The orthopalladation dinuclear [Pd(L1)(μ-OAc)]2, [Pd(L2)(μ-OAc)]2 and mononuclear [Pd(L3)2] complexes with [N, C, O] or [N, O] containing ligands: Synthesis, spectral characterization, electrochemistry and catalytic properties

Article abstract of DOI:10.1016/j.jorganchem.2009.12.003

Treatment of the salicylaldimine ligands (L₁H, L₂H, L₃H, L₄H and L₅H) with palladium(II) acetate in absolute ethanol gave the orthopalladation dinuclear [Pd(L₁)(μ-OAc)]₂, [Pd(L₂)(μ-OAc)]₂ and mononuclear [Pd(L₃)₂] with the tetradentate ligands [N, C, O] or [N, O] moiety. The ligands L₁H and L₂H are coordinated through the imine nitrogen and aromatic ortho carbon atoms, whereas the ligand L₃H coordinated through the imine nitrogen and phenolic oxygens atoms. The Pd(II) complexes have a square-planar structure and were found to be effective catalysts for the hydrogenation of both nitrobenzene and cyclohexene. These metal complexes were also tested as catalysts in Suzuki-Miyaura coupling of aryl bromide in the presence of K₂CO₃. The catalytic studies showed that the introduction of different groups on the salicyl ring of the molecules effected the catalytic activity towards hydrogenation of nitrobenzene and cyclohexene in DMF at 25 and 45 °C. The Pd(II) complexes easily prepared from cheap materials could be used as versatile and efficient catalysts for different C-C coupling reactions (Suzuki-Miyaura reactions). The structure of ligands and their complexes was characterized by UV-Vis, FT-IR, ¹H and ¹³C NMR, elemental analysis, molar conductivity, as well as by electrochemical techniques.

Full text of DOI:10.1016/j.jorganchem.2009.12.003

Journal of Organometallic Chemistry 695 (2010) 697–706
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The orthopalladation dinuclear [Pd(L1)(l-OAc)]2, [Pd(L2)(l-OAc)]2
and mononuclear [Pd(L3)2] complexes with [N, C, O] or [N, O] containing ligands:
Synthesis, spectral characterization, electrochemistry and catalytic properties
b
b
c
a
d
d
Ahmet Kilic a, , Dilek Kilinc , Esref Tas , Ismail Yilmaz , Mustafa Durgun , Ismail Ozdemir , Sedat Yasar
*
a Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey
b Department of Chemistry, University of Siirt, 56100 Siirt, Turkey
c Department of Chemistry, Technical University of Istanbul, 34469 Istanbul, Turkey
d Department of Chemistry, University of Inonu, 44280 Malatya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of the salicylaldimine ligands (L1H, L2H, L3H, L4H and L5H) with palladium(II) acetate in abso-
lute ethanol gave the orthopalladation dinuclear [Pd(L1)( -OAc)]2, [Pd(L2)( -OAc)]2 and mononuclear
Received 14 October 2009
Received in revised form 2 December 2009
Accepted 7 December 2009
Available online 6 January 2010
l
l
[Pd(L3)2] with the tetradentate ligands [N, C, O] or [N, O] moiety. The ligands L1H and L2H are coordinated
through the imine nitrogen and aromatic ortho carbon atoms, whereas the ligand L3H coordinated
through the imine nitrogen and phenolic oxygens atoms. The Pd(II) complexes have a square-planar
structure and were found to be effective catalysts for the hydrogenation of both nitrobenzene and cyclo-
hexene. These metal complexes were also tested as catalysts in Suzuki–Miyaura coupling of aryl bromide
in the presence of K2CO3. The catalytic studies showed that the introduction of different groups on the
salicyl ring of the molecules effected the catalytic activity towards hydrogenation of nitrobenzene and
cyclohexene in DMF at 25 and 45 °C. The Pd(II) complexes easily prepared from cheap materials could
be used as versatile and efficient catalysts for different C–C coupling reactions (Suzuki–Miyaura reac-
tions). The structure of ligands and their complexes was characterized by UV–Vis, FT-IR, 1H and 13C
NMR, elemental analysis, molar conductivity, as well as by electrochemical techniques.
Keywords:
Orthopalladation
Spectroscopy
Catalysis
Suzuki–Miyaura coupling reaction
Hydrogenation
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
so common. A few articles focused on the use of bridging bidentate
and tridentate salicyaldimine ligands have been described, but the
Nitrogen donor ligands, such as salicylaldimines and azines, are
prone to undergo cyclometallation reaction with formation of a
examples of self-assembly of cyclometallated units are scarce [12].
Also, the reactions of ortho-hydroxyazobenzenes with Pd(II) salts
to give mononuclear [N, O] [13], [C, N, O] [14] or dinuclear [C, N]
[15] cyclometallated complexes have been reported.
stable five-membered metallacycle containing a carbon–metal
r
bond; many examples of this reaction are known [1–4], and it is
important because of its potential use in, e.g., regiospecific organic,
organometallic synthesis [5], in catalytic materials [6] and as well
as promoting unusual coordination environments [7]. The more
common cyclometallated complexes encountered in the literature
are the Pd(II) five-membered ring species that have activated aro-
matic carbon (sp2) atoms. Analogous compounds containing alkly
carbon (sp3)–Pd bonds are also known although these are more
scarce [8–10]. Among all the examples reported so far those con-
taining square-planar Pd(II) units have attracted special attention
due to their potential applications in different areas, including
their ability as molecular receptors [11]. Nevertheless, the exam-
The catalytic hydrogenation of benzene and its derivatives gen-
erally requires more severe conditions than that of simple olefins
[16,17]. Even though a lot of success has been achieved with regard
to transition metal complexes in the catalytic hydrogenation of
olefins [18–20], there are only few reports on studies involving
hydrogenations of arenes using homogeneous metal complex cat-
alysts [21–23]. As a catalyst, palladium is very important in phar-
maceutical industry. However, it is expensive and toxic for large-
scale applications. It is particularly important to reduce both its
loss and presence in a product solution. The ability of transition
metal catalysts to add or remove hydrogen from organic substrates
by either direct or transfer hydrogenation process is a valuable
synthetic tool [24]. Transition metal-catalyzed coupling reaction
is one of the most important processes in organic chemistry and
has been extensively studied since they represent a powerful and
popular method for the formation of carbon–carbon bonds. This
ples involving Pd(II) blocks containing a
r (Pd–C) bond are not
* Corresponding author.
E-mail address: kilica63@harran.edu.tr (A. Kilic).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.12.003

698
A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
strategy has been applied to the synthesis of many organic com-
pounds, especially in complex natural products, in supramolecular
chemistry and in materials science [25–27]. The importance of
biaryl units [26] as components of many kinds of compounds,
mainly pharmaceuticals, herbicides and natural products, as well
as in the field of engineering materials, such as conducting poly-
mers, molecular wires and liquid crystals, has attracted enormous
interest from the chemistry community. Palladium and nickel-cat-
alyzed Suzuki–Miyaura cross-coupling [28–30] is the most impor-
tant and efficient strategy for the construction of unsymmetrical
biaryl compounds. This cross-coupling methodology allows the
use of organic solvents and inorganic bases, tolerates many func-
tional groups. It is not affected by steric hindrance of the sub-
strates, and is suitable for industrial processes [31].
2.1. Synthesis of the salicylaldimine ligands
3,5-Bis(trifloromethyl)aniline-3,5-dibromosalicylaldimine (L1H),
3,5-bis(trifloromethyl)aniline-4-methoxysalicylaldimine (L2H), 3,5-
bis(trifloromethyl)aniline-5-methoxysalicylaldimine (L3H), 3,5-bis-
(trifloromethyl)aniline-3,5-di-tert-butylsalicylaldimine (L4H) and
2,4-bis(trifloromethyl)aniline-3,5-dibromosalicylaldimine (L5H) li-
gands were synthesized by the reaction of 1.0 mmol (0.23 g) 3,5-
bis(trifloromethyl)aniline in 30 ml absolute ethanol with 1.0 mmol
(0.27 g) 3,5-dibromosalicyaldehyde for (L1H), 1.0 mmol (0.16 g) 4-
methoxysalicyaldehyde for (L2H), 1.0 mmol (0.16 g) 5-methoxysali-
cyaldehyde for (L3H), 1.0 mmol (0.24 g) 3,5-di-tert-butylsalicyalde-
hyde for (L4H) in 35 ml absolute ethanol and 1.0 mmol (0.23 g) 2,4-
bis(trifloromethyl)aniline in 30 ml absolute ethanol with 1.0 mmol
(0.27 g) 3,5-dibromosalicyaldehyde in 35 ml absolute ethanol. Also,
3–4 drops of formic acid (HCOOH) were added as catalyst. The mix-
tures were refluxed for 5–6 h, followed by cooling to room tempera-
ture. The resulting crystals were filtered under vacuum. Then, the
products were recrystallized from absolute methanol. The products
are soluble in common solvents such as EtOH, MeOH, CHCl3, DMF
and DMSO.
In this paper, we describe the synthesis of five new salicylaldi-
mine ligands and their orthopalladated dinuclear [Pd(L1)(
l-OAc)]2,
[Pd(L2)( -OAc)]2 and mononuclear [Pd(L3)2] metal complexes. We
l
have attempted to prepare Pd(II) metal complexes of ligands L4H
and L5H to no avail. However, we describe to the best of our knowl-
edge, the synthesis of the first cyclometallated Pd(II) complexes
derived from salicylaldimine containing free phenolic-OH tetra-
dentate ligands. In these complexes, the ligands are coordinated
through imine nitrogen and aromatic carbon and not through the
phenolic OH groups. The compounds have been identified by a
combination of 1H and 13C NMR spectra, FT-IR spectra, UV–Vis
spectra, elemental analysis, molar conductivity measurements,
and electrochemical techniques. The new orthopalladation dinu-
2.1.1. Ligand (L1H)
Color: orange; m.p:138 °C; Yield (%): 68; Anal. Calc. for
C15H7F6Br2NO: C, 36.69; H, 1.44; N, 2.85. Found: C, 37.02; H,
1.72; N, 2.97%. 1H NMR (CDCl3, TMS, d ppm): 13.93 (s, 1H, –OH,
D-exchangeable), 8.62 (s, 1H, CH@N), 7.87 (s, 1H, Ar–CH), 7.84 (d,
1H, J = 2.1 Hz, Ar–CH), 7.73 (s, 2H, Ar–CH), 7.59 (d, 1H, J = 2.4 Hz,
Ar–CH). 13C NMR (CDCl3, TMS, d ppm): C1(163.57), C2(157.05),
C3(148.76), C4(139.44), C5(134.17), C6(133.49), C7(133.04),
C8(121.56), C9(121.05), C10(120.12), C11(112.43) and C12(110.97).
clear [Pd(L1)(l-OAc)]2, [Pd(L2)(l-OAc)]2 and mononuclear
[Pd(L3)2] species exhibited very good catalytic activity towards
the hydrogenation of nitrobenzene and cyclohexene. Also, these
metal complexes exhibited catalytic activity in the Suzuki–Miyo-
ura coupling of aryl bromide in the presence of K2CO3.
IR (KBr pellets, cmÀ1): 3290
(OH), 3104 and 3005
*
1617 (C@N), 1277 t(C–O). UV–Vis [kmax/nm, ( :shoulder peak)]:
t
t(Ar–CH),
t
*
*
254, 274, 278, 302, 310, 326 , 363 (in CHCl3), 249, 297, 319 , 402
(in EtOH) and 260, 284, 309, 351, 446 (in DMSO).
2. Experimental
*
All reagents and solvents were of reagent-grade quality and ob-
tained from commercial suppliers (Fluka and Merck). Tetra-n-
butylammonium perchlorate (TBAP, Fluka) was used as received.
Elemental analysis were carried out on a LECO CHNS model 932
elemental analyzer. FT-IR spectra were recorded on a Perkin–Elmer
Spectrum RXI FT-IR Spectrometer using KBr pellets. The FT-IR spec-
tra of all compounds were carried out in the 4000–400 cmÀ1 range.
1H NMR spectra were recorded on Bruker-Avance 400 MHz spec-
trometers. UV–Vis spectra were recorded on a Perkin–Elmer Lamb-
da 25 PC UV–Vis spectrometer. Molar conductivities (KM) were
recorded on Inolab Terminal 740 WTW Series. The cyclic voltam-
mograms (CV) were carried out using CV measurements with
Princeton Applied Research Model 2263 potentiostat controlled
by an external PC. A three electrode system (BAS model solid cell
stand) was used for CV measurements in CH2Cl2 and consisted of
a 1.6 mm diameter of glassy carbon electrode as working electrode,
a platinum wire counter electrode, and an Ag/AgCl reference elec-
trode. Tetra-n-butylammonium perchlorate (TBAP) was used as a
supporting electrolyte. The reference electrode was separated from
the bulk solution by a fritted-glass bridge filled with the solvent/
supporting electrolyte mixture. The ferrocene/ferrocenium couple
(Fc/Fc+) was used as an internal standard but all the potentials in
the paper were referenced to the Ag/AgCl reference electrode.
The solutions containing ligands and mono- and dinuclear Pd(II)
metal complexes were deoxygenated by a stream of high purity
nitrogen for at least 5 min. before running the experiment and
the solution was protected from air by a blanket of nitrogen during
the experiment. GC analyses were performed on a HP 6890N Gas
Chromatograph equipped with capillary column (5% biphenyl,
2.1.2. Ligand (L2H)
Color: yellow; m.p:86 °C; Yield (%): 63; Anal. Calc. for
C16H11F6NO2: C, 52.90; H, 3.05; N, 3.86. Found: C, 52.86; H, 3.06;
N, 3.99%. 1H NMR (CDCl3, TMS, d ppm): 12.93 (s, 1H, –OH, D-
exchangeable), 8.58 (s, 1H, CH@N), 7.77 (s, 1H, Ar–CH), 7.68 (s,
2H, Ar–CH), 7.35 (d, 1H, J = 8.4 Hz, Ar–CH), 6.56 (t, 2H, J = 2.4 Hz,
Ar–CH), 3.88 (s, 3H, O–CH3). 13C NMR (CDCl3, TMS, d ppm):
C1(164.34), C2(150.34), C3(134.36), C4(128.55), C5(124.93),
C6(121.41), C7(121.32), C8(119.57), C9(117.70), C10(112.60),
C11(107.94), C12(101.06) and C13(55.54). IR (KBr pellets, cmÀ1):
3274–2385
t
(OHÁÁÁN), 3095 and 3004
t(Ar–CH), 2936–2848
t
(Aliph-CH), 1627
t
(C@N), 1279
t
(C–O). UV–Vis [kmax/nm,
*
*
*
( :shoulder peak)]: 247, 300 , 344 (in CHCl3), 245, 302 , 343 (in
*
EtOH) and 273, 304, 355, 376, 409, 451 (in DMSO).
2.1.3. Ligand (L3H)
Color: orange; m.p:89 °C; Yield (%): 71; Anal. Calc. for
C16H11F6NO2: C, 52.90; H, 3.05; N, 3.86. Found: C, 52.82; H, 2.98;
N, 3.93%. 1H NMR (CDCl3, TMS, d ppm): 12.04 (s, 1H, –OH, D-
exchangeable), 8.66 (s, 1H, CH@N), 7.81 (s, 1H, Ar–CH), 7.71 (s,
2H, Ar–CH), 7.19–6.95 (m, 3H, Ar–CH), 3.84 (s, 3H, O–CH3). 13C
NMR (CDCl3, TMS, d ppm): C1(165.42), C2(155.71), C3(152.58),
C4(150.21), C5(123.17), C6(132.72), C7(124.85), C8(122.20),
C9(121.55), C10(121.24), C11(119.57), C12(117.62) and C13(55.93).
IR (KBr pellets, cmÀ1): 3295
2966–2841 (Aliph–CH), 1621
t(OH), 3066 and 3019 t(Ar–CH),
t
t
(C@N), 1280
t
(C–O). UV–Vis
*
*
[kmax/nm, ( :shoulder peak)]: 247, 256, 279, 296, 394 (in CHCl3),
95% dimethylsiloxane) (30 m  0.32 mm  0.25
lm).
242, 284, 377 (in EtOH) and 263, 327, 362 (in DMSO).

A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
699
2.1.4. Ligand (L4H)
2.2.2. [Pd(L2)(l-OAc)]2
Color: yellow; m.p: 97 °C; Yield (%): 63; Anal. Calc. for
C23H25F6NO: C, 62.02; H, 5.66; N, 3.14. Found: C, 61.94; H, 5.73;
N, 3.27%. 1H NMR (CDCl3, TMS, d ppm): 13.05 (s, 1H, –OH, D-
exchangeable), 8.73 (s, 1H, CH@N), 7.80 (s, 1H, Ar–CH), 7.75 (s,
2H, Ar–CH), 7.57 (d, 1H, J = 2.4 Hz, Ar–CH), 7.32 (d, 1H, J = 2.1 Hz,
Ar–CH), 1.58 (s, 9H, C–CH3) and 1.38 (s, 9H, C–CH3). 13C NMR
Yield: (52%), color: green, m.p. P300 °C. Anal. Calc. for
C34H12Br4F12N2O6Pd2 (MW: 1169 g/mol): C, 36.98; H, 2.24; N,
2.40. Found: C, 36.48; H, 2.13; N, 2.31%. KM = 28
X ,
À1 cm2 molÀ1
1H NMR (CDCl3, TMS, d ppm): 8.60 (s, 2H, CH@N), 7.89 (s, 1H,
Ar–CH), 7.92 (s, 2H, CH@N), 7.80 (s, 3H, Ar–CH), 7.63 (s, 2H, Ar–
CH), 7.10 (d, 2H, J = 8.7 Hz, Ar–CH), 6.24 (dd, 2H, J = 2.4 Hz, Ar–
CH), 5.56 (d, 2H, J = 2.4 Hz, ÀOH, D-exchangeable), 3.67 (s, 6H, O–
CH3), 1.61 and 1.27 (s, 6H, CH3COO). 13C NMR (CDCl3, TMS, d
ppm): C1(167.19), C2(166.79), C3(161.95), C4(150.43), C5(135.94),
C6(131.84), C7(131.39), C8(125.86), C9(124.97), C10(121.36),
C11(113.63), C12(108.28), C13(100.78), C14(55.01) and C15(29.71).
(CDCl3, TMS,
d
ppm): C1(166.58), C2(158.53), C3(150.28),
C4(141.24), C5(137.40), C6(133.11), C7(132.67), C8(129.55),
C9(127.49), C10(124.92), C11(122.18), C12(121.54), C13(120.47),
C14(117.69) C15(35.16), C16(34.33), C17(31.52) and C18(29.63). IR
(KBr pellets, cmÀ1): 3432
(Aliph–CH), 1607
( :shoulder peak)]: 246, 291, 365 (in CHCl3), 231, 287, 363 (in
t
(OH), 3090
t(Ar–CH), 2960–2872
t
t
(C@N), 1280 (C–O). UV–Vis [kmax/nm,
t
IR (KBr pellets,
2925–2853 (Aliph-H), 1738
t t(OH), 3060 and 3025 t(Ar–H),
max/cmÀ1): 3410
*
t
t
(COO), 1614
t
(C@N), 1278
t
(C–O),
*
*
*
EtOH) and 274, 291, 310, 360 (in DMSO).
527
t
(Pd–N). UV–Vis [kmax/nm, ( :shoulder peak)]: 262, 270 ,
*
312, 406 (in CHCl3), 258, 309, 401 (in EtOH) and 269, 309, 399
(in DMSO).
2.1.5. Ligand (L5H)
Color: yellow; m.p: 74 °C; Yield (%): 66; Anal. Calc. for
C15H7Br2F6NO: C, 36.69; H, 1.44; N, 2.85. Found: C, 36.96; H,
1.52; N, 2.98%. 1H NMR (CDCl3, TMS, d ppm): 12.95 (s, 1H, ÀOH,
D-exchangeable), 8.57 (s, 1H, CH@N) ve 7.96–7.50 (m, 5H, Ar–
2.2.3. [Pd(L3)2]
Yield: (62%), color: brown, m.p. P300 °C. Anal. Calc. for
C32H20F12N2O4Pd (MW: 830.4 g/mol): C, 46.25; H, 2.43; N, 3.37.
Found: C, 46.29; H, 2.64; N, 3.20%. KM = 23
NMR (CDCl3, TMS, d ppm): 8.23 (s, 2H, CH@N), 7.02 (d, 6H,
J = 3.3 Hz, Ar–CH), 6.95 (d, 3H, J = 3.0 Hz, Ar–CH), 6.92 (d, 3H,
X ,
À1 cm2 molÀ1 1H
CH). 13C NMR (CDCl3, TMS,
d ppm): C1(163.70), C2(157.06),
C3(150.28), C4(146.73), C5(141.97), C6(139.64), C7(134.93),
C8(134.35), C9(127.84), C10(121.10), C11(120.30), C12(116.72),
C13(112.50), C14(111.54) and C15(110.88). IR (KBr pellets, cmÀ1):
J = 3.3 Hz, Ar–CH), 3.63 (s, 6H, O–CH3). IR (KBr pellets, tmax
cmÀ1): 3060 and 3025
(Ar–H), 2990–2837 (Aliph-H), 1593
(Pd–N), 461 (Pd–O). UV–Vis [kmax
/
t
t
3303–2397
t
(OHÁÁÁN), 3074
t(Ar–CH), 1629 t(C@N), 1266 t(C–O).
t
(C@N), 1284
t
(C–O), 544
t
t
/
*
*
*
UV–Vis [kmax/nm, ( :shoulder peak)]: 242, 255, 270, 356 (in CHCl3),
238, 263, 291, 328, 350 (in EtOH) and 262, 339, 440 (in DMSO).
nm, ( :shoulder peak)]: 250, 256, 297, 462 (in CHCl3), 241, 295,
461 (in EtOH) and 261, 296, 449 (in DMSO).
*
*
*
2.3. Hydrogenation procedure
2.2. Synthesis of the complexes
The hydrogenation of nitrobenzene and cyclohexene was car-
ried out in a thermostatic reaction flask (100 ml) at 25 and 45 °C
under 760 Torr H2 with vigorous stirring in dry and deoxygenated
50 ml DMF solution. A catalyst was added into 50 ml DMF and sat-
urated with H2 gas for 10–15 min. After addition of NaBH4, the
mixture was stirred for ca. 5 min. and then nitrobenzene and cyclo-
hexene were transferred into the vessel. Later, the H2 gas was bub-
bled again into the flask and the volume of the absorbed H2 gas was
measured periodically. Nitrobenzene and cyclohexene, as identi-
fied by means of FT-IR scanning, were completely reduced to ani-
line and cyclohexane.
A solution of palladium(II) acetate (1.0 mmol, 0.23 g) in abso-
lute ethanol (20 ml) was added to a solution of ligands L1H
(2.0 mmol, 0.98 g), L2H (2.0 mmol, 0.73 g), L3H (2.0 mmol, 0.73 g),
L4H (2.0 mmol, 0.89 g) and L5H (2.0 mmol, 0.98 g), in absolute eth-
anol (50 ml) at 50–60 °C. A distinct color change was observed
from the initial colorless solution under a N2 atmosphere with con-
tinuous stirring. After the mixture were evaporated to a volume of
15–20 ml under vacuum by heating 3 h in a water bath. After cool-
ing to room temperature, the solutions were filtered through Celite
to remove the all small amount of black palladium formed. Elution
with methanol afforded green or brown products after solvent
removals, which were recrystallized from methanol to give the de-
sired products as green or brown solids. Then products dried in va-
cuo at 35 °C. We have attempted to prepare Pd(II) complexes of
ligands L4H and L5H in different solvents, with no success.
2.4. General procedure for the Suzuki–Miyaura coupling reaction
The catalyst (1.0 mmol% of Pd complexes), aryl halides
(1.0 mmol), phenyl boronic acid (1.5 mmol), K2CO3 (2.0 mmol),
diethyleneglycol-di-n-butylether as internal standard (30 mg),
DMF (3 ml) were all added to a small Schlenk tube and the mixture
was heated at 100 °C for 6 h in an oil bath. Then, the mixture was
cooled, filtered and concentrated. The purity of the compounds
was checked by GC and TLC with NMR. The yields were based on
different aryl halides.
2.2.1. [Pd(L1)(l-OAc)]2
Yield: (56%), color: brown, m.p. = 261 °C. Anal. Calc. for
C34H12Br4F12N2O6Pd2 (MW: 1305 g/mol): C, 31.30; H, 0.93; N,
2.15. Found: C, 31.26; H, 1.10; N, 2.34%. KM = 21
X ,
À1 cm2 molÀ1
1H NMR (CDCl3, TMS, d ppm): 13.40 (s, 2H, ÀOH, D-exchangeable),
8.63 (s, 2H, CH@N), 7.95 (s, 2H, Ar–CH), 7.91 (s, 1H, Ar–CH), 7.86 (d,
1H, J = 2.4 Hz, Ar–CH), 7.73 (s, 2H, Ar–CH), 7.68 (s, 1H, Ar–CH), 7.60
(d, 2H, J = 2.1 Hz, Ar–CH), 7.34 (d, 1H, J = 2.4 Hz, Ar–CH), 1,60 (s, 6H,
CH3COO). 13C NMR (CDCl3, TMS, d ppm): C1(164.12), C2(163.59),
C3(150.19), C4(148.78), C5(141.58), C6(139.48), C7(135.88),
C8(134.17), C9(125.04), C10(121.59), C11(120.88), C12(116.61),
3. Results and discussion
3.1. Synthesis and characterization
The ligands employed provide several coordination approaches
as a function of the substitution pattern of the phenyl ring. The li-
gands L1H, L2H and L3H show available aromatic sp2 carbon atoms
suitable for cyclometallation, as well as OCH3 and OH substituents
liable to carbon–metal and oxygen–metal bond formation. On the
other hand, ligands L1H and L2H are found to bind to palladium
C13(106.63) and C14(26.93). IR (KBr pellets,
t
max/cmÀ1): 3415
t
t
(OH), 3068
(C@N), 1281
t
(Ar–H), 2917–2848 (COO), 1592
*
(C–O), 545 t(Pd–N). UV–Vis [kmax/nm, ( :shoulder
t
(Aliph-H), 1727
t
t
*
*
peak)]: 272, 300, 362, 459 (in CHCl3), 255 , 277, 357, 445 (in
EtOH) and 260, 302, 354, 434 (in DMSO).

700
A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
only through the imine nitrogen and aromatic carbon atoms, not
through the phenolic hydroxy groups [15]. Whereas, ligand L3H
coordinates imine nitrogen atom and phenolic hydroxy atom.
Hence, in L1H and L2H the palladium atom was found to be bonded
exclusively through the C6 atom, with no sign of metallation of the
C2 hydroxy group, resulting in the formation of a five-membered
palladated. Whereas, ligand L3H coordinates via the imine nitrogen
atom and phenolic hydroxy atom. The cyclopalladation reaction
with ligands L1H and L2H, which was carried out using palladium
acetate in ethanol as described earlier for some other azomethine
OCH3
Br
Br
O
O
O
O
HO
HO
Pd
Pd
HC
HC
2
N
N
2
ligands [32], afforded
l-acetato-bridged dimers [Pd(L1)(l-OAc)]2
F3C
F3C
Pd(L1)(µ-OAc)]2
CF3
CF3
and [Pd(L2)( -OAc)]2. In anhydrous ethanol, the phenolic OH func-
l
[Pd(L2)(µ-OAc)]2
tion therefore remains unreactive towards Pd(II), allowing the
ortho metallation reaction to take its own course – the Pd–C bond
being unreactive [33].
For the convenience of the reader the compounds and proposed
structures are shown in Schemes 1 and 2. The compounds de-
scribed in this paper were characterized by elemental analysis (C,
H, N), FT-IR, UV–Vis, 1H and 13C NMR spectroscopy, molar conduc-
tivity measurements, and electrochemical techniques. Also, the
catalytic activities of the new Pd(II) metal complexes have been
F3C
Pd
CF3
H3CO
O
N
HC
CH
N
O
investigated. The complexes [Pd(L1)(l-OAc)]2, [Pd(L2)(l-OAc)]2
and [Pd(L3)2] were synthesised by the reactions of the ligands with
Pd(AcO)2 in a 1:1 or 1:2 molar ratio in ethanol and shown in
(Scheme 2). Magnetic moment measurements of all metal com-
plexes were carried out at 25 °C. The results show that Pd(II) metal
complexes are diamagnetic as expected.
OCH3
CF3
F3C
[Pd(L3)2]
Scheme 2. The proposed structure for Pd(II) metal complexes.
The data of the 1H NMR spectra (in CHCl3) obtained for ligands
and their di- and mononuclear Pd(II) metal complexes are given in
the experimental section. The proton resonance is appearing as a
broad low intensity singlet at d = 13.93–12.04 ppm in the spectra
ligands (LnH) due to the OHÁÁÁN protons involved in intramolecular
D2O-exchangeable H-bonding. In contrast to expectation, in the 1H
with bridging acetate ligands. The chemical shifts of the acetate
protons are obtained at 1.60 ppm as singlets for [Pd(L1)( -OAc)]2
(Fig. 1) and at 1.61 and 1.27 ppm as singlets for [Pd(L2)( -OAc)]2.
Also, the chemical shifts of the acetate methyl carbons are obtained
at 26.93 ppm for [Pd(L1)( -OAc)]2 and at 29.71 ppm for [Pd(L2)(
OAc)]2. But, these characteristic acetate methyl protons and carbon
chemical shifts are not observed in the 1H and 13C NMR spectra of
[Pd(L3)2] complex. The 13C NMR data reveals the different field
shifts of the C nuclei relative to the free ligands, while the C@N,
C1 and C6 resonances reveal different field shifts of the Pd(II) metal
complexes, confirming metallation of the phenyl ring [4,35]. In the
13C NMR, the imine carbon resonance was found at 157.05 ppm for
(L1H), 150.34 ppm for (L2H), 155.71 ppm for (L3H), 158.53 ppm for
l
l
NMR spectra of [Pd(L1)(
glet peak in a low intensity appeared at d = 13.40–5.56 ppm due to
the presence of the OH protons involved in the dinuclear [Pd(L1)(
OAc)]2 and [Pd(L2)( -OAc)]2 (Fig. 1). This result indicates that the
l-OAc)]2 and [Pd(L2)(l-OAc)]2, a broad sin-
l
l-
l
-
l
(L1H) and (L2H) ligands bind palladium ions only through the imine
nitrogen and aromatic carbon atoms, not through phenolic hydro-
xy groups [15,34]. Whereas, this characteristic band are not ob-
served in the 1H NMR spectra of [Pd(L3)2] complex, which
indicates that the ligand (L3H) coordinates to palladium ion
through imine nitrogen atom and the deprotonated phenolic hy-
droxy group. Also, the appearance of acetate methyl groups in
the 1H and 13C NMR spectra maybe confirms the formulation of
(L4H), 157.06 ppm for (L5H), 150.19 ppm for [Pd(L1)(
150.43 ppm for [Pd(L2)( -OAc)]2. The other data of NMR spectra
l-OAc)]2 and
l
are given in experimental section.
[Pd(L1)(l-OAc)]2 and [Pd(L2)(l-OAc)]2 as typical dinuclear species
F3C
F3C
F3C
R
EtOH
N=CH
Reflux
+
NH2
CHO
R
OH
HO
F3C
R: 3,5-Br (L1H), 4-OCH3 (L2H), 5-OCH3 (L3H) and 3,5-C(CH3)3 (L4H)
CF3
CF3
R
EtOH
+
NH2
F3C
CHO
F3C
N=CH
Reflux
R
OH
HO
R: 3,5-Br (L5H)
Scheme 1. The synthesis route of proposed ligands.

A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
701
Fig. 1. The 1H NMR spectrum of L1H (a) and [Pd(L1)(
l-OAc)]2 (b).
The infrared spectra of ligands and their metal complexes have
been studied in order to characterize their structures. The main
bands in the infrared spectrum for all compounds are given in
experimental section. Characteristic absorptions in the spectra
FT-IR of ligands and their Pd(II) metal complexes are represented
by stretching vibrations of the CH@N group of ligands between
1629–1607 cmÀ1. The low frequency shift of the CH@N stretch
(in between 1614 and 1592 cmÀ1), in comparison with the free li-
gands, is consistent with N-coordination of Pd(II) ions to the azo-
methine ligands [32,36,37]. The coordination of the
salicylaldimine ligands to the Pd(II) center through the azomethine
nitrogen atom is expected to the reduce the electron density in the
acetate as the metal salt [38]. Furthermore, additional absorption
peaks from the stretching vibrations of the Pd–N, Pd–C and Pd–O
bonds appear in the low frequency in all complexes.
Electronic spectra of ligands and Pd(II) metal complexes have
been recorded in the 1100–200 nm range and their corresponding
data are given in experimental section. The electronic spectra of li-
gands and their Pd(II) metal complexes in CHCl3, EtOH and DMSO
solutions showed absorption bands at 231–462 nm. In the elec-
tronic spectra of the ligands and their Pd(II) complexes, the wide
*
*
range bands seem to be due to both the
p
?
p
and n ?
p
of
C@N chromophore or charge-transfer transition arising from
p
electron interactions between the metal and ligand which involves
either a metal-to-ligand or ligand to-metal electron transfer and d–
d transitions [39,40]. The absorption bands observed within the
range of 231–406 nm are most probably due to the transition of
azomethine link and lower the
spectra FT-IR of ligands are characterized by the appearance of a
band at between 3432 and 2385 cmÀ1 due to the free
(O–H) or
intramolecular
(O–HÁÁÁN) groups. In the spectra FT-IR of
[Pd(L1)( -OAc)]2 and [Pd(L2)( -OAc)]2 complexes, these bands
again appear at between 3415 and 3410 cmÀ1 due to free
(O–H)
t(C@N) absorption frequency. The
t
*
*
t
p ? p transitions in the benzene ring or n ? p of imine group
l
l
or acetate groups corresponding to the ligands and their Pd(II) me-
tal complexes [41]. The electronic spectra of Pd(II) metal com-
plexes show absorption bands at between 406–462 nm are
assigned to metal to ligand charge-transfer (MLCT) and
1A1g ? 1B1g transitions, respectively [42]. In this study, the elec-
tronic spectra of all compounds were recorded in the solution
state. Thus d–d bands could not be observed [16].
t
group. Whereas, in the spectra of [Pd(L3)2] complex, this band is
absent. These results show that ligands L1H and L2H are found to
bind the palladium ions only through the imine nitrogen and aro-
matic carbon atoms, not through the deprotonation phenolic hy-
droxy group, whereas, the ligand L3H binds the Pd(II) ion through
imine nitrogen atom and the deprotonation phenolic hydroxy
With a view to studying the electrolyte nature of the mono- and
dinuclear Pd(II) metal complexes, their molar conductivities were
measured in DMF (dimethyl formamide) at 10À3 M. The molar con-
ductivities (KM) values of these Pd(II) metal complexes are in the
atom. Also, the appearance of
at 1738–1727 cmÀ1 confirms the formulation of [Pd(L1)(
and [Pd(L2)( -OAc)]2 complexes as typical dinuclear species with
bridging acetate ligands as obtained previously when using Pd(II)
t(COO) bands in the FT-IR spectra
l-OAc)]2
l
range of 28–21
X
À1 cm2 molÀ1 at room temperature, indicating

702
A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
Table 1
their almost non-electrolyte nature. Thus, these metal complexes
are very poor in molar conductivity (Scheme 2) [43].
Conditions, initial rate of H2 absorption, catalytic activity of Pd(II) complexes at
760 Torr of H2 and 25 and 45 °C for nitrobenzene.
Compounds [Cat]
[Ph-NO2]
Initial rate of H2
Specific activity
mol H2/mol-cat
(min)
(10À4 mol/ (10À3 mol/ W absorption
3.2. Catalytic studies
L)
L)
(mmol/min)
3.2.1. Catalytic reduction of nitrobenzene and cyclohexene by Pd(II)
complexes
[Pd(L1)(
OAc)]2
l
-
16
16
16
2.73
0.04
0.06
0.05
10.23 (25 °C)
9.96 (45 °C)
10.08
The catalytic studies showed that the mono- and dinuclear
Pd(II) metal complexes exhibited a catalytic activity toward the
hydrogenation of nitrobenzene and cyclohexene under H2 gas
(760 Torr) in DMF solution at 25 and 45 °C (Schemes 3 and 4).
Nitrobenzene and cyclohexene, as identified by means of FT-IR
scanning, were completely reduced to aniline and cyclohexane.
Although addition of NaBH4 to the catalysts solution prior to the
introduction of H2 accelerated their catalytic activity, the hydroge-
nation did not generally need any preliminary activation (Tables 1
and 2, Figs. 2–7) The conditions, initial rate absorption of H2, and
specific activity for Pd(II) metal complexes are presented in Tables
1 and 2. The course of reduction for some mono- and dinuclear
Pd(II) catalysts are shown in Figs. 2–7. The initial rate of H2 absorp-
tion and the specific catalytic activity are approximately identical
for all mono- and dinuclear Pd(II) metal complexes. The results
indicate that introduction of different groups on in the salicylaldi-
mine ring increases the catalytic activity of all complexes. The elec-
tron-donating/withdrawing and other functional groups probably
increase the electron density on the Pd atom, which is subjected
to electrophilic attack by a nitro group of nitrobenzene and double
band of cyclohexene [13,34]. We carried out the same reaction un-
der identical conditions without the catalyst, with organic ligand,
with Pd(AcO)2. In all these three cases no hydrogenation product
was detected at the end of the reaction and the FT-IR spectra of
compounds have not been changed. These results show that the
hydrogenation was catalyzed by the added Pd(II) complex. Finke
and co-workers [44,45] reported that the hydrogenation of ben-
zene under vigorous conditions (50–100 °C) using metal com-
plexes proceeds through the formation of M(0) nanoclusters. We
have carried out the hydrogenation of nitrobenzene and cyclohex-
ene using the mono- and dinuclear Pd(II) metal complexes at the
temperature of 25 and 45 °C. We think that this hydrogenation
activity cannot be due to Pd(0), as it is not possible to reduce Pd(II)
metal complexes to Pd(0) at 25 and 45 °C. Further in the case of
Pd(II) metal complexes, at the end of the reaction there were no
particles of palladium metal in the reaction mixture and the addi-
tion of mercury, as a selective poison for colloidal/nanoparticle cat-
alysts, to the reaction system does not significantly affect the
percentage conversion of nitrobenzene and cyclohexene. From
(25 °C + NaBH4)
9.91
(45 °C + NaBH4)
7.50 (25 °C)
8.10 (45 °C)
4.45
(25 °C + NaBH4)
5.80
(45 °C + NaBH4)
7.11 (25 °C)
6.84 (45 °C)
4.25
16
0.06
[Pd(L2)(
OAc)]2
l
-
24
24
24
2.73
2.73
0.05
0.06
0.03
24
0.05
[Pd(L3)2]
24
28
24
0.05
0.05
0.04
(25 °C + NaBH4)
4.08
(45 °C + NaBH4)
28
0.04
Table 2
Conditions, initial rate of H2 absorption, catalytic activity of Pd(II) complexes at
760 Torr of H2 and 25 and 45 °C for cyclohexene.
Compounds [Cat]
[Cyclohexene] Initial rate of
H2
Specific activity
mol H2/mol-cat
(min)
(10À4 mol/ (10À3 mol/L)
W
L)
absorption
(mmol/min)
[Pd(L1)(
OAc)]2
l-
24
24
16
3.24
3.24
0.01
0.01
0.02
2.71 (25 °C)
2.76 (45 °C)
5.24
(25 °C + NaBH4)
5.61
(45 °C + NaBH4)
2.05 (25 °C)
2.74 (45 °C)
2.60
(25 °C + NaBH4)
1.90
16
0.02
[Pd(L3)2]
28
24
28
0.02
0.02
0.02
24
0.02
(45 °C + NaBH4)
these evidences, the hydrogenation of two organic compounds in
our system proceeds through a homogeneous mechanism and
not through the formation of Pd(0) nanoparticles [16]. From the re-
sults shown in Tables 1 and 2, it is evident that the [Pd(L1)(l-
OAc)]2 complex is more efficient than the other complexes for
the conversion of nitrobenzene to aniline at 25 °C temperature
and the [Pd(L1)(l-OAc)]2 complex is more efficient than the other
NH2
NO2
complexes for the conversion of cyclohexene to cyclohexane at
45 °C temperature and in the presence of NaBH4. The catalytic
activities are maybe correlated with the size of the Pd(II) metal
complexes, location of substituent, the temperature of reactions
and the absence/presence of NaBH4.
1) [Cat ], H2(g), 760 Torr,
2) [Cat + NaBH4], H2(g), 760 Torr,
, DMF
Scheme 3. Catalytic activity of Pd(II) complexes at 760 Torr of H2 and 25 and 45 °C
for nitrobenzene.
3.2.2. Suzuki–Miyaura coupling reaction by Pd(II) complexes
The nature of the initial precatalyst, solvent and base is crucial
for the success of the Suzuki–Miyaura coupling reaction. The reac-
tion condition screen demonstrated that the catalytic activity of
Pd(II) metal complexes in Suzuki–Miyaura coupling reaction is
highly solvent and base dependent (Scheme 5). Among palla-
dium-catalyzed cross-coupling processes, the Suzuki reaction of
aryl/vinyl halides with boronic acid is one of the most efficient
methods for C–C bond formation. We did the optimization exper-
iments and we found that when the reaction is carried out using
K2CO3 in mixture of DMF and water at 100 °C (6 h) the best result
1) [Cat ], H2(g), 760 Torr,
2) [Cat + NaBH4], H2(g), 760 Torr,
, DMF
Scheme 4. Catalytic activity of Pd(II) complexes at 760 Torr of H2 and 25 and 45 °C
for cyclohexene.

A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
703
250
200
150
100
50
300
a
a
b
c
b
c
250
200
150
100
50
0
0
50
100
150
200
250
0
0
50
100
150
200
250
Time (min)
Time (min)
Fig. 5. Hydrogenation of nitrobenzene as catalysts Pd(II) complexes + NaBH4 at
45 °C in DMF solution. (a = [Pd(L1)(l-OAc)]2, b = [Pd(L2)(l-OAc)]2, c = [Pd(L3)2]).
Fig. 2. Hydrogenation of nitrobenzene as catalysts Pd(II) complexes at 25 °C in DMF
solution. (a = [Pd(L1)(l-OAc)]2, b = [Pd(L2)(l-OAc)]2, c = [Pd(L3)2]).
300
a
300
a
b
c
250
d
b
250
200
150
100
50
c
200
150
100
50
0
0
50
100
150
200
250
Time (min)
Fig. 6. Hydrogenation of nitrobenzene as [Pd(L3)2] catalysts at 25 and 45 °C in DMF
solution. (a = nitrobenzene (25 °C), b = nitrobenzene (45 °C), c = nitroben-
zene + NaBH4 (25 °C) and d = nitrobenzene + NaBH4 (45 °C).
0
0
50
100
150
200
250
Time (min)
140
a
Fig. 3. Hydrogenation of nitrobenzene as catalysts Pd(II) complexes at 45 °C in DMF
b
c
120
solution. (a = [Pd(L1)(
l
-OAc)]2, b = [Pd(L2)(l-OAc)]2, c = [Pd(L3)2]).
d
e
100
f
g
80
h
250
a
60
b
40
20
0
c
200
0
50
100
150
200
250
Time (min)
150
100
50
Fig. 7. Hydrogenation of cyclohexene as [Pd(L1)(
25 and 45 °C in DMF solution. (a = 25 °C, b = 45 °C, c = NaBH4 + 25 °C and
d = NaBH4 + 45 °C for [Pd(L1)( -OAc)]2 and (e = 25 °C, f = 45 °C, g = NaBH4 + 25 °C
l-OAc)]2 and [Pd(L3)2] catalysts at
l
and h = NaBH4 + 45 °C for [Pd(L3)2].
is obtained. We initially tested the catalytic activity of the Pd(II)
metal complexes for the coupling of different aryl bromides with
phenylboronic acid and the control experiments showed that the
coupling reaction did not occur in the absence of the catalyst. Un-
der these conditions, different aryl bromides react cleanly with
phenylboronic acid in good yields (Table 3). The palladium-cata-
lyzed reaction of aryl halides with arylboronic acids (the Suzuki
0
0
50
100
150
200
250
Time (min)
Fig. 4. Hydrogenation of nitrobenzene as catalysts Pd(II) complexes + NaBH4 at
25 °C in DMF solution. (a = [Pd(L1)( -OAc)]2, b = [Pd(L2)( -OAc)]2, c = [Pd(L3)2]).
l
l

704
A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
100 oC, 6 h
[Cat],
B(OH)2
+
Br
R
R
DMF, K2CO3
Scheme 5. The Suzuki–Miyaura coupling of different aryl bromides with phenylboronic acid
Table 3
The Suzuki–Miyaura coupling of different aryl bromides with phenylboronic acid.
b
Entry
[Cat] (complex)
R
Product
Yield (%)a
TONb
TOF (hÀ1
)
1
2
3
4
5
6
7
8
[Pd(L1)(
[Pd(L2)(
[Pd(L3)2]
[Pd(L1)(
[Pd(L2)(
[Pd(L3)2]
[Pd(L1)(
[Pd(L2)(
[Pd(L3)2]
[Pd(L1)(
[Pd(L2)(
[Pd(L3)2]
l
l
-OAc)]2
-OAc)]2
H
H
H
93
70
88
82
93
97
98
97
95
88
75
95
69
59
87
93
70
88
82
93
97
98
97
95
88
75
95
69
59
87
16
12
15
14
16
16
16
16
16
15
13
16
12
10
15
l-OAc)]2
COCH3
COCH3
COCH3
CHO
CHO
CHO
NO2
NO2
NO2
OMe
OMe
OMe
l-OAc)]2
COCH3
CHO
l-OAc)]2
l-OAc)]2
9
10
11
12
13
14
15
l-OAc)]2
l-OAc)]2
NO2
[Pd(L1)(
[Pd(L2)(
[Pd(L3)2]
l-OAc)]2
l-OAc)]2
OMe
Reaction conditions: 1.0 mmol of p-R-C6H4Br; 1.5 mmol of phenylboronic acid; 2.0 mmol K2CO3; % 1.0 mmol Pd (Cat.) was used; DMF 3 mL; temperature 100 °C and at 6 h.
a
GC-yield using diethyleneglycol-di-n-butylether as internal standard
b
TON = mol product/mol cat. and TOF = (mol product/mol cat.) Â hÀ1
.
coupling reaction) is the most common method for C–C bond for-
mation [26,30,46]. The reactions are usually carried out homoge-
neously in the presence of a base under inert atmosphere. The
reactivity of the aryl halide component decreases sharply in the or-
der X = I > Br > Cl and electron-withdrawing substituents R are re-
quired for the chlorides to react [26,30,46–48]. The palladium-
catalyzed cross-coupling reaction of phenylboronic acid with aryl
bromides has been summarized in Table 3 and Scheme 5. The ob-
tained results indicate that catalytic activities are correlated with
the size of the Pd(II) metal complexes and location of bromo or
methoxy substituent. From the results shown in Table 3, it is evi-
0
-100
-200
-300
L H
1
L H
2
dent that the [Pd(L1)(l-OAc)]2 complex is more efficient than the
L H
3
other complexes for the product one and three. The [Pd(L3)2] com-
plex is more efficient than the other complexes for the product
two, four and five.
-2.0
-1.5
-1.0
-0.5
0.0
Potential / V vs. Ag/AgCl
Fig. 8. Cyclic voltammograms (CVs) of the ligands in CH2Cl2 containing 0.1 M TBAP.
Scan rate: 100 mV sÀ1. Working electrode: a 1.6 mm diameter of glassy carbon
electrode.
3.3. Electrochemistry
The electrochemistry of the ligands (L1H, L2H and L3H) and their
palladium complexes [Pd(L1)(l-OAc)]2, [Pd(L2)(l-OAc)]2 and
[Pd(L3)2] are studied by the cyclic voltammetry in the scan rates
of 25–500 mV sÀ1 in CH2Cl2 solution containing 0.1 M TBAP sup-
porting electrolyte. The cyclic voltammograms (CVs) of the ligands
are shown in Fig. 8 at 100 mV sÀ1 scan rate. As seen, (L2H) and
(L3H) ligands exhibited one irreversible reduction processes with-
out corresponding anodic waves. (L3H) ligand also showed second
reduction wave with corresponding anodic wave as well as the first
reduction process as observed for (L2H) and (L3H) ligands. The
reduction processes of the ligands are based on the reduction of
the azomethine nitrogen in the presence of the phenolic proton
in the molecule. Mostly, the process results in the reductive cou-
pling of the reduced species as previously observed for some Schiff
bases such as Salen or its derivatives, indicating irreversible fash-
ion of the processes [49–51]. However, the reduction process is
probably assigned to the electro-deprotonated ligand anion at
the platinum surface. The ligands (L1H), (L2H) and (L3H) displayed
the cathodic peak potentials at Epc = À1.19, À1.65, and À1.78 V
versus Ag/AgCl, respectively. The second reduction wave of (L1H)
appeared at Epa = À1.77 V. It is well known that the introduction
of electron-donating or electron-withdrawing groups to the elec-
troactive molecules is expected to lead to an increase or decrease
of electron density in the molecule, thereby making the molecule
easier to oxidise and harder to reduce or vice versa [52–56]. This
effect was clearly demonstrated by the fact that the cathodic peak
potential of (L1H) was shifted towards more positive potentials
compared to those of (L2H) and (L3H). It is strongly attributed to
the electron-withdrawing bromo group substituted on the benzene
ring. Therefore, the second reduction process for (L1H) could be ob-
served within the electrochemical scale studied. The cyclic voltam-
mogram (CV) of [Pd(L2)(l
-OAc)]2 in the scan rate of 100 mV sÀ1 is
exhibited in Fig. 9. Where the inset figure represents CVs of the
complex in the scan rates of 25–500 mV sÀ1 between 0 and 1.2 V
potential ranges. The data corresponding to the cyclic voltammo-
grams (CVs) of the ligands and their complexes are also listed in
Table 4. [Pd(L2)(l-OAc)]2 and [Pd(L3)2] showed two oxidation

A. Kilic et al. / Journal of Organometallic Chemistry 695 (2010) 697–706
705
4. Conclusions
-1
-1
500 mVs
250 mVs
100 mVs
50 mVs
25 mVs
Ia
50
100
50
0
-1
In this study, the five salicylaldimine ligands (L1H, L2H, L3H, L4H
and L5H) and their orthopalladation dinuclear [Pd(L1)( -OAc)]2,
[Pd(L2)( -OAc)]2 and mononuclear [Pd(L3)2] metal complexes were
-1
-1
IIa
l
l
0
synthesized and characterized by elemental analyses, FT-IR, UV–
Vis, 1H and 13C NMR spectra, molar conductivity, as well as by elec-
trochemical techniques. The L1H and L2H are coordinated through
the imine nitrogen and aromatic ortho carbon atoms, whereas the
ligand L3H is coordinated through the imine nitrogen and phenolic
oxygens atoms. The catalytic studies showed that the introduction
of different groups on the salicyl rings of the molecules increased
the catalytic activity towards hydrogenation of nitrobenzene and
cyclohexene in DMF at 25 and 45 °C. The electron-donating/with-
drawing and other functional groups probably increases the elec-
tron density on the Pd atom, which is subjected to electrophilic
attack by a nitro group of nitrobenzene and a double bond of cyclo-
hexene. Also, the obtained Suzuki–Miyaura coupling reaction re-
sults indicate that catalytic activities are correlated to the size of
the Pd(II) complexes and to the position of the bromo or methoxy
substituents.
Ic
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs. Ag/AgCl
Ia
IIc
Ic
0.0
0.5
1.0
1.5
Potential / V vs. Ag/AgCl
Fig. 9. Cyclic voltammogram (CV) of [Pd(L2)(
TBAP. Scan rate: 100 mV sÀ1. The inset figure is represents CVs of the complex in the
scan rates of 25–500 mV sÀ1 between
and 1.2 V potential ranges. Working
l-OAc)]2 in CH2Cl2 containing 0.1 M
0
electrode: a 1.6 mm diameter of glassy carbon electrode.
Acknowledgements
Table 4
Voltammetric data for the ligands and their palladium complexes vs. Ag/AgCl (Fc/Fc+):
values in parentheses vs. Fc/Fc+ in CH2Cl2–TBAP.
This work has been supported, in part, by the Research Fund of
Harran University (Sanliurfa, Turkey).
Complexes
L/LÀ
L/L+
Pd2+/Pd4+
D
Ed(V)
Ea (V)
Ec (V)
Eb (V)
pc
1/2
pa
(L1H)
À1.19 (À1.69)
À1.77e (À2.27)
À1.65 (À2.15)
À1.78 (À2.28)
À1.23 (À1.73)
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