Design, synthesis and characterization of bimetallic palladium complexes for terminal olefin epoxidation

Article abstract of DOI:10.1007/s10562-014-1315-4

A series of aromatic ring spaced binuclear Pd(II) complexes were synthesized from sterically tuned Schiff base ligands derived from 4,6-diacetylresorcinol and 2,6-dialkyl substituted anilines to provide a general and flexible set of complexes for terminal olefin epoxidation studies using "green" oxidant 30 % hydrogen peroxide as a terminal oxidant. All the compounds were fully characterized by analytical and spectrometric methods. Additionally, the ligands L1 and L2 were characterized by single crystal X-ray diffraction technique. The complexes C1 and C2 displayed high activity and substrate selectivity in the oxidation of 1-octene, whereas mixed products were observed in case of styrene with reasonable yields, arguably assignable to stringent steric and electronic factors of the complexes employed. Up to 98.5 % conversion is observed for epoxidation of 1-octene with high substrate selectivity, whereas up to 95 % conversion is observed for styrene with poor selectivity. Graphical Abstract: A series of Pd(II) complexes were synthesized from sterically tuned Schiff base ligands derived from 4,6-diacetylresorcinol and 2,6-dialkyl substituted anilines to provide a flexible set of complexes for olefin epoxidation studies using hydrogen peroxide as a terminal oxidant. The complexes C1-C3 showed substrate selectivity in the oxidation of 1-octene, where as mixed products were observed in case of styrene with reasonable yields.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1315-4

Catal Lett (2014) 144:1573–1583
DOI 10.1007/s10562-014-1315-4
Design, Synthesis and Characterization of Bimetallic Palladium
Complexes for Terminal Olefin Epoxidation
•
Sandeep P. Netalkar Anupama A. Nevrekar
•
Vidyanand K. Revankar
Received: 29 May 2014 / Accepted: 6 July 2014 / Published online: 24 July 2014
Ó Springer Science+Business Media New York 2014
Abstract A series of aromatic ring spaced binuclear
Pd(II) complexes were synthesized from sterically tuned
Schiff base ligands derived from 4,6-diacetylresorcinol and
2,6-dialkyl substituted anilines to provide a general and
flexible set of complexes for terminal olefin epoxidation
studies using ‘‘green’’ oxidant 30 % hydrogen peroxide as
a terminal oxidant. All the compounds were fully charac-
terized by analytical and spectrometric methods. Addi-
tionally, the ligands L1 and L2 were characterized by
single crystal X-ray diffraction technique. The complexes
C1 and C2 displayed high activity and substrate selectivity
in the oxidation of 1-octene, whereas mixed products were
observed in case of styrene with reasonable yields, argu-
ably assignable to stringent steric and electronic factors of
the complexes employed. Up to 98.5 % conversion is
observed for epoxidation of 1-octene with high substrate
selectivity, whereas up to 95 % conversion is observed for
styrene with poor selectivity.
essentially because epoxides are vital building blocks for
organic synthesis, both from the industrial and academic
points of interest [1] and at the same time, serve as key
intermediates, for the synthesis of a wide range of com-
mercial organic compounds such as glycols, carbonyl
compounds, alcohol, alkanolaomines and polymers such as
polyurethanes polyesters and epoxy resins with a variety of
commercial applications [2]. Perhaps the most celebrated
complex in such studies is Jacobsen’s catalyst, which has
been shown to achieve high enantioselectivity in epoxida-
tion of alkenes [3]. Subsequent work in this field has led to
accumulation of numerous unique and interesting metal
complexes in literature targeted specially for epoxidation
reactions [4–35]. Among them group VIII metals have
gained a special attention. Also the recent work by Stru-
kul’s group [20–29, 31, 32] have further reaffirmed the fact
that these metals specially Pd(II) and Pt(II) show unique
and interesting results not only in terms of efficiency and
enantioselectivity but also towards the activation of the
rather unreactive terminal alkenes (the lower electron
density of terminal olefins confines their reactivity towards
oxygen transfer), giving these metals a light of attraction.
Compared to the classical transition metal based systems,
the group VIII transition metals have been known to have
several distinctive and appealing features that has led these
group VIII transition metals, principally Pd (II) and Pt(II)
to penetrate deep into this important area of catalysis
[20–32, 36–39]. A few advantages of group VIII transition
metals to count, over classical d0 systems and others are,
they activate the alkenes towards the nucleophilic attack
and at the same time, through coordination to oxidants
especially, the hydroperoxides increase its nucleophilicity
[23]. Generally, the transformations of olefins into epox-
ides involve the use of catalyst coupled with oxidatants
such as molecular oxygen, hydrogen peroxides or organic
Keywords Schiff bases Á Catalytic epoxidation Á
Palladium (II) complex Á X-ray diffraction
1 Introduction
The interest pertaining to the transition-metal-mediated
alkene oxidation to epoxides has sustained for decades,
S. P. Netalkar Á V. K. Revankar (&)
Department of Chemistry, Karnatak University, Pavate Nagar,
Dharwad 580 003, Karnataka, India
e-mail: vkrevankar@rediffmail.com
A. A. Nevrekar
Department of Chemistry, Fergusson College,
Pune 411004, India
123

1574
S. P. Netalkar et al.
peroxides, but the use of H2O2 has become a choice of
preference over others in recent years due to stringent
environmental concerns. Merely the stern environmental
awareness do not favor the interest in the increased appli-
cation of H2O2 [33, 34] as preferred oxidant, but rather its
unique features and advantages such as high atom-effi-
ciency [30] moderate cost, safe handling and storage, and
production of water as the only byproduct [31, 32] make it
the most interesting oxidant only after molecular oxygen.
With continuously increasing research going on, in this
field of oxidation catalysis, in more recent years much
effort has been devoted towards designing new set of
complexes leading to improved catalyst performance than
already known ones with eventual objective of achieving
high efficiency, productivity and selectivity under milder
experimental conditions. Among these, multimetallic sys-
tems are rapidly gaining popularity with the hope of ben-
efiting from cooperative effects between active metal
centers present [35]. Both binuclear bridged and non-
bridged transition metal complexes with specific steric,
electronic and modulations currently draw considerable
attention in the catalysis community, with the expectation
to lead to unique activation modes and novel reactivity
patterns. From the standpoint of achieving increased effi-
ciency, the simplest incremental step that can be thought
of, from conventional designed single active metal site
complexes, would be insertion of additional active metal
centre in addition to one which is known to influence the
catalytic process, to the suitably designed ligand frame-
work, expecting combined influence of synergistic effects
of the active metal centers and also possible occurrence of
tandem reactions promoted by two active metal centers.
Focused in exploring alternative complexes as unique and
efficient catalytic systems for olefin epoxidation, benefiting
from cooperative effects between adjacent active metal
centers, in this work, we report synthesis, characterization
and catalytic performances of the series of aromatic spaced
binuclear palladium(II) complexes derived from the Schiff
bases of 4,6-diacetylresorcinol and 2,6-dialkylsubstituted
anilines in the epoxidation of 1-octene and styrene. The
aromatic spacer is expected to enhance the rigidity of the
ligand backbone and to enable fine-tuning of the electronic
character of the N or/and O-donors by ring substitutions,
thus expecting to broaden the outcome of possible epoxi-
dation reactions.
1H and 13C NMR spectra were recorded on a Bruker 400
and 100 MHz spectrometer respectively, in DMSO-d6 or
CDCl3 at room temperature using TMS as an internal
reference. IR spectra were recorded in a KBr disc matrix
using an Impact-410 Nicolet (USA) FTIR spectrometer
over the range of 4,000–400 cm-1. The electronic spectra
were measured on a Hitachi 150-20 spectrophotometer
over the range of 800–200 nm. Thermal analyses of the
metal complexes were carried out on a Universal V2.4F TA
instrument, keeping the final temperature at 1,000 °C and
the heating rate at 10 °C/min. The X-ray diffraction data
were obtained at 296 K on a Bruker SMART APEX2 CCD
area detector diffractometer using a graphite monochro-
˚
mated Mo-Ka (k = 0.71073 A) radiation source. The
frames were integrated with the Bruker SAINT Software
package using a narrow-frame algorithm. The structures
were solved by direct methods and refined by full-matrix
least-square on F2. In the absence of significant anomalous
scattering, Friedel pairs were merged. The H atoms were
all located in a difference map, but those attached to the
carbon atoms were repositioned geometrically. The H
atoms were initially refined with soft restraints on the bond
lengths and angles to regularize their geometry, after which
the positions were refined with riding constraints [40]. All
non-hydrogen atoms were refined anisotropically. Structure
solution and refinement were performed using Crystals,
SIR92 and SHELXL97 (Sheldrick, 2008) [41–43].
Molecular graphics was generated using ORTEP-3, Mer-
cury and Cameron [44–46]. Olefin oxidation products were
analyzed by SHIMADZU GC–MS-QP2010S gas chro-
matograph using an RtX5MS column.
2.2 Synthesis of the Precursor
4,6-Diacetylresorcinol was prepared by acetylating resor-
cinol (5.00 g, 45.5 mmol) with acetic anhydride (9.28 g,
91.0 mmol) in the presence of excess zinc chloride (10.0 g,
73.4 mmol) at 140 °C in a paraffin oil bath. The hot mix-
ture was cooled to room temperature and poured onto a
140 mL 50 % dilute hydrochloric acid. The Orange pre-
cipitate formed, increased on standing for 1 h. The orange
crude product was obtained by filtration with suction,
washed with distilled water till the color of the filtrate was
nearly colorless. Orange needle crystals were obtained by
crystallization using either ethanol or acetic acid/water
mixture. The yield of 4,6 diacetylresorcinol (DAR) was
6.88 g, 78 %, m.p. 179 °C [47].
2 Experimental
2.3 Synthesis of the Ligands
2.1 General Considerations
A 50 mL round bottom flask was charged with 2.5 g
(0.02 mol) of 2,6-dimethylaniline/3.0 g (0.02 mol) of 2,6-
diethylaniline/3.8 g (0.02 mmol) of 2,6-diisopropylaniline
All the reagents used in this study were purchased from
Sigma-Aldrich and used without further purification. The
123

Design, Synthesis and Characterization of Bimetallic Palladium Complexes
1575
Scheme 1 Synthetic route for
the preparation of ligands: L1H,
L2H and L3H
3
4
16
15
8
7
2
19
20
18
R
NH2
R
5
1
10
13
21
11
6
9
R
R
Toluene
17
22
N
O
O
N
R
+
1
2
R
14
12
Reflux, 60 h
HO
OH
HO
OH
2
1
R = Me
Et
iPr
and 1.94 g (0.01 mol) of 4,6-diacetylresorcinol along with
˚
0.5 g of 4 A molecular sieves in toluene (20 mL) and
104.0 (Ar, C13H), 111.34 (C9 and C11), 124.79 (Ar, C4H
and C20H), 126.16 (Ar, C3H, C5H, C19H and C21H),
132.66 (Ar, C10H), 133.88 (C2, C6, C18 and C22), 142.83
(C1 and C17), 166.80 (C7N and C15N), 171.93 (C12O1
and C14O2). kmax (nm): 270 p–p*, 333 n–p*. m/z: 456.
catalytic amount of p-toluene sulphonic acid (0.4 g). The
reaction mixture was heated to reflux for 60 h and cooled
to room temperature when yellow solid began to separate.
The precipitated ligand was filtered off, washed with cold
methanol, and then dried in vacuo. Slow evaporation of the
saturated solutions of L1 and L2 yielded yellow colored
crystals suitable for X-ray diffraction analysis. Schematic
representation for the synthesis of ligands and numbering
pattern for NMR assignment is given in Scheme 1.
2.3.3 4,6-Bis-(2,6-Diisopropylphenylimino)ethyl)benzene-
1,3-diol (L3)
Yield: 78 %; Anal. Calc. for C34H44N2O2 (%): C, 79.65; H,
8.65; N, 5.46. Found (%): C, 79.68; H, 8.69; N, 5.39. IR
(cm-1): (C=N) 1,624; (O–H) 3,434. 1H NMR (CDCl3,
ppm): 1.16 (d, 24H, J = 7.5 Hz, CH(CH3)2), 2.20 (s, 6H,
C8H3 and C16H3), 2.79 (sep, 4H, J = 7.5 Hz, CH(CH3)2),
6.58 (s, 1H, Ar, C13H), 7.19 (t, 2H, J = 7 Hz, Ar,
C4H and C20H), 7.25 (d, 4H, J = 7.5 Hz, Ar, C3H, C5H,
C19H and C21H), 7.94 (s, 1H, Ar, C10H), 15.86 (s, 2H,
O1H and O2H). 13C NMR (DMSO-d6 ppm): 17.00 (C8H3
and C16H3), 23.35 (CH(CH3)2), 27.90 (CH(CH3)2), 104.03
(Ar–C13H), 111.29 (C9 and C11), 122.99 (Ar, C4H and
C20H), 125.13 (Ar, C3H, C5H, C19H and C21H), 132.70
(Ar, C10H), 137.83 (C2, C6, C18 and C22), 141.21 (C1
and C17), 166.93 (C7N and C15N), 172.14 (C12O1 and
C14O2). kmax (nm): 268 p–p*, 333 n–p*. m/z: 512.
2.3.1 4,6-Bis-(2,6-Dimethylphenylimino)ethyl)benzene-
1,3-diol (L1)
Yield: 70 %; Anal. Calc. for C26H28N2O2 (%): C, 77.97; H,
7.05; N, 6.99; O, 7.99. Found (%): C, 77.89; H, 7.04; N,
1
7.03; O, 8.02. IR (cm-1): (C=N) 1,625; (O–H) 3,443. H
NMR (CDCl3, ppm): 2.11 (s, 12H, Ar, CH3), 2.20 (s, 6H,
C8H3 and C16H3), 6.60 (s, 1H, Ar, C13H), 7.05 (t, 2H,
J = 7 Hz, Ar, C4H and C20H), 7.13 (d, 4H, J = 7.5 Hz,
Ar, C3H, C5H, C19H and C21H), 7.96 (s, 1H, Ar, C10H),
15.63 (s, 2H, O1H and O2H). 13C NMR (DMSO-d6 ppm):
16.54 (Ar–CH3), 17.72 (C8H3 and C16H3), 103.86 (Ar,
C13H), 111.40 (C9 and C11), 124.38 (Ar, C4H and
C20H), 127.55 (Ar, C3H, C5H, C19H and C21H), 127.93
(Ar, C10H), 132.85 (C2, C6, C18 and C22), 144.17 (C1
and C17), 166.58 (C7N and C15N), 172.09 (C12O1 and
C14O2). kmax (nm): 269 p–p*, 331 n–p*. m/z: 400.
2.4 Syntheses of the Complexes
To the stirred solution of 0.2 g (0.5 mmol) of L1/0. 23 g
(0.5 mmol) of L2/0.58 g (0.256 mmol) of L3 in 20 mL
dichloromethane, triethylamine in methanol was added.
The solution was stirred for few minutes and 0.26 g
(1 mmol) of PdCl2(CH3CN)2 in dichloromethane was
added. The resulting suspension was stirred at room tem-
perature overnight. Then the orange solid formed was fil-
tered and washed several times with methanol and ether
and then dried in vacuum. Schematic representation for the
synthesis of the complexes and numbering pattern for
NMR assignment is given in Scheme 2.
2.3.2 4,6-Bis-(2,6-Diethylphenylimino)ethyl)benzene-
1,3-diol (L2)
Yield: 75 %; Anal. Calc. for C30H36N2O2 (%): C, 78.91; H,
7.95; N, 6.13. Found (%): C, 78.95; H, 7.83; N, 6.15. IR
(cm-1): (C=N) 1,626; (O–H) 3,438. 1H NMR (CDCl3,
ppm): 1.14 (t, 12H, J = 7.5 Hz, CH2CH3), 2.11 (q, 8H,
J = 7.5 Hz, CH2CH3), 2.19 (s, 6H, C8H3 and C16H3),
6.57 (s, 1H, Ar, C13H), 7.13 (t, 2H, J = 7 Hz, Ar,
C4H and C20H), 7.14 (d, 4H, J = 7.5 Hz, Ar, C3H, C5H,
C19H and C21H), 7.92 (s, 1H, Ar, C10H), 15.76 (s, 2H,
O1H and O2H). 13C NMR (DMSO-d6 ppm): 13.91
(–CH2CH3), 16.80 (C8H3 and C16H3), 24.29 (CH2CH3),
2.4.1 Pd2L1Cl2(CH3CN)2
Yield: 51 %; Anal. Calc. for C30H32Cl2N4O2Pd2 (%): C,
47.14; H, 4.22; N, 7.33. Found (%): C, 47.22; H, 4.18; N,
123

1576
S. P. Netalkar et al.
5
8
7
4
16
15
12
O
2
6
19
20
21
18
R
R
R
R
10
13
3
2
1
9
17
2
11
1
(Et)3N
N
N
22
N
N
R
Cl Pd
N
R
+ PdCl2(MeCN)2
R
R
14
O
1
Pd Cl
N
Dichloromethane
HO
OH
R = Me
C1 : R = Me
C2 : R = Et
C3 : R = iPr
Et
iPr
Scheme 2 Synthetic route for the preparation of sterically tuned dipalladium (II) complexes
1
7.29. IR (cm-1): (C=N) 1,591; (C:N) 2,334. H NMR
(DMSO-d6, ppm): 2.26 (s, 12H, Ar, CH3) 2.05 (s, 6H,
CH3CN), 2.20 (s, 6H, C8H3 and C16H3), 6.47 (s, 1H, Ar,
C13H), 7.10 (t, 2H, J = 7 Hz, Ar, C4H and C20H), 7.24
(d, 4H, J = 7.5 Hz, Ar, C3H, C5H, C19H and C21H),
8.10 (s, 1H, Ar, C10H). kmax (nm): 270 p–p*, 312 n–p*
and 410 (d–d transition). ESI mass, (m/z): 764.02 [M].
was charged with 10 mL of acetonitrile, 1.0 mmol of
alkene, 1.0 mmol of H2O2, and 0.02 mmol of complexes
C1, C2 and C3 as catalyst. The reaction was carried out in
an oil bath under stirring at 70 °C for time range of 2–4 h
depending on the substrate. At appropriate intervals, ali-
quots were removed and analyzed immediately by GC.
These aliquots were added to diethyl ether and centrifuged
to destroy the unreacted hydrogen peroxide, water and
complex. Further, the organic part was subsequently sep-
arated for GC analysis. Alkene conversion (%) and the
reaction products were quantified by GC and identified by
comparison with the retention time to those of an authentic
standard sample.
2.4.2 Pd2L2Cl2(CH3CN)2
Yield: 47 %; Anal. Calc. for C34H40Cl2N4O2Pd2 (%): C,
49.77; H, 4.91 N, 6.83. Found (%): C, 49.67; H, 4.93; N,
1
6.91. IR (cm-1): (C=N) 1,589; (C:N) 2,335. H NMR
(DMSO-d6, ppm): 1.1 (t, 12H, J = 7.5 Hz, CH2CH3), 2.05
(s, 6H, CH3CN), 2.27 (s, 6H, C8H3 and C16H3), 2.32 (q,
8H, J = 7.5 Hz, CH2CH3), 6.57 (s, 1H, Ar, C13H), 7.14 (t,
2H, J = 7 Hz, Ar, C4H and C20H), 7.23 (d, 4H,
J = 7.5 Hz, Ar, C3H, C5H, C19H and C21H), 8.12 (s, 1H,
Ar, C10H). kmax (nm): 270 p–p*, 314 n–p* and 408 (d–d
transition). ESI mass, (m/z): 820.04 [M].
3 Results and Discussion
3.1 Spectral Characterization of the Ligands and Their
Pd(II) Complexes
The FTIR spectra of all the ligands, L1–L3 exhibit char-
acteristic intense broad band around 1,589 cm-1, due to the
azomethine functionality and a typical –OH stretching
band at around 3,430 cm-1. The azomethine stretching
frequency has shifted to lower frequency by about 35 cm-1
in the FTIR spectra of corresponding Pd(II) complexes
C1–C3, indicating the involvement of azomethine nitrogen
atom in the coordination to the metal upon complexation.
In addition a less intense broad band, in the region,
3,440–3,500 cm-1 is observed compared to the free
ligands. Since the complexes are hygroscopic, due to
moisture, the H2O bands appear in the same region as
phenolic-OH, the reduction in intensity gives a hint that the
coordination of phenolic oxygen to the Pd(II) metal ion has
taken place and the broad band might be due to moisture.
Further, a new medium intense band appeared in all the
complexes around 2334, assigned to v(C:N), indicating
the involvement of acetonitrile functionality in coordina-
tion to the metal ion. In conclusion, these data suggest an
ON bidentate behavior of the ligands.
2.4.3 Pd2L3Cl2(CH3CN)2
Yield: 45 %; Anal. Calc. for C38H48Cl2N4O2Pd2 (%): C,
52.07; H, 5.52; N, 6.39. Found (%): C, 52.19; H, 5.48; N,
6.31. IR (cm-1): (C=N) 1,589; (C:N) 2,334. H NMR
(DMSO-d6, ppm): 1.11 (d, 24H, J = 7.5 Hz, CH(CH3)2),
2.06 (s, 6H, CH3CN), 2.24 (s, 6H, C8H3 and C16H3), 3.14
(sep, 4H, J = 7.5 Hz, CH(CH3)2), 6.48 (s, 1H, Ar, C13H),
7.15 (t, 2H, J = 7 Hz, Ar, C4H and C20H), 7.24 (d, 4H,
J = 7.5 Hz, Ar, C3H, C5H, C19H and C21H), 8.14 (s, 1H,
Ar, C10H). kmax (nm): 270 p–p*, 304 n–p* and 402 (d–d
transition). ESI mass, (m/z): 876.05 [M], 841.2 [M?-Cl].
1
2.5 Catalytic Epoxidation Studies
The epoxidation of terminal alkenes viz., 1-octene and
styrene, with 30 % aqueous hydrogen peroxide were per-
formed in a 25 mL round-bottom flask equipped with a
condenser. In a typical epoxidation experiment, the flask
123

Design, Synthesis and Characterization of Bimetallic Palladium Complexes
1577
1
The acquisition of the meaningful H and 13C NMR
7.24 ppm, J = 7.5 Hz; C2, 7.23 ppm, J = 7.5 Hz; and C3,
7.24 ppm, J = 7.5 ppm) and the triplet (C1, 7.10 ppm,
J = 7 Hz; C2, 7.14 ppm, J = 7 Hz; and C3, 7.15 ppm,
spectra of the ligands and their complexes supports their
respective structures. Detailed chemical shift values for all
the carbon and protons are given in the experimental sec-
tion. In 1H NMR Spectra of the ligands L1–L3, the methyl
protons of the methyl group of the azomethine linkage
resonated as singlets at around 2.2 ppm. The methyl/ethyl/
isopropyl signals of all the ligands are found in the
1
J = 7 ppm) signals appear in the H NMR spectra of the
complexes from the aromatic protons of the aniline part of
the complex. The aromatic protons of the resorcinol frag-
ment give two distinct singlets at around 8.1 and 6.48 ppm
respectively.
1
expected ranges in their H NMR spectra and are detailed
The mass spectra of the ligands, L1–L3 show molecular
ion peaks at 400, 456 and 512 respectively, corresponding
to their molecular weights. The ESI-Mass spectra of
complexes, C1–C3 show molecular ion peak with m/z val-
ues at 764.02, 820.04 and 876.05, respectively. These
values agree well with the proposed composition for the
complexes.
in the experimental section. The phenolic –OH proton
resonated as a singlet at around 15.7 ppm in all the three
ligands. A singlet at around 2.05 ppm in all the complexes
has been assigned to methyl protons of acetonitrile group,
indicating its involvement in coordination to the metal ion.
The aromatic signals composed of four distinct signals
owing to four magnetically distinct protons. A doublet
(L1, 7.13 ppm, J = 7.5 Hz; L2, 7.14 ppm, J = 7.5 Hz;
Electronic spectra of the ligands L1–L3, and its com-
plexes C1–C3, were recorded in DMF. Due to the struc-
tural similarity of all the ligands which differ only in
substitutions on phenyl ring at ortho-aryl position of the
anilines there is not much observable difference in the
spectra of all the ligands. All the ligands show two char-
acteristic bands in the range 255–290 and 310–350 nm
assigned to intramolecular p–p*and n–p* transitions orig-
inating from the CC, –OH and C=N modules [48, 49].
Upon metallation, these bands showed a red shift, indi-
cating the involvement of azomethine nitrogen (C=N) and
hydroxyl atoms (O–H) in the coordination. The bands
observed in the range 390–410 nm in the complexes
and L3, 7.25 ppm, J = 7.5 Hz) and
a
triplet
(L1, 7.05 ppm, J = 7 Hz; L2, 7.13 ppm, J = 7 Hz; and
L3, 7.19 ppm, J = 7 Hz) signals originated from the ani-
line part of the Schiff base in all the ligands. The two
aromatic protons in the resorcinol fragment of the Schiff
base gave two distinct singlets. The proton adjacent to the
hydroxyl group resonated at 6.60, 6.57 and 6.59 ppm in
case of L1, L2 and L3, respectively, and the other proton in
between the methyl group attached to azomethine linkages,
again resonated as singlets at 7.96, 7.92 and 7.94, respec-
tively, for L1, L2 and L3.
The 13C NMR spectral analyses agreed well with the 1H
NMR spectral data in demonstrating the architecture of the
binucleating scaffold ligands, L1–L3. In the 13C NMR
spectra of the ligands L1––L3, the signal observed around
*166 ppm is due to azomethine carbon at the lowest
downfield position of the carbon atoms. Another noticeable
signal at around *144 ppm is the carbon signal attached to
the azomethine nitrogen in the aniline fragment of the
ligands. The appearance of these signals confirms the
successful formation of the C=N linkage in the ligands,
L1–L3. The signals due to remaining carbons are found in
the predictable ranges in their 13C NMR spectra and are
detailed in experimental section.
In the 1H NMR spectra of Pd(II) complexes C1–C3, the
most prominent observation is the disappearance of the
phenolic –OH signal indicating its deprotonation and sug-
gesting the involvement of the oxygen in the coordination
to the palladium centers in all the complexes. The methyl
protons of the methyl group attached azomethine linkage
show a slight downfield shift, observed as singlets at
around 2.2 ppm in all the three complexes. The methyl
protons of –CH3CN appear at around 2.05 ppm in all the
complexes, suggesting the coordination of nitrogen of the
acetonitrile solvent. All the aromatic signals undergo a
downfield shift upon complexation, A doublet (C1,
1
C1–C3 have been assigned to A1g–1B1g transition con-
sistent with square-planar palladium (II) complexes [30,
31]. UV–Vis spectra of ligands and complexes are shown
in Fig. 1.
The complexes were studied for their thermal behavior
over the temperature range of 25–1,000 °C under a nitro-
gen atmosphere. The TG–DTA curve of C1 is shown as a
representative example in Fig. 2. Thermal studies (TGA
and DTA) of the complexes indicates that the complexes
are stable up to 230 °C and shows no weight loss in the
region 25–200 °C, indicating the absence of coordinated or
lattice held solvent molecules. Complex C1 shows thermal
decomposition in a single step demonstrating the weight
loss of 56 % in the temperature range of 240–360 °C,
attributed to the combined loss of two chlorides and the
ligand, with the respective DTA curve at 290 °C repre-
senting an exothermic process. Complex C2 and C3 show
thermal decomposition in two significant steps. The weight
loss of 8.5 % and 8 % in the first step is attributed to the
loss of two chlorides in the region 225–250 °C further
supported by an exothermic signal at 235 and 240 °C
respectively. In the second stage, the decomposition cor-
responds to a mass loss of 50 and 54 % in the temperature
range of 250–425 °C, showing the loss of the organic part
123

1578
S. P. Netalkar et al.
Fig. 1 UV–Vis spectra of ligands L1–L3 and complexes C1–C3
This is due to the presence of methyl/ethyl groups in 2,6
positions which otherwise would give rise to steric hin-
derance to methyl group of resorcinol ring. Furthermore,
the azomethine, C=N bond (N1–C5 and N2–C9), which
averages 1.283(3) and 1.290(3) (L1) and (N1–C11 and
N2–C19), 1.285(3) and 1.293(3) (L2) indicates the double
bond character, confirming the formation of azomethine
linkages in L1 and L2 respectively [50–52].
In the extended crystal structure of L1, the molecule is
stabilized by intramolecular hydrogen bonding with
˚
donorÁÁÁacceptor distance of H1–O1ÁÁÁN1 (2.478 A) and
˚
H2–O2ÁÁÁN2 (2.501 A). Neighboring molecules of the title
compound are stabilized by the formation of dimers
through
intermolecular
hydrogen
bonding
with
˚
donorÁÁÁacceptor distance of C22–H22CÁÁÁO1 (2.807 A),
Fig. 2 Representative TG-DTA curve of C1
˚
˚
C23–H23BÁÁÁO2 (2.607 A) and C25–H25BÁÁÁO2 (2.614 A)
respectively, forming a two dimensional network.
of the ligand. In all the cases the plateau observed beyond
In case of L2: C9, C10, C27, C28, C29, C30 carbon
atoms of ethyl substituent on either side of aniline are
disordered. In the extended crystal structure, the molecule
is stabilized by intramolecular hydrogen bonding between
420 °C is due to the formation of stable metal oxides.
3.2 Single Crystal X-Ray Diffraction Studies of L1
and L2
˚
˚
H1–O1ÁÁÁN1 (2.512 A) and H2–O2ÁÁÁN2 (2.516 A). Each
hydroxyl arm on either side of title compound stabilizes a
neighboring molecule through intermolecular hydrogen
Resorcinol derived imines L1 and L2 crystallizes in the
monoclinic crystal system with space groups P2(1)/n and
P21/c respectively. ORTEP representations of L1 and L2
showing 50 and 30 % displacement ellipsoids are shown in
Figs. 3 and 4 respectively. Summaries of the crystallo-
graphic data, bond lengths and bond angles of L1 and L3
are tabulated in the Tables 1–3 respectively. In the com-
pound crystals, L1 and L2, the C–C bond distances
˚
bonding via O1H1ÁÁÁO1H1 (2.658 A) and C18–H18CÁÁÁO2
˚
˚
(2.536 A), C20–H20BÁÁÁO2 (2.563 A) and C13–H13ÁÁÁO2
˚
(2.665 A) respectively.
3.3 Catalytic Epoxidation Studies
˚
The action of the binuclear palladium complexes C1, C2
and C3 were evaluated in the oxidation of 1-octene and
styrene. The catalyzed oxidation reactions of 1-octene and
styrene were carried out in presence of hydrogen peroxide
as oxidant to give a mixture of oxidized products in dif-
ferent ratio in the case of styrene, while a single epoxide
was observed in the case of 1-octene. The experiments
(1.38 A) and C–C–C bond angles (120°) of phenyl rings
are in accordance with the values reported in literature
[50–52]. An asymmetric unit of L1 and L2 consists of
azomethine modules derived from 2,6-dimethyl aniline/
2,6-diethyl aniline that lie almost perpendicular to resor-
cinol ring on either side making a dihedral angle of 83.28°
and 87.40° (L1) and 82.16° and 80.17° (L2) respectively.
123

Design, Synthesis and Characterization of Bimetallic Palladium Complexes
1579
Fig. 3 ORTEP projection of ligand L1 showing 50 % probability ellipsoids
Fig. 4 ORTEP projection of
ligand L2 showing 30 %
probability ellipsoids with
distorted ethyl groups
carried out under controlled conditions revealed that both
the oxidant (i.e. H2O2) and palladium complexes were
essential for the oxidation to proceed.
were almost in the comparable ranges with C3 being the
weedy of the all. The reason that can be easily linked to
this observation is the bulkiest group (isopropyl) that it
carries compared to C2 (ethyl) and C1 (methyl) remaining
all the factors being the same. The conversions of the
1-octene to its corresponding epoxides proceeded very
slowly at room temperature, and on refluxing in acetoni-
trile at 70 °C for the time range 1–3 h, the conversion
gained thrust and at the end of 3 h proceeded almost to
completion. The complex C1 showed conversion rates
(85.8 and 98.5 % after 2 and 3 h, respectively) more or
3.3.1 Epoxidation of 1-Octene
All the synthesized binuclear palladium complexes were
active in the oxidation of 1-octene to 1,2-epoxyoctane in
presence of 30 % hydrogen peroxide as the oxidant
(Scheme 3). As apparent from the Table 4, the percentage
conversion and the selectivity of all the three complexes
123

1580
S. P. Netalkar et al.
˚
Table 1 Crystal data and structure refinement details of the ligands
L1 and L2
Table 3 Selected bond distances (A) and angles (°) of ligand L2
˚
Bond distance (A)
Crystal data
L1
L2
C1–C2
1.465 (11)
1.381 (4)
0.9300
C15–O2
C17–O1
C19–N2
C21–N2
O1–H1
1.346 (3)
1.335 (3)
1.293 (3)
1.426 (3)
0.8200
C3–C8
Empirical formula
Formula weight
Crystal system
C26H28N2O2
400.52
C30H36N2O2
456.61
C4–H4
C7–C8
1.387 (4)
1.285 (3)
monoclinic
P21/n
monoclinic
P21/c
C11–N1
space group
Bond angle (°)
C1–C2–C3
C3–C2–H2A
C8–C3–C4
N1–C11–C12
Unit cell dimensions
116.4 (8)
108.2
C13–C12–C11
O2–C15–C16
O1–C17–C16
N2–C19–C14
121.6 (2)
118.5 (2)
118.3 (2)
118.0 (2)
˚
a (A)
8.085 (5)
15.603 (9)
14.544 (7)
12.024 (7)
90
˚
b (A)
11.046 (5)
117.6 (3)
117.4 (2)
˚
c (A)
25.362 (5)
a (°)
b (°)
c (°)
90
94.72 (5)
101.66 (2)
90
90
3
˚
C1/C2/C3
O
V (A )
2257.3 (18)
2672.5 (3)
4
Z
4
H2O2, MeCN
70 °C, 1-3h
Density (calcd) (Mg/m3)
1.178
1.135
1, 2- Epoxyoctane
Abs coeff (mm-1
)
0.075
0.071
1-Octene
F(000)
856
984
Scheme 3 Catalytic epoxidation of 1-octene by the complexes C1,
C2 and C3 in the presence of hydrogen peroxide
Crystal size (mm)
0.22 9 0.16 9 0.12
0.3 9 0.2 9 0.2
293 (2)
Temperature (K)
296
Limiting indices
h
-7 ? 7
0 ? 10
0 ? 24
1.6, 20.4
8,242
-18 ? 18
-17 ? 16
-14 ? 13
2.23, 25.00
26,289
Table 4 Catalytic performance of the palladium complexes C1, C2
and C3 in 1-octene epoxidation to 1,2-epoxyoctane with H2O2 as
oxidant
k
l
h Min, max (o)
Reflections collected
Unique reflections (Rint
Number of parameters
R1
Entry
Complex
Time
% Conversion
1
2
3
4
5
6
7
8
9
C1
C2
C3
C1
C2
C3
C1
C2
C3
1
1
1
2
2
2
3
3
3
53.6
38.4
24.8
85.8
79.3
69.4
98.5
94.5
84.4
)
2,231
4,716
279
395
0.1069
0.1930
0.1034
wR2 (all data)
0.1750
˚
Table 2 Selected bond distances (A) and angles (°) of ligand L1
˚
Bond distance (A)
H1–O1
0.827
C19–N2
C5–C6
1.422 (7)
1.389 (9)
1.505 (9)
1.498 (8)
1.401 (9)
1.377 (9)
Reaction conditions: complexes: 0.02 mmol, 1-octene: 1.0 mmol,
H2O2: 1.5 mmol, acetonitrile: 10.0 mL and temperature: 70 °C
H2–O2
0.822
C7–N1
1.431 (7)
1.307 (7)
1.332 (7)
1.334 (7)
C6–C8
C9–N1
C9–C10
C20–C21
C21–C22
respectively) were observed to less effective comparable
to C1 and C3 at 70 °C which further strengths the argu-
ment that increase in steric effects on going from C1–C2–
C3 (methyl–ethyl–isopropyl) present around the palladium
center might be the obvious factor. This can be briefly
explained in simplest terminology as the approach of the
olefin towards the catalysts might be hindered by the
presence of the bulky substituents, thus requiring addi-
tional amount of energy and time to form an intermediate
activated complex wherein the transfer of oxygen to olefin
is facilitated, thus reflected in terms of moderate conver-
sion rates in case of C3, although has not been directly
C14–O2
C16–O1
Bond angle (°)
C2–C1–H12
C1–C2–C3
C6–C7–N1
C2–C7–N1
111.3
C13–C14–O2
C11–C16–O1
C15–C16–O1
C7–N1–C9
121.5 (6)
121.8 (5)
118.5 (6)
123.4 (5)
122.5 (6)
118.1 (6)
117.5 (6)
less comparable to C2 (79.3 and 94.5 % after 2 and 3 h,
respectively). As evident from Table 4, the conversion
rates with C3 (69.4 and 84.4 % after 2 and 3 h,
123

Design, Synthesis and Characterization of Bimetallic Palladium Complexes
1581
OH
O
C1/C2/C3*
OH
OH
+
+
H2O2, MeCN
70 °C, 4h
Styrene
2-phenylethanol
1,2-Epoxystyrene
1-phenylethane-1,2-diol
Scheme 4 Catalytic epoxidation of styrene by the complexes C1, C2 and C3 in the presence of hydrogen peroxide. Asterisk indicated with
complex C3 only 1,2-epoxystyrene was obtained
100
90
80
70
60
50
40
30
20
10
0
selectivity for 1-octene and was selectively converted to
1,2-epoxy octane as a single product with no other
byproducts of oxidation. The conversion rates and % of
epoxides obtained is tabulated in Table 4.
3.3.2 Epoxidation of Styrene
The performance of binuclear Palladium complexes C1,
C2 and C3 were also evaluated for epoxidation with
styrene using 30 % hydrogen peroxide as a terminal
oxidant (Scheme 4). As evident from Fig. 5, the binuclear
palladium complexes were active in the conversion to
epoxides but when compared with oxidation results of
1-octene, seemed to be not as effective. The catalytic
oxidation of styrene by these palladium complexes did not
occur at room temperature even in the presence of excess
hydrogen peroxide and proceeded only at higher tem-
peratures. Unlike the substrate selectivity observed for
1-octene, with styrene, the complexes C1 and C2 showed
the formation of mixed products in poor to moderate yield
but C3 selectively gave only styrene epoxide as the
product of oxidation. The complexes C1 and C2 after
2 h showed conversion to epoxide accounting for about
0
1
2
3
4
5
Time (h)
C2
C1
C3
Fig. 5 Percentage conversion of styrene from epoxidation using
aqueous hydrogen peroxide at 70 °C catalyzed by palladium
complexes: C1, C2 and C3
proved with concrete substantiation. Nevertheless, the
activity of these binuclear palladium complexes was found
to be of comparable range with the heterogeneous cata-
lysts reported in literature in terms of the yields [53–56].
Interestingly, all the complexes show high substrate
Table 5 Catalytic performance of the palladium complexes C1, C2 and C3 in styrene epoxidation to 1,2-epoxystyrene, 2-phenylethanol,
1-phenylethane-1,2-diol with H2O2 as oxidant
Entry
Complex
Time
% Conversion
% Selectivity
1,2-Epoxystyrene
2-Phenylethanol
1-Phenylethane-1,2-diol
1
C1
C2
C3
C1
C2
C3
C1
C2
C3
C1
C2
C3
1
1
1
2
2
2
3
3
3
4
4
4
19.0
8.0
18.4
7.4
–
–
2
–
–
3
\5.0
39.0
25.0
11.0
57.0
50.0
30.0
95.2
90.1
46.0
3.0
–
–
4
38.4
20.0
11.0
52.9
45.6
29.9
77.8
75.8
46.93
–
–
5
–
–
6
–
–
7
3.5
3.3
–
–
8
–
9
–
10
11
12
14.4
11.8
–
2.9
3.0
–
Reaction conditions: complexes: 0.02 mmol, styrene: 1.0 mmol, 30 % H2O2: 1.5 mmol, acetonitrile: 10 mL and temperature: 70 °C
123

1582
S. P. Netalkar et al.
39 and 25 % conversion, but the complex C3 required a
much higher reaction time for conversion but showed
selectivity. The major product of oxidation formed was
styrene epoxide in case of C1 and C2. In case of the
complex C1, the product distribution accounted to styrene
epoxide ca. 77.8 %, 2-phenylethanol ca. 14.4 % and
1-phenylethane-1,2-diol ca. 2.9 % respectively, after re-
fluxing for 4 h in acetonitrile. With complex C2, the
product distribution accounted to styrene epoxide ca.
75.8 %, 2-phenylethanol ca. 11.8 % and 1-phenylethane-
1,2-diol ca. 3 % respectively, after refluxing for 4 h in
acetonitrile. The conversion in case of C3 was less than
50 % (46.3 %), amounting to the formation of only sty-
rene epoxide as a single major product with 46.93 %
efficiency, which reveals that the steric factor plays a vital
role in the performance of the complexes in the catalytic
epoxidation. Amusingly, only complex C3 with bulkiest
substituent (isopropyl) showed substrate selectivity
towards styrene while C1 and C2 gave mixed products.
Further, the exposure to prolonged reaction time did not
affect any further conversion even with varied H2O2
concentrations to the complex ratio in all the cases.
Nonetheless, the percentage conversion of styrene
achieved with the Pd(II) complexes was found to be in
comparative terms with reported heterogeneous catalysts
reported [57, 58], although the selectivity was compro-
mised. The % distribution of the products and conversion
rates is tabulated in Table 5.
noticeably better conversion over C3 indicating that the
steric factors had a vital role to play with the perfor-
mance of the complexes in the catalytic epoxidation. The
bulky groups hinder the approach of the olefin towards
the catalyst and hence are reflected in terms of poor
conversion rates in case of C3.
5 Supplementary Material
CCDC 994286 and CCDC 994287 contain the supple-
mentary crystallographic data for ligand L1 and L2
respectively. This data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (?44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments The authors are thankful to USIC, Karnatak
University, Dharwad, for providing spectral facilities. Recording of
NMR and IR spectra from IISc-Bangalore and IIT-Bombay is grate-
fully acknowledged. One of the authors (Sandeep P. Netalkar) is
thankful to Department of Science & Technology for providing
financial assistance under INSPIRE fellowship program.
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