Synthesis and characterization of palladium(II) complexes [PdX 2(p-diben)] (X = Cl, Br, I, N3, or NCO; P-diben = N,N′-bis(4-dimethylaminobenzylidene)ethane-1,2-diamine): Crystal structure of [Pd(N3)2(p-diben)]

Article abstract of DOI:10.1007/s11243-014-9824-y

Five new complexes of general formula [PdX₂(p-diben)], where p-diben = N,N′-bis(4-dimethylaminobenzylidene)ethane-1,2-diamine) (1) and X = Cl (2), Br (3), I (4), N₃ (5), or CNO (6), were synthesized and characterized by physicochemic

Full text of DOI:10.1007/s11243-014-9824-y

Transition Met Chem
DOI 10.1007/s11243-014-9824-y
Synthesis and characterization of palladium(II) complexes
[PdX2(p-diben)] (X 5 Cl, Br, I, N3, or NCO; p-diben 5 N,N0-
bis(4-dimethylaminobenzylidene)ethane-1,2-diamine): crystal
structure of [Pd(N3)2(p-diben)]
•
Luıs E. Sarto Iara M. R. Landre
•
´
•
•
Gabriela F. Bozza Claudia Torres
Eduardo T. de Almeida
Received: 29 January 2014 / Accepted: 24 March 2014
ꢀ Springer International Publishing Switzerland 2014
Abstract Five new complexes of general formula [PdX2(p-
diben)], where p-diben = N,N0-bis(4-dimethylaminobenzy-
lidene)ethane-1,2-diamine) (1) and X = Cl (2), Br (3), I (4),
N3 (5), or CNO (6), were synthesized and characterized by
physicochemical and spectroscopic methods. The crystal
structure of compound (5) was determined by single-crystal
X-ray diffraction. Complexes 2–6 were characterized as N,N-
chelated products. The crystal structure confirmed this for-
mulation for [Pd(N3)2(p-diben)], besides showing the isom-
erism inversion of one of the C=N bonds, caused by Pd(II)
coordination.
and characterization of new palladium(II) complexes of the
N,N0-bis(4-dimethylaminobenzylidene)ethane-1,2-diamine
(p-diben) ligand [19, 20], including halides (Cl-, Br-, and
I-) and pseudohalides (N3 and NCO-), synthesized by a
-
regiospecific substitution [21] of the chloride complex.
Substitution of chloride by other halides or pseudohalides
is generally highly stereospecific [22].
Experimental
All syntheses were carried out at room temperature. All
reagents were obtained from commercial suppliers and
used without further purification. Elemental analysis of
carbon, nitrogen, and hydrogen was performed using a
Leco Instruments Ltd—TruSpec CHNS-O instrument.
Infrared spectra were recorded as KBr pellets on a Shi-
madzu—Prestige-21 spectrophotometer in the spectral
range 400–4,000 cm-1. NMR spectra were recorded on a
Bruker DPX-300 spectrometer operating at 300 MHz (1H)
and 75 MHz (13C), using CDCl3 as solvent.
Introduction
Schiff bases have been widely used as chelating ligands in
the field of coordination chemistry [1–4]. These compounds
are easily prepared by the condensation reactions of alde-
hydes and imines [5, 6]. Over the last few decades, a great
deal of interest in the chemistry of transition metal Schiff
base chelates has resulted from their biologic activity as
antifungal, antibacterial, and antitumor agents [1, 7–11], and
also their catalytic activity in reactions, such as oxidation,
epoxidation, and hydrolysis [12–16]. In this context, the
complexes of palladium(II) with Schiff base ligands are of
particular interest. Such palladium(II) complexes have var-
ious applications, in addition to those already mentioned,
including photoluminescence [17] and materials science
(liquid crystals) [18]. The aim of this work was the synthesis
An orange parallelepiped single crystal of [Pd(N3)2(p-
diben)] with dimensions equal to 0.37 9 0.27 9 0.04 mm
was obtained from slow evaporation of a chloroform
solution of the complex. The single-crystal X-ray diffrac-
tion measurements were performed at 295 K on a Gemini
A-Ultra diffractometer equipped with an Atlas CCD
detector using graphite-monochromatized MoKa radiation
˚
(k = 0.71073 A). The program CrysAlisPRO [23] was
used for data collection, cell refinement, and data reduc-
tion. The structure was solved by direct methods using the
software Sir92 [24] and refined by full-matrix least squares
on F2 using the software SHELXL-2013 [25]. The non-
hydrogen atoms were refined with least squares of full
matrix on F2 with anisotropic parameters considered.
L. E. Sarto ꢀ I. M. R. Landre ꢀ G. F. Bozza ꢀ C. Torres ꢀ
E. T. de Almeida (&)
Instituto de Quımica, Universidade Federal de Alfenas-
UNIFAL-MG, Alfenas, MG 37130-000, Brazil
e-mail: tonon@unifal-mg.edu.br
´
123

Transition Met Chem
Synthesis of [PdCl2(p-diben)] (2)
To solution of lithium tetrachloropalladate(II),
Table 1 Crystallographic data obtained for [Pd(N3)2(p-diben)]
Parameters
Data
a
Chem formula
Fw
C20H26N10Pd
512.91
Li2[PdCl4], prepared from a one-pot synthesis by reaction
of PdCl2 and LiCl (PdCl2, 5.6 mmol) in methanol (70 mL),
was added the Schiff base (1). (p-diben, 5.6 mmol). The
reaction mixture was stirred for 8 h at room temperature.
The orange precipitate was filtered off, washed with water
and ethyl ether, and dried in vacuum. Yield: 93 %. Ele-
mental analysis: % calculated (found) for C24H24N2-
PdCl2C: 48.1 (48.1); H: 5.2 (5.1); and N: 11.2 (11.4). FTIR
(KBr cm-1): 1,595 (vs), 1,535 (s), 1,483 (vw), 1,444 (m),
1,408 (vw), 1,377 (s), 1,328 (w), 1,251 (w), 1,230 (w),
1,190 (s), 1,105 (vw), 1,168 (vw), 1,033 (w), 972 (vw), 945
(m), 813 (m), 792 (w), 734 (vw), 576 (vw), 516 (w), and
Space group
P21/n
˚
a (A)
13.9552(4)
8.2896(2)
20.0633(7)
106.738(4)
2222.65(11)
4
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
l (mm-1
qcalcd (Mg m-1
)
0.86
)
1.533
h-Range for data collection
Refl. collected/unique
Data/parameter/restraints
R indices [I [ 2d(I)]
GooF on F2
2.1–29.5ꢁ
17,233/5,544
5,544/280/0
R1 = 0.0425/wR2 = 0.1097
1.068
1
449 (vw). H NMR (CDCl3, d (ppm)): 3.08 (s, 12H), 3.80
(t, 2H, J = 6.63 Hz), 4.26 (t, 2H, J = 9.43 Hz), 6.70 (m,
4H, J = 9.28 Hz), 7.46 (dd, H, J = 28.87 and 8.76 Hz),
7.77 (d, H, J = 25.04 and 8.62 Hz), 8.63 (m, 2H,
J = 24.61 and 9.65 Hz), and 9.73 (s, 2H). 13C (CDCl3, d
(ppm)): 39.95, 57.60, 66.14, 111.00, 117.74, 133.26,
134.81, 167.40, 169.54, and 190.29.
-3
)
˚
Residual density (e A
0.56 and -0.65
Aromatic H atoms were observed in a difference Fourier
synthesis but were refined using a riding model. C–H atoms
were positioned and refined with individual parameters
Uiso(H) = 1,2Ueq (C). The main crystallographic data are
given in Table 1. These were generated with WingX [26],
and the figures were prepared with mercury [27]. Confor-
mation and molecular geometry were analyzed with the
software MOGUL [28].
Synthesis of [PdBr2(p-diben)] (3)
To a suspension of [PdCl2(p-diben)] (1.00 mmol) in ace-
tone (30 mL), was added a solution of KBr (2.1 mmol) in
distilled water (5 mL). The reaction mixture was stirred for
2 h at room temperature. The yellowish precipitate was
filtered off, washed with water and ethyl ether, and dried in
vacuum. Yield: 80 %. Elemental analysis: % calculated
(found) for C24H24N2PdBr2C: 40.8 (40.7); H: 4.4 (4.4); and
N: 9.5 (9.6). FTIR (KBr cm-1): 1,597 (vs), 1,534 (m),
1,465 (w), 1,437 (w), 1,409 (w), 1,376 (m), 1,254 (w),
1,231 (w), 1,190 (w), 1,166 (s), 1,057 (m), 943 (w), 815
(m), 729 (m), 596 (w), 554 (w), and 515 (w).1H NMR
(CDCl3, d (ppm)): 3.07 (s, 12H), 4.56 (s, 4H), 6.70 (d, 4H,
J = 8.89 Hz), 7.74 (d, 4H, J = 9.06 Hz), and 9.74 (s, 2H).
13C (CDCl3, d (ppm)): 40.09, 111.00, 125.21, 132.00,
154.35, and 190.33.
Synthesis of p-diben (1)
The title imine was synthesized according to previously
described methods [19, 20]. To a solution of 4-(dimethyl-
amino)benzaldehyde (13.4 mmol) in absolute ethanol
(50 mL) was added over for 1 h a solution of ethylenedi-
amine (6.7 mmol) in absolute ethanol (5 mL). The mixture
was stirred at room temperature (approximately 25 ꢁC) for
2 h. The white precipitate was separated by filtration,
washed with water, ethyl ether, and dried in vacuum.
Yield: 97 %. Elemental analysis: % calculated (found) for
C24H24N2: C: 74.5 (74.4); H: 6.1 (6.0); and N: 17.4 (17.1).
FTIR (KBr cm-1): 1,639 (s), 1,604 (vs), 1,526 (s), 1,443
(m), 1,430 (m), 1,363 (s), 1,340 (m), 1,308 (m), 1,229 (m),
1,187 (vs), 1,106 (w), 1,037 (w), (s), 1,005 (w), 939 (vw),
919 (vw), 862 (w), 809 (vw), 788 (m), 747 (vs), 711 (w),
653 (vw), 625 (vw), 580 (w), 563 (w), 514 (m), 474 (vw),
Synthesis of [PdI2(p-diben)] (4)
The synthesis of compound (4) followed the same method
as for compound (3). However, the salt used was KI
(2.1 mmol). The brown precipitate was filtered off, washed
with water and ethyl ether, and dried in vacuum. Yield:
72 %. Elemental analysis: % calculated (found) for C24-
H24N2PdI2C: 35.2 (35.1); H: 3.8 (3.9); and N: 8.2 (8.3).
FTIR (KBr cm-1): 1,591 (vs), 1,529 (s), 1,481 (vw), 1,438
(m), 1,409 (w), 1,375 (w), 1,325 (w), 1,247 (vw), 1,228
(w), 1,188 (s), 1,170 (s), 1,141 (vw), 1,124 (vw), 1,060 (w),
1,001 (vw), 945 (w), 813 (m), and 518 (w). 1H NMR
1
457 (vw), 445 (vw), and 425 (m). H NMR (CDCl3, d
(ppm)): 2.96 (s, 12H), 3.87 (s, 4H), 6.65 (d, 4H, J = 8 Hz),
7.57 (d, 4H, J = 8.33), and 8.15 (s, 2H). 13C NMR (CDCl3,
d (ppm)): 40.20, 62.11, 111.57, 124.53, 129.50, 151.97,
and 162.36.
123

Transition Met Chem
(CDCl3,
d
(ppm)): 3.08 (s, 12H), 4.18 (m, 4H,
CH3
N
J = 3.09 Hz), 6.73 (d, 4H, J = 8.91 Hz), 7.84 (d, 4H,
J = 9.36 Hz), and 9.52 (s, 2H). 13C (CDCl3, d (ppm)):
40.24, 65.19, 111.15, 130.22, 157.49, and 189.90.
CH3
N
N
Synthesis of [Pd (N3)2(p-diben)] (5)
H3C
N
CH3
The synthesis of compound (5) followed the same method
as for compound (3). However, the salt used was NaN3
(2.3 mmol). The orange precipitate was filtered off, washed
with water and ethyl ether, and dried in vacuum. Yield:
86 %. Elemental analysis: % calculated (found) for C24-
H24N8PdC: 46.8 (46.6); H: 5.1 (5.2); and N: 27.3 (27.5).
FTIR (KBr cm-1): 2,021 (vs), 1,587 (vs), 1,529 (s), 1,481
(vw), 1,438 (m), 1,411 (w), 1,377 (s), 1,325 (m), 1,284 (w),
1,234 (w), 1,190 (s), 1,174 (s), 1,128 (vw), 1,067 (w), 1,028
Fig. 1 Structure of N,N0-bis(4-dimethylaminobenzylidene)ethane-
1,2-diamine) [19, 20]
of the aldehyde precursor, the intense stretching band of
the m(C=O) bond characteristic of aromatic aldehyde
(*1,700 cm-1) is absent. These data indicate the forma-
tion of a Schiff base from the starting reagents.
The 1H and 13C NMR spectra of the Schiff base show a
symmetric structure [19, 20], with the formation of two
imine bonds as shown in Fig. 1.
1
(vw), 945 (m), 812 (m), 756 (w), and 524 (m). H NMR
(CDCl3, d (ppm)): 3.02 (s, 12H), 3.72 (t, 2H, J = 4.2 Hz),
4.08 (t, 2H, J = 5.89 Hz), 6.78 (m, 4H, J = 7.14 Hz), 7.58
(d, 2H, J = 9.13 Hz), 7.79 (d, H, J = 9.13 Hz), 8.24 (s, H),
8.31 (d, 2H, J = 8.98 Hz), and 9.63 (s, 2H). 13C (CDCl3, d
(ppm)): 39.86, 57.21, 65.78, 111.00, 118.41, 133.56,
134.15, 152.51, 154.27, 169.93, and 189.93.
In the IR spectra of the complexes 2–6, a shift in the
m(N=C) band to lower frequencies can be observed, over-
lapping a m(C=C) aromatic band (*1,597 cm-1). This
shift can be explained by coordination of the imine nitro-
gen to the metal, such that an electronic relocation occurs
in this bond. The replacement of halide Cl- by Br- (3) and
I- (4) cannot be discerned from the IR spectra; however,
terminal coordination of the azide group in complex (5) is
suggested by the masNNN band at 2,021 cm-1 [29–31]. In
the IR spectrum of complex (6), the presence of the mas-
NCO absorption at 2,196 cm-1 indicated the N-terminally
coordinated cyanate ligands [29].
Synthesis of [Pd (NCO)2(p-diben)] (6)
The synthesis of compound (6) followed the same method
as for compound (3). However, the salt used was KCNO
(2.2 mmol). The yellowish precipitate was filtered off,
washed with water and ethyl ether, and dried in vacuum.
Yield: 68 %. Elemental analysis: % calculated (found) for
C26H24N4O2Pd C: 51.5 (51.5); H: 5.1 (5.2); and N: 16.4
(16.3). FTIR (KBr cm-1): 2,196 (vs), 1,593 (vs), 1,533 (s),
1,483 (vw), 1,444 (m), 1,409 (w), 1,377 (s), 1,328 (m),
1,251 (w), 1,218 (w), 1,190 (s), 1,144 (vw), 1,066 (w),
1,033 (w), 1,002 (w), 945 (m), 813 (s), 792 (vw), 736 (w),
1
The H and 13C NMR spectra of complexes 2–6 also
indicate the formation of N,N-chelated products. Thus,
there is a displacement of the (–HC=N–) signal [d
(ppm) = 9.73, 9.74, 9.52, 9.84, and 9.68, respectively]
when compared with the free ligand [d (ppm) = 8.15],
indicating the coordination of the nitrogen atom to palla-
dium(II). This coordination deshields the imine hydrogen,
resulting in a higher chemical shift value. Furthermore, all
of the major signals can be observed in the spectra of the
complexes, indicating that the structure of the Schiff base
ligand is preserved in the complexes. On further analysis,
the spectra of complexes (2) and (5), one can note a sig-
nificant change in the aromatic region involving a com-
plicated series of signals observed at 6.70, 7.46, and
7.77 ppm for complex (2), and 6.82, 7.43, 7.74, and
7.86 ppm for complex (5). This can be explained by E/Z
isomerization [26–31] of one of the imines of the ligand.
Conjugation of the imine with the aromatic ring causes a
decrease in the double bond character of the C=N bond,
facilitating rotation to minimize the steric hindrance. The
1H NMR spectra of complexes (3), (4), and (6) show rel-
atively little change in the aromatic region (6.70 and
1
and 516 (m). H NMR (CDCl3, d (ppm)): 3.09 (s, 12H),
4.15 (s, 4H), 6.79 (d, 4H, J = 8.52 Hz), 7.72 (d, 4H,
J = 8.91 Hz), and 9.68 (s, 2H). 13C (CDCl3, d (ppm)):
39.95, 110.74, 115.62, 121.87, 131.57, 150.21, and 190.64.
Results and discussion
Spectroscopic characterization
The IR spectrum of the free Schiff base (1) exhibits the
m(N=C) band at 1,639 cm-1, indicating formation of the
imine linkage as expected. Compared with the spectrum of
the starting primary amine, the absence of bands corre-
sponding to the characteristic asymmetric (*3,300 cm-1
)
and symmetric (*3,200 cm-1) stretching (N–H) bond is
noticeable. Moreover, when compared with the spectrum
123

Transition Met Chem
Fig. 2 Representation of the
asymmetric unit of the complex
5 with ellipsoids with 50 % of
probability. The hydrogen
atoms have been omitted for
better view of the structure
˚
Table 2 Distance (A) and
angles (ꢁ) around the metallic
ion for the complex
7.74 ppm for (3); 6.73 and 7.84 ppm for (4); 6.79 and
7.72 ppm for (6)) when compared with the imine ligand
(6.65 and 7.57 ppm). This suggests that the two C=N bonds
are presented as the same isomers. As the ions Br-, I-, and
NCO- are bulky, the E/Z isomerization is favored.
Compound 5
Pd1–N2
Pd1–N3
Pd1–N5
Pd1–N8
2.033(3)
2.050(2)
2.110(3)
2.067(3)
The 13C NMR spectra of the complexes show a shift of
the imine carbon signal [d (ppm) = 190.29 (2), 190.33 (3),
189.90 (4), 189.93(5), and 190.64 (6)] when compared with
the free ligand (d = 162.36 ppm). In the 13C NMR spectra
of complexes (2) and (5), the aromatic region shows a
series of signals (110–170 ppm), indicating a difference in
aromatic ring carbons caused by different forms of the two
imines. The 13C NMR spectra of the compounds (3), (4),
and (6) show the expected signals in the aromatic region
(111.00, 125.21, 132.00, and 154.35 ppm for (3); 111.15,
130.22, and 157.49 ppm for (4); 110.74, 121.87, 131.57
and 150.721 ppm for (6)) when compared with the imine
ligand (111.57, 124.53, 129.50, and 151.97 ppm).
N2–Pd1–N3
N2–Pd1–N5
N3–Pd1–N8
N5–Pd1–N8
81.76(11)
90.84(11)
93.42(11)
93.83(12)
between ring B and N8 atom and between the azide groups,
respectively. The increase in the N5–Pd1–N8 angle may
also be attributed to participation of the azide groups in
nonclassical intermolecular hydrogen bonds, which tends
to cause a slight deviation in this parameter.
The molecular structure was analyzed with the software
MOGUL, which showed that all bond lengths and angles
were in agreement with the expected statistical values for a
good X-ray diffraction structure refinement and do not
reveal any deviations when compared with fragments of
structures deposited in the CSD. Comparing a selected bond
length of the parent ligand (polymorphs A and B) with the
coordinated one, it is possible to observe a slight lengthening
of the C9–N2, N2–C10, C11–N3, and N3–C12 bonds
(Table 3). This is expected due to the electronic relocation
caused during the metal coordination. The molecular struc-
ture clearly revealed a change in the conformation of the
ligand upon metal coordination, as predicted by the NMR
data, in which the ligands have an E/E configuration and the
complex has an E/Z one, as discussed above.
X-ray crystallographic determination
The crystal structure of complex (5) was studied by single-
crystal X-ray diffraction, and the data show that it consists
with a neutral molecule of [Pd(N3)2(p-diben)], as shown by
the representation given in Fig. 2.
The palladium(II) is four-coordinated with a character-
istic square-planar geometry, in which the nitrogen atoms
form a basal plane with the metal atom (RMSD of 0.067).
˚
The Pd–N bond lengths are in the range of 2.033–2.110 A,
and the angles around the metal center are all very close to
90ꢁ, as shown in Table 2. The variations in the angles can
be explained in terms of steric effects and presence of
intermolecular interactions. Thus, the reduced N2–Pd1–N3
angle can be attributed to the C10–C11 bond that forces the
angle to lower values, whereas the N3–Pd1–N8 and N5–
Pd1–N8 angles can be explained by the steric effect
The analysis of the crystal structure of the complex
revealed the absence of classical intermolecular hydrogen
bonds, since the potential donors and acceptors are coordi-
nated to the metal atom. However, the structure is stabilized
123

Transition Met Chem
˚
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Table 3 Distance (A) and angles (ꢁ) of nonclassical hydrogen bonds
D–H
H…A
D…A
\D–H…A
System A1
System B2
System C3
0.930
0.960
0.970
2.656
2.722
2.733
3.567
3.398
3.425
166.39
128.04
128.85
D donor, A acceptor
Symmetry code: (1) -0.5 ? x, 0.5 - y, -0.5 ? z; (2) 0.5 ? x,
0.5 - y, -0.5 ? z; and (3) 0.5 - x, 0.5 ? y, 0.5 - z
by nonclassical intermolecular hydrogen bonds, involving
the systems C17–H17…N7 (system A), C2–H2c…N10
(system B), and C10–H10b…N8 (system C) (Table 3).
These interactions arrange system A in an infinite chain
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Conclusion
In summary, we have described in this paper the synthesis
of a Schiff base ligand and five of its palladium(II) com-
plexes. Elemental analysis and IR, H, and 13C NMR data
1
of the complexes all indicate the formation of N,N-chelated
products. The IR spectra of complexes (5) and (6) indicate
N-terminal coordination of both pseudohalides (N3 and
NCO). Single-crystal X-ray diffraction revealed that in the
complex (5), there is a slightly distorted square-planar
geometry around the palladium atom, caused by repulsion
between the azide group and the Schiff base aromatic ring,
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Supplementary data
CCDC 984033 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments We thank CAPES, CNPq, FINEP, and FAP-
EMIG for financial support and scholarship: Sarto LE (FAPEMIG),
Landre IMR (FAPEMIG), and Bozza GF (CAPES). We thank Lab-
Cri-UFMG for the single-crystal diffraction measurements and Prof.
Dr. Carlos B. Pinheiro and Prof. Dr. Nilvado L. Speziali for support
during the single-crystal diffraction measurements.
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