Kinetic and mechanistic studies on the oxidation of pyrrolidine by bis(hydrogenperiodato)argentate(III) complex anion

Article abstract of DOI:10.1007/s11243-012-9655-7

The kinetics of oxidation of pyrrolidine by bis(hydrogenperiodato) argentate(III) complex anion ([Ag(HIO₆)₂]^5-) was studied in alkaline medium, with reaction temperatures in the range of 15.0-30.0 C. The experiments indicated that the oxidation follows an overall second-order reaction, being first-order in both Ag(III) and pyrrolidine. The observed second-order rate constants, k′, decreased with increasing [IO₄ ^-] but increased slightly with increasing [OH ^-]. The influence of ionic strength on the reaction rate was also investigated. The oxidation resulted in oxidative deamination of pyrrolidine, giving 4-hydroxybutyrate as the product. A reaction mechanism is proposed which includes an equilibrium between [Ag(HIO₆)₂]^5- and [Ag(HIO₆)₂(OH)(H₂O)]^2-; these two Ag(III) species are reduced by pyrrolidine in parallel rate-determining steps. The rate equation derived from the proposed mechanism can explain the experimental observations. The rate constants of the rate-determining steps, together with the associated activation parameters, were calculated accordingly.

Full text of DOI:10.1007/s11243-012-9655-7

Transition Met Chem (2013) 38:15–20
DOI 10.1007/s11243-012-9655-7
Kinetic and mechanistic studies on the oxidation of pyrrolidine
by bis(hydrogenperiodato)argentate(III) complex anion
•
Jiong Zhang Shuying Huo Hongmei Shi
•
•
•
Shigang Shen Yanli Shang
Received: 12 July 2012 / Accepted: 1 October 2012 / Published online: 16 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract The kinetics of oxidation of pyrrolidine by
bis(hydrogenperiodato)argentate(III) complex anion ([Ag-
(HIO6)2]5-) was studied in alkaline medium, with reaction
temperatures in the range of 15.0–30.0 °C. The experi-
ments indicated that the oxidation follows an overall sec-
ond-order reaction, being first-order in both Ag(III) and
pyrrolidine. The observed second-order rate constants, k0,
decreased with increasing [IO4-] but increased slightly
with increasing [OH-]. The influence of ionic strength on
the reaction rate was also investigated. The oxidation
resulted in oxidative deamination of pyrrolidine, giving
4-hydroxybutyrate as the product. A reaction mechanism
is proposed which includes an equilibrium between
[Ag(HIO6)2]5- and [Ag(HIO6)2(OH)(H2O)]2-; these two
Ag(III) species are reduced by pyrrolidine in parallel rate-
determining steps. The rate equation derived from the
proposed mechanism can explain the experimental obser-
vations. The rate constants of the rate-determining steps,
together with the associated activation parameters, were
calculated accordingly.
Introduction
In the last few years, studies on bis(hydrogenperioda-
to)argentate(III), [Ag(HIO6)2]5-, have led to two important
findings. Firstly, the Ag(III) complex can react with
luminol or other analytes to generate chemiluminescence;
this property has been utilized to determine analytes such
as hormones and drugs in biological samples with unusu-
ally high sensitivities [1–7]. Secondly, the complex has
shown some potential for the modification of peptides and
drugs [8–10]. At the same time, attention has been paid to
the kinetic and mechanistic aspects of Ag(III) oxidation
reactions in order to understand these processes in depth
[8–15]. So far, the reaction mechanisms for the Ag(III)
oxidations encompass two major types [12]. Briefly, in type
A mechanisms, [Ag(HIO6)2]5- is an oxidant precursor,
while [Ag(HIO6)2(OH)(H2O)]2- is the reactive species;
while in type B mechanisms, both [Ag(HIO6)2(OH)-
(H2O)]2- and [Ag(HIO6)2]5- are reactive, but [Ag(HIO6)2-
(OH)(H2O)]2- has higher reactivity [12]. These two types
of reaction mechanism have no obvious correlations with
the structures of the reductants [12].
Pyrrolidine is widely used as an important drug inter-
mediate [16, 17]; it has been employed for the synthesis of
drugs including procyclidine, bepridil and pyrrolidine
dithiocarbamate [18–22]. On the other hand, pyrrolidine
can also be used as a catalyst [23–27]. This wide range of
applications of pyrrolidine encouraged us to study its
oxidation kinetics in order to find out if its reaction follows
either of the two known mechanisms or a novel mecha-
nism. In this work, we report the kinetic results and
the proposed reaction mechanism for the oxidation of
pyrrolidine by [Ag(HIO6)2]5-. The structure of [Ag-
(HIO6)2]5- and its solution chemistry are well established
[9, 28–31].
J. Zhang ꢀ S. Huo ꢀ S. Shen (&) ꢀ Y. Shang
Key Laboratory of Analytical Science and Technology of Hebei
Province, Key Laboratory of Chemical Biology of Hebei
Province, College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, Hebei Province, People’s
Republic of China
e-mail: shensg@hbu.edu.cn
H. Shi
School of Public Health, Hebei Medical University,
Shijiazhuang 050017, Hebei Province, People’s Republic of
China
123

16
Transition Met Chem (2013) 38:15–20
Experimental
which is interpreted as MK?, where M corresponds to 4-
hydroxybutyric acid. Thus, the reaction stoichiometry can
be expressed by Eq. (1).
Pyrrolidine was purchased from Sigma-Aldrich. KOH,
KNO3, KIO4, NaIO4, AgNO3 and K2S2O8 were obtained
from the Kermel Chemical Reagent Company (Tianjin,
China). These reagents were of analytical grade or better
and were used without further purification. All solutions
were prepared with doubly distilled water.
2AgðIIIÞ þ C4H9N þ 3OHꢂ ! 2AgðIÞ þ HOðCH2Þ3CO2ꢂ
þ NH3 þ 2Hþ
ð1Þ
Bis(hydrogenperiodato)argentate(III) complex anion,
[Ag(HIO6)2]5-
Results and discussion
, was synthesized as Na5[Ag(HIO6)2]
according to the published procedure [28]. Electronic
spectra of aqueous solutions of the Ag(III) complex dis-
played two absorption bands at 362 and 253 nm, in
excellent agreement with those reported earlier [29]. Stock
solutions of the Ag(III) complex, prepared from the solid
compound [28], were used freshly and daily; their con-
centrations were determined spectrophotometrically at
362 nm by use of the known molar absorptivity of
e = 1.26 9 104 M-1 cm-1 [32].
UV–vis spectra were recorded on a UV 4802 Unic
spectrophotometer (Shanghai Instrument Co., Shanghai);
the same machine was used to follow the kinetic traces.
The cell compartment of the spectrophotometer was kept at
constant temperature ( 0.1°C) by circulating water from a
thermostat (BG-chiller E10, Baijing Biotech Inc., Beijing).
An Agilent 1200/6310 ion-trap mass spectrometer with
electrospray ionization (ESI) was used to run the mass
spectra.
Rate constants and kinetic order
Time-resolved spectra for the reaction between
[Ag(HIO6)2]5- and pyrrolidine under a constant set of reac-
tion conditions are displayed in Fig. 1. The absorbance
readings at 362 nm from Fig. 1 as a function of time are
shown in Fig. 2; the observed exponential decay was simu-
lated with Eq. (2) by use of a nonlinear least-squares method,
A ¼ ðA0 ꢂ A Þ expðꢂkobstÞ þ A
ð2Þ
1
1
where A, A0 and A? are the absorbances at times t, zero
and infinity, respectively. The simulation gave a good
quality fit, confirming that the reaction is indeed first-order
in Ag(III). From the fit, the observed first-order rate con-
stant, kobs, was obtained. For each set of concentrations,
three replicate experiments were run and the standard
deviation for kobs was usually less than 5 %.
The influence of [pyrrolidine] on the reaction rate was
studied in the range of 0.30 mM B [pyrrolidine] B 3.00 mM
at several temperatures; the results of these experiments are
shown in Fig. 3. Clearly, all the plots of kobs versus
Kinetic method
The reaction conditions employed were as follows: 10
[Ag(III)] B [pyrrolidine], [IO4-] ꢁ [OH-] and [pyrroli-
dine] ꢁ [OH-]. These conditions should ensure that the
Ag(III) is in the environment of a first-order reaction and
that [OH-] was kept unchanged. Operationally, the oxidant
solution containing a known amount of KIO4 and KOH and
the reductant solution containing a suitable amount of
KNO3 (adjusting the ionic strength) were thermally equil-
ibrated for at least 10 min. Then, the two solutions were
quickly mixed and transferred to the quartz cell, which was
pre-equilibrated in the cell compartment. The kinetic traces
were recorded at 362 nm.
Analysis of oxidation products
A solution of 10 mM pyrrolidine was mixed with an equal
volume of 1 mM Ag(III) solution; both solutions contained
0.02 M KOH. After the reaction was finished, the reaction
mixture was tested with Nessler’s reagent, which indicated
that deamination had occurred. The reaction mixture was
also subjected to mass spectrometric analysis. The mass
spectrum gave a clear and significant peak of m/z = 143
Fig. 1 Time-resolved spectra for reaction between [Ag(HIO6)2]5-
and pyrrolidine under the conditions: [Ag(III)] = 0.05 mM, [Pyrrol-
[OH-] = 0.050 M
-
idine] = 0.50 mM,
[IO4
]
= 1.00 mM,
tot
l = 0.30 M and 15.0 °C. The time between scans was 65 s
123

Transition Met Chem (2013) 38:15–20
17
Table 1 Influences of changing [OH-] and ionic strength (l) on the
observed second-order rate constants at 25.0 °C
[OH-] (M)
l (M)
k0 (M-1 s-1
)
0.030
0.050
0.100
0.150
0.200
0.030
0.030
0.030
0.030
0.030
0.030
0.030
0.30
15.0
15.4
16.8
17.1
17.2
20.2
20.2
18.1
17.3
16.2
15.5
15.0
0.30
0.30
0.30
0.30
0.031
0.051
0.101
0.151
0.201
0.251
0.301
Measured under the conditions of [Ag(III)] = 0.03 mM; [Pyrroli-
-
dine] = 0.50 mM and [IO4
]
= 1.00 mM
tot
Fig. 2 Absorbance at 362 nm as a function of time (data points);
reaction conditions are identical to those in Fig. 1. The solid line was
resulted from the curve fitting of Eq. (2) to the experimental data by a
nonlinear least-squares method
-
Influences of [OH-], ionic strength and [IO4
on the reaction rate
]
tot
The effect of [OH-] on the reaction rate was studied at
25.0 °C in the region 0.030 M B [OH-] B 0.20 M. The
values of k0 as a function of [OH-] are listed in Table 1.
Obviously, the k0 values increase only slightly with
increasing [OH-], and there is no linear relationship
between these parameters. The small change of k0 is
probably caused by a shift in the equilibrium between
[Ag(HIO6)2]5- and [Ag(HIO6)2(OH)(H2O)]2- due to the
change in periodate equilibria when [OH-] is varied, as
discussed below.
The impact of ionic strength on the rate was investigated
at 25.0 °C and in the region 0.031 M B l B 0.301 M.
Values of the second-order rate constant k0 as a function of
l are also listed in Table 1. Generally, when l is increased,
k0 decreases; but the overall decrease is small.
-
In the range of 0.20 mM B [IO4
-
]
B 2.00 mM, the
tot
impact of [IO4
]
on the oxidation rate was examined.
tot
-
Plots of k0 versus [IO4
]
tot
are illustrated in Fig. 4; it is
-
clear that k0 decreases when [IO4
four temperatures.
]
is increased for all
tot
Fig. 3 Pseudo first-order rate constants, kobsd, as a function of
[Pyrrolidine].
Reaction
conditions:
[Ag(III)] = 0.03 mM,
[IO4
]
tot
= 1.00 mM, [OH-] = 0.050 M and l = 0.30 M
-
Periodate anion in aqueous solution has several disso-
ciation equilibria. In the pH range we used in this work
[32–34], the major form of periodate is H2IO63-, whereas
2-
[pyrrolidine] are linear and pass through the origin; therefore,
the reaction is also first-order with respect to pyrrolidine. The
second-order rate law is expressed as Eq. (3), where k0 rep-
resents the observed second-order rate constant.
H3IO6
makes only a minor contribution to the total
-
concentration of periodate [IO4 ]tot. Equations (4a, 4b)
3-
describes the relationship between [H2IO6
]
and
e
-
[IO4 ]tot, where the subscript e represents the equilibrium
concentration, and b2 and b3 are known equilibrium
constants.
ꢂd½AgðIIIÞꢃ=dt ¼ kobs½AgðIIIÞꢃ ¼ k0½pyrrolidineꢃ½AgðIIIÞꢃ
ð3Þ
123

18
Transition Met Chem (2013) 38:15–20
_
_
5
2
OH
OH
OH
O
O
O
O
O
O
OH2
K1
_
3
I
Ag
I
I
Ag
+ 2H2O
+ H2IO6
O
O
O
O
O
O
OH
O
O
O
_
5
OH
I
OH
O
O
O
O
k1
Ag(I) + +CH2CH2CH2CH2NH+
+
I
Ag
Ag
NH
O
O
O
O
O
O
_
2
OH
O
O
OH2
OH
k2
Ag(I) + +CH2CH2CH2CH2NH+
I
+
NH
fast
O
O
O
_
+
+CH2CH2CH2CH2NH+ + OH
+
HOCH2CH2CH2CH-NH2
OH
_
fast
HOCH2CH2CH2CH-NH2 + OH
HOCH2CH2CH2CH-NH2
OH
fast
HOCH2CH2CH2CH-NH2
HOCH2CH2CH2CHO + NH3
_
_
fast
Ag(I) + HOCH2CH2CH2COO + 2H+
Ag(III) + HOCH2CH2CH2CHO + OH
-
Fig. 4 Second-order rate constants k0 as a function of [IO4
]tot at four
Scheme 1 Proposed reaction mechanism
temperatures. Reaction conditions: [Ag(III)] = 0.03 mM, [Pyrroli-
dine] = 0.50 mM, [OH-] = 0.050 M and l = 0.30 M. The solid
curves represent the best fits of Eq. (8) to the experimental data by use
of a nonlinear least-squares method
½H2IO36ꢂꢃe ¼ fð½OHꢂꢃÞ½IOꢂ4 ꢃtot
ð4aÞ
2
2
fð½OHꢂꢃÞ ¼ b3½OHꢂꢃ =ð1 þ b2½OHꢂꢃ þ b3½OHꢂꢃ Þ ð4bÞ
Reaction mechanism
In alkaline medium, Eq. (5) was proposed earlier [34] to
account for the observed dependence of the rate on
-
[IO4
]
tot and has subsequently been used to explain all the
-
relationships between kobs (or k0) and [IO4
]
for the
tot
oxidation kinetics of [Ag(HIO6)2]5- [8–15].
K1
½AgðHIO6Þ ꢃ5ꢂ þ 2H2O ꢀ ½AgðHIO6ÞðOHÞðH2OÞꢃ
2ꢂ
2
þ H2IO36ꢂ
ð5Þ
For some reductants [8–12], [Ag(HIO6)2(OH)(H2O)]2-
was the reactive species, while [Ag(HIO6)2]5- had a
negligible reactivity. In contrast, for some other reductants
[13–15], both [Ag(HIO6)2(OH)(H2O)]2- and [Ag(HIO6)2]5-
were reactive as judged by the relationships between kobs (or
Fig. 5 Eyring’s plots for the rate parameters, k1 and k2, as defined by
Eq. (8)
-
k0) and [IO4 ]tot.
pyrrolidine in parallel, and both attacks are rate-determining
steps. Strong support for this mechanism is provided by the
time-resolved spectra in Fig. 1; the absorption peak of the
Ag(III) complex at 362 nm had no observable shift during the
reaction course, suggesting that complex formation between
Ag(III)andpyrrolidinebeforetherate-determiningstepsisnot
likely. In both rate-determining steps, a carbon–nitrogen bond
in pyrrolidine is probably broken after being attacked by
-
Inthepresentwork,k0 decreaseswhen[IO4
]
tot isincreased,
and moreover, there is a nonlinear relation between 1/k0 and
-
[IO4 ]tot; these two features suggest that both [Ag(HIO6)2-
(OH)(H2O)]2- and [Ag(HIO6)2]5- are reactive toward pyr-
rolidine [13–15]. Thus, an analogous reaction mechanism
is proposed, as described in Scheme 1. In this mecha-
nism, [Ag(HIO6)2(OH)(H2O)]2- and [Ag(HIO6)2]5- attack
123

Transition Met Chem (2013) 38:15–20
19
activation entropies are in line with the usual bimolecular
collisions in the rate-determining steps. Moreover, the slight
variations of the rate constants with both [OH-] and ionic
strength are also coherent with the mechanistic picture. The
present reaction mechanism is similar to those proposed for
L-proline, L-glutamine and DL-pipecolinate [13–15]; in all
these reaction systems, the reactivity of [Ag(HIO6)2]5- is
lower than that of [Ag(HIO6)2(OH)(H2O)]2-. In some other
reaction systems, [Ag(HIO6)2(OH)(H2O)]2- was the reac-
tive species, whereas [Ag(HIO6)2]5- had negligible reac-
tivity [8–12]. Unfortunately, we have not found any
correlations between the substrate structures and the reaction
mechanisms. In this regard, other types of reaction mecha-
nisms could also be possible.
Table 2 Equilibrium constants, rate constants and activation
parameters for oxidation of pyrrolidine by the Ag(III) complex at
l = 0.30 M
t (°C)
k1 (M-1 s-1
)
k2 (M-1 s-1
)
15.0
20.0
25.0
30.0
7.4 0.2
10.2 0.2
12.2 0.4
14.7 0.4
12.0 0.2
15.8 0.2
19.3 0.4
28.0 0.5
DH=1 = 30 3 kJ mol-1
DH=2 = 37 4 kJ mol-1
DS=1 = - 123 12
DS=2 = - 95 12
J K-1 mol-1
J K-1 mol-1
[Ag(HIO6)2]5-/[Ag(HIO6)2(OH)(H2O)]2-, generating a con-
ceivable intermediate namely ?CH2CH2CH2CH2–NH?. A
couple of rapid reactions ensue after formation of this inter-
mediate, leading to deamination and the final product of
4-hydroxybutyrate, as shown in Scheme 1.
Acknowledgments This study was carried out and completed under
the guidance of Dr. Tiesheng Shi and was financially supported in part
by a grant from the Natural Science Foundation of Hebei Province
(F2011201102).
Rate Eq. (6) can be derived from the reaction
mechanism;
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ꢂd½AgðIIIÞꢃ ðk1K1 þ k2½H2IO36ꢂꢃeÞ½Pyrrolidineꢃ½AgðIIIÞꢃ
¼
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ð8Þ
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Conclusion
A detailed kinetic study was undertaken on the oxidation of
pyrrolidine by bis(hydrogenperiodato)argentate(III), and a
reaction mechanism was proposed. In the reaction described
in Scheme 1, [Ag(HIO6)2]5- and [Ag(HIO6)2(OH)(H2O)]2-
are both reduced by pyrrolidine in parallel rate-determining
steps. In fact, the relatively large negative values of the
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