Cyclopalladation of indole-3-carboxaldehyde aroylhydrazones

Article abstract of DOI:10.1016/j.jorganchem.2011.03.033

Reactions of PdCl₂, LiCl, indole-3-carboxaldehyde 4-R-benzoylhydrazones (H₂Lⁿ; n = 1, 2, 3 and 4 for R = H, Cl, OMe and NMe₂, respectively) and CH₃COONa· 3H₂O in 1:2:1:1 mol ratio in methan

Full text of DOI:10.1016/j.jorganchem.2011.03.033

Journal of Organometallic Chemistry 696 (2011) 2660e2664
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Journal of Organometallic Chemistry
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Note
Cyclopalladation of indole-3-carboxaldehyde aroylhydrazones
*
A.R. Balavardhana Rao, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Reactions of PdCl2, LiCl, indole-3-carboxaldehyde 4-R-benzoylhydrazones (H2Ln; n ¼ 1, 2, 3 and 4 for
R ¼ H, Cl, OMe and NMe2, respectively) and CH3COONa$3H2O in 1:2:1:1 mol ratio in methanol produce
the cyclopalladated species of general formula [Pd(HLn)Cl] in 65e85% yields. The complexes have been
characterized with the help of elemental analysis and spectroscopic (infrared, electronic and NMR)
measurements. The proton NMR spectra of the complexes suggest palladation at the peri position of the
indole moiety in (HLn)ꢀ. Molecular structure of a representative complex determined by X-ray crystal-
lography confirms the peri-palladation and formation of a distorted CNOCl square-plane around the
metal centre by the tridentate (HLn)ꢀ and the chloride.
Article history:
Received 12 February 2011
Received in revised form
18 March 2011
Accepted 23 March 2011
Keywords:
Palladium(II) complexes
Schiff bases
Ó 2011 Elsevier B.V. All rights reserved.
Indole
Peri-metallation
Crystal structure
1. Introduction
and acetylhydrazine [21]. With the bidentate system ortho-plati-
nation and with the tridentate system peri-palladation have been
Cyclometallated complexes are of immense contemporary
interest primarily because of their applications in a variety of
research areas such as organic synthesis, biological and pharma-
ceutical chemistry, catalysis and materials science [1e11]. We have
reported some such species with various Schiff bases prepared by
condensation of benzaldehyde and its derivatives with acid
hydrazides [12e15]. These amide-O coordinating Schiff bases can
provide only the ortho position of the benzaldimine moiety for
cyclometallation and act as tridentate C,N,O-donor ligands to form
5,5-membered fused chelate rings. On the other hand, indole-3-
carboxaldehyde 4-R-benzoylhydrazones (H2Ln, 2 H represent the
dissociable protons from the amide functionality and indole ring)
offer either of the two positions namely 2 or ortho and 4 or peri of
the indole moiety for cyclometallation and as C,N,O-donor it can
form either 5,5- or 6,5-membered fused chelate rings (Chart 1). It
may also be noted that in recent times indole containing ligands
and their metal complexes are receiving considerable attention for
their biological and clinical relevance [16e19]. There are two
reports on X-ray structurally characterized cyclometallated
complexes with such ligands. These ligands are bidentate alkaloids
gramine and tryptamine and (S)-triptophan methyl ester [20] and
the tridentate Schiff bases prepared by condensation of indole-3-
carboxaldehyde with semicarbazide, ethyl hydrazinecarboxylate
observed. We have tried to synthesize cylcopalladated species with
H2Ln to find out whether the aroyl moiety of H2Ln makes any
difference in the regioselectivity of the metallation site. Herein we
describe the synthesis, structure and properties of a family of peri-
metallated complexes having the general formula [Pd(HLn)Cl].
2. Experimental
2.1. Materials
Purification of solvents was performed by standard methods
[22]. All the chemicals used in this work were of analytical grade
available commercially and were used without further purification.
2.2. Physical measurements
A Thermo Finnigan Flash EA1112 series elemental analyzer was
used to collect the elemental (C, H, N) analysis data. A Shimadzu
LCMS 2010 liquid chromatograph mass spectrometer was used for
the purity verification. Solution electrical conductivities were
measured with a Digisun DI-909 conductivity metre. Infrared
spectra were recorded on a Jasco-5300 FT-IR spectrophotometer by
using KBr pellets. A PerkineElmer Lambda 35 UV/vis spectropho-
tometer was used to collect the electronic spectra. The proton NMR
spectra were recorded with the help of a Bruker 400 MHz NMR
spectrometer.
* Corresponding author. Tel.: þ91 40 2313 4829; fax: þ91 40 2301 2460.
E-mail address: spsc@uohyd.ernet.in (S. Pal).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.033

A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 696 (2011) 2660e2664
2661
Cl
H
N
6
O
Pd
5
7
Ar
C
N
4
8
7a
N
C
O
H
3a
H
9
N
HN
13
1
ortho-palladate
C
10
12
3
11
14
15
2
C
N
H
Cl
H
13a
R
Pd
14a
O
H2Ln
C
Ar
N
(n = 1- 4; R = H, Cl, OMe and NMe2)
HN
N
C
H
H
peri-palladate
Chart 1. Indole-3-carboxaldehyde aroylhydrazone and possible cyclopalladates.
The other three complexes namely [Pd(HL2)Cl] (2 (R ¼ Cl)),
[Pd(HL3)Cl] (3 (R ¼ OMe)) and [Pd(HL4)Cl] (4 (R ¼ NMe2)) were
synthesized from PdCl2, LiCl, the corresponding Schiff base (H2Ln)
and CH3COONa$3H2O in 65e85% yields by following procedures
very similar to that described above for 1 (R ¼ H).
Table 1
Crystal data for [Pd(HL1)Cl]$CH3CN (1$CH3CN).
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
PdC18H15N4OCl
445.22
Triclinic
P1
9.3308(10)
10.4842(11)
10.6176(11)
66.404(2)
84.781(2)
66.675(2)
871.23(16)
2
1.697
1.232
8421
3053
2854
227
0.0318, 0.0717
0.0352, 0.0730
1.105
0.500, ꢀ0.272
2.4. X-ray crystallography
b (Å)
c (Å)
A Bruker-Nonius SMART APEX CCD single crystal diffractometer,
equipped with a graphite monochromator and a Mo K
a fine-focus
a
sealed tube (
l
¼ 0.71073 Å) was used for determination of unit
b
g
(ꢁ)
cell parameters and intensity data collection at 298 K. The SMART
and the SAINT-Plus packages [23] were used for data acquisition
and data extraction, respectively. The absorption correction was
performed with the help of SADABS program [24]. The structure
was solved by direct method and refined on F2 by full-matrix least-
squares procedures. All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were included in the structure
factor calculation at idealized positions by using a riding model. The
SHELX-97 programs [25] were used for structure solution and
refinement. The ORTEX6a [26] and Platon packages [27] were used
for molecular graphics. Selected crystallographic data are summa-
rized in Table 1.
V (Å3)
Z
r
(g cmꢀ3
(mmꢀ1
)
)
m
Reflections collected
Reflections unique
Reflections [I ꢂ 2
s(I)]
Parameters
R1, wR2 [I ꢂ 2
s
(I)]
R1, wR2 [all data]
GOF on F2
Largest diff. peak and hole (e Åꢀ3
)
2.3. Synthesis of [Pd(HL1)Cl] (1)
3. Results and discussion
A mixture of PdCl2 (89 mg, 0.5 mmol) and LiCl (43 mg, 1 mmol)
was taken in 15 ml of dry methanol and refluxed with stirring for
1 h. It was then cooled to room temperature and filtered. H2L1
(132 mg, 0.5 mmol) and CH3COONa$3H2O (68 mg, 0.5 mmol) were
added to the filtrate and the mixture was stirred at room temper-
ature for 12 h. The complex separated as a green solid was collected
by filtration, washed with cold methanol and finally dried in air.
Yield: 170 mg (84%).
The Schiff bases (H2Ln) were prepared in 75e90% yields by
condensation reactions of equimolar amounts of indole-3-
carboxaldehyde and the corresponding 4-R-benzoylhydrazine
(R ¼ H, Cl, OMe and NMe2) in presence of a few drops of acetic acid in
ethanol [21]. Their molecular formulae and structures were
confirmed with the help of elemental analysis and spectroscopic
(mass and proton NMR) measurements. Reactions of H2Ln and
Table 2
Elemental analysis and electronic spectroscopica data.
Complex
Found (calc.) (%)
C
lmax (nm) (10ꢀ4
ꢃ
3
(Mꢀ1 cmꢀ1))
H
N
1
2
3
4
47.55 (47.21)
43.81 (43.54)
47.03 (47.12)
48.34 (48.03)
2.99 (3.05)
2.53 (2.38)
3.25 (3.36)
3.83 (3.72)
10.40 (10.55)
9.58 (9.35)
9.68 (9.59)
407 (5.92), 386 (7.89), 370 (5.92), 350b (3.38), 293b (2.53)
410 (5.78), 390 (7.84), 373 (5.87), 354b (3.43), 297b (2.48)
406 (6.40), 386 (8.18), 369 (5.97), 348b (3.41), 315b (2.36)
412 (7.06), 391 (8.77), 373 (6.32), 354b (3.84), 311 (4.56)
12.53 (12.37)
a
In dimethylformamide.
Shoulder.
b

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A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 696 (2011) 2660e2664
Table 3
Proton NMR dataa
(d (ppm) (J (Hz))) for complexes 1e4 in dmso-d6.
Complex
H(1)
H(2)
H(5)
H(6)
H(7)
H(8)
H(10)
H(13a,13a0)
H(14a,14a0)
R
1 (R ¼ H)
12.18 (s)
12.20 (s)
12.14 (s)
12.08 (s)
8.34 (d) (2)
8.33 (br,s)
8.30 (d) (3)
8.27 (d) (2)
7.80 (d) (8)
7.80 (d) (7)
7.79 (d) (8)
7.78 (d) (8)
6.98 (br,s)
6.99 (br,s)
6.98 (br,s)
6.96 (br,s)
7.21 (d) (7)
7.21 (br,s)
7.20 (d) (7)
7.19 (d) (6)
8.69 (s)
8.68 (s)
8.66 (s)
8.64 (s)
13.86 (s)
13.93 (s)
13.63 (s)
13.31 (s)
8.02 (d) (7)
8.04 (d) (8)
8.00 (d) (9)
7.87 (d) (9)
7.63 (t) (7)
7.71 (br,s)
7.16 (d) (8)
6.82 (d) (9)
7.72 (t) (6)
e
2 (R ¼ Cl)
3 (R ¼ OMe)
4 (R ¼ NMe2)
3.87 (s)
3.03 (s)
a
In dmso-d6.
in the ranges 333e323 and 289e268 nm. Thus the longer and the
shorter wavelength absorptions displayed by 1e4 are due to charge
transfer and ligand centred transitions, respectively [12,13,29,30].
The proton NMR spectra of the complexes and the free Schiff
bases were recorded in dmso-d6. The data for the complexes are
listed in Table 3. The H4 proton observed as a doublet within
Cl
C2
C3
C4
C1
C6
Pd
O1
C16
d
7.77e7.84 for the free Schiff bases is absent in the spectra of the
C10
C5
N1
C15
complexes. The absence of this resonance indicates peri-metal-
lation of the indole moiety in 1e4. In general, the resonances in the
complexes show a downfield shift compared to the corresponding
resonances observed for the free Schiff bases. However, the
C11
N2
N3
C7
C14
C13
C9
downfield shift by
compared to that (
shift of H5 corroborates peri-metallation of the indole ring in 1e4.
The singlets corresponding to indole NeH (H1 11.52e11.60) and
d
0.5e0.6 of the H5 is significantly larger
C8
C12
d
0.1e0.2) of the other CeH protons. This large
Fig. 1. Molecular structure of [Pd(HL1)Cl] (1) with the atom labelling scheme. All non-
hydrogen atoms are represented by their 50% probability thermal ellipsoids.
d
the amide NeH (H10 d 11.22e11.60) protons are very closely spaced
for the free Schiff bases. On the other hand, these signals are well
separated in the spectra of 1e4 due to disproportionate downfield
CH3COONa$3H2O with Li2PdCl4 (generated in situ from 1 mol
equivalent of PdCl2 and 2 mol equivalents of LiCl) in dry methanol
provide the cyclometallated species in moderate to good yields. The
elemental analysis data (Table 2) of the complexes (1e4) are
consistent with the general formula [Pd(HLn)Cl]. As expected for
palladium(II) species, 1e4 are diamagnetic. The solubility of all four
complexes in polar acetone, acetonitrile, dimethylformamide and
dimethylsulfoxide is moderate to high, while they are slightly
soluble in relatively much less polar halogenated solvents such as
dichloromethane and chloroform. In solution, 1e4 behave as non-
electrolyte. Thus the chloride is coordinated to the metal centre in
each complex.
shift of both. The indole NeH (H1) appears within
d 12.08e12.20,
while amide NeH (H10) appears in the range 13.31e13.93. The
d
much larger downfield shift of the latter indicates coordination of
the azomethine N-atom adjacent to the amide NeH and hence
Infrared spectra of 1e4 display the two NeH and the C]O
stretches in the frequency ranges 3195e3605 and 1597e1615 cmꢀ1
,
respectively [21,28]. The C]O stretches in 1e4 are about
15e25 cmꢀ1 lower than the C]O stretches displayed by the free
Schiff bases (H2Ln).
A strong band observed in the range
1600e1612 cmꢀ1 for H2Ln is attributed to the C]N stretch. The
corresponding band for 1e4 appears at a lower frequency range of
1560e1580 cmꢀ1 [12e15,21]. The lowering of both C]O and C]N
stretches indicates that the ligand (HLn)ꢀ coordinates to the metal
centre through the azomethine N-atom and the protonated amide
functionality O-atom forming a five-membered chelate ring.
The electronic spectra of 1e4 were collected in dimethylforma-
mide. Thespectral data arelisted inTable 2. The spectral profiles of 1e4
are verysimilarexceptforsomesmall shiftsinthebandpositions.They
display three strong peaks in the wavelength range 412e369 nm.
These are followed by two shoulders within 354e293 nm for 1e3 and
a shoulder and a peak at 354 and 311 nm, respectively for 4 (Table 2).
The free Schiff basesin dimethylformamide showonly two absorptions
Table 4
Selected bond lengths (Å) and angles (ꢁ) for 1$CH3CN.
PdeC(1)
PdeN(2)
C(10)eO(1)
C(1)ePdeO(1)
C(1)ePdeCl
O(1)ePdeCl
1.972(3)
2.016(3)
1.240(4)
172.57(11)
94.08(10)
93.18(6)
PdeO(1)
PdeCl
C(10)eN(3)
C(1)ePdeN(2)
O(1)ePdeN(2)
N(2)ePdeCl
2.156(2)
2.299(1)
1.342(4)
94.54(12)
78.26(9)
171.25(7)
Fig. 2. One-dimensional ordering of NeH/N connected [Pd(HL1)Cl]$CH3CN
(1$CH3CN) via NeH/Cl hydrogen bonds.

A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 696 (2011) 2660e2664
2663
formation of a six-membered metallacycle in each of 1e4 (Chart 1).
In the aroyl fragment, the meta protons (H13, H13a) with respect to
fused chelate rings is preferred over ortho-metallation and forma-
tion of 5,5-membered fused chelate rings (Chart 1).
the substituent (R) resonate as a doublet within
d
7.87e8.04. The
Cyclometallated species with a few ligands containing polycylic
aromatic moieties such as 1-naphthyl and 9-phenanthryl are
known [36e42]. These polycyclic moieties like indole can provide
either the ortho- or the peri-C for formation of 5- or 6-membered
metallacycle, respectively. Generally the bidentate ligands form
ortho-metallated species [36e39]. Both ortho- and peri-metallation
have been reported for a bidentate and a tridentate system [40,41],
while only peri-metallated complex has been isolated with a tri-
dentate ligand [42] as observed for (HLn)ꢀ. We are currently
engaged in exploring the regioselectivity of cyclometallation in
tridentate Schiff bases derived from acid hydrazides and a variety of
polycylic aromatic aldehydes.
ortho protons appear as a triplet for 1 (R ¼ H), broad singlet for 2
(R ¼ Cl) and doublet for 3 (R ¼ OMe) and 4 (R ¼ NMe2) in the range
d
d
6.82e7.71. The methyl protons of R in 3 and 4 appear as singlet at
3.87 and 3.03, respectively; while, the proton (H15) at the same
position in 1 appears as a triplet at
d 7.72.
The comparable spectral characteristics suggest that 1e4 have
similar molecular structures. The coordination of the chloride to the
square-planar bivalent palladium centre is confirmed by their non-
electrolytic and diamagnetic nature. Thus it becomes apparent that
the deprotonated Schiff base (HLn)ꢀ acts as a tridentate indole peri-C,
azomethine-N and amide-O donor ligand to form a 6,5-membered
fused chelate rings system and together with the chloride consti-
tutes a CNOCl square-plane around the metal centre in 1e4.
To confirm the above molecular structure we have tried to grow
single crystals of all the complexes. X-ray quality crystals could be
grown for only 1 by slow evaporation of its acetonitrile solution.
The complex crystallizes as 1$CH3CN in the space group P1. The
asymmetric unit contains one molecule each of the complex and
acetonitrile. The structure of 1 is illustrated in Fig. 1. Selected bond
parameters are listed in Table 4. As expected (HLn)ꢀ is indole peri-C,
azomethine-N and amide-O donor and the fourth coordination site
of the square-planar metal centre is satisfied by the chloride. The
CeO and CeN bond lengths in the eC(]O)eNHe fragment in
(HL1)ꢀ clearly indicate the protonated state of the amide func-
tionality [12,21,28,31,32]. The PdeC, PdeN and PdeO bond lengths
are within the range reported for bivalent palladium having similar
coordinating atoms [12,13,21,31e33]. The PdeCl bond length is
unexceptional [12,33e35]. The PdCNOCl square-plane is not ideal
with respect to the bond parameters associated with the metal
centre. The C(1)ePdeN(2) angle in the six-membered ring is
94.54(12)ꢁ, while the O(1)ePdeN(2) angle in the five-membered
ring is 78.26(9)ꢁ. However, the CNOCl square-plane including the
metal centre is perfectly planar (mean deviation 0.01 Å). In fact
except for the phenyl ring C(11)eC(16) rest of the molecule lie in
a single plane (Fig. 1). The maximum and the minimum deviations
from the mean plane constituted by Pd, Cl, O(1), N(1)eN(3) and
C(1)eC(10) are 0.077 and 0.003 Å, respectively. The phenyl ring
plane (mean deviation 0.006 Å) is slightly twisted along the C(10)e
C(11) bond. The dihedral angle between the phenyl ring plane and
the plane containing remaining atoms of 1 is 19.6(1)ꢁ.
Acknowledgements
Financial support provided by the Department of Science
and Technology (Grant No. SR/S1/IC-10/2007) is gratefully
acknowledged. Mr. A. R. Balavardhana Rao thanks the Council of
Scientific and Industrial Research for a fellowship.
Appendix A. Supplementary material
CCDC 811956 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.
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