RuIII(OTf)SalophenCH2-NHSiO2-Fe: An efficient and magnetically recoverable catalyst for trimethylsilylation of alcohols and phenols with hexamethyldisilazane

Article abstract of DOI:10.1002/aoc.3125

Efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by ruthenium(III) complex of chloromethylated Salophen supported on nanomagnetic materials is reported. First, the iron nanomagnets were silica coated, functi

Full text of DOI:10.1002/aoc.3125

Full Paper
Received: 1 October 2013
Revised: 24 November 2013
Accepted: 27 December 2013
Published online in Wiley Online Library: 27 February 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3125
III
Ru (OTf)SalophenCH2–NHSiO2–Fe: an efficient
and magnetically recoverable catalyst for
trimethylsilylation of alcohols and phenols
with hexamethyldisilazane
Maryam Torki, Shahram Tangestaninejad*, Valiollah Mirkhani*,
Majid Moghadam* and Iraj Mohammadpoor-Baltork
Efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by ruthenium(III) complex of
chloromethylated Salophen supported on nanomagnetic materials is reported. First, the iron nanomagnets were silica coated,
functionalized with amine and then ruthenium CM-Salophen was successfully bonded to their surface. The catalyst, RuIII(OTf)
SalophenCH2–NHSiO2–Fe, was characterized by elemental analysis, FT-IR and UV–visible spectroscopic techniques, transmission
electron microscopy and inductively coupled plasma (ICP). The RuIII(OTf)SalophenCH2–NHSiO2–Fe catalyzed trimethylsilylation
of primary and secondary alcohols as well as phenols, and the corresponding TMS ethers were obtained in high yields and short
reaction times at room temperature. This new heterogenized trimethylsilylation catalyst is easily recovered with a magnet and
showed no appreciable loss of activity even after five consecutive runs. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: magnetic nanoparticles; ruthenium Salophen; heterogeneous catalyst; silylation; hexamethyldisilazane
Schiff bases are the most attractive ligands for ruthenium
complexes for several reasons: (i) steric and electronic effects
Introduction
The protection of hydroxyl groups is often necessary during the
synthesis of fine chemicals and natural products. Several methods,
such as acetylation, tetrahydropyranylation, methoxymethylation
and trimethylsilylation, have been reported for the protection of
hydroxyl groups.[1,2] Silyl ether formation is an important process
not only for protection of reactive hydroxyl groups but also for
the synthesis of functional organosilicon compounds.[3–6] One of
the most common methods for the silylation of hydroxyl groups
is the use of hexamethyldisilazane (HMDS), which is a stable, cheap
and commercially available reagent, and its workup is not time
consuming because the by-product of the reaction is ammonia,
which is simple to remove from the reaction media. One disadvan-
tage of HMDS is its low silylating power in the absence of a
suitable catalyst. Therefore, for increasing the silylating power of
HMDS, a variety of catalysts including silica-supported perchloric acid
(HClO4.SiO2),[7] ZrCl4,[8] ZrO(OTf)2,[9] sulfonic acid-functionalized
nanoporous silica,[10] MgBr2.OEt2,[11] CuSO4.5H2O,[12] Fe(TFA)3,[13]
H3PW12O40,[14] (n-Bu4N)Br[15] and K-10 montmorilonite,[16] homoge-
neous and heterogeneous tin(IV) porphyrin,[17–19] V(IV) porphyrin[20]
and Ti(IV) Salophen[21] have been reported.
The metal Schiff base complexes are of great importance in the
coordination chemistry because of their stability, structural variabil-
ity, ease of preparation and variety of applications.[15–25] Salophen
and salen ligands bearing N and O donor atoms are important since
their metal complexes have extensive applications especially as
catalyst in various organic reactions.[26–28] Ruthenium complexes
have been known as versatile electron transfer compounds and
exhibit a wide range of applications such as oxidation catalysts,
probes for biological macromolecules, organic synthesis, etc.[25–29]
around the ‘Ru’ core can be finely tuned by changing the substit-
uents incorporated into the Schiff bases; (ii) in the chelated Schiff
base, the two donor atoms, N and O, have opposite electronic
effects: the phenolate oxygen is a hard donor and can stabilize
the higher oxidation state of the ruthenium atom, whereas the
imine nitrogen is a softer donor and, accordingly, will stabilize
the lower oxidation state of the ruthenium; and (iii) Schiff bases
are prepared through simple procedures via condensation of
common aldehydes with amines, in quantitative yields.[30]
Taking into account the highly desirable attributes of this type
of ligand, vast families of bidentate, tridentate and tetradentate
Schiff base-ligated ruthenium complexes, of wide applicability
as catalysts in numerous organic reactions, have been designed
and prepared. Some of them, e.g. tetradentate Ru–salen[31] and
Ru–porphyrin[32] complexes, show good to excellent activity
and high enantioselectivity in catalyzing a variety of organic
reactions.
Catalyst recovery and reuse are very important features in many
catalytic processes. Heterogeneous systems require a centrifuga-
tion or filtration step to separate and recover the catalyst. Due to
the higher specific surface area of nanoparticles in comparison with
*
Correspondence to: S. Tangestaninejad, V. Mirkhani and M. Moghadam,
Department of Chemistry, Catalysis Division, University of Isfahan,
Isfahan 81746–73441, Iran. E-mail: stanges@sci.ui.ac.ir; mirkhani@sci.ui.ac.ir;
moghadamm@sci.ui.ac.ir
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan
81746-73441, Iran
Appl. Organometal. Chem. 2014, 28, 304–309
Copyright © 2014 John Wiley & Sons, Ltd.

Synthesis, characterization and application of catalysts
Preparation of catalyst 1, RuIII(OTf)SalophenCH2–NHSiO2–Fe
their bulk counterparts, these materials have been attracted much
attention in catalysis; however, isolation and recovery of these tiny
nanoparticles from the reaction media is not a simple process.[33,34]
Therefore, the use of magnetic nanocatalysts has emerged as a
viable solution. Magnetically supported catalysts can be recovered
easily and efficiently with an external magnet because of the
paramagnetic character of the support, and subsequently reused
in another cycles.[35–37] Magnetic nanocatalysts have been used in
a wide range of reactions with desired activities. Their surface can
be functionalized during catalyst preparation.[38–41]
For preparation of the catalyst, the obtained solids in the previ-
ous step (5 g) were dispersed in acetonitrile (40 ml), and RuCl3
(0.5 g) was added to this mixture. The mixture was stirred at room
temperature for 48 h. The resulting solids were collected with a
magnet, washed with copious dichloromethane and methanol
and dried under vacuum. At the final step, the axial chlorine
was substituted with OTf by reaction of the resulting solid with
NaOTf in tetrahydrofuran.
In this paper, we describe the preparation, physicochemical
characterization and application of RuIII(Salophen)OTf supported
on silica-coated iron nanoparticles, RuIII(OTf)SalophenCH2–
NHSiO2–Fe (1), for trimethylsilylation of alcohols and phenols
with hexamethyldisilazane (Scheme 1).
General Procedure for Trimethylsilylation with HMDS Catalyzed
by 1
To a mixture of alcohol or phenol (1 mmol) and HMDS (0.7 mmol per
OH group) in CH2Cl2 (1 ml) was added 80 mg of 1 (0.04 mmol Ru)
and stirred at room temperature. The progress of the reaction was
monitored by GC or TLC. At the end of the reaction, Et2O (10 ml)
was added and the catalyst was separated by a magnet. The filtrates
were washed with brine, dried over Na2SO4 and concentrated under
reduced pressure to afford the crude product.
Experimental
Materials
All chemicals were purchased from Aldrich or Merck chemical
companies. All synthetic procedures were carried out under N2
atmosphere. All solvents were degassed and saturated with N2
before use.[42,43] The diffuse-reflectance UV–visible spectrum was
recorded by a solid UV–visible JASCO V-670 spectrophotometer.
FT-IR spectra were obtained as potassium bromide pellets in the
range of 350–4000 cmÀ1 with a JASCO 6300 spectrophotometer.
GC experiments were performed with a Shimadzu GC-16A instru-
ment using a 2 m column packed with silicon DC-200 or Carbowax
20m. In the GC experiments, n-decane was used as an internal
standard. Transmission electron micrographs of the magnetic
nanoparticles were taken on a Philips CM 120 TEM instrument.
Results and Discussion
Preparation and Characterization of the Catalyst 1
The surface modification of silica-coated nanomagnets with
amine to give NH2SiO2–Fe was carried out by the method
described by McKittrick and Jones.[45] The heterogeneous cata-
lyst 1 was prepared by the reaction of NH2SiO2–Fe with
H2Salophen, RuCl3 and NaOTf, respectively (Scheme 2). The
bonding of NH2SiO2–Fe and the metalloSalophen was so strong
that [RuIII(Salophen)OTf] was not eluted from the support in wa-
ter or common organic solvents.
The nitrogen content of NH2SiO2–Fe, measured by CHN
analysis, was about 1.8 mmol gÀ1. The Ru content of the
supported catalyst, determined by ICP, was 0.5 mmol gÀ1 of
the supported catalyst. The UV–visible spectrum provided
further evidence for the presence of RuIII(Salophen)OTf on
the support. The diffuse reflectance spectrum of the
supported catalyst in the BaSO4 matrix exhibited two bands
at 287 and 330 nm, which are attributed to d–d transitions
of the Ru(III) Salophen complex (Fig. 1). The spectrum of
the homogeneous Ru(Salophen)OTf showed the same peaks
in CH2Cl2 solution, but SiO2–Fe has no absorption peak in
this region.
Preparation Procedures
Preparation of amino-functionalized silica-coated nanomagnet
SiO2–Fe was prepared as reported in our previous work[44] and then
functionalized according to the literature.[45] The SiO2–Fe (5 g) was
dried in a vacuum oven for 12 h at approximately 120°C and then
suspended in dry toluene (250 ml). 3-Aminopropyltriethoxysilane
(APTES) (7 ml: 30.07 mmol) was slowly added and the mixture
was refluxed for 24 h. The materials (NH2–SiO2–Fe) were collected
by a magnet and washed with dry acetone.
Immobilization of H2Salophen on amino-functionalized silica-coated nanomagnet
The FT-IR spectrum of the heterogenized ruthenium Salophen
complex shows characteristic bands at 1605–1619 and 1355–
1467 cmÀ1 assigned to C N and C―O bonds, respectively. These
bands are not observed in the spectrum of pure silica-coated
nanoparticles. These observations indicated the successful
attachment of [RuIII(Salophen)OTf] complex to NH2SiO2–Fe
(Fig. 2). The bands observed in the region 407–474 cmÀ1 have
been assigned to Ru―O and Ru―N bonds. The TEM image of
catalyst 1 (Fig. 3) clearly shows the core–shell structure of the
support. On the other hand, this image shows that the particles
are nanosized.
5-Chloromethyl-2-hydroxybenzaldehyde and N,N′-bis(5-chloromethyl-
salicylidene)-1,2-phenylene diamine was synthesized according to the
literature.[46,47]
To a solution of H2Salophen (0.7 g) in acetonitrile (100 ml),
NH2–SiO2–Fe (5 g) and triethylamine (6 ml) were added and the
mixture was vigorously stirred under reflux conditions for 48 h.
After cooling, the solid was collected by a magnet, washed thor-
oughly with dichloromethane, methanol and ether, and dried un-
der vacuum at room temperature for several hours.[47]
RuIII(OTf)SalophenCH2-NHSiO2-Fe
H
CH2Cl2, R.T.
Trimethylsilylation of Alcohols and Phenols with HMDS
Catalyzed by 1
+
ROH
N
ROSiMe3 + NH3
Me3Si
SiMe3
Initially, the trimethylsilylation of benzyl alcohol with HMDS cata-
Scheme 1. Trimethylsilylation of alcohols and phenols with HMDS cata-
lyzed by RuIII(OTf)SalophenCH2–NHSiO2–Fe.
lyzed by RuIII(Cl)SalophenCH2–NHSiO2–Fe was carried out. The
Appl. Organometal. Chem. 2014, 28, 304–309
Copyright © 2014 John Wiley & Sons, Ltd.
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M. Torki et al.
(EtO)3SiCH2CH2CH2NH2
Toluene
O
O
O
Fe
SiO2
Fe
SiO2
Si
NH2
NEt3
CH3CN
N
N
OH HO
Cl
Cl
Fe
Fe
SiO2
SiO2
O
O
O
O
O
O
Si
Si
N
N
HO
NH
HN
OH
Figure 2. FT-IR spectrum of: (A) SiO2@Fe and (B) RuIII(OTf)SalophenCH2–
NHSiO2–Fe.
1. RuCl3
CH3CN
2. NaOTf
THF
Fe
Fe
The catalytic activity of 1 was investigated in the silylation of
benzyl alcohol with HMDS and the reaction parameters such as
amount of the catalyst and kind of solvent were optimized. When
the model reaction was carried out using different amounts of
the catalyst, the highest yield was obtained in the presence of 4
mol% of Ru. In order to choose the reaction media, the same
reaction was performed in different solvents. Among CH2Cl2,
CH3CN, EtOAc, n-hexane and chloroform, the highest yield was ob-
served in CH2Cl2. Therefore, dichloromethane was used as the solvent.
Under the optimized reaction conditions, a wide range of
alcohols including primary (benzylic and linear ones) and
secondary alcohols were successfully trimethylsilylated with
HMDS in the presence of 1. In the case of benzylic alcohols,
electron-withdrawing substituents require longer reaction times
for completion, while benzylic alcohols bearing electron-
donating substituents were completed in shorter reaction times
(Table 1). The results in Table 1 also show that steric hindrance
benzylic alcohols bearing ortho substituents did not affect
the reaction time. The only exception is 2-nitrobenzyl alcohol.
It seems that intramolecular hydrogen bonding between
hydroxyl and nitro groups in 2-nitrobenzyl alcohol reduces the
acidity of the alcohol and therefore the silylation require a longer
reaction time.
SiO2
SiO2
O
O
O
O
O
O
Si
Si
N
O
N
NH
HN
Ru
O
OTf
Scheme 2. Preparation route for catalyst.
Triphenylmethanol and 2-methyl-1-phenyl-propan-2-ol as
examples of tertiary alcohols were successfully converted to their
corresponding TMS ethers (Table 1, entries 11 and 12).
Figure 1. UV–visible spectrum of: (A) SiO2@Fe and (B) RuIII(OTf)
SalophenCH2–NHSiO2–Fe.
In the case of linear, secondary and tertiary alcohols, the
reaction times were longer in comparison with benzylic alcohols.
The trimethylsilylation of phenols was also investigated and the
corresponding TMS ethers were produced in high yields (Table 1).
The turnover frequencies (TOFs) were also calculated for some
of these alcohols and phenols at shorter reaction times. These
values were added to Table 1, which shows the high catalytic
activity of the catalyst in these reactions (entries 1, 5, 11 and 15).
Because of the high catalytic activity of RuIII(OTf)SalophenCH2–
NHSiO2–Fe in the trimethylsilylation of both alcohols and phe-
nols, we decided to investigate the selectivity of this method.
As shown in Table 2, primary alcohols are more reactive in the
presence of secondary alcohols and phenols. It is important to
note that, owing to the lesser nucleophilicity of sterically
hindered alcohols and phenols in comparison with primary
benzylic alcohols, only primary benzylic alcohols were silylated
with 0.7 mmol HMDS (Table 2, entries 1 and 2). However, in a
results show that only 21% of the corresponding silyl ether was
produced. Upon replacement of chlorine with OTf, the RuIII(Cl)
SalophenCH2–NHSiO2–Fe was produced which completely
converted the benzyl alcohol to silyl ether. These observations
clearly show the role of OTf in increasing the catalytic activity
of the Ru center. When the same reaction was carried out
under the same reaction conditions in the presence of homo-
geneous RuIII(Salophen)OTf, the conversion was 52%. The
higher catalytic activity of heterogeneous catalyst can be
attributed to the dispersion of catalyst on nanomagnets
having a high surface area, which increases the accessibility
of catalytic active sites.
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Appl. Organometal. Chem. 2014, 28, 304–309

Synthesis, characterization and application of catalysts
SalophenCH2–NHSiO2–Fe with some of those reported in
the
literature
(Table
3).
For
example,
sulfonic
acid@nanoporous silica[10] needs long reaction time (55
min); H3PW12O40 requires a high temperature[14]; ZrO(OTf)2
is carried out with high amounts of HMDS[9]
; ZrCl4 is
degraded in the presence of moisture[8]; MgBr2.OEt2 needs
a high amount of catalyst (5%),[11] metalloporphyrins are
expensive to purchase[17–20] and the recovery of homogeneous
Ti Schiff base[21] is not as easy as this heterogeneous Ru Schiff base
catalyst. Therefore, RuIII(OTf)SalophenCH2–NHSiO2–Fe /HMDS is a
very efficient catalytic system for trimethylsilylation of alcohols
and phenols.
Catalyst Reusability
Figure 3. TEM image of catalyst.
Despite the homogeneous nature of Ru(Salophen)OTf, the
binary mixture of benzyl alcohol and 1-octanol, the benzyl
alcohol was converted to the corresponding silyl ether in 84%
yield, while only 16% of the corresponding silyl ether was
observed for the linear alcohol (Table 2, entry 3). It seems that
selective trimethylsilylation of alcohols in the presence of
phenols is attributed to higher nucleophilicity of alcohols.
In order to show the effectiveness of the present system in the
silylation reactions, we compared the results of trimethylsilylation
of benzyl alcohol with HMDS catalyzed by RuIII(OTf)
nanomagnet-supported catalyst can be separated with a
magnet and reused several times without significant loss of its
activity. The reusability of 1 was studied for trimethylsilylation
of benzyl alcohol. After each run, the catalyst was separated from
the reaction mixture with a magnet. By placing a magnet outside
the reaction vessel, the nanomagnet-supported catalyst is fixed
at the wall of the vessel, while the solution is decanted. This
allows the complete separation of the catalyst from the reaction
mixture without significant loss. The catalyst was washed in the
Table 1. Trimethylsilylation of alcohols and phenols with HMDS catalyzed by RuIII(OTf)SalophenCH2–NHSiO2–Fe (1)
at room temperaturea
RuIII (OTf)SalophenCH2-NHSiO2-Fe
H
CH2Cl2, R.T.
N
ROSiMe3 + NH3
+
ROH
Me3Si
SiMe3
Entry
R
Time (min)
Yield (%)b
TOF (hÀ1
)
1
C6H5CH2
1
1
100 (95)c
100 (94)
100 (97)
100 (96)
100 (97)
96 (91)
100 (96)
100 (94)
100 (94)
100 (93)
100 (95)
93 (90)
95 (92)
99
1425 (1800)d
1410
2
3-MeOC6H4CH2
4-HOC6H4CH2
2-BrC6H4CH2
4-MeOC6H4CH2
2-MeOC6H4CH2
2,4-Cl2C6H3CH2
4-BrC6H4CH2
C7H15
3
1
1455
4
1
1440
1455 (3510)d
5
1
6
3
480
7
3
480
8
2
705
9
6
235
10
11
12
13
14
15
16
17
18
C8H17
8
175
109 (200)e
Cyclohexyl
Ph3C
13
15
15
5
93
C6H5CH2C(Me)2
C6H5
95
297
242 (260)f
2-ClC6H4
6
97
3-ClC6H4
5
99
297
2-H2NC6H4
2-O2NC6H4
10
10
90
135
92
138
aReaction conditions: alcohol or phenol (1 mmol), HMDS (0.7 mmol), catalyst (80 mg, 0.04 mmol Ru), CH2Cl2 (1 ml).
bGC yield.
cYields in parentheses refer to isolated yield.
dTurnover frequency (TOF) after 20 s.
eTOF after 5 min.
fTOF after 3 min.
Appl. Organometal. Chem. 2014, 28, 304–309
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M. Torki et al.
Table 2. Selective silylation of alcohols and phenols catalyzed by RuIII(OTf)SalophenCH2–
a
NHSiO2–Fe (1) in CH2Cl2
RuIII(OTf)SalophenCH2-NHSiO2-Fe
H
N
CH2Cl2, R.T.
ROSiMe3 + R'OSiMe3 + NH3
ROH +
R'OH +
Me3Si
SiMe3
Entry
R
R′
Time
(min)
Yield (%)b
R
R′
1
2
3
C6H5CH2
C6H5CH2
C6H5CH2
Cyclohexyl
C6H5
1
1
1
100
100
84
0
0
C8H17
16
aReaction conditions for a binary mixture: 1 mmol of each alcohol or phenol, HMDS (0.7
mmol), catalyst (1 mol%), CH2Cl2 (1 ml).
bGC yield.
Table 3. Comparison of the results obtained for the trimethylsilylation of benzyl alcohol catalyzed by RuIII(OTf)SalophenCH2–NHSiO2–Fe with those
obtained by the recently reported catalysts
CH2OH
CH2OSiMe3
Catalyst
Entry
Catalyst
HMDS (mmol) Catalyst (mol%) T (°C)
Time (min) Yield (%) TOF (hÀ1
)
Ref.
1
RuIII(OTf)SalophenCH2–NHSiO2–Fe
HClO4.SiO2
0.7
0.8
0.8
2
4
r.t.
r.t.
1
2
100
98
1425
1176
2850
11040
36
This work
[7]
2
2.5
2
3
ZrCl4
r.t.
1
95
[8]
4
ZrO(OTf)2
0.5
3
r.t.
1
92
[9]
5
Sulfonic acid@nanoporous silica
MgBr2.OEt2
0.6
0.6
0.7
0.7
0.8
0.5
0.7
r.t.
55
5
99
[10]
[11]
[12]
[13]
[14]
[19]
[21]
6
5
r.t.
98
235
7
CuSO4.5H2O
1
r.t.
12
5
98
490
8
Fe(TFA)3
2.5
1
r.t.
92
442
9
H3PW12O40
55–60
r.t.
23
2
90
235
10
11
[SnIV(TNH2PP)Cl2]@CMP
[TiIV(Salophen)(OTf)2]
1
100
100
3000
6000
1
r.t.
1
Table 4. Reusability of catalyst 1 in the trimethylsilylation of benzyl
alcohol with HMDS at room temperaturea
Run
TMS ether (%)b
Time (min)
Ru leached (%)c
1
2
3
4
5
100
100
100
100
100
1
1
1
1
1
0
0
0
0
0
aReaction conditions: alcohol (1 mmol), HMDS (0.7 mmol), catalyst
(80 mg, 0.04 mmol Ru), CH2Cl2 (1 ml).
bGC yield.
cDetermined by ICP.
Figure 4. UV–visible spectrum of recovered catalyst.
reaction vessel with CH3CN and EtOH and dried before use in the
subsequent run. After using the catalyst for five consecutive runs,
the TMS ether yield was still 100% (Table 4). The filtrates were col-
lected for determination of Ru leaching by ICP analysis. No
detectable leaching of Ru from the support was observed after
five consecutive reaction cycles. FT-IR (Fig. 4) and UV–visible
(Fig. 5) spectra of recovered catalyst show that the catalyst has
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Synthesis, characterization and application of catalysts
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Acknowledgment
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