Continuous flow hydrogenation with Pd complexes of pyridine-benzotriazole ligands

Article abstract of DOI:10.1002/aoc.6389

The use of continuous flow systems in chemical synthesis provides great advantages in terms of sustainability, efficiency, and safety. The ability to control reaction parameters such as temperature, pressure, and catalyst exposure in flow system enables rapid optimization of reaction conditions. In the present study, palladium complexes of 1-(piridin-2-il)-1H-benzo[d][1,2,3]triazol, N-((1H-benzo[d][1,2,3]triazol-1-il)metil)piridin-2-amin, and (1H-benzo[d][1,2,3]triazol-1-yl)(pyridin-2-yl)methanone ligands were synthesized and characterized. The catalytic activities of complexes are investigated in the hydrogenation of various alkenes such as styrene, cyclohexene, and 1-octene under continuous flow conditions. The complexes showed very high activity at 10-bar H₂ pressure and 50°C for short periods of 5–10?min. The catalysts reused for 10 cycles with no significant loss of catalytic activity.

Full text of DOI:10.1002/aoc.6389

Received: 26 May 2021
DOI: 10.1002/aoc.6389
Revised: 7 July 2021
Accepted: 14 July 2021
R E S E A R C H A R T I C L E
Continuous flow hydrogenation with Pd complexes of
pyridine-benzotriazole ligands
Filiz Yılmaz
| Deniz Hür
Faculty of Science Department of
Abstract
Chemistry, Yunusemre Campus, Eskisehir
Technical University, Eskisehir, Turkey
The use of continuous flow systems in chemical synthesis provides great
advantages in terms of sustainability, efficiency, and safety. The ability to
control reaction parameters such as temperature, pressure, and catalyst
exposure in flow system enables rapid optimization of reaction conditions. In
the present study, palladium complexes of 1-(piridin-2-il)-1H-benzo[d][1,2,3]
triazol, N-((1H-benzo[d][1,2,3]triazol-1-il)metil)piridin-2-amin, and (1H-benzo
Correspondence
Deniz Hür, Faculty of Science Department
of Chemistry, Yunusemre Campus,
Eskisehir Technical University, 26470
Tepebası, Eskisehir, Turkey.
Email: dhur@eskisehir.edu.tr
[
d][1,2,3]triazol-1-yl)(pyridin-2-yl)methanone ligands were synthesized and
Funding information
Eski s¸ ehir Technical University, Grant/
Award Number: 1701F025
characterized. The catalytic activities of complexes are investigated in the
hydrogenation of various alkenes such as styrene, cyclohexene, and 1-octene
under continuous flow conditions. The complexes showed very high activity at
ꢀ
1
0-bar H pressure and 50 C for short periods of 5–10 min. The catalysts
2
reused for 10 cycles with no significant loss of catalytic activity.
K E Y W O R D S
benzotriazole, continuous flow, hydrogenation, palladium
[
23]
1
| INTRODUCTION
Recently, Yu et al. published a minreview
to evaluate
innovations in continuous flow hydrogenation studies.
In this study, recent studies on continuous flow
homogeneous and heterogeneous catalysis of various
organic functional groups such as alkenes and
Heterogeneous catalytic hydrogenation (HCH) has wide
[1–8]
range application area
in different disciplines. Thus,
synthesis of new and effective heterogeneous catalysts
and efficiency investigation of these catalysts on various
organic molecules by cheaper, environmentally compati-
ble and rapid methods are among the issues that
remain important today. From this perspective, organic
conversion reactions carried out under continuous flow
conditions have been become increasingly important in
synthetic organic chemistry. Continuous flow processes
provide significant advantages in terms of safety, effi-
ciency, reproducibility, and waste reduction, as well
as practical, efficient, and environmentally/sustainable
organic transformations. Therefore, in recent years, con-
tinuous flow method has been an important tool for the
[
24–27]
[28,29]
[30,31]
alkynes,
imines and nitro,
and carbonyl
were examined. Recent studies show that continuous
flow hydrogenation has important advantages such as
providing more effective control of reaction conditions,
enabling industrial processes and laboratory-scale
computer-controlled automatic experimentation, easy
integration with instrumental analysis equipment such
[
32]
as FT-IR,
GC–MS, and benchtop NMR for faster
analysis, safer hydrogen source usage, more effective
catalyst-reactive interaction with good multiphase
mixing, higher enantio-selectivity, higher reusability of
catalysts, lower reaction temperature, and a shorter
reaction time compared to batch processes. Benzotriazole
is an important heterocyclic compound that can be
[
9–17]
efficiency and reusability of heterogeneous catalysts,
among many other synthetic organic applications.
[
18–22]
Appl Organomet Chem. 2021;e6389.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.6389

2
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YILMAZ AND HÜR
used in the development of new synthesis methods. It
is resistant to many synthetic conditions, nontoxic, inex-
pensive, and easily removable in both acidic and basic
7.61 (1H, dt, J = 8.09, 0.93 Hz), 7.46 (1H, dt, J = 8.13,
1
3
0.92 Hz), 7.33 (1H, ddd, J = 7.39, 4.88, 0.78 Hz) ppm;
C
NMR for PyrBt (100 MHz, CDCl ). δ = 114.8, 114.9,
3
[33]
medium.
In this study, pyridine and benzotriazole ring con-
120.0, 123.6, 125.8, 129.7, 131.3, 140.4, 146.5, 149.2,
151.2 ppm.
[33,34]
taining ligands
and their palladium complexes have
been synthesized. The catalytic activities of complexes
have been investigated in hydrogenation of various
alkenes in the continuous flow system. In catalytic
experiments, temperature, H2 pressure, and flow rate
parameters were changed to determine optimum reaction
conditions for each catalyst.
2.2.2 | Synthesis of N-((1H-benzo[d][1,2,3]
triazol-1-il)metil)piridin-2-amin, PyrNHCH Bt
2
PyrNHCH2Bt ligand was prepared according to literature
[
36]
procedure.
and 2-aminopyridine (1 eq.) in water was stirred for
min. Formaldehyde (37% w/w water, 1 eq.) was added
The solution of 1H-benzotriazole (1 eq.)
5
2
2
| EXPERIMENTAL
.1 | Materials
dropwise to this mixture. The reaction mixture was
stirred for 1 h. The precipitated solid is filtered and rec-
rystallized from an ethyl acetate/hexane mixture to give
the product. Yield: 76%; FT-IR (KBr): ν 3394 (s), 3093
(w), 3024 (m), 1610 (s), 1514 (s), 1453 (s), 1325 (s), 1296
All chemicals are used in analytical purity and purchased
from Merck. FT-IR spectra were recorded between
À1
1
(s), 1165 (s), 760 (s) cm ; H NMR (400 MHz, CDCl ).
3
À1
4
1
00 and 4000 cm
00 Spectrometer. The samples were incorporated with
using a Perkin Elmer Spectrum
δ = 8.20 (1H, s), 8.03–7.94 (2H, m), 7.49–7.38 (2H, m),
7.33 (1H, t, J = 14.33, 7.29 Hz), 6.72–6.65 (1H, m), 6.52
(1H, d, J = 8.2 Hz), 6.36 (2H, d, J = 6.91 Hz), 5.66 (1H, s,
1
KBr and pressed into a pellet. H NMR spectra were
measured on Agilent DDR2 400-MHz spectrometer
using TMS as the internal standard. Thermogravimetric/
differential thermal analyses were performed on a Perkin
Elmer Diamond TG/DTA instrument. Hydrogenation
13
br.) ppm; C NMR (100 MHz, CDCl ). δ = 54.0, 108.9,
3
111.2, 115.0, 119.5, 123.9, 127.3, 132.8, 137.8, 147.7,
155.9 ppm.
®
reactions were performed with ThalesNano H-Cube .
The product distributions in the catalytic experiments
were determined by Thermo Finnigan Trace GC
including FID detector and Permabond column
2.2.3 | Synthesis of (1H-benzo[d][1,2,3]
triazol-1-yl)(pyridin-2-yl)methanone, PyrCOBt
(SE-54-DF-0.25, 25 m  0.32 mm i.d.).
PyrCOBt ligand was prepared according to litera-
[
37]
ture.
A
solution of 2-picolinic acid (1 eq.),
mesitylbenzotriazole (1 eq.), and triethyl amine (2 eq.)
in 20 ml of dry THF was dissolved in an 80-ml
pressure tube under nitrogen atmosphere. The reaction
mixture was kept under 80-W microwave irradiation
for 1 h. When the reaction was completed, the solvent
was evaporated under vacuum. The reaction mixture
was dissolved in ethyl acetate and extracted with 20%
2
.2 | Synthesis
2
.2.1 | Synthesis of 1-(piridin-2-il)-1H-benzo
[
d][1,2,3]triazol, PyrBt
PyrBt ligand was prepared by the methods reported in
the literature.
[35]
2-Bromopyridine (1 eq.) and 1H-
Na CO3 (aq) solution (5 Â 20 ml). The organic phase
2
benzotriazole (2 eq.) was refluxed in toluene for 24 h.
After the reaction was completed, solvent was removed
under vacuum, and the residue was dissolved in ethyl
acetate, washed with saturated Na CO to remove excess
of benzotriazole. Obtained H NMR and C NMR results
are identical with those in literature. Yield: 82%; FT-IR
was dried with sodium sulfate, and the solvent was
evaporated under vacuum to give the product. Yield:
92%; FT-IR (KBr): ν 1706 (s), 1593 (m), 1484 (m), 1450
À1
1
(m), 1367 (m), 1217 (s), 1045 (s), 742 (s) cm
; H
2
3
1
13
NMR (400 MHz, CDCl3, TMS). δ = 8.81 (d,
J = 4.64 Hz, 1H), 8.33 (d, J = 8.83 Hz, 1H), 8.11 (d,
J = 7.68 Hz, 1H), 8.07 (d, J = 7.86 Hz, 1H), 7.91 (t,
J = 7.78 Hz, 1H), 7.66 (t, J = 7.69 Hz, 1H), 7.56–7.48
(KBr): ν 3110 (m), 3018 (m), 1590 (s), 1476 (s), 1441 (s),
À1
1
1
373 (w), 1285 (s), 1243 (w), 1065 (s), 758 (s) cm ; H
13
NMR (400 MHz, CDCl ). δ = 8.66 (1H, d, J = 8.41 Hz),
(m, 2H) ppm;
C NMR (100 MHz, CDCl , TMS).
3
3
8
.62 (1H, dd, J = 4.87, 0.7 Hz), 8.31 (1H, d, J = 8.32 Hz),
δ = 114.5, 120.2, 125.7, 126.3, 126.5, 126.9, 130.5, 131.8,
8
.13 (1H, d, J = 8.34 Hz), 7.95 (1H, dt, J = 8.46, 1.88 Hz),
136.7, 145.6, 149.8, 165.0 ppm.

YILMAZ AND HÜR
3 of 12
2
1
.2.4 | Synthesis of dichloro[1-(piridin-2-il)-
H-benzo[d][1,2,3]triazol]palladium(II), PyrBt-
2.3 | General catalytic hydrogenation
procedure in continuous flow system
Pd
Hydrogenation reactions were performed with Thal-
®
PyrBt ligand (1 eq.) was stirred with PdCl (1 eq.) in
esNano H-Cube in Figure 1. The hydrogen gas produced
2
®
acetonitrile at room temperature for 18 h. The yellow
solid was isolated by filtration and dried under vacuum
to give PyrBt-Pd complex (Scheme 1). Yield: 84%; FT-IR
by the H-Cube system in high purity by electrolysis of
water was directly supplied to the system at the desired
pressure. The catalyst (3 mg), which was grounded
together with silica (125 mg), was filled into the 5-cm-
long cartridge and placed on the port in the system. The
0.1 M substrate solution in ethanol was loaded into
the system with an HPLC pump. Reaction temperature
and H2 pressure were controlled through the system
and adjusted to the determined values. The reaction
(
(
(
(
(
KBr): ν 3113 (w), 1602 (m), 1473 (s), 1440 (m), 1309
m), 1253 (w), 1174 (m), 766 (s) cm
400 MHz, DMSO-d ). δ = 8.68 (1H, d, J = 3.9 Hz), 8.56
1H, d, J = 8.34 Hz), 8.23 (1H, d, J = 8.2 Hz), 8.21–8.10
2H, m), 7.70 (1H, t, J = 15.04, 7.39 Hz), 7.57–7.49 (2H,
À1
1
;
H NMR
6
13
m) ppm. C NMR (100 MHz, DMSO-d ). δ = 114.8,
6
À1
1
14.9, 118.3, 120.0, 123.6, 125.8, 127.9, 129.6, 140.4,
residual time was determined as 5 min for 1 ml min
À1
1
49.2 ppm.
and 10 min for 0.5 ml min in flow, respectively. The
product distributions in the samples taken at the end of
the reaction were determined by GC. After each catalytic
reaction in the system, the pure solvent was passed over
the cartridge to remove the substrate and product
residues from the catalyst bed. To determine the ideal
washing time, samples were taken from the system at
certain times and analyzed with the GC. According to the
GC analysis results, the ideal colon washing time was
determined as 40 min in all experiments.
2
.2.5 | Synthesis of dichloro[N-((1H-benzo
[
d][1,2,3]triazol-1-il)metil)piridin-2-amin]
palladium(II), PyrNHCH Bt-Pd
2
Solution of PdCl (1 eq.) in dry hot acetonitrile was added
dropwise to solution of ligand PyrNHCH Bt (1 eq.) in
acetonitrile under nitrogen atmosphere. The reaction
mixture was stirred 24 h at 60 C under nitrogen atmo-
2
2
ꢀ
sphere. Dark yellow precipitate was filtered and washed
with chloroform. Solid residue was dried under vacuum
3 | RESULT AND DISCUSSION
3.1 | Characterization
to give PyrNHCH Bt-Pd complex. Because the complex is
2
insoluble in NMR solvents, its structure was determined
using only the IR spectrum. Yield: 81%; FT-IR (KBr): ν
3
445 (broad), 3333 (s), 1617 (s), 1576 (s), 1523 (s), 1452
3.1.1 | Characterization of PyrBt and PyrBt-
Pd
À1
1
(
(
m), 1324 (s), 1270 (s), 1158 (s), 744 (s) cm . H NMR
400 MHz, DMSO-d6). δ = 9.60 (1H, s), 8.06 (d,
1
J = 8.22 Hz, 2H), 7.76 (t, J = 5.60 Hz, 1H), 7.46–7.32 (m,
H), 6.97 (s, 1H), 6.87 (d, J = 8.38 Hz, 1H), 6.62 (d,
J = 7.80 Hz, 1H), 6.24 (d, J = 7.67 Hz, 1H), 5.32 (s, 1H)
In the H NMR spectrum of PyrBt ligand, two doublet
2
signals recorded at 8.66 and 8.31 ppm and two triplet
signals recorded at 7.61 and 7.46 ppm are the signals
of the characteristic Bt-1 isomer. The doublet of
doublet recorded at 8.62 ppm, the doublet seen at
8.13 ppm, the doublet of triplet seen at 7.95 ppm, and
the doublet of the doublet recorded at 7.33 ppm are
13
ppm. C NMR (100 MHz, DMSO-d ). δ = 65.0, 111.3,
6
1
13.0, 114.9, 118.3, 119.3, 137.8, 139.6, 142.5, 148.4, 151.6,
1
56.2 ppm.
1
the signals of 2-substituted pyridine. Although the H
2
.2.6 | Synthesis of dichloro[(1H-benzo[d]
NMR spectrum of the PyrBt-Pd complex contains the
same signals as PyrBt as expected, shifts and widening
of the peaks were observed due to metal binding (see
Figures S1 and S2).
[
1,2,3]triazol-1-yl)(pyridin-2-yl)methanone]
palladium(II), PyrCOBt-Pd
Solution of PyrCOBt (1 eq.) and PdCl (1 eq.) in acetoni-
In the FT-IR spectrum of the ligand PyrBt
(see Figure S5), aromatic CH stretching vibrations of
benzotriazole and pyridine groups were recorded at
2
trile was refluxed 24 h. Yellow precipitates were filtered
and dried under vacuum to obtain PyrCOBt-Pd complex.
Yield: 85%; FT-IR (KBr): ν 3473 (broad), 3105 (s), 1753
À1
À1
3110 cm
and 3018 cm , aromatic CC + CN (ben-
À
(
(
m), 1684 (s), 1608 (s), 1459 (s), 1336 (s), 1226 (s), 1157
m), 1047 (m), 755 (s) cm . Because of insolubility no
zotriazole) stress at 1590, 1476, and 1441 cm 1. The C N
stretching of the pyridine ring was observed at 1373 and
À1
À1
NMR data were recorded.
1243 cm , and the N═N strain of the benzotriazole ring

4
of 12
YILMAZ AND HÜR
SCHEME 1 The schematic preparation of ligands and complexes (complex structures are proposed structures)
the FT-IR spectrum of the PyrBt-Pd complex (see
Figure S6), it was observed that the vibration frequency
of benzotriazole N═N stretching shifted from 1285 to
À1
À1
1
253 cm after coordination with metal. The reason for
this 30-cm frequency shift is that the coordination with
the metal through nitrogen atoms reduces the electron
density in the N═N bond. In addition, the fact that the
value of the C N (benzotrizale) stretching frequency,
À1
observed at 1441 cm , does not change after the forma-
tion of the complex reduces the possibility of the ben-
zotriazole ring coordinating with the metal over the N3
atom. If there is coordination from the N3 atom, there
should be a decrease in triazole C N vibrations. When
the changes in the stress vibrations of the pyridine ring
were examined, the C N stretching vibrations observed
®
FIGURE 1 Flow system (H-Cube ) using catalytic
À1
À1
at 1373 and 1243 cm decreased to 1367 and 1204 cm
experiments
after the complex formation. This decrease in frequency
indicates that the pyridine ring is in coordination with
the metal. For this reason, it was thought that metal is
bonded from benzotriazole to N2 nitrogen and pyridine
nitrogen is likely to be formed. These results are
À1
was observed at 1285 cm . The peaks observed at
À1
1
065 cm belong to C H bendings, while the strong
peak observed at 758 cm belongs to C C bendings. In
À1

YILMAZ AND HÜR
5 of 12
1
13
consistent with the results we obtained with spectro-
scopic and computational methods in our previous
study.
the pyridine nitrogen. H NMR and
C NMR of
PyrNHCH Bt-Pd are given in Figures S11 and S12,
2
[38]
respectively.
Thermal analysis (TG/DTA/DTG) of complexes was
performed under N2 atmosphere in the temperature
range from ambient to 900 C. The TG/DTA/DTG curve
In the FT-IR spectrum of the ligand PyrNHCH Bt,
2
C H stretching vibrations of the benzotriazole ring are
ꢀ
À1
observed at 3394 and 3337 cm , and the combination
of PyrBt-Pd complex is shown in Figure S7. The
thermal dehydration takes place between 30 C and
bands of C═N and N═N stretching are observed at
ꢀ
À1
1514 cm . The C H vibrations of the pyridine ring of
ꢀ
À1
1
92 C, with a mass loss 4.77% (calcd. 4.47). The thermal
the ligand are 3093 and 3024 cm , and the C═N tensile
ꢀ
ꢀ
À1
decomposition from 320 C to 410 C, with mass loss
4.47% (calcd. 45.29%) corresponds to the thermal
decomposition of the C H N fragment of PyrBt ligand.
vibrations are 1610 cm (see Figure S10) After coordina-
4
tion with the metal, the shift of C N vibrations in both
the pyridine ring and benzotriazole ring to high fre-
11
9 2
À1
The maximum rate of mass loss is indicated by the
quency (1617 and 1576 cm ) shows that the ligand coor-
ꢀ
DTG peak at 345.9 C. One endothermic peak with
dinates with the metal through the nitrogen donor atoms
in both the pyridine and benzotriazole ring. For this
reason, it is thought that metal binding formed via
pyridine nitrogen and N2 nitrogen of benzotriazole for
ꢀ
tmax = 348.7 C on the DTA curve was observed at this
stage. The second decomposition process attributable to
ꢀ
pyrolysis of the other fragments occurred at 400 C to
ꢀ
8
00 C with a mass loss 25.26%. Two exothermic peaks
the PyrNHCH Bt-Pd complex (see Figure S13).
2
ꢀ
ꢀ
with tmax = 427.2 C and 781.8 C on the DTA curve
were observed for this step. The overall mass losses are
observed to be 75.99%, which is in very good agreement
with the calculated value of 76.59%. The final residue,
The thermal decomposition of the PyrNHCH -Pd
2
complex occurs in multiple stages (see Figure S14). The
thermal dehydration of this complex takes place between
ꢀ
ꢀ
30 C and 192 C, with a mass loss of 4.77% (calcd. 4.47).
The first decomposition process occurred in the tempera-
0
estimated as Pd , has the observed mass 24.01% as
ꢀ ꢀ
against the calculated value of 23.41%.
ture range 195 C to 320 C, with a mass loss of 18.39%
(
calcd 19.38). The DTA peak corresponding to this stage
ꢀ
is a small endothermic peak at 258.5 C. The second
ꢀ
ꢀ
3
.1.2 | Characterization of PyrNHCH Bt and
decomposition between 320 C and 442 C with a mass
loss of 22.21% may be attributed to the removal of ben-
zotriazole part of ligand, and DTG peak corresponding to
this stage is found at 406.2 C with a maximum rate of
mass. An exothermic peak with tmax = 417.9 C on the
2
PyrNHCH Bt-Pd
2
1
ꢀ
The H NMR spectrum of the PyrNHCH Bt ligand has
2
ꢀ
mixed cleavage signals. This result is consistent with the
literature and the reason is that the free rotation of
Bt-1 and
NH groups around CH2 bridge. The singlet
signal seen at 8.20 ppm is the aromatic CH-proton neigh-
boring to the pyridine nitrogen. The multiple signals
seen at 8.03–7.94 ppm are benzene CH protons of
benzotriazole. On the other hand, the multiple peaks
seen in the 7.49- to 7.38-ppm range are the overlapping
signals of benzotriazole and pyridine protons. The
DTA curve was observed for this step. The mass of
the final residue corresponded to stable PdO, 28.91%
(calcd. 30.4).
3.1.3 | Characterization of PyrCOBt and
PyrCOBt-Pd
Characteristics of two doublet signals at around 8.10–
8.30 ppm and two triplets at 7.60–7.79 ppm are recorded
characteristic methylene ( CH2
)
signal between
1
NH and benzotriazole expected for this compound
was recorded as a doublet signal by interacting with the
NH proton at 6.36 ppm. In addition, the NH signal
recorded at 5.66 ppm was recorded as a broad signal
while waiting for a triplet, which is a result of the free
for N-acylbenzotriazole in H NMR spectra. Additionally,
disappearance of expected broad signal at around 10.0–
12.0 ppm for hydroxy signal of 2-picolinic acid supports
bonding of benzotriazole ring (see Figure S15). Unfortu-
nately, no NMR spectra could be recorded for PyrCOBt-
Pd complex, because it is not soluble in NMR solvents,
such as DMSO-d , CD3OD, and DMF-d solvents even
rotational motion (see Figure S8). PyrNHCH Bt-Pd com-
2
plex is slightly soluble in DMSO-d even when irradiated
6
6
7
with microwave. Some signal shifts and broadenings
were recorded in H NMR of this complex. Additionally,
the CH signal adjacent to the pyridine nitrogen shifted
to a significantly lower area than the same signal of
the ligand. This supports the binding of the metal to
under microwave irradiation.
1
When the FT-IR spectra of the ligand and the com-
plex are compared, the shift of the carbonyl (C═O)
stretching is observed. While C═O signal is recorded at
À1
1706 cm for PyrCOBt, after the complex formation, it

6
of 12
YILMAZ AND HÜR
À1
appeared at 1753 cm (see Figures S17 and S18). The
cyclohexene, ethyl benzene and cyclohexane are formed,
respectively. 1-Octene is selectively hydrogenated to
n-octane, but some isomer products such as 2-octene and
3-octene may be formed during the hydrogenation
(Scheme 2).
shifting of the C N stretching vibrations of the
benzotriazole ring from 1593 to 1608 cm
metal coordinate with carbonyl oxygen and N1 nitrogen
in the benzotriazole ring.
À1
supports
The thermogram of PyrCOBt complex is shown in
ꢀ
Figure S19. A mass loss of 2.26% between 35 C and
ꢀ
1
80 C is due to absorbed water. The second stage, which
3.2.1 | Catalytic activity of PyrBt-Pd
ꢀ
ꢀ
occurs in the temperature range of 180 C and 305 C,
corresponds to the decomposition of an organic part of
the ligand, with the DTG peak observed at 268.4 C (mass
loss of 24.47%). The next step from 305 C to 440 C with a
mass loss 23.37%, accompanied by an exothermic peak
with tmax = 420.6 C on the DTA curve, may be attributed
Experiments in styrene hydrogenation were carried out
ꢀ
ꢀ
ꢀ
at 25 C and 50 C. It was found that the increase in
temperature had a positive effect on the effectiveness of
PyrBt-Pd catalyst (Entries 1 and 2 in Table 1). On the
contrary, the increase in H2 pressure caused a small
decrease in the activity of catalyst (Entries 2 and 3). This
may be due to the catalyst being deactivated under high
ꢀ
ꢀ
ꢀ
to the pyrolysis of ligands. DTG peak corresponding to
this stage is found at 415.1 C with a maximum rate of
ꢀ
mass. Total mass loss was observed as 73.55% (calcd.
pressure. Best conversion (100%) in styrene hydrogena-
0
ꢀ
7
3.49). The final residue, estimated as Pd , has the
tion was obtained at 50 C, 10-bar H pressure, and
2
À1
observed mass 26.45% as against the calculated value of
6.39%.
1-ml min flow rate (TOF = 14,880). The substrate con-
2
centration is 0.1 M in the experiments carried out under
these optimum conditions. By keeping these conditions
constant, when the substrate concentration was increased
to 0.5 M, the ethylbenzene conversion was found to be as
81.62% (Entry 4). Although the number of substrate
molecules in the medium increased approximately five
times, the product conversion over 80% indicates that the
efficiency of PyrBt-Pd catalyst is very high. When flow
3
.2 | Catalytic activity of palladium
complexes in hydrogenation of various
alkenes
The efficiency of the synthesized palladium complexes
was tested in styrene, 1-octene, and cyclohexene
hydrogenations. In the hydrogenation of styrene and
À1
rate reduced to 0.5 ml min , the ethylbenzene conver-
sion increased to 97.85% (Entry 5). At low flow rate,
SCHEME 2 The catalytic hydrogenation of
styrene, cyclohexene, and 1-octene in flow
®
system using H-Cube

YILMAZ AND HÜR
7 of 12
TABLE 1 Catalytic activity of PyrBt-Pd complex
T
C
subst.
Flow rate
(ml min
H
(bar)
2
pres. Total conv.
Main product
(Distrib.%)
ꢀ
À1
Substrate
Entry ( C)
(M)
0.1
0.1
0.1
0.5
0.5
0.1
)
(%)
68.91
100
96.3
TON TOF
Styrene
1
2
3
4
5
6
25
50
50
50
50
25
1
10
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
n-Octane (80.2%)
854
1240
1194
506
10,248
14,880
14,328
6072
1
10
1
50
1
10
81.62
97.85
0.5
1
10
607
3642
1
-Octene
10
100
994
11,928
2
3
-Octene (15.7%)
-Octene (4.1%)
7
50
0.1
1
10
100
n-Octane (83.0%)
1029
12,348
2
3
-Octene (13.2%)
-Octene (3.8%)
8
9
0
50
50
50
0.1
0.1
0.5
0.5
1
10
50
10
100
100
n-Octane
1240
1240
518
7440
n-Octane
14,880
3108
1
0.5
99.3
n-Octane (83.5%)
2
3
-Octene (11.9%)
-Octene (3.9%)
Cyclohexene 11
25
50
50
50
50
50
50
0.1
0.1
0.1
0.1
0.1
0.1
0.5
1
10
10
10
20
30
50
50
40.8
49.5
82.5
97.06
99
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
506
614
6072
7366
6138
7221
7366
7440
3551
1
1
1
1
1
1
2
3
4
5
6
7
1
0.5
0.5
0.5
0.5
1023
1204
1228
1240
592
100
95.46
0.5
Note: Residual time is 10 and 5 min for 0.5- and 1-ml min flow rates, respectively. nsubst = 0.01, ncat. = 8.0315 Â 10À6
À1
.
substrates residence time in the column increases, so they
can interact with the catalyst for a longer time.
varying reaction parameters, the highest TOF value was
ꢀ
reached in 1-octene hydrogenation at 50 C temperature,
ꢀ
À1
In hydrogenation of 1-octene at 25 C with the PyrBt-
50-bar H pressure, and 1-ml min flow rate. In order to
2
Pd complex, the total conversion reached 100%; but
under these conditions, besides n-octane, other hydroge-
nation products, 2-octene and 3-octene, were also
detected. The distributions of 2-octene and 3-octane were
determined as 15.7% and 4.1%, respectively (Entry 6).
examine the effect of the substrate concentration on the
activity of the catalyst, the substrate concentration was
increased from 0.1 to 0.5 M. As can be seen in Entries
8 and 10 in Table 1, the TOF value decreases more than
half at 0.5 M concentration. This result is as expected
due to the increasing number of substrate molecules in
the mixture.
ꢀ
Although the temperature increased to 50 C, the
n-octane conversion increased from 80.2% to 83%, but
isomers were still observed in the mixture (Entry 7). One
hundred percent n-octane selectivity was achieved when
The efficiency of the PyrBt-Pd complex was also
tested in hydrogenation of cyclohexene, a cyclic alkene.
In cyclohexene hydrogenation, the reducing product is
À1
the flow rate was reduced to 0.5 ml min while keeping
ꢀ
ꢀ
temperature and pressure constant (50 C, 10 bar, Entry
cyclohexane. In the experiments conducted at 25 C and
ꢀ
À1
8
). Another way to increase n-octane selectivity is by
50 C (1-ml min flow, 10-bar H ), it was determined
2
increasing the pressure of hydrogen gas. It was deter-
mined that 1-octene was converted to n-octane without
that the cyclohexane conversion was below 50% (Entries
11 and 12). When the flow rate was reduced to
0.5 ml min , the conversion was increased to 82.5%.
À1
any isomer formation at 50-bar H pressure at a flow rate
2
À1
of 1 ml min (Entry 9). The reaction time is shorter at a
flow rate of 1 ml min . The important point in catalytic
Therefore, the ideal column flow rate was set at
À1
À1
0.5 ml min , and the H gas pressure was gradually
2
reactions is to reach high TOF values. According to
increased from 10 to 50 bar (Entries 13–16). It was

8
of 12
YILMAZ AND HÜR
determined that there was an increase in cyclohexane
conversion parallel to the increase in pressure. The
the effect of H pressure on catalytic activity, H pressure
2
2
was increased from 10 to 30 bar, and the ethylbenzene
conversion increased from 52.6% to 100% (Entry 4). In
summary, the highest TOF value in the styrene hydroge-
À1
highest TOF value was obtained at 0.5-ml min flow
ꢀ
rate, 50 C temperature, and 50-bar H2 pressure
(Entry 16).
nation of the PyrNHCH Bt-Pd catalyst was obtained at
5
2
ꢀ
0 C, 30-bar H , and 1-ml min flow rate conditions
2
À1
(
Entry 4).
3
.2.2 | Catalytic activity of PyrNHCH Bt-Pd
The efficiency of the PyrNHCH Bt-Pd catalyst in
2
2
1
-octene hydrogenation was tested first at low tempera-
ꢀ
The efficiency of PyrNHCH Bt-Pd complex in styrene
hydrogenation was initially investigated at 25 C, 10-bar
H2 pressure, and 1-ml min
ethylbenzene formation was determined under these
conditions (Entry 1 in Table 2). Keeping the temperature
constant and reducing the flow rate to 0.5 ml min only
provided a 1% increase in conversion (Entry 2). It was
observed that the conversion of ethylbenzene increased
to 50% when the temperature was increased to 50 C at
flow rate of 1 ml min (Entry 3). This result indicates
ture and then at 50 C (Entries 6 and 7). Almost no
conversion was observed at both temperatures. When
the pressure was gradually increased from 10 to 50 bar
2
ꢀ
À1
flow rate, and 21.3%
ꢀ
at 50 C, the total conversion increased to only 25%.
Although n-octane selectivity increased when H2
pressure was increased to 30 and 50 bar values,
respectively, the conversion amount remained below
30% (Entries 7–9). It is seen in Table 2 that the
most effective parameter in 1-octene hydrogenation is
flow rate. When the flow rate was reduced to
À1
ꢀ
À1
À1
that the temperature has a positive effect on the
0.5 ml min , 100% n-octane selectivity was obtained
PyrNHCH Bt-Pd catalyst activity. In order to determine
(Entry 10).
2
TABLE 2 Catalytic activity of PyrNHCH
2
Bt-Pd complex
T
C
subst.
Flow rate
(ml min
H
(bar)
2
pres. Total conv.
Main product
(Distrib.%)
ꢀ
À1
Substrate
Entry ( C)
(M)
0.1
0.1
0.1
0.1
0.5
0.1
0.1
)
(%)
TON TOF
Styrene
1
2
3
4
5
6
7
25
25
50
50
50
25
50
1
10
21.3
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
n-Octane (0.38%)
n-Octane (0.13%)
264
278
652
1240
218
4.7
3168
1668
7824
14,880
2616
56
0.5
1
10
22.4
52.6
10
1
30
100
35.2
0.38
1
10
1
-Octene
1
10
1
10
0.3
1.6
19
2-Octene (0.17%)
8
9
50
50
0.1
0.1
1
1
30
50
27.4
n-Octane (7.3%)
91
1092
3804
2
3
-Octene (14.8%)
-Octene (5.3%)
39.2
n-Octane (25.6%)
317
2
3
-Octene (10.1%)
-Octene (3.5%)
1
0
1
50
50
0.1
0.5
0.5
0.5
50
50
100
98
n-Octane (100%)
1240
531
7440
3186
1
n-Octane (85.7%)
2
3
-Octene (9.1%)
-Octene (3.2%)
Cyclohexene 12
25
50
50
50
50
0.1
0.1
0.1
0.1
0.5
1
1
1
10
10
30
30
30
3.35
8.45
90.4
100
96.3
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
42
504
1
1
1
1
3
4
5
6
105
1121
1240
597
1260
13,452
7440
3582
0.5
0.5
À1
Note: Residual time is 10 and 5 min for 0.5 and 1-ml min flow rates, respectively. nsubst = 0.01, ncat. = 7.45 Â 10À6
.

YILMAZ AND HÜR
9 of 12
ꢀ
In cyclohexene hydrogenation with PyrNHCH Bt-Pd
conversion of ethylbenzene was obtained at 25 C, 10-bar
H2 pressure, and 1-ml min
Table 3).
2
ꢀ
À1
complex, the highest TOF value was reached at 50 C,
flow rate (Entry 1 in
À1
3
0-bar H pressure, and 0.5-ml min flow rate condi-
2
tions (Entry 14). As can be seen in Entries 12 and 13 in
Table 2, temperature was not very effective on the activ-
PyrCOBt-Pd provided 63.7% and 78.2% n-octane selec-
tivity for 1-octene hydrogenation at 25 C and 50 C,
respectively (Entries 3 and 4). An increase in selectivity
ꢀ
ꢀ
ity of PyrNHCH Bt-Pd complex. The most significant
2
effect on the increase in catalytic activity in cyclohexene
was observed when H pressure was increased from 10 to
2
ꢀ
hydrogenation was obtained when the H pressure was
50 bar at 50 C (Entries 4–6). In these experiments carried
2
À1
increased from 10 to 30 bar (Entries 13–14). To examine
out with a flow rate of 1 ml min , small amounts of
the effect of the flow rate on the catalytic reaction, the
other isomer products besides n-octane were detected.
The n-octane selectivity reached 100% when the flow rate
À1
flow rate was decreased from 1 to 0.5 ml min
.
À1
Although 100% cyclohexane conversion was achieved at
low flow, the TOF value decreased due to the long
reaction time (Entry 15).
was reduced to 0.5 ml min (Entry 7).
It was found that PyrCOBt-Pd catalyst is also very
effective in cyclohexene hydrogenation. While the effi-
ꢀ
ciency of the catalyst was low at 25 C, a tremendous
increase in activity was observed when the temperature
ꢀ
3.2.3 | Catalytic activity of PyrCOBt-Pd
was raised to 50 C (Entries 9 and 10). Although the low
complex
flow rate increases the catalytic conversion, the longer
the reaction time causes a decrease in the TOF value
(Entry 12). Therefore, it can be said that the ideal
reaction conditions for the PyrCOBt-Pd complex in
The efficiency of PyrCOBt-Pd catalyst in styrene hydroge-
nation was found to be higher under milder conditions
than the other two complexes. One hundred percent
ꢀ
À1
cyclohexene hydrogenation are 50 C, 0.1-ml min flow
TABLE 3 Catalytic activity of PyrCOBt-Pd complex
T
C
(M)
subst.
Flow rate
(ml min
H
(bar)
2
pres. Total conv.
Main product
(Distrib.%)
ꢀ
À1
Substrate
Entry ( C)
)
(%)
TON TOF
Styrene
1
2
3
25
25
25
0.1
1
1
1
10
100
Ethylbenzene
Ethylbenzene
n-Octane (63.7%)
1240
503
14,880
0.5
10
81.1
6036
9480
1
-Octene
0.1
10
94
790
2
3
-Octene (20.8%)
-Octene (9.5%)
4
5
6
50
50
50
0.1
0.1
0.1
1
1
1
10
30
50
99.1
99.8
n-Octane (78.2%)
970
1166
1211
11,640
13,992
14,532
2
3
-Octene (15.5%)
-Octene (5.4%)
n-Octane (94%)
2
3
-Octene (4.4%)
-Octene (1.4%)
100
n-Octane (97.7%)
2
3
-Octene (1.7%)
-Octene (0.6%)
7
8
50
50
0.1
0.5
0.5
0.5
30
30
100
97.7
n-Octane (100%)
1240
1211
7440
7266
n-Octane (97.3%)
2
3
-Octene (0.05%)
-Octene (0.35%)
Cyclohexene
9
0
1
2
3
25
50
50
50
50
0.1
0.1
0.1
0.1
0.5
1
1
1
10
10
20
10
10
17.1
98.1
99.6
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
212
1216
1235
1240
870
2544
1
1
1
1
14,592
14,820
7440
0.5
0.5
À1
100
70.2
5220
Note: Residual time is 10 and 5 min for 0.5- and 1-ml min flow rates, respectively. nsubst = 0.01, ncat. = 7.47 Â 10À6
.

10 of 12
YILMAZ AND HÜR
rate, and 20-bar H pressure, with the highest TOF value
reduction in their activity. While PyrCOBt catalyst
maintained its effectiveness at the end of 10 cycles in
2
(Entry 11).
1-octene hydrogenation, 3% and 10% decreases were
Recycling experiments
observed in the activity of PyrBt-Pd and PyrNHCH Bt-
2
In the recycling experiments, the system was washed
with solvent after each reaction was completed. Wash-
ing procedure was carried out until no product and
substrate residue were observed in the GC analysis.
The cycle experiment was restarted by passing fresh
substrate solution through the same catalyst bed.
Recycling experiments were performed for 10 cycles
under optimal reaction conditions where the efficiency
of the catalysts was the highest for each substrate in
hydrogenation reactions. As can be seen from
Figure 2, all catalysts can be reused in styrene and
cyclohexene hydrogenation for 10 cycles without any
Pd catalysts, respectively.
The optimum reaction conditions in which catalysts
have the highest activity are presented in Table 4. It was
found that the activities of three catalysts were equal in
styrene hydrogenation. When evaluated according to the
reaction conditions, it was determined that the PyrCOBt-
Pd complex was effective at lower temperature and
pressure. The efficiency of PyrNHCH Bt-Pd catalyst in
2
1-octene hydrogenation was lower than the other two
complexes. In cyclohexene hydrogenation, the efficiency
of PyrCOBt-Pd catalysts was found to be higher than
other two complexes.
FIGURE 2 Recycling result for 10 cycles
TABLE 4 Optimum reaction conditions in hydrogenation of using substrate
Styrene 1-Octene
Cyclohexene
T
P
H2
Flow rate
T
P
H2
Flow rate
T
P
H2
Flow rate
ꢀ
À1
ꢀ
À1
ꢀ
À1
Catalysts
( C) (bar) (ml min
)
TOF
( C) (bar) (ml min
)
TOF
14,880 50
7440 50
( C) (bar) (ml min
)
TOF
PyrBt-Pd
50
10
30
1
1
14,880 50
14,880 50
50
50
1
50
30
0.5
1
7440
PyrNHCH
Pd
2
Bt- 50
0.5
13,452
PyrCOBt-Pd 25
10
1
14,880 50
50
1
14,532 50
20
1
14,820

YILMAZ AND HÜR
11 of 12
The efficiency of PyrBt-Pd catalyst in styrene hydroge-
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5
0 C temperature and 10-bar H pressure, it was deter-
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2
and PyrCOBt-Pd > PyrNHCH Bt-Pd > PyrBt-Pd in
2
45760.
cyclohexene hydrogenation.
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ACKNOWLEDGMENT
This study was supported by Eski s¸ ehir Technical
University (Project Number: 1701F025).
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Luque, ACS Sustainable Chem. Eng. 2019, 7, 14210.
AUTHOR CONTRIBUTIONS
Filiz Yılmaz: Data curation; formal analysis; investiga-
tion; methodology. Deniz Hür: Conceptualization;
data curation; formal analysis; methodology; project
administration.
[25] D. Geier, P. Schmitz, J. Walkowiak, W. Leitner, G. Francik,
ACS Catal. 2018, 8, 3297.
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[
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26] C. Moreno-Marrodana, F. Liguoria, P. Barbaro, H. Sawa, Appl.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are
available in the supporting information of this article.
Catal. A 2018, 558, 34.
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Res. 2019, 58, 16065.
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2020, 31, 147.
ORCID
29] A. K. Rathi, M. B. Gawande, V. Ranc, J. Pechousek, M. Petr,
Filiz Yılmaz https://orcid.org/0000-0002-6138-0460
Deniz Hür https://orcid.org/0000-0003-0704-1748
K. Cepe, R. S. Varma, R. Zboril, Catal. Sci. Technol. 2016,
6
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30] D. Suna, T. Saitoa, Y. Yamadaa, X. Chenb, S. Sato, Appl. Catal.
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How to cite this article: F. Yılmaz, D. Hür, Appl
Organomet Chem 2021, e6389. https://doi.org/10.
2
1002/aoc.6389