Syntheses, structures and efficient catalysis for C-C coupling of some benzaldehyde thiosemicarbazone complexes of palladium

Article abstract of DOI:10.1016/j.molcata.2011.05.003

Reaction of the 4-R-benzaldehyde thiosemicarbazones (denoted in general as L-R; R = OCH₃, CH₃, H, Cl and NO₂) with trans-[Pd(PPh₃)₂Cl₂] afforded a group of mixed-ligand complexes (denoted i

Full text of DOI:10.1016/j.molcata.2011.05.003

Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Syntheses, structures and efficient catalysis for C–C coupling of some
benzaldehyde thiosemicarbazone complexes of palladium
a
a
a
a
a
Piyali Paul , Sayanti Datta , Sarmistha Halder , Rama Acharyya , Falguni Basuli ,
b
c
c
d
e
Ray J. Butcher , Shie-Ming Peng , Gene-Hsiang Lee , Alfonso Castineiras , Michael G.B. Drew ,
Samaresh Bhattacharya
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, 188 Raja S C Mullick Road, Kolkata 700 032, India
Department of Chemistry, Howard University, Washington, DC 20059, USA
Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
Departamento de Quimica Inorganica, Universidade de Santiago de Compostela, E-15706 Santiago de Compostela, Spain
Department of Chemistry, University of Reading. Whiteknights, Reading RG6 6AD, UK
a,∗
a
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of the 4-R-benzaldehyde thiosemicarbazones (denoted in general as L-R; R = OCH , CH , H, Cl
3
3
Received 21 January 2011
Received in revised form 30 April 2011
Accepted 2 May 2011
and NO2) with trans-[Pd(PPh3)2Cl2] afforded a group of mixed-ligand complexes (denoted in general as
-R) incorporating a N,S-coordinated thiosemicarbazone, a triphenylphosphine and a chloride. Similar
reaction with Na2[PdCl4] afforded a family of bis-thiosemicarbazone complexes (denoted in general as
-R), where each ligand is N,S-coordinated. Crystal structures of 1-CH3, 1-NO2, 2-OCH3, 2-NO2 and L-
1
Available online 7 May 2011
2
NO2 have been determined. In all the complexes the thiosemicarbazones are coordinated to the metal
center, via dissociation of the acidic proton, as bidentate N,S-donors forming five-membered chelate
rings. With reference to the structure of the uncoordinated thiosemicarbazone, this coordination mode
is associated with a conformational change around the C N bond. All the 1-R and 2-R complexes display
intense absorptions in the visible region. Catalytic activity of the 1-R and 2-R complexes towards some
C–C coupling reactions (e.g. Suzuki, Heck and Sonogashira) has been examined and while both are found
to be efficient catalysts, 1-R is much better catalyst than 2-R.
Keywords:
Benzaldehyde thiosemicarbazones
Palladium complexes
Syntheses
Structures
Catalytic properties
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
plexes as catalysts over other species, and it is due to this unique
nature of palladium that we are witnessing the remarkable success
in catalytic C–C cross-coupling reactions [4]. Though complexes
with phosphine type ligands (such as [Pd(PPh ) ]) are always a
The chemistry of thiosemicarbazone complexes of the transi-
tion metal ions has been receiving significant current attention,
largely because of the bioinorganic relevance of these complexes
3
4
favorite choice as pre-catalyst for such reactions [5], complexes
with ligands other types are also gaining popularity with time
[6], and hence we decided to synthesize mixed-ligand (containing
PPh3 as the ancillary ligand) complexes as well as homoleptic
complexes of palladium with the selected thiosemicarbazones
(L-R) and test their catalytic efficiency towards C–C cross coupling
reactions. It may be relevant to mention here that though the
chemistry of palladium complexes of several thiosemicarbazones
has received considerable attention [2i,7], the binding of palladium
to the chosen ligands (L-R) has remained unexplored. In our earlier
studies we have observed that the 4-R-benzaldehyde thiosemi-
carbazones (L-R) usually bind to a metal center, via dissociation of
the acidic proton, as monoanionic bidentate N,S-donors forming
a rather unexpected four-membered chelate ring (I) [2b,2c]. Our
investigations have revealed that for these ligands (L-R), formation
of the four-membered chelate ring (I) is most favorable and
that of the five-membered chelate ring (II), which appears to be
very likely for the thiosemicarbazones in general, is not possible
[
1]. However, we have been exploring the chemistry of platinum
metal complexes of the thiosemicarbazones [2], mainly because
of the variable binding mode displayed by these ligands in their
complexes and also to gain a chemical control over the mode of
binding, and the present work has emerged out of this exploration.
The primary objective of the present study has been to scrutinize
the mode of binding of a group of 4-R-benzaldehyde thiosemicar-
bazones (denoted in general as L-R, R = OCH , CH , H, Cl and NO )
to palladium. This particular metal center has been chosen with an
aim to explore the catalytic activity, if any, of the resulting com-
plexes. Palladium complexes are well known to bring about C–C
cross coupling of different types (such as Suzuki, Heck, Sonogashira,
etc.) [3]. There are several advantages of using palladium com-
3
3
2
∗ Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 2414 6223.
E-mail address: samaresh b@hotmail.com (S. Bhattacharya).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.05.003

P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
63
because of the inevitable steric hindrance that develops between
2.2. Syntheses
the phenyl ring of the benzaldehyde thiosemicarbazone and the
metal center. It is worth noting here that we have not been able
to find a single example of a structurally characterized complex
of the 4-R-benzaldehyde thiosemicarbazones (L-R), where the
thiosemicarbazone is coordinated as in II. Though five-membered
chelate ring (II) formation by the 4-R-benzaldehyde thiosemi-
carbazones (L-R) has been found to be impossible, closeness of
the phenyl ring to the metal center in II points to the possibility
of its orthometallation (III) via C–H bond activation, which has
indeed been realized recently [2f]. A five-membered chelate ring
2.2.1. 1-R complexes
The 1-R (R = OCH , CH , H, Cl and NO ) complexes were prepared
3
3
2
by following a general procedure. Specific details are given below
for a particular complex.
1-OCH : To a solution of 4-methoxybenzaldehyde thiosemicar-
3
bazone (30 mg, 0.14 mmol) in hot ethanol (30 mL) triethylamine
(14 mg, 0.14 mmol) was added followed by trans-[Pd(PPh ) Cl ]
3
2
2
(100 mg, 0.14 mmol). The mixture was heated at reflux for 5 h
to yield an orange solution. Evaporation of this solution gave an
orangish-yellow solid, which was subjected to purification by thin
layer chromatography on a silica plate. With benzene as the eluant,
a yellow band separated, which was extracted with acetonitrile.
(
IV) formation by the 4-R-bezaldehyde thiosemicarbazones (L-R)
seems possible only via a conformational change across the C
bond, and this has indeed been observed in the present study.
Two different palladium compounds, viz. trans-[Pd(PPh ) Cl ]
N
Upon evaporation of the acetonitrile extract 1-OCH was obtained
3
2
2
3
and Na [PdCl ], have been utilized in this study as the source of
as a crystalline yellow solid. Yield: 60%. 1H NMR [9]: 3.85 (s, OCH3,
2
4
palladium. Reactions of the 4-R-benzaldehyde thiosemicarbazones
L-R) with these palladium compounds have afforded two families
3H), 6.05 (s, NH , 2H), 6.97 (d, J = 8.8 Hz, 2H), 7.43–7.78 (PPh ), 8.08
(d, J = 8.8 Hz, 2H), 8.56 (d, J = 4.0 Hz, azomethine 1H). Anal. Calc. for
2
3
(
of complexes, in both of which the thiosemicarbazones have been
found to coordinate palladium as bidentate N,S-donors forming
five-membered chelate rings (IV). This paper deals with the
chemistry of these two groups of palladium thiosemicarbazone
complexes, with special reference to their formation, structure
and catalytic efficiency towards C–C coupling reactions.
C27H25N OSClPPd: C, 52.98; H, 4.08; N, 6.86. Found: C, 52.45; H,
3
4.25; N, 6.64%.
1
1-CH : Yield: 62%. H NMR: 2.37 (s, CH , 3H), 4.79 (s, NH , 2H),
3
3
2
7.2 (d, J = 8.1 Hz, 2H), 7.40–7.79 (PPh ), 8.05 (d, J = 8.2 Hz, 2H), 8.53
3
(d, J = 4.3 Hz, azomethine 1H). Anal. Calc. for C27H25N SClPPd: C,
3
54.37; H, 4.20; N, 7.05. Found: C, 54.07; H, 4.41; N, 6.82%.
R
R = OCH , CH ,
3
3
R
H, Cl, NO2
N
NH2
S
H
N
N
H
NH2
S
H
N
M
L-R
I
R
R
M
M
M
H
N
S
N
S
N
S
H
N
H
N
N
NH2
NH2
NH2
II
III
IV
R
2
. Experimental
1-H: Yield: 65%. 1H NMR: 5.67 (s, NH2, 2H), 7.34–7.78
(
PPh +2H)*, 8.05–8.08 (3H), 8.64 (d, J = 4.0 Hz, azomethine 1H).
3
2.1. Materials
Anal. Calc. for C26H23N3SClPPd: C, 53.62; H, 3.95; N, 7.22. Found: C,
5
3.49; H, 4.15; N, 7.48%.
Palladium chloride was obtained from Arora Matthey, Kolkata,
India. The trans-[Pd(PPh ) Cl ] and Na [PdCl ] complexes were
1-Cl: Yield: 67%. 1H NMR: 4.87 (s, NH2, 2H), 7.36 (d, J = 8.6 Hz,
2H), 7.41–7.78 (PPh3), 8.11 (d, J = 8.6 Hz, 2H), 8.53 (d, J = 4.2 Hz,
azomethine 1H). Anal. Calc. for C26H22N3SCl2PPd: C, 50.61; H, 3.57;
N, 6.81. Found: C, 50.83; H. 3.34; N, 7.01%.
3
2
2
2
4
prepared by following reported procedures [8]. The 4-R-
benzaldehyde thiosemicarbazones 1 were prepared by reacting
equimolar amounts of thiosemicarbazide and the respective para-
substituted benzaldehyde in 1:1 ethanol–water mixture. All other
chemicals and solvents were reagent grade commercial materials
and were used as received.
1
1-NO2: Yield: 63%. H NMR: 4.96 (s, NH2, 2H), 7.35–7.72 (PPh3),
8.16 (d, J = 9.0 Hz, 2H), 8.25 (d, J = 8.94 Hz, 2H), 8.58 (d, J = 4.3 Hz,
azomethine 1H). Anal. Calc. for C26H22N4O2SClPPd: C, 49.77. H,
3.51; N, 8.93. Found: C, 49.91; H, 3.72; N, 8.66%.

6
4
P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
Fig. 1. Structure of 1-CH3.
entific L75FBAL magnet. 1H NMR spectra were recorded in CDCl3
and DMSO-d6 solutions on a Bruker Avance DPX 300 NMR spec-
trometer using TMS as the internal standard. Molecular orbital
calculations were carried out by density functional theory (DFT)
method using the Gaussian 03 (B3LYP/SDD-6-31G) package [10].
2
.2.2. 2-R complexes
The 2-R (R = OCH , CH , H, Cl and NO ) complexes were prepared
3
3
2
by following a general procedure. Specific details are given below
for a particular complex.
2
-OCH : To a solution of 4-methoxybenzaldehyde thiosemicar-
3
bazone (150 mg, 0.71 mmol) in hot ethanol (30 mL) triethylamine
(
(
80 mg, 0.79 mmol) was added followed by a solution of Na [PdCl ]
100 mg, 0.34 mmol) in ethanol (10 mL). The solution was then
2 4
2
2
.3. Application as catalysts
heated at reflux for 6 h. 2-OCH3 precipitated as a yellow crys-
talline solid, which was collected by filtration, washed thoroughly
.3.1. General procedure for the Suzuki coupling reactions
In a typical run, an oven-dried 10 mL round bottom flask was
with water, followed by ethanol and then dried in air. Yield: 79%.
charged with a known mole percent of catalyst, NaOH (1.7 mmol),
phenylboronic acid (1.2 mmol) and aryl halide (1 mmol) with the
appropriate solvents (4 mL). The flask was placed in a preheated
oil bath at required temp. After the specified time the flask was
removed from the oil bath and water (20 mL) added, followed by
extraction with ether (4 × 10 mL). The combined organic layers
were washed with water (3 × 10 mL), dried over anhydrous Na SO ,
1
H NMR: 3.82 (s, OCH , 3H), 6.96 (d, J = 8.6 Hz, 2H), 7.09 (s, NH ,
3
2
2
H), 7.22 (s, azomethine 1H), 8.18 (d, J = 8.6 Hz, 2H). Anal. Calc. for
C18H20N S O Pd: C, 44.04; H, 4.08; N, 17.13. Found: C, 44.39; H,
6
2
2
4
2
.27; N, 17.01%.
-CH : Yield: 77%. H NMR: 2.36 (s, CH , 3H), 7.22 (d, J = 11.3 Hz,
1
2
3
3
H), 7.26 (s, NH , 2H), 8.02 (d, J = 8.0 Hz, 2H). Anal. Calc. for
2
2
4
C18H20N S Pd: C, 44.04; H, 4.08; N, 17.13. Found: C, 44.31; H, 4.34;
N, 17.44%.
6
2
and filtered. Solvent was removed under vacuum. The residue was
dissolved in CDCl3 and analyzed by 1H NMR. Percent conversions
were determined against the remaining aryl halide [11].
2
-H: Yield: 81%. 1H NMR: 7.28 (s, NH , 2H), 7.31 (s, 1H),
2
7
.44–7.51 (3H), 8.12 (d, J = 7.6 Hz, 2H). Anal. Calc. for C16H16N S Pd:
6 2
C, 41.52; H, 3.46; N, 18.17. Found: C, 41.74; H, 3.23; N, 18.53%.
1
2.3.2. General procedure for the Heck coupling reactions
In a typical run, an oven-dried 10 mL round bottom flask
was charged with a known mole percent of catalyst, Cs CO3
2
-Cl: Yield: 76%. H NMR: 7.28 (s, 1H), 7.36 (s, NH , 2H),
2
7
.50 (d, J = 8.2 Hz, 2H), 8.17 (d, J = 8.7 Hz, 2H). Anal. Calc. for
C16H14N S Cl Pd: C, 36.13; H, 2.63; N, 15.81. Found: C, 36.38; H,
2
6
2
2
(1.7 mmol), n-butyl acrylate (1.2 mmol) and aryl halide (1 mmol)
2
.84; N, 15.67%.
-NO : Yield: 75%. H NMR: 7.40 (s, 1H), 7.66 (s, NH , 2H),
1
with polyethylene glycol (4 mL). The flask was placed in a preheated
2
2
2
◦
oil bath at 150 C. After the specified time the flask was removed
8
.20 (d, J = 8.8 Hz, 2H), 8.39 (d, J = 8.7 Hz, 2H). Anal. Calc. for
from the oil bath and water (20 mL) added, followed by extraction
with ether (4 × 10 mL). The combined organic layers were washed
with water (3 × 10 mL), dried over anhydrous Na SO , and filtered.
C16H14N O S Pd: C, 34.76; H, 2.53; N, 20.27. Found: C, 34.65; H,
8
4 2
2
.79; N, 20.42%.
2
4
Solvent was removed under vacuum. The residue was dissolved in
CDCl3 and analyzed by 1H NMR. Percent conversions were deter-
mined against the remaining aryl halide [11].
2.2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. IR spectra were obtained on a Shi-
madzu FTIR-8300 spectrometer with samples prepared as KBr
pellets. Electronic spectra were recorded on a JASCO V-570 spec-
trophotometer. Magnetic susceptibilities were measured using a
PAR 155 vibrating sample magnetometer fitted with a Walker sci-
2.3.3. General procedure for Sonogashira coupling reactions
To slurry of aryl halide (1 mmol), cuprous iodide (10 mol%) and
palladium catalyst (a known mol%) in an appropriate solvent (4 mL),
phenylacetylene (1.2 mmol) and NaOH (1.7 mmol) was added and

P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
65
heated at required temp. After completion of the reaction (moni-
tored by TLC), the flask was removed from the oil bath and water
between two layers of complex molecules. A closer inspection into
the network reveals that each water molecule is simultaneously
linked to three different 1-CH3 molecules through hydrogen-
bonding. These extended hydrogen-bonding interactions seem to
be responsible for holding the crystal together. It may be relevant
to note here that such non-covalent interactions are of significant
importance in molecular recognition processes as well as in crystal
engineering [13].
(
20 mL) added, followed by extraction with ether (4 × 10 mL). The
combined organic layers were washed with water (3 × 10 mL),
dried over anhydrous Na SO , and filtered. Solvent was removed
2
4
under vacuum. The residue was dissolved in CDCl and analyzed by
3
1
H NMR. Percent conversions were determined against the remain-
ing aryl halide [11].
Cl
PPh3
2.4. X-ray structure analysis
Pd
Single crystals of 1-CH , 1-NO and L-NO2 were grown by
3
2
H
slow evaporation of solvent from acetonitrile solutions of the
respective compounds. Single crystals of 2-OCH3 were obtained
by slow evaporation of solvents from a solution of the com-
plex in acetonitrile–ethanol (3:1) mixture and those of 2-NO2
were obtained by slow evaporation of solvent from an N,N-
dimethylformamide solution of the complex. Selected crystal data
and data collection parameters are given in Table 1. Data on the
crystals of 1-CH3 and 2-OCH3 were collected on a Bruker SMART
CCD diffractometer and those on the crystal of 1-NO2 were col-
lected on a Marresearch Image Plate system. Data on the crystal of
N
S
N
NH2
R
1
-R
2
-NO2 were collected on a Nonius Kappa CCD diffractometer. Data
on the crystal of L-NO2 were collected on an Enraf Nonius MACH3
automatic diffractometer. X-ray data reduction, structure solution
and refinement were done using the SHELXS-97 and SHELXL-97
packages [12]. The structures were solved by the direct methods.
The chloride of 1-CH3 was found to be disordered between two
locations, viz. Cl1A (59% occupancy) and Cl1B (41% occupancy).
However, all the reported crystallographic parameters of this par-
ticular complex include only Cl1A.
3
. Results and discussion
3.1. Syntheses and characterization
Five 4-R-benzaldehyde thiosemicarbazones (L-R), differing in
the substituent at the para position of the phenyl ring, have been
used in the present study. Reaction of these thiosemicarbazones
with trans-[Pd(PPh ) Cl ] proceeds smoothly in refluxing ethanol
3
2
2
in the presence of triethylamine to afford a group of mixed-ligand
complexes (denoted in general as 1-R) in decent yields. Preliminary
characterizations (microanalysis, NMR, IR, etc.) on these 1-R com-
plexes (vide infra) indicate the presence of a thiosemicarbazone,
a triphenylphosphine and a chloride in the coordination sphere.
In order to find out the stereochemistry of these complexes as
well as coordination mode of the thiosemicarbazones in them,
structure of one representative member of this family, viz. 1-CH3,
has been determined by X-ray crystallography. A selected view
of the complex molecule is shown in Fig. 1 and some relevant
bond parameters are listed in Table 2. The structure shows that
the thiosemicarbazone is coordinated to palladium as a bidentate
N,S-donor forming a five-membered chelate ring (IV) with a bite
◦
angle of 82.94(6) . A triphenylphosphine and a chloride are also
coordinated to the metal center. The triphenylphosphine is trans
to the nitrogen and the chloride is trans to the sulfur. Palladium is
thus nested in an NSPCl core, which is slightly distorted from ideal
square planar geometry, as manifested in the bond parameters
around the metal center. The Pd–N, Pd–P, Pd–S and Pd–Cl distances
are normal, as observed in structurally characterized complexes
of palladium containing these bonds [7]. In the crystal lattice of
1
-CH , there is one molecule of water per complex molecule. In
3
order to find out the link between this water molecule and the
complex molecule, packing pattern in the lattice has been scruti-
nized, which shows (Fig. 2) that a layer of water molecules lie in
Fig. 2. (a) Packing diagram of the lattice of 1-CH3·H2O viewed down the b axis and
b) the intermolecular hydrogen bonding.
(

6
6
P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
Table 1
Crystallographic data for 1-CH3, 1-NO2, L-NO2, 2-OCH3 and 2-NO2.
1
-CH3·H2O
1-NO2·CH3CN·H2O
L-NO2
2-OCH3
2-NO2·2DMF
Empirical formula
Mr
Crystal system
C27H27N3OPSClPd
614.43
Orthorhombic
Pbcn
C26H22N4O2PSClPd
684.44
Monoclinic
P21/n
C8H8N4O2S
224.24
Monoclinic
P21/c
C18H20N6O2S2Pd
522.92
Monoclinic
P21/c
C22H28N10O6S2Pd
699.06
Monoclinic
P21/n
Space group
a/A˚
b/A˚
16.1294(8)
17.1073(8)
18.6207(9)
90
90
90
5138.0(4)
8
1.589
9.4107(5)
35.6481(14)
9.7287(6)
90
114.541(7)
90
2968.9(3)
4
1.531
4.5686(15)
24.885(3)
8.9668(18)
90
98.557(17)
90
1008.1(4)
4
1.478
12.748(7)
5.561(3)
14.563(8)
90
102.659(13)
90
1007.3(10)
2
1.724
13.1237(3)
6.1481(2)
17.7804(5)
90
100.2035(10)
90
1411.94(7)
2
1.644
c/A˚
◦
˛
/
◦
◦
ˇ/
/
ꢀ
V/A˚
Z
3
−3
Dcalcd/mg m
F(0 0 0)
ꢁ
Crystal size/mm3
Temp./K
2496
1384
464
528
712
0.71073
0.35 × 0.50 × 0.60
93(2)
0.71073
0.03 × 0.22 × 0.22
150
0.71073
0.28 × 0.12 × 0.08
293(2)
0.71073
0.13 × 0.14 × 0.16
293
0.71073
0.35 × 0.20 × 0.02
150(1)
−
1
ꢂ/mm
0.996
0.878
0.306
1.157
0.862
Independent reflections
R int
6341
8415
0.064
15,617
0.0757
0.1749
1.05
1370
0.1171
1461
0.0489
0.0774
0.915
1301
0.066
6875
0.1187
0.3546
1.61
3227
0.0429
9705
0.0382
0.0896
1.167
0.0336
38,569
0.0348
0.0891
1.13
Collected reflections
a
R1
b
wR2
GOF
c
a
R1 = ||Fo| − |Fc||/ꢃ|Fo|.
ꢀ
ꢀ
ꢁ
ꢂ
1/2
2
w(F −F
o
2
2
[
) ]
c
b
ꢁ
Xxxxxxxxxxxxx wR2 =
.
2
o
2
[w(F
)
]
ꢁ
ꢂ
1/2
2
2
2
[
w(F −F
)
]
o
c
c
Xxxxxxxxxxxxx GOF =
, where M is the number of reflections and N is the number of parameters refined.
M−N
For an assessment of any changes in the structural features
in the thiosemicarbazone that might have occurred upon its
coordination to the metal, structure of an uncoordinated thiosemi-
attributable to delocalization of the negative charge, developed
upon dissociation of the acidic proton, over this N–C–S fragment,
which stabilizes the imino-thiolate form of the thiosemicarbazone.
Comparison of the geometry of the coordinated thiosemicarbazone
with that of the uncoordinated ligand reveals another signifi-
cant difference. In the free thiosemicarbazone, disposition of the
aryl ring and the N(H)C(S)NH2 fragment is trans with respect
carbazone, viz. 4-nitrobenzaldehyde thiosemicarbazone (L-NO ),
2
has also been determined by X-ray crystallography. The struc-
ture (Fig. 3) shows that the free ligand has the same symmetry
as shown in the drawing of L-R. Comparison of the bond lengths
(
Table 2) shows that upon complexation the C–S bond gets elon-
to the imine (C N) bond. In 1-CH , however, disposition of the
3
gated while the adjacent C–N bond, which forms part of the
chelate ring, gets contracted. These changes in bond lengths are
aryl ring and the NC(S)NH2 fragment across the imine (C N)
bond is cis. This indicates that coordination of the benzaldehyde
thiosemicarbazone to palladium has taken place associated with an
interesting conformational change around the imine (C N) bond.
It may be noted in this context that this particular binding mode
(IV) of the 4-R-benzaldehyde thiosemicarbazones is relatively less
common [14].
In the structure of 1-CH , interesting conformational change
3
around the C N bond, with respect to uncoordinated L-R ligand,
leading to formation of rather unusual five-membered ring has
been observed. In order to verify that the same structural features
also exist in the other 1-R complexes, crystal structure of another
complex of this group, viz. 1-NO , has been determined. Besides
2
small differences in the bond parameters, this structure (Fig. 4,
Table 2) is found to be similar to that of 1-CH3 (Fig. 1), with par-
ticular respect to binding mode of the thiosemicarbazone ligand.
Therefore, in view of the similarity in the method of synthesis of
the 1-R complexes, as well as similarity in their properties (vide
infra), the remaining three 1-R (R = OCH , H and Cl) complexes are
3
assumed to have similar structures as 1-CH3 or 1-NO2.
The observed conformational change of the thiosemicarbazone
upon binding to palladium was quite intriguing. Though the exact
driving force behind this phenomenon is unclear, steric bulk of
the triphenylphosphine seems to be one of the possibilities. To
check this, reaction of the selected 4-R-benzaldehyde thiosemi-
carbazones has also been carried out similarly as before with
another palladium starting material, viz. Na2[PdCl4], which does
Fig. 3. Structure of L-NO2.

P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
67
Table 2
Selected bond lengths ( A˚ ) and angles ( ) for 1-CH3, 1-NO2, L-NO2, 2-OCH3 and 2-NO2.
◦
1-CH3
Bond lengths ( A˚ )
Pd–N(1)
Pd–P
Pd–S
Pd–Cl1A
Bond angles ( )
2.111(2)
2.2707(7)
2.2303(8)
2.412(6)
C(7)–N(1)
C(8)–N(2)
N(1)–N(2)
C(8)–S
1.292(3)
1.300(3)
1.392(3)
1.753(3)
◦
N(1)–Pd–P
N(1)–Pd–S
174.15 (15)
82.94(6)
S–Pd–Cl1A
175.6(2)
L-NO2
Bond lengths ( A˚ )
C(16)–C(17)
N(12)–C(17)
C(18)–N(13)
1.473(8)
1.280(7)
1.370(8)
C(18)–N(14)
C(18)–S(1)
N(12)–N(13)
1.321(8)
1.670(7)
1.358(6)
◦
Bond angles ( )
N(14)–C(18)–S(1)
N(12)–N(13)–C(18)
N(12)–C(17)–C(16)
124.1(7)
118.8(6)
118.8(7)
N(14)–C(18)–N(13)
C(17)–N(12)–N(13)
116.1(7)
117.0(6)
1-NO2
Bond lengths ( A˚ )
Pd1–N14
Pd1–P1
Pd1–S11
Pd1–Cl1
2.113(5)
C15–N14
C12–N13
N13–N14
C12–S11
1.291(8)
2.2703(16)
2.2441(17)
2.3606(17)
1.309(9)
1.382(8)
1.749(7)
◦
Bond angles ( )
N14–Pd1–P1
N14–Pd1–S11
176.58(5)
83.72(15)
S11–Pd1–Cl1
176.34(7)
2-OCH3
Bond lengths ( A˚ )
Pd1–N1
Pd1–S1
C1–N3
C1–N2
2.020(13)
2.299(5)
1.35(2)
C2–N1
C1–S1
N1–N2
1.290(2)
1.741(17)
1.410(2)
1.31(2)
◦
Bond angles ( )
N1–Pd1–N1a
N1–Pd1–S1
180.00
82.80(4)
S1–Pd1–S1a
180.00
2-NO2
Bond lengths ( A˚ )
Pd–N(3)
Pd–S(1)
C(1)–N(1)
2.044(3)
2.2880(9)
1.327(4)
C(2)–N(3)
C(1)–S(1)
N(2)–N(3)
1.298(4)
1.743(3)
1.377(4)
◦
Bond angles ( )
N(3)–Pd–N(3A)
N(3)–Pd–S(1)
180.0(1)
82.61(8)
S(1)–Pd–S(1A)
180.0(1)
not have any bulky ligand. These reactions have afforded a group
of homoleptic bis-complexes (denoted as 2-R in general) in excel-
lent yields. Structure of a selected member of this family, viz.
intermolecular hydrogen-bonding interactions contribute to the
stability of the lattice.
R
2
-NO , has been determined by X-ray crystallography to ascertain
2
the binding mode of the thiosemicarbazone. The structure (Fig. 5)
shows that the thiosemicarbazones are coordinated to palladium
as before, forming five-membered chelate rings (IV). The two coor-
dinated nitrogens are trans and so are the two sulfurs. Formation
of the five-membered chelate ring (IV) by the 4-R-benzaldehyde
H N
2
N
S
N
H
thiosemicarbazones, even upon their reaction with Na [PdCl ],
2
4
Pd
clearly indicates that the steric bulk of the PPh ligands in the palla-
3
H
dium starting material, viz. trans-[Pd(PPh ) Cl ], had nothing to do
3
2
2
with the observed mode of coordination of the thiosemicarbazones
in the 1-R complexes. The observed bond parameters (Table 2) in
N
S
N
2
1
-NO2 compare well with those found in the Pd(N–S) fragment of
-NO . In the crystal lattice of 2-NO there are two molecules of
NH2
2
2
N,N-dimethylformamide per molecule of 2-NO , and the packing
2
pattern in the lattice (Fig. S1, Supplementary material) shows that
these solvent molecules have occupied strategic locations for effec-
tive hydrogen-bonding with the complex molecules. It is revealed
on zooming into the network that each N,N-dimethylformamide
molecule is hydrogen-bonded to two complex molecules, and these
R
2-R
Again, to ensure that in all the 2-R complexes the thiosemicar-
bazones are truly coordinated to palladium in the same fashion

6
8
P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
Fig. 4. Structure of 1-NO2.
(
IV), structure of another complex of this family, viz. 2-OCH3,
were obtained similarly and they show similar properties (vide
has been determined by X-ray crystallography. All the structural
features of this complex (Fig. 6, Table 2) are found to be sim-
infra), the other three 2-R (R = CH , H and Cl) complexes are
assumed to have the same structure as their R = NO2 or OCH3
analogue.
3
ilar to those of 2-NO , as expected. As all the 2-R complexes
2
Fig. 5. Structure of 2-OCH3.
Fig. 6. Structure of 2-NO2.

P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
69
Table 3
Table 4
Electronic spectral data of the complexes.
Composition of selected molecular orbitals of the complexes.
Complex
ꢁmax, nm (ε, dm3 mol cm
−1
−1
)
Complex
Contributing
fragments
%Contribution of
fragments to
1
1
1
1
1
2
2
2
2
2
-OCH3a
350(17,000), 306(14,400)
347(14,400), 300(16,600)
347(15,400), 297(19,300)
353(15,400), 302(19,100)
395(12,600), 338(11,300)
390(32,500), 330(35,700)
392(33,200), 320(42,800)
394(30,100), 316(43,800)
400(32,800), 326(42,300)
428(23,700), 390(25,200)
a
-CH3
HOMO
LUMO
a
-H
a
Pd
5.44
0
3.18
91.38
12.99
0
7.07
79.94
27.45
72.55
17.35
82.65
23.59
7.29
2.06
67.06
0.34
0
-Cl
-NO2a
PPh3
Cl
tsc-CH3
1-CH3
b
-OCH3
a
b
-CH3
b
Pd
-H
b
PPh3
Cl
tsc-NO2
-Cl
1-NO2
-NO2b
0
a
99.66
0.77
99.23
0
a
In dichloromethane solution.
In N,N-dimethylformamide solution.
Pd
tsc-OCH3
b
2-OCH3
a
Pd
tsc-NO2
2
-NO2
a
100.00
3
.2. Spectral properties
a
tsc-R = coordinated thiosemicarbazone.
1H NMR spectra of the 1-R complexes, recorded in CDCl3 solu-
tions, show all the expected signals. The phenyl protons of the PPh3
ligands show broad signals within 7.3–7.8 ppm. From the coordi-
nated thiosemicarbazone, the azomethine proton signal is observed
as a doublet, due to coupling with the proton at the ortho position
of the phenyl ring, within 8.5–8.7 ppm and the NH2 proton signal
is observed within 4.7–6.1 ppm. Most of the expected signals from
the phenyl ring of the coordinated thiosemicarbazone are clearly
observed in the aromatic region, while few could not be detected
bazone orbital (HOMO) to a vacant orbital on the same ligand
(LUMO).
The 2-R complexes are found to be insoluble in most
of the common organic solvents but readily soluble in N,N-
dimethylformamide and dimethylsulfoxide, producing intense
orange solutions [16]. Electronic spectra of these complexes in
N,N-dimethylformamide solutions show two intense absorptions
due to their overlap with other signals. Signals for the OCH3 and
1
CH substituents are observed respectively at 3.85 and 2.37 ppm. H
3
NMR spectra of the 2-R complexes, recorded in DMSO-d solutions,
6
show the azomethine proton signal within 8.2–8.6 ppm, the NH2
proton signal within 7.2–7.7 ppm and most of the expected aro-
matic proton signals are observed in the 6.9–8.6 ppm region. Signals
for the OCH3 and CH3 substituents are observed at 3.83 ppm and
2
.36 ppm respectively. In the infrared spectra of the 1-R complexes,
−
1
three strong bands near 520, 695 and 745 cm are observed due to
the coordinated PPh3 ligands. In these spectra several new bands
(
in comparison with the spectrum of the trans-[Pd(PPh ) Cl ]
3 2 2
−
1
complex) could be identified within 800–1600 cm , which are
attributable to the coordinated thiosemicarbazone ligands. Except
absence of the three bands due to the PPh3 ligands and small shifts
in the positions of the remaining bands, infrared spectra of the 2-R
complexes have been found to be similar to those of the respec-
tive 1-R complexes. The 1H NMR and infrared spectral data of the
1
-R and 2-R complexes are therefore in well accordance with their
compositions.
The 1-R complexes are soluble in common organic solvents like
methanol, ethanol, acetone, acetonitrile, dichloromethane, chlo-
roform, etc., producing intense yellow solutions [15]. Electronic
spectra of these complexes have been recorded in dichloromethane
solutions. Spectral data are presented in Table 3. Each com-
plex shows two intense absorptions within 400–250 nm. In
comparison with the spectra of the uncoordinated thiosemicar-
bazones L-R, which show an intense absorption near 300 nm, the
higher energy absorption is attributable to a transition within
the thiosemicarbazone orbitals. To have an understanding of
the origin of the lower-energy absorption (near 350 nm), DFT
calculations have been performed on the two structurally charac-
terized 1-R complexes, viz. 1-CH3 and 1-NO2 [10]. Compositions
of the highest occupied molecular orbital (HOMO) and the low-
est unoccupied molecular orbital (LUMO) are given in Table 4.
Contour plots of these molecular orbitals for 1-CH3 are shown
in Fig. 7 and the same for 1-NO2 are shown in Fig. S2 (Sup-
plementary material). In both 1-CH3 and 1-NO , the HOMO and
2
LUMO have major contribution from the thiosemicarbazone and
minor contribution from palladium. The lower energy absorption
is hence assignable to a transition from a filled thiosemicar-
Fig. 7. Contour plots of HOMO and LUMO of 1-CH3.

7
0
P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
within 500–250 nm (Table 3). The higher energy absorption in these
-R complexes near 320 nm is attributable, as before, to a transi-
tion within the thiosemicarbazone orbitals. DFT calculations on the
The observed ease of these catalytic coupling reactions,
with particular reference to low catalyst loading and relatively
low reaction temperature, prompted us to examine whether
the same coupling reaction of phenylboronic acid with p-
bromoacetophenone, using 1-OCH3 as the catalyst, can be carried
out at room temperature. The attempted reaction indeed proceeded
2
two structurally characterized 2-R complexes, viz. 2-OCH3 and 2-
NO [10], show (Figs. S3 and S4 (Supplementary material), Table 4)
2
that the HOMO has >70% contribution from the two coordinated
thiosemicarbazones, while the LUMO is almost entirely delocal-
ized over the same two ligands. Hence the observed lower-energy
absorption near 400 nm is attributable to a transition within the
filled and vacant orbitals of the thiosemicarbazone ligands.
◦
smoothly at 25 C showing high yield and turnover number as
before (entry 5 and Table 5). However, the time required for the
complete conversion was significantly higher (20 h). It is relevant
to mention here that reports of such Suzuki coupling at room tem-
perature are scarce in the literature [17]. Encouraged by this room
temperature Suzuki coupling, we got curious to see whether such
coupling can be brought about using water as a solvent. The cou-
pling did take place in aqueous medium, however the observed
catalytic efficiency was much less (entry 6 in Table 5). Though
according to the modern trend, use of water as solvent is very much
welcome, with particular reference to green chemistry, Suzuki cou-
pling in water has been rarely observed [18].
While 2-OCH3 is used as a catalyst in the Suzuki coupling
of phenylboronic acid and p-bromoacetophenone, the observed
turnover number was quite lower (entry 7 in Table 5) than that of
the reaction where 1-OCH3 is used as a catalyst (entry 1 in Table 5).
Using 2-NO2 for the same Suzuki coupling reaction almost similar
(compared to 2-OCH3) turnover number and yield were obtained
(entry 8 in Table 5). For Suzuki coupling of phenylboronic acid with
the other two aryl bromides using 2-OCH3 as the catalyst slightly
longer reaction time was needed for optimum yield of the product
(entries 9 and 10 in Table 5).
Encouraged by the facile Suzuki coupling reactions catalyzed
by the 1-R and 2-R complexes, we have further investigated the
catalytic activity of the same palladium complexes in Heck reac-
tion of different aryl bromides with butyl acrylate. All the members
from each family showed comparable reactivity, and the optimized
results for two representative complexes, viz. 1-OCH3 and 2-OCH3,
are shown in Table 6. A fairly good yield of 72% was obtained when
employing p-bromoacetophenone in the presence of 0.5 mol% of 1-
OCH3 (entry 1). A decrease in activity was observed in the cases
of p-bromobenzaldehyde (yield = 65%) and p-bromobenzonitrile
(yield = 57%) (entries 2 and 3). Whereas, for 2-OCH3 to obtain fairly
good yield higher catalyst loading (1 mol%) and higher temperature
but shorter time were required (entries 4–6).
3.3. Catalysis
The fact that palladium complexes are well known to serve as
efficient catalysts in bringing about C–C cross coupling reactions
of different types [3] has led us to explore such catalytic proper-
ties in the present series of 1-R and 2-R complexes. The catalytic
activity of all the five complexes of the two series 1-R and 2-R has
been examined for C–C bond formation reactions of three types,
viz. Suzuki, Heck and Sonogashira reactions.
Initially all the 1-R complexes were tested as catalysts in the
Suzuki coupling of phenylboronic acid and p-bromoacetophenone
to yield the biphenyl product. All these complexes showed remark-
able, as well as comparable, catalytic efficiency. For example,
using 1-OCH3 as catalyst the Suzuki coupling was achieved, under
relatively mild experimental condition, in high yield as well as
remarkably high turnover number (entry 1 in Table 5). The same
coupling reaction, when carried out using 1-NO2 as the cata-
lyst, showed slightly lower yield and turnover number (entry
2
in Table 5). The results of catalysis using 1-OCH3 and 1-NO2
are highlighted here as representative cases, because among the
five substituents (R = OCH , CH , H, Cl and NO ) used, OCH has
3
3
2
3
maximum electron donating power while NO has maximum elec-
2
tron withdrawing power. Though there are definite differences in
catalytic efficiency among the individual 1-R complexes, the mag-
nitude of difference is relatively small. In general, higher reaction
time and lower yield are observed with increase in electron with-
drawing character of R. For Suzuki coupling of phenylboronic acid
with the other two aryl bromides, viz. p-bromobenzaldehyde and
p-bromobenzonitrile, using 1-OCH3 as catalyst longer times were
required (entries 3 and 4 in Table 5) to achieve similar yield (as in
entry 1 in Table 5).
Finally we have scrutinized the catalytic efficiency of the 1-R
and 2-R complexes in Sonogashira coupling reaction between aryl
Table 5
Suzuki cross coupling of aryl halides with phenylboronic acid.a
Br
B(OH)2
1
-R or 2-R
+
NaOH,
solvent
R
1
R
1
.
◦
b
TONc
Entry
R1
Catalyst
Solvent
Temp,
C
Time, h
Amt of cat., mol%
Conversion , %
1
2
3
4
5
6
7
8
9
0
COCH3
COCH3
CHO
1-OCH3
1-NO2
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Water
Polyethylene glycol
Polyethylene glycol
Polyethylene glycol
Polyethylene glycol
95
95
95
95
25
100
110
110
110
110
9
12
12
12
20
12
12
12
13
15
0.001
0.001
0.001
0.001
0.001
1
0.01
0.01
0.01
0.01
>99
100,000
88,000
100,000
100,000
100,000
100
8800
8500
8100
8000
88
>99
>99
>99
>99
88
85
81
80
1-OCH3
1-OCH3
1-OCH3
1-OCH3
2-OCH3
2-NO2
CN
COCH3
COCH3
COCH3
COCH3
CHO
2-OCH3
2-OCH3
1
CN
a
b
c
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), NaOH (1.7 mmol), Pd catalyst, solvent (4 mL).
Determined by 1H NMR on the basis of residual aryl halide [11].
TON = turnover number ((mol of product)/(mol of catalyst)).

P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
71
Table 6
Heck cross coupling of aryl halides with butyl acrylate.
a
1
-R or 2-R
Cs2CO 3,
solvent
R1
Br
+
CO 2Bu
R1
CO 2Bu
.
◦
b
TONc
Entry
R1
Catalyst
Solvent
Temp,
C
Time, h
Amt of cat., mol%
Conversion , %
1
2
3
4
5
6
COCH3
CHO
CN
COCH3
CHO
CN
1-OCH3
1-OCH3
1-OCH3
2-OCH3
2-OCH3
2-OCH3
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Polyethylene glycol
Polyethylene glycol
Polyethylene glycol
110
110
110
150
150
150
24
48
48
12
12
14
0.5
0.5
0.5
1
1
1
72
65
57
80
78
62
144
130
114
80
78
62
a
b
c
◦
Reaction conditions: aryl halide (1.0 mmol), butyl acrylate (1.2 mmol), Cs2CO3 (1.7 mmol), Pd catalyst, solvent (4 mL), 150 C.
Determined by 1H NMR on the basis of residual aryl halide [11].
TON = turnover number ((mol of product)/(mol of catalyst)).
Table 7
a
Sonogashira cross coupling of aryl halides with phenylacetylene.
Br
1
-R or 2-R, CuI
+
H
Ph
R1
Ph
Solvent,
NaOH
R
1
.
◦
b
TONc
Entry
R1
Catalyst
Solvent
Temp,
C
Time, h
Amt of cat., mol%
Conversion , %
1
2
3
4
5
6
COCH3
CHO
CN
COCH3
CHO
CN
1-OCH3
1-OCH3
1-OCH3
2-OCH3
2-OCH3
2-OCH3
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Ethanol–toluene (1:1)
Polyethylene glycol
Polyethylene glycol
Polyethylene glycol
75
75
75
110
110
110
10
12
12
12
14
15
0.5
0.5
0.5
0.5
0.5
0.5
>99
76
74
>99
71
68
200
152
148
200
142
136
a
b
c
Reaction conditions: aryl halide (1.0 mmol), phenylacetylene (1.2 mmol), NaOH (1.7 mmol), Pd catalyst, CuI (10 mol%), solvent (4 mL).
Determined by 1H NMR on the basis of residual aryl halide [11].
TON = turnover number ((mol of product)/(mol of catalyst)).
bromides and phenylacetylene. As before, all the members from
each family showed comparable reactivity, and the results of 1-
OCH3 and 2-OCH3 are presented in Table 7 as representatives of
the substituents in the thiosemicarbazone ligand seemed to have
no predictable influence on the catalytic activities of the complexes.
the two families. Fairly good yields were obtained using 1-OCH
3
4
. Conclusions
◦
as catalyst (0.5 mol%) at 75 C (entries 1–3), whereas for 2-OCH
3
◦
(
0.5 mol%) a little higher temperature (110 C) was required (entries
The present study shows that the 4-R-benzaldehyde thiosemi-
4
–6).
The present studies thus demonstrate that palladium thiosemi-
carbazones (L-R) can readily bind to palladium and display a rather
uncommon coordination mode (IV) involving a restricted rotation
around the imine bond. The 1-R and 2-R complexes are also found
to be efficient catalyst for C–C coupling (viz. Suzuki, Heck and Sono-
gashira) reactions. In particular, the 1-R complexes have turned
out to be tremendously powerful catalysts towards Suzuki coupling
reaction.
carbazone complexes of both types 1-R and 2-R are efficient
catalysts for C–C coupling reactions. However, the 1-R complexes
are, in general, found to be much better catalysts for these cou-
pling reactions, especially for the Suzuki coupling. For example,
5
the high turnover numbers (∼10 ) exhibited by 1-OCH for the
3
coupling of aryl bromides with phenylboronic acid are substan-
tially greater than different reported palladium pre-catalysts [19].
Another noticeable aspect of the observed catalysis by these palla-
dium complexes 1-R and 2-R is that no additional ligands were
necessary for the cross-coupling reactions, and such ligand-free
catalysis is relatively less common [20]. 1-R not only exhibited
Suzuki coupling of very high turnover number even at room tem-
perature, but also exhibited Suzuki coupling in water with almost
Acknowledgements
Financial assistance received from the Department of Science
and Technology, New Delhi [Grant No. SR/S1/IC-29/2009] is grate-
fully acknowledged. The authors thank Dr. Kajal K. Rajak and Mr.
Dipravath Kumar Seth for their kind help with the DFT studies.
Thanks are also due to Dr. Subhas Samanta, Department of Inor-
ganic Chemistry, Indian Association for the Cultivation of Science,
Kolkata 700 032, for his help with the crystal structure determina-
tions. Michael G.B. Drew thanks EPSRC (UK) and the University of
Reading for funds for the Image Plate System. Piyali Paul and Sayanti
Datta thank the Council of Scientific and Industrial Research, New
Delhi, for their fellowship [Grant No. 09/096(0588)/2009-EMR-I
and 09/096(0563)/2008-EMR-I respectively].
1
00% yield. As in these cross-coupling reactions water can be used
as a solvent (or co-solvent) these cross-coupling reactions have
gained prominence in recent years at an industrial level, mainly
in the synthesis of pharmaceuticals and fine chemicals. The better
catalytic activity of the 1-R complexes, compared to their respec-
tive 2-R analogues, may be attributed to the presence of PPh3
ligand in the former complexes, which probably have supported
the palladium(0) state, generated in situ. The electronic nature of

7
2
P. Paul et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 62–73
Appendix A. Supplementary data
tner, R. Subashkumar, K. Natarajan, Eur. J. Med. Chem. 43 (2008) 268–273;
m) A.G. Quiroga, L. Cubo, P.J. Sanz Miguel, V. Moneo, A. Carnero, C. Navarro-
Ranninger, Eur. J. Inorg. Chem. (2008) 1183–1187;
n) R.V. Singh, M.K. Biyala, K. Sharma, M. Swami, N. Fahmi, Transit. Met. Chem.
33 (2008) 377–381;
o) L. Otero, C. Folch, G. Barriga, C. Rigol, L. Opazo, M. Vieites, D. Gambino,
H. Cerecetto, E. Norambuena, C. Olea-Azar, Spectrochim. Acta 70A (2008)
19–523;
(
CCDC 280187, 280188, 280189, 781792 and 781793 contain the
supplementary crystallographic data for this paper. Packing dia-
(
(
gram in the lattice of 2-NO ·2DMF (Fig. S1), contour plots of HOMO
2
and LUMO of 1-NO , 2-OCH and 2-NO (Figs. S2–S4) have been
2
3
2
5
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