Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes as Catalysts for Hydrogenation of Alkenes, Alkynes and Functionalized Benzenes

Article abstract of DOI:10.1007/s10562-020-03192-1

Abstract: This work reports the use of homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L1), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol) (L2), [L1-MCM-41] (L1im), and [L2-MCM-41] (L2im) as catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The homogeneous complexes [Pd(L1)₂] (Pd1), [Pd(L2)₂] (Pd2), [Pd(L1)(Cl₂)] (Pd3),?and [Pd(L2)(Cl₂)] (Pd4), and their respective silica immobilized?complexes [Pd(L1)₂]-MCM-41] (Pd1im), [Pd(L2)₂)-MCM-4] (Pd2im), [Pd (L1)(Cl₂)-MCM-41] (Pd3im) and [Pd(L2)(Cl₂)]-MCM-41] (Pd4im) formed active catalysts in?the molecular hydrogenation of these substrates. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed a pseudo-first order dependence on styrene substrate for both the homogeneous and immobilized catalysts. Significantly, the selectivity of both homogeneous and immobilized catalysts were comparable in the hydrogenation of both?alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum loss of catalytic activity and showed absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established that complexes Pd1–Pd4 were largely homogeneous in nature, while the immobilized complexes Pd1im–Pd4im formed Pd(0) nanoparticles as the main active species. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03192-1

Catalysis Letters
https://doi.org/10.1007/s10562-020-03192-1
Comparative Study of Homogeneous and Silica Immobilized N^N
and N^O Palladium(II) Complexes as Catalysts for Hydrogenation
of Alkenes, Alkynes and Functionalized Benzenes
1
1
1
Saphan O. Akiri · Nondumiso L. Ngcobo · Stephen O. Ojwach
Received: 6 December 2019 / Accepted: 18 March 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
This work reports the use of homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-
2
-((3(triethoxysilyl)propyl)imino)ethanol) (L1), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol) (L2), [L1-
MCM-41] (L1im), and [L2-MCM-41] (L2im) as catalysts in molecular hydrogenation of alkenes, alkynes and functionalized
benzenes. The homogeneous complexes [Pd(L1) ] (Pd1), [Pd(L2) ] (Pd2), [Pd(L1)(Cl )] (Pd3), and [Pd(L2)(Cl )] (Pd4),
2
2
2
2
and their respective silica immobilized complexes [Pd(L1) ]-MCM-41] (Pd1im), [Pd(L2) )-MCM-4] (Pd2im), [Pd (L1)
2
2
(
Cl )-MCM-41] (Pd3im) and [Pd(L2)(Cl )]-MCM-41] (Pd4im) formed active catalysts in the molecular hydrogenation of
2
2
these substrates. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the
substrate. The kinetic studies revealed a pseudo-ꢀrst order dependence on styrene substrate for both the homogeneous and
immobilized catalysts. Signiꢀcantly, the selectivity of both homogeneous and immobilized catalysts were comparable in
the hydrogenation of both alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four
times with minimum loss of catalytic activity and showed absence of any leaching from hot ꢀltration experiments. Kinetics
and poisoning studies established that complexes Pd1–Pd4 were largely homogeneous in nature, while the immobilized
complexes Pd1im–Pd4im formed Pd(0) nanoparticles as the main active species.
Graphic Abstract
Keywords Palladium · Immobilization · Hydrogenation · Alkenes · Benzenes · Recycling
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3

S. O. Akiri et al.
1
Introduction
containing more than two sites for electrophilic attack is
not a straightforward and obvious reaction. From these
accounts, it is clear that this is an area that requires intense
research in the design and development of more active and
selective catalysts.
Molecular hydrogenation reaction of saturated substrates
is among the vital catalytic processes in synthetic organic
chemistry, both in industrial and academic laboratories,
largely due its cleaner process [1]. The industrial applica-
tions of hydrogenation reactions range from pharmaceu-
ticals, petrochemical and ꢀne chemicals syntheses. Tra-
ditionally, platinum metal derivatives have been applied
as catalysts in the hydrogenation of alkenes, alkynes, and
other unsaturated hydrocarbons [2–4]. However, palladium
catalysts have been widely applied since they possess a
higher surface to volume ratio, than the platinum catalysts.
A classic example is the Lindlar’s catalyst, which has been
widely applied in food industries in the hydrogenation of
alkynes [5]. Phosphine-donor palladium catalysts are the
most studied systems, even though they suꢁer from air
and moisture sensitivity, and poor stability. Therefore,
nitrogen-donor catalysts have seen increased interest,
due to their improved stability and ease of synthesis [6,
These hydrogenation reactions are carried out under both
homogeneous and heterogeneous conditions, with varied
outcomes. While homogeneous catalysts display superior
selectivity [18–21], they suꢁer from lack of separation and
recycling of the active species [22–24]. On the other hand,
heterogeneous catalysts, are separable and recyclable [25],
but display poor selectivity [26]. Therefore, eꢁorts geared
towards exploiting the advantages of both catalyst systems
with the aim of achieving selective and recyclable systems
have been on the rise [27]. Various methods have since been
developed, such as using polymer supports [28–31], silica
[32–34], magnetic nanoparticles [35–37], and biphasic sys-
tems [38–40]. In our attempts to bridge the gap between
the heterogeneous and homogeneous catalysis, we recently
applied homogeneous and immobilized palladium(II) com-
pounds in the methoxycarbonylation of oleꢀns [41]. Both
catalyst systems display comparable catalytic activities
and regio-selectivity and the immobilized systems can be
recycled without signiꢀcant loss of catalytic activity [42].
Encouraged by these results in the methoxycarbonylation
of oleꢀns, we choose to extend the use of these systems
in molecular hydrogenation reactions of alkenes, alkynes,
and functionalized benzenes. We herein, report the com-
parative catalytic behaviour, reaction kinetics and catalyst
poisoning studies of these homogeneous and immobilized
palladium(II) complexes in these reactions.
7
]. In one of the reports, phosphine donor palladium(II)
complexes bearing (2,5-dimethylphospholano)benzene
applied by Drago & Pregosin aꢁorded a conversion of 40%
in the hydrogenation of 3-methyl-2-cyclohexene within
2
4 h [8]. Mixed P and N donor systems have also been
applied in hydrogenation reactions. Chelating P^N donor
palladium(II) systems bearing phosphino hydrazonic
ligands displayed conversion of 77% to 100% of styrene
to ethyl benzene within 48 h [9].
Hydrogenation of benzene and its derivatives is another
area that continues to attract interest both in industry
and academia [10]. For example, selective hydrogena-
tion of nitrobenzene is vital in the production of anilines,
an important intermediate in the pharmaceutical indus-
try [11]. The major challenge in this process is the low
reactivity of the benzene molecule and achieving good
selectivity, in multiple-functionalized benzene substrates
2
Experimental
2.1 Materials and Methods
[
12–14]. For example, the hydrogenation of 4-nitro ace-
All air sensitive reactions were manipulated using stand-
ard Schlenk techniques. All solvents obtained from Merck
were of analytical grade and were dried and distilled fol-
lowing standard protocols: methanol was dried through
heating over iodine activated with sodium, while toluene
was dried over sodium wire and benzophenone. (2-phe-
nyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L1) and
tophenone and 4-nitro benzaldehyde occur in multistep
complex pathways due to the competitive hydrogenation
reactions between the nitro and carbonyl groups [15]. In
one such example in the hydrogenation of 4-nitroacetophe-
none, using palladium catalyst supported on melamino-
formaldehyde polymer, Cao et al. reported selective hydro-
genation of the nitro group to give 4-aminoacetophenone
(
4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol)
[
13]. Using the same substrate, Hawkins and Makowski
observed a mixture of 4-aminoacetophenone (59%) and
-(4-aminophenyl) ethanol (34%) using Rh/Al O catalysts
(
L2), and their supported ligands, [L1-MCM-41] (L1im),
and [L2-MCM-41] (L2im) were synthesized following pre-
1
2
3
viously reported procedures [41, 42]. The respective homo-
[
16] Currall and Jackson on the other hand reported a mix-
.
geneous and immobilized palladium complexes [Pd (L1)
ture of 1-(4-aminophenyl) ethanol, 4-aminoacetophenone
and 1-(4-aminocyclohexyl) ethanol using Rh/silica cata-
lyst [17], an indication that the hydrogenation of benzenes
2
(
(
Pd1), [Pd (L2) ] (Pd2), [Pd (L1)(Cl )] (Pd3), [Pd (L2)
2
2
Cl )] (Pd4) and [Pd(L1) ]-MCM 41] (Pd1im); [Pd(L2) ]-
2
2
2
MCM 41(Pd2im); [Pd(L1)(Cl )]-MCM 41] (Pd3im) and
2
1
3

Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes…
[
Pd (L2)(Cl )]-MCM 41] (Pd4im) were synthesized follow-
and Pd1im–Pd4im respectively, (Fig. 1) were carried out
at 30 °C, 5 bar and at Pd: styrene ratio of 1:800 (Table 1).
All the complexes aꢁorded conversions of between 72 and
88% within 1 h under these reaction conditions. To deter-
mine if indeed the catalytic activities displayed were due to
the palladium complexes, we set up a typical hydrogenation
reaction without any palladium complex and obtained neg-
ligible conversions of 2% (Table 1, entry 9), conꢀrming that
the higher conversions obtained were due to the palladium
complexes.
2
ing our recently described procedures [42].
2
.2 Standard Procedure for the Hydrogenation
of Substrates
Hydrogenation reactions were carried out in a stainless steel
autoclave Parr reactor equipped with an internal cooling sys-
tem, a temperature control unit and a sampling valve. In a
typical experiment, the reactor was pre-heated, evacuated
and ꢂushed with nitrogen and cooled to room temperature.
A solution of styrene (0.25 ml, 2.10 mmol) and complex
Pd3 (0.0014 g, 0.0027 mmol) in toluene (20 ml) was then
introduced using a cannula. The reactor was then purged
with hydrogen gas several times, and then the desired pres-
sure and reaction temperature set and stirred for the speci-
3
.2 Kinetic Studies of the Catalytic Hydrogenation
Reactions
3
.2.1 Eꢀects of Complex Structure on the Kinetics
of Hydrogenation of Styrene
ꢀ
ed time. At the end of the reaction period, the reactor was
then cooled, the gas supply closed and excess hydrogen gas
vented oꢁ. The samples were then collected for GC anal-
yses using a syringe equipped with 0.45 µm micro ꢀlters
and analysed by a Varian CP-3800 GC (ZB-5HT Column
The inꢂuence of the complex structure on the kinetics of
hydrogenation reactions of styrene was carried out for both
the homogeneous (Pd1–Pd4) and immobilized complexes
(
Pd1im–Pd4im) at 5 bar and temperature of 30 °C. The
3
0 m×0.25 mm×0.1 µm) to establish percentage conver-
kinetic plots of time vs percentage conversion showed dif-
ferent proꢀles for the homogeneous and immobilized pal-
ladium catalysts (Fig. 2). The sigmoid curves obtained for
the immobilized complexes Pd1im–Pd4im depicted their
heterogeneous behaviour and possible formation of active
Pd(0) nanoparticles (discussed later). This is supported
by the induction periods, which signify conversion of the
supported complexes to active Pd(0) nanoparticles. On the
other hand, the smooth curves without the induction periods
obtained for complexes Pd1–Pd4 are typical of the presence
of mainly homogeneous active species [43, 44]. A similar
trend was observed by Gross et al., where the SBA-15 sup-
ported palladium catalyst gave sigmoid-shaped plots [45].
Nonetheless, the plots of ln[Styrene] /[Styrene] vs time for
sions. The percentage conversions were calculated by com-
parison of the substrate peak areas to those of the prod-
ucts, assuming 100% mass balance. The kinetics reactions
were done through monitoring the change in substrate con-
sumption over time at regular intervals. The rate constants
were then derived from graphical plots of ln[substrate] /
0
[
substrate] versus time. The catalysis products were identi-
t
ꢀ
ed using the standard authentic samples as well as GC–MS.
2
.3 Recycling of the Immobilized Catalysts
Procedure
The recycling experiments were carried using the immobi-
lized complexes (Pd1im – Pd4im) by centrifuging the reac-
tion mixture at 8500 RPM for 10 min after the preceding
reaction. The solid catalyst was recovered following a decan-
tation process of the supernatant liquid and reintroduced into
the reactor together with fresh substrate. The fresh reaction
was run under the pre-set reaction conditions and analyses
done with GC as outlined in Sect. 2.2.
o
t
both homogeneous and immobilized catalysts displayed lin-
ear relationships corresponding to pseudo-ꢀrst order kinet-
ics. From the linearity of both plots, the reaction rates with
respect to styrene substrate can thus be represented as given
in Eq. 1.
ꢀ
ꢁ1
Rate = k Styrene
(1)
3
Results and Discussion
The observed rate constants (k ) exhibited by the
obs
catalysts were calculated from the linear plots in (Figs.
S1 and S2) and used to evaluate the eꢁect of complex
structure on catalytic activities (Table 1). The catalytic
activities of the homogenous catalysts demonstrated
dependence on the coordination sphere of the metal center.
For instance, the mono(chelated) complex Pd3 gave a
3
.1 Preliminary Evaluation of the Homogeneous
and Immobilized Palladium Complexes
as Catalysts in Hydrogenation Reactions
of Styrene
−
1
Preliminary catalytic hydrogenation of styrene reactions
higher rate constant of 2.02 ± 0.05 h as compared to a
−
1
using homogeneous and immobilized complexes Pd1–Pd4
value of 1.86 ± 0.12 h obtained for the corresponding
1
3

S. O. Akiri et al.
Fig. 1 Structures of homo-
geneous (Pd1–Pd4) and
immobilized (Pd1im–Pd4im)
palladium complexes used as
catalysts in hydrogenation reac-
tions in this study
Table 1 Hydrogenation of styrene using homogeneous and immobi-
a
lized complexes
Conv (%)b
kobs (h−1
)
TOF (h )
−1 c
Entry
Catalyst
1
2
3
4
5
6
7
8
Pd1
86
81
88
84
78
72
83
76
2
1.86 (± 0.12)
1.62(± 0.09)
2.02 (± 0.05)
1.75 (± 0.09)
1.41 (± 0.12)
1.17 (± 0.10)
1.59 (± 0.14)
1.37 (± 0.12)
–
688
640
704
672
624
576
664
608
16
Pd2
Pd3
Pd4
Pd1im
Pd2im
Pd3im
Pd4im
–
9
0
1
d
1
Pd3
Pd3im
85
42
–
–
680
336
e
1
a
Reaction conditions: pressure, 5 bar; time: 1 h; temp 30 °C; Solvent,
toluene (20 ml); catalyst concentration, 0.125 mol% (Pd/substrate:
1
:800); styrene (0.5 ml, 4.4 mmol)
b
%
of styrene converted to ethyl benzene
c
−1
Fig. 2 Plots of percentage conversion of styrene to ethylbenzene
vs time showing smooth and sigmoid curves for the homogeneous
TOF (mol. sub/mol. Pd h
)
d,e
Reactions centrifuged and ꢀltered
(
Pd1–Pd4) and immobilized palladium catalysts (Pd1im–Pd4im)
respectively
1
3

Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes…
bis(chelated) Pd1 complex (Table 1, entries 1 and 3). Two
reasons can be fronted for this observation. First, the eas-
ier generation of the palladium-hydride species from the
Pd–Cl bond (Pd3), which is believed to be the active spe-
cies [46]. Secondly, we hypothesize that the bis(chelated)
complexes Pd1 and Pd2, may require dissociation of one
ligand unit to allow for styrene substrate coordination.
Similar trends were observed for the immobilized cata-
lysts Pd1im–Pd4im. However, this current observation
contradicts our recent reports using the same complexes
in the methoxycarbonylation of oleꢀns [41]. A plausible
reason is that the bis(chelated) complexes exhibit higher
thermal stability required for the methoxycarbonylation
reactions (90 °C), as opposed to room temperature condi-
tions employed in the current hydrogenation reactions. A
comparison of the catalytic activities of the homogene-
ous and the respective immobilized complexes revealed
diminished catalytic activities of the immobilized sys-
3.2.2 Dependence of Kinetics of Hydrogenation Reactions
on Catalyst Loading and Hydrogen Pressure
Having established that both the homogeneous and immo-
bilized complexes aꢁord active catalysts in the hydro-
genation of styrene, the most active homogeneous and
immobilized complexes Pd3 and Pd3im respectively
were employed in further investigations of the reactions
parameters such as catalyst loading, pressure and nature of
substrate. The dependence of the rate of the hydrogenation
reaction on catalyst concentration was examined by vary-
ing the styrene/Pd ratios from 600 to 1000 at a constant
styrene concentration. Plots of In[styrene] /[styrene] vs
0
t
time for complexes Pd3 and Pd3im at diꢁerent catalyst
concentration (Figs. S3 and S4) allowed us to determine
the rate constant at each catalyst concentration (Table S1).
While, the rates of the reactions appear to increase with
increase in catalyst concentration, a diꢁerent trend was
observed in the examination of the TOF values (Fig. S9).
For example, increasing the catalyst loading of Pd3im
from 0.10 mol% to 0.125 mol% led to an increase in both
−
1
tems. For example, rate constants of 1.86± 0.12 h and
−
1
1
.41 ± 0.12 h were reported for Pd1 and Pd1im respec-
tively (Table 1 entries 1 and 5). This is consistent with
the expected behaviour [47], and in good agreement with
the results observed in the methoxycarbonylation reactions
using the same catalysts [41, 42].
−
1
−1
−1
k
(from 0.85 h to 1.59 h ) and TOFs (from 580 h to
obs
−
1
664 h ) respectively (Table S1). However, beyond catalyst
loading of 0.125%, the TOF dropped signiꢀcantly (Fig.
S9). This clearly shows that, the apparent increase in the
rate of the reaction is of lower magnitude compared to
the amount of catalyst added. Thus, a lower catalyst load-
ing of 0.125% would be qualitatively preferred, from an
industrial perspective.
In comparison to the widely reported catalysts in hydro-
genation reactions of various substrates [48–51], the pre-
sent catalysts displayed moderate catalytic activities. For
example, the palladium(II) catalysts bearing perꢂuoro-
alkylated S^O– ligand reported by Kani et al. [52] display
−
1
TOFs of 372 h in the hydrogenation of styrene, which
The plots of – In(k ) vs – In[Pd3] and – In[Pd3im]
obs
−
1
is lower than the TOF of 704 h we observed using the
homogeneous catalyst Pd3. On the same vein, Altinel
et al.’s [53] reported perꢂuorinated Rh–BINAP catalysts
were used to obtain the order of the reactions with respect
−
1
−1
to Pd3 and Pd3im as 2.34 ± 0.18 h and 2.07 ± 0.15 h
respectively (Fig. 3). The reaction orders with reference to
catalysts Pd3 and Pd3im is an indication that the rates of
the hydrogenations reactions are sensitive to the changes
in catalyst concentration. A more interesting observa-
tion can be derived from the non-integer values of 2.34
and 2.07 for the homogeneous and immobilized Pd3 and
Pd3im catalysts, respectively. Since fractional orders are
associated with catalyst aggregation [59, 60], one would
expect a higher fraction in the order with respect to the
immobilized catalyst Pd3im, as the active species is likely
to be heterogeneous in nature. While the integer order of
2.07 can be attributed to a possible uniformly dispersed
palladium catalyst, it is not suꢃcient to account for this
discrepancy. The fractional order with respect to Pd3 may
be assigned to aggregation and or formation of Pd(0) nano-
particles as part of the active species [59, 60]. The rate
laws of the hydrogenation reactions of styrene with respect
to catalyst concentrations can therefore be expressed as
shown in Eq. 2.
−
1
to exhibit much lower TOF of 71.5 h in the hydrogena-
tion of styrene, when compared to our catalysts. In addi-
−
1
tion, the TOFs of 624 h observed for the immobilized
complex Pd3im (Table S4) is much higher compared to
−
1
the values of 1.33 h recorded for the soluble iron nano-
particles stabilized by MgCl reported by Phua et al. in
2
the hydrogenation of 1-hexene [54]. On the other hand,
palladium catalysts that display comparable to or higher
catalytic activities in comparison to the current immobi-
lized catalysts Pd1im – Pd4im complexes have also been
reported. For example, the palladium nanoparticles sta-
bilized by polyethylene glycol [55] and palladium nano-
particles anchored on uncoordinated carbonyl groups
−
1
−1
MOFs [56] exhibit TOFs of 660 h and 703 h respec-
tively. Similarly, Pd catalysts immobilized on polymer-
ized 2-(acetoacetoxy)ethyl methacrylate reported by Lan
et al. [57] and Gao et al.’s Pd–pyridyl catalytic ꢀlms [58]
−
1
display much higher catalytic activities of 1449 h and
1
6
944 h− respectively in the hydrogenation of styrene.
1
3

S. O. Akiri et al.
Fig. 3 Graphs of In(kobs) vs In[Pd3] (a) and In[Pd3im] (b) used in determining the reaction order with respect to complexes Pd3 and Pd3im
respectively
ꢀ
ꢁ
1
Rate = Styrene [ꢀꢁꢂ]
2
.34
3.2.3 Dependence of Kinetics of Hydrogenation of Styrene
on Reaction Temperature
ꢀ
ꢁ
(2)
1
Rate = Styrene [ꢀꢁꢂꢃꢄ]
2
.07
To determine the inꢂuence of reaction temperature on the
kinetics of hydrogenation of styrene using catalysts Pd3 and
Pd3im, the reaction temperatures were varied from 30 °C to
We also investigated the eꢁects of varying the hydrogena-
tion pressure on the kinetics of the hydrogenation of styrene
using complexes Pd3 and Pd3im by varying the hydrogen
pressure from 2.5 to 10 bar. Plots of In[styrene] /[styrene]
6
0 °C. By plotting a graph of In[styrene] /[styrene] vs time
0 t
at diꢁerent temperatures, the observed rate constants were
extracted. In general and expectedly, an increase in reaction
temperature was followed by a concomitant increase in the
0
t
vs time for complexes Pd3 and Pd3im at diꢁerent hydrogen
pressures (Figs. S5 and S6) aꢁorded linear relationships, from
which the rate constant at each pressure was determined. For
example, using complex Pd3 at 2.5 bar the observed rate
rate constants [63, 64]. For instance, using catalyst Pd3, at
−1
3
0 °C and 60 °C, the observed rate constants of 2.02 h
−1
and 5.95 h were obtained respectively (Table S3). The
activation parameters of complexes Pd3 and Pd3im were
calculated from the Arrhenius plots as 31.22± 0.3 kJ/mol
and 39.85 ± 0.8 kJ/mol, respectively (Fig. 4). These val-
ues are signiꢀcant in that they show that complex Pd3im,
indeed behaved largely as a heterogeneous catalyst, since
typical heterogeneous catalysts are known to exhibit higher
activation energies than homogeneous systems [65–67].
The results are in good agreement with those reported by
Semagina et al., where they obtained activation energies of
−
1
constant was 1.62 (± 0.10) h , a value which expectedly
−
1
increased dramatically to 3.74 (± 0.37) h at a pressure of
0 bar (Table S2). Similar increase trends were observed for
1
complex Pd3im. Kinetic plots of the observed rate constants
against pressure gave reaction orders with respect to pressure
of 0.28± 0.05 and 0.27± 0.04 for Pd3 and Pd3im respectively,
an indication of similar response to changes in hydrogen pres-
sure for both systems (Fig. S10). The lower reaction orders are
indicative of the formation of the monohydride compound as
the active intermediate [61, 62]. The overall rate law combin-
ing the substrate, catalyst concentration and hydrogen pressure
for complexes Pd3 and Pd3im can be represented as in Eq. 3.
3
0 kJ/mol and 36 kJ/mol for the homogeneous and supported
palladium catalysts respectively [65].
ꢀ
ꢁ
2.36ꢀPH2ꢁ0.28
1
Rate = styrene [ꢀꢁꢂ]
3.2.4 Investigations of Scope of Substrate
in the Hydrogenation Reactions
ꢀ
ꢁ
2.07ꢀPH2ꢁ0.27
(3)
1
Rate = styrene [ꢀꢁꢂꢃꢄ]
3
.2.4.1 Hydrogenation of Alkenes and Alkynes Complexes
Pd3 and Pd3im were used in the hydrogenation of 1-octyne,
-hexyne, 1-hexene, 1-octene, 1-nonene, 1-decene and phe-
nyl-acetylene to evaluate their viability in a wide range of
1
1
3

Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes…
2
Fig. 4 Arrhenius plot for the determination of activation param-
R =0.864; slope= −4817; Intercept=16 (Pd3im) for the hydrogen-
−1
eters. Values of E =31.22± 0.3 kJ mol (Pd3) and 39.95± 0.8 kJ/
ation of styrene were obtained
a
2
mol (Pd31m); R =0.9806; slope= −3762; Intercept=13 (Pd3),
Table 2 Eꢁect of alkene
−1
Entry
Substrate
k
(h
)
% Alkane
% Isomers
obs
and alkyne substrate on the
hydrogenation reactions
Pd3 Pd3im
Pd3
Pd3im
Pd3
Pd3im
1
2
3
4
5
1-Hexene
1-Octene
Styrene
Phenyl acetylene
1-Hexyne
1.75 (± 0.03)
1.57 (± 0.03)
2.02 (± 0.05)
2.82 (± 0.10)
3.85 (± 0.13)
1.39 (± 0.12)
1.22 (± 0.11)
1.59 (± 0.14)
2.06 (± 0.17)
2.70 (± 0.22)
76
65
>99
91
67
73
63
98
86
67
24
35
–
–
33
27
37
–
–
33
a
Reaction conditions: pressure, 5 bar; time: 1 h; temp 30 °C; solvent, toluene (20 ml); catalyst concentra-
tion, 0.125 mol % (Pd: substrate=1:800)
oleꢀns and alkynes (Table 2). Kinetic plots of In[substrate] /
substrate identity. We observed that terminal alkenes dis-
played considerable isomerization products in addition to
the hydrogenation products (Table 2, entries 1, 2 and 5).
For instance, Pd3 aꢁorded a chemoselectivity of 76% for
n-hexane, and isomerization products, cis-2-hexene (9%)
and trans-2-hexene (15%) in the hydrogenation of 1-hex-
ene (Fig. S11 and S13). More signiꢀcantly, the immobilized
complex Pd3im gave a similar chemo-selectivity of 73% for
n-hexane, indicating that the immobilization did not change
the selectivity of the resultant catalyst (Figs. S12 and S13).
The hydrogenation of phenyl acetylene was found to occur in
two steps; via the formation of styrene (Fig. S14). The ꢀnal
product composition comprised of 91% ethyl benzene and
9% styrene (Fig. S14), consistent with previous results [70].
0
[
substrate] vs time for all the studied substrates also showed
t
pseudo-ꢀrst order kinetics (Figs. S7 and S8). The study
revealed that the nature of substrate had a notable eꢁect
on the catalytic activities of the complexes. For instance,
using complex Pd3im, we observed that an increase in the
oleꢀn chain length from 1-hexene to 1-octene, resulted
−
1
in decreased rate of reaction from 1.39± 0.12 h and
−1
1
.22± 0.11 h respectively (Table 2, entries 1 and 2). This
is expected as increased chain length results in higher steric
hindrance and/or enhanced electron density and have the net
eꢁect of hindering substrate coordination to the metal cen-
tre [68, 69]. This hypothesis is supported by the higher rate
constants observed for the more electron deꢀcient alkyne
substrates (Table 2, entries 4 and 5).
We also studied the selectivity of the hydrogenation
reactions for the various substrates using complexes Pd3
and Pd3im (Figs. S13 and S14). The compositions of the
hydrogenation products were found to be dependent on the
3.2.4.2 Hydrogenation of Substituted Benzenes Using
Complexes Pd3 and Pd3im As highlighted in the introduc-
tion, the development of active and selective catalysts for
the hydrogenation of benzene and its derivatives to more
1
3

S. O. Akiri et al.
Table 3 Hydrogenation of various substituted benzenes using complexes Pd3 and Pd3im
Entry
Substrate
%Convb
TOFc
Major products
Pd3 Pd3im
Pd3 Pd3im Pd3 Pd3im
1
98
87
490 435
2
10
7
50
35
3
93
95
86
85
465 430
475 425
4d
5
93
83
465 415
6
90
88
80
77
450 400
440 385
7e
Reaction conditions: pressure, 5 bar; time: 1 h; temp 30 °C; Solvent, toluene (20 ml); catalyst concentration, 0.2 mol% (Pd/substrate:1:500)
a
%
of substrate converted to total products
b
c
d
−1
TOF (mol sub/mol Pd h
)
Minor products include 4 amino acetophenone (15% and 13%) and 4-nitrobenzylethanol (3% and 2%,) for Pd3 and Pd3im respectively
Minor product is 4 amino benzaldehyde 30% and 26% for Pd3 and Pd3im respectively
useful intermediates is currently attracting signiꢀcant
attention [11, 71–73]. We thus extended the use of palla-
dium complexes Pd3 and Pd3im in the hydrogenation of
a range of substituted benzenes (Table 3 and Figs. S15 and
S16). From Table 3, the complexes formed active catalysts,
with the catalytic activity being largely dependent on the
nature of the substrate. For example, while the hydrogena-
tion of nitrobenzene displayed conversions of 98% (TOF of
immobilized complex Pd3im, though with relatively lower
conversions in comparison to the homogeneous complex
Pd3. The higher reactivity of nitrobenzene is an expected
behaviour, since the nitro-group is more reactive and is
in line with previous results [17]. In addition, hydrogena-
tion of the benzaldehyde gave comparable conversions of
−
1
93% (TOF of 465 h ) to those observed for nitrobenzene.
Interestingly, introduction of electron-donating or electron-
withdrawing groups at the para-position of acetophenone
resulted in enhanced catalytic activities (Table 3, entries 2,
4–6). For example, 4-nitro-acetophenone and 4-methyl-ace-
−1
4
90 h ), acetophenone substrate aꢁorded only 10% con-
−1
version (TOF of 50 h ) within 1 h using complex Pd3
Table 3, entries 1 and 2). Similar trend was observed for the
(
1
3

Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes…
tophenone substrates recorded conversions of 95% and 90%
respectively, compared to 10% posted by acetophenone.
These reactivity trends could be attributed to resonance and
inductive eꢁects and are in good agreement with previous
reports [74–76].
both comparable and diꢁerent to the literature reports in
the hydrogenation reactions of bifunctional benzenes. For
example, Cao et al. observed selective hydrogenation of
4-nitroacetophenone substrate to give only 4-amino-aceto-
phenone using a Pd–MgO complex supported on melamino-
formaldehyde polymer [13]. On the other hand, Hawkins
and Makowski reported similar results to ours, where a mix-
ture of 4-amino-acetophenone (59%) and 1-(4-aminophe-
nyl) ethanol (34%) using Rh/Al O catalyst were observed
With respect to chemo-selectivity, hydrogenation reac-
tions of nitrobenzene (Fig. S15), acetophenone and benza-
ldehyde using Pd3 and Pd3im gave selectivity of > 99%
towards aniline, phenylethanol and phenylmethanol respec-
tively. However, the double-functionalized benzenes gave a
mixture of products (Table 3 and Fig. 5). For instance, from
the kinetics proꢀles (Fig. 5), hydrogenation of 4-nitroace-
tophenone initially formed 4-amino-acetophenone to give
a ꢀnal composition of 82% 4-aminophenylethanol and 15%
2
3
[16]. Interestingly, for 4-nitrobenzaldehyde, relatively lower
selectivity towards 4-(aminophenyl)methanol of 70% and
74% was observed for complexes Pd3 and Pd3im respec-
tively (Table 3, entry 7). The minor product being 4-amino-
benzaldehyde. In both cases, the results support the higher
reactivity observed for nitrobenzene. The homogeneous and
heterogeneous nature of complexes Pd3 and Pd3im was evi-
dent from the smooth and sigmoid curves of their kinetics
proꢀles [43, 44] given in Fig. 5.
4
1
-amino-acetophenone. Similar compositions of 85% and
3% for 4-aminophenylethanol and 4-amino-acetophe-
none were reported for the immobilized complex Pd3im
(
Table 3, entry 4). The observed chemo-selectivities are
Fig. 5 Selectivity/conversion vs. time proꢀle for hydrogenation of
4-nitroacetophenone, 4AACT 4-amino-acetophenone, 4APE 4-ami-
nophenylethanol, 4NE 4-nitroethanol
4
5
-nitroacetophenone using Pd3 (a) and Pd3im (b), H pressure,
2
bar, temperature, 30 °C; solvent, toluene (20 ml) time, 2 h. 4NACT
Fig. 6 A graph showing catalytic activities of complexes Pd1im–Pd4im with % conversions of subsequent cycles in the hydrogenation of sty-
rene (a) and nitrobenzene (b). Reaction conditions: time, 1.5 h; P : 5 bar; temp, 30 °C; solvent, toluene (20 ml)
H2
1
3

S. O. Akiri et al.
3.2.5 Investigation of Recovery and Re‑use
of the Immobilized Catalysts
Another important aspect of this work was to design easily
recoverable and recyclable catalysts in the hydrogenation
reactions. Thus, we investigated the propensity of the immo-
bilized complexes Pd1im–Pd4im to be recycled under opti-
mized conditions (Fig. 6). Signiꢀcantly, all the immobilized
complexes showed appreciable catalytic activities for ꢀve
consecutive cycles (Fig. 6) in the hydrogenation of styrene
and nitrobenzene. For example, complex Pd1im showed
conversions of 98%, 84% and 70% in the ꢀrst, fourth and
the ꢀfth cycles respectively in the hydrogenation of styrene.
The signiꢀcant drop witnessed between the fourth and ꢀfth
cycle showed that the catalyst remains stable within the
fourth runs, but undergo signiꢀcant deactivation thereaf-
ter. Similar trends were observed for the other complexes,
Pd2im–Pd4im (Fig. 6). In order to understand the factors
behind this loss of catalytic activities in the ꢀfth cycle, we
performed hot ꢀltration experiments using complex Pd3im
to establish if leaching might account for the observed loss
of catalytic activities in subsequent cycles. In a typical
experiment, the reaction was run for 30 min to achieve about
Fig. 7 Plot showing the eꢁect of varying number of drops of mercury
in the hydrogenation of styrene using Pd3 and Pd3im
is meant by excess mercury is not clear [43]. Thus we ꢀrst
used three drops of mercury and complex Pd3im and styrene
at the onset of the reaction and observed a drop in percentage
conversion from 83% (1.59± 0.14) to 54% (0.77± 0.04). On
the other hand, the use of seven drops saw the percentage
conversion drop to 31%, while ten drops led to near complete
poisoning of the catalyst achieving trace conversions of 4%
(Fig. 7). Thus in this work, ten drops of mercury could be
regarded as excess. We further studied the impact of adding
mercury at 50% conversion (once the active catalyst species
was generated). Similarly, we recorded complete poisoning
of the pre-formed catalyst (Fig. 7). This showed that addition
of mercury has the eꢁect of stopping the formation of the
active species as well as deactivating the pre-formed active
catalysts. The reduction in catalytic activities of complex
Pd3im upon addition of excess mercury thus implicates the
formation of Pd(0) nanoparticles as the active species [43].
Comparatively, addition of ten drops of mercury to the com-
plex Pd3 resulted in a slight drop in percentage conversion
from 88% (k = 2.02 ± 0.09), to 81% (k = 1.78 ± 0.09),
4
0% percentage conversion, then the reaction mixture was
cooled, ꢀltered and the supernatant liquid introduced into
the reactor and run for a further 1 h. We observed that, there
was no increase in percentage conversion (42%, representing
only 2% increase) indicating absence of leaching. Thus the
reductions in catalytic activities in subsequent runs may be
assigned to physical damage to the catalyst system, consist-
ent with the results we reported in the methoxycarbonylation
reactions [42]. The EDX data (Tables S5 and S6) showed
the existence of Pd in both fresh (1.26 to 16.04%) and used
catalysts (1.18 to 14.15%), and was indicative that the Pd
metal remained immobilized during and after post cataly-
sis experiments. These results also reꢂect those obtained
by Ziccarelli et al., where losses in catalytic activities drops
in the subsequent cycles in the alkoxycarbonylation of aryl
halides were associated with catalyst abrasion [77].
obs
obs
3.2.6 Investigation of the Nature of the Active Species
indicating that complex Pd3 forms mainly homogeneous
active species [45] with possible formation of trace amounts
of active Pd(0) nanoparticles. In order to verify this hypoth-
esis, we centrifuged the catalysis solution of complex Pd3
after 30 min of reaction time, and the supernatant solution
reintroduced into the reactor. The reaction was allowed to
proceed for a further 30 min under hydrogen pressure. Con-
versions of 85%, compared to 88% for the original reaction
(Table 1 entries 3 and 10) was realized. This corroborated
the homogeneous nature of complex Pd3. A similar proce-
dure with Pd3im led to a drastic drop in percentage conver-
sions from 83 to 42% (Table 1 entries 7 and 11), indicative
of the presence of Pd(0) nanoparticles as the main active
in the Hydrogenation Reactions
One of the main objectives of this study was to establish the
real active species of the homogeneous and immobilized Pd3
and Pd3im complexes by employing mercury poisoning [78]
and sub-stoichiometric poisoning agents [43, 79] as depicted
in Figs. 7 and S17. This was to determine the possible for-
mation of nanoparticles in both systems during the reducing
conditions. First, we employed mercury-poisoning experi-
ments by adding excess mercury in the using catalysts Pd3
and Pd3im. It is a common feature to use the term “excess
mercury” in the catalysis community, however, to date, what
1
3

Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes…
intermediates. To further gain insight into the nature of the
active species generated from complexes Pd3 and Pd3im,
we also used sub-stoichiometric poisoning agents PPh3 and
PCy3 (Fig. S17). We observed that addition of 20 and 100%
of PPh3 to the Pd3im complex system led to near complete
poisoning of the active catalyst, recording only 6% and 2%
percentage conversions respectively. On the other hand,
addition of 20 and 100% of the PPh3 to Pd3 catalyst system
resulted in very slight reductions in percentage conversion
from 88 to 83% and 79% respectively (Fig. S17).
5. Zhang Y, Liao S, Xu Y, Yu D (2000) Catalytic selective hydro-
genation of cinnamaldehyde to hydrocinnamaldehyde. Appl Catal
A 192:247–251
6
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van Laren MW, Duin MA, Klerk C, Naglia M, Rogolino D, Pel-
agatti P, Bacchi A, Pelizzi C, Elsevier CJ (2002) Palladium(0)
complexes with unsymmetric bidentate nitrogen ligands for the
stereoselective hydrogenation of 1-phenyl-1-propyne to (Z)-
1
-phenyl-1-propene. Organometallics 21:1546–1553
7
.
Chan K-T, Tsai Y-H, Lin W-S, Wu J-R, Chen S-J, Liao F-X, Hu
C-H, Lee HM (2009) Palladium complexes with carbene and
phosphine ligands: synthesis, structural characterization, and
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Drago D, Pregosin PS (2002) Palladium−Duphos structural
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Bacchi A, Carcelli M, Costa M, Leporati A, Leporati E, Pelagatti
P, Pelizzi C, Pelizzi G (1997) Palladium(II) complexes containing
a P, N chelating ligand Part II. Synthesis and characterisation of
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In summary, we have shown that homogeneous palladium
complexes (Pd1–Pd4) and their immobilized counterparts
(
Pd1im–Pd4im) form active catalysts in the hydrogenation
of alkenes, alkynes, and functionalized benzenes. In gen-
eral, the homogeneous complexes exhibited higher catalytic
activities than their corresponding immobilized complexes.
The use of multi-functionalized benzenes aꢁorded partial or
complete hydrogenation products, depending on the nature
of the substrate. Kinetics, mercury and sub-stoichiometric
poisoning data supported largely homogeneous nature
of complexes Pd1–Pd4 with formation of traces of Pd(0)
nanoparticles, while the supported Pdim1–Pdim4 formed
mainly active Pd(0) nanoparticles and small amounts of
active homogeneous intermediates. Signiꢀcantly, both the
homogeneous and immobilized catalysts displayed compa-
rable selectivities. The immobilized catalysts could be recy-
cled for ꢀve cycles in the hydrogenation reactions of styrene
and nitrobenzene without signiꢀcant loss of catalytic activity
and selectivity.
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Compliance with Ethical Standards
Conflict of interests The authors declare no conꢂict of interests.
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Aꢀliations
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Saphan O. Akiri · Nondumiso L. Ngcobo · Stephen O. Ojwach
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*
Stephen O. Ojwach
ojwach@ukzn.ac.za
School of Chemistry and Physics, University of KwaZulu-
Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209,
South Africa
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