Immobilization of Ir(I) complex on covalent triazine frameworks for C–H borylation reactions: A combined experimental and computational study

Article abstract of DOI:10.1016/j.jcat.2019.01.030

Metal-modified covalent triazine frameworks (CTFs) have attracted considerable attention in heterogeneous catalysis due to their strong nitrogen-metal interactions exhibiting superior activity, stability and hence recyclability. Herein, we report on a post-metalation of a bipyridine-based CTFs with an Ir(I) complex for C–H borylation of aromatic compounds. Physical characterization of the Ir(I)-based bipyCTF catalyst in combination with density functional theory (DFT) calculations exhibit a high stabilization energy of the Ir-bipy moiety in the frameworks in the presence of B₂Pin₂. By using B₂Pin₂ as a boron source, Ir(I)@bipyCTF efficiently catalyzed the C–H borylation of various aromatic compounds with excellent activity and good recyclability. In addition, XAS analysis of the Ir(I)@bipyCTF gave clear evidence for the coordination environment of the Ir.

Full text of DOI:10.1016/j.jcat.2019.01.030

Journal of Catalysis 371 (2019) 135–143
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Immobilization of Ir(I) complex on covalent triazine frameworks for CAH
borylation reactions: A combined experimental and computational study
a
b
c
d
c
Norini Tahir , Francesco Muniz-Miranda , Jonas Everaert , Pieter Tack , Thomas Heugebaert ,
a
d
c
b
a,
⇑
Karen Leus , Laszlo Vincze , Christian V. Stevens , Veronique Van Speybroeck , Pascal Van Der Voort
a
Center for Ordered Materials, Organometallics and Catalysis (COMOC), Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
Center for Molecular Modeling (CMM), Ghent University, Technologiepark 903, Zwijnaarde, Belgium
Department of Green Chemistry and Technology, Ghent University, Coupure Links 653, 9000 Ghent, Belgium
X-ray Micro-Spectroscopy and Imaging Group (XMI), Department of Chemistry, Ghent University, Krijgslaan 281-S12, 9000 Ghent, Belgium
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal-modified covalent triazine frameworks (CTFs) have attracted considerable attention in heteroge-
neous catalysis due to their strong nitrogen-metal interactions exhibiting superior activity, stability
and hence recyclability. Herein, we report on a post-metalation of a bipyridine-based CTFs with an Ir
Received 7 December 2018
Revised 17 January 2019
Accepted 23 January 2019
(
I) complex for CAH borylation of aromatic compounds. Physical characterization of the Ir(I)-based
bipyCTF catalyst in combination with density functional theory (DFT) calculations exhibit a high stabi-
lization energy of the Ir-bipy moiety in the frameworks in the presence of B Pin . By using B Pin as a
2
2
2
2
Keywords:
boron source, Ir(I)@bipyCTF efficiently catalyzed the CAH borylation of various aromatic compounds
with excellent activity and good recyclability. In addition, XAS analysis of the Ir(I)@bipyCTF gave clear
evidence for the coordination environment of the Ir.
Covalent triazine framework
Iridium complex
DFT calculations
CAH borylation
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
Aromatic boronic acids are highly important compounds that
well-established heterogeneous Ir(I)-based catalysts could catalyze
the CAH borylation of arenes exhibiting a high activity and selec-
tivity. However, their durability was somehow hampered and
showed a rather low tolerance for the presence of functional
groups. For example, the CAH borylation catalyzed by an Ir(I)-
based PMO showed a significant decreased catalytic performance
during recycling studies [13], whereas for the Ir(I)-based MOF cat-
alysts, the system required high reaction temperatures and long
reaction times (up to 72 h) for the CAH borylation of rigid and lar-
ger substrates [15].
Within this context, covalent triazine frameworks (CTFs) have
recently emerged as potential supports for heterogeneous catalysis
due to their high surface area having an abundant nitrogen content
[19,20]. CTFs are porous aromatic polymers which consist solely of
organic groups connected via strong covalent bonds. CTFs are typ-
ically produced through the reversible ionothermal trimerization
of polynitriles in molten zinc chloride which acts both as solvent
and as catalyst [21–23]. The most exciting feature of CTFs is their
flexibility for a specific structural design to control the porosity
and functionality, which is of utmost importance towards their
use as catalytic support [24]. CTFs also possess superior thermal
and chemical stability, hence, exhibiting high resistance to strongly
oxidizing and harsh catalytic reaction environments [25].
have been used in numerous synthetic procedures [1]. The transi-
tion metal-catalyzed CAH borylation of aromatic compounds is the
most efficient and convenient synthetic strategy for the synthesis
of organoboron compounds. It is a highly economical and environ-
mentally benign process that can be carried out in the absence of a
halogenated reactant [2–5]. Among the developed transition
metal-based catalysts to date, Ir complexes containing
bipyridine-based ligands have shown superior performance
exhibiting excellent activity and selectivity for the aromatic CAH
borylation under mild reaction conditions [6–8]. In recent decades,
intensive research efforts have resulted into the development of
Ir-based heterogeneous catalysts for CAH borylation in order to
overcome the drawbacks related to homogeneous systems such
as difficulties related to the recovery and reuse of the precious Ir
catalysts [9]. Various heterogeneous-based bipyridine supports
have been developed, including mesoporous silica [10,11], periodic
mesoporous organosilica (PMO) [12–14], metal-organic frame-
works (MOF) [15–17], and organosilica-nanotubes [18]. These
⇑
Corresponding author.
E-mail address: pascal.vandervoort@ugent.be (P. Van Der Voort).
https://doi.org/10.1016/j.jcat.2019.01.030
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

1
36
N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
Lotsch and co-workers synthesized a functionalized-CTF based
Hereafter, aqueous ammonia (25%) was added to make the mixture
alkaline and the resulting product was extracted with ethyl acetate
(3 ꢂ 50 mL). The combined organic layers were washed with a 5%
0
on 2,2 -bipyridine building blocks [26]. They demonstrated that
this bipyridine-based CTF (later known as bipyCTF) possess specific
and strong binding sites for transition metals ions including Co, Ni,
Pt, and Pd. Yoon et al. have further investigated the potential of the
bipyCTF as a host matrix for metal complexes. They demonstrated
that the bipyCTF-functionalized with either an Ir, Rh or Ru com-
plexes exhibited a good catalytic performance in the selective
hydrogenation of carbonyl compounds [27–29]. In another study,
they explored the incorporation of bimetallic Al-Co in the bipyCTF
for the carbonylation of propylene oxide to ß-butyrolactone [30].
These studies clearly demonstrate that the bipyCTF can act as a
chelating ligand stabilizing metal complexes for various catalytic
applications. In this study, we employed the bipyCTF as catalytic
4
aqueous LiCl solution and dried over MgSO , filtered and concen-
0
0
trated. 2,2 -bipyridine-5,5 -dicarbonitrile was obtained as a pale
brown powder in 91% yield (0.94 g). No further purification was
1
required for the next reaction step. H NMR (400 MHz, CDCl
3
): d
8.97 (2H, dxd, J = 2.0, 0.8 Hz), 8.64 (2H, dxd, J = 8.3, 0.8 Hz), 8.14
1
3
3
(2H, dxd, J = 8.3, 2.1 Hz) (Fig. S1). C NMR (100.6 MHz, CDCl ): d
157.0, 152.1, 140.5, 121.7, 116.5, 110.7 (Fig. S2).
2.2. Synthesis of the bipyCTF
The bipyCTF was synthesized according to the published
procedure [26]. Typically, a glass ampoule was charged with
,2’-bipyridine-5,5’-dicarbonitrile (100 mg, 0.48 mmol) and ZnCl
332 mg, 2.40 mmol) in a glove box. The ampoule was flame-
support for the anchoring of the [Ir(OMe)(cod)]
the borylation of aromatic CAH bonds. The immobilization of the Ir
I) complex onto the bipyCTF was investigated intensely both
2
complex towards
2
2
(
(
experimentally and computationally, showing a high reactivity in
the CAH borylation of various arenes and heteroarenes in the pres-
ence of B Pin as boron reagent.
2 2
sealed under vacuum and heated in an oven towards 400 °C with
a heating rate of 100 °C/h and held at this temperature for 48 h.
After cooling down to room temperature, the resulting black solid
was ground well and stirred in 250 mL of water for 4 h, filtered and
washed with water, acetone and refluxed in 250 mL of 1 M HCl
overnight, filtered, and washed subsequently with 1 M HCl
2
. Experimental section
All chemicals were purchased from commercial suppliers and
(
(
3 ꢂ 100 mL), H O (3 ꢂ 100 mL), THF (3 ꢂ 100 mL), and acetone
2
3 ꢂ 100 mL). The resulting product was dried under vacuum
used without further purification. Nitrogen adsorption analysis
was conducted at 77 K using an automated gas sorption system
Belsorp-mini II gas analyzer. Prior to sorption measurements, the
samples were dried under vacuum at 120 °C overnight to remove
overnight at 150 °C prior to use.
2.3. Post-synthetic metalation of bipyCTF with [Ir(OMe)(cod)]
2
ꢀ1
adsorbed water. FT-IR spectra in the region of 400–4000 cm
were recorded on a Thermo Nicolet 6700 FT-IR spectrometer
equipped with a nitrogen-cooled MCT detector and a KBr beam
splitter. Elemental analysis was measured on a Thermo Scientific
Flash 2000 CHNS-O analyzer equipped with a TCD detector. Pow-
der X-ray diffraction (XRPD) patterns were collected on a Thermo
Scientific ARL X’Tra diffractometer, operated at 40 kV, 40 mA using
To obtain the Ir(I)@bipyCTF catalyst, 140 mg of bipyCTF was
added to a solution of [Ir(OMe)(cod)] (10.0 mg, 0.015 mmol) in
2
30 mL anhydrous THF and the mixture was stirred at room temper-
ature. After 24 h, the solid was filtered and washed with THF
(3 ꢂ 25 mL) to remove the weakly bounded Ir complex and dried
under vacuum. It is important to note that all the handlings as
mentioned above were done under an inert atmosphere to prevent
oxidation of the Ir precursor.
Cu-Ka radiation (k = 1.5406 Å). TGA measurements were per-
formed using a Netzsch STA-449 F3 Jupiter. The samples were
heated in the temperature range 30–800 °C under an air atmo-
ꢀ
1
sphere at a heating rate of 10 °C min . The Ir loading was deter-
mined using an ICP-OES Optime 8000 atomic emission
spectrometer. HAADF-STEM and the EDX mapping analysis was
performed using JEOL JEM-2200FS High- Resolution STEM
equipped with an EDX spectrometer with a spatial resolution of
2
.4. Computational details
Density functional theory calculations have been performed
with the Gaussian 16 suite of programs [33]. Structural optimiza-
tions were carried out with the M06 [34] exchange-correlation
functional, employing the LanL2DZ combined pseudopotential
and basis set [35–37]. The M06 functional is known to yield accu-
rate structural parameters and thermochemical energies with Ir
cations [38]. The lack of negative frequencies in relaxed geometries
indicate that the optimized structures represent true energy min-
ima. Calculations for the anchoring models shown in Fig. 3 have
been carried out including the THF solvent via the polarizable con-
tinuum model (PCM) [39]. Calculations for the deprotonation ener-
gies have been carried out including the methanol solvent through
the PCM procedure. Dispersion interactions have been modelled
using the D3 version of Grimme’s dispersion with Becke-Johnson
damping [40].
0
.13 nm, image lens spherical aberration corrector, electron energy
loss spectrometer (filter) and an emission field gun (FEG) operating
at 200 KeV. X-ray Absorption Spectroscopy (XAS) measurements
were performed at beamline BM26A (Dutch-Belgian beamline,
DUBBLE) at the ESRF (Grenoble, France) [31]. The conversion of
the substrates was identified by a Finnigan Thermo Scientific Trace
GC Ultra equipped with an FID, and the yield formation of bory-
1
lated compounds was determined by means of H NMR measure-
ments on a Bruker Advance 300 MHz spectrometer.
0
0
2
.1. Synthesis of 2,2 -bipyridine-5,5 -dicarbonitrile
The nitrile based monomer was synthesized according to a
slightly adopted procedure reported by Duan et al [32]. In first
instance, NiCl O (0.12 g, 0.5 mmol) was dissolved in 20 mL
dry DMF. The resulting mixture was heated to 40 °C and 2-
2.5. General procedure for the aromatic CAH borylation
2
ꢁ6H
2
All the catalytic tests were carried out in a 25 mL Schlenk-tube
which was charged subsequently with bis(pinacolato)diboron
(31.75 mg, 0.125 mmol), an arene (0.125 mmol), 3 mL of dry hep-
tane and Ir(I)@bipyCTF (1.5 mol% Ir). The mixture was stirred at
90 °C for 8 h under a nitrogen atmosphere. At the end of the reac-
tion, the reaction mixture was analyzed by means of GC and GC–
MS using dodecane as internal standard and the product yield
bromo-5-cyanopyridine (1.83 g, 10.0 mmol), anhydrous LiCl
(
0.43 g, 10.0 mmol) and Zn powder (0.78 g, 12.0 mmol) were
added. After raising the temperature to 50 °C, a grain of I and
2
two drops of acetic acid were added into the mixture and stirred
for 30 min. Afterward, the mixture was cooled down to 0 °C before
adding 1 N HCl (15 mL) and stirring it for an additional 30 min.

N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
137
was determined by 1H NMR using mesitylene as the internal
standard.
(0.43 mmol g ) (Table S2). The nitrogen adsorption isotherms of
both the bipyCTF and Ir(I)bipyCTF material were subsequently
recorded at 77 K to determine the porosity of the networks. As
shown in Fig. 1, both materials exhibit a typical type I isotherms
ꢀ1
3
. Results and discussion
which is characterized by a sharp N
2
uptakes at low relative pres-
sure. The Brunauer-Emmett-Teller (BET) surface area and pore vol-
3
.1. Synthesis and characterization
2
ꢀ1
ume of the bipyCTF are calculated to be 714 m
g
and
3
ꢀ1
0
.39 cm g , respectively. These values decreased after the coordi-
The most commonly applied synthesis method to obtain CTFs is
2
ꢀ1
3
ꢀ1
nation of the Ir obtaining 494 m g and 0.27 cm g for the BET
surface area and total pore volume, respectively (Table S2). To eval-
uate the occupancy of Ir(I) complex inside the pore system of the
bipyCTF, we have calculated the theoretical occupied volume and
weight based on a 0.43 mmol g
amounts 0.043 cm g and 142 mg g , respectively. Starting from
through trimerization of nitrile monomers using ZnCl
4
dissolve the monomers in molten ZnCl
Lewis acid catalyst to promote the reversible trimerization
throughout the CTF formation. However, at temperatures above
2
at 400 °C for
0–48 h of reaction [23]. Notably, a high temperature is required to
in which Zn2 acts as a
+
2
ꢀ
1
Ir-complex loading which
3
ꢀ1
ꢀ1
4
00 °C, carbonization and structural degradation of the triazine
3
ꢀ1
the pore volume of the pristine bipyCTF being 0.39 cm g , and
taking into account only the weight increase, the theoretical pore
volume of the catalyst with all complexes on the outside would
ring can occur, whereas shorter reaction times (<40 h) result in
an incomplete polymerization [41]. In this study, the bipyCTF
was synthesized by heating the 2,2 -bipyridine-5,5 -dicarbonitrile
monomer in the presence of ZnCl at 400 °C for 48 h, in a vacuum
sealed glass ampoule (Scheme 1).
The Fourier Transform Infrared (FT-IR) analysis for the synthe-
sized bipyCTF was performed to evaluate the trimerization process
of the framework. The disappearing band of the ACN stretching
vibration at 2240 cm indicates the complete polymerization of
the framework (Fig. S3). Elemental analysis (EA) revealed an
increase in the C/N ratio of the bipyCTF in comparison to the the-
oretically calculated values due to the decomposition of nitrile
building blocks and the partial carbonization of the frameworks
during the synthesis reaction, which is commonly observed feature
for CTFs (Table S1) [42]. Furthermore, the Powder X-ray diffraction
0
0
3
ꢀ1
become 0.34 cm g . The occupied volume by the complex
2
3
ꢀ1
amounts 0.043 cm g , the theoretical pore volume considering
3
ꢀ1
all complexes inside the pores is then 0.29 cm g , which is
3
ꢀ1
slightly higher than the experimental pore volume (0.27 cm g ),
3
ꢀ1
but definitely lower than 0.34 cm g , which would be obtained
taking only the weight increase and not any pore filling into
account. These data are a good indication that most of the com-
plexes are inside the pores. The presence of the anchored Ir in
the Ir(I)@bipyCTF material was further confirmed by high angle
annular dark field scanning transmission electron microscopy
ꢀ1
(
HAADF-STEM) and the corresponding energy dispersive X-ray
spectroscopy (EDX) mapping images (Fig. S6). The main elements,
namely carbon and nitrogen are well dispersed and the Ir(I) com-
plex was uniformly distributed throughout the frameworks.
(
PXRD) measurement shows a broad diffraction peak representing
the structural ordering of the bipyCTF (Fig. S4) [43]. The thermal
stability of the studied bipyCTF was examined by means of Ther-
mogravimetric analysis (TGA) demonstrating that the bipyCTF
material starts to decompose at 450 °C, whereas approximately
3.2. Theoretical calculations
1
0% of weight loss below 100 °C can be assigned to adsorbed CO
2
Density functional theory (DFT) calculations have been per-
formed to investigate the anchoring of the Ir(I) complex onto the
bipyCTFs (see Section 2.4 for the complete computational details).
or solvents (Fig. S5). However, before the catalytic test, the samples
were preheated overnight at 150 °C in vacuo, which should remove
all adsorbed species.
Various anchoring models were studied including the possibility of
ꢀ
The Ir(I)bipyCTF was prepared by post-metalation of the syn-
deprotonation of bipy with the methoxy anion ( OCH
3
). This pro-
ꢀ
thesized bipyCTF with [Ir(OMe)(cod)]
2
in THF at room tempera-
cess can not a priori be ruled out because: (i) the OCH
3
anion,
ture, as shown in Scheme 1. ICP-OES analysis was performed to
determine the metal loading which amounts 8.26 wt% Ir
which has strong basic properties, is present in the [Ir(OMe)
(cod)] precursors complex; (ii) the CAH ionic dissociation
2
Scheme 1. Schematic representation of the synthesis of Ir(I)@bipyCTF.

1
38
N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
2
2
1
1
50
00
50
00
compounds, their relative energies have been computed in respect
to their subunits (namely the triazine-bipy-triazine unit, Ir(I)-cod,
ꢀ
and OCH
3
/methanol), according to the following equation:
ꢀ
D
ꢀ
G = G[model] ꢀ G[Ir(I)-cod] ꢀ G[ OCH
3
/methanol]
G½protonated=deprotonatedtriazine ꢀ bpy ꢀ triazineꢃ
This equation can be used because the three models have the
same number of atoms overall and the same charge (namely, zero),
even if their own subunits differ regarding this characteristic (for
example, the methoxy anion has a negative charge and one proton
less than methanol). The energies are reported in Fig. 2. The
DG
5
0
0
reported in the previous equation is referred to the constituents
of a specific model, whose details are reported in the Supporting
Information.
These data show that model 1 has the highest stabilization
energy; however, model 3 has a similar stabilization energy and
thus cannot be ruled out in principle. Hence, two further model
complexes were considered to study the possible coordination
bipyCTF
Ir@bipyCTF
0
.0
0.2
0.4
0. 6
0.8
1.0
Relative Pressure ( p/p0)
Fig. 1. Nitrogen adsorption isotherms of bipyCTF (black) and Ir(I)@bipyCTF (red)
measured at 77 K. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
2 2
with B Pin .
In fact, after addition of B
2 2
Pin , the Ir complex would be com-
pound A.2 if no deprotonation of the bipy ring occurs (‘‘anchoring
A”), whereas it would lead to complex B.2 if bipy is really deproto-
energies of the bipy embedded into CTF are close to the O-H ionic
dissociation energy of methanol (Fig. S7). In fact, regarding point
ꢀ
nated and Ir is anchored to a C anion (‘‘anchoring B”) (Fig. 3).
(
ii), an increase in the acidity of bipy was observed when it is
Model A.2 and B.2 involve coordination with three and two BPin
units, as their precursor compounds A.1 and B.1 had three and
two coordination bonds with the non-CTF ligands. Models A.2
and B.2 have the same charge but a different number of atoms,
so to compare their relative stability it is not possible to just
straightforwardly subtract the free energy of their constituent sub-
units from their own energy. In fact, to compare their relative
energy we considered the following formal reaction connecting
the two complexes A.2 and B.2:
embedded into the CTF environment. This increase has been traced
back to the effect of the nearby triazine residues, as shown in
Fig. S7, which makes the CAH bond in bipy less covalent and more
prone to deprotonation.
Various anchoring models have been considered which are
shown in Fig. 2, along with their stabilization free energy. Model
1
bipy, whereas in models 2 and 3 the anchoring is through one N
atom (of triazine in model 2, of bipy in model 3) and one deproto-
nated C atom of bipy, whereas methanol is basically dissociated
from the Ir(I) cation. To assess the stability of the various model
ꢀ
considers an anchoring of a OCH
3
anion to the two N atoms of
[
A.2] + [bpy syn ! anti isomerization]
½bpyringdeprotonationꢃ ꢀ ½BPinꢃ ! ½B:2ꢃ
+
Fig. 2. Structurally relaxed anchoring models (upper row) and their schematic formula (lower row). The stabilization energies are ꢀ444 kJ/mol (model 1), ꢀ414 kJ/mol
model 2), ꢀ439 kJ/mol (model 3).
(

N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
139
2 2
Fig. 3. Possible intermediate Ir(III) complexes formation after treatment with B Pin . Optimized structures are reported in Fig. S8 of the Supporting Information.
Table 1
Thus, in this case we have computed the free energy difference
G = G products ꢀ G reagents for this formal reaction as:
a
Ir(I)@bipyCTF catalyzed CAH borylation of 1,2-dichlorobenzene.
D
Ir(I)@bipyCTF
D
!
G = G products ꢀ Greagents = G[B.2] ꢀ G[A.2] ꢀ G[bpy syn
antiisomerizationꢃ ꢀ G½bpydeprotonationꢃ þ G½BPinꢃ
We found that in this case G = + 106 kJ/mole (see the details in
D
the Supplementary Information), thus indicating that the B.2 com-
plex (with Ir anchored to a deprotonated bipy ring) is significantly
less stable than complex A.2 (with Ir anchored to the two N atoms
of bipy). As a consequence, these calculations suggest that the Ir(I)
complex is indeed anchored to the CTF framework through the two
N atoms of bipy and that this compound would be the actual active
complex for the catalysis cycle.
Entry
DCB:B
2
Pin b
2
Solvent
T (°C)
Time (h)
Yield (%)c
1
2
3
4
5
1:0.5
1:0.5
1:0.5
1:0.5
1:1
THF
DMF
Heptane
Heptane
Heptane
70
70
70
90
90
24
24
24
24
8
0
0
18
70
95
a
2 2
Reaction conditions: Ir(I)@bipyCTF (1.5 mol% Ir), 1,2-dichlorobenzene, B Pin ,
3
.3. Aromatic CAH borylation catalyzed by Ir(I)@bipyCTF
dodecane (1 mmol).
b
Substrate to boron reagent mol ratio.
c
Yield was determined by 1H NMR.
The catalytic performance of the Ir(I)@bipyCTF was evaluated
for the CAH borylation of arenes and heteroarenes under mild
reaction conditions. Initially, the arene 1,2-dichlorobenzene (2
equiv.) was examined in the presence of B
2
Pin
2
(1 equiv.) at
separately, were carried out under the optimized reaction condi-
tions (Table 2, Entry 2 and 3). Apparently, no catalytic activity
was observed in both control experiments. In another control test,
7
0 °C for 24 h using Ir(I)@bipyCTF (1.5 mol% Ir) as catalyst in vari-
ous solvents. The results are summarized in Table 1. No borylated
products were observed when the borylation reaction was con-
ducted in polar solvents such as THF and DMF (Table 1, Entry 1
and 2). However, 18% yield was obtained in heptane at 70 °C
2
the bipyCTF and the [Ir(OMe)(cod)] were added together resulting
in a poor yield of only 10% after an extended reaction time of 24 h
(Table 2, Entry 4). This demonstrates that the Ir(I) complex can cat-
alyze the CAH borylation reaction in the presence of a bipy-based
support. Nevertheless, only when the Ir(I) complex is anchored
onto the bipyCTF, a high product yield of 95% was obtained after
8 h of reaction (Table 2, Entry 1). Additionally, the Ir(I)@bipyCTF
catalyst exhibited an initial slower reactivity compared to its
homogeneous analog, as can be seen in the kinetic profiles for both
catalysts (see Fig. 4).
(Table 1, Entry 3). The obtained results are in accordance to litera-
ture demonstrating that the solvent polarity has a substantial
effect on the conversion of the substrate [11,44]. When the reac-
tion temperature was increased to 90 °C, the yield significantly
rose to 70% (Table 1, Entry 4). The amount of the boron reagent also
plays a crucial role in the yield of the product [45]. For instance,
2 2
upon increasing of one equivalent of B Pin , the rate of the boryla-
tion reaction increases drastically, reaching 95% yield after 8 h of
reaction (Table 1, Entry 5).
We extended the scope of substrates using various arenes and
heteroarenes having electron-withdrawing and electron-donating
groups under the optimized catalytic reaction conditions. In gen-
eral, the Ir(I)@bipyCTF was able to catalyze the CAH borylation
Several control experiments were performed. The borylation of
1
2
,2-dichlorobenzene catalyzed by [Ir(OMe)(cod)] and bipyCTF,

1
40
N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
Table 2
Table 3
a
CAH borylation of arenes and heteroarenes catalyzed by Ir(I)@bipyCTF.a
Control experiments CAH borylation of 1,2-dichlorobenzene.
Entry
Catalyst
Yield (%)b
TON
Ir(I)@bipyCTF
1
2
3
4
Ir(I)@bipyCTF
[Ir(OMe)(cod)]
bipyCTF
95
0
64
0
0
B
2
Pin
2
2
2
0
10
Heptane, 90 °C, 8h
c
[Ir(OMe)(cod)]
+ bipyCTF
8
a
Reaction conditions: Ir(I)@bipyCTF (1.5 mol% Ir), 1,2-dichlorobenzene
1 equiv.), B Pin (1 equiv.), 90 °C, heptane, 8 h.
Entry
1
Substrate
Product
Yield (%)b
95
(
2
2
b
1
Yield was determined by H NMR.
c
Yield obtained after 24 h of reaction.
2
3
78
56
1
00
7
5
2
5
0
5
0
4
5
92
84
Ir(I)@bipyCTF
Homogeneous Ir-bipy
0
2
4
6
8
10
12
a
Reaction conditions: Ir(I)@bipyCTF (1.5 mol% Ir), arene (1 equiv.),
2 2
B Pin
Time (h)
(1 equiv.), 90 °C, heptane, 8 h.
b
Yield was determined by 1H NMR.
Fig. 4. Kinetic profiles for the CAH borylation of 1,2-dichlorobenzene catalyzed by
Ir(I)@bipyCTF and its homogeneous counterpart.
1
00
of various substrates and afforded good to excellent yield of the
corresponding borylated products, as summarized in Table 3. The
obtained results were in accordance with the literature, demon-
strating that the Ir catalyst favors the conversion of electron-poor
arenes in the CAH borylation [46,47]. For example, it can be seen
that for the borylation of 3- bromobenzotrifluoride, a higher cat-
alytic activity was observed in comparison to 3-(trifluoromethyl)
anisole using similar reaction conditions, for which a yield of 78%
and 56%, respectively was obtained (Table 3, Entry 2 and 3). More-
over, the regioselectivity of the borylation of arenes was controlled
by the steric effects, which means that the CAH boryation occurs at
the least sterically accessible CAH position of the arene. On the
other hand, the borylation of the heteroarenes is mainly deter-
mined by electronic effects which preferentially takes place at
7
5
0
5
25
0
the CAH bond
a-to the heteroatom [48]. As can be seen from
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Table 2 (Entry 4 and 5), the borylation of indole and pyrrole, gives
Fig. 5. Reusability of the Ir(I)@bipyCTF catalyst for CAH borylation of 1,2-
dichlorobenzene.
9
2% and 84% of the respective borylated products.
A hot filtration test was conducted to verify the heterogeneous
nature of the catalytic process. After the removal of the catalyst by
means of filtration, the catalyst-free reaction solution was stirred
for an additional 8 h (Fig. S9). No catalytic activity was observed
in the absence of the catalyst, which subsequently demonstrates
the heterogeneity of the Ir(I)@bipyCTF catalyst in nature. As for
the recycling tests, after the first cycle, the catalyst was recovered
by filtration, washed thoroughly with heptane and dried under
vacuum. The performance of the recycled catalyst in the borylation
of 1,2-dichlorobenzene up to five successive runs is shown in Fig. 5.
Only a slight loss of product yield was observed during the sequen-
tial tests. This loss in product yield is probably due to the loss of
some catalyst during the recovery process. The ICP-OES analysis,
performed after the first catalytic run, showed the presence of
8.09 wt% amount of Ir suggesting no significant leaching of the Ir
complex during the course of the reaction.
In addition, a comparison has been made between our Ir(I)
@bipyCTF catalyst to the other reported heterogeneous Ir(I)-
based catalysts (Table 4). Although a one-on-one comparison with
the PMO-based support catalyst by Inagaki et al. [13] and the MOF-
based support variant by Lin et al. [15] is not straightforward, due
to different operating conditions, we can conclude that the Ir(I)
@bipyCTF is a highly efficient catalyst. It adds to the high activity
also the larger pores (MOFs) and a very high stability.
An Ir-L edge XANES investigation of the Ir(I)@bipyCTF catalyst
3
before and after a catalytic run was carried out to elucidate the
coordination environment of the Ir center in the catalytic system.

N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
141
Table 4
Heterogeneous Ir-based catalysts catalyzed CAH borylation of 1,2-dichlorobenzene.
Catalyst
Reaction conditions
TON
Yield (%)
Ref.
Ir(I)@bipyCTF
Ir(cod)-BPy0.3-NT
Ir-Bpy-PMO
bpy-UiO-Ir
Ir-bpy-SBA-15
1.5 mol% Ir/90 °C/heptane/8 h
1.5 mol% Ir/80 °C/12 h
0.75 mol% Ir/80 °C/cyclohexane/12 h
3.0 mol% Ir/115 °C/heptane/72 h
1.5 mol% Ir/70 °C/hexane/48 h
64
64
34
NR
136
95
97
92
93
95
This study
[18]
[13,14]
[15,17]
[11]
*
*
TON was calculated for using benzene as substrate.
Scheme 2. A proposed plausible mechanism for CAH borylation of arene catalyzed by Ir(I)@bipyCTF [46].
The measurement displays a clear increase in the white line inten-
sity (11.22 keV) for the sample after catalysis (Fig. S10). This white
line corresponds to a 2p ? 5d electron transition. As the white line
intensity increases, the 5d electron density decreases, denoting a
shift from Ir(I) to Ir(III). This occasion can be explained by the cat-
alytic reaction mechanism proposed by Hartwig and Miyaura’s
the homogeneous counterpart. The heterogeneity test revealed
the stability and reusability of the developed heterogeneous-
based bipyCTF catalyst for at least five cycles without significant
loss of activity. More importantly, the catalyst performs effectively
under mild reaction conditions and tolerance to functional groups
which showed a similar trend to the typical homogeneous iridium-
based catalysts and displayed better performances than the con-
ventional iridium-based heterogeneous catalysts.
(
Scheme 2) [46]. During the catalytic reaction, an Ir(III) complex
is formed by the reaction of an Ir(I) complex with B pin . As the
reaction progresses, the active intermediate Ir(III) complex will
regenerate by the consumption of B pin , explaining the increase
2
2
2
2
Author contributions
in white line intensity. In-situ XANES experiments were also per-
formed to monitor the change in Ir state as the catalytic reaction
progresses (Fig. S11). A gradual increase in white line intensity
can be perceived, corresponding to a gradual increase in the pres-
ence of Ir(III)@bipyCTF. These results are in agreement with the
theoretical aforementioned earlier in this study stating that the Ir
N. Tahir synthesized, analyzed the catalyst and performed the
catalytic reactions under the supervision of P. Van Der Voort. J.
Everaert and T. Heugebaert synthesized the monomer under the
supervision of C. V. Stevens. P. Tack and K. Leus performed the
XAS analysis under the supervision of L. Vincze. F. Muniz-
Miranda performed the theoretical calculations under the supervi-
sion of V. Van Speybroeck. N. Tahir wrote the draft initial paper
while F. Muniz-Miranda contributed the theoretical calculations
section and P. Tack contributed the XAS analysis section. All
authors contributed to revising the paper and have given approval
to the final version of the manuscript.
state changes due to a decrease in B
progresses.
2 2
Pin as the reaction
4
. Conclusions
The straightforward post-synthetic metalation of bipyCTF with
Ir(OMe)(cod)] afforded a robust and efficient heterogeneous sys-
Notes: The authors declare no competing interest.
[
2
tem for borylation of aromatic CAH bonds. Ir(I)@bipyCTF exhibited
excellent catalytic activity for borylation of arenes and heteroare-
Acknowledgment
2 2
nes using B Pin as the borylating agent. The DFT calculations gave
strong evidence for Ir(I)@bipyCTF anchoring and elucidated its
local geometry, while XAS analysis confirmed the change of the
Ir state in the catalyst which has a similar environment to that of
NT would like to acknowledge the Universiti Malaysia Sabah
(UMS) and the Ministry of Higher Education of Malaysia (KPM)
for PhD funding. This work was supported by the Research Board

1
42
N. Tahir et al. / Journal of Catalysis 371 (2019) 135–143
of Ghent University (GOA010-17, BOF GOA2017000303). VVS and
FM-M acknowledge the Fund for Scientific Research – Flanders
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