Synthesis, spectral and structural characterization of cobalt(III) dithiocarbamato complexes: Catalytic application for the solvent free enamination reaction

Article abstract of DOI:10.1016/j.poly.2015.10.007

The syntheses, spectral, structural and catalytic properties of some new cobalt(III) complexes, [Co(mpcdt)₃] (1), [Co(ppcdt)₃] (2) and [Co(mppcdt)₃]·0.25CHCl₃ (3), derived from 4-methyl piperazine-1-carbodithioate (mpcdt), 4-phenyl piperazine-1-carbodithioate (ppcdt) and 4-(2-methoxyphenyl) piperazine-1-carbodithioate (mppcdt) have been described. A series of β-enaminoesters and β-enaminones were obtained in about 90% yield by the reactions of β-ketoesters and 1,3-diketones with aliphatic and aromatic amines using 1-2 mol% of the above cobalt(III) complexes as catalysts and these have been characterized by NMR, GC-MS and X-ray crystallography. Complexes 1, 2 and 3 are stabilized by intermolecular C-H?S interactions, leading to the formation of supramolecular architectures. Thermogravimetric analysis of complexes 1 and 2 have been investigated by TG-DTA, which indicate that cobalt sulfide is formed as the final product.

Full text of DOI:10.1016/j.poly.2015.10.007

Accepted Manuscript
Synthesis, spectral and structural characterization of cobalt(III) dithiocarbamato
complexes: catalytic application for the solvent free enamination reaction
Pooja Bharati, A. Bharti, P. Nath, M.K. Bharty, R.J. Butcher, N.K. Singh
PII:
S0277-5387(15)00587-2
DOI:
http://dx.doi.org/10.1016/j.poly.2015.10.007
POLY 11591
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
6 July 2015
3 October 2015
Please cite this article as: P. Bharati, A. Bharti, P. Nath, M.K. Bharty, R.J. Butcher, N.K. Singh, Synthesis, spectral
and structural characterization of cobalt(III) dithiocarbamato complexes: catalytic application for the solvent free
enamination reaction, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.10.007
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Synthesis, spectral and structural characterization of cobalt(III)
dithiocarbamato complexes: catalytic application for the solvent free
enamination reaction
a
b
a
a
c
a
Pooja Bharati , A. Bharti , P.Nath , M.K. Bharty *, R.J. Butcher , N. K. Singh *
a
Department of Chemistry, Banaras Hindu University, Varanasi-221005, India
Department of Chemistry, Kirori Mal College, University of Delhi, Delhi-110007, India
Department of Chemistry, Howard University, 525 College Street NW, Washington, DC20059USA
b
c
ABSTRACT
The syntheses, spectral, structural and catalytic properties of some new cobalt(III) complexes,
[
Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3), derived from 4-methyl
3 3 3 3
piperazine-1-carbodithioate (mpcdt), 4-phenyl piperazine-1-carbodithioate (ppcdt) and 4-(2-
methoxyphenyl) piperazine-1-carbodithioate (mppcdt) have been described. A series of β-
enaminoesters and β-enaminones were obtained in about 90 % yield by the reactions of β-
ketoesters and 1,3-diketones with aliphatic and aromatic amines using 1-2 mole % of the above
cobalt(III) complexes as catalysts and these have been characterized by NMR, GC-MS and X-ray
crystallography. Complexes 1, 2 and 3 are stabilized by intermolecular C-H···S interactions,
leading to the formation of supramolecular architectures. Thermogravimetric analysis of
complexes 1 and 2 have been investigated by TG-DTA, which indicate that cobalt sulfide is
formed as the final product.
Keywords: Dithiocarbamato complexes; Co(III) Catalysts; Solvent free enamination reaction;
Thermal studies; Single crystal X-ray structure.
*
Corresponding authors. Tel.: +91 542 6702447; fax: +91 542 2368127.
E-mail addresses: mkbharty@bhu.ac.in (M.K. Bharty); singhnk_bhu@yahoo.com (N.K.Singh)
1

1
. INTRODUCTION
Transition metal complexes of dithiocarbamates have attracted the attention of workers in
coordination chemistry due to their molecular electrical conductivity, optical and magnetic
properties and applications as single source precursors for the preparation of nano particle metal
sulfides. They also find applications in industry as rubber vulcanization accelerators, flotation
agents in metallurgy, petroleum additives and are of importance in biological processes [1-11].
Dithiocarbamates have a wide range of applications in the fields of medicine, industry and
agriculture and as antioxidants for increasing the longevity and photo-stability of a variety of
polymers, oils and other materials [12]. Several dithiocarbamates and N-substituted
dithiocarbamato complexes and their salts have been used as agrochemicals, mainly due to their
high efficiency in controlling plant fungal diseases with relatively low toxicity [13,14]. We have
explored the possibility of Co(III) dithiocarbamato complexes in chemical organic
transformation reactions as catalysts because cobalt exhibits variable oxidation states, it is less
expensive than other transition metals and is easily handled.
Enamination of 1,3-dicarbonyl and keto-esters to β-enamino ketones and β-enamino
esters are important organic transformations because they are used as intermediates in the
synthesis of heterocyclic compounds [15]. Several homogeneous catalysts such as
Ca(CF COO) , BF :OEt , Zn(ClO ) ·6H O, ceric ammonium nitrate and Bi(OTf) [16] have been
3
2
3
2
4 2
2
3
reported for the enamination of 1,3-dicarbonyl compounds, but they suffer from drawbacks
associated with homogeneous catalysis. The versatility of enaminones is mainly due to their
promptness to both electrophilic and nucleophilic attack [17]. For this reason, they have been
used in the synthesis of various heterocyclic [18,19], natural products [20] and hence several
methodologies have recently been developed for their synthesis [21], representing great
2

achievements compared to the original procedures [22]. The use of solid supports, ionic liquids
and different types of catalysts has been found to be effective. The reactions of open-chain
piperazine-containing ligands with metal ions such as palladium, nickel, copper, cobalt and iron
yield complexes of different coordination geometries, depending on the coordinated metal,
which further act as catalysts [23-27]. However, these catalysts are not reusable and the use of
NaOtBu as a base requires the use of dried toluene and an inert atmosphere. In view of the
above, we have synthesized new cobalt(III) dithiocarbamato complexes and examined their
catalytic behaviour towards the formation of β-enaminoesters/β-enaminones by the reactions of
β-ketoesters/1,3-diketones with aliphatic/aromatic amines. The cobalt(III) dithiocarbamato
complexes used as catalysts and the reaction products obtained by the enamination reactions
involving C-N bond formation have been authenticated and characterized by the crystal
structures of some representative compounds.
2
. Experimental
2
.1. Chemicals and starting materials
Commercial reagents were used without further purification and all experiments were
carried out in an open atmosphere. 1-Methyl piperazine, 1-phenyl piperazine, 4-(2-
methoxyphenyl) piperazine (Sigma Aldrich), CS (SD Fine Chemicals) and KOH (Qualigens)
2
were used as received. The solvents were dried and distilled before use following the standard
procedures.
2
.2. Physical measurements
Carbon, hydrogen, nitrogen and sulfur contents were estimated on a CHN Model CE-440
Analyser and on an Elementar Vario EL III Carlo Erba 1108. The electronic spectra were
recorded on a SHIMADZU 1700 UV-vis spectrophotometer. IR spectra were recorded in the
3

-1
1
4
000-400 cm region as KBr pellets on a Varian Excalibur 3100 FT-IR spectrophotometer. H
1
3
and C NMR spectra were recorded in DMSO-d on a JEOL AL300 FT-NMR spectrometer
6
using TMS as an internal reference. Thermogravimetric-differential thermal analyses (TG-DTA)
were completed on a Perkin Elmer-STA 6000 thermal analyzer at a heating rate of 15 °C/min in
a N atmosphere. Gas chromatogram-mass spectral data were obtained on a quadrupole Perkin
2
Elemer Clarus 500 MS coupled to a Perkin Elemer Clarus 500 GC with Elite-1 column and the
mass detector was operated at 70 eV.
2
.3. X-ray crystallography
The X-ray data collection for complexes 1, 2, 3 and compound 3b were performed on an
Oxford Diffraction Gemini diffractometer equipped with CrysAlis Pro., using a graphite
monochromated Mo Kα (λ = 0.71073 Å) radiation source. The structures were solved by direct
2
methods (SHELXL-2008) and refined against all data by full matrix least-squares on F using
anisotropic displacement parameters for all non-hydrogen atoms. All the hydrogen atoms were
included in the refinement at geometrically ideal positions and refined with a riding model [28].
The MERCURY package and ORTEP-3 programs were used for generating the molecular
structures [29,30].
2
4
.4. Syntheses of potassium 4-methyl piperazine [K(mpcdt)], 4-phenyl piperazine [K(ppcdt)] and
-(2-methoxy phenyl) piperazine [K(mppcdt)]-1 carbodithioates
The potassium salts of 4-methyl piperazine/4-phenyl piperazine/4-(2-methoxy phenyl)
piperazine-1-carbodithioate were prepared by adding CS (1.8 mL, 20 mmol) dropwise to a
2
suspension of 1-methyl piperazine (2.00 mL, 20 mmol), 1-phenyl piperazine (3.26 mL, 20 mmol)
and 1-(2-methoxyphenyl) piperazine (3.55 mL, 20 mmol) in methanol (20 mL) in the presence
of potassium hydroxide (1.2 g, 20 mmol). The reaction mixtures were stirred continuously for 30
4

min under cold conditions and the separated white solids were filtered, washed with EtOH, dried
under reduced pressure and recrystallized from a MeOH-CHCl mixture (50:50 v/v).
3
2
.4.1 [K(mpcdt)]: Yield: 70 %; m.p. 478 K. Anal. Found: C, 33.50; H, 5.15; N, 13.08; S, 29.80
. Anal. Calc. for C H N S K (215.00): C, 33.48; H, 5.11; N, 13.02; S, 29.76 %. IR (KBr, ν
%
6
11
2 2
-1
1
cm ): ν(C-N) 1469; ν(C=S) 970. H NMR (DMSO-d ; δ ppm): 4.38 (s, 3H, CH ), 2.20-3.06 (m,
6
3
1
3
8
H, CH ). C NMR (DMSO-d ; δ ppm): 200.10 (C=S), 154.05 (C-N), 51.80-54.25 (CH ), 45.25
2 6 2
(CH3).
2
.4.2 [K(ppcdt)]: Yield: 70 %; m.p. 475 K. Anal. Found: C, 47.79; H, 4.73; N, 10.11; S, 23.20
. Anal. Calc. for C H N S K (276.42): C, 47.80; H, 4.68; N, 10.12; S, 23.25 %. IR (KBr, ν
%
1
1
13
2 2
-1
1
cm ): ν(C-N) 1415; ν(C=S) 923. H NMR (DMSO-d , δ ppm): 6.85-7.24 (m, 3H, phenyl ring),
6
1
3
3
1
2
.20-4.54 (m, 8H, CH ). C NMR (DMSO-d , δ ppm): 209.57 (C=S), 150.86 (C-N), 116.2-
2
6
29.2 (phenyl ring), 44.93-50.37 (CH2).
.4.3 [K(mppcdt)]: Yield: 72 %; m.p. 458 K. Anal. Found: C, 47.06; H, 4.90; N, 9.11; S, 20.94
. Anal. Calc. for C H N OS K (306.44): C, 47.03; H, 4.92; N, 9.13; S, 20.92 %. IR (KBr, ν
%
1
2
15
2
2
-1
1
cm ): ν(C-O) 1503; ν(C-N) 1463; ν(C=S) 928. H NMR (DMSO-d , δ ppm): 6.85-7.25 (m, 3H,
6
1
3
phenyl ring), 3.88 (s, 3H, CH ), 3.08-3.46 (m, 8H, CH ). C NMR (DMSO-d , δ ppm): 209.56
3
2
6
(C=S), 152.20 (C-N), 111.12-123.68 (phenyl ring), 44.65-55.39 (CH2).
2
.5. Syntheses of [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3)
3
3
3
3
Co(OAc) (0.177 g, 1 mmol) and each of [K(mpcdt)] (0.645 g, 3 mmol), [K(ppcdt)]
2
(0.981 g, 3 mmol) and [K(mppcdt)] (1.071 g, 3 mmol) were dissolved separately in 10-20 mL
methanol, mixed together and stirred for 1 h. The green solids which separated were filtered,
washed successively with an ethanol:water mixture (50:50, v/v) and air dried. The solid
compounds were suspended in methanol, a few drops of pyridine were added and the mixture
5

was warmed. The resulting clear green solutions were filtered off and kept for crystallization.
Green colored single crystals of complexes 1, 2 and 3, suitable for X-ray analyses, were obtained
by slow evaporation of the above solutions over a period of 10-15 days (Scheme 1).
2
.5.1 [Co(mpcdt) ] (1): Yield: 0.65 g, 72 %; m.p. 438 K; Anal. Found: C, 36.89; H, 5.66; N,
3
1
4.39; S, 32.87 %. Calc. for C H CoN S (584.79): C, 36.93; H, 5.64; N, 14.36; S, 32.83 %. IR
18 33 6 6
-1
1
(
KBr, ν cm ): ν(C-N) 1487; ν(C-S) 996; ν(M-S) 521; H NMR (DMSO-d , δ ppm): 3.90 (s, 3H,
6
1
3
CH ), 2.47-2.32 (m, 8H, CH ). C NMR (DMSO-d , δ ppm): 203.06 (C=S), 152.20 (C-N),
3
2
6
4
2
1
2.34-44.42 (CH ), 38.16 (CH ).
2 3
.5.2 [Co(ppcdt) ] (2): Yield: 0.63 g, 68 %; m.p. 463 K; Anal. Found: C, 51.39; H, 5.06; N,
3
0.85; S, 24.97 %. Calc. for C H CoN S (770.99): C, 51.36; H, 5.05; N, 10.89; S, 24.90 %. IR
3
3
39
6 6
-1
1
(
KBr, ν cm ): ν(C-N) 1415; ν(C=S) 923; ν(M-S) 424. H NMR (DMSO-d , δ ppm): 6.85-7.24
6
13
(
m, 3H, phenyl ring), 3.20-4.54 (m, 8H, CH ). C NMR (DMSO-d , δ ppm): 209.57 (C=S),
2
6
1
2
5
50.86 (C-N), 116.2-129.2 (phenyl ring), 44.93-50.37 (CH2).
.5.3 [Co(mppcdt) ]·0.25CHCl (3): Yield: 0.73 g, 76 %; m.p. 573 K; Anal. Found: C, 48.78; H,
3
3
.06; N, 9.48; S, 21.57 %. Calc. for C H CoN O S ·0.25(CHCl ) (890.91): C, 48.82; H, 5.07;
3
6
45
6
3
6
3
-1
1
N, 9.42; S, 21.55 %. IR (KBr, ν cm ): ν(C-O) 1498; ν(C-N) 1433; ν(C=S) 925; ν(M-S) 424. H
NMR (DMSO-d , δ ppm): 6.90-7.25 (m, 3H, phenyl ring), 3.8-4.08 (m, 8H, CH ), 3.13 (s, 3H,
6
2
1
3
CH ). C NMR (DMSO-d , δ ppm): 204.34 (C=S), 152.16 (C-N), 111.28-123.77 (phenyl ring),
3
6
4
5.49-55.44 (CH2).
6

R
R
N
N
N
S
S
N
S
S
Co(OAc)
pyri
2
Co
CS , KOH
2
S
S
R N
NH
R N
N
S
S
EtOH
K+
dine
N
N
R
R= methyl, phenyl, 2-methoxy phenyl
Scheme 1. Preparation of the ligands and their complexes, [Co(mpcdt) ] (1), [Co(ppcdt) ] (2)
3
3
and [Co(mppcdt) ]·0.25CHCl (3)
3
3
2
.6. General procedure for the synthesis of the β-enaminones and β-enaminoesters
The β-ketoester (1.2 mmol) or 1,3-diketone (1.2 mmol) was added to a mixture of the aromatic
amine (1 mmol) or benzylamine (1 mmol) and 1 mol % of the cobalt catalyst under solvent free
conditions. The reaction mixture was stirred at room temperature for 30 min. After the specified
reaction time, the reaction mixture was extracted with EtOAc. The solvent was removed under
reduced pressure and the residue was purified by silica gel flash chromatography
(
hexane/EtOAc, 90:10) to afford the pure products (Scheme 2) (3a-l). All the compounds, except
h, are reported in the literature, so they were characterized by their melting points and spectral
analyses.
3
R
O
O
R
R
O
NH
N
N
S
N
N
S
O
Co
OCH2CH3
S
S
R
S
S
O
O
NH
H
N
R
NH
+
O
N
R
1
R
O
O
NH
Solvent free
O
2
7

Scheme 2. Enamination reaction of 1,3-dicarbonyl and β-ketoesters with amines
Characterization data of the compounds
1
2
.6.1 3-(Benzylamino)cyclohex-2-enone (3a): H NMR (CDCl , δ ppm): 2.02-2.04 (m, 8H), 4.21
3
(s, 2H), 5.01 (s, 1H), 5.81 (s, 1H), 6.97-7.70 (m, 3H), 8.68 (s, 1H).
+
1
2
.6.2 3-(Phenylamino)cyclohex-2-enone (3b): White solid; mp 457 K, GC-MS [M+H] : 377. H
2
NMR (CDCl , δ ppm): 2.04 (d, 2H), 2.35 (d, 2H), 2.49 (d, 2H), 5.57 (s, 1H), 6.28 (s, 1H), 7.13-
3
1
3
7
1
2
3
.32 (m, 3H). C NMR (CDCl , δ ppm): 21.83, 29.83, 36.46, 100.08, 123.90, 125.60, 141.83,
3
48.34, 161.76.
1
.6.3 Ethyl 3-(benzylamino)but-2-enoate (3c): H NMR (CDCl , δ ppm): 1.19 (t, 3H), 1.91 (s,
3
1
3
H), 4.05 (q, 2H), 4.61 (s, 2H), 6.61 (s, 1H), 6.99-7.23 (m, 5H), 10.29 (s, 1H). C NMR (CDCl ,
3
δ ppm): 14.51, 29.65, 45.94, 58.81, 86.09, 124.38, 124.87, 128.99, 129.22, 139.31, 158.89.
1
2
3
2
2
1
2
2
.6.4 Ethyl 3-(phenylamino)butanoate (3d): H NMR (CDCl , δ ppm): 1.06 (t, 2H), 2.29 (s, 3H),
3
1
3
.37 (s, 1H), 3.87 (s, 1H), 4.16 (q, 3H), 6.73-7.46 (m, 5H). C NMR (CDCl , δ ppm): 14.08,
3
2.43, 40.93, 82.99, 132.35, 133.09, 137.80, 152.46, 154.57.
1
.6.5 Ethyl 3-(2-methylpiperidin-1-yl)but-2-enoate (3e): H NMR (CDCl , δ ppm): 0.97 (s, 3H),
3
.41-1.71 (m, 6H), 1.56 (t, 3H), 2.27 (s, 3H), 3.44-3.54 (m, 2H), 4.19 (q, 2H), 4.94 (s, 1H).
1
.6.6 3-(Benzylamino)-5,5-dimethylcyclohex-2-enone (3f): H NMR (CDCl , δ ppm): 1.25 (d,
3
13
H), 1.28 (d, 2H), 1.44 (d, 2H), 4.62 (s, 1H), 6.28 (s, 1H), 7.43-6.47 (m, 4H), 8.03 (s, 1H). C
NMR (CDCl , δ ppm): 19.41, 34.63, 41.91, 141.83, 108.61, 113.86, 124.38, 137.76, 147.73,
3
1
2
6
47.83, 158.35, 196.75.
1
.6.7 3-(Benzylamino)-5,5-dimethylcyclohex-2-enone (3g): H NMR (CDCl , δ ppm): 0.91 (m,
3
H), 2.03 (s, 2H), 3.77 (s, 1H), 4.19 (s, 1H), 4.43 (s, 1H), 7.31-7.52 (m, 3H).
8

2
.6.8 5,5-Dimethyl-3-(pyridin-2-ylamino)cyclohex-2-enone (3h): Yellow solid; m.p. 451 K. GC-
+
1
MS [M ]: 216. H NMR (CDCl , δ ppm): 1.08 (s, 6H), 2.20 (s, 2H), 2.34 (s, 2H), 5.56 (s, 1H),
3
1
3
6
1
2
6
.82 (s, 1H), 7.33-7.12 (m, 4H). C NMR (CDCl , δ ppm): 28.30, 32.78, 43.44, 50.24, 98.27,
3
23.84, 125.47, 129.26, 138.16, 160.71, 160.76, 198.02.
1
.6.9 5,5-Dimethyl-3-(phenylamino)cyclohex-2-enone (3i): H NMR (CDCl , δ ppm): 1.02 (m,
3
1
3
H), 2.26 (s, 2H), 2.02 (s, 2H), 4.14 (s, 1H), 7.70-6.97 (m, 3H), 12.51 (s, 1H). C NMR (CDCl ,
3
δ ppm): 29.68, 30.04, 35.40, 45.12, 46.06, 87.20, 126.20, 138.91, 141.64, 147.16, 159.21,
1
2
2
2
1
91.80.
1
.6.10 3-(Piperidin-1-yl)cyclohex-2-enone (3j): H NMR (CDCl , δ ppm): 1.61-2.00 (m, 6H),
3
.26 (t, 2H), 2.99 (t, 2H), 3.34 (t, 2H), 3.36 (t, 2H), 4.50 (s, 1H).
1
.6.11 Ethyl 3-(piperidin-1-yl)but-2-enoate (3k): H NMR (CDCl , δ ppm): 1.28 (t, 3H), 1.89-
3
1
3
.97 (m, 6H), 2.28 (s, 3H), 3.45 (t, 2H), 3.47 (t, 2H), 4.20 (q, 2H), 7.32 (s, 1H). C NMR
CDCl , δ ppm): 13.96, 19.26, 21.06, 30.01, 46.67, 50.01, 61.27, 89.75, 126.59, 127.21, 128.65,
(
3
1
2
6
3
67.04, 200.60.
1
.6.12 5,5-Dimethyl-3-(piperidin-1-yl)cyclohex-2-enone (3l): H NMR (CDCl , δ ppm): 1.04 (m,
3
H), 1.62-1.77 (m, 6H), 1.95-2.12 (m, 4H), 2.99-3.01 (m, 4H), 6.31 (s, 1H).
. Results and discussion
The potassium salts of the new dithiothiocarbomato ligands 4-methyl piperazine-1-
carbodithioate [K(mpcdt)], 4-phenyl piperazine-1-carbodithioate [K(ppcdt)]
and 4-(2-
methoxyphenyl) piperazine-1-carbodithioate [K(mppcdt)] form the complexes [Co(mpcdt) ] (1),
3
[
Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3), respectively in the presence of warm
3 3 3
pyridine. It is observed that Co(II) is converted to Co(III) by aerial oxidation in the presence of
the dithio ligand, forming the [Co(carbodithioate) ] complexes. It has already been reported that
3
9

VO(acac) zinc nanoparticles, Ca(CF COO) based and other catalysts catalyse the formation of
2
,
3
2
β-enaminones [31,32]. In these reactions, β-enaminoesters/β-enaminones are obtained from β-
ketoesters and 1,3-diketones with aliphatic/aromatic amines using 10 mol% of
VO(acac) /Ca(CF COO) as a catalyst. The corresponding products were obtained in 82-90 %
2
3
2
ο
yield with a low environmental impact, whereas the same reaction requires 80 C in the presence
of zinc nanoparticles as a catalyst. Herein, we demonstrate that cobalt(III) based complexes
catalyse the enamination reactions of 1,3-dicarbonyls and β-ketoesters with amines in a very
efficient manner under solvent free conditions at room temperature.
3
.1. Catalytic activity of complexes 1, 2 and 3
The presence of the Lewis acidic Co(dithiocarbamato) complexes, which are able to
3
provide labile sites during reactions of the complexes, has resulted in their possible role in
organic transformation reactions. The enamination reaction, which requires the nucleophilic
addition of an amine to the carbonyl group mediated by a Lewis acidic center such as the
Co(dithiocarbamato) complexes, has been successfully carried out, demonstrating the possible
3
catalytic behaviour of the present complexes (Table 1).
Table 1. Optimization studies of Co(dithiocarbamato) catalysed enamination reactions of 1, 3-
3
a
dicarbonyls and β-ketoesters with amines
b
Entry
Catalyst used
No catalyst
Catalyst amount in mol % Reaction time (min)
Product
3d
Yield %
1
2
3
4
5
6
-
1
4 days
30
50
82
85
88
95
93
Co(III) complex
Co(III) complex
Co(III) complex
Co(III) complex
Co(III) complex
3d
2
25
3d
3
20
3d
5
15
3d
10
15
3d
a
The reactions were carried out with ethyl acetoacetate (2.4 mmol) and aniline (2.0 mmol)
Isolated yield.
b
1
0

In the above reactions, the 1,3-dicarbonyls and β-ketoesters were treated with amines in
the presence of 1-2 mol % of [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl
3
3
3
3
(3) under solvent free conditions for 30 min at room temperature; the corresponding β-
enaminones and β-enaminoesters were obtained in about 90 % yield (Table 2, entries 1-12).
Table 2. Catalytic activity of [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl
3
3
3
3
(
3) for the preparation of β-enaminones and β-enaminoesters by the reaction of 1,3-dicarbonyls
a
and β-ketoesters with amines
b
Entry
1
2
Time (min)/
Catalyst
Product (3)
Yield (Complex)
3
5
1
90
85
1
, 2, 3
3
a
2
40
1
, 3
3
b
3
4
40
, 3
88
1
3
c
30
8
0
1
, 2, 3
3
d
5
6
4
, 2, 3
0
88
1
1
3
e
40
8
0
, 2, 3
1
1

b
Entry
7
1
2
Time (min)/
Catalyst
Product (3)
Yield (Complex)
3
f
90
35
1
, 2
3
3
g
8
9
85
85
3
, 3
5
1
h
45
1
, 2, 3
3
i
1
1
0
1
90
85
4
, 2
0
1
3
j
30
1
1
, 2
3
k
O
1
2
80
30
, 2, 3
N
3
l
a
The reactions were performed using a β-ketoester (1.2 mmol) or 1,3-diketone (1.2 mmol) and an aromatic amine (1
mmol), aliphatic amine (1 mmol) or benzylamine (1 mmol).
b
Isolated yield.
The reaction products have been characterized by melting points and NMR studies. The
compound 2 3-(phenylamino)cyclohex-2-enone (3b) has also been characterized by GC-MS and
single crystal X-ray diffraction data. It has been observed that the cobalt(III) complexes 1, 2 and
3
, bearing a strongly π-accepting ligand, act as an efficient catalyst for the enamination reaction
1
2

of 1,3-dicarbonyls/β-ketoesters with amines and related compounds. Notable features of these
complexes are their outstanding stability in air and moisture in the solid state. Virtually no
decomposition of the complexes has been detected after extended (6 months) exposure to air.
The X-ray crystal structure of the complexes indicates that this may be due to the effective
shielding of the cobalt atom by three dithiocarbomato ligands and their stabilization by
intermolecular C-H···S hydrogen bonding. Complexes 1, 2 and 3 are able to promote the
reactions between an amine and the carbonyl moiety containing various substituents. Without the
complexes the reaction did not proceed at all, supporting the possible Lewis acid catalysed
activity of the networks. Further, when the catalyst is filtered off, the reaction was not promoted
any more. A control experiment using the building blocks of the complexes also did not result in
any product formation. However, the homogeneous reaction with a cobalt complex as a catalyst
resulted in <80% conversion, demonstrating the enhanced catalytic activity of networks of
complexes, like the heterogeneous conditions. These experiments clearly show that the soluble
and catalytically active species are not eluted at all from the networks of complexes under the
reaction conditions. Thus the reactions are expected to proceed by the heterogeneous catalysis of
the networks.
3
.2. Thermogravimetric analysis of complexes 1 and 2
The thermal properties of [Co(mpcdt) ] (1) were studied by TGA and DTA in the temperature
3
range 30-800 ºC under a nitrogen atmosphere. The thermogravimetric analysis of complex 1
shows that it starts decomposing at 185 ºC and the thermogram exhibits two distinct
decompositions steps at 185 and 386 ºC. The weight loss of 67.44 % in the temperature range
1
85 to 386 ºC could be ascribed to the loss of three methyl piperazine moieties (cal.: 65.08 %).
1
3

The second decomposition in the temperature range 386 to 789 ºC may be due to loss of the
remaining organic moieties, after which cobalt sulfide is formed as the final residue, 14.55 %.
The corresponding endo and exothermic peaks in the DTA were obtained at 232, 270, 500 and
6
50 ºC (Fig. 1).
The thermal properties of [Co(ppcdt) ] (2) were studied by TGA and DTA in the
3
temperature range 30-900 ºC under a nitrogen atmosphere. Thermogravimetric analysis of
complex 2 shows that it starts decomposing at 260 ºC and the thermogram exhibits two distinct
decompositions steps at 260 and 326 ºC. The weight loss of 76.46 % in the temperature range
2
60 to 326 ºC could be ascribed to the loss of three phenyl piperazine moieties (cal.: 79.86 %).
The second decomposition in the temperature range 326 to 900 ºC may be due to loss of the
remaining organic moieties, after which cobalt sulfide is formed as the final residue, 12.9 %.
The corresponding endo and exothermic peaks in the DTA were obtained at 245, 327, 640 and
8
10 ºC (Fig. 2).
3
.3. Crystal structure determination
Figs. 3, 5, 7 and 9 show molecular structure diagrams of complexes 1, 2, 3 and 3b, with the atom
numbering schemes, and the structural refinement data related to complexes 1, 2, 3 and
compound 3b are listed in Table 3. Hydrogen bonding structures of compounds 1, 2, 3 and 3b
are shown in Figs. 4, 6, 8 and 10, respectively.
1
4

Table 3. Crystallographic data table of complexes [Co(mpcdt) ] (1), [Co(ppcdt) ] (2),
3
3
[
Co(mppcdt) ]·0.25CHCl (3) and compound 3b
3 3
Compound
1
2
3
3b
Empirical formula
Formula weight
T (K)
C H CoN S
C H CoN S
C H CoN O S 0.25(CHCl )
C H NO
1
8
33
6
6
33 39
6
6
36 45
6
3
6·
3
12 13
584.79
770.99
890.91
187.23
293(2)
296(2)
293(2)
295(2)
Λ (Mo Kα) (Å)
Crystal system
Space group
a (Å)
0.71073
Trigonal
R-3c
0.71073
0.71073
0.71073
Monoclinic
P 21/c
Monoclinic
P 21/c
15.21(5)
6.23(3)
41.96(2)
90.00
Monoclinic
C 2/c
13.17(6)
13.17(6)
16.07(8)
18.60(10)
31.51(16)
90.00
6.12(7)
b (Å)
11.17(12)
14.44(10)
90.00
c (Å)
30.37(16)
90.00
90.00
120.00
4563.7(5)
6
α (º)
β (º)
96.62(4)
90.00
105.00(6)
90.00
90.95(8)
90.00
γ (º)
3
V (Å )
3956.5(3)
4
9102.7(8)
8
988.60(17)
4
Z
-
3
Dcalc(Mg m )
1.27
1.29
1.30
1.25
-
1
µ (mm )
0.99
0.78
0.73
0.08
F(000)
1836
1608
3716
357
3
Crystal size (mm )
0.30x0.27x0.25
3.09-28.87
0.30x0.27x0.24
3.14-29.20
-19≤ h ≤19
-8≤ k ≤8
0.37x0.25x0.18
3.35-30.30
-20≤ h ≤20
-25≤ k ≤24
-42≤ l ≤35
39509
0.37x0.27x0.24
3.65- 29.07
-4≤ h ≤8
-10≤ k ≤15
-17≤ l ≤19
1461
θ range (º)
Index ranges
-16≤ h ≤ 16
-
12≤ k ≤15
40≤ l ≤22
3336
-
-56≤ l ≤56
21760
Reflections collected
Independent reflections
Data/restraints/ parameters
1195
9198
11029
2654
1195/42/62
1.170
9198/0/443
1.043
11029/12/502
1.05
2654/0/127
0.82
2
Goodness-of-fit on F
Final R indices
0.06, 0.25
0.08, 0.18
0.07, 0.22
0.05, 0.18
wR [I>2σ(I)](R )
2
int
Final R indices (all data)
0.08, 0.26
0.16, 0.22
0.48, -0.36
0.12,0.28
0.090, 0.24
0.40, -0.24
-
3
Largest diff. peak/hole (e Å ) 1.26, -0.51
1.07, -0.91
a
R = Σ||F | – |Fc||Σ|F |.
1
o
o
b
2
o
2
2
2 2 1/2
R = [Σw(|F | – |F |) /Σw|F | ]
.
2
c
o
3
.4. Crystal structure descriptions of complexes 1, 2 and 3
1
5

The coordination sphere of complexes 1, 2 and 3 are fulfilled by two dithio-sulfur atoms from
each of the three bidentate carbodithioate moieties. The formation of three four membered
CS Co chelate rings, each with a bite angle of 76.71(7)º for complex 1, 76.56(6)º for complex 2
2
and 76.33(5)º for complex 3, represent a deviation from an ideal octahedral geometry. The Co-S
bond distances for the three CS Co chelate rings are equal, being 2.2683(15) Å, for complex 1
2
but they are unequal for complexes 2 and 3, being 2.2559(16), 2.2667(16), 2.2685(15),
2
2
.2758(17), 2.2788(17) and 2.2864(16) Å for complex 2 and 2.2561(12), 2.2685(15), 2.2733(13),
.2767(13) and 2.2778(13) Å (Table 4) for complex 3. The Co-S distances are comparable to the
bond lengths reported earlier for other similar cobalt dithio complexes, e.g.
Co{S CN(CH CH CH NMe ) } ] [33]. The carbon-sulfur distances within the chelate rings are
[
2
2
2
2
2 2 3
intermediate between single and double bond lengths. The average bond length, S(1)-C(1) =
.704(5) Å for complex 1, S(1)-C(1) = 1.704(6), S(2)-C(1) = 1.726(6), S(3)-C(12) = 1.724(5),
1
S(4)-C(12) = 1.713(5), S(5)-C(23) = 1.704(6) and S(6)-C(23) = 1.712(5) Å for complex 2 and
S(1C)-C(1C) = 1.720(4) and S(2C)-C(1C) = 1.699(5) Å (Table 4) for complex 3 suggest
considerable delocalization of charge [34]. In the complexes, the chelate rings and the piperazine
ring lie nearly in the same plane. The piperazine ring makes an extended coplanar system with
two chelate rings.
Table 4. Interatomic distances (Å) and angles (°) for [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and
3
3
[
Co(mppcdt) ]·0.25CHCl (3)
3
3
[
Co(mpcdt) ] (1)
3
Bond lengths (Å)
Bond angles (°)
Co(1)-S(1)
Co(1)-S(1)
2.26(15)
2.26(15)
S(1)-Co(1)-S(1)
76.71(7)
96.27(8)
166.58(8)
S(1)-Co(1)-S(1)
S(1)-Co(1)-S(1)
[
Co(ppcdt) ] (2)
3
1
6

Co(1)-S(1)
Co(1)-S(3)
Co(1)-S(5)
2.26(16)
2.26(15)
2.25(16)
S(5)-Co(1)-S(1)
S(5)-Co(1)-S(3)
S(1)-Co(1)-S(2)
94.23(7)
169.08(7)
76.56(6)
[
Co(mppcdt) ]·0.25CHCl (3)
3
3
Co(1)-S(1A)
Co(1)-S(1B)
Co(1)-S(1C)
2.27(13)
2.27(13)
2.26(15)
S(1B)-Co(1)-S(2A)
S(2A)-Co(1)-S(1C)
S(1C)-Co(1)-S(2C)
95.09(5)
167.16(5)
76.33(5)
Table 5. Hydrogen bonding parameters for [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3)
3
3
3
[
Co(ppcdt) ] (2)
3
D-H···A
D-H [Å]
0.93
H···A[Å]
2.95
D···A [Å]
3.61
∠ D-H···A [º] Sym Operator
C20-H1···S5
C27-H28B···S6
129.29
168.10
1-x,-y,-z
-x,1-y,-z
0.97
2.28
3.82
[
Co(mppcdt) ]·0.25CHCl (3)
3
3
D-H···A
D-H [Å]
0.96
H···A[Å]
D···A [Å]
∠ D-H···A [º] Sym Operator
C12A-H12A···S1C
C4A-H4AB···S1B
C9B-H9BA···S2C
2.85
2.76
2.86
3.79
3.51
3.77
165.3
134.9
167.74
-1/2+x,-1/2+y,z
1-x,y,1.5-z
0.96
0.93
-x,y,1.5-z
In the solid state, the complexes are stabilized via intermolecular C-H···S interactions between
the dithio sulfur and hydrogen atoms of the piperazine ring (Fig. 4, 6 and 8, Table 5).
3
.5. Crystal structure descriptions of 3-(phenylamino)cyclohex-2-enone (3b)
Fig. 9 shows the molecular structure diagram of 3-(phenylamino)cyclohex-2-enone (3b)
with the atomic numbering scheme. The carbon nitrogen bond distance C7-N1 (1.346 Å) is
shorter than C1-N1 (1.424 Å) because the C7 carbon is doubly and singly bonded to other two
carbons, C8 and C12, whereas C1 is bonded to others two carbons, C6 and C2, involved in
1
7

delocalized π-bonding. Compound 3b is stabilized via intermolecular N-H···O interactions
between the NH hydrogen atom of the phenyl amine and the 2-enone oxygen atom of other unit
of a 3-(phenylamino)cyclohex-2-enone molecule (Fig. 10).
4
. Conclusion
New complexes, [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3), derived
3
3
3
3
from
4-methyl
piperazine-1-carbodithioate/4-phenyl
piperazine-1-carbodithioate/4-(2-
methoxyphenyl)piperazine-1-carbodithioates have been synthesized and characterized by single
crystal X-ray. During complexation with the carbodithioate ligands, Co(II) is converted to
Co(III) by aerial oxidation, forming distorted octahedral [Co(carbodithioate) ] complexes. In
3
comparison with the reported metal catalysed enamination reactions of 1,3-dicarbonyls and
ketoesters with amines, the present work offers an effiicient alternative catalytic system for the
formation of a carbon-nitrogen bond. Most copper catalysed reactions usually require high
catalyst loading and high reaction temperatures. The cobalt(III) catalysed reactions presented
here employ simple dithio-piperazine ligands which offers a good complement to Pd(II), Ni(II)
as well as Co(II) catalysed reactions in terms of low environmental impact, easy preparation, less
cost and high activity, with a very low amount of catalyst loading. The new reaction system has
the potential of using a cobalt(III) complex as a very user-friendly, inexpensive and efficient
catalyst. TGA analysis of complexes 1 and 2 show that the complexes are stable up to 185 °C
and undergo two step decompositions, leading to the formation of cobalt sulfide as the final
product.
5
. Supplementary material
CCDC 917700, 902647, 982764 and 962738 contain the supplementary crystallographic data for
the complexes [Co(mpcdt) ] (1), [Co(ppcdt) ] (2), [Co(mppcdt) ]·0.25CHCl (3) and the
3
3
3
3
1
8

derivative
3b.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 IEZ, UK; fax: (+44)1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
6
. Acknowledgements
One of the authors (Pooja Bharati) thanks CSIR New Delhi for the award of a SRF. Dr. N.K.
Singh is thankful to UGC, New Delhi for the award of a UGC-Emeritus Fellowship (2014-2015).
References
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2
2

Fig. 1 (a) TGA; (b) DTA curve of [Co(mpcdt) ] (1)
3
Fig. 2 (a) TGA; (b) DTA curve of [Co(ppcdt) ] (2)
3
2
3

Fig. 3 Molecular structure of [Co(mpcdt) ] (1) with the atomic numbering scheme at the 30% probability level
3
Fig. 4 C-H···S hydrogen bonding and packing diagrams of [Co(mpdtc) ] (1)
3
Fig. 5 Molecular structure of [Co(ppcdt) ] (2) with the atomic numbering scheme at the 30% probability level
3
2
4

Fig. 6 C-H···S hydrogen bonding in [Co(ppdtc) ] (2) leading to the formation of a supramolecular architecture
3
Fig. 7 Molecular structure of [Co(mppcdt) ]·0.25(CHCl ) (3) with the atomic numbering scheme at the 30%
3
3
probability level
2
5

Fig. 8 C-H···S hydrogen bonding [Co(mppdtc) ]·0.25CHCl (3) leading to the formation of a supramolecular
3
3
architecture.
Fig. 9 Molecular structure of 2 3-(phenylamino)cyclohex-2-enone (3b) with the atomic numbering scheme at the
0% probability level
3
Fig. 10 Intermolecular N-H···O interactions in 2 3-(phenylamino)cyclohex-2-enone (3b) leading to a 1D-structure
2
6

Graphical Abstract (Picture)
2
8

Graphical Abstract (Synopsis)
Three new cobalt(III) complexes, [Co(mpcdt) ] (1), [Co(ppcdt) ] (2) and [Co(mppcdt) ]·0.25CHCl (3), of 4-
3
3
3
3
methyl piperazine-1-carbodithioate (mpcdt), 4-phenyl piperazine-1-carbodithioate (ppcdt) and 4-(2-
methoxyphenyl) piperazine-1-carbodithioate (mppcdt) have been synthesized and characterized by
various physicochemical techniques. The catalytic properties of the above complexes have also been
examined. A series of β-enaminoesters and β-enaminones were obtained by the reactions of β-
ketoesters and 1,3-diketones with aliphatic and aromatic amines, using the above cobalt(III) complexes
as catalysts, which have been characterized by NMR, GC-MS and X-ray crystallography.
Thermogravimetric analysis of complexes 1 and 2 have been investigated by TG-DTA, which indicate that
cobalt sulfide is formed as the final product.
2
9