Bench-Stable, Substrate-Activated Cobalt Carboxylate Pre-Catalysts for Alkene Hydrosilylation with Tertiary Silanes

Article abstract of DOI:10.1021/acscatal.6b00304

High-spin pyridine diimine cobalt(II) bis(carboxylate) complexes have been synthesized and exhibit high activity for the hydrosilylation of a range of commercially relevant alkenes and tertiary silanes. Previously observed dehydrogenative silylation is suppressed with the use of sterically unencumbered ligands, affording exclusive hydrosilylation with up to 4000 TON. The cobalt precatalysts were readily prepared and handled on the benchtop and underwent substrate activation, obviating the need for external reductants. The cobalt catalysts are tolerant of epoxide, amino, carbonyl, and alkyl halide functional groups, broadening the scope of alkene hydrosilylation with earth-abundant metal catalysts.

Full text of DOI:10.1021/acscatal.6b00304

Letter
pubs.acs.org/acscatalysis
Bench-Stable, Substrate-Activated Cobalt Carboxylate Pre-Catalysts
for Alkene Hydrosilylation with Tertiary Silanes
Christopher H. Schuster, Tianning Diao, Iraklis Pappas, and Paul J. Chirik*
Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: High-spin pyridine diimine cobalt(II) bis(carboxylate) complexes have been synthesized and exhibit high activity
for the hydrosilylation of a range of commercially relevant alkenes and tertiary silanes. Previously observed dehydrogenative
silylation is suppressed with the use of sterically unencumbered ligands, affording exclusive hydrosilylation with up to 4000 TON.
The cobalt precatalysts were readily prepared and handled on the benchtop and underwent substrate activation, obviating the
need for external reductants. The cobalt catalysts are tolerant of epoxide, amino, carbonyl, and alkyl halide functional groups,
broadening the scope of alkene hydrosilylation with earth-abundant metal catalysts.
KEYWORDS: cobalt, hydrosilylation, alkenes, tertiary silanes, redox active, silicone
catalyst precursors are challenges for their widespread
he platinum-catalyzed hydrosilylation of alkenes is one of
Tthe largest applications of homogeneous catalysis in implementation. In situ activation of more air-stable pyridine
diimine iron halides has been explored to overcome this
limitation; however, these methods often rely on strong
reductants such as Grignard reagents or NaBEt3H and result
in decreased activity and selectivity.10 This approach, while
offering improvements in a laboratory setting, is ultimately not
viable on scale as such additives are known to promote alkoxide
exchange in alkoxysilanes which can lead to impure product
formation, and in the case of trialkoxysilanes, formation of
pyrophoric silane gas.11 Here we describe a new substrate
activation approach that relies on readily prepared and bench-
stable pyridine diimine cobalt bis(carboxylate) complexes.
During the preparation of this manuscript, Nagashima and
co-workers described a similar approach using a mixture of
cobalt carboxylates and isocyanides.12 For the pyridine diimine
catalysts reported here, the alkene hydrosilylation activity and
anti-Markovnikov selectivity with a range of commercially
relevant alkenes and tertiary silanes is among the highest
reported with earth-abundant transition metal catalysts.
industry owing to its widespread use in the preparation of a
range of consumer goods.1 The high-activity, anti-Markovnikov
selectivity and robustness of platinum catalysts such as Speier’s2
(H2PtCl6·(H2O)2) and Karstedt’s3 catalysts (Pt2[(Me2SiCH
CH2)2O]3) has made these compounds the industry standard
for the past several decades. The relatively high and volatile
price of platinum, coupled with the environmental footprint
associated with its terrestrial extraction,4 inspires the discovery
of alternative catalysts that rely on earth-abundant transition
metals.
Recently, there has been considerable effort from our
laboratory and others exploring both iron and cobalt catalysts
for alkene hydrosilylation.5−9 Central to the development of
new catalyst systems is the importance of utilizing proper olefin
and silane combinations to reflect commercial product profiles.
Among recently reported base metal catalysts, pyridine diimine
iron dinitrogen complexes are notable due to their extremely
high activity and anti-Markovnikov selectivity for the hydro-
silylation of relevant terminal alkenes.5 For instance, the high
selectivity for the monohydrosilylation of the 4-position of
1,2,4-trivinylcyclohexane, an important component for low-
rolling resistance tires, exceeds the state-of-the-art precious
metal system.6 Importantly, this family of catalysts is reactive
with commercially relevant tertiary silanes, a distinguishing
feature from most other classes of alkene hydrosilylation
catalysts that rely on earth-abundant transition metals.7−9
Unfortunately, the extreme air- and moisture-sensitivity of the
Previously, our group reported the use of aryl substituted
pyridine diimine cobalt alkyl complexes for the dehydrogen-
ative silylation of terminal alkenes.13 Mechanistic studies
revealed rapid β-hydride elimination prior to turnover-limiting
reaction with silane as the origin of product selectivity. We
Received: January 29, 2016
Revised: March 12, 2016
© XXXX American Chemical Society
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Letter
reasoned that the use of ligands bearing smaller imine
substituents would accelerate the bimolecular reaction of the
silane with a cobalt alkyl relative to the unimolecular β-
hydrogen elimination pathway. If successful, rational catalyst
modification could switch selectivity between dehydrogenative
silylation and hydrosilylation. The N-methyl imine-substituted
pyridine diimine cobalt alkyl complex ((MeAPDI)Co-
(CH2SiMe3)) was prepared using standard methods and
evaluated for alkene hydrosilylation activity (Table 1). The
due to the insolubility of the complex in the reaction medium.
Performing the catalytic hydrosilylation at 80 °C resulted in
complete conversion of 1-octene, albeit with accompanying
dehydrogenative silylation, generating allylsilane as a minor
(13%) byproduct (entry 3). The poor solubility of (MeAPDI)-
Co(OAc)2 motivated exploration of other more soluble
cobalt(II) bis(carboxylate) sources. (MeAPDI)Co(2-EH)2 (EH
= ethylhexanoate) was prepared in a similar manner on the
benchtop in air and isolated as a brown powder in 94% yield.
The more soluble precatalyst proved to be highly active,
exhibiting high anti-Markovnikov selectivity and reaching
complete conversion in less than an hour at 23 °C with no
detectable evidence for dehydrogenative silylation (entry 4).
Importantly, (MeAPDI)Co(2-EH)2 exhibited no erosion in
catalytic hydrosilylation performance after exposure of the
precatalyst to air for 18 h (see Supporting Information for
details). The corresponding cobalt complexes of PyBox (entry
5) and its carbon analogue TFAPDI15 (entry 6) were also found
to give efficient hydrosilylation, with slightly reduced selectivity
with the use of PyBox.
Table 1. Evaluation of Substrate-Activated Pyridine Diimine
Cobalt Precatalysts for the Hydrosilylation of 1-Octene with
a
(EtO)3SiH
The high activity observed with (MeAPDI)Co(2-EH)2 and
(TFAPDI)Co(2-EH)2 prompted further evaluation for the
hydrosilylation of 1-octene with a variety of tertiary silanes
(Table 2). Although both catalysts proved to be highly effective
a
Table 2. Hydrosilylation of 1-Octene with Tertiary Silanes
a
All reactions were performed on 0.89 mmol scale using a 1:1 mixture
of olefin to silane, HS = hydrosilylation, DHS = dehydrogenative
b
silylation, EH = ethylhexanoate. Determined by 1H NMR analysis of
c
d
the crude reaction mixture. Yield of isolated material. Determined by
e
1H NMR analysis of isolated material. Reaction performed at 80 °C.
addition of triethoxysilane to 1-octene (neat) was chosen as a
model reaction as the resulting noctylsilane product is
manufactured annually on >6000 ton scale and finds
commercial applications in coatings such as weatherproofing
for masonry products, metal oxides, and glass.1 Notably, this
precatalyst resulted in high (>98:2) hydrosilylation selectivity
a
All reactions were performed on 1.00 mmol scale with a 1:1 mixture
b
c
of olefin and silane. Yield of isolated material. Determined from the
d
1H NMR spectrum of isolated material. 0.1 mol % catalyst loading.
e
Reaction performed at 80 °C.
n
and afforded OctSi(OEt)3 in 95% yield (entry 1).
with alkoxysilanes (entries 1−4), (TFAPDI)Co(2-EH)2 ex-
hibited superior performance with siloxanes (entries 5−8) and
triethylsilane (entries 9,10). High selectivity for hydrosilylation
over dehydrogenative silylation was observed in all cases, with
the exception of triethylsilane, where 56% of the product
distribution was dehydrogenative silylation.
Magnetic measurements and X-band EPR spectra of all
prepared (MeAPDI)Co(O2CR)2 and (TFAPDI)Co(O2CR)2
support S = 3/2, high-spin Co(II) complexes. Each compound
exhibited sharp but paramagnetically shifted resonances in the
CDCl3 1H NMR spectrum that proved useful for routine
characterization. The more crystalline derivatives (MeAPDI)-
Co(OPiv)2 and (TFAPDI)Co(OPiv)2 were prepared and
characterized by X-ray diffraction (Figure 1). Six-coordinate
cobalt complexes were observed with the carboxylate ligands
adopting both κ2 and κ1 hapticity similar to the coordination
geometry reported by Constable and co-workers with a related
Having identified a successful ligand to promote selective
hydrosilylation, attention was devoted to improving both the
bench stability and cost profile of the cobalt precatalysts.
Cobalt(II) carboxylates are perhaps the most attractive because
they are among the most inexpensive sources of cobalt, are
bench stable, and offer structural diversity by variation of the
carboxylate substituent. Our laboratory previously demonstra-
ted that treatment of phosphine-Co(OAc)2 mixtures with
pincolborane generated active catalysts for the hydroboration of
alkenes,14 suggesting that silane activation may also be
plausible. The desired pyridine diimine cobalt acetate complex,
(MeAPDI)Co(OAc)2 was isolated in 67% yield as a brown
powder following straightforward addition of the free ligand to
a THF slurry of anhydrous Co(OAc)2. Although (MeAPDI)-
Co(OAc)2 exhibited excellent bench stability, it proved
essentially inactive at ambient temperature (entry 2), likely
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Table 3. Hydrosilylation of Functionalized Terminal Alkenes
a
with (EtO)3SiH
Figure 1. Representations of the solid state molecular structures of
(
MeAPDI)Co(OPiv)2 (left) and (TFAPDI)Co(OPiv)2 (right) at 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
terpyridine example.16 Notably, for both (MeAPDI)Co(OPiv)2
and (TFAPDI)Co(OPiv)2, a single tert-butyl resonance was
observed by 1H NMR spectroscopy, suggesting rapid
equilibration of carboxylate coordination modes on the NMR
time scale. Significant differences in the pyridine nitrogen−
cobalt bond lengths (2.0442(8)Å and 2.112(4)Å for MeAPDI
and TFAPDI, respectively) as well as the chelate bite angle
(151° versus 144°) support a more open metal center in the
TFAPDI example, consistent with the improved catalytic
performance noted in Table 2. All of the ligand bond distances,
a diagnostic for redox activity, support neutral pyridine
diimines, and hence, the compounds are best described as
high-spin, Co(II).
The discovery of highly active, selective, and bench-stable
cobalt precatalysts enabled exploration of the scope of the
alkene substrates in the hydrosilylation reaction. Trialkoxysi-
lanes with functionalized alkyl groups are of interest due to the
improved adhesion properties between materials in silicone-
based caulks and sealants as well as imparting other beneficial
properties to coatings.1 Certain basic functional groups such as
amines are known to inhibit or poison platinum-based catalysts.
As presented in Table 3, (TFAPDI)Co(2-EH)2 was effective for
the hydrosilylation of a range of terminal alkenes bearing
functional groups using (EtO)3SiH. High to excellent isolated
yields were obtained after 1 h of reaction time at 23 °C using
0.25 mol % of the cobalt precatalyst. Notable findings include
tolerance of amides as well as tertiary and secondary amines.
Styrene, a notoriously challenging substrate for regioselective
hydrosilylation,17 yielded exclusively the anti-Markovnikov
product. Oxygen-based functional groups such as esters, ethers,
epoxides, and ketones were also well-tolerated by the cobalt
catalyst. Notably, the hydrosilylation of vinylacetate proceeded
efficiently in contrast to reactivity observed with (iPrPDI)Fe-
(N2)2, where rapid deactivation arising from C−O bond
cleavage was observed.18 (TFAPDI)Co(2-EH)2 also proved
tolerant of remote bromide functionality and large alkyl groups,
albeit with reduced efficiency. Unfortunately, allyl chloride
proved to be a catalyst poison.
a
All reactions performed on 2.00 mmol scale with a 1:1 mixture of
olefin and silane unless otherwise noted; products were obtained with
>98:2, HS:DHS selectivity. 0.5 mol % catalyst loading. 1.0 mol %
catalyst loading for 5 h, 1.00 mmol scale. 5 h reaction time. 24 h
reaction time. 0.1 mol % catalyst loading, isolated as a mixture of
b
c
d
e
f
59:41, anti-Markovnikov to Markovnikov products.
and odorous vapors, respectively. Reduced pyridine diimine
iron catalysts have been ineffective for this transformation due
to competing ring opening of the epoxide. To fully demonstrate
the performance of (TFAPDI)Co(2-EH)2, the hydrosilylation of
allyl glycidyl ether with triethoxysilane was conducted on 10 g
scale (Scheme 1). Rapid and exothermic reactions were
observed at catalyst loading as low as 0.025 mol %, resulting
in complete conversion in under 40 min with high anti-
Markovnikov selectivity and no evidence for epoxide ring
opening. This reactivity compares favorably with both
heterogeneous20 and homogeneous21 platinum catalysts,
which often promote competing alkene isomerization and
Markovnikov addition, leading to side products that must be
removed by distillation.
The cross-linking of silicone fluids by platinum-catalyzed
hydrosilylation produces silicone products that find application
as release coatings in everything from tapes to stamps and
labels. Due to the viscous morphology of the product, the
platinum catalyst becomes trapped in the final product and is
not recovered, accounting for approximately 30% of the cost of
the final silicone. Commercial standards mandate that this
curing process proceed rapidly with low catalyst loadings and
without leaving the product colored in any way. The cross-
linking activity of (TFAPDI)Co(2-EH)2 was examined at four
different catalyst loadings (Scheme 1). Rapid gel formation was
observed in each case, and effective silicon cross-linking was
observed as low as 1 ppm (based on Co metal, wt/wt). The
benefits of lower catalyst loading on the discoloration of the
product are illustrated in Scheme 1, where the 1−2 ppm levels
yielded colorless product.
Among functionalized terminal alkenes, there is considerable
interest in the hydrosilylation of allyl glycidyl ether using earth-
abundant transition metal catalysts. Trialkoxysilanes derived
from this substrate have found widespread application in
surface coatings as well as bonding agents in a wide range of
products from caulks to sealants to binders used by the
fiberglass industry.19 These products offer advantages to amine
and sulfur-based alternatives which can lead to discoloration
In summary, a new class of highly active and selective alkene
hydrosilylation catalysts based on the earth-abundant metal
cobalt has been discovered. Use of inexpensive cobalt(II)
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Letter
Scheme 1. Hydrosilylation of Allyl Glycidyl Ether and
Crosslinking of Silicone Fluids
ACKNOWLEDGMENTS
■
a
We thank Momentive Performance Materials Inc. for financial
support. We also acknowledge Drs. Kenrick Lewis (MPM),
Aroop Roy (MPM), Keith Weller (MPM), Julie Boyer (MPM),
and Jos Delis (MPM) for helpful discussions.
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Top: 10 g scale hydrosilylation of allyl glycidyl ether (AGE) with
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00304.
Experimental procedures and general protocols, as well
as characterization of all new compounds (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
■
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́
(c) Hyder, I.; Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P. Dalton
The authors declare the following competing financial
interest(s): Two patents related to this work have been filed:
(a) Diao, T.; Chirik, P. J.; Roy, A. K.; Lewis, K. M.; Nye, S.;
Weller, K. J.; Delis, J. G. P.; Yu, R.: US Patent Publication
US2015/0080536 A1 (Filed: November 19, 2014, published:
March 19, 2015) (b) Chirik, P. J.; Schuster, C. H.; Boyer, J. L.;
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