A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic Carbene Sites, for the Efficient Hydrogenation of CO2 to Formate

Article abstract of DOI:10.1021/acs.chemmater.7b01539

Functionalizing the recently developed porous materials such as porous organic frameworks and coordination polymer networks with active homogeneous catalytic sites would offer new opportunities in the field of heterogeneous catalysis. In this regard, a novel covalent triazine framework functionalized with an Ir(III)-N-heterocyclic carbene complex was synthesized and characterized to have a coordination environment similar to that of its structurally related molecular Ir complex. Because of the strong σ-donating and poor π-accepting characters of the N-heterocyclic carbene (NHC) ligand, the heterogenized Ir-NHC complex efficiently catalyzes the hydrogenation of CO₂ to formate with a turnover frequency of up to 16000 h^-1 and a turnover number of up to 24300; these are the highest values reported to date in heterogeneous catalysis for the hydrogenation of CO₂ to formate.

Full text of DOI:10.1021/acs.chemmater.7b01539

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Article
A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic
2
Carbene Sites, for the Efficient Hydrogenation of CO to Formate
Gunniya Hariyanandam Gunasekar, Kwangho Park, Vinothkumar Ganesan,
Kwangyeol Lee, Nak-Kyoon Kim, Kwang-Deog Jung, and Sungho Yoon
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01539 • Publication Date (Web): 18 Jul 2017
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Chemistry of Materials
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A Covalent Triazine Framework, Functionalized with Ir/N-
Heterocyclic Carbene Sites, for the Efficient Hydrogenation of
CO2 to Formate
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Gunniya Hariyanandam Gunasekar,† Kwangho Park,† Vinothkumar Ganesan,† Kwangyeol Lee,†
Nak-Kyoon Kim,‡ Kwang-Deog Jung,§ and Sungho Yoon*,†
†Department of Applied Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Republic of Korea.
‡Advanced Analysis Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Republic of Ko-
rea.
§Clean Energy Research Centre, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Republic of
Korea.
ABSTRACT: Functionalizing the recently developed porous materials such as porous organic frameworks and coordina-
tion polymer networks with active homogeneous catalytic sites would offer new opportunities in the field of heterogene-
ous catalysis. In this regard, a novel covalent triazine framework functionalized with an Ir(III)-N-heterocyclic carbene
complex was synthesized and characterized to have a coordination environment similar to that of its structurally related
molecular Ir complex. Because of the strong σ-donating and poor π-accepting characters of N-heterocyclic carbene (NHC)
ligand, the heterogenized Ir-NHC complex efficiently catalyzes the hydrogenation of CO2 to formate with a turnover fre-
quency of up to 16000 h−1 and a turnover number of up to 24300; these are the highest values reported to date in hetero-
geneous
catalysis
for
the
hydrogenation
of
CO2
to
formate.
Despite the fact that pyridine-functionalized CTF cata-
lysts have shown outstanding catalytic efficiencies, their
use in catalysis is limited because of the lack of func-
tional and selective sites in CTF materials. To date, in
coordination chemistry, apart from pyridine-ligated
complexes, a vast number of complexes, particularly
with strong electron-donating ligands, have been devel-
oped.23,24 In many cases, they have shown better catalyt-
ic efficiencies than pyridine-ligated complexes.25-27 Thus,
the design of novel functional CTF materials is essential
for the development heterogeneous catalysts. These
functionalized CTF materials would offer new opportu-
nities in emerging research fields such as CO2 hydro-
genation, C-H activation, H2 generation and frustrated
Lewis pair chemistry, etc.
In here, we focused on functionalizing CTF-based het-
erogenized catalyst for the efficient hydrogenation of
CO2 to formate. Although, previously prepared CTF-
based half-sandwich Ir-bipyridine complex (bpy-CTF-
[IrCp*Cl]Cl), reported by us,14 had shown the best TOF
(5300 h−1) among the heterogeneous catalysts, its effi-
ciency is still moderate compared to those of homoge-
neous catalysts.28-33 Similarly, a sphere-shaped IrCp*-
attached CTF, which was constructed using a 1:2 ratio of
2,6-pyridinedicarbonitrile and 4,4’-biphenyldicarbonitri-
le, had also shown very low catalytic activity for this
transformation (a maximum TON of 350 was ob-
served).13
1. INTRODUCTION
The need to improve the efficiencies of heterogeneous
catalysts is driving the development of heterogenized
catalysts across various catalytic processes. Several solid
supports, including silica, alumina, zeolite, and others,
have been both commercially and academically used to
heterogenize various active homogeneous catalysts.1-3
Recently, burgeoning classes of porous materials, such
as metal-organic frameworks (MOFs), covalent organic
frameworks (COFs) and covalent triazine frameworks
(CTFs), have been targeted as catalyst supports because
of their potential for incorporating functional organic
ligands on the pore surfaces.4-7 Among them, CTFs are
particularly interesting owing to their exceptional chem-
ical inertness toward various functional groups, espe-
cially in acidic and basic media, and outstanding stabil-
ity over a broad range of temperatures and pressures.8-12
The high surface area with rigid pore structures enable
the facile diffusion of substrates and products during
catalysis. In addition, the porous properties can be easily
tuned within the same array of monomers by changing
the synthesis conditions.11 Moreover, research into shap-
ing the CTFs for industrial applications has also been
initiated recently.13 Consequently, thus far, organic func-
tional ligands, such as pyridine and 2,2-bipyridine (bpy),
have been assembled in CTFs and employed as supports
to heterogenize different nitrogen-based transition-
metal complexes.14-22
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Scheme 1. Schematic representation of the synthesis of NHC-CTF-based Ir complex Ir-NHC-CTF
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N
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-
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-
-
CN
-
Cl
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n
n
Cl
Cl
Cl
N
Ir
-
Cl
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Br
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ii
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N
N
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-
N
-
N
N
CN
Cl
Cl
N
N
Ir-NHC-CTF
NHC-monomer
NHC-CTF
N
N
n
n
aReagents: (i) ZnCl2; (ii) [IrCp*Cl2]2 and Et3N.
Theoretical investigations on the hydrogenation of CO2
demonstrated that the ligands which improve the elec-
tron density on central metal ions would enhance the
efficiency of this catalytic reaction.34 Consequently, we
envisioned that N-heterocyclic carbene (NHC) may be a
good choice of ligand for an efficient catalysis because it
is well known for its strong σ-donating and poor π-
accepting characteristics.35-37 Accordingly, we focused on
employing NHC-functionalized CTF (NHC-CTF) as a
solid ligand motif for this catalysis. Utilizing 1,3-bis(5-
cyanopyridyl)-imidazolium bromide as the monomer
(NHC-monomer), a CTF with numerous NHC structural
units (NHC-CTF) is accessible (Scheme 1),38 and we pos-
tulate that it will allow the coordination of the {IrCp*}
unit through N^C coordination bond and form a half-
sandwich Ir(III) NHC-complex. Herein, a NHC func-
tionalized CTF was employed to immobilize the half-
sandwich Ir(III) complex via post-synthetic metalation
using an Ir precursor and a base (Scheme 1). The NHC-
CTF-based Ir complex [NHC-CTF-IrCp*Cl]Cl (Ir-NHC-
CTF) efficiently converts CO2 to formate via hydrogena-
tion with highest ever TOF of 16000 h−1 and TON of
24300. Surprisingly, to the best of our knowledge, this is
the first example of a NHC-functionalized porous organ-
ic framework (POF) employed as a support to hetero-
genize transition-metal complex through N^C coordina-
tion bond.
ylene chloride. CO2 (99.99%) and H2 (99.99%) were
purchased from Sinyang gas industries. The Ir precursor,
[IrCp*Cl2]2 was synthesized according to the previously
reported procedure.39
2.2 Characterization Techniques. Fourier transform
infrared (FT-IR) measurements were carried out on a
Nicolet iS 50 (Thermo Fisher Scientific). 13C cross-
polarization magic-angle spinning solid state nuclear
magnetic resonance (13C CP-MAS ssNMR) spectroscopic
data’s were acquired at ambient temperature on 400
MHz Solid state NMR spectrometer (AVANCE III HD,
Bruker, Germany) with an external magnetic field of 9.4
13
T. The operating frequency was 100.66 MHz for C and
the spectra were referenced relative to TMS. The sam-
ples were contained in HX CPMAS probe, with 4 mm
o.d. Zirconia rotor. Scanning electron microscopy (SEM)
and Energy-dispersive spectroscopy (EDS) measure-
ments were performed using JEOL LTD (JAPAN) JEM-
7610F operated at an accelerating voltage of 20.0 kV. X-
ray photoelectron Spectroscopy (XPS) measurements
were recorded on a ESCA 2000 (VG microtech) at a
pressure of ~3 x 10-9 mbar using Al-Ka as the excitation
source (hγ=1486.6 eV) with concentric hemispherical
analyzer. Iridium content in Ir-NHC-CTFs were ana-
lyzed by inductively coupled plasma optical emission
spectroscopy (ICP-OES) (iCAP-Q, Thermo fisher scien-
tific) using microwave assisted acid digestion system
(MARS6, CEM/U.S.A); CTFs (100.0 mg) were digested in
a mixture conc. HCl (20.0 mL) and conc. H2SO4 (10.0
mL) solution under microwave rays at 210 °C for 60 min
(ramp rate = 7 °C/min). N2 adsorption–desorption
measurements were conducted in an automated gas
sorption system (Belsorp II mini, BEL Japan, Inc.,) at 77
K; the samples were degassed for 2 h at 200 °C before
the measurements. The Barrett-Joyner-Halenda (BJH)
method was used for the pore size distribution. Powder
X-ray diffraction (PXRD) was measured on a RIGAKU
D/Max 2500V using Cu (40 kV, 30 mA) radiation. Solu-
2. EXPERIMENTAL SECTION
2.1 Materials. All chemicals and reagents were pur-
chased from commercial supplies and used as received
unless otherwise mentioned; 1-methyl-1H-imidazole,
zinc chloride, anhydrous methanol and chloroform,
triethylamine were purchased from Sigma Aldrich. Irid-
ium chloride hydrate, 1,2,3,4,5-pentamethyl cyclopenta-
diene were purchased from T.C.I. chemicals. 2-bromo-5-
cyanopyridine was obtained from AK Scientific Inc. and
purified by silica-column chromatography using meth-
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tion state H and C NMR were measured on 400 MHz
NMR spectrometer (ASCEND III HD, Bruker, Germany).
LC-MS measurements were performed on Agilent 6130
Single Quadrupole LC/MS spectrometer. HPLC was
measured on Waters Alliance 2695 (Waters Corpora-
tion) equipped with a binary pump, an online degasser,
an auto sampler, a thermostatically controlled column
compartment and a RI detector.
temperature for 30 minutes. A solution of [IrCp*Cl2]2
(the amount was varied to 0.410, 1.00 and 1.50 g, respec-
tively) in a methanol/chloroform mixture was then
slowly added and heated under N2 atmosphere for 24 h.
After cooling to room temperature, the black powder
was filtered, washed with dichloromethane (3x 250 mL)
and acetone (3x 250 mL). Finally, the solid was com-
pletely dried under vacuum for 24 h. The dried samples
were denoted as Irx-NHC-CTF (x denotes the amount
(wt%) of Ir loaded in NHC-CTF).
2.6 Synthesis of homogeneous [IrCp*(N^C)Cl]Cl. A
mixture of 1-(4-(tert-butyl)pyridin-2-yl)-3-methyl-1H-
imidazol-3-ium iodide (0.091 g, 0.265 mmol), designated
as N^C ligand, (see supporting information for the syn-
thesis) and Ag2O (0.036 g, 0.159 mmol) in CH2Cl2 (15.0
mL) was stirred at room temperature for 5 h under N2
atmosphere and filtered through celite bed and washed
with CH2Cl2 (2.0 mL). To the filtrate, [IrCp*Cl2]2 (0.105 g,
0.132 mmol) dissolved in CH2Cl2 (3.0 mL) was added
under N2 atmosphere and stirred for 10 h at room tem-
perature. The resulting white precipitate was filtered,
washed with CH2Cl2 (3.0 mL) and the filtrate was con-
centrated under reduced pressure. Finally, it was recrys-
tallized triturated with diethyl ether to afford homoge-
neous [IrCp*(N^C)Cl]Cl as a yellow solid. Yield = 0.140 g
(86%). 1H NMR (400 MHz, CDCl3): δ [ppm] = 9.34 (d, J =
1.4 Hz, 1H), 8.79 (d, J = 1.7 Hz, 1H), 8.34 (d, J = 6.2 Hz,
1H), 7.53 (d, J = 10.8 Hz, 1H), 7.33 (dd, J = 6.2, 1.9 Hz, 1H),
4.05 (s, 3H), 1.76 (s, 15H), 1.43 (s, 9H). 13C NMR (400
MHz, CDCl3) δ [ppm] = 168.5, 164.9, 152.8, 148.85, 125.6,
121.1, 119.9, 111.9, 91.8, 37.7, 36.4, 30.5, 9.62. LC/MS (ESI):
m/z 578.2 [η5-Cp*Ir(N^C)Cl]+
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2.3 Synthesis of NHC-monomer. The NHC-monomer
was prepared by following the procedure described in
the literature.40 A mixture of 2-bromo-5-cyanopyridine
(20.0 g, 0.109 mmol) and 1-methyl-1H-imidazole (7.47 g,
0.091 mmol) were taken in a 50 mL RBF equipped with a
vacuum adapter, and alternatively degassed with N2 and
vacuum for three times and closed under vacuum. Heat-
ed the mixture to 190 °C at a rate of 1 °C /min for 18 h
and cooled to room temperature. The resulting black
solid was purified in hot methanol, washed with CHCl3,
and dried. Finally, it was recrystallized in methanol to
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obtain pure NHC-monomer. Yield: 70%. H NMR (400
MHz, DMSO-d6) δ [ppm] = 11.07 (t, J = 1.66 Hz, 1H,
NCHN), 9.24 (dd, J = 0.77 Hz and 2.23 Hz, 2H, CH-6py),
8.86 (dd, J = 2.23 Hz, and 8.70 Hz, 2H, CH-4py), 8.84 (d,
J = 1.66 Hz, 2H, NCHCHN), 8.50 (dd, J = 0.77 Hz, and
13
8.70 Hz, 2H, CH-3py). C NMR (400 MHz, DMSO-d6) δ
[ppm] = 153.3 (s, CH-6py), 148.5 (s, CH-2py), 145.1 (s, CH-
4py), 136.3 (s, NCHN), 121.0 (s, NCHCHN), 116.5 (s, C-
5py), 115.7 (s, C-3py), 111.0 (s, CN). LC/MS (ESI): m/z
273.09.
2.4 Synthesis of NHC-CTFs. In an Ar filled glove box,
the NHC-monomer (1.00 g, 2.83 mmol) and zinc chlo-
ride (1.92 g, 14.1 mmol) were taken in a 5 mL ampule(s)
and closed with vacuum adapters. The ampule(s) were
taken out from glove box and sealed under vacuum, and
heated to 300 °C or 400 °C in a furnace at a heating rate
of 60 °C/h. After 48 h, the furnace was cooled to 200 °C
at a cooling rate of 10 °C/h. The resulting monolith was
ground well and stirred with 250 mL of water for 3 h and
filtered, washed with water (500 mL) and acetone (500
mL). The obtained solid was then refluxed in 1M HCl
(500 mL) for 12 h, and filtered, washed with 1M HCl
(3X100 mL), H2O (3X100 mL), tetrahydrofuran (3X100
mL) and acetone (3X100 mL). To a saturated aqueous
NaCl solution (3.0 M), the filtered solids were added and
stirred at room temperature for 24 h to metathesis all
Br− into Cl− ions. Then, the solid was filtered, washed
with water (3X500 mL), and acetone (3X100 mL). Final-
ly, the black powder was dried under vacuum at 200 °C
for 10 h. Yield: 0.800 g.
2.7 Representative procedure for the hydrogenation
of CO2 to formate
Hydrogenation was carried out in a 100 mL homemade
stainless steel reactor equipped with a glass vessel in-
sert. In a typical run, the Ir-NHC-CTF (10.0 mg) was
dispersed in a CO2 saturated aqueous triethylamine so-
lution and kept into the reactor, and tightly closed. Af-
ter flushing with CO2, the reactor was initially pressur-
ized with CO2 and then with H2 (1:1) to the desired pres-
sure at room temperature and heated at 80-160 °C. The
reaction was cooled to room temperature after appro-
priate time and slowly released the pressure. The con-
centration of formate was analyzed by HPLC on Aminex
HPX-87H column using 5.00 mM H2SO4 as an eluent. In
the recycling experiments, the catalyst was recovered
after each cycles by filtration, washed with water and
acetone, and dried under vacuum before next run. By
following the similar procedure, the solid was then used
for the successive runs.
2.5 General Procedure for the synthesis of Ir-NHC-
CTF. Ir-NHC-CTF was synthesized with three different
Ir loadings. In an oven-dried multi neck round bottom
flask equipped with a condenser, the NHC-CTF (0.610
g), synthesized at 400 °C, was suspended in a metha-
nol/chloroform mixture under N2 atmosphere. After 10
min, triethylamine (the amount was varied to 1.5, 4.5
and 6.0 mL, respectively) was added and stirred at room
3. RESULTS AND DISCUSSION
3.1 Synthesis and characterization of NHC-CTF: The
synthetic strategy for the preparation of the NHC-
functionalized CTF-based Ir complex (Ir-NHC-CTF) is
outlined in Scheme 1. The trimerization of the NHC
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Relative Pressure p/p0
Figure 1. Scanning electron microscopy (SEM) image of (a) NHC-CTF and (b) Ir0.68-NHC-CTF; (c) Energy-dispersive spec-
troscopic (EDS) mapping of the Ir atoms in Ir0.68-NHC-CTF. (d) N2 adsorption-desorption isotherms of NHC-CTF pre-
pared at 400 °C (red) and Ir0.68-NHC-CTF (blue).
synthesized at 400 °C (from here designated simply as
NHC-CTF) was subjected to further studies.
monomer was achieved at two different temperatures
(300 and 400 °C) via the ionothermal technique using
ZnCl2 as the catalyst and as a salt melt. The as-
synthesized CTFs were stable in air and insoluble in
nearly all common organic solvents (methanol, ethanol,
acetone, acetonitrile, dichloromethane, chloroform,
ethyl acetate, dimethylformamide, dimethylsulphoxide,
tetrahydrofuran and, diethylether) and water. X-ray
powder diffraction analysis revealed that the NHC-CTFs
were amorphous in nature (Figure S1). The absence of a
nitrile peak representing the monomer at 2236 cm−1 in
the FT-IR spectra indicates the complete conversion of
nitrile groups (Figure S2). The regional peaks at 115, 131,
and 140–165 ppm in the 13C CP-MAS ssNMR spectroscop-
ic analysis of NHC-CTFs confirm the presence of pyri-
dine, triazine, and imidazolium units (Figure S3). In
addition, the peak at 137 ppm clearly indicates the pres-
ence of carbene sites in NHC-CTFs. N2 adsorption–
desorption measurements revealed that the CTF synthe-
sized at 400 °C showed a type IV adsorption isotherm
with a hysteresis behavior (Figure 1d and S4), indicating
that the material exhibits both microporous (major) and
mesoporous structure (minor) with a total pore volume
of 0.38 cm3/g and the Brunauer-Emmett-Teller (BET)
surface area of 693 m2/g. Conversely, the CTF synthe-
sized at 300 °C showed very low porosity (total pore vol-
ume: 0.005 cm3/g and surface area: 2.4 m2/g); the behav-
ior of having low or no porosity in the CTFs synthesized
at lower temperature is reported previously.41 Since the
support porosity plays a key role in the diffusion of sub-
strates and products in heterogeneous catalysis, the CTF
3.2 Synthesis and characterization of Ir-NHC-CTF:
To synthesize Ir-NHC-CTF, the as prepared NHC-CTF
(synthesized at 400 °C) and [IrCp*Cl2]2 were refluxed in
a methanol/chloroform mixture with triethylamine un-
der N2 atmosphere for 24 h. The amount of Ir loaded on
NHC-CTF could be controlled by varying the concentra-
tion of [IrCp*Cl2]2, and three catalysts containing Ir
loadings of 0.68, 2.1 and 3.5 wt%) (determined by ICP-
OES) were prepared and are designated as Ir0.68-NHC-
CTF, Ir2.1-NHC-CTF and Ir3.5-NHC-CTF, respectively. The
Ir-NHC-CTFs were stable in air and insoluble in almost
all common organic solvents and water. SEM image of
Ir0.68-NHC-CTF shows that the original irregular mor-
phology of NHC-CTF remains unchanged during the
N
NH
N
N
N
NaH
N
Cl
Cl-
CH3I
Cl
Ir
Ag2O
[Cp*IrCl2]2
N
N
N
N
-
I
N
N
+
homogeneous
[IrCp*(N^C)Cl]Cl
N^C ligand
Scheme 2. Synthesis of homogeneous [IrCp*(N^C)Cl]Cl.
.
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N
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Binding Energy (eV)
Binding Energy (eV)
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Figure 2. Deconvoluted XPS C 1s spectra of N^C ligand (a); homogeneous [IrCp*(N^C)Cl]Cl (b); NHC-CTF (c); Ir0.68
NHC-CTF (d).
-
(a)
(c)
(b)
(d)
404
400
396
392
404
400
396
392
Binding Energy (eV)
Binding Energy (eV)
Figure 3. Deconvoluted XPS N 1s spectra of N^C ligand (a); homogeneous [IrCp*(N^C)Cl]Cl (b); NHC-CTF (c); Ir0.68
NHC-CTF (d).
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13
metalation (Figure 1a and 1b). SEM–EDS mapping
showed a homogeneous distribution of the C, N, Cl, and
Ir elements in the CTF matrix, evidencing that the meta-
lation has occurred uniformly throughout the frame-
work (Figure 1c and Figure S5). The formation of Ir-
NHC-CTF was confirmed by C CP-MAS ssNMR analy-
sis. As shown in Figure S6, compared to the 13C CP-MAS
ssNMR of NHC-CTF, the intensity of aromatic carbons
in the region of 90-145 ppm were increased and a new
peak at the aliphatic regions (~10 ppm) was observed.
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This new peak is assignable to the methyl carbon of the
Page 6 of 11
Cp* unit,42 and the increased intensity at the aromatic
region is attributed to the overlapped signals of aro-
matic carbons of NHC-CTF with the aromatic carbons of
Cp*. This result clearly indicates the presence of Cp*
unit in the Ir3.5-NHC-CTF and thus confirms the coordi-
nation of Ir precursor with NHC-CTF support. Finally,
the porous properties of Ir0.68-NHC-CTF were realized
by N2 adsorption–desorption analysis performed at 77 K
(Figure 1d and S7). The results revealed that Ir0.68-NHC-
CTF exhibit a type IV isotherm with hysteresis behavior,
suggesting that Ir-NHC-CTFs still exhibit both as mi-
croporous and mesoporous material. However, the sur-
face areas and total pore volumes were decreased ac-
cording to the metal loading amount This indicates that
the {IrCp*Cl2} units partially occupied the pores of
NHC-CTF and reduced the accessible pore volumes of
CTF. Notably, Ir-NHC-CTF is still a porous material and
might facilitate the diffusion of small molecules such as
CO2, H2, H2O, and formic acid through its pores.
1
2
3
4
5
6
7
8
(a)
(b)
Ir 4f5/2
65.0 eV
:
Ir 4f7/2:
62.0 eV
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68
64
60
56
3.3 Synthesis and characterization of homogeneous
[IrCp*(N^C)Cl]Cl: To understand the iridium interac-
tion with NHC-CTF support, a structurally related new
homogeneous iridium complex, [IrCp*(N^C)Cl]Cl (inset
in Scheme 1), was synthesized as a reference complex.
The synthetic scheme for the preparation of homogene-
ous [IrCp*(N^C)Cl]Cl was outlined in Scheme 2. The
N^C ligand was prepared through an addition-
elimination reaction of 4-tert-butylpyridyl-2-chloride
with sodium salt of imidazole, followed by N-
methylation of imidazole ring using methyl iodide. The
as prepared N^C ligand was treated with carbene trans-
fer reagent (Ag2O), followed by reaction with [IrCp*Cl2]2
Binding Energy (eV)
Figure 4. XPS Ir 4f spectra of (a) Ir0.68-NHC-CTF and (b)
homogeneous [IrCp*(N^C)Cl]Cl.
spectrum of N^C ligand showed two peaks at 398.7 and
400.7 eV binding energy (Figure 3a); the peak at 398.7
eV corresponds to pyridinic nitrogen species and the
peak at 400.7 eV corresponds to imidazolium nitrogen
species.43, 44 The C-1s deconvolution peaks (Figure 2a ) at
284.6 and 285.7 eV demonstrate that it has two kinds of
carbon species corresponds to C-C/C-H and C-N carbon
species, respectively.44 Upon complexation with iridium
metal center, the pyridinic nitrogen’s binding energy
was shifted from 398.7 to 399.8 eV (Figure 3a vs 3b) and
the intensity of C-N carbon species was decreased (Fig-
ure 2a vs 2b). This indicate that the electron density
around C-N and pyridinic nitrogen were reduced. The
diminution of electron density was attributed to the
consequences of electron donation from pyridinic nitro-
gen and C-N carbene species (NCHN unit) to the iridi-
um metal center. The behavior of pyridinic nitrogen’s
upon complexation with metal (i.e. the shift to higher
binding energy) was previously reported in Pt incorpo-
rated CTF by Schuth and co-workers.15 Ir-4f XPS spec-
trum showed that the binding energy for Ir-4f7/2 level
was 62.0 eV (Figure 4b), which was 0.7 eV lower than
the IrCl3 (62.7 eV).45 This indicates that the valence state
of Ir in homogeneous [IrCp*(N^C)Cl]Cl was Ir(III) and
the electron density around the iridium center was
much higher than the simple IrCl3. Higher electron den-
sity at the iridium metal center indicates that the N^C
and Cp* ligands donate electron densities to the iridium
cation.
to
yield
the
water-stable
yellowish
orange
[IrCp*(N^C)Cl]Cl in excellent yield. Absence of the peak
at 11.33 ppm, which corresponds to NCHN imidazolium
proton, and downfield shift of aromatic protons com-
1
pared to those of N^C ligand in H NMR spectrum of
[IrCp*(N^C)Cl]Cl indicates that the electron density
from the N^C ligand was donated to the iridium metal
center. In addition, the lack of characteristic set of two
distinct peaks for the 4-tert-butyl-pyridine ring confirms
the loss of symmetry and formation of a chelate. The
formation of [IrCp*(N^C)Cl]Cl was also evidenced by
mass spectrum through the presence of [η5-
Cp*Ir(N^C)Cl]+ ion (m/z= 578.2) with expected isotopic
distribution ratios.
3.4 XPS studies: Coordinative interaction between irid-
ium center and N^C site in Ir-NHC-CTFs and homoge-
neous [IrCp*(N^C)Cl]Cl were systematically studied by
XPS. To analyze how the electronic environments
around the C and N atoms of N^C ligand and NHC-CTF
changes upon iridium complex formation, XPS of N^C
ligand and NHC-CTF were compared with their respec-
tive iridium complexes.
Next, the iridium interaction with NHC-CTF support in
Ir0.68-NHC-CTF was established. As expected, the de-
convoluted N-1s spectrum of NHC-CTF showed the
presence of triazine/ pyridine (398.1 eV) and imidazoli-
Initially, the XPS of homogeneous [IrCp*(N^C)Cl]Cl and
N^C ligand were studied. The deconvoluted N-1s
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Chemistry of Materials
um (400.8 eV) nitrogen species (Figure 3c).15, 43 However,
Table 1: Catalytic Performances of Ir-NHC-CTFs under
various reaction conditionsa
1
a new peak at 399.8 eV was also observed (Figure 3c).
Similarly, apart from C-C/C-H and C-N carbon species, a
new peak at 288 eV was observed in the deconvoluted
C-1s spectrum of NHC-CTF (Figure 2c). The formation
of these additional species were attributed to the high
polymerization temperature used in the synthesis of
NHC-CTF; this behavior was often observed in the syn-
thesis of numerous CTFs.9,10,15,41 Coordinative interaction
of the NHC-CTF with iridium metal center result the
following changes in the C-1s and N-1s spectra. The de-
convoluted N-1s spectrum showed a decrease in the in-
tensity of the peak at 398.1 eV and an increase in the
intensity of the peak at 399.8 eV (Figure 3c vs 3d), which
can also be observed from the raw data (black). This
indicates that a fraction of nitrogen’s binding energies
was shifted from 398.7 to 399.7 eV, similar to the behav-
ior of N-1s spectra of N^C ligand vs homogeneous
[IrCp*(N^C)Cl]Cl. The deconvoluted C-1s spectrum
showed that the intensity of C-N species (the peak at
285.7 eV) was decreased (Figure 2c vs 2d), analogues to
the C-1s spectra of N^C ligand vs homogeneous
[IrCp*(N^C)Cl]Cl. These features indicate an interaction
of pyridinic and carbene (NCHN) sites of NHC-CTF with
iridium metal center as similar as that of the interaction
of N^C ligand with iridium metal center in homogene-
ous [IrCp*(N^C)Cl]Cl.
The Ir-4f spectrum of Ir-NHC-CTFs showed that the
binding energy of Ir-4f7/2 was 62.0 eV, (Figure 4a and
Figure S8), suggesting that Ir-NHC-CTFs and homoge-
neous [IrCp*(N^C)Cl]Cl have identical binding energy
values. All these results collectively confirm that the
formation and coordination environment Ir-NHC-CTFs
are similar to that of reference homogeneous
[IrCp*(N^C)Cl]Cl.
3.5 Catalytic Hydrogenation of CO2 into Formate:
The as-synthesized Ir-NHC-CTFs were used as catalysts
for CO2 hydrogenation. The hydrogenation is usually
carried out in amine solutions to trap formic acid as
formate-amine adduct.46 Thus, using Ir-NHC-CTFs the
hydrogenation was carried out in a 1.0 M aqueous tri-
ethylamine solution. Initially, the hydrogenation was
performed using Ir0.68-NHC-CTF catalyst at a stirring
speed of 300 RPM. As shown in Table 1, entry 1, at 80 °C
and under a total pressure of 4.0 MPa, Ir0.68-NHC-CTF
afforded a substantial TON of 800 within 2 h, which was
nearly ~1.5 times higher than the previously reported
bipyridine-based half-sandwich Ir complex (bpy-CTF-
[IrCp*Cl]Cl)14 under similar reaction conditions. This
indicates that the introduction of an electron donating
NHC ligand can significantly improve the efficiency of
CO2 hydrogenation catalysts as hypothesized.
2
3
4
5
6
7
8
9
Entry
T
P
Time
(h)
TONc
TOFd
(h-1)
(°C)
(MPa)b
1
2
80
100
120
160
160
160
120
120
120
120
120
120
120
120
120
120
4.0
4.0
4.0
4.0
6.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
2
2
800
2155
400
1077
3
2
4050
8450
11550
17330
4000
9200
14220
19655
24300
9230
9190
9250
6100
4250
2025
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
2
4225
5
2
5775
6
2
8665
16000e
7
0.25
2
8
-
-
-
-
-
-
-
-
-
9
5
10
11
10
15
2
12f
13g
14h
15i
16j
2
2
2
2
aReaction conditions: 10.0 mg of Ir0.68-NHC-CTF was
dispersed in 1.0 M aqueous Et3N solution (30 mL) and
stirred at a rate of 300 RPM (rotation per minute) under
desired pressure and temperature. bTotal pressure at
c
room temperature [CO2:H2 (1:1)]. TON = mole of for-
d
e
mate/mole of Ir. TOF = TON per hour. Initial TOF
calculated from the initial part of the reaction (after 15
f
g
h
i
min). RPM = 1000. RPM = 2000. RPM = 2750. Ir2.1-
NHC-CTF. jIr3.5NHC-CTF.
results shown in entries 2–4 show that the TON gradual-
ly increases at higher temperatures and reaches a TON
of 8450 at 160 °C. The effect of pressure was subsequent-
ly investigated at 160 °C for 2 h. When the total pressure
was increased from 4.0 to 6.0 or 8.0 MPa, a linear in-
crease in activity was observed (entries 4–6), and at-
tained a TON of 17330 at a pressure of 8.0 MPa in 2 h.
This increase in activity with respect to pressure indi-
cates that the rate of reaction is proportional to the
pressure of both H2 and CO2 gases; suggesting the hy-
drogenation follows a first order kinetics with respect to
both H2 and CO2.
Finally, the hydrogenation was monitored at different
time intervals (Table 1, entry 7-11 and Figure S5). Analyz-
ing the initial part of the reaction (15 min), it was re-
vealed that Ir0.68-NHC-CTF has an initial TOF of 16000
h−1 at 120 °C under a total pressure of 8.0 MPa, which is
the highest value reported to date for any
With this result in hand, the catalytic efficiency Ir0.68
-
NHC-CTF was further studied under different reaction
conditions to obtain the maximal activity. Initially, the
hydrogenation was studied at temperatures up to 160 °C
at a total pressure of 4.0 MPa (CO2/H2 = 1) for 2 h. The
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Page 8 of 11
on the external mass diffusion limitations in the current
12
8
1
2
3
4
reaction conditions. Finally, using Ir2.1-NHC-CTF and
Ir3.5-NHC-CTF catalyst the hydrogenation was per-
formed. As shown in entries 15-16, the catalysts demon-
strated TONs of 6100 and 4250, respectively at 120 °C
under 8.0 MPa total pressure in 2 h.
5
6
7
8
9
Before checking the recycling ability, the heterogeneous
nature of Ir-NHC-CTF was investigated by filtration
tests. To test this, the catalyst Ir0.68-NHC-CTF was fil-
tered after 1 and 5 h of hydrogenation and the resulting
colorless filtrates were used as the catalytic solution. As
shown in Figure S11, even after extended reaction time,
no increment in the formate concentration was ob-
served; indicates that the catalyst is purely working in a
heterogeneous manner. Finally, to examine the recycla-
bility of Ir0.68-NHC-CTF, the black solid after the initial
run was recovered by simple filtration and washed with
water, dried, and directly used in the next run. As shown
in Figure 5, the catalytic efficiency of Ir0.68-NHC-CTF
was slightly decreased in the successive runs and about
90% of the activity was retained in each recycles. XPS of
the recovered catalyst (after 3 cycles) showed no subtle
changes in the binding energies of Ir (Ir 4f7/2 = 62.4 eV
and Ir4f5/2 = 65.4 eV) compared to the binding energy of
Ir after 2 h of initial run (Figure S10), suggesting that the
coordination environment around the Ir metal center is
retained. However, ICP-OES analysis of the filtrate (in
each recycles) revealed that ca 6.4-7.2% of the loaded Ir
has been leached. These results suggest that the reduced
activity of Ir0.68-NHC-CTF at the successive runs may be
originated from the Ir leaching. The origin of Ir leaching
into the solution is currently under investigation.
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
1
2
3
Number of cycle
Figure 5. Recyclability of Ir0.68-NHC-CTF at 120 °C under
8.0 MPa total pressure.
heterogeneous catalytic system.46-56 As shown in Figure
S9, the generation of formate was gradually increased
with time and a formate concentration of 0.282 M with a
highest ever TON of 24300 was obtained in 15 h. Figure
S5 shows that the rate of formate generation was faster
at an early stage of the reaction (ca 2 h) compared to the
end stage of the reaction. The similar behavior was pre-
viously observed in the ruthenium (half-sandwich ox-
inato complexes) catalyzed CO2 hydrogenation.57 To
examine this behavior, the catalysts after 1, 5 and 20 h of
hydrogenation were analyzed by XPS. As shown in Fig-
ure S10, the Ir 4f binding energy of all the samples (1, 5
and 20 h samples) were same (Ir 4f7/2 = 62.4 eV and
Ir4f5/2 = 65.4 eV), indicating that the coordination envi-
ronment of Ir species after 1, 5 and 20 h of hydrogena-
tion remains same, and thus the slow rate at the end
stage of the reaction does not related to the coordina-
tion environment of Ir species. As shown in Figure S10,
compared to Ir0.68-NHC-CTF, the binding energies were
slightly increased by 0.4 eV, indicates a decrease in the
electron density at the Ir atom; this behavior might be
attributed to the chloride ligand substitution by the
more electron-withdrawing ligand such as generated
formate anions. Although, the exact reason for the slow-
er rate of formate generation at the final part of the re-
action is not clearly understood, we expect that it may
be attributed to the more competitive backward reac-
tion, i.e., the decomposition of formate back into CO2 to
H2, which usually promoted by most of such CO2 hydro-
genation catalysts.29,33,58,59 In addition, it could be due to
the changes in the properties of the reaction medium
such as dipole moment, pH,33 and etc., which expected
to have significant difference between the initial and
end stage of the reaction.
4. CONCLUSION
In conclusion, a novel covalent triazine framework-
functionalized with Ir-NHC complex (Ir-NHC-CTF) was
synthesized using a solid chelating ligand containing
NHC sites through post-synthetic immobilization tech-
nique. The coordination environment around the Ir(III)
center in Ir-NHC-CTF was closer to the reference ho-
mogeneous [IrCp*(N^C)Cl]Cl. The strong σ-donating
and poor π-accepting characters of NHC site in the CTF
enhances the electron density on central metal Ir(III)
cation. As a result, the Ir-NHC-CTF demonstrated an
excellent activity towards the hydrogenation of CO2 to
formate with TOFs of 16000 h-1 and TONs of 24300,
which are the highest values reported to date in hetero-
geneous catalytic systems. Thus, the result opens new
opportunities for improving the efficiency of heteroge-
neous catalysts across various catalytic processes.
ASSOCIATED CONTENT
Supporting Information
Details of the synthetic procedures and analytical data of
presented CTFs included. This material is available free of
charge via the Internet at http://pubs.acs.org
Then, to examine the external mass transfer effects on
the reaction rate, the hydrogenation was performed at
various stirring speed. The result showed that effect of
stirring speed on the reaction rate was negligible (en-
tries 12-14); indicating that the reaction rate may not rely
AUTHOR INFORMATION
8
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Chemistry of Materials
(14) Park, K.; Gunasekar, G. H.; Prakash, N.; Jung, K. D.; Yoon,
Corresponding Author
1
2
3
4
5
6
7
8
S. A Highly Efficient Heterogenized Iridium Complex for the
Catalytic Hydrogenation of Carbon Dioxide to Formate.
ChemSusChem 2015, 8, 3410-3413.
*yoona@kookmin.ac.kr
ACKNOWLEDGMENT
We acknowledged the financial support by grants from
Korea CCS R&D Centre, funded by the Ministry of Educa-
tion, Science and Technology of Korean government, and
also from Global Scholarship Program for Foreign Graduate
Students at Kookmin University in Korea.
(15) Soorholtz, M.; Jones, L. C.; Samuelis, D.; Weidenthaler, C.;
White, R. J.; Titirici, M.-M.; Cullen, D. A.; Zimmermann, T.;
Antonietti, M.; Maier, J.; Palkovits, R.; Chmelka B. F.; Schuth,
F. Local Platinum Environments in a Solid Analogue of the
Molecular Periana Catalyst. ACS Catal. 2016, 6, 2332-2340.
(16) Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Schuth,
F. Solid Catalysts for the Selective Low-Temperature Oxidation
of Methane to Methanol. Angew. Chem. Int. Ed. 2009, 48,
6909-6912.
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A covalent triazine framework, functionalized with Ir-NHC sites, has demonstrated a highest ever TOF of
16,000 h-1 and TON of 24,300 for the hydrogenation of CO2 to formate.
338x190mm (96 x 96 DPI)
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