalent, as the
signals in each spectrum represented the protons of both
phenoxyimine ligands on the cobalt atoms. Early studies of
homoleptic Schiff base cobalt(II) complexes showed that
octahedral, pyramidal, and tetrahedral species can exist as an
equilibrium mixture in solution.9 The electronic spectrum of 1
in the solid state showed a broad absorbance at approximately
1100 nm for one of the d−d* transitions in the octahedral
cobalt(II) complex,10 whereas the only distinct absorbance of
the dichloromethane solution of 1 was observed at 380 nm
(Figures S3 and S4). Therefore, a fast equilibrium of the
pendant group elimination in 1 could occur, at least in
solution. SQUID measurements revealed that the spin
quantum number (S) was 3/2, which was consistent with
the experimental values of 1 and 2 at 273 K (the magnetic
susceptibilities (χMT) were 2.12 and 1.98 cm3 K mol−1,
respectively) (Figures S5 and S6). The temperature depend-
Figure 2. Comparison of the geometries of complexes 1 and 2.
between the phenoxyimine nitrogen atoms and the Co were
2.045(4) Å (Co(1)−N(1)) and 2.046(3) Å (Co(1)−N(3)).
On the other hand, the distances between the cobalt and the
pyridine nitrogen atoms in complex 2, 2.154(2) Å (Co(1)−
N(2)) and 2.206(2) Å (Co(1)−N(4)), were approximately 0.2
Å longer than those between the cobalt and phenoxyimine
nitrogen atoms (Figure 2b). These differences between 2 and 1
may be due to the stronger π-acceptance of the pyridyl group,
in comparison to that of the dimethylamino group in 1.
Furthermore, the easy elimination of the labile pendant groups
may allow for a more straightforward study of the catalytic
activity, as the substrates can interact directly with the active
metal centers.
Catalytic Hydrosilylation of Aldehydes and Ketones.
As a model reaction, catalytic hydrosilylation of aromatic
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aldehydes was conducted using complexes 1 and 2 as catalyst
precursors. Benzaldehyde, 3 equiv of the methylphenylsilane,
and 3 mol % of complex 1 were dissolved in dichloromethane
alcohol in 70% yield. Although a higher reaction temperature
of 60 °C was required when polymethylhydrosiloxane
(PMHS) was used, hydrosilylation proceeded quantitatively
to provide the target benzyl alcohol in 70% yield after
hydrolysis with a fluoride anion (entry 2).
When complex 2 was used under similar conditions using
methylphenylsilane, it exhibited lower activity in comparison to
complex 1. For example, when benzaldehyde and 4-
trifluoromethylbenzaldehyde were used as the substrates,
higher temperatures were required (Table 1, entries 3 and
9). These results agree with the strong metal coordination
observed for the pyridyl group in L2, which would lower the
activity of complex 2.
1
and stirred at 40 °C for 18 h. The H NMR monitoring after
18 h showed the quantitative formation of the hydrosilylated
silyl ether, based on the internal standard hexamethylbenzene.
After the benzaldehyde was consumed, the resulting mixture
was treated with n-butylammonium fluoride solution, in THF,
to remove the silyl group. The benzyl alcohol was isolated in
84% yield after purification by silica gel column chromatog-
raphy (Table 1, entry 1). The reaction can also be conducted
using tertiary hydrosilanes. For instance, triethoxysilane,
(EtO)3SiH, had a similar result to give the corresponding
These hydrosilylations tolerated various substituents on the
aromatic group of the aldehydes. However, when the silyl
group was fluorinated, electron-donating substituents, such as
methyl and dimethylamino groups, were preferred and gave
benzyl alcohols in excellent yields (Table 1, entries 4 and 14).
In contrast, when the benzaldehyde derivatives had 4-
trifluoromethyl and 4-fluoro groups, the yields of the
corresponding alcohols were lower, 75% and 79%, respectively
(entries 8 and 13). Halogenated aldehydes did not disturb the
catalytic reaction (entries 6 and 10−12), likely because the
cobalt center was not reduced to Co(I) or Co(0) in these
systems. These reactions of ortho- and meta-substituted
analogues did proceed more slowly than the reactions of
para-substituted analogues. Furthermore, an o-hydroxyl group
did not deactivate the system, as the alcohol in entry 15 was
obtained in moderate yield (54%).11 Before obtaining this
product, we had imagined that silylation of the hydroxyl group
might occur, and the resulting, bulky siloxyl group would block
the addition of the aldehyde to the cobalt center. The product
formation seen in entry 15 indicates that complex 1 has a low
energy barrier, as it overcame the steric hindrance. Addition-
ally, the pyridyl group in 4-pyridinecarboxyaldehyde did not
poison the catalyst, giving the alcohol in high yield (82%, entry
16). Hydrosilylation of the aliphatic aldehyde 2,3-dihydrocin-
namaldehyde also proceeded, quantitatively forming the silyl
ether and producing the corresponding alcohol in good yield
(64%, entry 17).
Table 1. Reduction of Aldehydes with Hydrosilane
The reaction using acetophenone produced a mixture of
silylated products containing both the desired silyl ether and
dehydrogenated silyl enol ether in a 6:4 ratio (Scheme 2).
Scheme 2. Hydrosilylation of Acetophenone with H2SiMePh
(Table 2, Entry 1)
After the removal of the silyl moiety, 1-phenylethanol was
obtained in 40% yield. However, when propiophenone was
used as a substrate, the formation of the corresponding silyl
enol ether was completely suppressed, and the corresponding
alcohol was obtained almost quantitatively (Table 2, entry 1).
Other alkyl aryl ketones, such as isobutyrophenone (entry 2),
a
PMHS (polymethylhydrosiloxane) was used instead of methylphe-
b
nylsilane. Triethoxysilane was used instead of methylphenylsilane.
c
The yields were determined after purification by silica gel column
chromatography.
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Table 2. Reduction of Ketones with Hydrosilane
Scheme 3. Deuterium-Labeling Experiment Using
D2SiMePh
and Figure S13). The silyl methyl protons appeared as a
singlet, indicating that the other deuterium atom remained on
the deuterated hydrosilane. Furthermore, the 1H NMR
spectrum of the crude mixture did not show an increase in
the integral ratio of the Si−H proton signal, which
corresponded to the hydrogen in the remaining starting silane.
This indicated that H/D scrambling regarding D2SiMePh did
not occur during the reaction. The kinetic isotope effect (KIE),
kH/kD, was 2.5, obtained from the rates of the reactions with
each isotope (Figure S14 and Table S23). This indicates that
cleavage of the Si−H bond is the rate-determining step in this
catalytic reaction. Similar KIE values have been observed in
iron chemistry.6b,13 A possibility was that irreversible
coordination and cleavage of the Si−H bond occurred
simultaneously, via σ-bond metathesis. Thus, an alternative
pathway based on this could be proposed for Si−H bond
activation: specifically, a mechanism involving the interaction
of the silicon atom with the Lewis basic center of a ligand to
form a zwitterionic Si−−L−M+ structure, leading to the
activation of the Si−H bond.14 However, the plausibility of this
process can be supported by an inverse secondary H/D isotope
effect that was observed during a reaction involving deuterium-
containing silane and a rhenium-oxo complex.14a Therefore, we
believe that the σ-bond metathesis pathway is the most
probable mechanism occurring during the hydrosilylations
catalyzed by the cobalt(II) phenoxyimine complex in this
study.
a
The yields were determined after silica gel column chromatography.
Additionally, we obtained the following several experimental
results concerning the mechanism of hydrosilylation (Scheme
4). First, the presence of a radical-trapping agent, tetrame-
thylpiperidinyloxy (TEMPO), was added to the reaction
valerophenone (entry 3), hexanophenone (entry 4), octano-
phenone (entry 5), and 4′-fluoropropiophenone (entry 6),
were also converted into alcohols in high yields. Therefore, it
was revealed that the adjacent alkyl group can completely
block the removal of the α-proton, probably due to steric
hindrance. That is, the dehydrogenative hydrosilylation
pathway may be limited to acetophenone derivatives.
Furthermore, a CF3-substituted ketone was easily converted
to the corresponding silyl ether under conditions (40 °C)
milder than those required by the other ketones (60 °C),
giving the desired alcohol in 88% yield (entry 7). The efficient
conversion of 4,4′-dimethylbenzophenone afforded di-p-
tolylmethanol in 97% yield (entry 8). Dibenzyl ketone was
also converted into its corresponding alcohol in 61% yield
(entry 9). These results show that, with the exception of
methyl ketones, aromatic and aliphatic ketones can be
efficiently reduced to alcohols using the similar hydrosilylation
protocol.
Scheme 4. Experiments for the Mechanistic Studies
Mechanistic Studies. To investigate the mechanistic
aspects of the hydrosilylations using cobalt precatalyst 1,
deuterium-labeling experiments were conducted using a
deuterated silane, D2SiMePh. It was prepared from Cl2SiMePh
and LiAlD412 with 99% deuterium incorporation (Figure S11).
Benzaldehyde was reacted with D2SiMePh, and after 24 h the
1H NMR spectrum of the reaction mixture showed the
integration of the benzylic protons to be 1 H, indicating that
one of the deuterium atoms was incorporated here (Scheme 3
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media. Addition of 0.5 equiv of TEMPO did not affect the
catalysis using 1 and resulted in complete conversion of
benzaldehyde to its corresponding silyl ether (Scheme 4,
reaction 1, and Figure S15). In contrast, addition of 3.0 equiv
of TEMPO completely deactivated the reaction, resulting in
the recovery of the starting materials and the detection of some
silyl compounds (Figure S16). However, silylTEMPO was not
produced at all, indicating that the formation of the silyl radical
during the catalytic reaction was negated.15 Second, the
addition of the oxidizing agent ferrocenium cation, [FeCp2]-
(PF6), disrupted the reaction of benzaldehyde in 5 h at 40 °C
to result in only the starting compounds (Scheme 4 reaction 2,
and Figure S17). This suggested that the oxidized cobalt(III)
was not involved in the catalytic cycle of the reaction, although
there have been Co(III) catalysts for hydrosilylations.16 Third,
free salicylaldehyde or salicylimine derivatives derived from the
hydroxide along with H2 evolution from hydrosilane and water
has been proposed in the literature.19
On the basis of the above experimental results, we have
constructed a plausible mechanism for the catalytic hydro-
silylation of aldehydes (Scheme 5). Although we do not have
Scheme 5. Proposed Mechanism of the Hydrosilylation of
Aldehydes Using the Co(II) Phenoxyimine Catalyst: (a)
Catalytic Cycle; (b) Initiation Pathway in the Presence of
Water and Hydrosilane
1
ligand L1 were not detected in the crude H NMR spectrum
after the usual hydrosilylation reaction, suggesting that the pair
of L1 ligands are bound to the cobalt center in the active form
of the cobalt intermediate. This observation was supported by
a control experiment that employed CoCl2 instead of complex
1. As expected, the hydrosilylation of benzaldehyde using
CoCl2 was unsuccessful, leading to the complete recovery of
the starting materials (Scheme 4 reaction 3, and Figure S18).
In contrast, a ligand-free cobalt(II) salt in a previous study was
found to be active in the hydrosilylation of alkenes in the
presence of hydride sources.17 In addition, the results of our
control reaction confirmed that the catalytic reactions were not
mediated by cobalt nanoparticles generated in situ after
decomposition of the catalyst and the liberation of the
phenoxyimine ligands.18 Interestingly, it was found that the
presence of water accelerated the catalyst initiation pathway
and shortened the induction period and reaction time in the
presence of a high concentration of active catalyst. For
instance, when 10 mol % of water (ca. 3 equiv with respect to
complex 1) was added to the reaction media, the induction
period was reduced and it took only 2 h to complete the
reaction (Figure S19). In contrast, more than 5 h was required
for complete conversion without the addition of water. A
similar experiment in air was also performed because the
addition of water may have introduced air into the reaction.
The yield of the target silyl ether was reduced under these
conditions (71% NMR yield), suggesting that only water
promoted the catalytic reaction (Scheme 4, reaction 4).
enough information to discuss the detailed mechanisms,
including the structures of the intermediates, four-membered
transition states with Si−H and Co−O bonds, that do not
change the oxidation state of the cobalt(II), could occur,
following the elimination of the pendant groups from the
cobalt center (Scheme 5a). Water and hydrosilane play
significant roles in the activation of the precatalyst to form
cobalt(II) hydride species (Scheme 5b). In this stage, pendant
amino groups might be involved in the reaction, as basic
nitrogen atoms can accept protons from H2O molecules to
form zwitterionic cobalt species. Catalytic hydrogen gas
formation can occur in the presence of hydrides and protons.
An alternative process in the absence of water could involve
the transfer of a silyl group to the phenoxide ligand oxygen,
which was previously reported for an iron-catalyzed hydro-
boration.20 The σ-bond metathesis of Si−H bonds onto M−O
bonds was also proposed in studies on iron6 and cobalt21
chemistry. The coordination of the aldehyde and the liberation
of one of the pendant groups are fast processes, but Si−H
addition occurs slowly.
We sought to obtain experimental evidence for the cobalt
intermediates shown in Scheme 5b, but the cobalt compound
after the reaction in Scheme 4, reaction 5, was highly soluble in
nonpolar organic solvents and could not be separated by
recrystallization. Therefore, at present, there is no direct
evidence that a cobalt(II) species is an active key intermediate.
This also means that the reaction mechanism by which a
cobalt(I) species is active cannot be ruled out. However, it is
unlikely that the cobalt center was reduced without the use of a
reducing reagent under these experimental conditions. In
particular, cobalt(II) hydride complexes22 and hydroxo
complexes23 have been isolated and structurally determined.
To investigate the effect of water on the initiation pathway,
water was added to complex 1 in the presence of
methylphenylsilane. No reaction occurred when only methyl-
phenylsilane was added to complex 1 in benzene-d6 at 40 °C
for 24 h. However, as soon as excess water was added to the
mixture at the same temperature (40 °C), H2 gas evolution was
observed (Scheme 4, reaction 5). The generated hydrogen gas
could be detected, as shown in Figures S24 and S25. New
paramagnetic signals appeared in the 1H NMR spectrum
1
(Figure S22), and the integrated ratios of the H signals from
the new compounds increased in the presence of higher
amounts of hydrosilane. This suggests that a cobalt hydride
intermediate formed from 1, hydrosilane, and H2O during the
catalyst activation. The IR spectrum of the crude mixture
indicated the existence of a siloxane (Si−O−Si) moiety
(Figure S23). It is possible that the reaction of water with
cobalt(II) hydride formed cobalt(II) hydroxide, because a
similar transformation between a metal hydride and a metal
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N(CH3)2), 1.60 (s, 9 H, tBu). 13C NMR (C6D6): δ 166.35 (NCH),
161.17 (Ph), 137.70 (Ph), 129.89 (Ph), 129.57 (Ph), 119.27 (Ph),
118.02 (Ph), 60.00 (N−CH), 57.70 (N−CH), 45.61 (N(CH3)2),
35.12 (C(CH3)3), 29.65 (C(CH3)3). ESI-TOF MS (in methanol):
calcd for C15H24N2O + H+, m/z 249.1967; obsd, m/z 249.1921. Data
According to the literature, reduction of the cobalt(II) center
by the homolytic cleavage of the Co−H bond may be
unlikely.22a Thus, it is possible that divalent cobalt complexes
were formed as intermediates in this study as well.
1
for L2 are as follows. (0.23 g, 42%). H NMR (C6D6): δ 14.27 (s, 1
CONCLUSION
H, OH), 8.41 (d, J = 4.6 Hz, 1 H, Py), 7.91 (s, 1 H, NCH), 7.31 (d,
J = 7.7 Hz, 1 H, Ph), 7.02 (m, 1 H, Ph), 6.98 (t, J = 7.7 Hz, 1 H, Ph),
6.86 (d, J = 7.7 Hz, 1 H, Ph), 6.76 (t, J = 7.7 Hz, 1 H, Ph), 6.62 (m, 1
H, Ph), 4.59 (s, 2 H, N−CH2), 1.57 (s, 9 H, tBu). 13C NMR (C6D6):
δ 167.84 (NCH), 161.04 (Ph), 158.74 (Py), 149.60 (Py), 137.70
(Ph), 136.29 (Py), 130.32 (Ph), 129.92 (Ph), 121.99 (Py), 121.59
(Py), 119.28 (Ph), 118.20 (Ph), 65.12 (N−CH), 35.11 (C(CH3)3),
29.62 (C(CH3)3). ESI-TOF MS (in methanol): calcd for C17H20N2O
+ H+, m/z 269.1648; obsd, m/z 269.1693.
Synthesis of Cobalt Complexes 1 and 2. For complex 1, in a
50 mL Schlenk tube were placed the ligand L1 (198 mg, 0.795
mmol), sodium hydride (20.9 mg, 0.871 mmol), and THF (5 mL)
and the mixture was stirred for 10 min at room temperature. Then,
cobalt(II) dichloride (50.6 mg, 0.390 mmol) was added. After 3 h, the
volatiles were evaporated under vacuum pressure. The residual solid
was extracted with hexane and filtered through Celite. The resulting
solution was concentrated and cooled to −30 °C. Red crystals of
complex 1 were obtained (173 mg, 43%). Data for complex 1 are as
follows. 1H NMR (C6D6): δ 119.21 (2 H), 85.36 (2 H), 48.99 (2 H),
40.35 (12 H), 33.40 (2 H), 31.14 (2 H), 24.70 (2 H), 8.00 (18 H),
7.16 (2 H), −9.53 (2 H). Anal. Calcd for C30H46N4O2Co: C, 65.08;
H, 8.37; N, 10.12. Found: C, 65.20; H, 8.37; N, 10.03. Data for
complex 2 (250 mg, 59%) are as follows. 1H NMR (C6D6): δ 77.54 (2
H), 57.07 (2 H), 47.79 (2 H), 45.82 (2 H), 41.33 (2 H), 13.38 (2 H),
13.09 (22 H), 7.20 (2 H), −13.34 (2 H). Anal. Calcd for
C34H38N4O2Co·C4H8O: C, 68.56; H, 6.96; N, 8.42. Found: C,
68.53; H, 7.00; N, 8.59.
Synthetic Procedure of Alcohols from Aldehydes. In a typical
example, benzaldehyde (95 μL, 0.9 mmol), dihydromethylphenylsi-
lane (370 μL, 2.7 mmol), and dichloromethane (0.5 mL) were placed
in a 20 mL Schlenk tube. The mixture was stirred at room
temperature. Cobalt complex 1 (15 mg, 0.027 mmol) was added to
the solution, and the mixture was heated at 40 °C. After 18 h, the
reaction mixture was cooled, and dichloromethane (2.0 mL) and 1.0
M tetrabutylammonium fluoride solution in THF (1.0 mL, 1.0 mmol)
were added. The benzyl alcohol product was separated using silica gel
column chromatography with hexane/ethyl acetate (8/2) as eluent,
and the isolated yield was 84% (81 mg).
Synthetic Procedure of Alcohols from Ketones. In a typical
example, acetophenone (105 μL, 0.9 mmol), dihydromethylphenylsi-
lane (370 μL, 2.7 mmol), and toluene (0.5 mL) were placed in a 20
mL Schlenk tube. The mixture was stirred at room temperature.
Cobalt complex 1 (15 mg, 0.027 mmol) was added to the solution,
and the mixture was heated at 60 °C. After 42 h, the reaction mixture
was cooled, and dichloromethane (2.0 mL) and 1.0 M tetrabuty-
lammonium fluoride solution in THF (1.0 mL, 1.0 mmol) were
added. The 1-phenylethan-1-ol product was separated using silica gel
column chromatography with hexane/ethyl acetate (8/2) as eluent,
and the isolated yield was 40% (44.2 mg).
■
In summary, bench-stable, easy to handle, homoleptic
phenoxyimine cobalt(II) complexes were easily prepared and
determined. The successful reduction of aldehydes and ketones
was achieved via catalytic hydrosilylation using cobalt(II)
complexes 1 and 2 under mild conditions and subsequent
hydrolysis with fluoride anion. The complexes do not need any
additional base to activate the cobalt(II) center. Instead, water
promoted the initial activation of the cobalt complexes. With
these cobalt(II) catalysts, various functional groups could be
tolerated during hydrosilylation. Protic and basic molecules,
such as water, phenol, and amine, did not significantly reduce
the product yields. Preliminary mechanistic studies employing
deuterium-labeling experiments indicated that σ-bond meta-
thesis on cobalt(II) may form Si−O bonds to produce a silyl
ether during the final step of the catalytic process. This work is
the first account of divalent cobalt complexes catalyzing the
hydrosilylation of aldehydes and ketones without using basic
additives, enabling a functional-group-tolerant reduction
system. This methodology could be expanded to other
hydrofunctionalization reactions, and further mechanistic
studies are currently underway.
EXPERIMENTAL SECTION
■
General Considerations. All experiments were carried out under
an inert gas atmosphere using standard Schlenk techniques and a
glovebox (MBraun UniLab) unless otherwise noted. Supergrade
degassed and dried solvents, THF, toluene, dichloromethane, and
hexane, were used as purchased from Wako Chemical Industries, Ltd.
Organic reagents used for catalytic reactions were used as purchased.
Column chromatography of organic products was carried out using
silica gel (Kanto Kagaku, silica gel 60N (spherical, neutral)). 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were recorded on a
Bruker AVANCE III HD 400 MHz spectrometer at room
temperature in C6D6, CDCl3 or acetonitrile-d3. Chemical shifts (δ)
were recorded in ppm from the internal standard. IR spectra were
recorded in cm−1 on a Perkin-Elmer Spectrum Two spectrometer
equipped with a universal diamond ATR accessory. The magnetic
properties of the materials were investigated using a Quantum Design
MPMS-5S superconducting quantum interference device (SQUID)
magnetometer. The elemental analysis was carried out with J-
SCIENCE LAB MICRO CORDER JM11 and AUTO SAMPLER
JMA11 instruments. Electrospray ionization time-of-flight mass
spectrometry (ESI-TOF MS) was carried out on a JEOL JMS-T100
mass spectrometer. The sample solutions in THF (ca. 1 μmol L−1)
were directly infused using THF as the solvent stream. UV−visible
spectra were recorded on a Perkin-Elmer Lambda 35 ES UV/vis
spectrophotometer using a 10 mm quartz cell. The cyclic voltammo-
grams were recorded on an ALS/chi Model/610A electrochemical
analyzer with a platinum working electrode, a silver-wire reference
electrode, and a platinum-wire counter electrode at a scan rate of 100
mV s−1. The analyte solutions were prepared with a 0.1 M solution of
tetra-n-butylammonium perchlorate in acetonitrile.
X-ray Crystallography of 1 and 2. Single crystals of 1 and 2
suitable for X-ray diffraction were grown at −30 °C from THF/
hexane solutions. All of the data were collected at 163 K using a
Rigaku Saturn CCD diffractometer with a c0 onfocal mirror and
graphite-monochromated Mo Kα radiation (λ = 0.71070 Å). Data
reduction of the measured reflections was performed using the
software package CrystalStructure.24 The structures were solved by
direct methods (SHELXT-2014)25 and refined by full-matrix least-
squares fitting based on F2, using the program SHELXL-2014.26 All
non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were located at ideal positions and
included in the refinement but were restricted to riding on the atom
to which they were bonded. CCDC 1996671 and 1996672 contain
the supplementary crystallographic data of 1 and 2 for this paper.
Synthesis of Salicylimines L1 and L2. For the ligand L1, 3-tert-
butylsalicylaldehyde (0.89 g, 5.0 mmol) and N,N-dimethylethylenedi-
amine (0.44 g, 5.0 mmol) were placed in a Schlenk tube. The water
byproduct was removed under vacuum to yield a yellow oil (1.20 g,
1
96%). Data for L1 are as follows. H NMR (C6D6): δ 14.4 (s, 1 H,
OH), 7.83 (s, 1 H, NCH), 7.33 (d, J = 7.6 Hz, 1 H, Ph), 6.88 (d, J
= 7.6 Hz, 1 H, Ph), 6.77 (t, J = 7.6 Hz 1 H, Ph), 3.25 (t, J = 6.8 Hz, 2
H, N−CH2), 2.26 (t, J = 6.8 Hz, 2 H, N−CH2), 2.02 (s, 6 H,
1384
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pubs.acs.org/Organometallics
Article
Pawluc, P. Markovnikov hydrosilylation of alkenes: How an oddity
becomes the goal. ACS Catal. 2018, 8, 9865−9876.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00151.
■
sı
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Experimental details of mechanistic studies of hydro-
silylation and characterization details of the cobalt
complexes and organic products (PDF)
Accession Codes
CCDC 1996671−1996672 contain the supplementary crys-
tallographic 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.
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AUTHOR INFORMATION
Corresponding Author
Kouki Matsubara − Department of Chemistry, Fukuoka
University, Fukuoka 814-0180, Japan; orcid.org/0000-
0003-3909-7738; Email: kmatsuba@fukuoka-u.ac.jp
■
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performance of cobalt(I)−N-heterocyclic carbene complexes in
promoting the reaction of alkene with diphenylsilane: Selective 2,1-
hydrosilylation, 1,2-hydrosilylation, and hydrogenation of alkene. ACS
Catal. 2018, 8, 9637−9646. (h) Zong, Z.; Yu, Q.; Sun, N.; Hu, B.;
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Authors
Tomoaki Mitsuyama − Department of Chemistry, Fukuoka
University, Fukuoka 814-0180, Japan
Sayaka Shin − Department of Chemistry, Fukuoka University,
Fukuoka 814-0180, Japan
Momoko Hori − Department of Chemistry, Fukuoka
University, Fukuoka 814-0180, Japan
Ryuta Ishikawa − Department of Chemistry, Fukuoka
University, Fukuoka 814-0180, Japan; orcid.org/0000-
0002-1279-6283
Yuji Koga − Department of Chemistry, Fukuoka University,
Fukuoka 814-0180, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00151
Author Contributions
K.M. managed all of this study. S.S. made and determined the
complexes. T.M. conducted catalytic reactions and mechanistic
studies. M.H. studied mechanism in catalysis. R.I. analyzed
electronic properties of the complexes. Y.K. conducted
elemental analysis and other measurements.
Tarrino, S.; Concepción, P.; Ona-Burgos, P. Cobalt catalysts for
̃ ̃
alkene hydrosilylation under aerobic conditions without dry solvents
or additives. Eur. J. Inorg. Chem. 2018, 2018, 4867−4874. (j) Dong,
Y.; Zhang, P.; Fan, Q.; Du, X.; Xie, S.; Sun, H.; Li, X.; et al. The effect
of substituents on the formation of silyl [PSiP] pincer cobalt(I)
complexes and catalytic application in both nitrogen silylation and
alkene hydrosilylation. Inorg. Chem. 2020, 59, 16489−16499. (k) Kim,
D.; Chen, C.; Mercado, B. Q.; Weix, D. J.; Holland, P. L. Mechanistic
study of alkene hydrosilylation catalyzed by a β-dialdiminate cobalt(I)
complex. Organometallics 2020, 39, 2415−2424. (l) Xie, S.; Li, X.;
Sun, H.; Fuhr, O.; Fenske, D. [CNC]-Pincer cobalt hydride catalyzed
distinct selective hydrosilylation of aryl alkene and alkyl alkene.
Organometallics 2020, 39, 2455−2463.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research
19K05489) and JST ALCA-SPRING (JPMJAL1301). The
authors thank Dr. Shinji Kanegawa in IMCE, Kyushu
University, for SQUID measurements of the cobalt complexes.
(5) (a) Lyons, T. W.; Brookhart, M. Cobalt-catalyzed hydrosilation/
hydrogen-transfer cascade reaction: A new route to silyl enol ethers.
Chem. - Eur. J. 2013, 19, 10124−10127. (b) Niu, Q.; Sun, H.; Li, X.;
Klein, H.-F.; Flörke, U. Synthesis and catalytic application in
hydrosilylation of the complex mer-hydrido(2- mercaptobenzoyl)tris-
(trimethylphosphine)cobalt(III). Organometallics 2013, 32, 5235−
5238. (c) Yang, F.; Wang, Y.; Lu, F.; Xie, S.; Qi, X.; Sun, H.; Li, X.;
Fuhr, O.; Fenske, D. Preparation of hydrido [CNC]-pincer cobalt
complexes via selective C−H/C−F bond activation and their catalytic
performances. New J. Chem. 2018, 42, 15578−15586. (d) Nesbit, M.
A.; Suess, D. L. M.; Peters, J. C. E−H bond activations and
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