Synthesis and characterization of novel heterobinuclear mercury(II)-DTPA-M(II) complexes: Electrocatalytic and sensor applications

Article abstract of DOI:10.1080/15533170902784888

Hg(II) metal complex is synthesized using octadentate ligand diethylene triamine penta acetic acid (DTPA) and the interaction of second metal ions viz. Cu(II), Pb(II), Mg(II) and Ca(II) is studied for the formation of novel heterobinuclear complexes. Hete

Full text of DOI:10.1080/15533170902784888

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Synthesis and Characterization of Novel
Heterobinuclear Mercury(II)-DTPA-M(II) Complexes:
Electrocatalytic and Sensor Applications
Govind Krishna Mishra a , Vijay Krishna a & Rajiv Prakash b
a Department of Chemistry , Allahabad University , Allahabad, India
b School of Materials Science and Technology , Institute of Technology, Banaras Hindu
University , Varanasi, India
Published online: 03 Apr 2009.
To cite this article: Govind Krishna Mishra , Vijay Krishna & Rajiv Prakash (2009) Synthesis and Characterization of Novel
Heterobinuclear Mercury(II)-DTPA-M(II) Complexes: Electrocatalytic and Sensor Applications, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 39:3, 124-128
To link to this article: http://dx.doi.org/10.1080/15533170902784888
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 39:124–128, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533170902784888
Synthesis and Characterization of Novel Heterobinuclear
Mercury(II)-DTPA-M(II) Complexes: Electrocatalytic
and Sensor Applications
Govind Krishna Mishra,1 Vijay Krishna,1 and Rajiv Prakash2
1Department of Chemistry, Allahabad University, Allahabad, India
2School of Materials Science and Technology, Institute of Technology, Banaras Hindu University,
Varanasi, India
of multifunctional ligands have been reported to a lesser extent
for the technological applications. Apart from EDTA,[8,9] a few
reports are only available for mixed metal complexes, which are
mainly based on biofunctional ligands such as glutathionate, L-
histidine and dopamine.[10−11] Among various polyamino poly-
carboxylic acid ligands, diethylene triamine penta acetic acid
(DTPA) is the most studied for ligands for various applications
including detoxification and recently contrast enhancing agent
in biomedical magnetic resonance imaging (MRI).[12] DTPA is
an octadentate ligand possessing eight donor atoms (3N and 5O)
used for complexation of variety of metal ions.[13−16] The for-
mation and stability of binary (ML) complexes of some of tran-
sition metal ions with DTPA showed its potential towards cat-
alytic applications.[17] Mercury modified electrode is one of the
most popular in the analytical field due to its catalytic property
for various chemicals and toxicants. However, its low oxidation
potential, poor film stability and toxicity are major limitations
for sensor and electrocatalytic applications. In our recent stud-
ies, we found that single metal ion complexed DTPA possesses
uncoordinated sites that may bind to other metal ions and yield
heterobimetallic mixed metal complexes of higher stability and
electroactivity with very low solubility. An extensive study on
synthesis and characterization of heterobimetallic mixed metal
complexes of DTPA is carried out in the view of its potential
towards sensor and electrocatalytic applications. Electroactive
heterobimetallic mixed metal complexes are used for the devel-
opment of chemically modified electrodes and demonstrated for
the trace analysis of biologically important glutathione protein
in aqueous solution.
Hg(II) metal complex is synthesized using octadentate ligand
diethylene triamine penta acetic acid (DTPA) and the interaction
of second metal ions viz. Cu(II), Pb(II), Mg(II) and Ca(II) is studied
for the formation of novel heterobinuclear complexes. Heterobin-
uclear complexes are formed by taking 1:1 ratio of metal ions
with Hg(II)-DTPA in aqueous solution at pH above 7. Heterobinu-
clear Hg(II)-DTPA-M(II) complexes are characterized using elec-
trochemical technique and stability constant is calculated by pH-
titration technique. Electroactive and stable Hg(II)-DTPA-Pb(II)
complex is found to be suitable for the electrocatalytic application
and demonstrated for the detection of glutathione in the wide range
of concentration from 10 µg/L to 10 mg/L.
Keywords diethylene triamine penta acetic acid, electroactive com-
plex, electrocatalysis, glutathione detection, heterobinu-
clear complex, mixed metal complex
INTRODUCTION
The research on the complexation reactions has always been
of great interest because of the numerous applications of the
complexes, which have certain properties different from those
of its components. Metal complexes find their numerous appli-
cations in the area of analytical, environmental, electrochemical
chemistry and biological sciences.[1−5] However, poor stabil-
ity and solubility limit their extensive and diversified appli-
cations. In these developments, macro-ligands, functionalized
ligands and mixed ligands complexes with variety of metal ions
were synthesized and got intensive interest during the last two
decades.[5−7] However, the studies on mixed metal complexes
EXPERIMENTAL
Materials and Instruments
Received 25 November 2008; accepted 29 January 2009.
Address correspondence to Rajiv Prakash, School of Materials Sci-
ence and Technology, Institute of Technology, Banaras Hindu Univer-
sity, Varanasi, 221005, India. E-mail: rajivprakash12@yahoo.com
Sodium nitrate, nitric acid, sodium hydroxide and other
metal oxides were purchased from Merck, India. DTPA,
EDTA and glutathione (GSH MW 307) were purchased from
124

NOVEL HETEROBINUCLEAR MERCURY(II)-DTPA-M(II) COMPLEXES
125
Aldrich-Sigma, USA. All other reagents were of A.R. grade (A) NaNO3 + HNO3 + water.
and their solutions were prepared in deionized double distilled (B) NaNO3 + HNO3 + DTPA + water.
water.
(C) NaNO3 + HNO3 + DTPA + Hg(II) ion solution + water.
The absorption spectra were recorded on a Lambda 25 (D) NaNO3 + HNO3 + DTPA + Hg(II) ion solution + M(II)
Perkins Elmer UV visible spectrophotometer (obtained from
DAAD, Germany under instrument donation program). Titra-
tion was carried out using Century Cp 901 S pH meter. Ele-
mental analysis was carried out using atomic absorption spec-
trophotometer (Varian 250+) equipped with vapor generation
assembly (Varian VGA 77). All the electrochemical studies were
performed on potentiostat/ Galvanostat, CH Instruments, USA,
using a three-electrode system in a single compartment cell.
Glassy carbon, gold electrode and platinum disk electrodes,
Ag/AgCl reference electrodes and platinum rod counter elec-
trode were obtained from CH Instruments, USA.
ion soln.+ water.
where M(II) = Cu(II), Pb(II), Ca(II) and Mg(II).
Total volume was kept as 50 ml with ionic strength 0.1 M
(NaNO3), free acid concentration 0.02M (HNO3), metal ion
and ligand concentrations 0.01M for each. The molar ratio of
M(II):Hg(II):DTPA was kept as 1:1:1 where M(II) = Cu(II),
Pb(II), Ca(II) and Mg(II). All these solutions were individually
titrated with carbonate free 0.1M NaOH solution.[18]
The shifts from the pH titration curves were used to calculate
the n˜ (average number of ligands attached per metal ion), n˜H
(average number of protons attached per ligand) and pL (the
free ligand exponent) values as following:
METHODS
n˜H = Y – {(No+Eo) (V2–V1)}/(Vo+V1)Lo
n˜ = {(No+Eo) (V3–V2)}/ {(Vo+V1) n˜H Mo}
pL = log {(K1[H+] + K1K2[H+]2+–)/(Lo – n˜ × Mo)}
× {(Vo+V3)/Vo}
Synthesis of Complexes
The metal-DTPA complexes were synthesized by mixing the
appropriate amount (calculated for 10 mM) of the ligand DTPA
and metal oxides viz. mercury oxide (HgO) or other metal ox-
ides (CuO/PbO/CaO/MgO) in 1:1 mole ratio ligand:M(II) in
aqueous medium by controlled heating and continuous stir-
ring for 2–3 hours at 50◦C. The mixed-metal DTPA com-
plexes were synthesized by introducing the second metal ox-
ide (CuO/PbO/CaO/MgO) in the same mole ratio in the HgII -
DTPA complex solution. The reaction was carried out in aque-
ous medium by controlled heating and continuous stirring for
2–3 hours at 50◦C. The dissolution of DTPA with metal oxides
(initially insoluble in water), and appearance of clear solution
(with change in color of initial solution) confirmed the formation
of a new mixed-metal complex. Volume of reaction mixture was
reduced under controlled evaporation by putting it on water bath
at below 50◦C. Fine crystals of the complexes were appeared
at the bottom of the flask, when solution was kept for 1–2 days
at room temperature in the dark. Complex precipitates were re-
crystallized from water and alcohol mixture in 3:1 ratio. Finally
complexes were washed with ethanol and dried in an electric
oven at 50◦C. Air stable, non hygroscopic and crystalline solids
were stored in a bottle for further studies.
where,
Y is number of dissociable or replaceable protons attached to the
ligand
Lo is the total concentration of the ligand
No is the concentration of alkali
Eo is the total concentration of free acid
Vo is the total volume of reaction mixture
V1 is the volume of alkali needed to reach a specified pH for
solution “A” (i.e. acid)
V2 is the volume of alkali required to attain the same pH in the
(acid + ligand) curves
V3 is volume of alkali required to attain the same pH in the
(acid+ ligand + metal) curves.
Mo is the total concentration of metal present in solution
K1, K2, K3 are the protonation constants of the ligands and
H+ is the hydrogen ion concentration
Finally, the stability constants for binary (M(II)DTPA) and
bimetal (Hg(II)DTPA M(II)) complexes were evaluated using
following formula:
logK = [log(Lo × n˜) − log{Mo × (1 − n˜)} + pL]
log(Lo × n˜) represents the amount of complex formed, log {Mo
× (1 − n˜)} represents the amount of free metal ion and pL the
Calculation of Stability Constant
Irving and Rossotti[18] pH-titration technique (modified form concentration of free ligand (HgDTPA)3−
of the Bjerrum-Calvin[19]) was used for the calculation of sta-
The stability constants obtained for such binary and ternary
bility constant of the metal-DTPA and mixed metal DTPA complexes were refined with the aid of SCOGS computer
complexes. Metal nitrate solutions were standardized by EDTA program.[19] The calculated stability constant values as well
titrations.[20] The solution of the diethylene triamine penta acetic as literature values[20,21] were used as preliminary estimates in
acid (ligand) was prepared by dissolving it in two equivalents of SCOGS computer programme. Ionic product of water (Kw)
alkali in double distilled water. pH was measured with an accu- and activity coefficient of hydrogen ion under the experimental
racy 0.01pH at 25◦C. Four reaction mixtures viz. acid, ligand, conditions were obtained from literature.[21] Computer refined
mono metal complex and bimetal complex solutions were made values of the stability constants corresponding to minimum stan-
separately as follows:
dard deviation in the titer were taken.

126
G. K. MISHRA ET AL.
TABLE 1
Elemental Analysis
Stability/formation constant (log β) values for M-DTPA and
Hg-DTPA-M (1:1:1) type complexes in aqueous medium.
Ionic strength I = 0.1M (NaNO3), Temp.= 25 +1◦C.
Elemental analysis of the metal and mixed metal DTPA com-
plexes were carried out using atomic absorption spectropho-
tometer equipped with vapour generation assembly. The sample
preparation was done as per the standard method keeping in
view of volatile nature of mercury complexes.[22]
Complex species
log β
[HgDTPA]3−
[CuDTPA]3−
[PbDTPA]3−
[CaDTPA]3−
[MgDTPA]3−
[HgDTPACu]−
[HgDTPAPb]−
[HgDTPACa]−
[HgDTPAMg]−
27.00
21.10
19.60
09.30
09.10
31.44
29.18
27.86
27.49
Electrochemical Study and Development of Complex
Modified Electrode
Electrochemical studies of metal and mixed metal complexes
were carried out using CH Instruments, USA, in a single com-
partment cell equipped with three-electrodes. The cyclic voltam-
metry and differential pulse voltammetry were carried out either
dissolving the complexes in appropriate electrolytes or directly
coating over the metal or glassy carbon working electrodes.[5]
Metal or mixed metal complexes were electrochemically coated
over the Pt or glassy carbon electrodes by applying −0.6 V vs.
Ag/AgCl from1mg/mL complex solution in 0.2M Tris-HCl pH
7.5.
it has still some vacant coordination sites available for interac-
tion with other metal ions. The complexation equilibria in the
pH range 2.5–5.0 for the formation of mixed-metal complexes
with Pb(II) may be shown as below:
RESULTS AND DISCUSSION
Titration of the pentaprotic acid (H5L) DTPA ligand showed
pKH value 1.86, 2.79, 4.29, 8.02 and 10.49 corresponding to
successive deprotonation of five -COOH groups. The binary
complexes of this octadentate ligand was studied with various
metal ions viz. Hg(II), Cu(II), Pb(II), Ca(II) and Mg(II) metal
ions in aqueous solution. These metal-DTPA complexes were
studied for their stability as discussed under the experimental
section. Among these complexes Hg(II)-DTPA showed high-
est stability and therefore, it can be presumed that in a 1:1,
Hg(II):DTPA reaction mixture, Hg(II) remains fully complexed
and the reaction mixture mainly contains the species [Hg(II)-
DTPA] and [M] at pH lower than 2.5. When pH of the reaction
mixture was raised by adding alkali, the dissociable protons of
the uncoordinated groups of Hg(II)-DTPA were displaced as per
the following equilibria.
[HgDTPA]3− + Pb2+ ↔ [HgDTPAPb]−
Similarly other mixed metal complexes were synthesized
and stability constants were evaluated as discussed under the
experimental section and shown in Table 1.
The air and thermal stable crystalline complexes showed
water solubility, but were insoluble in most of the organic sol-
vents and sparingly soluble in DMSO. The elemental analy-
sis of the complexes also supported the formation of mixed
metal complexes in 1:1 mole ratio. The mixed metal complexes
showed higher stability in comparison to single metal DTPA
complexes. Thermal decomposition temperature of solid com-
plex samples was calculated and it was found to be highest for
Pb(II)-DTPA and Hg(II)-DTPA-Pb(II) among all the complexes.
Hg(II)-DTPA-Pb(II) complex was used for the development of
chemically modified electrodes and the electrocatalytic applica-
tions.
Amongst the various chemically modified electrodes, mer-
cury modified electrode is one of most popular in the analytical
field due to its unique catalytic property for the electro-oxidation
of various chemicals. However, the oxidation at low positive po-
tential and its hazardous nature are the major limitations for its
sensor or electrocatalytic applications. Moreover, the mercury
film coating is not stable over most of electrode materials. In
order to overcome these problems recently, various polymers
impregnated mercury electrodes are developed for quantifica-
[H3HgDTPA] + OH− ↔ [H2HgDTPA]− + H2O
[H2HgDTPA]− + OH− ↔ [HHgDTPA]2− + H2O
[HHgDTPA]2− + OH− ↔ [HgDTPA]3− + H2O
The interaction of other metal ions viz. Cu(II), Pb(II), Ca(II)
and Mg(II) with completely deprotonated ligand with rising pH
was studied and mixed-metal complexes [Hg(II)-DTPA-M(II)]
are formed as:
[HgDTPA]3−+Mn+ ↔ [MHgDTPA]n−3
The Hg(II)-DTPA complex is expected to involve four co- tion of the heavy metals in aqueous system.[23−25]
ordinate tetrahedral arrangement around Hg as this is the most
The electroactivity and the redox properties of the single
preferred coordination for Hg(II) metal ion.[23] In the complex metal-DTPA and mixed metal complexes were studied in aque-
species Hg(II)-DTPA, the Hg is expected to bind two amino ni- ous and non-aqueous solution using a three electrode system.
trogen and two carboxylic oxygens in a tetrahedral fashion and Hg(II)-DTPA-Pb(II) showed best electroactivity and stability

NOVEL HETEROBINUCLEAR MERCURY(II)-DTPA-M(II) COMPLEXES
127
range 10 to 1000 mV/s and the peaks potential is also not varying
much with scan rate, which shows pseudo reversible redox na-
ture and electroactivity of the complex. The complex modified
electrode was used for the electrocatalytic oxidation and esti-
mation of biological important glutathione protein. Glutathione
(GSH) is a small protein molecule formed from amino acid,
cysteine, glyceine and glutamic acid. It is an antioxidant and
acts as immune system and detoxifier in the body. Differen-
tial pulse voltammetry technique was used for the detection
of GSH (molecular wt. 307) using Hg(II)-DTPA-Pb(II) com-
plex modified electrode in the range of 10µg/L to 10mg/L in
0.1M phosphate buffer (6.7 pH). When the differential pulse
voltammetry was performed for Hg(II)-DTPA-Pb(II) modified
electrode in 0.1M phosphate buffer (6.7 pH) in the range of -
0.3V to 0.9V vs. Ag/AgCl, two oxidations peaks were obtained
as one at 0.22V due to oxidation of Hg(I) to Hg(II) and second a
weak peak at 0.5V vs. Ag/AgCl due to ligand DTPA, as shown
in Figure 3 of plot A. In addition to a small amount of GSH
(to get GSH total concentration of 10 µg/L), a considerable de-
cease in the Hg oxidation peak current was observed, as shown
in Figure 3 plot B. Further addition of GSH a new peak was
observed at 0.45V vs. Ag/AgCl. The peak current was found to
be proportional as the concentration of the GSH was increased
in the solution. The GSH was further estimated for another three
more concentrations as 0.1 mg/L, 1 mg/L and 10 mg/L, shown
in Figure 3 plot C, D and E. The GSH could not be detected at
the bare electrode when present below 10 mg/L. The decrease
in the mercury oxidation peak and appearance of a new peak on
addition of GSH probably indicates formation of some complex
of GSH with the Hg(II)-DTPA-Pb(II). The Hg(II)-DTPA-Pb(II)
complex showed a catalytic effect on the oxidation of GSH,
which facilitated oxidation of the GSH. The complexation or
interaction of the GSH with the Hg(II)-DTPA-Pb(II) modified
electrode was also supported as it was observed that the same
FIG. 1. Differential pulse voltammetric (25 mV/s) study of Hg(II)-DTPA-
Pb(II) coated Pt electrode in 0.2M Tris-HCl buffer at pH 7.5.
among all the complexes. Hg(II)-DTPA-Pb(II) complex was
coated over Pt or glassy carbon electrodes by applying -0.6 V
vs. Ag/AgCl from 1mg/mL solution in 0.2M Tris-HCl buffer
(pH 7.5) at room temperature. Hg(II)-DTPA-Pb(II) coated elec-
trode was studied under differential pulse voltammetry in 0.2M
Tris-HCl buffer (pH 7.5) at room temperature. Three oxida-
tions peaks at -0.5V due to oxidation of Pb metal, at 0.2 V
due to oxidation of mercury metal and a small hump at 0.55
V due to oxidation of DTPA were observed, as shown in
Figure 1. The electrochemical stability and redox property of
the Hg(II)-DTPA-Pb(II) complex was also studied for several re-
dox cycles and at various scan rates. A typical voltammogram is
shown in Figure 2 for Hg(II)-DTPA-Pb(II) coated Pt electrode in
0.2M Tris-HCl buffer (pH 7.5) in the range of -0.2V to 0.5 V vs.
Ag/AgCl for scan rates 10, 20, 50, 100, 200 500 and 1000 mV/s.
The peaks current are proportional to the scan rate in the scan
FIG. 3. Differential pulse voltammetric (25 mV/s) studies of (A) HgII-DTPA-
PbII modified Pt electrode in phosphate buffer 6.7pH (100 mM) as blank and
in presence of (B) 10 µg/L GSH, (C) 0.1 mg/L GSH (D) 1 mg/L GSH and (E)
10 mg/L GSH.
FIG. 2. Cyclic voltammetry of HgII-DTPA-PbII coated Pt electrode in 0.2M
Tris-HCl buffer at pH 7.5 at various scan rates 10, 20, 50, 100, 200, 500 and
1000 mV/s (lower to higher peak current).

128
G. K. MISHRA ET AL.
modified electrode after detection of GSH followed by proper
washing gave oxidation peak at 0.45 V (with decrease in the
peak current).
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CONCLUSION
Hg(II)-DTPA and Hg(II)-DTPA-M(II) heterobinuclear com-
plexes are synthesized using Cu(II), Pb(II), Mg(II) and Ca(II)
as second metal ions. Stability constants of the crystalline com-
plexes are calculated and Hg(II)-DTPA-Pb(II) complex is found
to be most stable among other complexes. Electroactive Hg(II)-
DTPA-Pb(II) complex is electrochemically coated over the Pt
or glassy carbon electrodes and studied for the electrocatalytic
applications for biomaterials. The Hg(II)-DTPA-Pb(II) modi-
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