Structural, spectral, electrochemical and catalytic reactivity studies of a series of N2O2 chelated palladium(II) complexes

Article abstract of DOI:10.1016/j.poly.2012.02.035

A series of neutral salicylaldiminato Pd(II) complexes have been synthesized and characterized. Spectral and electrochemical properties of the complexes in the solution phase have been investigated. The structure of complex 9 has been confirmed by X-ray a

Full text of DOI:10.1016/j.poly.2012.02.035

Polyhedron 38 (2012) 141–148
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Polyhedron
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Structural, spectral, electrochemical and catalytic reactivity studies of a series
of N2O2 chelated palladium(II) complexes
b
c
c
d
Mahmut Ulusoy a, , Özgül Birel , Onur Sßahin , Orhan Büyükgüngör , Bekir Cetinkaya
⇑
a Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey
b Mugla University, Faculty of Art and Science, Department of Chemistry, 48121 Mugla, Turkey
c Department of Physics, Ondokuz Mayis University, 55139 Kurupelit, Samsun, Turkey
d Department of Chemistry, Ege University, 35100 Bornova, Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of neutral salicylaldiminato Pd(II) complexes have been synthesized and characterized. Spectral
and electrochemical properties of the complexes in the solution phase have been investigated. The struc-
ture of complex 9 has been confirmed by X-ray analysis. Spectroscopic characteristics were determined
by visible absorption spectra. Cyclic voltammetry analysis has been performed to determine the energy
levels of the compounds. The coordination of the Schiff base, according to the spectroscopic techniques,
appears to occur through the two azomethine nitrogens and two o-OH groups. The salen type Pd(II) com-
plexes have been used as catalysts for the formation of cyclic organic carbonates from carbon dioxide and
liquid epoxides, which served as both reactant and solvent. The more electron donating substituents on
the backbone and the aryl ring of the ligand system, in particular, effectively promote carbon dioxide acti-
vation with liquid epoxides under homogeneous conditions.
Received 14 November 2011
Accepted 25 February 2012
Available online 19 March 2012
Keywords:
Pd(II) complex
Catalysis
Carbon dioxide
Cyclic carbonate
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
antifreeze, and as plasticizers [7–9]. Tetraalkylammonium salts,
phosphanes, main-group and transition-metal complexes, and al-
Carbon dioxide (CO2) is a particularly attractive alternative
feedstock as it is inexpensive, highly naturally abundant and the
by-product of many industrial applications, including combustion
[1]. Carbon dioxide is also attractive as an environmentally friendly
chemical reagent and is especially useful as a phosgene substitute
[2]. Carbon dioxide fixation has received much attention in recent
decades since carbon dioxide is the most inexpensive and infinite
carbon resource [3]. Cyclic carbonates are one kind of carbon diox-
ide fixation product and are widely used as monomers for polymer
synthesis, as an aprotic solvent and as a pharmaceutical intermedi-
ate [3–6]. The synthesis of cyclic carbonates includes three aspects:
catalytic reaction of CO2 and epoxide, electrochemical reaction of
CO2 and epoxide, and oxidative carboxylation of olefins. Various
catalyst systems were developed for the coupling of carbon dioxide
and epoxides in the so-called carbon dioxide fixation process,
including porphyrins, pathalocyanine, salen metal complexes, me-
tal oxides, zeolite, nanogold, alkali metal salts, ionic liquids and so
on [3].
kali metals convert epoxides and CO2 to cyclic carbonates [10].
According to previous studies, only a few metals are active for
the coupling of epoxides and CO2, including Al, Cr, Co, Mg, Li, Zn,
Cu and Cd [11–13].
Optimization of the catalytic activities for a given process is typ-
ically achieved through the methodical tailoring of the metal’s
ligand environment. Therefore, ligand frameworks, such as Schiff
base ligands, that can be sterically and electronically modified with
ease are very attractive [1]. Recently, Kleij reported detailed
knowledge of metal–salen complexes (metal: Cr, Co, Al, Zn, Cu,
Mn, Sn, Ru) as catalysts for the coupling of epoxides with CO2
[14]. To the best of our knowledge, Schiff bases containing Pd(II)
as the metal centre have not been used as catalysts for chemical
fixation of carbon dioxide [15]. In this study, six Pd(II) salen com-
plexes (five of them are novel) have been synthesized and investi-
gated as catalysts for the chemical coupling of CO2 with epoxides.
Cyclic carbonates are used industrially as polar aprotic solvents,
substrates for small molecule synthesis, antifoam agents for
2. Experimental
2.1. Materials and measurements
⇑
Corresponding authors. Tel.: +90 414 318 3000 1217; fax: +90 414 318 3541 (M.
Ulusoy). Tel.: +90 252 211 14 93; fax: +90 252 211 14 (O. Haklı-Birel).
All reagents and solvents were of reagent grade quality and ob-
tained from commercial suppliers (Aldrich or Merck). IR spectra
were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer
E-mail
addresses:
ulusoymahmut@harran.edu.tr,
ozgulbirel@mu.edu.tr
(M. Ulusoy).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.02.035

142
M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
as KBr pellets. 1H NMR spectra were recorded on a Varian AS-
400 MHz instrument in CDCl3 at room temperature. Chemical
shifts are given in parts per million from tetramethylsilane. Elec-
tronic spectral studies were recorded by an Analytic Jena Speed-
cord S-600 diod-array spectrophotometer. Melting points were
measured in open capillary tubes with an Electrothermal 9100
melting point apparatus, and are uncorrected. The electrochemical
properties of the palladium complexes were investigated by cyclic
voltammetry (CV). The cyclic voltammetry measurements were re-
corded using a CH 660B model potentiostat from CH Instruments.
The working electrode consisted of a glassy carbon electrode that
was polished before the experiments. A platinum wire was used
as a counter electrode. A silver wire served as a quasi-reference
electrode. Measurements were carried out in 0.1 M TBAPF6 as a
supporting electrolyte in acetonitrile. All solutions in the cell were
purged with argon before the measurements were taken. The
sweep rate was kept constant at 0.1 V/s. The oxidation potential
of the ferrocene/ferrocenium couple at about +0.45 V was used as
an internal reference. Catalytic tests were performed in a PARR
4843 50 mL stainless pressure reactor.
2.3.2. Synthesis of N,N’-2,2-dimethylpropylenebis(3,5-di-tert-
butylsalicylaldiminato)palladium(II), 9
The synthetic procedure was analogous to that used for 8. The
precipitated crude product was filtered via a cannula, washed with
hot methanol twice and then dried under vacuo. The product was
recrystallized from CH2Cl2-hexane. M.p.:>300 °C. Yield: 75%. 1H
NMR (400 MHz, CDCl3, d ppm): 1.08 (s, 6H, >C-(CH3)2); 1.26
(s, 18H, aryl-C(number-5)-(CH3)3); 1.49 (s, 18H, aryl-C(number-3)
-
(CH3)3); 3.36 (s, 4H, -CH2-C-(CH3)2); 6.9 (s, 2H, o-aryl-H); 7.4
(d, J = 2.4, p-aryl-H); 7.57 (s, 2H, N@CH); 13C NMR (100.56 MHz,
CDCl3, d ppm): 164.3 (N@CH); 163.3 (aryl-C-OH); 139.9 (aryl-
C(number-5)-(CH3)3); 135.9 (aryl-C(number-3)-(CH3)3); 130.5(o-aryl-C);
127.9 (p-aryl-C); 118.9 (aryl-C(number-1)); 71.9 (-CH2-C-(CH3)2);
36.1 (C-(CH3)3); 33.9 (C-(CH3)3); 31.5 (C-(CH3)3); 30.1 (>C-
(CH3)2); 24.5 (>C-(CH3)2). Anal. Calc. for C35H52N2O2Pd (MW:
639 g/mol): C, 65.76; H, 8.20; N, 4.38. Found: C, 65.43; H, 8.13;
N, 4.01%.
2.3.3. Synthesis of N,N0-ethylenebis(4-
diethlyaminosalicylaldiminato)palladium(II), 10
The synthetic procedure was analogous to that used for 9. M.p.:
275 °C (dec.). Yield: 70%. 1H NMR (400 MHz, CDCl3, d ppm): 1.10
(t, J = 7.2, 12H, -N-CH2-CH3); 3.27 (q, J = 6.4, 8H, -N-CH2-CH3);
3.55 (s, 4H, -N-CH2-CH2-N); 5.96 (d, J = 6.4, 2H, o-aryl-H); 6.31 (s,
2H, m-aryl-H); 6.84 (d, J = 8.4, 2H, m-aryl-H); 7.26 (s, 2H, N@CH);
13C NMR (100.56 MHz, CDCl3, d ppm): 167.3 (N@CH); 157.0
(aryl-C-OH); 135.5 (aryl-C(number-4)-N); 112.1 (o-aryl-C); 102.6
(aryl-C(number-1)); 101.0 (m-aryl-C); 59.7 (-CH2-CH2); 46.7 (-N-
CH2-CH3); 13.1 (-N-CH2-CH3). Anal. Calc. for C24H32N4O2Pd (MW:
515 g/mol): C, 55.98; H, 6.26; N, 10.88. Found: C, 56.21; H, 6.95;
N, 9.98%.
2.2. Synthesis of the ligands, 1–6
The previously reported salen type ligands 1–5 [1–3,5,6,8,7] and
the novel ligand 6 have been prepared from aliphatic diamines
(ethylenediamine, 1,3-propanediamine and 2,2-dimethyl-1,3-pro-
panediamine) with substituted salicylaldehydes in the presence
of alcoholic media (ethyl or methyl alcohol) in high yields.
2.2.1. Synthesis of N,N0-bis(4-diethylaminosalicylidene)-
2,2-dimethyl-propanediamine, 6
A mixture of 4-diethylaminosalicylaldehyde (2.0 mmoL), etha-
nol (20 mL) and 2,2-dimethyl-propanediamine (1.0 mmoL) was
stirred for 12 h at room temperature. Also, 3–4 drops of formic acid
were added as a catalyst. The resulting solution was concentrated
in vacuum. The oily product was crystallized by layering a satu-
rated CH2Cl2 solution with hexane (1:3). The desired ligand is sol-
uble in common solvents such as CHCl3, CH3CH2OH, DMF and
DMSO. M.p.: 103–105 °C. Yield: 70%. 1H NMR (400 MHz, CDCl3, d
ppm): 1.03 (s, 6H, >C-(CH3)2); 1.18 (t, J = 7.2, 12H, -N-CH2-CH3);
3.37 (q, J = 7.7, 8H, -N-CH2-CH3); 6.16 (t, J = 5.2, 2H, m-aryl-H);
6.25 (dd, 2H, m-aryl-H); 6.99 (d, J = 8.8, 2H, o-aryl-H); 7.96 (s, 2H,
N@CH); 13.65 (s, 2H, OH); 13C NMR (100.56 MHz, CDCl3, d ppm):
166.9 (aryl-C-OH); 163.8 (N@CH); 151.9 (aryl-C(number-4)-N);
133.1 (o-aryl-C); 108.4 (aryl-C(number-1)); 103.3 (m-aryl-C); 98.5
(m-aryl-C); 65.7 (-CH2-C-(CH3)2); 44.7 (-N-CH2-CH3); 36.5
(>C-(CH3)2); 24.2 (>C-(CH3)2); 12.9 (-N-CH2-CH3).
2.3.4. Synthesis of N,N0-propylenebis(4-
diethlyaminosalicylaldiminato)palladium(II), 11
The synthetic procedure was analogous to that used for 9. M.p.:
193 °C (dec.). Yield: 75%. 1H NMR (400 MHz, CDCl3, d ppm): 1.08–
1.12 (m, 12H, -N-CH2-CH3); 3.34–3.40 (m, 14H, -N-CH2-CH3, N-
CH2-CH2-CH2-N); 5.89 (d, J = 8.0, 2H, m-aryl-H); 6.25 (dd, 2H, m-
aryl-H); 7.24 (dd, 2H, o-aryl-H); 7.71 (s, 1H, N@CH); 7.78 (s, 1H,
N@CH); 13C NMR (100.56 MHz, CDCl3, d ppm): 165.9 (N@CH);
160.2 (aryl-C-OH); 153.1 (aryl-C(number-4)-N); 135.7 (o-aryl-C);
111.3 (aryl-C(number-1)); 102.7 (m-aryl-C); 99.1 (m-aryl-C); 60.2
(CH2-CH2-CH2); 55.3 (-N-CH2-CH3); 44.9 (CH2-CH2-CH2); 13.1 (-
N-CH2-CH3). Anal. Calcd. for C25H34N4O2Pd (MW: 529 g/mol): C,
56.76; H, 6.48; N, 10.59. Found: C, 57.11; H, 6.81; N, 9.66%.
2.3.5. Synthesis of N,N0-2,2-dimethylpropylenebis(4-
diethlyaminosalicylaldiminato)palladium(II), 12
2.3. Synthesis of the palladium(II) complexes 7–12
The synthetic procedure was analogous to that used for 8. M.p.:
278 °C (dec.). Yield: 81%. 1H NMR (400 MHz, CDCl3, d ppm): 0.94 (s,
6H, >C-(CH3)2); 1.09 (t, J = 7.0, 12H, -N-CH2-CH3); 3.19 (s, 4H, -CH2-
C-(CH3)2); 3.26 (q, J = 7.2, 8H, -N-CH2-CH3); 5.94 (d, J = 8.0, 2H, o-
aryl-H); 6.22 (s, 2H, m-aryl-H); 6.83 (d, J = 9.2, 2H, m-aryl-H);
7.14 (s, 2H, N@CH); 13C NMR (100.56 MHz, CDCl3, d ppm): 166.7
(N@CH); 160.5 (aryl-C-OH); 153.5 (aryl-C(number-4)-N); 135.4 (o-
aryl-C); 110.9 (aryl-C(number-1)); 102.6 (m-aryl-C); 99.8 (m-aryl-C);
71.9 (-CH2-C-(CH3)2); 44.6 (-N-CH2-CH3); 34.3 (>C-(CH3)2); 24.2
(-N-CH2-CH3); 13.1 (>C-(CH3)2). Anal. Calc. for C27H35N4O2Pd
(MW: 529 g/mol): C, 56.76; H, 6.48; N, 10.59. Found: C, 57.11; H,
6.81; N, 9.66%.
The Pd(II) complex 7 was prepared according to the literature
[10] by the reaction of an equivalent molar amount of the corre-
sponding ligand with Pd(OAc)2 in methanol.
2.3.1. Synthesis of N,N0-propylenebis(3,5-di-tert-
butylsalicylaldiminato)palladium(II), 8
A hot methanolic solution of N,N0-bis(3,5-di-tert-butylsalicylid-
ene)propanediamine (1 mmoL) and Pd(OAc)2 (1 mmoL) were stir-
red for 5 h under an argon atmosphere. The precipitated yellow
product was filtered via a cannula. It was then washed with a
CH2Cl2:EtOH (1:3) mixture several times. M.p.: >300 °C. Yield:
72%. The NMR data could not be obtained because of the low sol-
ubility of the product, even with chloroform-d, acetone-d6, di-
methyl sulfoxide-d6 and N,N-dimethylformamide-d7 as the NMR
solvent. Anal. Calc. for C33H48N2O2Pd (MW: 611 g/mol): C, 64.85;
H, 7.92; N, 4.58. Found: C, 65.08; H, 7.38; N, 4.27%.
2.4. General procedure for the cycloaddition of epoxides to CO2
A stainless pressure reactor (50 mL) was charged with complex
7–12 (4.5 ꢀ 10ꢁ5 moL), epoxide (4.5 ꢀ 10ꢁ3 moL), DMAP (11 mg,
9.0 ꢀ 10ꢁ5 moL) and CH2Cl2 (5.0 mL). The reaction vessel was

M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
143
placed under a constant pressure of carbon dioxide for 2 min to al-
low the system to equilibrate and CO2 was charged into the auto-
clave to the desired pressure, then the autoclave was heated to the
desired temperature. The pressure was kept constant during the
reaction. The vessel was allowed to cool in an ice bath after the
expiration of the desired reaction time. The pressure was released,
and then the excess gases were vented. The yields of epoxides to
corresponding cyclic carbonates were determined by comparing
the ratio of product to substrate in the 1H NMR spectrum of an ali-
quot of the reaction mixture.
and treated using a riding model with U = 1.2 times the U value of
the parent atom for CH, CH2 and CH3. Molecular diagrams were cre-
ated using ORTEP-III [20]. Geometric calculations were performed
with PLATON [21]. The unit cell has an accessible solvent volume of
2830 Å3 which is occupied by a severely disordered dichlorometh-
ane molecule. Therefore, this molecule was eliminated from the
refinement of the structure by means of the SQUEEZE subroutine
of PLATON [21] and the hkl intensities were modified accordingly.
3. Results and discussion
2.5. X-ray crystallography
3.1. Synthesis and characterization
An X-ray quality yellow crystal of 9 was obtained via layering a
saturated CH2Cl2 solution with hexane. A crystal of 9 suitable for
data collection was mounted on a glass fibre and data collection
was performed on a STOE IPDS II diffractometer with graphite
The synthetic route used for the preparation of the salen-
functionalized palladium(II) complexes 7–12 is shown in Scheme
1. The Schiff bases 1–5 [9–15] and 6 were prepared by conden
sation of 3,5-di-t-butylsalicylaldehyde [13] or 4-diethylamino
salicylaldehyde with commercially available aliphatic diamines
(ethylenediamine, 1,3-propanediamine and 2,2-dimethyl-1,3-pro-
panediamine). The salen ligands were isolated and then washed
with methanol to remove traces of the reactants. The Pd(II) com-
plex 7 was prepared according to the literature [10] by the reaction
of a molar equivalent of ligand 1 with Pd(OAc)2 in methanol. The
monochromated Mo Ka radiation at 296 K. The structure was solved
by direct-methods using SHELXS-97 [16] and refined by full-matrix
least-squares methods on F2 using SHELXL-97 [17] from within the
WINGX [18,19] suite of software. All non-hydrogen atoms were
refined with anisotropic parameters. Hydrogen atoms bonded to
carbon were placed in calculated positions (C–H = 0.93–0.97 Å)
R3
NH2
O
R2
A
+
NH2
R1
OH
ROH
R3
A
R2
N
O
N
N
Pd(OAc) 2
ROH
Pd
HO
HO
R1
R1
A
R3
R3
O
N
R2
R1
R1
R2
R2
R3
7 - 12
1 - 6
Schiff Bases
Pd(II) complexes
R1
tBu
tBu
R2
R3
tBu
tBu
R1
R2
R3
tBu
tBu
tBu
H
Compound
1
A
A
Compound
tBu
tBu
tBu
H
H
-CH2CH2-
-CH2CH2CH2-
-CH2CH2-
7
8
2
H
H
H
-CH2CH2CH2-
tBu
tBu
-CH2C(CH3)2CH2-
-CH2C(CH3)2CH2-
3
4
9
H
H
NEt2
-CH2CH2-
10
11
12
H
H
NEt2
-CH2CH2-
H
H
-CH2CH2CH2-
-CH2C(CH3)2CH2-
5
6
NEt2
NEt2
H
H
H
H
NEt2
NEt2
-CH2CH2CH2-
-CH2C(CH3)2CH2-
H
H
Scheme 1. Synthetic route to the salen–Pd(II) complexes.

144
M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
Table 1
Characteristic IR bands (cmꢁ1) of the synthesized compounds as KBr pellets.
Compounds
1
2
3
4
5
6
7
8
9
10
11
12
m
(O–H)
3419
1631
3390
1633
3428
1633
3436
1634
3435
1618
3461
1618
–
–
1609
–
–
1590
–
–
m(C@N)
1626
1614
1584
1589
Table 3
Crystal data and structure refinement parameters for 9.
0.3
0.25
0.2
Empirical formula
Formula weight
C35H52N2O2Pd
639.19
296
T (K)
0
0.71073
k (AÅ)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
0.15
0.1
23.6808(18)
31.743(3)
14.5164(13)
90
a
(°)
b (°)
125.795(6)
90
8850.7(14)
0.05
c
(°)
0
V (AÅ3)
0
Z
8
Dcalc (mg/mꢁ3
)
0.959
0.44
2704
240
290
340
390
440
490
Absorption coefficient (mmꢁ1
F(000)
)
Wavelength (nm)
Crystal size (mm)
h Range for data collection (°)
Independent reflection
0.48 ꢀ 0.32 ꢀ 0.07
1.2–26.8
8695
Fig. 1. UV–Vis absorption spectra of the palladium complex 9 in dichloromethane
solvent.
Collected reflection
56348
Absorption correction
Tmin
integration
0.522
Tmax
0.958
Rint
0.099
hmax(°)
26
h
k
l
ꢁ29 ? 29
ꢁ39 ? 39
ꢁ17 ? 17
Refinement method
Full-matrix least-squares on F2
Final R indices [I > 2
r
(I)]
0.049
0.093
0.98
wR(F2)
Goodness-of-fit on F2
(
D
/
r
)
0.001
0.32
max
0
D
D
qmax (e ÅAꢁ3
qmin (e ÅAꢁ3
)
0
ꢁ0.56
)
All the synthesized compounds 1–12 were fully characterized
by 1H and 13C NMR spectroscopy. In the 1HNMR spectra of the
ligands, obtained in CDCl3, singlet peaks appear at around 8 ppm,
which were assigned to the protons of azomethine [22]. The 1H
NMR (CDCl3) spectra of the ligands show proton signals at ca
Fig. 2. Cyclic voltammograms at a glassy carbon electrode for complex 9 (1 mM) in
MeCN containing 100 mM [TBA][PF6].
t
1 ppm, indicating the presence of the Bu substituents on the aryl
rings of the salt at position 3 and 5, indicating that the tBu protons
of these compounds are magnetically non-equivalent [23]. Also,
the elemental analysis results of the Pd(II) complexes are in agree-
ment with the proposed structure, as shown in Scheme 1. Details of
the synthesis and product characterization are given in the exper-
imental section.
other Pd(II) salen complexes were synthesized in a similar manner.
The obtained crude products were purified by re-crystallization
from CH2Cl2/MeOH (3:10) at 25 °C. The compounds were obtained
as pale yellow crystals.
Table 2
Molecular orbital energies of complex 9 with respect to the vacuum level.
E0 (V)
E0
(V)
ox1
E0ox2(V)/E0ox3(V)
LUMO (eV)
HOMO (eV)
Eg (eV)
red
Complex 9
ꢁ1.6
1.11
1.39
ꢁ2.72
ꢁ5.48
2.76
HOMO and LUMO energy levels of complex 9 were determined by the formulae:
E
LUMO = ꢁ(4.8 + Eredonset), Eredonset = E°redꢁE°ox(ferrocene), EHOMO = ELUMO ꢁ Egap
.

M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
145
The IR spectra of the salen ligands showed peaks in the
m
C@N
observed in the range 1.61/1.71 V for compound 9, and it exhibited
two or three anodic waves with the corresponding cathodic waves
and one cathodic wave with the corresponding anodic waves, indi-
cating a reversible process. The anodic wave showing the first
oxidation is assigned a ligand-based oxidation. The anodic peak
potentials of the complex were located in the 1.6–1.8 V range.
The measured redox potentials and energies of the highest occu-
pied molecular orbitals (HOMO) and lowest unoccupied molecular
orbials (LUMO) of the complex are listed in Table 2. In order to cal-
culate the LUMO energy level of the complex with respect to the
vacuum level, the redox data are standardized to the ferrocene/fer-
ricenium couple, which has a calculated absolute energy of -4.8 eV
[25]. Taking the reduction onsets into account, the LUMO energy
levels have been calculated and listed in Table 1. The HOMO energy
levels of complex 9, containing aromatic rings, are calculated to be
about ꢁ5.53 and ꢁ5.48 eV. The corresponding LUMO energy levels
stretching frequency region at 1634–1618 cmꢁ1. The bands at
3461–3419 cmꢁ1 are due to the O-H group of the salicylaldimine
units, as given in Table 1. The IR spectra of the palladium complexes
7–12 showed that the C@N stretching band shifted by ca 5–44 cmꢁ1
to lower frequency, compared to the salts as shown Table 1. This is
due to the effect of the metal on the ligand system [12].
UV–Vis absorption spectra of the Pd complexes taken in dichlo-
romethane solution are shown in Fig. 1. The spectra of the Pd(II)
complexes were recorded in the 200–800 nm range in CH2Cl2 solu-
tion. As given in Fig. 1, the electronic spectra of the Pd(II)
complexes in CH2Cl2 showed absorption bands in the 200–
500 nm range, exhibiting two absorption bands at 230–280 and
360–450 nm. The bands in the region 230–280 nm are attributed
to p–
p⁄ transitions of the aromatic ring of the ligands. The bands
in the region 360–450 are nm due to the n–p⁄ transition of the
non-bonding electrons present on the nitrogen of the azomethine
(C,N) groups in the ligands. While complex 8 has absorption peaks
at 256 and 412 nm, complex 9 has absorption peaks at 249 and
419 nm. Both 8 and 9 exhibit almost the same absorption proper-
ties in the UV–visible spectrum in the 200–500 nm range. These
are found to be ꢁ2.72 eV, using the formula EHOMO = ELUMO ꢁ Egap
The optical band gap value (Eg) was calculated as 2.76 eV.
.
3.2. Crystallography
complexes have conjugated
p electron system interactions
Important crystallographic data and the structure refinement
details of 9 are listed in Table 3, selected bond lengths and bond
angles are summarized in Table 4, and its ORTEP view is shown
in Fig. 3. The Pd(II) ion has a distorted square–planar environment,
in which the ligand is coordinated to the Pd(II) ion as a tetraden-
tate chelating ligand via the two phenolic oxygen atoms and two
imine nitrogen atoms, yielding three six-membered rings. The
two rings (Pd1/N1/C15/C6/C1/O1 and Pd1/N2/C35/C26/C21/O2)
are essentially planar and the third (Pd1/N1/C16/C17/C18/N2)
adopts a half-chair conformation, with the atom C17 displaced
from the Pd1/N1/C16/C18/N2 plane by 0.454(5) Å and with Cremer
and Pople (1975) puckering parameters [26]: Q = 0.594(4) Å,
between the Pd metal and the aromatic ligand, which can be attrib-
uted to the charge transfer bands (metal to ligand or ligand to
metal). There is a small bathochromic shift (ꢂ7 nm) in electronic
spectrum of complex 9 compared with complex 8. It has been re-
ported that absorption bands observed within the range 232–
408 nm in CH2Cl2 are most probably due to
p–
p⁄ transitions in
the aromatic benzene ring or n–p⁄ transitions of the imine group
of the ligands [24].
The cyclic voltammogram of complex 9 is given in Fig. 2. This
complex compound shows reversible reduction and oxidation.
A reversible reduction wave showing an electron process was
h = 59.3(4)° and
u = 186.4(5)°. The Pd(II) complex is coordinated
Table 4
0
in a cis-planar fashion by the two phenolic oxygen atoms and
two imine nitrogen atoms. The Pd–O distances are 1.993(3)–
2.012(2) Å with Pd–N distances 1.984(3)–1.994(3) Å, which are
typical for square–planar Pd(II) complexes of Schiff base ligands
[27]. The bond angles around the Pd(II) ion indicate that the
complex has a distorted square-planar geometry, O–Pd–O angle
Selected bond lengths (AÅ) and angles (°) for 9.
N1–Pd1
N2–Pd1
1.984(3)
1.994(3)
O1–Pd1
O2–Pd1
N1–Pd1–O1
O2–Pd1–O1
N2–Pd1–O1
2.012(2)
1.993(3)
92.01(11)
82.11(11)
172.75(11)
N1–Pd1–O2
N1–Pd1–N2
O2–Pd1–N2
172.78(11)
94.55(12)
91.56(12)
Fig. 3. The molecular structure of 9. Displacement ellipsoids are drawn at the 30% probability level.

146
M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
Table 5
Hydrogen bonds and C–Hꢃ ꢃ ꢃ
intermolecular C–Hꢃ ꢃ ꢃ
p interactions. In the first, atom C16 in the
p interactions for 9.
molecule at (x, y, z) acts as hydrogen-bond donor to the C21–C26
benzene ring in the molecule at (3/2 ꢁ x, 1/2 ꢁ y, 1 ꢁ z), so forming
a centrosymmetric R22(14) ring. In the second, atom C19 in the mol-
ecule at (x, y, z) acts as hydrogen-bond donor to the C1–C6 benzene
ring in the molecule at (1 ꢁ x, y, 1/2 ꢁ z), so forming a centrosym-
metric R22(16) ring. Details of these interactions are given in Table
0
0
0
D–Hꢃ ꢃ ꢃA
D–Hꢃ ꢃ ꢃA (°)
D–H (ÅA)
Hꢃ ꢃ ꢃA (ÅA)
Dꢃ ꢃ ꢃA (AÅ)
3.041(5)
3.000(4)
2.986(5)
3.010(6)
X–Hꢃ ꢃ ꢃCg (°)
C8–H8Bꢃ ꢃ ꢃO1
C9–H9Aꢃ ꢃ ꢃO1
C29–H29Bꢃ ꢃ ꢃO2
C30–H30Aꢃ ꢃ ꢃO2
Xꢃ ꢃ ꢃH(I)
0.96
0.96
0.96
0.96
Cg(J)
Cg(5)i
Cg(4)ii
2.40
2.36
2.35
124
124
123
123
2.38
0
5. The combination of the C–Hꢃ ꢃ ꢃ
p interactions form a chain running
Hꢃ ꢃ ꢃCg (AÅ)
C(16)–H(16B)
C(19)–H(19A)
3.3169
137.69
166.41
parallel to the [100] direction, as shown in Fig. 4.
2.9502
Cg(4): C1–C2–C3–C4–C5–C6, Cg(5): C21–C22–C23–C24–C25–C26.
Symmetry codes: (i) 3/2 ꢁ x, 1/2 ꢁ y, 1 ꢁ z; (ii) 1 ꢁ x, y, 1/2 ꢁ z.
3.3. Catalytic studies
82.11(11)° and N–Pd–N angle 94.55(12)°, deviating substantially
from that expected for a regular square–planar geometry. The
distortion can be attributed to the restricted bite angle of the Schiff
base ligand. Other bond lengths and angles observed in the struc-
ture are normal [28]. The two phenolate rings, (C1–C6) and (C21–
C26), are planar, the maximum deviations from the least-squares
planes being 0.0113(29) ÅA for atom C4 and 0.0214(33) ÅA for atom
C22. The dihedral angle between the two phenolate rings of the
tetradentate Schiff base ligand is 12.8(3)°.
The cycloaddition reaction of carbon dioxide with epoxides in
the presence of 1% 7–12 was conducted as a model reaction for test-
ing the use of these complexes as catalysts, and the results are com-
piled in Table 6. It is clear from Table 6 that bonding species to the
metal centre is influencing the yield of the cyclic carbonate (entries
1–12). Epichlorohydrin (EC) was found to be the most reactive epox-
ide, while propylene oxide (PO) exhibited low activity [29].
The salen Pd(II) complexes all showed catalytic efficiency only
while using EC as the substrate (Table 6, entries 7–12, 14). With
the other epoxides, such as PO and epoxybutane (EB), these com-
plexes did not show the desired activity. On increasing the catalyst
0
0
Within the selected asymmetric unit, intramolecular C–Hꢃ ꢃ ꢃO
hydrogen bonds define S(6) motifs. Compound 9 also contains two
Fig. 4. Packing arrangement of the molecules in the unit cell.

M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
147
Table 6
Table 6 (continued)
R = Propylene oxide (PO), epichlorohydrine (EC), and 1,2-epoxy butane (EB)
Entries
11
Catalyst
Product
Yield (%)a
93.2
TOFb
O
11
46.6
43.9
11.7
O
O
1% Cat., CH2Cl2, 100 oC
O
O
O
O
O
O
O
O
CO2
R
2% DMAP,
300 psi
R
Cl
Cl
.
12
13
12
12
87.7
23.4
O
O
Entries
1
Catalyst
Product
Yield (%)a
TOFb
7
27.5
17.2
12.8
13.5
27.0
28.3
13.8
O
O
O
CH
3
2
3
4
5
6
8
8.6
O
CH
O
O
3
14
15
12
82.4c
412
O
O
CH
3
O
O
O
O
9
6.4
O
Cl
O
O
–
5.0d
25.0
CH
3
10
11
12
6.8
O
CH
3
O
O
a
Yield of epoxides to the corresponding cyclic carbonates was determined by
comparing the ratio of product to substrate in the 1H NMR spectrum of an aliquot of
the reaction mixture.
CH
3
b
The rate is expressed in terms of the turnover frequency (TOF (mol of product
13.5
14.2
O
(mol of catalyst h)ꢁ1) = turnovers/h).
c
(4.5 ꢀ 10ꢁ5 mol) complex 12 as catalyst, and epichlorohydrine (4.5 ꢀ 10ꢁ2 mol)
O
O
was used as substrate.
d
Blank run, without any Pd(II) complex as a catalyst.
CH
3
O
loading to 1%, a high conversion (TON = 824) was obtained while
using catalyst 12 and EC as the substrate (Table 6, entry14).
According to the catalytic results, the more electron donating
substituents on the backbone and the aryl ring of the ligand sys-
tem, in particular, effectively promote carbon dioxide activation
with liquid epoxides under homogeneous conditions.
O
O
CH
Cl
3
7
7
92.0
97.5
96.6
96.3
46.0
48.8
48.3
48.2
O
O
O
O
O
O
O
O
O
4. Conclusion
8
8
O
O
O
Salen type Pd(II) complexes (7–12) have been synthesized and
spectroscopically characterized. The structure of complex 9 has
been confirmed by X-ray analysis. These Pd(II) complexes were
used as catalysts for the formation of cyclic organic carbonates
from carbon dioxide and liquid epoxides, which served as both
reactant and solvent. The salen Pd(II) complexes all showed cata-
lytic efficiency when using EC as the substrate.
Cl
9
9
Cl
Acknowledgments
10
10
_
The authors are grateful to TÜBITAK (106T364 and 110T655)
and Harran University (HUBAK-1110) for research funds and for
support. The authors wish to acknowledge the Faculty of Arts
and Sciences, Ondokuz Mayis University, Turkey, for the use of
the Stoe IPDS-II diffractometer (purchased from grant no. F279 of
the University Research Fund).
Cl

148
M. Ulusoy et al. / Polyhedron 38 (2012) 141–148
[14] A. Decortes, A.M. Castilla, A.W. Kleij, Angew. Chem., Int. Ed. 49 (2010) 9822.
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