Structural studies of complex compounds of 6,6'-diacetyl-2,2'-bipyridine dioxime with copper(I/II), platinum(II), and palladium(II) metal ions

Article abstract of DOI:10.3906/kim-1004-539

The tetradentate ligand 6,6'-diacetyl-2,2'-bipyridine dioxime (L) was obtained in high yield by the condensation reaction of 6,6'-diacetyl-2,2'- bipyridine with hydroxyl amine in alkaline solution. Density functional calculations and 3D modeling of the structure at the B3LYP/6-31G(d) level of theory for L revealed that the energy difference between the global trans conformer (trans-L), which possesses the C_2h point group, and the distorted cis conformer (cis-L) was 5.214 kcal/mol. Synthesis and characterization of the metal complexes with Cu(I), Cu(II), Pt(II), and Pd(II) metal ions were reported. L acted as a N_4- donor ligand to coordinate to the metal centers via N atoms of the 2,2'-bipyridine and the imine moieties to afford tetrahedral complex [Cu^I L]PF₆ (1), distorted octahedral complex [Cu^II L(OH₂)₂](NO ₃)₂ (2), and square-planar complexes [Pt^II L]Cl₂ (3) and [Pd^II L]Cl (4), respectively. The CHN analysis for 1 implies that L coordinated to 2 copper(I) ions as a bridging ligand to form a dinuclear metal complex, [Cu₂L₂] ²⁺, due to the twisting of the coordinated bonds exposed by the tetrahedral geometry preference for the metal ion. The ligand formed intramolecular hydrogen bonds between the oxime groups in 4, as revealed by the spectroscopic studies. The most stable conformations of the compounds were obtained by using the molecular mechanics optimization feature in CAChe software with an augmented MM2 force field. ¹.

Full text of DOI:10.3906/kim-1004-539

Turk J Chem
35 (2011) , 25 – 36.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1004-539
Structural studies of complex compounds of
6,6ꢀ-diacetyl-2,2ꢀ-bipyridine dioxime with copper(I/II),
platinum(II), and palladium(II) metal ions
1,∗
1
1
2
¨
˘
˘
Abdurrahman S¸ENGUL , Hakan AGAC¸ , Burak C¸ OBAN and Erol EROGLU
1Department of Chemistry, Faculty of Arts and Sciences, Karaelmas University,
67100, Zonguldak-TURKEY
e-mail: sengul@karaelmas.edu.tr
2Department of Physics, Faculty of Arts and Sciences, Harran University, Osmanbey Campus,
63300, S¸anlıurfa-TURKEY
Received 14.04.2010
The tetradentate ligand 6,6ꢀ -diacetyl-2,2ꢀ -bipyridine dioxime (L) was obtained in high yield by the
condensation reaction of 6,6ꢀ -diacetyl-2,2ꢀ -bipyridine with hydroxyl amine in alkaline solution. Density
functional calculations and 3D modeling of the structure at the B3LYP/6-31G(d) level of theory for L
revealed that the energy difference between the global trans conformer (trans-L), which possesses the C2h
point group, and the distorted cis conformer (cis-L) was 5.214 kcal/mol. Synthesis and characterization
of the metal complexes with Cu(I), Cu(II), Pt(II), and Pd(II) metal ions were reported. L acted as a N4 -
donor ligand to coordinate to the metal centers via N atoms of the 2,2ꢀ -bipyridine and the imine moieties to
afford tetrahedral complex [CuI L]PF6 (1), distorted octahedral complex [CuII L(OH2)2 ](NO3)2 (2), and
square-planar complexes [PtII L]Cl2 (3) and [PdII L]Cl (4), respectively. The CHN analysis for 1 implies
that L coordinated to 2 copper(I) ions as a bridging ligand to form a dinuclear metal complex, [Cu2 L2 ]2+
,
due to the twisting of the coordinated bonds exposed by the tetrahedral geometry preference for the metal
ion. The ligand formed intramolecular hydrogen bonds between the oxime groups in 4, as revealed by
the spectroscopic studies. The most stable conformations of the compounds were obtained by using the
molecular mechanics optimization feature in CAChe software with an augmented MM2 force field. 1
Key Words: Oxime, N-donor ligands, bipyridine, copper, platinum, palladium
∗Corresponding author
25

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Introduction
The coordination chemistry of oximes has been widely explored, though their use in supramolecular coordination
chemistry remains largely unexplored.2−4 The oxime moiety is potentially an ambidentate and can coordinate
either through the N or O atoms.2 The vast literature on structural studies of oxime complexes reveals some
interesting features of its coordination behavior. It may coordinate to one metal ion through the N atom and
another metal ion through the O atom. In the majority of complexes, only the nitrogen atoms coordinate to the
metal center, and the oxygen atoms remain uncoordinated.4 The hydrogen atom of the oxime =N–OH group can
form potentially strong intramolecular or intermolecular hydrogen bonds with other donor atoms or groups.5
Thus, metal complexes bearing nondeprotonated oximes may be considered as supramolecular synthons, capable
of forming extended supramolecular networks via intermolecular hydrogen bonds. The orientation of the oxime
group in these molecules plays a major role in determining the dimensionality of the extended network.6
Despite earlier studies establishing its capability to form hydrogen bonds,7−8 the oxime group has
received far less attention in supramolecular chemistry and crystal engineering9 than have other groups such
as carboxyl,10 amide,11 and alcohol.12 More specifically, molecular building blocks can be designed to carry
particular functional groups that are capable of recognition of other groups or self-recognition through the
formation of one or more hydrogen bonds. By such a synthetic approach, even quite complex molecular
aggregates (supermolecules) can be prepared in a designed manner.13
In the solid state, 6,6ꢀ -dimethyl-2,2ꢀ -bipyridine is a planar centrosymmetric molecule in which the
pyridine N atoms have a transoid arrangement by virtue of the symmetry.14 In general, 2,2ꢀ -bipyridines
substituted at the 6,6ꢀ -positions form double-stranded helical metal complexes with metal ions having preference
for tetrahedral geometry, i.e. Ag(I) and Cu(I), and mononuclear complexes with dicationic metal ions such as
Cu(II).15 This was exemplified by the tetradentate ligand, 6,6ꢀ -bis(3-pyrazolyl)-2,2ꢀ -bipyridine ligand, which
can behave either as a tetradentate chelate in a mononuclear Cu(II) complex, or as a bridging ligand in
a dinuclear complex with Ag(I), by adopting to the stereoelectronic requirements of both metal centers.16
Herein we present a structurally similar N4 -donor ligand, 6,6ꢀ -diacetyl-2,2ꢀ -bipyridine dioxime, and its complex
compounds with d10 copper(I), d9 copper(II), d8 platinum(II), and palladium(II) metal ions.
Experimental
Physical measurements
Melting points were determined with a Bu¨chi Melting Point B-540 Model digital melting point instrument.
Elemental analysis data were obtained with a LECO 932 CHNS analyzer. 1 H NMR spectra were recorded
on a Bruker DPX 400 spectrometer, using DMSO-d6 as a solvent and TMS as an internal standard. Infrared
spectra, as KBr pellets, were recorded using a JASCO FT-IR-300 E spectrophotometer. API-ESI mass spectra
were obtained with an LC/MS Agilent 1100 MSD mass spectrometer.
26

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Synthesis of 6,6ꢀ -diacetyl-2,2ꢀ-bipyridine dioxime (L)
The syntheses of N,Nꢀ -dioxide-2,2ꢀ -bipyridine,17 6,6ꢀ -dicyano-2,2ꢀ -bipyridine,18 and 6,6ꢀ -diacetyl-2,2ꢀ -bipyridi-
ne19,20 were previously reported. The 6,6ꢀ -diacetyl-2,2ꢀ -bipyridine dioxime (L) was synthesized according to
the literature,19,20 as depicted in Scheme 1. The reaction was carried out under a dry N2 atmosphere. An
excess amount of hydroxylamine hydrochloride (1.10 g, 15.6 mmol) was dissolved in ethanol (10 mL) and mixed
with an excess amount of sodium hydroxide (0.62 g, 15.6 mmol) in water (5 mL). To this solution, 6,6ꢀ -diacetyl-
2,2ꢀ -bipyridine (1.50 g, 7.1 mmol) dissolved in ethanol (5 mL) was added slowly. The solution was set to reflux
for 2 h. The solution was filtered and washed with water and ethanol, and dried under a vacuum. The white
precipitate was recrystallized from DMF to give 1.62 g (85%). Mp: 301 ◦ C. Anal. Calc. for C14 H14 N4 O2 : C,
62.21; H, 5.22; N, 20.73. Found: C, 62.0; H, 5.2; N, 20.7 (%). IR (ν cm−1): 3382 (O–H), 3143 (C–Har), 2838
(C–Hal), 1627 (C=N), 1560 (C=N), 1419 (C–C), 955 (N–O).
H2O2/ AcOH
N+
O-
N+
O-
N
N
DMS
KCN
n-BuLi/THF
N
N
N
N
O
O
NC
CN
NH2OH.HCl
N
N
N
HO
N
OH
L
Scheme 1.
Synthesis of copper(I) complex (1) [CuL]PF6 or [Cu2 L2 ](PF6)2
Since the geometrical isomers of the dioxime ligand could not be separated by column chromatography, L was
obtained as a mixture of (E) and (Z) isomers, and was used without separation. The establishment of the
conformational and configurational features of the ligand will be a future study, including X-ray analysis and
molecular mechanics investigations.
L (0.27 g, 1.0 mmol) was dissolved in acetonitrile (40 mL) under an inert nitrogen atmosphere.
A
few drops of trifluoroacetic acid were added to this solution with vigorous stirring to assure the dissolution
27

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Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
of the ligand. [Cu(CH3 CN)4 ]PF6 (0.372 g, 1.0 mmol) was added to the homogenous solution and it was
stirred for an additional 15 min. In order to prevent the oxidation of the metal center, as indicated by the
occurrence of a yellowish-green solution, hydrazine dihydrochloride (H2 NNH2·2HCl) was added with stirring.
The solution was concentrated under a vacuum and filtered, washed with hexane, and dried in a desiccator over
silica gel, 0.25g (75%). The complex was soluble in acetonitrile, ethanol, methanol, and DMF. Anal. Calc. for
[Cu2 L2 ](PF6)2·2H2 O: C, 33.85; H, 3.25; N, 11.28. Found: C, 33.2; H, 3.1; N, 11.4 (%). API-ES (m/z): 307.0
[L·2H2 O]; 334.0 [CuL] or [Cu2 L2 ]2+ . IR (ν cm−1): 3535 (H2 O), 3414 (O–H), 3125 (C–Har.), 1621 (C=N),
1500 (C=C), 960 (N–O).
Synthesis of copper(II) complex (2) [CuL](NO3)2
L (0.27 g, 1.0 mmol) was dissolved in aqueous methanol (35 mL). The pH of the solution was adjusted to
approximately 2 with 1 M HCl (10 mL) and set to reflux to obtain a clear solution. To this homogenous
solution, Cu(NO3)2 (0.187 g, 1.0 mmol) was added and the reaction was kept at 50 ◦ C with continuous stirring
for 1 h. The pH of the yellowish-green solution was adjusted to approximately 4 with a 1 M NaOH solution. The
solution was then concentrated on a rotary evaporator, filtered, and allowed to cool to room temperature. An
excess of a concentrated aqueous solution of NaNO3 was added. The precipitate was filtered off, washed with
methanol and water, and dried in a desiccator over silica gel, 0.22 g (59%). API-ES (m/z): 307.0 [L]·2H2 O;
334.0 [CuL]2+ ; 372.0 [CuL(H2 O)2 ]2+ ; 369.1[Cu(L)]2+. IR (ν cm−1): 3500 (H2 O), 3400 (O–H), 3150 (C–H.),
2800 (C–H), 1610 (C=N), 1500 (C–C), 1450 ( C=C), 965 (N–O).
Synthesis of platinum(II) complex (3) [PtL]Cl2
L (0.15 g, 0.5 mmol) was dissolved in 2 M HCl (10 mL), and K2 PtCl4 (0.21 g, 0.5 mmol) in water (10 mL)
was added with vigorous stirring. The solution was refluxed for 2 h. The dark solid was filtered and dried in
a desiccator, 0.11 g (60%). The crude product was slightly soluble in common polar organic solvents such as
MeCN, MeOH, CHCl3 , THF, and DMF. Anal. Calc. for [C14 H18 Cl2 N4 O4 Pt]: C, 29.38; H, 3.17; N, 9.79.
Found; C, 29.1; H, 3.3; N, 7.4 (%). 1 H NMR (DMSO-d6 , δ ppm): 11.60 (s, 2H, N–OH), 8.60 (t, J4,3 = 8.5 Hz,
2H, H4,4’), 8.53 (d, J3,4 = 8,5 Hz, 2H, H3,3’), 8.39 (d, J5,4 = 8,4 Hz, 2H, H5,5’). IR (ν cm−1); 3382 (N–H),
3143 (O–H), 2838 (C–H), 1627 (C=N), 1450 (C=C), 1419 (C–C), 966 (N–O).
Synthesis of palladium(II) complex (4) [PdL]Cl
L (0.15 g, 0.5 mmol) was dissolved in DCM/CH3 CN (15/10 v/v), and dichloro(1,5-cyclooctadine)palladium(II),
Pd(COD)Cl2 (0.5 mmol, 10 ml), in CH2 Cl2 (15 mL) was added with vigorous stirring. The solution was kept
at 70 ◦ C for 2 h. The resulting dark solid was filtered and dried in a desiccator, 0.12 g (66%). ESI-MS (m/z):
375.3 [Pd(L)]+ . 1 H NMR (DMSO-d6 , δ ppm); 7.38 (t, 2H, H4,4’), 7.23 (2H, H3,3’), 7.06 (d, 2H, H5,5’), 2.50
(s, 6H, CH3 ). IR (ν cm−1): 3400 (O–H), 3200 (C–Har), 2900 (C–H), 1580 (C=N), 1500 (C–C), 1460 (C=C),
1419 (C–C), 967 (N–O).
28

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Results and discussion
Synthesis and characterization of 6,6ꢀ -diacetyl-2,2ꢀ-bipyridine dioxime (L)
The synthesis of L is depicted in Scheme 1. The spectroscopic and physical data were identical to the reported
data in the literature19−21 and in good agreement with the molecular structure optimized by DFT as shown
in Figure 1. The intense OH-stretching vibrational band of the oximes lay in the expected region of 3450-3030
21
21,22
cm−1
.
The band of the C=N– group appeared at between 1615-1585 and 1625-1620 cm−1
.
Since the
aromatic ring vibrations occurred in the same region, the C=N band was usually not clearly recognizable. The
N–O stretching vibrational band lay in the region of 1055 and 870 cm−1 and often split into several maxima.21
In the IR spectrum, L showed major absorption bands at 2838 cm−1 , which were attributed to the C–H
vibrations of the methyl groups. The characteristic band at 3382 cm−1 was attributed to the O–H vibrations.
The aromatic stretching mode of the aromatic C–H groups appeared at 3143 cm−1 . The strong absorption band
at 1560 and 1419 cm−1 was the combination stretching of the C=N and C–C vibrations. The stretching mode
of the N–O bond was attributed to 955 cm−1 . The stretching mode of the oxime C=N bond was attributed to
1627 cm−1
.
Molecular structure of L
Calculations and 3D modeling for L were performed using the Gaussian 03 quantum chemistry package.23 To
save computational time, initial geometry optimizations were carried out with the molecular mechanics (MM)
method, using the MM+ force fields. The lowest energy confirmations of the molecules obtained by the MM
method were further optimized by the DFT method,24 by employing Becke’s 3-parameter hybrid functional
(B3LYP)25 and the 6-31G (d) basis set. Fundamental vibrations of L were also calculated using the same
method to check whether there were true minima. The possible stable conformers of L were determined by
potential energy scan calculations over the single rotatable bond between the 2 rings. In order to find the
final geometric parameters of the obtained stable conformers, each structure was optimized at the same theory
level. Via subsequent single point energy calculations by varying the dihedral angle of 18N–1C–6C–17N atoms,
2 stable low-energy conformers of L were obtained. The energy difference between the global trans conformer
(trans-L), which possesses the C2h point group, and the distorted cis conformer (cis-L) was 5.21 kcal/mol.
The pyridine rings in the cis-L had a dihedral angle of 40◦ . The optimized geometries of these conformers and
the numeration of the atoms of L are presented in Figure 1. The comparison of the geometric parameters of
the trans and the cis conformers of L are presented in Table 1.
Synthesis and characterization of copper(I) complex (1)
Copper(I) complex 1 was synthesized by the reaction of L with the [Cu(CH3 CN)4 ]·PF6 salt in CH3 CN and
a minimum amount of trifluoroacetic acid mixture, as illustrated in Scheme 2. The infrared spectra of the
transition metal oxime complexes were diagnostic of binding.26−30 All of the complexes displayed a strong
stretch near 3400-3480 cm−1 , indicating that the complexes were protonated in the solid state and that the
oxime protons were not hydrogen-bonded. While these ν (O–H) absorptions did occur in the typically broad
25
O–H stretching region, 3000-3500 cm−1
,
they were observed at higher energies than those that are observed
29

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
for most cis-configured bis(νic-dioximato) complexes (3180-3300 cm−1) or uncoordinated oxime ligands.26
Generally, values at approximately 3650 cm−1 are assigned to nonhydrogen-bonded ν (OH), while ν (NO· · ·H)
29,30
is assigned to values of approximately 3255 cm−1
.
Figure 1. The 2 low-energy stable conformers of free ligand 6,6ꢀ -diacetyl-2,2ꢀ -bipyridine dioxime (L): a) trans global
conformer, b) distorted cis conformer.
In the IR spectrum of 1, the bands in the mid-IR range were mostly due to the stretching vibrations of
the oxime moieties and in-plane vibrations of the pyridine rings. Generally, after coordination, the position of
some vibrations would change. Thus, in the case of 1, the strong and broad absorption band at 3535 cm−1
was attributable to intramolecular hydrogen-bonded O–H· · ·O groups. The aromatic C–H vibration appeared
at 3125 cm−1 . The C=N vibration of oxime moved to a lower frequency at 1621 cm−1 with a lower intensity.
The C=C vibrations appeared at 1500 cm−1 . The N–O vibration appeared at 960 cm−1
.
30

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Table. Selected structural parameters from the DFT/B3LTP method using the 6-31G(d) basis set.
˚
Bond lengths (A) Trans -L Cis -L
Bond angles (◦)
C19-C10-C9
Trans -L Cis -L
O31-N22
N22-C20
1.402
1.287
1.508
1.492
1.403
1.395
1.388
1.336
1.342
1.492
1.403
1.286
1.509
1.491
1.405
1.392
1.404
1.337
1.342
1.496
120.60
117.76
121.63
119.26
118.85
122.85
116.57
120.07
117.70
122.21
118.83
118.75
122.68
116.93
C19-C10-N17
C20-C24
N17-C10-C9
C20-C5
C10-N17-C6
C5-C4
C10-C9-C8
C4-C3
N17-C6-C7
C3-C2
N17-C6-C1
C1-N18
Dihedral Angles (◦)
O33-N21-C19-C10
O33-N21-C19-C23
C23-C19-C10-C9
N21-C19-C10-N17
C10-N17-C6-C7
C10-N17-C6-C1
N17-C6-C1-N18
C7-C6-C1-C2
N18-C5
-179.30
-0.04
-178.74
-0.05
C1-C6
Bond angles (◦)
H34-O33-N21
O33-N21-C19
N21-C19-C23
C23-C19-C10
N21-C19-C10
-11.18
-11.56
0.17
-23.91
-24.41
0.37
101.60
112.42
124.30
119.57
116.11
101.59
111.831
123.87
120.18
115.93
-179.89
-189.80
-178.94
179.91
-39.14
-40.05
Figure 2. Optimized structure of (1) [CuL]+ generated from the CAChe software package with augmented MM3
parameters.
In the API-ES mass spectrum, the molecular ion peak corresponding to 1 was observed at m/z 334.0,
which supports the formulation of [CuL]+1 as shown by the optimized molecular structure in Figure 2. This
value also corresponds to dinuclear complex [Cu2 L2 ]+2 , as illustrated in Scheme 2. These 2 species could not
be differentiated due to the low resolution of the mass spectrometer. However, the CHN analysis for 1 implies
that the ligand acted as a bridging ligand to coordinate to 2 copper(I) ions to form a dinuclear metal complex
31

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Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
[Cu2 L2 ]2+ , due to the twisting of the coordinated bonds exposed by the tetrahedral geometry preference of
the metal center, as revealed by the very similar ligand.16
+
N
N
Cu
N
N
OH
OH
TFA / CH3CN
, N2
[CuL]
L
+ [Cu(CH3CN)4]PF6
+2
OH
N
Cu
OH
N
N
N
N
Cu
N
N
HO
N
HO
[Cu2L2](PF6)2
Scheme 2.
Synthesis and characterization of copper(II) complex (2)
Copper(II) is a 6-coordinate elongated octahedral complex with CuN4 O2 , where the equatorial position is
defined by the N atoms of the ligand and the axial position by the water molecules, as shown in Figure 3.
Figure 3. Optimized structure of (2) [CuL(H2 O)2 ]2+ generated from the CAChe software package with augmented
MM3 parameters.
In the IR spectrum of complex 2, the coordinated H2 O molecules were identified by a broad O–H
absorption band at around 3400 cm−1 . The broad band at 3500 cm−1 was attributed to the N–H vibration.
32

¨
Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
The aromatic C–H vibration appeared at 3150 cm−1 and the aliphatic C–H vibration appeared at 2800 cm−1
.
The intensity of characteristic stretching and bending vibrations of the free ligand were shifted and lowered
on the complex formation, and new vibrational bands characteristic for complex 2 were observed. The N–O
vibration moved to a higher frequency at 965 cm−1 . The oxime C=N stretch decreased from 1627 cm−1 in the
free ligand to 1610 cm−1 in complex 2. The characteristic band due to (C–C) and (C=C) vibrations appeared
2,30
at 1500 and 1450 cm−1
.
In the API-ES mass spectrum, the molecular ion peak at m/z 334.0 corresponded
to the cationic fragment of [CuL]2+ . The simulated isotopic pattern was in good agreement with the measured
spectrum. The secondary peak at m/z 372.0 could be attributed to [CuL(H2 O)2 ]2+ in the solution, and was
identical to the calculated value at m/z 370.0. The tertiary peak at m/z 307.0 could be attributed to [L](H2 O)2
in the solution, showing that the complex underwent demetallation under the mass condition.
Synthesis and characterization of platinum(II) complex (3)
The optimized molecular structure of 3 is shown in Figure 4. In the IR spectrum of complex 3, a broad
O–H absorption peak appeared at 3382 cm−1 . The aromatic C–H vibration appeared at 3143 cm−1 and
the aliphatic C–H vibration appeared at 2838 cm−1 . The N–O vibration appeared at 966 cm−1 . The C=N
stretching vibrations of the oxime and the pyridine rings appeared at 1627 and 1582 cm−1 . The characteristic
band due to C–C and C=C vibrations appeared at 1500 and 1450 cm−1
.
Figure 4. Optimized structure of (3) [PtL]2+ generated from the CAChe software package with augmented MM3
parameters.
The CHN analysis showed the ratio of metal to ligand for complex 3 to be 1:1, which was consistent with
the formulation of [Pt(L)]Cl2 for [C14 H18 Cl2 N4 O4 Pt].
The 1 H NMR of the ligand in DMSO showed OH proton resonance at 11.60 ppm as a broad singlet due
to the exchange of protons with solvent. The aliphatic methyl group’s resonance was at 2.33 ppm as a singlet.
The aromatic protons of the ligand appeared in the expected region, at 8.60, 8.53, and 8.39 ppm, and were
assigned accordingly, almost identical to the reported oxime derivatives.31
33

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Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Synthesis and characterization of palladium(II) complex (4)
The optimized molecular structure of 4 is depicted Figure 5. In the IR spectrum of complex 4, a broad O–H
absorption peak appeared at 3400 cm−1 . The aromatic C–H vibration appeared at 3200 cm−1 and the aliphatic
C–H vibration appeared at 2900 cm−1 . The N–O vibration appeared at 967 cm−1 . The characteristic bands
for C–C and C=C vibrations appeared at 1500 and 1460 cm−1 . The C=N stretching vibration of the pyridine
rings appeared at 1580 cm−1 . The coordination of the oxime groups to the metal center through the N atoms
are expected to reduce the electron density and lower the C=N absorption frequency.
Figure 5. Optimized structure of (4) [PdL]1+ generated from the CAChe software package with augmented MM3
parameters.
ESI-MS measurements have been widely used to identify the species in solution as a very soft technique
that reduces demetallation in mass condition.32 The ESI-MS analysis was consistent with the formulation of
[PdL]+ for [C14 H13 N4 O2 Pd]+ . The molecular ion peak at m/z 375.3 corresponded to the cationic fragment.
The simulated isotopic pattern was in good agreement with the measured spectrum and was identical to the
calculated value at m/z 376.0. This result also supported the (O· · ·H–O) bonding, where one of the oximes was
deprotonated to act as a monoanionic ligand.
O–H· · ·O stretching and bending vibrations are usually observed in the ranges of 2200-2400 cm−1
and 1600-1800 cm−1 , respectively.2 The stretching vibrations between 1550-1650 cm−1 and 2250-2300 cm−1
supported the intramolecular O–H· · ·O hydrogen bond, as depicted in Figure 5.
The 1 H NMR spectrum of the 4 did not show any signal for the N–OH protons in DMSO-d6 , suggesting
that the complex underwent rapid H-D exchange with the solvent. The aliphatic methyl group’s resonance was
as a singlet at 2.50 ppm. The aromatic protons appeared at 7.38, 7.23, and 7.06 ppm. These values were in
accordance with the previously reported oxime derivatives and support the intramolecular hydrogen bonding.30
34

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Structural studies of complex compounds of..., A. S¸ENGUL , et al.,
Acknowledgements
The authors are grateful to the Scientific and Technological Research Council of Turkey [TBAG-2450 (104T060)],
and also to Zonguldak Karaelmas University for grant 2007/3-13-02-01.
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