Regioselective cyclometallation of 4-R-1-naphthaldehyde benzoylhydrazones: Palladium(II) complexes with CNO pincer like ligands

Article abstract of DOI:10.1016/j.jorganchem.2013.02.007

Reactions of Li₂PdCl₄, 4-R-1-naphthaldehyde benzoylhydrazones (H₂Lⁿ; n = 1 and 2 for R = H and OMe, respectively) and NaOAc·3H₂O in 1:1:1 mole ratio in methanol provide the cyclopalladated complexes w

Full text of DOI:10.1016/j.jorganchem.2013.02.007

Journal of Organometallic Chemistry 731 (2013) 67e72
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Journal of Organometallic Chemistry
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Regioselective cyclometallation of 4-R-1-naphthaldehyde benzoylhydrazones:
Palladium(II) complexes with CNO pincer like ligands
*
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 Li2PdCl4, 4-R-1-naphthaldehyde benzoylhydrazones (H2Ln; n ¼ 1 and 2 for R ¼ H and OMe,
respectively) and NaOAc$3H2O in 1:1:1 mole ratio in methanol provide the cyclopalladated complexes
with the general formula [Pd(HLn)Cl] (1 (R ¼ H) and 2 (R ¼ OMe)). Treatment of one mole equivalent of
[Pd(HLn)Cl] (1 and 2) with two mole equivalents of PPh3 in acetone results in the deprotonation of the O-
coordinated amide functionality and the replacement of the metal coordinated chloride with PPh3
leading to the formation of [Pd(Ln)(PPh3)] (3 and 4). All the complexes have been characterized with the
help of elemental analysis and spectroscopic (IR, UVevis and 1H NMR) measurements. NMR spectra
indicates the peri-metallation of the 1-naphthalenyl fragment of the tridentate ligand in 1e4. Molecular
structures determined by X-ray crystallography confirm the regioselective peri-metallation in each of 2, 3
and 4.
Article history:
Received 26 December 2012
Received in revised form
8 February 2013
Accepted 11 February 2013
Keywords:
Palladium(II)
Benzoylhydrazone
Cyclopalladation
Peri-metallation
Crystal structure
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
reactions [21]. Cyclometallated complexes with various bidentate
and tridentate ligands containing polycyclic aromatic fragments
The cyclometallation chemistry and cyclometallated complexes
continue to be an area of immense interest during the last few
decades because of their widespread applications in a variety of
research areas such as catalysis, organic synthesis, asymmetric
synthesis, photochemistry and materials chemistry [1e14]. As a
consequence, a vast literature on cyclometallated complexes with
various ligands containing a variety of aromatic rings that undergo
metallation is available [14e19]. Formation of the cyclometallated
intermediate is the key step in various alkylation, carbonylation and
annulation reactions [20e23]. In recent years, tridentate pincer li-
gands have attracted lot of attention for the design and synthesis of
new metallacycles which are more efficient in their applications in
the above mentioned areas [24e35]. We have been working on
cyclometallated complexes with tridentate Schiff bases derived
from acid hydrazides for quite some time now [36e41]. In these
complexes, the planar acid hydrazones act as tridentate aryl-C,
azomethine-N and amide-/amidate-O donor ligands and form
5,5- or 6,5-fused chelate rings. This type of coordination mode
makes these ligands unsymmetrical pincer like species [30e35].
Cyclometallation of a ligand that has two potential sites for met-
allation is of particular interest as it provides the scope for
controlled synthesis of a regioselective product [40,42,43]. Such
regioselectivity can play a decisive role in synthetic organic
such as indole-3, 1-naphthalenyl, 9-phenanthryl and 1-perylene
have been reported [43e59]. The aromatic part of these ligands
provide both ortho- and peri-positions as the potential sites for
metallation. Among the bidentate ligands, majority produce
exclusively ortho-metallated species [43e49], while few give both
ortho- and peri-metallated complexes [50e52]. On the other hand,
the tridentate ligands produce exclusively either peri-metallated
[40,53e56] or ortho-metallated species [47,57e59]. The dominance
of ortho-metallation over peri-metallation in bidentate ligands is
attributed to more stable 5-membered than less stable 6-
membered cyclometallated ring formation [51,54]. We have stud-
ied the chemistry of palladium with the potentially tridentate
C,N,O-donor 4-R-1-naphthaldehyde benzoylhydrazones (H2Ln,
n ¼ 1 and 2 for R ¼ H and OCH3, respectively) and found regiose-
lective peri-metallation of the 1-naphthalenyl fragment of the li-
gands. Herein, we describe syntheses, characterization and X-ray
crystal structures of these peri-metallated complexes.
2. Experimental
2.1. Materials
All chemicals used in this work were of analytical grade avail-
able commercially and were used as received. The solvents used
were purified by standard methods [60].
* Corresponding author. Tel.: þ91 40 2313 4829; fax: þ91 40 2301 2460.
E-mail addresses: spsc@uohyd.ernet.in, spscuh2000@yahoo.co.in (S. Pal).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.007

68
A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 731 (2013) 67e72
2.2. Physical measurements
(Hz)) ¼ 14.17 (s, 1H, NH), 8.76 (s, 1H, H9), 8.68 (br s, 1H, H2), 8.12 (8)
(d, 2H, H5, H7), 8.06 (7) (d, 2H, H12, H16), 7.71 (br s, 1H, H14), 7.62 (br
s, 2H, H13, H15), 7.31 (8) (t, 1H, H6), 7.20 (8) (d, 1H, H3), 4.11 (s, 3H,
OMe).
Elemental analysis data were obtained with a Thermo Finnigan
Flash EA1112 series elemental analyzer. Jasco-5300 and Thermo
Scientific Nicolet 380 FT-IR spectrophotometers were used to re-
cord the infrared spectra. Purity verifications of H2L1 and H2L2 were
performed with a Shimadzu LCMS 2010 liquid chromatograph mass
spectrometer. Solution electrical conductivities were measured
with a Digisun DI-909 conductivity meter. Electronic spectra were
collected on PerkineElmer Lambda 35 UV/vis and Shimadzu
UV3600 UV-vis-NIR spectrophotometers. The NMR spectra were
recorded with the help of Bruker 400 and 500 MHz NMR
spectrometers.
2.5. Synthesis of [Pd(L1)(PPh3)] (3)
Solid PPh3 (131 mg, 0.5 mmol) was added to a suspension of
[Pd(HL1)Cl] (1) (104 mg, 0.25 mmol) in acetone (10 ml) and the
mixture was stirred at room temperature for 24 h. The complex
[Pd(L1)(PPh3)] (3) separated as
a yellowish-green solid was
collected by filtration, washed with acetone and finally dried in air.
Yield: 110 mg (68%). Anal. calcd for PdC36H27N2OP: C, 67.45; H,
4.25; N, 4.37. Found: C, 67.58; H, 4.15; N, 4.26. UVevis in Me2NCHO:
lmax (nm) (10ꢀ3 ꢁ ε (Mꢀ1 cmꢀ1)) ¼ 428 (17.8), 403 (21.4), 383 (17.6),
360sh (11.0), 345sh (8.2), 319 (8.9), 272 (22.7). 1H NMR in (CD3)2SO:
2.3. Synthesis of H2L1
d
(ppm) (J (Hz)) ¼ 9.07 (s, 1H, H9), 8.65 (br, s, 1H, H2), 8.20 (br, s, 2H,
1-Naphthaldehyde (468 mg, 3 mmol) and a few drops of acetic
acid were added to an ethanol solution (50 ml) of benzoylhydrazine
(408 mg, 3 mmol). The mixture was boiled under reflux for 3 h. The
white crystalline solid separated was collected by filtration, washed
with ethanol and then dried in air. Yield: 660 mg (80%). Anal. calcd
for C18H14N2O: C, 78.81; H, 5.14; N, 10.21. Found: C, 78.96; H, 5.21;
N, 10.11. Mass in Me2NCHO: m/z ¼ 274. UVevis in Me2NCHO: lmax
(nm) (10ꢀ3 ꢁ ε (Mꢀ1 cmꢀ1)) ¼ 355sh (7.1), 336 (10.8), 320sh (9.5).
H
12, H16), 7.56 (m, 22H, H3, H4, H5, H7, H13e15, Hs of PPh3), 6.77 (br, s,
1H, H6). 31P NMR in (CD3)2SO:
d
(ppm) ¼ 25.51.
The yellowish-green [Pd(L2)(PPh3)] (4) was synthesized in 68%
yield from one mole equivalent of [Pd(HL2)Cl] (2) and two mole
equivalents of PPh3 using a very similar procedure described above.
Anal. calcd for PdC37H29N2O2P: C, 66.23; H, 4.36; N, 4.17. Found: C,
66.38; H, 4.41; N, 4.07. UVevis in Me2NCHO: lmax (nm) (10ꢀ3 ꢁ ε
(Mꢀ1 cmꢀ1)) ¼ 438 (14.0), 415 (20.8), 394 (17.8), 370sh (11.4), 355sh
1H NMR in (CD3)2SO:
d
(ppm) (J (Hz)) ¼ 11.99 (s, 1H, NH), 9.16 (s, 1H,
H9), 8.91 (8) (d, 1H, H2), 8.03 (m, 4H, H4, H5, H12, H16), 7.97 (7) (d,1H,
H8), 7.70 (8) (t, 1H, H14), 7.63 (m, 3H, H3, H13, H15), 7.58 (8) (t, 2H, H6,
H7).
(6.9), 315sh (7.2), 271 (21.2). 1H NMR in (CD3)2SO:
d (ppm) (J
(Hz)) ¼ 8.88 (s, 1H, H9), 8.69 (br s, 1H, H2), 8.16 (8) (d, 2H, H5, H7),
8.01 (6.4) (d, 2H, H12, H16), 7.73 (br s, 4H, H14, para 1Hs of PPh3), 7.63
(m, 2H, H13, H15), 7.45 (m, 12H, o,m 1Hs of PPh3), 7.21 (8.4) (d, 1H,
H3), 6.71 (br, s, 1H, H6), 4.11 (s, 3H, OMe). 31P NMR in (CD3)2SO:
H2L2 was prepared in 85% yield from equimolar amounts of 4-
methoxy-1-naphthaldehyde and benzoylhydrazine in presence of
acetic acid using the same procedure as described for H2L1. Anal.
calcd for C19H16N2O2: C, 74.98; H, 5.30; N, 9.20. Found: C, 74.81; H,
5.21; N, 9.36. Mass in Me2NCHO: m/z ¼ 304 mg. UVevis in Me2N-
CHO: lmax (nm) (10ꢀ3 ꢁ ε (Mꢀ1 cmꢀ1)) ¼ 370sh (13.6), 350 (22.3),
d
(ppm) ¼ 25.49.
2.6. X-ray crystallography
335sh (20.9), 280sh (6.8). 1H NMR in (CD3)2SO:
d (ppm) (J
Single crystals of 2 were grown by diethyl ether vapour diffusion
into its dimethylformamide solution, while single crystals of 3 and
4 were obtained by slow evaporation of their corresponding
(Hz)) ¼ 11.83 (s, 1H, NH), 9.02 (8) (d, 1H, H2), 9.00 (s, 1H, H9), 8.28
(8) (d, 1H, H5), 7.99 (7) (d, 2H, H12, H16), 7.87 (8) (d, 1H, H8), 7.71 (8)
(t, 1H, H14), 7.62 (7) (d, 2H, H13, H15), 7.57 (8) (t, 2H, H6, H7), 7.11 (8)
(d, 1H, H3) 4.05 (s, 3H, OMe).
acetonitrile solutions. Complex
2
crystallizes as [Pd(HL2)Cl]$
Me2NCHO (2$Me2NCHO). On the other hand, complexes 3 and 4
crystallize as it is without any solvent molecule. Determination of
2.4. Synthesis of [Pd(HL1)Cl] (1)
Table 1
A mixture of PdCl2 (178 mg, 1 mmol) and LiCl (86 mg, 2 mmol)
was taken in methanol (20 ml) and boiled with stirring under reflux
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 ml) of H2L1 (275 mg, 1 mmol) and NaOAc$3H2O (136 mg,
1 mmol). The mixture was stirred at room temperature for 48 h. The
complex precipitated as yellowish-green solid was collected by
filtration, washed with methanol and finally dried in air. Yield:
332 mg (80%). Anal. calcd for PdC18H13N2OCl: C, 52.07; H, 3.16; N,
6.75. Found: C, 52.15; H, 3.08; N, 6.85. UVevis in Me2NCHO: lmax
(nm) (10ꢀ3 ꢁ ε (Mꢀ1 cmꢀ1)) ¼ 428 (15.8), 403 (17.6), 382 (13.5),
360sh (8.0), 350sh (6.5), 320 (8.0), 271 (17.3). 1H NMR in (CD3)2SO:
Selected crystallographic data.
Complex
2$(CH3)2NCHO
3
4
Chemical formula
Formula weight
Crystal system
Space group
PdC22H22N3O3Cl
518.28
Monoclinic
C2/c
21.487(3)
8.9400(12)
23.968(4)
111.397(5)
4286.8(11)
8
PdC36H27N2OP
640.97
PdC37H29N2O2P
670.99
Monoclinic
P21/n
Monoclinic
P21/c
ꢀ
a (A)
16.0972(5)
15.4471(4)
23.2869(7)
102.977(3)
5642.5(3)
8
24.8618(14)
15.7887(9)
15.4990(9)
101.218(1)
5967.7(6)
8
ꢀ
b (A)
ꢀ
c (A)
b
(ꢂ)
3
ꢀ
V (A )
Z
r
(g cmꢀ3
(mmꢀ1
)
)
1.606
1.019
19,917
1.509
1.494
m
0.748
0.713
d
(ppm) (J (Hz)) ¼ 14.05 (s, 1H, NH), 8.92 (s, 1H, H9), 8.63 (br s, 1H,
Reflections
collected
Reflections
unique
27,787
42,467
H2), 8.23 (8) (d, 1H, H4), 8.13 (6) (d, 1H, H5), 8.08 (7) (d, 2H, H12, H16),
7.82 (8) (d, 1H, H7), 7.72 (8) (t, 1H, H14), 7.67 (8) (t, 1H, H3), 7.63 (m,
2H, H13, H15), 7.35 (8) (t, 1H, H6).
3779
2952
9937
6511
10,515
8226
Reflections
The yellowish-green [Pd(HL2)Cl] (2) was synthesized in 80%
yield from PdCl2, LiCl, NaOAc$3H2O and H2L2 (1:2:1:1 mole ratio) by
following a procedure very similar to that described for 1. Anal.
calcd for PdC19H15N2O2Cl: C, 51.26; H, 3.40; N, 6.29. Found: C, 51.36;
H, 3.51; N, 6.15. UVevis in Me2NCHO: lmax (nm) (10ꢀ3 ꢁ ε
(Mꢀ1 cmꢀ1)) ¼ 436 (15.9), 412 (20.3), 391 (16.8), 370sh (10.8),
[I ꢃ 2
s(I)]
Parameters
274
739
775
R1, wR2 [I ꢃ 2
s
(I)]
0.0560, 0.1107
0.0762, 0.1192
1.098
0.586, ꢀ0.315
0.0398, 0.0913
0.0694, 0.0998
0.907
0.0380, 0.0878
0.0539, 0.0943
1.023
R1, wR2 [all data]
GOF on F2
Largest diff. peak
and hole (e A
1.953, ꢀ0.612
0.575, ꢀ0.260
ꢀꢀ3
)
350sh (6.9), 315 (7.8), 270 (16.3). 1H NMR in (CD3)2SO:
d (ppm) (J

A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 731 (2013) 67e72
69
unit cell parameters and intensity data collection at 298 K for
2$Me2NCHO and 4 were performed with the help of a Bruker-
high solubility in dichloromethane and chloroform also. In solution,
all the complexes behave as non-electrolyte.
Nonius SMART APEX CCD single crystal diffractometer using
ꢀ
graphite monochromated Mo K
a
radiation (
l
¼ 0.71073 A). The
3.2. Spectroscopic characteristics
SMART and the SAINT-Plus packages [61] were used for data
acquisition and data extraction, respectively. The SADABS program
[62] was used for absorption corrections. Unit cell parameters and
Infrared spectra of the Schiff bases (H2L1 and H2L2) and the
corresponding complexes (1e4) have been collected using KBr
pellets. The Schiff bases display two medium intensity bands at
w3223 and w3062 cmꢀ1 due to the amide NeH and the aromatic
CeH stretches, respectively. The amide C]O stretch appears as a
strong band at 1645 and 1651 cmꢀ1 for H2L1 and H2L2, respectively.
The strong band observed at 1576 and 1580 cmꢀ1 for H2L1 and H2L2,
respectively is attributed to the C]N stretch [36e41]. As observed
for H2L1 and H2L2, both 1 and 2 also display two bands at w3180
and w3040 cmꢀ1 due to the amide NeH and the aromatic CeH
stretches, respectively. The O-coordinated amide C]O and N-co-
ordinated C]N stretches of the tridentate ligand (HLn)ꢀ in 1 and 2
appear at w1600 and w1560 cmꢀ1, respectively. Both 3 and 4 do
not display any band assignable to the NeH or C]O indicating the
deprotonation of the amide functionality. The band observed at
w3050 cmꢀ1 for 3 and 4 is because of the aromatic CeH moieties of
the ligands. Both complexes display a strong band at w1550 cmꢀ1
due to the conjugated C]NeN]C fragment of the tridentate (Ln)2ꢀ
[36e41].
the intensity data at 298 K for 3 were obtained using graphite
ꢀ
monochromated Mo K
a
radiation (
l
¼ 0.71073 A) on an Oxford
Diffraction Xcalibur Gemini single crystal X-ray diffractometer. The
CrysAlisPro software [63] was used for data collection, reduction
and absorption correction. The structures of all three complexes
were solved by direct method and refined on F2 by full-matrix
least-squares procedures. All non-hydrogen atoms were refined
anisotropically. The hydrogen atom of the NeH group of (HL2)ꢀ in
2$Me2NCHO was located in a difference map and refined isotropi-
cally with restrained thermal parameter. The remaining hydrogen
atoms in 2$Me2NCHO and all the hydrogen atoms in 3 and 4 were
included at ideal positions for structure factor calculation by using a
riding model. The SHELX-97 programs [64] available in the WinGX
package [65] were used for structure solution and refinement. The
Platon [66] and Mercury [67] packages were used for molecular
graphics. Selected crystallographic data are summarized in Table 1.
3. Results and discussion
Electronic spectra of the Schiff bases (H2L1 and H2L2) and the
complexes (1e4) were recorded using their dimethylformamide
solutions. Representative spectra are illustrated in Fig. 1. The
spectra of H2L1 and H2L2 are very similar and exhibit an intense
absorption at 336 and 350 nm, respectively. This absorption is
flanked by two shoulders at 355 and 320 nm and 370 and 335 nm
for H2L1 and H2L2, respectively. Thus there is a red-shift of the band
positions from the unsubstituted H2L1 to the substituted H2L2. The
spectral profiles of 1e4 are also very similar. All of them display
closely spaced three sharp absorptions followed by two shoulders
in the wavelength range 438e345 nm (Fig. 1). In addition to this
group of five absorptions, each complex shows two more absorp-
tion at w318 and w271 nm. It may be noted that the spectrum
reported for naphthalene in cyclohexane shows a similar group of
absorptions centred at w275 nm and a high energy shoulder at
w225 nm [68]. Thus the absorption bands of 1e4 are primarily due
to ligand centred transitions only. As observed for the Schiff bases,
here also the absorption features of 2 and 4 are red-shifted
compared to those of 1 and 3. The spectral profiles and the red-
shift of band positions observed for H2L1, H2L2 and the complexes
1e4 strongly resemble the spectral characteristics of the
3.1. Synthesis and characterization
The Schiff bases (H2Ln) were synthesized in 80e90% yields by
condensation of the corresponding 4-R-1-naphthaldehyde and
benzoylhydrazine in 1:1 mole ratio in presence of a few drops of
acetic acid in ethanol. The elemental analysis, LCMS and 1H NMR
spectra of H2L1 and H2L2 are consistent with their structures.
Treatment of one mole equivalent each of H2Ln and NaOAc$3H2O
with Li2PdCl4 (generated in situ from PdCl2 and LiCl in 1:2 mole
ratio) in methanol afforded the complexes of formula [Pd(HLn)Cl] in
very good yields (80%). Reaction of [Pd(HLn)Cl] (1 and 2) with PPh3
in 1:2 mole ratio in acetone produced [Pd(Ln)(PPh3)] (3 and 4) in
w70% yields (Scheme 1). The elemental analysis data of each of 1e4
support the corresponding molecular formula. All the complexes
are yellowish-green and diamagnetic. Both 1 and 2 are sparingly
soluble in halogenated solvents like dichloromethane and chloro-
form, moderately soluble in acetonitrile and acetone and highly
soluble in dimethylsulfoxide and dimethylformamide. Solubility
behaviour of 3 and 4 is very similar to that of 1 and 2 except for their
R
4
R
R
5
8
5a
6
7
3
2
8a
1
9
N
N
Pd Cl
O
N
Pd PPh3
O
(i)
(ii)
HN
10
HN
N
O
11
16
15
12
13
14
H2Ln
R = H (n = 1) and OMe (n = 2)
1 (R = H)
(R = OMe)
3 (R = H)
(R = OMe)
4
2
Scheme 1. (i) Li2PdCl4 and NaOAc$3H2O (equimolar amounts of each in methanol), (ii) PPh3 (2 mole equivalents in acetone).

70
A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 731 (2013) 67e72
Dimethylsulphoxide-d6 solutions of H2L1, H2L2 and the com-
plexes 1e4 were used to record the proton NMR spectra. The
spectra of 1 and 2 display all the protons of (HLn)ꢀ, while in the
spectra of 3 and 4 some of the aromatic protons of (Ln)2ꢀ could not
be assigned due to the overlapping signals of the PPh3 phenyl ring
protons. The H8 proton of H2L1 and H2L2 resonates as a doublet at
d
7.97 and
spectra of 1e4 indicates metallation at C8. The signal corresponding
to the NeH proton of H2L1 and H2L2 is observed at
11.99 and
11.83, respectively. A broad weak signal exhibited by 1 and 2 at
d 7.87, respectively. The absence of any such signal in the
d
d
d
14.05 and
d 14.17, respectively is assigned to the NeH proton of
(HLn)ꢀ. No such signal in the spectra of 3 and 4 indicates the
deprotonation of the amide functionality and the dianionic form
((Ln)2ꢀ) of the ligand. A singlet observed in the range
d 4.05e4.11 for
H2L2, 2 and 4 is attributed to the methyl protons of the eOMe
substituent. The singlet corresponding to the azomethine proton
(H9) resonates at
d
9.16 and
d
9.00 for H2L1 and H2L2, respectively,
and within
d
8.88e9.07 for 1e4. There is an upfield shift of H2, H6
and H9 protons of the complexes when compared with the corre-
sponding protons of the free Schiff bases. On the other hand, H3 and
H7 protons of the complexes are shifted downfield and H5 does not
show any significant change. In the benzoyl fragment of 1e4, ortho
protons (H12 and H16) are slightly downfield shifted, while no shift
is observed in case of meta and para protons (H13, H14 and H15). 31
P
NMR spectra of 3 and 4 display a singlet at d w25.5 indicating PPh3
coordination to the metal centre.
Fig. 1. Electronic spectra of H2L1 (d) and [Pd(L1)(PPh3)] (3) (——) in dimethylformamide.
3.3. X-ray structures
The molecular structures of 2, 3 and 4 are confirmed by single
crystal X-ray crystallography. Despite our best attempts X-ray
quality crystals of 1 could not be grown. The asymmetric unit of 2
contains one complex molecule and one dimethylformamide
corresponding 9-anthracenyl analogues [41]. Alteration of the
pe
p* energy gaps and hence red-shift of arene absorption bands
due to introduction of substituent and also metal coordination is
reported in literature [41,69e71].
Cl1
Table 2
C2
C3
ꢂ
ꢀ
Selected bond lengths (A) and angles ( ) for 2$Me2NCHO, 3 and 4.
O1
N2
Pd1
C1
C9
C8
Complex 2
Pd(1)eC(1)
Pd(1)eO(1)
C(1)ePd(1)eN(1)
C(1)ePd(1)eCl(1)
N(1)ePd(1)eCl(1)
C4
C15
C14
1.975(5)
2.132(3)
93.12(19)
98.06(15)
168.59(13)
Pd(1)eN(1)
Pd(1)eCl(1)
C(1)ePd(1)eO(1)
N(1)ePd(1)eO(1)
O(1)ePd(1)eCl(1)
1.963(4)
2.3090(15)
171.09(17)
78.58(15)
90.38(10)
C12
C10
C16
O2
C13
N1
C11
C19
C5
C6
C17
Complex 3
Molecule 1
Pd(1)eC(1)
Pd(1)eO(1)
C(1)ePd(1)eN(1)
C(1)ePd(1)eP(1)
N(1)ePd(1)eP(1)
Molecule 2
C18
2.020(4)
2.082(3)
92.59(14)
98.14(11)
168.43(9)
Pd(1)eN(1)
Pd(1)eP(1)
C(1)ePd(1)eO(1)
N(1)ePd(1)eO(1)
O(1)ePd(1)eP(1)
2.008(3)
2.2997(10)
169.15(12)
78.33(11)
91.41(7)
C7
(a)
Pd(2)eC(37)
Pd(2)eO(2)
C(37)ePd(2)eN(3)
C(37)ePd(2)eP(2)
N(3)ePd(2)eP(2)
2.021(4)
2.078(2)
92.78(13)
97.75(10)
168.96(9)
Pd(2)eN(3)
Pd(2)eP(2)
C(37)ePd(2)eO(2)
N(3)ePd(2)eO(2)
O(2)ePd(2)eP(2)
2.006(3)
2.2952(10)
169.78(12)
78.00(11)
91.74(7)
Complex 4
Molecule 1
Pd(1)eC(1)
Pd(1)eO(1)
C(1)ePd(1)eN(1)
C(1)ePd(1)eP(1)
N(1)ePd(1)eP(1)
Molecule 2
2.004(3)
2.077(2)
93.00(11)
99.02(9)
166.16(8)
Pd(1)eN(1)
Pd(1)eP(1)
C(1)ePd(1)eO(1)
N(1)ePd(1)eO(1)
O(1)ePd(1)eP(1)
1.998(2)
2.2923(8)
168.48(10)
78.18(9)
90.69(6)
Pd(2)eC(38)
Pd(2)eO(3)
C(38)ePd(2)eN(3)
C(38)ePd(2)eP(2)
N(3)ePd(2)eP(2)
2.010(3)
2.087(2)
93.05(12)
97.52(9)
166.97(8)
Pd(2)eN(3)
Pd(2)eP(2)
C(38)ePd(2)eO(3)
N(3)ePd(2)eO(3)
O(3)ePd(2)eP(2)
1.997(2)
2.2905(8)
169.68(10)
77.82(10)
92.19(6)
(b)
Fig. 2. (a) Molecular structure of 2 with the atom labelling scheme. All non-hydrogen
atoms are represented by their 50% probability thermal ellipsoids. (b) Ball and stick
diagram of the hydrogen bonded dimer of 2$Me2NCHO.

A.R.B. Rao, S. Pal / Journal of Organometallic Chemistry 731 (2013) 67e72
71
ꢀ
centre and there is essentially no deviation (0.01e0.04 A) of the
metal centre from this square-plane. The tridentate ligand forms
6,5-fused chelate rings in each of 2, 3 and 4. The two chelate rings
are slightly folded along the metal and azomethineeN bond (Figs. 2
and 3). The fold angles are within 3.1(2)e6.5(1)ꢂ. The CeO and CeN
bond lengths of the amide fragment of the tridentate ligand in 2 are
C24
C28
C25
C19
ꢀ
P1
1.241(6) and 1.341(6) A, respectively, while in 3 and 4 they are
C31
ꢀ
C2
C9
within 1.293(4)e1.298(4) and 1.298(4)e1.311(4) A, respectively.
C3
C35
Thus the amide functionality in 2 is protonated and that in 3 and 4
is deprotonated and the negative charge is delocalized over OCN
fragment [36e41]. This is also reflected in the significantly longer
PdeO bond length in 2 than that in both 3 and 4 (Table 2). The Pde
N bond length in 4 is longer compared to that in 2 due to the better
trans directing ability of PPh3 than chloride. The PdeC bond lengths
Pd1
C4
O1
C13
C14
C1
C10
C15
C5
C6
C12
N1
C8
C7
N2
C11
ꢀ
C16
(2.004(3)e2.021(4) A) in 3 and 4 are also longer than that
C18
ꢀ
(1.975(5) A) in 2. It is very likely that this difference in PdeC bond
C17
lengths is the result of larger trans effect of amidate-O in 3 and 4
than the trans effect of amide-O in 2 [41]. The PdeCl (in 2) and the
PdeP (in 3 and 4) bond lengths are unexceptional. In general, all the
bond lengths associated with the metal atom in each of 2, 3 and 4
are within the ranges observed for Pd(II) complexes having similar
coordinating atoms [36,37,40e42,48,51e53,55].
(a)
C25
C31
C26
4. Conclusions
C20
C1
Cyclometallated complexes [Pd(HLn)Cl] (1 and 2) and
[Pd(Ln)(PPh3)] (3 and 4) with the C,N,O-donor Schiff bases 4-R-1-
naphthaldehyde benzoylhydrazones (H2Ln, R ¼ H (n ¼ 1) and OMe
(n ¼ 2)) have been synthesized and characterized. In each species,
peri position in preference to the ortho position of 1-naphthalenyl
fragment of the tridentate ligand is metallated. This regioselective
metallation in 1e4 has been authenticated by proton NMR and
single crystal X-ray crystallography. Currently we are working on
synthesis and characterization of other platinum group metal
complexes with the present Schiff base system and analogous spe-
cies containing various polycyclic aromatic moieties to explore and
understand the regioselective cyclometallation process.
C32
C37
P1
C2
C3
Pd1
O1
C4
C14
C13
C10
C15
C9
O2
C12
N1
C19
C5
C6
N2
C16
C8
C18
C11
C17
C7
(b)
Acknowledgements
Fig. 3. Molecular structures of (a) 3 and (b) 4 with the atom labelling schemes. For
clarity selected C-atoms of the PPh3 in each structure are labelled. All non-hydrogen
atoms are represented by their 50% probability thermal ellipsoids.
Financial assistance for this work has been received from the
Department of Science and Technology (DST) (IYC Grant No. IR/S1/
CF-01E/2011). A. R. B. Rao thanks the Council of Scientific and In-
dustrial Research (CSIR) for a research fellowship. We thank the
DST and the University Grants Commission (UGC) for the facilities
provided under the FIST and the CAS programs, respectively.
molecule, while that of each of 3 and 4 contains two complex
molecules. The bond parameters involving the metal centre for all
three structures are listed in Table 2. There is no significant varia-
tion in the bond parameters of the two molecules present in the
asymmetric unit of each of 3 and 4. The solvated 2$Me2NCHO unit
dimerizes due to two types of hydrogen bonding. These hydrogen
bonds are between the tridentate ligand NeH and the solvent
amide-O and between the solvent CeH and the coordinated Cl. In
these NeH/O and CeH/Cl interactions, N/O and C/Cl distances
Appendix A. Supplementary material
CCDC 916284e916286 contain the supplementary crystallo-
graphic data for 2$Me2NCHO, 3 and 4, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ꢀ
are 2.709(6) and 3.620(8) A, respectively and NeH/O and CeH/Cl
angles are 163(5) and 146ꢂ, respectively. The structure of 2 and the
dimer of the solvate 2$Me2NCHO are illustrated in Fig. 2. For each of
3 and 4, the structure of only one of the two molecules present in
the corresponding asymmetric unit is shown in Fig. 3. The (HL2)ꢀ
binds the metal atom through the 1-naphthalenyl peri-C, the
azomethine-N and the amide-O atoms in 2, while the metal atom is
coordinated to the 1-naphthalenyl peri-C, the azomethine-N and
the amidate-O atoms of (Ln)2ꢀ in each of 3 and 4. The fourth co-
ordination site in 2 is occupied by a Cl-atom, whereas that in each of
3 and 4 is occupied by the P-atom of a PPh3 molecule. In each of the
three complexes, the four coordinating atoms form a satisfactory
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