Ruthenium(III) cyclometalates obtained by site-specific orthometallation and their reactivity with nitric oxide: Photoinduced release and estimation of no liberated from the ruthenium nitrosyl complexes

Article abstract of DOI:10.1002/ejic.201101026

Cyclometalated Ru^III complexes [Ru(L^SB1)(PPh ₃)₂Cl] (1), [L^SB1H₂ = 2-(3-nitrobenzylideneamino)phenol], and [Ru(L^SB2)(PPh ₃)₂Cl] (2), [L^SB2H₂ = 4-methyl-2-(3-nitrobenzylideneamino)phenol and H = dissociable protons], were synthesized by site-specific orthometallation and characterized by spectroscopic and electrochemical studies. Complexes 1 and 2 were treated with in situ generated nitric oxide (NO), derived from an acidified nitrite solution, which afforded the ruthenium nitrosyl complexes [Ru(L^SB3H)(PPh ₃)₂(NO)Cl](ClO₄) (1a) and [Ru(L ^BOX)(PPh₃)₂(NO)Cl](ClO₄) (2a), respectively, [L^SB3H₂ = 4-nitro-2-(3- nitrobenzylideneamino)phenol, L^BOXH = 5-methyl-7-nitro-2-(3- nitrophenyl)benzoxazole and H = dissociable protons]. Complexes 1a and 2a were found to be diamagnetic and were characterized by ¹H NMR and ³¹P NMR spectral studies. Both 1a and 2a exhibited V_NO in the range 1800-1835 cm^-1 in the IR spectra. Molecular structures of ω-aryl ruthenium nitrosyl complexes 1a·CH₃OH· 2CH₂Cl₂·H₂O and 2a·2CH ₂Cl₂ were determined by X-ray crystallography. Nitrosylation at the metal centre, oxidative cyclization and ligand nitration were authenticated from the crystal structures. The redox properties of the metal centre were investigated. In both the nitrosyl complexes 1a and 2a, coordinated NO was found to be photolabile. The amount of photoreleased NO was estimated by using the Griess reagent and the data was compared with that obtained from sodium nitroprusside (SNP). The role of the nitro group in the ligand frame was discussed with regard to orthometallation, NO reactivity and photolability. Ruthenium(III) cyclometalates were synthesized by site-specific orthometallation. The reactivity of nitric oxide was investigated and the photolability of nitric oxide in newly synthesized ruthenium nitrosyl complexes was examined. The amount of photoreleased NO was estimated by using a Griess reagent in UV as well as in visible light. The role of the nitro group in the ligand frame was scrutinized. Copyright

Full text of DOI:10.1002/ejic.201101026

FULL PAPER
DOI: 10.1002/ejic.201101026
Ruthenium(III) Cyclometalates Obtained by Site-Specific Orthometallation
and Their Reactivity with Nitric Oxide: Photoinduced Release and Estimation
of NO Liberated from the Ruthenium Nitrosyl Complexes
Kaushik Ghosh,*[a] Sushil Kumar,[a] Rajan Kumar,[a] and Udai P. Singh[a]
Keywords: Ruthenium / Redox chemistry / Metalation / Photolability
Cyclometalated RuIII complexes [Ru(LSB1)(PPh3)2Cl] (1),
[LSB1H2
2-(3-nitrobenzylideneamino)phenol], and [Ru-
(LSB2)(PPh3)2Cl] (2), [LSB2H2
4-methyl-2-(3-nitrobenzyl-
NMR and 31P NMR spectral studies. Both 1a and 2a exhibited
νNO in the range 1800–1835 cm–1 in the IR spectra. Molecular
structures of σ-aryl ruthenium nitrosyl complexes
1a·CH3OH·2CH2Cl2·H2O and 2a·2CH2Cl2 were determined
by X-ray crystallography. Nitrosylation at the metal centre,
oxidative cyclization and ligand nitration were authenticated
from the crystal structures. The redox properties of the metal
centre were investigated. In both the nitrosyl complexes 1a
and 2a, coordinated NO was found to be photolabile. The
amount of photoreleased NO was estimated by using the
Griess reagent and the data was compared with that ob-
tained from sodium nitroprusside (SNP). The role of the nitro
group in the ligand frame was discussed with regard to or-
thometallation, NO reactivity and photolability.
=
=
ideneamino)phenol and H = dissociable protons], were syn-
thesized by site-specific orthometallation and characterized
by spectroscopic and electrochemical studies. Complexes 1
and 2 were treated with in situ generated nitric oxide (NO),
derived from an acidified nitrite solution, which afforded the
ruthenium nitrosyl complexes [Ru(LSB3H)(PPh3)2(NO)-
Cl](ClO4) (1a) and [Ru(LBOX)(PPh3)2(NO)Cl](ClO4) (2a),
respectively, [LSB3H2 = 4-nitro-2-(3-nitrobenzylideneamino)-
phenol, LBOX
H = 5-methyl-7-nitro-2-(3-nitrophenyl)benz-
oxazole and H = dissociable protons]. Complexes 1a and 2a
1
were found to be diamagnetic and were characterized by H
1 and type 2 ligands. To the best of our knowledge, there is
no report in the literature where a type 1 or type 2 ligand
was used to provide more than one site for the metal centre
to establish a ruthenium–carbon bond during the synthesis
of ruthenium cyclometalates. Hence, in this endeavour, we
have chosen Schiff-base ligands derived from m-nitrobenz-
aldehyde and 2-aminophenol and substituted 2-amino-
phenol [shown in Scheme 1 (B)] to investigate the mode of
σ-aryl ruthenium–carbon bond formation.
Introduction
Organometallic ruthenium cyclometalates incorporating
trivalent ruthenium are scarce.[1–6] An investigation of the
literature of trivalent ruthenium organometallic complexes
revealed that the complexes are formed through C–H acti-
vation and the concomitant formation of a σ-aryl ruthe-
nium–carbon bond. It has been found that in the case of
ruthenium(III) cyclometalates derived from Schiff-base li-
gands, the metal–carbon bond is established through ortho-
metallation with the phenyl ring that has an aldehyde func-
tional group, see Scheme 1 (A), X = CH, Type 1.[3] On the
other hand, there is another group of structurally similar
complexes where instead of azomethine nitrogen, the azo
function is bound to the RuIII centre. In these types of com-
plexes, orthometallation is observed with the ring that has
been contributed by the aromatic amine during the azo cou-
pling reaction, as shown in Scheme 1 (A), X = N, Type
2.[4,6] In both cases either para-substituted benzaldehyde,
during Schiff-base formation, or para-substituted aromatic
amines were used during the ligand synthesis. Hence, for
the synthesis of the ruthenium complexes, a single site for
orthometallation was found to be available in both the type
[a] Department of Chemistry, Indian Institute of Technology,
Roorkee, Roorkee 247667, Uttarakhand, India
Fax: +91-1332-273560
E-mail: ghoshfcy@iitr.ernet.in
Scheme 1.
Eur. J. Inorg. Chem. 2012, 929–938
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K. Ghosh, S. Kumar, R. Kumar, U. P. Singh
FULL PAPER
Synthesis and characterization of ruthenium nitrosyl
Molecular structures of 1a·CH3OH·2CH2Cl2·H2O and
complexes and studies on the generation of nitric oxide 2a·2CH2Cl2 were determined by X-ray crystallography. The
(NO) on demand have recieved considerable attention.[7–9] redox properties of the metal centre in these complexes were
An investigation of the literature revealed that there is no investigated in order to better understand the stabilization
report on the reactivity of nitric oxide with ruthenium cy- of the ruthenium oxidation state after orthometallation.
clometalates incorporating trivalent ruthenium. To the best The photolability of coordinated NO was authenticated by
of our knowledge, a single report by Crutchley and cowork- UV/Vis spectral studies and liberated NO was trapped by
ers[5] is available in the literature, however, the nitrosyl com- reduced myoglobin. Photoreleased NO was not estimated
plex was not utilized for the generation of NO. This work in our previous reports,[10,11] however, in this report we esti-
stems from our interest to synthesize novel ruthenium ni- mated NO after photodissociation by a Griess reagent using
trosyl complexes derived from these ruthenium cyclomet- visible as well as UV light. Sodium nitroprusside (SNP),
alates. In recent reports, we have described the role of trans- which is utilized for the treatment of cardiovascular disor-
directing carbanion in the coordination and photolability der and blood pressure regulation through the release of
of NO.[10,11] Moreover, we have observed the effect of sub- NO,[12–14] was used as the standard.
stituents on the phenyl ring containing the phenolato func-
The role of the electron-withdrawing –NO2 group on the
tion. In our previous reports, we always used para-substi- phenyl ring, ligated to the metal centre through a σ-aryl
tuted benzaldehydes for the synthesis of Schiff bases and bond, will be discussed in the context of light-induced deliv-
the effect of the substituent on the phenyl ring containing ery of nitric oxide.
the aldehyde function was not discussed. Hence, meta-sub-
stituted benzaldehyde has never been used for the synthesis
of ruthenium cyclometalates (vide supra) as well as ruthe-
nium nitrosyl complexes derived from those cyclometalates.
Results and Discussion
Herein we report on the synthesis and characterization of
ruthenium cyclometalates [Ru(LSB1)(PPh3)2Cl] (1) [where
LSB1H2 is 2-(3-nitrobenzylideneamino)phenol] and [Ru-
(LSB2)(PPh3)2Cl] (2) [where LSB2H2 is 4-methyl-2-(3-nitro-
benzylideneamino)phenol] [H is the dissociable proton,
Scheme 1 (B)] derived from Schiff-base ligands LSB1H2 and
LSB2H2, respectively, and their site-specific orthometalla-
tion (Scheme 2).
Synthesis
The
complexes
[Ru(LSB1)(PPh3)2Cl]
(1)
and
[Ru(LSB2)(PPh3)2Cl] (2) were synthesized by the reaction of
Ru(PPh3)3Cl2 with the Schiff base ligands LSB1H2 and
LSB2H2, respectively, in ethanol.[3] Complexes 1 and 2
(Scheme 2) were red-brown in colour and both complexes
were isolated in good yield. They were highly soluble in
dichloromethane and benzene but were much less soluble in
polar solvents like water and methanol. Dichloromethane
solutions of 1 and 2 were treated with in situ generated
NO derived from an acidified nitrite (NaNO2) solution with
continuous stirring for 2 h.[10] The red-brown colour of the
solution disappeared and the formation of an orange-yel-
low colour was observed. A methanolic solution of NaClO4
–
was added to provide ClO4 ions to stabilize the large cat-
ionic ruthenium nitrosyl complexes. [Ru(LSB3H)(PPh3)2-
(NO)Cl](ClO4) (1a) and [Ru(LBOX)(PPh3)2(NO)Cl](ClO4)
(2a) (Scheme 3) were derived from 1 and 2, respectively.
Both the nitrosyl complexes 1a and 2a were recrystallized
from a dichloromethane/methanol mixture. Complexes 1a
and 2a were highly soluble in organic solvents like dichloro-
methane, acetonitrile, dimethylformamide and methanol,
however, lower solubility was observed in water.
Scheme 2.
The interaction of NO with these organometallic ruthe-
nium complexes was investigated and the resultant com-
plexes [Ru(LSB3H)(PPh3)2(NO)Cl](ClO4) (1a) and [Ru-
(LBOX)(PPh3)2(NO)Cl](ClO4) (2a) [LSB3H2 = 4-nitro-2-(3-
nitrobenzylideneamino)phenol, LBOXH = 5-methyl-7-nitro-
2-(3-nitrophenyl)benzoxazole and H = dissociable protons]
were synthesized and characterized (Scheme 3).
Description of Structures
The molecular structures of complexes [Ru(LSB3H)-
(PPh3)2(NO)Cl](ClO4)·CH3OH·2CH2Cl2·H2O (1a·CH3OH·
2CH2Cl2·H2O)
and
[Ru(LBOX)(PPh3)2(NO)Cl](ClO4)·
2CH2Cl2 (2a·2CH2Cl2) are depicted in Figure 1 and Fig-
ure 2, respectively. The matrix parameters of these com-
plexes are described in Table 4 and the selected bond
lengths and bond angles are given in Table 1.
Scheme 3.
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Eur. J. Inorg. Chem. 2012, 929–938

Ruthenium(III) Cyclometalates
Table 1. Selected bond lengths and bond angles of com-
plexes [Ru(LSB3H)(PPh3)2(NO)Cl](ClO4)·CH3OH·2CH2Cl2·H2O
(1a·CH3OH·2CH2Cl2·H2O) and [Ru(LBOX)(PPh3)2(NO)Cl]ClO4·
2CH2Cl2 (2a·2CH2Cl2).
Bond lengths [Å]
Bond angles [°]
1a·CH3OH·2CH2Cl2·H2O
Ru(1)–Cl(1) 2.3755(10)
N(2)–Ru(1)–C(1)
N(2)–Ru(1)–Cl(1) 100.64(8)
C(1)–Ru(1)–Cl(1) 88.38(8)
N(2)–Ru(1)–P(1)
N(1)–Ru(1)–P(1)
C(1)–Ru(1)–P(1)
P(1)–Ru(1)–P(2)
O(1)–N(2)–Ru(1)
170.81(10)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–C(1)
Ru(1)–N(2)
Ru(1)–N(1)
N(2)–O(1)
2.4651(11)
2.4425(10)
2.100(2)
1.799(2)
2.137(2)
1.152(3)
93.76(7)
95.66(6)
85.07(7)
168.04(3)
170.90(2)
2a·2CH2Cl2
N(1)–Ru(1)–C(1)
Ru(1)–Cl(1) 2.3534(11)
77.84(15)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–C(1)
Ru(1)–N(1)
Ru(1)–N(2)
N(2)–O(5)
2.4435(12)
2.4306(12)
2.108(4)
2.105(3)
1.786(4)
1.140(5)
N(2)–Ru(1)–Cl(1) 97.47(12)
C(1)–Ru(1)–Cl(1) 91.30(12)
C(1)–Ru(1)–P(1)
Cl(1)–Ru(1)–P(2)
P(1)–Ru(1)–P(2)
O(5)–N(2)–Ru(1)
Figure 1. ORTEP diagram (50% probability level) of the cation of
the complex [Ru(LSB3H)(PPh3)2(NO)Cl](ClO4)·CH3OH·2CH2Cl2·
H2O (1a·CH3OH·2CH2Cl2·H2O). All hydrogen atoms and solvent
molecules are omitted for clarity.
86.63(12)
90.23(4)
169.78(4)
170.90(4)
The molecular structure of the nitrosyl complex
2a·2CH2Cl2 is different from that of 1a·CH3OH·
2CH2Cl2·H2O in certain aspects. Similar to the previous
structure, it has been found out that the phenolato function
is not attached to the ruthenium centre and that the ligand
is chelated in a bidentate fashion, however, in the case of
2a·2CH2Cl2 oxidative cyclization[10] was observed.
The carbanion (C1) and imine nitrogen (N1) from the
substituted 2-phenylbenzoxazole moiety along with the Cl–
and NO ligands constitute the equatorial plane whereas the
two trans-PPh3 groups are found at the axial positions. The
P1–Ru1–P2, N1–Ru1–Cl1 and C1–Ru1–N2 angles indicate
a distorted octahedral geometry around the metal centre.
On the other hand in 2a·2CH2Cl2 oxidative cyclization and
benzoxazole formation was observed. Hence phenolato
–OH is not present in the molecule and the ring with the
phenolato function is found to be in the plane with the li-
Figure 2. ORTEP diagram (50% probability level) of the cation
of the complex [Ru(LBOX)(PPh3)2(NO)Cl](ClO4)·2CH2Cl2 (2a·
2CH2Cl2). All hydrogen atoms and solvent molecules are omitted
for clarity.
1
gand. For this we did not find any –OH peak in the H
In the molecular structure of 1a·CH3OH·2CH2Cl2·H2O, NMR spectrum. The presence of trans-PPh3 groups in both
carbanion (C1), Cl(1), NO and the imine nitrogen (N1) the complexes is consistent with the 31P NMR spectro-
constitute the equatorial plane whereas the phosphane scopic data (vide infra).[15,16]
groups occupy the axial positions trans to each other. So,
In both the nitrosyl complexes, Ru–NNO distances
the geometry around the metal centre is found to be dis- [1.799(2) Å in 1a·CH3OH·2CH2Cl2·H2O and 1.786(4) Å in
torted
octahedral.
Interestingly,
in
1a·CH3OH· 2a·2CH2Cl2] were found to be longer than those re-
2CH2Cl2·H2O, the phenolato oxygen is not bound to the ported,[17–19] which may be due to the trans effect of carban-
metal centre and protonation gave rise to a phenolic –OH ion (C1).[4,6] However, this value was close to the value re-
function, which was consistent with our H NMR spectro- ported by Crutchley and coworkers.[5] The N–O distance in
1
scopic data (vide infra). Hence, in this structure we ob- both the complexes was found to be consistent with the
served that the tridentate ligand in the precursor complex 1 values given in the literature.[16–18] All these data along with
became bidentate in the σ-aryl ruthenium nitrosyl complex Ru–N–O angles (≈171°) in 1a·CH3OH·2CH2Cl2·H2O and
1a·CH3OH·2CH2Cl2·H2O. The tilting of the phenyl ring 2a·2CH2Cl2 clearly show a {RuII–NO+}6 description[7] of
containing a phenolato function afforded a dihedral angle the {Ru–NO}6 moiety.[20] This is also supported by IR spec-
of ca. 47° with the ligand binding plane. Moreover, ligand troscopic data described in the next section.
nitration was observed on the phenyl ring containing the
The noncovalent interactions are very important in su-
phenolato function and the site of nitration was trans to the pramolecular chemistry and crystal engineering.[21] The
phenolato function.
interactions found in 1a·CH3OH·2CH2Cl2·H2O and
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K. Ghosh, S. Kumar, R. Kumar, U. P. Singh
FULL PAPER
2a·2CH2Cl2 are hydrogen bonding and are described in ence of a phenolic O–H proton. This peak disappeared after
Table S1 and the interactions are shown in Figure S5–Fig- shaking the NMR sample solution in D2O, which sup-
ure S7 in the Supporting Information.
ported the presence of an exchangeable proton in the –OH
Molecular structures of 1a·CH3OH·2CH2Cl2·H2O and functional group (Figure S14).[11,23] These data supported a
2a·2CH2Cl2 clearly authenticated that nitrosylation and the dissociation of the Ru–OPh bond during NO coordination
ligand nitration were two events that were common in both in 1a.
1
cases during our NO interaction studies.
The H NMR spectrum of 2a clearly shows the presence
The sites for nitration were different in the two cases; this of the methyl protons at ca. 2.5 ppm along with other pro-
occurs because the preferred para position (with respect to tons present in the complex. Interestingly, no peak was
the phenolato function)[22] for nitration was blocked by the found above 10.0 ppm, which indicates the absence of a
methyl group in complex 2a. However, for complex 2a the phenolic O–H proton in complex 2a. This data was consis-
oxidative cyclization and formation of the benzoxazole de- tent with our findings from structural calculations by X-ray
rivative showed a rare type of reactivity of NO with this crystallography and supported oxidative cyclization during
family of complexes (vide infra).
the NO interaction. 31P NMR spectra of the complexes 1a
and 2a in CD3CN indicate a single peak at about 20 ppm
and ca. 18 ppm, respectively, confirming the trans disposi-
tion of the PPh3 groups.[15,16]
Spectroscopic Studies
Complexes 1 and 2 are red-brown in colour and the elec-
tronic spectra in dichloromethane are displayed in Fig-
ure S8. It has been found[3,10] that organometallic ruthe-
nium complexes derived from type 1 ligands exhibit dif-
Site Specific Orthometallation: A Reaction Model
The reaction of Schiff-base ligands with Ru(PPh3)3Cl2 af-
ferent colours from the change of substituents on the forded organometallic ruthenium cyclometalates through
phenyl ring containing an aldehyde function. Complex 1 orthometallation. Considering the orientation of the phenyl
afforded two charge-transfer bands near 475 nm and ring of m-nitrobenzaldehyde (Scheme 1, B), there are two
585 nm. On the other hand, the electronic spectrum of 2 possible sites for orthometallation. A crucial point for this
gave rise to charge-transfer bands near 580 nm and 600 nm. reaction is that we ended up with a single product during
These bands are probably a result of the ligand-to-metal the preparation of ruthenium cyclometalates and interest-
charge transfer (LMCT) transition.[1–3,6] The electronic ab- ingly, after orthometallation the nitro group was found to
sorption spectra of complexes 1a and 2a (shown in Fig- be at a para position with respect to the Ru–C bond. It is
ure S9) exhibit one strong band near 312 nm along with a reported in the literature that during orthometallation the
shoulder near 385 nm. These bands are probably from the metal centre could act as a nucleophile or an electro-
charge-transfer transitions.[11]
The infrared spectra of all the complexes exhibit a band seems to proceed by electrophilic attack on the aryl ring by
near 1590 cm–1 corresponding to the azomethine (νC=N
the metal centre. The –I as well as –R effect of the –NO2
stretching frequency, which is similar to the value reported group will decrease the electron density at the ortho and
by Chakravorty and coworkers.[3]
para positions, however, the effect at the para position will
phile.[24–26] In the present study, the C–H bond activation
)
The presence of the {Ru–NO}6 moiety[10,11] in both the be lower compared to that at the ortho position. We specu-
nitrosyl complexes was also confirmed by the characteristic late that the orthometallation occurred at the position para
peaks in the infrared spectra. Complexes 1a and 2a have to the –NO2 group, which had better electron density com-
N–O stretching frequencies (νNO) near 1810 cm–1 and pared with the ortho position. Moreover, stability of the re-
1835 cm–1, respectively. These values clearly indicate the sultant complexes 1 and 2 may be obtained by keeping the
presence of NO+ bound to the RuII centre and a description –NO2 group at the para position with respect to the Ru–C
of {RuII–NO+}6 for the {Ru–NO}6 moiety present in 1a bond (vide infra).
and 2a is proposed.[7] This is also supported by the data
obtained from their crystal structures (vide supra). The
Electrochemistry
peaks around 1090 cm–1 and 623 cm–1 show the presence of
perchlorate ion in the complexes 1a and 2a, respectively.
The redox property of the metal centre was measured by
These complexes also afforded new bands in the range be- cyclic voltammetry (Figure 3) in a dichloromethane solu-
tween 1290 and 1380 cm–1 and these bands are probably tion using 0.1 m TBAP as the supporting electrolyte. Com-
from the ring nitration (shown in the Supporting Infor- plex 1 exhibits quasi-reversible redox couples and E1/2 val-
mation). In all the complexes, the peaks near 745 cm–1, ues are found to be at –0.42 V and +0.80 V vs. Ag/AgCl.
695 cm–1 and 520 cm–1 show the presence of PPh3 However, these two values for complex 2 are at –0.44 V and
groups.[1,2,6,11]
+0.72 V vs. Ag/AgCl [Figure 3 (a)]. The redox couples at
The nitrosylation of the metal centre in the ruthenium the negative potential in 1 and 2 are assigned to the RuIII
–
cyclometalates 1a and 2a afforded clean NMR spectra that RuII couple and those at the positive potential in both the
confirmed the diamagnetic behaviour of complexes with an complexes are described as a RuIII–RuIV couple.[1–4] The cy-
S = 0 ground state. In the 1H NMR spectrum of 1a (shown clic voltammetric data for the complexes are given in
in Figure S10), the peak at δ = 11.84 ppm indicates the pres- Table 2.
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Ruthenium(III) Cyclometalates
Ag/AgCl, respectively. Hence in the nitrosyl complexes we
found better stabilization of RuII. In the precursor com-
plexes (1 and 2) we have determined the effect of the posi-
tion of the nitro group on the phenyl ring, which was at-
tached to the ruthenium centre through a σ-aryl Ru–C
bond. This prompted us to examine the stabilization of ru-
thenium(II) in complexes 1a and 2a resulting from the
change in position of the –NO2 group in the ligand frame.
A small decrease in the negative potential was obtained for
complex 2a compared with a similar complex reported re-
cently.[10] After examination of electrochemical data of this
report and our previous report,[10] we found that E1/2 values
for the RuII/RuIII couple were near –0.20 V vs. Ag/AgCl for
those complexes where oxidative cyclization has occurred.
Other complexes, where we did not find the cyclization, af-
forded the same value in the range –0.30 V to –0.40 V vs.
Ag/AgCl.
Photolysis Experiments of Nitrosyls
The photolability of coordinated NO of σ-aryl ruthe-
nium nitrosyl complexes was examined in dichloromethane
(for 1a) and acetonitrile solutions (for 2a). Solutions of the
Figure 3. Cyclic voltammograms of 10–3 m solutions of (a) complex
1 (solid line) and complex 2 (dashed line). (b) Complex 1a and (c)
complex 2a in dichloromethane, in the presence of 0.1 m tetra-
butylammonium perchlorate (TBAP), using a working electrode
(glassy-carbon), reference electrode (Ag/AgCl) and auxiliary elec-
trode (platinum wire), scan rate = 0.1 Vs–1.
Table 2. Cyclic voltammetric data for complexes at 298 K. Condi-
tions: solvent dichloromethane; supporting electrolyte TBAP
(0.1 m); working electrode glassy carbon; reference electrode Ag/
AgCl; scan rate 0.1 Vs–1; concentration 10–3 m.
Complex
E1/2 [V][a] (ΔEp [mV])[b]
RuIII–RuII
RuIII–RuIV
1
–0.42 (117)
–0.44 (109)
–0.32 (102)
–0.18 (190)
+0.80 (118)
+0.72 (114)
2
1a
2a
–
–
[a] E1/2 = 0.5 (Epa + Epc) and [b] ΔEp = (Epa – Epc), where Epa and
Epc are the anodic and cathodic peak potentials, respectively.
We have compared these E1/2 values with the data re-
ported by Chakravorty and coworkers (especially for com-
plexes 5 and 5a in that report).[3] A comparison of the E1/2
values for the RuIII–RuII redox couple clearly indicate better
stabilization of the RuII in complexes 1 and 2. It is imporant
to note here that in this report because of orthometallation,
the nitro group was situated para to the carbanion that was
involved in Ru–C bond formation. On the other hand, in
the above report[3] the nitro group was found to be at the
meta position with respect to the carbanion. Hence, the
presence of the –NO2 group in the para position with re-
spect to the Ru–C bond is probably responsible for the bet-
ter stabilization of ruthenium(II) in this class of organome-
tallic ruthenium cyclometalates.
Figure 4. Photodissociation of (a) complex 1a (ca. 1.1ϫ10–5 m) in
dichloromethane and (b) complex 2a (ca. 1.8ϫ10–5 m) in CH3CN
under illumination with a 100 W tungsten lamp. Repetitive scans
were taken at 30 s intervals for 1a and at 1 min intervals for 2a.
Inset: Time-dependent changes in absorbance at λ = 311 nm and λ
In both the nitrosyl complexes 1a and 2a, quasi-revers-
ible cyclic voltammograms are obtained [shown in Fig-
ure 3(b) and (c)] with E1/2 values of –0.32 V and –0.18 V vs. = 314 nm for 1a and 2a, respectively.
Eur. J. Inorg. Chem. 2012, 929–938
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933

K. Ghosh, S. Kumar, R. Kumar, U. P. Singh
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complexes were stable in the dark, however, when these important to mention here that in the dark we observed
solutions were exposed to visible light (100 W tungsten little change in absorbance at 538 nm, which indicated the
lamp), a rapid change in colour from orange-yellow to red- loss of NO from 1a and 2a. The loss of NO was found to be
brown was observed.
ca. 1.5% with respect to the concentration of the complexes
Spectral changes upon irradiation of a solution of 1a (50 μm) in solution. Using 50 μm of both of the complexes
(≈10–5 m) in dichloromethane with a 100 W tungsten lamp 1a and 2a, we found that in the dark the concentration of
are shown in Figure 4(a) and the spectral changes give rise
to three clear isosbestic points near 265 nm, 280 nm and
350 nm. Interestingly we observed that after the photolysis
experiment of 1a, the electronic absorption spectrum of the
resulting solution was similar to the spectrum of 1. This
prompted us to characterize the resultant complex after the
photocleavage reaction. The resultant complex was charac-
terized by spectroscopic and electrochemical studies (shown
in the Supporting Information, Figure S15). The data indi-
cated the formation of ligand nitrated 1 (1b). This complex
(1b) also reacts with nitric oxide and liberates NO under
photolytic conditions (Scheme S1). Similar spectral changes
were also observed with complex 2a in an acetonitrile
[shown in Figure 4(b)] solution. In this case, the isosbestic
points are near 278 nm and 360 nm. A comparison of the
data dispalyed in the inset clearly shows that in the presence
of visible light, the release of NO in 2a is slower than that
for 1a.
Figure 5. Electronic spectra of the formation of dye when the
Griess reagent (100 μL) was treated with complex 1a (50 μm) in the
presence of light (100 W tungsten lamp). Repetitive scans were
taken at 1 min intervals. Inset: Time-dependent change in ab-
sorbance at λ = 538 nm.
Transfer of NO to Myoglobin
The photocleavage of the coordinated NO was also con-
firmed by trapping the liberated NO by reduced myoglobin
(shown in Figure S16). When an acetonitrile solution of
complex 1a was added to a buffer solution of reduced myo-
globin under dark conditions no reaction was observed.
However, when the same mixture was exposed to a tungsten
lamp (100 W) for 2–3 min, its absorption spectrum at
420 nm showed the formation of a Mb–NO adduct.[11,27]
Determination of the Concentration of NO by Griess
Reaction
The amount of NO released by the complexes 1a and 2a,
synthesized in this report, was determined by using the
Griess reagent.[28] We prepared the standard curve with dif-
ferent concentrations of NaNO2 (5–50 μm) in solution
(shown in Figure S18). From the molar extinction coeffi-
cient of dye (at 540 nm, ε = 41000 m–1 cm–1)[29] the NaNO2
standard curve predicted that the concentration of NO gen-
erated from an acidified nitrite solution was the concentra-
tion of NaNO2 (Ϯ5 μm). The change in the absorbance of
the dye (produced by the Griess reaction) in the dark as
well as in the presence of light (UV and visible) clearly ex-
pressed the presence of photolabile NO in the nitrosyl com-
plexes and this observation also supported our previous ex-
periments, namely electronic absorption spectral studies
(Figure 4) and trapping of NO by reduced myoglobin (Fig-
ure S16). The generation of a peak near 538 nm clearly indi-
cates the formation of NO in solution. Hence, we could
predict that photolabile coordinated NO was responsible
Figure 6. Bar diagrams showing the amount of photoreleased NO
(dye formation) from the complexes under exposure to (a) visible
for the change in absorbance at the same wavelength. It is light and (b) UV light for 15 min.
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Ruthenium(III) Cyclometalates
produced dye was ca. 0.8 μm. Shining of visible light (100 W phenolato function was not necessary for the cyclization re-
tungsten lamp) onto the 50 μm solution of 1a for 10–15 min action for the complexes obtained from p-nitrobenzalde-
gave rise to the formation of about 23.0 μm of dye (Fig- hyde [described in Scheme 4 (B)].
ure 5), however, the solution of 2a with the same concentra-
tion produced only ca. 6 μm of azo dye in visible light [Fig-
ure 6(a), Table 3]. We have compared our data with the data
obtained from sodium nitroprusside (SNP), a donor of ni-
tric oxide.[12] In visible light [Figure 6 (a), Table 3], a 50 μm
solution of sodium nitroprusside produced a very small
amount of nitric oxide (ca. 0.2 μm in 15 min), however, in
ultravoilet light [Figure 6 (b), Table 3], the same solution
provided about 4.0 μm of nitric oxide.
Table 3. Determination of the amount of dye produced from the
complexes 1a, 2a and sodium nitroprusside (SNP) on reaction with
the Griess reagent in the dark, visible light and UV light.
Complex Complex
Concentration of dye produced [μm][a]
conc. [μm]
In the
dark
Exposure to Exposure to UV
visible light
light
1a
50
50
50
0.73
0.84
0.04
23.00
5.74
0.22
24.0
12.0
3.4
2a
SNP
[a] Average of three experiments.
These data afforded the formation of NO in solution and
the concentration of photoreleased NO in complexes 1a
and 2a was found to be greater in comparison with the NO
released by SNP.
The quantum yield (φ) values for complexes 1a and 2a
(λirr = 365 nm) were found to be 0.017Ϯ0.001 and
0.012Ϯ0.001, respectively, in acetonitrile solutions. These
data showed a slightly higher NO donor capacity for 1a
than for 2a.
Role of the –NO2 Group in Oxidative Cyclization and NO
Delivery
It is important to note here that in this study of NO
reactivity we have found oxidative cyclization and concomi-
tant formation of the substituted 2-phenylbenzoxazole de-
rivative, only when a methyl group was present in the posi-
tion trans to the phenolato function [described in Scheme 4
(A)]. We have compared these results with our previous ob-
servations.[10] Moreover, to better understand the findings,
we studied the synthesis of another ruthenium nitrosyl com-
Scheme 4.
Conclusions
The following are the principle findings of the present
plex 3s (described in the Supporting Information, study. The organometallic ruthenium(III) cyclometalates 1
Scheme S2) in order to observe the positional effect of the and 2 were synthesized by site-specific orthometallation. In
–NO2 group. The following points are important in this re- these complexes, the metal–carbon bond was established in
1
gard: First, the H NMR spectrum of 3s (Figure S22) did such a way that the electron-withdrawing –NO2 group was
not afford the characteristic peak for the –OH function. found to be at the para position with respect to the Ru–C
Second, electrochemical data for 3s (Figure S23) afforded bond. The interaction of nitric oxide with these complexes
the E1/2 redox couple for RuII/RuIII at –0.21 V vs. Ag/AgCl. was investigated and complexes 1a and 2a were obtained
Third, UV/Vis spectroscopic data (Figure S24) for the pho- from 1 and 2, respectively. The trans effect of carbanion
tocleavage experiment of NO in 3s resembles that of 4s gave rise to dissociation of the phenolato function and NO
(complex 2 of ref.[10]). Four, the amount of liberated NO was coordinated to the metal centre. Molecular structures
from 3s and 4s was similar to that of 2a (Table S2, Fig- of 1a·CH3OH·2CH2Cl2·H2O and 2a·2CH2Cl2 were deter-
ure S25 and S26). All these data clearly indicate that the mined by X-ray crystallography. Interestingly, structure de-
presence of the methyl group on the ring containing the termination clearly authenticated three important events of
Eur. J. Inorg. Chem. 2012, 929–938
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935

K. Ghosh, S. Kumar, R. Kumar, U. P. Singh
FULL PAPER
vents with an Evolution 600, Thermo Scientific UV/Vis spectro-
photometer. Infrared spectra were obtained as KBr pellets with a
Thermo Nicolet Nexus FTIR spectrometer, using 16 scans and
were reported in cm–1. 1H and 31P NMR spectra were recorded
with a Bruker AVANCE, 500.13 MHz spectrometer in the deuter-
ated solvents. Cyclic voltammetric studies were performed with a
CH-600 electroanalyser in dichloromethane with 0.1 m tetrabu-
tylammonium perchlorate (TBAP) as the supporting electrolyte.
The working electrode, reference electrode and auxiliary electrode
were glassy carbon electrode, Ag/AgCl electrode and Pt wire,
respectively. The concentration of the compounds was in the order
the NO rectivity studies. First, nitrosylation of the metal
centre and synthesis of the ruthenium nitrosyl complexes.
Second, ligand nitration was observed in both nitrosyl com-
plexes. Third, oxidative cyclization and formation of the be-
noxazole ring in 2a.
Photolability of NO was authenticated by UV/Vis spec-
tral studies and trapping experiments. In 1a, the ruthe-
nium–phenolato bond was reestablished after the photore-
lease of coordinated NO. The Ru–N–O angle and N–O
bond distances clearly indicated the presence of
a
of 10–3 m. The ferrocene/ferrocenium couple occurred at E1/2
=
{RuIINO+}6 moiety in both the nitrosyl complexes. Both 1a
+0.51 (102) V vs. Ag/AgCl (scan rate 0.1 Vs–1) in dichloromethane
under the same experimental conditions. Quantum yields were de-
termined by actinometry studies using a ferric oxalate solution. The
intensity of the light (λirr = 365 nm) was determined using a ferriox-
alate actinometer (0.006 m solution of potassium ferrioxalate in
0.1 n H2SO4).[31–33] Quantum yields (φ) of the NO photorelease for
complexes 1a and 2a were determined from the decrease in their
absorption bands near 310 nm and 314 nm when irradiated with
365 nm light and were calculated by following the procedure re-
1
and 2a exhibited H NMR as well as 31P NMR spectra.
These findings were supported by the diamagnetic behav-
iour of 1a and 2a and NO stretching frequencies in the IR
spectral studies. An electrochemical investigation indicated
RuII–RuIII and RuIII–RuIV redox couples for 1 and 2. On
the other hand, only a RuII–RuIII couple was obtained for
complexes 1a and 2a.
We found that the methyl group was necessary on the
ring containing the phenolato function for oxidative cycliza- ported earlier.[7,33,34]
tion in the complexes described in this report. However, for
Preparation of Complexes
the ruthenium cyclometalates obtained from p-nitrobenzal-
Caution: Perchlorate salts of metal complexes with organic ligands
are potentially explosive. Only a small amount of material should
be prepared and handled carefully.
dehyde[10] the presence of a methyl group was not impor-
tant.
We have estimated the amount of available NO in solu-
tion during these photolytic reactions and we compared our
data with the same obtained from sodium nitroprusside. It
has been found that complex 1a was equally efficient in NO
delivery in visible and UV light and that the amount of NO
produced by 1a was higher than that of 2a. Complexes 3s
and 4s were similar to 2a in terms of NO generation in
solution. Ruthenium cyclometalates have never been used
for photoinduced delivery of nitric oxide[30] and complexes
described in this report demand their application in photo-
dynamic therapy.
Synthesis of Ligands: The Schiff base ligands LSB1H2 [2-(3-nitro-
benzylideneamino)phenol] and LSB2H2 [4-methyl-2-(3-nitrobenzyl-
ideneamino)phenol] were synthesized by the condensation of 3-ni-
trobenzaldehyde with 2-aminophenol and 2-amino-4-methylphen-
ol, respectively, in ethanol by following the procedure reported by
Chakravorty and his coworkers.[3]
The precursor complex [Ru(PPh3)3Cl2] was prepared by the pro-
cedure reported earlier.[35]
[Ru(LSB1)(PPh3)2Cl] (1): A batch of LSB1H2 (0.036 g, 0.15 mmol)
and ethanol (5 mL) was added to a warm solution of Ru(PPh3)3Cl2
(0.096 g, 0.10 mmol) in ethanol (30 mL). The mixture was heated
under reflux for 2 h and was then allowed to cool to obtain a pre-
cipitate of a red-brown colour. The solid was filtered out and was
washed thoroughly with ethanol and diethyl ether and was then
dried. Complex 1 (0.062 g, 0.069 mmol) was eluted on an alumina
A study of the effect of the electron-donating group of
the phenyl ring containing an aldehyde function in the co-
ordination and photolability of NO is under progress. Bio-
logical applications of these complexes along with the syn-
thesis of substituted benzoxazole are also being investi- column by a dichloromethane/hexane (1:1) mixture; yield 69%. IR
(KBr disk): ν = 1580 (νC=N), 1482, 1434, 1332, 1310 (νNO2), 1284,
˜
gated.
742, 695, 515 (νPPh3) cm–1. UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) =
267 (38182), 371 (13636), 475 (5636), 585 (2273) nm.
[Ru(LSB2)(PPh3)2Cl] (2): This was prepared by the same procedure
as that used for 1, with Ru(PPh3)3Cl2 (0.096 g, 0.10 mmol) and
Experimental Section
LSB2H (0.038 g, 0.15 mmol); yield 63%. IR (KBr disk): ν = 1595
Materials: All the solvents used were of reagent grade. The analyti-
cal grade reagents; sodium nitrite, (Sigma Aldrich, Steinheim, Ger-
many), RuCl3·3H2O, triphenylphosphane (SRL, Mumbai, India),
2-aminophenol, 2-amino-4-methylphenol, 3-nitrobenzaldehyde, so-
dium perchlorate monohydrate, sulfanilamide, naphthylethylenedi-
amine dihydrochloride (NED) (Himedia Laboratories Pvt. Ltd.,
Mumbai, India), disodium hydrogen phosphate anhydrous (RFCL
Ltd. New Delhi, India) and sodium dihydrogen phosphate (Chem-
port India Pvt. Ltd. Mumbai, India) were used as obtained.
Double-distilled water was used in all of the experiments. Equine
skeletal muscle myoglobin was obtained from Sigma Aldrich,
Steinheim, Germany.
˜
2
(νC=N), 1570, 1482, 1433, 1330, 1309 (νNO2), 1298, 1266, 740, 693,
519 (νPPh3) cm–1. UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) = 270
(39231), 376 (12308), 580 (5385), 598 (2308) nm.
[Ru(LSB3H)(PPh3)2(NO)Cl]ClO4 (1a): [where LSB3H2 = 4-nitro-2-
(3-nitrobenzylideneamino)phenol]: A batch of complex 1 (0.016 g,
0.018 mmol) was dissolved in dichloromethane (25 mL) to obtain
a brownish-red coloured solution in a round-bottomed flask
(100 mL). Acidified distilled water (20 mL) was then layered over
this solution. Sodium nitrite (0.19 g, 2.7 mmol) was added to the
bilayer solution and the mixture was stirred at room temperature
for 1 h to obtain a yellowish-orange coloured solution of complex
1a. A dichloromethane layer was separated out and NaClO4 (in
excess) with methanol (5 mL) was added to this solution. Stirring
Physical Measurements: Electronic absorption spectra of all the
complexes were recorded in dichloromethane and acetonitrile sol-
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Eur. J. Inorg. Chem. 2012, 929–938

Ruthenium(III) Cyclometalates
of this solution was continued for another 1 h. A yellowish-orange
solid was precipitated out by the evaporation of the solvent. In
order to remove the excess NaClO4, the compound was further
dissolved in dichloromethane and was filtered off. Complex 1a
(0.012 g, 0.011 mmol) was eluted on an alumina column by a
dichloromethane/methanol (9:1) mixture. Single crystals of the
complex for X-ray crystallography were obtained within 2 d upon
slow evaporation of a dichloromethane/methanol mixture; yield
were performed with a Bruker Kappa Apex-II CCD diffractometer
by using graphite-monochromated Mo-Kα radiation (λ
=
0.71073 Å) at 273 K for 1a·CH3OH·2CH2Cl2·H2O and at 296 K
for 2a·2CH2Cl2 (Table 4). Crystal structures were solved by direct
methods. Structure solutions, refinement and data output were car-
ried out with the SHELXTL program.[36,37] All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were placed in
geometrically calculated positions and refined using a riding model.
Images were created with the DIAMOND program.[38]
61.11%. IR (KBr disk): ν = 1811 (νNO), 1590 (νC=N), 1480, 1434,
˜
1345, 1304 (νNO2), 1089, 623 (νClO4), 748, 693, 518 (νPPh3) cm–1.
UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) = 385 (5273), 311 (27455) nm.
Table 4. Crystal data and structural refinement parameters for
complexes 1a·CH3OH·2CH2Cl2·H2O and 2a·2CH2Cl2.
31P NMR (CD3CN, 500 MHz):
δ =
20.35 ppm. 1H NMR
[(CD3)2SO, 500 MHz]: δ = 11.84 (s, 1 H), 8.83 (s, 1 H), 8.47 (s, 1
H), 8.06 (d, 1 H), 7.46–7.21 (m, 31 H), 6.97 (s, 1 H), 6.95 (d, 1 H)
and 6.84 (d, 1 H) ppm.
[Ru(LBOX)(PPh3)2(NO)Cl]ClO4 (2a): {where LBOXH = 5-methyl-7-
nitro-2-(3-nitrophenyl)benzoxazole}: Complex 2a was prepared by
the same procedure as that used for 1a. Single crystals of the com-
plex 2a for X-ray crystallography were obtained within 3 d in the
dark upon slow evaporation of a dichloromethane/methanol mix-
1a·CH3OH·2CH2Cl2·H2O2a·2CH2Cl2
Empirical formula C52H48Cl6N4O14P2Ru C52H42Cl6N4O10P2Ru
Mr [gmol–1]
Temperature [K]
λ [Å] (Mo-Kα)
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
γ [°]
1328.65
273(2)
1258.61
296(2)
0.71073
monoclinic
P21/n
0.71073
triclinic
¯
P1
12.505(3)
13.488(3)
18.220(4)
93.11(3)
101.47(3)
106.87(3)
2861.5(13)
2
13.4992(4)
23.2857(7)
17.6737(5)
90
ture; yield 59%. IR (KBr disk): ν = 1837 (νNO), 1590, 1380, 1350,
˜
1233, 1091, 623 (νClO4), 750, 695, 517 (νPPh3) cm–1. UV/Vis
(CH3CN): λmax (ε, m–1 cm–1) = 384 (6722), 314 (26667) nm. 31P
90
1
NMR (CD3CN, 500 MHz): δ = 18.12 ppm. H NMR [(CD3)2SO,
β [°]
101.980(10)
5434.5(3)
4
V [Å3]
500 MHz]: δ = 8.49 (d, 1 H), 8.15 (s, 1 H), 7.69 (d, 1 H), 7.37–7.25
(m, 32 H), and 2.45 (s, 3 H) ppm.
Z
ρcalcd. [gcm–3]
F(000)
1.542
1352.0
1.538
2552.0
Transfer of NO to Myoglobin
Preparation of the Phosphate Buffer Solution: A phosphate buffer
solution (50 mm) of pH 6.8 was prepared by adding
NaH2PO4·2H2O (0.4192 g) and anhydrous Na2HPO4 (0.3283 g) to
MilliQ water (50 mL) and making the volume to 100 mL in a volu-
metric flask.
Theta range [°]
Index ranges
1.18–25.00
–14ϽhϽ14,
–16ϽkϽ16,
–21ϽlϽ21
1.47–28.35
–18ϽhϽ18,
–31ϽkϽ31,
–23ϽlϽ23
13337 / 0 / 677
Data / restraints / 10077 / 0 / 719
parameters
GOF[a] on F2
1.190
1.317
Preparation of the Myoglobin Stock Solution: Equine skeletal mus-
cle myoglobin (5 mg) was dissolved in the above prepared buffer
solution (5 mL).
[b]
R1 [IϾ2σ(I)]
0.0595
0.1394
0.1574
0.2141
0.0636
0.1143
0.1802
0.2155
R1 [all data]
[c]
wR2 [IϾ2σ(I)]
Binding of the Photoreleased NO with Myoglobin: Electronic ab-
sorption spectra were obtained against the phosphate buffer as ref-
erence. A myoglobin stock solution (100 μL) was diluted up to
1000 μL in a quartz cuvette with an optical length of 1 cm and
sealed with a rubber septum. Its UV/Vis spectrum showed an in-
tense band at 409 nm (Soret band). The UV/Vis spectrum of re-
duced myoglobin at 433 nm was obtained by the addition of excess
sodium dithionite to the same cuvette. When complex 1a was added
to the buffer solution of reduced myoglobin under dark conditions,
no reaction was observed. However, when the same mixture was
exposed to a tungsten lamp (100 W) for 2 min, its absorption spec-
trum at 420 nm showed the formation of a Mb–NO adduct.[11,27]
wR2 [all data]
[a] GOF = {Σ[w(Fo2 – Fc ) ]/M – N}1/2 (M = number of reflections,
2 2
N = number of parameters refined). [b] R1 = σ||Fo| – |Fc||/Σ|Fo|.
[c] wR2 = {Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2]}1/2
.
2
2 2
Supporting Information (see footnote on the first page of this arti-
cle): Characterization of complexes by IR, UV/Vis and NMR spec-
tral studies. The standard curve of NaNO2 for the Griess reaction.
X-ray crystallographic data of complexes 1a·CH3OH·2CH2Cl2·
H2O and 2a·2CH2Cl2 in CIF format.
Estimation of NO Production by the Griess Reaction: The pro-
duction of NO by complexes 1a and 2a was estimated using the
Griess reagent (GR).[28] It was prepared fresh by mixing equal vol-
umes of 1% sulfanilamide in 5% orthophosphoric acid and 0.1%
naphthylethylene diamine dihydrochloride (NED) in water. To esti-
mate the production of NO or NO2, the absorbance was measured
at 538 nm to determine the formation of azo dye. Aqueous solu-
tions of sodium nitrite with different concentrations (5–50 μm) were
used to prepare standard curves for the determination of nitrite.
Acknowledgments
K. G. is thankful to the Council of Scientific and Industrial Re-
search (CSIR), New Delhi for financial assistance [01(2229)/08/
EMR-II, dated 06 March, 2008]. S. K. and R. K. are thankful to
the CSIR for financial assistance. U. P. S. is thankful for support
by single-crystal X-ray facility of the IIT Roorkee.
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Eur. J. Inorg. Chem. 2012, 929–938
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937

K. Ghosh, S. Kumar, R. Kumar, U. P. Singh
FULL PAPER
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