STABILITIES OF TRIVALENT METAL-ION COMPLEXES OF THE TETRAACETATE DERIVATIVES OF 12-MEMBERED, 13-MEMBERED AND 14-MEMBERED TETRAAZAMACROCYCLES

The stabilities of metal complexes of the trivalent metal ions La(III), Gd(III), Fe(III), Ga(III) and In(III) with the tetraazamacrocyclic tetraacetates, 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA), 1,4,7,10-tetraazacyclotridecane-N,N',N'',N'''-tetraacetic acid (TRITA) and 1,4,8,11-tetraazacyclotetradecane-N,N',N'',N'''-tetraacetic acid (TETA), have been determined in KCl electrolyte (I = 0.100 M, 25.0 (n)C) by potentiometric p[H] methods and by spectrophotometric determination of exchange with a competing ligand. All of the trivalent metal ion complexes of DOTA, TRITA and TETA form precipitates at neutral and higher p[H] but are soluble in acidic solutions. The log stability constants (log K(ML) = [ML]/[L4-][M3+]) of the La(III), Gd(III), Ga(III) and In(III) complexes of DOTA (L) (log K(LaL) = 21.7; log K(GeL) = 24.0; log K(GaL) = 21.33; log K(Inl) = 23.9) are low in comparison to that of Fe(III)-DOTA (log K(FeL) = 29.4). The stabilities of the trivalent metal complexes with TRITA and TETA are lower than those of the DOTA complexes. In(III) has slightly higher affinities for these ligands than does Ga(III), while the affinity of Gd(III) is considerably higher than that of La(III).