Lanthanide(III) hexaaza macrocyclic Schiff-base complexes and their decomposition in aqueous solution

The kinetics of decomposition of several lanthanide(III) hexaaza Schiff-base macrocyclic complexes is examined in aqueous solution. For a given macrocycle, decomposition of the Schiff-base into component amine and carbonyl compound is dependent on which lanthanide ion is used. For the L-1 complexes (L-1 = C22H26N6) the rate of decomposition increases as: Eu(L-1)(3+) < La(L-1)(3+) < Gd(L-1)(3+) < Tb(L-1)(3+) << Lu(L-1)(3+). Decomposition of La(L-1)(3+) is accelerated 78-fold in the presence of a 10-fold excess of ethylenediaminetetraacetic acid and 10-fold in the presence of a 10-fold excess of diethylenetriaminepentaacetic acid (DTPA). In contrast Eu(L-1)(3+) decomposes twofold more rapidly in the presence of a 10-fold excess of DTPA. Other La(III) Schiff-base macrocycles which feature larger chelate rings, more rigid macrocyclic backbones and removal of two of the imine linkages all decompose more rapidly than does the parent La(L-1)(3+) complex. This data is useful in the design of kinetically inert macrocyclic Ln(III) complexes for biologically relevant applications. (C) 2001 Elsevier Science B.V. All rights reserved.