Studies on Chelates of 5-oximino barbituric acid(L₁) and

4-amino 2,6 –dihydroxy pyrimidine (L₂) with Cr(III) ,

 Mo(VI) , UO₂ (II) , Th(IV), La(III) and Ce(III) .

 

Nndia. A. Abdalla and Alla A .M . Gad

Chemistry Department , Faclty of Science , Aswan , Egypt .

 

 

 ABSTRACT

              Chelates of 5- oximino barbituric acid ( V.A ) and 4- amino 2,6 –dihydroxy pyrimidine  ( ADHP) with Cr(III) , Mo(VI) , UO₂ (II) , Th(IV), La(III) and Ce(III) have been  Characterized on the basis of conductance ,

 pH-titration .The stabitity constants of the chelates have been determined at

 1.0 mol dm^-3( NaNO₃ ) . The species distribution diagrams of the metal complex species are discussed .

 

 

 INTRODUCTION

 

          The pyrimjdine ring systems are importants as structural units in natural products and compounds of pharmaceutical interest. The chemical and physical natures of these molecules have been exlensively studied  ( Hawkins et al  2002 ; Marques  et al 2002 ; Fidel et al  1990 ; Enrique et al 1988 and Katritzky et al 1960 ). They Are also

a well-known class of ligand , capable of forming stable complexes with metal jons (Juan et al  1994 ; Awadallah et al 1988 ; Angustias et al 1990 ;Cartwright et al 1977 , Battistuzzi et al 1978 , Aboul-Kasim et al 1990 and Battistuzzi et al 1979  ) However , studies on their complexation equllibria with heavy metals in oqueous modiam are still scarce .

 

            The work reportiod here concerns detailed pH-metric studies on the interaction of 5-oximino barbituric acid(V.A) and 4- amino 2,6 –dihydroxy pyrimidine (ADHB) with Cr(III) , Mo(VI) , UO₂ (II) , Th(IV), La(III) and Ce(II)

 

             The dissociantion constants of the ligands , stoichiometry and stabillity constants of their complexes are calculated at 25^o C and 1.0 mol dm^-3 (NaNO₃)

 ionic strength, using the computation methods of lriving and Rossotti(lriving et al 1954 ) and the computer analysis if the calculated data by the programme MINIPOT  ( Gaizer et al 1981 ) . The study involves characterization of the chlates by pH-titration and conductance data .

 

 

EXPRIMENTAL

 

              The ligands were prepared by the method previously reportes( Enrique et al 1988 )  1.0 × 10^-2 mol dm^-3  stock solutions of the langads and the metal ion(extrapure)  were prepared by dissolving the needed amount in doubly distilled water . Carbonte - free NaOH (DBH) (0.1 mol dm^-3  ), NaNO₃ and HNO₃ ( E.Merck) solutions were prepared in freshly boiled doubly distilled water  and standarlized bythe usual method( Vogel 1961 )  .The electrolytic  conductance measurements  were made at 30^o C  in absolute ethanol , using kinck conductance bridge and conducting cell of the dipping type

 

Potentiometric titration were performed at 2 5^o C with 0.1 mol dm^-3  NaOH in aqueous medium . The ionic strength of all titrated solutions was adjusted to 1.0 mol dm^-3 NaNO₃  as supporting electrolyte . An pH –meter Model,MV

digital pH –messgerate , accurate  to ±0.005 unit ,was used with combined glass and reference electrodes immersed in the test solution.The analyzer in the pH mode was calibrated before and after each titration using two standadrad  buffers at pH's  6.98  and  4.03 In all titrations purified nitrogen was surged in the solution before and during titration . Multiple titration were carried out for each system . The conversion of pH-meter reading into [ H⁺ ] was done by the early suggested method( Van uitert et al 1953 ) The    pH-metric   titration measurements were made at 30^oC  in doubly distilled water by titrating 30 ml of 1.0×10^-2 mol dm^-3  of ligand with 1.0×10^-3 mol dm^-3

Of each metal ions in 0.5 ml increasments .

 

RESULTS AND DISCUSSION

         

Potentiometric studies

           Figure (1) shows the potentiometric  titration curves for free ligand and complexes for  V.A as representative . it is worthmentioning that the ligands investigates do not undergo hydrolysis under the experimental conditions .This indicated by the rapid attainment equilibrium during the time of titration .

           The titration curves of the free ligand (V.A) shows one inflection at a=1 ( a = number of moles of base added per mole of ligand ) . The (ADHP) shows two  inflection at a = 1 and a = 2. It is expected that heterocyclic nitrogen atom of the pyrimidine since its not protonated under these conditions since its

pk = 2.3 ^{( 17)} .This suggests that the first inflection is due to the dissociation of enolic hydroxy group proton . The second one observed in case of the OH^- group of the pyrimidine moiety .

 

               Examination of the titration curves of the subject ligands in the presence of the studied metal cations (Fig . 1 ) indicates that, the addition of matel ion to the ligand solutions causes a decrease in the pH value . This reveal that , the complexation reaction proceeds by the replacement of protons from such ligands . The titration curves of the complexed ligand exhibit two inflections at  m =1 and 2 ( m = moles of base added per mole of metal ) ; corresponding to formation of ML₁ and ML₂  complex species represented by the following equilibria .

(i) For V.A

 

(ii) For ADHP

The presence of two separate inflection in the titration curves of the complexed ligands , as well as the high values of n^- (~ 1.9) indicate that ML and ML₂ of complex species are formed ( Fig . 2 and 3 ) . The first inflection is observed in the pH range  4.6- 6.0 corresponding to libration of one proton. The second inflection indicate the liberation of two protons on complexation in the pH rang ( 6.8- 8.9 ). In case of ADHP complexes step 3,5 are the most probable and step 2 may be excluded . This is based on the suggestion that hydroxyl group for ADHP is involved in coordination . Morever , the slight lowering in the first buffer region of ADHP ligand between m = 0 and m= 1 support that the hydroxyl groups of ADHP since the lowering of the second buffer region between m= 1 and m= 2 , is more pronounced .

 Based on these results and the fact that these compounds are exist mainly in the enol – keto forms: 

 

 

 The overall equilibrium constants for the free ligands and their metal complexes were carried out using standard procedures based on the calculation

of th average numbr of ligand bounded per metal  ion n^-, the  average numbr of protons associated with Irving the residuals  in the  relation :

 

 

 

 

           The experimentally observed independent variables are n^-, [ H⁺] , and [L] .It chose to omit points having  n  within 0.1 of  0,  0.15 , 1 or  0.11 of 2.0. By applying  the   linear least squares method on eqution (6) , it gives a best linear relations  where  values of equilibrium constants B₁  and  B₂  were obtained from the intercept and slope .

            The numerical method ( Block et al 1953 ) , which solved the Bjerrum's formation of the individual  formation constants for a coordination number  of  2, is also used to calculate B₁  and  B₂  . In all cases there  is good agreement between  the values obtained by the two method . The dissociation and  stability constant values of the ligand  (V.A ) can form either stable six membered ring systems on chelation. Since, the results reveal that the stabilities of ( V.A)  chelates are considerably higher than those of  the  corresponding (ADHB) chelates . It can concluded  that  ADHP complexes are formed through four-four  membered and not six membered ring .  This is clearly evident from the higher pk values for V.A than those for ADHP . The structure suggested for these two complexes are :

 

 

 

 

 

 

 

 

 

 

 

 

The order of  stability constants of the chelates is formed  be

 

Mo⁶⁺ > Cr³⁺  > TH ⁴⁺  > Uo₂²⁺  > La³⁺  > Ce ³⁺

 

The lower stability of la (III) and Ce (III) chelates relative to other  metal chelates  is inagreement  with  the  fact  that Ianthanides  are much less prone complex formation than the actinides ( Cotton et al 1972 ) .This can be attributed to the hydrolytic tendencies  of  the  hydrolytic tendencies of the lanthanide chelates  in  aqueous  solution( Cotton et al 1975 ).   

             The pH-metric titration curves (Fig . 4 as representative ) obtained for the metal ions with ligand ( V . A) show distinct inflections at 1 :1 for  Cr (III)

And UO₂(II) chelates 1:2  for Ce(III) ,while 1:1 and 1:2 chelates for La (III), Th(IV) and Mo(VI) . This confirms the stepwise formation   ML and ML₂ species However , the sharp decrease in the pH values of all complexes suggests the possible libration of protons from the ligand during complexation

Which decreases  the pH of the solution.

               The above  results are also confirmed by condctometric titration .  The sharp increase of the titration curves of V.A and  ADHP with metals confirmed the liberation of hydrogen ions during complexation  . Generally , on plotting the specific conductance values as a function of the volume of each ligand added , the graphs shown in Fig . 5 for V.A as representative are obtained .

The relationships  obtained show  well defined breaks corresponding to the stoichimetric ratios of 1:1 and 1:2  metal to ligand .

 

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