Tuning of the catalytic properties of PdCl2(X nPy)2 complexes by variation of the basicity of aromatic ligands

Article abstract of DOI:10.1016/j.molcata.2014.01.020

The position and number of substituents in pyridine ligands (X _nPy) were correlated with structural, physical, and chemical properties of PdCl₂(X_nPy)₂ complexes applied as catalysts for the carbonylation of aromatic nitrocompounds (phosgene-free method of carbamates production). Thermal stability and catalytic activity of PdCl₂(X_nPy)₂ complexes without steric hindrance increases with increasing X_nPy's basicity whereas a decrease of thermal stability and catalytic activity of the complexes is observed for sterically crowded complexes (with the ortho-substituted X_nPy). The complexes with X = Cl in meta- position of X_nPy decompose to a mixture of PdCl₂ and metallic Pd (similarly to complexes with Me _nPy) whereas complexes with ortho-chlorine (in X_nPy) decompose to the organopalladium products. Therefore, two different mechanisms of thermal decomposition are proposed for PdCl₂(Cl _nPy)₂ and PdCl₂(Me_nPy)₂. The results of complex thermal and structural analysis of a series of PdCl ₂(X_nPy)₂ complexes allow us to get insight into the mechanism of nitrobenzene (NB) carbonylation catalyzed by PdCl ₂(X_nPy)₂ at 150-180 °C. We conclude that the electron transfer from Pd(0) to nitrobenzene is the rate determining step of catalytic cycle of NB carbonylation.

Full text of DOI:10.1016/j.molcata.2014.01.020

Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Tuning of the catalytic properties of PdCl2(XnPy)2 complexes by
variation of the basicity of aromatic ligands
Agnieszka Krogul∗, Jadwiga Skupin´ ska, Grzegorz Litwinienko
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The position and number of substituents in pyridine ligands (XnPy) were correlated with structural,
physical, and chemical properties of PdCl2(XnPy)2 complexes applied as catalysts for the carbonylation
of aromatic nitrocompounds (phosgene-free method of carbamates production). Thermal stability and
catalytic activity of PdCl2(XnPy)2 complexes without steric hindrance increases with increasing XnPy’s
basicity whereas a decrease of thermal stability and catalytic activity of the complexes is observed for ste-
rically crowded complexes (with the ortho-substituted XnPy). The complexes with X = Cl in meta- position
of XnPy decompose to a mixture of PdCl2 and metallic Pd (similarly to complexes with MenPy) whereas
complexes with ortho-chlorine (in XnPy) decompose to the organopalladium products. Therefore, two
different mechanisms of thermal decomposition are proposed for PdCl2(ClnPy)2 and PdCl2(MenPy)2. The
results of complex thermal and structural analysis of a series of PdCl2(XnPy)2 complexes allow us to get
insight into the mechanism of nitrobenzene (NB) carbonylation catalyzed by PdCl2(XnPy)2 at 150–180 ◦C.
We conclude that the electron transfer from Pd(0) to nitrobenzene is the rate determining step of catalytic
cycle of NB carbonylation.
Received 13 November 2013
Received in revised form 22 January 2014
Accepted 26 January 2014
Available online 4 February 2014
Keywords:
Palladium complexes
Substituted pyridines
Thermal analysis
Crystal structure
Carbonylation
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
involves CO (without the step of phosgene synthesis from CO and
Cl2) giving an additional reduction of the costs [3–6].
Polyurethanes belong to the most common polymers and their
global production is still increasing, from 13.65 mln tonnes in
2010 to 17.95 mln tonnes expected by 2016 (with market value
$55,480 mln) [1]. The polyurethanes are produced from methylene
diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). Cur-
rently, the main method of industrial synthesis of isocyanates is
a process employing phosgene – a toxic and relatively expensive
gas produced from carbon monoxide and chlorine. The phosgene
employing method is a potential source of species depleting the
ozone layer and, therefore, needs to be eliminated [2]. The efforts
to develop environmental friendly methods have been directed
toward phosgene-free processes and one of the most promising
method is the carbonylation of aromatic nitrocompounds catalyzed
by transition metal complexes, leading to production of isocyanates
and their precursors–carbamates (Scheme 1).
There is a great interest in studies of more effective catalysts and
Pd complexes receive much attention because of their relatively
low cost (comparing to Rh and Ru compounds) and high effective-
ness [7–15]. Recently, our research group have reported the system
based on PdCl2(XnPy)2 complexes (where: Py = pyridine, X = Me or
Cl, n = 0–2) as the effective catalyst for carbonylation of nitroben-
zene (NB) to ethyl N-phenylcarbamate (EPC) [16]. The current work
aims to connect physical/chemical properties of PdCl2(XnPy)2 with
their catalytic activity and this is an extention of the research
on Pd(II) complexes with substituted pyridines and their toxicity
[17,18]. We also proposed the mechanism of carbonylation of NB
with electron transfer from the palladium atom to nitrobenzene as
the rate determining step as well as we determined spectral prop-
erties of PdCl2(XnPy)2 in solution to confirm their square planar
coordination geometry [16,17].
Although the yield and costs of catalytic carbonylation still
cannot compete with the phosgene method, the carbonylation
of nitro-compounds has additional advantage because it directly
Carbonylation of NB to ethyl N-phenylcarbamate is carried out
at relatively high-temperatures (150◦–180 ◦C) and thermal stability
of a catalyst is of crucial importance for its activity. Therefore, from
the engineering point of view it is necessary to correlate the phys-
ical properties (particularly thermal stability) of Pd(II) complexes
applied as catalysts with their catalytic activity in carbonylation.
Thermal analysis (DSC and TG) seem to be the most appropriate for
such studies of PdCl2(XnPy)2.
∗
Corresponding author. Tel.: +48 22 8220211x378; fax: +48 22 8225996.
E-mail address: akrogul@chem.uw.edu.pl (A. Krogul).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.020

142
A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
2.3. X-ray structure determinations
All measurements of crystal structure were performed on
a KM4CCD ꢀ-axis diffractometer with graphite-monochromated
MoK␣ radiation. The crystals were positioned at 62 mm from
the CCD camera. 1600 frames were measured at 0.5◦ intervals
with a counting time of 11 s. The data were corrected for Lorentz
and polarization effects. Empirical correction for absorption was
applied [22]. Data reduction and analysis were carried out with
the Oxford Diffraction programs [23]. The structures were solved
by direct methods [24] and refined using SHELXL [25]. The refine-
ment was based on F2 for all reflections except those with very
negative F2. Weighted R factors wR and all goodness-of-fit S val-
ues are based on F2. Conventional R factors are based on F with
F set to zero for negative F2. The F02 > 2ꢁ(F02) criterion was used
only for calculating R factors and is not relevant to the choice of
reflections for the refinement. The R factors based on F2 are about
twice as large as those based on F. All hydrogen atoms were located
geometrically and their positions were refined. Temperature fac-
tors for some hydrogen atoms were fixed. Scattering factors were
taken from Tables 6.1.1.4 and 4.2.4.2 [26]. The crystallographic data
for the complexes are summarized in Table S1 (in Supplementary
Material).
Scheme 1. Products of carbonylation of nitrobenzene in the presence of CO.
There are many examples of studies that deal with substituent
effect in aromatic ring on properties of transtion metal (i.e. Fe, Cu)
complexes with pyridine ligands [19]. Knowledge on the relation-
ship between thermal stability of palladium(II) complexes and their
catalytic activity will be also helpful in elucidation of mechanism
of NB carbonylation in more details. Catalytic carbonylation can
be described as a cyclic sequence of steps initiated by reaction of
PdCl2(XnPy)2 catalyst with aniline replacing the XnPy ligands to
form PdCl2(PhNH2)2 that reacts with CO and XnPy to give dipheny-
lurea and Pd0(XnPy)2 [16]. Subsequently, Pd0(XnPy)2 complex
reacts with nitrobenzene molecule and with CO. Rearrangements
in cyclic intermediates lead to the formation of palladium–nitrene
complex consecutively reacting with aniline. It is difficult to clearly
distinguish whether the carbonylation occurs by a direct attack of
aniline on the already coordinated molecule of CO or by a coordi-
nation of aniline to the metal, followed by attack of aniline on the
adjacent coordinated CO. According to the literature both mecha-
nisms are possible [4,20,21]. On the basis of calculation made by
Hong et al. and on the basis of our own studies we suggest that
both molecules, aniline and CO, are coordinated to Pd and then
react [21]. After addition of another molecule of carbon monoxide,
the complex decomposes with diphenylurea reacting immediately
with ethanol to produce aniline and ethyl N-phenylcarbamate and
the complex Pd0(XnPy)2 is recovered and able to start next catalytic
cycle [16]. All these processes form a catalytic cycle but at first, a
replacement of XnPy in PdCl2(XnPy)2 complex by aniline initiates a
series of carbonylations.
2.4. Thermal analysis, XRD and SEM measurements
Thermal stability of PdCl2(XnPy)2 complexes was measured by
Differential Scanning Calorimeter DSC 910 Du Pont Instruments
(USA) connected to the thermal analyzer 9900 Computer/Thermal
Analyzer and GBIP interface. The termogravimetric (TG) measure-
ments were performed by means of termogravimeter Du Pont TGA
951 also connected to a thermal analyzer 9900 Computer/Thermal
Analyzer and GBIP interface. All DSC and TG curves were recorded
under nitrogen flow (6 dm3/h), heating rate = 10 K/min. Thermal
stability of the complexes was determined in temperature range
80–400 ◦C and mass of each sample was about 4 mg. Weight loss
during thermal decomposition of PdCl2(XnPy)2 was determined in
temperature range 80–600 ◦C. TG measurements were performed
in platinum cells and the average weight of sample was about
5–8 mg. Each result presented in this paper is the arithmetic mean
of 3 repetitions and the difference of results in a series of deter-
minations of the sample is up to 2%. Analysis of the products of
thermal decomposition of PdCl2(XnPy)2 complexes was carried out
using powder diffractometer, working with copper lamp (radiation
length 1.54) with a snap allowing to measure in the temperature
range from 180 to 350 ◦C. The camera was equipped with a table for
reflectometric measurements. Scanning electron microscope (SEM,
model LEO 435 VP, Zeiss) equipped with an energy dispersive spec-
troscopy (EDS, Roentec) was used to examine the composition of
the residue obtained after thermal decomposition of PdCl2(XnPy)2
complexes.
In this report we present thermal data for
a series of
PdCl2(XnPy)2 complexes in order to correlate them with their cat-
alytic activity. We hope the obtained data will be helpful during
elucidation of detailed mechanism of the carbonylation process and
during design and development of new Pd(II)-based catalysts.
2. Experimental
2.1. Materials
PdCl2 was used as received. Pyridine (Py), substituted pyridines
(2-MePy; 3-MePy; 4-MePy; 2,6-Me2Py; 2,4-Me2Py; 3,5-Me2Py; 2-
ClPy, 3-ClPy; 2,4-Cl2Py), acetonitrile and acetone were distilled
(or fractionally distilled) over the drying agent and stored under
argon. Substituted pyridines: 2,6-Cl2Py and 3,5-Cl2Py were used as
received.
2.5. Carbonylation procedure
2.2. Synthesis of PdCl2(XnPy)2 (compounds I–XII)
The procedure has been described elsewhere [16]. Briefly, the
reaction was carried out in a 200 mL stainless-steel autoclave
equipped with magnetic stirrer. Before experiment, the autoclave
was heated at 120 ◦C for 3 h and cooled down to room temperature.
Subsequently, 0.056 mmol of catalyst PdCl2(XnPy)2 and 2.68 mmol
of Fe powder were placed in the autoclave, the air was evacuated
and the system was filled with purified argon. Then, under an argon
stream, other reagents and solvents were added: 0.12 mmol of I2,
6.2 mmol of Py or XnPy, 81 mmol of nitrobenzene or 27 mmol of
nitrobenzene and 54 mmol of aniline; 20 ml ethanol (solvent). The
amounts of nitrobenzene and aniline (27 and 54 mmol) are related
to carbonylation of mixture of NB/AN in stoichiometric molar ratio
The procedure has been described elsewhere [16]. Palladium
chloride complexes with pyridines were prepared under argon.
PdCl2 (1.128 mmol) was placed in 10 mL flask equipped with mag-
netic stirrer and 2.26 mmol of Py or substituted XnPy in 10 mL
acetonitrile were added. Reaction was carried out at room tem-
perature for 24 h. The elemental analyses of complexes I, II, V,
IX, XI and XII were performed by a conventional method [16].
Single yellow crystals of III, IV and VIII obtained by a slow evap-
oration of their acetone solutions, were characterized by X-ray
diffraction [17] and crystals of VI, VII and X are described in this
work.

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
143
Table 1
Temperatures >of extrapolated start of decomposition Ton and temperatures of maximal heat release Tmax for complexes I–XII together with their pKa.
No.
PdCl2(XnPy)2
Symbol
pKa of XnPya
Ton (◦C)
Tmax (◦C)
1
2
3
4
5
6
7
8
9
PdCl2(2,6-Me2Py)2
PdCl2(2,4-Me2Py)2
PdCl2(3,5-Me2Py)2
PdCl2(4-MePy)2
PdCl2(2-MePy)2
PdCl2(3-MePy)2
PdCl2(Py)2
PdCl2(3-ClPy)2
PdCl2(2-ClPy)2
PdCl2(3,5-Cl2Py)2
PdCl2(2,4-Cl2Py)2
PdCl2(2,6-Cl2Py)2
V
VI
VII
IV
II
6.20
6.10
5.5
5.43
5.38
5.17
4.09
2.6
254(292)b
258
268
291(300b
285
300
252
283
224(260)b
226(248)b
234
247(278)b
235(266)b
256
III
I
IX
VIII
XII
XI
X
232
184
230
181
273
207
276
206
2.0
1.22
1.4
10
11
12
0.97
128
162
a
Basicity of XnPy from Ref. [15].
Two peaks are observed on DSC curves for complexes V, II, III.
b
1:2. The cover was closed and autoclave was directly filled with
carbon monoxide (4 MPa), fixed, placed in a hot oil bath and kept
at 180 ◦C for 1 or 2 h, depending on the reaction rate (see footnotes
for Tables 2 and 3). After the reaction was completed, the auto-
clave was cooled in a water bath, then vented and a liquid sample
of the reaction mixture was analyzed. The yield of the reaction was
calculated on the basis of GC–FID and GC–MS analyses.
curves are observed as one peak as in Fig. 1A, or two, more or less
separated peaks, as for complexes II, III and V, Fig. 1B. For III the
first peak (in DSC curve) is not accompanied by the weight loss
(in TG curve) indicating a phase transition (melting). Peaks for the
complexes II and V overlap with temperature range of the weight
loss (observed in TG curves), thus, the complexes II and V melt
and decompose at the same time. Table 1 presents temperatures
of the extrapolated start of decomposition Ton and temperatures of
maximal heat release Tmax for complexes I–XII.
3. Results and discussion
The TG curves for PdCl2(XnPy)2 complexes show one or two
decomposition stages with the weight loss of 40.9–63.8%. Table 2
presents the data for complexes I–XII together with calculated
mass losses and the assignments of degrading part of the com-
plex for each decomposition step. The results depend on the nature
of substituent(s) in the pyridine ring as well as on their positions.
Observed differences might be a result of various mechanisms of
decomposition of complexes with methyl- and chloropyridines.
TG curves indicate two-step decomposition of PdCl2(MenPy)2,
except for complexes VI and VII (TG curves have no inflec-
tion, so they correspond to a single-step decomposition). For
all PdCl2(MenPy)2 complexes the experimental weight loss is
slightly larger than the expected for the elimination of two MenPy
molecules and such data are in agreement with the results obtained
by House [27], who suggested (on the basis of TG measurements,
without chemical analysis of the residue) that decomposition of
Thermal stability was determined for twelve PdCl2(XnPy)2 com-
plexes (where: Py = pyridine; X = Me or Cl; n = 0–2) by means
of differential scanning calorimetry (DSC) and thermogravimetry
(TG). All DSC and TG curves were recorded under the same condi-
tions (nitrogen atmosphere, heating rate = 10 K/min). The products
of thermal degradation were analyzed by X-ray diffraction (XRD).
Selected DSC and TG curves are shown in Fig. 1 (curves for other
complexes are included in the Supplementary Material). Data
obtained for all complexes are collected in Table 1 (DSC data) and
Table 2 (TG).
Fig. 1 presents DSC and TG curves for PdCl2(3-MePy)2 (III)
and PdCl2(3-ClPy)2 (IX). Comparing DSC and TG results it can be
distinguished whether the effects observed on DSC curves corre-
spond to a phase transition (mass = const.) or to the decomposition
(heat =/ const.; mass =/ const.). The endothermic effects on DSC
Table 2
TG data: temperature range, experimental weight loss, calculated weight loss and assignments of eliminating part of PdCl2(XnPy)2 for complexes I–XIIa.
No.
1
PdCl2(XnPy)2
Symbol
Temperature range (◦C)
Weight loss exp. (%)
Weight loss calc. (%)
Assignments
V
263–330
263–360
260–368
265–377
258–352
258–435
238–318
238–362
238–336
238–481
250–340
250–465
256–363
26.0
63.8
63.0
61.7
51.5
57.5
49.6
56.4
41.0
51.3
40.0
50.5
59.4
27.4
63.8
63.8
63.8
51.2
61.0
51.2
61.0
25.6
51.2
23.6
47.2
56.2
64.9
17.5
45.6
62.5
15.0
46.3
15.0
46.3
-Me2Py
-2Me2Py; -Cl
–2Me2Py; -Cl
-2Me2Py; -Cl
-2MePy
-2MePy; -Cl
-2MePy
-2MePy; -Cl
-MePy
PdCl2(2,6-Me2Py)2
2
3
4
PdCl2(2,4-Me2Py)2
PdCl2(3,5-Me2Py)2
PdCl2(4-MePy)2
VI
VII
IV
5
6
7
8
9
II
PdCl2(2-MePy)2
PdCl2(3-MePy)2
PdCl2(Py)2
III
I
-2MePy
-Py
-2Py; -Cl
-2ClPy
-2ClPy; -Cl
-2Cl
-2Cl;-ClPy
-2Cl2Py
-2Cl
-2Cl; -Cl2Py
-2Cl
IX
VIII
PdCl2(3-ClPy)2
PdCl2(2-ClPy)2
171–255
171–320
258–341
188–261
188–345
131–204
131–252
24.0
40.9
60.4
27.0
46.8
21.5
41.5
10
11
PdCl2(3,5-Cl2Py)2
PdCl2(2,4-Cl2Py)2
XII
XI
12
X
PdCl2(2,6-Cl2Py)2
-2Cl; -Cl2Py
a
The products of thermal degradation were analyzed by X-ray diffraction (XRD). The presence of metallic palladium was detected in all complexes. For complexes IX and
XII PdCl2 were also detected in the residue.

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A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
-1
TGA
DSC
100
90
80
70
60
50
40
30
TGA
DSC
-1
-2
-3
-4
-5
-6
100
90
80
70
60
50
40
-1,5
-2
-2,5
-3
-3,5
-4
80
180
280
380
480
580
80
180
280
380
480
580
Temperature [°C]
Temperature [°C]
Fig. 1. DSC and TG curves for complexes: PdCl2(3-ClPy)2 (IX) and PdCl2(3-MePy)2 (III).
PdCl2(2,6-Me2Py)2 and PdCl2(3,5-Me2Py)2 gives a mixture of PdCl2
and metallic Pd. Indeed, our XRD analysis of PdCl2(MenPy)2 decom-
position products indicated the presence of metallic palladium,
however, the residue was partially soluble in aqueous HCl giving a
brown solution and it was a proof that other compounds apart from
elemental Pd(0) were present in the products of thermal decom-
position at 600 ◦C. Chlorine in the residue was detected by means
of SEM measurements and we suggest that some polymeric struc-
ture of PdCl2 was obtained, not identified by XRD as any known
polymorph of PdCl2. Summing up, our experiments with HCl, com-
bined with the results of SEM and powder X-ray analysis are in
agreement with the results described by other authors for decom-
position of PdCl2 complexes [28,29], therefore, we can state that
the residue contains also palladium compounds other than Pd(0).
On the basis of literature data as well as the results obtained in
this work we propose that thermal decomposition of PdCl2 com-
plexes with methylpyridines is at least two-step process with initial
loss of two MenPy molecules followed by loss of chloride anion
to form dimeric or polymeric palladium chloride. Decomposition
of the complexes with 3-ClnPy proceeds in a similar way as the
decomposition of PdCl2(MenPy)2 complexes: metallic palladium
and PdCl2 were found by XRD analysis of the residue and the
polymorphic structure of residual PdCl2 differs from a structure
of PdCl2 used in the synthesis. A different decomposition pattern
can be observed for PdCl2(2-ClnPy)2 complexes (containing ortho
substituted 2-ClnPy ligands). Data presented in Table 2 indicate
that the first step corresponds to the elimination of two chlo-
rine atoms followed by decomposition of one molecule of mono-
or dichloropyridine (2-ClnPy), depending on the type of ligand.
We suggest that during the first step a chlorine atom is removed
from PdCl2 and another Cl comes from 2-ClnPy. Removal of two
chlorine atoms from PdCl2 is hardly possible, otherwise the same
mechanism should take place for complexes with 3-ClnPy, but the
data listed in Table 2 show that the next step is the removal of a
Table 3
´
˚
◦
a
Selected bond distances (A) and angles ( ) for complexes V–VII and X .
a
VI. VII. X – this work
V – literature data
Bond lenghts
Pd-N(1)
VI
2.017(2)
VII
2.0176(12)
X
V
2.0322(15)
2.0322(15)
2.3037(5)
2.3037(5)
1.345(2)
1.341(2)
1.345(2)
1.341(2)
2.0410(18)
2.0410(18)
2.3118(7)
2.3118(7)
1.355(3)
1.353(3)
1.355(3)
1.353(3)
Pd-N(1A)
Pd-Cl(1)
2.017(2)
2.2970(7)
2.2970(7)
1.329(3)
1.320(4)
1.329(3)
1.320(4)
2.0176(12)
2.3163(4)
2.3163(4)
1.3456(19)
1.3438(19)
1.3456(19)
1.3438(19)
Pd-Cl(1A)
N(1)-C(1)
N(1)-C(5)
N(1A)-C(1A)
N(1A)-C(5A)
Bond Angles
N(1)-Pd-N(1A)
N(1)-Pd-Cl(1)
N(1)-Pd-Cl(1A)
N(1A)-Pd-Cl(1)
N(1A)-Pd-Cl(1A)
Cl(1)-Pd-Cl(1A)
C(1)-N(1)-Pd
180.00
180.00
89.78(4)
90.22(4)
90.22(4)
89.78(4)
179.999(1)
89.78(4)
90.22(4)
90.22(4)
89.78(4)
176.17(4)
121.45(13)
121.90(13)
121.45(13)
121.90(13)
180.00
89.57(6)
90.43(6)
90.43(6)
89.57(6)
89.69(5)
90.31(5)
90.31(5)
89.69(5)
180.00
118.9(2)
122.8(2)
118.9(2)
122.8(2)
180.00
179.9
120.18(10)
120.42(10)
120.18(10)
120.42(10)
119.87(15)
120.12(15)
119.87(15)
120.12(15)
C(5)-N(1)-Pd
C(1A)-N(1A)-Pd
C(5A)-N(1A)-Pd
a
Schematic picture showing the order of the atoms.

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
145
280
260
240
220
200
180
160
140
120
70
65
60
55
50
y = 1,1235x- 218,55
R² = 0,889
45
40
complexes with
3- and 4-XnPy
complexes with
2-XnPy
35
30
0,5
2,5
4,5
6,5
228
238
248
pKa
T
on
Fig. 2. Plots of temperatures of extrapolated start of thermal decomposition (Ton
of PdCl2(XnPy)2 complexes versus pKa values of XnPy ligands with (ꢀ) and without
(ꢁ) ortho substituents.
)
Fig. 4. Conversion of nitrobenzene (NB conv.) in the presence of PdCl2(XnPy)2 versus
temperatures >of extrapolated start of decomposition (Ton) of >PdCl2(XnPy)2 com-
plexes. >Conversion of nitrobenzene was determined for the reaction conditions
described in the caption to Fig. 3. Ton was determined based on DSC measure-
ments: under nitrogen flow (6 dm3/h), in the temperature range 80–400 ◦C, heating
rate = 10 K/min >and mass of each sample was about 4 mg.
whole 2-ClnPy ligand. Such sequence of decomposition indicates
that there is still one molecule of organic ligand in the final prod-
uct and we suggest the formation of organopalladium compound
with C Pd bond instead of C Cl bond. Structures of mono- and
dinuclear organopalladium compounds are described in the litera-
ture [30–34]. It can be concluded that position of substituent in the
chloropyridine ring has a significant influence on the mechanism
of the decomposition of PdCl2(ClnPy)2, a phenomena not observed
for the complexes with methylpyridines.
As can be seen in Fig. 2 the increase of thermal stability
(expressed as initial temperatures of decomposition, Ton) of com-
plexes PdCl2(XnPy)2 with the increase of pKa of XnPy ligands is
observed. Significantly lower thermal stability of PdCl2(2-XnPy)2
complexes is due to a steric hindrance in the complexes with ortho-
substituted ligands. Such correlation between thermal stability of a
complex and basicity of XnPy ligand is accompanied by other rela-
tionships. The basicity of XnPy ligand is strongly correlated with the
catalytic activity of PdCl2(XnPy)2 during nitrobenzene carbonyla-
tion to ethyl N-phenylcarbamate, see Fig. 3 where the activity of
the complex is expressed as conversion of NB [16].
can be easily predicted that thermal stability of the complex will
be correlated with its activity. Indeed, Fig. 4 demonstrates that
catalytic activity of PdCl2(XnPy)2 complexes increases with increas-
ing thermal stability. Concluding all three Figs. 2–4, the activity of
PdCl2(XnPy)2 depends on basicity and substitution pattern in XnPy
ligands, therefore, the strategies of design of new catalysts should
be based on introduction of more electron-donating substituents
into 3- or 4-position of pyridine ring.
We also investigated the substituent effects on the crystal struc-
tures of PdCl2(XnPy)2 complexes in order to combine the results
with thermal stability and catalytic activity. The crystallographic
data for complexes of Pd(II) with Py and monosubstituted XPy (I–IV,
VIII and IX) as well as for PdCl2(2,6-Me2Py)2 are already known
(structures of III, IV and VIII were characterized by our research
group) [17,35–37], however, it was necessary to solve the crystal
structures of complexes with disubstituted X2Py (VI, VII and X) that
have never been done before. We compared our results with avail-
able data for the whole series of complexes. Single yellow crystals
of VI, VII and X were obtained by slow evaporation of the acetone
solutions. All three crystalline compounds were found to be neutral
mononuclear palladium complexes. The crystallographic data and
details of structure refinements for these new complexes are pre-
´
Taking into account the relationships between pKa and ther-
mal stability (Fig. 2) and between pKa and conversion (Fig. 3) it
70
65
60
55
50
45
40
35
30
˚
sented in Supplementary Material, the selected bond distances (A)
and angles (◦) collected for all complexes V–VII and X are collected
in Table 3.
In complex VI (Fig. 5A) 2,4-dimethylpyridine rings show a
trans disposition of methyl groups in ortho position and this phe-
nomenon is the opposite to the disposition of ortho substituent in
complexes with monosubstituted 2-XPy’s (X = Me or Cl) [17]. The
resulting crystals of PdCl2(2,4-Me2Py)2 belong to monoclinic space
group P 21/c. The angles formed between both 2,4-Me2Py rings
and the coordination plane are 85.05◦. The square planar coordi-
nation geometry is confirmed by angles [N–Pd–Cl] = 89.69(5)◦ and
90.31(5)◦. Crystals of complex VII belong also to monoclinic space
group P 21/c, the crystallographic data for this complex are pre-
sented in Supplementary Material and its structure is shown in
Fig. 5B. The ligand (3,5-dimethylpyridine) ring in VII is not ortho-
gonal to the coordination plane and the dihedral angle between the
coordination plane and the ligand ring plane is 57.05◦. The square
planar coordination geometry is confirmed by [N–Pd–Cl] angles,
89.78(4)◦ and 90.22(4)◦, respectively (see Table 3). In contrast to VI
and VII, the crystals of complex X belong to triclinic space group
0,5
2,5
4,5
6,5
pKa
Fig. 3. Conversion of nitrobenzene (NB conv.) in the presence of PdCl2(XnPy)2
versus pKa of XnPy for complexes without ortho substituted ligands. Con-
version of nitrobenzene was determined for the following reaction condi-
tions: PdCl2(XnPy)2/Fe/I2/XnPy = 0.056/2.68/0.12/6.2 mmol; NB = 81 mmol; solvent
(ethanol) = 20 ml; T = 180 ◦C; p(CO) = 4 MPa; time = 120 min. Py = pyridine (X = CH3,
Cl; n = 0–2).

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A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
Fig. 5. SHELXL93 drawings of the molecular structures of: (A) PdCl2(2,4-Me2Py)2 (complex VI), (B) PdCl2(3,5-Me2Py)2 (complex VII), (C) PdCl2(2,6-Cl2Py)2 (complex X). All
drawings represent 50% probability.
P -1. As it was observed for VI, the aromatic rings in X are also
orthogonal to the coordination plane and dihedral angle between
coordination plane and ring plane is 86.14◦ and angles [N–Pd–Cl]
are 90.22(4) and 89.78(4)◦, respectively.
(with trans disposition of substituents in the XnPy ring) as well as
compounds with 4-Me (IV), 2,6-Me2Py (complex V), 3,5-Me2Py
(complex VII), 2,6-Cl2Py (complex X) and Py itself (I) have a
centrosymmetric structure – a structural feature not observed
for complexes with ortho monosubstituted ligands, i.e. 2-MePy
(complex II) and 2-ClPy (complex VIII).
Our results obtained for VI, VII and X can be combined with the
results for I–V, VIII and IX (accessible in database CSD: H. Allen,
Acta Crystallogr., 2002, B58, 380) [17,33–35]. In the whole series
of complexes the palladium ion coordinates two nitrogen atoms
of pyridine ligands and two chlorine atoms in trans positions,
exhibiting a square planar coordination geometry. We observe
either cis or trans disposition of methyl/chlorine substituents
in pyridine rings. For complexes with ortho monosubstituted
XPy ligands the X is in cis disposition, whereas for sterically
very similar disubstituted 2,4-Me2Py ligands (complex VI) the
ortho methyl groups are in trans position. Trans disposition of
substituents is also observed in complexes with 3-MePy (complex
III) and 3-ClPy (complex IX). Consequently, complexes III, VI, IX
We did not find a direct correlation between ligand basicity
and Pd N bond length, similar in all investigated complexes
´
˚
(2.010–2.043 A). The only observation is that Pd N distances are
slightly longer for complexes with MenPy comparing to complexes
with ClnPy perhaps due to a greater contribution of back-donation
of electrons from central atom into ClnPy than into MenPy, related
to inductive effect of chlorine atoms [38]. A presence of ortho
substituent in pyridine ring is manifested by increasing Pd–N dis-
tance in PdCl2(2-XnPy)2. Except for this “effect of ortho group”, the
introduction of ortho-methyl or ortho-chlorine in the pyridine
ligand causes differences in the Pd
N C bond angles. Another

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 141–148
147
difference between the complexes with ortho substituted pyridines
and the rest of complexes is that the ring plane is almost orthogonal
to the coordination plane in PdCl2(2-XnPy)2 – and this is observed
for complexes with mono- and disubstituted ligands.
Because of low solubility of PdCl2(2,4-Cl2Py)2 (XI) we did not
determine the crystal structure of this compound. We have not
found any literature data on cis/trans disposition of ortho chlorine
atoms in the pyridine rings in XI. However, the comparative ther-
mal analysis might be helpful method of indirect determination of
structural features: the similar kinetic profiles of thermal decompo-
sitions of XI and PdCl2(2-ClPy)2 (complex VIII, with cis disposition,
as well as complex X) support the hypothesis on the cis disposition
of ortho substituents in XI.
The results of thermal analysis of PdCl2(XnPy)2 complexes allow
us to get insight into the mechanism of nitrobenzene (NB) carbony-
lation in the presence of PdCl2(XnPy)2. Complexes with chloro-
and dichloropyridines are thermally less stable than complexes
with methyl- and dimethylpyridines. This observation is in agree-
ment with our previous report that MePy’s are better complexing
agents than ClPy’s [16]. Lability of a ligand has capital meaning
because the initial step during nitrobenzene carbonylation requires
a replacement of XnPy ligands in PdCl2(XnPy)2 complex by ani-
line. The ClnPy ligands can be replaced faster than MenPy ligands
(better complexing agents). If this initial step limited the rate of car-
bonylation, the PdCl2(ClnPy)2 complexes would be more effective
catalysts, however, complexes with ClnPy’s are not better than the
ones with MenPy’s [16]. It means that a cleavage of Pd(II) N(from
XnPy) bond is not the rate determining step (RDS) in the mechanism
of carbonylation. Higher activity observed for more thermally sta-
ble complexes (with methylpyridines) might be caused by stronger
pyridine coordination and these complexes remain catalytically
active for a longer time. It also should not be excluded that lower
catalytic activity of PdCl2(ClnPy)2 catalysts is caused by their lower
thermal stability and different mechanism of thermal decompo-
sition. In our previous paper we reported that another step, the
electron transfer from Pd(0) to nitrobenzene is the RDS [16] and
the complexation of XnPy to Pd(0) seems to be more important
for the reaction rate. In the present manuscript we exluded the
pyridine substitution as the RDS, therefore, this observation is
enforcing our previous hypothesis about nitrobenzene activation
as RDS.
decomposition. Thermal stability and mechanism of degradation
of PdCl2(ClnPy)2 complexes do not depend on the number of sub-
stituents in the pyridine ring but on their position in XnPy: the
complexes with chlorine in meta- position of Py decompose to
mixture of PdCl2 and metallic Pd (similarly to complexes with
MenPy) whereas complexes with ortho-chlorine decompose to
the organopalladium products. Thermal analysis can be helpful
for indirect characterization of crystal structure of non-soluble
PdCl2(XnPy)2.
Finally, we can state that the replacement of XnPy ligands in
PdCl2(XnPy)2 complex by aniline (the initial step of NB carbony-
lation process) is not a rate determining step. These results are
consistent with the proposed elsewhere mechanism of catalytic
carbonylation of nitrobenzene with the activation of nitrobenzene
as the RDS.
Acknowledgement
This project was funded by the Ministry of Science and Higher
Education (research project no. IP2011027071).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.2014.
01.020.
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