Syntheses of selected mono- and di-esters of phosphoric acid and stability constants of their copper (II) complexes
A number of different routes were attempted to the syntheses of organic phosphate models (R₁O)(R₂O)PO; where R₁ = 4-nitrophenyl group and R₂ = benzyl, 2-pyridylmethyl, 1-naphthylor 8-quinolyl group. However, preparative difficulties and side reactions, due mainly to the susceptibility of the 4-nitrophenoxy moiety to nucleophilic displacement reactions, resulted in simplification of the target models to a pair of monoesters (R₃O)PO₃H₂ and a pair of diesters (R₃O)(R₄O)P(O)O⁻Na⁺ where R₃ = 1-naphthyl group (monoester (A) or diester (C)) or 8-quinolyl group (monoester (B) or diester (D)) and R₄ = methyl group. Such a set of substrates allowed comparison of the reactivities of phosphate monoesters and diesters and as the components of each pair differed only by the presence or absence of a tertiary amine function in one of the ester linkages, the effect on the reactivity of such a compound, due to the presence of a heterocyclic atom, could be gauged. Complex formation between ligands (A), (B), (C) and (D) and copper(II) ions in aqueous solution at 25° C and I = 0,15 mol dm⁻³ (Na)[Cl⁻] was investigated by means of glass electrode potentiometry and computational analysis. Mononuclear and hydroxo complexes were found to occur with ligands (A), (B) and (D). No complexation was detected with ligand (C). The copper(II) ion was found to be only singly coordinated to ligands (A) and (D) through a terminal phosphate group and the quinolyl nitrogen respectively,while with ligand (B) a chelate structure was formed involving the quinolyl nitrogen and a terminal phosphate group to form a seven-membered ring. Formation constants are given for two protonation and four copper(II) complexes of ligand (B) and for one protonation and four copper(II) complexes, each, of ligands (A) and (D).
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