(Pyridyl)benzoazole palladium(II) complexes as homogeneous catalysts in hydrogenation of alkenes and alkynes

Article abstract of DOI:10.1039/c5cy02204e

Reactions of 2-(2-pyridyl)benzimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with either [PdCl₂(NCMe)₂] or [PdClMe(COD)] produced complexes [(PdCl₂(L1)] (1), [(PdCl₂(L2)] (2), [(PdCl₂(L3)] (3) and [(PdClMe(L1)] (4) in good yields. Treatment of 1 with PPh₃ gave the cationic complex [(Pd(L1)ClPPh₃]Cl (5), while reactions of 4 with one equiv. of PPh₃ and NaBAr₄ (Ar = 3,5-(CF₃)₂C₆H₃) produced the cationic complex [(Pd(L1)MePPh₃]BAr₄ (6). Single-crystal X-ray analysis has been used to elucidate the structure of complex 6. The complexes formed active catalysts in hydrogenation reactions of alkenes and alkynes. Hydrogenation of terminal alkenes was accompanied by isomerization to the internal isomers, while alkyne reactions involved a two-step process producing alkenes and the respective alkanes. While the kinetics data and formation of inactive palladium nanoparticles support the homogeneous nature of the active species, mercury drop experiments indicated a possible role of the nanoparticles in the re-generation of the active species.

Full text of DOI:10.1039/c5cy02204e

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(Pyridyl)benzoazole palladium(II) complexes as homogeneous catalysts in
hydrogenation of alkenes and alkynes
Stephen O. Ojwach,* Aloice O. Ogweno, Matthew P. Akerman
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville,
Pietermaritzburg, 3209, South Africa.
Abstract:
Reactions of (2ꢀ(2ꢀpyridyl)benzimidazole (L1), 2ꢀ(2ꢀpyridyl)benzothiazole (L2) and
2ꢀ(2ꢀpyridyl)benzoxazole (L3) with either [PdCl2(NCMe)2] or [PdClMe(COD)] produced
complexes [(PdCl2(L1)] (1), [(PdCl2(L2)] (2), [(PdCl2(L3)] (3) and [(PdClMe(L1)] (4) in
good yields. Treatment of 1 with PPh3 gave the cationic complex [(Pd(L1)ClPPh3]Cl (5),
while reactions of 4 with one equiv. of PPh3 and NaBAr4 (Ar = 3,5ꢀ(CF3)2C6H3) produced the
cationic complex [(Pd(L1)MePPh3]BAr4 (6). Single Xꢀray analysis has been used to elucidate
the structure of complex 6. The complexes formed active catalysts in hydrogenation reactions
of alkenes and alkynes. Hydrogenation of terminal alkenes was accompanied by
isomerization to the internal isomers, while alkyne reactions involved a twoꢀstep process
producing alkenes and the respective alkanes. While the kinetics data and formation of
inactive palladium nanoparticles support homogeneous nature of the active species, mercury
drop experiments indicate possible role of the nanoparticles in reꢀgeneration of the active
species.
Keywords: Palladium; Alkenes; Hydrogenation; Isomerization; Homogeneous;
*Corresponding author: Tel.: +27 (33) 260 5239; Fax: +27 (33) 260 5009
Eꢀmail: ojwach@ukzn.ac.za (S. O. Ojwach)
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1.
Introduction
Alkenes and alkynes have a range of useful applications such as building blocks or
components of petrochemical, fine chemical, agrochemical and pharmaceutical products
[1,2,]. As a result, transformation of these unsaturated CꢀC multiple bonds to other chemicals
have received a lot of interest both in academic and industrial research [3]. One of the key
transformations of alkenes and alkynes to afford a number of useful products is via
hydrogenation reactions using molecular hydrogen in the presence of transition metal
catalysts [1,2,4].
Several platinum group metal complexes have been vastly studied for catalytic
hydrogenation of alkenes and alkynes and other unsaturated hydrocarbons under both
heterogeneous and homogeneous reaction conditions [1,2,5]. For example, Lindlars’ catalyst,
is highly effective in the hydrogenation of alkynes in food industries [6]. In general,
palladium catalysts are preferred in most hydrogenation reactions of organic compounds
because they have high surfaceꢀtoꢀvolume ratio compared to other platinum group metal
complexes [2]. However, few homogeneous palladium(II) catalysts have been applied in high
pressure hydrogenation as opposed to heterogeneous catalysts [7]. Recently, Yilmaz et al.
reported the application of perflouroalkylated S^Oꢀchelated palladium acetate complexes as
homogeneous catalysts for high pressure hydrogenation of styrene and 1ꢀoctene [8].
The catalytic activities of the respective metal complexes in hydrogenation reactions
have been shown to greatly depend on the nature of the donor atoms within the ligand
framework [9]. In the last few decades, phosphineꢀdonor palladium(II) complexes have been
the most widely applied in homogeneous transitionꢀmetal catalysed hydrogenation of olefins.
For example (2,5ꢀdimethylphospholano)benzene palladium(II) complexes were shown to
catalyze the hydrogenation of
3ꢀmethylꢀ2ꢀcyclohexenone displaying moderate conversions of
2

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40% after 24 h [10]. Other palladium(II) phosphine catalysts investigated for homogeneous
hydrogenation reactions include binuclear palladium(0) complexes bearing 1,1ꢀ
Bis(diphenylphosphino)methane ligands [11]. Mixed Nꢀ and Pꢀ donor ligands have also been
used in designing homogeneous palladium catalysts for the hydrogenation of unsaturated
hydrocarbons. Bacchi et al. reported the catalytic activities of hydrazonic phopshine
palladium(II) complexes for hydrogenation of styrene with promising results at mild reaction
conditions [12, 13].
Despite the success of phosphineꢀdonor palladium catalysts in olefin hydrogenation
reactions, most of these systems suffer from lack of stability and sensitivity to moisture and
air. Thus nitrogenꢀdonor palladium catalysts are gaining a lot of interest in the hydrogenation
of unsaturated organic compounds due to their enhanced stability and ease of syntheses [14,
15, 16, 17, 18]. Herein, we report the syntheses of palladium(II) complexes bearing
(pyridyl)benzoazole ligands and their applications as catalysts in high pressure hydrogenation
of alkenes and alkynes. Detailed studies on the kinetics, chemoꢀselectivity and nature of the
active species of these hydrogenation reactions have been performed and are herein
discussed.
2
Experimental section
2.1
Materials, instrumentation and methods
All reactions were carried out under nitrogen atmosphere using standard Schlenk line
techniques. Solvents were dried and distilled under nitrogen in the presence of suitable drying
agents. PdCl2.xH2O (99.9%), triphenylphosphine, (99%) were purchased from SigmaꢀAldrich
whileNaBAr4 {Ar4 = (3,5ꢀ(CF3)2C6H3)4} was obtained Boulder Scientific and used as
received. Ligands, 2ꢀ(2ꢀpyridyl)benzimidazole (L1), 2ꢀ(2ꢀpyridyl)benzothiazole (L2) and 2ꢀ
3

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(2ꢀpyridyl)benzoxazole (L3) and complexes [Pd(L1)Cl2] (1), [Pd(L2)Cl2] (2), [Pd(L3)Cl2]
(3) and [Pd(L1)MeCl] (4) were prepared according to literature procedures [19, 20]. NMR
spectra (1H, 400 MHz and 31P{1H}, 162 MHz) were recorded on a Bruker 400 Ultrashield
instrument at room temperature in CDCl3 and DMSOꢀd6 solvents. The chemical shifts are
1
reported in δ (ppm) and referenced to the residual proton in the solvents for H and 85%
H3PO4 for 31P nuclei respectively. All coupling constants (J) are measured in Hertz (Hz).
Elemental analyses were performed on Thermal Scientific Flash 2000 and mass spectra were
recorded on LC Premier microꢀmass Spectrometer. Xꢀray data were recorded on a Bruker
Apex Duo diffractometer equipped with an Oxford Instruments Cryojet while electron
microscopy analyses were carried out on a JEOL JEMꢀ1400X transmission electron
microscope at the University of KwaZuluꢀNatal.
2.2
Synthesis of cationic palladium(II) complexes 5 and 6
The new cationic complexes 5 and 6 were prepared following literature procedures [21]. The
synthetic procedures for the synthesis of the cationic complexes 5 and 6, together with the
analytical, spectroscopic and crystallographic data are given as supplementary materials.
2.3 Density Functional Theoretical (DFT) calculations of 1-6
DFT calculations were performed in gas phase for complexes 1-6 using a wellꢀ
established approach for the third row transition metal complexes, to identify the energyꢀ
minimized structures based on B3LYP/LANL2DZ [22,23] (Los Alamos National Laboratory
2 double ζ) level theory, with inner core electrons of Pd atom replaced by relativistic
effective core potential (ECP). The Gaussian09 suite of program was used for all the
computations [24].
4

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2.4
High pressure hydrogenation reactions of alkenes and alkynes
In a typical experiment, styrene (0.73 mL, 8.00 mmol), catalyst 1 (6 mg, 0.02 mmol)
S/C 400 and toluene (50 mL) were introduced into stainless steel Parr autoclave (400 ml)
fitted with internal stirring and cooling system. The solution mixture was purged with
hydrogen (three times) before the autoclave was finally charged with hydrogen and the
pressure maintained at 5 bar at a constant temperature of 30 °C. The stirring speed was set to
600 rpm and the stirring started when the temperature reached equilibrium. After the reaction,
period, excess pressure vented off and samples withdrawn, filtered using 0.45 µm micro
filters and analyzed by Varian CPꢀ3800 GC (ZBꢀ5HT column 30m×0.25 mm×0.10µm) to
determine the consumption of styrene. Ethyl benzene from SigmaꢀAldrich was used as an
authentic standard to determine identity of ethyl benzene. Percentage conversion of styrene to
ethyl benzene was determined by comparing the peak intensities of styrene and ethyl
benzene.
2.5
TEM analyses
The catalytic samples after the hydrogenation experiments were analyzed using JEOL
JEMꢀ1400X electron microscope. In a typical experiment, the solvent was evaporated after
the reaction period and the residue solids (catalytic mixture) dispersed onto suspension by
sonicating in ethanol. A drop of the catalytic suspension was mounted on a copper grid with
carbon lacey, evaporated in air and introduced into the microscope.
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3.
Results and discussion
3.1
Synthesis and characterization of complexes 1-6
The benzoazole ligands (2ꢀ(2ꢀpyridyl)benzimidazole (L1), 2ꢀ(2ꢀpyridyl)benzothiazole
(L2) and 2ꢀ(2ꢀpyridyl)benzoxazole (L3) and their neutral palladium(II) complexes
[Pd(L1)Cl2] (1), [Pd(L2)Cl2] (2), [Pd(L3)Cl2] (3), and [Pd(L1)MeCl] (4) were prepared
according to the literature procedures [19, 20]. Treatment of the neutral complexes 1 and 4
with equimolar amounts of PPh3 and PPh3/NaBAr4 respectively gave the respective cationic
complexes [Pd(L1)PPh3Cl]Cl (5) and [Pd(L1)PPh3Me]BAr4 (6) in good yields (Scheme 1).
H
N
Cl
Pd(MeCN)2Cl2
Pd(COD)MeCl
CH2Cl2, 24 h
X
N
N
N
X
N
PPh3
Pd
N
CH2Cl2, 24 h
Cl
(5)
PPh3
N
Pd
Cl
L
X = NꢀH
X = S
(
L1)
(
L2)
)
PPh3, NaBAr4
CH2Cl2, 12 h
X = NꢀH, L= Cl (1)
X = S, L= Cl (2)
X = O, L= Cl (3)
X = NꢀH, L =Me (4)
X = O
,
(
L3
H
N
BAr4
N
N
Pd
Me
(6)
PPh3
Scheme 1: Synthesis of neutral and cationic benzoazole palladium(II) complexes 1-6.
The identities of the new cationic complexes 5 and 6 were established by a
1
combination of H, 31P{H} NMR spectroscopy, microꢀanalyses, mass spectrometry and
1
single crystal Xꢀray analysis for complex 6. H NMR spectra of the complexes showed
downfield shifts relative to the corresponding ligands (Figure S1). 31P NMR spectra of
6

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complexes 5 and 6 showed peaks at 28.89 ppm and 30.17 ppm respectively (Figures S2 and
S3) which are within the range of 25.4ꢀ40.7 ppm reported for similar palladium(II)
compounds [25, 26, 27]. ESIꢀMS spectra of 5 recorded in the positive ion mode, showed a
base peak at m/z = 600.07 amu corresponding to the [Pd(L1)PPh3Cl]+ fragment (Figure S4a)
and was in good agreement with the calculated isotopic distribution (Figure S4b). The microꢀ
analyses data of the new cationic compounds 5 and 6 were in good agreement with the
proposed structures and confirmed the purity of the isolated complexes.
3.2
Molecular structure of complex 6
Single crystals suitable for Xꢀray analyses for complex 6 were grown by slow
evaporation of a DCM/hexane solution at room temperature. Figure 1 shows the molecular
structure of 6 and selected bond parameters while the data collection and structural
refinement parameters are provided as supplementary information in Table S1. In the solid
state structure of complex 6, ligand L1 adopts a bidentate coordination mode to the
palladium(II) metal center (Figure 1). The bond angles for N(1)ꢀPd(1)ꢀC(13) of 93.29° and
N(1)ꢀPd(1)ꢀN(2) of 77.84(8)° deviate from the 90° typical of squareꢀplanar complexes. Thus
the coordination geometry of the palladium ion in compound 6 could be best described as
distorted square planar. The acute N1ꢀPd1ꢀN2 bond angle is ascribed to the limited bit of the
ligand and resulting fiveꢀmembered chelation ring. The N2ꢀPd1ꢀP1 bond angle (102.15(6)°)
is significantly more obtuse than the ideal bond angle. This is seemingly required to relieve
nonꢀbonded repulsion between the triphenylphosphine moiety and benzimdazole ligand. The
PdꢀNpy bond length of 2.156(2) Å is longer the average of 2.0442 Å for a related 2ꢀ(2ꢀ
pyridyl) benzoazoleꢀPdCl2 complex reported by Haneda and coꢀworkers [26c] indicative of a
stronger transꢀeffect of the PPh3 ligand in 6.
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Figure 1: Molecular structure of 6 drawn at 50% probability ellipsoids. Bond lengths (Å):
PdꢀN(1) 2.156(2); PdꢀN(2), 2.174(2); Pd ꢀP(1) 2.2441(6); Pd ꢀC(13) 2.028(3). Bond angles
(°): N(1)ꢀ Pd (1)ꢀN(2) 77.84(8); P(1)ꢀ Pd (1)ꢀC(13) 87.53(8); N(1)ꢀ Pd (1)ꢀC(13) 93.3(1);
N(2)ꢀ Pd (1)ꢀP(1) 102.15(6); C(13)ꢀ Pd (1)ꢀN(2) 168.9(1); N(1)ꢀ Pd (1)ꢀP(1) 172.44(6).
3.3
1-6
Molecular hydrogenation reactions of alkenes and alkynes catalysed by complexes
Preliminary investigations of the propensity of complexes 1-6 to catalyse high pressure
hydrogenation of alkenes were performed using styrene as a model substrate. In a typical
reaction, styrene (0.74 mL, 8.00 mmol), complexes 1-6 (0.02 mmol, 0.25 mol%,
substrate/catalyst ratio of 400), H2 pressure (5 bar) in toluene (50 mL) were used at stirring
speed of 600 rpm (Scheme 2).
1-6 H2
Toluene, 5 bar, 30 oC, 600 rpm
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Scheme 2: Catalytic molecular hydrogenation of styrene using palladium(II) complexes 1-6.
All the complexes formed active catalysts in the hydrogenation of styrene to 100% ethyl
benzene giving conversions ranging between 17%ꢀ98% within 1 h (Figure S5). Control
experiments conducted without the use of any complex under similar reaction conditions
displayed conversions of 3% and 6% after 1 h and 6 h respectively, indicating that complexes
1-6 were responsible for the higher catalytic activities reported. Upon establishing that
complexes form 1-6 effective catalyst in high pressure hydrogenation of alkenes, we carried
out detailed kinetics, chemoꢀselectivity and theoretical studies to evaluate the effect of
catalyst structure, substrate and reaction parameters on these reactions.
3.3.1 Kinetics of hydrogenation of styrene reactions
3.3.1.1 Effect of catalyst structure on hydrogenation of styrene
In order to investigate the effect of the complex/ligand structure, hydrogenation
reactions were carried out for all complexes 1-6 using styrene as a model substrate (Table 1).
Plots of ln[Sty]o/[Sty]t vs time was used to establish the initial rate constants for complexes 1-
6 (Figure S6) and order of reactions with respect to styrene. The linearity of the plots
established pseudo-first order kinetics of the hydrogenation reactions with respect to styrene.
Thus the dependency of the rate of the hydrogenation reaction on styrene concentration can
be expressed as shown in equation 1.
………………………………………..…………… eq. 1
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The observed rate constants for the complexes were extracted from the gradients of
the plots in Figure S6. Table 1 contains the observed rate constants (kobs) and the
corresponding TOFs for complexes 1ꢀ6. The catalytic activities of the complexes were found
to be dependent on the nature of the ligand backbone. For example, complex 1, bearing the 2ꢀ
(2ꢀpyridyl)benzoimidazole ligand (L1) recorded the highest catalytic activity (kobs of 6.24 hꢀ1)
while the analogous thiazole and oxazole complexes 2 and 3 exhibited kobs of 3.02 hꢀ1 and
4.26 hꢀ1 respectively.
Table 1: Effect of catalyst structure on the hydrogenation of styrene by complexes 1-6a
Conversion
(mol%)b
98
Initial rates
kobs(hꢀ1)
6.24
Entry
Catalyst
TOF(hꢀ1)c
1
2
3
4
5
6
523
389
464
91
1
2
3
4
5
6
73
87
17
28
17
3.02
4.26
0.30
0.72
0.21
149
91
aConditions: styrene, substrate/catalyst = 400; substrate, 8.00 mmol; catalyst; 0.02 mmol (0.25 mol%); solvent,
toluene; pressure, 5 bar; toluene, temperature, 30 oC;, time 45 min. bDetermined by GC. cTOF in mol substrate.
molꢀ1 catalyst.hꢀ1
The higher catalytic activity of 1 could be attributed to presence of the NꢀH moiety
which is likely to generate a more electrophilic metal center due to its greater acidity. We
recently reported a similar trend in the transfer hydrogenation of ketones using
ruthenium(II)/(III) complexes of ligands L1ꢀL3 [28]. The catalytic activity trend observed
were also in good agreement with those previously reported by Haneda et al. [20c], where the
10

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most labile and less stable benzothiazole ligand L2 formed the least active catalyst in the
MizorokiꢀHeck coupling reactions. Thus, increased stability of the complexes also played a
major role in controlling the catalytic activities of the complexes. Indeed, from our DFT
calculations, we observed that the activity trends of complexes 1-6 was dependent on the
average bond lengths {(NPy +Nbz)/2} and the HOMOꢀLUMO energy gap (eV). Generally,
shorter average PdꢀNpy/bz bond distances and larger HOMOꢀLUMO energy gaps resulted in
higher catalytic activities. For example, complex 1, bearing the shortest average bond length
{(NPy +Nbz)/2} of 2.0857 Ǻ (Table S2, entry 1) was found to be the most active with an
observed rate constant of 6.24 hꢀ1 while complex 6 ({(NPy +Nbz)/2}of 2.1953 Ǻ) was the
least active with a rate constant of 0.21 hꢀ1 (Table S2, entry 6).
We also observed that the ancillary ligands (Me and Cl) affected the catalytic
activities of the respective complexes. For example, the observed rate constants of 6.24 hꢀ1
and 0.30 hꢀ1 were recorded for complexes 1 and 4 bearing Cl and Me ligands respectively
(Table 1, entries 1 and 4). The ease of formation of Pdꢀhydride, believed to be the active
species, for PdꢀCl systems, compared to PdꢀMe might account for this trend [29]. Similarly,
the cationic complexes [Pd(L1)PPh3Cl]Cl (5) and [Pd(L1)PPh3Me]BAr4 (6) displayed kobs of
0.72 hꢀ1 and 0.21 hꢀ1 respectively, which were significantly lower than the kobs values of 6.24
hꢀ1 and 0.30 hꢀ1 obtained for the corresponding neutral complexes 1 and 4 respectively (Table
1, entries 1, 4ꢀ6). The poor lability (poor leaving group) of PPh3 in addition to its steric
demands might hinder the formation of active Pdꢀhydride species and/or block the
coordination of an incoming alkene substrate [29,30] under homogeneous conditions.
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3.3.1.2 Effect of catalyst concentration on the kinetics of hydrogenation reactions
Further kinetic experiments were performed to establish the effects of catalyst
concentrations on the hydrogenation of styrene using complex 1. The concentrations of 1
were varied from 1.0 x 10ꢀ4 M (substrate/catalyst ratio of 1 200) to 3.0 x 10ꢀ4
M
(substrate/catalyst ratio of 400). The kobs derived from the plots of ln[sty]0/[sty]t vs time
(Figure S7) were used to establish the dependency of the rates of the reactions on catalyst
concentration.
From Table 2, it is evident that increase in catalyst concentration (lower
substrate/catalyst ratios) resulted in higher rates of hydrogenation reactions. For example, rate
constants of 6.24 hꢀ1 and 2.18 hꢀ1 were observed at substrate/catalyst ratios of 400 and 1 200
respectively (Table 2, entries 1 and 4). However, increase in kobs at higher catalyst
concentrations (lower substrate/catalyst ratios) were marked by decrease TOF. For example,
while kobs of 6.24 hꢀ1 and 3.86 hꢀ1 were observed at substrate/catalyst ratios of 400 and 800
respectively, TOFs of 523 hꢀ1 and 565 hꢀ1were obtained at ratios of substrate/catalyst ratios
400 and 800 respectively. This indicates that the increase in the reaction rates was not
proportional to the increase in catalyst concentration and thus higher substrate/catalyst ratio
of 800 could be taken as the optimum reaction conditions.
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Table 2: Effect of reaction parameters on the hydrogenation of styrene using catalyst 1a
Entry
Sub/Cat
PH2 (bar)
Conversion
(mol%)a
kobs (hꢀ1)
TOF (hꢀ1)b
1
400
600
800
1200
400
400
400
400
400
400
400
ꢀ
5
98
66
53
15
91
92
96
06
85
81
55
10
3
6.24
4.94
3.86
2.18
6.76
8.77
9.59
ꢀ
523
528
565
240
485
491
512
4.8
680
1296
110
ꢀ
2
5
3
5
4
5
5
7.5
10
12.5
5
6
7
5c
6d
7e
8f
9g
10h
5
5.28
5.34
ꢀ
5
5
5
ꢀ
ꢀ
5
ꢀ
ꢀ
aConditions: styrene, 8.00 mmol; pressure, 5 bar; solvent, toluene, time, 45 min; temperature, 30 oC .
Determined by GC. bTOF in mol substrate. molꢀ1 catalyst.hꢀ1
ctemperature, 25 °C; Time 5 h; d50 °C, time 0.5 h, e60 °C, time 0.25 h
fMercury drop test (5 drops of mercury added to the reaction mixture)
gisolated nanoparticles as the catalyst from catalyst 1
hcontrol experiment, no catalyst used
A plot of ꢀln(kobs) vs ꢀln [1] was used to establish the order of reaction with respect
to catalyst 1 (Figure 2). From the gradient of the plot of 0.9±0.14644, a pseudoꢀfirst order
kinietics of the reaction with respect to catalyst 1 can be assigned. The nonꢀinteger value of
reaction order could be due to aggregation of the active species during the hydrogenation
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reactions [31]. In addition, possible formation of the palladium nanoparticles as the active
species (discussed in section 3.3.3) supports the observed fractional order of reaction with
respect to catalysts 1. The overall rate law with respect to styrene substrate and catalyst 1
could therefore be represented by equation 2. In addition, the nonꢀzero yꢀintercept implies
that the hydrogenation reactions can occur spontaneously in the absence of a catalyst
consistent with our experimental observations and literature reports [32].
Figure 2: Plot of ln(kobs) versus ln[1] for the detemination of order of reaction with respect
to catalyst 1.
……………………eq. 2
3.3.1.3 Effect of hydrogen pressure on the kinetics of hydrogenation of styrene
The influence of H2 pressure on the kinetics of hydrogenations of styerene was also
investigated by varrying the pressure from 5 bar to 12.5 bar using catalyst 1. Plots of
ln[Sty]0/[Sty]t vs time at different H2 pressures displayed linear relationships (Figure S8).
The observed rate constants (kobs) were found to increase with increase in H2 pressure. For
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instance, kobs of 9.59 hꢀ1 and 6.24 hꢀ1 were reported at 12.5 bar and 5 bar respectively (Table 2,
entries 1 and 7). A plot of kobs vs H2 pressure was used to determine the order of reaction with
respect to the H2 concentration (Figure S9). From the plot in Figure S9, the order of reaction
on H2 concentration was found as 0.104. The fractional order of reaction coupled with the
relatively insignificant dependence of hydrogenation reactions on H2 concentration was
indicative of fast dissociative adsorption and rapid activation of the He molecule. This is
followed by hydrogenation reactions on the surface of the catalyst, which is likely to be the
rate determining step [33]. Thus, the overall rate law for the hydrogenation of styrene by
catalyst 1 can be represented as given in equation 3.
……… eq. 3
3.3.1.4 Effect of temperature on the kinetics of hydrogenation reactions of styrene
The effect of temperature on the hydrogenation kinetics of styrene was also studied by
varying the reaction temperature between 25 °C and 60 °C at S/C of 400 using complex 1
(Figure S10). At 25 °C, only 6% conversion was achieved after 5 h (TOF = 4.8 hꢀ1), while at
o
60 C, percentage conversions of 81% was realized within 15 min translating to a TOF of 1
296 hꢀ1 (Table 2, entries 5 and 7). Generally, the kobs increased with increase in reaction
temperatures. For example, kobs of 3.58 hꢀ1 and 5.34 hꢀ1 were obtained at 30 °C and 60 °C
respectively (Table 2, entries 6 and 7). This is expected as higher reaction temperatures lead
to higher collisions of the reacting molecules [34]. Most significantly, was the greater thermal
stability of catalyst 1, since no decomposition to palladium(0) black was observed even at 60
oC.
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3.3.1.5 Effects of alkene and alkyne substrates on the kinetics of hydrogenation reactions
We also evaluated the effects of different substrates on the hydrogenation behavior of
complex 1. The substrates, 1ꢀhexene, 1ꢀoctene, 1ꢀhexyne, 1ꢀoctyne and phenyl acetylene
were investigated under similar reaction conditions. Plots of ln[substrate]0/[substrate]t vs
time (Figure S11) were used to determine the initial rate constants (kobs) with respect to each
substrate. From the data obtained, higher reaction rates were reported for alkyne substrates
compared to the corresponding alkenes. For example, kobs of 0.49 hꢀ1 and 6.38 hꢀ1 were
obtained for 1ꢀhexene and 1ꢀhexyne respectively. This difference could be attributed to the
higher reactivity of alkynes compared to alkenes [35,36]. The catalytic activity of complex 1
was also found to be dependent on the alkyl chain length of the substrate. As an illustration,
kobs of 0.49 hꢀ1 and 0.36 hꢀ1 were observed for 1ꢀhexene and 1ꢀoctene substrates respectively.
Similarly, kobs of 6.38 hꢀ1 and 6.06 hꢀ1 were reported of 1ꢀhexyne and 1ꢀoctyne respectively
(Table 3, entries 1 and 2). A more interesting observation was the higher kobs of 6.24 hꢀ1
reported for styrene compared to kobs of 0.49 hꢀ1 and 0.36 hꢀ1 obtained for the aliphatic 1ꢀ
hexene and 1ꢀoctene substrates respectively (Table 3, entries 4ꢀ6). This higher reactivity of
styrene could be largely attributed to the piꢀdelocalization of electrons in the aromatic
benzene ring [37].
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Table 3: Effect of substrates on the catalytic activity of catalyst 1a
TOF (hꢀ1)b
Entry
Substrate
1ꢀhexyne
(kobs) (hꢀ1)
%Alkanec
1
2
3
4
5
6
6.38
278
90
52
17
1ꢀoctyne
4.06
Phenyl acetylene
Styrene
5.03
532
523
341
245
100
98
6.24
1ꢀhexene
0.49
64
1ꢀoctene
0.36
46
aConditions: substrate, substrate/catalyst = 400; substrate, 8.00 mmol; catalyst; 0.02 mmol (0.25 mol%); time,
45 min for determination of activity/percentage conversion; pressure, 5 bar; solvent, toluene; temperature, 30
b
c
oC. Determined by GC. TOF in mol substrate. molꢀ1 catalyst.hꢀ1; Selectivity towards alkane hydrogenation
products after 2 h.
3.3.2 Chemo- and regio-selectivity of the hydrogenation reactions
The selectivity of catalyst 1 in hydrogenation reactions of 1ꢀhexene was extensively
studied by monitoring the product distributions over time. It was noted that the hydrogenation
reactions of terminal 1ꢀhexene were accompanied by isomerization reactions to produce the
corresponding internal hexenes (Figure 3a). Typical GC chromatogram of the product
compositions of hydrogenation reactions of 1ꢀhexene and 1ꢀhexyne are given in Fig. S12.
Tandem isomerization and hydrogenation reactions are typical of palladium catalysts and
have been attributed to the existences of empty dꢀorbitals which interact with the πꢀorbitals of
alkenes to activate the adjacent CꢀH bond [38]. A detailed time profile of the product
distributions showed final compositions of 64% for hexane, 24% (transꢀ2ꢀhexene) and 12%
(cisꢀ2ꢀhexene) within 2 h (Figure 3a).
We also extensively studied the product distributions in the hydrogenation of
phenylacetylene as a function of time using catalyst 1. From Figure 3b, it was evident that
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the hydrogenation of phenylacetylene occurred in two steps; partial hydrogenation to styrene
and finally to the saturated ethyl benzene (Scheme 3). From Figure 3b, it is clear that the
concentration of ethyl benzene in the reaction mixture was minimal within 0.5 h, a stage that
can be assigned to the formation of mainly styrene. Indeed the concentration of styrene
reached a maximum of about 92% within this period. Upon complete conversion of phenyl
acetylene to styrene, production of ethyl benzene is evident from the gradual increase of its
composition to 100% followed by near complete consumption of the styrene intermediate.
These results clearly demonstrate the twoꢀstep step hydrogenation process consistent with
literature reports [39, 40]. One signficant aspect of these catalysts is that, one can selectively
perfom partial hydrogenation of alkynes to alkenes at the expense of alkanes by ensuring
continued supply of the alkyne substrate or running the hydrogenations reactions for shorter
periods.
CH
H2
H2
Scheme 3: A two step hydrogenation of phenylacetylene to ethylbenzene via styrene
intermediate.
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(b)
(a)
Figure 3: Conversion/selectivity vs. time profile for hydrogenation reactions of (a) 1ꢀhexene
and (b) phenylacetylene using catalyst 1. H2 pressure, 5 bar; temperature, 30 °C; solvent,
toluene (50 mL); stirring speed, 600 rpm; time, 2 h.
3.3.3 Determination of the formation of palladium nanoparticles and their possible role
in the hydrogenation reactions.
To understand the nature of the active species and possible formation and role of
palladium nanoparticles in the hydrogenation reactions catalyzed by complexes 1-6, we
carried out mercury poison test [41] by adding five drops of mercury to the reaction mixture
of styrene
and catalyst 1. A significant drop of catalytic activity from 98% (TOF = 523 hꢀ1) to 55%
(TOF = 110 hꢀ1) was observed (Table 2, entries 1 and 8). To further ascertain the formation of
palladium nanoparticles/colloids, transition electron microscopy (TEM) was used to analyze
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the residues of the catalytic residue. Figure 4 represents the TEM images obtained from the
reaction mixtures of catalysts 1 and 5 after the hydrogenation reactions.
Figure 4: TEM image of palladium(II) nanoparticles isolated from the reaction mixtures
using (a) catalyst 1 and (b) catalyst 5 . Conditions: styrene, 8.00 mmol; catalyst; 0.02 mmol
(0.25 mol%); pressure, 5 bar; solvent, toluene; temperature, 30 oC; time 45 min.
The particles sizes in Figure 4(b) showed a wide range from 40 nm ꢀ330 nm possibly due
to catalyst aggregation during the hydrogenation reactions or solvent evaporation [42,
43,44]. The high carbon content in the EDX spectral data (Figures S13) indicated the
presence of the ligand in the nanoparticles and hence formation of ligand stabilized palladium
nanoparticles. To unambiguously confirm if the palladium nanoparticles were active in the
hydrogenation reactions, nanoparticles formed were isolated from the reaction mixture of
catalyst 1 after 4 h by filtration and used in the hydrogenation of styrene substrate. From the
GC chromatogram obtained (Figure S14), only trace amounts of the product (percentage
conversion of 10%) was observed (Table 2, entry 9) compared to the initial conversion of
98% (Table 1, entry 1).
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From the kinetics data obtained (absence of induction periods), the lack of complete
poisoning of the catalysts by mercury and the inactive nature of the nanoparticles isolated
thus indicate a homogenous catalytic system. However, the significant drop in catalytic
activity upon mercury addition points to some possible role of the nanoparticles in the
catalytic cycle. Thus, as previously reported in literature by Ananikov and Beletskaya and coꢀ
workers [45], these systems can be best described as a “cocktail” of catalysts in which the
active species are homogeneous while the palladium nanoparticles serve as reservoir to
generate the active species.
3.4. Proposed mechanistic pathway for the homogeneous catalysed
hydrogenation of alkenes
Having established that the active species of complexes 1ꢀ6 in the hydrogenation
reactions of alkenes and alkynes were mainly homogeneous in nature, a possible mechanism
via apalladium hydride intermediates was proposed (Scheme 4).46 Since complexes 1-6 are
coordinatively saturated, dissociation of a Clꢀ ligand would be necessary prior to the
formation of the Pdꢀhydride intermediate 1a (Scheme 4). This is believed to occur via
heterolytic cleavage leading to the formation of HCl byproduct.47 Coordination of the styrene
substrate to 1a gives the Pdꢀalkyl species 1b.48 A Markovnikov migration of the hydride to
the coordinated substrate leads to the formation of the palladium alkyl intermediate 1c, which
then undergoes oxidative addition in the presence of H2 to give the alkyl Pd(IV) dihydride
compound 1d. Hydride migration to the coordinated alkyl ligand in 1d followed by reductive
elimination produces a monohydride Pd(II) intermediate 1e and elimination of ethylbenzene
product and subsequent regeneration of the active hydride species 1a.
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H
N
N
N
Pd
Cl
Cl
H2
1
ꢀHCl
H
N
N
N
Pd
H
Cl
Cl
Cl
1a
H
N
H
N
N
N
N
N
Pd
Pd
H
H
1e
1b
Reductive elimination
Cl
H
N
Cl
H
N
N
N
N
(IV)
N
H
Pd
Pd
H
1c
1d
H2
Oxidative addition
Scheme 4: Proposed mechanism for the high pressure hydrogenation of styrene by 1.
4.
Conclusions
A series of neutral and cationic Pd(II) complexes of (2ꢀ(2ꢀpyridyl)benzoazole ligands
were evaluated as catalsyts in hydrogenation reactions of alkenes and alkynes under mild
conditions. The catalysts showed 100% selectivity in the hydrogenation of styrene to form
ethyl benzene. While hydrogenation of terminal alkenes was accompanied by isomerization
reactions to form internal alkenes, the alkyne reactions produced alkenes as intermediates
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prior to the formation of alkanes. The kinetics of hydrogenation reactions were pseudoꢀfirst
order with respect to the catalysts and substrates. The absence of induction periods coupled
with the inactivity of the isolated palladium nanoparticles establishes the homogenous nature
of the active species.
Supporting information
Supplementary materials contain analytical, spectroscopic and singleꢀcrystal Xꢀray
characterization data of complexes 1ꢀ6, kinetics plots, GCꢀMS spectra of the products and
TEM images of the nanoparticles formed. CCDC number 1404729 contains the
supplementary crystallographic data for compound 6. These data can be obtained free of
charge
from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.Acknowledgments
Acknowledgement
The authors are grateful for the financial support received from the University of KwaZuluꢀ
Natal.
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Table of contents (TOC)
Complexes 1-6 form “cocktail” catalysts in the hydrogenation and isomerization of alkenes
and alkynes where the active species are homogeneous.