Cyclopalladated complexes with N-(benzoyl)-N′ -(2,4-dimethoxybenzylidene)hydrazine: Syntheses, characterization and structural studies

Article abstract of DOI:10.1016/j.jorganchem.2003.10.022

Cyclopalladated complexes with the Schiff base N-(benzoyl)-N′ -(2,4-dimethoxybenzylidene)hydrazine (H₂L, 1) have been described. The reaction of 1 with Li₂[PdCl₄] in methanol yields the complex [Pd(HL)Cl] (2). [Pd(HL) (CH₃CN)Cl] (3) has been prepared by dissolving 2 in acetonitrile. In methanol-acetonitrile mixture, treatment of 2 with two mole equivalents of PPh₃ produces [PdL(PPh₃)] (4) and that with one mole equivalent of PPh₃ produces [Pd(HL)(PPh₃)Cl] (5). Crystallization of 2 from dmso-d₆ results into isolation of [Pd(HL)((CD₃)₂SO)Cl] (6). In 2, the monoanionic ligand (HL^-) is C,N,O-donor and the Cl-atom is trans to the azomethine N-atom. In 3, 5 and 6, HL^- is C,N-donor and the Cl-atom is trans to the metallated C-atom. The remaining fourth coordination site is occupied by the N-atom of CH₃CN, the P-atom of PPh₃ and the S-atom of (CD₃) ₂SO in 3, 5 and 6, respectively. Thus on dissolution in acetonitrile and dmso and in reaction with stoichiometric PPh₃ the incoming ligand imposes a rearrangement of the coordinating atoms on the palladium centre. On the other hand, in presence of excess PPh₃ deprotonation of the amide functionality in 2 occurs and the Cl-atom is replaced by the P-atom of PPh₃ to form 4. Here the dianionic ligand (L^2-) remains C,N,O-donor as in 2. The compounds have been characterized with the help of elemental analysis (C, H, N), infrared, ¹H NMR and electronic absorption spectroscopy. Molecular structures of 3, 4, and 6 have been determined by X-ray crystallography.

Full text of DOI:10.1016/j.jorganchem.2003.10.022

Journal of Organometallic Chemistry 689 (2004) 352–360
www.elsevier.com/locate/jorganchem
Cyclopalladated complexes with N-(benzoyl)-
N0-(2,4-dimethoxybenzylidene)hydrazine: syntheses,
characterization and structural studies
*
Sunirban Das, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
Received 27 August 2003; accepted 20 October 2003
Abstract
Cyclopalladated complexes with the Schiff base N-(benzoyl)-N0-(2,4-dimethoxybenzylidene)hydrazine (H2L, 1) have been de-
scribed. The reaction of 1 with Li2[PdCl4] in methanol yields the complex [Pd(HL)Cl] (2). [Pd(HL)(CH3CN)Cl] (3) has been pre-
pared by dissolving 2 in acetonitrile. In methanol–acetonitrile mixture, treatment of 2 with two mole equivalents of PPh3 produces
[PdL(PPh3)] (4) and that with one mole equivalent of PPh3 produces [Pd(HL)(PPh3)Cl] (5). Crystallization of 2 from dmso-d6 results
into isolation of [Pd(HL)((CD3)2SO)Cl] (6). In 2, the monoanionic ligand (HLꢀ) is C,N,O-donor and the Cl-atom is trans to the
azomethine N-atom. In 3, 5 and 6, HLꢀ is C,N-donor and the Cl-atom is trans to the metallated C-atom. The remaining fourth
coordination site is occupied by the N-atom of CH3CN, the P-atom of PPh3 and the S-atom of (CD3)2SO in 3, 5 and 6, respectively.
Thus on dissolution in acetonitrile and dmso and in reaction with stoichiometric PPh3 the incoming ligand imposes a rearrangement
of the coordinating atoms on the palladium centre. On the other hand, in presence of excess PPh3 deprotonation of the amide
functionality in 2 occurs and the Cl-atom is replaced by the P-atom of PPh3 to form 4. Here the dianionic ligand (L2ꢀ) remains
C,N,O-donor as in 2. The compounds have been characterized with the help of elemental analysis (C, H, N), infrared, 1H NMR and
electronic absorption spectroscopy. Molecular structures of 3, 4, and 6 have been determined by X-ray crystallography.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Palladium; Benzoylhydrazone; Ortho-cyclometallation; Crystal structures
1. Introduction
ortho-metallation and can produce cyclometallated
complexes having two fused rings at the metal centre
[12–23]. Thiosemicarbazones and semicarbazones of
aromatic aldehydes belong to this type of ligands [18–
21]. Thiosemicarbazones can coordinate the metal ion as
monoanionic or bianionic tridentate C,N,S-donor li-
gand [18,19]. On the other hand, semicarbazones act
only as monoanionic C,N,O-donor ligand [20,21]. It has
been also found that in these complexes the metal–
oxygen bond cleaves more easily than the metal–sulfur
bond [18]. There is no report on species containing
bianionic C,N,O-donor semicarbazone ligand. Like
semicarbazones, aroyl- or acyl-hydrazones of aromatic
aldehydes are also potential tridentate C,N,O-donor li-
gands. In principle, such a ligand can be either mono-
anionic or dianionic. However structurally characterized
cyclometallated species of this class of ligands are scarce.
To the best of our knowledge the lone example reported
Cyclometallated complexes form an important class
of organometallic species [1–7]. Formation of such
complexes involves ligand C–H activation. Interest on
this activation process and the corresponding metallated
complexes stems from their utilization in insertion re-
actions, regio- and stereo-selective reactions and as
catalysts in organic synthesis [8–12]. Thus there is a
continuing quest for new cyclometallated species with
different types of ligands. Compounds containing two
non-carbon donor atoms are well known to act as tri-
dentate ligands for palladium(II) or platinum(II) via
* Corresponding author. Tel.: +91-40-2301-0500; fax: +91-40-2301-
2460.
E-mail address: spsc@uohyd.ernet.in (S. Pal).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.10.022

S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
353
is a cationic palladium(II) complex with a monoaninoic
C,N,O-donor acetylhydrazone having triphenylphos-
phine as the ancillary ligand [22]. A couple of
cyclopalladated complexes are reported with ben-
zoylhydrazones of phenyl 2-pyridyl ketone [23] and 2,6-
diacetyl pyridine [24]. In both cases, the metal centre is
bound to the pyridine-N and the deprotonated amide-N.
The ortho-metallation occurs at the phenyl ring of the
benzoyl group. In this work, we report new palla-
dium(II) complexes with N-(benzoyl)-N0-(2,4-dimeth-
oxybenzylidene)hydrazine (H2L (1), two H represent the
amide proton and the 6-H of the 2,4-dimethoxy phenyl
ring). Unlike the previously reported complexes with
benzoylhydrazones [23,24], in the present complexes the
cyclometallation occurs at the benzylidene ring. In the
cationic cyclopalladated compounds of semicarbazone
[20,21] and acetylhydrazone [22] containing monoden-
tate PPh3, the ligands act as monoanionic C,N,O-donor.
In contrast the analogous compound with H2L contains
dianionic C,N,O-donor ligand (L2ꢀ). In the following
account, we describe the syntheses, characterization and
structures of these cylcopalladated compounds of H2L.
2.3. Synthesis of N-(benzoyl)-N0-(2,4-dimethoxybenzy-
lidene)hydrazine (H2L, 1)
To a methanol solution (20 cm3) of benzoylhydr-
azine (0.545 g, 4.0 mmol) 2,4-dimethoxybenzaldehyde
(0.665 g, 4.0 mmol) and a few drops of acetic acid were
added. The mixture was refluxed for 6 h and then
cooled to room temperature. The white crystalline solid
separated was collected by filtration, washed with cold
methanol and finally dried in air. Yield, 0.966 g (85%).
M.p. 196–198 °C. Anal. Calc. for C16H16N2O3: C,
67.59; H, 5.67; N, 9.85. Found: C, 67.4; H, 5.5; N,
9.7%. Selected IR bands (cmꢀ1): 3175m, 3009m, 2843m,
1642s (mC@O), 1603s (mC@N), 1503m, 1466s, 1416m,
1360s, 1281s, 1208s, 1159m, 1105m, 1026s, 968m, 939m,
1
909m, 822s, 700s, 505m. H NMR (200 MHz) data in
(CD3)2SO (d (J (Hz))): 3.81 (s, 3H, –OCH3); 3.83 (s,
3H, –OCH3); 6.42 (s, 1H, H3); 6.51 (8) (d, 1H, H6); 7.48
(m, 3H, H11, H12, H13); 7.89 (6) (d, 2H, H10, H14); 8.05
(8) (d, 1H, H5); 8.56 (s, 1H, –HC@N–); 9.40 (s, 1H,
–NH–). Electronic spectral data in CH3OH (k (nm) (e
(Mꢀ1 cmꢀ1))): 331 (30 200), 298 (16 500), 287 (15 000),
231 (18 800).
2.4. Synthesis of [Pd(HL)Cl] (2)
O
A mixture of PdCl2 (89 mg, 0.5 mmol) and LiCl (43
mg, 1.0 mmol) was taken in 15 cm3 methanol and ref-
luxed with stirring for 1 h. It was then cooled to room
temperature and filtered. The filtrate was added drop-
wise with stirring to a methanol solution (20 cm3) of
H2L (141 mg, 0.5 mmol). The mixture was then stirred
at room temperature for 3 days followed by another 3
days of refluxing. The red solid separated was collected
by filtration, washed with methanol and finally dried in
air. Yield, 100 mg (47%). Anal. Calc. for C16H15
N2O3ClPd: C, 45.20; H, 3.55; N, 6.59. Found: C, 44.9;
H, 3.5; N, 6.4%. Selected IR bands (cmꢀ1): 3206w,
2926w, 1641s (mC@O), 1566s (mC@N), 1506m, 1449m,
1373w, 1319w, 1252s, 1217s, 1142s, 1026s, 802m, 723w,
689m, 528w. Electronic spectral data in CH3OH (k (nm)
(e (Mꢀ1 cmꢀ1))): 476sh (7100), 448 (10 970), 428sh
(9900), 360sh (6700), 347 (7400), 271 (42 100).
N
C
C
H
N
H
OCH
3
2. Experimental
2.1. Materials
The chemicals and solvents used in this work were of
analytical grade available commercially and were used
as received.
2.2. Physical measurements
Elemental (C, H, N) analysis data were obtained with
a Perkin–Elmer Model 240C elemental analyzer. Room
temperature solid state magnetic susceptibilities were
measured by using a Sherwood Scientific magnetic sus-
ceptibility balance. Solution electrical conductivities
were measured with a Digisun DI-909 conductivity
meter. Infrared 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 of the complexes in (CD3)2SO solutions were
recorded on a Bruker 200 MHz spectrometer using
Si(CH3)4 as an internal standard.
2.5. Synthesis of [Pd(HL)(CH3CN)Cl] (3)
[Pd(HL)Cl] (2) (50 mg, 0.12 mmol) was taken in 50
cm3 of acetonitrile and then refluxed for 10 h. The
mixture was then filtered to remove the undissolved
[Pd(HL)Cl]. The filtrate was concentrated to 15 cm3 on a
rotary evaporator and left for slow evaporation in air at
room temperature. The orange crystalline solid sepa-
rated was collected by filtration and dried in air. Yield,
40 mg (73%). Anal. Calc. for C18H18N3O3ClPd: C,
46.37; H, 3.89; N, 9.01. Found: C, 46.1; H, 3.6; N, 8.8%.
Selected IR bands (cmꢀ1): 3219w, 2942w, 2168w (mCBN),

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S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
1672m (mC@O), 1570s (mC@N), 1508m, 1487w, 1466m,
1412m, 1348m, 1262s, 1202m, 1153s, 1028s, 831m,
789w, 702s, 592w. 1H NMR (200 MHz) data in
(CD3)2SO (d (J (Hz))): 2.08 (s, 3H, CH3CN); 3.80 (s, 3H,
–OCH3); 3.83 (s, 3H, –OCH3); 6.28 (s, 1H, H3); 7.22 (s,
1H, H5); 7.54 (m, 3H, H11, H12, H13); 7.90 (7) (d, 2H,
H10, H14); 8.37 (s, 1H, –HC@N); 8.74 (s, 1H, –NH–).
Electronic spectral data in CH3CN (k (nm) (e
(Mꢀ1 cmꢀ1))): 487sh (1500), 458sh (2500), 391 (11 300),
358sh (10 900), 267sh (18 000), 242sh (27 200), 217
(46 000).
spectral data in CH3OH (k (nm) (e (Mꢀ1 cmꢀ1))): 485sh
(6800), 457 (10 700), 438sh (9700), 352 (6900), 274
(39 600).
2.8. Isolation of [Pd(HL)((CD3)2SO)Cl] (6)
A (CD3)2SO solution of 2 was used to collect the
NMR (200 MHz) data. However, the solution gave
crystals of [Pd(HL)((CD3)2SO)Cl] (6). Selected IR bands
(cmꢀ1): 3289m, 3094w, 2823w, 1661s (mC@O), 1578s
(mC@N), 1514s, 1487s, 1452m, 1416m, 1354s, 1265s,
1206s, 1159s, 1119s, 1030s, 939m, 853w, 826s, 783w,
1
2.6. Synthesis of [PdL(PPh3] (4)
698s, 588m, 552w, 501m. H NMR data (d (J (Hz))):
3.80 (s, 3H, –OCH3); 3.84 (s, 3H, –OCH3); 6.43 (s, 1H,
H3); 7.13 (s, 1H, H5); 7.65 (m, 3H, H11, H12, H13); 7.90
(8) (d, 1H, H14); 8.03 (7) (d, 1H, H10); 8.36 (s, 1H,
–HC@N–); 8.74 (s, 1H, –NH–). Electronic spectral data
in (CH3)2SO (k (nm) (e (Mꢀ1 cmꢀ1))): 487sh (5800), 458
(8800), 434sh (7700), 390sh (10 200), 358 (12 900), 273
(30 100).
[Pd(HL)Cl] (2) (43 mg, 0.1 mmol) was taken in a 1:1
mixture of methanol and acetonitrile (50 cm3) and
stirred at room temperature for 3 h followed by the
addition of triphenyl phosphine (58 mg, 0.22 mmol).
The mixture was stirred at room temperature for 2 days.
The orange microcrystalline solid formed was collected
by filtration, washed with methanol and then dried in
air. Yield, 52 mg (80%). Anal. Calc. for C34H29
N2O3PPd: C, 62.73; H, 4.49; N, 4.30. Found: C, 62.4;
H, 4.4; N, 4.0%. Selected IR bands (cmꢀ1): 1574s
(mC@N), 1478s, 1431s, 1414m, 1360m, 1339s, 1296m,
1260m, 1242s, 1221w, 1200m, 1148s, 1098m, 1051m,
1028s, 910w, 843m, 793m, 745s, 706s, 692s, 590w, 530s,
2.9. X-ray crystallography
Single crystals of 3 were obtained by slow evapora-
tion of an acetonitrile solution of the complex and that
of 4 were grown by slow evaporation of a dimethyl-
sulfoxide-acetonitrile (1:2) solution of the complex.
Single crystals of [Pd(HL)((CD3)2SO)Cl] (6) were ob-
tained by slow evaporation of the dmso-d6 solution of 2.
X-ray data were collected on an Enraf–Nonius Mach-3
single crystal diffractometer using graphite monochro-
1
513s, 498m. H NMR (200 MHz) data in (CD3)2SO (d
(J (Hz))): 3.14 (s, 3H, –OCH3); 3.74 (s, 3H, –OCH3);
5.31 (s, 1H, H5); 6.09 (s, 1H, H3); 7.40 (m, 3H, H11, H12,
H13); 7.58 (m, PPh3 protons); 7.80 (10) (d, 2H, H10,
H14); 7.86 (8) (d, 1H, –HC@N–). Electronic spectral
data in CH3OH (k (nm) (e (Mꢀ1 cmꢀ1))): 487sh (5800),
457 (9400), 438sh (8500), 360sh (5700), 348 (5900), 275
(34 400).
ꢀ
mated Mo Ka radiation (k ¼ 0:71073 A) by x-scan
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 three check re-
flections were measured after every 1.5 h during the data
collection to monitor the crystal stability. In all the three
cases, there is no significant change in the intensities of
the check reflections. Empirical absorption correction
was applied to each of the three datasets based on the w-
scans [25] of four reflections (v 68.82–70.97° and h 5.66–
17.89°) for 3, eight reflections (v 83.22–89.13° and h
4.25–19.31°) for 4 and six reflections (v 82.13–87.53° and
h 3.90–15.64°) for 6. The structures were solved by di-
rect methods and refined on F 2 by full-matrix least-
squares procedures. In each case, all non-hydrogen
atoms were refined using anisotropic thermal parame-
ters. Hydrogen and deuterium atoms were included in
the structure factor calculation at idealized positions by
using riding model, but not refined. The programs of
WinGX [26] were used for data reduction and absorp-
tion correction. Structure solution and refinement were
performed with the SHELX-97 programs [27]. The
ORTEX6a package was used for molecular graphics
[28]. Selected crystal and refinement data for 3, 4 and 6
are listed in Table 1.
2.7. Synthesis of [Pd(HL)(PPh3)Cl] (5)
To a suspension of [Pd(HL)Cl] (2) (64 mg, 0.15
mmol) in 1:1 methanol–acetonitrile (25 cm3) solid PPh3
(39 mg, 0.15 mmol) was added and the mixture was
stirred at room temperature. In about 10 min a clear
deep yellow solution was obtained. The stirring was
continued for 10 h and a orange microcrystalline solid
separated. The compound was collected by filtration,
washed with methanol and finally dried in air. Yield, 50
mg (48%). Anal. Calc. for C34H29N2O3PPd: C, 59.40; H,
4.40; N, 4.07. Found: C, 59.2; H, 4.2; N, 3.9%. Selected
IR bands (cmꢀ1): 3233m, 1658s (mC@O), 1576s (mC@N),
1510m, 1487m, 1435s, 1346s, 1267s, 1209s, 1155s,
1096m, 1030s, 829s, 752s, 690s, 638m, 532s, 513s, 496m,
436w. 1H NMR (200 MHz) data in (CD3)2SO (d (J
(Hz))): 3.01 (s, 3H, –OCH3); 3.79 (s, 3H, –OCH3); 5.50
(s, 1H, H5); 6.12 (s, 1H, H3); 7.50 (m, PPh3 protons);
7.68 (m, 3H, H11, H12, H13); 7.87 (7) (d, 2H, H10, H14);
8.20 (s, 1H, –HC@N–); 9.46 (s, 1H, –NH–). Electronic

S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
355
Table 1
Crystal data for [Pd(HL)(CH3CN)Cl] (3), [PdL(PPh3)] (4) and [Pd(HL)((CD3)2SO)Cl] (6)
Complex
3
4
6
Chemical formula
Formula weight
Crystal system
Space group
PdC18H18N3O3Cl
466.20
PdC34H29N2O3P
650.96
Triclinic
PdC18H15D6N2O4ClS
509.31
Monoclinic
P21/n
Monoclinic
P21/c
ꢁ
P1
ꢀ
a (A)
7.3111(11)
23.570(3)
11.0760(14)
90
9.755(4)
12.198(2)
13.995(4)
113.75(2)
101.29(4)
93.00(3)
1478.8(8)
2
16.968(3)
6.1762(11)
20.455(4)
90
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
97.750(13)
90
111.203(13)
90
3
ꢀ
V (A )
1891.2(4)
4
1.637
1998.5(6)
4
1.693
Z
q (g cmꢀ3
l (mmꢀ1
)
1.462
0.719
)
1.145
4424
1.193
4986
Reflections measured
Reflections unique
Reflections I > 2rI
Parameters
Goodness-of-fit on F 2
R1, wR2 (I > 2rI )
R1, wR2 (all data)
8533
8533
3952
2385
4555
3172
6766
372
238
1.009
248
1.033
1.035
0.0431, 0.0788
0.1009, 0.0948
1.033
0.0357, 0.0804
0.0542, 0.0884
0.344
0.0374, 0.0797
0.0682, 0.0932
0.567
ꢀ3
ꢀ
Largest peak (e A
)
3. Results and discussion
SO)Cl] (6). The structure of 6 determined by X-ray
crystallography reveals the same coordination mode of
HLꢀ as in 3. The Cl-atom occupies the trans position
with respect to the metallated C-atom. The (CD3)2SO
coordinates the metal ion through the S-atom. The
molecular structures of 3 and 6, and the comparison of
the infrared and electronic spectral properties (vide in-
fra) of 2, 3, 5 and 6 suggests that HLꢀ is possibly
C,N,O-donor in 2 and C,N-donor in 5. In acetonitrile or
dmso and in reaction with one equivalent of PPh3, the
solvent or the PPh3 molecule comes in between the C-
and Cl-atom and binds the metal ion in 2 followed by
the Pd–O bond cleavage to form the product (Scheme
1). Thus in each of 3, 5 and 6 the incoming ligand is
situated trans to the azomethine N-atom. The N-atom
of acetonitrile, P-atom of PPh3 and the S-atom of dmso
are softer than the amide keto-O. Thus the protonated
amide keto-O of HLꢀ is replaced by the N-donor ace-
tonitrile, P-donor PPh3 and the S-donor dmso but not in
a stereo-retentive way. During the formation of 4 by
treating 2 with excess PPh3, most likely 5 is formed first
followed by deprotonation of the amide functionality by
the remaining unreacted PPh3 and then the Cl-atom is
replaced by the amide O-atom. Here the O-atom of the
deprotonated amide functionality can replace the Cl-
atom and bind the metal ion possibly due to the better
r-bonding ability of the enolate-O than that of the
chloride and chelate effect (Scheme 1).
3.1. Synthesis and characterization
The Schiff base N-(benzoyl)-N0-(2,4-dimethoxyben-
zylidene)hydrazine (H2L, 1), was obtained in high yield
by condensation reaction of one mole equivalent each of
2,4-dimethoxy benzaldehyde and benzoylhydrazine in
methanol in presence of acid. All the ortho-metallated
species synthesized using 1 are shown in Scheme 1. In
methanol, the reaction of 1 with Li2[PdCl4] (generated in
situ by treatment of one mole equivalent of PdCl2 with
two mole equivalents of LiCl) produces the ortho-
metallated species [Pd(HL)Cl] (2). When dissolved in
acetonitrile, 2 produces [Pd(HL)(CH3CN)Cl] (3). Re-
action of 2 with excess triphenyl phosphine results into
the formation of [PdL(PPh3)] (4). On the other hand,
reaction of 2 with PPh3 in 1:1 mole ratio produces
[Pd(HL)(PPh3)Cl] (5). The elemental analysis data of all
the compounds are consistent with the molecular for-
mulae given above. The complexes are orange to red in
the solid state and sparingly soluble in the common
organic solvents. All the four complexes are diamagnetic
and electrically non-conducting in solutions. The mo-
lecular structures of 3 and 4 have been determined by X-
ray crystallography (vide infra). Both complexes are
square-planar. In 3, the monoanionic ligand (HLꢀ) acts
as bidentate C,N-donor and the Cl-atom is at the trans
position with respect to the metallated C-atom. In 4, the
dianionic ligand (L2ꢀ) acts as C,N,O-donor. We were
unable to grow X-ray quality single crystals of 2.
However, (CD3)2SO solution of 2 used to collect the
proton NMR spectrum gave crystals of [Pd(HL)((CD3)2
3.2. Spectral properties
Infrared spectra of all the compounds have been
collected using KBr pellets. The free Schiff base (1)

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S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
H
C
N
Pd NCCH3
Cl
O
N
H
H3CO
H
OCH3
(3)
C
N
Pd PPh3
(iii)
N
O
3
H3CO
H
OCH3
2
H3CO
H
OCH3
4
5
1
C
6
C
7
(ii)
N
(4)
(i)
N
Pd Cl
O
N
8 O
N
H
H
9
H3CO
OCH3
14
13
10
11
(iv)
H
C
12
(2)
(1)
N
Pd PPh3
Cl
O
N
(v)
H
(5)
H3CO
OCH3
H
C
N
Pd S(=O)(CD3)2
Cl
O
N
H
Scheme 1. (i) Li2[PdCl4] (1:1 in methanol), (ii) PPh3 (1:2 in methanol–acetonitrile), (iii) in acetonitrile, (iv) PPh3 (1:1 in methanol–acetonitrile), (v) in
dmso-d6.
shows three medium intensity bands at 3175, 3009 and
2843 cmꢀ1. These are assigned to the N–H, aromatic C–
H and methyl C–H stretches, respectively. Two strong
bands observed at 1642 and 1603 cmꢀ1 are likely to be
due to the C@O and C@N stretches [20–22]. The spectra
of 2, 3, 5 and 6 display a weak peak in the range 3289–
3206 cmꢀ1. The origin of this peak is most likely the N–
H group of the protonated amide functionality in the
ligand HLꢀ. The appearance of this stretch at higher
wavenumbers for the complexes as compared to that
observed for 1 is most likely due to the involvement of
the N–H proton of 1 in intermolecular hydrogen
bonding. Crystal structures of 3 and 6 reveal no inter-
molecular hydrogen bonding involving the amide func-
tionality of HLꢀ. All the complexes display weak
multiple bands in the range 3100–2800 cmꢀ1 due to the
aromatic and methyl C–H stretches. Complex 3 displays
a weak band at 2168 cmꢀ1 ascribable to the CBN of the
coordinated acetonitrile. In the spectra of 2, 3, 5 and 6,
the highest wavenumber strong band appears at 1641,
1672, 1658 and 1661 cmꢀ1, respectively. This band is
assigned to the C@O stretch of the ligand in each
complex. For 2 the position of this band is essentially
identical with that of 1. On the other hand, C@O stretch
appears at higher wavenumber for 3, 5 and 6. In 3 and 6,
the ligand HLꢀ is bidentate C,N-donor (vide infra). The
higher wavenumber shift of the C@O stretch for 5 in-
dicates a similar coordination behavior of HLꢀ in 5. It is
known that the attachment of electron withdrawing
group to the amide-N causes higher energy shift of the
C@O stretch [29]. The C,N coordination of HLꢀ to the
metal ion is very likely to cause a similar electronic effect
on the amide functionality. Possibly for this reason the
C@O stretch appears at higher wavenumbers for 3, 5
and 6 compared to that for 1. There is an interesting
trend in the C@O stretching frequencies in 3, 5 and 6.
The observed values are decreasing in the order
3 > 6 > 5. The trans directing ability is expected to be in
the order PPh3 > dmso > CH3CN and hence the trans
Pd–N(azomethine) bond strength is expected to decrease

S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
357
in the order 3 > 6 > 5. Indeed this bond length is signif-
icantly shorter in 3 than that in 6 (vide infra). As a result
the electron withdrawing effect on the amide-N due to
metal coordination to the adjacent azomethine N-atom
will be decreasing in the order 3 > 6 > 5. Therefore the
values of the amide C@O stretching frequencies should
be also decreasing in the same order. It is not very clear
whether HLꢀ is C,N- or C,N,O-donor in 2 due to the
appearance of the C@O stretch at the same position
(1641 cmꢀ1) for both 1 and 2. If the ligand is C,N-donor,
2 has to be a dichloro bridged species. In such a situa-
tion, as observed for 3, 5, and 6 the C@O stretch for 2
should appear at a higher frequency than that in 1. In an
unlikely situation of the bridging chlorine atom being a
stronger trans directing group than PPh3, the C@O
stretch will appear at a lower frequency compared to
that in 5. On the other hand, if HLꢀ is C,N,O-donor in
2, the amide C@O stretch should shift to the lower
wavenumber [20–22] compared to that for the free Schiff
base (1) due to the O-coordination to the metal. The
formation of 3, 5, and 6 from 2 does not differentiate the
dichloro bridged or a mononuclear structure as both
structures can give the same products either by cleavage
of Pd–Cl (bridging) bond or by cleavage of Pd–O bond.
Considering the appearance of the C@O stretch at the
same position for both 1 and 2 as a consequence of
possible intermolecular hydrogen bonding involving the
O-atom of the C@O in 1 and metal coordination of the
same in 2 we have tentatively assigned the C,N,O-co-
ordinating mode of HLꢀ in 2 (Scheme 1). It also may be
noted that analogous complexes of Pd with semi-
carbozanoes and acetylhydrazone are mononuclear and
the ligand is monoanionic C,N,O-donor [20–22]. The
spectrum of 4 does not display any band assignable to
either the C@O or the N–H group. This observation is
consistent with the deprotonation of the amide func-
tionality and the C,N,O binding mode of the dianionic
ligand (L2ꢀ) in 4. All the five ortho-metallated species
display a strong band in the range 1566–1578 cmꢀ1. This
band is assigned to the azomethine (–HC@N–) fragment
of the ligand. The shift of the C@N stretch to lower
wavenumbers as compared to that (1603 cmꢀ1) of the
free Schiff base is expected due to the N-coordination of
the azomethine in all the complexes [17,30].
singlets. There is no significant variation in the chemical
shifts (d 3.80–3.84) of these two signals due to ortho-
metallation. The H5 proton of 5 resonates at d 5.50. This
indicates that the PPh3 is cis to the metallated C-atom
(C6) and the significant upfield shift of the H5 resonance
compared to that of 1, 3 and 6 is due to the shielding by
the phosphine phenyl rings [17,19,22]. For the same
reason the 4-methoxy group is expected to be observed
at lower d value. Indeed the two singlets due to the two
methoxy group protons are relatively well separated for
5 as compared to 1, 3 and 6 and one of them shows a
significant upfield shift (d 3.79 and 3.01). The signal at
the lower d value is assigned to the 4-methoxy group.
The –HC@N– and –NH– protons of 1 appear as singlets
at d 8.56 and 9.40, respectively. For 3, 5 and 6 the
–HC@N– proton appears as singlet. But these are shif-
ted upfield as compared to 1. These shifts are by d 0.2–
0.3. The upfield shift of the azomethine proton is con-
sistent with the weakening of the C@N bond due to the
N-coordination and the trans disposition of the Pd and
the proton around the C@N [12,16,31,32]. The –NH–
proton of 3 and also of 6 is observed at d 8.47. However,
the –NH– proton for 5 appears as a broad singlet at d
9.47 which is very close to that of 1. The methyl protons
of the acetonitrile in 3 are observed as a singlet at d 2.08.
In the spectrum of 4, there is no signal attributable to
the –NH– proton. Thus the amide functionality is de-
protonated and the dianionic ligand is C,N,O-donor in
4. As observed for 3, 5 and 6 the –HC@N– proton in 4 is
upfield shifted (d 7.86) as compared to 1 due to the N-
coordination and the trans disposition of the Pd and the
proton around the C@N. However, for this complex it
appears as a doublet due to coupling with the 31P nu-
cleus that is trans (vide infra) to the N-atom [16,18,31].
Due to the cis arrangement of the metallated C-atom
and the PPh3 in 4, the H5 proton is considerably upfield
shifted (d 5.31) and the signals due to the 2-methoxy (d
3.74) and 4-methoxy (d 3.14) groups are well separated
as observed for 5.
The electronic spectral data of all the compounds are
listed in Section 2. The free Schiff base displays four
intense absorptions in the range 331–231 nm. On the
other hand, the complexes show six to seven absorptions
within 491–217 nm. The lower energy absorptions are
likely to be due to ligand-to-metal charge transfer
transitions and the absorptions below 360 nm involve
intraligand transitions. The electronic spectral profiles
of 2, 3 and 6 in methanol solutions are identical. On the
other hand, the spectral profiles of 2 and 3 in acetonitrile
solutions are same. Similarly the spectra of 2 and 6 in
dimethylsulfoxide solutions are alike. These observa-
tions suggest that in acetonitrile 2 is transformed to 3
and in dimethylsulfoxide 2 is transformed to 6. Similarly
both 3 and 6 in methanol are converted to 2. The
spectral profiles of 1, 4 and 5 in methanol are very
similar. Unlike 2, 3 and 6, the spectral profile of 4 is
1
The H NMR spectra of 1, 3, 4, 5 and 6 in dmso-d6
have been recorded. Compound 6 is generated by dis-
solving 2 in dmso-d6 (vide supra). The H6 resonance is
observed as a doublet at d 6.51 for 1. The absence of this
proton in the spectra of 3, 4, 5 and 6 is in agreement with
the ortho-metallation of the 2,4-dimethoxybenzylidene
ring in these complexes. The signal corresponding to the
H5 is observed at d 8.05, 7.22 and 7.13 for 1, 3 and 6,
respectively. This upfield shift of the H5 resonance for 3
and 6 further supports the metallation of the C6 atom in
these two complexes. For 1, 3 and 6, the methyl protons
of the two methoxy groups appear as two closely spaced

358
S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
unaffected by the change of solvent. Thus the C,N,O,P
square-plane around the metal ion in 4 remains intact in
the above mentioned solvents.
3.3. Description of structures
The molecular structures of 3, 4 and 6 are depicted in
Figs. 1–3, respectively. The selected bond parameters
associated with the metal ion are listed in Table 2. In 3
and 6, the ligand HLꢀ binds the palladium(II) centre
through the aryl C-atom and the azomethine N-atom
forming a five-membered metallacycle. The Cl-atom
occupies the trans site with respect to the metallated C-
atom in both complexes. An acetonitrile N-atom and the
S-atom of dmso-d6 satisfies the fourth coordination site
in 3 and 6, respectively. In 4, the planar tridentate ligand
L2ꢀ coordinates the metal ion through the aryl C-atom,
azomethine N-atom and the O-atom of the deprotonated
amide functionality forming two five-membered rings.
The fourth coordination site is satisfied by the P-atom of
PPh3. The N2–C8 and C8–O1 bond distances in 3 and 6
are significantly larger than the distances observed for
the same bonds in 4 (Table 2). These values clearly sug-
gest the deprotonation of the amide functionality in 4
[33,34]. In each complex, the palladium(II) centre is in a
slightly distorted square-planar environment. The aver-
age deviations of the trans atoms from the mean plane
Fig. 1. Molecular structure of [Pd(HL)(CH3CN)Cl] (3) with the atom-
labeling scheme. All non-hydrogen atoms are represented by their 40%
probability thermal ellipsoids.
ꢀ
are 0.07 and –0.07, 0.02 and –0.04 and 0.06 and –0.07 A
for 3, 4 and 6, respectively. In 4, the O(1)–Pd–N(1) angle
in the five-membered ring formed by the O-atom of the
deprotonated amide functionality and the azomethine
N-atom is 76.92(8)°. In the five-membered metallacycle,
the C(1)–Pd–N(1) angles in 3 (80.90(19)°) and 6
(80.32(14)°) are very similar but slightly smaller than that
(82.14(9)°) in 4. The Pd–C(1) bond lengths are in the
Fig. 2. Molecular structure of [PdL(PPh3)] (4). Selected C-atoms of the
PPh3 phenyl rings are labeled for clarity. All non-hydrogen atoms are
represented by their 40% probability thermal ellipsoids.
ꢀ
range 1.987(5)–2.011(3) A in these complexes. These C–
Pd–N angles and Pd–C bond lengths are comparable
with the values reported for similar ortho-metallated
complexes with Schiff bases [12,13,17–22,32]. The Pd–N1
ꢀ
bond lengths are 2.011(4) and 2.059(3) A in 3 and 6,
respectively. The better trans directing ability of dmso
compared to acetonitrile is likely to be the reason for the
lengthening of the Pd–N1 bond in 6. This bond length
ꢀ
(1.9945(19) A) in 4 is expected to be the largest as the
trans directing ability of PPh3 is higher than that of
dmso. Most likely the rigidity [34] of the tridentate L2ꢀ
causes the observed shortening of the Pd–N1 bond
length in 4. As expected the Pd–O1 bond length (2.102(2)
ꢀ
A) in 4 is shorter than the bond lengths (2.153(2)–
ꢀ
2.168(4) A) observed for palladium(II) complexes of
C,N,O-donor ligands in which the O-coordinating amide
functionality is protonated [20–22]. However, this bond
length is longer than the Pd–O distance (2.016(3)–
ꢀ
2.033(4) A) observed in palladium(II) complexes with
N,N,O-donor ligand containing deprotonated amide
functionality [23]. The reason for this is most likely the
Fig. 3. Molecular structure of [Pd(HL)((CD3)2SO)Cl] (6) with the at-
om-labeling scheme. All non-hydrogen atoms are represented by their
40% probability thermal ellipsoids.

S. Das, S. Pal / Journal of Organometallic Chemistry 689 (2004) 352–360
359
Table 2
ꢀ
Selected bond lengths (A) and angles (°) for 3, 4, and 6
[Pd(HL)(CH3CN)Cl] (3)
Pd–C(1)
Pd–N(3)
1.987(5)
2.010(5)
1.361(6)
80.90(19)
93.3(2)
Pd-N(1)
Pd–Cl
2.011(4)
2.4418(14)
1.211(4)
N(2)–C(8)
C(1)–Pd–N(1)
C(1)–Pd–N(3)
C(1)–Pd–Cl
C(8)–O(1)
N(1)–Pd–N(3)
N(1)–Pd–Cl
N(3)–Pd–Cl
172.45(17)
92.76(12)
93.33(13)
172.62(14)
[PdL(PPh3)] (4)
Pd–C(1)
Pd–O(1)
2.011(3)
2.102(2)
1.332(3)
82.14(9)
159.05(9)
97.71(8)
Pd–N(1)
Pd–P
1.9945(19)
2.2659(8)
1.292(3)
N(2)–C(8)
C(8)–O(1)
N(1)–Pd–O(1)
N(1)–Pd–P
O(1)–Pd–P
C(1)–Pd–N(1)
C(1)–Pd–O(1)
C(1)–Pd–P
76.92(8)
175.75(6)
103.20(6)
[Pd(HL)((CD3)2SO)Cl] (6)
Pd–C(1)
Pd–S
2.003(4)
2.2582(11)
1.351(5)
Pd–N(1)
Pd–Cl
2.059(3)
2.4089(12)
1.207(4)
N(2)–C(8)
C(8)–O(1)
N(1)–Pd–S
N(1)–Pd–Cl
S–Pd–Cl
C(1)–Pd–N(1)
C(1)–Pd–S
C(1)–Pd–Cl
80.32(14)
97.49(11)
170.64(11)
176.68(9)
91.62(9)
90.77(4)
better trans directing ability of the aryl C-atom in 4 than
that of the pyridine N-atom in the reported complexes.
The Pd–Cl bond lengths observed in 3 and 6 are within
the range reported for ortho-palladated species
[12,16,35–38] having the Cl-atom at the trans position
with respect to the metallated C-atom. The Pd–N(3)
[23,39], Pd–P [16–18,31] and Pd–S [40] bond lengths in 3,
4 and 6, respectively are unexceptional.
numbers are CCDC-218178, CCDC-218179 and
CCDC-218180 for 3, 4 and 6, respectively). Copies of
this information may be obtained from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK.
Acknowledgements
X-ray crystallographic studies were performed at the
National Single Crystal Diffractometer Facility, School
of Chemistry, University of Hyderabad (funded by the
Department of Science and Technology, New Delhi). We
thank the University Grants Commission, New Delhi for
the facilities provided under the UPE program. Mr. S.
Das thanks the Council of Scientific and Industrial Re-
search, New Delhi for a research fellowship.
4. Conclusion
In conclusion, we have synthesized and characterized
ortho-palladated complexes with N-(benzoyl)-N0-(2,4-
dimethoxybenzylidene)hydrazine (H2L, 1). In one of the
complexes (2), HLꢀ acts as a tridentate C,N,O-donor
ligand. In presence of coordinating solvents such as
acetonitrile or dimethylsulfoxide or one mole equivalent
of PPh3 the Pd–O bond in 2 cleaves and HLꢀ behaves as
a bidentate C,N-donor ligand (3, 5 and 6). However, in
presence of excess PPh3 the deprotonation of the amide
functionalty in 2 occurs and L2ꢀ remains as a tridentate
C,N,O-donor ligand in the new complex (4), which is
stable in coordinating solvents. Thus the dianionic li-
gand can tightly uphold three coordination sites making
the remaining position available for other ligands. We
are currently involved in synthesizing such new com-
plexes with this and similar Schiff bases.
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