Synthesis, characterization and structural studies of palladium(II) complexes with N-(aroyl)-N′-(2,4-dimethoxybenzylidene)hydrazines

Article abstract of DOI:10.1016/j.jorganchem.2006.01.052

Reactions of Li₂[PdCl₄] with N-(aroyl)-N′-(2,4-dimethoxybenzylidene)hydrazines (H₂L = (4-R-C₆H₄)C({double bond, long}O)NHN{double bond, long}CH(2,4-(CH₃O)₂C₆H₃

Full text of DOI:10.1016/j.jorganchem.2006.01.052

Journal of Organometallic Chemistry 691 (2006) 2575–2583
www.elsevier.com/locate/jorganchem
Synthesis, characterization and structural studies of palladium(II)
complexes with N-(aroyl)-N0-(2,4-dimethoxybenzylidene)hydrazines
*
Sunirban Das, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
Received 4 January 2006; received in revised form 26 January 2006; accepted 26 January 2006
Available online 6 March 2006
Abstract
Reactions of Li2[PdCl4] with N-(aroyl)-N0-(2,4-dimethoxybenzylidene)hydrazines (H2L = (4-R–C6H4)C(@O)NHN@CH(2,4-(CH3O)2-
C6H3)) in presence of PPh3 have provided cyclopalladated complexes having the general formula [PdL(PPh3)] (1, 2 and 3 where R = OCH3,
CH3 and Cl, respectively) and a coordination complex trans-[Pd(HL)(PPh3)2Cl] (4 where R = NO2). The complexes have been characterized
by elemental analysis, infrared, 1H NMR and electronic absorption spectroscopy. X-ray structures of all the complexes have been deter-
mined. In 1, 2 and 3, the C,N,O-donor dianionic ligand (L2ꢀ) forms two fused five-membered chelate rings at the metal centre. On the other
hand, the monoanionic ligand (HLꢀ) acts as deprotonated amide N-donor in 4. The strong electron withdrawing effect of the nitro group on
the aroyl fragment is possibly responsible for the monodentate amide N-coordinating behavior of HLꢀ in 4. The orientation of the 4-meth-
oxy group in 1 is different than that in 2 and 3 due to intramolecular C–Hꢁ ꢁ ꢁp interaction in the last two complexes. The structure of 4 shows
an apical C–Hꢁ ꢁ ꢁPd interaction involving the azomethine (–CH@N–) group of HLꢀ. In the crystal lattice of all the structures, various types
of intermolecular non-covalent interactions are present. The self-assembly of the molecules of 1, 2 and 3 leads to two-dimensional networks.
The same network observed for 2 and 3 reflects the interchangeability of the chloro and methyl groups due to their similar volumes. In the
case of 4, the complex and the water molecules present in the crystal lattice form parallel homo-chiral helices and finally interhelical inter-
actions lead to a three-dimensional network.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Cyclopalladation; Aroylhydrazones; Substituent effect; Crystal structures; Self-assembly
1. Introduction
complexes, the metal–oxygen bond cleaves more easily
than the metal–sulfur bond. Aroyl- or acylhydrazones of
aromatic aldehydes which are similar to semicarbazones
can also act as tridentate C,N,O-donor ligands via cyclo-
metallation at the benzylidene ring [5,6]. Structurally char-
acterized cyclometallated complexes of such ligands are
very rare [6]. Recently we have reported the synthesis, reac-
tivities and structures of some cyclopalladated complexes
with N-(benzoyl)-N0-(2,4-dimethoxybenzylidene)hydrazine
(H2L, R = H) which falls in this class of ligands [7]. We
have shown that the ligand can behave as mono- as well
as bianionic C,N,O-donor. When it is monoanionic
(HLꢀ) the metal–oxygen bond is labile to the nucleophiles.
On the other hand, when it is bianionic (L2ꢀ) the metal–
oxygen bond is inert. The electronic nature of the substitu-
ent (R) at the para position of the aroyl fragment is
Thiosemicarbazones and semicarbazones of aromatic
aldehydes are known to act as tridentate ligands for Pd(II)
and Pt(II) via ortho-metallation and can yield cyclometal-
lated complexes having two fused five-membered chelate
rings at the metal centre [1–4]. Thiosemicarbazones can
coordinate the metal ion as mono- or bianionic C,N,S-
donor ligand. On the other hand, semicarbazones are
reported to act only as monoanionic C,N,O-donor ligand.
No complex containing bianionic C,N,O-donor semicar-
bazone as ligand is reported so far. In these cyclometallated
*
Corresponding author. Tel.: +91 40 2313 4756; fax: +91 40 2301 2460.
E-mail address: spsc@uohyd.ernet.in (S. Pal).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.01.052

2576
S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
expected to affect the acidity of the amide functionality
present in H2L and hence the r-bonding ability of the coor-
dinating atom (N or O) of the deprotonated amide. If the
deprotonated amide functionality binds the metal ion via
the N-centre, ortho-metallation if any has to occur at the
aroyl fragment instead of the benzylidene fragment of the
ligand [8,9]. In this work, we have studied the effect of
the substituents (R) having different polar effects on the
cyclopalladation reactions with N-(4-R-benzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazines (H2L). The synthesis,
characterization and crystal structures of the complexes
isolated in this effort are described in the present account.
2.3.1. N-(4-methoxybenzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazine
White solid (86%): M.p. 188–190 °C; Anal. Calc. for
C17H18N2O4: C, 64.96; H, 5.77; N, 8.91. Found: C, 64.9;
H, 5.7; N, 8.5%. IR (cmꢀ1): 3167 m (mN–H), 1636s (mC@O),
1
1599s (mC@N). H NMR (400 MHz) data in CDCl3 (d (J
(Hz))): 3.83, 3.84, 3.86 (3s, 9H, 3 OCH3); 6.42 (s, 1H,
H3); 6.53 (8) (d, 1H, H5); 6.94 (8) (d, 2H, H11, H13); 7.83
(8) (d, 2H, H10, H14); 8.07 (8) (d, 1H, H6); 8.50 (s, 1H,
H7); 9.12 (s, 1H, NH). Electronic spectral data in CH3CN
(k (nm) (e (Mꢀ1 cmꢀ1))): 330 (28200), 297 (18300), 286
(17700), 238 (14800).
2.3.2. N-(4-methylbenzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazine
H3CO
O
White solid (86%), M.p. 202–204 °C; Anal. Calc. for
C17H18N2O3: C, 68.44; H, 6.08; N, 9.39. Found: C, 68.4;
H, 5.9; N, 9.2%. IR (cmꢀ1): 3202 m (mN–H), 1639s (mC@O),
N
C
C
H
N
H
OCH3
1
1597s (mC@N). H NMR (400 MHz) data in CDCl3 (d (J
R
(Hz))): 2.42 (s, 3H, CH3), 3.84 (s, 6H, 2 OCH3), 6.43 (2)
(d, 1H, H3), 6.54 (8) (d, 1H, H5), 7.26 (7) (d, 2H, H11,
H13), 7.75 (7) (d, 2H, H10, H14), 8.09 (8) (d, 1H, H6), 8.50
(s, 1H, H7), 9.06 (s, 1H, NH). Electronic spectral data in
CH3CN (k (nm) (e (Mꢀ1 cmꢀ1))): 330 (28100), 297
(16900), 287 (16000), 236 (19300).
(H L, R = OCH , CH , Cl, NO )
2
3
3
2
2. Experimental
2.1. Materials
2.3.3. N-(4-chlorobenzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazine
The solvents were purified by standard methods [10]. All
the chemicals used in this work were of analytical grade
available commercially and were used without further
purification.
White solid (92%): M.p. 180–182 °C. Anal. Calc. for
C16H15N2O3Cl: C, 66.10; H, 5.20; N, 9.64. Found: C,
66.1; H, 5.1; N, 9.3%. IR (cmꢀ1): 3185 m (mN–H), 1649s
1
(mC@O), 1597s (mC@N). H NMR (400 MHz) data in CDCl3
(d (J (Hz))): 3.83, 3.84 (2s, 6H, 2 OCH3); 6.42 (s, 1H, H3);
6.52 (bs, 1H, H5); 7.43 (8) (d, 2H, H11, H13); 7.80 (8) (d, 2H,
H10, H14); 8.04 (bs, 1H, H6); 8.53 (s, 1H, H7); 9.21 (s, 1H,
NH). Electronic spectral data in CH3CN (k (nm) (e
(Mꢀ1 cmꢀ1))): 330 (26800), 298 (14600), 287 (14000), 236
(20500).
2.2. Physical measurements
Microanalytical (C, H, N) data were obtained with a
Thermo Finnigon Flash EZ1112 series elemental analyzer.
Room temperature solid state magnetic susceptibilities
were measured by using a Sherwood Scientific magnetic
susceptibility balance. Solution electrical conductivities
were measured with a Digisun DI-909 conductivity meter.
IR spectra were collected by using KBr pellets on a
Jasco-5300 FT-IR spectrophotometer. A Shimadzu 3101-
PC UV/vis/NIR spectrophotometer was used to record
the electronic spectra. Proton NMR spectra were recorded
on a Bruker 400 MHz spectrometer using Si(CH3)4 as an
internal standard.
2.3.4. N-(4-nitrobenzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazine
Orange solid (88%): M. p. 222–224 °C. Anal. Calc. for
C16H15N3O5: C, 58.36; H, 4.59; N, 12.76. Found: C, 58.3;
H, 4.5; N, 12.4%. IR (cmꢀ1): 3184 m (mN–H), 1651s (mC@O),
1
1599s (mC@N). H NMR (400 MHz) data in CDCl3 (d (J
(Hz))): 3.85 (s, 6H, 2 OCH3); 6.44 (s, 1H, H3); 6.50 (8)
(d, 1H, H5); 7.58 (8) (d, 1H, H6); 8.10 (8) (d, 2H, H10,
H14); 8.31 (8) (d, 2H, H11, H13); 8.57 (s, 1H, H7); 9.20 (s,
1H, NH). Electronic spectral data in CH3CN (k (nm) (e
(Mꢀ1 cmꢀ1))): 335 (19100), 287sh (18300), 279 (19400),
236 (16100).
2.3. General procedure for the synthesis of Schiff bases
To a methanol solution (20 cm3) of the appropriate
substituted benzoylhydrazine (4.0 mmol) and 2,4-dimeth-
oxybenzaldehyde (4.0 mmol) a few drops of acetic acid
were added. The mixture was refluxed for 6 h and then
cooled to room temperature. The crystalline solid sepa-
rated was collected by filtration, washed with cold metha-
nol and finally dried in air.
2.4. General procedure for the synthesis of complexes (1–4)
A mixture of PdCl2 (0.5 mmol) and LiCl (1.0 mmol)
was taken in 15 cm3 dry methanol and refluxed with stir-
ring for 1 h. It was then cooled to room temperature and

S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
2577
filtered. The filtrate was added dropwise with stirring to a
2.4.3. [Pd{(4-Cl–C6H4)C(O)@NN@CH(2,4-
(CH3O)2C6H2)}(PPh3)] (3)
methanol–acetonitrile (1:2) solution (20 cm3) of the appro-
priate Schiff base (H2L) (0.5 mmol). The mixture was then
stirred at room temperature for 3 days followed by
another 1 day after addition of an acetonitrile solution
(10 cm3) of PPh3 (1.0 mmol). The orange solid separated
was collected by filtration, washed with methanol and
finally dried in air.
Orange solid (75%): Anal. Calc. for C34H28N2O3ClPPd:
C, 59.58; H, 4.12; N, 4.09. Found: C, 59.5; H, 4.1; N, 3.9%.
IR (cmꢀ1): 1574s (mC@N). 1H NMR (400 MHz) data in
CDCl3 (d (J (Hz))): 3.03 (s, 3H, OCH3); 3.72 (s, 3H,
OCH3); 5.41 (s, 1H, H5); 5.93 (s, 1H, H3); 7.28 (9) (d,
2H, H11, H13); 7.42–7.69 (m, PPh3 protons), 7.88 (9) (d,
2H, H10, H14); 7.92 (9) (d, 1H, H7). Electronic spectral data
in CH3CN (k (nm) (e (Mꢀ1 cmꢀ1))): 493sh (4500), 462
(7000), 440sh (6200), 361 (6200), 347sh (6300), 275sh
(26300), 219 (49400).
2.4.1. [Pd{(4-CH3O–C6H4)C(O)@NN@CH(2,4-
(CH3O)2C6H2)}(PPh3)] (1)
Orange solid (70%): Anal. Calc. for C35H31N2O4PPd: C,
61.73; H, 4.59; N, 4.11. Found: C, 61.6; H, 4.5; N, 4.0%. IR
1
(cmꢀ1): 1576s (mC@N). H NMR (400 MHz) data in CDCl3
2.4.4. trans-[Pd{(4-NO2–C6H4)C(@O)NN@CH(2,4-
(CH3O)2C6H3)}(PPh3)2Cl] (4)
(d (J (Hz))): 3.03 (s, 3H, OCH3); 3.72 (s, 3H, OCH3); 3.82
(s, 3H, OCH3), 5.41 (s, 1H, H5); 5.93 (s, 1H, H3); 6.82 (9)
(d, 2H, H11, H13); 7.43–7.73 (m, PPh3 protons); 7.90 (9)
(d, 1H, H7), 7.91 (9) (d, 2H, H10, H14). Electronic spectral
data in CH3CN (k (nm) (e (Mꢀ1 cmꢀ1))): 489sh (4200), 460
(6500), 438sh (5900), 358 (6500), 343sh (6800), 276sh
(25800), 221 (45800).
Orange solid (77%): Anal. Calc. for C52H44N3O5ClP2Pd:
C, 62.79; H, 4.46; N, 4.22. Found: C, 62.4; H, 4.4; N, 4.0%.
IR (cmꢀ1): 1603s (amide), 1593s (mC@N). 1H NMR
(400 MHz) data in CDCl3 (d (J (Hz))): 3.89 (s, 3H,
OCH3); 3.91 (s, 3H, OCH3); 6.49 (s, 1H, H3); 6.63 (8) (d,
1H, H5); 7.06 (8) (d, 1H, H6); 7.32–7.83 (m, PPh3 protons);
8.11 (9) (d, 2H, H11, H13); 8.75 (9) (d, 1H, H10); 9.03 (9) (d,
1H, H14); 9.05 (s, 1H, H7). Electronic spectral data in
CH3CN (k (nm) (e (Mꢀ1 cmꢀ1))): 392 (8500), 355sh
(18100), 315 (32700), 291sh (30600), 276sh (28900), 220
(52200).
2.4.2. [Pd{(4-CH3–C6H4)C(O)@NN@CH(2,4-
(CH3O)2C6H2)}(PPh3)] (2)
Orange solid (77%): Anal. Calc. for C35H31N2O3PPd: C,
63.21; H, 4.70; N, 4.21. Found: C, 63.1; H, 4.7; N, 3.9%. IR
1
(cmꢀ1): 1562s (mC@N). H NMR (400 MHz) data in CDCl3
(d (J (Hz))): 2.35 (s, 3H, CH3); 3.03 (s, 3H, OCH3); 3.72 (s,
3H, OCH3); 5.41 (s, 1H, H5); 5.93 (s, 1H, H3); 7.12 (8) (d,
2H, H11, H13); 7.43–7.74 (m, PPh3 protons); 7.85 (8) (d, 2H,
H10, H14); 7.93 (9) (d, 1H, H7). Electronic spectral data in
CH3CN (k (nm) (e (Mꢀ1 cmꢀ1))): 490sh (3600), 460 (5700),
439sh (5200), 360 (6100), 347sh (6200), 257 (28500), 221
(43700).
2.5. X-ray crystallography
Single crystals were obtained by slow evaporation of
dimethylsulfoxide-acetonitrile (1:2) solutions of the com-
plexes. X-ray data were collected on an Enraf-Nonius
Mach-3 single crystal diffractometer using graphite mono-
˚
chromated Mo Ka radiation (k = 0.71073 A) by x-scan
Table 1
Crystallographic data for 1, 2, 3 and 4 Æ H2O
Complex
1
2
3
4 Æ H2O
Chemical formula
Formula weight
Crystal system
Space group
PdC35H31N2O4P
680.99
Triclinic
PdC35H31N2O3P
664.99
Triclinic
PdC34H28N2O3ClP
685.40
Triclinic
PdC52H46N3O6ClP2
1012.71
Orthorhombic
P212121
15.8207(18)
15.9343(19)
18.931(2)
90.00
90.00
90.00
4772.5(9)
4
1.409
ꢀ
ꢀ
ꢀ
P1
P1
P1
˚
a (A)
10.020(2)
10.902(2)
14.356(3)
92.04(3)
97.89(3)
90.47(3)
1552.4(5)
2
10.9484(7)
10.707(4)
13.466(1)
95.220(14)
100.002(6)
98.391(15)
1526.9(5)
2
10.8011(11)
10.8132(18)
13.507(4)
94.52(2)
100.873(16)
98.355(10)
1523.6(6)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
q (g cmꢀ3
)
1.457
0.691
1.446
0.698
1.494
0.787
l (mmꢀ1
)
0.565
Reflections unique
Reflections I P 2rI
Parameters
GOF on F2
5426
4656
391
1.132
5359
4581
382
1.067
5351
4656
381
1.103
4643
2721
594
1.191
R1, wR2 (I P 2rI)
R1, wR2 (all data)ꢀ3
0.0395, 0.0949
0.0507, 0.1195
0.875
0.0312, 0.0737
0.0417, 0.0779
0.520
0.0280, 0.0616
0.0371, 0.0748
0.371
0.0640, 0.1537
0.1384, 0.1783
0.547
˚
Largest peak (e A
)

2578
S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
method at 298 K. In each case, unit cell parameters were
determined by least-squares fit of 25 reflections having h
values in the range 9–12°. Intensities of 3 check reflections
were measured after every 1.5 h during the data collection
to monitor the crystal stability. No decay was observed for
any of the four crystals. Empirical absorption correction
was applied to each dataset based on the w-scans [11] of
selected reflections. The structures were solved by direct
methods and refined on F2 by full-matrix least-squares pro-
cedures. The asymmetric unit of each of 1, 2, and 3 con-
tains a single complex molecule while that of 4 contains a
complex molecule and a water molecule. For all the struc-
tures, non-hydrogen atoms were refined using anisotropic
thermal parameters. Hydrogen atoms were included in
the structure factor calculation at idealized positions by
using a riding model, but not refined. The programs of
WINGX [12] were used for data reduction and absorption
correction. Structure solution and refinement were per-
formed with the SHELX-97 programs [13]. The ORTEX6a
[14] and PLATON [15] packages were used for molecular
graphics. Selected crystal and refinement data for 1, 2, 3
and 4 Æ H2O are listed in Table 1.
Schiff bases are consistent with their formulae and struc-
tures. In methanol–acetonitrile, the reaction of H2L with
Li2[PdCl4] (generated in situ by treatment of one mole
equivalent of PdCl2 with two mole equivalents of LiCl)
and PPh3 produces the ortho-metallated species having
the general formula [PdL(PPh3)] when R = OCH3 (1),
R = CH3 (2) and R = Cl (3) in 70–77% yields (Scheme
1). In each of these three complexes (1–3), the bianionic
planar ligand (L2ꢀ) acts as tridentate C,N,O-donor and
the PPh3 satisfies the fourth coordination site. However,
under the same reaction condition ortho-metallation does
not occur when R = NO2 and the complex isolated in
77% yield is trans-[Pd(HL)(PPh3)2Cl] (4) where the mono-
anionic HLꢀ acts as monodentate deprotonated amide
N-donor (Scheme 1). The molecular structures of all the
complexes (1–4) have been confirmed by X-ray crystallog-
raphy (vide infra). The elemental analysis data are consis-
tent with their molecular formulae found in the X-ray
structures. All the four complexes are diamagnetic and
electrically non-conducting in solutions.
A plausible explanation for the lack of ortho-metallation
in the case of nitro substituted Schiff base and the forma-
tion of trans-[Pd(HL)(PPh3)2Cl] (4) is as follows. The
strong electron withdrawing effect of the nitro group at
the para position with respect to the amide functionality
is expected to increase the acidity of the amide proton com-
pared to that in the other three Schiff bases. As a result the
corresponding conjugate base will be the weakest. In other
words the r-bonding ability of the coordinating centre of
the deprotonated amide functionality will be the lowest
or more soft for the nitro substituted ligand than the other
ligands. In general, the N-centre is softer than the O-centre
in a deprotonated amide group. The Pd(II) being a soft
metal ion the difference in softness of the coordinating
3. Results and discussion
3.1. Synthesis and characterization
The Schiff bases N-(4-R-benzoyl)-N0-(2,4-dimethoxyb-
enzylidene)hydrazines (H2L, R = OCH3, CH3, Cl and
NO2) were obtained in high yields (86–92%) by condensa-
tion reactions of 2,4-dimethoxybenzaldehyde and the 4-
substituted benzoylhydrazines (1:1 mole ratio) in methanol
in presence of catalytic amount of acetic acid. The micro-
analytical data and the spectroscopic features of all the
H3CO
H
OCH3
C
1 (R = OCH3)
2 (R = CH3)
3 (R = Cl)
N
Pd PPh3
3
H3CO
H
OCH3
2
N
O
4
5
1
C
6
7
N
Li2[PdCl4], PPh3
in CH3OH-CH3CN
R
N
8 O
H
Cl
9
14
13
10
11
Ph3P Pd PPh3
O2N
H
OCH3
R
N
C
N
(H2L)
O
OCH3
4
Scheme 1.

S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
2579
atoms can play a major role in coordinating behavior of
the ligand. Possibly for this reason amide–N coordination
instead of amide–O coordination occurs in 4. The amide–
N coordination does not facilitate the ortho-metallation
at the benzylidene ring. However, ortho-metallation could
have taken place at the aroyl fragment ring with the forma-
tion of a five-membered ring [8,9]. It is quite likely that the
presence of the electron withdrawing nitro group at the
meta position with respect to the metallation site of
the aroyl fragment ring does not help this process also.
as a singlet in the range d 9.06–9.21. The H3 resonance is
observed as a singlet within d 6.42–6.44. Both H5 and H6
resonate as doublets in the ranges d 6.50–6.54 and 7.58–
8.09, respectively. The azomethine proton (H7) is observed
as a singlet within d 8.50–8.57. Two doublets are observed
in the ranges d 6.94–7.43 and 7.75–7.83 for the Schiff bases
with R = OCH3, CH3 and Cl. The former is attributed to
the H11 and H13 and the latter is assigned to the H10 and
H14 protons. The doublet due to H11 and H13 shows a clear
up-field shift with the progressive increase in the electron
releasing ability of the R group. In contrast, for the Schiff
base containing the most electron withdrawing NO2 substi-
tuent the doublet due to H11 and H13 appears at d 8.31 and
that due to H10 and H14 appears at d 8.10. The spectrum of
none of the complexes shows any signal attributable to the
N–H proton. Thus the amide functionality of the ligand is
deprotonated in each complex. The absence of the H6 res-
onance in the spectra of 1–3 and the appearance of H5 pro-
ton as a singlet instead of a doublet at d 5.41 are consistent
with the ortho-metallation of the 2,4-dimethoxy benzyli-
dene ring. The significant up-field shift of the H5 resonance
compared to that in the free Schiff bases indicates shielding
by the phenyl rings of the PPh3 which is cis to the metal-
lated C-atom (vide infra) [2,6,7]. For the same reason the
singlet due to the 4-methoxy group also shows up-field
shift. As a result the singlets due to the methoxy groups
of the benzylidene ring are relatively well separated (4-
OCH3 at d 3.03 and 2-OCH3 at d 3.72) for 1–3 when com-
pared with those for the free Schiff bases. The H3 proton is
observed as a singlet with a small up-field shift (d 5.93). The
azomethine proton (H7) in 1–3 is also up-field shifted (d
7.90–7.93). This up-field shift is consistent with the weaken-
ing of the C@N bond due to the N-coordination and the
trans disposition of the Pd and the proton around the
C@N [7,19–22]. However, unlike the free Schiff bases it
appears as a doublet due to the coupling with the 31P
nucleus which is trans (vide infra) to the N-atom
[4,7,21,22]. Two doublets observed in the ranges d 6.82–
7.28 and 7.85–7.91 correspond to the aroyl ring protons.
The former is assigned to the H11 and H13 protons and
the latter is assigned to the H10 and H14 protons. In the
spectra of all the complexes (1–4) the PPh3 phenyl ring pro-
tons appear as multiplets in the range d 7.32–7.83. Not sur-
prisingly the other features in the spectrum of 4 are
different compared to those in the spectra of the ortho-met-
allated species (1–3). Due to deprotonation of the amide
functionality the N–H proton is not observed. The singlets
corresponding to the two methoxy groups of the benzyli-
dene ring appear at d 3.89 and 3.91. The signal at slightly
up-field is assigned to the 2-OCH3 group as it is closer to
one of the two PPh3 ligands (vide infra). The singlet
observed at d 6.49 is assigned to the H3 proton. The H5
and H6 protons appear as two doublets at d 6.63 and
7.06, respectively. The doublet corresponding to two pro-
tons observed at d 8.11 is assigned to the H11 and H13 pro-
tons. Each of the two doublets observed at d 8.75 and 9.03
corresponds to single proton. The X-ray structure of 4
3.2. Spectroscopic properties
Infrared spectra of all the compounds have been collected
using KBr pellets. The free Schiff bases (H2L) display the
characteristic bands due the N–H and the C@O groups of
the amide functionality in the ranges 3167–3202 and 1636–
1651 cmꢀ1, respectively [16,17]. The C@N stretch appears
as a strong band near 1598 cmꢀ1 [1,7]. The spectra of 1–3
do not display any band assignable to the N–H or the
C@O group of the amide functionality. Thus the amide func-
tionality is in the enolate form and the tridentate ligand (L2ꢀ
)
coordinates the metal centre via the amide–O, the imine–N
and the ortho C-atom of the benzylidene ring. The X-ray
structures confirm this coordination mode of L2ꢀ in 1–3
(vide infra). The strong band observed in the range 1562–
1576 cmꢀ1 is assigned to the C@N stretch of the tridentate
ligand. The low-energy shift of the C@N stretch as compared
to that in the free Schiff bases is consistent with the azome-
thine N-coordination to the metal ion [7,18]. The X-ray
structure shows that there is no ortho-metallation and HLꢀ
acts monodentate deprotonated amide–N coordinating
ligand in 4 (vide infra). Accordingly the spectrum of 4 does
not display any band due to the amide N–H stretch. Two clo-
sely spaced strong bands observed at 1603 and 1593 cmꢀ1 are
attributed to the deprotonated N-coordinating amide [17]
and the uncoordinated azomethine (–HC@N–) fragments
of HLꢀ, respectively.
The electronic spectral data for the Schiff bases and the
complexes are listed in the experimental section. The spec-
tra of the Schiff bases are very similar and display four
intense absorptions in the range 335–236 nm. The spectra
of 1–3 are also very similar and display seven absorptions
in the range 493–219 nm. In contrast the spectrum of 4 is
very different and display six absorptions within 392–
220 nm. The lower energy additional absorptions for all
the complexes are likely to be due to ligand-to-metal charge
transfer transitions and the higher energy absorptions are
due to ligand based transitions [7].
1
The H NMR spectra have been recorded using CDCl3
solutions of all the compounds. For the free Schiff bases the
protons of the two methoxy groups on the benzylidene
fragment appear as two closely spaced singlets when
R = OCH3 and Cl and as a singlet when R = CH3 and
NO2 near d 3.84. The protons of the methoxy and the
methyl substituents on the aroyl moiety appear as singlets
at d 3.86 and 2.42, respectively. The N–H proton resonates

2580
S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
shows that the H14 is very close to the uncoordinated
imine–N (vide infra). Possibly due to a similar situation
in solution the signal due to H10 and H14 gets separated
and appear as two doublets at d 8.75 and 9.03, respectively.
For the free nitro substituted Schiff base the azomethine
proton (H7) resonates as a singlet at d 8.57. In the spectrum
of 4, a singlet corresponding to a single proton is observed
at d 9.05 The solid state structure of 4 reveals that the
azomethine hydrogen of HLꢀ is very close to the apical
coordination site of the metal centre (vide infra). The reso-
nance at d 9.05 is assigned to the azomethine proton (H7)
considering its down-field appearance [23,24]. A further
down-field shift of this signal by about d 0.1 on cooling
to ꢀ60 °C supports this assignment and suggests that the
C–Hꢁ ꢁ ꢁPd interaction in 4 is possibly more of hydrogen
bond type than the agostic bond type [24].
Table 2
˚
Selected bond lengths (A) and angles (°) for 1, 2, and 3
Complex
1
2
3
Pd–C(1)
Pd–N(1)
Pd–O(1)
Pd–P
2.006(5)
1.983(4)
2.107(3)
2.2491(12)
2.015(3)
1.982(2)
2.121(2)
2.2638(10)
2.007(3)
1.982(2)
2.112(2)
2.2648(10)
C(1)–Pd–N(1)
C(1)–Pd–O(1)
C(1)–Pd–P
N(1)–Pd–O(1)
N(1)–Pd–P
82.79(18)
81.58(11)
81.69(11)
159.15(16)
99.62(14)
76.38(14)
176.32(11)
101.23(9)
158.06(10)
100.92(9)
76.56(9)
174.00(7)
100.68(6)
158.08(10)
100.63(9)
76.49(9)
174.63(7)
100.94(6)
O(1)–Pd–P
mentary material) is identical with that of 2. The selected
bond parameters associated with the metal ions are listed
in Table 2. In each complex, the planar tridentate ligand
L2ꢀ coordinates the metal ion via the aryl C-atom, the
azomethine N-atom and the O-atom of the deprotonated
amide functionality forming two fused five-membered
rings. The P-atom of PPh3 occupies the fourth coordina-
tion site and completes a CNOP square-planar environ-
ment around the Pd(II) centre. The N–N, N–C and C–O
bond lengths in the @N–N@C(Oꢀ)– fragment of L2ꢀ are
consistent with the enolate form of the amide functionality
[7,25]. The bond lengths and bond angles associated with
the metal ion in 1–3 are very similar (Table 2). The Pd–C
and Pd–N bond lengths and the C–Pd–N angles are com-
parable with the values reported for similar ortho-palla-
dated complexes with Schiff bases [1–4,6,7,19,20]. The
Pd–O bond lengths are comparable to that observed for
the analogous complex in which R = H [7]. As expected
these are shorter than the Pd–O bond lengths observed
for Pd(II) complexes of C,N,O-donor ligands having O-
coordinating protonated amide functionality [1,3,6]. The
Pd–P [1–4,6,7,19–22] bond lengths are unexceptional.
There is a major difference in the molecular conformations
of 1–3 with respect to the orientation of the methyl group
of the 4-methoxy substituent on the benzylidene ring
(Fig. 1 and Fig. S1). In 2 and 3, the methyl group is close
to one of the phenyl rings of PPh3. However, it is away
from the same phenyl ring in 1. It may be noted that the
conformation of 1 is identical to that of the analogous
complex where R = H [7]. Possibly intramolecular C–
Hꢁ ꢁ ꢁp interaction [26] involving the methyl C–H and the
phenyl ring is primarily responsible for the observed orien-
tation of 4-methoxy methyl group in 2 and 3. The C15ꢁ ꢁ ꢁcg
(centroid of the phenyl ring) and Hꢁ ꢁ ꢁcg distances are 4.05
3.3. Description of molecular structures
The molecular structures of 1 and 2 are depicted in
Fig. 1. In every respect the structure of 3 (Fig. S1 in supple-
C20
C19
C26
C21
C22
C34
C27
C28
C25
C33
C35
C18
C23
C24
C30
C32
P
C31
O2
C2
C29
C15
C1
Pd
C3
C4
O1
C14
C9
C13
C6
C5
C8
N1
C17
C7
N2
C10
C12
O3
C16
O4
C11
(a)
C26
C27
C28
C22
C21
C23
C18
C25
C32
C24
C29
C31
C33
C15
C20
C2
C30
P
C19
C34
C35
O2
C1
C3
Pd
O1
C4
˚
˚
and 3.31 A for 2 and 4.04 and 3.30 A for 3. The C15–
Hꢁ ꢁ ꢁcg angles are 135.5° and 135.1° for 2 and 3,
respectively.
C14
C5
C6
C9
N1
C13
C8
C7
C16
N2
O3
The structure of 4 is shown in Fig. 2. The bond para-
meters related to the metal ion are listed in Table 3. The
Pd(II) centre is in square-planar NP2Cl coordination geom-
etry assembled by the deprotonated amide–N donor HLꢀ,
the chloride and the two PPh3 which are trans to each
other. The Pd–Cl bond length is within the range reported
for chloride ligated Pd(II) species [4,7,21,22]. The Pd–N
C10
C12
C17
C11
(b)
Fig. 1. Molecular structures of [PdL(PPh3)] (a) 1 (R = OCH3) and (b) 2
(R = CH3). All non-hydrogen atoms are represented by their 25%
probability thermal ellipsoids.

S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
2581
C31
with three neighboring molecules. The ortho C–H and the
meta C–H of two phenyl rings are close to aroyl ring meth-
oxy O-atoms of two molecules and the meta C–H of the
third phenyl ring is close to the imine–N of the third mol-
ecule. Thus the O-atom of the aroyl ring methoxy group of
each molecule is involved in two C–Hꢁ ꢁ ꢁO interactions.
The C31ꢁ ꢁ ꢁO4 and C26ꢁ ꢁ ꢁO4 distances are 3.441 and
C32
C33
C30
C15
C26
C25
C23
C29
C27
C4
C16
P1
C24
O3
C34
C28
C3
C22
O2
C21
C17
C5
C7
C6
˚
3.292 A, respectively. The corresponding C–Hꢁ ꢁ ꢁO angles
C1
C2
C13
C20
C18
are 150.6 and 148.4°, respectively. The C22ꢁ ꢁ ꢁN1 distance
C14
C9
C19
O5
O4
˚
N2
and the C22–Hꢁ ꢁ ꢁN1 angle are 3.467 A and 145.5°, respec-
C12
N1
Cl
tively. As this sp2 N-atom is metal coordinated this close
contact is most probably due to the C–Hꢁ ꢁ ꢁp interaction
[26] involving the C@N p-electrons. These C–Hꢁ ꢁ ꢁO and
C–Hꢁ ꢁ ꢁp interactions lead to a two-dimensional layered
structure of 1 in the solid state (Fig. 3). In all likelihood
the C–Hꢁ ꢁ ꢁO interactions are primarily responsible for
the two-dimensional network of the molecules which is fur-
ther supported by the C–Hꢁ ꢁ ꢁp interactions.
Pd
C8
N3
O1
C52
C10
C11
C51
P2
C47
C35
C48
C40
C46
C41
C42
C50
C45
C39
C38
C44
C49
C43
C36
It is known that the crystal structures of chloro and
methyl group containing molecules which are otherwise
identical do not change due to the similar sizes of these
two groups [28]. The molecules of 2 and 3 provide examples
of such species. Indeed the crystal packing of 2 (Fig. 4) and
3 (Fig. S2 in supplementary material) are found to be
essentially identical. In both cases, the metal uncoordinated
N-atom (N2) of the deprotonated amide functionality par-
ticipates in C–Hꢁ ꢁ ꢁN interaction with the para C–H (C27–
H27 for 2 and C26–H26 for 3) of one of the phenyl rings of
PPh3 belonging to a neighboring molecule. The Cꢁ ꢁ ꢁN dis-
C37
Fig. 2. Molecular structure of [Pd(HL)(PPh3)2Cl] (4, R = NO2). All non-
hydrogen atoms are represented by their 25% probability thermal
ellipsoids. Except the azomethine hydrogen atom all other hydrogen
atoms are omitted for clarity.
Table 3
˚
Selected bond lengths (A) and angles (°) for 4 Æ H2O
Pd–Cl
Pd–P(1)
2.391(7)
2.327(4)
Pd–N(2)
Pd–P(2)
2.039(11)
2.322(4)
˚
Cl–Pd–N(2)
Cl–Pd–P(2)
N(2)–Pd–P(2)
178.3(3)
89.1(2)
91.8(3)
Cl–Pd–P(1)
86.21(19)
92.7(3)
171.98(16)
tances and the C-Hꢁ ꢁ ꢁN angles are 3.295 A and 150.2° for 2
˚
N(2)–Pd–P(1)
P(1)–Pd–P(2)
and 3.296 A and 146.9° for 3. These intermolecular C–
Hꢁ ꢁ ꢁN interactions lead to a one-dimensional ordering of
the molecules in each case. The same phenyl ring that is
involved in the C–Hꢁ ꢁ ꢁN interaction is also close to the
methyl (for 2) or chloro (for 3) substituent on the aroyl
group of another neighboring molecule. The C17ꢁ ꢁ ꢁC28
bond length is longer than the bond lengths reported for
deprotonated amide–N coordinated Pd(II) complexes
[8,9]. Possibly steric constraints imposed by the two PPh3
ligands and the monodentate nature of HLꢀ causes the
lengthening of the Pd–N bond in 4. Two mutually trans
Pd–P bond lengths in 4 are very similar and as expected
they are longer than the Pd–P bond lengths observed in
1–3 (Table 2). Interestingly the azomethine (–HC@N–)
proton is very close to the metal centre at the apical site
(Fig. 2) indicating a C–Hꢁ ꢁ ꢁPd interaction [23,24,27]. The
Pdꢁ ꢁ ꢁC7 and Pdꢁ ꢁ ꢁH7 distances and the C7–H7ꢁ ꢁ ꢁPd angle
˚
distance and C12–C17ꢁ ꢁ ꢁC28 angle are 3.329 A and
177.4°, respectively, for 2, while the Clꢁ ꢁ ꢁC27 distance
˚
and C12-Clꢁ ꢁ ꢁC27 angle are 3.197 A and 171.6°, respec-
tively, for 3. These geometrical parameters indicate that
˚
are 3.142(13) and 2.56 A and 121°, respectively. It may be
noted that the plane containing HLꢀ is roughly orthogonal
to the plane containing rest of the molecule of 4 due to the
steric constraint forced by the two bulky trans oriented
PPh3 ligands (Fig. 2). It is very likely that this steric con-
straint facilitates the observed C–Hꢁ ꢁ ꢁPd interaction in 4.
3.4. Intermolecular non-covalent interactions and
self-assembly
Fig. 3. Two-dimensional network of 1 in the crystal lattice. The methoxy
groups on the 2,4-dimethoxybenzylidene fragment and all the hydrogen
atoms except those of the three phenyl ring C–H groups involved in
intermolecular interactions are omitted for clarity.
In the crystal lattice of 1, one C–H group from each of
the three phenyl rings of the ancillary ligand PPh3 of each
molecule are involved in weak intermolecular interactions

2582
S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
In the crystal lattice of 4 Æ H2O, the water molecule and
the O-atom (O1) of the N-coordinated amide functionality
of HLꢀ are involved in a reasonably strong O–Hꢁ ꢁ ꢁO
hydrogen bond. The O6ꢁ ꢁ ꢁO1 distance and the O6–
˚
Hꢁ ꢁ ꢁO1 angle are 2.736 A and 156.3°, respectively. In addi-
tion to this, the water O-atom also acts as acceptor in a
weak C–Hꢁ ꢁ ꢁO interaction where the C–H group belongs
to one of the phenyl rings of PPh3. The O6ꢁ ꢁ ꢁC31 distance
˚
and the C31–Hꢁ ꢁ ꢁO6 angle are 3.424 A and 167.9°, respec-
tively. These O–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁO interactions lead to a
left-handed helical structure that propagates along the
crystallographic a-axis (Fig. 5). The parallel homo-chiral
helices are connected by a second C–Hꢁ ꢁ ꢁO interaction
which involves a PPh3 phenyl ring C–H group and one of
the O-atoms of the nitro substituent on the aroyl fragment
of HLꢀ. The O4ꢁ ꢁ ꢁC19 distance and the C19–Hꢁ ꢁ ꢁO4 angle
˚
are 3.259 A and 130.3°, respectively. Due to these interhe-
lical C–Hꢁ ꢁ ꢁO interactions a three-dimensional network of
4 Æ H2O is formed in the crystal lattice (Fig. 5).
Fig. 4. Two-dimensional network of 2 in the crystal lattice. Two phenyl
rings of the PPh3 and the methoxy groups of the 2,4-dimethoxybenzylid-
ene fragment are omitted for clarity.
4. Conclusion
C–Hꢁ ꢁ ꢁp interaction [26] in 2 and C–Clꢁ ꢁ ꢁp interaction
[29,30] in 3 cannot be ruled out. As these C–Hꢁ ꢁ ꢁp and
C–Clꢁ ꢁ ꢁp interactions are roughly orthogonal to the C–
Hꢁ ꢁ ꢁN interactions, a two-dimensional layered structure
is formed in both cases (Fig. 4 and Fig. S2).
The synthesis, properties and X-ray structures of four
new square-planar Pd(II) complexes with the Schiff bases
N-(4-R-benzoyl)-N0-(2,4-dimethoxybenzylidene)hydrazines
(H2L) are described. The Schiff bases differ by the substitu-
ent (R = OCH3, CH3, Cl and NO2) present in the aroyl
fragment. Cyclopalladated species of general formula
[PdL(PPh3)] are isolated when R = OCH3, CH3 and Cl.
On the other hand, for R = NO2 the coordination complex
trans-[Pd(HL)(PPh3)2Cl] is obtained. In [PdL(PPh3)], L2ꢀ
acts as a tridentate C,N,O-donor ligand, while HLꢀ in
trans-[Pd(HL)(PPh3)2Cl] is a monodentate deprotonated
amide–N donor ligand. The structure of trans-
[Pd(HL)(PPh3)2Cl] reveals that the azomethine (–CH@N–)
hydrogen is very close to the apical site of the metal centre
suggesting a C–Hꢁ ꢁ ꢁPd interaction. The solution proton
NMR data also indicate the presence and predominantly
hydrogen bond character of this C–Hꢁ ꢁ ꢁPd interaction.
The self-assembly of all the three cyclopalladated com-
plexes via intermolecular non-covalent interactions such
as C–Hꢁ ꢁ ꢁN, C–Hꢁ ꢁ ꢁO, C–Hꢁ ꢁ ꢁp and C–Clꢁ ꢁ ꢁp generates
two-dimensional layered structures. The crystal packing
patterns of the two complexes where R = CH3 and Cl are
identical but different than that of the complex where
R = OCH3. This observation is in accordance with the
chloro-methyl exchange rule. The hydrated molecules of
trans-[Pd(HL)(PPh3)2Cl] form a helical structure via
O–Hꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁO interactions and finally inter-helical
C–Hꢁ ꢁ ꢁO interactions lead to a three-dimensional network.
Acknowledgements
Fig. 5. (a) The helical packing of 4 Æ H2O along the a-axis. For a better
view only the (@N–N(PdPPh)–C(@O)–) Æ H2O fragment is shown. (b) The
projection of the three-dimensional network of 4 Æ H2O onto the bc-plane.
For clarity 2,4-dimethoxybenzylidene fragment, one PPh3 and one phenyl
ring of the second PPh3 are not shown.
Financial support for this work was provided by the
Council of Scientific and Industrial Research (CSIR),
New Delhi (Grant No. 01(1880)/03/EMR-II). X-ray crys-
tallographic studies were performed at the National Single

S. Das, S. Pal / Journal of Organometallic Chemistry 691 (2006) 2575–2583
[7] S. Das, S. Pal, J. Organomet. Chem. 689 (2004) 352.
2583
Crystal Diffractometer Facility, School of Chemistry, Uni-
versity of Hyderabad (funded by the Department of Sci-
ence and Technology, New Delhi). We thank the
University Grants Commission, New Delhi for the facilities
provided under the UPE and CAS programs. Mr. S. Das
thanks the CSIR, New Delhi for a research fellowship.
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Appendix A. Supplementary data
Figures depicting the molecular structure of 3 (Fig. S1)
and the two-dimensional network of 3 (Fig. S2). Crystallo-
graphic data have been deposited with the Cambridge
Crystallographic Data Centre (deposition numbers are
CCDC-294177, CCDC-294178, CCDC-294179 and
CCDC-294180 for 1, 2, 3 and 4 Æ H2O, respectively). Copies
of this information can be obtained free of charge from
The Director, CCDC, 12, Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk). Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2006.01.052.
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