Indian Journal of Chemistry Vol. 42A, July 2003, pp. 1666-1670

Molecular complexes of paraquat and chloranil with phenylhydrazones

T Charan Singh, P Venkateshwar Rao, T Veeraiah & G Venkateshwarlu*

Department of Chemistry, Nizam College, Hyderabad 500 001 , India

Received 17 August 200] ; revised 18 December 2002

Molecular complexes of paraquat and chloranil with phenylhydrazones have been studied speclrophotometrically in methanol together with those of paraquat in aqueous SDS micellar media. All the complexes exhibit one charge transfer band each in the region where neither of the components have any absorption. The ionisation potentials of the donors have been determined from the positions of the CT bands. The positions of CT bands and the stabilities of the compiexes are affected by the substituents. A drastic enhancement in the stabilities of the complexes of paraquat is observed in SDS micellar media, which is attributed to the increase in local concentrations of the components in SDS media.

I, I' -Dimethyl-4,4' -bipyridinium dichloride (Paraquat, PQ) is shown to form charge transfer complexes with inorganic anions lA , organic anions5-? as well as with neutral organic donors8- 11

, owing to its strong electron acceptor property . In continuation of our study on the CT complexes of PQ with neutral organic donors (anilines)ll, it is considered interesting to use benzaldehyde phenylhydrazones (BPHs) as donors. The title donors were chosen not only because they are structurally related to ani lines but also due to the fact that BPH is shown to form molecular complexes with chloranil (CA) and TCNEI2.15

• Since the electron affinity of CA (1.35eV) and PQ (1.24eV) are close, the molecular interaction between the PQ and BPHs is hopefully expected. Although the CT spectra of CA with BPH was reported, the study of CT complexes of its substituted derivatives have not been reported yet. The characters of charge transfer complexes viz., the CT spectra, stabilities and thermodynamic properties of CA-BPHs and PQ-BPHS in methanol, together with the effect of sodium dodecyl sulphate, SDS, on the stabilities of the CT complexes of PQ BPHs in aqueous medium have been investigated and are reported.

Experimental Paraquat was prepared by the dimerisation of

pyridine to 4,4' -bipyridyl, followed by quarternisation with methyl chloride and isolation as dihydrate8.

Commercial sample of chloranil (Aldrich) was recrystallised from chloroform and was vacuum sublimed till TLC pure. Phenylhydrazones were prepared by the condensation between substituted benzaldehydes and phenylhydrazine l6 and were repeatedly recrystallised till TLC pure. High quality SDS (BDH Analar) and spectrograde (BDH) methanol were used without further purification. Triply distilled water was used to prepare aqueous SDS solutions. The UV -Vis spectra of the complexes were recorded on a Shimadzu 240 double beam spectrophotometer. In methanol, the concentrations of the donors were held constant at 2.5xlO-2 M and that of PQ varied between 2.5xlO-3 and 1.25xlO- l M so as to produce the complexes with measurable absorbance. The concentration of CA was held constant at 2.5xlO-3 M while those of donors varied between 0.2 M and 0.8 M . The solutions of complexes in SDS were prepared by injecting concentrated methanolic solutions of BPHs into solutions of different concentrations of SDS which already contained required quantity of PQ. In SDS media the concentrations of any donor is 3xlO-3 M and that of PQ varied between 2xlO-3 and lxlO-l M. The strength of methanol was kept below 1 % v/v since higher concentration of methanol is known to alter the characters of micelle lO

• The formation constants of the complexes were determined by (1 ) Benesi­Hildebrand l? and Rose- Drago methods l8. The composition of the complexes was determined by Job's continuous variation method and all other experimental details are reported elsewhere?·ll.

[Dol/d=1/[AolK£+ 1/£ .. . (1)

K- l =d/£-([ Aol+[DoD+[ Aol [Dol£/d . .. (2)

Results and discussion The methanolic solutions of PQ and PH turned

brown red immediately after mixing. The BPHs too, produced characteristic colours, stable for several hr, on mixing with PQ indicating the formation of CT complexes. These complexes exhibited one CT band each in the range of 450-700 nm where neither of the components have any absorption. The CA-BPHs complexes too exbibited one CT band each in the region 600 to 800 nm. A linear relationship is

NOTES 1667

observed between the VCT of CA-BPHs and the VCT of PQ-BPHs indicating that same molecular orbital of donor is involved in complexation with both the acceptors. The appearance of CT bands is attributed to the excitation of an electron from the highest occupied molecular orbital (HOMO) of the donors to the lowest unoccupied molecular orbital (LUMO) of the acceptors. It is interesting here to note that BPH has three 1t-donor sites viz., benzaldehyde moiety (ring A), anilino moiety (ring B) and the hydrazone group (>C=N-N<). The donor ability of ring A is expected nearly equal to that of benzaldehyde and that of ring B is lower than the donor ability of aniline as >C=N group competes for the non bonded electrons of adjacent N atom. Aniline forms CT complex with CA as well as with PQ and exhibits CT bands at 560 nm 420 nm respectively, while benzaldehyde does not. The fact that BPH exhibit CT band at 640 nm with CA and at 528 nm with PQ implies that neither the benzaldehyde nor the aniline ring is the donor site and donation from hydrazone group is more plausible as envisaged by Giorgio et al. 15 in case of chloranil­BPH complex.

The characters of 1t-HOMO of phenyl hydrazones have been theoretically calculated by Pacansky et al. 19. The authors report that the HOMO consists of three pairs of p- orbitals two of which form the C=N bond and the third contains the lone pair of electrons on amine nitrogen. From the orbital plot they show that, although the extended 1t-system is delocalised over entire molecular frame work, the most part of the charge of HOMO is localised on or in the immediate vicinity of the hydrazone group. It is therefore inferred that donation takes place from the HOMO consisting of hydrazone group (Structure I)

Effect of substituents on the CT bands The position of CT band of CA-BPH at 640 nm and

PQ-PH at 528 nm are significantly affected by substituents present in ring A. The electron releasing substituents viz., p-N,N-(CH3)2' P-OCH3 and P-CH3 groups shifted the CT band to longer wavelengths while p-CI o-OH produced a hypsochromic shift. A plot of VCT versus Hammett cr constant is linear but for p-Cl and o-OH which exhibited a scatter. The bathochromic shifts caused by the substituents may be attributed to the mesomeric (+M) effect of p-N,N'­dimethyl and p-methoxy groups and to hyperconjugative effect of p-methyl group which boost up the energy of the HOMO of donor and bring it closer to tife LUMO of acceptor. The chloro group

which is expected as electron releasing due to the involvement of p-electrons, can also act as electron withdrawing due to its inductive effect with a Hammett constant of + 0.23 can qmse a large bathochromic shift, and the combined effect of these two oppositely acting factors cannot be evaluated a priori. However, the small hypsiochromic shift ' observed may be due to the predominance of -I effect of the Cl group over mesomeric effect. The hypsochromic shift due o-hydroxy group may be attributed to hydrogen bonding between the o-OH and the lone pair of electrons on the N atom which reduces the electron density of the N atom which in turn reduces the electron density of HOMO that essentially consists of p-orbitals of hydrazone moiety l9.

Ionisation potentials of the donors The ionisation potentials of the donors have been

determined from the positions of the CT bands using the relationship between the energy of the CT band (ECT) and the ionisation potentials of the donors (10) viz.,

ECT = 0.90 10 - 4.19 for PQ complexes8

and are reported in Table 1. The ionisation potentials are very close not only to those reported for a few phenylhydrazones (6.94-7.3eV) from their photoelectron spectra20 but also to the values reported from the CT spectra of a few structurally related PHs with chloranil23

. The ionisation potentials could not be determined from the positions of CT bands of CA­BPHs since an appropriate equation for the same in methanol is not available in literature. However the E CT of CA-BPHs are linearly related to the ionisation potentials obtained from the E CT of PQ-BPHs with a slope of 0.685 and an intercept of -3.04 eV.

Formation constants and thermodynamic parameters The formation constants of complexes increase

with increasing electron releasing ability of the

I3PHs

I

1668 INDIAN J CHEM, SEC A, JULY 2003

substituents in the ring A and are in the order:

the logarithmic functions of the stability constants bear a linear relationship with Hammett (J constants except p-CI and o-OH. A small decrease in the stability may be due to the predominance of inductive effect over the mesomeric effect of CI group as stated earlier. The o-OH group decreased the stability of the complex, contrary to the expectation. This may be due to decrease in the electron density in donor site due to H. bonding as discussed earlier. The thermodynamic parameters viz.,MI, LiS and Lie of the complexes have been determined from the variation of K with temperature. A plot of log K vs liT gave a straight line, from the slope and intercept of which the MI and .1S have been evaluated. The Lie values are calculated using the equation

Lie = MI-T .1S

and are reported at 25°C Tables I and 2. The MI values are found to be about 10 k cal mol- ' a characteristic feature of CT complexes. The negative enthalpies show that the complex formation is spontaneous while negative entropies indicate a decrease in the degree of feedom of the components upon complexation. The plot of MI vs LiS is a straight line indicating that the complex formation is sterically unhindered by the substituents. This favours a sandwitch structure for the complexes in which the n orbital of donor and n* orbital of acceptor can have greater possible overJap22.

CT complexes of paraquat with phenylhydrazones in SDS micellar media

Colourless solutions of paraquat in aqueous SDS produced characteristic colours when a small volume of methanolic solutions of donors were introduced. The colours of the solutions are comparable to those of PQ-BPHs in methanol. Colouration was not

Table I- Charge transfer spectra, stability constants and thermodynamic parameters of PQ-BPHs complexes in methanol (Values in the parenthesis are of PQ anilines given for comparison)

Substituent A. £ IP K (M'I) at -/!"H -.15 - L1G (nm) (M'I eV 10°C 15°C 20°C 25°C 30°C 35°C (kcal (cal (kcal

cm - I) mol- I) deg- I mor l)

mor l)

p- NN(CH) 2 645 500 6.84 8.65 7.31 6.22 5.33 4.57 3.95 5.43 14.9 0.989 (500) (540) (7.45) (4.8)

p- OCH) 563 400 7.13 6.84 5.84 5.01 4.29 3.76 3.28 5.11 14.2 0.868 (495) (510) (7.46) (4.45)

p- CH) 545 360 7.20 6.46 5.50 4.75 4.14 3.58 3.13 5.01 14.0 0.838 (469) (480) (7.61) (3.16)

p- H 528 350 7.27 5.86 5.03 4.35 3.75 3.29 2.89 4.90 13.8 0.787

(440) (400) (7.78) ( 1.90) p- Cl 525 340 7.30 4.82 4.18 3.64 3.16 2.80 2.47 4.62 13.2 0.686

(429) (360) (7.91) (0.9)

0- OH 518 350 7.32 3.60 3.15 2.77 2.44 2.17 1.93 4.30 12.6 0.545

Table 2---Charge transfer spectra, stability constants and thermodynamic parameters of CA-BPHs complexes

Substituent A. £ K (M'I) at -L1H -.15 -L1G (nm) (M'I 10°C 15°C 20°C 25°C 30°C 35°C (kcal (cal (kcal

cm- I) mol-I) deg-I mol-I) mol-I)

p- NN(CH)2 769 1480 22.55 17.55 13.78 10.9 8.73 7.00 8.12 22.5 1.415

p- OCH) 680 1240 13.33 10.6 8.51 6.92 5.60 4.58 7.40 .20.6 1.260

p- CH) 655 1120 11.77 9.43 7.62 6.21 5.08 4.19 7.16 20.4 1.082

p- H 640 1050 9.55 7.82 6.53 5.45 4.58 3.87 6.25 17.6 1.007

p- Cl 637 980 9.00 7.44 6.21 5.18 4.36 3.70 6.16 17.4 0.975

0- OH 630 800 7.44 6.21 5.22 4.37 3.74 3.15 5.94 17.1 0.874

NOTES 1669

Table 3-Charge transfer spectra, stability constants of PQ-BPHs complexes in SDS micellar medium. (Values in the parenthesis are of PQ~nilines given for comparison)

Substituent Ie E

(nm) (lfl cm - I) [SDS] = 0.02

p-NN(CH3h 640 520 1600

p- OCH3 560 420 1503

(1400)

P-CH3 542 380 1451

(1052)

p-H 532 350 1278

(601 )

p-CI 525 360 1052

0- OH 518 350 526

observed when donors were introduced into simple SDS solution (without PQ). The colour changes in these systems are therefore attributed to the formation of CT complexes between PQ and BPHs in micellar pseudophase. Each of the solutions exhibits an absorption band characteristic of CT complex. The positions of the CT bands in SDS are close to those of the CT bands in methanol. The substituents in donors shifted the positions of CT band and these are parallel to those observed in methanol. Hence the effect of substituents may be explained in terms of mesomeric, inductive effects of substituents and H bonding as discussed earlier.

The plausible site of CT complex formation in SDS may be the polar end of the micelle. Bertolotti et al. 3

showed that PQ strongly binds (K = 1700 M-1) to the

polar end of the micelle while neutral organic donors are simply solubulised in the non polar end of the micelle 10 It is reasonable to think that the donors can diffuse towards the PQ and form CT complexes at the polar end of the micelle.

The formation constants observed in SDS are hundred to thousand folds more than those observed in methanol (Table 3). The drastic enhancement in the stabilities of the complexes in SDS cannot be due to the increased affinity of components to form complexes either by decrease in the ionisation potential of donor or by increase in the electron affinity of acceptor in SDS. In such cases the AcT and c should also have changed. As there is not much change observed either in ACT or c, we infer the remarkably high stabilities of the complexes to the increase in local concentration of the components in micellar medium3

,lO. The apparent stability constants, K (SDS) of complexes decreased with increasing concentration of SDS from 0.02 to 0.1 M. Studies

K (lfl) apparent 0.04 0.06 0.08 0.1 Kcr

840 630 458 364 5.6

732 484 361 288 4.2

(650) (360) (300) (202) (3.7)

706 467 349 278 3.86

(512) (338) (253) (202) (2.8)

622 411 307 245 3.4

(293) (193) ( 144) ( liS) ( 1.6)

512 338 253 202 2.8

256 169 126 101 1.4

could not be made below this concentration as precipitation occurred sometimes. The decrease in the apparent formation constants with increasing concentration of SDS may be attributed to the increase in the effective volume of the residence i. e., the volume of micelle in which the components reside and this results in a decrease in the local concentrations of the components and hence in apparent formation constants. The micellar volume independent formation constants (KCT) have been calculated from the relation put forward by Bertolotti et aP.

K - \SDS) = V ([SDS] - cmc)IKcT

Plots of K - \SDS) versus ([SDS] - cmc) gave straight lines with slope equal to V/KCT where V is a constant (0.14 M-1

) called molar volume of micellised detergene3

. The KCT values in micellar pseudophase are somewhat smaller than those in methanol. Comparison of positions of CT bands and stabilities of PQ - anilines complexes with those of PQ- BPHs, either in the methanol or in SDS micellar media, indicate that BPHs are better donors.

Acknowledgement The authors are thankful to the Principal, Nizam

College and Head, Department of Chemistry for providing facilities. One of us (T V) is thankful to B Yadagiri Reddy of SAP college for encouragement They wish to thank B. Subrahmanyam for helpful discussion and suggestions.

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1670 INDIAN J CHEM, SEC A, JULY 2003

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