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 L¹ complexes (L¹ = C₂₂H₂₆N₆) the rate of decomposition increases as: Eu(L¹)³⁺ < La(L¹)³⁺ < Gd(L¹)³⁺ < Tb(L¹)³⁺ ≪ Lu(L¹)³⁺. Decomposition of La(L¹)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(L¹)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(L¹)³⁺ complex. This data is useful in the design of kinetically inert macrocyclic Ln(III) complexes for biologically relevant applications.