KINETICALLY AND THERMODYNAMICALLY STABLE ISOMERS OF THORIUM CHELATES OF POLYAZA POLYCARBOXYLIC MACROCYCLES

The solution conformation of the thorium(IV) complexes of two polyaza polycarboxylic macrocycles, DOTA and HEHA 10-tetraazacyclododecane-N,N ',N'',N'''-tetraacetic acid and 1,4,7,10,13,16-hexaazacyclooctadecane- N,N',N'',N''',N'''',N'''''-hexaacetic acid), was investigated by one- and two-dimensional nuclear magnetic resonance spectroscopy. ThHEHA(2 -) forms a kinetically stable topomer of C-2 symmetry and a thermodyna mically stable topomer of S-6 symmetry. Both complexes are assigned an icosahedral geometry. The activation energy for the intermolecular ex change is very high (214 kJ mol(-1)). The behavior of ThHEHA(2-) contr asts with the properties of the other Th(IV) chelates that are known t o be fluxional.