Indian Journal of ChemistryVol. 27A. December 1988, pp. 11.00-1101

Ternary Complexes of Copperfll) InvolvingBarbituric Acid and Some of Its Derivatives•

ROHITASHVA M TRIPATHI, (Mrs) RANJANA GHOSE &ANIMESH K GHOSE*'

Department of Chemistry, Faculty of Science, Banaras HinduUniversity, Varanasi 221 005

Received 15 December 1987; revised and accepted21 March 1988

Stability constants of ternary complexes of Cu(II) with 2,2'-bipyridyl as primary ligand and barbituric acid or its 5,5-sub-stituted derivatives as secondary ligand in 50% (v/v) aq. etha-nol have been determined pH-metrically at IA = 0.1 M(NaNO,) and temp = 30° ± 1°C. The equilibrium constants ofternary species follow the acidity order of barbiturates.

Barbiturates, which are widely used in medicine',are among the most frequently encountered drugsin the field of toxicology. Moreover, barbituratesare weak acids and can form metal complexes? - 4.The study of stabilities of mixed complexes areimportant in biological systems as many metabolicand toxicological functions are to a large extentdependent upon them.

In the present note the stability constants ofmixed ligand complexes of barbituric acid (BBA),5,5-diethylbarbituric acid (BABL), 5-ethyl-5-phen-ylbarbitunc acid (PHEN BABL), 5,5-diallylbarbit-uric acid (ALLOBABL) and with Cu(IJ) in thepresence of 2,2 -bipyridyl are reported.

All chemicals used were of AR grade and dou-bly distilled, CO2-free water was used for prepar-ing solutions.

The following five mixtures were prepared in50% (v/v) aqueous ethanol medium (total volume50 ml), and titrated individually against standardalkali using Global digital pH meter (DPH 500)with a glass calomel electrode assembly: (i) Nitricacid; (ii) nitric acid+ bipy; (iii) nitric acid + bipy "metal ion; (iv) nitric acid + secondary ligand; and(v) nitric acid + bipy + secondary ligand + metalion. The final concentrations of nitric acid, metalion and bipy or secondary ligand in all these mix-tures were 2.0 x 10- 3 M, 1.0 x 10 - 3 M and1.0 x 10-3 Mrespectively.

The electrode system was calibrated by directtitration with HN03 and the observed pH in 50%(v/v) aq. ethanol medium was corrected for actual[H + 1 following the procedures of Uitert er a1.5•

The calculation of stability constants of proton-liand and mixed ligand complexes were according

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Table 1- Stability Constants of Ternary Complexes in50% (v/v) Aq Ethanol ~ = 0.10 M NaN03;

temp. = 30° ± 1°C[Ionic strength = 0.1 M (NaNO)); temp. = 30° ± 1°C)

Barbituric acid pK I pK 2 log Kcu bipy LParent acid 12.39 4.69 9.405,5-Diethyl 12.84 9.30 9.865,5-Diallyl 12.53 9.20 9.835-Ethyl-5-phenyl 12.29 8.76 9.80

to Martell" and Thompson et aU respectively.In acid medium barbituric acid is present in the

trioxo form, the three O-atoms and the two N-at-oms have lone electron pairs. So the moleculecould act as a good complexing agent.

The higher pKa values of substituted barbituricacids over that of parent compound (vide Table 1)are due to the fact that 5,5'-disubstitution forcesthe substituted compounds to exist in 2,4,6-triketoform.

The titration of a solution containing Cu(ll)-bi-py in equimolar quantities gives one iMlexion inthe lower pH region indicating the formation of a1:1 complex. Turbidity appears during the laterpart of the titration where probably a hydroxocomplex is formed.

Although bipy is relatively weak base, it formschelates with metal ions in which there are presu-mably strong a donor bonds, supplemented bysome moderate degree of Jt bonding. The result-ing complexes have a high order of stability andthe complexes are formed at lower pH. Thus bipybehaves as a suitable primary ligand. Therefore,when 1:1 (M:A) mixtures are taken, obviously bipyoccupies two of the coordination positions aroundthe metal ion leaving the other positions to be oc-cupied by H20 molecules. Interactions with barbi-turates would result in the replacement of H20 byL.

In the titration of 1:1:1 system, Cu(IJ)-bipyBBA, two intlextions are observed. In the lowerpH region the curve for the mixed systems over-laps the curve for the 1:1 Cu(II)-bipy system uptopH - 3.5. It indicates that initially 1:1 Cu(ll)-pri-mary ligand (bipy) complex is formed prior to theaddition of the secondary ligand. Further additionof alkali neutralizes the protons liberated from thesecondary ligand due to ternary complex forma-tion.

The values of the stability constants of the ter-nary complexes studied are given in Table 1.

Inspite of the fact electrometric studies cannotestablish coordination site conclusively, from theliterature reports's", it is presumed that metal ionis bonded through the nitrogen of the barbiturateanion in the ternary complexes.

The stabilities of the ternary complexes followthe increasing order of protonation constant (pK2)of the barbiturates.

The authors are thankful to the Bureau of Po-lice Research & Development, Ministry of HomeAffairs, Government of India, New Delhi for a fel-lowship to one of them (RMT ).

NOTES

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