kinetics of cetyl trimethyl ammonium bromide catalyzed substitution of tris(sulfonated...
TRANSCRIPT
Kinetics of cetyl trimethyl ammonium bromide catalyzedsubstitution of tris(sulfonated triazine)iron(II) complexes by1,10-phenanthroline, 2,20-bipyridine and 2,20,6,200-terpyridine
Rajesh Bellam • Nageswara Rao Anipindi
Received: 11 December 2013 / Accepted: 27 January 2014
� Springer International Publishing Switzerland 2014
Abstract The kinetics of substitution of tris(3-(2-pyridyl)-
5,6-bis(4-phenyl-sulfonic acid)-1,2,4-triazine)-iron(II),
FeðPDTSÞ4�3 , and tris(3-(4-(4-phenylsulfonic acid)-2-pyri-
dyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine)-iron(II),
FeðPPDTSÞ7�3 , by two polypyridyls, namely 2,20-bipyridine
(bpy) and 2,20,60,200-terpyridine (terpy), have been studied
at 25–55 �C under pseudo-first-order conditions, i.e.,
[ppy] � ½FeðPDTSÞ4�3 � or ½FeðPPDTSÞ7�3 � in acetate buffers
over the pH range 3.6–5.6 (ppy = bpy, terpy or phen). The
reactions are first order in FeðPDTSÞ4�3 or FeðPPDTSÞ7�3 . The
reaction rates increase with [ppy] and pH. Plots of kobs versus
[ppy] and 1/[H?] are linear with positive intercepts on the rate
axes, indicating that the reactions proceed by both ppy- and
hydrogen ion-dependent and independent paths. Ionic
strength has no influence on the rate of reaction. Cetyl
trimethyl ammonium bromide (CTAB) catalyzes these sub-
stitution processes, including substitution by 1,10–phenan-
throline (phen). The micelle-catalyzed reactions essentially
follow the same general pattern as the uncatalyzed reactions.
Micellar catalysis is ascribed to the binding of the anionic
substrate on the surface of the cationic surfactant by hydro-
philic and/or electrostatic interactions and significant elec-
trostatic contribution to the binding of the positively charged
quaternary ammonium head group to p-electron-rich poly-
pyridyls. Kinetic data have been obtained at three different
temperatures, and the specific rate constants (k1 and k2) and
thermodynamic parameters (Ea, DS# and DG#) have been
computed. The binding constants between FeðPDTSÞ4�3 /
FeðPPDTSÞ7�3 and CTAB have been evaluated. The near-
equal values of DG# obtained in aqueous and CTAB media
suggest that these reactions occur by essentially the same
mechanism in either medium.
Introduction
A common feature of bpy, phen, terpy, 3-(2-pyridyl)-5,6-
bis(4-phenyl-)-1,2,4-triazine (PDT), 3-(4-(4-phenyl)-2-pyri-
dyl)-5,6-bis(4-phenyl)-1,2,4-triazine (PPDT), 3-(2-pyridyl)-
5,6-bis(4-phenyl-sulfonic acid)-1,2,4-triazine (PDTS) and
3-(4-(4-phenylsulfonic acid)-2-pyridyl)-5,6-bis(4-phenyl-
sulfonic acid)-1,2,4-triazine (PPDTS) is the possession of an
a,a-diimine moiety, constituting part of the aromatic system
and capable of forming five-membered chelate rings with
iron(II) (Scheme 1). The iron(II) complexes of triazines and
their sulfonated analogues are structurally similar to the
FeðbpyÞ2þ3 ion, with three planar bidentate N-ligands coor-
dinated with the central iron, in an octahedral geometry
(Scheme 2). In the case of PDT and PPDT (and their sulfo-
nated analogues), the two least sterically hindered donor
atoms for chelation are the pyridyl N-atom and the N-atom in
position 2 of the triazine ring. The intense absorptivity of these
complexes indicates that they enjoy pronounced electron
delocalization, suggestive of a planar conformation for each
ligand to provide electronic conjugation between its various
rings. Kinetic studies of substitution of FeðTPTZÞ2þ2 by bpy
Electronic supplementary material The online version of thisarticle (doi:10.1007/s11243-014-9804-2) contains supplementarymaterial, which is available to authorized users.
R. Bellam � N. R. Anipindi (&)
Department of Physical and Nuclear Chemistry and Chemical
Oceanography, Andhra University, Visakhapatnam 530 003,
India
e-mail: [email protected]
R. Bellam
e-mail: [email protected]
123
Transition Met Chem
DOI 10.1007/s11243-014-9804-2
and phen [1], terpy [2] and PDTS and PPDTS [3] have been
carried out based on the principle that the log b value of the
reactants and products determines the possibility of occur-
rence of a particular reaction. We have reported the substi-
tution of FeðPDTÞ2þ3 , FeðPPDTÞ2þ3 , FeðPDTSÞ4�3 and
FeðPPDTSÞ7�3 by phen to form Feðphen)2þ3 [4] in aqueous
acetate buffers. We noticed that bpy and terpy also react with
FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 (hereafter referred to as
Fe(stz)3) to form the corresponding iron(II)-polypyridyl
complexes, namely FeðbpyÞ2þ3 and FeðterpyÞ2þ2 , respectively.
It has also been observed that these substitution processes,
including substitution by phen, are catalyzed in CTAB med-
ium. We have now studied the kinetic and mechanistic details
of these substitution reactions in aqueous and CTAB media,
and the results are presented in this paper.
Experimental
Bpy and terpy obtained from GFS Chemicals Inc., USA,
were used without any further purification. Standard bpy
and terpy solutions of 1.0 9 10-3 mol dm-3 were pre-
pared by dissolving the requisite quantities of bpy and
terpy in 1.0 9 10-3 mol dm-3 HNO3. CTAB of 99 %
purity (Sigma–Aldrich) was used as received. All other
solutions were prepared as previously described [4].
A Shimadzu UV-1800 UV-visible spectrophotometer fitted
with CPS 240A containing a six-cell holder in which the
temperature could be controlled to ±0.1 �C by the Peltier
effect was used to record the absorption spectra and
monitor the reaction kinetics.
Product analysis and kinetic procedure
The visible absorption spectra of the products obtained in
aqueous and CTAB media for the substitution of
FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 by bpy and terpy show
absorption maxima at 522 and 552 nm, respectively. The
molar absorptivities for each of these substitution products
on the basis of iron(II) content of the substrates correspond
to those of FeðbpyÞ2þ3 or FeðterpyÞ2þ2 as appropriate. Sim-
ilarly, the visible absorption spectra of the products in the
presence of phen show an absorption maximum at 510 nm
corresponding to that of FeðphenÞ2þ3 . The spectra of
FeðPDTSÞ4�3 and its reaction products with bpy, terpy and
phen are shown in Fig. 1. The visible absorption spectra of
FeðPPDTSÞ7�3 and its substitution reaction products are
shown in the Supplementary data (S1).
The critical micellar concentration (CMC) values of ppy–
surfactant mixtures were determined conductometrically
using a Control Dynamics conductivity meter. Ppy–surfactant
mixtures with 2 9 10-4 mol dm-3 of ppy and with varying
[CTAB] from 2 9 10-5 to 5 9 10-3 mol dm-3 at pH = 4.0
were prepared and their specific conductance values measured
at 25, 35 and 45 �C. The point of intersection of the two
straight lines in the plot of specific conductance of ppy–CTAB
mixtures against [CTAB] corresponds to the CMC. The CMC
values for bpy, terpy and phen mixtures at 35 �C are
1.2 9 10-3, 8.5 9 10-4 and 1.8 9 10-3, respectively. The
kinetic runs were performed following the same procedure as
previously described [4]. The substitution reactions in both
Scheme 1 Structures of PDTS and PPDTS
Scheme 2 Structures of
iron(II)-PDTS/PPDTS
complexes
Transition Met Chem
123
aqueous and CTAB media obey pseudo-first-order kinetics as
indicated by the exponential decrease in (A? - At) (A? and
At are absorbance values at infinite time and at time ‘t,’
respectively) with time and also straight plots of log
(A? - At) versus time (Fig. 2). The pseudo-first-order rate
constants, kobs, were evaluated from the slopes of these plots.
The kinetic runs were performed at four temperatures (25, 35,
45 and 55 �C). About 50 % of the kinetic runs were per-
formed in duplicate. The rate constants were reproducible
within ±8 %. Values of kobs in aqueous and CTAB media are
referred to as kw and kw, respectively.
Results and discussion
The reactions in aqueous medium were carried out under
pseudo-first-order conditions with [ppy] � [Fe(stz)3] in
the pH range 3.6–5.6 in acetate buffers. Kinetic runs were
performed at different initial concentrations of [Fe(stz)3],
keeping the concentrations of ppy, pH and l constant. A
fourfold increase in the initial concentration of Fe(stz)3
does not alter the kw (kobs in aqueous media) values, indi-
cating that the reactions are first order in [Fe(stz)3]. The
reactions were also carried out at different [ppy], which
revealed that the rate constant increases with increase in
[ppy]. These observations suggest that the reactions occur
by two distinct pathways, namely associative (ppy depen-
dent) and dissociative (ppy independent). Plots of kw versus
[bpy] are presented in Fig. 3. Plots for the terpy systems
are given in the supplementary data (S2). The values of kw
increase with increasing pH. The plots of kw versus 1/[H?]
are linear with positive slopes and positive intercepts on the
ordinate. This suggests that these reactions occur by both
pH-dependent and pH-independent paths. The bpy substi-
tution reaction plots are shown in Fig. 4, and those of other
systems are given in the supplementary data (S3). The
kinetic data for ppy (the data for phen substitution studies
from ref. [4] are also presented for comparison, wherever
necessary) at 35 �C are presented in Table 1. The data
obtained at other temperatures, namely 25, 45 and 55 �C,
are presented in supplementary data (S4).
Proposed mechanism
Bpy has a pKa value of 4.54 at 25 �C [5], whereas terpy has
pKa and pKb values of 4.71 and 3.57, respectively [6], at
this temperature. Using these values, the concentrations of
the diprotonated form of terpy and mono- and unprotonated
forms of bpy and terpy at different pH values in the range
of 3.6–5.6 have been computed using a simulation pro-
gram. The data indicate that in this pH range, both bpy and
terpy exist mostly in their monoprotonated forms. How-
ever, the unprotonated forms are considered to be the
reactive species.
ppyHþ �Ka
ppyþ Hþ ð1Þ
Fig. 2 Plots of (At - A?) versus time (a) and log (At - A?) versus
time (b). [bpy] = 1.0 9 10-3 mol dm-3, pH = 4.0, l = 0.1,
½FeðPPDTSÞ7�3 � = 2.0 9 10-5 mol dm-3, Temperature = 25 �C
Fig. 1 Visible absorption spectra of FeðPDTSÞ4�3 (a), and its reaction
products with terpy (b), phen (c) and bpy (d)
Transition Met Chem
123
It was considered that these iron(II)–stz complexes
undergo reversible dissociation in the rate-determining step
of the ppy-independent path. The iron(II)-sulfonated tria-
zines undergo reversible dissociation in the rate-limiting
step with a rate constant k1 to give a five-coordinate
intermediate, Fe(stz)2(stz-g1):
FeðstzÞ3 �k1
k�1
FeðstzÞ2ðstz-g1Þ ð2Þ
where stz-g1 is monodentate sulphonated triazine and
k1 ››› k-1. This five-coordinate intermediate reacts further
with ppy molecules in a series of fast steps to give the final
products:
FeðstzÞ2ðstz-g1Þ þ ppy�!fastFeðstzÞ2ðppy-g1Þ þ stz ð3Þ
FeðstzÞ2ðppy-g1Þ þ ðn� 1Þppy�!fastFeðppyÞ2þn þ 2stz ð4Þ
where n = 3 for bpy/phen and 2 for terpy.
In the ppy–dependent pathway, a ppy molecule binds to
the iron(II) center in Fe(stz)3 in the rate-determining step to
give a seven-coordinate intermediate, Fe(stz)3(ppy-g1):
FeðstzÞ3 þ ppy�!slow
k2
FeðstzÞ3ðbpy-g1Þ=FeðstzÞ3ðterpy-g1Þ
ð5Þ
Subsequently, Fe(stz)3(bpy-g1) rearranges with the
monodentate bpy switching to chelation of the iron(II)
Fig. 3 Effects of bpy on the rate. FeðPDTSÞ4�3 or FeðPPDTSÞ7�3 = 2.0 9 10-5 mol dm-3, pH = 4.0 and l = 0.1
Fig. 4 Effects of hydrogen ion on the rate. FeðPDTSÞ4�3 or FeðPPDTSÞ7�3 = 2.0 9 10-5 mol dm-3, [bpy] = 4.0 = 1.0 9 10-3 mol dm-3 and
l = 0.1
Transition Met Chem
123
center after which the monodentate sulfonated triazine
dissociates:
FeðstzÞ3ðbpy-g1Þ�!fastFeðstzÞ2ðstz-g1ÞðbpyÞ ð6Þ
FeðstzÞ2ðbpyÞ�!fastFeðstzÞ2ðbpyÞ þ stz ð7Þ
Similarly, Fe(stz)3(terpy-g1) rearranges to give
Fe(stz)2(terpy-g2) (terpy-g2 is bidentate terpy) and mono-
dentate sulfonated triazine. These intermediate species,
Fe(stz)2(bpy)/Fe(stz)3(terpy-g1), subsequently react with
two bpy or one terpy molecule(s), respectively, in a series
of fast steps to give the final products.
This mechanism leads to the rate law:
Rate ¼ k1½FeðstzÞ3� þ k2½FeðstzÞ3�½ppy�e ð8Þ
As already stated, under the present experimental
conditions, ppy exists mostly in monoprotonated form.
Therefore,
½ppy�t ¼ ½Hppyþ�e þ ½ppy�e ð9Þ
Hence,
½Hppyþ�e ¼½ppy�e½Hþ�e
Ka
ð10Þ
Therefore
Table 1 kw Values for
substitution of Fe(stz)3 by ppy
at 35 �C
a Data from ref. [4]
[Fe(stz)3] 9 105
(mol dm-3)
[ppy] 9 103
(mol dm-3)
pH l kw 9 104 (mol dm-3)
PDTS PPDTS
Bpy Terpy Phena Bpy Terpy Phena
1.0 1.0 4.0 0.1 1.12 1.02 1.05 0.74 0.71 0.71
2.0 1.0 4.0 0.1 1.14 1.03 1.03 0.72 0.72 0.71
3.0 1.0 4.0 0.1 1.14 1.05 1.01 0.75 0.74 0.69
4.0 1.0 4.0 0.1 1.15 1.04 1.04 0.73 0.72 0.69
5.0 1.0 4.0 0.1 1.14 1.01 1.06 0.75 0.71 0.71
6.0 1.0 4.0 0.1 1.11 1.06 1.03 0.76 0.70 0.71
7.0 1.0 4.0 0.1 1.12 1.04 1.04 0.74 0.73 0.70
8.0 1.0 4.0 0.1 1.14 1.04 1.03 0.73 0.72 0.72
2.0 0.5 4.0 0.1 1.06 1.01 1.00 0.70 0.67 0.65
2.0 1.5 4.0 0.1 1.18 1.04 1.10 0.74 0.77 0.72
2.0 2.0 4.0 0.1 1.20 1.08 1.19 0.78 0.78 0.76
2.0 2.5 4.0 0.1 1.24 1.11 1.21 0.80 0.79 0.77
2.0 3.0 4.0 0.1 1.25 1.13 1.27 0.83 0.86 0.78
2.0 3.5 4.0 0.1 1.32 1.19 1.37 0.84 0.89 0.79
2.0 4.0 4.0 0.1 1.33 1.20 1.41 0.87 0.90 0.81
2.0 4.5 4.0 0.1 1.39 1.26 1.46 0.88 0.92 0.86
2.0 1.0 3.6 0.1 1.10 1.03 1.03 0.71 0.69 0.69
2.0 1.0 3.8 0.1 1.11 1.03 1.03 0.72 0.70 0.70
2.0 1.0 4.2 0.1 1.20 1.05 1.04 0.76 0.74 0.72
2.0 1.0 4.4 0.1 1.22 1.06 1.05 0.84 0.74 0.74
2.0 1.0 4.6 0.1 1.27 1.06 1.07 0.87 0.76 0.77
2.0 1.0 4.8 0.1 1.54 1.10 1.08 0.91 0.86 0.78
2.0 1.0 5.0 0.1 1.65 1.18 1.02 1.25 0.94 0.86
2.0 1.0 5.2 0.1 2.22 1.28 1.20 1.47 1.16 0.95
2.0 1.0 5.4 0.1 2.53 1.34 1.28 1.94 1.28 1.21
2.0 1.0 5.6 0.1 3.47 1.49 1.35 2.51 1.71 1.37
2.0 1.0 4.0 0.05 1.14 1.03 1.03 0.70 0.72 0.71
2.0 1.0 4.0 0.15 1.13 1.04 1.03 0.74 0.73 0.72
2.0 1.0 4.0 0.20 1.15 1.03 1.04 0.75 0.70 0.70
2.0 1.0 4.0 0.25 1.11 1.02 1.04 0.76 0.72 0.70
2.0 1.0 4.0 0.30 1.17 1.05 1.04 0.76 0.73 0.73
2.0 1.0 4.0 0.35 1.13 1.06 1.03 0.75 0.73 0.73
2.0 1.0 4.0 0.40 1.12 1.04 1.05 0.74 0.72 0.68
Transition Met Chem
123
½ppy�e ¼Ka½ppy�t
Ka þ ½Hþ�eð11Þ
Substituting the value of [ppy]e in Eq. (8), we get
Rate ¼ k1½FeðstzÞ3� þk2Ka½FeðstzÞ3�½ppy�t
Ka þ ½Hþ�eð12Þ
¼ ½FeðstzÞ3� k1 þk2Ka½ppy�tKa þ ½Hþ�e
� �ð13Þ
The terms [ppy]t and [H?]e can be replaced by the initial
concentrations of ppy and hydrogen ion, respectively,
written as [ppy] and [H?]. Hence,
kw ¼ k1 þk2Ka½ppy�Ka þ ½Hþ�
ð14Þ
Since [H?] � Ka, the denominator in the above equation
may be reduced to [H?]. Hence, Eq. (14) can be written as
kw ¼ k1 þk2Ka½ppy�
Hþð15Þ
From the slopes and intercepts of the plots of kw versus
1/[H?], the specific rate constants k1 and k2 at all the four
temperatures of the study have been evaluated and are
presented in Table 2 (Ka values at 35, 45 and 55 �C have
been evaluated using the Perrin equation).
Anipindi [3] found that a plot of kobs values of
FeðTPTZÞ2þ2 –ppy reactions versus log b values of the
corresponding product is linear. Similar observations were
made by Goswami et al. [7] for the divalent metal ion
(Cu2?, Ni2?, Co2? and Mn2?) catalyzed dissociation of
cis-diaquobis(oxalato)chromate(III). The log b values of
FeðbpyÞ2þ3 , FeðterpyÞ2þ2 and FeðphenÞ2þ3 are 17.45, 21.26
and 21.30, respectively [3]. On the basis of these values, it
is expected that the kw values in substitution by terpy/phen
should be equal to or higher than those noticed in substi-
tution by bpy. However, in the present study, it is noticed
that the rate of substitution follows the order bpy [ ter-
py & phen (cf data in Table 1). This may be attributed to
steric factors wherein the approach of the less bulky bpy to
the metal center is more facile than for terpy or phen.
Moreover, terpy and phen are more rigid than bpy.
The mechanism suggested requires that the k1 values
should be the same for a particular substrate irrespective of
the incoming ppy, since the ppy-independent path is
common. The data in the above Table, which includes the
values for phen substitution from ref. [4], are in accordance
with this requirement. The k2 values, however, differ
because of their dependence on the nature of incoming ppy
molecule. The k2 values for terpy or phen substitution are
of same magnitude and lower than those observed in cor-
responding bpy reactions.
Using these specific rate constants (k1 and k2), Ea, DS#
and DG# values for both the reaction paths have been
evaluated and are presented in Table 3. Generally, low
activation energy and large negative entropy values suggest
bimolecular substitutions, as well as a significant associa-
tive character for these reactions. The nature of the entering
ppy influences the activation parameters through the for-
mation of a new bond in the activated state. In the asso-
ciative mechanism, both the incoming ppy and departing
stz are coordinated with the metal center. Hence, the
movement of the molecule is restricted in the activated
complex. This results in a favorable entropy, which is
generally a large negative value.
We have also performed the substitution of FeðPDTÞ2þ3and FeðPPDTÞ2þ3 by bpy and terpy (hereafter PDT and
PPDT are referred to as tz and their iron(II) complexes as
Fe(tz)3). These results are very similar to those obtained in
the corresponding sulfonated analogues, and the reactions
follow the same mechanism. The kw values at 35 �C are
given in supplementary data, S5. The specific rate constants
and activation parameters are presented in Table 4.
The data in Tables 2 and 4 suggest that the specific rate
constants for substitution of Fe(stz)3 are higher than those
of Fe(tz)3. This can be attributed to the weakening of the
iron(II)–nitrogen bonds as a result of the electron-with-
drawing effects of the sulfonate groups. Because of this,
the Ea values in the substitution of sulfonated triazines are
slightly lower for both the dissociative and associative
paths of the reaction. In the dissociative path, the entropies
of activation for substitution of Fe(stz)3 are lower than
those of their unsulfonated analogues, indicating more
Table 2 Specific rate constants and activation parameters for Fe(stz)3–ppy reactions
ppy Dissociative path, k1 9 104 (mol dm-3 s-1) Associative path, k2 9 102 (mol dm-3 s-1)
PDTS PPDTS PDTS PPDTS
25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C
Bpy 0.26 1.08 3.85 12.50 0.15 0.69 2.36 8.65 0.66 1.52 4.00 8.35 0.42 1.20 2.92 7.44
Terpy 0.25 1.02 3.85 12.31 0.16 0.69 2.36 8.64 0.14 0.45 1.17 3.38 0.17 0.89 1.94 6.60
Phena 0.25 1.02 3.85 12.31 0.16 0.69 2.36 8.64 0.10 0.41 1.24 3.75 0.17 0.87 1.89 6.68
a Data from ref. [4]
Transition Met Chem
123
ordered transition states in the case of the former systems.
The large negative values of entropy for the ppy-dependent
path support the assignment of an associative path.
Burgess [8] reported that the reaction
FeðbpyÞ2ðCNÞ2 þ 3 phen! FeðphenÞ2þ3 þ 2 bpyþ CN�
ð16Þ
occurs essentially by a dissociative mechanism. The kinetic
results suggested that the reaction is independent of phen
concentration, i.e., the nature of the entering ligand.
Burgess proposed a dissociative mechanism in which the
rate-determining step is loss of the first bpy. This leads to
the formation of an aquo intermediate Fe(bpy)2(CN)2
(H2O)2, which is of high-spin type. This in subsequent fast
steps gives FeðH2OÞ2þ6 , which is substitution labile (high-
spin type) and reacts with phen to form ferroin in a series of
fast steps:
FeðbpyÞ2ðCNÞ2�!r:d:s:
FeðbpyÞ2ðCNÞ2ðH2OÞ2 þ bpy ð17Þ
FeðbpyÞ2ðCNÞ2ðH2OÞ2�!fast
Fe2þaq �!
phenFeðphenÞ2þ3 ð18Þ
It is well known that when a water molecule coordi-
nates with the iron(II) center, the resulting species
becomes kinetically labile. This may be due to the pos-
sible excitation of the complex from low-spin ðt62gÞ to
high-spin ðt42ge2
gÞ. Broomhead and Dwyer [9] considered
that Fe(terpy)(H2O)2þ3 and FeðphenÞðH2OÞ2þ4 are labile
because they are high-spin compared to the inert low-
spin complexes FeðterpyÞ2þ2 , FeðphenÞ2þ3 and FeðbpyÞ2þ3 .
Acid dissociation [10] of FeðphenÞ2þ3 and FeðbpyÞ2þ3follows a dissociative mechanism, i.e., the loss of the
first ligand is rate determining. After breaking of one Fe–
N bond, a water molecule moves into the coordination
sphere of iron(II). This would lead to a change of spin
from low [FeðphenÞ2þ3 ] to high [FeðphenÞ2ðH2OÞ2þ2 ] and
[FeðphenÞðH2OÞ2þ4 ].
In the present studies, the kinetic results indicate that in the
substitution of Fe(stz)3 by bpy, terpy or phen, the reaction
rates are dependent on the concentration of the incoming ppy.
This suggests that these reactions occur essentially by an
associative mechanism. Evidence in favor of this also comes
from the activation parameters.
The reaction between FeðphenÞ2þ3 and CN- in aqueous
medium was investigated by Margerum and Morgethaler
[11], and their results are consistent with the following rate
equation
kobs ¼ k1½CN�� þ k2 ð19Þ
A similar rate law was derived by Burgess [12] for the
FeðbpyÞ2þ3 –CN- reaction. Both these reactions occur by
dissociative as well as associative paths. Burgess [13]
also made a detailed kinetic study of the reactions of
Table 3 Activation parameters for dissociative path for the substitution of Fe(stz)3 by ppy
Parameter Dissociative path Associative path
PDTS PPDTS PDTS PPDTS
Bpy Terpy Phena Bpy Terpy Phena Bpy Terpy Phena Bpy Terpy Phena
Ea (kJmol-1) 105 105 104 109 108 108 69 86 94 78 85 93
DS# (Jmol-1 K-1) 14 16 14 20 18 18 -62 -23 -39 -39 -22 -35
DG# (Jmol-1 K-1) 99 99 99 100 100 100 86 89 83 87 88 82
a Data from ref. [4]
Table 4 Specific rate constants and activation parameters for the substitution of Fe(tz)3 by ppy
Parameter Dissociative path Associative path
PDT PPDT PDT PPDT
Bpy Terpy Phena Bpy Terpy Phena Bpy Terpy Phena Bpy Terpy Phena
k1 9 105 (mol dm-3 s-1) 1.42 1.41 1.41 0.51 0.51 0.51 – – – – – –
k2 9 103 (mol dm-3 s-1) – – – – – – 6.33 0.79 0.61 2.58 1.15 1.00
Ea (kJmol-1) 112 114 111 114 113 114 83 97 104 84 98 103
DS# (Jmol-1 K-1) 22 27 23 19 18 20 -27 -7 -8 -21 -2 -7
DG# (Jmol-1 K-1) 104 104 104 107 107 107 88 94 111 91 94 111
a Data from ref. [4]
Transition Met Chem
123
tris(3-(2-pyridyl)-5,6-bis(4-phenylsulfonato)-1,2,4-triazine)
iron(II) with hydroxide, cyanide and peroxidisulfate. All
these reactions obey the general rate law:
kobs ¼ k1½L� þ k2 ð20Þ
where L = OH-, CN- or S2O2�8 . The value of k2 is of the
order of 10-4 s-1 at 35 �C for all these reactions. This
value compares well with the acid dissociation constant of
Fe(stz)3. In the substitution of Fe(tz)3/Fe(stz)3 by bpy,
terpy and phen, we obtained a similar rate law and the
dissociative step is independent of the incoming ppy, i.e., it
should be same for all reactions. The specific rate constants
for the dissociative step for the Fe(tz)3 and Fe(stz)3 reac-
tions compare well within experimental error and also
agree well with those reported by Burgess [13].
Thompson and Mottola [14] also studied the kinetics of
complexation of iron(II) with ferrozine in the pH range
2.80 to 5.50 by stopped-flow techniques:
Fe2þ þ 3ferrozine�k1
k�1
FeðferrozineÞ3 ð21Þ
The complex formation and dissociation rates are
3.08 9 1011 dm3 mol-1 s-1 and 4.25 9 10-5 s-1,
respectively, at 25 �C and l = 0.1. The dissociation of
Fe(stz)3 is very slow [14] (i.e., the dissociation rate coef-
ficient (k-1) for the reaction is 4.25 9 10-5 s-1). The k1
values obtained in the present studies are of this order. This
provides further evidence for suggested mechanism.
In general, the substitution reactions at iron(II) centers
are considered to occur by dissociative mechanisms.
However, in the present studies, the specific rate constants
for the associative step (k2) are much greater than the rate
constants for the dissociative path (k1). The low activation
energy and high negative entropy favor an associative
mechanism. It is considered that the substitution of Fe(tz)3
and Fe(stz)3 by ppy also occurs through the formation of
ternary complexes of the type Fe(tz)2(ppy) or Fe(stz)2
(ppy), which have been characterized by ion-exchange
studies in substitution of Fe(stz)3 [4]. The reaction inter-
mediates, FeðPDTSÞ2ðppyÞ2�, FeðPDTSÞðppyÞ2, etc., are
devoid of aqua ligands, and hence, there is no possibility of
formation of high-spin species. Thus, at no stage of the
reaction is the low-spin character of iron(II) disturbed.
Hence, we conclude that whenever the low-spin character
is not disturbed, the substitution reactions at iron(II) occur
by an associative mechanism.
Ion-exchange studies
Reaction mixtures containing 59Fe-labeled FeðPDTSÞ4�3 or
FeðPPDTSÞ7�3 were subjected to ion-exchange separation
to characterize the reaction intermediates, adopting the
same procedure used for phen substitution [4]. The reaction
mixtures were prepared and allowed to stand for about
45 m, and then the reaction was quenched by adding ice.
The solutions were successively passed through Dowex
50W-X8 cation-exchange (H? form) and Dowex 1X8
anion-exchange (NO�3 form) resin columns of dimensions
30 cm length 9 1 cm dia. In the FeðPDTSÞ4�3 –bpy system,
the solutions obtained after passing successively through
cation- and anion-exchange columns showed c-activity,
indicating the presence of neutral iron(II) species. This
species may be Fe(PDTS)(bpy)2. The cation-exchange
resin column was washed with NaNO3 solutions of dif-
ferent concentrations. Gamma activity was noted in the
solutions obtained on elution with 2.0 mol dm-3 NaNO3.
The visible absorption spectrum of this solution showed an
absorption maximum at 522 nm. These results suggest that
this species is dipositive, and its spectral characteristics
correspond to those of FeðbpyÞ2þ3 . The anion-exchange
resin bed was washed with NaNO3 solutions of various
concentrations. The solutions obtained with 2.1 and
4.5 mol dm-3 NaNO3 showed c-activity. These results
suggest that the species eluted with 2.1 mol dm-3 NaNO3
has a charge of 2 [probably Fe(PDTS)2(bpy)2-]. The spe-
cies separated with 4.5 mol dm-3 NaNO3 may be unre-
acted substrate, FeðPDTSÞ4�3 . Its visible absorption
spectrum showed kmax at 562 nm, supporting this assign-
ment. The visible absorption spectra of the neutral species
and that of charge 2 are shown in Fig. 5.
A reaction mixture containing inactive FeðPDTSÞ4�3(1.0 9 10-4 mol dm-3) and bpy (2.0 9 10-3 mol dm-3)
was prepared and allowed to stand at ambient temperature
for about 45 m and then quenched by the addition of ice
cubes. The solution obtained (neutral species) on passing
these reaction mixtures successively through cation- and
anion-exchange resin columns was allowed to stand for
sufficient time to attain ambient temperature. To an aliquot
of this solution, bpy was added such that
[bpy] = 1.0 9 10-3 mol dm-3 and the visible absorption
spectrum recorded after 2 h showed a single peak at
522 nm, corresponding to FeðbpyÞ2þ3 . The visible absorp-
tion spectra of the intermediates and the product solutions
were recorded using a 10 cm quartz cell because of very
low concentrations of iron. The visible absorption spectra
of the neutral species, Fe(PDTS)(bpy)2 and Fe(PDTS)2
(bpy)2-, show absorption maxima at 582 and 580 nm,
respectively (Fig. 5). To three other aliquots (20 ml) of this
dinegative species solution, bpy and acetate buffer were
added such that the solutions were 1.0, 2.0 and
3.0 9 10-3 mol dm-3 in bpy and the pH was 4.0. The
absorbance changes of these solutions (in 10 cm cuvettes)
were recorded with time at 580 nm. Plots of log (At - A?)
versus time, where At and A? are the absorbance values at
Transition Met Chem
123
time t and after completion of the reaction, respectively,
were perfect straight lines, and from their slopes, the
pseudo-first-order rate constants were evaluated. The
solution obtained on elution with 2.1 mol dm-3 NaNO3
[tentatively identified as Fe(PDTS)2(bpy)2-] was allowed
to attain ambient temperature, and to a portion of this
solution, bpy was added and allowed to stand for about 2 h,
whereupon the solution turned to pale orange red. To three
other aliquots of this solution, bpy was added and similar
kinetic runs, by following the changes at 582 nm, as
described above were performed in order to obtain the
pseudo-first-order rate constants. Similar ion-exchange/
kinetic studies were carried out with FeðPPDTSÞ7�3 –bpy
reaction mixtures. The results gave evidence for
FeðPPDTSÞðbpyÞ�2 and Fe(PPDTS)2(bpy)4- intermediates.
These intermediates show absorption maxima at 589 and
591 nm, respectively (Fig. 5). The pseudo-first-order con-
stants for the reactions of these two intermediates with bpy,
evaluated by following the absorbance changes at kmax of
their respective species, are presented in Table 5.
Reaction mixtures, 2.0 9 10-5 mol dm-3 in 59Fe-
labeled FeðPDTSÞ4�3 and 1.0 9 10-3 mol dm-3 in terpy,
were prepared, allowed to stand for about 1 h and then
quenched. These solutions were successively passed
through cation- and anion-exchange resin columns. The
solutions obtained after passing through both exchange
columns showed c-activity, indicating the presence of
neutral iron(II) species, probably Fe(PDTS)(terpy)2. The
cation-exchange resin column was washed with NaNO3
solutions of different concentrations. A purple band slowly
moved down the column on elution with 2.0 mol dm-3
NaNO3 and completely desorbed from the resin column.
The visible absorption spectrum of this solution showed
kmax at 552 nm. These results suggest that this species is
dipositive and its spectral characteristics correspond to
those of FeðterpyÞ2þ2 . The anion-exchange column was also
eluted with NaNO3 solutions of various concentrations.
The solution obtained on elution with 2.0 mol dm-3
NaNO3 showed c-activity, indicating that the species has a
charge of 2 [probably FeðPDTSÞ2ðterpyÞ2�]. Similar ion-
exchange studies were also carried out using 59Fe-labeled
FeðPPDTSÞ7�3 –terpy reaction mixtures. The solutions
obtained on passing the reaction mixture successively
through both exchange columns did not show any c-
activity, confirming the absence of any neutral intermediate
species. The anion-exchange column was eluted with
NaNO3 solutions of different concentrations. The solutions
obtained on elution with 1.0 and 4.0 mol dm-3 showed c-
activity, indicating that the species separated have charges
of 1 [probably FeðPPDTSÞðterpyÞ�2 ] and 4 [probably
FeðPPDTSÞ2ðterpyÞ4�], respectively.
To a solution containing 1.0 9 10-4 mol dm-3 of
inactive FeðPDTSÞ4�3 , terpy was added such that [ter-
py] = 2.0 9 10-3 mol dm-3. This was allowed to stand at
ambient temperature for about 45 m and then quenched by
Fig. 5 Visible absorption spectra of reaction intermediates and final products in Fe(stz)3–bpy reactions. a FeðPDTSÞ4�3 , b Fe(PDTS)2(bpy)2-,
c Fe(PDTS)(bpy)2, d FeðbpyÞ2þ3 , e FeðPPDTSÞ7�3 , f Fe(PPDTS)2(bpy)4- and g FeðPPDTSÞðbpyÞ�2
Transition Met Chem
123
adding ice. The neutral solution obtained on passing the
reaction mixture successively through cation- and anion-
exchange columns was allowed to attain ambient temper-
ature. The solution turned pale pink on attaining ambient
temperature, and its visible absorption spectrum showed a
kmax value at 552 nm. The neutral species may be
FeðPDTSÞðterpyÞ2, which is slowly converted to
FeðterpyÞ2þ2 . The solution obtained from the anion-
exchange column on elution with 2.0 mol dm-3 NaNO3
contains a dinegative ion, probably FeðPDTSÞ2ðterpyÞ2�. It
shows an absorption maximum at 553 nm (Fig. 6). To
three 20 ml aliquots of this solution, terpy and acetate
buffer were added and kinetic runs were performed at
553 nm as described above to obtain the pseudo-first-order
rate constants. Similar ion-exchange results were obtained
with FeðPPDTSÞ7�3 –terpy reaction. The mononegative
species obtained on elution with 1.0 mol dm-3 NaNO3
turned pale pink on attaining room temperature, and its
visible absorption spectrum with kmax at 552 nm was
similar to that of FeðterpyÞ2þ2 . The solution obtained on
elution with 4.0 mol dm-3 NaNO3 showed an absorption
maximum at 556 nm (Fig. 6). To three aliquots of this
solution, terpy and acetate buffer were added and kinetic
runs were carried out by following the absorption changes
at 556 nm in order to evaluate the pseudo-first-order rate
constants. Similar studies have been carried out with
FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 –phen systems, and similar
results to those for the corresponding bpy reactions were
obtained. The spectra of the intermediate complexes are
shown in supplementary data, S6. The rate constants
evaluated with various intermediate species are presented
in Table 5.
Substitution in CTAB medium
We have carried out studies on the effect of various sur-
factants (sodium dodecyl sulfate (SDS), dioctyl sodium
sulfosuccinate or Aerosol OT (AOT), CTAB and Triton X-
100) on the reactions between Fe(tz)3 or Fe(stz)3 and ppy.
Catalysis was observed in Fe(stz)3–ppy reactions in CTAB
medium only. The kinetics of the CTAB-catalyzed reac-
tions were studied under identical conditions to those
employed in aqueous medium, except that in CTAB
medium [ppy] was maintained at 2.0 9 10-4 mol dm-3 as
against 1.0 9 10-3 mol dm-3 in aqueous medium, for the
reasons discussed below. The kw (kobs in micellar media)
values were evaluated by varying the concentrations of
substrate, ppy, pH and ionic strength at the CMC of the
ppy–CTAB solutions. The kw values were also evaluated at
different [CTAB] keeping the concentrations of all otherTa
ble
5R
ate
con
stan
tsfo
rsu
bst
itu
tio
no
fin
term
edia
teco
mp
lex
esis
ola
ted
fro
mio
n-e
xch
ang
em
eth
od
by
pp
yat
35
�C
[pp
y]
91
03
(mo
ld
m-
3)
Rat
eco
nst
ant
91
03
(mo
ld
m-
3)
Feð
PD
TSÞ4�
3F
e(P
DT
S) 2
(pp
y)
Fe(
PD
TS
)(p
py
) 2F
eðP
PD
TSÞ7�
3F
e(P
PD
TS
) 2(p
py
)F
e(P
PD
TS
)(p
py
) 2
Bp
yT
erp
yP
hen
Bp
yT
erp
yP
hen
Bp
yP
hen
Bp
yT
erp
yP
hen
Bp
yT
erp
yP
hen
Bp
yP
hen
1.0
0.1
14
0.1
03
0.1
03
0.4
86
0.1
74
0.8
26
0.8
47
16
.21
0.0
71
0.0
72
0.0
73
0.4
59
0.0
91
0.7
39
0.6
75
10
.05
2.0
0.1
20
0.1
08
0.1
19
0.6
98
0.1
92
0.9
64
1.4
18
20
.78
0.0
79
0.0
78
0.0
77
0.5
62
0.1
01
0.9
31
0.9
03
13
.51
3.0
0.1
25
0.1
13
0.1
27
0.8
89
0.2
09
1.2
21
1.6
34
24
.27
0.0
84
0.0
86
0.0
79
0.6
15
0.1
16
1.1
73
1.0
82
16
.13
Th
era
ted
ata
ind
icat
eth
atth
era
teco
nst
ants
for
the
inte
rmed
iate
–p
py
reac
tio
ns
are
fast
erth
anth
ek w
val
ues
for
the
corr
esp
on
din
gF
e(st
z)3–
pp
yre
acti
on
s.T
hes
eo
bse
rvat
ion
sar
eco
nsi
sten
tw
ith
the
mec
han
ism
sug
ges
ted
Transition Met Chem
123
reactants constant. The kinetic results suggest that the
CTAB-catalyzed reactions are essentially similar to those
in aqueous medium in all respects, indicating that these
substitution reactions follow the same mechanism as sug-
gested for the reactions in aqueous medium. Hence, they
obey a similar rate law:
kw ¼ k1 þk2Ka½ppy�½Hþ�
� �ð22Þ
where k1, k2 and Ka stand for the same terms defined for the
reactions in aqueous medium. The kw values for substitu-
tion by ppy at 35 �C are presented in Table 6, and data
obtained at other temperatures are presented in the sup-
plementary data, S7. The k1 and k2 values at 35 �C for
CTAB media are presented in Table 7.
From a comparison of kw and kw values (Tables 1, 6), it can
be inferred that in CTAB medium the rate of reaction is much
higher than in aqueous medium, even though [ppy] is fivefold
lower. The near-equal values of k1 in aqueous and CTAB
media show that the dissociative path predominates in the bulk
phase. The rate enhancement in CTAB medium is essentially
due to higher k2 values than those observed in the aqueous
medium. This shows that in CTAB medium the reactions
occur only by the associative path. The activation parameters
for both dissociative and associative paths for the Fe(stz)3–
ppy reactions in CTAB medium are presented in Table 8.
The Ea values for the dissociative path are the same in
aqueous and CTAB media, whereas for the associative path,
these values are lower in the latter (cf data in Tables 3, 8).
This shows that the associative path takes place essentially in
the micellar phase. The lowering of Ea values in the asso-
ciative path can be ascribed to lowering of the activation
barrier. The DS# values for the associative path are lower in
CTAB medium, indicating that the transition state is more
ordered in the micellar phase. A comparison of DS# values
for FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 shows that in both
aqueous and CTAB media, higher -DS# values are observed
in the former and this can be ascribed to the bulkiness of the
PPDTS ligand whereby the transition state is more ordered in
FeðPDTSÞ4�3 systems. The near-equal DG# values observed
in aqueous and CTAB media clearly indicate that these
substitution reactions follow essentially the same mecha-
nism in either medium.
Effect of CTAB
The effect of CTAB on the rate of reaction has been studied
at 25, 35 and 45 �C by varying the concentration of CTAB
from 5.0 9 10-5 to 8.0–10-3 mol dm-3 and keeping the
concentrations of all other reactants constant. The kw val-
ues obtained at 35 �C are presented in Table 9. The kw
Fig. 6 Visible absorption spectra of reaction intermediates in Fe(stz)3–terpy reactions. a FeðPDTSÞ4�3 , b Fe(PDTS)2(terpy)2-, c FeðterpyÞ2þ3 ,
d FeðPPDTSÞ7�3 , e Fe(PPDTS)2(terpy)4-
Transition Met Chem
123
values were plotted against [CTAB]. In the FeðPDTSÞ4�3 –
bpy or phen systems, the plot of kw versus [CTAB] shows
that in low concentrations of CTAB the data fit into Pis-
zkiewicz positive cooperativity model [15, 16] and in high
concentrations of CTAB, the data obey Raghvan and
Srinivasan’s model [17]. The kw - [CTAB] profile for the
FeðPDTSÞ4�3 –bpy system is shown in Fig. 7 and that of the
FeðPDTSÞ4�3 –phen is presented in the supplementary data,
S8. According to the Piszkiewicz model, kw is expressed as
a function of concentration of surfactant by
kw ¼kM½D�n þ kWKD
KD þ ½D�nð23Þ
where kM is the maximum rate constant in CTAB medium,
D is detergent, KD = dissociation constant of detergent–
substrate binary species and n = number of detergent
molecules associated with each substrate molecule. This
equation can be rearranged as
logkw � kW
kM � kw
� �¼ n log½D�� log KD ð24Þ
Table 6 kw Values for
substitution of Fe(stz)3 by ppy
at 35 �C in CTAB medium
[Fe(stz)3] 9 105
(mol dm-3)
[ppy] 9 104
(mol dm-3)
pH l kw 9 104 (mol dm-3)
PDTS PPDTS
Bpy Terpy Phen Bpy Terpy Phen
1.0 2.0 4.0 0.1 2.15 1.43 1.23 0.90 0.79 0.76
2.0 2.0 4.0 0.1 1.92 1.52 1.22 0.87 0.77 0.75
3.0 2.0 4.0 0.1 1.89 1.37 1.24 0.88 0.77 0.75
4.0 2.0 4.0 0.1 2.22 1.63 1.24 0.88 0.75 0.76
5.0 2.0 4.0 0.1 2.09 1.47 1.23 0.86 0.78 0.76
6.0 2.0 4.0 0.1 2.16 1.55 1.21 0.90 0.77 0.75
7.0 2.0 4.0 0.1 1.90 1.49 1.25 0.89 0.74 0.75
8.0 2.0 4.0 0.1 2.29 1.37 1.23 0.88 0.76 0.75
2.0 1.0 4.0 0.1 1.48 1.22 1.10 0.78 0.75 7.05
2.0 1.5 4.0 0.1 1.75 1.33 1.20 0.87 0.78 7.28
2.0 2.5 4.0 0.1 2.17 1.51 1.27 0.95 0.82 7.69
2.0 3.0 4.0 0.1 2.24 1.66 1.39 1.00 0.83 7.96
2.0 3.5 4.0 0.1 2.50 1.68 1.43 1.02 0.85 8.14
2.0 4.0 4.0 0.1 2.64 1.83 1.46 1.09 0.87 8.22
2.0 4.5 4.0 0.1 2.73 1.86 1.56 1.15 0.88 8.48
2.0 5.0 4.0 0.1 2.79 1.92 1.65 1.16 0.90 8.58
2.0 5.5 4.0 0.1 3.06 2.04 1.72 1.23 0.94 8.72
2.0 2.0 3.6 0.1 1.59 1.23 1.09 0.75 0.73 0.71
2.0 2.0 3.8 0.1 1.75 1.32 1.18 0.86 0.75 0.72
2.0 2.0 4.2 0.1 2.41 1.92 1.43 0.99 0.86 0.79
2.0 2.0 4.4 0.1 3.31 2.05 1.70 1.17 0.88 0.82
2.0 2.0 4.6 0.1 4.75 2.74 2.28 1.51 0.90 0.92
2.0 2.0 4.8 0.1 6.83 3.37 2.70 1.96 1.04 1.12
2.0 2.0 5.0 0.1 9.44 5.25 3.88 2.46 1.70 1.24
2.0 2.0 5.2 0.1 15.81 7.98 5.75 3.42 1.61 1.60
2.0 2.0 5.4 0.1 20.80 9.88 8.69 5.97 2.47 2.18
2.0 2.0 5.6 0.1 36.61 16.62 12.37 8.23 3.78 2.90
2.0 2.0 4.0 0.05 1.90 1.57 1.22 0.86 0.76 0.73
2.0 2.0 4.0 0.15 2.04 1.40 1.22 0.89 0.77 0.73
2.0 2.0 4.0 0.20 2.19 1.60 1.21 0.86 0.78 0.75
2.0 2.0 4.0 0.25 2.29 1.56 1.23 0.90 0.77 0.73
2.0 2.0 4.0 0.30 1.96 1.60 1.25 0.86 0.75 0.75
2.0 2.0 4.0 0.35 2.10 1.56 1.22 0.87 0.76 0.76
2.0 2.0 4.0 0.40 2.23 1.47 1.23 0.89 0.77 0.76
Transition Met Chem
123
This implies that a plot of log{(kw - kw)/(kM - kw)}
versus log[D] should be linear, and the kinetic data are in
agreement with this (supplementary data, S9). From the
slopes and intercepts of these plots, the values of n and KD
have been evaluated. At high concentrations of CTAB,
kw ¼kw þ kMK1K2½D�n
1þ K1½D�n 1þ K2½S�T� � ð25Þ
where K1 is the binding constant between the surfactant
and substrate, K2 is the equilibrium constant for the
formation of the ternary complex from the binary complex
(of surfactant and substrate) and nucleophile ppy and S is
substrate. Equation (25) can be rearranged as
kw � kW
kw
� �1
½D�n ¼ K1K2
kM
kw
� ��K1 1 þ K2½S�T
� �ð26Þ
Using the value of n obtained from the Piszkiewicz model,
the binding constants for binding of FeðPDTSÞ4�3 to CTAB
were evaluated from the linear plots of {(kw - kw)/kw}
(1/[D]n) versus (kM/kw), where [D] = [CTAB] - CMC
(supplementary data, S10). The binding constants for the
FeðPDTSÞ4�3 –bpy and phen systems are 23.19 9 105 and
5.77 9 105, respectively, at 35 �C.
The kw - [CTAB] profile for the FeðPDTSÞ4�3 –terpy
system (Fig. 8) fits Berezin’s model [18]. According to this
model, the rate expression is
kw ¼kMKSKLC þ kW
ð1þ KSCÞð1þ KLCÞ ð27Þ
where KS is the binding constant between the surfactant
and substrate, KL is the binding constant between the
surfactant and terpy, and C = [CTAB] - CMC. This
equation can be rearranged as
1
kw¼ 1
kMKSKLCþ KS þ KL
kMKSKLCþ C
kM
ð28Þ
The binding constants between CTAB and FeðPDTSÞ4�3(KS) were evaluated from the plots of 1/kw versus C (sup-
plementary data, S11). The KS value at 35 �C is 5.53 9 105.
The kinetic data for the FeðPPDTSÞ7�3 –bpy, terpy and
phen fit systems into the Menger and Portnoy model [19].
The kw versus [CTAB] profile for the FeðPPDTSÞ7�3 –bpy
reaction in CTAB medium is shown in Fig. 8, and the
others are given in the supplementary data. The rate
equation according to the Menger and Portnoy model is
kw ¼kMKSC þ kW
1þ KSCð29Þ
The rearranged form of this equation is
1
kw � kW
¼ 1
KACðkM � kWÞþ 1
kM � kW
ð30Þ
The values of KS are evaluated from the straight line plots
of 1/kw versus 1/C (see supplementary data, S12 for these
plots). The binding constants for the FeðPPDTSÞ7�3 –bpy,
terpy and phen systems are 1.66 9 104, 5.18 9 104 and
8.73 9 103, respectively (Fig. 9).
Micellar effects of bimolecular reactions involving ionic
reactants follow the Hartley principle [20], i.e., cationic
reactions are accelerated by anionic micelles and inhibited by
Table 7 Specific rate constants for Fe(stz)3–ppy reaction in CTAB medium
Ppy Dissociative path, k1 9 104 (mol dm-3 s-1) Associative path, k2 (mol dm-3 s-1)
PDTS PPDTS PDTS PPDTS
25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C 25 �C 35 �C 45 �C 55 �C
Bpy 0.26 1.14 3.71 12.71 0.16 0.69 2.36 8.64 0.51 1.13 2.05 4.10 0.10 0.25 0.50 1.17
Terpy 0.26 1.07 4.00 12.71 0.16 0.70 2.29 8.73 0.29 1.74 1.34 2.91 0.05 0.13 0.33 0.67
Phen 0.25 0.97 3.87 12.04 0.16 0.68 2.41 8.68 0.27 0.71 1.27 2.81 0.06 0.15 0.34 0.69
Table 8 Activation parameters for the substitution of Fe(stz)3 by ppy in CTAB medium
Parameter Dissociative path Associative path
PDTS PPDTS PDTS PPDTS
Bpy Terpy Phen Bpy Terpy Phen Bpy Terpy Phen Bpy Terpy Phen
Ea (kJmol-1) 105 105 105 108 108 108 78 63 63 80 68 68
DS# (Jmol-1 K-1) 16 16 16 18 16 18 -72 -59 -57 -53 -49 -49
DG# (Jmol-1 K-1) 99 99 99 100 100 100 80 76 76 91 80 80
Transition Met Chem
123
cationic micelles, and vice versa. Counterions are effectively
concentrated at surfaces of ionic micelles where they partially
neutralize the head group charges. Polar substrates are located
at micellar surfaces, so it is reasonable 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 [21]. The rate enhancement in CTAB medium is not
only due to lowering of the activation barrier, but also due to
increased encounter probability [22] between the reactants.
Micellar catalysis is often rationalized in terms of bringing
together the reactants in the Stern layer surrounding the
micellar surface. It is likely that the anionic substrate mole-
cules, FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 , are attracted to the
micellar surface due to ion–ion interactions. The accumula-
tion of reactants in a small volume of micellar surface will
enhance the encounter probability compared to that in the bulk
homogeneous solution, resulting in enhancement of the
reaction rate in CTAB medium. The binding constants
increase with increasing temperature. The CTAB and other
molecules are highly hydrated at lower temperatures. How-
ever, as the temperature increases, the increased thermal
motion of the ions and molecules destabilizes the hydration
structure, resulting in the loss of water molecules. This will
increase the interaction between substrate molecules and
CTAB, giving higher binding constants at higher tempera-
tures. The higher binding constants obtained for the
FeðPDTSÞ4�3 –ppy reactions compared to the FeðPPDTSÞ7�3system can be ascribed to the small size of FeðPDTSÞ4�3 .
Attempts to correlate the kw values and binding constants were
futile.
In CTAB medium, the binding of substrate to quaternary
ammonium ion is essentially due to hydrophobic and
electrostatic interactions. Even though the ppy molecules
are neutral, the electrostatic contribution to the binding
from the positively charged quaternary ammonium head
group and p-electron-rich ppy is significant. We expected
Fig. 7 Profile of kw versus [CTAB] for the Fe(PDTS)4�3 –bpy reaction
at 35 �C. ½FeðPDTSÞ4�3 � = 2.0 9 10-5 mol dm-3, [bpy] = 2.0 9
10-4 mol dm-3, pH = 4.0, l = 0.1
Fig. 8 Profile of kw versus [CTAB] for the FeðPDTSÞ4�3 –terpy
reaction at 35 �C. ½FeðPDTSÞ4�3 � = 2.0 9 10-5 mol dm-3, [ter-
py] = 2.0 9 10-4 mol dm-3, pH = 4.0, l = 0.1
Table 9 kw Values for substitution of Fe(stz)3 by bpy in CTAB
medium at 35 �C [Fe(stz)3] = 2 9 10-5 mol dm-3,
[ppy] = 1 9 10-4 mol dm-3, pH = 4.0 and l = 0.1
[CTAB] 9 103
(mol dm-3)
kw 9 104 mol dm-3
PDTS PPDTS
Bpy Terpy Phen Bpy Terpy Phen
0.80 0.616 1.230 0.705 0.588 0.458 –
0.84 – 1.410 – – 1.023 –
0.88 – 1.110 – – 0.697 –
0.90 – 0.936 – – 0.605 –
0.92 – 0.942 – – 0.441 –
0.94 – 0.946 – – 0.248 –
0.96 – 0.957 – – 0.187 –
1.00 0.765 0.964 – 0.765 0.143 0.575
1.10 0.828 0.982 – 0.837 0.107 –
1.20 0.871 – 0.887 0.879 0.098 0.625
1.40 1.085 – 0.976 0.748 – 0.672
1.60 1.217 – 1.075 0.628 – 0.721
1.80 1.274 – 1.172 0.538 – 0.751
2.00 1.392 – 1.270 0.490 – 0.711
3.00 1.590 – 1.636 0.465 – 0.514
3.50 1.749 – 1.800 0.464 – 0.506
5.00 – – – 0.460 – 0.485
Transition Met Chem
123
micellar catalysis for substitution of FeðPDTÞ2þ3 and
FeðPPDTÞ2þ3 by ppy in the anionic surfactants SDS and
AOT, but surprisingly, no catalysis was observed. This
may be due to the fact that the ppy molecules are not
soluble in these surfactants, even though cationic
FeðPDTÞ2þ3 and FeðPPDTÞ2þ3 bind to the anionic surfactant
by electrostatic interactions. Summarizing, the large kw
values observed in CTAB solutions can be attributed to the
concentration of both reactants, Fe(stz)3 and ppy, in the
small volume of the micelles.
The substitution of FeðPDTSÞ4�3 and FeðPPDTSÞ7�3 by
bpy, terpy or phen in aqueous medium was carried out at
[ppy] = 5.0 9 10-4 to 4.5 9 10-3 mol dm-3. At these
[ppy], the reactions are complete in about 6 h at ambient
temperature. At lower [ppy], e.g., about 2.0 9 10-4, the
reaction takes not less than 24 h for quantitative comple-
tion, whereas in CTAB medium, the reactions are complete
in about 2 h. Under the former experimental conditions, no
significant change in the reaction rates was noticed in
CTAB medium. As already discussed, the catalysis of these
substitution reactions in CTAB medium is not only due to
electrostatic interactions between the cationic quaternary
ammonium ion and anionic iron(II) substrate molecules but
also due to significant binding of the positively charged
quaternary ammonium head group to p-electron-rich ppy,
which results in enhancement of [ppy] in micellar medium.
The absence of catalysis at [ppy] of about 1.0 9 10-3
mol dm-3 is probably due to equal concentrations of ppy in
both bulk aqueous and micellar phases, because of satu-
ration of ppy in the micellar medium. Hence, micellar
catalysis is observed only at low [ppy].
Conclusions
The substitution of iron(II)–triazine complexes by ppy
ligands proceeds via two paths. The ppy-independent path
is common for a particular substrate, irrespective of the
incoming ligand or ppy. The substitution reactions at the
iron(II) center, in general, are considered to occur by a
dissociative mechanism. The low activation energy and
high negative entropy obtained in the present reactions
show that the reactions at the iron(II) center also occur by
an associative mechanism. The substitution of Fe(tz)3 and
Fe(stz)3 by ppy occurs through the formation of ternary
complexes of the type Fe(tz)2(ppy) or Fe(stz)2(ppy). Thus,
at no stage of the reaction is the low-spin character of
iron(II) disturbed. It can be concluded that whenever the
low-spin character is not disturbed, the substitution reac-
tions at iron(II) occur predominantly by an associative
mechanism.
The near-equal values of k1 in aqueous and CTAB
media show that dissociative path predominantly takes
place in the bulk phase. The rate enhancement in CTAB
medium is essentially due to higher k2 values than those
observed in the aqueous medium. The Ea values for the
associative path in CTAB medium are lower than those in
aqueous medium. The near-equal values of DG# for the
reactions in aqueous and micellar media indicate that these
substitution reactions occur essentially by a similar mech-
anism in either medium. The rate enhancement in CTAB
medium is not only due to lowering of the activation barrier
but also due to increased encounter probability between the
reactants and enhancement of ppy concentration in the
micellar medium.
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Transition Met Chem
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