Synthesis of a novel six membered CNS palladacycle; TD-DFT study and catalytic activity towards microwave-assisted selective oxidation of terminal olefin to aldehyde

Article abstract of DOI:10.1016/j.jorganchem.2016.08.006

This article documented synthesis of a Schiff base, E-2-(benzylthio)-N-{(2-methoxynaphthalene-1-yl)methylene}benzenamine (HL) by the treatment of 2-meyhoxynaphthaldehyde and 2-(benzylthio)aniline. Reaction of this synthesized Schiff base with Na₂PdCl₄has been investigated, which gave a novel six membered Pd(II) complex (PdLCl) through C,N,S-donor atom of Schiff base. These two newly synthesized compounds were characterized by¹H NMR, FTIR and UV–Vis spectra and structure of the Pd(II) complex was confirmed by X-ray crystallography. The catalyst (PdLCl) displayed considerable reactivity (up to 99% selectivity and 90% yield) in the selective oxidation of terminal [Formula presented] bond in aryl substituted olefin to aldehyde. The method shows good functional groups compatibility, no ketone byproducts and is operationally simple. Time dependent density functional study (TD-DFT) of representative cyclopalladated complex has been undertaken. The simulated optical spectrum of the complex is in good agreement with the experimentally observed spectrum.

Full text of DOI:10.1016/j.jorganchem.2016.08.006

Accepted Manuscript
Synthesis of a novel six membered CNS palladacycle; TD-DFT study and catalytic
activity towards microwave-assisted selective oxidation of terminal olefin to aldehyde
Kuladip Sarma, Namita Devi, Dipankar Sutradhar, Bipul Sarma, Asit K. Chandra,
Pranjit Barman
PII:
S0022-328X(16)30327-8
10.1016/j.jorganchem.2016.08.006
JOM 19585
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 4 June 2016
Revised Date: 2 August 2016
Accepted Date: 3 August 2016
Please cite this article as: K. Sarma, N. Devi, D. Sutradhar, B. Sarma, A.K. Chandra, P. Barman,
Synthesis of a novel six membered CNS palladacycle; TD-DFT study and catalytic activity towards
microwave-assisted selective oxidation of terminal olefin to aldehyde, Journal of Organometallic
Chemistry (2016), doi: 10.1016/j.jorganchem.2016.08.006.
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

ACCEPTED MANUSCRIPT
Synthesis of a novel six membered CNS palladacycle; TD-DFT
study and catalytic activity towards microwave-assisted selective
oxidation of terminal olefin to aldehyde
[
a]
[a]
[b]
[c]
[b]
Kuladip Sarma , Namita Devi , Dipankar Sutradhar , Bipul Sarma , Asit K Chandra and
[
a]
Pranjit Barman*
[
[
[
a] Department of Chemistry, National Institute of Technology, Silchar, Assam-788010, India
b] Department of Chemistry, North Eastern Hill University, Shillong-793022, Meghalaya, India
c] Department of Chemical sciences, Tezpur University, Napaam, Tezpur-784028, Assam, India.
E-mail: kuladipsrm88@gmail.com (K. Sarma), namitadevi14@gmail.com (N. Devi), dipankarsutradhar09@gmail.com (D.
Sutradhar), bcsarma@tezu.ernet.in (B. Sarma), akchandra.bwn@gmail.com (A.K. Chandra) and
barmanpranjit@yahoo.co.in (P.Barman).
Abstract: This article documented synthesis of a Schiff base, E-2-(benzylthio)-N-{(2-methoxynaphthalene-1-
yl)methylene}benzenamine (HL) by the treatment of 2-meyhoxynaphthaldehyde and 2-(benzylthio)aniline. Reaction
2 4
of this synthesized Schiff base with Na PdCl has been investigated, which gave a novel six membered Pd(II)
complex (PdLCl) through C,N,S-donor atom of Schiff base. These two newly synthesized compounds were
1
characterized by H-NMR, FTIR and UV-Vis spectra and structure of the Pd(II) complex was confirmed by X-ray
crystallography. The catalyst (PdLCl) displayed considerable reactivity (up to 99% selectivity and 90% yield) in the
selective oxidation of terminal C=C bond in aryl substituted olefin to aldehyde. The method shows good functional
groups compatibility, no ketone byproducts and is operationally simple. Time dependent density functional study
(
TD-DFT) of representative cyclopalladated complex has been undertaken. The simulated optical spectrum of the
complex is in good agreement with the experimentally observed spectrum.
1
. Introduction
The Schiff base derived C,N,S-palladacycles are one of the interesting classes of palladacycles
due to their relevance as precatalyst in C-C and C-heteroatom bond forming reactions [1].
Besides, palladacycles are also utile in optical resolution [2] and design of advanced
metallomesogenes [3]. The vast majority of these complexes possess anionic four-electron

ACCEPTED MANUSCRIPT
(bidentate) or six-electron (tridentate) donor ligands, with five-membered nitrogen-containing
rings being the most common due to their thermodynamic stability [4]. The studies on
cyclopalladation reported till date, reveals that majority of palladacyclic species are five-
membered [5]. A scant literature is accessible on the synthetic details of C,N,S-palladacycles that
possess a six membered ring.
The Pd(II)/Cu(I) lead oxidation of alkenes to ketone or aldehyde is a continuation of the
traditional Wacker method, also known as Tsuji-Wacker method [6]. The Pd(II) catalyst have
been substantially used till date on oxidation of alkenes and usually they give ketone as the prime
product [7]. Formation of aldehyde is overturned by ketone, yet in some cases aldehydes are the
major products [8]. Nonetheless, higher selectivity and yield always have been a matter of
concern. Original Wacker method entails a transition metal catalyzed oxidation of ethylene to
acetaldehyde in a solvent system of H O cum HCl. However, aromatic and high carbon number
2
alkenes show poor solubility in water and in presence of HCl, carry out many side reactions [9].
Thus, in current scenario, microwave-assisted organic synthesis (MAOS) has received a
considerable recognition with its ability to lessen possible side reactions and improves the yield
of its outcome. It also minimizes chemical reaction time from hours to minutes. MAOS are more
effective in ionic liquids [10], as ionic liquids interact very efficiently in microwaves through
ionic conduction mechanism [11]. Moreover, ionic liquids are excellent substitute to various
possible “green” solvents in catalytic and other possible reactions.
In this study, details on synthesis of a Schiff base (HL) from 2-methoxynaphthaldehyde
and 2-(benzylthio)aniline and its cyclopalladated complex (PdLCl) from Pd(II) ions has been
reported. The overall study reveals formation of a less common six membered cyclopalladated
complex. This complex was used as a catalyst in microwave-promoted selective oxidation of

ACCEPTED MANUSCRIPT
4 2
substituted styrenes to aldehydes. The reactions were performed in [bmim]BF -H O medium and
ammonium peroxosulfate was used as an oxidant, which avoids the use of Cu(I). The reaction
offers excellent yield, and useful in terms of selectivity towards aldehyde, up to 99% (Scheme
1
).
Pd(II) complex,
Ammonium peroxosulfate
O
o
MW, [bmim]BF -H O, 70 C
4
2
R
R
>
99% selectivity
upto 90% yield
Scheme 1: Oxidation of terminal alkene
2
. Results and Discussion
The synthesis of ligand, HL involves the condensation of 2-(Benzylthio)aniline and 2-
meyhoxynaphthaldehyde in absolute methanol as reported in the Scheme 2. The ligand, HL is a
yellow solid, stable in air and soluble in most organic solvents.
H
NH2
C
MeOH
N
S
Ph
H CO
3
H CO
3
o
Reflux, 60 C, 2 h
CHO
S
Scheme 2. Synthesis of ligand, HL
The reported Pd(II) complex was achieved from the synthesized ligand by reaction
between Na PdCl and HL in a 1:1 molar ratio (Scheme 3), obtained as an orange crystal. Both
2
4
1
these synthesized compounds were characterized by UV-Vis, FT-IR, H-NMR, and elemental
analysis. Molecular structure of the Pd(II) complex was established by the single crystal X-ray
diffraction technique.

ACCEPTED MANUSCRIPT
H CO
3
HC
OCH3
N
2
Na PdCl4
Pd
S
N
Cl
MeOH, Reflux, 1 h
S
Scheme 3. Synthesis of Pd(II) complex, PdLCl
-1
The FT-IR spectrum for this ligand shows a peak at a frequency of 1618 cm and it
-1
confirms the formation of an azomethine group (>C=N-). The strong peaks at 1462 cm and 749
-1
cm are attributed to C=C and C-S respectively. The spectrum of the complex however, exhibits
-1
-1
a band at 1601 cm for Ʋ(C=N) and shifted by 17 cm compared to corresponding band of the
ligand, indicating the co-ordination via nitrogen atom of azomethine moiety. Moreover, C-S
-1
band is also shifted from 741 cm compared to the ligand and gives evidence of its coordination
via sulfur atoms [12].
1
The H-NMR spectra of the ligand and complex discloses the formation of Pd-N, Pd-S
and Pd-C bonds. The ligand exhibits a singlet peak at δ 9.24 ppm for the imine proton (N=CH).
The aromatic (Ar–H) protons appear at δ 7.2–7.9 ppm, the benzylic protons (PhCH ) appear as a
2
3
singlet at δ 4.19 ppm and methoxy protons (O-CH ) give a singlet at δ 4.0 ppm. However, the
1
H-NMR spectra of Pd(II) complex shows an up field shift of the singlet peak for the imine
proton. The shift occurs due to the N-coordination of palladium (Pd-N) and appears at δ 8.6 ppm.
Besides, one doublet peak near δ 7.6 ppm, correspond to aromatic (Ar–H) protons is missing and
a slightly downfield shift of benzylic protons (PhCH ) at δ 4.2 ppm reveals the coordination of
2
palladium through C (Pd-C) and S (Pd-S) [13].

ACCEPTED MANUSCRIPT
The electronic spectra of the ligand and the complex were investigated in acetonitrile
-5
solution at 10 M concentration. The spectral observation of the ligand, HL shows bands at 254,
20, 345, and 376 nm. We ascribe these bands as the π–π* transitions in phenyl ring and n–π*
transitions of the imine moiety. The Pd(II) complex shows peaks at 230, 280, 321, 367, 451, and
73 nm. For the complex, the bands that appear in the region 230–376 nm are attributed to a π–
3
4
π* and n–π* transition within the ligand, and the other bands appeared in 451-473 nm are due to
the fusion of LMCT and ILCT transition [14].
2
.1. X-ray crystallography
The crystal of Pd(II) complex was collected by using diffusion recrystallization technique
through slow diffusion of hexane into DCM solution. Prospective view of the molecular structure
of the complex is shown in Figure 1.
Figure 1. Molecular structure of the Pd(II) complex, shown with 30% probability ellipsoids
The Pd(II) complex crystallizes in the orthorhombic space group P2(1)2(1)2(1) and it
contains one molecule in the asymmetric unit. The crystallographic evidence substantiate the
spectroscopic results. It reveals that, the compound is consisted of a Pd(II) metal ion arranged in
a distorted square-planar environment. The metal atom is surrounded by N, S, and C atom from

ACCEPTED MANUSCRIPT
the Schiff base ligand (HL) and one -Cl atom. Compared to our earlier reported Pd(II) complex
[12], the 2-position of the naphthalene ring is blocked by one methoxy group, leading to the
th
coordination of Pd metal center via C-H activation at 8 position. As a result, the complex
formed is a six member palladacycle, which is rare in case of N containing ligand. In general, the
metallated CY-type palladacycles (anionic four-electron donor) are preferably stable five
member rings. Reports on other six-member palladacycles from the Schiff bases are very few,
3
and in most cases, the metallated carbon is sp , in a CH
2
group. Only two previous cases verify
2
the presence of a metallated aromatic (sp ) carbon atom, included in a polyaromatic systems,
such as perylene [15] as well as, an endocyclic six-membered cyclopalladated imine derived
from anthracene [16]. The formation of a six member ring via tail of the ligand is usual where
the ligand has electronegative atoms like phosphorus, oxygen etc. But, formation of a six
membered ring by C-H activation at peri position of a naphthalene ring including C=N within the
ring is not explored. Presence of a softer sulfur in the co-ordination sphere may strengthen the
binding of the ligand to the palladium center, resulting in the formation of a stable six membered
ring. All important bond lengths and bond angles of the complex are given in Table 1. The Pd—
S, Pd—N, and Pd—C bond lengths are 2.013(4), 2.3893(13), and 2.3444(12). The bite angles
C(51)—Pd(1)—N(1), C(51)—Pd(1)—Cl(1), N(1)—Pd(1)—S(2) and Cl(1)—Pd(1)—S(2) are
9
0.89(19), 94.36(15), 85.24(11) and 89.49(5) respectively. The Pd, N, S, and C (coordinated)
atoms lie in same plane. Again, angles for N(1)—Pd(1)—Cl(1) and C(51)—Pd(1)—S(2) are
71.51(12) and 176.5(13) confirms that geometry is slightly distorted square planner. It should,
1
however, be noted that the benzyl moiety of the ligand lies perpendicular to the plane.
Table 1. Selected bond lengths (Å) and bond angles (˚) for Pd(II) complex, PdLCl
Bond lengths
Pd(1)—N(1)
Expt
Calc
2.05
Bond angles
Expt
Calc
2.013(4)
C(51)—Pd(1)—N(1)
90.89(19)
92.18

ACCEPTED MANUSCRIPT
Pd(1)—S(2)
Pd(1)—Cl(1)
Pd(1)—C(51)
S(2)—C(1)
S(2)—C(17)
O(1)—C(9)
O(1)—C(24)
N(1)—C(7)
N(1)—C(6)
C(6)—C(1)
C(7)—C(8)
C(13)—C(8)
C(13)—C(51)
2.3893(13)
2.3444(12)
2.000(5)
1.766(5)
1.841(5)
1.357(6)
1.444(6)
1.302(6)
1.439(6)
1.399(7)
1.426(7)
1.432(7)
1.447(7)
2.47
2.38
2.01
1.79
1.87
1.36
1.42
1.30
1.42
1.41
1.43
1.44
1.42
C(51)—Pd(1)—Cl(1)
N(1)—Pd(1)—Cl(1)
C(51)—Pd(1)—S(2)
N(1)—Pd(1)—S(2)
Cl(1)—Pd(1)—S(2)
C(1)—S(2)—C(17)
C(17)—S(2)—Pd(1)
94.36(15)
171.51(12)
176.13(16)
85.24(11)
89.49(5)
95.17
169.01
175.49
84.00
88.94
104.38
107.17
101.0(2)
104.40(18)
2
.2. Computational Study
The molecular geometry of the palladium complex is fully optimized in the gas phase
employing DFT based B3LYP method along with the 6-31G** basis set for lighter atoms (H, C,
N, O, S and Cl) and LanL2DZ for heavier atom (i.e. Pd). The calculated bond distances and bond
angles along with the corresponding experimental data are listed in Table 1 and the optimized
structure of the complex is shown in Figure 2. The theoretically calculated structural parameters
of the Pd(II) complex are very similar to those found in crystal, indicating a substantial stability
of the complex even inside the crystal lattice. The TD-DFT method is used to calculate the
electronic absorption spectra of the complex under the same basis set. The charge distribution
analysis is carried out by Natural Bond Orbital, (NBO) analysis.
Figure 2. Optimized structure of the Pd(II) complex, PdLCl

ACCEPTED MANUSCRIPT
2
.3. Charge distribution
The charge distribution analysis is an important tool to elucidate the pattern of electron
delocalization from ligand to complex. In the ligand HL, the charges on N and S are obtained as -
0
.454 e and 0.241 e respectively. However, on complex formation, the negative charge on N
reduces to -0.449 e and the positive charge on S increases to 0.429 e. The charge on Pd is found
to be 0.212 e which is much lower than its formal charge of +2. All these results confirm that, the
ligand transfers its negative charge to the Pd metal on complex formation. In free Pd(II) state, the
2
2
2
population of 4dxy, 4dxz, 4dyz, 4dx -y and 4d
respectively. But on complex formation, the population on 4dxy, 4dxz, 4dyz, 4dx -y and 4d
orbitals changes and attains a value of 1.976, 1.955, 1.906, 1.268 and 1.941 e respectively. It is
z
orbitals are 1.999, 1.999, 1.999, 1.4986 and 0.499 e
2
2
2
z
2 2
x -y
2
clearly visible that the population is slightly reduced in 4d
orbital and it has increased in 4dz
orbital upon complex formation. The population of the remaining three orbitals are less affected
on interacting with the metal (Pd).
2
.4. Absorption spectra and Frontier Molecular Orbital
The electronic absorption spectrum was calculated using the TD-DFT method in
acetonitrile solvent employing the PCM model as implemented in the Gaussian 09W programs.
The calculated and the experimental absorption data, HOMO-LUMO energy gaps, and the
character of electronic transitions are listed in Table 2. The HOMO→LUMO transition for the
complex is found to possess ILCT character. Two MLCT transitions at around 412 and 374 nm
has been observed which corresponds to dπ(Pd)→π*(L) transfer. A few LLCT transitions for
PdLCl complex at around 360 and 358 nm can be assigned to the Pπ(Cl)→π*(L) transition. This
is however in good agreement with the experimental data. The high energy absorptions of the
complex that occur in a range between 228-279 nm correspond to the ILCT character, i.e. π→π*

ACCEPTED MANUSCRIPT
transitions. The UV-Vis spectra based on TD-DFT calculations is seen to be in good agreement
with the experimental graph (Figure 14 in SI).
Table 2. Electronic transitions calculated by the TD-DFT/B3LYP method and experimental absorption bands of
Pd(II) complex, PdLCl
The most important
orbital excitations
λ(nm)
E (eV)
f
Character
Experimental
λ (nm)
HOMO→LUMO
461.11
412.45
374.03
360.35
358.31
320.86
315.98
284.85
283.26
279.37
230.31
228.04
3.02
3.13
4.55
3.60
3.60
4.37
4.33
4.94
4.73
4.73
5.72
5.83
0.1912 Ligand→Ligand(ILCT)
0.0112 Pd/Cl→Ligand(MLCT/LLCT)
0.0191 Pd/Cl→Ligand(MLCT/LLCT)
0.0276 Cl→Ligand (LLCT)
473,451
HOMO-1→LUMO
HOMO-1→LUMO+1
HOMO-3→LUMO
HOMO-2→LUMO
HOMO-6→LUMO
HOMO→LUMO+2
HOMO-10→LUMO
HOMO-1→LUMO+2
HOMO→LUMO+3
HOMO-14→LUMO
HOMO-5→LUMO+3
367
321
280
0.0819 Cl→Ligand (LLCT)
0.0265 Ligand→Ligand(ILCT)
0.0258 Ligand→Ligand(ILCT)
0.0017 Pd/Ligand→Ligand(MLCT/ILCT)
0.0036 Pd/Lligand→Ligand(MLCT/ILCT)
0.0203 Ligand→Ligand(ILCT)
0.0132 Ligand→Ligand(ILCT)
0.0930 Ligand→Ligand(ILCT)
230
The HOMO-LUMO energy gap is known to be a good measure for the chemical
reactivity and kinetic stability of a molecule. Molecule with a lower HOMO-LUMO gap can be
excited easily and thereby enhances the reactivity of the molecule. The higher energy gap leads
to kinetic stability but less reactivity. The present work reports the comparison of our
synthesized complex PdLCl with the earlier reported Pd complexes, 2 and 3 (Figure 3). Thus,
we would like to compare its kinetic stability and chemical reactivity in terms of HOMO-LUMO
energy gap and to further compare their composition of molecular orbitals. The structure of
complex 2 and 3 is almost similar; hence we compare PdLCl with the complex 2. The HOMO-
LUMO energy gap for PdLCl and complex 2 are found to be 3.02 and 3.23 eV respectively

ACCEPTED MANUSCRIPT
(Figure 4). It is evident that the energy gap is slightly less for PdLCl complex indicating its
enhanced reactivity over the complex 2. From the % contribution of molecular orbital analysis as
shown in Table 1 of Supplementary Information, it is clearly visible that for complex 2 and
PdLCl, most percentage of HOMO as well as LUMO is located on the ligand itself. But for
HOMO-1 level in PdLCl, the maximum percentage (about 41%) of molecular orbital is located
on the metal, whereas similar pattern has not been encountered for the complex 2.
Br
H CO
3
O
O
N
Pd
S
N Pd
S
N
Pd
S
Cl
Cl
Cl
1
2
3
Figure 3. Structure of three Pd catalyst, 1 (PdLCl), 2 (Ref.12) and 3 (Ref.17)

ACCEPTED MANUSCRIPT
Figure 4. HOMO-LUMO energy gap comparison of PdLCl and complex 2
2
.5. Catalytic Activity study
The PdLCl complex mediated transformation of terminal olefin to aldehyde is performed
on various substituted styrene in microwave (Table 3). The reaction condition is standardized by
performing a series of experiments with various solvents, oxidant and catalysts. The optimized
reaction conditions and the results for oxidation of styrene are shown in Table 4. The Table 5
however monitors the optimum microwave parameter for the reaction.
Table 3. Oxidation of terminal olefin to aldehyde catalyzed by Pd(II) complex in [bmim]BF -H O-(NH4) S O
8
4
2
2
2
system
Pd(II) complex,
Ammonium peroxosulfate
O
[
bmim][BF ]-H O
4 2
R
R
MW, 15 min

ACCEPTED MANUSCRIPT
Entry
1
Substrate
Product
Yield
90
O
H
2
3
O
86
86
H
Cl
Cl
O
H
H
Cl
Cl
4
5
O
82
82
Cl
Cl
O
H
H
O N
2
O N
2
6
7
8
O
91
85
86
H C
3
H C
3
O
H
H CO
3
H CO
3
O
H
Table 4. Optimized reaction condition for conversion of styrene to 2-phenyl acetaldehyde
O
O
Catalyst,Oxidant
+
Solvent, MW
a
b

ACCEPTED MANUSCRIPT
Sl.
No.
Catalyst
Oxidant
Solvent
DMF
Yield (%)
a
Yield (%)
b
1
2
3
PdLCl
PdLCl
PdLCl
(NH ) S O
35
38
48
10
25
40
4
2
2
8
8
8
(NH
4
)
2
S
2
O
3
CH CN
(NH ) S O
NMP
DMSO
4
2 2
4
PdLCl
(NH ) S O
55
32
4
2
2
8
5
PdLCl
(NH ) S O
[bmim]BF -H O
90
Trace
4
2
2
8
8
8
8
4
2
6
7
2
3
(NH ) S O
[bmim]BF -H O
83
81
70
10
12`
25
4
2
2
4
2
(NH
4
)
2
S
2
O
[bmim]BF
4
-H
2
O
8
9
PdLCl
(NH ) S O
[bmim]BF4
[bmim]Br
4
2
2
PdLCl
None
(NH
4
)
2
S
2
O
8
58
-
32
-
1
1
1
1
0
1
2
3
(NH ) S O
[bmim]BF -H O
4
2
2
8
4 2
PdCl2
(NH ) S O
[bmim]BF -H O
50
40
35
30
56
31
4 2
2
8
4
2
Pd complex
Benzoquinone
[bmim]BF -H O
4 2
Pd complex
H O
[bmim]BF -H O
2
2
4
2
a,b
Isolated yield. Reaction condition: 1 mmol styrene, 0.5 mol% Pd(II) complex, 10 ml [bmim]BF
4
-H
2
O, 1.5 mmol ammonium
o
persulfate, 80 W,70 C, 15 min. The significance of bold values shows the optimized reaction conditions for styrene.
Table 5. Optimized microwave parameter for conversion of styrene to 2-phenyl acetaldehyde
b
Entry
MW (W)
T (
̊
Time (min)
Yield (%)
1
2
80
80
70
70
5
45
67
10
3
80
70
15
90
4
80
100
15
82
5
6
7
80
100
-
50
70
70
15
15
24 h
75
89
73
c
b
c
Isolated yield. Conventional heating (open vessel reflux condition).
The reported reaction was performed depending on higher loss factors [11] for some
common organic solvents that follows the trend [bmim]BF > DMSO> NMP> DMF> CH CN.
4
3
The solvent screening showed that, DMF and acetonitrile leads to an incomplete conversion of
olefin and the outcome obtained is a mixture of aldehyde and ketone. When treated in NMP
solvent, the styrene showed good conversion but it offers the aldehyde and ketone in almost 1:1
ratio. DMSO however gives approx. 3:2 ratio of aldehyde and ketone. Then we performed the

ACCEPTED MANUSCRIPT
4 4
same reaction with ionic liquid, [bmim]BF and we observed that a pure [bmim]BF solvent
increases the selectivity towards aldehyde with a minimum amount of ketone. However,
increasing the water content in the solvent, at a ratio of 6:1 subsequently led to higher selectivity
towards aldehyde formation. The maximum selectivity observed is ~99% when [bmim]BF -H O
4
2
ratio is 2:1.
The miscibility of ionic liquid with water is strongly dependent upon the nature of
counter anion. [bmim]BF was selected for this study since it has an excellent miscibility with
O. The method avoids the use of any acid as an additive contrary to the classical Wacker
4
H
2
-
4
process. This may be due to the hydrolysis of BF liberating HF during the reaction, which may
act as the acid source [9]. The chemical structure of [bmim]BF is presented in Figure 5.
4
F
N
N
F
F
B
F
Figure 5. Structure of [bmim]BF4
The optimization of a suitable oxidant in this reaction shows that, ammonium persulfate
in presence of a transition metal is a very good oxygen source. Presence of ammonium persulfate
led to avoid the use of Cu(I) necessitate to complete the catalytic cycle. The reaction remained
incomplete when performed with H O . However, benzoquinone showed a very good conversion
2
2
of styrene although discarded due to equivalent mixture of aldehyde and ketone.
The Pd center has a notable effect on this catalytic conversion as the reaction stops in its
2
absence. The use of the salt, PdCl enhances the reaction significantly, accompanying mixture of
keto derivatives. However, the Pd metal complex is seemed to be extremely selective. The
synthesized Pd complex, PdLCl trigger the progress of the reaction unlike PdCl . The
2

ACCEPTED MANUSCRIPT
accumulation of electrons surrounding the Pd-centre from S, N atom of the ligand increases the
efficiency of the complex as a catalyst. The high reactivity and selective nature of this complex
leads to the significant yield of aldehyde in almost 90%. To understand, whether this selective
nature is solely the property of our synthesized complex, PdLCl, or all pd-complexes follow the
same behavior, we performed the reaction with our previously reported Pd Complexes, 2 [12]
and 3 [17] (Figure 3) maintaining others parameters as constant. The observation showed that,
other two complexes are also effective towards terminal oxidation, though leads to a significantly
lower yield along with ketone as minimum by-product. The anomaly in reactivity of these
complexes has been established by FMO, HOMO-LUMO energy gap. One of those structurally
variant complexes was taken as a model and computational investigation was compared with our
newly reported complex, PdLCl. The lower energy gap of complex PdLCl leads to lower kinetic
stability enhancing its reactivity. Moreover, Pd centre of the complex, PdLCl accumulates more
electron cloud (41%) in HOMO-1 level compared to the other one (11%) which is another reason
of higher reactivity.
Table 3 explains that the reaction is tolerant to a range of substituent from donating to
2
highly withdrawing (-NO ) group. However, there is a little effect of substituent towards isolated
yield, except highly withdrawing -NO substituent lowers the yield substantially. Oxidation of 1-
2
phenyl-1,3-butadiene was also performed to examine the selectivity of the catalyst toward
oxidation of terminal double bonds.
4
The plausible mechanism (Scheme 4) involves PdLCl as a catalyst. The η nature of
3
styrene acts as a pseudo-diene to form the η -palladium intermediate followed by nucleophilic
4
attack of H
2
O [18]. This η -interaction between the palladium and substrate apparently
3
contributes to the anti-Markovnikov behavior. The η -palladium intermediate offer desired

ACCEPTED MANUSCRIPT
aldehyde and releases HPdL to the solution. The high selectivity towards aldehyde formation is
probably due to the favorable path A. The catalytic cycle of Pd is completed in presence of
+
-
oxygen from ammonium persulfate, H from hydrolysis of BF , and HCl from the solution.
4
H O
2
LPdCl
HCl + 1/ O2
+
H
2
(
NH S O ) (Hydrolysis
4 2 8
of BF4)
HO
PdL
PdL
Cl
HPdL
X
B
OH
PdL
O
A
H O
2
H
HCl
Scheme 4. Plausible reaction mechanism
Thus, we have seen that our synthesized palladacycle, PdLCl is more selective compared
to readily available Pd-complexes. Many synthetic processes accompany limitations of moderate
aldehyde selectivity, requirement of hetero polyacid or stoichiometric palladium [18a]. In some
cases, they are restricted to activated olefins, that is, olefins containing a chelating group located
at an appropriate position, such as a heteroatom or unsaturation, in order to result in the
regioselectivity [7c]. The advantage of our method is, it minimizes these limitations under a
greener approach which avoids use of any acid. However, use of catalytic amount of Pd (0.5
mol%) limits toxic metal wastes with enhanced regioselectivity towards aldehyde.

ACCEPTED MANUSCRIPT
3
. Conclusions
Here, we have reported a new cyclometalated complex by activating the inert C-H bond,
and presented a combined experimental and theoretical study. We have validated the DFT results
of Pd complex through crystallographic confirmation of structural calculations. The complex
formed is a new six membered palladacycle, having slightly distorted square planar geometry.
TD-DFT calculation is used to elaborate the electronic structure and to provide band assignments
in UV-Vis spectra of the complex. The Pd(II) complex shows effective catalytic behavior
towards selective oxidation of terminal alkenes to aldehyde by a microwave assisted Wacker
oxidation method. The reaction is efficient in terms non-hazardous solvent selection, high yield
and selective conversion to aldehyde almost up to 99%.
4
4
. Experimental Section
.1. Experimental Details
4
.1.1. Materials
All chemicals were used without further purification. 2-Methoxynaphthaldehyde and
Sodium tetrachloropalladate were purchased from Alfa Aesar. 2-(Benzylthio)aniline were
prepared by reported method [19]. Solvent used were extra pure grade purchased from Merck
India and were dried according to the reported procedure [20].
4
.1.2. Methods

ACCEPTED MANUSCRIPT
The infrared spectra of the ligand and complexes were recorded with FT-IR-3000
Hyperion Microscope (Bruker, Germany). Elemental analyses were performed on a Flash 2000
1
Thermo Scientific instrument. H NMR spectra of the ligand, Pd(II) complex and substituted 2-
phenylacetaldehyde were performed on Mercury-400 and JNM-ECS400 spectrometer using
CDCl as solvent. The electronic spectra of the compounds were carried out with Cary 100 Bio
3
UV-visible spectrometer in acetonitrile solvent.
4
.2. Synthesis of Ligand, HL
2
-(Benzylthio)aniline (0.215 g, 1 mmol) and 2-meyhoxynaphthaldehyde (0.186 g, 1
mmol) was dissolved in methanol separately. Then to the methanolic solution of 2-
Benzylthio)aniline, solution of 2-meyhoxynaphthaldehyde was added drop wise with continuous
(
stirring. The resulting solution was stirred under reflux (60 °C) condition for 2 h. Color of the
solution changed to dark yellow. Progress of the reaction was monitored by TLC. The solution
was kept in 0 °C; after 2-3 days a yellow crystalline product was formed. Filtered off the product
-2
and washed with 25% methanol and dried in vacuum (10 torr). Yield was almost quantitative.
M. P: 190 °C. Elemental analysis calculated for (C H NOS): %C 78.3, %H 5.52, %N 3.65, %S
25
21
-
1
8
1
7
.36, %O 4.17; found: %C 77.98, %H 5.81, %N 3.72, %S 8.44, %O 4.30. FT-IR (KBr, cm ):
1
618, 1531, 1432, 1315, 1158, 1062, 749. H-NMR (CDCl , 400 MHz): δ 9.24 (1H, s, CH=N),
3
.07-7.96 (15H, m, Ar-H), 4.189 (2H, s, -SCH -), 4.00 (3H, s, -OCH ). UV-Vis [CH CN, λmax,
2
3
3
-1
-1
nm (εmax, Lmol cm )]: 254 (36231), 320 (18720), 345 (16450), 376 (21320) nm.
4
.3. Synthesis of Pd(II) complex, PdLCl

ACCEPTED MANUSCRIPT
A solution of ligand (0.191 g, 0.5 mmol) in 10 mL of methanol was slowly added in a
dropwise manner to a solution of Na PdCl (0.147 g, 0.5 mmol) in 10 mL of methanol. The
2
4
resulting reaction mixture was stirred under reflux condition for 1 h, and kept at 0 °C. After a
couple of days, an orange precipitate formed was filtered off and recrystallized from DCM and
hexane. The orange red needle like crystals suitable for single crystal X-ray diffraction analysis
was washed with 25% methanol-water solution for several times to remove impurities and dried
-2
under vacuum (10 torr) (Purity was checked by TLC). Yield 72%; MP: > 300 °C. Elemental
analysis calculated for (C25 20ClNOPdS): %C 57.26, %H 3.84, %N 2.67, %S 6.11, %O 3.05;
H
-1
found: %C 57.36, %H 3.60, %N 2.55, %S 5.96, %O 3.14. FT-IR (KBr, cm ): 1601, 1516, 1447,
1
1
378, 1158, 1062, 741. H-NMR (CDCl , 400 MHz): δ 8.61 (1H, s, CH=N), 7.01-8.40 (14H, m,
3
-1
-
Ar-H), 4.27 (2H, s, -SCH -), 4.14 (3H, s, -OCH ). UV-Vis [CH CN, λmax, nm (εmax, Lmol cm
2
3
3
1
)]: 230 (38450), 280 (18750), 321 (6723), 367 (5420), 451 (7891), 473 (9750).
4
.4. Crystallography
X-ray diffraction experiment was carried out with a Bruker APEX-II CCD diffractometer
for the Pd(II) complex using graphite monochromated Mo-Kalpha radiation (l ¼ 0.71073_A, u-
scans) at 100 K temperature. Structure was determined by using SHELXS-97 program [21]. Cell
measurement and data reduction were done by Bruker SAINT [22]. Multi-scan method
2
(
SADABS) was used for absorption corrections [22]. Full matrix least-squares on F were
performed using the SHELXS-97 program. All non-H atom were refined in anisotropic
approximation using reflector with I>2σ (I). Crystal data and structure refinements of Pd(II)
Complex are given in Table 6.
Table 6. Crystal structure and structure refinement details of the Pd(II) complex , (PdLCl)

ACCEPTED MANUSCRIPT
Compound
Pd(II) complex
1428239
CCDC entry no.
Empirical formula
Formula weight
T (K)
C25 H Cl N O Pd S
20
524.33
100(2)
λ (Ǻ)
0.71073
Crystal system
Space group
Unit cell dimensions
a (Ǻ)
Orthorhombic
P2(1)2(1)2(1)
9.5822(3)
11.3805(3)
18.9062(5)
90.00
90.00
90.00
2061.73(10)
4
1.689
b (Ǻ)
c (Ǻ)
o
α ( )
o
β ( )
o
γ ( )
V (Ǻ )
3
Z
3
Dcalc (Mg/m )
ꢀ
-1
(mm )
1.149
F (0 0 0)
1056.0
3
Crystal size (mm )
0.36 x 0.14 x 0.13
2.38-26.02
-10 ≤ h ≤ 11
o
θ( )
Index ranges
-
-
14 ≤ k ≤ 14
21 ≤ l ≤ 22
Reflections collected
8880
Independent reflections (Rint)
Completeness (%)
4069(0.0407)
1.72/0.98
Absorption correctio
Max and min transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F
multi-scan(SADABS)
0.8199 and 0.6825
Full matrix least-squares on F
4069/0/272
2
2
1.138
Final R indices [I>2σ(I)]
R indices (all data)
Largest difference in peak and hole(eÅ )
R1=0.0347, wR2=0.0826
R1=0.0402, wR2=0.1053
0.631/-1.253
-3
4
.4. Computational method
All computational studies were performed by using Gaussian 09W [23]. Full geometry
optimizations of the ligand and palladium complex were carried out using the density functional
nd
theory based B3LYP method and with basis set 6-31G(d,p) for all 2 row elements and
hydrogen and LanL2DZ basis set for Pd. TD-DFT calculation for singlet-excitations in
acetonitrile and the calculation of vibrational spectrum of the complex in the gas phase were

ACCEPTED MANUSCRIPT
performed by using the same basis set. The charge distribution analysis was carried out by
Natural Bond Orbital (NBO) analysis [24]. The percentage composition of the molecular orbital
analysis was done by Chemissian software package [25].
4
.5. Catalytic activity studies
4
.5.1. Microwave assisted palladium complex catalyzed selective oxidation of terminal
olefin to aldehyde
To a homogeneous solution of Pd(II) complex (0.5 mol%) in 10 ml [bmim]BF
4 2
-H O
(2:1), substituted styrene (1 mmol) was added, followed by ammonium persulfate (1.5 mmol) as
oxidant were taken in glass tubes, sealed and placed in the cavity of microwave apparatus. Then
set parameters are as follows: microwave irradiation power 80 W, increasing time 5 min, target
temperature 70 °C, standing time 15 min, standing temperature 70 °C. A maximum irradiation
power of 80 W and 70 °C were applied for 15 min. After the temperature reached 70 °C, the
instrument was automatically adjusted to maintain a constant temperature. After completion of
the reaction (monitored through TLC), the reaction mixture was cooled to room temperature then
2 4
diluted with 2x10 ml water, then extracted with ethyl acetate and dried over Na SO . The
resulting solution was concentrated under vacuum. Product was further purified by
chromatographic separation with a mixture of ethyl acetate:hexane (1:10).
Acknowledgements
The authors gratefully acknowledge Tezpur University, Tezpur and IIT Bombay,
Mumbai, IIT Guwahati, Guwahati for spectral and analytical data. Also, thanks to Director of

ACCEPTED MANUSCRIPT
NIT, Silchar, for financial assistance (K. S and N. D). A. K. Chandra thanks Computer center
NEHU, for computational facilities.
Keywords: Schiff Base • Six membered Palladacycle • TD-DFT Study • Olefin Oxidation •
MAOS
References
[
1] (a) K. Karami, N. Rahimi, C. Rizzoli, Polyhedron 59 (2013) 133-137; (b) H.X. Wang, H.F. Wu, X.L.
Yang, N. Ma, L. Wan, Polyhedron 26 (2007) 3857-3864; (c) K. Gopi, P. Saxena, M. Nethaji, N.
Thirupathi, Polyhedron 52 (2013) 1041-1052; (d) C. Sinha, D. Bandyopadhyay, A. Chakravorty, Inorg.
Chem. 27 (1988) 1173-1178; (e) P. Wadhwani, D. Bandyopadhyay, Organometallics 19 (2000) 4435-
4
436; (f) A. McNally, B. Haffemayer, B.S.L. Collins, M.J. Gaunt, Nature 510 (2014) 129-133; (g) J.
Dupont, C.S. Consorti, J. Spencer, Chem. Rev. 105 (2005) 2527-2571; (h) J. Dupont, M. Pfeffer, J.
Spencer, Eur. J. Inorg. Chem. 2001 (2001) 1917-1927.
[
[
[
2] J.S.L. Yap, H.J. Chen, Y. Li, S.A. Pullarkat, P.H. Leung, Organometallics 33 (2014) 930-940.
3] M. Curic, D. Babic, A. Visnjevac, K. Molcanov, Inorg. Chem. 44 (2005) 5975-5977.
4] (a) X. Riera, C. Lopez, A. Caubet, V. Moreno, X. Solans, M.J. Font-Bardia, Inorg. Chem. 2001 (2001)
2
135-2141; (b) F. Raoof, M. Boostanizadeh, A.R. Esmaeilbeig, S.M. Nabavizadeh, R.B. Aghakhanpour,
K.B. Ghiassi, M.M. Olmstead, A.L. Balch, RSC Adv. 5 (2015) 85111-85121; (c) C.S. Consorti, G.
Ebeling, F. Rodembusch, V. Stefani, P.R. Livotto, F. Rominger, F.H. Quina, C. Yihwa, J. Dupont, Inorg.
Chem. 43 (2004) 530-536.
[
[
5] M.T Chen, C.A Huang, C.T Chen, Eur. J. Inorg. Chem. 2008 (2008) 3142-3150.
6] (a) J. Tsuji, Synthesis 1984 (1984) 369-384; (b) C.N. Cornell, M.S. Sigman, Org. Lett. 8 (2006) 4117-
4
120; (c) K.M. Gligorich, M.S. Sigman, Chem. Commun. 2009 (2009) 3854-3867; (d) P. Teo, Z.K.
Wickens, G. Dong, R.H. Grubbs, Org. Lett. 14 (2012) 3237-3239.
[
7] (a) Q. Cao, D.S. Bailie, R. Fu, M.J. Muldoon, Green Chem. 17 (2015) 2750-2757; (b) G. Zhang, X. Xie,
Y. Wang, X. Wen, Y. Zhao, C. Ding, Org. Biomol. Chem. 11 (2013) 2947-2950; (c) J. Muzart

ACCEPTED MANUSCRIPT
Tetrahedron 63 (2007) 7505–7521; (d) R.J. DeLuca, J.L. Edwards, L.D. Steffens, B.W. Michel, X. Qiao,
C. Zhu, S.P. Cook, M.S. Sigman, J. Org. Chem. 78 (2013) 1682-1686; (e) M.G. Kulkarni, S.M. Bagale,
M.P. Shinde, D.D. Gaikwad, A.S. Borhade, A.P. Dhondge, S.W. Chavhan, Y.B. Shaikh, V.B. Ningdale,
M.P. Desai, D.R. Birhade, Tetrahedron Lett. 50 (2009) 2893-2894; (f) G.J. ten Brink, I.W.C.E. Arends,
G. Papadogianakis, R.A. Sheldon, Chem. Commun. 1998 (1998) 2359-2360; (g) B. Morandi, Z.K.
Wickens, R.H. Grubbs, Angew. Chem. Int. Ed. 52 (2013) 1-5.
[
[
[
8] J. Guoa, P. Teo, Dalton Trans. 43 (2014) 6952-6964.
9] I.A. Ansari, S. Joyasawal, M.K. Gupta, J.S. Yadav, R. Gree, Tetrahedron Lett. 46 (2005) 7507-7510.
10] (a) R. Singh, D.S. Raghuvanshi, K.N. Singh, Org. Lett. 15 (2013) 4202-4205; (b) N. Singh, R. Singh,
D.S. Raghuvanshi, K.N. Singh, Org. Lett. 15 (2013) 5874-5877.
[
[
[
11] C.O. Kappe, Angew. Chem. Int. Ed. 43 (2004) 6250-6284.
12] K. Sarma, N. Devi, M. Kalita, B. Sarma, P. Barman, J. Coord. Chem. 68 (2015) 3685-3700.
13] T. Bhattacharjee, M. Kalita, D. Chakravarty, P. Barman, V.G. Puranik, J. Coord. Chem. 67 (2014) 1702-
1
714.
[
[
[
[
[
14] M.S. Jana, A.K. Pramanik, T.K. Mondal, Polyhedron 76 (2014) 29-35.
15] S. Lentijo, J.A. Miguel, P. Espinet, Dalton Trans. 40 (2011) 7602-7609.
16] J. Zhou, Q. Wang, H. Sun, Acta Crystallogr. Sect. E: Struct. Rep. Online 64 (2008) m688-m688.
17] M. Kalita, P. Gogoi, P. Barman, B. Sarma, A.K. Buragohain, R.D. Kalita, Polyhedron 74 (2014) 93-98.
18] (a) J.A. Wright, M.J. Gaunt, J.B. Spencer, Chem.-Eur. J. 12 (2006) 949-955; (b) J. Guo, P. Teo, Dalton
Trans.43 (2014) 6952–6964.
[
[
19] Q.X. Shi, R.W. Lu, Z.X. Zhang, D.F. Zhao, Chin. Chem. Lett. 17 (2006) 1045-1047.
20] D.D. Perrin, W.L.F. Armarego, Purification of laboratory Chemicals, 3rd ed., Pergamon, New York,
1
988.
[
[
21] (a) G.M. Sheldrick, Acta Crystallogr. A. 64 (2008) 112-122; (b) G.M. Sheldrick, SHELXS-97 and
SHELXL-97, Fortran Programs for Crystal Structure Solution and Refinement, University of Gottingen,
Gottingen, Germany, 1997.
22] SMART, SAINT and SADABS; Bruker AXS Inc., Madison, Wisconsin, USA 2007.

ACCEPTED MANUSCRIPT
[
23] (a) M.J. Frisch et al., GAUSSIAN 09, Revision C.01, Gaussian, Inc., Walling-ford, CT 2009; (b) A.D.
Becke, J. Chem. Phys. 98 (1993) 5648-5652; (c) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1998) 785-
7
89; (d) E. Cances, B. Mennucci, J. Tomasi, J. Chem. Phys. 107 (1997) 3032-3041.
[
[
24] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899-926.
25] L. Skripnikov, Chemissian v4. 36, A computer program to analyse and visualize quantum-chemical
calculations, 2015.

ACCEPTED MANUSCRIPT
Highlights
•
•
The complex formed is a new six membered palladacycle, having slightly distorted
square planar geometry.
The electronic absorption spectrum of the complex was calculated using the TD-DFT
method in acetonitrile solvent employing the PCM model as implemented in the Gaussian
0
9W programs.
•
The Pd(II) complex shows effective catalytic behavior towards selective oxidation of
terminal alkenes to aldehyde by a microwave assisted Wacker oxidation method.
The reaction is efficient in terms non-hazardous solvent selection, high yield and
selective conversion to aldehyde almost up to 99%.
•