Complexes of Uracil (2,4-Dihydroxypyrimidine) DerivativesPart III. pH-Metric, ISE, and Spectrophotometric Studies on Co(II), Ni(II), and Zn(II) Complexes

Equilibrium studies have been carried out on complex formation of M2+ ions (M=Co, Ni, and Zn) with L = thymine, 6-chloromethyluracil, 5-hydroxymethyluracil, uracil, 6-methyluracil, and 6-umpm (dimethyl 6-uracilmethylphosphonate) in aqueous solution, at 25 ∘;C and an ionic strength of I=0.1 mol⋅L−1 (KNO3). Potentiometric results indicate the formation of ML species (coordination via N3) for Co(II) and Ni(II) as well as a hydroxo complex MLH−1—with a deprotonated water of the inner coordination sphere. Titrations of Zn(II) confirmed both the existence of ML and MLH−1 species with 6-chloromethyluracil as well as 5-hydroxymethyluracil, whereas for uracil, thymine, and 6-methyluracil the only species accepted pH-metrically was MLH−1. For all the ligands under study the complexation with Zn(II) was reinvestigated by means of ion-selective electrodes (ISE). The role of substituents is discussed.