Catalytic activity of PdCl2 complexes with pyridines in nitrobenzene carbonylation

Article abstract of DOI:10.1016/j.molcata.2011.01.007

Synthesis of square planar palladium(II) complexes of general structure PdCl₂(X_nPy)₂ (where: Py = pyridine; X _nPy = 2-MePy; 3-MePy; 4-MePy; 2,4-Me₂Py; 2,6-Me ₂Py; 2-ClPy; 3-ClPy and 3,5-Cl₂Py) has been performed in order to study activity of these complexes as catalysts of nitrobenzene (NB) carbonylation - a process of industrial importance leading to production of ethyl N-phenylcarbamate (EPC). Electron withdrawing/electron donating properties of X_nPy ligands (described by experimentally determined acidity parameter pK_a) have been correlated with activities of PdCl ₂(X_nPy)₂ complexes during NB carbonylation in presence of catalytic system PdCl₂(X_nPy) ₂/Fe/I₂/X_nPy. We observed that conversions of substrates and yields of EPC increase within increasing basicity of X _nPy ligand (for not sterically hindered X_nPy's). On the basis of current work and our previous studies a detailed mechanism of catalytic carbonylation of NB is proposed.

Full text of DOI:10.1016/j.molcata.2011.01.007

Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic activity of PdCl2 complexes with pyridines in nitrobenzene
carbonylation
Agnieszka Krogul∗, Jadwiga Skupin´ ska, Grzegorz Litwinienko
Faculty of Chemistry, University of Warsaw, 02-093 Warsaw, Pasteura 1, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of square planar palladium(II) complexes of general structure PdCl2(XnPy)2 (where:
Py = pyridine; XnPy = 2-MePy; 3-MePy; 4-MePy; 2,4-Me2Py; 2,6-Me2Py; 2-ClPy; 3-ClPy and 3,5-Cl2Py)
has been performed in order to study activity of these complexes as catalysts of nitrobenzene (NB) car-
bonylation – a process of industrial importance leading to production of ethyl N-phenylcarbamate (EPC).
Electron withdrawing/electron donating properties of XnPy ligands (described by experimentally deter-
mined acidity parameter pKa) have been correlated with activities of PdCl2(XnPy)2 complexes during NB
carbonylation in presence of catalytic system PdCl2(XnPy)2/Fe/I2/XnPy. We observed that conversions of
substrates and yields of EPC increase within increasing basicity of XnPy ligand (for not sterically hin-
dered XnPy’s). On the basis of current work and our previous studies a detailed mechanism of catalytic
carbonylation of NB is proposed.
Received 9 November 2010
Received in revised form 5 January 2011
Accepted 6 January 2011
Available online 15 January 2011
Keywords:
Carbonylation
Reduction of nitrobenzene
Palladium complexes
Carbamates
© 2011 Elsevier B.V. All rights reserved.
Substituted pyridines
1. Introduction
to complexes of other metals [40–43]: cis-[Rh(CO)2(XnPy)2]PF6,
[Rh(COD)(XnPy)2]PF6 and [Ir(COD)(XnPy)2]PF6 where COD is 1,5-
cyclooctadiene and XnPy is pyridine itself or 2-methylpyridine;
3-methylpyridine; 4-methylpyridine; 2,6-dimethylpyridine; 3,5-
dimethylpyridine and those complexes were tested as catalysts in
reduction of nitrobenzene to aniline by CO/H2O. There is a general
agreement that catalytic activity of complexes depends on basicity
of a pyridine ligand [18]. Another important factor is a localization
of methyl group at pyridine ring – a strong connection of catalyst
activity with ligand structure has been reported by Halligudi et al.
[38] during their studies on Pd complexes as active catalysts of car-
bonylation of nitrobenzene and aliphatic amines to ureas in the
presence of FeCl3 or CuCl2 as co-catalysts.
In our previous studies we applied and described
PdCl2/Fe/I2/pyridine system as catalyst for carbonylation of
nitro-compounds and aromatic amines [44–46]. Concentration of
each component, the molar ratio, CO pressure and other parame-
ters were examined in order to explain their influence on the rate,
conversion, and selectivity of nitrobenzene (NB) carbonylation to
ethyl N-phenylcarbamate (EPC) [44,45,47].
In several reports two mechanisms of NB carbonylation were
considered: direct carbonylation or indirect carbonylation of NB
via stage of aniline [29,38,48,49]. One of us previously reported
a detailed study of mechanism via an intermediate stage of ini-
tially formed aniline. The aniline is carbonylated to urea that
subsequently reacts with ethanol yielding EPC and, again, ani-
line [45]. The initial amount of aniline is formed as a product of
nitrobenzene reduction in the presence of traces of water or ethanol
[34,45,50–52].
Over the last few decades square planar palladium(II) complexes
with nitrogen donor ligands have received much attention. These
complexes can be applied as catalysts for the carbonylations [1–5],
polymerizations [6–8], and other reactions in synthetic organic
chemistry [9–12]. Moreover, the cytotoxic activity of palladium
complexes leads to great interest in potential application of these
compounds as anticancer agents [13–17].
The catalytic activity of metal complexes with nitrogen con-
taining ligands depends on electron density of the palladium
atom [18]. Taking into account the steric and electronic effects,
the strategy to design (and to obtain) more active catalysts is
based on incorporation of new, more electron donating ligands
in the palladium complexes. Typical electron donating ligands
contain nitrogen: pyridine, bipyridine, phenanthroline and their
derivatives [18–25], other ligands containing phosphorus are
very common P-donor phoshines [22,23,26]. Palladium(II) com-
plexes with these ligands are used as active catalysts, particularly
in the carbonylation of nitro-compounds [27–31] and amines
[32–36] to form carbamates, isocyanates and ureas, etc. The
application of Pd(II) complexes with pyridine ligands as cata-
lysts in carbonylation of nitro-compounds and amines was a
subject of a few reports only [37–39]. The studies were limited
∗
Corresponding author. Tel.: +48 22 8220211x378; fax: +48 22 8222380.
E-mail address: akrogul@chem.uw.edu.pl (A. Krogul).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.007

10
A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
Overall process of NB carbonylation is described by general
equation:
KOH in water) were subsequently added to 200 ml of substi-
tuted pyridine or pyridine solution at a concentration of 10−4 M
in methanol:water (1:1). A precision pH meter was used with a
combined pH glass electrode calibrated on standards for methanol:
water solution. After each addition of titrant and when pH value
was stable, small 0.5 ml samples of titrated solution were trans-
ferred to quartz cuvettes (optical path 10 mm), and UV–vis spectra
in the range 200–400 nm were recorded. Each time the samples
were returned to the main titrated solution. The pKa’s were cal-
culated by means of DATAN 3.1 (MultiD Analyses). The software
analyzes complete spectra (instead of single analytical wavelength)
and calculates protolytic equilibria as a function of pH and pro-
tolytic constants on the basis of principal component analysis.
PhNO2 + 3CO + EtOH → PhNHC(O)OEt + 2CO2
(1)
However, the presence of catalytic amount of aniline is essential
for NB to be reduced:
2PhNH2 + PhꢀNO2 + 3CO → PhNHC(O)NHPh + PhꢀNH2 + 2CO2 (2)
Diphenylurea reacts with ethanol to yield aniline and ethyl N-
phenylcarbamate:
PhNHC(O)NHPh + EtOH → PhNHC(O)OEt + PhNH2
(3)
Thus, aniline generated in reactions (2) and (3) is, in turn, consumed
in reaction (2). Both reactions are complex, multistage processes.
Reaction (2) represents a sequence of reactions (4)–(6).
2.3. Synthesis of PdCl2(XnPy)2 (compounds I–IX)
[Pd–ON(O)Phꢀ] + CO → [Pd–ONPhꢀ] + CO2
[Pd–ONPhꢀ] + CO → [Pd NPhꢀ] + CO2
[Pd NPhꢀ] + 2H+ → [Pd2+] + PhꢀNH2
(4)
(5)
(6)
Palladium chloride complexes with pyridines were prepared
under argon. A known amount (1.128 mmol) of PdCl2 was placed
in 10 ml flask equipped with magnetic stirrer and 2.26 mmol of Py
or substituted XnPy in 10 ml acetonitrile were added. Reaction was
carried out at room temperature for 24 h. After the reaction was
completed, yellow precipitate was crystallized from acetone.
Elemental analysis of the PdCl2(XnPy)2 complexes was carried
out by a conventional method (see Table S1 in Supporting Material).
Single yellow crystals of PdCl2(2,4-Me2Py) (II), PdCl2(4-MePy)2 (III),
PdCl2(3-MePy)2 (V) and PdCl2(2-ClPy)2 (VIII), obtained by slow
evaporation of their acetone solution, were characterized by X-ray
diffraction measurements [56].
Removal of the first oxygen atom from nitrobenzene can be
assumed to be the rate determining step in the nitrobenzene car-
bonylation over PdCl2/Fe/I2/Py catalyst [46], however, according
to our knowledge, no evidence has been reported about electron
transfer from palladium to nitrobenzene during the reaction cat-
alyzed by palladium complexes with monodentate N-donor ligands
like pyridine.
Our studies have long been concerned with the substituent
effects in the nitrobenzene and aniline ring on the yield of EPC in the
carbonylation of either NB or mixture NB/AN (where AN is aniline),
respectively [46,53]. A presence of an electron-withdrawing sub-
stituent in nitrobenzene ring causes an increase of the nitrobenzene
conversion [46], however, we did not observe any significant effect
of the substituent in the aniline ring on the yield of the substituted
carbamates formed [53]. Our previous studies were limited to PdCl2
and PdCl2Py2 complexes without detailed research on the nature
of a ligand and the structure of a catalyst, therefore, in this work
we try to get knowledge on the role of ligand structure in the cata-
lyst activity in the mechanism of carbonylation of NB. We decided
to investigate the complexes of general structure PdCl2(XnPy)2,
where XnPy = pyridine, 2-methylpyridine; 3-methylpyridine; 4-
methylpyridine; 2,4-dimethylpyridine; 2,6-dimethylpyridine; 2-
chloropyridine; 3-chloropyridine; 3,5-dichloropyridine to get
knowledge on the role of XnPy ligand during the catalysis of NB car-
bonylation. In this report we are trying to investigate substituent
effects of XnPy in PdCl2(XnPy)2 catalyst for both kinds of substrates:
for NB as well as for mixture of NB/AN. The second system has been
chosen in order to eliminate the initial step of aniline formation.
2.4. Synthesis of PdCl2(PhNH2)2
PdCl2Py2 complex (0.56 mmol), aniline (10.8 mmol), pyridine
(1.24 mmol) and ethanol (4 ml) were heated at 180 ◦C for 60 min. On
completion of the reaction golden-yellow flakes of PdCl2(PhNH2)2
complex were obtained. The precipitate was crystallized from ace-
tone, mp = 240–245 ◦C. Anal. Calcd. for PdCl2(PhNH2)2 (%): C, 39.66;
H, 3.86; N, 7.71; Cl, 19.56. Found: C, 39.98; H, 3.97; N, 7.74; Cl, 19.04.
IR (KBr): 3287, 3202, 3117, 1594, 1573, 1111, 756 cm−1
.
2.5. The reaction of PdCl2(PhNH2)2 complex with carbon
monoxide
PdCl2(PhNH2)2 complex (0.7 mmol), in chlorobenzene (10 ml)
was heated at 150 ◦C under CO pressure of 4 MPa for 60 min.
On reaction completion a white product and palladium black
precipitated. On dissolution and crystallization from ethanol,
white needles were identified as diphenylurea using HPLC.
mp = 235–237 ◦C, IR (KBr): 3328, 3198, 1649 cm−1
.
2.6. Carbonylation procedure
2. Experimental
The procedure has been described elsewhere [46]. 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
1:2 (see Eq. (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
2.1. Materials
All operations were carried out using standard Schlenk tech-
niques under oxygen-free and water-free argon. Carbon monoxide
(99.9%), PdCl2, iodine and iron powder were used as received.
Pyridine (Py), substituted pyridines (2-MePy; 3-MePy; 4-MePy;
2,4-Me2Py; 2,6-Me2Py; 2-ClPy; 3-ClPy; 3,5-Cl2Py), nitrobenzene,
aniline and ethanol were distilled (or fractionally distilled) over
CaH2 and stored under argon.
2.2. pKa measurements
Spectrophotometric titrations were performed as described by
Albert and Sergeant [54,55]. Briefly, small volumes of titrant (1 M

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
11
Table 1
Structures, abbreviations and acidity parameters (pKa’s) of nine pyridine ligands XnPy, the symbols of the PdCl2(XnPy)2 complexes.
XnPy
PdCl2(XnPy)2
Symbola
Structure (abbreviation)
pKa
b
c
d
e
6.81–7.06
5.07–6.98
6.63
14.4
6.2f
I
(2,6-Me2Py)
N
6.80
6.05
15.4
14.0
6.05f
5.43
II
(2,4-Me2Py)
N
6.04
III
(4-MePy)
(2-MePy)
(3-MePy)
N
N
N
N
5.76
5.67
5.21
5.97
5.69
5.23
13.5
13.4
12.5
5.38
5.17
4.09
IV
V
VI
(Py)
Cl
Cl
2.74
0.49
–
2.83
0.61
–
10.8
7.1
–
2.6f
VII
VIII
IX
(3-ClPy)
N
N
2.0f
(2-ClPy)
Cl
Cl
1.22f
(3,5-Cl2Py)
N
a
b
c
d
e
f
Symbol of complex formed by ligand with PdCl2.
From Ref. [58].
From Ref. [59].
From Ref. [60].
This work. pKa values (with error 0.05 unit) for ligands measured in methanol/water (50/50), see Section 2.
For these ligands pKa values are determined with error 0.10.
footnotes for Tables 2 and 3). After the reaction was completed, the
autoclave was cooled in a water bath then vented and a liquid sam-
ple of the reaction mixture was analyzed. The yield of the reaction
was calculated on the basis of GC–FID and GC–MS analyses.
the central Pd atom. In order to describe electron donor proper-
ties of ligands we determined experimentally acidity parameter,
pKa. Results compared with other accessible in the literature
[58–60] are collected in Table 1. The basicity of pyridine derivatives
decreases in the following order: 2,6-Me2Py > 2,4-Me2Py > 4-
MePy > 2-MePy > 3-MePy > Py > 3-ClPy > 2-ClPy > 3,5-Cl2Py (as it is
shown in Table 1).
3. Results
3.1. Ligand acidity
3.2. Carbonylation reactions catalyzed by PdCl2(XnPy)2
The presence of substituents in pyridine ring influences the
electron density on nitrogen atom and, thus, the basicity of the
pyridine. The basicity of pyridine ligands is linearly related to
their nucleofilicity [57] and directly affects the electron density on
Palladium chloride complexes with XnPy ligands (where:
Py = pyridine, X = –CH3, –Cl; n = 0–2) were used as cat-
alysts for reductive carbonylation of NB to form ethyl

12
A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
Table 2
Conversion and selectivity of the carbonylation of nitrobenzene in presence of complex PdCl2(XnPy)2 as catalyst (X = –CH3, –Cl; n = 0–2)a.
No.
Catalyst
PdCl2(XnPy)2
Conversion NB
(mmol)
Yield EPC
(mmol)
Yield AN
(mmol)
Selectivityb EPC
(%)
TOFc
(mmol mmol−1 h−1
)
I
II
PdCl2(2,6-Me2Py)2
PdCl2(2,4-Me2Py)2
PdCl2(4-MePy)2
PdCl2(2-MePy)2
PdCl2(3-MePy)2
PdCl2Py2
PdCl2(3-ClPy)2
PdCl2(2-ClPy)2
PdCl2(3,5-Cl2Py)2
24
41
54
39
45
40
33
28
30
15
36
48
32
39
34
26
19
23
1
5
6
5
6
6
5
5
4
63
88
89
82
87
85
79
68
77
134
321
429
286
348
304
232
170
205
III
IV
V
VI
VII
VIII
IX
a
Reaction conditions: PdCl2(XnPy)2/Fe/I2/XnPy = 0.056/2.68/0.12/6.2 mmol; NB = 81 mmol; ethanol = 20 ml; T = 180 ◦C; p(CO) = 4 MPa; time = 120 min. Py = pyridine;
(X = –CH3, –Cl; n = 0–2); AN = aniline; NB = nitrobenzene; EPC = ethyl N-phenylcarbamate.
b
Selectivity of EPC expressed as mmol EPC × (mmol converted NB)−1
.
c
Turnover frequency expressed in (mmol EPC × (mmol Pd)−1 × h−1).
N-phenylcarbamate, Eq. (1). Conversion of NB, yield of EPC
and selectivity of catalysts presented in Table 2 indicates that
for PdCl2(XnPy)2 catalysts the conversion of NB increases
in the range: chloropyridines < unsubtituted pyridine ≈ 2-
methylpyridine < methylpyridines and the observed tendency
is in agreement with increasing basicities of XnPy ligands (the
lower pKa of protonated ligand the lower conversion) with one
exception, for 2,6-Me2Py ligand, assigned to steric crowding
caused by two ortho-methyl groups (see below). Despite EPC,
aniline was detected as a by-product of the reaction (see Table 2).
Also found among the reaction products were 2-methylquinoline
(up to 3%), diphenylurea, diphenyldiazene, diphenyldiazene oxide
below 1% and diethyl carbonate in trace amounts.
version of AN decreases within decreasing XnPy ligand basicity
(the lower pKa of protonated ligand the lower conversion), in
similar way as it was observed for carbonylation of NB with-
out initially added AN. We also monitored the conversion of NB
in the systems containing a mixture NB/AN as starting mate-
rial. The data collected in Table 3 shows similar correlation
between basicity of used XnPy ligand and conversion of NB,
as for conversion of aniline. As it was observed for carbony-
lation of NB, 2-methylquinoline, diphenylurea, diphenyldiazene,
diphenyldiazene oxide and diethyl carbonate were identified as by-
products of the NB/AN carbonylation. Additionally, N-ethylaniline
and N-phenylformamide were found among the products of the
reaction.
Among all investigated complexes, compound III (with 4-
methylpyridine) was the most active catalyst (TOF = 429), and for
this complex the conversion of NB reached 54 mmol, the yield
of ethyl N-phenylcarbamate was 48 mmol and selectivity 89%.
Also high conversion, yield, and selectivity were observed for
PdCl2(3-MePy)2 catalyst (see Table 2). The results obtained for
other complexes with electron withdrawing XnPy ligands are much
lower – for chloropyridines, the conversions and yields are almost
half of the values obtained for III (see Table 2). The decrease of the
catalyst activity is accompanied by its declining selectivity. When
complexes with ortho-substituted pyridines: PdCl2(2,6-Me2Py)2,
PdCl2(2,4-Me2Py)2, PdCl2(2-MePy)2, were used, the lower conver-
sions of nitrobenzene and lower yields of EPC were obtained than it
would be expected according to basicities of XnPy ligands (Table 2,
compounds I, II, IV). The lowest conversion (24 mmol), EPC yield
(15 mmol) and selectivity (63%) were observed for the complex
with 2,6-dimethylpyridine.
4. Discussion
Our former research concerned the role of each component of
the PdCl2/Fe/I2/Py catalytic system as well as activity and selec-
tivity in carbonylation of NB to ethyl N-phenylcarbamate [44,47].
We investigated the substituent effect in NB and AN rings on the
reductive carbonylation of NB [46] and oxidative carbonylation of
AN [53], respectively. In order to understand the mechanism a
detailed explanation of each step of catalytic process of carbony-
lation is necessary and the purpose of this work was to study the
effects of structural and electronic changes in the pyridine ligand on
the activity of PdCl2(XnPy)2 complexes as catalyst of NB carbony-
lation. The pyridine derivatives were modified by incorporation
of electron-withdrawing or electron-donating substituents. Due to
some limitations (X should not undergo the carbonylation reac-
tion), we excluded NH2 and NO2 groups from our considerations,
and X is chlorine atom or methyl group.
Results shown in Table 3 refer to carbonylation of NB with
initially added AN. For not sterically hindered catalysts the con-
Table 3
Conversion and selectivity of the carbonylation of nitrobenzene and aniline in presence of PdCl2(XnPy)2 as catalyst (X = –CH3, –Cl; n = 0–2)a.
No
Catalyst
PdCl2(X2Py)2
Conversion ANb
(mmol)
Conversion NB
(mmol)
Yield EPC
(mmol)
TOFc
(mmol mmol−1 h−1
)
I
II
PdCl2(2,6-Me2Py)2
PdCl2(2,4-Me2Py)2
PdCl2(4-MePy)2
PdCl2(2-MePy)2
PdCl2(3-MePy)2
PdCl2Py2
PdCl2(3-ClPy)2
PdCl2(2-ClPy)2
PdCl2(3,5-Cl2Py)2
19
25
35
24
27
25
23
19
20
8
15
23
13
20
18
16
9
18
23
27
23
26
24
22
18
19
321
411
482
411
464
429
392
321
339
III
IV
V
VI
VII
VIII
IX
12
a
Reaction conditions: PdCl2(XnPy)2/Fe/I2/XnPy = 0.056/2.68/0.12/6.2 mmol; NB = 27 mmol; AN = 54 mmol; ethanol = 20 ml; T = 180 ◦C; p(CO) = 4 MPa; time = 60 min.
Py = pyridine; (X = –CH3, –Cl; n = 0–2); AN = aniline; NB = nitrobenzene; EPC = Ethyl N-phenylcarbamate.
b
Amount of aniline is a result of calculation made according to the following equation: amount of AN in the reaction mixture = unreacted aniline in reaction (2) + amount
of AN formed in the NB reduction – AN consumed in the EPC formation.
c
Term and units explained in footnote “c” in Table 2.

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
13
4.1. pKa values
1.7
4Me
A
1.65
Basicity of ligands directly affects the electron density on the
central Pd atom, therefore, we decided to study how electron-
withdrawing (EW) and electron-donating (ED) substituents X in
XnPy ligands are connected with catalytic activity of PdCl2(XnPy)2
complexes. We considered acidity (pKa defined for deprotonation
equilibria of a pyridinium salt: [XnPyH]+ ꢀ XnPy + H+) as reasonable
and convenient parameter to get knowledge on EW/ED properties
of XnPy. Basicity of a pyridine nitrogen (or acidity of protonated
pyridines) was studied by several authors, however, the litera-
ture data are not consistent because of a number of methods
and conditions employed. According to our literature search, it
was not possible to find pKa values measured by one method for
the whole series of XnPy compounds used in our work (where
X = –Cl and –CH3). Thus, it was necessary to re-measure acidi-
ties for all the compounds. Since the pKa values measured in
non-aqueous solutions are sometimes questionable, we decided to
range the acidities of XnPy ligands in 1:1 water/methanol (v/v) solu-
tion by spectrophotometric titration method [54,55], which can
be successfully used for compounds of low solubility and having
extremely low or extremely high pKa’s. The measured values for
nine XnPy ligands are collected in Table 1, listed together with liter-
ature data. The presented parameters are in rather good agreement
with the previously published ones, however, for 2-chloro- and
3,5-dichloropyridine the less satisfying agreement was obtained,
perhaps because of their very weak basicity. Strong acidity of their
conjugated acids (the extremely low pKa) might be a reason for sig-
nificant differences observed in literature data. Our experimental
pKa’s for 2-chloro- and 3,5-dichloropyridine are also obtained with
larger errors, however, values for both pyridines (as well as pKa’s
for other XnPy ligands) perfectly correlate with thermodynamic
pKHB scale for hydrogen bond formation in XnPy· · ·HO–CH3 systems
based on IR measurements of OH stretching published by Berth-
y = -0.6483x + 1.5531
R2 = 0.9742
3Me
1.6
1.55
1.5
H
3Cl
1.45
1.4
1.35
-0.2
-0.1
0
0.1
0.2
0.3
Hammett sigma
1.44
1.42
1.4
3Me
B
4Me
y = -0.2273x + 1.3917
R2 = 0.958
1.38
1.36
1.34
1.32
H
3Cl
-0.2
-0.1
0
0.1
0.2
0.3
Hammett sigma
Fig. 1. Hammett type plots for carbonylation of nitrobenzene (A), and mixture of
nitrobenzene/aniline (B) to carbamates. For reaction conditions see footnotes “a” in
Tables 2 and 3.
elot et al. [61]. Taking into account that both scales (pKa and pKHB
)
are based on EW/ED properties of XnPy ligand, we believe that our
series of pKa values reasonably represent acidities of all nine com-
pounds and can be applied for assessment of EW/ED properties of
XnPy ligand.
line ligands used under carbonylation conditions in presence of
Pd(phenantroline)2[BF4]2 catalysts because of oxidative addition
of halogenated ligand to palladium. We did not observe that reac-
tion in our PdCl2(XnPy)2 complexes. It was expected that increased
nucleophilicity of the metal center would increase the catalytic
activity of the catalyst: the presence of ED groups on the pyridine
ring was supposed to facilitate electron transfer by increasing the
nucleophilicity of Pd, and, contrary, the EW groups should retard
carbonylation. To examine the impact of ED and EW groups we
used a modified Hammett equation [64] connecting the yield of
the carbonylation process with the Hammet ꢀ constants of X in
XnPy ligands:
4.2. Substituent effect on the rate determining step
Previous studies, described in the Introduction, have demon-
strated that carbonylation reaction of NB to EPC is a multistep
reaction, with aniline as essential intermediate. Initial small
amount of AN is formed as a product of hydrogen atom trans-
fer from ethanol to NB and once aniline is generated, it reacts
with nitrobenzene and carbon monoxide to form ethyl N-
phenylcarbamate with AN being recycled (general reactions (2) and
(3)). We showed that in the PdCl2/Fe/I2/Py system invented in our
laboratory [45] a palladium complex PdCl2Py2 is an active center
of carbonylation and we concluded that the abstraction of the first
oxygen atom from nitrobenzene (reaction (4)) is a rate determin-
ing step [46]. Literature survey indicates that only two catalytic
systems (both based on phenanthroline) have been investigated in
depth [62] and according to our knowledge, no conclusive evidence
of electron transfer from Pd to nitrobenzene during the carbonyla-
tion catalyzed by Pd complexes with monodentate N-donor ligands
has been reported. Therefore, we decided to explore the mechanism
of that reaction. If electron transfer from palladium to NB is a rate
determining step – any change in XnPy ligand nucleophilicity would
be manifested by increase or decrease of catalyst activity. There
are some limitations in X selection (vide supra) and X is limited to
chlorine atom and methyl group. During his research on catalysts
containing phenanthroline derivatives, Ferretti et al. [63] suggested
that halogen atoms are not suitable substituents in phenanthro-
log(yield) = ꢁꢀ + const
(7)
A linear dependence with a good correlation R2 = 0.97 and ꢁ = −0.65
have been obtained for our PdCl2(XnPy)2/Fe/I2/XnPy system with
pyridine and substituted pyridines in carbonylation of nitroben-
zene. Additionally, R2 = 0.96 and ꢁ = −0.23 have been obtained for
NB/AN carbonylation. (Fig. 1).
The steric effects of substituents in position 2- are strong enough
in comparison to inductive effects and for these substituents
the Hammett equation fails. The negative ꢁ-value indicates that
higher electron density on the palladium center in PdCl2(XnPy)2
accelerates the carbonylation. The results confirm our previous
hypothesis that electron transfer is the rate determining step
[46] and are also in agreement with mechanisms of carbonyla-
tion proposed for other catalysts based on Pd and Ru complexes
containing phenanthroline or phosphine as a ligand [48,65–70].
Some of these mechanisms have been confirmed by isolation

14
A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
and identification of intermediates. Skoog et al. [67] investigated
carbonylation of nitroarenes catalyzed by Ru(dppe)(CO)3, where
dppe = bis(diphenylphosphino)ethane and they provided kinetic
evidence that the initial deoxygenation proceeds via rate determin-
ing inner-sphere electron transfer. Leconte et al. [65] isolated and
characterized a metallacyclic complex, which appears to be directly
involved in the catalytic process of nitrobenzene carbonylation:
added aniline to omit the first step (i.e. initial formation of aniline).
We observed similar correlation of the carbonylation yield (or con-
version, see Table 3) with ligand basicity in PdCl2(XnPy)2 catalyst
as it was observed for carbonylation of NB without added aniline.
Therefore, in both experiments (with and without initial addition of
AN) the same process is a rate determining step and carbonylation
of NB can be considered as carbonylation of AN with nitrobenzene
being an oxidant, as suggested previously by one of us [46]. How-
ever, for carbonylation of NB/AN mixture (Table 3) the quantities of
EPC are higher than the conversions of NB (see Introduction, reac-
tions (1) and (2)). Thus, there must be another agent responsible for
the oxidation of AN. As we showed earlier [53], molecular iodine (a
catalyst component) can react with aniline in the presence of the
palladium catalyst (Eq. (8)) and that reaction explains why carba-
mate is formed in amount exceeding the NB conversion (reactions
(1) and (2)).
O
Ph
N
N
N
O
Pd
O
Low ꢁ-value indicates that the carbonylation of nitrobenzene
is probably a multistep reaction [64]. In order to find what reac-
tion (initial formation of AN or electron transfer from Pd to NB) is
a rate determining step we decided to carry out experiments with
PhNH2 + I2 + CO + C2H5OH → PhNHC(O)OC2H5 + 2HI
(8)
(XnPy)2PdCl
2 + 2PhNH2
(9)
PhNHCOOEt + PhNH2
(PhNH2)2PdCl2
(18)
PhNH
CO
2XnPy
EtOH
(10)
2HCl+(PhNH)2CO
Ph'NO2
O
PhNH
Pdo(XnPy)2
CO
(13)
O
C
O
PhNH
PhNH
P2d+(XnPy)2
O
2+
2Pd
(XnPy)
N
O
Ph'
(14)
CO
(17)
CO2
CO
2+
(PhNH)2Pd(XnPy)2
Ph'
N
2+
(XnPy)2Pd
O
Ph'NH2
2PhNH2
(16)
C
(15)
CO2
2+
Pd
O
(XnPy)2
NPh'
Ph =Ph' = C6H5
Scheme 1. Summaric scheme of the carbonylation reaction of nitrobenzene with carbon monoxide. The numbers in parentheses refer to the reactions described in the text.
Symbols Phꢀ and Ph (both mean C6H5) were introduced to distinguish phenyl group from nitrobenzene (designed as PhꢀNO2) and from aniline (PhNH2). However, there is no
chemical difference between Phꢀ and Ph and first molecules of aniline are formed from nitrobenzene. Previous experiment [45] with 3-chloroaniline allowed us to distinguish
Ph and Phꢀ for low conversions, because 3,3ꢀ-dichloro-N,Nꢀ-diphenylurea was detected.

A. Krogul et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 9–16
15
Results obtained for carbonylation of NB (Table 2) and for car-
bonylation of NB/AN mixtures (collected in Table 3) show that for
catalysts containing ligands with orto-methyl group, i.e. I, II and
IV, the conversion of NB is significantly smaller. These observa-
tions support the hypothesis, that during catalysis a molecule of
NB is coordinated to Pd and this oxidative addition process is rate
determining step.
At the same time the nitrene accepts two hydrogen atoms, one
H atom from each aniline and new aniline molecule is formed
(Scheme 1, reaction (16)).
Two molecules of AN coordinated to Pd2+ react with carbon
monoxide forming diphenylurea as leaving molecule (Scheme 1,
reaction (17)) and the complex Pd0 (XnPy)2 is recovered being
able to another cycle of NB reduction, whereas diphenylurea reacts
with ethanol to form ethyl N-phenylcarbamate (Scheme 1, reaction
(18)).
4.3. Mechanism of the process
Our previous studies on carbonylation [44,45] taken together
with the results of this work allow to propose a detailed mechanism
of the catalytic cycle of NB carbonylation and the proposed mecha-
nism includes the changes in the oxidation state of central Pd atom.
Catalytic amount of aniline generated by reaction of ethanol with
NB (in the presence of PdCl2(XnPy)2/Fe/I2/XnPy system) [34,50], ini-
tiates a series of carbonylations. Two molecules of AN react with
PdCl2(XnPy)2 to form PdCl2(PhNH2)2, reaction (9).
5. Conclusions
We synthesized a series of palladium chloride complexes with
XnPy2 ligands (where: Py = pyridine, X = −CH3, −Cl; n = 0–2). EW
and ED properties of these XnPy ligands have been character-
ized on the basis of their experimental pKa values measured by
spectrophotometric titration. The synthesized PdCl2(XnPy)2 com-
plexes have been used in catalytic system PdCl2(XnPy)2/Fe/I2/XnPy
for carbonylation of nitrobenzene and nitrobenzene/aniline mix-
ture. In both reactions the conversion of nitrobenzene and aniline
and yield of ethyl N-phenylcarbamate were monitored in order
to investigate the impact of substituent effects in XnPy ligands
on the catalytic activity of PdCl2(XnPy)2 catalyst. We found that
activity of PdCl2(XnPy)2 is correlated with the basicity of XnPy
derivatives: in both carbonylations (NB alone or NB/AN mixtures)
for more basic XnPy ligands the higher conversion of NB and
yield of ethyl N-phenylcarbamate was observed. Catalytic car-
bonylation can be described as a cyclic sequence of reactions
represented in Scheme 1. The process is initiated by reaction of
PdCl2(XnPy)2 catalyst with aniline replacing XnPy ligands to form
PdCl2(PhNH2)2 that reacts with CO and XnPy to give diphenyl urea
and Pd0(XnPy)2. The Pd0(XnPy)2 complex starts the catalytic cycle
by reacting with nitrobenzene molecule and CO. Rearrangements
in cyclic intermediates (Scheme 1, steps 14–15) lead to formation
of palladium–nitrene complexes consecutively reacting with two
molecules of aniline (steps 16). After addition of a molecule of
carbon monoxide (Scheme 1, step 17), the complex decomposes
with diphenyl urea reacting immediately with ethanol to produce
aniline and ethyl N-phenylcarbamate. At this step the complex
Pd0(XnPy)2 is recovered and able to start next catalytic cycle.
PdCl2(XnPy2)2 + 2PhNH2 → PdCl2(PhNH2)2 + 2XnPy
(9)
The presence of PdCl2(PhNH2)2 complex has been detected and
characterized by us during the reaction carried out with no carbon
monoxide at 180 ◦C (see Section 2.4).
The aniline ligands react with CO to form urea (reaction (10))
and in this step a central atom is reduced Pd2+ → Pd0.
PdCl2(PhNH2)2 + CO+2XnPy→(PhNH)2CO+Pd0(XnPy)2 + 2HCl
(10)
One of the proofs of the Pd0 presence in the system is partial pre-
cipitation of inactive Pdblack as reported by Ragaini and Cenini [5]
(see also Section 2.5). The transformation of oxidation state of Pd
taken together with a change of chemical nature of a new ligand
(aniline → urea) generates weak complex and urea is immediately
replaced by XnPy ligands forming Pd0(XnPy)2 complex. The role
of XnPy is to prevent the precipitation of Pdblack and methyl pyri-
dine derivatives (stronger bases) are better complexing agents than
chloro- and dichloropyridines (weaker bases). Inactive Pdblack is
recovered to Pd2+ form in the process:
Fe2+ + Pd0 + 5CO → Fe(CO)5 + Pd2+
(11)
where cations Fe2+ are generated from co-catalysts containing Fe
(powder) and I2 [47].
Acknowledgement
Complex Pd0(XnPy)2 formed in reaction (10) reacts with
nitrobenzene and carbon monoxide, thus, central atom Pd0 is oxi-
dized to Pd2+, according to Eq. (12).
A. Krogul tkanks for support from Mazovian Doctoral Scholar-
ship 2009.
PhꢀNO2 + Pd0(XnPy)2 + 2CO → Phꢀ–N Pd2+(XnPy)2 + 2CO2
(12)
Appendix A. Supplementary data
The formation of nitrene intermediate state in the carbonyla-
tion of NB using palladium complex with 1,10-phenanthroline was
also proposed by Halligudi et al. [48]. In fact, Eq. (12) can be consid-
ered as a sequence of three reactions including cyclic five and four
membered rings (Scheme 1, reaction (13)–(15)).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.01.007.
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The reduction of nitrobenzene to nitrene state requires six elec-
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as additional ligand (reaction (12)). When Pd0(XnPy)2 complex
contains ortho-methylated pyridines, the steric crowd around the
central Pd0 atom is too big to make an access for a nitrobenzene
molecule.
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