Highly efficient large bite angle diphosphine substituted molybdenum catalyst for hydrosilylation

Article abstract of DOI:10.1021/cs4004276

Treatment of the complex Mo(NO)Cl₃(NCMe)₂ with the large bite angle diphosphine, 2,2′-bis(diphenylphosphino)diphenylether (DPEphos) afforded the dinuclear species [Mo(NO)(PaP)Cl ₂]₂[μCl]₂ (PaP = DPEph

Full text of DOI:10.1021/cs4004276

Research Article
pubs.acs.org/acscatalysis
Highly Efficient Large Bite Angle Diphosphine Substituted
Molybdenum Catalyst for Hydrosilylation
Subrata Chakraborty, Olivier Blacque, Thomas Fox, and Heinz Berke*
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Treatment of the complex Mo(NO)-
Cl3(NCMe)2 with the large bite angle diphosphine, 2,2′-
bis(diphenylphosphino)diphenylether (DPEphos) afforded
the dinuclear species [Mo(NO)(P∩P)Cl2]2[μCl]2 (P∩P =
DPEphos = (Ph2PC6H4)2O (1). 1 could be reduced in the
presence of Zn and MeCN to the cationic complex
[Mo(NO)(P∩P)(NCMe)3]+[Zn2Cl6]2− (2). In a metathetical reaction the [Zn2Cl6]2− counteranion was replaced with
1/2
1/2
NaBArF4 (BArF4 = [B{3,5-(CF3)2C6H3}4]) to obtain the [BArF ]− salt [Mo(NO)(P∩P)(NCMe)3]+[BArF ]− (3). 3 was found to
4
4
catalyze hydrosilylations of various para substituted benzaldehydes, cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and
2-furfural at 120 °C. A screening of silanes revealed primary and secondary aromatic silanes to be most effective in the catalytic
hydrosilylation with 3. Also ketones could be hydrosilylated at room temperature using 3 and PhMeSiH2. A maximum turnover
frequency (TOF) of 3.2 × 104 h−1 at 120 °C and a TOF of 4400 h−1 was obtained at room temperature for the hydrosilylation of
4-methoxyacetophenone using PhMeSiH2 in the presence of 3. Kinetic studies revealed the reaction rate to be first order with
respect to the catalyst and silane concentrations and zero order with respect to the substrate concentrations. A Hammett study
for various para substituted acetophenones showed linear correlations with negative ρ values of −1.14 at 120 °C and −3.18 at
room temperature.
KEYWORDS: homogeneous catalysis, hydrosilylation, molybdenum, diphosphine, aldehydes, ketones
the Si−H bond, and Lewis acid9 catalyzed hydrosilylation
reactions are also well documented by several authors.
INTRODUCTION
■
The hydrosilylation of unsaturated carbonyl functionalities has
received considerable attention as a convenient reduction
method both in industry and in academia. In addition the
alkoxy silane products obtained by atom-economical hydro-
silylation are valuable intermediates for the synthesis of
organosilicon polymers. Metal mediated hydrosilylation reac-
tions generally operate according to the Chalk−Harrod
mechanism for alkenes and alkynes1 or for carbonyl
compounds2,3 on the Ojima mechanism showing the following
key steps: (a) oxidative addition of a Si−H bond to a metal
center, (b) coordination of the unsaturated substrate, (c)
migration of the hydride (or in rare cases of the silyl group) to
the unsaturated substrate, and finally (d) reductive elimination
of a C−O−Si (or C−C−Si) bond to form the hydrosilylated
product. Early transition metal systems deviate from this
scheme and catalyze hydrosilylation reactions via σ bond
metathesis.4 Toste5a et al demonstrated that hydrosilylation of
organic carbonyls could take place via initial insertion into the
Re−H bond of the Re(O)(OSiPhMe2) (PPh3)2(I)(H) complex
obtained via the reaction of Re(O)2(PPh3)2I and PhMe2SiH. In
2011, Nikonov5d et al. provided evidence that the hydride
transfer to carbonyl from Re(O)(OSiPhMe2)(PPh3)2(I)(H)
complex is not the dominant reaction pathway of the catalytic
cycle. A pathway with a silylene intermediate was also proposed
by Tilley et al.6 Asymmetric hydrosilylation with high
enantioselectivities,7 ionic hydrosilylations8 with heterolysis of
Nevertheless, to date the most common and efficient
catalysts involved in hydrosilylation reactions are based on
platinum group transition metals, which suffer from high cost
and recognized toxicities,10 since in many cases the catalytic
metal content cannot be removed from the product. To
overcome this problem, the development of iron catalysts11 and
early transition metal catalysts12 have attracted significant
attention. Pursuing the issue of “Cheap Metals for Noble
Tasks”,13 catalysis based on molybdenum is also quite
appealing. There are several reports on Mo catalyzed
hydrosilylation reactions of carbonyl compounds including in
depth mechanistic investigations.14,15 Despite the ability of
these catalysts to perform hydrosilylation reactions under mild
conditions, they lack high activity and catalyst stability.
Our group has a long-standing interest in rhenium and group
VI nitrosyl chemistry. Recently we had discovered excellent
catalytic performance of ligand tuned rhenium nitrosyl
complexes in hydrogenations16 comparable or even superior
to precious metal catalysts. Hydrosilylations,17 dehydrogenative
silylations,18 and dehydrogenative aminoborane coupling19
reactions and also of molybdenum catalyzed hydrogenation
reactions20 could be accomplished. We were particularly
Received: June 4, 2013
Revised: August 7, 2013
© XXXX American Chemical Society
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Research Article
interested in developing an efficient molybdenum nitrosyl
based catalyst for hydrosilylation reactions. We sought
inspiration particularly from the earlier work of our group
with rhenium compounds21 where large bite angle chelating
diphosphine ligands had dramatically influenced the ability of
ligand exchanges of the metal centers and directed these toward
catalysis of the hydrogenation of alkenes. For this reason, we
have initially chosen the simplest of aryl substituted
diphosphines, the large bite angle 2,2′-bis(diphenylphosphino)-
diphenylether ligand (DPEphos), and prepared molybdenum
nitrosyl derivatives and applied them in efficient hydrosilylation
reactions of various substituted aromatic aldehydes and
ketones.
MeCN at room temperature led to formation of the red
cationic [Mo(NO)(P∩P)(NCMe)3]+[Zn2Cl6]2− complex 2
in excellent yield (84%) (Scheme 1). The 311/P2{1H} NMR
spectra revealed a single resonance at δ = 53.5 ppm, which
spoke for the equivalence of the phosphorus atoms of the
DPEphos ligand. A νNO band was observed at 1593 cm−1. One
1
broad signal at δ = 2.01 ppm in the H NMR spectra was
attributed to the methyl protons of the coordinated MeCN
ligands. The signals of the CMe and CCN atom appeared in the
13C{1H} NMR spectra at δ = 0.65 and 123 ppm, respectively.
The complex was further characterized by C,H correlation,
long-range C,H correlation experiments, elemental analysis, and
finally by a single crystal X-ray diffraction study. 2 is soluble in
MeCN, THF, and sparingly soluble in toluene.
Single crystals for a X-ray diffraction study could be obtained
by layering pentane onto a concentrated toluene solution of 2
at −30 °C. The X-ray diffraction study of 2 revealed a pseudo
octahedral geometry around the metal center (see Supporting
Information). Two coordinated MeCN ligands occupy the
plane of the DPEphos ligand. The average Mo−NMeCN bond
length is 2.172(5) Å. The asymmetric unit contains one
cationic molybdenum molecule, one toluene as solvate, and half
of the dianionic [Zn2Cl6]2− species.
RESULTS AND DISCUSSION
■
Preparation of Molybdenum Nitrosyl Complexes. The
preparation of the large bite angle DPEphos ligand (DPEphos
= (Ph2PC6H4)2O, Bite angle =102.2°)22a containing dinuclear
species [Mo(NO)(P∩P)Cl2]2[μCl]2 (1) was achieved by ligand
substitution reactions starting from the Mo(NO)Cl3(NCMe)2
precursor complex at room temperature in tetrahydrofuran
(THF) (Scheme 1). 1 precipitated out from THF solution as a
We chose the bulky noncoordinating [B{3,5-
Scheme 1. Synthetic Access to Various Molybdenum
Complexes with the DPEphos Ligand
(CF3)2C6H3}4]− anion (BArF −) to replace the [Zn2Cl6]2−
4
1/2
specie since exchange of the counteranion could help to
increase the solubility of this complex.
The reaction of 2 with Na+[B{3,5-(CF3)2C6H3}4]− (Na-
BArF ) at room temperature in MeCN produced the highly
4
electrophilic cationic complex [Mo(NO)(P∩P)-
(NCMe)3]+[BArF ]− (3) with concomitant formation of
4
Na2[Zn2Cl6] in high yield (90%). Complex 3 is quite soluble
in THF, toluene, benzene, and chlorobenzene. The 31P{1H}
NMR spectra exhibited a single resonance at δ = 52 ppm owing
to the equivalence of the phosphorus atoms of the DPEphos
ligand. The 31P{1H} NMR signal of 3 did not display a
significant shift when compared to 2. This could presumably be
due to a rather loose ion pairing interaction in 2 and 3.
Nevertheless, the exchange of the counterion was confirmed
from 13C{1H} NMR spectra and X-ray diffraction studies. Red
single crystals suitable for an X-ray diffraction study of 3 were
obtained from a toluene/pentane mixture at −30 °C. The X-ray
structure analysis of 3 (Figure 1) exhibited pseudo octahedral
coordination around the metal center similar to 2. The
phosphorus atoms of the DPEphos ligand and the two
MeCN ligands were located in the “equatorial” plane.
The average Mo−N bond length of the acetonitrile ligands
was found to be 2.196(2) Å, which is a little longer than the
corresponding values of 2. Similar to 2 the “axially” coordinated
MeCN ligand trans to NO was found to be at a slightly longer
distance than the “equatorially” coordinated ones.
Catalytic Hydrosilylation Reactions of Aldehydes and
Ketones. Various attempts were made to use 3 as a catalyst in
olefin hydrogenations, but all of them were unsuccessful.
However, 3 was found to catalyze a surprisingly great variety of
hydrosilylations. In an optimization series 3 was tested using
benzaldehyde and triethylsilane in a 1:1.2 ratio and a loading of
1 mol % of 3. The reaction was carried out at 120 °C in C6D5Cl
over 3 h to obtain the corresponding silylether, triethyl-
(benzyloxy)silane in 100% yield as revealed from the 1H NMR
spectra (Table 1, entry 1). However, the achieved initial
turnover frequency (TOF) was quite low (54 h−1).
Acetophenone was found to be even less active and required
gray material in 86% yield. It is insoluble in organic solvents.
Therefore, it could be characterized only by elemental analysis
and solid state IR spectroscopy. The IR spectrum showed a
sharp signal at 1664 cm−1 attributed to the νNO vibration. An
attempt to cleave this chloride dinuclear species 1 by heating it
in the coordinating solvent MeCN failed, which resulted in an
inseparable mixture of decomposition products including the
presence of the free DPEphos ligand as indicated by 31P NMR
spectroscopy. It should be mentioned that an attempt to
prepare a similar large bite angle complex using the
diphosphine 4,6-bis(diphenylphosphino)-10,10-
dimethylphenoxasilin(Sixanthphos) ligand22a from the Mo-
(NO)Cl3(NCMe)2 precursor failed completely, even at higher
temperatures as revealed by the 31P NMR spectra.
Despite its insolubility, the Mo(ll) complex 1 could be
reduced by Zn to obtain a low-valent molybdenum nitrosyl
complex. The reduction of 1 with excess zinc (10 equiv) in
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Research Article
promote the catalytic transformations. Addition of 1 mol % of
catalyst 3 to a solution of PhCHO and Et3SiH (1:1.2) in
C6D5Cl at room temperature did not furnish any hydrosilylated
products after 20 h of reaction time. When eventually the
temperature was raised to 80 °C, 16% conversion was observed
after 3 h with an initial TOF of 2 h−1 (Table 1, entry 7). In
addition to these efforts to improve the activity, various silanes
were then screened.
The primary aromatic silane PhSiH3 was found to be highly
effective in the hydrosilylation of benzaldehyde. A loading of
0.05 mol % of the catalyst 3 led to 100% conversion of
benzaldehyde to the corresponding silylether with a maximum
TOF of 3520 h−1. However, because of the presence of more
than one HSi atoms, formation of PhCH2OSiPhH2 (17%) and
(PhCH2O)2SiPhH (83%) products were observed as revealed
1
by the GC-MS and the H NMR spectra. The secondary
silanes, Ph2SiH2 and PhMeSiH2, were also found to be quite
active in the hydrosilylations of benzaldehyde. Nevertheless, in
both cases monosilylated products were predominantly formed
(eq 1, monosilylated, a; disilylated, b).
Figure 1. Molecular structure of the cationic part of 3. Thermal
ellipsoids are drawn at the 50% probability level. All hydrogen atoms,
solvent molecule, and BArF counteranion are omitted for clarity.
4
Selected bond distances (Å) and bond angles (deg): Mo1−N1
1.7691(19), Mo1−N2 2.236(2), Mo1−N3 2.1839(18), Mo1−N4
2.1675(19), Mo1−P1 2.4794(5), Mo1−P2 2.5156(5), N1−O1
1.221(2), N1−Mo1−N2 177.22(8), N3−Mo1−N4 79.24(7), P1−
Mo1−P2 96.857(18), P1−Mo1−N3 170.16(5), P1−Mo1−N2
90.77(5).
a longer reaction time for full conversion (Table 1, entry 2). To
improve the activity we screened various solvents. Toluene,
THF, and dimethylsulfoxide (DMSO) could not increase the
catalytic activity significantly. When MeCN was used as a
solvent, conversion of benzaldehyde to the corresponding
silylether could not be observed as revealed by 1H NMR (Table
1, entry 5). Temperature was suspected to play a crucial role to
Among the tertiary silanes, trimethoxysilane appeared to be
more active compared to other silanes. A loading of 0.2 mol %
of catalyst 3 to a mixture of benzaldehyde and HSi(OMe)3
(1:1.2) in chlorobenzene at 120 °C, revealed full conversion in
less than 2 h with an initial TOF of 560 h−1 (Table 1, entry 15).
The sterically hindered aromatic silanes diphenylmethylsilane
Table 1. Optimization Reactions for the Hydrosilylation of Benzaldehyde Catalyzed by 3
a
b
c
c
entry
1
cat (mol %)
silane
Et3SiH
solvent
TOF (h‑1)
time (h)
conv (%)
yield (%)
1
C6D5Cl
C6D5Cl
THF
54
22
46
38
3
100
100
78
82
0
100
100
78
82
0
d
2
1
Et3SiH
11
12
3
3
4
5
6
1
Et3SiH
1
Et3SiH
toluene
MeCN
DMSO
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
THF
1
Et3SiH
4
1
Et3SiH
10
4
59
16
100
100
100
20
20
100
15
100
0
59
16
e
7
1
Et3SiH
2
3
f
8
0.05
0.05
0.05
0.05
0.5
0.2
0.2
0.2
1
Ph2SiH2
PhSiH3
1860
3520
2512
800
2.5
1
83/17
f
9
1
33/67
f
10
11
PhMeSiH2
PhMeSiH2
PhMeSiH2
PhMe2SiH
Ph2MeSiH
HSi(OMe)3
iPr3SiH
1.5
0.5
16
3
67/33
f
20/0
g
12
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
C6D5Cl
20
100
15
100
0
13
14
15
16
17
18
290
30
17
<2
0.5
0.5
3
560
0.2
0.2
0.05
HSi(SiMe3)3
Ph3SiH
0
0
0
0
0
h
19
PhMeSiH2
0
0
a
Unless otherwise stated 3 mg of the catalyst 3, benzaldehyde as substrate (given ratio with respect to the catalyst according to Table 1), 1.2 equiv of
b
the silane with respect to substrate, 0.4 mL of solvent, and 120 °C temperature were used as reaction conditions. Initial TOFs were determined
c
d
e
from the 1H NMR spectra after 30 min. Conversions and yields were determined from 1H NMR spectra. Acetophenone was used as substrate. 80
f
g
°C. Ratio of monosilylated and disilylated products (a/b) determined by GC/MS. The reaction was carried out at room temperature, and average
TOF was calculated after 16 h. No catalyst was used.
h
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and phenyldimethylsilane turned out to be less active toward
hydrosilylation of benzaldehyde (Table 1, entries 13 and 14).
Catalysis was suppressed with the more sterically encum-
bered triphenylsilane, triisopropylsilane, and tris(trimethylsilyl)-
silane (Table 1, entries 16, 17, and 18) even at higher
temperatures. A blank experiment using PhMeSiH2 and
benzaldehyde at 120 °C did not show any hydrosilylation
Table 2. Hydrosilylation of Various Aldehydes Catalyzed by
3 Using PhMeSiH2
1
product as checked by H NMR and GC/MS. The reaction of
benzaldehyde with PhMeSiH2 using 3 was then carried out in
THF at 120 °C, which resulted in a decrease of the TOF value
(800 h−1, Table 1, entry 11) in comparison with chlorobenzene
as the solvent. However it should be mentioned here that at
room temperature a higher catalyst loading (0.5 mol % 3) led
to some activity in the hydrosilylation of benzaldehyde using
PhMeSiH2, and after 16 h only 20% conversion was observed
with the low TOF value of 2.5 h−1 (Table 1, entry 12).
Nevertheless, at optimized conditions using chlorobenzene
as a solvent and phenylmethylsilane at a temperature of 120 °C,
the catalytic activities of 3 toward various other aldehydes were
tested. Several para substituted benzaldehydes were converted
to their corresponding silyl ethers with excellent rates and
yields (Table 2). One hundred percent conversions could be
achieved running the reactions for longer times. But in the case
of 4-methoxy and 4-cyanobenzaldehydes, longer reaction times
led to as yet unidentified byproducts. The electron donating 4-
methoxybenzaldehyde was found to be less active than the
electron withdrawing para fluoro and para chlorobenzalde-
hydes (Table 2, entries 1, 3 and 4). 2-Chlorobenzaldehyde
showed the highest activity among the various substituted
benzaldehydes to obtain a maximum TOF of 4864 h−1. 3-
Cyclohexenecarboxaldehyde and cinnamaldehyde could be
hydrosilylated by 3 without attack on the alkene double bond
to obtain a maximum TOF of 960 h−1 and 800 h−1 respectively
(Table 2, entries 6 and 11). The catalytic performance of 3 in
the hydrosilylation of heterocyclic aromatic aldehydes was also
tested. The 2-thiophenecarboxaldehyde and 2-furfural were
easily converted to their corresponding hydrosilylated products
with high turnover frequencies and yields (Table 2, entries 7
and 8). It should be noted that within an initial period of 30
min of the reactions, disilylated products could not be observed
by GC/MS nor by 1H NMR spectroscopy in most cases of the
reactions (For entries 7, 8, and 9 where 15% of disilylation were
detected within 30 min). Quite naturally as the reaction
progressed the amount of disilylation products increased at a
rate slower than the monosilylation products. In some cases
monosilylated products were obtained exclusively (Table 1,
entries 1, 5, 10, and 11). Thus, the performance of 3 in
hydrosilylations of various other substrates were additionally
screened. Surprisingly, hydrosilylation of ketones turned out to
be much more effective with catalyst 3. For instance
acetophenone was hydrosilylated with PhMeSiH2 with a
loading of 0.01 mol % of 3 at 120 °C in 53% conversion
within 15 min with an initial TOF of 21232 h−1 (Table 3, entry
1) as revealed by GC/MS. 3 was found to also catalyze the
hydrosilylation of acetophenone at room temperature. When
0.025 mol % of 3 was loaded to a mixture of acetophenone and
PhMeSiH2 in chlorobenzene, 38% conversion was observed
after 1 h with a TOF of 1520 h−1 and within 5 h full conversion
of acetophenone to the corresponding hydrosilylated products
(Table 3, entry 1) could be achieved. Thus, we tested various
other para substituted acetophenones at room temperature and
at 120 °C. 4-Methoxyacetophenone turned out to be the most
active substrate in hydrosilylations catalyzed by 3. At 120 °C a
a
Unless otherwise stated 3 mg of the catalyst 3, substrate/catalyst ratio
according to Table 2, 1.2 equiv of the silane with respect to the
b
substrate, 0.4 mL of C6H5Cl and 120 °C temperature. Initial TOFs
were calculated by GC/MS after 30 min of reaction time.
c
d
Conversions were determined by GC/MS. Yields on the basis of
the substrate consumption determined by GC/MS and a/b is the ratio
of mono and disilylated products. TOF was determined after initial 15
min.
e
maximum TOF of 3.2 × 104 h−1 was obtained when 0.01 mol %
catalyst 3 was loaded to a mixture of 4-methoxyacetophenone
and PhMeSiH2 (1:1.2) in chlorobenzene. At room temperature,
4-methoxyacetophenone revealed a maximum TOF of 4420 h−1
with a catalyst loading of 0.02 mol % (Table 3, entry 3).
Continuing the series of catalyzes with para substituted
acetophenones as substrates, efficient conversions to the
corresponding hydrosilylated products were found to also
reach excellent yields within a few hours. Electron withdrawing
groups on the para position of the acetophenone were slightly
less active than para electron donating groups on the para
substituted acetophenones (Table 3, entries 2, 3, 4, and 5).
However, unlike the cases of aldehyde hydrosilylation, ketone
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α,β unsaturated aromatic ketones turned out to be sluggish with
a maximum TOF of 240 h−1.
Table 3. Hydrosilylation of Various Other Substrates
Catalyzed by 3 Using PhMeSiH2
It turned out that imines could be hydrosilylate as well. N-
benzylideneaniline, and N-(4-methoxybenzylidene)aniline were
fully converted to their corresponding silylamines within 7 h
with a loading of 0.2 mol % of 3 (Table 3, entries 11 and 12).
Hydrosilylation could not be observed when styrene and
benzonitrile were used as substrates. Applying hydrosilylation
reactions with 3 to aliphatic ketones these showed that acetone
and 3-pentanone could be converted to the corresponding
hydrosilylated products at 120 °C within 7 h in 100% yields as
1
revealed by H NMR and 13C NMR spectroscopy (Table 3,
entries 9 and 10). With a loading of 0.1 mol % catalyst 3, 3-
pentanone was found to be hydrosilylated at room temperature
sluggishly showing an initial TOF of 150 h−1, and the reaction
went to completion in just 24 h. On the other hand, acetone
did not show any activity in hydrosilylation with 3 at room
temperature even after 14 h of reaction time and a loading of
0.1 mol % of the catalyst (Table 3, entries 9 and 10).
Kinetic Studies. We carried out three initial kinetic
experiments to determine the rate of the hydrosilylation
reactions of acetophenone catalyzed by 3 at room temperature
varying the concentrations of acetophenone, the silane, and the
catalyst. The initial reaction rates were found to be first order
with respect to the concentration of the catalyst and the silane.
However, the rate was zeroth order with respect to the
substrate concentration indicating that catalyst saturation
already takes place at low concentration of the substrates.
Exemplary kinetic plots are given in Figure 2 (see also
Supporting Information).
Based on the kinetic experiments (Table 4) the initial rate
equation was established as
rate = 1.131(M−1min−1)[catalyst][silane]
Hammett Correlation Study. To understand the influence
of various para substituted acetophenones, Hammett23
correlations were established applying catalyst 3 in hydro-
silylations of para-methoxy, -methyl, -H, -fluoro, and -chloro
substituted acetophenones at room temperature and at 120 °C
(Figure 3). The initial rates (s−1) were determined from the
initial TOF values (s−1). A Hammett plot of ln(rate) vs
substituent constant (σ) showed linear correlations with a
negative slope (ρ) of −1.14 at 120 °C and −3.18 at room
temperature. The Hammett parameters (ρ) indeed reflect the
substituent susceptibility of the hydrosilylation processes. A
negative ρ value greater than unity indicates a positive charge
build up in the transition states and vice versa. The rate limiting
step of the hydrosilylation reactions is apparently greatly
affected at room temperature by the presence of the electron
donating groups in the para position of the acetophenone.
Naturally, at higher temperature the rate was less sensitive to
para substitution of the acetophenones (Table 3, entries 1, 2, 3,
4, and 5).
a
Unless and otherwise stated 0.3 mL of chlorobenzene, 0.5 mg of 3 as
catalyst, substrate/catalyst ratio (according to Table 3), 1.2 equiv of
PhMeSiH2 with respect to the substrate, and 120 °C temperature were
used as reaction conditions. For room temperature reactions, 1 mg of
catalyst 3 was used, and TOFs were calculated from the initial 1st h.
b
TOFs were determined after the initial 15 min of reaction time by
c
GC/MS. Yields by GC/MS on the basis of substrate consumption.
2
d
e
Deuterium Labeling Study. A H NMR experiment was
THF solvent. Isolated yields.
performed to confirm that hydride was transferred selectively
from the silane moiety. For this purpose, benzaldehyde and
Et3SiD were mixed and heated to 120 °C for 1 h in the
hydrosilylation brought about exclusively monosilylated prod-
ucts as revealed by GC/MS.
In further studies extension of the scope of the hydro-
silylation reactions catalyzed by 3 was explored. Benzophenone,
cyclohexanone, 2-acetylthiophene could indeed be converted to
their corresponding silylether with high activities and yields
(Table 3, entries 7, 8, and 13). However, the hydrosilylation of
2
presence of catalyst 3 (1 mol %). H NMR in chlorobenzene
revealed the appearance of a doublet signal at δ 4.8 ppm {2J(1H,
2H) = 1.8 Hz} corresponding to 27% conversion, which
2
became a singlet in the proton decoupled H NMR spectra.
This clearly spoke for the transfer of deuterium from the Et3SiD
moiety and formation of PhCHDOSiEt3.
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Figure 2. Left: Kinetic plot of [PhCOCH3] vs time. Right: Kinetic plot of ln[PhMeSiH2] vs time. [PhCOCH3] = 2.075 M, [cat] = 0.002 M,
[PhMeSiH2] = 2.43 M. C6D5Cl solvent.
acetonitrile which supports the coordination of this ketone to
the catalyst 3 via exchange of the labile acetonitrile ligands.
When another 80 equiv of acetophenone were added, an
additional singlet appeared at δ 52 ppm at the expense of the
signal of 3 in the 31P NMR spectra. This could be assigned to a
η2-acetophenone species [Mo(NO)(P∩P)(η2-OCPhMe)-
Table 4. Dependence of the Initial Rate of Hydrosilylations
on the Concentrations of Substrate, Catalyst, and Silane
a
PhMeSiH2
(M)
PhC(O)CH3
(M)
initial rate
TOF
(h−1
runs 3 (M)
(M min−1
)
)
−3
1
2
3
4
0.002
0.002
0.002
0.001
2.43
4.86
2.43
2.43
2.075
2.075
4.14
5.5 × 10
165
320
165
170
1.04 × 10−2
5.5 × 10−3
2.9 × 10−3
(NCMe)2]+[BArF ]−. However further addition of acetophe-
4
none did not lead to complete disappearance of the complex 3.
This indicated reversible binding of the acetophenone to the
molybdenum center. In a similar way, addition of 25 equiv of
benzaldehyde to 3 (6 mg) in chlorobenzene (or THF) led to
2.075
a
Reactions were carried out at room temperature. Initial rates were
determined after 1 h from 1H NMR integration. Runs 1 and 2,
Variation of [PhMeSiH2]; Runs 1 and 3, Variation of [PhC(O)CH3];
Runs 1 and 4, Variation of [Catalyst].
two new doublet signals in the 31P NMR spectra at δ 37 (JPP
=
98 Hz) and 30 ppm (JPP = 98 Hz) with complete disappearance
of the 31P NMR resonance of 3 indicating stronger
coordination of aldehydes to the molybdenum center of 3
liberating acetonitrile as also observed in the 1H NMR spectra.
This stronger binding could be the reason for the lower
reactivity of aldehydes in the reported hydrosilylation reactions,
although the structure of this new species could not be fully
established. Nevertheless we assume that a [Mo(NO)(P∩P)-
(η1-OCHPh)(NCMe)2]+[BArF ]− species had formed where
4
the coordinated aldehyde resides in the equatorial plane trans
to one of the phosphorus atoms of DPEphos ligand. However
addition of excess of PhMeSiH2 to a solution of 3 in the
absence of the ketone or the aldehyde at room temperature did
not reveal changes in the 31P NMR which may exclude initial
coordination of the silane to the catalytic center. But when 3
and excess of PhMeSiH2 were heated at 120 °C, the 31P NMR
spectra showed new signals at δ 44 ppm along with signals at δ
−9 and δ −17 ppm due to the formation of an as yet
unidentified species.
The room temperature hydrosilylation of acetophenone
catalyzed by 3 (1 mol %) using PhMeSiH2 revealed in the 31P
NMR spectra the presence of the signals mentioned above (δ
52.9, 52, and 50.8 ppm) throughout the hydrosilylation process
and after completion of the hydrosilylation reaction only the
signal of 3 (50.8 ppm) was observed. In a similar way when 3,
acetophenone (300 equiv with respect to the catalyst), and
PhMeSiH2 were heated at 120 °C for one minute, the 31P NMR
instaneously revealed the presence of 3 along with the δ 52.9
and 52 signals before the reaction went to completion.
However at the end of the reaction a weak unidentified signal
at δ 49 ppm (10%) appears (presumably a product of the
reaction of excess silane with 3), the intensity of which
Figure 3. Hammett plot for the hydrosilylation of various para
substituted acetophenones catalyzed by 3 extracted from initial TOF
values. Squares, with ρ = −1.14, hydrosilylations at 120 °C. Circles
with ρ = −3.18, at room temperature. Black: p-MeO, red: p-CH3,
green: p-H, blue: p-F, cyan: p-Cl groups. PhMeSiH2 was used as silane.
Search for Catalytic Intermediates and Mechanistic
Proposal. Additional mechanistic investigations were directed
toward detection of catalytic intermediates. When a solution of
catalyst 3 (6 mg) in chlorobenzene (or THF), was mixed with
25 equiv of acetophenone at room temperature, the 31P NMR
spectra showed appearance of a new singlet signal at δ 52.9
ppm (15%) along with the signal of 3 (δ 50.8 ppm, 85%). This
new signal at δ 52.9 ppm could be attributed to the formation
of a ketone species of type [Mo(NO)(P∩P)(η1-OCPhMe)-
1
(NCMe)2]+[BArF ]− (4). The H NMR spectra in C6D5Cl of
4
the added acetophenone solution revealed the presence of free
2213
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ACS Catalysis
Research Article
increases with heating for longer period of times with
concomitant decrease of the 31P NMR signal of 3. Therefore
it became obvious that catalyst 3 is the resting state of the
catalytic cycle, and 4 (Scheme 2) could be the active species
correlation study by the rate enhancement using electron
donating para substituted acetophenone since the electron
donating groups makes the metal center more electron rich
facilitating the oxidative addition of the silane to form species 5
(Scheme 2).
On the basis of the Hammett correlation, kinetic study, and
our mechanistic observations described just before, an Ojima
type2,3 mechanism is proposed (Scheme 2) for the hydro-
silylation of ketones and aldehydes catalyzed by 3 where
oxidative addition of the silane is assumed to be rate limiting. 3
takes the role of a precatalyst and resting state. Then initial
exchange of one MeCN ligand by the substrate takes place via a
4 type structure followed by rate limiting oxidative addition of a
silane molecule replacing another acetonitrile ligand. A
consecutive silyl transfer and H-shift with the subsequent
reductive elimination generates the silyl ethers.
Scheme 2. Proposed Ojima Type Mechanism for the
Hydrosilylation Reactions Catalyzed by 3
CONCLUSION
■
In conclusion, we have demonstrated that a low-valent cationic
molybdenum nitrosyl complex bearing a large bite angle
diphosphine and labile MeCN ligands is a highly efficient
catalyst in the hydrosilylation of ketones and aldehydes
particularly in the presence of the secondary silane PhMeSiH2.
A great variety of silyl ethers derived from ketones and aryl
aldehydes were accessed with good activities and in excellent
yields using catalyst 3 at 120 °C. Ketones were found to show
excellent catalytic performance even at room temperature.
Kinetic studies revealed that the rates of the reactions show a
first order dependence on the concentration of the catalyst and
the silane and are independent of the substrate concentration,
which is in agreement with a Ojima type hydrosilylation
mechanism.
before the rate limiting step which may therefore accumulate
during the catalysis. To further support this observation
another kinetic run was also pursued by 31P NMR using
benzophenone as the substrate. Similar to the acetophenone
experiment, when 100 equiv of benzophenone was added to a
C6D5Cl solution of the catalyst 3 (6 mg) the red solution
immediately becomes dark green and the 31P NMR spectra of
the green solution revealed appearance of two new singlet
signals at δ 47.6 and 53.3 ppm (20%) along with a signal for 3.
Further addition of benzophenone does not increase the
intensity of these above-mentioned signals as indicated by 31P
NMR spectra. Then PhMeSiH2 was added (1.2 equiv with
respect to benzophenone) to this dark green solution and
monitored by 31P NMR spectroscopy at room temperature.
Similar to the acetophenone case, the above-mentioned two
signals at δ 47.6 and 53.3 ppm along with 3 persist throughout
the catalysis, and at the end of the reaction only the signal of 3
was observed regenerating the red color of the catalyst.
Furthermore, to get insight into the rate limiting step, a
deuterium kinetic isotope effect experiment was also carried out
for the hydrosilylation of acetophenone using triethylsilane at
120 °C in chlorobenzene. A loading of 1 mol % of the catalyst 3
with acetophenone in presence of Et3SiH (1.2 equiv) led to
formation of a hydrosilylated product corresponding to a
conversion of 52% after 3 h. Under similar conditions use of
Et3SiD gave 25% of hydrosilylated product after 3 h as
indicated by GC/MS resulting in a DKIE (kH/kD) value of 2.1.
This relatively high DKIE value led us to propose oxidative
addition of the silane to 4 type complexes as the rate limiting
step. This was in fact also supported by the Hammett
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried
out under an atmosphere of nitrogen using either standard
Schlenk techniques or in a glovebox. All reagent grade solvents
were dried according to standard laboratory procedure and
distilled prior to use under N2 atmosphere. Deuterated solvents
were dried with sodium benzophenone ketyl (THF-d8, toluene-
d8, C6D6) and calcium hydride (C6D5Cl and CD2Cl2) and
distilled via freeze−pump−thaw cycle prior to use. The
DPEphos ligand and Mo(NO)Cl3(NCMe)2 were prepared
according to literature procedures.22b All other chemicals were
purchased from commercially available sources and used
without further purifications. NMR spectra were measured
1
with a Varian Mercury 200 spectrometer (200.1 MHz for H,
81.0 MHz for 31P), with Varian Gemini-300 instrument (1H at
300.1 MHz, 13C at 75.4 MHz), with a Bruker-DRX 500
1
spectrometer (500.2 MHz for H, 202.5 MHz for 31P, 125.8
MHz for 13C), and a Bruker-DRX 400 spectrometer (400.1
1
MHz for H, 162.0 MHz for 31P, 100.6 MHz for 13C). All
1
chemical shifts for H and 13C{1H} are expressed in ppm
relative to tetramethylsilane (TMS) and for 31P{1H} relative to
85% H3PO4 as an external standard reference. Signal patterns
are as followed: s, singlet; d, doublet; t, triplet; q, quartet; m,
multiplet. IR spectra were obtained either ATR or KBr methods
using Biorad FTS-45 instrument. Elemental analyses were
carried out at Anorganisch-Chemisches Institut of the
University of Zurich. The GC/MS spectra were recorded on
̈
a Varian Saturn 2000 spectrometer equipped with Varian 450-
GC chromatograph.
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ACS Catalysis
Research Article
[Mo(NO)(P∩P)Cl2]2[μCl]2 (P∩P = DPEphos) (1). Mo(NO)-
Cl3(NCMe)2 (0.200 g, 0.636 mmol) was dissolved in 10 mL of
THF. A solution of DPEphos (0.342 g, 0.635 mmol) in 5 mL of
THF was added into that solution. The resulting mixture was
kept stirring for 3 h at room temperature. The formed
precipitate was filtered off and was washed first with minimum
amount of THF and then with pentane. Finally the solid was
dried in vacuo to obtain 1 in 86% yield (0.420 g). IR (cm−1,
ATR): 1664 (νNO). Anal.Calcd for [C36H28Cl3MoNO2P2]2: C,
56.09; H, 3.66; N, 1.82. Found: C, 56.21; H, 3.75; N, 1.68.
[Mo(NO)(P∩P)(NCMe)3] [Zn2Cl6]1/2 (P∩P = DPEphos) (2).
To a suspension of 1 (0.20 g, 0.129 mmol) in 10 mL of MeCN,
zinc granules (10 equiv) were added. The resulting mixture was
kept overnight in acetonitrile with constant stirring at room
temperature. After completion of the reaction the solution was
filtered off, the solvent was removed in vacuo, and washed with
pentane. The solid residue was dissolved in THF and filtered
off. The pure product was obtained as an orange powder after
removal of the THF in vacuo in 84% yield (0.209 g). IR
(cm−1): 1593 (s, ν(NO)). 1H NMR (500 MHz, THF-d8,
293K): δ 7.52- (broad singlet, Ph), 7.42−7.39 (m, Ph), 7.34−
7.30 (m, Ph), 7.26−7.22 (m, Ph), 7.21−7.17 (m, Ph), 7.1 (m,
Ph), 7.04−7.02 (m, Ph), 6.95−6.92 (m, DPEphos H), 6.58
(broad singlet, DPEphos H), 2.01 (broad singlet, 9H, CH3)
ppm. 31P{1H} NMR (125 MHz, THF-d8, 20 °C): δ 53.5 (s).
13C{1H} NMR (125.8 MHz, THF-d8, 293K): δ = 159.3 (m,
Ph), 135 (m, Ph), 134 (m, Ph), 132(m, Ph), 130 (m, Ph), 129
(m, Ph), 128.8 (m, Ph), 124 (m, DPEphos), 123 (m, CN), 120
(m, DPEphos), 0.75 (m, CH3) ppm. Anal. Calcd for
C42H37Cl3MoN4O2P2Zn (959.41): C, 52.58; H, 3.89; N, 5.84.
Found: C, 52.53; H, 3.97; N, 5.64.
direct methods using SHELXS97.27 The structure refinements
were performed by full-matrix least-squares on F2 with
SHELXL97.27 PLATON28 was used to check the result of
the X-ray analysis. All programs used during the crystal
structure determination process are included in the WINGX
software.29 For more details about the data collections and
refinements parameters, see the Crystallographic Information
files (Supporting Information). Crystallographic data have been
deposited with the Cambridge Crystallographic Data Center as
CCDC-921393 (for 2) and CCDC-921394 (for 3); they
contain the supplementary crystallographic data (excluding
structure factors) for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request/cif
General Procedures for the Catalytic Hydrosilylation
Reactions. Table 1: Appropriate amounts of the catalyst 3 (3
mg) and substrate at a given ratio with the catalyst 3, 1.2 equiv
of the silane with respect to the substrate in deuterated solvents
(0.4 mL) were placed in a Young tap NMR tube. The reaction
mixture was heated to 120 °C. The TOF was determined after
initial 30 min of reaction times by 1H NMR spectroscopy. After
the reaction (reaction times according to Table 1), the resulting
solution was analyzed by 1H NMR spectroscopy. The
resonances were in accord with those of the literature. The
1
yields were determined by integration of the H NMR spectra
on the basis of substrate consumption.
Table 2: The reactions were carried out in C6H5Cl in a
Young Schlenk tube with stirring. The TOF values (after initial
30 min.) and the final yields were determined by GC/MS on
the basis of substrate consumption.
Table 3: A fresh stock solution of catalyst 3 (6.5 mg in 650
μL C6H5Cl) was prepared. An aliquot (50 μL) of that stock
solution was added to a mixture of the ketone and the silane
(amount of ketone with respect to catalyst 3 according to Table
3, ketone: silane = 1:1.2) in 250 μL C6H5Cl in a Young tap
Schlenk tube. Then the Schlenk tube was kept in a preheated
oil bath (120 °C) with stirring. After 15 min of reaction time
the Schlenk tube was immediately taken out and cooled to
room temperature and taken into the glovebox. Without further
purification a GC/MS analysis was carried out to determine the
yield of the product (on the basis of the substrate
consumption). The same reaction mixture was taken out
from the glovebox and heated for longer period of reaction
times (Table 3) to achieve complete conversions. For room
temperature reactions: A fresh stock solution of catalyst 3 (10
mg in 1 mL of C6H5Cl) was prepared. An aliquot (100 μL) of
that stock solution was added to a mixture of ketone and silane
(amount of ketone with respect to catalyst 3 according to Table
3, ratio of ketone: silane = 1:1.2) in 300 μL of C6H5Cl in a
Young Schlenk tube and kept stirring. The TOFs were
determined after the initial 1 h by GC/MS.
[Mo(NO)(P∩P)(NCMe)3][BArF ] (P∩P = DPEphos) (3). Mo-
4
(NO)(DPEphos)(NCMe)3] [Zn2Cl6]1/2 (0.070 g, 0.072
mmol) was dissolved in 10 mL of acetonitrile. NaBArF
4
(0.080 g, 0.09 mmol) dissolved in 10 mL of acetonitrile was
added to that solution with constant stirring and kept stirring
overnight. Then the solution was filtered off, the solvent was
removed in vacuo and washed with pentane. The obtained
product was extracted with toluene, concentrated, and kept
refrigerated (layering with pentane) to reveal pure red colored
1
crystals of 3 in 90% yield. IR (cm−1): 1595 (NO). H NMR
(500 MHz, THF-d8, 293K): δ 7.79 (s, BArF4), 7.58 (s, BArF4),
7.38 (m, Ph), 7.28 (m, Ph), 7.22 (m, Ph), 7.12 (m, Ph), 7.0 (m,
Ph), 6.7(m, DPEphos H), 2.16 (s, 3H, CH3), 1.41 (s, 6H,
CH3), ppm.31P{1H} NMR (162 MHz, THF-d8, 20 °C): δ 52
(s). 13C{1H} NMR (100.8 MHz, THF-d8, 293K): δ = 163 (q, i-
BArF , 1JBC = 50.0 Hz,), Ph 159 (m, Ph), 135 (m, Ph), 133 (m,
4
Ph), 132 (m, Ph), 130 (m, Ph), 129 (m, Ph), 128.8 (m, Ph),
127 (m, BArF ), 125 (m, DPEphos), 123 (s, CN), 121 (s,
4
BArF ),), 120 (m, CN), 118 (m, DPEphos), 3.06 (s, CH3), 0.62
4
( s ,
C H
)
p p m .
A n a l .
C a l c d
f o r
3
General Procedures for the Kinetic Experiments. For
kinetic experiments of Figure 2: In a glovebox, catalyst stock
solution was freshly prepared in C6D5Cl (5 mg in 1.5 mL of
C6D5Cl) and an aliquot (0.3 mL) amount was added to a
mixture of acetophenone and phenylmethylsilane in a Young
NMR tube (see Supporting Information). Then the tube was
sealed and left at room temperature. The reaction was
monitored by 1H NMR after each certain time intervals. Yields
and product concentrations were determined from the
C74H49BF24MoN4O2P2(1650.89): C, 53.84; H, 2.99; N, 3.39.
Found: C, 53.75; H, 2.92; N, 3.20.
X-ray Diffraction Analyses. Single-crystal X-ray diffraction
data were collected at 183(2) K on an Agilent Technologies
Xcalibur Ruby area-detector diffractometer using a single
wavelength Enhance X-ray source with MoKα radiation (λ=
0.71073 Å).24 The selected suitable single crystals were
mounted using polybutene oil on a flexible loop fixed on a
goniometer head and immediately transferred to the diffrac-
tometer. Pre-experiment, data collection, data reduction, and
analytical absorption correction25 were performed with the
program suite CrysAlisPro.26 The structures were solved by
1
integration of the H NMR spectra.
GC/MS Data for the Hydrosilylated Products of
Various Substituted Benzaldehydes and Acetophe-
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ACS Catalysis
Research Article
(4) Berc, S. C.; Kreutzer, K. A.; Buchwald, S. L. J. Am. Chem. Soc.
1991, 113, 5093.
nones (Other Products See Supporting Information).
PhCHO: rt = 3.727 min, m/z = 106; PhCH2OSiPhH2: rt =
8.519 min, m/z = 213; (PhCH2O)2SiPhH: rt = 13.924 min, m/
z = 320; PhCH2OSiPh2H: rt = 12.534 min, m/z = 289;
(PhCH2O)2SiPh2: rt = 24.231 min, m/z = 395;
PhCH2OSiPhMeH: rt = 8.808 min, m/z = 227;
(PhCH2O)2SiPhMe: rt = 14.109 min, m/z = 333; p-
MeOC6H4CHO: rt = 6.127 min, m/z = 135; p-MeOPh-
CH2OSiPhMeH: rt = 10.346 min, m/z = 257; p-MeC6H4CHO:
rt = 4.718 min, m/z = 119; p-MePhCH2OSiPhMeH: rt = 9.420
min, m/z = 241; (p-MePhCH2O)2SiPhMe: rt = 16.757 min, m/
z = 361; p-ClC6H4CHO: rt = 5.128 min, m/z = 139; p-
ClPhCH2OSiPhMeH: rt = 10.039 min, m/z = 261; (p-
ClPhCH2O)2SiPhMe: rt = 22.023 min, m/z = 402; p-
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FC6 H4 CHO: rt
= 3.652 min, m/z = 124; p-
FPhCH2OSiPhMeH: rt = 8.746 min, m/z = 245; (p-
FPhCH2O)2SiPhMe: rt = 13.741 min, m/z = 369; p-
CNC6H4CHO: rt = 6.019 min, m/z = 131; p-
CNPhCH2OSiPhMeH: rt = 10.933 min, m/z = 252; o-
ClC6H4CHO: rt = 5.033 min, m/z = 139; o-
ClPhCH2OSiPhMeH: rt = 9.863 min, m/z = 261; PhC(O)-
CH3: rt = 4.560 min, m/z = 120; PhCH(CH3)OSiPhMeH: rt =
8.634 min, 8.725 min, m/z = 241; p-MeOC6H4C(O)CH3: rt =
6.863 min, m/z = 150; p-MeOC6H4CH(CH3)OSiPhMeH: rt =
10.115 min, 10.233 m/z = 272; p-CH3C6H4C(O)CH3: rt =
5.556 min, m/z = 134; p-CH3C6H4CH(CH3)OSiPhMeH: rt =
9.209 min, 9.320 min, m/z = 255; p-FC6H4C(O)CH3: rt =
4.461 min, m/z = 138; p-FC6H4CH(CH3)OSiPhMeH: rt =
8.581 min, 8.864 min, m/z = 259; p-ClC6H4C(O)CH3: rt =
5.967 min, m/z = 154 ; p-ClC6H4CH(CH3)OSiPhMeH: rt =
9.808 min, 9.930 min, m/z = 275; p-CNC6H4C(O)CH3: rt =
6.830 min, m/z = 145 p-CNC6H4CH(CH3)OSiPhMeH: rt =
10.696 min, 10.836 min, m/z = 266.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Hogerl, M. P.; Gigler, P.; Kuhn, F. E. ACS Catal. 2012, 2, 613−621.
̈
̈
Molecular structure of 2, the GC/MS data of various other
substrates and products, and crystallographic information files
of 2 and 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hberke@aci.uzh.ch.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Swiss National Science Foundation
and the funds of the University of Zurich are gratefully
acknowledged.
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