Determination and evaluation of acid dissociation constants of some novel benzothiazole Schiff bases and their reduced analogs by spectroscopy

The acid dissociation constants (K a ) of eight novel biologically active benzothiazole Schiff bases were determined by UV-vis spectroscopic technique at 25 ± 0.1 °C. The calculated acidity constants, pK a values , were evaluated in structure elucidation and protonation – deprotonation mechanisms. The prototropic tautomerism was also studied.


Introduction
2][13] It has been suggested that certain types of cancers are caused by viruses.The interaction between the metal ion and their ligands with cancer-associated viruses might represent an important route in designing, a new anticancer therapies for tumors that become resistant to the conventional drugs. 14A recent methodology to design novel antiviral therapies is achieved by coordinating a metal ion from an important bio-molecule, such as a zinc finger protein, with the antiviral agent, usually containing sulfur functionalities with good complex forming behavior. 157][18][19] Following our work on the synthesis of some novel benzothiazole Schiff bases we now are reporting on the acid dissociation constants of them to elucidate structure-reactivity relation-ships of these novel compounds. 20

Results and Discussions
There are two series of molecules in this work.The first series consists of four compounds 1 -4 and they are all of them are novel Schiff bases.The second series contains the reduced forms of these novel Schiff bases and they are compound 5 -8 (Table 1).
The first four compounds exhibit proton tautomerism.Therefore, we have concentrated on the proton tautomerism and intramolecular H-bond forming of compounds 1-4.These two phenomena can have pronounced effect on acid-base behaviours as well as structural properties of organic molecules.
The formation of B and C in acidic or basic media seems to be very unlikely due to loss of aromatic property of the phenol ring (Scheme 2).

Acidity constants
Considering the structures of the Schiff bases 1-4 we can deduce that they have two protonation sites the first one is the aza-nitrogen atom of the benzothiazole ring and imino-nitrogen atom which is substituted at 2C position of the benzothiazole ring.These compounds, however, possess one deprotonation site (i.e.OH group of the phenol ring) and they are in enol-imino form (Scheme 2).On the other hand when they are in keto-imino tautomeric form (B) they have two protonation sites (i.e.imino nitrogen atom and ketonic oxygen atom of the phenol ring) and one deprotonation site (i.e.-NH-group of the benzothiazole ring).However, when they are in ketoamine form (C), they have three protonation sites (i.e.aza nitrogen atom =NH of benzothiazole ring, amine group -NH-which is substituted at 2C position of benzothiazole ring and ketonic oxygen atom =O of the phenol ring) and they possess one deprotonation site (i.e.-NH-group which is located at 2C position of the benzothiazole ring) (Scheme 2).

Deprotonation (pK a1 values)
When we arrange the studied Schiff bases and their reduced derivatives in an increasing acidity order for deprotonation process we get the following sequence; Molecule If we take the ionization constant of phenol (i.e.pK a =10) as reference, we can say that the big groups (i.e. S and R) act as ortho substituent on the ionization of Schiff bases and their reduced derivatives (Scheme 4 and 7) respectively.It seems that in molecules 1 -4 both the benzothiazole ring and phenol ring are on the same plane and the through conjugation lead to proton transfer from OH group to N atom of imino group, forming the zwitter ion iminium form (B), followed by a subsequent rearrangement into the keto-amine form (C).Under these conditions, the deprotonation may occur from NH group and rearrangement of anion (D) leads to anion (E).This means that deprotonation mechanism of these four molecule 1 -4 is different from the other molecules 5 -8 (Scheme 3 and Table 2).

ISSN 1551-7012
Page 200 © ARKAT USA, Inc.In molecule 4, the two methyl groups at 5 and 6 position of benzothiazole ring make S group more electron-donating than the other molecules of 1-3 and keeping the proton more firmly make the molecule 4 more acidic (less basic) among the others.In molecule 1, however, there is no methyl group on benzothiazole ring and the S group becomes less electron-donating and consequently molecule 1 becomes less acidic (more basic).
In molecules 5 -8, there is no possibility of thorough conjugation.Therefore formation of keto forms are not feasible and presumably the deprotonation occurs at phenolic OH group.In here also dimethyl groups in molecule 8 were effective by making the R group more electron withdrawing but in opposite direction of molecule 4 as expected.Since there is no thorough conjunction, the only effect of group R is inductive electron donation and making the whole molecule more basic or less acidic.If we think the position of methyl group in molecule 7, the unusual effect of the R group can be expected (Scheme 3).

First protonation (pK a2 values)
When we arrange the molecules with increasing acidity power for the first protonation, we get the following order: If we compare the obtained half protonation values of the studied molecules (Table 3) with the literature pK a values of 1.84 and 4.51 for the aza nitrogen atom protonation of benzothiazole ring 22,23 and 2-aminobenzothiazole 24 molecules respectively , due to similarity of the obtained pK a values of this study we can conclude that the studied molecules were protonated with the same mechanism of these two molecules and that is the aza nitrogen atom protonation of the benzothiazole ring.The reduced derivatives of the corresponding Schiff bases are less basic (i.e.pK a (5,6,7,8) < pK a (1,2,3,4) 5<1; 6<2; 7<3 and 8<4).This is an expected result because there are a through conjugation between the two rings namely benzothiazole and phenol ring in Schiff bases 1 -4 which don't exist in their reduced derivatives 5 -8 (Scheme 4).

Second protonation (pK a3 values)
When we are arrange the studied molecules with increasing acidity power for the second protonation, we get the following order: Molecule: 4 2 3 1 Half protonation (H 1/2 ) 3.10 2.18 1.86 1. 30  increasing acidity The half protonation values of 1.86 and 1.30 for compounds 3 and 1 are comparable with benzothiazole aza nitrogen atom protonation (i.e.1.84) .So obviously the protonation had taken place at aza nitrogen atom of benzothiazole ring of these two compounds.Obviously strong electron -withdrawing effect of protonated imine group makes the ring nitrogen electron deficient.This means that molecules become more acidic.Following the second protonation, a subsequent mezomerization seems to occur to form a more stable carbonium ion (Scheme 5).
By considering the pK a value of 3.10 for molecule 4 which is quite different from the other three molecules 1 -3 which contains one methyl or no methyl group, we can predict that this bigger pK a value of this compound is expected because of inductively electron donating effect of dimethyl groups on benzothiazole ring.
Possible second protonation patterns for molecules 1 and 3.

Experimental Section
General Procedures.The studied compounds (Table 1) were synthesized and the procedures of synthesis are described elsewhere. 20Methanol, ethanol, glycine, KOH, H 2 SO 4 , HCl, CH 3 COOH, CH 3 COONa, NaOH, KH 2 PO 4 , Na 2 CO 3 , NaHCO 3 , phenolphthalein indicator, and standard buffer solutions were from Aldrich and were not purified further.
Apparatus.pH measurements were performed using a glass electrode.pH values of the standard buffer solutions were 1, 7, and 14 they were used in the calibration of the Orion pH/ion analyzer.The Ohaus Advanturer balance; a Scimadzu UV2450 PC UV-vis scanning spectrometer was used for measurements.
Procedure.Acid solutions were prepared with H 2 SO 4 (w/w) (0.0049 to 96 % H 2 SO 4 ) in water. 21he CO 2 -free NaOH solutions were prepared with NaOH pellets (1 to 16.4 mol.dm -3 ) in water. 25uffer solutions were prepared with procedures well described by using Perrin's desciription. 26pectrometry is an ideal method 27 when a substance is not soluble enough for potentiometry or when its pK a value is particularly low or high (e.g., less than 2 or more than 11).The protonation of a weak base can be defined as follows: 21 HX+SH X -+ SH 2 + (1) where SH is the solvent, then the equilibrium constant might be expressed in terms of concentration and activity coefficient: where a= cγ; a= activity constant; γ= activity coefficient; c= concentration: Therefore, the Eq. 2 can be rearranged as follows: where H x is an acidity function.The H o scale is defined such that, for the uncharged primary aniline indicators used, the plot of log I (i.e., log([BH + ]/[B])) against H 0 has unit slope.It was observed from work on bases other than the Hammett type that the slopes of the plots of log I against H o , donated by m, were not always unity.Thus, series of structurally similar bases, like triarylmethanols 28 , primary amides 29,30 and tertiary aromatic amines 31  rearranging the last expression we will produce Eq.5: Where H 1/2 describes the half protonation value.The general procedure applied as follows: a stock solution of the compound under investigation was prepared by dissolving the compound (about 10 to 20 mg) in water or sulfuric acid of known strength (25 mL) in a volumetric flask.Aliquots (1 mL) of this solution were transferred into 10 mL volumetric flask and diluted to the mark with sulfuric acid solutions of various strengths or buffers of various pH.The total mass of solution in each flask was measured, and the mass percent of sulfuric acid in each solution was than calculated knowing the mass of sulfuric acid added and the total mass of the final solution.In the case of buffer solutions, the pH was measured before and after addition of the new solution.The optical density of each solution was then measured in 1 cm cells, against solvent blanks, using a constant temperature cell-holder Scimadzu UV2450 PC UV-Vis.A scanning spectrometer was thermostated at 25 °C (to within ± 0.1 °C).The wavelengths were chosen such that the fully protonated form of the substrate had a much greater or a much smaller extinction coefficient than the neutral form.The analytical wavelengths, the half-protonation values, and the UV absorption maxima for each substrate were depicted in Tables 2 and 4.
Calculations of half-protonation values were carried out as follows; the sigmoid curve of optical density or extinction coefficients at the analytical wavelength (OD,λ) was first obtained (Figure 1).The optical density of the fully protonated molecule (OD ca , optical density of conjugated acid) and the pure free base (OD fb , optical density of free base) at an acidity were then calculated by linear extrapolation of the arms of the curve.Eq. 6 provides the ionization ratio where the OD obs (the observed optical density) was in turn converted into molar extinction ∈ obs using Beers law of OD=∈bc (b=cell width, cm; c= concentration, mol.dm -3 ): The linear plot of log I against Hx or pH, using the values -1.0<log I<1.0, had slope m, yielding half-protonation value as H x 1/2 or pH 1/2 at log I= 0 The pK a values were calculated by using Eq.7 (Figure 2).

Scheme 6 .
Scheme 6. Possible deprotonation, first and the second protonation pathways for the enol and keto forms of studied benzothiazole Schiff bases [1 -4].

7 .
Scheme 7.Possible deprotonation, the first protonation and the second protonation pathways for the amine and imin forms of the studied reduced benzothiazole Schiff bases[5 -8].

Table 1 .
Nomenclature and formulae of the studied molecules[1-8] 32fined individual acidity functions (H R , H A , and H_ ), which have a linear relationship to H o with m values of 2.0, 0.6, and 1.3, respectively.Yates proposed that any acidity function H x would be proportional to H o over the entire acidity range, that is, H x= mH o , with a common point H o =0.32As explained in the previous paragraph plot of log I against H o does not yield the pK a at log I=0, unless it is an Hammett base, but yields the H o at half-protonation value (H o 1/2 ).The general

Table 2 .
The UV Spectral data and the acidity constants, pK a1 values, of compounds 1-8 for a Measured in pH= 7 buffer solution.b Measured in pH=13 buffer solution.c The analitical wavelength for pK a determination.d Half-protonation values.e Correlations for log I as a function of pH graph.

Table 3 .
The UV Spectral data and the acidity constants, pK a2 values, of compounds 1-8 for the Measured in pH=7 buffer solution.bMeasured in pH=1 buffer solution.cTheanalytical wavelength for pK a determination.d Half-protonation value.e Correlation for log I as a function of pH graph.

Table 4 .
The UV Spectral data and acidity constants, pK a3 values, of compounds 1-4 for the a Measured in pH= 1 buffer solution.b Measured in %50 H 2 SO 4 .c The analytical wavelength for pK a determination.d Half-protonation value.e Slopes for log I as a function of pH (or acidity function H o ) graph. 27f Acidity constant value g Correlation for log I as a function of H o graph.