Complex Formation of Nickel(II) and Copper(II) with Barbituric Acid

Equilibrium studies have been carried out on complex formation of M2+ ions (M？=？Ni and Cu) with L？=？barbituric acid (BA) in aqueous solution at °C and with an ionic strength of ？M (KNO3) in aqueous medium. The basicity of the ligand was also assessed by the determination of the dissociation constants of the ligand. The experimental pH titration data were analyzed with the help of the BEST computer program in order to evaluate the stability constants of the various species formed. The stability constants of the binary systems decrease in the order of Cu(II)？>？Ni(II). Distribution diagrams for the species were drawn showing the concentrations of individual species as a function of pH by the SPE software program. 1. Introduction Organotransition-metal complexes formed with biologically active ligands have attracted great attention in recent years. Studies of such complexes in biological systems can lead to a better understanding of the roles of these ligands and can also contribute to the development of metal-based chemotherapeutic agents. Pyrimidine ring-containing compounds can be found in nucleic acids, various vitamins, and coenzymes and they play an important role in many biological systems [1, 2]. In nucleic acids, they are related to anticancer chemotherapeutic antimetabolites [3]. Pyrimidine metal complexes have been studied in recent years because of their great variety of biological activities such as antimalarial, antibacterial, antitumoral, antiviral activities, and so forth [4–10]. Despite the multitude of coordination complexes of pyrimidines, the organometallic chemistry of these ligands received little attention, mostly from Beck and coworkers [11, 12]. Barbiturates are the derivatives of barbituric acid (2,4,6-trioxypyrimidine) (Scheme 1). These drugs are used for many reasons such as hypnotics, sedatives, or anesthetics [13, 14]. They also affect the motor and sensory functions, and they are used as cures for anxiety, epilepsy, and other psychiatric disorders [15, 16]. The production process of plastics and pharmaceuticals requires the uses of barbituric acid. Introduced as one of the first medical use of barbiturates, barbital diethylbarbituric acid is known as veronal, or diemal [17]. The first psychologically active drug, barbital or veronal was introduced in 1903 by E. Fischer [18]. Only a few barbiturates have anticonvulsant properties, although many of them have sedative-hypnotic attributes. However, most barbiturates cause convulsions at large doses. Phenobarbital (5 ethyl-5 phenyl barbituric acid) is used for the treatment of