Cu3(BTC)2 catalyzed oxidation of silane to silanol using TBHP or water as oxidants

Article abstract of DOI:10.1016/j.apcata.2017.07.028

In the present work, a series of metal organic frameworks are examined for the conversion of Si-H to Si-OH using either t-butylhydroperoxide (TBHP) or water as oxidants. The reaction is optimized using dimethylphenylsilane (1) as a model substrate. It is observed that Cu₃(BTC)₂ (BTC: 1,3,5-benzenetricarboxylate) exhibits a comparable activity with Zr(BDC) (BDC: 1,4-benzenedicarboxylate) while the activity of Fe(BTC) is lower than Cu₃(BTC)₂ using TBHP as oxidant. On the other hand, the reaction of 1 with water in the presence of Cu₃(BTC)₂ as a catalyst showed complete conversion of 1 with 99% selectivity to the corresponding silanol, but other MOFs like Fe(BTC) and Zr(BDC) are inactive under identical reaction conditions. A series of control experiments indicate that Cu²⁺ is essential to convert 1 to 2 under the present experimental conditions. Further, Cu²⁺ in Cu₃(BTC)₂ acts as redox centre with TBHP whereas it behaves as a Lewis acid using water as oxidant. High conversion and selectivity is observed for all the silanes studied under the present experimental conditions. The catalyst stability is assessed by powder XRD, FT-IR and SEM images and observing no structural deterioration of Cu₃(BTC)₂ either in TBHP or water as oxidants. Furthermore, hot filtration test indicated the absence of copper under the present reaction conditions, thus confirming the stability of Cu₃(BTC)₂.

Full text of DOI:10.1016/j.apcata.2017.07.028

Accepted Manuscript
Title: Cu3(BTC)2 Catalyzed Oxidation of Silane to Silanol
using TBHP and Water as Oxidants
Authors: Nagaraj Anbu, Amarajothi Dhakshinamoorthy
PII:
S0926-860X(17)30337-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.028
APCATA 16333
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Applied Catalysis A: General
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Accepted date:
3-4-2017
30-6-2017
17-7-2017
Please cite this article as: Nagaraj Anbu, Amarajothi Dhakshinamoorthy, Cu3(BTC)2
Catalyzed Oxidation of Silane to Silanol using TBHP and Water as Oxidants, Applied
Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.028
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Cu3(BTC)2 Catalyzed Oxidation of Silane to Silanol using TBHP and Water as Oxidants
Nagaraj Anbu, Amarajothi Dhakshinamoorthy*
School of Chemistry, Madurai Kamaraj University, Madurai-21.
E-mail:admguru@gmail.com
Graphical Abstract
Highlights

To develop a simple and convenient water stable catalyst for the oxidation of silanes to silanols in
replacing precious noble metal based catalysts.


To avoid the use of strong oxidizing agents such as permanganate, peroxides, ozone, and peracids.
To develop a highly stable and reusable catalyst for the oxidation of silane to silanol in water.
Abstract
In the present work, a series of metal organic frameworks are examined for the
conversion of Si-H to Si-OH using either t-butylhydroperoxide (TBHP) or water as oxidants.
The reaction is optimized using dimethylphenylsilane (1) as a model substrate. It is observed
that Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylate) exhibits a comparable activity with Zr(BDC)
(BDC: 1,4-benzenedicarboxylate) but however the activity of Fe(BTC) is lower than Cu3(BTC)2
using TBHP as oxidant. On the other hand, the reaction of 1 with water in the presence of
Cu3(BTC)2 as a catalyst showed complete conversion with 99% selectivity to the corresponding
silanol while other MOFs like Fe(BTC) and Zr(BDC) are inactive under identical reaction
conditions. A series of control experiments indicate that Cu2+ is essential to convert 1 to 2
under the present experimental conditions. Further, Cu2+ in Cu3(BTC)2 acts as redox centre

while the use of TBHP whereas it behaves as a Lewis acid using water as oxidant. High
conversion and selectivity is observed for all the silanes studied under the present
experimental conditions. The catalyst stability is monitored by powder XRD, FT-IR and SEM
images and observing no structural deterioration of Cu3(BTC)2 either in TBHP or water as
oxidants. Furthermore, hot filtration test indicated the absence of copper under the present
reaction conditions, thus confirming the stability of Cu3(BTC)2.
Keywords: Metal Organic Frameworks; TBHP; Silane oxidation; Si-H activation; Water
1. Introduction
Silanols are compounds containing Si-OH bonds and are silicon analogues of alcohols
[1], which have found important applications in various fields [2], especially in the production
of silicon-based polymeric materials [3]. Furthermore, several organosilanols have shown to
exhibit bioactivity [4, 5] and some of them were applied as structural components in the
transition-state analogues of metalloprotease inhibitors [6, 7]. In addition, silanols are useful
synthons in various fields of organic synthesis as well as in materials science [8-11]. In organic
synthesis, they have been employed as a nucleophilic coupling partner in metal catalyzed
carbon-carbon cross-coupling reactions [12-14], hydrogen bond donors in organocatalysis
[15-17] and directing groups in C-H bond activation [18-21]. A variety of procedures of have
been developed for the synthesis of silanols which include hydrolysis of chlorosilanes [22],
oxidation of organosilanes with stoichiometric amounts of oxidants[23] and treatment of
siloxanes with alkali metal reagents [24]. However, some of the main issues associated with
these methods are limited scope, formation of toxic by-products and generation of
disiloxanes as side products consequent from the condensation of silanols. On the other hand,
catalytic oxidations of the Si-H bond of organosilanes have been reported with Ni, Cr, Rh, and
Cu complexes [25-28]. However, these protocols showed a rather limited selectivity to certain

substrates and they produce disiloxanes in high amounts in many cases. In order to overcome
these drawbacks, there is a huge demand to develop an efficient catalyst for the conversion
of silanes into silanols.
The development of efficient synthetic methods, which minimize environmental
problems, is a great challenge for chemists in both industrial and academic research. In this
context, homogeneous transition metal catalysts have emerged to meet these requirements
mentioned above, but, however their use accompany with a difficulty of residual metal
impurities in the final products and they suffer from reusability standpoint. Subsequently,
heterogeneous catalytic systems have emerged as potential alternatives, because of the
inherent operational advantages such as ease of handling, separation and recovery of the
catalyst from the reaction mixture for reusability [29]. However, one of the major issues of
heterogeneous catalysis are gradual leaching of transition metals from the solid supports [30,
31].
During the last few decades, metal organic frameworks [32, 33] (MOFs) have emerged
as potential candidates for variety of applications in the field of gas storage [34], gas
separation [35], drug delivery [36], sensor[37] and heterogeneous catalysis [38]. MOFs are a
class of crystalline porous solid whose structure is constructed from metal nodes or cluster
interconnected by rigid bi- or multipodal organic linkers [39-41]. They possess large surface
area, high porosity, structural diversity and high metal density [42]. In this regard, Cu3(BTC)2
is one of the well-known MOF, which is composed from dimeric cuprate tetra carboxylate
units [43]. Furthermore, Cu3(BTC)2 has been used as heterogeneous catalysts for a wide range
of reactions including oxidation of benzylic alcohol [44], cyanosilylation of benzaldehyde [45],
ring opening of epoxide [46], benzylic oxidation [47], Knoevenagel condensation [48] and

dehydrogenative coupling of silane [49]. The above reactions suggested that Cu3(BTC)2 is a
stable catalyst in promoting these organic transformations. However, certain MOFs showed
poor stability in polar solvents like water and DMF and in particular with water due to their
high dielectric constant separating ion-pairs can in principle cause deterioration of the crystal
structure [50]. Hence, it is always challenging to develop a catalytic system for organic
transformation using water as solvent or oxidant. Hence, the present study aims to report the
activity of Cu3(BTC)2 as a stable and an efficient reusable heterogeneous catalyst for oxidation
of silanes to silanols either in water or with TBHP as oxidants. On the other hand, the activity
of Cu3(BTC)2 is compared with analogous heterogeneous solid MOFs. Moreover, the stability
of Cu3(BTC)2 is also surveyed under these experimental conditions. One of the main objectives
of the present work is to develop a simple and convenient catalyst in replacing strong
oxidizing agents that have been frequently used for the conversion of silanes to silanols such
as permanganate, peroxides, ozone, and peracids [51-53]. However, these protocols usually
lead to significant amounts of byproducts, such as siloxanes that are produced from silanol
condensation. In addition, the present work could be an alternative protocol for the
replacement of the use of precious noble metals like gold, silver, palladium nanoparticles [54-
56] and bimetallic nanoparticles [57] which have been used as catalyst for the oxidation of
hydrosilanes in the presence of water as oxidant.
2. Experimental
2.1 Materials
Cu3(BTC)2 and Fe(BTC) were purchased from Sigma Aldrich under the commercial
trade Basolite C300 and Basolite F300 respectively. Zr(BDC) was synthesized by following the
protocol reported in an earlier precedent [58]. A dimethylphenylsilane, triphenylsilane and
diphenylsilane used in the present study were purchased from Sigma Aldrich and used as

received. tert-Butylhyroperoxide and acetone were purchased from Sigma Aldrich and used
as received. Tetrahydrofuran (THF), ethyl acetate and sodium sulfate were also purchased
from Sigma Aldrich and used as received.
2.2 Instrumentation
Powder XRD diffraction patterns were measured in the refraction mode in a Philips
X’Pert diffractometer using the CuK radiation ( = 1.54178 Å) as the incident beam,
PW3050/60 (2 theta) as Goniometer, PW 1774 spinner as sample stage, PW 3011 as detector,
incident mask fixed with 10 mm. PW3123/10 for Cu was used as a monochromator.
PW3373/00 Cu LFF was used as X-ray tube with power scanning of 45 kV and 40 mA current.
The sample powder was loaded into a holder and leveled with a glass slide before mounting
it on the sample chamber. The specimens were scanned between 2o and 70o with the scan
rate of 0.02o/s. Scanning electron microscope (SEM) studies were performed using a Hitachi
S-3000H scanning electron microscope.
2.3 Reaction procedure
In a typical experimental procedure, 25 mg of Cu3(BTC)2 was charged into a 50 mL
round bottom flask followed by the addition of 1 mmol of silane and 3 mmol of TBHP. This
reaction mixture was stirred at 60 oC for 20 h. The progress of the reaction was monitored by
gas chromatography and after completion of the reaction, the mixture was washed twice with
acetonitrile and filtered. Then, the product is analyzed by gas chromatography for its purity
and selectivity. Conversion and selectivity were determined by Agilent gas chromatography
using internal standard method.
2.4 Reaction procedure using water as nucleophile
Typically, 25 mg of Cu3(BTC)2 was loaded into a 50 mL round bottom flask following
the addition of 1 mmol of silane, 20 mmol deionized H2O and 1 mL of THF. This reaction

mixture was stirred at 65 oC for 4 h. It was observed that the reaction mixture turns from blue
to brown and returned back to original colour after 4 h with the evolution of hydrogen gas.
The reaction progress was monitored by gas chromatography and after completion of the
reaction, the mixture was extracted with ethyl acetate followed by acetonitrile, filtered and
then dried with sodium sulfate. Then, the product is analyzed by gas chromatography for its
purity and selectivity. Conversion and selectivity were determined by Agilent gas
chromatography using internal standard method. The same procedure is followed for the
reusability experiments.
3. Results and Discussion
3.1. Structural aspects of MOFs
The crystal structure of Fe(BTC) is constituted between Fe(III) as a nodal metal ion and
tripodal BTC as ligand. It is believed that the crystal structure of Fe(BTC) resembles with MIL-
100(Fe) which is an iron(III) carboxylates built up from trimers of iron octahedra sharing a
common vertex μ3-O. These trimers are linked by the BTC ligands in such a way that this leads
to the formation of hybrid supertetrahedra thus resulting in two types of mesoporous cages
of free apertures of around 25 and 29 Å, accessible through microporous windows of around
5.5 and 8.6 Å. The BET surface area of Fe(BTC) was 1300 m2/g. The second MOF in the present
work is Cu3(BTC)2 whose crystal structure is constituted by the clusters of two Cu(II) ions
coordinated with four BTC ligands defining a paddle-wheel building unit that acts as a
structural node held in place by the tripodal BTC linkers. It has a surface area around 1050
m2/g with 11, 16 and 6 Å pore dimensions. Furthermore, the crystal structure of UiO-66
[Zr(BDC)] is constructed between cationic Zr6O4(OH)4 nodes formed in situ via hydrolysis of
ZrCl4 and BDC linkers with a cubic framework having high crystallinity. The BET surface area
and pore dimensions of Zr(BDC) is 950 m2/g and 8.4, 7.4 Å respectively.

3.2. Oxidation of silanes with TBHP
In the initial stage of our study, dimethylphenylsilane (1) was chosen as a model
substrate for the conversion of silane to silanol (Scheme 1) using TBHP as oxidant and the
attained results are given in Table S1. The reaction was optimized with respect to the
concentration of TBHP and observing that the conversion of 1 increased gradually by
increasing TBHP from 1 to 3 mmol in the presence of Cu3(BTC)2 as solid catalyst at 60 oC (Figure
1, Table S1). The maximum conversion of 1 was achieved to be 52, 92 and 96% with 1, 2 and
3 mmol of TBHP as oxidant, respectively (Table S1). Therefore, further studies were
performed with 3 mmol of TBHP as oxidant. In contrast, Cu3(BTC)2 afforded 20% conversion
of 1 with complete selectivity of 2 at room temperature after 20 h (Table S1).
Scheme 1. Oxidation of silane into silanol using TBHP or water as oxidant in the presence of
MOFs as heterogeneous solid catalysts.

Figure 1. Time conversion plot for the conversion of 1 using Cu3(BTC)2 as heterogeneous solid
catalyst with (a) 1, (b) 2 and (c) 3 mmol of TBHP as oxidant. Reaction conditions: 1 (1 mmol),
TBHP (mmol), Cu3(BTC)2 (25 mg), 60 oC.
After optimizing the concentration of TBHP with Cu3(BTC)2 for the oxidation of 1 to 2,
a series of MOFs were studied as heterogeneous solid catalysts which include, Fe(BTC) and
Zr(BDC) under the optimized reaction conditions. The observed results are given in Table S2.
A maximum conversion of 1 was achieved to be 96, 90 and 97% using Cu3(BTC)2, Fe(BTC) and
o
Zr(BDC) as catalysts, respectively, after 20 h with 3 mmol TBHP as oxidant at 60 C. The time
conversion plots for the conversion of 1 to 2 using different catalysts are given in Figure 2. On
the other hand, a blank control experiment in the absence of catalyst showed 32% conversion
of 1 after 20 h under identical reaction conditions, thus clearly establishing the role played by
the MOFs in promoting the oxidation of 1 to 2. In all these cases, it was observed that the
selectivity of 2 is much higher in the initial reaction time, but, however, the formation of 3 is
noticed upon reaction progress resulting from the condensation of 2, which is predominant
along with reaction time (Figure 2). It has been reported that Cu3(BTC)2 can convert 1 to the
corresponding siloxane [59] and thus, in the present case, it is believed that self-condensation
of 2 occurs at long reaction time thus reducing the selectivity of 2. In order to understand the
superior activity of Cu3(BTC)2 for the conversion of 1 to 2, a series of control experiments were
performed using homogeneous catalysts such as Cu(NO3)2.3H2O, Fe(NO3)3.9H2O and
ZrOCl2.8H2O. The observed results are given in Table S3. The conversion of 1 to 2 was gradually
increased with all these catalysts, but, high conversion of 1 is achieved with Cu(NO3)2.3H2O as
a homogeneous catalyst which suggest that Cu2+ is the active site in promoting the conversion
of 1 to 2. Furthermore, comparing the selectivity of 2 between Cu3(BTC)2 and Cu(NO3)2.3H2O
as catalyst suggest that the confined environment and the porous nature of Cu3(BTC)2 favors
to achieve better selectivity of 2. Also, these experiments indicate that the reaction takes

place within the pores of Cu3(BTC)2 but however, one cannot avoid the reaction occurring on
the surface of Cu3(BTC)2. In a way to understand whether Cu2+ in Cu3(BTC)2 acting as a Lewis
acid site or redox centre, a control experiment was undertaken in the presence of pyridine as
a catalyst poison. The observed results are given in entry 4, Table S3. The conversion of 1
catalyzed by Cu3(BTC)2 using TBHP as oxidant has no influence over the use of pyridine. If one
assumes that Cu2+ in Cu3(BTC)2 is acting as a Lewis acid site, the conversion of 1 should have
been reduced or completely suppressed under identical reaction conditions. Furthermore,
the color of Cu3(BTC)2 was retained during the course of reaction and no color change is
observed. These experimental data indicate that Cu2+ is behaving as a redox centre than a
Lewis acid site when TBHP is used as oxidant.
Reusability experiments were performed for the oxidation of 1 to 2 using Cu3(BTC)2 as
solid catalyst with TBHP as oxidant under the optimized reaction conditions. The conversion
of 1 was 98%, 92%, 91% and 90% after 20 h for the fresh, first, second and third reuses
respectively. These data indicate that the catalyst maintains its activity for three recycles
without any decay in its activity. Furthermore, the catalyst stability was also investigated by
comparing the fresh Cu3(BTC)2 with the recovered Cu3(BTC)2 for the conversion of 1 to 2 using
TBHP as oxidant by different analytical and morphological studies. Analysis of the powder XRD
patterns of the fresh Cu3(BTC)2 with the recovered Cu3(BTC)2 for the oxidation of 1 to 2
revealed that the structural integrity of Cu3(BTC)2 is maintained during the course of the
reaction (Figure 3). This observation was also holds good for other substrates like
diphenylsilane and triphenylsilane under identical conditions (Figure 3). Furthermore,
comparison of FT-IR spectra of fresh Cu3(BTC)2 with the recovered Cu3(BTC)2 for the oxidation
of 1 to 2 with TBHP as oxidant showed no significant changes in the stretching frequencies,
thus indicating the retainment of the solid framework (Figure 4). On the other hand, SEM

images of fresh and the recovered Cu3(BTC)2 for the reaction of 1 to 2 with TBHP indicate that
the particle size is considerably reduced after the reaction.

Figure 2. Conversion of 1 (black line) and selectivity of 2 (red line) using (a) Cu3(BTC)2, (b)
Fe(BTC), (c) Zr(BDC)as MOF solid heterogeneous catalysts and (d) blank control experiment.
Oxidation of triphenylsilane to the corresponding silanol using Cu3(BTC)2 as solid catalyst (e).
Reaction conditions: 1 (1 mmol), TBHP (3 mmol), catalyst (25 mg), 60 oC.
Figure 3. Powder XRD pattern of (a) fresh, (b) the recovered MOF after the reaction of 1 with
TBHP, (c) the recovered MOF from triphenylsilane with TBHP and (d) the recovered catalyst
from diphenylsilane with TBHP Cu3(BTC)2.

Figure 4. FT-IR spectrum of (a) fresh and (b) the recovered Cu3(BTC)2 after the reaction of 1
with TBHP.

Figure 5. SEM images of (a) fresh and (b) the recovered Cu3(BTC)2 after the reaction of 1 with
TBHP.
The scope of Cu3(BTC)2 was further studied with various silanes like diphenylsilane and
triphenylsilane. The observed conversion and selectivity data are given in Table S4. The
conversion of diphenylsilane to the corresponding silanol increased as a function of time but,
however, the selectivity of silanol is 39% after 20 h. This may be due to the presence of two
hydrogens and it is expected to give more than one silanol. This observation is in good
agreement with earlier procedure showing higher reactivity of this substrate with Cu3(BTC)2
[49]. On the other hand, Cu3(BTC)2 promoted the oxidation of triphenylsilane to
triphenylsilanol with 98% conversion and a complete selectivity of silanol under identical
conditions (Figure 2).

3.3. Oxidation of silanes with water
In recent years, the use of MOFs as heterogeneous catalysts for various organic
transformations has become a intensive research area. A highly challenging task in
heterogeneous catalysis by MOFs is to develop catalytic systems involving MOFs as
heterogeneous solid catalysts in the presence of water without compromising the stability
under the experimental conditions. The chemical reactions using water as a medium has
many advantages from environmental benignity, low cost, abundance, non-explosive, non-
flammable and non-toxic. Recently, the catalytic transformation of silanes into silanols, using
water as a green oxidant has been reported with many catalytic systems like nanoporous
carbon [60], Au/SiO2 [61], Au/KCC-1-APTS [62], Au/HAP [63], Au-MPBen [64], Ag-Hap [55],
[RuCl2(p-cymene)]2 [65], [IrCl(C8H12)]2 [66], [Re(O)(hoz)2(Solv)]B(C6F5)4 [67], Au NP decorated
o-CNT [68], [Re(O)(hoz)2(CH3CN)]B(C6F5)4 [69], [RuCl2(p-cymene)]2/C [70] and
Ru2(CO)4(PPh3)2Br2 [71]. These reports triggered us to examine the activity of Cu3(BTC)2 for
oxidation of silane to silanol using water as oxidant. The observed results are given in Table
1. Among a series of MOFs tested as catalysts for the conversion of 1 to 2 using water as an
oxidant, Cu3(BTC)2 exhibited complete conversion of 1 with 99% selectivity of 2 (Table S5). On
the other hand, Fe(BTC) and Zr(BDC) did not afford any conversion of 1 to 2 under identical
reaction conditions. This observation is in accordance with earlier report in where Cu3(BTC)2
promotes dehydrogenative coupling between 1 and n-butanol [49]. Also, the blank control
experiment in the absence of catalyst showed no oxidation of 1 to 2 under identical
conditions. This observation was further supported by performing a series of control
experiments with homogeneous catalysts like Cu(NO3)2.3H2O, Fe(NO3)3.9H2O and
ZrOCl2.8H2O. The observed results are given in Table S6. These data indicate that the
conversion of 1 to 2 is effective only in the presence of Cu(NO3)2.3H2O while other

homogeneous catalyst showed negligible conversions of 1. These experiments clearly confirm
that Cu2+ is essential to promote the conversion of 1 to 2 under the present experimental
conditions. Further, comparing the product selectivity of 2 between Cu(NO3)2.3H2O and
Cu3(BTC)2 indicates that the selectivity is always more than 98% in the presence of Cu3(BTC)2
and it is not influenced by the amount of water present in the reaction medium. In contrast,
the selectivity of 2 decreased gradually from 72% at 30 min to 65% after 4 h, thus establishing
the importance of the porosity of Cu3(BTC)2 to achieve very high selectivity. This can be
considered as an indirect proof that the reaction takes place inside the pores of Cu3(BTC)2 but
however, one cannot exclude the contribution from external active sites. On the other hand,
another control experiment was performed using pyridine as a poison for the Lewis acid sites
in Cu3(BTC)2. It was observed that the conversion of 1 was significantly reduced from 99% to
0% after 1 h in the presence of pyridine (Table S7, Figure 7). These experiment indicate that
Cu2+ in Cu3(BTC)2 is acting as a Lewis acid site rather than redox centre. Furthermore, the color
of Cu3(BTC)2 is changed from blue to pale brown by simultaneously liberating hydrogen gas.
However, the color of Cu3(BTC)2 return back to its original color after 4 h. As it can be seen in
Table 1, the conversion of 1 to 2 is very much influenced by the presence of water. A complete
conversion of 1 was achieved after 1 h using Cu3(BTC)2 as catalyst either with 15 or 20 mmol
of water with very high selectivity (~ 99%) of 2. In contrast, the conversion of 1 was 98% after
4 h in the presence of Cu3(BTC)2 in THF without any water and the selectivity of 2 is 33% and
this lower selectivity due to the formation of the corresponding disiloxane with 67%. These
results clearly indicate the active role played by water as an oxidant/nucleophile under the
present experimental conditions. Furthermore, the conversion of 1 and selectivity of 2 did not
have any influence on catalyst loading. Under identical conditions, a complete conversion of
diphenylsilane was observed with 76% selectivity of the corresponding silanol after 1 h (entry

2, Table S8). Further, the oxidation of diphenylsilane to the corresponding silanol showed 93%
conversion with 76% selectivity at room temperature. On the other hand, efforts to convert
triphenylsilane to triphenylsilanol was unsuccessful and this is in contrast with the data using
TBHP as oxidant using Cu3(BTC)2 as catalyst (entry 4, Table S8). This result indicates that
triphenylsilane encounter some steric hindrance to reach the Lewis acid site either at
external/internal pores of Cu3(BTC)2. It is relevant to mention that the color of Cu3(BTC)2 is
not changed during the course of the reaction.
a
Table 1. Oxidation of 1 into 2 using Cu3(BTC)2 as catalyst under various conditions.
Entry
Water
content
(mmol)
Time (h)
Conversion of 1 (%)b
Selectivity of
2 (%)b
30 min
1
2
4
30 min
1
2
4
1
30
95
98
3
12
45
98
100
100
37(63)
33(67)
100
100
100
94
1
2
3
4
5
-
5
30 min
1
2
4
30 min
1
2
4
13
91
98
100
96
>99
100
100
97
>99
100
100
100
98
97
96
99
99
99
99
99
10
15
30 min
20
1
2
4
99
99
99

aReaction conditions: 1 (1 mmol), H2O, THF (1 mL), Cu3(BTC)2 (25 mg), 65 oC.
bConversion and selectivity were determined by GC.
One of the issues to be addressed in heterogeneous catalysis is the stability of catalyst
under the optimized reaction conditions. Hence, the stability of Cu3(BTC)2 was ascertained by
reusing it in consecutive cycles, performing leaching experiment (hot filtration test) and
analyzing the spent catalyst by powder XRD, SEM and FT-IR techniques. The catalyst was
recovered from the reaction mixture after the required time, washed with ethyl acetate
o
followed by acetonitrile and dried at 100 C for 4 h. Then, this catalyst was reused in
consecutive cycles with the fresh reactants under identical conditions. As it can be seen in
Table S9, the catalyst was reused in four consecutive cycles without any decay in conversion
of 1 (Figure 6). On the other hand, hot filtration test indicated the absence of copper in the
reaction mixture, thus confirming the heterogeneity of the process under the present
experimental conditions (Figure 7, Table S7). In addition, comparison of the chemical analysis
data between fresh and three times reused Cu3(BTC)2 did not show any difference in the
copper content, thus supporting the results of leaching experiment. Furthermore, comparison
of the powder XRD patterns of fresh Cu3(BTC)2 with those recovered after the reaction with 1
and diphenylsilane indicated no noticeable changes in the crystalline nature of these samples,
thus supporting the retainment of structural integrity of Cu3(BTC)2 using water as oxidant
(Figure 8). In addition, comparison of the fresh Cu3(BTC)2 with the recovered after the reaction
with 1 and four times reused Cu3(BTC)2 by FT-IR analysis showed no significant differences in
the stretching frequencies (Figure 9). On the other hand, comparison of the structural
morphology of fresh, the recovered catalyst after the reaction with 1 in water as oxidant and
four times reused Cu3(BTC)2 by SEM show significant changes in their strutural morphology
(Figure 10). It can be clearly seen that the crystallite size is reduced or surface is etched in the

recovered as well as reused catalyst. This observation is in accordance with an earlier
precedent in where Cu3(BTC)2 undergoes a structural deterioration over the exposure with
water [72]. However, in any case, the activity of this catalyst is retained as evidenced from
the reusability experiments. In addition, the catalyst stability was further ascertained by
performing the reaction with 5 mmol under identical conditions. It was interesting to note
that more than 99% conversion of 1 is achieved after 2 h with 99% selectivity of 2 (entry 5,
Table S8). As the conversion and selectivity was almost quantitative, the reaction mixture was
allowed to stir with ethylacetate and filtered. The removal of the solvent afforded 94% yield
of 2. As commented earlier, the conversion of 1 to 2 has been achieved with many catalytic
systems involving the use of metal nanoparticles supported on different supports exhibiting
high turnover numbers. However, this work reports the activity of Cu3(BTC)2 as a reusable
heterogeneous MOFs catalyst for this reaction and the activity achiveved here is lower than
with metal nanoparticles. All these data firmly support that Cu3(BTC)2 can be conveniently
used as heterogeneous solid catalyst for the conversion of 1 to 2 using water as oxidant.

Figure 6. Reusability of Cu3(BTC)2 as heterogeneous solid catalyst for the oxidation of 1 to 2
using water as oxidant. Reaction conditions: 1 (1 mmol), H2O (20 mmol), THF (1 mL) and
Cu3(BTC)2 (25 mg), 65 oC, 4 h.
Figure 7. Leaching test for the oxidation of 1 to 2. In the presence of catalyst (▲), the catalyst
filtered after 15 minutes (●) and the poison test with pyridine under the optimized conditions
(■).Reaction conditions: 1 (1 mmol), H2O (20 mmol), THF (1 mL) and Cu3(BTC)2 (25 mg), 65 oC,
4 h.

Figure 8. Powder XRD pattern of Cu3(BTC)2 (a) fresh, (b) recovered after the reaction of 1 with
H2O, (c) recovered after the reaction of diphenylsilane with H2O.
Figure 9. FTIR spectrum of (a) fresh, (b) recovered after the reaction from 1 and (c) four times
used Cu3(BTC)2.

Figure 10. SEM images of (a) fresh, (b) recovered after the reaction of 1 in water and (c) four
times used Cu3(BTC)2.
3.4. Proposed mechanism
Based on our findings, we propose a tentative mechanism for the oxidation of 1 to 2
using Cu3(BTC)2 using water as an oxidant. Cu3(BTC)2 interacts with 1 by changing its blue color

to pale brown as shown in Scheme 2. Then, water reacts with 1 to form an intermediate which
later gives the desired product 2. It is experimentally observed that the liberation of hydrogen
stops at lower reaction time (15 min). Further, the liberation of hydrogen from this reaction
is confirmed by gas chromatography. In addition, the color of Cu3(BTC)2 slowly returns back
to its original color.
Scheme 2. A proposed mechanism for the oxidation of 1 to 2 with water using Cu3(BTC)2 as
heterogeneous solid catalyst.
4. Conclusions
A series of MOFs were examined for the conversion of 1 to 2 either in the presence of
TBHP or with water as oxidants. It was noticed that Cu3(BTC)2 shows superior activity but
however Fe(BTC) and Zr(BDC) did not show any activity with H2O as oxidant. On the other
hand, quantitative conversion of 1 was achieved using water as an oxidant in the presence of

Cu3(BTC)2 as a catalyst with very high selectivity to 2. The catalyst stability was studied by
powder XRD, FT-IR and SEM using TBHP and water as oxidants and observing no structural
changes were observed under the present experimental conditions. However, SEM images of
the fresh and reused Cu3(BTC)2 showed a considerable change in their crystallite size both in
TBHP and H2O as oxidants. Finally, hot filtration test showed the absence of Cu2+ in the
solution under the present experimental conditions, thus, further confirming the stability of
Cu3(BTC)2 and heterogeneity of the process. A series of control experiments indicate that Cu2+
is essential to convert 1 to 2 under the present experimental conditions. Further, Cu2+ in
Cu3(BTC)2 acts as redox centre while the use of TBHP whereas it behaves as a Lewis acid using
water as oxidant.
Acknowledgements
AD thanks University Grants Commission, New Delhi, for the award of Assistant
Professorship under its Faculty Recharge Program. AD also thanks the Department of Science
and Technology, India, for the financial support through Extra Mural Research funding
(EMR/2016/006500). Authors would like to thank DST-PURSE for financial support rendered
to purchase SEM instrument.

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List of Captions
Scheme Captions
Scheme 1. Oxidation of silane into silanol using TBHP or water as oxidant in the presence of
MOFs as heterogeneous solid catalysts.
Table Caption
a
Table 1. Oxidation of 1 into 2 using Cu3(BTC)2 as catalyst under various conditions.
Figure Captions
Figure 1. Time conversion plot for the conversion of 1 using Cu3(BTC)2 as heterogeneous solid
catalyst with (a) 1, (b) 2 and (c) 3 mmol of TBHP as oxidant. Reaction conditions: 1 (1 mmol),
TBHP (mmol), Cu3(BTC)2 (25 mg), 60 oC.
Figure 2. Conversion of 1 (black line) and selectivity of 2 (red line) using (a) Cu3(BTC)2, (b)
Fe(BTC), (c) Zr(BDC)as MOF solid heterogeneous catalysts and (d) blank control experiment.
Oxidation of triphenylsilane to the corresponding silanol using Cu3(BTC)2 as solid catalyst (e).
Reaction conditions: 1 (1 mmol), TBHP (3 mmol), catalyst (25 mg), 60 oC.
Figure 3. Powder XRD pattern of (a) fresh, (b) the recovered MOF after the reaction of 1 with
TBHP, (c) the recovered MOF from triphenylsilane with TBHP and (d) the recovered catalyst
from diphenylsilane with TBHP Cu3(BTC)2.
Figure 4. FT-IR spectrum of (a) fresh and (b) the recovered Cu3(BTC)2 after the reaction of 1
with TBHP.
Figure 5. SEM images of (a) fresh and (b) the recovered Cu3(BTC)2 after the reaction of 1 with
TBHP.
Figure 6. Reusability of Cu3(BTC)2 as heterogeneous solid catalyst for the oxidation of 1 to 2
using water as oxidant. Reaction conditions: 1 (1 mmol), H2O (20 mmol), THF (1 mL) and
Cu3(BTC)2 (25 mg), 65 oC, 4 h.

Figure 7. Leaching test for the oxidation of 1 to 2. In the presence of catalyst (▲), the catalyst
filtered after 15 minutes (●) and the poison test with pyridine under the optimized conditions
(■).Reaction conditions: 1 (1 mmol), H2O (20 mmol), THF (1 mL) and Cu3(BTC)2 (25 mg), 65 oC,
4 h.
Figure 8. Powder XRD pattern of Cu3(BTC)2 (a) fresh, (b) recovered after the reaction of 1 with
H2O, (c) recovered after the reaction of diphenylsilane with H2O.
Figure 9. FTIR spectrum of (a) fresh, (b) recovered after the reaction from 1 and (c) four times
used Cu3(BTC)2.
Figure 10. SEM images of (a) fresh, (b) recovered after the reaction of 1 in water and (c) four
times used Cu3(BTC)2.

Scheme 2. A proposed mechanism for the oxidation of 1 to 2 with water using Cu3(BTC)2 as
heterogeneous solid catalyst.