C-Nitrosation of a β-diketiminate ligand in copper(ii) complex

Article abstract of DOI:10.1039/c4ra08281h

A copper(ii) complex, 1 of a β-diketiminate ligand {L = (4-(2,6-dimethylphenylimino)pentane-2ylidene)-2,6-dimethylbenzeneamine} has been synthesized and characterized. Single crystal X-ray structure reveals the presence of a mononuclear copper(ii) unit with a distorted square planar geometry. In acetonitrile solution of the complex, the β-diketiminate ligand undergoes oxidative degradation to the corresponding ketone diimine under aerobic condition. Addition of nitric oxide to the acetonitrile solution of complex 1 affords the corresponding oxime diimine ligand. All the modified ligands have been isolated and characterized. This journal is

Full text of DOI:10.1039/c4ra08281h

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C-Nitrosation of a b-diketiminate ligand in
copper(II) complex†
Cite this: RSC Adv., 2015, 5, 643
*
Aswini Kalita, Vikash Kumar and Biplab Mondal
A copper(II) complex, 1 of a b-diketiminate ligand {L ¼ (4-(2,6-dimethylphenylimino)pentane-2ylidene)-2,6-
dimethylbenzeneamine} has been synthesized and characterized. Single crystal X-ray structure reveals the
presence of a mononuclear copper(II) unit with a distorted square planar geometry. In acetonitrile solution
of the complex, the b-diketiminate ligand undergoes oxidative degradation to the corresponding ketone
diimine under aerobic condition. Addition of nitric oxide to the acetonitrile solution of complex 1 affords
the corresponding oxime diimine ligand. All the modified ligands have been isolated and characterized.
Received 7th August 2014
Accepted 21st November 2014
DOI: 10.1039/c4ra08281h
www.rsc.org/advances
Introduction
b-Diketiminate derivatives with bulky aromatic groups as N-
substituents have attracted considerable research interest
from coordination chemists.1–3 These mono-anionic bidentate
ligands are isoelectronic to cyclopentadienyl anions; their steric
and electronic properties can easily be altered by switching to
appropriate amine and diketone derivatives for synthesis.4,5
They afford coordinatively unsaturated and stable mono-/
binuclear complexes of transition and lanthanide metal
ions.6,7 These complexes nd extensive applications as catalysts
as well as models for active sites of metallo-enzymes.8–10
In general b-diketiminate frameworks are obtained from the
reaction of acetylacetone with appropriate aniline derivative.
They are stable and allow the synthesis and characterization of a
wide range of metal complexes.11 However, examples of ligand
modication during catalysis reactions are also known in some
cases.12,13 Itoh and coworkers demonstrated that b-diketiminate
ligand with mesityl substituents at N-atom in copper(II) complex
in methanol solution undergoes easy oxidative transformation
to the corresponding keto-diimine under aerobic condition.14
When the reaction mixture was allowed to stand for few days,
the crystals of copper(II) complex of the corresponding hemi-
acetal derivative of the ketone diimine were obtained. However,
the demetallation of this complex was reported to afford the
corresponding ketone diimine. The similar reaction was
reported with complex of Zn(II), also.15 In anaerobic condition,
the reaction did not occur. The labelling experiment with 18O2
conrmed the involvement of molecular oxygen. The reaction
was proposed to proceed through the formation of bridged
Fig. 1 Ligand, L, used for the present study.
peroxo intermediate.14 In spite, there has not been much study
on the degradation of b-diketiminate ligand frameworks in
presence of metal ions.
This reaction instigates to study the reactivity of copper(II)
complex of a b-diketiminate ligand, (L), with molecular oxygen
and nitric oxide (NO) (Fig. 1). The ligand undergoes oxidative
degradation to the corresponding ketone diimine (L1) in case of
reaction with molecular oxygen and C-nitrosation during reac-
tion with NO to afford corresponding oxime di-imine frame-
work, L2.
Results and discussion
The ligand L was prepared by reacting acetylacetone with 2,6-
dimethyl aniline using a reported procedure.15 The copper(II)
complex, 1 used for the present study was prepared by stirring
the free ligand with equivalent amount of copper(II) acetate in
acetonitrile at room temperature. Complex 1 was characterized
by various spectroscopic techniques as well as by single crystal
structure determination. The ORTEP diagram is shown in
Fig. 2. The crystal structure reveals the mononuclear metal
center with a distorted square planar geometry where the
anionic b-diketiminate and acetate act as bidentate ligands. The
crystallographic data, important bond angles and distances are
listed in Tables 1–3, respectively. The average Cu–N distance is
Department of Chemistry, Indian Institute of Technology Guwahati, Assam – 781039,
India. E-mail: biplab@iitg.ernet.in; Fax: +91-361-258-2339; Tel: +91-361-258-2317
† Electronic supplementary information (ESI) available: The detail experimental
processes and all characterization data are included. CCDC 1006415, 1007056
and 1006417. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ra08281h
˚
1.900 (3) A. C–N and C–C distances of the diketiminate
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intra-ligand transitions in UV-range. In X-band EPR spectra,
complex 1 exhibits four line spectrum characteristics to the
square planar copper(II) complexes with dx2ꢁy2 ground state
(Fig. 4).16
The colour of the acetonitrile solution of complex 1 changed
to green when heated to 50 ꢀC for 2 h under aerobic condition.
This led to the oxidation of the ligand to corresponding ketone
diimine analogue, L1. Similar observation was reported earlier
by Itoh et al.14 The reaction presumably proceeds through the
formation of a bridged peroxo intermediate as proposed by Itoh
et al. (Scheme 1).14 The modied ketone diimine ligand, L1 was
isolated quantitatively from the reaction mixture aer removal
of copper(II) ion using aqueous Na2S (experimental section). L1
was characterized using various spectral analyses. We could not
grow the X-ray quality crystals of it. However, when the crude
reaction mixture was kept at room temperature, dark green
crystals of complex 2 appeared (ESI†).
The ORTEP diagram of complex 2 is shown in Fig. 5b. The
crystal data and bond angle and distances are shown in Tables
1–3, respectively. The crystal structure reveals the heptanuclear
structure of complex 2 and the formation of corresponding diol
diimine which actually dehydrates while demetallated to afford
the ketone diimine. PLATON/SQUEEZE was performed to rene
the framework for complex 2 along with the water molecules in
the crystal by excluding the disordered solvent electron
Fig. 2 ORTEP diagram of complex 1 (50% thermal ellipsoid plot; H-
atoms are removed for clarity).
˚
˚
framework are 1.340(4) A and 1.387 A, respectively. Average
˚
Cu–Oacetate distance is 2.018 A. In acetate, the C–O distance is
ꢀ
˚
1.255 A. The bite angle of the diketiminate ligand is 96.30 ;
whereas for acetate, this is 64.22ꢀ. For copper(II) complex of the
b-diketiminate ligand having mesityl substituents at N atom,
˚
the average Cu–N and Cu–O distances are 1.915 and 2.028 A,
respectively.14
Complex 1 in acetonitrile solution displays d–d transitions at
772 nm and 559 nm in the visible spectrum (Fig. 3) along with
Table 1 Crystallographic data for complex 1, complex 2 and L2
Complex 1
Complex 2
L2
Formulas
Mol. wt.
C
23H28Cu1N2O2
C77H129Cu7N7O38
2205.65
C21H25N3O1
335.44
428.01
Crystal system
Space group
Temperature/K
Triclinic
P1
296(2)
0.71073
7.0615(3)
12.4388(6)
13.4851(8)
70.511(5)
83.469(4)
76.344(4)
1084.26(10)
2
1.311
1.027
Multi-scan
450
Cubic
P213
296(2)
0.71073
22.7589(3)
22.7589(3)
22.7589(3)
90.00
90.00
90.00
11788.3(2)
4
1.243
1.310
Multi-scan
4588
Mononclinic
I2/c
296(2)
0.71073
21.2119(14)
7.8518(4)
23.5414(15)
90.00
95.420(6)
90.00
3903.3(4)
8
1.142
0.071
None
1440
ꢀ
˚
Wavelength/A
˚
a/A
˚
b/A
˚
c/A
a/deg
b/deg
g/dꢁeg3
˚
V/A
Z
Density/mg mꢁ3
Abs. coeff/mmꢁ1
Abs. correction
F(000)
Total no. of reections
Reections, I > 2s(I)
Max. 2q/deg
3822
3330
25.00
6992
5668
25.24
3442
2194
25.00
Ranges (h, k, l)
ꢁ8 # h # 8
ꢁ14 # h # 21
ꢁ10 # k # 27
ꢁ18 # l # 22
99.7
ꢁ24 # h # 25
ꢁ14 # k # 8
ꢁ9 # k # 9
ꢁ16 # l # 15
ꢁ27 # l # 27
Complete to 2q (%)
Renement method
GoF (F2)
R indices [I > 2s(I)]
R indices (all data)
Flack parameter
99.9
99.8
Full-matrix least-squares on F2
Full-matrix least-squares on F2
1.065
Full-matrix least-squares on F2
0.966
0.0478
0.1357
—
1.011
0.0573
0.1615
—
0.0648
0.1303
ꢁ0.010(19)
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Fig. 3 UV-visible spectrum of complex 1 in acetonitrile.
Fig. 5 (a) Complex 2 and (b) ORTEP diagram of complex 2 (50%
thermal ellipsoid plot; H-atoms and solvent molecules are removed
for clarity).
Fig. 4 X-band EPR spectrum of complex 1 in acetonitrile at 77 K.
from methanol solution. Demetallation of this complex using
aqueous ammonia afforded the ketone diimine. A similar
ligand oxygenation reaction was reported in Co(II) complexes of
tetraaza macrocyclic dibenzotetramethyltetraaza-[14]annulene
dianion.18
Addition of nitric oxide to the degassed acetonitrile solution
of complex 1 resulted in colour change from brown to deep
green through a transient intermediate step. The absorptions
densities.17 These calculations amount to 601 electrons may be
attributed to the water molecules.
This oxidative transformation is similar to what Itoh et al.
reported earlier with mesityl substuted b-diketiminate. The
crystal structure of the copper(II) complex with intermediate
hemiacetal of the corresponding diketiminate was reported
Scheme 1
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Table 2 Selected bond length (A˚) of complex 1, complex 2 and L2
˚
˚
˚
Complex 1
Bond length (A)
Complex 2
Bond length (A)
L2
Bond length (A)
Cu1–O2
Cu1–O1
Cu1–N1
Cu1–N2
O2–C22
O1–C22
N1–C8
2.015(2)
2.022(2)
1.899(3)
1.910(3)
1.254(5)
1.257(4)
1.443(5)
1.340(4)
1.434(3)
1.336(4)
1.394(5)
Cu2–O5
Cu2–O6
Cu1–O5
Cu1–N1
Cu1–O4
Cu1–O2
Cu1–O3
Cu3–O6
Cu3–N2
Cu3–O7
Cu3–O8
2.102(3)
2.108(4)
1.937(3)
2.001(4)
2.009(2)
1.964(4)
2.326(4)
1.945(3)
1.993(4)
1.977(2)
2.345(6)
O1–H1
O1–N3
N1–C9
0.819(2)
1.381(3)
1.267(3)
1.442(3)
1.281(3)
1.426(3)
1.279(3)
1.502(3)
1.492(4)
1.477(3)
1.495(4)
N1–C8
N2–C12
N2–C14
N3–C11
C9–C11
C9–C10
C11–C12
C12–C13
N1–C9
N2–C14
N2–C12
C11–C12
559 nm and 772 nm were diminished and a new band at 580 nm
appeared (Fig. 6) for a short while suggesting the formation of
the transient intermediate. This is presumably due to the
formation of Cu(II)-nitrosyl intermediate. The frozen reaction
mixture appeared silent in X-band EPR as expected for a Cu(II)-
nitrosyl intermediate. FT-IR spectrum of the intermediate could
not be recorded owing to its very less stability. The 580 nm
absorption band diminished within few seconds and resulted in
the appearance of an absorption band at 640 nm (Fig. 6) which
is attributed to the d–d transition of copper(II) complex of
modied oxime diimine ligand, 3.
Complex 3 has been isolated as solid and characterized
(experimental section and ESI†).
In FT-IR spectroscopy, complex 3 exhibits new stretching
frequency at ꢂ1627 cmꢁ1. This band is assigned as the coor-
dinated oxime stretching frequency (ESI†).19
Removal of copper(II) ion from the solution of complex 3
afforded the corresponding modied oxime diimine ligand, L2.
The formation of L2 has been conrmed by various spectral
analyses as well as by X-ray single crystal structure determina-
tion. The ORTEP diagram of L2 is shown in Fig. 7.
Fig. 7 ORTEP diagram of the oxime diimine, L2 (50% thermal ellipsoid
plot; H-atoms are removed for clarity).
Crystallographic data, important bond distances and angles
are listed in Tables 1–3, respectively. The C–Nimine distances are
˚
˚
1.267(3) and 1.281(3) A. C–Noxime distance is 1.279(3) A and
˚
N–Ooxime is 1.381(3) A.
It should be noted that C-nitrosation did not occur while
nitric oxide is purged to the acetonitrile solution of the free
ligand. On the other hand, addition of NOClO4 in the acetoni-
trile solution of L in presence of NaOEt, afforded corresponding
C-nitrosation. Thus, the formation of copper(II)-nitrosyl with an
electrophilic nitrosyl centre is the key step for the C-nitrosation
reaction.
Though the detail mechanism of the C-nitrosation in the
present case is not yet fully understood, presumably NO rst
binds to the metal center to form Cu(II)-nitrosyl intermediate as
indicated by UV-visible and EPR studies. In the next step,
nucleophillic attack to N-atom of NO by the diketiminate ligand
Fig. 6 UV-visible spectra of complex 1 before (black trace) and after
(red trace) purging NO in acetonitrile. Blue represents the final
compound (complex 3).
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Table 3 Selected bond angles (ꢀ) of complex 1, complex 2 and L2
Complex 1
Bond angles (ꢀ)
Complex 2
Bond angles (ꢀ)
L2
Bond angles (ꢀ)
O2–Cu1–O1
O2–Cu1–N1
O2–Cu1–N2
O1–Cu1–N1
O1–Cu1–N2
N1– Cu1–N2
Cu1–O2–C22
Cu1–O1–C22
Cu1–N1–C8
Cu1–N1–C9
C8–N1–C9
Cu1–N2–C14
Cu1–N2–C12
C14–N2–C12
—
—
—
—
—
—
—
—
—
—
—
—
64.2(1)
164.0(1)
99.7(1)
99.7(1)
163.8(1)
96.3(1)
89.3(2)
89.0(2)
116.7(2)
123.8(2)
119.5(3)
116.9(2)
123.4(2)
119.7(2)
—
—
—
—
—
—
—
—
—
O6–Cu2–O5
O5–Cu2–O6
O5–Cu1–N1
O5–Cu1–O4
O5–Cu1–O2
O5–Cu1–O3
N1–Cu1–O4
N1–Cu1–O2
N1–Cu1–O3
O4–Cu1–O2
O4–Cu1–O3
O2–Cu1–O3
Cu2–O6–Cu3
Cu2–O5–Cu1
O6–Cu3–N2
O6–Cu3–O7
O6–Cu3–O9
O6–Cu3–O8
N2–Cu3–O7
N2–Cu3–O9
N2–Cu3–O8
O7–Cu3–O9
O7–Cu3–O8
O9–Cu3–O8
O4–Cu1–O5
O4–Cu1–N1
154.6(1)
65.8(1)
82.7(2)
92.1(1)
170.1(2)
94.6(2)
158.7(1)
93.0(2)
103.0(2)
88.8(1)
98.0(1)
95.0(2)
108.8(2)
110.3(2)
82.9(2)
93.0(1)
172.5(2)
92.7(2)
164.3(2)
91.9(2)
104.6(2)
90.6(2)
90.7(2)
94.0(2)
92.1(1)
158.7(1)
H1–O1–N3
C9–N1–C8
C12–N2–C14
O1–N3–C11
N1–C9–C11
N1–C9–C10
N3–C11–C9
N3–C11–C12
N2–C12–C11
N2–C12–C13
N1–C8–C6
N1–C8–C2
N2–C14–C20
N2–C14–C15
—
—
—
—
—
—
—
—
—
—
—
—
109.5(2)
120.6(2)
120.3(2)
112.4(2)
117.1(2)
126.7(2)
124.4(2)
117.2(2)
115.4(2)
125.8(2)
121.4(2)
116.8(2)
119.7(2)
118.6(2)
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
takes place with successive loss of proton resulting to the oxime
diimine moiety.
Single crystals were grown by slow diffusion followed by slow
evaporation technique. The intensity data were collected using a
Bruker SMART APEX-II CCD diffractometer, equipped with a
˚
ne focus 1.75 kW sealed tube MoKa radiation (l ¼ 0.71073 A)
at 273(3) K, with increasing u (width of 0.3ꢀ per frame) at a scan
speed of 3 s per frame. The SMART soware was used for data
acquisition. Data integration and reduction were undertaken
with SAINT and XPREP soware.20 Multi-scan empirical
absorption corrections were applied to the data using the
program SADABS.21 Structures were solved by direct methods
using SHELXS-97 and rened with full-matrix least squares on
F2 using SHELXL-97.22 All non-hydrogen atoms were rened
anisotropically. Structural illustrations have been drawn with
ORTEP-3 for Windows.23
Experimental procedures
Materials and methods
All reagents and solvents were purchased from commercial
sources and were of reagent grade. Acetonitrile was distilled
from calcium hydride. Deoxygenation of the solvent and solu-
tions were effected by repeated vacuum/purge cycles or
bubbling with Ar for 30 minutes. NO gas was puried by passing
through KOH and P2O5 column. UV-visible spectra were recor-
ded on a Perkin Elmer lamda 25 UV-visible spectrophotometer.
FT-IR spectra were taken on a Perkin Elmer spectrophotometer
with either sample prepared as KBr pellets or in solution in a
sodium chloride cell. Solution electrical conductivity was
checked using a Systronic 305 conductivity bridge. 1H-NMR
Synthesis
Synthesis of ligand L. The ligand L was synthesized by a
400 MHz Varian FT- reported procedure.15 A mixture of 20.3 ml 2,6-dimethylaniline
spectra were obtained with
a
spectrometer. Chemical shis (ppm) were referenced either (20 g, 0.17 mol), 8.5 ml 2,4-pentanedione (8.25 g, 0.08 mol) and
with an internal standard (Me4Si) for organic compounds or to 14.2 g p-toluenesulphonic acid in 250 ml of dry toluene was
the residual solvent peaks. The X-band Electron Paramagnetic reuxed for 24 h. The toluene was decanted, and to the solid
Resonance (EPR) spectra of the complexes and of the reaction residue 200 ml diethyl ether was added followed by 150 ml water
mixtures were recorded on a JES-FA200 ESR spectrometer with and 36 g Na2CO3$10H2O. Aer stirring for 25 min, the ether
frequency, 9431.848 MHz; center eld, 336.0 mT; power, 0.998 layer was separated, dried with MgSO4, and the solvent was
mW; modulation width, 0.80 mT; time constant, 0.03 s and removed in vacuum. The residue was dried in vacuum at 100 ꢀC
sweep time, 30.0 s.
for 6 h to remove any remaining free 2,6-dimethylaniline, giving
Elemental analyses were obtained from a Perkin Elmer crystalline solid of L. Yield: 18.8 g (74%). FT-IR: 3009, 2918,
Series II Analyzer. The magnetic moment of complexes are 2852, 1625, 1551, 1487, 1465, 1432, 1376, 1278, 1181, 1090,
measured on a Cambridge Magnetic Balance.
1024, 767 cmꢁ1. 1H-NMR: (600 MHz, CDCl3): dppm: 1.80 (6H, s),
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2.28 (12H, s), 4.99 (1H, s), 7.04–7.06 (2H, t), 7.13–7.15 (4H, d), purged till the colour of the solution changed to deep green. The
12.32 (1H, s). 13C-NMR: (150 MHz, CDCl3): dppm: 18.5, 20.4, 93.6, solution was allowed to stand for 1/2 h. The excess NO was
124.4, 127.9, 132.2, 143.9, 160.9.
removed by applying vacuum and purging Ar. The volume of the
Synthesis of complex 1. CuII(CH3COO)2$2H2O (0.997 g, 5 solution was reduced to ꢂ1 ml. To this saturated aqueous
mmol) was dissolved in 10 ml of acetonitrile. Chloroform solution of Na2S was added and the mixture was stirred for 1/2
solution (10 ml) of L (1.53 g, 5 mmol) was added slowly with h. The black precipitate thus appeared was ltered out. To the
constant stirring. The colour of the solution turned to brown ltrate 20 ml of water was added. Few drops of acetic acid were
from green. The stirring was continued for 2 h at room added to neutralize the solution and then the organic part was
temperature. The solution was dried in rotary evaporator to extracted with chloroform (3 portions ꢃ 25 ml). Extracted
obtain a brown solid. The solid was then washed with distilled organic portion was dried under vacuum and pure L2 was
water and dried to get crystalline solid of complex 1. Yield: 2.02 obtained. Yield, 52 mg (ꢂ65%). FT-IR (in KBr) 3144, 3017, 2852,
g (80%). UV-vis (acetonitrile): lmax (3, Mꢁ1 cmꢁ1): 772 nm (55) 1666, 1625, 1591, 1470, 1368, 1337, 1206, 1092, 1069, 1001, 830,
559 nm (160), lmax, 359 nm (2105). X-band EPR (in acetonitrile 817, 771, 764 cmꢁ1. 1H-NMR: (600 MHz, CDCl3): dppm: 1.92 (3H,
at 77 K): g , 2.189; gt, 2.056. FT-IR (KBr pellet): 3016, 2975, s), 1.96 (3H, s), 2.01 (6H, s), 2.13 (6H, s), 6.90–6.95 (2H, m), 7.02–
k
2919, 1556, 1529, 1470, 1393, 1261, 1182, 1024, 945, 859, 761, 7.04 (4H, t), 10.93 (1H, s). 13C-NMR: (150 MHz, CDCl3): dppm
692, 609 cmꢁ1. Complex 1 behaves as non-electrolyte in aceto- 16.2, 18.1, 18.5, 20.7, 123.3, 124.1, 125.2, 128.0, 164.6. The X-ray
nitrile solution. The observed magnetic moment is 1.57 BM. quality single crystals were grown from the chloroform solution
:
Synthesis of complex 2. Complex 1 (100 mg) was dissolved in of the modied ligand, L2.
20 ml acetonitrile in a round bottom ask. The solution was
heated to 50 ꢀC with constant stirring for 2 h under aerobic
condition. The colour of the solution changed to dark green.
Conclusion
The reaction mixture was cooled to room temperature and In the copper(II) complex, the b-diketiminate ligand,
L
allowed to stand for 24 h to obtain green crystals of complex 2. undergoes oxidative degradation in acetonitrile solution under
The crystals were ltered out and characterized. Yield, 53 mg aerobic condition to afford corresponding ketone diimine. The
(ꢂ80%). UV-visible (acetonitrile): lmax (3, Mꢁ1 cmꢁ1): 725 nm reaction possibly proceeds through a peroxo intermediate. On
(158), 241 nm (4473). FT-IR (KBr pellet): 3419, 2921, 1647, 1590, the other hand, addition of nitric oxide to the acetonitrle solu-
1469, 1440, 1204, 1092, 1036, 766 cmꢁ1
.
tion of complex 1 results in the C-nitrosation of the ligand
Synthesis of complex 3. To a degassed acetonitrile (10 ml) framework and affords corresponding oxime diimine. The
solution of complex 1 (100 mg), nitric oxide was purged till the modied ligands are isolated and characterized. The formation
colour of the solution changed to deep green from light brown. of the oxime diimine is further supported by X-ray single crystal
The solution was allowed to stand at room temperature for 1/2 structure determination. The reaction, presumably, proceeds
h. The excess nitric oxide was removed by applying several through the formation of copper(II)-nitrosyl intermediate.
cycles of vacuum followed by purging Ar. The volume of the
solution was reduced to 2 ml and benzene (10 ml) was added to
make a layer and kept in freezer for overnight. The complex 3
Acknowledgements
was obtained as green precipitate. It was ltered off and washed The authors sincerely thank the Department of Science and
with diethylether. Yield, 80 mg (ꢂ74%). UV-visible (acetoni- Technology, India for nancial support; DST-FIST for X-ray
trile): lmax (3, Mꢁ1 cmꢁ1): 640 nm (189), 246 nm (2643). FT-IR: diffraction facility. AK and VK would like to thank CSIR, India
3444, 2921, 1627, 1566, 1470, 1439, 1335, 1198, 1094, 768 cmꢁ1
.
for providing the scholarship.
Isolation of ligand L1. Complex 1 (300 mg) was dissolved in
20 ml acetonitrile in a round bottom ask. The solution was
heated to 50 ꢀC with constant stirring for 2 h under aerobic
condition while the colour of the solution changed to dark
green. Aer cooling down to room temperature, the volume of
the solution was reduced to 2 ml. To this, 5 ml saturated
aqueous solution of Na2S was added with constant stirring for 1/
2 h. The black precipitate thus appeared was ltered off and the
ltrate was diluted with 25 ml water. Then the organic part was
extracted with chloroform (20 ml ꢃ 3 portions). The combined
chloroform layer was dried and subjected to column chroma-
tography to get L1. Yield, 135 mg (ꢂ60%). FT-IR: 3419, 2922,
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