Thermochimica Acta, 88 (1985) 355-361 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands 355 zyxwvutsrq STABILITY AND THERMODYNAMIC PARAMETERS OF THIOPYRIMIDINES, 2-THIOXANTHINE AND THEIR COMPLEXES WITH Zn(I1) AND Cd(I1) M.P. SANCHEZ-SANCHEZ, J.M. SALAS-PEREGRIN *, M.A. ROMERO-MOLINA and A. MATA-ARJONA Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, 18071 - Granada (Spain) (Received 10 December 1984) ABSTRACT The thermodynamic protonation constants of 4-amino-1,6-dihydro-2-methylthio-5-nitroso- 6-oxo-pyrimidine (MTH), 4-amino-5-nitroso-6-oxo-l,2,3,6,-tetrahydro-2-thiopyrimidine (TANH) and 2-thioxanthine (TXH) in aqueous media at 25, 30,40 and 50 f O.l”C, have been determined potentiometrically by Bjerrum and Robinson’s methods. The ionic strength was maintained constant by using 0.1 M NaNO, as the supporting electrolyte. The stability constants and thermodynamic functions for Zn(I1) and Cd(H) complexes of MTH and TXH, at different ionic strengths and temperatures, have been calculated. INTRODUCTION Interactions between metal ions and pyrimidine and purine derivatives have attracted much attention in recent years [l-7], particulary after the discovery of the anti-neoplastic nature of some platinum(I1) complexes [g-lo]. On the other hand, complexes containing Ru(II), Ru(II1) and Pt(I1) ions show carcinostatic properties [ll-131 and ions such as Zn(I1) and Cd(I1) have interesting importance in most of the biological processes involving enzymatic, reactions [14]. Information about the structures of the metal complexes of pyrimidine and purine derivatives has been obtained by spectroscopic and crystallo- graphic techniques [15,16] and some thermodynamic functions have been reported [17-191. In this paper we account the results concerning the effect of temperature and ionic strength on the stability constants of some Zn(I1) and Cd(I1) complexes of several pyrimidine and purine derivatives, and the variations of acidity constants for these heterocyclic compounds. * To whom correspondence should be addressed. 0040-6031/85/$03.30 0 1985 Elsevier Science Publishers B.V.