Mode of activation of cobalt(II) amides for catalytic hydrosilylation of alkenes with tertiary silanes

Article abstract of DOI:10.1021/jacs.6b12938

Cobalt(II) complexes capable of catalyzing alkene hydrosilylation in the absence of external activators are rarely known, and their activation mode has remained poorly understood. We present here that cobalt(II) amide complexes, [Co(N(SiMe₃)₂)₂] and its NHC adducts [(NHC)Co(N(SiMe₃)₂)₂] (NHC = N-heterocyclic carbene), are effective catalysts for the hydrosilylation of alkenes with tertiary silanes. Mechanistic studies revealed that cobalt(II) amides can react with hydrosilane to form cobalt(I) species, silylamide, and hydrogen, which serves as the entry to the genuine catalytically active species, presumably cobalt(I) species, for the cobalt-catalyzed hydrosilylation reaction.

Full text of DOI:10.1021/jacs.6b12938

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Communication
Mode of Activation of Cobalt(II) Amides for Catalytic
Hydrosilylation of Alkenes with Tertiary Silanes
Yang Liu, and Liang Deng
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12938 • Publication Date (Web): 25 Jan 2017
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Mode of Activation of Cobalt(II) Amides for Catalytic Hydrosi-
lylation of Alkenes with Tertiary Silanes
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Yang Liu and Liang Deng*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China
Supporting Information Placeholder
alkenes with tertiary silanes. Intriguingly, the system allows
ABSTRACT: Cobalt(II) complexes capable of catalyzing al-
the establishment of the activation mode of the precatalyst
that entails the reaction of cobalt(II) bis(amide) with hy-
drosilane to form cobalt(I) amide, silylamide, and hydrogen.
The resulting cobalt(I) amide further reacts with hydrosilane
to form cobalt(I) hydride. The comparable catalytic activity
of the isolated cobalt(I) amide complex to its cobalt(II) pre-
cursor suggests that cobalt(I) species could be the genuine
catalytically active species in the cobalt-catalyzed hydrosi-
lylation reaction.
kene hydrosilylation in the absence of external activators are
rarely known, and their activation mode has remained poorly
understood. We present here that cobalt(II) amide complex-
es,
[Co(N(SiMe3)2)2]
and
its
NHC
adducts
[(NHC)Co(N(SiMe3)2)2] (NHC = N-heterocyclic carbene), are
effective catalysts for the hydrosilylation of alkenes with ter-
tiary silanes. Mechanistic studies revealed that cobalt(II)
amides can react with hydrosilane to form cobalt(I) species,
silylamide, and hydrogen, which serves as the entry to the
genuine catalytically active species, presumably cobalt(I)
species, for the cobalt-catalyzed hydrosilylation reaction.
Chart 1. Cobalt(II) Amide Catalysts
There has been great interest in the development of iron-
and cobalt-based hydrosilylation catalysts as alternatives for
the applied noble metal catalysts.1 As the traditional design
of the alkene hydrosilylation catalysts has focused on low-
valent iron and cobalt complexes,2 a new development in the
field is the direct use of iron(II) and cobalt(II) complexes as
pre-catalysts.3,4 Initially, this type of divalent metal catalyst
Table 1. Catalytic Performance of Cobalt(II) Amide
Complexes in the Reaction of 1-Octene with
HSi(OEt)3
was
restricted
to
the
(PDI
alkyl
=
complexes,
e.g.
(PDI)Fe(CH2SiMe3)2
2,6-diiminopyridine),4a
(tpy)Fe(CH2SiMe3)2 (tpy = terpyridine),4a and Co(IMes’)2
(IMes’ = cyclometallated IMes ligand, Chart 1),4b that pro-
mote the hydrosilylation of alkenes with primary and sec-
ondary silanes. Very recently, Nagashima, Chirik, and their
coworkers found that the carboxylate complexes
M(Opiv)2/CNBut (M = Fe, Co; Opiv = pivalate)4c and
(PDI)Co(2-EH)2 (EH = 2-ethylhexanonate)4d are also effective,
effecting the hydrosilylation of alkenes with tertiary silanes.
The divalent metal complex-based catalysts obviate the use
of external reducing reagents, e.g. NaBHEt3 and alkyl lithium
reagents, to generate active species, which is highly desired
from the view of catalyst’s practical usage. The new devel-
opment also brings fundamental mechanistic questions as to
the activation mode of the precatalysts and the nature of the
genuine catalytic species, which are unsolved yet. In this
study, we report a new type of cobalt(II) complex-based hy-
drosilylation catalyst, the simple binary complex
[Co(N(SiMe3)2)2] and its NHC adducts (Chart 1), which does
not require the use of external activator. These cobalt(II)
amides can catalyze the anti-Markovnikov hydrosilylation of
GC Yield (%)a
En-
try
Time
(h)
Conv.
(%)
b
Cat.
HS
DS C8H16
n-
C8H18
1
5.0 mol% 1
0.5
24
> 99
> 99
> 99
> 99
93
26
79
5
10
3
1
52
<1
90
<1
75
<1
10
1
12
18
4
2
3
5.0 mol% 2
5.0 mol% 3
0.5
24
68
11
0
1
32
6
0.5
36
> 99
> 99
> 99
76
86
99
3
0
0
21
3
4
5
0.1 mol% 4
0.1 mol% 5
24
0.5
0
aYields were based on the alkene. bIsomers of 1-octene.
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Table 2. Hydrosilylation of Various Alkenes with Tertiary Silanesa
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b
aConditions: alkene (1.0 mmol), silane (1.2 mmol), complex 5 (1 mol%) at room temperature in neat conditions. Isolated yields.
c0.1 mol % catalyst loading. dAt 60 oC. e1.05 mmol HSi(OEt)3 were used. GC yield, and alkene isomers were observed.
f
Inspired by Tilley’s recent report of Fe(N(SiMe3)2)2-
catalyzed hydrosilylation of ketones with H2SiPh2,5 we per-
ceived the capability of [Co(N(SiMe3)2)2] (1) in promoting
alkene hydrosilylation. The speculation was verified by the
reaction of 1-octene with HSi(OEt)3 (1.2 equiv. ). Using 1 (5
mol%, Chart 1) as the catalyst, the reaction could furnish the
hydrosilylation product n-C8H17Si(OEt)3 (HS) in 26% and 79%
GC yields in 0.5 and 24 h, respectively. Interestingly, in addi-
tion to the hydrosilylation product, the alkene isomers of 1-
C8H16 and the dehydrogenative silylation product n-
C6H13CH=CHSi(OEt)3 (DS) were also formed at the early
stage of the reaction. At the late stage of the reaction, these
side products, however, fully converted into n-C8H17Si(OEt)3
and n-octane (entry 1 in Table 1). The change of product dis-
tribution over time suggests that, under the reaction condi-
tions, 1 serves as an effective precatalyst not only for alkene
hydrosilylation but also for alkene isomerization and hydro-
genation.6 In pursuit for a more selective hydrosilylation
catalyst, N-heterocyclic carbene (NHC) ligands were applied
to tune the reaction selectivity. As shown in Table 1, the reac-
the hydrosilylation of mono-substituted aliphatic alkenes to
selectively give anti-Markovnikov addition products in mod-
erate to excellent yields (Table 2).8 High chemo-selectivity
was observed in the reactions of the aliphatic alkenes bearing
ester, epoxide, and secondary amine functional groups,
wherein the hydrosilylation reaction took place selectively on
the C-C double bonds (entries 3-6 in Table 2). The catalytic
system is sensitive to the steric properties of alkenes, as the
hydrosilylation of 4-vinyl-cyclohexene took place exclusively
on the vinyl group (entry 9), the reaction with the tert-butyl
substituted alkene afforded the hydrosilylation product in a
decreased yield (entry 10) than that with 1-octene, and the
reaction of norbornene requires elevated temperature (60 oC,
entry 11). Probably due to the coordination effect of arene
toward the in-situ generated low-coordinate cobalt species
(vide infra), the cobalt(II) amide catalyst is incapable in pro-
moting the hydrosilylation of styrene. The cobalt-catalyzed
hydrosilylation reaction is also sensitive to the steric proper-
ties of hydrosilanes. The reactions with the tertiary hy-
drosilanes HSiMe(OEt)2 and HSiPh3 gave the corresponding
hydrosilylation products in lower yields (entries 12 and 13)
and the sterically hindered hydrosilanes HSiEt3 and MD’M
(MD’M = 1,1,1,3,5,5,5-heptamethyltrisiloxane) are inapplicable
for the cobalt-catalyzed hydrosilylation reaction (entries 14
and 15). It should be noted that, in addition to the steric ef-
fect, the different electronic property of the Si-H bonds in
these hydrosilanes might also exert influence.
7
tions employing 5 mol% [(IPr)Co(N(SiMe3)2)2] (2, Chart 1)
7
and [(IMes)Co(N(SiMe3)2)2] (3, Chart 1) have similar out-
come with that of 1, giving the hydrosilylation product in 68%
and 76% yields in 24 and 36 h, respectively (entries 2 and 3 ).
On the other hand, decreasing the steric bulk of the NHC
ligand by using [(IMesPr)Co(N(SiMe3)2)2] 8 (4, Chart 1) as the
precatalyst, the yield of n-C8H17Si(OEt)3 could be increased to
86% (entry 4). In a marked contrast, the reaction utilizing
The cobalt(II) amide complexes 1-5 represent a new cate-
gory of cobalt catalyst for alkene hydrosilylation obviating
the use of external activator after the aforementioned alkyl
and carboxylate complexes.4 Noting the very limited
knowledge on the activation mode of these divalent metal
complex-based precatalysts, the stoichiometric reactions of
cobalt(II) amide complexes with hydrosilanes and alkenes
were studied. These cobalt(II) amide complexes are inert
toward 1-octene under ambient conditions, but they can
8
[(IMesMe)Co(N(SiMe3)2)2] (5, Chart 1) has nearly full con-
version to the hydrosilylation product in 0.5 h even with a 0.1
mol% catalyst loading (entry 5). Notably, further adding 1-
octene and HSi(OEt)3 to the mixture can trigger the catalytic
hydrosilylation reaction though with slightly decreased con-
version (Figure S11), demonstrating the high activity of the
catalytic system.
Using 5 (1 mol%) as the catalyst and HSi(OEt)3 as the
silane source, a preliminary substrate scope study on the
cobalt-catalyzed alkene hydrosilylation reaction was per-
formed. The cobalt(II) amide catalyst can effectively promote
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Scheme 1. The Reactions of Cobalt Amide Complexes with HSi(OEt)3
1
2
3
4
5
N
N
+ HSi(OEt)3
N
+ HSi(OEt)3
n-hexane, r.t.
N
N
N
Dipp
Dipp
Dipp
H
Dipp
Dipp
Dipp
C6H6, 60 o
C
CoI
N''
CoII
CoI
- Si(OEt)3(N'')
- 1/2 H2
- Si(OEt)3(N'')
N''
N''
6
6
7
2
7
8
9
(N'' = N(SiMe3)2)
N
N
Dipp
Dipp
+ HSi(OEt)3
N
N
Dipp
Dipp
+ HSi(OEt)3
- Si(OEt)3(N'')
+ C6H6
CoI
CoII
- [H]
- Si(OEt)3(N'')
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B
N''
H
A
H
readily react with HSi(OEt)3. While the attempts to access
reactive cobalt species from the reaction of
[(IMesMe)Co(N(SiMe3)2)2] (5) with HSi(OEt)3 were unsuc-
cessful. The interaction of HSi(OEt)3 with
[(IPr)Co(N(SiMe3)2)2] (2) that has a more bulky NHC ligand
in n-hexane at room temperature led to the clean formation
of
the
two-coordinate
cobalt(I)
amide
complex
9
[(IPr)Co(N(SiMe3)2)] (6)
that can further react with
o
HSi(OEt)3 at 60 C in benzene to furnish the cobalt(I) hy-
dride complex [(IPr)Co(ƞ6-C6H6)H] (7) in high yield (Scheme
1).8 Notably, the latter reaction could also take place at room
temperature though with decreased rate (Figure S14). Along
with
the
cobalt
complexes,
the
silylamide
Si(OEt)3(N(SiMe3)2) was also formed in both reactions.8
Complexes 6 and 7 have been characterized by NMR spectro-
scopic methods, elemental analyses, and X-ray crystallo-
graphic studies (Figures S2 and S3). Complex 6 is a rare ex-
ample of heteroleptic two-coordinate cobalt(I) complex after
Jones’ [(IPr2Me2)Co(NSiPh3Ar*)].10 Complex 7 is diamagnetic.
The characteristic signal at -21.6 ppm in its 1H NMR spectrum
supports the presence of the metal-bounded hydride ligand.
Figure 1. Reaction profiles of the reaction of 1-octene (1 mmol)
with HSi(OEt)3 (1.2 mmol) using 5 mol% of 2 (black), 6 (red),
and 7 (blue) as the precatalysts. Solid lines show the change
of the total yield of the HS and DS products versus time. The
dashed lines show the change of the net yield of the mixture
of alkene isomers and n-octane.
The attainment of these cobalt(I) complexes prompted fur-
ther studies on their catalytic performance in the reaction of
1-octene with HSi(OEt)3. The reaction using 6 as the catalyst
shows similar selectivity as that using 2, wherein alkene hy-
drosilylation took place after alkene isomerization, but both
reactions have faster rates. As depicted in Figure 1, the reac-
tion using 5 mol% 6 merely requires 4 h to reach the plateau,
whereas that with 2 needs 14 h (Figure 1). Despite this, the
reaction still shows a short induction period, implying that 6
is an intermediate complex en route to in-cycle catalytic spe-
cies. Complex 7 can effectively catalyze alkene isomerization,
but is an inefficient catalyst for alkene hydrosilylation (blue
lines in Figure 1). The fine performance of 7 in catalyzing
alkene isomerization implies that low-coordinate cobalt(I)
hydride species formed from the reaction of 6 with HSi(OEt)3,
presumably (IPr)Co(H), should be responsible for the alkene
isomerization process in the cobalt(II) amide-catalyzed reac-
tion. The poor performance of 7 in promoting alkene hydros-
ilylation might be due to the coordinative saturation of its
metal center, which renders the coordination of the hy-
drosilane and its subsequent activation difficult. To certain
degree, it also provides explanation for the observations that
benzene is not a suitable solvent and styrene is not applica-
ble for the cobalt(II) amide complex-catalyzed hydrosilyla-
tion reaction, since in these cases the in-situ formed reactive
low-valent cobalt species could be readily trapped by these
aromatic molecules. On the other hand, the successful hy-
drosilylation of N-allyl aniline and 4-phenylbutene (entries 5
and 7 in Table 1) might benefit from their electron-rich arene
fragments that might have weaker affinity toward cobalt(I)
center as compared to benzene.
The reaction of HSi(OEt)3 with 2 results in the reduction of
the cobalt(II) species to cobalt(I), which is different from the
known oxidative addition or σ-bond metathesis reaction of
hydrosilanes with cobalt species,11 and also differs from the
existing cobalt(II)-to-cobalt(I) reduction reactions that gen-
erally employ strong reducing reagents, e.g. alkali metals,
alkali earth metals, and boron hydrides.12 The formation of 6
might result from the homolytic Co-H bond cleavage reac-
tion
of
a
cobalt(II)
hydride13
intermediate
(IPr)Co(H)(N(SiMe3)2) (A) that can be formed from the σ-
bond metathesis reaction of 2 with HSi(OEt)3 (Scheme 1). In
support of this proposal, the by-products Si(OEt)3(N(SiMe3)2)
and H2 were detected in NMR-scale reactions (Figures S6-S9).
The formation of 7 implies the production of the low-
coordinate cobalt(I) hydride (IPr)Co(H) (B) in the reaction of
6 with HSi(OEt)3 (Scheme 1). This reaction outcome is dis-
tinct from that of the cobalt(I) alkyl complex
[(IAd)Co(PPh3)(CH2SiMe3)] with HSiPh3, wherein cobalt(I)
silyl complex [(IAd)Co(PPh3)(SiPh3)], rather than a cobalt(I)
hydride, was formed.14 The difference might root in the selec-
tivity of the cobalt(III) intermediate (NHC)Co(H)(SiR3)(X) (X
= CH2SiMe3, N(SiMe3)2), which can be formed from the oxi-
dative addition of cobalt(I) species with hydrosilanes, to per-
form either H-X or R3Si-X bond-forming reductive elimina-
tion. The large Si-N bond enthalpy15 could drive the Si-N
bond-forming reductive elimination, giving the hydride spe-
cies.
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Page 4 of 5
6, 290.
As the results have pointed out that the genuine catalytic
species for the hydrosilylation reactions might be produced
from the reaction of cobalt(I) amide species with one (or
more) equivalent of HSi(OEt)3, one could speculate its iden-
tity as a low-coordinate cobalt(I) hydride or a cobalt(I) silyl
species. The latter could be formed from the reaction of co-
balt(I) hydride with one equivalent of HSi(OEt)3 after elimi-
nating H2.16 At this stage, the exact identity of the genuine
catalytically active species is difficult to confirm. However,
the observation of the dehydrogenative silylation product n-
C6H13CH=CHSi(OEt)3 in these cobalt-catalyzed reactions
(Table 1, entries 1-3) implies the involvement of cobalt silyl
intermediates,1 whose interaction with n-octene could give β-
silylalkyl cobalt species that could further undergo β-H elim-
ination to give the dehydrogenative silylation product. On
the other hand, it is noted that the low-coordinate cobalt(0)
(2) For examples, please see: (a) Bart, S. C.; Lobkovsky, E.; Chirik, P.
J. J. Am. Chem. Soc. 2004, 126, 13794. (b) Tondreau, A. M.; Atien-
za, C. C. H.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.;
Chirik, P. J. Science 2012, 335, 567. (c) Sunada, Y.; Noda, D.;
Soejima, H.; Tsutsumi, H.; Nagashima, H. Organometallics 2015,
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1133. (e) Seitz, F.; Wrighton, M. S. Angew. Chem., Int. Ed. Engl.
1988, 27, 289. (f) Atienza, C. C. H.; Diao, T, Weller, K. J.; Nye, S.
A.; Lewis, K. M.; Delis, J. G. P.; Boyer, J. L.; Roy, A. K.; Chirik, P. J.
J. Am. Chem. Soc. 2014, 136, 12108. (g) Chen, C.; Hecht, M. B.;
Kavara, A.; Brennessel, W. W.; Mercado, B. Q.; Weix, D. J., Hol-
land, P. L. J. Am. Chem. Soc. 2015, 137, 13244. (h) Ibrahim, A. D.;
Entsminger, S. W.; Zhu, L.; Fout, A. R. ACS Catal. 2016, 6, 3589.
(i) Rivera-Hernández, A.; Fallon, B. J.; Ventre, S.; Simon, C.;
Tremblay, M.; Gontard, G.; Derat, E.; Amatore, M.; Aubert, C.;
Petit, M. Org. Lett. 2016, 18, 4242. (j) Chu, W.; Gilbert-Wilson,
R.; Rauchfuss, T. B. Organometallics 2016, 35, 2900.
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complex [(IPr)Co(η2-vtms)2] (8) (vtms
=
vinyltrime-
(3) There are also recent reports using nickel(II) complexes as cata-
lysts for hydrosilylation. For examples, see: (a) Buslov, I.;
Becouse, J.; Mazza, S.; Montandon-Clerc, M.; Hu, X. Angew.
Chem., Int. Ed. 2015, 54, 14523. (b) Pappas, I.; Treacy, S.; Chirik, P.
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thylsilane)17 is ineffective in promoting the hydrosilylation
reaction, which excludes the involvement of cobalt(0) species
as the genuine catalytically active species.
In summary, we found that cobalt(II) amide complexes can
serve as catalysts for the hydrosilylation of alkenes with ter-
tiary silanes, obviating the use of external activator. Mecha-
nistic studies disclosed that hydrosilane can react with co-
balt(II) bis(amide) complex to form cobalt(I) amide and co-
balt(I) hydride species, and that low-coordinate cobalt(I)
species could be the genuine catalysts for the alkene hydrosi-
lylation reaction. New catalyst design for alkene hydrosi-
lyation employing the unique catalyst-activation mode is
ongoing.
(4) (a) Tondreau, A. M.; Atienza, C. C. H.; Darmon, J. M.; Milsmann,
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Delis, J. G. P.; Lobkovsky, E.; Chirik, P. J. Organometallics 2012,
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52, 10845. (c) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. J.
Am. Chem. Soc. 2016, 138, 2480. (d) Schuster, C. H.; Diao, T.;
Pappas, I.; Chirik, P. J. ACS Catal. 2016, 6, 2632. (e) Du, X.;
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: xxxxxxxxx.
Crystallographic data (CIF)
Experimental procedures, characterization data of the com-
pounds, and NMR spectra (PDF)
(7) Day, B. M.; Pal, K.; Pugh, T.; Tuck, J.; Layfield, R. A. Inorg. Chem.
2014, 53, 10578.
(8) For experimental details on the preparation and characterization
data of new compounds, please see Supporting Information.
(9) During the reviewing process of this work, Danopoulos and
Braunstein reported the preparation of 6 via the reaction of
[(IPr)Co(N(SiMe3)2)Cl] with KC8: Danopoulos, A. A.; Braunstein,
P.; Monakhov, K. Y.; van Leusen, J.; Kögerler, P.; Clémancey, M.;
Latour, J.-M.; Benayad, A.; Tromp, M.; Rezabalh, E.; Frison, G.
Dalton Trans. 2017, DOI: 10.1039/c6dt03565e.
(10) Hicks, J.; Jones, C. Organometallics 2015, 34, 2118.
(11) (a) Corey, J. Y. Chem. Rev. 2011, 111, 863. (b) Corey, J. Y. Chem.
Rev. 2016, 116, 11291.
(12) For examples, see: (a) Aresta, M.; Rossi, M.; Sacco, A. Inorg.
Chim. Acta 1969, 3, 227. (b) Ung, G.; Peters, J. C. Angew. Chem.,
Int. Ed. 2015, 54, 532.
(13) For homoleptic cobalt(II)-H bond cleavage, please see: Krafft, M.
J.; Bubrin, M.; Paretzki, A.; Lissner, F.; Fiedler, J.; Záliš, S.; Kaim,
W. Angew. Chem., Int. Ed. 2013, 52, 6781.
(14) Mo, Z.; Xiao, J.; Gao, Y.; Deng, L. J. Am. Chem. Soc. 2014, 136,
17414.
(15) The Si-N bond in Me3Si–N(SiMe3)2 has a bond enthalpy of 108
kcal/mol, whereas that of the N-H bond in H-NMe2 is ca. 95
kcal/mol. For the data, see: Luo, Y.-R. Comprehensive Handbook
of Chemical Bond Energies; Boca Raton: CRC Press, 2007.
(16) Analogous transformations have been known for cobalt(I) phos-
phine complexes, see: (a) Archer, N. J.; Haszeldine, R. N.; Parish,
R. V. J. Chem. Soc. D 1971, 524. (b)Archer, N. J.; Haszeldine, R. N.;
Parish, R. V. J. Chem. Soc., Dalton Trans. 1979, 695.
AUTHOR INFORMATION
Corresponding Author
* E-mail: deng@sioc.ac.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The work was supported by the National Key Research and
Development Program (2016YFA0202900), the National Nat-
ural Science Foundation of China (Nos. 21690062, 21421091
and 21432001), and the Strategic Priority Research Program of
the Chinese Academy of Sciences (XDB20000000).
REFERENCES
(1) For reviews, please see: (a) Marciniec, B.; Maciejewski, H.; Pie-
traszuk, C.; Pawluć, P. In Hydrosilyaltion: A Comprehensive Re-
view on Recent Advances; Marciniec, B., Ed.; Springer: Berlin,
2009; Chapter 1. (b) Zhang, M.; Zhang, A. Appl. Organometal.
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