Synthesis and characterization of chromium(III) Schiff base complexes: Antimicrobial activity and its electrocatalytic sensing ability of catechol

Article abstract of DOI:10.1016/j.saa.2014.12.012

A series of acyclic Schiff base chromium(III) complexes were synthesized with the aid of microwave irradiation method. The complexes were characterized on the basis of elemental analysis, spectral analysis such as UV-Visible, Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopies and electrospray ionization (ESI) mass spectrometry. Electrochemical analysis of the complexes indicates the presence of chromium ion in +3 oxidation state. Cr (III) ion is stabilized by the tetradentate Schiff base ligand through its nitrogen and phenolic oxygen. From the spectral studies it is understood that the synthesized chromium(III) complexes exhibits octahedral geometry. Antimicrobial activity of chromium complexes was investigated towards the Gram positive and Gram negative bacteria. In the present work, an attempt was made to fabricate a new kind of modified electrode based on chromium Schiff base complexes for the detection of catechol at nanomolar level.

Full text of DOI:10.1016/j.saa.2014.12.012

Accepted Manuscript
Synthesis and Characterization of Chromium (III) Schiff base complexes: An-
timicrobial activity and its Electrocatalytic sensing ability of catechol
S. Praveen Kumar, R. Suresh, K. Giribabu, R. Manigandan, S. Munusamy, S.
Muthamizh, V. Narayanan
PII:
S1386-1425(14)01783-1
http://dx.doi.org/10.1016/j.saa.2014.12.012
SAA 13053
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 August 2014
16 October 2014
9 December 2014
Please cite this article as: S. Praveen Kumar, R. Suresh, K. Giribabu, R. Manigandan, S. Munusamy, S. Muthamizh,
V. Narayanan, Synthesis and Characterization of Chromium (III) Schiff base complexes: Antimicrobial activity and
its Electrocatalytic sensing ability of catechol, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.12.012
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Synthesis and Characterization of Chromium (III) Schiff base complexes:
Antimicrobial activity and its Electrocatalytic sensing ability of catechol
S. Praveen Kumar, R. Suresh, K. Giribabu, R. Manigandan,
S. Munusamy, S. Muthamizh and V. Narayanan*
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras,
Guindy campus, Chennai- 600 025.*Corresponding author E-mail: vnnara@yahoo.co.in
Abstract:
A series of acyclic Schiff base chromium (III) complexes were synthesized with the aid of
microwave irradiation method. The complexes were characterized on the basis of elemental
analysis, spectral analysis such as UV-Visible, Fourier transform infrared (FT-IR), nuclear
magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopies and
electrospray ionization (ESI) mass spectrometry. Electrochemical analysis of the complexes
indicates the presence of chromium ion in +3 oxidation state. Cr (III) ion is stabilized by the
tetradentate Schiff base ligand through its nitrogen and phenolic oxygen. From the spectral studies
it is understood that the synthesized chromium (III) complexes exhibits octahedral geometry.
Antimicrobial activity of chromium complexes was investigated towards the Gram positive and
Gram negative bacteria. In the present work, an attempt was made to fabricate a new kind of
modified electrode based on chromium Schiff base complexes for the detection of catechol at
nanomolar level.
Key words: Microwave synthesis, chromium (III) Schiff base complex, antimicrobial activity,
electrocatalytic sensor, catechol.
1. Introduction
The great interest was shown to synthesis Schiff base and their transition metal complexes
due to its numerous applications such as metal chelating agents, heterogeneous and homogenous
1

catalysts, antimicrobial agents, in trace metal analysis, pesticides, antitumor agents,
spectrophotometric and fluorimetric agents and in solar cells [1-14]. Chromium Schiff base
complexes has received much attention because of the +3 oxidation state of the chromium metal
ion, since the +3 oxidation state of chromium ion is more stable with d3 configuration and less
toxic than +6 chromium ion [15, 16]. Chromium metal has a vital role for maintaining the glucose
tolerance factor (GTF) in the normal carbohydrate and lipid metabolism. Insufficient amount of
chromium(III) ion consumption may lead to type II diabetes and cardiovascular diseases [17].
In chemical synthesis, the use of microwave irradiation becoming popular when compared to
conventional synthesis, owing to its advantages such as shorter reaction time, high yield, low
energy consumption and ecofriendly nature. The reactants are slowly activated by external heat
energy in conventional heating method, due to this the heat energy is first passing through the
walls of the vessel and then it reaches the reagents and reactants. In this way the energy that
reaches the reacting substance is inefficient and slow. But in microwave irradiation, the
microwaves are directly coupled with the reacting molecules and leads to rapid rise in temperature.
As a result, in microwave irradiation reaction occurs at faster rate, because of two types of motion
(dipole rotation and ionic conduction). The time duration of a chemical reaction is reduced from
hours to minutes, i.e., rate of the reaction is very fast. In microwave method, the amount of solvent
required for a chemical reaction is very less when compared with the conventional method.
Considering all these facts in our work, we have carried out our reactions with the aid of
microwave irradiation [18, 19].
In this present work, we have prepared five salen derivatives of chromium complexes and
investigated the antibacterial activities and electrocatalytic sensing ability towards catechol.
Electrochemistry of the complexes have direct relation with the structure and oxidation states of
2

the metal ions. Catechol is mainly found in wastewater discharged from industries, which may
produce serious environmental pollution with mutagenesis alteration, decreased liver function and
renal tube degeneration in humans. It is also present in human urine and one of the byproduct of
the cell metabolism. Hence it becomes important to determine the levels of catechol present in
environment and as well as in human body. In literature, so many analytical methods are available
for sensing of catechol, to name a few, fluorimetric, spectrophotometric, chromatography and
electrochemical methods [20], the electrochemical method is preferred owing to its advantages like
cost effective, sensitivity and selectivity. In the present study for the first time, we have utilized the
synthesized salen chromium complexes as a sensing platform for catechol determination.
Experimental
2.1 Reagents
All chemicals and reagents used were of AR grade and were used without purification. The
CrCl3.6H2O was purchased from Alfa Aesar. 5-methylsalicylaldehyde was synthesized by
following the literature method [21]. All the diamines, [N,N-bis-(3-aminopropyl)ethylenediamine,
diethylenetriamine, N,N-bis-(3-aminopropyl)piperazine, tris-(2-aminoethyl)amine, N,N-bis-(3-
aminopropyl)-1,3-propanediamine] were purchased from Sigma Aldrich. TBAP was purchased
from Fluka and recrystallized in hot methanol. All other solvents and reagents were obtained
commercially and were used without further purification.
2.1.1 General synthesis of 5-methylsalicylaldehyde
A mixture of glycerol (150 g, 1.63 mol) and boric acid (35 g, 0.57 mol) was heated for 30
o
min at 170 C to expel water. Then a mixture of 4-methylphenol (25 g, 0.23 mol) and
hexamethylenetetramine (25 g, 0.18 mol) was added. The mixture was stirred for 15 min. The
o
thick brown liquid obtained was allowed to cool to 110 C. A solution of concentrated sulphuric
3

acid (30 mL) in water (70 mL) was added and the whole mixture was boiled in a current of steam.
The product was collected by steam distillation. The product was recrystallized from 80% ethanol.
Yield: 8.2 g (26%), M.P.: 55 oC [21].
2.2 Instrumentation
Elemental analyses were carried out on a Perkin–Elmer CHN- 2400 elemental analyzer.
FT-IR measurements (KBr Pellets) were carried out on a Bruker-Tensor27 FT-IR. Electronic
absorption spectra were measured on a Perkin-Elmer Lambda 600 UV–Visible spectrophotometer.
NMR measurements were performed on Bruker 300 MHz NMR, CDCl3 as a solvent and TMS as
an internal reference. Electrospray ionization mass spectra of the complexes were performed on a
Quan-Tof (Q-Tof) mass spectrometer. Electron paramagnetic resonance (EPR) studies were
carried out in the Bruker X-Band CW EPR spectrometer (9.4GHz). The electrochemical analysis
were carried out using CHI 1103A electrochemical workstation, with three electrode system, using
glassy carbon electrode as working electrode, platinum wire as counter electrode, Ag/AgCl as a
reference electrode, tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. Cyclic
voltammetry was carried out in acetonitrile as solvent. All voltammetric measurements were
carried out after nitrogen purging for 20 minutes.
2.3 Preparation of Chromium Schiff base complex modified GCE
Prior to electropolymerization, the GCE was subjected to the pre–treatment and
sequentially polished with alumina slurries of different sizes to attain mirror finish. Then the
polished GCE was ultrasonicated in double distilled water for 15 min. The electrochemical pre-
treatment of the electrode was carried out, first by applying a fixed potential of +0.5 V versus SCE
for 300 s, followed by potential cycling between -1.5 and +1.5 V at a scan rate of 50 mVs-1 for 20
cycles in 0.1 M sulphuric acid and acetonitrile solution. After that, the chromium Schiff base
4

complex film was successfully generated on the surface of the clean GCE via the following
electrochemical polymerization procedure. 0.1 M of chromium complex was prepared in
acetonitrile and tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. After the
successful electropolymerization on the GCE, the film was washed with double distilled water and
the electrocatalytic response of the modified electrode was evaluated using 0.1mM of catechol in
0.1 M Phosphate buffer solution (PBS) at pH 7 as the background electrolyte.
2.4. Preparation of the Schiff bases
Schiff bases were prepared by microwave assisted method [16]. All the diamines [N,N-bis-
(3-aminopropyl)ethylenediamine, diethylenetriamine, N,N-bis-(3-aminopropyl)piperazine, tris-(2-
aminoethyl)amine, N,N-bis-(3-aminopropyl)-1,3-propanediamine] were condensed with 5-
methylsalicylaldehyde by the following procedure (Scheme-1). 1mM methanolic solution of
diamine was added drop wise to a stirred solution of 2 mM 5-methylsalicylaldehyde in 5 ml
methanol. This mixture was then irradiated in the microwave oven at 320 W for 10-12 minutes.
After the irradiation, the mixture was cooled to room temperature, then it was kept for 12 h at 10
0C. The obtained coloured solid products were filtered and washed with cold ethanol and dried
overnight in a desiccator. All the Schiff bases were recrystallized using 1:1 methanol and
dichloromethane as solvent.
H2L1 = N,N'-bis(5-methylsalicylaldimine)-N,N’-bis-(2-aminoetyl)ethane-1,2-diamine;
H2L2 = N,N'-bis(5-methylsalicylaldimine)-N-(2-aminoethyl)ethylenediamine;
H2L3=N,N'-bis(5-methylsalicylaldimine)-N,N’-bis-(3-aminopropyl)piperazine;
H2L4 = N,N'-bis(5-methylsalicylaldimine)-tris(2-aminoethyl)amine;
H2L5 = N,N'-bis(5-methylsalicylaldimine)-N,N’-bis-(3-aminopropyl)ethylenediamine.
5

Scheme-1: Synthesis of Schiff bases.
2.3.1. Synthesis of Schiff base ligand [H2L1]
Greenish yellow powder, Yield: 0.689 g, 79.9 %. m.p.: 140 °C. Molecular formula
C24H34N4O2; Elemental Analysis Calc. (%): C =70.21; H =8.35; N =13.65; Exp(%): C=70.15; H =
8.28; N = 13.54. Calc av.MS (m/z) 410.27, [MH+] 411.27. Selected IR data (KBr) (ν, cm-1): 3750
ν(OH), 2925 ν(OH••••N=C), 2812 ν(C-H), 1624 ν(C=N), 1583 ν(C=C), 1357 ν(C-N), 1275 ν(C-
1
O). H NMR (ppm in CDCl3 300 MHz ): δ 1.86 (s, 3H), 2.27 (s, 4H), 2.72 (s, 4H), 3.64 (s, 3H),
6.84–6.86 (d, 2H), 7.01-7.26 (m,3H), 8.29 (s, 1H), 8.12(s, 2H), 13C NMR (CDCl3, 300 MHz):
20.35, 28.00, 53.25, 55.92, 57.61, 76.74, 77.05, 116.75, 118.46, 127.51, 131.18, 132.19, 158.98,
164.97. UV–Vis spectrum in methanol: 219 nm, 256 nm, 325 nm.
2.3.2. Synthesis of Schiff base ligand [H2L2]
Yellow powder, Yield (0.763 g, 84%). m.p.: 156 °C. Molecular formula C20H25N3O2;
Elemental Analysis Calc (%): C = 70.77; H = 7.42; N = 12.38. Exp(%) : C = 70.57; H = 7.25; N =
6

12.57. Calc av.MS (m/z) 339.43, [MH+] 340.43. Selected IR data, (KBr) (ν, cm-1): 3243 ν(OH),
1
2946 v(OH••••N=C), 2823 ν(C-H), 1633 ν(C=N), 1593 ν(C=C), 1490 ν(C-N), 1275 ν(C-O). H
NMR (ppm in CDCl3 300 MHz ): δ 2.27(s,3H ), 3.68-3.71 (m, 5H), 3.92 (s,1H), 6.82 (s, 1H),
6.85 (s, 1H), 7.01 (s,1H), 8.31 (s, 1H), 13.05 (s,1H). 13C NMR (CDCl3, 300 MHz): δ 49.75, 59.71,
59.87, 76.71, 77.03, 77.34, 116.68, 118.37, 127.63, 131.38, 131.53, 133.08, 133.18, 158.83,
166.47. UV–Vis spectrum in methanol: 220 nm, 257 nm, 330 nm.
2.3.3. Synthesis of Schiff base ligand [H2L3]
Yellow powder, Yield (0.7823, 89.9%). m.p.: 145 °C. Molecular formula C26H36N4O2;
Elemental Analysis Calc (%): C = 71.53; H = 8.31; N = 12.8. Exp(%) :C = 71.85; H = 8.19; N =
12.80. Calc av. MS (m/z) 436.59, [MH+] 437.55. Selected IR data (KBr) (ν, cm-1): 3325 ν(OH),
1
2916 v(OH••••N=C), 2802 ν(C-H), 1625 ν(C=N), 1583 ν(C=C), 1490 ν(C-N), 1275 ν(C-O). H
NMR (ppm in CDCl3 300 MHz): δ 1.84–1.91 (m, 5H), 2.40–2.47(m,6H), 3.60–3.63 (s, 2H), 6.84–
6.86 (s, 1H), 7.02–7.09 (s, 2H), 8.29 (s,1H), 13.26 (s, 1H). 13C NMR (CDCl3, 300 MHz): δ 20.35,
31.24, 47.46, 57.50, 116.73, 127.54, 131.21, 132.92, 158.93, 164.88. UV–Vis spectrum in
methanol: 224 nm, 257 nm, 327 nm.
2.3.4. Synthesis of Schiff base ligand [H2L4]
Yellow powder, Yield (0.792 g, 86.6%). m.p.: 148 °C. Molecular formula C22H30N4O2;
Elemental Analysis Calc (%): C = 69.08; H = 7.91; N = 14.65. Exp. (%): C = 69.16; H = 7.82; N =
14.76. Calc av. MS (m/z) 382.50, [MH+] 383.57. Selected IR data (KBr) (ν, cm-1): 3427 ν(OH),
1
2925 v(OH••••N=C), 2822 ν(C-H), 1645 ν(C=N), 1583 ν(C=C), 1490 ν(C-N), 1286 ν(C-O) . H
NMR (ppm in CDCl3 300 MHz): δ 1.55 (s, 3H), 2.27 (s, 3H), 6.82-6.85 (d, 1H), 7.00-7.20 (m, 2H),
8.31 (s, 1H), 12.96 (s, 1H). 13C NMR (CDCl3, 300 MHz): δ 20.94, 31.26, 33.80, 47.45, 49.39,
7

49.94, 56.61, 57.51, 76.73, 77.37, 116.57, 166.73, 118.34, 127.53, 131.43, 132.93, 158.94, 164.96.
UV–Vis spectrum in methanol: 215 nm, 257 nm, 331 nm.
2.3.5. Synthesis of Schiff base ligand [H2L5]
Yellow powder, Yield (0.732 g, 84.6%). m.p.: 150 °C. Molecular formula C25H36N4O2;
Elemental Analysis Calc (%): C = 70.72; H = 8.55; N = 13.20. Exp. (%): C = 70.52; H = 8.36; N =
13.65. Calc av. MS (m/z) 424.58, [MH+] 425.59. Selected IR data (KBr) (ν, cm-1): 3625 ν(OH),
1
2915 v(OH••••N=C), 2812 ν(C-H), 1634 ν(C=N), 1583 ν(C=C), 1480 ν(C-N), 1275 ν(C-O) . H
NMR (ppm in CDCl3 300 MHz): δ 1.84-1.87 (m, 3H), 2.27 (s, 4H), 2.71 (s, 4H), 3.63 (s, 2H),
6.84-6.86 (m, 1H), 7.01-7.10 (s, 2H), 8.29 (s, 1H). 13C NMR (CDCl3, 300 MHz): δ 28.94, 31.26,
33.80, 47.45, 49.39, 56.61, 57.51, 76.73, 77.37, 116.57, 118.45, 131.43, 132.93, 158.94, 164.90.
UV–Vis spectrum in methanol: 213 nm, 256 nm, 326 nm.
2.4. Preparation of the Chromium Schiff base complexes
An absolute methanolic solution of chromium chloride (CrCl3.6H2O) [0.266 g 1 mmol] was
gradually added to the stirred methanolic solution of (1mmol) Schiff base, then the solution was
irradiated in a microwave oven at 320 W for 12-15 minutes. The crude product was dissolved in
methanol and the solution was filtered. The filtrate was allowed to stand at room temperature for
12 h, which upon filtration yield a solid product. The resulting solid compound was recrystallized
with methanol and dichloromethane in 1:1 ratio.
2.4.1. Preparation of [Cr(L1)(H2O)2]+
o
Dark green powder, Yield: 0.628 g, 85.67%. m.p.: 225-230 C decomposition
(uncorrected). Molecular formula for the complex [C24H36N4O4Cr]Cl. Elemental Analysis Calc.
(%): C = 54.18; H = 6.82; N = 10.53; Cr =9.77; Cl = 6.66. Exp. (%): C = 54.13; H = 6.75; N =
10.56; Cr = 9.81; Cl = 6.56. Selected IR data (KBr, cm-1): 3416 v(H2O•••Cr), 2946 ν(C-H), 1614
8

ν(C=N), 1479 ν(C=C), 1306 ν(C-N), 1151 ν(C-O), 527 ν(O•••Cr), 470 ν(N•••Cr). Mass spectrum:
[Calc. m/z]: 496.21, [MH+] m/z: 497.22; ESI/MS, in methanol [Exp. m/z]:514, [MH+] m/z: 515.
UV–Vis spectrum in methanol: 289 nm, 325 nm, 400 nm, 570 nm. µeff (298 K): 3.76 BM.
2.4.2. Preparation of [Cr(L2)(H2O)2]+
Pale green powder, Yield: 0.594 g, 84.6%. m.p.: 237-240 oC decomposition (uncorrected).
Molecular formula for the complex [C20H27N3O4Cr]Cl. Elemental Analysis Calc. (%): C = 52.12;
H = 5.90; N = 9.12; Cr =11.28; Cl = 7.69. Exp. (%): C = 52.20; H = 6.01; N = 9.05; Cr = 11.16; Cl
= 7.72. Selected IR data (KBr, cm-1): 3411 v(H2O•••Cr), 3010 ν(C-H), 1622 ν(C=N), 1475 ν(C=C),
1300 ν(C-N), 1157 ν(C-O), 533 ν(O•••Cr), 478 ν(N•••Cr). Mass spectrum; [Calc. m/z]: 425.24,
[MH+] m/z: 426.14. ESI/MS, in methanol [Exp. m/z]: 443, [MH+] m/z: 444. UV–Vis spectrum in
methanol: 276 nm, 320 nm, 403 nm, 560 nm. µeff (298 K): 3.81 BM.
2.4.3. Preparation of [Cr(L3)(H2O)2]+
o
Brown powder. Yield: 0.625 g, 84.1%. m.p.: 218-220 C decomposition (uncorrected).
Molecular formula for the complex [C26H38N4O4Cr]Cl. Elemental Analysis Calc. (%): C = 55.96;
H = 6.86; N = 10.04; Cr =9.32; Cl = 6.35. Exp. (%): C = 56.02; H = 6.84; N = 9.98; Cr = 9.28; Cl
= 6.29. Selected IR data (KBr, cm-1): 3411 v(H2O•••Cr), 2940 ν(C-H), 1618 ν(C=N), 1464 ν(C=C),
1320 ν(C-N), 1157 ν(C-O), 522 ν(O•••Cr), 446 ν(N•••Cr). Mass spectrum; [Calc. m/z]: 522.23,
[MH+] m/z: 523.23. ESI/MS, in methanol [Exp. m/z]: 550, [MH+] m/z: 551. UV–Vis spectrum in
methanol: 284 nm, 323 nm, 409 nm, 550 nm. µeff (298 K): 3.69 BM.
2.4.4. Preparation of [Cr(L4)(H2O)2]+
o
Green powder, Yield: 0.643 g, 89.1%. m.p.: 224-228 C decomposition (uncorrected).
Molecular formula for the complex [C22H32N4O4Cr]Cl. Elemental Analysis Calc. (%): C = 52.43;
H = 6.40; N = 11.12; Cr = 10.32; Cl = 7.03. Exp. (%): C = 52.40; H = 6.36; N = 11.05; Cr =
9

10.38; Cl = 7.00. Selected IR data (KBr, cm-1): 3402 v(H2O•••Cr), 2919 ν(C-H), 1625 ν(C=N),
1496 ν(C=C), 1280 ν(C-N), 1167 ν(C-O), 572 ν(O•••Cr), 455 ν(N•••Cr). Mass spectrum; [Calc.
m/z]: 468.18, [MH+] m/z: 469.19. ESI/MS, in methanol [Exp. m/z]:486, [MH+] m/z: 487. UV–Vis
spectrum in methanol: 253 nm, 324 nm, 410 nm, 565 nm. µeff (298 K): 3.78 BM.
Scheme-2: Synthesis of chromium Schiff base complexes.
2.4.5. Preparation of [Cr(L5)(H2O)2]+
o
Green powder, Yield: 0.496 g, 80.6%. m.p.: 230-232 C decomposition (uncorrected).
Molecular formula for the complex [C25H38N4O4Cr]Cl. Elemental Analysis Calc .(%): C = 54.99;
H = 7.01; N = 10.97; Cr =9.52; Cl = 6.49. Exp. (%): C = 55.03; H = 6.98; N = 10.24; Cr = 9.55; Cl
= 6.50. Selected IR data (KBr, cm-1): 3422 v(H2O•••Cr), 2940 ν(C-H), 1618 ν(C=N), 1474 ν(C=C),
1310 ν(C-N), 1157 ν(C-O), 532 ν(O•••Cr), 478 ν(N•••Cr). Mass spectrum; [Calc. m/z]: 496.21,
[MH+] m/z: 497.22. ESI/MS, in methanol [Exp. m/z]: 514, [MH+] m/z: 515. UV–Vis spectrum in
methanol: 275 nm, 319 nm, 405 nm, 550 nm. µeff (298 K): 3.72 BM.
10

3. Results and discussion
Chromium (III) complexes are more stable in both solution and solid states. The yields of
the synthesized complexes varied from 80 % to 89 %. The reason for yield variation may be the
solubility differences between the ligand and final crude product. The resulting complexes were
soluble in methanol, ethanol, DMF and DMSO. The final chromium complexes were brown and
green in colour. Chromium (III) complexes have three unpaired electrons and d3 configuration so
that it shows the magnetic moment in the range of 3.69-3.81 BM. The experimental values were in
good agreement with the calculated values of the complexes.
The elemental analysis was carried out for the chromium Schiff base complexes, [C,H,N]
values are given in supplementary data as Table-S1, with the molecular formula, colour, melting
point and yield of the complexes. The observed results have good agreement with theoretically
calculated values and the given molecular formula.
3.1 IR-spectra
The significant FT-IR spectra of the free ligands and their metal complexes show various
bands in the region of 400-4000 cm-1, and their tentative assignments were used for the
establishment of the mode of coordination. The typical FT-IR spectra of Schiff base and their
metal complexes are shown in the Fig. 1(A) and Fig. 1(B) respectively. Schiff bases are capable to
form coordinate bonds with many metal ions through both imine nitrogen group and phenolic
oxygen group. The O–H stretching frequency of the ligand is expected from the 3500–3800 cm-1
region, but this frequency is displaced to the 2900–2950 cm-1 region due to the internal hydrogen
bond between OH----N = C. These ligands are planar in structure and hence they have sufficient
intramolecular distance for forming hydrogen bond [22]. In chromium(III) complexes, the band at
2900-2950 cm-1 broadens due to the formation of metal oxygen bond in the complex when
11

compared to that of Schiff base ligands. In the metal complexes a broad band at 3400-3425 cm-1
establish the bond formation between the chromium metal with water molecule.
The C=N stretching vibration shows a peak in the region of 1592–1640 cm-1 as reported
for same kind of ligands [23]. The free Schiff ligands exhibit the characteristic band of C=N in the
region of 1620-1650 cm-1, while in the complexes the C=N stretching vibration were observed in
the region of 1600-1630 cm-1. The shift of C=N stretching vibration in lower region, shows that
decreasing the C=N bond order and the coordination of imine nitrogen lone pair with the metal ion.
The stretching frequency of phenolic C–O exhibits a weak band within the range of 1100 –1240
cm-1 as reported for acyclic Schiff ligands [24].This C – O band for the Schiff bases present in the
region of 1080 – 1200 cm-1 and for the complexes this band exhibits in the region of 1162 – 1250
cm-1 . This higher shift in the stretching frequency of C – O generally shows the formation of a
bond between phenolic oxygen with the chromium metal ion. The bands in the region of 400–600
cm-1 gives evidence for the bond between chromium metal ion with the nitrogen and phenolic
oxygen atom of Schiff bases. The band at 520–580 cm-1 is established to Cr–O bond formation in
complex at the mean while a band at 440-480 cm-1 is established to Cr–N bond formation in the
complex. These bands are newly exhibited in the complexes, which are not present in the spectrum
of the free ligand [24].
12

Fig-1 (A): FT-IR spectra of Schiff base ligands (a) [H2L1]; (b) [H2L2]; (c) [H2L3];
(d) [H2L4]; (e) [H2L5].
Fig-1(B): FT-IR spectra of chromium Schiff base complexes (a) [CrL1(H2O)2]+;
(b) [CrL2(H2O)2] +; (c) [CrL3(H2O)2] +; (d) [CrL4(H2O)2] +; (e) [CrL5(H2O)2] +.
13

3.2 Electronic spectra
The electronic spectra of the Schiff base and its complexes are shown in Fig-2. In these
spectra the benzene ring shows characteristics observation band at 210-220 nm due to π→π*
transition in C=C in free ligands, but the auxochromes and chromophores in substituted benzene
ring shows red shift, this band appears at 280-285 nm in complex. A band exhibit in the region of
255-257 nm shows the n→π* transition of the C=N chromophores, but this band appeared in
complexes in the region of 320-325 nm. The red shift explains that the non-bonded electrons
having a weak electrostatic interaction with the central metal ion. In the free ligands at 325-330 nm
an absorption was appeared, this band exhibit due to the π→π* transition of the C=N
chromophores. In the electronic spectra a low intensity broad band obtained in the region of 400 -
410 and 550 - 570 nm, it is attributed to the d–d transitions of the Cr(III) ion. In the chromium
complexes two transitions bands are observed, they are 4T2g ← 4A2g (ν1), and 4T1g ← 4A2g (ν2), the
4
4
third transition T1g (P) ← A2g (F) which could not be identified, it lies in the ligand field. The
corresponding energy levels for the transitions are 17544-18182 and 24390–25000 cm-1
respectively. The spectral transitions suggest that the geometry of the complex is octahedral
around the central Cr3+ ion [25].
14

Fig-2: (A) UV-Visible spectra of Schiff base, (B) UV-Visible spectra of chromium Schiff base
complexes, (C) Comparison UV-Visible spectra of Schiff base and its chromium complex and
(D) UV- Visible spectrum for d-d transition.
3.3 Nuclear magnetic resonance spectra
1
The investigation of H NMR spectra of the free Schiff base ligands were recorded in
CDCl3. The 1H and 13C NMR spectrum of the Schiff base ligand was shown in Fig. 3. It appeared
in down field from the internal reference of tetramethylsilane. In the spectra a peak appeared in the
range of 12.92-13.27 ppm, which attributed to phenolic protons. Signals for the aromatic protons
are appeared in the range between 6.8-7.80 ppm. The proton which present next to the imine
carbon shows more downfield when compare with the other protons in the aromatic ring. In imine
group, the methine protons, –N = C(H) – which exhibit the peak in between 8.28-8.32 ppm. The
15

peaks were appeared in the range of 2.0-4.8 ppm which is due to the presence of aliphatic protons
in the substituted methyl group in phenyl ring.
The interpretation of the 13C NMR spectra shows the peaks for the various carbon atoms in
the Schiff base as follows; the carbons which are present in diamine –N─(CH2)n ─N─, shows
peaks in the range of 30 ppm to 60 ppm. The imine group carbon ─N=CH─ show peak at 164
ppm. The carbon which bonded to the imine group shows peak at 118 ppm. The carbon bonded
with the hydroxyl group exhibit a peak at 158 ppm. The aromatic carbons in the benzene rings
show peak in the range of 115-132 ppm. The substituted methyl group in the phenyl ring exhibits a
peak at 25 – 28 ppm. These assignments were carried out based on the tetradentate Schiff base
ligands [26- 28].
.
Fig. 3: (A) and (B) are 1H and 13C NMR Spectra of H2L2
3.4 ESI-MASS spectra
The ESI-mass spectra of chromium Schiff base complexes [Cr(L1-5)(H2O)2] were recorded
in the methanol solution at room temperature. The ESI-mass spectra of chromium complexes were
16

shown in Fig-4 (a) and (b). The mass spectrum exhibits multiple peaks represent the degradation of
chromium complex and the proposed molecular formula of the chromium complexes were
confirmed from m/z value in the mass spectrum. The molecular ion peaks of the complexes are as
follows [CrL1(H2O)2] m/z = 514, [CrL2(H2O)2] m/z =443, [CrL3(H2O)2] m/z =550, [CrL4(H2O)2]
m/z= 486 and [CrL5(H2O)2] m/z=514. The mass spectra of chromium complex support the
nitrogen rule, and also confirm the stoichiometry of the metal ion and the free ligand. The
molecular ion peaks show higher molecular weight than the theoretically calculated molecular
weight of the complexes. The spectra shows 18 units of mass higher than the theoretical weight, it
shows that the one water molecule bonded with the phenolic oxygen by hydrogen bond. The
spectral data has good agreement with the proposed molecular formula of the chromium
complexes, and gives the more details about the intermediates and reaction products [29, 30].
Fig-4 (a): ESI-Mass spectrum of [CrL3(OH2)2]+ complex.
17

Fig-4 (b): ESI-Mass spectrum of [CrL3(OH2)2]+ complex.
3.5 EPR Spectral analysis
The EPR spectra of chromium complexes were recorded in methanol solution at room
temperature on X-band 9.4 GHz frequency under the magnetic field strength of 3400 G. The EPR
spectra show a single broad peak. The EPR spectrum was interpreted by using the simple
Hamiltonian Ĥ operator.
Ĥ= β B. g. S + S.D.S
Where, S=1, g and D are tensors, the contribution of hyperfine interaction is neglected [31].
The Lande splitting factor ‘g’ was calculate by the following method
g = 2.0024(1─4λ/10Dq)
Where λ is the orbit spin coupling constant for the metal ion in the complex [32].
The g factor for the chromium complexes were obtained 1.9784 -1.9991, with giso (2.0024) at
ground state. The EPR spectra have no features characteristic signals for a binuclear complex.
18

From the g factor value, the octahedral geometry is thus confirmed for the aforesaid chromium
complex [33, 34]. The magnetic moment of the chromium Schiff base complexes at the room
temperature is 3.69 – 3.81 BM. It is understood from EPR results that the complex possess the
central metal ion in d3 electronic configuration. The magnetic momentum is also support the
mononuclear and the octahedral geometry of the complex. Besides with it confirms the electronic
state of the central metal ion. The typical EPR spectra are shown in Fig-5.
Fig-5: EPR spectra of Chromium Schiff base complexes (A) [CrL1(H2O)2]+;
(B) [CrL3(H2O)2]+; (C) [CrL4(H2O)2]+.
19

3.6 Electrochemical studies of chromium complexes:
The electrochemical behavior of chromium(III) complexes were investigated in acetonitrile
as solvent with TBAP as supporting electrolyte, at scan rate of 50 mVs−1. The cyclic
voltammogram of the chromium complexes show only reduction peak. The acyclic Schiff base
ligands were also examined at the same condition it does not show any characteristic peaks. When
we compare these voltammograms of ligands and complexes, the complexes only gives cathodic
peaks, at the potential of -0.85 to -1.10 V, this clearly reveals that the reduction of Cr(III)/Cr(II)
metal ion has occurred in the complexes. In the complexes CrL1, CrL4 and CrL5 the
voltammograms show an additional reduction peak at the potential range of -0.73 to -0.80 V, it
may be due to the reduction of ligand center. From the cyclic voltammogram we conclude that all
the chromium complexes were electrochemically active and show response in the potential range
of 0.0 to ‒1.50 V vs Ag/AgCl. The reduction potential of the Cr(III)/Cr(II) is dependent on number
of factors such as coordination number of the complex, bulkiness and hard/soft nature of the ligand
[35, 36]. The corresponding cyclic voltammograms are shown in Fig-6.
Fig-6: Cyclic voltammograms of chromium complexes in
acetonitrile at the scan rate of 50 mVs-1.
20

3.7 Antimicrobial activity:
The in-vitro antimicrobial activity of chromium Schiff base complexes were tested against
the bacteria; Staphylococcus aureus (Gram positive), Escherichia coli (Gram negative) and
Pseudomonas aeruginosa (gram negative) in agar medium. For the antimicrobial testing these
complexes were dissolved in DMSO at the concentration of 5 mg/ml, 10 mg/ml, 50 mg/ml, 100
mg/ml and 500 mg/ml. In general procedure 50 mL of the agar was added to a petri dishes and it
was allowed to solidify. The testing organism was spread evenly on the agar medium. These Petri
0
dishes were incubated for 48 h at 30 C. The pH range was maintained between 7.0 and 7.5. The
result was observed, from the observation we conclude that the growth of microorganism were
inhibited by the chromium complex solution the inhibition zones developed on the medium were
measured. The antibacterial activity of the chromium complexes were determined by disc diffusion
method. The inhibition values for chromium complexes are summarized in Table-1-3. From the
study of antimicrobial activity we conclude that the [CrL3(OH2)2]+ has a weak activity against E.
coli and P. aeruginosa. The [CrL5(OH2)2]+ has low inhibition activity for the S. aureus bacteria.
The complex [CrL1(OH2)2]+ has better activity in contrast with [CrL3(OH2)2]+, especially against
E. coli and P. aeruginosa bacteria. This activity varied due to the different ligand center in the
chromium complexes. For the S. aureus [CrL1(OH2)2]+ has better activity in contrast with
[CrL5(OH2)2]+. As the concentration of chromium complex increases it will increase the inhibition
activity of bacterial growth. After these studies, we compare the inhibition activities of the
chromium complexes with standard drug like ampicillin, its shows that the chromium complexes
are not strong active antibacterial agents than the ampicillin. The lower antimicrobial activity of
chromium complexes may be ascribed to the lower electronegativity and small radius of the central
21

metal ion [Cr(III)]. It shows that the higher electronegativity and large atomic radius will increase
the activity, due to decrease in the effective positive charge on the metal complex [37].
Table- 1: Antimicrobial activity of the chromium complexes against E.coli
E.coli
5 mg/ml
Zone of inhibition (mm)
Complexes
[CrL1]+
[CrL2]+
[CrL3]+
[CrL4]+
[CrL5]+
SD
10 mg/ml
27 0.1
23 0.2
-
50 mg/ml
32 0.2
25 0.1
17 0.3
27 0.2
28 0.1
43 0.4
100 mg/ml
36 0.1
30 0.2
21 0.2
32 0.1
30 0.2
49 0.2
500 mg/ml
41 0.4
34 0.3
26 0.2
36 0.2
34 0.2
56 0.5
23 0.4
20 0.2
-
20 0.2
22 0.2
32 0.3
24 0.1
25 0.1
38 0.2
SD- Ampicillin drug.
Table- 2: Antimicrobial activity of the chromium complexes against S. aureus
S. aureus
5 mg/ml
Zone of inhibition (mm)
Complexes
[CrL1]+
[CrL2]+
[CrL3]+
[CrL4]+
[CrL5]+
SD
10 mg/ml
30 0.2
26 0.2
19 0.1
28 0.3
-
50 mg/ml
34 0.2
28 0.1
24 0.2
33 0.1
18 0.1
46 0.2
100 mg/ml
39 0.1
34 0.1
28 0.2
36 0.3
24 0.3
53 0.2
500 mg/ml
45 0.4
39 0.2
30 0.1
38 0.3
29 0.2
61 0.3
26 0.1
24 0.3
15 0.1
23 0.3
-
32 0.2
39 0.3
SD - Ampicillin drug.
22

Table- 3: Antimicrobial activity of the chromium complexes against P. aeruginosa
P. aeruginosa
5 mg/ml
Zone of inhibition (mm)
Complexes
[CrL1]+
[CrL2]+
[CrL3]+
[CrL4]+
[CrL5]+
SD
10 mg/ml
21 0.2
20 0.3
-
50 mg/ml
26 0.3
24 0.2
-
100 mg/ml
30 0.1
27 0.2
-
500 mg/ml
36 0.4
33 0.3
14 0.1
30 0.2
24 0.2
70 0.5
18 0.2
16 0.1
-
-
19 0.2
-
24 0.2
-
27 0.1
18 0.2
61 0.5
-
35 0.3
42 0.3
53 0.5
SD- Ampicillin drug.
3.8 Electrochemical Sensing
3.8.1 Electrochemical behaviors of Catechol at Cr-SBC/GCE
A solution of 0.1mM of chromium Schiff base complexes was taken for the
electropolymerization in the presence of TBAP as supporting electrolyte. Chromium Schiff base
complexes were polymerized on the glassy carbon electrode surface. Polymerized chromium
Schiff base complexes modified GCE (Cr-SBC/GCE) was used for the detection of catechol. Fig. 7
shows the electrochemical behaviors of 0.1 mM catechol (CT) at the bare GCE and chromium
complex modified /GCE in 0.1 M PBS (pH = 7). It can be observed that at bare GCE, there is a
broad redox peak with ∆EP (159 mV for CT). This indicates that the electrochemical reaction of
CT at bare GCE is irreversible. For complex modified/GCE the redox peak of CT is also found to
be irreversible in nature. In addition, the redox peak current has increased by about three times
compared with that observed at bare GCE. It is evident that the chromium Schiff base complexes
(Cr-SBC) could be an effective electrocatalyst for the redox reaction of dihydroxyphenyl
compounds. This electron transfer between the electrode and analyte is facilitated by the complex
and is found to be a good electrocatalyst. The modified GCE has large surface area to interact with
23

the analyte and thereby it enhances the electrocatalytic activity [38]. All these five complexes
[CrL2(H2O)2]+ show better activity for the sensing of catechol. The difference in the catalytic
activity of all the complexes may be due to the ligand size and nature. We have found out that the
less number of subituted ligand has more sensing ability towards catechol. From this work it
clearly indicates that the Cr-SBC/GCE is favorable for determination of catechol.
Fig-7: Cyclic voltammograms of (a) bare GCE, (b) [CrL1(H2O)2]+/GCE,
(c) [CrL2(H2O)2]+ /GCE, (d) [CrL3(H2O)2]+/GCE, (e) [CrL4(H2O)2]+/GCE and
(f) [CrL5(H2O)2]+/GCE in presence of 1 × 10−4 M CT at scan rate of 50 mV s−1
3.8.2 Effect of scan rate
CVs of [CrL2(H2O)2]+/GCE were recorded at different scan rates in 0.1 M PBS (pH 7.0)
containing of 0.1 CT. The CVs of [CrL2(H2O)2]+/GCE shows well-defined peaks, Ipa and Ipc
increased with the increasing the scan rate from 10 to 500 mVs-1 (Fig - 8). The Ipa and Ipc of CT is
directly proportional to the square root of the scan rate. Plots of the anodic or cathodic peak current
for CT against the scan rate showed linear relationship over the range above with the regression
24

coefficient of 0.998 for Ipa & 0.997 for Ipc. It suggests that the redox reaction of CT is diffusion
controlled. Ipa(µA) = 0.7393 ν1/2(mV s−1)1/2 - 0.3658 (R2 = 0.998),
Ipc(µA) = – 0.8738 ν1/2(mV s−1)1/2 + 0.7264 (R2 = 0.997).
Fig-8: Cyclic voltammograms of [CrL2(H2O)2]+ /GCE in 1 × 10−4 M CT (pH 7.0) at the scan rate
of (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, (f) 60, (g) 70, (h) 80, (i) 90, (j) 100, (k) 125, (l) 150,
(m) 175, (n) 200, (o) 225, (p) 250, (q) 275, (r) 300, (s) 325, (t) 350, (u) 375, (v) 400,
(w) 425, (x) 450, (y) 375 and (z) 500 mVs−1.
3.8.3 Effect of pH for the electrochemical detection of CT
pH effect of the electrochemical behavior of 0.1 mM CT in 0.1 M PBS solution at
[CrL2(H2O)2]+ /GCE was investigated by CV in a wide range of pH from 1 to 11, was shown in
Fig. 9. From this we can found that the Epa and Epc values are negatively shifted with increase in
the pH value. It shows that the Epa and Epc values of CT is inversely proportional to pH value. If
we plot Epa and Epc vs pH we get the linear relationship with the regression coefficients R2 0.998
and 0.999 respectively (Epa = 0.8276 + 0.067pH & Epc = 0.3828 – 0.055pH). According to the
25

formula [39], dEp/dpH = 2.303mRT/nF, in which, m and n are the number of proton and electron,
respectively; the calculated proton and electron ratio m/n is 1.03 for CT redox reaction. It shows
that the number of proton and electron involved in the redox process of CT is equal. In addition, to
this the slopes of the regression equations are close to the theory value of 67.1 mV pH−1 for two
electrons and two protons process [40]. The possible electrocatalytic reaction is shown in Scheme-
3.
Scheme – 3: Electrocatalytic redox reaction of catechol (CT)
Fig – 9: CVs of 1 × 10−4 M CT in different pH value from 1 to 11 on [CrL2(H2O)2]+ /GCE.
26

3.8.4 Determination of catechol
Fabricated electrode was also evaluated for sensing of catechol quantitatively by
chronoamperometric technique [41]. The chronoamperometry (CA) has higher sensitivity than CV,
hence it was utilized for the determination of CT. In CA, each addition of 5 µL CT the current
peak was increased that we observed in Fig. 10. A calibration graph of current against
concentration gives a linear relationship in a range of 5 µL to 45 µL, with the R2 value is 0.9986.
I(µA) = 0.2079 + 1.97 CCT. It reveals that the concentration is proportional to the peak current (Ip).
From the linear relation we can calculate the limit of detection by using the following equation
[42].
LOD = 3σ / q
Where, σ is the standard deviation and q is the slope of the calibration curve.
From the calculation we conclude that the Cr-L2/GCE has a detection limit of 519 nM
towards catechol, which clearly indicated us that this method is more sensitive for the catechol
determination.
Fig - 10: Chronoamperometric responses of [CrL2(H2O)2]+/GCE for the successive additions of
5 µL of 1 × 10−4 M CT to 30 mL of 0.1 M PBS. Applied potential is 0.38 V.
27

4. Conclusion:
In this work, we have used a green synthesis method for the synthesis of all symmetrical
tetradentate Schiff bases and their chromium(III) complexes. Microwave (MW) irradiation method
is an efficient and environmentally friendly method to use for the various inorganic syntheses with
higher yields in shorter reaction periods. The complexes were spectrally characterized and
electrochemical redox behavior also tested. The geometry of the synthesized Cr(III) complexes
have been proposed as octahedral based on the spectral studies. The biological activity of the
Cr(III) complexes have been investigated. From the result we conclude that the complex
[CrL1(OH2)2]+ has the better activity in contrast with [CrL3(OH2)2]+1. The synthesized Cr(III)
complexes successfully was used for determination catechol. It gives good response for the
electrochemical detection of phenolic compounds with high sensitivity. The Cr-SBC/GCE is a new
platform for the detection of catechol by using Schiff base complexes. In this method we can
detect at nanomolar level catechol.
Acknowledgment
One of the authors (S.P) wishes to thank Department of Science and Technology (DST),
Government of India for the financial assistance in the form of INSPIRE fellowship (Inspire
Fellow no: 130032) under the AORC scheme. The authors are grateful to the IIT Madras and
CLRI-Chennai for ESI - MASS, EPR spectral studies respectively.
28

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