Unexplored analytics of some novel 3d-4f heterometallic Schiff base complexes

Article abstract of DOI:10.1039/c4ra04531a

Three heterometallic Schiff-base complexes of Cu having Pr, Nd and Sm as the heteroatoms have been synthesized. The compounds have also been characterized by their IR spectra and CHN analysis. The single crystal structures of these compounds have been stu

Full text of DOI:10.1039/c4ra04531a

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Unexplored analytics of some novel 3d–4f
heterometallic Schiff base complexes†
Cite this: RSC Adv., 2014, 4, 40794
a
b
Partha Pratim Chakrabarty,ab Sandip Saha, Kamalika Sen, Atish Dipankar Jana,c
*
*
Debarati Dey,d Dieter Schollmeyere and Santiago Garcıa-Grandaf
´
Three heterometallic Schiff-base complexes of Cu having Pr, Nd and Sm as the heteroatoms have been
synthesized. The compounds have also been characterized by their IR spectra and CHN analysis. The
single crystal structures of these compounds have been studied from the X-ray crystallographic data. To
the best of our knowledge the article describes the possibility of application of these compounds in the
field of species dependent anion sensing for the first time. Amongst a number of anionic species, certain
sulphur species were found to have greater reactivity towards a Schiff-base complex as they can incur
probable changes in the molecular complexity. The S2O82ꢀand S2O3 species could modify the spectral
2ꢀ
Received 14th May 2014
Accepted 14th August 2014
features of the Schiff-base complex containing Nd as the heteroatom. This particular complex was
found to exhibit changes in its absorbance and fluorescence spectral features upon interaction with the
anionic species S2O82ꢀand S2O32ꢀ. The results give a strong platform for the Schiff base complexes for
their analytical applications.
DOI: 10.1039/c4ra04531a
www.rsc.org/advances
13
˚
0.85 A). A large number of 3d–4f complexes have been
synthesized using Schiff bases with acyclic diaminoalkanes
Introduction
such as ethylene diamine or propane diamine or compart-
mental type Schiff bases with cyclohexane-1,2-diamine or
orthophenylene diamine.14,15 Among the different dia-
minoalkanes 1,2-propane diamine has rarely been used along
with 3-methoxy/ethoxy salicylaldehyde to synthesize these types
of 3d–4f heterometallic complexes. To the best of our knowl-
edge only one Fe–Gd complex was reported using the hex-
adentate Schiff base obtained by the 1 : 2 condensation of 1,2-
propane diamine and 3-methoxy salicylaldehyde.16
The synthesized compounds having practical applications
are most welcome to the chemical world as they impart solu-
tions to solving different chemical problems. Speciation anal-
ysis and anion sensing are widely popular problems in the
world of analytical sciences. Little attention has been paid to
solve the speciation of different anions which play vital roles in
biological and environmental systems. Presence of halides in
living cells induces changes in the conformation of steady state
cells, anion exchange, active transport and electrodiffusion.17
Nitrates present in natural water at higher concentrations are
generally associated with human activities and can cause
adverse health effects on animals, human beings and plants.
The toxicity of high nitrate concentrations arises from the
capability of the human body to reduce it in the stomach or in
the lower intestine to nitrite, which leads to methemoglobi-
nemia.18,19 Inorganic arsenates and arsenites are well known for
their chronic toxicities. Sulphate is an important anion involved
in many physiological processes, having numerous biosynthetic
and pharmacological functions.20 It is involved in a variety of
In the past two decades, a great number of acyclic and
compartmental hexadentate Schiff base ligands derived from
the condensation of 3-methoxy/ethoxy salicylaldehyde with
diaminoalkanes have been used for synthesis of hetero-
dinuclear 3d–4f metal complexes.1–5 They have also been well
documented due to their potential applications in the eld of
magnetism,6–10 luminescence11 and asymmetric catalysis.12
Hexadentate Schiff base with two different cores afford a
straightforward route to synthesize heterodinuclear 3d–4f
complexes. This is due to the fact that these Schiff bases contain
an inner site with N- and O-donor chelating centers suitable for
˚
complexation with d-block ions (ionic radii 0.75–0.6 A). In
addition the outer coordination sites with four O-donors are
larger than the inner one and are able to incorporate large
oxophilic ions, such as 4f lanthanide ions (ionic radii 1.06–
aDepartment of Chemistry, Acharya Prafulla Chandra College, New Barrackpur,
Kolkata-700131, India. E-mail: sandipsaha2000@yahoo.com
bDepartment of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700009,
India. E-mail: kschem@caluniv.ac.in
cDepartment of Physics, Behala College, Parnasree, Kolkata 700 060, India
dDepartment of Chemistry and Environment, Heritage Institute of Technology,
Chowbaga Road, Anandapur, Kolkata 700107, India
eInstitut fur Organische Chemie, Universit at Mainz, Duesbergweg 10-14 55099, Mainz,
Germany
f
´
´
´
´
´
Departamento de Quımica Fısicay Analıtica, Universidad de Oviedo, C/Julian Claverıa
8, 33006 Oviedo, Spain
† Electronic supplementary information (ESI) available. CCDC 973931, 973932
and 973930. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4ra04531a
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activation and detoxication processes. Sulfate ions are Counting (TCSPC) Lifetime Spectroscopy. The UV visible spectra
important for proper cell growth and development of living were obtained using an Agilent 8453 diode array spectropho-
organisms. It is involved in synthesis of cell matrix and main- tometer. Total independent data for compound 1 was collected
tenance of cell membrane. Sultes are used as food preserva- on a STOE IPDS2T diffractometer equipped with a graphite
˚
tives or enhancers. Sultes occur naturally in wines and are monochromator Mo Ka radiation (l ¼ 0.71073 A). Total inde-
commonly introduced to arrest fermentation at a desired time. pendent data for compounds 2 and 3 were collected on a Bruker
High sulte content in the blood and urine of babies can be Smart Apex II CCD Area Detector equipped with a graphite
˚
caused by molybdenum cofactor deciency disease which leads monochromator Cu Ka radiation (l ¼ 1.54184 A).
to neurological damage and early death unless treated.21 Thio-
sulfate occurs naturally and is produced by certain biochemical
processes. It is worthy for its use to halt bleaching in the paper-
Synthesis
making industry. Sodium thiosulfate, is widely used in
photography.22 Perdisulphates are used as food additives and it
is used in organic chemistry as an oxidizing agent. It takes also
an important role as initiator for emulsion polymerization.23
The presence of these sulphur species therefore affects many
systems of practical importance. Speciation of such anions and
their sensing has been rarely examined. Report on chemical
speciation of sulfur in heavy petroleums, using X-ray absorption
near-edge structure (XANES) spectroscopy is available. This was
used for approximate quantitation of different classes of sulfur-
containing compounds (e.g., sulfur, suldes (including disul-
des and polysuldes as a group), thiophenes, sulfoxides,
sulfones, sulnic acids, sulfonic acids, and sulfate) in a series of
petroleums and petroleum source rocks.24 Speciation of sulphur
as S2ꢀ, S42ꢀ and S62ꢀ in lithium batteries were performed using
operand X-ray absorption spectroscopy.25 Analytical procedures
for the determination of selected species of sulphur were
proposed for natural water samples using the catalytic, chro-
matographic, ion-selective methods and absorption spectrom-
etry.26 Anion sensing using different chemical compounds is an
upcoming eld. Quite a few numbers of reports can be found in
the literature.27 However, in the present scenario, speciation
based anion sensing solely based on chemical methods is still
lacking and the large potential class of metal-Schiff base
complex is highly unexplored for this purpose. In this report we
have studied the possibility of application of the newly synthe-
sized heterometallic Schiff base complexes in species depen-
dent anion sensing of sulfur containing anions using spectral
methods.
Synthesis of the ligands. The Schiff base ligands H2L
(Scheme 1) was synthesized by reuxing 1,2-propane diamine
(0.074 mL, 1 mmol) and 3-ethoxy salicylaldehyde (0.332 g, 2
mmol) in methanol (10 mL) for two h. The ligand was not iso-
lated; instead the resulting yellow methanolic solution was
subsequently used for complex formation in every case.
A clear solution of Cu(CH3COOH)2$H2O (0.199 g, 1 mmol) in
methanol (10 mL) was added to a 10 mL methanolic solution of
the ligand, and the mixed solution was reuxed for two hours.
The resulting green colored complex was isolated and charac-
terized by IR and CHN analysis. This complex was further used
for the 3d–4f heterometallic compound preparations.
Synthesis of complexes [Cu(L2ꢀ)Pr(NO3)3] (1), [Cu(L2ꢀ
)
Nd(NO3)3(CH3OH)] (2), Cu(L2ꢀ)Sm(NO3)3(H2O)] (3). Prepared
Cu-complex (0.1124 g, 0.25 mmol) was dissolved in to 10 mL
acetone and mixed with 10 mL methanolic solution of praseo-
dymium nitrate (0.113 g, 0.25 mmol) for complex 1, neodymium
nitrate (0.1095 g, 0.25 mmol) for complex 2, and samarium
nitrate (0.113 g, 0.25 mmol) for complex 3 with constant stirring
for 2 h. The brown solution was ltered. On slow evaporation
the dark brown block shaped single crystals of the complex 1–3
was separated out in a few days. The crystals were ltered and
washed with methanol and dried in air.
Complex [CuLꢀ2]$CH3OH: yield 65%. Anal. cacld for
C
23H31CuN2O5: C: 51.67; H: 6.52; N: 5.85%. Found: C: 51.00; H:
7.0; N: 5.90%. IR (KBr pellets, cmꢀ1): n(CH3OH) 3400, n(C]N)
1631.
Complex 1: yield 65%. Anal. cacld for PrCuC21H27CuN5O14:
C: 32.42; H:3.49; N:9.00%. Found: C: 33.0; H: 3.60; N: 8.60%. IR
(KBr pellets, cmꢀ1): n(C]N) 1634, n(NO3) 1384.
Complex 2: yield 65%. Anal. cacld for NdCuC22H28CuN5O14:
C: 33.32; H: 3.55; N: 8.82%. Found: C: 33.31; H: 3.60; N: 8.83%.
Experimental section
Materials
The lanthanide salts Pr(NO3)3$6H2O, Nd(NO3)3$6H2O,
Sm(NO3)3$6H2O and solvents were purchased from Sigma-
Aldrich and were used as received. All other chemicals used
were of analytical grade.
Apparatus
IR spectra were recorded using KBr pellets within the range
4000–400 cmꢀ1 on a Perkin-Elmer Spectrum 65 FTIR Spec-
trometer. Elemental analyses were carried out using a Heraeus
CHN-O-Rapid elemental analyzer. Horiba Jobin Yvon Fluo-
rocube 01-NL and 291 nm Horiba nanoLED, IBH DAS-6 decay
analysis soware were used for Time Correlated Single Photon
Scheme 1
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IR (KBr pellets, cmꢀ1): n(CH3OH) 3400, n(C]N) 1632, n(NO3) complex 1, complex 2 and complex 3. Absorbance and uores-
1384. cence spectral measurements were done with the solutions of
Complex 3: yield 65%. Anal. cacld for SmCuC21H22N5O14: C: the complexes as well as with added anionic species.
32.29; H:2.83; N: 8.97%. Found: C: 32.80; H: 2.90; N: 8.72%. IR
(KBr pellets, cmꢀ1): n(H2O) 3392, n(C]N) 1629, n(NO3) 1384.
Time Correlated Single Photon Counting (TCSPC) Lifetime
Spectroscopy
X-ray crystal structure determination of complex 1–3
Fluorescence lifetimes of Schiff base, Schiff base-Cu complex,
2ꢀ
2ꢀ
complex 2, and S2O8 and S2O3 treated complex 2 were
determined by the method of time correlated single-photon
counting (TCSPC) using a nanosecond diode laser as the light
source at 291 nm. The IBH DAS-6 decay analysis soware was
used to deconvolute the uorescence decays.
Crystal data for the compound 1–3 are given in Table 1. The
structure of the complex was solved by SIR 97 (ref. 28) for
compounds 1–3. The structure renement was also performed
by full-matrix least squares based on F2 with SHELXL-2014.29 All
non-hydrogen atoms were rened anisotropically. The C-bound
H atoms were constrained to ideal geometry and were included
in the renement in the riding model approximation. Data for
molecular geometry, intermolecular interactions and pictures
were produced using Platon-2009 (ref. 30) and ORTEP3.2 (ref.
31) programs. All three structures possess structural disorder
for the diamino moiety of the produced ligand.
Results and discussion
Crystal structure descriptions
The X-ray crystal structures of the three complexes show that all
the complexes are diphenoxo-bridged dinuclear neutral
complexes of copper(II) and lanthanides(III) (Pr, Nd and Sm).
ORTEP representations of 1–3 along with atom labels are shown
in Fig. 1, 2 and 3 respectively. The selected bond lengths and
angles are listed in Tables S1–S3.†
Preparation of reagent for anion sensing studies using
spectral methods
10 mM methanolic solutions of the Cu-Schiff base complex,
The inner salen-type N2O2 cavity is occupied by copper(II),
complex 1, complex 2 and complex 3 were prepared separately. while lanthanide(III) is present in the open and larger O4
10 mM solutions of NaNO3, NaNO2, NaF, NaCl, NaBr, NaI, compartment of the dinucleating hexadentate ligand L2ꢀ. The
Na2SO4, Na2SO3, Na2S2O3, Na2S2O8, NaAsO2, Na2HAsO4$7H2O copper(II) centre in the compound 2 and 3 are penta coordi-
were each prepared by weighing. 0.5 mL solutions of different nated by the two imine nitrogen atoms, two bridging phenoxo
anionic species were added to 2.5 mL methanolic solution of oxygen atoms and one oxygen atom in the apical position
Table 1 Crystal data and refinement parameters
Complex 1
Complex 2
Complex 3
Crystal data
Formula
Pr Cu C21H27 Cu N5 O14
777.92
Monoclinic
Nd Cu C22H28 Cu N5 O14
794.27
Monoclinic
Sm Cu C21H26 N5 O14
786.36
Monoclinic
Formula weight
Crystal system
Space group
P21/c (no. 14)
P21/c (no. 14)
P21/c (no. 14)
˚
a, b, c [A]
9.1341(8), 21.0076(12), 14.4523(14) 9.2033(5), 20.5226(10), 15.6347(12) 9.0452(6), 21.2587(14), 14.5234(14)
90, 94.088(7), 90
2766.1(4)
a, b, g [ꢁ]
90, 92.680(6), 90
2949.8(3)
90, 91.628(8), 90
2791.6(4)
3
˚
V [A ]
Z
4
4
4
D (calc) [g cmꢀ3
m (CuKa) [mm ]
F(000)
]
1.872
2.588
1549
1.786
14.833
1580
1.866
17.266
1552
Crystal size [mm]
0.06 ꢂ 0.09 ꢂ 0.46
0.19 ꢂ 0.26 ꢂ 0.30
0.15 ꢂ 0.19 ꢂ 0.26
Data collection
Temperature (K)
193
MoKa 0.71073
2.4, 28.0
ꢀ12:10; ꢀ27:27; ꢀ19:19
26837, 6667, 0.169
3912
293
CuKa 1.54184
3.6, 70.8
ꢀ10:11; ꢀ24:25; ꢀ13:19
18842, 5563, 0.120
3268
293
CuKa 1.54184
3.7, 71.0
ꢀ10:11; ꢀ17:25; ꢀ15:17
10976, 5205, 0.113
2418
˚
Radiation [A]
Theta Min–Max [ꢁ]
Dataset
Tot., Uniq. Data, R(int)
Observed data [I > 2.0 s(I)]
Renement
Nref, Npar
6667, 443
5563, 401
5205, 391
R, wR2, S
Max. and Av. Shi/Error
Min. and Max. Resd. Dens. [E/A ] ꢀ1.88, 1.49
0.0805, 0.2390, 1.03
0.00, 0.00
0.0610, 0.1214, 0.99
0.00, 0.00
ꢀ0.60, 0.62
0.0797, 0.1773, 1.00
0.24, 0.01
ꢀ0.89, 0.92
3
˚
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Fig. 1 The ORTEP diagram with 30% ellipsoidal probability and atom
numbering scheme for complex 1 (the disorder of the diamino-
propane moiety of the ligand has been omitted for clarity).
Fig. 3 The ORTEP diagram with 30% ellipsoidal probability and atom
numbering scheme for complex 3 (the disorder of the diamino-
propane moiety of the ligand has been omitted for clarity).
adopts an approximate square-planar coordination environ-
ment in 1. The lanthanide(III) center is ten-coordinated with two
bridging phenoxo oxygen atoms, two ethoxy oxygen atoms, and
two oxygen atoms of each of the three chelating nitrates in all
three compounds.
In 1, the nitrate ion as well as the central propane diamine
part of the ligand is disordered over two positions (Fig. 1).
Various similar bond distances of all the complexes have been
jotted down in Table 2 for an easy comparison. In 1, the Cu–O
˚
bond distances fall in the range 1.904(8)–1.926(8) A and Cu–N
˚
˚
bond distances are 1.886(11) A and 1.923(11) A, respectively.
˚
The Pr–Oligand distances remain in the range 2.424(7)–2.653(9) A
and the Pr–Onitrate distance fall in the range 2.616(9)–2.626(17)
˚
A. Similarly in 2, the observed Cu–O bond distances vary in the
˚
range 1.912(7)–2.346(11)A and Cu–N bond distances are
˚
˚
1.897(9)A and 1.913(10) A, respectively. The Nd–Oligand
˚
distances are in the range 2.411(6)–2.554(8) A and the Nd–
˚
O
nitrate distance are observed between 2.471(9)–2.533(9) A. In 3,
˚
the Cu–O bond distances fall in the range 1.893(11)–1.901(10) A
Fig. 2 The ORTEP diagram with 30% ellipsoidal probability and atom
numbering scheme for complex 2 (the disorder of the diamino-
propane moiety of the ligand has been omitted for clarity).
˚
˚
and Cu–N bond distances are 1.903(14) A and 1.930(15) A,
respectively. The Sm-Oligand distances are observed between
˚
2.566(13)ꢀ2.653(9) A and the Sm–Onitrate distances fall in the
˚
coming from methanol molecule for compound 2 and the same range 2.465(18)ꢀ2.515(15) A. A systematic variation is not
oxygen atom coming from water molecule for compound 3. observed for the Ln–O(nitrate) bond distances in three Cu(II)–
Thus copper(II) centre adopts an approximate square-pyramidal Ln(III) complexes. In the compounds 1 and 2, the Ln–O(phe-
coordination environment for these two compounds. In noxo) bond distances are shorter than the bond lengths
contrast, copper(II) centre is tetra-coordinated by two imine involving ethoxy and nitrate oxygen atoms but this trend is not
nitrogen atoms and two bridging phenoxo atoms and thus observed for compound 3 (see Table 2).
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Table 2 A summary of the geometrical parameters of the complexes 1–3
Cu(II)–O distance
range (A)
Cu(II)–N distance
range (A)
Ln–O distance
range (A)
Cu/Ln
distance range (A)
Dihedral angle between
ꢁ
˚
˚
˚
˚
Complex
the bridging cores (d,
)
1 (Ln ¼ Pr)
2 (Ln ¼ Nd)
3 (Ln ¼ Sm)
1.904(8)–1.926(8)
1.912(7)–2.346(11)
1.901(10)–1.893(11)
1.886(11), 1.923(11)
1.897(9), 1.913(10)
1.930(15), 1.903(14)
2.424(7)–2.653(9)
2.411(6)–2.554(8)
2.465(18)–2.515(15)
3.485
3.487
3.451
3.8(5)
8.7(4)
5.5(6)
˚
Intermolecular copper/lanthanide distances are 3.485 A,
˚
˚
3.487 A and 3.451 A, for 1–3 respectively. These distances are
again indicating that 3d–4f dinuclear cores are not well sepa-
rated and these systems are not discrete dinuclear cores as the
copper/lanthanide separations are less than 7 A.15 Thus we can
˚
consider the planarity of these complexes which give some idea
about the stability of these complexes in solution. The extent of
planarity of the internal cores can be understood from the
dihedral angles (d) between the CuO(phenoxo)2 and
LnO(phenoxo)2 planes. The dihedral angle between the plane
consisting of Cu1–O10–O23 and Pr1–O10–O23 is 3.8(5)ꢁ where
O10 and O23 are bridging phenoxo O atoms in 1. The dihedral
angle between the plane consisting of Cu1–O1–O2 and Nd1–O1–
O2 is 8.7(4)ꢁ where O1 and O2 are bridging phenoxo O atoms in
2. The dihedral angle between the plane consisting of Cu1–O2–
O3 and Sm1–O2–O3 is 5.5(6)ꢁ where O2 and O3 are bridging
phenoxo O atoms in 3 (Table 2). From these values, it is indi-
cated that the bridging moeity is more twisted in 2 compared to
1 and 3.15 This renders the complex more susceptible to changes
upon addition of foreign ions in solution.
Fig. 4 Absorption spectra of the Schiff base and its mono-metallic and
hetero-metallic complexes.
higher intensities i.e., 225 nm peak for n–p* transition, 280 nm
peak for p–p* transition and 360 nm peak for LMCT transition.
Complex 2 with Nd as the hetero atom behaves in a different
way with new positions of absorption peaks. For complex 2, the
absorption maxima are found at 225 nm for p–p* transition,
263 nm peak for n–p* transition and 350 nm peak for LMCT
transition. This indicates a slightly different nature of the
complex containing Nd as the heteroatom.
All the ve compounds were excited at 350 nm to study their
uorescence emission properties. Only the Schiff base was
found to be highly uorescent and upon metal complexation
there was a huge loss of emission intensity (Fig. 5). Metal
complexation induces rigidity to the structure and prohibits
delocalization of labile electrons in the molecule and hence the
observed loss of emission intensity.33 Upon addition of different
anionic species to complexes 1–3, the absorption spectra with
complexes 1 and 3 containing Pr and Sm as the hetero-atoms
respectively show negligible changes (Fig. 6 and 8). However,
complex 2 with Nd as the heteroatom (Fig. 7) shows maximum
Spectral studies for anion sensing analysis
The UV/Vis spectra of the Schiff base ligands and their
complexes were recorded at 300 K in methanol medium. The
observed bands in the absorbance spectrum are listed in Table
3, and the spectrum of the complex in CH3OH medium is shown
in Fig. 4. The spectrum of the free ligand exhibits two inter
ligand charge-transfer (CT) bands with lmax 227 nm which can
be attributed to p–p* transition.32 The lmax of 261 nm is
attributed to n–p* transition. The Cu(L2ꢀ)$CH3OH complex
also shows a peak at 225 nm for p–p* transition and the other
two distinct peaks at 280 and 360 nm are observed for n–p* and
LMCT transitions. Complex 1 and 3 at same concentration with
Schiff base-Cu complex show similar absorption peaks but with
changes in the absorption spectral properties only in presence
of S2O8
Table 3 The observed bands in the absorbance spectrum of the Schiff
base ligand and its complexes at 300 K in methanol medium
2ꢀ
and S2O32ꢀ. This indicates that there are some
structural changes in this particular complex in presence of
Wavelength (nm) and 3 (Mꢀ1 cmꢀ1
)
these sulphur species.34 With slowly increasing concentrations
2ꢀ
of S2O8
ions in a solution of complex 2, the absorption
Complex
p–p*
n–p*
LMCT
maximum of 350 nm rst suffers a red shi and then a new
peak at 335 nm arises with a subsequent reduction of the rst
peak (at 385 nm) (Fig. 9). Perdisulphate being a highly oxidizing
species (eqn (11)) interacts with the p electron system of the
ligand surrounding the metal core which affects the optical
properties of the ligand.35 Spectral changes of a solution of
H2L
227 (1140)
225 (3130)
225 (3710)
225 (3130)
225 (3710)
261 (670)
280 (1820)
280 (3200)
263 (670)
280 (3200)
—
Cu (L2ꢀ)$CH3OH
Complex 1
Complex 2
Complex 3
360 (1100)
360 (1100)
350 (1200)
360 (1100)
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Fig. 8 Absorption spectra of the complex 3 in presence of different
anions.
Fig. 5 Fluorescence spectra of the Schiff base and its mono-metallic
and hetero-metallic complex excited at 350 nm.
2ꢀ
Fig. 9 Absorption spectra of complex 2 with increasing S2O8
concentration.
Fig. 6 Absorption spectra of the complex 1 in presence of different
anions.
2ꢀ
Fig. 10 Absorption spectra of complex 2 with increasing S2O3
concentration.
Fig. 7 Absorption spectra of the complex 2 in presence of different
anions.
particular species also.36 This is again due to the positive
oxidation potential of this particular species in solution (eqn
(12)) compared to the low negative values of the other anionic
species. S2O3 species is therefore also responsible to create
changes in the structural environment of compound 2 and thus
affect its optical properties. The appearance of new absorbance
2ꢀ
complex 2 were also observed with increasing concentrations of
S2O32ꢀ (Fig. 10). A red shied absorbance spectrum with higher
S2O32ꢀ concentrations indicates sensitivity of complex 2 for this
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2ꢀ
the complex 2 also enhances in presence of the anions S2O8
and S2O32ꢀ (Fig. 13 and 14) indicating that the complex suffers
specic changes in the uorophore environment in presence of
these two particular species. There are hardly any changes of
emission spectra in presence of SO42ꢀ and SO32ꢀ species (gure
not shown). Fig. 15 shows the emission life times of the uo-
rescence active compounds, i.e., the Schiff base ligand itself
(4.378 ns), Cu-Schiff base complex (2.55 ns), complex 2 in
presence of S2O82ꢀ (1.13 ns) and in presence of S2O32ꢀ (0.25 ns).
The results are again indicative towards formation of different
geometrical orientation which is responsible for the existence of
different uorescence active species. All these observations may
also be explained on the basis of the redox potentials of these
anionic species and hence their possibility to break down the
hetero-metallic complex in their presence.
2ꢀ
Fig. 11 Absorption spectra of complex 2 with increasing SO4
concentration.
ꢀ
AsO2 + 2H2O + 3eꢀ $ As + 4OHꢀ Eꢁ ¼ ꢀ0.68 V
(1)
2ꢀ
3ꢀ
ꢀ
AsO4 + 2H2O +2eꢀ $ AsO2 + 4OHꢀ Eꢁ ¼ ꢀ0.71 V (2)
peak at 335 nm (Fig. 9) and the red shi (Fig. 10) with S2O8
2ꢀ
and S2O3 respectively may be due to the formation of new
2ꢀ
species of neodymium in the complex.37 In presence of SO4
F2 + 2eꢀ $ 2Fꢀ Eꢁ ¼ ꢀ2.87 V
(3)
2ꢀ
and SO3 there are no observable spectral changes excepting
the dilution effect (Fig. 11 and 12). The uorescence emission of
2ꢀ
Fig. 14 Fluorescence spectra of complex 2 with increasing S2O3
concentration.
2ꢀ
Fig. 12 Absorption spectra of complex 2 with increasing SO3
concentration.
2ꢀ
Fig. 13 Fluorescence spectra of complex 2 with increasing S2O8
Fig. 15 Emission life times of the fluorescence active compounds in
concentration.
study.
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Cl2(g) + 2eꢀ $ 2Clꢀ Eꢁ ¼ ꢀ1.358 V
Br2(aq) + 2eꢀ $ 2Brꢀ Eꢁ ¼ ꢀ1.087 V
I2 +2eꢀ $ 2Iꢀ Eꢁ ¼ ꢀ0.535 V
(4)
(5)
(6)
(7)
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NO2 + H2O + eꢀ $ NO+2OHꢀ Eꢁ ¼ ꢀ0.46 V
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2ꢀ
The above redox potentials indicate that only S2O82ꢀ/SO4
2ꢀ
and S2O3 /S38 systems have higher oxidation potentials
compared to the other redox systems. The complex containing
Nd as heteroatom was only found susceptible to such attack. It
is evident that the complex 2 containing Nd3+ suffers encoun-
ters observable spectral changes with S2O82ꢀand S2O32ꢀ species
which results in a subtle geometrical reorganization of the
molecule. Fluorescence activity reappears and the resulting
species have different emission life time than the parent
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like S2O82ꢀand S2O3 having high redox potentials. All the
2ꢀ
results of absorbance, uorescence spectra and uorescence
life time measurements indicate that complex 2 containing Nd
heteroatom is more prospective towards speciation based anion
2ꢀ
sensing of sulphur containing anion species S2O82ꢀand S2O3
in particular.
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Acknowledgements
S. S. thankfully acknowledges UGC and DST, New Delhi for
funding of the total work done in the Dept of Chemistry, A. P. C.
College. S. S. also acknowledges DST-FIST for funding of
Instruments. K. S. acknowledges UGC (Sanction no. 41-248/
2012 (SR)) for funding.
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40802 | RSC Adv., 2014, 4, 40794–40802
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