Indi an Journal of Chemistry Vol. 44A. November 2005. pp. 221 8-2227

Stopped-flow kinetic investigations of one-electron transfer reactions of 4,4'-diaminodiphenylmethane and its radical cation in aqueous solution

C San karl , P Aravindan I *, S Anandan2 & P Maruthamuthu2.*

I Department o f Chemistry, Annamalai Unive rsity. Annamalai Nagar 608002, India

2Department of Energy, University of Madras. Guindy Campus, Chennai 600025. India

Email : aravindan02@yahoo.com

Received 8 FebruGlY 2005; revised 23 August 2005

The oxidation of 4,4'-diaminodiphenylmethane (DADPM) by metal ion (Ce4+); oxoanions [Mn04' and Cr20 / ' ). perox ides [peroxomonosulfate (PMS), peroxodisulfate (PDS) and H20 2], and halogens [CI l , Br2 and 12], to the rad ical ca tion DADPM"+. and further oxidat io n to the product mo nocation DADPM+ by the above mentioned oxidants has been investigated by stopped- tlow technique. A probable mechanism has been proposed. The reactions follow a total second­order kinetics, first-order each with respect to the [DADPMj or [DADPMo+j and [ox idant] . In addition, the radical cation DADPM

o+ ox idi zes sulfite (SO/ ), thiosulfate (S20 {), dithionite (S2042,) and metabisulfite (S20t ) to regenerate the parent

compo und DADPM. The rate constants for these react ions have al so been estimated. Reactivity for rhe formation o f DADPM follow s the decreasing order in the case of oxoani ons (M n04' > CrzO/, > Ce(IV)) and peroxides (S20 / > HS05' > H20 2) which is in agreement wi th the redox potential?, while the react ivity trend for hal ogens is found to be 12 > Br2 > C I2 which is not in agreement with the redox potentials. The observed rate constants for e lec tron transfer have been corre lated theoretically using Marcus theory.

[PC Code: Int.CI.? C07B33/o0

The electron transfer reactions have been extensively studi ed within the past decade to develop systems for long-lived charge separation, which may be useful for light energy storage. Light absorption by a chemical system may cause electron transfer from a donor species to an acceptor species, generating pai rs of high-energy intermediates that are usually radical ions'. The world ' s largest and the most important energy conversion process is the photosynthetic reactions, which proceed through radical cation intermediates2

. In general, aromatic amines undergo a variety of photophysical and photochemical processes based on electron transfer reactions3

. It is also clear that the organic compounds containing nitrogen atom play an important role in photocatalytic reactions3

.

Based on this, the radiation chemistry4 and kinetic studiess.6 have been carried out to reveal the formation of a radical cation and a monocation (for example Michler' s hydrol) , In continuation of our previous studiess.6, we are interested in 4,4'­diaminodiphenylmethane (DADPM), because it is used as an important intermediate in many chemical and biological reactions7. Moreover, information on the kinetics of the formation and decay of the radical cation (DADPMo +) and the cation (DADPM+) is not

available up to now. It is interesting to know whether the reactions occur by successive one-electron transfer or not when a multi-electron oxidant reacts with DADPM. The stopped-flow technique has been used to study the formation and decay of the radical cation DADPM

o+ by one-electron as well as multi­

electron oxidizing agents. In order to prove one electron oxidation involved in this system, we have compared the results with our previous kinetics studies with 4,4'-(dimethylamino )diphenylmethane (DMADPM). In addition, the observed radical cation formation (DADPM

o+) is evidenced by ESR studies .

To our knowledge, this system is not reported previously even though only the luminescence properties of the starting compound (DADPM) are known8 (Scheme I) .

Materials and Methods 4,4'-diaminodiphenylmethane (DADPM) was

purchased from Sigma (USA) (recrystallized from benzene) and potassium peroxomonosulfate (PMS) in the form of a triple salt 2KHSOs.KHS04.K2S04 under the trade name OXONE was received from DuPont de Nemours (USA). Other chemicals such as peroxodisulphate (PDS), KMn04, K2Cr20 7,

SANKAR et al. : ONE-ELECTRON TRANSFER REACTIONS OF DADPM 2219

-e -Fairly stable DAD PM "+

-e lH'N-Q-CH'-ONHT -DADPM"+

Scheme 1

Ce(NH4h(S04h, and molecular h, 8r2, and Cl2 were obtained from E.Merck (India) and Sarabhai M. Chemicals. Doubly distilled w2ter was used to prepare all the reagent solutions .

The absorbance of pure DADPM (I x 10-5 M) In

aqueous solution shows two peaks at 241 nm (I:; = 4.2x104 M-'cm-') and 287 nm (£ = 4.3x103 M-'cm-');

and it changes to 480 nm for DADPMo+ (£ = S.9x l03

M-'cm·'). All kinetic measurements were made with the Applied Photophysics 170S stopped-tlow spectrophotometer by monitoring the optical absorption of the radical cation DADPM

o+ at 480 nm.

pH of the reaction medium (PH 3-10) was maintained, using a universal buffer (potassium hydrogen phthalate, potassium dihydrogen phosphate and borax with added acid or base depending upon the pH requiredt Also, constant ionic strength (0.2S M) was maintained by adding sodium perchlorate solutions as required. To determine the effect of the temperature on the formation and decay of the radical cation, experiments were performed at lS-3S°C. Duplicate experiments were carried out to check the reproducibility of the results (error found to be ±S%).

The kinetics of the formation and decay of the radical cation DADPM

o+ by various oxidants

mentioned above were carried out under pseudo-first­order condition with [oxidant] at least ten times greater than [DADPM] or [DADPM o+] at temperatures IS, 20, 2S and 3S°C. The changes in the absorption of the transient (A) with time were followed at different time scales depending upon the reactivity of the oxidants with DADPM or DADPM o

+.

The plots of log A against time were linear fot all the reactions (correlation coefficient ~ 0.99) . For different initial concentrations of DADPM or DADPM o

+ and at a fixed concentration of oxidant (the half life is

independent of [DADPM] or [DADPMo+] at constant

[oxidant]), the plots of log A~-AI versus time (for the formation of DADPM

o+) or log AI versus time (for the

disappearance of DADPMo+) were linear. It indicated

that the rates of the reactions were independent of the initial concentrations of DADPM or DADPM

o+. From

the linear plots, the calculated pseudo-first-order rate constants (k'f or k'd, sol) were found to be almost constant indicating the first-order dependence of the rate on [DADPM] for the formation of DADPM

o+ or

first order dependence of the rate 011 [DADPMo+] for

the disappearance of DADPMo+. Moreover, at a fixed

concentration of DADPM or DADPM o+ and at different concentrations of the oxidant at constant temperature, the pseudo-first-order rate constants were found to vary. The plots of k'f or k'd, sol versus the [oxidant] were found to be linear (Figs 1 and 2) , indicating a first-order dependence of the rate on [oxidant]. From the slopes of these plots, the second­order rate constants, k2f or k2d , M-' sol was determined (Table I). From the above results, the following rate laws can be proposed for the formation and decay of the radical catio:1, DADPMo+:

d[DADPMO+]/dt=k/f [DADPM]; k2f = k/f/[oxidant]

... (1 )

-d[DADPMO+]ldt= k'd[DADPM] ; k2d =k/d/[oxidant] . . . (2)

For evaluating the rate constants for the decay of the radical cation (DADPM

o+) to the monocation of

DADPM (DADPM+), experiments were carried out with the pre-prepared DADPM

o+ by reacting DADPM

and peroxodisulfate (PDS) in the ratio of 2: 1 and keeping it at 30°C for 2 h for the reaction to complete.

2220 IND IAN J CHEM . SEC A, NOVEMBER 2005

T he radi cal cation was found to be stable for several hours in the absence of any oxidizing or reducing agents and at low temperature (0-4°C).

The formation of radicul cation was conformed using Varian E-109 X-band spectrometer at room temperature (9.396 GHz microwave frequency and

I

o -w- cri

LLUc:i<! ttl t

0.08.4 .8.4.0.160

0 .07 ,4 .2.3.5,140

0 .06 .3.6 .3.0.120

0.05 .3.0.2.5. 100

~ 0.04 ,2.4 .2.0 ,80 ~

0.03 .1.8 ,1.5.60

0 .02 ,1.2.1.0.40

0 .01.0 .6 .05.20

A

G

,0

o 2 4 6 8 10 12

[Oxidant] x 104 . M ______ A ,B. I [Oxidant] x 103 , M _____ C,O,E,G,H [OXidant] x 102 . M ----- F

Fig. I - First-order dependence of the rate on 10xidant] for the formation of DADPM' + at 25°C (B and C are measu red in 0.2 M H ~S04)' fA. Mn04'; B. Crp/" . C. Ce4+; D. HSO:\. E, S ,08~' ; F. H ~02 ; G, C1 ~; H. Br2'; I. 121.

0.335 T mag .letic field) and the g -val ue (2 .0029) was determi ned with an uncertainty of ± 0.000 I usi ng a marker contatnIng I , l-diphenyl-2-picry lhydrazy l (DPPH) built into the spectrometer. No ESR signals were noticed for pure DADPM solution.

l? u:

w cO <{ c:i c..i It '

16.0.08 ,3 ..

14 ,0.07 ,2.8

12,0.06 ,2.4

"7 10 ,0 .05 .2. V>

U -"

8 ,0.04 .1.6

6 ,0.03. 1.2

4 ,0.02,0.8

2,0.01 ,0.4

o

D

F

/ A

~' , . ,

2 6 8 10 A ,B .C.D. E . F.G

(Oxidant] x 103 . M ___ G (Oxidant] x 102 • M ___ p .. a, D ,E. F .G

Fig, 2 - First-order dependcnce of the rat e on 10xidant] for the decay of DADPM'+at 25°C (B and C are measured in 0.2 M H,S04)' [A, Mn04'; B, Cr20/', C, Ce4~ ; D, HS05-. E. S20 / ; F. Cll ; G, Br2]'

Table ] - Kinetic data for the formatio n and deeay of DADPM'+ (in M·I 5-1) at different temperatures ( IS-35°C)

Oxidant" 15°C 20°C 25°C 35°C

k2f kld klf k2d klf k~d k2f k2d Metal ion Ce4+ 3.6x l02 L1 x IO' SAx I 0' 1.6x I 02 7.2x lO' 2. lx I0' lAx I 0' 4.2x I0'

Oxoanions Mn04' 1.4x I 0" 4,2x 1OI 2. IXI05 6.5x 101 2.9x I0" 8,6x 101 5.7x I 05 1.7 x I 02

Cr,0 72. 1.5x 10'1 6.6 2.3x l 04 9.7 3.0X 104 I.3x lOI 5.9x 10'1 2.6x 101

Perox ides HSO; 6.9 0.7 10.3 1.1 1.4x 101 1.5 2.8xlOl 3.0 s20 l 1.0x I 0' 0.5 1.5 x I 02 0.8 2.0x I0' 1.0 4.0x 102 2. 1 H2O, 0.5 0.7 1.0 2. 1

Halogens CI 2 I.Sx I 02 I.2x lO I 2.2x 102 1.9x l01 2.8x I 02 2.6x 101 5.8><. 10' 5. 1x 1OI Br2 8.7x lO' 1.8 I.3x 103 2.8 1.7x 10' 3.8 3.4x I 03 7.5 12 2.4x 104 3.7x 104 5.1><104 9.9x l04

"The reacti CJ ns of Ce~+ and Cr20 / are in 0.2 M H,SO.\. All other rcac tions are carried out at pH 3.0 adjusted with potassium hydrogen phtha late buffer

SANKAR el al.: ONE-ELECTRON TRANSFER REACTIONS OF DADPM 2221

Results and Discussion

DADPM and DADPM·+-peroxides

The reactions of PDS and H20 2 with DADPM and DADPM o+ were carried out at pH 2.8 (adjusted with potassium hydrogen phthalate buffer) and that of PMS were performed at pH 2.8, 7A and 9.6.

The peroxides, PMS, PDS and H20 2 reacted with DADPM, generating the radical cation, DADPM

o+, on

a shorter time scale (2-20 ms). The DADPMo+ was

further oxidized on a longer time scale (10-50 ms) only by PMS and PDS and not by H20 2 whatever be the concentrations of H20 2 and DADPM

o+. The

second-order rate constants for the formation and decay of DADPM

o+ by PMS were respectively

1.4xl01 ± 5 M-I S-I and 1.5 ± 5 M-I S-I at 25°C (Table I). Hence, the rate of oxidation of DADPM to DADPM

o+ was 9 times faster than that of DADPM

o+

to DADPM+ by PMS. From the magnitudes of the rate constants for the form ation of DADPM

o+ (Table I) the

reactivity trend of the peroxides with DADPM was found to be s20 l- > HSOs- > H20 2, which was in agreement with the redox potential s of these oxidants [2 .0 V versus Normal Hydrogen Electrode (NHE) S20 t > (1.8 V versus NHE) HSOs- > (1.7 V versus NHE) H20 2• The peroxodisulfate ion S20 t is a well known and strong oxidizing agent lO whose thermal, photochemical and radiolytic or redox reactions provide the radical anion S04°-. As reported earl ier, many inorganic and organic compoundss.6 were oxidized by PMS and the one electron reduction of it generated °OH/SO/- radicals as the reactive intermediates.6. From the magnitude of the rate constants (Table I), it is seen that the rate constants of PDS are approximately 14 times higher than those of PMS for the formation of DADPM

o+ from DADPM .

Even though these peroxides are two electron oxidants, they are found to undergo step-wise one electron oxidation. The reactions with DADPM may be written as:

DADPM + S20 8 2~ DADPM o+ + SO/- + S042

-

... (3)

DADPM + SO/-~ DADPM o+ + sot . .. (4)

DADPM + HSOs' ~ DADPM o+ + (OH/S04°-)

+ (OH'/SOt) ... (5)

DADPM+ °OH/ SO/-~ DADPM o++ OH'/SO/­.. . (6)

DADPM + H202~ DADPMo+ + °OH + OH-

... (7)

DADPM + °OH ~ DADPMo+ + OH' .. . (8)

The reaction species formed after the one electron reduction of S20 t, HSOs- and H20 2 may be S04°', °OH/SO/- and °OH, respectively as shown by the reactions. However, it is immaterial whether the species mentioned above are formed as the intermediate in the present investigation, since the rad icals react with DADPM [Eqs (4), (6) and (8)] generating DADPM

o+ at a rate, which cannot be

monitored by the stopped-flow technique. In the case of decay of DADPM

o+, the second-order rate

constants for DADPMo+ and S20 t give a product that

is 0.5 times lower than that for the corresponding reaction of PMS (Table I) .

ElTect of pH on the formation and decay of DADPMH by PMS The reactivity of amino compound and oxidant

varies at different pHs, since the redox potentials of the protonated species are different from those of the deprotonated species. From the general reactivity of PMS with organic and inorganic compounds, it was observed that PMS could undergo ionic (nucleophi lic or electrophilic) or free radical reactions depending upon the pH of the mediums.6 and the nature of the substrate used. ]n the present investigation, PMS was found to undergo step-wise one-electron transfer with DADPM, through free radical mechanism.

The pKa value of HOOS03- is 9A (ref. I I ). Hence. at pHs 2.8 and 7A, PMS exists as HOOSO)-; and at pH 9.6, it exists as 40% HOOS03- and 60% -OOSO)-. It was seen that the rate constants increased as the pH was increased ; > 214 times atpH 7A (k2f = 3.0x 103± 4 M-Is- I) and> 385 times at pH 9.6 (k2f = 5Ax103 ± 5 M-Is- I) compared to the rate constant at pH 2.8 ( k 2f = lAx 101 ± 5 M-Is-I). Even though PMS existed as HOOS03- between pH 2.8 and 7A, the higher reactivity at pH 7A indicated more reactive form of 4,4'-diaminodiphenylmethane (DADPM), form the acid-base equilibrium as:

DADPMH+ ~ DADPM + H+ .. . (9)

For DADPM, the pKa value of DADPMW was not available. However, from the pKa value of aniline cations (PKa = 4.87)9, it was assumed that the DADPMW might be around 5.0 (i.e., the pKa value for the proton transfer reaction could not be calculated

2222 INDIAN J CHEM, SEC A, NOVEMBER 2005

for thi s system because there is no constancy in the isobestic point. So, it is mentioned it is closer to that of aniline due to the presence of a CH 2 group between the rings8). Hence, DADPM would be in the form of DADPMH+ at pH 2.8 and as DADPM at pH 7.4 and 9.6. DAD PM ex isted as 100% DADPMW at pH 2.8. At p\-l 7.4, it ex isted as a mixture of DADPM and DADPMH+. Since deprotonated amines were more reac tive than protonated amines with oxidants, the hi gher rate constant at pH 7.4 than at pH 2.8 was due to hi gher reacti vity of DADPM with PMS and the rate constant observed was a combination of rate constants for the steps:

DADPMW + HOOS03' -7DADPMH2+0

+ (S04°' + OH') or (SO/ · + OHO

) •• • (10)

DADPM + HOOSOJ' -7 DADPM o+ + (SO/· + 0\-1') or (S042

. + OH O

) • •• (1J)

At pH 9 .6, PMS existed as 60% 'OOSOJ' and 40% HOOSOJ' and the substrate ex isted fully as DADPM . It was seen that the reactivity at pH 9.6 was only 1.8 times higher than that at pH 7.4 even though the substrate ex isted as 100% DADPM which was more reactive than DADPMH+. Thi s was due to the fact that PMS ex isted as 60% 'OOSOJ' and 40% HOOS03' at pH 9.6 and '00S03' was less reactive than HOOS03' in electron transfer reactions.6. If the HSOs' is highly reactive compared to its anion, it may react entirely via Eq .( 12). The rate constant at pH 9.6 is a combination of rate constants for the reactions via Eqs (12) and (13) :

DADPM + HOOS03' -7 DADPMo+ . .. (12)

... (13)

Experiments could not be conducted successfully at pH > 9.6, to estimate the rate constant for the reaction (13) because of spontaneous decomposition of '00S03' (ref. 5,6).

The pKa value of an iline radical cations.6 is 7.05 and it is assumed that DADPM

o+ may be around 7.

Hence DADPM o + would be in the form of DADPMH2+0 at pH 2.8 and as DADPM

o+ at pH 7.4

and 9.6. The reaction at pH 2.8 (k2d = 1.5 ± 5 M·ls· l) is slow compared to those at pH 7.4 (k2d = 1.8xlO2 ± 4 M·ls· l) and 9.6 (k2d = 2. 1 X 103 ± 4 M·ls· I

). The slowest rate at the lowest pH 2.8 suggests that the radical cation may exist as DADPMH2+0

, which will be more

difficult to ox idize than the deprotonated species, DADPM

o+. Increas ing the pH to 7.4 and 9.6 gives ri se

to DADPMo+ according to the acid-base equilibrium :

DADPMH2+0 F DADPM o+ + W ... ( 14)

At pH 7.4, the reaction is between DADPMo+ and

HOOS03' (100%). The rate is 1.8x 102 ± 4 M· I S· I. At

pH 9.6 , the reaction is between DADPMo+ and

HOOS03' (60 % '00S0 3' and 40% HOOS03·). The rate is 2. lxI03± 4 M· I

S· I. The peroxomonosulfate dian ion is well known for its behaviour to undergo nucleophilic reactionss.6 . Hence both the reactions,

DADPM O + + HOOS03' -7 Product .. . ( IS)

and

. . . ( 16)

are responsible for the disappearance of DADPMo+ at

pH 9.6. From the reactions at pH 9.6 and at pH 7.4, the rate constant for the reaction (16) is 1.9x 10" ± 4 M· I

S·I [2.lxI03 ±4M·1 S· I (rateatpH9.6)-1.8xI02 ±

4 M· I S· I (rate at pH 7.4) = 1.9x 103 ± 4 M· I

S·I], which is one order of magnitude higher than the reaction ( IS).

DADPM and DADPMH-metal ion and oxoanions

All the oxidants undergo successive one-electron transfer reaction with DADPM and DADPM

o+, even

though Mn04' and Cr20l were performed in 0.2 M H2S04 medium are multi-electron transferring ox idants. From the magnitude of the second-order rate constants , the reactivity of these metal ions and oxoal1lons with DADPM is found to be Mn04' > Cr20 /" > Ce4+, wh ich is in agreement with the oxidation potentialss.6 of these oxidants, 1.7 V (Mn04') > 1.4 V (Cr20l) =; J.4 V Ce4+. Permanganate is a powerful and 111 some cases a drastic oxidant for organic substrates and its oxidation rate is slower in neutral pH than in alkaline conditions. The rate is faster in acidic medium l2. Even though Cr2072. (1.4 V, 0.2 M H2S04) and Ce4+ (1.4 V, 0.2 M H2S04) have the same oxidation potential , for the formation of DADPM

o+, the former shows a

higher reactivity than the latter. The reason is probably [hat Ce4+ has higher ability of complex ings.6

with groups containing electron donors. In the case of reaction of DADPM

o+, the reactivity trend (Table I) is

found to be Ce4+ > Mn04' > Cr20 / and the second-

SANKAR el (II .: ONE-ELECTRON TRANSFER REACTIONS OF DADPM 2223

order rate constants are not in line with the oxidation potentials of these oxidants.

DADPM and DADPMH-halogens

Electron transfer reactions and formation of charge­transfer complexes between a variety of electron donors and halogens are widely documented in literatures.fl . In general, the halogens are two electron oxidants and it is also known that they are capable of abstracting a single electron from one electron reductant. In the present investigation , at low concentrations of the reactants ([DADPM] = I .OxO-5

M; [halogen] = 0.2 -1O.OxlO-3 M) and at shorter time scales, formation of radical cation is observed. At higher concentrations of halogen ([halogen] = (l.0 -10.0) X 10.2 M) except h, at longer time scales, the radical cation is found to decay. The formation and decay of the radical cation, DADPM

o+ are found to

follow a total second-order kinetics, first-order each with respect to [DADPM] or [DADPM

o+] and

[halogen] (Figs I and 2). In the present study, the reactivity trend for the formation of DADPM

o+ with

halogens is Jz> Br2>CI2. h (0.5 V), having the lower oxidation potential than the other two oxidants CI2 (1.4 V) and Br2 (1.1 V) which possess the highest reactivity. In the case of reaction of Jz, the signal intensity remains the same even at the highest concentration of lz (10.0 x 10-3 M). The observed transient may be a charge-transfer complex between Iz and DADPM and not the product of complete electron transfer reaction. This observation is confirmed by the absorption spectra of DADPM: h, Amax = 434 nm. The absorbance is entirely different from the molar absorption spectrum of DADPM

o+ in aqueous soluti on

C)"max = 480 nm) as well as h solution (Amax = 458 nm). Iodine is well known to form such charge transfer complexs.6 and plays an important role in chemical, photochemical and biological systemsS

.6

.13

. The estimated rate constant for this process at 25°C is 5.1 x 104 ± 5 M-I

S- I, which is greater than those of Br2 (1.7x)03 ± 5 M·I S-I) and Ciz (2.8xI02 ± 5 M-I S· I).

From the magnitudes of the second-order rate constants for the decay of the radical cation, the reactivity of the halogens is Ciz > Br2, which is in agreement with the redox potentials of the oxidants ).4 V (Cl2) > ).1 V (Br2).

Reduction of DADPMH by various reducing agents

The radical cation DADPM o+ was converted back to

the parent compound, DADPM , by various reducing agents like su lfite (SO{ ), thiosulfate (S20 / ),

dithionite (S20 / ) and metabisulfite (S20 /), suggesting many room temperature oxidations by DADPM o

+, DADPM o+ +e-. DADPM . The kinetics

of these reactions were performed under pseudo-first­order condition, [reducing agent] » [DADPM

o+] by

monitoring the disappearance of DADPMo+ at 480

nm. ([DADPMo+] = 1.0x 1O-4M; [reducing agent] =

(0.5-1O.0)x 1O·2M) . The reactions were studied out at

25°C and conducted at pH 6.0. The pseudo-first-order decay rate constant k'r (S- I)

values are found to increase with increasing concentrations of reducing agents and the plots of k r versus [reducing agent] are linear (Fig. 3) passing through the ong1l1, indicating the first-order dependence of the rate on [reducing agent]. The overall second-order rate constants k2r 1M-I

S-I are evaluated from the slopes of the above plots and are reported in Table 2. The general rate law for the reduction of the radical cation can be given as:

-d [DADPMo+]ldt = k'r [reducing agent];

k2r = k'r I [redtring agent].

0

U a:i <i

16 f

14

12

~

'", 10 -~ ""

8

6

4

2

0 2 0.5

4 1.0

C

6 1.5

8 2.0

[reducing agent] 103 , M _ A,B

[reducing agent] 102 , M - C,D

... ( 17)

10 B.C.D 2.5A

Fig. 3 - First-order dependence of the rate on [reducing agent] for the reduction of DADPMH at 25°C. [A, S20/' ; B, Sp/"; C, HSO)', D, S20 / "].

2224 INDI AN J CHEM, SEC A, NOVEMB ER 2005

Table 2 - Kinetic data fo r the reduction of DADPMH and DM ADPMH at 25°C

Reductant pH k2r (M-I sol)

DADPM o+ DMADPM o

+

AH,/A H- 3.0 2.6 x 104

S 0 2- 6.0 LO x 101 1.5 x 101 2 1

S20 / - 6.0 2. 1 x 101 1.0 X 101

HSO; 6.0 2.2 x 102 5.2 X 102

S 20 / 6.0 1.5 x 10' 3. 1 X 101

With the excepti on of sot, the reducing sulfur compounds and DADPM

o+ exi st as such without

protonation at pH 6.0. The pKa value of HS03- is 7 .2 (refs 5,6) and hence at pH 6.0 sulfite ion exi sts as a mi xture of HS03- (approxi mately 94%) and S03"­(approximately 6%). Hence, the rate constant measured may be a combinat ion of those due to the reactions, DADPM

o+ + sot and DADPM

o+ + HS03-,

since the species present in large excess (HS03-) need not be the acti ve species. However, separate experiments carried out at pH 4 .5 ( 100% HS03-) exhibit a lmost the same rate constant as that observed at pH 6.0 , proving the reaction to be as Eq.(18) .

In all the reactions, the rad ical cation DADPMo+ is

reduced to the parent compound DADPM . The fo llow ing mechanisms are proposed for the e lectron transfer reactions of DADPM

o+ with the reducing

agents:

DADPM O+ + HS03- ~DADPM + H+ + S03°-

2S0 o _~S 0 2-32 6

DADPM+ S20 no-~DADPMo+ + S20 t

2S20 oo-~ S40 t + (n-3) O2

where n = 3, 4 and 5

. . . (1 8)

. . . ( 19)

. . . (20)

.. . (2 1)

In all these reactions, the radical anions formed are proposed to undergo self-terminati on. Although some are mild oxidizing agentsS

.6

, no equilibrium reaction such as DADPM

o+ + R2-F DADPM + RO

- (where R2

- represents a reducing agent) is observed under the present experimental conditions, [reducing agent] » [DADPM

o+]. Such an equilibrium reaction is observed

in the pulse radiolytic reaction by Huie and Neta l4

between chloropromazine and S03°-. From the second-order rate constants estimated, the

reacti vity of the reduc ing agents with DADPMo+ is

fo und to vary in the order S20 / > S20 / > sot> S20 4

2-.

Even though DA DPMo+ behaves as a mild

oxidizing agent , the reduction of DADPMo+ by

various reducing agents reveals the reversibility of the reaction. DADPM

o+ == DADPM as long DADPM

o+

is not oxidized further. Thi s observation supports the fact that DADPM may be used as a potenti al e lectron sacrificial agent.

Effect of temperature (correlation of r ate and activation parameters)

The reaction temperature is varied in the range 15-

35°C for both the formati on and decay of the radical

cation DADPMo+, for a ll the ox idants. The energy of

activation, Ea (kJ mor l) fo r each reaction is evaluated from the slope o f the pl ot o f log k 2f versus liT or log k2d versus liT. The va lues of pre-exponenti al factor A and the transition state parameters j'j.f{" , j'j.s* and j'j.C"l­

are calculated and presented in Table 3. Kinetic data for the formation and decay of

DMADPMo+ at 25°C are given in Table 4 . All the rate

constan ts are collectively presented in Table I . From the tables, it is seen that for the fo rmation of radi cal cation (k2r), the j'j.s* values are all negati ve except Mn04- and lz reactions. The positi ve j'j.S"l- favours the reaction of Mn04- and Iz to proceed at faster rates in the fo rmation of radical cation. T he high negati ve values of j'j.s* may be indicati ve of more po lar nature of the acti vated complex than the reactants and the in vo lvement of high e lectrostric tion of the solven t

molecules. The reac tions with high negati ve j'j.s* values, especially the decay of the radical cati on (k2d)' are associated with large positive LI,.C"l- values and found to be slow.

Comparative study of DADPM with DMADPM The present study discusses the formati on and

decay of DADPMo+. We have already reported the

kineti c study of the 4 ,4' -(Dimethylamino)diphenyl­methane radical cation (DMADPM'+)s in aqueous solution by several oxidizing agents. The compari son of the properti es of DADPM (Table 1) wi th th at of DMADPM (Table 4)shows that the reacti vity is higher in both the formation and decay of DMADPM"+ than in DADPM

o+. The higher reacti vity

in DMADPMo+ may be due to the methyl group,

which has more electron releas ing power; bes ides, the available electron density at nitrogen atom increases the reaction rate .

Even though ox idation by H20 2 may be preferred pathway due to favourable kinetics and thermodynamics, in the group of peroxi des, no decay of the transient is observed in H20 z even at hi gher

SANKAR el al.: ONE-ELECTRON TR ANSFER REACTIONS OF DADPM 2225

Table 3 - Calcu lated kinet ic and tran sit ion-state parameters for the formation and decay of DADPM ' +

Oxidant* A (M' IS' I) E" (kJ mol' I) /', H' (kJ mo l' l) /',5' OK ' I mo r l) 298 K /',C'(kJmorl)

k 2f k 2d k 2f k 2d k 2f k 2d k 2f k 2d k 2f k aJ

Metal ion Ce4+ 2.1 x lO" 5.2 X 1010 48.4 47.9 45.9 45.4 -36. 1 -47.9 56.7 59.7

Oxoanions Mn04- 1.8 x 1014 4 .2 X 10 10 50.3 49.6 47 .8 47.2 19.9 -49.8 41.9 6 1.9

Crl0/ 9.8 x 1011 5. l x 109 48 .6 49.0 46. 1 46.6 -4 .5 -67.2 47 .5 66.7

Peroxides HSOo' 8.4 x 109 1.7 X 109 48.2 51.7 47.7 49.3 -63.2 -76.3 66.5 72.1 S20 g2, 9.1 X 10 10 7.1 X log 49.4 50.4 47.0 48.0 -43 .2 -83.6 59.9 72.9 H20 2 1.3 x 109 52. 1 49.7 -78.4 73.0

Halogens CI 2 7.8 x 10 10 3.3 x 1010 48.1 52.1 45.7 48.9 -44.5 -53.8 58.9 65.0 Br2 5.5 x lO" 2.9 x 109 48 .5 50.8 46.1 48.3 -28.4 -7 1.9 54.6 69.8 12 3.6 x 101, 50.5 48.1 6.3 46.2

*The reactions o f Ce4+ and Cr20 / are in 0 .2 M H2S04, All other reJctions are carried out at pH 3.0 adjusted with potassium hydrogen phthalate buffer

Table 4 - Kinetic data for the formatio n and decay o f DMADPM' + Jt 25°C

Oxidant* k2f (M, I S· I) k2d (M' I S' I)

DMADPM' + DMADPM' + Metal ion Ce4+ 5.8 x 10' 3.2x 102 Oxoanions Mn04' 1.6x I 06 5.0x104

Cr20 72, I.3x I 00 9.0x I0' Peroxides H,SOo' 5.lxI0' 2 .0 S20 / 3.4x I 02 1.0 H20 2 5.2 0.8

Halogen CI2 1.8x I 0' 3.8x 102 Br2 6.6x 104 12 5.3x 1OO

*The reJc tions of Ce4+ and Cr20 / Jre in 0.2 M H2S04, All other

react ions are carried out at pH 3.0 adj usted with potassium hydrogen phthalate buffer

concentration. It implies that it has not been able to

oxidize the hi ghly stabl e DADPMo+, whereas for

DMADPM o+, a very low value of 0.8 ± 4 M' I S' I is observed.

An excellent agreement to note is that iodine, which has lower oxidation poten tial among the oxidants used in the studies, has no reaction during the decay of both DADPM

o+ and DMADPM o+. This

may be due to the poss ible formation of charge trasfer complex between DADPM and DMADPM with iodi ne respectively, which may stabilize the free radicals DADPM

o+ and DMADPM

o+. Further, a very

low value of the rate 3.8 ± 3 M-I S' I is obse rved for Br2

with DADPMo+. But, in the case of DMADPM

o+,

the re is no detectable reaction either with Br2 or B(. The radical cations DADPM

o+ and DMADPM' +

were found to undergo one-electron reduction with the reducing agents. The rate constants for both are

tabulated (Table 2). The reaction of DMADPMo+ with

ascorbic acid was very fast and found to occur within the mixing time of the instrument (2 ms). However,

DADPMo+ did not react with ascorbic acid. Even by

making changes in pH for the reaction, we could not achieve the rate constant. Ascorbic ac id was found to

act as a strong reductant for DMADPMo+, but no

transient was observed for DADPMo+. DADPM

o+ was

able to react with other reducing agents such as SO/" S20 /- , S20 / and SzO/' and prove the reversibility of the reaction to form the parent compound DADPM .

The reduction of the radical cations, DADPMo+ and

DMADPMo+ by reducing agents proves the

reversibility of the reactions as long as the radical cations are not oxidized further to DADPM+ and DMADPM+, the reaction of which is very similar to that of the methylviologen radical cation IS. Both DADPM and DMADPM are also less photosensitive. This observation supports the fact that substrates DADPM and DMADPM may be used as a potential e lectron sacri ficial agent.

Comparison of oxidation values of DADPM: Marcus theory The influence of Marcus theory o n the kinetics o f

e lectron-transfe r processes has received considerable . 161 7 Th I . I . . attentton ' . e tleory requIres t le reorganlzallo n

term lS, the so lvent reorgani zatio n energy '?c' which is

2226 INDI AN J CHEM. SEC A. NOVEMBER 2005

Table 5 - The ex perimen tal and ca lcul ated rate constnnts for e1cctron transfer reac ti on between DADPM (- 1. 12 V versus HE) and ox idants for the formation of DADPMo+ at 25° C

Oxidant* kH k2f(ca l) Ie Oxidation potenti al (Volt) (M-I S- I ) (M-I S· I) (kJ 1110 rl ) (One e lectron potenti al)

Metal ion Ce4 + 7 .2 x I O~ 3.3 x 1 0~ 25 1.0 1.4 Oxoanions MnO; 2.9 x 10' :U x 10' 22 1.4 1.7 Cr20/ 3.0 x 104 6.0 X 104 197. 1 1.4 Perox ides HSO; 1.4 x 104 3.5 X 101 340.0 1.8 S20 8

2. 2.0 x 102 l.4 x 102 352. 1 2.0

H20 2 1.0 1.6 360.6 1.7

Halogens Cil 2.8 x 102 5.7 X 102 229.8 1.4 Br2 1.7 x 10' 4.4 X 103 155.4 1.1 12 5. 1 X 104 1.8 101.6 0.5

*The reactions of Ce4+ and Cr2072- are in 0.2 M H2S04 , All other reactions nre cnrried out nt pH 3.0 adjusted wi th potassium hydrogen

phthalnte buffer

considered equal to .'J..,' (assuming 'Ai ' is very small when compared to Ao 19).

... (24)

where 'e' is the electronic charge, Eo is the permittivity of free space, rl and r 2 are the radii of the two reacting species which are assumed to be spherical , and rl 2 = rl + r 2, Do is the optical relative permittivity (equal to the square of the refractive index) and Ds is the static relative permittivity of the solvent (78.4 for water) .

The Gibbs activation energy is calculated from the

A values (Table 5) using Eq. (25):

(A + ~Go)2 ~G~ =-----

4A .. . (25)

Using the reported potentials, the values of ~Go are calculated (~Go = -nFt-<J ) and the expression for the theoretical rate constant (kca l) of a reaction is represented by :

... (26)

Here, Z is the collision number ( lO") 18.20 and kB is

the Boltzman constant. The theoretically calculated va lues agree well with

the ex perimentally observed rate constants for all the oxidants except [2. For [2, the observed higher rate constants may be due to the charge transfer complex formation between DADPM and hS

.6 as mentioned

earli e r. [n addition , the following ex press ion is used

12

10 0 0 0 0

0 0

8 0 0

" "'

0 Ci.o .Q 6 'b g

=s 4 ~ 0>

0 ;;,

2 ~ 0 .!!.

0

=s a 0 0> .Q

-2 0

-4 6

·6 0

-8

-400 -300 -200 -100 a 100

Fi g. 4 - Plot of log [ktcalll (0) and log [k(obs)1 (1';) versus I';C" for the formation of DADPMo+.

to tes t the present experimental results with Marcus plot:

k obs or

k = k(] cal I+Aexp({W +~G" (O)[+~Go /4~G " (0)] 2 }/ RT)

... (27)

A and W (work function) are assumed as 1 and 0 respectively and the diffusion controlled limit kd for waterS is taken as 1.1x10 10

. ~G* (0) is calculated to be 46 kJ mor 1 for DAD PM. [n Fig. 4, the va lues of k obs

versus ~Go and ken I versus ~Go are plotted. k obs is the rate constant obtained from the experiments concerni ng the reactions of DADPM with oxidants,

SANKAR et al.: ONE-ELECTRON TR ANSFER REACTIONS OF DADPM 2227

/).GO is the standard Gibbs energy change for the respective redox pair and kcal is the value calcul ated for /).Go ranging from -300 to +75 kJ mOrl . The trend of the plots shows that the experimentally obtained rate constants are in excellent agreement with the calculated values .

Acknowledgement Sincere thanks are due to UGC, India for the award

of project fell owship to CS.

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