Reactivity of manganese, technetium and rhenium aquacarbonyl-complexes as potential precursors for radiopharmaceuticals

Technetium is an artificial element and its isotope 99mTc is now used for several years as radio-imaging agent. Much research effort is directed towards enhanced targeting of the imaging agents in order to improve accuracy of the diagnosis and reduce the radiation doses delivered to the patient. Technetium(I) complexes are promising in this field and especially those with the fac-[(CO)3Tc]+ entity. Rhenium which is closely related to technetium, is used as model compound, but has also its own radiopharmaceutical potential with the isotopes 186Re and 188Re. The preparative chemistry of 99mTc has to be performed in water. Therefore, it is of importance to know its actual form in solution and understand its reactivity. Thus the goal of the present work was to investigate the reactivity of fac- [(CO)3Mn(H2O)3]+, fac-[(CO)3Tc(H2O)3]+, fac-[(CO)3Re(H2O)3]+ et fac- [(CO)2(NO)Re(H2O)3]2+ by multinuclear magnetic resonance. The water exchange rates on these complexes have been determined as a function of acidity and for tow of them (Mn and Tc) as a function of temperature. The water exchange rate decreases when going from manganese to technetium and then to rhenium. In the latter case it was found that the replacement of a CO ligand by an NO+ ligand decreases again the lability of the water molecules. For the two rhenium complexes an increase of the water exchange rate with decreasing acidity has been observed. Water molecules on fac-[(CO)3Mn(H2O)3]+, fac-[(CO)3Tc(H2O)3]+, fac- [(CO)3Re(H2O)3]+ complexes are easily substituted. This reaction has been investigated with selected ligands of various types. The complexes show a stronger affinity for soft ligands such as dimethylsulfide than for harder ligands like acetonitrile. Variable pressure experiments for the ligand substitution on fac- [(CO)3Re(H2O)3]+ provided activation volumes. The substitution by the soft ligand was found to be associatively activated whereas the substitution by hard ligands is dissociatively activated. Although activation volumes could not be determined for the substitution reaction on fac-[(CO)3Mn(H2O)3]+ and fac-[(CO)3Tc(H2O)3]+ their similar reactivity suggests that they exhibit the same mechanistic behaviour as fac- [(CO)3Re(H2O)3]+.