Synthesis of borasiloxanes by oxidative hydrolysis of silanes and pinacolborane using Cu3(BTC)2 as a solid catalyst

Article abstract of DOI:10.1039/c7cc05221a

A convenient method for the synthesis of borasiloxanes from silanes and pinacolboranes using Cu₃(BTC)₂ as a heterogeneous catalyst in acetonitrile at 70 °C is reported. This procedure is more convenient than Ru and Pd based homogeneous catalysts because it avoids the use of noble metals, easy handling of starting materials and the catalyst can be reused.

Full text of DOI:10.1039/c7cc05221a

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DOI: 10.1039/C7CC05221A
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Synthesis of Borasiloxanes by Oxidative Hydrolysis of Silanes and
Pinacolborane using Cu3(BTC)2 as Solid Catalyst
Amarajothi Dhakshinamoorthy*a, Abdullah M. Asiri,b Patricia Concepcion,c Hermenegildo
Received 00th January 20xx,
Accepted 00th January 20xx
Garcia*b,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
case silanols can form easily disiloxanes and the vinyl moiety of
the boronated reagent is not incorporated in the final product.
In another method, the synthesis of Me3SiOBpin by the reaction
of Me3SiOH and B2pin2 in the presence of [Ir(OMe)(cod)]2 (cod
= 1,5-cyclooctadiene) as catalyst was reported.18 Alternatively,
preparation of borasiloxanes was also carried out by using
palladium acetate as catalyst for the dehydrogenative O-
borylation of a variety of silanols with easily available
diborons.19 Later, preparation of poly(borasiloxane) was
promoted by polycondensation of Ph2Si(OH)2 with ArBH2 (Ar =
mesityl) in the presence of rhodium or palladium catalysts at
room temperature,3 but again silanediols are highly reactive
and prone to undergo self-condensation to polysiloxanes. On
the other hand, the reaction of hydrosilanes with
bisboryloxide/boroxine led to the selective formation of
borasiloxanes in the presence of Mo, W and Fe complexes.20
More related to the present study, it has been recently reported
an alternative protocol for the synthesis of borasiloxanes using
Ru-catalysed reaction of hydroboranes (or boronic acids) with
silanes and water at 125 oC in toluene.21
A convenient method for the synthesis of borasiloxanes from
silanes and pinacolboranes using Cu3(BTC)2 as heterogeneous
catalyst in acetonitrile at 70 oC is reported. The procedure is more
convenient than others based on the use of Ru and Pd
homogeneous catalysts, since in addition to avoiding noble metals
the starting materials are easy to handle and the catalyst can be
recovered from the reaction mixture and reused.
Borasiloxanes are attracting increasing attention due to the
unique properties of derived polymers, including heat
resistance and notable chemical stability.1-3 In addition,
borasiloxanes are also intermediates in organic synthesis and
they have application as electrolytes and in material chemistry.4
This type of compounds can be prepared by reaction of boranols
11
with chloro,5-8 alkoxysilanes9 or silanols10,
(Scheme 1).
Alternatively, borasiloxanes can also be prepared by reaction
between silanols with borane derivatives12-16 (Scheme 1).
However, the above mentioned conventional methods suffer
from some drawbacks, such as the use of moisture-sensitive
starting materials, high reaction temperature, low to moderate
selectivity to the wanted products and formation of undesired
byproducts such as HCl, water and disiloxanes. Hence, there is
still room for the development of clean and selective syntheses
of borasiloxanes using readily available starting materials. In
this regard, a more convenient approach is the catalytic three
component reaction of silanes, boranes and water, whereby
hydrogen is, in principle, the only by-product formed and the
substrates are stable under ambient conditions in the absence
of catalyst.
X=
X=
+
R
Cl, -OR, -OH
R2B-OH
3Si-O-BR2
3Si-O-BR2
R
R
3SiX
+
R2B-X
B
H
R
Cl,
-OH,
3SiOH
3SiOH
3SiOH
+
R
3Si-O-Bpin
R
2pin2
R
3Si-O-Bpin
+
H
2O
+
R
HBpin
Scheme 1. Methods for the synthesis of borasiloxanes.
For the sake of sustainability and affordability, it would be
important to develop other catalysts for this process, based on
abundant metals and also to develop heterogeneous catalyst
that can simplify the work-up of the reaction mixture and
eventually even allow them to be reused. In this context, metal
organic frameworks (MOFs) have shown a wide range of
Marciniec and co-workers have reported the versatility of
Ru complexes to produce borasiloxanes by the reaction
between silanols with vinylboronates in high yields,17 but in this
applicability as solid catalysts in liquid phase reactions under
a.School of Chemistry, Madurai Kamaraj University, Tamil Nadu, India 625 021..
b.Centre of Excellence for Advanced Materials Research, King Abdulaziz University,
Jeddah, Saudi Arabia.
23
conditions compatible with their structure.22,
Among the
various advantages of MOFs as solid catalysts, those that have
been shown to be more important are their large surface area,
accessibility to the active sites and their versatility in the
synthesis and design of these materials, allowing the selection
of these components that are suitable as active sites.
cInstituto Universitario de Tecnología Química CSIV-UPV, Universitat Politècnica de
València, Av. De los Naranjos s/n, 46022, Valencia, Spain.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Among the MOFs that have shown high catalytic activity in Table 1. Hydrolytic coupling of
a large variety of reactions, one that has been frequently reaction conditions.a
studied for a wide range of reactions is Cu3(BTC)2 (BTC: 1,3,5-
1
with HBpin uVniedweArrtivclaerOionluinse
DOI: 10.1039/C7CC05221A
benzenetricarboxylate).24-28 This MOF is constituted by dimeric
Cu2 units with paddle wheel shape forming the nodes
coordinated to the carboxylic groups of BTC. The structure of
Cu3(BTC)2 defines large cages of 11, 16 and 6 Å diameter.29 One
Run Catalyst
Time Conversion Selectivity
of the positions of each octahedrally coordinated Cu atom is not
compromised with the crystal structure and it is typically
occupied by water or solvent molecules that can be exchanged
with reagents and substrates under reaction conditions.
Recently, we have reported that Cu3(BTC)2 can promote the
dehydrogenative coupling of silane with alcohols30 and later,
the hydrogenation of acetophenone to 1-phenylethanol by
silanes as reducing agents.31 Continuing with this research and
considering the current interest of borasiloxanes previously
commented, it was of interest to test the activity of Cu3(BTC)2
to promote the hydrolytic coupling of silanes and pinacolborane
(HBpin). In the present manuscript, it would be presented that
Cu3(BTC)2 is also a suitable catalyst for the selective formation
of borasiloxanes by oxidative hydrolysis of silanes followed by
coupling with boranes exhibiting wide scope. It was observed
that the material becomes partly deactivated during the course
of this coupling, probably due to the partial reduction of Cu2+ to
Cu+ by HBpin.
(h)
1
(%)b
(%)b
2
3
-
-
-
-
10
3
-
67
4
-
1
-
4
4
4
4
4
4
4
1
4
4
6
6
6
4
4
-
-
c
2
3
4
5
6
7
8
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
10
40
70
12
100
10
100
85
43
-
100
100
100
90
97
100
33
96
100
-
d
e
e,f
g
h
9
Cu(NO3)3.3H2O
Cu2O
MIL-101(Cr)
MIL-53(Al)
Fe(BTC)
10
11
12
13
14
15
-
-
-
-
-
-
6
14
-
-
Cu/G
Cu2O/G
100
100
-
In the first stage of our work, dimethylphenyl silane (
HBpin were selected to determine their hydrolytic coupling to
the corresponding borasiloxane ( ). A blank control experiment
between and HBpin in the absence of any catalyst in
1) and
aReaction conditions:
1 (0.5 mmol), HBpin (0.5 mmol), catalyst
(30 mg), CH3CN (2 mL), 70 oC, nitrogen atmosphere;
bDetermined by GC; cAt room temperature; dToluene as solvent;
2
1
eMoist ambient atmosphere; With 0.5 mmol of B2pin2; With
0.1 mL of pyridine; hReaction with 0.5 mmol of
dimethylphenylsilanol.
f
g
acetonitrile at 70 oC showed no product at all (entry 1, Table 1).
In contrast, Cu3(BTC)2 was able to promote the oxidative
coupling in different conversion and selectivity depending on
the material and reaction conditions. The observed results are
summarized in Table 1. As it can be seen in this Table 1,
In contrast to the lack of activity of MOFs not containing Cu,
a soluble Cu salt, namely, Cu(NO3)3.3H2O exhibits a remarkable
activity in promoting the hydrolytic three components coupling,
Cu3(BTC)2 promotes the formation of 2 over 10 % conversion at
although conversion of
1 was not complete and a small amount
room temperature after 4 h (entry 2, Table 1). It was observed
of was also detected under the optimized conditions after 4 h
3
that heating at 70 oC, either in toluene or particularly in
(entry 9, Table 1). On the other hand, Cu2O displayed much
lower activity for borasiloxane formation compared to
Cu(NO3)3.3H2O, probably due to the lack of porosity, lower
surface area or the absence of Cu2+ (entry 10, Table 1).
Furthermore, Cu3(BTC)2 was more efficient than Cu metal or
Cu2O supported on graphene (entries 14-15, Table 1). These
data indicate that Cu3(BTC)2 is the most active catalyst among
the various Cu-containing catalysts studied here in promoting
acetonitrile, increased the conversion of
1 (entries 3 and 4,
Table 1). Conversion of was 70 % under ambient air
1
atmosphere, probably due to the unwanted influence ambient
moisture. It will be commented below that the reaction rate is
negatively influenced by water. A very low conversion of
%) was noticed when the reaction is performed with diborane
B2pin2 and enhanced selectivity towards unwanted disiloxane
is observed after 4 h under identical conditions (entry 5, Table
2). A complete conversion of was achieved with 97%
selectivity of using Cu3(BTC)2 as catalyst in acetonitrile under
1 (12
3
the oxidative hydrolytic coupling of
borasiloxane.
1 and HBpin to form
1
2
In agreement with the presumed reaction mechanism for
this hydrolytic coupling involving as the first step hydrolysis of
silane to silanol, dimethylphenylsilanol also gives rise to the
o
nitrogen atmosphere at 70 C after 4 h (entry 6, Table 1). The
other product that could be detected under these optimal
conditions was disiloxane
3
formed by the oxidative coupling of
formation of
less selectivity due to the formation of
2
in the presence of Cu3(BTC)2, although with much
by condensation as the
1
.
3
To elucidate the role of Cu3(BTC)2 as catalyst in this reaction,
predominant product in 67% selectivity after 1 h (entry 8, Table
1). On the other hand, the evolved gas from the reaction
mixture arising by the hydrolytic coupling of 1 with HBpin under
the optimized reaction conditions was collected by a micro
syringe and analyzed by gas chromatography. This analysis
confirmed the formation of hydrogen, being consistent with the
formation of silanol as an intermediate from silane. It seems
that the presence of large amount of silanol at the initial
a control experiment was performed by adding pyridine to
Cu3(BTC)2 prior to check its activity. It was observed that
pyridine quenched almost the catalytic activity of Cu3(BTC)2,
suggesting that Lewis acidity of Cu2+ ions plays a key role in this
reaction (entry 7, Table 1). The catalytic activity of Cu3(BTC)2
sharply contrasts with the lack of activity of other MOFs such as
commercially available Fe(BTC), MIL-53(Al) and MIL-101(Cr) in
spite that it is known that some of these MOFs exhibit catalytic
activity in many different reaction types requiring Lewis acidity
(entries 11-13, Table 1).
reaction time does not favour achieving high selectivity of
due to the tendency of silanols to undergo condensation.
2,
2 | J. Name., 2012, 00, 1-3
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According to the reaction shown in Scheme 1, the formation Cu3(BTC)2 catalyst occurs. It is well known that boranes can be
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of
2 requires the presence of water as reagent. In the present strong reducing agents and this colour change of Cu (BTC) is
3 2
DOI: 10.1039/C7CC05221A
case, it is believed that the amount of water for the reaction compatible with the partial reduction of Cu2+ to Cu+. However,
must come from the solvent, from the ambient or from the no boranol was detected under this experimental condition.
moisture adsorbed on the catalyst. It should be noted that the
Accordingly, the most likely reaction mechanism of the
water content in acetonitrile (~1 wt%) and that of Cu3(BTC)2 oxidative coupling appears to involve the combined activity of
(See TGA in Fig.S1) is more than enough than the water needed Cu(II) as Lewis acid activating water and silanol with that of Cu(I)
to meet the stoichiometry of the reaction under the present activating borane. Silanol will be an intermediate that will react
conditions.
promptly to form the desired product. This reaction illustrates
To address the origin of water molecules, an additional the process that is compatible with the colour change observed
experiment was carried out in where Cu3(BTC)2 was submitted in the catalyst during the reaction, indicating the presence of
o
to dehydration by evacuation at 150 C under vacuum for 3 h. Cu+, and the recovery of the characteristic Cu2+ visual
Then, the activated Cu3(BTC)2 was used as catalyst to promote appearance when the reaction completed.
the reaction under optimized conditions. It was observed that
In order to understand the role of Cu3(BTC)2 in this reaction,
the evacuated Cu3(BTC)2 catalyst exhibited a clear induction a series of FT-IR spectroscopic studies were performed in the
period in the initial reaction time that can be interpreted as the presence of both
1 and HBpin as reactants from room
time required to regain water on the active sites of Cu3(BTC)2 to temperature to 70 oC. In the pre-reacted sample a band at 3537
promote the hydrolytic coupling. Figure 1 shows the temporal cm-1 is observed, which could be associated to the hydroxy
profile for the conversion of
1
under optimized reaction groups of (Cu2+-OH) species (Figure 2a). XPS and FT-IR spectra
conditions catalyzed by Cu3(BTC)2 and the thermally evacuated using NO as a probe molecule also agree with the existence of
Cu3(BTC)2 sample. Moreover, if an additional amount of water Cu2+-OH species (Figs. S2 and S3). This Cu2+-OH band clearly
is added on purpose to the reaction mixture at initial reaction disappeared after the adsorption of
1
(Figure 2b). Besides these
time, then, also an induction time in the conversion of
1 was characteristic bands of 1
, two new bands appeared at 2044 cm-
observed, indicating again that too much water can have a 1 and 3627 cm-1. The IR band at 2044 cm-1 corresponds to Cu-H
negative effect, due to the blocking of active sites. This species,32 while the band at 3627 cm-1 is characteristic for
explanation is also in agreement with the observation that silanol groups. Thus one can assume that the Si-H bond in
performing the reaction under nitrogen atmosphere results in converted to Si-OH, probably involving Cu2+-OH species,
faster conversion and higher selectivity of than the reaction resulting in the formation of Cu-H species. Hence, a control
1 is
2
performed under ambient conditions, where moisture is experiment was performed in where water and
1
were co-
present. Thus, an excess or defect of water appears to influence adsorbed on the same catalyst. After adsorption of
1, heating to
o
negatively the initial reaction rate, the ambient equilibrated 70 C led to the appearance of new IR bands at 1284, 1188,
Cu3(BTC)2 being the most efficient material to promote the 1029, 818 and 797 cm-1 attributable to the disilanoxy compound
hydrolytic coupling.
3 (see also Figs. S4 and S5). When after adsorption of 1, co-
adsorption of HBpin was performed and then the solid heated,
besides the bands precisely indicated corresponding to
100
compound 3
, a new band at 976 cm-1, not observed before was
80
60
40
20
0
recorded and attributed to Si-O-B bonds. In parallel to the new
IR bands, the IR band at 2044 cm-1 decreased in its intensity at
increasing temperature which may be related to hydride
recombination to give H2 formation.
0,2
0,2
0
1
2
3
4
Time (h)
Figure 1. Time-conversion plot for the conversion of 1 under optimal
reaction conditions (black square), with addition of 0.5 mmol of
water (rose triangle), thermal activated Cu3(BTC)2 at 150 C for 3 h
under vacuum (red circle) and hot filtration of catalyst after 1 h (blue
triangle).
To gain further information on the reaction mechanism, we
performed a reaction starting with dimethylphenylsilanol and
2 0 4 4
2 0 4 4
o
e
e
d
d
c
c
b
b
a
a
4000
4000
3500
3500
3000
3000
2500
2500
2000
2000
1500
1500
1000
1000
observing the formation of
2 and 3, even at a faster reaction
-1
-1
Wavenumber (cm
)
)
Figure 2. IR spectra ofWparvee-nruemabcetre(cdmCu3(BTC)2 sample (a), after 2.8
rate, thus indicating that silanol can be a possible reaction
intermediate, its presence being mostly undetectable due to its
faster reaction compared to silane. This was further supported
mbar 1 adsorbed at 25 oC (b), 2.6 mbar HBpin co-adsorbed at 25 C
o
(c), and followed by increasing temperature to 50 oC (d) and at 70 oC
(e).
by another experiment in which
of HBpin and formation of silanol and
conversion of was low. In addition, it will be commented later
that bulky triphenylsilane affords mixture of the
1
was added first in the absence
3
was observed, but
The heterogeneous nature of the process was confirmed by
performing a hot filtration test (See SI for more details). In addition,
ICP analysis showed that less than 2 ppm of copper leached to the
solution. The catalyst stability was further assessed by reusing
Cu3(BTC)2 in two consecutive reuses. It was observed that the
conversion of 1 to be 100, 59 and 39% for fresh, first and second
1
a
corresponding triphenylborasiloxane and triphenylsilanol. On
the other hand, if HBpin is added first to the catalyst in the
absence of
1, a visually observable colour change of the
This journal is © The Royal Society of Chemistry 20xx
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reuses. This decrease in activity is associated to the appearance of that can be blocked by pyridine and Cu+ generated by HBpin as
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new peaks at low angle in the powder XRD patterns of the two times reducing agent.
DOI: 10.1039/C7CC05221A
used Cu3(BTC)2 catalyst (Figure 3). However, in spite of the
AD thanks the University Grants Commission (UGC), New Delhi,
appearance of new peaks, it seems that the crystallinity of Cu3(BTC)2 for the award of an Assistant Professorship under its Faculty
is retained during the reuse experiments.
Recharge Programme. AD also thanks the Department of Science and
Technology, India, for the financial support through Extra Mural
Research Funding (EMR/2016/006500). Financial support by the
Spanish Ministry of Economy and Competitiveness (Severo Ochoa
and CTQ2015-69153-CO2-1) is gratefully acknowledged.
3000
3000
2000
2000
1000
1000
Notes and references
(c)
(c)
(b)
(b)
(a)
(a)
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Time Conversion Selectivity
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2
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7
8
Dimethylphenylsilane
Triethylsilane
Methylphenylsilane
Di-t-butylsilane
4
4
7
4
8
5
21
5
4
100
95
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62
64
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100
40(58)c
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68(32)d
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24
aReaction conditions: Silane (0.5 mmol), HBpin (0.5 mmol), Cu3(BTC)2
(30 mg), CH3CN (2 mL), 70 oC, nitrogen atmosphere; bDetermined by
GC and the products were confirmed by GC-MS; cSelectivity of 2,4,6-
trimethyl-2,4,6-triphenyl-1,3,5,2,4,6-trioxatrisilinane; dSelectivity of
bis addition of HBpin; eSelectivity of triphenylsilanol.
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The scope of Cu3(BTC)2 to promote hydrolytic coupling of silanes
and boranes was screened using HBpin as reagent for a variety of
silanes having one or two Si-H bonds. The results are presented in
Table 2. They can be summarized considering that conversion and
selectivities are lower as the size of the silanes and the number of Si-
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H
bonds increases. For instance, methylphenylsilane can be
converted to the corresponding borasiloxane accompanied by the
cyclic trioxasiloxane derivative (Scheme S1). Furthermore, di-t-
butylsilane exhibits lower conversion due to the bulkiness of two t-
butyl groups, undergoing mono and bis boronation by HBpin and
increasing the selectivity of bis derivative as the reaction progresses.
Also, triisopropylsilane requires long times to reach high conversion
due to the bulkiness of this substrate. The reactivity patterns of
triisopropylsilane, triphenylsilane and di-t-butylsilane is compatible
with the reaction occurring on the external surface of the crystallites
due to their large molecular dimensions, but at much slower rates
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In conclusion, in the present study, it has been shown that Cu-
containing MOF is a suitable catalyst for the oxidative hydrolytic
coupling of HBpin with silanes to the corresponding borasiloxanes.
This reaction works under much milder reaction conditions than
those reported with Ru21 and Pd19 based homogeneous catalysts. The
reaction seems to occur on the external and internal surface of the
crystallites and to involve combined action of Cu2+ as Lewis acid sites
TOC
4 | J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
View Article Online
DOI: 10.1039/C7CC05221A
O
B
Ph
Si
O
Si
Ph
+
H
HB
O
O
O
CH3CN, 70 o
N
2 atm
C,
This work reports the synthesis of borasiloxanes from silanes and
pinacolboranes using Cu3(BTC)2 as heterogeneous catalyst in acetonitrile
under milder conditions.
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