Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1527
2.2 Instruments Used
catalysts in various redox processes has attracted consid-
erable interest [11]. Although the mechanism of catalysis
depends on the nature of the substrate, oxidant and
experimental conditions, it has been shown [12] that metal
ions act as catalysts by one of these different paths such as
the formation of complexes with reactants or oxidation of
the substrate itself or through the formation of free radicals.
Osmium(VIII) (Os(VIII)) and ruthenium(III) (Ru(III))
catalysis in redox reactions involves different degrees of
complexity, due to the formation of intermediate com-
plexes and different states of osmium/ruthenium, etc. The
uncatalysed reaction of oxidation of L-val by DPA has been
studied [13]. We have observed that Os(VIII) and Ru(III)
catalyses the oxidation of L-val by DPA in alkaline medium
in micro amounts. In order to understand the active species
of oxidant and catalyst, to compute the activity of the
catalyst and to propose the appropriate mechanism, the title
reaction is investigated in detail. An understanding of the
mechanism allows the chemistry to be interpreted, under-
stood and predicted.
(i) For kinetic measurements, a Peltier Accessory (tem-
perature control) attached to Varian CARY 50 Bio
UV–Visible spectrophotometer (Varian, Victoria-
3170, Australia) was used.
(ii) For product analysis, Nicolet 5700-FT-IR spectrom-
1
eter (Thermo, USA), 300 MHz H NMR spectropho-
tometer (Bruker, Switzerland) were used.
(iii) For pH measurements ELICO pH meter model LI
120 was used.
2.3 Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of KIO4
as described elsewhere [17]: the mixture of 28 g of KOH
and 23 g of KIO4 in 100 cm3 of water along with 8.5 g
AgNO3 was heated just to boiling and 20 g of K2S2O8 was
added in several lots with stirring and then allowed to cool.
It was filtered through a medium porosity fritted glass filter
and 40 g of NaOH was added slowly to the filtrate,
whereupon a voluminous orange precipitate agglomerates.
The precipitate is filtered as above and washed three to four
times with cold water. The pure crystals were dissolved in
50 cm3 water and heated to 80 °C with constant stirring
thereby some solid was dissolved to give a red solution.
The resulting solution was filtered when it was hot and on
cooling at room temperature, the orange crystals separated
out and were recrystallised from water.
2 Experimental and Methods
2.1 Materials and Reagents
All chemicals used were of reagent grade and Millipore
water was used throughout the work. A solution of L-valine
(S. D. Fine Chem.) was prepared by dissolving an appro-
priate amount of recrystallised sample in Millipore water.
The purity of L-valine was checked by comparing its
melting point 294 °C with the literature data [Lit. m.p.
296 °C]. The required concentration of L-valine was
obtained from its stock solution. The Os(VIII) solution was
prepared by dissolving OsO4 (Johnson Matthey) in
0.50 mol dm-3 NaOH. The concentration was ascertained
[14] by determining the unreacted [Fe(CN)6]4- with stan-
dard Ce(IV) solution in an acidic medium. A standard stock
solution of Ru(III) was prepared by dissolving RuCl3 (S.D.
Fine Chem.) in 0.20 mol dm-3 HCl. The concentration
was determined [15] by EDTA titration. KNO3 and KOH
(BDH) were used to maintain ionic strength and alkalinity
of the reaction, respectively. An aqueous solution of
AgNO3 was used to study the product effect, Ag(I). A stock
The complex was characterized from its UV spectrum,
which exhibited three peaks at 216, 255, and 362 nm.
These spectral features were identical to those reported
earlier for DPA [12]. The magnetic moment study revealed
that the complex is diamagnetic. The compound prepared
was analysed for silver and periodate by acidifying a
solution of the material with HCl [18], recovering and
weighing the AgCl for Ag and titrating the iodine liberated
when excess of KI was added to the filtrate for IO4-. The
stock solution of DPA was used for the required [DPA]
solution in the reaction mixture.
2.4 Kinetic Measurements
The kinetic measurements were performed on a Varian
CARY 50 Bio UV–Visible spectrophotometer. The kinet-
ics was followed under pseudo-first-order condition where
[L-val] ꢁ [DPA] at 25 0.1 °C, unless specified. The
reaction was initiated by mixing the DPA to L-valine
solution which also contained required concentrations of
Os(VIII) or Ru(III), KNO3, KOH and KIO4. The progress
of reaction was followed spectrophotometrically at 360 nm
by monitoring decrease in absorbance due to DPA with the
-
standard solution of IO4 was prepared by dissolving a
known weight of KIO4 (Riedel-de-Haen) in hot water and
used after keeping for 24 h to attain the equilibrium. Its
concentration was ascertained iodometrically [16] at neu-
tral pH maintained using phosphate buffer. The pH of the
medium in the solution was measured by ELICO (LI 120)
pH meter. t-Butyl alcohol (S.D. Fine Chem.) was used to
study the dielectric constant of the reaction medium.
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