Arene borylation through C–H activation using Cu3(BTC)2 as heterogeneous catalyst

Article abstract of DOI:10.1016/j.cattod.2020.06.034

C–H borylation by diborane is an important process to access organoboron compounds. Noble metals, including Ir and Rh-based complexes either in the form of homogeneous or heterogeneous catalysts, have been reported to promote arene C–H borylation. Recently, metal organic frameworks (MOFs) having Ir and Co as active sites have been used as catalysts, but they require co-catalysts. In the present study, commercially available Cu₃(BTC)₂ (BTC: 1,3,5-benzenetricarboxylate) MOF is reported as an effective catalyst to promote borylation of arenes through C–H activation employing bis(pinacolato)diboron (1) as reagent leading to benzylic and aromatic borylation products. Interestingly, other related MOFs like MIL-101(Cr) and Al(OH)(BDC) (BDC: 1,4-benzenedicarboxylate) do not exhibit catalytic activity under identical conditions. Mechanistic studies using in-situ IR spectroscopy reveal that Cu ions play a crucial role in activating the arene and B–B bond in 1.

Full text of DOI:10.1016/j.cattod.2020.06.034

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Arene borylation through C-H activation using Cu3(BTC)2 as
heterogeneous catalyst
´
Amarajothi Dhakshinamoorthy, Cristina Valles Garc´ıa, Patricia
Concepcion, Hermenegildo Garcia
PII:
S0920-5861(20)30420-X
DOI:
https://doi.org/10.1016/j.cattod.2020.06.034
CATTOD 12952
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5 June 2020
13 June 2020
Please cite this article as: Dhakshinamoorthy A, Garc´ıa CV, Concepcion P, Garcia H, Arene
borylation through C-H activation using Cu3(BTC)2 as heterogeneous catalyst, Catalysis
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Arene borylation through C-H activation using Cu3(BTC)2 as heterogeneous catalyst
Amarajothi Dhakshinamoorthy*a, Cristina Vallés Garcíab, Patricia Concepcionb and
Hermenegildo Garcia*b,c
aSchool of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India.
E-mail: admguru@gmail.com
bDepartamento de Quimica and Instituto Universitario de Tecnologia Quimica (CSIC-UPV),
Universitat Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain. E-mail:
hgarcia@qim.upv.es
cCenter of Excellence in Advanced Materials Research, King Abdulaziz University, Saudi
Arabia.
Graphical abstract
TOC Graphic
1

Highlights





Cu3(btc)2 is a suitable catalyst for borylation by pinacolyl diborane
Cu3(btc)2 exhibits a wide scope promoting aromatic and benzylic borylation
Cu3(btc)2 can be reused without deactivation
Other metal-organic frameworks were unable to promote the C-B formation reaction
Toluene can be borylated at the aromatic and benzylic carbon
Abstract
C-H borylation by diborane is an important process to access organoboron
compounds. Noble metals, including Ir and Rh-based complexes either in the form of
homogeneous or heterogeneous catalysts, have been reported to promote arene C-H
borylation. Recently, metal organic frameworks (MOFs) having Ir and Co as active sites have
been used as catalysts, but they require co-catalysts. In the present study, commercially
available Cu3(BTC)2 metal organic framework is reported as an effective catalyst to promote
borylation of arenes through C-H activation employing bis(pinacolato)diboron as reagent
leading to benzylic and aromatic borylation products. Interestingly, other related MOFs like
MIL-101(Cr) and Al(OH)(BDC) do not exhibit catalytic activity under identical conditions.
Mechanistic studies using in-situ IR spectroscopy reveal that Cu ions play a crucial role in
activating the arene and B-B bond.
Keywords: Heterogeneous Catalysis; Metal organic frameworks as catalysts; Cu3(BTC)2;
Borylation Reaction
2

1. Introduction
Aryl and alkyl boronates are important starting materials in transition-metal
catalyzed cross-coupling reactions, resulting in the formation with high yield and
selectivity of new C-C and C-heteroatom bonds.[1, 2] Typically, the reaction
conditions of cross-couplings involving boronates are compatible with the presence of
a large variety of functional groups including hydroxyl, carbonyl, nitro and amino,
among others.[3, 4] Due to the importance of organoboron compounds as substrates,
development of preparation procedures for this type of substrates is of considerable
interest.[4]
One of the general strategies for the preparation of organoboron compounds is
based on the catalytic substitution of halides by boronates using copper and other
transition metals as catalysts.[5, 6] For the sake of atom economy, it will be much
more convenient to obtain the wanted organoboronate by C-H activation of the
precursor. In this context, it has been recently reported that iridium and rhodium
complexes are homogeneous catalysts able to promote C-H activation of
methyaromatics to form a mixture of C-(sp2)-B (aromatic) and C-(sp3)-B products
(benzylic) products.[7] Rh-based complexes have been reported for the borylation of
alkanes, arenes, amines, ethers, ketals, and haloalkanes.[7-9] Although the activity of
these Rh-based complexes is lower for alkyl C-H bonds, the combination of
bipyridines and iridium(I)-olefin complexes has been reported to be more effective for
the borylation of arenes.[10] A highly stable and recyclable single-site solid catalyst
was prepared by post-synthetic Ir metalation of the 2,2′-bipyridyl-derived UiO-67 to
obtain bpy-UiO-67-Ir and its activity was tested in the borylation of aromatic C–H
bonds using B2pin2.[11] Very recently, Zr-based metal organic framework (MOF) has
been used to stabilize low-coordinate Ir complex to promote active methane borylation
3

to its monoborylated product.[12] These examples have shown the possibility to use
MOFs as hosts of active Ir complex developing in this way a heterogeneous catalyst
for borylation.
Moving towards less costly transition metal catalysts, noble metal-free catalysts
for promoting the formation of borylated products have been subsequently developed,
although the reaction mechanism still remains unknown. Cobalt-based homogeneous
catalysts are also able to activate borylation at the benzylic positions, but it results in
polyborylation through the multiple activation of sp3-H bonds.[13] MOFs can also be
used as hosts of Co2+ complexes, obtaining solid catalysts for borylation. In this way,
metal-organic layers stabilized with Co-terpyridine diradical complexes promoted
borylation of arenes.[14] Also, cobalt-metalated PCN-308 (PCN: porous coordination
network) has been reported as noble metal free, air-stable heterogeneous solid catalyst
for the C-H borylation of arenes.[15] In addition, UiO-68-Co was shown to promote
C-H borylation of arenes under solvent-less conditions.[16]
Considering the interest of organoboron compounds, it is herein reported that a
readily available commercial Cu3(BTC)2 is an efficient heterogeneous catalyst to
activate C-H bond in arenes (sp2) and benzylic positions (sp3 carbons) leading to the
formation of the corresponding organoboranes. Some of the salient features of this
method are the absence of noble metal, catalyst availability, no need of post-synthetic
modification or external reducing agent and stability of catalyst under the present
experimental conditions. Although it has been frequently observed a poor Cu3(BTC)2
stability as catalyst, it has been found that Cu3(BTC)2 can be reused as catalyst for C-
H borylation.
The structure of Cu3(BTC)2 (commercially known as Basolite C 300) is
constructed by clusters of two Cu2+ ions coordinated with four carboxylate groups of
4

1,3,5-benzenetricarboxylate (BTC) forming a paddle-wheel building unit that acts as
structural nodes interconnected by tripodal BTC linkers. The pore dimensions of
Cu3(BTC)2 are around 16 Å, with pore windows of around 11 and 6 Å. Further, each
Cu2+ ion is coordinated to one water molecule which can be removed upon thermal
activation of the material, generating a Lewis acid site. These coordinatively
unsaturated Cu2+ ions exhibit catalytic activity for a large variety of reactions
including Knoevenagel condensation,[17] ring opening of epoxides,[18] heterocycle
synthesis,[19-21] oxidation reactions[22, 23] and others.[24-26] As it will be shown
below, these Cu2+ ions are also responsible for borylation with B2pin2.
2. Experimental Section
As previously commented, C-H borylation of arenes promoted by transition metals is
becoming increasingly important due to its synthetic versatility to synthesize borylated
aromatic compounds under mild reaction conditions.[27-32] In this aspect, MOFs have been
employed as heterogeneous catalysts in different ways, like as a scaffold to stabilize metal
nodes within the framework, as a host to immobilize organometallic complex and to anchor
organometallic complex in the satellite position of the linkers.
2.1 Materials
Cu3(BTC)2 also commercially known as Basolite C300 and Al(OH)(BDC) also
commercially known as A100 were supplied by Sigma Aldrich. MIL-101(Cr) was
synthesised following a reported procedure.[33] CuCl, Cu(NO3)2.3H2O and
bis(pinacoloto)diboron (1) were obtained from Sigma Aldrich. All the starting materials
employed in Table 2, including solvents like toluene and acetonitrile were also purchased
from Sigma Aldrich and used as received without further purification.
2.2 Raman spectra measurements
5

Raman spectra were recorded with a Renishaw Raman Spectrometer (“Reflex”)
equipped with a CCD detector. A 514 nm laser operating at 25 mW and acquiring a total of
20 acquisitions for each spectra has been used. For the in situ studies a Linkam CCR 1000
catalytic cell has been used. The sample has been ex situ impregnated with B2Pin2 at 100 C
and dried at 60C. Raman spectra has been acquired at 100C in N2 flow, followed by
flowing a N2 feed saturated with toluene. At similar acquisition conditions, reference spectra
of toluene, B2Pin2 and BPin have been acquired.
2.3 FT-IR spectra measurements
The FTIR spectra were collected with a Thermo “Nexus” IR spectrometer equipped
with a DTGS detector (4 cm–1 resolution, 32 scans). A home-made IR cell allowing in situ
treatments in controlled atmospheres and temperatures has been connected to a vacuum
system with gas dosing facility. Self-supporting pellets (ca. 10 mg cm–2) were prepared from
the sample powders and treated in vacuum (10-4 mbar) at 100 C for 0.5 h. 5mbar of toluene
has been adsorbed at 100 C , kept 0.5h and evacuated at 10-4 mbar at the same temperature.
After it, the sample temperature has been decreased to 25 C, and CO has been adsorbed at
4mbar. IR spectra were recorded after each dosage.
2.4 Experimental Procedure
In a 25 mL round bottom flask, toluene (1 mL), Cu3(BTC)2 (30 mg), 1 (127 mg, 0.5
mmol) were charged and this slurry was stirred at 100 oC. The borylation of toluene was
monitored by gas chromatography (GC) and the conversion of 1 was determined at different
time intervals. Similar experimental procedures were performed with radical quenchers like
TEMPO and 2,6-di-t-butyl-4-methylphenol with appropriate amount mentioned in the table.
The product identity was confirmed by analyzing the reaction mixture by GC-MS.
6

Conversion of the borylation reaction and selectivity of the products were determined by
Agilent gas chromatography.
2.5 Product Analysis
Aliquots of the reaction mixture (100 μL) were collected at increasing time intervals
and diluted with 2 mL of acetonitrile. Later, this slurry was subjected to filtration with Nylon
filter of 0.2 μm adapted to a syringe and the clear filtrate analyzed by GC using a flame
ionization detector and a methyl-phenyl silicone column (TRB-5MS;30m x0.32mm x 0.25
μm) with the following conditions: initial column temperature, 30 °C; initial dwelling time, 3
min, rate of temperature ramp, 10 °C/min, final temperature 280 °C. Quantification was
performed with calibration curves to obtain the relative response factors (RF) of the products
and starting materials compared to the nitrobenzene as the internal standard. The product
identities were ascertained by injecting the reaction mixture in a GC-MS (Agilent 5973)
model. The percentage of metal leaching from the catalyst to the liquid phase at the end of the
reaction was determined by quantitative ICP-OES analysis. In brief, the catalyst was filtered
at the end of the reaction by using 0.2 μm Nylon filter. Then, 30 mL of 3 M aqueous nitric
acid solution was added to the organic phase and stirred for 20 h at 80 oC. Then, the aqueous
phase was analyzed by ICP-OES to determine the leached Cu from the solid to solution.
Furthermore, the conversion of 1 and selectivity of products formed, namely
compounds 2, 3 and 4, were determined using the following formulae:
RF was obtained with the following equation
RF= Area of nitrobenzene X moles of 1/Area of 1 X moles of nitrobenzene
Moles of remaining 1 and product were calculated using the following equation
Moles of 1 = RF X moles of nitrobenzene X Area of 1/ Area of nitrobenzene
7

Conversion (%) = moles of 1 reacted/initial moles of 1
Selectivity (%) = moles of 2 formed/total moles of 2, 3 and 4
3. Results and discussion
Recently, we have shown that borasiloxanes can be facilely synthesised by
oxidative hydrolysis of silanes with pinacolborane using Cu3(BTC)2 as a
heterogeneous solid catalyst.[25] Continuing with this research line to develop a
simple and readily available transition metal based catalyst, Cu3(BTC)2 is used as solid
catalyst for the promotion of C-H borylation of arenes.
The activity of Cu3(BTC)2 in the C-H borylation of arenes was screened by
selecting toluene as a model compound. The C-H borylation of toluene was performed
by
1
under solvent-free conditions at 100 oC. It was observed that C-H borylation of
toluene occurs at four different positions, namely at benzylic and aromatic positions
either in ortho, meta or para. The observed results are summarized in Table 1. A blank
control experiment using
1 and toluene in the absence of catalyst resulted in no
conversion at 100 oC. On other hand, the reaction between toluene and
1
using
Cu3(BTC)2 as catalyst did not provide any product at 60 and 80 oC under identical
conditions. Both control results are in agreement with earlier reports.[14, 32]
Interestingly, Cu3(BTC)2 reached 27 % conversion of
1 with 68 % of 2 and 26 % of 3
at 100 oC after 5 h (entry 1, Table 1). Under identical conditions, the conversion of
1
reached to 87 % after 24 h with a selectivity of 70 and 22 % for compounds
2 and 3,
respectively (entry 2, Table 1). Figure 1 shows the time-conversion profile for the C-H
borylation of toluene using Cu3(BTC)2 at 100 oC. Further, the selectivities of the
products (2, 3 and 4) remained constants with the conversion. When the C-H
8

borylation of toluene was performed under similar experimental conditions with
HBpin instead of , conversion was negligible (4 %) without observing the formation
1
of any desired product (entry 3, Table 1). It should be commented that Cu3(BTC)2 is
not stable in the presence of HBpin that apparently causes a prompt chemical
reduction of Cu2+ under these experimental conditions.
After identifying Cu3(BTC)2 as a suitable catalyst to promote C-H borylation of
toluene, a series of reactions were performed to gain insight into the nature of
intermediates involved in the C-H borylation. Initially, the C-H borylation of toluene
with
1 using Cu3(BTC)2 as solid catalyst was performed in the presence of TEMPO as
radical scavenger. The experimental results shown in Table 1 indicated that the
addition of TEMPO to the reaction medium completely arrests the reaction, no
borylation occurring neither at benzylic nor at aromatic positions (entries 4-5, Table
1). This hypothesis was further confirmed by another control experiment. The C-H
borylation of toluene with
conditions and 20 mg of TEMPO was added to the reaction mixture at around 27 %
conversion of after 5 h. The time-conversion profile indicated that the progress of
1
using Cu3(BTC)2 was performed at 100 oC under identical
1
the reaction is significantly quenched upon addition of TEMPO, thus supporting the
involvement of C-centred radical intermediates. These results are shown in Figure 1.
Furthermore, the C-H borylation of toluene was also performed using 2,6-di-t-butyl-p-
cresol as radical quencher, no C-H borylation products being observed (entry 6, Table
1). The C-H borylation of toluene with
1 using Cu3(BTC)2 as heterogeneous catalyst in
inert atmosphere was also very low (3%) after 24 h (entries 7-8, Table 1). These series
of control experiments in the presence of radical scavengers clearly suggest that the
reaction mechanism involves as reaction intermediate benzyl radicals originated from
toluene by Cu3(BTC)2. Furthermore, the C-H borylation of toluene with
1 using
9

Cu3(BTC)2 was also quenched by the addition of methanol suggesting that
electrophilic species intercepted by methanol are also formed.
In order to gain understanding in the role of Cu in Cu3(BTC)2, and particularly
the oxidation state involved in the process, a series of control experiments were
performed under identical conditions using CuCl and Cu(NO3)2.3H2O as
homogeneous catalysts. The observed results are also given in Table 1. CuCl as
homogeneous catalyst for the C-H borylation of toluene with
1 reached complete
conversion with 70 and 26 % selectivity to
2 and 3
, respectively after 8 h at 100 oC
(entry 10, Table 1). Under identical conditions, the C-H borylation of toluene using
Cu(NO3)2.3H2O as homogeneous catalyst afforded also complete conversion of with
68 and 37 % selectivity to compounds and , respectively after 5 h (entry 11, Table
1
2
3
1). Comparing the initial reaction rates of CuCl and Cu(NO3)2.3H2O as homogeneous
catalysts that are related to the nature of the initial Cu+ or Cu2+ ion, the later catalyst
exhibits much higher activity. These experiments suggest that C-H borylation can
occur both with Cu(I) or Cu(II) salts as active sites, although the latter is more
efficient. These results with partially soluble Cu salts can serve to rationalize the
activity observed for Cu3(BTC)2 that may contain in its framework both Cu(II) and
Cu(I) sites.[34] On other hand, the activity of CuCl and Cu(NO3)2.3H2O was
comparatively much higher with Cu3(BTC)2 that is mainly due to the easy accessibility
of active sites in the former soluble Cu catalysts, while the reactants encounter some
diffusion limitations with Cu3(BTC)2 as catalyst, resulting in lower activity. This
relative catalytic activity of homogeneous versus heterogeneous catalysts has been
frequently observed when the catalytic active sites in the mechanism are similar and
diffusion is the main differential factor. On the other hand, solid catalysts offer a
10

simple separation from the reaction mixture that usually requires a more elaborated
workup for homogeneous catalysts.
Further, the activity of Cu3(BTC)2 was also compared with other related MOFs
that are frequently employed as catalysts for liquid phase reactions. Hence, the C-H
borylation of toluene with
(BDC: 1,4-benzenedicarboxylate). These two solids afforded negligible conversion of
even after prolonged time (entries 12-15, Table 1). These results clearly indicate that
C-H borylation of toluene with is highly dependent on the nature and coordination of
1 was also studied with MIL-101(Cr) and Al(OH)(BDC)
1
1
metal ion. Furthermore, these data imply that Cu2+ plays a catalytic role activating
and toluene to promote C-H borylation of arene.
1
Heterogeneity of the catalytic process under the present reaction conditions was
ascertained by performing the hot-filtration test. The C-H borylation of toluene was
started in the presence of Cu3(BTC)2 and after 5 h at 27 % conversion of 1, the solid
catalyst was removed by filtration at the reaction temperature. Then, the reaction in the
absence of solid was continued for 24 h. The experimental results indicated that upon
removal of solid the conversion of
1 increased to 32 % after 24 h which suggest that
the progress of the reaction is much slower in the absence of Cu3(BTC)2. These results
are consistent with the reaction being mainly catalysed by the active sites on the solid,
confirming the heterogeneity of the reaction (Figure 1). Further, ICP-OES analysis of
the fresh and the recovered Cu3(BTC)2 solids showed identical copper content, thus
supporting the absence of Cu leaching. Furthermore, the activity of Cu3(BTC)2 was
retained for two consecutive runs without change in the product distribution. The
structural integrity of reused solid was almost identical to the fresh solid based on
XRD (Figure 2).
11

In order to understand the role of Cu3(BTC)2 in promoting the C-H borylation
of toluene, in-situ IR spectroscopy studies were performed. The results are given in
Figure 3. Initially, toluene was adsorbed on dehydrated Cu3(BTC)2 observing the
appearance of a species at 1944 cm-1 that can be attributed to Cu-H (Figure 3, blue
spectra).[35] Formation of Cu(I), by hydride reduction of Cu(II) was further confirmed
by CO titration measurements that indicate that Cu(II) is reduced to Cu(I) as it can be
deduced from the wavenumber of the CO stretching band at 2124 cm-1.[36]
Spontaneous formation of less active Cu(I) justifies the beneficial effect of the
atmospheric oxygen promoting reoxidation of possible Cu(I) to Cu(II). Thus, FT-IR
spectra indicate that Cu3(BTC)2 produced benzyl radicals from toluene by undergoing
the reduction of the oxidation state of copper from divalent to monovalent, which can
be detected by CO titration. As commented earlier, these benzyl radical intermediates
were trapped with radical scavengers like TEMPO and BHT that quench the reaction
due to the radical scavenging.
Besides adsorption of toluene, it was also observed that activated Cu3(BTC)2 can also
interact with 1 (Figure 4). The interaction between Cu3(BTC)2 with 1 leads to the formation
of Bpin appearing in the Raman spectra at a wavenumber of 289 cm-1 (Figure 4). This
monomer becomes adsorbed on the MOF structure as evidenced by the shift of the 502, 449
and 190 cm-1 bands to 506, 460 and 229 cm-1, respectively (compare green and blue IR
spectra in Figure 4). The initial band at 190 cm-1 is shifted to 229 cm-1 after Bpin interaction
with the lattice. This band has been ascribed to Cu-Cu stretching vibration of the two Cu(II)
ions of the Cu2C4O8 nodes[37] and the remarkable shift indicates the involvement of the Cu
atoms in the process resulting in the cleavage of 1. The shift in the characteristic Cu-Cu
vibration band suggests that the Cu-Cu bond distance is shortened upon interaction with
12

Bpin. Furthermore, the two bands at 506 and 460 cm-1 are attributed to Cu-O stretching
modes.[38] They are also shifted from the original ones at 502 and 449 cm-1 to higher
frequencies after Bpin interaction. A new band at 596 cm-1 is clearly formed, which
disappears after toluene adsorption (Figure 4, red spectra), suggesting the formation of a
reaction intermediate species. In addition, after toluene adsorption, the band at 229 cm-1
decreased slightly by restoring partially the band at 190 cm-1 band. This change would agree
with the partial release of the Bpin adsorbed on the Cu(II) ions and the restoring of the initial
Cu-Cu interaction.
The activity of Cu3(BTC)2 on the borylation of toluene with 1 encouraged us to
expand the scope of this solid catalyst to other arenes. The observed results are shown in
Table 2. The borylation of diphenylmethane with 1 using Cu3(BTC)2 exhibited complete
conversion with 50 % benzylic and 42 % aromatic borylations after 24 h (entry 1, Table 2).
On other hand, the reactivity of ethylbenzene with 1 was also complete within 24 h with 29
and 62 % for benzylic and aromatic borylations, respectively under identical conditions
(entry 2, Table 2). Furthermore, the borylation of anisole with 1 afforded exclusively
aromatic borylation under similar reaction conditions (entry 3, Table 2). In contrast,
borylation of p-xylene resulted in low conversion of 1 with 6 and 4 % of benzylic and
aromatic borylation, respectively (entry 4, Table 2). This notably lower reactivity of p-xylene
compared to toluene under the present experimental conditions may be due to the close
packing of p-xylene within the pores of Cu3(BTC)2 that can cause diffusion limitations to 1 to
reach the active sites. Similarly, 4-bromotoluene also afforded low conversion with almost
equal selectivity between benzylic and aromatic positions (entry 5, Table 2). An attempt to
borylate aniline using Cu3(BTC)2 was unsuccessful under the present experimental
conditions. This may be ascribed due to the coordination of amine groups to the active Cu(II)
sites. Finally, the borylation of benzyl bromide using Cu3(BTC)2 was attempted, but no
13

borylated products were detected. However, the solid catalyst turned its colour to black,
suggesting the decomposition of Cu3(BTC)2 by benzyl bromide. One of the probable reasons
for this deactivation may be the high reactivity of this benzylic halide with Cu(II) that could
result in formation of copper halides and favouring the reduction to lower oxidation state or
metallic state.
4. Conclusions
In conclusion, it has been shown that Cu3(BTC)2 is able to activate C-H bond
in arenes to promote borylation at benzylic and aromatic positions using
1 as
borylating agent. This work demonstrates the ability of Cu(II) to act as active sites for
C-H bond activation under mild reaction conditions as evidenced by IR study. The
present study illustrates C-H borylation with readily available MOF without the
requirement of noble metals. Among the series of analogous MOFs tested under
identical conditions for C-H borylation, Cu3(BTC)2 is the only solid exhibiting
activity. The activity of Cu3(BTC)2 was maintained for two runs with no changes in
the conversion and product distribution. Cu3(BTC)2 as catalyst offers some
advantages, like the absence of noble metal catalysts, ready availability, no need of
post-synthetic modification or addition of external reducing agent and catalyst
stability.
Author’s contribution:
Experimental work and catalytic tests were carried out by Amarajothi Dhakshinamoorthy,
and Cristina Vallés García. Sample characterization was performed by Patricia Concepcion.
The concept of the paper and writing were carried out by Amarajothi Dhakshinamoorthy and
Hermenegildo Garcia. All the authors read and agreed on the manuscript.
14

Conflicts of interest
There are no conflicts to declare.
The authors declare that they do not incur in any conflict of interest.
Acknowledgements
A.D. thanks the University Grants Commission, New Delhi, for the award of an
Assistant Professorship under its Faculty Recharge Programme. A.D. also thanks the
Department of Science and Technology, India, for the financial support through
Extramural Research Funding (EMR/2016/006500). Financial support by the Spanish
Ministry of Economy and Competitiveness (Severo Ochoa and RTI2018-089231-
CO2-R1) and Generalitat Valenciana (Prometeo 2017-083) is gratefully
acknowledged.
15

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17

100
80
60
40
20
0
(a)
(c)
(b)
0
5
10
15
20
25
Time (h)
Figure 1. (a) Time conversion plot for the C-H borylation of toluene with
1 using
Cu3(BTC)2 as heterogeneous solid catalyst, (b) hot filtration test upon removal of
Cu3(BTC)2 after 5 h at reaction temperature and (c) addition of TEMPO as radical
scavenger after 5 h.
Figure 2. Powder XRD of (a) fresh and (b) two times reused Cu3(BTC)2 solid.
18

2124
0.01
1944
2400
2200
2000
1800
wavenumber (cm-1)
Figure 3. IR spectra of dehydrated Cu3(BTC)2 (black spectra), after toluene adsorption
at 100 oC followed by evacuation (blue spectra) and after CO titration at 25 oC (red
spectra).
toluene
BPin
B2Pin2
506
460
*
229
*
289
596
200
400
600
Wavenumber (cm-1)
19

Figure 4. Raman spectra of Cu3(BTC)2 (green spectra), after adsorbing 1 (blue spectra) and
after adding toluene (red spectra), and the reference Raman spectra of B2Pin2, HBpin and
toluene (black spectra). (*) bands of toluene
20

Table 1. Optimization of reaction conditions for the borylation of toluene using
as reagent.a
diboronate
1
S.No Catalyst
Time Conversion Selectivity
(h)
(%)b
(%)b
2
3
4
1
Cu3(BTC)2
5
27
87
4
68 26 6
70 22 8
2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
CuCl
24
2c
3
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
4
5d
-
5
24d
24e
5f
-
6
-
7
-
8
24f
5g
3
50 49 1
70 27 3
70 26 4
68 37 3
9
63
100
100
<1
3
10
11
12
13
14
15
CuCl
8g
Cu(NO3)2.3H2O 5h
MIL-101(Cr)
MIL-101(Cr)
Al(OH)(BDC)
5
-
-
-
24
5
72 22 6
<1
4
-
-
-
Al(OH)(BDC) 24
72 23 5
21

aReaction conditions: toluene (1 mL), catalyst (30 mg),
presence of air.
1
(127 mg, 0.5 mmol), 100 oC,
bDetermined by GC. Conversion represents of B2pin2.
cHBpin (0.150 mL)
dTEMPO (20 mg) was added.
e2,6-di-t-butyl-4-methylphenol (BHT) (50 mg) was added.
fReaction performed under inert atmosphere.
g15 mg was used in respect to the Cu content in Cu3(BTC)2.
h37 mg was used in respect to the Cu content in Cu3(BTC)2.
22

Table 2. Scope of Cu3(BTC)2 as catalyst for borylation using
1
as reagent.a
S.No Arene
Conversion Selectivity (%)b
(%)
100
Benzylic
50
Aromaticc
42(8)d
1
2
100
29
62(9) d
3
4
5
100
10
-
92(8) d
4(90) d
40(9) d
6
50
51
aReaction conditions: arene (1 mL),
bDetermined by GC.
1
(0.5 mmol), Cu3(BTC)2 (30 mg), 100 oC, 24 h.
cBorylation includes ortho/meta/para isomers.
dValues in parentheses indicate the selectivity to Bpin-O-Bpin.
23