Cobalt-Catalyzed Selective Synthesis of Disiloxanes and Hydrodisiloxanes

Article abstract of DOI:10.1021/acscatal.9b00305

Selective syntheses of symmetrical siloxanes and cyclotetrasiloxanes are attained from reactions of silanes and dihydrosilanes, respectively, with water, and the reactions are catalyzed by a NNN^HtBu cobalt(II) pincer complex. Interestingly, when phenylsilane was subjected to catalysis with water, a siloxane cage consisting 12 silicon and 18 oxygen centers was obtained and remarkably the reaction proceeded with liberation of 3 equiv of molecular hydrogen (36 H₂) under mild experimental conditions. Upon reaction of silane with different silanols, highly selective and controlled syntheses of higher order monohydrosiloxanes and disiloxymonohydrosilanes were achieved by cobalt catalysis. The liberated molecular hydrogen is the only byproduct observed in all of these transformations. Mechanistic studies indicated that the reactions occur via a homogeneous pathway. Kinetic and independent experiments confirmed the catalytic oxidation of silane to silanol, and further dehydrocoupling processes are involved in syntheses of symmetrical siloxanes, cyclotetrasiloxanes, and siloxane cage compounds, whereas the unsymmetrical monohydrosiloxane syntheses from silanes and silanols proceeded via dehydrogenative coupling reactions. Overall these cobalt-catalyzed oxidative coupling reactions are based on the Si-H, Si-OH, and O-H bond activation of silane, silanol, and water, respectively. Catalytic cycles consisting of Co(II) intermediates are suggested to be operative.

Full text of DOI:10.1021/acscatal.9b00305

Research Article
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 5552−5561
Cobalt-Catalyzed Selective Synthesis of Disiloxanes and
Hydrodisiloxanes
Sandip Pattanaik and Chidambaram Gunanathan*
School of Chemical Sciences, National Institute of Science Education and Research, HBNI, Bhubaneswar 752050, India
S
* Supporting Information
ABSTRACT: Selective syntheses of symmetrical siloxanes and
cyclotetrasiloxanes are attained from reactions of silanes and
dihydrosilanes, respectively, with water, and the reactions are
catalyzed by a NNNHtBu cobalt(II) pincer complex. Interestingly,
when phenylsilane was subjected to catalysis with water, a
siloxane cage consisting 12 silicon and 18 oxygen centers was
obtained and remarkably the reaction proceeded with liberation
of 3 equiv of molecular hydrogen (36 H2) under mild
experimental conditions. Upon reaction of silane with different
silanols, highly selective and controlled syntheses of higher order
monohydrosiloxanes and disiloxymonohydrosilanes were achieved by cobalt catalysis. The liberated molecular hydrogen is the
only byproduct observed in all of these transformations. Mechanistic studies indicated that the reactions occur via a
homogeneous pathway. Kinetic and independent experiments confirmed the catalytic oxidation of silane to silanol, and further
dehydrocoupling processes are involved in syntheses of symmetrical siloxanes, cyclotetrasiloxanes, and siloxane cage
compounds, whereas the unsymmetrical monohydrosiloxane syntheses from silanes and silanols proceeded via dehydrogenative
coupling reactions. Overall these cobalt-catalyzed oxidative coupling reactions are based on the Si−H, Si−OH, and O−H bond
activation of silane, silanol, and water, respectively. Catalytic cycles consisting of Co(II) intermediates are suggested to be
operative.
KEYWORDS: disiloxanes, silicones, cobalt, hydrogen, catalysis, sustainable chemistry
Condensation of silanols with chlorosilanes and alkoxysilane
and acid- or base-mediated ring opening of cyclic oligosiloxane
methods are employed in classical synthesis to access the
higher order siloxanes (Scheme 1a).10 In recent years, limited
synthetic methods have been developed toward siloxanes of
desired structures.11 Mono- and dihydrosiloxanes (siloxanes
containing Si−H and SiH2 functionalities) are useful building
blocks for silicone polymer, polysiloxane materials, as they
assimilate the siloxane motifs to various chemical compounds
upon activation of Si−H bond.12−14 Catalytic oxidation of
monohydrosilanes with water to produce silanols and siloxanes
is attained by transition metals such as ruthenium, palladium,
rhodium, iridium, and gold.15 These investigations have in
general not been applied to the dihydro- and trihydrosilanes
(vide infra) due to the complexity of the reactions leading to
the formation of a mixture of products.16 Currently such
studies are limited to two reports in the literature in which
rhodium- and gold-catalyzed dehydrogenative coupling of
silanols with dihydro- and trihydrosilanes resulted in selective
synthesis of mono- and dihydrosiloxanes, respectively (Scheme
1b).17
INTRODUCTION
■
Siloxanes and hydrosiloxanes are classes of compounds having
−Si−O−Si− and −Si−O−SiH− motifs, respectively, and they
have displayed widespread applications in organic synthesis
and inorganic and material chemistry.1 These silicones display
inherent molecular properties such as flexible pressure sensors,
liquid crystals, pharmacological activity, high gas permeability,
high thermal stability, and low entropy of dilution and are
extensively used in aerospace, automotive, paints and coatings,
pharmaceuticals, textiles, and leather industries.2,3 In particular,
unsymmetrical siloxanes have important applications as liquid
crystalline polymers and low-dielectric-constant materials.3
Given the extensive applications of silicones as functional
materials, the synthesis of siloxanes directly from readily
available compounds without producing chemical waste is very
important.4,5 Such a synthesis of siloxanes with predictable size
and selectivity by conventional methods is difficult, and
potential synthetic methods remain to be explored.6 For
example, cyclosiloxanes are building blocks of various silicon-
based polymeric materials.3,7 However, the conventional
synthesis of cyclosiloxanes involves hydrolysis of chlorosilane
and alkoxysilane and self-condensation of siloxane diols and
siloxane polyols.1a,7,8 In general, these reactions require high
temperature and excess amounts of base and lead to the
formation of a number of side products such as silanols,
cyclosiloxanes, and oligo- and polysiloxanes.9
Received: January 22, 2019
Revised: May 7, 2019
© XXXX American Chemical Society
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a
Scheme 1. Traditional Approach and Recent Advances in
Catalytic Synthesis of Siloxanes
Scheme 2. Synthesis and Solid-State Structure of Catalyst 1
a
Ellipsoids are drawn at the 50% probability level.
Reactions of Hydrosilanes with H2O. The reaction of
triethylsilane with water was investigated using complex 1 as a
catalyst. Initial experiments indicated that catalytic coupling
leads to the formation of symmetrical disiloxanes. The reaction
conditions have been further optimized, and selected reactions
are presented in Table 1. Heating a dioxane solution
containing 2 mol % of catalyst 1, triethylsilane (1 mmol),
and water at 60 °C resulted in 92% conversion of the silane to
provide disiloxane 2a in 54% yield. The reaction of catalyst 1
with silane under neutral conditions is remarkable; however,
36% triethylsilanol (conversion based on GC analysis) was also
The sustainable development of synthetic methods catalyzed
by naturally abundant, low-toxicity, and cheap base-metal
complexes is becoming one of the principal objectives in
homogeneous catalysis.18 Currently, the base-metal-catalyzed
synthesis of siloxanes remains underexplored. We have
developed highly efficient and selective hydroboration
reactions19 and hydrosilylation of aldehydes20 in which we
have uncovered the involvement of hitherto unknown
intermediates. Further, we have developed the highly selective
synthesis of borasiloxanes directly from borane, silanes, and
water, catalyzed by simple ruthenium catalysts.21 In a
continuation of these endeavors with hydroelementation
processes, herein we report a new and simple NNNHtBu cobalt
pincer complex catalyzed selective synthesis of symmetrical
disiloxanes, cyclosiloxanes, and cagelike siloxanes from silanes
and water, and unsymmetrical monohydrodisiloxanes from
silanes and silanols (Scheme 1c).
Table 1. Optimization of Reaction Conditions for Cobalt-
Catalyzed Siloxane Synthesis
Cat.
(mol %)
base
(mol %)
conversion
yield
c
b
entry
Cat.
(%)
(%)
d
1
1
2
1
1.5
2
2
0
2
3
4
4
92
49
68
96
89
0
0
0
13
54
41
53
94
63
0
0
0
13
RESULTS AND DISCUSSION
2
3
4
1
1
1
1
■
Synthesis of Co Catalyst. The design and development of
new catalysts that are stable to air and moisture and readily
accessible from simple synthetic preparations remain highly
attractive. Thus, addition of the ligand tBuH‑NNN (N,N′-
(pyridine-2,6-diylbis(methylene))bis(2-methylpropan-2-
amine)) to CoBr2 in ethanol at room temperature resulted in
formation of the new pincer complex 1 in 85% yield. The
e
5
6
7
8
9
4
0
4
CoBr2
CoBr2
2
2
1
paramagnetic complex 1 has been fully characterized; its H
a
Reaction conditions unless specified otherwise: triethylsilane (1
NMR spectrum displays a broad singlet at δ 9.53 ppm
corresponding to two NH protons. A broad peak at 3207 cm−1
in the IR spectrum of 1 also confirmed the presence of NH
functionalities. Further, a DCM/toluene solution of 1 provided
single crystals suitable for X-ray analysis, which established a
distorted-square-pyramidal geometry around the cobalt center
(Scheme 2).
mmol), degassed water (1 mmol), catalyst 1, base, and dioxane (2
mL) were placed in a sealed tube and heated at 60 °C under closed
b
conditions. Conversion was determined by gas chromatography
c
using dodecane as an internal standard. Isolated yield after column
d
e
chromatography. 36% of triethylsilanol also formed (GC). Reaction
performed at room temperature; as per GC analysis, 22% of
triethylsilanol was also formed as a side product.
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present in the reaction mixture, indicating incomplete reactions
(entry 1, Table 1). Further, a similar catalytic reaction using 1
mol % of catalyst 1 and 2 mol % of KOtBu resulted in 49%
conversion of the silane to disiloxane 2a and H2 (entry 2, Table
1). Increasing the catalyst load to 1.5 mol % provided
moderate conversion and yield (entry 3, Table 1). Upon using
2 mol % of catalyst 1 and 4 mol % of base, 96% conversion of
silane occurred to provide the siloxane 2a in 94% yield in 2 h
(entry 4, Table 1). Performing a similar reaction at room
temperature resulted in diminished conversion of silane (89%)
and product formation (63%, entry 5, Table 1); The presence
of the corresponding silanol (22%) was also found in the GC
analysis. Further, control experiments without catalyst 1 and
base and with base alone failed to provide any siloxanes
(entries 6 and 7, Table 1), confirming that the catalyst is
essential for the siloxane synthesis from silane and water.
Moreover, catalytic experiments using simple CoBr2 (2 mol %)
provided no reaction under neutral conditions; however, when
CoBr2 was used together with base, only 13% conversion of
silane to siloxane 2a was observed (entries 8 and 9, Table 1).
These results clearly reflect the efficient reactivity of pincer
catalyst 1 in the oxidative coupling of silanes with water.
Following the optimized reaction conditions, alkyl, aryl, and
bulky silanes were subjected to synthesis of symmetrical
disiloxanes, which provided the products in very good yields
(Table 2). When trioctylsilane was subjected to catalysis,
Scheme 3. Cobalt-Catalyzed Oxidative Dehydrocoupling of
Dihydrosilanes to Siloxane Macrocycles
tetrasiloxane 3b, which was isolated in 96% yield, and the
formation of 2 equiv of hydrogen was quantified (see the
Supporting Information). Single-crystal X-ray analysis clearly
confirmed the cyclic structure of 3b (see the Supporting
Information). Dehydrogenative oxidation of dihydrosilane with
water leads to formation of a silanediol intermediate, which
further undergoes cyclocondensation to provide the observed
cyclotetrasiloxanes. Alternatively, formation of cyclosiloxanes
may also directly occur from the dehydrogenative self-coupling
of a monohydrosilanol intermediate. An appropriate molecular
geometry also plays an important role in the formation of these
products. When naphthylphenyldihydrosilane was reacted with
water, formation of an unidentified mixture of products was
observed, perhaps due to the increased steric hindrance.
When phenylsilane reacted with water under optimized
conditions using catalyst 1, the siloxane compound 4 was
obtained in 69% yield (Scheme 4). Interestingly, single-crystal
X-ray analysis revealed that compound 4 is a three-dimensional
highly ordered siloxane cage. Notably, compound 4 is
commercially available, obtained from a tedious multistep
procedure,22 and its X-ray structure has also been reported.23
In addition to the formation of product 4, significant liberation
of hydrogen was also observed. Further, to confirm the amount
of molecular hydrogen liberated from this reaction, we have
rerun the experiment three times using 0.5 mmol of
phenylsilane with water and collected the hydrogen produced
from the reaction using a graduated buret. An average of 37
mL of hydrogen gas (from three experiments) was collected in
the buret, which corresponds to 1.4 mmol against the
theoretically expected 1.5 mmol of molecular hydrogen. This
observation indicates that 3 equiv of molecular hydrogen is
liberated from the reaction. Overall the cobalt-catalyzed
reaction of phenylsilane with water provided a simple and
direct access to siloxane cage 4 with liberation of 3 equiv of
molecular hydrogen (36H2). Presumably, as in the formation
of cyclotetrasiloxane 3, dehydrogenative oxidation of phenyl-
silane with water produced the phenyltrihydroxysilane
intermediate that underwent a self-condensation reaction
leading to the formation of 4.
Table 2. Cobalt-Catalyzed Synthesis of Siloxanes from
Silane and Water
trialkylsiloxane 2b was obtained in 86% yield. Dimethylphe-
nylsilane reacted with water to provide the dimethylphenylsi-
loxane 2c in quantitative yield. When triphenylsilane was
employed in the reaction, 1,1,1,3,3,3-hexaphenyldisiloxane 2d
was isolated in 89% yield. Notably when sterically hindered
tris(trimethylsilyl)silane was employed in hydrolytic oxidation,
it provided the corresponding siloxane product 2e in 92%
yield. These results indicate the ability of the cobalt metal
center to coordinate and catalyze the bond activations of
sterically hindered silanes in the coordination sphere.
Next, cobalt-catalyzed oxidative dehydrocoupling of dihy-
drosilanes with water was investigated. When diethylsilane was
reacted, quantitative conversion was observed and the
cyclotetrasiloxane 3a was isolated in 97% yield (Scheme 3).
Diphenylsilane also provided the similar octaphenyl cyclo-
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the anticipated product but resulted in formation of 5m.
Catalytic coupling experiments involving the isolated mono-
hydrosiloxane 5a and monohydrodisiloxane 5m with dime-
thylphenylsilanol failed to provide the desired products,
perhaps due to the higher steric hindrance of siloxanes.
Preliminary Mechanistic Studies. A series of experi-
ments was performed to decipher the mechanistic insight of
this interesting catalysis by cobalt complex 1. The catalytic
reaction of diphenylsilane with water in the presence of
mercury provided the corresponding cyclosiloxane 3b in 87%
yield (against the 96% yield in the absence of mercury; see
Scheme 3), indicating that the reaction proceeds with
molecular intermediates (Scheme 5a). Reaction of complex 1
with KOtBu leads to the formation of complex 6. Although the
EPR spectra of 1 and 6 were similar, as both complexes contain
Co(II) metal centers, the IR spectrum of complex 6 confirmed
the disappearance of the NH peak (which appeared at 3207
cm−1 for 1). Similarly, the 1H NMR spectrum of 6 also
confirmed the disappearance of the NH signal that appeared
for 1 at 9.53 ppm (Scheme 5b). Unfortunately, repeated
attempts to get the single-crystal X-ray structure of the three-
coordinated complex 6 remain unsuccessful. When isolated 6
was employed as a catalyst, the formation of siloxanes 2a,d
with enhanced yields was observed (see Table 2). Remarkably,
the use of base was no longer required and the activated
catalyst 6 alone catalyzed these reactions under neutral
conditions (Scheme 5c,d), which indicates that the role of
base is debromination and deprotonation of 1. Further,
reaction of diphenylsilanediol and dimethylphenylsilanol with
catalyst 1 at room temperature provided the corresponding
siloxanes in poor yields. However, the same reactions at 60 °C
provided the corresponding siloxanes 3b and 2c in quantitative
yields, confirming that the reactions are occurring via silanol
intermediacy and reiterating the importance of optimized
experimental conditions (Scheme 5e,f). Further, dehydrogen-
ative coupling of silanaol with triethylsilane was performed
both at room temperature and at 60 °C. While less than 2% of
siloxane 2a formed at room temperature, the same product 2a
was obtained quantitatively at elevated temperature. Appa-
rently, the reaction proceeded via self-coupling of triethylsila-
nol, as the triethylsilane remained unreacted (GC analysis).
These results clearly confirm that siloxane formation is
sensitive to the steric hindrance of monohydrosilanes and
proceeds via the dehydrative coupling of silanol intermediates
(Scheme 5g).
As performed in the synthesis of unsymmetrical siloxane in
Table 3, when triethylsilanol reacted with dimethylphenylsila-
nol using the catalyst 1 at room temperature, no product
formation was observed and both silanols remained unreacted,
which proves the formation of unsymmetrical siloxanes 5 is
proceeding via the dehydrogenative coupling of silane and
silanol (Scheme 5h). When the same reaction was performed
at 60 °C, only self-coupling of silanols resulted in the
formation of symmetrical disiloxanes and the products 2a,c
were obtained in 59% and 78% yields, respectively. Although
we were unable to ascertain the reasons, no cross-coupling of
silanols occurred and the unsymmetrical siloxane was not
observed (GC analysis, Scheme 5i). Attempted thermal
condensation of triethylsilanol with dimethylphenylsilanol
even at 120 °C failed to provide any product (Scheme 5j).
Thus, the formation of siloxanes is essentially a catalytic
process. GC analysis of the gas phase from the catalytic
reaction of triethylsilanol with diphenylsilane confirmed the
Scheme 4. Cobalt-Catalyzed Dehydrogenative and
Dehydrocoupling of Phenylsilane with Water
Reactions of Silanols with Hydrosilanes. In an attempt
to develop a highly controlled and selective synthesis of higher
order hydrosiloxane compounds, further we have examined the
cross-coupling of silanol with dihydro- and trihydrosilanes.
When dihydrosilanes reacted with silanol in the presence of
catalyst 1, they underwent dehydrogenative coupling reactions
with silanol and provided monohydrosiloxanes. Moreover,
upon reactions of a trihydrosilane with different silanols,
double dehydrogenative coupling occurred, leading to the
exclusive formation of monohydrosiloxanes.
When triethylsilanol reacted with diphenyl- and methyl-
phenylsilane, the corresponding siloxanes 5a,b were obtained
in 82% and 88% yields, respectively (Table 3). Similarly
tripropylsilanol, triisopropylsilanol, and triisobutylsilanol re-
acted with various dihydrosilanes and provided the corre-
sponding monohydrosiloxane products 5c−g in very good
yields. Further, when sterically hindered silanols such as
tris(trimethylsilyl)silanol and dimethyloctadecylsilanol were
employed in cobalt-catalyzed dehydrogenative coupling, the
corresponding monohydrosiloxane products 5h−j were
obtained in good yields. When an aromatic silanol such as
dimethylphenylsilanol was employed in the reaction, the
corresponding cross-coupling products 5k,l were isolated in
80% and 82% yields. Further, when phenylsilane reacted with
an assortment of silanols, the corresponding double-cross-
coupling products 5m−p were obtained. Notably, when CoBr2
was used as a catalyst in the reaction of triethylsilanol with
diphenylsilane and phenylsilane, no formation of the
corresponding products 5a,m was observed (Table 3, entries
2 and 15), reiterating the importance of the cobalt pincer
catalyst. The attempted catalytic synthesis of dihydromonosi-
loxane from phenylsilane and tritheylsilanol did not provide
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a
Table 3. Cobalt-Catalyzed Cross-Coupling Reaction of Silanol with Dihydro- and Trihydrosilanes
a
Reaction conditions unless specified otherwise: silane (1 mmol), silanol (1 mmol), catalyst 1 (2 mol %), and base (4 mol %) were stirred at room
b
c
d
temperature. Yield corresponds to isolated product. Reaction performed using CoBr2 (2 mol %) as a catalyst. 2 mmol of silanol was used.
presence of molecular hydrogen (Scheme 5k), which clearly
indicates that the formation of unsymmetrical monohydrosi-
loxanes occurs from the dehydrogenative coupling.
Further, in an attempt to interrupt catalysis, methanol was
added to the catalytic reaction of phenylsilane with
triethylsilanol, in which the predominant formation (71%) of
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the desired monohydrosiloxane product 5m was observed
(Scheme 5l) together with a minor amount of dimethox-
yphenylsilane (7, 13%, GC analysis). However, upon reaction
of phenylsilane with methanol under standard catalysis, 7 was
obtained in 99% yield (Scheme 5m). These observations
indicate that dehydrogenative coupling of silane with silanol is
much more favored under catalytic conditions than coupling of
silane and alcohol. Further, addition of styrene and aryl halide
to catalysis was tested, which failed to interrupt the catalytic
process. The competition experiment involving the reaction of
diphenylsilane and phenylsilane with triethylsilanol resulted in
monohydrodisiloxane 5m and monohydrosiloxane 5a in 76%
and 11% yields (GC analysis), respectively (Scheme 5n),
indicating that less bulky silanes are preferably consumed over
the sterically hindered diarylsilanes.
Scheme 5. Mechanistic Studies on the Catalytic Synthesis of
Disiloxanes and Monohydrodisiloxanes
EPR analyses of catalyst 1 and intermediate 6 revealed
identical spectral lines (Figure 1, also see the Supporting
Information), indicating the presence of similar Co(II) metal
centers in both complexes.
Figure 1. Overlaid EPR spectra of 1 and 6.
The reaction progress for the symmetrical and unsym-
metrical siloxane processes was monitored using GC. The
formation of a diphenylsilanediol intermediate was transient,
and it transformed to the product upon a condensation
reaction (red line in Figure 2a(i)). The decreasing
concentration of diphenylsilane (black line in Figure 2a(i))
authenticates the increasing concentration of product 3b
(Figure 2a(ii)). The reaction progress for the formation of
unsymmetrical siloxane 5a is shown in Figure 2b.
Although more data are required, a plausible mechanism for
the synthesis of symmetrical disiloxanes using monohydrosi-
lane and dihydrosilane with water catalyzed by 1 is presented
in Scheme 6. Catalyst 1 undergoes debromination and
deprotonation upon reaction with base to provide coordina-
tively unsaturated intermediate 6. Coordination of silane to
unsaturated complex 6 provides the intermediate I. Nucleo-
philic attack by water on the silane coordinated to cobalt
generates II. The Si−H and O−H bond activation at the metal
center leads to the formation of dihydrogen-coordinated III
and silanol. Reaction of silanol with intermediate III liberates
the coordinated molecular hydrogen and provides silanol-
ligated intermediate IV.24 Free silanol reacts with metal-
coordinated silanol ligand, resulting in condensation, perhaps
occurring via Si−O bond metathesis and leading to the
formation of disiloxane-ligated V.25 Further dissociation of
disiloxanes from intermediate V regenerates 6 and completes
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Scheme 6. Proposed Mechanism for the Cobalt-Catalyzed
Synthesis of Symmetrical Siloxanes
these catalytic processes. Similar amine−amide metal−ligand
cooperation is operative in catalysis by the Ru-MACHO pincer
catalyst.27
Similarly, a reaction mechanism for cobalt pincer complex
catalyzed synthesis of unsymmetrical siloxanes from silane and
silanol is proposed in Scheme 7. The in situ generated active
intermediate 6 reacts with dihydrosilane, resulting in formation
Scheme 7. Proposed Catalytic Cycle for Cross-Coupling of
Silane with Silanol
Figure 2. Monitoring the reaction progress of (a) symmetrical
cyclotetrasiloxane 3b with (i) conversion of diphenylsilane and
formation of a diphenylsilanediol intermediate and (ii) formation of
product 3b and of (b) unsymmetrical siloxane 5a.
the catalytic cycle. Alternatively, bond activation involving
amine−amide metal−ligand cooperation26 may also operate in
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Symposium Series 729. (c) Zeigler, J. M.; Fearon, F. W. G. Silicon-
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(e) Longenberger, T. B.; Ryan, K. M.; Bender, W. Y.; Krumpfer, A.-
K.; Krumpfer, J. W. The Art of Silicones: Bringing Siloxane Chemistry
to the Undergraduate Curriculum. J. Chem. Educ. 2017, 94, 1682−
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Publishing: Carol Stream, IL, 2008. (b) Advances in Silicones and
Silicone-Modified Materials; Clarson, S. J., Owen, M. J., Smith, S. D.,
Van Dyke, M. E., Eds.; American Chemical Society: Washington, DC,
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Synthesis of Branched Alkoxysiloxane Oligomers Si[OSiH(OR)2]4
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Alkoxysilylated-Derivatives of Double-Four-Ring Silicate as Novel
Building Blocks of Silica-Based Materials. Chem. Mater. 2008, 20,
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of coordinated intermediate VI, to which silanol undergoes
nucleophilic attack leading to the formation of VII and
subsequent bond metathesis resulting in monohydrosiloxanes
and III. Liberation of dihydrogen coordinated to Co in III
regenerates 6 to complete one loop on a catalytic cycle.
However, the involvement of other mechanistic pathways
cannot be ruled out.
CONCLUSION
■
In summary, an air-stable and easily accessed cobalt pincer
complex was prepared. This simple NNN cobalt pincer
complex catalyzed highly facile and efficient processes for the
synthesis of symmetrical siloxanes and cyclotetrasiloxanes from
silanes and disilanes, respectively, using water. The liberated
molecular hydrogen is the only byproduct in these interesting
transformations. Reaction of a trihydrosilane with water
resulted in formation of a siloxane cage in which formation
of 3 equiv of dihydrogen was observed, indicating that it can be
a potentially useful transformation in hydrogen storage.
Moreover, when the dihydrosilanes and trihydrosilanes were
subjected to catalysis with silanol at room temperature, the
higher order monohydrosiloxanes and disiloxymonohydrosi-
lanes were obtained in very good yields and the reactions
proceeded with liberation of molecular hydrogen. The
mechanistic studies indicated that the reactions involve
molecular intermediates. Accordingly, catalytic cycles for the
synthesis of siloxanes and unsymmetrical higher order silanes
are proposed, incorporating Co(II) intermediates. Bond
activation reactions such as Si−H, −O−H of water, and Si−
OH bonds are operative in catalysis, which provided the
dehydrative silanol condensation and dehydrogenative cou-
pling of silanes and silanols.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.9b00305.
■
S
1
Experimental procedures, spectral data, and H NMR,
13C NMR, and 29Si NMR spectra of the products (PDF)
Single-crystal X-ray data for complex 1 (CIF)
(6) Yoshikawa, M.; Tamura, Y.; Wakabayashi, R.; Tamai, M.;
Shimojima, A.; Kuroda, K. Protecting and Leaving Functions of
Trimethylsilyl Groups in Trimethylsilylated Silicates for the Synthesis
of Alkoxysiloxane Oligomers. Angew. Chem., Int. Ed. 2017, 56, 13990−
13994.
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.G.: gunanathan@niser.ac.in.
■
(7) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W.
Hetero- and Metallasiloxanes Derived from Silanediols, Disilanols,
Silanetriols, and Trisilanols. Chem. Rev. 1996, 96, 2205−2236.
(8) For recent examples, see: (a) Yoshikawa, M.; Shiba, H.;
Kanezashi, M.; Wada, H.; Shimojima, A.; Tsuru, T.; Kuroda, K.
Synthesis of a 12-Membered Cyclic Siloxane Possessing Alkoxysilyl
Groups as a Nanobuilding Block and its Use for Preparation of Gas
Permeable Membranes. RSC Adv. 2017, 7, 48683−48691. (b) Cho,
H. M.; Lee, J.-E.; Lee, M. E.; Lee, K. M. Synthesis of 6-, 7- and 8-
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met. Chem. 2011, 696, 2754−2757.
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ysilanes with Halosilanes for Selective Synthesis of Unsymmetrical
ORCID
Chidambaram Gunanathan: 0000-0002-9458-5198
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the SERB New Delhi (EMR/2016/002517), DAE,
and NISER for financial support. C.G. thanks Prof. Herbert W.
Roesky and Prof. Pradyut Ghosh for helpful discussions. S.P.
thanks the CSIR for a research Fellowship. We thank Prof. J. V.
Yeldho and Ashish Kar for their kind help.
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