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ISSN 1061933X, Colloid Journal, 2010, Vol. 72, No. 4, pp. 530–537. © Pleiades Publishing, Ltd., 2010.
Studies on Outer Sphere Electron Transfer Reactions of Some
SurfactantCobalt(III) Complexes with Ferrocyanide Anion1
K. Sasikala and S. Arunachalam
School of chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
email: [email protected]
Received July 16, 2009
Abstract—The kinetics and mechanism of reduction of the surfactantcobalt(III) complex ions, cis
[Co(bpy)2(C12H25NH2)2]3+ and cis[Co(phen)2(C12H25NH2)2]3+ (bpy = bipyridyl, phen = 1,10phenan
throline, C12H25NH2 = dodecylamine) by Fe(CN6)4– in selfmicelles were studied at different temperatures.
Experimentally the reaction was found to be second order and the electron transfer postulated as outer
sphere. The rate constant for the electron transfer reaction for both the complexes was found to increase with
increase in the initial concentration of the surfactantcobalt(III) complex. This peculiar behaviour of depen
dence of secondorder rate constant on the initial concentration of one of the reactants has been attributed
to the presence of various concentration of micelles under different initial concentration of the surfactant
cobalt(III) complexes in the reaction medium. The effect of inclusion of the long aliphatic chain of the sur
factant complex ions into βcyclodextrin on these reactions has also been studied.
DOI: 10.1134/S1061933X10040149
1
INTRODUCTION
The outersphere electron transfer between transi
tion metal complexes plays an essential role both in
vivo [1] and in operation of molecular scale devices,
such as molecular wires and logic gates [2–4]. The
alteration of the outersphere environment of metal
complex caused by the variation of concentration of
the counter ions [5] has an influence on electron
transfer reactions. Gaswick et al. have reported that
the hexacyanoferrate(II) anion can reduce some pen
tamminecobalt(III) complexes to cobalt(II) via an
outersphere electron transfer step [6] and also they
have reported that the substituted pentammineco
balt(III) complexes could be reduced by hexacyanof
errate(II) with the formation of an ion pair [7].
Numerous studies have been performed addressing
the dependence of electron transfer on different envi
ronments such as micelles [8, 9], vesicles [10] and
DNAs [11–13]. The redox processes occurring in bio
logical systems are controlled both by specific geome
try of the inner coordination sphere, which mainly
controls the operation potential of the metal center,
and by the hydrophobic effect offered by the pseudo
biological interfaces. Sanchez et al. have studied the
electron transfer reactions by micellar pseudophase
and macromolecules, in general referred to as
restricted geometry conditions, as a way of modifica
tion of redox processes [14–19]. Cyclodextrins (CDs)
are cyclic polysugars composed of glucose units linked
by 14α glycoside bonds [20–22]. The cyclic structure
1 The article is published in the original.
forms a hydrophobic cavity and CDs can include a
variety of guest molecules in their cavity [23–25]. The
effects of CD inclusion on the kinetics and mechanism
of ligand substitution [26, 27] and electron transfer
reactions of transition metal complexes in aqueous
solution [28–31] have received considerable attention
in recent years.
We have been interested in synthesis, micelles
forming properties and electron transfer reactions
of many surfactantmetal complexes for a long
time [32–36]. In all these surfactantmetal com
plexes the coordination complex containing a cen
tral metal ion with surrounding ligands coordi
nated to metal acts as surfactant. Like any other
wellknown surfactant, for example, sodium dode
cyl sulphate, these surfactantmetal complexes
also form micelles at a specified concentration
called critical micelle concentration in aqueous
solution. Recently in one of our previous reports
we have studied the outersphere electron transfer
reactions of cis[Co(bpy)2(C12H25NH2)2]3+ and
cis[Co(phen)2(C12H25NH2)2]3+ (bpy = bipyridyl,
phen = 1,10phenanthroline, C12H25NH2 = dode
cylamine) with Fe(II) ion [37] where both the oxi
dant and reductant are cations. As these complexes
themselves form micelles we have conducted the
reactions in micelles created by the surfactant
cobalt(III) complex molecules themselves. In this
report we present our interesting results on the
outersphere electron transfer reactions between
530
STUDIES ON OUTER SPHERE ELECTRON TRANSFER REACTIONS
the same surfactantcobalt(III) complexes with
Fe(CN)64 −
in the self micelles of these surfactant
cobalt(III) complexes. The novelty of the present
work is that both the oxidant (surfactant
cobalt(III) complex) and the reductant have oppo
site charges. Also the present report includes the
effect of βcyclodextrin, which is a good structure
breaker of micelles, on the same electrontransfer
reactions.
N
EXPERIMENTAL
Materials
All reagents were of analytical grade (SigmaAld
rich and Merck). MilliQ water was used to prepare
the solutions. The surfactantcobalt(III) complexes,
cis[Co(bpy)2(C12H25NH2)2](ClO4)3
and
cis
[Co(phen)2(C12H25NH2)2](ClO4)3 were synthesised as
reported [23]. The structures of the phenanthroline,
bipyridine and the surfactantcobalt(III) complexes in
cyclodextrin cavity are shown in Scheme 1.
N
1,10phenanthroline
2,2'bipyridine
3+
N
531
3+
N
N
Co
CO
N
N
N
H2N N2H
NH2
[Co(phen)2(DA)2]3+
N
Co
CO
N
NH2
[Co(bpy)2(DA)2]3+
Surfactantcobalt(III) complexes in the cyclodextrin cavity
Scheme1.
Kinetic Measurements
The rate of the reaction was measured spectropho
tometrically using a Varian Gary 500 scan UVVis
NIR spectrophotometer equipped with the water Pelt
ier system (PCB 150). The temperature was controlled
within ±0.01°C. A solution containing the desired
concentration of potassium ferrocyanide, sodium
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nitrate and disodium ethylenediamine tetraacetate
(Na2H2EDTA) in oxygen free water was placed in a 1
cm cell which was then covered with a serum cap fitted
with a syringe needle. This cell was placed in a thermo
stated compartment in the spectrophotometer, the
solution containing the surfactantcobalt(III) com
plex was added anaerobically using the syringe, and
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SASIKALA, ARUNACHALAM
then the increase in absorbance of the oxidant was fol
lowed at 423 nm of the oxidant.
All kinetic measurements were performed under
pseudofirst order conditions with the Fe(CN6)4– in
excess over cobalt(III) complex. The concentration of
Fe(CN6)4– used was 0.01 mol dm–3 and the concentra
tion of surfactantcobalt(III) complex was always chosen
much above their CMC values in the 3 × 10–4 mol dm–3 to
7 × 10–4 mol dm–3 region. The ionic strength was
maintained at 1.0 mol dm–3 in all runs using NaNO3.
The secondorder rate constant, k, for the reduction of
the cobalt(III) complex by Fe(CN6)4– given as
d[Co(III)]/dt = k[Co(III)][Fe(CN)64 − ], was calculated
from the concentration of Fe(CN)64 − and the slope of
the log(At – Aα) versus time for the pseudo first order
plot, which is equal to –k[Fe(CN)64 − ]/2.303, where At
is the absorbance at time t, Aα is the absorbance after
all the cobalt(III) complex has been reduced to
cobalt(II), and k is the secondorder rate constant.
Usually the value of Aα a was measured at times corre
sponding to 10 halflives. All the firstorder plots were
substantially linear for at least five halflives. Each rate
constant reported was the average result of triplicate
runs. Rate constants obtained from successive halflife
values within a single run agreed to within ±5%. No
trends indicative of systematic errors were noted, and
the average values did not differ significantly from
those obtained from least square treatment of logarith
mic plots of absorbance difference against reaction
time.
Uniqueness of the SurfactantCobalt Complexes
The uniqueness of the surfactantcobalt(III) coor
dination complex lies in the fact that the bond between
the head group and the tail part of the complex is a
coordinate bond and the surfactant contains a higher
charge on the head group unlike common surfactants
like sodium dodecyl sulfate. At the same time like the
common surfactants, this surfactantcobalt(III) coor
dination compound forms foam in aqueous solution
when mechanically disturbed like shaking, and this
complex dissolves slowly in water, though sometimes
we have to sonicate the solution to get a homogeneous
solution.
RESULTS AND DISCUSSION
Nature of Reaction
On mixing Fe(CN)64 − and surfactantcobalt(III)
complex in aqueous solution a precipitate was formed
and therefore homogeneous kinetic measurements
were precluded. When Na2H2EDTA was present in the
solution to sequester the cobalt(II), no precipitate was
formed during the reaction and therefore all the exper
iments were carried out in the presence of
Absorbance
2
1
400
450
500
550
Wavelength, nm
Fig. 1. A repetitive scan of the spectrum during the reduc
tion of cis[Co(phen)2(C12H25NH2)2]3+ by Fe(CN)64 at
25.0°C. [complex] = 4 × 104 mol dm–3, [Fe(CN)64] =
0.01 mol dm3, cycle time = 60 s.
Na2H2EDTA [6]. Na2H2EDTA acted as a sequestering
agent to remove cobalt(II) and prevent its precipita
tion as a hexacyanoferrate salt. A repetitive scan of the
spectrum during the reaction time at 25°C is shown in
Fig. 1 where an increase in absorbance was observed.
The reaction is represented as
surfactantcobalt(III) complex + Fe(CN)64 −
3−
Co 2+
aq + Fe(CN ) 6 + protonated amines
and the rate is given by
rate = k[surfactantcobalt(III) complex][Fe(CN)64 − ],
where k is the second order rate constant.
Effect of initial concentration of surfactantcobalt(III)
complexes
The reduction of cis[Co(LL)2(C12H25NH2)2]3+
(LL = bipyridine or phenanthroline) by Fe(CN)64 − is
postulated as outersphere in comparison to such
type of reactions in the literature [37] involving or
dinary lower primary amine coordinated cobalt(III)
complexes similar to our surfactantcobalt(III)
complexes. Accordingly, the mechanism is delineat
ed in Scheme 2. The observed secondorder rate
constants k, are given in Table 1 for the above reac
tion, under various initial concentrations of the sur
factantcobalt(III) complexes, at 298, 303, and
308 K in aqueous solution. As seen from this table,
the rate constant of the reaction goes on increasing
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STUDIES ON OUTER SPHERE ELECTRON TRANSFER REACTIONS
533
4–
Table 1. Secondorder rate constants for the reduction of cobalt complex ions by Fe(CN ) 6 in aqueous solution under var
4–
ious temperatures. [Fe(CN ) 6 ] = 0.01 mol dm–3, μ = 1.0 mol dm–3
Oxidizing agent
[Complex]
× 104
cis[Co(bpy)2(C12H25NH2)2]3+
cis[Co(phen)2(C12H25NH2)2]3+
(mol
k × 102 (mol–1 dm3 s–1)
dm–3)
298 K
303 K
308 K
3
2.0
2.5
3.5
4
2.9
3.9
5.0
5
3.8
4.8
6.0
6
6.0
7.2
7.8
7
7.6
8.5
12.5
3
1.8
2.0
2.7
4
2.5
3.1
3.8
5
3.6
4.0
4.5
6
4.5
5.0
5.7
7
6.0
6.5
7.2
Complex: surfactantcobalt(III) complex ion.
tween micellized cobalt(III) complex and
Fe(CN)64 − ). So in our case we have encountered
such behaviour of dependence of rate constant on
the initial concentration of one of the reactants.
The increase in the rate constant of the outer
sphere electron transfer reactions with increase in
concentration of these surfactant complexes
(Fig. 2) can be attributed to the aggregation of these
metal complexes in their own selfmicelles. (With
increase in initial concentration of the surfactant
cobalt(III) complexes the number of micelles
present in the medium also increases. The reactants
are encountered in a small volume of Stern layer of
the micelles leading to enhancement of concentra
tion of reactants at the microlevel and as a result to
higher rate and lower activation energy).
with increase in the initial concentration of the
complex from 3 × 10–4 mol dm–3 to 7 × 10–4 mol dm–3. As
this concentration range is very much higher than
the critical micelle concentration values (cis
[Co(bpy)2(C12H25NH2)2]3+ = 9.4 × 10–5 mol dm–3, cis
[Co(phen)2(C12H25NH2)2]3+ = 8.1 × 10–5 mol dm–3) [23]
of these surfactant complexes, all these rate con
stant values correspond to the rate constant values
in selfmicelles formed from these metal complex
molecules themselves. (We tried to perform the ki
netics of the same reaction at below the cmc values
of our surfactantcobalt(III) complexes also, but
the reaction was so slow that we didn’t observe any
change in the absorbance values. So we conclude
that the rate constants we have calculated in the
present work correspond only to the reaction be
KIP
{[Co(LL)2(DA)2]3+, Fe(CN)64– }
{[Co(LL)2(DA)2]3+, Fe(CN)64– }
ket
{[Co(LL)2(DA)2]2+, Fe(CN)63– }
{[Co(LL)2(DA)2]2+, Fe(CN)63– }
Fast
[Co(LL)2(DA)2]3+ + Fe(CN)4–
6
Products
LL = bipyridyl or phenanthroline, DA: Dodecylamine
Scheme 2.
Effect of βcyclodextrin
βcyclodextrin has the ability to form complex with
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host molecules, the complex forms when a suitable
hydrophobic molecule displaces water from the cavity
534
SASIKALA, ARUNACHALAM
k × 102, mol–1 dm3 s–1
14
k × 10–3, mol dm–3
5.5
(а)
3
12
5.0
4.5
10
2
1
8
4.0
(a)
3.5
6
(b)
3.0
4
2.5
2
2.0
4
3
5
k × 102, mol–1 dm3 s–1 (b)
8
6
7
[Co(III)] × 104, M
3
2
1
6
2
3
4
5
6
7
8
[βCD] × 10–4, mol dm–3
Fig. 3. Plots of k against βCD concentration for (a) cis
and
(b)
cis
[Co(bpy)2(C12H25NH2)2]3+
[Co(phen)2(C12H25NH2)2]3+.
aliphatic hydrophobic chain present in one of the
ligands of our surfactantcobalt(III) complexes into
βCD cavity which ultimately breaks the micelles
formed from ou surfactantcobalt(III) complexes
leading to lowering of rate constant. This effect of β
CD on the rate constant supports our observation of
the dependence of initial concentration of our com
plexes on secondorder rate constant.
4
2
3
4
5
6
7
[Co(III)] × 104, M
Fig. 2. Plots of k against initial concentration of cobalt
complex ion for (a) cis[Co(bpy)2(C12H25NH2)2]3+ and
(b) cis[Co(phen)2(C12H25NH2)2]3+ at different tempe
ratures: (1) 303 K, (2) 308 K, (3) 313 K. [Fe(CN)64] =
0.01 mol dm3, μ = 1.0 mol dm3.
[38]. The effects of presence of CD in the medium on
the kinetics of the same electron transfer reactions
between the surfactantcobalt(III) complexes and
Fe(CN)64 − have also been investigated. In the presence
of CD media also the reduction of the surfactant
cobalt(III) complexes by Fe(CN)64 − proceed with sec
ondorder reaction and the results are listed in the
Table 2. As seen from this table and Fig. 3, the addition
o increasing concentrations of CD has resulted in sig
nificant decrease in the secondorder rate constant. It
is well known fact that βCD is a good structure
breaker of micelles. So in our case the decrease of rate
constant with increase in the concentration of βCD
in the media can be attributed to the inclusion of long
Activation Parameters (ΔS≠ and ΔH≠)
The effect of temperature on reaction rate was
studied at three different temperatures (298, 303, and
308 K) for each initial concentration of the surfactant
cobalt(III) complexes (Table 1), in order to obtain the
activation parameters for the reaction.
Using the Eyring equation shown below the values
of ΔS≠ and ΔH≠ were determined by plotting ln(k/T)
vs 1/T
ln(k/T) = ln(kB/h) + ΔS≠/R – ΔH≠/RT.
The results are shown in Table 3.
Though we expected an increase of entropy in the
transition state due to charge neutralization process
(union of a positive charged oxidant and negatively
charged reductant), our ΔS≠ values reveal that the
entropy has decreased (with a slight increase at lower
concentration in the case of bpy complex). This may
be due to released hydration water, on union of the
reactants, still binding on the Stern layer of the micel
lar surface.
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STUDIES ON OUTER SPHERE ELECTRON TRANSFER REACTIONS
Isokinetic Plot
Table 2. Secondorder rate constants for the reduction of co
4–
balt complex ion by Fe(CN ) 6
in aqueous solution in the pres
ence of [βCD], μ = 1.0 mol dm–3. Temperature = 303 K,
[complex] = 3 × 10–4 to 7 × 10–4 mol dm–3
[βCD] × 104
k × 103
( mol dm–3) (mol–1 dm3 s–1 )
Oxidizing agent
535
We get a straight line for the plot between enthalpy
of activation versus entropy of activation values for the
series of initial concentration of the two complexes
(Fig. 4) indicating that a common mechanism exists in
all the initial concentrations of the complexes studied.
Comparison to Fe(II) as Reductant
3+
cis[Co(bpy)2(C12H25NH2)2]
cis[Co(phen)2(C12H25NH2)2]3+
2
5.1
4
3.6
6
3.1
8
2.4
2
3.5
4
3.0
6
2.7
8
2.2
In our earlier work we have reported the kinetics of
reductions of these complexes by iron(II) ion in self
micelles. On comparing the rate for these reactions
with the results obtained in the present study, the rate
constants for the reactions with ferrocyanide ion is
greater by one order of magnitude. This may be attrib
uted to the negative charge (–4) present in the reduc
tant molecules which can be attracted towards the self
micelles of surfactantcobalt(III) complexes contain
ing a sheath of negative charges on the surfaces of
micelles, whose effect increases with the increase in
the initial concentration of surfactantcobalt(III)
complexes.
CONCLUSIONS
The present work explains the outersphere elec
tron transfer reaction between Fe(CN)64 − and
Table 3. Activation parameters for the reduction of cis[Co(bpy)2(C12H25NH2)2]3+ and cis[Co(phen)2(C12H25NH2)2]3+,
μ = 1.0 mol dm–3. Temperature = 308 K
[Complex] × 104 (mol dm–3)
Oxidizing agent
cis[Co(bpy)2(C12H25NH2)2]3+
cis[Co(phen)2(C12H25NH2)2]3+
Complex: surfactantcobalt(III) complex ion.
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ΔH# (kJ mol–1)
ΔS# (J K–1 mol–1)
3
22.03
9.41
4
21.2
5
17.8
6
12.4
–13.6
7
10.4
–18.4
3
17
–8.0
4
16
–11.0
5
9
–27.0
6
8
–28.9
7
7
–32.1
10.0
0.68
536
SASIKALA, ARUNACHALAM
ΔH ⫽, kJ mol–1
tration of βCD in the medium. This is attributed to
the inclusion of the long aliphatic chain present in one
of the ligands of our complexes into the cavity of β
CD thereby breaking of micelles leading to lowering of
rate constant.
(а)
20
ACKNOWLEDGEMENTS
We are grateful to the UGCSAP & COSIST and
DSTFIS. Financial assistance from the CSIR
(01(2075)/06/EMRII) and UGC (F. No. 32
274/2006 SR) sanctioned to S. Arunachalam are also
gratefully acknowledged.
15
10
–18
–12
ΔH ⫽, kJ mol–1
18
–6
(b)
REFERENCES
0
6
12
⫽
–1 –1
ΔS , J mol K
15
12
9
6
–35
–30
–25
–20
–15 –10
–5
⫽
–1
ΔS , J mol K–1
Fig.
4.
Isokinetic
plots
for
and
[Co(bpy)2(C12H25NH2)2]3+
[Co(phen)2(C12H25NH2)2]3+.
(a)
(b)
cis
cis
cis[Co(bpy)2(C12H25NH2)2](ClO4)3,
cis[Co(phen)2(C12H25NH2)2](ClO4)3 in the self
micelles formed from these surfactantcobalt(III)
complex molecules themselves. The rate constant of
the outersphere electron transfer reaction increases
with increase in initial concentration of these surfac
tant complexes. This can be due to the aggregation of
these metal complexes in their own selfmicelles. With
increase in initial concentration of the surfactant
cobalt(III) complexes the number of micelles present
in the medium also increases. In these micelles the
reactants are encountered in a small volume of Stern
layer of the micelles leading to enhancement of con
centration of reactants at the microlevel and as a result
to higher rate and lower activation energy. The reac
tion has also been carried out in the presence of β
cyclodextrin. We have observed that the second order
rate constant decreased with increase in the concen
1. Babich, O.A. and Gould, E.S., Inorg. Chim. Acta, 2002,
vol. 336, p. 80.
2. Hopfield, J.J., Onuchic, J.N., and Beratan, D.N., J.
Phys. Chem., 1989, vol. 93, p. 6350.
3. Szacilowski, K., Eur. J., 2004, vol. 10, p. 2520.
4. Andersson, M., Linke, M., Chambron, J.C., et al., J.
Am. Chem. Soc., vol. 124, p. 4347.
5. Pfeiffer, J., Kirchner, K., and Wherland, S., Inorg.
Chim. Acta, 2001, vol. 313, p. 37.
6. Gaswick, D. and Haim, A., J. Am. Chem. Soc., 1971,
vol. 93, p. 7347.
7. Miralles, A.J., Szecsy, A.P., and Haim, A., Inorg.
Chem., 1982, vol. 21, p. 697.
8. Weidemaier, K., Tavernier, H.L., and Fayer, M.D., J.
Phys. Chem. B, 1997, vol. 101, p. 9352.
9. Tavernier, H.L., Barzykin, A.V., Tachiya, M., and
Fayer, M.D., J. Phys. Chem. B, 1998, vol. 102, p. 6078.
10. Hammarstrom, L., Norrby, T., Stenhangen, G., et al.,
J. Phys. Chem. B, 1997, vol. 101, p. 7494.
11. Wang, X.L., Chao, H., Li, H., et al., J. Inorg. Biochem.,
2004, vol. 98, p. 1143.
12. Ji, L.N., Zou, X.H., and Liu, J.G., Coord. Chem. Rev.,
2001, vol. 216, p. 513.
13. Srinivasan, S., Annaraj, J., and Athappan, P.R., J.
Inorg. Biochem., 2005, vol. 99, p. 876.
14. PradoGotor, R., Jimenez, R., Lopez, P., et al., Lang
muir, 1998, vol. 14, p. 1539.
15. PradoGotor, R., Jimenez, R., PerezTejeda, P., et al.,
Chem. Phys., 2001, vol. 263, p. 139.
16. LopezCornejo, P., PradoGotor, R., GomezHerrera, C.,
et al., Langmuir, 2003, vol. 19, p. 5991.
17. De la Vega, R., PerezTejeda, P., LopezCornejo, P.,
and Sanchez, F., Langmuir, 2004, vol. 20, p. 1598.
18. LopezCornejo, P., Perez, P., Garcia, F., et al., J. Am.
Chem. Soc., 2002, vol. 124, p. 5154.
19. LopezCornejo, P., PredoGotor, R., GarciaSantana, A.,
et al., Langmuir, 2003, vol. 19, p. 3185.
20. Jones, C.A., Wearner, L.E., and Mackay, R.A., J. Phys.
Chem., 1980, vol. 84, p. 1495.
21. Saenger, A.Q., Angew. Chem., Int. Ed. Engl., 1980,
vol. 19, p. 344.
22. Bender, M. and Kamiyama, L., Cyclodextrin Chemistry,
Berlin: Springer, 1978.
COLLOID JOURNAL
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No. 4
2010
STUDIES ON OUTER SPHERE ELECTRON TRANSFER REACTIONS
23. Comprehensive Supramolecular Chemistry, Ed. by
Attwood, J., Davies, J.E.D., MacNicol, D.D., et al.,
Oxford: Elsevier, 1993, vol. 3.
24. Fujita, K., Ejima, S., and Imoto, T., J. Chem. Soc.,
Chem. Commun., 1984, p. 469.
25. Yamamura, H., Yamada, S., Kohno, K., et al., J. Chem.
Soc., Perkin Trans., 1999, vol. 1, p. 2943.
26. Imonigie, J.A. and Macartney, D.H., Inorg. Chim. Acta,
1994, vol. 225, p. 51.
27. Macartney, D.H., Roszak, A.W., and Smith, K.C.,
Inorg. Chim. Acta, 1999, vol. 291, p. 365.
28. Stephen Wylie, R. and Macartney, D.H., Inorg. Chem.,
1993, vol. 32, p. 1830.
29. Shortreed, M.E., Stephen Wylie, R., and Macartney, D.H.,
Inorg. Chem., 1993, vol. 32, p. 1824.
30. Zwickel, A. and Taube, H., J. Am. Chem. Soc., 1961,
vol. 83, p. 793.
COLLOID JOURNAL
Vol. 72
No. 4
2010
537
31. Bansch, B., Martinez, P., and Van Eldick, R., J. Phys.
Chem., 1992, vol. 96, p. 234.
32. Arumugam, M.N., Santhakumar, K., and Arunacha
lam, S., Asian J. Chem., 2003, vol. 15, p. 191.
33. Arumugam, M.N. and Arunachalam, S., Indian J.
Chem., 1997, vol. 36A, p. 84.
34. Santhakumar, K., Kumaraguru, N., Arunachalam, S.,
and Arumugam, M.N., Trans. Met. Chem., 2006,
vol. 31, p. 62.
35. Kumaraguru, N., Santhakumar, K., Arunachalam, S.,
and Arumugam, M.N., Polyhedron, 2006, vol. 25,
p. 3253.
36. Santhakumar, K., Kumaraguru, N., Arunachalam, S.,
and Arumugam, M.N., Int. J. Chem. Kinet., 2006,
vol. 38, p. 98.
37. Sasikala, K. and Arunachalam, S., Colloids Surf., A,
2009, vol. 335, p. 98.
38. Szejtli, J., Cyclodextrin Technology, Dordrecht: Kluwer,
1988.