Kinetics and mechanism of oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) complex anion

Article abstract of DOI:10.1007/s11243-010-9434-2

Oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) ([Ag(HIO₆)₂]^5-) in alkaline medium results in demethylation, giving rise to formaldehyde and ethylamine as the oxidation products. The oxidation kinetics has been followed spectrophotometrically in the temperature range of 20.0-35.0 °C, and shows an overall second-order character: being first-order with respect to both Ag(III) and N-methylethylamine. The observed second-order rate constants k′ increase with increasing [OH^-] of the reaction medium, but decrease with increasing the total concentration of periodate. An empirical rate expression for k′ has been derived as: k′ = (k _a + k _b[OH^-])K ₁/{f([OH^-])[IO₄ ^-]_tot + K ₁}, where k _a and k _b are rate parameters, and K ₁ is an equilibrium constant. These parameters have been evaluated at all the temperatures studied, enabling calculation of activation parameters. A reaction mechanism is suggested to involve two pre-equilibria, leading to formation of an intermediate Ag(III) complex, namely [Ag(HIO₆)(OH)(MeNHEt)]^2-. In the subsequent rate-determining steps, this intermediate undergoes inner-sphere electron transfer from the coordinated amine to the metal center via two distinct routes, one of which is spontaneous while the other is mediated by a hydroxide ion.

Full text of DOI:10.1007/s11243-010-9434-2

Transition Met Chem (2011) 36:59–64
DOI 10.1007/s11243-010-9434-2
Kinetics and mechanism of oxidation of N-methylethylamine
by bis(hydrogenperiodato)argentate(III) complex anion
Yukun Guo
Shigang Shen
•
Hongmei Shi
•
Shuying Huo
•
Received: 10 August 2010 / Accepted: 29 September 2010 / Published online: 16 October 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Oxidation of N-methylethylamine by bis(hy-
of which is spontaneous while the other is mediated by a
5
drogenperiodato)argentate(III) ([Ag(HIO ) ] ) in alkaline
-
hydroxide ion.
6
2
medium results in demethylation, giving rise to formalde-
hyde and ethylamine as the oxidation products. The oxi-
dation kinetics has been followed spectrophotometrically
in the temperature range of 20.0–35.0 °C, and shows an
overall second-order character: being first-order with
respect to both Ag(III) and N-methylethylamine. The
Introduction
For the last few years, we have carried out systematic studies
on the oxidations of biologically and medically important
0
observed second-order rate constants k increase with
-
increasing [OH ] of the reaction medium, but decrease
molecules by the bis(hydrogenperiodato)argentate(III)
5-
complex anion ([Ag(HIO ) ] ) in order to find more
6 2
with increasing the total concentration of periodate. An
0
applications for this complex [1–8]. Our studies to date have
led two new areas of application. First, the complex can
oxidize luminol in alkaline medium to generate chemilu-
minescence (CL). Moreover, the CL reaction has been
utilized to determine cortisol [1] and dopamine [2] in bio-
logical samples with extreme sensitivities due to the drastic
enhancement of the CL reaction by trace amounts of these
analytes. Second, the complex has the potential to modify
some peptides/proteins and some drugs [3–5].
empirical rate expression for k has been derived as:
0
-
k = (k ? k [OH ])K /{f([OH ])[IO ]tot ? K }, where
-
-
a
b
1
4
1
k and k are rate parameters, and K is an equilibrium
a
b
1
constant. These parameters have been evaluated at all the
temperatures studied, enabling calculation of activation
parameters. A reaction mechanism is suggested to involve
two pre-equilibria, leading to formation of an intermediate
2
-
Ag(III) complex, namely [Ag(HIO )(OH)(MeNHEt)] . In
6
the subsequent rate-determining steps, this intermediate
undergoes inner-sphere electron transfer from the coordi-
nated amine to the metal center via two distinct routes, one
These and other studies [3–8] have provided kinetic
data and mechanistic pictures, which in turn can help us
understand the oxidation reactions in depth. In the present
work, we have chosen N-methylethylamine (MeNHEt) as a
reductant and have undertaken a kinetic and mechanistic
investigation in continuation of our research interest in this
Ag(III) complex. The reasons for the choice are twofold.
First, there seems to be no report on the oxidation of sec-
ondary amines by the complex. Second, the MeNHEt moiety
is involved in some pharmaceutical compounds such as the
adrenergic drugs phenylpropylmethylamine [9] and pro-
pylhexedrine [10] and the antihypotensive drug pholedrine
Y. Guo ꢀ S. Huo ꢀ S. Shen (&)
College of Chemistry and Environmental Science, Hebei
University, 071002 Baoding, Hebei, People’s Republic of China
e-mail: shensg@hbu.edu.cn
[
11]. In this paper, the kinetic results for the oxidation of
MENHEt by the Ag(III) complex are reported, and a reac-
tion mechanism is proposed. Some generalizations on the
mechanistic aspects of the Ag(III) oxidations are attempted.
H. Shi
School of Public Health, Hebei Medical University,
0
50017 Shijiazhuang, Hebei, People’s Republic of China
123

6
0
Transition Met Chem (2011) 36:59–64
Experimental
MeNHEt was obtained from Sigma–Aldrich. AgNO3,
KIO , K S O , KOH, KNO and 2,4-dinitrophenylhydrzine
4
2
2
8
3
(
DNPH) were purchased either from Beijing Chemical
Reagent Company or from Tianjin Chemical Reagent
Company. All the reagents were of either analytical or
reagent grade and used as received without further purifi-
cation. All solutions were prepared with doubly distilled
water.
5-
The synthesis of [Ag(HIO ) ]
2
was carried out
6
according to the procedure described in the literature [12].
The electronic spectrum of the Ag(III) complex in aqueous
solution showed two maxima at 362 and 253 nm, which are
in excellent agreement with those reported earlier [13].
Stock solutions were freshly prepared from solid samples
of the synthesized complex and used on the same day.
Their concentrations were determined spectrophotometri-
cally at 362 nm by use of the molar absorptivity of e =
4
-1
-1
1
.26 9 10 M cm [13].
Electronic spectra were recorded on a UV–visible
spectrophotometer (TU-1901, Beijing Puxi Inc., China);
the spectrometer had a cell compartment which was kept at
constant temperature (± 0.2 °C) by circulating water from a
thermostat (BG-chiller E10, Baijing Biotech Inc., Beijing).
Time-resolved spectra and kinetic measurements were also
carried out on the same spectrometer. Mass spectra were
recorded on a Fourier Transform Ion Cyclotron Resonance
mass spectrometer with an Electrospray Ionization mode
(
Apex-Ultra 7.OT, Bruker, Germany).
Kinetic methods
Fig. 1 Time-resolved spectra for the reaction between Ag(III) and
MeNHEt at 25.0 °C and the corresponding kinetic trace at 362 nm.
Reaction conditions: [Ag(III)] = 0.05 mM, [MeNHEt] = 6.0 mM,
Pseudo-first-order conditions, namely [Ag(III)] ꢁ [MeN-
-
-
-
HEt] ꢁ [OH ] and [Ag(III)] ꢁ [IO
]
ꢁ [OH ], were
4
tot
-
-
[
4
OH ] = 0.020 M, [IO ] = 1.0 mM, and l = 0.30 M. a The time-
employed. The reaction was initiated by mixing equal
volumes of the oxidant and reductant solutions which
resolved spectra. The first spectrum was recorded about 10 s after
mixing and the time between scans was 60 s. b Kinetic trace at 362 nm
(data points from absorbance readings on the time-resolved spectra)
-
contained the required concentrations of [OH ], [IO ] ,
-
4
tot
and [KNO ]; KNO was used to adjust the ionic strength of
3
3
the reaction medium. Both the oxidant and reductant
solutions were thermally equilibrated for at least 15 min
before mixing. Figure 1a shows time-resolved spectra for
the reaction between the Ag(III) and MeNHEt under the
reaction conditions given in the figure legend. The absor-
bance at 362 nm (absorption peak of the Ag(III) complex)
from Fig. 1a as a function of time is displayed in Fig. 1b
that the reaction is first-order in [Ag(III)]. Thus, the kinetic
traces were followed by monitoring the absorbance
decrease at 362 nm. Pseudo-first-order rate constants, kobsd
were evaluated from the simulations. Values of kobsd
,
,
representing the average of three repeated runs, were
generally reproducible to ± 5% within standard deviations.
(
data points). Simulation of the kinetic trace in Fig. 1b
Analysis of oxidation products
using Eq. 1 resulted in an excellent fit, where A , A and
t
0
A? refer to
A solution containing 0.5 mM Ag(III) and 0.08 M KOH
A ¼ ðA ꢂ A ÞexpðꢂkobsdtÞ þ A1
ð1Þ
absorbances at times t, zero and infinity, respectively. The
t
0
1
(20 mL total) was mixed with a solution containing 5 mM
MeNHEt and 0.08 M KOH (20 mL). After completion of
the reaction, as marked by disappearance of the yellow
excellent fit by the single exponential equation indicates
1
23

Transition Met Chem (2011) 36:59–64
61
color, slight excesses of KI and Na SO were added to the
2
order in [MeNHEt]. Therefore, an overall second-order rate
0
3
reaction mixture, followed by adjustment of the pH to 5–6
with HCl. Under these conditions, periodate was rapidly
law as expressed in Eq. 3 is established, where k stands for
0
the observed second-order rate constants and k = kobsd/
[MeNHEt];
reduced to iodate with formation of I [14], which was
2
quickly reduced to iodide by sulfite. After aging the solu-
tion for a while, precipitates (presumably AgI and AgCl)
appeared. The precipitates were filtered off, and the filtrate
was collected. An excess of saturated DNPH solution in
0
ꢂd[Ag(III)]=dt ¼ kobsd½AgðIIIÞꢃ ¼ k ½MeNHEtꢃ½AgðIIIÞꢃ
ð3Þ
-
The influence of [OH ] on the reaction rate was
-
investigated for [OH ] concentrations between 0.020 and
2
M HCl was added to this filtrate. After aging the mixture
0
-
.150 M; the variation of k as a function of [OH ] is
0
in a refrigerator for several hours, the yellow precipitate
was filtered off and washed with water a couple of times;
then dried in air. The ESI mass spectrum in the positive
mode of the precipitate sample showed a dominant peak of
illustrated in Fig. 3. It is obvious that linear relationships
0
-
between k and [OH ] exist at all temperatures studied
with well-defined positive intercepts. Previously, two kinds
of reactivity trends have been observed for the Ag(III)
?
m/z = 233 (interpreted as MNa = 233). Thus, formal-
0
oxidation reactions: (1) cases where k is essentially inde-
-
pendent of [OH ], as noticed in the oxidations of L-proline
dehyde was one of the oxidation products, resulting from
the demethylation of MeNHEt. Ethylamine is most likely
another oxidation product after demethylation of MeNHEt,
leading to a stoichiometry expressed by reaction (2);
[
6], L-pipecolinate [7], and L-glutamine [8]; (2) cases where
0
-
k increases linearly with the increase in [OH ], as found in
the reactions of L-serine/L-threonine [3], mephenesin [4]
and 3-(4-methoxyphenoxy)-1,2-propanediol [5]. The
oxidation of MeNHEt follows the second of these rela-
ꢂ
Ag(III) þ CH CH NHCH þ OH ! Ag(I)
3
2
3
þ
þ CH3CH2NH2 þ HCHO þ H
ð2Þ
0
-
tionships between k and [OH ], which can be expressed as
0
-
k = a ? b[OH ], where a and b are constants. This
two-term expression may suggest that the oxidation reaction
proceeds via two distinct routes: one that is independent on
Results and discussion
-
Influences of [MeNHEt], [OH ] and [IO ]tot
-
-
-
4
OH , and a second that is facilitated by OH .
Observed rate constants were also measured for
-
IO4 ]tot concentrations between 0.30 and 2.00 mM; plots
Observed pseudo-first-order rate constants, kobsd, were
studied as a function of [MeNHEt] at several temperatures.
Plots of kobsd versus [MeNHEt] are shown in Fig. 2; it is
clear that these plots are linear at all these temperatures and
pass through the origin, suggesting that the reaction is first
[
0
-
of k versus [IO ]tot are shown in Fig. 4. Data analysis
4
0
reveals that good linear correlations between 1/k (or 1/
-
kobsd) and [IO4 ]tot exist at all temperatures studied (figures
0
-
Fig. 3 k as a function of [OH ] at four temperatures. Reaction
Fig. 2 Pseudo first-order rate constants, kobsd, as a function of
-
MeNHEt]. Reaction conditions: [Ag(III)] = 0.03 mM, [OH ] =
-
[
conditions: [Ag(III)] = 0.03 mM, [MeNHEt] = 6.00 mM, [IO
1.0 mM and l = 0.30 M
4
] =
-
0
4
.080 M, [IO ]tot = 1.0 mM, and l = 0.30 M
123

6
2
Transition Met Chem (2011) 36:59–64
simulations gave good curve fittings, providing the K1
values at different temperatures listed in Table 1. Refor-
mulating Eq. 4 leads to Eq. 6.
È
É
k fð½OH ꢃÞ½IO4ꢂꢃ þ K1 =K ¼ k þ k ½OH ꢃ
0
ꢂ
ꢂ
ð6Þ
tot
1
a
b
0
-
By use of the K values in Table 1, values of k {f([OH ])
1
-
0
IO4 ]tot ? K }/K were calculated. Plots of k {f([OH ])
1 1
-
[
[
-
-
IO4 ]tot ? K }/K against [OH ] are linear, as anticipated
1
1
from Eq. 6, and are displayed in Fig. 5. From the plots,
values of k and k were evaluated and are summarized in
a
b
Table 1. Eyring plots for k and k were obtained subse-
a
b
quently, see Fig. 6, and the corresponding activation
parameters were computed, and are also listed in Table 1.
Mechanistic interpretation
It is of interest to notice that the general kinetic features
found in the present reaction system are similar to those
observed in the oxidations of L-serine/L-threonine [3],
mephenesin [4], and 3-(4-methoxyphenoxy)-1,2-propane-
diol [5], although these various reactants are not structurally
0
-
Fig. 4 k as a function of [IO
conditions: [Ag(III)] = 0.03 mM, [MeNHEt] = 6.00 mM, [OH ] =
.10 M, and l = 0.30 M. The solid lines represent the best fit of
4
]tot at four temperatures. Reaction
-
0
-
Eq. 4 to the experimental data, where f([OH ]) = 0.966
not shown). The correlations for the present reaction
resemble those observed in some other oxidation reactions
[
3–5]. The empirical rate expression, i.e. Eq. 4 derived
earlier [3–5], is thus expected to be valid in the present
reaction system, where k and k are rate parameters and K
1
a
b
denotes a pre-equilibrium constant (cf. discussions below).
È
É
0
ꢂ
ꢂ
ꢂ
tot
k ¼ ðka þ kb½OH ꢃÞK1= fð½OH ꢃÞ½IO4 ꢃ þK1
ð4Þ
-
In Eq. 4, f([OH ]) is a fraction and is defined by Eq. 5,
where the subscript e denotes the equilibrium concentration,
and b and b are the known equilibrium constants of
2
3
periodate in alkaline medium [3, 15].
Ã
Â
ꢂ
3ꢂ
=½IO4ꢂꢃ
fð½OH ꢃÞ ¼ H IO
2
6
tot
eꢀ
ꢁ
ꢂ 2
ꢂ
ꢂ 2
¼
Equation 4 was utilized to simulate the relationship
b ½OH ꢃ = 1 þ b ½OH ꢃ þ b ½OH ꢃ
:
ð5Þ
3
2
3
0
-
between k and [IO ]tot and as shown in Fig. 4, by a
4
nonlinear least-squares routine with K and the term
1
-
-
-
0
4 1 1
Fig. 5 Plots of k {f([OH ])[IO ]tot ? K }/K versus [OH ] at four
-
k ? k [OH ] as two adjustable parameters while
a
b
temperatures. Reaction conditions are the same as those described in
Fig. 3
-
f([OH ]) = 0.966 (calculated from Eq. 5) was fixed. The
Table 1 Equilibrium constants,
rate constants, and activation
parameters for oxidation of
MeNHEt by the Ag(III)
-1 -1
s
-2 -1
s
4
t (°C)
k
a
(M
)
k
b
(M
)
1
10 K (M)
2
2
3
3
0.0
5.0
0.0
5.0
0.50 ± 0.05
0.90 ± 0.09
1.21 ± 0.10
13.1 ± 0.6
15.4 ± 1.0
17.5 ± 1.1
5.1 ± 0.2
5.7 ± 0.5
6.3 ± 0.2
6.9 ± 0.4
complex at l = 0.30 M
1.97 ± 0.07
à
21.6 ± 0.9
à
-1
-1
DH
a
= 64 ± 5 kJ mol
DH
b
= 22 ± 2 kJ mol
à
a
-1
= -33 ± 18 J K mol
-1
à
-1
-1
DS
b
DS = -149 ± 6 J K mol
à
Activation parameters
1
23

Transition Met Chem (2011) 36:59–64
63
the coordinated amine to the metal center (typically inner-
sphere electron transfer), is expected to take place for both
rate-determining reactions except that the first case is
spontaneous, whereas the second is facilitated by a
hydroxide ion.
Based on this mechanism, the rate Eq. 7, has been
derived in terms of the total concentration of Ag(III);
ꢂ
ꢂd½AgðIIIÞꢃ ðk1K2 þ k2K2½OH ꢃÞK1½MeNHEtꢃ½AgðIIIÞꢃ
¼
3
ꢂ
dt
½H2IO6 ꢃ þ K1 þ K1K2½MeNHEtꢃ
ð7Þ
Equation 7 can be simplified to Eq. 8 when
e
K [MeNHEt] ꢁ 1, meaning K K [MeNHEt] ꢁ K ;
2
1
2
1
ꢂ
ꢂd½AgðIIIÞꢃ ðk1K2 þ k2K2½OH ꢃÞK1½MENHEtꢃ½AgðIIIÞꢃ
¼
3ꢂ
dt
½H2IO6 ꢃ þK1
e
ð8Þ
When Eq. 8 is compared with Eq. 3, Eq. 9 is obtained.
Fig. 6 Eyring’s plots for the rate parameters k
Eq. 4
a
and k
b
as defined by
ÀÂ
Ã
Á
0
ꢂ
3ꢂ
k ¼ ðk1K2 þ k2K2½OH ꢃÞK1= H2IO6
þK1
ð9Þ
into Eq. 9
Á
e
3
Substitution of [H IO ] = f([OH ])[IO
-
-
-
]
tot
2
6
e
4
correlated. By analogy to the reaction mechanisms proposed
earlier for other oxidations by the Ag(III) complex [3–5], a
reaction mechanism is depicted in Scheme 1 for the present
gives rise to Eq. 10;
À
0
ꢂ
ꢂ
ꢂ
tot
k ¼ ðk1K2 þ k2K2½OH ꢃÞK1= fð½OH ꢃÞ½IO4 ꢃ þK1 :
case in which a pre-equilibrium (denoted by K ) is incor-
1
ð10Þ
porated. In this mechanism, the initial Ag(III) complex,
5
Ag(HIO ) ] , is essentially unreactive and serves only as a
-
If we assume that k K = k and k K = k , Eq. 10 is
1
[
2
a
2
2
b
6
2
2
-
identical to the empirical rate expression given by Eq. 4,
demonstrating that the proposed mechanism is reasonable.
The time-resolved spectra in Fig. 1a show no significant
shift of the 362 nm absorption band, suggesting that the
concentrations of any reaction intermediates could be very
low. This observation supports the assumption that
K [MeNHEt] ꢁ 1. Moreover, the K value at 25.0 °C
precursor for the reactive species, [Ag(HIO )(H O)(OH)]
6
.
2
2
-
In another pre-equilibrium, [Ag(HIO )(H O)(OH)]
2
6
reacts with the amine to form an intermediate complex
2
-
[
Ag(HIO )(OH)(MeNHEt)] , whose possible structure is
6
illustrated in Scheme 1, followed by two parallel rate-
determining reactions. One-step two-electron transfer from
2
1
derived in the present reaction system is identical, within the
experimental errors, to those evaluated from other reaction
systems [3–5, 15], strongly supporting the proposed reaction
mechanism.
_
_
2
5
OH
OH
OH
O
O
O
O
O
O
OH2
K1
3_
I
Ag
Ag
I
+ 2H O
I
Ag
+ H IO6
2
2
O
O
O
O
O
O
OH
O
O
O
_
2
_
2
OH
OH
CH3
O
O
OH2
OH
CH3
K2
O
O
NH
I
+
NH
+ H2O
I
Ag
C2H5
Conclusion
O
O
C2H5
O
O
OH
O
O
The mechanistic picture outlined in Scheme 1 can explain
convincingly all the kinetic observations. This mechanism
is similar to those proposed for Ag(III) oxidations of
L-serine/L-threonine [3], mephenesin [4] and 3-(4-
methoxyphenoxy)-1,2-propanediol [5].We refer to this and
similar mechanisms [3–5] as Type A mechanisms. The
reductants in Type A mechanisms do not appear to have
any common structural features. On the other hand, the
proposed mechanisms for oxidations of L-proline [6],
L-pipecolinate [7], and L-glutamine [8] by the Ag(III)
complex are also similar (we refer to these as Type B
_
2
OH
CH3
O
O
k1
NH
OH
Ag(I) + CH3CH2NHCH2+ + H+
I
Ag
Ag
C H
2
5
O
O
O
_
2
OH
CH3
O
O
_
k2
NH
OH
+
+
OH
Ag(I) + CH CH NHCH + H O
3 2 2 2
I
C2H5
O
O
O
_
fast
+
CH CH NHCH + OH
CH CH NH + HCHO
3 2 2
3
2
2
Scheme 1 Suggested reaction mechanism
123

6
4
Transition Met Chem (2011) 36:59–64
mechanisms), but substantially different from Type A
mechanisms. It is worth mentioning that Type A and Type
B mechanisms cannot be correlated with the structures of
the reductants. In this regard, it is hard to predict at present
if a new Ag(III) oxidation reaction follows Type A or Type
B mechanisms, or neither of them.
3. Shi H, Shen S, Sun H, Liu Z, Li L (2007) J Inorg Biochem
1
4
5
6. Sun H, Shi H, Shen S, Kang W, Guo Z (2008) Chin J Chem
26:615
01:165
. Shen S, Shi H, Sun H (2007) Int J Chem Kinet 39:440
. Shi H, Kang W, Shen S (2009) Transit Met Chem 34:91
7
8
9
. Shi H, Liu S, Shen S, Huo S, Kang W (2009) Transit Met Chem
4:821
. Shen S, Du R, Shi H, Hao Z, Peng S (2009) J Hebei Univ 29:311
Natural Science Edition)
. Sinsheimer JE, Kauhnelian AM (1966) J Pharm Sci 55:1240
3
Acknowledgments This work was carried out under the guidance
of Dr. Tiesheng Shi and was financially supported in part by a
grant from the Natural Science Foundation of Hebei Province
(
1
1
0. Alm S, Jonson S, Karlsson H, Sundholm EG (1983) J Chromatogr
54:179
(
B2006000962).
2
1. R o¨ mhild W, Krause D, Bartels H, Ghanem A, Sch o¨ ning R,
Wittig H (2003) Foren Sci Int 33:101
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