Formation and phosphodiesterolytic activity of lanthanide(III) N,N-bis(2-hydroxyethyl)glycine hydroxo complexes

Potentiometric titrations of N,N-bis(2-hydroxyethyl)glycine (bicine) in the presence of Ln(III) cations (Ln=La, Pr, Nd and Eu) in the pH range extended to ca. 9.5 reveal formation of two types of binuclear hydroxo complexes Ln2(bic)2(OH)4 and Ln2(bic)(OH)4 + (bicH=bicine) in addition to previously reported mononuclear mono- and bis-complexes Ln(bic)2+ and Ln(bic)2 +, which predominate at pH below 8. 1H NMR titrations of La(III)–bicine mixtures in D2O show that the complex formation with bicine is slow in the NMR time scale and confirm formation of hydroxide rather than alkoxide complexes in basic solutions. Formation of a different type of hydroxide species under conditions of an excess of metal over ligand is confirmed by studying the absorption spectra of the Nd(III)–bicine system in the hypersensitive region. The binuclear hydroxide complexes are predominant species at pH above 9 and their stabilities increase in the order La < Pr ≈ Nd < Eu. They show fairly high catalytic activity in the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) at room temperature. Comparison of concentration and pH-dependences of the reaction rates with the species distribution diagrams shows that the catalytic hydrolysis of BNPP proceeds via a Michaelis–Menten type mechanism, which involves the Ln2(bic)(OH)4 + complex as the reactive species. The values of the catalytic rate constants and the Michaelis constants are in the range 0.002–0.004 s−1 and 0.35–1.5 mM, respectively, for all lanthanides studied. The half-life for the hydrolysis of BNPP is reduced from 2000 years to ca. 10 min at 25 °C and pH 9.2 in the presence of 5 mM La(III) and 2.5 mM bicine.