Kinetics and mechanism of base hydrolysis of tris(3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine)iron(II) in aqueous and micellar media

Article abstract of DOI:10.1007/s11243-015-0018-z

The kinetics of base hydrolysis of tris(3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine)iron(II), Fe(PDT) 3 2 + has been studied in aqueous, cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) media at 25, 35 and 45 °C under pseudo-first-or

Full text of DOI:10.1007/s11243-015-0018-z

Transition Met Chem
DOI 10.1007/s11243-015-0018-z
Kinetics and mechanism of base hydrolysis of tris(3-(2-pyridyl)-
5,6-diphenyl-1,2,4-triazine)iron(II) in aqueous and micellar media
Rajesh Bellam1 G. Ganga Raju2 Nageswara Rao Anipindi2 Deo Jaganyi1
•
•
•
Received: 30 October 2015 / Accepted: 11 December 2015
Ó Springer International Publishing Switzerland 2015
Abstract The kinetics of base hydrolysis of tris(3-(2-pyr-
idyl)-5,6-diphenyl-1,2,4-triazine)iron(II), Fe(PDT)23þ has
been studied in aqueous, cetyltrimethyl ammonium bromide
(CTAB) and sodium dodecyl sulphate (SDS) media at 25, 35
and 45 °C under pseudo-first-order conditions, i.e.
½OHÀŠ ꢀ ½Fe(PDT)23þŠ. The reactions are first order in both
The ionic strength effect in SDS medium suggests that the
rate-determining step involves an ion and a neutral species.
The results in this study are compared with those obtained for
other iron(II)-bipyridine complexes.
of substrate Fe(PDT)23þ and hydroxide ion. The rates
decrease with increasing ionic strength in aqueous and
CTAB media, whereas SDS medium shows little ionic
strength effect. The rate also increases with CTAB concen-
tration but decreases with SDS. The specific rate constant, k
and thermodynamic parameters (Ea, DH#, DS# and DG#)
have also been evaluated. The near equal values of DG#
obtained in aqueous and CTAB media suggest that these
reactions occur essentially by the same mechanism such that
Fe(PDT)23þ reacts with OH- in the rate-determining step.
Introduction
The formation and dissociation kinetics of the low-spin iro-
n(II) diimine complexes, tris(1,10-phenanthroline)iron(II) [1]
and tris(2,20-bipyridyl)iron(II) [2, 3] has been of interest to
researchers for many years. Initially, the kinetics of these
reactions were studied in aqueous solutions; later studies were
also extended to organic solvents [4–6], micellar [7] and
reversemicellarmedia[8]. Ofthese,thenucleophilicattackby
hydroxide ion, i.e. base hydrolysis at the iron(II) centre, has
been the most extensively studied [9–11]. The ligand 3-(2-
pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT) is capable of
forming stable five-membered chelate rings with iron(II). Its
iron(II) complex, Fe(PDT)23þ is structurally similar to
Fe(bpy)23þ, with three planar bidentate N-ligands coordinated
tothecentralironatominanoctahedralgeometry(Scheme 1).
PDT has two phenyl rings which are known for their electron
donating tendency. This results in increased electron density
on the iron(II) centre. Burgess and Prince studied the base
hydrolysis of iron(II) 4,40- and 5,50-dimethyl-substituted
bipyridine complexes and observed decreased rates. This was
attributedtotheelectrondonating natureofthe methyl groups.
It is observed that Fe(PDT)23þ undergoes base hydrolysis in
aqueous medium. This reaction is catalysed by cationic
cetyltrimethyl ammonium bromide (CTAB) and inhibited by
anionic sodium dodecyl sulphate (SDS). We have taken up
this work to understand the reactivity of hydroxide ion with
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-015-0018-z) contains supplementary
material, which is available to authorized users.
& Nageswara Rao Anipindi
anipindinr@gmail.com
Rajesh Bellam
rajeshchowdarybellam@gmail.com
G. Ganga Raju
ganji.gangaraju@gmail.com
Deo Jaganyi
jaganyi@ukzn.ac.za
1
School of Chemistry and Physics, University of KwaZulu-
Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209,
South Africa
2
Department of Physical and Nuclear Chemistry and Chemical
Oceanography, Andhra University, Visakhapatnam 530 003,
India
123

Transition Met Chem
Varian Cary 100 Bio UV–Visible spectrophotometer ther-
mostatted with a Varian peltier temperature controller to
within 0.05 °C. The data obtained from the UV–VS
instrument were fitted by nonlinear least-squares to the
absorbance-time data according to Eq. (1), using Origin
7.5Ò graphical analysis software. All kinetic traces were
fitted with exponential functions to generate the pseudo-
first-order rate constants, kobs (for convenience the kobs
values in aqueous medium are referred to as kw and those in
CTAB/SDS media as kw).
obstÞ
At ¼ A1 þ ðAo À A1ÞexpðÀk
ð1Þ
where Ao, At and A? represent the absorbance of the
reaction mixture at zero time, at time t and at the end of the
reaction, respectively. The kinetic runs were performed at
three different temperatures (25, 35 and 45 °C). Half of the
runs were performed in duplicate. The rate constants were
reproducible to within 8 %.
Scheme 1 Structural formula of Fe(PDT)32þ
Results and discussion
Fe(PDT)23þ and compare the results with iron(II) bipyridine,
4,40- and 5,50-dimethyl-substituted bipyridinecomplexes. The
results of our studies are presented in this paper.
Base hydrolysis in aqueous medium
The kinetic runs were carried out under pseudo-first-order
conditions with ½OHÀŠ ꢀ ½Fe(PDT)23þŠ (i.e., [OH-] was in
50-fold excess over ½Fe(PDT)23þŠ) at different initial concen-
trations of [OH-] in the range 1.0 9 10-2 - 20.0 9 10-2
mol dm-3, keeping the concentration of Fe(PDT)23þ at
Experimental
Reagents
2.0 9 10-5 mol dm-3 and ionic strength at 0.2 mol dm-3
.
The effects of Fe(PDT)23þ and ionic strength on the rate have
also been studied. Figure 1 shows the spectral changes during
the reaction; the inset is a kinetic trace of time dependence of
the absorbance at the kmax of Fe(PDT)23þ: The rate data
(Table 1) show that the rate constants are linearly dependent
on [OH-] (see Supplementary Fig. 1), indicating that the
order with respect to hydroxide ion is unity. Furthermore, the
rate of reaction is found to decrease with ionic strength,
indicating that the rate-limiting step involves ions of opposite
charge. The rate equation can be expressed as
PDT (GFS Chemicals Inc., USA) and SDS (Fluka) were
used as such without recrystallization. A 1.0 9 10-2
mol dm-3 solution of PDT was prepared by dissolving the
requisite quantity in methanol (Merck, GR) and stored in
an amber coloured bottle. 1.0 9 10-2 mol dm-3 solutions
of SDS and NaOH were prepared by dissolving the req-
uisite quantities in Millipore water. All other solutions
were prepared as described earlier [12]. A 2.0 9 10-4
mol dm-3 solution of Fe(PDT)23þ was prepared by mixing
ferrous ammonium sulphate and PDT in 1:3 ratio. The kmax
and emax values for this solution were 555 nm and
2.34 9 104 mol-1 dm3 cm-1, respectively, in agreement
with the literature [13].
2þ
d½FeðPDTÞ3
dt
Š
Rate ¼ À
¼ fk[OHÀŠg½Fe(PDT)23þ
Š
ð2Þ
or
Kinetic procedure
kobs ¼ k½OHÀŠ
ð3Þ
Base hydrolysis of the magenta coloured Fe(PDT)23þ gives
a colourless solution of Fe(OH)3. The kinetic runs in
aqueous and surfactant media were performed at 555 nm,
i.e. the kmax of Fe(PDT)23þ by the procedure described
earlier [12], and the product has negligible absorbance at
this wavelength. Kinetic runs were performed using a
Hereafter the rate constant, kobs in aqueous medium is
referred to as kw. The slope of a plot of kw versus [OH-]
gives the specific rate constant, k (Table 2).
Hydroxide ion is capable of nucleophilic attack on
Fe(II)-diimine complexes and gives an intermediate,
[Fe(diimine)2(diimine-g1)(OH)]? in which OH- is bonded
123

Transition Met Chem
Fig. 1 UV–Vis spectral
changes observed during the
reaction; Inset is a typical
kinetic trace at 555 nm.
½Fe(PDT)23þŠ ¼
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.30
0.25
0.20
0.15
0.10
0.05
2:0 Â 10À5mol dmÀ3
½OHÀŠ ¼
;
5:0 Â 10À2 mol dm-3
,
l = 0.2 mol dm-3 and
temperature = 25 °C
0
100
200
300
400
500
Time (min)
350
400
450
500
550
600
650
Wavelngth (nm)
Table 1 kobs values in the base
hydrolysis of Fe(PDT)23þ in
aqueous and micellar media
where CTAB = 1.5 9 103,
mol dm-3 and
[OH-] 9
102 (mol dm-3
l
kobs 9 104 (mol dm-3
)
½Fe(PDT)23þŠ 9
105 (mol dm-3
)
)
Aqueous medium
Micellar medium
CTAB
SDS
SDS = 1.0 9 103, mol dm-3
temperature = 35 °C
,
1.0
2.0
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
0.4
0.5
0.6
0.7
0.8
0.50
1.23
2.11
2.61
3.75
4.54
5.72
6.72
7.67
2.18
2.21
2.08
2.16
1.97
2.03
2.15
3.09
1.19
0.82
0.60
0.47
0.39
0.32
0.97
2.34
4.04
5.41
7.53
8.79
11.44
12.85
14.71
3.28
3.48
3.38
3.50
3.47
3.36
3.52
5.17
2.92
2.24
1.88
1.37
1.08
0.87
0.06
0.17
0.28
0.40
0.55
0.69
0.89
1.03
1.20
0.24
0.29
0.25
0.19
0.22
0.31
0.27
0.21
0.35
0.41
0.28
0.33
0.27
0.30
3.0
5.0
7.0
10.0
12.0
15.0
17.0
20.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
3.0
4.0
5.0
6.0
7.0
8.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Similarly, base hydrolysis of Fe(phen)23þ or Fe(bpy)32þ
gives Fe(OH)3 in the presence of dissolved oxygen. In the
present studies, the solutions were optically clear up to
to the metal centre and one dimimine has become mon-
odentate. This ultimately gives Fe(OH)2 which is subse-
quently oxidised to Fe(OH)3 in the presence of O2.
123

Transition Met Chem
Table 2 Specific rate constants
and activation parameters for
the base hydrolysis of
Fe(PDT)23þ in aqueous and
Temperature/parameter
k 9 103 (mol dm-3 s-1
)
Aqueous medium
Micellar medium
CTAB
SDS
micellar media
25 °C
35 °C
45 °C
0.80 0.015
3.85 0.024
13.66 0.11
104.9 1.7
102.4 2.1
35.7 6.4
2.51 0.16
8.83 0.21
25.09 0.43
80.6 1.2
0.12 0.01
0.50 0.01
1.93 0.02
122.4 3.0
113.6 2.3
15.2 7.0
Ea (kJ mol-1
)
DH# (kJ mol-1
–DS# (J K-1 mol-1
DG#35 °C (kJ mol-1
)
78.0 1.8
)
109.2 5.6
111.6 0.24
)
113.4 0.13
118.3 0.22
Scheme 2 Mechanism for the base hydrolysis of Fe(PDT)32þ
*80 % of the reaction. At the end of each run, a red
colloidal precipitate was observed. Hence, we assume
that Fe(PDT)23þ reactsþ with hydroxide to give
determined by conductometric methods at fixed
[OH-] = 5.0 9 10-2 mol dm-3 varying the concentration
of CTAB/SDS. The CMC is the point of intersection of
two straight lines observed in plots of [CTAB]/[SDS]
versus specific conductance. The CMC values obtained for
CTAB-OH- and SDS-OH- mixtures were 1.5 9 10-3 and
1.0 9 10-3 mol dm-3, respectively. The results show that
the pseudo-first-order rate constants in micellar media, kw,
increase with increasing OH- concentration (see Table 1)
and follow the same trend as observed in aqueous medium
(Fig. 2), suggesting that the reactions in micellar media
occur by a similar mechanism to that proposed for aque-
ous medium. Hence, the final rate equation can be
expressed as
½Fe(PDT)2ðPDT-g1ÞðOH)Š : This step is considered to be
þ
rate-limiting. ½Fe(PDT)2ðPDT-g1ÞðOH)Š loses the mon-
odentate PDT by subsequent nucleophilic attack of another
OH- to give ½Fe(PDT)2ðOH)2Š which decomposes to
Fe(OH)2 plus PDT. Under aerobic conditions,
Fe(OH)2 rapidly oxidises to Fe(OH)3. The overall mecha-
nism for the base hydrolysis of Fe(PDT)23þ is outlined in
Scheme 2.
Base hydrolysis in micellar media
kw ¼ k½OHÀŠ
ð4Þ
When a surfactant is dissolved in water, its molecules have a
tendency to aggregate spontaneously into thermodynamically
stable particles of colloidal dimensions, due to hydrophobic
interactions [14, 15]. At lower concentrations, the surfactant
molecules behave like ordinary solutes. After attaining a
certain concentration, the individual surfactant molecules
assemble into spherical aggregates known as micelles [16].
The minimum concentration for the onset of micellization is
known as critical micelle concentration, CMC.
Comparison of the rate data in Table 1 shows that the rate
is enhanced in CTAB medium, but significantly retarded in
SDS. Representative plots of rate constants versus [OH-]
at 35 °C in aqueous and micellar media are presented in
Fig. 3.
The specific rate constants, k, have been evaluated from
the slopes of the plots of kobs versus [OH-]. Using these
specific rate constants, the activation parameters (Ea, DH#,
DS# and DG#308K) for aqueous, CTAB and SDS media
were computed using the Arrhenius and Eyring equations
(Table 2).
The electrical charge of the surfactant generally influ-
ences the rate of reaction. Hence, the title reaction has also
been carried out in cationic CTAB and anionic SDS
micellar media at their respective CMC values. These
CMC values of CTAB-OH- and SDS-OH- mixtures were
The Ea values in Table 2 suggest catalysis of the reac-
tion in CTAB medium and inhibition in the presence of
123

Transition Met Chem
(a)
(b)
45
5
4
3
2
1
0
45 0C
45 0C
40
35
30
25
20
15
10
5
35 0C
25 0C
35 0C
25 0C
0
0
5
10
15
-3
20
0
5
10
15
20
[OH- ] x 102 mol dm
2
[OH - ] x 10 mol dm-3
Â
Ã
Fig. 2 Effect of hydroxide ion on the base hydrolysis of Fe(PDT)23þ in (a) CTAB and (b) SDS media. Fe(PDT)32þ ¼ 2:0 Â 10À5 mol dm-3
,
l = 0.2 mol dm-3, [CTAB] = 1.5 9 10-3 mol dm-3, [SDS] = 1.0 9 10-3 mol dm-3
CTAB media, suggesting that the rate-determining step
involves ions of opposite charge. However, in SDS med-
ium the ionic strength has little effect on the reaction which
indicates that the rate-determining step involves an ion and
neutral species. Formation of a neutral ion pair between
Fe(PDT)23þ and two DS- ions is likely, and this reacts with
OH- in the rate-determining step. This explains the ionic
strength effect and the slightly higher DG# value in SDS
medium.
20
15
10
5
CTAB
Aqueous
SDS
Effect of CTAB
0
The effect of CTAB on the reaction was also studied by
varying the concentration of CTAB from zero to
1.0 9 10-3 mol dm-3 at [OH- = 5.0 9 10-2 mol dm-3
and l = 0.2 mol dm-3 (see Supplementary Table 1). The
kw versus [CTAB] profile shows that the rate of the reaction
increases sharply with increasing [CTAB] and reaches a
maximum at 1.5 9 10-3 mol dm-3 (CMC) and then
decreases (Fig. 4). This is the characteristic feature of
bimolecular micellar catalysis [18–20].
0
5
10
15
20
-3
[OH- ] x 102 mol dm
Fig. 3 Effect of hydroxide ion on the base hydrolysis of Fe(PDT)32þ
Â
Ã
in aqueous and different surfactant media at 35 °C. Fe(PDT)32þ
¼
2:0 Â 10À5 mol dm-3, l = 0.2 mol dm-3, [CTAB] = 1.5 9 10-3
mol dm-3, [SDS] = 1.0 9 10-3 mol dm-3
SDS. The large negative value of entropy and low-positive
enthalpy of activation are consistent with base hydrolysis
[17]. The large negative entropy value obtained in CTAB
indicates that the transition state is more rigid and ordered
than that in aqueous medium. Further the high values of
free energy and enthalpy of activation suggest that the
transition state is highly solvated. The very similar DG#
values observed in aqueous and CTAB media clearly
indicate that these reactions occur essentially by a similar
mechanism in both cases. The inhibition noticed in SDS
medium may be due to repulsion of OH- by anionic
dodecyl sulphate, DS- aggregates. The rate of the reaction
decreases with increasing ionic strength in aqueous and
These observations can be explained by a pseudo-phase
kinetic model proposed by Menger and Portnoy [21].
According to this model, the total volume of surfactant or
micelle is considered as a separate phase which is uni-
formly distributed in the aqueous phase. Base hydrolysis
occurs in both bulk aqueous and micellar phases
(Scheme 3).
The terms S, Dn and Ks in Scheme 3 denote substrate
(Fe(PDT)23þ), micellised surfactant (Dn = [total surfac-
tant] - CMC) and binding constant of substrate with
micelle (Ks = [SM]/[SW][Dn]), respectively. The constants
kM and kW are the first-order rate constants in the micellar
123

Transition Met Chem
14
12
10
8
Table 3 Binding constants, KS and rate constants in pure micellar
medium, kM for base hydrolysis of Fe(PDT)23þ in CTAB medium
Temperature (°C)
kM 9 104 (s-1
)
KS 9 10-5 (mol dm-3
)
45 oC
25
30
35
0.85 0.03
3.64 0.01
9.70 0.02
4.53
1.32 11
0.76
4
9
6
35 oC
25 oC
4
1/(kw - kw) and 1/[Dn] could be observed, Eq. (6) is
modified (by neglecting kw) as
2
1
1
1
0
¼
þ
ð8Þ
kW kM kMKS½DnŠ
0
2
4
6
8
10
104 x [CTAB], mol dm
-3
From the present experimental data, good straight line plots
are obtained at all three temperatures employed when 1/kw
is plotted against 1/[Dn] (see Supplementary Fig. 2). The
slope and intercept of these plots are equal to 1/kMKS and
1/kM, respectively. The resulting constants are given in
Table 3.
Fig. 4 Profile of kw versus [CTAB] for base hydrolysis of
Fe(PDT)23þ ¼ 2:0  10À5 mol dm-3, ½OHÀŠ ¼ 5:0 Â
Â
Ã
Fe(PDT)23þ
.
10À2 mol dm-3 and l = 0.2 mol dm-3
Micellar effects of bimolecular reactions involving ionic
reactants follow the Hartley principle [23], i.e. cationic
reactions are accelerated by anionic micelles and inhibited
by cationic micelles and vice versa. Counter-ions are
effectively concentrated at surfaces of ionic micelles where
they partially neutralise the head group charges. Polar
substrates are located at micellar surfaces, so it is reason-
able to assume that the reactions involving ions or polar
molecules occur in the region occupied by the surfactant
head groups, the so-called Stern layer [24]. Most micellar
catalysed reactions are supposed to occur in the Stern layer
[21, 25]. Micellar catalysis is often rationalised in terms of
bringing together the reactants in the Stern layer sur-
rounding the micellar surface.
Scheme 3 Menger and Portnoy pseudo-phase kinetic model
and aqueous phases, respectively. According to this model,
kw for the overall reaction is given by Eq. (5).
The catalytic effect in the presence of CTAB is due to
electrostatic and hydrophobic interactions between
hydroxide anions and CTA? cations. The positively
charged cetyltrimethylammonium and substrate ions can-
not easily approach each other in the Stern layer. However,
the high electron density of the three PDT ligands in the
substrate should lead to their orientation around the Stern
layer, facilitating interaction between Fe(PDT)23þ and
OH-. The two opposing effects, namely concentration
effect and dilution effect, explain the profile of kw versus
[CTAB] which shows a maximum at the CMC. At lower
[CTAB], the number of ionic or polar reactant species
going into the Stern layer increases, leading to enhance-
ment of the rate. After saturation in the micellar phase,
further increase in the concentration of CTAB favours an
increase in the number of micelles, leading to a decrease in
effective concentrations of both Fe(PDT)23þ and OH- ions
around the Stern layer. This is probably one of the reasons
for the decrease in rate at higher [CTAB].
kMKS½DnŠ þ kW
kw ¼
ð5Þ
1 þ KS½DnŠ
For kM [ kW, the rate of the reaction increases with
increase in [Dn] and ultimately reaches the limiting value
kM. Conversely, if kM \ kW, an increase in [Dn] produces a
decrease in kw and kw tends to attain the limiting value kW.
Equation (5) can be rearranged into its reciprocal form as
1
1
1
kW
¼
¼
þ
ð6Þ
kW À kW KS½DnŠðkM À kWÞ kM À kW
or
kW À kW
kM À kW
¼ KS½DnŠ
ð7Þ
Bunton and Cerichlli [22] have pointed out that the treat-
ment of Eq. (6), i.e. a plot of 1/(kw - kW) versus 1/[Dn], is
very sensitive to the values of CMC that may be affected
by the reaction media. As no linear relationship between
123

Transition Met Chem
2.5
2.0
1.5
1.0
0.5
45 oC
35 oC
25 oC
0.0
Scheme 4 Berezin’s pseudo-phase model
0
2
4
6
8
10
103 x [SDS], mol dm
-3
Table 4 Binding constant, KS ? KOH and rate constants in aqueous
medium, kw for base hydrolysis of Fe(PDT)23þ in SDS medium
Fig. 5 Plot of kw versus [SDS] for base hydrolysis of Fe(PDT)23þ in
Â
Ã
SDS medium. Fe(PDT)23þ ¼ 2:0 Â 10À5 mol dm-3, [OH-] = 5.0 9
Temperature
(°C)
kw 9 105
(s-1
-(KS ? KOH)
(mol-1 dm3)
10-2 mol dm-3 and l = 0.2 mol dm-3
)
25
30
35
0.54 0.01
1.25 0.03
7.31 0.01
31.07 0.47
32.18 0.55
31.07 0.24
Effect of SDS
The rate data in Table 1 show that SDS inhibits base
hydrolysis of Fe(PDT)23þ. The effect of SDS on the rate of
reaction was also studied by varying the concentration of
SDS in the range from 1.0 9 10-5 to 1.0 9 10-2
mol dm-3, at fixed concentration of OH- (5.0 9 10-2
mol dm-3) and ionic strength (0.2 mol dm-3) (see Sup-
plementary Table 2). The kw versus [SDS] profile shows
that kw initially decreases with increasing [SDS], and then
becomes nearly saturated at higher concentrations (Fig. 5).
The profile in Fig. 5 can be explained by using Bere-
zin’s pseudo-phase model [25–27] (Scheme 4). According
to this model, the micellar catalysis or inhibition is based
on the assumption that a bimolecular interaction between
the reactants may occur in both the micellar pseudo-phase
and aqueous phase.
and [SDS] = kM/V (where V is the molar volume of the
micelle). Since the hydroxide ion is a charged species and
the substrate is a large molecule, the hydrophobic and
electrostatic interactions are significant and give high val-
ues of KS and KOH. Since [SDS] is small it may be possible
that kW ꢀ kMKSKOH½DnŠ so that Eq. (9) takes the form
kW
kw ¼
ð10Þ
2
1 þ fðKS þ KOHÞ½DnŠg þ KSKOH½DnŠ
Further, since [Dn] is very small and thus the terms con-
taining [Dn]2 may be neglected. The inverse of Eq. (10)
then simplifies to
In this scheme, S denotes substrate (Fe(PDT)23þ), whilst
the subscripts M and W denote a pseudo-phase associated
with the surfactant and the bulk solvent, respectively. KS
and KOH are the equilibrium constants for partition of
substrate and hydroxide between the aqueous and micellar
phases. The constants kM and kw are the rate constants for
the micellar and aqueous phases, respectively. In such a
two phase system, it has been shown that for dilute sur-
factant solutions, in which the volume fraction of the
micellar phase is small, the quantitative expression for the
pseudo-first-order rate constant for a bimolecular reaction
occurring only in aqueous and micellar phase is given by
1
1
ðKS þ KOHÞ½DnŠ
¼
þ
ð11Þ
kw kW
kW
A plot of 1/kw versus Dn gives a straight line with
negative slope and positive intercept (see Supplementary
Fig. 3). The value of kw has been evaluated from the
intercept of this plot. Substituting this value into the slope
of the plot gives the sum of the binding constants,
(KS ? KOH) which are presented in Table 4.
The rate inhibition in the presence of SDS is attributed to
selective partitioning of Fe(PDT)23þ in the micellar region.
According to this, Fe(PDT)23þ is preferentially bound to the
micelle whereas OH- is repelled. Thus, the partition of
Fe(PDT)23þ between different regions decreases the proba-
bility of interaction with OH-, resulting in a decrease in rate.
Ionic strength has no effect on the reaction in SDS medium,
indicating that the rate-determining step involves an ion and
kW þ kMKSKOH½DnŠ
kW
ð1 þ KS½DnŠÞð1 þ KOH½DnŠÞ
where Dn = [total surfactant] - CMC, KS and KOH are the
¼
ð9Þ
association constants of Fe(PDT)23þ and OH- with SDS
123

Transition Met Chem
neutral molecule. Since OH- ion is one of the reactants, the
other reactant namely the iron(II)-PDT complex should be
in neutral form. This is possible only when Fe(PDT)32þ
forms an ion pair with anionic dodecyl sulphate anion,
DSÀ; fFe(PDT)23þ Á 2DSÀg:
to the Stern layer of the micelle and thus distributes itself
between aqueous and SDS phases. This decreases the
probability of interaction between the two reactants in the
aqueous phase, resulting in a decrease in rate.
The base hydrolysis of Fe(II)-bipyridine, 4,40- and 5,50-
dimethyl-substituted bipyridine and PDT all follow essen-
tially the same mechanism. At 25 °C, the specific rate
constant (9103) values for substitution of Fe(II)-bipyridine,
4,40-dimethyl bipyridine, 5,50-dimethyl bipyridine and PDT
complexes by hydroxide are 7.0, 6.0, 5.67 [9] and 0.80 [this
work] mol dm-3 s-1, respectively. Hence, the substitution
of Fe(II)-PDT complex by hydroxide is extremely slow
compared to analogous complexes. This can be explained
as follows. The methyl and phenyl groups are weakly
activating, i.e. these groups release electrons to the nitro-
gen, resulting in increased electron density on the iron
atom. Comparing Fe(PDT)23þ with dimethyl-substituted
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Fe(bpy)23þ; the former has more electron density on the
iron(II) centre due to the presence of triazine N-atoms.
Hence, Fe(PDT)23þ is less able to react with hydroxide ion.
Therefore, the order of the reactivity towards base
hydrolysis is bipyridine › dimethyl bipyridine » PDT. The
DH# values for the base hydrolysis of Fe(PDT)23þ
=
Fe(bpy)23þ in aqueous and SDS medium are 102/75 and
114/82 kJ mol-1, respectively [28]. These values also
confirm the above order of reactivity.
Conclusions
The catalytic effect observed for the title reaction in the
presence of CTAB is due to electrostatic and hydrophobic
interactions between hydroxide ion and positively charged
CTAB. The positively charged cetylammonium and sub-
strate ions are not expected to come close in the Stern
layer. However, the high electron density of three PDT
molecules in Fe(PDT)23þ leads to their orientation around
the Stern layer, facilitating the reaction between
Fe(PDT)23þ and OH-. In SDS medium hydroxide ion
cannot easily approach negatively charged DS- aggre-
gates. The other reactant Fe(PDT)23þ is preferentially bound
123