Syntheses, structures and catalytic activities of two cyclopalladated complexes derived from N'-(2-naphthylidene)benzohydrazide

Article abstract of DOI:10.1016/j.jorganchem.2017.08.002

Condensation reaction of 2-naphthaldehyde with one equivalent of benzohydrazide in presence of acetic acid in refluxing methanol yielded the Schiff base N'-(2-naphthylidene)benzohydrazide (H₂L) in 86% yield. Reaction of equimolar amounts of Li₂PdCl₄ (generated in situ from PdCl₂ and LiCl taken in 1:2 mol ratio), H₂L and NaOAc·3H₂O in methanol at room temperature provided the complex [Pd(HL)Cl] (1) in 82% yield. Treatment of 1 mol equivalent of 1 with 2 mol equivalents of PPh₃ in acetone at room temperature produced the complex [Pd(L)(PPh₃)] (2) in 72% yield. The Schiff base and the two complexes were characterized by elemental analysis, mass spectrometric and various spectroscopic (IR, UV–Vis and ¹H NMR) measurements. The molecular structures of both complexes (1 and 2) were determined by single crystal X-ray crystallography. In each square-planar complex, the tridentate ligand (HL^? in 1 and L^2? in 2) acts as pincer-like CNO-donor. The fourth coordination site is occupied by chloride in 1, while that in 2 is satisfied by the P-atom of PPh₃. The 2-naphthyl fragment of both HL^? and L^2? is palladated at the 3-position. Complex 2 was found to be an effective catalyst for one-pot Suzuki-Miyaura double cross-coupling reactions of 3,5-dihalosalicylaldehydes with a variety of arylboronic acids to provide the corresponding triaryl products in moderate to excellent yields.

Full text of DOI:10.1016/j.jorganchem.2017.08.002

Accepted Manuscript
Syntheses, structures and catalytic activities of two cyclopalladated complexes
derived from N'-(2-naphthaldimine)benzohydrazide
G. Narendra Babu, A.R. Balavardhana Rao, Srinivas Keesara, Samudranil Pal
PII:
S0022-328X(17)30482-5
10.1016/j.jorganchem.2017.08.002
JOM 20055
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 31 May 2017
Revised Date: 2 August 2017
Accepted Date: 3 August 2017
Please cite this article as: G.N. Babu, A.R.B. Rao, S. Keesara, S. Pal, Syntheses, structures and
catalytic activities of two cyclopalladated complexes derived from N'-(2-naphthaldimine)benzohydrazide,
Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.08.002.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract

ACCEPTED MANUSCRIPT
Syntheses, structures and catalytic activities of two cyclopalladated
complexes derived from N'-(2-naphthaldimine)benzohydrazide
G. Narendra Babu, A. R. Balavardhana Rao, Srinivas Keesara, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
ABSTRACT
Condensation reaction of 2-naphthaldehyde with one equivalent of benzohydrazide in
presence of acetic acid in refluxing methanol yielded the Schiff base N'-(2-
naphthaldimine)benzohydrazide (H2L) in 86% yield. Reaction of equimolar amounts of
Li2PdCl4 (generated in situ from PdCl2 and LiCl taken in 1:2 mole ratio), H2L and
NaOAc·3H2O in methanol at room temperature provided the complex [Pd(HL)Cl] (1) in 82%
yield. Treatment of one mole equivalent of 1 with two mole equivalents of PPh3 in acetone at
room temperature produced the complex [Pd(L)(PPh3)] (2) in 72% yield. The Schiff base and
the two complexes were characterized by elemental analysis, mass spectrometric and various
1
spectroscopic (IR, UV-Vis and H-NMR) measurements. The molecular structures of both
complexes (1 and 2) were determined by single crystal X-ray crystallography. In each square-
planar complex, the tridentate ligand (HL− in 1 and L2− in 2) acts as pincer-like CNO-donor.
The fourth coordination site is occupied by chloride in 1, while that in 2 is satisfied by the P-
atom of PPh3. The 2-naphthyl fragment of both HL− and L2− is palladated at the 3-position.
Complex 2 was found to be an effective catalyst for one-pot Suzuki-Miyaura double cross-
coupling reactions of 3,5-dihalosalicylaldehydes with a variety of arylboronic acids to
provide the corresponding triaryl products in moderate to excellent yields.
Keywords: Cyclopalladation; Regioselective; N′-(2-naphthaldimine)benzohydrazide; Crystal
structures; Spectroscopic properties; Double cross-coupling
1. Introduction
Syntheses of new cyclometallated complexes continue to attract immense interest
because of the potential applications of such species in a diverse range of research areas that
include materials science, biological studies, pharmaceutical chemistry and organic synthesis
[1−10]. We have been working on cyclometallated platinum group metal ion complexes with
a variety of Schiff bases prepared by condensation reactions of mono- or polycyclic aromatic
aldehydes with aroylhydrazides, thiosemicarbazides and thioaroylhydrazides during the past
several years [11−20]. In all these complexes, the ligands have the pincer like CNO- or CNS-
coordination mode. Based on the position of the azomethine group, the Schiff bases derived
* Corresponding author.
E-mail address: spal@uohyd.ac.in (S. Pal)
1

ACCEPTED MANUSCRIPT
from polycyclic aromatic aldehydes may provide the prospect for cyclometallation of the
polycyclic moiety in a regioselective way and isolation of the corresponding cyclometallates
[13−18]. Regioselective cyclometallation reaction is an important tool in organic synthesis
reactions as it can assist in functionalization of a specific site among several available. The
Schiff base N'-(2-naphthaldimine)benzohydrazide (H2L, 2 Hs represent the amide-H and the
H-atom at the 1- or 3-position of the 2-naphthyl group) is one such Schiff base. It is expected
that the chelation of the Schiff base via the azomethine-N and the amide-O atom will bring
the 2-naphthyl moiety in close proximity of the metal center for the activation of its C−H
bond either at the 1- or 3-position both being ortho to the azomethine group and eventual
cyclometallation at one of the two C-atoms. In the present work, we have studied the
palladium(II) coordination chemistry of H2L to examine how the cyclopalladation of the 2-
naphthyl fragment occurs. In this effort, we have isolated two complexes, [Pd(HL)Cl] (1) and
[Pd(L)(PPh3)] (2), in which the 3-position of the 2-naphthyl fragment of each of the two
pincer-like tridentate ligands (HL− and L2−) is regioselectively metallated (Scheme 1). In the
following account, we report the syntheses, X-ray crystal structures and spectroscopic
properties of these two complexes. The catalytic activities of both complexes in the one-pot
Suzuki-Miyaura double cross-coupling reactions of 3,5-dihalosalicylaldehydes with various
arylboronic acids have been also evaluated.
2. Experimental
2.1. Materials
All chemicals used in this work were of analytical grade available commercially and
were used as received without any further purification. All solvents used were purified by
standard procedures [21].
2.2. Physical measurement
Elemental (CHN) analysis measurements were performed using a Thermo Finnigan
Flash EA1112 series elemental analyzer. High resolution mass spectra were recorded with a
Bruker Maxis (ESI-TOF analyzer) spectrometer. A Sherwood Scientific balance was used for
room temperature magnetic susceptibility measurements. The solution electrical
conductivities were measured with the help of a Digisun DI-909 conductivity meter. A
Thermo Scientific Nicolet 380 FT-IR spectrophotometer was used to collect the infrared
spectra. The electronic absorption spectra were recorded by employing a Shimadzu UV3600
2

ACCEPTED MANUSCRIPT
1
UV-Vis-NIR spectrophotometer. A Bruker spectrometer was used to record the H (400
31
MHz, SiMe4 as an internal standard) and P{1H} (160 MHz, 85% H3PO4 as an external
standard) NMR spectra.
2.3. Synthesis of H2L
Few drops of acetic acid were added to a methanol solution (40 ml) of
benzohydrazide (681 mg, 5 mmol) and 2-naphthaldehyde (786 mg, 5 mmol) and the resulting
mixture was boiled under reflux for 5 h. The reaction mixture was then cooled to room
temperature. The white solid precipitated was filtered, washed with methanol (2 x 15 ml) and
finally dried in vacuum. Yield: 1.18 gm (86%). Anal. Calcd for C18H14N2O (%): C, 78.81; H,
5.14; N, 10.21. Found (%): C, 78.65; H, 5.21; N, 10.12. HRMS in CHCl3 m/z found (calcd)
for [M+H]+: 275.1187 (275.1184). Selected IR bands:
ν
(cm−1) = 3199 (N−H), 1638 (C=O),
1605 (C=N). UV-Vis in Me2NCHO: λmax (nm) (ε (104 M−1 cm−1)) = 347sh (0.65), 333sh (1.74),
1
315 (2.48), 282 (2.01), 274 (1.86). H NMR in (CD3)2SO:
δ (ppm) = 11.99 (s, 1H), 8.64 (s,
1H), 8.16 (s, 1H), 8.03−7.93 (m, 6H), 7.63−7.53 (m, 5H).
2.4. Synthesis of [Pd(HL)Cl] (1)
To a methanol (30 ml) solution of Li2[PdCl4] (generated in situ from PdCl2 (178 mg, 1
mmol) and LiCl (85 mg, 2 mmol)) were added H2L (275 mg, 1 mmol) and NaOAc·3H2O
(137 mg, 1 mmol) and the mixture was stirred at room temperature (298 K) for 24 h. The
yellow solid separated was collected by filtration, washed with methanol (2 x 10 ml) and then
dried in vacuum. Yield: 340 mg (82%). Anal. Calcd for C18H13N2OClPd (%): C, 52.07; H,
3.16; N, 6.75. Found (%): C, 52.21; H, 3.09; N, 6.87. HRMS in CHCl3-MeCN (1:1) m/z
found (calcd) for [M−Cl+MeCN]+: 420.0407 (420.0328). Selected IR bands:
ν
(cm−1) = 3178
(N−H), 1612 (C=O), 1596 (C=N). UV-Vis in Me2NCHO: λmax (nm) (ε (104 M−1 cm−1)) = 442
1
(0.44), 419sh (0.57), 394 (1.21), 374 (1.17), 356 (0.95). H NMR in (CD3)2SO:
δ (ppm) =
12.32 (br s, 1H), 8.85 (s, 1H), 8.33 (s, 1H), 8.11−7.99 (m, 2H), 7.83−7.33 (m, 6H), 7.25−7.16
(m, 1H), 7.01−6.94 (m, 1H).
2.5. Synthesis of [Pd(L)(PPh3)] (2)
Solid PPh3 (263 mg, 1 mmol) was added to a suspension of [Pd(HL)Cl] (1) (208 mg,
0.5 mmol) in acetone (25 ml) and the mixture was stirred at room temperature (298 K) for 24
h. The light yellow solid precipitated was collected by filtration, washed with acetone (2 x 10
ml) and finally dried in air. Yield: (230 mg, 72%). Anal. Calcd for C36H27N2OPPd (%): C,
3

ACCEPTED MANUSCRIPT
67.45; H, 4.25; N, 4.37. Found (%): C, 67.31; H, 4.18; N, 4.46. HRMS in CHCl3 m/z found
(calcd) for [M+H]+: 641.0971 (641.0974). Selected IR bands:
Vis in Me2NCHO: λmax (nm) (ε (104 M−1 cm−1)) = 448 (0.63), 422 (0.78), 393 (1.26), 374
(1.18), 357 (0.84). 1H NMR in CDCl3:
(ppm) (J (Hz)) = 8.04 (s, 1H), 8.02 (s, 1H), 7.91 (7)
ν
(cm−1) = 1587 (C=N). UV-
δ
(d, 1H), 7.79−7.75 (m, 6H), 7.59 (6) (d, 1H), 7.56−7.52 (m, 4H), 7.49−7.46 (m, 6H), 7.42 (6)
(d, 1H), 7.37 (6) (t, 2H), 7.26 (6) (t, 1H), 7.19 (6) (t, 1H), 6.80 (6) (d, 1H), 6.32 (3) (d, 1H).
31P{1H} NMR in CDCl3:
δ (ppm) = 34.06.
2.6. General procedure for double cross-coupling reactions
In a 10 ml round bottom flask, 3,5-dihalosalicylaldehyde (1 mmol), arylboronic acid
(2.2 mmol for diiodo precursor or 2.5 mmol for dibromo precursor), LiOH·H2O (2 mmol)
and complex 2 (0.01 mol% for diiodo precursor or 0.05 mol% for dibromo precursor) in 0.1
ml dimethylformamide were taken in 2 ml of water-methanol (1:1) mixture. The reaction
mixture was then heated at 80 °C under aerobic condition. Progress of the reaction was
monitored by TLC. On completion, the reaction mixture was cooled to room temperature and
extracted with two 10 ml portions of ethyl acetate. The combined extract was washed with
water (2 x 10 ml), dried over anhydrous Na2SO4 and finally the solvent was removed under
reduced pressure. The solid obtained was subjected to column chromatography over silica gel
using ethyl acetate and hexane mixture as the eluent to afford the pure product. All the
1
functionalized triaryl products obtained were characterized by H-NMR spectroscopy and
high resolution mass spectrometry.
2.7. X-ray crystallography
Single crystals of [Pd(HL)Cl] (1) were grown by diethyl ether vapor diffusion into its
dimethylformamide solution, whereas single crystals of [Pd(L)(PPh3)] (2) were obtained by
slow evaporation of its chloroform-methanol (1:1) solution. 1 crystallizes as 1·Me2NCHO
and 2 crystallizes as it is without any solvent molecule. Unit cell determination and intensity
data collection for 1·Me2NCHO were performed on an Oxford Diffraction Xcalibur Gemini
single crystal X-ray diffractometer using graphite monochromated Mo Kα radiation (λ =
0.71073 Å) at 298 K. Data collection, reduction and absorption correction were performed
using the CrysAlisPro software [22]. In the case of 2, a Bruker D8 QUEST diffractometer
equipped with a PHOTON 100 CMOS area detector and an INCOATEC microfocus source
for graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) was used for determination of
4

ACCEPTED MANUSCRIPT
the unit cell and intensity data collection at 298 K. The APEX3 software package [23] was
used for data collection, data integration, reduction and absorption correction. The structures
of both complexes were solved by direct method and refined on F2 by full-matrix least-
squares procedures. The structure of 2 was refined as an inversion twin using the
TWIN/BASF procedure [24]. The non-hydrogen atoms of both structures were refined
anisotropically. In the case of 2, four carbon atoms (two from benzoyl phenyl ring and two
from PPh3 phenyl ring) were refined with restraints on anisotropic displacement parameters.
All hydrogen atoms were included in the structure factor calculation at idealized positions
using a riding model. The SHELX-97 programs [24] available in the WinGX software suite
[25] were used for structure solution and refinement. The Mercury package [26] was used to
prepare the molecular illustrations. Selected crystallographic data are listed in Table S1
(Supplementary material).
3. Results and discussion
3.1. Synthesis and some properties
The Schiff base N'-(2-naphthaldimine)benzohydrazide (H2L) was prepared in good
yield (86%) by condensation reaction of equimolar amounts of 2-naphthaldehyde and
benzohydrazide in methanol in presence of a few drops of acetic acid. The purity and the
identity of H2L have been confirmed by elemental analysis, mass spectrometric and
1
spectroscopic (IR, UV-Vis and H NMR) measurements. The palladium(II) complex
[Pd(HL)Cl] (1) was synthesized in 77% yield by reacting in situ generated Li2PdCl4,
NaOAc·3H2O and H2L in 1:1:1 mole ratio in methanol at room temperature. The second
complex [Pd(L)(PPh3)] (2) was synthesized in 73% yield by treating 1 with two equivalents
of PPh3 in acetone (Scheme 1). The excess PPh3 is expected to facilitate the deprotonation of
the amide functionality of HL− in 1. The elemental analyses data of 1 and 2 are in good
agreement with their corresponding molecular formulas. In chloroform and dichloromethane,
2 is highly soluble but 1 is sparingly soluble. Both complexes are highly soluble in
dimethylformamide and dimethylsulfoxide, sparingly soluble in acetonitrile and methanol
and insoluble in hexane and toluene. Due to solvent restrictions and the solubility problem
the positive ion ESI mass spectrum of 1 was collected in CHCl3-MeCN (1:1) mixture, where
its solubility is slightly better when compared with that in the pure solvents. The spectrum
shows the base peak at 420.0407 corresponding to the cation where the chloride is replaced
5

ACCEPTED MANUSCRIPT
by acetonitrile, i.e., [1−Cl+MeCN]+. On the other hand, the spectrum of 2 in pure CHCl3
displays the usual [2+H]+ as the base peak at 641.0971. As expected for square-planar
complexes of palladium(II), both 1 and 2 are diamagnetic. The electrically non-conducting
nature of 1 and 2 in solution is consistent with their molecular formulas as neutral species.
3.2. X-ray molecular structures
The structures of 1·Me2NCHO and 2 have been solved in the monoclinic space
groups C2/c and P21, respectively. The asymmetric unit of 1·Me2NCHO consists of one
molecule each of the complex and the solvent dimethylformamide, while that of 2 contains
two complex molecules. In 1·Me2NCHO, the solvent molecule is hydrogen bonded with the
complex molecule. The structure of the solvated species is illustrated in Fig. 1. The O-atom
of the solvent molecule and the amide-NH of the ligand HL− are involved in this hydrogen
bonding. The N(2)···O(2) distance and the N(2)−H···O(2) angle are 2.734(5) Å and 165°,
respectively. In contrast, there is no significant interaction between the two complex
molecules present in the asymmetric unit of 2. Both molecules are very similar with respect
to intramolecular bond lengths and bond angles. Thus the structure of one of these two
independent molecules of 2 is depicted in Fig. 2. Selected bond parameters for 1·Me2NCHO
and 2 are listed in Table 1. In each complex, the tridentate ligand (HL− in 1 and L2− in 2) acts
as pincer-like CNO-donor and forms fused 5,5-membered chelate rings. The structures reveal
that the 2-naphthyl fragment of each of HL− and L2− is palladated at the 3-position instead of
the 1-position. There are very few reports on cyclometallated 2-naphthyl containing chelating
ligands [27−30]. As observed in the present study, generally 3-position gets regioselectively
metallated when both 1- and 3-positions of the 2-naphthyl fragment are available [27−29]. In
such cases, it is thought that the 1-position is not accessible for metallation due to steric
hindrance caused by the H-atom at the peri-C (8-position). However, it has been shown that
the 1-position of the 2-naphthyl group can be metallated when the 3-position is blocked by a
substituent [30]. In the amide functionality (−C(=O)NH−) of HL− in 1, the C(12)−N(2) and
the C(12)−O(1) bond lengths are 1.358 (5) and 1.246 (5) Å, respectively. These values are
consistent with the neutral keto form of the amide functionality [13−15,17,19]. On the other
hand, for 2 the average of C(12/12′)−N(2/2′) bond lengths and that of C(12/12′)−O(1/1′) bond
lengths are 1.335(6) and 1.285(6) Å, respectively (Table 1). The shorter C(12/12′)−N(2/2′)
and longer C(12/12′)−O(1/1′) bond lengths in 2 compared to the corresponding bond lengths
6

ACCEPTED MANUSCRIPT
in 1 clearly indicate that the amide functionality of L2− in 2 is deprotonated and their values
suggest that the negative charge is delocalized over the amidate fragment [11−17,19,20]. The
HL− and the chloride constitute a CNOCl square-plane around the metal center in 1, while the
L2− and the PPh3 assemble a square-planar CNOP coordination environment for the metal
center in 2. In both complexes, the metal center has no deviation from the square-plane
formed by the four coordinating atoms. The rms deviations from the mean plane constituted
by the metal and the four coordinating atoms are within 0.01−0.02 Å. The amidate-O is
expected to be a better σ-donor than the amide-O. As a consequence, the Pd−O(amidate)
bond length in 2 is significantly shorter (by ~0.1 Å) than the Pd−O(amide) bond length in 1.
In contrast, the Pd−C bond length in 1 is shorter by ~0.05 Å than that in 2. This lengthening
of the Pd−C bond in 2 is very likely due to the better trans-directing ability of the amidate-O
than the amide-O (Figs. 1 and 2). Similarly the stronger trans-effect of PPh3 than Cl− causes a
longer (by ~0.02 Å) Pd−N(azomethine) bond length in 2 compared to that in 1 (Table 1).
Overall, all the bond lengths and the bond angles associated with the metal centers in 1 and 2
are within the ranges observed for palladium(II) complexes with similar ligands
[11,13−15,17,19].
3.3. Spectroscopic properties
Infrared spectra of the Schiff base (H2L) and the complexes (1 and 2) were recorded
by employing KBr pellets in the range 4000−400 cm−1. A large number of bands have been
observed in each of the three spectra. Except for few characteristic bands we have not
attempted to assign the remaining bands. The free Schiff base (H2L) displays the typical
bands for the amide N−H and C=O stretching vibrations at 3199 and 1638 cm−1, respectively.
A medium intensity band observed at 1605 cm−1 has been assigned to the azomethine C=N
stretching vibration. The spectrum of 1 displays the amide N−H stretch at 3178 cm−1. Two
medium intensity bands observed at 1612 and 1596 cm−1 are assigned to the amide C=O and
the azomethine C=N stretching frequencies, respectively. Appearance of these two bands at
lower frequencies compared to those of the free Schiff base is consistent with the
coordination of both amide-O and azomethine-N to the metal center in 1. On the other hand,
the spectrum of 2 does not display the amide N−H and C=O stretching bands due to
deprotonation of the amide functionality. A strong band observed at 1587 cm−1 is most likely
associated with the one end coordinated diazine (C=N−N=C) like fragment of L2− in 2.
7

ACCEPTED MANUSCRIPT
Electronic absorption spectra of H2L, 1 and 2 were recorded in dimethylformamide.
The spectrum of H2L displays a broad band centered at 315 nm preceded by a couple of
shoulders and followed by a relatively narrow band at 282 nm and a shoulder (Fig. S1,
Supplementary material). This type of spectral profile is typical of organic compounds
containing naphthyl group [15,31,32]. Pure naphthalene is known to display similar
absorptions at higher energy region with more pronounced vibrational structure [33,34]. The
spectral profile of naphthalene consists of a group of absorption bands centered at ~275 nm.
The absorption profiles of the two complexes 1 and 2 are very similar to that of the
naphthalene except for a large red-shift. A group of five bands are observed in the range
448−356 nm for both 1 and 2. The spectrum of 2 is illustrated in Fig. S1 (Supplementary
material). Thus the absorption bands observed for H2L as well as its complexes 1 and 2 are
due to transitions predominantly centered at their corresponding 2-naphthyl fragments.
1
1H NMR spectra of H2L and 1 were recorded in (CD3)2SO solution and the H and
31P{1H} spectra of 2 were recorded in CDCl3 solution. The amide N−H proton of H2L
resonates as a singlet at δ 11.99 ppm. The azomethine proton and the proton at the 1-position
of the 2-naphthyl fragment of H2L appear as two one-proton singlets at δ 8.64 and 8.16 ppm,
respectively. The remaining aromatic protons are observed as a six-proton multiplet and
another five-proton multiplet having the chemical shifts centered at δ ~7.98 and ~7.58 ppm,
respectively. A broad singlet at δ 12.32 ppm due to the amide N−H proton of the ligand HL−
is observed in the spectrum of 1. As in the free Schiff base, here also two one-proton singlets
corresponding to the azomethine proton and the proton at the 1-position of the 2-naphthyl
fragment of HL− are observed at δ 8.85 and 8.33 ppm, respectively. Slight downfield shifts of
these three resonances in 1 when compared with those in free H2L are not unusual
considering the deshielding caused by coordination of the amide-O and the azomethine-N to
the palladium(II) center. The remaining ten aromatic protons belonging to the 2-naphthyl and
the phenyl rings of HL− appear as four multiplets centered at δ ~8.05, ~7.58, ~7.21 and ~6.98
ppm. As expected the spectrum of 2 does not display the signal corresponding to the amide
N−H due to deprotonation. The spectrum shows two closely spaced one-proton singlets at δ
8.04 and 8.02 ppm followed by a one-proton doublet at δ 7.91 ppm. The singlets are very
likely to be due to the protons at the 1- and 4-positions of the 2-naphthyl fragment of L2−. The
one-proton doublet is assigned to the azomethine proton. The doublet splitting is attributed to
8

ACCEPTED MANUSCRIPT
the PPh3 P-atom coordinated at the trans-position with respect to the azomethine-N. There is
a significant upfield shift of the azomethine proton in 2 compared to that in H2L and 1. It is
very likely that the stronger σ-donor PPh3 facilitates better π-backdonation from metal to
azomethine in 2 and hence causes the upfield shift of the azomethine proton. The remaining
nine aromatic protons of L2− and fifteen protons of the PPh3 phenyl rings appear in the range
δ 7.79−6.32 ppm with various splitting patterns such as doublet, triplet and multiplet. In
general, the aromatic protons in the complexes are observed over a broader chemical shift
range than in the free Schiff base due to shielding/deshielding caused by metal coordination
and presence of the monodentate ancillary ligands. The singlet observed at
δ 34.06 ppm in
the 31P{1H} NMR spectrum of 2 confirms the presence of PPh3 as the ancillary ligand.
3.4. Catalysis studies
Functionalized polyaryls and polyheteroaryls are of considerable interest because of
their potential applications in natural products, biological, agrochemical, pharmaceutical and
materials research [35−39]. Hence, there has been a continuous effort to develop new and
efficient methods for the synthesis of such species [40−43]. Among the various strategies
used so far, the simplest and most versatile approach is the palladium catalyzed Suzuki-
Miyaura poly cross-coupling reactions of polyhaloaromatic compounds with arylboronic
acids [44−47]. In the present work, we have attempted the one-pot synthesis of functionalized
triaryls via double cross-coupling reactions of 3,5-dihalosalicylaldehydes with various
arylboronic acids using the cyclopalladated complexes 1 and 2 as catalysts. The reaction of
3,5-diiodosalicylaldehyde and phenylboronic acid in presence of 2 as catalyst was chosen as
the model reaction to screen the bases and the solvents (Table S2, Supplementary Material).
Among the various inorganic bases used in water-methanol (1:1) as the reaction solvent
(entries 1−6), LiOH·H2O was found to provide the best yield (94 %) of the double cross-
coupling product 3,5-diphenylsalicylaldehyde at a reaction temperature of 80 °C (entry 4).
No coupling product was detected in the absence of base (entry 7). Using LiOH·H2O as the
base the reaction was then performed in various water-solvent (1:1) mixtures (entries 8−13).
In all these reaction solvents, no yield to very poor yields were observed. Subsequently the
reaction was conducted under the identical conditions as described in entry 4 except for the
use of 1 instead of 2 as catalyst (entry 14). The change of the catalyst caused a significantly
low yield (80%) of the coupled product and indicated that 2 is a better catalyst than 1. Thus
9

ACCEPTED MANUSCRIPT
the optimal reaction conditions were found to be the use of 3,5-diiodosalicylaldehyde (1
mmol), phenylboronic acid (2.2 mmol), LiOH·H2O (2 mmol) and 2 (0.01 mol%) in water-
methanol (1:1) at 80 °C for 15 h (entry 4).
To examine the substrate scope of the catalyst 2, one-pot double cross-coupling
reactions of 3,5-diiodosalicylaldehyde with various arylboronic acids were performed under
the above mentioned optimal conditions. The results are summarized in Table 2. As observed
with phenylboronic acid (entry 1), high yields (88−92%) of the double cross-coupled
products were also obtained with p-methoxy-, p-fluoro-, and m-chlorophenylboronic acids
and 2-naphthylboronic acid but in 5 to 9 h longer reaction times (entries 2–5). Comparatively
rather low yield (74%) was obtained with o-anisolylboronic acid even in longer reaction
duration (entry 6).
The scope of the catalyst 2 was further extended to double cross-coupling reactions of
3,5-dibromosalicylaldehyde with the same group of arylboronic acids which was used for
3,5-diiodosalicylaldehyde. The reaction of 3,5-dibromosalicylaldehyde with phenylboronic
acid under the same optimal conditions described above for the diiodo precursor provided the
product 3,5-diphenylsalicylaldehyde but in significantly low yield (40%). However, increase
of the catalyst loading from 0.01 to 0.05 mol% and the amount of the phenylboronic acid
doubled the yield to 82% in 24 h of reaction time (Table 3, entry 1). The reactions with the
remaining arylboronic acids were then performed with the higher (0.05 mol%) catalyst
loading and arylboronic acid to dibromo precursor mole ratio (2.5:1) (entries 2–6). The
corresponding double cross-coupled products were isolated in moderate to good yields
(60−83%). In general, despite the use of higher catalyst loading and mole ratio of the
reactants and comparable or longer reaction times, the yields of the coupled products from
the dibromo precursor are less (Table 3) than the yields obtained in the case of the diiodo
precursor (Table 2). This difference in the reactivities is attributable to the fact that the C−Br
bond is stronger than the C−I bond [48].
4. Conclusions
Synthesis, characterization and spectroscopic properties of the Schiff base N'-(2-
naphthaldimine)benzohydrazide (H2L) and the cyclometallated palladium(II) complexes
[Pd(HL)Cl] and [Pd(L)(PPh3)] with it have been described. The X-ray molecular structures of
the two complexes demonstrate pincer-like CNO coordination mode of both HL− and L2−and
10

ACCEPTED MANUSCRIPT
regioselective cyclopalladation at the 3-position of the 2-naphthyl group of each ligand. The
spectroscopic characteristics of the two complexes are consistent with their molecular
structures. Catalytic properties of both complexes in one-pot Suzuki-Miyaura double cross-
coupling reactions of 3,5-dihalosalicylaldehydes with a variety of arylboronic acids have
been investigated. Between the two complexes, [Pd(L)(PPh3)] has been found to be the better
perfoming catalyst . The catalytic reactions exhibited reasonable substrate scope and provided
moderate to excellent yields of the functionalized triaryls.
Acknowledgements
G. N. Babu thanks the Council of Scientific and Industrial Research, New Delhi for a
research fellowship (No. 09/414(1025)/2012-EMR-I). Financial and infrastructure supports
from the University Grants Commission, New Delhi (through the UPE and CAS programs)
and the Department of Science and Technology, New Delhi (through the PURSE and FIST
programs) are gratefully acknowledged.
Appendix A. Supplementary material
CCDC 1553122 and 1553123 contain the supplementary crystallographic data for
1·Me2NCHO and 2, respectively. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, by e-mailing to data_request@ccdc.cam.ac.uk or by
contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44)1223-336-033. Supplementary Tables S1 and S2 and Fig. S1 related to
this article can be found in the online version.
References
[1] G. Zhou, W.–Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706−1727.
[2] C.–H. Siu, C.–L. Ho, J. He, T. Chen, P. Majumda, J. Zhao, H. Li, W.–Y. Wong,
Polyhedron 82 (2014) 71–79.
[3] Y. You, W. Nam, Chem. Soc. Rev. 41 (2012) 7061–7084.
[4] Q. Zhao, C. Huang, F. Li, Chem. Soc. Rev. 40 (2011) 2508−2524.
[5] N. Cutillas, G.S. Yellol, C. de Haro, C. Vicente, V. Rodríguez, J. Ruiz, Coord. Chem.
Rev. 257 (2013) 2784–2797.
[6] I. Omae, Coord. Chem. Rev. 280 (2014) 84–95.
[7] X. Jiang, N. Zhu, D. Zhao, Y. Ma, Sci. China Chem., 59 (2016) 40–52.
11

ACCEPTED MANUSCRIPT
[8] L. Cuesta, E.P. Urriolabeitia, Comm. Inorg. Chem. 33 (2012) 55−85.
[9] K. Godula, D. Sames, Science 312 (2006) 67−72.
[10] Y.–F. Han, G.–X. Jin, Chem. Soc. Rev. 43 (2014) 2799−2823.
[11] S. Das, S. Pal, J. Organomet. Chem. 689 (2004) 352–360.
[12] R. Raveendran, S. Pal, J. Organomet. Chem. 692 (2007) 824–830.
[13] A.R.B. Rao, S. Pal, J. Organomet. Chem. 696 (2011) 2660−2664.
[14] A.R.B. Rao, S. Pal, J. Organomet. Chem. 701 (2012) 62−67.
[15] A.R.B. Rao, S. Pal, J. Organomet. Chem. 731 (2013) 67−72.
[16] K. Nagaraju, S. Pal, J. Organomet. Chem. 737 (2013) 7−11.
[17] A.R.B. Rao, S. Pal, J. Organomet. Chem. 762 (2014) 58−66.
[18] R.N. Prabhu, S. Pal, J. Chem. Sci. 127 (2015) 589−596.
[19] G.N. Babu, S. Pal, J. Organomet. Chem. 805 (2016) 19−26.
[20] G.N. Babu, S. Pal, J. Organomet. Chem. 824 (2016) 42−47.
[21] D.D. Perrin, W.L.F. Armarego, D.P. Perrin, Purification of Laboratory Chemicals,
second ed., Pergamon, Oxford, 1983.
[22] CrysAlisPro Version 1.171.36.28, Agilent Technologies, Yarnton, Oxfordshire, UK,
2013.
[23] Bruker (2016). APEX3, SAINT-Plus and SADABS. Bruker AXS Inc., Madison,
Wisconsin, USA.
[24] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112−122.
[25] L.J. Farrugia, J. Appl. Crystallogr. 45 (2012) 849−854.
[26] F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. M. Cabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Crystallogr. 41
(2008) 466−470.
[27] J.–M. Valk, F. Maassarani, P. van der Sluis, A.L. Spek, J. Boersma, G. van Koten,
Organometallics 13 (1994) 2320−2329.
[28] M. Crespo, M. Font-Bardía, S. Pérez, X. Solans, J. Organomet. Chem. 642 (2002)
171−178.
[29] S. Fuertes, H. García, M. Perálvarez, W. Hertog, J. Carreras, V. Sicilia, Chem. Eur. J.
21 (2015) 1620−1631.
12

ACCEPTED MANUSCRIPT
[30] J.–M. Valk, R. van Belzen, J. Boersma, A.L. Spek, G. van Koten, J. Chem. Soc. Dalton
Trans. (1994) 2293−2302.
[31] J.R. Cardinal, P. Mukerjee, J. Phys. Chem. 82 (1978) 1614−1620.
[32] P.K. Lekha, T. Ghosh, E. Prasad, J. Chem. Sci. 123 (2011) 919–926.
[33] J.M. Dixon, M. Taniguchi, J.S. Lindsey, Photochem. Photobiol. 81 (2005) 212−213.
[34] J. Krausko, J.K. Malongwe, G. Bičanová, P. Klán, D. Nachtigallová, D. Heger, J. Phys.
Chem. A 119 (2015) 8565−8578.
[35] J.–K. Liu, Chem. Rev. 106 (2006) 2209−2223.
[36] F. Bellina, R. Rossi, Tetrahedron 62 (2006) 7213–7256.
[37] G. Hervé, C. Len, Sust. Chem. Proc. 3 (2015) 3.
[38] J.E. Anthony, Chem. Rev. 106 (2006) 5028−5048.
[39] A.R. Murphy, J.M.J. Fréchet, Chem. Rev. 107 (2007) 1066−1096.
[40] D. Pérez, E. Guitián, Chem. Soc. Rev. 33 (2004) 274–283.
[41] X. Feng, W. Pisula, K. Müllen, Pure Appl. Chem. 81 (2009) 2203–2224.
[42] D. Pérez, D. Peña, E. Guitián, Eur. J. Org. Chem. (2013) 5981–6013.
[43] T. Truong, M. Mesgar, K.K.A. Le, O. Daugulis, J. Am. Chem. Soc. 136 (2014)
8568−8576.
[44] L. Liu, Y. Zhang, B. Xin, J. Org. Chem. 71 (2006) 3994−3997.
[45] F. Beaumard, P. Dauban, R. H. Dodd, Org. Lett. 11 (2009) 1801−1804.
[46] A.R. Chaudhary, A.V. Bedekar, Appl. Organometal. Chem. 26 (2012) 430–437.
[47] S. Reimann, P. Ehlers, M. Sharif, A. Spannenberg, P. Langer, Tetrahedron 72 (2016)
1083−1094.
[48] Y.–R. Luo, Handbook of Bond Dissociation Energies in Organic Compounds, CRC
Press LLC, Boca Raton, 2003.
13

ACCEPTED MANUSCRIPT
Table 1
Selected bond lengths (Å) and angles (°) for 1·Me2NCHO and 2
[Pd(HL)Cl]·Me2NCHO (1·Me2NCHO)
Pd(1)−C(3)
1.953(4)
2.208(3)
1.358(5)
81.89(15)
98.89(13)
178.89(9)
Pd(1)−N(1)
1.977(3)
Pd(1)−O(1)
Pd(1)−Cl(1)
2.2929(12)
1.246(5)
C(12)−N(2)
C(12)−O(1)
C(3)−Pd(1)−N(1)
C(3)−Pd(1)−Cl(1)
N(1)−Pd(1)−Cl(1)
C(3)−Pd(1)−O(1)
N(1)−Pd(1)−O(1)
O(1)−Pd(1)−Cl(1)
158.13(14)
76.26(12)
102.97(8)
[Pd(L)(PPh3)] (2)
Molecule 1
Pd(1)−C(3)
2.008(5)
2.107(3)
1.333(6)
82.25(18)
98.94(13)
178.55(13)
Pd(1)−N(1)
1.996(4)
Pd(1)−O(1)
Pd(1)−P(1)
2.2594(11)
1.288(6)
C(12)−N(2)
C(12)−O(1)
C(3)−Pd(1)−N(1)
C(3)−Pd(1)−P(1)
N(1)−Pd(1)−P(1)
Molecule 2
C(3)−Pd(1)−O(1)
N(1)−Pd(1)−O(1)
O(1)−Pd(1)−P(1)
158.52(15)
76.40(15)
102.38(10)
Pd(1′)−C(3′)
2.005(5)
2.113(3)
1.337(6)
82.62(19)
98.61(15)
178.18(14)
Pd(1′)−N(1′)
2.002(4)
Pd(1′)−O(1′)
Pd(1′)−P(1′)
2.2586(11)
1.282(6)
C(12′)−N(2′)
C(12′)−O(1′)
C(3′)−Pd(1′)−N(1′)
C(3′)−Pd(1′)−P(1′)
N(1′)−Pd(1′)−P(1′)
C(3′)−Pd(1′)−O(1′)
N(1′)−Pd(1′)−O(1′)
O(1′)−Pd(1′)−P(1′)
158.62(16)
76.17(16)
102.53(9)
14

ACCEPTED MANUSCRIPT
Table 2
Reactions of 3,5-diiodosalicylaldehyde with arylboronic acidsa
Entry
Ar
Time (h)
Yieldb (%)
1
2
3
4
5
6
phenyl
15
24
20
24
24
24
94
88
90
92
90
74
p-anisolyl
p-fluorophenyl
m-chlorophenyl
2-naphthyl
o-anisolyl
a
Conditions: 3,5-Diiodosalicylaldehyde (1 mmol), arylboronic
acid (2.2 mmol), LiOH·H2O (2 mmol), 2 (0.01 mol%) in 0.1 ml
of dimethylformamide, H2O-MeOH in 1:1 volume ratio (2 ml),
80 °C.
b Isolated yield.
15

ACCEPTED MANUSCRIPT
Table 3
Reactions of 3,5-dibromosalicylaldehyde with arylboronic acidsa
Entry
Ar
Time (h)
Yieldb (%)
1
2
3
4
5
6
phenyl
24
24
24
30
30
24
82
80
84
60
62
83
p-anisolyl
p-fluorophenyl
m-chlorophenyl
2-naphthyl
o-anisolyl
a
Conditions: 3,5-Dibromosalicylaldehyde (1 mmol), arylboronic
acid (2.5 mmol), LiOH·H2O (2 mmol), 2 (0.05 mol%) in 0.1 ml
of dimethylformamide, H2O-MeOH in 1:1 volume ratio (2 ml),
80 °C.
b Isolated yield.
16

ACCEPTED MANUSCRIPT
Figure Captions
Scheme 1. Conversion of [Pd(HL)Cl] (1) to [Pd(L)(PPh3)] (2).
Fig. 1. ORTEP representation of [Pd(HL)Cl]·Me2NCHO (1·Me2NCHO) with the atom
numbering scheme. All non-hydrogen atoms are shown by their 50% probability thermal
ellipsoids.
Fig. 2. ORTEP representation of one of the two molecules of [Pd(L)(PPh3)] (2) present in the
asymmetric unit. The thermal ellipsoids for the non-hydrogen atoms are drawn at the 50%
probability level. All non-carbon atoms and a few selected carbon atoms are labeled for
clarity.
17

ACCEPTED MANUSCRIPT
Scheme 1
18

ACCEPTED MANUSCRIPT
Figure 1
19

ACCEPTED MANUSCRIPT
Figure 2
20

ACCEPTED MANUSCRIPT
Research Highlights
• Cyclopalladation chemistry of N'-(2-naphthaldimine)benzohydrazide (H2L) was examined.
• Mononuclear cyclopalladates [Pd(HL)Cl] and [Pd(L)(PPh3)] were isolated.
• Structures of both complexes were studied by X-ray crystallography and spectroscopy.
• 2-naphthyl of each of HL− and L2− is regioselectively cyclometallated at 3-position.
• [Pd(L)(PPh3)] efficiently catalyzed Suzuki-Miyaura double cross-coupling reactions.