New acyclic Schiff-base copper(II) complexes and their electrochemical, catalytic, and antimicrobial studies

Article abstract of DOI:10.1080/00958972.2011.553221

A new series of acyclic mononuclear copper(II) complexes have been prepared by Schiff-base condensation derived from 5-methylsalicylaldehyde, diethylenetriamine, tris(2-aminoethyl) amine, triethylenetetramine, N,N-bis(3-aminopropyl)ethylene diamine, N,N-b

Full text of DOI:10.1080/00958972.2011.553221

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New acyclic Schiff-base copper(II)
complexes and their electrochemical,
catalytic, and antimicrobial studies
A. Vijayaraj a , R. Prabu a , R. Suresh a , C. Sivaraj b , N. Raaman b
& V. Narayanan a
a Department of Inorganic Chemistry , School of Chemical
Sciences, University of Madras , Guindy Campus, Chennai 600 025,
India
b Centre for Advanced Studies in Botany, University of Madras ,
Guindy Campus, Chennai 600 025, India
Published online: 18 Feb 2011.
To cite this article: A. Vijayaraj , R. Prabu , R. Suresh , C. Sivaraj , N. Raaman & V.
Narayanan (2011) New acyclic Schiff-base copper(II) complexes and their electrochemical,
catalytic, and antimicrobial studies, Journal of Coordination Chemistry, 64:4, 637-650, DOI:
10.1080/00958972.2011.553221
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Journal of Coordination Chemistry
Vol. 64, No. 4, 20 February 2011, 637–650
New acyclic Schiff-base copper(II) complexes and their
electrochemical, catalytic, and antimicrobial studies
A. VIJAYARAJy, R. PRABUy, R. SURESHy, C. SIVARAJz,
N. RAAMANz and V. NARAYANAN*y
yDepartment of Inorganic Chemistry, School of Chemical Sciences, University of Madras,
Guindy Campus, Chennai 600 025, India
zCentre for Advanced Studies in Botany, University of Madras, Guindy Campus,
Chennai 600 025, India
(Received 26 May 2010; in final form 17 November 2010)
A new series of acyclic mononuclear copper(II) complexes have been prepared by Schiff-base
condensation derived from 5-methylsalicylaldehyde, diethylenetriamine, tris(2-aminoethyl)
amine, triethylenetetramine, N,N-bis(3-aminopropyl)ethylene diamine, N,N-bis(aminopropyl)
piperazine, and copper perchlorate. All the complexes were characterized by elemental and
spectral analyses. Electronic spectra of the complexes show a d–d transition in the range 500–
800 nm, electrochemical studies of the complexes show irreversible one-electron-reduction
process around ꢀ1.10 to ꢀ1.60 V. The reduction potential of the mononuclear copper(II)
complexes shifts toward anodic direction upon increasing the chain length of the imine
compartment. ESR spectra of the mononuclear copper(II) complexes show four lines,
characteristic of square-planar geometry, with nuclear hyperfine spin 3/2. The copper(II)
complexes show a normal room temperature magnetic moment value ꢀeff ¼ 1.72–1.76 BM, close
to the spin-only value of 1.73 BM. Electrochemical and catalytic studies of the complexes were
compared on the basis of increasing the chain length of the imine compartment. All the
complexes were screened for antifungal and antibacterial activities.
Keywords: Schiff-base ligands; Copper(II) complexes; Cyclic voltammetry; Catecholase activ-
ity; Antimicrobial activity
1. Introduction
Coordination metal complexes have long been recognized as materials with unusual
properties. The ability of transition metals to possess a variety of coordination
geometries is largely responsible for their unique biological actions [1]. Study of certain
transition metals containing compounds relate to the crucial roles of metal ions in
nature; looking at the known structures of metallobiosites and then designing analogues
can initiate new areas of coordination chemistry.
Acyclic compounds are important and powerful ligands, ubiquitous in transition
metal coordination chemistry. They mimic important biological ligands [2]. The
chemistry of compartmental ligands capable of forming acyclic complexes with similar
*Corresponding author. Email: vnnara@yahoo.co.in
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2011.553221

638
A. Vijayaraj et al.
or dissimilar metal ions [3] has grown rapidly. Acyclic ligands can impose high degree of
preorganization on metal complex formation and are used as models [4] for protein–
metal binding sites in biological systems, as synthetic ionophores, as therapeutic
reagents in chelate therapy for the treatment of metal toxication, in catalysis, and to
investigate the mutual influence of metal centers on their physicochemical properties [5].
Schiff bases are easily prepared by condensation between aldehydes and amines.
Stereogenic centers of chirality (planes, axes) can be introduced in the synthetic design.
Schiff-base ligands are able to coordinate [6–10] and stabilize various oxidation states.
Schiff bases can be used to obtain optical materials and conducting polymers [11]. Thus,
new optical and conducting materials can be produced by these compounds. Schiff-base
ligands have been used in the construction of membrane sensors [12–19] and as models
for biological systems [20, 21]. Schiff bases have been often used as chelating ligands of
transition metals, as radiopharmaceuticals for cancer targeting, as agrochemicals, as
catalysts, and as dioxygen carriers. Copper is an important trace element for plants and
animals and is involved in a number of biological processes. Copper complexes
containing Schiff bases are of great interest since they exhibit numerous biological
activities, such as antitumor [22], anticandida [23], antibacterial [24], and antimicrobial
[25] activities. Structure–activity relationships of the metal complexes are determined by
the oxidation state of metals, the type and number of donors, and the relative
disposition within the ligand [26]. Parashar et al. [27] reported that copper(II),
cobalt(II), and nickel(II) complexes of Schiff-base ligands obtained from 2-substituted
anilines and salicylaldehyde exhibit good antifungal and antibacterial activities.
This work deals with the influence of ligand modification on spectral, electrochem-
ical, magnetic, and catalytic studies, reporting the synthesis and characterization of
acyclic mononuclear copper(II) complexes.
2. Experimental
2.1. Physical measurements
Elemental analyses of the complexes are obtained using a Haereus CHN rapid analyzer.
Conductivity measurements of the complexes are obtained using an Elico digital
conductivity bridge model CM-88 using freshly prepared solution of the complex in
DMF. IR spectra were recorded on a Perkin Elmer FT-IR 8300 series spectro-
photometer on KBr discs from 4000 to 400 cmꢀ1. 1H NMR spectra were recorded using
a JEOL GSX 400 MHz NMR spectrometer. Mass spectra were obtained on a JEOL
DX-303 mass spectrometer. Electronic spectral studies were carried out on a Perkin
Elmer 320 spectrophotometer from 200 to 1100 nm. Cyclic voltammograms were
obtained on a CHI-600A electrochemical analyzer under oxygen-free conditions using a
three-electrode cell in which a glassy carbon electrode was the working electrode, a
saturated Ag/AgCl electrode was the reference electrode, and a platinum wire was the
auxiliary electrode. A ferrocene/ferrocenium couple was used as an internal standard
and E1/2 of the ferrocene/ferrocenium (Fc/Fcþ) couple under the experimental
condition was 470 mV. Tetra(n-butyl)ammonium perchlorate (TBAP) was used as the
supporting electrolyte. Room temperature magnetic moments were measured on a PAR
vibrating sample magnetometer (VSM) Model-155. X-band EPR spectra were recorded

Acyclic Schiff-base copper(II)
639
at 25ꢁC on a Varian EPR-E 112 spectrometer using diphenylpicrylhydrazine (DPPH) as
the reference. Catalytic oxidation of catechol to o-quinone by the copper complexes was
studied in 10ꢀ3 mol Lꢀ1 DMF solutions. The reactions were followed spectrophotome-
trically with the strongest absorption of o-quinone at 390 nm and monitoring the
increase in absorbance. A plot of log (Aꢁ/Aꢁ ꢀ At) versus time was made for each
complex and rate constants for the catalytic oxidations were calculated.
2.2. Chemical and reagent
5-Methylsalicylaldehyde [28] was prepared following the literature method. Analytical
grade methanol, acetonitrile, and DMF were purchased from Qualigens. TBAP, used as
supporting electrolyte in electrochemical measurements, was purchased from Fluka and
recrystallized from hot methanol. N,N-bis-(3-aminopropyl) piperazine, N,N-bis-(3-
aminopropyl) ethylene diamine, and tris-(2-aminoethyl)amine were purchased from
Aldrich. Triethylenetetramine and diethylenetriamine were purchased from Qualigens.
2.3. Synthesis of ligands
2.3.1. Synthesis of bis(5-methylsalicylaldehyde)diethylenetriamine, L1 [29]. An absolute
methanol solution (10 mL) containing diethylenetriamine (0.1030 g, 1 mmol) was added
dropwise to a stirred solution of 5-methylsalicylaldehyde (0.2720 g, 2 mmol) and the
solution was refluxed for 7 h. The resulting yellow solution was cooled in ice. The yellow
precipitate was filtered, washed with hexane, and dried under vacuum. The crude
product on recrystallization from THF gave yellow crystals. Yield (54.8%). m.p.:
163ꢁC. Anal. Calcd for C20H25N3O2 (%): C, 70.77; H, 7.42; N, 12.38. Found (%): C,
70.72; H, 7.31; N, 12.30. ESI MS: (m/z) 339.19, Calcd av. m/z 338.10. IR data (KBr) (ꢂ,
cmꢀ1): 3405 ꢂ [b, (OH)], 1637 [s, ꢂ(C¼N)], 1H NMR (ppm in CDCl3 400 MHz):
2.94(2CH3, s, 6H), 3.24(N–CH2, d, 8H, J ¼ 12 Hz), 6.84(Ar–H, t, 6H, J ¼ 8 Hz),
7.81(N–H, d,1H, J ¼ 8 Hz), 8.29(–CH¼N, d, 2H, J ¼ 12 Hz), 9.78(2Ar–OH, s, 2H).
Ligands L2, L3, L4, and L5 were synthesized by following the above procedure using
tris-(2-aminoethyl)amine (0.1490 g, 1 mmol), triethylenetetramine (0.1420 g, 1 mmol),
N,N-bis-(3-aminopropyl)ethylenediamine (0.1740 g, 1 mmol), and N,N-bis-(3-
aminopropyl)piperazine (0.2000 g, 1 mmol) instead of diethylenetriamine.
2.3.2. Synthesis of L2. Yield (35.2%). m.p.: 138ꢁC. Anal. Calcd for C22H30N4O2 (%):
C, 69.08; H, 7.91; N, 14.65. Found (%): C, 69.01; H, 7.34; N, 14.61. ESI MS: (m/z)
382.24, Calcd av. m/z 382.10. Selected IR data (KBr) (ꢂ, cmꢀ1): 3485[b, ꢂ(OH)],
1635[s, ꢂ(C¼N)], 1H NMR (ppm in CDCl3 400 MHz): 2.35(2CH3, s, 6H), 3.49(N–CH2,
d, 10H, J ¼ 8 Hz), 5.82(NH2, s, 2H), 6.94(Ar–H, t, 6H, J ¼ 8 Hz), 7.93(2CH2, s, 4H),
8.24(–CH¼N, d, 2H, J ¼ 12 Hz), 9.89(2Ar–OH, s, 2H).
2.3.3. Synthesis of L3. Yield (41.5%). m.p.: 125ꢁC. Anal. Calcd for C22H30N4O2 (%):
C, 69.08; H, 7.81; N, 15.65. Found (%): C, 69.01; H, 7.64; N, 15.61. ESI MS: (m/z)
383.24, Calcd av. m/z 383.10. Selected IR data (KBr) (ꢂ, cmꢀ1): 3445 ꢂ(OH), 1630[s,
1
ꢂ(C¼N)], H NMR (ppm in CDCl3 400 MHz): 2.32(2CH3, s, 6H), 3.77(2N–(CH2)2, d,

640
A. Vijayaraj et al.
12H, J ¼ 12 Hz), 6.94(Ar–H, t, 6H, J ¼ 8 Hz), 7.98(2N–H, d, 2H, J ¼ 8 Hz),
8.32(–CH¼N, d, 2H, J ¼ 8 Hz), 9.87(2Ar–OH, s, 2H).
2.3.4. Synthesis of L4. Yield (29.5%). m.p.: 141ꢁC. Anal. Calcd for C24H34N4O2 (%):
C, 70.21; H, 8.34; N, 13.69. Found (%): C, 70.13; H, 8.29; N, 13.53. ESI MS: (m/z)
410.30, Calcd av. m/z 410.10. Selected IR data (KBr) (ꢂ, cmꢀ1): 3475 ꢂ(OH), 1637 [s,
1
ꢂ(C¼N)]. H NMR (ppm in CDCl3 400 MHz): 2.60(2CH3, s, 6H), 3.71(N–(CH2)2, d,
4H, J ¼ 8 Hz), 5.32(2N–H, d, 2H, J ¼ 8 Hz), 7.09(Ar–H, s, 6H), 7.81(CH2, s, 12H),
8.16(–CH¼N, s, 2H), 9.81(2Ar–OH, s, 2H).
2.3.5. Synthesis of L5. Yield (29.5%). m.p.: 171ꢁC. Anal. Calcd for C26H36N4O2 (%):
C, 71.53; H, 8.32; N, 12.80. Found (%): C, 71.41; H, 8.24; N, 12.77. ESI MS: (m/z)
436.35, Calcd av. m/z 435.20. Selected IR data (KBr) (ꢂ, cmꢀ1): 3435 ꢂ(OH), 1631[s,
1
ꢂ(C¼N)]. H NMR (ppm in CDCl3 400 MHz): 2.28(2CH3, s, 6H), 3.54(2N–(CH2)2, d,
8H, J ¼ 12 Hz), 5.71(CH2, s, 12H), 7.24(Ar–H, s, 6H), 8.25(–CH¼N, s, 2H), 9.79(2Ar–
OH, s, 2H).
2.4. Synthesis of metal complexes
An absolute methanol solution containing Cu(ClO4)2 ꢂ 6H2O (0.037 g, 0.1 mmol) was
added dropwise to a stirring solution of L1 (0.0339 g, 0.1 mmol) in 20 mL of absolute
methanol and refluxed for 8 h. On cooling the solution, green microcrystals formed,
were filtered, washed with methanol followed by diethyl ether, and dried in vacuum.
The crude product was recrystallized from methanol and acetonitrile (1 : 3, v/v).
[Cu(II)L2], [Cu(II)L3], [Cu(II)L4], and [Cu(II)L5] were synthesized by following the
above procedure using L2 (0.038 g, 0.1 mmol), L3 (0.0382 g, 0.1 mmol), L4 (0.0426 g,
0.1 mmol), and L5 (0.0468 g, 0.1 mmol).
2.4.1. Synthesis of [Cu(II)L1]. Yield: (65%). m.p.: 322ꢁC (dec). Anal. Calcd for
C20H23CuN3O2 (%): C, 59.91; H, 5.78; N, 10.49; Cu, 15.85. Found (%): C, 59.69; H,
5.72; N, 10.22; Cu, 15.61 ESI MS: (m/z) 400.12, Calcd av. m/z 400.02. Conductance
(L Scm2 molꢀ1) in DMF: 65. Selected IR data (KBr) (ꢂ, cmꢀ1): 3375 ꢂ(NH), 1624 [s,
ꢂ(C¼N)], 616 [s, ꢂ(M–N)], 436 [s, ꢂ(M–O)], UV-Vis [ꢃmax, nm (",(mol L
)
ꢀ1 ꢀ1 cmꢀ1)] in
DMF: 585 (140), 355 (16,450).
2.4.2. Synthesis of [Cu(II)L2]. Yield: (81%). m.p.: 331ꢁC (dec). Anal. Calcd for
C22H28CuN4O2 (%): C, 59.51; H, 6.36; N, 12.62; Cu, 14.31. Found (%): C, 59.32, H,
6.25; N, 12.49; Cu, 14.26. ESI MS: (m/z) 443.15, Calcd av. m/z 442.10. Conductance
(L Scm2 molꢀ1) in DMF: 71. Selected IR data (KBr) (ꢂ, cmꢀ1): 3417 ꢂ(NH2), 1614 [s,
ꢂ(C¼N)], 426 [ꢂ(M–N)], 441 [s, ꢂ(M–O)], UV-Vis [ꢃmax, nm (",(mol L
)
ꢀ1 ꢀ1 cmꢀ1)] in
DMF: 600 (123), 360 (18,000).
2.4.3. Synthesis of [Cu(II)L3]. Yield: (71%). m.p.: 325ꢁC (dec). Anal. Calcd
for C22H28CuN4O2 (%): C, 59.51; H, 6.36; N, 12.62; Cu, 14.31. Found (%): C,
59.42; H, 6.20; N, 12.43; Cu, 14.21. ESI MS: (m/z) 443.15, Calcd av. m/z 445.02.

Acyclic Schiff-base copper(II)
641
Conductance (L Scm2 molꢀ1) in DMF: 75. Selected IR data (KBr) (ꢂ, cmꢀ1): 3360
ꢂ(NH), 1620 [s, ꢂ(C¼N)], 567 [ꢂ(M–N)], 446 [s, ꢂ(M–O)], UV-Vis [ꢃmax, nm
(", (mol L )
ꢀ1 ꢀ1 cmꢀ1)] in DMF: 680 (138), 381 (18,400).
2.4.4. Synthesis of [Cu(II)L4]. Yield: (79%). m.p.: 335ꢁC (dec). Anal. Calcd for
C24H32CuN4O2 (%): C, 61.06; H, 6.84; N, 11.86; Cu, 13.45. Found (%): C, 61.01; H,
6.76; N, 11.73; Cu, 13.31. ESI MS: (m/z) 475.21, Calcd av. m/z 473.18. Conductance
(L Scm2 molꢀ1) in DMF: 79. Selected IR data (KBr) (ꢂ, cmꢀ1): 3330 ꢂ(NH), 1628
[s, ꢂ(C¼N)], 560 [ꢂ(M–N)], 430 [s, ꢂ(M–O)], UV-Vis [ꢃmax, nm (", (mol L
)
ꢀ1 ꢀ1 cmꢀ1)] in
DMF: 630 (128), 390 (19,600).
2.4.5. Synthesis of [Cu(II)L5]. Yield: (61%). m.p.: 341ꢁC (dec). Anal. Calcd for
C26H34CuN4O2 (%): C, 62.69; H, 6.87; N, 11.25; Cu, 12.74. Found (%): C, 62.58; H,
6.80; N, 11.17; Cu, 12.69. ESI MS: (m/z) 499.25, Calcd av. m/z 497.12. Conductance
(L Scm2 molꢀ1) in DMF: 82. Selected IR data (KBr) (ꢂ, cmꢀ1): 3113 ꢂ(N–CH2), 1626
[s, ꢂ(C¼N)], 569 [s, ꢂ(M–N)], 455 [s, ꢂ(M–O)], UV-Vis [ꢃmax, nm (", (mol L
)
ꢀ1 ꢀ1 cmꢀ1)]
in DMF: 730 (130), 340 (18,200).
3. Results and discussion
A series of acyclic mononuclear copper(II) complexes were synthesized by Schiff base
condensation of the precursor compounds with diamines in the presence of copper as
shown in schemes 1 and 2. Conductivity measurements (L 73–97 Scm2 molꢀ1) show they
are 1 : 1 electrolytes in DMF [30]. All attempts to grow single crystals of the complexes
(e.g. by the diffusion of diethyl ether vapor into DMF solutions or recrystallization of
the complexes from acetonitrile) have failed and only green powder or microcrystals
were obtained. Spectral, electrochemical, catalytic, and antimicrobial studies of the
complexes were carried out.
3.1. Spectroscopic studies
The FT-IR spectrum of 5-methy1salicylaldehyde shows a peak at 1655 cmꢀ1
corresponding to (–CHO). The ligands as well as 5-methy1salicylaldehyde show a
band at 3400 cmꢀ1 corresponding to (–OH). Ligands and complexes show a sharp band
at 1620–1640 cmꢀ1 due to C¼N. The presence of this new peak and the absence of
(C¼O) and (–OH) in the complexes indicate Schiff-base condensation [31–36]. For the
complexes, bands at 430–466 cmꢀ1 could be assigned to –(M–O) bond. Other weak
bands at lower frequency could be assigned to –(M–N) [37]. The spectra of all the
complexes are dominated by bands at 3150–3070 cmꢀ1 due to the aromatic C–H
stretching vibration. A strong band at 1260 cmꢀ1 in the free Schiff bases has been
assigned to phenolic C–O stretch. Upon complexation, this band shifts to higher
frequency (1300 cmꢀ1) showing coordination through phenolic oxygen [38]. The
presence of uncoordinated perchlorate anions in all the mononuclear complexes are
inferred from a single broad band around 1100 cmꢀ1 (ꢂ3-antisymmetric stretch) which is

642
A. Vijayaraj et al.
CH3
CH3
OH
OH
CH3OH
N
N
NH2
NH2
R
2
1
+
R
Reflux 8 h
O
OH
N
CH3
NH
NH
NH
NH
N
R
NH
N
NH2
L1
L2
L5
L3
L4
Scheme 1. Schematic diagram for the synthesis of ligands.
not split and a band around 650 cmꢀ1 (ꢂ4-antisymmetric bending). The band around
930 cmꢀ1 (ꢂ2-symmetric stretching) due to coordinated perchlorate is not observed and
this indicates that no perchlorate ions are coordinated in the complexes.
3.2. Electronic spectra
Electronic spectra of all the complexes obtained in DMF show a single weak d–d band
2
2
at 450–800 nm due to B1g ! A1g associated with square-planar [39] geometry around
[Cu(II)L]. A red shift in the ꢃmax value of d–d band [40] with increase in the chain length
between imine nitrogen has been observed. This red shift may be due to distortion from
planar geometry as the chain length increases. Moderately intense band at 280–450 nm
is associated with ligand to metal charge transfer. An intense band observed at
220–260 nm is associated with intra ligand transition.
3.3. ESR spectra
Solid-state ESR spectra of the copper(II) complexes recorded in the X-band region
at room temperature show four lines [41] due to hyperfine splitting (nuclear spin 3/2).

Acyclic Schiff-base copper(II)
643
CH
3
CH
3
OH
OH
Cu(ClO ) .6H O
N
N
4 2
2
O
O
N
N
R
R
Cu
CH OH
3
Reflux 8h
CH
CH
3
3
N
N
NH
NH
NH
NH
N
R
NH
NH
2
1
2
5
3
4
L
L
L
L
L
Scheme 2. Schematic diagram for the synthesis of complexes.
Table 1. ESR and magnetic moment data of mononuclear copper(II) complexes.
ESR
Complexes
Magnetic moment ꢀeff BM
g||
g
?
A||
[Cu(II)L1]
[Cu(II)L2]
[Cu(II)L3]
[Cu(II)L4]
[Cu(II)L5]
1.72
1.73
1.73
1.75
1.76
2.31
2.28
2.30
2.21
2.19
2.09
2.10
2.12
2.14
2.16
160
169
171
180
186
The observed g values are less than 2.3 indicating considerable covalent character in the
M–L bonds [42]. The ESR data of [Cu(II)L] complexes are given in table 1. g|| and g
?
values indicate square-planar geometry. The absence of the half-field signal at 1600 G,
corresponding to DM ¼ ꢃ2 transition, ruled out Cu–Cu interaction [43], consistent
with mononuclear complexes. The hyperfine A|| splitting falls in the range
150–165 ꢄ 10ꢀ4 cmꢀ1, indicative of an electron interacting with only one copper
nucleus. The relation g|| 4 g is typical of d9 copper(II) complexes in a ground state
?

644
A. Vijayaraj et al.
Table 2.
activity data for the complexes.
Electrochemicala reduction (cathodic potential) and catecholaseb
Rate constant (k) ꢄ 103 minꢀ1
Complexes
Epc(V vs. Ag/AgCl)
Catecholase activity
[Cu(II)L1]
[Cu(II)L2]
[Cu(II)L3]
[Cu(II)L4]
[Cu(II)L5]
ꢀ1.55
ꢀ1.50
ꢀ1.43
ꢀ1.38
ꢀ1.15
3.66
3.83
4.29
4.75
5.22
aMeasured by CV at 50 mV sꢀ1. E vs. Ag/AgCl condition, GC working electrode, and
Ag/AgCl. Reference electrode: supporting electrolyte TBAP, concentration of complexes is
1ꢄ10ꢀ1 mol Lꢀ1
.
bMeasured spectrophotometrically in DMF. Concentration of the complexes is
1 ꢄ 10ꢀ3 mol Lꢀ1
.
doublet with the unpaired electron in a dx2ꢀy2 orbital [44–47]. The order g|| 4 g 4 2
?
and the ESR parameters coincide with related systems, suggesting square-planar
geometry [46]. In the axial spectra, the g values are related to the exchange interaction
term G by the expression
G ¼ ð g ꢀ 2:0023Þ=ð g ꢀ 2:0023Þ:
jj
?
According to Hathaway and Tomlinson [46] if G 4 4.0, exchange is negligible
because the local tetragonal axes are aligned parallel or slightly misaligned. If G 5 4.0,
exchange is considerable and the local tetragonal axes are misaligned. The observed G
values for the complexes are higher than 4.0.
Room temperature (at 298 K) magnetic moments of the complexes, determined
by VSM, show ꢀeff from 1.72 to 1.76 BM, close to the spin-only value of copper(II) [48].
The room temperature magnetic data of the copper(II) complexes do not provide
conclusive evidence for the geometry; however, the electronic spectral data coupled
with magnetic moment values indicate square-planar geometry for the copper(II)
complexes.
3.4. Electrochemistry of the complexes
Electrochemical properties of the mononuclear complexes were studied by cyclic
voltammetry in DMF containing TBAP as supporting electrolyte in the potential range
ꢀ0.4 to ꢀ2.0 V. The electrochemical data of copper(II) complexes are given in table 2.
Generally the electrochemical properties depend on chelate ring/size, axial ligation
degree, distribution of unsaturation, and substitution pattern in the chelate ring. Each
voltammogram shows a one electron irreversible reduction wave at ꢀ1.10 to ꢀ1.60 V.
Controlled potential electrolysis carried out at 100 mV, more negative than the
reduction wave, shows the consumption of one electron per molecule. The reduction
potential shifts in the anodic direction for [Cu(II)L1] to [Cu(II)L5] from ꢀ1.55 V to
ꢀ1.15 V, as the number of methylene groups increases [49, 50]. This shows that, as the
number of methylene groups between the imine nitrogen (chain length) increases, the
entire acyclic ring becomes more flexible, causing a distortion of the complexes, which
stabilizes Cu(I).

Acyclic Schiff-base copper(II)
645
OH
OH
O
+
1/2 O
2
+
H O
2
O
Scheme 3. Oxidation of catechol.
4. Kinetic studies
4.1. Oxidation of pyrocatechol (catecholase activity)
The catecholase activity of the copper(II) complexes was carried out using pyrocatechol
as the model substrate for the identification of functional models for metalloenzymes
(scheme 3) [51]. For this purpose, 10ꢀ3 mol dmꢀ3 solutions of complexes in DMF were
treated with 100 equivalents of pyrocatechol in air. The course of the reaction was
followed spectrophotometrically at 390 nm for 45 min at regular time intervals of 5 min.
The slope was determined by the method of initial rates by monitoring the growth of the
390 nm band of the product o-quinone. A linear relationship for initial rate and the
complex concentration obtained for the copper(II) complexes shows a first-order
dependence on the complex concentration.
The plots of log (Aꢁ/Aꢁ ꢀ At) versus time for catecholase activity of the copper(II)
complexes are shown in figure 1. The inset of figure 1 shows the time-dependent growth
of o-quinone chromophore in the presence of [Cu(II)L5]. The observed initial rate
constants for copper(II) complexes are given in table 2. [Cu(II)L5] has higher catalytic
activity (5.22 ꢄ 10ꢀ3 minꢀ1) than [Cu(II)L4] (4.75 ꢄ 10ꢀ3 minꢀ1) which in turn is higher
than [Cu(II)L1] (3.66 ꢄ 10ꢀ3 minꢀ1). The order of activity of the complexes is shown
below.
½CuðIIÞL5ꢅ 4 ½CuðIIÞL4ꢅ 4 ½CuðIIÞL3ꢅ 4 ½CuðIIÞL2ꢅ 4 ½CuðIIÞL1ꢅ:
The rate of oxidation of catecholase to o-quinone increases as the chain length
increases, due to the flexibility resulting from the distortion of the coordination sphere.
Increase in the chain length causes a greater distortion of the geometry of the
complexes, which favors the reaction.
4.2. Antimicrobial activity
4.2.1. Microorganisms. Microorganisms, Staphylococcus aureus (ATCC 12600),
Micrococcus luteus, Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 11775),
Klebsiella pneumonia (ATCC 13883), and Candida albicans (ATCC TW10637) were
stored in a refrigerator at the microbiology lab, Centre for Herbal Sciences, and used
for the antimicrobial study.
4.2.2. Reference and control. Tetracycline was chosen as the reference compound for
all the bacterial species used, S. aureus, M. luteus, B. subtilis, E. coli, and K. pneumonia.

646
A. Vijayaraj et al.
0.055
0.050
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(e)
(d)
300
400
500
600
700
Wave number (nm)
(c)
(b)
(a)
0
10
20
Time (min
30
40
50
–1
)
Figure 1. Catecholase activity of the copper(II) complexes: (a) [Cu(II)L1], (b) [Cu(II)L2], (c) [Cu(II)L3],
(d) [Cu(II)L4], and (e) [Cu(II)L5]. The inset is the time-dependent growth of the o-quinine chromophore in the
presence of [Cu(II)L5].
Itraconazole was chosen as the reference compound for C. albicans. The control
consists of plates of solidifying agar onto which pure solvent was placed.
4.2.3. Agar preparation. Two types of agar were used, Mueller Hinton agar (MHA),
(HIMEDIA-M173-500G) to make up the medium for bacteria and Sabouraud dextrose
agar (SDA), (HIMEDIA-M063-500G) to make up the medium for fungi.
4.2.4. MHA (bacteria). MHA was purchased from HiMedia Laboratories Pvt. Ltd.,
Mumbai. MHA of 60.8 g was suspended in 1600 mL of distilled water in a 2 L flask,
stirred, boiled to dissolve, and then autoclaved at 15 lbs and at 121ꢁC for 15 min.
The pH range was maintained between 7.0 and 7.5. Incubation was maintained at
37ꢁC for 24 h.
4.2.5. SDA (fungi). SDA was purchased from HiMedia Laboratories Pvt. Ltd.,
Mumbai. SDA of 9.75 g was suspended in 150 mL of distilled water in a 250 mL flask,
stirred, boiled to dissolve, and then autoclaved at 15 lbs and at 121ꢁC for 15 min. The
pH range was between 6.0 and 6.5. Incubation was maintained at 37ꢁC for 24 h [52].
4.2.6. Results. The data for antimicrobial activity of the copper(II) complexes against
six microorganisms are shown in table 3. [Cu(II)L5] showed maximum zone of
inhibition against E. coli, [Cu(II)L2] showed maximum zone of inhibition against M.
luteus, [Cu(II)L4] showed maximum zone of inhibition against B. subtilis, [Cu(II)L3]
showed maximum zone of inhibition against K. pneumoniae, [Cu(II)L4] showed
maximum zone of inhibition against S. aureus, and [Cu(II)L3] showed maximum zone
of inhibition against C. albicans. All compounds inhibited the growth of

Acyclic Schiff-base copper(II)
647

648
A. Vijayaraj et al.
microorganisms with values ranging between 250 and 750 mg mLꢀ1 [53]. This difference
in inhibition may be due to the differences between the cell structures of bacteria and
does not depend upon the number of alkyl group present in the complex. While the cell
walls of fungi contain chitin, the cell walls of bacteria contain murein [54]. In addition,
fungi contain ergosterol in their cell membranes instead of the cholesterol found in the
cell membranes of animals [55, 56]. The difference in antibacterial activity of copper
complexes probably is associated with ligand type and its space distribution around the
complex core [57].
5. Conclusion
Five Schiff-base copper(II) complexes have been synthesized and their coordination
chemistry and antibacterial activity have been investigated. The electronic spectra of
[Cu(II)L] indicate square-planar geometry, and there is a red shift due to the increase in
chain length. Cyclic voltammograms exhibit one-electron quasi-reversible process with
reduction potential shifting anodically on increasing chain length. All [Cu(II)L]
complexes show good catecholase activity. Increase in chain length causes a greater
distortion of the geometry of the complexes which favors higher rate of reaction. The
complexes showed remarkable activity against all microorganisms tested. [Cu(II)L4] has
the highest activity against B. subtilis and S. aureus. [Cu(II)L5] has the highest activity
against C. albicans. All these studies of the complexes agree well with established trends.
Supplementary material
The ESR spectrum of complex CuL1 and the cyclic voltammograms of complexes
CuL1, CuL2, CuL3, CuL4, and CuL5 are given as supplementary material.
Acknowledgments
Financial support from CSIR, New Delhi is gratefully acknowledged.
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