Chalcogenated Schiff bases: Complexation with palladium(II) and Suzuki coupling reactions

Article abstract of DOI:10.1007/s12039-012-0323-4

Chalcogenated Schiff bases of 5-chloroisatin (L1-L3), 2-(methythio) benzaldehyde (L4), 2-acetylpyridine (L5) and benzaldehyde (L6-L7) have been synthesized. Both the carbonyl groups of 5-chloroisatin appear to be reactive (noticed for the first time) for

Full text of DOI:10.1007/s12039-012-0323-4

c
J. Chem. Sci. Vol. 124, No. 6, November 2012, pp. 1245–1253. ꢀ Indian Academy of Sciences.
Chalcogenated Schiff bases: Complexation with palladium(II)
and Suzuki coupling reactions
PRADHUMN SINGH, G K RAO, MOHD SALMAN KARIM and AJAI K SINGH∗
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110 016, India
e-mail: aksingh@chemistry.iitd.ac.in
Abstract. Chalcogenated Schiff bases of 5–chloroisatin (L1–L3), 2–(methythio)benzaldehyde (L4),
2–acetylpyridine (L5) and benzaldehyde (L6–L7) have been synthesized. Both the carbonyl groups of 5–
chloroisatin appear to be reactive (noticed for the first time) for making >C=N bond, of course one at a
time only. The 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectroscopy have been used to establish the co-
existence of two products, which were found in the ratio 53:47 (E = S), 55:45 (E = Se) and 81:19 (E = Te).
The larger amount is of the one in which C=O group away from NH is derivatized. The two products are
not separable. Palladium complexes (1–4) of Schiff bases of other three aldehydes were synthesized. The
ligands as well as complexes were characterized by multinuclear NMR spectroscopy. The crystal structures of
[Pd(L4/L5)Cl][ClO4] (1/2) have been solved. The Pd–Se bond lengths are 2.4172(17) and 2.3675(4) Å, respec-
tively for 1 and 2. The Pd–complexes (3–4) of L6–L7 were explored for Suzuki–Miyaura coupling and found
promising as 0.006 mol % of 3 is sufficient to obtain good conversion with TON up to 1.58 × 104.
Keywords. Chalcogenated Schiff base; palladium; Suzuki coupling; crystal structure.
1. Introduction
studies we have synthesized some new chalcogenated
Schiff bases (see scheme 1) and palladium complexes
of some of them. The possibilities of Suzuki coupling
are explored with some representative complexes. In
case of 5–chloroisatin observed Schiff base formation
is not on reported lines.12 Both carbonyl groups appear
to be reactive, of course not together. Thus, mixture
of two Schiff bases is formed. Their ratio varies with
chalcogen. These results are presented in this paper.
Schiff bases and related compounds continue to
be of current interest for catalyst designing. They
are extensively studied as ligands but chalcogenated
Schiff bases1 known so far, particularly those hav-
ing selenium and tellurim donor sites are not many.
Some tellurated Schiff bases reported include 1,6-bis-
2-butyltellurophenyl-2,5-diazahexa-1,4-diene,2 Schiff
base derived by reacting bis(o-formylphenyl) tel-
luride and o-(butyltelluro)benzaldehyde with chiral
amines (R)-(+)-(1-phenylethylamine) and (1R,2S)-(-)-
norephedrine, respectively3 and macrocyclic Schiff
bases.4 Selenated Schiff bases5–7 are known as li-
gands suitable for designing efficient palladium catalyst
for Heck and Suzuki reactions under aerobic condi-
tion. Recently some chalcogenated Schiff bases as li-
gands and applications of their metal complexes have
been reported by our group.8,9 Half sandwich com-
plexes of ruthenium(II) with chalcogenated Schiff bases
and their derivatives are very efficient transfer hydro-
genation catalysts for ketones as well as suitable cata-
lysts for oxidation of alcohols.10 The palladacycle of
a Schiff base related ligand has been found efficient
for Suzuki coupling of ArCl.11 In continuation of these
2. Experimental
2.1 Physical measurement
Perkin–Elmer 2400 Series II C, H, N analyzer was used
for elemental analyses. The 1H, 13C{1H}, 77Se{1H} and
125Te{1H} NMR spectra were recorded on a Bruker
Spectrospin DPX-300 NMR spectrometer at 300.13,
75.47, 57.24 and 94.69 MHz respectively. IR spectra in
the range 4000–400 cm−1 were recorded on a Nicolet
Protége 460 FT-IR spectrometer. The diffraction data
on single-crystals of 1 and 2 were collected on a Bruker
AXS SMART Apex CCD diffractometer using Mo–Ka
(0.71073 Å) radiations at 298(2) K. The software SAD-
ABS13 was used for absorption correction (if needed)
and SHELXTL for space group, structure determina-
tion and refinements.14 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included
∗For correspondence
1245

1246
Pradhumn Singh et al.
6
5
3
2
E
4
R
N
13
10
1
14
9
Cl
12
7
O
8
11
N
H
L1a: E = S, R = H; L2a: E = Se, R = H;
L3a: E = Te, R = OMe
O
Cl
EtOH
R.T.
E
R
O
H2N
+
+
N
O
7
6
H
5
13
3
2
14
9
Cl
12
N
E
R
4
8
1
11
N
H
10
L1b: E = S, R = H; L2b: E = Se, R = H;
L3b: E = Te, R = OMe
5
6
2
3
13
10
8
9
O
N
1
Se
MeOH
R.T.
12
11
7
4
Se
H2N
+
S
S
Me
O
Me
L4
5
6
2
3
Me
7
Me
N
1
Se
MeOH
R.T.
12
4
8
Se
H2N
+
+
11
N
N
L5
9
10
5
6
2
3
7
N
E
R
O
13
1
EtOH
R.T.
4
8
E
R
H2N
9
12
11 10
L6: E = Se, R = H; L7: E = Te, R = OMe
Scheme 1. Synthesis of ligands L1–L7.
in idealized positions with isotropic thermal parame- 2.3 Synthesis of L1–L3
ters set at 1.2 times that of the carbon atom to which
2–(Phenylsulphanyl)ethylamine (0.76 g, 5.0 mmol)/2–
(phenylseleno)ethylamine (1.0 g, 5.0 mmol)/2–(4–
methoxyphenyltelluro)ethylamine (1.39 g, 5.0 mmol)
was stirred in dry ethanol (20 mL) at room temperature
for 1 h. 5–Chloroisatin (0.77 g, 5.0 mmol,) dissolved in
dry ethanol (20 mL) was mixed drop-wise with stirring.
The mixture was stirred further at room temperature for
2–3 h. The solvent was evaporated on rotary evaporator
resulting in a yellow or orange (in case of Te) precipi-
tate of L1–L3. The precipitate was filtered, washed
with cold ethanol and dried in vacuo.
they are attached. The least-square refinement cycles
on F2 were performed until the model converged. The
conductivity measurements were made in CH3CN (con-
centration ca. 1 mM) using ORION conductivity meter
model 162. Melting points determined in open capillary
are reported as such.
2.2 Chemicals and reagents
2–(Phenylthio)ethylamine, 2–(phenylseleno)ethylamine
and 2–(4–methoxyphenyltelluro)ethylamine were syn-
thesized by reported methods.15–18 2–(Methylthio)-
benzaldehyde, 2–acetylpyridine, benzaldehyde, PdCl2,
Na2[PdCl4], AgClO4, bromobenzene or its derivatives,
phenylboronic acid and Cs2CO3 were procured from
L1a–b: Yield 1.42 g, 90%. Anal. Calc. for
C16H13ClN2OS: C, 60.66; H, 4.14; N, 8.84%. Found:
1
C, 61.06; H, 4.26; N, 8.97%. H NMR (CDCl3, 25◦C,
vs Me4Si): (δ, ppm): 3.36–3.40 (t, 3 JH−H = 6.9 Hz, 2H,
3
H5a), 3.50–3.55 (t, JH−H = 7.5 Hz, 2H, H5b), 4.20 (t,
Sigma–Aldrich (USA) and used as received. All the sol- 3 JH−H = 7.5 Hz, 2H, H6b), 4.61 (t, JH−H = 6.9 Hz,
3
3
vents were dried and distilled before use by well-known 2H, H6a), 6.75–6.78 (d, JH−H = 8.4 Hz, 1H, H10a),
3
standard procedures.19
6.87–6.89 (d, JH−H = 8.4 Hz, 1H, H10b), 7.48 (s, 1H,

Pd-complexes in Suzuki coupling
1247
H
13b), 7.56 (s, 1H, H13a), 7.15–7.43 (m 12H, H1a, H1b, (s, 1H, H7). 13C{1H} NMR (CDCl3, 25◦C, vs Me4Si):
H2a, H2b, H3a, H3b, H11a, H11b), 8.24 (s, 1H, NHa), 9.19 (δ, ppm): 16.9 (SMe), 28.5 (C5), 61.5 (C6), 125.5–
(s, 1H, NHb). 13C{1H} NMR (CDCl3, 25◦C, vs Me4Si): 132.8 (C1, C2, C3, C4, C10, C11, C12, C13), 134.1 (C8),
(δ, ppm): 34.2 (C5a), 34.9 (C5b), 52.1 (C6b), 53.9 (C6a), 139.3 (C9), 160.3 (C7). 77Se{1H} NMR (CDCl3, 25◦C,
111.6–164.7 (CAr).
vs Me2Se): (δ, ppm): 278.3. IR (KBr, νmax/cm−1): 3059
L2a–b: Yield 1.64 g, 90%. Anal. Calc. for (m; νC−H(aromatic)), 2921 (s; νC−H(aliphatic)), 1637, 1585 (s;
C16H13ClN2OSe: C, 52.85; H, 3.60; N, 7.70%. Found:
ν
C=N), 1197 (m; νC−N), 746 (m; νC−H(aromatic)).
1
1
C, 53.15; H, 3.61; N, 7.90%. H NMR (CDCl3, 25◦C,
L5: Yield 1.48 g, 90%. H NMR (CDCl3, 25◦C, vs
vs Me4Si): (δ, ppm): 3.35 (t, 3 JH−H = 6.9 Hz, 2H, H5a), Me4Si): (δ, ppm): 2.32 (s, 3H, Me), 3.32 (t, JH−H
=
3
3
3
3.46 (t, JH−H = 7.5 Hz, 2H, H5b), 4.27 (t, JH−H
=
6.9 Hz, 2H, H5), 3.85 (t, 3 JH−H = 6.9 Hz, 2H, H6), 7.20–
3
7.5 Hz,2H, H6b), 4.71 (t, JH−H = 6.9 Hz, 2H, H6a), 7.29 (m, 6H, H1, H2, H10, H11, H12), 7.50–7.71 (m,
6.75–6.78 (d, JH−H = 8.4 Hz, 1H, H10a), 6.87–6.90 2H, H3), 8.59 (d, JH−H = 7.5 Hz, 1H, H9). 13C{1H}
3
3
(d, JH−H = 8.4 Hz, 1H, H10b), 7.22–7.57 (m 14H, NMR (CDCl3, 25◦C, vs Me4Si): (δ, ppm): 14.4 (Me),
3
H1a, H1b, H2a, H2b, H3a, H3b, H11a, H11b, H13a, H13b), 34.6 (C5), 52.0 (C6), 121.0–129.2 (C1, C2, C3, C4, C10,
8.24 (s, 1H, NHa), 9.19 (s, 1H, NHb). 13C{1H} NMR C11, C12), 136.3 (C8), 148.3 (C9), 167.7 (C7). 77Se{1H}
(CDCl3, 25◦C, vs Me4Si): (δ, ppm): 27.49 (C5a), 28.75 NMR (CDCl3, 25◦C, vs Me2Se): (δ, ppm): 279.7. IR
(C5b), 52.85 (C6b), 54.75 (C6a), 111.49–164.57 (CAr). (KBr, νmax/cm−1): 3055 (m; νC−H(aromatic)), 2924 (s;
77Se{1H} NMR (CDCl3, 25◦C, vs Me2Se): (δ, ppm): νC−H(aliphatic)), 1636, 1588 (s; νC=N), 1194 (m; νC−N),
287.1 (a), 289.5 (b).
746 (m; νC−H(aromatic)).
L3a–b: Yield 1.99 g, 90%. Anal. Calc. for
C17H15ClN2O2Te: C, 46.16; H, 3.42; N, 6.33%. Found:
2.5 Synthesis of L6–L7
1
C, 44.59; H, 3.40; N, 6.20%. H NMR (CDCl3, 25◦C,
2-(Phenylseleno)ethylamine (1.00 g, 5 mmol) or 2-
(aryltelluro)ethylylamine (1.39 g, 5 mmol) was stirred
in dry ethanol (20 mL) at room temperature for 0.5 h.
Benzaldehyde (0.53 g, 5.0 mmol), dissolved in dry
ethanol (20 mL), was added drop-wise with stirring.
The mixture was stirred further at room temperature for
2 h. The solvent was evaporated on a rotary evaporator
which resulted in L6–L7 as yellow oil.
vs Me4Si): (δ, ppm): 3.25 (t, 3 JH−H = 6.9 Hz, 2H, H5a),
3.33 (t, 3 JH−H = 7.5 Hz, 2H, H5b), 3.78 (s, 6H, OMe-a,
3
OMe-b), 4.36 (t, JH−H = 7.5 Hz, 2H, H6b), 4.83 (t,
3 JH−H = 6.9 Hz, 2H, H6a), 6.75 (m, Hz, 2H, H10a,
H10b),
3
6.77–6.80 (d, JH−H = 6.9 Hz, 2H, H2a), 6.86–6.89
3
3
(d, JH−H = 8.4 Hz, 2H, H2b), 7.32–7.35 (d, JH−H
=
3
8.4 Hz, 1H, H11a), 7.38–7.42 (d, JH−H = 8.1 Hz, 1H,
H11b), 7.53 (s, 1H, H13a), 7.58 (s, 1H, H13b), 7.71–7.74
1
L6: Yield 1.24 g, 86%. H NMR (CDCl3, 25◦C, vs
(d, 3 JH−H = 8.7 Hz, 2H, H3a), 7.77 (s, 2H, H3b), 8.00 (s,
1H, NHa), 8.79 (s, 1H, NHb). 13C{1H} NMR (CDCl3,
25◦C, vs Me4Si): (δ, ppm): 7.6 (C5a), 10.2 (C5b), 54.0
(OMe-Ca, OMe-Cb), 55.1 (C6b), 56.3 (C6a), 99.4–164.4
(CAr). 125Te{1H} NMR (CDCl3, 25◦C, vs Me2Te): (δ,
ppm): 452.3 (a), 456.1 (b).
3
Me4Si): (δ, ppm): 3.28 (t, JH−H = 6.9 Hz, 2H, H5),
3
3.96 (t, JH−H = 6.9 Hz, 2H, H6), 7.26–7.76 (m, 10H,
H1, H2, H3, H9, H10, H11), 8.27 (s, 1H, H7). 13C{1H}
NMR (CDCl3, 25◦C, vs Me4Si): (δ, ppm): 28.2 (C5),
61.2 (C6), 126.6 (C10), 128.0 (C1), 128.8 (C2), 128.9
(C4), 129.8 (C3), 130.6 (C9), 132.4 (C8), 135.7 (C11),
162.1 (C7). 77Se{1H} NMR (CDCl3, 25◦C, vs Me2Se):
(δ, ppm): 278.5.
2.4 Synthesis of L4–L5
1
L7: Yield 1.66 g, 89%. H NMR (CDCl3, 25◦C, vs
2–(Methylthio)benzaldehyde (0.76 g, 5 mmol) or 2–
acetylpyridine (0.61 g, 5 mmol) dissolved in 15 mL
of dry CH3OH was stirred at room tempera-
ture for 0.5 h and mixed with a solution of 2–
(phenylseleno)ethylamine (1.00 g, 5 mmol) made in
10 mL of dry CH3OH with constant stirring. The mix-
ture was further stirred at room temperature for 24 h.
The solvent was evaporated on a rotary evaporator to
obtain ligands L4 or L5 as yellow oil.
Me4Si): (δ, ppm): 3.14 (t, 3 JH−H = 7.5 Hz, 2H, H5), 3.79
3
(s, 3H, OMe), 4.01 (t, JH−H = 7.5 Hz, 2H, H6 ), 6.74
3
(d, JH−H = 6.9 Hz, 2H, H3), 7.39–7.42 (m, 3H, H10,
H11, H12), 7.67–7.72 (m, 4H, H2, H9, H13), 3.24 (s, 1H,
H7). 13C{1H} NMR (CDCl3, 25◦C, vs Me4Si): (δ, ppm):
9.9 (C5), 54.9 (OMe), 62.6 (C6), 100.4 (C4), 114.9 (C2),
128.0–135.7 (C8, C9, C10, C11, C12, C13), 140.8 (C3),
159.5 (C1), 161.3 (C7). 125Te{1H} NMR (CDCl3, 25◦C,
vs Me2Te): (δ, ppm): 437.8.
1
L4: Yield 1.48 g, 90%. H NMR (CDCl3, 25◦C, vs
3
Me4Si): (δ, ppm): 2.46 (s, 3H, SMe), 3.25 (t, JH−H
=
2.6 Synthesis of Pd(II)–Complexes (1–2)
3
6.9 Hz, 2H, H5), 3.95 (t, JH−H = 6.9 Hz, 2H, H6),
7.16–7.39 (m, 6H, H1, H2, H11, H12, H13), 7.52–7.56 The CH3CN (20 mL) and solid PdCl2 (0.18 g, 1 mmol)
3
(m, 2H, H3), 7.80 (d, JH−H = 7.8 Hz, 1H, H10), 8.72 were mixed and the mixture was refluxed with stirring

1248
Pradhumn Singh et al.
until a clear light yellow coloured solution was
3: Yield 0.18 g, 79%. M.p. 149.0◦C. ꢀM = 9.2 S
obtained. The solution of a ligand L4 / L5 (0.34 / 0.30 g, cm2 mol−1. Anal. Calc. for C15H15Cl2NPdSe: C, 38.70;
1 mmol) made in CH3OH (5 mL) was added to it and H, 3.25; N, 3.01%. Found: C, 38.64; H, 3.19; N, 3.11%.
the mixture was refluxed for 5 h. Thereafter AgClO4 1H NMR (DMSO-d6, 25◦C, vs Me4Si): (δ, ppm): 2.67–
(0.21 g, 1 mmol) was added and the mixture further 2.77 (m, 1H), 3.59–3.73 (m, 2H), 4.51–4.59 (m, 1H),
refluxed for 3 h. It was cooled to room temperature and 6.67 (d, 3 JH−H = 8.7 Hz, 1H), 7.20–7.45 (m, 6H), 7.95
turbidity of AgCl was filtered off. The filtrate was con- (s, 1H), 8.16–8.19 (m, 2H). 13C{1H} NMR (DMSO-d6,
centrated to ∼5 mL on a rotary evaporator and mixed 25◦C, vs Me4Si): (δ, ppm): 32.3, 64.8, 124.9, 126.6,
with diethyl ether (10 mL) to obtain 1/2 as orange solid, 127.5, 129.7, 129.9, 130.4, 130.9, 133.5, 133.4, 164.2.
which was filtered and dried in vacuo. The single crys- 77Se{1H} NMR (DMSO-d6, 25◦C, vs Me2Se): (δ, ppm):
tals of 1/2 were grown by slow evaporation of their solu- 439.4.
tions in CH3CN–CH3OH mixture (3:2). Caution: per-
4: Yield 0.23 g, 82%. M.p. 157.0◦C. ꢀM = 7.9 S
chlorate is potentially explosive and therefore should be cm2 mol−1. Anal. Calc. for C16H17Cl2NOPdTe: C,
handled carefully.
35.31; H, 3.15; N, 2.57%. Found: C, 35.29; H, 3.17;
1
1: Yield 0.54 g, 85%. M.p. 150.5◦C. ꢀM
=
N, 2.57%. H NMR (DMSO-d6, 25◦C, vs Me4Si): (δ,
145.2 S cm2 mol−1. Anal. Calc. for C16H17ClNPdSSe·ClO4: ppm): 2.27–2.35 (m, 1H), 3.62–3.74 (m, 2H), 3.88 (s,
C, 33.39; H, 2.98; N, 2.43%. Found: C, 33.40; H, 3H, OMe), 4.62–4.78 (m, 1H), 6.84 (d, 3 JH−H = 8.4 Hz,
1
3
2.96; N, 2.40%. H NMR (CD3CN, 25◦C, vs Me4Si): 2H), 8.16–8.39 (m, 5H), 8.71 (d, JH−H = 8.1 Hz).
(δ, ppm): 2.99 (s, 3H, SMe), 3.04–3.27 (m, 2H, H5), 13C{1H} NMR (DMSO-d6, 25◦C, vs Me4Si): (δ, ppm):
4.32–4.80 (m, 2H, H6), 7.54–8.17 (m, 9H, H1, H2, H3, 15.65, 57.34, 63.09, 104.36, 115.66, 128.13, 128.16,
H10, H11, H12, H13), 8.51 (s, 1H, H7). 13C{1H} NMR 129.71, 129.99, 138.69, 161.1, 167.9. 125Te{1H} NMR
(CD3CN, 25◦C, vs Me4Si): (δ, ppm): 26.2 (SMe), 32.4 (DMSO-d6, 25◦C, vs Me2Te): (δ, ppm): 679.1.
(C5), 72.3 (C6), 126.3–134.6 (C1, C2, C3, C4, C10, C11,
C12, C13), 137.2 (C8), 139.5 (C9), 167.7 (C7). 77Se{1H}
NMR (CD3CN, 25◦C, vs Me2Se): (δ, ppm): 455.1.
IR (KBr, νmax/cm−1): 3056 (m; νC−H(aromatic)), 2917 (s;
νC−H(aliphatic)), 1630, 1578 (s; νC=N), 1190 (m; νC−N),
841 (m; νP−F), 746 (m; νC−H(aromatic)).
2.8 Procedure for catalytic Suzuki reaction
Bromobenzene or its derivative (1 mmol), phenyl-
boronic acid (0.18 g, 1.5 mmol), Cs2CO3 (0.65 g,
2.0 mmol), distilled water (0.5 mL), DMF (4 mL) and
catalyst were stirred together under reflux on an oil bath
for 24 h under ambient conditions. The reaction mix-
ture was cooled to room temperature and mixed with
20 mL of water. The product was extracted from the
aqueous mixture with diethyl ether (25×50 mL). The
solvent was evaporated on a rotary evaporator and the
resulting residue was purified by column chromatogra-
2: Yield 0.44 g, 80%. M.p. 152.8◦C. ꢀM = 143.7 S
cm2 mol−1. Anal. Calc. for C15H16ClN2PdSe·ClO4: C,
33.08; H, 2.96; N, 5.14%. Found: C, 33.10; H, 2.99; N,
1
5.10%. H NMR (CDCl3, 25◦C, vs Me4Si): (δ, ppm):
2.84 (s, 3H, Me), 3.34–3.74 (m, 2H, H5), 4.24–4.47 (m,
2H, H6), 7.55–8.34 (m, 8H, H1, H2, H3, H10, H11, H12),
8.86–8.89 (m, 1H, H9). 13C{1H} NMR (CDCl3, 25◦C,
vs Me4Si): (δ, ppm): 22.1 (Me), 37.9 (C5), 62.2 (C6),
123.5–133.3 (C1, C2, C3, C4, C10, C11, C12), 138.4 (C8),
149.1 (C9), 176.5 (C7). 77Se{1H} NMR (CDCl3, 25◦C,
vs Me2Se): (δ, ppm): 464.9. IR (KBr, νmax/cm−1): 3054
(m; νC−H(aromatic)), 2928 (s; νC−H(aliphatic)), 1631, 1577 (s;
1
phy on silica gel. The H and 13C{1H} NMR spectra
identified the products.
3. Results and discussion
νC=N), 1188 (m; νC−N), 748 (m; νC−H(aromatic)).
The syntheses of ligands L1–L7 and Pd(II)–complexes
(1–4) of L4–L7 are summarized in schemes 1 and 2,
respectively. L1–L7 and Pd(II)-complexes (1–4) are
2.7 Synthesis of Pd(II)-Complexes (3–4)
The Na2[PdCl4] (0.15 g, 0.5 mmol) was dissolved in stable and can be stored under ambient conditions up
5 mL of water. The solution of ligand L6 (0.14 g, to six months. The molar conductance values in ace-
0.5 mmol) / L7 (0.18 g, 0.5 mmol) made in 10 mL of tonitrile indicate 1:1 electrolyte nature of both com-
acetone was added to it with vigorous stirring. The mix- plexes 1–2, whereas for complexes 3–4 molar con-
ture was further stirred for 2 h. The orange red precipi- ductance values in acetonitrile indicate non-electrolytic
tate was extracted with chloroform. The chloroform nature. The complexes 1 and 2 show good solubility in
layer was washed with water, dried with anhydrous CH3OH, CH3CN, CH2Cl2 and CHCl3. On other hand,
Na2SO4 and evaporated to dryness in vacuo to obtain complexes 3–4 have good solubility in DMSO and
3/4 as orange coloured solid.
DMF. Each ligand shows good solubility in all common

Pd-complexes in Suzuki coupling
1249
+
5
6
2
3
5
6
13
10
2
3
8
9
N
1
Se
PdCl2 / AgClO4
12
11
[ClO4]
7
13
10
4
8
9
1
N
Se
Pd
12
11
7
4
CH3CN / Reflux / 8h
S
Cl
S
Me
1
Me
L4
+
5
6
2
3
Me
5
6
2
3
Me
7
7
PdCl2 / AgClO4
N
1
Se
[ClO4]
12
11
4
1
N
Se
8
12
11
4
Pd
CH3CN / Reflux / 8h
8
N
Cl
N
9
10
L5
2
9
10
5
6
5
2
6
3
2
3
7
7
N
E
R
N
E
R
Na2[PdCl4] / R.T.
13
1
13
4
1
4
8
8
Pd
Water / Acetone
9
12
9
12
Cl
Cl
11 10
11 10
L6: E = Se, R = H; L7: E = Te, R = OMe
3: E = Se, R = H; 4: E = Te, R = OMe
Scheme 2. Synthesis of Pd(II)-complexes (1–4).
organic solvents. In diethyl and petroleum ether the unusual reaction noticed for the first time in the con-
complexes 1–4 were found to be nearly insoluble. The text of preparation of a Schiff base by isatin or its any
solutions of complexes 1 and 2 in DMSO showed signs derivative.12 The possibility of formation of a com-
of decomposition within few hours.
pound having both C=O groups derivatized in the same
molecule, was ruled out by the results of elemental
1
analyses. The H, 13C{1H}, 77Se{1H} and 125Te{1H}
NMR spectra of the reactions products of scheme 1
were found characteristic (see figures S1–S4 in online
3.1 NMR spectra
The 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spec- supplementary material showing aliphatic regions of
tra of ligands L1–L7 have been found characteristic. 1H and 13C{1H} NMR spectra and relevant sections of
However, in the course of reactions of 5–chloroisatin 77Se{1H} and 125Te{1H} NMR spectra). They show sig-
with chalcogenated amines, H2NCH2CH2E–C6H4–4–R nals which indicate the products as mixtures of L1a and
(E = S, Se, or Te), we have observed that and pro- L1b or L2a and L2b or L3a and L3b. In 13C{1H} NMR
bably both C=O groups react with amines as shown spectrum of L1a, L2a or L3a signal of C6 is expected
in scheme 1, resulting always in a mixture of Schiff to be at higher frequency than that of L1b, L2b or L3b,
bases L1a and L1b or L2a and L2b or L3a and L3b. as there is no possibility in the former of tautomeriza-
The chromatographic separation of the pairs L1a–L1b tion, which can reduce partly double bond character of
or L2a–L2b or L3a–L3b did not succeed. This is an >C=N group.
The possibility that Schiff base is formed from not observed as expected from tautomer-II, (iii) the
C(7)=O and multiple signals arise from the ene-imine multiplicity of =CH-CH2- system of tautomer-II was
1
tautomerism of the Schiff base or tautomer-I was ruled not observed. In fact, in H NMR (see figures S5–
out on the basis of following: (i) no OH peak expected S7 in supplementary material) two sets of similar sig-
from tautomer-I was observed, (ii) CH signals were nals appear. Therefore, the possibility of reaction of

1250
Pradhumn Singh et al.
both the C=O groups appears to be convincing, as in them. The signals in 77Se{1H} NMR spectra of 1,
77Se{1H} and 125Te{1H} NMR spectra also have two 2 and 3 appear shifted to higher frequencies by 176.8,
signals. Further, the ratio of two species varies drasti- 185.2 and 160.9 ppm with respect to those of free L4,
cally with chalcogen, which does not favour tautomer- L5 and L6, respectively as Se is coordinated to palla-
1
ization. On the basis of H NMR (figure S1) the ratios dium centre. Similarly, in 125Te{1H} NMR spectrum of
of L1a:L1b and L2a:L2b (figure S2) has been found to 7 the signal appears at a higher frequency by 241.3 ppm
be 53:47 and 55:45 for sulphur and selenium contain- relative to that of free L7, which is coordinated to pal-
1
ing Schiff bases, respectively whereas L3a:L3b (fig- ladium through Te. In H and 13C{1H} NMR spectra
ure S3) ratio was found to be 81:19. Schiff bases con- of 1–4 signals generally appear at higher frequencies
taining Se and Te exhibit two signals in their 77Se{1H} relative to those of corresponding free ligands L4–L7,
and 125Te{1H} NMR spectra, respectively (figure S4) which coordinate with palladium (tri-dentate mode in
supporting the reaction of both C=O groups of 5- case of L4–L5 and bi-dentate in case of L6–L7). How-
chloroisatin with selenated or tellurated amines. The ever, magnitude of shift to higher frequency is high for
presence of tellurium in amine probably makes nitrogen C5 to C7 (up to 10.8 ppm in 13C{1H} NMR) and protons
electron rich, which makes the condensation reaction attached to them (up to 0.71 ppm in 1H NMR). The sig-
faster and consequently higher yield of L3a. As Schiff nals of SMe in 13C{1H} and 1H NMR spectra of 1 also
bases of isatin derivative were inseparable mixtures, appear at higher frequency (9.3 and 0.53 ppm, respec-
their complexation was not attempted.
tively) relative to those L4. Similarly, the signals of Me
1
The NMR spectra of complexes1–4 have been found in 13C{1H} and H NMR spectra of 2 appear at higher
consistent with their molecular structures shown in frequency (7.7 and 0.52 ppm, respectively) with respect
scheme 2 and support the presence of ligands L4–L7 to those of corresponding free ligand L5.
Table 1. Crystal data and structural refinements for complexes 1–2.
Compounds
1
2
Empirical formula
Formula weight
Crystal size [mm]
Crystal system
C16H17ClNPdSSe·ClO4
575.64
C15H16ClN2PdSe·ClO4
544.56
0.48 × 0.23 × 0.18
0.46 × 0.22 × 0.17
Monoclinic
Monoclinic
Space group
C 2/c
P 21/n
Unit cell dimensions
a = 18.326(14)Å
b = 14.414(10)Å
c = 15.634(10)Å
α = 90.00◦
a = 10.2179(9)Å
b = 16.8940(15)Å
c = 11.3698(10)Å
α = 90.00◦
β = 105.817(11)◦
γ = 90.00◦
β = 107.776(2)◦
γ = 90.00◦
Volume [Å3]
Z
3973.0(5)
1869.0(3)
8
4
ρcalcd[g/cm3]
1.925
3.162
2256.0
1.82–25.00
1.935
3.248
1064.0
2.23–25.00
μ(MoKα) [mm−1
F(000)
]
θ range [◦]
Index ranges
−21 ≤ h ≤ 21
−17 ≤ k ≤ 17
−18 ≤ l ≤ 18
45118
−12 ≤ h ≤ 12
−20 ≤ k ≤ 20
−13 ≤ l ≤ 13
17308
Reflections collected
Independent reflections (Rint.)
Completeness to max. θ [%]
Max./min. Transmission
Data/restraints/parameters
Goodness-of-fit on F2
4618 (0.0409)
99.4
0.574/0.427
3496/0/236
3287 (0.0274)
99.7
0.574/0.427
3287/0/227
1.055
1.036
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0499, wR2 = 0.1328
R1 = 0.0794, wR2 = 0.1576
0.867/ − 0.502
R1 = 0.0287, wR2 = 0.0777
R1 = 0.0328, wR2 = 0.0800
0.497/ − 0.353
Largest diff. peak / hole [e.Å−3
]

Pd-complexes in Suzuki coupling
1251
3.2 Crystal structures
The crystal structures of 1–2 have been solved and their
crystal and refinement data are given in table 1. The
ORTEP diagrams of cations of 1 and 2 are given in
figures 1 and 2 with selected bond lengths and angles
(more values are given in supplementary material;
table S1). The geometry around Pd in the cations of 1–2
is nearly square planar and the ligands are coordinated
with Pd in a tri-dentate (S, N, Se) or (N, N, Se) mode
forming one six- and one five-membered or two five-
membered rings, respectively. The Pd–S bond length
of 1, 2.304(2) Å is consistent with normal reported
range.5,7,20,21 Pd–Se bond length of 1, 2.4172(17) Å is
greater than that of 2 (2.3675(4) Å), but both are nor-
mal.5,7,20,21 The Pd–N and Pd–Cl bond lengths of com-
plexes 1–2, 1.992(3)–2.041(3) and 2.2895(9)–2.392(2)
Å, respectively are also normal.20,21 The bond angles at
the coordinating S, Se and N atoms are as expected for
nearly trigonal–pyramidal and tetrahedral geometries,
respectively.
Figure 2. ORTEP diagram of the cation of 2 with ellip-
soids at the 30% probability level. The ClO−4 anion has
been omitted for clarity. Selected bond lengths (Å): Pd(1)–
Se(1) 2.3675(4), Pd(1)–N(1) 1.992(3), Pd(1)–N(2) 2.041(3),
Pd(1)–Cl(1) 2.2895(9); bond angles (◦): Se(1)–Pd(1)–N(1)
88.18(9), Se(1)–Pd(1)–N(2) 168.71(8), Se(1)–Pd(1)–Cl(1)
93.68(3), N(1)–Pd(1)–Cl(1) 177.99(9), N(2)–Pd(1)–Cl(1)
97.60(8), N(1)–Pd(1)–N(2) 80.54(11).
with phenylboronic acid the reaction conditions used
were similar to those used for analogous phosphine-
free systems.11,22 Firstly, the coupling reaction of bro-
mobenzene with phenylboronic acid in the presence
of 3 was studied to optimize the reaction conditions.
The Cs2CO3 has been found most appropriate base
as with alternatives viz. potassium carbonate, triethyl-
amine, sodium acetate or sodium carbonate longer reac-
tion time was necessary for reasonable conversions.
Furthermore, the reaction was also influenced by the
solvent. DMF and water mixture was found to be the
best choice of solvent for this reaction among the seve-
ral solvents investigated. The complex 3 was found to
show promising activity11 for several aryl bromides
(table 2) including electron-rich ones as very low cata-
lyst loading (up to 0.006 mol%) was found sufficient for
good conversion in several cases. The highest turnover
number (TON) found in case of 4-bromobenzonitrile is
1.58 × 104 with 95% yield. This value indicates that
efficiency of present catalyst is of the same order as that
of Pd(II)-N–{2–(phenylseleno)ethyl}pyrrolidine com-
plex (Yield up to 82%, TON up to 8.2 × 104).22 The
conversions with present catalyst are comparable with
those reported for other Pd(II)-selenated Schiff base
(Se, N, O−) complexes (Yield up to 95%). However,
with high catalyst loading 0.5 mol% only such conver-
sions were realized earlier.23 The catalytic activity of
tellurium analogue, 4 which differs from 3 only in type
of chalcogen donor site is significantly lower (table 3)
than that of 3. This may probably be attributed to the
large size and metallic character of Te. The catalytic
3.3 Catalytic Suzuki reactions
The Suzuki reactions between different aryl bromides
and phenylboronic acid were carried out using com-
plexes 3 and 4 as catalyst (summarized in scheme 3).
The detailed results of catalysis are given in tables 2–3.
In view of air and moisture sensitivity of palla-
dium complexes of phosphorus ligands, there is an
interest in the development of phosphine-free ligands
for the Suzuki–Miyaura coupling reaction. Therefore,
complexes 3–4 were explored for such coupling. For
carrying out Suzuki–Miyaura reactions of aryl halide
Figure 1. ORTEP diagram of the cation of 1 with ellip-
soids at the 30% probability level. The ClO−4 anion
has been omitted for clarity. Selected bond lengths (Å):
Pd(1)–Se(1) 2.4172(17), Pd(1)–S(1) 2.304(2), Pd(1)–N(1)
2.021(6), Pd(1)–Cl(1) 2.292(2); bond angles (◦): Se(1)–
Pd(1)–S(1) 176.97(6), Se(1)–Pd(1)–Cl(1) 90.08(7), Se(1)–
Pd(1)–N(1) 86.76(18), S(1)–Pd(1)–Cl(1) 92.65(8), S(1)–
Pd(1)–N(1) 90.51(19), N(1)–Pd(1)–Cl(1) 176.83(19).

1252
Pradhumn Singh et al.
Catalyst
R
R
X
(HO)2B
+
Cs2CO3, aq. DMF
Scheme 3. Suzuki–Miyaura coupling reaction.
Table 2. Suzuki coupling reactions catalysed by 3a.
>C=O groups of isatin has been observed for the first
time. The ratio of two co-existing products was found
to vary with chalcogen; 53:47 (E = S), 55:45 (E =
Se) and 81:19 (E = Te). Pd(II)–complexes (1–4) were
synthesized and characterized. The crystal structures
of 1–2 have been solved. The Pd–Se bond lengths are
2.3675(4)–2.4172(17) Å. The Pd–complexes (3–4) of
L6–L7 were explored for Suzuki–Miyaura coupling
and found promising as 0.006 mol% of 3 is sufficient to
obtain good conversion with TON up to 1.58 × 104.
Entry no.
1.
Aryl halide
Mol% % Yield TON
4-Bromobenzaldehyde
0.1
95
70
50
95
95
58
45
39
67
39
35
950
7000
8333
15833
1583
58
45
39
67
39
0.01
0.006
0.006
2.
3.
4.
5.
6.
7.
8.
9.
4-Bromobenzonitrile
1-Bromo-4-nitrobenzene 0.06
Bromobenzene
4-Bromotoluene
4-Bromoanisol
3-Bromobenzoic acid
3-Bromopyridine
2-Bromopyridine
1.0
1.0
1.0
1.0
1.0
1.0
Supplementary material
35
aReaction conditions: 1.0 equiv. of aryl bromide, 1.3 equiv. of
phenylboronic acid and 2 equiv. of base (Cs2CO3), aqueous
DMF as solvent and temperature of bath 100◦C, reaction time
24 h, isolated yield after column chromatography
CCDC numbers: 863907 and 863908 contain the sup-
plementary crystallographic data for 1 and 2, respec-
tively. These data can be obtained free of charge
at www.ccdc.cam.ac.uk/data.request/cif or from The
Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: +44 1223
336033; E-mail: deposit@ccdc.cam.ac.uk. Supplemen-
tary data associated with this article (figures S1–S7 and
table S1) are available at http://www.ias.ac.in/chemsci.
Table 3. Suzuki coupling reactions catalysed by 4a.
Entry no.
Aryl halide
Mol% % Yield TON
1.
2.
3.
4.
5.
6.
4-Bromobenzaldehyde
1-Bromo-4-nitrobenzene 1.0
4-Bromobenzonitrile
4-Bromotoluene
4-Bromoanisol
1.0
55
57
62
42
59
48
55
57
62
42
59
48
1.0
1.0
1.0
1.0
Acknowledgements
Authors thank the Council of Scientific and Industrial
Research (CSIR), New Delhi, India for the project no.
01(2421)10/EMR-II and the Department of Science and
Technology (DST), New Delhi, India for partial finan-
cial assistance given to establish single crystal X–ray
diffraction facility at Indian Institute of Technology
Delhi, India under its FIST program. PS thanks the Uni-
versity Grants Commission (UGC), New Delhi, India
for the award of Junior and Senior Research Fellowships.
4-Bromobenzoic acid
aReaction conditions: 1.0 equiv. of aryl bromide, 1.3 equiv.
of phenylboronic acid and 2 equiv. of base (Cs2CO3), aque-
ous DMF as solvent and temperature of bath 100◦C, reaction
time 24 h, isolated yield after column chromatography
activity appears to be dependent on the substituent
groups present on the aryl ring. The additional advan-
tage of using these catalysts is that they are air stable
and also not moisture sensitive.
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The reaction of H2NCH2CH2E–C6H4–4–R (R = H;
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