Role of mononuclear rare earth metal complexes in promoting the hydrolysis of p-nitrophenyl phosphate (NPP)

Two long-chain multidentate ligands: 2,9-di-(n-2',5',8'-triazanonyl)-1,10-phenanthroline (L-1) and 2,9-di-(n-4',7',10'- triazaundecyl)-1,10-phenanthroline (L-2) were synthesized. The hydrolytic kinetics of p-nitrophenyl phosphate (NPP) catalyzed by complexes of L-1 and L-2 with La(III) and Gd(III) have been studied in aqueous solution at 298 K, I = 0.10 mol dm(-3) KNO3 at pH 7.5-9.1, respectively. The study shows that the catalytic effect of GdL1 was the best in the four complexes for hydrolysis of NPP. Its k(LnLH-1), k(LnL) and pK(a) are 0.0127 mol(-1) dm(3) s(-1), 0.000022 mol(-1) dm(3) s(-1) and 8.90, respectively. This paper expounds the result from the structure of the ligands and the properties of the metal ions, and deduces the catalysis mechanism.