Coordination abilities of N-methyl alkylaminomethane-1,1-diphosphonic acids towards zinc(II), magnesium(II) and calcium(II) metal ions. Equilibrium studies in aqueous solutions

Abstract

Complex-formation abilities of a series of N-methyl alkylaminomethane-1,1-diphosphonic acids (1–7) with a common tertiary nitrogen atom (CH3−N−R) bearing linear or branched alkyl, cycloheptyl or phenylalkyl R substituents towards zinc(II), magnesium(II) and calcium(II) in aqueous solution have been studied by means of pH-potentiometry, ESI-MS spectrometry, 1H NMR and 31P NMR methods. The obtained results indicate that in the all presented systems mononuclear protonated equimolar and bis complexes are formed. In acidic and neutral solution, the formation of the protonated dinuclear species has been confirmed. The comparative analysis of the potentiometric and NMR data reveal that the nitrogen atom of (CH3)−NH+−R group remains protonated in the whole pH range studied and the Zn(II), Mg(II) and Ca(II) ions are coordinated exclusively through oxygen atoms of the phosphonate groups in all detected complex species.