Pd-Cl cleavage of dichloro-[1-alkyl-2-(naphthylazo)imidazole]palladium(II) complexes by picolinic acid: Kinetic and mechanistic studies

Article abstract of DOI:10.1016/j.poly.2007.06.016

Picolinic acid (picH) reacts with [Pd(α-/β-NaiR)Cl₂] [α-/β-NaiR = 1-alkyl-2-(naphthyl-α-/β-azo)imidazole] in acetonitrile (MeCN) medium to give [Pd(α-/β-NaiR)(pic)](ClO₄). The products are characterized by spectroscopic techniques (F

Full text of DOI:10.1016/j.poly.2007.06.016

Polyhedron 26 (2007) 4841–4848
www.elsevier.com/locate/poly
Pd–Cl cleavage
of dichloro-[1-alkyl-2-(naphthylazo)imidazole]palladium(II)
complexes by picolinic acid: Kinetic and mechanistic studies
*
Pradip Kumar Ghosh, Sushanta Saha, Ambikesh Mahapatra
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Received 18 May 2007; accepted 15 June 2007
Available online 28 June 2007
Abstract
Picolinic acid (picH) reacts with [Pd(a-/b-NaiR)Cl2] [a-/b-NaiR = 1-alkyl-2-(naphthyl-a-/b-azo)imidazole] in acetonitrile (MeCN)
medium to give [Pd(a-/b-NaiR)(pic)](ClO4). The products are characterized by spectroscopic techniques (FT-IR, UV–Vis, NMR).
The reaction kinetics show first order dependence of rate on each of the concentration of Pd(II) complex and picH. Addition of LiCl
to the reaction decreases the rate of reaction and has proved the cleavage of Pd–Cl bond at the rate-determining step. Thermodynamic
parameters (DàHꢀ and DàSꢀ) are determined from variable temperature kinetic studies. The magnitude of the second order rate constant,
k2 increases as in the order: Pd(NaiEt)Cl2 < Pd(NaiMe)Cl2 < Pd(NaiBz)Cl2 as well as Pd(b-NaiR)Cl2 < Pd(a-NaiR)Cl2.
ꢁ 2007 Elsevier Ltd. All rights reserved.
Keywords: Palladium(II); Substitution; Naphthylazoimidazole; Picolinic acid; Pd–Cl cleavage; p-acidity
1. Introduction
[15–22]. The reaction of DNA bases with Pd(II)/Pt(II)
complexes of chelating N,N0–donors having cis-MCl2 con-
figuration constitutes a model system which may allow for
exploration of the mechanism of the anti-tumor activity of
cisplatin. The kinetics and mechanism of the reactions
involving Pd(II) complexes [23–27] stimulates us to study
the kinetic and mechanistic aspects of palladium(II) com-
plexes of cisplatin analogues. In this work we now explore
the reaction of picolinic acid with Pd (N,N0)Cl2. The kinet-
ics and mechanism of the substitution reactions involving
Pd(II) complexes of 1-alkyl-2(arylazo)imidazoles (i) with
adenine [28], cytosine [29], 2-mercapto-pyridine [30], 2-
amino-pyrimidine [31], picolinic acid [32], and 8-hydroxy
quinoline [33,34] was reported. We are intending to incor-
porate higher steric crowding around the target metal cen-
tre by using different ligands with azoimine chelating mode
(–N@N–C@N–), which will open an avenue to find out
mechanistic aspect of nucleophilic interaction with the
metal centre under different local environments. Naphthy-
lazoimidazoles (ii) are chemical analogues to phenylazoim-
idazoles (i) but with a higher degree of steric crowding and
The anticancer properties of cis-Pt(NH3)2Cl2 [1–9] gave
an impetus to the research in the field of platinum metal
chemistry. This activity is due to the cis-PtCl2 fragment
that binds with DNA bases. However, the drug also inter-
acts with non-cancer cells, mainly with molecules having –
SH groups. This causes nephrotoxicity. The anticancer
properties of biologically important palladium(II) complex
molecules [10–14] has aroused interest in the design of com-
plexes of platinum(II) and palladium(II) [4] and ruthe-
nium(II/III) [8,9] of better activity and lower toxicity.
Mono-dentate ligands can bind in both cis- and trans-
arrangements around the metal and the stability of the iso-
mers depend on several factors. Consequently, bidentate
ligands are more reliable for the preparation of cis-com-
plexes, in particular with palladium(II) and platinum(II)
*
Corresponding author. Tel.: +91 33 2432 4586; fax: +91 33 2414 6584.
E-mail addresses: ambikesh_ju@yahoo.com, amahapatra@chemistry.
jdvu.ac.in (A. Mahapatra).
0277-5387/$ - see front matter ꢁ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.06.016

4842
P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
electron donating ability. Herein we present the kinetic and
mechanistic studies of the reaction of picolinic acid (picH)
with Pd(a/b-NaiR)Cl2. Picolinic acid which is a naturally
occurring degradation product of tryptophan [35], a
DNA base, is a cheap and safe drug facilitating Zn/Cu
ion absorption from intestine because of its metal ion
chelating activity. Also picolinic acid has an extra clinical
antimicrobial chemo-theraupteic application. Phenylazo-
imidazole (i) Naphthylazoimidazole (ii)
(picH) to the solution of Pd(NaiR)Cl2 in MeCN the orange
solution changes to yellow–orange. The influence of the
addition of picolinic acid (picH) on the spectra of Pd(a-
NaiEt)Cl2 is shown in the Fig. 1. The change proceeds
through a single isosbestic point at ca. 489 nm. The
decrease in absorbance of the reaction mixture was
recorded automatically at ca. 530 nm as a function of time
(s) for at least three halves of the reaction and up to max-
imum 1000 s. A was measured after ꢁ 24 h of mixing,
1
when the absorbance became constant. In all experiments,
initial molar concentration of picH, [picH]0, was kept at
least ten times in excess so as to maintain pseudo-first-
order kinetic condition. Pseudo-first-order rate constants,
kobs, were obtained from the slopes of the plots of
N
N
R
N
N
R
N
N
N
R
N
N
N
(At ꢀ A ) versus time (s) (Fig. 2) where At is the absor-
1
N
N
bance of the reaction mixture at time, t (s) after mixing
of picH solution and A is the absorbance of same reac-
1
tion mixture after completion of the reaction.
2.2. Synthesis of [Pd(a-/b-NaiR)(pic)](ClO4) (a-NaiR,3;
b-NaiR,4)
α-NaiR
β-NaiR
Naphthylazoimidazole ( ii )
Phenylazoimidazole (i)
A representative synthesis of [Pd(a-NaiEt)(pic)](ClO4)
(3a) is given below.
2. Experimental
2.1. Reagents/materials
To a MeCN solution (15 ml) of the complex, Pd(a-
NaiEt)Cl2 (0.025 g, 5.84 · 10ꢀ2 mmol), picH (0.0216 g,
17.52 · 10ꢀ2 mmol) was added and the orange colored
solution was stirred for about 24 h. It was filtered and
allowed to evaporate to dryness at room temperature.
The mass was then dissolved in MeOH and NaClO4 (aq)
(1 g) was added to it. A brown precipitate was filtered
and washed with water. Dried product was then chromato-
graphed over silica gel and MeCN eluted orange band.
Yield: 0.021 g (65.22%).
The complexes were prepared by a reported procedure
[36,37]. Picolinic acid (picH) (SRL, Mumbai) was purified
by recrystallization from boiling toluene. Acetonitrile
(MeCN) (SRL, Mumbai) was purified by a known proce-
dure [38,39]. All kinetic and spectroscopic measurements
were recorded on an Agilent 8453E UV-Vis Spectroscopy
System. Quartz cells (1.0 cm path length) from Hellma
were used. IR spectra (KBr pellet) were recorded on a
FT-IR Spectrophotometer Spectrum RX1, Perkin–Elmer.
1H NMR spectra were drawn from 300 MHz Bruker
NMR instrument in CD3CN using TMS as an internal
standard. Microanalyses were carried out on a Perkin–
Elmer 2400 CHN elemental analyzer. The specific conduc-
tance was measured on a JENCONS 4010 Conductivity
Meter. The pH was measured on a SYSTRONICS 361
Digital pH meter. Rate constants and standard deviations
were calculated by linear regression using a PC-based pro-
grammed Microcal-origin version Origin-6.1.
All other complexes were prepared by the same proce-
dure and yield varied 60–70%.
1.4
1.2
1.0
Isosbestic
0.8
0.6
0.4
0.2
2.1.1. Kinetic measurements
For kinetic measurements, stock solutions of the com-
plexes (ca. 10ꢀ3 mol dmꢀ3) and of picolinic acid (picH)
(ca. 10ꢀ2 mol dmꢀ3) were prepared in dry MeCN. Solu-
tions of different concentrations were prepared by quanti-
tative dilution of stock solutions using dry MeCN. All
experiments were performed at 298 K (unless otherwise
mentioned) by mixing required volumes of the thermo-
stated reactants and transferring the mixture to the absorp-
tion cell (1.0 cm length). On addition of picolinic acid
400
420
440
460
480
500
520
540
560
580
600
WaveLength (nm)
Fig. 1. Spectra of Pd(a-NaiEt)Cl2(ꢂ ꢂ ꢂ) in MeCN and a reaction mixture of
Pd(a-NaiEt)Cl2 and picH in MeCN solution at 298 K. The arrows indicate
decrease and increase of band intensities as reaction proceeds.

P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
4843
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
12.12%. For [Pd(b-NaiBz)(pic)](ClO4) (4c): Anal. Calc.
for C26H20O6N5PdCl: C, 48.76; H, 3.13; N, 10.94. Found:
C, 48.72; H, 3.16; N, 10.89%.
Ab At4-Af4
ExpDec1 fit of Data1_C
Data: Data1_C
Model: ExpDec1
Equation: y = A1*exp(-x/t1) + y0
Weighting:
3. Results and discussion
y
No weighting
3.1. Kinetics and mechanism
Chi^2/DoF
R^2 = 0.99887
= 4.1184E-6
Two classes of naphthylazoimidazole complexes of
palladium(II) have been used in this work. They are
Pd(a-NaiR)Cl2 (1) (a-NaiR = 1-alkyl-2-(naphthyl-a-azo)-
imidazole) and Pd(b-NaiR)Cl2 (2)(b-NaiR = 1-alkyl-2-
(naphthyl-b-azo)imidazole) [where R = Me (a)/Et(b)/
Bz(c)]. The ligands belong to the unsymmetric bidentate
N,N0-donor type and form dichloropalladium(II) com-
plexes, here after we abbreviate the complex as
Pd(N,N0)Cl2 (Scheme 1). The reaction kinetics between
Pd(N,N0)Cl2 and picolinic acid (picH) was studied spectro-
photometrically. The reaction is first order in Pd(II)
complex as kobs-values are almost constant when all
variants are constant except the complex concentration.
The kobs-values (Table 1) and the linear plots for the kobs
versus [picH]0 (Figs. 3 and 4) indicate the reaction is first
order in picH. The slope of the plot is second order rate
constant (k2) of the reaction. The presence of intercept
(k0) indicates the existence of a minor solvent assisted path
to the overall reaction products (MeCN is a coordinating
solvent).
y0
A1
t1
0.00608
0.17878
241.30021
0.00137
0.0013
5.53471
0
200
400
Time (s)
600
800
1000
Fig. 2. Plot of (At ꢀ A ) vs. time (s) for reaction: Pd(a-NaiEt)Cl2 + picH
1
in MeCN. [Pd(a-NaiEt)Cl2]0 = 1.00 · 10ꢀ4 mol dmꢀ3, [picH]0 = 7 · 10ꢀ3
mol dmꢀ3 and temperature = 298 K.
Microanalytical data of the complexes are as follows:
For [Pd(a-NaiMe)(pic)](ClO4) (3a): Anal. Calc. for
C20H16O6N5PdCl: C, 42.56; H, 2.84; N, 12.41. Found: C,
42.50; H, 2.96; N, 12.36%. For [Pd(a-NaiEt)(pic)](ClO4)
(3b): Anal. Calc. for C21H18O6N5PdCl: C, 43.61; H, 3.11;
N, 12.11. Found: C, 43.56; H, 3.10; N, 12.13%. For
[Pd(a-NaiBz)(pic)](ClO4) (3c): Anal. Calc. for C26H20O6-
N5PdCl: C, 48.76; H, 3.13; N, 10.94. Found: C, 48.72; H,
3.15; N, 11.01%. For [Pd(b-NaiMe)(pic)](ClO4) (4a): Anal.
Calc. for C20H16O6N5PdCl: C, 42.56; H, 2.84; N, 12.41.
Found: C, 42.52; H, 2.81; N, 12.44%. For [Pd(b-NaiEt)-
(pic)](ClO4) (4b): Anal. Calc. for C21H18O6N5PdCl: C,
43.61; H, 3.11; N, 12.11. Found: C, 43.68; H, 3.16; N,
The rate increases with rise of temperature, as expected
from the Eyring equation. The second order rate constants
(k2) were measured at different temperatures in the range of
293–313 K for each reaction. Activation parameters, stan-
dard enthalpy of activation (DàHꢀ) and standard entropy
of activation (DàSꢀ) were calculated from Eyring plots
Cl
Cl
N
N
N
N
Cl
R
Pd
Pd
R
N
Cl
N
N
Cl
N
N
Pd
/
N
Cl
/
Pd(α-NaiR)Cl2(1)
Pd(β-NaiR)Cl (2)
Pd(N,N )Cl2
2
5
+
4
5
4
+
O
O
O
O
N
N
N
N
R
R
Pd
Pd
d
c
N
N
d
c
N
N
N
N
a
a
15
8
b
10
11
9
b
14
13
15
14
10
12
11
12
13
R = Me (a), Et (b), CH Ph (c)
2
[Pd(α-NaiR)(pic)](ClO4) (3)
[Pd(β-NaiR)(pic)](ClO4) (4)
Scheme 1.

4844
P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
12
Table 1
2a
2b
2c
Pseudo-first-order rate constants (kobs) for reactions of picolinic acid with
Pd(R0aiR)Cl2 in MeCN at 298 K
10
Pd(R0aiR)Cl2
103 [picH]0
103kobs 10k2
103k0
(sꢀ1
(mol dmꢀ3
)
(sꢀ1
)
(dm3 molꢀ1 sꢀ1
)
)
8
Pd(a-NaiMe)Cl2 (1a)
1.0
3.0
5.0
7.0
10.0
0.88
2.20
3.62
4.68
6.43
6.2
0.36
0.17
0.21
0.174
0.168
0.16
6
4
2
0
Pd(a-NaiEt)Cl2 (1b)
Pd(a-NaiBz)Cl2 (1c)
Pd(b-NaiMe)Cl2 (2a)
Pd(b-NaiEt)Cl2 (2b)
Pd(b-NaiBz)Cl2 (2c)
1.0
3.0
5.0
7.0
10.0
0.81
1.79
2.90
4.14
5.79
5.6
7.4
0
2
4
6
8
10
1.0
3.0
5.0
7.0
10.0
0.96
2.42
3.97
5.42
7.65
-3
3
10 [picH] (mol dm
)
0
Fig. 4. Plots of kobs vs. [picH]0 for the reactions: Pd(b-NaiR)Cl2 + picH in
MeCN. [Pd(b-NaiR)Cl2]0 = 1.00 · 10ꢀ4 mol dmꢀ3 and temperature =
298 K.
1.0
3.0
5.0
7.0
10.0
1.12
2.91
4.78
6.65
9.40
9.2
5.4
1a
5.6
1b
1.0
3.0
5.0
7.0
10.0
1.00
2.59
4.25
5.90
8.33
8.2
1c
5.8
6.0
6.2
6.4
6.6
1.0
3.0
5.0
7.0
10.0
1.20
3.19
10.2
5.32
7.29
10.40
[Pd(R0aiR)Cl2]0 = 1.00 · 10ꢀ4 mol dmꢀ3
.
3.20
3.25
3.30
3.35
3.40
3
-1
10 (1/T) (K
)
1a
8
1b
Fig. 5. Plots of ln(k/T) vs. (1/T) for the reactions: Pd(a-NaiR)Cl2 + picH
1c
in MeCN.
6
5.4
2a
2b
4
2
0
5.5
2c
5.6
5.7
5.8
5.9
6.0
6.1
0
2
4
6
8
10
3
-3
10 [picH]0 (mol dm
)
Fig. 3. Plots of kobs vs. [picH]0 for the reactions: Pd(a-NaiR)Cl2 + picH in
MeCN. [Pd(a-NaiR)Cl2]0 = 1.00 · 10ꢀ4 mol dmꢀ3 and temperature =
298 K.
3.20
3.25
3.30
3.35
3.40
3
-1
10 (1/T) (K
)
(Figs. 5 and 6) and are recorded in Table 2. The activation
parameter values corroborate with the experimental k2-val-
ues. The order for DàHꢀ and DàSꢀ are: Pd(NaiEt)Cl2>
Fig. 6. Plots of ln(k/T) vs. (1/T) for the reactions: Pd(b-NaiR)Cl2 + picH
in MeCN.

P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
4845
Table 2
picH. Kinetic studies in the presence of externally added
Clꢀ (LiCl) reveal that the rate as well as kobs decreases
inversely with [Clꢀ]0 (Table 3), which supports the mecha-
nism given in Scheme 2. The dissociation of the second Pd–
Cl bond is the rate-determining step and the dissociation of
the first Pd–Cl bond is affected by externally added Clꢀ ion.
The product was isolated and characterized as [Pd(N,N0)-
(pic)](ClO4). The nucleophilic substitution process involves
direct displacement of 2 Clꢀ ions by picolinic acid (Eq. (1)).
In the MeCN medium picH will exist largely in the undis-
sociated form [40].
Second order rate constants (k2) at different temperatures and activation
parameters of the reactions
Pd(R0aiR)Cl2
Temperature 10k (dm3 DàHꢀ
(K)
molꢀ1 sꢀ1) (kJ molꢀ1
DàSꢀ (J Kꢀ1
molꢀ1
)
)
Pd(a-NaiMe)Cl2 293
(1a)
4.92
6.20
7.96
9.93
12.30
32.62( 4)
ꢀ139.35( 25)
ꢀ125.46( 30)
ꢀ159.29( 18)
ꢀ184.99( 15)
ꢀ162.70( 20)
ꢀ201.28( 12)
298
303
308
313
Pd(a-NaiEt)Cl2 293
(1b)
4.25
5.60
7.28
9.40
12.00
37.05( 5)
26.27( 3)
18.05( 2)
24.99( 3)
12.95( 2)
298
303
308
313
O
+
O
N
N
N
N
Cl
Cl
-
Cl
HCl
+
Pd
+
+
/
Pd
/
N
N
C
O
Pd(a-NaiBz)Cl2 293
(1c)
6.05
7.40
8.93
10.75
12.90
OH
298
303
308
313
NaClO
4
O
O
N
N
Pd
/
-
+
(ClO )
+ Na Cl
+
4
N
Pd(b-NaiMe)Cl2 293
(2a)
8.00
9.20
10.55
12.00
13.75
298
303
308
313
ð1Þ
Pd(b-NaiEt)Cl2 293
(2b)
6.80
8.20
9.83
11.75
14.00
298
303
308
313
Table 3
Effect of externally added LiCl on pseudo-first-order rate constants (kobs
)
for reactions of picolinic acid with Pd(R0aiR)Cl2 in MeCN at 298 K
Pd(R0aiR)Cl2
103 [LiCl]0 (mol dmꢀ3
)
103kobs (sꢀ1
)
Pd(b-NaiBz)Cl2 293
(2c)
9.20
10.20
11.35
12.50
13.80
Pd(a-NaiMe)Cl2 (1a)
0.2
0.4
0.6
0.8
1.0
2.05
1.16
0.82
0.65
0.57
298
303
308
313
Pd(a-NaiEt)Cl2 (1b)
Pd(a-NaiBz)Cl2 (1c)
Pd(b-NaiMe)Cl2 (2a)
Pd(b-NaiEt)Cl2 (2b)
Pd(b-NaiBz)Cl2 (2c)
0.2
0.4
0.6
0.8
1.0
1.71
0.93
0.65
0.52
0.45
Pd(NaiMe)Cl2 > Pd(NaiBz)Cl2 and Pd(a-NaiR)Cl2 >
Pd(b-NaiR)Cl2. The iso-kinetic plot (Fig. 7) supports an
identical mechanism for the reaction of Pd(N,N0)Cl2 with
0.2
0.4
0.6
0.8
1.0
2.35
1.28
1.00
0.75
0.62
-10
2c
-15
2a
0.2
0.4
0.6
0.8
1.0
2.78
1.56
1.12
0.85
0.73
-20
2b
-25
1c
0.2
0.4
0.6
0.8
1.0
2.49
1.35
0.98
0.74
0.62
-30
1a
-35
1b
0.2
0.4
0.6
0.8
1.0
3.02
1.55
1.14
0.87
0.75
-40
120
130
140
150
160
170
180
190
200
210
-Δ ‡So (J K-1mol-1
)
Fig. 7. Plot of DàHꢀ vs. DàSꢀ i.e., iso-kinetic plot for the reactions:
[Pd(R0aiR)Cl2]0 = 1.00 · 10ꢀ4 mol dmꢀ3, [picH]0=3.00 · 10ꢀ3 mol dmꢀ3
.
Pd(R0aiR)Cl2 + picH in MeCN.

4846
P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
+
N
O
Cl
Cl
N
N
N
O
O
N
N
N
(fast)
K
N
N
1
k
+
-
Pd
Pd
Pd
/
+ Cl
/
/
-HCl
H
(slow)
Cl
(C)
(A)
(B)
Scheme 2.
The observed rate is
for about 24 h. It was filtered and allowed to evaporate
almost to dryness at room temperature. Mass so produced
was then dissolved in MeOH and NaClO4 (aq) was added.
A brown precipitate formed which was filtered and then
washed with water and cold MeCN. Dried product was
chromatographed over silica gel and MeCN eluted an
orange-red band. Finally the dried product was character-
ized by micro-analytical data (experimental section), 1H
NMR (Table 4) spectral data. The molar conductance data
in MeCN solution (KM = 90–106 S cm2 molꢀ1) show 1:1
electrolytic nature of the complexes. The IR spectra of
the complexes display a sharp stretch at 1380–1410 cmꢀ1
which corresponds to m(N@N) and is red shifted by 5–
10 cmꢀ1 from that of Pd(NaiR)Cl2 [36]. This may be attrib-
uted to better charge delocalization from coordinated picꢀ
to the chelated NaiR motif than that of Cl in precursor.
The binding is indirectly established by the disappearance
of two m(Pd–Cl) bands corresponding to the cis-PdCl2 con-
figuration [21,36,41] and appearance of new bands at 1450–
1460 and 460–500 cmꢀ1 due to m(COO) and m(Pd–O),
respectively. All the complexes exhibit a structure less band
at 1050–1100 cmꢀ1 corresponding to m(ClO4) suggesting
lack of any significant interaction in the solid state [42].
The 1H NMR spectra of the complexes were recorded in
CD3CN and the data are collected in Table 4. The proton-
numbering pattern is shown in Scheme 1. Data reveal that
the signals in the spectra of the complexes are shifted
downfield relative to the free ligand values [33]. This sup-
ports the coordination of the ligand to Pd(II). An impor-
tant feature of the spectra is the shifting of imidazole
protons 4H and 5H to lower d-values, in general, relative
to the naphthyl protons (6H–13H). The imidazole protons
suffer downfield shifting by 0.1–0.2 ppm compared to the
free ligand position. This supports the strong preference
of imidazole-N binding to Pd(II). The naphthyl protons
appear as multiplets except a broad singlet of 6H and dou-
blet of 8H (in [Pd(b-NaiR)(pic)](ClO4)) in the complexes
Rate ¼ ½k0 þ k2½picHꢃ g½PdðR0aiRÞCl2ꢃ
0
¼ kobs½PdðR0aiRÞCl2ꢃ
kK1
(where k2 ¼
, which is constant at constant [Clꢀ]0 and
½Clꢀꢃ0
kobs = k0 + k2[picH]0).
The k0 and k2 are the intercept and the slope of the plot
of kobs versus [picH]0 respectively. It is probable that the
highly negative O donor will coordinate first with the pos-
itively charged metal centre, with fast dissociation of the
first Pd–Cl bond, and finally forms a stable four coordi-
nated square planar complex (B). Then picolinate-N
(which remains free in B) slowly coordinates with Pd(II)
as a compulsion of chelate stability. Obviously the later
rate is slower than the former rate. Therefore the second
Pd–Cl bond cleavage is the rate-determining step. This is
also supported by the effect of externally added Clꢀ ions
(LiCl). The plausible mechanism is given in Scheme 2.
There are two parallel pathways: one is the existence of a
minor solvent assisted path where MeCN substitutes Clꢀ
initially, then MeCN is displaced by picH and finally forms
the reaction product (C) (Scheme 3).
Although the steric crowding of a-/b-naphthyl group in
the N,N0 chelating ligand is significant, its electron with-
drawing property, due to conjugation, stabilizes the
Pd(N,N0) chelated species in preference to de-chelation.
The kinetic data in Table 1 reveal that the magnitude of
k increases in the order: Pd(a-NaiEt)Cl2 < Pd(a-Nai-
Me)Cl2 < Pd(a-NaiBz)Cl2. This is because of the electron
withdrawing tendency of Bz group is greater than Me
which in turn is slightly greater than Et.
3.2. Product characterization
To MeCN solution of the complex, Pd(N,N0)Cl2, picH
was added and the orange colored solution was stirred
+
NCMe
Cl
N
N
O
N
N
N
N
N
O
Cl
Cl
+ N
-H+
Pd
/
+ MeCN
Pd
Pd
/
/
Cl
-
-Cl
-MeCN
(B)
-
-
Cl
+
O
N
N
Pd
/
N
(C)
Scheme 3.

P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
4847
and a doublet of 15H in case of all the complexes. The
1-alkyl group appears as singlet for 1-Me in [Pd(a/b-
NaiMe)(Q)](ClO4), a quartet to –N–CH2– and a triplet
for the –CH3 group in [Pd(a/b-NaiEt)(pic)](ClO4) and
a
singlet for –CH2– and benzyl protons in [Pd-
(a/b-NaiCH2Ph)(pic)](ClO4) complexes. The pic protons
(a–d) appear at higher d, 8–9 ppm.
4. Conclusion
The kinetics of the reactions between Pd(N,N0)Cl2 and
picolinic acid (picH) were studied spectrophotometrically
at 530 nm under pseudo-first-order condition in MeCN.
The reactions are first order in Pd-complex and picH.
The rate of reaction is largely influenced by the p-acidity
of the chelating ligand; substitution in the naphthylazoim-
idazole backbone influences much the rate of the substitu-
tion process. Activation parameters Dà Hꢀ and DàSꢀ of
reactions were measured which corroborates the kinetic
rate data. The products isolated from the reaction between
Pd(NaiR)Cl2 and picH in MeCN have been characterized
by spectral data and support the composition [Pd(NaiR)-
(pic)](ClO4).
Acknowledgements
Financial help from Jadavpur University is gratefully
acknowledged. We thank Dr. C. Sinha, Professor of Chem-
istry, Jadavpur University for his sincere all round help and
support.
References
[1] S.J. Lippard, in: I. Bertini, H.B. Gray, S.J. Lippard, J.S. Valentine
(Eds.), Bioinorganic Chemistry, University Science Books, Mill
Valley, CA, 1994, p. 505.
[2] P.D. Beer, Acc. Chem. Res. 31 (1998) 71.
[3] Y.S. Yoo, Y.S. Sohn, Y.K. Do, J. Inorg. Biochem. 73 (1999) 187.
[4] J. Li, L.M. Zheng, I. King, T.W. Doyle, S.H. Chan, Curr. Med.
Chem. 8 (2001) 121.
[5] P. Banerjee, Coord. Chem. Rev. 19 (1999) 190.
[6] B. Lippert, Coord. Chem. Rev. 18 (1999) 263.
[7] M. Kato, J. Takahashi, Y. Sugimoto, C. Kosuge, S. Kishi, S. Yano, J.
Chem. Soc. Dalton Trans. (2001) 747.
[8] H. Chen, J.A. Parkinson, S. Pearsons, R.A. Coxall, R.O. Gould, P.J.
Sadler, J. Am. Chem. Soc. 124 (2002) 3064.
[9] A.H. Velders, H. Kooijman, A.L. Spek, J.G. Hassnoot, D. Vos, D.
Reedijk, J. Inorg. Chem. 39 (2000) 2966.
[10] D. Kovala-Demertzi, A. Boccarelli, M.A. Demertzis, M. Coluccia,
Chemotherapy 53 (2007) 148.
[11] A. Divsalar, A.A. Saboury, H. Mansoori-Torshizi, B. Hemmatinejad,
Bull. Korean Chem. Soc. 27 (2006) 1801.
[12] J.L. Butour, S. Wimmer, F. Wimmer, P. Castan, Chemico-Biol.
Interact. 104 (1997) 165.
[13] D.S. Umreiko, D.N. Kachurina, I.E. Chernikova, G.G. Novitskii,
N.M. Sinitsyn, T.M. Buslaeva, V.I. Efanov, Zdravookhranenie
Belorussii 12 (1986) 22.
[14] M.M.L. Fiallo, A. Garnier-Suillerot, Biochemistry 25 (1986) 924.
[15] T.K. Misra, D. Das, C. Sinha, P.K. Ghosh, C.K. Pal, Inorg. Chem.
37 (1998) 1672.
[16] P. Byabartta, P.K. Santra, T.K. Misra, C. Sinha, C.H.L. Kennard,
Polyhedron 20 (2001) 905.

4848
P.K. Ghosh et al. / Polyhedron 26 (2007) 4841–4848
[17] P.K. Santra, T.K. Misra, D. Das, C. Sinha, A.M.Z. Slawin, J.D.
Woollins, Polyhedron 18 (1999) 2869.
[30] S. Saha, T. Majumdar, A. Mahapatra, Transition Met. Chem. 31
(2006) 1017.
[18] S. Pal, D. Das, P. Chattopadhyay, C. Sinha, K. Panneerselvam, T.H.
Lu, Polyhedron 19 (2000) 1263.
[31] S. Saha, T. Majumdar, A. Mahapatra, Indian J. Chem. 45A (2006)
877.
[19] G.K. Rauth, S. Pal, D. Das, C. Sinha, A.M.Z. Slawin, J.D. Woollins,
Polyhedron 20 (2001) 363.
[32] S. Saha, P.K. Sarkar, A. Mahapatra, Transition Met. Chem. 31
(2006) 389.
[20] R. Roy, P. Chattopadhyay, C. Sinha, S. Chattopadhyay, Polyhedron
15 (1996) 3361.
[21] D. Das, M.K. Nayak, C. Sinha, Transition Met. Chem. 22 (1997) 172.
[22] D. Das, C. Sinha, Transition Met. Chem. 23 (1998) 309.
[33] S. Saha, A. Mahapatra, Inorg. React. Mech. 6 (2006) 71.
[34] P.K. Ghosh, S. Saha, A. Mahapatra, Polyhedron 26 (2007) 2655.
[35] G.W. Evans, P.E. Johnson, Characterization and quantization of Zn-
binding ligand in human milk, Pediatr. Res. 14 (1980) 876.
[36] J. Dinda, P.K. Santra, C. Sinha, L.R. Falvello, J. Organomet. Chem.
629 (2001) 28.
ˇ
´
[23] Z.D. Bugarcˇic, S.T. Nandibewoor, M.S.A. Hamza, F. Heinemann, R.
van Eldik, Dalton Trans. (2006) 2984.
[24] E. Breet, R. van Eldik, Berichte der Bunsen-Gesellschaft 102 (1998)
1418.
[37] S. Pal, D. Das, P. Chattopadhyay, C. Sinha, K. Panneerselvam, T.H.
Lu, Polyhedron 19 (2000) 1263.
[25] A. Shoukry, T. Rau, M. Shoukry, R. van Eldik, J. Chem. Soc. Dalton
Trans. (1998) 3105.
[38] R. Roy, T.K. Misra, C. Sinha, A. Mahapatra, A. Sanyal, Transition
Met. Chem. 22 (1997) 453.
[26] H. Hohmann, S. Suvachittanont, R. van Eldik, Inorg. Chim. Acta
177 (1990) 51.
[39] R. Roy, D. Das, M. Banerjee, A. Mahapatra, Transition Met. Chem.
26 (2001) 205.
[27] J. Berger, M. Kotowski, R. van Eldik, U. Frey, L. Helm, A.E.
Merbach, Inorg. Chem. 28 (1989) 3759.
[40] T. Suganuma, A. Tanada, H. Tomizawa, M. Tanaka, E. Miki, Inorg.
Chim. Acta 320 (2001) 22.
[28] S. Saha, P.K. Ghosh, A. Mahapatra, Transition Met. Chem. 30
(2005) 706.
[41] C.K. Pal, S. Chattopadhyay, C. Sinha, D. Bandyopadhyay, A.
Chakravorty, Polyhedron 13 (1994) 999.
[29] S. Saha, T. Majumdar, A. Mahapatra, Inorg. React. Mech. 6 (2006)
19.
[42] S. Goswami, W. Kharmawphlang, A.K. Deb, S.M. Peng, Polyhedron
15 (1996) 3635.