S.V. Nipane et al. / Inorganic Chemistry Communications 14 (2011) 1102–1106
1103
2
.8
Table 1
Effect of concentration of oxidant and perchloric acid on the oxidation of pyridoxine by
−
3
enneamolybdomanganate(IV) at 25 °C. [Pyridoxine]=0.01 mol dm
and I=0.1 mol
−
3
dm
.
1
03 [MnIVMo
9
O
32]6−
10[HClO
4
]
102
s−1
k
obs
mol dm−
3
mol dm−3
2.7
0
0
0
1
1
1
1
1
1
1
1
1
.2
.6
.8
.0
.2
.4
.6
.2
.2
.2
.2
.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.1
0.3
0.5
0.7
1.0
2.5
2.6
2.6
2.6
2.6
2.5
2.5
1.1
2.6
3.9
5.0
7.1
2.6
1.2
1.3
1.4
1.5
02(1/D)
1.6
1.7
1
Fig. 2. Plot of log kobs against (1/D) the oxidation of pyridoxine by enneamolybdomanga-
3
IV
6−
−3
−3
nate(IV) at 25 °C. 10 [Mn Mo
9 32
O ]
=1.2 mol dm , [Pyridoxine]=0.01 mol dm , 10
3
−3
[
HClO
4
]=0.3 mol dm− and I=0.1 mol dm
.
out in order to understand the mechanism of oxidative transforma-
tion of pyridoxine, the nature of interaction and the probable
intermediates.
characteristic of 4-pyridoxic acid, thus confirming 4-pyridoxic acid as
product of reaction [19] as shown in Eq. (1).
h
i
h
i
All chemicals were of reagent grade and double-distilled water
was used throughout the work. A solution of pyridoxine hydrochlo-
ride (vitamin B6)(Hi Media) was freshly prepared by dissolving an
appropriate amount of sample in double-distilled water. Standard
solution of perchloric acid was prepared in double distilled water. The
IV
6−
II
8−
2
Mn Mo O
+ C H11NO3 + H O→2 Mn Mo O
ð1Þ
9
32
8
2
9
32
þ
+
C H NO + 2H
8 9 4
IV
IV
ammonium salt of Mn complex, (NH
4
)
6
[Mn Mo
9
O32] was prepared
The reaction was also studied in presence of added acrylonitrile to
by reported method [18]. The oxidant was characterized by FTIR and
AAS analysis as reported earlier [16,17].
Kinetic measurements were performed on Elico SL-177 spectro-
photometer. The kinetics was followed under pseudo-first order
conditions keeping large excess of [vitaminB6] over [oxidant] at
constant temperature 25± 0.1 °C. The reaction was initiated by mixing
understand the intervention of free radicals. There was no effect of
added acrylonitrile on the reaction and also no precipitate due to the
polymerization of the added acrylonitrile was observed thus con-
firming the absence of any free radical formation in the reaction.
The reaction was carried out under pseudo-first-order conditions
and the plots of log [oxidant] against time were found to be linear in all
the runs up to three half lives of the reaction. The values of pseudo-first-
order rate constants were constant between the range of [oxidant] of
4 6
the previously thermostated solutions of vitaminB6 and (NH )
IV
[
Mn Mo
9
O32] which also contained the required amount of perchloric
−
4
−3
−3
acid and doubly distilled water. The progress of reaction was followed
2.0×10
to 1.6×10 mol dm
(Table 1) therefore, the order in
3
−1
−1
spectrophotometrically at 468 nm (ε=360± 2 dm mol cm ) by
monitoring the decrease in absorbance of oxidant. The pseudo-first
order rate constants were determined from the log [oxidant] versus
time plots and the rate constants were reproducible within ± 5% and
the reaction was studied up to 80% completion.
[oxidant] is unity. The pseudo-first-order rate constants were found to
−
3
increase (Table 2) as [vitamin B6] increases from 5.0×10
5.0×10 mol dm
to
.
−
2
−3
at a constant [oxidant] of 1.2×10 mol dm−3
−3
The order in [vitamin B6] was found to be 0.28 as determined from the
log kobs against log [vitamin B6]. Since, the order in [vitamin B6] was
fractional which indicates the formation of a complex therefore, the
The stoichiometry was studied by keeping concentration of
IV
6−
−3
−3
9 32
[Mn Mo O ]
constant at 1.0×10 mol dm
and varying con-
kinetic data were used to obtain Michaelis–Menten plot of (1/kobs)
−
3
−4
−3
centration of vitamin B6 from 1.0×10 to 2.0×10 mol dm these
reaction mixtures also contained required amount of perchloric acid.
The concentration of unreacted [Mn Mo
against (1/[vitamin B6]). Such a plot was found to be linear with an
intercept supporting the formation of a complex between the
reactants. In order to evaluate thermodynamic parameters the effect
of [vitamin B6] was studied at five different temperatures (Table 2).
IV
6−
9 32
O ]
was determined after
2
4 h spectrophotometrically. The stoichiometry was found to be 2 mol
IV
6−
+
of [Mn Mo
9 32
O ]
per mole of vitamin B6 indicating 4-pyridoxic acid
The effect of [H ] on the reaction was studied by varying the
−
3
as the product. Further, the fluorescence spectra of reaction mixture
after 24 h was examined which shows emission at 418 nm (Fig. 1b)
perchloric acid concentration between 0.01 and 0.1 mol dm
at a
−
3
constant ionic strength of 0.1 mol dm . The rate of reaction is
+
+
accelerated (Table 1) by increase in [H ] and the order in [H ] was
found to be 0.8.
The effects of ionic strength and solvent polarity were studied
IV
6−
keeping concentration of [Mn Mo
9
O
32
]
, vitamin B6 and perchloric
Table 2
− 3
− 3
− 3
acid constant at 1.2 × 10
mol dm
,
0.01 mol dm
and
Effect of concentration of Pyridoxine on the oxidation of pyridoxine by enneamolyb-
−
3
3
IV
6−
−3
−3
0.03 mol dm
respectively at 25 °C. Sodium perchlorate was used to
domanganate(IV). 10 [Mn Mo
9
O
32
]
=1.2 mol dm
4
, 10[HClO ]=0.3 mol dm
and I=0.1 mol dm−
3
.
vary the ionic strength. The rate of the reaction was unaffected with
varying ionic strength and the rate of reaction decreases as percentage of
acetonitrile increases from 0 to 40% v/v. The plot of log kobs vs. (1/D) is
linear with a negative slope (Fig. 2).
02
103
obs s−1
k
1
[
Pyridoxine]
mol dm
−
3
288 K
293 K
298 K
303 K
308 K
Enneamolybdomanganate(IV) is one of the stable heteropolymo-
lybdate containing Mn(IV) as a hetero atom and is noncentrosym-
metric. The central Mn(IV) is surrounded octahedrally by six oxygen
0
0
1
3
5
.5
.8
.0
.0
.0
1.3
1.5
1.7
2.1
2.4
1.5
1.8
2.0
2.4
2.9
1.8
2.1
2.5
3.0
3.5
2.1
2.5
3.0
3.5
3.9
2.4
3.0
3.3
4.0
4.5
atoms with a structure of D
3
symmetry. It shows a characteristic
4
IV
charge transfer band at 468 nm due to A2g → T2g transition of Mn