Carboxylic acids promoted speiers catalyst for the hydrosilylation of styrene with triethoxysilane: Activity, selectivity, and mechanism

Article abstract of DOI:10.1080/10426507.2013.855772

(Equation present) Catalytic performances of Speier's catalyst with carboxylic acids were studied in the hydrosilylation reaction of styrene with triethoxysilane. The effect of carboxylic acids (RCOOH) with different pKa values and size was investigated, and the temperature and n(COOH)/n(Pt) ratios were also studied. The catalytic activity of the hydrosilylation reaction is related to the acid strength of the carboxylic acid, temperature, and n(COOH)/n(Pt) ratio, while the selectivity for the β-adduct is related to the size of carboxylic acid and the n(COOH)/n(Pt) ratio. Moreover, the influence of carboxylic acid on the hydrosilylation reaction was explained with the help of density functional theory calculations, and a reasonable mechanism was proposed. 2014 Copyright Taylor & Francis Group, LLC.

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Carboxylic Acids Promoted Speier’S
Catalyst for the Hydrosilylation of
Styrene with Triethoxysilane: Activity,
Selectivity, and Mechanism
a
a
a
a
Shunmin Ding , Shengjun Deng , Ning Zhang & Longhua Yang
a
Department of Chemistry, Nanchang University, Nanchang, 330031,
China
Accepted author version posted online: 23 Jan 2014.Published
online: 12 Jun 2014.
To cite this article: Shunmin Ding, Shengjun Deng, Ning Zhang & Longhua Yang (2014) Carboxylic
Acids Promoted Speier’S Catalyst for the Hydrosilylation of Styrene with Triethoxysilane: Activity,
Selectivity, and Mechanism, Phosphorus, Sulfur, and Silicon and the Related Elements, 189:6, 803-811,
DOI: 10.1080/10426507.2013.855772
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Phosphorus, Sulfur, and Silicon, 189:803–811, 2014
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Taylor & Francis Group, LLC
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.855772
CARBOXYLIC ACIDS PROMOTED SPEIER’S CATALYST
FOR THE HYDROSILYLATION OF STYRENE WITH
TRIETHOXYSILANE: ACTIVITY, SELECTIVITY,
AND MECHANISM
Shunmin Ding, Shengjun Deng, Ning Zhang,
and Longhua Yang
Department of Chemistry, Nanchang University, Nanchang 330031, China
GRAPHICAL ABSTRACT
Abstract Catalytic performances of Speier’s catalyst with carboxylic acids were studied in the
hydrosilylation reaction of styrene with triethoxysilane. The effect of carboxylic acids (RCOOH)
with different pKa values and size was investigated, and the temperature and n(COOH)/n(Pt)
ratios were also studied. The catalytic activity of the hydrosilylation reaction is related to
the acid strength of the carboxylic acid, temperature, and n(COOH)/n(Pt) ratio, while the
selectivity for the β-adduct is related to the size of carboxylic acid and the n(COOH)/n(Pt)
ratio. Moreover, the influence of carboxylic acid on the hydrosilylation reaction was explained
with the help of density functional theory calculations, and a reasonable mechanism was
proposed.
Keywords Hydrosilylation; carboxylic acids; DFT calculations
INTRODUCTION
Hydrosilylation, a reaction for Si–C bond formation, is very important for the syn-
thesis of organofunctional silanes which have found many commercial applications, among
Received 23 February 2013; accepted 11 October 2013.
Address correspondence to Ning Zhang, Department of Chemistry, Nanchang University, Nanchang 330031,
China. E-mail: nzhang.ncu@163.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
8
03

804
S. DING ET AL.
Scheme 1
others as adhesion promoters for inorganic-material–polymer systems and modifiers of min-
1
,2
eral surfaces. Generally, Group VIII metals and their complexes are known to be good
3
catalysts for the title reaction, in particular the Pt-based catalysts. As a Pt-based catalyst,
4
Speier’s catalyst is efficient and commonly employed in industrial production. Neverthe-
less, it is still confronted with two main problems that are the relatively long induction
time and low selectivity. Recently, a number of studies have revealed that the introduction
of activators such as alkadienes, norbornenes, and cycloolefins into the Pt catalyst systems
5
–10
can improve the catalytic activity and selectivity for hydrosilylation of alkenes.
More
recently, Lai, et al. demonstrated that carboxylic acids could significantly improve the
11
catalytic activity and selectivity of Speier’s catalyst for the hydrosilylation of alkenes.
However, the reason for the influence of carboxylic acid on the catalytic performance of
Speier’s catalyst is still not clear.
In this work, we endeavored to provide insight into the role of carboxylic acids in the
Speier’s catalyst catalyzed hydrosilylation of alkenes. The model reaction, hydrosilylation
of styrene with triethoxysilane, was selected to check the catalytic performance (Scheme 1).
The pKa values and size of carboxylic acids (RCOOH), temperature, and n(COOH)/n(Pt)
ratio were all investigated to reveal the influence on the hydrosilylation reaction. The effect
of carboxylic acids was further studied with the help of theoretical calculations performed
with the density functional theory (DFT) approach. Furthermore, a reasonable mechanism
for the reaction was proposed.
RESULTS AND DISCUSSION
Effect of n(COOH)/n(Pt) on the Hydrosilylation Reaction
Speier’s catalyst solutions with differing acid contents were applied to the hydrosi-
◦
lylation reaction at 60 C, and the results are listed in Table 1 (using CH3COOH as an
example). Speier’s catalyst without the addition of acid shows a 100% styrene conver-
sion and 65% β-adduct selectivity after 360 min. When introducing the carboxylic acids
to Speier’s catalyst, the catalytic activity and selectivity of the β-adduct produced are
11–13
obviously improved, which is consistent with previous reports.
Table 1 also reveals
that the catalytic activity and selectivity of the β-adduct increase with the acid content.
This is probably attributed to the increase of Pt–carboxylic acid species, which might be

CARBOXYLIC ACIDS PROMOTED SPEIER’S CATALYST
805
Table 1 Effect of n(COOH)/n(Pt) ratios on the hydrosilylation reaction (using CH3COOH as an example)
β-adduct
selectivity (%)
α-adduct
selectivity (%)
Phenylethane
selectivity (%)
a
Entry
n(COOH)/n(Pt)
t100 (min)
n(β)/n(α)
1
2
3
4
5
6
7
8
0
360
20
7
5
4.5
4
65.4
72.4
73.9
75.6
76.4
80.8
92.7
92.3
32.0
24.7
23.0
20.9
20.1
15.6
4.0
2.6
2.9
3.1
3.5
3.5
3.6
3.3
3.6
2.04
2.93
3.21
3.62
3.80
5.18
23.17
19.60
100: 10
100: 8
100: 6
100: 4
100: 2
100: 1
150: 1
2
2
4.4
◦
Reaction conditions: n(Pt)/n(styrene) = 1:4000, n((EtO)3SiH)/n(styrene) = 1.1:1, 60 C.
a
The time needed for 100% styrene conversion.
more active than the Pt species in Speier’s catalyst (see the theoretical calculation and
proposed mechanism section). It is noteworthy that the hydrosilylation reaction rate and
selectivity of β-adduct remain at steady levels when n(COOH)/n(Pt) is above 100:1, in-
dicating that the Pt–carboxylic acid species is saturated at n(COOH)/n(Pt) = 100:1. In
addition, to better examine the influence of other factors on the catalytic activity and se-
lectivity, the n(COOH)/n(Pt) ratio of 100: 8 was selected, due to its moderate reaction rate
(
Table 1).
Effect of Temperature on the Hydrosilylation Reaction
The effect of temperature on catalytic activity and selectivity of the hydrosilylation
reaction is shown in Figure 1 (using Speier’s catalyst with acetic acid as an example).
With the increase of temperature, the time needed for 100% styrene conversion (t100) is
shortened, while the selectivity for the β-adduct is kept at around 73%. This indicates
that the temperature promotes the catalytic activity but has only a slight influence on the
β-adduct selectivity (Figure 1(b)).
Figure 1 Effect of temperature on the hydrosilylation reaction: (a) catalytic activity and (b) selectivity to β-adduct.

806
S. DING ET AL.
Table 2 Effect of pKa values and size of carboxylic acids on the hydrosilylation reaction
t100 a
β-adduct
α-adduct
Phenylethane n(β)/
Entry Carboxylic acid
pKa
Size(Å2)
(min) selectivity (%) selectivity (%) selectivity (%) n(α)
1
2
3
4
5
6
7
8
HCOOH
Benzoic acid
CH3COOH
CH3(CH2)3COOH 4.81 7.821 × 2.453
CH3(CH2)4COOH 4.84 8.705 × 2.459
CH3(CH2)5COOH 4.89 9.748 × 2.452
3.77 3.085 × 2.011
4.19 7.094 × 4.933
4.76 4.143 × 2.455
4
5
7
8
8
8
360
360
69.1
82.3
73.9
77.5
79.4
81.7
65.4
75.2
28.0
14.2
23.0
19.2
17.0
15.0
32.0
22.4
2.9
3.5
3.1
3.3
3.6
3.3
2.6
2.4
2.47
5.80
3.21
4.04
4.67
5.47
2.04
3.35
CH3COONa
◦
Reaction conditions: n(Pt)/n(styrene) = 1:4000, n((EtO)3SiH)/n(styrene) = 1.1:1, 60 C, n(COOH)/n(Pt) =
00:8.
1
aThe time needed for 100% styrene conversion.
Effect of pKa values and Size of Carboxylic Acids on the Hydrosilylation
Reaction
Table 2 summarizes the catalytic results of Speier’s catalyst with carboxylic acids
with different pKa values and size. It can be seen that t100 decreases from 8.0 to 4.0 min
when the pKa ranged from 4.89 to 3.77, indicating that increasing the acid strength of
the carboxylic acids can improve the catalytic activity of the hydrosilylation reaction.
From Table 2, we can also find that the high selectivity for the β-adduct is obtained.
The large-sized carboxylic acid indicates that the size of carboxylic acid has a significant
influence on the selectivity to β-adduct. It should be pointed out that the different size
acids CH3(CH2)3COOH, CH3(CH2)4COOH, and CH3(CH2)5COOH that possess similar
pKa values show the same t100 (8.0 min) but display different selectivities for the β-adduct
(77.5%, 79.4%, 81.7%). This suggests that the acid strength of the carboxylic acid has
a negligible influence on the selectivity for the β-adduct, and the size of carboxylic acid
has a slight influence on the catalytic activity. Consequently, it can be concluded that the
improvement of the catalytic activity and selectivity are attributed to the acid strength and
size of carboxylic acids, respectively. This result might be helpful in the design of an
efficient promoter for the Speier’s catalyst. Noticeably, upon the addition of CH3COONa to
Speier’s catalyst, the catalytic activity is not promoted (Table 2). The above result reveals
that the activated hydrogen of the promoter is important for hydrosilylation activity. To
obtain further insight into the influence of carboxylic acid, theoretical calculations were
performed.
Theoretical Calculation and Proposed Mechanism
The Chalk–Harrod mechanism is commonly proposed to explain the Pt-catalyzed
hydrosilylation of alkenes,1
4–16
which involves elementary steps such as oxidative
addition of Si–H to the metal center, insertion of the alkene into the M–H bond and
reductive elimination of the Si–C product. Based on this mechanism, we investigated the
role of carboxylic acid (HCOOH was taken as an example) by a theoretical calculation
method.

CARBOXYLIC ACIDS PROMOTED SPEIER’S CATALYST
807
Scheme 2 Proposed mechanism for hydrosilylation of styrene with triethoxysilane catalyzed by (a) Speier’s
catalyst and (b) Speier’s catalyst with carboxylic acid.

808
S. DING ET AL.
i
Figure 2 UV–VIS absorption of samples: (a) PrOH, (b) HCOOH, (c) Speier’s catalyst, (d) Speier’s catalyst with
HCOOH, and (e) addition of HSi(OEt)3 in the Speier’s catalyst with HCOOH.
In this calculation, the Pt(0)17–18 was adopted as the model catalysts since the low-
1
5,19–21
valence Pt species are generally considered as the active sites of Speier’s catalyst.
According to the theoretical calculations, a reasonable mechanism is proposed and shown in
Scheme 2. Scheme 2 depicts four elementary steps: (a) the Pt interacted with carboxylic acid
to form the Pt–carboxylic acid intermediated I’1 with the reaction energy δG = −755.3218
kJ/mol, which was supported by the shift of UV–VIS absorption (Figure 2) of the Pt species
from 245 to 215 nm);1
7,18,22
(b) the addition of triethoxysilane to the Pt center of I’1 (I’2)
(
with δG = −11.6715 kJ/mol, which is consistent with the slight shift of UV–VIS absorption
as compared with I’1; (c) insertion of the styrene into the Pt–H bond of I’2 produces I’3
and I’4 with δG = −39.8462 J, −10.5420 kJ/mol, respectively; (d) reductive elimination
of α- and β-adducts. In step (d), the Pt species may eliminate in the forms of Pt(0) or I’1.
As revealed by the theoretical calculation results, the elimination of Pt(0) exhibits a higher
reaction energy than the elimination of I’1 (Scheme 2). This suggests that the Pt species in
step (d) is probably eliminated in the I’1 form. Moreover, the Si–C reductive elimination of
Scheme 2b displays a lower reaction energy than that of Scheme 2a, where carboxylic acids
are not introduced. It may explain why the carboxylic acids can significantly improve the
catalytic activity of Speier’s catalyst. To get the relationship between the size of carboxylic
acid and the selectivity of β-adduct, step (c) was examined. The steric hindrance of I’2
increases with the size of carboxylic acid, which facilitates the formation of I’4 when
inserting styrene. This explains why using the larger sized carboxylic acid can result in
higher selectivity β-adduct.
CONCLUSION
In this work, the effect of carboxylic acids with different pKa values and size on the
Speier’s catalyst-catalyzed hydrosilylation of alkenes was carried out on the hydrosilylation
of styrene with triethoxysilane at different temperatures and n(COOH)/n(Pt) ratios. The

CARBOXYLIC ACIDS PROMOTED SPEIER’S CATALYST
809
results show that the acid strength of the carboxylic acid is related to the catalytic activity
of the hydrosilylation reaction, the size of carboxylic acid has a significant influence on
the selectivity for the β-adduct, n(COOH)/n(Pt) affects both of the catalytic activity and
selectivity, and the temperature promotes the catalytic activity but has only a slight impact on
selectivity to β-adduct. Additionally, DFT calculations were performed to better understand
the influence of carboxylic acid on the hydrosilylation reaction, and a reasonable mechanism
was proposed. As revealed by the proposed mechanism, step (d) displays a lower reaction
energy than the Si–C reductive elimination step of Speier’s catalyst without carboxylic
acids. It may be the reason that the carboxylic acids can significantly improve the catalytic
activity. In step (c), the large size of carboxylic acid facilitates the formation of I’4. This
explains why using the larger sized carboxylic acid can result in higher β-adduct selectivity.
These results might be helpful in the design of novel Pt catalysts with high catalytic activity
and selectivity for the hydrosilylation of alkenes.
EXPERIMENTAL
Materials
All reactants were obtained commercially and used without further purification.
Styrene (AR), H2PtCl6·6H2O (AR), and phenylethane (AR) were purchased from
Sinopharm Chemical Reagent Co., Ltd. HSi(OEt)3 (99%) was purchased from Qufu
Chenguang Chemical Co., Ltd. All carboxylic acids (AR) were purchased from Aladdin
Reagent Co., Ltd. Speier’s catalyst was prepared by dissolving chloroplatinic acid (1 g) in
isopropanol (50 mL) at room temperature for 7 days.
Catalyst Characterization
Absorption spectra were recorded on UV–VIS spectrophotometer (“Hitachi” U4100,
Japan) using a 1.0 cm quartz cell in the range of 500–200 nm (n-hexane as solvent and
reference).
Products Characterization
GC–MS confirmation was carried out using an Agilent Technologies 7890A network
GC system equipped with a mass-selective detector (MSD) model 5975C network, and a
3
0 m × 0.25 mm HP-5 MS fused-silica capillary column (0.25 μm film thickness). The test
−1
◦
conditions: split, 50: 1; flow, 1 mL min ; injector temperature, 260 C; column temperature
◦
◦
−1
◦
program: 50 C, held for 1 min; increased at a rate of 15 C min up to 260 C, held for
0 min.
1
1
H-NMR spectra were recorded on a Bruker AV400 MHz spectrometer using TMS
23,24
as an internal standard in CDCl3:
1
β-adduct, H-NMR (CDCl3, 400 MHz) δ (ppm): 1.01 (t, J = 9.0 Hz, 2H, Si-CH2),
1
.25 (t, J = 7.0 Hz, 9H, CH3), 2.75 (t, J = 8.6 Hz, 2H, Ph-CH2); 3.86 (q, J = 7.0 Hz, 6H,
O-CH2), 7.18–7.29 (m, 5H, Ph).
1
α-adduct, H-NMR (CDCl3, 400 MHz) δ (ppm): 1.18 (t, J = 7.0 Hz, 9H, CH3), 1.42
(
d, J = 8.0 Hz, 3H, CH3) 2.34 (q, J = 8.0 Hz, 1H, Ph-CH); 3.73 (q, J = 7.0 Hz, 6H,
O-CH2), 7.12–7.19 (m, 5H, Ph).
1
Phenylethane, H-NMR (CDCl3, 400 MHz) δ (ppm): 1.16 (t, J = 8.0 Hz, 3H, CH3),
2.66 (q, J = 8.0 Hz, 2H, Ph-CH2), 7.13–7.27(m, 5H, Ph).

810
S. DING ET AL.
Catalytic Test
Typical hydrosilylation reaction procedures were as follows: styrene (20 mmol) and
Pt-catalysts (0.005 mmol; Speier’s catalyst with or without carboxylic acids) were combined
and stirred at reaction temperature for 10 min in a 20-mL round flask equipped with a
mechanical stirrer. Then, triethoxysilane (22 mmol) was added to the reaction mixture. The
products were detected by a GC with FID detector.
Conversion of styrene and selectivity of all products were calculated with the follow-
ing equations:
ꢀ
ꢁ
moles of styrene consumed
moles of styrene initial
Styrene conversion(%) =
× 100;
ꢀ
ꢁ
moles of phenylethane produced
moles of styrene consumed
Phenylethane selectivity(%) =
× 100;
ꢀ
ꢁ
moles ofβ − adduct produced
ß − adduct selectivity(%) =
× 100
moles of styrene consumed
ꢀ
ꢁ
Aβ
Aα+Aβ
=
[100 − Phenylethane selectivity(%)] × ;
;
α−adduct selectivity(%) = 100 − Phenylethane selectivity(%)ß − adduct selectivity(%).
Aα: the GC area of α-adduct, Aβ: the GC area of β-adduct, the response factors of
the two isomers (α- and β-adduct) were the same in the GC, which was verified by proton
NMR integration. The styrene and phenylethane have their own standard curves by the
standard samples.
All results were reproducible within experimental error.
Theoretical Calculation
Geometries of reactants, transition states, and products were optimized via DFT
calculations using the Becke3LYP (B3LYP) functional within the Gaussian 09 suite of ab
initio programs.2
5–28
For Pt atom, the effective core potential basis set Lanl2dz (Los Alamos
ECP plus double-ζ) was used.2
9–31
For Si, O, C, and H atoms atoms, the all-electron split
valence basis set 6-31G (d, p) was used. Frequency calculations at the same level of theory
have also been performed to identify all of the stationary points as minima. The energy
difference between products and reactants (δG) is calculated with the following equation:
δG = energy of product – energy of reactants.
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