Solvent and pH effects on the electronic spectra of some

Academic Open Internet Journal

www.acadjournal.com

Volume 14, 2005

Solvent and pH effects on the electronic spectra of some

azo benzimidazoles

A . H . Amrallah , N.A .Abdalla and E.y.El – Haty

Faculty of Science South Valley University , Aswan , Egypt

Pure and mixed organic solvents effects on the spectra of some azo benzimidazoles

have been examined. The different absorption bands displayed in the electronic spectra of these compounds are assigned and the possible solute-solvent molecular complexes are investigated. The pH effect is included with the aim of elucidating the acid- base behaviour. The pK values have been explained on the basis of the relative contribution of acidic and basic character of respective species.

Key words: Azo Benzimidazoles, Solvents .effect, Molecular complexes, pH effect, pK values.

Introduction:

Benzirnidazole nucleus annears to be especially toxic to fungi. Recently large number of benzimidazole derivatives have been developed and patented as fungicides ( Mosquera et al 1993 and srivastava 1980 ) .Although the recent development of the chemistry of heterocycic azodyes has been remarkable (Robets 1981 ) , the chemical properties of azo benzirnidazoles have not been fully investigated.

In view of the above observations we considered it worthwhile to study the spectral behaviour of some azo compounds in pure and mixed organic solvents of various polarities. The pH effect is also included with the aim of elucidating the acid - base behaviour as well as of calculating the acidity constants of the compounds under investigation.

Experimental :

A-preparation of the ArylaLo cyanomethyf Beiizimidazoles : -

All the chemicals used in the preparation of the azo dyes were A.R. products from BDH and Merck. 2-cyanomethyl benzimidazole was prepared using the procedure described previously ( Schams et al 1989 ) .The product so formed was crystallized from ethanol as pale yellow crystals. Azo compounds were prepared in similar manner as described previously

( ELNagdi et al 1979 and Vogel 1961 ) . The separated product was filtered off and crystallized from ethanol. The analysis y compounds by elemental analysis, IR and

H¹ NMR spectra gave good results.

B- Solutions:

All organic solvents used in the present investigation were of spectral grade (BDH or Merck) products. Stock solutions were prepared by dissolving the accurate weight of the compound in the appropriate volume of the required solvent.

The modified aqueous universal buffer series (pH 1.87-13.1) were prepared according to

method of Britton (Britton 1952) . The measured pH values were corrected using the relation pH^*= pH(R)-δ ,where pH^* is the corrected reading and pH(R) is the meter reading obtained in water-organic solvent mixtures. The values of δ for aqueous buffer solution containing organic solvents used was determined according to Douheret (Douheret 1967 ) The measurements of pH were carried out using pH-meter Model, MV 87 digital pH- Messgerate, accurate to ± 0.005 unit. The electrode system was calibrated using standard buffer solutions of pH’s 4.0, 7.0, and 9.0. The absorption spectra in the uv and visible regions were recorded on a Shimadzu UV-visible 240 recording spectrophotometer using 1 cm matched cells. The azo derivatives synthesized can be represented as follows:

where: X = -H (I_a ,I_b), X 0 -OH (II_a , II_b (,

X = p-OH ( III_a , III_b (,

X = p-COOH, rn-OH (IV_a,IV_b) , X = p-N0₂ (V_a ,V_b).

Results and Discussion

(A) Electronic Spectra In Pure Ethanol :

The recorded electronic spectra of the 2-(X -2-cyanomethyl-2- benzimidazoles (I_aV_a) and

(I_b-V_b) in absolute ethanol are shown in Fig 1. The band maxima, λ_max and the molar extinction coefficients, values of the absorptIon bands appeared in the spectra of the studied compounds in different organic solvents used are compiled in Table 1 , It is clear from the data that the spectra of the compounds comprise mainly three to four bands in the UV and visible regions depending upon their molecular structure and the nature of the solvent. The two UV bands located in the regions 2 12-220 and 232-258 nm can be attributed to the localization of it-electrons in benzene ring and imidazole ring of benzimidazole moiety respectively ( Srivaastava et al 1989 ). The little influence of the positions of the two UV bands by the nature of the substituent(X) , in addition to their high molar absorptivities substantiate their π – π ^* nature.

Moreover, the location of the two UV bands at longer wavelengths than that reported for substituted imidazole ( Perston 1981 ) can be attributed to the high delocalization of π –electrons in the azo compounds under investigation .

The band observed at around 270 nm in all compounds ,(except the p-NO derivatives), may be ascribed to the excitation of the π –electrons which localized on the phenyl ring. Such assignment is achieved from the observed influence of both Its position and molar extinction coefficient by the nature of the polar substituent (X) attached to the phenyl moiety.

The lower intensity shoulder observed in the range 280-312 mu is assignable to an n– π^* electronic transition involving the lone pair-of electrons of the tertiary nitrogen atom and /or the lone pair of the azomethin nitrogen atom. This assignment is supported by the spectral behaviour of the compounds in aqueous buffer solutions of varying pH’s, where the n– π^* band disappeared by protonation in acidic medium and thus, the excitation of n-electrons is expected to be hindered. However the absence of the n– π^* in the spectra of other compounds can be presumably ascribed to Its hiding under the envelop of the strong intramolecular charge transfer (CT) band of these compounds.

The main visible band located in the region 384-430 nm is an intense one with λ_max much affected by the molecular structure of azo compounds. Thus, it can be assigned to

π–π^* irtransition involvng it-electromc system of the compounds. This transition is associated with considerable intramolecular charge transfer (CT) character. The CT nature of this band is substantiated by its broadness and its sensitivity to the nature of solvent used and is confirmed by its spectral behaviour in buffer solution of different pH values, The observed large red shift in alkaline solutions compared to acidic ones indicates that π– electrons of the N=N group is influenced by an intramolecular CT involving the lone pair of tertiary nitfogen

Except for the p-NO derivatives, the compounds displayed only a single main CT band

as fairly intense in polar and non polar solvents. The appearance of this band at shorter wavelengths than that expected for azo compounds ( Mahmoud et al 1983 ), is due to the existance of the compounds in the hydrazone form in solution which having a limited electronic path. Further, since the spectra of true azo compounds in EtOH are characterized by low excitation energy n-it band (— 400 nm) ( Pilpenko et al 1978 ), the absence of this band in the spectra of the compounds studied confirms the presence of these compounds in true hydrazone form. This assignment agrees with that previously reported for structurally related compounds ( Morsi et al 1982 ) further factor in the stabilization of the hydrazone structure could arise from the lability of the hydrogen atom of the cyanomethylene group

( Abed et al 1986 ) , which tend to rearrange giving rise to hydrazone form. This form would be stabilized through hydrogen bonding which undoubtedly enhance its relative structure as given below:

X= -H compounds(a), = CH compounds (b(

Other possible hydrogen bonds in the arylhydrazone cyanomethyl benzimidazoles can be established between the cyano nitrogen atom and the N-H nitrogen of imidazole moiety as represented schematically as follows:

In the visible spectra of p-NO₂ compounds in ethanol ,the main intramolecular CT band is by a broad shoulder at longer wavelengths (c.f .Table 1, Fig 1 ) . This shoulder has a similar feature to the main CT band. This behaviour can be explained on the basis that p-NO₂ compounds exist in an azo-hydrazone tautomeric equilibrium in solution. Such an equilibrium can be represented as follows:

Accordingly, it is possible to assign the longer wavelength visible band to a CT interaction within the azo form due to the increased conjugation in this form. The shorter wavelength is attributed to CT transition within the hydrazone form, in which the electron delocalization is somewhat restricted relative to the true azo form olcompounds. This is reasonable, since the longer conjugate double bonds in the azo form tend to facilitate the CT transition i.e.lower energy is required in azo form relative to the hydrazone form

Based on the fact that the imidazole ring is characterized by a low basicity, one can expect that the benzimidazole moiety acts as an electron acceptor and the hydrazone N-I-I nitrogen or X-phenyl moiety act as electron donor for the CT. The high acidic character of the

imidazole N-H (pk_b - 2.3 ) ( Mahmoud et al 1984 ) ,can be consider as further evidence for the electron withdrawing character of the imidazole ring. On the other hand the withdrawing character of the cyano group can be directed the CT from X-phenyl moiety to the C ≡ N group as an acceptor center as suggested previously ( Mataga et al 1971 ), Accordingly the intramolecular CT can be represented schematically as given below.

The data in Tables 1,2 clearly indicate that the intramolecular CT exhibits a slight blue shift ifl its Xmax when the substituent (X) is an electron donor (compounds X= -H, o-OH, p-OH and rn-OH, p-COOH) compared to in case of(X) is an electron acceptor (compound X p NO₂ This behaviour can be interpreted due to the influence of the high electron withdrawing character of p-NO₂ group, it is expected that a strong intramolecular H-bond existing between the hydrazo hydrogen atom and the tertiary nitrogen atom of imidazole ring or cyano nitrogen as represented schematically in structure (a) and (b). Such interaction leaves a residual negative charge on the hydrazo nitrogei and positive charge on imidazole or cyano nitrogen atom. Thus, these compounds can be behaved as donor- acceptor conjugated system where the CT takes place between these two centers. Accordingly the electron , withdrawing power of p-NO₂ will results in an easier CT within H-bond hydrazo form ( Matga et al 1971 ) , i.e. low excitation energy is required.

(B)Effect of the Nature of the Organic Solvent :

The electronic absorption spectra of the title compounds are investigated in DMSO, DMF, MeOH, EtOH, CHCI₃ and CCI₄solvents λ _max and. c values obtained are given in Table 1 . The data clearly reveal that the UV bands are slightly affected by changing the polarity of the solvent. This confirm the local excitation nature, i.e. the π- π^* character of these bands.

The n-π^* band appeared as a shoulder at λ _max =280-312 nm in almost the spectra of all compounds in polar solvents, while in non polar solvents it is well defined. The low molar

absorptivity and the red shift of the band when the solvent polarity is decreased supports its n-π^* nature, since in polar and hydroxylic solvents, the n-electrons are expected to be blocked by the solvent H⁺ ions through intermolecular H-bonds and consequently the excitation of n-electrons is difficult i.e. n. band will appear as an ill defined band in polar solvents.

The most interesting effects of changing the solvent are observed with the main intramolecular CT band. In general, this band exhibits an appreciable red shift in its λ _max on

changing the solvent from EtOH to DMF or DMSO. This behaviour indicate that the excitestate of these compounds are more polar than their ground state nd thus, this red shift can be ascribed to stabilization of the polar excited state as the polarity of the solvent increased i.e., lower excitation energy is required in DMSO or DMF relative to in EtOH. This substantiate the CT character of this band. On the other hand, although CHCI₃ and CCI₄ are characterized by smaller dielectric constant than EtOH, the CT band shows a small blue shift in its λ _max in EtOH compared to that in CHCI₃ or CCI₄ This shift can be explained on the basis of the amphiprotic character of EtOH, i.e. the alcohol molecules can behave as proton es,d donor or acceptor toward the hydrazo NH-N group through H-bonding. These two types of bonding would compensate each other,indicating that the small blue shift observed in alcohol is the net effect of increased solvent polarity. Moreover, CHCI₃ and CCI₄ can associate only with the proton of the hydrazo NH group through a weak intermolecular

H-bond. This type el of bonding facilitates the intramolecular CT and thus, leading to the observed red shift of the CT band in CHCI₃ and CCI₄ compared to in alcohol.

The low excitation energy of intermolecular charge transfer (CT) band observed in the visible spectra of compound I_a (X= H ), II_a,(X= o –OH ) and pNO₂ derivatives (V_a and V_b) exhibits a red shift in its λ _max in DMF or DMSO relative to in EtOH mean while, it disappears in CHCI₃ or CCI₄. This red shift as well as its disappearance in non polar solvents can mainly

attributed to the low ionization potential of DMF (9. 12 ev) compared to that of EtOF{(10.49

ev), CHCI (11.42 ev), and CCI₄ (11.47 ev) ( Weast 1978 ). This results in an easier electron transfer from DMF molecule to the antibonding σ^* molecular orbital of N-H bond of hydrazo group.

It appears from the data in Tables 1,2 that in case of p-NO₂ derivatives the intermolecular charge transfer (CT) band in DMF shows a high red shift (λ _max =558 nm) and its molar extinction coefficient is getting very high (ε_max =36.O) relative to the same band in compound L_a (X=H, λ_max = 460 nm).This can be attributed to the expected weakening of the N-Hσ bond under the high -I (Inductive effect) of NO₂ group attached to the phenyl moiety. Thus, low excitation energy is required for electron transfer from DMF molecule to the antibonding orbital of N-H bond of the hydrazo group.

The possibility of the formation of a H-bonded solvated molecular complex between the molecules of p-NO₂ derivatives and proton acceptor solvents (DMSO and DMF) molecules was investigated by studying the visible spectral behaviour of the compounds in polar- nonpolar mixed organic solvents. Thus, a set of absorption spectra was measured taking the three component system viz., the solute-proton acceptor solvent (EtOH, DMF, DMSO) and

( CHCI₃ or CCI₄ ) .The visible spectra of the compounds in pure non-polar solvent display

only one intramolecular CT band (Fig 2 Table 2). On adding DMF in solution mixture, a new band is developed at a longer wavelength, which can be ascribed to the gradual formation of

the complex species through intermolecular H-bonds. The absorbance of the new band increases as the molarity of DMF is increased as a result of increasing in the solvation energy

in the excited state, as well as H-bond strength on addition of polar solvent ( Bayless et al 1964) .A fine isosbestic point is observed in the spectrum of each molecular complex spectrum, which clearly indicates 1:1, solute-solvent H-bond formation.The interaction of DMF molecules with proton of hydrazo group leaves a residual negative charge on the nitrogen atom of H-N hydrazo group. Accordingly the CT is enhanced within the solute molecule, leading to the observed red shift in the proton acceptor solvent, DMF.

A support for the H-bonded formation could be derived from the plot of the absorbance values of the intermolecular CT band against the molarity or mole fraction of the polar solvent which exhibits a typical dissociation curve or non linear relationship, respectively. This can be considered a convincing evidence for the formation of H-bonding solvated molecular complex.

The effect of dielectric constant (D) of the medium is investigated by plotting the absorbance of the H – banded band as a function of (D) for all systems studied . The non – line relations obtained indicate that, the spectral changes are governed by other factor besides the dielectric constant of the medium. These factors include specific solute-solvent interaction H- bond formation which leads to the formation of solvated molecular complex.

The formation constant values (K_f )of the solute -solvent molecular complex is determined from the absorbance with increasing the polar solvent (DMF) concentration at a given λ_max as

described previously ( Issa 1971 ) .

Further support for H-bond formation can be obtained from the data of heat formation (ΔG) of the solvated molecular complex which calculated using the relation ΔG = - RT In K_f The mean values of K_f and ΔG for the investigated solvated complex are reported in Table 3 . The data indicate that K_f values of p-NO₂ derivatives are high (6.92-9.38). This is in accordance with the expected weakening of the hydrazo N-H bond under the effect of the strong electron withdtawing character of 1 attached to the phenyl moiety. This reflects itself in an easier electron transfer from DMF molecule to the antibonding orbital of the N-H bond, i.e. high stability of the formed complex.

Moreover, the results suggest that molecular complex is formed through a weak intermolecular H-bond and has the stoichiometric ratio 1:1 solute: solvent. The heat of formation for the different solute-DMF systems amounts to (1.2-1.93 k.cal/mole). This low values confirming that the molecular complex under investigation are weak in nature, where this ΔG values lies within the range for energy of H-bonding.

(D) Spectra in buffer solution :

The electronic absorption spectra of the studied compounds have been recorded in

aqueous buffer solutions of varying pH’s ranging from 1.87-13.1, each containing 20% (v/v)

EtOH to insure the complete solubility of the compounds in each media. The recorded

spectra are shown in Figs 3 (as representative ) .

Generally, it is evident that the recorded spectra of the all compounds in the pH range

1.87 – 5.28 , show mainly one band in the visible region (390-415)nm). This band represents the absorption of the protonated form of these compounds. Such protonation takes place at the tertiary nitrogen atom of imidazole ring. Increasing the pH of the medium results in decreasing in its intensity, where in solutions of pH’s >5.0 the band attains a more or less constant λ_max value. This indicate that protonation leads to an increase of the acceptor character of these compounds and hence, it is expected an increase in CT interaction within the protonated form of the compound. As the pH values of the medium is increased to>5.0, the withdrawing power of the acceptor center is decreased as a result of deprotonation of the

tertiary nitrogen atom and consequently decreases in the CT band intensity is observed or it display a blue shift in its λ_max (compound IV_a ). This behaviour can be considered as evidence for the assignment of this band to CT transition as given before.

Except for compound I_a , the recorded spectra show a a clear isosbestic point in the pH range 1.87-5.28, which indicates the presence of cation-neutral equilibrium as represented below:

Moreover, the protonation of the imidazole tertiary nitrogen atom in high acidic media leads to disappearance of the n-π^* This is due the blocking of the lone pair of tertiary nitrogen atom and thus the excitation of these electrons is expected to be difficult in the protonated solute. On increasing the pH, these lone pairs become free and consequently their excitation to the antibonding π^* orbital become easier. The above behaviour is further prove for the n-π^* nature of this band as assigned before.

In solutions of pH ranging from 5.28-8.1, the visible spectra of the compounds comprise one main band with λ_max ranging from 425 nm to 510 nm which represents the spectra of neutral form. The intensity of this band decreases by increasing the pH of the medium and mean while a new band is developed at longer wavelengths, where its intensity increases by increasing the pH value.

In higher pH media (pH >10.5), the λ_max of the new band reaches a more or less constant value where the band located at shorter wavelength disappears. The observed behaviour of the band located at 425- 510 nm region can be attributed to the increase in the CT interaction within the solute molecule as a result of ionization of N-H group of the imidazole ring (compounds I_a -V_a) as well as the ionization of the substituent (X) attached to the phenyl ring. This reflects itself in facilitation of CT interaction as a result of increase of participation of n-electrons of imidazole nitrogen and increase of the mesomeric interactions of ionized (X) with the rest of solute and thus, a red shift is observed in this band as the acidity of the medium is decreased. This behaviour can considered as further convincing evidence for the CT nature of the visible band.

Except for compound I. (X= -H) ,the recorded spectra of the compounds II_a - V_a display two clear isosbestic points in the wavelength regions 325-360 nm and 390-469 urn within the pH range 4.3-9.6 and 8.3-12.7 respectively. This represents two equilibria in solutions, the first one is established between the neutral form and monoanionic form formed from the ionization of substituent (X) while the second one is between the monoanionic and dianionic form resulted from the ionization of N-H imidazole moiety.

In case of arylhydrazo methyl benzimida.zole (l_b -V_b), only one fine isosbestic point is observed in alkaline solutions, which represents an equilibrium between neutral and ionic form of these compounds.

The CT band in the spectra of the alkaline solution of compounds IV_b and V_b is broad with two main maxima. This behaviour indicates that those two compounds exist in an azo hydrazone tautomeric equilibrium in solution. Therefore, the shorter wavelength band (390- 410 nm) corresponds to CT within the hydrazone form, whereas that at 440-505 nm is due to CT transition within the azo form. Support for these assignment is the presence of a fine isosbestic point between the two forms which confirmed the presence of the suggested tautomeric equilibrium.

(E) Determination of Basicity and Acidity Constants :

The absorbance- pH curves at the selected wavelengths within the low pH range as well as the high pH range are typical dissociation curves of S-shape. This denotes an acid-base character of the equilibria established within the pH ranges studied. A least square refinement

of the pK values obtained by applyiqg the half-wave height ( Issa 1971 ), the limiting absorbance and the modified Colleter (Colleter 1960 ), methods is made and are recorded in Table 3 .

In the high of the obtained pK values, the following conclusion can be drawn:

a) Comparing the values of basicity constants of the tertiary nitrogen atom belonging to the

benzimida.z ring of the studied compounds (pK_b, =3.62-5.15), with their analogous

imidazole ( Albert et al 1962 ) (pK_b =7.03, 25 °C), it is obvious that the pKb values of

the studied compounds is lower than those of imidazoles This is in accordance with the

previous observation that the fusion of benzene ring to the imidazole moiety reduces the

basicity of pyridine-like nitrogen as a result of increase in the resonance interaction

involving the lone pair electrons of the nitrogen atom of imidazole ring . This behaviour

further proves the electron withdrawing character of the benzimidazole moiety of the

investigated compounds.

b)The data show that the pK_b values of the azo bezimidazoles are lower than that of

unsubstituented benzimidazole (5.48) ( Person et al 1981 ). This indicates that the studied

compound are more acidic than the parents compound . This can be attributed to

stabilization of the compound by delocalization of charge over the whole molecule

( Abed et al 1986 ). The charge is also affected by the arylazo moiety in these molecules.

c) The acidity constant pK_a2 values of the hydrazo N-H are of high values. This is due to

possible interaction of the hydrogen atom of the hydrazo group and the nitrogen atom of

cyano group through intramolecular H-bond. This results in difficult ionization of N-H

proton .

d) As expected the electron donating power of the substituent on the aryl moiety increases the

bond strength of N-H and hinders its ionization (i.e.higher pK_a2 values). The effect of

substituent is more apparent in casey of p-NO derivatives. The low pK_NH values (high

ionization constant) in p-NO derivatives(V_a and V_b ) can be attributed to the antagonizing

character of NO group on the CT interaction that takes place in solute molecule. The

strong - I effect of the NO group results in decrease the electron density on the β –nitrogen

atom of hydra.zo group. This leading to destabIlIzation of the N-H bond and subsequently

pK decreases as represented below:

e) The acidity constant of the rn-OH group (pK_a1 ) of the compounds IV_a and IV_b (X=m- OH;

p-COOH) are of higher values than that of other compounds. This can be attributed to the

ease of ionization of a proton from COOH group at lower pH values (~ 4.0) forming

anionic species .

References

Albert A., and Serjeant E. P., “Ionization constants of Acids and Bases” .J Wiley and-sons

Inc., New York , 1962 .

Abed N. M., Nashed B. , Fahmy H.M. , and Abdel Azzem M. ,Monatsh. Chem.,117, 1986 .

Britton H.T.S., “Hydrogen ions” 4 Ed. Chapmann and Hall London ,I952 .

Bayless N. S., and Macrase E. G., J .Phs. Chem., 56 1002 , 1964 .

Colleter J.C, Chem. (Paris), 5, 415 ,1960 .

Douheret G., Bull: Soc. Chim. Fr. 1413 , 1967 .

EL-Nagdi M. H., EL-Moghayar M. R. H., Hammam A.G., and Khallaf S.A , J Heterocyclic

Vogel, A.L.” A text book of practical organic chemistry “, 3 rd Ed. Longmans.

London , 1961 .

Issa R. M., Z . Physik. Chem. NF, 74, 1971 ; Egypt. J. Chem., l4,113 , 1971 .

Issa R., M.,; Ghoniem M. M., Idriss K. A., and Harfoush A. A., Z Phys. Chem.,94, 135,1975.

Mahmoud M. R., Ibrahim SA., and Hamed M. M, SpectrochimActa, 39 A ,927 , 1983 .

Morsi M. A., Fahmy H. M., EL-Nagdi M. H, and Abdel Azzem M., J . Electronal. Chern

133 , 269 , 1982 .

Mahmoud M.R , .Hamed A .M, and Ibrahim S. A., Phys. Chem. Leipzig, 265, 2, 302 ,1984 .

Mataga M., and Kubota T., “Molecular interaction and electronic spectra” Marcel Dekker

Inc.,New York , 1971.

Novo.Mosquera M. and Rodriguez prieto F. J.Chem. Soc. Faraday Trans., 89, 885, 1993 .

Chem., 16, 1541 ,1979 .

Preston P. N., “Benzimidazoles and congeneric tricyclic compounds “, part 1, Wiley,

New_ York, 64 , 1981 .

Person P. N., Smith D. M., and tenant C., “benzimidazoles and congeneric tricyclic

Compounds “John Wifely & Sons, mc, New York, .63 , 1981

.451 , 1978 Pilpenko, A. T. and Savransky L.L.:Talanta, 25,

Robets Y.Y.: Met.Ions Genet Inform. Trans., 3,273 , 1981 .

Schams H. Z., Khalifa F. A., Barsum B. N., and Naoum M. M , Bull. Soc .Chim.

Fr.N.1,119, 1989 .

Srivastava R.S, lnorg. Chim, Acta 46, L 43 , 1980.

Srivaastava S., and Dogra S., md. J. Chem.,284, 11 , 1989 .

Weast C. R., “Hand Book Chemistry and Physics” ,55^th Ed., E74-CRS press , Cleveland

Ohio 1978 .

Technical College - Bourgas,