Synthesis, spectral and electrochemical studies of mononuclear samarium (II), europium (II) and ytterbium (II) complexes with a NNO donor Schiff base derived from 4-methyl-2,6-dibenzoylphenol and propane-1,3-diamine

Article abstract of DOI:10.1080/24701556.2019.1606828

Three mononuclear divalent lanthanide complexes, [Sm(L)₂(THF)₂] (1), [Eu(L)₂(THF)₂] (2) and [Yb(L)₂(THF)₂] (3), (where HL^Me = 2-{N-(3-aminopropyl)benzimidoyl}-6-benzoyl-4-methylp

Full text of DOI:10.1080/24701556.2019.1606828

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: https://www.tandfonline.com/loi/lsrt21
Synthesis, spectral and electrochemical studies
of mononuclear samarium (II), europium (II) and
ytterbium (II) complexes with a NNO donor Schiff
base derived from 4-methyl-2,6-dibenzoylphenol
and propane-1,3-diamine
Sanjay Kumar & Sushil K Gupta
To cite this article: Sanjay Kumar & Sushil K Gupta (2019): Synthesis, spectral and
electrochemical studies of mononuclear samarium (II), europium (II) and ytterbium (II) complexes
with a NNO donor Schiff base derived from 4-methyl-2,6-dibenzoylphenol and propane-1,3-
diamine, Inorganic and Nano-Metal Chemistry, DOI: 10.1080/24701556.2019.1606828
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2019.1606828
Synthesis, spectral and electrochemical studies of mononuclear samarium (II),
europium (II) and ytterbium (II) complexes with a NNO donor Schiff base
derived from 4-methyl-2,6-dibenzoylphenol and propane-1,3-diamine
Sanjay Kumar and Sushil K Gupta
School of Studies in Chemistry, Jiwaji University, Gwalior, India
ABSTRACT
ARTICLE HISTORY
Received 26 October 2018
Accepted 19 March 2019
Three mononuclear divalent lanthanide complexes, [Sm(L)2(THF)2] (1), [Eu(L)2(THF)2] (2) and
[Yb(L)2(THF)2] (3), (where HLMe ¼ 2-fN-(3-aminopropyl)benzimidoylg-6-benzoyl-4-methylphenol)
have been synthesized and characterized by elemental analyses, conductivity measurements, mass
1
KEYWORDS
spectrometry, IR, electronic and H NMR spectroscopy and cyclic voltammetry. The spectral studies
Divalent lanthanides; NNO
Schiff base; NMR; cyclic
voltammetry; DNA binding
suggest that the LnN4O4 coordination polyhedron is a distorted dodecahedral. The electrochemical
behavior of 1–3 shows irreversible metal centered oxidation wave at ca. 0.544–0.643 V and ligand
centered reduction at ca. ꢀ1.085 to ꢀ1.260 V versus Ag/Agþ. Interaction of 1, 2 and 3 with calf
thymus DNA suggests an intercalative binding mode.
Introduction
Experimental
Tridentate ligands such as phenol based compartmental Schiff Materials
base are capable of forming complexes that have important
All chemicals were of reagent grade and used as received.
Solvents were purified by standard methods[22] and freshly
distilled prior to use. Samarium(II) iodide, europium(II)
bromide, ytterbium(II) iodide and propane-1,3-diamine were
obtained from Aldrich. Calf thymus DNA was obtained from
Sigma and used as received. The sodium salt of CT DNA was
stored at 4 ꢁC. Precursor, 4-methyl-2,6-dibenzoylphenol
(mdbpH) was synthesized by a previously reported method.[17]
applications in several areas of research. Lanthanide-Schiff base
complexes are receiving great attention for important catalytic
and biological applications.[1–6] Generally, the tridentate Schiff
base ligands with a flexible donor atom, in comparison with
bidentate ligands, can provide the protective shield for catalyt-
ically active metal centers leading to a more stable active spe-
cies by the coordination of the additional donor atom. As a
result, the activity of the complex with a tridentate Schiff base
ligand is much higher than that of complex with bidentate
ligands.[7,8] Various divalent lanthanide complexes with cyclo-
pentadienyl and pyrazolylborate ligands have been
reported.[9–14] However, lanthanide complexes with phenol
based tridentate Schiff base still remain poorly explored.[15] In
repertoire, monomeric lanthanocene aryloxide complexes with
a tridentate Schiff base ligands, N-1-(o-methoxyphenyl)salicyli-
deneamine have been described.[16] As part of our research
Physical measurements
Elemental analysis were carried on Elemental Vario EL
analyzer model CARLO ERBA model DP 200. Molar
conductance of the complex in 10ꢀ3 mol Lꢀ1 CH3CN
solution was measured by use of a Century CC-601 digital
conductivity meter. Melting point was determined on
work on phenol-based Schiff bases,[17–21] we herein report the a LABTRONICS model LT-110 Digital Automatic Melting
synthesis and characterization of divalent lanthanide complexes
with tridentate NNO Schiff base, 2-fN-(3-amino-
propyl)benzimidoylg-6-benzoyl-4-methylphenol (LMeH).
Point Apparatus in an open capillary and are reported with-
out further correction. The electrospray ion mass spectra
(ESIMS) were recorded on a MICROMASS QUATTRO II
triple quadrupole mass spectrometer. Infrared spectra were
recorded on a KBr pellets using SHIMADZU Prestige-21 FT
IR Instruments. Electronic spectra of 10ꢀ3 mol Lꢀ1 solutions
in CH3CN were obtained on a PerkinElmer Lambda 35
1
N
OH
O
UV–vis spectrophotometer. H NMR spectra were recorded
on a 500 MHz JEOL AL-500 FT NMR spectrometer in
DMSO-d6/CDCl3 relative to SiMe4. The magnetic moment
was measured on a Sherwood susceptibility instrument
model MSB MK1 at room temperature.
NH2
LMe
H
CONTACT Sushil K Gupta
skggwr@gmail.com
School of Studies in Chemistry, Jiwaji University, Gwalior, 474 011, India.
ß 2019 Taylor & Francis Group, LLC

2
S. KUMAR AND S. K. GUPTA
Electrochemical experiments
Synthesis of complexes
Cyclic voltammetry
[Bis(2-fN-(3-aminopropyl)benzimidoyl-NCg-6-benzoyl-4-
methylphenolato-j3 N1N2O1)(tetrahydrofuran- j O)
samarium(II)] [Sm(LMe)2(THF)2] (1)
Cyclic voltammetric measurements were carried out with an
Advanced Electrochemical System, PARSTAT 2253 instru-
ment equipped with a three-electrode system. The micro-cell To a solution of SmI2 (0.328 g; 0.812 mmol) in THF
model KO264 consisted of a platinum working electrode, (30 cm3) was added drop-wise solution of LMe
a
H
platinum wire as auxiliary electrode and a non-aqueous Ag/ (0.603 g, 1.623 mmol) in THF (30 cm3) at room tempera-
Agþ reference electrode with 0.1 mol Lꢀ1 AgNO3 in aceto- ture and the mixture was slowly heated at 50 ꢁC for 1 hr.
nitrile as a filling solution. Tetrabutylammonium perchlorate After cooling, a yellow precipitate was obtained, filtered
(0.1 mol Lꢀ1 solution in CH3CN) was used as the supporting and washed with cold THF and dried in vacuum. Yield:
electrolyte. Cyclic voltammograms with scan speeds of 0.464 g (64%), m.p. 265 ꢁC (dec.). Anal. Calc. for
50–500 mV sꢀ1 were run in 10 ꢂ 10ꢀ3 mol Lꢀ1 CH3CN solu- C56H62N4O6Sm (%): C, 64.83; H, 6.02; N, 5.40. Found: C,
tion under nitrogen. Under these conditions the ferroce- 64.95; H, 5.96; N, 5.51. ESI-MS: m/z (relative abundance,
nium/ferrocene (fcþ/fc) couple shows E1= at 0.060 V.
%): 728 (8) [M–2THF–NH2(CH2)3NH2–PhCH3]þ, 671
(100)
[M–2THF–NH2(CH2)3NH2–PhCH3–NH(CH2)3]þ],
2
437 (8) [Sm(LH)ꢀN(CH2)3NH2ꢀCH3]þ, 373 (12)
[LH þ H]þ, 356 (22) [LH–CH4]þ, 317 (25) [(2,6-
PhCO)2(4-Me)C6H2OH þ H]þ, 301 (20) [MH ꢀ N(CH2)3
NH2ꢀCH2]þ. FT IR (selected, cmꢀ1): 3056 [ꢀ(N ꢀ H)];
1637 [ꢀ(C ¼ O)]; 1572 [ꢀ(C ¼ N)/ring]; 1523 [ꢀ(C… O)/
DNA binding studies
Electronic absorption spectroscopy
The experiments involving the interaction of the complex
with CT-DNA were carried out in doubly distilled water
buffer containing 5 mM Tris [tris(hydroxymethyl)-aminome-
thane] and 50 mM NaCl and adjusted to pH 7.2 with hydro-
chloric acid. Solution of CT-DNA gave ratios of absorbance
at 260 and 280 nm of about 1.8–1.9, indicating that the
DNA was sufficiently free of protein contamination.[23] The
DNA concentration per nucleotide was determined spectro-
photometrically by employing a molar absorption coefficient
of 6600 Mꢀ1 cmꢀ1 at 260 nm after 1:100 dilutions.[24] The
complex was dissolved in a solvent mixture of 1% DMSO
and 99% Tris-HCl buffer (5 mM Tris-HCl, 50 mM NaCl, pH
7.2) at concentration 3.0 ꢂ 10ꢀ5 M. An absorption titration
experiment was performed by maintaining 30 mM compound
and varying the concentration of nucleic acid. While meas-
uring the absorption spectra, an equal amount of CT-DNA
was added to both the compound solution and the reference
solution to eliminate the absorbance of CT-DNA itself.
Titration curves were constructed from the fractional change
in the absorption intensity as a function of DNA concentra-
tion. The intrinsic binding constant can be obtained by the
phenolate];
1443
[d(C–H)
bending/CH2];
1040
[ꢀ(C–O–C)]; 768 [q(CH2) rocking]; 522 [ꢀ(Sm ꢀ N)]; 496
[ꢀ(Sm ꢀ O)]. UV-Vis (nm) (e/L molꢀ1 cm–1) (CH3OH):
413 (432), 262 (1213), 218 (2500). 1H NMR (500 MHz,
dmso-d6): d 1.54 (t, J ¼ 6.5 Hz, CH2); 2.70 (t, J ¼ 6.5 Hz,
CH2); 3.60 (m, CH2); 2.35 (s, CH3), 2.50 (CH2);
7.02 ꢀ 7.74 (m, aromatic); 8.11 (s, NH2). KM (10ꢀ4 M,
MeOH, 298 K): 19 S cm2molꢀ1
.
[Bis(2-fN-(3-aminopropyl)benzimidoylg-6-benzoyl-4-
methylphenolato-j3N1N2O1) (tetrahydrofuran- j O)
europium(II)] [Eu(LMe)2(THF)2] (2)
A solution of EuBr2 (0.316 g, 1.02 mmol) in THF (35 cm3) was
added drop-wise a solution of LMeH (0.752 g, 2.02 mmol) in
THF (30 cm3) at room temperature and the mixture was slowly
heated at 50 ꢁC for 1 hr. After cooling, a pale yellow
precipitate was obtained, filtered and washed with cold THF
and dried in vacuum. Yield: 0.532 g (59%), m.p. 277 ꢁC (dec.).
Anal. Calc. for C56H62N4O6Eu (%): C, 64.73; H, 6.01; N, 5.39.
Found: C, 64.65; H, 5.90; N, 5.46. ESI-MS: m/z (relative
abundance, %): 805 (10) [M–2THF–NH2(CH2)3NH2–CH4]þ,
728 (22) [M–2THF–NH2(CH2)3NH2–Ph ꢀ CH4]þ, 709
(100) [M–2THF–C4H5N2ꢀPhCO]þ, 671 (40) [MH–NH2
following equation[25]
:
½
ꢃ
½
ꢃ
DNA = e
ð
DNA = e
e
¼
e
þ 1=K e
bð
e
bꢀ
(1)
ð
f Þ
f Þ
f Þ
aꢀ
bꢀ
In plots of [DNA]/(ea – ef) versus [DNA], Kb is given by
the ratio of the slope to intercept.
(CH2)3NH2–PhCH3–NH(CH2)3]þ,
429
(8)
[EuLH ꢀ Ph ꢀ H2O]þ, 356 (22) [LH ꢀ CH4], 317 (25) [(2,6-
PhCO)2(4-Me)C6H2OH þ H]þ,
301
(20)
Synthesis of tridentate Schiff base ligand
[MH ꢀ N(CH2)3NH2ꢀCH2]þ. FT IR (selected, cmꢀ1): 3056
[ꢀ(N ꢀ H)]; 1636 [ꢀ(C ¼ O)]; 1573 [ꢀ(C ¼ N)]; 1531
[ꢀ(C ¼ O)/phenolate]; 1464 [d(C–H) bending/CH2]; 1046
[ꢀ(C–O–C)]; 704 [q(CH2) rocking]; 526 [ꢀ(Eu ꢀ N)]; 498
Synthesis of [2-fN-(3-aminopropyl)benzimidoylg-6-
benzoyl-4-methylphenol] (LMeH)
The ligand precursor, LMeH, was synthesized by a previously [ꢀ(Eu ꢀ O)]. UV-Vis (nm) (e/L molꢀ1cm–1) (CH3OH): 407
reported method[17] using 4-methyl-2,6-dibenzoylphenol and (713), 259 (1998), 208 (2748). KM (10ꢀ4 M, MeOH, 298 K): 15
propane-1,3-diamine in dry THF at 0 ꢁC.
S cm2molꢀ1
.

INORGANIC AND NANO-METAL CHEMISTRY
3
[Bis(2-fN-(3-aminopropyl)benzimidoylg-6-benzoyl-4-
methylphenolato-j3N1N2O1) (tetrahydrofuran- j O)
ytterbium(II)] [Yb(LMe)2(THF)2] (3)
pattern for samarium and europium. Other important peaks
have similar fragmentation pattern as observed in the ligand.
The ESI mass spectrum of 3 shows the expected molecular ion
peak at m/z 916 with correct isotopic pattern of ytterbium.
Other features are same as observed in case of 1 and 2.
A solution of YbI2 (0.326 g, 0.763 mmol) in THF (30 cm3)
was added drop-wise
a
solution of LMe
H
(0.565 g,
1.52 mmol) in THF (30 cm3) at room temperature and the
mixture was slowly heated at 50 ꢁC for 1 hr. After cooling, a
yellow precipitate was obtained, filtered and washed with
cold THF and dried in vacuum. Yield: 0.489 g (70%), m.p.
256 ꢁC (dec.). Anal. Calc. For C56H62N4O6Yb (%): C, 63.44;
H, 5.89; N, 5.28. Found: C, 63.30; H, 5.72; N, 5.34. ESI-MS:
IR spectra
The absence of O – H stretching and d(O – H) vibration
(1270 cmꢀ1) in the IR spectra of complexes 1, 2 and 3 indi-
cated deprotonation of OH group.[27] The bands observed
in the region 3020–3056 cmꢀ1 in the complexes are assigned
to ꢀ(N–H) stretching. The lower value of ꢀ(N–H) is attrib-
uted to the fact that the NH2 group is coordinated to the
metal. Further, the ꢀ(C ¼ N) vibration is shifted to lower
frequency suggesting the coordination of azomethine N with
lanthanide ions. However, the ꢀ(C ¼ O) band is found to be
almost at the same position as in the precursor. This has
been attributed to noninvolvement of benzoyl O in the
coordination with lanthanide ion. The band at ca.1527 cmꢀ1
has been assigned to ꢀ(C ¼ O) vibration of the phenol[28] as
a consequences of the delocalization of the double bond.
The presence of strong band in the region 1040–1050 cmꢀ1
in 1, 2 and 3 indicates coordinated THF [ꢀ(C–O–C)].[29]
This band appears at higher frequency in free THF
(1086 cmꢀ1). The ligand coordination to the metal center
m/z (relative abundance, %): 917 (12) [MH–2THF] þ, 804
þ
(48) [M–2N(CH2)3]þ, 723 (22) [M–2N(CH2)3–C4þH5N2]
,
671
(40)
[M–2N(CH2)3–C4H5N2–C4H4]
,
429
(12) [YbLH ꢀ CPhCO]þ, 357 (10) [LH ꢀ CH3]þ, 317
(20)
[(2,6-PhCO)2(4-Me)C6H2OH þ H]þ,
301
(10)
[MH ꢀ N(CH2)3NH2ꢀCH2]þ. FT IR (selected, cmꢀ1): 3020
[ꢀ(N ꢀ H)]; 1633 [ꢀ(C ¼ O)]; 1573 [ꢀ(C ¼ N)/ꢀ(C–C) ring];
1526 [ꢀ(C ¼ O)/phenolate]; 1424 [d(C–H) bending/CH2];
1050 [ꢀ(C–O–C)]; 760 [q(CH2) rocking]; 625 [ꢀ(Yb ꢀ N)];
489 [ꢀ(Yb ꢀ O)]. UV-Vis (nm) (e/L molꢀ1 cm–1) (CH3OH):
407 (456), 266 (1220), 218 (3022). 1H NMR (500 MHz,
CDCl3): d 2.77 (s, CH2); 2.94 (s, CH2); 3.34 (s, CH2);
2.12 (s, CH3), 6.01 ꢀ 7.38 (m, aromatic); 9.60 (s, NH2).
KM (10ꢀ4 M, MeOH, 298 K): 14 S cm2molꢀ1
.
results in bands at 522–625 cmꢀ1
489–496 cmꢀ1, ꢀ(Ln–O).
, ꢀ(Ln–N) and at
Results and discussion
Synthesis and properties of lanthanide (II)-Schiff
base complexes
Electronic spectra
The partially filled f-shell can participate in transitions in
the visible region of the spectrum. For trivalent species, the
The reactivity of the monoanionic tridentate ligand towards
divalent lanthanide derivatives has been studied by reacting
them in 1:2 molar ratio (Scheme 1). The products obtained
are found to have composition [M(L)2(THF)2] [M ¼ Sm(1);
Eu(2); Yb(3)] in good yield. All the complexes are stable
at room temperature, soluble in acetonitrile, chloroform,
methanol, DMSO and DMF and melt with decomposition in
the temperature range 256 ꢀ 277 ꢁC.
colors arise from Laporte-forbidden 4f
!
4f transi-
tions.[30–32] In contrast, the colors of the divalent lanthanide
ions are attributed to Laporte-allowed 4f ! 5d transi-
tions.[15] In the present investigations, the divalent lanthan-
ide complexes 1, 2, and 3 in CH3CN exhibit absorptions in
the visible region. These low energy transitions
(407–417 nm) are ascribed to ligand to metal charge transfer
(LMCT)[33] transitions. In addition, all the complexes show
very intense absorptions in the UV region (259–266;
208–218 nm), possibly due to intra-ligand transition (ILCT)
as observed in the free ligands at similar energies.
Scheme 1. Preparation of complexes.
1H NMR spectra
The molar conductance value in methanol is very low
compared to the value reported[26] for 1:1 electrolyte
(KM 120 ꢀ 160 S cm2 molꢀ1) suggesting that these are
non-electrolytes.
Samarium (Sm) provides the only Ln(II)–Ln(III) system in
which both oxidation states have complexes which are NMR
accessible.[34] Despite room-temperature magnetic moments
of 3.4–3.8 BM for Sm(II) (4f6) and 1.3–1.9 BM for Sm(III)
(4f5), 1H NMR resonances are reasonably sharp and are
Mass spectra
found within
10 ppm of the normal 0–10 ppm region
All peaks in the electrospray mass spectra are identified by the
most intense peak in the isotopic mass distribution. In com-
pounds 1 and 2 the molecular ion peak was not observed.
where diamagnetic resonances are located. This is not true
for Eu(II) (4f7), Eu(III) (4f6) or Yb(III) (4f13), which have
Peaks at m/z 728 (8%) and 883 (6%) are resulted from the loss room temperature magnetic moment in the ranges 7.4–8.0,
of amine, toluene and carbon, respectively with typical isotopic 3.3–4.2 and 4.2–4.9 BM, respectively.[35–37] Of course,

4
S. KUMAR AND S. K. GUPTA
diamagnetic Yb(II) (4f14) (171Yb, I ¼ 1= , abundance 14.27%)
ꢄ
Table 1. Cyclic voltammetric data for 1, 2 and 3.
2
provides NMR-observable complexes.[38]
Compound
1
Scan rate (mV/s)
Epa (V)
ipa (mA)
Epc (V)
ipc
1
50
100
150
200
250
300
350
400
450
500
50
0.472
0.512
0.544
0.552
0.576
0.560
0.560
0.552
0.560
0.552
0.504
0.560
0.584
0.600
0.608
0.624
0.648
0.656
0.672
0.688
0.512
0.576
0.584
0.624
0.632
0.672
0.696
0.704
0.696
0.736
2.268
3.874
5.092
6.789
7.436
8.640
9.781
10.67
11.69
12.65
1.573
2.704
3.679
4.796
5.522
6.321
7.165
7.790
8.521
9.194
2.058
3.370
4.219
5.049
5.940
6.683
7.521
8.347
9.080
9.659
ꢀ1.112
ꢀ1.072
ꢀ1.024
ꢀ1.024
ꢀ1.048
ꢀ1.088
ꢀ1.080
ꢀ1.096
ꢀ1.160
ꢀ1.152
ꢀ1.112
ꢀ1.160
ꢀ1.112
ꢀ1.176
ꢀ1.104
ꢀ1.152
ꢀ1.128
ꢀ1.128
ꢀ1.112
ꢀ1.160
ꢀ1.160
ꢀ1.184
ꢀ1.232
ꢀ1.248
ꢀ1.280
ꢀ1.264
ꢀ1.280
ꢀ1.296
ꢀ1.296
ꢀ1.352
ꢀ2.504
ꢀ4.136
ꢀ6.062
ꢀ7.695
ꢀ9.059
ꢀ10.07
ꢀ11.42
ꢀ12.10
ꢀ12.93
ꢀ14.03
ꢀ1.729
ꢀ2.883
ꢀ3.866
ꢀ5.076
ꢀ5.719
ꢀ6.603
ꢀ7.356
ꢀ7.919
ꢀ8.331
ꢀ9.197
ꢀ2.092
ꢀ3.068
ꢀ4.126
ꢀ5.134
ꢀ5.762
ꢀ6.561
ꢀ7.208
ꢀ7.792
ꢀ8.398
ꢀ8.828
The H NMR spectra of complexes 1 and 3 do not show
any phenolic proton signal suggesting its displacement and
coordination of phenolic oxygen with Sm(II) and Yb(II). The
complexed THF proton appears at 0.83–0.88 and
2.43–2.58 ppm. The sharp spectrum of 3 compared to 1 con-
firmed the diamagnetic nature of ytterbium complex. Though
the complex 1 is paramagnetic, however peaks have been
assigned tentatively. The peaks for NH2, CH2, CH3 and phenyl
protons are observed in these complexes (see experimen-
tal section).
2
100
150
200
250
300
350
400
450
500
50
3
100
150
200
250
300
350
400
450
500
ꢄComplex ¼ 10 ꢂ 10ꢀ3 M in CH3CN solution, supporting electrolyte: TBAP
0.1 M; working electrode: glassy carbon; auxiliary electrode: Pt wire;
reference electrode: Ag/0.1 mol Lꢀ1 AgNO3.
1
2
Figure 2. A plot of cathodic peak current (ipc) vs square root of scan rate (ꢀ =
)
Figure 1. Cyclic voltammograms of 1, 2 and 3 (0.1 mM) in CH3CN at 400 mV/s.
of 1, 2 and 3.

INORGANIC AND NANO-METAL CHEMISTRY
5
1=2
leff ¼ g½JðJ þ 1Þꢃ
(1)
Cyclic voltammetry
The cyclic voltammograms of 1, 2 and 3 showed a single
wave at Epa ¼ 0.552 to 0.643 V versus the Ag/Agþ (Figure 1)
in the 50–500 mV/s scan range (Table 1) attributable to an
irreversible Ln2þ/Ln3þ one electron oxidation.[33,39,40] No
cathodic peak could be observed even after applying a
500 mV/s scan rate indicating the irreversible nature of the
electrode process. The linear plot of the cathodic peak cur-
1
=
2
rent (Ipc) versus the square root of the scan rate (ꢀ ) passes
close to the origin, indicating that the electrode reaction is
mainly diffusion controlled (Figure 2). An irreversible cath-
odic peak at Epc ¼ ꢀ1.085 to ꢀ1.259 V could be assigned to
the reduction of the ligand[41] on the electrode surface (free
ligand at ca. ꢀ1.173 V). The observed oxidation potential for
the complexed Schiff base moiety with respect to the free
ligand shows that the coordination to the electropositive lan-
thanide center facilitates the ligand reduction.
Efforts to grow a single crystal did not succeed, and this
precluded a single crystal X-ray determination.
DNA binding properties
Electronic spectral studies
The UV-Vis spectra of the complexes 1, 2 and 3 for a buf-
fered solution at pH 7.2 in the presence of increasing quan-
tities of CT DNA are shown in Figure 3. Absorption spectra
exhibited hypochromism by 6 to 15% and moderate batho-
chromic shift in the charge transfer band of the complex
(Table 2). The calculated values of binding constant (Kb) for
the neutral complex [Sm(L)2(THF)2] 1, [Eu(L)2(THF)2] 2
Figure 3. Absorption spectra of 30 lM [Complex] in the absence (a) and
presence (b) of increasing amount of DNA (30–120 lM) for 1, 2 and 3. Inset
shows the plot of [DNA]/(ea – ef) versus [DNA].
and [Yb(L)2(THF)2] 3 at 30 mM are 1.63 ꢂ 104 M21
,
3.20 ꢂ 104 M21 and 1.67 ꢂ 104 Mꢀ1, respectively. Addition
of a 10–15 fold excess of CT DNA after saturation caused
no change in the absorption spectrum of the final equilib-
rium mixture. It indicates an intercalative binding mode of
complexes, possibly by electrostatic interaction with phos-
phate residues.[4–6,42]
Table 2. Absorption spectroscopic data of complexes 1, 2 and 3 upon
addition of CT DNA.
kmax (nm)
Binding constant,
Complexes Free Bound Dk Hypochromisma (%)
Kb (Mꢀ1
)
1
2
3
414
420
411
416
422
413
2
2
2
5.5
15.0
9.6
1.63 ꢂ 104
3.20 ꢂ 104
1.67 ꢂ 104
Proposed structure of the complex
aHypochromism ¼ [(A
– A bound)/A free] ꢂ 100%.
free
The analytical, spectral and electrochemical evidences sug-
gest that complexes 1, 2 and 3 have distorted dodecahedral
geometry around the metal with tridentate coordination
through amine N, imine N and phenolic O with tetrahydro-
furan molecules occupying apical position in LnN4O4 core.
Similar structure for complex [Dy(bpdc)(Hbpdc)]ꢅ3H2O] has
earlier been reported.[43]
Magnetic moment studies
The magnetic susceptibilities of lanthanide(II) complexes 1
and 2 in the solid state were determined by the Sherwood
susceptibility instrument model MSB MK1 at room tempera-
ture (298 K). The experimental magnetic moments of the
complex 1 is 3.65 BM and that of 2 is 8.20 BM which are
comparable to the literature values.[34–37] Magnetic moment
data reveal involvement of higher excited J states which
made actual magnetic moments higher than those calculated
by the use of Equation (1)[37] in Bohr magneton, with the
value of J for the ground state (for Sm(II), 4f6, meff ¼ 0 and
for Eu(II), 4f7, meff ¼ 7.94).
Funding
We are thankful to the Council of Scientific and Industrial Research
(CSIR), New Delhi for financial support (Project no.01(1858)/03/EMR-
II). SK acknowledges the CSIR for the award of JRF and SRF. We
acknowledge CDRI, Lucknow for mass, NMR and CIF, JU for mag-
netic susceptibility measurements.

6
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