Synthesis and characterization of a stable paramagnetic hexacoordinated oxochromium(IV) complex with dianionic tetradentate Schiff base ligand salen

Article abstract of DOI:10.1016/j.ica.2010.07.035

A paramagnetic octahedral oxochromium(IV) complex with dianionic tetradentate ligand salen (where H₂salen is N,N′- bis(salicylidene)-1,2-ethylenediamine) has been synthesized. This compound [CrO(OH₂)(salen)] (1) is characterized by e

Full text of DOI:10.1016/j.ica.2010.07.035

Inorganica Chimica Acta 363 (2010) 3798–3802
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Synthesis and characterization of a stable paramagnetic hexacoordinated
oxochromium(IV) complex with dianionic tetradentate Schiff base ligand salen
a,
Manjuri K. Koley a, Periakaruppan T. Manoharan b,c, , Aditya P. Koley
*
*
a Chemical Engineering and Chemistry Groups, Birla Institute of Technology and Science-Pilani, K. K. Birla Goa Campus, Goa 403 726, India
b Department of Chemistry and SAIF, Indian Institute of Technology-Madras, Chennai 600 036, India
c School of Sciences, Indira Gandhi National Open University, Maidan Garhi, New Delhi 110 068, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A paramagnetic octahedral oxochromium(IV) complex with dianionic tetradentate ligand salen (where
H2salen is N,N0-bis(salicylidene)-1,2-ethylenediamine) has been synthesized. This compound [CrO(OH2)
(salen)] (1) is characterized by elemental analysis, magnetic moment measurement, IR, UV–Vis and
EPR spectroscopic studies. Measured room temperature (RT) magnetic moment value is 2.96 BM for 1
indicates a d2 system with a triplet ground state. The magnetic moment value rules out a large spin–orbit
coupling. The RT and LNT powder EPR spectra of 1 in X-band clearly shows two lines, one around
g = 1.965 and the other with larger intensity at g = 4.26 0.10. The first line at g = 1.965 corresponds to
the |0> M | 1> transition from the Kramers doublet | 1>, while the broad and intense line at low field
with the g-value of 4.26 0.10 is due to the forbidden transition |ꢀ1> M |+1>. Compound 1 displays
two successive reductions at ꢀ0.76 and ꢀ1.63 V (versus Ag/AgCl), respectively, while it undergoes only
one irreversible oxidation as evident from the well-defined anodic wave at +1.48 V in its cyclic
voltammogram.
Received 10 March 2010
Received in revised form 7 July 2010
Accepted 12 July 2010
Available online 13 August 2010
Keywords:
Oxochromium(IV) complex
Schiff base ligand
Salen
EPR and electronic spectra
Cyclic voltammetry
Electrochemistry
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
chromium(III)(salen) complexes. Some of these [CrVO(salen)]+
complexes are synthesized from the corresponding [CrIII(salen)]+
Schiff base complexes of Cr(III) involving the ligand H2salen
(N,N0-bis(salicylidene)-1,2-ethylenediamine) and related ligands
derived from different substituted salen are widely used in the
catalysis of various organic reactions [1–12]. Also these Cr(III)
Schiff base complexes are used in the construction of artificial
metalloenzymes [13–15] in bioinorganic chemistry. These Cr(III)
Schiff base compounds are found to be capable of catalyzing oxy-
gen atom transfer from reagents such as iodosylbenzene, H2O2,
etc. to various organic substrates [2,3,16–18]. From the mechanis-
tic studies of these oxygen atom transfer reactions it has been
shown that initially a [CrVO(Schiff base)]+ complex is formed from
the reaction of e.g., iodosylbenzene and the Cr(III) Schiff base com-
plex and then there is an electrophilic attack on the substrate by
this oxochromium(V) cation. Adam et al. [16] have suggested that
a [CrIV(OAc)(salen)]+ species other than the [CrVO(salen)]+ complex
may also be involved in the catalytic oxidation based on their
mechanistic studies of chemoselective oxidation of alcohols to car-
bonyl products with iodosobenzene diacetate mediated by
complexes and their properties have been studied [2,3,16–18].
Srinivasan and Kochi have reported [3] the synthesis and molecu-
lar structures of a number of stable oxochromium(V)salen cations
from the corresponding Cr(III)salen cations treated with a slight
excess of iodosylbenzene. This oxochromium(V)salen compound
on one-electron reduction either electrochemically or chemically
produced the corresponding neutral oxochromium(IV)salen spe-
cies which was found to be transient character [3]. We have been
able to stabilize and isolate an oxochromium(IV)salen complex
from methanol solution following a similar procedure for the prep-
aration of the recently reported oxochromium(IV)salophen (where
H2salophen is N,N0-bis(salicylidene)-1,2-phenylenediamine) com-
plex, [CrO(OH2)(salophen)] (2) [19]. Here, we report the magnetic,
spectroscopic and electrochemical properties of this compound.
2. Results and discussion
2.1. Infrared spectra
* Corresponding authors. Address: Chemistry Group, Birla Institute of Technology
and Science, Pilani-K. K. Birla Goa Campus, Goa 403 726, India. Fax: +91 832 255
7033.
E-mail addresses: ptm@iitm.ac.in (P.T. Manoharan), koleyap@yahoo.co.in (A.P.
Koley).
The infrared and far infrared spectra of the chromium complex
[CrO(OH2)(salen)] (1) have been recorded using KBr and polyethyl-
ene pellet, respectively. The broad band around 3450 cmꢀ1 region
indicates the presence of water [20] in complex 1. The very strong
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.035

M.K. Koley et al. / Inorganica Chimica Acta 363 (2010) 3798–3802
3799
and sharp band at 1630 cmꢀ1 due to
m
(C@N) in the IR spectrum of
1.5
1.0
0.5
0.0
the free ligand is found to be shifted to 1620 cmꢀ1 in complex 1.
3
2
1
0
The medium but prominent band at 1025 cmꢀ1 in the spectrum
of complex 1 may be attributed to m(Cr@O) [19–21]. This band is
absent in the IR spectrum of the corresponding trans-[CrIII(OH2)2
(salen)]+ reported by Srinivasan and Kochi [3]. Two medium inten-
sity bands at 477 and 457 cmꢀ1 in the far infrared spectrum of 1
500 600 700 800
may be assigned as
m
(Cr–N) while a medium intensity peak at
Wavelength (nm)
316 cmꢀ1 may be assigned as
m
(N–Cr–N) [20,22]. The bands at
421, 395 and 384 cmꢀ1 may be originating due to
m(Cr–O) while
200 300 400 500 600 700 800
a medium intensity peak at 213 cmꢀ1 may be assigned as
Cr–O) [20].
m(O–
Wavelength (nm)
Fig. 2. Electronic spectrum of [CrO(OH2)(salen)] (1) in CH3CN.
2.2. Magnetic moment
intense compared to the weak d–d transition observed at 485 nm
= 248) for 1. This could be due to substitution of the ethylenedi-
amine bridge of 1 with o-phenylenediamine in 2 which causes a
larger charge delocalization in the latter resulting in a more in-
tense band which is CT in nature. The second and third expected
d–d transitions for 2 were obscured by the strong CT transitions
[19].
It may be pointed out here that in the UV–Vis spectrum of the
reported chromium(III) complex with the same ligand salen,
[Cr(OH2)2(salen)]ClO4, the lowest energy band was reported by
The room temperature magnetic susceptibility measurement
shows that the eff value for complex 1 is 2.96 BM (at 25 °C). Since
(e
l
the spin–orbit coupling constant value reported for Cr(IV) in the
literature [23] is around 160 cmꢀ1, we can interpret this room tem-
perature susceptibility value for 1 as indicating two unpaired elec-
trons and 3F to be the ground state for the free ion and hence
supposed to be a 3T1g(F) for the complex (in Oh) [24–26]. However,
the complex as indicated by its electronic and EPR spectra is ex-
p
3
pected to be tetragonal with low symmetry D4h, and the T1g (in
Oh) will be split into Eg and A2g (in D4h) [25]. Thus, in D4h symmetry,
dxz, dyz transform as Eg containing an electron in each orbital.
The brown compound 1 is readily soluble in methanol, toluene,
CCl4, CH3CN, CH2Cl2, DMF; poorly soluble in diethyl ether, and
insoluble in water. Conductivity measurement of 1 in CH3CN solu-
tion shows the compound is non-electrolyte. Thus, based on its ele-
mental analysis, IR spectrum, magnetic moment value and
conductivity measurement, and in analogy to trnas-[Cr(OH2)2(sa-
len)]+, the proposed structure for 1 is shown in Fig. 1 below.
Kanthimathi et al. [30,31] at 485 nm (e = 275) while that for other
[Cr(OH2)2(salen)]+ cations reported by Coggon et al. [32] were ob-
served in the range 482–500 nm; and for the five coordinate oxo-
chromium(V) cation, [CrO(salen)]+ [3] the lowest energy band
was observed at 566 nm (e = 1200), respectively, clearly reflecting
the differences in their electronic structures with that of com-
pound 1 presumably containing a Cr(IV) ion.
2.4. EPR results
2.3. Electronic spectra
The powder EPR spectra for the complex 1 were recorded at X-
band frequency both at RT and LNT (Fig. 3a–c) and also at Q-band
frequency at RT (Fig. 4). When this compound is dissolved in DMF,
this solution does not exhibit any EPR signal at RT. However, when
this DMF solution is frozen as a glass at LNT, strong EPR signal is
observed (Fig. 3d). It may be pointed out here that the powder
EPR spectrum displayed by compound 1 at X-band frequency is
found to be drastically different from the X-band EPR spectrum
of [Cr(OH2)2(salen)]Cl reported by Koner et al. which displayed
an intense line at g = 1.965 [33].
The electronic spectrum of [CrO(OH2)(salen)] (1) has been re-
corded in CH3CN solution. The spectrum is shown in Fig. 2, and
the electronic spectral band positions are listed in Table 1. The
electronic spectral band positions of the corresponding reported
[CrO(OH2)(salophen)] (2) [19] are also presented for comparison.
The two new lower energy bands at 560 nm (
e = 131) and
485 nm ( = 248) appeared in the visible region of the electronic
e
spectrum of complex 1 in CH3CN are absent in the spectrum of
the free ligand H2salen. These two bands may be attributed to d–
d transitions [26–28]. The other expected higher energy d–d tran-
sition [26,28] is probably masked by the strong CT transitions. The
higher energy bands in the region between 218 and 315 nm are
comparable to those in the free ligand spectrum, and are most
likely associated with intraligand
It is to be noted here that the lowest energy d–d transition for
the corresponding reported [19] [CrO(OH2)(salophen)] (2) in
The EPR of the chromium (IV) complexes can be interpreted by
the simple Spin Hamiltonian,
ꢀ
H ¼ bB ꢁ g ꢁ S þ S ꢁ D ꢁ S
ð1Þ
p ?
p* transitions [29].
with S = 1, g and D being tensors and the contributions from hyper-
fine interaction being neglected. Compound 1 has been studied
using powder both at RT and LNT (Fig. 3a and c) and frozen DMF
solution (Fig. 3d) at LNT in X-band, and powder at RT in Q-band
(Fig. 4). Measurement at two different frequencies gives us a clear
idea on the understanding of the origin of these unusually broad
lines. The RT and LNT powder spectra in X-band clearly present
two lines, one around g = 1.965 and the other with larger intensity
at g = 4.26 0.10 what could be termed as a half-field line. The first
line at g = 1.965 corresponds to the |0 > M| 1> transition from the
Kramers doublet | 1>. However, the broad and intense line at low
field, i.e., with the g-value of 4.26 0.10 is due to the forbidden
transition |ꢀ1 > M|+1>. While the large intensity is caused by fairly
large D value, substantial broadening could be caused by dipolar
interaction between nearby paramagnetic species noting that the
powder is a condensed paramagnetic lattice. The intensities of such
CH3CN was observed at 600 nm (
band appeared at 480 nm ( = 1267) exhibited a large solvatochro-
mic shift to 464 nm ( = 1874) in DMF and was attributed to a
e = 116). The next higher energy
e
e
charge transfer (CT) transition [19]. This band is found to be more
Fig. 1. Structure of compound 1.

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M.K. Koley et al. / Inorganica Chimica Acta 363 (2010) 3798–3802
Table 1
Electronic spectral band positions in nm (cmꢀ1) of (a) [CrO(OH2)(salen)] (1) in CH3CN and (b) [CrO(OH2)(salophen)] (2) in CH3CN [19].
(a) [CrO(OH2)(salen)] (1) in CH3CN
(b) [CrO(OH2)(salophen)] (2) in CH3CN
Band position
e
, Mꢀ1 cmꢀ1
Assignments
Band position
nm
e
, Mꢀ1 cmꢀ1
Assignments
nm
(cmꢀ1
)
(cmꢀ1
)
560
485
391
315
272
250
218
(17,857)
(20,618)
(25,575)
(31,746)
(36,765)
(40,000)
(45,872)
131
248
2649
3607
8259
d ? d
d ? d
CT
600
480
373
330
316
292
207
(16,667)
(20,833)
(26,810)
(30,303)
(31,646)
(34,247)
(47,619)
116
1267
3588
6405
7074
8683
20,025
d ? d
CT
CT
p
p
p
p
?
?
?
?
p*
p*
p*
p*
n ? p*
p
p
p
?
?
?
p*
p*
p*
16,543
18,918
Fig. 3. EPR spectra of [CrO(OH2)(salen)] (1) recorded at X-band frequency (a)
Powder at RT with DPPH, (b) Powder at RT without DPPH, (c) Powder at LNT
without DPPH, and (d) Frozen glass in DMF at LNT without DPPH.
Fig. 5. Cyclic voltammogram of [CrO(OH2)(salen)] (1) in CH3CN containing 0.1 M
TEAP at a scan rate of (a) 20 mV sꢀ1, (b) 100 mV sꢀ1
Fig. 4. Powder EPR spectrum of [CrO(OH2)(salen)] (1) recorded at Q-band frequency
at RT without DPPH.
.
2
transfer process for this step. For an initial positive scan, on the
other hand, a well-defined oxidative wave is observed at +1.48 V.
No corresponding cathodic peak is observed on scan reversal
(Fig. 5b). Moreover, this anodic wave is shifted to +1.65 V in the
second cycle and is found to be very broad (figure not shown) indi-
cating oxidative degradation of the compound. Similar oxidative
degradation was also observed for the corresponding salophen
complex 2 [19].
forbidden transitions (
D
Ms = 2) are proportional to (D/h
m
)
and
from its intensities it can be guessed that the 2D value could be just
comparable to the X-band frequency. The Q-band powder spectrum
at RT very clearly supports the above interpretation: (i) the clearly
separated anisotropic lines at higher frequency due to gII = 1.734
and g\ = 1.997 giving a g-average of 1.91, close to 1.965 decided
from an unresolved spectrum in X-band; (ii) as expected, the above
referred forbidden line (
D
Ms = 2) at g ꢂ 4.3 has a much reduced
In the electrochemical studies of [CoII(salen)] in DMSO using
cyclic voltammetry and coulometry by Ortiz and Park [34] two
nearly reversible redox couples were observed at 0.03 and
ꢀ1.13 V (versus Ag/AgCl) corresponding to Co(III)/Co(II) and
Co(II)/Co(I), respectively. Thus the first reduction observed at
ꢀ0.76 V (versus Ag/AgCl) for 1 in the present case is associated
with metal-centered Cr(IV)/Cr(III) reduction. It should be noted
here that the free ligand H2salen is reduced only at a potential
ꢀ1.77 V (versus SCE) as observed by Isse et al. [35] from their elec-
trochemical studies for the reduction of H2salen in DMF. The
Cr(IV)/Cr(III) couple for the reported tetraphenylporphyrinato oxo-
chromium(IV) compound [CrO(TPP)] in CH2Cl2 was observed at
ꢀ1.12 V (versus SCE) [36]. The second reduction waves for 1 and
2 are surprisingly found to be at the same potential ca. ꢀ1.63 V
(versus Ag/AgCl) under similar experimental condition strongly
intensity in correspondence to measurement at higher frequency.
We estimate from the relative intensities of this line at two different
frequencies a D value of ꢂ0.3 cmꢀ1
.
2.5. Electrochemical results
The redox behavior of compound 1 has been studied in CH3CN
containing 0.1 M Et4NClO4 (TEAP) at a glassy carbon working elec-
trode, platinum auxiliary electrode, and a Ag/AgCl reference elec-
trode using cyclic voltammetry (CV). The cyclic voltammogram
shows two cathodic waves at ꢀ0.76 and ꢀ1.63 V, respectively
(Fig. 5a). The reductive response at ꢀ0.76 V is found to be coupled
to a weak and broad anodic peak (figure not shown) around
ꢀ0.65 V (
DEP = 110 mV) suggesting a quasi-reversible electron

M.K. Koley et al. / Inorganica Chimica Acta 363 (2010) 3798–3802
3801
suggesting that the cathodic wave observed at ꢀ1.63 V (versus Ag/
AgCl) for 1 is also associated with a metal-centered reduction and
is most likely due to the Cr(III)/Cr(II) couple. The Cr(III)/Cr(II) cou-
ple for the reported trans-[Cr(OH2)2(salen)]+ in DMSO was ob-
served at ꢀ1.34 V (versus SCE) [30,31], for [CrCl(TPP)] in CH2Cl2
it was observed at ꢀ1.10 V and for [CrO(TPP)] in CH2Cl2 it was
found at ꢀ1.51 V (versus SCE), respectively [36].
compound 1 is clearly evident from the magnetic moment and its
other properties which are highly inconsistent with that of the cor-
responding reported Cr(III) compound and oxochromium(V) com-
pound. Several attempts to prepare single crystal for this Cr(IV)
compound was unsuccessful, so X-ray structural determination
has not been possible. So, all the studies were confined to powder
samples and solutions.
The oxidative degradation of 1 may be explained as follows. It
should be mentioned here that for the recently reported
[CrO(OH2)(salophen)] (2) an initial positive scan yielded two irre-
versible oxidation waves at +1.03 and +1.47 V (versus Ag/AgCl),
respectively, but the second cycle yielded only a very broad oxida-
tion wave around +1.50 V with large anodic current indicating oxi-
dative degradation of the compound followed by electrode
pollution [19]. It appears from the cyclic voltammogram of 2 that
it undergoes two successive metal centered oxidations leading to
a Cr(VI) species probably followed by a chemical change, thereby
making the process completely irreversible and possibly with the
electrode depositions of this oxidized species. The oxidation wave
observed for 1 is found to be even at a slightly higher positive po-
tential than that of the second oxidation peak for 2. Thus it is pos-
sible that compound 1 undergoes an irreversible two-electron
oxidation, as evident from high positive potential with large anodic
current, directly to a Cr(VI) species followed by some chemical
change and electrode deposition. The postulation of a single step
two-electron oxidation for 1 is also rationalized from the fact that
the reported CrVO/CrIVO potential of the salen complex, [CrO(sa-
len)]+ in CH3CN, (E1/2 = +0.47 V versus SCE), is at much less positive
potential [3], and it is highly unlikely that the coordination of a
water molecule trans to the oxo group in the corresponding Cr(IV)
compound (1) will cause such a drastic change in the CrIVO/CrVO
oxidation potential, though it is apparent that the presence of a
water molecule trans to the oxo group has dramatically changed
the stability of 1 in the present case as evident from the following
observations.
The electrochemical behavior of the oxochromium(V)salen cat-
ion in dimethyl sulfoxide solution containing 0.1 M tetra-n-butyl-
ammonium hexafluorophosphate as the supporting electrolyte
was studied by Srinivasan and Kochi using cyclic voltammetry
(CV). The compound displayed a well-defined quasi-reversible
one-electron reduction, but the cathodic wave was found to dimin-
ish upon repetitive CV scans, indicative of the gradual disappear-
ance of the oxochromium cation. The reductive degradation of
oxochromium(V) was also observed in acetonitrile solution and
the large anodic current observed on the reverse scan was symp-
tomatic of electrode pollution and a brown deposit caused this
electrode deposition. They also found a similar brown precipitate
that was formed during the chemical reduction of the oxochromi-
um(V) cation with ferrocene. This transient brown species was ten-
tatively ascribed to a neutral chromium(IV) derivative by these
researchers. Thus it appears that the oxoCr(IV) species generated
electrochemically or chemically from the corresponding oxochro-
mium(V)salen cation was unstable under those experimental
conditions.
4. Experimental
4.1. Chemicals
Salicylaldehyde and ethylenediamine (reagent grade) were ob-
tained from Aldrich and were used without further purification.
Absolute ethanol (GR) was obtained from Merck, Germany. Potas-
sium dichromate (GR) was obtained from Sarabhai Chemicals, In-
dia. Methanol (GR), dichloromethane (GR), toluene (GR), DMF
(GR) and acetonitrile (HPLC) were obtained from Merck, India. All
other chemicals were of reagent grade and were used as such. Tet-
raethyl ammonium perchlorate (TEAP) was prepared using a meth-
od described in literature [37].
4.2. Preparation of the Schiff base ligand H2salen
This ligand was prepared by the reaction of 6.1 g (0.05 mol) of
salicylaldehyde with 1.5 g (0.025 mol) of ethylenediamine in dry
ethanol, and the yellow compound was purified by recrystalliza-
tion from ethanol. Anal. Calc. for C16H16N2O2: C, 71.62; H, 6.01;
N, 10.44. Found: C, 71.80; H, 6.08; N, 10.52%. Mp 126 °C. UV–Vis
(CH2Cl2) [kmax, nm]: 257, 318. IR (KBr pellet, cmꢀ1): 3450br
m(O–
H), 1635s
m(C@N), 1283s m(C–O).
4.3. Preparation of [CrO(OH2)(salen)] (1)
A 0.294 g (1 mmol) sample of solid potassium dichromate was
added to 0.536 g (2 mmol) of H2salen ligand in 50 mL of methanol
with constant stirring at room temperature (RT). Stirring was con-
tinued for 90 h while the solution turned from yellow to red
brown. The solution was filtered through a G4 sintered glass cruci-
ble and the red brown filtrate was evaporated to dryness in vac-
uum. The brown solid thus obtained was stirred at RT with
100 mL of toluene for 1 h in a stoppered conical flask and filtered.
The red brown filtrate was kept for slow evaporation at RT when a
brown compound was obtained. This was filtered and washed
thoroughly with diethyl ether. This compound was then recrystal-
lized from CH2Cl2 and n-hexane. Anal. Calc. for C16H16N2O4Cr: C,
54.53; H, 4.58; N, 7.95. Found: C, 55.02; H, 4.61; N, 8.13%.
4.4. Physical measurements
Elemental analyses (C, H and N) were performed in a Perkin–El-
mer 240 CHNS/O analyzer. Infrared spectra were measured with a
Jasco IR report-100 spectrophotometer using KBr pellet. Far infra-
red spectra were recorded with a Bruker IFS 66 V spectrometer
using polyethylene pellet. Static susceptibility measurements were
made with the help of a Princeton Applied Research Vibrating Sam-
ple Magnetometer Model 155. Electronic spectra were recorded
with a Jasco V-570 UV/Vis/NIR spectrophotometer using a pair of
matched quartz cell of path length of 1 cm. EPR spectra were re-
corded using a Varian E-112 X/Q-band spectrometer. RT solution
EPR spectra were recorded using an aqueous cell. Frozen glass
EPR spectra were recorded in liquid nitrogen temperature (LNT)
using a quartz dewar. Diphenylpicrylhydrazyl (DPPH) was used
as internal field marker. Electrochemical measurements were done
with the help of a PAR Versastat-II electrochemistry system at
3. Conclusions
It has been possible to stabilize a paramagnetic oxoCr(IV) com-
plex in the solid state with the Schiff base ligand H2salen. Complex
1 is remarkably stable and no apparent decomposition or transfor-
mation to other compounds is observed for this compound. This
observation is consistent with its low reduction potential. It is very
puzzling that the coordination of a water molecule trans to the oxo
group has resulted in such a dramatic change in its stability as well
as in its redox behavior. However, the Cr(IV) oxidation state in the

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A.P.K. and M.K. thank Professor Sakti Prasad Ghosh, Department
of Inorganic Chemistry, Indian Association for the Cultivation of
Science, Kolkata for help with the elemental analysis and magnetic
moment measurement. A.P.K. thanks Dr. Shyamal Kumar Chatto-
padhyay of the Department of Chemistry, Bengal Engineering and
Science University, Shibpur, Howrah for help with the electro-
chemical studies. We thank the SAIF, IIT-Madras for providing
EPR and far-infrared facilities. P.T.M. thanks the DST, Govt. of India,
respectively for the Research Scheme (SP/S1F18/2000) and Raman-
na Fellowship (SR/S1/RFIC-02 /2006).
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