Neutral and cationic (pyrazolylmethyl)pyridine palladium(II) complexes: kinetics and chemoselectivity studies in hydrogenation of alkenes and alkynes

Article abstract of DOI:10.1007/s11243-016-0050-7

Reactions of (3,5-dimethylpyrazolylmethyl)pyridine (L1) and (3,5-diphenylpyrazolylmethyl)pyridine (L2) with either [PdCl₂(NCMe)₂] or [PdClMe(COD)] afforded the respective neutral palladium complexes, [PdCl₂(L1)] (1), [PdCl₂(L2)] (2) and [PdClMe(L1)] (3). Treatment of complex 1 with equimolar amounts of PPh₃ or PPh₃/NaBAr₄ produced the corresponding cationic complexes [Pd(L1)ClPPh₃]Cl (4) and [Pd(L1)ClPPh₃]BAr₄ (5), respectively. Complexes 1–5 formed active catalysts in hydrogenation of alkenes and alkynes. Isomerization reactions were predominant in the hydrogenation reactions of terminal alkenes, while hydrogenation of alkynes involved a two-step process via alkene intermediates prior to the formation of the respective alkenes. The lack of induction periods in the hydrogenation reactions in addition to pseudo-first-order kinetics with respect to the substrates established the homogeneous nature of the active species.

Full text of DOI:10.1007/s11243-016-0050-7

Transition Met Chem
DOI 10.1007/s11243-016-0050-7
Neutral and cationic (pyrazolylmethyl)pyridine palladium(II)
complexes: kinetics and chemoselectivity studies in hydrogenation
of alkenes and alkynes
Stephen O. Ojwach1 Aloice O. Ogweno1
•
Received: 5 March 2016 / Accepted: 1 April 2016
Ó Springer International Publishing Switzerland 2016
Abstract Reactions of (3,5-dimethylpyrazolylmethyl)-
pyridine (L1) and (3,5-diphenylpyrazolylmethyl)pyridine
(L2) with either [PdCl2(NCMe)2] or [PdClMe(COD)]
afforded the respective neutral palladium complexes,
[PdCl2(L1)] (1), [PdCl2(L2)] (2) and [PdClMe(L1)] (3).
Treatment of complex 1 with equimolar amounts of PPh3 or
PPh3/NaBAr4 produced the corresponding cationic com-
plexes [Pd(L1)ClPPh3]Cl (4) and [Pd(L1)ClPPh3]BAr4 (5),
respectively. Complexes 1–5 formed active catalysts in
hydrogenation of alkenes and alkynes. Isomerization reac-
tions were predominant in the hydrogenation reactions of
terminal alkenes, while hydrogenation of alkynes involved a
two-step process via alkene intermediates prior to the for-
mation of the respective alkenes. The lack of induction
periods in the hydrogenation reactions in addition to pseudo-
first-order kinetics with respect to the substrates established
the homogeneous nature of the active species.
This is mainly due to their high reactivity and ability to be
transformed to wide range of relevant products [2–7]. One
of the most common transformations employed both in
industry and research to convert alkenes and alkynes to
other industrial products is hydrogenation using metal-
based catalysts and molecular hydrogen, commonly refer-
red to as high pressure hydrogenation [8, 9].
Several platinum group metal complexes of Ru, Rh, Ir, Pt
and Pd have been extensively studied for the catalytic
molecular hydrogenation of alkenes and alkynes under both
heterogeneous and homogeneous conditions [5, 10, 11] Pal-
ladium complexes, in particular the Lindlar catalyst, have
been known as effective catalysts for the hydrogenation of
alkenes and alkynes [1, 11, 12]. The preference of palladium
complexes in hydrogenation reactions is mainly due to their
high surface-to-volume ratio compared to other platinum
group metal complexes [13]. To date, most hydrogenation
catalystsbasedonpalladium are heterogeneous in nature, with
few reports of promising homogeneous catalysts [14–16].
One group of palladium(II) complexes that have been
widely used as homogeneous catalysts in alkene and alkyne
hydrogenation reactions are those derived from phosphine-
donor ligands [17–19]. For example, Drago and Pregosin
[20] reported bidentate (2,5-dimethylphospholano)benzene
palladium(II) complexes as effective hydrogenation cata-
lysts of alkenes. Despite the success of phosphine-donor
palladium catalysts in olefin hydrogenation reactions, a
number of these catalysts are relatively unstable and sen-
sitive to moisture and air [19]. Thus, the design and
development of alternative ligand systems that could offer
more stability as well as comparable catalytic activity to
the phosphine compounds are gaining momentum. In one
such report, Yilmaz et al. [21] employed S^O-chelated
palladium acetate complexes as homogeneous catalysts in
high pressure hydrogenation of styrene and 1-octene.
Introduction
Alkenes and alkynes constitute some of the most useful
building blocks or components of the petrochemical, fine
chemical, agrochemicals and pharmaceutical industries [1].
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0050-7) contains supplementary
material, which is available to authorized users.
& Stephen O. Ojwach
ojwach@ukzn.ac.za
1
School of Chemistry and Physics, University of KwaZulu-
Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209,
South Africa
123

Transition Met Chem
Synthesis of cationic palladium(II) complexes
Nitrogen-donor palladium complexes also represent
another promising alternative to the well-established
phosphine-donor complexes due to their ease of syntheses,
stability and lower sensitivity to moisture and air [22–27].
Examples of such complexes reported in the literature
include bis(arylimino)acenaphthene palladium(0) [28] and
pyridine-2-carbaldimine Pd(0) complexes [22] which have
been shown to be very stable under hydrogen pressure and
display excellent selectivity in the hydrogenation of a wide
range of alkenes and alkynes. In this current contribution,
we report the syntheses of palladium(II) complexes sup-
ported by (pyrazolylmethyl)pyridine ligands and their
applications as catalysts in molecular hydrogenation of
alkenes and alkynes. Detailed studies on the kinetics and
chemoselectivity of these hydrogenation reactions have
been performed and are herein discussed.
Synthesis of [{2-(3,5-dimethylpyrazol-1-
ylmethyl)pyridine}PdPPh3Cl]Cl (4)
To a suspension of 1 (0.09 g, 0.24 mmol) in CH2Cl2
(5 mL), a solution of PPh3 (0.07 g, 0.27 mmol) in CH2Cl2
(5 mL) was added to give a light yellow precipitate. The
mixture was stirred for 12 h and filtered to isolate com-
1
pound 4 as light yellow solid. Yield = 0.10 g (62 %). H
NMR (DMSO-d6): d 2.40 (s, 3H, CH3, pz); 2.43 (s, 3H,
CH3, pz); 5.79 (d, 2H, py-CH2-pz) 6.16 (d, 2H, pz,
3JHH = 8.0 Hz); 7.66–7.61 (m, Ph); 7.94 (t, 1H, py,
3
3JHH = 7.2 Hz); 8.12 (d, 1H, py, JHH = 8.0 Hz); 8.79(d,
3
1H, py, JHH = 8.4 Hz. 31P {H} NMR (DMSO-d6): d
28.88 (s, 1P, PPh3). Anal. Calcd. For C29H28Cl2N3PPd: C,
55.6; H, 4.5; N, 6.7 %. Found C, 55.7; H, 4.0; N, 5.3 %.
Positive mode (ESI–MS) m/z 525.94 (M?–ClMe2, 100).
Synthesis of [{2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine}
Experimental section
PdClPPh3]BAr4 (5)
Materials and instrumentation
Toasuspensionof1 (0.10 g, 0.27 mmol)inCH2Cl2 (10 mL),
PPh3 (0.08 g, 0.30 mmol) and NaBAr4 (Ar4 = 3,5-(CF3)2-
C6H3) (0.22 g, 0.27 mmol)were added andstirredunder inert
atmosphere for 12 h. The mixture was filtered and concen-
trated to approximately 3 mL. Hexane (10 mL) was then
added to precipitate 5 as a yellow crystalline solid. Recrys-
tallization of 5 using CH2Cl2/Hexane solvent mixture affor-
ded single crystals suitable for X-ray analysis.
All reactions were carried out under nitrogen atmosphere
using a dual vacuum/nitrogen line and standard Schlenk
techniques unless stated otherwise. Solvents were dried and
distilled under nitrogen in the presence of suitable drying
agents. PdCl2 (59 % Pd) and PPh3 (99 %) were purchased
from Sigma-Aldrich while the sodium salt, NaBAr4
{Ar4 = (3,5-(CF3)2C6H3)4} (95 %), was obtained from
Boulder Scientific and used without any further purification.
All solvents were of analytical grade and were purchased
from Merck Chemicals and dried using appropriate tech-
niques. The compounds (2-(3,5-dimethylpyrazol-1-yl-
methyl)pyridine (L1), 2-(3,5-diphenylpyrazol-1-ylmethyl)
pyridine (L2) [29] and complexes [Pd(L1)Cl2] (1),
[Pd(L2)Cl2] (2) and [Pd(L1)MeCl] (13) were prepared
according to the literature procedures [30]. NMR spectra
were recorded on a Bruker 400 Ultrashield instrument at
room temperature in CDCl3 and DMSO-d6 solvents. The 1H
(400 MHz) and 31P{1H} (162 MHz) chemical shifts are
reported in d (ppm) and referenced to the residual proton in
1
Yield = 0.26 g (65 %). H NMR (CDCl3): d 2.21 (s, 3H,
CH3, pz); 2.28 (s, 3H, CH3, pz); 5.27 (d, 2H, py-CH2-pz) 6.06
(d, 2H, pz, 3JHH = 8.0 Hz); 7.48–7.53 (m, Ph); 7.51 (s, 8H,
BAr4) 7.71 (s, 4H, BAr4); 7.89 (t, 1H, py, 3JHH = 7.2 Hz);
3
8.65 (d, 1H, py, JHH = 8.0 Hz); 8.93(d, 1H, py,
3JHH = 8.4 Hz. 31P {H} NMR (CDCl3): d 27.85 (s, 1P,
PPh3). Anal. Calcd. For C61H40ClF24N3PPd: C, 50.4; H, 2.8;
N, 2.9 %. Found: C, 50.4; H, 2.9; N, 3.1 %. Positive mode
(ESI–MS) m/z 555.36 (M?, –Cl, 40).
Hydrogenation reactions of alkenes and alkynes
In a typical experiment, styrene (0.73 mL, 8.00 mmol),
catalyst 1 (6 mg, 0.02 mmol) equivalent to a substrate-to-
catalyst ratio of 400 and toluene (50 mL) were introduced
into a stainless steel Parr autoclave (400 mL) fitted with an
internal stirring and cooling system. The solution mixture
was purged with hydrogen (three times) before the auto-
clave was finally charged with hydrogen and the pressure
maintained at 5 bar at a constant temperature of 30 °C. The
1
the solvents for H and 85 % H3PO4 for 31P nuclei. All
coupling constants (J) are measured in Hertz (Hz). Elemental
analyses were performed on a Thermal Scientific Flash 2000,
and mass spectra were recorded on an LC Premier micro-
mass Spectrometer. Electron microscopy analyses were
done on a JEOL JEM-1400X transmission electron micro-
scope at the school of life sciences, University of KwaZulu-
Natal.
123

Transition Met Chem
stirring speed was set to 600 rpm and stirring started to
initiate the reaction. After the reaction period, excess
pressure was vented off, samples withdrawn, filtered using
0.45-lm micro-filters and analyzed by Varian CP-3800 GC
(ZB-5HT column 30 m 9 0.25 mm 9 0.10 lm) to deter-
mine the consumption of styrene. Ethyl benzene (97 %),
cis-2-hexene (98 %), trans-2-hexene (97 %), octane
(98 %) were received from Sigma-Aldrich and used as
authentic standards to establish the identity of hydrogena-
tion products. Percentage conversion of styrene to ethyl
benzene was determined by comparing the peak intensities
of styrene and ethyl benzene. Kinetics of the hydrogenation
reactions were performed by monitoring the consumption
of the substrates at regular time intervals. The rate con-
stants for each experiment were derived from plots of
ln[substrate]0/[substrate]t versus time (where [sub-
strate]o = initial concentration of substrate at time 0 and
[substrate]t = concentration of styrene at time t.
(5) were synthesized as shown in Scheme 1. Thus, treatment
of a suspension of the neutral complex 1 with one molar
equivalent of either PPh3 or PPh3/NaBAr4 afforded the cor-
responding cationic compounds 4 and 5, respectively
(Scheme 1).
The new cationic palladium(II) complexes 4 and 5 were
characterized by 1H NMR, 13C{H} NMR and 31P{H}
NMR spectroscopies (Figures S1 and S2) and elemental
analyses. For example, the 1H NMR spectrum of complex 5
(Figure S1) showed pyridine (N–CH) protons at 8.93 ppm
compared to 8.53 ppm for the corresponding ligand, L1.
31P NMR spectra of complex 5 showed a singlet peak at
27.85 ppm (Figure S2) consistent with a coordinated PPh3
ligand [31, 32]. Micro-analyses data of compounds 4 and 5
were in good agreement with the proposed empirical for-
mulae and confirmed their purity.
Hydrogenation reactions of styrene catalyzed
by complexes 1–5
Results and discussion
Preliminary investigations of the (pyrazolylmethyl)-
pyridine palladium(II) complexes 1–5 as catalyst precur-
sors in hydrogenations of alkenes were performed using
styrene as a model substrate. In a typical reaction, styrene
Synthesis and characterization of new cationic
palladium(II) 4 and 5 complexes
(0.73 mL, 8.00 mmol), complex
1
(0.02 mmol,
The compounds (2-(3,5-dimethylpyrazol-1-ylmethyl)
pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)
pyridine (L2) and their respective neutral palladium(II)
complexes, [Pd(L1)Cl2] (1), [Pd(L2)Cl2] (2) and [Pd(L1)
MeCl] (3) were synthesized following reported literature
procedures [29, 30]. The corresponding cationic palladium(II)
complexes [Pd(L1)PPh3Cl]Cl (4) and [Pd(L1)PPh3Cl]BAr4
0.25 mol %, substrate-to-catalyst ratio of 400), H2 pressure
(5 bar) in toluene (50 mL) were used at stirring speed of
600 rpm (Scheme 2).
All the complexes showed significant catalytic activities
in the hydrogenation of styrene to afford 100 % ethyl
benzene and conversions between 17 and 98 % within 2 h
(Figure S3). Control experiments conducted without the
R
Cl
N
N
N
Pd
R
L
PPh3
h3
P
P
h
4
2
,
l
C
2
R= Me, L = Cl (4)
H2
R
C
Pd(MeCN)2Cl2/
Pd(COD)MeCl
CH2Cl2, 12 h
R
N
N
N
N
N
N
P
R
Pd
P
R
h
,
,
3
R
L
C
Na
Cl
H
BAr4
B
Ar
2 C
l
R = CH3 (L1)
R = Ph (L2)
N
4
2
1
2
h
R= Me, L= Cl (1)
R= Ph, L= Cl (2)
R= Me, L= Me (3)
N
N
R
Pd
L
PPh3
R= Me, L = Cl (5)
Scheme 1 Synthetic protocol of neutral and cationic (pyrazolylmethyl)pyridine palladium(II) complexes 1–5
123

Transition Met Chem
catalytic activity, exhibiting kobs of 1.508 h-1 compared to
its neutral analog, complex 1 (kobs of 0.787 h-1). Inter-
estingly, the cationic complex 4, bearing the Cl- counter
anion showed lower catalysis (kobs = 0.935 h-1) compared
to 5, containing the BAr4 counter anion. This trend clearly
demonstrated the significance of complex solubility in
controlling their respective catalytic activities. Complex 5
showed the highest solubility in most organic solvents in
comparison with complexes 1–4. Indeed, the methylated
palladium(II) complex 3 showed better solubility than 1, 2
and 4, and concomitant higher catalytic activity (Table 1,
entries 1–5). In addition, replacing the smaller pyrazolyl
methyl substituent in 1, with a bulkier phenyl group in 2,
resulted in an increase in kobs from 0.787 h-1
(TOF = 152 h-1) to 1.027 h-1 (TOF = 174 h-1).
1-5, H2
Toluene, 5 bar, 30 oC, 600 rpm
Scheme 2 Catalytic hydrogenation of styrene using Pd(II) com-
plexes 1–5
use of any complex under similar reaction conditions
afforded conversions of 3 and 6 % within 1 and 6 h,
respectively, confirming that complexes 1–5 were respon-
sible for the observed higher conversions. After establish-
ing that complexes 1–5 form active catalysts in
hydrogenation of alkenes, detailed kinetics and chemose-
lectivity studies were conducted to evaluate the effect of
catalyst structure, identity of substrate and reaction
parameters on these hydrogenation reactions. The subse-
quent sections systematically present the findings of these
investigations.
Generally the complexes showed good stability under
the specified hydrogenation conditions as shown by the
minimum amount of zero-valent palladium nanoparticles
formed. Indeed, attempts to isolate the nanoparticles to
catalyze the hydrogenation of styrene did not give any
catalytic activity. This was, however, consistent with the
pseudo-kinetics of the reactions and lack of induction
periods which largely supported homogeneous nature of
the active species.
Effect of catalyst structure on the kinetics
of hydrogenation of styrene
In order to investigate the effect of catalyst structure,
hydrogenation reactions were carried out for all the com-
plexes 1–5, using styrene as a model substrate over a 2 h
period (Table 1). To determine the rate constants (kobs) for
each complex and the order of reactions with respect to
styrene substrate, plots of ln[Sty]0/[Sty]t versus time were
constructed (Figure S4). The linearity of the graphs
established that the hydrogenation reactions using com-
plexes 1–5 as catalysts obey pseudo-first-order kinetics
with respect to styrene substrate as shown in Eq. 1.
1
Effect of catalyst concentration and hydrogen
pressure on the kinetics of hydrogenation reactions
of styrene
In order to establish the effects of catalyst concentration
and pressure on the hydrogenation reactions of styrene, the
hydrogenation reactions were carried out at different sub-
strate-to-catalyst ratios and hydrogen pressures using
complex 2. The substrate-to-catalyst ratio was thus varied
from 400 to 1200 at constant initial concentration of styr-
ene (Table 1, entries 2, 6–8). Plots of ln[Sty]0/[Sty]t vs time
at various catalyst concentrations were linear (Figure S5).
Rate ¼ k½StyreneŠ
ð1Þ
The observed rate constant (kobs) for each complex was
thus derived from the gradients of the plots in Figure S4
(Table 1). The cationic complex 5 showed the highest
Table 1 Summary of styrene
hydrogenation data using
complexes 1–5
Entry
Catalyst
Sub/cat
Time (h)
Conversion (mol%)a
Initial rates
kobs (h-1
TOF (h-1 b
)
)
1
2
3
4
5
6
7
8
1
2
3
4
5
2
2
2
400
400
400
400
400
600
800
1200
2
2
2
2
2
2
2
2
76
87
86
84
95
84
78
55
0.787
1.027
1.043
0.935
1.508
0.857
0.713
0.420
152
174
172
168
190
252
312
330
Conditions: styrene, 8.00 mmol; catalyst; 0.02 mmol; time, 2 h; 5 bar; toluene, temperature, 30 °C
a
Determined by GC
TOF in mol substrate. mol-1 catalyst h-1
b
123

Transition Met Chem
From the data in Table 1, it was evident that the rates of
hydrogenation reactions were dependent on the substrate-
to-catalyst ratio (Table 1, entries 2, 6–8). For example, kobs
As an illustration, kobs of 1.027 and 2.353 h-1 were
reported at hydrogen pressures of 5 and 10 bar, respec-
tively (Figure S6). Construction of a plot of ln kobs versus
hydrogen pressures allowed us to determine the order of
reaction with respect to hydrogen concentration as
0.16 0.01 using catalyst 2 (Fig. 1b). Fractional and very
low reaction orders with respect to hydrogen concentration
are indicative of a fast dissociative adsorption process
followed by surface reaction, which is likely to be the rate
determining step [34, 35]. Thus, the overall rate law for the
hydrogenation of styrene catalyzed by complex 2 can be
represented by Eq. 3.
of 1.027 h-1 (TOF = 174 h-1) and 0.420 h-1 (330 h-1
)
were observed at substrate-to-catalyst ratios of 400 and
1200, respectively (Table 1, entries 2 and 8). However, it is
important to note that higher TOFs were observed at lower
catalyst concentrations (higher substrate-to-catalyst ratios)
despite the lower rate constants. This observation indicated
that increasing catalyst loading did not increase the cat-
alytic activity by a similar magnitude. Thus, from these
experiments, lower catalyst loading (high substrate-to-cat-
alyst ratios of 800 and 1200) was qualitatively superior
since they give higher turnover frequencies.
1
1
0:16
Rate ¼ ½StyreneŠ ½2Š ½PH2Š
ð3Þ
Further plots of the -ln(kobs) versus -ln[2] enabled us
to establish the order of reaction with respect to catalyst 2
(Fig. 1a). From the slope of the graphs, the order of the
reaction with respect to catalyst was obtained as
1.025 0.101 h-1. Therefore, the hydrogenation reactions
of styrene follow pseudo-first-order kinetics with respect to
catalyst 2 and the rate law may be represented as given in
Eq. 2. In addition, the nonzero y-intercept at -6.83 is
consistent with the hydrogenation reactions taking place in
the absence of any catalyst [33].
Influence of alkene and alkyne substrates
on the kinetics of hydrogenation reactions
The scope of alkene and alkyne substrates that could
effectively undergo hydrogenation using complex 2 as a
catalyst was studied using 1-hexene, 1-octene, 1-hexyne,
1-octyne and phenylacetylene (Table 2). To determine the
kobs for each substrate, plots of ln[substrate]0/[substrate]t
versus time were constructed (Figure S7). From Table 2, it
was evident that the nature of the alkene/alkyne substrate
affected both the catalytic activity and product distribution
of catalyst 2 in the respective hydrogenation reactions.
Generally, alkynes showed higher reactivity compared to
the corresponding alkenes. For example, kobs of 6.657 and
1.631 h-1 were recorded for 1-hexyne and 1-hexene,
respectively (Table 2, entries 1 vs. 5). This was expected
and has been attributed to the high reactivity of the triple
1
1:025
Rate ¼ ½StyreneŠ ½2Š
ð2Þ
The effect of hydrogen pressure on the kinetics of the
hydrogenation reactions of styrene was also investigated
using complex 2 by varying the pressure from 5 to
12.5 bars. A plot of ln [Sty]0/[Sty]t versus time at different
hydrogen pressures gave linear graphs (Figure S6). The kobs
were found to increase with increase in hydrogen pressure.
(a)
(b)
Y = A + B * X
9.2
Y = A + B * X
Parameter
Value Error
3.2
2.8
2.4
2.0
1.6
B
1.02496
SD
0.10071
P
A
B
1.05 0.11619
0.16 0.01265
9.0
8.8
8.6
8.4
8.2
R
N
R
SD
N
P
0.99048
0.06895
4
0.00952
0.99381
0.07071
4
0.00619
0.0
0.2
0.4
0.6
0.8
1.0
4
6
8
10
12
14
pH2 (bar)
-ln[2]
Fig. 1 a Plot of observed rate constants (kobs) versus catalyst concentration and b plot of observed rate constants (kobs) versus hydrogen pressure
using catalyst 2
123

Transition Met Chem
Table 2 Effect of substrates on
the catalytic activity of catalyst 2
Entry
Substrate
%Conv
kobs (h-1
)
TOF (h-1 a
)
%Alkane
1
2
3
4
5
6
1-Hexyne
1-Octyne
99
99
98
87
98
90
6.657
6.593
4.840
1.190
1.631
1.150
198
198
196
174
196
180
50
58
Phenylacetylene
Styrene
86
100
40
1-Hexene
1-Octene
30
Conditions: substrate, substrate/catalyst = 400; substrate, 8.00 mmol; catalyst; 0.02 mmol (0.25 mol %);
time, 2 h; pressure, 5 bar; solvent, toluene; temperature, 30 °C. Determined by GC
a
TOF in mol substrate. mol-1 catalyst h-1
Fig. 2 Conversion/selectivity versus time profile for hydrogenation of a 1-hexene b phenylacetylene using catalyst 2. Substrate-to-catalyst 2
ratio = 400. H2 pressure: 5 bar; temperature: 30 °C; solvent: toluene (50 mL); stirring speed: 600 rpm; time: 2 h
bonds in alkynes compared to double bonds in alkenes [36–
39]. The substrate reactivities were also found to be
dependent on the alkene chain length. For instance, kobs of
1.631 and 1.150 h-1 were obtained for 1-hexene and
1-octene substrates, respectively (Table 2, entries 5 vs. 6).
This phenomenon is largely associated with the poor
coordination ability of the longer chain alkenes to the
active metal center [4, 5]. Interestingly, for the alkyne
respectively, within the 2 h period investigated (Fig. 2a). It
is therefore clear that catalyst 2 favored isomerization
(60 % internal isomers) over hydrogenation (40 % hex-
anes) reactions. The larger composition of the trans-2-
hexene could be assigned to the steric restrictions present
in the cis-isomer [40, 41]. Tandem hydrogenation and
isomerization reactions are typical of palladium(II) based
catalysts due to the presence of empty d-orbitals which
interact with the p-orbitals of the alkenes in activating the
adjacent C–H bonds [42].
series, both 1-hexyne (6.657 h-1) and 1-octyne (6.593 h-1
)
showed similar rate constants (Table 2, entries 1 vs. 2).
The chemoselectivity and regio-selectivity of the
hydrogenation reactions catalyzed by complex 2 were also
extensively investigated. Hydrogenation reactions of ter-
minal alkenes were observed to produce both the respective
alkenes and internal isomers. Thus, it was evident that there
occurred tandem hydrogenation and isomerization reac-
tions (Figure S8). For example, hydrogenation reactions of
1-hexene produced final compositions of hexanes, cis-2-
hexene and trans-2-hexene in yields of 40, 18 and 42 %,
We also used phenylacetylene substrate to study the
product distribution of alkynes over time using complex 2.
The choice of phenylacetylene was due to the inability of
its alkene intermediate (styrene) to undergo isomerization
reactions. Two main products ethyl benzene and styrene
were produced over the 2 h reaction period (Fig. 2b). From
Fig. 2b, it was clear that the concentration of ethyl benzene
in the reaction mixture was minimal until all the pheny-
lacetylene was reduced to styrene. This was followed by
123

Transition Met Chem
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