ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2011, 8(3), 1102-1107

Kinetic Study on Induced Electron

Transfer Reaction in Pentaamminecobalt(III)

Complexes of αααα-Hydroxy Acids by Pyridinium

Fluorochromate in Micellar Medium

B. MOHAMMED NAWAZ*, K.SUBRAMANI and MANSUR AHMED

PG and Research Department of Chemistry

Islamiah College, Vaniyambadi-635 752, Tamilnadu, India

nawazalam70@gmail.com

Received 24 November 2010; Accepted 22 January 2011

Abstract: Pyridinium fluorochromate (PFC) oxidation of pentaamminecobalt(III)

complexes of α-hydroxy acids in micellar medium yielding nearly 100% of

carbonyl compounds are ultimate products. The decrease in UV-visible

absorbance at λ=502 nm for Co(III) complex corresponds to nearly 100% of the

initial absorbance. The stoichiometry of unbound ligand and cobalt(III) complex

is accounting for about 100% reduction at the cobalt(III) centre. The kinetic and

stoichiometric results have been accounted by a suitable mechanism.

Keywords: Pentaamminecobalt(III) complexes, Induced electron transfer reaction, Triol, PFC.

Introduction

Pyridinium fluorochromate (PFC) is an efficient reagent for oxidation of primary and

secondary alcohols to carbonyl compounds. A large class of organic compounds were

oxidized by PFC has been reported1-5

. Since induced electron transfer in pentaamminecobalt(III)

complexes of α-hydroxy acids with various oxidants have been studied6-11

. The extent of

PFC oxidation of pentaamminecobalt(III) complexes of α-hydroxy acids in micellar medium

as an oxidisable hydroxyl group is separated from carboxyl bound to Co(III) centre by a

saturated fragment namely C-C bond12

.The cation radical is formed due to the oxidation of

hydroxyl group by PFC is nearly in a synchronous fashion of electron transfer resulting in a

carbon-carbon, oxygen-hydrogen bonds scission and reduction at cobalt(III) center.

Experimental

184 mL of 6 M HF was added to 100 g of CrO3 with stirring, the resulting solution was

cooled and 79.1 g of pyridine was added over 100 min. The solid pyridinium fluorochromate

was filtered and recrystallised with suitable solvent. α-Hydroxy acids employed as ligands

(Aldrich products) were used as obtained.

1103 B. M. NAWAZ et al.

The monomeric cobalt(III) complexes of α-hydroxy acids were prepared as their

perchlorates by the method of Fan and Gould13

. The tris (µ-hydroxo) complex; (NH3)3 Co (OH)3

Co (NH3)3 (ClO4)3 (triol) has been prepared by the procedure of Siebert and Co workers14,15

.

And unbound ligands in the presence of micelles were carried out at 35±0.2 0C in an

electrically operated thermostat bath. The concentrations of unreacted PFC was determined

iodometrically. The disappearance of Co(III) was followed spectrophotometrically by following

the decrease in absorbance at 502 nm. (For the monomeric Co(III) complex). Ionic strength was

maintained by the addition of suitable quantities of NaClO4. The specific rates estimated from

the optical density measurements agree with the values from the volumetric procedure within

±7% curiously, the change in absorbance observed at 502 nm Co(III) complexes of α-hydroxy

acids corresponds to nearly 100% of the initial concentration of Co(III), while the change in

optical density at 374 nm for PFC corresponds to nearly 100% of [Co(III)] initial.

Co(II) was estimated after completion of reaction, by diluting the reaction mixture

10-fold with concentrated HCl, Allowing the evolution of chlorine gas to cease and then

measuring the absorbance of blue fluoro complex of Co(II) at 69 2 nm (ε =560 dm3 mol

-1

cm-1

)16,17

. The amount of Co(II) estimated in all these cases corresponds to nearly 100% of

[Co(II)]initial.

After 48 h, the product was extracted with diethyl ether and analyzed iodometrically for the

amount of benzaldehyde formed was determined by measuring absorbance at 250 nm

[ε=11,400 dm3 mol

-1 cm

-1]

18,19. The yield of benzaldehyde in all these cases was nearly

100% [Co(III)]initial Table 1 & 2.

Table 1. Stoichiometric data for PFC oxidation of Co(III) bound and unbound α-hydroxy

acids in presence of NaLS at 35±0.2 0C

103[Compound]

mol dm-3

102[PFC]initial

mol dm-3

102[PFC]final

mol dm-3

∆10

3[PFC]

mol dm-3

[Compound]:

∆[PFC] Mandelic acid

4 2 1.74 2.63 1.00 : 0.65 5 2 1.67 3.24 1.00 : 0.64 6 3 2.6 3.96 1.00 : 0.66

Lactic acid 4 2 1.73 2.68 1.00 : 0.67 5 2 1.66 3.2 1.00 : 0.64 6 3 2.62 3.9 1.00 : 0.65

Glycolic acid 4 2 1.72 2.52 1.00 : 0.63 5 2 1.68 3.25 1.00 : 0.65 6 3 2.6 3.96 1.00 : 0.66

Co(III)-Mandelato 4 2 1.86 1.32 1.00 : 0.33 5 2 1.84 1.6 1.00 : 0.32 6 3 2.8 1.98 1.00 : 0.33

Co(III)-Lactato 4 2 1.86 1.36 1.00 : 0.34 5 2 1.82 1.75 1.00 : 0.35 6 3 2.79 2.04 1.00 : 0.34

Co(III)-Glycolato 4 2 1.86 1.32 1.00 : 0.33 5 2 1.83 1.7 1.00 : 0.34 6 3 2.78 2.1 1.00 : 0.35

[HClO4]=1.00 mol dm-3, [NaLS]=1.00x10-4 mol dm-3, Temperature=35±0.2 oC

Kinetic Study on Induced Electron Transfer Reaction 1104

Table 2. Stoichiometric data for PFC oxidation of Co(III) bound and unbound α-hydroxy

acids in presence of CTAB at 35±0.2 0C

103[Compound]

mol dm-3

102[PFC]initial

mol dm-3

102[PFC]final

mol dm-3

∆ 10

3[PFC]

mol dm-3

[Compound]:

∆ [PFC]

Mandelic acid

4 2 1.73 2.68 1.00 : 0.67

5 2 1.68 3.25 1.00 : 0.65

6 3 2.61 3.96 1.00 : 0.66

Lactic acid

4 2 1.74 2.63 1.00 : 0.65

5 2 1.67 3.21 1.00 : 0.64

6 3 2.62 3.92 1.00 : 0.67

Glycolic acid

4 2 1.74 0.53 1.00 : 0.63

5 2 1.68 3.27 1.00 : 0.64

6 3 2.61 3.96 1.00 : 0.66

Co(III)-Mandelato

4 2 1.86 1.33 1.00 : 0.34

5 2 1.83 1.62 1.00 : 0.32

6 3 2.79 2.1 1.00 : 0.35

Co(III)-Lactato

4 2 1.85 1.36 1.00 : 0.34

5 2 1.83 1.75 1.00 : 0.35

6 3 2.8 1.98 1.00 : 0.33

Co(III)-Glycolato

4 2 1.87 1.34 1.00 : 0.33

5 2 1.84 1.73 1.00 : 0.34

6 3 2.79 2.12 1.00 : 0.35

[HClO4] = 1.00 mol dm-3, [CTAB] =1.00x10-4 mol dm-3, Temperature=35±0.2 oC

After neutralization of the reaction mixture with sodium bicarbonate, the pH of the

aqueous layer was adjusted to about 6.0 and the aqueous layer was separated by filtration in

the case of both free ligands and corresponding complexes. On evaporation of water under

reduced pressure, the product separated and the percentage yield was calculated. Though the

yield of cobalt(II) was 100%, the estimation of cobalt(II), Cr(V) and carbonyl compounds

were quantitative, In both the cases the IR spectra of the product agreed with IR spectra of

authentic samples.

Results and Discussion

Table 3 summarizes the kinetic data for the PFC oxidation of free α-hydroxy acids with 1

N HClO4 in presence of anionic and cationic micelles at 35±0.2 0C. The reaction exhibits

total second order dependence on [Cobalt(III)] as well as [α-hydroxy acids]. Based on the

oxidation of PFC with α-hydroxy acids the following rate law has been deduced.

Rate=k[α-hydroxy acid] [PFC]

Table 4 lists the formation constants for PFC co-complexes of α-hydroxy acid along

with the specific rates. Such complex formation seems to be absent when the carboxyl and

it is tied up by Co(III) and the reaction between PFC and Co(III) complexes of α-hydroxy

acids exhibit uncomplicated second order kinetics.

1105 B. M. NAWAZ et al.

From a comparison, the specific rates for PFC oxidation of the respective Co(III)

complexes and the dimeric cobalt(III) glyoxalato complex, one can infer that the oxidation

rates of α-hydroxy acids are not significantly affected by complex formation. This may be due

to the point of attack lies away from the Co(III) centre so that its electrostatic influence is less

felt. There is, however a considerable change in the specific rate of PFC oxidation of the Co(III)

keto acid complex as the two Co(III) centers can exert greater electrostatic influence over the

reacting centre. This suggests that PFC attacks the O-H centre in the slow step of the reaction,

leading to ligand oxidation takes place. The rate of the reaction is increased by the addition of

both NaLS and CTAB. A plot of specific rate constant versus micellar concentration is

sigmoidal in shape the catalytic effect is more in CTAB than NaLS.

The specific rate of the lactato complex is more when compared to both the rate of

unbound ligand and mandelato complexis due to the ligation of lactic acid to cobalt(III) centre

has probably increased its reactivity towards PFC and this effect seems to be more specific for

this ligands only. In NMR spectrum of lactato complex the alpha methine proton has

undergone considerable downfield shift compared to the alpha C-H proton of the unbound

ligand [δ C-H=1.73 ppm in lactic acid and δ=C-H 2.30 ppm in lactato complex whereas

δ= C-H 4.75 ppm in mandelic acid δC-H=3.85 ppm in the respective complex]. Suggesting an

increase in acidic nature of methine proton of lactic acid is due to ligation to metal centre. If

the reaction proceeds through a performed chromate ester, then the rate of alpha C-H will be

enhanced, resulting in an increased rate of oxidation of lactato complex such a precursor

complex may be sterically hindered in the case of mandelato and glycolato complexes.

The stoichiometric results indicate that for 1 mole of cobalt(III) complex, about 0.65 mole

of PFC is consumed, whereas with the unbound ligands for 1 mole of α-hydroxy acids about

0.92 mole of PFC is consumed (Table 3 & 4). The stoichiometric results coupled with kinetic

data and product analysis can be accounted for by the following the reaction Scheme 1.

Table 3. First order rate constants for PFC oxidation of α-hydroxy acids at 35±0.2 0C

102[α-hydoxy

acids] mol dm-3

104k1, s

-1

NaLS

102 k2dm

3

mol-1

s-1

NaLS

104 k1, s

-1

CTAB

102 k2dm

3

mol-1

s-1

CTAB

Lactic acid 1 4.2301 4.2301 4.513 4.513

1.5 6.345 4.23 6.7692 4.5128 2 8.46 4.23 9.0261 4.513

2.5 10.5749 4.229 11.2823 4.5129 3 12.6897 4.2299 13.5387 4.5129

Mandelic acid 1 3.2846 3.2846 3.3633 3.3633

1.5 4.9266 3.2844 5.0448 3.3632 2 6.569 3.2845 6.7264 3.3632

2.5 8.2113 3.2845 8.4081 3.3632 3 9.8536 3.2845 10.0898 3.3632

Glycollic acid 1 2.1276 2.1276 2.3524 2.3524

1.5 3.1912 2.1274 3.5284 2.3522 2 4.2551 2.1275 4.7046 2.3523

2.5 5.3187 2.1274 5.8808 2.3523 3 6.3826 2.1275 7.0569 2.3523

[PFC]=2.00x10-3 mol dm-3, [HClO4] =1M, [NaLS] =1.00x10-4 mol dm-3, [CTAB] =1.00x10-4 mol dm-3

Temperature=35±0.2 0C

Kinetic Study on Induced Electron Transfer Reaction 1106

Table 4. First order rate constants for PFC oxidation of Co(III) complexes of α-hydroxy

acids at 35±0.2 0C

103[(NH3)5Co

III-L]

mol dm-3

104k1, s

-1

NaLS

102k2dm

3

mol-1

s-1

NaLS

104 k1, s

-1

CTAB

102 k2 dm

3

mol-1

s-1

CTAB

L = Lactato

1 3.5552 3.5552 3.6477 3.6477

1.5 5.3326 3.555 5.4713 3.6475

2 7.1104 3.5552 7.2951 3.6475

2.5 8.8879 3.5551 9.119 3.6476

3 10.6655 3.5551 10.943 3.6476

L = Mandelato

1 2.3833 2.3833 2.605 2.605

1.5 3.575 2.3833 3.9072 2.6048

2 4.7669 2.3834 5.2098 2.6049

2.5 5.9586 2.3834 6.5123 2.6049

3 7.1504 2.3834 7.8148 2.6049

L = Glycolato

1 1.3528 1.3528 2.007 2.007

1.5 2.0289 1.3526 3.0103 2.0068

2 2.7053 1.3526 4.0139 2.0069

2.5 3.3818 1.3527 5.0174 2.0069

3 4.0582 1.3527 6.0209 2.0069

[PFC]=2.00x10-3mol dm-3, [HClO4] =1M, [NaLS] =1.00x10-4mol dm-3, [CTAB] =1.00x10-4mol dm-3

Temperature=35±0.2 oC

The reaction scheme proposes that PFC oxidizes OH center of the cobalt(III) bound α-

hydroxyacids at a rate of comparable to that of the unbound ligand and there is 100%

reduction at the cobalt(III) centre, forms a chromate ester with cobalt(III) glyoxalato

complex which can decompose in a slow step, proceeds through C-C bond fission leading to

the formation of cobalt(II), carbonyl compounds and carbon dioxide. As 1 mole of

cobalt(III) glyoxalato complex consumes 0.65 mole of PFC yielding nearly 100% of Co(II)

and 100% carbonyl compounds. Similarly 1 mole of unbound α-hydroxy acid consumes

nearly 0.92 mole of PFC, yielding 100% of carbonyl products and CO2.

Scheme 1

1107 B. M. NAWAZ et al.

Conclusion

It has been found that the rate of oxidation of Co(III) complexes of unbound and bound

moieties are enhanced more in the presence of CTAB when compared to the NaLS. The

micelles act as a positive catalyst in the present study.

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