Indian Journal of Chemistry Vol. 43B, December 2004, pp. 2653 -2660

Synthesis, characterization and electrochemical behaviour of some substituted 2-phenyl-4-( 4'-ary lazophenyl)-3-thioxo-2H-3,4-dihydro-2,4,9, lO-tetra­

azaphenanthren-l-ones

Pratibha Sharma*, Ashok Kumar, Manisha Sharma & Siya Upadhyay

School of Chemical Sciences, Devi Ahi lya Univers ity, Indore 452 017 , Indi a

E-mail: drpratibhasharma@yahoo.com

Received 25 Novell/ber 2003; accepted (re vised) 24 May 2004

A new faci Ie method for the sy nthesis of 2-phenyl-4-( 4' -arylazpheny l)-3- thioxo-2H-3,4-dihydro-2,4,9, 10-tetra­azaphe nanthren- I-ones has been developed. Structures of a ll the sy nthes ized compounds have been estab li shed o n the basis of their consistent elemental, IR and NMR speclra l clata. The electroc he mi ca l reductions of synthesized compounds have been studied over a w ide pH range in B.R. buffers at dropping-mercury and g lassy carbon e lec trodes. All the compounds are found to undergo catt>odic rcduction by the uptake of two-electrons in diffusion-controlled ancl irrevers ible manner. On the basi s o f polarography, cyc lic vo ltammetry and coulometry, a plausible reduction mechanism is suggested to account for the

reduction of azo site in the compounds . Kinetic parameters i.c., charge-transfer coeffic ient (an), forward rate constant (K\,,), diffusion-coefficient (Do"\ and d iffusion current constan t (I) have al so been calc ulated . An inte resting corre lation between half-wave potential (E I12) values of va ri ous substitucn ts and Hammet t substi tuents constan t (0) have a lso bee n interpreted.

IPC: Int.CI.7 C 07 D 221110

1n recent years the number of investi gation within electrochemistry has been increased tremendously. A number of analytical methods can be used to elucidate the mechanism of the electrochemica l reaction. The electron is a non-polluting agent and electrochemica l reactions are easy to control automat ically . A systematic perusal of carlier literature reveals that inspite of the variegated importance associated with heterocyc li c system compri ses phenanthrene nucleus l

.

7 d . 8· 10 I . I j' . an azo mOiety, re atlve y aew reports eXist on the electrochemical behaviour of azo compounds having heterocycl ic moiety attached at one of the ends of azo group . However, electrochemical studi es on aromatic azo compounds have been the subj ect of

. .. I ' I I f II - IR many 1I1ves tl ga tlons t unng 1 1C astew years .

In continuation of our recent work on electro­chemical mcthods 1'1.20 and synthesis of heterocycl ic systems2 I

' 23 , efforts have been laid down to undertake comprehensive viz. , polarographic, cycli c voltammetric and coulometri e studies on some hitherto uninvestigated synthes ized compounds containing azo functional ity nan ked bet ween phenanethrene nucleus and aryl group. Keeping this ract under consideration and to broaden the knowledge of the electrochemistry

and possible electroanalytical assays, it was considered worthwhile to study critically the electrochemical behaviour of some 2-phenyl-4-(4'-arylazophenyl)-3-thioxo-3 ,4-d ihydro-2H-2,4,9, 10-tetraazaphenanthren- l­ones with a view (i) to decide about the rate of electro­reduction process,(ii) to elucidate the mechani sm of elect ro-reduction, and (iii ) to find out the effect of various experimental conditions. The electrochemical behaviour of these compounds was studi ed at dropping-mercury and glassy carbon electrodes. The effect of substituent is interpreted in terms of the Hammett eq uation . The kinetic parameters, i.e., charge­transfer coefficient (an) and forward rate constant (KOCh) have also been calculated.

Experimental Section

IR spectra (cm· l) of the sy nthesized compounds

were sca nned in KBr pellets on a Perkin-Elmer Ff-IR program 1000 spectrophotometer; and I H N M R spectra (in DMSO-d6/CDCl J) on Varian 270 MHz instrument using TMS as an internal standard. All the reported melting points were taken in open capillaries on an electri c melting point appara tus and are uncorrected. Purity of all the sy nthesi zed compounds

2654 INDIAN J. CHEM., SEC B, DECEMBER 2004

was ascertained by recrystallization and TLC (ethyl acetate-xylene; 4:6 v/v). Potentiometric studies were carried out using an expanded scale pH meter with glass electrode, which was previousl\y standardized with buffer of known pH.

Synthesis of 2-phenyl-4-(4' -arylazophenyl)-3-thioxo-2H-3, 4-dihydro-2,4,9,10-tetraazaphenanth­rene-1-ones. To a diazotized solution of pertinent anilines (0.01 M), an equimolar quantity of diethylmalonate was added at freezing temperature and thoroughly stirred to obtain coupled hydrazono product diethyl [(E)-phenyldiazenyl] malonate 1, which in turn was cyclized in the presence of phosphoryl chloride (5.0 mL) ll si ng reflux ing conditions to yield yellowish crystals of ethyl 4-chlorocinnoline-3-carboxylate 2. Derivative 2 was refluxed with p-aminoazobenzene (1.96 g, 0.01 M) for 30 min under alcoholic medium and then inverted on crushed-ice to yield beautiful reddish crystals of

compound ethyl 4-( {4-[(E)-phenyldiazenyl]phenyl}­amino)cinnoline-3-carboxylate 3. It was further stirred with phenyl isothiocyanate (1 .35 g, 0 .01 M) for 2 hr, which resulted in the formation of an inter­mediate precursor compound 1-(3-methyl-cinnoline-4 -yl)-3-phenyl-I-( 4-phenylazo-phenyl)thiourea 4 . Precursor 4 thus obtained was heated on sand-bath at 190°C for 30 minutes to generate finally the crystals of fused phenanthrene heterocycl ic derivatives 2-pheny 1-4-(4' -ary lazopheny 1)-3-thioxo-2H-3 ,4-dihydro-2,4,9,1 O-tetraazaphenanthrene- L -ones 5.

Characterizations of all the synthes ized compounds are given in Table I and schematic representation of synthetic pathway is g iven in Scheme I.

Electrochemical Studies The polarographic measurements were made with

Toshniwal Polarograph. 1.0 x 10.3 M stock solutions of azo compounds were prepared in dimethyl

Table I-Characterization data of 2-phenyl-4-(4'-arylazophenyl)-3-thioxo-2H-3,4- dihydro-2,4,9, 10·· tetraazaphenanthren-I-ones

Compd R m.p. Yield Found(Calcd) (%) iH NMR (8, ppm) °c (%) C H N S

Sa H 120-22 70 69. 10 3.74 17.28 6.06 7.4(5, 10H, C6Hs x2), 7.6-7 .9(m, 8H, Ar-H) (69. 12 3.73 17.27 6.59)

5b 4-CI 126-38 60 64.49 3.26 16.12 6.14 7.5(5, 5H, C6Hs), 7.7-7.9 (m, 12H, Ar-H) (64.55 3.29 16.13 6.15)

5c 4-0H 115-17 72 66.85 3.58 16.71 6.36 7.3 (5, 5H, C6Hs), 7.5-7 .7 (m, 12H, Ar-H), 12.5 (66.92 3.61 16.72 6.38) (bs, OH, 0 20 exchangeable).

5d 3-C2Hs 122-24 67 69.95 4.27 16.32 6.21 1.20 (t. 3H. CH). J = 9.0 Hz). 2.61 (q. 2H . CH2•

(70.02 4.31 16.33 6.23) J = 9.0 Hz), 7.5- 7.6 (bs. 5H. C6Hs), 7.8-8.0 (m. 12H. Ar-H)

CI H,N----O-N -OR

c6COOC H ~ "N Ij -.....:::: -.....:::: 2 5

I h- ~N -

N Stirring. 30 min (2)

.. Cyclization

(4) (3)

Compd Sa 5b 5c 5d R H 4-CI 4-0H 3-C2Hs

Scheme I

SHARMA et al.: ELECTROCHEMICAL BEHAVIOUR OF TETRAAZAPHENANTHRENES 2655

formamide (AnalaR). To study the effect of pH on E I12, Britton-Robinson buffer24 was used. Saturated calomel electrode and dropping mercury electrode (DME) were used as reference and working electrodes, respectively. Capillary characteristics were determined at zero applied potential in 1.0 M potassium chloride solution unless otherwise stated and polarographic curves were recorded at height of 40.0 cm of mercury reservoir column at room temperature.

Cyclic voltammetric studies were carried out on computerized VSM/EC/30-S potentiostat. The solu­tion was deoxygenated by bubbling with purified nitrogen gas for about IS min and then a blanket of nitrogen gas was maintained throughout the solution. The reference electrode used was Ag/ AgCl electrode, and the auxiliary electrode was a platinum electrode. In this study a glassy carbon (GC) electrode was used as the working electrode. The reproducibility and the activity of the electrode was tested by measuring the cyclic voltamogram response for ferricyanide/ferro­cyanide system in 0.1 M KCI2s .

The aliquots for electrochemical measurements were prepared by taking 4 mL of compound solution, 2 mL of dimethylformide, 2 mL of supporting electrolyte and 2 mL of B.R. buffer of appropriate pH in a electrolyte cell. The number of electrons (n) involved in the reduction was determined by comparing the waves with the waves

of azobenzene in B.R. buffer at different pH by the method of DeVries et a1. 26

. The values of 'n' were also determined by controlled potential electrolysis27

• The temperature coefficient of the electrode process was calculated by the method of Meites28 (Table II).

As the products of controlled potential electrolysis (CPE) have been known to differ, electrolysis was carried out in a microce1l29

. Preparative electrolysis was carried out in 2.0xlO·3 M solution at d.m.e. keeping a blanket of nitrogen over the surface of the solution. The product of analysis was identified by comparison of properties with the expected product.

The pKa value of all the compounds under investigation were determined by the measurement of absorbance parameters29

.30

. The pKa values of all these compounds calculated lie between 7.4 to 8.4, and are almost in the same range as evaluated by polarographic measurements (Table II).

Results and Discussion

(a) Controlled Potential Electrolysis (CPE) and Product Analysis. Controlled potential electrolysis (CPE) with DME of 2-phenyl-4- (4'-arylazo phenyl)-3-thioxo-2H-3,4-dihydro-2,4,9, 10-tetraazaphenanthren­I-ones was carried out at the plateau potential -1.2V. The value of number of electron (n) involved was found to be 2 as confirmed by the diffusion-current

Table II - Electrochemical characleristics of 2-pheny 1-4-(4' -arylazopheny 1)-3-thioxo-2H-3,4-dihydro-2,4,9, 10-tetraazaphenanthren- l-ones

{m213t"6 = 3.34 mg213 sec· I12 , n = 2, C = 0.2xI0·3M, pH = 6.2, t"2 = 1.73 sec, h = 40.0 cm}

S. R -E1I2 (V) id (/JA) Slope (V) an Do"2x 10.3 K\h Temp. pK(*) No. (cms· l ) (cm2s· l ) coeff. (%)

I H 0.42 2.42 0.075 0.722 2.56 7.1 7xlO·9 3.62 1.16 8.1 (7.9) 2 4-CI 0.35 2.47 0.077 0.703 2.61 5.67xI0·s 3.69 1.10 8.4 (8.2) 3 4-0H 0.52 2.37 0.084 0.645 2.50 9.77xlO· 'O 3.55 1.32 7.6 (7.4) 4 3-C2Hs 0.40 2.40 0.086 0.630 2.53 2.2IxI0·s 3.59 1.50 8.2 (8.0)

*Spectrometric values

Table III-Values of half-wave potential (-E II2' V) and diffusion-current (id' /JA) for 2-phenyl-4-(4'­ary lazophcny l)-3- thioxo-21!-3,4-di hydro- 2, 4, 9, I O-Iclraazaphenanlhrcn-I-one at diffcrent pH va lucs

{C = 0.2xI0·3 M, h = 40.0 cm}

S. No. pH 4-CI 4-0H 3-C2Hs -E1I2 (V) id (/JA) -EI I2 (V) id (/JA) -E1I2 (V) id (/JA)

2.4 0.10 2.50 0.18 2.28 0.12 2.36 2 4.0 0.19 2.48 0.32 2.26 0.22 2.42 3 5.2 0.28 2.48 0.43 2.26 0.31 2.40 4 6.2 0.35 2.47 0.52 2.37 0.40 2.40 5 8.0 0.44 2.50 0.69 2.40 0.51 2.40 6 9.4 0.50 2.52 0.72 2.42 0.55 2.38 7 10.0 0.50 2.50 0.72 2.38 0.55 2.38 8 11.2 0.50 2.48 0.72 2.29 0.55 2.40

2656 INDIAN J. CHEM., SEC B, DECEM BER 2004

constant31 (l ::= 1.67n) values given in Table III. The progress of the electrolysis was monitored by recording UY -vis spectra at di ffe rent intervals of time. With the progress of the electrolys is, absorbance at 360 nm systematicall y decreases and fin all y dis­appears. These observations show that no inter­mediate with sufficient half- life was generated during electro reducti on of the compound . The structure of the fin al product was confirmed by IR spectra owing to simultaneous emergence of a band at 3325 cm-I and di sappearance of a band at 1580 cm-I due to the reducti on of 920 moiety. IR (cm- I): 3325 (N-H ), 3050 (C-H, sp\ 1705 (C=O), 1990 (C=S ).

(b) D.C. Polarography. All the synthesized 2-phenyl-4-( 4'-arylazo phenyl)-3-thioxo-2H-3,4-dihydro-2,4,9, 10-tetraazaphenanthrene- I-ones gave well defined, two-electron transfer wave in the pH range 3.0-11.0 at dropping mercury electrode corresponding to the reduction of azo group (Table III, F igure 1). The diffusion character of the limiting current has been confirmed by the linear dependence of the limiting current on the concentration of depolarizer (Table IV)

O.B

0.7 4-CI •

• 4-0H 0.6 ... 3-C

2Hs

0.5

> ~0.4 l-';l •

0.3

0.2

0. 1

4 6 8 10 12

pH

Figure I-EII2 liS pH plots for 2-phcny l-4-(4'-ary lazo phcnyl) -3-th ioxo-211-3,4-dihydro-2,4,9, 10- lctraazaphcnanthrcnc- I -oncs

and square root of the mercury reservoir height (Table V). The irreversible nature32.33 of the electrode process was establ ished by the log plots. It was fo und that the slope value of the plots of Ed.c vs log i/id-i exceeded appreciably by 59.2/a n mY . However, the numerical value of E3/4 - E1 /4 was more than 56.4/a n mY. These facts confi rmed the irreversible nature of the electrode process. The fact that Et /2 shifted towards negative potential with increasing depolari zer concentrati on (Table V) further confirmed the irreversible nature of the waves. The low va lue of temperature coeffici ent (1. \0- 1.50%) further verified the diffusion-controll ed nature of the waves.

The half- wave potenti als are dependent on pH and shifted towards more negative potential with increase in pH. It is deduced from thi s behaviour th at a protonation takes pl ace before first electron uptake. Simi lar dependence of half-wave potenti al on pH has also been fo und by other workers in the case of azo compounds34-36

. The plots of EI /2 liS pH are li near with the slopes in the range 0.071 to 0.09 1 Y pH-I.

Keeping in view the criteria of diffusion-controlled nature and irreversib il ity the kinetic parameters, i.e., charge transfer coeffici en t (product of transfer coeffi cient and number of electrons involved per molecul e of the reactant) and forward rate constant (k\h) have been calculated37 by the method of Meites

3~ et al. and are reported in Table II . The number of protons in volved per molecule of the reactant in the rate-determining step are ca lcul ated using the f II ' . 39 o oWll1g equatlOll' .

P = (d EI/2/d pH) . a n/0.05916

As the number of electrons involved in the reduction is two and the number of proton in vo lved in the rate determining step is one, a mechani sm can be proposed fo r the reduction of these compounds whi ch is anal ogous fo r the reducti on of azobenzene and

40-42 other azo compounds . (c) Cycl ic Yoitammetry . The cyclic vo lta mm­

ograms of the these compounds were recorded at

T able I V - Va lucs of hal f-wavc potcn tia l (- E I /2 , V) and di ffusion-currcnt (id , ~lA) for 2-phcny l-4-(4'-ary lazophcnyl)-3-Ih ioxo-2H-3,4-di hydro-2,4,9, I O- l c traazaphcn ~lIl l hrc n- I-o nc at di f'('crcnl conccntrati ons of dcpolarizcr

{pH = 6.2 ± O. I ,h = 40.0 em)

S. No. R Dc[!olarizcr (M ) 1.0x I 0-4 2.0x I0-4 3.0x IO-4 4.0x I 0-4 S.Ox IO-·'

-E1I2 (V) i" (~lA ) -E1I2 (V) i" ( ~lA ) -E1I2 (V) i" (~ lA ) -E 1I2 (V) i" ( ~lA ) -E 1;2 (V) i" (~lA )

4-CI 0.32 1.1 6 0.35 2.47 0.38 4.97 0.40 9.98 0.42 11.85 2 4-0 1-1 0.50 1.1 8 0.52 2.37 0.54 5.44 0.56 7.22 0.58 9.06 3 3-C2H5 0. 38 1.20 0.40 2.40 0.43 3.62 0.45 4.82 0.47 6.04

SHARMA et al.: ELECTROCHEMICAL BEHAVIOUR OF TETRAAZAPHENANTHRENES 2657

different scan rates (20 m V /s to 200 m V /s and concentrations (1.0xlO-4 to 5.0x l0-4 M). All the compounds exhibit well-defined cathodic peak in the pH range 3.0-11 .0. The linear dependence of the peak current (ip) on the square root of the sweep rate (V/2) (Table VI) and on concentration (Table VII) indicate diffusion-controlled nature. The shift of peak potential towards negative side with the increase of concentration has been assigned due to irreversible reduction. Hence, kinetic parameters i.e. , an and KOr.h

Table V - Effect of varying mercury pressure on some 2-phenyl-4-( 4' -ary lazopheny 1)-3-thioxo-3,4-dihydro-

2H,2,4,9,10- tetraazaphenanthren-I-ones(pH = 6.2 ± 0.1, C = 0.2x I0·) M)

SI No.

R '-Ihcff id ()..LA) Idl'-lhcff ()..LA/cnf ln )

4-CI 30 27 .6 5.25

35 32.6 5.70

40 37.6 6.13

45 42.6 6.52

50 47.6 6.89

55 52.6 7.25

60 57.6 7.58

2 4-0H 30 27.6 5.25

35 32.6 5.70

40 37.6 6.13

45 42.6 6.52

50 47.6 6.89

55 52.6 7.25

60 57.6 7.58

3 3-C2Hs 30 27.6 5.25

35 32.6 5.70

40 37.6 6.13

45 42.6 6.52

50 47 .6 6.89

55 52.6 7.25 60 57.6 7.58

2.12

2.26

2.47

2.65

2.73

2.91

3.03

2.04

2. 19

2.37

2.51

2.67

2.78

2.96

2.05

2.25

2.40

2.54

2.73

2.86 2.97

0.404

0.397

0.403

0.406

0.396

0.401

0.400

0.389

0.384

0.387

0.385

0.388

0.383

0.391

0.391

0.395

0.392

0.390

0.396

0.394 0.392

have been calculated, which are in the almost same range as evaluated by polarographic measurements and reported in Table VIII.

(d) Redox Mechanism. Keeping in view the feasibilities of the sites of the reduction and on the basis of OCP, CV and CPE, it is concluded that two­electrons and one protons are involved in the rate­determining step. The solution after CPE gives negative test for amino group, thereby showing that after reduction of -N=N- to -NH-NH- , further reduction does not take place. The max imum absorbance at 350 nm disappears in solution after complete electrolysis. Since only one product is obtained after exhaustive electrolysis, the mechani sm as shown in Scheme II is suggested. Furthermore, the above mentioned electrolyzed solution does not give

Table VI-Values of peak potential (-Ep, V) and peak current (lp' )..LA) of 2-pheny l-4-( 4' -arylazophenyl)-3-thioxo-3,4dihydro-

2H, 2, 4, 9, 10 -tetraazaphenanthrene-I-ones at square root of various scan rates

(pH = 6.2 ± 0.1, C = 0.2x I0·) M, n = 2, A = 0.02704 cm2)

S. No. R v (V/Sec) v ln -Ep (V) ip ()..LA)

4-CI 0.02 0.141 0.46 1.20

0.05 0.224 0.48 1.90

0.10 0.314 0.51 2.70

0.15 0.387 0.53 3.30

0.20 0.447 0.44 3.82

2 4-0H 0.02 0.141 0.54 1.13

0.05 0.224 0.57 1.80

0.10 0.314 0.59 2.56

0.15 0.387 0.62 3. 12

0.20 0.447 0.64 3.61

3 0.02 0. 141 0.50 1.24

0.05 0.224 0.52 1.99

0.10 0.314 0.54 2.82

0. 15 0.387 0.57 3.45

0.20 0.447 0.60 3.98

Table VIJ-Values of (-Ep,V) and (Ip' )..LA) for 2-phenyl-4-(4'-arylazo phenyl)-3-thioxo-2H-3,4dihydro-2, 4, 9, 10 tetraaza phenanthren-I-ones at different concentrations of depolari zer

(pH = 6.2 ± 0. 1, A = 0.02704 cm2, v = 0.0 I V Isec )

S. R DC[~o lari zer (M) No. I.Ox I0·4 2.0xI0·4 3.0x lO·4 4.0x I0·4 5.0x I0·4

-Ep (V) ip ()..LA) -Ep (V) ip ()..LA) -Ep (V) ip ()..LA) -EI> (V) ip ()..LA) -Ep (V) ip ()..LA) I 4-CI 0.48 1.20 0.51 2.70 0.53 4.98 0.56 7.14 0.59 9.24 2 4-0H 0.57 1.09 0.59 2.56 0.61 4 .16 0.63 6.60 0.65 8.82 3 3-C2Hs 0.51 1.40 0.54 2.82 0.57 5.05 0.60 7.25 0.62 9.32

265 8

S. No.

2

3

INDI AN J. CHEM., SEC B, DECEMB ER 2004

Table VIII - Diffusion coefficient (an) and forward rate constant ( KOu,) for 2-phenyl-4-(4'-a ry lazopheny l )-3-thi oxo-2 H-3,4-dihydro-2, 4, 9, 10- tetraazaphenanth ren- I-ones

R Conc. (111M)

a n Po larography Cyc li c voltam metry

-Ep (V) ip (),lA)

Polarography Cyc lic vo ltammetry Do 1/2X I 0.3 K"r.h

(cms· l ) (cm2s' l)

D ol/2 x I 0.3 KOr.h (cms· l) (cm2s· l

)

4-CI 0. 1 0.732 0.32 1.2 1 0.48 1.20 2.56 9.62x I0·8 2.72 l.l 4x 10.8

0.2 0.703 0.35 2047 0.51 2.70 2.6 1 5.67x I0·x 3. 13 9.75x I0·9

004 0.678 0.38 4 .84 0.53 4.98 2.6 1 2A5x I O'x 2.94 8.7 Ix I0·9

0.6 0.66 1 0040 7.26 0.56 7. 14 2.56 104 1 x I 0.9 2.84 5A6x I 0.9

0.8 0.638 0042 9.69 0.59 9.24 2.56 8. 18x I0·9 2.80 4.1 6x I0·9

1.0 0.623 0045 12. 11 0.63 11.0 I 2.56 2.59x I0·9 2.70 2. 12x I0·9

4-0H O. I 0.660 0.50 l.l 8 0.57 1.09 2.50 1.60x I 0.9 2.60 3.95x I 0.9

0.2 0.645 0.52 2.37 0.59 2.56 2.50 9.77x I0· IO 3.09 3.92x I 0.9

004 0.630 0.55 4.73 0.6 1 4. 16 2.50 4.59x I0' 1O 2.57 3.93x I 0.9

0.6 0.6 15 0.57 7. 10 0.63 6.60 2.50 2.78x 10. 10 2.72 2.57x I 0.9

0.8 0.602 0.59 9046 0.65 8.82 2.50 1.68x I 0.10 2.70 2.22x I 0.9

1.0 0.589 0.6 1 11.82 0.68 10.02 2049 1.0 1x 10·1O 2.5 1 1.11 x I 0.9

3-C2Hs 0. 1 0.7 13 0.38 1.1 9 0.5 1 l AO 2.5 1 3.58x I0·8 3.22 I.09x I0·x

0.2 0.630 0040 2.70 0.54 2.82 2.53 2.2 Ix IO·8 3045 2.07x I0's

004 0.660 0043 4.79 0.57 5.05 2.62 1.06x I 0.8 3.02 4.58x lO·9

0.6 0.637 0045 7. 18 0.60 7.25 2.53 6A7x 10·9 2.94 3A7x I 0.9

0.8 0.6 15 0047 9.58 0.62 9.32 2.53 3.96x I0·9 2.88 3AOx 10·9

1.0 0.595 0.50 11.97 0.65 11.24 2.53 1.89x I0·9 2.83 2.70x I0" J

Q s Q s N~ R N~ R

oCS-O-N=N-<) . oCS-O-N_~---O • N,\ _

N ~ /; N ~ /;

H+

Fast

Q ~ /; s Q s . N~ H H R

°CS-O-~-~-O ~ H R

N0- -2e', H+ 0i:S-O-L~-<) , .

N ~ /; Slow N" _ rate determining step

N ~ /;

R = As reported in Table I

Scheme II

SHARMA et al.: ELECT ROCH EMICAL BEHAVIOUR OF T ETRAAZAPHENANTHR ENES 2659

0.54

0.52 4·0H

0.50

0.48

0.46

:;- 0.44

~ 0.42 ~

0.40 3·C,H,

0.38

0.36 4·CI

0.34

·0.4 ·0.3 ·0.2 ·0.1 0.0 0. 1 0.2 0.3

Figure 2--Plot of -E 1dy) liS (J for some 2-pheny l-4-(4'-arylazo pheny l)-3-th ioxo-2H-3,4-d ihydro-2,4,9, IO-tetraza

phenanthrene- I-ones

any polarographic and cyclic voltammetric peak, thereby confirming the reduction mechanism. This mechanics is further supported by the work of others40

.42

.

(e) Structural Effects and Correlation of El/2 with the Hammett Substituent Constant (0'). It is apparent from the data that the values of dEl l2ldpH and a n are practically in the same range for the entire reaction series. Thus, it was possible to di scuss the effects of subst ituents quantitati vely in terms of the Hammett eq uati on. The values of the Hammett constant (cr) were used from the literature43. Figure 2 gives the plot of EI/2 liS cr. It is observed from thi s plot that artha substituents viz., 2-CI and 2-CH3 show deviation from regression line. Polarographic arfh a

shift (L10) is expressed by the foll owing relat ion.

L10 = (E II2)0.R-(E I12)P. R

It was determined fo r chloro and methyl substi­tuents and is found to be 0.02 and 0.05, respecti vely. The positive values of the arfha shi ft for these substi­tuents provide the support to the observat ion that reduction is facilitated in the arfha derivative44 in comparison to the para substituents. Similar va lues of the polarographic orlflU shi rt s have al so been reported by other workers45 in the rcd ucti on of azo compounds.

The influence of substituents has been justifi ed by Hammett cq uat ion i.e., log K/ KII = pcr given by Okamoto-Brown46

. The plot of log KlKH aga inst cr is linear with slope eq ual to 3.0 1.

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