Kinetics and mechanism of uncatalyzed and ruthenium(III)-catalyzed oxidation of formamidine derivative by hexacyanoferrate(III) in aqueous alkaline medium

Article abstract of DOI:10.1007/s12039-016-1067-3

The catalytic effect of ruthenium(III) on the oxidation of N, N-dimethyl- N^′-(4H-1,2,4-triazol- 3-yl) formamidine (ATF) by hexacyanoferrate(III) (HCF) was studied spectrophotometrically in aqueous alkaline medium. Both uncatalyzed and catalyzed reactions showed first order kinetics with respect to [HCF], whereas the reaction orders with respect to [ATF] and [OH ^?] were apparently less than unity over the concentration range studied. A first order dependence with respect to [Ru^III] was obtained. Increasing ionic strength increased the rate of uncatalyzed reaction and decreased the rate of the catalyzed one Plausible mechanistic schemes of oxidation reactions have been proposed. In both cases, the final oxidation products are identified as aminotriazole, dimethyl amine and carbon dioxide. The rate laws associated with the reaction mechanisms are derived. The reaction constants involved in the different steps of the mechanisms were calculated. The activation and thermodynamic parameters have been computed and discussed. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1067-3

J. Chem. Sci. Vol. 128, No. 5, May 2016, pp. 733–743. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-016-1067-3
Kinetics and mechanism of uncatalyzed and ruthenium(III)-catalyzed
oxidation of formamidine derivative by hexacyanoferrate(III) in aqueous
alkaline medium
a,b
AHMED FAWZY
a
Chemistry Department, Faculty of Applied Sciences, Umm Al-Qura University, 21955 Makkah, Saudi Arabia
Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
b
e-mail: afsaad13@yahoo.com
MS received 4 January 2016; revised 17 February 2016; accepted 21 February 2016
ꢁ
Abstract. The catalytic effect of ruthenium(III) on the oxidation of N, N-dimethyl-N -(4H-1,2,4-triazol-
3-yl) formamidine (ATF) by hexacyanoferrate(III) (HCF) was studied spectrophotometrically in aqueous alka-
line medium. Both uncatalyzed and catalyzed reactions showed first order kinetics with respect to [HCF],
−
whereas the reaction orders with respect to [ATF] and [OH ] were apparently less than unity over the concen-
III
tration range studied. A first order dependence with respect to [Ru ] was obtained. Increasing ionic strength
increased the rate of uncatalyzed reaction and decreased the rate of the catalyzed one Plausible mechanistic
schemes of oxidation reactions have been proposed. In both cases, the final oxidation products are identified
as aminotriazole, dimethyl amine and carbon dioxide. The rate laws associated with the reaction mechanisms
are derived. The reaction constants involved in the different steps of the mechanisms were calculated. The
activation and thermodynamic parameters have been computed and discussed.
Keywords. Kinetics; mechanism; oxidation; ruthenium(III) catalysis; formamidine; hexacyanoferrate(III).
fewcases.16–18 Itisreported thatsuchcomplexesarethe
16
1
. Introduction
most promising complexes for anticancer chemotherapy
Hexacyanoferrate(III) (HCF) is an efficient one-electron
oxidant especially in alkaline media1 due to its high
stability, water solubility and its moderate reduction
potential, 0.45 V, leading to its reduction to hexacya-
noferrate(II), a stable product. It adds less error to
the experimental results, and the data can be analyzed
meticulously to establish the reaction path.
(activity). Furthermore, transition metal ions are used
–5
as catalysts in various oxidation-reduction reactions
19–22
because they have various oxidation states.
Literature survey reveals that no work has been
reported about the kinetics and mechanism of oxidation
of ATF by HCF oxidant. This observation prompted
me to investigate the title reactions. The aims of the
6
Formamidines have achieved considerable impor- present study were to establish the optimum conditions
tance in the last decades because of their biological affecting oxidation of ATF by HCF in aqueous alkaline
7
,8
activity. The N, N-dialkyl derivatives are highly effec- medium, to understand the active species of the reac-
tive acaricides and the most rewarding of these studies tants in such medium, to examine the catalytic activity
III
resulted in the discovery of the acaricide insecticide of Ru catalyst and finally to elucidate plausible oxida-
chlordimeform. Oxidation of formamidines9 is very tion reaction mechanisms.
,10
important, since the N, N-dialkyl formamidine group
is one of the most versatile protecting groups, especially
2
. Experimental
11
in biosynthetic applications. Formamidines form
complexes with transition metal ions1 and such com- 2.1 Materials
2,13
plexes exhibit remarkable biological activity against
certain microbes, viruses and tumors.1
4,15
The presence The chemicals used in the present study were of reagent
of heteroatom in such ligands plays a key role when grade andtheir solutionswerepreparedbydissolving the
coordinated with these metal ions such as 1,2,4-tria- requisite amounts of the samples in doubly distilled
zole derivatives which act as bidentate ligands. Studies on water. The solution of aminotriazole formamidine (ATF)
ruthenium triazole complexes are scarce and limited to a was freshly prepared by dissolving the sample in dou-
733

734
Ahmed Fawzy
bly distilled water. A solution of hexacyanoferrate(III) 3. Results and Discussion
was prepared by dissolving potassium hexacyanofer-
3
.1 Time-resolved spectra
rate(III) (BDH) in water and its concentration was
ascertained by iodometric titration.2
3(a)
Hexacyanofer-
The spectral scans during the oxidation of ATF by HCF
in alkaline medium in the absence and presence of Ru
rate(II) solution was obtained by dissolving potassium
III
hexacyanoferrate(II) (S.D fine Chem.) in water and stan-
dardizing with cerium(IV) solution.2
3(b)
catalyst are shown in figures 1a and 1b, respectively. In
both cases, the scanned spectra indicate gradual decay
of HCF(III) band with time as a result of its reduction
to HCF(II). It can be observed that the decay of HCF
at its absorption maximum during the catalyzed reac-
tion is significantly faster than that of the uncatalyzed
one. Furthermore, a careful examination of the spectral
scans of the ruthenium(III)-catalyzed oxidation con-
firms growth of a new peak located at about 480 nm as
shown in figures 2a,b with appearance of an isosbestic
The solution
of ruthenium(III) chloride was prepared as described
24
previously. Sodium hydroxide and sodium perchlo-
rate were used to vary the alkalinity and ionic strength,
respectively.
2.2 Kinetic measurements
All kinetic runs were followed under pseudo-first order point at 465 nm suggesting formation of an intermediate
III
conditions with [ATF] »[HCF]. The progress of the complex between ATF and Ru .
III
uncatalyzed and Ru -catalyzed reactions was followed
by measuring the decay of HCF absorbance as a func-
tion of time at its absorption maximum (λ = 420 nm),
whereas the other constituents of the reaction mix- 3.2 Stoichiometry and product analysis
tures do not absorb significantly at this wavelength. The
The stoichiometry was determined spectrophotometri-
absorbance measurements were made on a Shimadzu
UV-VIS-NIR-3600 double-beam spectrophotometer.
cally at λ = 420 nm at fixed alkalinity, ionic strength
The oxidation of ATF by HCF in alkaline medium was and temperature. The results indicate the consump-
III
found to proceed with a slow rate in the absence of Ru
tion of two moles of HCF for one mole of ATF to
catalyst and the catalyzed reaction is suggested to occur yield the oxidation products as shown in the following
in parallel paths, with contributions from both catalyzed scheme 1, where the compounds (I), (II) and (III) are
and uncatalyzed paths. Therefore, the total rate constant ATF, aminotriazole and dimethyl amine, respectively.
(
kT) is equal to the sum of the rate constants of the The above stoichiometric relation is consistent with the
uncatalyzed (k ) and catalyzed (k ) reactions, so k = results of product analysis. The product, aminotriazole,
U
C
C
k – k . The rate constants were determined as the gra- was confirmed by spectral data. The IR spectrum
T
U
dients of ln(absorbance) versus time plots. Average val- showed the presence of two bands due to NH
group
2
−1
ues of at least two independent runs of the rate con- near 3415 and 3370 cm . Dimethylamine was
25
stants were taken for the analysis. The rate constants identified by spot test, and carbon dioxide by lime
were reproducible to within 4%.
water.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
HCF
HCF
0.8
0.6
0.4
ATF
ATF
0.2
0.0
200
200
250
300
350
400
450
500
550
250
300
350
400
450
500
550
(
a)
Wavelength, nm
(b)
Wavelength, nm
Figure 1. Time-resolved spectra for: (a) uncatalyzed, and (b) ruthenium(III)-catalyzed oxi-
−
3
−4
dation of ATF by HCF in alkaline medium. [ATF] = 0.03 mol dm , [HCF] = 7.0 × 10 mol
−3
−
−3
−3
◦
III
−6
dm , [OH ] = 0.5 mol dm and I = 1.0 mol dm at 25 C. [Ru ] = 5.0 × 10 mol
−3
dm . Scan time intervals = 5 min.

Uncatalyzed and ruthenium(III)-catalyzed oxidation of formamidine
735
0.05
0.04
0.03
0.02
0.01
0.00
0.06
0.05
HCF
Intermediate
0.04
0.03
0.02
0.01
HCF
0.00
300
440
460
480
500
520
540
560
350
400
450
500
550
600
(a)
(b)
Wavelength, nm
Wavelength, nm
Figure 2. (a) Spectra taken from figure 1b, and b) New spectra where reference cell contains
−
HCF and OH of the same reaction mixture concentrations.
N
N
N
N
H
+
2[Fe(CN)6] + 2OH-
3
-
+ 2[Fe(CN)6] + CO + HNMe
4-
(1)
2
2
N
H
N
NMe2
N
H
NH2
(
I)
(II)
(III)
Scheme 1. Stoichiometric relation for the oxidation of ATF by HCF in alkaline medium.
3.3 Order of reactions
The reaction rates were measured at constant [ATF],
HCF], [Ru ] (for the catalyzed reaction), ionic strength
III
[
The orders of uncatalyzed and catalyzed reactions with
respect to the reactants were determined from the slopes
and temperature but with various concentrations of the
−3
alkali ranging from 0.1 to 0.9 mol dm . The rates
of the reactions were found to increase with increas-
of the log kU and log k versus log(concentration)
C
−
−
plots by varying the concentrations of substrate, alkali
and catalyst, in turn, while keeping other conditions
constant.
ing [OH ]. Plots of k and k versus [OH ] were
U
C
linear with positive intercepts (figure S2 in SI) confirm-
ing fractional-first order dependences with respect to
−
Hexacynoferrate(III) oxidant was varied in both
uncatalyzed and catalyzed reactions in the concentra-
[
OH ].
The ruthenium(III) catalyst concentration was varied
4
−4
−3
−
6
−6
−3
tion range of 3.0 × 10 to 11.0 × 10 mol dm while
other variables such as the concentration of reductant,
ruthenium(III) catalyst and sodium perchlorate were
kept constant. The pH (13.7) and temperature were also
kept constant. It has been observed that the increase
in the oxidant concentration did not alter the oxida-
tion rates of ATF (table 1). This indicates that the oxi-
dation rates are independent of oxidant concentration
and the order of reactions with respect to the oxidant is
confirmed to be one.
from 1.0 × 10 to 9.0 × 10 mol dm at constant
−
[
ATF], [HCF], [OH ] and at constant ionic strength and
temperature. Reaction rate was found to increase with
III
increasing [Ru ] (table 1). The order with respect to
III
[
Ru ] was found to be unity as the slope of log k
C
III
versus log[Ru ] plot (figure S3 in SI).
3.4 Effect of ionic strength
The observed rate constants (k and k ) were deter- The effect of ionic strength on both uncatalyzed
U
C
mined at different initial concentrations of the reductant and catalyzed reactions was studied by varying the
ATF keeping others constant. The rates of both uncat- NaClO concentration. The rate of uncatalyzed reaction
4
alyzed and catalyzed reactions were found to increase was found to increase with increasing ionic strength,
with increasing [ATF] as listed in table 1. The orders whereas the rate of catalyzed reaction decreases with
with respect to [ATF] were found to be 0.79 and 0.71 the increase of ionic strength. Thus, the plot of ln k
U
1
/2
1/2
for uncatalyzed and catalyzed reactions, respectively, as and ln k versus I /(1+I ) are linear with a positive
C
the slopes of plots of log k and log k versus log[ATF] slope for uncatalyzed path and with a negative slope for
U
C
(
figure S1 in Supplementary Information).
the catalyzed one as shown in figure 3.

736
Ahmed Fawzy

Uncatalyzed and ruthenium(III)-catalyzed oxidation of formamidine
737
This indicates that the reactions proceeded through free
radical paths.
-
-
-
-
-
7.2
7.6
8.0
8.4
8.8
3.8 Mechanism of the uncatalyzed oxidation reaction
uncatalyzed reaction
catalyzed reaction
HCF oxidation of ATF was found to occur in a slow
III
rate in the absence of Ru catalyst in alkaline medium.
The reaction has a stoichiometry of 2:1, i.e., two moles
of HCF consumed one mole of ATF. The reaction
exhibits first order dependence with respect to [HCF]
and less than unit order each with respect to both [ATF]
0.50
0.52
0.54
0.56
0.58
0.60
−
−
1/2
1/2
and [OH ]. The less than unit order in [OH ] sug-
gests deprotonation of ATF by the alkali in a pre-
I
/ (1+I )
Figure 3. Debye-Huckel plots for the uncatalyzed and equilibrium step to form a more reactive species of
ruthenium(III)-catalyzed oxidation of ATF by HCF in alka-
the reductant. The rate is not significantly affected
by HCF(II) suggesting absence of any fast equilib-
rium with the products in the rate-determining step.
The latter should be irreversible as is generally the
case for one-electron oxidants and the oxidation takes
place through generation of a free radical as obtained
−3
−4
line medium. [ATF] = 0.03 mol dm , [HCF] = 7.0 × 10
−
3
−
.
−3
◦
III
mol dm and [OH ] = 0.5 mol dm at 25 C. [Ru ] =
6
−3
5
.0 × 10 mol dm
26
3.5 Effect of initially added product
The effect of added hexacyanoferrate(II) product was experimentally. The rate of reaction increases upon
−4
studied also in the concentration range 3.0 × 10 to increasing the ionic strength of the medium suggesting
−
4
−3
1
1.0 × 10 mol dm at fixed concentrations of the that the reaction occurs between two similarly charged
27,28
oxidant, reductant, alkali and catalyst. It was found that
HCF(II) did not have any significant effect on the rates
of reactions.
ions.
On the other hand, the less than unit order in [ATF]
suggests formation of a complex (C ) between HCF
1
and deprotonated ATF species which may be as a result
of an outer sphere association through sodium cation
3
.6 Effect of temperature
−
+
3−
bridge, i.e., between ATF and Na [Fe(CN) ] i,e.,
6
2
−
The rates of the uncatalyzed and ruthenium(III)- [NaFe(CN) ] . Complex formation was proved kinet-
6
catalyzed reactions were measured at five different ically by the non-zero intercept of the 1/kU versus
temperatures, namely, 288, 293, 298, 303 and 308 K 1/[ATF] plot (figure 4) in favor of possible formation
under varying ATF, substrate and alkali concentrations. of an intermediate complex between the oxidant and
29
Both rates increased with the rise of temperature. The substrate. The formed complex (C ) was slowly
1
activation parameters of the rate constants of the slow decomposed in the rate-determining step giving the
steps of both uncatalyzed and ruthenium(III)-catalyzed initial oxidation products as the substrate radical
.
reactions (k and k ) along with thermodynamic param- (ATF ) and HCF(II). The substrate radical reacts with
1
2
eters of the equilibrium constants involved in the reac- another HCF species in a subsequent fast step to yield
tion mechanisms were evaluated and were listed in an intermediate product, 1,1-dimethyl-2-hydroxy-3-
tables 3.
(4H-1,2,4-triazol-3-yl) formamidine (or 1,1-dimethyl-
-(4H-1,2,4-triazol-3-yl) urea). In a further fast step,
3
the intermediate product is hydrolyzed to give the
final oxidation products as given in Scheme 2. Since
3.7 Polymerization test
The involvement of free radical species in the reactions scheme 2 is in accordance with the principle of non-
was assayed by a polymerization test. A known quan- complementary oxidations taking place in a sequence
tity of acrylonitrile monomer was added to the reaction of one-electron steps, the reaction between ATF and
mixtures in an inert atmosphere, with the result of for- HCF would afford a radical intermediate as obtained
mation of white precipitates in the reaction mixtures experimentally. Scheme 2 depicts the following
suggesting presence of free radicals during reactions. equations:
When the experiments were repeated in the absence of
The relationship between the reaction rate and the
ATF under similar conditions, the tests were negative. substrate, oxidant and alkali concentrations can be

738
Ahmed Fawzy
N
N
N
N
H
H
_
K
+
OH
_
+ H O
-
2
N
H
N
NMe2
N
N
NMe
2
(
(
2)
3)
_
(
ATF )
N
N
N
N
H
H
K1
_
N
+
2-
-
N
-
+ Na [Fe(CN) ]
N
NMe2
6
N
NMe
2
_
+
2-
Na Fe(CN)6
(
ATF
)
(
C )
1
N
N
-
N
N
H
.
k1
+
3-
(4)
(5)
+ Na [Fe(CN) ]
NMe2
6
N
N
NMe2
N
H
N
slow
+
2-
Na Fe(CN)
6
N
N
N
N
N
OH
.
_
fast
3
-
+ [Fe(CN)6]4-
NMe
2
+
[Fe(CN)6] + OH
N
H
N
N
NMe2
N
H
N
N
OH
N
N
OH
fast
+
H O
+
2
N
H
NMe2
fast
N
H
(6)
(7)
NH2
O
NMe2
OH
CO2
+ HNMe2
O
NMe2
Scheme 2. Mechanism of the uncatalyzed oxidation of ATF by HCF in alkaline medium.
Table 2. Values of the rate constants of the slow step (k1) and the equilibrium constants
(K and K1) in the uncatalyzed oxidation of ATF by HCF in alkaline medium at different
temperatures.
Temperature (K)
298
Constant
288
293
303
313
4
−1
)
1
1
1
0 k1(s
3.11
5.11
4.87
4.88
6.98
4.21
6.51
8.61
3.38
9.30
11.72
2.44
11.69
19.96
1.15
2
3
−1
0 K(dm mol
)
−
2
3
−1
0
K1(dm mol
)
Table 2a. Activation parameters associated with the slow step (k1).
ꢂ
=
−1 −1
ꢂ
=
−1
ꢂ
=
−1
ꢂ
=
−1
−1 −1
ꢀ
S J mol
K
ꢀH kJ mol
ꢀG 98kJ mol
Ea kJ mol
A mol
s
2
5
−
131.31
56.14
95.27
58.64
2.17 × 10
Table 2b. Thermodynamic parameters associated with the equilibrium constants
Kand K1).
(
−1
−1
−1 −1
ꢀS J mol K
Equilibrium Constant
ꢀH kJ mol
ꢀG 298kJ mol
K
K1
40.95
−33.24
6.08
−14.43
117.01
−63.12
Experimental error ± 4%.

Uncatalyzed and ruthenium(III)-catalyzed oxidation of formamidine
739
deduced (see Appendix A) to give the following rate- obtained as reciprocal of the intercepts of 1/k ver-
U
law equation,
Rate = 1
sus 1/[ATF] plots are listed in table 2. The activation
parameters of k are calculated from the Arrhenius and
−
1
k KK [ATF][HCF][OH ]
1
1
(8) Eyring plots and are listed in table 2a. Values of the
equilibrium constants associated with the mechanistic
−
−
+ K[OH ] + KK [ATF][OH ]
1
scheme 2 (K and K ) at different temperatures are cal-
1
The above rate law is consistent with all the observed
orders with respect to different species. Under pseudo-
first order condition, the rate-law can be expressed by
Eq. (9),
culated from the slopes and intercepts of such plots and
are also inserted in table 2. The thermodynamic param-
eters of K and K are evaluated from van’t Hoff plots
1
and these are also inserted in table 2b.
−
d[HCF]
Rate =
= k [HCF]
(9)
U
dt
Comparing Eqs. (8) and (9), the following relationship
is obtained,
3.9 Mechanism of ruthenium(III)-catalyzed oxidation
−
reaction
k KK [ATF][OH ]
1
1
kU = 1
(10)
−
−
+ K[OH ] + KK [ATF][OH ]
It was reported30,31 that in alkaline solutions ruthe-
nium(III) is mostly present as the hydroxylated species
in the form of [Ru(H O) OH] . The reaction between
1
Equation (9) can be verified by rearranging to Eqs.
11) and (12),
(
2+
2
5
ꢀ
ꢁ
HCF and ATF in alkaline medium in the presence
of traces of ruthenium(III) catalyst is similar to the
uncatalyzed reaction with respect to stoichiometry and
reaction orders. The reaction is first order with respect
to ruthenium(III) catalyst. In the contrary to the uncata-
12) lyzed reaction, increasing ionic strength was found to
1
1
1
1
1
=
+
+ k (11)
−
kU
k KK [OH ] k1K1 [ATF]
1
1
1
ꢀ
ꢁ
1
kU
1
1
1
1
=
+
+
−
k KK [ATF] [OH ] k K [ATF] k1
1
1
1
1
(
Equations (11) and (12) require that plots of 1/k
decrease the rate. The observed less than unit order with
U
−
versus 1/[ATF] at constant [OH ], and 1/k versus respect to [ATF] presumably results from a complex
U
−
1
/[OH ] at constant [ATF] to be linear with positive formation between the substrate ATF and the catalyst
intercepts and these are found to be so as shown in ruthenium(III) in a pre-equilibrium step before the
figures 4 and 5, respectively. The values of the rate reaction with the oxidant. Spectral evidence for the
constant of the slow step (k ) at different temperatures complex formation was obtained from UV–Vis spectra
1
2
2
1
1
5
0
5
0
5
0
4
0
288 K
288 K
293 K
298 K
303 K
308 K
2
2
3
3
93 K
98 K
03 K
08 K
30
2
1
0
0
0
0
20
40
60
80
100
0
2
4
6
8
10
3
-1
-
3
-1
1
/[ATF], dm mol
1/[OH ], dm mol
−
Figure 4. Plots of 1/kU versus 1/[ATF] for the uncatalyzed Figure 5. Plots of 1/kU versus 1/[OH ] for the uncat-
oxidation of ATF by HCF in alkaline medium at different alyzed oxidation of ATF by HCF in alkaline medium at dif-
−
4
−3
−
−4
−3
temperatures. [HCF] = 7.0 × 10 mol dm , [OH ] = ferent temperatures. [HCF] = 7.0 × 10 mol dm , [ATF]
−3
−3
−3
−3
.
0
.5 mol dm and I = 1.0 mol dm
.
= 0.03 mol dm and I = 1.0 mol dm

740
Ahmed Fawzy
shown in figures 1b and 2a,b. The formation of the com- Therefore,
plex was also proved kinetically by a non-zero intercept
III
−
Rate
k KK [ATF][Ru ][OH ]
III
2
2
−
of [Ru ]/k versus 1/[ATF] plot (figure 6).
C
kC =
=
−
(
[HCF]
1+K[OH ] + KK [ATF][OH ]
2
In view of the above-mentioned aspects, deproto-
21)
III
nated ATF is suggested to combine with Ru species,
Equation (21) can be rearranged to the following
forms, which are suitable for verification,
2+
[
Ru(H O) OH] , to form an intermediate complex
2 5
C prior to the rate-determining step. Then, the oxi-
2
ꢀ
ꢁ
III
dant HCF attack the formed complex in a slow (rate-
determining) step to yield ATF free radical and HCF(II)
[Ru ]
1
1
1
1
=
+
+
−
k
C
k KK [OH ] k2K2 [ATF] k2
2
2
III
with regeneration of the catalyst Ru . The radical
(22)
1
ꢀ
ꢁ
intermediate reacts with another mole of the oxi-
dant in a subsequent fast step to yield the final oxi-
dation products. The results are accommodated in
scheme 3.
The suggested mechanism leads to the following rate
law expression,
III
[
Ru ]
1
1
1
=
+
+
(
−
k
k KK [ATF] [OH ] k K [ATF] k2
C
2
2
2
2
23)
III
Regarding Eqs. (22) and (23), plots of [Ru ]/k versus
/[ATF] at constant [OH ] and [Ru ]/k versus
/[OH ] at constant [ATF] should be linear with posi-
C
−
III
1
1
C
−
III
−
tive intercepts. The experimental results satisfied these
requirements as shown in figures 6 and 7, respec-
tively. The values of the rate constant of the slow step
k KK [ATF][HCF][Ru ][OH ]
2
2
Rate = 1
)
(19)
−
−
+ K[OH ] + KK [ATF][OH ]
2
(
k ) at different temperatures obtained as reciprocal of
Under pseudo-first order condition,
2
III
intercepts of [Ru ]/k versus 1/[ATF] plots are listed
C
in table 3. The activation parameters of k are cal-
2
−
N
d[HCF]
culated and are inserted in table 3a. Also, the plots
Rate =
= k [HCF]
(20)
C
III
−
dt
of [Ru ]/k
C
versus 1/[OH ] yield straight lines with
N
N
N
H
H
_
K
+
OH
_
N
+ H O
-
2
N
H
N
NMe
2
N
NMe
2
(13)
_
(
ATF )
N
N
N
N
N
H
H
H
K2
_
N
2+
-
-
+ [Ru(H O) OH]
+ H O
(14)
(15)
N
NMe
2
5
N
N
NMe
2
2
2
_
2+
[
Ru(H O) OH]
2 4
(
ATF
)
(
C )
2
N
-
N
N
k2
.
+ [Fe(CN)6]4-
+ [Fe(CN) ]3 + H O
-
N
N
NMe
2
6
2
slow
N
N
NMe2
2+
H
+ [Ru(H O) OH]
2+
2
5
[
Ru(H O) OH]
2
4
N
N
N
N
N
OH
.
_
fast
-
+ [Fe(CN)6]4-
+
[Fe(CN) ]3 + OH
6
N
H
N
NMe
2
N
H
N
NMe
2
(16)
N
OH
N
N
OH
fast
+
H O
+
2
N
H
N
NMe
2
N
H
NH2
O
NMe
(17)
(18)
2
OH
fast
CO2 + HNMe2
O
NMe2
Scheme 3. Mechanism of ruthenium(III)-catalyzed oxidation of ATF by HCF in alkaline medium.

Uncatalyzed and ruthenium(III)-catalyzed oxidation of formamidine
741
ꢂ
=
their slopes and intercepts equal to 1/k KK [ATF] and
The obtained large negative values of ꢀS (listed in
/k K [ATF]+1/k , respectively Thus, the values of tables 2 and 3) suggest compactness of the formed com-
2
2
1
2
2
2
the equilibrium constants associated with the mechanis- plexes and such complexes are more ordered than the
3
2
tic scheme 3 (K and K ) at different temperatures as reactants due to loss of degrees of freedom. Also, the
2
ꢂ
=
well as their thermodynamic parameters of K and K2 obtained values of ꢀS are within the range of radical
are evaluated and are also listed in table 3b.
reactions.
3
2
2
1
1
00
50
00
50
00
3
50
300
50
200
2
2
2
3
3
88 K
93 K
98 K
03 K
08 K
288 K
293 K
298 K
303 K
308 K
2
1
1
50
00
50
50
0
0
0
20
40
60
80
100
0
2
4
6
8
10
3
-1
-
3
-1
1
/[ATF], dm mol
1/[OH ], dm mol
III
III
−
Figure 6. Plots of [Ru ]/kC versus 1/[ATF] for the Figure 7. Plots of [Ru ]/kC versus 1/[OH ] for the
ruthenium(III)-catalyzed oxidation of ATF by HCF in alka- ruthenium(III)-catalyzed oxidation of ATF by HCF in alka-
−4
−4
line medium at different temperatures. [HCF] = 7.0 × 10
line medium at different temperatures. [HCF] = 7.0 × 10
−3
−
−3
−3
−3 −3 −3
mol dm , [OH ] = 0.5 mol dm and I = 1.0 mol dm
.
mol dm , [ATF] = 0.03 mol dm and I = 1.0 mol dm
.
Table 3. Values of the rate constants of the slow step (k2) and the equilibrium constants
(
K and K2) in the ruthenium(III)-catalyzed oxidation of ATF by HCF in alkaline medium at
different temperatures.
Temperature (K)
298
Constant
288
293
303
313
k2(s 1)
−
19.97
6.74
8.44
31.31
7.99
7.27
43.17
10.09
6.11
57.89
11.97
4.36
73.74
15.88
2.28
2
3
−1
1
0 K(dm mol
)
−
2
3
−1
10
K2(dm mol
)
Table 3a. Activation parameters associated with the slow step (k2).
ꢂ
=
−1 −1
ꢂ
=
−1
ꢂ
=
−1
ꢂ
=
−1
−1 −1
ꢀ
S J mol
K
ꢀH kJ mol
ꢀG 98kJ mol
Ea kJ mol
A mol
s
2
9
−
62.21
42.20
60.73
43.69
9.71 × 10
Table 3b. Thermodynamic parameters associated with the equilibrium constants (K and
K2).
o
−1
o
−1
o
−1 −1
Equilibrium Constant
ꢀH kJ mol
ꢀG 98kJ mol
ꢀS J mol K
2
K
K2
31.19
−31.18
5.70
−15.89
85.54
−51.29
Experimental error ± 4%.

742
Ahmed Fawzy
4
. Conclusions
Substituting Eqs. (A7) and (A9) into Eq. (A4) (and
omitting ‘T’ and ‘F’ subscripts) leads to,
ꢁ
The reaction between N, N-dimethyl-N -(4H-1,2,4-
triazol-3-yl) formamidine and hexacyanoferrate(III) in
alkaline medium was found to proceed with a slow
−
k KK [ATF][HCF][OH ]
1
1
−
Rate = (
1 + K[OH ])(1 + KK [ATF][OH ])
−
1
III
(A10)
rate in the absence of Ru catalyst. Plausible mech-
Under pseudo-first order condition, the rate-law can
be expressed by Eq. (A11),
anistic schemes of oxidation reactions in the absence
and presence of the catalyst have been proposed. The
final oxidation products are identified as aminotriazole,
dimethyl amine and carbon dioxide.
−d[HCF]
Rate =
= k [HCF]
(A11)
U
dt
Comparing Eqs. (A10) and (A11), the following
relationship is obtained.
Supplementary Information (SI)
−
All additional information pertaining to the order with
respect to substrate (figure S1), alkali (figure S2) and
catalyst (figure S3) are given in the supporting informa-
tion, available at www.ias.ac.in/chemsci.
k KK [ATF][OH ]
1
1
k =
U
−
−
−
2
2
1
+K[OH ]+KK [ATF][OH ]+K K [ATF][OH ]
1
1
(A12)
2
− 2
The term K K [ATF][OH ] in the denominator of
1
Eq. (A12) is negligibly small compared to unity in
view of the low concentration of ATF used. Therefore,
this term can be deleted and with rearrangement, the
following equations are obtained.
Appendix A. Derivation of the rate-law expression
for the uncatalyzed oxidation reaction.
ꢀ
ꢁ
1
1
1
1
1
=
=
+
ꢁ
+ k (A13)
According to the suggested mechanistic scheme 2,
−
kU
k KK [OH ] k1K1 [ATF]
1
1
1
ꢀ
−
d[HCF]
1
kU
1
1
1
1
Rate =
= k [C ]
(A1)
1
1
+
+
dt
−
k KK [ATF] [OH ] k K [ATF] k1
1
1
1
1
−
(A14)
[
ATF ]
−
−
K =
, [ATF ] = K[ATF][OH ] (A2)
−
[
ATF][OH ]
[
C1]
−
−
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=
=
, [C ] = K [ATF ][HCF]
1
1
[
ATF ][HCF]
−
KK [ATF][HCF][OH ]
(A3)
1
2
3
4
5
6
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1
1
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(A5)
(A6)
T
F
1
7
595
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F
−
−
1
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1
2
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−
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F
1
1
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HCF]
T
1
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HCF] = 1
(A9)
F
−
+ KK [ATF][OH ]
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