Effects of Concentration of Some Lanthanide(III) Complexes of 1,7‐Bis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane on Bis( p ‐nitrophenyl)phosphate Hydrolysis

A detailed study of the effects of the concentration of some lanthanide(III) complexes [LnDO2A + ; DO2A = 1,7‐bis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane] on the hydrolysis of the phosphodiester bond of the compound BNPP [sodium bis(4‐nitrophenyl)phosphate] is reported at 25 °C and pH 7.90 (Ln = Eu), 9.35 (Ln = Eu), and 9.90 (Ln = Eu, Er and Yb). The reaction is found to be first order with respect to BNPP concentration at pH 7.90 and 9.35 and to increase to 1.78 ± 0.09, 1.64 ± 0.07, and 1.63 ± 0.09 with respect to [EuDO2A + ] concentration at pH 7.90, 9.35, and 9.90, respectively, thereby indicating that dinuclear species are involved in the reactions. Similar observations were found for the other two LnDO2A + complexes at pH 9.90 [1.63 ± 0.11 (Er) and 1.49 ± 0.09 (Yb)]. The k obs data obtained by the initial rate method could be fitted to a monomer‐dimer reaction model for all LnDO2A + complexes at pH 7.90, 9.35, and 9.90 in the [LnDO2A + ] concentration range 1.0–20.0 m M . The dinuclear species, namely the mono‐hydroxo‐bridged complexes [Ln 2 (DO2A) 2 (μ‐OH)(OH)(H 2 O) n = 1–3 ] and the di‐hydroxo‐bridged complexes [Ln 2 (DO2A) 2 (μ‐OH) 2 (H 2 O) n = 0–2 ] are found to be more reactive, with reactivities in the order Eu > Yb > Er. The overall formation constants for [Ln 2 (DO2A) 2 (μ‐OH) 2 (H 2 O) n ]‐BNPP are around 10 4 M –1 . The EuDO2A + ‐promoted BNPP hydrolysis rates are similar to, or up to two orders of magnitude faster than, most of those reported for other lanthanide and transition‐metal complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)