Kinetics and chemoselectivity studies of hydrogenation reactions of alkenes and alkynes catalyzed by (benzoimidazol-2-ylmethyl)amine palladium(II) complexes

Article abstract of DOI:10.1016/j.ica.2018.08.004

A series of (benzoimidazol-2-ylmethyl)amine palladium(II) complexes have been employed as catalysts in the homogeneous hydrogenation of alkenes and alkynes under mild conditions. A correlation between the catalytic activity and the nature of the ligand was established. Kinetic studies of the hydrogenation reactions of styrene established pseudo-first-order dependence on styrene substrate. On the other hand, partial orders with respect to H₂ and catalyst concentrations were obtained. The nature of the solvent used influenced the hydrogenation reactions, where coordinating solvents resulted in lower catalytic activities. Kinetics and mechanistic studies performed were consistent with the formation of palladium monohydride intermediates as the active species.

Full text of DOI:10.1016/j.ica.2018.08.004

Inorganica Chimica Acta 483 (2018) 148–155
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Research paper
Kinetics and chemoselectivity studies of hydrogenation reactions of alkenes
and alkynes catalyzed by (benzoimidazol-2-ylmethyl)amine palladium(II)
complexes
⁎
Thandeka A. Tshabalala, Stephen O. Ojwach
School of Chemistry and Physics, Pietermaritzburg Campus, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa
A R T I C L E I N F O
A B S T R A C T
Keywords:
Palladium
Alkenes
Alkynes
Hydrogenation
Kinetics
A series of (benzoimidazol-2-ylmethyl)amine palladium(II) complexes have been employed as catalysts in the
homogeneous hydrogenation of alkenes and alkynes under mild conditions. A correlation between the catalytic
activity and the nature of the ligand was established. Kinetic studies of the hydrogenation reactions of styrene
established pseudo-first-order dependence on styrene substrate. On the other hand, partial orders with respect to
H and catalyst concentrations were obtained. The nature of the solvent used influenced the hydrogenation
2
reactions, where coordinating solvents resulted in lower catalytic activities. Kinetics and mechanistic studies
performed were consistent with the formation of palladium monohydride intermediates as the active species.
1. Introduction
palladium(II) catalysts are emerging as suitable alternatives due to their
better stability and ease of synthesis in comparison to the phosphine-
Hydrogenation reactions of alkenes and alkynes are currently one of
donor palladium(II) complexes. For example, the pyridine-2-carbaldine
palladium(0) [12] and bis(arylimino)acenaphthene palladium(0) [13]
complexes have been shown to exhibit good selectivity and stability in
the homogeneous hydrogenation of alkynes.
We recently reported the use of palladium(II) complexes anchored
on (benzoimidazol-2-ylmethyl)amine ligands [14] to give active cata-
lysts in the methoxycabonylation of higher olefins. Due to the pro-
mising results obtained in the methoxycabonylation reactions by these
systems, we chose to explore their propensity to catalyze the molecular
hydrogenation of selected alkenes and alkynes. In addition, detailed
kinetics, mechanistic and theoretical studies have been performed and
are herein discussed.
the dominant industrial processes used for the reduction of unsaturated
organic compounds to produce a wide range of relevant products [1–3].
Several metal-based catalysts derived from nickel, palladium, ruthe-
nium, rhodium, iridium and platinum have been employed in the cat-
alytic hydrogenation of alkenes and alkynes under both homogeneous
[
4,5] and heterogeneous [6,7] conditions. Currently, the major focus in
transition metal catalyzed homogeneous molecular hydrogenation re-
actions has been on ligand design; and the insights gained so far show
that the ability to control the catalytic behavior of these catalyst lies in
the coordination environment around the metal atom.
In particular, palladium(II) catalysts are currently receiving much
attention in the hydrogenation of alkenes and alkynes due to their su-
perior catalytic activities and selectivity [8]. Numerous reports have
appeared on the homogeneous hydrogenation of alkenes and alkynes
using palladium(II) catalysts supported on phosphine-donor ligands.
For example, Bacchi et al [9] and Drago et al [10] employed hydazonic
phosphine palladium(II) and bidentate (2,5-dimethylphospholano)
palladium(II) complexes as effective catalysts in the hydrogenation of
alkenes. Even though the phosphine-donor palladium(II) catalysts have
been successfully used in the homogeneous hydrogenation reactions of
alkenes and alkynes, these systems suffer from lack of stability and
sensitivity to moisture and air [11]. As a result, nitrogen-donor
2. Experimental section
2.1. Materials and methods
All moisture and air sensitive reactions were performed using
standard Schlenk line techniques. Methanol (ACS reagent, ≥99.8%),
toluene (ACS reagent, ≥99.5%), dichloromethane (ACS reagent,
≥99.8%), absolute ethanol (ACS reagent, ≥98%) and tetrahydrofuran
(anhydrous, ≥99.9%) were purchased from Merck. Solvents were dried
and distilled under nitrogen in the presence of suitable drying agents:
⁎
Corresponding author.
E-mail address: ojwach@ukzn.ac.za (S.O. Ojwach).
https://doi.org/10.1016/j.ica.2018.08.004
Received 22 May 2018; Received in revised form 7 August 2018; Accepted 7 August 2018
Available online 11 August 2018
0020-1693/ © 2018 Elsevier B.V. All rights reserved.

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
Fig. 1. Neutral and cationic (benzoimidazol-2-ylmethyl)amine palladium (II) complexes 1–6 [14] used as catalysts in the hydrogenation reactions.
Toluene and acetone were dried over anhydrous calcium chloride,
methanol and absolute ethanol over calcium oxide, dichloromethane
over phosphorus pentoxide and stored over 4 Å molecular sieves. The
ligands N-(1H-benzoimidazol-2-ylmethyl-2-methoxy)aniline (L1), N-
3. Results and discussion
3.1. Hydrogenation reactions of alkenes and alkynes using palladium (II)
complexes 1–6 as catalysts
(
1H-benzoimidazol-2-ylmethyl-2-bromo)aniline (L2), N-(1H-benzoimi-
dazol-2-ylmethyl)benzenamine (L3) and N-(1H-benzoimidazol-2-yl-
methylamino)benzenethiol (L4), were synthesized following the pub-
Preliminary evaluations of complexes 1–6 (Fig. 1) in the hydro-
genation of styrene were performed at 5 bar of H2 pressure, 30 °C and
[styrene]/catalysts] = 600:1. Under these conditions, all the complexes
showed catalytic activities to afford 100% ethylbenzene with conver-
sions ranging from 54% to 99% within 1.5 h (Fig. S1). In order to fully
account for the role of complexes 1–6 in the observed catalytic hy-
drogenation reactions, control experiments were conducted without the
use of the palladium(II) complexes and also in the presence of the li-
gand L2 only under similar reaction conditions. The low percentage
conversions of 4% and 6% obtained respectively within 10 h (Table 1,
entries 7 and 8) confirmed that complexes 1–6 were responsible for the
observed higher catalytic activities. We thus further carried out ki-
netics, selectivity, theoretical and mechanistic studies of hydrogenation
reactions of alkenes and alkynes using complexes 1–6 as catalysts.
lished literature method [15]. The palladium complexes [Pd(L1)Cl
1), [Pd(L2)Cl ] (2) [Pd(L3)Cl ] (3), [Pd(L4)Cl ] (4), [Pd(L2)ClMe] (5)
and [Pd(L2)ClPPh ]BAr (6), were prepared following our recently
published procedure [14].
2
]
(
2
2
2
3
4
2.2. Density Functional Theory (DFT) studies
DFT calculations were performed in a gas phase to identify the
energy-minimized structures based on B3LYP/LANL2DZ (Los Almos
National Laboratory 2 double ζ) level theory [16]. A split bases set,
LANL2DZ for palladium and 6-311G for all other atoms was used to
optimize the geometries and energies of the complexes. The Gaussian09
suite of programs was used for all the computations.
3.2. Kinetics of styrene hydrogenation reactions
2
.3. General procedure for the hydrogenation reactions of alkenes and
3.2.1. Effect of complex structure on catalytic hydrogenation of styrene by
1–6
alkynes
Kinetics of the hydrogenation reactions of styrene were investigated
for complexes 1–6 by monitoring the reactions using GC chromato-
graphy. Table 1 contains the initial rate constants derived from the
The catalytic hydrogenation reactions were performed in a stainless
steel autoclave equipped with temperature control unit and a sample
valve. In a typical experiment, styrene (0.5 mL, 4.20 mmol) and com-
plex 2 (3.47 mg, 0.007 mmol, S/C 600) were dissolved in toluene
Table 1
(
50 mL). The reactor was evacuated, flushed with nitrogen and the
a
Effect of catalyst structure on the hydrogenation of styrene by complexes 1-6.
catalytic solution was introduced to the reactor via a cannula. The re-
actor was purged three times with hydrogen, and then set at the
equipped pressure, heated to the desired temperature and the reaction
stirred at 500 rpm. At the end of the reaction time, the reactor was
cooled, excess hydrogen vented off. Samples for GC analyses were
drawn via a syringe, filtered using 0.45 µm micro filters and analyzed by
Varian CP-3800 GC (ZB-5HT column 30 m × 0.25 mm × 0.10 µm) GC
instrument to determine the percentage conversion of styrene to
ethylbenzene. The percentage conversions were determined by com-
paring the peak areas of the alkene/alkyne substrate and respective
products, assuming 100% mass balance. For example, comparison of
peak areas of styrene and ethylbenzene at regular time intervals al-
lowed the determination of the rate of conversion of styrene to ethyl-
benzene. Standard authentic samples; ethylbenzene (97%), trans-2-
hexene (97%), cis-2-hexene (98%), trans-2-octene (98%) and octane
Entry
Catalyst
Conversionb (mol%)
k
obs (h−1
)
TOFc (h−1
)
1
2
3
4
5
6
1
2
3
4
5
6
__
__
74
92
78
86
54
99
4
0.91 ( ± 0.03)
1.67 ( ± 0.01)
0.98 ( ± 0.05)
1.38 ( ± 0.07)
0.56 ( ± 0.04)
2.93 ( ± 0.1)
__
296
368
312
344
215
396
__
d
7
e
8
6
__
__
aConditions: styrene (0.41 g, 4.00 mmol); [styrene]/[catalyst], 600; substrate,
catalyst (0.007 mmol); solvent, toluene (50 mL); pressure, 5 bar; temperature,
0 °C; time, 1.5 h.
Determined by GC by comparing the peak areas of styrene substrate to
ethylbenzene at regular time intervals.
TOF in molsubstratemolcatalyst
3
b
c
−1 −1
(h−1).
h
(
98%) were purchased from Sigma-Aldrich and used to confirm the
d
e
Control experiment, no catalyst used, time, 10 h.
Control experiment, in the presence of the ligand L2; time, 10 h.
presence and composition of hydrogenation products.
149

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
cationic complex 6. Another factor that appeared to control the cata-
Table 2
Effect of catalyst concentration and pressure on the hydrogenation of styrene
lytic activity was the Pd-Cl/Me bonds on the complex structure. For
a
−1
−1
using catalysts 2 and 6.
example, rate constants of 1.67 h
complexes 2 and 5 bearing Pd-Cl and Pd-Me groups respectively
Table 1, entries 2 and 5). This can be attributed to enhanced stability
and 0.56 h
were observed for
obs (h−1)
TOF (h−1)b
Entry [sub]/[cat]
P
H2 (bar)
K
(
2
6
2
6
of the dichloride complex 2, compared to the Pd-Me complex 5. Gen-
erally, metal alkyls are known to readily undergo deactivation due the
higher reactivity of metal-alkyl bonds [19].
1c
2
200
400
600
800
1000
600
600
600
600
5
5
5
5
5
7.5
10
12.5
5
3.84 ( ± 0.02) 6.32 ( ± 0.01) 158
2.66 ( ± 0.02) 3.56 ( ± 0.03) 267
1.67 ( ± 0.01) 2.92 ( ± 0.02) 368
1.45 ( ± 0.03) 1.79 ( ± 0.04) 448
0.91 ( ± 0.05) 1.05 ( ± 0.05) 466
2.02 ( ± 0.02)
5.45 ( ± 0.13)
6.84 ( ± 0.06)
198
316
396
501
d
3
4
5
6
3
.2.2. Effect of catalyst concentration and hydrogen pressure on the kinetics
of hydrogenation reactions of styrene
Kinetic experiments were further conducted to establish the effects
of catalyst concentrations on the hydrogenation reactions of styrene
using complexes 2 and 6. The concentrations of catalysts 2 and 6 were
thus varied from 200:1 to 1000:1 at constant initial concentration of
513
_
_
__
392
475
594
c
__
__
__
__
7
d
8
e
9
0.69 ( ± 0.03) 1.81 ( ± 0.03) 268
376
aConditions: styrene, (4.00 mmol); solvent; toluene (50 mL); temperature,
0 t
styrene (Table 2, entries 1–5). Plots of In[styrene] /[styrene] vs time
3
0 °C; time, 1.5 h.
b
c
TOF in molsubstratemolcatalyst−1 h−1
(Fig. S3) gave linear relationships from which the observed rate con-
stants (kobs) were derived (Table 2). It was observed that an increase in
catalyst concentration resulted in an increase in catalytic activity. For
.
Time, 1.0 h
d
Time, 1.25 h.
e
−1
−1
Mercury drop test (5 drops of mercury were added to the reaction mixture).
instance kobs of 1.67 h
and 0.91 h
were obtained at [styrene]/[2]
ratios of 600 and 1000 respectively. However, a closer examination of
the TOF values for both 2 and 6 at different catalyst loadings paint a
different picture. For example, increased [styrene]/[6] ratio (decrease
in catalyst concentration) from 600 to 1000 was marked by an increase
in TOF from 396 h and 513 h respectively (Table 2, entries 3–5). It
is therefore evident that increasing catalyst concentration did not in-
crease the catalytic activity by a similar magnitude and thus higher
plots of In[styrene] /[styrene] vs time (Fig. S2). A linear relationship
0
t
was established consistent with a pseudo-first order kinetics with respect
to styrene for all the complexes. The dependence of the rate of the
hydrogenation reactions on styrene substrate can therefore be re-
presented as given in Eq. (1).
−1
−1
1
Rate = k[styrene]
(1)
[
substrate]/[catalyst] ratios (lower catalyst loadings) is not only re-
From the rate constants observed, the cationic complex 6 was the
most active (Table 1, entries 2 and 6). This higher catalytic activity
could be attributed to the improved solubility of complex 6 (due to the
bulky Ar groups) compared to complexes 1–5 [11]. Another plausible
explanation could be a higher positive charge on the palladium(II)
metal atom in the cationic complex 6 relative to the neutral analogues,
thus better substrate coordination [8]. A similar trend in the hydro-
genation of 1-hexene was reported where higher catalytic activity
commended but would also be industrially beneficial with these sys-
tems.
The orders of reaction with respect to catalysts 2 and 6 were ex-
tracted from the plots of –In(kobs) vs –In[2] and –In[6] (Fig. 2) and
obtained as 0.73 ± 0.08 and 1.03 ± 0.06 respectively (Eqs. 2 and 3).
From our previous work, we reported fractional orders with respect to
catalyst concentration in hydrogenation reactions of styrene catalyzed
by palladium(II) complexes [20,21] and was associated with possible
catalyst aggregation during the hydrogenation reactions and/or for-
mation of palladium(0) nanoparticles as the active species [22,23]. On
the other hand, the integer order with respect to the cationic complex 6
thus shows minimum aggregation of the active species, in good agree-
−
1
+
(
TOF = 4000 h ) for the cationic complex ([Rh(PPh
3
)
2
COD] ) com-
for the neutral complex [Rh(PPh Cl] was
−
1
pared to TOF = 700 h
3 3
)
observed [17,18]. We also observed that the ligand motif had an in-
fluence on the catalytic activity. For instance, complex 2, bearing
electron withdrawing Br group on the phenyl ring showed higher cat-
−
1
ment with the value of 1.08 h
reported by Kluwer et al. [24]
−
1
alytic activity, (kobs of 1.67 h ) than the analogues complex 1 (kobs of
1
0.73
Rate = k[styrene] [2]
(2)
(3)
−1
0
3
.91 h ), containing the electron donating OCH substituent (Table 1,
entries and 2). This can also be rationalized from electrophilic metal [8]
center in 2 compared to 1, consistent with the argument fronted for the
1
1.03
Rate = k [styrene] [6]
Fig. 2. Plot of In(kobs) vs In[2] (a) and In[6] (b) for the determination of the order of reaction with respect to catalyst 2 and 6 in the hydrogenation of styrene.
2
2
R = 0.98237; slope = 0.73, Intercept = 5.56 (a) R = 0.96186; slope = 1.03, Intercept = 7.41 (b).
150

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
To shed some light on the possible formation of palladium(0) na-
noparticles, mercury poisoning test was conducted for catalysts 2 and 6
by adding few drops of mercury to the reaction solutions [22]. While
appreciable drop in catalytic activity from 92% (kobs = 1.67 h-1) to
6
7% (kobs = 0.69 h-1) was observed for catalyst 2, catalyst 6 exhibited
minimal decline in catalytic activity from 99% (kobs = 2.92) to 94%
obs = 1.81), (Table 2, entries 3 and 9). This showed possible formation
(
k
of more nanoparticles in catalyst 2 compared to catalyst 6 [21] in good
agreement with kinetic Eqs. (2) and (3). In general, the absence of in-
duction periods (Figs. S1–S6) point to a largely homogeneous catalyst
systems for both complexes 2 and 6.
The effect of H
genation reactions was also investigated at different H
2
concentration on the kinetics of styrene hydro-
pressures of
bar to 12.5 bar (Table 2, entries 3, 6–8). From the data obtained, it
2
5
2
was evident that an increase in H pressure resulted in an increase in
−
1
the observed rate constants (kobs). For example, kobs of 1.67 h
and
pressures of 5 and 7.5 bar (Table 2, en-
tries 3 and 6). Linear relationships were observed from the plot of In
Sty] /[Sty] vs time at different H pressures (Fig S4) to generate a rate
order of 0.7 ± 0.1 with respect to H concentration (Fig. 3). This
fractional and lower order indicates a complex reaction mechanism
with respect to [H ] and possibly, the formation of a Pd-monohydride
−
1
2.02 h
were recorded at H
2
[
0
t
2
Fig. 3. Plot of kobs vs PH2 (bar) for the determination of the order of reaction
2
with respect to H pressure in the hydrogenation of styrene using catalyst 2.
2
2
R = 0.98271; slope = 0.73, Intercept = 2.26.
2
species as the active species [25]. The rate law for the hydrogenation
reactions of styrene using catalyst 2 can therefore be expressed as given
in Eq. (4).
Table 3
Effect of temperature and solvent on the hydrogenation of styrene using catalyst
a
2
.
Rate = k[styrene] [2] [PH2]0.7
1
0.7
(4)
0
Conv (%)b
k )
obs (h−1
TOF (h−1)c
Entry
Solvent
T ( C)
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Toluene
Methanol
DCM
30
40
50
60
30
30
30
30
92
99
> 99
> 99
49
68
86
35
1.67 ( ± 0.01)
4.19 ( ± 0.03)
5.46 ( ± 0.1)
6.30 ( ± 0.07)
0.61 ( ± 0.08)
0.90 ( ± 0.04)
1.40 ( ± 0.03)
0.52 ( ± 0.06)
368
475
480
600
196
272
343
183
d
d
e
3.2.3. Effect of temperature and solvents on styrene hydrogenation kinetics
The effect of temperature on the kinetics of hydrogenation of
styrene using catalyst 2 was investigated by comparing the catalytic
activities of 2 from 30 °C to 60 °C (Table 3, entries 1–4). The observed
rate constants at different temperatures in Table 3 were extracted from
THF
DMSO
the plot of In[styrene]
nificant increase in the rate constant from 1.67 h
0 t
/[styrene] vs time (Fig. S5). Expectedly, a sig-
−1
−1
to 6.30 h
was
aConditions: styrene (0.41 g, 4.00 mmol); solvent; toluene (50 mL); [styrene]/
recorded with an increase in reaction temperature from 30 °C to 60 °C.
The overall activation energy (Ea) of the hydrogenation of styrene using
[
2] = 600; time, 1.5 h, pressure, 5 bar.
b
Determined by GC by comparison of peak areas of styrene and ethylbenzene
2
3
4
was calculated from the Arrhenius plot of Inkobs vs 1/T (Fig. 4a) as
product.
−1
5.61 ± 1.6 kJ mol . This value is comparable to the value of
c
TOF in molsubstratemolcatalyst−1 h−1 (h−1).
Time, 1.25 h
Time. 1.0 h.
−1
−1
2.05 ± 0.01 kJ mol
(10.05 ± 0.01 kcal.mol ) reported by Pela-
d
e
gatti et al. [26] in the hydrogenation reaction of alkenes at 40 °C and are
−
1
−1
in good agreement with the TOFs of 600 h and 580 h obtained for
−
1
≠
32.98 ± 1.9 kJ mol−1
≠
Fig. 4. Arrhenius plot (a) and Eyring plot (b) for the determination of the
E
a
= 35.61 ± 1.6 kJ mol
ΔH
=
,
and ΔS
=
−
1
−1
2
2
−
127.9 ± 1.9 J mol
K
for the hydrogenation of styrene using catalyst 2. R = 0.9645; slope = −42823; Intercept = 553 (a), R = 0.9724; slope = −4006;
Intercept = 611 (b).
151

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
Table 4
a
Effect of substrate on the catalytic performance of complexes 2 and 6.
Entry
Substrate
K
obs (h−1)
TOF (h−1)b
%Alkanesc
2
6
2
6
2
6
1
2
3
4
5
6
7
8
Styrene
1-hexene
1-Octene
1-Nonene
1-Decene
Phenyl-acetylene
1-Hexyne
1-Octyne
1.67 ( ± 0.01)
0.69 ( ± 0.06)
0.58 ( ± 0.13)
0.56 ( ± 0.05)
0.50 ( ± 0.07)
2.40 ( ± 0.05)
2.92 ( ± 0.04)
2.29 ( ± 0.02)
2.93 ( ± 0.01)
0.97 ( ± 0.03)
0.73 ( ± 0.02)
0.62 ( ± 0.11)
0.54( ± 0.05)
3.10 ( ± 0.04)
3.17 ( ± 0.07)
2.91 ( ± 0.06)
368
284
268
236
220
400
260
208
396
312
276
248
232
400
372
356
92
42
40
37
33
100
59
52
99
71
67
59
55
100
65
52
b
a
TOF in molsubstratemolcatalyst−1 h−1
Conditions: substrate, substrate/catalyst = 600; solvent, toluene; pressure, 5 bar; temperature, 30 °C; time, 1.5 h.
Selectivity towards alkane hydrogenation products after 1.5 h.
.
c
2
and the Pelagatti catalyst respectively. The Eyring plot in Fig. 4b was
abilities of the solvents; where strongly coordination solvents are
known to compete with the alkene substrate, for the active site resulting
in diminished activities [29,30]. Consistent with our observations,
Unver and Yilmaz recently reported 27% and 63% conversions in
DMSO and toluene solvents respectively in the hydrogenation of 1-oc-
tene catalyzed by ruthenium complexes [31].
≠
used to obtain the enthalpy of activation (ΔH
±
±
meters is that they support largely homogeneous nature of catalyst 2 as
has been previously reported by others [27,28]. Typical heterogeneous
catalysts in which the hydrogenation reactions are diffusion controlled
=
32.98
−127.9
). The significance of these thermodynamic para-
−
1
≠
1.9 kJ mol
)
and entropy of activation (ΔS
=
−
1
−1
1.9 J mol
K
−1
−1
a
display lower values of E between 8 kJ mol to 17 kJ mol [27,28].
4. Effect of alkene and alkyne substrates on styrene
We also studied the effects of solvents using toluene, THF, di-
chloromethane, methanol and DMSO in the hydrogenation reactions of
styrene using complex 2 (Table 3, entries 1, 5–8). The order of re-
activity was established as follows: DMSO < methanol < THF <
toluene. For example, higher catalytic activities were obtained in to-
hydrogenation kinetics and selectivity
Complexes 2 and 6 were further used to investigate the hydro-
genation reactions of a range of alkene and alkyne substrates: 1-hexene,
1-octene, 1-nonene, 1-decene, phenyl-acetylene, 1-hexyne and 1-oc-
tyne. It was observed that the catalytic performance of complexes 2 and
−1
−1
−1
luene (kobs of 1.67 h , TOF = 368 h ) than DMSO (kobs of 0.52 h
TOF = 183 h ). This trend is in line with different coordinating
,
−1
6
were controlled by the nature of the substrate (Tables 4). The initial
Fig. 5. Product distribution over time in the hydrogenation of (a) 1-hexene and catalyst 2 (b) 1-hexene and catalyst 6 and (c) phenyl-acetylene using catalyst 2.
152

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
Fig. 5 that catalyst 2 favored isomerization (58% internal isomers) over
hydrogenation (42% hexanes), in comparison to catalyst 6 (71% hex-
anes and 29% internal alkenes). Lower isomerization reactions ob-
served for catalyst 6 can be easily apportioned to the presence of
Table 5
DFT calculated data for palladium (II) complexes using B3LYP/LANL2DZ level
of theory.
bulkier PPh
3
groups in the metal coordination sphere as has been re-
cently reported by Smarun et al. [35].
4.1. Theoretical insights of the hydrogenation reactions of alkenes
Density Functional Theory (DFT) calculations were conducted in
order to have an understanding of the effect of the ligand motif and
catalyst structure on the catalytic activities of complexes 1–4. The
geometries-optimized structures and frontier orbital energy (HOMO
and LUMO) maps are summarized in Table 5 and Fig. S7, respectively.
The charge on the metal ion was also observed to influence the catalytic
activities of complexes 1–4 (Fig. S8). For instance, catalyst 2 carrying a
positive charge of 0.392 was more active than catalyst 1, with a charge
of 0.326 on the palladium (II) atom. This trend is in agreement with a
more facile substrate coordination to an electrophilic palladium atom
[36].
Parameter
1
2
3
4
LUMO (eV)
HOMO (eV)
−1.87
−7.13
5.26
121
0.392
296
−1.83
−6.38
4.55
−1.77
−6.75
4.98
115
0.354
312
−1.81
−6.35
4.54
105
0.358
344
ΔEL-H
Δɛ[kcal mol−1]
104
NBO charges (Pd) 0.326
−
1 a
TOF (h
k
)
368
1.67
obs (h−1)
0.91
0.98
1.38
aTOF in molsubstrate molcatalyst−1 h−1
4.2. Proposed mechanism of the hydrogenation of styrene
.
The dependency of the rate of hydrogenation reactions on styrene,
catalyst and hydrogen pressure (Eq. (2)) is consistent with equations (3)
rate constants (kobs) of each substrate were determined from the plot of
In[substrate] /[substrate] vs time (Fig. S6). The results obtained gen-
erally showed that alkynes were more reactive compared to the corre-
0
t
and (4). The partial and lower order of reaction with respect to [H
2
] of
0.7 ± 0.1 support the formation of a monohydride species as the rate
−1
sponding alkenes [32,33]. For example, kobs of 0.69 h
and kobs of
determining step. This points to mechanism (3) as the most probable
pathway [37] .
−1
2.92 h
were obtained for 1-hexene and 1-hexyne (Table 4, entries 2
and 7). The alkyl chain length had a profound effect on the reactivity of
In order to fully establish if the pathway given in Eq. (3) is indeed
the preffered one, we used ESI-MS (Figure S9) to detect the reaction
intermediates (Scheme 1). This was done by sampling of the reaction
mixture of complex 2 at various intervals and analysing the aliquots
using ES-MS. The mono-hydride intermediate (2a) as observed from MS
(m/z = 445 amu) is believed to occur via heterolytic cleavage leading
to the formation of an HCl by-product [38] pointing to Eq. (3) as the
operatitve mechanism. Coordination of the styrene substrate to 2a af-
fords the Pd-styrene adduct (2b) as deduced from the base peak at m/
z = 514 amu. A Markovnikov migration of the hydride to the co-
ordinated substrate to form a 14-electon Pd-alkyl species (2c) was es-
tablished from the m/z signal at 510 amu. Subsequent oxidative addi-
the substrates, in which shorter chains were more reactive. For in-
−
1
−1
stance, kobs of 0.97 h and 0.54 h were observed for 1-hexene and 1-
decene respectively (Table 4, entries 2 and 5). The decrease in catalytic
activity with alkene chain has been attributed to poor coordinating
abilities of higher alkenes to the active metal center [11]. With respect
to alkenes, the best catalytic activity for both catalysts 2 and 6 was
−
1
−1
obtained using styrene to give kobs of 1.67 h
and 2.93 h
respec-
tively. This trend has largely been associated with the formation of the
more reactive benzylic palladium intermediate, which has a lower en-
ergy than alkyl palladium intermediates [34]. The product distribution
of terminal alkenes and alkynes was similar to our recent reports
[
20,21]; where hydrogenation reactions of terminal alkenes were fol-
2
tion in the presence of H to give the Pd(IV) dihydride compound (2d)
lowed by isomerization reactions to give the corresponding internal
alkenes, while alkyne hydrogenation reactions occurred in two steps to
produce the respective alkenes and alkanes (Fig. 5). It was clear from
was confirmed from the signal at m/z = 515 amu. Hydride migration to
give the coordinated alkyl ligand in 2d followed by reductive elim-
ination to produce the monohydride Pd(II) intermediate 2e (m/
153

T.A. Tshabalala, S.O. Ojwach
Inorganica Chimica Acta 483 (2018) 148–155
Scheme 1. Mechanism for the hydrogenation of styrene catalyzed by 2 as established from ESI-MS, m/z corresponds to the cationic palladium fragments.
z = 409) and elimination of the ethylbenzene product was also con-
firmed. Finally, it is believed that regeneration of the active hydride
species 2a (m/z = 450) completes the catalytic cycle. Attempts to
confirm the formation of the Pd-hydride intermediates using 1H NMR
spectroscopy has not been successful even at elevated temperatures
Conflict of interest statement
The authors declare no conflict of interests in this manuscript with
any other third party, individual or organization.
(
(
Fig. S10). We did not observe any signal associated with the Pd-H
around −0.5 ppm) and this may be due to low pressures (atmospheric)
Appendix A. Supplementary data
employed in the NMR studies as opposed to higher pressures of
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.ica.2018.08.004.
5
–30 bar used in the catalytic experiments.
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