Silicon containing new salicylaldimine Pd(II) and Co(II) metal complexes as efficient catalysts in transformation of carbon dioxide (CO2) to cyclic carbonates

Article abstract of DOI:10.1016/j.jorganchem.2011.01.005

A new series of metal complexes of salicyladimine ligands with Pd(II) and Co(II) have been prepared and characterized by different techniques (elemental analysis, UV-vis, FT-IR, ¹H NMR spectra, magnetic susceptibility measurements). Electronic

Full text of DOI:10.1016/j.jorganchem.2011.01.005

Journal of Organometallic Chemistry 696 (2011) 1372e1379
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Silicon containing new salicylaldimine Pd(II) and Co(II) metal complexes as
efficient catalysts in transformation of carbon dioxide (CO2) to cyclic carbonates
,
a
,
a
b
b
Mahmut Ulusoy a , Ahmet Kilic , Mustafa Durgun , Zeynep Tasci , Bekir Cetinkaya
**
*
a Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey
b 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 new series of metal complexes of salicyladimine ligands with Pd(II) and Co(II) have been prepared and
characterized by different techniques (elemental analysis, UVevis, FT-IR, 1H NMR spectra, magnetic
susceptibility measurements). Electronic spectra and magnetic susceptibility measurements reveal
square planar geometry for Pd(II) metal complex and tetrahedral geometry for Co(II) metal complex. The
synthesized Pd(II) and Co(II) complexes were also tested as catalysts for the formation of cyclic organic
carbonates from carbon dioxide and liquid epoxides which served as both reactant and solvent. The
results showed that the [M(L3)2] (M ¼ Pd or Co) complexes bearing 5-methyl substituent on the aryl ring
are more efficient than the other Pd(II) and Co(II) metal complexes for the formation of cyclic organic
carbonates from carbon dioxide. These catalysts, [Pd(L3)2] and [Co(L3)2] complexes and location
(p-position of phenoxy) of electron donating methyl substituent in particular, effectively promote the of
carbon dioxide activation with liquid epoxides under solvent-free homogeneous conditions. Further-
more, [Pd(L3)2] can be reused more than eight times with a minimal loss of its original catalytic activities.
Ó 2011 Elsevier B.V. All rights reserved.
Received 29 November 2010
Received in revised form
3 January 2011
Accepted 3 January 2011
Keywords:
Pd(II) complex
Catalyst
Carbon dioxide
Salicylaldimine
Cyclic carbonate
1. Introduction
of cyclic carbonate includes three aspects: catalytic reaction of CO2
and epoxide, electrochemical reaction of CO2 and epoxide, and
In the current global climate, with high oil prices, increasing
concern over global warming, and depleting petroleum resources,
the development of renewable carbon sources is of the utmost
importance. Carbon dioxide (CO2) is a particularly attractive alter-
native feedstock as it is inexpensive, highly naturally abundant, and
the by-product of many industrial processes, including combustion
[1]. Also, utilizing renewable resources is a prerequisite for
a sustainable society. One easily available renewable carbon
resource is carbon dioxide (CO2), which has the advantages of being
nontoxic, abundant and economical. Carbon dioxide (CO2) 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 last decades since carbon
dioxide is the most inexpensive and infinite carbon resource [3,4].
The cyclic carbonates are one kind of carbon dioxide fixation
products and widely used as monomer for polymer synthesis, an
aprotic solvent, a pharmaceutical intermediate [3e7]. The synthesis
oxidative carboxylation of olefin. Various catalysts systems were
developed for the coupling carbon dioxide and epoxides in the
so-called carbon dioxide fixation process, including porphyrin,
phthalocyanine, salen metal complexes, metal oxides, zeolite,
nanogold, alkali metal salts, ionic liquid, and so on [3].
Cyclic carbonates are used industrially as polar aprotic solvents,
substrates for small molecule synthesis, additives, antifoam agents
for antifreeze, and plasticizers [8,9]. The five-membered ring cyclic
carbonates (1,3-dioxolan-2-ones) are generally incapable of ring-
opening polymerization due to their thermodynamic stability, but
do undergo polymerization with partial loss of CO2 to yield
macromolecules with both ether and carbonate linkages [10]. Due
to such uses, a number of syntheses of cyclic carbonates have been
described over the last 30 years. For example, tetraalkylammonium
salts, phosphanes, main-group and transition-metal complexes,
and alkali metals convert epoxides and CO2 to cyclic carbonates
[11]. As observed in early studies, only a few metals are active for
the coupling of epoxides and CO2, including Al, Cr, Co, Mg, Li, Zn, Cu,
and Cd [12,13,19]. Studies have shown that large differences in
catalytic efficacy result from the organic frameworks surrounding
these metals. Accordingly, subsequent studies have largely focused
on empirical modification of ligands to generate improved cata-
lysts. To the best of our knowledge, Schiff bases containing Pd(II) as
* Corresponding author. Tel.: þ90 414 318 3587; fax: þ90 414 318 3541.
** Corresponding author. Tel.: þ90 414 318 3000/1267; fax: þ90 414 318 3541.
E-mail addresses: ulusoymahmut@harran.edu.tr (M. Ulusoy), kilica63@harran.
edu.tr (A. Kilic).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.005

M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
1373
the metal centre have not been used as catalysts for chemical
room temperature. The crystals were filtered in vacuum. Then the
fixation of carbon dioxide.
products were recrystallized from hot ethanol.
O
O
O
O
Cat. / co-cat.
O
O
1
CO2
O
R2
R1
R2
n
R1
R1
R2
Polycarbonate
Cyclic carbonate
Darensbourg and others noted that coupling of epoxides and
carbon dioxide produced either corresponding cyclic carbonates or
polycarbonates depending on the nature of epoxide, the metal
catalyst and the reaction conditions such as temperature and
pressure [14e20] (Eq. (1)).
The development of efficient recoverable and reusable catalysts
that can be easily prepared, recovered and reused without losing their
activities has received much attention from a practical and environ-
mental point of view [21]. In this study, four salicyclaldimine ligands
and their Pd(II) and Co(II) metal complexes have been synthesized
and characterized by elemental analysis, UVevis, FT-IR, 1H NMR
spectra, and magnetic susceptibility measurements. We firstly
reported novel and efficient catalysts of silicon-containing Pd(II) and
Co(II) Schiff base complexes in conjunction with 4-dimethylamino-
pyridine (DMAP) for the conversion of CO2 to cyclic carbonates.
2.1.1. For (L1H) ligand
Color: yellow; m.p: 220 ꢀC; yield (%): 85. Anal. Calc. for
C16H28N2O4Si (F.W: 340.5 g/mol): C, 56.44; H, 8.29; N, 8.23. Found:
C, 56.28; H, 8.38; N, 8.14%. 1H NMR (400 MHz, DMSO, Me4Si, ppm):
d
¼ 13.66 (s, 1H, eOH, D-exchangeable),
(s, 1H, HC]N),
¼ 7.25e7.20 (m, 2H, AreCH),
AreCH), ¼ 2.51e2.49 (m, 8H, CeCH2),
¼ 3.93 (s, 9H, OeCH3),
¼ 2.15 (s, 3H, CeCH3) and ¼ 1.91 (s, 2H, SieCH2). IR (KBr pellets,
nmax/cmꢁ1): 3520e2504
(OH), 3054 (AreCH), 2946e2705 (Alif-
CH),1631 (C]N),1493e1348 (C]C),1264 (CeO) and 1128e1087
d
¼ 8.60 (s, 1H, NH),
d
¼ 8.17
d
d
¼ 6.81e6.77 (m, 1H,
d
d
d
d
n
n
n
n
n
n
*
n
(SieO). UVevis (lmax, nm, ¼shoulder peak): 260, 325, 424* (in
EtOH), 264, 322, 403* (in MeOH) and 268, 322 (in DMF).
2.1.2. For (L2H) ligand
Color: yellow; m.p: 260 ꢀC; yield (%): 83. Anal. Calc. for
C16H28N2O5Si (F.W: 356.5 g/mol): C, 53.91; H, 7.92; N, 7.86. Found:
C, 53.76; H, 7.85; N, 7.91%. 1H NMR (400 MHz, DMSO, Me4Si, ppm):
2. Experimental
d
¼ 13.34 (s,1H, eOH, D-exchangeable),
(s, 1H, HC]N),
¼ 7.27e7.25 (d, 1H, J ¼ 8.8 Hz, AreCH),
¼ 6.40e6.37 (m, 1H, AreCH), ¼ 6.32 (d, 1H, J ¼ 2.4 Hz, AreCH),
¼ 3.82 (s, 3H, SieOeCH3), ¼ 3.74 (s, 9H, OeCH3), ¼ 2.51e2.49
¼ 1.90 (s, 2H, CeCH2) and ¼ 1.23 (s, 2H, SieCH2).
IR (KBr pellets, nmax/cmꢁ1): 3534e2545
(OH), 3060 (AreCH),
2937e2764 (Alif-CH), 1631 (C]N), 1494e1345 (C]C), 1276
(CeO) and 1116e1025
d
¼ 8.42 (s, 1H, NH),
d
¼ 8.19
All reagents and solvents were of reagent-grade quality and
obtained from commercial suppliers. Elemental analysis was
carried out on a LECO CHNS model 932 elemental analyzer. FT-IR
spectra were recorded on a Perkin Elmer Spectrum RXI FT-IR
spectrometer as KBr pellets. 1H NMR spectra were recorded at
297 K using a Varian AS-400 MHz spectrometer. UVevis spectra
were recorded on a Perkin Elmer Lambda 25 PC UVeVis spec-
trometer. Melting points were measured in open capillary tubes
with a Stuart Scientific melting point apparatus and uncorrected.
Magnetic susceptibilities were determined on a Sherwood Scien-
tific Magnetic Susceptibility Balance (Model MK1) at room
temperature (20 ꢀC) using Hg[Co(SCN)4] as a calibrant; diamagnetic
corrections were calculated from Pascal’s constants [22e24].
Catalytic tests performed in a PARR 4843 50 mL stainless steel
pressure reactor.
d
d
d
d
d
d
(m, 6H, CeCH2),
d
d
n
n
n
n
n
n
*
n
(SieO). UVevis (lmax, nm, ¼shoulder
peak): 278, 305*, 378, 427* (in EtOH), 252, 280, 301, 379* (in MeOH)
and 278, 306, 383* (in DMF).
2.1.3. For (L3H) ligand
Color: yellow; m.p: 230 ꢀC; yield (%): 84. Anal. Calc. for
C16H28N2O4Si (F.W: 340.5 g/mol): C, 56.44; H, 8.29; N, 8.23. Found:
C, 56.38; H, 8.32; N, 8.42%. 1H NMR (400 MHz, DMSO, Me4Si, ppm):
d
¼ 13.08 (s,1H, eOH, D-exchangeable),
(s, 1H, HC]N),
¼ 7.19 (s, 1H, AreCH),
¼ 6.75e6.73 (d, 1H, J ¼ 8.8 Hz, AreCH),
d
¼ 8.50 (s, 1H, NH),
d
¼ 8.19
d
d
¼ 7.12e7.10 (d, 1H,
2.1. Synthesis of the ligands (L1H, L2H, L3H and L4H)
J ¼ 8.8 Hz, AreCH),
d
d
¼ 3.88 (s, 9H, OeCH3),
d
¼ 2.49e2.48 (m, 8H, CeCH2), ¼ 2.03 (s,
d
N-[N-(3-(trimethoxysilyl)propyl)-ethylenediamine]-3-methylsali-
3H, CeCH3) and
d
¼ 1.22 (s, 2H, SieCH2). IR (KBr pellets, nmax/cmꢁ1):
cylaldimine
(L1H),
N-[N-(3-(trimethoxysilyl)propyl)-ethylene-
3546e2574
n
(OH), 3019
n
(AreCH), 2938e2797
n(Alif-CH), 1632
n
diamine]-4-methoxysalicylaldimine (L2H), N-[N-(3-(trimethoxysilyl)
propyl)-ethylenediamine]-5-methylsalicylaldimine (L3H) and
N-[N-(3-(trimethoxysilyl)propyl)-ethylenediamine]-5-methoxysali-
cylaldimine (L4H) ligands were synthesized by the reaction of
5.0 mmol (1.11 g) N-(3-(trimethoxysilyl)propyl)-ethylenediamine in
50 ml absolute ethanol and 5.0 mmol (0.68 g) 3-methylsali-
cylaldehyde for (L1H), 5.0 mmol (0.76 g) 4-methoxysalicylaldehyde
for (L2H), 5.0 mmol (0.68 g) 5-methylsalicylaldehyde for (L3H) and
5.0 mmol (0.76 g) 5-methoxysalicylaldehyde for (L4H), in 30 ml
absolute ethanol. Also, 3e4 drops of formic acid was added as
catalyst. The mixtures were refluxed for 6 h, followed by cooling to
(C]N), 1493e1347
n
(C]C), 1280
n
(CeO) and 1134e1040
n
(SieO).
UVevis (lmax, nm, *¼shoulder peak): 259, 334 (in EtOH), 263, 327,
413* (in MeOH) and 262, 326 (in DMF).
2.1.4. For (L4H) ligand
Color: yellow; m.p: 225 ꢀC; yield (%): 88. Anal. Calc. for
C16H28N2O5Si (F.W: 356.5 g/mol): C, 53.91; H, 7.92; N, 7.86. Found:
C, 53.78; H, 7.86; N, 7.71%. 1H NMR (400 MHz, DMSO, Me4Si, ppm):
d
¼ 12.68 (s,1H, eOH, D-exchangeable),
(s, 1H, HC]N),
¼ 7.00 (s, 1H, AreCH),
AreCH),
¼ 6.79e6.77 (d, 1H, J ¼ 8.4 Hz, AreCH),
d
¼ 8.53 (s,1H, NH),
¼ 6.93e6.90 (m, 1H,
¼ 3.89 (s, 3H,
d
¼ 8.15
d
d
d
d

1374
M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
SieOeCH3),
d
¼ 3.68 (s, 9H, OeCH3),
¼ 1.21 (s, 2H, SieCH2). IR (KBr pellets,
(OH), 3048 (AreCH), 2936e2746 (Alif-
(C]N), 1494e1347 (C]C), 1273 (CeO) and 1158e1032
d
¼ 2.49e2.47 (m, 6H, CeCH2),
2.2.4. For [Pd(L4)2]
d
¼ 1.89 (s, 2H, CeCH2) and
nmax/cmꢁ1): 3517e2533
CH), 1632
d
Color: green; m.p: >300 ꢀC; yield (%): 73. Anal. Calc. for
C32H54N4O10Si2Pd (F.W: 817 g/mol): C, 47.02; H, 6.66; N, 6.85.
Found: C, 47.08; H, 6.58; N, 6.94%. 1H NMR (400 MHz, DMSO, Me4Si,
n
n
n
n
n
n
*
n
(SieO). UVevis (lmax, nm, ¼shoulder peak): 258, 352 445* (in
ppm):
d
¼ 10.12 (s, 2H, NH),
d
¼ 8.06 (s, 2H, HC]N),
¼ 7.12e6.88 (m, 2H, AreCH),
¼ 6.76e6.74 (d, 2H, J ¼ 8.3 Hz, AreCH), ¼ 3.81 (s, 6H, SieOeCH3),
¼ 2.48e2.45 (m, 12H, CeCH2), ¼ 1.22 (s,
¼ 0.83e0.81 (m, 4H, SieCH2). IR (KBr pellets,
(AreCH), 2958e2854 (Alif-CH), 1616 (C]N),
(CeO), 1122e1039 (SieO), 520 (PdeN),
d
¼ 7.71e7.68
EtOH), 264, 343, 456* (in MeOH) and 268, 344 (in DMF).
(d, 2H, J ¼ 12 Hz, AreCH),
d
d
d
d
¼ 3.66 (s, 18H, OeCH3),
d
d
2.2. Synthesis of the Pd(II) and Co(II) metal complexes
4H, CeCH2) and
d
nmax/cmꢁ1): 3052
n
n
n
The metal(II) complexes were prepared by the same general
method: L1H (0.34 g, 1.0 mmol), L2H (0.36 g, 1.0 mmol), L3H
(0.34 g, 1.0 mmol) and L4H (0.36 g, 1.0 mmol) were dissolved in
40 ml methanol at room temperature. The suspended solution
of the [Pd(Ac)2] (0.11 g, 0.5 mmol) or [Co(Ac)2$4H2O] (0.13 g,
0.5 mmol) in 20 ml in methanol was added under a nitrogen
atmosphere with continuous stirring. The stirred mixtures were
then heated to the reflux temperature for 8 h and were main-
tained at this temperature. Then, the mixture was evaporated to
a volume of 10 ml under vacuum and left to cool to room
temperature. The precipitated compounds were filtered in
vacuum. The crude products were dissolved in chloroform and
filtered via cannula. Then the products were recrystallized from
chloroformeethanol.
1489e1387
463
n(C]C), 1287
n
n
n
*
n
(PdeO). UVevis (lmax, nm, ¼shoulder peak): 275, 321*, 413
(in MeOH) and 281, 330*, 431 (in DMF).
2.2.5. For [Co(L1)2]
Color: dark brown; m.p: >300 ꢀC; yield (%): 68. Anal. Calc. for
C32H54N4O8Si2Co (F.W: 738 g/mol): C, 52.09; H, 7.38; N, 7.59.
Found: C, 52.16; H, 7.27; N, 7.64%. IR (KBr pellets, nmax/cmꢁ1): 3055
n
(AreCH), 2923e2871
n
(Alif-CH), 1612
n
(C]N), 1455e1397 (C]C),
n
1281 (CeO), 1124e1044
n
n
(SieO), 516
n
(PdeN), 468 n(PdeO).
*
meff ¼ 2.24 [B.M]. UVevis (lmax, nm, ¼shoulder peak): 264, 301,
396, 639 (in MeOH) and 268, 392 (in DMF).
2.2.6. For [Co(L2)2]
Color: brown; m.p: >300 ꢀC; yield (%): 73. Anal. Calc. for
C32H54N4O10Si2Co (F.W: 770 g/mol): C, 49.92; H, 7.07; N, 7.28.
2.2.1. For [Pd(L1)2]
Color: green; m.p: >300 ꢀC; yield (%): 76. Anal. Calc. for
C32H54N4O8Si2Pd (F.W: 785 g/mol): C, 48.94; H, 6.93; N, 7.13. Found:
C, 48.79; H, 6.98; N, 7.06%. 1H NMR (400 MHz, DMSO, Me4Si, ppm):
Found: C, 49.84; H, 7.13; N, 7.14%. IR (KBr pellets, nmax/cmꢁ1): 3061
n
(AreCH), 2930e2871
n
(AlifeCH), 1608
n
(C]N), 1488e1405 (C]C),
n
1280 (CeO), 1122e1026
n
n
(SieO), 521
n
(PdeN), 464 n(PdeO).
*
d
¼ 8.52 (s, 2H, NH),
AreCH),
¼ 6.82 (s, 2H, AreCH),
¼ 2.49e2.46 (m, 16H, CeCH2), ¼ 2.11 (s, 6H, CeCH3) and
(s, 4H, SieCH2). IR (KBr pellets, nmax/cmꢁ1): 3042
(AreCH),
2939e2805 (Alif-CH), 1602 (C]N), 1455e1361 (C]C), 1282
(CeO), 1135e1023 (SieO), 515 (PdeN) and 462 (PdeO). UVevis
d
¼ 8.16 (s, 2H, HC]N),
¼ 3.96 (s, 18H, OeCH3),
¼ 1.86
d
¼ 7.23e7.19 (m, 4H,
meff ¼ 3.72 [B.M]. UVevis (lmax, nm, ¼shoulder peak): 281, 289*,
d
d
364, 478* (in MeOH) and 273, 364, 472* (in DMF).
d
d
d
n
2.2.7. For [Co(L3)2]
n
n
n
n
Color: dark brown; m.p: >300 ꢀC; yield (%): 76. Anal. Calc. for
C32H54N4O8Si2Co (F.W: 738 g/mol): C, 52.09; H, 7.38; N, 7.59.
n
n
n
*
(
lmax, nm, ¼shoulder peak): 268, 334, 408, 502* (in MeOH) and
Found: C, 52.12; H, 7.31; N, 7.54%. IR (KBr pellets, nmax/cmꢁ1): 3013
n
266, 328 (in DMF).
(AreCH), 2924e2865
n(Alif-CH), 1614
n
(C]N), 1470e1391 (C]C),
n
1289
n
(CeO), 1138e1047
n
(SieO), 514
n
(PdeN), 464 n(PdeO).
2.2.2. For [Pd(L2)2]
meff ¼ 2.18 [B.M]. UVevis (lmax, nm, *¼shoulder peak): 260, 394 (in
Color: green; m.p: >300 ꢀC; yield (%): 78. Anal. Calc. for
C32H54N4O10Si2Pd (F.W: 817 g/mol): C, 47.02; H, 6.66; N, 6.85.
Found: C, 47.12; H, 6.59; N, 6.92%. 1H NMR (400 MHz, DMSO, Me4Si,
MeOH) and 270, 395* (in DMF).
2.2.8. For [Co(L4)2]
ppm):
2H, J ¼ 8.8 Hz, AreCH),
(m, 2H, AreCH),
¼ 3.79 (s, 6H, SieOeCH3),
OeCH3),
d
¼ 8.58 (s, 2H, NH),
¼ 7.28e6.23 (m, 2H, AreCH),
¼ 3.72 (s, 18H,
d
¼ 8.13 (s, 2H, HC]N),
d
¼ 7.62e7.60 (d,
Color: dark brown; m.p: >300 ꢀC; yield (%): 70. Anal. Calc. for
C32H54N4O10Si2Co (F.W: 770 g/mol): C, 49.92; H, 7.07; N, 7.28.
Found: C, 49.96; H, 7.02; N, 7.21%. IR (KBr pellets, nmax/cmꢁ1): 3025
d
d
¼ 6.56e6.19
d
d
d
¼ 2.57e2.52 (m,12H, CeCH2),
d
¼ 1.22 (s, 4H, CeCH2) and
n(AreCH), 2925e2853
n(Alif-CH),1604
n
(C]N),1470e1393 (C]C),
n
d
¼ 0.83 (s, 4H, SieCH2). IR (KBr pellets, nmax/cmꢁ1): 3042
n
(AreCH),
1286 (CeO), 1122e1034
n
n
(SieO), 521
n
(PdeN), 468 n(PdeO).
*
2936e2847
n
(Alif-CH), 1605
n
(C]N), 1456e1361
n
(C]C), 1290
n
meff ¼ 2.26 [B.M]. UVevis (lmax, nm, ¼shoulder peak): 257, 302*,
(CeO), 1121e1023
n
(SieO), 519
n
(PdeN), 465
n
(PdeO). UVevis
409 (in MeOH) and 274, 294, 305, 412 (in DMF).
*
(
lmax, nm, ¼shoulder peak): 278, 306*, 376 (in MeOH) and 279,
306*, 373, 514 (in DMF).
2.3. Reactions of carbon dioxide and epoxides
2.2.3. For [Pd(L3)2]
All the catalytic reaction experiments were performed in
a stainless steel pressure reactor of 50 mL with a stirrer. The reactor
was charged with complexes (4.5 ꢂ10ꢁ5 mol), epoxide
(4.5 ꢂ10ꢁ2 mol), DMAP (11 mg, 9.0 ꢂ 10ꢁ5 mol). The reaction vessel
was placed under a constant pressure of carbon dioxide for 2 min to
allow the system to equilibrate and CO2 was charged into the
autoclave with desired pressure then heated to the desired
temperature. The pressure was kept constant during the reaction.
The vessel was then cooled to 5e10 ꢀC in ice bath after the expi-
ration of the desired time of reaction. The pressure was released,
then, the excess gases were vented.
Color: green; m.p: >300 ꢀC; yield (%): 70. Anal. Calc. for
C32H54N4O8Si2Pd (F.W: 785 g/mol): C, 48.94; H, 6.93; N, 7.13.
Found: C, 48.81; H, 6.82; N, 7.18%. 1H NMR (400 MHz, DMSO,
Me4Si, ppm):
¼ 7.70e7.69 (d, 2H, J ¼ 3.2 Hz, AreCH),
AreCH), ¼ 3.81 (s, 18H, OeCH3),
¼ 6.74e6.61 (m, 2H, AreCH),
¼ 2.48e2.22 (m, 18H, CeCH2), ¼ 2.12 (s, 6H, CeCH3) and ¼ 1.09
(s, 4H, SieCH2). IR (KBr pellets, nmax/cmꢁ1): 3013
(AreCH),
2954e2859 (Alif-CH), 1603 (C]N), 1493e1381 (C]C), 1285
(CeO), 1123e1039 (SieO), 512 (PdeN), 467 (PdeO). UVevis
lmax, nm, ¼shoulder peak): 278, 323*, 402, 514 (in MeOH) and
270, 318, 406 (in DMF).
d
¼ 10.10 (s, 2H, NH),
d
¼ 8.12 (s, 2H, HC]N),
d
d
¼ 7.13e6.92 (m, 2H,
d
d
d
d
d
n
n
n
n
n
n
n
n
*
(
For recycling studies, the catalyst [Pd(L3)2] and base(DMAP)
were separated from the reaction mixture by vacuum distillation at

M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
1375
(M ¼ Pd or Co, n ¼ 1e4) (Scheme 2). The elemental analyses and
other analytical data were consistent with the proposed formulae.
The Pd(II) and Co(II) metal complexes were observed to precipitate
from solution during the reaction as orange-brown solids for Co(II)
metal complexes and as pale-green solids for Pd(II) metal
complexes, which were purified by recrystallisation. All metal
complexes were found to be stable in the solid state at room
temperature for months. The metal complexes were also fairly
stable in different solution for days even in the absence of nitrogen,
showing no visible signs of decomposition.
CH3
O
H3C
O
O
R
H
N
Si
H3C
CHO
NH2
OH
CH3
H3C
The 1H NMR spectral results obtained for ligands (L1H, L2H, L3H
and L4H) and their mononuclear Pd(II) metal complexes in DMSO-
d6 did not give any signal corresponding to amine group and
different salicylaldehyde protons. The ligands and their mono-
nuclear Pd(II) metal complexes give well-defined 1H NMR spectra,
which permit unambiguous identification and assessment of
purity. The proton chemical shifts are assigned with the influence
of steric, inductive and conjugative effects and compared with
those of similar compounds [25e27]. The mononuclear Pd(II) metal
complexes of different ligands are in different magnetic environ-
ments. The spectrum of Pd(II) complexes were compared with that
of the parent salicylaldimine ligands. The OH signal found between
13.66 and 12.68 ppm as singlet in the spectrum of the salicylaldi-
mine ligands due to intramolecular OH/N hydrogen bonding
completely disappeared in the spectrum of the Pd(II) metal
complexes. These signals are D2O exchangeable. This result indi-
cates the involvement of the OH group in chelation with Pd(II)
through displacement of ortho hydroxyl proton [28,29]. The singlets
corresponding to azomethine group were observed between 8.60
and 8.42 ppm for ligands. The azomethine group has undergone
a downfield shift of 0.47e0.29 ppm indicating the participation of
azomethine nitrogen in coordination [30]. The peaks at range
7.25e6.77 ppm for (L1H), at range 7.27e6.37 ppm for (L2H), at range
7.19e6.73 ppm for (L3H) and at range 7.00e6.77 ppm for (L4H) are
assignable to the protons of AreCH as singlet, doublet or multiplet
peaks. These peaks are observed at range 7.71e6.19 ppm for all Pd
(II) metal complexes, as singlet, doublet or multiplet peaks. The
other results of the ligands and their Pd(II) metal complexes are
given in Experimental section.
O
O
H
N
R
Si
H3C
N=CH
O
HO
R: 3-CH3, 4-OCH3, 5-CH3 and 5-OCH3
Scheme 1. Synthesis route of ligands.
80 ꢀC. Reuseability of [Pd(L3)2]/DMAP catalytic system was per-
formed using propylene oxide as substrate which is available for
many applications and easier to separate propylene carbonate from
the reaction mixture by vacuum distillation. After distillation of the
propylene carbonate and propylene oxide, DMAP and [Pd(L3)2]
stayed in the reaction pot. Adding the same amount of PO, the
second reuse was performed. The yields of epoxides to corre-
sponding cyclic carbonates were determined by comparing the
ratio of product to substrate in the 1H NMR spectrum of an aliquot
of the reaction mixture.
3. Results and discussion
3.1. Synthesis and spectral properties
The chemical structures of silicon-containing ligands used in
this work and the synthetic paths to the Pd(II) and Co(II) metal
complexes created with these ligands are illustrated in Schemes 1
and 2. The compounds are yellow ligands, green or dark green Pd
(II) metal complexes and brown or dark brown Co(II) metal
complexes solids at room temperature. Ligands (L1H), (L2H), (L3H),
and (L4H) were prepared by condensation between different sali-
cylaldehyde and N-(3-(trimethoxysilyl)propyl)-ethylenediamine
using the general method described for the preparation of
salicylaldimine ligands [23,24]. Its characterization data (see
Experimental section) agreed with the proposed formulae.
The infrared spectra of ligands (L1H, L2H, L3H and L4H) and their
Pd(II) and Co(II) metal complexes have been studied order to
characterize their structures. The FT-IR spectra of all compounds
were carried out in the 4000e400 cmꢁ1 range. The characteristic
infrared spectrum data are given in Experimental section. The
n
(OeH) vibration of the free ligands is observed in the range
3546e2504 cmꢁ1. The occurrence of this band at such a relatively
low wavenumber is characteristic of these ligands and is an indi-
cation of fairly extensive hydrogen bonding [31]. In the FT-IR
spectra of Pd(II) and Co(II) metal complexes, this band does not
appear as expected. Characteristic absorptions in the FT-IR spectra
of ligands and their Pd(II) and Co(II) metal complexes are repre-
Treatment of 2:1 molar ratio amounts of ligands and Pd(AcO)2 or
Co(AcO)2$4H2O in refluxing methanol for 8 h, followed by the
removal of small amounts other residues, produced [M(Ln)2]
sented by stretching vibrations of the
n
(C]N) group of ligands
(C]
R
between 1632 and 1631 cmꢁ1. The low frequency shift of the
n
CH3
H3C
O
N) stretch (in between 1616 and 1602 cmꢁ1 for Pd(II) and 1614 and
1604 cmꢁ1 for Co(II) complexes), in comparison with the free
ligands, is consistent with N-coordination of Pd(II) and Co(II) ions to
the azomethine ligands [32,33]. The coordination of the salicy-
laldimine ligands to the Pd(II) or Co(II) centre through the azo-
methine nitrogen atom is expected to the reduce the electron
O
H
N
H3C
Si
O
N
CH
O
M
O
HC
N
O
N
H
Si
O
CH3
O
CH3
CH3
density in the azomethine link and lower the
n
(C]N) absorption
(CeO)
R
frequency. Coordination is further confirmed by shift in the
n
stretching vibration of the phenoxy group from the region
1280e1264 cmꢁ1 to a higher frequency range for Pd(II) and Co(II)
metal complexes which indicates MeO coordination [31,34,35]. The
coordination of the azomethine nitrogen and phenolic oxygen is
M: Pd(II) or Co(II)
R: 3-CH3, 4-OCH3, 5-CH3 and 5-OCH3
Scheme 2. The proposed structure for the Pd(II) and Co(II) complexes.

1376
M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
further supported by the appearance of two peaks at 521e512 cmꢁ1
for (MeO), due to stretching
(MeN) and 468e462 cmꢁ1 for
Table 1
Synthesis of styrene carbonate from styrene oxide and CO2 catalyzed by synthesized
complexes.
n
n
vibrations that are not observed in the infrared spectra of the
ligands [36]. Thus, it is clear that the ligands are bonded to the
palladium and cobalt metal ion in an N2O2 fashion through
the deprotonated phenolate oxygen, and azomethine nitrogen. This
peak is not observed in the free ligands. These results agree with
the expected structures. The results show that Pd(II) and Co(II)
complexes indicate deprotonation and coordination of the phenolic
OH group. It also confirms that nitrogen atom of the azomethine
group contributes to the complexation.
O
O
0.1% cat., 0.2% DMAP
O
O
CO2
100 oC,
2 h,
1.6 Mpa
Entries
Catalysts
Yielda
TONb
TOFc (hꢁ1
)
Electronic spectra of ligands (L1H, L2H, L3H and L4H) and their
mononuclear Pd(II) and Co(II) metal complexes have been recorded
in the 1100e200 nm range in EtOH, MeOH, and DMF (for ligands)
and in MeOH and DMF (for metal complexes) solutions and their
corresponding data are given in Experimental section. The absorp-
tion bands in the visible region can be safely assigned to spin-
allowed MLCT transitions, whereas the absorption bands in the
ultraviolet region are assigned to LC transitions. The electronic
spectra of the free ligands in EtOH, MeOH, and DMF showed strong
absorption bands in the UVevis region (301e456 nm), attributed to
1
2
3
4
5
6
7
8
[Pd(L1)2]
[Pd(L2)2]
[Pd(L3)2]
[Pd(L4)2]
[Co(L1)2]
[Co(L2)2]
[Co(L3)2]
[Co(L4)2]
74
81
82
79
58
47
81
42
740
810
820
790
580
470
810
420
370
405
410
395
290
235
405
210
a
Yield of styrene oxide to styrene carbonate was determined by comparing the
ratio of product to substrate in the 1H NMR spectrum of an aliquot of the reaction
mixture (see Fig. 2).
the p/
p* or n/p* transitions. Also, the free ligands exhibited a UV
b
Moles of cyclic carbonate produced per mole of catalyst.
bands attributed to the n/p* or n/s* transitions [25,26]. The
spectra of the Pd(II) and Co(II) metal complexes showed modifica-
tions with respect to the position and intensity of the characteristic
bands to the free ligands as compared. They also new bands which
were attributed to the formed metal complexes [37]. The absorption
bands in the range of 394e472 nm were attributed to the d/p*
c
The rate is expressed in terms of the turnover frequency (TOF (mole of product
(mole of catalyst h)ꢁ1) ¼ turnovers/h).
The results shown that the Pd(II) and Co(II) complexes bearing
5-methyl substituent on the aryl ring ([M(L3)2]) are more efficient
than the other Pd(II) and Co(II) metal complexes. The presence of
solvent such as dichloromethane did not use for all catalytic run
due to redundant according to previous study [39]. The catalyst [Pd
(L3)2] shows low activity (15% yield) without addition of DMAP that
can enhance the Lewis acidity of Pd2þ. While using only the DMAP
as catalyst without absence of metal species gave only 5% conver-
sion at the same catalytic conditions. As a result of this conclusion it
charge-transfer transitions, which are overlapping with the p/
p*
or n/p* transitions of the ligands. The spectrum of the free ligands
was compared with the spectra of the metal complexes, a new broad
but weak peak is observed at range 514e639 nm in different
solvents for Pd(II)and Co(II) metalcomplexes, whichcan beassigned
to the metal-ligand charge-transfer (MLCT) absorption between the
central Pd(II) and Co(II) ion and ligands. These modifications of the
shifts and intensity of the absorption bands, confirmed the coordi-
nation of the ligand to the Pd(II) or Co(II) central metallic ion. A
square planar geometry for Pd(II) and tetragonal geometry for Co(II)
complexes with four donor atoms (N2O2) in the same plan was
proposed for the metal complexes in solid state [38].
Table 2
Coupling of CO2 and various epoxides catalyzed by complex [Pd(L3)2].
Entries
Product
Yield (%)
Selectivity (%)
TON
TOF (hꢁ1
)
Magnetic susceptibility measurements provide sufficient data to
characterize the structure of the metal complexes. Magnetic
moments measurements of Co(II) metal complexes are carried out
in the solid state at room temperature and results are given in
Experimental section. The magnetic moments of the Co(II)
complexes are 2.24 B.M. (low-spin) for [Co(L1)2], 3.72 B.M. (high-
spin) for [Co(L2)2], 2.18 B.M. (low-spin) for [Co(L3)2] and 2.26 B.M.
(low-spin) for [Co(L4)2], respectively which suggests tetragonal
geometry for Co(II) metal complexes. Magnetic moments
measurements of Pd(II) metal complexes carried out at 25 ꢀC show
that all Pd(II) complexes are diamagnetic, indicating the low-spin
(S ¼ 0) square planar d8-systems, as expected.
O
O
O
1
82
99
820
410
O
O
O
O
2
3
4
74
99
69
99
98
97
740
990
690
370
495
345
CH
3
3.2. Catalytic properties
O
O
The coupling of carbon dioxide and epoxides catalyzed by Pd
(II) and Co(II) (Scheme 2) was investigated under different reac-
tion conditions. The results were shown in Tables 1 and 2. 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 aliquot of the reaction mixture (Fig. 1). It
can be seen that the catalysts of [Pd(L3)2] and [Co(L3)2] have more
effective activities than the other Pd(II) and Co(II) metal complexes
for the formation of cyclic organic carbonates from carbon dioxide
(Fig. 2).
Cl
O
O
O
CH
3
Reaction conditions:
6
(4.5 ꢂ l0ꢁ5 mol), DMAP (9 ꢂ l0ꢁ5 mol), epoxide (4.5 ꢂ
l0ꢁ2 mol), CO2 (1.6 MPa), 100 ꢀC, 2 h.

M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
1377
Fig. 1. A typical 1H NMR spectra of the reaction mixture (Table 1, entry 3).
can be understood that metal catalyst must be used with a Lewis
base such as DMAP. A comparative study with DMAP and NEt3 was
investigated. DMAP is more active base with 82% yield, the yield is
decreased to 23% when NEt3 was replaced as base.
When the reaction temperature and the time rise, the yield of
this reaction was augmented by using catalyst [Pd(L3)2]/DMAP
(Fig. 3) [39,44]. Meanwhile the electron-withdrawing groups at the
2-position of the epoxide activated the substrate, while electron
donating substituents deactivate the epoxide (see Table 2).
Epichlorohydrin was found to be the most reactive epoxide, while
propylene epoxide exhibited the lowest activity of the epoxides
surveyed.
The reaction conditions such as reaction time, temperature and
CO2 pressure were shown to have an influence on the catalytic
activity of transformation of SO to its related cyclic carbonate using
[Pd(L3)2] as catalyst (see Fig. 3). The influence of temperature on the
yield of styrene carbonate (SC) was investigated at CO2 pressures of
1.6 MPa. The yield of SC strongly depended on reaction temperature
(Fig. 3c). In the lower temperature range, the yield increased with
increasing temperature. However, the yield decreased slightly in
Regarding the reaction mechanism we envisioned the occur-
rence of a similar pathway to the one proposed previously [40], i.e.
activation of the epoxide by a Lewis acid and attacks of Lewis base
(DMAP) to the sterically less hindered carbon atom to open the
epoxide ring. The generated oxy anion species then reacts with CO2
to give the cyclic carbonate. Lewis base such as DMAP, NEt3 and
DBU is required as co-catalyst for the catalytic cycle. The catalytic
system is co-catalyzed by a Lewis base amine and Lewis acid metal
centre. The Lewis base and Lewis acid work together to open the
epoxy ring and then react with CO2 to give the corresponding cyclic
carbonate via a ring opening and recyclization process [41e43].
Fig. 2. The comparison of metal catalysts for the formation of styrene oxide to corresponding cyclic carbonate (styrene carbonate) at the same catalytic conditions.

1378
M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
a
b
c
Fig. 3. Conversion of styrene oxide as a function of time (a), pressure (b) and temperature (c) with active complexes.
the high-temperature region. Time is also effective parameter of
the conversion. SO reached 94% yield within 6 h, at 100 ꢀC (Fig. 3a).
Fig. 4b shows the dependence of SC yield on pressure at 100 ꢀC with
a reaction time of 2 h. An increase in pressure resulted in an
increase in the SC yield in the low-pressure region up to 1.8 MPa
and a decrease of the yield in the high-pressure region. A similar
effect was reported by Darensbourg et al. [45], catalytic trans-
formation of CO2 to polycarbonate increased with increasing the
pressure, but the value of TOF was decreased. The main reason for
this phenomenon was reported by Han et al. [44] as phase
behaviour of the substrates and CO2. In all catalytic runs, the
selectivity for the chemical fixation of CO2 into cyclic carbonate
form is over 98%. One of the active catalyst [Pd(L3)2] is reused more
than eight times with a minimal loss of its original catalytic activity
(Fig. 4). According to Fig. 4, we optimized the best fit catalytic
conditions (1.8 MPa CO2, 110 ꢀC and 2 h) for reuse studies. During
the catalytic recycling of [Pd(L3)2]/DMAP, there was a small
reduction of the propylene carbonate since, at the end of the eighth
cycle the value was, respectively, 14% lower than initial value
(Fig. 4).
4. Conclusions
100
90
90
88
A new series of metal complexes of salicyladimine ligands with
Pd(II) and Co(II) have been prepared and characterized by different
techniques. The new complexes, synthesized in good yields,
generate an active and useful catalyst for cycloaddition of CO2 to
epoxides in good to excellent yields. The electron donating
substituent on the p-position of aryl ring of the ligand system
accelerates the chemical fixation of CO2 into cyclic carbonates. The
active catalyst [Pd(L3)2] was reused more than eight times with
a minimal loss of its original catalytic activity.
86
86
90
80
70
60
50
83
78
76
Acknowledgment
1
2
3
4
5
6
7
8
Reuse
This work has been supported by the Research Fund of Harran
University (Sanliurfa, Turkey).
Fig. 4. Recycling studies of propylene oxide by [Pd(L3)2] catalyst.

M. Ulusoy et al. / Journal of Organometallic Chemistry 696 (2011) 1372e1379
1379
[22] A. Earnshaw, Introduction to Magnetochemistry. Academic Press, London,
1968, p. 4.
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