Synthesis and structure of mononuclear copper(II) complexes with acyclic Schiff-base ligands containing organotellurium substituents: A comparative study with selenium analogs

Article abstract of DOI:10.1080/00958972.2010.529497

Tellurium-bearing acyclic Schiff bases, 2,6-bis({N-[2-(phenyltellurato) ethyl]}benzimidoyl)-4-methylphenol (HL³) and 2,6-bis({N-[3- (phenyltellurato)propyl]}benzimidoyl)-4-methylphenol (HL⁴) of the Te₂N₂O type have been prepared by condensation of 4-methyl-2,6-dibenzoylphenol (mdbpH) with the appropriate phenyltellurato(alkyl) amine. HL³ and HL⁴ have been characterized by mass spectrometry, IR, electronic and 1H-NMR spectroscopies and cyclic voltammetry. Their reactions with Cu(II) acetate monohydrate in a 2 : 1 molar ratio in methanol yield [(C₆H₂(O)(Me){(C₆H ₅)C=N(CH₂)nTe(C₆H₅)}{(C ₆H₅)C=O})₂Cu] (3 (n = 2), 4 (n = 3)) as suggested by analytical and spectroscopic data and single crystal X-ray crystallography of 3. In both complexes, one arm of the ligand undergoes hydrolysis at the C=N position and two molecules of the partially hydrolyzed ligand coordinate to Cu(II) through imido nitrogen and the phenolic oxygen. The telluriums do not form part of the copper(II) distorted square planar coordination sphere which has a trans-CuN₂O₂ core. Electrochemical studies of 3 and 4 indicate quasi-reversible reductions (E' = -1.113 V (3) and -1.149 V (4)) corresponding to the reduction of copper(II) to copper(I). The interactions of 3 and 4 with calf thymus DNA, investigated by spectrophotometry and cyclic voltammetry, indicate that 3 and 4 bind to DNA via intercalation, and the binding affinity of 3 is lower than that of its selenium analog.

Full text of DOI:10.1080/00958972.2010.529497

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Synthesis and structure of mononuclear
copper(II) complexes with acyclic
Schiff-base ligands containing
organotellurium substituents: a
comparative study with selenium
analogs
S. Nair a , V.K. Verma a , C.S. Verma a , T.A. Jain a , S.K. Tripathi b
, R. Singh c , S.K. Gupta d & R.J. Butcher e
a Department of Chemistry , Disha Institute of Management and
Technology , Raipur, India
b MOCVD Division, Defence Materials and Stores Research and
Development Establishment , Kanpur, India
c ER and IPR Cell, Defence Research and Development
Organization , New Delhi, India
d School of Studies in Chemistry, Jiwaji University , Gwalior, India
e Department of Inorganic Chemistry , Howard University ,
Washington, DC 20059, USA
Published online: 29 Oct 2010.
To cite this article: S. Nair , V.K. Verma , C.S. Verma , T.A. Jain , S.K. Tripathi , R. Singh ,
S.K. Gupta & R.J. Butcher (2010) Synthesis and structure of mononuclear copper(II) complexes
with acyclic Schiff-base ligands containing organotellurium substituents: a comparative
study with selenium analogs, Journal of Coordination Chemistry, 63:23, 4088-4103, DOI:
10.1080/00958972.2010.529497
To link to this article: http://dx.doi.org/10.1080/00958972.2010.529497
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Journal of Coordination Chemistry
Vol. 63, No. 23, 10 December 2010, 4088–4103
Synthesis and structure of mononuclear copper(II) complexes
with acyclic Schiff-base ligands containing organotellurium
substituents: a comparative study with selenium analogs
S. NAIRy, V.K. VERMA*y, C.S. VERMAy, T.A. JAINy, S.K. TRIPATHIz,
R. SINGHx, S.K. GUPTA{ and R.J. BUTCHER?
yDepartment of Chemistry, Disha Institute of Management and Technology, Raipur, India
zMOCVD Division, Defence Materials and Stores Research and Development
Establishment, Kanpur, India
xER and IPR Cell, Defence Research and Development Organization, New Delhi, India
{School of Studies in Chemistry, Jiwaji University, Gwalior, India
?Department of Inorganic Chemistry, Howard University, Washington, DC 20059, USA
(Received 25 June 2010; in final form 10 September 2010)
Tellurium-bearing acyclic Schiff bases, 2,6-bis({N-[2-(phenyltellurato)ethyl]}benzimidoyl)-4-
methylphenol (HL3) and 2,6-bis({N-[3-(phenyltellurato)propyl]}benzimidoyl)-4-methylphenol
(HL4) of the Te2N2O type have been prepared by condensation of 4-methyl-2,6-dibenzoylphe-
nol (mdbpH) with the appropriate phenyltellurato(alkyl)amine. HL3 and HL4 have been
characterized by mass spectrometry, IR, electronic and 1H-NMR spectroscopies and cyclic
voltammetry. Their reactions with Cu(II) acetate monohydrate in a 2 : 1 molar ratio in
methanol yield [(C6H2(O)(Me){(C6H5)C¼N(CH2)nTe(C6H5)}{(C6H5)C¼O})2Cu] (3 (n ¼ 2),
4 (n ¼ 3)) as suggested by analytical and spectroscopic data and single crystal X-ray
crystallography of 3. In both complexes, one arm of the ligand undergoes hydrolysis at the
C¼N position and two molecules of the partially hydrolyzed ligand coordinate to Cu(II)
through imido nitrogen and the phenolic oxygen. The telluriums do not form part of the
copper(II) distorted square planar coordination sphere which has a trans-CuN2O2 core.
Electrochemical studies of 3 and 4 indicate quasi-reversible reductions (Eꢀ0 ¼ ꢁ1.113 V (3) and
ꢁ1.149 V (4)) corresponding to the reduction of copper(II) to copper(I). The interactions of 3
and 4 with calf thymus DNA, investigated by spectrophotometry and cyclic voltammetry,
indicate that 3 and 4 bind to DNA via intercalation, and the binding affinity of 3 is lower than
that of its selenium analog.
Keywords: Hybrid ligands; Copper(II) complexes; X-ray structure; Cyclic voltammetry; DNA
binding
1. Introduction
Organochalcogen chemistry has developed rapidly over the past few decades [1–3], and
is attributed to potential applications of organochalcogen compounds in transition
metal catalyzed organic synthesis [4–12], as single source precursors in metalorganic
chemical vapor deposition (MOCVD) processes for developing materials [13–19],
*Corresponding author. Email: verma_vinay2002@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.529497

Synthesis and structure of mononuclear copper(II) complexes
4089
in ligand chemistry [3, 20–23] and biochemistry [24–29]. Applications of organochalco-
gen compounds have, however, been restricted by difficulties in synthetic methodol-
ogies, purification and instabilities of certain derivatives. To overcome these difficulties,
sterically bulky substituents and chelating groups in close proximity to selenium and
tellurium have been used. Internal chelation has been extended to design and synthesize
heterofunctionalized chalcogen-bearing ligands containing both ‘‘hard’’ and ‘‘soft’’
donors [30, 31]. Such a ligand framework is anticipated to exhibit selective coordination
leading to coordinatively unsaturated metal centers and may exhibit new chemistry
from their heterofunctionalized nature [32–38]. A number of acyclic hybrid ligands with
(O, N, Se/Te), (N2Se/Te), and (N3Se2/Te2) have been recently reported [39–46].
We have very recently reported [47] synthesis of phenol-based Schiff bases,
[PhSe(CH2)nN¼CPh}2C6H2(4-Me)(OH)] (n ¼ 2, HL1; n ¼ 3, HL2) and their reactions
with Cu(II) which yielded [Cu{(PhSe(CH2)nN¼CPh}C6H2(4-Me)C(O)Ph}2)] (n ¼ 2, 1;
n ¼ 3, 2). The present investigation deals with the synthesis and characterization of
potentially pentadentate Te2N2O-type acyclic Schiff bases, 2,6-bis({N[2-phenyltellur-
ato)alkyl]}benzimidoyl)-4-methylphenol,
[{PhTe(CH2)nN¼CPh}2C6H2(4-Me)(OH)]
(n ¼ 2, HL3; n ¼ 3, HL4) and their reactions with Cu(II). A comparative study of
Cu(II) complexes (3, 4), thus formed, with those of their selenium analogs (1, 2) is
reported herein. The structure of ligands HL3 and HL4 and those of complexes 1 and 3
are shown below.
CH3
CH3
N
N
OH
N
N
OH
Te
Te
Te
Te
HL3
4
HL
CH3
CH3
N
N
O
O
O
O
Se
Te
Cu
Cu
Se
Te
O
O
O
O
N
N
H3C
H3C
1
3

4090
S. Nair et al.
2. Experimental
All chemicals were of reagent grade. Solvents were purified by standard methods [48]
and freshly distilled prior to use. The precursors, 4-methyl-2,6-dibenzoylphenol
(mdbpH) [49] and phenyltellurato(alkyl)amines [50], were synthesized following
reported methods. Calf thymus (CT) DNA was obtained from Sigma and used as
received. The sodium salt of CT-DNA was stored at 4ꢀC.
2.1. Physical measurements
Elemental analyses were carried out on a Carlo Erba Model DP 200 analyzer. Melting
points of compounds in capillaries are uncorrected. Molar conductances in
10ꢁ3 mol Lꢁ1 CH3CN solution were measured using a Global DCM-900 digital
conductivity meter. The electrospray ion mass spectra (ESIMS) were recorded on a
MICROMASS QUATTRO II triple quadrupole mass spectrometer. Infrared spectra
were recorded on a Nicolet Magna 750 FT-IR spectrophotometer with KBr pellets
(400–4000 cmꢁ1). Electronic spectra in 10ꢁ4 mol Lꢁ1 CH3CN were obtained using
a Perkin Elmer Lambda 35 UV-Vis spectrometer. 1H-NMR spectra were recorded on a
Brucker AMX 400 FT-NMR spectrometer in CDCl3 with chemical shifts relative
to SiMe4.
2.1.1. Electrochemical measurements. Cyclic voltammetric measurements were carried
out with an Advanced Electrochemical System, PARSTAT 2253 instrument equipped
with a three-electrode system. The micro-cell model KO264 consisted of platinum/
glassy carbon as a working electrode, Pt wire as auxiliary electrode and a non-aqueous
Ag/Agþ reference electrode with 0.1 mol Lꢁ1 AgNO3 in acetonitrile as filling solution.
Tetrabutylammonium perchlorate (0.1 mol Lꢁ1 solution in CH3CN) was used as
supporting electrolyte. Cyclic voltammograms with scan speeds of 50–400 mV sꢁ1 were
run in 10ꢁ4 mol Lꢁ1 CH3CN under nitrogen.
2.2. Synthesis of HL3 and HL4
A
solution of 4-methyl-2,6-dibenzoylphenol (1.26 g, 4.00 mmol Lꢁ1
)
in
acetonitrile (150 mL) was added dropwise to a solution of the appropriate
phenyltellurato(ethyl)amine (1.98 g, 8.00 mmol Lꢁ1)/phenyltellurato(propyl)amine
(2.10 g, 8.00 mmol Lꢁ1) in acetonitrile (200 mL) in a 1 : 2 molar ratio with stirring for
7–8 h. Progress of the reaction was monitored by thin layer chromatograpy (TLC). It
took about 45 h for completion of the reaction. The organic solvent was removed under
reduced pressure and the viscous, oily product, thus obtained, was purified by column
chromatography (silica–gel, 60–120 mesh, second fraction) using a solvent mixture of
hexane : ethylacetate in 80 : 20 or 70 : 30 ratio. The characteristics of these compounds
are given below.
2.2.1. 2,6-Bis({N-[2-(phenyltellurato)ethyl]}benzimidoyl)-4-methylphenol (HL3), [2,6-
{PhTe(CH2)2N¼CPh}2C6H2(4-CH3)(OH)]. Yellow viscous liquid; Yield: 2.68 g
(86%). Positive ES–MS: m/z 779 [MHþ]. FT-IR (cmꢁ1): ꢀ(OH) 3441, ꢀ(C¼N) 1598,

Synthesis and structure of mononuclear copper(II) complexes
4091
ꢀ(CꢁO) 1252. UV-Vis ꢁmax (nm) (CH3CN): 423 (PhOꢁ ! Ph). 1H-NMR (CDCl3,
ꢂ ppm): 16.01 (s, 1H, OH), 7.10–7.90 (m, 22H, C6H5 and C6H2), 3.64 (t, 4H, N–CH2),
2.99 (t, 4H, Te–CH2), and 2.27 (s, 3H, CH3).
2.2.2. 2,6-Bis({N-[3-(phenyltellurato)propyl]}benzimidoyl)-4-methylphenol (HL4), [2,6-
{PhTe(CH2)3N¼CPh}2C6H2(4-CH3)(OH)]. Yellow viscous liquid; Yield: 2.93 g
(91%). Positive ESꢁMS: m/z 807 [MHþ]. FT-IR (cmꢁ1): ꢀ(OH) 3416, ꢀ(C¼N) 1577,
ꢀ(CꢁO) 1253. UV-Vis ꢁmax (nm) (CH3CN): 429 (PhOꢁ ! Ph). 1H-NMR (CDCl3,
ꢂ ppm): 16.26 (s, 1H, OH), 7.06–7.93 (m, 22H, C6H5 and C6H2), 3.31 (t, 4H, N–CH2),
2.82 (t, 4H, Te–CH2), 2.05 (q, 4-H, mid-CH2 (CH2 between N–CH2 and Te–CH2)), 2.13
(s, 3H, CH3).
2.3. Reactions of HL3 and HL4 with Cu(II)
A degassed solution of Cu(OCOCH3)2ꢂH2O (0.10 g, 0.5 mmol Lꢁ1) in freshly distilled
methanol (5 mL) added to degassed solution of HL3 (0.68 g, 1.0 mmol Lꢁ1)/HL4 (0.71 g,
1.0 mmol Lꢁ1) in methanol (15 mL) at room temperature gave a dark blue solid almost
immediately. The reaction mixture was stirred for 12 h and the precipitate thus obtained
was filtered, washed with cold methanol and diethylether, and dried under vacuum. The
characteristics of these complexes are given below.
2.3.1. Bis(2-[N-{2-(phenyltellurato)ethyl}benzimidoyl]-6-benzoyl-4-methylphenolato)-copper(II)
(3); [(C6H2(O)(4-CH3){PhC¼N(CH2)2TePh}PhCO)2ECu]. Dark blue crystalline solid;
Yield: 0.39 g (71%); m.p. 250ꢀC. Anal. Calcd for C58H48N2O4Te2Cu: C, 60.27; H, 4.19;
N, 2.42. Found: C, 60.18; H, 4.30; N, 2.48%. Positive ES–MS: m/z 1157 [MHþ]. FT-IR
(selected, cmꢁ1): ꢀ(C¼O) 1654, ꢀ(C¼N) 1533, ꢀ(C–O) 1246, ꢀ(Cu–N) 570, ꢀ(Cu–O) 437.
UV-Vis ꢁmax (nm) (CH3CN): 639. LM (10ꢁ3 mol Lꢁ1, CH3CN, 298 K): 6 S cm2 mol Lꢁ1
.
2.3.2. Bis(2-[N-{3-(phenyltellurato)propyl}benzimidoyl]-6-benzoyl-4-methylphenolato)-copper(II)
.
(4); [(C6H2(O)(4-CH3){PhC¼N(CH2)3TePh}PhCO)2 Cu]. Light blue crystalline
solid; Yield: 0.36 g (67%); m.p. 180ꢀC. Anal. Calcd for C60H52N2O4Te2Cu: C, 60.87;
H, 4.43; N, 2.37. Found: C, 60.89; H, 4.41; N, 2.42%. Positive ES–MS: m/z 1185
[MHþ]. FT-IR (selected, cmꢁ1): ꢀ(C¼O) 1664, ꢀ(C¼N) 1533, ꢀ(C–O) 1244, ꢀ(Cu–N)
570, ꢀ(Cu–O) 440. UV-Vis ꢁmax (nm) (CH3CN): 613. LM (10ꢁ3 mol Lꢁ1, CH3CN,
298 K): 8 S cm2 mol Lꢁ1
2.4. Crystal structure determination
Crystals of 3 were grown by slow evaporation of a solution in chloroform. Data were
collected with an Oxford Diffraction Gemini R CCD area detector using CrysAlisPro
˚
software and graphite-monochromated Mo-Kꢃ radiation (ꢁ ¼ 0.71073 A) at 200(2) K.
The structure solution and refinements were made by SHELXS 97 [51]. Hydrogens were
placed in their calculated positions and included in the refinement using the riding
mode. An absorption correction was performed using CrysAlis RED and all

4092
S. Nair et al.
calculations were performed using SHELXTL 2000 [52]. ORTEP-3 [53] was used to
prepare the molecular drawings.
2.5. DNA binding studies
Experiments involving interaction of the complex with CT-DNA were carried out in
doubly distilled water buffer containing 5.0 mmol Lꢁ1 tris [tris(hydroxymethyl)-
aminomethane] and 50 mmol Lꢁ1 NaCl and adjusted to pH 7.2 with hydrochloric
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 free of protein contamination [54]. The DNA
concentration per nucleotide was determined spectrophotometrically by employing a
molar absorption coefficient of 6600 (mol L )
ꢁ1 ꢁ1 cmꢁ1 at 260 nm after 1 : 100 dilution
[55]. The complex was dissolved in 1% DMSO and 99% tris-HCl buffer (5.0 mmol Lꢁ1
tris-HCl, 50 mmol Lꢁ1 NaCl, pH 7.2) at 3.0 ꢃ 10ꢁ5 mol Lꢁ1. An absorption titration
was performed on 30 mmol Lꢁ1 compound by varying the concentration of nucleic acid.
While measuring the absorption spectra, an equal amount of CT-DNA was added to
both the compound solution and the reference solution to eliminate absorbance of
CT-DNA itself. Titration curves were constructed from the fractional change in the
absorption intensity as a function of DNA concentration. The intrinsic binding
constant, Kb of complex with CT-DNA was determined according to the following
equation [56] through a plot of [DNA]/("a ꢁ "f) versus [DNA].
½DNAꢄ=ð"a ꢁ "fÞ ¼ ½DNAꢄ=ð"b ꢁ "fÞ þ 1=Kbð"b ꢁ "fÞ
ð1Þ
where [DNA] is the concentration of DNA in base pairs, the apparent absorption
coefficients "a, "f, and "b correspond to Aobserved/[Cu], the extinction coefficient for the
free copper complex and the extinction coefficient for the bound copper complex,
respectively. In plots of [DNA]/("b ꢁ "f) versus [DNA], Kb is given by the ratio of the
slope to intercept.
For cyclic voltammetry, the supporting electrolyte was 50 mmol Lꢁ1 NaCl,
5 mmol Lꢁ1 tris, pH 7.2. The aqueous Ag/AgCl reference electrode was filled with
filling solution containing 3 mol Lꢁ1 NaCl and saturated AgCl.
3. Results and discussion
3.1. Synthesis and properties of the Schiff bases
The monoprotic hybrid acyclic ligands HL3 and HL4 were synthesized via one-step
dipodal condensation of 4-methyl-2,6-dibenzoylphenol with appropriate phenyltellur-
ato(alkyl)amine in 1 : 2 molar ratio in acetonitrile and isolated as yellow viscous
liquids (scheme 1).
The mass spectra of HL3 and HL4 exhibit the molecular ion corresponding
to [2,6-{PhTe(CH2)2N¼CPh}2C6H2(4-Me)(OH)] and [2,6-{PhTe(CH2)3N¼CPh}2C6H2
(4-Me)(OH)], respectively.
The IR spectrum of HL3 shows an absorption at 1598 cmꢁ1 and that of HL4 at
1577 cmꢁ1 due to ꢀ(C¼N). The vibration at 3441 cmꢁ1 is due to ꢀ(OH) and appears
almost at the same position as in the precursor 4-methyl-2,6-dibenzoylphenol.

Synthesis and structure of mononuclear copper(II) complexes
4093
1 : 2
[2,6-{PhTe(CH2)2N=CPh}2C6H2(4-Me)(OH)]
HL3
H2N(CH2)2TePh
(2,6-PhCO)2(4-Me)C6H2OH
1 : 2
[2,6-{PhTe(CH2)3N=CPh}2C6H2(4-Me)(OH)]
H2N(CH2)3TePh
HL4
Scheme 1. Preparation of ligands.
Table 1. CV data of a 0.1 mmol Lꢁ1 solution of HL3 and HL4 in CH3CN/0.1 mol Lꢁ1 NBu4ClO4 at a
platinum electrode vs. Ag/0.1 mol Lꢁ1 AgNO3 at different scan rates.
Compounds
Scan rate
Epa (V)
ipa (mA)
Epc (V)
ipa (mA)
HL3
100
200
300
400
500
0.176
0.188
0.194
0.200
0.194
05.95
25.09
37.17
47.02
55.15
HL4
100
200
300
400
0.230
0.225
0.205
0.200
8.81
29.72
52.04
73.44
0.825
0.830
0.955
0.995
ꢁ44.43
ꢁ61.32
ꢁ93.85
ꢁ116.80
Electronic absorption spectra of ligands consist of an intense band at 423–429 cmꢁ1
attributable to charge transfer from phenolate oxygen to phenyl [57]. The ꢄ–ꢄ*
transitions associated with azomethine chromophore [58] may also be responsible for
this band.
1H-NMR spectra exhibit a significant downfield shift of phenolic OH (signals
between 16.01 and 16.26 ppm) in comparison to mdbpH (ꢂ 12.2 ppm) due to
intramolecular hydrogen bonding between the benzimidoyl nitrogen and the phenolic
OH [59, 60]. In fact, the downfield in them is more and so the intramolecular hydrogen
bonding is greater, than in the corresponding selenium analogs (ꢂ 15.9 ppm) [47].
The cyclic voltammogram of HL3 (Supplementary material; table 1) shows one
irreversible oxidation at 0.190 V whereas HL4 exhibits one quasi-reversible oxidation at
0.558 V versus Ag/0.1 mol Lꢁ1 Agþ with 100–500 mV sꢁ1 scan rates. The peak
separation, DEp, in HL4 varies as a function of scan rate. Also the ratio of peak
current (ipa/ipc ¼ 0.45) is less than 1.0, which indicates that electron transfer is followed
by a slow chemical reaction. Under similar experimental conditions, the observed
oxidation potentials in tellurium-bearing ligands are shifted to more negative potentials
than selenium analogs (0.778 V (HL1), 0.567 V (HL2)). These observations suggest that
tellurium bearing ligands are more sterically hindered than selenium analogs, consistent
with X-ray observations.
3.2. Properties of 3 and 4
The addition of methanolic solution of copper acetate monohydrate to HL3 and HL4 in
1 : 2 molar ratio resulted in precipitation of air stable blue 3 and 4 (scheme 2).

4094
S. Nair et al.
MeOH
2[2,6-{PhTe(CH2)nN=CPh}2C6H2(4-Me)(OH)] + Cu(OCOCH3)2·H2O
[(C6H2(O)(4-Me){PhC=N(CH2)nTePh}PhCO)2Cu] + 2PhTe(CH2)nNH2 + 2 CH3COOH
n=2, 3 ; n=3, 4
Scheme 2. Preparation of the complexes.
The analytical data (sections 2.3.1 and 2.3.2) of the complexes suggest their
formulation as [(C6H2(O)(4-Me){PhC¼N(CH2)nTePh}PhCO)2.Cu]; n ¼ 2, 3; n ¼ 3, 4.
The formula of the complexes indicates hydrolysis of one arm of the ligands at C¼N
and release of acetic acid due to combination of an acetate with phenolic proton of the
ligands. The complexes are non-electrolytes, as suggested by conductivity data.
Mass spectra of 3 and 4 exhibit a molecular ion peak corresponding to a mass of
double the hydrolyzed ligand plus one Cu(II), suggesting complexes of 2 : 1 ligand to
metal stoichiometry. Absorptions at 1654 (3) and 1664 cmꢁ1 (4) in IR spectra can be
due to ꢀ(C¼O) which are almost at the same position as in the precursor 4-methyl-2,6-
dibenzoylphenol. Further, a band at 1533 cmꢁ1 in both complexes may be due to
ꢀ(C¼N). The appearance of ꢀ(C¼O) as well as ꢀ(C¼N) suggests cleavage of one arm
of the ligand due to hydrolysis at C¼N during complexation. The unchanged position
of ꢀ(C¼O) in spectra suggest non-involvement of benzoyl oxygen in coordination.
However, the ꢀ(C¼N) band shows a red shift of 44–66 cmꢁ1 which may be attributed
to participation of azomethine nitrogen in coordination [61]. Further, the absence of
ꢀ(OH) in the spectra indicates deprotonation of phenolic proton and bonding of
phenolic oxygen with copper. Appearance of medium intensity bands at 570 cmꢁ1 and
440–437 cmꢁ1 for both complexes is due to ꢀ(Cu–N) and ꢀ(Cu–O), respectively [61].
Electronic spectra of paramagnetic Cu(II) complexes in CH3CN show only one band
in the 613–639 nm region other than charge transfer at higher frequency. This band is
eg ! b1g transition characteristic of a distorted square planar geometry [62, 63].
Thus, the spectroscopic data suggest that the two molecules of the partially
hydrolyzed ligand coordinate through azomethine N and phenolic O to one Cu(II) to
give N2O2 coordination with two stable six-membered rings around Cu(II). The
structure is confirmed by the spectroscopic data and a single crystal X-ray study.
3.3. Crystal structure of 3
The molecular structure (figure 1) and the crystal data (tables 2 and 3) of 3 show that,
like 1, it has no molecular symmetry. The two monoanionic ligands are bidentate
coordinating through imido nitrogen and phenolic oxygen with nitrogen donors and
phenolic oxygen donors trans to each other. The replacement of Se by the larger Te
results in significant changes in the overall shape of the molecule. The phenyl groups
attached to Se in 1 stick out on either side whereas in 3 they are folded over in the same
direction. The distortions from square planar toward tetrahedral geometry are common
with Cu(II)N2O2 complexes when there are interactions within or between bulky ligands
˚
[64]. The mean Cu–N and Cu–O distances in 3 [1.996(17), 1.878(14) A] differ
˚
significantly from those in 1 [1.973(3), 1.898(2) A] and the O–Cu–O and N–Cu–N

Synthesis and structure of mononuclear copper(II) complexes
4095
Figure 1. Molecular structure of 3.
angles in 3 [160.4(6), 152.5(7)] are smaller than those in 1 [167.4(11), 164.5(12)ꢀ].
Complex 3 is more distorted from square planar toward tetrahedral geometry than 1.
Pulkkinen et al. [65] suggested that ring–ring interactions affect the crystal packing and
this in turn affects coordination at copper. The distortion toward tetrahedral geometry
is in general associated with longer Cu–N and Cu–O bonds; however, in this case, it is
associated with longer Cu–N but smaller Cu–O bonds in a trans configuration.
Intraligand steric interactions are enhanced by replacement of Se by the larger Te,
shown by narrowing of the angle C–Se/Te–C at the chalcogen by the usual inert
pair (relativistic) effect [cf C–Se–C 100ꢀ, 99ꢀ; CTe–C 94ꢀ, 97ꢀ]. Further, there are
significant differences in the C–C–Se–C and C–C–Te–C torsion angles [cf 60ꢀ, 61ꢀ (1);
105ꢀ, 84ꢀ (3)] which suggest that distortions are driven by steric interactions in the
ligand system.
3.4. Electrochemical studies
Cyclic voltammetric (CV) data (tables 4 and 5) for 3 and 4 have been collected in
0.1 mmol Lꢁ1 solution of CH3CN/0.1 mol Lꢁ1 [NBu4][ClO4] under nitrogen with
50–400 mV sꢁ1 scan rates; potentials are reported with reference to Ag/0.1 mol Lꢁ1
Agþ. In the cyclic voltammograms of 3 and 4 (Supplementary material), quasi-
reversible waves at Epc ꢁ 0.857 V, Epa ꢁ 1.368 V for 3 and Epc ꢁ 0.826 V, Epa ꢁ 1.473 V

4096
S. Nair et al.
Table 2. Crystal data and structure refinement for 3.
Empirical formula
Formula weight
Temperature (K)
C58H48CuN2O4Te2
1155.72
200(2)
˚
Wavelength (A)
0.71073
Monoclinic
C 2/c
Crystal system
Space group
Unit cell dimensions (A, )
ꢀ
˚
a
b
c
ꢃ
ꢅ
ꢆ
1232(13)
13.9603(4)
20.0461(6)
90
105.694(4)
90
10001.6(5), 8
1.535
1.629
3
˚
Volume (A ), Z
Calculated density (Mg m3)
Absorption coefficient (mmꢁ1
F(000)
)
4600
0.52 ꢃ 0.47 ꢃ 0.18
Crystal size (mm3)
q range for data collection (ꢀ)
Index ranges
4.63–34.86
ꢁ59 ꢅ h ꢅ 40;
ꢁ19 ꢅ k ꢅ 22;
ꢁ31 ꢅ l ꢅ 30
46999
Reflections collected
Independent reflections
Completeness to q ¼ 25.00 (%)
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I 4 2s(I )]
R indices (all data)
20,078 [R(int) ¼ 0.0395]
99.1
Semi-empirical from equivalents
1.00000 and 0.58997
Full-matrix least-squares on F 2
20,078/0/604
0.909
R1 ¼ 0.0441, wR2 ¼ 0.0837
R1 ¼ 0.1000, wR2 ¼ 0.0951
1.814 and ꢁ0.651
ꢁ3
˚
Largest difference peak and hole (e A
)
for 4 versus Ag/Agþ are observed. These are attributed to CuII to CuI reduction [66].
The cathodic peak potential values for 3 and 4 [ꢁ0.857 V, ꢁ0.826 V] being more positive
than those of 1 and 2 [ꢁ1.422 V, ꢁ1.550 V] suggest that copper in 3 and 4 has
greater distortion, consistent with the X-ray structure. The electrochemical behavior of
3 and 4 has also been studied on glassy carbon working electrode (table 5), where
observed reduction potentials [ꢁ0.778 V; ꢁ0.304 V for 3 and 4] are more positive
compared to those with platinum working electrode [ꢁ0.857 V; ꢁ0.826 V]. This suggests
that reduction of Cu(II) to Cu(I) is faster on the glassy carbon electrode surface as
compared to that on the platinum electrode.
3.5. DNA-binding properties of 3 and 4
3.5.1. Electronic absorption spectra. Complexes binding with DNA through interca-
lation usually result in hypochromism and bathochromism due to intercalative mode
involving a strong stacking intercalation between an aromatic chromophore and the
DNA base pairs. The extent of the hypochromism commonly parallels the intercalative
strength. Absorption spectra (figure 2) of 3 and 4 have been studied in the absence or

Synthesis and structure of mononuclear copper(II) complexes
4097
ꢀ
˚
Table 3. Selected bond lengths (A) and angles ( ) for 3.
Bond lengths
Te(1A)–C(24A)
Te(1A)–C(23A)
Te(1B)–C(24B)
Te(1B)–C(23B)
Cu–O(1B)
Cu–O(1A)
Cu–N(1B)
Cu–N(1A)
O(1A)–C(13A)
O(2A)–C(7A)
O(1B)–C(13B)
O(2B)–C(7B)
N(1A)–C(15A)
N(1A)–C(22A)
N(1B)–C(15B)
N(1B)–C(22B)
2.116(3)
2.150(2)
2.113(2)
2.154(2)
1.8658(16)
1.8834(16)
1.988(2)
1.995(2)
1.320(3)
1.214(3)
1.307(3)
1.211(3)
1.291(3)
1.482(3)
1.294(3)
1.476(3)
Bond angles
C(24A)–Te(1A)–C(23A)
C(24B)–Te(1B)–C(23B)
O(1B)–Cu–O(1A)
O(1B)–Cu–N(1B)
O(1A)–Cu–N(1B)
O(1B)–Cu–N(1A)
O(1A)–Cu–N(1A)
N(1B)–Cu–N(1A)
C(15A)–N(1A)–Cu
C(22A)–N(1A)–Cu
C(15B)–N(1B)–Cu
C(22B)–N(1B)–Cu
97.19(10)
93.72(9)
160.43(8)
91.67(8)
95.26(8)
90.97(8)
91.26(8)
152.54(8)
124.41(17)
114.06(16)
125.87(16)
112.48(15)
Table 4. CV data of 0.1 mmol Lꢁ1 solutions of 3 and 4 in CH3CN/0.1 mol Lꢁ1 NBu4ClO4 at a platinum
electrode vs. Ag/0.1 mol Lꢁ1 AgNO3 at different scan rates.
Complexes
Scan rate
Epa/V (ipa)/mA
Epc/V (ipc)/mA
DEp (mV)
Eꢀ0 (V)
3
50
100
200
300
400
ꢁ1.304 (11.52)
ꢁ1.310 (10.49)
ꢁ1.370 (41.88)
ꢁ1.406 (54.69)
ꢁ1.454 (72.69)
ꢁ1.004 (ꢁ4.98)
ꢁ0.968 (ꢁ12.25)
ꢁ0.812 (ꢁ10.46)
ꢁ0.776 (ꢁ16.83)
ꢁ0.728 (ꢁ26.90)
300
342
558
630
726
ꢁ1.154
ꢁ1.139
ꢁ1.091
ꢁ1.091
ꢁ1.091
4
100
200
300
400
ꢁ1.480 (16.33)
ꢁ1.428 (27.53)
ꢁ1.496 (36.49)
ꢁ1.488 (43.07)
ꢁ0.920 (ꢁ2.93)
ꢁ0.824 (ꢁ11.09)
ꢁ0.792 (ꢁ17.73)
ꢁ0.768 (ꢁ24.59)
560
604
704
720
ꢁ1.200
ꢁ1.126
ꢁ1.144
ꢁ1.128
DEp ¼ (Epa ꢁ Epc); Eꢀ0 ¼ ½ (Epa þ Epc); Supporting electrolyte, NBu4ClO4; Working electrode, Pt; Reference electrode,
Ag/Agþ.
presence of CT-DNA. In the presence of CT-DNA, absorption of 3 at 423 nm exhibits
hypochromism of about 45% and bathochromism of about 2 nm (table 6).
Complex 4 at 416 nm exhibits hypochromism of about 51% and bathochromism of
4 nm. The binding constants Kb for these complexes are 1.26 ꢃ 104 and
1.0 ꢃ 104 (mol L
)
ꢁ1 ꢁ1, respectively, quite comparable to other Schiff-base square

4098
S. Nair et al.
Table 5. CV data of 0.1 mmol Lꢁ1 solutions of 3 and 4 in CH3CN/0.1 mol Lꢁ1
NBu4ClO4 at a glassy carbon electrode vs. Ag/0.1 mol Lꢁ1 AgNO3 at different scan
rates.
Complexs
Scan rate
Epa (V)
ipa (mA)
Epc (V)
ipc (mA)
3
100
200
300
400
ꢁ1.664
ꢁ1.636
ꢁ1.125
ꢁ1.104
24.24
32.95
13.10
15.26
ꢁ1.034
ꢁ1.006
ꢁ0.537
ꢁ0.536
ꢁ1.826
ꢁ3.809
ꢁ11.22
ꢁ10.70
4
100
200
300
400
ꢁ1.280
ꢁ1.4567
ꢁ1.648
ꢁ1.712
16.14
26.69
34.27
41.26
ꢁ0.536
ꢁ0.280
ꢁ0.272
ꢁ0.128
ꢁ7.172
ꢁ19.28
ꢁ25.62
ꢁ24.26
Table 6. Absorption spectroscopic data of 3 and 4 upon addition of CT-DNA.
ꢁmax (nm)
Binding constant,
Kb (Mꢁ1
)
Complexes
Free
Bound
Dꢁ
Hypochromisma (%)
3
4
423
416
425
420
2
4
45
51
1.26 ꢃ 104
1.00 ꢃ 104
aHypochromism ¼ [(Afree ꢁ Abound)/Afree] ꢃ 100%.
planar Cu(II) complexes reported in the literature [67, 68]. These results suggest an
association of the compounds with CT-DNA via intercalation [69–72]. The observed
binding constants for 3 and 4 are lower than that of 1 (1.46 ꢃ 104 (mol L
)
ꢁ1 ꢁ1). These
results suggest that 1, in comparison to 3 and 4, binds more strongly with DNA. The
higher binding affinity of selenium containing complex can be attributed to its more
planar structure than the tellurium analogs [73–78] (figure 2).
3.5.2. Cyclic voltammetry. Cyclic voltammetric technique has been employed to study
the interaction of the redox active Cu(II) complex with DNA to confirm the DNA
binding mode suggested by the spectral studies. Typical CV (figure 3, table 7) behavior
of copper complex has been studied in the absence (curve A) and presence (curve B) of
CT-DNA. In the absence of CT-DNA, Epc and Epa are 0.180 V and ꢁ0.028 V for 3 and
0.193 V and ꢁ0.032 V for 4 versus Ag/AgCl. The formal potentials, Eꢀ0 (or
voltammetric E½), taken as the average of Epc and Epa, are 0.076 and 0.080 V,
respectively. The separation of anodic and cathodic peak potentials, DEp, 152 mV for 3
and 160 mV for 4, indicates a quasi-reversible one-electron redox process [79, 80]. In the
presence of 30 mmol Lꢁ1 DNA, at the same concentration of the complex, the Epc and
Epa are 0.200 and ꢁ0.023 V for 3 and 0.196 and ꢁ0.026 V for 4 versus Ag/AgCl. Thus,
the peak potentials, Epc and Epa as well as Eꢀ0 (0.088 V (3) and 0.085 V (4)), are shifted
to more positive values as compared to a solution without DNA. The values of DEp in
the presence of DNA are 177 and 170 mV, showing that reversibility of the electron
transfer is maintained under these conditions. The apparent Eꢀ0 shifts to more positive
potentials by 25 and 10 mV, respectively, in the presence of DNA. In addition to

Synthesis and structure of mononuclear copper(II) complexes
4099
0.035
0.03
2
0.025
0.02
0.015
0.01
0.005
0
R
=0.9868
40
DNA
0
20
60
80
100
(a)
0.21
0.21
[DNA]/mM
0.20
0.19
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.10
0.09
0.08
390 400
420
440
460
480
500
520
540 550
nm
0.26
0.25
(b)
2
R
=0.9996
0.25
0.2
0.15
0.1
0.05
0
0.24
0.23
0.22
0.21
DNA
0
200
400
600
800
1000
[DNA]/mM
0.20
0.19
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.10
350
400
450
500
550
600
650
700
nm
Figure 2. Absorption spectra of 3 (30 mmol Lꢁ1) (a) and 4 (139 mmol Lꢁ1) (b) at 7.2 pH in the presence
of increasing amount of CT–DNA (0–210 mmol Lꢁ1). Inset: plot of [DNA]/("a ꢁ "f) vs. [DNA] for the titration
of DNA with 3 and 4.

4100
S. Nair et al.
Figure 3. Cyclic voltammograms of 3 (30 mmol Lꢁ1) in the absence (A) and the presence (B) of CT–DNA
(90 mmol Lꢁ1) in 50 mmol Lꢁ1 NaCl, 5 mM Tris, pH 7.2 at scan rate 200 mV sꢁ1
.
Table 7. Cyclic voltammetric behavior of 3 and 4 in the presence of CT–DNA.
Complexes
Scan rate mV sꢁ1
100
R
Epa/V (ipa)/mA
Epc/V (ipc)/mA
DEp (mV)
Eꢀ0 (V)
3
0
ꢁ0.028 (33.82)
ꢁ0.023 (38.89)
ꢁ0.020 (37.11)
ꢁ0.017 (41.24)
ꢁ0.047 (73.52)
ꢁ0.044 (73.30)
ꢁ0.044 (77.00)
ꢁ0.041 (81.30)
0.180 (ꢁ23.95)
0.200 (ꢁ16.20)
0.199 (ꢁ15.40)
0.199 (ꢁ17.90)
0.205 (ꢁ31.16)
0.208 (ꢁ30.64)
0.208 (ꢁ31.40)
0.208 (ꢁ25.50)
152
177
179
182
158
164
164
167
0.076
0.086
0.089
0.091
0.078
0.081
0.082
0.083
30
60
90
0
30
60
90
200
4
100
200
0
30
60
90
0
30
60
90
ꢁ0.032 (36.32)
ꢁ0.026 (36.04)
ꢁ0.026 (38.50)
ꢁ0.023 (34.04)
ꢁ0.056 (69.83)
ꢁ0.050 (71.60)
ꢁ0.050 (71.90)
ꢁ0.050 (74.60)
0.193 (ꢁ31.00)
0.196 (ꢁ30.70)
0.196 (ꢁ25.10)
0.199 (ꢁ25.60)
0.199 (ꢁ48.73)
0.202 (ꢁ44.10)
0.202 (ꢁ48.70)
0.203 (ꢁ34.35)
161
170
170
176
143
152
152
153
0.080
0.084
ꢁ0.085
ꢁ0.087
0.071
0.076
ꢁ0.075
ꢁ0.076
DEp ¼ (Epa ꢁ Epc); Eꢀ0 ¼ ½ (Epa þ Epc); Complex 30 mmol Lꢁ1; R ¼ [DNA]/[Complex] supporting electrolyte, 50 mmol Lꢁ1
NaCl þ 5 mmol Lꢁ1 Tris, pH 7.2; Working electrode, Pt; Reference electrode, Ag/AgCl.
changes in formal potential upon addition of DNA, the cathodic peak current, ipc,
decreased to 33% for 3 and 13% for 4 compared to that in the absence of DNA for a
solution with R ¼ 30 (100 mV sꢁ1). Similar observations have recently been reported in
the literature [67, 68]. Bard and co-workers [79] suggested that the electrochemical
potential of small molecules would shift in a positive direction when intercalated into
DNA and it would shift in a negative direction if bound to DNA by electrostatic
interaction. Thus, the observed results imply that 3 and 4 bind to DNA by intercalation
involving stacking interaction between the aromatic chromophore of the complex and
the base pairs of DNA.

Synthesis and structure of mononuclear copper(II) complexes
4101
4. Conclusion
HL3 and HL4 have been synthesized by single step dipodal condensation of 4-methyl-
2,6-dibenzoylphenol with the appropriate phenyltellurato(alkyl)amine in 1 : 2 molar
ratio. The reactions of these ligands with copper acetate monohydrate result in partial
hydrolysis of ligands and formation of [(C6H2(O)(Me){(C6H5)C¼N(CH2)n
Te(C6H5)}(C6H5)C¼O)2Cu]; n ¼ 2 (3); 3 (4). The partially hydrolyzed monoanionic
ligands are bidentate coordinating through the imido N and phenolic O to one Cu(II) to
give N2O2 coordination. The X-ray structure of 3 suggests that it is more distorted from
square planar toward tetrahedral than is 1. In cyclic voltammetry, a quasi-reversible
reduction peak from ꢁ0.857 to ꢁ0.826 V (vs. Ag/Agþ) is assigned to the Cu(II)/Cu(I)
couple. The cathodic peak potential value for 3 and 4 being more positive than those of
their selenium analogs 1 and 2 suggests that the copper is somewhat more distorted,
consistent with the X-ray structures. Binding of the complexes to CT-DNA has been
investigated by absorption and cyclic voltammetric measurements, which indicate that
the complexes can intercalate into DNA base pairs; the binding affinity of selenium
containing complex is higher than for tellurium analogs.
Supplementary material
CCDC 739452 contains the supplementary crystallographic data for 3. These data can
be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (þ44) 123-336-033; or E-mail:
deposit@ccdc.cam.ac.uk. Figures of cyclic voltammogram of the ligands (HL3 and
HL4) and complexes (3 and 4) can be found in the online supplementary material at
informaworld.com
Acknowledgments
The authors are grateful to Disha Academy of Research and Education (DARE),
Raipur, for providing financial assistance. We thank Professor B.L. Khandelwal,
DIMAT, Raipur, and Dr J.D. Smith, University of Sussex, UK, for discussion and
Professor Sandeep Verma, IIT Kanpur, for recording mass spectra. Authors are also
thankful to the reviewer for his constructive suggestions.
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