Synthesis and structural chemistry of N-{2-(arylthio/seleno)ethyl} morpholine/piperidine-palladium(II) complexes as potent catalysts for the Heck reaction

Article abstract of DOI:10.1016/j.ica.2012.07.029

The reactions of in-situ generated PhS^-, PyS^- and PhSe^- with 4-(2-chloroethyl)morpholine hydrochloride and 1-(2-chloroethyl)piperidine hydrochloride in N₂ atmosphere have resulted in N-{2-(arylchalcogeno)ethyl}morpholine/piperidine ligands (L1-L6), which on reaction with Na₂PdCl₄ at room temperature result in complexes of composition [PdCl₂(L)] (1-6; L = L1-L6). All the complexes except 2 have been characterized by X-ray crystallography. The geometry around Pd is nearly square planar in all the cases. The Pd-S and Pd-Se bond lengths are in the ranges 2.2598(10)-2.2795(14) and 2.3541(6)-2.3632(15) ?, respectively. In ⁷⁷Se{¹H} NMR spectra of Pd(II)-complexes with L5 and L6 (Se ligands), the signals appear at higher frequency (up to 187 ppm) with respect to those of corresponding free ligands. Pd(II)-complexes (2, 4 and 6) have been found promising for Heck reaction of aryl bromides with methyl acrylate. The TON values are up to 18 200 and TOF up to 758 h^-1 (with complex 6 as catalyst). The Pd-complexes of piperidene derivatives show little higher catalytic efficiency than those of morpholine ones. The positive Hg-test has implied that nano-sized Pd(0) species probably catalyze Heck reactions.

Full text of DOI:10.1016/j.ica.2012.07.029

Inorganica Chimica Acta 394 (2013) 77–84
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Inorganica Chimica Acta
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Synthesis and structural chemistry of N-{2-(arylthio/seleno)ethyl}morpholine/
piperidine–palladium(II) complexes as potent catalysts for the Heck reaction
⇑
Pradhumn Singh, Dipanwita Das, Om Prakash, Ajai K. Singh
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
The reactions of in-situ generated PhSꢀ, PySꢀ and PhSeꢀ with 4-(2-chloroethyl)morpholine hydrochloride
and 1-(2-chloroethyl)piperidine hydrochloride in N2 atmosphere have resulted in N-{2-(arylchalco-
geno)ethyl}morpholine/piperidine ligands (L1ꢀL6), which on reaction with Na2PdCl4 at room tempera-
ture result in complexes of composition [PdCl2(L)] (1–6; L = L1ꢀL6). All the complexes except 2 have
been characterized by X-ray crystallography. The geometry around Pd is nearly square planar in all the
cases. The PdꢀS and PdꢀSe bond lengths are in the ranges 2.2598(10)ꢀ2.2795(14) and 2.3541
(6)ꢀ2.3632(15) Å, respectively. In 77Se{1H} NMR spectra of Pd(II)-complexes with L5 and L6 (Se ligands),
the signals appear at higher frequency (up to 187 ppm) with respect to those of corresponding free
ligands. Pd(II)-complexes (2, 4 and 6) have been found promising for Heck reaction of aryl bromides with
methyl acrylate. The TON values are up to 18200 and TOF up to 758 hꢀ1 (with complex 6 as catalyst). The
Pd-complexes of piperidene derivatives show little higher catalytic efficiency than those of morpholine
ones. The positive Hg-test has implied that nano-sized Pd(0) species probably catalyze Heck reactions.
Ó 2012 Elsevier B.V. All rights reserved.
Article history:
Received 18 February 2012
Received in revised form 5 July 2012
Accepted 25 July 2012
Available online 14 August 2012
Keywords:
Palladium
Sulfur/selenium ligands
Morphopline
Piperidine
Crystal structure
Heck reaction
1. Introduction
for Heck–Mizoroki and Suzuki–Miyaura coupling both. Thiosemi-
carbazones and thioamide form palladium complexes active for
The morpholine and piperidine skeleton containing species
important in the synthesis of organic compounds [1], including
pharmaceuticals [2] have selective enzyme inhibition, antimicro-
bial and antifungal activities [3]. Antioxidant and hypocholestero-
lemic activities of 2-biphenylmorpholine derivatives [4] have also
been reported. The complexes of morpholine and its derivatives
with Ni(II) [5], Cu(II) [6], Zn(II) [7], Cd(II) [7], Hg(II) [8], Os(II)
[9], Re(III) [10] and Ag(I) [11] have been reported. Platinum(II)-N-
(2-aminoethyl)morpholine complexes have exhibited antitumor
activities [12]. Ruthenium-carbonyl-piperidine complexes cata-
lyze the aerial oxidation of cinnamaldehyde and show antibacte-
rial and antifungal activity [13]. 99mTc-nitrido dithioformate
complexes containing piperidine rings can act as brain imaging
agents [14]. Singh and co-workers [15] have reported the ligation
of N-{2-(4-methoxyphenyltelluro)ethyl}morpholine with palla-
dium(II), half-sandwich ruthenium(II) complexes of N-{2-(arylch-
alcogeno)ethyl}morpholine as useful catalysts for oxidation of
alcohols [16] and rhodium(III) complexes of N-{2-(arylseleno/
telluro)ethyl}morpholine as catalysts suitable for transfer hydro-
genation of ketones [17]. Some other Pd-complexes of hybrid
organochalcogen ligands relevant in the present context are as
follows. Palladium-complexes of 2-organochalcogenomethylpyri-
dines and their derivatives [18–20] have been found promising
the two couplings reactions [21]. Canovese et al. have studied
various aspects of allyl amination of Pd(II) allyl complexes
containing chelating chalcogenated pyridine derivatives [22].
Recently we have synthesized [23] palladium(II) complexes of
N-{2-(4-methoxyphenyltelluro)ethyl}morpholine/piperidine and
observed their unprecedented conversion into PdTe nano-parti-
cles when catalysis of C–N coupling reaction was attempted with
them. In continuation of these studies we have designed palla-
dium(II) complexes of N-{2-(arylthio/seleno)ethyl}morpholine/
piperidine ligands (L = L1ꢀL6). These bidentate ligands have been
envisaged to give appropriate stability to the palladium complex
catalyst for Heck coupling reaction. The high yield syntheses of
L1ꢀL6 were carried out by reacting appropriate heterocyclic
halides with chalcogenated nucleophiles. Their palladium(II) com-
plexes [PdCl2(L)] (L = L1ꢀL6; 1–6) synthesized at room tempera-
ture were structurally characterized (except 2) and some of
them (2, 4 and 6) explored for heck coupling reaction. These re-
sults are reported in the present paper.
2. Experimental
2.1. Physical measurement
Perkin-Elmer 2400 Series II C, H, N analyzer was used for ele-
mental analyses. The 1H, 13C{1H} and 77Se{1H} NMR spectra were
recorded on a Bruker Spectrospin DPXꢀ300 NMR spectrometer at
⇑
Corresponding author. Tel.: +91 11 26591379; fax: +91 11 26581102.
E-mail addresses: ajai57@hotmail.com, aksingh@chemistry.iitd.ac.in (A.K. Singh).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.07.029

78
P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
300.13, 75.47 and 57.24 MHz, respectively. The atom numbering
used for NMR data is as shown in Scheme 1. IR spectra in the range
4000–400 cmꢀ1 were recorded on a Nicolet Protége 460 FTꢀIR
spectrometer. The diffraction data on single crystals of 1 and 3–6
mixture further refluxed for 1 h. 1-(2-Chloroethyl)piperidine
hydrochloride (ꢁ0.37 g, 2.0 mmol) dissolved in 20 cm3 of ethanol
was added to it and the reaction mixture refluxed further for 3 h.
Thereafter, it was poured into cold water (30 cm3). The L2 was ex-
tracted with chloroform (4 ꢂ 25 cm3). The extract was washed
with water (3 ꢂ 40 cm3) and dried over anhydrous sodium sulfate.
The solvent was evaporated off under reduced pressure on a rotary
evaporator to get L2 as pale yellow oil. Yield: 0.35 g, 80%. 1H NMR
(CDCl3, 25 °C, vs Me4Si): d (ppm): 1.41–1.44 (m, 2H, H9), 1.53–1.60
were collected on a Bruker AXS SMART Apex CCD diffractometer
0
using Mo–Ka (0.71073 ÅA) radiations at 298(2) K. The software
SADABS [24] was used for absorption correction and SHELXTL for space
group, structure determination and refinements [25]. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included in idealized positions with isotropic thermal param-
eters set at 1.2 times that of the carbon atom to which they are
attached. The least-square refinement cycles on F2 were performed
until the model converged. The conductivity measurements were
made in CH3CN (concentration ca. 1 mM) using ORION conductiv-
ity meter model 162. Melting points determined in open capillary
are reported as such.
3
(m, 4H, H8), 2.38–239 (m, 4H, H7), 2.57 (t, JH–H = 8.4 Hz, 2H, H5),
3
3.04 (t, JH–H = 7.8 Hz, 2H, H6), 7.11–7.33 (m, 3H, H1, H2), 7.47 (d,
3JH–H = 8.1 Hz, 2H, H3). 13C{1H} NMR (CDCl3, 25 °C, vs Me4Si): d
(ppm): 24.1 (C9), 25.7 (C8), 25.7 (C7), 54.3 (C5), 58.4 (C6), 125.6
(C1), 127.3 (C2), 128.9 (C4), 136.4 (C3).
2.4. Synthesis of L4
2.2. Chemicals and reagents
2,20-Dipyridyldisulfide (Aldrithiol) (0.22 g, 1 mmol) dissolved in
30 cm3 of ethanol was stirred and treated under nitrogen atmo-
sphere with a solution of sodium borohydride (0.08 g, ꢁ2 mmol)
made in 5 cm3 of aqueous NaOH (5%) dropwise till pale yellow
color appeared due to the formation of PySNa. 1-(2-Chloro-
ethyl)piperidine hydrochloride (ꢁ0.37 g, 2.0 mmol) dissolved in
20 cm3 of ethanol was added to the pale yellow solution with con-
stant stirring and the mixture stirred further for 3 h. It was poured
into cold water (30 cm3). The L4 was extracted with chloroform
(4 ꢂ 25 cm3). The extract was washed with water (3 ꢂ 40 cm3)
and dried over anhydrous sodium sulfate. The solvent was evapo-
rated off under reduced pressure on a rotary evaporator to get L4
as pale yellow oil. Yield: 0.31 g, 70%. 1H NMR (CDCl3, 25 °C, vs Me4-
Si): d (ppm): 1.42 (m, 2H, H10), 1.59 (m, 4H, H9), 2.47 (m, 4H, H8),
Diphenyldiselenide, 2,20-dipyridyldisulfide (Aldrithiol), thio-
phenol, 4-(2-chloroethyl)-morpholine hydrochloride, 1-(2-chloro-
ethyl)piperidine hydrochloride, Na2PdCl4 methyl acrylate, aryl
bromides and n-butylamine were procured from Sigma–Aldrich
(USA) and used as received. All the solvents were dried and
distilled before use by well known standard procedures [26]. The
ligands L1 (CAS-registry number 364739-42-4) and L2 (CAS-regis-
try number 1209-11-6) both are available commercially in Aurora
Fine Chemicals Ltd. (Austria) with the product number
K00.451.488 and K07.496.805, respectively. The L1, L3 and L5 used
here have been synthesized according to procedure reported
earlier by us [16].
3
3
2.67 (t, JH–H = 7.2 Hz, 2H, H6), 3.32 (t, JH–H = 6.9 Hz, 2H, H7), 6.93
3
2.3. Synthesis of L2
(m, 1H, H2), 7.15 (d, JH–H = 7.2 Hz, 1H, H4), 7.42 (m, 1H, H3), 8.39
3
(d, JH–H = 7.2 Hz, 1H, H1). 13C{1H} NMR (CDCl3, 25 °C, vs Me4Si):
Thiophenol (0.2 cm3, 2 mmol) was added to sodium hydroxide
(0.16 g, 4 mmol) refluxed in 30 cm3 of ethanol for 0.5 h and the
d (ppm): 24.3 (C10), 25.8 (C9), 26.9 (C8), 54.3 (C6), 59.0 (C7), 119.1
(C4), 122.0 (C2), 135.7 (C3), 149.3 (C1), 158.9 (C5).
PhSH ; Py2S2 / Ph2Se2
NaOH
EtOH
BH4
EtOH
N2
N2
PhS Py S
;
PhSe
/
Cl
Cl
5
7
6
5
7
2
6
5
3
2
3
N
N
8
8
9
1
O
O
1
1
N
N
E
E
4
4
6
8
7
7
6
8
1
9
9
L1: E = S; L5: E = Se
L2: E = S; L6: E = Se
10
N
N
4
O
2
2
N
N
S
S
5
Na2PdCl4
Na2PdCl4
Acetone / H2O
4
3
3
L4
L3
Acetone / H2O
Acetone / H2O
Na2PdCl4 Acetone / H2O
Na2PdCl4
6
7
7
8
7
5
1
6
8
6
6
5
3
2
7
2
1
3
3
8
9
8
9
N
9
O
N
4
10
O
2
N
1
2
1
N
N
N
S
5
E
E
S
4
4
5
Pd
Pd
Pd
Pd
4
3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1: E = S; 5: E = Se
3
4
2: E = S; 6: E = Se
Scheme 1. Synthesis of ligands (L1–L6) and their Pd(II)-complexes (1–6).

P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
79
2.5. Synthesis of L6
(C6), 59.8 (C7), 121.1 (C4), 125.7 (C2), 139.2 (C3), 151.4 (C1), 158.0
(C5). IR (KBr,
m
max/cmꢀ1): 3023 (
m
C–H(aromatic)), 2812 (mC–H(aliphatic)),
The PhSeNa was obtained from diphenyldiselenide (0.32 g,
1 mmol) as described for PySNa and reacted with 1-(2-chloro-
ethyl)piperidine hydrochloride (ꢁ0.37 g, 2.0 mmol) in a manner
similar to that of L4. The L6 was isolated as yellow oil by work-
up used for L4. Yield: 0.32 g, 60%. 1H NMR (CDCl3, 25 °C, vs Me4Si):
d (ppm): 1.40 (m, 2H, H9), 1.52–1.57 (m, 4H, H8), 2.39 (m, 4H, H7),
2.63 (t, 3JH–H = 8.1 Hz, 2H, H5), 3.02 (t, 3JH–H = 7.5 Hz, 2H, H6), 7.20–
1651 ( C–C(aromatic)), 1178, 1104 (
m
m
C–N), 737 ( C–H(aromatic)).
m
Complex 5: Orange solid. Yield: 0.032 g, 70%. m.p. 152.0 °C.
KM = 7.2 S cm2 molꢀ1. Anal. Calc. for C12H17Cl2NOPdSe: C, 32.21;
H, 3.83; N, 3.13%. Found: C, 32.23; H, 3.84; N, 3.12%. 1H NMR (CD3-
CN, 25 °C, vs Me4Si): d (ppm): 2.99–3.08 (m, 4H, H8), 3.38–3.45 (m,
4H, H7), 3.79–3.97 (m, 2H, H5), 4.21–4.62 (m, 2H, H6), 7.56–7.58
(m, 3H, H1, H2), 8.20–8.25 (m, 2H, H3). 13C{1H} NMR (CD3CN,
25 °C, vs Me4Si): d (ppm): 30.1 (C8), 54.4 (C7), 60.8 (C5), 65.5 (C6),
128.3 (C1), 130.9 (C2), 132.7 (C3), 136.0 (C4). 77Se{1H} NMR (CD3CN,
3
7.27 (m, 3H, H1, H2), 7.48 (d, JH–H = 7.2 Hz, 2H, H3). 13C{1H} NMR
(CDCl3, 25 °C, vs Me4Si): d (ppm): 22.2 (C9), 24.8 (C8), 25.7 (C7),
54.4 (C5), 59.1 (C6), 126.4 (C1), 128.8 (C2), 130.3 (C4), 132.0 (C3).
77Se{1H} NMR (CDCl3, 25 °C, vs Me2Se): d (ppm): 278.7.
25 °C, vs Me2Se): d (ppm): 466.6. IR (KBr,
H(aromatic)), 2990, 2858 (s; C–H(aliphatic)), 1628 (m;
1192, 1112 (w; C–N), 741 (m; C–H(aromatic)).
m
max/cmꢀ1): 3053 (m; mC–
C–C(aromatic)),
m
m
m
m
2.6. Synthesis of palladium(II) complexes (1–6)
Complex 6: Orange solid. Yield: 0.035 g, 80%. m.p. 156.0 °C.
KM = 7.2 S cm2 molꢀ1. Anal. Calc. for C13H19Cl2NPdSe: C, 35.05; H,
4.30; N, 3.14%. Found: C, 35.03; H, 4.33; N, 3.15%. 1H NMR
(DMSO-d6, 25 °C, vs Me4Si): d (ppm): 1.38–1.47 (m, 4H, piperi-
dine-H), 2.66–4.47 (m, 10H, piperidine-H, H5, H6), 7.51–7.53 (m,
3H, H1, H2), 8.21–8.24 (m, 2H, H3). The solubility of 6 in solvents
other than DMSO-d6 was inadequate for recording 13C{1H} NMR
and stability of its DMSO-d6 solution was not enough so 13C{1H}
NMR spectrum could not be recorded. 77Se{1H} NMR (DMSO-d6,
The solution of a ligand from L1 to L6 (0.1 mmol) made in
10 cm3 of acetone was added to Na2PdCl4 (0.0294 g, 0.1 mmol) dis-
solved in 10 cm3 of deoxygenated water. The resulting mixture was
stirred for 30 min at room temperature and poured into 100 cm3 of
water. The complex was extracted into chloroform (2 ꢂ 50 cm3).
The extract was dried over anhydrous sodium sulfate, concentrated
to ꢁ10 cm3 with a rotary evaporator and mixed with hexane
(20 cm3) to obtain 1–6 as yellow-orange colored solid, which was
filtered and dried in vacuo. Single crystals of 1 and 3–5 were grown
by slow evaporation of their solutions in CH3CNꢀCH3OH mixture
(3:2) whereas of 6 by slow evaporation of its solution in CH3CN,
CH3OH, CHCl3 and hexane mixture (1:1:1:1).
25 °C, vs Me2Se): d (ppm): 417.0. IR (KBr,
C–H(aromatic)), 2939, 2860 (s; C–H(aliphatic)), 1625 (m;
1191, 1110 (w; C–N), 735 (m; C–H(aromatic)).
m
max/cmꢀ1): 3051 (m;
C–C(aromatic)),
m
m
m
m
m
2.7. Procedure for catalytic Heck reaction
Complex 1: Yellow solid. Yield: 0.030 g, 75%. m.p. 147.0 °C.
KM = 8.0 S cm2 molꢀ1. Anal. Calc. for C12H17Cl2NOPdS: C, 35.98; H,
4.28; N, 3.50%. Found: C, 35.96; H, 4.25; N, 3.54%. 1H NMR (CD3CN,
25 °C, vs Me4Si): d (ppm): 2.93–3.08 (m, 4H, H8), 3.38–3.51 (m, 4H,
H7), 3.76–3.94 (m, 2H, H5), 4.19–4.56 (m, 2H, H6), 7.59–7.61 (m,
3H, H1, H2), 8.21–8.25 (m, 2H, H3). 13C{1H} NMR (CD3CN, 25 °C,
vs Me4Si): d (ppm): 30.9 (C8), 56.1 (C7), 61.6 (C5), 65.7 (C6), 130.2
The mixture of methyl acrylate (0.13 g, 1.5 mmol), aryl bromide
(1 mmol), n-butylamine (0.146 g, 2.0 mmol), p-xylene (ꢁ3 cm3)
and complex 2/4/6 (0.005 mol%) was stirred for 24 h at 100–
110 °C on an oil bath. It was cooled to room temperature, treated
with chloroform (40 cm3) and filtered. The chloroform extract
was washed with acidified water, dried over anhydrous sodium
sulfate and its solvent was evaporated on a rotary evaporator to
obtain the product, which was purified by column chromatography
and further characterized by 1H NMR. All the Heck products are
known compounds [27].
(C1), 130.9 (C2), 132.1 (C3), 134.2 (C4). IR (KBr,
(m; C–H(aromatic)), 2965, 2862 (s; C–H(aliphatic)), 1585 (m; mC–C(aro-
matic)), 1187, 1114 (w; C–N), 751 (m; C–H(aromatic)).
m
max/cmꢀ1): 3055
m
m
m
m
Complex 2: Yellow solid. Yield: 0.031 g, 79%. m.p. 145.0 °C.
KM = 7.6 S cm2 molꢀ1. Anal. Calc. for C13H19Cl2NPdS: C, 39.17; H,
4.84; N, 3.51%. Found: C, 39.18; H, 4.86; N, 3.49%. 1H NMR (CD3CN,
25 °C, vs Me4Si): d (ppm): 1.01–1.37 (m, 2H, H9), 2.46–3.52 (m, 8H,
H7, H8), 3.68–3.90 (m, 2H, H5), 4.15–4.45 (m, 2H, H6), 7.58–7.61
(m, 3H, H1, H2), 8.19–8.24 (m, 2H, H3). 13C{1H} NMR (CD3CN,
25 °C, vs Me4Si): d (ppm): 19.5 (C9), 22.1 (C8), 54.6 (C7), 55.1 (C5),
58.7 (C6), 128.0 (C1), 129.6 (C2), 132.0 (C3), 133.9 (C4). IR (KBr,
3. Results and discussion
3.1. General
The Scheme 1 summarizes the syntheses of L1ꢀL6 and their
palladium complexes (1–6). The ligands (L1ꢀL6) have been syn-
thesized by the reactions of in situ generated PhSꢀ, PySꢀ and
PhSeꢀ with 4-(2-chloroethyl)morpholine hydrochloride/1-(2-chlo-
roethyl)piperidine hydrochloride in nitrogen atmosphere. The
treatment of ligands (L1ꢀL6) with Na2PdCl4 in ratio of 1:1 at room
temperature yielded the corresponding complexes (1–6). The L2
has been prepared by synthetic procedure different from the one
reported earlier [28]. Instead of reacting piperidene with a chloro
compound having rest of the desired skeleton [28] we have reacted
1-(2-chloroethyl)piperidine with PhSH directly. All the ligands
were found stable for a month at ꢁ5 °C and soluble in common or-
ganic solvents except hexane. The molar conductance values in
acetonitrile indicate non-electrolyte nature of all complexes 1–6.
The complexes 1–4 are soluble in CH3OH, CH3CN, CH2Cl2 and CHCl3
whereas 6 has poor solubility in these solvents. The solutions of
1–6 in DMSO show sign of decomposition within few hours.
m
max/cmꢀ1): 3016 (m;
(m; C–C(aromatic)), 1177, 1101 (w;
m
C–H(aromatic)), 2829 (s; C–H(aliphatic)), 1645
m
m
mC–N), 734 (m; mC–H(aromatic)).
Complex 3: Yellow solid. Yield: 0.032 g, 80%. m.p. 151.0 °C.
KM = 7.8 S cm2 molꢀ1. Anal. Calc. for C11H16Cl2N2OPdS: C, 32.90;
H, 4.02; N, 6.98%. Found: C, 32.88; H, 4.00; N, 7.01%. 1H NMR (CD3-
CN, 25 °C, vs Me4Si): d (ppm): 2.71 (m, 4H, H8), 3.01–3.34 (m, 2H,
H6), 3.43–3.78 (m, 2H, H7), 3.82–4.37 (m, 4H, H9), 7.47–7.51 (m,
3
1H, H3), 7.85–7.91 (m, 1H, H2), 8.14 (d, 1H, JH–H = 7.8 Hz, H4),
8.65 (d, 1H, 3JH–H = 7.5 Hz, H1). 13C{1H} NMR (CD3CN, 25 °C, vs Me4-
Si): d (ppm): 31.2 (C8), 57.6 (C6), 60.7 (C7), 66.5 (C9), 124.2 (C3),
128.1 (C4), 139.0 (C2), 151.1 (C1), 158.0 (C5). IR (KBr,
3025 (m; C–H(aromatic)), 2832 (s; C–H(aliphatic)), 1653 (m; mC–C(aro-
matic)), 1187, 1114 (w; C–N), 736 (m; C–H(aromatic)).
m
max/cmꢀ1):
m
m
m
m
Complex 4: Yellow solid. Yield: 0.029 g, 75%. m.p. 149.0 °C.
KM = 7.5 S cm2 molꢀ1. Anal. Calc. for C12H18Cl2N2PdS: C, 36.06; H,
4.54; N, 7.01%. Found: C, 36.08; H, 4.55; N, 7.00%. 1H NMR (CD3CN,
25 °C, vs Me4Si): d (ppm): 1.26–2.21 (m, 6H, H9, H10), 3.08–3.59 (m,
3.2. NMR Spectra
3
8H, H6, H7, H8), 7.12 (m, 1H, H2), 7.33 (d, JH–H = 7.8 Hz, 1H, H4),
7.49 (m, 1H, H3), 8.15 (d, 3JH–H = 7.8 Hz, 1H, H1). 13C{1H} NMR (CD3-
CN, 25 °C, vs Me4Si): d (ppm): 24.0 (C9), 24.6 (C10), 53.8 (C8), 58.1
The 1H and 13C{1H} NMR spectra of complexes 1–6 have been
found consistent with their molecular structures and support the

80
P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
Table 1
Chemical shifts in NMR spectra of L1–L6 (d1 ppm) and 1–6 (d2 ppm).
L1/1 (E = S)
L2/2 (E = S)
L3/3 (E = S)
L4/4 (E = S)
L5/5 (E = Se)
L6/6 (E = Se)
1H NMR
E–CH2
d1/d2
d2ꢀd1
d1/d2
d2ꢀd1
2.65/3.85
1.20
3.06/4.38
1.32
2.57/3.79
1.22
3.04/4.30
1.26
2.69/3.18
0.49
3.34/3.61
0.27
2.67/3.34
0.67
3.32/3.34
0.02
2.68/3.88
1.20
3.02/4.42
1.40
2.63/3.57
0.94
3.02/3.57
0.55
a
N–CH2
a
13C{1H} NMR
E–CH2
d1/d2
d2ꢀd1
d1/d2
d2ꢀd1
53.5/61.6
8.1
58.0/65.7
7.7
54.3/55.1
0.8
58.4/58.7
0.3
53.5/57.6
4.1
58.2/60.7
2.5
54.3/58.1
3.8
59.0/59.8
0.8
53.1/60.8
7.7
58.5/65.5
7.0
54.4/–
–
59.1/–
–
a
N–CH2
a
77Se{1H} NMR
E–CH2
d1/d2
d2ꢀd1
279.5/466.6
187.1
278.7/417.0
138.3
a
a
d2ꢀd1 is deshielding and indicates that ligands are bound with Pd in (E,N) mode.
presence of ligands L1–L6 in them. The signals in 77Se{1H} NMR
spectra of 5 and 6 appear shifted to higher frequency (187 and
138 ppm, respectively) with respect to those of free L5 [16] and
L6 as, Se is coordinated to palladium (Table 1). In 1H and 13C{1H}
NMR spectra of 1–6 also, signals of all protons and carbon atoms
appear at higher frequency relative to those of free ligands which
coordinate with palladium in a bidentate mode. The chemical
shifts of NCH2– and ECH2– (E = S/Se) of ligands L1–L6 and the cor-
responding complexes 1–6 in 1H, 13C{1H} and 77Se{1H} (L5/L6 and
5/6 only) NMR spectra have been compared in Table 1. The higher
frequency shifts in all cases support the coordination of Pd in 1–6
through N and S/Se donor sites as found in solid state by single
crystal structures. However on complexation highest shift to high-
er frequency (ꢁ36 ppm) in 13C{1H} NMR spectra was observed for
C8 in case of morpholine analog.
3.3. Crystal structures
The single crystal structures of complexes 1 and 3–6 have been
solved and their crystal data and refinements are given in Tables 2.
The crystals suitable for X-ray diffraction could not be grown for 2.
The ORTEP diagrams of 1 and 3–6 are given in Fig. 1 while their
selected bond lengths and angles are given in Table 3 (more values
are given in Table S1 of on-line Supplementary material). The
geometry around Pd in 1 and 3–6 is nearly square planar and the
ligands are coordinated with Pd in a bidentate (S/Se, N) mode
forming five membered chelate ring. The PdꢀS bond lengths of
complexes 1, 3 and 4 are 2.2598(10)–2.2795(14) Å, consistent with
earlier reported values [18–20,29–32] 2.2765(14)ꢀ2.2540(5) Å for
[PdCl(Oꢀ, N, S)ligand] and [PdX2(N, S)ligand] (X = Cl and I). The
present PdꢀS bond lengths were found shorter than the reported
Table 2
Crystal data and structural refinement parameters for 1 and 3–6.
Complexes
1
3
4
5
6
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
Unit cell dimension
a (Å)
C
12H17Cl2NOPdS
C11H16Cl2N2OPdS
401.63
C12H18Cl2N2PdS
399.65
C12H17Cl2NOPdSe
447.55
0.46 ꢂ 0.22 ꢂ 0.17
orthorhombic
pbca
C13H19Cl2NPdSe
445.57
0.46 ꢂ 0.22 ꢂ 0.17
orthorhombic
Pca2(1)
400.64
0.46 ꢂ 0.21 ꢂ 0.17
orthorhombic
Pbca
0.47 ꢂ 0.21 ꢂ 0.19
0.06 ꢂ 0.15 ꢂ 0.20
triclinic
triclinic
ꢀ
ꢀ
P1
P1
7.739(19)
9.812(2)
37.950(9)
90°
90°
8.246(3)
9.628(4)
7.9462(13)
8.5467(14)
11.3279(18)
94.118(3)°
101.201(2)°
101.223(2)°
735.4(2)
7.8485(15)
9.9150(18)
38.212(7)
90°
90°
90°
7.949(3)
b (Å)
c (Å)
19.348(6)
10.049(3)Å
90°
90°
90°
10.664(4)
110.868(7)°
110.368(6)°
97.375(9)°
710.2(5)
a
(°)
b (°)
c
(°)
90°
2881.7(11)
V (Å3)
2973.6(10)
1545.5(9)
Z
8
2
2
8
4
Dcalc (Mg mꢀ3
)
1.847
1.790
1600.0
1.878
1.818
400.0
1.805
1.750
400.0
1.999
4.067
1744.0
1.915
3.884
872.0
Absorption coefficient (mmꢀ1
F(000)
)
h range (°)
Index ranges
2.15–25.00
ꢀ9 6 h 6 9
ꢀ11 6 k 6 11
ꢀ45 6 l 6 45
13,411
2.26–25.00
ꢀ9 6 h 6 9
ꢀ11 6 k 6 11
ꢀ12 6 l 6 12
2832
2.67–28.30
ꢀ9 6 h 6 9
ꢀ10 6 k 6 10
ꢀ13 6 l 6 13
3677
2.81–24.99
ꢀ9 6 h 6 9
ꢀ11 6 k 6 11
ꢀ45 6 l 6 45
19884
2.11–25.00
ꢀ9 6 h 6 9
ꢀ23 6 k 6 23
ꢀ11 6 l 6 11
13540
Reflections collected
Independent reflections (Rint
)
2517 (0.0341)
0.635/0.749
2517/0/163
1.189
2490 (0.0214)
0.711/0.641
2490/0/164
1.152
2594 (0.0194)
0.902/0.740
2340/0/163
1.184
2599 (0.0472)
0.507/0.363
2599/0/163
1.119
2715 (0.0723)
0.713/0.470
2715/1/163
1.209
Maximum/minimum transmission
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R1 = 0.0343,
wR2 = 0.0853
R1 = 0.0376,
wR2 = 0.0871
1.428 and ꢀ0.583
R1 = 0.0547,
wR2 = 0.1726
R1 = 0.0575,
wR2 = 0.1836
2.065 and ꢀ1.060
R1 = 0.0334,
wR2 = 0.0857
R1 = 0.0380,
wR2 = 0.1078
0.853 and ꢀ0.791
R1 = 0.0374,
wR2 = 0.0838
R1 = 0.0439,
wR2 = 0.0862
0.990 and ꢀ0.670
R1 = 0.0606,
wR2 = 0.1205
R1 = 0.0698,
wR2 = 0.1246
1.657 and ꢀ1.004
R indices (all data)
Largest difference in peak and hole [e Åꢀ3
]

P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
81
Fig. 1. ORTEP diagram of 1 and 3–6 with ellipsoids at the 30% probability level.
Table 3
Selected bond lengths and angles in complexes 1 and 3–6.
Bonds
1 (E = S)
3 (E = S)
4 (E = S)
5 (E = Se)
6 (E = Se)
Lengths (Å)
Pd(1)–E(1)
Pd(1)–N(1)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
2.2598(10)
2.102(3)
2.3282(10)
2.2931(10)
2.2795(14)
2.117(4)
2.2976(16)
2.3146(16)
2.2619(14)
2.094(4)
2.3248(14)
2.2951(13)
2.3541(6)
2.104(4)
2.2874(13)
2.3307(13)
2.3632(15)
2.114(7)
2.349(3)
2.293(3)
Angles (°)
Cl(1)–Pd(1)–E(1)
Cl(2)–Pd(1)–E(1)
N(1)–Pd(1)–E(1)
N(1)–Pd(1)–Cl(1)
N(1)–Pd(1)–Cl(2)
Cl(2)–Pd(1)–Cl(1)
169.01(4)
89.31(6)
176.87(4)
87.96(11)
177.11(9)
93.46(12)
89.32(6)
172.58(5)
86.20(4)
169.71(4)
89.73(10)
174.29(11)
93.26(11)
91.44(5)
168.99(10)
87.02(9)
89.1(2)
93.5(20)
174.02(2)
91.15(11)
88.10(4)
89.09(9)
92.71(9)
174.28(9)
90.99(4)
86.56(5)
87.79912)
94.89(12)
173.89(12)
91.01(5)

82
P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
Fig. 2. C–Hꢃ ꢃ ꢃp interaction in 1.
Fig. 3. Clꢃ ꢃ ꢃH inter-molecular and intra-molecular H-bonding in 4.
In the crystal structure of all five complexes, Clꢃ ꢃ ꢃH inter-molec-
ular and intra-molecular H-bonding (2.70–2.91 and 2.56–2.92 Å,
respectively) and CꢀHꢃ ꢃ ꢃ interactions (3.03–3.29 Å) have been
interactions in case
of 1 and Fig. 3, Clꢃ ꢃ ꢃH interactions in case of 4. These secondary
interactions result in chains and 2-D sheets. Figs. S1ꢀS8 and Table
S2 in on-line Supplementary material give secondary interaction of
other complexes.
Ar
Catalyst 2/4/6
Ar
Br
+
n-butylamine
p-xylene, N2 atm
COOMe
p
COOMe
observed. For example Fig. 2 shows CꢀHꢃ ꢃ ꢃ
p
Scheme 2. Catalytic Heck coupling reaction with complexes 2, 4 and 6.
values 2.283(2) Å for MePd(II) complexes of bidentate
pyridylthioether ligand [33] and 2.306(3) Å for [PdI(NSt-Bu)
(C(Tol)=NR))] (where NSt-Bu = 2-(tert-butylthiomethyl)pyridine,
R = 2,6-Me2C6H3) [34]. The PdꢀSe bond lengths of complex 5,
3.4. Catalytic Heck coupling reaction
2.3541(6) and 6, 2.3632(15) Å are similar but shorter than the
One of the most important facets of electronic properties of
selenium is its strong electron donating ability which has been
responsible for the efficiency of Pd(II)–selenium ligand complexes
for Heck coupling reactions [37]. There is a current interest in
phosphine free catalysts for Heck coupling [38–39]. Thus present
palladium complexes have been first explored for Heck coupling
reaction between phenyl bromide and methyl acrylate at 100–
110 °C, using n-butylamine as base, p-xylene as solvent,
0.005 mol% of Pd-complex and reaction time 24 h (Scheme 2).
The% yields obtained as 23% (with 1), 26% (with 3) and 31% (with 5)
0
sum of covalent radii 2.44 ÅA. They are consistent with earlier re-
ported values [19,29,35–36] 2.3537(7)–2.3669(11) Å for [PdCl(Oꢀ,
N, Se)ligand] and [PdCl2(N, Se)ligand]. The PdꢀN bond lengths of
complexes 1, 3–6 are from 2.094(4) to 2.117(4) Å consistent with
the sum of covalent radii 2.03 Å for Pd and N and normal. The
PdꢀCl bond lengths in 1 and 3–6 are also normal (in the range
2.2874(13)–2.349(3) Å). The bond angles at the coordinating S/Se
and N atoms are as expected for nearly trigonal–pyramidal and tet-
rahedral geometries, respectively.

P. Singh et al. / Inorganica Chimica Acta 394 (2013) 77–84
83
Table 4
Heck coupling reactions of aryl bromide with methyl acrylate catalyzed by 2, 4 and 6.a
Ar–Br
2
4
6
% Conversion (TON)
29 (5800)
TOF (hꢀ1
241
)
% Conversion (TON)
30 (6000)
TOF (hꢀ1
250
)
% Conversion (TON)
36 (7200)
TOF (hꢀ1
300
)
Br
23 (4600)
80 (16000)
84 (16800)
86 (17200)
191
666
700
716
25 (5000)
84 (16800)
87 (17400)
90 (18000)
208
700
725
750
29 (5800)
87 (17400)
90 (18000)
91 (18200)
241
725
750
758
Me
Br
Br
NC
OHC
O2N
Br
Br
a
Reaction conditions: 1.0 equiv. of aryl bromide, 1.5 equiv. of methyl acrylate, 2 equiv. of base (n-butylamine), p-xylene as solvent, temperature of bath 100–110 °C, reaction
%
ꢀ1
time 24 h, % conversion of products compared to the starting material based on NMR analysis. TON ¼ ConversionꢂSubstrateðMolÞ :TOF ¼
h
.
TON
CatalystðMolÞ
Reaction time
somewhat lower than those obtained with 2, 4 and 6. In view of the
fact that the catalytic performances of morpholine based Pd(II)-
complexes in comparison to piperidine analogs are not better the
further investigations with other substrates (Table 4) were re-
stricted to 2, 4 and 6 only. In Table 4 substrates, percentage conver-
sions, turnover numbers (TON) and values of turn over frequencies
(TOF) are given. The palladium complexes 2 and 4, which are sulfur
analog of 6, have been found to be only marginally less efficient
than 6 (Table 4). High yields up to 91% were observed even with
deactivated aryl bromides. The TON values are satisfactory (up to
Pd is nearly square planar in all cases. The palladium(II)-complexes
(2, 4 and 6) have been found promising for Heck coupling reaction
(TON values up to 18200 and TOF up to 758 hꢀ1). The catalytic effi-
ciency of Pd-complexes of morpholine derivatives is somewhat
lower than those of piperidines. The Heck reaction was probably
catalyzed by nano-sized palladium species as Hg-test was found
to be positive.
Acknowledgements
ꢁ18200) and maximum TOF value has been found to be 758 hꢀ1
;
Authors thank Council of Scientific and Industrial Research
(CSIR) New Delhi (India) for the Project No. 01(2421)10/EMRꢀII
and Department of Science and Technology (India) for partial finan-
cial assistance given to establish single crystal X-ray diffraction and
mass spectral facilities at IIT Delhi, New Delhi (India) under its FIST
program. P.S. D.D. and O.P. thank University Grants Commission
(India) for the award of Junior and Senior Research Fellowship.
both show good efficiency of these complexes as catalyst for Heck
coupling. The catalytic efficiency of the three piperidine complexes
follows the order: 6 > 4 ꢄ 2.
The efficiency of complexes 2, 4 and 6 as catalyst, as revealed by
TON values, was found to be higher for Heck reaction than the
Pd(II)-complexes of some tridentate chalcogenated Schiff bases
previously reported by us [29–30]. These complexes not only show
promise for Heck coupling but are also air stable and moisture
insensitive. In the course of catalysis of Heck reaction black parti-
cles (probably of Pd(0)) were instantaneously formed just when
the reaction was started. Therefore Hg-test was performed as
detailed in on-line Supplementary material. Even after 24 h no
product was formed in the Heck coupling reaction in the presence
of Hg which implied that the reaction was heterogeneously cata-
lyzed most probably by in-situ generated Pd(0) nano-particles.
There are lengthy discussions in the literature about nano-particles
released from catalyst degradation being the true catalysts, partic-
ularly in Pd-catalyzed Heck coupling reactions [40]. Under these
conditions where Pd(0) is not sufficiently protected by ligand, the
addition of metallic mercury leads to the amalgamation of nano-
particles, thereby decelerates or even shuts down the reaction
[41]. Thus, the involvement of nano-particles in the present cata-
lytic process has to be considered, as the mercury test has been
found positive [42].
Appendix A. Supplementary material
CCDC 778774, 778775, 778776, 778777 and 709837 contain the
supplementary crystallographic data for 1, 3, 4, 5 and 6, respec-
tively. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.ica.
2012.07.029.
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