Cobalt(II) and (I) Complexes of Diphosphine-Ketone Ligands: Catalytic Activity in Hydrosilylation Reactions

Article abstract of DOI:10.1002/ejic.201801221

The hydrosilylation of unsaturated compounds homogeneously catalyzed by cobalt complexes has gained considerable attention in the last years, aiming at substituting precious metal-based catalysts. In this study, the catalytic activity of well-characterized Co^II and Co^I complexes of the ^pToldpbp ligand is demonstrated in the hydrosilylation of 1-octene with phenylsilane. The Co^I complex is the better precatalyst for the mentioned reaction under mild conditions, at 1 mol-% catalyst, 1 h, room temperature, and without solvent, yielding 84 % of octylphenylsilane. Investigation of the substrate scope shows lower performance of the catalyst in styrene hydrosilylation, but excellent results with allylbenzene (84 %) and acetophenone (> 99 %). This catalytic study contributes to the field of cobalt-catalyzed hydrosilylation reactions and shows the first example of catalysis employing the dpbp ligand in combination with a base metal.

Full text of DOI:10.1002/ejic.201801221

A Journal of
Accepted Article
Title: Cobalt(II) and (I) complexes of Diphosphine-Ketone Ligands:
Catalytic Activity in Hydrosilylation Reactions
Authors: Dide G.A. Verhoeven, Joost Kwakernaak, Maxime A. C. van
Wiggen, Martin Lutz, and Marc-Etienne Moret
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801221
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
Cobalt(II) and (I) complexes of Diphosphine-Ketone Ligands:
Catalytic Activity in Hydrosilylation Reactions
Dide G. A. Verhoeven,[a] Joost Kwakernaak,[a] Maxime A. C. van Wiggen,[a] Martin Lutz,[b] Marc-Etienne
Moret*[a]
Abstract: The hydrosilylation of unsaturated compounds
homogeneously catalyzed by cobalt complexes has gained
considerable attention in the last years, aiming at substituting precious
metal-based catalysts. In this study, the catalytic activity of well-
characterized Co(II) and Co(I) complexes of the pToldpbp ligand is
demonstrated in the hydrosilylation of 1-octene with phenylsilane. The
Co(I) complex is the better precatalyst for the mentioned reaction
under mild conditions, at 1 mol% catalyst, 1 h, room temperature, and
without solvent, yielding 84% of octylphenylsilane. Investigation of the
substrate scope shows lower performance of the catalyst in styrene
hydrosilylation, but excellent results with allylbenzene (84%) and
acetophenone (>99%). This catalytic study contributes to the field of
Co-catalyzed hydrosilylation reactions, and shows a first example of
catalysis employing the dpbp ligand in combination with a base metal.
The scope for the unsaturated substrate is versatile, including
alkene,[1,11,20-28] alkyne,[29-33] carbonyl[34,35] and carboxylic
derivative[36] groups. Additionally, a number of different silanes
can be used: much research is performed with phenylsilane, and
other silanes such as diphenylsilane,[24] alkyl silanes,[20] and
alkoxy silanes[24,37] can be employed as well. Prominent examples
for hydrosilylation of alkenes with phenylsilane are the b-
diketiminate cobalt complexes reported by Holland,[22] the
phosphine-iminopyridine cobalt complexes used by Huang[11] and
the bis(carbene)cobalt-dinitrogen complex by Fout (Figure 1).[24]
Ge and coworkers developed efficient Co-based systems for the
hydrosilylation of dienes, employing an in situ generated catalyst
from commercially available Co(acac)2 in combination with
common
diphosphine
(e.g.
xanthphos,
binap,
1,2-
(diphenylphospine)ethane)
or trinitrogen (e.g. substituted
pyridinediimine) ligands.[32,38] The first step in the catalytic cycle of
hydrosilylation reactions is commonly the oxidative addition of the
silane substrate, forming metal-hydrides.[16] To assist this two-
electron step on cobalt – a first row transition metal with the
tendency to undergo one-electron steps – the incorporation of
cooperative ligands is of interest.[34,39,40] An instructive example
was recently reported by the group of Peters, wherein
diphosphine-borane cobalt-dinitrogen complexes cooperatively
activate the silane substrate in the catalytic hydrosilylation of
carbonyl compounds (Figure 1).[34]
Introduction
The hydrosilylation of unsaturated compounds catalyzed by
cobalt complexes has received significant attention in recent
years.[1-4] This reaction is highly important in the silicon industry
as it is used in the synthesis of silicon polymers, oils and resins,
as well as in the production of organosilicon reagents for fine
chemicals.[5] Generally, extremely active and selective Pt-based
homogeneous catalysts, such as Speier’s or Karstedt’s
catalysts,[6-8] are used but the ongoing quest for more sustainable
processes leads to interest in employing base metals iron,[9-11]
cobalt and nickel[12-15]. While hydrosilylation catalyzed by the low-
valent Co(0) complex Co2(CO)8 was reported as early as 1965 by
Chalk and Harrod,[16-19] the number of cobalt catalysts increased
explosively in recent years.
We previously described the coordination of the
diphosphine ketone ligand Phdpbp (1Ph, Figure 2) to base metals
Fe, Co, Ni and Cu.[39,41] Especially, exploration of the cobalt
complexes in hydrosilylation reactions is of interest given the
recent success in the field. Co-complexes (Phdpbp)CoCl2 (2Ph
)
and (Phdpbp)CoCl (3Ph) illustrate the particular hemilabile behavior
of the ketone moiety upon changing the oxidation state from Co(II)
to Co(I), resulting in an η2(C,O)-coordination in the latter complex
(Figure 2).[39,42] The adaptivity of the ligand in combination with the
accessibility of low oxidation state such as Co(I) prompted us to
evaluate the activity of these complexes for hydrosilylation.
Herein, the catalytic activity of these Co(II) and Co(I) complexes
is described. The diphosphine ketone ligand pToldpbp is employed,
equipped with bis(p-tolylphosphino) functionalities to improve
solubility of the complexes during reactions with respect to the
previously reported diphenylphosphino analogues.
Figure 1. Examples of Co-catalysts for hydrosilylation reactions. P in most right
structure: PiPr2.
[a]
[b]
Dr. Dide G. A. Verhoeven, Joost Kwakernaak, Maxime A. C. van
Wiggen, Dr. Marc-Etienne Moret
Organic Chemistry & Catalysis, Debye Institute for Nanomaterials
Science, Faculty of Science, Utrecht University
Universiteitsweg 99, 3584 CG, Utrecht, The Netherlands
E-mail: M.Moret@uu.nl
https://www.uu.nl/en/research/organic-chemistry-catalysis
Dr. Martin Lutz
Crystal and Structural Chemistry, Bijvoet Center for Biomolecular
Research, Faculty of Science, Utrecht University
Padualaan 8, 3584 CH, Utrecht, The Netherlands
Figure 2. Structures of Rdpbp (left), (Rdpbp)CoCl2 (middle) and (Rdpbp)CoCl
(right). 1Ph, 2Ph, 3Ph: R = Ph; 1pTol, 2pTol, 3pTol: R = p-tolyl.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
Results and Discussion
geometry around Co with pToldpbp bound in a k4(P,P,C,O)-fashion,
including an η2(C,O)-coordination of the ketone, and coordination
of both phosphine groups (Figure 3, bottom). Next to this, a
chloride is bound to cobalt. The C–O bond is elongated due to π-
backdonation in the π* antibonding orbital of the ketone moiety
from 1.2262(19) Å in 2pTol to 1.307(6) Å in 3pTol. This is similar to
the elongation as found in the Fe(I) and Ni(I) complexes (C–O
bond distance for Fe(II): 1.218(3), Fe(I): 1.3296(14), Ni(II):
1.2288(16) and Ni(I): 1.310(2) Å).[39]
Synthesis of cobalt complexes
The synthesis of Co(I) complex (Phdpbp)CoCl (3Ph) was described
previously.[39] An initial experiment with 3Ph in the hydrosilylation
reaction of 1-octene with phenylsilane showed promising results:
applying a high catalyst loading of 10 mol% in C6D6 in a Young-
type NMR tube resulted in full conversion of the substrates and
formation of octylphenylsilane as the major product in 30 min.
However, the low solubility of the precatalyst resulted in a turbid
mixture during catalysis. To improve solubility, the phenyl groups
of Phdpbp were replaced by p-tolyl groups, with the additional
benefit of providing a convenient handle for NMR analysis of the
complex. The ligand pToldpbp (1pTol) was synthesized in two steps
similarly to its phenyl substituted analogue, via first a Pd-
catalyzed cross coupling reaction to yield the o-bromo-substituted
phenyldi-p-tolylphosphine precursor which was subsequently
used in
a
double nucleophilic substitution reaction on
dimethylcarbamylchloride, resulting in the diphosphine-ketone
ligand (Scheme 1).[39, 41]
Scheme 1. Synthesis of 1pTol
.
A Co(II) complex was formed by addition of pToldpbp to CoCl2 in
CH2Cl2 resulting in (pToldpbp)CoCl2 (2pTol) after workup. The H
1
Figure 3. X-ray crystal structures of 2pTol (top) and 3pTol (bottom)[39]. Hydrogen
atoms are omitted for clarity and the ellipsoids are shown at 50% probability
level. In 2pTol: the co-crystallized THF molecule was omitted for clarity.
NMR spectrum shows the formation of a species with signals from
18 to –2 ppm, and no peak was observed in the corresponding
31P NMR spectrum, indicating the formation of a paramagnetic
species. A distorted tetrahedral cobalt center is observed in the
crystal structure of 2pTol with the two phosphine tethers of the
ligand coordinated to the metal center, next to two chloride ligands
(Figure 3, top). The P–Co–P angle is 115.734(16)o, akin to 2Ph
(Table 1).[39] The ketone moiety of the ligand backbone has a C=O
bond distance of 1.2262(19) Å and Co–C and Co–O distances of
3.3599(15) Å and 3.0474(13) Å, respectively, and is not
coordinated to cobalt. Reduction of the Co(II) complex with
sodium naphthalide in THF results in the Co(I) complex
Table 1. Crystal structure comparison of the cobalt complexes. Selected
distances (Å) and angles (°).[39]
Co–O1
Co–C7
C7–O1
P1–Co–P2 Dihedral IR
angle*
C=O
cm-1
2Ph
2.9255(13) 3.2896(18) 1.227(2)
116.676(18) 36.16(9) 1647
2pTol 3.0474(13) 3.3599(15) 1.2262(19) 115.734(16) 34.03(8) 1663
(
pToldpbp)CoCl (3pTol) after precipitation from the mixture with
hexanes, followed by extraction of the product with THF and
evaporation of the solvents. 1H NMR analysis indicates once more
the formation of a paramagnetic species with signals from 33 to –
17 ppm. Distinctive signals for the p-tolyl methyl groups are
present at 9.24 and 17.67 ppm, identified by integration and
comparison to 3Ph, and the magnetic moment of the complex as
measured by Evans method (µeff = 2.8) indicates a spin state of S
= 1. Whereas the phenyl-substituted analogue 3Ph was reluctant
to crystallization, slow vapor diffusion of toluene/hexanes with
3pTol resulted in single crystals suitable for X-ray diffraction. The
crystal structure of 3pTol was reported earlier[39] and is discussed
here for comparison with that of 2pTol. It reveals a tetrahedral
3pTol 1.947(3)
2.071(5)
1.307(6)
109.62(6)
64.2(2)
~
1300
* The dihedral angle is calculated between the two phenyl rings connected to
the carbonyl group.
Hydrosilylation reactions
The reactivity of Co(I) complex 3pTol with phenylsilane was
explored. Addition of 3 eq of PhSiH3 to 3pTol in C6D6 lead to an
exothermic reaction, resulting in a mixture of species. The 1H
NMR spectrum shows a diamagnetic spectrum containing three
broad signals around –12 ppm, of which one exhibits apparent
triplet multiplicity, consistent with the presence of one or more
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
diamagnetic Co(I)-hydride species (supporting info, Figure S14,
S15).[43,44] Attempts to perform further analyses were
unsuccessful due to the high reactivity of the obtained Co–H
species, therefore the structure has not been elucidated.
Nevertheless, the high reactivity prompted for further investigation
on catalytic hydrosilylation by 3pTol
.
Pre-catalyst 3pTol is active in the hydrosilylation reaction of
1-octene with phenylsilane, resulting in 98% conversion of both
substrates with 84% octylphenylsilane as product (Table 2, Entry
1).[45] Initial optimization lead to the following standard conditions:
1 mol% 3pTol, a reaction time of 1 h at room temperature under
neat conditions. The catalyst exhibits full selectivity toward the
anti-Markovnikov, i.e. linear, product, as shown by APT 13C NMR
after its isolation. Only a single signal containing the opposite
phasing was observed in the aliphatic region of the 13C NMR
spectrum, corresponding to the single CH3 group in the resulting
product (Figure S8). Furthermore, analyses by 1H NMR and GC-
MS confirmed the presence of a single product isomer.
Scheme 2. Observed side-products and possible side-reaction pathways.
Analysis was performed by GC-MS and NMR for product
characterization and GC for quantitative analysis. Besides
remaining starting materials and formed product, three main side-
products were observed. Ph2SiH2 was formed with a yield of 5%,
which is explained by a scrambling reaction of PhSiH3 (Scheme
1). Besides GC and GC-MS analyses, Ph2SiH2 was also
characterized using NMR techniques. The SiH2 moiety gives rise
to a distinctive peak in 1H NMR at 5.04 ppm (JHSi = 198 Hz) and
in 29Si NMR a peak was observed at –33.75 ppm, which were
confirmed to belong to Ph2SiH2 by comparison to an authentic
sample. Completing the stoichiometry in this reaction would result
in the formation of one equivalent of SiH4, which however could
not be detected directly, presumably due to its high volatility
(Scheme 2, top). To account for the incorporated silane in this
byproduct, it is assumed that the amount is equal to the amount
of Ph2SiH2. This silane scrambling is a known side-reaction in
hydrosilylation reactions[46-48] and it hampers product formation as
it converts PhSiH3 to less reactive side-products. A slight excess
of 1.1 equivalents of PhSiH3 was therefore used to maximize the
yield of hydrosilylation reactions.
With the benchmark hydrosilylation of 1-octene with phenylsilane
established, a number of control experiments were performed to
validate the need of the pre-catalyst. Experiments without the
addition of a catalyst showed no conversion of the substrates and
no product formation (Table 2, Entry 2). Using 1 mol% of CoCl2
as catalyst without addition of a co-ligand lead to low conversion
of the substrates, i.e. 4% conversion of 1-octene and 16%
conversion of phenylsilane, but no formation of the product was
observed. Conversion of the substrates is attributed to ongoing
heterogeneous processes (Table 2, Entry 3), as a result of the
poor solubility of CoCl2 in organic solvents. Addition of PPh3 in
combination with CoCl2 again lead to low conversion of 1-octene
(3%), but high conversion of phenylsilane was obtained (95%)
(Table 2, Entry 4). The products of 1-octene conversion were
analyzed as the isomerization (3%) and hydrogenation (traces)
products. The product of phenylsilane conversion was not
identified, but it is likely that a minor fraction was used in the
formation of hydride species, as these are needed for the 1-
octene based side reactions (vide supra).
Furthermore, formation of octane (2%) was observed, which
can tentatively be explained by a reduction of the double bond,
and formation of 2- and 3-octene (both 3%). These latter products
likely originate from isomerization of the carbon-carbon double
bond of 1-octene, which may proceed via a separate catalytic
cycle, catalyzed by 3pTol. A test reaction for the conditions of
isomerization was performed. 1-Octene was mixed with 1 mol%
of 3pTol and stirred for 4 h at room temperature, which did not lead
to any side-product formation. To proceed, the reaction likely
needs a hydride source, i.e. PhSiH3, to form the active Co–H
catalyst (Scheme 2, bottom). The Co–H catalyzed reaction is
assumed to follow a mechanism as proposed by Cramer in
1966,[49] in which the double bond of 1-octene initially coordinates
in an η2-fashion to Co, followed by addition of the hydride to Cα.
Next, the hydride positioned at Cγ is added to Co, and the
substrate dissociates from the metal resulting in 2-octene and the
Co–H catalyst (Scheme 2).
Comparison of Co(II) complex 2pTol to Co(I) complex 3pTol
showed lower conversion of the substrates when using 2pTol: 42%
for 1-octene and 60% for phenylsilane with a lower selectivity
toward octylphenylsilane, resulting in a yield of 32% (Table 2,
Entry 5). Low amounts of the 1-octene based side products were
obtained and no silane scrambling was observed. The higher yield
when using 3pTol could be explained by the reduced Co center,
the η2(CO)-coordination of the ligand, or a combination of these
two. It is also plausible that the same active species is generated,
albeit less efficiently, via reduction of the Co(II) complex by the
silane. Comparison of 3pTol to its phenyl substituted analogue 3Ph
showed a somewhat lower product yield of 75% caused by lower
solubility of 3Ph as performing the reaction in THF resulted in 86%
yield (Table 2, Entry 6-7, respectively).
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
Table 2. Hydrosilylation of 1-octene with PhSiH3 with the Co-complexes.
Table 3. Substrate scope of the hydrosilylation reactions with 3pTol
.
PhSiH3 (1.1 eq)
Cat (1 mol%)
H
Neat
RT, 1 h
SiPhH2
[A]
Cat
Conv
Conv
Prod
Isom
Octane Ph2SiH2
Silane
Alkene Yield
1-oct
[A]
Time Substrate
Conv
Silane
Conv
Substrate
Prod
Yield
Ph2SiH2
1
2
3
4
3pTol
98
-
98
-
84
-
6
-
2
5
-
No cat
CoCl2
-
1
2
3
4
4 h
4 h
Allylbenzene
Styrene
97
31
39
96
84
4
5
7
4
16
95
4
-
-
-
-
17
7 (3:4)[B]
7 (3:4)[B]
>99[C]
CoCl2 +
PPh3
3
-
3
Traces
-
24 h Styrene
4 h
20
5
6
7
2pTol
3Ph
60
99
99
42
98
99
32
75
86
4
5
6
1
2
1
-
Acetophenone 92
>99
3
3
All amounts are all given in percentage. Conversion is determined by GC
(see supporting Info). [A] Amount of SiH4 is considered equimolar to Ph2SiH2.
[B] Markovnikov:anti-Markovnikov product, ratio determined by NMR and
GC. [C] Analysis of 1-phenylethanol.
3Ph
THF
+
All amounts are all given in percentage. Conversion is determined by GC
(supporting Info). [A] Amount of SiH4 is considered equimolar to Ph2SiH2.
Substrate scope
The activity of 3pTol with a ketone was explored in the
hydrosilylation of acetophenone, a substrate that is generally
known to be a more reactive due to the polarized nature of the
carbon-oxygen double bond. The reaction of acetophenone with
phenylsilane at RT for 4 h in THF catalyzed by 1 mol% 3pTol leads
to an excellent yield of the hydrosilylated product phenyl(1-
phenylethoxy)silane (>99%) (Table 3, Entry 4). The product was
quantified by GC and GC-MS analyses after acidification, forming
the alcohol species 1-phenylethanol.
Extending the alkene substrate scope, allylbenzene was explored
in a hydrosilylation reaction with PhSiH3 catalyzed by 3pTol. The
reaction conditions were slightly changed to enhance product
formation, to an extended reaction time of 4 h, and THF was used
as the solvent instead of neat conditions, resulting in a 96%
conversion of allylbenzene, 97% conversion of phenylsilane and
a
product yield of 84%. Full selectivity toward the anti-
Markovnikov product was obtained over the possible Markovnikov
product formation (Table 3, Entry 1; Figure S9), and
diphenylsilane was again obtained as a byproduct (4%). Styrene,
however, resulted in a low yield of only 7% (Table 3, Entry 2).
Product determination by GC-MS, 1H NMR, 13C NMR and 2D 1H-
1H COSY NMR (Figure S10-13), and subsequent quantitative
analysis by GC showed the formation of both the Markovnikov
and anti-Markovnikov product with respective yields of 3% and
4%. The formation of both products is more commonly observed
when using styrene as the substrate.[10,50] The catalytic selectivity
of styrene hydrosilylation with 3pTol was significantly lower
compared to allylbenzene, with styrene conversion of 17% and
silane conversion of 31% after 4 h. Besides silane scrambling, a
number of unidentified byproducts were observed in small
quantities. Increasing the reaction time to 24 h did not improve the
reaction significantly, as only the conversion improved poorly and
larger amount of byproduct diphenylsilane was obtained (Table 3,
Entry 3).[51] Low conversion of styrene is presumably due to
conjugation with the aromatic π-system, resulting in a less
reactive C=C bond. The significantly higher product yield when
using allylbenzene shows that the extra CH2 group in the
substrate makes for a great difference in reactivity, explained by
the lesser extent of the conjugated π-system of allylbenzene.
Chemoselectivity
Combining both an olefinic and ketone functionality in one
substrate gives insight in possible chemoselectivity of the catalyst,
for which the hydrosilylation of 5-hexen-2-one with PhSiH3 was
performed. After the reaction, an acidic workup was performed
before analysis by GC. Addition of 2.1 equivalents of silane results
in full conversion of both the olefin and ketone functionality after
24 h (Figure 4, product C), showing the catalysts capability of
double addition on one substrate. Addition of a limiting amount of
silane, i.e. 1 equivalent, is more indicative of the catalysts
behavior regarding chemoselectivity. After reaction for 4 h the
substrates did not yet reach full conversion and GC analysis
showed, next to 5-hexen-2-one and PhSiH3, the three products of
hydrosilylation (Figure 4) on a) the ketone, b) the olefin, c) both
the ketone and olefin, in a 1:1:1 ratio, forming products A, B and
C, respectively. Hence, the catalyst does not discriminate
between a terminal olefin and a ketone under the chosen
conditions, and the second addition seems to have a higher rate
as this is obtained in similar ratio.
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
room temperature and without the need of a solvent, resulting in
84% of octylphenylsilane. Extending the scope of the alkene
substrate was performed to a small extent, showing that the
hydrosilylation of styrene leads to a low yield, whereas full
conversion was reached using allylbenzene. The hydrosilylation
of a polar double bond was shown to be the most facile reaction,
with full substrate conversion and high selectivity for the product
with >99% yield. The observed high catalytic reactivity combined
with excellent product selectivity adds to the quickly developing
field of Co-catalyzed hydrosilylation reactions.
Figure 4. Products of the chemoselectivity experiment. Acidic workup was
applied before analysis.
Experimental Section
General considerations
Discussion of the mechanism
Unless stated otherwise, all reagents were purchased from commercial
sources and used as received. Reactions are performed in a N2-filled
glovebox at room temperature with dry, degassed solvents. Deuterated
benzene (C6D6) and deuterated dichloromethane (CD2Cl2) were degassed
using the freeze-thaw-pump method (4x) and subsequently stored over
molecular sieves. Dry diethylether, toluene and hexane were taken from
an MB SPS-800 solvent purification set-up and degassed by bubbling N2
through the solvent for 30 min and were further dried overnight over
molecular sieves. CH2Cl2 was distilled over CaH2 and tetrahydrofuran
(THF) was distilled over sodium/benzophenone before use, both were
degassed by bubbling N2 through it for 30 min and stored over molecular
sieves. THF, ether or petroleum ether (40-60 °C) used for column
chromatography and calibration curve preparation were wet solvents of
technical purity. Diphenylphosphine, Pd(PPh3)4, CoCl2, metallic sodium
and naphthalene were placed in the glovebox without additional
purification. o-Bromoiodobenzene, Et3N and all substrates were degassed
by bubbling N2 for 30 min and dried over molecular sieves before use in
the glovebox.
Cobalt-catalyzed hydrosilylation is a growing field with a number
of highly efficient catalysts reported in recent years. The Co-
catalyzed hydrosilylation reactions reported by Holland,[22]
Huang[11] and Fout[24] (vide supra, Figure 1) are proposed to follow
the Chalk-Harrod mechanism, in agreement with an in-depth
investigation on mechanistic aspects, using
a Co-bound
phosphine substituted 2-iminopyridine ligand, as published by
Rauchfuss, van Gastel and co-workers.[52] A rare example in
which the specific role of a cooperative ligand is determined was
shown by Peters and co-workers with a diphosphine-borane
cobalt-dinitrogen complex (Figure 1, right), where the silane
substrate is activated between the metal and the ligand backbone
by addition of –SiHPh2 to Co and insertion of the hydride into the
Co–B bond.[34]
Investigations of the reaction of the Co-precatalyst with
phenylsilane suggest activation by formation of Co–H species,
consistent with the Chalk-Harrod mechanism.[16] The role – if any
– of the ketone moiety in cobalt catalysts presented herein cannot
be established with certainty from the data at hand. Possible
reactivity involving the ketone moiety include: a) hemilabile
behavior upon changing the oxidation state of the metal center,[39]
b) reversible addition of hydride to the moiety under catalytic
conditions in analogy to the borane in Peters’ catalyst, or c) its
hydrosilylation to generate a catalytically active species. Recent
experimental and computational investigations on similar Phdpbp
complexes with Co or Rh by Young and co-workers showed the
ability of the ketone to reversibly accept a hydride equivalent to
become an sp3 hydroxyalkyl ligand.[42,53] Related processes are
plausible under hydrosilylation conditions, but the exact nature of
the involved species is unclear, and further investigations are
ongoing to address details of the mechanism.
1H, 13C, 29Si and 31P NMR spectra (respectively 400, 100, 80 and 161 MHz)
were recorded on an Agilent MRF400 or a Varian AS400 spectrometer at
25 °C. 1H and 13C NMR chemical shifts are reported in ppm relative to TMS
using the residual solvent resonance as internal standard. 31P NMR
chemical shifts are externally referenced to 85% aqueous H3PO4 and 29Si
NMR chemical shifts are externally referenced to TMS. Infrared spectra
were recorded using a Perkin Elmer Spectrum One FT-IR spectrometer
equipped with a N2 flow over the crystal. UV-Vis spectra were measured
using a Lambda 35 UV-Vis spectrometer. The UV-Vis samples were
prepared under a N2 atmosphere and sealed with a Teflon cap after which
the spectra were recorded directly. ESI-MS spectra were recorded on a
Walters LCT Premier XE KE317 Micromass Technologies spectrometer.
Compounds of which elemental analysis is reported were either
recrystallized or precipitated and dried under high vacuum overnight
before submission and analysis was performed by the Mikroanalytisches
Laboratorium Kolbe, Mülheim an der Ruhr, Germany. Gas
chromatography mass spectrometry was carried out on a PerkinElmer
Clarus 680 Gas Chromatograph with a PE Elite 5MS 15 m x 0.25 mm x
0.25 μm column, connected to a PerkinElmer Clarus SQ 8 T Mass
Spectrometer. Gas chromatography was carried out on a PerkinElmer
Clarus 500 Gas chromatograph with an Alltech EconocapTM ec TM 30.0
m x 0.32 mm ID x 0.25 μm, 5% phenyl and 95% methylpolysiloxane
column. In general method 1 was used: 50 °C for 3 min, 40 °C/min increase
for 5.75 min to reach 280 °C, 280 °C for 3 min (total 11.75 min). For
analysis of reactions using 5-hexen-2-one, method 2 was used: 50 °C for
3 min, 5 °C/min increase for 46 min to reach 280 °C, 280 °C for 3 min (total
52 min). Details on X-ray crystal structure determination and selected
Conclusions
The investigated cobalt complexes equipped with a diphosphine-
ketone ligand (Rdpbp) are active catalysts in hydrosilylation
reactions with phenylsilane, showing a first example of catalysis
with this family of complexes. Both, 2pTol and 3pTol are active in the
hydrosilylation of 1-octene with phenylsilane, but 3pTol is superior.
The Co(I) catalyst proceeds under very mild conditions of 1 h,
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
spectra are given in the supporting information. CCDC 1871266 contains
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
vacuum resulting in a green powder (88.2 mg, 0.12 mmol, 72%). 1H NMR
(CD2Cl2): δH 18.82, 13.63, 11.77, 9.99, 7.88, 0.54, 0.10, –0.55. ATR-IR ν:
1634, 1499, 1286, 1096, 805. Solution cell IR (THF, cm-1): 1662, 1598
(weak) cm-1. ESI-MS m/z: [M-Cl]+ obs. 700.1297, calcd. 700.1262. UV-Vis
(CH2Cl2) λmax, nm: 603, 650, 734, 760. Elemental Analysis: calcd: C 66.02,
H 5.42, found: C 66.86, H 4.93.
Synthesis
o-Bromophenyl-di-p-tolylphosphine:[39,54]
Bromo-iodobenzene (6 mL, 46.7 mmol) was
dissolved in toluene (10 mL) in a Schlenk tube under
N2 atmosphere. In consecutive order, a mixture of di-
p-tolylphosphine (10.1 g, 46.7 mmol) in toluene (15
mL), dry Et3N (10 mL, 70.1 mmol) and Pd(PPh3)4
(296 mg, 0.26 mmol) in toluene (20 mL) were added
resulting in a yellow solution. The mixture was heated to 80 °C for 12 h,
after which the deep brown mixture was washed with brine (2 x 65 mL).
The aqueous phase was extracted with Et2O (2 x 75 mL, 4 x 35 mL). The
combined organic layers were dried with MgSO4, filtered and the solvent
was evaporated. Cold MeOH (3 x 15 mL) was used to wash the crude
o-
(
pToldpbp)CoCl (3pTol):[39] Naphthalene (79.9 mg, 0.62 mmol) was
dissolved in THF (4 mL). A lump of Na0 was added and the mixture was
stirred for 3 h, after which a dark green solution was obtained. CoCl2 (65.0
mg, 0.50 mmol) and pToldpbp (300.6 mg, 0.49 mmol) were suspended in
THF (25 mL) and cooled to –78 °C. The naphthalide solution was filtered
and added to the cooled Co-mixture. This final mixture was stirred further
for 1 h at –78 °C and for 18 h at room temperature, after which it was
concentrated in vacuum to ~1.5 mL. Hexane was added (5 mL) and the
precipitated solids were collected by filtration. The product was extracted
with THF and dried in vacuum, resulting in (pToldpbp)CoCl as a brown
powder with a yield of 86% (300.3 mg, 0.43 mmol). 1H NMR (C6D6): δH
31.59, 20.00, 17.64 (s, 3H, –Me), 14.34, 13.54, 9.21 (s, 3H, –Me), –0.06,
–0.98 (broad), –6.07, –7.97, –16.38. ATR-IR ν: 2919, 2863, 1599, 1498,
1434, 1397, 1258, 1241, 1189, 1096, 1019, 917, 803, 777, 693, 626, 510
cm-1. UV-Vis: λmax: 525 nm. Elemental analysis: calcd: C 70.24, H 5.18,
found: C 70.00, H 5.37.
product. The product was dried in vacuum and isolated as an off-white
powder (14.7 g, 39.7 mmol, 85%). 1H NMR (C6D6): δH 7.34 (ddd, JHH
3
=
7.9, 3JHP = 3.6 Hz, 4JHH = 1.2 Hz, 1H, H4), 7.29 (t, 3JHH = 7.8 Hz, 3JHP = 7.8
Hz, 4JHH = 1.7 Hz, 4H, H5), 6.93 (dt, 3JHH = 7.7 Hz, 3JHP 2.1 Hz, 4JHH = 2.1
3
Hz, 1H, H1), 6.87 (d, JHH = 7.7 Hz, 4H, H6), 6.76 (td, 3JHH = 7.5 Hz, 4JHH
= 1.2 Hz, 1H, H3), 6.64 (td, 3JHH = 7.7 Hz, 4JHH = 1.7 Hz, 1H, H2), 1.97 (s,
6H, 7). 13C NMR (C6D6): δC 138.61, 134.43, 134.25, 134.05, 132.94 (d, J
Catalysis
= 2.2 Hz), 129.69, 129.48, 129.41, 127.05, 20.78. 31P NMR (C6D6): δP
–
All catalytic reactions were performed in duplo and given values are the
average of the two runs, unless stated otherwise. Conversion and yield
were determined by GC analysis, for which calibration curved were
prepared for all substrates and products. The yields of product arising from
1-octene isomerization were determined using the calibration curve for 1-
octene. Next to the characterized and quantified products, a number of
small signals (GC area >1%) were generally obtained in GC analysis.
These products could not be identified, and are therefore not described in
the analysis. Further analysis and characterizations are described in the
Supporting information.
6.31.
pToldpbp (1pTol):[39] o-Bromophenyl-di-p-tolyl-
phosphine (4.93 g, 13.4 mmol) was suspended
in Et2O (30 mL). The light-yellow suspension
was cooled to -50 °C using an acetone dry ice
bath. n-BuLi (1.6 M in hexane, 16 mmol, 10
mL) was added drop wise while stirring,
resulting in a rapid color change form yellow to brown. Within 30 min, the
reaction mixture was allowed to warm up to room temperature and was
directly cooled down again to –50 °C, after which a solution of N,N-
dimethylcarbamoylchloride (0.61 mL, 6.5 mmol) in Et2O (15 mL) was
added drop wise over 20 minutes. During addition, the temperature of the
bath was kept between –35 °C and –45 °C, after which the suspension
was stirred overnight allowing the mixture to warm up to room temperature.
The reaction was cooled down again and treated at 0 °C with a degassed
2.5 M NH4Cl solution in water (38 mL, 4.5 g, 85 mmol NH4Cl) turning the
suspension orange-yellow. The product was extracted with Et2O and dried
with MgSO4 and solvents were evaporated. The obtained yellow
compound was washed with MeOH, resulting in the product as a yellow
powder after solvent removal (2.41 g, 4.0 mmol, 61%). 1H NMR (C6D6): δH
7.37 (t, J = 7.6 Hz, 8H, H5), 7.37 (m, 2H, H4), 7.31 (dd, J = 7.7, 2.4 Hz,
2H, H1), 6.95 (td, J = 7.5, 1.1 Hz, 2H, H2/3), 6.91 (d, J = 7.7 Hz, 8H, H6),
6.83 (td, J = 7.5, 1 Hz, 2H, H3/2), 2.03 (s, 12H, H7). 13C NMR (CDCl3): δC
197.35 (t, J = 3 Hz, C=O), 144.64 (dd, J = 24.4, 1 Hz, CqAr), 140.68 (J =
26.1, 2.2 Hz, CqAr), 138.21 (CqAr), 135.56 (dt, J = 3, 11 Hz, CqAr), 135.00
(CHAr), 134.45 (m, CHAr), 131.03 (t, J = 3.3 Hz, CHAr), 130.66 (CHAr),
129.48 (m, CHAr), 128.00 (CHAr), 21.21 (–CH3). 31P NMR (C6D6): δP –9.77.
ATR-IR ν: 3012, 2956, 2920, 2863, 1910, 1738, 1664, 1598, 1495, 1446,
1395, 1296, 1271, 1243, 1185, 1090, 1018, 928, 805, 747, 637, 507 cm-1.
ESI-MS (MeCN AgNO3/H2O) m/z: [2M+Ag]+: 1319.3529, calc: 1319.3534.
Elemental analysis: calcd: C 81.17, H 5.98, found: C 80.84, H 6.05.
Hydrosilylation method 1 (1-Octene): In a glovebox, catalyst (0.01 mmol)
was added to a 6 mL vial. 1-Octene (0.160 mL, 1.0 mmol) was added.
Addition of PhSiH3 (0.135 mL, 1.1 mmol) while stirring caused bubbling of
the mixture, and resulted in a clear brown solution after 1 minute. The
solution was stirred for 1 h at room temperature and then taken out of the
glovebox, where it was opened in air. The mixture was filtered over a plug
of silica (~1 cm) using THF as eluent (total amount of mixture: 25 mL) to
remove cobalt. From this a GC-MS sample was taken. For GC analysis, 1
mL of the prepared solution was added to 2.5 mL of a mesitylene (Mes)
solution (internal standard, 0.017 M Mes solution: 102.4 mg Mes in 50 mL
THF) and THF was added to a total volume of 10 mL.
Hydrosilylation method 2 (Allylbenene and styrene): In a glovebox, 3pTol
(7.0 mg, 0.01 mmol) was dissolved in THF (1 mL) resulting in a clear brown
solution. A mixture of two substrates was added with a syringe while
stirring. The solution was stirred for 4 h at room temperature and then
taken out of the glovebox and opened in air. The mixture was filtered over
a silica plug (~1 cm) using THF as eluent (total amount of mixture: 25 mL)
to remove cobalt. From this a GC-MS sample was taken. For GC analysis,
1 mL of the prepared solution was added to 2.5 mL of a mesitylene (Mes)
solution (internal standard, 0.017 M Mes solution: 102.4 mg Mes in 50 mL
THF) and THF was added to a total volume of 10 mL.
(
pToldpbp)CoCl2 (2pTol): pToldpbp (101.0 mg, 0.17 mmol) and CoCl2 (65.0
mg, 0.16 mmol) were suspended in CH2Cl2 (5 mL) and stirred for 17 h,
resulting in green solution. The product was precipitated from
CH2Cl2/hexane, collected via filtration, extracted using CH2Cl2 and dried in
Hydrosilylation method 3 (Acetophenone and 5-hexen-2-one): In a
glovebox, 3pTol (7.0 mg, 0.01 mmol) was added to a small vial and
dissolved in THF (1 mL) resulting in a clear brown solution. A mixture of
a
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10.1002/ejic.201801221
European Journal of Inorganic Chemistry
FULL PAPER
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Acknowledgements
DV and MEM would like to thank the Sectorplan Natuur- en
Scheikunde (Tenure-track grant at Utrecht University) for financial
support. The X-ray diffractometer was financed by The
Netherlands Organization for Scientific Research (NWO). This
work was sponsored by NWO Exacte en Natuurwetenschappen
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Keywords: Silane chemistry • Cobalt • Hydrosilylation •
Homogeneous catalysis • Cooperative catalysis
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The catalytic activity of Co(II) and
Co(I) complexes of the diphosphine
ketone ligand pToldpbp is
demonstrated in the hydrosilylation
of 1-octene with phenylsilane at mild
conditions, showing the first catalytic
application of dpbp ligands in
Cobalt-catalyzed hydrosilylation*
Dide G. A. Verhoeven, Joost
Kwakernaak, Maxime A. C. van
Wiggen, Martin Lutz, Marc-Etienne
Moret*
Page No. – Page No.
combination with base metals.
Cobalt(II) and (I) complexes of
Diphosphine-Ketone Ligands:
Catalytic Activity in Hydrosilylation
Reactions
*one or two words that highlight the emphasis of the paper or the field of the study
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Author(s), Corresponding Author(s)*
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Title
Text for Table of Contents
*one or two words that highlight the emphasis of the paper or the field of the study
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