Olefin hydrosilylation catalyzed by cationic nickel(II) allyl complexes: A non-innocent allyl ligand-assisted mechanism

Article abstract of DOI:10.1039/c6cc01665k

The arene-supported cationic nickel allyl complexes serve as good catalysts for olefin hydrosilylation at room temperature. Detailed mechanistic studies based on experiments and DFT calculations support the novel mechanism, which includes the facile Si-H bond cleavage and Si-C bond formation, assisted by a non-innocent allyl ligand.

Full text of DOI:10.1039/c6cc01665k

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Olefin Hydrosilylation Catalyzed by Cationic Nickel(II) Allyl
Complexes: Non-Innocent Allyl Ligand-Assisted Mechanism
J. Mathew,a Y. Nakajima,*b Y.-K. Choe,a Y. Urabe,b W. Ando,b K. Satob and S. Shimadab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The arene-supported cationic nickel allyl complexes serve as way to a new selective hydrosilylation reaction.
good catalysts for olefin hydrosilylation at room temperature.
As seen from above mentioned studies, identification of a
Detailed mechanistic studies based on experiments and DFT new mechanism for catalytic hydrosilylation can bring an
calculations support the novel mechanism, which includes the important breakthrough to the conventional hydrosilylation
facile Si-H bond cleavage and Si-C bond formation, assisted by a chemistry, and thus is of great importance for the design of new
non-innocent allyl ligand.
catalysts, which enables to develop more efficient and selective
systems. In this study, it was revealed that cationic nickel allyl
The hydrosilylation reaction is an efficient method for the
formation of organosilicon compounds and represents one of
the most important reactions in the silicon chemistry. Thus far,
active studies have been continuously performed in this field
using various transition metal catalysts such as precious metals
(Pt, Rh, Ru etc.)1,2 as well as earthꢀabundant nonꢀprecious
metals (Fe3, Co4, Ni5,6, etc.). Through these studies, several
hydrosilylation mechanisms have been disclosed until now.
One of the most important mechanism is the Chalkꢀ
Harrod/modified ChalkꢀHarrod mechanism, which normally
operates in precious metalꢀcatalyzed systems.7 Although the
utility of these systems are nowadays widely recognized both in
the laboratory and in largeꢀscale industrial applications, it is
also reported that the system often accompanied by side
reactions, leading to the decrease in the reaction selectivity. For
complexes 1aꢀd (Figure 1) successfully attain selective
monoalkylation of secondary silanes, R2SiH2. Detailed
mechanistic study using DFT methods suggested the novel
Ni(II) catalyzed hydrosilylation mechanism including a unique
function of a nonꢀinnocent allyl ligand, which account for the
observed unique reaction selectivity.
Me
+
Me
+
–
–
[BArF
]
[BArF
]
4
Ni
tBu
Ni
4
t
tBu
tBu
Bu
OH
OH
1a
1b
tBu
tBu
+
tBu
+
[BArF
]
Ni
tBu
–
[BArF
]
–
Ni
4
4
earlyꢀtransition metal and lanthanide catalyzed systems, σꢀbond
t
Bu
metathesis process is proposed.8 Since this mechanism proceeds
with fewer byꢀproducts, the development of this system
significantly contributed to the design of highly selective
1c
1d
BArF4 = B(3,5-(CF3)2(C6H3))4
Figure 1. Complexes 1a-d.
hydrosilylation processes. Recently,
a novel mechanism
including a cationic ruthenium silylene complex as a key
species was proposed by Tilley and Glaser.9 The system
successfully demonstrated the selective monoalkylation of
primary silanes RSiH3. As a result, the study has opened up the
Complex 1b, which has been established as a good diene
polymerization catalyst, is easily prepared by the reaction of
[Ni(
(CF3)2(C6H3)}4).10 Complexes 1a
synthesized using a commercially available nickel precursor
[Ni(
3ꢀCH2C(CH3)CH2)Cl]2 (Scheme 1).
η
3ꢀC3H5)Cl]2
with NaBArF
(BArF
=
B{3,5ꢀ
4
4
,
1c, and 1d were similarly
η
In the presence of 0.5 mol% of the catalyst 1a, 1ꢀoctene was
successfully hydrosilylated with various silanes at room
temperature. The results of the reactions are summarized in
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Et2(
nOct)SiH (92%) along with small amounts of byproducts
when using 1d
.
Table 1. Hydrosilylation of 1-octene with various silanes catalysed by 1[a]
entry
silane
catalys
t
time
Product yields[b]
(isolated yield)
(mol%)
1a
Scheme 1. Synthesis of 1a, 1c, and 1d.
1
2
3
(nHex)SiH3
(nOct)SiH3
Et2SiH2
24 h
24 h
(nHex)(nOct)SiH2 29% (25%)
(nHex)(nOct)2SiH 11% (6%)
(nOct)2SiH2 19% (17%)
(nOct)3SiH 18% (9%)
Et2(nOct)SiH 75% (71%)
Et2(nOct)2Si 1%
(0.5)
1a
Table 1. Hydrosilylation of 1ꢀoctene with RSiH3 (R =
nHex,
nOct) slowly proceeded to form the corresponding
(0.5)
1a
monohydrosilylated product R(nOct)SiH2 and dihydrosilylated
product R2(nOct)SiH in the total yield of ca 40% at room
30
(0.5)
min
temperature after 24 h (entries 1,2). In these reactions, 1ꢀoctene
also underwent isomerization to give a mixture of internal
olefins and was completely consumed after the reaction,
whereas unreacted RSiH3 remained. In contrast, hydrosilylation
of 1ꢀoctene with secondary silanes R2SiH2 quickly proceeded.
For example, hydrosilylation of 1ꢀoctene with Et2SiH2
completed within 30 min under the same reaction conditions to
Et(nOct)2SiH 2%
4
Et2SiH2
1a
2.5 h
Et2(nOct)SiH 68% (58%)
Et2(nOct)2Si 2%
Et(nOct)2SiH trace
(nPen)2(nOct)SiH
(0.1)
5
6
(nPen)2SiH2
(nHex)2SiH2
Et2SiH2
1a
(0.5)
1a
2 h
2 h
6 h
90% (86%)
(nHex)2(nOct)SiH
(0.5)
1a
96% (95%)
selectively form monohydrosilylated product Et2(
75% yield as well as trace amounts of Et2( Oct)2Si (1%) and
Et( Oct)2SiH (2%) (entry 3). Et( Oct)2SiH is likely to be
nOct)SiH in
7[c]
8
1-(PhC2H4)Et2SiH 68% (68%)
2-(PhC2H4)Et2SiH 32% (14%)
Et2(nOct)SiH 76% (72%)
Et2(nOct)2Si 1%
n
(0.5)
1b
n
n
Et2SiH2
30
formed by the redistribution of the Et groups on the Si atom.6 In
this reaction, olefin isomerization was also detected. It should
be noted that 1a also catalysed hydrosilylation of 1ꢀoctene with
Et2SiH2 even at lower catalyst loading of 0.1 mol% to
(0.5)
min
Et(nOct)2SiH 2%
9
Et2SiH2
Et2SiH2
1c
30
Et2(nOct)SiH 76% (75%)
Et2(nOct)2Si 4%
(0.5)
min
selectively give Et2( Oct)SiH (68%) in 2.5 h (entry 4). Using
n
Et(nOct)2SiH 1%
(
n
Pen)2SiH2 and (nHex)2SiH2, with bulkier substituents,
10
1d
9 h
Et2(nOct)SiH 92% (91%)
Et2(nOct)2Si 2%
hydrosilylation of 1ꢀoctene proceeded slowly but resulted in the
formation of the corresponding monohydrosilylated products,
(0.5)
Et(nOct)2SiH trace
(
n
Pen)2(
nOct)SiH and (nHex)2(nOct)SiH with high selectivities
[a] ]1 (0.0042 mmol, or 0.00084 mmol for entry 4), silane (0.84 mmol), olefin
in high yields (90% and 96%, respectively; entries 5,6).
(0.84 mmol) in CH2Cl2 (4 mL) at 25 °C. [b] GC yield ( Isolated yield). [c]
Unfortunately, reactions using bulky tertiary silanes did not Styrene was used as an olefin substrate.
proceed. Furthermore, the hydrosilylation reactions were not
successful to quantitatively recover 1ꢀoctene when using either
Ph2SiH2 or PhSiH3, which probably deactivates the catalyst by
It is likely that the bulky 1,3,5ꢀtriꢀtertꢀbutylbenzene kinetically
stabilizes 1d and thus suppresses the formation of other Ni
species, which could catalyse unwanted sideꢀreactions.
coordinating with the active Ni center via
ring. Olefins with coordinating amino or sulphide groups (
dimethylallylamine, butꢀ3ꢀenꢀ1ꢀyl(phenyl)sulfane)
πꢀbonding of the Ph
One of the characteristic features of this system is that the
catalysts exhibit extremely high activity when secondary
silanes were utilized as substrates and achieve the selective
monoalkylation of secondary silanes. It was also confirmed that
the observed unique selectivity did not change when using an
excess amount of 1ꢀoctene. Motivated by these results,
hydrosilylation of 1ꢀoctene with Et2SiH2 in the presence of 5
N,Nꢀ
and
coordinating solvents (acetonitrile, THF) could not be utilized,
either. On the other hand, styrene was successfully
hydrosilylated with Et2SiH2 to quantitatively give a mixture of
α
ꢀ and
To check the effect of the ligands on the catalytic activity,
the hydrosilylation reaction of 1ꢀoctene with Et2SiH2 was
performed using catalysts 1b . Complex 1b, which has an
allyl group instead of methallyl group, catalysed the
βꢀsilylated products (entry 7).
1
mol% of 1a was monitored by H NMR spectroscopy to shed
light on the reaction mechanism. Considering 18e count of 1a
,
ꢀ
d
dissociation of the arene ligand and/or elimination of the
methallyl ligand as an isobutene are expected for the Si–H bond
a
hydrosilylation in a similar manner as 1a; i.e. 1ꢀoctene was
successfully hydrosilylated with Et2SiH2 in 30 min at 0.5 mol%
cleavage step in the conventional ChalkꢀHarrod and
σꢀbond
methathesis mechanisms. However, surprisingly, complex 1a
was continuously observed without any deterioration through
the reaction; i.e. complex 1a was fully recovered even after
further addition of both Et2SiH2 (200 equiv) and 1ꢀoctene (200
equiv), and the expected hydrosilylated product Et2(Oct)SiH
was obtained as a major product. It was also confirmed that the
presence of Hg did not bring any deleterious effect on the
catalyst loading, yielding Et2(
nOct)SiH (76%), Et2(nOct)2Si
(1%), and Et( Oct)2SiH (2%) (entry 8). Complex 1c supported
n
by 1,4ꢀdiꢀtertꢀbutylbenzene also exhibited a similar catalytic
activity under the same reaction conditions, leading to the
formation of Et2(
nOct)SiH (76%), Et2(nOct)2Si (4%), and
Et( Oct)2SiH (1%) (entry 9)On the other hand, the reaction
n
proceeded rather slowly albeit with higher selectivity to form
2 | J. Name., 2012, 00, 1-3
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hydrosilylation reaction catalysed by 1a. Thus, a possibility of
To investigate the mechanism, DFT calculations have been
the formation of active heterogeneous species was excluded.
Figure 1. Summary of the structural changes of the reaction.
performed. In this calculation, a hydrosilylation reaction of 1ꢀ the reaction proceeds via a novel mechanism other than either
butene with Et2SiH2 was studied using a model catalyst [Ni( ꢀbond methathesis
3ꢀ conventional ChalkꢀHarrod mechanism or
CH2C(CH3)CH2)( Pr2(C6H4)] (1’), which possesses 1,4ꢀ mechanism, where bond formations between the Ni center and
6ꢀ1,4ꢀ
η
σ
η
i
diisopropylbenzene as an arene ligand.
the Si/H atom are expected (vide infra).
The reaction proceeds via ten transition states where the
overall reaction energy is exothermic by –12.9 kcal/mol.11
Figure 1 summarizes the important reaction steps of the
mechanism, and all stationary points are depicted in Figure S4
in the supporting information. The reaction starts with the
incorporation of 1ꢀbutene, which proceeds via dissociation of
Table 2. Mayer bond order of INT4, TS4-5, and INT5
INT4
0.50
0.01
–
TS4-5
0.01
0.45
0.24
0.49
0.42
INT5
–
Si–H1
C1-H1
Si–C2
Ni–Si
0.63
0.61
0.16
0.30
0.27
0.46
the arene ligand, to form
πꢀallyl olefin complex INT2. In the
Ni-H1
next step, Et2SiH2 is incorporated via the Si–H bond to form
σ
ꢀ
silane complex INT3. When going to INT4, the butene rotates
for the better access of the silane H1 atom to the butene C1
atom. Then, the H1 atom approaches the C1 atom (C1–H1 =
1.37 Å), and the Si–H1 bond is lengthened to be 2.05 Å in TS4-
5, supporting the late transition state with a weak Si–H1
bonding interaction. Accompanied with this geometrical
change, the methallyl C2 atom accesses the Si atom. As a
result, the Si–C2 bond length becomes 2.56 Å, indicating the
formation of a weak Si–C2 bonding interaction in TS4-5. The
single bond formation of the C1–H1 and Si–C2 bonds
completed in INT5; i.e. the C1–H1 and Si–C2 bond lengths are
1.18 Å and 2.01 Å, respectively. The energy barrier of this step
INT5 further undergoes successive Si–C2 and Ni–C3 bond
rotations to form INT10, in which the Ni atom possesses an
agostic interaction with a Si–H bond. The process includes five
transition states with relative energy barriers of 14 – 23
kcal/mol. Upon going to INT11 from INT10, the Si–H agostic
interaction dissociates from the Ni atom. As a result, this step is
endothermic by 14.8 kcal/mol and proceeds via TS10-11 with the
highest energy barrier (23.5 kcal/mol) through the whole
reaction steps. The free energy of activation is reasonably
comparable to the experimantal results, in which
hydrosilylation reactions smoothly proceed at room
temperature. In the structure of INT11, the Si–C2 bond is
slightly elongated by 0.08 Å to be 2.04 Å and the Ni–C2
become shortened (2.15 Å). In the next step, the C3 atom of the
butyl group approaches the Si atom. As a result, the Si–C3
distance becomes 2.89 Å, and the Si–C2 bond is simultaneously
further elongated to be 2.21 Å in TS11-12. The Si–C3 bond
formation and Si–C2 bond cleavage occur upon going from
TS11-12 to INT12 in which the Si–C3 bond length is 2.04 Å.
The energy barrier of this process is 22.7 kcal/mol. It is to be
noted that the Ni and Si atoms again keep a certain distance
(2.5 – 2.8 Å) through these reaction steps. After the dissociation
is 11.4 kcal/mol, and the free energy (
It is to be noted that the Ni–H and Ni–Si distances exhibit
significantly longer values (1.5ꢀ1.7 and 2.4ꢀ2.8 Å,
ꢀG) is 15.2 kcal/mol.
Å
respectively) than the normal Ni–H (1.39ꢀ1.54 Å)12,13 and Ni–Si
(ca. 2.2 Å)13,14 single bond lengths, through the Si–H bond
cleavage process. The Mayer bond order analysis also supports
the observed unique changes in the bonding nature along the
reaction (Table 2). Upon going to INT5 from INT4, the bond
order of the Si–H1 bond decreases, whereas the bond orders of
the C1–H1 and Si–C2 bonds increase. The rather small bond
orders of the Ni–H1 and Ni–Si bonds (0.30 and 0.16,
respectively) support no significant bonding interaction for
those atom pairs in INT5. These results strongly indicate that
of Et2(nBu)SiH and recoordination of the arene ligand, 1’ is
regenerated. We tentatively ascribe the observed unique
mechanism to the cationic nature of the Ni center of 1’; the
NBO charge of the Ni in 1’ is 0.32, whereas the Pt atom in
[Pt(SiH4)(PPH3)2], which is calculated to undergo facile
oxidative addition of a Si–H bond,15 exhibits negative NBO
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and the reaction selectivity are highly dependent on the type of
the arene ligands in this system. Thus, the precise design of the
reaction space using various arene ligands will be performed in
due course for the further improvement of this system.
This work was supported by the "Development of
Innovative Catalytic Processes for Organosilicon Functional
Materials" project (PL: K.S.) from the New Energy and
Industrial Technology Development Organization (NEDO).
charge of –0.57. Since the cationic Ni atom preferably keeps a
distance from the electropositive Si atom whose NBO charge is
1.31, the Si–H bond cleavage proceeds without any direct
participation of the Ni(II) atom. Another notable feature of this
process is the involvement of the allyl group as a nonꢀinnocent
ligand. Orbital analysis suggested the involvement of C2 (2p)
orbital for these steps. Figure 2a exhibits the 33th orbital of
INT4, which includes σꢀbonding interaction of Si (3s) and H1
(1s). This interaction is weakened, and the Si 3s orbital access
the C2 2p orbital to form bonding interaction upon going to
INT5 from INT4 (Figure 2b).
Notes and references
1
B. Marciniec, H. Maciejewski, C. Pietraszuk, P. Pawluć in
Hydrosilylation: A Comprehensive Review on Recent
Advances, Springer, Berlin, 2009.
2
B. Marciniec, Coord. Chem. Rev., 2005, 249, 2374; A. K.
Roy, Adv. Organomet. Chem., 2008, 55, 1; D. Troegel and J.
Stohrer, Coord. Chem. Rev., 2011, 255, 1440; Y. Nakajima
and S. Shimada, RSC Adv., 2015, 5, 20603.
3
4
5
J. Y. Wu, B. N. Stanzl and T. Ritter, J. Am. Chem. Soc., 2010,
132, 13214; A. M. Tondreau, C. C. H. Atienza, K. J. Weller,
S. A. Nye, K. M. Lewis, J. G. P. Delis and P. J. Chirik,
Science, 2012, 335, 567.
Figure 2. (a) 33th orbital of INT4 (b) 34th orbital of INT5. Insets show schematic
pictures of major orbital contributions.
M. Brookhart and B. E. Grant, J. Am. Chem. Soc., 1993, 115
2151; C. Chen, M. B. Hecht, A. Kavara, W. W. Brennessel,
B. Q. Mercado, D. J. Weix and P. L. Holland, J. Am. Chem.
Soc., 2015, 137,13244.
,
The involvement of the C2 2p orbital is also supported on
the Si–C3 bond forming step. Thus, as the Si atom access the
Y. Kiso, M. Kumada, K. Tamao and M. Umeno, J.
C3 atom, the
σꢀbonding interaction between C2 (2p) and Si
Organomet. Chem., 1973 50, 297; Y. Chen, C. SuiꢀSeng, S.
,
(3p) is concomitantly weakened upon going to INT12 from
INT11 (Figure S8). It is likely that the cooperative bond
forming and dissociating actions of the C2 atom compensate
the energy change along the reaction. As a result, the Si–H
cleavage and Si–C forming steps proceed with moderate energy
barriers.
Boucher and D. Zargarian, Organometallics, 2005, 24, 149;
L. B. Junquera, M. C. Puerta and P. Valerga,
Organometallics, 2012, 31, 2175; I. Buslov, J. Becouse, S.
Mazza, M. MontandonꢀClerc and X. Hu, Angew. Chem. Int.
Ed., 2015, 54, 14523.
V. Srinivas, Y. Nakajima, W. Ando, K. Sato and S. Shimada,
Catal. Sci. Technol., 2015, 5, 2081; V. Srinivas, Y. Nakajima,
W. Ando, K. Sato and S. Shimada, J. Organomet. Chem.,
DOI 10.1016/j.jorganchem.2016.02.025.
A. J. Chalk and J. F. Harrod, J. Am. Chem. Soc., 1965, 87, 16.
P.ꢀF. Fu, L. Brard, Y. Li and T. J. Marks, J. Am. Chem. Soc.,
1995, 117, 7157; G. A. Molander and J. A. C. Romero, Chem.
Rev., 2002, 102, 2161.
P. B. Glaser and T. D. Tilley, J. Am. Chem. Soc., 2003, 125,
13640.
6
In the proposed mechanism, agostic Si–H and C–H bond
interactions with the Ni atom play an important role. For
examples, agostic interactions of Si–H and C–H bonds
successfully stabilize the electron deficient Ni center in TS10-11
.
7
8
Furthermore, the agostic interactions facilitate the rotation of
Si–C2 and Ni–C3 bonds. Therefore, it is reasonable that the
system is not applicable to the hydrosilylation using tertially
silanes, which cannot form stable intermediates through the Si–
H agostic interaction after the Si–H bond cleavage. Indeed,
optimization of INT10 and TS10-11 using Et3SiH was not
successful due to their high instability. On the other hand, the
effect of the agostic interactions does not explain the low
reactivity of the catalytic system towards the hydrosilylation
using primary silanes. To clarify this point, a DFT study on
hydrosilylation of 1ꢀbutene with EtSiH3 was performed. It was
revealed that the rateꢀlimiting step including the agostic Si–H
bond dissociation proceeds via TS10ꢀ11* with a higher energy
barrier compared with that in the Et2SiH2 system by 1 kcal/mol
probably due to the strong agostic interaction between the less
sterically hindered EtSiH2 group and the Ni center. Indeed, the
Ni–Si and Ni–H bond lengths in the transition state structure
are 2.66 and 2.28 Å, respectively, which are 0.15 – 0.3 Å
shorter than those in TS10-11. The energy difference might be
one of the reasons for the low reactivity of monoalkylsilane in
this system.
9
10 J. Cámpora, M. del Mar Conejo, M. L. Reyes, K. Mereiter
and E. Passaglia, Chem. Commun., 2003, 78.
11 Figures S2 and S3 in the supporting information show the
details.
12 W. Chen, S. Shimada, M. Tanaka, Y. Kobayashi and K. Saigo
J. Am. Chem. Soc., 2004, 126, 8072; J. A. Hatnean, R. Beck,
J. D. Borrelli and S. A. Johnson, Organometallics, 2010, 29
6077; P. Fischer, K. Gӧtz, A. Eichhorn and U. Radius
Organometallics, 2012, 31, 1374.
,
13 T. Zell, T. Schaub, K. Radacki and U. Radius, Dalton Trans.,
2011, 40, 1852.
14 S. N. MacMillan, W. H. Harman and J. C. Peters, Chem. Sci.,
2014, 5, 590.
15 S. Sakaki, N. Mizoe, M. Sugimoto and Y. Musashi, Coord.
Chem. Rev., 1999, 190-192, 933.
In summary, this study demonstrated the utility of cationic
nickel allyl complexes as an olefin hydrosilylation catalyst. It is
to be noted that the system exhibits high selectivity towards
reactions using dialkylsilanes. The DFT study provides the
evidence for the novel mechanism, which is assisted by the
nonꢀinnocent allyl ligand for both the Si–H bond cleavage and
the Si–C bond formation steps. Such a mechanism is different
from either the conventional oxidative addition/reductive
elimination or σꢀbond metathesis process. The catalytic activity
4 | J. Name., 2012, 00, 1-3
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Graphical and textual abstract for the Table of Contents
Cationic nickel allyl complexes catalyse selective
monohydrosilylation of αꢀolefins with secꢀsilanes via a
unique mechanism assisted by a nonꢀinnocent allyl ligand.
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