Square-planar Ni(II) thiosemicarbazonato complex as an easily accessible and convenient catalyst for Sonogashira cross-coupling reaction

Article abstract of DOI:10.1016/j.tetlet.2016.09.049

A simple route for the synthesis of a new square-planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-N-phenylhydrazinecarbothioamide ligand has been described. The composition of the complex has been established by elemental analysis, F

Full text of DOI:10.1016/j.tetlet.2016.09.049

Accepted Manuscript
Square-planar Ni(II) thiosemicarbazonato complex as an easily accessible and
convenient catalyst for Sonogashira cross-coupling reaction
Rupesh Narayana Prabhu, Rengan Ramesh
PII:
S0040-4039(16)31207-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.09.049
TETL 48119
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
29 August 2016
12 September 2016
14 September 2016
Please cite this article as: Prabhu, R.N., Ramesh, R., Square-planar Ni(II) thiosemicarbazonato complex as an easily
accessible and convenient catalyst for Sonogashira cross-coupling reaction, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.09.049
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Graphical Abstract
A new square planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-N-phenylhydrazinecarbothioamide
was synthesized and characterized by elemental analysis, FT-IR, NMR and single crystal X-ray crystallographic technique.
The complex acts as an effective homogeneous catalyst for the Sonogashira reaction of phenylacetylene with various aryl
halides (iodides and bromides) under optimized conditions.
Leave this area blank for abstract info.
Square-planar Ni(II) thiosemicarbazonato complex as an easily accessible
and convenient catalyst for Sonogashira cross-coupling reaction
Rupesh Narayana Prabhu, Rengan Ramesh*
H
S
Complex 1
CuI, DMF
H
N
N
S
S
N
+
X
Ni
Et
3
N, 80 o
C
N
R
R
N
H
N
X = I, Br
S
H
1

1
Tetrahedron Letters
journal homepage: www.els evier.com
Square-planar Ni(II) thiosemicarbazonato complex as an easily accessible and
convenient catalyst for Sonogashira cross-coupling reaction
§
‡
Rupesh Narayana Prabhu , Rengan Ramesh ∗
§
Department of Chemistry, Srimad Andavan Arts and Science College, Tiruchirappalli 620 005, Tamil Nadu, India
Centre for Organometallic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
‡
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple route for the synthesis of a new square-planar nickel(II) complex bearing 2-((3-
methylthiophen-2-yl)methylene)-N-phenylhydrazinecarbothioamide ligand has been described.
The composition of the complex has been established by elemental analysis, FT-IR, NMR and
single crystal X-ray crystallographic technique. The complex acts as an active homogeneous
catalyst for the Sonogashira reaction of phenylacetylene with electron deficient (activating) and
electron rich (deactivating) aryl halides (iodides and bromides) under optimized conditions.
Available online
Keywords:
2
009 Elsevier Ltd. All rights reserved.
Thiosemicarbazone ligand
Nickel(II) complex
Crystal structure
Sonogashira reaction
nickel(II) thiosemicarbazonato complexes have been developed
10
as effective catalysts for various cross-coupling reactions, there
is still ample scope for developing new air-stable nickel(II)
thiosemicarbazonato complexes that can act as efficient catalysts
for the Sonogashira reaction of the highly reactive aryl iodides,
moderately reactive aryl bromides and the more challenging aryl
chlorides. In addition, to the best of our knowledge the use of
square planar nickel(II) thiosemicarbazonato complex as catalyst
for the Sonogashira coupling reaction has not been explored so
far.
Diaryl acetylenes play an important role as intermediates for
1
2
substituted alkynes, dendrimers, optical and electrical
3
4
materials, intermediates in natural products, pharmaceuticals,
5
conjugated oligomers and polymers, etc. Among the various
cross-coupling reactions, palladium catalyzed Sonogashira
reaction of aryl halides with phenylacetylenes has emerged as a
practical and efficient method for the preparation of diaryl
6
acetylenes. Though palladium complexes show high catalytic
activities, one obvious drawback is the prohibitive cost of
palladium which has resulted in the exploration of less costly
transition metals as alternatives to palladium systems for cross-
coupling reactions while still preserving high levels of efficiency
and functional group tolerance. Nickel appears to be the most
promising alternative for palladium as catalysts in Sonogashira
In continuation of our research on the synthesis, structural
characterization and catalytic applications of transition metal
11
complexes containing multidentate ligands, herein, we have
chosen a new square-planar nickel(II) thiosemicarbazonato
complex to assess the suitability of the complex as a
homogeneous catalyst in the synthesis of substituted diaryl
acetylenes by the Sonogashira reaction of phenylacetylene with
aryl halides (iodides and bromides) containing different
electronic effect. The Schiff base, 2-((3-methylthiophen-2-
yl)methylene)-N-phenylhydrazinecarbothioamide (HL, where H
represents the dissociable proton) was prepared by the
condensation of 4-phenyl-3-thiosemicarbazide with 3-methyl-
7
coupling reaction. The low cost, low toxicity and high reactivity
of nickel is attractive, and a range of substrates has been shown
to undergo nickel–catalyzed C–C coupling reactions, although so
far no nickel catalyst has been able to rival palladium for the
Sonogashira coupling reaction.
Thiosemicarbazones are unique and multifaceted N- and S-
donor Schiff bases for transition metal ions that possess synthetic
flexibility and exhibit a wide range of coordination modes in
12
thiophene-2-carboxaldehyde. The new neutral square-planar
8
nickel(II) complex, [Ni(L)
by the reaction of Ni(OAc)
2
] (1), was synthesized in ~85% yield
⋅4H O and HL in 1:2 ratio in ethanol-
their metal complexes. Though there are several reports related
2
2
to the biological applications of thiosemicarbazone complexes of
9
dichloromethane at room temperature (Scheme 1). The complex
is sparingly soluble in methanol, ethanol, chloroform,
dichloromethane or acetonitrile and is readily soluble only in
nickel, research into their role in catalysis however, is still a
relatively new area of interest. In recent years, though a few
—
——
∗
Corresponding author. Tel.: +91-431-2407053; fax: +91-431-2407045; e-mail: ramesh_bdu@yahoo.com (R. Ramesh)

2
Tetrahedron Letters
solvents such as dimethylformamide, dimethyl sulphoxide etc.,
The molecular structure of the complex 1 has been determined
by single crystal X-ray diffraction to confirm the coordination
mode of the ligand and geometry of the complex. The ORTEP
view of 1 is shown in Figure 1. The nickel(II) ion is
tetracoordinated in a square-planar geometry by two ligand
molecules, each acting as monoanionic bidentate N,S-donor. The
ligands are in trans position with respect to the azomethine
nitrogen and the complex is centrosymmetric around the nickel
centre. The N(2)−C(7) and C(7)−S(2) bond lengths of 1.303(3)
and 1.735(2) Å respectively are consistent with the deprotonation
of the thioamide functionality in the coordinated
producing intense brown solutions. The complex is electrically
non-conductive in solution which is consistent with its molecular
formula as neutral species. The complex is non-hygroscopic and
air stable in both the solid and the liquid states at room
temperature. The observed elemental analysis data of 1 (Calc. C,
5
1.41; H, 3.98; N, 13.83; S, 21.11%. Found: C, 51.32; H, 4.06;
N, 13.80; S, 21.07%) is consistent with its composition and it
appears from the formulation that each Schiff base coordinate to
the nickel(II) ion in a monobasic bidentate fashion.
10a,15
thiosemicarbazonate ligand.
The bond lengths and bond
angles are in good agreement with those found in related
structurally
thiosemicarbazonato complexes.
characterized 0a,15square-planar
nickel(II)
1
H
H
S
N
S
H
N
2
HN
Ni(OAc)2..4H2O
N
S
S
N
S
NH
Ni
EtOH-CH2Cl2
RT, 3 h
N
a
N
H
N
Table 1. Optimization of reaction conditions
S
H
COCH3
HL
1
Complex 1
COCH3
Scheme 1. Synthesis of Ni(II) thiosemicarbazonato complex
+
Solvent, base
I
-
1
In the FT–IR spectroscopy, the νC=S (842 cm ) and νN–H (3206
b
Entry
Solvent
Base
Temp. (°C)
Yield (%)
–
1
cm ) of the N–NH–C=S group present in the free HL
disappeared in suggesting enolization and subsequent
coordination through the thioamidate sulfur to the Ni(II) ion. The
C=N absorption is observed at a lower frequency in the 1 (1583
cm ) when compared to that of the free HL (1605 cm ),
indicating the coordination of azomethine-N to the metal
1
2
3
4
5
6
7
8
Toluene
Acetone
THF
MeCN
DMSO
DMF
DMF
DMF
DMF
DMF
Et
Et
3
3
N
N
110
55
65
80
80
80
80
80
80
80
50
30
80
80
<20
<20
<20
54
80
>99
33
<20
50
61
1
3
Et N
ν
Et
Et
Et
3
3
3
N
N
N
–
1
−1
1
0a,13
1
NaOAc
KOH
NaHCO3
centre.
In the H NMR spectra, the signal due to the
azomethine proton in 1 (δ 9.1 ppm) is slightly downfield when
compared to that of the free Schiff base (δ 8.2 ppm), suggesting
deshielding on coordination of the azomethine-N to Ni(II) ion.
The singlet that appeared for the N–NH–C=S proton of the free
HL (δ 11.4 ppm) is absent in 1, supporting enolisation and
9
10
K
2
CO
3
1
1
1
1
1
2
3
4
DMF
DMF
DMF
DMF
Et
Et
Et
Et
3
N
N
N
N
90
3
3
3
40 c
N.R.
d
<20
1
0a,14
coordination of the thioamidate-S to the metal centre.
In 1,
a
4
'-Iodoacetophenone (1.0 mmol), phenylacetylene (1.5 mmol), 1 (121.5
atm, 2 h.
the multiplets observed in the region δ 7.6-6.9 ppm are assigned
to the aromatic protons (phenyl and thiophenyl ring) of the
coordinated ligand. The singlet at δ 2.3 ppm is due to the
resonance of the methyl protons of the thiophenyl ring.
ppm), CuI (5 µmol), Solvent (5 ml), Base (3.0 mmol), N
2
b
c
d
Isolated yield after column chromatography.
Without 1, for 6 h. N.R. = No reaction.
Without CuI, for 6 h.
.
In recent years, nickel complexes have attracted immense
attention as catalysts for various carbon-carbon cross-coupling
reactions and this prompted us to explore the catalytic efficacy of
1
in the Sonogashira cross-coupling reaction. The initial tests
involved the optimization of solvent, base and reaction
temperature. The reaction between 4'-iodoacetophenone and
phenylacetylene to furnish 1-(4-(phenylethynyl)phenyl)ethanone
in the presence of 1 as catalyst and CuI as cocatalyst was first
selected and various reaction conditions such as solvent, base and
temperature were sequentially varied to ascertain the optimum
reaction condition for this coupling reaction (Table 1). In the
beginning, various solvents were screened (entries 1-6). From the
inspection of the obtained results, it can be inferred that the
reaction proceeded relatively well in polar solvents and DMF
was found to be the solvent of choice (entry 6) with excellent
isolated yield of the expected internal olefin. In DMF, significant
sensitivity to base was also noted. Among the various bases
Figure 1. ORTEP of 1 with 50% probability. The solvent
screened (entries 6-10), Et N gave excellent results. Inorganic
3
(
9
DMF) is omitted for clarity. Selected bond lengths (Å): Ni−Ni
050(17), Ni−S2 2.1720(9), N1−N2 1.391(2), N2−C7 1.303(3),
S2−C7 1.735(2); Selected bond angles (°): N1−Ni−N1a
bases such as NaOAc, KOH, NaHCO or K CO resulted in much
3
2
3
lower yields under the studied conditions. In DMF-Et N,
3
excellent isolated yield of the desired product was obtained at 80
1
9
80.000(1), S2−Ni−S2a 180.0, N1−Ni−S2 85.17(5), Ni−S2−C7
6.03(7), Ni−N1−N2 121.23(12), N1−N2−C7 112.04(16),
°C (entry 6). Decrease in temperature, resulted in reduced yield
of the product (entries 11,12). Hence, further reactions were
N2−C7−S2 122.64(15).
carried out using DMF as solvent and Et N as base at 80 °C. In
3

3
addition, control experiments revealed that the reaction
completely shut down without the addition of 1 (entry 13),
whereas the yield fell to below 20% if CuI was not added (entry
present on the ortho position (entry 10), a slightly reduced
reactivity than the corresponding para substituted derivative
(entry 6) was observed which may be due to steric effect.
1
4).
Under the above optimized conditions the coupling reaction
was then performed at different catalyst loadings in order to
discover the effectiveness of 1 (Table 2). Good isolated yields of
desired coupling product were obtained when 121.50 or 60.75
ppm of 1 (entries 1,2) was used. The yield of the product
significantly dropped when a lower catalyst loading of 12.15 or
Table 3. Sonogashira reaction of aryl iodides with
phenylacetylene
a
6
.08 ppm were used (entries 3,4). Interestingly, the coupling
Complex 1
CuI, DMF
reaction can be carried out even with a very low catalyst loading
of 0.12 ppm (entry 5) with high turnover number (TON). The
isolated yields are good with appreciable TON when 60.75 ppm
of the catalyst was used and consequently, this catalyst loading
was used for further studies.
+
I
Et3N, 2 h
R
R
b
c
Entry
1
Product
Yield (%)
TON
99
98
96
94
91
89
87
85
86
1980
NO
2
2
3
4
5
6
7
8
9
1960
1920
1880
1820
1780
1740
1700
1720
COCH
3
a
Table 2. Effect of catalyst loading
CHO
H
COCH3
Complex 1
COCH3
+
CuI, DMF
Et3N, 80 o
C
I
CH
3
b
c
Entry
1 (ppm)
Yield (%)
TON
OCH
3
1
2
3
4
5
121.50
60.75
12.15
6.08
>99
98
990
OH
1960
8700
12600
330000
87
NH
2
63
0.12
33
a
4
'-Iodoacetophenone (1.0 mmol), phenylacetylene (1.5 mmol), 1 (121.50-
.12 ppm), CuI (5 µmol), DMF (5 ml), Et N (3.0 mmol), 80 °C, N atm, 2 h.
Isolated yield after column chromatography.
OCH3
0
3
2
b
10
83
1660
c
TON = Turnover number = ratio of moles of product formed to moles of
catalyst used.
H CO
3
a
Aryl iodide (1.0 mmol), phenylacetylene (1.5 mmol), 1 (60.75 ppm), CuI (5
N (3.0 mmol), 80 °C, N atm, 2 h.
Isolated yield after column chromatography.
µmol), DMF (5 ml), Et
3
2
b
c
After establishing the optimal reaction conditions, the
Sonogashira coupling reaction of phenylacetylene with activated
electron-withdrawing substituents), unsubstituted (electron-
TON = Turnover number = ratio of moles of product formed to moles of
catalyst used.
(
neutral) and deactivated (electron-donating substituents) aryl
iodides was investigated to examine the electronic and steric
effects of aryl iodides on the efficiency of the catalytic reaction
(
Table 3). All the reactions were carried out under identical
Encouraged by the facile Sonogashira reactions of various aryl
conditions to allow comparison of results. It was inferred that
aryl iodides with varying electronic nature of the substituents
coupled smoothly with phenylacetylene and the corresponding
internal alkynes were obtained in good to excellent yields. In
general, it was observed that aryl iodides containing electron-
withdrawing groups (4-nitro, 4-acetyl, 4-formyl) coupled
effectively with phenylacetylene (entries 1-3) and the
corresponding products were obtained in excellent isolated yield.
Iodobenzene (entry 4) and aryl iodides containing electron-
donating groups (4-methyl, 4-methoxy, 4-hydroxy, 4-amino)
iodides with phenylacetylene catalyzed by 1 in presence of CuI,
the catalytic efficacy of 1 in the coupling of phenylacetylene with
aryl bromides was then evaluated (Table 4). As observed in the
case of aryl iodides, the electronic nature and the steric effect of
the substituents on the aryl ring affected the yield of the products.
However, it was observed that when compared to the iodo
analogue, the coupling reactions of the bromo derivative was
slower and a higher catalyst loading (121.50 ppm) as well as
longer reaction time (8 h) were required to obtain good to
excellent isolated yield of the product. The stronger C–X bond in
the aryl bromide than in the aryl iodide is expected to be the main
reason for the variation of their reactivities. The ultimate
objective in designing the catalyst for Sonogashira coupling
reaction is to form an active species that can catalyze the
(
entries 5-8) gave good amount of the desired product when
coupling with the phenylacetylene. Aryl iodides with methoxy
substituent on the meta position (entriy 9) underwent coupling
reaction efficiently, whereas when the methoxy substituent was

4
Tetrahedron Letters
coupling of the more difficult aryl chlorides. However, no
phenylhydrazinecarbothioamide ligand has been synthesized and
characterized by elemental analysis and spectral (FT-IR, NMR)
methods. Single crystal X–ray diffraction study evidenced the
coordination of each thiosemicarbazonate ligand through the
azomethine-N and the thioamidate-S atoms and reveals the
presence of a distorted square planar geometry around the Ni(II)
ion. The utility of the new complex as excellent catalyst for the
Sonogashira reaction has been highlighted by the coupling
reaction of activating, neutral and deactivating aryl halides
(iodides and bromides) with phenylacetylene under the optimized
conditions of solvent, base, reaction temperature and catalyst
loading. When compared with the aryl iodides, the reaction was
slower in the case of the corresponding aryl bromo analogues,
consistent with the strength of the C-X bond in Ar-I and Ar-Br.
The desired products were obtained in good to excellent isolated
yields.
catalytic activity was observed in the coupling of 4'-
chloroacetophenone with phenylacetylene even after using larger
amount of 1 (2400 ppm) in DMF/Et N after 24 h. This trend is
3
consistent with the increasing strength of carbon–halide bonds
from aryl bromide to aryl chloride. Though nickel(II)
thiosemicarbazone complexes have been used earlier as effective
catalysts for the Sonogashira reaction, a direct comparison with
the present nickel(II) thiosemicarbazonato complex (1) is
difficult due to the difference in the reaction conditions such as
solvents, base, reaction temperature, reaction time and catalyst
loading.
1
0
Table 4. Sonogashira reaction of aryl bromides with
phenylacetylene
a
Complex 1
CuI, DMF
Acknowledgments
+
Br
Et3N, 8 h
R
R
The use of the single crystal diffractometer facility at the
School of Chemistry, University of Hyderabad, Telangana, India,
is gratefully acknowledged. We also express sincere thanks to
DST-FIST, India for the use of the Bruker 400 MHz
spectrometer at the School of Chemistry, Bharathidasan
University, Tiruchirappalli.
b
c
Entry
1
Product
Yield (%)
TON
99
97
94
91
88
87
85
84
92
990
NO
2
2
3
4
5
6
7
8
9
970
940
910
880
870
850
840
920
COCH
3
CHO
H
Supplementary Material
Crystallographic data for the structural analysis have been
deposited with Cambridge crystallographic center, CCDC No.
8
97455. Copies of this information may be obtained free of
CH
3
charge from The Director, CCDC, 12 Union roads, Cambridge
CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk). The summary
of the data collection and refinement parameters for the complex;
OCH
3
1
1
Experimental procedures; H NMR spectra of the complex; H
OH
13
and C NMR data for all the coupling products.
NH
2
References and notes
CHO
1
2
3
.
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6
1
Tetrahedron Letters
Highlights
. New Ni(II) thiosemicarbazone complex is
synthesised
2
. X-ray structure indicates the presence of a
square-planar geometry
3
. The catalytic activity of the complex is explored
for the Sonogashira reaction