Synthesis, crystal structure investigation, DFT analyses and antimicrobial studies of silver(i) complexes with N,N,N′,N′′-tetrakis(2- hydroxyethyl/propyl) ethylenediamine and tris(2-hydroxyethyl)amine

Article abstract of DOI:10.1039/c3nj00729d

The synthesis, crystal structures, DFT and antimicrobial studies of three novel complexes of silver: [Ag(THEEN)]₂(PIC)₂ (1), [Ag(THPEN)]₂(PIC)₂ (2) and [Ag(TEAH₃) ₂](PIC) (3), have been reported in the present work, where THEEN/THPEN (N,N,N′,N′′-tetrakis(2-hydroxyethyl/propyl) ethylenediamine) are tetrapodal ligands and TEAH₃ (tris(2-hydroxyethyl)amine) is a tripodal ligand. Complexes (1) and (2) are dinuclear, whereas (3) is mononuclear. Complex (1) adopted a see-saw geometry with coordination number four, whereas (2) and (3) are five coordinated. Complex (2) acquired distorted square-pyramidal geometry, whereas complex (3) acquired distorted trigonal-bipyramidal geometry. Extensive hydrogen bonding interactions have been found in all three complexes. The primary coordination sphere of the newly synthesized silver(i) complexes has been optimized, structural parameters have been calculated and the energy gaps of the frontier orbitals have been predicted with the B3LYP/6-31G/LANL2DZ level of theory. Structural parameters from the crystallographic and DFT studies are in good agreement with each other. The relatively smaller calculated HOMO-LUMO energy gaps (HLG) suggest charge transfer transitions. Antimicrobial studies have been performed with the new silver(i) complexes against gram +ve bacteria (Staphylococcus aureus), gram -ve bacteria (Serratia marcescens, Sphingobium japonicum and Stenotrophomonas maltophilia) and fungal species (Candida albicans, Aspergillus niger and Saccharomyces cerevisiae). Dinuclear complexes (1) and (2) exhibited remarkable results. All the synthesized silver(i) complexes have been proven to be better antibacterial and antifungal agents, even than their standard drugs (ciprofloxacin and fluconazole) and can be used as effective antimicrobial agents and potential drugs in the future.

Full text of DOI:10.1039/c3nj00729d

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Synthesis, crystal structure investigation, DFT analyses
and antimicrobial studies of silver(I) complexes with
N,N,N0,N00-tetrakis(2-hydroxyethyl/propyl)
Cite this:DOI: 10.1039/c3nj00729d
ethylenediamine and tris(2-hydroxyethyl)amine†
Rakesh Kumar,a Sangeeta Obrai,*a Amanpreet Kaur,b Maninder Singh Hundal,*b
Harmanpreet Meehnianc and Asim Kumar Janac
The synthesis, crystal structures, DFT and antimicrobial studies of three novel complexes of silver:
[Ag(THEEN)]2(PIC)2 (1), [Ag(THPEN)]2(PIC)2 (2) and [Ag(TEAH3)2](PIC) (3), have been reported in the
present work, where THEEN/THPEN (N,N,N0,N00-tetrakis(2-hydroxyethyl/propyl)ethylenediamine) are
tetrapodal ligands and TEAH3 (tris(2-hydroxyethyl)amine) is a tripodal ligand. Complexes (1) and (2) are
dinuclear, whereas (3) is mononuclear. Complex (1) adopted a see-saw geometry with coordination
number four, whereas (2) and (3) are five coordinated. Complex (2) acquired distorted square-pyramidal
geometry, whereas complex (3) acquired distorted trigonal-bipyramidal geometry. Extensive hydrogen
bonding interactions have been found in all three complexes. The primary coordination sphere of the
newly synthesized silver(I) complexes has been optimized, structural parameters have been calculated
and the energy gaps of the frontier orbitals have been predicted with the B3LYP/6-31G/LANL2DZ level
of theory. Structural parameters from the crystallographic and DFT studies are in good agreement with
each other. The relatively smaller calculated HOMO–LUMO energy gaps (HLG) suggest charge transfer
transitions. Antimicrobial studies have been performed with the new silver(I) complexes against gram
+ve bacteria (Staphylococcus aureus), gram ꢀve bacteria (Serratia marcescens, Sphingobium japonicum
and Stenotrophomonas maltophilia) and fungal species (Candida albicans, Aspergillus niger and Saccharomyces
cerevisiae). Dinuclear complexes (1) and (2) exhibited remarkable results. All the synthesized silver(I) complexes
have been proven to be better antibacterial and antifungal agents, even than their standard drugs (ciprofloxacin
and fluconazole) and can be used as effective antimicrobial agents and potential drugs in the future.
Received (in Montpellier, France)
3rd July 2013,
Accepted 29th November 2013
DOI: 10.1039/c3nj00729d
www.rsc.org/njc
The construction of oligopyridines and oligophenanthroline
with Ag(I) generally results in the formation of single, double
1 Introduction
A unique feature of the coordination chemistry of silver(I) is its or triple standard helicates.6–13 The coordination chemistry of
ability to form complexes with a variety of ligands that have all silver(I) with aromatic ligands has extended p-systems and also
types of donor atoms resulting in the formation of supramolec- leads to the formation of supramolecular structures having p–p
ular compounds.1 Silver(I) complexes with monocarboxylic acid interactions.14–16 Hydrogen bonding interactions are also
antibiotics monensin, lasalocid, and D-lactobionate have been responsible for supramolecular interactions, as are formed in
reported.2–5 Silver(I) compounds with N-donor ligands adopt amide moieties having infinite chains through N–Hꢁ ꢁ ꢁO inter-
novel structures, such as multi dimensional helical, honeycomb actions belonging to some silver complexes.17,18 The supra-
channel, interwoven diamondoid, graphite framework etc. molecular structure has both p–p and H-bonding interactions
in {1,2,4,5-C6H2[CH2OCH2C(pz)3]4 Ag2(BF4)2}n.19
Silver(I) has a relatively low affinity for oxygen atoms,
although a few complexes with b-diketonates, carboxylates
and crown ethers are well known.1 The complex formation of
noncyclic polyethers, crown ethers and large crown ethers have
been reported.20,21 The most frequently used crown ether is the
dibenzo-18-crown-6-ether for which silver(I) prefers an octa-
hedral coordination and forms close dinuclear complexes
based on cation–p interactions in an Z2 fashion.22
a Department of Chemistry, Dr B R Ambedkar National Institute of Technology,
Jalandhar-144011, India. E-mail: obrais@nitj.ac.in
b Department of Chemistry, Guru Nanak Dev University, Amritsar, India.
E-mail: hundal_chem@yahoo.com
c Department of Biotechnology, Dr B R Ambedkar National Institute of Technology,
Jalandhar-144011, India
† Electronic supplementary information (ESI) available. CCDC 919841, 936204
and 929555. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3nj00729d
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Silver(I) complexes not only have a beautiful architecture, activities and also the ligand – exchangeability of the silver(I)
but also possess useful applications. For instance, b-diketonates complexes plays a significant role in the magnitude of anti-
derivatives of silver(I) can be easily converted into metallic silver microbial activities. Many water soluble and light sensitive
by metal organic chemical vapour deposition (MOVCD) thermal silver(I) complexes with amino acids26,28,37 and with chiral and
decomposition techniques.23 The complexes of silver(I) with racemic camphanic acid show potential antimicrobial activity
polyether tetraglymes are useful in the fabrication of silver- against a number of bacterial strains. Silver(I) complexes contain-
based films.24
ing various types of ligands, such as carboxylic acids,42 amino
Silver and its complexes are known for their effective anti- acids,43 nitrogen,44 phosphorus or sulphur45 donor ligands also
bacterial, antifungal,25–32 antiseptic,33 anti-inflammatory34 and exhibit selectivity against a variety of cancer cells. The present
antitumour35,36 activities. The antimicrobial properties of silver(I) work describes the synthesis, crystal structures, DFT ana-
complexes depend upon the nature of the atoms coordinated to lyses and antimicrobial activities of three novel silver(I) com-
the silver ion and its bonding properties, rather than the solubi- plexes: [Ag(THEEN)]2(PIC)2 (1), [Ag(THPEN)]2(PIC)2 (2) and
lity, charge, chirality or degree of polymerization.37 The coordi- [Ag(TEAH3)2](PIC) (3). Previously the complex of silver(I) ion
nating ligands of the silver(I) complexes play the role of carrier with ligand THEEN has been studied under the influence of
for the silver(I) ions to the biological system.38 In silver(I) sacchrinate anion.46a
complexes, Ag–O and Ag–N bond strengths also play an impor-
tant role in exhibiting a wider spectrum of antimicrobial and
antifungal activities and are the potential target sites for the
2 Results and discussion
2.1 Description of crystal structures (1), (2) and (3)
inhibition of bacterial and yeast growth.37,38 Ag(I)–S com-
plexes39 possess a narrower spectrum of antibacterial activity
than silver(I)–N26,28 and silver(I)–O complexes,27,29,38,40 whereas The crystal data and the refinement details of the silver(I)
silver(I)–P41 based coordination complexes have shown no complexes are given in Table 1. Selected bond distances, bond
activity against bacterial strains, yeast and molds. The effective angles and hydrogen bonding geometry interactions are listed
antimicrobial activities of Ag(I)–N and Ag(I)–O bonded com- in Tables 2 and 3 respectively.
plexes are due to the weak silver(I)–N and silver(I)–O bonds and
2.1.1 [Ag(THEEN)]2(PIC)2 (1). The molecular structure of
can be easily replaced with biomolecules, especially with those the dinuclear silver complex (1) contains two Ag(I) ions, two
of thiol groups.37 The antimicrobial activities of silver(I)–O tetrapodal ligands (THEEN) with polar hydroxyl groups and two
complexes support the fact that the coordination geometry picrate anions (Fig. 1a). Each silver(I) ion has coordination
around the silver(I) influences the strength of antimicrobial number four and see-saw geometry (Fig. 1b) is proposed for it
Table 1 Crystal data and structure refinement for complexes (1), (2) and (3)
Complexes
[Ag(THEEN)]2(PIC)2 (1)
[Ag(THPEN)]2(PIC)2 (2)
[Ag(TEAH3)2](PIC) (3)
Empirical formula
Formula weight
Crystal system
Crystal size
C32H52Ag2N10O22
1144.58
Monoclinic
C40H68Ag2N10O22
1256.78
Monoclinic
C18H32AgN5O13
634.36
Monoclinic
0.13 ꢂ 0.10 ꢂ 0.09
0.14 ꢂ 0.10 ꢂ 0.08
0.13 ꢂ 0.11 ꢂ 0.09
Color
Yellow
Yellow
Yellow
Shape
Space group
Prism
P21/c
Needle
P21/n
Needle
P21/c
Unit cell dimensions (Å), (1)
a = 8.886(14), a = 90
b = 19.167(3), b = 91.75(5)
c = 13.127(2), g = 90
2234.7(6), 2
1.701
0.969
a = 12.796(10), a = 90
b = 15.621(2), b = 94.29(2)
c = 13.283(3), g = 90
2647.6(7), 2
1.577
0.826
a = 9.194(5), a = 90
b = 20.9520(5), b = 108.31(5)
c = 13.625(5), g = 90
2491.7(17), 4
1.691
0.884
1304.0
Volume (Å3), Z
rcalc. g cmꢀ3
m (cmꢀ1
F(000)
)
1168.0
1296
T (K)
296(2)
296(2)
296(2)
y Range of data collection
Limiting frequency
1.88 to 32.78
ꢀ13 r h r 13
ꢀ28 r k r 28
ꢀ19 r l r 19
24 944
2.02 to 26.51
ꢀ16 r h r 16
ꢀ13 r k r 19
ꢀ16 r l r 16
21 660
1.85 to 24.83
ꢀ10 r h r 10
ꢀ24 r k r 24
ꢀ15 r l r 15
29 302
Total reflections
Independent reflections
Data/restraints/parameters
Completeness to y = 25.00
Refinement method
Goodness of fit on F2
Final R indices [I > 2(I)]
R indices (all data)
7859 [R(int) = 0.0409]
7859/4/310
5477 [R(int) = 0.0248]
5477/5/351
4245 [R(int) = 0.0828]
4245/12/352
95.0%
99.7%
98.7%
Full matrix least squares on F2
1.150
Full matrix least squares on F2
1.012
Full matrix least squares on F2
1.089
R1 = 0.0769, wR2 = 0.1553
R1 = 0.1245, wR2 = 0.1786
2.298 and ꢀ1.288
919841
R1 = 0.0347, wR2 = 0.0828
R1 = 0.0449, wR2 = 0.0881
0.600 and ꢀ0.387
936204
R1 = 0.0422, wR2 = 0.0985
R1 = 0.0671, wR2 = 0.1145
0.849 and ꢀ0.408
929555
Largest diff. peak & hole (e Å3)
CCDC no.
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Table 2 Selected bond distances (Å) and bond angles (1) for complexes
(1), (2) and (3)
Bond lengths
Bond angles
Complex (1)
Ag–N1
Ag–N2
Ag–O1
Ag–O4B
Agꢁ ꢁ ꢁAgB
2.452 (4)
2.438 (4)
2.595 (5)
2.386 (4)
3.236 (9)
N1–Ag–N2
N1–Ag–O1
N1–Ag–O4B
N2–Ag–O4B
N2–Ag–O1
O4B–Ag–O1
78.56 (14)
71.20 (14)
134.27 (14)
144.09 (14)
125.35 (13)
85.30 (14)
Complex (2)
Ag–N1
Ag–N2
Ag–O1
Ag–O3B
Ag–O4
2.479 (2)
2.437 (2)
2.558 (3)
2.471 (2)
2.639 (2)
3.233 (5)
N1–Ag–N2
N1–Ag–O1
N1–Ag–O3B
N2–Ag–O3B
N2–Ag–O1
N2–Ag–O4
O3B–Ag–O1
O1–Ag–O4
O4–Ag–N1
78.03 (9)
69.70 (9)
137.26 (8)
141.33 (7)
127.92 (8)
69.61 (8)
82.52 (7)
161.12 (8)
125.97 (8)
Agꢁ ꢁ ꢁAgB
Complex (3)
Ag–N1
Ag–N2
Ag–O1
Ag–O2
2.346 (4)
2.276 (3)
2.675 (4)
2.717 (5)
2.660 (4)
N1–Ag–O1
N1–Ag–O2
N1–Ag–O3
N2–Ag–N1
N2–Ag–O3
N2–Ag–O2
O1–Ag–O2
O1–Ag–O3
O1–Ag–N2
O2–Ag–O3
72.94 (12)
71.30 (15)
73.99 (12)
174.46 (12)
106.38 (12)
103.43 (14)
107.42 (13)
121.63 (11)
110.87 (12)
105.55 (13)
Ag–O3
Symmetry transformations used to generate equivalent atoms:
[Symmetry code complex (1) B = 2 ꢀ x, 1 ꢀ y, 2 ꢀ z: complex (2) B =
2 ꢀ x, 1 ꢀ y, 2 ꢀ z].
Table 3 Selected H-bond interactions (Å, 1) for complexes (1), (2) and (3)
Hydrogen bonding geometry
D–Hꢁ ꢁ ꢁA
d(D–H)
d(H–A)
d(D–A)
+(DHA)
Complex (1)
O1–H1ꢁ ꢁ ꢁO3B
O3–H3ꢁ ꢁ ꢁO5A
O3–H3ꢁ ꢁ ꢁO6A
0.83 (5)
0.84 (4)
0.84 (4)
1.99 (5)
2.13 (5)
2.32 (5)
2.78 (6)
2.81 (6)
2.97 (7)
158 (5)
137 (3)
134 (4)
Complex (2)
O1–H1ꢁ ꢁ ꢁO4B
O4–H4ꢁ ꢁ ꢁO5A
O4–H4ꢁ ꢁ ꢁO11A
0.81 (2)
0.79 (3)
0.79 (3)
1.94 (2)
2.00 (3)
2.40 (4)
2.76 (3)
2.71 (3)
3.00 (4)
176 (2)
149 (3)
133 (3)
Complex (3)
O1–H1ꢁ ꢁ ꢁO7B
O3–H3ꢁ ꢁ ꢁO4B
O4–H4ꢁ ꢁ ꢁO5C
O5–H5ꢁ ꢁ ꢁO7B
O5–H5ꢁ ꢁ ꢁO13B
O6–H6ꢁ ꢁ ꢁO1A
0.83 (3)
0.82 (2)
0.83 (3)
0.81 (5)
0.81 (5)
0.84 (4)
1.94 (3)
2.03 (2)
1.89 (3)
2.00 (5)
2.34 (4)
1.99 (3)
2.76 (4)
2.83 (5)
2.68 (5)
2.77 (5)
2.90 (6)
2.78 (5)
170 (4)
167 (2)
157 (2)
158 (4)
127 (3)
156 (3)
Fig. 1 (a) ORTEP diagram of [Ag(THEEN)]2(PIC)2 complex with atom label-
ling scheme. Picrate anions and hydrogen attached to the carbon atoms
have been omitted for clarity. [Symmetry code: B = 2 ꢀ x, 1 ꢀ y, 2 ꢀ z] (b)
see-saw geometry of the complex. (c) Molecular structure of the complex
with hydrogen bonding interactions [Symmetry code: A = x, 1/2 ꢀ y, 1/2 + z,
B = 2 ꢀ x, 1 ꢀ y, 2 ꢀ z].
Complex (1): A = x, 1/2 ꢀ y, 1/2 + z, B = 2 ꢀ x, 1 ꢀ y, 2ꢀz; complex (2):
A = 1 ꢀ x + 1/2, y ꢀ 1/2, 1 ꢀ z + 1/2, B = 2 ꢀ x, 1 ꢀ y, 2 ꢀ z: complex (3):
A = ꢀx, ꢀy, 2 ꢀ z, B = x, 1/2 ꢀ y, z + 1/2, c = x, 1/2 ꢀ y, ꢀ1/2 + z.
on the basis of the simple geometry index tau (t = 0.62) for a through its two amine nitrogens and one hydroxyl oxygen O1.
four coordinate complex.46b See-saw geometry has been reported The fourth coordination site around each silver(I) ion is occupied
in the literature for some of the silver and copper complexes.46 In by hydroxyl oxygens from another THEEN ligand present in
complex (1), each THEEN ligand is coordinated to the silver ion the unit cell. The other two hydroxyl oxygens of each ligand
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are non-coordinating and are involved only in hydrogen bonding positions forming one plane (Fig. 2b). The hydroxyl oxygens
interactions. The fourth hydroxyl oxygen of each THEEN ligand O3 and O3B belonging to both the THPEN ligands bridge
asymmetrically bridges between the two Ag+ ions. The picrate between two silver(I) centres. The hydroxyl oxygens O2 and
anions do not directly interact with the metal ion but remain O2B of each THPEN ligand are non-coordinating and are
outside the coordination sphere in the present complex. There involved only in the hydrogen bonding interactions. The picrate
are intermolecular hydrogen bonding interactions between the anions do not directly interact with the metal ions but remain
donor polar hydroxyl groups of tetrapodal THEEN and acceptor outside the coordination sphere in the title complex. There are
picrate anions through oxygens of the phenolic and the nitro intermolecular hydrogen bonding interactions involving the
groups in the crystal structure (Fig. 1c) (Table 3). The hydroxyl hydroxyl, phenolic and oxygens of the nitro groups of the
oxygen O3 is extensively involved in hydrogen bonding inter- picrate anions in the crystal structure (Fig. S3, ESI†). The polar
actions as it is non-coordinated. The oxygen also acts as a double hydroxyl groups of the tetrapodal ligand (THPEN) act as H-bond
donor of hydrogen bonds to the oxygen atoms O5 and O6 donors towards the acceptor picrate anions through phenolic
belonging to the phenolate and nitro group of the picrate anion and nitro oxygens (Table 3). The hydroxyl oxygen O4 is exten-
respectively, as well as an acceptor of hydrogen bonds from the sively involved in hydrogen bonding interactions as it is non-
hydroxyl oxygen of the neighbouring molecule. Overall, the picrate coordinated.
anions join together several binuclear units of [Ag(THEEN)]2 (Fig. S1,
The hydroxyl oxygen O4 acts as a double donor of hydrogen
ESI†) resulting in the formation of a zig-zag polymer chain. Fig. S2 bonds to the oxygen atoms O5 and O11 belonging to the
(ESI†) shows a top view of the zig-zag polymer chain of complex phenolate and nitro group of the picrate anion respectively,
[Ag(THEEN)]2(PIC)2, extended along the crystallographic ‘c’ axis.
as well as an acceptor of hydrogen bonds from the hydroxyl
2.1.2 [Ag(THPEN)]2(PIC)2 (2). An ORTEP representation of oxygen of the neighbouring molecule (Fig. S3, ESI†). Overall,
the structure, including the atomic numbering scheme is given the picrate anions join together several binuclear units of
in Fig. 2a. The molecular structure of the dinuclear complex (2) [Ag(THPEN)]2 (Fig. S4, ESI†) resulting in the formation of a
consists of two silver(I) ions, two tetrapodal ligands (THPEN) zig-zag polymer chain. Fig. S5 (ESI†) shows a top view of zig-zag
and two picrate anions. Each silver(I) ion is coordinated polymer chain of complex [Ag(THPEN)]2(PIC)2, extended along
through two nitrogen and two oxygen atoms of one THPEN the crystallographic ‘c’ axis. The distance between two Agꢁ ꢁ ꢁAg
ligand and a third oxygen atom belonging to another THPEN in complexes (1) and (2) are 3.236 Å and 3.233 Å, which are
ligand of the dinuclear unit (Fig. 2a). The geometry around each longer than the Ag–Ag separation in metallic silver (2.88 Å)47
silver ion can be described as distorted square-pyramidal based and indicate the existence of van der Waals contact between
on the angular structural parameter (t = 0.33) for five coordi- these two silver(I) ions. This Ag–Ag distance is longer than
nate molecules.46c O4 is the axial donor atom lying above the {[Ag(othf)]2}n (2.781(1) to 2.823(2) Å and {[Ag(hpyrrld)]2}n
plane and is tetragonally elongated, whereas the N1, N2, O1 and (2.875(2)–2.9022(7) Å),48 {[Ag2(S-ca)2]}n, 3.0963 (11) and
O3B donor atoms of the THPEN ligand are in equatorial {[Ag2(ca)2(Hca)2]}n, 3.1087(7) but smaller than earlier reported
Fig. 2 (a) ORTEP diagram of [Ag(THPEN)]2(PIC)2 complex with atom labelling scheme. Picrate anions and hydrogen attached to the carbon atoms have
been omitted for clarity. [Symmetry code: B = 2 ꢀ x, 1 ꢀ y, 2 ꢀ z] (b) distorted square-pyramidal view of the complex.
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in {[Ag2(R-ca)2(R-Hca)2]}n 3.4167(6) and {[Ag2(S-ca)2(S-Hca)2]}n, All the polar hydroxyl groups of the tripodal ligand (TEAH3) and
3.44025(14)41 complexes. The number of dinuclear Ag(I) com- oxygens of phenolic, as well as nitro groups belonging to the
plexes [Ag2(sac)2(pen)2],49 [Ag2(sac)2(pyet)2],50 [Ag(sac)(mpr)]2,51 picrate anion, are involved in hydrogen bonding interactions in
[Ag2(sac)2(dmen)2],52 [Ag2(sac)2(m-aepy)2]53 with different ligands complex (3). The tripodal ligand donates hydrogen bonds
in the presence of saccharinate anion has been reported earlier. through its polar hydroxyl groups, whereas the picrate accepts
Complexes (1) and (2) are of a charge-separated nature as the hydrogen bonds through its oxygen. The non-coordinated hydro-
counter anions, picrates are present outside the coordination xyl oxygen O5 of monocoordinated TEAH3 is more extensively
sphere of both silver ions, whereas in the earlier reported involved in hydrogen bonding interactions rather than other
complex of silver saccharinate with tetrapodal THEEN has an hydroxyl oxygens and acts as a double donor, as well as an
ion-paired nature, as the counter anion saccharinate directly acceptor of hydrogen bonds (Fig. S6, ESI†). Picrate anions join
interacts with silver(I) ion.46a
together several mononuclear units of [Ag(TEAH3)2] (Fig. S7,
2.1.3 [Ag(TEAH3)2] (PIC) (3). [Ag(TEAH3)2] (PIC) (3) is a ESI†) resulting in the formation of a chain structure. Fig. S8
mononuclear complex. The molecular structure consists of (ESI†) shows a top view of the polymer chain of complex
one silver ion, two triethanolamine (TEAH3) ligands and one [Ag(TEAH3)2] (PIC), extended along the crystallographic ‘c’ axis.
picrate anion (Fig. 3a). One of the tripodal ligand acts as
tetradentate, while another acts as a monodentate ligand. This
mode of coordination of the TEAH3 ligand is unique and is
3 NMR spectral discussion
found only in the present complex, although a number of
NMR spectroscopy can be useful for studying the coordination of
complexes of TEAH3 with different modes of coordination with
various ligating sites in solution phase. However in the present
some s-block,54–56 d-block57,58 metal ions and lanthanoids59
complexes, the ligands THEEN, THPEN and TEAH3 do not seem
have been structurally characterized and reported.
to be very strong complexing agents in solution form and might
The coordination number of the silver ion in complex (3) is
get dissociated, as is evident from the appearance of multiplet
five and it has a distorted trigonal-bipyramidal geometry
signals corresponding to methyl and methylene protons in their
(Fig. 3b). The geometry around the five coordinate silver ion
1H NMR spectra. The signal corresponding to picrate is more
can be described as distorted trigonal-bipyramidal according to
apparent and appears at 8.60 ppm in complexes (1) and (3) while
Addison’s approach46c of angular structural parameter (t = 0.88).
at 8.61 ppm in complex (2) (Fig. S9–S11, ESI†).
O3, N2 and O1 donor atoms of the TEAH3 ligand define the
trigonal plane and occupy the equatorial positions, whereas the
axial positions are occupied by N1 and N2. The bond angle of
N1–Ag–N2 is 174.46(12)1. The nature of [Ag(TEAH3)2] (PIC) (3) is
4 Computational discussion
charge–separated as the picrate anion is present outside the It is appropriate here to correlate the structural parameters
coordination sphere and interacts indirectly by coordinating obtained from the crystal structure studies with computational
through its amine nitrogen and all three hydroxyl oxygen atoms. data. The structural parameters have been calculated, and the
Fig. 3 (a) ORTEP diagram of the complex (3) with an atom labelling scheme. Picrate anion and the hydrogen attached to the carbon atoms have been
omitted for clarity. (b) Distorted trigonal-bipyramidal view of the complex.
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Fig. 4 Optimized geometries: (a) [Ag(THEEN)]2(PIC)2 (1), (b) [Ag(THPEN)]2(PIC)2 (2) (c) [Ag(TEAH3)2](PIC) (3).
HOMO–LUMO energy gaps have been evaluated by the DFT (LUMO) is completely delocalized on both the silver(I) ions,
method. Fig. 4 shows the optimized molecular structures for amine nitrogens and hydroxyl oxygens for complexes (1) and
complexes (1), (2) and (3). Only the primary coordination (2). The HOMO in complex (3) is concentrated on the metal ion
sphere of the metal ions are optimized. Comparison of the as well as on the coordinated hydroxyl oxygen atoms, whereas
selected bond lengths and bond angles are given in the the LUMO is completely delocalized on the whole complex.
supplementary information (Table S1, ESI†) as space restric- The energy gap between the highest occupied and the lowest
tions do not allow the complete listing of calculated bond unoccupied molecular orbitals is a critical parameter in deter-
lengths, bond angles for the new silver(I) complexes. It is mining molecular electrical transport properties because it is a
important to mention here that all of the geometric structures measure of electron conductivity. The energy gaps observed for
were identified and after optimization, tend to reorganise close dinuclear complexes (1) and (2) are 2.530, 2.640 eV respectively
to the experimental structures.
and is 1.061 eV for mononuclear complex (3) (Table 4). It
The optimized geometry analysis reveals that the observed is pertinent to mention here that these gaps are smaller than
and calculated positions of the metal, ligand, the coordination the previously reported analogous copper complexes60 and
number and the molecular geometry are in agreement. The [Ag(sac)(Ph2SNH)]61 (Table 4). In general, molecules with smaller
energy minimized structures reproduced the observed X-ray energy gaps have unusual optoelectronic properties and electron–
structures and a maximum coherence in bond lengths is transfer phenomena.62 The title complexes can be made a desir-
observed in complex (2) (Fig. S13, ESI†). Deviation in the able target for physical studies and electronics applications.
experimental and theoretical bond lengths is less than 0.20 Å
for complexes (1) and (2) of the coordinating atoms, whereas
it is slightly higher for complex (3) (0.22 Å). The deviation in
experimental and theoretical bond distances between two
5 Antimicrobial activities
Agꢁ ꢁ ꢁAg in complex (1) is 0.36 Å, whereas in complex (2) All of the synthesized silver(I) complexes and ligands have been
deviation is very small (0.03 Å). The maximum deviation in screened on different pathogenic gram ꢀve (S. marcescens,
the L–Ag–L bond angles is 13.51 for complex (1), whereas S. japonicum, S. maltophilia) and gram +ve (S. aureus) bacterial
for complexes (2) and (3) it is relatively less (6.71 and 4.31) strains using the microbroth dilution method and exhibited
effective activity against selected bacterial strains with MIC
(Table S1, Fig. S12–S14, ESI†). The small discrepancies in bond
lengths and bond angles are attributable to H-bonding and values ranging from 0.7 to 50 mg mlꢀ1, as shown in Table 5.
packing interactions within the lattice, which are not modelled A comparative study of the ligands, metal salt and its complexes
during computational studies.
indicates that the latter exhibit more antimicrobial activity than
the metal salt and ligands against all of the aforementioned
bacterial strains, except complex (3) against S. aureus. It has
been found that the ligand TEAH3 and the complex [Ag(TEAH3)2]
4.1 HOMO–LUMO analyses
The frontier orbitals, HOMO and LUMO take part in chemical (PIC) (3) have equal MIC values against S. aureus. Complexes (1)
reactions. Fig. 5 shows the highest occupied molecular orbitals and (2) have been proven to be more effective and even better
(HOMO) and lowest unoccupied molecular orbital (LUMO) for antimicrobial agents against S. aureus, S. marcescens and
silver complexes (1), (2) and (3). It has been found that the S. japonicum than free picric acid, ligands: THEEN, THPEN,
highest occupied molecular orbital (HOMO) is largely dis- TEAH3 and silver picrate. This proves the fact that the chelation
tributed over the amine nitrogens, hydroxyl oxygen atoms and of ligands with metal ions increases the biological activity
also on the silver(I) ions in complexes (1) and (2), whereas the as reported earlier.63 Complex (2) has been proven to be more
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Fig. 5 Frontier molecular orbital surfaces and energy gaps for complexes (1), (2) and (3).
Table 4 HOMO–LUMO energy gaps for the title and previously reported complexes
HOMO–LUMO
energy gap (eV)
HOMO–LUMO
energy gap (eV)
Complexes
Complexes
[Cu(THEEN)(H2O)](PIC)2 (1)
[Cu(THPEN)](PIC)2C3H8O (2)
[Cu(TEAH3)(PIC)](PIC)ꢁH2O (3)
[Ag(sac)(Ph2SNH)]61
3.537
3.467
3.619
4.470
[Ag2(THEEN)2] (PIC)2 (1)
[Ag2(THPEN)2] (PIC)2 (2)
[Ag(TEA)2] (PIC) (3)
2.530
2.640
1.061
Table 5 Minimum inhibitory concentrations for the ligands, metal salt and
the complexes (mg mlꢀ1
reasons: (i) S. aureus is a gram +ve bacteria which is less
susceptible to silver ions than gram ꢀve bacteria66 because
the former has more peptidoglycan due to its thicker cell wall
than the latter. (ii) Complex (3) has a mononuclear metal
centre, the dinuclear centres are more active than mono-
nuclear centres. As antimicrobial agents the silver(I) picrate
complexes follow the order: (2) > (1) > (3). They have exhibited
better antimicrobial activity than the previously reported ana-
logues copper picrate complexes viz., [Cu(THEEN)(H2O)](PIC)2,
[Cu(THPEN)](PIC)2 C3H8O against the aforementioned bacterial
strains.60 [Ag(TEAH3)2] (PIC) (3) is relatively less effective than
the corresponding complex of copper i.e., [Cu(TEAH3)(PIC)]-
)
Complexes
S. aureus S. marcescens S. maltophilia S. japonicum
THEEN
THPEN
TEAH3
PIC
25
50
50
12.5
25
25
25
50
25
50
25
25
25
25
50
50
25
25
Ag(PIC)
6.2
6.2
Complex (1)
Complex (2)
Complex (3)
Ciprofloxacin
6.2
6.2
50
3.1
0.7
12.5
3.2
12.5
1.5
3.1
6.2
6.2
6.2
12.5
1.5
3.2
effective and even better antimicrobial agents against (PIC)ꢁH2O towards gram +ve (S. aureus), gram ꢀve (S. marcescens,
S. marcescens and S. maltophilia than standard ciprofloxacin, S. japonicum) and is equally effective towards gram ꢀve
whereas complex (1) is an equally good antimicrobial agent (S. maltophilia) bacterial strains.
against S. marcescens as standard ciprofloxacin (Fig. 6). This
The ligands, silver picrate and novel complexes of silver(I),
may be due to the increased lipophilicity which enhances the have been found to be effective antifungal agents when tested
penetration of complexes into the lipid membranes and blocks on fungi against C. albicans, A. niger and S. cerevisiae (Table 6,
the metal binding sites in the enzymes of microorganisms Fig. 6b). Silver picrate and its complexes have been proven to be
and they also disturb the respiration process of the cell and better antifungal agents than the ligands. The former have
thus block the synthesis of the proteins that restricts the relatively lower MIC values than the standard drug (flucon-
further growth of organisms.64,65 Complex (3) has a lesser anti- azole) against the aforementioned fungal strains except complex
microbial activity against S. aureus which might be due to two (3) against S. cerevisiae.
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Fig. 6 (a) Antimicrobial and (b) antifungal activities of the ligands, metal salt and complexes with the minimum inhibitory concentrations (MIC) in (mg mlꢀ1).
Table 6 Minimum inhibitory concentrations for the complexes and the
contact between the two silver(I) ions. Extensive hydrogen bond-
metal salt (mg mlꢀ1
)
ing interactions have been found in all three complexes. Struc-
tural parameters from crystallographic and DFT studies have
been put together and found to be coherent with each other. The
accuracy of the results predicts that the DFT studies performed
at the B3LYP/LANL2DZ level is the appropriate quantum
chemical method for reproducing the experimental results for
the title complexes. The small discrepancies in geometric para-
meters are attributable to H-bonding and packing interactions
within the lattice which are not modeled during computational
study. The calculated HOMO–LUMO energy gaps (HLG) suggest
the charge transfer transitions are taking place within the
Candida
albicans
Aspergillus
niger
Saccharomyces
cerevisiae
Complexes
THEEN
THPEN
TEAH3
Ag(PIC)
Complex (1)
Complex (2)
Complex (3)
Fluconazole
50
25
50
6.2
6.2
6.2
12.5
25
25
50
25
3.2
1.5
0.7
1.5
6.2
50
25
50
1.5
0.7
0.7
3.2
1.5
Silver picrate salt has been proven to be better or an equally complexes. The HLG are smaller for the mononuclear complex
effective antimicrobial agent as a standard drug against the (3) relative to the dinuclear complexes (1) and (2).
selected fungal species. Silver carbene and water–soluble silver
All three synthesized silver(I) complexes can be used as
complexes {[Ag(Hhis)]0.2Et(OH)}2, [Ag(Hpyrrld)]2 have been antimicrobial agents and potential drugs. In the present work,
tested earlier on the same fungal species.27,28 Silver(I) com- the chelation of ligands with metal ions has not only increased
plexes have been found to be very effective antifungal (Table 6, the biological activity of the title complexes but made them
Fig. 6b) and antibacterial agents, but are better antifungal than better and more effective antimicrobial agents than standard
antibacterial agents.
drugs against some of the selected bacterial strains and fungal
species. Dinuclear complexes (1) and (2) exhibited better anti-
microbial activity than the mononuclear complexes. Complexes
(1) and (2) are effective antimicrobial agents even against gram
+ve bacteria (S. Aureus). All of the three reported silver(I)
6 Conclusions
The synthesis, crystal structures, DFT, and antimicrobial stu- complexes have the potential to be used as precursors to
dies of complexes [Ag2(THEEN)2](PIC)2 (1), [Ag2(THPEN)2](PIC)2 produce nanomaterials for materials science, biomaterials for
(2) and [Ag(TEAH3)2](PIC) (3) have been reported. Complexes (1) therapeutic actions and can be made desirable targets for
and (2) are dinuclear, whereas complex (3) is mononuclear. physical studies and electronics applications in future.
Complex (1) has adopted a see-saw geometry with coordination
number four, whereas complexes (2) and (3) have adopted
distorted square-pyramidal and trigonal-bipyramidal geometry
with coordination number 5. The three novel complexes of
silver(I) have been found to be charge-separated in nature. The
7 Experimental section
7.1 General
longer distances between the two Agꢁ ꢁ ꢁAg in the dinuclear
7.1.1 Equipment and materials. All of the reagents and
complexes (1) and (2) indicates the existence of van der Waals chemicals were purchased from commercial sources and used
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as received without further purification. The ligands THEEN, agreement between the calculated and experimental structure
THPEN and TEAH3 were supplied by Sigma Aldrich and was obtained throughout the entire range of available struc-
Loba Chemie. Picric acid was used after recrystallization. tures. The geometries of the complexes are also described by
The IR absorption spectra were recorded in the range of 450– quantum chemical approach using the input coordinate from
4000 cmꢀ1 by means of a Perkin Elemer FT-IR RX I spectro- the crystal structure. The primary coordination sphere of all of
meter with KBr pellets (Sigma Aldrich). C H N elemental the silver(I) complexes has been optimized, structural para-
analyses were performed using Flash 2000 Organic Elemental meters have been calculated, and energy gaps of the frontier
Analyzer. 1H NMR were recorded in DMSO–CDCl3 on the FT orbitals (HOMO–LUMO) have been determined with the B3LYP/
NMR Spectrometer model Avance-II 400 (Bruker UK Limited, 6-31G/LANL2DZ level of theory. The theoretical and crystallo-
Banner Lane, Coventry, CV4 9GH) using tetramethylsilane as an graphic analyses are put together and found to be in good
internal standard. Antimicrobial studies of the complexes were consistency with each other. The previously reported examples
carried out on pathogenic bacterial strains Serratia marcescens of some silver(I) complexes have used DFT/B3LYP/6-31G/
(MTCC-97), Sphingobium japonicum (MTCC-6362), Stenotrophomonas LANL2DZ (Gaussian software) to optimize the molecular geo-
maltophilia (MTCC-2446) and Staphylococcus aureus (MTCC-3160), metries of complexes of silver with tryptophan,43a n-acetyl-
whereas antifungal studies were carried out on Candida albicans l-cysteine,43b S-S-diphenylsulfimide,61 chloroquine.77 The same
(183), Aspergillus niger (872) and Saccharomyces cerevisiae (3090) basis set has been used to elucidate the structure of the silver(I)
procured from the Microbial Type Culture Collection (MTCC), complex with L-buthionine sulfoximine78 using GAMESS
Institute of Microbial Technology, Chandigarh.
software.
7.1.4 Antimicrobial and antifungal studies. The anti-
7.1.2 X-ray crystallography. X-ray crystallographic data
was collected on a ‘‘Bruker APEX-II CCD’’ area detector dif- microbial activity of all of the synthesized silver(I) complexes were
fractometer using a graphite-monochromatized MoKa (l = investigated against bacterial strains gram ꢀve (S. marcescens,
0.71073 Å) radiation source. All of the crystal structures were S. japonicum, S. maltophilia) and gram +ve bacteria (S. aureus)
solved by direct methods67 using the program ‘SIR-92’ and using the microbroth dilution79 method. A broth microdilution
refined by full-matrix least-squares refinement methods based susceptibility test was carried out in a 96 well microtitre plate by
on F2 using the SHELXL-97 programme.68 Absorption correc- distributing 240 ml of sterile media and 30 ml of metal complexes
tions were applied using multi–scan.69 All non hydrogen atoms solutions ranging from (0.197 mg mlꢀ1 to 500 mg mlꢀ1) taking
were refined anisotropically. The hydrogen atoms of the hydroxyl appropriate blanks and controls. Then 30 ml bacterial inocula
groups were located from the difference Fourier synthesis and (organism 107 cells per ml) was poured to each well and incubated
were refined isotropically with Uiso values 1.2 times that of their at 30 1C for 48 hours at 150 rpm in an incubator shaker. The
carrier oxygen atoms, with their distances fixed as 0.84(2) Å. In growth was observed in the form of turbidity with naked eyes
complex 2, one of the hydroxyl groups (O2) was disordered. The under light background by comparing the clarity of blank media
disorder could be resolved by splitting the oxygen atom over two and the turbidity of the controls. The MICs of the metal complexes
atomic positions with their site occupancies and thermal para- were recorded as the lowest concentration where no viability of
meters refined as free variables, with a fixed C–O distance of bacteria was observed in the wells of 96-microwell plates after
1.46(2) Å. All of the other hydrogen atoms were fixed geometri- incubation of 48 h. The growth of the bacteria was measured by
cally with their Uiso values 1.2 times that of the carrier atoms for observing the minimum inhibitory concentration. DMSO was
methylene and phenylene carbons and 1.5 times for the methyl used as a control, ciprofloxacin and fluconazole as standard drugs
group. The geometry of the complexes and hydrogen bonding for the antimicrobial and antifungal studies.
were calculated using the PARST70 programme. All of the drawings
of the complexes were made using the ORTEP71 and MERCURY
programs.72
7.1.5 General methods of synthesis
General Synthesis. Silver(I) picrate was synthesised by making
a solution of silver nitrate and picric acid separately in ethanol
7.1.3 Computational details. The quantum chemical solution. These solutions were then mixed slowly and warmed.
calculations (DFT calculations) giving the molecular geometries The ratio of silver nitrate and picric acid was kept at 1 : 1. The
of the minimum energies and molecular orbitals (HOMO– solution was stirred for 30 minutes and bright yellow coloured
LUMO) were performed using the Gaussian 03 package.73 The crystals of silver(I) picrate were obtained. The melting point of
molecular orbitals were visualized using ‘‘Gauss view’’. The silver(I) picrate was found to be 155 1C. All three silver(I)
method used was Becke’s three-parameter hybrid-exchange complexes were prepared by the reaction between silver(I)
functional, the nonlocal correlation was provided by the Lee, picrate and the corresponding ligands (THEEN, THPEN and
Yang and Parr expression, and the Vosko, Wilk, and Nuair 1980 TEAH3) in equimolar quantities (1 : 1) using ethanol as the
local correlation functional (III) (B3LYP)74 in gaseous phase. solvent with stirring for 50 minutes. Then the solution was
The 6-31G basis set was used for C, N and O. The LANL2DZ allowed to evaporate slowly at room temperature. Suitable light
basis set75 and pseudopotentials of Hay and Wadt were used for yellow colour crystals were obtained after one week for X-ray
the Ag atom.76 The input coordinates were obtained from the crystal analysis.
crystal structure data. Only the primary coordination sphere of
the metal ion in complexes (1), (2) and (3) has been optimized.
[Ag(THEEN)]2(PIC)2 (1). Melting point: 110 1C with grey
The structural parameters were adjusted until an optimal metallic silver. Yield: 85%. Elemental analysis: anal. calc.
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1991, 10, 127–132; (c) E. Constable and J. V. Walker,
J. Chem. Soc., Chem. Commun., 1992, 884–886.
(found) for C32H52Ag2N10O22: C, 33.58 (33.49); H, 4.58 (4.50); N,
12.24 (12.15)%. IR (KBr) cmꢀ1: 3368br (OH), 2951br (C–H),
1631s (CQC), 1561s (NO2), 1336s (NO2), 1437br (QCH), 1277s
(C–O), 1161s (ꢀOH), 1079 (C–O). 1H NMR (DMSO-d6–CDCl3
(1 : 1)): 2.64–2.67 (m, 12H, 6CH2), 3.48–3.54 (m, 8H, 4CH2), 8.60
(s, 2H, ArCH).
8 (a) E. C. Constable, A. J. Edwards, P. R. Raithby and J. V.
Walker, Angew. Chem., Int. Ed. Engl., 1993, 32, 1465–1467;
(b) Y. J. Fu, J. Sun, Q. Li, Y. Chen, W. N. Dai, D. F. Wang,
T. C. W. Mak, W. X. Tang and H. W. Hu, J. Chem. Soc., Dalton
Trans., 1996, 2309–2313; (c) P. K. K. Ho, S. M. Peng, K. Y.
Wong and C. M. Che, J. Chem. Soc., Dalton Trans., 1996,
1829–1834; (d) T. M. Garrett, U. Koert, J. M. Lehn, A. Rigault,
D. Meyer and J. Fischer, J. Chem. Soc., Chem. Commun., 1990,
557–558.
9 (a) L. Carlucci, G. Ciani, D. W. v. Gudenberg and D. M.
Proserpio, Inorg. Chem., 1997, 36, 3812–3813; (b) C. Provent,
S. Hewage, G. Brand, G. Bernardinelli, L. J. Charbonniere
and A. F. Willians, Angew. Chem., Int. Ed. Engl., 1997, 36,
1287–1289; (c) M. A. Withersby, A. J. Blake, N. R.
Champness, P. Hubberstey, W. S. Li and M. Schroder, Angew.
Chem., Int. Ed. Engl., 1997, 36, 2327–2329; (d) G. Baum,
E. C. Constable, D. Fenske, C. E. Housecroft and T. Kulke,
Chem. Commun., 1998, 2659–2660.
10 (a) G. Baum, E. C. Constable, D. Fenske, C. E. Housecroft,
T. Kulke, M. Neuburger and M. Zehnder, J. Chem. Soc.,
Dalton Trans., 2000, 945–959; (b) P. K. Santra, U. Ray, S. Pal
and C. Sinha, Inorg. Chem. Commun., 2001, 4, 269–273;
(c) H. L. Zhu, Y. X. Tong and X. M. Chen, J. Chem. Soc.,
Dalton Trans., 2000, 4182–4186; (d) O. Mamula, F. J.
Monlien, A. Porquet, G. Hopfgartner, A. E. Merbach and
A. von Zelewsky, Chem.–Eur. J., 2001, 7, 533–539; (e) G. Baum,
E. C. Constable, D. Fenske and T. Kulke, Chem. Commun., 1997,
2043–2044; ( f ) L. Carlucci, G. Ciani, D. M. Proserpio and
A. Sironi, Inorg. Chem., 1998, 37, 5941–5943.
11 (a) L. M. Tong, X. M. Chen, B. H. Ye and S. W. Ng, Inorg.
Chem., 1998, 37, 5278–5281; (b) S. Sailaja and M. V.
Rajasekharan, Inorg. Chem., 2000, 39, 4586–4590;
(c) H. Oshio, M. Yamamoto and T. Ito, J. Chem. Soc., Dalton
Trans., 1999, 2641–2643; (d) H. Oshio, T. Yaginuma and
T. Ito, Inorg. Chem., 1999, 38, 2750–2752; (e) H. Oshio,
T. Hikichi, T. Yaginuma, H. Onodera, M. Yamamoto and
T. Ito, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1999, 334,
479–509; ( f ) H. Zhang, J. Cai, X. L. Feng, J. Z. Liu, X. Y. Li
and L. N. Ji, Inorg. Chem. Commun., 2001, 4, 241–244;
(g) S. P. Yang, H. L. Zhu, X. H. Yin, X. M. Chen and
L. N. Ji, Polyhedron, 2000, 19, 2237–2242.
[Ag(THPEN)]2(PIC)2 (2). Melting point: 140 1C. Yield: 65%.
Elemental analysis: anal. calc. (found) for C40H68Ag2N10O22: C,
38.23 (38.15); H, 5.45 (5.37); N, 11.15 (11.06)%. IR (KBr) cmꢀ1
:
3319br (OH), 2973w (C–H) br), 1610s (CQC), 1567s (NO2), 1343
vs. (NO2), 1430w (QCH), 1275s (C–O), 1163s (–OH), 1084s
(C–O). 1H NMR (DMSO-d6–CDCl3 (1 : 1)): 1.10–1.19 (m, 12H,
4CH3), 3.84–3.86 (m, 4CH), 2.00–2.98 (m, 12H, 6CH2), 8.61
(s, 2H, ArCH).
[Ag(TEAH3)2](PIC) (3). Melting point: 105 1C. Yield: 70%.
Elemental analysis: anal. calc. (found) for C18H32AgN5O13: C,
34.08 (34.01); H, 5.08 (5.03); N, 11.04 (10.94)%. IR (KBr) cmꢀ1
:
3380br (OH), 2953br (CH), 1631s (CQC), 1561s (NO2), 1337s
(NO2), 1438br (CH), 1277s (C–O), 1161s (OH), 1075s (CO).
1H NMR (DMSO-d6-CDCl3 (1 : 1)): 2.52–2.54 (m, 6CH2), 8.60
(s, 2H, ArCH).
Acknowledgements
The authors are highly thankful to the Ministry of Human
Resource and Development (MHRD), New Delhi for providing
research assistantship to Mr Rakesh Kumar, one of the authors
and Dr B R Ambedkar National Institute of Technology, Jalandhar
for providing research infrastructure. Rakesh Kumar is also
thankful to Joyee Mitra for their kind help during the course of
this present work.
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